Phase Transition Mechanism of Poly(l-lactic acid) among the α, δ, and β Forms on the Basis of the Reinvestigated Crystal Structure of the β Form

Abstract

The crystal structure of poly(l-lactic acid) β form has been reinvestigated on the basis of the 2-dimensional X-ray diffraction diagram measured for the sample obtained by stretching the highly crystalline α form at a high temperature. The six helical chains of (3/1) conformation are packed in a rectangular unit cell of the space group P1 with the complicated but systematic packing mode of the upward and downward chains. This structural model is different from the previously reported trigonal model [Puiggali Polymer 2000, 41, 8921]. The structural phase transition mechanism from the α form to the β form via the δ form has been proposed by assuming the cooperative displacements of the upward and downward helical chains as well as the conformational change. To support this mechanism, the two types of experiments were performed: (i) The highly oriented regular α form was stretched at about 165 °C to the various drawing ratios and cooled to the room temperature with the sample length fixed constantly. The X-ray diffraction data of these samples revealed the transition from the α to the mechanically deformd α (αd), to the δ form, and then to the β form depending on the drawing ratio. (ii) The α sample was suspended vertically with a constant load in the heating process, and the X-ray diffraction pattern was measured as a function of time (temperature). The original α form was found to melt at about 200 °C via the transition to the αd form and then to the δ form, followed by the recrystallization into the highly oriented β form. From these two experiments, the tension-induced structural transition from the α to the β form was found to occur via the disordering of the α form to the αd and to δ form under tension. The structural change process was derived on the basis of all of the knowledge collected from the X-ray structure analysis and the ex-situ and in-situ X-ray diffraction measurements: the molecular chains experience the cooperative translational slippage along the diagonal directions in the ab-plane to give the complicatedly mixed packing of the upward and downward chains to give the characteristic structure of the β form. At the same time, the chain conformational changes from the regular (10/3) to the disordered (10/3) and then to the (3/1) form during the disordering in the chain packing mode.