Photopolymerization rate

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

Li et al. [87,88] found that aniline will process the photopolymerization of AN either in N,N-dimethylformamide (DMF) solution or in bulk with a fair rate of polymerization only next to DMT. From UV spectra it is proved that aniline will form a CTC with AN. Using 313-nm radiation that CTC is excited to an exciplex and polymerization proceeds. N-methylaniline will polymerize AN similarly. The following mechanism was proposed ... 

The well-known photopolymerization of acrylic monomers usually involves a charge transfer system with carbonyl compound as an acceptor and aliphatic tertiary amine, triethylamine (TEA), as a donor. Instead of tertiary amine such as TEA or DMT, Li et al. [89] investigated the photopolymerization of AN in the presence of benzophenone (BP) and aniline (A) or N-methylaniline (NMA) and found that the BP-A or BP-NMA system will give a higher rate of polymerization than that of the well-known system BP-TEA. Still, we know that secondary aromatic amine would be deprotonated of the H-atom mostly on the N-atom so we proposed the mechanism as follows ... 

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. 

A variety of photo-initiators and water-soluble diluents were used to study the photopolymerization of 2-hydroxyethyl methacrylate (HEMA). The rate of polymerization can be correlated to the structures as well as the concentration of initiators and diluents. It was found that, in terms of rate of polymerization,... 

Table 2 summarizes the peak times for the photopolymerization of HEMA. Among initiators with structures known, the ranking of more active initiators, in terms of decreasing polymerization rate was ... 

The objective of the present work was to determine the influence of the light intensity on the polymerization kinetics and on the temperature profile of acrylate and vinyl ether monomers exposed to UV radiation as thin films, as well as the effect of the sample initial temperature on the polymerization rate and final degree of cure. For this purpose, a new method has been developed, based on real-time infrared (RTIR) spectroscopy 14, which permits to monitor in-situ the temperature of thin films undergoing high-speed photopolymerization, without introducing any additive in the UV-curable formulation 15. This technique proved particularly well suited to addressing the issue of thermal runaway which was recently considered to occur in laser-induced polymerization of divinyl ethers 13>16. 

Figure 1 shows a RTIR comparison of the photopolymerizations of the above seven model compounds. The initial slopes of the curves were calculated and are shown in Table 1. Based on this data, the monomers may be ranked in the following order with respect to their rates of polymerization. 

Photopolymerizations were initiated with either ultraviolet or visible blue light of varying intensity (1-150 mW/cm2). In general, the high concentration of double bonds in the system and the multifunctional nature of the monomer (two double bonds per monomer molecules) led to the formation of a highly crosslinked polymer system in a period of a few seconds, depending on the initiation rate. 

Figure 5 contains experimental profiles of the reaction temperature at the bottom of the sample as a function of time for nearly adiabatic photopolymerizations of Derakane resins containing between 0 and 60 wt.% of the glass fibers. The figure illustrates that for all fiber loadings, upon illumination the temperature exhibits an initial increase from room temperature to a final plateau value around 130°C. Moreover, the figure illustrates that as the fiber loading is increased, both the rate of the initial temperature increase, and the final plateau value, are reduced. These trends are easily explained by the reduction in the reactive fraction of the sample... 

Dual Photo/Thermal Initiation Studies. A series of studies were performed using reactive formulations containing both a photoinitiator and a thermal initiator dissolved in the Derakane resin. The objective of these studies was to investigate a dual cure strategy in which the heat liberated by the photo-induced polymerization leads to the production of additional active centers by the dissociation of a thermal initiator. In this way, the dual cure strategy could offer both the temporal control of the start of the reaction afforded by the photopolymerization, as well as enhanced reaction rate and completeness of cure provided by the thermal initiation. 

Photopolymerizations of the bis-pyrimidine derivatives were carried out in solid film by irradiation with UV light from a spectroirradiator. The rate of the photopolymerization was found to depend on the wavelength of light. The highest reactivity was observed at 300 nm (Figure 7a), while the highest reactivity for photoreversal of the photodimer was obtained by irradiation at 240 nm (Figure 7b). The polymers... 

Ranby and Shi also studied hyperbranched methacrylated polyesters and their use in photopolymerizations of films and fiber-reinforced polymer composites. The resins were found to have low viscosities and higher curing rates than those of corresponding linear unsaturated polyesters [131-133]. 

We chose to modify the anhydride monomers with photopolymerizable methacrylate functionalities. Methacrylate-based polymers have a long history in biomedical applications, ranging from photocured dental composites [20] to thermally cured bone cements [21]. Furthermore, photopolymerizations provide many advantages for material handling and processing, including spatial and temporal control of the polymerization and rapid rates at ambient temperatures. Liquid or putty-like monomer/initiator... 

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