A local segmental mobility was determined by electron spin resonance (ESR) spin-label method for a series of polystyrene (PS) with various molecular weights. Each PS specimen was selectively spin-labeled with stable nitroxide radicals at a chain end or inside sites. Molecular motion at the inside of the chain was compared with that at the chain end from the temperature dependence of ESR spectra of the nitroxide radicals. The transition temperature of molecular motion, T 5.0mT, at which the extreme separation width due to 14 N anisotropic hyperfine splitting is 5.0 mT, increased with an increase in molecular weight. The WLF equation confirmed that the T5.0mT correlated with a glass transition temperature, Tg, of PS. The T5.0mT for the spin-labeled PS at the chain end was ca. 5 K lower than that for the spin-labeled PS at the inside sites due to the enrichment of the specific free volume around the chain end. The transition temperature, T5.0mT, for both labeled PS depended on the molecular weight in accordance with the Unberreiter-Kanig equation for a glass transition. The T5.0mT for the spin-labeled PS at the chain end had a strong dependence on the molecular weight as compared with that at the inside sites because the molecular motion of the chain end was accelerated by an encounter of more than two chain ends. From the molecular weight dependence, we determined the short correlation time for segmental motion of the chain end, ca. 40 s, and the segment size undergoing the segmental motion at the T g. The obtained segment size agreed well with the general segment size reported by others, 5-10 monomeric unit size.
Individual effective glass transitions of components in miscible blends of polystyrene/poly(vinyl methyl ether), poly(o-chlorostyrene)/poly(vinyl methyl ether), and poly(styrene-block-n-butyl acrylate)/polystyrene were detected by temperature-modulated differential scanning calorimetry. These blends apparently showed broad single glass transitions in their heat capacity (C p ) curves. This result has been widely accepted as a criterion of miscible mixing of both components. However, the derivative C p curves of some blends showed bimodal peaks. This indicates the coexistence of two relaxations in the miscible blend. Moreover, the ratio of the two peaks was related to the compositions of the blends. The temperatures at the peaks of the derivative C p curve were taken to be effective glass transition temperatures (T g,A and T g,B) of the components in the blend. T g,A and T g,B were much different from the average glass transition temperature (T g,av) at which the endothermic shift was half in the C p curve. On the other hand, T g,av and T g,A of homopolymers and poly(styrene-random-n-butyl acrylate)s were almost coincident with each other. T g,A and T g,B of the blends were in excellent agreement with the effective glass transition temperatures predicted by the model, taking into account the self-concentration effect.
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