Synthesis and Characterization of Novel HyperbranchedPoly(imide silsesquioxane) Membranes

CHUNQING LIU, NATHANIEL NAISMITH, YONGQING HUANG, JAMES ECONOMY

Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801

Received 10 May 2003; accepted 26 July 2003

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 opticalflat, 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 significant effect on the properties of the mem-branes. N2 adsorption–desorption isotherms for these membranes showed surfaceareas of 6–16 m2/g, whereas CO2 adsorption–desorption isotherms showed muchhigher surface areas in the range of 106–127 m2/g. © 2003 Wiley Periodicals, Inc. J PolymSci Part A: Polym Chem 41: 3736–3743, 2003Keywords: hyperbranched; poly(imide silsesquioxane)s; polycondensation; mem-branes

INTRODUCTION

The synthesis of organic–inorganic hybrid compos-ites by a sol–gel process is a rapidly developing fieldof investigation that has made possible the tailoringof new materials combining the properties of inor-ganic materials and organic polymers.1–7 The sol–gel process consists of two steps, the first being thehydrolysis of a metal alkoxide {e.g., tetraethoxysi-lane, tetramethoxysilane, or organotrialkoxysilane[R�Si(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

poly(tetramethylene oxide),10,11 polysulfone,12 andpolyimides1–4] and dendritic polymers13–17 [e.g.,poly(amidoamine)].

There are several methods for the preparationof organic–inorganic hybrids in the literature.The simplest one is to mix an organic polymerwith a silicon alkoxide, and this is followed by asol–gel reaction.18–21 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

Polymeric membranes are becoming increas-ingly important for water purification and gas

Correspondence to: J. Economy (E-mail: jeconomy@uiuc.edu)Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 41, 3736–3743 (2003)© 2003 Wiley Periodicals, Inc.

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separation22,23 because of their low cost and theease of fabricating them into compact hollow-fibermodules 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 oforganic–inorganic hybrid materials 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

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 organic–inorganic hybrid membranes.

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 sol–gel 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,26 and CO2adsorption and desorption.26

EXPERIMENTAL

Materials

Most of the solvents and all the reagents were ob-tained from commercial suppliers and were usedwithout further purification. 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 (10 wt %). 2,2-Bis(3,4-dicarboxyphenyl-

)hexafluoropropane dianhydride (6FDA) was puri-fied by sublimation in vacuo before use. N,N-Dimethylacetamide (DMAc) was distilled underreduced pressure and dehydrated with 4-Å molecu-lar sieves.

Measurements

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.

Synthesis of Amine-Terminated HPI25

In a 1000-mL, three-necked flask 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 Dean–Starkapparatus. After cooling to room temperature, themixture was poured into 3000 mL of methanol,and a yellow powder was precipitated. The crudeproduct was collected by filtration, 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 filteredthrough a 0.2-�m polytetrafluoroethylene (PTFE)membrane filter with pressure filtration at 20 psi.The filtrate was poured into 1000 mL of metha-nol, and the precipitate was collected by filtration.The precipitate was further purified with 1 L ofmethanol in a Soxhlet extraction apparatus for

HYPERBRANCHED POLY(IMIDE SILSESQUIOXANE) 3737

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%.

Synthesis of HPIS Casting Solutions

The HPIS casting solutions were prepared withamine in HPI (referring to free amine groups that

Scheme 1

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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 final product (HPIS) in solution isshown in Scheme 1.

Preparation of the HPIS Membranes

The HPIS casting solution (HPIS-1, HPIS-2, orHPIS-3) was filtered through a 0.2-�m PTFEmembrane filter. The filtrate was cast onto thesurface of a NaCl optical flat 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 flat via dip-ping in H2O and were further dried at 150 °C for12 h in vacuo. Scheme 2 illustrates the fabricationprocedure.

RESULTS AND DISCUSSION

Syntheses

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 film-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 filmsare formed. Thus, using a difunctional crosslink-ing agent in which the two functional groups havedifferent reactivities may effectively solve thisproblem.

In this work, novel HPIS membranes were pre-pared via a sol–gel process with 3-glycidoxypro-

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 films 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 first 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 flat for thepreparation of the HPIS membrane (Scheme 2).Finally, the HPIS membrane was obtained by thecuring of the film on NaCl at 80 °C for 24 h and itsdetachment from the NaCl support.

Scheme 2

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The introduction of polysilsesquioxane seg-ments into HPI macromolecules through covalentbonds facilitated the formation of homogeneoussoft and flexible HPIS membranes. In addition,tertiary amine moieties formed from the reactionbetween the amine and epoxy groups could favor-ably interact with CO2,28 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 flexible.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.

Characterization

The HPIS membranes were insoluble in all theorganic solvents, and this suggested thatcrosslinking reactions occurred. The crosslinkingreaction was confirmed by FTIR measurements.

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 cm�1

can be observed. Moreover, the appearance of thebroad bands around 3480 cm�1 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 1102cm�1,29,30 indicating the occurrence of the sol–gelcrosslinking reaction.

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

Table 1. Effect of the Molar Ratios of GPTMS to Amine Groups in HPI on the Properties of HPIS Membranes

MembraneMolar Ratio of

GPTMS to Amine Appearance Toughnessd-Spacing (Å)

from XRD

HPIS-1 0.5 Transparent, yellow Relatively brittle 5.54HPIS-2 2 Transparent, yellow Soft, flexible 5.37HPIS-3 4 Opaque, yellow Soft, flexible —

Figure 1. FTIR spectra of (a) an HPI powder and (b)an HPIS-2 membrane.

