Novel Liquid Crystal Resins for Stereolithography: Mechanical and Physical Properties

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1 Nvel Liquid Crystal Resins fr Sterelithgraphy: Mechanical and Physical Prperties J. W. Schultz, J. S. Ullett, R. P. Chartff, R.T. Pgue Rapid Prttype Develpment Labratry and Ohi Rapid Prttype Prcess Develpment Cnsrtium The University f Daytn Daytn, Ohi USA ABSTRACT This paper cnsiders phtcurable liquid crystal (LC) mnmers which are a new class f sterelithgraphy resins. These resins frm plymers with high upper-use temperatures. The rdlike mlecules can be aligned by an external frce. When cured in an aligned state, the aligned structure is "lcked in" resulting in materials with anistrpic physical and mechanical prperties. FTIR spectrscpy, therm-mechanical analysis (TMA), dynamic mechanical analysis (DMA), and large strain mechanical tests were applied t liquid crystal pht-plymers, bth in the green state and after pstcure. These measurements shwed that the pht-plymerizatin reactin lcked in the mlecular rder. Elastic mdulus in the glassy state, revealed apprximately a factr f tw difference between the directins parallel and perpendicular t the alignment. Thermal expansin measurements shwed an anistrpic linear expansin that was very small, and smetimes negative in the alignment directin. Finally, these resins demnstrated high glass transitin temperatures which culd be advanced t as high as 150 C by pstcuring. INTRODUCTION T expand the applicatins f sterelithgraphy, new resins must be develped that have better mechanical prperties and higher use temperatures. Current sterelithgraphy resins have sftening temperatures well under 100 C. As a result, they have limited usefulness in high temperature applicatins such as direct injectin mlding and under-the-hd autmtive applicatins. Liquid crystal (LC) mnmers are a new class f sterelithgraphy resins that can prduce plymers with glass transitin temperatures exceeding 100 C and appraching 200 C. These resins are typically made up f rigid, rd-shaped mlecules. Unlike cmmercial resins, LC resins are ften crystalline at rm temperature and must be heated t exist in a liquid crystalline r istrpic liquid state. As a result, cure temperature becmes an imprtant variable as it affects bth prcessing parameters and green state prperties. In additin, LC resins can be aligned unidirectinally using an external magnetic field. Curing the aligned mnmer results in frmatin f an anistrpic netwrk with a preferred directin, as shwn in Figure 1. The resulting mechanical and physical prperties f such a netwrk are anistrpic. Fr example, the thermal expansin cefficient is much smaller in the directin parallel t alignment than perpendicular t the alignment. Thus parts can be directinally ptimized fr dimensinal stability ver brad temperature ranges. In additin, prper alignment allws ptimizatin f 325

2 strength in a part. Finally, it may be pssible t increase fracture tughness in these materials by varying the alignment frm layer t layer. Figure 1. Schematic f aligned netwrk frigid rds frmed by liquid crystal resins In rder t take advantage f these ptentially useful prperties, it is imprtant t have a gd understanding f the effects f plymerizatin cnditins and pstcure n the netwrk frmatin and resulting anistrpic prperties. A previus paper[1] cnsidered sme f the prcessing parameters f these resins, including rientatin dynamics and wrking curve parameters. The present paper examines sme f the anistrpic mechanical and physical prperties f these new sterelithgraphy resins. The fcus f this paper is n the extent f anistrpy that can be achieved in the mdulus and strength, and n the effect f different pstcure cnditins n the glass transitin temperature. In additin, sme linear thermal expansin data are als presented. EXPERIMENT Materials The liquid crystal mnmers used in this study, designated N1 and N2, are bth diacrylate mnmers with 'rigid rd' cres made f armatic rings. The N1 mnmer has 6-carbn aliphatic spacers n each side f the cre while the N2 has shrter, 3-carbn spacers. N1 has been studied extensively by Brer et al.[2]. The mlecular structures fr the tw mnmers are, JO(c,-b-c- -O(]("'> J O(rn:z),-Q-c,-b-c- -O(]("'> The crystalline melting transitins fr these mnmers are 88 C and 72 C fr N1 and N2 respectively. Abve these temperatures, they are in a liquid crystalline phase. In the liquid crystal phase, the rigid cres fthe mlecules can be aligned twards a preferred directin. At higher temperatures (118 C and 126 C fr N1 and N2 respectively) these resins underg a secnd phase change int an istrpic liquid. In the istrpic phase, these resins lse their mlecular rder, s that their rigid cres are riented randmly. Fr cmparisn purpses, cmmercially available resins als were cnsidered. The cmmercial resins used were SL5149 and SL5170 (Ciba-Geigy). (N1) (N2) 326

