Storrs, Connecticut Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269

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1 Struture Property Relationship of Polyimides Based on Pyromelliti Dianhydride and Short-Chain Aliphati Diamines for Dieletri Material Appliations Aaron F. Baldwin, 1 Rui Ma, 1 Chenhen Wang, 2 Rampi Ramprasad, 2 Gregory A. Sotzing 1,3 1 The Polymer Program, Institute of Materials Siene, University of Connetiut, Storrs, Connetiut Department of Chemial, Materials and Biomoleular Engineering, Institute of Materials Siene, University of Connetiut, Storrs, Connetiut Department of Chemistry, University of Connetiut, Storrs, Connetiut Correspondene to: G. A. Sotzing (E- mail: sotzing@mail.ims.uonn.edu) ABSTRACT: Most polyolefins that are used for dieletri materials exhibit a low dieletri onstant and operating temperatures up to 70 C. Polyimides offer a means to a higher dieletri onstant material by the introdution of a polar group in the polymer bakbone and are thermally stable at temperatures exeeding 250 C. A ommon dianhydride, pyromelliti dianhydride (PMDA), is reated with various short-hain diamines to produe polymers with high imide density. Homopolymers and opolymers synthesized had dieletri onstants ranging from 3.96 to These materials exhibit a dieletri onstant twie that of biaxially oriented polypropylene and therefore a twofold inrease in apaitane as well as maintaining low dissipation fators that are aeptable for this appliation. The experimental dieletri onstants of these materials are also ompared to density funtional theory alulations and exhibit a lose relationship. VC 2013 Wiley Periodials, In. J. Appl. Polym. Si. 000: , 2013 KEYWORDS: dieletri properties; polyimides; struture-property relations Reeived 10 January 2013; aepted 26 February 2013; published online DOI: /app INTRODUCTION There is a urrent push by the industrial omplex to move away from traditional pneumati, hydrauli, and mehanial devies to eletrohemial ones whih are ontrolled eletrially. 1 Thus, there is a need for a high-energy storage system to operate these new devies. The best solution to this problem is the inorporation of high-energy density apaitor banks in whih the eletrial energy an be stored over long harging times and released as needed through short pulses. 2 For this system to operate effiiently and to redue the size of the overall apaitor bank, an insulating material is needed that possesses a large dieletri onstant while still maintaining a low dieletri loss. The two benhmark polymer dieletri materials, biaxially oriented polypropylene (BOPP) and polyvinylidene fluoride (PVDF), have dieletri onstants of 2.2 and 10, respetively. 3 6 Polyimides are attrative materials for this appliation as they have a more polar bakbone and thus a higher dieletri onstant than polyolefins. The fat that make polyimides even more desirable is that they are thermally stable at temperatures exeeding 250 C, twie the operating temperature of most ommon dieletri materials, making them able to withstand the heat generation that these apaitor banks will give rise to and there is less need for ooling the entire apaitor bank. 7 Most ommon polymers used as dieletri materials exhibit severe derease in dieletri strength around 70 C. 8 Interestingly, a searh of reent literature yields little work being done in this field. Instead, polyimides are being researhed as a possible replaement of silion dioxide in appliations suh as the insulating material in semiondutors, printed miroeletronis, and so forth Most studies of polyimides in eletroni appliations have looked at ways to redue the dieletri onstant of the material. The redution of the dieletri onstant has been ontrolled by lowering the polarizability of the polymer through modifiation of the bakbone with the inorporation of bulky, spae-filling groups, the replaement of hydrogen with fluorine atoms, or a ombination of both. The introdution of spae-filling groups, suh as aromatis, inreases the free volume and thus dereasing the dipolar and atomi polarizability, whereas fluorine replaement will redue the total polarizability through the heightening of the hydrophobiity of the polymer VC 2013 Wiley Periodials, In. WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: /APP

