Crystal transformation from the a- to the b-form upon tensile drawing of poly(l-lactic acid)

A film of poly(l-lactic acid) (PLLA) consisting of highly oriented *a crystals was uniaxially drawn by tensile force. The effects of the draw ratio (DR), draw temperature (Td), and draw stress on the crystal/crystal transformation from the *a- to the *b-form crystals were studied. At the initial stage of drawing, the highly oriented *a crystals of the starting film transformed into a broader orientation distribution, and significant crystal disorder was introduced. Upon further drawing, the *a crystals steadily transformed into *b crystals with increasing the DR. For the drawing at a constant Td, the crystal transformation proceeded more efficiently at a higher draw rate and, hence, at a higher draw stress. Furthermore, for the drawing at a constant draw rate, the transformation proceeded with DR most efficiently for the tensile draw at a Td around 140 DGC, although the draw stress increased with decreasing the Td. The present result combined with the previous finding in the drawing of PLLA by solid-state extrusion [Macromolecules 36 (2003) 3601] suggests that there is a Td of around 140 DGC at which the crystal transformation proceeds most efficiently with DR, suggesting that there are two factors that have opposite effects on the efficiency of the crystal transformation with increasing the Td. However, as a result of the combined effects of the Td and DR on the crystal transformation and the ductility increase with the Td, an oriented film consisting predominantly of *b crystals was obtained by tensile drawing at a Td in the range of 140-170 DGC to the highest DR achieved at each Td.