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Mechanical Properties and Microstructure of Triblock Copolymer Network: DPD Simulation Study

Yelena Sliozberg1, Jan Andzelm1, Mark VanLandingham1, John K. Brennan1, Victor Pryamitsyn2, and Venkat Ganesan2. (1) U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005, (2) Chemical Engineering, University of Texas at Austin, 1 University Station, Code C0400, Austin, TX 78712

Amphiphilic triblock ABA copolymers undergo microphase separation in a middleblock-selective solvent that results in a formation of three-dimensional network. The polymer network consists of the junctions of the solvent-incompatible end blocks connected by the middle blocks. However, both end blocks of one chain could be incorporated in the same micelle core that leads to formation of the loop conformation of the middle block, excluding it from participation in the network. The loop/bridge ratio depends on several factors such as copolymer concentration, length of the blocks and addition of corresponding diblock copolymer. The main focus of this study was to explore different methods to control the loop/bridge ratio of a gel formed by association of ABA copolymer in B selective solvent to achieve the desirable mechanical properties of the material, described by a computational rheological experiment.

In this work, we employed both equilibrium and non-equilibrium mesoscale computer simulations of block copolymers in the explicit solvent. The dissipative dynamics method (DPD) was used with the non-equilibrium time dependent Lees-Edwards boundary conditions. We obtained dynamic shear storage (G′) and loss (G″) moduli for various ratios of the blocks length and polymer concentration. The behavior of a mix of triblock and diblock copolymers with different concentration of diblock was also investigated. We have observed formation of the polymer network with the diverse loop/bridge ratio. We have shown that reduction of relative length of the middle block leads to close association of the chains and causes the significant decrease in a number of the middle blocks participating in the polymer network because of the loop formation. This phenomenon manifests itself in reduction G′(ω) for low frequency region with comparison with the network with low loop/bridge ratio and ultimately leads to the principally different viscoelastic behavior of the system, where G′(ω) is proportional to ω showing an evident terminal zone. G′(ω) for the network with low loop/bridge ratio approaches the constant value. We obtained good agreement with other theoretical and experimental results. We extended this approach to predict viscoelastic properties of the block copolymer as function of polymer architecture and solvent selectivity.