Formation and growth of cavities in tensile deformation of Poly(ε-caprolactone) and its miscible blends

Microstructural development of poly(ε-caprolactone) (PCL) and its miscible blends with poly(vinyl methyl ether) (PVME) and poly(styrene-co-acrylonitrile) (SAN) were investigated as a function of strain in the process of uniaxial stretching using in situ synchrotron ultrasmall- and small-angle X-ray scattering (USAXS and SAXS) techniques. The addition of amorphous polymer SAN gives rise to a distinct reduction in the entanglement density of the amorphous phase of PCL, whereas an introduction of PVME does not affect the entanglement density in the blend. At small strains, the initial cavities in pure PCL are oriented perpendicular to the drawing direction, while the cavities at the onset of their formation are oriented along the extensional direction for the PCL/PVME blend. Nevertheless, no cavity scattering is observed in PCL/SAN blend during the process of tensile deformation. These results confirm that the cavitation is directly linked to the lamellar crystallites with their orientation parallel to the drawing direction at small extension ratios. For the two cavitated samples, the volume fraction of cavities was assessed from the integrated scattering intensity based on a three phase model. A critical stress value, above which the lamellar mosaic blocks begin to be pulled apart and the cavitation just sets in, was found to be nearly constant at around 15 MPa. Furthermore, the dimensions of cavities were evaluated at different deformations via direct cylinder model fitting to the USAXS patterns. The height of cavities is much larger than the long spacing of lamellar stacks even at the early stages of cavity formation, which indicates the cavities emanating from the lamellae into several amorphous phases at small elongations. Analysis of the SAXS patterns shows an independence of lamellar long spacing on the sample composition at large strains, which strongly supports the deformation scheme of stress-induced fragmentation and recrystallization.