Understanding Mass Dependence of Glass Formation in Ring Polymers.

Having highly tunable molecular topology is one of the most important characteristics of polymers that provides these materials with a wide range of interesting and unique properties. In particular, ring polymers exhibit a number of properties that are markedly distinct from their linear counterparts. Here, we compare and contrast the glass formation of unknotted, nonconcatenated ring and linear polymer melts having variable molecular mass based on molecular dynamics simulations of a coarse-grained model. After revealing an unusual property in the structure of small rings, we discuss the mass dependence of the structural relaxation time determined from the self-intermediate scattering function over a wide range of temperatures in both ring and linear polymers. As a general trend, we find that the characteristic temperatures (e.g., the glass transition temperature) and fragility of glass formation increase with increasing molecular mass in linear polymers, but the mass dependences of these properties are rather weak in the family of ring polymer models considered, in broad accord with experimental measurements. Importantly, we show that the glass formation of ring polymers can quantitatively be described by the string model, a model that is broadly consistent with the entropy theory of glass formation and that takes the mass of string-like clusters as a molecular realization of the abstract cooperatively rearranging regions. This opens the possibility of applying the configurational entropy-based theories to describe the glass formation of ring polymers, once the ring topology is taken into account. [ABSTRACT FROM AUTHOR]