[acs symposium series] radiation curing of polymeric materials volume 417 || laser-initiated...

Chapter 30

Laser-Initiated Polymerization of Multifunctional Acrylates

Repetition Rate Effects on Percent Conversion

Charles E. Hoyle and Martin A. Trapp

Department of Polymer Science, University of Southern Mississippi, Southern Station Box 10076, Hattiesburg, MS 39406-0076

The pulsed laser-initiated polymerization of multifunctional acrylates has been investigated using a photocalorimeter. In the case where a limited number of pulses are delivered to the sample, the extent of conversion decreases as the laser repetition rate increases. However, for a large number of pulses, higher repetition rates do not lead to decreases in the overall degree of conversion.

The use of lasers to initiate the polymerization of both monofunctional and multifunctional monomers has been reported in a number of papers during the past decade. Decker (1) was the first to demonstrate that pulsed lasers could be effectively used to obtain relatively high degrees of polymerization for trimethylolpropane triacrylate. He showed that even for pulsed lasers which deliver up to gigawatts of peak power, polymerization could be effectively carried out over a wide range of conditions (1).

In recent reports (2-7), it has been shown that it is important to consider the effect of such laser operating parameters as pulse repetition rate on the polymerization kinetics. It was clearly demonstrated that pulsing the laser at narrow time intervals on the order of the lifetime of growing polymer radical chains resulted in a premature chain termination due to injection of small molecule "terminator" radicals into the system. In this paper we focus on the effect of pulse repetition rate on the polymerization of multifunctional acrylates, in particular 1,6-hexanediol diacrylate (HDODA) and trimethylolpropane triacrylate (TMPTA).

Experimental

1,6-hexanediol diacrylate (HDODA) (Aldrich), trimethylolpropane triacrylate (TMPTA) (Radcure Specialties) and hexyl acrylate (HA) (Scientific Polymer Products) were used as received. The photoinitiator, 2,2-dimethoxy-2-phenyl

O097-6156/90/O417-O429$06.00/0 ο 1990 American Chemical Society

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In Radiation Curing of Polymeric Materials; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

430 RADIATION CURING OF POLYMERIC MATERIALS

acetophenone (Irgacure 651, Ciba Geigy), was recrystallized several times from methanol. Two microliter samples were placed onto a DSC sample pan and an empty sample pan was used as the reference. Polymerization exotherms were monitored with a modified Perkin Elmer differential scanning calorimeter (DSC-1B) (8). The 2 second response time of the instrument results in a convoluted response function for the exotherm curves. Each sample was degassed for 10 minutes prior to irradiation. Copper tubing was used for all the nitrogen degassing lines. A 10 second baseline was collected before pulsing the laser. The laser source was a Lumonics Hyper-EX 440 operating with a xenon-fluorine fill gas with output at 351 nm (~15-ns per pulse fwhm). An IBM-AT was used to fire the laser and collect data from the DSC. Intensity measurements were accomplished using a Scientech Joule Meter (Model # 360401). The average power intensity was approximately 35 mJ/cnr /pulse with no appreciable rolloff at the higher repetition rates.

