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3. RESULTS AND DISCUSSION

3.1. AMPHIPHILIC POLYSTYRENE BLOCK COPOLYMERS

This chapter will focus on the synthesis and characterization of amphiphilic block copolymers prepared via atom transfer radical polymerization (ATRP).

In this framework polystyrene based block copolymers containing both oxyethylene (PEG) and fluoroalkyl segments in the amphiphilic side chains were chosen as materials of interest, potentially applicable as fouling release coatings against both hydrophobic and hydrophilic adhesives of marine organisms.

3.1.1. Synthesis of the monomer

The amphiphilic styrene monomer (Sz) that we designed for the preparation of block copolymers is not a commercially available product. It was prepared by the esterification reaction of 4-vinylbenzoic acid (1) with a commercially available fluorinated PEG surfactant, Zonyl FSO-100 (2). The esterification was carried out at room temperature using N,N’-dicyclohexylcarbodiimide (DCC) and 4-(dimethylamino)pyridine (DMAP) (Scheme 3.1). The reaction proceeded with high yield (> 80%), even if the by-product N,N’-dicyclohexylurea was difficult to remove from the desired product. After purification by repeated extractions with NaHCO

3

and HCl solutions, Sz was obtained as a yellow-orange semisolid.

COOH

HO(CH2CH2O)xCH2CH2(CF2CF2)yF +

COO(CH2CH2O)xCH2CH2(CF2CF2)yF 2

x ! 5, y ! 4

1 Sz

x ! 5, y ! 4 CH2Cl2, r. t.

DCC, DMAP

Scheme 3.1. Synthetic procedure for the preparation of the amphiphilic monomer Sz.

The fluorinated PEG (2), F(CF

2

CF

2

)

y

(CH

2

CH

2

)

x

CH

2

CH

2

OH, has a broad distribution of

molecular weights, with x = 0−15 and y = 1−7 as specified by the supplier. On the basis

of the areas of −CF

2

−CH

2

− protons and −CF

2

fluorines in

1

H and

19

F NMR spectra,

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46

respectively, the average composition was determined to be y = 3.5 and x = 5.5. On the other hand, MALDI-TOF spectrometry showed an oligomer species with mass-to-charge ratios of 695 g/mol (y = 5, x = 3) as the major component.

As a result of NMR and MALDI-TOF analyses, we considered the average polymerization degree of the hydrophilic PEG segment to be x = 5 and that of the hydrophobic-lipophobic fluorinated segment to be y = 4.

3.1.2. Homopolymerization and random copolymerization experiments

The perfluoro-ethoxylated monomer was polymerized by AIBN initiated free radical polymerization in solution (Scheme 3.2). The monomer concentration was ~ 0.2−0.35 M and the polymerizations were carried out in different conditions. The polymers were purified by several precipitations in n-hexane, since they were soluble in polar solvents such as methanol and tetrahydrofurane.

COO(CH2CH2O)xCH2CH2(CF2CF2)yF

Sz x ! 5, y ! 4

COO(CH2CH2O)xCH2CH2(CF2CF2)yF

P(Sz) x ! 5, y ! 4 AIBN, 65°C

Scheme 3.2. Free radical polymerization of Sz.

The random copolymers of Sz with styrene (S) were prepared in solution of either diglyme or trifluorotoluene using AIBN as initiator (Scheme 3.3). The concentration of the comonomers in the reaction mixtures was ~ 0.5−1.5 M. The reaction conditions are reported in Table 3.1.

1

H NMR,

19

F NMR and GPC analyses confirmed the actual copolymerization and incorporation of the two different comonomer units in the copolymers P(S-co-Sz).

COO(CH2CH2O)xCH2CH2(CF2CF2)yF Sz

x ! 5, y ! 4 +

S

AIBN, 65°C

COO(CH2CH2O)xCH2CH2(CF2CF2)yF P(S-co-Sz)

x ! 5, y ! 4

Scheme 3.3. Free radical copolymerization of Sz with S.

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47

The GPC curves were monomodal when the reaction time was relatively low and became multimodal as the reaction time increased. At longer polymerization time (ca. 72 h) the polymer product became hardly soluble in most of the organic solvents. As a consequence, the molecular weight and polydispersity of these random copolymers also increased with the reaction time.

Table 3.1. Free radical copolymerization experiments of Sz.

Sample Sz t

reac.a)

Yield

b)

feed copolymer

(mol %) (h) (%)

P(S-co-Sz)74 10 26 14 42

P(S-co-Sz)65 25 35 8 21

P(S-co-Sz)34 50 66 72 75

a) Reaction time. b) Yield calculated as [polymer weight/(S weight + Sz weight)]⋅100.

The copolymer composition was evaluated from the integrated areas of the

1

H NMR signals at 4.4 ppm (COOCH

2

of Sz) and at 6−8 ppm (aromatic rings of S and Sz).

The homopolymers and random copolymers were used as model compounds to compare their surface and bulk properties with those of the respective block copolymers.

3.1.3. Synthesis of polystyrene macroinitiators

The block copolymers were prepared by ATRP of the perfluorinated-ethoxylated styrene monomer Sz starting from a bromo-terminated polystyrene macroinitiator P(S).

This was prepared by ATRP of styrene using 1-phenylethyl bromide (1-(PE)Br) as

initiator and CuBr/2,2’-bipyridine (Bipy) complex as catalyst system (Scheme 3.4). The

polymerizations were carried out in bulk at 110 °C, using a constant CuBr/Bipy molar

ratio (1:3), different styrene/1-(PE)Br molar ratios, and different reaction times. The last

two parameters were conveniently changed in order to obtain polystyrene macroinitiators

with different molecular weights, which could be then used in the subsequent step of

block copolymerization.

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48

S

CuBr/Bipy, bulk, T=110°C

Br n

P(S) Br

Scheme 3.4. ATRP synthesis of polystyrene macroinitiator.

At the end of the reaction, the copper complex was removed by filtration over neutral alumina and the polymer was purified by repeated precipitations into methanol.

To maintain the highest possible chain-end functionality, the polymerization reactions were generally stopped at less than 90% conversion. Table 3.2 summarizes the results of the macroinitiator formation.

Table 3.2. Experimental conditions and chemical physical properties of polystyrene macroinitiators.

