Analysis of molecular orientation in polylactic acid injection moldings using microscopic polarized laser Raman spectroscopy

Abstract

We analyzed the molecular orientation in polylactic acid injection moldings using microscopic polarized laser Raman spectroscopy. Two intensity ratios of Raman peaks at 877 and 1,458 cm ⁻¹ were examined, I ₈₇₇ / I ₁₄₅₈ (0°, P), measured with the flow direction parallel to the laser polarization plane under a parallel Nicols configuration, and I ₈₇₇ / I ₁₄₅₈ (45°, C), measured at 45° under a crossed Nicols configuration. Raman mapping in the thickness direction of the injection moldings showed that I ₈₇₇ / I ₁₄₅₈ (0°, P) varied little with thickness and was insensitive to the injection rate, in contrast to the optical strain measured by polarized optical microscopy (POM), which varied both with thickness and injection rate. The distribution of I ₈₇₇ / I ₁₄₅₈ (45°, C) in the thickness direction showed qualitative agreement with the POM optical strain distribution and similarly reflected the dependence on the injection rate. Furthermore, I ₈₇₇ / I ₁₄₅₈ (45°, C) showed that residual molecular orientation in the core layer of the injection moldings can relax even below the glass transition temperature. In situ Raman observations during tensile tests indicated that I ₈₇₇ / I ₁₄₅₈ (45°, C) can better detect local molecular orientation than I ₈₇₇ / I ₁₄₅₈ (0°, P).