Formation Of Α-form Crystals Research Articles

Concurrent solvent induced crystallization and hydrolytic degradation of PLA by water-ethanol solutions

Abstract Poly(lactic acid) (PLA) films were immersed in pure water, 50% and 95% ethanol solutions for up to 180 days. The change in molecular weight, sorption of water, sorption of ethanol, and lactic acid released were monitored. Glass transition temperature and percent crystallinity as a function of ethanol content were also measured. PLA experienced faster hydrolytic degradation in contact with 50% than with 95% ethanol or pure water. NMR methodologies were developed to measure sorption of deuterated water and ethanol in PLA. More water was sorbed in 50% ethanol, explaining the higher hydrolysis. During exposure, PLA experienced solvent induced crystallization. Higher percent crystallinity was found in films exposed to 50% ethanol with the formation of α-form crystals. The hydrolysis of PLA was correlated with the release of lactic acid during exposure. Mathematical models are proposed to explain the concurrent solvent induced crystallization and hydrolytic degradation of PLA.

Effect of Uracil on the Isothermal Melt Crystallization Kinetics and Polymorphic Crystals Control of Biodegradable Poly(butylene adipate)

Biodegradable poly(butylene adipate) (PBA) and uracil composites were prepared via a solution and casting method at low uracil loadings in this work. The isothermal melt crystallization rate has been increased apparently in the composites by the addition of uracil, which acts as a nucleating agent for the crystallization of PBA. PBA may crystallize in the α- or β-form under different conditions. During the nonisothermal melt crystallization, the α-form crystals of neat PBA are formed at only relatively slow cooling rate; however, the formation of α-form crystals can be induced by uracil in the composites at relatively faster cooling rates. The formation of polymorphic crystals of PBA may be regulated by changing cooling rate and the uracil loading, which provides an efficient way of controlling the polymorphic crystals formation and biodegradation behaviors.

Regulation of polymorphic behavior of biodegradable poly(butylene adipate) by multi-walled carbon nanotubes

Biodegradable poly(butylene adipate) (PBA) may crystallize in the α- or β-form under different conditions. Relative to the β-form, the α-form crystals are thermodynamically more stable and show a faster biodegradation rate; however, they are formed during the nonisothermal melt crystallization at only a relatively low cooling rate. In the present work, carboxyl-functionalized multi-walled carbon nanotubes (f-MWCNTs) were incorporated into the PBA matrix at low f-MWCNTs loading to regulate the polymorphic behavior of PBA crystals. It is found that the formation of polymorphic crystals of PBA is affected not only by cooling rate during the crystallization process but also by the f-MWCNTs loading. The formation of α-form crystals can be induced by f-MWCNTs at a high cooling rate, providing an effective way of controlling the polymorphic crystal formation and biodegradation rate of PBA for its wider practical application.

Polymorphic Crystals and Melting Behavior of sPS/iPP Blends Prepared by Polymerization

Blends of sPS/iPP of various compositions from 55/45 to 82/18 were prepared with a novel catalyst by polymerization. Crystal forms, crystallinity and melting behavior of sPS/iPP blends have been studied by FTIR spectroscopy, XRD and DSC. The results obtained show that the polymerization blending and the processing at 290 °C are favorable for formation of α-form crystals in neat sPS and in sPS/iPP blends. Melting points (T m ) and crystallinity of sPS and iPP in blends are reduced. The reflection peaks of sPS in the blends are diffuse and not easily depicted in X-ray patterns. The crystallization rate of iPP is slightly affected by blending, while the crystallization rate of sPS is strongly reduced.

Fiber Structure Formation in High-Speed Melt Spinning of Polyamide 6/Clay Hybrid Nanocomposite

Polyamide 6 (PA 6)/clay hybrid (NCH) nanocomposites containing 2 and 5 wt% of clay were melt spun at take-up velocities from 1 to 5 km/min, and the effect of clay on the fiber structure formation was investigated. As the reference, neat PA 6 was spun at take-up velocities from 1 to 7 km/min. The NCH fibers showed higher crystallinity in the whole take-up velocity range, although there was no remarkable crystallinity dependence on the clay content. The birefringence of NCH fibers exceeded that of neat polymer only in the low take-up velocity region (i.e., up to 3 km/min). The orientation-induced crystallization was observed to start at about 2 km/min for NCH and at 4 km/min for neat PA 6. Also, at these take-up velocities, the peaks of α-form crystals appeared in the wide-angle X-ray diffraction equatorial (WAXD) scans, indicating that the orientation-induced crystallization was related to the direct formation of α-form crystals in the spinline. The NCH fibers were superior in Young's modulus; however, their tenacity was higher only in the low take-up velocity region in which the crystallization in the spinline of neat PA 6 did not occur yet. That variation of tenacity was attributed to the molecular orientation, whereas the modulus was supposed to be determined by the stiffness of intercrystalline regions.