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ABSTRACT: In the present investigation, blends of poly(vinyl alcohol) (PVA) and poly(ethylene oxide) (PEO) were prepared to develop membranes for wound care applications. Carboxymethyl cellulose (CMC) was added as compatibilizer to PVA- PEO blend and the influence of CMC on the compatibility of PVA and PEO was studied. It was found that addition of CMC led to the stabilization of PVA and PEO system. The preparation of PVA/PEO/CMC blends having CMC content 5, 10, and 20 wt% was carried out. The blends were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and attenuated total reflectance-fourier transform infra-red (ATR-FTIR) techniques. The swelling behavior of the blend hydrogel at different pH was also studied. From the experimental results it has been found that the partial miscibility exists between PVA, PEO and CMC because of the formation of hydrogen bonds between the polymers. Swelling ratio of hydrogels increases with the increase in concentration of CMC and blends also show pH responsiveness. These blends have good prospect to be used as wound dressing because of their outstanding swelling behavior and good biocompatibility.
Key words: Polyvinyl alcohol; Polyethylene oxide; Carboxymethyl cellulose; Miscibility, Hydrogels.
*Corresponding author: Tel.: +91 11 26591416; Fax: +91 11 26581061
E-mail address: email@example.com (B. Gupta)
Hydrogels belong to one of the most significant polymeric materials being used for various biomedical applications. [1,2] These can be made by irradiation, freeze-thawing or blending. Blending of polymers is a good method for obtaining materials having required mechanical and thermal properties in comparison to pure components. It is also a simple method by which suitable shapes, such as films, sponges and hydrogels can be obtained for various biomedical applications. This method also helps to prepare improved materials exhibiting advantage of all the different materials. [3-5]
Poly(vinyl alcohol) (PVA) is a water-soluble semicrystalline polymer, used in many biomedical applications as it can be easily prepared by the hydrolysis of poly(vinyl acetate). It has good chemical resistance and appropriate physical properties. [6,7] It is widely utilized in many medical and chemical industries because of its nontoxicity, biocompatibility, hydrophilicity and good fibre and film forming ability.  The chemical structure of PVA favors the formation of intramolecular [9,10] hydrogen bonding, thus it is used in the preparation of various membranes and hydrogels. PVA blends can be cast as films and can be used as dialysis membranes, wound dressing, artificial skin, cardiovascular devices and drug delivery vehicles. [7,11-15] On the other hand, poly(ethylene oxide) (PEO) is hydrophilic semicrystalline polyether which exhibits biocompatibility, nontoxicity, non polarity, non antigenicity, and non immunogenicity.  For these reasons, PEO hydrogels are used in number of biomedical applications like dressings, hemodialysis membrane and drug delivery vehicles. [9,17-19]
Mishra et al.  found that miscibility of PVA and PEO likely to exist over only a small range of compositions and the mixtures otherwise seems to form only microscopically immiscible blends. It is also found that PVA and PEO are immiscible and incompatible blends which does not possess a tendency for extensive mutual solubility [9,20]. So in order to prepare a blend of PVA and PEO we found that if a cellulose derivative is used as a compatibilizer between these two polymers then the blend prepared can be miscible. Xiao et al.  found that a cellulose derivative, CMC and PVA in different ratios can be mixed homogeneously in an aqueous solution and Kondo et al.  proposed the mechanism for the development of interaction in the cellulose/PEO blend and showed that the hydrogen bonding between the C6 position hydroxyls of cellulose and skeletal oxygen of PEO is more favorable. At first, the two polymers cellulose and PEO are trapped to form a large adduct or a complex and their movement is restricted. Another PEO molecule interacts with the adduct either by hydrogen bonding between the remaining free hydroxyls in cellulose and oxygen in PEO, or by Vander Waals bonding between PEO molecules.  Thus it can be concluded that a cellulose derivative such as CMC can be used as a compatibilizer between PVA and PEO.
