Characterization of Linear Poly(N-isopropylacrylamide) and Cloud Points in its Aqueous Solutions

The second virial coefficient A2 and intrinsic viscosity [η] were determined in methanol at 25.0 °C for poly(N-isopropylacrylamide) (PNIPA) samples synthesized by living anionic polymerization in the range of weight-average molecular weight Mw from 4.91 × 103 to 7.23 × 104, which are called L samples, and also for those by radical polymerization in tert-butanol and benzene by the use of azobis(isobutyronitrile) as an initiator in the range of Mw from 1.23 × 104 to 7.83 × 104, which are called T and B samples, respectively. It is found for both A2 and [η] that their values for the three kinds of samples agree well with each other in the range of Mw ≲ 3 × 104 but deviate from each other as Mw is increased from 3 × 104, the value for the L sample being the largest and that for the B sample the smallest. The result is consistent with the fact that the average chain dimension is the largest for the L sample having no branch point and the smallest for the B sample having the largest number of branch points. From a simultaneous analysis of A2 and [η] for the L samples on the basis of the Kratky-Porod wormlike chain with excluded volume, the stiffness parameter λ-1 is estimated to be 18 Å, which is almost the same as those determined for typical flexible polymers. For the L samples, the cloud point was also determined in their aqueous solutions in the range of the weight fraction w of PNIPA from ca. 0.5 to ca. 10%. It is found that the cloud point in the range of w≳ 2% decreases from ca. 32 °C to ca. 18 °C as Mw is decreased from 7.23 × 104 to 5.47 × 103. Such behavior may be regarded as arising from effects of hydrophobic chain end groups of the L samples.