Effect of Methanol/Water Mixtures on the Lower Critical Solution Temperature of Poly(N-isopropylacrylamide)
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Effect of Methanol/Water Mixtures on the Lower Critical Solution Temperature of Poly(Nisopropylacrylamide) Sanket A. Deshmukh1, Ganesh Kamath3, Derrick C. Mancini2, and Subramanian K.R.S. Sankaranarayanan1 1 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, U.S.A 2 Physical Sciences and Engineering, Argonne National Laboratory, Argonne, IL 60439, U.S.A 3 Department of Chemistry, University of Missouri-Columbia, Columbia 65211, U.S.A ABSTRACT Poly(N-isopropylacrylamide) (PNIPAM) is a thermo-sensitive polymer that exhibits a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water and above this temperature polymer chains collapse prior to aggregation. In the presence of methanol, electron paramagnetic resonance (EPR) spectroscopy suggests that, LCST of PNIPAM is depressed up to certain mole fraction of methanol (0.35 mole fraction) and it is speculated that addition of methanol affects the PNIPAM-water interactions. Above 0.35 mole fraction of methanol, LCST gets elevated to temperatures above ~305 K (32qC) and cannot be detected up to 373 K (100 qC). The atomistic origin of this co-solvency effect on the LCST behavior is not completely understood. In the present study, we have used molecular dynamics (MD) simulations to investigate the effect of methanol-water mixtures on conformational transitions and the LCST of PNIPAM. We employ two different force fields i.e. polymer consistent force-field (PCFF) and CHARMM to study solvation dynamics and the PNIPAM LCST phase transition in various methanol-water mixture compositions (0.018, 0.09, 0.27, 0.5, and 0.98 mole fractions). Simulations are conducted at fully atomistic level for three different temperatures (260, 278, and 310 K) and radius of gyration (Rg) of PNIPAM chains was computed for determination of LCST behavior of PNIPAM. INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAM) is a thermo-sensitive polymer that is well-known for its lower critical solution temperature (LCST) around 305K [1]. Below the LCST, PNIPAM is soluble in water, and above this temperature, polymer chains collapse and tend to aggregate. This coil-to-globule transition of PNIPAM is important in several applications, which includes drug delivery, peptides separation, surface modifications, and tissue engineering [2]. There are several variables such as tacticity, temperature, pressure, and pH which can be used to tune the conformational dynamics of PNIPAM [3]. For example, in addition to temperature, the coil-toglobule transition in PNIPAM can be altered by controlling the solvent composition [4]. At low temperature water and methanol, on their own, are good solvents for PNIPAM. However, experiments by Winnik et al. suggest that the addition of methanol to aqueous PNIPAM solution results in depression of LCST up to certain mole fraction of methanol (0.35 mole fraction) [4]. It is also speculated that addition of methanol affects the PNIPAM-water interactions resulting in alteration of LCST. Above 0.35 mole fraction of methano
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