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Free radical macrophotoinitiators: an overview on recent advances

This paper provides an overview of the recent advances on polymeric photoinitiators for UV curing. During the last decade, significant developments have been achieved in the synthesis of macrophotoinitiators, due to the advantages derived of their macromolecular nature, in comparison with their corresponding low molecular weight analogues. In particular, a variety of macromolecules containing the two main types of free radical photoinitiators: hydrogen-abstracting (thioxanthone, benzil, anthraquinone, camphorquinone) and photofragmenting chromophores (benzoin ether, acylphosphine oxides) are described. For hydrogen-abstracting photoinitiator, the photoinitiation activity have been examined in terms of volume and nature of substituent in the polymeric coil, and their influence to prevent the recombination of radicals favouring their reaction with the monomer. Also, copolymers bearing chromophore and amine groups with potential synergistic effects of activity are reported. It has been found that the approach of the tertiary amine to the chromophore to produce the corresponding exciplexes is dependent on both the monomeric or polymeric nature of chromophore and the tertiary amine. Type II polymeric photoinitiators, such as benzoin ether derivatives having the benzoin methyl ether moieties connected to the main chain through the benzyl aromatic are reported. And a fragmentation mechanism involving the formation of an stable quinoid structure and aliphatic acyl radical is proposed for the above copolymers, which would justified their lower initiating efficiency than the corresponding low molecular weight model. In addition, polymers bearing phosphine oxide moieties are described. The efficiency in the polymerisation of all photoinitiators was found to be similar and irrespective of the presence of flexible spacer in their structure. However, it was found that the flexible oligomethylene spacer enhanced the compatibility of the new polymeric photoinitiators in acrylic adhesive formulations. Finally, polysilanes as photoinitiators are reported. Under UV irradiation, polysilanes undergo main-chain scission leading to free silyl radicals capable of reacting with olefinic monomers. The silyl radicals generated by photolysis can be oxidised by appropriate onium salts to yield cationic initiating species (photoinitiated radical promoted cationic polymerisation). The photoinitiation efficiency of polysilanes having different aliphatic and aromatic side groups has been investigated and compared with commercial low molecular weight photoinitiators as benzoin.

Photooxidative and thermal degradation of polyethylenes: interrelationship by chemiluminescence, thermal gravimetric analysis and FTIR data

Abstract The thermal and photooxidative behaviour of polyethylenes of different manufacturing histories (linear low, metallocene and high density) have been investigated by FTIR spectroscopy and the data related to the chemiluminescence (CL) and thermal analysis of the polymers. The CL intensity and activation energies for thermal decomposition were found to be dependent on the rates of oxidation of the polymers under light and heat. On thermal oxidation, the activation energies were found to decrease with time and followed the order HDPE>m-PE>LLDPE, which correlated with the results obtained by means of CL analysis, where a higher intensity of CL at low temperature was found for HDPE. On photooxidation, the order followed their instability, i.e. m-PE>HDPE>LLDPE. The rates of thermal oxidation in each case were found to be the same irrespective of the nature of the oxidation product, i.e. hydroperoxide, vinyl and carbonyl groups. The influence of branching in the polymer appears to be important, showing a decrease of thermal stability as branching in polyethylene increases.

Ageing and stabilisation of filled polymers: an overview

The durability of filled polymers is a scientific and technological problem involving a complex interplay of phenomena and mechanisms many of which are little understood. There are a number of issues that require addressing in this regard. The first relates to the stability of the filler itself, especially if it is a coloured pigment. In this regard organic pigments may fade or change colour to an unacceptable shade. The second involves complex filler-polymer interactions and these, in turn, are further complicated by the nature of the environmental ageing conditions such as temperature, atmosphere, humidity and light source. The third problem relates to filler-stabiliser interactions where chemical and physical adsorption processes or sensitisation reactions can control the nature of both the oxidation and stabilisation processes. In this regard fillers with high surface areas and pore volumes can markedly influence such mechanisms, especially hindered phenolic antioxidants and hindered piperidine light stabilisers (HALS). This paper presents an overview of our current understanding of the complexities of filler-polymer and additive interactions with a major emphasis on carbon black, silica and titanium dioxide pigments.

