Dielectric relaxation studies of polyvinylpyrrolidone (PVP) and polyvinylbutyral (PVB) in benzene solutions at microwave frequency

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1 Indian Journal of Pure & Applied Physics Vol. 5, September 014, pp Dielectric relaxation studies of polyvinylpyrrolidone (PVP) and polyvinylbutyral (PVB) in benzene solutions at microwave frequency A Rawat*, H K Mahavar, A Tanwar & P J Singh Department of Physics, M S J College, Bharatpur , India * anandrawat15oct@gmail.com Received November 013; revised 19 February 014; accepted 10 April 014 Microwave dielectric relaxation measurements on polyvinylpyrrolidone (PVP), polyvinylbutyral (PVB) and their have been done in dilute solutions of benzene at GHz at 303K temperature. The heterogeneous molecular interactions and intra-molecular rotations of these polymers in dilute solutions have been studied by comparing the evaluated values of average relaxation time ( 0 ), relaxation times corresponding to overall molecular rotation ( 1 ) and intra-molecular rotation ( ). The values of different dielectric parameters namely; static permittivity ( 0 ), dielectric constant ( ), dielectric loss ( ) and optical permittivity ( ) have been determined for five different mole fractions of PVP-PVB. The dipole moment () and free energy of activation F were also calculated. It is observed that the value of decreases, whereas F increases with the increase of mole fraction of PVP in the mixture. The higher values of the distribution parameter () indicate that besides the overall rotation, there is a large contribution of segmental reorientation and group rotation to the relaxation processes. Keywords: Dielectric relaxation, Binary mixture, Dipole moment, Distribution parameter, Polyvinylpyrrolidone, Polyvinylbutyral 1 Introduction During last several years, investigations of dielectric relaxation phenomena have provided an important approach to explore the structural behaviour of polar molecules in non-polar solvents. To study the structural behaviour of molecules, it is necessary to determine various dielectric parameters which are related with inter and intra-molecular association and internal rotation. Several researchers 1-3 have studied dielectric relaxation behaviour of binary mixtures in dilute solutions of non-polar solvents and observed interesting behaviour due to formation of complexes in the mixture of polar molecules. When polar molecules are subjected to electromagnetic waves in microwave frequency region, they absorb considerable microwave energy thereby perturbing dipole moment of molecules due to molecular rotation. The study of dielectric relaxation of polar molecules in non-polar solvents from the microwave absorption gives valuable information about various types of the molecular associations present in the solutions as microwaves can detect weak molecular interactions 4-9. The dielectric studies of small polar molecules and polymers in pure liquid state and dilute solutions at microwave frequencies provide vital information on the molecular configuration of a system. Polymer solutions show very complex behaviour, as a consequence of the combination of free volume and energetic contributions of the constituent components. The changes originating in macromolecules due to solute-solvent interaction have been studied by many researchers 10,11. Microwave dielectric relaxation studies in non-polar solvents are very useful in determining the flexibility of chains, mobility of the polymer segment, internal group rotation and steric hindrance to the internal rotation due to hydrogen bonding. Several researchers 15,17 have studied the dielectric behaviour of polymers in dilute solutions. In the present paper, an attempt has been made to understand the molecular dynamics of polyvinylpyrrolidone (PVP), polyvinylbutyral (PVB) and their in dilute solutions of benzene by comparing their microwave relaxation times. Experimental Details PVB (Mw: 30500) was purchased (LR grade) from HiMedia Laboratories Pvt Ltd., Mumbai, India. PVP (Mw: 40000) and benzene (AR grade) were purchased from Central Drug House Pvt. Ltd., New Delhi, India. Both PVP and PVB were used as received and benzene was used as a solvent. The X-band

