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

The second virial coefficient A 2 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 M w from 4.91 × 10 3 to 7.23 × 10 4 , 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 M w from 1.23 × 10 4 to 7.83 × 10 4 , which are called T and B samples, respectively. It is found for both A 2 and [ η ] that their values for the three kinds of samples agree well with each other in the range of M w ≲ 3 × 10 4 but deviate from each other as M w is increased from 3 × 10 4 , 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 A 2 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 M w is decreased from 7.23 × 10 4 to 5.47 × 10 3 . Such behavior may be regarded as arising from effects of hydrophobic chain end groups of the L samples.