Reports: AC7
46666-AC7 Modification of the Fragility of Polymer Melts with Structured Solvents
Despite recent claims in The New York Times that the glass transition problem 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]
The entropy theory of glass formation has been extended to model the thermodynamic and dynamic properties of poly(α-olefins). By combining this thermodynamic theory with the Adam-Gibbs model (which relates the configurational entropy to the rate of structural relaxation), we provide systematic computations for all four characteristic temperatures (TA, Tc, Tg, T0), governing the position and breadth of the glass transition, and the fragility parameters (D, m) describing the strength of the temperature dependence of the structural relaxation time, where TA is the temperature below which the relaxation is non-Arrhenius, Tc is the crossover or empirical mode-coupling temperature, Tg is the glass transition temperature, and T0 is the temperature at which the extrapolated relaxation time diverges. These temperatures and fragility parameters are evaluated as a function of molar mass, pressure, and the length n of the alpha-olefin side chains. The nearest neighbor interaction energy and local chain rigidities are found to strongly influence the four characteristic temperatures and the low temperature fragility. We also observe an "internal plasticization" of the poly(α-olefins) wherein the fragility decreases as the number n of "flexible" side group units increases. Our computations provide solid support for a pressure counterpart of the Vogel-Fulcher-Tammann relation. The entropy theory of glass formation predicts systematic changes in fragility with chain stiffness, cohesive energy, polymerization index, and side chain length, and qualitative trends in these parameters are discussed. By determining the parameters of the theory by fitting the equation of state data for polypropylene and the glass transition temperatures of polyethylene and polypropylene, predictions are made for the glass transition temperature and fragility as a function of n, and good quantitative agreement with experiment is obtained.
We are have also extended 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. Calculations for various additive molecular structures enable assessing which additional molecular characteristics promote anti-plasticization. The opposite effect, i.e. an increase in fragility, occurs 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).
[3]. E.B. Stukalin, J. F. Douglas, K. F. Freed, Application of the Entropy Theory of Glass Formation to Poly(alpha-olefins), J. Chem. Phys., submitted.
