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 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. —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 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. The Raman spectrum is particularly rich in value of the dynamical matrix. The PED contributions 23 the lower frequency range . The CF2 rocking vibra- corresponding to each of the observed frequencies tion occurs at 417 cm–1 and 59% of the corresponding over 10% are alone listed in the present work. The potential energy distribution is taken by that motion. observed frequencies along with the assignments of This CF2 rocking vibration mixes with CF2 deforma- various modes of vibration, calculated frequencies tion (26%). The strong band at 287 cm–1 is assigned to and potential energy distribution are presented in CH2 twisting vibration. The CCC deformation vibra- Table 1. tions are assigned to the weak bands at 205 cm–1 and 220 cm–1. 4 Results and Discussion 4.2 Assignments for the infrared active modes 4.1 Assignments for Raman active modes In contrast to , Raman spec- The frequencies due to stretching in the CH2 group generally occur in the region 2800-3000 cm–1. The troscopy yields bands of low wave number as readily –1 –1 as bands of higher wave number20. If a molecule pos- infrared bands at 3027 cm and 2986 cm are as- sesses a centre of symmetry, the fundamentals are signed respectively to asymmetric and symmetric active in Raman only or in IR only but not in both. modes of CH2 group. As expected, these modes are pure stretching modes. The CH2 bending mode ex- Thus, Raman data provide more spectroscopic infor- –1 mation which cannot be provided by IR analysis pected in the 1450 cm region is identified with 1456 cm–1 band. The very strong band at 1405 cm–1 is as- alone. From the Table 1, it is evident that CH2 asym- signed to CH2 wagging mode. Of the two frequencies metric stretching, CH2 symmetric stretching and CH2 –1 –1 deformation are mainly from pure modes and they are 1405 cm and 976 cm , the higher one is assigned to assigned to the bands at 2986 cm–1, 2972 cm–1, 1432 the wagging mode and the lower one to the twisting –1 mode of the CH2 group. The CH2 rocking mode is cm , respectively. The CH2 wagging modes are weak –1 in Raman spectra of polymers. The very weak Raman identified with the strong infrared band at 845 cm . bands at 1406 cm–1 and 1384 cm–1 are assigned to In making the assignments of CC stretching vibra- CH2 wagging modes. The sharp and intense Raman tions, guidance has been taken from the results dis- 9 band at 799 cm–1 (calculated – 789 cm–1) is assigned cussed by previous authors . The CC stretching mode to CH2 rocking vibration. CC modes dominate in Ra- has little change in dipole moment and is expected to man spectrum. The intense Raman lines are due to the be weak in the infrared spectra. The very weak band –1 symmetric stretching of CC bonds21. In line with the at 1026 cm is associated with the CC symmetric –1 above conclusion, the strong band at 878 cm–1 is as- stretching vibration. The strong band at 1074 cm is signed to CC stretching vibration. assigned to CC asymmetric stretching mode which is in agreement with Enomoto et al.8. The distinct band The CF stretching motions have large change in di- –1 pole moment and small changes in polarizability9. at 880 cm is determined by the CC asymmetric Hence, they are expected to be weak in the Raman stretching vibration (78% of PED). spectra. The weak bands at 1149 cm–1 and 1190 The very strong band appearing at 1184 cm–1 is –1 cm are assigned to CF2 symmetric stretching modes. mainly composed of the CF2 symmetric stretching The medium Raman bands at 1297 cm–1 mode (87% of PED). The band at 1219 cm–1 (calcu- –1 –1 and 840 cm are mixtures of CF2 stretching, lated – 1212 cm ) represents coupling of the CC stretching and CH2 rocking. In accordance with CF2 asymmetric stretching mode and CC symmetric NALLASAMY & MOHAN: CHARACTERIZATION OF FORM II POLY(VINYLIDENE FLUORIDE) 825

Table 1 – Observed and calculated frequencies (cm–1) and potential energy distribution of form II poly(vinylidene fluoride)

Species Observed frequency Calculated Vibrational mode (PED%) (cm–1) and intensity frequency Raman Infrared (cm–1)

2986 VS 2981 CH2νas 96 νas(CH2) 2972 S 2970 CH2νs 94νs(CH2) 1432 S 1424 CH2δ 89δCH2 1406 VW 1401 CH2ω 74ω(CH2) + 10τ(CH2) 1297 M 1287 CF2νas 70νas(CF2) + 14νCC + 10ρ(CH2) Ag 1149 W 1139 CF2νs 87νs(CF2) +18νCC 1055 M 1050 CCνas 84νas(CC) + 10ν(CH) 976 W 981 CH2τ 49τ(CH2) + 30ωCH2 878 S 870 CCνs 74νs(CC) + 12νCF 840 M 832 CH2ρ 49ρ(CH ) + 22νCC + 24νCF 2 2 610 S 608 CF2ω 71ω(CF2) + 21τCH2 488 M 474 CF2δ 61δ(CF2) + 30ρCF2 417 S 411 CF2ρ 59ρ(CF2) + 26δCF2 287 S 274 CF2τ 52τ(CF2) +31ωCF2 205 W 198 CCC δ 49δ(CCC) + 21δCF2+ 24δCH 170 VW 161 CF τ 61τ + 21ω 2 (CF2) (CF2)