Results and Discussion

Figure 1 shows the polymerization exotherms for the LIP of HDODA (2 wt percent photoinitiator) as a function of laser pulsing frequency (repetition rate ranging from 1 Hz to 80 Hz) for a total of 20 pulses delivered to the sample. It is quite obvious that the sample (curve a) exposed to a total of 20 pulses at a 1 second time interval between pulses (laser operation at 1 Hz) reaches a much higher total degree of polymerization, as indicated by the integrated area under the exotherm curve, than samples receiving pulses at shorter time intervals (curves b-f). The results for HDODA in Figure 1 were obtained by placing a neutral density filter of 3.0 in the laser path limiting the output of the laser to 0.035 mJ/cnr /pulse . Figure 2 (curve a) shows a plot of percent conversion versus laser repetition rate for HDODA. In contrast to the results for HDODA, polymerization exotherms for hexyl acrylate (HA), also shown in Figure 2 (curve b), have integrated areas which are essentially independent of the laser repetition rate. [We should mention that at much higher pulse densities and photoinitiator concentrations, we would expect to see H A also display a dependence of percent conversion on the laser repetition rate (see reference 2)]. Furthermore, at any given repetition rate, the double bond percent conversion is significantly greater for HDODA than for H A despite the fact that the H A samples were exposed to the full output of the laser. The use of the full output of the laser was necessary for the H A samples since only very weak exotherms could be generated if a neutral density filter of 3.0 was used as in the case of HDODA. The results for HDODA and H A lead us to speculate that the very rapid polymerization rate of HDODA is influenced to a large extent by its difunctionality, i.e., two reactive acrylate groups. Indeed, the rate enhancement for bifunctional acrylates has been previously observed and postulated to arise from inhomogeneties and related consequences in the polymerizing medium (9-11). One question is left unanswered: why do longer time intervals (pulses greater than 50 msec apart-repetition rates less than 20 Hz) provide for enhanced conversion efficiency? A simple explanation can be offered. Since the radical lifetimes for difunctional monomers are very long relative to the lifetimes for

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30. HOYLE & TRAPP Polymerization of Multifunctional Acrylates 431

5.0-1

100

Time (sec)

Figure 1: Plot of Exotherm Rate vs. Time for HDODA at 358 K, 2 wt% photoinitiator, neutral density filter 3.0 used, 20 pulses at different laser repetition rates:

(a) 1Hz (b) 5 Hz (c) 10 Hz (d) 20 Hz (e) 40 Hz (f) 80 Hz — X — X

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RADIATION CURING OF POLYMERIC MATERIALS

25H

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154

10H

5H

a

b

1 1 1 1 r 20 40 60 80 100

Laser Repetition Rate (Hz)

Plot of Percent Conversion vs. Laser Repetition Rate for ( a ) HDODA and ( b ) H A at 325 Κ (2 wt% photoinitiator, neutral density filter 3.0 used for HDODA, no neutral density filter used for HA, 20 pulses total). D

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polymer radicals generated from monofunctional monomers, premature injection of small molecule terminator molecules prior to or during the formation of gels (or microgels) greatly inhibits the polymerization, i.e., at high repetition rates (above 20 Hz) small molecule terminator radicals are injected into the polymerization medium and terminate the rapidly growing polymer kinetic chain. [A complete discussion of the premature termination process induced in pulsed laser-initiated polymerization appears in several recent publications (2-7)]. Another consequence of laser-initiated polymerization of HDODA is illustrated in Figure 3 which shows results for percent conversion versus repetition rate for a total number of pulses ranging from 20 to 3000. Upon initial inspection, it is seen that an increase in the total number of pulses delivered to the sample results in a dramatic rise in the percent conversion obtained at all repetition rates. From 50 to 3000 pulses, the percent conversion at higher repetition rates increases disproportionately. This result can be attributed to higher polymerization rates attained at the higher repetition rates which allows polymerization to actually precede the contraction process which accompanies crosslinking. Indeed, higher conversions at higher average powers using steady state lamp sources has previously been reported by Kloosterboer et al (4) and attributed to creation of a temporary excess free volume resulting in increased diffusion rates of reacting acrylate groups.