2 RAWAT et al.: DIELECTRIC RELAXATION STUDIES OF POLYVINYLPYRROLIDONE 633 microwave bench was used to measure wavelength in the dielectric medium and voltage standing wave ratio (VSWR) using a short-circuiting plunger. The set-up was tuned at microwave frequency GHz. The s of the required mole fractions are prepared by mixing PVP and PVB in the calculated proportions. Dilute solutions of PVP, PVB and the s of PVP-PVB in benzene were then prepared. For each mole fraction, several solutions having different weight fractions were prepared. The static permittivity ( 0 ) values of the solutions were measured at 100 Hz frequency using LCR meter (Aplab, India, Make: M 4080A) by directly measuring the capacitance and calibrating it for standard liquids. The accuracy in the measurement of capacitance is up to third place of decimal in pico farad order. The high frequency dielectric permittivity ( ) is obtained by squaring the refractive indices (n) of the sodium D-lines, measured with the help of an Abbe s refractometer. The accuracy of measurement of refractive indices for sodium light is up to third place of decimal. The real ( ) and imaginary ( ) parts of the complex dielectric permittivity ( * = -i ) were determined by X-band microwave bench at GHz by the method described by Heston et al 18. The experimental set-up employed for the dielectric measurements is schematically shown in Fig. 1. The set-up includes a reflex klystron (K-7, III-78), its power supply, a ferrite isolator, a direct reading frequency meter, two slide screw tuners, one variable attenuator, a slotted section, liquid dielectric cell with E-bend, movable plunger and a spot reflecting galvanometer arranged as shown in Fig. 1. The reflex klystron is fitted on the klystron mount and energized by a regulated power supply. The frequency of electromagnetic waves travelling in the waveguide is measured by the direct reading frequency meter which has resolution of 5 MHz. The output of the standing wave detector is fed through a crystal detector to the spot reflecting galvanometer. The liquid cell fitted with a micrometer plunger has a least count of cm. The electromagnetic waves pass through the liquid and then reflected back by the shortcircuited plunger. As a result of super position of direct and the reflected electromagnetic waves, a standing wave pattern is formed inside the dielectric liquid. After properly arranging the X-band microwave bench, the guide wavelength ( g ) and the wavelength in waveguide filled with dielectric liquid ( d ) have been measured. 3 Theory The dielectric constant ( ) and the dielectric loss ( ) were measured for dilute solutions of PVP, PVB and of PVP-PVB in benzene solutions at 303 K containing 0, 0.3, 43.3, 69.6 and 100 mol % of PVP. For dilute solutions in non-polar solvents,, 0 and can be expressed as linear functions of concentrations 19,0 in the following manner: = 1 +a W (1) =a W () 0 = 10 +a 0 W = 1 +a W (3) (4) where subscript 0 refers to the static or low frequency case, 1 to the pure solvent, to the solute and refers to the infinite frequency or optical frequency case, W is taken as the weight fraction of the solute. a, a, a 0 and a are the slopes of the above mentioned linear equations. According to Heston et al 18., the values of and are given by: λ0 λ0 = + λc λd λ0 λg d ρ = π λd λ d dn (5) (6) Fig. 1 Experimental setup in X-band for low loss liquids where 0, c, g and d are the free space wavelength, the cut-off wavelength, the guide wavelength and the wavelength in the waveguide filled with dielectric medium, respectively. Parameter is the inverse

3 634 INDIAN J PURE & APPL PHYS, VOL 5, SEPTEMBER 014 voltage standing wave ratio (VSWR) and d/dn is the slope of the curve versus n, where n is the number of minima. The precision of measurement of wavelength with the variable X-band microwave test bench is ±0.001 cm. Corresponding to this accuracy, the error in the measurement of and are about ±1% and ±5%, respectively. The linear slopes a 0, a, a and a have been used for the determination of relaxation times and molecular dipole moments. The most probable relaxation time ( 0 ) and distribution parameter () were calculated by Higasi s 0 single frequency measurement equations: 1 (1 α ) 1 A + B τ 0 = ω C (7) 1 A where (1 α) = tan, is the angular π B frequency selected for the measurements and A=a (a 0 a ), B= (a 0 a )(a a ) (a ), C=(a a ) +(a ). The relaxation times corresponding to overall molecular rotation ( 1 ) and intra-molecular rotation ( ) were calculated using the equations of Higasi et al 1., proposed for dilute solutions: a τ1 = ω( a a ) a0 a τ = ωa (8) (9) The molecular dipole moment () of these molecules have been calculated by using the methods proposed by Higasi (Eq. 10) and Guggenheim 3 (Eq. 11): 7kTM µ H = ( a 0 a ) 4 π N( ε01 ) d + 1 (10) where M is the molecular weight of solute, d 1 density of solvent, k the Boltzman constant, N the Avogadro s number and T is the