CMC is ether derivative of cellulose in which H atoms of hydroxyl groups are replaced by carboxymethyl groups (-CH2COOH). It is often used as its sodium salt, Na-CMC and exhibits pH- sensitivity. It is used primarily due to its high viscosity, good water solubility, non-toxicity and biocompatibility. [23,24] In the present work, PVA and PEO blends were prepared by using CMC as the compatibilizer. The influence of concentration of CMC on the blends of PVA and PEO was determined. The characterization of PVA/PEO/CMC blends by various analytical techniques such as X-Ray diffraction (XRD), attenuated total reflectance-Fourier transform infra red spectroscopy (ATR-FTIR), differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA). Swelling behavior of the blend hydrogel was also studied.
Poly(vinylalcohol) (PVA) having degree of polymerization 1700-1800 and molecular weight 1,15,000 and Carboxymethyl cellulose (CMC) sodium salt of high viscosity was received from Loba Chemie Pvt. Ltd., Mumbai, India. Poly(ethylene oxide) (PEO) of molecular weight 3,00,000 was supplied by Sigma Aldrich. Deionised water was used throughout the study.
Preparation of Blends
Various concentrations of PVA, PEO and CMC solutions were prepared by dissolving PVA, PEO and CMC in deionized water at 70-80°C for about 8 h with stirring to become homogeneous mixtures. Ensuring that the total polymer concentration is 5% by weight, four different compositions were prepared. A known amount of solution is pourd in petridish and kept for drying firstly at room temperature for 24 h and then dried in air oven at 80°C for 2h so that the solvent is completely removed. Similar procedure of casting and drying was used to prepare films of different compositions.
Attenuated Total Reflectance-Fourier Transform Infra Red Spectroscopy (ATR-FTIR)
ATR-FTIR spectroscopy of thin films of samples was recorded on a Perkin-Elmer spectrophotometer in the wave number range of 650-4000 cm−1 using transmittance mode.
Density measurements of the film samples were carried out by taking into account the thickness of membranes of specific size by measuring thickness of the film by thickness tester and by measuring the weight of the sample on analytical balance. Weight in gram per cubic centimeter was represented as the density of the membranes.
X-Ray diffraction (XRD)
X-ray diffraction (XRD) patterns of the samples were recorded in the 2θ range of 5-40° on a Phillips X-ray diffractometer equipped with a scintillation counter. CuKα radiation (wavelength, 1.54 Çº; filament current, 30 mA; voltage, 40 kV) is used for the generation of X-rays. The degree of crystallinity of various samples was evaluated from the X-ray diffraction pattern by separating the crystalline and amorphous portions under the diffraction pattern using the following expression (1). 
Degree of Crystallinity = (ACr/Acr+ Aam ) X 100 (1)
where, Acr is the area under crystalline peak and Aam is the area under amorphous part.
Differential Scanning Calorimetry (DSC)
The DSC studies on the samples were carried out with a Perkin-Elmer DSC-7 system, in aluminium pans under nitrogen atmosphere. For this vacuum-dried samples were loaded, and the thermograms were run in the below mentioned temperature range under nitrogen atmosphere at a heating rate of 10°C/min. The weight of sample used in DSC was in the range of 5-10 mg. The melting temperature was obtained as the peak of the thermogram. The heat of fusion (ΔHf) is obtained from the area under melting thermograms. The heat of crystallization (ΔHf(crys)) of 100% crystalline pure PVA is obtained from the literature. The crystallinity of samples is obtained by the following expression (2): 
Crystallinity (%) =ΔHf/ΔHf(crys) X 100 (2)
where ΔHf is the heat of fusion of the sample obtained from the melting thermogram and ΔHf(crys) Is the heat of fusion of 100% crystalline PVA and Is taken as 161 J/g and heat of fusion of 100% crystalline PEO Is taken as 197 J/g. 
In high temperature DSC, all samples were heated from 50 to 150°C at a heating rate of 10°C/min, kept 5 min at 150°C, cooled to 50°C at the same rate, and kept 5 min at 50°C. Then, the samples were heated from 50 to 350°C at the same rate to record DSC curves. The thermal properties of the polymer blends were determined using two scans. The first heating scan, which was conducted to eliminate the residual water and solvent. The results reported in this work correspond to the second heating scan.
