Che-Min Chou

Solvent effects on aggregation behavior of polyvinyl alcohol solutions

Abstract The present study elucidates the effects of solvent quality and polymer concentration on the aggregation behavior of polyvinyl alcohol (PVA) solutions from the dilute solution to semi-dilute solution regions. Combination of the results in static and dynamic light scattering indicates that in dilute solution the thermodynamic driving forces primarily dominate the dynamic behavior of PVA/N-methylpyrrolidone (PVA/NMP) solution. In contrast with PVA/water solution, the hydrodynamic interaction would dominate the dynamic behavior of the solution. The concentration dependence on the dynamic behavior of semi-dilute solution has also been studied through DLS measurement. For PVA/water solutions, the hydrodynamic correlation length of fast mode has a concentration dependence given by ξD=[η]C−0.42, and the value of exponent is close to the predicted value from scaling theory in the marginal solvent. On the other hand, for PVA/NMP solutions the value of exponent is close to the predicted value from the scaling theory in good solvent limit: −0.75. However, contrary to the expectation, the gelation behavior could not occur in PVA/water solutions at the same condition of gelation in PVA/NMP solution even though the affinity of water to PVA is much lower than that of NMP. In this work, we considered that the PVA/NMP solution might possess a character that could form a homogeneous network structure. The intermolecular associations must play a dominant role as soon as the chains start to overlap in PVA/NMP solution. On the other hand, combining the DLS result with our previous work, the gelation of PVA/NMP solutions is considered due to complex formation in the transient network junctions, i.e. the formation of molecular complex is crucial to the physical gelation in a good solvent system.

Phase separation and gelation behaviors in poly(vinylidene fluoride)/tetra(ethylene glycol) dimethyl ether solutions

Abstract In this work, the phase separation and gelation behaviors in poly(vinylidene fluoride)/tetra(ethylene glycol) dimethyl ether (PVDF/TG) solutions were studied through the time-resolved light scattering and the gelation kinetic analyses. Combination of the phase diagram and the results of gelation kinetics shows that the gelation behaviors in PVDF/TG solutions could be separated into three diverse regions. According to the initial thermodynamic conditions, one can divide the gelation processes into two major parallel reactions. At the gelation temperature above the spinodal temperature (T>Ts), the gelation should be virtually independent of phase separation and then the gelation should occur in terms of bimolecular association and pure crystalline nucleation. When T

Light scattering studies on aggregation behavior of polyvinyl chloride/dioxane solutions

In this work, the aggregation behavior of polyvinyl chloride/dioxane (PVC/DOA) solution was discussed through light scattering and viscometric measurements. From the static light scattering results, PVC/DOA solution has a negative value of the second virial coefficient, A2 value (ca. 23:65 £ 10 24 cm 3 mol g 22 U at 308C, implying that a repulsive force appeared between PVC chains and DOA. The dynamic light scattering results indicate that the dynamic behavior of PVC solution could be classified into three regions by increasing the concentration of PVC. In the infinite dilution region,âhaC , 1; the single relaxation mode is considered related to the translational diffusion of the individual PVC coil. In the hydrodynamic screen region, 1 ,âhaC , 4; the relaxation time distribution was divided into two major relaxation modes; i.e. the fast mode resembled that of the individual PVC coil in the dilute solution, and the slow mode is considered related to the cluster formed from the aggregation of several individual coils. At higher concentrations, âhaC . 4; the polymer coils start to overlap and entangle further, then the third relaxation mode due to the transient gel network originated from the aggregation of the clusters in the concentrated solution was observed. On the other hand, the relative amplitudes of middle and slow modes for semi-dilute solutions decrease with increasing temperature. At a temperature of about 508C, only a broad relaxation mode was observed indicating that intermolecular aggregation is disintegrated with increasing temperature, but the intramolecular aggregation of an isolated chain still exists due to the poor affinity of DOA to PVC chains in this temperature region. The poor solubility of the solvent used should be more favorable to the intramolecular aggregation of PVC chains, resulting in comparatively complex dynamic behaviors in semi-dilute solutions. q 2000 Elsevier Science Ltd. All rights reserved.