Journal of Membrane Science 278 (2006) 83–91

Silica filled poly(1-trimethylsilyl-1-propyne) nanocomposite membranes:Relation between the transport of gases and structural characteristics

Kristien De Sitter a,b,∗, Petra Winberg c, Jan D’Haen d,e, Chris Dotremont a, Roger Leysen a,Johan A. Martens b, Steven Mullens a, Frans H.J. Maurer c, Ivo F.J. Vankelecom b

a Flemish Institute for Technological Research (VITO), Boeretang 200, B-2400 Mol, Belgiumb Centre for Surface Chemistry and Catalysis (COK), Faculty of Bioscience Engineering, Katholieke Universiteit Leuven,

Kasteelpark Arenberg 23, B-3001 Leuven, Belgiumc Department of Polymer Science & Engineering, Lund Institute of Technology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

d Institute for Materials Research, Hasselt University, Wetenschapspark 1, B-3590 Diepenbeek, Belgiume IMEC vzw, Division IMOMEC, Wetenschapspark 1, B-3590 Diepenbeek, Belgium

Received 15 July 2005; received in revised form 19 October 2005; accepted 28 October 2005Available online 27 December 2005

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bstract

The performance of poly(1-trimethylsilyl-1-propyne) (PTMSP)/silica nanocomposites was studied for membranes with a filler content betweenand 50 wt%. An increase in permeability and a decrease in vapor selectivity was measured with increasing filler content. The free volume sizes

nd interstitial mesopore sizes of the composites were determined by use of positron annihilation lifetime spectroscopy (PALS). In addition to anncrease in large free volume size with increasing filler content, interstitial mesopores were observed in all PTMSP/silica nanocomposites. The sizef these interstitial cavities, located between the particles of a silica agglomerate, was increasing with increasing filler concentration. The presencef these agglomerates was visualized by TEM. The existence of the cavities was confirmed by nitrogen adsorption measurements. The hydrogen,itrogen and propane permeability was clearly correlated with the size of the interstitial mesopores.

2005 Elsevier B.V. All rights reserved.

eywords: PTMSP; Nanocomposites; PALS; TEM; Membrane performance

. Introduction

Over the past decades, there has been growing attention forolymer membranes in the gas separation industry, due to theirotential energy saving capacity compared with more tradi-ional separation techniques. Thanks to its unique structure androperties, poly(1-trimethylsilyl-1-propyne) (PTMSP) is one ofhe most studied polymers for applications in this field [1–4].espite its glassy nature at room temperature, PTMSP shows

he highest known permeability for several gases. This high per-eability can be attributed to the extremely high free volume

f the polymer matrix. Even more important, PTMSP is moreermeable to large condensable vapors than to small perma-ent gases. This reversed selectivity makes PTMSP membranesttractive for several industrial gas separations, like the removal

∗ Corresponding author. Tel.: +32 14 335614; fax: +32 14 321186.E-mail address: kristien.desitter@vito.be (K. De Sitter).

of higher hydrocarbons from hydrogen streams and the recoveryof organic vapors from process streams [5].

To be able to compete with conventional gas separationmethods, a gas separation membrane has to manifest simulta-neously high permeability and selectivity. Usually, the increasein gas permeability leads to a decrease in selectivity and viceversa [6]. Attempts to overcome this fundamental limitation ledto the synthesis of so-called mixed matrix membranes. In thistype of membranes, inorganic particles like zeolites and carbonblack are incorporated in polymer matrices. During the lastdecades, many articles about the addition of molecular sieves torubbery and glassy polymers, especially polydimethylsiloxane(PDMS) and polyimide (PI), have been published [7–10].Despite all the attempts, development and implementation ofmixed matrix membranes is hindered, mainly due to difficultiesin preparing mixed matrix membranes without defects at theparticle–polymer interface.

