Milla Shah

Thermal and photooxidation of polypropylene influence of long-term ambient oxidation: spectroscopic, thermal and light scattering studies

Abstract The influence of long-term ambient storage conditions in the presence of air has been examined on the thermal and photooxidation of unstabilized polypropylene. Samples of polypropylene have been aged under ambient storage for 12 years together with a control sample kept under refrigeration at −20°C. The samples have been examined using second order-derivative absorption, fluorescence and phosphorescence spectroscopic techniques, thermal analysis (d.s.c.), light scattering and hydroperoxide analysis. All the polymer samples stored under ambient conditions exhibit significant increases in luminescent species and hydroperoxide concentrations as a consequence of slow oxidation processes. The d.s.c. and light scattering analysis show that these polymers exhibit an increase in crystallinity compared with that which has been stored by refrigeration. The data are related to the thermal and photooxidative behaviour of the polymers using Fourier transform infra-red spectroscopy (FTi.r.) and compared with that of a modern suspension grade polypropylene.

Photochemistry and photoinitiator properties of 2-substituted anthraquinones: 2. Photopolymerization and flash photolysis

Abstract The photoinduced polymerization activities of fifteen 2-substituted anthraquinones have been determined in different monomers and prepolymers using real-time Fourier-transform infra-red spectroscopy and photocalorimetry. The relative order in photoinitiator efficiency is found to be highly dependent upon the method used, the nature of the light source, amine co-synergist and monomer being crucial factors. Oxygen quenching in all cases indicates that the triplet state is the active precursor. Without an amine co-synergist, absolute quantum-yield measurements show that anthraquinones with electron-withdrawing groups are more active than those which are electron-donating. However, in the presence of a tertiary amine and under polychromatic illumination, the effects are different. In the latter case amido derivatives with long-wavelength charge-transfer bands and mixed n π ∗ ππ ∗ triplet states tend to be more reactive. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state. The relative positions of the lowest excited singlet ππ ∗ and second excited n π ∗ triplet states play an important role in determining their photoactivities, as shown in paper 1. Semiquinone radical and radical-anion intermediates are observed using microsecond flash photolysis and the data interrelated to their spectroscopic and photopolymerization activities. The haloanthraquinones are shown to undergo an additional mechanism involving dehalogenation.

Photochemistry and photoinitiator properties of 2-substituted anthraquinones 1. Absorption and luminescence characteristics

The photophysical and photochemical properties of 15 2-substituted anthraquinones were determined in various solvents. The absorption spectra revealed the presence of two types of structure: (1) a structure with a long-wavelength band around 360 nm associated with the electron-donating effects of the substituent; (2) a structure with a long- wavelength band around 320 nm associated with the electron-withdrawing effects of the substituent. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state, with the relative positions of the lowest excited singlet ππ∗ and second excited triplet nπ∗ states playing an important role in determining the luminescence behaviour and subsequent photoactivity. The electron-withdrawing and electron-donating abilities of the 2-substituent are important in this regard. Two of the anthraquinones, 2-(4-phenyl)thiazoleanthraquinone and 2-chloroamidoanthraquinone, give anomalous dual phosphorescence emissions from both the triplet ππ∗ and nπ∗ states depending on the wavelength of excitation. The data are discussed in relation to the expected photoinitiation activities of the compounds.

The mode of action of metal stearate stabilisers in poly (vinyl chloride). III. Influence of pre-heating on polyene formation and secondary reactions

Abstract Spectroscopic evaluations of thermally degraded samples of PVC stabilised with CaSt 2 and ZnSt 2 were carried out in order to explain the anomalous behaviour of the metal stearates when pre-heated. At pre-heating temperatures of 100, 130, 150 and 180 °C, samples at 5, 10, 15 and 20 min of degradation were analysed by visible reflectance spectroscopy. Samples preheated at 180 °C were used to determine polyene formation by UV spectroscopy. Visible reflectance traces showed the development of the long conjugated double bond sequences, the appearance of pale coloured compounds and dark colouration of the polymer throughout the degradation time. UV absorptions were used to analyse the formation of short, long and total polyenes, as well as the ratio of the short/long polyenes. The trends observed here consolidate our previous findings and suggest that the preheated metal soaps direct the stabilisation mechanism through different routes.

