pH Responsive Properties of Methyl Red/PVA Polymers for Smart Food Packaging Applications

Responsive polymers in particular have gained increasing interest during the last decades due to their unique properties. Their Ability to exhibit sharp and reversible changes in response to environmental pH condition have made them suitable for various applications. The pH responsive polymeric materials are being developed by the researcher in the recent year for the improvement of food safety issue in the packaging materials. In this study organic methyl red (MR) dye is used as a pH indicator induced with polyvinyl alcohol (PVA) films prepared by solution casting method. The prepared samples were characterized for microstructural, morphological, mechanical, water absorption and pH responsive properties using various analytical techniques. The XRD analysis has shown better crystallinity property which is in good agreement with the mechanical strength of the material. The morphological studies have shown homogeneous dispersion of MR on PVA matrix and change in the morphology of the MR/PVA polymer composites after acid treatment. The MR/PVA films were treated with rotten papaya fruit and change occurred in the samples were identified using UV-vis spectrometric analysis. The prepared MR/PVA pH-responsive polymers have shown good pH reactivity in the range of pH 4.5 to 6.5. Overall, the prepared samples have good pH responsive properties with additional qualities like good optical, structural, and mechanical characteristics.


Introduction
The pH responsive polymers, also known as smart polymers or stimuli-responsive polymers, are a class of materials that can undergo reversible changes in their properties in response to changes in the pH of their surrounding environment [1][2][3].These polymers are designed to be sensitive to variations in the acidity or alkalinity of their surroundings and can exhibit significant changes in their structure, solubility, and other physical or chemical properties in response to pH changes [4].This unique ability makes them valuable in a wide range of applications, including drug delivery, sensing, textiles, agriculture, environmental remediation, and food packaging applications [2].
Halochromic chemicals are introduced sparingly to the polymer matrix as pH indicators so that the colour changes caused by its reaction with bases or acids can be seen.The pH indicators are weak acids that are found as natural dyes that can change colour to show how many H + (H3O + ) ions are present in a solution [5].The pH value of the substance under evaluation, is determined using the logarithm of this concentration, serves as an indicator to its acidity, basicity, or neutrality.Indicators work on the basis that they react with water to produce the hydrogen cation H + or hydronium ion H3O + [5].The indicator molecule's colour changes as a result of the reaction.While some indicators alternate between coloured and colourless states, others change from one colour to another.
Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer that has a wide range of applications in various fields, including the development of pH-responsive polymers [6].The pH responsive polymers, when incorporated into food packaging materials, can help in extending the shelf life of perishable products.PVA-based films or coatings can be engineered to respond to changes in the pH of the packaged food, indicating the freshness of the product [7].This application ensures that consumers are aware of the quality of the food they are purchasing.In the context of food packaging applications, MR could potentially be used as a pH indicator to monitor the acidity of food products.The change in pH could indicate spoilage.For instance, if a food product becomes more acidic due to spoilage, the colour change in MR can indicate this change.
In this context, a MR/PVA composite polymer was prepared using simple solution casting method, without the aid of any special equipment or any toxic compound involving in it.The MR/PVA polymers were characterized for structural, morphological, mechanical, swelling, and pH responsive properties using suitable analytic techniques.

