Molecular Dynamics Investigation of the Pressure Dependence of Glass Formation in a Charged Polymer Melt

ZY Yang and XL Xu and JF Douglas and WS Xu, MACROMOLECULES, 56, 4049-4064 (2023).

DOI: 10.1021/acs.macromol.3c00317

Mostcommodity polymers are derived from petroleum raw materials,and correspondingly, these materials are normally uncharged and nonpolar,attributes that make this class of material relatively insoluble inwater and relatively slow to breakdown in the environment. Naturaland synthetic charged polymer materials often do not exhibit thesedrawbacks, thereby offering the potential for being more sustainable.As with all polymer materials, properties related to glass formationplay a central role in the design and characterization of materials.Here, we investigate the influence of a crucial processing variable,the pressure P, on the glass formation of a coarse- grainedcharged polymer melt using molecular dynamics simulation. We findthat the temperature (T) dependence of the thermodynamicsand segmental dynamics under variable P conditionslargely resemble the trends observed before for uncharged polymerliquids. In particular, we are able to organize all our segmentalrelaxation data as functions of both T and P in terms of the conventional phenomenology for unchargedpolymer melts, namely, the Vogel-Fulcher-Tammannn temperaturedependence of the structural relaxation time tau(alpha) and a pressure analog of this equation, etc. Moreover, we can quantitatively describe all our simulation data for bothcharged and uncharged polymer melts with the string model of glassformation. Importantly, our results indicate that the main effectof charge on the dynamics of polymeric glass-forming liquids is to"renormalize" the cohesive interaction strength. Thisopens the possibility of applying theories of neutral polymer meltsto describe the glass formation of synthetic and natural charged polymermelts, ionic fluids, and possibly even polar polymer melts, once anaccounting is made for how charge modifies the effective cohesiveinteraction strength.

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