Ep17 Chain models and DSC - NANO 134 Darren Lipomi UCSD

The video segment introduces the concept of polymer chain dimensions and how they form microstructures in both amorphous and crystalline regions of a polymer film. It discusses the importance of understanding these structures to measure bulk properties in the lab. The presenter begins by explaining the two main dimensions of a polymer chain: the end-to-end distance (R) and the radius of gyration (S). The segment uses the analogy of a random coil to describe these dimensions and introduces the root mean square (RMS) values to define them more precisely. The explanation also touches on the nomenclature used in polymer science to denote averaging due to polydispersity and the averaging of many conformations for chains of the same degree of polymerization.

This paragraph delves into various models used to describe polymer chain dimensions. It starts with the random coil model, also known as the drunkard's walk, which assumes no constraints on the direction of the steps taken by the 'drunkard', symbolizing the polymer chain. The model is used to derive the relationship between the end-to-end distance and the number of steps (or bonds) in the chain. The explanation then moves to the freely jointed chain model, which considers bond angles but not bond rotations, and the freely rotating chain model with fixed bond angles, which accounts for the directional persistence in polymer chains. The segment concludes with the hindered rotation chain model, which introduces the possibility of bond rotation in and out of the plane, affecting the dimensions of the polymer chain.

The video script discusses how the interaction of polymer chains with solvents affects their dimensions. It introduces the concept of the expansion factor (alpha), which quantifies how a polymer chain expands or contracts in different solvents. The script differentiates between good solvents, which cause the polymer to expand due to favorable interactions, and poor solvents, which lead to contraction as the polymer prefers self-interaction. The special case of a Theta solvent is highlighted, where the solvent's effect on the polymer chain dimensions is negligible. The Theta temperature is mentioned as the temperature at which any solvent becomes a Theta solvent. The segment also briefly touches on the characterization techniques like NMR and IR spectroscopy that can provide information on polymer chain conformations.

The script continues with a discussion on the characterization of polymer chain conformations, focusing on spectroscopic techniques such as NMR and IR spectroscopy. It explains how these methods can reveal information about the chemical environment and fluidity of chains in solid samples. The explanation then shifts to thermal transitions, introducing differential scanning calorimetry (DSC) as a technique to study these transitions. The DSC method is described in detail, including how it measures the heat flow between a sample and a reference to detect thermal events such as glass transitions, crystallization, and melting. The segment uses the example of isotactic polypropylene to illustrate how DSC can identify different thermal transitions in a semicrystalline polymer sample.

This final paragraph focuses on the thermal transitions of polymers, as observed through differential scanning calorimetry (DSC). It explains the process of heating a polymer sample past its melting point and then quenching it to form a glassy solid, which results in distinct peaks during the DSC analysis. The segment details the identification of the glass transition temperature (Tg), the crystallization temperature (Tc), and the melting temperature (Tm) from a DSC trace. The explanation emphasizes the exothermic nature of crystallization, where heat is released as new bonds form, and the endothermic process of melting, where heat is absorbed as the crystallites break down. The segment concludes with a promise to discuss crystalline morphologies and glass transitions in more detail in future classes.