2 In our researh, we fous on the opposite end of the polyimide hemial spae in whih an inrease of the dieletri onstant of the polymer is sought. This is done by the polymerization of a ommon aromati dianhydride with various alkyl diamines produing an overall redution of the free volume as well as maintaining a high-imide funtional group density in the polymer bakbone. The experimental results are ombined with theoretial dieletri alulations done through density funtional theory (DFT) to give a better understanding of the struture property relationship of these polyimides. A omparison of theoretial and experimental data will then lead to a next generation of polyimides for the ones reported here. EXPERIMENTAL Materials PMDA, 1,3-diaminopropane (1,3-DAP), 1,2-diaminopropane (1,2-DAP), dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) were purhased from Aldrih Chemial (St. Louis, Missouri). Jeffamines (Figure 1) EDR-104, HK511, and D230 were provided by the Huntsman (The Woodlands, Texas). PMDA was rerystallized from aeti anhydride, whereas the diamines were used as reeived. Shim stoks were purhased from MMaster Carr (Prineton, New Jersey) with a diameter of 2 00, a thikness of , and ASTM A666 stainless steel. General Proedure for Homopolymerization A three-nek flask equipped with a Teflon TM -oated magneti stir bar was fitted with a reflux ondenser, a glass stopper, and a glass apparatus for trapping evolved water. The entire apparatus is flame dried under vauum to remove moisture and plaed under an inert atmosphere. To the flask, the appropriate amount of diamine, 0.02 ml of isopropylamine (IPAm) and 50 ml of NMP, was added. The solution was stirred for 30 min at room temperature to disperse the amine equally in solution. To the solution, the appropriate amount of dianhydride was added and this mixture was stirred at C for 2 h. One all of the solids were dissolved, the temperature was inreased to 180 C and the solution was stirred for 10 h. The polyimide was preipitated out of solution by the addition of methanol. The solvent was deanted off and the polymer was dried at 75 C in vauo. Polymer 4. FTIR (m 1 ): 1774, 1717, 1664, 1395, and Differential sanning alorimetry (DSC) results: T g ¼ not observed; T m ¼ not observed. Thermogravimetri analysis (TGA) (10 C min 1 ): N 2 (onset): 258 C. Polymer 7. 1 H NMR ((CD 3 ) 2 S¼O, ppm): d ¼ 1.23 (m, 12H), 2.94 (m, 11H), 4.23 (m, 2H), 8.11 (m, 2H). FTIR (m 1 ): 2976, 2878, 1771, 1716, 1460, 1380, 1355, 1265, 1105, and 917. DSC results: T g ¼ 75 C; T m ¼ not observed. TGA (10 C min 1 ): N 2 (onset): 262 C. General Proedure for Copolymerization A three-nek flask equipped with a Teflon TM -oated magneti stir bar was fitted with a reflux ondenser, a glass stopper, and a glass apparatus to trap evolved water and a rubber septum stopper. The apparatus was then flame dried under vauum to remove moisture and plaed under an inert atmosphere. To the flask, the appropriate amount of the two diamines, 0.04 ml of IPAm and 25 ml of DMF, was added. The solution was stirred for 30 min Figure 1. Struture of Jeffamines EDR-104 (top), D230 (middle), and HK511 (bottom). at room temperature to disperse the diamines equally in DMF. The appropriate amount of PMDA was added to an Erlenmeyer flask and stored under inert atmosphere. To the flask, 75 ml of DMF was added. The flask is heated slightly until all of the PMDA is dissolved. The amine solution is heated to 50 Cand the PMDA/DMF solution is then anulated into the three-nek flask. After the addition of PMDA/DMF solution is omplete, the temperature is raised to 150 C and the solution is stirred until the polyami aid is dissolved. After omplete dissolution of the polyami aid, the temperature is raised to 180 Candthesolution is refluxed for h to form the polyimide. Polymer 9. 