Unique Crystalline Orientation of Poly[(R)-3-hydroxybutyrate]/Cellulose Propionate Blends under Uniaxial Drawing

Abstract

Blends of poly[(R)-3-hydroxybutyrate] (PHB) and cellulose propionate (CP) with various compositions were prepared by the solvent-casting method, and their uniaxial drawing behavior was studied by means of two-dimensional wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Although this system has been reported to be entirely miscible, the DSC results revealed that the glass transition temperatures of the blends were not composition-dependent in the whole composition range. Furthermore, these transitions were quite broad or even double in the blends with high CP content. These features are the result of differing molecular motions attributed to the difference in chemical structure between the two components of the blend. The feasible maximum draw ratio decreased with increasing CP content due to restraint of chain slippage and necking caused by the stiffness and high friction coefficient of the CP component. A remarkable feature during uniaxial drawing was that the manner of crystallization of the PHB component under strain varied from the c-axis-oriented to the a-axis-oriented crystal growth with increasing CP content. WAXD analysis demonstrated that drawn pure PHB exhibited an ordinary c-axis-oriented pattern, indicating that the (110) and (020) main reflections of PHB were concentrated on the equatorial line. On the contrary, when the CP content was 50 wt %, the diffraction texture was transformed to the a-axis orientation where the (110) reflection was located near the meridian and the (020) and (002) reflections lay on the equatorial line. In the SAXS patterns of the blends containing 30−50 wt % CP, instead of meridional spots, the equatorial streaks were clearly observed, providing direct evidence for lamellar stacking perpendicular to the stretching direction. Such a change of crystalline orientation was found to be strongly dependent on the draw ratio and annealing temperature in addition to the blend composition. In the present paper, a mechanism based on the intramolecular nucleation and confined crystal growth model was suggested to interpret this uncommon orientation behavior.