The Woodruff School

SUMMARY

We develop a mesoscale computational model for permanently cross-linked polymer networks and use it to study several practical problems involving these soft materials. Specifically, we analyze the permeability and diffusivity of polymer networks under mechanical deformations, we examine the release of encapsulated solutes from microgel capsules during volume transitions, and we explore the complex tribological behavior of elastomers. We show that our model can successfully simulate the convective and diffusive transport of fluid and solutes through polymer networks. Additionally, we demonstrate that our model is able to capture key features of polymer networks micromechanics. The results of our simulations reveal that the transport properties of mechanically loaded networks are defined by the network porosity and orientation of network filaments. We characterize the latter by a second order orientation tensor, and show that the permeability along the principal directions of a deformed network is directly related to the magnitudes of the corresponding tensor components. Moreover, our calculations indicate that responsive microcapsules can be effectively utilized for steady and pulsatile release of encapsulated solutes. We illustrate that swollen gel capsules allow steady, diﬀusive release of nanoparticles and polymer chains, whereas gel deswelling causes burst-like discharge of solutes driven by an outward ﬂow of the solvent enclosed within a shrinking capsule. We demonstrate that this hydrodynamic release can be regulated by introducing rigid microscopic rods in the capsule interior. We also probe the effects of sliding velocity, temperature, and normal load on the friction of elastomers. Our friction simulations predict a bell-shaped curve for the dependence of the friction coefficient on the sliding of elastomers on smooth and rough substrates. We find that, depending on the surface interactions, the maximum friction velocity is determined by the temperature and ratio between the thermal energy of the system and adhesion energy of the counter surface. Furthermore, our calculations illustrate that at low sliding velocities, friction decrease with an increase in the temperature.

SUBJECT:	Ph.D. Dissertation Defense

BY:	Hassan Masoud

TIME:	Thursday, July 26, 2012, 9:00 a.m.

PLACE:	MRDC Building, 4211

TITLE:	Polymer Networks: Modeling and Applications

COMMITTEE:	Dr. Alexander Alexeev, Chair (ME) Dr. Richard F. Salant (ME) Dr. Paul M. Goldbart (PHYS) Dr. David L. Hu (ME) Dr. Alberto Fernandez-Nieves (PHYS)

SUMMARY We develop a mesoscale computational model for permanently cross-linked polymer networks and use it to study several practical problems involving these soft materials. Specifically, we analyze the permeability and diffusivity of polymer networks under mechanical deformations, we examine the release of encapsulated solutes from microgel capsules during volume transitions, and we explore the complex tribological behavior of elastomers. We show that our model can successfully simulate the convective and diffusive transport of fluid and solutes through polymer networks. Additionally, we demonstrate that our model is able to capture key features of polymer networks micromechanics. The results of our simulations reveal that the transport properties of mechanically loaded networks are defined by the network porosity and orientation of network filaments. We characterize the latter by a second order orientation tensor, and show that the permeability along the principal directions of a deformed network is directly related to the magnitudes of the corresponding tensor components. Moreover, our calculations indicate that responsive microcapsules can be effectively utilized for steady and pulsatile release of encapsulated solutes. We illustrate that swollen gel capsules allow steady, diﬀusive release of nanoparticles and polymer chains, whereas gel deswelling causes burst-like discharge of solutes driven by an outward ﬂow of the solvent enclosed within a shrinking capsule. We demonstrate that this hydrodynamic release can be regulated by introducing rigid microscopic rods in the capsule interior. We also probe the effects of sliding velocity, temperature, and normal load on the friction of elastomers. Our friction simulations predict a bell-shaped curve for the dependence of the friction coefficient on the sliding of elastomers on smooth and rough substrates. We find that, depending on the surface interactions, the maximum friction velocity is determined by the temperature and ratio between the thermal energy of the system and adhesion energy of the counter surface. Furthermore, our calculations illustrate that at low sliding velocities, friction decrease with an increase in the temperature.