Run S:I:CuBr:Bipy

a)

t

reacb)

(min)

Yield

c)

(%)

M

nd)

(g/mol)

M

w

/M

ne)

DP

n

P(S)8.4 160:1:1:3 330 59 8400 1.27 81

P(S)5.3 80:1:1:3 330 73 5300 1.35 51

P(S)4.1 60:1:1:3 180 68 4100 1.38 39

P(S)2.7 60:1:1:3 90 50 2700 1.52 26

a) Styrene/1(PE)Br/CuBr/Bipy mole ratio. b) Reaction time. c) Yield calculated as [polymer weight/(monomer weight + 1(PE)Br weight)]⋅100. d) Number average molecular weight evaluated by GPC with UV detector. e) Polydispersity evaluated with UV detection.

The prepared macroinitiators were characterized by molecular weights in the range from

2700 g/mol (P(S)2.7) to 8400 g/mol (P(S)8.4), which correspond to polymerization

degrees (DP

n

) of 26 and 81, respectively. In any case, the macroinitiator polydispersities

were quite narrow (M

w

/M

n

~ 1.4) and well agreed with the living character of the

mechanism.

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49

3.1.4. Synthesis of block copolymers

Block copolymers with semifluorinated side chains have been previously synthesized by ATRP polymerization in order to obtain polymers with a well-defined architecture and potentially applicable as low surface energy materials capable of preventing sticking and adhesion. In particular, bromine-end-capped polystyrene macroinitiators have been used to polymerize either fluorinated (meth)acrylate [119−121] or fluorinated styrene [122, 123] monomers.

In this work fluorinated amphiphilic block copolymers P(S-b-Sz) were synthesized by ATRP of the ethoxylated-fluoroalkyl monomer Sz at 115 °C with PS as macroinitiator, the CuBr/Bipy complex as catalyst and anisole as solvent (Scheme 3.5). In particular, we used four different polystyrene macroinitiators, having molecular weights of 2700, 5300, 6200 and 8400 g/mol, for the preparation of different sets of samples, in which the first block was constant while the amphiphilic second block was appropriately varied either by changing the monomer/macroinitiator molar ratio while keeping constant the reaction time, or by keeping constant the ratio and changing the reaction time (Tab. 3.3). These different experiment conditions allowed us to vary both the copolymer composition and the relative lengths of the two blocks over a broad range of values.

The block copolymers will be labelled as P(Sa-b-Sz)b to underline the molar mass of the polystyrene block in kg/mol (a) and its mole percentage (b).

Br

COO(CH2CH2O)xCH2CH2(CF2CF2)yF

Sz x ! 5, y ! 4 +

P(S)

CuBr/Bipy

COO(CH2CH2O)xCH2CH2(CF2CF2)yF P(S-b-Sz)

x ! 5, y ! 4 Anisole, T=115°C

n m

Br n

Scheme 3.5. Synthesis of block copolymers P(S-b-Sz) via ATRP.

The catalyst was removed either by filtration of the polymer mixture on neutral alumina

(for P(S5.3-b-Sz) and P(S6.2-b-Sz)) or by repeated extractions with distilled water until it

became colorless (for P(S2.7-b-Sz) and P(S8.4-b-Sz)). The polymers were soluble in

most of the organic solvents (THF, CHCl

3

, trifluorotoluene, hexafluorobenzene) and were

purified by numerous precipitations into methanol. This kind of purification allowed the

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50

removal of the homopolymer P(Sz) eventually formed during the reaction with a non- controlled mechanism.

Table 3.3. Experimental conditions for the synthesis of P(S-b-Sz) block copolymers.

Run P(S)

(g/mol)

Sz:P(S)

a)

t

reac.b)

(h)

Yield

c)

(%)

P(S8.4-b-Sz)95 8400 5:1 66 86

P(S8.4-b-Sz)93 8400 10:1 66 85

P(S8.4-b-Sz)88 8400 20:1 66 75

P(S8.4-b-Sz)81 8400 50:1 66 44

P(S8.4-b-Sz)68 8400 100:1 66 31

P(S6.2-b-Sz)89 6200 10:1 66 67

P(S5.3-b-Sz)90 5300 10:1 64 45

P(S5.3-b-Sz)75 5300 20:1 24 39

P(S5.3-b-Sz)72 5300 20:1 66 71

P(S2.7-b-Sz)88 2700 5:1 66 66

P(S2.7-b-Sz)87 2700 5:1 24 66

P(S2.7-b-Sz)78 2700 10:1 24 67

P(S2.7-b-Sz)67 2700 20:1 66 51

P(S2.7-b-Sz)66 2700 10:1 66 60

P(S2.7-b-Sz)58 2700 100:1 66 20

P(S2.7-b-Sz)53 2700 50:1 66 46

a) Sz/P(S) mole ratio. b) Reaction time. c) Yield calculated as [polymer weight / (monomer weight + P(S) weight)]⋅100.

The actual copolymerization, by the incorporation of the fluorinated-pegylated block, was confirmed by GPC,

1

H NMR and

19

F NMR spectroscopy analyses (Fig. 3.1).

As in the case of random copolymers, the block copolymer composition was calculated on the basis of the integrated area of the

1

H NMR signal at 4.4 ppm (COOCH

2

) relevant to the Sz block.

Since the polymerization degree (DP

n

) of P(S) was known, it was possible to calculate the

polymerization degree (DP

m

) of the second block Sz.

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51

Figure 3.1.

19

F spectrum of P(S-b-Sz)72 (in CDCl

3

/CF

3

COOH).

Figure 3.2 shows the GPC traces of P(S)8.4 and the resultant block copolymers P(S8.4-b- Sz)95, P(S8.4-b-Sz)81 and P(S8.4-b-Sz)68. The elution time for the block copolymers was shorter than that for P(S)8.4, indicating that their molecular weight was larger than that of P(S)8.4. Moreover, the elution time for block copolymers decreased, and thus the molecular weight rose, as the monomer/macroinitiator ratio increased.

The monomodal shape on the GPC plots of the block copolymers suggested the absence

of a homopolymer composed of either styrene or Sz and the complete initiation of the

macroinitiator during the ATRP process. However, the polydispersity values were high

(1.35 ≤ M

w

/M

n

≤ 1.77) (Tab. 3.4), probably because of the inherent high polydispersity of

the side chain in the monomer Sz (M

w

/M

n

= 1.3). The M

n

values tested by GPC should be

considered as an approximation of the actual values, because the block copolymers do not

have the same hydrodynamic volume as the polystyrene standards used for calibration.

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52

90x103 80 70 60 50 40 30 20 10 0

Intensity (a.u.)

