Synthesis and characterization of novel hyperbranched poly(imide silsesquioxane) membranes

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<ul><li><p>Synthesis and Characterization of Novel HyperbranchedPoly(imide silsesquioxane) Membranes</p><p>CHUNQING LIU, NATHANIEL NAISMITH, YONGQING HUANG, JAMES ECONOMY</p><p>Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801</p><p>Received 10 May 2003; accepted 26 July 2003</p><p>ABSTRACT: A new family of hyperbranched polymers with chemical bonds between thehyperbranched polyimide and polysilsesquioxane network was synthesized by thereaction of an amine-terminated aromatic hyperbranched polyimide with 3-glyci-doxypropyl trimethoxysilane, followed by hydrolysis and polycondensation in the pres-ence of an acid catalyst. The hyperbranched poly(imide silsesquioxane) membraneswere fabricated by the casting the aforementioned polymer solution onto a NaCl opticalat, which was followed by heating at 80 C for 24 h. The membranes were character-ized by Fourier transform infrared, X-ray diffraction, thermogravimetric analysis,scanning electron microscopy, N2 adsorption and desorption, and CO2 adsorption anddesorption. The presence of covalent bonds between the hyperbranched polyimide andpolysilsesquioxane segments had a signicant effect on the properties of the mem-branes. N2 adsorptiondesorption isotherms for these membranes showed surfaceareas of 616 m2/g, whereas CO2 adsorptiondesorption isotherms showed muchhigher surface areas in the range of 106127 m2/g. 2003 Wiley Periodicals, Inc. J PolymSci Part A: Polym Chem 41: 37363743, 2003Keywords: hyperbranched; poly(imide silsesquioxane)s; polycondensation; mem-branes</p><p>INTRODUCTION</p><p>The synthesis of organicinorganic hybrid compos-ites by a solgel process is a rapidly developing eldof investigation that hasmade possible the tailoringof new materials combining the properties of inor-ganic materials and organic polymers.17 The solgel process consists of two steps, the rst being thehydrolysis of a metal alkoxide {e.g., tetraethoxysi-lane, tetramethoxysilane, or organotrialkoxysilane[RSi(OR)3, where the R group is unreactive duringthe hydrolysis]} and the second being the polycon-densation of the hydrolysis products. The polymersused as organic phases include linear polymers[poly(methyl methacrylate),8 poly(vinyl acetate),9</p><p>poly(tetramethylene oxide),10,11 polysulfone,12 andpolyimides14] and dendritic polymers1317 [e.g.,poly(amidoamine)].</p><p>There are several methods for the preparationof organicinorganic hybrids in the literature.The simplest one is to mix an organic polymerwith a silicon alkoxide, and this is followed by asolgel reaction.1821 However, in this case, un-desirable phase separation occurs at higher silicacontents, and this results in decreased mechanicalstrength. Therefore, the control of phase separationis critical to obtaining the morphologies that givethe best combinations of properties. One way ofreducing the extent of phase separation (increasingthe compatibility) and improving the mechanicalproperties is introducing covalent bonds betweenthe organic and inorganic segments.3,4</p><p>Polymeric membranes are becoming increas-ingly important for water purication and gas</p><p>Correspondence to: J. Economy (E-mail: of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 37363743 (2003) 2003 Wiley Periodicals, Inc.