Enhancement and investigation of biodegradability of poly (methyl methacrylate) and poly (vinyl chloride) by blending with biodegradable polymer

Presently, society needs an eco-friendlier alternative for non-biodegradable polymers, nonetheless, synthetic polymers have established the market because of cost and easy to manufacture. To address the challenge of reducing the lifetime of degradation of these polymers, the scope of blending natural biopolymers is effective. This paper focuses on confirming the effectiveness of biodegradation in the molecular level of polymer blends between synthetic polymers and biopolymers. The synthetic polymers such as poly (methyl methacrylate) (PMMA) and poly (vinyl chloride) (PVC) were blended with varying compositions of biodegradable cellulose acetate butyrate (CAB). Using dimethylformamide (DMF) the films of PMMA/CAB, PVC/CAB blends were prepared by the solution casting method. Four different methods for studying biodegradability of these blends, namely soil burial test, enzymatic degradation, activated sludge degradation followed by microbial degradation were performed. The confirmation of degradation was done by NMR, FTIR, and Gel Permeation Chromatography (GPC) studies. Moreover, degradation analyses were determined by the weight loss method. Sufficient biodegradability was shown with an increase in CAB content in the blend. This work provides an approach for bringing about the degradation of synthetic polymers without much compromise on their properties. Also, the type of microorganisms that effectively degrades these polymer bends can be known.


Introduction
Common polymers like polypropylene, PMMA, PVC, polyethylene are extensively used, and reusing or recycling them is a serious task. Even their composites add on to the non-biodegradable wastes throughout the world. The alternative is the use of biodegradable polymers which mainly originates from renewable sources. However, their properties of brittleness, low thermal stability, water vapor permeability, and uncontrolled molecular weight are the main challenges that are yet to be overcome when compared to synthetic polymers. Moreover, there are no single biodegradable polymers that can be used as such in applications such as carry bags, biomedical, and food packages. Therefore, it is desirable to modify existing synthetic polymers by blending with a wide range of biopolymers, fillers and bring about enhanced degradation [1]. Some researchers have attempted for preparing these kinds of composite polymers by blending biopolymers with synthetic polymers [2][3][4]. Optimum balancing of physical and biodegradation properties of biopolymer/synthetic polymer blends will minimize the waste disposal problems. Biopolymer nanocomposites have a scope in industrial sectors as it is easy to process and have slightly greater biodegradation property [1]. Cellulose is a biopolymer that is found in abundance in wood and bacterial origin. Untreated cellulose behaves as a thermosetting polymer and has less solubility. On the other hand, cellulose derivatives are thermoplastics in nature and easy to process which makes them blend with synthetic polymers. Most of the cellulose derivatives are readily biodegradable by microorganisms that utilize cellulose enzymes, but due to the additional acetyl groups cellulose requires the presence of esterases. Thereby, the blending of the CAB may induce biodegradation in the synthetic polymer. PMMA a versatile synthetic polymer has enormous useful properties required in industrial and pharmaceutical applications. The literature survey on PMMA/Cellulose shows that a higher concentration of cellulose improves the bonding within them. PMMA/Cellulose blends [5] were subjected to dynamic mechanical analysis showed greater interaction at a high concentration of cellulose with PMMA. The nanocomposites with PMMA molecules covalently grafted to cellulose exhibited much higher optical transparency, thermal stability, and hygro-mechanical properties than the control samples [6]. PMMA/2,2,6,6tetramethylpiperidyl-1-oxyl oxidized CNFs (PMMA/TOCN) nanocomposites with high transparency, controllable birefringence, and enhanced mechanical properties exhibit great potential for applications in the optical devices and engineering field [7]. Porosity and stability can be seen in membranes based on CA/ PMMA which was used for the separation of proteins and toxic heavy metals [8].
To effectively modify the synthetic polymer, a few grafted copolymers based on cellulose-g-PMMA were found to be more moisture resistant and also exhibited better chemical and thermal resistance [9]. Carboxymethyl cellulose-g-PMMA showed thermal stability better than that of PMMA [10]. Nevertheless, none of the literature discussed the biodegradation of these fibers/films. Hence, this paper focuses on the aspects of PMMA/CAB degradation. One more synthetic polymer PVC which is dominated in pipe and automobile industries is of major concern as this polymer is non-degradable. The preparation of PVC and biopolymer composites is challenging and requires more study. Cellulose acetate (CA) and PVC nanofiber mats were electrospun into nanofibers which showed smooth fibers at 14% CA and solution concentration had a significant influence on the tensile strength of the nanofiber mats [11]. It was found that there are no biodegradable studies on PVC/cellulose derivative films. CAB as one of the biodegradable and biocompatible cellulose materials has attracted much attention in both academic and industrial fields ranging from a food packages, biomedical products, clothing to automotive materials [12]. Hence, this paper reports the preparation and degradation of PVC/CAB film in various standard conditions. Different ratios of films PMMA/CAB, PVC/CAB blends were prepared and characterized using chemical, spectroscopic, and chromatographic techniques. A comparison of the mode of degradation was made to understand the microbial degradation as well as the physical degradation of the polymer. This paper has been split into two parts, the first part shows biodegradation studies such as soil-burial, activated sludge, and enzyme-based methods using ASTM D6954, D5988, D5209-92, D5247-92 [13,14]. In the second part, the degradation of polymer to smaller elements was monitored by microbial degradation method and verified using FTIR, NMR, GPC techniques [15,16].

