Dynamic Heterogeneity and Collective Motion in Star Polymer Melts
Jack F. Douglas, Jinpeng Fan, Hamed Emamy, Francis W. Starr, Alexandros Chremos
While glass formation of linear chain polymers has been widely explored, comparatively little is known about glass formation in star polymer melts. We study the segmental dynamics of star polymers via molecular dynamics simulations and contrast the segmental dynamics to those of the star core. We further examine the dynamical heterogeneity and cooperative nature of segmental motion. In particular, we quantify how the molecular architecture of star polymers, i.e., the number of arms and the length of those arms, aﬀect the glass transition temperature Tg, the non-Gaussian nature of molecular displacements, the collective string-like motion of monomers, and the role of chain connectivity on the cooperative motion. Although varying the number of star arms f and molecular mass Ma of the star of arms can signiﬁcantly inﬂuence the average molecular shape (as quantiﬁed by the radius of gyration tensor of the molecule), the glass transition temperature and even activation energy for relaxation at elevated temperatures where relaxation has an Arrhenius temperature dependence, all our relaxation data can be quantitatively described in a uniﬁed way by the string model of glass formation, an activated transport model consistent with many of the assumptions of the Adam-Gibbs model where the degree of cooperative motion is identiﬁed with length of string-like particle exchange motions observed in our simulations. Previous work has shown the consistency of the string model with simulations of linear polymers at constant volume and constant pressure, as well as for thin supported polymer ﬁlms and nanocomposites with variable polymer-surface interactions where there are likewise large mobility gradients as in the star polymers studied in the present paper.