The first part will be concerned with the conformational rearrangement in nematic melts of a polymer comprising rigid and flexible sequences in alternating succession. This represents the last important project of Paul Flory concerning the statistical mechanics of chain molecules, that he left unfinished in 1985. His original draft manuscript and my extension/computation of his theoretical treatment were published in 1989 in the Materials Research Society Symposium Proceedings dedicated to him (Volume 134, 3-19). Detailed comparisons with previous experimental results show the validity and importance of this theory. The basic concept of this theory is then applied to understand the recently reported liquid-crystalline characteristics of organic semiconducting polymers, poly(3-alkylthiopenes) with hexyl, 2-ethylhexyl, and dodecyl substituents, respectively.<br/>The second part will be concerned with the interface characteristics of polystyrene melts in free-standing thin films and on graphite substrates, obtained from the molecular dynamics simulations employing an explicit all-atom force field. The calculated surface tension is in good agreement with experiment, which provides a support for the force field parameters. The predicted density profile exhibits an enrichment of phenyls relative to the backbone alkyls in the top surface, wherein the phenyl planea are preferentially oriented normal to the film surface, both in agreement with experiments. Moreover, this top surface is followed by a layer of relatively depleted phenyl density of ca. 1 nm thickness. The presence of phenyl depleted region near the free surface of polystyrene films seems to be a key factor in understanding the most pronounced decrease of glass transition temperatures in thin polystyrene films and the observed lack of such characteristics in the case of poly(α-methyl styrene) films.

Statistical mechanics of chain molecules in nematic melts of semiflexible polymers and thin polymer films

2019 Fall National Meeting

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