Indian Journal of Pure & Applied Physics Vol. 43, November 2005, pp. 821-827 Vibrational spectroscopic characterization of form II poly(vinylidene fluoride) P Nallasamy Department of Physics, Bharathidasan Govt. College for Women, Pondicherry 605 003 and S Mohan Raman School of Physics, Pondicherry University, Pondicherry 605 014 Received 28 December 2004; revised 20 June 2005; accepted 8 September 2005 The Fourier transform Raman (FT-Raman) and infrared absorption (FTIR) spectra of form II poly(vinylidene fluoride) have been recorded and analysed. A complete spectral analysis, assignments to observed bands, normal coordinate analysis and discussion of the spectra are presented. The computed spectrum is in excellent agreement with experiment. The poten- tial energy distribution (PED) is evaluated from respective potential constants to analyse the purity of the modes. Keywords: Infrared and Raman spectra; Normal coordinate analysis, Poly(vinylidene fluoride), Vibrational spectroscopy IPC Code: G01J3/00 1 Introduction carried out a vibrational analysis of two forms (α and Vibrational spectroscopy has significant contribu- β) of PVDF using Urey-Bradley type force field. tions towards the studies of structure and physico– Boerio and Koenig9 reported the Raman spectra of chemical properties of crystals and molecular sys- form II PVDF and observed some unique bands that tems1-3. Raman spectroscopy—among spectroscopic are not observed in the IR spectra. Cortili and Zerbi10 techniques that provided detailed information about discussed the chain conformation of PVDF by analys- molecular structure—is the most appropriate tool be- ing the infrared spectra of form I and form II. Infrared cause it is simple, quick and powerful to perform the and Raman spectra of form I PVDF have been studied vibrational assignment and to elucidate the structure extensively by Boerio and Koenig11, Cessac and 12 13 14 and conformation of the molecule. In the present Curro , Lauchlan and Rabolt , Tashiro et al. and study, both the techniques—infrared and Raman, have Armengaud et al.15. Molecular vibrations of three been applied to obtain the maximum amount of in- crystal forms of PVDF were investigated by Kobaya- formation from the vibrational spectra of the title shi et al.16 on the basis of group theoretical considera- compound. tions and normal coordinate analysis. A Fourier trans- form infrared spectroscopic study of form III PVDF The substitution of fluorine for hydrogen in organic 17 polymers resulted in materials with remarkable char- was reported by Bachmann et al. They have also acteristics4. Poly(vinylidene fluoride) (PVDF) is an proposed possible models for the crystal structure of important polymer, exhibiting piezoelectric, pyroelec- the three forms of PVDF and compared the spectra of 5 form I and form II with the spectra of form III. Mohan tric and ferroelectric properties . X-ray diffraction 18 studies identified four different polymorphs for PVDF et al. carried out a normal coordinate analysis for (α, β, γ and δ)6. Natta et al.7 proposed the third form two forms of PVDF employing general quadratic va- lence force field and verified the vibrational assign- for PVDF. It was revealed by IR and XRD studies 19 that the PVDF homopolymer predominantly consists ments. Hsu et al. used the FTIR method to study the of form II crystalline structure. crystallization behaviour of PVDF from the melt in the presence of a weak electric field. More recently, For the last two decades, PVDF has been the centre 6 8 Kim et al. examined the morphology, crystalline of attention of several publications. Enomoto et al. structure, thermal, mechanical and electrical proper- —————— ties of PVDF using FTIR spectra and X-ray diffrac- Email: [email protected] tion techniques. In this paper, we report for the first 822 INDIAN J PURE & APPL PHYS, VOL 43, NOVEMBER 2005 time, a complete vibrational assignments for the form The crystal lattices of forms I and III differ from each II PVDF on the basis of FT-Raman and FTIR spectra other in the relative height of the two chains in the coupled with the normal coordinate calculations. unit cell. The form I (β-phase) which is planar is known to have one monomer in a repeat distance of 2 Experimental Details 2.57 Å. The form II (α-phase) which is non-planar The infrared spectra of solid poly(vinylidene fluo- has two monomers in the repeat unit with a repeat ride) is recorded employing a Brucker IFS 66V FTIR distance of about 4.64 Å. The repeat distance in form spectrometer in the range 4000 – 200 cm–1. The scan- III PVDF is 9.18 Å. The molecule assumes planar ning speed was held at 30 cm–1 min–1 with a spectral zigzag conformation in