1 H NMR ((CD 3 ) 2 S¼O, ppm): d ¼ 1.01 (m, 10H), 1.28 (m, 8H), 3.64 (m, 10H), 4.43 (m, 3H), 8.14 (m, 4H). FTIR (m 1 ): 2975, 2936, 2873, 1771, 1717, 1458, 1358, 1266, 1157, 1110, 1067, 1040, 916, 828, and 731. DSC results: T g ¼ 54 C; T m ¼ not observed. TGA (10 C min 1 ): N 2 (onset): 349 C. Polymer H NMR ((CD 3 ) 2 S¼O, ppm): d ¼ 1.08 (m, 16H), 4.18 (m, 10H), 8.17 (m, 4H). FTIR (m 1 ): 2975, 2936, 2873, 1771, 1717, 1458, 1358, 1266, 1157, 1110, 1067, 1040, 916, 828, and 731. DSC results: T g ¼ 54 C; T m ¼ not observed. TGA (10 C min 1 ): N 2 (onset): 349 C. Polymer H NMR ((CD 3 ) 2 S¼O, ppm): d ¼ 0.93 (m, 10H), 1.28 (m, 12H), 3.56 (m, 10H), 4.43 (m, 4H), 8.15 (m, 4H). FTIR (m 1 ): 2867, 1770, 1716, 1355, 1105, 1036, and 728. DSC results: T g ¼ 58 C; T m ¼ not observed. TGA (10 C min 1 ): N 2 (onset): 350 C. Instrumentation Solution 1 H NMR was performed on a Bruker DMX500 highresolution digital NMR spetrometer. All hemial shifts were referened to (CD 3 ) 2 S¼O ( 1 H ¼ 2.50 ppm). DSC was done with a TA instruments DSC Q series with a heating rate of 10 C min 1. Samples were sealed in an aluminum pan with an empty pan used as a referene. TGA was performed using a TA instruments TGA Q500. Dieletri measurements were made with an Imass Time Domain Dieletri Spetrometer. General Proedure for Generation of Polyimide Thin Film An appropriate amount of polyimide and DMF is added to a sintillation vial to produe a 10% wt/wt mixture. The mixture is heated until the polyimide dissolves and the solution is then filtered. The filtered solution is drop asted onto a stainless steel 2 J. APPL. POLYM. SCI. 2013, DOI: /APP WILEYONLINELIBRARY.COM/APP

3 Dieletri Properties. The insoluble polymers were pressed into a pellet to test the dieletri properties of the material with polymer 4 being ohesive enough and did not fall apart when plaed in the sample holder. Figure 3 (top) shows the dieletri spetra of polymer 4. The dieletri onstant is stable above 5.5, whereas the loss fator remains below 2% even as the temperature is inreased to 100 C. Polymer 4, exhibits a dieletri onstant that is 2.5 times larger than BOPP while still being well under the 5% dieletri loss threshold whih onstitutes apaitor failure. 23 When the longer ether hains are added instead of the short alkyl hains, the dieletri onstant is redued as there is an inrease in free volume stemming from the protruding methyl groups. The loss fator, whih is below 1% at room temperature, inreases signifiantly when the polymer is heated to T g and the instrument short iruits as soon as the polymer is heated above T g. Figure 2. Strutures of synthesized polyimides. shim stok and overed with a glass petri dish slightly opened to the air and allowed to dry overnight to produe a taky film. The shim stok with the taky film is then plaed into a vauum oven and heated at C for 24 h to evaporate all of the solvents. Isothermal TGA is run at 170 C on a dried part of the film to determine the level of solvent remaining RESULTS AND DISCUSSION Homopolymers Solubility and Thermal Properties. The homopolymers shown in Figure 2 were all synthesized from the same ondensation proedure. Polymers 4, 5, and 6 proved to be insoluble in most organi solvents with the exeption of m-resol. However, even after extensive drying under vauum, the minute amount of m- resol remaining aused the films asted from this solvent to be too ondutive. Polymers 7 and 8 proved to be more soluble stemming from the inrease in distane between rigid aromati funtionalities in the polymer bakbone. The DSC data summarized in Table I show that polymers 4, 5, and 6 do not exhibit a glass transition temperature even down to 90 C. A melting temperature is either absent or is above the point of thermal degradation and thus not observed. The inorporation of a longer alkyl ether hain between aromati imide groups gives rises to a T g but again there is an absene of a melting temperature. Copolymers As the polymers that ontain the longer ether hains are more soluble in high boiling solvents, it was deided to opolymerize them with the shorter alkyl diamines. The introdution of the shorter alkyl diamine served a twofold purpose (1) by inreasing the perent funtionality of imide there should be an inrease in dieletri onstant and (2) the loss fator should also be redued through an inrease in rigidity of the polymer, that is less response by the polymer to the swithing eletri field. Thermal Properties and Film Formation. Combining the short alkyl diamine with the long ether diamines aused a redution in the T g versus a polymer solely made with the ether linkage. The solubility of these opolymers, made with 20% of