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Water uptake and transport studies in PVP-PMMA hydrogels
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ScienceDirect Materials Today: Proceedings 17 (2019) 430–441
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6th International Conference on Functional Materials & Devices (ICFMD 2017)
Water uptake and transport studies in PVP-PMMA hydrogels S.N.A. Wafaa, L.N. Simc, Z. Radzic, N.A. Yahyaa, N.H.A. Kassima, M.T. Rahmana, J.T. Czernuszkab, A.K. Arofc* a Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Materials, University of Oxford, Parks Road, OX1 3PH, United Kingdom c Centre for Ionics University of Malaya, Department of Physics, University of Malaya, 50603, Kuala Lumpur, Malaysia b
Abstract Hydrogels comprising 90 wt.% polyvinylpyrrolidone and 10 wt.% poly(methyl methacrylate) have been soaked in distilled water. By taking the mass change for different soaking times the number of water molecules entering the hydrogel can be calculated. A maximum in conductivity has been observed in the conductivity-soaking time plot. The increase and decrease in conductivity over time imply that some water molecules in the hydrogel are mobile (infrared band at 1598 cm-1) and immobile water band observed at 1687 cm-1. These can also be implied from the plot for number density of conductivity contributing water molecules versus soaking time. From the results shown water molecules diffuses at a faster rate when there are less conductivity contributing water molecules. © 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th International Conference on Functional Materials & Devices 2017 (ICFMD 2017). Keywords: Water uptake; Transport studies; Hydrogel; Ionic conductivity; Fourier transform infrared spectroscopy
1. Introduction A hydrogel comprises a physically or chemically crosslinked polymer network that can absorb water without being dissolved. This characteristic has led to various applications of hydrogel materials in clinical surgery whereby the hydrogel function by absorbing surrounding water molecules and expand the soft tissues around targeted skin
* Corresponding author. Tel.: +603-7967 4085. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th International Conference on Functional Materials & Devices 2017 (ICFMD 2017).
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and palate, providing extra tissues for reconstruction purposes [1-3]. In addition, it has also been utilised in drug delivery, tissues engineering and regenerative medicine [4]. The presence of water molecules can affect the dielectric behaviour of a polymer network. Dielectric constant and loss as a function of salt content in polymer electrolytes exhibit the same trend as conductivity versus salt concentrations [5-7] and the similarity of this trend is more obvious in the dielectric constant versus frequency plot as frequency decreases. Siu et al.[8] have studied the effect of water on the conductivity of polymer electrolytes between 25 and −37 °C. Their results indicate that proton conductivity is achievable via strongly bound and highly polarized water. From infrared studies, changes to the water molecule in a complex can be studied by observing the OH stretching (ν(OH)) and bending of water (δ(H2O)) modes of water that can be found in the IR region of 3000-3700 cm-1 and around 1640 cm-1respectively [9]. The IR envelope of both ν(OH) and δ(H2O) modes can be separated into several component bands which indicate distinct water environments that depend on the degree of hydrogen bonding. Generally, a higher degree of hydrogen bonding will exhibit a lower wavenumber. According to Shen and Wu [10], there are three types of water molecules that can be investigated from observing the IR wavenumbers of OH bending belonging to water viz., the band of aggregated water with strong hydrogen bond that is located at 1676 cm-1, the band of aggregated water with hydrogen bond of moderate strength at 1645 cm-1 and free water band at 1592 cm-1of the ATR-FTIR spectrum. Free water is water molecule without hydrogen bonding. These water molecules do not interact or only weakly interacts with each other. Water is reported to diffuse in a polymer matrix in these forms. As for the OH stretching mode of water, researchers have reported three, four or six components obtained from the deconvolution of the IR mode that correspond to different hydrogen bonding environments [9, 11]. From these findings, one may hypothesize that when subjected to an applied electric field, water molecules will be polarized. A highly polarized water molecule can be likened to a salt in a solvent with the positive and negative ions separated at a certain distance. Depending on the state of polarity of the electric field, the free water molecules will be forced to move in the direction of the field. Impedance of a hydrogel as a function can be detemined by electrochemical impedance spectroscopy. In this work, the xerogel (dry state of hydrogel) which is obtained via the evaporation of the liquid phase in gel, consists of two polymers viz., polyvinylpyrrolidone abbreviated as PVP and poly(methyl methacrylate), PMMA. The conductivity of a PVP-PMMA xerogel soaked in distilled water (becoming hydrogel) has been calculated using impedance spectroscopy. The number density of water molecules that contribute towards conductivity and their diffusion coefficient has been calculated using the method developed by Arof et al.[12]. 2. Experimental and theory 2.1. Materials and sample preparation Xerogel samples in the form of a long cylindrical rod (14 cm) were obtained from Polymeric Science Ltd. (UK). The purpose of xerogel in this study is to expand the soft tissue skin and palate for tissue reconstruction [1,4]. The gels were prepared by co-polymerization of pharmaceutical grade (ISO 13488) methyl methacrylate (MMA) and vinylpyrrolidone (VP) in a weight percentage ratio of 10:90 and designated as X2. Alkyl methacrylate (0.2 wt.%) was used as the cross-linking agent and azo-bis-isobutyronitrite (0.2 wt.%) as the initiator to facilitate the polymerization reaction. The rod-shaped xerogels were washed in an excess of pure water to remove any unreacted reactants and dried before being cut into discs of 10 mm diameter and 2 mm thick and weighing 0.3780g. The discs were polished with 60, 600 and 1000 grit sandpaper and were finally stored in airtight packages with silica desiccant until required. 2.2. Materials characterization 2.2.1. Swelling measurements The time-dependent swelling behaviour (St) of isotropic X2 hydrogels was determined. Each swelling experiment was repeated three times. St was determined from the mass change as a function of time following the equation
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St
(M t M d ) Md
(1)
where Md represents the mass of the xerogel and Mt is the mass of the hydrogel at time t. All swelling experiments were performed at (37±1)C, with measurements continued until an equilibrium swelling ratio (S∞) was achieved. Each mass measurement was determined by extracting the sample from a solution bath and removing the free solution with filter paper before weighing. The samples were designated as Sx, where x represents the soaking time in minutes. In EIS, a small sinusoidal potential was applied across the sample and the current through the sample measured. Hence, an electric field will polarize the water molecules in the hydrogel. From these, the impedance was obtained for every operating frequency. The applied voltage was set small (30 mV) in order to minimize possible charge carrier concentration changes and the time average of the changes during the measurements. Xerogels were soaked in excess amount of distilled water (DW). At every 30 min interval, the hydrogel was taken out and its mass, thickness and diameter were recorded. After drying, the hydrogel was then sandwiched between two stainless steel disc electrodes and the impedance of the hydrogel measured with a HIOKI 3520 LCR Hi-Tester in the frequency range from 50 Hz to 100 kHz at room temperature. A typical Nyquist plot obtained from the soaked hydrogels is as shown in Fig. 1. It can be seen that the Nyquist plot consists of a partially depressed or tilted semicircle and a tilted spike. From the Nyquist plot, the bulk impedance can be obtained and with it the conductivity of the soaked hydrogel can be calculated following the equation reported in literature [13].
