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

Abstract

The crystallization-driven gelation kinetics of semidilute poly(vinyl alcohol) (PVA) aqueous solution at 20 °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-fieldgradient (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 ξ, 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 ξ 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 ν values of 0.55 and 0.52. This suggests that the present temperature is close to the θ temperature. The concentration/solid content of time-dependent mesh size ξ 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-ξ 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.