Structural and physical studies of PVC/PVDF doped Nano lithium salt for electrochemical applications

AbstractPolymer blend of poly (vinyl chloride) and poly(vinylidene fluoride) of nominal composition (30PVC/70PVDF) wt:wt were prepared in the form of thin films using casting technique. Samples of the same composition doped with gradient concentration of nano lithium salt (LTO) were prepared and studied. Proposed filler was characterized using Fourier transform infrared spectroscopy (FTIR), UV/vis. optical absorption, X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Electron diffraction (ED).  Obtained data approve the crystalline nano structure of filler with a cubic structure of average size (25-30 nm). Prepared nano composites were then investigated using different spectroscopic methods. XRD reveals the amorphous nature of the base polymer blend with tendency for increase in crystallinity with increasing the content of lithium salt. FTIR shows a preservation of the main vibrational spectral bands in their position with small variation in the area and intensity of some spectral bands related to the interaction between polymer and filler.

PVDF is a semi-crystalline polymer that contain more than two crystalline forms. β-phase that encloses transplanar (TT) chains responsible for piezoelectricity, while α-phase composed of random combination of trans-gauche isomers [7]. PVDF have a wide range of technological and biomedical applications in addition to uses in photo recording and microwave modulation due to their chemical resistance, high piezoelectricity, mechanical strength and process ability [8].
Polyvinyl chloride (PVC) is an amorphous linear thermoplastic polymer that have enormous industrial and commercial applications results from their unique properties and their accessibility to basic raw materials [9].
Power density and high energy are combination characterized Li ion batteries categorize them as unique technology for portable electronics that reduce green-house gas emission [10]. Lithium-metal alloys, e.g., lithium-tin alloys, or lithium salt alloy are among the most promising negative electrode materials to replace the conventional carbonbased compounds in advanced lithium ion batteries [11]. A promising anode material of high-rate lithium-ion batteries among the LTO phases is the spinal Li4Ti5O12 and it was studied by several researchers in the lase decade [12].
Present work aims to introduce a new material based in polymeric matrices and containing lithium salt in the nano-scale suitable of lithium batteries application combined with investigation of the structural changes results for investigate structural and physical characteristic variations results from the process of preparation of nanocomposite and role of each constituent.

Materials
High molecular weight PVC and PVDF with average molecular weights 150,000 was supplied by Fluka and used as received without further purification. Lithium titanium oxide Li4Ti5O12 (LTO) was purchased from Aldrich, USA, tetrahydrofuran (THF) was purchased from Fisher Chemicals UK. 70:30 weight/weight ratio of PVDF/PVC was dissolved in THF separately. Obtained solutions were then mixed and stirred vigorously and continuously together for 24 h until a clear homogenous liquid was achieved. Pre-determined concentrations of LTO were added to polymer blend and sonicated to about 3h to ensure uniform distribution of LTO. Obtained mixture was then poured into glass Petri dishes and kept at embient temperature to evaporate solvent. Dry films peeled from the glass dishes and stored in a vacuum dissector until use.

Methods
FTIR spectra were measured in the range 4000-400 cm -1 using Nicolet is10 single beam spectrometer to identify the main vibrational bands and the nature of complexation behavior between polymeric matrix constituents and that between polymeric matrix and filler. XRD spectra were recorded using PANalytical X'Pert PRO XRD system using Cu Kα radiation (where, λ= 1.540Ȧ, the tube operated at 30 kV, the Bragg's angle (2θ) in the range of (5-70°).
The SEM micrographs of thin films were recorded using JXA-840 SEM (JEOL, Japan) to study the surface topography of the prepared films while TEM supported by electron diffraction was used to determine LTO filler size and to examine crystallinity of the filler.

