| Abstract |
“Smart” polymer coatings enable to tune the interfacial physico-chemical properties of a variety of organic and inorganic materials, at will. Polymer brushes, are ideal coatings for numerous applications, ranging from “smart”, controllable adhesive, biosensing and antimicrobial surfaces. This thesis presents the synthesis and characterization of novel, well-defined polymer brushes, bearing desirable functionalities, via surface-initiated atom transfer radical polymerization (SI-ATRP). The surface properties and the antimicrobial performance of the brushes were studied rendering them attractive for use as lubricants with responsive behavior and/or dual-functional antimicrobial surfaces in the solid state. Homopolymer brushes based on 2-(dimethylamino)ethyl metahcrylate, (DMAEMA), or fluorinated methacrylates, were synthesized, via SI-ATRP, on glass and silicon substrates. PDMAEMA brushes comprise a convenient model-system to investigate the surfaces properties upon a facile post-modification reaction introducing different alkyl chain lengths (ACL) on the side groups of the end-grafted polymer chains. Three fluorinated methacrylates, with different fluorinated alkyl chain lengths (FCLs = 1, 4 and 6 fluorocarbon atoms) in their side-groups, referred to as TFEMA, OFPMA and TDFOMA, respectively, were utilized. The variations in the hydrophilicity/hydrophobicity and the surface free energy of the brush, as a function of the FCL of the side group and the ACL of the quaternization agent, were determined. A hydrophilic to hydrophobic transition of the surfaces and a significant decrease of the degree of quaternization of the DMAEMA moieties was found upon increasing the ACL of the quaternization agent above six carbon atoms, allowing to tune the wettability, the thickness and the pH-response of the brushes. Next, the adhesion and friction properties of the polymer brushes in the solid state against a sliding inorganic surface were examined. Finally, the hydrophilic, PDMAEMA and quaternized PDMAEMA, brushes are shown to be unstable in water due to the degrafting of the polymer chains, by the hydrolysis of the labile ester or siloxane bonds of the surface-bound initiator, that is mechanically driven by the tension on the chains. On the other hand, all fluorinated brushes were stable due to the inhibition of the penetration of water molecules at the polymer-substrate interphase. In the second part of the present study, amphiphilic diblock copolymer and binary mixed polymer brushes were prepared, comprising PDMAEMA and PTFEMA, POFPMA or PTDFOMA chains. The reorganization of the polymer chains and the switching of the film wettability, upon exposure to selective solvents for the two polymers, were observed. In addition, the mixed brushes exhibited tunable friction and surface energies, in response to external stimuli, which renders them attractive for use as “smart” surfaces in the dry state.
Quaternization of the DMAEMA groups, diminished the responsive behavior of the brushes,
which as attributed to the large χ value between the two very dissimilar blocks (charged
PQDMAEMA and semi-fluorinated polymethacrylates). Finally, evidence of unwanted chain
degrafting of the diblocks was found again, attributed to hydrolysis, after exposure of the
brushes in aqueous media for prolonged time periods, whereas the amphiphilic mixed polymer
brushes exhibited a remarkable stability in aqueous media with the fluorinated polymer acting
as a barrier to shield the labile initiator bonds from hydrolysis.
In the final part of this thesis, quaternized PDMAEMA brushes bearing quaternary ammonium
groups of different ACLs were assessed as biocidal coatings. The effect of the ACL of the
quaternary ammonium groups on the contact killing efficiency of the surfaces, against E. coli
and B. cereus bacteria, was investigated. Antimicrobial tests revealed that the hydrophilic
polymer brushes exhibited enhanced bactericidal activity, whereas the hydrophobic surfaces
showed a significant deterioration of the in vitro bactericidal performance. In another approach,
the antifouling activity of the semi-fluorinated homopolymer brushes, bearing different FCLs
on the polymer side groups, was found to increase with the number of fluorocarbon atoms.
These results elucidate the antimicrobial action of the quaternized polymer brushes and the lowsurface
energy fluorinated brushes, dictating the appropriate choice of the ACL or FCL, for the
development of coatings that effectively inhibit biofilm formation on surfaces either by killing
or by releasing the bacteria. Finally, dual functional coatings, comprising mixed polymer
brushes of the bacterial-releasing fluorinated chains, PTFEMA, POFPMA or PTDFOMA, and
the bactericidal PQDMAEMA chains, were shown to possess significantly improved
antimicrobial performance, against both E. coli and B. cereus, due to their combined antifouling
and bacteria killing action.
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