Photocatalytic Degradation of Poly (Acrylamide-co-acrylic Acid)

Jul 3, 2008 - R. Vinu and Giridhar Madras* ... The copolymers were statistical with a relatively high percentage of acrylamide units, as determined by...
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J. Phys. Chem. B 2008, 112, 8928–8935

Photocatalytic Degradation of Poly(Acrylamide-co-acrylic Acid) R. Vinu and Giridhar Madras* Department of Chemical Engineering, Indian Institute of Science, Bangalore, 560012, India ReceiVed: March 04, 2008; ReVised Manuscript ReceiVed: April 29, 2008

Poly(acrylamide-co-acrylic acid) copolymers of different compositions were synthesized and characterized. The copolymers were statistical with a relatively high percentage of acrylamide units, as determined by 13C NMR. Reactivity ratios calculated by the Finemann-Ross and Kelen-Tudos methods showed that the copolymers were random with a reactivity ratio of rAM ) 3.76 and rAA ) 0.28. The photolytic and photocatalytic degradation of the copolymers and the homopolymers was conducted in the presence of combustion-synthesized nano anatase titania. The degradation of the copolymer in the presence of combustion-synthesized titania was significantly higher than that observed in the presence of commercial titania, Degussa P-25. The degradation was modeled by using continuous distribution kinetics by following the time evolution of molecular weight distribution. The degradation follows a two step mechanism, wherein the rapid first step comprises the scission of weak acrylic acid units along the chain which is followed by the breakage of relatively strong acrylamide units. The rate constants for the weak and strong links follow a linear trend with the percentage of acrylic acid and acrylamide in the copolymer, respectively. This linear variation can be correlated with a similar trend observed for the activation energies obtained for the pyrolytic degradation of the polymers. Introduction Poly(acrylamide-co-acrylic acid) [p(AM-co-AA)] is a water soluble copolymer, which has nonionic acrylamide (AM) and anionic acrylic acid (AA) units. The water solubility of p(AMco-AA) is due to the hydrophilic amide and carboxylic acid moieties in its backbone, which impart sufficient polarity, charge, and hydrogen-bonding ability for hydration. P(AM-coAA) is a classical copolymer the comonomer reactivity ratio of which is very sensitive to pH. At highly acidic pH values, AA exists in its undissociated form, whereas at basic pH values, it exists as acrylate anion. It was shown1 that the reactivity of AA is high at sufficiently low pH and low at high pH owing to its dissociation. Thus, by tweaking the pH, the sequence length distribution of AA and AM can be tailored according to their potential application. Few reports are available on the synthesis,1–4 hydrolysis,5,6 and thermal degradation characteristics7 of p(AMco-AA). In the past decade, the polychelatogenic properties of p(AM-co-AA) in complexing different metal ions8 and its flocculation efficiency in tannery waste water9 have been reported. Hence, photodegradation of the copolymer assumes importance in the disposal of the residual waste after the above water purification processes. Recently, spatially cross-linked p(AM-co-AA) hydrogels with superior antiburn properties have been synthesized, and their swelling, network parameters, and rheological properties have been evaluated.10,11 Photolysis of the TiO2 semiconductor results in the generation of valence band holes and conduction band electrons, which produce hydroxyl (OH•) and superoxide radicals (O2-•), which oxidize the organic matter present in water. Recently, we have developed combustion-synthesized nano anatase titania (CS TiO2),12,13 which has high specific surface area, high surface hydroxyl content, and low band gap compared to commercially available Degussa P25 TiO2 (DP 25). It is well proven that CS TiO2 can degrade dyes12 and organics14 at a faster rate compared * Corresponding author. Tel: +91-80-2293-2321. Fax: +91-80-23600683. E-mail: [email protected].

