Lee, Seung Geol and Pascal, Tod A. and Koh, Wonsang and Brunello, Giuseppe F. and Goddard, William A., III and Jang, Seung Soon (2012) Deswelling Mechanisms of Surface-Grafted Poly(NIPAAm) Brush: Molecular Dynamics Simulation Approach. Journal of Physical Chemistry C, 116 (30). pp. 15974-15985. ISSN 1932-7447 http://resolver.caltech.edu/CaltechAUTHORS:20121010-144500177
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Technologies ranging from solvent extraction and drug delivery to tissue engineering are beginning to benefit from the unique ability of “smart polymers” to undergo controllable structural changes in response to external stimuli. The prototype is poly(N-isopropylacrylamide) (P(NIPAAm)) which exhibits an abrupt and reversible hydrophilic to hydrophobic transition above its lower critical solution temperature (LCST) of ∼305 K. We report here molecular dynamics simulations to show the deswelling mechanisms of the hydrated surface-grafted P(NIPAAm) brush at various temperatures such as 275, 290, 320, 345, and 370 K. The deswelling of the P(NIPAAm) brush is clearly observed above the lower critical solution temperature below which the P(NIPAAm) brush is associated with water molecules stably. By simulating the poly(acrylamide) brush as a reference system having the upper critical solution temperature (UCST) behavior with the same conditions employed in the P(NIPAAm) brush simulations, we confirmed that the deswelling of P(NIPAAm) brush does not take place at a given range of temperatures, which validates our simulation procedure. By analyzing the pair correlation functions and the coordination numbers, we found that the dissociation of water from the P(NIPAAm) brush occurs mainly around the isopropyl group of the P(NIPAAm) above the LCST because of its hydrophobicity. We also found that the NH of the amide group in NIPAAm does not actively participate in the hydrogen bonding with water molecules because of the steric hindrance caused by the attached isopropyl group, and thereby the hydrogen bonding interactions between amide groups and water molecules are significantly weakened with increasing temperature, leading to deswelling of the hydrated P(NIPAAm) brush above the LCST through favorable entropic change. These results explain the experimental observations in terms of a simple molecular mechanism for polymer function.
|Additional Information:||© 2012 American Chemical Society. Received: February 17, 2012; Revised: July 4, 2012; Published: July 11, 2012. The authors thank Grace M. Kim for her help with data analysis. Partial support of this research was provided by Dow Chemical Company. The MSC computational facilities were provided by ARO−DURIP and ONR-DURIP. Additional figures of experimental data (Figures 1S−7S). This material is available free of charge via the Internet at http://pubs.acs.org. These authors contributed equally to this work. The authors declare no competing financial interest.|
|Official Citation:||Deswelling Mechanisms of Surface-Grafted Poly(NIPAAm) Brush: Molecular Dynamics Simulation Approach Seung Geol Lee, Tod A. Pascal, Wonsang Koh, Giuseppe F. Brunello, William A. Goddard, III, and Seung Soon Jang The Journal of Physical Chemistry C 2012 116 (30), 15974-15985|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Aucoeur Ngo|
|Deposited On:||10 Oct 2012 22:24|
|Last Modified:||27 Dec 2012 02:51|
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