Biodegradation behaviors and water adsorption of poly(vinyl alcohol)/starch/carboxymethyl cellulose/clay nanocomposites
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The focus of this work is to study the effect of sodium montmorillonite (MMT-Na) clay content on the rate and extent of enzymatic hydrolysis polyvinyl alcohol (PVA)/starch (S)/carboxymethyl cellulose (CMC) blends using enzyme cellulase. The rate of glucose production from each nanocomposite substrates was most rapid for the substrate without MMT-Na and decreased with the addition of MMT-Na for PVA/S/CMC blend (51.5 μg/ml h), PVA/S/CMC/1% MMT (45.4 μg/ml h), PVA/S/CMC/3% MMT (42.8 μg/ml h), and PVA/S/CMC/5% MMT (39.2 μg/ml h). The results of this study have revealed that films with MMT-Na content at 5 wt.% exhibited a significantly reduced rate and extent of hydrolysis. Enzymatic degradation behavior of MMT-Na containing nanocomposites of PVA/S/CMC was based on the determinations of weight loss and the reducing sugars. The degraded residues have been characterized by various analytical techniques, such as Fourier transform infrared spectroscopy, scanning electronic microscopy, and UV-vis spectroscopy.
KeywordsNanocomposites Carboxymethyl cellulose Scanning electron microscopy Cellulase
However, the enzymatic degradation of PVA/starch (S)/xcarboxymethyl cellulose (CMC) nanocomposites and sodium montmorillonite clay has never been studied.
The current paper studies the cellulase action on PVA/S/CMC composite film containing sodium montmorillonite nanoparticle at temperature 37°C ± 1°C. The modifications induced by the enzymatic treatment were evidenced by determination of weight loss, water absorption capacity, sugars released during biodegradation, as well as by UV spectroscopy, and total sugars were estimated by dinitrosalicylic acid (DNS) method .
Starch (S) was provided by Merck Company (Whitehouse Station, NJ, USA), and PVA (Mn = 72,000) and glycerol (Mn = 92/10, 78% purity) were purchased from the same company. Carboxymethyl cellulose sodium salt, with an average molecular weight of Mn = 295,225 was purchased from Fluka Company (Buchs, Switzerland). Sodium montmorillonite (Cloisite Na+) with a cation exchange capacity of 92.6 meq/100 g clay was supplied by Nanocor Inc. (Arlington Heights, IL, USA). Reagent DNS was used for the determination of sugars released during degradation. Cellulase from Aspergillus niger (specific activity 111 U mg−1) and hydroxide sodium were provided by Fluka Company.
The present work analyzes the enzymatic degradation behavior of some montmorillonite-containing nanocomposites of PVA/starch with CMC based on the determinations of weight loss and the reducing sugars. The nanocomposites have been prepared from 50 wt.% PVA-30 wt.% S-20 wt.% CMC containing small amounts of plasticizers, stabilizers, and destructuring agents (stabi-lizers or destructuring agents such as sodium montmorillonite clay and plasticizer such as glycerol) [28, 29]. Biodegradation studies were carried out at 37°C ± 1°C, pH = 4.8, using cellulase for 72 h.
Enzymatic degradation test
The enzymatic reaction mixture, comprising 1 ml of cellulase and 25 ml of 0.1 M acetate buffer, was placed in clean conical flasks. The dried samples were cut into 4 × 4 cm2 specimens, weighed, and immersed in the conical flasks. The flasks were placed in a shaking incubator with a rate of 70 rpm for 72 h at 37°C ± 1°C. After 1, 2, 3, 5, 7, 9, 12, 18, 24, 30, 36, 40, 48, 56, 60, and 72 h, the samples were removed, rinsed with distilled water to remove the enzyme, dried, and weighed, respectively.
where W0 and Wd represents the initial and final weights (before and after degradation, respectively) of the blends.
Water absorption test
where Wwet represents the weight of the wet specimen and Wdry is the weight of the dry specimen.
Detection of reducing sugars
The reducing sugars in the degradation solutions were quantified by the Nelson-Somogyi method (dinitrosalicylic acid method): 1 ml of reagent DNS was added to 1 ml of the sample to be analyzed  using 1 mg/ml glucose stock solution as a standard. At the same time, the blank was prepared using 1 ml of control sample. The mixture was heated at 90°C to 100°C for 10 min. After cooling to room temperature, 5 ml of distilled water was added, and the absorbance at 540 nm was measured. The respective carbohydrate concentration was obtained by comparison with a standard curve. Concentration of glucose produced for nanocomposites after 72 h and the first 4 h of enzymatic degradation is due to the action of cellulase at temperature 37°C ± 1°C.
Infrared spectral analysis of the blends, before and after degradation, was carried out by Shimadzu FT-IR RF50 spectrometer (Kyoto, Japan). The samples were prepared using KBr pellets, and spectra were recorded at a resolution of 4 cm−1.
Scanning electronic microscopy
Surface morphology of the films, before and after enzymatic degradation, was investigated using a scanning electronic microscope of XL30 type (FEI, Eindhoven, The Netherlands). The films were coated with pure metallic Ag. The laying down of Ag was carried out by evaporation of the metal under a high vacuum to give a thickness of around 100 Å.
Results and discussion
Degradability of polymers is a critical functionality for their application. Currently, no official standard method was established in determining biodegradability of polymers. The enzyme method , the microbiological method , and the soil burial method  have been used by different researchers. Moreover, biodegradability is also recorded by diverse indexes even using the same methods. The present study shows the role of cellulase in PVA/S/CMC/MMT-Na degradation.
Weight loss and water uptake
Rate and extent of glucose production
Rates of glucose production due to the action of 1-mg cellulase for each substrate
Rate (μg⁄ml h)
Scanning electronic microscopy
This present study reports the role of cellulase enzyme in the degradation of nanocomposites. The sodium montmorillonite clay content significantly impacted on the rate of CMC solubilization. Biodegradation behavior of films depends on the content of nanoparticles, and the decrease of the degradation rate observed in the final stage can be explained to the lower degradability of the MMT-PVA-S-CMC domains that remain in the material. After 4 to 72 h, the variation is almost negligible, nearly zero, as no saccharides and other compounds leached to the solution, as demonstrated before. The reduction of the degradation rate is also influenced by the water uptake ability of these polymers. MMT (0% to 5% w/w film) could decrease the WAC% and DED% of PVA/S/CMC/MMT nanocomposite films compared to the PVA/S/CMC films. Considering these results, it seems that the PVA/S/CMC/MMT bionanocomposite films show better physico-chemical properties than PVA/S/CMC films, and they can potentially replace PVA/S/CMC films.
We are grateful to University of Tabriz Research Council for the financial support of this research.
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