Ep19 Crystallization and crystalline microstructure - UCSD NANO 134 - Darren Lipomi

This paragraph delves into the concepts of crystallization in semicrystalline polymer samples, explaining the distinctions between amorphous and crystalline domains. It covers their different thermal and mechanical properties, emphasizing the density differences due to van der Waals interactions. Key points include the glass transition temperature for amorphous domains, the crystallization and melting temperatures for crystalline domains, and the factors influencing crystallization, such as symmetrical chains and specific intermolecular forces like hydrogen bonding.

This paragraph focuses on the role of molecular motion in crystallization, highlighting the importance of temperature in providing sufficient molecular motion. It explains the temperature ranges for optimal crystallization, the impact of molecular weight on crystallization, and the two stages of crystallization: nucleation and growth. The discussion also touches on how impurities and thermal fluctuations can trigger nucleation events, and the competition between nucleation and chain redispersion during crystallization.

This paragraph describes an experiment involving supercooling of water in a smooth glass flask to illustrate nucleation. It explains how the absence of nucleation sites prevents crystallization despite being below freezing temperature and how introducing density fluctuations can induce crystallization. The paragraph further discusses the growth of crystallites, the factors influencing their growth rate, and the conditions under which crystallization occurs optimally.

This paragraph examines the effect of thermal history on crystallization, explaining that complete crystallization is rarely achieved due to factors like polydispersity and branching. It discusses the role of small crystallites in melting at lower temperatures and the broadening of melting temperature due to size confinement effects. The paragraph also covers the importance of annealing in achieving nearly complete crystallization and how thermal history influences crystallization kinetics.

This paragraph addresses the relationship between melting and volume changes in polymers, noting exceptions like water. It explains how thermal history affects melting behavior, the necessity of annealing for optimal crystallization, and the influence of branching on melting temperature. The discussion includes an example involving specific volume versus temperature for polyethylene samples, highlighting the impact of branching on crystallization.

This paragraph explores the thermodynamic principles of melting in polymers, defining the equilibrium melting temperature and the relationship between enthalpy and entropy changes upon melting. It compares the melting behaviors of polyethylene and poly(tetrafluoroethylene) (Teflon), explaining the influence of intermolecular forces and entropy on their melting temperatures. The paragraph highlights the greater entropy change in polyethylene due to its flexible structure compared to Teflon.

This paragraph contrasts the molecular structures of polyethylene and Teflon, focusing on their behavior in the crystalline and melt phases. It explains how the presence of fluorine atoms in Teflon results in a less flexible structure compared to the hydrogen atoms in polyethylene. The discussion includes the impact of these structural differences on the entropy of the melt state and the resulting melting temperatures of the two polymers.

This paragraph delves into the relationship between molecular flexibility and melting entropy, using the example of polyethylene and Teflon. It explains how the covalent radius of fluorine in Teflon restricts molecular flexibility, leading to a higher energy barrier for rotational freedom compared to polyethylene. The paragraph also discusses the implications of this difference for the entropy of the melt state and the melting temperatures of the polymers.

This paragraph describes the formation and structure of crystalline morphologies in polymers, particularly the nucleation and radial growth of spherulites. It explains how these spherulites form under cross-polarized light, resulting in distinctive patterns due to anisotropy. The discussion covers the internal structure of spherulites, including the arrangement of crystalline and amorphous regions and the role of molecular axes in determining crystallite orientation.

This final paragraph explores the different conformations of polymer chains in crystallites, comparing regular folding with the disordered switchboard model. It explains how these conformations affect the structure of spherulites, with adjacent re-entry being more common in solution-grown crystals and the switchboard model favored in spherulites. The paragraph concludes by summarizing the key points of the discussion on polymer crystallization and thanking the audience.