Heterogeneity of the Segmental Dynamics in Lamellar Phases of Diblock Copolymers

Abstract

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.

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

“…This result evidences that the effect of the presence of the, much more flexible and faster, PDMS block on the α-dynamics of the PI block is restricted to a few segments close to the junction point. This is consistent with recent computational investigations on the α-relaxation in bead−spring copolymers with lamellar, 18 cylindrical, and spherical morphologies. 19 A thorough experimental characterization of the segmental dynamics in the different morphologies of the PI−PDMS copolymers is beyond the scope of this work and will be shown elsewhere.…”

“…29 In the following, time and length units in the simulations can be qualitatively mapped to picoseconds and nanometers. 28 The procedure for equilibration of the lamellar structure in the ordered phases, and other simulation details, are described in ref 18. D. Small-Angle X-ray Scattering (SAXS).…”

“…15,17 Recent simulations confirmed this picture. 18,19 A recent study by Sanz et al 16 addressed the effect on the α-relaxation when the confining matrix crosses its glass transition temperature, T g , i.e., when going from a "hard" to a "soft" confinement. It was found that the low-T g (fast) component did not exhibit any discontinuity upon crossing the T g of the confining (slow) phase.…”

Influence of molecular weight and degree of segregation on local segmental dynamics of ordered block copolymers

“…In brief, our results suggest that the heterogeneities in dynamics are enhanced near interfaces and that such characteristics become more pronounced with increasing mobility disparity and degree of segregation between the A and B blocks. These results broadly mirror the trends reported by Slimani et al…”

An Emerging Unified View of Dynamic Interphases in Polymers

“…[ 9 ] While this fi eld has most heavily focused on polymers [ 4,6 ] and small molecules liquids [ 7,10,11 ] confi ned in thin fi lm or pore geometries, polymers with internal nanostructure, such as those shown [ 12 ] in Figure 1 , have been reported to exhibit similar alterations in dynamics for half a century or more. These observations include the following: deviations of T g from pure-state values in block copolymer nanodomains, noted as early as the 1960s; [13][14][15][16][17][18][19][20][21] shifts in segmental dynamics around particles in nanocomposites [22][23][24][25][26][27][28][29] and fi lled rubbers, [30][31][32][33][34] which are believed to contribute to mechanical reinforcement effects in these systems; observation of a rigid amorphous fraction with suppressed segmental dynamics and enhanced T g around crystalline domains in semicrystalline polymers; [ 35 ] and evidence of suppressedmobility domains surrounding ionic aggregates in ionomers. [36][37][38] Despite strong parallels in the phenomenology of near-interface dynamics in these materials, they have commonly been treated as fundamentally distinct systems, each with unique underlying physics.…”