A few years ago, remarkable results were obtained with thedispersion of non-porous nanoparticles in high free volume

376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2005.10.046

84 K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91

polymers [11]. Until that moment, non-porous particles wereonly successfully added to polymers to enhance conductivity,mechanical toughness and optical and catalytic activity. Almostevery attempt to add such non-porous inorganic particlesto improve the membrane performance led to reduction ofpermeability without an important change in selectivity. Thisdecrease in permeability is caused by an increase in the diffusionpath length that penetrants experience when they traverse themembrane. In contrast with this theory, Merkel et al. reportedan increase in permeability by adding nanostructured fumedsilica to several glassy high free volume polymers [12–14].This increase resulted in their opinion from the capacity ofthe silica particles to disrupt the polymer chain packing andto increase the free volume available for molecular transport.For the high free volume polymer poly(4-methyl-2-pentyne)(PMP) also an increase in vapor selectivity was reported[14]. For PTMSP, opposite results on selectivity have beenpublished [12]. According to Gomes et al., two different typesof transport can occur in filled high free volume polymers[15]. The incorporation of fumed silica can increase the freevolume without creating non-selective defects or it can createfree volume elements large enough to permit non-selectiveKnudsen transport. Which mechanism prevails, is determinedby the original polymer free volume.

In the light of the promising but not fully understood increasein permeability and sometimes selectivity, it is important to gainmmdmnAetmaPpbnfi

2

2

UCmhsddd

2.2. Membrane preparation

Dense films of pure PTMSP were prepared by casting atoluene solution (5 wt%) on a glass plate. The films were driedat ambient conditions for 4 days. To reach a total evaporation ofthe solvent, this step was followed by further drying under vac-uum at 80 ◦C for 2 h. The cast membranes were around 100 �mthick.

Hydrophobic TS-530 silica particles were used as filler for thesystem. Mixed matrix membranes were prepared by using a threestep solvent casting procedure. First, 10–50 wt% silica (based onpolymer) was dispersed in toluene at room temperature applyingultrasonic treatment for 30 min followed by magnetic stirring for3 h. Secondly, the PTMSP was dissolved in the silica/toluenedispersion by stirring with a magnetic stirrer for 4 days. Afterthis, the solution was cast on a glass plate and dried as describedfor the pure PTMSP. Films with a thickness between 70 and100 �m were obtained.

Pure and mixed matrix films were stored under atmosphericconditions. Before using them for permeability measurements,the membranes were kept in methanol for at least 24 h to recoverthe original permeability [17]. The permeabilities reported inthis study are the coefficients measured 3 days after removalfrom methanol. At that moment, the first strong decrease of thepermeability was finished and the permeability nearly stabilized.

2

fipwmmdpsl

P

wta1

r

B

α

bpta

ore insight in the structure and performance of such mixedatrix membranes. Recently, our research team published a

etailed report about the free volume in silica filled PTMSPembranes [16]. Interstitial mesopores were observed in the

anocomposites and in the as-received fumed silica as well.lso an increase in the mean size of the larger free volume

lements in the PTMSP matrix was observed. It seems likelyhat interstitial mesopores affect the performance of the mixed

atrix membranes. This study reports, apart from PALS results,lso permeability and selectivity measurements of silica filledTMSP. The effect of the interstitial cavities on the separationroperties is investigated and discussed. The mixed matrix mem-ranes are also further characterized with techniques such asitrogen sorption, TEM and TGA. In this way, this work is aurther contribution to the interpretation of the transport of gasesn nanostructured silica filled high free volume polymers.

. Experimental

.1. Materials

PTMSP used in this study was obtained from Gelest Inc.,SA. The fumed silica incorporated in the polymer films,abosil TS-530, was purchased from Cabot Corporation, Ger-any. TS-530 is a hydrophobic silica due to a treatment with

examethyldisilazane. The reported density is 2.2 g/cm3, thepecific surface area 220 m2/g. Based on the surface area andensity and assuming a spherical shape, the individual particleiameter can be estimated to be about 12 nm. The toluene waselivered by Merck (>99% purity).

.3. Permeability and selectivity measurements

Pure-gas transport properties of PTMSP and mixed matrixlms were determined by exposure to nitrogen, hydrogen andropane [18]. For permeability measurements, flat sheet discsith a diameter of 3.3 cm were cut from the membrane. Theeasurements were carried out at room temperature. The per-eate side was kept at atmospheric pressure, while the pressure

ifference over the membrane was 1.5 × 105 Pa (1.5 bar). Theermeate flow was measured with a digital flowmeter. Whenteady-state conditions were achieved, permeability was calcu-ated as follows [1]:

= Nl

(pf − pp)A(1)

here N is the permeate flow in cm3/s, l the membranehickness in cm, A the membrane area in cm2 and pfnd pp are, respectively, the feed and permeate pressure in.333224×103 Pa (cmHg). The permeability is expressed in bar-

er(

1 barrer = 10−10 cm3 cmcm2 s cmHg

).