3020 M 3011 CH2νas 96νas(CH2) 2978 Sh 2967 CH2νs 93νs(CH2) 1440 VW 1434 CH2δ 87δ (CH2) 1384 VW 1382 CH2ω 74ω (CH2) + 15τ (CH2) 1200 M 1194 CF2νas 72νas(CF2) + 13ν(CC) + 10ρ(CH2) 1190 VW Sh 1186 CF2νs 81νs(CF2) + 15ρ (CH2) 1080 Sh 1070 CCνs 82νs(CC) + 15ν (CF) 937 VW 925 CH2τ 59τ (CH2) + 29ω(CH2) 885 Sh 876 CCνas 76νas(CC) + 21ν(CH) 799 VS 789 CH2ρ 59ρ(CH2) + 22δ (CH2) Bg 769 VW 754 CF2δ 61δ(CF2) + 24ρ(CF2) 538 M 531 CF2δ 81δ (CH2) + 11ρ(CH2) 369 W 364 CCC δ 54δ(CCC) + 23δ(CF) + 11δ(CH) 360 W 348 CF2τ 49τ(CF2) + 30ω(CF2) 220 VW 211 CCC δ 49δ(CCC) + 19δ(CH) + 12δ(CF) 177 VW 161 CF τ 62τ + 24ω 2 (CF2) (CF2) 3027 W 3024 CH2νas 94νas(CH2) 2986 W 2984 CH2νs 91ν s(CH2) 1456 W 1448 CH2δ 88δ (CH2) Au 1380 Sh 1371 CH2ω 70ω(CH2) + 11τ(CH2) 1219 Sh 1212 CF2νas 84ν as(CF2) + 12ν(CC) 1184 VS 1181 CF2νs 87νas(CF2) + 12ρ(CF2) 1026 VW 1022 CCνs 86ν s(CC). 946 VW 938 CH2τ 64τ(CH2) + 28ω(CH2) 907 Sh VVW 900 CH2τ 67τ(CH2) + 21ω(CH2) 880 VS 868 CCνas 78νas(CC) + 12ν(CH) + 10ν(CC) (Contd) 826 INDIAN J PURE & APPL PHYS, VOL 43, NOVEMBER 2005

Table 1 – Observed and calculated frequencies (cm–1) and potential energy distribution of form II poly(vinylidene fluoride)

Species Observed frequency Calculated Vibrational mode (PED%) (cm–1) and intensity frequency Raman Infrared (cm–1)

798 W 786 CH2ρ 71ρ(CH2) + 19δ(CH2) 766 S 761 CF2δ 69δ(CF2) + 22ρ(CH2) Au 531 W 528 CF2δ 74δ(CF2) + 21ρ(CH2) 390 VW 381 CF2ρ 64ρ(CF2) + 25δ(CH2) 357 W 352 CF2τ 58τ(CF2) + 17 ω(CF2) 215 W 206 CCC δ 69δ (CCC) + 24δ(CH)

3027 W 3021 CH2νas 95νas(CH2) 2986 W 2984 CH2νs 90νs(CH2) 1420 Sh 1411 CH2δ 87δ(CH2) 1405 VS 1394 CH2ω 72ω(CH2) + 18τ(CH2) 1280 M 1274 CF2νas 74νas(CF2) + 14νCC 1150 Sh 1148 CF2νs 76νs(CF2) + 18νCC 1074 S 1067 CCνas 79νas(CC) + 11νCC 976 M 972 CH2τ 64τ(CH2) + 26ω(CH2) Bu 856 Sh VVW 854 CCνs 51νs(CC) + 19νCC + 16νCF + 12δCH2 845 S 840 CH2ρ 59ρ(CH2) + 32δ(CH2) 612 S 604 CF2ω 72ω(CF2) + 19τ(CH2) 488 M 492 CF2δ 56δ(CF2) + 29ρ(CF2) 435 W 421 CF2ρ 60ρ(CF2) + 32δ(CF2) 410 M 401 CF2ρ 66ρ(CF2) + 19δ(CH2) 292 W 294 CF2τ 59τ(CF2) + 31ω(CF2) 208 VW 202 CCC δ 59δ(CCC) + 24ρ(CF2)

