Chain-length-dependent correlated molecular motion in polymers

Abstract

We show how dynamic heterogeneities (DH), a hallmark of glass-forming materials, depend on chain flexibility and chain length in polymers. For highly flexible polymers, a relatively large number of monomers ($N_c\sim 500$) undergo correlated motion at the glass transition temperature $T_g$, independent of molecular weight ($M$). In contrast, less flexible polymers show a complex $N_c(M)$ behaviour divided into three regimes, consistent with observation in both $T_g(M)$ and chain conformational structure. For short oligomers ($\lesssim$ 2 Kuhn steps), a transition from mainly $inter$molecular correlations and $N_c\sim 200$, to strongly $intra$molecular correlations and $N_c<50$ (roughly the molecular size) is observed; for longer chains, $N_c$ increases weakly, before saturating. For poly(methyl methacrylate), a remarkable similarity is found between $N_c(M)$ and the $M$-dependent ratio of the activation barriers of the structural ($\alpha$) and secondary ($\beta$) relaxations. Our results suggest a link between the DH length-scale and the number of $\beta$ relaxation events jointly-activated to facilitate the $\alpha$ relaxation.

Main manuscript: 8 pages, 5 figures; Supplementary: 9 pages, 8 figures and 1 table

References in corpus (20)

• Theoretical perspective on the glass transition and amorphous materials
• Direct experimental evidence of a growing length scale accompanying the glass transition
• The Excess Wing in the Dielectric Loss of Glass-Formers: A Johari-Goldstein beta-Relaxation?
• Spatial correlations in the dynamics of glassforming liquids: Experimental determination of their temperature dependence
• Fifth-order susceptibility unveils growth of thermodynamic amorphous order in glass-formers
• Excitations are localized and relaxation is hierarchical in glass-forming liquids
• Glassy timescale divergence and anomalous coarsening in a kinetically constrained spin chain
• Spatially Heterogeneous Dynamics in a Metallic Glass Forming Liquid Imaged by Electron Correlation Microscopy
• Thirty milliseconds in the life of a supercooled liquid
• Universal features of dynamic heterogeneity in supercooled liquids
• Modern computational studies of the glass transition
• Microscopic origin of excess wings in relaxation spectra of supercooled liquids
• Calorimetric glass transition explained by hierarchical dynamic facilitation
• Short and long range correlated motion observed in colloidal glasses and liquids
• Predicting dynamic heterogeneity in glass-forming liquids by physics-inspired machine learning
• Cooperative intramolecular dynamics control the chain-length-dependent glass transition in polymers
• On the Spectral Shape of the Structural Relaxation in Deeply Supercooled Liquids
• Effects of hydrogen bonding on supercooled liquid dynamics and the implications for supercooled water
• Collective relaxation dynamics in a three-dimensional lattice glass model
• Modeling non-linear dielectric susceptibilities of supercooled molecular liquids