Fig. 1. Typical Nyquist plot for the hydrogel obtained in this work.
Following Linford [14], the Nyquist plot of the hydrogel can be the same as that of an equivalent circuit consisting of a parallel connection of a resistor and a leaky capacitor in series with another leaky capacitor. The impedance of the equivalent circuit can be written as [12]:
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Zr =
p 1 R R 2 k1 cos 1 2 p1 p 2 1 2 Rk1 p1 cos 1 R 2 k1 2 2
+
(2)
Zi =
p 1 R 2 k1 p1 sin 1 2 p1 p 2 1 2 Rk1 cos 1 R 2 k1 2 2 1
p1
p cos 1 2 1
k2 2
1
+
433
p 2 p1
p sin 2 2 1
k2 2
p 2 p1
(3)
The value of p1 and p2 can be obtained from the Nyquist plot and their meaning is explained in Arof et al. (2014). R is the value of the bulk resistance that can be also obtained from the plot and k1-1 and k2-1 are the capacitance of the leaky capacitor in parallel with the resistor representing the bulk and the second serial leaky capacitor respectively. The angular frequency is represented by. It is to be noted that the value of k1 and k2 for every hydrogel sample were obtained by trial and error. With these values, the transport parameters such as number density of free water molecules contributing to conductivity (n) and diffusion coefficient of the free water molecules (D) can be calculated through the equations [12]:
= =
(
(4)
)
(
)
(5)
A is the surface area of the hydrogel, T is room temperature in kelvins, o is vacuum permittivity and 2 is the reciprocal of at the bottom of the spike.r is present in all the above equations and is the dielectric constant of the hydrogel. The value of r is obtained from the corresponding impedance data using the equation [15]:
=
(
+
)
(6)
The value of r for use in the equations for n and D was obtained in the frequency region where log r is constant, Fig. 2. 2.2.2. Fourier transform infrared spectroscopy Fourier transform infrared (FTIR) spectroscopy was performed to identify the carbonyl functional group and the C-O-C band. The shift in wavenumber of these bands can indicate interaction between the polymer and the water molecules. The identification of these bands was done using the Thermo Scientific model Nicolet iS10 spectrometer at 4 cm-1 resolution. The spectrum was recorded in the transmittance mode from 650 to 4000 cm-1. Deconvolution of the several IR regions were conducted by fixing the number and line shape, and allowing band parameters such as full width at half maximum, area, intensity and band shape to vary without constraints during the iteration. The Gaussian/Lorentzian function was used to fit the selected bands of the hydrogels and all the deconvoluted spectra was best-fitted with constant baseline.
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Fig. 2. Dielectric constant, r variation with frequency.
3. Results and discussion As shown in Fig. 1, by drawing a circle through the partially depressed semicircle, the tilt of the semicircle becomes more obvious and the diameter of the depressed semicircle is observed below the real impedance axis. It may also be implied that the Nyquist plot will pass through the origin. All log r versus log f for samples soaked at different times show similar profiles i.e. log r decreases with frequency increase and is constant at very high frequencies in the frequency range studied. The decrease in r in the frequency region between 50 and 10 000 Hz may be attributed to the tendency of the polarized water molecules to align themselves in the direction of the applied electric field. As frequency increases (> 104 Hz), the polarized water molecules could not align themselves with the applied field and the dielectric constant decreased less rapidly. In other words, due to the very fast periodic reversal of the electric field the value of log εr is almost frequency-independent since there is almost no excess water molecule diffusion in the direction of the field [16]. The trend of log r versus log f in this work is similar to that for polymer electrolytes [17,18]. The Zr and Zi are used to calculate the r values. The r values, together with other parameters for use in the equations of n and D are listed in Table 1. Table 1. Parameter values used in the equations for Zr and Zi and that needed to calculate n and D. Sample S30 S60 S90 S120 S150 S180 S210 S240 S270 S300 S330 S360 S390 S410 S440 S500 S530 S590 S650 S710
Conductivity, × 10-5 (S cm-1) 3.10 2.30 3.23 3.47 4.34 5.19 4.91 5.58 4.58 3.86 6.11 5.73 6.92 6.33 6.44 4.37 4.66 4.40 3.60 3.10
p1
p2
k1-1 (pF)
k2-1 (µF)
r
0.85 0.94 0.95 0.95 0.95 0.96 0.96 0.97 0.96 0.96 0.95 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.97
0.79 0.75 0.69 0.82 0.83 0.79 0.79 0.78 0.81 0.79 0.80 0.80 0.82 0.78 0.79 0.79 0.77 0.80 0.78 0.79
0.41 0.10 0.17 0.19 0.18 0.16 0.13 0.15 0.14 0.14 0.16 0.14 0.13 0.15 0.15 0.14 0.16 0.14 0.15 0.12
10.00 11.49 22.73 28.57 25.00 30.30 30.53 35.71 28.98 30.30 28.57 27.40 26.32 29.41 27.78 24.39 28.57 20.83 21.74 20.00
56 48 91 99 99 100 94 118 99 95 90 96 88 105 103 100 109 105 105 106
2
× 10-5 (s) 3.97 3.18 3.18 8.38 5.31 3.98 3.98 3.98 5.31 5.31 3.18 3.98 2.84 3.18 3.18 5.31 3.98 5.31 5.31 5.31
