Properties of Hyperbranched   Poly(e-caprolactone) Copolyesters

Properties of Hyperbranched Poly(e-caprolactone) Copolyesters

Abstract:
Hyperbranched poly (e-caprolactone) copolyesters (hb-PCL) have been prepared by combination of ring opening polymerization (ROP) of e-caprolactone (e-CL) as cyclic AB monomer and concurrent polycondensation of 2,2´-bis(hydroxymethyl)butyric acid (BHB) as branching AB2 comonomer unit, using a supported lipase (Candida Antarctica Lipase B, CALB; Novozym 435) as catalyst. By systematic variation of the BHB fraction, hyperbranched copolyester samples with systematically varied degrees of branching (DB) have been obtained. 1H-NMR and 13C-NMR spectra provided detailed information on the DBs. Molecular weights for the hb-PCLs were determined by means of SEC and VPO and exhibited a decreasing tendency with increasing fraction of branching comonomer. Thermal properties of the hb-PCL samples were investigated with respect to the DB. As expected, Tm and DHm were found to decrease with increasing content of BHB (DSC). The crystalline morphology of the samples was investigated by WAXS, revealing the DB-dependent existence of two crystalline polymorphs with different density and chain packing. Light microscopy studies demonstrated both a different crystal habitus for the hyperbranched copolymers, as well as a decrease of the crystalline fraction for the hb-PCL samples in comparison to the linear analogue. With increasing DB, the crystallites became small and irregularly shaped, indicating phase segregation in crystallizable and non-crystallizable material during crystallization. Solution viscosimetry showed low specific and intrinsic viscosity for the branched materials and demonstrated that the hyperbranched samples assume an increasingly compact solution structure with increasing extent of branching. For both CHCl3 and THF solutions, an increase of the temperature from 20 to 50oC results in a decrease of for all analyzed hb-PCLs. Keywords: copolyesters, hyperbranched polymers, degree of branching, polymer morphology

Issue: 2006 Volume 43, Issue 3

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