Introduction
1.1
Photochemistry of azobenzene
The azobenzene history comes from the XIX century when William Henry Perkin during his research against the malaria on the synthesis of quinine obtained a coloured solution from the oxidation of the aniline. This dye was the first synthetic one and is known as Perkin’s Mauve from its color. During the past century industry developed azo dyes for their versatility and strong colors. The presence of azo chromophore (-N=N-) and of copla-nar aromatic rings allows good electronic delocalization that is responsible for this property of vivid dyes. Another important step for the history of azobenzene was in 1957 when Teitel [1] studied the effect of polarized light on the dye ’congo red’. In the ’80s Todorov et al. [2] were the first who investigated birefringence and induced dichroism by irradiation with a linear polarized light on films of methyl orange dispersed in polyvinyl alcohol. Interest in azobenzene compounds has rapidly increased and azobenzene containing materials are nowadays a topic in which significant efforsts have been made. This is mainly due to the capacity of azobenzene to isomerize in two different isomers [Fig.1.1] [3], trans and cis, with different absorption spectra [Fig.1.2]. The trans isomer is thermodynamically more stable than the cis isomer. The isomerization from trans to cis takes place with light of proper wavelength, while the back isomerization can be induced either by temperature or by light or could be obtained spontaneously. The trans to cis photoisomerization results in a change in the molecular structure, geometry and dipole moment.
0.90 nm
0.55 nm
λ1 λ2 o Δ
Figure 1.1: Trans and cis isomers of azobenzene molecule.
Figure 1.2: Absorption spectrum of a solution of an azobenzene containing com-pound.
Looking at the absorption spectra of a compound containing azoben-zene, it exhibits two main absorption bands: a π → π∗ transition in the UV region (∼ 360 nm) and an n →π∗ transition in the visible regions (∼ 460 nm) of lesser intensity. The probability of the first absorption is higher in the trans state while the n →π∗ transition is more probable in a cis isomer [4]. e μ polarization vector dipole transition moment e μ θ e μ e μ (a) (b) (c) max
probability intermediate probability probabilityzero
Figure 1.3: Probability of absorption in function of the angle θ between the direc-tion of the dipole transidirec-tion moment µ and the direcdirec-tion of the polarizadirec-tion vector e. The probability is maximum (a), intermediate (b) and zero (c).
To induce optical anisotropy linear polarized light is applied in the green-blue region where absorption takes place for both isomers in the n → π∗ absorption region. The trans isomer dipole corresponding of this transition is parallel to the molecular axis and the cis isomer is almost parallel to the direction of the double bond N=N [5]. When the molecules interact with linearly polarized light in this region of the spectrum, they are selectively excited depending on the angle θ between the direction of the transition dipole moment and the light polarization (more precisely, the probability of absorption is proportional to cos2θ) [Fig.1.3].
Starting from an isotropic distribution of trans isomers, linearly polar-ized light is absorbed mostly by the trans isomers with the molecular axis
parallel to the polarization vector. Molecules absorbing light can isomerize from trans to cis and go back again to trans thermally or by optical excita-tion. During these trans–cis–trans cycles the axes of the molecules reorient themselves until the dipole moment is perpendicular to the light polarization and they do not absorb light anymore. Accordingly, after various cycles of photoisomerization, the azobenzene molecules are aligned preferentially per-pendicular to the polarization of the incident light [Fig.1.4]. The anisotropy of the polarizability of azobenzene molecule leads to the anisotropy of the refraction index or birefringence [6]. This process is known as the Weigert effect [7].
Isotropic state anisotropic state
e
Figure 1.4: Photoinduced birefringence in azobenzene compounds. The irradiated region with linearly polarized light is delimited by the circle.
1.1.1 Mass migration and reliefs
Polymer materials containing azobenzene units exhibit a variety of photome-chanical responses. In particular, matter motion over distances much larger than the molecular size and polymer chain length is photoinduced upon the illumination in the absorption band of the chromophores and gives rise to the formation of surface reliefs [8, 9, 10] [Fig.1.5]. In the mechanism that leads to the surface reliefs formation various phenomena, not fully understood yet, are involved [11]. Forces generated between spatially non-homogeneous electromagnetic field and dipoles of the molecules involved could play an important role. Indeed, these forces could be responsible for the material displacement allowed by viscoelastic properties of the material.
In the literature theories are reported on the free volume variations and on pressure gradients during the trans-cis photoisomerization, [12] but they
Figure 1.5: Images captured by Atomic Force Microscopy on the azobenzene poly-mers [12]
do not explain why the Surface Reliefs Formation SRF depends on the po-larization [13, 14]. However, this writing process for data storage seems less applicable than the purely optical one based on the photoinduced anisotropy. In fact, topographic writing entails mass migration which is a slower and more energy requiring process than the purely optical writing.
1.1.2 Other molecules or systems for optical writing
Organic photochromic compounds are regarded as highly promising mate-rials for applications in photonics and optical storage. The host-guest tochromic materials preparation could involve the dissolution of the pho-tochromic dyes in the polymeric host or processes to chemically link the molecule to the polymer backbone (click chemistry or copolymer synthesis [15, 16, 17, 18]). These molecules transform themselves by the interaction with light from one form to another one which differs in terms of molecular structure and of their absorption spectra (photochromism). Every pho-tochromic molecule has its advantages and disadvantages for application in data storage. For example, spiropyrans, benzopyrans, heterocyclic aryls, fulgides, quinone dyes, etc.[19] are some molecules that are studied in this field. As is shown in Fig.1.6, the ultraviolet radiation on the spiropyran molecule 2a promotes the opening of its pyran ring and the formation of merocyanine 2b [20]. The photoinduced isomer has an extended π-system able to strongly absorb in the visible region and is accompanied by the ap-pearance of colour due to the transformation of 2a into 2b. Once again, like for the azobenzene, after the thermal closing of the pyran ring the coloured species 2b revert to their original colorless form 2a. The diarylethene 3a and fulgide 4a also undergo coloration upon ultraviolet irradiation [21, 22].
Indeed, the photoinduced closing of a six-membered ring at the core of their structures results in the formation of the coloured isomers 3b and 4b, re-spectively. However, these compounds are thermally stable and revert to the original and colorless forms only after the visible irradiation. Under these conditions, the central six-membered ring of 3b and 4b opens back to regenerate 3a and 4a, respectively.
Figure 1.6: Photoinduced and reversible interconversion from a to b form for azobenzene (1), spiropyran (2), diarylethene (3), fulgide (4).[19]
The properties of an excellent material for optical data storage are: - Thermal stability
- High fatigue resistance (optica cycling) - High sensitivity to light and fast rensponse
- Capacity to be read without destroying the recorded information - Facility to erase, preferentially with temperature also with light
Among these properties, thermal stability and fatigue resistance are very important. The most common photoresponsive molecules are shown in Fig.1.6, however none of them fulfill all the above requirements.
1.2
Application of azobenzene
The possibility to induce optical anisotropy in azobenzene containing ma-terials has gained the attention of many research groups all over the word. These materials are studied to develop devices for data storage and in other fields where the interaction between materials and light led to a change of physical-chemical properties (sensors, membranes, etc.). Optical writing is one of the azobenzene materials research fields. Here, our work is focused on the development of new materials able to support the incoming writing technology. Holography is applied since many years to obtain optical writing trying to reach high capacity of storage information in the volume by multi-plexing information pages at different angles. Beyond the interest to develop new materials and technology to write information, the light responsive ma-terials as azobenzene mama-terials are studied in many fields [23]. Azobenzene photoisomerization gives rise to changes of dipole moment, polarity and shape. Thus, azobenzene chromophores open up limitless utilizations in the intelligence materials synthesis, such as liquid crystals with great promise in actuators, sensors, micromachines, artificial muscles, [24, 25, 26, 27], supramolecular helix for photodynamic systems [28] and photochromic lig-ands for cell signaling [29]. Thanks to the property of azobenzene to change its molecular shape by photoisomerization there are some studies to improve molecular motors, Fig1.7 [30]. Moreover, other studies develop surfaces able to change their wettability properties thanks to the azobenzene dipole mo-ment increase from trans to cis isomer (µ ∼5 debye for the cis form vs µ ∼1 debye for the trans isomer) which causes a lowering of surface energy, [31].