30 25

20 15

10 5

0

time (min) P(S)8.4

P(S8.4-b-Sz)95 P(S8.4-b-Sz)81 P(S8.4-b-Sz)68

Figure 3.2. GPC traces of P(S)8.4, P(S8.4-b-Sz)95, P(S8.4-b-Sz)81, P(S8.4-b-Sz)68.

Table 3.4. Physical-chemical properties of P(S-b-Sz) block copolymers.

Run Sz

a)

(mol %)

Sz

a)

(wt %)

M

nb)

(g/mol) (GPC)

M

nb)

(g/mol) (NMR)

DP

mc)

Sz

DP

nd)

S

M

w

/M

ne)

P(S8.4-b-Sz)95 5 28 13500 11700 4 81 1.48

P(S8.4-b-Sz)93 7 37 17400 13300 6 81 1.62

P(S8.4-b-Sz)88 12 52 21700 17300 11 81 1.70

P(S8.4-b-Sz)81 19 65 21300 23900 19 81 1.50

P(S8.4-b-Sz)68 32 79 27300 40100 39 81 1.73

P(S6.2-b-Sz)89 11 49 11100 11900 7 60 1.50

P(S5.3-b-Sz)90 10 47 13300 10200 6 51 1.50

P(S5.3-b-Sz)75 25 72 16000 19100 17 51 1.51

P(S5.3-b-Sz)72 28 75 22800 21600 20 51 1.77

P(S2.7-b-Sz)88 12 52 9300 6000 4 26 1.65

P(S2.7-b-Sz)78 22 69 13200 9200 8 26 1.69

P(S2.7-b-Sz)67 33 79 10100 13300 13 26 1.35

P(S2.7-b-Sz)66 34 80 13200 14100 14 26 1.63

P(S2.7-b-Sz)58 42 85 19100 18200 19 26 1.69

P(S2.7-b-Sz)53 47 87 20500 21400 23 26 1.54

a) Mole and weight percentage of Sz. b) Calculated by GPC and 1H NMR. c) Polymerization degree of Sz calculated by 1H NMR. d) Polymerization degree of S calculated by GPC. e) Polydispersity index calculated by GPC.

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53

Amphiphilic block copolymers of ethylene oxide and fluorinated methacrylate have been recently studied by Hussain et al [124]. Also, Vaidya and Chaudhury [125] have investigated the surface properties of amphiphilic polyurethanes prepared by reacting fluorinated diols with isocyanate-terminated poly(ethylene oxide)-b-poly(dimethyl siloxane)-b-poly(ethylene oxide). Guidipati et al. [126] have studied the biological properties of an amphiphilic network containing hyperbranched fluoropolymers crosslinked with poly(ethylene glycol). Moreover, examples have been reported recently about the synthesis of block copolymers of styrene and methyl methacrylate [127, 128]

and poly(propylene glycol) methacrylate [129] starting from an amphiphilic Zonyl based macroinitiator. All those structures would require mesoscale rearrangement of the hydrophobic and hydrophilic domains, leading to a complex topography with a low adhesion towards some marine organisms, e.g. Ulva.

On the other hand, we hypothesized that the polymers of this work could favour environment-dependent surface reconstruction by simple flipping of the fluorinated- pegylated side chains. Moreover, one would expect that if the surface is covered with a thin layer of the ethoxylated fluoroalkyl side chains, any change in surface polarity would occur uniformly throughout the surface, without the complex topographic changes observed by others [130, 131].

After this PhD work was started, we became aware that Ober et al. [42] were developing amphiphilic block copolymers similar to those of our interest:

COO(CH2CH2O)xCH2CH2(CF2CF2)yF

n m

However, in that case the synthesis was based on the grafting of Zonyl chains onto a

preformed polystyrene-b-poly(acrylic acid) copolymer. That synthetic route, although

versatile, leads to a number of structural defects in the final block copolymer. In addition,

copolymers with a polystyrene backbone appeared better suitable than did polystyrene-

polyacrylate copolymers to promote phase segregation in the block copolymer, either

alone or blended with a thermoplastic SEBS elastomer for creating low surface energy

and low elastic modulus materials.

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54

3.1.5. Thermal properties

The thermal behavior of block copolymers was studied by differential scanning calorimetry (DSC), with special attention to detection of their thermal transitions.

In any case the polymers were amorphous and showed only glass transitions (Fig.3.3).

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Heat Flow (W/g)

120 100

80 60

40 20

0 -20 -40

-60

Temperature (°C)

P(S2.7-b-Sz)67 P(S5.3-b-Sz)72 P(S8.4-b-Sz)93 P(S6.2-b-Sz)89 Tg

Tg Tg

Tg Tg

exo> Tg

Figure 3.3. DSC traces of different block copolymers.

In particular, only the copolymers with a mole content of Sz ≤ 11% showed two clear glass transition temperatures at ~ −35 °C, typical of the amphiphilic polystyrene block, and at ~ 90 °C, typical of the polystyrene block. The presence of two glass transitions was due to the phase segregation of the chemically incompatible blocks in separate domains.

For all the other block copolymers, only one glass transition temperature was detected in the range between −35 °C and −40 °C, in correspondence of the amphiphilic block.

Consistent with the thermal behavior of the richest block copolymers in S units, we

assume that the two blocks were also micro-separated in different structures in the

copolymers having higher molar percentages of Sz, even though it was not possible to

detect the T

g

of the polystyrene block by DSC (Tab. 3.5).

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55

Table 3.5. Glass transition temperature of some of the amphiphilic polymers.

Polymer T

g

(°C)

a)

Sz S

P(Sz) −36

P(S-co-Sz)65 −40

P(S-co-Sz)34 −39

P(S)8.4 103

P(S8.4-b-Sz)95 −40 90

P(S8.4-b-Sz)93 −39 85

P(S8.4-b-Sz)88 −38 n.d.

b)

P(S8.4-b-Sz)81 −37 n.d.

b)

P(S8.4-b-Sz)68 −38 n.d.

b)

P(S6.2-b-Sz)89 −38 84

P(S5.3-b-Sz)72 −35 n.d.

b)

P(S2.7-b-Sz)78 −39 n.d.

b)

P(S2.7-b-Sz)67 −40 n.d.

b)

P(S2.7-b-Sz)53 −38 n.d.

b)

a) Heating rate 10 °C/min. b) Not detected by DSC.

To evaluate the thermal stability of the amphiphilic polymers, thermal gravimetric analysis (TGA) was performed under nitrogen atmosphere starting from room temperature up to 700 °C with a heating rate of 10 °C/min. Moreover, the thermal properties were compared with those of polystyrene-b-poly(ethylene-co-butylene)-b- polystyrene (SEBS) triblock thermoplastic elastomers and SEBS grafted with 1.4−2 wt % of maleic anhydride (SEBS-MA), which are the main components for the preparation of bilayer films for marine biological testing (see § 3.1.16).