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PMMA and PVP are low dielectric constant materials. PVP has a dielectric constant of 2.3 [19] and PMMA has a dielectric constant of 4.9 at 100 Hz [20]. DW has dielectric constant of ~80. At early soaking times, the dielectric constant of the hydrogel is less that that of DW. However as more water molecules entered the hydrogel, the dielectric constant increased and is greater than that of DW after 90 min (i.e. after three 30 min intervals) of soaking. This implied that polarization has increased with soaking time and more water molecules have entered the hydrogel. Fig. 3 (a) and (b) show typical FTIR spectra for the hydrogel in the region from 1510 to 1800 cm-1 and from 2600 to 3800 cm-1. These two IR envelopes were deconvoluted in order to determine the existence of free water and aggregated water with strong and moderately strong hydrogen bonds. Free water molecules are water molecules not attached to the heteroatoms of the hydrogel polymers. In this work, it could be the oxygen atom in the carbonyl group of PVP and PMMA and the C-O-C group in PMMA. From Fig. 3 (a), four peaks were deconvoluted; the peak located at 1639 cm-1 belongs to the carbonyl stretching [ν(C=O)] of PVP. Three other peaks found at 1598, 1687 and 1713 cm-1are attributable to the free water, hydrogen bonded water and carbonyl group of PMMA respectively. In Fig. 3 (b), six component bands located at 3096, 3210, 3295, 3390, 3495 and 3610 cm-1were best-fitted upon the deconvolution of the IR envelope attributable to O-H stretching mode of water. The presence of the IR bands due to water indicates that water is contained within the PVP-PMMA-based hydrogels.
Fig. 3. Typical deconvolutions of the FTIR spectrum in the wavenumber region of (a) 1510 to 1800 cm-1and (b) 2600-3800 cm-1.
The mass of water entering the xerogel for a period of almost 12 h is as shown in Fig. 4. In the figure, the mass of water is represented as the number of water molecules entering the hydrogel. The conductivity profile is also shown in the figure. Conductivity is attributed to the movement of polarized free water in the direction of the applied electric field. The water molecules that are attached to the polymers that make up the hydrogel through strongly and moderately strong hydrogen bonds are immobile. From the profile obtained, it may be implied that the rate of diffusion of water molecules will reach an asymptote and the hydrogel swelling will reach a maximum. The conductivity can be considered as being quadratically related to the soaking time via the equation:
( )=
3
10
+ 2
10
+2
10
(7)
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Fig. 4. Profile of water intake (in terms of number of water molecules) into hydrogel and conductivity of hydrogel as a function of soaking time.
By this equation, the maximum conductivity occurred at 333 1/3 min of soaking time after which conductivity decreased. Hence it can be observed that although water uptake continued to increase, conductivity decreased after the optimized soaking time. To further strengthen the attachment of water molecules to the polymers consider the report by Hodge et al. [21] that have explained water molecule attachment to the polar groups of a polymer and in the case of poly(vinyl alcohol) or PVA, it is with oxygen in the hydroxyl group. Kitano et al. [22] have also reported that water can form hydrogen bond to the oxygen atoms in the ether and ester groups of poly(methoxyethylacrylate) or PMEA side chains. In this work water attachments to the PVP and PMMA polymer can occur at the carbonyl oxygen and in PMMA it can also occur at the oxygen of C-O-C group. It may therefore be inferred that after 333 1/3 min, most of the water molecules will attach themselves to the heteroatoms of the polymers that make up the hydrogel. Hence there is less polarized free water molecules that contribute to conductivity. These results have shown what can happen to the water molecules when subjected to an applied voltage as in impedance spectroscopy. From the above discussion, we have differentiated between water molecules that may have attached themselves to the polymer through the oxygen atom in the carbonyl functional group of both polymers or through the oxygen in the C-O-C group of PMMA and free water molecules that can move in the direction of the electric field and contributing towards conductivity. Therefore, the number of the conductivity contributing water molecules should at anytime be less than the amount of water diffusing into the hydrogel. Our calculations on the number density of conductivity contributing water molecules are illustrated in Fig. 5 and the number density of conductivity contributing water molecules is compared to the number of water molecules entering the hydrogel. It can be observed that the number density of conductivity contributing water molecules is a maximum at 300 min. This implies that the quadratic equation relating conductivity (σ) and soaking time t is reasonably accurate since it predicts that a maximum in conductivity to occur at 333 1/3 min. The number of water molecules entering the hydrogel is observed to increase at a decreasing rate. This implies that the diffusion of water into the gel decreased with time. Comparison of the number of water molecules entering the gel and the diffusion coefficient of water molecules in the hydrogel is as illustrated in Fig. 6.
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Fig. 5. Profile of water uptake into hydrogel and the number density of charge carriers in hydrogel. The dashed line serves as a guide to the eye.
Fig. 6. Profile for intake of water molecules into hydrogel and diffusion coefficient of charge carriers as a function of soaking time. Dashed line serves as guide to the eye.
