Effect of Polar Interactions on Polymer Dynamics

Abstract

Despite significant experimental and theoretical efforts, a fundamental understanding of how the chemical structure influences various dynamic processes in glass-forming materials and polymers remains a topic of active discussion. The present study analyzes the influence of polar interactions on the temperature dependences of segmental and chain dynamics in polymers. We found that segmental dynamics slow down (the glass transition temperature T g increases) and have steeper temperature dependence (higher fragility index m) when a polar group is attached directly to the polymer backbone. However, when a polar group is separated from the backbone by a side group, both T g and m become complex functions of the monomer's polarity and the relative position of the polar group. Our analysis revealed unexpected effect of polar interactions on chain dynamics: chain modes in polar polymers are coupled to the segmental dynamics stronger than in nonpolar polymers with similar fragilities. This results in a steeper temperature dependence of chain dynamics in polar polymers. How the polar interactions affect the coupling of chain and segmental modes remains unclear. ■ INTRODUCTIONFor the past several decades, a significant amount of research was aimed at understanding the effect that chemical structure has on various dynamic processes in polymeric systems. 1−7 It is well-known that parameters like the glass transition temperature, T g , and the steepness of the temperature dependence of segmental relaxation (fragility index, m) in polymers strongly depend on backbone/side group chemical structure. 1,2,4,6,8 However, most of the research in the field of segmental dynamics was focused either on materials with weak van der Waals interactions or relatively polar polymers were grouped together with nonpolar polymers in the analysis. 1,2,4,6 Limited attention was paid to the influence that polar interactions themselves may have on properties such as T g and m. The presence of additional intermolecular interactions does, without a doubt, shift the delicate balance between the entropic and enthalpic variables that control the segmental relaxation process. A naive picture suggests that the presence of polar interactions would lead to slower segmental dynamics due to higher friction between relaxing units; i.e., the glass transition temperature would rise. An increase in the strength of the intermolecular interactions could also result in a higher degree of cooperativity/heterogeneity of the segmental dynamics, potentially leading to a higher fragility index value for a polymer. However, the presence of a polar group may have a different effect on the volumetric and energetic activation barriers for segmental motion, making it rather difficult to predict the exact behavior of T g and m.The effect that strong intermolecular interactions impose on chain dynamics is even less clear. Several recent studies revealed an intriguing aspect of polymer dynamics: chemical structure seems to have very little impact on the steepness of temperature d...

Energy Renormalization for Coarse‐Graining Polymers with Different Fragilities: Predictions from the Generalized Entropy Theory

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