Simulation studies on polymer gel-polymer solution interfaces and molecular motor-polymer conjugates
Loading...
Date issued
Authors
Editors
Journal Title
Journal ISSN
Volume Title
Publisher
Reuse License
Abstract
This dissertation discusses two major projects, focusing on the computational modeling of core-shell microgels and molecular motor gels. The first project involves investigating the interface of core-shell microgels formed through micro-fluidic experiments. A deeper understanding of the processes at the boundaries of these two phase can be developed by using molecular dynamic simulations, which could help in optimizing the stratergy for synthesizing core-shell particles. Here in our study, we use a simple and idealized model for a gel which has the topology of a diamond lattice, with the free polymer and strand length chosen to be equal, and to consist of the same type of monomers. In the simulations, the concentration of the polymer phase is varied to study the interpenetration of the gel and the polymer solution phases. The density profiles reveals that there are two time regimes, an initial compression of the gel followed by swelling of the gel. An analysis on the interpenetrating chains at the interface, given by "degree of interfacial integration", reveals that the interface locally equilibrates after around 100 chain relaxation times. At times greater than the local equilibration time for the interface, the free polymer chain configuration within the gel region reveals emerging percolating clusters, only if the chain concentration exceeds a certain threshold. This threshold concentration was found to be of the same order of magnitude as the overlap concentration of the chains. Finally, we studied the structures formed at the interface by applying the capillary wave theory on the locally equilibrated interfaces, which revealed a positive correlation between interfacial width and interfacial tension.
The second topic focused on the molecular machines, created by incorporating uni-directionally rotating light responsive rotors as cross-linkers in a polymer matrix. The winding of the chains, due to motor rotation leads to contraction of these gels. Because of this property, they are expected to be used as artificial muscles. Here in this study, we present the first coarse-grained molecular dynamics simulation of such gels, assuming a model motor that does not unwind even under large loads. We focus on simulating the limiting behaviour of these gels as observed in the contraction experiments, conducted by X.Yao et al. A regular diamond network with the model rotors as cross-linkers was chosen to represent the quasi-ideal network studied in the experiments. We demonstrate the success of our model, by qualitatively replicating the limiting behavior observed in the experiments, using a free regular gel and a periodically cross-linked regular gel under different loads. The simulation, in agreement with the experiments confirmed that the contraction ratios are independent of the strand-length and is limited mainly by the winding of the chain pairs attached to the motor. We examined the torque needed by these motors to sustain the contracted state and concluded that it is independent of the loads in the Hookean regime. However, the average torque for loads in the same regime shows a slight dependence on strand length. Further, the Gaussian linking number was used to analyze the local winding of the chain pairs that are connected to the same motors, revealing the regions of different linking. Finally, we studied the size ratio of "stiff gels", which showed a non-monotonic behavior as we varied the stiffness.