Ep21 The glassy state and the glass transition - UCSD NANO 134 Darren Lipomi

The script begins with a discussion on the glass transition (Tg) and its impact on the mechanical properties of materials. It explains that materials exhibit different behaviors below and above Tg, affecting their stress-strain characteristics. The glassy state is described as kinetically trapped, with local bending and unbending of bonds as a means of strain accommodation. Sub-Tg relaxation mechanisms contribute to the toughness of polymers, which is the energy absorbed before fracture. The talk also touches on the difference between amorphous and crystalline regions in semicrystalline samples, emphasizing the importance of Tg in understanding material behavior.

This paragraph delves into the sub-Tg relaxation mechanisms that allow polymer chains to dissipate energy without deforming the entire sample. It uses the analogy of a 'crankshaft mechanism' to describe bond rotations in polymers like polyethylene, which contribute to their toughness. The discussion also covers how these mechanisms are not present in inorganic glasses due to the difference in atomic bonding. The paragraph further explores the conditions under which these relaxation mechanisms occur, including the influence of mechanical force and the transition to more energetically favorable states.

The script uses an analogy of a semicrystalline polymer to explain the glass transition. It describes how, as temperature increases above Tg, the material transitions from a stiff, spring-like state to a weaker, more damping state due to the increased mobility of polymer chains. The analogy of a rubber hose wound into a helix is used to illustrate the mechanical properties of the material at different temperatures relative to Tg and Tm (melting temperature). The paragraph highlights the changes in mechanical properties, such as damping and time dependence, as the material warms through the Tg.

This section discusses the time-dependent mechanical behavior of viscoelastic solids, contrasting it with the immediate response seen in ideal solids. It uses the example of chewing gum to illustrate how materials can exhibit different mechanical properties over time, with the restoring force weakening as molecules realign. The paragraph also touches on the concept of mechanochemistry, where mechanical energy can break chemical bonds over time, as demonstrated by the degradation of chewing gum.

The script provides a detailed look at the glass transition, describing it as a second-order phase transition with significant changes in mechanical properties, specific volume, modulus, heat capacity, dielectric constant, and refractive index. It emphasizes the absence of a discontinuity in entropy or volume at Tg, unlike first-order phase transitions, and discusses the technological importance of Tg. The paragraph also explores factors that influence Tg, such as polymer flexibility and chemical structure.

This paragraph examines the structural characteristics of polymers that affect their Tg, such as the flexibility of the polymer backbone and the presence of bulky side groups. It provides examples of different polymers, like PDMS, polyethylene, and PEO, and their respective Tg values. The discussion also covers the impact of steric effects, configurational isomerism, and the importance of efficient packing on Tg, with examples of polymers like PPO, PP, and PMMA to illustrate these concepts.

The script explains the concept of free volume and how it relates to the Tg of a polymer. It describes the van der Waals volume, packing volume, and the role of molecular shape and size in determining free volume. The paragraph discusses how factors like polydispersity, plasticizers, branching, and residual solvents can increase free volume, thereby lowering Tg. It also touches on the idea that more free volume results in a lower Tg due to reduced intermolecular forces.

The final paragraph introduces the basic concepts of stress and strain in the context of polymer mechanics. It defines strain as the change in length relative to the original length and stress as the force per unit area causing deformation. The paragraph also introduces the Poisson's ratio, which describes the transverse contraction of a material under axial strain. The discussion sets the stage for further exploration of stress-strain curves and viscoelasticity in subsequent lessons.