to DP 25. CS TiO2 was also shown to catalyze the degradation of poly(bisphenol-A-carbonate),15 poly(ethyleneoxide) (PEO), and PAM16 more effectively than only ultraviolet radiation and DP 25. In the former, the mechanism of photocatalytic degradation was formulated on the basis of the photo-Fries rearrangement and nonconcerted cage recombination. In the latter, the photocatalytic degradation was attributed to the •OH radicals attacking the R-carbon atom of both PEO and PAM, thus enabling the oxidative scission of the macroradical. Continuousdistribution kinetics provides an elegant way to model the macromolecular scission by assuming the molecular weight to be a continuous variable. Polymer fission is assumed to take place by intramolecular hydrogen abstraction. The hydrogenabstracted radical produced then degrades into a stable polymer radical of lesser molecular weight and another radical, which again undergoes degradation. The rate coefficients for the polymer degradation, which are assumed to be linear functions of chain length,17 are calculated from the moment solution to the population balance equation (PBE). Although many studies exist on the thermal and photodegradation of the homopolymers AM and AA,16,18–21 investigations on the degradation of p(AM-co-AA) are relatively scarce.7 In the present study, p(AM-co-AA) of different compositions have been synthesized by solution polymerization and characterized by 13C NMR spectroscopy. The reactivity ratio of AM was kept high so that the effect of small amounts of AA units in the copolymer on photodegradation can be evaluated. The mechanism of chain scission based on the photogenerated hydroxyl radical has been proposed, and the photodegradation was modeled by using continuous distribution kinetics. The degradation rate coefficients of the homopolymers and of the copolymer of different compositions in the presence of UV radiation alone and in the presence of two catalysts, CS TiO2 and DP 25, have been determined. The dependence of rate coefficients on pH of the reaction system has also been evaluated.

10.1021/jp801887t CCC: $40.75  2008 American Chemical Society Published on Web 07/03/2008

Photocatalytic Degradation of p(AM-co-AA)

J. Phys. Chem. B, Vol. 112, No. 30, 2008 8929

Figure 2. Finemann-Ross plot (inset: Kelen-Tudos plot). Figure 1. Instantaneous copolymer composition (F) versus comonomer feed composition (f).

Experimental Section Materials. Poly(acrylic acid) (Mv ) 450 000) was purchased from Sigma Aldrich. The monomers, AM (S. D. Fine Chem., India) and AA (Merck, India) and the initiator potassium persulfate (S. D. Fine Chem., India) were purified prior to polymerization by standard techniques. Titanium isopropoxide (Lancaster, U.K.) and glycine (Merck, India) used in the preparation of the catalyst were used as received. Doubledistilled, Millipore filtered water was used for all purposes. Polymer Synthesis and Characterization. P(AM-co-AA) was prepared by free radical polymerization in aqueous medium. Because a high percentage of AM units in the copolymer is desired, copolymerization was carried out at a basic pH because AM is more reactive toward itself than toward AA at high pH values. Five different compositions (85/15, 75/25, 60/40, 50/ 50, and 30/70 by mol%) of AA and AM were taken in 30 mL culture tubes with 5% potassium persulfate as the initiator. The total solution concentration was 1 mol L-1. The solutions were initially purged with nitrogen for 10 min and maintained at 50 ( 0.1 °C in a water bath for 2 h. After the reaction, the copolymers were precipitated twice in methanol and dried at 60 °C until constant weight was obtained. These were then powdered and used for degradation experiments. For all the copolymers synthesized, the conversion was less than 5% for the applicability of the copolymer equation. Characterization of the copolymers was carried out by using 13C NMR spectroscopy. The proton decoupled 13C spectra were collected in AMX 400 MHz multinuclear FT-NMR spectrometer, with D2O as the solvent. The peak assignment for the various carbon atoms in the copolymer was as reported by Zurimendi et al.6 The composition of the individual monomer units in the copolymer was calculated from the integration areas of the resonance peaks corresponding to methylene carbon of the carboxylic and amide moiety appearing at δ (ppm) 34.54 and 36.01, respectively. Figure 1 shows the dependence of instantaneous copolymer composition F on the initial comonomer composition f. It is clear that the copolymers are statistical with a high percentage of AM units. Figure 2 shows the Finemann-Ross and Kelen-Tudos plots for calculating the reactivity ratios. The reactivity ratios using both methods were rAM ) 3.76 and rAA ) 0.28. The number average molecular

