Polymeric materials and plastics have become an indispensable part of the day-to-day life. However, the discarded plastics pose a potential threat to the environment, as they take a very long time to degrade under natural conditions. Plastic wastes contribute to about 9% of the total 1.20 lakh tons per day of the municipal solid waste generated in India.175Besides affecting the aesthetics of the environment, the ingestion of waste plastics, usually the packaging materials, results in the death of aquatic organisms, animals and birds. Many plastics are also non-biodegradable, and hence contribute to white-pollution. Historic waste disposal technique by landfilling is ineffective, as plastic bags at landfill sites take several decades to photodegrade. Hence, stringent regulations on the plastic waste disposal have led to the development of novel techniques for the degradation of the polymers. One of the conventional methods is by the incineration of the plastic waste. This method involves high temperatures and results in the generation of toxic gases before the polymer is fully mineralized.
Therefore, polymer photodegradation has emerged as a non-conventional mode of degrading and recycling the waste polymers and plastics.
Photodegradation is an important mode of polymer degradation wherein the incident light radiation itself induces the scission of the macromolecule by the initiation of polymer radicals, or results in the generation of hydroxyl (.OH) radicals in presence of oxidizing agents and/or catalysts, which then attack the polymer backbone to initiate the radicals. Photodegradation of the polymers can be carried out either in the solid or liquid phase. In the solid state, the polymeric materials are exposed to UV radiation or sunlight in the form of thin films or sheets, while in the liquid phase, the polymers are dissolved in aqueous or organic solvents. Although the degradation of polymers in the solid state is more realistic from a practical viewpoint, liquid phase degradation is faster, owing to the homogeneous medium which offers enhanced mass transfer of the reactants and products. Moreover, liquid phase degradation is useful in studying the kinetics of chain scission of the polymers, by following the time evolution of the polymer molecular weight by gel permeation chromatography (GPC). Solid state degradation is monitored by different techniques, viz., reduction in weight of the sample with time, monitoring the changes in the surface morphology using scanning electron microscopy (SEM) and by monitoring the transmittance of the films by UV/vis or FT-IR spectroscopy.
Figure 13: Network mechanism for the dye sensitized degradation of phenolic compounds in presence of nano-TiO2.
Figure 14: Mechanism of generation of hydroxyl radicals by the excitation of TiO2using UV radiation (left half), and the excitation of the adsorbed dye (Eosin Y) using visible radiation (right half). The mechanism of electron injection and the formation of dye radical cations are also shown.
Recent focus is laid on the photocatalytic degradation of the polymers, where a semiconductor photocatalyst like TiO2 is used to enhance the degradation. In the solid state degradation, a
composite of the polymer and TiO2is used, while in the liquid state, the TiO2particles are suspended in the polymer solution. However, the mechanism of photodegradation is unaffected by the state in
Figure 15: Reduction of normalized molecular weight, during the photocatalytic degradation of PAM, P(AM-co-AA) and PAA in presence of CS TiO2. The inset shows the magnified portion of the degradation curve till 60 min. The values in the inset figure indicate the percentage of AA in the copolymer. It is evident that the copolymer degrades at a higher rate with the inclusion of more AA units. (Redrawn from ref.
177.)
which the polymer is degraded. Table 845,46,176–185
presents a listing of the different polymers that were photocatalytically degraded in both solid and in solution. It is clear that a wide variety of polymers based on acrylates, methacrylates, styrene, poly(olefins), poly(carbonate), and a few biodegradable polymers have been degraded, mostly in presence of TiO2.
The first step in the photocatalytic degradation of the polymers is the formation of the hydroxyl radicals (.OH) from the TiO2 surface. In the presence of organic solvents or in the absence of moisture, surface hydroxyl groups present in TiO2 serve as the key source of hydroxyl radicals. The UV photon and/or the.OH radicals generated according to reactions (1)–(14) (section 2) attacks the polymer, resulting in the generation of polymer α-radical [P.]. Theseα-radicals are the precursors of chain breakage. The next step is the reaction of [P.] with atmospheric oxygen to form polymer peroxy radicals [POO.]. These combine bimolecularly with one another and form [POO-OOP] species.
Thus, with the exclusion of oxygen, polymer oxy radicals are formed [PO.]. Finally, the scission of the polymer oxy radical produces a radical and a non-radical fragment. A detailed discussion on the various aspects of the mechanism of chain scission
of the different class of polymers is provided by Rabek.169
Sivalingam and Madras46 have studied the photocatalytic degradation of poly(bisphenol-A- carbonate) and have shown that bond cleavage occurs by Photo-Fries rearrangement of the aromatic carbonate unit, resulting in the formation of phenyl salicylate and dihydroxybenzophenone.
The time evolution of molecular weight of this polymer indicated the presence of weak and strong links, which degraded at faster and slower rates, respectively. Recently, we have shown that acrylic acid units in poly(acrylamide-co-acrylic acid) (p(AM-co-AA)) form weak linkages and acrylamide units form stong linkages, which degrade at faster and slower rates, respectively.177Figure 15 shows the reduction in molecular weight of p(AM-co- AA) of different comonomer composition, when degraded in aqueous medium. It is clear that the inclusion of acrylic acid units reduce the photostability of the copolymer. It was shown that the scission rate coefficients corresponding to the weak and strong links exhibit linearity with respect to the acrylic acid content in the copolymer.177 Figure 16 shows the possible mechanism of scission of the acrylic acid units in the p(AM-co-AA) copolymer based on the above discussion. It is evident that the scission product includes the formation of carbonyl moiety (aldehyde or ketonic groups), which was verified by UV/vis spectroscopy.
Chiantore et al.186and Kaczmarek et al.187have proposed alternate pathways for the formation of carbonyl groups, carboxylic acid groups and lactones during the photodegradation of alkyl acrylate and alkyl methacrylate polymers.
Zan et al.180 have observed the formation of cavities and cracks in the polystyrene-TiO2
nanocomposite films, when exposed to UV radiation and sunlight illumination. At long exposure periods, whitening of the composite films was observed, which was confirmed by the reduction in transmittance of the films. After 400 h of exposure in air atmosphere, nearly 30% weight loss was observed for the composite films with 2% TiO2under UV irradiation, and 20% weight loss under sunlight illumination. Similar morphological changes were also observed for the degradation of low density polyethylene (LDPE)-TiO2 composites.181 The weight loss of the LDPE-2% TiO2films was 60% and the number average molecular weight reduction was nearly 94% after 400 h of irradiation. Similar studies on the degradation of other polymer composite films have unequivocally shown that the composite films show a higher reduction in weight loss and significant morphological changes compared to the pure polymer films without TiO2.
Figure 16: Mechanism of main chain scission of the copolymer P(AM-co-AA) in presence of UV radiation. The by-products involve the formation of aldehydes. (Redrawn from ref. 177.)
9. Photocatalytic degradation of gaseous