Figure 1.7: Schematic of a single-molecule pulling setup and of the DNA-based molecular motor. The molecule is attached by its ends to a surface at the terminal O atom and an AFM tip at the terminal O atom. The distance between the surface and the cantilever (L) is controlled, while the deflection of the cantilever from its equilibrium position measures the instantaneous applied force on the molecule by the cantilever F = -k(ξ - L). Here ξ is the fluctuating molecular end-to-end distance and k the cantilever stiffness. The structure of the photoresponsive DNA that provides the basis for the motor is shown in the right panel. It consists of two guanine-cytosine base pairs connected by an azobenzene linker. Light is used to induce cis-trans isomerization in the azobenzene, thus modifying the molecular elasticity properties [30].
1.3
Polymers containing azobenzene
Polymer systems containing azobenzene can be classified into two main groups: guest-host where azobenzene molecules are dispersed in polymeric matrix and polymers where azobenzene containing groups are covalently linked to the polymer chain.
Guest-host systems were the first systems to be studied. The first study of photoinduced birefringence and dichroism was carried out on films of methyl orange (guest) dispersed into poly vinyl alcohol (host) [3]. Starting from this study, scientists have investigated different matrices and different azobenzene molecules [32, 33]. The polymeric matrix plays an important role to final properties because it influences both the isomerization kinetic and the isomer life time [34, 35]. Despite the advantages of these materials as the facility of preparation, there are restrictions due to aggregate forma-tion, that negatively influences the photoisomerization process. Moreover, in these guest-host systems, one must comply with the low thermal stability of the photoinduced anisotropy. Polymers where azobenzene is a part of the macromolecule can overcome this drawback. According to the architecture of the polymer we can classify polymers in linear, branched, cross-linked and network as shown in Fig.1.8. The side chain azobenzene polymers, that belong to the branched polymers, are the most investigated polymers for optical data storage.
Another classification can be introduced for the morphology and ther-mal properties, especially when dealing with polymers where it is possible to induce anisotropy by light. Here, amorphous and liquid crystalline poly-mers exhibit different behaviours [36]. A liquid crystalline phase (with the isotropic transition above room temperature) can enhance the photoinduced birefringence because the interaction among azobenzene chromophores is higher than in the amorphous phase. Morphology is another relevant fac-tor that increases and stabilizes birefringence. In fact, structured materials with azobenzene moieties confined into micro or nano domains immersed in a rigid matrix [37] behave, in general, significantly better than materials where azobenzene moieties are randomly dispersed. Structured materials could be obtained by the synthesis of block copolymers that are able to self-assemble in different morphologies according to various chemical-physical parameters [38].
LINEAR BRANCHED
CROSS-LINKED NETWORK
Figure 1.8: Schematic classification of polymer architectures.
1.4
Block copolymers
Block copolymers are widely investigated for their capacity to segregate in different architectures. A block copolymer is a polymer that could be thought as two or more different homopolymers connected to each other. It is like a mixture of two or more components but covalently linked at the end of each chain. Block copolymers are able to self-assemble if the strength of repulsive forces between blocks are sufficiently large [38]. For these rea-sons they are widely studied for numerous applications [39]. The number of distinct homopolymer homogeneous sections determines the molecular ar-chitecture of block copolymer: diblock, triblock, and multiblock copolymers, as well as star block copolymers are possible [Fig.1.9].
Diblock copolymers which contain two distinct homopolymers are the simplest molecular architecture of block copolymers and have been stud-ied most extensively for generating a variety of nanostructures. Polymer mixtures will separate into different phases while two blocks of a diblock copolymer tend to de-mix. The covalent bond linking the blocks, however, prevents the macroscopic phase separation observed in binary mixtures of the homopolymers and results in the nanoscale structural organization of
Figure 1.9: Schematic illustration of several (A − B)n type block copolymer
ar-chitectures. Solid and dashed lines represent A and B block chains. The n = 1 and n = 2 architectures are commonly referred to as diblock and triblock copolymers, while n = 4 are denoted star block copolymers.
each block. Formation of different morphologies is possible on the basis of volumetric fraction of the minority component. For instance, diblock copolymers generally exhibit shapes like spheres, cylinders, lamellae inside the matrix of the most abundant block polymer [Fig.1.10]. In a diblock copolymer A-B, by changing the volumetric fraction of one of the two com-ponents, for example of A (φA), one obtains:
- Spheres of A in the B matrix: φA≤ 0.18-0.23
- Cylinders of A in the B matrix: φA≤ 0.3-0.35
- Lamellae of A and B: φA≈ φB
- phase inversion: φA φB
Figure 1.10: Schematics of thermodynamically stable diblock copolymer phases. The A–B diblock copolymer, such as the PS-b-PMMA molecule represented at the top, is depicted as a simple two-color chain for simplicity. The chains self-organize such that contact between the immiscible blocks is minimized, with the structure determined primarily by the relative lengths of the two polymer blocks fA, [40].
Volumetric fraction is not the only parameter which affects morphol-ogy. Other parameters arising from VdW forces and described by Flory-Huggins theory play an important role in the segregation process. Block copolymers can exhibit micro or nano separation depending on volumetric fraction and/or interactions between blocks which can eventually separate if the thermodynamics allows it. Translation entropy and interaction energy are correlated in Eq.1.1 by the interaction Flory-Huggins parameter χ that quantifies the tendency of the blocks to separate and in particular a positive and rising value of χ would imply an increased tendency to micro-separate.
∆f kBT = φAln φA νANA + φBln φB νBNB + χ ν · φAφB (1.1) ∆f free mixing energy per volume unit; φA, φB volumetric fraction of A
and B; νA, νB occupied volumes by A and B molecules; ν volume reference,
ν = (νA· νB) 1
2; NA, NB number of occupied reticular sites; χ Flory-Huggins
interaction parameter.
Ordered phase in a block copolymer depends on the product between Flory-Huggins interaction parameter χ and N, polymerization degree (num-ber of repeat units in the diblock copolymers).
An Order-disorder transition occurs when χ· N falls below a critical value [Fig.1.11]. For a fixed polymerization degree N (sufficiently high to permit a separation) there is a critical value χODT (order-disorder transition),
be-low which the material goes towards a disordered state from an ordered one [Eq.1.2]. This transition can occur at very low molecular weight (N is small) or at high temperature for a given molecular weight:
χ ≈ α + β
T (1.2)
where α and β are constants.
To sum up, χ · N product determines the microphase separation de-gree and the volumetric fraction of the minority component determines the particular morphology of the block copolymer [41].
The Segregation process is classified in three categories on the basis of the force segregation intensity [38]:
- Weak segregation (Weak Segregation Limit) (WSL): Nχ 10 - Intermediate segregation (Intermediate Segregation Region) (ISR):
Figure 1.11: Schematic representation of order and disorder in a symmetric diblock copolymer showing lamellar order.
- Strong segregation (Strong Segregation Limit) (SSL): Nχ → ∞
1.5
Liquid Crystal Polymers
The liquid crystal state (LCS) shows order in one or two dimensions; it lacks the three-dimensional long-range order of the crystalline state. LCS has characteristics intermediate between those of the crystalline and the dis-ordered amorphous states; for example many of them can flow like ordinary liquids but they display birefringence and other properties characteristic of crystalline solids. In liquid crystal phases the molecules can move but the orientational order is preserved at least in one direction. The LCS can be displayed by small molecules and by polymers, but in both cases a char-acteristic chemical component is needed. The existence of the liquid crys-tal state is related to the molecular asymmetry and the presence of strong anisotropic intermolecular interactions [42, 43, 44]. Thus, molecules with a rigid rod structure can form highly ordered states in concentrated solutions or in the molten state.