It should be noted that the diffusion coefficient is high when the number of water molecules entering the hydrogel is small. Thus, on entering the hydrogel, the water molecules diffuse at a faster rate. The diffusion coefficient decreases as more water molecules entered the hydrogel, until a minimum is reached, which corresponds to the maximum in number density of water molecules contributing towards conductivity at 300 min. It can be observed from Fig. 5 that the minimum diffusion coefficient occurs at 300 min. After 300 min, there are lesser number of water molecules contributing to conductivity probably due to more incoming water molecules attaching to the heteroatoms of the polymer. Conductivity decreased, but diffusion coefficient increased. This again makes it clear that diffusion is high when there are less mobile and polarized water molecules in the hydrogel.
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The diffusion coefficient for distilled water into the gel as reported by Swan et al. [1] was 1.63 × 10-3 cm2 s-1. It is to be noted that the results shown in the above figure is the diffusion coefficient of water molecules entrapped in the hydrogel. It can be observed that the diffusion coefficient is between four to five orders of magnitude lower than the results reported by Swan and co-workers. A plot of conductivity versus water molecule uptake is as shown in Fig. 7. It can be inferred that in the early soaking hours, there may be competition between water molecules attaching themselves to the polymers comprising the hydrogel and non-attaching or free water molecules. When 2 × 1022 water molecules have entered the hydrogel, most of the water molecules attach themselves to the polymer resulting in the decrease in conductivity. As more water molecules diffuse into the gel, a bigger portion did not attach to the polymer and conductivity increased. A maximum in conductivity occurred when about 6 × 1022 water molecules are in the hydrogel after which conductivity decreased that can be attributed to increased water molecules attached to the polymer.
Fig. 7. Room temperature conductivity versus water uptake. (Dashed line serves as guide to the eye).
As mentioned above, water attachment in PMMA could take place via hydrogen bonding to the oxygen in the carbonyl group and/or through the oxygen in the C-O-C component of PMMA. Since PVP also has a carbonyl band, hydrogen bonding can also be expected at the oxygen atom of PVP. It is thus useful to know at which oxygen atom water attachment occurred. Fig. 8 depicts the IR region between 1500 and 1800 cm-1 of the hydrogels which shows the C=O stretching (ν(C=O)) bands belonging to PVP and PMMA. The ν(C=O) band belonging to PVP for the S0 sample containing no diffused water is located at 1639 cm-1. When soaked, it can be observed from the plots that this band did not show any significant shift which may imply that water does not form any hydrogen bond with the oxygen in the carbonyl group. Ericson et al.[23] also reported no carbonyl peak shift in the FTIR spectrum of their samples. On the other hand, theν (C=O) band of PMMA which was originally located at 1713cm-1 was not observed when soaked in water. Soaking was prolonged to1260 min or 21 h to ensure whether there is shifting or otherwise. Fig. 9 depicts the IR spectra of PVP-PMMA-based hydrogels between 1250 and 1350 cm-1. As observed in Fig. 9 (a), the IR spectrum of S0 displayed three peaks; the CH2 wagging of PVP was located at 1318 cm-1 whereas the CO stretching (ν(CO)) of PMMA were found at 1276 and 1289 cm-1. When soaked into water, only the ν(CO) mode of PMMA displayed significant wavenumber shifting from 1289 cm-1 to 1294-1295 cm-1 in samples soaked at various times. This indicates that water has interacted with the C-O group of PMMA.
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Fig. 8. IR spectra in the region between 1500 and 1800 cm-1of (a) S0, (b) S60, (c) S120, (d) S240, (e) S360 and (f) S1260 which show the ν(C=O) band of PVP.
Fig. 9. IR spectra in the region between 1250 and 1350 cm-1 of (a) S0, (b) S60, (c) S120, (d) S240, (e) S360 and (f) S1260.
Fig. 10 shows the variation of area % of free water and hydrogen-bonded water as a function of soaking time. It was observed that the area % of free water increased up to 240 min before decreasing beyond that soaking duration. Hydrogen-bonded water on the other hand decreased with increasing soaking time up to 240 min before increasing above that duration. This is within reasonable agreement with the decrease in conductivity as depicted in Fig. 4. The
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increase in area % of hydrogen-bonded water shows that there is increase in attachment of the water molecules to the hydrogel.
Fig. 10. Area % of free water and hydrogen-bonded water as a function of soaking time.
4. Conclusions In this work, the hydrogel ratio of 90 wt.% polyvinylpyrrolidone and 10 wt.% poly(methyl methacrylate) were soaked in distilled water. The number of water molecules entering the hydrogel was calculated based on the mass change for different soaking times. Since highly polarized water molecules can exhibit conductivity when subjected to an electric field, a maximum in conductivity could be observed in the conductivity-soaking time plot. The increase and decrease in conductivity imply that some water molecules in the hydrogel are mobile. The infrared mobile waterband was observed at 1598 cm-1 and the immobile water bands can be observed at 1687 cm-1. The plot for number density of conductivity contributing water molecules versus soaking time can also imply the presence of these types of water. The immobile water molecules have attachment to the polymers at the oxygen atom of the CO-C group in PMMA. From the results shown water molecules diffuses at a faster rate when there were less conductivity contributing water molecules. Acknowledgements This work is supported by the High Impact Research MoE Grant UM.C/625/1/HIR/MOHE/ DENT/21from the Ministry of Education (MoE) Malaysia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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R.G. Linford, in: B.V.R. Chowdari, S. Radhakrishna (Eds.), Solid State Ionics Devices, World Scientific, Singapore, 1988, pp. 551–571. S.L. Agrawal, M. Singh, M. Tripathi, M.M. Dwivedi, K. Pandey, J. Mat. Sci. 44 (2009) 6060-6068. M.E. Londono, J.M. Jaramillo, R. Sabater, J.M. Velez, Revista EIA 18 (2012) 105-114. A. Karmakar, A. Ghosh, Curr. Appl. Phys. 12 (2012) 539-543. S.K. Tripathi, A. Gupta, M. Kumari, B. Mat. Sci. 35 (2012) 969-975. A. Rawat, H.K. Mahavar, A. Tanwar, P.J. Singh, Indian J. Pure Ap. Phy. 52 (2014) 632-636. P. Thomas, R.S. Ernest Ravindran, K.B.R. Varma, presented in part at 2012 IEEE 10th International Conference on the Properties and Applications of Dielectric Materials, Bangalore, India, July, 2012. [21] R.M. Hodge, G.H. Edward, G.P. Simon, Polymer 37 (1996) 1371-1376. [22] H. Kitano, K. Ichikawa, M. Fukuda, A. Mochizuki, M. Tanaka, J. Colloid and Interface Sci. 242 (2001) 133-140. [23] H. Ericson, C. Svanberg, A. Bordin, A.M. Grillone, S. Panero, B. Scrosati, P. Jacobson, Electrochim. Acta 45 (2000) 1409-1414.
