The sample of poly (4-vinyl biphenyl), (PVBP) was obtained from Sigma Aldrich Chem. Co. The average molecular weight for the polymer is 115,000, and was used as received. Dichloroethane (DCM) solvent as a spectroscopic grade was obtained from Fluka GMBH, and showed no absorption or emission in the range of 293 - 550 nm. The used plasticizers, dimethyl terephthalate (DMT), diethyl terephthalate (DET), dioctyl terephthalate (DOT), dibutyl phthalate (DBP), and dioctyl phthalate (DOP), were obtained from Sigma Aldrich Co., and were used without further purifications. These were of high purity of about (99.8%), and were found to give no detectable absorption in the range (265 - 400 nm).

Poly (4-vinyl biphenyl) thin films with thickness of approximately 0.02 mm were prepared by solution casting of the polymer in spectrograde dichloroethane on a spectroscopic window (quartz plate of 1.0 mm × 20 mm diameter). Moreover, about 0.02 mm thick poly (4-vinyl biphenyl) - plasticizer films, containing different wt% plasticizers were prepared by solution casting [polymer + (5 - 15 wt%) added plasticizer], using spectrograde dichloroethane solvent. These films were used as to gain better insight into the possibility of photodegradation of the polymer during irradiation. The films were dried in a vacuum oven at 293 K for 6 hours, as to ensure the complete removal of solvent traces [41] .

Irradiation processes of solid films were exposed to different intervals of irradiation time in air, from (0 - 120 min), Using JASCO- Spectrofluorometer with a built in Hydrogen-Xenon lamp (6808-J007A-model number ESC-333) supported with monochromator of holographic grating with 1800 groves/mm. All solid films were exposed to the same rate of irradiation intensity for different intervals of irradiation time in air, from (0.0 - 120 min). The intensity of incident radiation was (4.9 mW/cm²), and the distance between light source and samples was (5 cm).

UV-Vis absorption spectra of non-irradiated PVBP and the used plasticizers were obtained using an Agilent 8453 UV-Vis spectrophotometer.

Fluorescence spectra of pure and blended polymer solid films were recorded on JASCO-FP 6500 Spectrofluorometer. The parameters were constant for all measurements, and the excitation wavelength was 288 nm. The emission wavelength range was 290 - 550 nm, and all fluorescence spectra for solid films were obtained using a thermostated solid sample holder at 298 K.

Infrared spectra were collected using a NICOLET-MAGNA-IR-560 spectrometer, (the manufacturer Thermo Nicolet Corporation, Madison, USA). The working wavenumber range of the spectrometer was from (4000 to 500 cm⁻¹) at (4 cm⁻¹) optical resolution and 3 scan repetitions per analysis. The FT-IR spectra were recorded for the irradiated and the non-irradiated polymer films, whereas the transmittance was plotted as function of the wavenumber.

The UV-absorption spectra of both, poly (4-vinyl biphenyl), the phthalate and terephthalate plasticizers in dichloroethane solutions at room temperature are shown in Figure 1(a) and Figure 1(b) respectively.

Poly (4-vinyl biphenyl) molecular structure.

As can be seen from Figure 1(a), the absorption spectrum of poly (4-vinyl biphenyl) was found to absorb radiation in the UV-region in the range of (270 - 330 nm) with a maxima at (288 nm), approximately similar to those of polystyrene and other substituted polystyrenes. The small changes in the position and the shape of absorption spectrum may be attributing to the chemical structure and the position of substitution in phenyl ring [6] . The excitation wavelength that has been used for PVBP films was 288 nm for fluorescence spectra and FT-IR spectra measurements. Both phthalate and terephthalate plasticizers absorb radiation in the range of (215 - 262 nm) and are far away from the absorption region of PVBP films, as shown in Figure 1(b).

