Improving the toughness of poly(lactic acid) (PLA) through co-continuous, immiscible, biodegradable blends with PHA

Poly(lactic acid) (PLA) and Polyhydroxyoctanoate (PHO) were melt blended using a Haake torque rheometer in the ratios of 80/20, 60/40, 50/50, 40/60, and 20/80. Due to the large difference in the melt viscosity in the binary blends, 0.55 wt % Hexamethylene diisocyanate was added to the PHO to improve the processability, which resulted in the formation of urethane linkages by the reaction of the isocyanate with the hydroxyl group in the PHO. The number average and weight average molecular weight of the modified PHO with 0.55 wt % isocyanate were increased 314% and 275%, respectively, as compared with those of pure PHO. In this study, thermal, mechanical, rheological, biodegradable, and morphological properties of the blends were investigated. Differential scanning calorimetry showed that the two components in these blends were immiscible.; It is concluded from the mechanical tests that the tensile strength at break and Young's modulus decreased with increasing levels of modified PHO with 0.55 wt % isocyanate content in the blends. The addition of the modified PHO in PLA increased the toughness of the PLA and increased the elongation to break more than 158%. The mechanical properties of aging PLA/modified PHO blends with 0.55 wt % isocyanate at 38°C after thirty-eight days were decreased remarkably, as compared with those of the original samples. Results obtained from the rheological tests indicate that the melt viscosities of the modified PHOs were dependent on the isocyanate content and crystallization time. The melt viscosities of PLA/modified PHO blends decreased with an increase in the content of the modified PHO with 0.55 wt % isocyanate. The hydrolytic degradation results during forty days of testing time showed that the weight loss for all of the blends has no significant change.; The fracture morphology of immiscible PLA and modified PHO blends was changed by the relative amounts of the two polymer components. PLA/modified PHO 50/50 blends composition showed a co-continuous morphology. With increasing modified PHO content, a minor phase with small spherical particles was dispersed in the major polymer matrix.