SURFACE MOLECULAR AGGREGATION STRUCTURE AND SURFACE MOLECULAR MOTIONSOF HIGH-MOLECULAR-WEIGHT POLYSTYRENE LOW-MOLECULAR-WEIGHT POLY(METHYLMETHACRYLATE) BLEND FILMS

Surface molecular aggregation structure and surface molecular motions of high-molecular-weight polystyrene/low-molecular-weight poly(methyl methacrylate) (HMW-PS/LMW-PMMA) blend films were investigated on the b asis of X-ray photoelectron spectroscopic measurements and scanning fo rce microscopic observations. Monodisperse PS with M-n = 1450k, where M-n denotes the number-average molecular-weight, and monodisperse PMMA s with M-n 1.2k, 4.2k, 40.5k, 144k, and 387k were used as HMW-PS and L MW-PMMAs, respectively. Static contact angle measurements revealed tha t the magnitudes of surface free energy, gamma, of PMMAs for all M(n)s studied here were higher than that of PS with M-n = 1450K. In the cas e of the (HMW-PS/LMW-PMMA) blend films, in which the M-n for each PMMA was less than 144K, PMMA was preferentially segregated at the air-pol ymer interface, even though PMMA had a main chain with a higher gamma compared with that of PS. It was found from scanning viscoelasticity m icroscopic measurements that the surface molecular motion of the (PS w ith M-n = 1450k/PMMA with M-n = 4.2k) blend film was fairly activated in comparison with that of the bulk one due to the surface segregation of LMW-PMMA. The surface enrichment of LMW-PMMA can be explained by e nthalpic and entropic terms as follows. (1) Since the magnitudes of ga mma of both chair! end groups of a polymer chain synthesized by an ord inary living anionic polymerization are smaller than that of the main chain part, the chain end groups are preferentially segregated at the surface. Therefore, the chain end effect at the air-polymer interface becomes more remarkable with a decrease of M,, due to an increases in the number density of chain end groups. (2) Since polymeric chains exi sting in a surface region are compressed along the direction perpendic ular to the film surface, the surface chains take smaller conformation al entropy in a confined state compared with that of bulk chains. The difference in conformational entropy between the surface chain and the bulk one, that is, the conformational entropic penalty of the polymer ic chain at the surface, decreases nith a decrease in M-n. Then, when the enthalpic and entropic effects mentioned above overcome the gamma difference of main chain parts between PS and PMMA, PMMA with higher g amma is stably enriched at the blend film surface.