Hello, A new project on the LCRC cluster has been requested. Please forward the information on to the LCRC Allocation sub-committee. Applicant's name: Sanket Deshmukh Applicant's institution: ANL Applicant's division: CNM Project Name: MD_of_pnipam_brush Project title: Structural and dynamic properties of thermo-sensitive polymer brushes: Molecular Dynamics study. Associated funding: Other Systems: Carbon (CNM) Science: Grafting of polymer chains on the surface is challenging but has great importance in many areas of science and engineering e.g. colloidal stabilization, biotechnology, rheology etc.1 Such a grafted structure is called as “polymer brush” and is of both practical and theoretical interest. Theoretical interest in such systems arises due to the fact that single/free chains have different configurations to the chains grafted on the surface or chains near the surface. For most of these systems, in most situations, a solvent is present and grafted polymeric chains are not at melt density. The conformation of such polymers in a solvent can dramatically change with graft density; at low densities, these chains assume a “mushroom” conformation with the coil dimension similar to that of free chains. However, brushes with high grafting density and height (polymer chain length) have characteristic structure and properties quite different and unpredictable from those of low graft density brushes. Polymer brushes can be divided into two concentration densities, namely “semi-dilute” and “concentrated”. Poly(N-isopropylacrylamide) PNIPAM is a thermo-sensitive polymer that has a lower critical solution temperature (LCST) around 305K in aqueous solution. As the LCST of PNIPAM is close to human body temperature, it is a serious contender for a controlled drug delivery system. Coil-to-globule transition occurs in PNIPAM when temperature is raised above the LCST. It is believed that this occurs due to the change in hydrophilic interaction to the hydrophobic interaction. Concentration and molecular weight of PNIPAM affects the LCST by less than 1K (304–306K). Lower molecular weight samples appear to have slightly higher LCSTs. Presence of salts ions in the solution lowers the LCST significantly.3 In the case of PNIPAM brushes, LCST depends upon the grafting density and molecular weight. In this work, we propose to use molecular dynamics (MD) simulation techniques to understand how the grafting density and local forces acting on the polymer bonds affect PNIPAM brushes at fully atomistic level. Behavior of a single chain in an aqueous solution, across the LCST, in the brush will be studied using MD simulations. This information can be useful for the development of composite system, where functional group or nano-particles are attached to the free ends of chain. It is also proposed to study the structural properties and change in the LCST of PNIPAM, if any, when two polymer chains, namely PNIPAM and poly(ethylene oxide) (PEO), with different LCSTs are grafted together in presence of water. Project description: In this work we propose to use the Consistent Valence force field (CVFF) to understand the structural changes in the polymer brush at global as well as local level using an atomistic level model. The preliminary studies we carried out with CVFF on a single chain made up of 30 monomer units of PNIPAM in presence of pure water predicted the LCST close to 305K, which is in good agreement with experimentally observed value. To carry out MD simulations we will be using LAMMPS MD simulation package. Fully atomistic polymer chains with ~25 monomer units will be generated. Each chain will be end-grafted to form a 5 x 5 grid on flat surface, which will define the x-y plane of the system. Grafting density will be varied from 0.05 to 0.5 to study the structural changes with respect to grafting density. Depending upon the number of polymer chains TIP4P water molecules (~20,000 to ~100,000) will be added to simulation cell. To study the structural changes across the LCST of PNIPAM, simulation system will then be equilibrated up to 40-50ns using LAMMPS at 278, 310 and 360K. To study the change in the LCST of PNIPAM in presence of another thermo-sensitive polymer, if any, we propose to use PNIPAM and PEO grafted and equilibrated in similar manner as mentioned above. The computational requirements for the proposed study will be substantial. To study the phase transition across the LCST in the case of single PNIPAM chain, simulations should be carried out for ~50ns. From preliminary results of the simulations we carried out on the CNM’s cluster Carbon, we observed a clear phase transition across the LCST, for polymer chains with 30 monomer units (572 atoms) in presence of 9,000 water molecules. This system with ~30,000 atoms required ~145 hrs of simulation time for 10ns on 96 cores. In the proposed work we plan to study, PNIPAM ~25 mer (475 atoms) brush made up of ~20 polymer chains (~9000 atoms) in presence of ~30,000 water molecules. Simulations will be carried out at 278, 310 and 360K for ~50ns (total simulation time = 150ns). We also propose to study PNIPAM ~25 mer (475 atoms) and PEO ~25 mer (150 atoms) polymer chains. In the initial stages of this project we plan to use 5 and 10 chains of both PNIPAM (2375 and 4750 atoms) and PEO (1500 and 3000 atoms) in presence of 30,000 water molecules. Total system will be consisted of ~100,000 atoms and simulations will be carried out at 278, 310 and 360K for ~50ns (total simulation time = 300ns). The estimated time to carry out the mentioned calculations in this particular work would be ~500,000 core hours. Project URL: Requested allocation: 600000 Justification: To study the phase transition across the LCST in the case of single PNIPAM chain, simulations should be carried out for ~50ns. From preliminary results of the simulations we carried out on the CNM’s cluster Carbon, we observed a clear phase transition across the LCST, for polymer chains with 30 monomer units (572 atoms) in presence of 9,000 water molecules. This system with ~30,000 atoms required ~145 hrs of simulation time for 10ns on 96 cores. The requester has used 0 hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. 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