Mesoscale Modeling of Controlled Degradation and Erosion of Polymer Networks
Professor Olga Kuksenok, Clemson University
 April 9, 2021 at 3pm
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 Controlled degradation of hydrogels plays a vital role in a variety of applications ranging from regulating growth of complex tissues and neural networks to controlled drugs and biomolecules delivery. Further, of a particular interest is photo-controlled degradation of polymer networks, which permits spatially-resolved dynamic control of physical and chemical properties of the materials. Notably, in a number of practical applications, either the characteristic features of degradable gels or the dimensions of the entire degradable gel particles range between nanometers to microns scales, the length scales referred to as mesoscopic. We develop a Dissipative Particle Dynamics (DPD) approach to capture degradation of polymer networks at the mesoscale. DPD is a mesoscale approach utilizing soft repulsive interactions between beads representing collections of atoms, thereby allowing low computational cost of simulations. To overcome unphysical topological crossings of bonded polymer chains (a known limitation of DPD), we adapted a modified Segmental Repulsive Potential (mSRP) formulation to model gels with degradable crosslinks. We track the progress of the degradation process via measuring the fraction of degradable bonds intact. Further, we track mass loss from the hydrogel films, along with the number, sizes, and spatial distributions of clusters (bonded beads) formed during the degradation process. The figure below shows representative snapshots of degradation from the three-dimensional film (side view); for clarity, solvent beads are not shown. The cluster size distribution enables us to calculate the evolution of the weight average degree of polymerization, DPw, during degradation. The evolution of DPw depends on the crosslink density, polymer volume fraction, and solvent quality. As degradation proceeds, the hydrogel film undergoes reverse gelation and the percolating network disappears. We quantify the point at which this reverse gelation occurs (reverse gel point) from the maximum of the reduced DPw, which excludes the largest cluster in the system, and compare our measured value with previous analytical theories and experimental results. Our measured value agrees well with the value obtained from the bond percolation theory on a diamond lattice.