Influence of Chain End and Molecular Weight on Molecular Motion of Polystyrene, Revealed by the ESR Selective Spin-Label Method

Abstract

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.

Spin Labeling and Molecular Dynamics

“…As an example, Kajiyama et al [6] reported that polymer surface enriched with segregated chain ends exhibited higher molecular mobility, in other words, lower glass-rubber transition temperature (T g ) as evidenced by atomic force measurement. Miwa et al [7] spin-labeled selectively at particular sites and detected the molecular mobility of each site. Figure 8.1 shows chemical structures of spin-labeled polystyrene (PS) at the inside and the end of chain segments.…”

“…In general, the splitting between Chemical structure of spin-labeled PS at inside (left) and end (right) of chain. The figure is adapted from [7] with permission from the American Chemical Society the outer-most lines of the main triplet spectrum due to the nitrogen hyperfine coupling becomes narrower with an increase in the mobility of the radicals because of the motional averaging of the anisotropy. The extreme separation width shown with arrows in Fig.…”

“…Separation between arrows shows extreme separation width. The figure is adapted from [7] with permission from the American Chemical Society Fig. 8.3 Temperature dependence of extreme separation width for spin-labeled PS at chain end (solid) and at inside site (open).…”

“…8.3 Temperature dependence of extreme separation width for spin-labeled PS at chain end (solid) and at inside site (open). The figure is adapted from [7] with permission from the American Chemical Society than that of the spin-labeled PS at the inside site. This is caused by the large free volume at the chain end in comparison with that at the inside of the chain.…”

Ag-Nylon Nanocomposites by Dynamic Emulsion Polycondensation

“…Second, as discussed in Section 4.3, the AgNWs act as heterogeneous nucleating sites and lead to shorter polymer chains. As a result, the segmental mobility of the chains increases accounting for lower T g [64]. …”