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ABSTRACT Appropriate cooling conditions in melt processing were found to provide crystalline poly(lactic acid) (PLA) with greater mechanical toughness in tensile tests. PLA films cooled near its glass transition temperature Tg showed ductile behavior, whereas those obtained by a quenching process exhibited brittle fracture. The content of gauche-gauche (gg) conformer, which leads to low critical onset stress for shear yielding, increased in the films cooled near Tg. The crystallinity of the films hardly affected their mechanical toughness and proportion of gg conformer except for that with a high degree of crystallinity (>50%). © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44960. [ABSTRACT FROM AUTHOR]

The isothermal crystallization kinetics and morphology of the poly(lactic acid) (PLA) blends containing three different sizes of both spherical and fibrous poly(butylene terephthalate) (PBT) domains have been comparatively investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The dynamic DSC measurement reveals that PBT domains significantly increase the degree of crystallinity of the PLA. Furthermore, the Avrami model is employed to evaluate the crystallization kinetics under isothermal conditions and it is found that PBT acts as nucleating agent, leading to a high overall crystallization rate constant k and shortened crystallization half time t1/2. Furthermore, the crystallization rate of PLA is promoted with the incorporation of PBT with a large specific surface area. The average Avrami index n of all samples lies within the range of 3.3 − 4.0, suggesting that morphologies of PBT do not affect the nucleation mechanism; however, the depression of equilibrium melting temperature in the blends ascribes the reductions of perfectness and size of the PLA crystallites. Besides, the nucleation of PLA crystallites around PBT fibers is probably faster than those around PBT spheres because the PBT chains oriented at the fiber surface as a result of flow-induced crystallization during melt stretching may serve as the primary nuclei for PLA chains to drastically crystallize at the fiber surface. POLYM. ENG. SCI., 56:258-268, 2016. © 2015 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

The present paper describes a novel method for reinforcing isotactic polypropylene (PP) by adding poly(vinyl alcohol) (PVA) fibers prepared by melt-stretching. Low-viscosity PVA droplets were deformed into fibers in the molten PP by melt-stretching them beyond the melting point of PVA. When the obtained composite was extruded below the melting point of PVA, the fibers became oriented toward the flow direction. The PVA demonstrated marked nucleating activity with regard to PP crystallization, which was further promoted by enlargement of the surface area by fibrillization. As a result, there was virtually no orientation relaxation of the PP chains with an increase in crystallinity, which explains the improvement in rigidity of the reinforced composite. [Display omitted] • PVA fibers were well dispersed in PP by melt-mixing. • PVA fibers act as a nucleating agent for PP. • Chain orientation and crystallinity of PP were enhanced by the PVA addition. • Modulus of the extruded sample was enhanced by PVA fibers. [ABSTRACT FROM AUTHOR]