In this study we prepared a poly(lactic acid)/ethyl cellulose (PLA/EC) bioplastic blend film by melt pressing after solvent casting and investigated the optical, thermal, and mechanical properties in relation to the structural changes revealed by various microscopies and scattering measurements. Despite the immiscibility of PLA and EC, the blend film exhibited good optical transparency due to the close match in their refractive indices. A second run of differential scanning calorimetry during heating revealed a decrease in the glass transition temperature (TgPLA) and melting point (TmPLA) of PLA with EC although EC had the higher values. Thermomechanical analysis suggested that these decreases were due to the negative pressure generated due to the suppression of PLA shrinkage below TgEC and above TgPLA during cooling. Furthermore, ductile drawing of the blend film (70/30) at an optimal temperature of 70 °C above TgPLA resulted in a uniform whitening near 100% strain. Delamination at the interface between the PLA matrix and EC domain led to the formation of micrometer-sized oriented voids, which enhanced the refractive index difference in the phase-separated structure, thereby scattering light and achieving the uniform and oriented white blend film.
