Rate of photoinitiation

Independent estimates of these quantities can be obtained from the temperature coefficients of photopolymerization. If the rate of photoinitiation is assumed to be independent of the temperature, the increase in rate must be due entirely to the change in kp/k] (see Eq. 13), hence the slope of the plot of log Rp against 1/T for the photochemical polymerization should yield Ep — Et/2. Burnett reported the value 5.5 kcal./mole for styrene, and Burnett and Melville found 4.4 kcal./mole for vinyl acetate, in satisfactory agreement with the values given above. 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

Figure 8. Dependence of the rate of photoinitiated polymerization on the free energy for photo-induced electron transfer from borate to the excited state of cyanine dyes, listed in Table 2.

The same trend is observed for the rate of photoinitiated polymerization (Figure 22). This suggests that ... 

Gosh and Gosh [105] studied photoinitiated polymerization of methyl methacrylate initiated by the BP-TV,A-dimethylaniline couple, and Clarke and Shanks [106] tested the influence of a variety of amines on benzophenone-initiated polymerization. That amino radicals resulted during the initiation the polymerization by benzophenone-tertiary aromatic amines was shown by Li through the use of ESR and spin-trapping methods [107]. It was shown that the rate of photoinitiated polymerization depends on the structure of the amine. More recently [108] benzophenone-tertiary aromatic amines were studied as initiators of the free-radical polymerization of polyol acrylates. Illustrative kinetic curves recorded during photoinitiated polymerization of TMPTA are shown in Figure 23. 

A variety of amines (Table 8) and various xanthene dyes (Figure 25) were tested as visible-light photoinitiators of free-radical polymerization [108, 121]. The rate of photoinitiated polymerization depends on the type of amine used as the electron donor (Figure 26) [122, 123]. 

Photopolymerization experiments were carried out to compare the efficiency of the photoinitiated polymerization as a function of the SCCA structures. The rates of polymerization determined from the efficiencies of the photopolymerization after 6 min of irradiation are also presented in Table 11. A reference sample containing CB and other components, but without any co-initiator, did not show any polymerization. The results in Table 11 show that the rate of photoinitiated polymerization depends on the SCCA used. 

There is another interesting feature of Eqs. (54) and (55). They also show that the rate of photoinitiated polymerization is proportional to the square root of the quantum yield of free-radical formation (<1>r )- It is evident from Scheme 21 that there are two types of processes that could initiate polymerization. The quantum yield of a-CBR radical production can be taken to be equal to the quantum yield of CO2. The quantum yield of a-SR radical could be estimated by... 

Quantitative study of kinetics of radicals accumulation has required solution of auxiliary problem - definition of the rate of photoinitiation Win. In the case of solid-phase reactions there are experimental difficulties in solution of this problem. Measurement of Win according to consumption of inhibitor is complicated by possible photochemical reactions of inhibitor itself and specific solid-phase effects of kinetic stop type and so on [10]. Measurement of Win according to initial rate of radicals accumulation is also tactless in solid polymer, as the latter may be much lower than Wm [164]. 

Obtained result allows to explain dependence of efficiency on concentration of HAC -XLIX (Figure 2.9). Photosensitizing is absent and HAC acts only as ultraviolet absorber at small concentrations of CC (0-1,5%), where the rate of photoinitiation, caused by HAC, is much lower than the rate of photoinitiation, caused by pure CDA. Light stability of polymer increases by iq, times over this range of concentration A=L Photosensitizing becomes visible (A<1) at relatively higher concentration of HAC. 

The results of these photochemical studies form guidelines for the choice of sensitizers, onium salts and other additives potentially useful in the cationic curing of coatings. The sensitized photochemistry of diphenyliodonium hexafluoroarsenate and triphenylsulfonium hexaflurorarsenate was investigated at 366 nm. Product quantum yields are compared to relative rates of photoinitiated cationic polymerization of an epoxy resin. 

In photoinitiated polymerization it is possible to commence the generation of radicals abruptly by exposure of the polymerization cell to the light source, and the time required for temperature equilibration in an otherwise initiated polymerization can be avoided. The rate of photoinitiation is given by Eq. (6.77) and Eq. (6.100) then becomes... 

