Electrochemical devices based on natural polymers are of low cost and have excellent conductive properties. The ionic conduction and electrochemical stability of biopolymer-based electrical devices have many applications like in bioelectrodes and gas sensors (1–4). Chitin [poly-b(1–4)-N-acetyl-d-glucosamine] and chitosan [poly-b(1–4)-d-glucosamine] (CS), which are naturally occurring polysaccharides, have been shown to be valuable resources and efficient materials for modern times. Chitosan (cationic polyelectrolyte) is a biocompatible and biodegradable natural polymer extracted from chitin. It shows excellent biological, electrical and chemical properties. The presence of free amino and hydroxyl groups on its backbone has given a vital prospective for the preparation of functional polymers. Biopolymers including CS have good conducting properties for electrode coatings but have poor mechanical properties. These biopolymers have properties fairly different from those of synthetic polymers, and efforts have also been made to evaluate the conductivities of CS-based membranes (5–7). Blending is used to adjust the properties of polymers. Thus, smart polymeric blends appropriate for specific applications can be obtained, which is not possible from individual polymers (8–11). Biopolymer-based electrolytes have attracted much attention owing to their low cost and exceptional environmental protection. In this regard, CS (highly hydrophilic with better chemical properties), a cost-effective natural biopolymer, is believed to be a promising contender as a conducting polymer electrolyte. It can be modified chemically by the hydroxyl (-OH) and amine (-NH2) groups. The polysaccharide ring of CS helps conserve the thermal stability (1).
Both natural and synthetic polymers are used for blending purposes to get the collective benefits of natural and synthetic polymers. The blends of CS with other synthetic polymers have attracted much interest (12, 13). These blends showed excellent properties such as improved thermal, mechanical and surface properties (10). Poly(vinyl alcohol) (PVA) is a biocompatible and biodegradable polymer that improves the mechanical properties of the blends (9, 14–16).
Chitosan and PVA are considered to be a group of materials that have characteristic properties including thermal, electrical and mechanical, and optic-like conductive metals and inorganic semiconductor materials. The blends of CS with PVA have good mechanical, thermal and surface properties (8–10, 16–19). The use of biopolymer membranes helps in the formulation of an economically smart method for bioelectric applications. Biopolymer-based membranes are considered as smart materials with potential for commercial applications owing to their high selective and responsive properties; acid, base and solvent resistance; and a wide pH range with decent mechanical strength when blended with biocompatible synthetic polymers like PVA. PVA cross-linked with CS in this study was used to enhance their conductive capability, thermal stability, mechanical strength and surface behavior owing to the film-forming capability, significant constancy against chemicals and greater hydrophilic characteristics of PVA.
In this work, cross-linked CS and PVA blends of varying compositions were freshly prepared to analyze their various properties (16). The aim of this work was to determine the effect of temperature and silane cross-linker content on the electrical properties of the blended membranes and provide a clear direction for selecting the appropriate cross-linker amount. The effect of cross-linker content on the thermal, mechanical and surface properties was also investigated, which could be helpful for different biomedical applications like drug delivery systems for targeted/controlled release of drugs, tissue engineering for orthopedics, separation membranes for dialysis, artificial corneas for eyes, skin regeneration for burn, cell adhesive for wound dressing, ionic conductive batteries for cardiovascular pacemakers and health-care products like weight loss pills, dietary fiber, etc.
2 Materials and methods
Chitosan from crab shells having a degree of deacetylation >75% and a bulk density between 150 and 300 kg/m3, PVA (Mw: 146,000–186,000; 98–99% hydrolyzed), acetic acid, tetraethoxysilane (TEOS), methanol and sodium hydroxide were purchased from Sigma-Aldrich (Milwaukee, WI, USA). All of these chemicals were of analytical grade and used as received.
2.2 Method: preparation of the blends
The blends of CS and PVA cross-linked by TEOS were prepared by published method (16). CS solution was prepared in acetic acid (0.5 mol/l), whereas that of PVA was prepared in deionized water at 80°C. Both solutions were blended together at a ratio of 95:5 (CS/PVA) with dropwise addition of TEOS under constant stirring. After 1 h, the solution was poured into plastic plates for air drying. The resulting membranes were vacuum dried at 60°C for 8 h. The codes of the blends were HG2, HG4, HG6, HG8 and HG10, where the numbers 2, 4, 6, 8 and 10 represent the concentrations of the cross-linker used, respectively.