Materials and Methods
The Polyvinyl alcohol (PVA) (~89,000 Mw, SDFCL, India) was used as base polymer material.Methyl Red (MR) (0.1% solution, indicator grade, Loba Chemicals, India) was used as a pH sensitive die for identifying the food spoilage.Glutaraldehyde (25% aqueous solution, Aldrich, USA) was used as crosslinking agent for the MR/PVA polymer composites.Double distilled water (DDW) was used throughout the experiments.All the chemicals were of analytical grade and used without further purification.
The MR/PVA polymer composites were synthesized using solution casting method.Initially 15 wt% of PVA aqueous solution were prepared by dissolving 15 gm of PVA into 100 ml of double distilled water at 70 C under constant stirring and a small amount of glutaraldehyde was added to the prepared solution as a crosslinking agent and cooled to room temperature.Further, different concentration of MR namely, 0 ml, 1 ml 2 ml and 3 ml were added separately to the prepared PVA solution.Then the solutions were poured to the petri dishes separately and dried overnight at 50 C.The dried pure PVA polymer and 1 ml MR/PVA (MRPVA1), 2 ml MR/PVA (MRPVA2) and 3 ml MR/PVA (MRPVA3) polymer composites were used for further characterization.The pH response of the prepared samples were studied by treating them in the acidic environment of pH 4.5 to 6.5.The prepared polymer composites were also studied for pH responsive property with papaya fruit.
The prepared sample were tested for microstructural, morphological, mechanical, water absorbance and pH responsive properties using various analytical technique.The microstructural characteristics were carried out using powder X-ray diffractometer (powder XRD) (Rigaku; Miniflex, USA) within the diffraction angle (2Ɵ) range of 10-70 ̊ at a scan rate of 3/min.The surface morphology of the prepared samples were characterized using Field Emission Scanning Electron Microscopy (FESEM) (ULTRA 55; Carl Zeiss, Germany).The mechcanical strength and elongation properties were studied using Universal Testing Machine (UTM) (LRX Plus; Lloyd Instruments, UK).The water absorption characteristics of the prepared samples were observed by immersing the samples in the DDW.The saturation swelling percentage were recorded after 24 hrs.The pH responsive properties of the prepared samples against different pH solution and with the papaya fruit were studied using UV-visible spectrophotometer (UV-vis) (UV-2600; Shimadzu, Japan).

Result and Discussion
The crystallinity of MR/PVA polymer were investigated using powder XRD analysis (figure 1).The pure PVA and the untreated MR/PVA have resembled same XRD pattern revealing the predominant diffraction angle at 2Ɵ=19.8 o .This peak has a d spacing of 4.57, which corresponds to the crystalline PVA's (101) reflection plane [8,9].This pattern represents the semi-crystalline nature of PVA ascribing to the strong interaction between the PVA chains through intermolecular and intramolecular hydrogen bonding [10].The acid treated MR/PVA polymer composite demonstrated slight increment in the crystallinity by showing increase in the intensity of the diffraction angle at 2Ɵ=19.8 o .The powder XRD results shows that the acid treatment has marginally increased the crystallinity of the MR/PVA polymers.This result demonstrated that the interaction of acid with the MR/PVA polymer composite caused decrease in the amorphous nature of the films showing brittleness.To study the way molecules are arranged in the MR/PVA polymer composites, the surface morphology of the Pure PVA, MRPVA3, and MRPVA3 treated with buffer solution of pH 4.5 were investigated using FESEM analysis (figure 2).The morphology of pure PVA [figure 2(a)] has shown smooth morphology with no cracks of inhomogeneity on the surface confirming the amorphous nature complementing to the powder XRD analysis [8].Whereas the morphology of the MRPVA3 [figure 2(b)] has also shown smooth morphology with small crystalline phases spread inhomogeneously throughout the surface ascribing to the presence of MR within the PVA matrix [11].This indicates slight increment in the crystallinity after the addition of MR to PVA matrix.The acid treated MRPVA3 sample has shown major cracks throughout the surface and shown drastic increase in the crystallinity of the samples.The mechanical analysis of the MR/PVA polymer composites before and after the acid treatment were obtained from the stress vs strain curve and are provided in figure 3. The pure PVA polymer has shown the tensile strength of 49.89±1.2MPa.The MR/PVA polymer composites have shown small decrement in the tensile strength with increase in the MR concentration as shown in the figure 3. Also, it was observed that the tensile strength has significantly decreased for MR/PVA polymer composites after the acid treatment.The % elongation of the samples has also shown decreasing trend with increase in the MR concentration.The % elongation of the MR/PVA polymer composites has considerably decreased after the acid treatment as shown in the figure 3. The mechanical studies of the prepared sample justifies increase in the crystallinity percentage with the acid treatment which can be correlated to the powder XRD and FESEM analysis results.The values of tensile strength, Young's modulus and % elongation were within the range of 40.5±0.5 to 49.89±1.2,59.21±0.99 to 89.29±0.5 and 112.1±10.56 to 197.1±15.25 respectively.By considering the values of tensile strength, Young's modulus, and %elongation the developed MR/PVA polymer composites exhibit acceptable load bearing capacity, sufficient stiffness, and good elastic characteristics, which could make them suitable for the use in food packaging applications.
The swelling properties were analysed by studying the % swelling characteristics of prepared MR/PVA polymer composites.The samples were swollen to saturation and the equilibrium degree of swelling (%EDS) was recorded by soaking them in double distilled water for 24 hours.The %EDS of all the samples were shown in figure 3. The pure PVA has shown %EDS 192.5 ±21.2.The MR/PVA polymer composites have shown small decrement in the %EDS value with addition of MR concentration.The %EDS of MRPVA3 samples after treating with buffer solution of pH 4.5 and 6.5 were found to be decreasing significantly.The decrement in the %EDS is may be due to increase in the crystallinity of the sample after treating it with acids.The UV-Vis spectroscopic analysis of the MR/PVA polymer composites were shown in figure 4 (a).The pure PVA polymer has shown an absorption peak at ~215 nm which can be attributed to the carbonyl group or the presence of residual acetate group of PVA.The MR/PVA polymers have shown a significant absorption peak at 283 nm and 408 nm along with the PVA characteristic peak [3,12].The MR/PVA polymers treated with buffer solution of pH 6.5 have shown maximum absorption peak at 435 nm confirming the yellowish appearance after the treatment.Whereas, MR/PVA polymers treated with buffer solution with pH 4.5 have shown maximum absorption peak at 525 nm confirming reddish appearance after treatment [12].The intensity of the characteristic absorbance peak of the samples against both 6.5 and 4.5 pH buffer solutions found to be increased with increasing MR concentration confirming better pH response of MRPVA3 sample compared to others.
The pH response of the MR/PVA polymers were also tested against papaya fruit.The papaya fruit was allowed to rot for 9 days and the pH of the rotten papaya was recorded.The MRPVA3 sample was tested for pH response against rotten papaya at pH 4.3 and 6.6.The photographs representing the MRPVA3 sample after the treatment were shown in figure (b).The MRPVA3 appeared to be pale yellowish and red in appearance with respect to pH 6.5 and 4.5 respectively.The results obtained are in good agreement with the UV-vis spectrometric analysis of MR/PVA polymer composites treated with the range of buffer solutions.The pH response trails showed that the MRPVA3, which has the highest MR concertation, demonstrated the most noticeable colour change that is easily visible to the human eye.

