Carboxymethyl Cellulose On Blends Of Polyvinyl Alcohol Environmental Sciences Essay

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1st Jan 1970 Environmental Sciences Reference this

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ABSTRACT

Poly(vinyl alcohol) (PVA) – Poly(ethylene oxide) (PEO) blends were prepared and found that Poly(vinyl alcohol) – Poly(ethylene oxide) are inherently immiscible and therefore incompatible. So, a compatibilizer Carboxymethyl cellulose (CMC) is added to PVA and PEO and the influence of CMC is studied on the compatibility of blends of PVA and PEO. It is found that on adding CMC, PVA and PEO become partially miscible. Here, we describe the preparation of PVA/PEO/CMC blends having weight percentage of CMC 5, 10, 20 wt% and the influence of concentration of CMC on the blends of PVA and PEO is studied and the miscibility of the blends was characterized by using wide-angle X-ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Attenuated Total Reflectance-Fourier Transform Infra-red (ATR-FTIR) techniques. Also, swelling ratio of the different blends is studied.

Keywords: Hydrogels; Polyvinyl alcohol; Polyethylene oxide; Carboxymethyl cellulose; Miscibility, Immiscible.

*Correspondence to: Prof. Bhuvanesh Gupta, Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India. E-mail: [email protected]

INTRODUCTION

Designing new materials with improved properties is one of the main goals of the chemists. Two common ways are chemical synthesis and blending which mainly used to get a material with improved or new properties. Chemical synthesis is an unlimited method to get new substances with well-defined properties but it is often time consuming and not seldom costly. On the other side, blending is a well-known and simple method to combine the advantages of different materials [23,65], efficient way to prepare new materials with improved properties. [8] The blending of hydrophilic/hydrophobic polymers produce phase-separated composite hydrogels. Polymer blends exhibit superior and rare properties, unexpected from homopolymers. The physical, chemical and radiant methods can be applied to prepare polymer blends. [6] Polymer blends are physical mixtures of structurally different polymers or co-polymers, which interact through secondary forces such as hydrogen bonding, dipole-dipole forces and charge transfer complexes for homopolymer mixtures with no covalent bonding [34,36-38] that are miscible at molecular level. Polymer blend hydrogels are composed of water-soluble or swellable polymers, such as poly(ethylene oxide) (PEO) [25,26] poly(vinyl alcohol) (PVA) [24] and, other synthetic water-soluble polymers and degradable or nondegradable water-insoluble or swellable polymers, such as poly(lactic acid) (PLA) [25], poly(lactic acid-co-glycolic acid) (PGLA). [24] The most common method used to blend polymers is through solvent-casting techniques. In this process, two or more polymers are dissolved in a mutual solvent and the blends are obtained by evaporating the solvent. The resulting materials have a microphase – separated structure [25,26] and often improved miscibility via hydrogen bonding among polymers [24], resulting in transparent materials.

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Poly(vinyl alcohol) (PVA) is a water-soluble polyhydroxy polymer, used in practical applications because of its easy preparation, excellent chemical resistance and physical properties, appropriate mechanical properties [68,71], and it is completely biodegradable and cheap and the -OH groups can be a source of hydrogen bonding (H-bonding) and hence of assistance in the formation of polymer blends. Polyvinyl alcohol has excellent film forming, emulsifying, and adhesive properties. It is also resistant to oil, grease and solvent. It is odorless, nontoxic and has high tensile strength, flexibility, as well as high oxygen and aroma barrier. The chemical structure of PVA favors the formation of intramolecular [1] hydrogen bonding because of favorable disposition of relatively small -OH groups attached to alternate carbon atoms of PVA [20], thus it is used in the preparation of various membranes and hydrogels. Hydrogels are hydrophilic polymers having three-dimensional networks [27], and are most often defined as two-component systems where one of the components is a hydrophilic polymer and the second one is water. These have the ability to swell in the presence of water without dissolution because of a three-dimensional network joining as chains. The interactions responsible for water absorption by hydrogels include the processes of hydration, which is connected to the presence of such chemical groups as -OH, -COOH, -CONH2, -CONH-, and -SO3H, and the existence of capillary areas and differences in osmotic pressure. [67]

PVA blends can be cast as films and applied as functional materials including biomedical materials such as dialysis membranes, wound dressing, artificial skin, cardiovascular devices and as vehicles to release active substances in a controlled manner. [69-71] PVA hydrogels have been studied extensively but their properties need to be improved further for special applications. [2,6,7] In order to improve or modify the properties of PVA hydrogels, PEO is used to blend with PVA to form hydrogels which is hydrophilic semicrystalline polyether with a glass transition temperature below room temperature, biocompatible, non toxic, non polar, non antigenic and non immunogenic [45] and is highly desirable in most biomedical applications requiring contact with physiological fluids. For these reasons, PEO hydrogels are applied as wound coverings, drug delivery systems, hemodialysis membrane [1], as a component of a tissue sealant [15,16] and as a coating for medical devices [17], both poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) are industrially important polymers [75] and their blends can be of significant practical utility, but it is found that PVA and PEO are immiscible and incompatible blends [1, 75] which do not possess a tendency for extensive mutual solubility. [1] Also it is found that hydroxyl-containing polymers are self-associated and hence the competition between self association and interpolymer interaction plays an important role in determining the miscibility behavior of their blends. For example, poly(vinyl alcohol) (PVA) is miscible with three tertiary amide polymers poly(N-vinyl-2-pyrrolidone) (PVP) [54-58], poly(N,N-dimethylacrylamide) [59] and poly(2-methyl-2-oxazoline) [60], but is immiscible with another tertiary amide polymer poly(2-ethyl-2-oxazoline) (PEOx). [61] PEO is etheric in nature. However formation of weak H-bonds between PEO and PVA cannot be ruled out. The C-O-C bond angle in PEO is normally 108° and when a -OH group from a neighbouring PVA chain approaches the etheric oxygen atom in order to form a H-bond, the C-O-C bond angle deviates from 108° so that the lone pair of the etheric oxygen is positioned nearer to the approaching OH from PVA. It would therefore be interesting to investigate the structure and thermal properties of the composites formed in the PEO-PVA system with different proportions of the components. We have found that mutual miscibility of PEO and PVA is likely to exist over only a small range of compositions. The mixtures otherwise seem to form only microscopically immiscible blends which do not possess a tendency for extensive mutual solubility. They are referred to as incompatible polymer blends or simply blends. [77] To make them compatible, a compatibilizer i.e. Carboxymethyl cellulose (CMC) is added.

