Poly(N-isopropylacrylamide) (PNIPAM) is a popular polymer widely used in smart hydrogel synthesis due to its thermo-responsive behavior in aqueous medium. Aqueous PNIPAM hydrogels can reversibly swell and collapse below and above their lower critical solution temperature (LCST), respectively. The present work used molecular dynamics simulations to explore the behavior of water molecules surrounding the side chains of a NIPAM pentamer in response to temperature changes (273−353 K range) near its experimental LCST (305 K). Results suggest a strong inverse correlation of temperature with water density and hydrophobic hydration character of the first coordination shell around the isopropyl groups. Integrity of the first and second coordination shells is further characterized by polygon ring analysis. Predominant occurrence of pentagons suggests clathrate-like behavior of both shells at lower temperatures. This predominance is eventually overtaken by 4membered rings as temperature is increased beyond 303 and 293 K for the first and second coordination shells, respectively, losing their clathrate-like property. It is surmised that this temperature-dependent stability of the coordination shells is one of the important factors that controls the reversible swell-collapse mechanism of PNIPAM hydrogels.
Bio-based flame retardant (FR) resins typically exhibit diminished mechanical properties compared with their petroleum-based counterparts. Recent experiments identified a promising FR phosphorus-bearing vanillin-based epoxy resin, EP2, that exhibited superior thermomechanical properties compared to that of petroleum-based diglycidyl ether of bisphenol A. However, the structure/property relationships of such phosphorus-containing bio-based resins are relatively under-explored and cannot be resolved via experiments alone. Here, molecular simulations are used to explore these relationships for a resin comprising EP2 cured with 4,4-diaminodiphenylmethane. The predicted thermomechanical properties are consistent with experimental observations, and critically, the structural analysis reveals the importance of the pendant phosphite group in the monomer as central to maintaining extensive hydrogen-bonding networks, giving rise to the excellent Young's modulus. This work provides the foundation for knowledge-based strategies to systematically improve the structure/property relationships in FR bio-based epoxy resins.
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