Surface reordering of stretched polyurethane block copolymer films studied by sum frequency generation vibrational spectroscopy

A molecular-level study of polymer surface behavior during elongation was performed for relatively short (710-1450 g/mol) soft and hard polyurethane (PUR) block copolymers. Two copolymers differing only in the length of the hard segment were investigated. Surface-specific sum frequency generation (SFG) vibrational spectroscopy was used to observe the backbone methylene group orientation as PUR films were stretched to elongations up to 100-150% of the original length. The soft and hard copolymer segments are distinguishable by SFG, enabling the analysis of changes in surface composition and molecular orientation caused by elongation. The difference between the surface compositions of the two copolymers is distinguishable by SFG. However, both as-cast PUR copolymer films are enriched with the lower surface tension soft segments. The hard segment length controls the surface composition. With elongation, the intensity of the methylene symmetric stretch increases relative to that of the antisymmetric stretch, suggesting an increase in the upward orientation of the backbone methylene groups. Under fixed elongation, the symmetric to antisymmetric stretch intensity ratio of methylene decreases, and the ether methylene symmetric stretch intensities decrease below their original value, indicating a permanent change in the surface composition.