Figure 4 illustrates the effect of decreasing the photoinitiator concentration for a plot of the percent conversion versus laser repetition rate (shown for total pulses ranging from 20 to 3000 as in Figure 3 for direct comparison). The most obvious, but not necessarily the most important, effect of lowering the photoinitiator concentration is a decrease in the percent conversion for a given repetition rate. However, probably the most profound difference for a given pair of curves (curves a-f in Figure 3 versus the corresponding curves a-f in Figure 4) is realized when comparing the maximum percent conversion attained at low repetition rates to the percent conversion at high repetition rates. In each case in Figure 4, the difference in the conversion attained at lower repetition rates and the conversion attained at higher repetition rates is much less than the corresponding differences (for a given number of pulses) in Figure 3. For instance, the ratio of the maximum percent conversion at 1 Hz for 2.0 wt % photoinitiator for 20 pulses total (curve a, Figure 3) to the percent conversion at 40 Hz is 5.0 compared to a value of 1.6 for the same ratio (curve a, Figure 4) with 0.1 wt % photoinitiator. [In the latter case, the maximum percent conversion was obtained at 5 Hz and not 1 Hz], These results can be attributed to the unique design of the laser-initiated polymerization experiment. Since the ability of the small molecule radicals produced at a given delay period to effect premature termination of growing polymer radicals depends on the photoinitiator concentration (see reference 11 for a complete discussion of this phenomenon), the systems with the highest photoinitiator concentration show the largest suppression in the conversion efficiency at higher repetition rates where premature chain termination is most effective.

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100H

100 Laser Repetition Rate (Hz)

Figure 3: Plot of Percent Conversion vs. Laser Repetition Rate for HDODA at 358 Κ (2 wt% photoinitiator, neutral density filter 3.0 used) ( a ) 20 pulses, ( b ) 50 pulses, ( c ) 100 pulses, ( d ) 200 pulses, ( e ) 500 pulses, ( f ) 3000 pulses. D

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30. HOYLE & TRAPP Polymerization of Multifunctional Aery 435

100-j

Repetition Rate (Hz)

Figure 4: Plot of Percent Conversion vs. Laser Repetition Rate for HDODA at 358 Κ (0.1 wt% photoinitiator, neutral density filter 3.0 used) ( a ) 20 pulses, ( b ) 50 pulses, ( c ) 100 pulses, ( d ) 200 pulses, ( e ) 500 pulses, ( f ) 3000 pulses. D

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A second point should be noted for the curves in Figure 4. For curves a-e, unlike the corresponding systems in Figure 3 where the maximum conversion is obtained at 1 Hz, the maximum percent conversion occurs at 5 Hz (curves a-c), 10 Hz (curve d), and 20 Hz (curve e). This is due to a lack of effective premature termination of growing polymer chains, as previously discussed, which occurs when using lower photoinitiator concentrations. Apparently, if premature termination reactions did not occur, we would expect to observe higher extents of polymerization with increasing repetition rate for even low total numbers of pulses.

In summarizing the results in Figures 3 and 4, we note that two factors are operable which are opposite in their effect on the overall polymerization efficiency of laser-initiated polymerization of HDODA. The first factor is a result of efficient premature termination due to coupling of small molecule radicals produced by firing laser pulses at intervals less than 50 msec: this decreases the overall yield. The second factor is a consequence of carrying out the photopolymerization/photocrosslinking at higher overall light intensity: as the light intensity or overall average power of the laser increases an increase in the percent conversion is expected since polymerization precedes the conversion induced shrinkage of the film. At higher photoinitiator concentrations (Figure 3) the first factor dominates resulting in especially low percent conversions at high repetition rates (greater than 20 Hz). At a low photoinitiator concentration (Figure 4), the premature termination process is diminished and percent conversions at high repetition rates (while less than at low repetition rates) are only marginally lower than at low repetition rates.

An interesting comparison to the results in Figure 3 and 4 for HDODA is shown in Figure 5 for trimethylolpropane triacrylate (TMPTA). First, note that the percent conversion for TMPTA curve at 1 Hz for 20 pulses (curve a, Figure 5) is slightly greater than 20 percent, while for HDODA under similar experimental conditions it is almost 30 percent (curve a, Figure 3). Yet, at higher repetition rates the reverse is true, i.e., at 80 Hz the percent conversion for TMPTA is about 16 compared to 5 for HDODA. Apparently, for an identical photoinitiator concentration premature termination is greatly reduced for TMPTA at higher repetition rates. This is indicative of a lower termination rate for TMPTA, and/or an enhanced polymerization rate compared to HDODA. This premise is borne out by single pulse experiments for HDODA and TMPTA (13). Finally, TMPTA proceeds to approximately 50 percent conversion compared to about 80 percent for HDODA. This phenomenon is most assuredly a result of extensive crosslinking for trifunctional acrylate systems which prohibits attainment of high percentages of conversion.

