Abstract
The non-equilibrium thickness of lamellar crystals in semicrystalline polymers varies significantly between different polymer systems and depends on the crystallization temperature Tc. There is currently no consensus on the mechanism of thickness selection. Previous work has highlighted the decisive role of intracrystalline chain diffusion (ICD) in special cases, but a systematic dependence of lamellar thickness on relevant timescales such as that of ICD and stem attachment has not yet been established. Studying the morphology by small-angle X-ray scattering and the two timescales by NMR methods and polarization microscopy respectively, we here present data on poly(oxymethylene), a case with relatively slow ICD. It fills the gap between previously studied cases of absent and fast ICD, enabling us to establish a quantitative dependence of lamellar thickness on the competition between the noted timescales.
Highlights
IntroductionDifferent mechanisms have been suggested—without reaching a final agreement—to limit reorganization to a certain thickness, such as thickness-dependent stability of different crystal phases[9] or mesophases[10,11] or thickness-dependent intracrystalline chain diffusion (ICD)[12,13,14]
The non-equilibrium thickness of lamellar crystals in semicrystalline polymers varies significantly between different polymer systems and depends on the crystallization temperature Tc
The characteristic morphological feature of semicrystalline polymers crystallized from the melt is a nanoscopic twophase structure of thin lamellar crystals separated by disordered amorphous layers, which contain the entanglements retained during crystallization
Summary
Different mechanisms have been suggested—without reaching a final agreement—to limit reorganization to a certain thickness, such as thickness-dependent stability of different crystal phases[9] or mesophases[10,11] or thickness-dependent intracrystalline chain diffusion (ICD)[12,13,14] All these models primarily aim at an explanation of the temperature dependence of the crystal thickness of a given semicrystalline polymer. Detailed analysis of NMR data reflecting the timescale of ICD in the temperature range of crystallization showed that for PEO the ICD is so fast that it can cause reorganization over a very small nanometre-sized reorganization zone directly behind the growth front and practically simultaneously with crystal growth[21] From these results, we concluded that in crystal-mobile polymers the morphology is controlled by a minimum value of the amorphous thickness related to the entanglement density in the amorphous regions
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