| dc.description.abstract | The graft copolymerization of methyl acrylate, ethyl acrylate and acrylonitrile onto gelatin in aqueous medium was necessarily heterogeneous and was not simple enough for studying the kinetic and mechanistic aspects of the reactions involved. Quantitatively, three features are discernible from the results obtained. Firstly, the characteristic feature of grafting is the small number of grafting sites and high molecular weight of the grafted branches.
Secondly, the time of polymerization has an important effect. At 60°C, the efficiency of grafting and the percent grafting are lower in the early stages of polymerization. Significant increase both in the efficiency and percent grafting are observed with progress of polymerization in both the systems. The results seem to indicate that, in the presence of K?S?O?, the grafting occurs by chain transfer at 60°C. The low efficiency of grafting in the early stages can be due to ineffectiveness of the primary radicals to produce radical sites on the protein backbone, which can also explain the simultaneous formation of more homopolymer. A relatively short polymerization time (30 min) is preferable to obtain high molecular weight grafted branches.
Lastly, the temperature effect on the polymerization is also highly interesting. One of the possible interpretations for the observed increase in grafting efficiency (at 70°C, 30 min) may be the combined effect of both primary radicals (SO??) and growing macroradical of synthetic moiety in producing active sites on the protein by hydrogen abstraction. As far as grafting is concerned, there does not seem to be any essential difference in the grafting behaviour of the monomers tested.
All the gelatin graft copolymers were practically insoluble in almost all organic solvents and hence could not be fabricated by solution casting technique. In general, the melt flow of these samples was low and it was difficult to get films of uniform thickness by the compression moulding technique. Due to non-availability of Brabender extension equipment, it was not possible to establish the feasibility of using extrusion equipment. These materials may have applications as biodegradable plastic mulches.
The selected samples, inoculated and incubated with the test organisms, did show an initial rapid weight loss during the first week, followed by a period (to the conclusion of the study) of nearly uniform rate at an appreciably reduced level. It was also found that the rate of degradation progressively diminished with the increase in grafting efficiency. Parallel sets of experiments carried out employing the same samples as the only sources of both carbon and nitrogen showed marginal but definite increase in the utilization of the copolymers.
The relationship between the [log(rate)] (during the first week) and the percentage of synthetic polymer in the grafted samples is linear. The first-order dependence of rates may be indicative of the unimolecular nature of degradation. In fact, it is expected that such rate-composition plots can be used to predict, with a reasonable degree of accuracy, the rate of degradation of a copolymer sample of known composition under given conditions. But, the rate of degradation appears to be considerably influenced by other factors resulting in substantially low rates of degradation. The long-range objective can be set for the development of a predictive scheme that would allow the properties of yet unsynthesized polymer to be predicted.
The observed phenomena of rapid weight loss during the first week and the concurrent cooperative changes such as exponential increase in bacterial population and pH of the culture medium, superimpose well and indicate rapid and early exhaustion of the polymer samples. The change in pH of the medium or catabolite repression may explain the retarded rate of degradation after the first week. Scanning electron micrographs and estimation of total nitrogen content of samples, before and after testing, also confirmed biodegradation.
A limited comparison of gelatin-ethyl acrylate and gelatin-methyl acrylate copolymers, under identical conditions, reveals that –CH?– and –C?H?– offer approximately equal resistance against enzymatic attack. This is in full conformity with the reported stability of carbon-chain polymers, especially of those having side groups on the alternate carbon atoms. It is explained that these pendant groups prevent the ?-oxidation of carbon-chain backbone. The total resistance of these grafted branches is not surprising considering the fact that no enzyme system has been found so far which is capable of even removing the substituents in modified cellulose.
Degradation tests coupled with the enrichment techniques carried out over a longer period might offer the test organisms the possibility of adapting themselves and may provide more affirmative answer to biodegradability.
In contrast to the above systems, gelatin-g-poly(acrylonitrile) exhibited a greater inhibitory effect probably due to the presence of cyano groups. A calculative control of efficiency of grafting may, perhaps, result in total resistance of these copolymers to microbial attack. The gelatin–acrylonitrile graft compositions can be considered for glaze finishing on leathers where the microbial resistance is a desirable factor.
The selected bacterial strains seem to show a certain degree of antibiosis when inoculated as mixed inoculum. In some cases, the decrease of the optical density of the culture media after 20–25 days, incubated with the mixed inoculum is suggestive of this phenomenon. Consequently, mixed bacterial inoculum of the above bacterial strains is less effective than their individual activities. A more critical selection of organisms whose symbiotic relationship would result in more effective degradation might be the right angle of attack for achieving higher rates of biodegradability.
Interact in a unique way which is in marked contrast to the behaviour of pure cultures studied in the laboratory. The relatively slower degradation observed in soil can be due to the availability of other easily accessible carbon sources in the soil which will be preferentially attacked. It is also reasonably well-established that a certain period of time is required before the causative organisms reach a certain population level, and thereafter any fresh introduction of the sample will result in more effective degradation. The compositions of the microflora of any habitat is governed by the biological equilibrium created by the association and interactions of all individuals found in the community. Environmental changes temporarily upset the equilibrium, but it is re-established, possibly in modified form, as the community shifts to become acclimated to the new circumstances and hence better results are expected.
Also, one can extend the imagination to speculate that grafting of conventional monomers e.g., ethyl acrylate etc., with the incorporation of photosensitive groups i.e., copolymerization with alkyl vinyl ketones, would render these copolymers both photo- and biodegradable.
In any case, a time-limited investigation, such as the present attempt, cannot clarify all the questions related to the complexity and variability of the interrelationships between the microorganisms and the synthetic materials. Endeavours Can be made to utilize the laboratory and field testing data to employ a judicious combination of various factors, to predict the effective life of a plastic in a given usage environment.
To conclude, it may be added that it has been made quite obvious that we are concerned with a subject of very active interest, in which, as yet, it has been possible to put forward more speculative ideas than conclusions based on reliable and reproducible experimental data. The present investigation may be a stimulus for new and further-reaching research on biodegradable polymers. | |
