Long Two-Dimensional Folding Chain Structure Formation of Poly(vinylidene fluoride) in Solutions of a Polar Solvent, N-Methylpyrrolidone

The conformation and structure of poly(vinylidene fluoride) (PVDF; (-CH2CF2-)(n)) samples with wide-range weight average molar masses (M-w) ranging from 100 to 3300 kg mol(-1) dissolved in a highly polar solvent, N-methylpyrrolidone (NMP), were fully reconsidered based on the newly obtained results using small- to wide-angle X-ray scattering (S-WAXS), static light scattering (SLS), and viscometric techniques. The dependencies of scattering intensities on the scattering vector (q) observed in the S-WAXS experiments showed the characteristic local structure of long columnar particles with rather an extended rectangular or long two-dimensional folding chain structure in the solution. The q dependencies of the concentration reduced excess Rayleigh ratios for each PVDF sample determined in the SLS experiments provided the M-w dependence of the radius of gyration (R-g) approximately described in the manner R-g proportional to M-w(similar to 0.6) in the solution and were similar to those of flexible polymer chains dissolved in good solvents as reported previously. Using a rectangular columnar (parallelepiped) particle model, the global structure of PVDF molecules in the solution was analyzed, and the particle length (L) of each PVDF sample was carefully determined. The determined L value was not proportional to M-w or the weight average contour length (l) but demonstrated the relationship L proportional to M-w(similar to 0.6). Then, the relationship R-g similar to root 12 L was finally obtained over the entire examined M-w range, which implies that PVDF molecules behave as long rigid rectangular columnar particles in the solution. Consequently, the obtained SLS data revealed that the folding number, lL(-1), gradually increases from 5 to 24 with increasing M-w value. The M-w dependence of intrinsic viscosity ([eta]) also clearly demonstrated the rigid rectangular columnar particle behavior for all of the PVDF samples in the NMP solution. The obtained results in this study would be extremely important to improve the performance of commercial lithium ion batteries.