Effect of polyvinyl acetals on non-isothermal crystallization behaviour and mechanical properties of poly(ε-caprolactone)

Polyvinyl acetaldehyde (PVAC) and polyvinyl butyral (PVB), as nucleating agents, were melt mixed with poly(ε-caprolactone) (PCL), and the non-isothermal crystallization behavior and crystalline morphology of PCL was characterized using differential scanning calorimetry (DSC) and polarizing microscopy (POM). It was found that melt shearing made the crystallinity of control PCL increase from 37.94% (as-received) to 41.15% and the crystallization temperature raised by 7.7 °C. As the mass fraction of polyvinyl acetals increased from 0.1 wt% to 10 wt%, both crystallization temperature and crystallinity of PCL were decreased; the spherulite size of PCL was increased from a few microns to 200 μm and a fast cooling procedure facilitated the homogeneity of spherulite size. When the content of nucleating agent was less than 1 wt%, no obvious microphase separation was observed under scanning electron microscopy; the tensile strength, yield strength and elongation at break of PCL increased slightly; and the Young's modulus of PCL was improved by 67% at a 0.1 wt% mass fraction of PVB which is beneficial to PCL in high load bearing applications. When the contents of nucleating agents were higher than 1 wt%, the microphase separation occurred obviously and a large amount of fine spherulites appeared with the deterioration of mechanical properties. The modified Avrami and Tobin models could well describe the non-isothermal crystallization kinetics.