{"66345":{"#nid":"66345","#data":{"type":"event","title":"MSE Ph.D. Defense - Seung Geol Lee","body":[{"value":"\u003Cp\u003E\u003Cstrong\u003ETitle: \u003C\/strong\u003ESTRUCTURE-PROPERTY RELATIONSHIP OF HYDROGEL: MOLECULAR\nDYNAMICS SIMULATION APPROACH\n\n\u003C\/p\u003E\n\n\u003Cp\u003E\u003Cstrong\u003ESummary: \u003C\/strong\u003EWe have used a molecular modeling of both random and\nblocky sequence hydrogel networks of poly(N-vinyl-2-pyrrolidone-co-2-hydroxyethyl\n\n\u003C\/p\u003E\u003Cp\u003Emethacrylate) (P(VP-co-HEMA)) with a composition of\nVP:HEMA = 37:13 to investigate the effect of the monomeric sequence and the\nwater content on the equilibrium structures and the mechanical and transport\nproperties by full-atomistic molecular dynamics (MD) simulations.\u0026nbsp; The degree of randomness of the monomer\nsequence for the random and the blocky copolymers, were 1.170 and 0.104,\nrespectively, and the degree of polymerization was fixed at 50.\u0026nbsp; The equilibrated density of the hydrogel was\nfound to be larger for the random sequence than for the blocky sequence at low\nwater contents (\u0026lt; 40 wt %), but this density difference decreased with\nincreasing water content.\u0026nbsp; The pair\ncorrelation function analysis shows that VP is more hydrophilic than HEMA and\nthat the random sequence hydrogel is solvated more than the blocky sequence\nhydrogel at low water content, which disappears with increasing water content.\u003C\/p\u003E\n\n\u003Cp\u003ECorrespondingly, the water structure is more disrupted by\nthe random sequence hydrogel at low water content but eventually develops the\nexpected bulk-water-like structure with increasing water content.\u0026nbsp; From mechanical deformation simulations, the\nstress-strain analysis showed that the VP is found to relax more efficiently,\nespecially in the blocky sequence, so that the blocky sequence hydrogel shows\nless stress levels compared to the random sequence hydrogel.\u0026nbsp; As the water content increases, the stress\nlevel becomes identical for both sequences.\u0026nbsp;\nThe elastic moduli of the hydrogels calculated from the constant strain\nenergy minimization show the same trend with the stress-strain analysis.\u0026nbsp; Ascorbic acid and D-glucose were used to\nstudy the effect of the monomeric sequence on the diffusion of small guest\nmolecules within the hydrogels.\u0026nbsp; By\nanalyzing the pair correlation functions, it was found that the guest molecule\nhas greater accessibility to the VP units than to the HEMA units with both\nmonomeric sequences due to its higher hydrophilicity compared to the HEMA\nunits.\u0026nbsp; The monomeric sequence effect on\nthe P(VP-co-HEMA) hydrogel is clearly observed with 20 wt % water content, but\nthe monomeric sequence effect is significantly reduced with 40 wt % water\ncontent and disappears with 80 wt % water content.\u0026nbsp; This is because the hydrophilic guest\nmolecules are more likely to be associated with water molecules than with the\npolymer network at the high water content.\u0026nbsp;\nBy analyzing the mean square displacement, the displacement of the guest\nmolecules and the inner surface area, it is also found that the guest molecule\nis confined in the system at 20 wt % water content, resulting in highly\nanomalous subdiffusion.\u0026nbsp; Therefore, the\ndiffusion of the guest molecules is directly affected by their interaction with\nthe monomer units, the monomeric sequence and the geometrical confinement in\nthe hydrogel at a low water content, but the monomeric sequence effect and the\nrestriction on the diffusion of the guest molecule are significantly decreased\nwith increasing the water content.\u003C\/p\u003E\n\n\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\n\n\u003Cp\u003EWe also investigated the de-swelling mechanisms of the\nsurface-grafted\u003C\/p\u003E\n\n\u003Cp\u003Epoly(N-isopropylacrylamide) (P(NIPAAm)) brushes\ncontaining 1300 water molecules at 275 K, 290 K, 320 K, 345 K, and 370 K.\u0026nbsp; We clearly observed the de-swelling of the\nwater molecules for P(NIPAAm) above the lower critical solution temperature\n(LCST) (~305 K).\u0026nbsp; Below the LCST, we did\nnot observe the de-swelling of water molecules.\u0026nbsp;\nUsing the upper critical solution temperature (UCST) systems\n(poly(acrylamide) brushes) for comparison purposes, we did not observe the\nde-swelling of water molecules at a given range of temperatures.\u0026nbsp; By analyzing the pair correlation functions\nand the coordination numbers, the de-swelling of the water molecules occurred\ndistinctly around the isopropyl group of the P(NIPAAm) brush above the LCST\nbecause C(NIPAAm) does not offer sufficient interaction with the water\nmolecules via the hydrogen bonding type of secondary interaction.\u0026nbsp; We also found that the contribution of the\u003C\/p\u003E\n\n\u003Cp\u003EN(NIPAAm)-O(water) pair is quite small because of the\nsteric hindrance of the isopropyl group.\u0026nbsp;\nBy analyzing the change in the hydrogen bonds, the hydrogen bonds\nbetween polar groups and water molecules in the P(NIPAAm) brushes weaken with\nincreasing temperature, which leads to the de-swelling of the water molecules\nout of the brushes above the LCST.\u0026nbsp; Below\nthe LCST, the change in the hydrogen bonds is not significant.\u0026nbsp; Again, the contribution of the\nNH(NIPAAm)-water pairs is insignificant; the total number of hydrogen bonds is\n~20, indicating that the interaction between the NH group and the water\nmolecules is not significant due to steric hindrances.\u0026nbsp; Lastly, we observed that the total surface\narea of the\u003C\/p\u003E\n\n\u003Cp\u003EP(NIPAAm) brushes that is accessible to water molecules\nis decreased by collapsing the brushes followed by the de-swelling of water\nmolecules above the LCST.\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003E\u003Cstrong\u003ETitle:\u003C\/strong\u003E STRUCTURE-PROPERTY RELATIONSHIP OF HYDROGEL:\nMOLECULAR DYNAMICS SIMULATION APPROACH\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"MSE Ph.D. Defense - Seung Geol Lee"}],"uid":"27388","created_gmt":"2011-06-01 11:06:20","changed_gmt":"2016-10-08 01:55:03","author":"Bill Miller","boilerplate_text":"","field_publication":"","field_article_url":"","field_event_time":{"event_time_start":"2011-06-06T11:00:00-04:00","event_time_end":"2011-06-06T13:00:00-04:00","event_time_end_last":"2011-06-06T13:00:00-04:00","gmt_time_start":"2011-06-06 15:00:00","gmt_time_end":"2011-06-06 17:00:00","gmt_time_end_last":"2011-06-06 17:00:00","rrule":null,"timezone":"America\/New_York"},"extras":[],"groups":[{"id":"1238","name":"School of Materials Science and Engineering"}],"categories":[],"keywords":[{"id":"10802","name":"MSE_Interal_Event"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[{"id":"1791","name":"Student sponsored"}],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[],"email":[],"slides":[],"orientation":[],"userdata":""}}}