The ideal selectivity of a polymer film for components A tois the ratio of the permeabilities of both pure components:

(A/B) = PA

PB(2)

Mixed-gas permeation properties of filled and unfilled mem-ranes were determined with a feed mixture containing 25 vol%ropane and 75 vol% hydrogen. The pressure difference overhe membrane is 3×105 Pa (3 bar), the permeation side was keptt atmospheric pressure. The ratio of the permeate to the feed

K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91 85

flow was less than 1%. The composition of the permeate streamis determined with a mass spectrometer (OmnistarTM, Pfeif-fer Vacuum, Asslar, Germany). The total permeate flow wasmeasured with a digital flowmeter. The permeability of eachcomponent can be calculated with the following expression:

Pmixed = xpNl

A

1

pfxf − ppxp(3)

where xp and xf are the molar fraction of the gas components in,respectively, the permeate and the feed stream [1].

The separation factor of the membrane is the ratio of thepermeabilities of the components and can be calculated by usingEq. (2).

2.4. Nitrogen sorption

Low-temperature (77 K) nitrogen adsorption/desorptionmeasurements of silica particles and of unfilled and filled mem-branes were made using the CoulterTM OmnisorpTM 100 fromMicromeritics. Samples were degassed overnight at 120 ◦Cunder high vacuum prior to analysis. Sorption equilibrium wasreached in a few hours. Nitrogen adsorption isotherms provideinformation on size distributions in the micro- (pore diameterd < 2 nm), meso- (2 < d < 50 nm) and macroporosity range. TheHorvath–Kawazoe model is used for micropore size calcula-tH

2

TtbTdFcu

2

nbiiaPT

T

pa

spherical potential well with a radius R0 = R + �R, where �R isan electron layer with a thickness of 0.166 nm [20,21].

The positron source used in this study was 22Na encapsu-lated between sheets of Kapton. The source gave a count rate of60–80 s−1 and each spectrum, which were all recorded at ambi-ent conditions, consisted of about 2.5 million counts. The spectrawere recorded using a fast–fast coincidence system with CsFcrystals. Two evaluation methods, POSITRONFIT and MELT,were used for extracting positron and positronium lifetimes andintensities from the measured spectra. POSITRONFIT fits themeasured spectra with a model function consisting of a sum ofdecaying exponentials convoluted with the resolution functionof the lifetime spectrometer plus a constant background. A five-component analysis was used to evaluate the spectra of PTMSPnanocomposites, while a four-component analysis was used forspectra of the unfilled PTMSP and fumed silica. MELT resolvesthe spectra in a lifetime distribution consisting of a number ofpeaks. The average lifetimes and the corresponding intensitiesare calculated as the mass center and relative area under thepeaks. The number of peaks is not fixed. The entropy weightwas varied between 10−4 and 10−7 and the maximum probabil-ity was chosen. The cut-off value was 10−3.

2.7. Thermogravimetric analysis

Thermogravimetric analysis (TGA) experiments were carriedosa

3

3

mTa

FaM

ions. In the meso- and macrodomain, the Barret, Joyner andalenda (BJH) method is applied.

.5. TEM and STEM

The TEM samples are cryo-ultramicrotome cut samples.hey are investigated with a Philips CM12 transmission elec-

ron microscope. The lines in the TEM images are causedy the diamond knife used during the specimen preparation.he BF images in the TEM mode allow only absorption andiffraction contrast. By using a STEM detector in a Quanta 200EG-SEM scanning electron microscope besides the absorptionontrast, also atomic number contrast is obtainable for the cryo-ltramicrotome cut samples.

.6. Positron annihilation lifetime spectroscopy

The free volume and interstitial cavity sizes in PTMSPanocomposite membranes and neat materials were investigatedy Positron Annihilation Lifetime Spectroscopy (PALS). PALSs a technique which probes the free volume cavities by measur-ng the lifetime of ortho-Positronium (o-Ps) before annihilationt the free volume sites of the polymer. The lifetime (τ3) of o-s is a direct measure of the free volume size, according to theao–Eldrup equation [19,20]:

3 = 0.5

(1 − R

R0+ 1

2πsin

2πR

R0

)−1

(4)

The Tao–Eldrup equation assumes that the free volume inolymers consists of spherical cavities with a radius R. o-Ps inspherical free volume cavity is described as a particle in a

ut in STA 449C Jupiter (Netzsch GmbH, Selb, Germany). Theamples were heated from 25 to 1200 ◦C under 50 ml/min dryir at 5 ◦C/min.

