CAB was found to be miscible in both PVC and PMMA solutions [14] indicating CAB chains have specific interaction with PVC and PMMA.
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 reduced to 2.4%. The blend with more CAB showed high degradation as seen in Fig. 1a and 1b. The polymers exposed to soil might have initially experienced biodegradation, where microorganisms consume the natural cellulose component. Consequently, the oxygen can attack the newly generated surface with the formation of peroxides, hydroperoxides, oxides etc., which promote the scission of polymeric chains into small fragments. These chains are more susceptible to microorganism attack [17, 18]. Soil bacteria and fungi might be responsible for degradation [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 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. 1c and 1d). Within fifteen days the blends showed biodegradation to an extent pure CAB. Therefore, enzymes have a major contribution to the biodegradation of the blends.
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 polymer blend with higher CAB content showed maximum degradation (Fig. 2a and 2b). The results comparatively showed higher degradation than any other methods used maybe because of the combined effect of microbial degradation and hydrolysis [20, 21].
Tensile properties
Figure 2c and 2d show the tensile properties of PMMA-CAB, PVC-CAB blend films for different days. In this study also the tensile strength of the blend films decreases with increasing days. This result strongly proves the degradation of the synthetic polymers in the presence of CAB.
Enumeration of bacteria during degradation
The control and test samples containing polymer blends were examined at intervals of five days for 30 days (Fig. 3). The bacteria recovery in control showed exponential growth than test samples. The bacterial count was about 5.1×105 CFU/mL for control on the fifth day while test samples remained below 2.1×105 CFU/mL. A gradual increase in bacterial count in test samples was noticed with an increase in days. Killing efficiency between the test sample and control showed no significant difference. This point out that present blend polymer is comparatively stable up to fifteen days, thereafter due to regeneration capability of bacteria the bacterial count increases in remaining days. 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.
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. As the enumeration of bacteria data during degradation the enumeration of the enzyme showed a similar increase in the bacterial count with an increase in days. The overall mechanism of enzyme and bacteria degradation of polymer blends is shown in scheme 1.
FTIR analysis of blend polymer degradation
FTIR spectra of pure CAB at different stages of degradation are shown in Fig. 4a. Before biodegradation the spectrum (a) shows the cellulose characteristic bands at 3484 cm− 1, 2984 cm− 1 (C-H aliphatic stretch); 1743 cm− 1 (C = O carbonyl group), 1092 cm− 1 (C = O stretch for C-O-C alicyclic anhydride group); 911 cm− 1 (C-H stretch for substituted benzene). Spectrum (b) for bacteria degradation, the characteristic bands at 3607 cm− 1 (OH stretch); 2924 (C-H aliphatic stretch); 2111 (C-H aliphatic stretch); 1282 (C-O stretch for alicylic anhydride group), 787 (mono substituted benzene) diminished. Spectrum (c) for enzyme degradation, all the characteristic stretching frequency showed less intensity indicating polymer has reduced to simpler elements [22, 23]. FTIR spectra of PMMA/CAB (30:70) at different stages of degradation are shown in Fig. 4b. 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 def 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. 4c. 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 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. 4d. 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 [24].
1 H NMR analysis of blend polymer degradation
Table 1 shows the functional groups of the before and after biodegradable polymers, respectively. The 1HNMR spectra of before and after degradation samples are shown in Fig. 5a for pure CAB, Fig. 5b for 30:70 PMMA/CAB, Fig. 5c for 50:50 PMMA/CAB, Fig. 5d 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 multiplet peaks between 0.10 to 2.5 ppm. The CH2 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 (CH2) 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, 23, 24]. After 15 days of biodegradation plenty of small peaks are observed in the region of 1.5 to 5, because of the breakage of long-chain polymers and hydrolytic degradation of esters into CO2 and H2O.
Table 1
NMR Chemical shift values of before and after degradation of polymer blends
Compound
|
Position
|
δ (ppm)
Before degradation
|
δ (ppm)
After degradation
|
Remarks
|
A: Pure CAB
|
7
6
2
3
-CH3
|
3.7
2.0
1.9
1.9
0.9–1.2
|
2.3
2.2
1.2
1.7
0.9–1.2
|
Due to the splitting of carbonyl compounds, CH3, CH2, C = C the 3–5 small peaks are observed.
|
B: PMMA/CAB (30:70)
|
7
6
2
3
-CH3
|
3.7
2.0
1.9
1.9
0.9–1.2
|
2.3
2.2
1.2
1.7
0.9–1.2
|
Due to the splitting of carbonyl compounds, CH3, CH2, C = C the 3–5 small peaks are observed.
|
C:
PMMA/CAB (50:50)
|
7
6
2
3
-CH3
|
3.7
2.0
1.0
1.4
0.9–1.2
|
2.3
2.2
1.2
1.7
0.9–1.2
|
Due to the splitting of carbonyl compounds, CH3, CH2, C = C the 3–5 small peaks are observed.
|
D
PVC/CAB (30:70)
|
7
6
2
3
-CH3
|
3.7
2.0
1.0
1.4
0.9–1.2
|
2.3
2.2
1.2
1.7
0.9–1.2
|
Due to the splitting of carbonyl compounds, CH3, CH2, C = C the 3–5 small peaks are observed.
|
E
PVC/CAB (50:50)
|
7
6
2
3
-CH3
|
3.7
2.0
1.0
1.4
0.9–1.2
|
5.0
3.7
2.0
1.0
1.4
0.9–1.2
|
Due to the splitting of carbonyl compounds, CH3, CH2, C = C the 3–5 small peaks are observed.
|
GPC characterization
GPC calibrated was PEO/PEG standards [12] and molecular weights 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. Figure 6a to 6d shows GPC response of PMMA/CAB (30:70), PVC/CAB (30:70), PMMA/CAB (50:50), PVC/CAB (50:50), respectively. The decrease in molecular weight shown in spectral data matched with NMR and FTIR studies. Thus, it implies that biodegradability increases when synthetic polymers are modified using natural polymer like CAB.
Table 2
GPC characterization and molecular weight values of before and after degradation of polymer blends
Sample
|
Molecular weight before degradation
|
Molecular weight after degradation
|
Difference
|
A (CAB)
Peak 1
Peak 2
|
92186
6421
|
77993
5238
|
14193
1065
|
B
PMMA/CAB (50:50)
Peak 1
Peak 2
|
92462
6473
|
84791
5900
|
7491
573
|
C PMMA/CAB (30:70)
Peak 1
Peak 2
|
92186
6421
|
80910
5477
|
11276
944
|
D
PVC/CAB (50:50)
Peak 1
Peak 2
|
85878
6475
|
79176
5574
|
6702
905
|
E
PVC/CAB (30:70)
Peak 1
Peak 2
|
99440
6271
|
78348
5639
|
21092
632
|