Carboxymethyl cellulose (CMC) obtained from natural cellulose by chemical modification is a water soluble cellulose ether derivate [3] and is formed by its reaction with sodium hydroxide and chloroacetic acid. It has a number of sodium carboxymethyl groups (CH2COONa), introduced into the cellulose molecule, which promote water solubility. The various properties of CMC depend upon three factors: molecular weight of the polymer, average number of carboxyl content per anhydroglucose unit, and the distribution of carboxyl substituents along the polymer chains. The most important properties of CMC are viscosity building and flocculation. Among all the polysaccharides, CMC is easily available and it is also very cheap. It has high shear stability. The structure of CMC is shown in Figure 1. [78]

Figure 1 Structure of (a) Poly(vinyl alcohol), (b) Poly(ethylene oxide) and (c) Carboxymethyl cellulose

CMC has good water solubility, broadly used due to its low cost, biodegradability, biocompatibility [51] and lack of toxicity. [8,29-33] CMC is an ionic polyelectrolyte [30] that contains carboxyl groups and exhibits pH sensitivity as it has lot of carboxylic groups. [48-50] It has been used in several medical applications [10] and more recently as a component of an antiadhesion gel. [11,12] CMC and PVA in different ratios can be mixed homogeneously in an aqueous solution without evident phase separation, and this can be attributed to the interaction between the components. [49] The hydrogen bonds that form between the carboxylic groups of CMC and hydroxyl groups of PVA, and form semi-interpenetrating polymer networks [49] while with PEO, CMC undergoes micro phase separation to form a two-phase system. [9]

Berg et. al. [9] found that the turbidity results of CMC/PEO gels are demonstrated by transparency data. It is found that gels prepared either from CMC alone or from PEO alone were transparent. However, for CMC/PEO composite gels, the transparency of gels changed as the ratio of the two components changed. 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 so 20% CMC concentration is taken as the optimized concentration for further studies.

The polymer-polymer interaction for the miscibility is thought to be due mainly to hydrogen bonding between three hydroxyl groups in the anhydroglucose unit of CMC and the functional groups of the synthetic polymers PVA and PEO. However, since each of the three hydroxyl groups in the repeating unit of the cellulose is quite different in terms of regiochemistry and polarity, the hydrogen bond formation is not easily clarified. Kondo et.al. [47] proposed the mechanism for the development of interaction in the cellulose/PEO blend and showed that the hydrogen bonding between the C6 position hydroxyls and skeletal oxygen of PEO is more favourable, at first the two polymers are trapped to form a large adduct, which is a complex between cellulose and PEO, by the hydrogen bond, and the mobility of the molecules is restricted. Then 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. [79]

The purpose of the present paper is to investigate the influence of concentration of CMC on the blends of PVA and PEO. In this article, we report the characterization of PVA/PEO/CMC blends by various techniques such as X-Ray diffraction (XRD), infrared (ATR-FTIR) spectroscopy, Differential scanning Calorimetry (DSC) and Thermal gravimetric Analysis (TGA).

EXPERIMENTAL

Materials

Poly(vinylalcohol) (PVA) of Loba Chemie Pvt. Ltd., Mumbai, India having degree of polymerization 1700-1800 and molecular weight 1,15,000, Poly(ethylene oxide) (PEO) of Sigma Aldrich of molecular weight 3,00,000 were used. Carboxymethyl cellulose (CMC) sodium salt of high viscosity was received from Loba Chemie Pvt. Ltd., Mumbai, India. Distilled water was used for all experiments.

Preparation of Blends of PVA and PEO

Preparation of the pure film of PVA and blends of PVA and PEO were carried out in the following manner. PVA was dissolved in distilled water under constant mechanical stirring at temperature 60 -70 °C to get 5% PVA solution and then about 15 g. of PVA solution is poured to form layers 2 mm thick in a petridish at room temperature. The solution was first dried in air for 2 days and then in a vacuum oven at 100°C to remove solvent from it. Then, the blends of PVA/PEO/CMC were prepared by dissolving different concentrations of each polymer in distilled water, the total polymer concentration in the solvent remains 5% by weight. Water constitutes a suitable reaction medium, because PVA, PEO and CMC are soluble in water.

Each polymer having concentration as shown in Table 1 were added in distilled water one by one and then dissolved under constant mechanical stirring at temperature 60 -70°C. As shown in Figure 2, it was found that blend solutions formed with CMC shows compatibility as compared to the solution having no CMC i.e. solution (a). It is clear from the Figure 2 that compatibility in the blend increases as the CMC concentration increases from 5% to 20%. These blend solutions were then poured in petridishes at room temperature. The solutions were first dried in air for 2 days and then in a vacuum oven at 100°C to remove solvent from it. The films so obtained are then characterized by XRD, TGA, DSC and ATR-FTIR techniques to determine miscibility.

Table 1 Samples taken for characterization

Figure 2 Solutions prepared from the polymer sample to test compatibility

Swelling Ratio (%)

All the samples (a), (b), (c) and (d) in film form were accurately weighed and placed in a beaker having fixed volume i.e. 50 ml PBS (pH 7.4) and then kept in a water bath undisturbed for a fixed interval i.e. 24 h. The samples were removed after 24 h., and the excess surface water is removed by pressing gently between filter paper and weighed. The Swelling ratio(%) i.e. SR (%), was calculated as indicated in Equation given below.

SR (%) = (Ws – Wd) / Wd x 100

where Wd is the weight of dry film, and Ws is the weight of swollen film.