. Results and discussion

.1. Permeability and selectivity measurements

Fig. 1 presents pure-gas nitrogen, hydrogen and propane per-eability coefficients for PTMSP as function of filler content.he permeability for the three tested gases is increasing as soons more than 10 wt% of silica is added. For example, at a pressure

ig. 1. Pure-gas permeability coefficients for nitrogen, hydrogen and propanes function of filler content. The dashed line represents the prediction of theaxwell model for nitrogen.

86 K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91

difference of 1.5 × 105 Pa (1.5 bar), the nitrogen permeabilityof a membrane containing 30 wt% silica is almost 200% of thepermeability of an unfilled PTMSP membrane. Only for a mem-brane with 10 wt% silica, the permeability is lower than for theoriginal one.

The increase of permeability is in contrast with the expec-tations. The addition of non-porous nanometer scale particlestypically reduces polymer permeability, originating from anincreasing tortuosity caused by the filler particles. Also the effec-tive cross sectional area available for transport is decreasing.A frequently used model that describes this transport behavioris the Maxwell model [22,23]. Following this model, the per-meability of a composite consisting of a polymer matrix andnon-porous and impermeable fillers can be calculated with theexpression:

P = Pp1 − φf

1 + φf2

(5)

where Pp is the polymer permeability and Φf is the filler vol-ume fraction. The dashed line in Fig. 1 represents the nitrogenpermeability calculated by the Maxwell equation. Only for amembrane with 10 wt% silica, this model is followed.

Fig. 2 presents the effect of the filler particles on the mixed-gas selectivity of the nanocomposites. According to the lit-erature, the transport in high free volume PTMSP occurs bycdasi[

shTmfmb

Ff

Table 1Radii of small (r3) and large (r4) free volume elements and interstitial cavities(r5)

Φf (wt%) r3 (A) r4 (A) r5 (A)

0 3.0 5.3 –10 2.9 5.4 10.120 2.9 5.5 10.430 3.1 5.7 10.850 2.8 5.8 11.5

filler content, propane seems no longer able to block as muchhydrogen as in the unfilled polymer.

3.2. Positron annihilation lifetime spectroscopy

Table 1 summarizes the sizes of free volume elements mea-sured by PALS for unfilled and filled PTMSP membranes. PALSstudies of the unfilled membrane suggest a bimodal distribu-tion of free volume elements. This is uncommon for amorphouspolymers, but already mentioned in literature for PTMSP andother high free volume polymers [2,24,25]. The two measuredcomponents represent two kinds of free volume holes, namelylarger cages (r4) connected with channel-like holes (r3) [2,26].The values reported for pure PTMSP for both the small (3 A)and the larger free volume elements (5.3 A) are in good agree-ment with previous studies [2]. From the data represented inTable 1, it can be concluded that the mean size of the larger freevolume cavities (r4) is increasing significantly with increasingfiller content. Such an increase in free volume sizes in PTMSPupon silica addition has been reported previously by Merkel etal. and was ascribed to disruption of the polymer chain packingdue to presence of nanoparticles [12].

Fig. 3 represents the cavity size distribution of an unfilledPTMSP membrane (a) and a PTMSP membrane filled with50 wt% silica (b), both calculated with MELT. Compared withpure PTMSP, larger cavities with a radius of 11.5 A are observedia

FPc

ompetitive sorption and surface diffusion [3,4]. Due to con-ensation of the larger vapor molecules, the pores in PTMSPre blocked for smaller permanent gases, which leads to reversedelectivity. The measured propane/hydrogen mixed gas selectiv-ty for pure PTMSP agrees with values mentioned in literature1].

The data presented in Fig. 2 show a decrease in this reversedelectivity with increasing filler content. On the right Y-axis, theydrogen blocking ratio of the composite membranes is showed.he blocking ratio is the hydrogen permeability in mixed-gaseasurements divided by the hydrogen permeability measured

or pure gas [1]. For unfilled PTMSP, the blocking ratio is 0.10,eaning that 90% of the potential hydrogen stream is blocked

y condensed propane. This ratio is increasing with increasing

ig. 2. Mixed-gas propane/hydrogen selectivity and hydrogen blocking ratio asunction of filler content (feed contains 25 vol% propane and 75 vol% hydrogen).

n the filled membrane. Such mesopores are also measured ins-received silica. In pure silica, this cavity is reported to be a

ig. 3. Distribution of interstitial and free volume cavity radius in (a) a pureTMSP membrane and (b) a PTMSP membrane containing 50 wt% silica, bothalculated with MELT program.