Liquid crystals are classified into two groups known as thermotropics and lyotropics. Thermotropics are those that are formed after melting crystalline solids, and they can remain in the liquid crystal mesophase without decom-position, passing to the isotropic liquid state when subsequently heated.
Figure 1.12: Comparison of the one-dimensional composition profiles characteriz-ing the weak (WSL) and strong (SSL) segregation limits, f refers to the local and stoichiometric A-block volume fractions.
Lyotropic LCs form mesophases in concentrated solution when the concen-tration exceeds a critical value. Not all crystalline polymers pass through liquid crystal states. Only if they are modified by introducing mesogenic groups could they form liquid crystal phases. The mesogen groups confer the liquid crystal characteristic; they have the form of rigid rods or discs, Fig. 1.13 shows the possible locations of mesogenic groups giving rise to what are called main-chain and side-chain liquid crystalline polymers [42, 43, 44, 45].
Figure 1.13: (a) Some typical mesogenic groups. (b) Schematic representation of types of liquid crystal polymers according to the location of the mesogenic groups: in the main chain (right) or as side substituents (left).
1.5.1 LC mesophases
The liquid crystal state can be found in different forms known as mesophases as shown in Fig.1.14 [42, 43, 44, 45]. In the mesophases known as smectic, the molecules are oriented in one direction and ordered in parallel layers. Nematic mesophase lacks ordering in layers and retain only the orientational order. The optical anisotropy and the response to electric fields of nematic phases are the basis for their use in screens for calculators, watches, and electronic devices. In the cholesteric mesophase the molecules are ordered in layers, with the direction of orientation slightly changing in consecutive layers, giving rise to helical structures. The periodicity of the helical layers depends on temperature; as a result cholesterics display different colours depending on temperature and can be used as temperature sensors. Finally, discotic mesophases are typical of disc-shaped molecules and can display ne-matic or columnar mesophases. Liquid crystal polymers can exist in different mesophases, depending on temperature and pressure conditions. When the temperature rises, they can pass from one mesophase to another, or they can reach the isotropic liquid state. The transitions solid-LC mesophase, LC mesophase-LC mesophase, and LC mesophase-isotropic liquid are generally first-order thermodynamic transitions. On such phase transitions enthalpy undergo discontinuous change [46], thus they can be detected by differential scanning calorimetry.
1.5.2 Main-Chain LC polymers
Main-chain LC polymers are characterized by their low solubility and by high melting temperatures, and in many cases they decompose before reach-ing the molten state. These characteristics are a consequence of their rigid chain structure with strong intermolecular interactions. The melting or dis-solving of the crystals requires the breaking of many interactions and is therefore highly endothermic. On the other hand, in the solid-liquid transi-tion very little entropy is originated, since the chains continue to maintain their stretched conformation. This is in contrast with the behavior of flexible macromolecules, which have higher melting entropy as they have much lower melting points. Main-chain LC polymers, such as aromatic polyamides, usu-ally form lyotropic LCs with nematic mesophases. Lyotropic main-chain LC polymers are not used in the liquid crystalline state. Nevertheless, this is the state in which they are processed and once the solvent is eliminated, they possess excellent mechanical properties such as rigidity, tensile strength, and extraordinary chemical resistance. For this reason, they are used in advanced
Figure 1.14: Side-chain LCPs organized in different mesophases.
applications in the aerospace, telecommunication, and electrical industries. For example, Kevlar is processed by dissolving in sulfuric acid to form high resistance fibers. A favorable characteristic for the processing is that the in-crease in the viscosity of the solution with the concentration ceases when the critical concentration is reached and the mesophase is formed. The ordering of the chains in the direction of flow reduces the viscosity, which facilitates the processing of the fibers.
1.5.3 Side-Chain LC Polymers
Side-chain liquid crystal polymers are obtained by fixing mesogenic units as side substituents of flexible polymer chains, commonly polyacrylates and polysiloxanes. The bonding of rigid rod type mesogenic groups directly onto the main chain does not lead to LC mesophases, because, due to steric hindrance, the arrangement of mesogenic moieties does not take place. Nev-ertheless, the attachment of mesogens to the main chain via flexible spacers stabilizes the ordered mesophase [45] [see Fig.1.14]. When alkyl chain spac-ers are used and the main chain is sufficiently long, smectic mesophases can be observed, which, when the temperature is increased, can pass through a nematic mesophase to the isotropic liquid state. In side-chain LCs, the order of the mesophase can persist when the mesophase is cooled quickly to
a temperature below the glass transition temperature; thereby it is possible to obtain a nematic (or smectic) glass, i.e., a rigid phase displaying the order and anisotropic properties of the LC phase. This characteristic makes these polymers interesting as materials for storing information. Unlike main-chain LC polymers, side-chain LCs do not possess such good mechanical proper-ties, but their optical behavior - particularly their response to electric and magnetic fields - makes them useful as materials for non-linear optics (mix-ers, amplifi(mix-ers, and frequency modulators), optical storage etc.
1.6
Polymers: Mechanics and Dynamics
In this section some aspects regarding the dynamics in amorphous polymers and their mechanical behavior are illustrated. The relaxation processes and dynamics at different time length scale are relevant parameters factors to optical writing stability.
1.6.1 Mechanical behaviour
The mechanical properties are influenced by molecular weight, degree of crystallinity, glass transition, crosslinking and so on. Among them, glass transition (amorphous polymers) is one of the most important to consider. In this brief section we present an overview of mechanical and thermal prop-erties of polymers below and above their glass transition temperature. Starting with the classification of polymers depending on their mechanical behaviour, five regions of mechanical behaviour for linear amorphous poly-mers [47, 48, 49, 50] are shown in Figure 1.15.
- The Glassy Region. Young’s modulus for glassy polymers just be-low the glass transition temperature is constant over a wide range of temperatures, with value of approximately 3×109 Pa [Region 1, Fig.1.15]. In the glassy state, molecular motions are largely restricted to vibrations and short-range rotational movements.
- The Glass Transition Region. Typically the modulus drops by about a factor 103 in a 20 to 30 ◦C range [Region 2, Fig.1.15]. The behavior of polymers in this region is best described as leathery. - The Rubbery Plateau Region. After the sharp drop in the glass
transition region modulus undergoes almost constant again in the rubbery plateau region, with typical values of 2×106 Pa [Region 3,
Figure 1.15: Five regions of viscoelastic behavior for a linear, amorphous polymer. Also illustrated are effects of crystallinity (dashed line) and cross-linking (dotted line) [26].
Fig.1.15]. In the rubbery plateau region, polymers exhibit long-range rubber elasticity, which means that the elastomer can be stretched, perhaps up to several hundred percents, and snaps back to substan-tially its original length on being released. Two cases in region 3 need to be distinguished:
1.The polymer is linear. In this case the modulus will drop off slowly, as indicated in Figure 1.15. The width of the plateau is governed primar-ily by the molecular weight of the polymer; the higher the molecular weight, the longer the plateau is.
2.The polymer is cross-linked. In this case the dotted line in Figure 1.15 is followed, and improved rubber elasticity is observed.
