Molecular chain properties of poly(N-isopropyl acrylamide)

Abstract

A series of poly(N-isopropyl acrylamide) (PNIPAM) samples with molecular weight ranging from 2.23 × 104 to 130 × 104 and molecular weight distributionMw/Mn ≤1. 28 were obtained by free radical polymerization and repeat precipitation fractionation. The molecular weightMw, second virial coefficientA2 as well as the mean-square-root radius of gyration for PNIPAM samples in tetrahydrofuran (THF) were determined by light scattering, and the relations were estimated atA2 ∞ - 0.25 and S2 1/2 = 1.56 × 10-9 Mw 0.56. The intrinsic viscosity for THF solution and methanol solution of PNIPAM samples was measured and the Mark-Houwink equations were obtained as [η] =6.90x 10-5 M0. 73(THF solution) and [η] = 1.07 × 10-4 M0.71(methanol solution). The above results indicate that both THF and methanol are good solvents for PNIPAM. The limit characteristic ratioC ∞ for PNIPAM in the two solutions was determined to be 10.6 by using Kurata-Stockmayer equation, indicating that the flexibility of PNIPAM chain is close to that of polystyrene. The intrinsic viscosity for the aqueous solution of PNIPAM was measured at 25-30.5°C. The result demonstrates that PNIPAM chain begins to shrink at 25°C, and from about 30°C, onward the molecular chain shrinks much more sharply till the solution becomes macroscopically unstable at 32°C.