The mode of action of metal stearate stabilisers in poly(vinyl chloride)-IV. The application of fluorescence spectroscopy to characterise chromophoric species

Fluorescence emission spectroscopy has been used to monitor the development of polyenes in poly(vinyl chloride) [PVC] which has been stabilised with mixtures of pre-heated (180 °C) calcium stearate (CaSt2) and zinc stearate (ZnSt2). Treatment with NaBH4 has been used to distinguish polyenes attached to carbonylic groups. Emission from excitations at 245 and 360 nm enabled the development of short polyene sequences and carbonylic polyenes to be followed. CaSt2 when pre-heated and added to ZnSt2 and PVC produced high amounts of carbonylic polyenes and lead to secondary reactions yielding large amounts of black material attributed to the action of the prodegradant ZnCl2 formed. Pre-heated ZnSt2 produced small quantities of longer polyene sequences as shown by a slight yellowing of the PVC, and a pre-heated mixture of the soaps led to small quantities of carbonylic groups and an initial yellow discoloration. For the pre-heated mixture of soaps and pre-heated ZnSt2 the formation of a complex is suggested which interacts with the PVC chain from the beginning of degradation; the nature of the interaction being ionic and determining the specific mechanism of the stabilisation.

Photochemistry and photoinitiator properties of novel 1-chloro-substituted thioxanthones. III: Preliminary study of the photoacid generation

The dehalogenation of a selected number of 1-chloro-4-acyloxy/oxy derivatives of thioxanthone have been studied by a spectrophotometric method, developed using the sodium salt of tetrabromophenol blue as the acid probe. The method has allowed the in situ monitoring of the photoacid generation by absorption spectroscopy. This has consequently lead to the evaluation of absolute quantum yields of acid generation through steady state photolysis, and the investigation of the derivatives as potential photoacid generators, able to initiate cationic polymerisation of cyclohexene oxide.

Photophysical properties and photoinduced polymerisation activity of novel 1-chloro-4-oxy/acyloxythioxanthone initiators

The excited state characteristics of 13 novel 1-chloro-4-oxy/acylox derivatives ofthioxanthone were determined using micro-, nano- and picosecond flash photolysis techniques. Triplet energy levels have also been determined using phosphorescence analysis while photoinitiated polymerisation activities were measured using photocalorimetry. All the initiators exhibit high photopolymerisation activity except the 4-hydroxy model and 2-methyl-4-n-propoxy derivatives. The triplet energies are found to be fairly insensitive to solvent polarity with an observed spectral broadening from non-polar to polar solvents. This is consistent with close lying mixed triplet states of 3ππ* and 3nπ* character. Both the least active 4-hydroxy and 2-methyl-4-propoxy derivatives exhibit lower triplet energies suggesting the presence of a less active lower triplet 3ππ* state. Triplet-triplet absorption spectra are obtained with all the thioxanthones in the range 600-680 nm with a marked blue shift from non-polar to polar solvents owing to stabilisation of the lowest triplet state by solvent reorganisation. In photo-reductive solvents such as methyl alcohol and 2-propanol a longer lived species is observed absorbing in the region 400-500 nm associated with the formation of the ketyl radical. This observation is highly solvent dependent and further supported by microsecond flash spectroscopy in 2-propanol. The 4-hydroxy derivative gave only weak transient absorption and is consistent with its much lower initiation activity. In the presence of a tertiary amine no ketyl radical is observed. Triplet lifetimes increase with solvent polarity confirming the presence of mixed 3ππ* and 3nπ* states where vibronic coupling influences the rate of intersystem crossing to the ground So state. Bimolecular triplet quenching rate constants indicate all the thioxanthones, except the 2-methyl-4-n-propoxy and 4-hydroxy, interact strongly with a range of tertiary amines, DABCO, triethylamine and methyldiethanolamine. The lower triplet quenching constants for the 2-methyl4-n-propoxy and 2-hydroxy derivatives (an order of magnitude) indicates weaker interaction by the amine and is consistent with their lower photoinitiation activities. Low triplet quenching rates are also observed in the presence of monomer (methyl methacrylate). Bimolecular triplet quenching rates are also measured with naphthalene and are similar to those for benzophenone except the 2-methyl-4-n-propoxy and 4-hydroxy, derivatives which are an order of magnitude less. From this data triplet molar extinction coefficients are determined and found to be higher than that for benzophenone owing to the presence of mixed states. Again, the 2-methyl-4-n-propoxy and 4-hydroxy derivatives exhibited lower values as did the 4-benzoyloxy derivative. Relatively high quantum yields of intersystem crossing are observed ( < 0.6) but are all lower than that of benzophenone with the side chain substituents having no significant effect on the rate. The growth rate of the triplet state could be measured and triplet maxima accurately determined via picosecond flash photolysis. Within the growth time of the triplet state (6-18 ps) there is a concurrent formation of the ketyl radical except for the 4-acetyloxy and 4-acryloxy derivatives. The importance of this technique in gaining valuable information on the relative rates of concurrent excited state reactions for thioxanthones is discussed.

