Exploring the Interphase Structure of Poly(vinyl alcohol) during Sol-Gel Transition Using Selective Probes with Varied Molecular Sizes

The crystallization-driven gelation kinetics of semidilute poly(vinyl alcohol) (PVA) aqueous solution at 20 degrees C was investigated by time-resolved time-domain (TD) NMR, i.e., magic sandwich echo free induction decay (MSE-FID), single-evolution-time proton double-quantum (SET-DQ), and pulsed-field-gradient (PFG) NMR. The crystallization-driven gelation of PVA (c = 0.28-0.47 g/mL >> c* = 0.037 g/mL) is first confirmed by the linear correlation between PVA crystallinity index obtained by MSE-FID and the normalized SET-DQ intensity. The three-phase structure, namely, crystalline, interphase, and mobile amorphous domains, was further elucidated by the combination of MSE-FID and PFG-NMR. The key structural parameter of the percolation network, the correlation length xi, or the mesh size, is obtained by PFG-NMR. To probe the interphase structure after PVA crystallizes, two molecules with different hydrodynamic radii R-H were used. Fitted by Phillies' model, the slopes of the reference curves between xi and c on a double log scale are -0.85 and -0.93 for probe molecule glycerol and solvent water, respectively, corresponding to the Flory exponent. values of 0.55 and 0.52. This suggests that the present temperature is close to the theta temperature. The concentration/solid content of time-dependent mesh size xi during gelation is further corrected by the fractions of crystalline and interphase domains obtained from in situ MSE-FID. The corrected concentration/solid content considering the crystallinity index converges in the master c-xi curve for water, whereas for glycerol, the fractions of both crystalline and interphase domains need to be considered (except for the highest concentration c = 0.47 g/mL). Such a phenomenon is attributed to the selective plasticization of water and glycerol with different R-H: the interphase is selectively plasticized by solely water molecule, whereas the mobile amorphous domain is plasticized by both molecules. The above methodology also provides a new strategy to access the network structure of the interphase domain.