weight of the copolymers (henceforth indicated by the percentage of AA units in the copolymer), C10.3%, C20.4%, C29.3%, C47. 8%, and C58.9% were 4.4 × 105, 4.2 × 105, 4.2 × 105, 4.6 × 105, and 4.2 × 105 g mol-1, respectively. PAM was synthesized by the method reported elsewhere,16 and the number average molecular weight was 4.6 × 105 g mol-1. Catalyst Preparation and Characterization. Nanosize anatase titania was prepared by solution combustion synthesis that involves the combustion of aqueous solutions containing stoichiometric amounts of titanyl nitrate and glycine.12 A pyrex dish (300 mL) containing an aqueous redox mixture of stoichiometric amounts of titanyl nitrate (2 g) and glycine (0.8878 g) in 30 mL of water was introduced into the muffle furnace, which was preheated to 350 °C. The solution initially undergoes dehydration, and a spark appearing at one corner spreads throughout the mass and yields anatase-phase TiO2. The catalyst was characterized by various techniques such as XRD, XPS, TEM, BET, TG-DTA, FT-IR, and UV spectroscopy, as described elsewhere.12,13 The XRD pattern of CS TiO2 was indexed to the pure anatase phase of TiO2 in the space group I41/amd. The crystallite size determined by using the Scherrer formula was 8 nm. XPS showed the presence of Ti in +4 state. TEM studies showed that the crystallites of TiO2 are homogeneous with a mean size of 8 ( 2 nm, which agrees well with the XRD measurements. The BET surface area of the catalyst was 240 m2 g-1, which is higher than that of Degussa P-25 (50 m2 g-1). Figure SI 1 in the Supporting Information shows the UV-vis absorption spectra and the TG-DTA of CS TiO2 and DP 25. It is clear that CS TiO2 exhibits two optical absorption thresholds at 570 and 467 nm that correspond to band gap energies of 2.18 and 2.65 eV, respectively. This is significantly different from the band gap of Degussa P-25 (3.1 eV) because of the carbide ion substitution for oxide ion of the form TiO2-2xCxRx, where R is the oxide ion vacancy.14 The TG thermograms of the catalysts indicate that there is a 15.6% total weight loss for CS TiO2, indicating more surface hydroxyl groups, thus confirming the FT-IR study. Photochemical Reactor. The photochemical reactor used in the present study consisted of a jacketed quartz tube of 3.4 cm i.d., 4 cm o.d., and 20 cm length. A high-pressure mercury vapor lamp of 80 W (Philips, India) was placed inside the reactor. This reactor assembly was held concentrically inside a jacketed pyrex glass container of 5 cm i.d., 7.5 cm o.d., and 19 cm height.

8930 J. Phys. Chem. B, Vol. 112, No. 30, 2008

Vinu and Madras SCHEME 1: Mechanism of Formation of Hydroxyl Radicals from TiO2

Figure 3. Variation of Mn with time for the copolymers when degraded with CS TiO2.

Cold water was circulated in the annulus of both the reactor and the container to maintain the solution temperature below 35 °C because excess temperature might deplete the dissolved oxygen in the solution. The distance between the source and the bottom of the vessel was 2 cm to aid better stirring by using a magnetic stirrer. The lamp radiated predominantly at 365 nm (3.4 eV). The intensity and photon flux calculated by onitrobenzaldehyde actinometry22 were 4.64 × 10-6 einstein s-1 and 8.43 mW cm-2, respectively. The degradation was performed in an open system wherein the top surface of the photoreactor was open to the atmosphere. This facilitates atmospheric air to provide enough oxygen for oxidative degradation of the polymer solution. Further details are provided elsewhere.12 Degradation Experiments in Solution. The synthesized copolymers and homopolymers were degraded in aqueous solution at its natural pH, which was between 3.4 and 4.4. pH. It was monitored during the course of the reaction, and no significant changes in were observed. No degradation of the polymers was observed without UV radiation. Every time, 100 mL of 1 g L-1 solution was exposed to UV light with an optimal catalyst concentration of 1 g L-1.12 The degraded polymer sample collected at every time interval was centrifuged to remove the catalyst particles prior to analysis of molecular weight by HPLC-GPC. GPC Analysis. The HPLC-GPC consisted of Waters 515 isocratic pump, Rheodyne 7725 injector (sample loop, 200 µL), Waters Ultrahydrogel linear column measuring 7.8 mm × 300 mm (maintained at 50 °C), Waters R 401 differential refractometer, and a data acquisition system. The eluent used was 0.1 N NaNO3 at a flow rate of 1 mL min-1. The sample volume injected was 500 µL, and the chromatogram was converted to molecular weight distribution by using PEO narrow standards (Polymer Standards, U.S.A.). Degradation Experiments using TGA. The pyrolytic degradation of the polymers was carried out in a Perkin-Elmer PyrisDiamond thermogravimetric-differential thermal analyzer in the presence of inert nitrogen at a flow rate of 150 mL min-1. Different heating rates (2-20 °C min-1) were employed to find the activation energy of thermal decomposition calculated by Friedmann, Chang, and First Kissinger techniques.23

