Comparison of photoinitiators

Table XI. Comparison of Photoinitiators (PM, DPI, TPS)—Influence of Temperature and Humidity on Tack-Free Time...

Schneider F, Piletsky S, Piletska E, Guerreiro A, Ulbricht M. Comparison of thin-layer and bulk MlPs synthesized by photoinitiated in situ crosslinking polymerization from the same reaction mixtures. J Appl Polym Sci 2005 98 362-372. 

During the past decade, the study of photoinitiated reactive and inelastic processes within weakly bound gaseous complexes has evolved into an active area of research in the field of chemical physics. Such specialized microscopic environments offer a number of unique opportunities which enable scientists to examine regiospecific interactions at a level of detail and precision that invites rigorous comparisons between experiment and theory. Specifically, many issues that lie at the heart of physical chemistry, such as reaction probabilities, chemical branching ratios, rates and dynamics of elementary chemical processes, curve crossings, caging, recombination, vibrational redistribution and predissociation, etc., can be studied at the state-to-state level and in real time. 

For the purpose of visualizing the role of the crystal lattice ( matrix effect ) during the reaction, a comparison of the chemical behavior in solution and in the molten state is meaningful. Photoinitiated vinyl-type polymerization has been observed with p-PDA Et in the molten state (see Sect. Ill.a.)19. Photopolymerizability in solution has been investigated with DSP series, p-PDA Et, and p-CPA nPr, including non-photopolymerizable compounds in the crystalline state34,36 39). 

The comparison of the kinetic data reported in Tables 26 and 28 clearly shows that poly(VBPO-co-MMA)s are more active in the photoinitiated polymerization of the HDDA/BA equimolar mixture than poly(MAPO-co-MMA)s, notwithstanding their uncomplete solubility in the acrylic formulation. In particular, the remarkable shortening of the induction period (to) causes a sharp increase of the overall polymerization efficiency in poly(VBPO-co-MMA)s, as indicated by ti/2 values which are about one third of those for poly(MAK)-co-MMA)s. 

Bulk semiconductors and powders have been used as initiators for radical polymerization reactions [140-144], Recently the study has been extended to semiconductor nanoclusters [145-147]. It was found that polymerization of methyl methacrylate occurs readily using ZnO nanoclusters. Under the same experimental conditions, no polymerization occurred with bulk ZnO particles as photoinitiators [145], In a survey study, several semiconductor nanoclusters such as CdS and Ti02, in addition to ZnO, were found to be effective photoinitiators for a wide variety of polymers [146], In all cases nanoclusters are more effective than bulk semiconductor particles. A comparison of the quantum yields for polymerization of methyl methacrylate for different nanoclusters revealed that Ti02 < ZnO < CdS [146]. This trend is parallel with the reduction potential of the conduction band electron. The mechanism of polymerization is believed to be via anionic initiation, followed by a free-radical propagation step. 

Such simplifications are less valid for the comparison of different photoinitiators. However, experience has suggested that calculations of PIA provide a reasonable guide to understanding photolnltlator performance. The power of curing lamps, reflector geometry, sample position and belt speed are also Important. In this study we have maintained these at fixed levels. 

Figure 3. Comparison of the differential scanning photocalorimeter UV cure response of epoxy-terminated silicone oligomers with different chain lengths. The oligomers were cured using 0.5 mole % (4-octyloxyphenyl) phenyliodonium hexafluoroantimonate as photoinitiator.

Equation (2.26) leads to a solution for from available knowledge of the rate R, the concentration of monomer in the monomer-polymer particles [M], and the number of particles, N. This method has been applied to several monomers and has been especially useful in the case of the dienes, where the classical method of photoinitiation poses difficulties. Some of these results are shown in Table 2.4 in the form of the usual kinetic parameters. The results obtained for styrene by photoinitiation techniques are included for comparison. It can be seen that the agreement is remarkably good, considering the widely different experimental methods used. Recent studies of the emulsion polymerization of butadiene have shown that the rate constant for propagation is even higher than previously estimated (see Table 2.1) (Weerts et al., 1991). 

While cure time is of substantial importance, other variables have also been found to result in major property changes. A semiquantitative comparison of the importance of three factors - multifunctional acrylate concentration, photoinitiator concentration, and cure time - is given in Table 9 for peel strength, shear, and the adhesive/cohesive peel mode. We have assigned relative values of high, medium, and low to each of the independent variables for their effect on the dependent properties. 

Table 1 shows a comparison of physical properties for the 11 free radical photoinitiators reviewed in this paper. This comparison includes the physical state, color and comments on solubility in typical multifunctional acrylate diluents used in UV-curing. Also included is a list of photoinitiator abbreviations used throughout this report. 

Comparison of Commercial Photoinitiators in Aliphatic Urethane Acrylate Formula... 

Three different techniques have been presented that all measure different flavors of photoinitiated CT distances. Indeed, a comparison of the techniques in Table 1 indicates many differences between them. Notably, the difference between vector and scalar dipole moment differences must be considered with respect to the geometry of the analyte, since scalar measurements do not necessarily yield the actual CT distances. Also, the analyte spectrum components must be clearly resolved. For example. Stark absorption on (bpy)Re(CO)3Cl would be very difficult due to overlapping ligand TTjTT and MLCT bands. Alternatively, its emission spectrum exhibits only the MLCT band, allowing Stark emission to easily measure the CT distance. The need for Stark spectroscopy samples... 

The main focus in this monograph is on models of the kinetics of photoinitiated polymerization of mono- and bifunctional monomers up to the high conversion state, their derivation, analysis and comparison with the experimental data. 

The development of photoinitiators for cationic polymerization is an important one, with both practical and fundamental applications. Practical uses are primarily in the area of light-induced curing of coatings, a process which is expected to be energy-conservative in comparison with conventional thermal curing (4). 

Comparison of obtained averaged kinetic curves of copolymerization of di(meth)acrylates HDDA and TGM-3 at different intensities of UV irradiation depending on composition of copolymerizing system is shown in figures below.Kinetic regularities of photoinitiated copolymerization of the system HDDA - TGM-3 till high conversion were described and analyzed in [18],... 

COMPARISON OF THE EXPERIMENTAL AND CALCULATED DATA ON THE KINETICS OF PHOTOINITIATED COPOLYMERIZATION OF HDDA-TGM-3 SYSTEM TILL HIGH CONVERSION... 

Comparison of the experimental integral kinetic curves and their differential anamorphoses of photoinitiated copolymerization of HDDA -TGM-3 system depending on its composition and intensity of UV radiation (a) and the corresponding calculated kinetic curves (b) is shown in Figs. 2-4. 

Figure 3 shows representative single-wavelength absorbance transients for three dyes electrostatically bound to colloidal Sn02. The transients correspond to photoinitiated bleaching and recovery of the respective MLCT absorbances. From Fig. 3, it is clear that (1) injection is rapid in comparison to backET, (2) back ET is complex kinetically, but (3) the complex recovery rates depend strongly on the identity of the dye, at least in the first few hundred nanoseconds of the recovery. In order to isolate the shorter-time recovery kinetics, transients were fit, somewhat pragmatically, to a bi-exponential decay function ... 

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