Intracrystalline chain diffusion, semicrystalline morphology and mechanical modulus of selected aliphatic polyesters

Abstract

Recently, based upon experiments on three model polymers, we concluded that semicrystalline polymers exhibit different morphological features depending on the timescale of intracrystalline chain diffusion (ICD) relative to the kinetics of crystal growth. In this study, we investigated a series of selected aliphatic polyesters to test the generality of this concept. ICD and crystallization kinetics of the selected polyesters were characterized by nuclear magnetic resonance (NMR) and optical microscopy, respectively. Small-angle X-ray scattering (SAXS) was used to reveal the semicrystalline morphology, and to differentiate ICD of polyesters with rather slow ICD (undetectable by NMR) through comparing crystal-thickening slopes during crystallization. Our results confirm that the observed relation between morphology and ICD is valid for all investigated polyesters. The formation of ester layers in the polyester crystals results in slowing down of ICD and a reduction of the enthalpy of melting (𝛥𝐻𝑚,100), which is determined via a reliable combination of differential scanning calorimetry with 1H low-resolution NMR and further confirmed by SAXS. In addition, we propose a further refinement and expansion of the classification of the crystal-fixed polymers, a term that is commonly used in the literature. Furthermore, the shear modulus of the selected polyesters with slow ICD shows a common exponential dependence on their crystallinity.