3 Apparatus and Prcedures The samples were made in an experimental table-tp sterelithgraphy apparatus (TTSLA)[3]. The TTSLA includes a small heated aluminum vat and an external magnet with a field strength f apprximately 0.32 Tesla. The magnet induces unidirectinal alignment in the liquid crystal resin. The mnmers were mixed with phtiniatr and phtplymerized in the liquid crystalline phase. T reduce warpage during cure, single layer samples were sandwiched between glass slides. UV curing was accmplished with an argn in laser delivering apprximately 33 mw f pwer. Bth single and multi-layer parts were made. The single layer parts were scanned multiple times with narrw line spacings t insure a unifrm cure. The multilayer parts were made using a typical sterelithgraphy prcess using supprts. Infrared dichrism was measured using Furier transfrm infrared (FTIR) spectrscpy measurements made n a Niclet 20DXB spectrmeter. The transmittance f varius plymer samples was measured bth parallel and perpendicular t the alignment directin by inserting an infrared plarizer at different rientatins int the ptical path. T ensure sufficient signal strength, the sample thickness was kept belw 15 micrns. Dynamic mechanical measurements were cnducted n a Rhemetrics slids analyzer (RSA IT), using a thin film fixture. The measurements were made in tensin at a frequency f 1 Hz, and a strain f apprximately 0.05 percent. Sample dimensins were typically 22 mm lng, 3 mm wide, and 0.17 mm thick. T ensure a unifrm thermal histry, the samples were heated abve their glass transitin temperatures and quickly cled befre testing. Fr large strain testing, a Rhemetrics Minimat tester was used in cnjuctin with a high reslutin digital camera fr accurate strain measurement. Specimens were dgbne shaped with gauge dimensins f apprximately 25 mm length, 5 mm width, and 1 mm thickness. The testing prcedures were dne in accrdance with ASTM D638M. Linear expansin as a functin f temperature was measured n a TA instruments TMA 2940 thermmechanical analyzer using a thin film fixture. The measurements were made with a linear temperature ramp f 2 C/minute and with a lad f 0.5 grams. Sample dimensins were typically 13 mm lng, 4 mm wide, and 0.22 mm thick. RESULTS Mlecular Order T better understand the rigin f the anistrpy in the varius physical prperties f these plymers, FTIR analysis was used t measure the degree f mlecular rder. T describe the amunt f mlecular rder in the liquid crystal state, a generally accepted quantity is the mlecular rder parameter, S. The rder parameter is defined s that fr a cmpletely rdered material, S =1, and fr an istrpic material (cmpletely disrdered), S =O. In the FTIR technique, an infrared plarizer is used t measure the sample's absrbance as a functin f rientatin angle. T btain rder parameter frm the absrbance data, the dichric rati is calculated, which is the rati f absrbances parallel and perpendicular t the alignment directin. The mlecular rder parameter is then calculated frm the dichric rati. Figure 2 shws the rder parameter as a functin f cure temperature fr plymer N 1. In this plt, the rder parameter frm tw different absrbance bands are shwn. The 1605 cm