Ultrafast reversible phase change in GeSb films for erasable optical storage

Amorphous‐to‐crystalline and crystalline‐to‐amorphous transformations are triggered in GeSb thin films by irradiation with femtosecond and picosecond laser pulses. Phase changes are accompanied with optical contrast and therefore the feasibility of phase‐change optical recording at ultrafast rates is demonstrated for the first time. The phase reversal by ultrashort pulses seems to be related to the dependence of the degree of undercooling prior to solidification on the irradiation energy density.

Photochemical study and photoinitiation activity of macroinitiators based on thioxanthone

In this work we studied the photopolymerization of methyl methacrylate using polymeric initiators based on thioxanthone chromophore. As co-initiator were used low molecular weight tertiary amines and bound to a polymer chain. The photoinitiation efficiency of these systems was compared with that of the corresponding low molecular weight analogue. A higher efficiency was obtained with the system comprising the thioxanthone bound to a polymer chain and the free amine. The results carried out with low molecular weight TX and the polymeric co-initiator showed that the efficiency is independent of the amount of amine units in the polymer chain. The photophysics of the different systems show that the polymer chain does not affect the emission characteristics of the thioxanthone chromophore. Bimolecular rate constants for the singlet and triplet quenching of thioxanthone by the amine were determined using fluorescence and laser flash photolysis techniques. The dependence of the photoinitiation efficiency with the amine concentration was simulated from the measured quenching rate constants, and shows that the active radicals are produced from the interaction of the ketone triplet with the amine. The ketyl radical yield was slowly higher for the thioxanthone bound to the polymer. The difference between the photoinitiation activity of the polymeric system and the low molecular weight models is explained in terms of the effect of the polymer chain on the reactivity of the amino radicals.

Following in situ photoinitiated polymerization of multifunctional acrylic monomers by fluorescence and photocalorimetry simultaneously

The photoinitiated polymerization of multifunctional (meth)acrylic monomers was simultaneously monitored in real time by fluorescence and differential scanning calorimetry. As the curing proceeds, the fluorescence emission changes due to an increase in the viscosity of the microenvironment. A good correlation between fluorescence intensity and degree of conversion was established by using two different fluorescent probes, each having the same fluorophore, dialkylamino derivatives of 7-nitro-2-oxa-1,3-diazol. One of the probes contains an acrylic moiety which can react with the monomers labelling the formed network. The use of the first moment of fluorescence is presented as a suitable methodology to avoid experimental fluctuations. Furthermore, the influence of the length of the spacer between the acrylic groups in the polymerization reaction kinetics and fluorescence changes has been also discussed in terms of reaction diffusion controlled termination kinetics and free volume fraction. It was concluded that the fluorescence method is a powerful tool to study the kinetics of photopolymerization of multifunctional monomers.

Stabiliser interactions in the thermal and photooxidation of titanium dioxide pigmented polypropylene films

The thermoxidative (oven ageing) and photooxidative degradation of polypropylene films (200 μm thick) containing a range of antioxidants and light stabilisers, together with anatase and rutile titanium dioxide pigments are studied by Fourier Transform infra-red (FTIR) spectroscopy. Rates of thermal and photooxidative degradation are determined by measuring the formation of non-volatile carbonyl and hydroperoxide oxidation products which absorb in the infra-red region of the spectrum with maxima at 1710 and 3410cm−1, respectively. During photooxidation the rutile pigment is synergistic in stabilisation with phenolic antioxidants and hindered piperidine stabilisers (HAS) but antagonistic with benzotriazole and benzophenone absorbers. In the case of anatase marked autocatalytic oxidation is observed giving strong antagonistic effects. With mixed antioxidant/stabiliser combinations, synergistic stabilisation is not significantly influenced by rutile but markedly antagonised by the presence of the anatase pigment. The most effective light stabiliser in retarding the catalytic oxidative effect of anatase is the polymeric HAS, Chimassorb 944. During thermal oxidation the polymeric HAS exhibit strong thermal antioxidant activity compared with the non-polymeric HAS. Both antioxidants and the benzophenone absorber are strongly antagonistic with rutile while the HAS and benzotriazole stabilisers display weak synergism. The anatase strongly accelerated and catalysed the thermal stabilisation effects with all the stabilisers and antioxidants. For the stabiliser combinations, the HAS are more effective at inhibiting oxidative degradation than the absorbers in the presence of both pigment types. The thermal catalytic effects of anatase are strongly suppressed by antioxidant/stabiliser combinations.

