Surface Structure Formation And Construction Of Fluorinated Acrylate Copolymer

The ultimate performance of materials in many traditional and modern applications depends not only on their bulk properties,but also relies heavily on their surface microstructure and interfacial behavior.Up to the present time,there existed few researches in the fields of formation mechanism of polymer surface structure.Progress in polymer surface and interface science and engineering will have been attributed to the ability of controlling and tailoring surface structure.Now days,fluorinated materials are widely used because of their low surface energy,chemical inertness,thermal stability,and low friction coefficient.Applications enclose low dielectric constant polymers in electronic industry,friction modifiers in lubrication oils, antifouling coating,optical fiber claddings and membranes.Since the costly fluorinated monomers were in used and the excellent surface performance of copolymer was lost when exposing in polar environment induced by surface reconstruction,it is challenging to fabricate fluorine-containing polymer surface with low-energy properties and superior long-lasting barrier properties and lower fluorine content,In this paper,the copolymers composed of methyl methacrylate(MMA) and 2-perfluorooctylethyl methacrylate(FMA) with various chain structures were synthesized by ATRP or free radical polymerization.The effects of chain structure,film formation methods and solvents on the polymer surface formation by solution solidness were investigated in detail using contact angle measurement,X-ray photoelectron spectroscopy(XPS),sum frequency generation spectra(SFG),atomic force microscopy(AFM) and X-ray diffraction.The difference of film-forming techniques on surface structure and performance of fluorinated methacrylate block copolymer and random copolymer were completely different.It was found that the enrichment of fluorinated moieties at spin-coated film surface was higher than that of cast film for low fluorine content random copolymer,but there was reverse results for high fluorine content copolymer.It surprisingly exhibited the figure like "8" that the tendency of surface segregation of fluorine at cast and spin-coated films surface with increasing of fluorine content.When the fluorinated monomer content in the copolymer was low,the perfluoroalkyl group was unable to migrate to the solution/air interface and became buried in the random-coil chain conformation as a result of entropic forces dominating the interfacial structure.When the films were prepared by spin-coating,the copolymer chains in solution were likely extended due to the centrifugal force,and the entropy effect of the polymer chains was thus weakened,resulting in the segregation of the fluorinated moieties on the film surface. However,perfluoroalkyl groups could not segregate at the surface of films prepared by casting and were thus buried in the random-coil chains.For the copolymers with high FMA contents, the migration of perfluoroalkyl groups at the solution/air interface or polymer/air interface was controlled by enthalpic forces.However,the cast film surface of diblock copolymer (PMMA-b-PFMA) has more fluorinated moieties than that of spin-coated film.The micelles was exposed at the surface during spin-coated which result in the decreasing of water and oil-repellence due to fluorinated diblock copolymer forms a micelle composed of the PMMA corona and the PFMA core in cyclohexanone or toluene.With comparing,the fluorinated moieties have enough time to be disentangled from micelles and could completely segregate at the outermost surface by using solution cast.The end-capped poly(methyl methacrylate) with several FMA units possessed a special push-me/pull-you architecture could be used for fabricating stable low free energy surface.The films of the end-capped PMMA with only about 3 or 4 FMA units(0.38mol%,FMA) have the same water and oil-repellence with PFMA homocopolymer,and exhibited better stability and resistance to polar environments for the perfluorinated moieties which can self-assemble to form an ordered and close-packed structure at surface.With increasing of FMA block lengths,water and oil repellency decreased,the ratio of F/C increased with increasing film depth,and the degree of ordered packing of the perfluoroalkyl side chains at the surface decreased.The PMMA segments preliminarily occupied the film surface even when FMA units were 10.It was attributed to the PFMA block length affecting molecular aggregation structure of the copolymer in the solution and the interracial structure at the air/liquid interface,which affects surface structure formation during solution solidification in turn.The results suggest that the proper PFMA block length play an important role in structure formation on the solid surface.At the same time,the longer PMMA block enhanced both the enrichment of fluorinated moieties and the order of packing orientation of the perfluoroalkyl side chains on the surface.It seems that the longer PMMA block(n=355) in triblock fluorinated copolymer (PFMA-b-PMMA-b-PFMA) was in favor of the fluorinated moieties segregating to outer surface and the perfluoralkyl side chain self-assemble into double-layer packing,single-layer packing and hexagonal packing structure,and the bulk formed lameUar structure.Accordingly, this film exhibited better stability and resistance to polar environments.As for triblock copolymer with shorter PMMA block(n=91),the perfluoralkyl side chain only self-assemble into hexagonal packing structure and the bulk formed cylinder structure.The proper solvent can promote the segregation of the perfluoroalykl side chain of fluorinated block copolymer at surface,forming a relatively perfect close-packed and well-ordered structure of the perfluoroalkyl side chains at the surface.The films own excellent stability and resistance to polar environments.Films of PMMA₈₅₇-b-PFMA_3.3 with about four 2-perfluorooctylethyl methacrylate units casted with benzotrifluoride solution exhibited excellent resistance to polar environments compared with those cast by cyclohexanone and toluene solutions.When the film was cast with benzotrifluoride solution,in which only unimers existed.The perfluoroalkyl side chains can segregate easily on the surface and self-assemble into an ordered structure.While the perfluoroalkyl side chains could not be easily disentangled and they could not completely segregate at the outermost surface and form a well-ordered packing structure,casting with cyclohexanone and toluene solution,in which micelle existed. After annealing the extent of segregation of fluorinated moieties increased,but the ordered structure of the perfluoroalkyl side chains at the PMMA857-b-PFMA3.3 film surface was destroyed which result in the decreasing in the surface stability of the films.