Synthesis, Characterization And Crystallization Of PEG/PCL Block Copolymers

The aliphatic polyesters and their block copolymers are established biocompatible, biodegradable materials used for a variety of applications in the pharmaceutical and biomedical field.Especialy,Poly(ε-caprolactone)(PCL) presents great interest for carrier materials and body short term plants of medicine control system,however,the potential applications of PCL are considerabely restrained by the slow biodegradation rate and the high hydrophobicity.PCL backbone attached by hydrophilic poly(ethylene glycol)(PEG) block could improve hydrophilic property and the slow biodegradation rate.Firstly,three types of biodegradable-biocompatible amphiphilic block copolymers have been synthesized.PCL-PEG-PCL triblock copolymers(or PEG-PCL copolymers) were prepared in good yields by ring opening polymerization ofε-caprolactone initiated by poly(ethylene glycol)(PEG)(or monomethoxy- terminated poly(ethylene glycol)(mPEG)) catalyzed with stannous octoate(Sn(Oct)₂).Then,coupling of PEG-PCL copolymers with hexamethylene diisocyanate(HDI) afforded PEG-PCL-PEG(formally PEG-PCL-PCL-PEG) triblock copolymers.Secondly,Characterization of copolymers was determined by ¹HNMR,IR and GPC analysis.The results showed that three types of biodegradable-biocompatible amphiphilic block copolymers were prepared successfully.Finally,their thermal behavior and crystal formation were studied by DSC,XRD.Especialy, one sample(wt%_PEG=0.13) of was selected PCL-PEG-PCL for Nonisothermal crystallization and subsequent melting behavior.Polymer crystallinity has a great influence on the phys-chemical properties and the biodegradability of aliphatic polyesters.The results illustrated that the crystallization of the copolymers depended on the relative length of each block in copolymers,and topological variation.When the length of PCL blocks was relatively small,the PCL crystals disappear due to confinement of the PEG block.When the length of PCL blocks was relatively large,the PCL crystals appear and restrict the PEG's crystallization. Even,PEG was uncr- -ystallizable,due to the hindrance of the lateral PCL blocks,the crystallization among three type copolymers.PEG's crystallization restricted by PCL, PEG-PCL-PEG＞PEG-PCL＞PCL-PEG-'PCL;PCL's crystallization restricted by PEG, PCL-PEG-PCL＞PEG-PCL＞PEG-PCL-PEG..The nonisothermal crystallization data at various cooling rates are analyzed with the Ozawa,modified Avrami,and Mo models.The experimental data showed no agreement with Ozawa's theoretical predictions in the whole crystallization process,while the modified Avrami and Mo models were found to describe the experimental data for the nonisothermal crystallization processes fairly well.The amorphous PEG block acts as nucleating agents for the PCL block crystallization but increases the activation energy for nonisothermal crystallization,due to its covalent bond with.the PCL block.