Nondestructive Characterization of Li+ Ion-Doped Multifunctional Poly(vinylidene fluoride)-g-poly(dimethyl amino ethyl methacrylate) by Impedance Spectroscopy

Poly(vinylidene fluoride) (PVDF)-graft-poly-(dimethyl amino ethyl methacrylate) (PDMAEMA) (PD copolymer) is produced via atom transfer radical polymerization from PVDF solution in N-methyl-2-pyrrolidone. PD copolymer is doped with 1% and 5% (w/w) Li+ ion to produce PDLi1 and PDLi5 samples, respectively. In PD copolymer, the crystalline structure of PVDF changes from a polymorph to a mixture of a and beta polyrnorph, and it transforms completely to piezoelectric beta polymorph on doping with 1% (w/w) Li+ ion. The impedance behavior of PVDF changes on grafting, and that of the PD graft copolymer also changes with increasing Li+ ion dopant concentration. In the Nyquist plots, PVDF exhibits a straight line character, and a curvature has appeared in the PD graft copolymer; on doping the latter with Li+ ion (1% w/w), the curvature increases and a semicircle is completed on 5% Li+ doping. Fitting the data from the Z-view program, the Ohmic resistance of PDLi1 is found to be 78 M Omega having capacitance with constant phase element (CPE) = 1.38 nF while for the PDLi5 sample the resistance decreases to 16.1 M Omega with a small increase in CPE to 1.46 nF. The modulus plane plots for PDLi1 and PDLi5 samples also exhibit only one peak supporting the presence of only one equivalent resistance capacitance circuit with constant phase element in both PDLi1 and PDLi5 samples. Both the impedance and modulus vs frequency plots of PDLi1 and PDLi5 samples exhibit a single Debye peak suggesting isotropic nature of the samples. For PVDF and PDMAEMA, ac-conductivity increases linearly with angular frequency, but in the case of PDLi1 and PDLi5 samples, it remains at first invariant in the frequency range 1-10(2) Hz, and above 10(2) Hz, an increase in conductivity with frequency occurs obeying the double power law. In the temperature variation of conductivity, PVDF exhibits its typical insulating nature, and in the PD graft copolymer, the conductivity decreases with increase of temperature (metallic-like behavior) due to gradual breaking of supramolecular interaction. The temperature variation of ac-conductivity of the Li+-doped PD graft copolymer suggests that both the ionic and supramolecular contributions of conductivity operate; the former increases and the latter decreases with rise in temperature showing a maximum. The temperature-dependent FTIR spectra of PDLi1 and PDLi5 samples support the gradual breaking of supramolecular interactions with increase of temperature.