4 band arises frm an armatic stretch, and therefre crrespnds t the stiff central cre f the mnmer. The 2940 cm- 1 band is due t an aliphatic stretch and crrespnds t the flexible spacer. The rder parameter f the aliphatic spacer is cnsiderably lwer than that f the rigid rd cre. This is expected because f the increased flexibility f the spacer and because it must stretch and cntrt t accmmdate the crsslinked netwrk. The data in Figure 2 als shw that there is little variatin in the rder parameter as a functin f cure temperature (except fr very near the liquid crystal t istrpic temperature). Thus when ptimum cure cnditins are being determined fr these resins, the rder parameter is nt an imprtant cnsideratin, since it is nly weakly dependent n cure temperature. 1, ; I- * I- 'E 0.2 -"" 1605 cm- 1 band 2940 cm- 1 band ; I I I I - I I I I Cure Figure 2. FTIR measured rder parameter f Nl plymer as a functin f cure temperature. Mechanical Behavir The DMA data shw curves typical fnetwrk plymers with high crsslink densities. Because f the apparently high crsslink densities, the elastic mdulus abve Tg is t high t be called rubbery -- greater than 10 8 Pa. Figure 3 cmpares the elastic mdulus f sme cmmercial resins t the liquid crystal resins after they have been UV pstcured. The LC resins shw nt nly higher glass transitin temperatures, but als the mdulus abve the glass transitin is a factr f 5 t 15 higher than the cmmercial resins. Thus the usefulness f these liquid crystalline resins may extend well beynd their glass transitin temperatures. The data in Figure 3 als shw that there is a significant difference in the mdulus parallel and perpendicular t the alignment directin in these liquid crystal resins. This anistrpic mdulus is shwn in mre detail in Figure 4. These data are fr bth single-layer and 2-layer parts made frm resin N2. The mdulus in the directin parallel t the alignment is 50 t 100% greater than in the perpendicular directin. In general the mdulus f multi-layer parts is lwer than that f the single layer films. This is expected, since multi-layer parts have a nnunifrm cure. Other cmmercial resins have als shwn similar differences in mdulus when cmparing multilayer parts t idealized single layer films[4]. 328

5 10-, N2 (parallel) '" N2 (perpendicular) Nl (parallel) t:. Nl (perpendicular) --XB5149 SL ' _ Figure 3. Cmparisn felastic mdulus f cmmercial resins and liquid crystal resins , a) single layer parts b) 2-layer parts,-., ; -...; fj::l 1 fj::l , / Figure 4. Anistrpic elastic mdulus fn2 resins Since the linear elastic mdulus is anistrpic, the tensile strength wuld be expected t behave similarly. This is indeed the case as shwn fr UV pstcured samples fplymer N2 in Figure 5. While the strength is higher in the directin parallel t alignment, the strain at break is greater perpendicular t the alignment. N2 has tensile strengths f ver 60 MPa parallel t alignment and 45 MPa perpendicular. Mdulus values fr N2 are abut 3 GPa parallel t alignment and 2 GPa perpendicular. Because f the rtatable magnet, alignment can be varied n a layer t layer basis leading t cmpsite-like structures. Thus mechanical mdulus and strength can be directinally ptimized in a part; thugh the amunt f mdulus anistrpy is still much less than in fiber reinfrced cmpsites

6 , parallel 60 ' perpendicular O-F l----l strain (%) Figure 5. Typical tensile stress-strain curves fr plymer N2 after UV pstcure Pstcure Effects While the glass transitin temperature fthe green plymer is already higher than current cmmercial resins, cnsiderable advancement f the glass transitin temperature is pssible by pstcuring. Tw appraches t pstcure have been studied: UV pstcure at lwer temperature and dark pstcure at higher temperature. Figure 6a shws the effect f UV pstcure n plymer N2. Fr these samples, a 100 W mercury vapr lamp was used t pstcure with the temperature held at 150 C. The data shw that as pstcure advances, the glass transitin temperature shifts upward twards an ultimate Tg f apprximately 120 C as measured by the peak: in the lss mdulus at 1 Hz. In additin the intensity fthe glass transitin lss peak: decreases as shwn by the tan () data. This decreasing intensity f the glass transitin is an indicatin f the increasing crsslink density which results in an even stiffer netwrk abve the glass transitin. a) UV pstcure b) thennal pstcure green : *ii.im green 5min 0.2 -, 0.2 t c. g 0.1 I:i g 0.1 I:i.s.s f:a f:a L t----j =::!! a a Figure 6. Pstcure effects n N2 resin. 330

