Composite films with excellent mechanical, antioxidant and UV-shielding properties prepared from oligomeric proanthocyanidin nanospheres and poly(vinyl alcohol)

Almost 90,000 tons of larch bark, which contains 10-16 % anthocyanins by weight, is produced as waste in China each year. Here, proanthocyanidins from the bark of Larix gmelinii were used to prepare oligomeric proanthocyanidin nanospheres (OPCM). The OPCMs were then blended with poly(vinyl alcohol) (PVA) to produce OPCM/PVA nanocomposite films. Incorporation of OPCM not only improves the performance of the film but also expands the applications of proanthocyanidins and provides a way to reduce the environmental impact of waste bark. Scanning electron microscopy images showed that the OPCM were uniformly dispersed in the PVA, without macroscopic phase separation. OPCM/PVA nanocomposite film had improved mechanical and thermal properties compared with pure PVA film because of intermolecular hydrogen bonding between the OPCM and PVA. The tensile strength of nanocomposite film containing 3 wt% OPCMs was 95 % higher than that of PVA film and its toughness and elongation at break were also enhanced to some extent. Compared with PVA film, the glass transition temperature (Tg) and peak decomposition temperature (Tmax) of nanocomposite film containing 10 wt% OPCM were increased by 10.4 degrees C and 20.2 degrees C, respectively. The addition of OPCMs also overcomes a major disadvantage of PVA film, which is that the melting temperature is close to Tmax, and thus facilitates melt processing of PVA composite materials. The composite film also had increased resistance to oxidation and better surface wettability than PVA film. Although the composite film had enhanced UV-blocking properties compared with PVA film, even films with the highest proportion of OPCM still showed good transmittance of visible light. Because both OPCs and PVA are biodegradable, the resulting nanocomposites are also biodegradable and can be regarded as suitable candidates for agricultural film and packaging materials.