Abstract: Dimensionally stable, pigmentable polyester molding compositions comprise:(A) a polymerizable polyester component; and(B) an additive component comprising discrete particles of less than about 50 microns diameter of a lightly cross-linked styrene terpolymer having a glass transition temperature of below about 100.degree. Celsius and which is chemically inert with respect to the polyester component and is swellable but insoluble therein. The additive component is the reaction product of styrene, an unsaturated monofunctional comonomer and a polyfunctional monomer such as divinyl benzene.

Abstract: An improved polyester filament (i.e., a principally polyethylene terephthalate filament) suitable for use in commercial applications is provided having a unique internal structure. The filament possesses an interconnected highly oriented crystalline microstructure coextensive with its length coexisting with an interdispersed substantially disoriented non-crystalline phase. The filament microstructure imparts inter alia a propensity for the filament to undergo a low degree of shrinkage under a high degree of force at an elevated temperature as evidenced by a modulus ratio (as defined) of at least 0.1. The filament exhibits a relatively high initial modulus, coupled with a relatively high crystalline orientation function, and a relatively low amorphous orientation function.