</p><p>3736</p></li><li><p>separation22,23 because of their low cost and theease of fabricating them into compact hollow-bermodules with high separation area/volume ratios,but they are less stable against chemicals andtemperature. In contrast, inorganic membraneshave high thermal and chemical stabilities, whichmake them attractive for separations at high tem-peratures and in aggressive environments. How-ever, inorganic membranes still have technical lim-itations and suffer from problems such as brittle-ness and lack of surface integrity. The use oforganicinorganic hybridmaterials for the design ofinnovative membranes is an open and promisingstrategy that can combine the properties of bothorganic polymeric and inorganic membranes andcontribute to solving some problems connected toeach of them. However, they have not been studiedin any great detail until now. Hyperbranched poly-mers such as polyphenylene24 and polyimide,25</p><p>which possess many open and accessible cavi-ties24,25 and can be simply prepared by the directone-step polymerization of multifunctional mono-mers, have never been explored as organic phasesin organicinorganic hybrid membranes.</p><p>In this study, new hyperbranched poly(imidesilsesquioxane) (HPIS) membranes were pre-pared with covalent bonds between the polyimideand silsesquioxane segments. First, hyper-branched polyimide (HPI)/silane precursors forthe solgel process were synthesized by the reac-tion of an epoxytrialkoxysilane and amine-termi-nated HPI (Scheme 1). The precursor was thenhydrolyzed and polycondensed in the presence ofwater and an acid catalyst to produce HPIS mem-brane materials. The HPIS membranes werecharacterized with Fourier transform infrared(FTIR), X-ray diffraction (XRD), thermogravimet-ric analysis (TGA), scanning electron microscopy(SEM), N2 adsorption and desorption,</p><p>26 and CO2adsorption and desorption.26</p><p>EXPERIMENTAL</p><p>Materials</p><p>Most of the solvents and all the reagents were ob-tained from commercial suppliers and were usedwithout further purication. 3-Glycidoxypropyltri-methoxysilane (GPTMS) was purchased fromGelest, Inc. Tris(4-aminophenyl)amine (TAPA) wassynthesized by the reduction of tris(4-nitrophe-nyl)amine with hydrazine monohydrate in the pres-ence of Pd/C (10wt%). 2,2-Bis(3,4-dicarboxyphenyl-</p><p>)hexauoropropane dianhydride (6FDA) was puri-ed by sublimation in vacuo before use. N,N-Dimethylacetamide (DMAc) was distilled underreduced pressure and dehydrated with 4- molecu-lar sieves.</p><p>Measurements</p><p>FTIR spectra were recorded on an FTS60 spec-trometer. 1H and 13C NMR spectra were recordedon a Varian Mercury 400 spectrometer operatingat 400.2 (1H) and 100.6 MHz (13C). The XRDpatterns were obtained with a Rigaku D-MAX Adiffractometer with Cu K radiation. The surfaceareas of all the membranes were determined byN2 adsorption at 77 K and CO2 adsorption at 291K, respectively, with an Autosorb-1 volumetricsorption analyzer controlled by Autosorb-1 forWindows 1.19 software (Quantachrome). All sam-ples were outgassed at 100 C until the test of theoutgas pressure rise was passed by 10 Hg/minbefore their analysis. SEM images were recordedwith a Hitachi S-4700 scanning electron micro-scope. The thermal stabilities of the polymerswere measured on a Hi-Res TA Instruments 2950thermogravimetric analyzer at a heating rate of10 C/min under nitrogen.</p><p>Synthesis of Amine-Terminated HPI25</p><p>In a 1000-mL, three-necked ask equipped with amagnetic stirrer, 5.22 g (18 mmol) of TAPA wasdissolved in 240 mL of DMAc under nitrogen toform a purple solution at room temperature.6FDA (8 g, 18 mmol) was dissolved in 120 mL ofDMAc and added dropwise to the TAPA solutionover 12 h. The reaction was allowed to continuefor an additional 10 h. m-Xylene (200 mL) wasadded to the reaction mixture, and the mixturewas heated to 150 C for 18 h with a DeanStarkapparatus. After cooling to room temperature, themixture was poured into 3000 mL of methanol,and a yellow powder was precipitated. The crudeproduct was collected by ltration, washed with500 mL of methanol, and dried in vacuo at 60 Covernight to yield 14.8 g (94.1%) of a yellow pow-der. The raw product was dissolved in 130 mL ofDMAc to make a 10 wt % solution and was lteredthrough a 0.2-m polytetrauoroethylene (PTFE)membrane lter with pressure ltration at 20 psi.The ltrate was poured into 1000 mL of metha-nol, and the precipitate was collected by ltration.The precipitate was further puried with 1 L ofmethanol in a Soxhlet extraction apparatus for</p><p>HYPERBRANCHED POLY(IMIDE SILSESQUIOXANE) 3737</p></li><li><p>the removal of any residual DMAc. After 15 h ofextraction, the powder was then dried in vacuo at50 C for 20 h. The pure product (13.2 g) wasobtained in a yield of 85%.