All the amphiphilic polymers showed a relatively high thermal decomposition

temperature around 400 °C (T

max

) and a weight loss lower than 1% until 230−260 °C (T

1

)

(Tab. 3.6). The TGA traces of block copolymers, as well as those of P(S) and SEBS,

showed a single step of weight loss basically due to the degradation of the polystyrene

backbone (Figs. 3.4 and 3.5).

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56

Table 3.6. Thermogravimetric data of amphiphilic polymers and SEBS.

Polymer T

1a)

(°C)

T

maxb)

(°C)

P(Sz) 147 390

P(S-co-Sz)34 148 402

P(S8.4-b-Sz)93 229 394

P(S6.2-b-Sz)89 255 398

P(S5.3-b-Sz)72 263 399

P(S2.7-b-Sz)67 233 394

P(S2.7-b-Sz)53 241 391

P(S) 238 414

SEBS

c)

384 445

SEBS-MA

c)

368 451

SEBS/SEBS-MA

d)

377 449

a) 1 wt % loss temperature. b) Temperature of the maximum weight loss rate. c) SEBS and SEBS-MA contain ∼ 30 wt % poly(styrene). d) Blend containing 44 wt % SEBS and 66 wt % SEBS-MA.

100

80

60

40

20

0

Residual Weight (%)

600 500

400 300

200

100 Temperature(°C)

P(S) P(Sz) P(S-co-Sz)34 P(S8.4-b-Sz)93 P(S6.2-b-Sz)89 P(S5.3-b-Sz)72 P(S2.7-b-Sz)67 P(S2.7-b-Sz)53

Figure 3.4. TGA traces of block copolymers, polystyrene P(S) and amphiphilic

polystyrene P(Sz).

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57

100

80

60

40

20

0

Residual Weight (%)

600 500

400 300

200

100 Temperature (°C)

SEBS SEBS-MA SEBS/SEBS-MA

Figure 3.5. TGA traces of SEBS, SEBS-MA and a blend of SEBS/SEBS-MA with 44 wt

% of SEBS.

It can be noticed that the introduction of polystyrene block seemed to stabilize the copolymer to thermal degradation with respect to the homopolymer and the random copolymer (T

1

∼ 147 °C). Such findings confirmed the general stability of those samples, ensuring that the subsequent thermal and thermo-mechanical treatments (e.g. annealing and moulding) would not damage the polymer films to any significant extent.

3.1.6. Preparation of polymer films

In order to study the morphology and wetting properties of the amphiphilic polymers, films were prepared according to two different methods (Fig. 3.6):

a) a thin layer of the active polymer was deposited on a glass slide by spin-coating or dip coating (monolayer geometry).

b) a thin layer of the active polymer was spry coated on a bottom layer consisting of

SEBS (bilayer geometry).

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58

Figure 3.6. Schematic representation of two different polymer film geometries: (a) monolayer and (b) bilayer.

According to the latter strategy, it was possible to deposit a low surface energy top layer on a low elastic modulus bottom layer.

The coatings were annealed at 120 °C for 12−15 h in order to promote the formation of an equilibrium structure.

The presence of two layers with different thicknesses and the strength of adhesion between each other were analyzed by SEM on cryogenic fracture sections. SEM images confirmed the actual deposition of a thin film of fluorinated polymer (ca. 300 nm thick) on the SEBS layer (ca. 200 µm thick) (Fig. 3.7 and Fig. 3.8).

Figure 3.7. SEM image of the bilayer film of P(S5.3-b-Sz)72 before annealing.

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59

Figure 3.8. SEM image of the bilayer film of P(S5.3-b-Sz)72 after annealing at 120 °C for one night.

The two layers appeared to be partially welded together and had a tendency to delaminate at the interface, this phenomenon being less marked for the annealed samples. However, it is worth noting that the fractures were obtained by submission of the samples to drastic treatments, such as deformation under liquid nitrogen, which could have induced different mechanical behavior on the two layers, leading to delamination.

EDX elemental analysis carried out on bilayer films showed the uniform presence of fluorine at the polymer-air interface, whereas it was completely absent in the SEBS layer.

This finding indicated that the fluorinated polymer was really segregated in the top layer without penetrating significantly into the bottom layer (Tab. 3.7).

Even within the resolution limitations of the EDX, the amount of fluorine in the top layer

was higher in the annealed films than in equivalent samples, which were not submitted to

thermal treatment (4 wt % against 2 wt % of fluorine). Thus thermal annealing promoted

the formation of an equilibrium structure where the fluorinated chains tend to migrate to

the outermost surface, owing to their low surface energy, and to segregate in nano-

domains uniformly distributed at the surface (Fig. 3.9).

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60

Table 3.7. Chemical elemental analysis of the top and the bottom layers of the polymer films used for SEM analysis.

atom

wt % not annealed

top layer

wt % annealed top surface

wt % not annealed bottom layer

wt % annealed bottom layer

C 86 94 93 88

O 12 2 7 12

F 2 4 - -

Figure 3.9. Field emission SEM image of P(S8.4-b-Sz)88 deposited by spin coating on silicon wafer and annealed at 120 °C (the red spots are the fluorinated species).

3.1.7. Wetting behavior and surface energy

3.1.7.1. Static contact angle measurements and surface tension

Static contact angle measurements were performed on thin (200−300 nm thickness) polymer films, which were spin coated onto glass slides from a 3 wt % solution and then annealed for one night at 120 °C to achieve equilibrium morphologies. In fact, XPS and NEXAFS measurements carried out on similar amphiphilic surfaces by Ober et al. [42]

showed that annealing influenced the chemical composition of the surface. In particular,

they observed that the fluorinated carbon atom (−CF

2

, −CF

3

) concentration increased with

increasing annealing temperature.

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61

AFM root mean square R

q

= (Σ

i

Z

i2

/N)

0.5

ranged between 2 and 4 nm, where Z

i

are the height values and N the number of points measured on the analyzed surface. Those values were very low, therefore the effect of surface roughness on contact angle measurements could be neglected [132].

Deionized water, artificial seawater (ASW), n-hexadecane, diiodomethane and ethylene glycol were used as interrogating liquids (Tab. 3.8), in order to obtain contact angle values ( θ ), which were used for the calculation of polymer surface tension according to the additive component approaches. Moreover, we performed measurements with n- heptane, n-octane, n-decane on three samples (Tab. 3.9), to evaluate the surface tension with the equation of state method.