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6th International Conference on Functional Materials & Devices (ICFMD 2017)
Water uptake and transport studies in PVP-PMMA hydrogels S.N.A. Wafaa, L.N. Simc, Z. Radzic, N.A. Yahyaa, N.H.A. Kassima, M.T. Rahmana, J.T. Czernuszkab, A.K. Arofc* a Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Materials, University of Oxford, Parks Road, OX1 3PH, United Kingdom c Centre for Ionics University of Malaya, Department of Physics, University of Malaya, 50603, Kuala Lumpur, Malaysia b
Abstract Hydrogels comprising 90 wt.% polyvinylpyrrolidone and 10 wt.% poly(methyl methacrylate) have been soaked in distilled water. By taking the mass change for different soaking times the number of water molecules entering the hydrogel can be calculated. A maximum in conductivity has been observed in the conductivity-soaking time plot. The increase and decrease in conductivity over time imply that some water molecules in the hydrogel are mobile (infrared band at 1598 cm-1) and immobile water band observed at 1687 cm-1. These can also be implied from the plot for number density of conductivity contributing water molecules versus soaking time. From the results shown water molecules diffuses at a faster rate when there are less conductivity contributing water molecules. © 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th International Conference on Functional Materials & Devices 2017 (ICFMD 2017). Keywords: Water uptake; Transport studies; Hydrogel; Ionic conductivity; Fourier transform infrared spectroscopy
1. Introduction A hydrogel comprises a physically or chemically crosslinked polymer network that can absorb water without being dissolved. This characteristic has led to various applications of hydrogel materials in clinical surgery whereby the hydrogel function by absorbing surrounding water molecules and expand the soft tissues around targeted skin
* Corresponding author. Tel.: +603-7967 4085. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of 6th International Conference on Functional Materials & Devices 2017 (ICFMD 2017).
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and palate, providing extra tissues for reconstruction purposes [1-3]. In addition, it has also been utilised in drug delivery, tissues engineering and regenerative medicine [4]. The presence of water molecules can affect the dielectric behaviour of a polymer network. Dielectric constant and loss as a function of salt content in polymer electrolytes exhibit the same trend as conductivity versus salt concentrations [5-7] and the similarity of this trend is more obvious in the dielectric constant versus frequency plot as frequency decreases. Siu et al.[8] have studied the effect of water on the conductivity of polymer electrolytes between 25 and −37 °C. Their results indicate that proton conductivity is achievable via strongly bound and highly polarized water. From infrared studies, changes to the water molecule in a complex can be studied by observing the OH stretching (ν(OH)) and bending of water (δ(H2O)) modes of water that can be found in the IR region of 3000-3700 cm-1 and around 1640 cm-1respectively [9]. The IR envelope of both ν(OH) and δ(H2O) modes can be separated into several component bands which indicate distinct water environments that depend on the degree of hydrogen bonding. Generally, a higher degree of hydrogen bonding will exhibit a lower wavenumber. According to Shen and Wu [10], there are three types of water molecules that can be investigated from observing the IR wavenumbers of OH bending belonging to water viz., the band of aggregated water with strong hydrogen bond that is located at 1676 cm-1, the band of aggregated water with hydrogen bond of moderate strength at 1645 cm-1 and free water band at 1592 cm-1of the ATR-FTIR spectrum. Free water is water molecule without hydrogen bonding. These water molecules do not interact or only weakly interacts with each other. Water is reported to diffuse in a polymer matrix in these forms. As for the OH stretching mode of water, researchers have reported three, four or six components obtained from the deconvolution of the IR mode that correspond to different hydrogen bonding environments [9, 11]. From these findings, one may hypothesize that when subjected to an applied electric field, water molecules will be polarized. A highly polarized water molecule can be likened to a salt in a solvent with the positive and negative ions separated at a certain distance. Depending on the state of polarity of the electric field, the free water molecules will be forced to move in the direction of the field. Impedance of a hydrogel as a function can be detemined by electrochemical impedance spectroscopy. In this work, the xerogel (dry state of hydrogel) which is obtained via the evaporation of the liquid phase in gel, consists of two polymers viz., polyvinylpyrrolidone abbreviated as PVP and poly(methyl methacrylate), PMMA. The conductivity of a PVP-PMMA xerogel soaked in distilled water (becoming hydrogel) has been calculated using impedance spectroscopy. The number density of water molecules that contribute towards conductivity and their diffusion coefficient has been calculated using the method developed by Arof et al.[12]. 2. Experimental and theory 2.1. Materials and sample preparation Xerogel samples in the form of a long cylindrical rod (14 cm) were obtained from Polymeric Science Ltd. (UK). The purpose of xerogel in this study is to expand the soft tissue skin and palate for tissue reconstruction [1,4]. The gels were prepared by co-polymerization of pharmaceutical grade (ISO 13488) methyl methacrylate (MMA) and vinylpyrrolidone (VP) in a weight percentage ratio of 10:90 and designated as X2. Alkyl methacrylate (0.2 wt.