In low temperature DSC, all samples were heated from 30 to 120°C at a heating rate of 10°C/min, kept 5 min at 150°C, cooled to -50°C at the same rate, and kept 5 min at -50°C. Then, the samples were heated from -50 to 230°C at the same rate to record DSC curves. The thermal properties of the polymer blends were determined using two scans. The first heating scan, which was conducted to eliminate the residual water and solvent. The results reported in this work correspond to the second heating scan.
Thermogravimetric Analysis (TGA)
The thermal stability of the prepared samples is evaluated by Thermogravimetric analysis (TGA) performed on a Perkin-Elmer TGA-7, using a nitrogen stream as purge gas, at a heating rate of 10°C/min within the range of 50- 600°C. For this, the prepared samples are firstly vaccum dried at 100° C and then loaded in the crucible and the thermograms are run under nitrogen atmosphere from 50- 600°C.
All the blend samples were accurately weighed and placed in separate beakers having 50 ml phosphate buffer (PBS) pH 7.4 and then kept in a water bath undisturbed for 24 h at 37°C. The samples were removed after 24 h., and the excess surface water is removed by pressing gently between filter paper and weighed on an analytical balance. The swelling ratio was calculated using the following formula (3): 
Swelling Ratio (%) = (Ws - Wd) / Wd x 100 (3)
where Wd and Ws are the weight of dry and wet films, respectively.
The blends show pH responsiveness so the effect of pH of swelling medium on the swelling behavior was also studied by immersing the samples in solutions having varying pH (3, 5, 6, 7.4, 8, 9, 10) for 24h at 37°C. The samples were removed after 24 h., and the excess surface solution is removed by pressing gently between filter paper and weighed on an analytical balance. The swelling ratio was calculated using the formula (3)
RESULTS AND DISCUSSION
The blend membranes of PVA/PEO/CMC were prepared. The chemical structures of three components are presented in Figure 1. In PVA and PEO blend phase separation is clearly visible (Figure 2a). However, addition of CMC leads to significant changes in the miscibility of these two components. The compatibility in the blend increases as the CMC concentration increases from 5% to 20%. Our observations are also supported from the literature. [21, 22, 28] It is found that CMC and PVA can form homogeneous solution and PEO shows microphase separation with CMC. Also, the gel composed of equal amounts of CMC and PEO had the highest turbidity while the gel having 20% CMC has more than 90% transparency, data as given by Berg et al.  So if 20% CMC is taken as the optimized concentration then the membrane so formed is transparent to some extent.
The FTIR spectrum of pure samples along with blend is shown in Figure 3. The FTIR spectra of PVA shows C-H broad alkyl stretching band at 2933 cm-1 and typical strong hydroxyl bands for intermolecular and intramolecular hydrogen bonded band at 3287 cm-1. This vibrational band at 1140 cm-1 is mostly attributed to the crystallinity of the PVA,  related to carboxyl stretching band (C-O). The band at 1140 cm-1 has been used as an assessment tool of poly(vinyl alcohol) structure because it is a semicrystalline synthetic polymer able to form some domains depending on several process parameters.  The band at 1420 cm-1 is due to -CH2 group and at 1087 cm-1 is due to C-O-C group, the rocking vibration peaks of -CH2 appeared at 913 and 843 cm-1. The FTIR spectrum of pure PEO shows the stretching of ether groups from 1057-1143 cm-1. It also shows the characteristic C-H alkyl stretching band at 2881 cm-1 thus showing the major peaks associated with PEO. From the IR spectra of CMC, it is evident that it shows a broad absorption band at 3302 cm-1, due to the stretching frequency of the -OH group. The band at 2915 cm-1 is due to C-H stretching vibration. The presence of a strong absorption band at 1582 cm-1 confirms the presence of COO- group. It is worth to remark that in the CMC a part of the carboxylic groups are in acid form and a part in ionic form. The bands around 1411 and 1318 cm-1 are assigned to -CH2 scissoring and -OH bending vibration, respectively. The band at 1055 cm-1 is due to =CH-O-CH2 stretching.
