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 11 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 11 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.