%) was used as the cross-linking agent and azo-bis-isobutyronitrite (0.2 wt.%) as the initiator to facilitate the polymerization reaction. The rod-shaped xerogels were washed in an excess of pure water to remove any unreacted reactants and dried before being cut into discs of 10 mm diameter and 2 mm thick and weighing 0.3780g. The discs were polished with 60, 600 and 1000 grit sandpaper and were finally stored in airtight packages with silica desiccant until required. 2.2. Materials characterization 2.2.1. Swelling measurements The time-dependent swelling behaviour (St) of isotropic X2 hydrogels was determined. Each swelling experiment was repeated three times. St was determined from the mass change as a function of time following the equation
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St
(M t M d ) Md
(1)
where Md represents the mass of the xerogel and Mt is the mass of the hydrogel at time t. All swelling experiments were performed at (37±1)C, with measurements continued until an equilibrium swelling ratio (S∞) was achieved. Each mass measurement was determined by extracting the sample from a solution bath and removing the free solution with filter paper before weighing. The samples were designated as Sx, where x represents the soaking time in minutes. In EIS, a small sinusoidal potential was applied across the sample and the current through the sample measured. Hence, an electric field will polarize the water molecules in the hydrogel. From these, the impedance was obtained for every operating frequency. The applied voltage was set small (30 mV) in order to minimize possible charge carrier concentration changes and the time average of the changes during the measurements. Xerogels were soaked in excess amount of distilled water (DW). At every 30 min interval, the hydrogel was taken out and its mass, thickness and diameter were recorded. After drying, the hydrogel was then sandwiched between two stainless steel disc electrodes and the impedance of the hydrogel measured with a HIOKI 3520 LCR Hi-Tester in the frequency range from 50 Hz to 100 kHz at room temperature. A typical Nyquist plot obtained from the soaked hydrogels is as shown in Fig. 1. It can be seen that the Nyquist plot consists of a partially depressed or tilted semicircle and a tilted spike. From the Nyquist plot, the bulk impedance can be obtained and with it the conductivity of the soaked hydrogel can be calculated following the equation reported in literature [13].
Fig. 1. Typical Nyquist plot for the hydrogel obtained in this work.
Following Linford [14], the Nyquist plot of the hydrogel can be the same as that of an equivalent circuit consisting of a parallel connection of a resistor and a leaky capacitor in series with another leaky capacitor. The impedance of the equivalent circuit can be written as [12]:
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Zr =
p 1 R R 2 k1 cos 1 2 p1 p 2 1 2 Rk1 p1 cos 1 R 2 k1 2 2
+
(2)
Zi =
p 1 R 2 k1 p1 sin 1 2 p1 p 2 1 2 Rk1 cos 1 R 2 k1 2 2 1
p1
p cos 1 2 1
k2 2
1
+
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p 2 p1
p sin 2 2 1
k2 2
p 2 p1
(3)
The value of p1 and p2 can be obtained from the Nyquist plot and their meaning is explained in Arof et al. (2014). R is the value of the bulk resistance that can be also obtained from the plot and k1-1 and k2-1 are the capacitance of the leaky capacitor in parallel with the resistor representing the bulk and the second serial leaky capacitor respectively. The angular frequency is represented by. It is to be noted that the value of k1 and k2 for every hydrogel sample were obtained by trial and error. With these values, the transport parameters such as number density of free water molecules contributing to conductivity (n) and diffusion coefficient of the free water molecules (D) can be calculated through the equations [12]:
= =
(
(4)
)
(
)
(5)
A is the surface area of the hydrogel, T is room temperature in kelvins, o is vacuum permittivity and 2 is the reciprocal of at the bottom of the spike.r is present in all the above equations and is the dielectric constant of the hydrogel. The value of r is obtained from the corresponding impedance data using the equation [15]:
=
(
+
)
(6)
The value of r for use in the equations for n and D was obtained in the frequency region where log r is constant, Fig. 2. 2.2.2. Fourier transform infrared spectroscopy Fourier transform infrared (FTIR) spectroscopy was performed to identify the carbonyl functional group and the C-O-C band. The shift in wavenumber of these bands can indicate interaction between the polymer and the water molecules. The identification of these bands was done using the Thermo Scientific model Nicolet iS10 spectrometer at 4 cm-1 resolution. The spectrum was recorded in the transmittance mode from 650 to 4000 cm-1. Deconvolution of the several IR regions were conducted by fixing the number and line shape, and allowing band parameters such as full width at half maximum, area, intensity and band shape to vary without constraints during the iteration. The Gaussian/Lorentzian function was used to fit the selected bands of the hydrogels and all the deconvoluted spectra was best-fitted with constant baseline.
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Fig. 2. Dielectric constant, r variation with frequency.
