Thiol-Ene Photopolymerization of Polymer-Derived CeramicPrecursors

NEIL B. CRAMER,1 SIRISH K. REDDY,1 HUI LU,1 TSALI CROSS,2 RISHI RAJ,2 CHRISTOPHER N. BOWMAN1,3

1Department of Chemical and Biological Engineering, University of Colorado, Engineering Center, ECCH 111, 424 UCB,Boulder, Colorado 80309

2Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309

3Department of Restorative Dentistry, University of Colorado Health Sciences Center, Denver, Colorado 80045

Received 2 September 2003; accepted 30 November 2003

ABSTRACT: The liquid, ceramic precursor monomer VL20 was copolymerized with athiol monomer in a traditional radical thiol-ene photopolymerization. Polymerizationoccurred via addition of the thiol functional group to the vinyl silazane functional groupin a 1:1 ratio consistent with a step-growth polymerization. Gelation occurred at a highconversion of functional groups (70%) consistent with an average molecular weight andfunctionality of 560 and 1.7, respectively, for VL20 monomers. Initiatorless photopoly-merization of the thiol-VL20 system also occurred upon irradiation at either 365 or 254nm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1752–1757, 2004Keywords: kinetics (polym.); photopolymerization; step-growth polymerization

INTRODUCTION

Microelectromechanical systems (MEMS) are be-ing investigated for applications such as sensorand actuator systems, microcombustors, heat ex-changers, microfluidic devices, and microopticssystems. Applications for MEMS requiring hightemperatures or harsh environments such as mi-crocombustors, heat exchangers, and opticalMEMS for high-energy applications often requirethe use of ceramic materials.1–4 Polymer-derivedceramics (PDCs) for MEMS devices have excel-lent thermal and mechanical properties for thesetypes of applications. PDCs use polymerization ofceramic precursor monomers to form polymer

structures, which upon pyrolysis become ceramicstructures of a self-similar shape.4–7

Photopolymerization of liquid, ceramic precur-sor monomers allows for spatial as well as tem-poral control of the reaction and is performedunder ambient conditions. Furthermore, litho-graphic techniques can be used to fabricate free-standing three-dimensional structures (in con-trast, thermal polymerizations require molds).4–7

Material properties can be controlled by tailoringthe chemistry of the organic precursors.4

MEMS-scale ceramic structures made of sili-con carbonitride were fabricated from the prece-ramic liquid monomer VL20 (a polysilazane). Thepreceramic monomers were typically homopoly-merized via a radical photopolymerization; how-ever, this polymerization method was extremelyinefficient and slow relative to typical radical pho-topolymerizations. Even with large amounts ofphotoinitiator, the curing rates attainable in theformation of these structures were low as com-pared with other typical photopolymerizations,

Correspondence to: C. N. Bowman, Department of Chemi-cal and Biological Engineering, University of Colorado, Engi-neering Center, ECCH 111, 424 UCB, Boulder, Colorado80309 (E-mail: bowmanc@stripe.colorado.edu)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 42, 1752–1757 (2004)© 2004 Wiley Periodicals, Inc.

1752

for example, those of acrylates and methacry-lates. High initiator concentrations also led to aninability to polymerize thick structures (�500�m) because of light absorption and attenuationby the photoinitiator molecules. In this article, wedescribe polymerization of the preceramic mono-mer VL20 with a thiol-ene photopolymerization.A thiol-ene polymerization has been shown8 toovercome the significant limitations of low curedepths and slow polymerization rates. Polymer-ization rates with a thiol-ene system were in-creased by several orders of magnitude, and re-quired concentrations of initiator molecules weresignificantly reduced or even eliminated, facilitat-ing the fabrication of thick structures with highaspect ratios. Shrinkage and ultimate ceramicproperties also appeared to be relatively unaf-fected.

