Cellulose- graft-poly[poly(ethylene glycol) methyl ether acrylate] (Cell- g-PPEGA) and poly[poly(ethylene glycol) methyl ether acrylate] (PPEGA) comb-like copolymers were synthesized by electron transfer atom transfer radical polymerization (ARGET ATRP). PPEGA/PLA and Cell- g-PPEGA/PLA pairs are both immiscible as indicated by slightly decreasing glass transition temperatures of PLA in the blends measured by differential scanning calorimetry (DSC) and phase separation morphologies observed by a phase contrast optical microscope (PCOM). Cell- g-PPEGA/PLA and PPEGA/PLA double-layer films and neat PLA film were prepared to measure the nucleation and spherulitic growth rates during isothermal crystallization at various temperatures above the melting points of Cell- g-PPEGA and PPEGA layers by using a polarized optical microscope (POM). In contrast to our previous results on the miscible polymer pairs, covering the immiscible molten PPEGA layer can greatly accelerate the spherulitic growth rates for PLA and the Cell- g-PPEGA layer shows a similar but less significant effect. Nevertheless, nucleation density is much higher for the Cell- g-PPEGA/PLA film than for the PPEGA/PLA and neat PLA films. A significant finding is that although the whole double-layer film systems represent phase separated ones, the densely grafted PEGA chains in the comb-like copolymers can simultaneously take action to amplify the segmental mobility of PLA chains through local contacts at interfacial layers between phase separated domains, which significantly enhances the formation of chain folding lamellae of the PLA, resulting in obvious enhancements of the crystallization kinetics.