46666-AC7
Modification of the Fragility of Polymer Melts with Structured Solvents
Despite recent claims in The New York Times that the Òglass transition problemÓ has been solved, many fundamental questions persist concerning the influence of molecular structure, interactions, packing, etc., on central properties of glass formers, such as the glass transition temperature and the fragility, the latter being a crucial material property for determining whether the material can be processed by extrusion, casting, jet spray, etc. methods. We have combined the lattice cluster theory [1] with the Adam-Gibbs model to produce the first molecular theory for the fragility of glass forming polymers as well as for the influence of monomer molecular structure on the glass transition temperature, the relaxation dynamics, etc. Glass fragility is shown to arise due to the frustration in packing of polymers with irregular local monomer structures and/or longer range stiffness. [2] We are now extending the lattice cluster theory to deduce the molecular characteristics that promote anti-plasticization, a phenomenon in which the addition of a small molecule additive reduces the glass transition temperature as in plasticization but instead toughens the glass. Our previous one-component theory of glass forming polymers has been extended to multi-component systems and has been applied to glass formation in polymers with 10 to 20% of a small molecule additive. The structures of the additives correspond to oligomers of the polymers but different stiffness and cohesive energy parameters are used for the additives. Calculations have been performed over a wide range of energy parameters for stiffness and cohesion. Flexible additives uniformly depress the glass transition temperature, and the shift in the glass transition temperature is greater for the smaller additives. The calculations exhibit both phenomena, plasticization and anti-plasticization of the polymers, depending on the characteristics of the additive. Anti-plasticization is promoted by additives that are more flexible and have higher cohesive energy than that of the host polymers, and the change in toughness appears to be independent of the size of the oligomeric additive. The stiffness of the additive less important when the additive is less extended, so even stiff, yet small enough additives reduce the glass transition temperature and can promote either plasticization or anti-plasticization depending on the cohesive energy parameters. Additionally, anti-plasticized fluids seems to exhibit less packing frustration since the mixtures have larger densities at low temperatures than the pure polymer melts. Computations show that both plasticized and anti-plasticized polymers are less fragile. We plan to vary the molecular structure of the additive to assess which additional molecular characteristics promote anti-plasticization. The opposite effect, i.e. an increase in fragility, is expected to occur upon the addition of stiff molecules to the host polymer. Thus, mixtures of either flexible or strong glass-formers with stiff small molecule additives provide good candidates for future studies. [1] Influence of Monomer Molecular Structure on the Miscibility of Polymer Blends. K. F. Freed and J. Dudowicz, Adv. Polym. Sci. 183, 63-126 (2005). [2] Generalized Entropy Theory of Polymer Glass-Formation. J. Dudowicz, K. F. Freed, and J. F. Douglas, Adv. Chem. Phys. 137, 125-222 (2008).