In certain cases, vinyl-containing monomers,which do not readily radically homopolymerize,polymerize readily in a step-growth thiol-ene re-action. Excellent examples are polymerizations ofallyl and vinyl ethers and norbornene functionalgroups.9–15 Thiol-ene photopolymerizations arebased on the radical addition of a thiol to a vinylfunctional group with the final product resultingfrom a combination of the thiol and ene functionalgroups:10

RSH � R�CHACH23 RSCH2CH2R�

The polymerization proceedes by a step-growthaddition mechanism via sequential propagationof a thiyl radical through a vinyl functional groupand the subsequent chain transfer to the thiol,regenerating the thiyl radical.9,10,13,16,17 This suc-cessive propagation/chain-transfer mechanism isthe basis for the thiol-ene step-growth mecha-nism and is presented in the subsequent scheme:

propagation RS� � R�CHACH23

R�C�HOCH2SR step 1

chain transfer R�C�HOCH2SR � RSH3

R�CH2OCH2SR � RS� step 2

Radicals are most commonly introduced intothe system or initiated via typical radical photo-initiators, such as benzophenone or 2,2-dime-thoxy-2-phenyl acetophenone (DMPA),9,10,13 al-though radicals may also be generated withoutthe presence of any added photoinitiator spe-

cies.13,17–19 Radical termination occurs via bimo-lecular radical–radical recombinations.9,20

The thiol-ene polymerization mechanism leadsto unique polymerization and material propertiesrelative to other radical vinyl homopolymeriza-tions. Because of their step-growth mechanism,thiol-ene polymerizations exhibit a very slow mo-lecular weight buildup and delayed gelation rela-tive to chain-growth polymerizations.21 Thiol-enepolymerizations are not inhibited by oxygen andcan be initiated with little or no added photoini-tiator.9,10,13,18,20,22,23

This work examines the copolymerizationmechanism of thiol monomers with the vinyl-con-taining preceramic monomer VL20. Specifically,significant evidence is presented that showed thatthis polymerization behaves in a manner consis-tent with a traditional thiol-ene step-growth po-lymerization.

EXPERIMENTAL

Materials

The monomers used in this study were pentae-rythritol tetra(3-mercaptopropionate) (tetrathiol)donated by Bruno Bock (Marschacht, Germany)and the vinyl-containing ceramic precursor mono-mer VL20 donated by Kion Corp. (New York, NY).The monomer structures are portrayed in Figure1. The photoinitiator DMPA was purchased fromCiba-Geigy (Hawthorne, NY). All monomers andthe photoinitiator were used as received.

Methods

Fourier transform infrared (FTIR) studies wereconducted with a Nicolet 750 Magna FTIR spec-trometer with a KBr beam splitter and an MCT/Adetector. Series scans were recorded, taking spec-tra at the rate of approximately 5 scans/s. TheFTIR sample chamber was continuously purgedwith dry air. Samples were irradiated until thereaction was complete, as indicated by the func-tional group absorption peaks remaining con-stant. Thiol-functional group conversion wasmonitored with the S–H absorption peak at 2570cm�1, and vinyl conversions were monitored withthe carbon–carbon double-bond absorption peakat 1593 cm�1. Conversions were calculated withthe ratio of peak areas to the peak area beforepolymerization. Analyses and apparatus for theFTIR experiments have been discussed in de-

THIOL-ENE PHOTOPOLYMERIZATION 1753

tail.24,25 All samples were irradiated with 365 nmlight unless otherwise specified.

Monomer samples were placed between NaClcrystals in a horizontal transmission apparatus.24

Polymerizations with 365 nm light were initiatedvia an EXFO Acticure light source (EXFO, Mis-sissauga, Ontario) with a 320–500-nm filter. Po-lymerizations with 254 nm light were initiatedvia a UVP low-pressure PenRay lamp (Ace GlassInc., Vineland, NJ). Irradiation intensities weremeasured with an International Light, Inc. modelIL1400A radiometer (Newburyport, MA). All re-actions were performed under ambient condi-tions.