The IR peak of blend having 20% CMC is shown in Figure 3. The IR peak ranges which are of interest in spectrum are C-O-C asymmetric stretch at 1080-1095 cm-1 i.e. 1087 cm-1, -OH broad peak in the range 3260-3295 cm-1 i.e. 3295 cm-1 and -CH stretching vibration in the range 2900-2920 cm-1 i.e. 2907 cm-1. These peaks in the spectrum of blend have been shown to shift due to interactions like hydrogen bonding between PVA, PEO and CMC. So, it can be concluded that blend shows characteristic peaks of all the components present.
The influence of concentration of CMC is evaluated and found that from Figure 4 that there is not appreciable difference in the density of air dried films with the increase of the concentration of CMC from 0 to 20%. However, as the concentration of CMC increases, the density of air dried films slightly increases from 0.947 to 1.192 g cm-3 as the hydrogen bonding between three hydroxyl groups in the anhydroglucose unit of CMC and the functional groups of the synthetic polymers PVA and PEO increases, this makes the system more interacting and compact, thus making the blend more dense.
X-ray diffraction (XRD) patterns of the blends and the pure components are shown in Figure 5. It may be seen that pure PVA exhibits three major peaks characteristic of polymer crystallinity at 2θ values of 14.1°, 16.9° and 19.0°, indicating the presence of low-degree crystalline ordering. PEO has two well-defined reflections at 2θ values 18.9° and 23.2°. These reflections are consistent with literature reports on crystalline PEO. [31-33] The blend (d) having PVA/PEO 90/10 shows only one reflection at 2θ values 19.7°. XRD analysis showed that CMC exhibits a very small crystallinity 17.4% and shows a peak at 2θ value 20.0°. It can also be seen from the Figure 5 that as CMC is added to the blends of PVA and PEO all the crystalline peaks of pure components get merged and show only one single diffraction at around 19.5° proving that CMC helps in increasing compatibility between the blends thus intermolecular interaction destroyed the regularity of pure polymers. Therefore, it can be concluded that the partial miscibility existed between PVA, PEO and CMC due to the formation of hydrogen bond between the polymers in the blends. The individual polymers interfere with the crystallization process and disrupt the crystallinity. The degree of crystallinity as calculated by the equation (1) is presented in Figure 6. As the concentration of CMC increases, the percentage crystallinity firstly increases then slightly decreases.
Thermal properties and crystallinity of the prepared samples were investigated by DSC method (Figure 7 and 8). PVA exhibited a relatively large and sharp endothermic peak at 225.4°C and PEO at 65.5°C. It is observed from Figure 7 that the melting point and melting enthalpies of the samples a, b, c, d are somewhat decreased from the pure PVA and PEO samples. This decrease in melting temperature might be related to a decrease in the crystallinity of the sample and proper alignment of the chains due to the interference of other polymers present in the blend. It was found that the melting temperature of PVA and PEO shifts towards a lower temperature when the PVA is added to the PEO, the change in Tm shows the change from semi crystalline to amorphous phase. The melting points of the blends show that the interaction between CMC and PVA weakens the interaction between PVA chains and hinders the crystallization of PVA. The decrease in crystallinity (%) of blends with the CMC content is presented in Figure 8. This reduction is also due to the decrease in the proper alignment of the chains due to the interference of other polymers present in the blend.
