Light-scattering study of coil-to-globule transition of a poly(N-isopropylacrylamide) chain in deuterated water

Using a nearly monodisperse (M-w/M-n < 1.1) high molar mass (1.3 x 10(7) g/mol) poly(N-isopropylacrylamide) (PNIPAM), we studied the conformation change of individual PNIPAM chains between an extended random coil and a fully collapsed thermodynamically stable single chain globule in an extremely dilute D2O solution (similar to 6.3 x 10(-7) g/mL) by a combination of static and dynamic laser light scattering (LLS). In comparison with PNIPAM in H2O, the Theta-temperature of PNIPAM in D2O is slightly higher, shifting from 30.6 to 32.1 degrees C. Moreover, the coil-to-globule transition in D2O showed no molten globular state, different from PNIPAM in H2O. However, the molten globular state exists in the globule-to-coil transition in D2O. The size of the PNIPAM chains near the Theta-temperature in the globule-to-coil transition is smaller than that in the coil-to-globule transition; i.e., there exists a hysteresis, similar to PNIPAM in H2O. The hysteresis can be attributed to the intrachain interaction formed in the globular state. Our results indicate that the PNIPAM chains are stiffer in D2O than in H2O.