A number of studies on the photodegradation of polystyrene and para-substituted polystyrene have received a considerable attention, mainly due to their industrial importance [18] [20] . However, the photodegradation of some substituted polystyrene has been studied extensively but no works have been reported on the photodegradation of puce and plasticized PVBP by fluorescence and FT-IR spectroscopic techniques.

The fluorescence spectrum of PVBP film composes from a minor band as monomer fluorescence at (λ_max = 342 nm) and a major band at longer wavelength as excimer fluorescence at ((λ_max = 372 nm). The fluorescence spectrum of pure PVBP films irradiated at (λ_ext = 288 nm) is shown in Figure 2.

Blending of PVBP films with phthalates (DBP and DOP), and terephthalates (DMT, DET and DOT) plasticizers at (5.0%, 10% and 15.0% wt.), showed a gradual decrease in the intensity of polymer fluorescence with increasing of amount of added plasticizer and decreases with increasing of irradiation times. The fluorescence spectra of blended PVBP films with (5% wt, 10% wt and 15% wt) DOT is shown in Figure 3.

The fluorescence spectra of blended PVBP films with, (5% wt, 10% wt and 15% wt) DOP are shown in Figure 4.

As can be seen from Figure 3 and Figure 4, the fluorescence spectra for blended PVBP films, photo-irradiated for (0.0 - 120 min) was obtained at room temperature and excitation wavelength of 288 nm. Irradiation of the PVBP film at different exposure time was accompanied by the gradual quenching in the intensity of the excimer band, as well as a small red shift in the (λ_max) of the excimer fluorescence band of (372 - 380 nm), as shown in Table 1.

It is more likely that this red shift occurs as result of photodegradation of the polymeric chains, and can be attributed to the formation of conjugated double bonds along the polymer chain during the photodegradation by hydrogen abstraction process [36] [42] . The shift in the excimer fluorescence band is in good agreement with that reported with irradiated blended polystyrene [8] , poly (4-methoxystyrene) [30] and poly (4-methylstyrene) [32] . Tovborg and Kops [36] pointed out that quenching of polymeric emission might be attributed to the energy transportation of the oxides formed by the photo-oxidation of polymeric chromophores, or to the quenching effect of the peroxide formed during UV-irradiation.

Blending of PVBP solid films with different percentages of DMT, DET, DOT, DBP and DOP plasticizers, showed only decrease in the intensity of the fluorescence bands without formation of exciplex fluorescence at longer wavelength. Most likely, the energy transfer complex formed between the polymer chromophore and the plasticizer molecule is not stable enough to give exciplex fluorescence emission [41] . The decrease in excimer fluorescence may be attributed to; first, quenching of excited polymer chromophores by plasticizer molecules [41] ; second, to the distraction of polymeric chains by photochemical reaction through formation of free radical reactions [5] [7] .

From the quenching process for the intensity of excimer fluorescence of irradiated PVBP films for different intervals of time and by blending with phthalate and terephthalate plasticizers, some kinetic parameters can be collected by applying Al Ani and Hawi relationship [30] . The intensity of excimer fluorescence of non irradiated polymer film is denoted by ( I E X o ), whereas ( I E X ) represents the fluorescence intensity of irradiated pure and blended PVBP films at different intervals of time. The quenching ratio is denoted by [ ( I E X o ) / ( I E X ) ] , and can be represented by the following relationship [30] :

( I E X o I E X ) * = 1 + [ { ( k ′ P Q ) ( A ) } ( t ) ] (1)

where [A] = number of (photons/s) absorbed by polymeric chromophores.

t = time of irradiation in s.

k ′ P Q = rate constant.