Density of blended films

Density measurements of the samples (a), (b), (c) and (d) 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. Weight in gram per cubic centimeter was represented as the density of the membranes.

Wide angle X-Ray diffraction (XRD)

X-ray diffraction (XRD) patterns of the samples are recorded in the 2θ range of 5-40° on a Phillips X-ray diffractometer equipped with a scintillation counter. CuKα radiation (wavelength, 1.54 Ǻ; filament current, 30 mA; voltage, 40 kV) is used for the generation of X-rays. A polymer can be considered partly crystalline and partly amorphous. The crystallinity parts give sharp narrow diffraction peaks and the amorphous component gives a very broad peak. The ratio between these intensities can be used to calculate the amount of crystallinity in the material.

Crystallinity (%) = (AC/AT ) X 100

Where AC is the area of crystalline part of the samples and AT is the total area of crystalline and amorphous part of prepared samples.

Thermogravimetric Analysis (TGA)

The thermal stability of the prepared samples is evaluated by Thermogravimetric analysis (TGA) performed on a Perkin- Elmer TGA, 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.

Attenuated Total Reflectance- Fourier Transform Infra Red Spectroscopy (ATR- FTIR)

Attenuated Total Reflectance-Fourier-transform infra-red (ATR-FTIR) spectroscopy is one of the most powerful techniques to investigate multicomponent systems because it provides information on the blend composition as well as on the polymer-polymer interaction. Infrared spectra of both the blends and the pure components were obtained using the films on an ATR-FTIR spectrometer. It is used to characterize the presence of specific chemical groups in the materials. IR spectroscopy of the thin films of samples are recorded on a Perkin-Elmer spectrophotometer in the wave number range of 650-4000 cm−1 using transmittance mode.

Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is done to study thermal properties such as melting temperature, glass transition temperature and melting enthalpies of dry samples. The DSC studies on the samples are 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 following 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:

Crystallinity (%) =ΔHf/ΔHf(crys) X 100

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 150 J/g.[64]

in high temperature DSC, all samples as shown in table 1 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 as shown in table 1 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.

RESULTS AND DISCUSSION

Swelling Ratio (%)

Figure 3 Effect of concentration of CMC on the Swelling Ratio (%) of the blends in PBS (pH 7.4) in 24 h.

Figure 3 clearly 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. It is because as the concentration of CMC increases in the blends number of hydroxyl group increases thus increasing the interaction.

Density Measurements

Table 2 Comparison of influence of concentration of CMC on the density of air dried films

Figure 4 Comparison of influence of concentration of CMC on the density of air dried films

As shown in Table 2 and Figure 4, it can be clearly concluded 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%. But as the concentration of CMC increases in the blends the density of air dried films slightly increases 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, thus making the blend more dense. Also it can be seen that the density of pure CMC is more as compared to pure PVA and PEO.

X-ray diffraction

Figure 5 X-Ray diffraction patterns of pure PVA, pure PEO and sample (a)

Figure 6 X-Ray diffraction patterns of pure CMC, samples (b), (c) and (d)

X-ray diffraction (XRD) patterns of the blends and the pure components are shown in Figure 5 and 6. It may be seen that pure PVA exhibits only a broad and shallow diffraction feature around the 2θ value of 16.9°, 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. The blend (a) having PVA/PEO 90/10 shows only one reflection at 2θ values 19.8°. XRD analysis showed that CMC exhibits a very small crystallinity which can be seen in the Table 3 given below.

Table 3 Percentage crystallinity calculated by XRD of samples

In samples (a), (b), (c) and (d) as the concentration of CMC increases, the % crystallinity shows not much difference as shown in Figure 7 given below. But as shown in Figure 6 the merging of all the peaks of pure PVA, PEO and CMC shows that on adding CMC to the blend of PVA and PEO, the compatibility increases.

Figure 7 Graph of percentage crystallinity vs concentration of CMC by XRD

Thermogravimetric Analysis (TGA)

Figure 8 TGA of thin films of samples for studying the effect of concentration of CMC on the thermal stability of the samples

The thermal stability of the dry superabsorbent hydrogels was determined from 50°C to 600°C. Figure 8 shows the thermograms for different hydrogel compositions at various temperatures. Generally, 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. From the figure 8, 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 weight loss process correspond to the degradation of PVA by a dehydration reaction on the polymer chain and the third weight loss process is due to the degradation of the polyene residues to yield carbon and hydrocarbons while PEO undergoes one step degradation. In samples a, b, c, d two step degradation process takes place.

The hydrogels having concentrations equal to 100% CMC showed a single-step thermogram, whereas the major weight loss of ~ 50% occurred from 250 to 350°C. This weight loss was attributed mainly to the thermal degradation of the two component polymers of the hydrogel, whereas the weight loss up to 600°C was ~ 70%. This means that hydrogels having 100% CMC showed high thermal stability. On the other hand, the thermogram of blends is two-step thermogram. The first step was from 200 to 300°C, which was also attributed to thermal degradation of the side chains. The second step took place from 350 to 450 ° 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 hydrogel components.

TGA of CMC showed two distinct zones where the weight is being lost. 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. As there are COO- groups in the case of CMC, it is decarboxylated.

Attenuated Total Reflectance- Fourier Transform Infra Red Spectroscopy (ATR- FTIR)

Figure 9 ATR-FTIR of thin films of samples pure PVA and PEO

Figure 10 FTIR of pure CMC powder

Figure 11 Comparison of ATR-FTIR of blend (d) with pure samples

From Figures 9, 10 and 11, in the IR spectra of the CMC, we can notice the characteristic bands of COO- at 1610, 1419 cm-1, COOH groups at 1055.9 cm-1, -OH at 1419, 1319.54 cm-1 and the ether groups at 1055.9 cm-1. 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 spectrum of CMC shows the stretching vibrations of at -CH-O-CH2 1055.9 cm-1. The band at 1610 cm-1 and 2878.37 cm-1 are assigned to the stretching vibration of the carboxyl group (COO-) and the stretching vibration of methine (C-H), respectively. Pure CMC displays two characteristic absorption bands at 1610 cm-1 and 1419 cm-1, which represents symmetry stretching and asymmetry stretching of COO− group, respectively. It shows a broad band at 3433.59 cm-1, due to the stretching frequency of the -OH group. The band at 2878.37 cm-1 is due to C-H stretching vibration. The presence of a strong absorption band at 1610 cm-1 confirms the presence of COO- group. The bands around 1419 and 1319.54 cm-1 are assigned to -CH2 scissoring and -OH bending vibration, respectively.

