Photopolymerization times

Fig. 9 Top Rose-bengal-stained fluorescence images of photograft-polymerized regions of Models Ay B and C as a function of photopolymerization time or copolymer composition of CMS (see Fig. 8 legend). These indicate that stem-chain length (Model A), daughter-chain length (Model B) and daughter-chain density (Model C) are well-controlled. Bottom Change in fluorescence intensity as a fimction of photopolymerization time or copolymer composition of CMS CMS Chloromethyl styrene...

Figure 6. Change of the photopolymerization times with resin composition.

In 1957, Otsu and coworkers reported that the polymer obtained from St with 13 could induce the radical polymerization of second monomers leading to block copolymers [70-74]. Poly(St)-hZock-poly(MMA), poly(St)-hZock-poly (AN), poly(St)-Z Zock-poly(VAc), and poly(St)-hZock-poly(VA) were prepared from the end-functional poly(St) [75], In the photopolymerization of St and MMA with 13, it was also confirmed that the molecular weight of the polymers produced linearly increased with the reaction time, although the reaction mechanism was not ascertained at that time. Thereafter, the poly(St) produced with 13 was confirmed to have two DC end groups, which can further dissociate pho-tochemically [76]. 

Figure 2 shows the time-conversion and time-molecular weight relationships in the photopolymerization of St and MMA with 13 at 30°C [16, 76,157]. The yields and molecular weight of the polymer increased with polymerization time. From the analysis of the end groups of the polymer chain, it was confirmed that the number of the DC groups remained at two during polymerization (Table 2) [76,156]. 

It was confirmed that the resulting polymers obtained from the St polymerization with 13 induced further photopolymerization of MMA to produce a block copolymer, and the yield and molecular weight increased as a function of the polymerization time, similar to the results for the polymerization of MMA with 13, indicating that this block copolymerization also proceeds via a living radical polymerization mechanism [64]. Similar results were also obtained for the photoblock copolymerization of VAc. Thus, various kinds of two- or three-component block copolymers were prepared [157,158]. 

Recently, Kondo and coworkers reported on the polymerization of St with diphenyl diselenides (37) as the photoiniferters (Eq. 39) [ 162]. In the photopolymerization of St in the presence of 37a and 37b, the polymer yield and the molecular weight of the polymers increased with reaction time. The chain-end structure of the resulting polymer 38 was characterized. Polymer 38 underwent the reductive elimination of terminal seleno groups by reaction with tri-n-butyltin hydride in the presence of AIBN (Eq. 40). It also afforded the poly(St) with double bonds at both chain ends when it was treated with hydrogen peroxide (Eq. 41). They also reported the polymerization of St with diphenyl ditelluride to afford well-controlled molecular weight and its distribution [163]. 

The time-conversion and time-molecular weight relationships in the photopolymerization of St with 7 and 8 are shown in Fig. 3, in which the concentration of the DC group as an iniferter site in these iniferters was identical, i.e., [7]/2-[8]. 

After photopolymerization of a certain monomer for a given time, the polymerization mixture was poured into excess precipitant to isolate the polymer, which was then extracted with a solvent to separate the polymer grafted onto the PSG from the homopolymer. 

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. 

Fig.3 Influence of the light intensity on the photopolymerization of a polyurethane-acrylate (PUA) film. — IR response time...

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... 

Figure 5. Reaction temperature as a function of time for nearly adiabatic photopolymerizations of vinyl esters with various fiber loadings.

It is interesting that there is little change in the time required to cure the polymer as the concentration of BP is increased from 0.2 to 1.0 wt.%. These results suggest that the reaction proceeds to completion even before the light penetrates deep into the sample, perhaps by the propagation of a thermal front created by the initial photopolymerization at the leading surface. In any case, these results illustrate that the addition of a thermal initiator drastically reduces the polymerization time for thick systems. 

Studies were performed to demonstrate the effects of process variables such as light intensity, cure time, initiator concentration, and fiber loading on the evolution of the mechanical properties of the polymers and composites. Even with moderate incident light intensities (less than 500 mW/cm2) and high fiber loadings (60 wt.% random fibers) the photopolymerizations proceed to completion in minutes and exhibit mechanical properties equivalent to samples prepared by traditional... 

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