Conclusions

The results in this paper demonstrate the utility of employing a pulsed-laser source to initiate and probe the kinetics of photopolymerization of

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multifunctional acrylates. Specific results derived from this study are: 1. For low numbers of pulses delivered to a di- or trifunctional

acrylate, higher degrees of conversion are obtained at lower pulse repetition rates.

2. Suppression of polymerization for the difunctional acrylate at higher repetition rates is enhanced at increased photoinitiator concentrations.

60H

10H

OH 1 1 1 1 Γ Ο 20 40 60 80 100

Laser Repetition Rate (Hz)

Figure 5: Plot of Percent Conversion vs. Laser Repetition Rate for TMPTA at 358 K, (2 wt% photoinitiator, neutral density filter 3.0 used) ( a ) 20 pulses, ( b ) 50 pulses, ( c ) 100 pulses, ( d ) 200 pulses, ( e ) 500 pulses, ( f ) 3000 pulses. D

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These results point to two processes, premature radical chain termination and film shrinkage, which compete in determining the ultimate polymerization conversion efficiency of multifunctional acrylates. It is obvious that critical attention must be paid to the pulse repetition rate, photoinitiator concentration, and acrylate functionality in developing any photopolymerizable system for laser-initiated polymerization. Future publications on laser-initiated polymerization of multifunctional acrylates will deal with monomer extraction of partially polymerized films, mechanical properties of laser polymerized films, and the kinetics of single-pulsed systems.

Acknowledgment

This research is supported by The National Science Foundation under Grant DMR 85-14424 (Polymers Program). Acknowledgement is also made to NSF for assistance in purchasing the laser system utilized in the course of this investigation (Grant CHE-8411829-Chemical Instrumentation Program).

Literature Cited

1. C. Decker, ACS Symposium Series 266; Materials for Microlithography: Radiation Sensitive Polymers, 9, 207 (1984).

2. C. E. Hoyle, M . A. Trapp, C. H. Chang, D. D. Latham and K. W. McLaughlin, Macromolecules, 22, 35 (1989).

3. C. E. Hoyle, M . A. Trapp and C. H . Chang, Polym. Mat. Sci Eng., 57, 579 (1987).

4. C. E. Hoyle, C. P. Chawla, P. Chattertion, M . Trapp, C. H . Chang and A. C. Griffin, Polymer Preprints, 29, (1), 518 (1988).

5. D. D. Latham, K. W. McLaughlin, C. E. Hoyle and M. A. Trapp, Polymer Preprints, 29, (2), 328 (1988).

6. C. E. Hoyle, C. H. Chang and M . A. Trapp, Macromolecules, accepted for publication.

7. C. E. Hoyle, M . A. Trapp and C. H. Chang, J. Polym. Sci., Polym. Chem Ed., 27, 1609 (1989).

8. C. E. Hoyle, R. D. Hensel and M . B. Grubb, J. Polym. Sci., Polym. Chem. Ed., 22, 1865 (1984).

9. J. G. Kloosterboer and G. J. M . Lippits, Journal of Imaging Science, 30, 177 (1986).

10. J. G. Kloosterboer, G. M . M . van de Hei, R. G. Gossink and G. C. M . Dortant, Polym. Comm., 25, 322 (1984).

11. J. G. Kloosterboer, G. M . M . van de Hei and H. M . J. Boots, Polym. Comm., 25, 354 (1984).

12. C. E. Hoyle and M . A. Trapp, J. Imaging Sci., accepted for publication. RECEIVED October 5, 1989

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[acs symposium series] radiation curing of polymeric materials volume 417 || laser-initiated...

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