Investigation of the Temperature Dependence of Activation Volume in Glass-Forming Polymer Melts under Variable Pressure Conditions

Abstract

Motivated by empirical observations suggesting that changes in the activation volume ΔV # upon varying temperature (T), determined from the pressure (P) dependence of the segmental structural relaxation time τ α , might be related to changes in the extent of collective motion in glass-forming liquids, we investigate the temperature dependence of ΔV # and the extent of collective motion in a coarse-grained polymer melt using molecular dynamics simulation. After confirming the validity of the string model of the dynamics of glass-forming liquids in predicting the T and P dependence of τ α , we find that there is unfortunately no direct relation between ΔV # and the average string length L quantifying the extent of collective motion. This negative finding is also in qualitative accord with predictions from the generalized entropy theory of glass formation, in which the extent of collective motion is quantified indirectly through the configurational entropy of polymer melts. By drawing an analogy between the definitions of ΔV # and the fragility of glass formation, we instead find that both ΔV # and fragility are complex quantities involving a convolution of energetic effects associated with the variation of activation free energy, along with changes in the extent of collective motion with T and P. We further demonstrate that the extent of collective motion can be estimated quantitatively by normalizing the activation free energy by its value at the onset temperature of glass formation. It is suggested that this normalized measure of the change of collective motion with T should be superior to fragility in quantifying the role of collective motion in condensed materials giving rise to non-Arrhenius relaxation.

Impact of High Pressure on Reversible Structural Relaxation of Metallic Glass

“…[29,30] As our approach has successfully described the glass transition of glass-forming liquids having simple and complex structures in both experiments [7][8][9][14][15][16]26,34] and simulation [21] in the past, in future work, it will be possible to extend our theoretical framework to study more complex polymer glasses, such as those in the previous studies. [37,38]…”

Molecular Origin of Strain‐Induced Chain Alignment in PDPP‐Based Semiconducting Polymeric Thin Films

“…[19,[46][47][48][49] Over the last few decades, the CG-MD approaches have been broadly applied to explore the influences of fundamental molecular parameters on the thermomechanical behaviors of polymers with different segmental/chain structures at bulk and confined states. [50][51][52][53] The semi-crystalline morphology of PE under uniaxial tension has been explored via CG-MD methods. It was shown that polymer chains tend to align in the deformation direction, where the large extension and orientation of the chains were found to affect the crystallization and facilitate the primary nucleation.…”

Toward a Better Understanding of Activation Volume and Dynamic Decoupling of Glass‐Forming Liquids under Compression

“…The activation volume monotonically grows with cooling and overall this variation is qualitatively consistent with prior experiments [30][31][32] and simulations. [31,33,34] Our activation volumes are the same order of magnitude as simulations in ref. [33,34].…”

“…[31,33,34] Our activation volumes are the same order of magnitude as simulations in ref. [33,34]. On one hand, we roughly observe two linearly decreasing trends of log 10 ΔV # occurring in ternidazole, glycerol, and probucol.…”