the alkyl and 80% of the ether diamines, in high boiling solvents was maintained ompared to the homopolymers and the films produed looked like a sheet of glass over the shim stok. Changing the feed of the diamines to a 50/50 mixture reated polymers with redued solubility in DMF. Instead of a smooth, lear film, a dispersion in DMF was obtained at a muh redued onentration (1 5%). Upon ooling the film, raks appeared and the film peeled off of the shim stok. Dieletri Properties. Figure 3 (bottom) shows the typial dieletri results for the opolymers. There is an inrease in the dieletri onstant with opolymers onstruted from 1,3-DAP and either Jeffamine 2 or 3. Using 1,2-DAP instead aused a redution in the dieletri onstant whih an be explained by the free volume inrease from the methyl group. Even with the inorporation of alkyl diamine, the loss fator remains on the same order as the homopolymer, whereas inreasing the temperature to the T g of the polymer does not ause as signifiant inrease in the loss as ompared to the homopolymer. Raising the temperature above the T g of the polymer auses the loss fator to inrease well above 5% at high frequenies and above 10% at low frequenies. Table I. Thermal and Dieletri Properties of Polyimides Polymer T g ( C) a T m ( C) T d ( C) b k al k 60 Hz k 1 khz tan(d) 60 Hz tan(d) 1kHz 4 d n/o n/o n/o n/o 349 TBD e n/o 350 TBD e n/o 378 TBD e a Values taken from the midpoint, b Values taken from onset, Values at room temperature, d Pressed pellet, e To be determined. WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: /APP

4 Figure 3. Temperature effets on dieletri properties. [Color figure an be viewed in the online issue, whih is available at wileyonlinelibrary.om.] Comparison to Theoretial Calulations Theoretial Calulations. Given that the synthesized polyimides have omplex strutures, it is diffiult and time-intensive to perform DFT alulations. Therefore, we onsidered a few polyimides (Figure 4) with simpler but similar strutures ompared to those synthesized polyimides. In this way, we an give a qualitative view on the struture property relationship of synthesized polyimides. To mirror the experimental work, the repeat unit of A-1 is exatly the same as synthesized polymer 4, and polymer A-3 whih is very similar to synthesized polymer 7. By onsidering A-3, we an identify the effet of inorporating methyl groups and inreasing the number of ACH2A units in the repeat unit. DFT, as implemented in the Vienna ab initio simulation pakage (VASP), was used to determine the strutural and eletroni properties of the polymers. 24 The Perdew, Burke, and Ernzerhof funtional (PBE), projetor-augmented wave frozen-ore potentials, and a utoff energy of 400 ev for the plane wave expansion of the wave funtions were used The PBE-optimized geometry was then used to determine the dieletri onstant tensor using density funtional perturbation theory (DFPT). 28 In this study, we onsidered an isolated infinite hain of the polyimides. The true dieletri permittivity of the polymer hain alone was then extrated by ombining the DFPT omputation of the superell (ontaining a signifiant amount of vauum) with effetive medium theory, using a reently developed method. 29,30 The band gap and dieletri onstant results of polyimides are shown in Figure 5 in whih the filled symbol represents the eletroni Figure 4. Repeating units of polyimides alulated using DFT. Figure 5. Band gap and dieletri onstant results. 4 J. APPL. POLYM. SCI. 2013, DOI: /APP WILEYONLINELIBRARY.COM/APP

5 dieletri onstant, whereas the open symbol is for total dieletri onstant. When hanging the linker from ACH2A to AOA, the band gap remains almost the same, but both the eletroni and the total dieletri onstant inrease, but when the linker is hanged to ACOA, both the band gap and the dieletri onstant derease. Also, in Figure 5, when omparing A-3 with A-2, we an see that inorporating methyl groups and inreasing the number of ACH2A units in the repeating unit will derease the band gap and the dieletri onstant. Polymer 4 has an experimentally higher, about a fator of 1.4, than the alulated dieletri onstant, whereas the alulated and experimental dieletri onstant is the same for polymer 7. The differene in auray for the two