Huge optical rotatory power of uniaxially oriented film of poly-L-lactic acid

Research on synthesized polymers which are easily degraded under natural environments, namely biodegradable polymer materials, is being actively performed from the viewpoint of environmental preservation [1]. Aliphatic polyester is an example of such polymers; in particular, expectations for polylactic acid (PLA), a polyester with asymmetric carbon atoms in its primary chains, are high, since it has high transparency, its degraded product is safe, and it is a recyclable resource [1, 2]. Now, PLA is attracting attention as a medical material which will be used, for example, for forming microcapsules for the controlled release of drugs [1–3]. On the other hand, crystallographic research on polyL-lactic acid (PLLA), which is one of the optical isomers of PLA, has been conducted for many years [4–6]. In addition, Kobayashi et al. [7] and Masuko et al. [8] have performed advanced research on the detailed structures of a PLLA crystal. They clarified that a PLLA crystal consists of two 10/3 helical molecular chains packed in a pseudo-orthorhombic system. In contrast, polymers in which molecules containing asymmetric carbon atoms have a helical orientation have been expected, for a long time, to exhibit optical activity with significantly higher optical rotary power in their solid state, compared with inorganic low-molecular-weight crystals [9, 10]. On the other hand, PLLA films can be uniaxially drawn even though PLLA is rarely seen in helical polymer films [9]. Therefore, the systemic symmetry of uniaxially drawn PLLA films becomes D∞ [9]. Namely, in the solid state, it is expected that helical polymer films such as PLLA will exhibit large optical rotatory power. However, reports on the optical rotatory power of polymer films have been extremely few [9, 11, 12]. Recently, however, Kobayashi et al. [7] attempted to evaluate the huge intrinsic rotatory power in the parallel direction to the fiber axis of a uniaxially drawn PLLA film, which had been, so far, impossible to measure. In order to solve this problem, they measured the small amount of rotatory power in the perpendicular direction to the fiber axis using an expensive high-accuracy universal polarimeter developed by them, and calculated the intrinsic rotatory power in the parallel direction to the fiber axis from the very slight rotatory power observed in the perpendicular direction [7]. In practice, their experiments were executed according to the following procedure; a uniaxially drawn PLLA film is rotated around the fiber axis while inclining the fiber axis slightly with respect to the incident laser beam [7]. As a result, they reported that the optical rotatory power of the PLLA film was more than 1000 times that of α-quartz crystals [7]. However, the measurement method used requires accurate measurements of the distribution of crystal orientation and complex calculation to obtain the gyration tensor; in addition, with this method, the optical rotatory power of the film in the fiber axis direction is not observed directly [7, 9]. Therefore, unfortunately, their results were accepted with reservation by many researchers [9]. Therefore, in order to practically use the helical polymers as new optical elements which utilize their huge rotatory power, more direct evaluation of their large rotatory power is necessary. In this study, using a uniaxially drawn PLLA sample, we attempted to perform direct measurement of a large amount of rotatory power by introducing laser light in the fiber axis direction. To prepare a sample for measurement, we first uniaxially drew the cylindrical PLLA rod (diameter: 10 mm), cut it perpendicular to the drawing-axis direction using a microtome, polished it using sandpaper until it exhibited a certain thickness, under conditions of dryness, then polished it further using a buff. The thickness d(μm) of the PLLA films was measured using a touch-type thickness meter (Ono Sokki Co. Ltd., ST-022). The measurement error of film thickness was ±0.1μm. We then measured the orientation of the fiber axis in a uniaxial drawn PLLA film from the X-ray image. On the basis of the X-ray image, the crystallographic c axis (fiber axis) was oriented uniaxially in the parallel direction to the drawn axis and the crystallographic a and b axes were randomly distributed in the plane perpendicular to the drawn axis [7, 8]. Accordingly, using the PLLA films, we measured the rotatory power in the fiber axis direction directly using the developed system [13]. It is possible to measure the optical activity, namely, the optical rotation and the circular dichroism [10, 12]. The developed system