Stress-development forces arising from shrink-age during polymerization versus conversion ex-periments were performed with in situ monitor-ing of the polymerization by guiding a near-IRbeam through the sample, which was mounted ona tensometer. The near-IR beam was refocused tothe near-IR detector. The tensometer (AmericanDental Association Health Foundation, Gaithers-burg, VA) is based on cantilever beam-deflectiontheory: shrinkage force generated by the sampleduring curing causes the beam to bend, and thedeflection is measured with a linear variable-dif-ferential transformer. The shrinkage force is thencalculated with the beam constant of the cantile-ver beam. The shrinkage stress value is obtainedby dividing the shrinkage force by the samplecross-sectional area. Further detail on this appa-ratus and experimental technique is presentedelsewhere.26

RESULTS AND DISCUSSION

A series of experiments was performed to demon-strate that the copolymerization of VL20 and thiol

monomers is consistent with a traditional, radi-cal, thiol-ene step-growth polymerization. Theconversions of thiol- and vinyl-functional groupswere determined for monomer systems with vary-ing thiol:VL20 weight ratios [Fig. 2(a)]. VL20monomers are a disperse mixture of polysilazanes(Fig. 1), and thus, the exact vinyl functionalityand molecular weight are not known. To analyzethe polymerization results, the vinyl-functionalgroup conversions were normalized with respectto the weight ratio of thiol monomers [Fig. 2(b)].Specifically, the vinyl conversions were divided bythe weight ratio of thiol monomer in each sample.Normalized conversions in these systems werenearly identical, indicating that thiol- and vinyl-functional groups react in equimolar amounts.Thus, the vinyl-functional group conversion is di-rectly correlated to the concentration of thiol-functional groups, where the number of vinylgroups consumed is nearly equal to the number ofthiol-functional groups consumed as would be ex-pected in a step-growth polymerization.

For a thiol:VL20 weight ratio of 1:2.7, both thethiol- and vinyl-functional groups achieve nearly100% conversion (Fig. 3), indicating that the mo-lar ratio of thiol- and vinyl-functional groups isapproximately 1:1. Given that thiol:VL20 mix-tures with a 1:2.7 mass ratio yield a 1:1 stoichio-metric mixture of functional groups, the 1:5 and1:8 weight ratios would correspond to 1.9:1 and3:1 stoichiometric ratios of thiol- to vinyl silazane-functional groups. Vinyl silazane-functionalgroups are thus in excess in both of these systemsand should achieve conversions of 53 and 34%,respectively, which agreed with the results in Fig-ure 2(a).

Because of their step-growth mechanism, thiol-ene polymerizations do not exhibit gelation untilrelatively high conversions. Gel point conversions

Figure 1. Chemical structures of (a) tetrathiol and (b) VL20, where R � H orCH2ACH2 and n � 1–20.

1754 CRAMER ET AL.

were accurately predicted for thiol-ene and otherstep-growth polymerizations with the Flory–Stockmayer theory of gelation:

Xc �1

�r�fthiol � 1��fvinyl � 1�1/2 (1)

Here, r is the stoichiometric ratio of thiol- tovinyl-functional groups and fthiol and fvinyl are theaverage functionalities of the thiol and vinylmonomers, respectively.14,21 Significant elasticmodulus development does not occur until afterthe gel point conversion has been exceeded.14 Todetermine gel point conversions, we used in situshrinkage-stress measurements as a function ofconversion. Shrinkage-stress evolution for a stoi-

chiometric thiol-VL20 polymerization is pre-sented in Figure 4. The stress evolution indicatesthat gelation occurs at approximately 70% con-version of functional groups. From the Flory–S-tockmayer theory, this value corresponds to anaverage vinyl functionality of approximately 1.7for the VL20 monomers. Given that a 1:1 stoichi-ometric ratio of functional groups exists for a1:2.7 mass ratio of thiol to VL20 monomers, anaverage molecular weight of 560 g/mol was calcu-lated for VL20 monomers.

Thiol-ene systems exhibit radical–radical bi-molecular termination. This is determined by thepolymerization rate (Rp) scaling to the initiationrate (Ri) by the 1/2 power:9 Rp Ri

1/2. To demon-strate this feature, thiol-VL20 systems were po-

Figure 2. (a) Conversion of vinyl-functional groups versus time for thiol-VL20 poly-merizations with thiol:VL20 weight ratios of 1:2.7 (�), 1:5 (�), and 1:8 (E). (b) Nor-malized vinyl-functional group conversions for thiol:VL20 weight ratios of 1:2.7 (�), 1:5(�), and 1:8 (E). Vinyl-functional group conversions are normalized by dividing by theinitial mass ratio of thiol to VL20 monomer. All samples contained 0.02 wt % DMPA asthe photoinitiator and were irradiated at 2.0 mW/cm2.