If a polymer is semicrystalline, the dashed line in Figure 1.15 is fol-lowed. The height of the plateau is governed by the degree of crys-tallinity. This is so because of two reasons: first, the crystalline re-gions tend to behave as a plasticizer phase, and second, because the crystalline regions also behave as a type of physical cross-link, tying the chains together. The crystalline plateau extends until the melting point of the polymer.
rubbery plateau region for linear amorphous polymers, the rubbery flow region is reached [Region 4 of Fig.1.15]. In this region the polymer is marked by both rubber elasticity and flow properties, depending on the time-scale of the experiment. For short time-scale experiments, the physical entanglements are not able to relax, and the material still behaves as rubbery. For longer times, the molecular motion, increased by the increased temperature, allows assemblies of chains to move in a coordinated manner (depending on the molecular weight), and hence to flow.
- The Liquid Flow Region. At still higher temperatures, the liquid flow region is reached (Region 5 of Figure 1.15). The polymer flows readily. In this region, as an idealized limit, the Newton law holds and viscosity η linearly depends on the shear stress τ . The increased en-ergy allotted to the chains permits them to rapidly reptate out through entanglements and flow as individual molecules. For semicrystalline polymers, the modulus depends on the degree of crystallinity. The amorphous portions go through glass transition, but the crystalline portion remains hard. Thus a composite modulus is found. As the melting temperature is reached on heating, the modulus drops rapidly to that of the corresponding amorphous material in the liquid flow region.
1.6.2 Relaxation phenomena in the glassy state
Glass transition in non-crystalline polymers, under ordinary experi-mental conditions, occurs on cooling when the characteristic time of molecular motion responsible for structural rearrangements toward the equilibrium state becomes longer than the time-scale of the experi-ment. As a result, structural relaxation is arrested below the glass transition temperature Tg, and the polymer is in the glassy state. For
example, the viscosity, like any other transport coefficient, reflects the underlying movement of the molecules in the system. The variation of η is essentially proportional to the time-scale τ of the relaxation process, and hence an increase in τ by several orders of magnitude is observed upon cooling. These strongly temperature-dependent slow processes are referred to structural relaxation processes. As a result, it is possible to detect structural relaxation phenomena in the exper-imental observation of many different physical quantities. Structural
relaxation phenomena have been studied in great detail during the past decades by means of various techniques. The results are reviewed in the books by Ferry, by McCrum et al and by Wong and Angell [90, 52, 53]. The glass-forming materials can show different tempera-ture dependences as Tg is approached from above. Angell proposed a
useful classification from strong to fragile behavior [54]. Fragile glass-formers show a bent curve in the normalized activation plot logτ versus Tg/T , while strong ones exhibit a straight one like [55]. The relaxation
times and viscosity of the former behave in Arrhenius fashion:
τArrh(T ) = τ∞· exp(
Ea
kBT
) (1.3)
where Ea is the so-called activation energy and Ea/kB the activation
temperature
Fragile liquids show marked deviations from Arrhenius behavior; in many cases, the temperature dependence of relaxation times (or the viscosity) is described by the Vogel-Fulcher (VF) equation [56, 57, 58]:
τV F(T ) = τ0· exp(
Tb
T − T0
) (1.4)
where Tb is the pseudo-activation temperature, and T0 is the so-called
Vogel temperature.
For T0 = 0, the familiar Arrhenius equation results. In this case, the
constant Tb is equal to Ea/kB, where Ea is the activation barrier in
the Arrhenius equation. When T0 > 0, the temperature dependence
is non-Arrhenius, and relaxation time is predicted to become infinite at T0.
Relaxation processes over different time-length scales are fundamen-tal to understand the interplay between homogeneity at the molecular level, bit stability and working temperature range. Today, the research field of azobenzene-functional materials remains extremely active, fo-cusing on the characteristics that make it possible to devise a suitable photochromic system, i.e. high sensitivity to laser pulse, low fatigue, rapid response, non-destructive readout capability. Among these, a special role is represented by the presence of thermal stability and
homogeneity at the molecular level. Heterogeneities at the molecu-lar level may substantially affect bit stability, thus seriously limiting the effectiveness of the azobenzene-based polymer matrices as erasable storage devices on the nanometer length scale. Relaxation processes, effective on different time and length scales, can have an influence on the writing stability bit, even if backbone conformational rearrange-ments can stabilize stored information over a larger temperature range. Therefore, spectroscopic techniques covering different time and length scales must be employed in order to fully characterize the matrices of interest. For a deeper understanding of the reorientation processes and their correlation with the local environment in side group polymers, electron spin resonance (ESR) can play a major role. Indeed, X band ESR experiments are sensitive to rotational dynamics for microscopic times in the interval 10−12 s - 10−6 s. The ESR characterizations are in turn correlated with the structural relaxation of the polymers, as re-vealed by rheological measurements as function of temperature. The comparison between microscopic and macroscopic data may help to identify the process leading the paramagnetic tracer rotational diffu-sion, thus making it possible to locate the molecular reorienting sites and to understand the relation between dynamics and optical writing.
Results and Discussion
2.1
Synthesis and characterization
This chapter focuses on the synthesis and characterization of new azobenzene polymers, mainly block copolymers synthesized via atom transfer radical polymerization (ATRP) [59, 60]. In this framework azobenzene based block copolymers were chosen as materials of inter-est, potentially applicable as supports for high density optical writing. Based on a specifically designed azobenzene monomer, 3-methyl-4-[6-(methylacryloyloxy)-hexyloxy]-4’-pentyloxy azobenzene (MA4), we prepared and studied the corresponding homoplolymer H and two sets of copolymers with methyl methacrylate (MMA), random copolymers Rz and block copolymers Bz (where z is the mole percentage of the MA4 counits).
2.1.1 Azobenzene monomer MA4
Figure 2.1: The azobenzene monomer MA4.
The MA4 monomer presents two-fold character of being both photore-sponsive and nematic.The aliphatic spacer -O(CH2O)6O- allows the
mobility of the azobenzene group in the polymers of MA4 (homopoly-mer, random and block copolymers), thereby permitting ordering in a mesophase and responding to external stimuli.
The synthetic scheme of the monomer consists of three steps reaction [61] and it is shown in Figure 2.2. The monomer forms a nematic mesophase with a clearing temperature of 39◦C.
Figure 2.2: Synthetic scheme of the monomer MA4.
Optical and dielectric measurements
Optical and dielectric investigations on the monomer were carried out at the Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences with the collaboration of Dr.Nandor Eber, Dr.Peter Salamon and Dr.Katalin Fodor-Csorba during a COST Short Term Scientific Mission.
The monomer MA4 was filled in a custom-made cell with a planar orientation. The connecting wires were soldered to the ITO electrodes on the glasses. The two glasses are separated by spacers of 20 µm [Fig. 2.3] and the planar orientation of the nematic liquid crystal are confirmed by observing the samples by the optical microscope as is shown in Fig. 2.4.
Figure 2.3: Custom made cells of the monomer (left) and homopolymer (right).
Firstly, observations of the monomer with a polarizing microscope (POM) on heating and cooling confirmed that the clearing temper-ature is 39◦C (monotropic nematic).
Figure 2.4: Scheme of the polarized light and the brightest position for a nematic liquid crystal with planar alignment (left) and POM images of the monomer: the brightest (center) and the darkest position (right) differ each other for 45◦.
– Dielectric Measurements
Dielectric spectroscopy measurements were performed in the fre-quency range of 100 Hz to 100 kHz. The complex impedance of the samples was recorded as the function of frequency and the temperature was kept constant. In order to obtain the proper ge-ometries to measure the parallel and perpendicular components of the complex permittivity, our compounds were aligned by mag-netic field, therefore the hot stage with the sample was placed in the electromagnet with the help of a rotatable mount. We measured the real (ε’) and imaginary (ε”) parts of the parallel and perpendicular dielectric permittivity for the monomer. The results obtained show that there is no relaxation in the studied frequency range because ε’ is approximately constant and the loss ε” does not show any peak [Figs. 2.5, 2.6]. At low frequencies,
the decrease in ε” is due to the dc conductivity of the material. On the other side, at high frequencies the increase in ε” is most probably due to a parasite effect, namely the onset of the so-called ITO- relaxation. It means that the serial resistance of the ITO becomes comparable with the impedance of the liquid crystal at high frequencies. These considerations are valid for the parallel as well as for the perpendicular components of ε.