K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91 87

Fig. 4. Nitrogen, hydrogen and propane permeability as function of the intersti-tial cavity size.

consequence of o-Ps annihilating in the interstitial cavities ofthe silica agglomerates [27]. In our opinion, the observed meso-pores in the PTMSP/silica composites have the same origin.More details are reported in a previous publication [16].

The data presented in Table 1 show that the interstitial meso-pores are observed in the whole range (10–50 wt%) of nanocom-posites. The mean radius of those mesopores is increasing from10 A for a membrane filled with 10 wt% silica to 11.5 A fora filler content of 50 wt%. This can be explained by a partialfilling of interstitial cavities in silica aggregates by polymer seg-ments. Such filling can be expected to be more efficient at lowerfiller concentration since more polymer segments are availableper filler surface area and the membrane casting solution has alower viscosity.

3.3. Effect of interstitial cavities on membrane performance

From the data presented in Table 1 and Fig. 1, the conclusioncan made that the pure-gas permeabilities of the membranes areincreasing with increasing radius of the large free volume ele-ments (r4). This confirms the correlation between permeabilityand polymer free volume mentioned in previous studies [12]. Inaddition, it seems likely that also the larger interstitial cavities(10–11.5 A in radius) have a considerable effect on the perfor-mance of the PTMSP/silica membranes. In Fig. 4, the nitrogen,hoToFgMiob

tv5

where gas transport can occur by both pore flow mechanisms andsolution-hindered diffusion [3]. Vapor selectivity is only possi-ble because the micropores are small enough for blocking of freevolume elements to occur due to sorption of condensable com-ponents [1]. It is imaginable that also the enlarged microporesin filled PTMSP membranes can be blocked, but its sure that themesopores are too large to be blocked. These interstitial cavitiesoffer a faster, non-selective route of transportation to the pene-trants, leading to a decrease in selectivity.Recently, Moore andKoros presented the importance of the organic–inorganic inter-face in mixed matrix materials [28]. In this study, they describedthe types of morphologies that are believed to be present at theinternal interfaces within such materials, especially for poly-mers filled with zeolites. Five cases and their consequence forthe transport properties were shown. In the first case, the polymerchains near the filler surface have a lower segmental mobility,leading to a rigidified layer with reduced permeability. In thesecond and third cases, there are voids at the interface with anincrease in permeability as consequence. If the voids are small(case II), the selectivity is untouched, but if the size of the voidis in the order of the size of the gaseous penetrants (case III), theselectivity is decreased. In the last two cases, the outer pores ofthe zeolites are blocked (case IV) or decreased in size (case V)due to sorption of strongly held molecules. Both cause a decreasein permeability. Moore and Koros believe that the PTMSP/silicananocomposites are a special case of the type III morphology andppts

3

mafftbppbsatTpgBbt

3

s

ydrogen and propane permeabilities are plotted against the sizef the interstitial mesopores. A clear correlation is observed.he interstitial cavities seem to have a significant positive effectn the permeability of gasses in PTMSP/silica nanocomposites.or filler contents above 10 wt%, this effect dominates over theenerally negative effect of reduced diffusivity presented by theaxwell model. For the lowest filler content, the higher tortuos-

ty seems to be the dominant factor, what is probably the sourcef the decrease in permeability. Independent measures of solu-ility and diffusivity need to be made to confirm this statement.

Also the decrease of the selectivity with increasing filler con-ent can be explained by the existence of interstitial cavities. Pre-ious studies suggested already that PTMSP, with pores betweenand 15 A in diameter can be considered as a transition material

ossess an effective void or high free volume region between theolymer and the particle. Our PALS measurements show indeedhe existence of mesopores. In our opinion, these cavities areituated between the particles of an aggregate.

