We present computer simulations of concentrated solutions of unknotted nonconcatenated semiflexible ring polymers. Unlike in their flexible counterparts, shrinking involves a strong energetic penalty, favoring interpenetration and clustering of the rings. We investigate the slow dynamics of the centers-of-mass of the rings in the amorphous cluster phase, consisting of disordered columns of oblate rings penetrated by bundles of prolate ones. Scattering functions reveal a striking decoupling of self- and collective motions. Correlations between centers-of-mass exhibit slow relaxation, as expected for an incipient glass transition, indicating the dynamic arrest of the cluster positions. However, self-correlations decay at much shorter time scales. This feature is a manifestation of the fast, continuous exchange and diffusion of the individual rings over the matrix of clusters. Our results reveal a novel scenario of glass formation in a simple monodisperse system, characterized by self-collective decoupling, soft caging, and mild dynamic heterogeneity.

By means of computer simulations, we investigate the segmental dynamics in the lamellar phase of a simple bead–spring model of diblock copolymers. We characterize the dynamic heterogeneity in the mean-squared displacements and bond reorientations. This characterization is made as a function of both the position of the monomers along the chain and the distance to the nearest interface between consecutive domains. Both characterizations of the dynamic heterogeneity reveal moderate gradients of mobility in the investigated temperature range, which qualitatively probes relaxation time scales of up to hundreds of nanoseconds. Namely, the obtained distribution of relaxation times spreads over about 1 decade. However, the extrapolation of the former analysis to lower temperatures leads to an increasing spread over several time decades. The spread mostly arises from monomers located at the immediate neighborhood of the interface. Beyond such distances the structural relaxation approaches that of the homopolymer. Thus, the observed dynamic heterogeneity is esentially an interfacial effect. It does not originate from gradients of density over the domains. Indeed such gradients are absent, and the local density within the domains is identical to that of the corresponding homopolymers.

By means of molecular dynamics simulations, we investigate the structural relaxation in disordered random copolymers and lamellar phases of gradient copolymers, containing chemical species of very different mobilities. Two models have been investigated: a generic bead–spring system and a MARTINI coarse-grained model of a polyester resin. The lamellar phase of the gradient copolymer is formed by domains rich in one species and poor in the other one, which are separated by broad interfaces. Unlike in strongly segregated block copolymers, there is a finite probability of finding monomers of a given species at any position within the domains rich in the other species. A direct consequence of this feature is that monomers can probe very different chemical environments, and because of the strong dynamic asymmetry between the two components, their relaxation are characterized by an extreme dynamic heterogeneity. This is confirmed by an analysis of dynamic correlators as a function of the distance to the interface. In the case of random copolymers long-range ordering is not possible, and local microsegregation results in a much weaker dynamic heterogeneity. The former features are consistent with the experimental observation of narrow glass transitions in random copolymers but extremely broad ones in lamellar gradient copolymers.

By using dielectric spectroscopy, we investigate the chain dynamics of nonentangled polyisoprene (PI) under soft confinement in lamellar domains of block copolymer melts with polydimethylsiloxane (PDMS). The data show a dramatic difference in the end-to-end vector dynamics of the PI blocks as compared not only with that of the corresponding homopolymer PI chains but also with respect to previous results for the same blocks under soft confinement in cylindrical domains. Two contributions to the dielectric normal mode relaxation are detected. The data are analyzed by means of a model including contributions from internal chain modes (accounting for the fastest component) and a slow component attributed to the junction point dynamics. The contribution of the internal chain modes is modeled according to the analysis of the Rouse modes obtained from simulations of a generic bead−spring model for strongly segregated symmetric diblock copolymers. In this way it is shown that the internal chain modes of the blocks have time scales close to those expected from the homopolymer chain independently of the structural details. In contrast, the contribution attributed to the junction point dynamics depends critically on minor structural differences. We interpret these findings as a result of the presence of fast moving defects and/or grain boundaries in the lamellar structures formed by these relatively short, nonentangled diblock copolymers.

We have performed simulations of a simple bead-spring model for cylindrical and spherical phases of diblock copolymers. We have analyzed in detail the dynamic heterogeneity of the structural α-relaxation of the component confined in the minority domains. In analogy with previous investigations on the lamellar phase of the same bead-spring model, the analysis reveals moderate gradients of mobility in the investigated temperature range, which qualitatively probes time scales up to 100 ns. Thus α-relaxation times measured at different distances from the domain center spread over less than one decade. The spatial extension of the gradients of mobility is apparently consistent with that previously observed in the lamellar phase of the same model. Gradients of mobility do not seem to be related to gradients of density within the domains, which are indeed absent. We have performed an analysis of selfand effective concentrations, a concept usually invoked to explain the α-relaxation in polymer mixtures and that has been recently adapted to the analysis of experimental data of diblock copolymers in ordered phases. The simulation results reveal a strong disagreement with the proposed empirical relation between self-and effective concentration, as well as with the Flory-Fox mixing rule for the dynamics. This suggests that the use of these relations may bias the analysis of experimental data.