Investigations On The Morphology And Melt Crystallization Of Poly(ε-caprolactone)-Poly(Ethylene Glycol) Diblock Copolymers

During the past three decades,biodegradable polymers have attracted much interest in both basic research and chemical industry.Aliphatic polyesters such as polylactide(PLA),polyglycolide(PGA) and poly(e-caprolactone)(PCL) present excellent degradation properties,with degradation by-products harmless to the environment and animal body.Up to now,a great deal of work has been reported on biomedical applications of PCL and its copolymers,in particular in the fields of controlled drug release and tissue engineering due to the outstanding biocompatibility,biodegradability and drug permeability All these applications strongly depend on the specific physical properties. For example,high molecular weight(MW) and highly condensed structure are required for higher-modulus PCL used as tissue engineering scaffolds,while drug delivery systems generally require amorphous copolymers of relatively low MW. Hydrolytic degradation of PCL proceeds very slowly as PCL is a highly crystalline andhydrophobic polymer.The total degradation time is about 2-4 years,which strongly limits its application as drug carrier.Introduction of hydrophilic chains such as poly(ethylene glycol)(PEG) is a means to enhance the degradability and hydrophilicity of the materials.The crystallization behavior and morphology greatly influence the degradation performance of polymers.In literature,researchers have reported on the crystallization of PCL-PEG diblock and triblock copolymers.A specific concentric circle of crystals was observed.However,the crystallization of PCL-PEG copolymers with various PEG and CL/EO ratios has not been investigated in detail,so far.This work aims at investigating the morphology and crystallization of PCL-PEG diblock copolymers,including the thermodynamics,crystallization kinetics, condensed structure,morphology,and the influence of MW.Three series of PCL-PEG copolymers with various CL/EO ratios from 0.2 to 3.0 were synthesized by ring opening polymerization ofε-caprolactone in the presence of monomethoxy PEG with molar masses of 5000,2000 and 1000,using low toxic Zn(LA)₂ as catalyst.The copolymers were characterized by using analytical techniques including NMR,DSC,WAXD,and SEC.The thermodynamics of isothermal crystallization,crystal structure,and morphology were investigated in detail. The main contents of our work are shown as follows.1.PCL-PEG copolymers with various molar masses of PEG and CL/EO ratios were synthesized and characterized.The basic thermal behaviors and morphologies were determined.Avrami equation was used to analyze nucleation and growth type, deducing the kinetic parameters of crystallization.2.The influences of molar mass of PEG and CL/EO ratio on the thermodynamics and kinetics of PCL crystallization were investigated.Explanation on concentric circle of crystals was given.In particular,rod-like crystals were observed as one dimensional growth.The molar mass has great influence on the growth rate.