Proving Partial Miscibility in Poly(L-lactic acid)/Ethylene-Vinyl Acetate Copolymer Blends Using the Spherulite Observation Method

Poly(L-lactic acid) (PLLA) was blended with an ethylene-vinyl acetate (EVA) copolymer, which is generally recognized as a phase-separated system. The interactions between these polymer species were examined via spherulite observation. The PLLA/EVA blend was concluded to be a partially miscible system. The onset temperature for the crystallization of PLLA, as the crystalline polymer, systematically changed when PLLA was blended with EVA at various ratios. The glass transition behavior of EVA was almost absent in the thermogram when the PLLA:EVA blend ratio was greater than 2:1. The spherulite size distribution of PLLA became finer as the PLLA:EVA ratio was changed from 3:1 to 2:1 to 1:1, and observing spherulites was difficult when the blend ratio was 1:2. Because the nucleation position was different each time during the repeated melting/crystallization of spherulites, this system exhibited homogeneous nucleation. In addition, in a plot of the spherulite size versus the crystallization time, the inclination angle changed between the PLLA/EVA = 3:1 and 2:1 blends, and the critical ratio at which the crystallization behavior changed was estimated.