νs = symmetric stretch; νas = asymmetric stretch; δ = deformation; ρ = rock; ω = wag; τ = twist / torsion; VS – very strong; S – strong; M – medium; W – weak; VW – very weak; VVW- very very weak, Sh – shoulder stretching mode. The calculation actually shows that very weak in the PVDF (Ref.11). In polyethylene all 74% of PED for the frequency of 1280 cm–1 is con- carbon atoms move with same amplitude during the nected with CF2 asymmetric stretching. The band at vibrations but in form I, PVDF, the carbon atoms to 856 cm–1 is assigned to the symmetric stretching vi- which the fluorines are bonded move with considera- bration of the CC bond (51% of PED) with small con- bly smaller amplitude than the carbon atoms bonded tributions from stretching of CF bond (16%) and de- to the hydrogen atoms. This leads to smaller derived formation of the HCH angle. The CF2 bending vibra- polarizabilities and intensities in PVDF. The CH2 tion is assigned to the strong band at 766 cm–1. The rocking mode is identified with the weak IR band at –1 –1 CF2 wagging mode is associated with strong 890 cm in polyethylene whereas the strong 845cm band at 612 cm–1. The medium bands at 357 cm–1 and band in form II PVDF is associated with this vibra- –1 292 cm are assigned to CF2 twisting vibrations. The tion. The CH2 twisting modes are strong in the IR –1 weak band at 435 cm corresponds to the CF2 rocking spectrum of polyethylene whereas the very weak IR vibration. band at 946 cm–1 in form II PVDF is assigned to the

4.3 IR and Raman spectra of pvdf and polyethylene twisting vibration of this group. The lack of intensity in modes such as symmetric The profiles of the Raman scattering curves show a and asymmetric stretching of CC bonds in form I similarity in the intensities of the CH2 bending modes PVDF is remarkable. These modes possess the intense of both the compounds. The strong Raman band at lines in the Raman spectra of polyethylene but appear 1453 cm–1 in polyethylene and at 1432 cm–1 in form II NALLASAMY & MOHAN: CHARACTERIZATION OF FORM II POLY(VINYLIDENE FLUORIDE) 827

PVDF are assigned to the bending vibration of the 8 Enomoto S, Kawai Y & Sugita M, J Polymer Sci, A–2, 6 (1968) 861. CH2 group. 9 Boerio F J & Koenig J L, J Polymer Sci, A-2, 7 (1969) 1489. 5 Conclusion 10 Cortili G & Zerbi G, Spectrochim Acta, 23A (1967) 285. 11 Boerio F J & Koenig J L, J Polym Sci, A-2, 9 (1971) 1517. A complete assignment of the vibrational frequen- 12 Cessac G L & Curro J G, J Poly Sci, Polym Phys Ed, 12 cies in the FTIR and FT-Raman spectra and the nor- (1974) 695. mal coordinate analysis of form II PVDF is presented 13 Lauchlan L & Rabolt J F, Macromolecules, 19 (1986) 1049. in the present work. The experimental results obtained 14 Tashiro K, Itoh Y, Kobayashi M & Tadokoro H, Macro- molecules, 18 (1985) 2600. are in good agreement with the predictions of theory. 15 Armengaud A, Ramonja J L & Abenoza M, J Raman Spec- The potential energy distribution faithfully reflects the trosc, 18 (1987) 109. purity of the modes. 16 Kobayashi M, Tashiro K & Tadokoro H, Macromolecules, 8 (1975) 158. References 17 Bachmann M A, Gordon W L, Koenig J L & Lando J B, J 1 Edwards H G M, Farwell D W, de Faria D L A et al., J Ra- Appl Phys, 50(10) (1979) 6106. man Spectrosc, 32(2001) 17. 18 Prabhakaran A R, Spectroscopic Investigations of some 2 Hu J, Moigno D, Kiefer W et al., Spectrochim Acta, 56A Polymers and Organic Molecules, Ph D Thesis, Pondicherry (2000) 2365. University, Pondicherry, 1990. 3 Stidham H D, Duffy D J, Hsu S L et al., Spectrochim Acta, 19 Lu F J & Hsu S L, Macromolecules, 19 (1986) 326. 57A (2001) 1567. 20 Long D A, Raman Spectroscopy (McGraw Hill, New York), 4 Emeleus H J, The Chemistry of Fluorine and its Compounds 1977. (Academic Press, London), 1969. 21 Kuptsov A H & Zhizhin G N, Handbook of Fourier Trans- 5 Kroschwitz J I, Encyclopedia of Polymer Sci and Engg, Vol form Raman and Infrared Spectra of Polymers (Elsevier, 17 (John Wiley, New York), 1989. Amsterdam), 1998. 6 Kim J N, Cho W J & Ha C S, J Poly Sci Poly Phy Ed, 40 22 Hannon M J, Boerio F J & Koenig J L, J Chem Phy, 50 (2002) 19. (1969) 2829. 7 Natta G, Allegra G, Bassi I W et al., J Polym Sci, A 3 (1965) 23 Koenig J L, Spectroscopy of Polymers (Elsevier, New York), 4263. 1999.