5 4 3 2 1 0 ε Õ, ε ÕÕ parallel 102 2 3 4 5 6 7 103 2 3 4 5 6 7 104 2 3 4 5 f [Hz] εÕparallel εÕÕparallel
Figure 2.5: Dielectric spectra of monomer MA for parallel ε components. Tem-perature is 34◦C and electric field 0.2 V.
After collecting values to estimate the sign of the dielectric and conductivity anisotropy at measuring voltage of 1 kHz, 0.2V, we obtained:
∆ε = εk− ε⊥= 0; ∆σ = σk− σ⊥ = 0.005 S/m (2.1)
From these data it can be assumed that there could be a Freed-ericksz transition in this geometry. To locate this transition we performed measurements by transmitted light.
– Transmitted light measurements
The temperature dependence of the dielectric and optical proper-ties of our compound was investigated by the method described
5 4 3 2 1 ε Õ, ε ÕÕperpendicular 50x103 40 30 20 10 f[Hz] εÕperpendicular εÕÕperpendicular
Figure 2.6: Dielectric monomer spectra of monomer MA for perpendicular ε com-ponents. Temperature is 34◦C and electric field 0.2 V.
next. The cell was taken in a heat stage controlled by a tem-perature controller. The hot stage was placed between crossed polarizers, while the rubbing direction of the planar cell was ad-justed to be at 45 degrees with respect to the polarizer to obtain maximum intensity. The cell was illuminated by a red (λ = 650 nm) diode laser and the transmitted intensity was measured by a photoelectron-multiplier connected to a digital multimeter unit. At the same time, the complex impedance of the sample was measured by a bridge. Utilizing slow ramp rates (± 0.07 K/min), the temperature dependence of the complex permittivities and of the transmitted light could be recorded by a LabView program. After modifying the setup described before by exchanging the LCR bridge with a voltage source at a constant temperature and frequency (f = 1 kHz) we measured the voltage dependence of the transmitted light. The data obtained from the temperature ramp experiments show that ε’ decreases with increasing T, with a transition occurring at around 40◦C [Fig. 2.7]. This transi-tion, is the isotropic transitransi-tion, which is also confirmed by DSC measurements and optical microscopy [Fig. 2.8]
Looking at the experiments carried out by recording the transmit-ted intensity versus the applied voltage of the field [Fig. 2.9] we could make some considerations. There is a big jump around 85V
4.02 4.00 3.98 3.96 3.94 3.92 3.90 ε ' 45 40 35 30 T[¡C] 2.0 1.5 1.0 0.5 Intensity (a.u.) Heat Cool Intensity
Figure 2.7: Variation of ε0 with temperature for the monomer
M MA4
which could be due to the voltage threshold of electroconvection. The behaviour of the oscillation of light suggests the presence of a Freedericksz transition with the threshold around 10-20 V. The Freedericksz threshold is connected to the minimum volt-age that induces orientational deformation in the liquid crystal. In theory, below the threshold, there is no deformation, there is no change expected in the phase difference between the ordinary and extraordinary rays, and the intensity should be constant. The transmitted intensity is proportional with the sine square of the half of the phase difference, therefore above the threshold, the decreasing phase difference (in our geometry) results in the oscillations of the intensity. Closer to the threshold, the change in the phase difference is larger, thus we expect oscillations with smaller periods. It is difficult to analyze the region from 6V to 30V because the molecules need more time to align themselves with the field according to their positive anisotropy. The director relaxation time is proportional to the viscosity and to the square of the thickness. The slow response time of this high viscosity compound in a relatively thick cell is also affected by the slow-ing dynamics near the threshold of Freedericksz transition. We tried to find the threshold carrying out a slow measurement in a small range of voltage values but unfortunately in this region the fluctuations of the temperature affected the measurement too much. Therefore the threshold voltage could not be determined precisely and was estimated to be between 10 and 25V. One can estimate the threshold from the theoretical formula:
Uth= π K 1 ε0εa 12 (2.2)
where ε0 is the dielectric constant of vacuum, εais the anisotropy
of the dielectric constant of the liquid crystal, and K1 is the splay
elastic constant. With an elastic constant K1 between 1 and 10
pN, and εa = 0.01 we obtain a threshold Uth between 10 and
33V, consistent with the voltage that we could expect from the graph in Fig.2.9.
3.5 3.0 2.5 2.0 1.5 1.0 Intensity [a.u.] 100 90 80 70 60 50 40 30 20 10 V
Figure 2.9: Intensity versus applied voltage at 34◦C in the nematic phase.
– Microscopy with applied voltage
Microscopy observations with polarized light were performed ap-plying fields voltage on the sample, changing frequency and volt-age. However a profile of electroconvection in different conditions was detected because the sample was not homogeneous enough. Applying an electric field of 18V and 10 Hz to the monomer perpendicular to the plane where the director lies [Fig. 2.10], a pattern due to electroconvection was obtained [Fig. 2.11 left]. This pattern is made of rolls, but it is not homogeneous because there are some defects affect the formation of the pattern. On increasing the voltage up to 64V, dynamic scattering took place [Fig. 2.11 right].
In Fig. 2.12 we can see that there is a Freedericksz transition because in a proper position of the rotating stage the sample appeared bright with a very low voltage (1V, 1000Hz), while it became dark on applying a very high voltage for several minutes (106V, 1000Hz). It could mean that the director aligns itself with the field because it has a very small positive anisotropy.
E
n
⟨
Figure 2.10: Scheme of the planar cell with the director that lies in the plane and the perpendicular electric field.
Figure 2.11: Optical image of the monomer at 37◦C with an electric field of 18V and 10Hz (left) and of 64V and 10Hz (right).
Figure 2.12: Images obtained by polarized optical microscope at 34◦C. On the left there is a bright image of monomer with an electric field of 1V, 1000Hz and on the right another one with an electric field of 106V and 1000Hz.
2.1.2 Homopolymer and random copolymers
Figure 2.13: Chemical structure of the H homopolymer.
The H homopolymer was synthesized by a free radical polymerization of the monomer MA4 with AIBN. The chemical-physical properties of H are shown in Tab.[2.1].
Table 2.1: Number average and weight average molecular weights and polydisper-sity (by GPC) of the H homopolymer
Mn (kg·mol−1) Mw (kg·mol−1) Mw/Mn
18.6 59.0 3.17
Figure 2.14: Chemical structure of the Rz random copolymers.
The different random copolymers [2.14] were prepared in four poly-merization batches in which the experimental conditions (feed compo-sition, concentration of monomer, initiator and polymerization time) were adjusted to yield samples with different molar masses and molar mass distributions [Table 2.2].
The homopolymer and the random copolymers rich in azobenzene units (down to 30 mol %) formed a nematic phase [Table 2.3] as detected by differential scanning calorimetry (DSC). In the random copolymer with 10% mol of azobenzene units, the liquid crystalline
be-Table 2.2: Number average and weight average molecular weights and polydisper-sity (by GPC) of the random copolymers Rz.
Copolymer Mn (kg·mol−1) Mw (kg·mol−1) Mw/Mn
R90 53.0 18.0 3.40
R80 49.0 17.7 3.61
R70 33.0 11.7 3.54
R10 28.7 64.8 2.26
havior was not detected because the amount of azobenzene mesogens is not enough to permit the formation of this mesophase [Fig.2.15]. Be-side the nematic-isotropic transition, the homopolymer exhibited one further transition pointing to the occurrence of a conformational tran-sition of the polymer backbone at an intermediate temperature (Tc).
