Impact of Molecular Architectures on the Thermal and Mechanical Properties of Multi-Phase Polymer Networks

Abstract

Multiphase copolymer networks (CLEG) composed of crystallizable poly(ε-caprolactone) (PCL) and crystallizable poly(ethylene glycol) (PEG) segments can exhibit a pronounced triple-shape effect in the dry state or a dual-shape effect in the water swollen, hydrogel state. We hypothesize that by adjusting the network architecture of CLEG copolymers networks the swelling behavior, the thermal and mechanical properties, as well as the crystal structure of PEG and PCL domains can be tailored. Here, we studied CLEG materials with a fixed PCL/PEG weight ratio but different polymer network architectures, whereby the PEG segments were incorporated as grafted side chains or as network chains connecting two netpoints. The prepared CLEGs were analyzed regarding their gel contents, swelling behavior, thermal and mechanical properties and finally the crystallinity of the polymer networks were determined by differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS) measurements. High gel content values of G  ≥ 98% were achieved for all copolymer network samples, indicating an almost complete conversion of the reaction precursors. The degree of swelling (Q) determined with water increased from Q ≈ 150% to 223% with increasing poly(ethylene glycol) monomethyl monomethacrylate (PEGMMA) content to create more PEG grafted side chains. Only when PEGMMA moieties were present in the CLEG networks two separated melting transitions related to the PCL and PEG segments were obtained, whereby Tm,PEG was found to increase with increasing the PEGMMA weight fraction. Based on this observation it can be assumed that the incorporated poly(ε-caprolactone) diisocyanoethyl methacrylate PEGDIMA segments forming the main network were not able to form crystallites in CLEG networks. An almost two-fold decrease in the Younǵs modulus was observed with increasing amounts of grafted PEG side chains, while the elongation at break increased significantly. Based on Mooney-Rivlin Equations, the crosslinking density increased from 0.10% to 0.21% when PEG segments switched from grafted chains to the network. WAXS and DSC investigations revealed an increase in the degree of crystallinity (DOC) of PEG segments with increasing PEGMMA moieties, while the DOC related to the crystalline PCL domains remained almost constant. The obtained results clearly demonstrated the importance of the molecular architecture in designing polymer networks.