Parameters Affecting the Lower Critical Solution Temperature of Linear and Crosslinked Poly(N‐ethylacrylamide) in Aqueous Media

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Abstract
A series of linear poly(N‐ethylacrylamide) (PEA) samples varying in molar mass has been prepared by free radical polymerization in the absence and presence of a chain transfer agent and a hydrogel of PEA has been prepared using N,N‐methylenebisacrylamide (BIS) as crosslinker. The lower critical solution temperatures (LCST) of the linear polymers in water and aqueous media were determined turbidimetrically as a function of molecular weight, concentration, heating/cooling rate, and concentrations of KCl and anionic surfactant sodium dodecyl sulfate (SDS) in the aqueous solution. The corresponding LCST for the hydrogel was determined from the gravimetric swelling ratios (r). In pure water the values of LCST for linear polymer and hydrogel are 73 °C and 62 °C, respectively. The LCST of linear PEA increases with decreasing molecular weight. The swelling ratio for gels and the LCST for solutions and gels increased with the inclusion of SDS into water. The opposite effects prevailed on inclusion of KCl into water or incorporation of crosslinker. Additionally, the rates of heating/cooling play a significant role in the measured value of LCST. Hence the swelling ratio of hydrogel or the LCST can be adjusted via 1) addition of either SDS or KCl into water; 2) use of different molecular weight samples; 3) incorporation of crosslinker into polymer chain. magnified image

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