temperature at which the experiment is performed. After knowing the values of a 0 and a, H is obtained from Eq. (10) at constant temperature T. 9kT 3 µ G = 4 π N ( ε + )( ε + ) C (11) where =(ε 0 ε ) (ε 01 ε 1 ) where 0 and are the static and high frequency limiting permittivity of solution and 01 and 1 are the static and high frequency limiting permittivity of solvent, respectively, C is the concentration of the solute in the solution in terms of moles/cc. The relaxation time has been utilized to obtain the value of molar free energy of activation (F ). From the theory of relaxation time as a rate process developed by Eyring 4, the relaxation time is given by: h Fε τ = exp kt RT from Eq. (1) we obtain: F RT ln kt τ ε = h (1) (13) 4 Results and Discussion The dielectric parameters ( 0,, and ), their slopes (a 0, a, a and a ) for different weight fractions, distribution parameter (), relaxation times 0, 1 and, dipole moments ( H and G ) and molar free energy of activation (F ) are reported in Tables 1-3 at constant temperature 303 K. Table 1 Table 1 Values of dielectric parameters ( 0,, and ) for different mole and weight fractions of solute in benzene at 303 K % Mole fraction of PVP in PVB-PVP Weight fraction (W ) 0 ' "

4 RAWAT et al.: DIELECTRIC RELAXATION STUDIES OF POLYVINYLPYRROLIDONE 635 Table Values of slopes (a 0, a', a" and a ) and distribution parameter () for different mole fractions of solute in benzene at 303 K % Mole fraction of PVP in PVB-PVP a 0 a' a" a Table 3 Values of relaxation time ( 0, 1, ), dipole moment ( H and G ) and free energy of activation (F ) for different mole fractions of solute in benzene at 303 K 0 % Mole fraction (ps) of PVP in PVB-PVP 1 (ps) (ps) H G F (in Debye) (in Debye) (kcal mole 1 ) presents that for each mole fraction of PVP, the values of 0, and increase with the increase in weight fraction of solute in the solution. The slight changes in the values of 0,, and for other compounds in dilute solutions have been observed in literature,5-9. The large values of indicate the presence of more than one relaxation processes. In our investigations, the value of given in Table varies between 0.86 to The value of for PVP is of the same order as reported in literature 30. These higher values suggest that besides the overall rotation, there is a large contribution of segmental reorientation and group rotation to the relaxation processes. Hence, more than one relaxation processes are present in the molecules. This is further confirmed by the values of different relaxation times. For the molecules under investigation, the value of systematically increases as the mole fraction of PVP is increased in the mixture. This suggests that the contribution of processes other than the molecular relaxation is the least for PVB and highest in the case of PVP. Table 3 presents the difference in the values of 1 and is significant at all mole fractions. The larger value of suggests that there is hindrance to the group rotation, which is due to the hydrogen bonding. This implies that more than one mechanism are present i.e., the dielectric absorption by its molecules is not solely contributed by their rotation as a single unit, but also contains contribution from the intra-molecular relaxation process in addition to the overall relaxation process. From these observations, it may be concluded that in these samples the intra-molecular rotations are dominant as compared to the molecular rotations. The average relaxation time ( 0 ) is found to increase with the increase of PVP mole fraction in the solution which confirms that there is a large breaking of the homogeneous hydrogen bonded structure of PVB molecules and these PVB molecules form the heterogeneous with the long chain length PVP molecule species due to complex formation between the carbonyl group of PVP monomer units and the poly alcohol group of PVB. Thus, the increase in the value of 0 on increasing the PVP concentration in the mixture is due to the decrease in the number of poly alcohol groups in comparison to the number of carbonyl groups in the mixture. From Table 3, it has also been observed that the value of is higher than the average relaxation time 0. The higher values of in comparison with 0 suggest that intra-molecular H-bonding exists in the folded structure of the chains of these molecules in dilute solutions. The higher values of also confirm that the presence of side groups increase hindrance to the segmental reorientation to a greater extent. The dipole moment values of the dilute solutions under investigation, listed in Table 3, show that H is nearly