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.
Molecular motion in an interfacial region of microdomains of polystyrene-block-poly(methyl acrylate) (PS-block-PMA) was studied by electron spin resonance (ESR) technique. The junction points between the blocks, which are located in the interfacial region, were labeled with stable nitroxide radicals. Mobility of the spin-labels reflected the dynamic environments in the interfacial region. The transition temperature of the motion of the spin-labels, T 5.0mT, at which the extreme separation width due to 14N anisotropic hyperfine splitting is 5.0 mT, was estimated, and it reflects a glass transition of the region around the labels. The T 5.0mT of the PS-block-PMA labeled at the junction point was almost the mean value of those of the spin-labeled PS and PMA homopolymers, and the distribution of the motional correlation times (τc) in the interfacial region was much broader than that in the homopolymers. These results are considered to be caused by the heterogeneous mixture of the each segment in the interfacial region at a certain length scale. The molecular weight strongly influenced the interfacial thickness and the segmental mobility and the width of the distribution of the τc in the interfacial region as well as on the glass transition temperatures (T g's) of the microdomains. It was revealed that the width of the distribution of the τc depended on not the interfacial thickness so much as the difference between the mobilities of the block chains in the microdomains. On the other hand, extremely small effects of the overall composition and the morphology of the PS-block-PMA on the segmental mobility in the interfacial region were observed. From these results, it was considered that the dynamic environment in the interfacial region was strongly affected by the gradient of the segmental concentration in the interfacial region and the mobility of the block chains in the microdomains.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.