3740 LIU ET AL.

at high temperatures. The silsesquioxane seg-ments in the HPIS membranes may undergo re-organization over 180 °C.

To investigate the morphology of the preparedHPIS membranes, we carried out SEM and wide-angle XRD pattern measurements. It can be ob-served from the SEM photographs of the surfacesof HPIS membranes with different contents ofsilsesquioxane that each of these membranes hada homogeneous and continuous surface and thatno phase separation occurred; this indicated theexcellent compatibility between the polyimideand polysilsesquioxane segments. The connectionbetween the globular highly branched polyimideand polysilsesquioxane via chemical bonds ex-plains the high compatibility and absence ofphase separation. Figure 3 illustrates the wide-angle XRD patterns of HPIS membranes. All themembranes showed completely amorphous struc-tures. The d-spacing was calculated according tothe Bragg equation from the scattering angles(2�) in the center of the broad peak, as listed inTable 1. The HPIS membranes prepared withdifferent molar ratios of HPI to GPTMS showedalmost the same d-spacing values, which sug-gested the independence of d-spacing from thesilsesquioxane crosslinking segment. The inten-sity of the broad peak decreased with the increasein the crosslinking silsesquioxane content in themembranes, and this suggested that the existence

of the broad peak was due to the rigid HPI seg-ments in the membranes.

The surface areas of the prepared HPIS mem-branes were measured with both N2 and CO2adsorption isotherms. Figures 4 and 5 illustratethe representative CO2 adsorption–desorptionand N2 adsorption–desorption isotherms of theHPIS-2 membrane, respectively. Table 2 lists thesurface areas of the HPIS membranes calculatedwith both the N2 Brunauer–Emmet–Tellermethod and the CO2 Dubinin–Radushkevich

Figure 2. TGA curves of HPIS membranes with different contents of silsesquioxane:(a) HPIS-1, (b) HPIS-2, and (c) HPIS-3.

Figure 3. XRD patterns of HPIS membranes withdifferent contents of silsesquioxane: (a) HPIS-1, (b)HPIS-2, and (c) HPIS-3.

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method. Interestingly, the surface areas of theHPIS membranes measured from CO2 adsorptionisotherms (106–127 m2/g) were much higher thanthose measured from N2 adsorption isotherms(6–16 m2/g), and this suggested that a muchhigher amount of CO2 than N2 could be adsorbedby an HPIS membrane under the same pressure.One possible reason is that there were many openand accessible nanoporous cavities in the rigidHPI segments of the HPIS membrane, and thisresulted in a relatively high surface area and atype I CO2 adsorption–desorption isotherm ob-tained at 291 K (Fig. 4). However, when N2 wasused as the adsorbate, the N2 adsorption isothermwas measured at 77 K, at which temperature theopen and accessible nanoporous cavities mayhave undergone collapse or were closed by the

silsesquioxane segments on the external surfaceof the globular HPI, which gave rise to a lowsurface area and a type III N2 adsorption iso-therm (Fig. 5). Another possible reason is that theimide and tertiary amine28 moieties in the HPISmembrane favorably interacted with CO2,whereas they had very weak interactions or nointeraction with N2. This would result in a muchhigher CO2 adsorption by the HPIS membraneunder the same conditions.

HPIS membranes might have promising appli-cations in gas separation, especially for the sepa-ration of CO2 and N2, because of the presence ofHPI segments, which are known to be quite se-lective for CO2 and N2, and the tertiary amine,which could favorably interact with CO2, as wellas the silsesquioxane segments, which may con-trol the swelling of membranes during perme-ation and improve the toughness of membranes.Further work on the gas-separation applicationsof these new membranes is in progress now.

CONCLUSIONS

A new class of HPIS membranes was developedby the sol–gel reaction and characterized. Themembranes were obtained by the reaction of anamine-terminated aromatic HPI with epoxysilanes in a DMAc solution, followed by hydrolysisand condensation. The introduction of silsesqui-oxane segments into the HPI backbones led to theconnection of globular hyperbranched macromol-ecules via covalent bonds, and this improved theproperties of the membranes. The membraneswith lower silsesquioxane contents were yellowand transparent, whereas the membranes withhigh silsesquioxane contents were yellow andopaque.

FTIR analysis confirmed the chemical struc-ture of the HPIS materials. TGA results indicatedthat these membranes had thermal decomposi-

Table 2. Surface Areas of HPIS Membranes withDifferent Contents of GPTMS

Membrane

Surface Area (m2/g)

N2 BET Method CO2 DR Method

HPIS-1 6.3 106.2HPIS-2 9.8 120.6HPIS-3 15.9 126.7

Figure 4. CO2 adsorption–desorption isotherm of anHPIS-2 membrane.

Figure 5. N2 adsorption–desorption isotherm of anHPIS-2 membrane.

3742 LIU ET AL.

tion temperatures at a 5% weight loss of over 300°C in N2. SEM spectra confirmed that these mem-branes were homogeneous. CO2 adsorption–des-orption isotherms of these membranes showedrelatively high surface areas.

This work was supported by the Science and Technol-ogy Center program of the National Science Founda-tion under agreement number CTS-0120978.

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