Table 3.8. Static contact angle values for the calculation of surface tension with additive component methods.

Polymer θ

wa)

(°)

θ

ASWb)

(°)

θ

hc)

(°)

θ

DId)

(°)

θ

EGe)

(°) P(S8.4-b-Sz)93 106 ± 1 99 ± 1 67 ± 1 87 ± 1 92 ± 1 P(S8.4-b-Sz)88 108 ± 1 101 ± 0 69 ± 1 84 ± 2 94 ± 1 P(S8.4-b-Sz)81 107 ± 1 101 ± 1 66 ± 1 92 ± 2 92 ± 1 P(S8.4-b-Sz)68 104 ± 1 105 ± 1 63 ± 1 81 ± 2 94 ± 1 P(S5.3-b-Sz)72 100 ± 1 100 ± 1 66 ± 1 84 ± 1 93 ± 2 P(S2.7-b-Sz)88 103 ± 1 104 ± 1 66 ± 1 - 92 ± 1 P(S2.7-b-Sz)78 108 ± 1 100± 1 69 ± 1 91 ± 1 92 ± 1 P(S2.7-b-Sz)67 102 ± 1 104 ± 1 66 ± 1 81 ± 2 95 ± 1 P(S2.7-b-Sz)53 106 ± 1 109 ± 1 66 ± 1 84 ± 2 92 ± 1

a) Contact angle measured with water. b) Contact angle measured with artificial seawater. c) Contact angle measured with n-hexadecane. d) Contact angle measured with diiodomethane. e) Contact angle measured with ethylene glycol.

The values of θ with water and n-hexadecane are conventionally regarded as estimations

of hydrophobicity ( θ

w

> ∼ 90°) and lipophobicity ( θ

h

> ∼ 60°), respectively. According to

this criterion all the amphiphilic polymers resulted to be both hydrophobic ( θ

w

≥ 99°) and

lipophobic ( θ

w

≥ 61°). The values of θ

w

were not markedly affected when the

interrogating liquid was ASW. A preferential surface segregation of the fluorinated

species was implicated, in agreement with numerous similar findings on fluorinated

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62

polymer surfaces. We note that the values of θ associated to each wetting liquid did not change significantly and univocally for polymers which contained different amounts of fluorinated moieties, not even when their concentration was very low (e.g. 7 mol % in P(S8.4-b-Sz)93). Thus, we can suppose that a threshold of surface concentration of fluorinated chains existed, above which an increase in Sz units did not lead to an increase in contact angle.

The role of fluorinated chains in determining the surface properties of polymer films is particularly clear when one compares the wettability behavior of the block copolymers with that of SEBS based films and that of SEBS/P(S-b-Sz) blends (Tab. 3.10).

Table 3.9. Static contact angle values for the calculation of surface tension with the equation of state method.

Liquid P(Sz)

θ (°)

P(S-co-Sz)34 θ (°)

P(S6.2-b-Sz)89 θ (°)

water 103 ± 1 99 ± 1 101 ± 1

ASW 101 ± 2 101 ± 1 104 ± 1

n-hexadecane 64 ± 1 61 ± 1 66 ± 2

diiodomethane 81 ± 3 79 ± 2 82 ± 2

ethylene glycol 88 ± 2 85 ± 2 90 ± 2

n-heptane 52 ± 2 49 ± 2 51 ± 1

n-octane 52 ± 1 51 ± 2 55 ± 2

n-decane 58 ± 2 58 ± 1 59 ± 2

n-dodecane 60 ± 1 60 ± 2 62 ± 1

Table 3.10. Static contact angle values for SEBS blends with block copolymers.

Polymer θ

wa)

(°)

θ

ASWb)

(°)

θ

hc)

(°)

θ

DId)

(°)

θ

EGe)

(°)

SEBS 103 ± 1 101 ± 2 20 ± 1 64 ± 3 74 ± 2

SEBS/SEBS-MA 101 ± 1 102 ± 1 19 ± 0 65 ± 1 74 ± 1 P(S5.3-b-Sz)72/SEBS

f)

113 ± 2 108 ± 1 71 ± 1 85 ± 1 99 ± 1 P(S6.2-b-Sz)89/SEBS

f)

107 ± 1 106 ± 1 67 ± 1 82 ± 2 94 ± 1

a) Contact angle measured with water. b) Contact angle measured with artificial seawater. c) Contact angle measured with n-hexadecane. d) Contact angle measured with diiodomethane. e) Contact angle measured with ethylene glycol. f) Blend composition: 10 wt % SEBS, 90 wt % block copolymer.

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63

Although all those films resulted to be hydrophobic, only block copolymer based blends showed a clear lipophobic character ( θ

h

≥ 67°). By contrast, SEBS films were quite lipophilic being wetted by n-hexadecane ( θ

h

∼ 20°). This suggests that the blends of SEBS with the amphiphilic block copolymer were partially compatible and therefore the segregation of the fluorinated block at the outer polymer/air interface was promoted.

Measurements of liquid-solid contact angles are commonly used to evaluate solid surface tension ( γ

sv

). However, the correlation between θ and γ

sv

is still a controversial question and none of the different methods proposed are generally accepted [52]. Accordingly, we followed two alternative approaches to extract the solid surface tension from experimental θ values, namely i) the surface tension component approaches of Owens- Wendt-Kaelble (OWK) [59, 60] and van Oss-Chaudhury-Good (vOCG) [133] and ii) the equation of state method (ES) [64]. The calculated surface tensions γ

sv

are collected in Table 3.11.

The values of the solid surface energy obtained by the surface tension component methods, γ

svOWK

and γ

svvOCG

, well agreed with each other, while they were always greater than the respective values evaluated with the equation of state γ

ES

. This effect seems to be inherent in the system and derives from the difficulty in correlating contact angle values with vastly different surface tensions of the wetting liquids (Fig. 3.10). Moreover, more liquids with intermediate value of γ

lv

should be used to determine the best fit values of γ

sv

and β (see Eq. 8).

In any case, the values of γ

sv

for polymers containing Sz units, were consistent with a low surface energy of the polymer film, being lower than 19 mN/m.

Block copolymers generally showed γ

sv

values lower than those of the homopolymer P(Sz) and the random copolymer P(S-co-Sz)34, suggesting that in the latter polymers the fluorine segregation was less effective than in the block copolymers, where the chemical incompatibility between the two blocks enhanced migration of the fluorinated moieties towards the outer surface.

