Influence of biaxial stretching mode on the crystalline texture in polylactic acid films

Structural evolution during simultaneous (SB) and sequential rubbery state biaxial stretching (SEQ) of polylactic acid (PLA) films from cast amorphous precursors was investigated. Simultaneous biaxial stretching always leads to films with in-plane isotropy and poor crystalline order. In the first stage of sequential biaxial stretching, oriented crystallization gradually develops while transverse isotropy is maintained. Application of transverse stretching to these films possessing semicrystalline structure gradually destroys the crystalline structure oriented in MD during this realignment while establishing a second population of oriented but poorly ordered crystallites in TD. This destruction is caused primarily by splaying action under transverse stretching as evidenced by the decrease of crystallite sizes in MD.