Theoretical Model. The experimental variation of the number average molecular weight (Mn) without any catalyst and with CS TiO2 and DP 25 indicate that the reduction in Mn is significant in the initial time period, and as time progresses, the reduction is moderate. Hence, it is logical to assume that there are weak links in the polymer chain that get depleted in the initial period, followed by the degradation of the strong links. The presence of weak and strong links has been observed for the degradation of polystyrene.24 The conclusion on the presence of weak and strong links is based on the comparison of the degradation profiles of the copolymers with the respective homopolymers, as will be discussed later. Adsorption of the byproducts of the degradation on the catalyst surface can also prevent further degradation. However, this is not the case here because similar trends of degradation are observed even for the photodegradation of the copolymers without the catalyst. The following continuous distribution model is based on the understanding that weak links are more susceptible to photodegradation in the initial time period, after which the strong links degrade. A similar treatment has been applied for the thermal degradation of poly(styrene) in solution by Madras et al.,24 where the scission of the weak links was attributed to the head-to-head or head-to-tail configuration of the phenyl group and the photocatalytic degradation of poly(bisphenol-A-carbonate) by using CS TiO2 by Sivalingam and Madras,15 where the degradation of weak links was attributed to photo-Fries rearrangement and concerted cage recombination. No oligomeric species, as a result of chain-end scission, was observed in the GPC chromatogram. The peak area corresponding to the polymer mass concentration was constant for all the samples, suggesting a random scission of the copolymers. The reaction for the binary cleavage of a copolymer of molecular weight x can be written as24 k(x)

p(x) 98 p(x ′ ) + p(x - x ′ )

(I)

where, p(x) is the time-dependent molar concentration of the copolymer. The total concentration of the copolymer is the sum of weak (w) and strong (s) links.

p(x, t) ) pw(x, t) + ps(x, t)

(1)

Thus, at any time t, the total molar concentration of the copolymer is given by

p(0)(t) ) pw(0)(t) + p(0) s (t)

(2)

The PBE for the random degradation of the copolymer in a well stirred batch reactor for the weak and strong links are15,24

Photocatalytic Degradation of p(AM-co-AA)

J. Phys. Chem. B, Vol. 112, No. 30, 2008 8931 pw(1) ) pw0(1) and ps(1) ) ps0(1). The time rate of change of molar concentration can be obtained by solving the two independent variable ODEs with the corresponding initial conditions pw(0)(t ) 0) ) pw0(0) and ps(0)(t ) 0) ) ps0(0). The molar concentration of the weak and strong links are given by (1) pw(0) - p(0) w0 ) kw pw t

(10)

(0) (1) p(0) s - ps0 ) ks ps t

(11)

Adding eqs 10 and 11 and rewriting the expression in terms of total molar concentration and Mn yields

(

Figure 4. UV-vis absorption spectra of the copolymers during degradation.

∫x∞ kw(x ′ )pw(x ′ , t) Ω(x, x ′ )

∂ps(x, t) ) -ks(x) ps(x, t) + 2 ∂t

∫x

∞

dx’(3)

ks(x ′ ) ps(x ′ , t) Ω(x, x ′ ) dx’ (4)

kw(x) and ks(x) are the strong and weak-link rate coefficients, which are linearly dependent on the molecular weight.17 Hence, kw(x) ) kwx and ks(x) ) ksx. Ω(x,x′) is the stoichiometric kernel that defines the product size distribution when a polymer of molecular weight x′ breaks into two fragments of molecular weight x and (x′ - x). Because the mode of scission in the present case is random, Ω(x,x′) ) 1/x′.24 The two integro-differential eqs 3 and 4 are solved by applying moment operation to the above equations defined by:

p(j)(t) )

∫0∞ x(j) p(x, t) dx

(5)

The zeroth and first moment, which represent the total molar and mass concentrations, are obtained by setting j ) 0 and 1, respectively. The number (Mn) and weight average molecular weights (Mw) are defined as p(1)/p(0) and p(2)/p(1), respectively. The PBEs after applying moment operation get transformed to

dpw(j)(t) j-1 k p(j)(t) )dt j+1 w w

( ) dp (t) j-1 k p (t) ) -( dt j + 1) (j) s

s

(j) s

(6) (7)

The zeroth and first moment are given by

dpw(0)(t) ) kw pw(1)(t) dt dp(0) s (t) ) ks p(1) s (t) dt

(12)

Mn0 - 1 ) (kw′ + k′s)t Mn

(13)

The rate constants kw’ and ks’ can be found from the following equation by assuming that the weak links degrade after a finite time td; that is, no weak links are present after time td.