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The FTIR spectrum of pure PVA reference sample is shown in figure 9 and 11. It clearly reveals the major peaks associated with poly(vinyl alcohol). For instance, it can be observed C-H broad alkyl stretching band 2933.33 cm-1 and typical strong hydroxyl bands for intermolecular and intramolecular hydrogen bonded band at 3286.66 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 1087.11 cm-1 is due to C-O-C group.

The IR peak of interest in the C-O-C asymmetric stretch is at 1095.88 cm-1. This peak in the spectrum of blends has been shown to shift due to hydrogen bonding to PVA and CMC. The spectra obtained for blends are shown in Figure 12.

Figure 12 ATR-FTIR of thin films of samples (a), (b), (c) and (d)

From Figure 12 it can be concluded that all the blends show characteristic peaks of all the polymers present.

Differential Scanning Calorimetry (DSC)

The melting temperatures were determined from maximum in the melting endotherm, the glass transition temperatures were taken as the mid point of the heat capacity change.

One of the most commonly used methods to estimate polymer-polymer miscibility is the determination of the Tm of the blend compared with the Tms of the two components separately. In the case where one component is crystalline, observation of a melting point depression of this polymer may also be used as evidence to support the miscibility of the polymer pair.

Figure 13 DSC curves showing the melting peaks of PVA, PEO and CMC

Thermal properties and crystallinity of the prepared samples are examined by DSC method (Figure 13 and Table 4). PVA exhibited a relatively large and sharp endothermic peak at 222.2°C, PEO at 70.2°C and CMC at 265.9°C. It is observed from Figure 14 that the melting point and melting enthalpies of the samples a, b, c, d are somewhat decreased from the pure PVA sample. 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. Figure 15 shows the glass transition temperature i.e. Tg of the pure PVA sample. 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.

Figure 14 DSC curves showing the melting peaks of PVA, samples (a), (b), (c) and (d)

Figure 15 DSC curves showing the glass transition peak of PVA

Figure 16 DSC curves showing the melting temperature peaks of PEO and samples (a), (b), (c) and (d)

PEO exhibited a relatively large and sharp endothermic peak at 65.5°C. It is observed from Figure 16 that the melting point and melting enthalpies of the samples a, b, c, d are somewhat decreased from the pure PEO sample and the melting peaks are widened. This decrease in melting temperature is also 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 as shown in Table 4. It was found that the melting temperature of 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.

Table 4 Percentage crystallinity calculated by DSC of samples

Figure 17 Graph of Percentage Crystallinity vs Concentration of CMC

In Figure 17, the percentage crystallinity data obtained by DSC for different polymer compositions (a), (b), (c) and (d) are plotted against compatibilizer CMC concentration, to clarify the effect of the CMC content on the crystallinity of the present system. This is also clear from the Table 4 given above that as the concentration of CMC increases in the blend the crystallinity % decreases this is due to the decrease in the proper alignment of the chains due to the interference of other polymers present in the blend.

Figure 18 Graph of Melting Temperature (Tm) vs Concentration of CMC

In Figure 18 and table 4, the Tm data obtained by DSC for different polymer compositions (a), (b), (c) and (d) are plotted against compatibilizer CMC concentration, to clarify the effect of the CMC content on the thermal property of the present system. It is clear from the figure that as the concentration of CMC increases in the blend the melting temperature firstly increases then decreases.

CONCLUSIONS

We have effectively produced PVA/PEO/CMC hydrogels via aqueous route. These hydrogel blends were properly characterized by using XRD, FTIR spectroscopy, TGA and DSC techniques.

FIGURES CAPTIONS

Figure 1 Structure of (a) Poly(vinyl alcohol), (b) Poly(ethylene oxide)

and (c) Carboxymethyl cellulose

Figure 2 Solutions prepared from the polymer sample to test compatibility

Figure 3 Effect of concentration of CMC on the Swelling Ratio (%) of the blends in PBS (pH 7.4) in 24 h.

Figure 4 Comparison of influence of concentration of CMC on the density of air dried films

Figure 5 X-Ray diffraction patterns of pure PVA, pure PEO and sample (a)

Figure 6 X-Ray diffraction patterns of pure CMC, samples (b), (c) and (d)

Figure 7 Graph of percentage crystallinity vs concentration of CMC by XRD

Figure 8 TGA of thin films of samples for studying the effect of concentration of CMC on the thermal stability of the samples

Figure 9 ATR-FTIR of thin films of samples pure PVA, PEO and CMC

Figure 10 FTIR of pure CMC powder

Figure 11 Comparison of ATR-FTIR of blend (d) with pure samples

Figure 12 ATR-FTIR of thin films of samples (a), (b), (c) and (d)

Figure 13 DSC curves showing the melting peaks of PVA, PEO and CMC

Figure 14 DSC curves showing the melting peaks of PVA, samples (a), (b), (c) and (d)

Figure 15 DSC curves showing the glass transition peak of PVA

Figure 16 DSC curves showing the melting temperature peaks of PEO and samples (a), (b), (c) and (d)

Figure 17 Graph of Percentage Crystallinity vs Concentration of CMC

Figure 18 Graph of Melting Temperature (Tm) vs Concentration of CMC

TABLES CAPTIONS

Table 1 Samples taken for characterization

Table 2 Comparison of influence of concentration of CMC on the density of air dried films