</p><p>Synthesis of HPIS Casting Solutions</p><p>The HPIS casting solutions were prepared withamine in HPI (referring to free amine groups that</p><p>Scheme 1</p><p>3738 LIU ET AL.</p></li><li><p>were added in an excess of anhydride groups dur-ing HPI synthesis) to epoxy (in GPTMS) molarratios of 2/1 (HPIS-1), 1/2 (HPIS-2), and 1/4(HPIS-3). The HPI and GPTMS were mixed for24 h at room temperature in DMAc, according tothe reaction shown in Scheme 1. An aqueous so-lution of HCl (0.15 M) was added in a stoichio-metric amount (GPTMS/H2O 1:3), and the so-lution was mixed for an additional 24 h at roomtemperature and for 2 h at 80 C. The chemicalstructure of the nal product (HPIS) in solution isshown in Scheme 1.</p><p>Preparation of the HPIS Membranes</p><p>The HPIS casting solution (HPIS-1, HPIS-2, orHPIS-3) was ltered through a 0.2-m PTFEmembrane lter. The ltrate was cast onto thesurface of a NaCl optical at and dried at 80 C inan air oven for 24 h. The resulting membranes (anHPIS-1 membrane from an HPIS-1 solution, anHPIS-2 membrane from an HPIS-2 solution, andan HPIS-3 membrane from an HPIS-3 solution)were detached from the NaCl optical at via dip-ping in H2O and were further dried at 150 C for12 h in vacuo. Scheme 2 illustrates the fabricationprocedure.</p><p>RESULTS AND DISCUSSION</p><p>Syntheses</p><p>Recently, a new kind of HPI has been reported inthe literature.25 The highly rigid structure of thisaromatic HPI is favorable for the formation ofopen and accessible cavities, which result in bet-ter solubility in solvents than the correspondinglinear ones and may enhance the gas-separationproperties. However, these HPIs have poor lm-forming ability because of the lack of chain entan-glement. Okamoto et al.27 successfully preparedHPI membranes with difunctional organiccrosslinking agents. Nevertheless, the crosslink-ing reaction rate should be controlled carefully toavoid crosslinking that is too fast between thedifunctional crosslinking agents and polyimide,which will result in an insoluble gel before lmsare formed. Thus, using a difunctional crosslink-ing agent in which the two functional groups havedifferent reactivities may effectively solve thisproblem.</p><p>In this work, novel HPIS membranes were pre-pared via a solgel process with 3-glycidoxypro-</p><p>pyl trimethoxysilane (GPTMS) as the crosslink-ing agent for the amine-terminated aromatic HPI.Tough HPIS membranes were obtained easily inthree steps without the formation of an insolublegel before lms were formed because of the pres-ence of the GPTMS crosslinking agent, which hastwo functional groups, epoxy and trimethoxysi-lane, in one molecule with different reactivities.In the rst step, the amine terminal groups onHPI reacted readily with the epoxy group onGPTMS in anhydrous DMAc at room tempera-ture, whereas the trimethoxysilane group onGPTMS was unreactive during this reaction pro-cess, as shown in Scheme 1. The resulting HPISprecursor had good solubility in DMAc. Three dif-ferent HPIS precursor solutions were preparedthrough changes in the molar ratio of GPTMS toHPI, as listed in Table 1. The second step was thehydrolysis of trimethoxyl groups on the HPIS pre-cursor in the presence of water and with HCl asan acid catalyst, followed by polycondensation at80 C for 2 h. The formed HPIS polymer withsilsesquioxane segments remained soluble inDMAc. The last step included the casting of theHPIS solution onto a NaCl optical at for thepreparation of the HPIS membrane (Scheme 2).Finally, the HPIS membrane was obtained by thecuring of the lm on NaCl at 80 C for 24 h and itsdetachment from the NaCl support.