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: pnipam_agglomeration Project title: Agglomeration dynamics in thermo-sensitive polymers across the lower critical solution temperature: A molecular dynamics simulation study Associated funding: LDRD Other Systems: Carbon (CNM) Science: PNIPAM is a thermo-sensitive polymer that has a lower critical solution temperature (LCST) around 305K in aqueous solution.1 As the LCST of PNIPAM is close to human body temperature, it is a serious contender for a controlled drug delivery system.2,3 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.1 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.1,4 It is believed that, when PNIPAM is dissolved in solvent like water, PNIPAM has to arrange it self in a specific orientation to form hydrogen bond with the already somewhat arranged water molecules.1 In the case of water molecules, which are near to hydrophobic group of polymer, -CH3 of the PNIPAM side chains, these water molecules must reorient being unable to form hydrogen bond with the non-polar groups in PNIPAM. This reorientation leads to formation of clathrate-like structures, this phenomenon is known as hydrophobic effect, results in decreased entropy upon mixing (negative ΔS).1 When temperature is raised, the entropy term dominates the otherwise exothermic enthalpy of the hydrogen bonds formed between the polymer polar groups and water molecules that is the initial driving force for dissolution. Once the free energy change (ΔG) becomes positive upon mixing, the consequence is phase separation above a LCST as seen in the case of PNIPAM.1 If the concentration of the PNIPAM is high enough, this replacement of PNIPAM-water contacts with PNIPAM-PNIPAM and water-water contacts is manifested by precipitation.1 Since the discovery, that PNIPAM changes its solubility in water as a function of temperature, numerous reports deal with the origin of the change from solubility to insolubility at elevated temperatures.5-7 Based on the study of diluted aqueous PNIPAM solutions by static and dynamic light scattering (DLS), a coil-to-globule transition caused by dehydration of polymer chains during heating was postulated.6,7 For concentrated aqueous PNIPAM solutions, phase separation at LCST occurs as a macroscopic manifestation of the coil-to-globule transition followed by aggregation.8 To validate this theory and to understand the exact atomistic scale origin of the phase transition at the LCST, we propose to use MD simulation techniques. Two PNIPAM chains, consisting of 30 monomer units each, will be placed in a simulation cell and will subsequently be solvated. We plan to carry out simulations of two 30-mer 20 Å and 50 Å apart from each other. Simulation cell will be solvated with ~24,000 water molecules. Simulations will be carried out below and above the LCST, namely at 278, 295, and 310K for 50ns. Simulated trajectories will be analyzed for structural and dynamical properties of PNIPAM (e.g. radius of gyration (Rg), radial distribution functions (rdf)) and diffusion properties of water. The agglomeration and coil-to-globule transition, if any, will be studied at 278, 295, and 310K. Project description: Accuracy of the model and MD simulation results mainly depends upon the method of structure generation and force-field used for MD simulations.9 In this work we propose to use the Polymer Consistent force field (PCFF) to understand the structural changes in the polymer brush at global as well as local level using an atomistic level model.10 The preliminary studies we carried out with PCFF 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. Two 30-mer will be placed in simulation cell (20 Å and 40 Å apart from each other). Simulation cell will be solvated with ~24,000 water molecules (72,000 atoms). Simulations will be carried out below and above the LCST, namely at 278, 295, and 310K for ~50ns.Transport properties of water will be studied using Einst ein relation. Results will be compared with the ongoing experiments in the APS. The computational requirements for the proposed study will be substantial. For the proposed calculations, to study the phase transition across the LCST simulations should be run for ~50ns. From preliminary results of the simulations we carried out on the Fusion cluster, 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 ~135 hrs of simulation time for 10ns on 96 cores. In the proposed work we plan to study, two 30-mer chains of PNIPAM in presence of ~24,000 water molecules (~72,000 atoms). All the simulations will be carried out over a temperature range across the LCST, namely 278, 295, and 310 (3*4 = 12 simulations). Each simulation will be run for ~50 ns (total simulation time = ~600ns). The estimated time for this particular work, to carry out the mentioned calculations, would be ~750,000 core hours. References: 1. Y. Tsujii, K. Ohno, S. Yamamoto, A. Goto, T. Fukuda, Adv. Polym. Sci. 2006, 197, 1-45. 2. D. I. Dimitrov, A. Milchev, K. Binder, J. Chem. Phy. 2007, 127, 084905-084909. 3. H. G. Schild, Prog. Polym. Sci. 1992, 17, 163-249. 4. D. Schmaljohann, Adv. Drug Del. Rev. 2006, 58(15), 1655-1670. 5. American Cyanamid Company 1963, N-Isopropylacrylamide, Brochure C3-1354-500-4/63. 6. K. Kubota, S. Fujishige, I. Ando, J. Phys. Chem. 1990, 94, 5154. 7. C. Wu, S. Q. Zhou, Macromolecules 1995, 28, 8381. 8. K. Chan, R. Pelton, J. Zhang, Langmuir 1999, 15, 4018. 9. M.P. Allen, D. J. Tildesley, Computer Simulations of Liquid, Clarendon Press, Oxford, 1987. 10. Accelrys, Inc. Cerius2 and Discover (programs) and Discover User Guide, version 96.0/4.0.0; Molecular Simulations, Inc.: San Diego, CA, 1996. Project URL: Requested allocation: 750000 Q1: 550000 Q2: 200000 Q3: 000000 Q4: 000000 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. On the Fusion cluster same calculations took ~135 hrs. The requester has used undetermined amount hours of their initial startup project. In addition to approving an initial amount, please specify a Category and Subcategory for this project. For a list of the current selection of approved categories, please see: https://wiki.lcrc.anl.gov/wiki/Processes/Categories Once the Allocation committee has approved the project, please go to the Project Management page to create it: https://accounts.lcrc.anl.gov/projects.php Thank You, The LCRC Accounts System