Table 3 Percentage crystallinity calculated by XRD of samples

Table 4 Percentage crystallinity calculated by DSC of samples

ABSTRACT

Poly(vinyl alcohol) (PVA) – Poly(ethylene oxide) (PEO) blends were prepared and found that Poly(vinyl alcohol) – Poly(ethylene oxide) are inherently immiscible and therefore incompatible. So, a compatibilizer Carboxymethyl cellulose (CMC) is added to PVA and PEO and the influence of CMC is studied on the compatibility of blends of PVA and PEO. It is found that on adding CMC, PVA and PEO become partially miscible. Here, we describe the preparation of PVA/PEO/CMC blends having weight percentage of CMC 5, 10, 20 wt% and the influence of concentration of CMC on the blends of PVA and PEO is studied and the miscibility of the blends was characterized by using wide-angle X-ray Diffraction (XRD), Thermal Gravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) and Attenuated Total Reflectance-Fourier Transform Infra-red (ATR-FTIR) techniques. Also, swelling ratio of the different blends is studied.

Keywords: Hydrogels; Polyvinyl alcohol; Polyethylene oxide; Carboxymethyl cellulose; Miscibility, Immiscible.

*Correspondence to: Prof. Bhuvanesh Gupta, Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India. E-mail: [email protected]

INTRODUCTION

Designing new materials with improved properties is one of the main goals of the chemists. Two common ways are chemical synthesis and blending which mainly used to get a material with improved or new properties. Chemical synthesis is an unlimited method to get new substances with well-defined properties but it is often time consuming and not seldom costly. On the other side, blending is a well-known and simple method to combine the advantages of different materials [23,65], efficient way to prepare new materials with improved properties. [8] The blending of hydrophilic/hydrophobic polymers produce phase-separated composite hydrogels. Polymer blends exhibit superior and rare properties, unexpected from homopolymers. The physical, chemical and radiant methods can be applied to prepare polymer blends. [6] Polymer blends are physical mixtures of structurally different polymers or co-polymers, which interact through secondary forces such as hydrogen bonding, dipole-dipole forces and charge transfer complexes for homopolymer mixtures with no covalent bonding [34,36-38] that are miscible at molecular level. Polymer blend hydrogels are composed of water-soluble or swellable polymers, such as poly(ethylene oxide) (PEO) [25,26] poly(vinyl alcohol) (PVA) [24] and, other synthetic water-soluble polymers and degradable or nondegradable water-insoluble or swellable polymers, such as poly(lactic acid) (PLA) [25], poly(lactic acid-co-glycolic acid) (PGLA). [24] The most common method used to blend polymers is through solvent-casting techniques. In this process, two or more polymers are dissolved in a mutual solvent and the blends are obtained by evaporating the solvent. The resulting materials have a microphase – separated structure [25,26] and often improved miscibility via hydrogen bonding among polymers [24], resulting in transparent materials.

Poly(vinyl alcohol) (PVA) is a water-soluble polyhydroxy polymer, used in practical applications because of its easy preparation, excellent chemical resistance and physical properties, appropriate mechanical properties [68,71], and it is completely biodegradable and cheap and the -OH groups can be a source of hydrogen bonding (H-bonding) and hence of assistance in the formation of polymer blends. Polyvinyl alcohol has excellent film forming, emulsifying, and adhesive properties. It is also resistant to oil, grease and solvent. It is odorless, nontoxic and has high tensile strength, flexibility, as well as high oxygen and aroma barrier. The chemical structure of PVA favors the formation of intramolecular [1] hydrogen bonding because of favorable disposition of relatively small -OH groups attached to alternate carbon atoms of PVA [20], thus it is used in the preparation of various membranes and hydrogels. Hydrogels are hydrophilic polymers having three-dimensional networks [27], and are most often defined as two-component systems where one of the components is a hydrophilic polymer and the second one is water. These have the ability to swell in the presence of water without dissolution because of a three-dimensional network joining as chains. The interactions responsible for water absorption by hydrogels include the processes of hydration, which is connected to the presence of such chemical groups as -OH, -COOH, -CONH2, -CONH-, and -SO3H, and the existence of capillary areas and differences in osmotic pressure. [67]

PVA blends can be cast as films and applied as functional materials including biomedical materials such as dialysis membranes, wound dressing, artificial skin, cardiovascular devices and as vehicles to release active substances in a controlled manner. [69-71] PVA hydrogels have been studied extensively but their properties need to be improved further for special applications. [2,6,7] In order to improve or modify the properties of PVA hydrogels, PEO is used to blend with PVA to form hydrogels which is hydrophilic semicrystalline polyether with a glass transition temperature below room temperature, biocompatible, non toxic, non polar, non antigenic and non immunogenic [45] and is highly desirable in most biomedical applications requiring contact with physiological fluids. For these reasons, PEO hydrogels are applied as wound coverings, drug delivery systems, hemodialysis membrane [1], as a component of a tissue sealant [15,16] and as a coating for medical devices [17], both poly(ethylene oxide) (PEO) and poly(vinyl alcohol) (PVA) are industrially important polymers [75] and their blends can be of significant practical utility, but it is found that PVA and PEO are immiscible and incompatible blends [1, 75] which do not possess a tendency for extensive mutual solubility. [1] Also it is found that hydroxyl-containing polymers are self-associated and hence the competition between self association and interpolymer interaction plays an important role in determining the miscibility behavior of their blends. For example, poly(vinyl alcohol) (PVA) is miscible with three tertiary amide polymers poly(N-vinyl-2-pyrrolidone) (PVP) [54-58], poly(N,N-dimethylacrylamide) [59] and poly(2-methyl-2-oxazoline) [60], but is immiscible with another tertiary amide polymer poly(2-ethyl-2-oxazoline) (PEOx). [61] PEO is etheric in nature. However formation of weak H-bonds between PEO and PVA cannot be ruled out. The C-O-C bond angle in PEO is normally 108° and when a -OH group from a neighbouring PVA chain approaches the etheric oxygen atom in order to form a H-bond, the C-O-C bond angle deviates from 108° so that the lone pair of the etheric oxygen is positioned nearer to the approaching OH from PVA. It would therefore be interesting to investigate the structure and thermal properties of the composites formed in the PEO-PVA system with different proportions of the components. We have found that mutual miscibility of PEO and PVA is likely to exist over only a small range of compositions. The mixtures otherwise seem to form only microscopically immiscible blends which do not possess a tendency for extensive mutual solubility. They are referred to as incompatible polymer blends or simply blends. [77] To make them compatible, a compatibilizer i.e. Carboxymethyl cellulose (CMC) is added.

