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An analysis of glass formation for polymer melts that are diluted bystructured molecular additives is derived by using the generalized entropytheory, which involves a combination of the Adam-Gibbs model and the directcomputation of the configurational entropy based on a lattice model of polymermelts that includes monomer structural effects. Antiplasticization isaccompanied by a "toughening" of the glass mixture relative to the purepolymer, and this effect is found to occur when the diluents are small specieswith strongly attractive interactions with the polymer matrix. Plasticizationleads to a decreased glass transition temperature T_g and a "softening" of thefragile host polymer in the glass state. Plasticization is prompted by smalladditives with weakly attractive interactions with the polymer matrix. Theshifts in T_g of polystyrene diluted by fully flexible short oligomers areevaluated from the computations, along with the relative changes in theisothermal compressibility at T_g to characterize the extent to which theadditives act as antiplasticizers or plasticizers. The theory predicts that adecreased fragility can accompany both antiplasticization and plasticization ofthe glass by molecular additives. The general reduction in the T_g andfragility of polymers by these molecular additives is rationalized by analyzingthe influence of the diluent's properties (cohesive energy, chain length, andstiffness) on glass formation in diluted polymer melts. The description ofglass formation at fixed temperature that is induced upon change the fluidcomposition directly implies the Angell equation for the structural relaxationtime as function of the polymer concentration, and the computed "zero mobilityconcentration" scales linearly with the inverse polymerization index N.

Plasticization and antiplasticization of polymer melts diluted by low molar mass species