Abstract
The cold crystallization behaviors and structure evolutions of amorphous PLA stretched within a high-temperature range (100–150 °C) were investigated via in situ synchrotron WAXS and SAXS measurements. The PLA stretched at 120 °C exhibits excellent mechanical properties with the highest modulus, stretching strength and moderate elongation, which is attributed to the coordination effect of thermally-induced initial crystallinity and moderate chain segment mobility. The WAXS results showed that the diffraction peaks of (200/110) and (203) planes shift to higher 2θ values, which indicates the formation of more ordered α′ and α forms during PLA stretching within 100–150 °C. More α forms can be formed from the stretched α′ form and/or from the amorphous phase at 100 °C. The phase diagram of α and α′ forms in stretched PLA with stretching temperature was proposed according to the obtained data of lattice spacing. The crystallinity of stretched PLA increases faster than that which underwent quiescent cold crystallization at the same temperature. The SAXS results indicated that the long period and lamellar thickness along and vertical to the stretching direction of PLA stretched within 100–150 °C exhibit distinct variations, which is attributed to the difference of strain-induced crystallization and lamellar orientation.
Highlights
The cold crystallization behaviors and structure evolutions of amorphous PLA stretched within a hightemperature range (100–150 °C) were investigated via in situ synchrotron WAXS and SAXS measurements
The α and α′ forms of PLA have been identified according to wide-angle X-ray diffraction (WAXD) and infrared spectroscopy (IR) measurements
It is shown that the intensity of the diffraction peak of the (203) plane of PLA stretched at 100 °C increases with strain and the position shifts to that of PLA annealed at 120 °C related to the α form only, which implies that the structural ordering of the PLA crystal increases gradually[9] and more α phases are formed from the stretched α′ phase and/or from the amorphous phase directly the α′ phase still exists
Summary
CrystEngComm content of the α form will enhance Young's modulus and the barrier to water vapor of PLA films.[16]. Stretching amorphous PLA samples at different temperatures and drawing rates can induce different ordered structures. The strain-induced ordered structures (mesocrystal and crystal) and crystal orientation of amorphous PLA stretched at a wide temperature range have been investigated extensively. The strain-induced variations of crystalline structures (containing lamellae and forms of the crystal) during stretching at different temperatures have been rarely reported. The effects of strain on the variation of crystal structures (α and α′ forms) and cold crystallization behavior together with the mechanical properties of stretched PLA were investigated within 100–150 °C. Based on the in situ synchrotron WAXS/SAXS results, the straininduced variation of the stacked lamellar structure of PLA during stretching within 100–150 °C was discussed
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