4. CONCLUDING REMARKS
The current challenge of the prevention or limitation of biofouling in the aquatic environment demands innovation. On one side, the great diversity of foulants and their mechanisms of adhesion on a surface impose the use of protective coatings that are sufficiently versatile and robust to resist adhesion and settlement under different conditions without recurring to toxic biocides. On another side, the complexity of the interfacial interactions and processes between the living organisms and the sensed structure and morphology of the outermost surfaces of the material requires a multidisciplinary approach focusing on the ability of the film to respond at micro- and nano-size levels. Thus, a comprehensive strategy to develop non-toxic, environmental benign antifouling/fouling release coatings should include collaborative efforts of synthetic chemists, materials scientists, surface physicists and marine and freshwater biologists to deepen fundamental understanding and fill the ‘technology gap’.
Most recent and innovative advances encompass use of hybrid polymer nanocomposites, biomimetic polymer-analogue platforms, lithographically patterned monolayers and plasma assisted chemical vapor deposition of metal surfaces. Although highly diversified in their individual architecture and function, all such thecnologies share the underpinning concept to exploit the interactions intervening between the fouling organisms and the specific surface features at the nano-scale. Surface segregation of low surface tension polymers may be an additional powerful tool to nanostructure a coating in such a way to comply with the nanosized cues of the foulants, thereby effecting an antifouling activity or favouring removal of those foulants that do adhere.
In keeping with this rationale, we designed and engineered novel coatings with potential antifouling/fouling release properties based on macromolecular architectures of fluorinated polymers capable to self-assemble in segregated surface nanostructures. Polymers incorporating relatively long perfluorinated side chains are known to segregate to the outer surface of thin films in order to minimize the energy of the system driven by their intrinsic high immiscibility and low surface energy.
The polymer systems of choice where block copolymers with polystyrene and amphiphilic fluorinated polystyrene blocks in view of the anticipated multiple (nano)structuring effects operating in such type of materials. Other fluorinated polymers of interest were either block copolymers with siloxane blocks or random copolymers with
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siloxane grafts, which we choosed with the aim to assess the role of the primary chemical constituents, including low modulus component.
It was possible to synthesize different copolymer systems in which the lengths of the blocks and their contents were modulated by the adopted polymerization conditions. By taking advantage of the controlled process of the atom transfer radical polymerization, any given polystyrene macroinitiator was employed to generate families of block copolymers by the sequential insertion of the second amphiphilic block. Although precise tuning of the molar mass and content of the latter block was difficult, because of the oligomer nature of the monomer itself, the diblock copolymers were characterized by well diversified macromolecular chemical parameters. This in turn enabled detailed and reliable structural and surface investigations to be performed.
We emphasize that introduction of the oxyethylene-tetrafluoroethylene side groups provided an amphiphilic extra-character to the otherwise hydrophobic polystyrene backbone. The different capabilities of the hydrophilic oxyethylene chain segments and the hydrophobic/lipophobic tetrafluoroethylene chain segments to interact with the surronding medium resulted in a responsive character of the material, which was expressed in a kind of multifold or ‘ambiguous’ nature of the surface of films derived therefrom.
Consistent with this chemical prerequisite, the wetting behavior with different interrogating liquids and, most significant, with water proved to depend on the treatment history of the film, such us immersion time under static conditions and advancement/recession under dynamic experiments. Nevertheless, in any case the films exhibited quite distinct low energy properties, as identified by their surface tension values, owing to the preferential outer segregation of the fluorinated units to both polymer-air and polymer-water interfaces. Thus, the overall surface energy was dictated by the predominant fluorine effect.
According to the XPS and NEXAFS findings the perfluorinated chains protruded out of the surface, although orientationally disordered, and apparently created a homogeneous and uniform cover onto it, which afterwards underwent a marked reconstruction upon contact with water. Two processes for the structural reorganization appeared to take place at different time length scales, which possibly involved flipping of the amphiphilic chain substituents at shorter times and migration of the polystyrene block towards the inner regions at longer times. To what extent the underwater reconstruction may affect the
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actual structure and performance in terms of wetting and antifouling activities is not well understood at present because of experimental difficulties and remains to be better established.
While surface enrichment of the fluorinated species was striking in the block copolymers with large differences in the surface and interface tensions of the immiscible blocks, the propensity of the lowest surface energy fluorinated chains to surface segregation was pronounced in all the copolymers, that we investigated, even when in combination with other widely recognized low energy polymer components, such as siloxane blocks and grafts. Irrespective of the copolymer architecture, the fluorinated chains were found to populate the outer surface, with their placement and organization depending on the details of the chemical structure.