Materials and methods
PMMA, PVC, and CAB were obtained from commercial sources. The viscosity average molecular weight of PMMA, PVC, and CAB were 75,000, 101,000, 70,000 g mol −1 , respectively. Bushnell-Hass broth and Bushnell-Hass agar (BH, Hi-Media) were used as the medium. All other chemicals used were of analytical reagent grade.

Preparation of PMMA/CAB and PVC/CAB films
Dimethylformamide (DMF) was used as a common solvent for preparing polymer solutions of CAB and PMMA and the preparation of PVC/CAB polyblends. We have previously prepared synthetic/natural polymer blends and studied their biodegradability. The procedure is the same as Krishna et al. [14]. Briefly, 1 g of polymers was dissolved in 50 mL of DMF at ambient temperature and used as stock solutions. Composites were prepared by mixing different ratios of stock solutions. Mainly, CAB content were 0%, 30%, 50%, 70%, and 100% in PMMA/ CAB and PVC/CAB blends. The solutions were stirred well for an hour and the solution was cast on the Petri dish. Films thicknesses ranging from 0.2-0.4 mm were obtained. Polarizing microscope (SDTECHS make) was used for analyzing the pure polymers and their interaction in blends. [17,18] As reported in the literature about the soil nature [14] the blend films were subjected to soil burial tests by maintaining 25% moisture weight. The films were taken out, washed with water, dried, and weighed in the intervals of 5, 10, and 15 days. In the case of soil adhering to the blend films after the tenth day, the films were washed and extracted with DMF. Using filtration, the soil was separated after evaporating the solvent residue. The experiment was repeated thrice and results were reproducible with ± 2% error.

Activated sludge degradation study
Activated sludge was provided by BASF Company. The characteristics of the sludge have been reported in [14]. Aeration was provided for jars containing fixed volumes of activated sludge and blend films. The degraded films were tested after 5, 10, 15 days after washing with distilled water and drying at 60 °C. The experiment was repeated thrice and results were within ± 3% error.

