Melt crystallization and morphology of poly(ε‐caprolactone)‐poly(ethylene glycol) diblock copolymers with different compositions and molecular weights

Ring opening polymerization of ε‐caprolactone was realized in the presence of monomethoxy poly(ethylene glycol) with M_n = 1000 and 2000, using Zn(La)₂ as catalyst. The resulting PCL‐PEG diblock copolymers with CL/EO repeat unit molar ratios from 0.2 to 3.0 were characterized by using DSC, WAXD, SEC, and ¹H NMR. The crystal phase of PCL blocks exist in all polymers, and the crystallization ability of PCL blocks increases with CL/EO ratio. PEG blocks are able to crystallize for copolymers with CL/EO below 1.0 only. Melt crystallization results were analyzed with Avrami equation. The Averami exponent n is around 3.0 in most cases, in agreement with heterogeneous nucleation with three dimensional growth. The morphology of the crystals was observed by using POM. Rod‐like crystals were found to grow in 1, 3 or 2, 4 quadrants for samples with low molecular weights. In the case of a copolymer with M_n,PEG = 2000 and M_n,PCL = 800, PEG blocks could crystallize and grow on PCL crystals after PCL finished to form rod‐like crystals, leading to formation of poorly or well structured spherulites. The spherulite growth rate (G) was determined at different crystallization temperatures (T_c) ranging from 9 to 49 °C. All the copolymers present a steady G decrease with increasing crystallization temperature due to lower undercooling. On the other hand, increase of CL/EO ratio leads to increase of G in the same T_c range. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 286–293, 2010