Left- or Right-Handed Lamellar Twists in Poly[(R)-3-hydroxyvalerate] Banded Spherulite: Dependence on Growth Axis

Novel morphology of banded spherulite was observed in poly[(R)-3-hydroxyvalerate) (PHV) during its isothermal crystallization at temperatures between 50 and 80 degrees C. A novel optical technique, Polscope was applied to obtain both the birefringence retardation and azimuth angle of the slow optical axis at each pixel in the micrograph of the polycrystalline specimen. It is revealed that two different morphological regions, normal and eye-like regions, are derived from different growth axes of the lamellae along the radial direction. In the normal region, lamellae grow and twist along b axis, leading to the alternative strong and weak positive birefringence. In the eye-like region, they grow and twist along a axis, resulting in the alternative positive and negative birefringence. It is of particular interest to note that PHV lamellae in the two regions exhibit the opposite twist senses. To the best of our knowledge, it is the first report that an optically pure chiral polymer contains two lamellar twist senses in one spherulite. We attribute the driving force of lamellar twist in PHV spherulite to the unbalanced anisotropic surface stresses. A mechanical model is proposed to interpret the opposite twisting senses along the two orthogonal growth axes. The results demonstrate that the growth axis is one of the factors determining the twist handedness of the lamellae of chiral polymers.