This tendency was further increased in SEBS/P(S-b-Sz) blends, which showed surface

tension values ( γ

svOWK

≤ 14.9 mN/m) markedly lower than that of pristine SEBS ( γ

svOWK

=

26.4 mN/m) and comparable with those of the amphiphilic block copolymers ( γ

svOWK

17.4 mN/m).

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64

Table 3.11. Comparison of surface tension values calculated with different methods.

Polymer γsvd

mN/m γsvp

mN/m

γsvOWKa)

mN/m

γsvLW

mN/m γsv

mN/m

γsv+

mN/m

γsvvOCGb)

mN/m

γsvESc)

mN/m

P(Sz) 14.4 2.2 16.6 16.7 3.04 7.6⋅103 17.0 15.1

P(S-co-Sz)34 15.1 3.0 18.1 17.9 4.22 6.3⋅103 18.2 15.7 P(S8.4-b-Sz)93 13.3 1.7 15.0 14.1 2.52 1.8⋅102 14.5

P(S8.4-b-Sz)88 12.7 1.4 14.1 15.6 2.08 2.1⋅102 16.0 P(S8.4-b-Sz)81 13.7 1.3 15.0 11.8 2.30 1.1⋅101 12.9 P(S8.4-b-Sz)68 14.5 1.9 16.4 17.1 4.14 1.5⋅101 18.6

P(S6.2-b-Sz)89 13.7 3.0 16.7 16.1 5.00 6.1⋅103 16.5 14.8 P(S5.3-b-Sz)72 13.5 3.9 17.4 15.6 8.58 1.5⋅101 17.9

P(S2.7-b-Sz)78 12.7 1.5 14.2 12.4 2.21 8.5⋅102 13.2 P(S2.7-b-Sz)67 13.6 2.7 16.3 16.6 6.20 2.7⋅101 19.2 P(S2.7-b-Sz)53 13.7 1.6 15.3 15.5 2.62 1.4⋅103 15.6

SEBS 26.0 0.4 26.4 26.1 0.27 1.0⋅101 26.5

SEBS/MA 26.2 0.7 26.9 25.5 0.77 1.0⋅101 26.1

P(S5.3-b-Sz)72/

SEBSd) 12.0 0.7 12.7 15.1 1.30 9.6⋅102 15.8

P(S6.2-b-Sz)89/

SEBSd) 13.5 1.4 14.9 16.4 2.50 7.8⋅102 17.3

a) Calculated with the Owens-Wendt-Kaelble method: γsvd dispersion component, γsvp polar component. b) Calculated with the van Oss-Chaudhury-Good method: γsv+ Lewis acid contribution, γsv Lewis basic contribution, γsvLW Lifshitz- van der Waals contribution. c) Calculated according to the equation of state of Neumann (β = 0.0001234 (m/mN)2). d) Blend composition: 10 wt % SEBS, 90 wt % block copolymer.

As expected of non-polar, non-hydrogen bonding surfaces such as fluorinated surfaces, the dispersion contribution ( γ

svd

≈ 12−15 mN/m) to γ

svOWK

was largely dominant, with γ

svp

being minimal ( γ

svh

≈ 1−4 mN/m). However, the polar component was significantly higher than that calculated for other fluorinated block copolymers [134], probably because PEG segments of Sz units were involved in polar interactions to a greater extent.

Similarly, the apolar parameter γ

svLW

provided the major contribution to γ

svvOCG

with

respect to the acid-basic component γ

sv±

. It can also be emphasized that the electron

donating basic contribution was higher ( γ

sv

≈ 2−9 mN/m) than the electron withdrawing

contribution ( γ

sv+

≈ 1.4⋅10

3

−1.5⋅10

1

mN/m). This result suggests that the polar

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65

interactions although weak, took essentially place with the basic units −CH

2

CH

2

O− more than with acid units −CH

2

CF

2

−, which were very few and practically unaccessible to the wetting liquid.

On the other hand, there was no trend of γ

sv

with the nominal content of oxyethylene units in the copolymer which evidently populated the outer surface layers in an uneven fashion, possibly dependent upon the specific phase segregated morphology.

0.8

0.6

0.4

0.2

0.0

-0.2

Cos!

55 50

45 40

35 30

25 20

Surface Tension "lv (mN/m)

P(Sz) P(S-co-Sz)34 P(S6.2-b-Sz)89

Figure 3.10. Cos θ values fitted with the equation of state for polymers P(Sz), P(S-co- Sz)34 and P(S6.2-b-Sz)89.

3.1.7.2. Contact angle dependence on time immersion in water

Contact angle measurements were performed after relatively short and long periods of immersion in water in order to estimate the stability of polymer films in contact with the wetting liquid. The former kind of measurements (advancing and receding dynamic contact angle) was carried out on films dip coated in a 1.5 wt % toluene solution of the amphiphilic block copolymer (or THF for the homopolymer and the random copolymer);

after slow evaporation of the solvent, the films were dried under vacuum and annealed for

one night at 120 °C (∼ 500 nm thickness). The latter kind of measurements was carried

out on films deposited by the same method as those for the measurements of static contact

angle.

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66

3.1.7.2.1. Advancing and receding contact angle measurements

Experiments on polymer films were carried out to assess the effect of the wetting medium, water, on surface stability.

The first set of measurements consisted of three immersion cycles at 6 mm immersion depth with dwell times between immersion and withdrawal of 10 s. In the second set of measurements the coated slide was advanced by 6 mm and kept immersed for 1000 s.

Then the slide was reversed to the start position and finally advanced for 12 mm, so that an additional 6 mm of fresh surface was exposed to water. Therefore, we could determine the advancing ( θ

a

) and receding contact ( θ

r

) angles and contact angle hysteresis (Δ = θ

a

− θ

r

), for quite short immersion times (∼ 2 min).

In most cases neither θ

a

nor θ

r

changed significantly during the three advancing and receding cycles, even if they tended to decrease. As one such example, P(S8.4-b-Sz)81 showed θ

a

values of 107° and 105° and θ

r

values of 52° and 49°, in the first and third cycles, respectively (Fig. 3.11). Since most of the polymers did not show this behavior, average values of the three cycles are reported in Table 3.12.

0.25

0.20

0.15

0.10

0.05

0.00

-0.05

-0.10

-0.15

Weight (g)

7 6

5 4

3 2

1 0

Immersion Depth (mm)

1st Cycle 2nd Cycle 3rd Cycle

Figure 3.11. Weight-immersion depth curves for the sample P(S8.4-b-Sz)81.

