Dynamics and thermodynamics of small molecule glass formers, polymers and organic crystals




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Dynamics and thermodynamics of small molecule glass formers, polymers and organic crystals are addressed in this thesis. Dynamics of glass formers in the vicinity of glass transition are investigated in the dynamic part, while the thermodynamics of rubber swelling and the melting of the energetic materials PETN are discussed in the thermodynamics section. The glass transition, where liquids transform into metastable, non-crystallized solids below the melting temperature, is a subject of intense research despite decades of investigation. Among all the problems in glass transition, the dynamics of glass-forming systems in the vicinity of glass transition, which affects the viscoelastic properties of glass formers, is of great significance and has drawn considerable attention. The dynamic divergence, where the dynamics of the glass formers, e.g. viscosity and diffusion coefficient, goes to infinity at finite temperature, is conventially depicted by the Vogel-Fulcher-Tammann (VFT) or Williams-Landel-Ferry (WLF) expressions. However, recent study suggests that the dynamic divergence may not exist. Furthmore, there is no solid-therotical foundation to explain the reason of the dynamic divergence. Both of these two factors inspire us to find a new model that does not have dynamic divergence to study the dynamics response near Tg. In this dissertation, the Dyre shoving model, an elastic model which does not have dynamic divergence was evaluated. The dynamic response near Tg of small molecule glass formers were found to deviate from the dynamic divergence and can be described by the Dyre shoving model, which provides a new perspective to the study of the dynamics of glass formers. Investigations of the dynamics of a rubbery polymer in the glass-rubber transition region are present in Chapter 3. There is usually one shoulder in the dynamic data in terms of relaxation spectrum H(τ) and loss tangent (tanδ) for glass formers. In the dynamic mechanical measurements in the glass-rubber transition zone of polyisobutylene (PIB), an additional shoulder beside the major shoulder was observed in the relaxation spectrum H(τ) and loss tangent (tanδ). We attributed this additional shoulder to the sub-Rouse modes. This interpretation was further tested by studying the change of the mechanics by adding plasticizers to PIB, which should disrupt the effective chain packing of undiluted PIB. It was found by adding plasticizers, the softening dispersion becomes narrower, and the additional shoulder disappears eventually, confirming our assumption that the additional shoulder in tanδ is due to the sub-Rouse mode of motion. Detailed study of the thermodynamics of rubber swelling, especially the explanation of the nonexistence of the peak in the swelling activity parameters, are presented in Chapter 4. This problem has puzzled the rubber community for half a century. A new avenue of thought combined with simulated results were used to investigate the swelling activity parameter. It was found that the calculation method used by the previous rubber researchers exaggerate the experimental uncertainties and trigger the confusion concerning the purported peak in S. Swelling activity parameter obtained by our new calculation method shows consistent trend with changing temperature, crosslinking density, and more importantly, the prediction of classic Freckle-Flory-Renner (FFR) theory. The fifth part of this dissertation is a continuation of the fourth part. The new methodology developed in the second part to calculate S was used to analyze the swelling data for PDMS and polyisoprene networks. Consistent correlations between S and temperature and crosslinking density were found for the first time. Meanwhile, the thermodynamics of the melting behavior of nanocrystals of energetic materials were investigated systematically in this work. Controlled pore glasses were used to control the size of the nanocrystals. It was found that the melting temperature of nanocrystals depcreases as the pore size of the controlled pore glasses decreases, which is consistent with the prediction of Gibbs-Thomson equation. The value of liquid-solid interface energy, calculated according to the Gibbs-Thomson equation, is also reasonably consistent with the estimation from the empirical Turnbull equation.



Flory-Huggins, Glass transition, Rheology, Polymer physics, Thermal analysis, Nano-explosive, Pentaerythritol tetranitrate (PETN), Dyre shoving model, Dynamic divergence, Gibbs-Thomson equation, Melting temperature depression, Frenkel-Flory-Rehner (FFR) hypothesis, Kohlrausch-Williams-Watts (KWW), BSW (function), Small molecule glass former