Conclusion:
The pH responsive MR/PVA polymer composites were developed through solution casting method by physically blending MR in PVA matrix.The prepared samples were tested for pH responsiveness in the range from 4.5 to 6.5 pH values.The study demonstrates the feasibility of pH responsive MR/PVA polymer composites with the functionality of the colour change within the studied range of pH values.Change in microstructure and morphology of the MR/PVA polymers with addition of MR and acid treatment were studied using powder XRD and FESEM analysis.Both the results compliment the fact that the acidic environment has certainly increased the crystallinity of the polymers.Though, mechanical properties of the MR/PVA polymer composites has significantly varied after acid treatment, the material retained good mechanical strength, flexibility and ability to protect the food products.The colour changes occurred to the samples after the acid treatment were visually identified and also change in the wavelength were confirmed by the UV-vis spectroscopic analysis.The results of pH response of samples against papaya fruit during 9 days of rotting was in agreement with the UV-vis spectrometric analysis against range of acidic environment.The ability of the MR/PVA polymer composites to change colour in response to rotten papaya fruit allows consumers to determine the fruit's freshness using their naked eyes.Also, with better microstructural and mechanical stability the prepared MR/PVA polymer composites have the high potential to be used as a smart food packaging material.

Figure 3 :
Figure 3: Mechanical and swelling analysis of MR/PVA polymer composites.

Figure 4 :
Figure 4: a) UV-vis spectroscopic studies of MR/PVA polymers treated with different pH buffer solutions, b) pH response study of MR/PVA polymer composites against papaya fruit.