Photochemical Stability and Photostabilizing Efficiency of Poly(methyl methacrylate) Based on 2-(6-Methoxynaphthalen-2-yl)propanoate Metal Ion Complexes

1. Introduction

2. Experimental Section

2.1. Materials

Several metal ions of 2-(6-methoxynaphthalen-2-yl)propanoate complexes were prepared from the reaction of potassium salt of 2-(6-methoxynaphthalen-2-yl)propanoic acid (2 mole equivalents) and metal acetate or chloride (1 mole equivalent) in ethanol under reflux conditions for 30 min [21]. The solid obtained upon cooling was collected by filtration, washed with ethanol and recrystallized from ethanol to give the colored complexes that have been dried under vacuum at 50 °C. The metal contents were determined by the use of flame atomic absorption techniques and were in agreement with the calculated ones. The FT infrared spectra for the complexes (Table 1 and Figure 1) show three characteristics bands that have appeared within the regions of 432–480, 1545−1606 and 1392−1460 cm⁻¹, which have been attributed to ν for M–O, COO⁻ asymmetrical and COO⁻ symmetrical bonds, respectively. The magnetic moment was zero for ZnL₂ (L, ligand), CdL₂ and SnL₂ and 2.91 and 1.53 Bohr Magnetons (B.M.), B.M. = 9.27 × 10⁻²⁴ Amperes/Joules (J.T⁻¹), for NiL₂ and CuL₂, respectively. The conductivity of complexes was measured, at room temperature in dimethylformamide, to be in the range of 10–15 ohm⁻¹ cm² mol⁻¹, which is an indication that all complexes are non-electrolytes [21].

Table 1. Structure of metal ligand complexes (ML₂) used along with a ligand (L).

Table 1. Structure of metal ligand complexes (ML₂) used along with a ligand (L).

Symbol	Name
L	Potassium 2-(6-methoxynaphthalen-2-yl)propanoate
NiL2	bis[2-(6-Methoxynaphthalen-2-yl)propanoate] nickel(II)
CuL2	bis[2-(6-Methoxynaphthalen-2-yl)propanoate] copper(II)
ZnL2	bis[2-(6-Methoxynaphthalen-2-yl)propanoate] zinc(II)
SnL2	bis[2-(6-Methoxynaphthalen-2-yl)propanoate] tin(II)

Figure 1. Structure of ML₂ used.

3. Results and Discussion

3.1. Photodegradation Mechanism for PMMA

The PMMA photochemical degradation steps are summarized as shown below (Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8):

(1) The formation of free radicals, A₁ and A₂, via homolytic cleavage of the C–CH₃ and C–COOCH₃ bonds, respectively (Figure 2).

Figure 2. Formation of radicals A₁ and A₂.

(2) The formation of peroxy radicals B₁ and B₂ via reaction of A₁ and A₂ free radicals with oxygen, respectively (Figure 3).

Figure 3. Formation of radicals B₁ and B₂.

(3) The abstraction of the hydrogen atom from (P−H) polymer chain by the peroxy radicals B₁ and B₂ afforded the hydroperoxide polymers C₁ and C₂, respectively (Figure 4)).

Figure 4. Formation of hydroperoxide polymers C₁ and C₂.

(4) The homolytic cleavage of the O−OH bonds of the hydroperoxy polymers C₁ and C₂ to produce the polymer oxy radicals D₁ and D₂, respectively (Figure 5).

Figure 5. Formation of polymer oxy radicals D₁ and D₂.

(5) The alkoxy radicals D₁ and D₂ further react via abstraction of the hydrogen atom to produce polymeric alcohols E₁ and E₂, respectively (Figure 6), or the β-scission reaction to produce ketonic polymers F₁ and F₂, respectively (Figure 7).

Figure 6. Formation of polymeric alcohols E₁ and E₂.

Figure 7. Formation of ketonic polymers F₁ and F₂.

The carbonyl groups in the polymeric chain play an essential role in the formation of acyl radicals (Figure 8) via oxidative degradation of the main polymeric chain.

