Understanding microscopic parameters that control steepness of the temperature variations of segmental relaxation (fragility) and the glass transition phenomenon remains a challenge. We present dielectric and mechanical relaxation studies of segmental dynamics in various polymers with different side groups and backbone structures. The results have been analyzed in terms of flexibility of backbone and side groups of polymeric molecules, as suggested by the recent theoretical works by Dudowicz et al. A comparison of structures with identical backbones and varying side groups and identical side groups but different backbones reveals that the flexibility of side groups relative to the flexibility of the backbone is the most important factor controlling fragility in polymers, while the glass transition temperature T g depends primarily on the backbone flexibility and the side group bulkiness (occupied volume). Based on these results and analysis of literature data we formulated a modified approach to understand the role of chemical structure in segmental dynamics: (i) Polymers with stiff backbones always have high T g and fragility, while (ii) polymers with flexible backbones and no side groups are the strongest; (iii) however, for the most common type of polymeric structure, C-C or Si-O backbone with side groups, fragility increases with increasing "relatiVe" stiffness of side groups versus the backbone. In this class of polymers, lowest fragility is expected when the side groups are of similar chemical structure (or flexibility) as the backbone, as in the case of polyisobutylene, one of the strongest polymers known.

The influence of molecular weight, M, on the fragility and fast dynamics in polyisobutylene (PIB) was studied using dielectric and mechanical relaxation spectroscopies, calorimetry, and Raman spectroscopy. The measurements indicate a decrease in fragility with increasing M for shorter chains, in the range of M where T g is M-dependent. Such behavior is not observed for other polymers and is at odds with traditional theoretical models that predict an increase in fragility with chain length. These results confirm the unusual character of PIB, as evident in various properties including extremely low gas permeability, a low fragility, and a segmental relaxation spectrum much broader than expected for a low-fragility material. The reason for this anomalous behavior remains unclear, but might be related to the symmetric structure of the PIB repeat unit, together with comparable flexibility of both structural components, the backbone and side groups.

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...

Dissipation in a nano-mechanical resonator under the application of a nearly uniform strain field is investigated using Molecular Dynamics (MD) simulations. Under the application of a uniform strain field and in the frequency range studied, we expect Akhiezer damping to be the dominant loss mechanism. The scaling of energy dissipation rate with frequency for the bulk case and a finite sized nanostructure are studied and the results are explained by Akhiezer damping. The size effect on the dissipation rate is also investigated. The results show a significant role of the surface on the dissipation rate. An increase in the Q factor with a decrease in thickness of the structure is observed for a certain range. Below some critical thickness, the trend reverses indicating multiple roles of the surface contributing to the dissipation process.