Enzymatic degradation study
The films were tested as reported in [14] using enzyme lipase from the porcine pancreas in buffer solution. The test was reproduced thrice with ± 3% error. Briefly, Bushnell-Hass agar (BH, Hi-Media) and Bushnell-Hass broth were used for detecting the biodegradation process. The pH of the medium was set to 7. The mixture of bacteria culture containing Pseudomonas, Escherichia coli, Bacillus, and Klebsiella was used in this study. Commercially available enzyme lipase was obtained from Hi-Media. As the sole carbon source about 10 mL of CAB, PMMA/CAB (30:70), PMMA/CAB (50:50), and PVC/CAB (30:70), solutions with control was taken in Erlenmeyer flasks and dissolved in dichloromethane (DCM). 50 ppm of mixed culture (Pseudomonas, Escherichia coli, Bacillus, Klebsiella) was inoculated. The test was carried as reported in [14]. After 30 days of the incubation period, the residual blend films were extracted with an equal volume of DCM. The resultant solute of polymer blends (about 1 µL) after evaporating the solvent was analyzed by using FTIR, (PerkinElmer, Paragon 500 model), NMR (Bruker 300 MHz, deuterated chloroform as solvent, and 20 µL of samples) and Gel Permeation Chromatography (GPC). Tetrahydrofuran (THF) was used as an eluent with a flow rate of 1 ml min −1 at 60 °C for GPC studies. The column was 3 × p1 gel 10 µm MIXED-B 300 × 7.5 mm with differential refractometer as detector and PI caliber GPC software. The preparation of samples was done by dissolving 1-2 mg ml −1 of blends and taking in a different aliquot of the eluent. 0.45 mm membrane was used to filter these solutions and 100 µL was injected for analysis.

Result and discussions
CAB was found to be miscible in both PVC and PMMA solutions [14] indicating CAB chains have specific interaction with PVC and PMMA. Figure 1 shows polarized optical micrograph of (a) PMMA, (b) PMMA/CAB (70:30), (b) (50:50), and (c) (30:70). With an increase in CAB content in the blend homogenous morphology was intact in all the blends wherein pure PMMA had a plane morphology while PMMA/CAB showed wrinkles at higher CAB content. In (e) PVC, (f) PVC/CAB (70:30), (g) (50:50), (h) (30:70), and CAB, the pure PVC had a clear morphology and with increasing CAB content more wrinkles were observed without losing the homogenous characteristics. The wrinkles may be caused due to miscible interaction between synthetic polymer and CAB. In CAB (i) the image shows wavy morphology wherein the film formed is not as smooth as its blends. So, due to the presence of miscibility between synthetic polymers and CAB, the films formed were uniform, had less crystalline region, and no spherulites regions.

Soil burial degradation
Soil degradation test of PMMA/CAB blend films showed 0.3 to 2.95% of degradation order, while the PVC/CAB blend films comparative less as 2.4% as seen in Fig. 2a and b. It is visible that 100% CAB had shown more degradation than other blends. The polymers exposed to soil might have initially experienced Consequently, the oxygen can attack the newly generated surface with the formation of peroxides, hydroperoxides, oxides, etc., which promote the scission of PMMA and PVC polymeric chains into small fragments. These chains are further susceptible to microorganism attacks [17,18]. Soil bacteria and fungi might be responsible for biodegradation [19]. Moreover, introducing CAB (30%, 50%, 70%, and 100%) into PMMA and PVC showed an increase of almost 0.8% in the overall degradation of the blends. This suggests that CAB had a major role in initiating the biodegradation in the blend. Nevertheless, PVC/CAB weight loss is less as compared to pure CAB which suggests that the breakdown of PVC is quite difficult. It is noticeable that with an increase in days of the test, the biodegradation of each blend ratio increased up to 1% every 5 th day. This biodegradation might be due to bacteria and fungi present in the soil [19].

Enzymatic degradation
In the presence of phosphate buffer, the blends are initially hydrolyzed and then followed by enzymatic degradation. Lipase from porcine pancreas under a controlled environment was able to degrade about 0.9-4.95% for PMMA/CAB blend and 0.6-4.42% for PVC/CAB blends (Fig. 2c and d). Within fifteen days, the blends showed biodegradation to an extent similar to 100% CAB. Therefore, enzymes have a major contribution to the biodegradation of the blends. However, the pure PMMA lost 0.4 mg and PVC lost 0.03 mg of weight after fifteen days of estimation period.