Carboxymethyl cellulose (CMC) obtained from natural cellulose by chemical modification is a water soluble cellulose ether derivate [3] and is formed by its reaction with sodium hydroxide and chloroacetic acid. It has a number of sodium carboxymethyl groups (CH2COONa), introduced into the cellulose molecule, which promote water solubility. The various properties of CMC depend upon three factors: molecular weight of the polymer, average number of carboxyl content per anhydroglucose unit, and the distribution of carboxyl substituents along the polymer chains. The most important properties of CMC are viscosity building and flocculation. Among all the polysaccharides, CMC is easily available and it is also very cheap. It has high shear stability. The structure of CMC is shown in Figure 1. [78]

Figure 1 Structure of (a) Poly(vinyl alcohol), (b) Poly(ethylene oxide) and (c) Carboxymethyl cellulose

CMC has good water solubility, broadly used due to its low cost, biodegradability, biocompatibility [51] and lack of toxicity. [8,29-33] CMC is an ionic polyelectrolyte [30] that contains carboxyl groups and exhibits pH sensitivity as it has lot of carboxylic groups. [48-50] It has been used in several medical applications [10] and more recently as a component of an antiadhesion gel. [11,12] CMC and PVA in different ratios can be mixed homogeneously in an aqueous solution without evident phase separation, and this can be attributed to the interaction between the components. [49] The hydrogen bonds that form between the carboxylic groups of CMC and hydroxyl groups of PVA, and form semi-interpenetrating polymer networks [49] while with PEO, CMC undergoes micro phase separation to form a two-phase system. [9]

Berg et. al. [9] found that the turbidity results of CMC/PEO gels are demonstrated by transparency data. It is found that gels prepared either from CMC alone or from PEO alone were transparent. However, for CMC/PEO composite gels, the transparency of gels changed as the ratio of the two components changed. 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 so 20% CMC concentration is taken as the optimized concentration for further studies.

The polymer-polymer interaction for the miscibility is thought to be due mainly to hydrogen bonding between three hydroxyl groups in the anhydroglucose unit of CMC and the functional groups of the synthetic polymers PVA and PEO. However, since each of the three hydroxyl groups in the repeating unit of the cellulose is quite different in terms of regiochemistry and polarity, the hydrogen bond formation is not easily clarified. Kondo et.al. [47] proposed the mechanism for the development of interaction in the cellulose/PEO blend and showed that the hydrogen bonding between the C6 position hydroxyls and skeletal oxygen of PEO is more favourable, at first the two polymers are trapped to form a large adduct, which is a complex between cellulose and PEO, by the hydrogen bond, and the mobility of the molecules is restricted. Then 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. [79]

The purpose of the present paper is to investigate the influence of concentration of CMC on the blends of PVA and PEO. In this article, we report the characterization of PVA/PEO/CMC blends by various techniques such as X-Ray diffraction (XRD), infrared (ATR-FTIR) spectroscopy, Differential scanning Calorimetry (DSC) and Thermal gravimetric Analysis (TGA).

EXPERIMENTAL

Materials

Poly(vinylalcohol) (PVA) of Loba Chemie Pvt. Ltd., Mumbai, India having degree of polymerization 1700-1800 and molecular weight 1,15,000, Poly(ethylene oxide) (PEO) of Sigma Aldrich of molecular weight 3,00,000 were used. Carboxymethyl cellulose (CMC) sodium salt of high viscosity was received from Loba Chemie Pvt. Ltd., Mumbai, India. Distilled water was used for all experiments.

Preparation of Blends of PVA and PEO

Preparation of the pure film of PVA and blends of PVA and PEO were carried out in the following manner. PVA was dissolved in distilled water under constant mechanical stirring at temperature 60 -70 °C to get 5% PVA solution and then about 15 g. of PVA solution is poured to form layers 2 mm thick in a petridish at room temperature. The solution was first dried in air for 2 days and then in a vacuum oven at 100°C to remove solvent from it. Then, the blends of PVA/PEO/CMC were prepared by dissolving different concentrations of each polymer in distilled water, the total polymer concentration in the solvent remains 5% by weight. Water constitutes a suitable reaction medium, because PVA, PEO and CMC are soluble in water.

Each polymer having concentration as shown in Table 1 were added in distilled water one by one and then dissolved under constant mechanical stirring at temperature 60 -70°C. As shown in Figure 2, it was found that blend solutions formed with CMC shows compatibility as compared to the solution having no CMC i.e. solution (a). It is clear from the Figure 2 that compatibility in the blend increases as the CMC concentration increases from 5% to 20%. These blend solutions were then poured in petridishes at room temperature. The solutions were first dried in air for 2 days and then in a vacuum oven at 100°C to remove solvent from it. The films so obtained are then characterized by XRD, TGA, DSC and ATR-FTIR techniques to determine miscibility.

Table 1 Samples taken for characterization

Figure 2 Solutions prepared from the polymer sample to test compatibility

Swelling Ratio (%)

All the samples (a), (b), (c) and (d) in film form were accurately weighed and placed in a beaker having fixed volume i.e. 50 ml PBS (pH 7.4) and then kept in a water bath undisturbed for a fixed interval i.e. 24 h. The samples were removed after 24 h., and the excess surface water is removed by pressing gently between filter paper and weighed. The Swelling ratio(%) i.e. SR (%), was calculated as indicated in Equation given below.

SR (%) = (Ws – Wd) / Wd x 100

where Wd is the weight of dry film, and Ws is the weight of swollen film.

