Design of acrylamide-based thermoresponsive copolymer with potential capability for physical network formation in water: a molecular dynamics study

Abstract

In this work, a new smart copolymer of poly(N-isopropylacrylamide)-b-polyacrylamide-b-poly(N-isopropylacrylamide) (PNIPAm-b-PAM-b-PNIPAm) was designed as an efficient thermo-sensitive polymer with a potential for formation a physical network structure above a certain temperature in water. To design the desired copolymer, molecular behaviors of the copolymer blocks were investigated in water at different blocks fractions and temperatures via atomistic simulation. The blocks were evaluated from the interactional aspects via estimating the electrostatic and van der Waals contributions of their interaction energies with the media to consider the amount of hydrophilicity variation of the blocks with temperature variation. To find the origin of changes in blocks hydrophilicity, the structure and orientation of interfacial water were determined via calculating coordination number and order parameters of the water molecules in the first and second hydration shell of the blocks. In this regard, a novel parameter based on water ordering and hydration level was presented. The PAM block in the copolymer with shorter PNIPAm blocks, particularly 8 mol% (percentage by mole) PNIPAm, enforced the thermoresponsive end blocks to behave like a water-soluble polymer above their lower critical solution temperature. In contrast, increasing the PNIPAm content to 32 mol% caused an interactive competition between the blocks. Finally, the copolymer with the PNIPAm/PAM=0.19 was found as the most efficient copolymer composition for the mentioned goal via using the analysis of variance (ANOVA) of the calculated responses.