Polymer networks are critical components of many modern materials, from epoxy coatings to the rubbery soles of your shoes, and understanding how the structure of these materials affects the way they respond to force is critical for designing materials with the right properties for each application. In our group, we use force-responsive molecules called mechanophores to probe the molecular-scale force responses in polymer networks with different structures.  We then correlate the molecular-scale responses to the macroscopic mechanical properties to build a comprehensive picture of how network structure affects force transmission throughout complex polymeric materials.

We currently study molecular-scale force responses in two types of polymer networks: self-assembled block copolymers, and covalently-crosslinked elastomers.

Block copolymers are made by self-assembly of polymers containing two or more immiscible chains that are covalently linked together.  They are a fascinating platform for probing the relationship between macroscopic and microscopic force responses of polymer networks, because both the macroscopic mechanical properties and the nano-scale structure and connectivity of the material can be changed by changing the relative fractions of the different components of the polymer.  To investigate the relationship between polymer structure and composition and molecular-scale force distributions in these materials, we synthesized a series of triblock copolymers with different compositions containing a force-responsive mechanophore in the middle of the midblock chains.  We found that increasing the fraction of the glass end-blocks decreased the strain necessary to activate the mechanophores, but increased the stress.

To understand the molecular origins of these trends, we worked with Antonia Statt (UIUC) to carry out molecular dynamics simulations of the activation process in block copolymers.  We found that the activation occurs primarily in the tie chains connecting different glassy domains, and that the activation is not spatially uniform, but instead is concentrated in the regions of the self-assembled morphology that undergo the most extreme spatial deformations as the sample is strained.

In addition to our work on block copolymers, we are interested in understanding how forces are distributed in covalently-crosslinked elastomers, and particularly, how the distributions of crosslinks in these materials affect their responses to stress.  We are currently focusing on understanding the difference between randomly-crosslinked networks, in which the spatial distribution of crosslinks and the molecular weights of the chains between crosslinkers are both heterogeneous, and regularly-crosslinked networks, in which the synthesis is controlled such that both the molecular weight between crosslinks and their spatial distributions are uniform.

Together, these efforts are helping us develop a comprehensive picture of how macroscopic forces in polymeric materials are distributed at the molecular scale, and how these force distributions change with the network structure.  While our primary goal is to improve our fundamental understanding of these materials, we anticipate that our results will also help inform design of high-efficiency platforms for mechanochemical activation and networks with precisely tailored mechanical properties for demanding applications.

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Our work on mechanochemistry and polymer networks is currently supported by the National Science Foundation under award no. 1846665 (DMR-POL).