Mn0 - 1 ) kw′ td + k′st Mn

∂pw(x, t) ) -kw(x) pw(x, t) + ∂t 2

)

pw(1) p(1) s p(0) - 1 ) kw (0) + ks (0) t ) (kw′ + k′s)t (0) p0 p0 p0

and

dpw(1)(t) )0 dt

(8)

and

dp(1) s (t) )0 dt

(9)

The rates of change of first moment for the weak and strong links show that their mass concentrations are constant, and hence

(14)

Results and Discussion Figure 3 shows the evolution of Mn with time when the copolymers were degraded in the presence of CS TiO2. It is evident that almost 70% of the degradation occurs within a time period of 20 min. A comparison of the Mn profiles suggests that PAM and PAA exhibit the slowest and the fastest degradation, respectively. The degradation profiles of the copolymers lie in-between the homopolymers and exhibit higher degradation rates with increasing AA content. The initial fast degradation of the copolymers can be due to AA, which form weak links in the copolymer chain compared to AM. Hence, we propose that AA units in the chain form weak links that are highly unstable to UV radiation and degrade within the initial 20 min. After this, AM units degrade at a slower rate, owing to their strong links. This is also consistent with the observation of Smets and Hesbain5 for the acid catalyzed hydrolysis of p(AM-co-AA), where the hydrolysis proceeds through two steps, in which the first step was 50 times faster than the second one. The mechanistic description of the chain scission is provided in the following section. Mechanism of Photocatalytic Degradation. The mechanism of degradation of p(AM-co-AA) incorporates the faster depletion of AA units and the slower degradation of the AM units. UV irradiation of the copolymer solution without any catalyst results in the generation of polymeric R-radicals, which undergo degradation reactions, albeit with a slower rate. In the presence of the catalysts, CS TiO2 and DP 25, hydroxyl radicals (•OH) are generated by the mechanism depicted in Scheme 1.12 When TiO2 is irradiated with UV light of energy greater than or equal to its band gap, valence band holes and conduction band electrons are produced. The valence band holes react with water in the solution and hydroxyl anions and form hydroxyl radicals. The electron pathway proceeds with the formation of superoxide radicals, which react with protons to form hydrogen peroxide and hydroxyl radicals. These hydroxyl radicals can react with the copolymer by hydrogen abstraction at the R and β positions to the carboxyl or amide moieties to form radicals. Ulanski et al.25 have confirmed by EPR spectroscopy that β-to-R radical conversion

8932 J. Phys. Chem. B, Vol. 112, No. 30, 2008

Vinu and Madras

SCHEME 2: Mechanism of Scission of AA Units in the Copolymer

SCHEME 3: Mechanism of Scission of AM Units in the Copolymer

is significantly faster for PAA radicals at low pH values, which correspond to the present experimental investigations. Furthermore, for the PAM radicals, Grollmann and Schnabel26 have reported that the reactivity of the methine (R-) carbon to form radicals is four times that of the methylene (β-) carbon and eight times that of amide nitrogen. Hence, it is reasonable to assume that R radicals [P•] are the precursors of chain breakage. Once these primary radicals are formed, the next step is the formation of polymer peroxy radicals [POO•] by the reaction of [P•] with oxygen. This reaction is very fast and diffusion-controlled.27 These peroxy radicals can combine bimolecularly with another radical to form [POO-OOP]. This results in the formation of

polymer oxy radical [PO•] with the exclusion of oxygen. The final step in photo-oxidative degradation is the formation of scission products from the oxy radical. The scission produces a fragment radical and a nonradical fragment. In the case of PAA degradation, Kaczmarek et al.21 have confirmed the carboxyl radical abstraction in the initial step by UV-vis absorption studies. This results in the formation of carbonyl group by the β-scission reaction. This was also confirmed in our experiments by the presence of a shoulder in the 250-300 nm range in the UV-vis absorption spectra of the C20.4% and C29.3% copolymer solutions during degradation (Figure 4). This corresponds to the forbidden transition of the

Photocatalytic Degradation of p(AM-co-AA)

J. Phys. Chem. B, Vol. 112, No. 30, 2008 8933

Figure 6. Variation of weak-link rate coefficient, kw with percentage AA content.