Figure 3. Conversion of thiol (�) and vinyl (�) func-tional groups in a 1:2.7 weight ratio of thiol:VL20monomers. The sample contained 0.02 wt % DMPA andwas irradiated at 2.0 mW/cm2.

Figure 4. Normalized shrinkage stress versus vinylconversion for a stoichiometric thiol-VL20 polymeriza-tion. The sample was mixed with a 1:2.7 mass ratio ofthiol to VL20 monomers, contained 0.02 wt % DMPA,and was irradiated at 60 mW/cm2.

THIOL-ENE PHOTOPOLYMERIZATION 1755

lymerized with different rates of initiation byvarying the irradiating light intensity from 2 to25 mW/cm2 [Fig. 5(a)]. The polymerization timewas then normalized by the initiation rate (i.e.,the light intensity) to the 1/2 power [Fig. 5(b),normalized time � time � I0

1/2]. The polymeriza-tions were all nearly identical for normalized cur-ing times, indicating bimolecular radical termina-tion dominates throughout the polymerization.

One of the more unique polymerization proper-ties of thiol-enes is that they are photopolymeriz-able without added photoinitiator molecules.13,18,20

Here, the thiol-VL20 system photopolymerizedreadily without added photoinitiators. Initiator-less thiol-VL20 photopolymerizations were per-formed by irradiating with 365 nm of light at 50mW/cm2 [Fig. 6(a)] and 254 nm of light at approx-imately 0.05 mW/cm2 [Fig. 6(b)]. In accordance

with previous results,20 initiatorless polymeriza-tion rates for thiol-VL20 systems with 254 nm oflight were approximately twice that when 365 nmof light are used despite a three-order-of-magni-tude decrease in light intensity. Although poly-merizations with 254 nm of light are extremelyrapid, cure depths are limited to thin films be-cause of high light attenuation, whereas irradia-tion with 365 nm of light allows for the curing ofthick samples because of low light attenua-tion.8,20

CONCLUSIONS

The vinyl silazane-functional group on VL20 ce-ramic precursor monomers participated in a tra-ditional, radical, thiol-ene step-growth photopoly-merization reaction. Thiol- and vinyl-functionalgroups are equivalently consumed during pho-topolymerization. VL20 monomers are a mixturewith varying molecular weights and functional-ity. A 1:2.7 weight ratio of thiol to VL20 yields a1:1 stoichiometric mixture of thiol- and vinyl-functional groups. Shrinkage-stress evolution in-dicated a gel point conversion of approximately70%, consistent with an average functionality of1.7 for VL20 monomers. Polymerization ratesscaled with the initiation rate to the 1/2 power,indicating bimolecular radical termination. Poly-merizations also proceeded readily in the absenceof added photoinitiators via irradiation with both365 and 254 nm of light.

The authors acknowledge DARPA, a Department ofEducation GAANN fellowship, the Fundamentals andApplications of Photopolymerizations National Science

Figure 5. Vinyl-functional group conversion at three different light intensities as afunction of (a) polymerization time and (b) polymerization time normalized by lightintensity to the 1/2 power (time � LI1/2). Samples were a 1:2.7 weight ratio of thiol:VL20,contained 0.05 wt % DMPA, and were irradiated at 2 (E), 10 (�), and 25 (�) mW/cm2.

Figure 6. Photopolymerization of 1:2.7 weight ratioof thiol-VL20 without added photoinitiator molecules.Samples were irradiated at 50 mW/cm2 with 365 nm oflight (�) and approximately 0.05 mW/cm2 with 254 nmof light (�).

1756 CRAMER ET AL.

Foundation (NSF) IUCRC, an NSF Tie Grant(0120943), NSF grant DMR-9796100, and a NationalInstitutes of Health grant (DE10959) for funding thiswork.

REFERENCES AND NOTES

1. Riedel, R.; Passing, G.; Schonfelder, H.; Brook, R. J.Nature 1992, 355, 714.

2. Mehregany, M.; Zorman, C. A. Thin Solid Films1999, 518, 355.

3. Yasseen, A. A.; Zorman, C. A.; Mehregany, M. JMicroelectromech S 1999, 8, 237.

4. Liew, L.-A.; Zhang, W.; An, L.; Shah, S.; Luo, R.;Liu, Y.; Cross, T.; Dunn, M. L.; Bright, V.; Daily,J. W.; Raj, R. Am Ceram Soc Bull 2001, 80, 25.

