Cold Crystallization of Glassy Polylactide during Solvent Crazing.

Abstract

Uniaxial tension accompanied by the orientation and crystallization of polymer chains is one of the powerful methods for the improvement of mechanical properties. Crystallization of amorphous isotropic polylactide (PLA) at room temperature is studied for the first time during the drawing of films in the presence of liquid adsorption-active media (ethanol, water-ethanol mixtures, and n-heptane) by the solvent crazing mechanism. The crystalline structure arises only under simultaneous actions of a liquid medium and a tensile stress and does not depend on the nature of the environment. The degree of polymer crystallinity increases nearly linearly with the growth in the fraction of the fibrillar material and reaches a maximum value of 42-45%. It has been stated that polymer crystallization happens in crazes involving nanofibrils with a diameter of about 10-20 nm without affecting the bulk polymer parts. Wide-angle X-ray scattering has been used to confirm that the crazing-induced crystallization is accompanied by the formation of the α'-crystalline phase with crystallite sizes (X-ray coherent scattering region) of 3-5 nm, depending on the nature of the liquid medium. After stretching in liquid media to a high tensile strain, the strength of a PLA film has increased to 200 MPa.