The photochemistry and photopolymerisation activity of eight novel aromatic carbonyl photoinitiators have been examined. Five of these were t-butyl perester derivatives, three of 9-fluorenone and these were compared with fluorenone itself and the other two of benzophenone and this was used for comparison also. The remaining three compounds incorporated acetophenone type chromophores bound onto a polysiloxane backbone and these were compared to diethoxyacetophenone. The photochemistry was examined using UV/Vis absorption spectroscopy, luminescence, and flash photolysis, both conventional microsecond and laser nanosecond. The photopolymerisation activity was studied using different model monomers and concentration levels and analysis of the cure rate was measured using a gravimetric method. Investigation of the resulting polymers were undertaken using gel permeation chromatography, UV absorption spectroscopy and, where appropriate, fluorescence for end group analysis. For the t-butylperester derivatives differential photocalorimetry, real time FfIR and pendulum hardness testing was used. Spectroscopic and fluorescence data showed that the perester substitutions on the fluorenone chromophore had an electron withdrawing effect and increased the charge transfer content of the lowest excited singlet state. Absorption and phosphorescence spectra for the benzophenone derivatives compared to benzophenone itself were similar although the lower quantum yields and longer life times were indicative of the presence of an increase in charge transfer content of the lowest excited triplet state with increasing substitution. For the polysiloxane compounds the extinction coefficients and phosphorescent quantum yields tended to increase with increasing molecular weight. Photoreduction quantum yields were higher for the substituted fluorenones than fluorenone itself, that for the di substituted being twice that of the mono suggesting homolysis of the peroxyester groups to yield reactive benzoyloxy and t-butoxy radicals. For the benzophenone derivatives, the benzophenone had much higher values than the peresters indicating that benzophenone undergoes hydrogen abstraction more efficiently whereas the derivatives again undergo photoc1eavage about the peroxyester groups. Conventional microsecond flash photolysis showed that all the t-butylperester initiators were capable of forming a ketyl radical via hydrogen abstraction. For the benzophenone compounds this formation follows the order benzophenone>mono->tetraperester and is consistent with the phosphorescence measurements and other data. For the polysiloxanes conventional flash data indicated the formation of benzoyl radicals via photofragmentation and again the intensity of the transient absorption was proportional to the molecular weight of the photoinitiator. On nanosecond laser flash photolysis of the fluorenone derivatives in nitrogen saturated non-hydrogen donating solvents, transient absorptions corresponding to the triplet states were seen and residual absorption probably due to benzoyloxy radicals. Triplet-triplet absorption occurs for the benzophenone initiators under the same conditions but this is much stronger for benzophenone than the derivatives. In oxygenated solutions the triplet absorptions were effectively quenched but an interesting phenonemon occurs for all the t-butylperester derivatives in that a long lived transient growth was observed which is tentatively assigned to that of aromatic peroxy radical formation. On laser flash photolysis of the polysiloxane compounds very long lived transients were observed in both oxygenated and nitrogen saturated solutions. When nitrogen saturated, stable benzoyl radicals appear to have formed whereas when oxygen saturated some type of complexation seemed to occur with the oxygen. For all compounds the photopolymerisation activity was shown to be highly dependent on the structure of the initiator, the extinction coefficient in the relevant region of excitation and the degree of self termination reactions which can occur. All the tbutylperester derivatives and polysiloxane compounds were effective at photoinitiating the polymerisation of acrylate monomers. The derivatives of fluorenone initiated polymerisation without the use of a tertiary amine co-initiator unlike fluorenone itself. The derivatives of benzophenone also initiated polymerisation effectively without the presence of an amine co-synergist with the overall order being monoperester>tetraperester>benzophenone. Consistent with other results the efficiency of the polysiloxane compounds to initiate polymerisation was proportional to the molecular weight of the initiator.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:359296 |
| Date | January 1994 |
| Creators | Hardy, Stella Jane |
| Publisher | Manchester Metropolitan University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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