Block copolymers are one of the most widely investigated categories of associative polymers, forming the basis of materials as diverse as rheology modifiers in consumer products, new hydrogels for tissue engineering or drug delivery, and pressure sensitive adhesives when they are swollen or gelled with a variety of oils and solvents.  Their rheological response is critical to engineering in all of these applications and represents complex effects of deformation on chain stretching and perturbation of the block copolymer nanostructure.  In our own lab, we have developed injectable triblock copolymer gel systems based on associative protein midblocks and thermoresponsive protein endblocks, allowing a responsive transition from a shear-thinning gel at low temperature to a toughened gel at elevated temperature.  Depending upon the identity of the endblocks, these gels may also exhibit tacky behaviour over specific temperature ranges.<br/> <br/>In this work, a combination of transient network theory, rheology, and simultaneous rheology and small-angle X-ray scattering (rheo-SAXS) experiments are used to understand the response of these associative polymers to deformation.  Experimentally, rheology and tack testing are used to identify specific thixotropic signatures of structure deformation and recovery during shear and recovery and to identify regions where the gels are adhesive.  Rheo-SAXS provides insight into the specific structural transitions that occur as a function of temperature, concentration, and molecular design.  A variant of transient network theory is also developed specifically to model block copolymers by allowing dynamic bonds to disengage in stages, mimicking stepwise pull-out of an endblock from a micellar structure.  We show how these systems are related to single-bond systems with different effective bond strengths.  In particular, stepwise bond breakage allows pull-out at lower chain extensions and allows dissipation of substantial bond energy with only moderate applied force on polymer chains, providing a mechanism for controlling dissipation in the materials.<br/>

Simulations and experiments to understand the rheological response of dual associative block copolymer gels

254th National Meeting (2017)

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