7 Figure 6b shws the effect f high temperature (250 C), dark pstcure n plymer N2 after varius lengths ftime. As in the UV pstcure, the glass transitin shifts upward, hwever the Tg's btained frm thermal pstcure can be cnsiderably higher. Instead f the 120 C ultimate Tg frm the UV pstcure, thermal pstcure can result in Tg' s as high as 150 C fr plymer N2 and 105 C fr plymer N1. Hwever, while the glass transitin becmes weaker and brader during UV pstcure (i.e. the tan 0 peak), the thermal pstcure results in glass transitins that are just as intense as in the green part. This indicates that with a high temperature thermal pstcure, the increase in Tg may arise frm a different chemical reactin than ccurs in the UV pstcure. Linear Expansin Just as the aligned plymer samples have anistrpic mechanical mdulus and strength, they als have anistrpic thermal expansin. Figure 9 cmpares the linear expansin f plymer N2 with the cmmercial resin, SL5170. In the directin perpendicular t the alignment (i.e. in the y directin), the linear expansin f N2 lks much like the cmmercial plymer, with the linear thermal expansin cefficient ging frm apprximately 110 mm/mc belw Tg t 220 mm/mc abve Tg. In the directin parallel t alignment, the liquid crystal plymer has a very small psitive cefficient f expansin belw Tg which changes t a negative cefficient f expansin abve the glass transitin regin. This type frespnse has been cited in the literature fr ther LC thermsetting systems[5]. Thus, mlecular alignment results in anistrpic thermal expansin, and by varying the layer t layer alignment, cmpsite like structures can be manufactured t directinally ptimize the linear expansin prperties , :-:::;;,-." '--' 1.5 a u l:l.r;: 0.5 is N2, parallel t i----t---j """'f l Figure 7. Linear expansin fn2 liquid crystal resin and XB5170 cmmercial resin. 331

8 CONCLUSIONS Tw liquid crystal mnmers were prcessed in a table-tp sterelithgraphy apparatus. A magnet placed utside the vat aligned the mnmer befre phtplymerizatin. FI'IR spectrscpy shwed that the alignment was lcked in by the phtplymerizatin, and that the amunt f rder in the plymer was relatively independent f plymerizatin temperature. The aligned plymer specimens shw anistrpic mechanical and physical prperties. Glassy state elastic mduli f samples tested parallel and perpendicular t the alignment directin differ by a factr f tw. The tensile strength shws a similar anistrpy. Linear expansin is similar t that f the cmmercial SLA resins in the directin perpendicular t alignment, while in the directin parallel t alignment, the expansin cefficient is very small and even becmes negative at temperatures abve the glass transitin. In additin t the anistrpic prperties, these new sterelithgraphy resins shw high glass transitin temperatures. Green specimen TgS ranged frm 50 t 82 C while pstcured specimens had TgS ranging up t 150 C. ACKNOWLEDGMENTS The authrs are pleased t acknwledge that this research was supprted by the U.S. Natinal Science Fundatin thrugh NSF Grant # DMR , frm the Plymer Prgram, Divisin f Materials Research. Dr. Andrew Lvinger was prgram manager. Frm the University fdaytn, we als thank Mary Galaska and Katy Weaver fr assistance in sample preparatin and setting up experiments. REFERENCES 1. Ullett, J.S., Schultz, J.W., Chartff, R.P.. (1997) "Advanced High Temperature Resins fr Sterelithgraphy," Prceedings, 1 h Intematinal Cnference n Rapid Prttyping. 2. Brer, D.J., Ml, G.N., Challa, G. (1991) "In Situ Phtplymerizatin f an Oriented Liquid-Crystalline Acrylate, 5, Influence fthe Alkylene Spacer n the Prperties f the Mesgenic Mnmers and the Frmatin and Prperties f Oriented Plymer Netwrks," Makrml. Chem., 192, Ullett, J.S., Chartff, R.P., Schultz, J.W., Bhatt, J.C., Dtrng, M., Pgue, R.T., (1996) "Lw Shrinkage, High Tg Liquid Crystal Resins fr Sterelithgraphy," Slid Freefrm Fabricatin Prceedings, Univ. Of Texas, Austin, TX September 1996, Herskwits, V., Meller, A. (1993) "Translatin fthe RP Prcess int Labratry Characterizatin f RP Materials," Prceedings, 4 th Internatinal Cnference n Rapid Prttyping, Univ. f Daytn, Daytn, OH, Barclay, G.G., McNamee, S.G., Ober, C.K., Papathmas, KJ., Wang, D.W., (1992) "The Mechanical and Magnetic Alignment fliquid Crystalline Epxy Thermsets," Jurnal fplymer Science: Part A: Plymer Chemistry, 30,

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