In the photo-quenching processes, if we assume that the number of photons released from the light source and absorbed by polymer chromophores is constant (A), Then according to (Equation (1)), we can write:

k P Q = { ( k ′ P Q ) ( A ) }

where ( k P Q ) is the photo-quenching rate constant, and the efficiency of quenching ratio [ I [ E X ] ∗ o / I [ E X ] ∗ ] is given by the following equation:

( I E X o I E X ) * = 1 + [ ( k P Q ) ( t ) ] (2)

According to Equation (2), [ I [ E X ] ∗ o / I [ E X ] ∗ ] -values were plotted against time of irradiations (t). From the plots, the photo-quenching rate constant (k_PQ) values and quenching efficiency ratio [ I [ E X ] ∗ o / I [ E X ] ∗ ] were calculated and used to estimate the efficiency of photodegradation processes for pure and blended irradiated polymer. The plots of the ratio of excimer fluorescence [ I [ E X ] ∗ o / I [ E X ] ∗ ] against the different irradiation times of pure and blended PS films with 0.0% wt, 5.0% wt, 10.0% wt and 15.0% wt of DMT, DET and DOT plasticizers are presented in Figure 5.

As can be noticed from Figure 5, an increase in the amount of added terephthalate plasticizers resulted in the decrease in the degradation ratio of [ I [ E X ] ∗ o / I [ E X ] ∗ ] with the increase of exposure time, and decrease in the photo-

quenching rate constant. On the other hand, the increase in the amount of added phthalate plasticizers resulted in a higher decease in the photo-quenching rate constant (k_PQ) as shown in Figure 6. The same results were obtained in the photodegradation of blended Poly (4-tert-butylstyrene) and Poly (4-methylstyrene) films [10] [32] . Blending with DBP showed a higher photo quenching rate constant than in blending with other phthalate or terephthalate plasticizers.

As can be seen from Figure 4 and Figure 5, the photo-quenching efficiencies of excimer fluorescence ( I [ E X ] ∗ o / I [ E X ] ∗ ) of pure and blended P4VBP were plotted at different intervals of exposure time and at different amount of added plasticizers and found to increase by the increase in exposure times, and to decrease by the increase in the amount of blended plasticizers.

From Figure 5 and Figure 6, the photo quenching rate constants in blending with phthalate plasticizers are higher than that obtained in blending with terephthalates plasticizers. The order of increase in photo quenching by blending with terephthalates plasticizers is DOT, DET and DMT, whereas in blending with phthalate plasticizers, DOP showed a higher efficiency of photo-quenching

of polymeric fluorescence.

The increase in irradiation times to PVBP films caused an increase to both the excimer fluorescence ratio and to the photo-quenching rate constant (kPQ), this indicates that fluorescence quenching by added plasticizers may be accompanied by photochemical reaction in the polymer backbone. Also, there is a possible reaction, between the free radicals formed by the chain scission of polymeric chains and the added plasticizer molecules, which increase the efficiency of the quenching process. It was reported that chains scissions and changes in the molecular weight distribution, as well as, photo-oxidation process that was interpreted as a reaction involving benzene ring-opening photo-oxidation was also the process that resulted from irradiation of polystyrene and substituted polystyrene in the presence of polymeric additives [35] [37] . It has been reported that the photo stability of PS was reduced by the addition of polymeric additives, which depends on the chemical structure of the additive. Chain scissions, and changes in the molecular weight distribution, were the main reactions resulted from the photo-oxidation of irradiated plasticized polystyrene films [3] .

From the data in Figure 5 and Figure 6, and by applying the Al Ani-Hawi relationship [30] , where the ratio ( I [ E X ] ∗ o / I [ E X ] ∗ ) was plotted against different intervals of the exposure time, from the slope of the obtained lines, (kPQ) values were calculated for pure and blended PVBP films with different percentages of DMT, DET, DOT, DBP and DOP, and are presented in Table 2.

As can be seen from Table 2, blending PVBP films with phthalate and terephthalate plasticizers, gave a lower values for the photo-quenching rate constant in comparison with that for pure polymer. It was found that the para- substituted group in the phenyl ring of the monomer and the structure and bulkiness of the used plasticizer molecule play a big role in the photo-quenching and photo-stability of plasticized polymers [9] [10] . Furthermore, the photo- stability of polymer chains was found to depend on the above mentioned factors as well as the UV-exposure time. It has been reported previously that the presence of plasticizer molecules in polymer matrix caused lowering in the activation energy during the photodegradation of poly (fluorostyrene) isomers films [23] [24] , and was attributed to the lowering in the charge transfer character of the excimer conformation. It was also found that the main process resulted from irradiation of blended polymer film in presence of air, were photo-oxidation of polymeric chromophores as well as chain scission and crosslinking processes [33] [34] .

