Structural and Rheological Properties of Temperature-Responsive Amphiphilic Triblock Copolymers in Aqueous Media

This Thermoresponsive amphiphilic biodegradable block copolymers of the type poly(-caprolactone-co-lactide)-poly(ethylene glycol)-poly(-caprolactone-co-lactide) (PCLA-PEGn-PCLA) have great potential for various biomedical applications. In the present study, we have surveyed the effects of PEG-spacer length (n=1000 and 1500), temperature, and polymer concentration on the self-assembling process to form supramolecular structures in aqueous solutions of the PCLA-PEGn-PCLA copolymer. This copolymer has a lower critical solution temperature, and the cloud point depends on both concentration and PEG-length. Thermoreversible hydrogels are formed in the semidilute regime; the gel windows in the phase diagrams can be tuned by concentration and length of the PEG-spacer. The rheological properties of both dilute and semidilute samples were characterized; especially the sol-to-gel transition was examined. Small-angle neutron scattering (SANS) experiments reveal fundamental structural differences between the two copolymers for both dilute and semidilute samples. The intensity profiles for the copolymer with the long PEG-spacer could be described by a spherical core-shell model over a broad temperature domain, whereas the copolymer with the short hydrophilic spacer forms rod-like species over an extended temperature range. This finding is supported by cryo-TEM images. At temperatures approaching macroscopic phase separation, both copolymers seem to assume extended rod-like structures

Keywords:

Copolymer , self-assembling , structure , temperature responsive