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2. OBJECTIVES OF THE WORK

The design and the control of a polymer surface structure and/or nanostructure are topical subjects. Indeed, the comprehension of the correlation between the structure and the properties of a material surface and the capability of appropriately tuning the chemical-physical properties at a molecular level can lead to novel developments in all those applicative fields, where interfacial interactions, operating within a few nanometers of a surface, are critical.

Biofouling is a prime example of a nanoscale adhesion process, occurring at the interface between fouling organisms and all the man-made surfaces, in both marine and freshwater environments. It is an issue of high practical and economical relevance and its control has imposed environmental burdens through the use of toxic antifouling paints, which are now banned. Since current non-biocidal coatings are unsuitable for most applications, there is a ‘technology gap’ demanding innovation. Hence the ultimate motive of the present research project was to develop innovative, non-biocidal coatings for the use in aquatic environment as an alternative to traditional, vastly used toxicant coatings based on cuprous oxide or organotin compounds. Once incorporated into a coating, such material should be intrinsically fouling resistant and allow the formation of only a weak bond with fouling organisms, being at the same time environmentally benign.

Since low surface energy and low elastic modulus are regarded as two of the most important characteristics for future generation coatings, the main aim of this work was to develop novel materials which combined these properties into one polymeric system and a formulated coating therefrom in a synergistic way. To pursue this objective, we studied three different polymeric systems, all of them being characterized by the presence of semifluorinated side chains capable of minimizing the surface tension and, in principle, of enriching the polymer surface of fluorine thereby effecting a non-stick behaviour of the material.

Amphiphilic polystyrene block copolymers, (Fig. 2.1) synthesized by copper-mediated controlled radical polymerization (ATRP) starting from a polystyrene macroinitiator, appeared to be suitable materials as they can generate a well ordered nanostructured morphology, because of the chemical incompatibility of the two blocks associated with a selective segregation of the fluorinated tails to the surface. Furthermore, the amphiphilic character owing to the ethoxylated fluoroalkyl side chains could be expected to lead to a

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hydrophobic surface in an air environment, which could rearrange in a responsive way to a more hydrophilic structure upon immersion in water. This ‘smart’ compliance to the external environment was useful to create an “ambiguous”, “chaotic” surface able to confuse aquatic organisms during settlement and adhesion, resulting in improved antifouling/fouling release properties of the coating.

m n

COO(CH2CH2O)xCH2CH2(CF2CF2)yF

Figure 2.1. The structure of amphiphilic polystyrene based block copolymers.

In order to impart to this low surface polymer a good elastomeric character, we devised bilayer films, in which the top layer contained the fluorinated amphiphilic block copolymer and the bottom layer was a thermoplastic elastomer (SEBS).

One further objective of the work was to evaluate, on a laboratory scale, the antifouling/fouling release performances of the bilayer films, thus assessing their potential for practical application, e.g. ship hulls and pleasure boats, aquaculture and fishing nets and optical windows of immersed instrumentation. Therefore, they were subjected to biological testing against different marine and freshwater organisms, such as the alga Ulva linza, the diatom Navicula perminuta, the bacteria Pseudomonas fluorescens, Cobetia marina, Marinobacter hydrocarbonoclasticus and Vibrio alginolyticus, and the barnacle Balanus amphitrite.

Fluorinated/siloxane graft random copolymers (Fig. 2.2) were synthesized by free radical polymerization of a bicycloacrylate monomer, bearing a perfluorinated side chain, with a polysiloxane methacrylate graft. The advantage of these copolymers was to combine in the same structure the low surface energy properties of the fluorinated side chains and the low elastic modulus imparted by the polydimethylsiloxane grafts. Moreover, this class of materials was expected to self-assemble in a liquid crystalline smectic mesophase, thanks to the long rigid-rod-like semifluorinated tails. The presence of a well organized structure in the bulk and at the surface should give to the entire material a greater stability against reconstruction upon exposure to water. Indeed, the fluorinated side-groups of a polymer residing in a smectic mesophase and covering the surface of a polymer film should pay a

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high enthalpy penalty to rearrange upon exposure to water escaping from the surface and hiding into the bulk of the coatings.

O(CH2)3 (SiO) SiC4H9

10

O COO(CH2)2(CF2)10F

Figure 2.2. The structure of fluorinated/siloxane graft copolymers.

Fluorinated-polysiloxane based block copolymers were prepared by controlled radical polymerization (ATRP) of living polydimethylsiloxane macroinitiators in the presence of either a fluorinated ethoxylated styrene monomer (Fig. 2.3) or a fluorinated acrylate monomer (Fig. 2.4) in order to obtain an amphiphilic or a liquid crystalline elastomeric block copolymer, respectively.

SiO SiCH2CH2CH2OCH2CH2O C4H9SiO

O

COO(CH2CH2O)xCH2CH2(CF2CF2)yF

n

Figure 2.3. The structure of polysiloxane based amphiphilic block copolymers.

SiO SiCH2CH2CH2OCH2CH2O

C4H9SiO

O

O OCH2CH2(CF2CH2)4F

n

Figure 2.4. The structure of polysiloxane based liquid crystalline block copolymers.

Fluorinated liquid crystalline block copolymers possess two extra levels of self-assembly in addition to the assembly of blocks into periodic microdomain morphologies commonly formed by linear coil-coil diblock copolymers. Indeed, in the liquid crystalline domains, the fluorinated mesogenic side chains are organized in a lamellar structure of a smectic mesophase, and in addition to this level of organization, the individual side chains could be structured within each smectic lamella.

Thanks to the presence of polydimethilsiloxane chains, the latter two classes of copolymers could be easily dispersed in commercial PDMS matrix. Anyway, the

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preparation of PDMS based blends containing fluorinated surface active copolymers was beyond the aim of this thesis and will be studied in another work.

All the above polymeric materials were subject of a careful investigation of their bulk and surface characteristics. The characterization took into account not only classical aspects such as molecular structure and composition and thermal behaviour, but also surface composition, wettability and orientation (XPS, SIMS, CA, and NEXAFS) with particular attention to the study of surface micromorphologies (AFM and GISAXS) of the block copolymers.

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