Density of blended films

Density measurements of the samples (a), (b), (c) and (d) 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. Weight in gram per cubic centimeter was represented as the density of the membranes.

Wide angle X-Ray diffraction (XRD)

X-ray diffraction (XRD) patterns of the samples are recorded in the 2θ range of 5-40° on a Phillips X-ray diffractometer equipped with a scintillation counter. CuKα radiation (wavelength, 1.54 Ǻ; filament current, 30 mA; voltage, 40 kV) is used for the generation of X-rays. A polymer can be considered partly crystalline and partly amorphous. The crystallinity parts give sharp narrow diffraction peaks and the amorphous component gives a very broad peak. The ratio between these intensities can be used to calculate the amount of crystallinity in the material.

Crystallinity (%) = (AC/AT ) X 100

Where AC is the area of crystalline part of the samples and AT is the total area of crystalline and amorphous part of prepared samples.

Thermogravimetric Analysis (TGA)

The thermal stability of the prepared samples is evaluated by Thermogravimetric analysis (TGA) performed on a Perkin- Elmer TGA, 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.

Attenuated Total Reflectance- Fourier Transform Infra Red Spectroscopy (ATR- FTIR)

Attenuated Total Reflectance-Fourier-transform infra-red (ATR-FTIR) spectroscopy is one of the most powerful techniques to investigate multicomponent systems because it provides information on the blend composition as well as on the polymer-polymer interaction. Infrared spectra of both the blends and the pure components were obtained using the films on an ATR-FTIR spectrometer. It is used to characterize the presence of specific chemical groups in the materials. IR spectroscopy of the thin films of samples are recorded on a Perkin-Elmer spectrophotometer in the wave number range of 650-4000 cm−1 using transmittance mode.

Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is done to study thermal properties such as melting temperature, glass transition temperature and melting enthalpies of dry samples. The DSC studies on the samples are 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 following 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:

Crystallinity (%) =ΔHf/ΔHf(crys) X 100

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 150 J/g.[64]

in high temperature DSC, all samples as shown in table 1 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 as shown in table 1 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.

RESULTS AND DISCUSSION

Swelling Ratio (%)

Figure 3 Effect of concentration of CMC on the Swelling Ratio (%) of the blends in PBS (pH 7.4) in 24 h.

Figure 3 clearly 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. It is because as the concentration of CMC increases in the blends number of hydroxyl group increases thus increasing the interaction.

Density Measurements

Table 2 Comparison of influence of concentration of CMC on the density of air dried films

Figure 4 Comparison of influence of concentration of CMC on the density of air dried films

As shown in Table 2 and Figure 4, it can be clearly concluded 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%. But as the concentration of CMC increases in the blends the density of air dried films slightly increases 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, thus making the blend more dense. Also it can be seen that the density of pure CMC is more as compared to pure PVA and PEO.

X-ray diffraction

Figure 5 X-Ray diffraction patterns of pure PVA, pure PEO and sample (a)

Figure 6 X-Ray diffraction patterns of pure CMC, samples (b), (c) and (d)

X-ray diffraction (XRD) patterns of the blends and the pure components are shown in Figure 5 and 6. It may be seen that pure PVA exhibits only a broad and shallow diffraction feature around the 2θ value of 16.9°, 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. The blend (a) having PVA/PEO 90/10 shows only one reflection at 2θ values 19.8°. XRD analysis showed that CMC exhibits a very small crystallinity which can be seen in the Table 3 given below.

Table 3 Percentage crystallinity calculated by XRD of samples

In samples (a), (b), (c) and (d) as the concentration of CMC increases, the % crystallinity shows not much difference as shown in Figure 7 given below. But as shown in Figure 6 the merging of all the peaks of pure PVA, PEO and CMC shows that on adding CMC to the blend of PVA and PEO, the compatibility increases.

Figure 7 Graph of percentage crystallinity vs concentration of CMC by XRD

Thermogravimetric Analysis (TGA)

Figure 8 TGA of thin films of samples for studying the effect of concentration of CMC on the thermal stability of the samples

The thermal stability of the dry superabsorbent hydrogels was determined from 50°C to 600°C. Figure 8 shows the thermograms for different hydrogel compositions at various temperatures. Generally, 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. From the figure 8, 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 weight loss process correspond to the degradation of PVA by a dehydration reaction on the polymer chain and the third weight loss process is due to the degradation of the polyene residues to yield carbon and hydrocarbons while PEO undergoes one step degradation. In samples a, b, c, d two step degradation process takes place.

The hydrogels having concentrations equal to 100% CMC showed a single-step thermogram, whereas the major weight loss of ~ 50% occurred from 250 to 350°C. This weight loss was attributed mainly to the thermal degradation of the two component polymers of the hydrogel, whereas the weight loss up to 600°C was ~ 70%. This means that hydrogels having 100% CMC showed high thermal stability. On the other hand, the thermogram of blends is two-step thermogram. The first step was from 200 to 300°C, which was also attributed to thermal degradation of the side chains. The second step took place from 350 to 450 ° 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 hydrogel components.

TGA of CMC showed two distinct zones where the weight is being lost. 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. As there are COO- groups in the case of CMC, it is decarboxylated.

Attenuated Total Reflectance- Fourier Transform Infra Red Spectroscopy (ATR- FTIR)

Figure 9 ATR-FTIR of thin films of samples pure PVA and PEO

Figure 10 FTIR of pure CMC powder

Figure 11 Comparison of ATR-FTIR of blend (d) with pure samples

From Figures 9, 10 and 11, in the IR spectra of the CMC, we can notice the characteristic bands of COO- at 1610, 1419 cm-1, COOH groups at 1055.9 cm-1, -OH at 1419, 1319.54 cm-1 and the ether groups at 1055.9 cm-1. 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 spectrum of CMC shows the stretching vibrations of at -CH-O-CH2 1055.9 cm-1. The band at 1610 cm-1 and 2878.37 cm-1 are assigned to the stretching vibration of the carboxyl group (COO-) and the stretching vibration of methine (C-H), respectively. Pure CMC displays two characteristic absorption bands at 1610 cm-1 and 1419 cm-1, which represents symmetry stretching and asymmetry stretching of COO− group, respectively. It shows a broad band at 3433.59 cm-1, due to the stretching frequency of the -OH group. The band at 2878.37 cm-1 is due to C-H stretching vibration. The presence of a strong absorption band at 1610 cm-1 confirms the presence of COO- group. The bands around 1419 and 1319.54 cm-1 are assigned to -CH2 scissoring and -OH bending vibration, respectively.