Those variations were not due to contamination effects of the wetting liquid, since no

significant change in water surface tension before and after the experiment was observed.

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67

Table 3.12. Advancing and receding contact angle values for amphiphilic block polymers.

Sample θ

a

(°)

a)

θ

r

(°)

a)

Δ (°)

b)

P(Sz) 101 ± 1 34 ± 2 66

P(S-co-Sz)34 103 ± 1 50 ± 2 53

P(S8.4-b-Sz)93 97 ± 1 54 ± 1 43

P(S8.4-b-Sz)81 106 ± 1 51 ± 2 55

P(S8.4-b-Sz)68 103 ± 3 50 ± 3 53

P(S6.2-b-Sz)89 102 ± 2 40 ± 1 62

P(S5.3-b-Sz)72 106 ± 1 41 ± 1 65

P(S2.7-b-Sz)88 96 ± 4 46 ± 1 50

P(S2.7-b-Sz)78 103 ± 1 42 ± 1 61

P(S2.7-b-Sz)67 106 ± 1 44 ± 2 62

P(S6.2-b-Sz)72/SEBS

c)

107 ± 1 43 ± 1 61

P(S6.2-b-Sz)89/SEBS

c)

98 ± 2 46 ± 1 52

P(S6.2-b-Sz)89/SEBS

d)

99 ± 1 44 ± 1 55

a) Advancing and receding contact angle values calculated as an average on the three immersion cycles. b) Hysteresis: Δ = θa − θr. c) Blend composition: 10 wt % SEBS, 90 wt % block copolymer. d) Blend composition: 30 wt % SEBS, 70 wt % block copolymer.

The θ

a

values were quite high and ranged between 97° and 100°, while θ

r

were lower and generally less than 50°. As a consequence, very high values of hysteresis (50° < Δ < 70°) were found.

Hysteresis of contact angles is usually due to the existence of metastable states at the solid-liquid-vapor interface and in terms of energetics may also be defined as γ (cos θ

r

– cos θ

a

), where γ is the surface tension of the wetting liquid. It implies that the free energy required to separate the liquid from the solid is greater than the energy released during contact. It is mainly connected to three factors: surface roughness, chemical heterogeneity of the topmost layer and surface reconstruction of the polymer film after contact with the liquid [135].

Since, the effect of roughness was negligible, the hysteresis phenomenon typical of these

amphiphilic copolymers was probably due to a combination of chemical heterogeneity

and restructuring of the surface.

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68

While the θ

a

values seemed to be determined by the hydrophobic fluorinated segment of amphiphilic side chains in the Sz block, the θ

r

values were especially sensitive to the hydrophilic polyethylene glycol present in the same Sz block [43].

To confirm the dependence of the contact angle values on the film history in immersion/withdrawal cycles a second set of measurements was carried out. In fact, the curves of weight vs immersion depth showed that in the first 6 mm of the second cycle (after 1000 s of immersion in water) θ

a

and θ

r

values were lower than those related to the second 6 mm, when fresh surface was exposed, thereby causing a net discontinuity in the graph, e.g. for P(S8.4-b-Sz)81 θ

a

= 107° and θ

r

= 52° for the fresh portion and θ

a

= 102°

and θ

r

= 49° for the immersed surface. Moreover, the advancing contact angle of the fresh surface turned out to be the same as for the first cycle (Fig. 3.12). These findings confirm the hypothesis of structural reorganization of the amphiphilic side chains at the polymer- air interface. These results suggest that a reconstruction of the outer molecular layers happened upon short immersion times in water, thanks to the migration of the PEG segments to the water-polymer interface. In fact this phenomenon can occur by flipping of the amphiphilic side chains, which facilitates the enthalpically favourable interaction of PEG with water, while simultaneously minimizes the water contact angle of the hydrophobic fluoroalkyl segments (Fig. 3.13).

0.25 0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15

Weight (g)

14 12

10 8

6 4

2 0

Immersion Depth (mm)

1st Cycle 2nd Cycle 3rd Cycle

old surface

fresh surface

Figure 3.12. Weight-immersion depth curves for the polymer P(S8.4-b-Sz)81: 1

st

cycle 6

mm depth/1000 s dwell time, 2

nd

and 3

rd

cycles 12 mm depth/10 s dwell

time.

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69

(a) (b)

Figura 3.12. Proposed mechanism for surface reconstruction of the ethoxylated fluoroalkyl side chains upon immersion of the surface in water: orientation of side chains in air (a) and effect of water immersion (b).

3.1.7.2.2. Static contact angle measurements as a function of immersion time

Films of the amphiphilic polymers were completely immersed in water and the contact angles were measured at different immersion times up to a maximum of 14 days (Fig.

3.14). For all samples, θ decreased as the immersion time increased; this trend was more pronounced at short immersion times and then proceeded more gradually until it reached a plateau value after about 7 days.

At any time, contact angle values were higher for the films of block copolymers with respect to homopolymer and random copolymer, the highest θ values being recorded for P(S6.2-b-Sz)89 (e.g. θ = 85° after 14 days) upon the whole immersion period (Fig. 3.14).

This experiment suggests that another reorganization process existed spanning over a longer time scale than did the previous one. Such a molecular reorganization of the surface could occur by at least two different mechanisms: (i) the migration of the polystyrene block away from the interface and (ii) the riorientation of Sz chains by the flipping (Fig. 3.12). In the first case the surface reconstruction led to a strong segregation of the polystyrene block into the bulk and of the ethoxylated-fluorinated active block to the surface of the polymer film.

Such processes likely occur simultaneously, but with different kinetics since the first one

needed long immersion times (days according to the static contact angle measurements),

while the second one was a very quick process which took place after short immersion

times (minutes according to θ

a

and θ

r

measurements).

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70

105

100 95 90 85 80 75 70

! (°)

20000 18000 16000 14000 12000 10000 8000

6000 4000 2000 0

time (min)

P(Sz)

P(S-co-Sz)34 P(S6.2-b-Sz)89 P(S5.3-b-Sz)72

Figure 3.14. Contact angle θ vs time of immersion in water for P(Sz), P(S-co-Sz)34, P(S6.2-b-Sz)89 and P(S5.3-b-Sz)72.

3.1.8. Chemical surface analysis

A detailed study of the morphology and chemical composition of the polymer surfaces was started using both spectroscopy (XPS, GISAXS, NEXAFS, SIMS) and microscopy (AFM) methods. They were available thanks to national and international collaborations.