By contrast, such a transition was not detected in any of the copoly-mers investigated, probably because of the spreading of the azoben-zene units by the non-mesogenic MMA units. Copolymerization also affected both TN I , that was weakly depressed, and Tg, that was
slightly increased, relative to the respective values of the homopoly-mer.
Table 2.3: Transition temperatures of the random copolymers and H homopolymer measured by DSC thermal analysis.
Sample Tg (◦C) Tc(◦C) TN I (◦C) H 21 47 80 R90 33 - 79 R80 35 - 75 R70 41 - 73 R10 83 - -2.1.3 PMMA-Br macroinitiator
Poly(methyl methacrylate) macroinitiators PMMA-Br [Fig. 2.16] were synthesized by ATRP of MMA monomer [18] in different experimen-tal conditions to obtain different products characterized by different
R10
Figure 2.15: DSC heating curve of R10.
molecular weight [Tab. 2.4]. In the PMMA-Br-H, H stands for high molecular weight while in the PMMA-Br-L, L stands for low molecular weight.
Figure 2.16:Bromo-terminated poly methyl methacrylate macroinitiators PMMA-Br.
Table 2.4: Molecular weight (GPC) of PMMA-Br macroinitiators
Run Mn (kg·mol−1) Mw/Mn
PMMA-Br 20.9 1.3
PMMA-Br-H 21.4 1.2
PMMA-Br-L 13.1 1.2
PMMA-Br-H and PMMA-Br-L are the methyl methacrylate compo-nents of the blends with Bz copolymers, while PMMA-Br is involved in the Bz synthesis.
2.1.4 Block Copolymers
Block copolymers have been synthesized by ATRP. In fact, the ATRP synthesis allows the attainment of polymers with a well-defined archi-tecture and composition. The block copolymers are labelled as Bz to underline the molar percentage of the azobenzene block (z) [Fig. 2.17].
Figure 2.17: Block copolymers with H and PMMA.
In this work block copolymers were synthesized by ATRP of the MA4 monomer with PMMA-Br as macroinitiator, the CuCl, HMTETA cat-alytic system and anisole as solvent (Scheme 2.18) [37].
Figure 2.18: Reaction scheme for ATRP of block copolymers with MA4 and PMMA-Br.
Table 2.5: Synthesis of the block copolymers Bz. The DPn of PMMA is 200 for
every block copolymers.
Polymer MA4 DPn Mn Mw/Mn
(%mol) MA4 (kg·mol−1)
B3 3 5 29.0 1.2
B10 10 20 27.0 1.3
B20 20 48 34.0 1.3
DSC analyses performed on the block copolymers show two glass tran-sitions associated with the PMA4 block (around 50◦C) and to the PMMA block (around 120◦C) respectively. The first glass transition was measured by the first heating cycle [Figs. 2.19, 2.20]. The PMA4 block is made up by several repeating units of the MA4 monomer. Moreover, the nematic-isotropic transition can be detected around 80◦C. In the block copolymer containing 3% mol of the azobenzene block, the content of this azobenzene block is too small to be detected by DSC. The presence of a nematic liquid crystalline phase was ob-served by the optical polarized microscope. The two polymer blocks were microphase separated underwent their individual thermal transi-tions. Accordingly, the presence of a nematic mesophase was possible even in the block copolymers with a minimal amounts of the azoben-zene mesogens. This is in marked contrast with the phase behavior of the random copolymers. In Table 2.6 are reported the results about the thermal characterization of the block copolymers.
Table 2.6: Thermal characterization of block copolymers Bz. aDetected by POM
microscopy.
Polymer Tg1(◦C) Tg2(◦C) TN I (◦C)
B10 50 124 80
B20 50 120 85
B3 B10 B20
Figure 2.19: Second heating cycle of DSC analyses of block copolymers.
B20 1st
2.1.5 Blends
Blends of Bz block copolymers with PMMA-L and PMMA-H (poly-methylmetacrylates reported in the Tab. 2.4]) have been prepared by dissolving the corresponding amounts of both components in dried dichloromethane. The final azobenzene content in the blends was 1% and 5% by weight [Table 2.7]. The Blends that contain 1% wt and 5% wt are named A and B respectively. As an example, 10BH means a blend with B10 and PMMA-H, with an azobenzene content of 5% wt.
Table 2.7: Blends with PMMA and azobenzene block copolymers
Blend Copolymer PMMA azo %wt
10BH B10 H 5 10BL B10 L 5 10AH B10 H 1 10AL B10 L 1 20BH B20 H 5 20BL B20 L 5 20AH B20 H 1 20AL B20 L 1
2.2
Morphology
In this section we discuss the results of the study on the surface mor-phology carried by AFM measurements on spin-coated films of block copolymers and on cast films of blends with PMMA and block copoly-mers. Moreover TEM images are collected on slices of bulk material. The aim was to obtain a nanostructured material to increase the pho-toresponse of the azobenzene chromophores and the optical writing stability. Moreover, with a nanostructured material where azobenzene domains are in the range of tens of nanometers, the resolution of the writing in terms of diameter of the spots, might be increased. Indeed, even if the diameter size of the blue laser spot on the sample is larger than 50 nm, one could write only an azobenzene domain of 30 nm or even less as is shown in Fig. 2.21.
written dot PMMA PMMA laser spot azo domains nanost ructure d azo blo ck copol ymer
Figure 2.21: Schematic mechanism of writing due to a nanostructured azobenzene block copolymer with a dimension of azobenzene domains smaller than a writing blue laser spot.
2.2.1 Atomic Force Microscopy Imaging
Block copolymers, give rise to a phase separation in nanodomains due to the different nature of each block. This segregation is stronger when the differences in the chemical nature of the blocks is higher. According to the volume fraction of each block, the microphase separation leads to different morphologies spreading from spheres to lamellae [38, 41].
Block Copolymers
In a block copolymer, liquid-crystalline order of one block could ham-per self-assembly because liquid crystalline structure could be incom-patible with the natural morphology of the material. Shah et al. have found that the morphology for polymers similar to ours is not influ-enced by liquid-crystalline order. We assume that the all morphologies mainly derive from the volumetric fractions of the blocks [62].
AFM measurements were carried at room temperature on B3, B10 and B20. The roughness of all samples, evaluated as root main square roughness Rq (Rq = (ΣiZi2/N )0.5), is from 2 up to maximum 5 nm.
In the block copolymer with 3% mol of azobenzene block (10 % wt) a spherical morphology was observed [Fig. 2.22]. The diameter of the spheres was in the range from 20-25nm to 40-50nm.
1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) -15 -10 -5 0 5 10 x10 3 Height [T] (nm) 1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 6.0 5.5 5.0 4.5 4.0 3.5 3.0 Phase [T] (degree)
Figure 2.22: AFM maps of B3. Topography (left) and phase (right) maps.
B10 that contains 35% wt of azobenzene exhibits elongated domains tentatively associated to lamellae of 15-20nm of lateral size dimension [Fig.2.23] and B20, 56% wt of azobenzene, also exhibits lamellae of 30-35nm of lateral size dimension [Fig. 2.24].
1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 8 6 4 2 0 Height [T] (nm) 1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 0.20 0.18 0.16 0.14 0.12 0.10 Phase [R] (degree)
Figure 2.23: AFM maps of B10. Topography (left) and phase(right) maps.
1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 6 4 2 0 Height [R] (nm) 1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 34 32 30 28 26 24 22 20 Phase [R] (degree)
Blends
The films of the polymer blends observed here are deposited by casting and their roughness is low (2-5 nm) except for the 10BH that is around 15nm. All of these blends, with a 5 % of azobenzene amount, exhibit a spherical morphology. The sphere-like morphology of the blends with the copolymer B10, 10BH and 10BL, is constituted by spheres of a diameter around 8-10 nm [Figs. 2.25, 2.26].