.4. TEM imaging and size measurement of aggregates

With the formation of interstitial cavities as one of the deter-ining factors for membrane performance, gaining insight into

ggregate size and structure seems an important issue. There-ore, TEM analysis of the mixed matrix membrane was per-ormed. Fig. 5a shows a cross section of the membrane con-aining 30 wt% silica resulting from a STEM analysis. It cane concluded that the silica particles are homogeneously dis-ersed throughout the whole depth of the membrane. Fig. 5b–dresent a global TEM picture of the aggregates in PTMSP mem-ranes containing, respectively, 10, 30 and 50 wt% silica. Theilica particles are clearly aggregated in the polymer matrix withggregate sizes of several hundred nanometers. The concentra-ion of aggregates is increasing with increasing filler content.he higher this concentration, the more difficult it is for theolymer to fill all the space between the particles in an aggre-ate, which leads to more interstitial cavities in the membrane.ecause of the importance of the interstitial cavities in the mem-rane permeation, more research about the size and structure ofhe silica aggregates is needed.

.5. Nitrogen adsorption/desorption

Determination of the nitrogen adsorption isotherm is a clas-ical method to characterize the surface area and the pore diam-

88 K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91

Fig. 5. (a) STEM image of the cross section of a PTMSP membrane containing 30 wt% silica and (b–d) TEM pictures of PTMSP membranes filled, respectively,with 10, 30 and 50 wt%.

eters of porous materials [29]. Until now, this method is onlyrarely used for polymers, since they have mostly extremelylow specific surfaces. But because PTMSP is ultrapermeableand known as a “microporous” polymer [3,4], applying this

technique can be useful. Nitrogen adsorption isotherms of as-received silica, pure PTMSP and PTMSP filled with 50 wt%fumed silica are displayed in Fig. 6. From the shape of theisotherm of silica, one can conclude that Cabosil TS-530 con-

eived

Fig. 6. Nitrogen adsorption/desorption isotherms for as-rec silica, pure PTMSP and PTMSP filled with 50 wt% silica.

K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91 89

Fig. 7. BET specific surface as function of filler content. The dashed line rep-resents the theoretical decrease you would expect without interaction betweenpolymer and filler.

tains mainly larger mesopores. Both polymer samples take up ahigh amount of nitrogen at low relative pressures (<0.1) indica-tive of the presence of micropores. Adsorption and desorptionbranches exhibit a broad hysteresis loop. Hysteresis is associ-ated with capillary condensation in mesopores. Theoretically,the shape and the position of hysteresis loops are dependent onthe pore shape, size distribution and connectivity. For unfilledPTMSP, hysteresis occurs over the whole relative pressure rangewithout loop closure indicating unstable, shapeable porosityaltering upon adsorption. The nitrogen adsorption isotherm ofPTMSP filled with 50 wt% silica shows a type H2 hysteresisloop. This isotherm type suggests the presence of a complexpore structure made up of interconnected networks of poresof different size and shape [30]. Nitrogen adsorption shows

that the incorporation of silica particles causes a change inmembrane structure. The filler acts as a framework makingthe structure of the membrane more solid and the porositypermanent.

In Fig. 7, the BET specific surface of the unfilled and filledmembranes is plotted against the filler content. In absence ofinteraction between the polymer matrix and the silica particles,a linear decrease of the specific surface with increasing fillercontent is expected (represented by the dashed line). For mem-branes with low filler content, the measured specific surfaceis significantly lower than expected, confirming the formationof interfaces between PTMSP and silica leading to an over-all loss of specific surface area. This supports the hypothesismade in Section 3.2 about partial filling of interstitial cavitiesby polymer segments. For the higher filler contents, less poly-mer segments are available per silica surface, with less fillingcapacity and a specific surface closer to the theoretical one asconsequence.

For determining the mesopore size distribution, the BJHmethod is used. Fig. 8 presents a detail of the BJH curve of bothunfilled and 50 wt% filled PTMSP. The unfilled sample displaysa wide variation of small mesopore sizes. The pore size distri-bution according to BJH analysis of the desorption branch ofthe isotherm of the silica filled sample displays a peak at 3.8 nmpore diameter, corresponding to the step in the isotherm at P/P00.42 (Fig. 6). Nitrogen desorptions at this pressure are due totmtdsb

d PTM

Fig. 8. Mesopore size distribution of unfille

he tensile strength effect [31], limiting the application of theethod to pore diameters of 3.8 nm and wider. It is concluded

hat the silica filled membrane contains extra mesopores with aiameter less than ca. 3.8 nm or 38 A. This interpretation is con-istent with the existence of the holes in the composite measuredy PALS.