The photo-quenching rate constant (kPQ) of excimer fluorescence increases with the increase in molar mass of the used phthalate and terephthalate plasticizers; a fact that correlates well with that obtained for the thermal degradation of substituted polystyrenes [1] [4] . The plot of the increase in the change of efficiency of excimer fluorescence quenching (Δk_PQ) with the increase in molar mass is shown in Figure 7.

As can be observed from Figure 7, phthalate plasticizers showed a higher efficiency of photodegradation in comparison with that obtained for the photodegradation in blending the polymer with terephthalate plasticizers. The photodegradation process was found to increase by the presence of air during polymer irradiation process. It depends upon the rate of oxygen diffusion into the polymer matrix, and to the stability of the formed energy transfer complex between oxygen molecules and polymer structures [42] [43] [44] [45] .

Furthermore, it is more likely that the increase in the bulkiness of plasticizer molecule caused enhancement to the degradation process through the decrease

in the stability of the energy transfer complex between excited polymer chromophores and the ground state plasticizer molecules [46] . It was reported that the photodegradation process is dependent upon the chemical structure of the substituent and also on the position of substitution in phenyl ring of the styrene chromophores [42] [43] .

The effect of the increase of percentage of added plasticizers to the photodegradation process and to the photo-quenching rate constant is shown in Figure 8.

As can be seen from Figure 8, the increase in amount of added plasticizer caused a decrease the photo-quenching rate constant of irradiated polymer chains and found to depend upon the structure of the used plasticizers, and also on the amount of added plasticizers.

From the data obtained for irradiated pure and blended PVBP solid films, we can notice that the photostability of PVBP is similar to that of polystyrene. This fact can be explained as phenyl ring substitution (as an inert group) in the para position of the styrene monomer increases the photostability of PVBP polymer towards irradiation. On the other hand, the phenyl substitutions make the hydrogen abstracting process during the irradiation process as a difficult process.

In order to confirm the presence of photo degradation in the irradiated pure and blended PVBP films with different amount of doped phthalate and terephthalate plasticizers, FT-IR spectra were recorded of non-irradiated and irradiated films under atmospheric oxygen, and at 298 K.

Figure 9 shows the FT-IR spectra of non-irradiated and irradiated for (0.0 and 120 min) of pure PVBP solid film, and were recorded immediately before and after irradiation.

As can be seen from Figure 9, chemical changes were assessed by FT-IR spectra for photo-irradiated pure solid film. These changes of decrease in absorbance for some bands, increase in other bands and red shift in the absorption of

many bands, are corresponding to the photo-oxidation and photodegradation of PVBP polymeric chains. It was noted that the most demonstrated trend is the increase of the absorbance bands that were resulting from polymer chains photodegradation, are shown in Table 3.

As can be seen from Figure 9 and Table 3, modifications occur in the intensity of different absorption bands in the FT -IR spectra of the irradiated pure and blended PVBP films. These changes correspond to photo-oxidation and photodegradation of polymeric chains [47] [48] . Transformation is clearly demonstrated by the decrease in absorbance of band at (3800 - 3400 cm⁻¹), and by the increase in the absorption band at 3027 cm⁻¹, 2919 cm⁻¹, 2842 cm⁻¹, 1644 cm⁻¹, 1480 cm⁻¹, 1111 cm⁻¹, 825 cm⁻¹ and 753 cm⁻¹, resulting from the polymer chains photodegradation. Rabek and Ranby have assigned the absorption band at 3440 cm⁻¹ to the increase in the photo-oxidation of benzene rings and to the decrease in aromacity in the polymer sample [3] . They have reported during irradiation of polystyrene films, three cases of oxidation of the benzene ring being observed. The formation of benzyl acetophenone chromophores; acetophenone structures; and conjugated double bonds, were the main products resulted from the photo- oxidation of polystyrene films. The same observations were observed in the UV-irradiation of Substituted polystyrene in solid films [19] [44] .

