Effect of miscibility on spherulitic growth rate for double-layer polymer films

The spherulitic growth rates for the lower polylactide (PLA) layer in poly(epsilon-caprolactone)/polylactide (PCL/PLA), poly(ethylene oxide)/polylactide (PEO/PLA), and poly(ethylene glycol)/polylactide (PEG/PLA) double-layer films during isothermal crystallization at various temperatures above the melting points of PCL, PEO and PEG layers have been measured by using polarized optical microscopy (POM), with the particular results compared with those for neat PLA films. The PCL/PLA, PEO/PLA and PEG/PLA double-layer films were in situ prepared by covering PCL, PEO and PEG films, respectively on PLA films at 180 degrees C and holding for 5 min before quenching to isothermal crystallization temperatures for POM observations. It is interesting to find that the covering molten PEG layer can greatly accelerate the spherulitic growth rates for the lower PLA layer and a PEO layer shows a similar effect for PEO/PLA double-layer films. However, the effect of the PEO layer is weaker than that of PEG layer for acceleration of PLA spherulitic growth rates in the double-layer films. Covering a molten PCL layer on a PLA layer can only slightly increase the spherulitic growth rate of PLA. Different spherulitic morphologies in the neat PLA film and PCL/PLA, PEO/PLA and PEG/PLA double-layer films can be observed. The miscibility of these polymer pairs was investigated by using differential scanning calorimetry (DSC) and phase contrast optical microscopy (PCOM), which is proposed to play a key role for the observed accelerating effects.