Rigid amorphous fraction and multiple melting behavior in poly(butylene terephthalate) and isotactic polystyrene

Abstract

The multiple melting behavior of many semicrystalline polymers is generally attributed to fusion of original crystals reorganized/recrystallized upon heating at conventional scanning rates. In the present study, the triple and double melting behavior of poly(butylene terephthalate) (PBT) and isotactic polystyrene (iPS), which is observed after isothermal crystallization at low and high T c s, respectively, is put into relation with the presence and absence of rigid amorphous/crystal interphase around the original primary crystals. Such constrained amorphous interphase has been recognized as decisive for the definition of the mechanical, thermal, and barrier properties of semicrystalline polymers. The method for the approximate determination of the limit temperature for the presence of this rigid amorphous fraction (RAF), already applied to poly[(R)-3-hydroxybutyrate] (PHB) and poly(ethylene terephthalate) (PET), is here utilized for PBT and iPS. The complex melting behavior displayed by all these polymers is explained by assuming that two distinct morphologies of primary crystals develop during crystallization at temperatures lower than the RAF limit temperature, in a percentage that is a function of the crystallization temperature. Simultaneously with the RAF devitrification, these distinct morphologies differently reorganize/recrystallize upon heating at low scanning rate, originating the triple melting behavior. On the contrary, after crystallization at high T c s, a single reorganization route is followed by all the primary crystalline structures, characterized by the same morphology made of tightly chain-folded lamellae and absence of vitrified RAF at the amorphous/crystal interphase.