Meso-scale Simulations of Poly(N-isopropylacrylamide) Grafted Architectures
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Meso-scale Simulations of Poly(N-isopropylacrylamide) Grafted Architectures Sanket A. Deshmukh1, Ganesh Kamath2, Derrick C. Mancini3, Subramanian K.R.S. Sankaranarayanan1, Wei Jiang4 1 Center for Nanoscale Materials, 3Physical Sciences and Engineering, 4Leadership Computing Facility, Argonne National Laboratory, Argonne, IL 60439 2 Department of Chemistry, University of Missouri-Columbia, Columbia 65211 ABSTRACT Poly(N-isopropylacrylamide) (PNIPAM) is a thermosensitive polymer that is well-known for its behavior at a lower critical solution temperature (LCST) around 305 K. Below the LCST, PNIPAM is soluble in water, and above this temperature, polymer chains collapse and transform into a globule state. The conformational dynamics of single chains of polymer in a solution is known to be different from those of grafted structures that comprise of an ensemble of such single chains. In this study, we have carried out MD simulations of a mesoscopic nanostructure of PNIPAM polymer chains consisting of 60 monomer units grafted onto gold nanoparticles of different diameters, to study the effect of temperature and core particle size on the polymer conformations. Additionally, we have also studied the effect of grafting density on the coil-toglobule transition exhibited by PNIPAM through the LCST. The systems investigated consisted of ~3 and ~6 million atoms. Simulations were carried out below and above the LCST of PNIPAM, at 275K and 325K. Simulation trajectories were analyzed for radius of gyration of PNIPAM chains. INTRODUCTION Poly(N-isopropylacrylamide) (PNIPAM) undergoes a coil-to-globule transition when the temperature is raised from below to above its lower critical solution temperature (LCST), typically near 305 K.[1] Coil-to-globule transitions shown by such macromolecules can have a number of practical applications, including drug delivery, medical diagnostics, biomolecule separations, etc.[2] Recent advancements in synthetic chemistry have allowed us to alter the LCST of the these polymers by copolymerizing these temperature sensitive polymers with other polymers that are hydrophilic or hydrophobic in nature, by modifying the end groups of these polymers with different ionic groups or attachment of biomolecules, for example. Addition of salts and electrolytes in solvent, such as salt ions or co-solvents can also modify the LCST of these polymers.[3] With the aid of recent synthetic techniques, these macromolecules can be built up into mesoscopic nanostructures with arbitrary architectural complexity, which allows us to incorporate building blocks of different chemical functionality or biomolecule (DNA, peptides etc.) in well-controlled positions.[4] A “polymer brush” structure is a classic example of such a mesoscopic macromolecular nanostructure. Polymer brushes are essentially flexible macromolecular chains that are strategically anchored by special linkers to a substrate at sufficiently high grafting density such that different chains interact.[5] A precise control of the make-up of such macromolecular chains o
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