The thermal stability of the dry superabsorbent hydrogels was determined from 50°C to 600°C. The TGA results for pure components and blend sample (d) are shown in Figure 9. In the initial stage of the thermograms from 50°C to 200°C, the weight loss was due to the dehydration process of the water contained in the hydrophilic hydrogels. Three degradation steps can be observed in PVA sample. The first weight loss process is associated with the loss of absorbed moisture and/or with the evaporation of trapped solvent, the second degradation step mainly involves the elimination reactions, while the third one is dominated by chain-scission and cyclization reactions while PEO undergoes one step degradation. The thermal degradation of PEO occurs at a lower temperature when compared to other polymers without an ether backbone.  The C-O bonds are weaker and more susceptible for breakage.  PEO and PVA degrade without leaving any or very less residual mass. The decomposition products so formed were not tried to confirm as it is well known that the decomposition leads to evolution of lower molecular weight alkanes, alkenes, aldehydes, ketones, etc. and also acetaldehyde and acetic acid in the case of PVA. In sample d, two step degradation processes takes place. Pure CMC showed a two-step thermogram, where the major weight loss of ~50% occurred from 350 to 400°C. TGA of CMC showed two distinct zones where the weight is being lost those can be seen in Figure. The initial weight loss is due to the presence of small amount of moisture in the sample. The second loss is due to the loss of CO2 from the polysaccharide. The TGA profile of blend sample d shows that the regions of decomposition can be broadly divided into two regions where the first region of 200-300°C which was also attributed to thermal degradation of the side chains and the second region of 350 - 400°C with a major weight loss equal to 80%. This weight loss was attributed to some thermal degradation of the main chain C-C- bond of the blend components.
The effect of concentration of CMC in blends in PBS (pH 7.4) at 37°C is presented in Figure 10. It shows that as the concentration of CMC increases from 0 to 20% in the blends of PVA/PEO/CMC, the swelling ratio (%) of blends increases from 400% to 850%. The results can be explained by the fact that increasing the ratio of hydrophilic polymer in the blend increases the affinity for water and thus resulting in increased swelling ratio. As we know that blood pH is around 7.4 the blend formed will swell when it will come in contact with the blood and because of which voids will be created in the polymer network. These voids will help in entrapping the exudates that will come out from the wound surface so we can conclude here that these blends have good prospect to be used as wound dressing.
CMC is a kind of natural polyelectrolyte, which has many carboxylic groups in its molecular chain structure. The dissociation degree of carboxyl group is closely related to the pH value of the medium. So in order to investigate the influence of pH value of the medium on the swelling ratios of the hydrogels, the pH of the external solution has been varied in the range 3.0- 10 and the effect on the swelling ratio has been observed. The results are shown in Figure 11 which indicates that swelling increases upto pH 9 while beyond this pH a drop in the swelling ratio is observed. The reason for the observed decrease at higher pH may be that at higher pHs the ionic concentration becomes large in the external medium and, therefore, swelling decreases. 
In this work, attempts were made to prepare PVA and PEO blends using CMC as compatibilizer. These hydrogel blends were characterized by using XRD, FTIR, TGA and DSC techniques. Swelling behaviour of the samples were also measured. From the experimental data it can be concluded that CMC increases the compatibility in the blends of PVA and PEO. Single diffraction peak in the XRD of blends make us to conclude that the partial miscibility existed between PVA, PEO and CMC due to the formation of hydrogen bond between the polymers in the blends. FTIR of all the blends show characteristic peaks of all the components present. The decrease in melting peaks of the blend in DSC results shows that there is some interactions present in blends that result in decrease in crystallinity of the blends. Swelling behavior of the blends in PBS (pH 7.4) increases on increasing CMC concentration. The blends also exhibit pH responsiveness. Thus it is found that these blends are appropriate for wound dressing applications. The requirements for an ideal wound dressing are transparency, thermally stable, flexibility, biocompatibility and enough moisture absorption ability. From this point, PVA/PEO/CMC blends have good transparency, flexibility, biocompatibility, thermal stability and moisture absorption ability so these blends have good future to be used as wound dressings. Out of all the blends prepared, blend having 20% CMC shows highest swelling ratio in PBS (pH 7.4) which is an important criteria for an ideal wound dressing so 20% CMC concentration in the blend is taken as the optimized concentration for further studies of using this blend as advanced wound care product.