In highly selective solvents, block copolymers (BCPs) form association colloids, while in solvents with poor selectivity, they exhibit a temperature-controlled (de)mixing behavior. Herein, it is shown that a temperature-responsive self-assembly behavior emerges in solvent mixtures of intermediate selectivity. A biocompatible poly-ethylene(oxide)-block-poly-ϵ-caprolactone (PEO-PCL) BCP is used as a model system. The polymer is dissolved in solvent mixtures containing water (a strongly selective solvent for PEO) and ethanol (a poorly selective solvent for PEO) to tune the solvency conditions. Using synchrotron X-ray scattering, cryogenic transmission electron microscopy, and scanning probe microscopy, it is shown that a rich temperature-responsive behavior can be achieved in certain solvent mixtures. Crystallization of the PCL block enriches the phase behavior of the BCP by promoting sphere-to-cylinder morphology transitions at low temperatures. Increasing the water fraction in the solvent causes a suppression of the sphere-to-cylinder morphology transition. These results open up the possibility to induce temperature-responsive properties on demand in a wide range of BCP systems.
Bibliographical noteFunding Information:
The authors thank the European Synchrotron Radiation Facility (Grenoble, France) for the provision of synchrotron radiation facilities and the ID02 beamline staff for the excellent support. Furthermore, the authors thank Lisa Timmers for help with the SAXS measurements. Cryo-TEM images were obtained using the instrumentation at the IMRI (ps.uci.edu/imri/) facilities at UC Irvine. M.V. acknowledges the Netherlands Organisation for Scientfic Research (NWO) for a Veni grant (no. 722.017.005). The authors thank M. Chi for kindly performing the DSC characterization of PEO and PCL homopolymers.
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