Nonlinear mechanooptical behavior of uniaxially stretched poly (lactic acid): Dynamic phase behavior

The mechanooptical behavior of melt-cast amorphous poly(l-lactic acid) (PLA) films in the rubbery state was investigated using an apparatus that allows for direct measurement of true stress, true strain, and birefringence in real time under well-controlled temperature over a wide range of stretching rates. Three distinct regimes of stress−optical behavior are observed during uniaxial deformation of PLA films in the rubbery state. Regime I deformation is characterized by adherence to the stress optical rule; within this regime, birefringence remains linearly proportional to stress with a stress optical constant of 3.1 GPa-1. This is followed by either a positive deviation from linearity into regime II at higher temperature and/or lower rates or a negative deviation into regime IIIa at lower temperatures and/or higher rates. Films exhibiting regime II behavior eventually deviate into regime IIIc behavior at higher levels of deformation. The appearance of regime II is associated with strain-induced crystallization. In the absence of regime II behavior, the polymer remains uncrystallized, yet becomes highly oriented, exhibiting “nematic-like” order. This stable nematic-like order prevails at all large deformation levels with no sign of crystallization. When nematic-like order is present, the strain optical behavior was found to exhibit linear or near-linear behavior in a wide deformation range. Conversely, this behavior is nonlinear with the development of strain-induced crystallization. On the basis of the structural and true mechanical measurements, a dynamic phase diagram was constructed for defining the structure development during the rubbery state uniaxial deformation of PLA