Direct Detection of High Mobility around Chain Ends of Poly(methyl methacrylate) by the Spin-Labeling

Abstract

Effects of chain ends on the dynamics of atactic poly(methyl methacrylate) (PMMA) was studied by the spin-label technique. PMMA's with well-defined molecular weights were synthesized by atom transfer radical polymerization and selectively labeled at the end or inside sites of the chain with a stable nitroxide radical. The WLF treatment demonstrated that a transition temperature, T 5.0mT, at which an extreme separation width was 5.0 mT, reflected the glass transition observed at a frequency of electron spin resonance (ESR). The T 5.0mT of the PMMA labeled at the chain end (T 5.0mT,e) was ca. 8 K lower than that of the PMMA labeled at the inside sites (T 5.0mT,i). From the comparison of some polymers, the difference between the T 5.0mT,i and T 5.0mT,e of a rigid polymer was found to be larger than that of a flexible polymer. The T 5.0mT,i and T 5.0mT,e decreased with a decrease in the molecular weight as well as a glass transition temperature determined by calorimetric measurements. This result supports that the T 5.0mT reflects the glass transition of the PMMA and suggests that the cooperative motion with neighboring segments is necessary for not only inside segments but also chain ends to undergo a rotational relaxation. The high mobility around chain ends brought a shorter correlation time (τc) at the region around chain ends. Moreover, a model on the basis of an arrangement of segments suggested that an encounter of chain ends was unable to be ignored in the case of low molecular weights, and the encounter of chain ends locally induced a much shorter τc around them.

Control of Polymer Properties by Entanglement: A Review

“…Some studies suggest that lower entanglement means a higher concentration of chain ends, which lead to lower T g . [ 107–109 ] Others indicate that entanglements influence the remaining shift and physical properties near T g . [ 110 ] Bernazzani et al.…”

Extreme Ductility in Freestanding Polystyrene Thin Films

“…Based on the provided areal densities, the microtomed surface consists of roughly 70% chain ends by area. Chain ends are known to reduce the local density and friction as well as enhance local segmental mobility. ,− For example, depression of T g with decreasing molecular weight is thought to be due to the increasing chain end concentration . Therefore, the chains at the fractured surfaces of the microtomed thin films can be expected to have a segmental mobility that is much higher than the native surface chains of bulk polystyrene.…”

Selective determination of glass transition temperature and vibrational properties at the chain end of polystyrene by Fourier transform infrared measurement in combination with deuterium-labeling

“…In particular, the segmental mobility at the chain end of the PS in the bulk state has been intensively and quantitatively studied by Lund et al [22e24] using dielectric spectroscopy with the cyanolabeling and by Miwa et al [13,14,16] using electron spin resonance (ESR) spectroscopy with spin-labeling. The segmental motion detected by these techniques is the a relaxation mode of a PS and the time scale is z10 À6 s around T g þ 50 K. These studies revealed that the segmental mobility at the chain end is roughly double compared to that at the midchain sites at T g þ 50 K because of reduced intermolecular constraint and higher degree of freedom in motion at the chain end.…”