Dr. Sanket' Talk

Jan 06, 2010 - 15:00

Jan 06, 2010 - 16:30

Creativity Hall (Room No 118)

Seminars

Dr. Sanket Deshmukh (PhD Chemical Engineering University College Dublin 2009)

Abstract: Thermo-sensitive hydrogels are a class of stimuli-sensitive hydrogels which can undergo a significant volume transition in response to change in the temperature of the surrounding environment. Most thermo-sensitive hydrogels exhibit a characteristic temperature called Lower Critical Solution Temperature (LCST) at which they experience the volume transition. In this work we have studied a thermo-sensitive hydrogel poly(N-isopropylacrylamide) (PNIPAM) that shows an LCST at 32C. The PNIPAM hydrogel is hydrophilic (swollen) below the LCST (< 32C) while it is hydrophobic (contracted) above the LCST (> 32C). As the LCST of PNIPAM lies close to body temperature it has great potential as a controlled drug delivery system. The exact molecular mechanisms that govern the LCST behaviour are not well understood. In the present work we have performed atomistic level Molecular Dynamics (MD) simulation of PNIPAM hydrogels mainly to i) Understand the exact origin of the LCST at molecular level ii) Study the effect of LCST on the diffusion coefficient of the solvent (water). A number of computer models of structures of the PNIPAM hydrogels with N N' methylene bisacrylamide (BIS) and ethylene glycol dimethacrylate (EGD) crosslinkers were generated using the principles of directed self-avoiding random walk. The structures were further equilibrated using the GROMOS96 force field followed by insertion of SPC/E water molecules into the simulation cell. MD simulations of a thermo-sensitive hydrogel PNIPAM in presence of water were performed below at and above its LCST (namely 300 305 and 310K) using the GROMOS96 force field.The effect of crosslinking density on the structural and dynamical properties of PNIPAM across the LCST was studied using different number of BIS crosslinkers in the model structure of PNIPAM. The MD simulations in NPT ensembles were conducted at 300 305 and 310K. From the analysis of the radial distribution function of key atoms and pore size distribution it was observed that for all the crosslinking densities the hydrogel structures were contracted at 310K as compared to 300K. The diffusion coefficients for water were also calculated and it was observed that there was very little effect of the LCST and crosslinking density on the diffusion coefficient of water. Hydrogen bonding interactions between polymer-polymer and water-polymer were also studied. To study the effect of different crosslinkers on the PNIPAM hydrogel structure two different crosslinkers namely BIS and EGD were used to generate a model structure. The MD simulations were performed in the NPT ensemble at 300 305 and 310K. From an analysis of the radial distribution functions of key atoms as well as pore size distribution it can be concluded that hydrogel structures for both crosslinkers showed LCST around 305K. The hydrogel structures with both BIS and EGD crosslinkers were collapsed at 310K as compared to 300K. Comparing the two crosslinkers it was observed that the hydrogel structures with EGD crosslinkers were more swollen as compared to BIS crosslinked hydrogels. This is an effect of the longer chain length of EGD making it swell more than the BIS crosslinker. The diffusion coefficients of water were slightly lower in the presence of EGD crosslinked hydrogel than for the BIS crosslinked hydrogel. This is because the EGD crosslinked hydrogel is more swollen which inhibits the motion of the water molecules. Hydrogen bonding interactions between polymer-polymer and water-polymer were also studied for both rosslinkers. This study will provide some insight into design of the hydrogels themselves and how one can manipulate the accessibility of water to the hydrogel as a means of controlling collapse.