SP and PTMSP containing 50 wt% silica.

90 K. De Sitter et al. / Journal of Membrane Science 278 (2006) 83–91

Fig. 9. TGA thermogram of pure PTMSP and several filled samples.

3.6. Thermal stability

The thermal properties of pure PTMSP and PTMSP/silicananocomposites are characterized by DSC and TGA. Fig. 9presents a TGA thermogram of pure PTMSP and several filledmembrane samples. For all samples, thermal degradation hap-pens in two stages. The first degradation step reaches its highestrate around 300 ◦C, the second step starts at a temperatureabove 400 ◦C. Following literature, the thermal PTMSP degra-dation in air begins at the carbon–carbon double bonds in themain chain [2]. Several kinds of carbonyl groups are producedon the degraded chain ends. As degradation continues, vari-ous bonds are broken to form carbonyl groups and siloxanelinkages. The same happens with PTMSP/silica nanocompos-ites. At 750 ◦C, all the polymer is degraded. The residues afterpolymer degradation are the silicon oxides originated fromthe silylgroups in PTMSP and the fillers added to the poly-mer matrix in mixed matrix membranes. The beginning tem-peratures of the decomposition are determined from the DSCcurves and listed in Table 2. The onset of thermal degra-dation of PTMSP, an exothermal process, is situated around155 ◦C. The PTMSP sample filled with 50 wt% silica startsto decompose around 185 ◦C. Such retarding effect of parti-cles on the decomposition of polymers is a known phenomenon[32,33].

The silica contents in the PTMSP/silica nanocomposites aredmbg

TT

4. Conclusions

PTMSP membranes filled with different concentrations offumed silica are studied. The addition of the hydrophobic sil-ica to the polymer matrix leads to an increase in permeabilityand a decrease in selectivity. Also the free volume of the mem-brane is changed: the large free volume elements increase insize and interstitial cavities are formed. The size of the pores isincreasing with increasing filler content. In our opinion, the inter-stitial cavities are situated between the particles of an aggregate.The existence of aggregates in the mixed matrix membranes isconfirmed by TEM images. Besides the earlier mentioned cor-relation between the change in large free volume size and theincrease in permeability, our data confirm that also the meso-pores have an important influence on the performance of themembrane. A clear correlation between the mesopore size andthe permeability is observed. The presence of mesopores in themixed materials explains also the decrease in selectivity com-pared with unfilled PTMSP membranes. Nitrogen adsorptionmeasurements confirm the presence of extra porosity in thehighly filled membranes. Finally, thermal analysis of filled andunfilled membranes shows that the fumed silica has a retardingeffect on polymer decomposition.

Acknowledgements

PaWeGsBoar1

R

etermined from the residual weight at 750 ◦C in the TGA ther-ograms and summarized in Table 2. The standard deviation

etween the different measurements is small, indicating a homo-eneous dispersion over the whole membrane.

able 2hermogravimetric analysis of filled and unfilled PTMSP membranes

Φf (wt%) Onset degradation(◦C)

Real filler content(wt%)

Standard deviation(wt%)

0 157 – –10 161 10.3 1.120 – 19.6 0.330 169 30.7 0.950 185 49.9 0.5

The authors would like to thank John Algers for help with theALS measurements and for fruitful discussions. K. De Sittercknowledges the VITO for a grant as doctoral research fellow.im Bouwen, Louis Willems and Jos Cooymans are acknowl-

dged for their help with the experimental setup at VITO andina Vanbutsele for performing the nitrogen adsorption mea-

urements at the COK. The COK research was supported by theelgian Federal Government in the frame of an IAP-PAI grantn Supramolecular Catalysis and the Flemish Government forgrant from the Concerted Research Action (GOA). The TEM

esearch was (partly) performed in the framework of the projects.2.13/D2/841 and 1.2.14/PO/841.

eferences

[1] I. Pinnau, C.G. Casillas, A. Morisato, B.D. Freeman, Hydrocar-bon/hydrogen mixed gas permeation in poly(1-trimethylsilyl-1-propyne)(PTMSP), poly(1-phenyl-1-propyne) (PPP), and PTMSP/PPP blends, J.Polym. Sci., Part B: Polym. Phys. 34 (1996) 2613.

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