Degradation in activated sludge
The blends subjected to activated sludge for 15 days showed degradation of about 1.1 to 5.21% for PMMA/CAB blends and 0.9 to 4.87% for PVC/CAB blends. The activated sludge method involves the use of rich inoculated biodegrading microbes and constant stirring with sufficient aeration. The polymer blend with higher CAB content showed maximum degradation ( Fig. 3a and b). The results comparatively showed higher degradation than any other methods under study. Moreover, there is a notable increase of 2% in weight loss percentage compared to the soil degradation method. This strongly suggests that the combined effect of microbes and hydrolysis enhanced the biodegradation of PMMA/CAB and PVC/ CAB blends. The pure PMMA lost 0.8 mg while PVC lost 0.2 mg of weight in this analysis. Nonetheless, the sign of the impact of hydrolysis on blend shows that the CAB is bleaching out of the intramolecular network, thereby exposing synthetic polymer strands vulnerable to microbes and other means of degradation [20,21].

Enumeration of bacteria during degradation
The control and test samples containing polymer blends were examined at intervals of five days for 30 days (Fig. 4). The bacteria recovery in control showed exponential growth than test samples. The bacterial count was about 5.1 × 10 5 CFU/mL for control on the fifth day while test samples remained below 2.1 × 10. 5 CFU/mL. A gradual increase in bacterial count in test samples was noticed with an increase in the number of days. Killing efficiency between the test sample and control showed no significant difference. This indicates that the blend polymer is comparatively stable up to fifteen days, thereafter the bacterial count increased due to the regeneration capability of bacteria. Even significant changes in the pH of the test solutions were observed. The initial pH value of 7.0 ± 0.2 was reduced to 6.8 in 30:70 (PMMA/CAB) systems. Similar results were obtained for PVC/CAB blends. Pseudomonas and Bacillus would be the main reason for the biodegradation of PVC [22]

Enumeration of the enzyme during degradation
The BH medium was taken in five sets of Erlenmeyer flasks and commercially available enzyme lipase was used for the efficiency studies. 250 mL of BH medium + 250 units per 10 mL of lipase was dissolved in phosphate buffer. The bacterial count was done at a regular interval of five days. While comparing the enumeration of bacterial studies with the enumeration of the enzyme studies

Scheme 1. Preparation and biodegradation investigation of polymer blends
both showed a similar increase in the bacterial count with an increase in exposed days. Nevertheless, the lipase enzyme attacks the ester bonds of CAB wherein the products formed to interact with PVC/PMMA chains to either liberate chlorine from PVC or peroxides from PMMA. This breakdown of long chains into smaller chains exposes the vulnerable function group in polymers and hence, boosting the bacteria growth. Moreover, the presence of hydrophobic regions in synthetic polymer provides a surface to effectively catalyze the ester bond linkage. The overall process pathway of enzyme and bacteria degradation of polymer blends is shown in scheme 1.  the characteristic stretching frequency showed less intensity indicating polymer has reduced to simpler elements [23,24]. FTIR spectra of PMMA/CAB (30:70) at different stages of degradation are shown in Fig. 5b. Before biodegradation the spectrum (a) shows the characteristic bands at 3446 cm −1 (OH), 2936 (C-H aliphatic stretch), 1743 cm −1 (C=O carbonyl group), 1365 cm −1 (C-H for methyl group), 576 cm −1 , (C-H stretch for monosubstituted benzene). Spectrum (b) shows signs of degradation having the characteristic bands at 3421 cm −1 (OH), 2931 (C-H aliphatic stretch), 1743 cm −1 (C=O carbonyl group) reduced. The same diminishing of characterization stretching frequency in the spectrum (c) for enzyme degradation is observed. FTIR spectra of PMMA/CAB (50:50) at different stages of degradation are shown in Fig. 5c. Spectrum (a) shows the characteristic bands at 3471 cm −1 (OH), 2832 (C-H aliphatic stretch); 2017 cm −1 (C=O carbonyl group) 1741 cm −1 (C=O carbonyl group), 1446 cm −1 (C=C stretch in aromatic nuclei) 887 cm −1 (C-H stretch for substituted benzene). In spectrum (b) characteristic stretching frequency is diminished in 400 to 600 cm −1 . In enzymatic degradation spectrum (c), stretching frequency is observed but the intensity of the peak is less. This implies that the degradation has occurred in the blend film. FTIR spectra of PVC/CAB (30:70) at different stages of degradation are shown in Fig. 5d. Spectrum (a) shows the characteristic bands at 3495 cm −1 (OH), 2974 (C-H aliphatic stretch); 2104 cm −1 (C=O carbonyl group), 1756 cm −1 (C=O carbonyl group), 1408 cm −1 (C \=C stretch in aromatic nuclei) 887 cm. −1 (C-H stretch for substituted benzene). Spectrum (b) shows the bacteria degradation wherein all the stretching frequency is in less intensity, whereas in enzyme degradation all the frequency is diminished [25]. Similar structural molecular changes have been observed by several researchers in polystyrene-starch blends [26] 1 H NMR analysis of blend polymer degradation Table 1 shows the functional groups of the before and after biodegradable polymers, respectively. The 1 HNMR spectra of before and after degradation samples are shown in Fig. 6a for pure CAB, Fig. 6b for 30:70 PMMA/CAB, Fig. 6c for 50:50 PMMA/CAB, Fig. 6d for 30:70 PVC/CAB. PMMA is a proton acceptor polymer and CAB is a proton donor polymer and they are found to be miscible due to hydrogen bonding. Before degradation, as per the CAB structure point of view, four ester groups are present in the 6, 3, 2, and 7 positions. One more ester group is formed due to blending with PMMA. These ester groups show multiple peaks between 0.10 to 2.5 ppm. The CH 2 aliphatic protons can be observed between 2.19 and 2.29 ppm. Another acetylenic proton peak is observed between 2.19 and 2.29 ppm. The aliphatic methylene (CH 2 ) peaks are noticed at 1.1 and 1.6 ppm and the methyl protons peaks are observed at 0.86 ppm. The peak at 3.5 ppm is due to vinyl compounds [13,16,24,25]. After 15 days of biodegradation, plenty of small peaks is observed in the region of 1.5 to 5, because of the breakage of long-chain polymers and hydrolytic degradation of esters into CO 2 and H 2 O.