300 250 200 150 100 50 0 Size (nm) 300 200 100 0 Size (nm) -30 -20 -10 0 10 20 x10 3 Height [R] (nm) 300 250 200 150 100 50 0 Size (nm) 300 200 100 0 Size (nm) 4 3 2 1 0 Phase [R] (degree)
Figure 2.25: AFM maps of 10BL. Topography (left) and phase (right) maps.
250 200 150 100 50 0 Size (nm) 250 200 150 100 50 0 Size (nm) 22.0 21.5 21.0 20.5 20.0 19.5 19.0 Height [R] (nm) 250 200 150 100 50 0 Size (nm) 250 200 150 100 50 0 Size (nm) 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Phase [R] (degree)
Figure 2.26: AFM maps of 10BH. Topography (left) and phase (right) maps.
The blends 20BL and 20BH, with the copolymer B20, show spheres with a diameter around 20-30nm for 20BL [Fig.2.27] and around 30nm for the 20BH respectively[Fig.2.28]. Thus, the size of these nanodomains
was similar for the different blends. About the differences between 20BL, 20BH and 10BL,10BH we suggest that these differences could be due to the different polymerization degree of the azobenzene block in the two block copolymers. The number of azobenzene monomer in B20 is 50 units and 22 for B10. The length of azobenzene block in B20 is almost twice the length in B10.
400 300 200 100 0 Size (nm) 400 300 200 100 0 Size (nm) 2.5 2.0 1.5 1.0 0.5 0.0 -0.5 Height [R] (nm) 400 300 200 100 0 Size (nm) 400 300 200 100 0 Size (nm) 3.5 3.0 2.5 2.0 1.5 Phase [R] (degree)
Figure 2.27: AFM maps of 20BL. Topography (left) and phase (right) maps.
Figure 2.28: AFM maps of 20BH. Topography (left) and phase(right) maps.
2.2.2 Transmission Electron Microscopy
In order to support the AFM morphology data, TEM measurements have been done. Block copolymers and B-blends (with 5% wt
azoben-zene) were observed with a transmission electron microscope. Block copolymers show a lamellar morphology: lamellae of azobenzene blocks are approximately 25 nm of width in B10 and 30 nm in B20 [Fig.2.29].
Figure 2.29: TEM micrographs of B10 (left) and B20 (right).
Blends of B10 exhibit a spherical morphology with spheres of a diam-eter around 20 nm [Fig.2.30] while the diamdiam-eter of the spheres of the blends with B20 is around 30 nm [Fig.2.31].
Figure 2.30: TEM micrographs of blends 10BL(left) and 10BH (right).
The results obtained by TEM microscopy are consistent with those obtained by AFM. Hovewer, one should remember that TEM investi-gates across the bulk, while AFM probes investigated the surface of a sample that could explain the differences observed.
Figure 2.31: TEM micrographs of blends 20BL(left) and 20BH (right).
2.2.3 Order Disorder Transition (ODT)
AFM microscopy was used in investigations of morphology as a func-tion temperature in samples with different chemical composifunc-tion. Since the block copolymer is used as a recording medium at the nanoscale, the structure should remain intact in the range of temperatures used for writing and for reading. There are two approaches towards studies of polymer phase transitions. In the first, the polymer sample is heated or cooled, and the images at different temperatures are recorded. In the other, the AFM probe, which stays in permanent contact with the sample, is heated and the probe deflection is monitored. The probe deflection changes substantially when the probe temperature exceeds glass transition or the melting temperature of the sample. In this way, the polymer material under the probe can be recognized on the base of its thermal characteristics. In earlier AFM applications to polymers, when measurements at different temperatures were not possible, the effect of thermal annealing on polymer structures was examined at room temperature (RT) condition. After the initial imaging at RT, the sample was heated to the target temperature for some time and then cooled back to RT. This approach is quite laborious since it re-quires repeated location of the same spot on the sample surface with high precision. In addition, this procedure is not always acceptable because of the changes that can occur in the polymer structure on cooling to RT. With the development of AFM incorporating heating accessories, these films are examined at various temperatures. In par-ticular in our studies an hot stage can be exploited at temperatures
ranging from RT to 250◦C. The temperature is increased from room temperature to 70◦C (above the glass transition of azobenzene block but below the other glass transition and the nematic-isotropic tran-sition), to 100◦C (in the isotropic phase and below the PMMA glass transition) and to 130◦C (above all temperature transitions). Above 130◦C the measurements are intensified (130◦C, 135◦C, 137◦C). AFM maps are here shown only for B20 as a representative behav-ior of order-disorder transition. At room temperature (25◦C) sample appears well nanostructured [Fig. 2.32], but when the temperature arises to 130-135◦C the nanostructures are less defined [Figs. 2.33, 2.34]. Structured morphology disappears at the order-disorder transi-tion temperature (TODT) of 140◦C as is shown in Fig.2.36.
1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 6 4 2 0 Height [R] (nm) 1.0 0.8 0.6 0.4 0.2 0.0 Size ( µ m) 1.0 0.8 0.6 0.4 0.2 0.0 Size (µm) 34 32 30 28 26 24 22 20 Phase [R] (degree)
Figure 2.32: AFM maps of B20 at room temperature. Topography (left) and phase (right) maps.
All samples exhibit different order-disorder transition temperature val-ues [Tab.2.8].
Table 2.8: TODT values obtained by AFM measurements.
Sample TODT (◦C)
B3 160
B10 150
B20 140
Figure 2.33: AFM maps of B20 at 130◦C. Topography (left) and phase (right) maps.
Figure 2.34: AFM maps of B20 at 135◦C. Topography (left) and phase (right) maps.
Figure 2.35: AFM maps of B20 at 137◦C. Topography (left) and phase (right) maps.
Figure 2.36: AFM maps of B20 at 140◦C. Topography (left) and phase (right) maps.
2.3
Electron Spin Resonance (ESR)
Electron spin resonance spectroscopy, due to the anisotropy of the magnetic tensor involved, has turned out to be a powerful tool to inves-tigate molecular dynamics, especially rotational dynamics, in simple and complex liquids [63, 64, 65, 66]. In the case of the nitroxide para-magnetic radical [63], often used to investigate diapara-magnetic systems, X band ESR experiments are sensitive to rotational dynamics for mi-croscopic time in the interval 10−12s<τ <10−7s. This Section investi-gates the reorientational dynamics of the cholestane molecule dissolved in different polymeric matrices [Fig. 2.37]. This paramagnetic probe has proved to be an excellent molecular tracer, especially in the study of reorientational processes of liquid-crystal polymers [67, 68]. The probe dynamics falls in the slow-motion regime, due to its geometry and the high viscosity of the polymeric host matrix [69]. The analysis of the ESR line shape allowed us to evaluate the heterogeneities and cooperativity degrees that are induced by the thermal annealing in the isotropic phase of polymers with azobenzene side groups. For the sake of clarity, the case of the azobenzenic block copolymers will be treated separately from the random copolymers, due to the different features of these samples.
Figure 2.37: Cholestane spin probe: chemical structure.
2.3.1 Homopolymer and random copolymers
It is well-known that the ESR line shape is sensitive to local environ-ments that modulate the diffusional process of molecular tracers in simple fluids [63]. ESR studies have also shown their ability to probe local order and dynamics in polymers [68, 70, 71, 72, 73] even though an accurate evaluation of the reorientational processes of the para-magnetic molecules in highly heterogeneous systems has very seldom
been performed [74]. Moreover, memory effects due to thermal his-tory substantially complicate the analysis. Therefore, special care was devoted in this work to singling out suitable experimental conditions for achieving reproducible responses of the polymer samples during the measuring time with the aim of identifying memory effects on the polymer structural relaxation and, therefore, on the relaxation on the time-scale of the probe.