The FTIR spectrum of pure PVA reference sample is shown in figure 9 and 11. It clearly reveals the major peaks associated with poly(vinyl alcohol). For instance, it can be observed C-H broad alkyl stretching band 2933.33 cm-1 and typical strong hydroxyl bands for intermolecular and intramolecular hydrogen bonded band at 3286.66 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 1087.11 cm-1 is due to C-O-C group.

The IR peak of interest in the C-O-C asymmetric stretch is at 1095.88 cm-1. This peak in the spectrum of blends has been shown to shift due to hydrogen bonding to PVA and CMC. The spectra obtained for blends are shown in Figure 12.

Figure 12 ATR-FTIR of thin films of samples (a), (b), (c) and (d)

From Figure 12 it can be concluded that all the blends show characteristic peaks of all the polymers present.

Differential Scanning Calorimetry (DSC)

The melting temperatures were determined from maximum in the melting endotherm, the glass transition temperatures were taken as the mid point of the heat capacity change.

One of the most commonly used methods to estimate polymer-polymer miscibility is the determination of the Tm of the blend compared with the Tms of the two components separately. In the case where one component is crystalline, observation of a melting point depression of this polymer may also be used as evidence to support the miscibility of the polymer pair.

Figure 13 DSC curves showing the melting peaks of PVA, PEO and CMC

Thermal properties and crystallinity of the prepared samples are examined by DSC method (Figure 13 and Table 4). PVA exhibited a relatively large and sharp endothermic peak at 222.2°C, PEO at 70.2°C and CMC at 265.9°C. It is observed from Figure 14 that the melting point and melting enthalpies of the samples a, b, c, d are somewhat decreased from the pure PVA sample. 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. Figure 15 shows the glass transition temperature i.e. Tg of the pure PVA sample. 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.

Figure 14 DSC curves showing the melting peaks of PVA, samples (a), (b), (c) and (d)

Figure 15 DSC curves showing the glass transition peak of PVA

Figure 16 DSC curves showing the melting temperature peaks of PEO and samples (a), (b), (c) and (d)

PEO exhibited a relatively large and sharp endothermic peak at 65.5°C. It is observed from Figure 16 that the melting point and melting enthalpies of the samples a, b, c, d are somewhat decreased from the pure PEO sample and the melting peaks are widened. This decrease in melting temperature is also 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 as shown in Table 4. It was found that the melting temperature of 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.

Table 4 Percentage crystallinity calculated by DSC of samples

Figure 17 Graph of Percentage Crystallinity vs Concentration of CMC

In Figure 17, the percentage crystallinity data obtained by DSC for different polymer compositions (a), (b), (c) and (d) are plotted against compatibilizer CMC concentration, to clarify the effect of the CMC content on the crystallinity of the present system. This is also clear from the Table 4 given above that as the concentration of CMC increases in the blend the crystallinity % decreases this is due to the decrease in the proper alignment of the chains due to the interference of other polymers present in the blend.

Figure 18 Graph of Melting Temperature (Tm) vs Concentration of CMC

In Figure 18 and table 4, the Tm data obtained by DSC for different polymer compositions (a), (b), (c) and (d) are plotted against compatibilizer CMC concentration, to clarify the effect of the CMC content on the thermal property of the present system. It is clear from the figure that as the concentration of CMC increases in the blend the melting temperature firstly increases then decreases.

CONCLUSIONS

We have effectively produced PVA/PEO/CMC hydrogels via aqueous route. These hydrogel blends were properly characterized by using XRD, FTIR spectroscopy, TGA and DSC techniques.

FIGURES CAPTIONS

Figure 1 Structure of (a) Poly(vinyl alcohol), (b) Poly(ethylene oxide)

and (c) Carboxymethyl cellulose

Figure 2 Solutions prepared from the polymer sample to test compatibility

Figure 3 Effect of concentration of CMC on the Swelling Ratio (%) of the blends in PBS (pH 7.4) in 24 h.

Figure 4 Comparison of influence of concentration of CMC on the density of air dried films

Figure 5 X-Ray diffraction patterns of pure PVA, pure PEO and sample (a)

Figure 6 X-Ray diffraction patterns of pure CMC, samples (b), (c) and (d)

Figure 7 Graph of percentage crystallinity vs concentration of CMC by XRD

Figure 8 TGA of thin films of samples for studying the effect of concentration of CMC on the thermal stability of the samples

Figure 9 ATR-FTIR of thin films of samples pure PVA, PEO and CMC

Figure 10 FTIR of pure CMC powder

Figure 11 Comparison of ATR-FTIR of blend (d) with pure samples

Figure 12 ATR-FTIR of thin films of samples (a), (b), (c) and (d)

Figure 13 DSC curves showing the melting peaks of PVA, PEO and CMC

Figure 14 DSC curves showing the melting peaks of PVA, samples (a), (b), (c) and (d)

Figure 15 DSC curves showing the glass transition peak of PVA

Figure 16 DSC curves showing the melting temperature peaks of PEO and samples (a), (b), (c) and (d)

Figure 17 Graph of Percentage Crystallinity vs Concentration of CMC

Figure 18 Graph of Melting Temperature (Tm) vs Concentration of CMC

TABLES CAPTIONS

Table 1 Samples taken for characterization

Table 2 Comparison of influence of concentration of CMC on the density of air dried films

Table 3 Percentage crystallinity calculated by XRD of samples

Table 4 Percentage crystallinity calculated by DSC of samples

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