Figure 8. Formation of acyl radicals via degradation of the main polymeric chain.

Photostabilization of poly(vinyl chloride) (PVC) films having different concentrations (0.1%–0.5% by weight) of various additive metal complexes has shown that a complex concentration of 0.5% by weight gave the best results [19]. Such a concentration has been applied successfully for photostabilization of PVC and polystyrene films with other additive metal complexes [27,28]. Therefore, additive metal complexes have been used in a concentration of 0.5% by weight.

2-(6-Methoxynaphthalen-2-yl)propanoate complexes were used as additives for the PMMA films’ photostabilization. The hydroxyl group index was monitored with irradiation time using IR spectrophotometry to allow the photochemical activity of these additives for the photostabilization of PMMA films to be studied. The PMMA films was irritated with a UV light (λ = 313 nm) and produced a clear change in the FTIR spectrum. The absorption bands appearing at 3430 cm⁻¹ was attributed to the formation of the OH groups [29]. Such an absorption band was used to follow the progress of the degradation of the polymer with irradiation and has been calculated as the hydroxyl index. It is fair to link the polymer degradation degree with the growth of the hydroxyl index. However, in Figure 9, the I_OH of SnL₂, ZnL₂, CuL₂ and NiL₂ indicated a lower OH growth rate with irradiation time for the PMMA control film in the absence of any additives. Since the growth of the hydroxyl index with irradiation time was lower than the PMMA blank (Figure 9), it was conclude that such additives could be considered as photostabilizers for the PMMA polymer. In principle, a longer induction period is a sign of an efficient photostabilizer. Therefore, the NiL₂ is considered to be the most efficient photostabilizer, followed by CuL₂, ZnL₂ and SnL₂.

Figure 9. The relationship between the hydroxyl index and irradiation time for PMMA films (40-μm thickness) with different additives (0.5% by weight).

3.2. Variation of PMMA Molecular Weight during Photolysis in the Presence of bis[2-(6-Methoxynaphthalen-2-yl)propanoate] Complexes

A versatile test for random chain scission was provided from the analysis of the relative changes in viscosity average molecular weight $\left({\overline{M}}_{\text{V}}\right)$. Figure 10 shows the changes in $\left({\overline{M}}_{\text{V}}\right)$ versus irradiation time for PMMA film (0.5% by weight) in the absence of selected additives at a light absorption intensity of 1.121 × 10⁻⁸ ein dm⁻³ s⁻¹. Equation 3 was used to measure $\left({\overline{M}}_{\text{V}}\right)$ in benzene at room temperature. It is worth mentioning that insoluble film traces with additives indicate that some degree of PMMA branching or cross-linking took place during the photolysis process [30]. To prove that, Equation (7) was used to calculate the number of average chain scissions (average number of cuts per single chain; S) [31].

$$\text{S}=\frac{\overline{Mv},o}{\overline{Mv},t}$$

(7)

where $\overline{Mv},o$ and $\overline{Mv}, t$ = the viscosity average molecular weight at initial (zero) and t irradiation time, respectively. Figure 11 shows the plot of S versus time and indicates an increase in the branching degree that might arise from cross-linking. The insoluble residue formed during the irradiation process provides additional evidence for the cross-linking occurrence. Randomly-distributed weak bonds [32] break rapidly in the initial stages of photodegradation. Equation 8 was used to calculate the degree of deterioration (α).

$$\alpha =\frac{m.s}{\overline{Mv}}$$

(8)

where m = the initial molecular weight. Figure 12 shows the relationship between α with irradiation time. It was found that the α values for the irradiated samples are higher when no additive was used and lower in the presence of additives. The value of α increases rapidly with time in the initial stages of photodegradation of PMMA, which is an indication of random polymeric chain bond breaking. Other evidence for the degradation reaction could be obtained from the quantum yield of the chain scission (Φ_cs) measurements [19]. Equation (6) was used to calculate the quantum yield for chain scission for PMMA films in the absence or presence of additive (0.5% by weight). The low values of Φ_cs can be explained by the fact that in the PMMA macromolecule, the energy was absorbed at one site, and then, the electronic excitation was distributed over various bonds, so that the probability of a single bond breaking was low, or the absorbed energy was dissipated by non-reactive processes. The Φ_cs values for complexes are given in Table 2.