forms I and III and has a width 20 cm–1. Raman spectra of PVDF is also re- TGTG’ conformation in form II. corded on the same instrument with FRA 106 Raman The PVDF compounds have many applications and module equipped with Nd:YAG laser source operat- they possess rich and versatile stereochemistry. The ing at 1.06μm line with 200mw power. The frequen- optically active normal modes of form I are classified –1 cies for all sharp bands were accurate to ± 1 cm . The under the point group C2v and the vibrations of form observed infrared and Raman spectra are given in III are distributed under the point group C2. The IR Figs 1 and 2. band around 1250 cm–1 and of the Raman scattering around 810 cm–1 show a remarkable difference be- 3 Theoretical Considerations tween forms I and III. The corresponding peaks ap- The structures of various forms of PVDF are pear at similar frequencies, but the relative intensities shown in Figs 3 and 4. The repeat unit is –CH2–CF2–. are quite different from each other in the two crystal Fig. 1 – FTIR Spectrum of poly(vinylidene fluoride) Fig. 2 – FTR Spectrum of poly(vinylidene fluoride) NALLASAMY & MOHAN: CHARACTERIZATION OF FORM II POLY(VINYLIDENE FLUORIDE) 823 Fig. 3—Experimental and theoretical SPR reflectance curves for surface plasmon modes excited along the Ag film (545Å)— water interface and form II (Ref. 8). The CH2 bending mode of the form I appears at 1431 cm–1 in IR and this band is split into 1456 cm–1 and 1420 cm–1 bands in the spectrum of form II. The strong absorption at 1235 cm–1 that is useful in characterizing form III is –1 assigned to CF2 asymmetric stretching. The 976 cm IR band in form II is absent in form III and form I (Ref. 17). The vibrational spectrum of form II, PVDF has been analysed assuming C2h point group symmetry for the molecule and the fundamental modes are classi- fied as: Fig. 4—Experimental SPR reflectance curves for surface plasmon modes excited along the interface of Ag film (545Å) and sugar solutions of different concentrations Γvib = 16 Ag + 16 Bg + 16 Au + 16 Bu –1 forms. The 1230 cm band in form III is stronger Ag, Bg modes are inactive in IR while Au, Bu modes –1 than the 1273 cm band which is assigned to the A1 are inactive in Raman. fundamental in form I. Form III gives higher fre- quency for the librational lattice mode as well as the 3.1 Normal coordinate analysis CF2 deformation as compared units form I. This is due The normal coordinate analysis for one finite re- 16 to stronger intermolecular forces in form III then peating unit of form II PVDF has been carried out for in form I. The IR bands due to bending and the purpose of the complete assignment of the vibra- wagging modes of CF2 group in form III appear at tional frequencies. Several X-ray diffraction studies frequencies higher than those in form I. The absorp- on the structure of the polymer have formed the basis –1 tion at 442 cm in form I is assigned to the CF2 rock- for this analysis. The simple general valence force ing mode. The corresponding band in form III is field has been adopted. The initial set of force con- rather obscure. stants is taken from polyethylene and polytetrafluoro- The Rule of mutual exclusion is realised for the ethylene and small alterations are made in few inter- normal modes of form II since the two chains in the action constants to obtain a close fit between the ob- unit cell correlate with each other by the operation of served and calculated frequencies. The values of bond the centre of symmetry. The IR band at 1150 cm–1 is lengths and bond angles have been taken from Sut- unique to the form II and is well separated from the ton’s table. Wilson’s F-G matrix method is used for band at 1180 cm–1 which is common to both form I normal coordinate calculations. 824 INDIAN J PURE & APPL PHYS, VOL 43, NOVEMBER 2005 3.2 Potential energy distribution the normal coordinate calculations, the band at To analyze the purity of the modes, the potential 1200 cm–1 is determined by the stretching vibration of energy distributions associated with each normal the CF2 bond (72% of PED) with a small contribution modes are calculated using the relation: from stretching of CC bond and deformation of HCH –1 2 angle. The medium band appears at 538 cm is Fii Lik PED = mainly composed of the CF2 deformation mode (81% λk of PED). The results of the normal coordinate analysis –1 predict a CF2 wagging mode at 608 cm . The Raman where Fii are the force constants defined by damped band observed at 610 cm–1 is assigned to the above least square technique, Lik the normalised amplitude mode in close agreement with the earlier literature of the associated element (i,k) and λk, the k-th eigen values22.