Photochemistry and photoinitiator properties of novel 1-chloro-substituted thioxanthones Part I: Influence of 4-acyloxy substitution

Abstract The photoinduced polymerisation activities of five novel 1-chloro-4-acyloxy substituted thioxanthones have been determined and compared to that of the 4-hydroxy and 4-n-propoxy derivatives in different monomers and prepolymers using RTIR and photocalorimetry methods. Absorption, fluorescence and phosphorescence analysis and photoreduction studies have also been undertaken on the compounds and the data inter-related to their photopolymerisation activities. Fluorescence and phosphorescence analysis indicates a high rate of intersystem crossing to the triplet state. The latter is essentially ππ ∗ in nature but strongly mixed with a close lying n π ∗ as indicated by the phosphorescence lifetime data. Compared with the 1-chloro-4-hydroxy derivative used as a comparative model all 4-acyloxy derivatives exhibit much higher photoinitiation activity. The results are also consistent with a marked enhancement in photoreduction quantum yields, especially in the presence of an amine cosynergist. The enhanced photoactivity of the 4-acyloxy derivatives is associated with reduced fluorescence quantum yields and consequent increased n π ∗ activity of the triplet state through the electron withdrawing effect of the acyl group. High photoconversion during polymerisation is observed in the presence of oxygen indicating the predominance of photodehalogenation which is enhanced by the presence of a tertiary amine cosynergist. Using RTIR higher degrees of conversion are observed under visible light irradiation than under UV exposure.

Photochemistry and photoinitiator properties of 4-substituted amidobenzophenones and imidobenzophenones

Abstract The photoinduced polymerization activities of ten novel 4-substituted amidobenzophenones and imidobenzophenones were determined in different monomers and prepolymers using real-time IR (RTIR) and data related to their spectroscopic activities. The relative order of the photoinitiator efficiency is found to be highly dependent on the method used, the amine co-synergist and the monomer. Absorption, phosphorescence analysis and photoreduction studies were also undertaken on the compounds and the data were interrelated to the photopolymerization activities. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state. The latter is strongly mixed with a close-lying ππ∗ state due to substitution and electron donation by the nitrogen atom. Transient absorption spectra on microsecond flash photolysis are assigned to the radical anion species formed by electron transfer. The photoinduced polymerization activities of the benzophenones, measured by RTIR, appear to be closely related to the formation of such transient species. Oxygen quenching in all cases indicates that the triplet state is the active precursor.

Photochemistry and photoinitiator properties of novel 1-chloro-substituted thioxanthones—II. Influence of 4-oxy and 1-phenylthio substitution

Abstract The photoinduced polymerization activities of six novel 1-chloro-4-oxy substituted thioxanthones have been determined and compared to that of the 4-hydroxy and 4- n -propoxy derivatives in different monomers and prepolymers using RTIR and photocalorimetry methods. Absorption, fluorescence and phosphorescence analysis, as well as photoreduction studies have been undertaken on the compounds and the data is inter-related to their photopolymerization activities. In the case of the 4-propoxy derivative the 1-chloro group has been replaced with a 1-phenylthio substituent. Fluorescence and phosphorescence analysis indicates a high rate of intersystem crossing to the triplet state. The latter is essentially excited ππ ∗ in nature as indicated by the phosphorescence lifetime data. Compared with the 1-chloro-4-hydroxy and 1-phenylthio-4-propoxy derivatives used as comparative systems, all the other 4-oxy derivatives exhibit much higher photoinitiation activity. The results are also consistent with a marked enhancement in the photoreduction quantum yields, especially in the presence of an amine cosynergist. High photoconversion during polymerization is observed in the presence of oxygen for all the 1-chloro derivatives, except that of the 4-hydroxy, indicating the predominance of photodehalogenation which is enhanced by the presence of a tertiary amine cosynergist. Using RTIR photopolymerization is more effective under polychromatic visible irradiation than under UV light. Alkoxy substitution in the 4-position appears to enhance this mechanism as evidenced by the low quantum yields of photoconversion for the 4-hydroxy derivative. This is further supported by removal of the 1-chloro group by 1-phenylthio, which significantly reduces photoinitiation activity. The latter exhibits a high degree of charge-transfer character in its lowest exicted singlet state and has significantly lower phosphorescence quantum yield.