Characterization of LTO nanoparticles
FTIR Spectra of dopant LTO in the spectral range 4000-400 cm -1 were recorded using KBr tablet technique (100:1) KBr to sample in the form of transparent disk pressed in hydrolic press (5 ton/cm 2) . Figure      Where A  and A  represent the relative areas corosponding to  and  phases shown and indexed in Figure (4) of PVDF is the sum of relative area corresponding to  and  phases respectively as reported by Abdelghany et al. [13]. Noticeable gradual change in the diffraction pattern of tested samples indicates not only a homogenous distribution of LTO dopant within the polymeric matrix but also indicate a complete dissolution of salt crystalline phase within the matrix without any evidence for aggregation or phase separation.

FTIR absorption spectra
FTIR spectroscopy is a powerful nondestructive tool for structural investigation of prepared thin films of virgin and blended polymeric matrices. Figure (7) shows FTIR spectra of base constituent polymeric films combined with their blend films.

Figure (7) FTIR spectra of pristine polymers and their blend
FTIR spectra of PVC consists mainly of set of peaks in the spectral range 3000-2800 cm -1 attributed the C-H stretching vibrations (symmetric stretch at higher wavenumber and asymmetric stretching at lower one) followed by a peak around 1400 cm -1 due to aliphatic C-H bonding. The peak at about 1250 cm -1 assigned to the bending C-H vibration near Cl, C-C stretching bond vibration of the backbone chain structure located at 1100-1000 cm -1 . Finally, the vibration correspond to the C-Cl gauche bond was observed in the range 650-600 cm -1 [14]. Spectrum of PVDF includes C-H symmetrical stretching vibration observed in the range 3150 -3030 cm -1 . A series of weak combination and overtone bands appears and the pattern of the overtone bands reflects the substitution pattern of the benzene ring in the spectral range 2000 -1670 cm -1 region. α-phase of PVDF is evidenced particularly by the peaks 435, 486, 603 and 768 cm -1 , while the β-phase is characterized by the peaks at 510 and 838 cm −1 [15]. The peak at 510 cm −1 , belonging to the β-phase, are assigned to CF2 bending and wagging modes. The band at 836 cm -1 is assigned to CH2 bending deformation, belonging to the β-phase [8]. Table (3) list the FTIR band position of constituent pristine polymer and in combination with their assignment previously discussed by different authors [8,14,15]. The presence of the low intensity broad band at about 1200-1300 cm -1 assigned to the hydrogen bonding between C-H groups from PVC to CF2 groups of PVDF. In addition, a noticeable change in band intensity and position in comparison of the base poly blend sample were observed especially, for the vibrational bands located at 849, 879 and 1405 cm -1 assigned for the high polarizable γ-phase of PVDF. Mentioned band observed to growth in intensity with increasing content of LTO. The change in bands shape and position of the peak around 1400 may be assigned to interaction, complexation and dissolution of the salt crystalline phase in polymeric matrices results from progressive addition of LTO [16].

Conclusions
Samples of PVDF/PVC polymer blend in addition to other samples that contain gradual increasing content of LTO salt up to 40 wt% were successfully prepared. FTIR of dopant materials shows the presence of characteristic broad band due to hydroxyl groups (Ti-OH) in addition strong band at 668 cm -1 orgininating from Ti-O bonds and TiO6 octahedron. XRD approve the crystalline nature of used dopant and calculated value of crystallite size was found to be ranged (25-30 nm) and in agreement with data obtained from TEM and electron diffraction.
XRD of prepared nanocomposite reveals a decrease in crystallinity indicating complete dissolution, miscibility or gradient dispersion of LTO phase in the polymeric matrix phase without any evidence for aggregation or phase separation combined with a decrease of -phase fraction F() fitted to a quadratic equation with three degree of freedom.
Low intensity FTIR absorption band at 1200-1300 cm -1 assigned to hydrogen bonding between C-H groups from PVC to CF2 groups of PVDF. The change in bands shape and position of the peak around 1400 was attributes to the interaction, complexation and dissolution of the salt crystalline phase in polymeric matrices results from progressive addition of LTO.