In this context, it is worth reminding that the rigid-rod −(CF2)8F segments self-assembled
in a smectic-like order which exhibited a relatively high orientational order parameter. Such a surface order could be a manifestation of the bulk mesophase order which was demonstrated for both block and random copolymers carrying relatively long perfluorinated chains −(CF2)nF (n = 8, 10). The formation of this mesophase state is due
to the intramolecular phase segregation of the strongly incompatible hydrocarbon-fluorocarbon constituents of the polymer repeat units that spontaneously organize in an orderly fashion. Therefore, in these polymers a hierarchy of structures extends over several spatial length scales from the morphological level typical of block copolymers down to molecular dimensions distinctive of liquid crystalline mesophases.
The GISAXS and AFM analyses of the amphiphilic block copolymers helped to confirm that morphologies with periodic arrays were maintained at the surface of the thin films. The minor polystyrene component was in fact segregated into discrete spherical or cylindrical nanodomains embedded in the amphiphilic polystyrene matrix. Moreover, bicontinuous lamellar nanostructures occurred when the content of the polystyrene block was increased to reach a nearly equal weight fraction to the other block. Tuning of the surface morphology was, therefore, achieved in thin films by a chemical control of the macromolecular structural parameters of the bulk polymeric system.
In order to produce test coatings for biological assays of the antifouling/fouling release performances we devised a preferred approach by depositing films in a bilayer geometry. While the top layer contained the block copolymer either alone or blended with SEBS in varied compositions, the bottom layer consisted of a SEBS/SEBS-MA mixture.
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According to this method not only were we able to produce robust films with improved adhesion to the glass substrate without delamination, but we also combined into one coating system both the low surface energy and the low elastic modulus properties of the individual polymeric components. The anticipated elastomeric character was preserved by incorporation of small amounts of the cost-effective amphiphilic fluorinated material, as demonstrated by the mechanical stress-strain findings. However, whilst such elastomeric range extended over a very broad range of temperatures across room temperature, their overall tensile modulus seemed to be possibly not sufficiently low to allow for the best suited elastomeric mechanical response. This particular feature may be expected to be relevant in the detachment and release of the macrofouling organisms, primarily barnacles, and one should aim to optimise the formulation and the engineering of the bilayer coatings for further improvement of the biological performance.
Indeed, the fouling phenomenon is very complex and development of the novel materials to combat it requires extensive testing under lab-scale and field-trial conditions. For an initial, but nonetheless comprehensive, evaluation of the potential of the block copolymers and their test surfaces as antifouling/fouling release coatings we screened a number of micro- and macro-organisms, vastly different in fouling behavior, including their chemistry and biology of settlement and growth.
The resultant activities, whether antifouling or fouling release of the surfaces, were rather well diversified, as partly expected. In fact, whereas a universal coating appears to be totally unrealistic, novel materials can achieve special goals against foulant species in view of specific end-use applications. A general overview of the reported evaluations with all the organisms is summarized in Table 4.1, with the individual tests being marked (excellent, positive, equivocal, negative), with reference to our internal controls (T2 Silastic, Epikote, glass).
Inferior performances are recognized for the attachment tests with the cyprid Balanus, against which many coatings appear not satisfactory. By considering that Balanus
amphitrite is one of the most common and aggressive marine macrofoulers, the coatings
assayed will not be appropriate for seawater applications. It may in fact be foreseen that it would be hard to dislodge the basal plate of the attached barnacle once it metamorphosed to its juvenile and adult stages without failure. By contrast, we note that essentially all the samples exhibit removal properties as regards the organisms investigated, excellent release being detected against the alga Ulva and the bacterium Pseudomonas.
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Table 4.1. Antifouling/fouling release properties of the tested coatings.
Sample Ulva Navicula Pseudomonas Marine bacteriaa) Balanus
Rb) Rb) Ac) Rb) Ac) Rb) Ac) UniPi9 ++ + + + + UniPi17 ++ + + ++ + ++ − UniPi18 − ++ + + − UniPi21 ++ ± + ++ + ++ + UniPi22 ++ + − ++ + + + UniPi23 ± ++ + + + UniPi26 ++ + − UniPi27 ++ + − UniPi28 ++ + −
a) Consortium of Cobetia marina, Marinobacter hydrocarbonoclasticus, and Vibrio alginolyticus. b) Removal. c)
Attachment.
Bacteria and algae are normally found in freshwater as the most frequent biofouling agents, and control and reduction of the burden they impose on structures in those environments are questions of concern and socio-economical impact. Therefore, exploitation of fouling release coatings in such fields as protection of pleasure crafts, aquaculture fish nets and heat exchangers is highly desirable.
In this connection, the coatings developed in this work show good promise for more potential application and deserve indepth investigation. The most suitable coatings seem to be UniPi21 and UniPi 22 which share the common feature of incorporating the block copolymer richest in amphiphilic block component in their top layer. While these findings may offer general guidance to the design of fouling release surfaces, extending the case studies to more coatings with varied compositions and formulations of the fundamental constituents will be really worthwhile to the final aim of providing sound knowledge-based relationships between chemical structure and chemical-physical properties and biological functionfor anti-biofouling materials.
