Correlation between fragility and free volume void size at glass transition temperature

Abstract

Despite the tremendous endeavors devoted to exploiting the nature of glass transition, the factors that control the steepness index of viscosity near glass transition, that is, fragility, remain unclear. In this study, we demonstrate that, for polymeric and small molecular weight organic glass formers, fragility increases upward with increasing size of the free volume void at the glass transition temperature. This changing trend indicates that fragility is governed by the properties of the segments or molecular clusters in the free volume void rather than by the properties of the entire polymer chains. The physics behind the relationship between fragility and free volume void at the glass transition temperature is consistent with the physics behind the relationship between fragility and the molecular weight as well as the mechanics of the relationship between fragility and size of the cooperative units. This relationship also provides new insights into the understanding of the nature of the glass transition of polymeric and small molecular weight organic glass formers.

Quantifying methyl tunneling induced (de)coherence of nitroxides in glassy ortho-terphenyl at low temperatures

“…From our analysis we cannot conclude whether this narrowing of the rotation barrier distribution is characteristic for a given glassy matrix, since we lack experimental reference data to determine the sensitivity of methyl tunneling ESEEM to quantify the temperature-related increase in free volume of a glass. Moreover, free volume void size at the glass transition temperature varies quite strongly between different glass formers 57 and the ratio between glass transition temperature and measurement temperature might also play a role. To gain more insight into this phenomenon a larger sample set consisting of the same methyl group containing nitroxide in various glassy matrices would need to be studied in the low-temperature regime.…”

Modified Free Volume Theory for Self-Diffusion of Small Molecules in Amorphous Polymers

“…The fragility-based interpretation of ξ 2 is nonetheless adopted in this study because it is soundly founded on the underlying theory of spatially heterogeneous dynamics near glass transition and is in agreement with the experimental diffusion data of single-component glass-formers . Congruently, the fragility of macromolecules and small organic glass-formers has additionally been linked to the average free volume at T g . By contrast, the jumping unit approach uses an empirical relation of the polymer’s jumping unit with its T g , as well as the geometrical analysis of the diffusant molecule in a cuboid (see the Supporting Information Part III).…”

“…47 Congruently, the fragility of macromolecules and small organic glass-formers has additionally been linked to the average free volume at T g . 93 By contrast, the jumping unit approach uses an empirical relation of the polymer's jumping unit with its T g 65,94 as well as the geometrical analysis of the diffusant molecule in a cuboid 91 (see the Supporting Information Part III). To further inspect the two approaches to determine the value of ξ 2 and its influence on D 1 , an example is provided in Figure 6.…”

Open Questions that Can Bridge Intermolecular Interactions and Macroscopic Wetting/Dewetting Behaviors of Thin Films

“…[92] These pronounced effects in the crosslinked system support the large critical thicknesses for contact angle dependence on crosslinked ultrathin PS/PMMA films observed by Petek et al [36] This review invites future work to combine the additive vdW potential with the cooperative nature of polymers to aid in the understanding of ultrathin polymer film wettability. Experimentation that assesses variables known to impact the cooperative motion of polymers, such as bulky side groups [91] or fragility, [93,94] would be insightful to determine if the unexplained changes in contact angle and surface energy presented within are truly driven by cooperativity.…”