2.3 Characterization of the blended membranes
High-temperature (323–403 K) impedance measurements were carried out using an Agilent E4980A (20 Hz–2 MHz) Precision LCR Meter (Santa Clara, CA, USA). The AC signal amplitude used for all measurements was 1 V. Appropriate sized membranes (2.5×2.5 cm) were cut, connected through wires on the membranes by indium/gallium paste and cured at 60–65°C for 1 h. The thickness of the blended membranes was between 135 and 140 μm, and the area between the contacts was about 1.0 cm2 as determined by a micrometer at different positions and the average value was reported. ZView software (Scribner Associates Inc., Southern Pines, NC, USA) was used for fitting the measured results (20).
A Perkin-Elmer (Waltham, MA, USA) Pyris 1 differential scanning calorimetry (DSC) instrument was used to perform thermal scanning measurements. The samples were scanned from -90°C to 190°C at a heating rate of 10°C/min.
The mechanical properties were evaluated using a mechanical tester (BOSE, Model ELF 3200; BOSE Corporation, Eden Prairie, MN, USA). The size of the specimen was 20×10 mm, and the distance between the two holders was 12 mm. The experiments were carried out at room temperature (∼22°C) and 50% relative humidity. Five specimens were tested for each membrane, and the average value was recorded.
For surface analysis/wettability, the membranes were freshly synthesized 1 week before analysis, dried at room temperature, then vacuum dried and stored in an inert atmosphere. The surface wettability of the membranes was calculated by measuring the static water contact angles at room temperature. A contact angle goniometer (model 100-00-220, Ramé-Hart Instrument Company, NJ, USA) was used to measure the contact angle by using the drop method. The membranes were placed on the sample stage of the instrument. A steady drop of deionized water was deposited on the membranes, and the contact angle was measured for a specific interval of time from 0 to 32 min until the drop disappeared completely. Each measurement was performed three times, and the average value was calculated and reported.
3 Results and discussion
3.1 Impedance spectroscopy
Impedance spectroscopy measurements of the CS/PVA membranes were performed in the frequency range of 25 Hz–2 MHz at different temperature ranges (323–403 K). Figure 1A–E shows the imaginary part of the impedance (Bode plots) of all membranes. The figure shows that the conductivity increased (Z″ decreased) with an increase in temperature for all samples. Also, the conductivity was frequency dependent, showing a peak in Z″. The peak shift and reduction in height toward higher frequency are indicated by an arrow, showing an increase in conductivity with an increase in temperature. The conductivity increased up to 393 K in the HG2, HG4 and HG6 samples, whereas it increased up to 373 K in the HG8 and HG10 samples. This behavior elucidates that the conductivity increased up to a smaller temperature range in the highest cross-linked membranes. This phenomenon shows that the number of effective charge carrier ions as well as their mobility increased up to 6% of cross-linked membranes at higher temperature ranges. At higher amounts of cross-linker contents (>6%, as in HG8 and HG10) and at higher temperatures (>373 K), the number of effective charge carriers decreased, exhibiting a decrease in the conductivity at higher frequency. The silane formed silanol containing the -OH groups and was condensed with the OH group of CS and PVA via siloxane (Si-O-Si) linkage (16). This shows a decrease in the available number of free charge carriers in the system (cross-linker content from 8% to 10%) even at higher temperatures. This phenomenon can also be elucidated by the hydrogen bonding between NH and CO in the system. The presence of a single peak shows the occurrence of one relaxation process except in the HG8 sample at 343 K. The relaxation frequency at each peak value of
The Nyquist plots of cross-linked CS/PVA membranes are shown in Figure 2A–E, where the arrow shows the direction of increase in frequency. The plot exhibits one semicircular arc at higher frequency for each blend and a low frequency spike (big loop), which might be due to the contact wire between the two electrodes. In a complex plane, a spike corresponds to the highest resistivity toward the ions. The ions cannot pass through the area between the two electrodes. Consequently, the ions have to face the highest resistivity and, hence, the lowest ionic conductivity is observed. In HG2, the length of the spike increased up to 343 K, decreased up to 373 K and then remained constant up to 403 K. In HG4 and HG6, it increased up to 353 and 333 K, respectively, decreased up to 393 K in HG4, remained constant up to 353 K and then decreased up to 403 K in HG6. In HG8, the length of the spike increased up to 373 K as compared to the HG2, HG4 and HG6 samples, owing to the higher amount of the cross-linker. In HG10, no spike was observed up to 313 K, which started at 323 K, increased up to 343 K, decreased at 353 K and finally remained constant up to 373 K. The intersection of the curve at high frequency (left-hand side) passes through the origin, whereas the intersection at low frequency exhibits the total resistance of the membranes. The relaxation time can be calculated using Equation 1:
where Rb is the bulk resistance and C is the capacitance related to that phase (21). Figure 2A–E indicates that Rb (Z″ ohms) decreased up to 403 K in HG2, HG4 and HG6 toward a higher frequency and then increased again at higher temperature toward a lower frequency. In HG8, an anomalous behavior was observed at 323 and 343 K; resistance first increased and then decreased up to 363 K and finally increased up to 373 K, whereas in HG10, resistance decreased up to 373 K. At higher temperature, free charge carriers were in saturated form, where more siloxane linkages formed between the active sites of the CS/PVA membranes. HG10 was the most resistive membrane at higher temperature, which can be seen in Figure 2A–E.