alulations ould be owing to how well the polymer hains are ordered as polymer 4 is a pressed pellet and polymer 7 is a thin film. The ordering of polymer hains ould be different and this differene is not taken into aount in DFT alulations. CONCLUSIONS A series of high-funtionality polyimide homopolymers and opolymers featuring PMDA with various short-hain aliphati diamines were synthesized and dieletri properties were measured. The homopolymers with the densest funtionality proved to be insoluble, whereas the soluble homopolymers had a large inrease in dieletri loss and ould not be measured at temperatures near or above T g. A redution in loss of the soluble homopolymers is aomplished by opolymerization with a shorter alkyl diamine whih also allows for the measurements to be made above T g, but again the loss is quite signifiant above this temperature. Still, the dieletri onstant of all of the polymers was higher than biaxially oriented polypropylene and possessed operating temperatures well above that of BOPP. Future work will look at proessing a film of polymer 4 from the more soluble poly(ami) aid form and then performing thermal imidization. ACKNOWLEDGMENTS This study is supported through a multidisiplinary university researh initiative (MURI) grant through the Offie of Naval Researh. The authors aknowledge JoAnne Ronzello for performing all of the dieletri measurements. A. F. Baldwin, R. Ma, and C. Wang drafted the artile from aquired data. G. A. Sotzing and R. Ramprasad gave final approval of the submitted version of the manusript. REFERENCES 1. Beah, F.; MNab, I. In IEEE Conferene on Pulsed Power, Monterey, California, USA, June 13 17, 2005, Sarjeant, W.; Zirnheld, J.; MaDougall, F. IEEE Trans. Plasma Si. 1998, 26, Barshaw, E.; White, J.; Chait, M.; Cornette, J.; Bustamante, J.; Folli, F.; Bilthik, D.; Borelli, G.; Pii, G.; Rabuffi, M. IEEE Trans. Magn. 2007, 43, Guan, F.; Pan, J.; Wang, J.; Wang, Q.; Zhu, L. Maromoleules 2010, 43, Dang, Z. M.; Wang, H. Y.; Xu, H. P. Appl. Phys. Lett. 2006, 89, Anderson, E. W.; MCall, D. W. J. Polym. Si.: Part A: Polym. Chem. 1958, 31, Rabuffi, M.; Pii, G. IEEE Trans. Plasma Si 2002, 30, MaDougall, F. W.; Ennis, J. B.; Cooper, R. A.; Bates, J.; Seal, K. In 14th IEEE International Pulsed Power Conferene Digest of Tehnial Papers, Dallas, Texas, USA, June 15 18, 2003, Hergenrother, P. M. High Perform. Polym. 2003, 15, Rebeiz, G. M. In RF MEMS: Theory, Design, and Tehnology; Wiley Intersiene: Hoboken, New Jersey, 2003, p Chen, C.-J.; Yen, H.-J.; Chen, W.-C.; Liou, G.-S. J. Polym. Si.: Part A: Polym. Chem. 2001, 49, Lee, C.; Sundar, S.; Kwon, J.; Han, H. J. Polym. Si.: Part A: Polym. Chem. 2004, 42, Gonzalo, B.; Vilas, J. L.; Brezewski, T.; Pèrez-Jubindo, M. A.; De La Fuente, M. R.; Rodriguez, M.; Lèon, L. M. J. Polym. Si.: Part A: Polym. Chem. 2009, 47, Chisa, S.; Musteata, V.; Sava, I.; Bruma, M. In Semiondutor Conferene, 2010 International, Sinaia, Romania, Otober 11 13, 2010, 2, Houghman, G.; Tesoro, G.; Viehbek, A.; Chapple-Sokol, J. D. Maromoleules 1994, 27, Mo, X.; Wang, C. Y.; Li, G.; Jiang, J. M. J. Maromol. Si. B Phys. 2012, 51, Popovii, D.; Hulubei, C.; Cozan, V.; Lisa, G.; Bruma, M. High Perform. Polym. 2012, 24, Wang, C.; Zhao, X.; Li, G.; Jiang, J. Colloid. Polym. Si. 2011, 289, Othman, M. B. H.; Ming, N. A. S.; Akil, H. M.; Ahmad, Z. J. Appl. Polym. Si. 2011, 121, Wu. S.-Y.; Yuen, S.-M.; Ma, C.-C. M.; Huang, Y.-L. J. Appl. Polym. Si. 2009, 113, Banerjee, S.; Madhra, M. K.; Kute, V. J. Appl. Polym. Si. 2004, 93, Sroog, C. E.; Endrey, A. L.; Abramo, S. V.; Berr, C. E.; Edwards, W. M.; Olivier, K. L. J. Polym. Si.: Part A: Polym. Chem. 1965, 3, MaDonald, J. R.; Shneider, M. A.; Ennis, J. B.; MaDougall, F. W.; Yang, X. H.; In IEEE Eletrial Insulation Conferene, Montreal, Canada, May 31 June 3, 2009, Kresse, G.; Furthmuller, J. Phys. Rev. B 1996, 54, Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, Blohl, P. E. Phys. Rev. B 1994, 50, Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, Baroni, S.; de Gironoli, S.; Dal Corso, A. Rev. Mod. Phys. 2001, 73, Pilania, G.; Ramprasad, R. J. Mater. Si. 2012, 47, Wang, C. C.; Pilania, G.; Ramprasad, R. Phys. Rev. B 2013, 87, WILEYONLINELIBRARY.COM/APP J. APPL. POLYM. SCI. 2013, DOI: /APP