Large Optical Rotatory Power and Light Modulation in Polylactic Acid Film

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Polylactic acid, molecules of which contain asymmetric carbon atoms and have a helical orientation, have long been expected to exhibit optical activity (OA) with significantly high optical rotatory power in the solid state, compared with inorganic low-molecular-weight crystals. We examined the OA and light modulation (LM) properties of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) films of which the higher order structure is achieved by a forging process. We confirmed that the PLLA and PDLA films possess levorotatory power and dextrorotatory power, respectively. The optical rotatory power ρ of the PLLA film is 7200°/mm which is approximately 300 times greater than that of α-quartz. This ρ value is an outstanding record. It may be impossible for us to identify every factor affecting the value of ρ because there are so many. Thus, we selected two conceivably important factors, a molecular weight M_w and isomer (D-type) content (ID). ρ decreases with increasing ID. ρ also decreases with decreasing M_w. Thus, we found that ID and M_w are important factors affecting the value of ρ. The higher order structure of PLLA films is greatly influenced by ID and M_w. Namely, the unprecedented large ρ of the PLLA sample is due to its higher order structure. Based on the above results, we conclude that this forging technology is useful for producing films with a controlled higher order structure. We then confirmed LM of the PLLA film up to 10 MHz. Since the PLLA film provides a fast response, we believe that a LM speed in the GHz range is possible using a more modified PLLA sample.