3. Results and discussion As shown in Fig. 1, by drawing a circle through the partially depressed semicircle, the tilt of the semicircle becomes more obvious and the diameter of the depressed semicircle is observed below the real impedance axis. It may also be implied that the Nyquist plot will pass through the origin. All log r versus log f for samples soaked at different times show similar profiles i.e. log r decreases with frequency increase and is constant at very high frequencies in the frequency range studied. The decrease in r in the frequency region between 50 and 10 000 Hz may be attributed to the tendency of the polarized water molecules to align themselves in the direction of the applied electric field. As frequency increases (> 104 Hz), the polarized water molecules could not align themselves with the applied field and the dielectric constant decreased less rapidly. In other words, due to the very fast periodic reversal of the electric field the value of log εr is almost frequency-independent since there is almost no excess water molecule diffusion in the direction of the field [16]. The trend of log r versus log f in this work is similar to that for polymer electrolytes [17,18]. The Zr and Zi are used to calculate the r values. The r values, together with other parameters for use in the equations of n and D are listed in Table 1. Table 1. Parameter values used in the equations for Zr and Zi and that needed to calculate n and D. Sample S30 S60 S90 S120 S150 S180 S210 S240 S270 S300 S330 S360 S390 S410 S440 S500 S530 S590 S650 S710
Conductivity, × 10-5 (S cm-1) 3.10 2.30 3.23 3.47 4.34 5.19 4.91 5.58 4.58 3.86 6.11 5.73 6.92 6.33 6.44 4.37 4.66 4.40 3.60 3.10
p1
p2
k1-1 (pF)
k2-1 (µF)
r
0.85 0.94 0.95 0.95 0.95 0.96 0.96 0.97 0.96 0.96 0.95 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.96 0.97
0.79 0.75 0.69 0.82 0.83 0.79 0.79 0.78 0.81 0.79 0.80 0.80 0.82 0.78 0.79 0.79 0.77 0.80 0.78 0.79
0.41 0.10 0.17 0.19 0.18 0.16 0.13 0.15 0.14 0.14 0.16 0.14 0.13 0.15 0.15 0.14 0.16 0.14 0.15 0.12
10.00 11.49 22.73 28.57 25.00 30.30 30.53 35.71 28.98 30.30 28.57 27.40 26.32 29.41 27.78 24.39 28.57 20.83 21.74 20.00
56 48 91 99 99 100 94 118 99 95 90 96 88 105 103 100 109 105 105 106
2
× 10-5 (s) 3.97 3.18 3.18 8.38 5.31 3.98 3.98 3.98 5.31 5.31 3.18 3.98 2.84 3.18 3.18 5.31 3.98 5.31 5.31 5.31
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PMMA and PVP are low dielectric constant materials. PVP has a dielectric constant of 2.3 [19] and PMMA has a dielectric constant of 4.9 at 100 Hz [20]. DW has dielectric constant of ~80. At early soaking times, the dielectric constant of the hydrogel is less that that of DW. However as more water molecules entered the hydrogel, the dielectric constant increased and is greater than that of DW after 90 min (i.e. after three 30 min intervals) of soaking. This implied that polarization has increased with soaking time and more water molecules have entered the hydrogel. Fig. 3 (a) and (b) show typical FTIR spectra for the hydrogel in the region from 1510 to 1800 cm-1 and from 2600 to 3800 cm-1. These two IR envelopes were deconvoluted in order to determine the existence of free water and aggregated water with strong and moderately strong hydrogen bonds. Free water molecules are water molecules not attached to the heteroatoms of the hydrogel polymers. In this work, it could be the oxygen atom in the carbonyl group of PVP and PMMA and the C-O-C group in PMMA. From Fig. 3 (a), four peaks were deconvoluted; the peak located at 1639 cm-1 belongs to the carbonyl stretching [ν(C=O)] of PVP. Three other peaks found at 1598, 1687 and 1713 cm-1are attributable to the free water, hydrogen bonded water and carbonyl group of PMMA respectively. In Fig. 3 (b), six component bands located at 3096, 3210, 3295, 3390, 3495 and 3610 cm-1were best-fitted upon the deconvolution of the IR envelope attributable to O-H stretching mode of water. The presence of the IR bands due to water indicates that water is contained within the PVP-PMMA-based hydrogels.
Fig. 3. Typical deconvolutions of the FTIR spectrum in the wavenumber region of (a) 1510 to 1800 cm-1and (b) 2600-3800 cm-1.
The mass of water entering the xerogel for a period of almost 12 h is as shown in Fig. 4. In the figure, the mass of water is represented as the number of water molecules entering the hydrogel. The conductivity profile is also shown in the figure. Conductivity is attributed to the movement of polarized free water in the direction of the applied electric field. The water molecules that are attached to the polymers that make up the hydrogel through strongly and moderately strong hydrogen bonds are immobile. From the profile obtained, it may be implied that the rate of diffusion of water molecules will reach an asymptote and the hydrogel swelling will reach a maximum. The conductivity can be considered as being quadratically related to the soaking time via the equation:
( )=
3
10
+ 2
10
+2
10
(7)
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Fig. 4. Profile of water intake (in terms of number of water molecules) into hydrogel and conductivity of hydrogel as a function of soaking time.
By this equation, the maximum conductivity occurred at 333 1/3 min of soaking time after which conductivity decreased. Hence it can be observed that although water uptake continued to increase, conductivity decreased after the optimized soaking time. To further strengthen the attachment of water molecules to the polymers consider the report by Hodge et al. [21] that have explained water molecule attachment to the polar groups of a polymer and in the case of poly(vinyl alcohol) or PVA, it is with oxygen in the hydroxyl group. Kitano et al. [22] have also reported that water can form hydrogen bond to the oxygen atoms in the ether and ester groups of poly(methoxyethylacrylate) or PMEA side chains. In this work water attachments to the PVP and PMMA polymer can occur at the carbonyl oxygen and in PMMA it can also occur at the oxygen of C-O-C group. It may therefore be inferred that after 333 1/3 min, most of the water molecules will attach themselves to the heteroatoms of the polymers that make up the hydrogel. Hence there is less polarized free water molecules that contribute to conductivity. These results have shown what can happen to the water molecules when subjected to an applied voltage as in impedance spectroscopy. From the above discussion, we have differentiated between water molecules that may have attached themselves to the polymer through the oxygen atom in the carbonyl functional group of both polymers or through the oxygen in the C-O-C group of PMMA and free water molecules that can move in the direction of the electric field and contributing towards conductivity. Therefore, the number of the conductivity contributing water molecules should at anytime be less than the amount of water diffusing into the hydrogel. Our calculations on the number density of conductivity contributing water molecules are illustrated in Fig. 5 and the number density of conductivity contributing water molecules is compared to the number of water molecules entering the hydrogel. It can be observed that the number density of conductivity contributing water molecules is a maximum at 300 min. This implies that the quadratic equation relating conductivity (σ) and soaking time t is reasonably accurate since it predicts that a maximum in conductivity to occur at 333 1/3 min. The number of water molecules entering the hydrogel is observed to increase at a decreasing rate. This implies that the diffusion of water into the gel decreased with time. Comparison of the number of water molecules entering the gel and the diffusion coefficient of water molecules in the hydrogel is as illustrated in Fig. 6.