5. Raj, R.; Riedel, R.; Soraru, G. D. J Am Ceram Soc2001, 84, 2158.

6. Liew, L.-A.; Liu, Y.; Luo, R.; Cross, T.; An, L.;Bright, V. M.; Dunn, M. L.; Daily, J. W.; Raj, R.Sens Actuators 2002, 95, 120.

7. Cross, T.; Liew, L.-A.; Bright, V. M.; Dunn, M. L.;Daily, J. W.; Raj, R. Technical Digest of the Fif-teenth IEEE International Conference on MICROELECTRO MECHANICAL SYSTEMS (MEMS -2002), Jan 20–24, 2002, Las Vegas, NV.

8. Reddy, S. K.; Cramer, N. B.; Cross, T.; Raj, R.;Bowman, C. N. Chem Mater, 2003, 15, 4257.

9. Morgan, C. R.; Magnotta, F.; Ketley, A. D. J PolymSci Polym Chem Ed 1977, 15, 627.

10. Jacobine, A. F. Radiation Curing in Polymer Sci-ence and Technology III. In Polymerisation Mech-anisms; Fouassier, J. D.; Rabek, J. F., Eds.;Elsevier Applied Science: London, 1993; Vol. 3, pp219.

11. Jacobine, A. F.; Glaser, D. M.; Nakos, S. T. InPhotoinitiated Cross-Linking of Norbornene Res-

ins with Multifunctional Thiols. ACS SymposiumSeries 417; American Chemical Society: Washing-ton, DC, 1990; pp 160–175.

12. Jacobine, A. F.; Glaser, D. M.; Grabek, P. J.; Man-cini, D.; Masterson, M.; Nakos, S. T.; Rakas, M. A.;Woods, J. G. J Appl Polym Sci 1992, 45, 471.

13. Cramer, N. B.; Bowman, C. N. J Polym Sci Part A:Polym Chem 2001, 39, 3311.

14. Bor-Sen, C.; Saad, A. K. Macromolecules 1997, 30,7322.

15. Muller, U.; Kunze, A. J Macromol Sci Pure ApplChem 1996, 33, 439.

16. Kharasch, M. S.; Read, J.; Mayo, F. R. Chem Ind(London) 1938, 57, 752.

17. Cramer, N. B.; Davies, T.; O’Brien, A. K.; Bowman,C. N. Macromolecules 2003, 36, 4631.

18. Hoyle, C.; Cole, M.; Bachemin, M.; Yoder, B.;Nguyen, C.; Kuang, W.; Bowman, C.; Cramer, N.;Jonsson, S.; Viswanathan, K. RadTech Japan 2000Technical Proceedings, Tokyo, Japan, Dec. 2000, p211.

19. Kuhne, G.; Diesen, J. S.; Klemm, E. MakromolChem 1996, 242, 139.

20. Cramer, N. B.; Scott, J. P.; Bowman, C. N. Macro-molecules 2002, 35, 5361.

21. Odian, G. Principles of Polymerization, 3rd ed.;Wiley: New York, 1991.

22. Woods, J. G. Radiation Curable Adhesives in Radi-ation Curing: Science and Technology; Pappas,S. P., Ed.; Plenum: New York, 1992; p 333.

23. Kharasch, M. S.; Nudenberg, W.; Mantell, G. J. JOrg Chem 1951, 16, 524.

24. Lovell, L. G.; Berchtold, K. A.; Elliot, J. E.; Lu, H.;Bowman, C. N. Polym Adv Technol 2001, 12, 335.

25. Berchtold, K. A.; Hacioglu, B.; Lovell, L.; Nie, J.;Bowman, C. N. Macromolecules 2001, 34, 5103.

26. Lu, H.; Eichmiller, F. C.; Dickens, S. H.; Stansbury,J. W.; Bowman, C. N. Biomaterials, submitted De-cember 2003.

THIOL-ENE PHOTOPOLYMERIZATION 1757