Dynamic viscoelasticity study of the phase transition of poly(N-isopropylacrylamide)

Communication: The phase transition of poly(N-isopropylacrylamide) (poly(NI PAM)) solutions was characterized by a dynamic fluid rheometer. Two critica l points were observed below the tower critical solution temperature (LCST; approximate to 31.5 degrees C): (1) a critical point resembling sol-gel tr ansition occurs at approximate to 28.5 degrees C, not far beyond the onset of elasticity (approximate to 28.2 degrees C), and (2) a second critical po int; appears at approximate to 30.4 degrees C corresponding to the reported Flory temperature (Theta). These findings suggest that intermolecular asso ciation and coil-globule transition of poly(NIPAM) occur below the LCST, Th e fractal dimension of association of poly(NIPAM) chains which was calculat ed based upon the rheological method (approximate to 1.49) is close to that determined by static light scattering.