Optical photothermal infrared (O-PTIR) and Raman spectroscopy and imaging was used to explore the spatial distributions of molecular constituents of a laminate sample consisting of the bioplastics, polyhydroxyalkanoate (PHA) and polylactic acid (PLA), near the interfacial boundary. Highly spatially resolved simultaneous IR and Raman spectra were sequentially collected at 100 nm increments along a line traversing the interface. The set of spectra were subjected to 2D-COS analysis to extract the detailed nature of the spatial distribution of the laminate constituents. It was revealed that the laminate is not a simple binary system of two non-interacting polymers, but consists of different constituents with more complex spatial distributions. Some portion of PLA seems to penetrate into the PHA layer. The crystallinity of PHA near the interface is reduced compared to the rest of the PHA layer. The result suggests the existence of some partial molecular mixing even for these seemingly immiscible polymer pairs. The mixing probably occurs at the segmental level confined to only several hundred nanometers of space at the interface. Such partial mixing may explain the high compatibility between the two bioplastics.
Keywords: 2D-COS; Bioplastics; O-PTIR; Optical photothermal infrared spectroscopy; Raman; Spectral imaging; Two-dimensional correlation spectroscopy.