Surface enrichment in a miscible random copolymer blend: Influence of polydispersity and architecture

Neutron reflectivity and low-energy forward-recoil spectrometry were used s imultaneously to monitor surface segregation in blends of poly(styrene-ran- acrylonitrile), dSAN23 and SAN27, having 23 and 27 wt % AN, respectively. B ecause of its lower AN content, dSAN23 was found to partition at the polyme r/air interface, in agreement with the work of Mansfield et al. (Physica B 1991, 173, 207) and Kim et al. (Polymer 1995, 36, 2427. In contrast to the previous studies, this work explores surface segregation at high dSAN23 bul k volume fractions, phi--- (i.e., phi--- > 0.50). New results include the o bservation of a maximum dSAN23 surface excess (similar to 75 Angstrom) near phi--- approximate to 0.35 and a dSAN23 surface volume fraction that appro aches 1, rather than 0.75, as phi---, increases to 1. The dSAN23 profile de viates from the exponential profile predicted by Schmidt-Binder and self-co nsistent mean-field (SCMF) models. For phi---, > 0.50, the profile displays a surface flattening not observed at low phi---,. The SCMF model is extend ed to account for polymer polydispersity. Although thicker than the monodis perse profile, the polydisperse profile does not capture the experimentally detected surface flattening. Coil deformation along the surface is given a s a possible explanation.