3.2 Differential scanning calorimetry
The amounts of different states of water such as free water, intermediate water and bound water in the membrane network were determined by DSC. The water that does not form a hydrogen bond with polymer molecules is free water. Intermediate water (freezing bound water) forms a weak hydrogen bond with polymer molecules, whereas bound water (non-freezing water) forms a hydrogen bond with polymer molecules (22–24). The DSC curves and enthalpy of free water were same as those of pure water. The thermogram of swollen membranes is shown in Figure 3. The bound water exhibited no endothermic peak in the range of -70°C–0°C. The melting endothermic heat of fusion of pure water was 79.9 cal/g. The endothermic peak area of the corresponding swollen membrane divided by the heat of fusion for pure water provided the amount of free water in the total water (22). The endothermic peak of swollen membrane appeared between 5°C and 10°C. The amount of bound water, free water and intermediate water can be calculated by the following equations (22–24):
where Wb is the amount of non-freezing bound water, Wt is the equilibrium water content (EWC, %), Qendo is the heat of fusion of free water in the sample and Qf is the heat of fusion of ice. Wfb and Wf are the amounts of freezing bound water and free water, respectively.
The calculated values of non-freezing bound water (Wb), free water (Wf) and EWC (%) are given in Table 1. EWC values were calculated from swelling data reported elsewhere (16). The amount of bound water increased with the increase in cross-linker content as the hydrogen bonded water molecules increased with the -OH groups of silanol and hence decreased the free water, which had no interaction via hydrogen bonding. This indicates that the amount of free water decreased with the increase in cross-linker content and had no contact with polymer molecules, whereas the bound water formed hydrogen bonding with polymer molecules.
Different states of water of the CS/PVA blend membranes calculated from DSC.
|Sample||EWC (%)||Qendo/Qf (%)||Free water (Wf) (%)||Bound water (Wb) (%)|
3.3 Mechanical properties
The tensile strength (TS) and elongation at break (Eb) of the silane cross-linked CS/PVA membranes were determined, and the results are shown in Figure 4. It was observed that TS increased gradually (from 7.7 to 8.7 MPa) as the cross-linker content increased. CS/PVA membranes with the maximum cross-linker content had the highest TS, i.e. 8.70 MPa. The enhancement in TS was due to the interaction between the -NH2 and the -OH groups of CS and between the -OH groups of TEOS and PVA (10, 18, 25).
It was also observed that all membranes had lower elongation at break with the increase in cross-linker content. This confirms that the silane cross-linked membranes had shown less flexibility and more brittle behavior (18).
3.4 Water contact angle
The water contact angle of CS/PVA membranes plotted against time is shown in Figure 5. The figure shows that the water contact angle decreased with time, which indicates that the surface wettability increased. Initially, the contact angle of the membranes increased with the increase in the amount of cross-linker. This exhibits the less hydrophilicity of the membranes, and, also, the value of the contact angle increased at higher cross-linker content. This confirms that the amount of water penetrating into the network was reduced and, hence, the hydrophilicity of the membrane surfaces decreased. The membranes exhibited a contact angle of <90°, showing the hydrophilic character of the membranes (10).
The electrical, thermal, mechanical and surface properties of silane cross-linked CS/PVA blend membranes were investigated. The conductivity of the membranes increased with the increase in temperature. Results have shown the quantitative determination of freezing bound water and non-freezing water. It was observed that the free water decreased and the bound water increased with the increase in cross-linker content. The tensile strength increased and the elongation at break decreased with the increase in cross-linker content. The surface properties showed that, with the increase in cross-linker content, a small amount of water penetrated into the network. Additionally, the surface wettability of the blend membranes increased, exhibiting the hydrophilic character of the blend membranes.
So, from the results obtained, it is obvious that the increase in concentration of the cross-linker provides the maximum efficiency for improving the characteristic properties of polymeric membranes and, also, a successive improvement in the conductive properties with the increase in temperature was observed.
Atif Islam is very grateful to the Higher Education Commission of Pakistan for providing him with the IPFP fellowship to work at the Department of Polymer Engineering and Technology, University of the Punjab, Lahore, Pakistan.
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