Figure 7. Variation of strong-link rate coefficient, ks with percentage AM content.

Figure 5. Variation of [(Mn0/Mn) - 1] for the homopolymers and copolymers (a) without any catalyst, (b) with CS TiO2, and (c) with DP-25.

π-π* orbital and is characteristic of the formation of aldehyde and ketonic moiety, which is consistent with the mechanism. Scheme 2a,b depicts the mechanism of photodegradation of AA units in the initial 20 min time period. In the scission of the polymer oxy radical [PO•], two situations can arise. When there is an AA unit next to the primary radical, the fragment radical can either be an R-radical of the subsequent AA or AM unit. This is based on the sequence length distribution of the dyads consisting of AA and AM units. From statistical calculations using the reactivity ratios and feed compositions,28 it is evident that the probability of the formation of AA-AM or AM-AA dyad is greater than that of AA-AA dyad. For the C47.8% copolymer, P(AM-AM) is 30%, P(AA-AA) is 20%, and P(AA-AM) is 50%. It is also found that P(AA-AM) increases with and increase in AA content in the copolymer. Hence, it is more probable to find an AM unit next to AA, and therefore, the formation of an AM R-radical is more possible. The degradation mechanism after the initial period is given by Scheme 3. The polymer oxy radical of PAM degrades by two processes. The first one is the scission fragment radical

8934 J. Phys. Chem. B, Vol. 112, No. 30, 2008

Figure 8. Variation of thermal activation energy with percentage of AA content.

Figure 9. Effect of solution pH on the rate constants for the degradation of PAM, PAA, and copolymer C47.8%.

produced by the abstraction of a proton from the adjacent AM unit. Because the probability of the formation of AM-AM dyad is high (67% for C18.7%), proton abstraction occurs from the AM unit only. The second one is the hydrolysis of the amide functional to form carboxylic acid with the abstraction of NH3. Effect of Catalysts. Figure 5a-c shows the variation of [(Mn0/Mn) - 1] with the time of irradiation for the homo- and copolymers without the catalyst, in the presence of CS TiO2 and DP 25, respectively. The strong and weak-link rate coefficients calculated from eq 14 suggest that CS TiO2 is 40 and 60% more efficient compared to the uncatalyzed and catalyzed DP 25 systems, respectively. It is found that for all the copolymers, the rate of photodegradation with DP 25 is slightly less than that of the uncatalyzed system. This may be due to the fact that DP 25, with its 37 nm anatase and 90 nm rutile grains,29 may block the available photoenergy incident on the polymer species because of its high dispersion characteristics. Hence, because of the reduced radiation flux density in presence of DP 25, lower photoactivity is observed. The high photoactivity of CS TiO2 compared to the uncatalyzed and DP 25 systems is due to the high surface hydroxyl content and reduced band gap of CS TiO2,13 compared to that of DP 25. In addition to the above observations, the specific surface area of CS TiO2 is five times that of DP 25, and hence,

