Agglomeration Dynamics In Thermo-Sensitive Polymers Across The Lower Critical Solution Temperature: A Molecular Dynamics
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Agglomeration Dynamics In Thermo-Sensitive Polymers Across The Lower Critical Solution Temperature: A Molecular Dynamics Simulation Study Sanket A. Deshmukh1, Subramanian K.R.S. Sankaranarayanan1 and Derrick C. Mancini2 1 2
Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439 Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439
ABSTRACT Poly(N-isopropylacrylamide) (PNIPAM), a classic thermo-sensitive polymer, has a lower critical solution temperature (LCST) at ~32°C. In this work we have used molecular dynamics simulations to understand the origin of the LCST and agglomeration of PNIPAM chains of 5 and 30 monomer units (5-mer and 30-mer). Experimentally, when the concentration of PNIPAM is >1 ppm, polymer chains after undergoing coil-to-globule transition above the LCST aggregates to yield a stable colloidal dispersion.In our study two PNIPAM chains, consisting of 30 monomer units each, were placed in a cubic simulation cell and were subsequently solvated. Simulations were carried out below and above the LCST, namely at 278 and 310K for 10ns. Simulated trajectories were analyzed for structural and dynamical properties of both PNIPAM and water. We observe coil-to-globule transition in PNIPAM above the LCST. We also find that the PNIPAM chains agglomerate above the LCST. We also observe entanglement in PNIPAM chains below the LCST. We also study agglomeration of 5 PNIPAM chains each consisting of 5 monomer units. There was no significant difference in polymer agglomeration behavior across the LCST for these short chain oligomers. The agglomeration behavior is thus strongly correlated to the size of the polymer chains. These results provide fundamental insight into the atomistic scale mechanism of PNIPAM agglomeration across the LCST. INTRODUCTION Understanding the phase transition in dilute solutions of short linear polymer chains is a fundamental problem in the field of polymer science[1, 2]. Thermo-sensitive polymers and hydrogels, such as Poly(n-isopropylacrylamide) (PNIPAM) represent an important class of materials which undergo phase transition across the lower critical solution temperature (LCST), which is around 305K[3]. The LCST behavior is a common characteristic of all PNIPAM oligomers in aqueous solution, irrespective of their degree of polymerization (Dp). Interestingly, however, it has been speculated that the atomic scale mechanism by which the short chain oligomers undergo phase transition across LCST might be very different from the collapse mechanism that exists in the case of solutions having longer polymer chains [4]. There have been experimental reports about the LCST behavior of N-isopropyl propionamide (NIPPA) and of PNIPAM oligomers at different DP’s[5-7]. It has been suggested that an LCST phase behavior is shown even by aqueous solutions of N-isopropyl propionamide (NIPPA), the monomer unit which corresponding to the repeating unit of PNIPAM. The LCST behavior of NIPPA and short oligomers solutions appears to be initiated by intermolecular aggregat
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