Figure 9 shows the FT-IR spectra of non-irradiated and irradiated for (0.0 and 120 min) of blended PVBP with (10% DOT), (10% DET) and (10% DMT) in solid films at irradiation wavelength of 288 nm.

As can be seen from Figure 10 and Figure 11, irradiation of PVBP blended films in the presence of air with (10% added plasticizers), caused photodegradation

and photo-oxidation of polymeric chains leading to the increase the absorption intensity in many regions in the spectrum. Chemical changes were assessed by FT-IR spectra for irradiated blended PVBP solid films, and show typical functional group, in the hydroxyl (3800 - 3000 cm⁻¹); C-H stretching vibration in the aliphatic chain and in aromatic rings (2800 - 3100 cm⁻¹); carboxyl stretching vibration (1800 - 1600 cm⁻¹); and deformation vibration (1400 - 600 cm⁻¹) regions.

The increase in the intensity, decrease in the intensity and red shift in position of FT-IR absorption bands for pure and blended PVBP with (10.0% wt.) DOT, DET and DMT are summarized in Table 4 and Table 5.

As can be seen from Table 5, the increase in intensity of the absorption bands and the small red shift in these bands upon blending with these plasticizer can give good indication that there is a change in the photo-product resulted from irradiations.

As can be seen in Figure 10 and Figure 11, the photo-oxidation of polymeric chains of pure and blended PVBP films, leads to increase in the absorption intensity in the bands at 1716 cm⁻¹, 1649 cm⁻¹, 1480 cm⁻¹ and 1404 cm⁻¹. The bands found at 1649 cm⁻¹ and 1480 cm⁻¹, can be ascribed to stretching vibration of the substituted benzene rings; it means that the aromatic rings lose their symmetry through the photo-oxidation processes, and are almost in the same

position as observed in polystyrene spectrum [44] . The increase in absorption intensity for the bands 1412 cm⁻¹, 1090 cm⁻¹ and 1074 cm⁻¹ in blended films in comparison with that of pure PVBP films, indicates that there is an increase in the number of (C = C) that resulted from hydrogen abstraction during chains- scission process. The presence of the absorption band at 825 cm⁻¹, may suggest that the formation of conjugated double bond sequences in the main polymer chain.

As can be seen from Table 4 and Table 5, many FT-IR absorption bands increase their intensities, other decrease their intensities while some new bands appeared in irradiated blended PVBP films, and these changes increases with increase in amount of added plasticizers and with the increase in molar mass of the used plasticizers. These changes correlate well with obtained for thermal degradation of plasticized poly (para-substituted styrene) solid films [1] [4] . The trend of increase in the efficiency of photo-degradation was found, DOT > DET > DMT. Phthalate plasticizers, DOP and DBP showed higher efficiency of photo quenching than what was obtained by terephthalate plasticizers. This behavior may indicate that there is higher stability for the [terephthalate-PVBP]* excited complex than that of [phthalate-PVBP]* excited complex.

It was reported by Szarka et al, that the thermo-oxidation of plasticization of poly (vinyl chloride) (PVC) in dioctyl phthalate (DOP), showed an increase in degradation process. Thermo-oxidation leads to PVCs with certain extent of internal plasticization by DOP chemically bound to the PVC chains and by the oxidized chain segments as well. The added plasticizer can also degrade during photo-oxidation and forming free radicals. The resulting radicals can combine with the polymeric radicals, and thus the plasticizers can be chemically bound to the polymer chain [49] [50] , and hence accelerate the photogegradation processes in blended polymeric matrix.