Table 2. Quantum yield (Φcs) for the chain scission for PMMA films (40-μm thickness) with and without additive (0.5% by weight) after irradiation (250 h).

Table 2. Quantum yield (Φcs) for the chain scission for PMMA films (40-μm thickness) with and without additive (0.5% by weight) after irradiation (250 h).

Additive (0.5 wt%)	Quantum Yield of Main Chain Scission (Φcs)
PMMA +NiL2	5.96 × 10−5
PMMA + CuL2	5.35 × 10−5
PMMA + ZnL2	7.11 × 10−5
PMMA + SnL2	7.21 × 10−5
PMMA (blank)	4.55 × 10−6

The Φ_cs values for PMMA films with additives are less than those for the corresponding ones in the absence of additives (blank) and increase in the order: NiL₂ > CuL₂ > ZnL₂ > SnL₂.

Figure 10. Changes in the viscosity average molecular weight $\left(\overline{M_{\text{V}}}\right)$during irradiation of PMMA films (40 μm; control) and additives (0.5% by weight).

Figure 11. Changes in the main chain scission (S) during irradiation of PMMA films (40 μm; control) and additives (0.5% by weight).

Figure 12. Changes in the degree of deterioration (α) during irradiation of PMMA films (40 μm; control) and additives (0.5% by weight).

3.3. Suggested Mechanisms of Photostabilization of PMMA by the bis[2-(6-Methoxynaphthalen-2-yl)propanoate] Complexes

The results obtained showed that the efficiency of bis[2-(6-methoxynaphthalen-2-yl)propanoate] complexes as stabilizers for PMMA films can be ordered according to the change in the hydroxyl content as a reference for comparison (Figure 9), where NiL₂ > CuL₂ > ZnL₂ > SnL₂ > PMMA.

Each additive in the polymeric material plays a role in the photostability of such a material. Therefore, various mechanisms for the photostabilization could take place, which were mainly based on the additive chemical structure, the functional groups present, in particular the ones that resonate, such as aromatic and heteroaromatics moieties, and the compatibility of the additive with the polymer matrix. For example, the metal complexes might stabilize PMMA by dissipation of energy as heat via direct absorption of the UV radiation [33]. However, obviously, the nature of the metal will have a significant effect on the efficiency of the additive as a stabilizer. Clearly, both the metal and the ancillary ligands could play a role in absorbing the UV light [34,35,36].

In a similar way, the aromatic rings in complexes play an important role in the stabilization mechanism by acting as UV absorbers. The UV light absorption by such additives containing aromatic rings dissipates the UV energy to harmless heat energy (Scheme 1). Moreover, the aromatic rings play a role in the resonating structure conjugation of the radical in the peroxide decomposer, which supports this compound as being a photostabilizer [37].

Scheme 1. Possible photostabilization of aromatic moiety as a UV absorber.

Hydroperoxides play an important role in the degradation of polymeric materials by photooxidation. The use of typical UV stabilizers protecting polymer films, through peroxide decomposition, has been reported [38]. Metal chelate complexes are known as PMMA photostabilizers through both a peroxide decomposer and excited state quencher. Therefore, it was expected that such complexes could act as peroxide decomposers (Scheme 2) [19].

Scheme 2. Possible photostabilization of ML₂ complexes as peroxide decomposers.

These metal chelate complexes also act as radical scavengers through energy transfer, form un-reactive charge transfer complexes between the metal chelate and excited state of the chromophore (POO^●) and stabilize through resonating structures, as shown in Scheme 3 [26].

Scheme 3. Possible photostabilization of carboxylates complexes as radical scavengers through energy transfer.

4. Conclusions

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