End-to-End Vector Dynamics of Nonentangled Polymers in Lamellar Block Copolymer Melts: The Role of Junction Point Motion

Abstract

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.

Component dynamics in nanostructured PI-PDMS diblock copolymers with PI segregated in lamellas, cylinders, and spheres

“…In fact, the end-to-end vector reorientation process in the lamella phases can be decomposed in two parts. The fastest one is dominated by internal PI modes, as confirmed by molecular dynamics simulations [18]. The raw data of PI4-PDMS4 in Fig.…”

“…1), would also account for the faster normal mode peak time as compared to the better-ordered lamellar structure in PI4-PDMS4. Finally, it is worthy of remark that the fast component of the normal mode in the symmetric copolymers reflects the internal chain dynamics of PI block [18]. From the careful deconvolution of the slow and fast components performed in [18], it becomes clear that the internal PI chain dynamics in the segregated lamella phases has a time scale close to that of the whole normal mode in the asymmetric copolymers with cylindrical and spherical PI segregation.…”

“…11b). However, it has been found that the NM for the lamellas accounts for the complete reorientation of the PI-block end-to-end vector [18]. In fact, the end-to-end vector reorientation process in the lamella phases can be decomposed in two parts.…”

“…The tunable morphologies of block copolymers provide a variable range of geometrical constraints for fundamental studies devoted to the polymer dynamics under confinement [15][16][17]. In this context, the PI dynamics in segregated phases formed in PI-PDMS block copolymers have been subjected to recent studies by broadband dielectric [16,18] and neutron spin echo spectroscopy [17]. Block copolymers with PI volume fractions, f PI , in the range of 0.16-0.19 form hexagonally ordered PI cylinders or spheres in a continuous matrix of PDMS polymer.…”

Dielectric relaxations of Acrylic-Polyurethane hybrid materials

“…For instance, this is the case of the terminal relaxation in polymers that in some cases is also detectable dielectrically as the so-called normal mode relaxation. This dielectric relaxation is the result of the fact that in these polymers (1,4 cis-polyisoprene as a prototype) [17] there is a dipole moment proportional to the end-to-end chain vector and therefore a detectable dielectric relaxation reflecting these fluctuations. However, there is not evidence of the presence of an end-to-end dipole moment in PU polymers.…”