A brief paragraph, that includes the main practical aspects of each technique, is reported in the Appendix.

3.1.8.1. XPS analysis

Information about the chemical composition of the surface was obtained by means of X- ray photoelectron spectroscopy (XPS). Polymer films were prepared by spin-coating a diluted solution of the polymer (∼ 3 wt %) either on glass slide or on silicon wafer. Then they were dried and annealed at 120 °C for one night.

The films of the polymers reported in Table 3.13 were analyzed before (‘dry’ films) and

after 7 days of immersion in water (‘wet’ films); in any case the XPS analyses were

carried out in high vacuum conditions, therefore in equilibrium conditions different from

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71

those actually reached after the treatments mentioned above. XPS spectra were acquired at different electron emission angles φ (the angle between the surface normal and the path taken by the electrons toward the detector) which corresponded to sampling depths of approximately 1−7 nm [43].

For all the investigated sampling depths, the survey spectra showed three different peaks due to C (1s) at ∼ 285 eV, O (1s) at ∼ 533 eV and F (1s) at ∼ 689 eV (Fig. 3.15).

It was also possible to detect a peak at ∼ 40 eV attributed to the (2s) transition, even if its intensity was too low to be considered for quantitative assessment. Moreover, survey spectra showed peaks at binding energies higher than 800 eV, connected with Auger electrons (not reported in Fig. 3.15).

8000

6000

4000

2000

0

Intensity (a.u.)

800 600

400 200

Binding Energy (eV)

20°

50°

70°

C (1s) O (1s)

F (1s)

Figure 3.15. Survey XPS spectra of P(S2.7-b-Sz)53 at various emission angles φ .

The elemental analysis data for the different emission angles φ are summarized in Table 3.13, where they are also compared with the corresponding values calculated from the known stoichiometric ratios of the block components. The atomic percentage dependence on angle φ showed that there was a composition gradient normal to the film surface into the bulk.

Since the emission efficiency was not constant, but decreased with increasing distance

from the outer layers, the reported values of composition at different emission angles

cannot be used to define a concentration profile of the atomic species along the surface

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72

normal. Anyway semi-quantitative considerations are in order to establish a realistic chemical composition of the film surface.

Table 3.13. XPS atomic composition of ‘dry’ and ‘wet’ block copolymers alone and blended with SEBS.

Polymer φ ‘dry’ ‘wet’

C O F C O F

% % % % % %

P(S5.3-b-Sz)72 stoichiometric

a)

67 10 23

70° 50.2 11.3 38.5 60.0 16.6 23.4

50° 55.4 12.7 31.9 55.1 14.9 30.0

20° 64.1 13.3 22.6 65.3 14.5 20.2

P(S2.7-b-Sz)78 stoichiometric

a)

70 9 21

70° 51.1 11.4 37.5 58.2 16.7 25.1

50° 57.2 11.6 31.2 63.7 15.1 21.2

20° 64.5 12.1 23.4 67.6 15.1 17.3

P(S2.7-b-Sz)67 stoichiometric

a)

65 10 25

70° 50.7 10.5 38.8 49.4 13.7 36.9

50° 53.9 12.5 33.6 53.0 15.6 31.4

20° 61.9 13.1 25.0 59.8 15.1 25.1

P(S2.7-b-Sz)53 stoichiometric

a)

61 11 28

70° 45.0 12.2 42.8 50.3 14.3 35.4

50° 49.3 14.8 35.9 54.8 16.3 28.9

20° 54.2 16.3 29.5 63.8 17.5 18.7

P(S2.7-b-Sz)78/SEBS

b)

70° 52.4 10.7 36.9 63.5 13.9 22.6

50° 60.0 12.3 27.7 67.2 14.5 18.3

20° 65.4 11.8 22.8 73.2 13.5 13.3

a) Calculated on the basis of the known molar composition of the polymer. b) Blend composition: 50 wt % SEBS and 50 wt % P(S2.7-b-Sz)78.

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73

The values in Table 3.13 show that the experimental C atomic percentage was lower than the stoichiometric one and increased with increasing sampling depth, for instance, from 51.1% to 64.5% in passing from φ = 70° to φ = 20° for P(S2.7-b-Sz)78. On the other hand, the F atomic percentage followed the opposite trend, being in general significantly higher than the stoichiometric percentage. For instance, it decreased from 37.5% at φ = 70° to 23.4% at φ = 20°. This means that the polymer surface was enriched in fluorine content thanks to the tendency of the fluorinated chains to migrate to the surface because of their low surface energy. The same remarks are also valid for a SEBS-based blend containing 50 wt % P(S-b-Sz)78, which had a fluorine content (36.9% at φ = 70°) in the topmost layer very similar to that of the amphiphilic block copolymer alone (37.5% at φ = 70°). The O atomic percentage was slightly higher than the stoichiometric percentage and generally increased with decreasing φ . This finding is consistent with the increment of oxyethylene segments in the bulk of the segregated amphiphilic block copolymer film.

In the ‘wet’ samples the atomic percentage of fluorine was significantly lower as compared to the respective ‘dry’ samples and generally decreased with increasing sampling depth, e.g. from 25.1% to 17.3% in going from φ = 70° to φ = 20° for P(S2.7-b- Sz)78. On the contrary, the O atomic percentage was higher in ‘wet’ samples and quite independent of φ , for instance it increased from 11.4% to 16.7% at φ = 70° for the same block copolymer. Therefore, the concentration of PEG segments at the surface was higher after exposure to water, as a consequence of the flipping of the amphiphilic side chains.

This hypothesis agreed very well with the results obtained from contact angle measurements and was confirmed by XPS and NEXAFS analyses.

Moreover one can note that also the C atomic percentage increased in passing from ‘dry’

to ‘wet’ samples, being at φ = 70° 51.1% and 58.2%, respectively. It was more likely that this increase depended on both the pegylated chains and the styrene backbone in the Sz block, which were more exposed at the surface as a consequence of the flipping process, than on the polystyrene block, which tended to migrate into the bulk because of its hydrophobic nature.

In all the XPS spectra, the C (1s) peak revealed a complex shape and the fitting procedure

indicated the presence of at least five main contributions as shown for P(S5.3-b-Sz)72 at

φ = 50° in Figure 3.16. The signals at ∼ 292 eV and ∼ 294 eV corresponded to the −CF

2

and −CF

3

moieties of the perfluorinated chains. The peak at ∼ 289 eV was attributed to

C=O of the ester group, whereas the partially resolved peaks at ∼ 285 eV and ∼ 287 eV

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