Abstract
Poly(1,4-oxybenzoate) (POB) and poly(2,6-oxynaphthoate) (PON) and their copolymers which have a well-established phase diagram have been studied with temperature-modulated differential scanning calorimetry (TMDSC). All the analyzed polymers have more than one disordering transition between the glass transition (from 400 to 430 K) and decomposition (starting at ≈700 K). Above the glass transition, the reversible heat capacity, Cp, increases beyond that calculated from the crystallinity and the known Cp of the solid and melt. This is likely due to an increase of mobility within the crystals and/or a possible rigid-amorphous fraction (mainly for the copolymers). The disordering transitions are largely irreversible, supporting the observation that semicrystalline, linear macromolecules show decreasing amounts of locally reversible melting with increasing rigidity and crystal perfection.