Tie molecules, morphology and confinement effects in semi-crystalline poly(ethylene naphthalene-2,6-dicarboxylate) (PEN)

Abstract

The glass transition dynamics and confinement effects in poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) were studied by broadband dielectric relaxation spectroscopy (DRS) in relation with semi-crystalline morphology. PEN samples were obtained by cold crystallization at different crystallization temperatures (Tc ranging from 165 to 240°C) and crystallization times (tc from 30min to 24h). Differential scanning calorimetry and X-ray diffraction showed that the crystallinity ratio (Xc) increases when Tc and tc increase. The glass transition relaxation is shifted to higher temperatures as tc increases but this confinement effect decreased with increasing Tc. The origin of this anomalous dynamics can be related to (i) the crystalline lamellar stack morphology revealed by small angle X-ray scattering (SAXS) and (ii) chain scission occurring during annealing at the crystallization temperature. As a result, the density of tie-chain molecules (chain portions bridging crystallites) can be considered as the key factor for the understanding of confinement effects in semicrystalline polymers. Accordingly the confinement effects on the glass transition dynamics can be optimally rationalized as a function of a characteristic length intermediate between the interlamellae thickness la and that of the theory of Brown and Huang 2l c+la where lc is the crystallite thickness.