Evolution of phase behavior and orientation in uniaxially deformed polylactic acid films

The development of crystalline structure and orientation during uniaxial stretching of cast amorphous linear and branched lactic acid films were investigated in the rubbery temperature ranges that spans between glass transition temperature and cold crystallization temperature. This material exhibited almost ideal stress-strain behavior in the temperature range 65–80°C. Because of its strain crystallizability, films with uniform thickness can be obtained at high deformation levels as a result of self-leveling. Branching was found to retard this self-leveling through its slightly detrimental effect on the strain hardening. Upon stretching the material undergoes rapid orientation in the amorphous state and beyond a critical level very sharp and highly oriented β crystalline form chains with −3/1 helix. If the temperature is at or below Tg, with additional stretching, the films were found to revert to a highly oriented amorphous state through the destruction of the crystalline domains. At higher temperatures, further stretching results in continuation of improvement in crystalline order