</p><p>Scheme 2</p><p>HYPERBRANCHED POLY(IMIDE SILSESQUIOXANE) 3739</p></li><li><p>The introduction of polysilsesquioxane seg-ments into HPI macromolecules through covalentbonds facilitated the formation of homogeneoussoft and exible HPIS membranes. In addition,tertiary amine moieties formed from the reactionbetween the amine and epoxy groups could favor-ably interact with CO2,</p><p>28 and this may play animportant role in the future application of HPISmembranes for CO2/N2 gas separation. As sum-marized in Table 1, both HPIS-1 and HPIS-2membranes with lower silsesquioxane contentswere transparent and yellow because of thecrosslinking of the globular hyperbranched mac-romolecules with silsesquioxane networks bychemical bonds. In contrast, an HPIS-3 mem-brane prepared from GPTMS and HPI with amolar ratio of 4 to 1 (epoxy to amine) was opaqueand yellow, although it was still a tough mem-brane. A possible reason is that all the amineterminal groups (from theoretical calculations) onHPI could completely react with the epoxy groupson GPTMS with a molar ratio of 1 (amine) to 2(epoxy). Thus, when too much GPTMS was used,homopolymerization occurred in the excessGPTMS in addition to polycondensation betweenthe HPIS precursor and GPTMS. The scatteringof large polysilsesquioxane homopolymer do-mains, which had phase-separated, resulted inthe opaqueness of the membrane. Both HPIS-2and HPIS-3 membranes were soft and exible.However, the HPIS-1 membrane was relativelybrittle, possibly because of the lack of enoughsilsesquioxane crosslinking segments among theHPI globular segments. The HPIS-2 membranehad the best properties among the prepared mem-branes because of the existence of a suitable con-tent of silsesquioxane crosslinking segments.</p><p>Characterization</p><p>The HPIS membranes were insoluble in all theorganic solvents, and this suggested thatcrosslinking reactions occurred. The crosslinkingreaction was conrmed by FTIR measurements.</p><p>Figure 1 shows the FTIR spectra of an HPI pow-der and an HPIS membrane (HPIS-2). In theFTIR spectrum of the HPIS-2 membrane [Fig.1(b)], the characteristic absorption bands of theimide group near 1783, 1720, 1381, and 722 cm1</p><p>can be observed. Moreover, the appearance of thebroad bands around 3480 cm1 indicates the for-mation of OH groups, which was due to the reac-tion between NH2 groups and epoxy groups. Ad-ditionally, Fig. 1(b) shows typical absorptionbands for SiOOOSi network vibrations at 1102cm1,29,30 indicating the occurrence of the solgelcrosslinking reaction.</p><p>The thermal stabilities of the HPIS mem-branes were studied by TGA. The TGA measure-ments of these new membranes are shown in Fig-ure 2. All the membranes showed good thermalstability. The thermal stability decreased a littlebit with an increase in the silsesquioxane contentfrom the HPIS-1 membrane to the HPIS-3 mem-brane. Nevertheless, the thermal decompositiontemperatures at a 5 wt % weight loss for all theHPIS membranes were over 300 C, and thismakes them promising candidates for separations</p><p>Table 1. Effect of the Molar Ratios of GPTMS to Amine Groups in HPI on the Properties of HPIS Membranes</p><p>MembraneMolar Ratio of</p><p>GPTMS to Amine Appearance Toughnessd-Spacing ()from XRD</p><p>HPIS-1 0.5 Transparent, yellow Relatively brittle 5.54HPIS-2 2 Transparent, yellow Soft, exible 5.37HPIS-3 4 Opaque, yellow Soft, exible </p><p>Figure 1. FTIR spectra of (a) an HPI powder and (b)an HPIS-2 membrane.</p><p>3740 LIU ET AL.</p></li><li><p>at high temperatures. The silsesquioxane seg-ments in the HPIS membranes may undergo re-organization over 180 C.</p><p>To investigate the morphology of the...</p></li></ul>