As discussed in a previous section, a number of studies have been conducted to increase the rate of cationic polymerization of epoxides. In curing applications, polymerization should be rapid enough for high output of production. In a recent work, the effect of addition of tetraethylene glycol (TEG) or polyEPB on the rate of photoinitiated cationic polymerization of CY179, limonene dioxide (LDO), and 1,2,7,8-diepoxyoctane (DEO) has been investigated [150]. These hydroxyl containing additives were shown to obviously accelerate the polymerization, increase the total epoxide conversion and decrease the induction period. 

The advantage of photoinitiation lies in that the generation of radicals can be commenced (or stopped) instantly by turning on (or off) light to the polymerization cell and, moreover, time needed for temperature equilibration (when thermal initiators are used) can be avoided. Using Eq. (6.61) for the rate of photoinitiation, Eq. (6.79) becomes... 

The irritation of air enriched solely with NO or NO2 did not produce photo-oxidants this only occurred when hydrocarbons were also present in the polluted urban atmosphere. This leads to a build-up of tropospheric ozone, and hence to faster rates of photoinitiation through photodissociation of ozone, and then to a further build-up of ozone, and so on. Besides ozone, which is toxic at low concentrations (0.1-1 ppmv), other intermediates responsible for adverse effects include aldehydes and organic nitrates, such as peroxyacetyl nitrate (PAN). 

Polyamic acids are useful resists especially when containing 2,2 -dinitrodi-phenylmethane segments, while a Ti sapphire laser has been found to be effective for 3D curing and microfabrication. On a theoretical note, a direct correlation has been found between the calculated Boltzmann-averaged dipole moment and the measured maximum rate of photoinitiated radical polymerization of acrylic monomers. ... 

Third, allowing that the thickness of layer / for photopolymerising composition and its optical densities ecgl are small in our experiments. Let us also assume, that the kinetic model will be approximately adequate for photoinitiated polymerisation in a case when we use average rate upon layer instead of differential rate of photoinitiator decomposition according to Equation (4.51). Under the definition of average values of Equation (4.51) we obtain ... 

Let us again use the expression obtained in Refs. [60. 61] for the differential rate of photoinitiation as a function of layer coordinate x and time t ... 

Effect of Temperature and Photoinitiator Anion. The polymerization temperature has a significant effect on both the rate of polymerization and the final limiting conversion. For example. Figure 9 contains a series of plots of reaction rate as a function of time for photopolymerization of an epoxide monomer. This figure illustrates that as the temperature is increased, the peak reaction rate increases and the reaction time decreases. The increase in reaction rate with increasing temperature ultimately arises from the effect of temperature on the propagation rate constant (which increases with increasing temperature as described by the Arrhenius equation for the rate constants). The rate of photoinitiation is... 

Initiation. In both thermal- and photopol5unerizations, the rate of initiation depends on two processes the dissociation of the initiator and the initiation of the propagating chain. The decomposition rate Ua) of thermal initiators strongly depends on temperature, with the half-life of many thermal initiators at the reaction temperature on the order of minutes or hours. In contrast, for photopolymerizations, the rate at which photons are absorbed at a specific wavelength will determine the decomposition rate of photoinitiators. This process is not temperature-dependent. Thus, in the classic initiation mechanism, the interaction between light of a specific wavelength and a photoinitiator molecule is considered. For a unimolecular photoinitiator, this reaction step can be written as follows ... 

The rate of photoinitiator decomposition (i.e. the rate of radical production) is given by ... 

As one can see in Figure 2 4, kinetic curves of polymerization in the presence of SGS have typical S-like shape. However, the rate of photoinitiated polymerization of diaciylate composition with increase of SGS content decreases as compared with initial polymerizing composition. Maximal rate of polymerization w at SGS content of 70% V. diminishes approximately by 2 times in comparion with w for initial composition, whereas, time of achievement of maximal rate increases by 2 times. Possible explanation of this fact may be that an inorganic constituent forms steric limitations for the process of polymerization of diaciylate monomer. An additional spatial network of nanoparticles of silica phase which appears as a result of sol-gel process leads to macroradicals decay and accordingly, to deceleration ofpolymerization process. 

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