Photophysics and photoreactivity of substituted thioxanthones

The photophysics and the interaction of the excited states of substituted thioxanthones have been studied in order to be able to predict their behaviour as photoinitiators in photopolymerization systems. Electron-donor groups in position 2 in the thioxanthone ring lead to high fluorescence quantum yields, red-shifts in the emission spectra and long singlet lifetimes, explained by the separation of the nπ* and ππ* excited states induced by the higher electron density of the aromatic system. For some of the derivatives a biexponential decay due to slow relaxation of the solvent has been observed. Rate constants for the quenching of the singlet state by amines are near the diffusional limit and are not modified by substituents. On the other hand, substitution of electron-donor groups on the thioxanthone ring in position 2 decreases the triplet reactivity towards amines and alkenes. The incorporation of double bonds on the side chains of the compounds does not affect the lifetimes and reactivity of the thioxanthones.

Ageing and spectroscopic properties of polyethylenes : comparison with metallocene polymer

The thermal and photooxidative stabilities of high, linear low and metallocene grade polyethylenes are compared using FTIR and hydroperoxide concentration analysis and thermal methods (DSC, TGA). The thermal ageing of solid films follow the order LLDPE > metallocene > HDPE while for light stability the order is LLDPE > HDPE > Metallocene. Derivative UV and FTIR spectroscopic analysis of the three polyethylene types show that the vinyl types and concentrations are different and that they reflect the thermal stability of the polymer materials. Hydroperoxide levels also appear to be important and correlate with the carbonyl index oxidation rates. Differences in catalyst residues play a less prominent role although the effect of the Ti levels may be important. The fluorescence and phosphorescence excitation and emission spectra of the three types of polyethylene are similar confirming earlier identities on the role and presence of unsaturated carbonyl species. There are however, significant differences in a long wavelength component in the fluorescence emission indicating the presence of other active chromophores. These were found to be significant in the case of the metallocene polymer. These chromophores may contribute to the differences in the light stability of the polymers with the metallocene exhibiting the greatest intensity and least stability followed by HDPE and LLDPE.

Efficient biodegradation of common ionic liquids by Sphingomonas paucimobilis bacterium

The biodegradation of ionic liquids (ILs) was evaluated by an indirect impedance technique, through which carbon dioxide production was measured during bioassay time. The biodegradation study was focused on finding a microorganism able to efficiently degrade common IL compounds. For the first time, a bacteria strain of Sphingomonas paucimobilis was employed in biodegradability tests of ILs, carried out for 37 commercial imidazolium-, pyridinium-, pyrrolidinium-, ammonium- and phosphonium-based ILs, including in the sample 12 different anions and 14 different cations. Remarkably, more than half of ILs studied (54% of the sample) exhibited a biodegradation percentage ≥60% after a 28-day incubation period with S. paucimobilis at 45 °C; therefore, they behave as easily biodegradable compounds from the indirect impedance test. In summary, current results suggested the possibility of biotreatment for the rapid and ultimate mineralisation of widely used ILs, such as BmimNTf2, BmimPF6, etc., which were noted as recalcitrant to biodegradation in previous standard tests with other microorganisms.