Memory effects and annealing
The H homopolymer and the R90, R80, R70 copolymer samples proved to reach an electron spin resonance equilibrium spectrum at fixed tem-perature after a variable time [69, 75, 76], depending on the differ-ent thermal histories to which the differdiffer-ent polymers were submitted. The study of the rotational dynamics of the paramagnetic probe in the polymeric host matrix thus required the identification of a suit-able annealing procedure. In fact, the strong memory effects of the local polymeric structure would have prevented measurement repeata-bility. All the equilibrating samples were annealed in the isotropic phase at a fixed temperature Ta for an annealing time tA, after which
a stable ESR spectrum was recorded, which then showed no changes after longer annealing times. The samples were then cooled to a pre-determined temperature, and ESR spectra were recorded over a set of decreasing temperatures in such a way that the overall period of recording time tR was 1.5 h. After this set was completed, the
sam-ple was taken back to Ta and annealed there for a time tw of 2 h,
which ensured attainment of equilibrium conditions before recording over the following set of decreasing temperatures was started. By this procedure, the temperature range Ta – 280 K was fully
investi-gated. Spectra in the upper temperature region were simply recorded by slowly heating the sample from Ta to at least 420 K. The evolution
of the experimental spectra as obtained at the annealing temperature Ta = 358 K is illustrated in Figure 2.38. For each sample, Figure
2.38 also reports the annealing temperature Ta and a comparison of
some of the recorded ESR spectra during the annealing time Ta. ESR
investigations depending on the thermal history were also performed on the H homopolymer and R70 copolymer samples, choosing a differ-ent annealing temperature (Ta= 383 K; see Fig.2.38 (d) and 2.38(e)).
homopolymer at the annealing temperature Ta = 383 K. It should be
noted that this line shape reaches its equilibrium after some hours. This finding is unexpected, because the temperature Ta is well above
Tg. The time dependence of the ESR spectrum suggests a high degree
of localization of the cholestane probe in the matrix (the topic will be reconsidered later).
The ESR equilibrium spectrum for the R60 copolymer was obtained quickly and no particular thermal procedure was needed to obtain re-producible results. This feature may be a manifestation of the non mesogenic character of the R60 copolymer. Representative ESR spec-tra of the R60 copolymer recorded at different annealing times and temperatures are reported in Figure 2.39.
ESR line shape reproduction by simulations
The cholestane probe has a cylindrical shape [Fig.2.40], so its relax-ation time consists of two components, τk and τ⊥, respectively parallel
and perpendicular to the rotational axis of cholestane.
Every attempt aimed at reproducing the experimental spectra by a single dynamic component failed, due to the presence of the double-peaked structure in the low-field spectral region. The line shape is best simulated by a two-component distribution ρ(τ ∗) that mimics the bi-modal character of the probe dynamics by setting:
ρ(Dk) = A · δ(Dk− D1) + B · δ(Dk− D2) (2.3)
where δ(x) is the Dirac function and A and B are the weight coeffi-cients with A + B = 1.
According to this choice of simulation, τ components are related with the diffusion coefficient components by relations illustrated in the equation 2.4.
τ⊥ ≡
1 6Dk
τ⊥∼= 15τk (2.4)
where Dk is the parallel component of the diffusion coefficient of the
PMA4 S1 P(MA4-r-MMA)90 H R90 P(MA4-r-MMA)80 P(MA4-r-MMA)70 R80 R70 P(MA4-r-MMA)70 PMA4 S1 H R70
Figure 2.38: Time evolution of the ESR spectra of the H homopolymer sample (a) and R90 (b), R80 (c) and R70 (d) copolymer samples during annealing at Ta =
358 K. The annealing time tA for each sample is also reported. As a comparison,
the time evolution at Ta = 383 K is reported of the ESR spectra of the H (e) and
P(MA4-r-MMA)60 R60P(MA4-r-MMA)60 R60
Figure 2.39: Time stability of the ESR line shape of the R60 during annealing (left) at 360 K and (right) 399 K [78, 79].
τ
τ
cholestane
Figure 2.40: Scheme of a cylindrical shape of cholestane probe with two τ com-ponents.
Figure 2.41 (a) shows the best fit obtained by the distribution function of Equation 2.3 to reproduce the ESR equilibrium spectrum of the ho-mopolymer sample annealed and recorded at 383 K. The fairly good agreement between theory and experiment substantiates the bimodal character of the distribution function of the molecular sites. There-fore it seems that two discrete dynamic components are sufficient in order to capture the essential details of the experimental line shape. According to this observation, from now on the ESR line shape as a function of temperature is calculated by assuming a two − δ distri-bution, because this choice satisfies the requirement of the minimum number of free parameters.
(a)
S1H
Figure 2.41: Experimental ESR spectra and best simulations for the H homopoly-mer after annealing at Ta = 383 K in accordance with the (a) distribution function
of Equation 2.3 with the following parameters: D1 = 0.49 Gauss, D2 = 5.6 Gauss,
A = 0.37, Dk/D⊥ = 15 for both D1 and D2 [40].
Spin probe dynamics in homopolymer and random copoly-mers
The rotational dynamics of the paramagnetic probe dissolved in the H homopolymer sample, and in the R90, R80, R70 and R60 random copolymer samples was studied. The dynamics of the paramagnetic
probe in the H homopolymer and in the random copolymers under study was heterogeneous and completely characterized in both fast and slow sites; the relative populations were evaluated in a large tem-perature interval. The temtem-perature dependence of the spinning corre-lation time τ parallel in the fast and slow molecular sites of cholestane dissolved in the samples investigated is shown in Figures 2.42 and 2.43. For each sample, in the inset the percentage population is also reported of the fast sites as a function of the temperature.
In order to understand whether the copolymer composition may affect the bit stability and the working temperature above Tg, the
homopoly-mer and the R70 random copolyhomopoly-mer were the first samples investigated [69, 75, 77]. ESR investigations on these samples were carried out at two different annealing temperatures, with the aim of singling out the optimal thermal procedure to obtain suitable substrates for optical writing. Following the chronological evolution of the research project, our attention will be primarily focused on describing the results found for these two matrices. The scenario that they depict has been inves-tigated, by successively characterizing the R90, R80, R60 samples by means of ESR spectroscopy. This has made possible to improve the comprehension of the physical mechanisms that can limit or prevent the application of such materials for optical information storage. The site populations of the fast component as a function of 1000/T in H and R70 samples are shown in the insets of Figure 2.42. Let us examine in detail the fast component behavior of the H homopolymer annealed at 358 K (Figure 2.42 (b); see [80]). A clear decrease in the molecular fast site component is observed in the low temperature region. Thus, the thermal treatment performed at 358 K provided a homogeneous substrate. At 378 K the population shows a relative maximum around 50%, and subsequently increases with temperature up to about 430 K, which was the upper temperature limit investi-gated. In Figure 2.42 (a) the fast population of the H sample annealed at 383 K is also reported [69]. Its stability over all the investigated temperature range denotes the sample heterogeneity, due to the differ-ent thermal history: the annealing procedure in the isotropic phase at 383 K rendered the polymer matrix rich in defects on the nanometer length-scale [69], which is detected by ESR studies with the cholestane probe and which would be unsuitable in devising nanorecording on the polymer.
an-S1H S1H
P(MA4-r-MMA)80
P(MA4-r-MMA)70R70 R70P(MA4-r-MMA)70
Figure 2.42: Temperature dependence of ESR correlation times in the H
ho-mopolymer annealed at Ta = 383K (a) and at Ta = 358K (b), and in the R70
copolymer sample for the annealing procedures at Ta = 383K (c) and Ta= 358K
P(MA4-r-MMA)90 P(MA4-r-MMA)80
R90 R80
P(MA4-r-MMA)60R60
Figure 2.43: Temperature dependence of ESR correlation times in the R90 (a), R80 (b) and R60 (c) copolymer samples. The insets show the population of the fast component.