equal to G. Both H and G decrease as the PVP mole fraction increases in the solution. This may possibly be due to the shifting of charge centers in the dilute solutions. The dipole moment of a molecule is one of the various factors which affects its relaxation mechanism. The relaxation time values do not indicate a direct relationship to the dipole moment, suggesting the influence of other factors as well, such as shape and size of the molecules and the intramolecular interactions. The value of dipole moment for PVP reported in this paper agrees well with the value reported in literature 30. The values of molar free energy of activation (F ) for different compositions are listed in Table 3. The value of F is the least for PVB and highest for PVP, while for the binary mixture of PVP-PVB it lies in between these two values. This indicates that force of hindrance experienced by the molecules for dipolar rotation is a factor depending on the nature of molecules. The

5 636 INDIAN J PURE & APPL PHYS, VOL 5, SEPTEMBER 014 values of free energy of activation F are found to be in the range kcal mol 1. The value of free energy of activation for PVP is.14 kcal mol 1, which is comparable with the reported value 30. These values are of the order of the energy required for the breaking of hydrogen bonds in dilute solutions. This also suggests that in these dilute solutions there is continuous breaking and reforming of hydrogen bonds. 5 Conclusions The molecular dynamics of PVP, PVB and their s in the dilute solution form has been explored using the dielectric measurements at microwave frequency. The large values of distribution parameter () indicate the contribution of segmental reorientation and group rotation to the relaxation processes. Hence, the presence of more than one relaxation processes is confirmed. From the measurements of relaxation times 0, 1 and, it is observed that the intra-molecular and overall orientations are present in the dilute solutions. Both dipole moment and free energy of activation F depend on the PVP mole fraction in the dilute solution. Inter-molecular and intra-molecular H-bonding affects the relaxation times for both the polymers. The solute solvent interaction is present in dilute solutions of PVP, PVB and their binary mixtures in benzene. Acknowledgement Two of the authors (AR and HKM) would like to thank University Grants Commission (UGC), Bhopal for awarding the teacher research fellowship. The authors are also thankful to the Head, Department of Physics, M S J College, Bharatpur and the Principal, M S J College, Bharatpur for providing experimental facilities. References 1 Singh P J & Sharma K S, Indian J Pure & Appl Phys, 34 (1996) 388. Kalaivani T & Krishnan S, Indian J Pure & Appl Phys, 47 (009) Mohan T M, Sastry S S & Murthy V R K, Indian J Pure & Appl Phys, 48 (010) Thakur Nagesh & Sharma D R, Indian J Pure & Appl Phys, 38 (000) Vyas A D & Rana V A, Indian J Pure & Appl Phys, 40 (00) Rangra V S & Sharma D R, Indian J Phys, 78B (004) Chaudhari A, Ahire S & Mahrotra S C, J Mol Liq, 94 (001) Rewar G D & Bhatnagar D, Indian J Pure & Appl Phys, 40 (00) Rangra V S & Sharma D R, Indian J Pure & Appl Phys, 4 (004) Sengwa R J & Chaudhary R, Polymer International, 50 (001) Tanwar A, Gupta K K, Singh P J & Vijay Y K, Indian J Pure & Appl Phys, 44 (006) Gupta K K, Bansal A K, Singh P J & Sharma K S, Indian J Pure & Appl Phys, 4 (004) Singh P J & Sharma K S, Indian J Pure & Appl Phys, 34 (1996) Gupta K K & Singh P J, Indian J Phys, 77B (003) Sengwa R J, Abhilasha & More N M, Polymer, 44 (003) Block H & North A M, Advan Mol Relaxation Processes, 1 (1970) Murthy V R K, Kadaba P K & Bhagat P K, J Polym Sci: Polym Phys, 117 (1979) Heston W M, Franklin A D, Hennely E J & Smyth C P, J Amer Chem Soc, 7 (1950) Franklin A D, Heston W M, Hennely E J & Smyth C P, J Amer Chem Soc, 7 (1950) Higasi K, Bull Chem Soc Japan, 39 (1966) Higasi K, Koga Y & Nakamura M, Bull Chem Soc Japan, 44 (1971) 988. Higasi K, Bull Inst Phys Chem Research, (1943) Guggenheim E A, Trans Faraday Soc, 45 (1949) Glasstone S, Laider K J & Eyring H, Theory of Rate Processes (McGraw Hill Book Co, New York, USA), Kumar R, Rangra V S, Sharma D R, Thakur N & Negi N S, Indian J Pure & Appl Phys, 45 (007) Kumar R, Sharma V & Rangra V S, Indian J Pure & Appl Phys, 48 (010) Kumar R, Chaudhary R K & Rangra V S, Indian J Pure & Appl Phys, 49 (011) 4. 8 Jain R, Bhargava N, Sharma K S & Bhatnagar D, Indian J Pure & Appl Phys, 49 (011) Gupta K K, Bansal A K, Singh P J & Sharma K S, J Mol Liq, 108, 1-3 (003) Sengwa R J, Sonu Sankhla & Abhilasha, Indian J Phys, 79 (005) 879.