GPC characterization
PEO/PEG standards were used to calibrate GPC [12] and molecular weights were calculated before and after bacterial degradation are shown in Table 2. The blend film containing high CAB showed greater degradation as observed in molecular weight.   the presence of CAB polymer chains as intermolecular network with PMMA and PVC demonstrated enhanced polymer degradation as enzymes were not only able to hydrolyze surface but also able to penetrate the matrix. Microplastics are yet another problem polluting coastal and marine habitats [27,28]. Because of enhanced degradation in these synthetic polymers in our blend polymers, the generation of microplastics is minimized. As performed as per ASTM methods [29][30][31][32][33], we propose a probable mechanism of degradation of PMMA and PVC in the presence of CAB in Scheme 2. The CAB being vulnerable to microbial attack starts degrading first which generates peroxides and hydroperoxides groups on PMMA and PVC. These peroxides groups cleave the carbon backbone of the synthetic polymers. These smaller fragments further undergo depolymerization to give carboxylic end groups and finally carbon dioxide and water. The extent of 4-5% weight loss in a matter of 15 days is remarkable as pure synthetic polymer takes 100 years for the same.
In PVC/CAB blends, the major degradation may be due to CAB rather than PVC, as observed in FTIR and NMR studies. But the formation of ester groups in NMR studies suggests that the dechlorination is promoted to some extent in the presence of CAB. Nonetheless, these synthetic polymers need organic solvents for extraction and analysis at the starting analysis stages, which later were soluble in water indicating the breakdown of long chains into smaller fragments.

Conclusion
The problem of the least understood biodegradation of synthetic polymer when blended with natural polymer is addressed in this paper. The weight loss, bacterial, enzymatic methods were used to study biodegradation of blend polymers. The Scheme 2. Probable mechanism of degradation pathway of PMMA/CAB and PVC/CAB blends in bacterial and enzymatic conditions results obtained from FTIR, NMR, and GPC indicated a positive response for biodegradation of blend films in which the content of CAB is more. Moreover, pure CAB partially degraded probably because the reduced number of free hydroxyls enhanced the interaction with water and inhibited enzymatic hydrolysis. While, in a less water-containing solid culture medium, synthetic polymer blended CAB showed biodegradation.
Funding Open access funding provided by Manipal Academy of Higher Education, Manipal.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.