Spinodal Decomposition, Nucleation Growth, and Arrested Macrophase Separation of Physical Gels of Atactic Poly(N-isopropylacrylamide) in Water

Aqueoussolutions of atactic poly(N-isopropylacrylamide)(a-PNIPAM) with 5-12 wt % concentrations exhibit physical gelationat temperatures lower than the binodal temperature (T (b)). This phenomenon is clearly demonstrated by the hysteresisof dynamic storage modulus G & PRIME;(T) during repeated heating and subsequent cooling protocol at T < T (b) because of the differencein the T-dependent rate of interchain associationand dissociation. Herein, we performed a temperature-sweep test ata low heating rate and at a low frequency to obtain dynamic rheologicalcurves of the loss modulus and storage modulus. From these curves,two transition temperatures are derived, namely, the macroscopic geltemperature (T (gel)) and the temperatureat which G & PRIME; starts to rise (T (1)) owing to the enhanced interchain associations developingthe self-similarly branched structures of associated chains. On heating,the one-phase a-PNIPAM solution undergoes pronounced concentrationfluctuations at a temperature above T (1) to enter the pregel regime followed by the formation of a macroscopicgel network at T (gel). The temperature sequencefor the phase transitions is T (1) + 2 & DEG;C approximately equal to T (gel) < T (b), independent of concentration (& phi;(w)). Thespinodal temperatures of the a-PNIPAM solution (T (s,sol)) and a-PNIPAM gel (T (s,gel)) are obtained from small-angle X-ray scattering based on a theoreticalmodel derived for the associated chains to account for the presenceof gel junctions. The derived T (s,gel) isslightly higher than T (s,sol); the gap isca. 0.9-1.6 & DEG;C depending on & phi;(w). The derived T (s,sol) and T (s,gel) marginally decrease with increased & phi;(w). Using time-resolvedlight scattering (TRLS), the nucleation and growth (NG) and spinodaldecomposition (SD) of the gel are validated at temperatures above T (b). The growth of the SD structure is dramaticallyhindered (or pinned down) within a very short period (<5 s). Wefind that Cahn's linear theory for SD phase separation is inapplicableto derive T (s,gel) in the present dynamicallyasymmetric system. On the basis of phase transformation from the pinnedSD structure into the pinned NG structure, T (s,gel) could be alternatively determined from the steady-statelight-scattering profiles as a function of T obtainedfrom TRLS with a stepwise cooling protocol. The estimated T (s,gel) in this manner agrees with that derivedfrom rigorous analyses of static SAXS as a function of temperature.On the basis of the present results, a phase diagram of semidilutea-PNIPAM/water solutions is constructed. At temperatures above T (b), the arrest of the macroscopic phase separationthrough NG and SD is found to develop pinned NG and SD structure dependingon T and is thermoreversible. We propose that thistemperature-dependent pinned NG and SD underlie the basic principlefor the arrested macrophase separation and the thermoreversible changein the phase-separated structures of the physical gel as discussedin the text.