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Fig. 5. Profile of water uptake into hydrogel and the number density of charge carriers in hydrogel. The dashed line serves as a guide to the eye.
Fig. 6. Profile for intake of water molecules into hydrogel and diffusion coefficient of charge carriers as a function of soaking time. Dashed line serves as guide to the eye.
It should be noted that the diffusion coefficient is high when the number of water molecules entering the hydrogel is small. Thus, on entering the hydrogel, the water molecules diffuse at a faster rate. The diffusion coefficient decreases as more water molecules entered the hydrogel, until a minimum is reached, which corresponds to the maximum in number density of water molecules contributing towards conductivity at 300 min. It can be observed from Fig. 5 that the minimum diffusion coefficient occurs at 300 min. After 300 min, there are lesser number of water molecules contributing to conductivity probably due to more incoming water molecules attaching to the heteroatoms of the polymer. Conductivity decreased, but diffusion coefficient increased. This again makes it clear that diffusion is high when there are less mobile and polarized water molecules in the hydrogel.
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The diffusion coefficient for distilled water into the gel as reported by Swan et al. [1] was 1.63 × 10-3 cm2 s-1. It is to be noted that the results shown in the above figure is the diffusion coefficient of water molecules entrapped in the hydrogel. It can be observed that the diffusion coefficient is between four to five orders of magnitude lower than the results reported by Swan and co-workers. A plot of conductivity versus water molecule uptake is as shown in Fig. 7. It can be inferred that in the early soaking hours, there may be competition between water molecules attaching themselves to the polymers comprising the hydrogel and non-attaching or free water molecules. When 2 × 1022 water molecules have entered the hydrogel, most of the water molecules attach themselves to the polymer resulting in the decrease in conductivity. As more water molecules diffuse into the gel, a bigger portion did not attach to the polymer and conductivity increased. A maximum in conductivity occurred when about 6 × 1022 water molecules are in the hydrogel after which conductivity decreased that can be attributed to increased water molecules attached to the polymer.
Fig. 7. Room temperature conductivity versus water uptake. (Dashed line serves as guide to the eye).
As mentioned above, water attachment in PMMA could take place via hydrogen bonding to the oxygen in the carbonyl group and/or through the oxygen in the C-O-C component of PMMA. Since PVP also has a carbonyl band, hydrogen bonding can also be expected at the oxygen atom of PVP. It is thus useful to know at which oxygen atom water attachment occurred. Fig. 8 depicts the IR region between 1500 and 1800 cm-1 of the hydrogels which shows the C=O stretching (ν(C=O)) bands belonging to PVP and PMMA. The ν(C=O) band belonging to PVP for the S0 sample containing no diffused water is located at 1639 cm-1. When soaked, it can be observed from the plots that this band did not show any significant shift which may imply that water does not form any hydrogen bond with the oxygen in the carbonyl group. Ericson et al.[23] also reported no carbonyl peak shift in the FTIR spectrum of their samples. On the other hand, theν (C=O) band of PMMA which was originally located at 1713cm-1 was not observed when soaked in water. Soaking was prolonged to1260 min or 21 h to ensure whether there is shifting or otherwise. Fig. 9 depicts the IR spectra of PVP-PMMA-based hydrogels between 1250 and 1350 cm-1. As observed in Fig. 9 (a), the IR spectrum of S0 displayed three peaks; the CH2 wagging of PVP was located at 1318 cm-1 whereas the CO stretching (ν(CO)) of PMMA were found at 1276 and 1289 cm-1. When soaked into water, only the ν(CO) mode of PMMA displayed significant wavenumber shifting from 1289 cm-1 to 1294-1295 cm-1 in samples soaked at various times. This indicates that water has interacted with the C-O group of PMMA.
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Fig. 8. IR spectra in the region between 1500 and 1800 cm-1of (a) S0, (b) S60, (c) S120, (d) S240, (e) S360 and (f) S1260 which show the ν(C=O) band of PVP.
Fig. 9. IR spectra in the region between 1250 and 1350 cm-1 of (a) S0, (b) S60, (c) S120, (d) S240, (e) S360 and (f) S1260.
Fig. 10 shows the variation of area % of free water and hydrogen-bonded water as a function of soaking time. It was observed that the area % of free water increased up to 240 min before decreasing beyond that soaking duration. Hydrogen-bonded water on the other hand decreased with increasing soaking time up to 240 min before increasing above that duration. This is within reasonable agreement with the decrease in conductivity as depicted in Fig. 4. The
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increase in area % of hydrogen-bonded water shows that there is increase in attachment of the water molecules to the hydrogel.
Fig. 10. Area % of free water and hydrogen-bonded water as a function of soaking time.
4. Conclusions In this work, the hydrogel ratio of 90 wt.% polyvinylpyrrolidone and 10 wt.% poly(methyl methacrylate) were soaked in distilled water. The number of water molecules entering the hydrogel was calculated based on the mass change for different soaking times. Since highly polarized water molecules can exhibit conductivity when subjected to an electric field, a maximum in conductivity could be observed in the conductivity-soaking time plot. The increase and decrease in conductivity imply that some water molecules in the hydrogel are mobile. The infrared mobile waterband was observed at 1598 cm-1 and the immobile water bands can be observed at 1687 cm-1. The plot for number density of conductivity contributing water molecules versus soaking time can also imply the presence of these types of water. The immobile water molecules have attachment to the polymers at the oxygen atom of the CO-C group in PMMA. From the results shown water molecules diffuses at a faster rate when there were less conductivity contributing water molecules. Acknowledgements This work is supported by the High Impact Research MoE Grant UM.C/625/1/HIR/MOHE/ DENT/21from the Ministry of Education (MoE) Malaysia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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