Poly (vinylidene fluoride), PVDF, has different crystalline phases, such as alfa, beta, gamma, delta, epsilon, depending on film processing methods. Among them, alfa, beta, and gamma are the major phases, particularly the beta phase which determines PVDF’s pyro-, piezo-, and ferroelectric properties.
The crystalline phases can be determined by XRD, DSC, and FTIR. FTIR method is useful when the PVDF thin films are under study, since XRD might not be sensitive in this case. DSC can be used to determine the total crystalline degree.
Figure 1 shows the major difference in molecular chain conformations among the major three crystalline phases, alfa, beta, and gamma.
Based on the FTIR spectra (reference 2,3), characteristic IR absorbing peaks are allocated to different phases. Then based on Beer’s law, the concentration of different phases in the film can be calculated by the following equations (reference 4):
In the above calculation, the 840cm-1 peak is allocated to pure beta phase’s absorbance. However, there do have different opinions that the peak is related to both beta and gamma phases. Another method was proposed to calculate the fractions of different phases.
As shown in Figure 1, 762 and 1275 peak are allocated to the alfa and beta phase, respectively. 835 peak is considered as a combined effects of beta, gamma, and amorphous alfa phase. By different film deposition, annealing, and stretching methods, PVDF films with different phases can be prepared and then the absorbance coefficients will be measured or calculated by the above equations. The film thickness can also be calculated by the absorbance of 1070 cm-1 peak by the above equation. (reference 2).
Reference:
- J. Mater. Chem. A, 2017, 5, 3091
- Journal of Applied Polymer Science, Vol. 89, 1093–1100 (2003)
- Prog. Polym. Sci. 2014, 39, 683-706
- Journal of Polymer Science: Part B: Polymer Physics, Vol. 32,859-870 (1994)