Vinu and Madras CS TiO2 shows a marked increase in photoactivity compared to DP 25 for the photodegradation of PAM, p(AM-co-AA), and PAA. Furthermore, in a previous work,14 it was proved that the higher photocatalytic efficiency of CS TiO2 is unaltered by the heat treatment of the catalyst to 400 °C that reduces the surface area of the catalyst to 63 m2/g and increases the particle size to 16 nm. Effect of Copolymer Composition. Figures 6 and 7 show the linear variation of the weak-link and strong-link rate coefficients with the percentage AA and AM content in the copolymer, respectively. It is clear that the weak link coefficient increases with an increase in AA content in the copolymer, which is due to the high photoinstability of AA units. However, the strong-link rate coefficient reduces with increase in the AM content. It was earlier suggested that after 20 min, the degradation is mostly due to the scission of the AM units, which are more stable to photooxidation compared to AA units. The observed reduction in rate coefficient can be attributed to the lower probability of finding the AM-AA dyad with an increase in AM content in the copolymer. Hence, for low AM content, it is more probable to find a subsequent AA unit, which can degrade at a faster rate because of its inherent photoinstability. This is supported by the TG weight loss profiles of the polymers. All the polymers exhibit three distinct stages of weight loss corresponding to dehydration, decarboxylation, and/or inter- and intramolecular imidization and main chain scission.19,30 It is evident from Figure SI 2 in the Supporting Information that the inclusion of AA units in the copolymer results in faster degradation with AA homopolymer exhibiting maximum weight loss. More interestingly, the activation energy of thermal decomposition for the polymers calculated for the final stage (420-600 °C) corresponding to main chain scission decreases linearly with an increase in AA content in the copolymer (Figure 8). This suggests that the thermal energy barrier for degradation decreases with an increase in AA content and hence faster degradation. Figures SI 3 and SI 4 in the Supporting Information show the Friedmann and Chang plot for the pyrolytic degradation of the polymers at a heating rate of 5 °C min-1. Although the activation energy calculated by the above single heating rate methods are close to each other, activation energy by the Kissinger method is nearly twice as large for all the polymers, although the linear variation with the percentage composition of AA unit is the same in all the methods. The above observations are further confirmed by the glass transition temperature (Tg) of the copolymer, reported by Klein and Heitzmann,2 where the Tg of the copolymers decreases with an increase in AA content. This means that the mobility (due to bond rotation) and hence the flexibility of the chain increase with an increase in AA content in the copolymer. Thus, oxygen diffusion occurs more rapidly, resulting in the higher degradation rates observed for the copolymers with higher AA content. Effect of Solution pH. In order to ascertain whether pH has any role in the rate of degradation of the homopolymers and copolymers evaluated thus far, the rate coefficients were evaluated at neutral and basic pH regimes. Figure 9 shows the variation of the weak- and strong-link rate coefficients for PAM, PAA, and C47.8% copolymer degraded without any catalyst. At neutral pH, the rate coefficient is slightly higher for PAM and the copolymer compared to that of the natural (acidic) pH condition. The rate of degradation of PAA is doubled at neutral pH, and at basic pH regime, the rate is the lowest for all the polymers. This is consistent with the observation of Gurkaynak et al.20 for the high-temperature degradation of PAA in solution,

Photocatalytic Degradation of p(AM-co-AA) where high rates observed at low pH were attributed to the polarization induced in the carbon-carbon bond by the uncharged hydroxyl substituents. Hence, our investigation of the rate coefficients reflect the highest possible photocatalytic degradation of the copolymers. Conclusions P(AM-co-AA) has been synthesized by free radical copolymerization with high reactivity of AM units. To our knowledge, this is the first work that reports the photolytic and photocatalytic degradation of p(AM-co-AA) with titania catalysts. The degradation mechanism based on the hydroxyl radical attack, hydrogen abstraction from the R-carbon and β-scission, has been proposed. The copolymer exhibits a peculiar mode of degradation, wherein AA and AM units form weak and strong links that photodegrade at a faster and slower rate, respectively. The degradation proceeds at a faster rate with the inclusion of more AA units in the chain. The strong- and weak-link rate coefficients are found to be linear functions of the composition of comonomers, which is analogous to the linear reduction of the thermal activation energy with increasing AA content in the copolymer. CS TiO2 exhibits a higher rate of degradation for all the copolymers compared to the systems without any catalyst and with DP 25. This observed high photoactivity of CS TiO2 can be attributed to its high specific surface area and high surface hydroxyl content. Acknowledgment. The authors thank the NMR Research Center, Indian Institute of Science, for the analysis of the copolymer samples. R.V. thanks the General Electric Foundation for a scholarship. G.M. thanks the department of science and technology for financial support and Swarnajayanthi fellowship. Supporting Information Available: SI 1, UV-vis absorption spectra of CS TiO2 and Degussa P 25 TiO2. (inset: TG profiles of the catalysts); SI 2, normalized TG traces for the copolymers and homopolymers at a heating rate of 5 °C min-1 in N2 atmosphere at a flow rate of 150 mL min-1; SI 3, Friedmann plot for the copolymers and homopolymers at a heating rate of 5 °C min-1 in N2 atmosphere at a flow rate of 150 mL min-1; SI 4, Chang plot with a unit order (n ) 1) of decomposition for the copolymers and homopolymers at a heating rate of 5 °C min-1 in N2 atmosphere at a flow rate of 150 mL min-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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