Crystallization of ultrathin poly(3-hydroxybutyrate) films in blends with small amounts of poly(l-lactic acid): correlation between film thickness and molecular weight of poly(l-lactic acid)

Abstract

The crystallization behavior of poly(3-hydroxybutyrate) (PHB) ultrathin films in blends with small amounts of poly(L-lactic acids) (PLLAs) was investigated using grazing incidence X-ray diffraction (GIXD) and infrared-reflection absorption spectroscopy (IRRAS). Ultrathin films of PHB/PLLA with the same blend ratio of 80/20 (w/w) and two different film thicknesses (30 and 13 nm) were prepared using PHB (M_w = 650 000 g mol⁻¹) and PLLA with M_w's ranging from 300 000 to 710 g mol⁻¹ to explore the effects of molecular weight on the crystallization of PHB in different confined environments. In the 30 nm thick films, the intensity of PHB crystalline Bragg reflections was strikingly affected by the blended PLLA molecules. Middling molecular weight PLLAs (M_w = 23 000–13 100 g mol⁻¹) significantly inhibited the crystallization of PHB compared with higher molecular weight PLLAs (M_w ≥ 50 000 g mol⁻¹). In the 13 nm thick films, thickness confinement was revealed to play an important role in crystallization inhibition as both higher and middling molecular weight PLLAs inhibited the crystallization of PHB. Furthermore, IRRAS showed that, for the 30 nm thick films, the addition of a small amount of PLLA (M_w ≥ 13 100 g mol⁻¹) only altered the crystalline structure of PHB in the highly ordered state. In contrast, such PLLAs greatly affected the PHB crystals in both intermediate and highly ordered states in the 13 nm thick films. Unlike the behavior of PLLA in bulk PHB, the lower molecular weight PLLAs (M_w ≤ 3600 g mol⁻¹) showed limited effects on the crystallinity and crystalline structure of PHB in both thicknesses of thin films. Several factors, such as phase separation caused by free surface and interface effects, entanglement of PLLA chains, and molecular size of PLLA, are very likely to be responsible for the particular crystallization behavior of PHB in the studied blends.