Polymer 51 (2010) 3784e3792. Contents lists available at ScienceDirect. Polymer. journal homepage:

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1 Polymer 51 (010) 3784e379 Contents lists ville t ScienceDirect Polymer journl homepge: Influence of previous history on physicl ging in thin glssy polymer films s gs seprtion memrnes Brndon W. Rowe, Benny D. Freemn, D.R. Pul * Deprtment of Chemicl Engineering, Texs Mterils Institute nd Center for Energy nd Environmentl Resources, The University of Texs t Austin, Austin, TX 7871, United Sttes rticle info strct Article history: Received 1 April 010 Received in revised form 31 My 010 Accepted 3 June 010 Aville online 9 June 010 Keywords: Physicl ging Gs permeility Memrnes The physicl ging ehvior of thin glssy polysulfone (PSF) films (w15 nm) with different previous histories ws trcked using gs permeility mesurements. The initil sttes of these mterils were modulted y therml nneling t fixed tempertures elow the glss trnsition or y exposure to high pressure (800 psig (56. r)) CO for vrious times. Regrdless of the previous history, the nture of the ging response in these smples ws consistent with the ging ehvior of n untreted film tht ws freshly quenched from ove T g, i.e., permeility decresed nd pure gs selectivity incresed with ging time. However, the extent of ging-induced chnges in trnsport properties of these mterils depended strongly on previous history. The ging ehvior ws descried using Struik s ging model y llowing the initil conditions to depend on ech smple s previous history. Ó 010 Elsevier Ltd. All rights reserved. 1. Introduction All non-equilirium glsses, including glssy polymers, undergo physicl ging, process tht cuses property chnges s the mteril spontneously relxes towrd n equilirium stte, nd the influence of these chnges cn e profound. Physicl ging hs een studied extensively for ulk mterils to id in understnding nd predicting their long-term ehvior [1,]. More recently, ttention hs turned to thin glssy films where significnt nd unpredicted devitions from ulk ehvior hve een reported [3e5]. For exmple, the physicl ging ehvior of thin glssy polymer films hs een shown to depend on film thickness, exhiiting gretly ccelerted ging in films elow w1 micron in thickness [5]. Interest in thin glssy films is due to their growing pplictions in microelectronics, seprtion memrnes, opticl mterils, lithogrphy, etc. While studies show intriguing devitions from ulk ehvior in thin films, the polymer physics underlying thin film phenomen, including ging, re not well understood [6e8]. The stte of glssy polymer nd, correspondingly, its ging ehvior, depend not only on the immedite environment, ut lso on the mteril s previous history (i.e., therml, stress, vpor exposure, etc.) [9e11]. This sensitivity to processing conditions nd smple preprtion is known to cuse discrepncies in reported * Corresponding uthor. Tel.: þ ; fx: þ E-mil ddress: drp@che.utexs.edu (D.R. Pul). properties of glssy polymers [1]. For instnce, the rte t which polymer is cooled from ove the glss trnsition temperture, T g, hs profound effects on the mteril s physicl properties [13]. These differences rise from the greter displcement from equilirium in smples rpidly quenched from ove T g s compred to slowly cooled smples. As n exmple, Cngilosi et l. showed higher initil free volume nd ccelerted physicl ging in polycronte with incresing cooling rtes from ove T g [14]. The ccelerted ging in these mterils ws ttriuted to the initilly higher free volume stte of the rpidly quenched smples s compred to the slowly cooled smples, which creted lrger driving force to equilirium nd thus incresed the rte of ging. After polymer vitrifies, vrious conditioning tretments re known to lter its thermodynmic stte. For instnce, in experiments with ulk glssy polymers, exposure to high pressure CO cuses the glssy polymer structure to swell, nd upon removl of the CO, the system is unle to immeditely return to its initil stte, thus, it hs lower density thn efore the exposure [11,15e17]. While the effects of this conditioning tretment re not immeditely reversile, the polymer my return to its originl stte y physicl ging over time (i.e., chnges due to conditioning re only semi-permnent). This effect cn lso e cused y the uptke of vpors nd liquids including hexne nd wter vpor [18e1]. Conversely, therml nneling t elevted tempertures, yet elow the glss trnsition, increses the density of glssy polymers [11,]. Mechnicl stresses cn lso lter the stte of glssy polymers nd ffect ging ehvior [3]. A study y Berens nd Hodge illustrtes the effects of pplied stress nd vpor exposure efore /$ e see front mtter Ó 010 Elsevier Ltd. All rights reserved. doi:1016/j.polymer

2 B.W. Rowe et l. / Polymer 51 (010) 3784e ging of poly(vinyl chloride), s proed y differentil scnning clorimetry (DSC) [9]. Enthlpy relxtion ws enhnced similrly y the exposure to mechnicl (tensile nd compressive) nd sorptive stresses [9]. While these effects hve een demonstrted in ulk polymer smples, it is not well understood how thin films respond to the sme conditioning tretments. Since previous history hs strong influence on glssy polymer properties, it is importnt tht ging studies employ smples with identified nd reproducile histories. Typiclly, controlled quench from ove the glss trnsition temperture is used to define n experimentl strting time. This technique ws used in previous studies on physicl ging in thin films; however, ctul pplictions of these mterils involve much more complex histories. For exmple, thin glssy polymer films used in gs seprtion memrnes re creted through n intricte phse inversion process, nd they my undergo dditionl tretment steps efore pckging nd testing cn egin. Additionlly, myrid of storge times nd conditions exist efore these mterils rech their ultimte destintion for use in the field. Becuse of the complex nd illdefined history of glssy polymers in vrious pplictions, it is criticl to understnd how ging ehvior chnges s function of the mteril s previous history to ccurtely predict their long-term performnce. As n initil step towrd understnding how previous history ffects the physicl ging ehvior of thin glssy polymer films, vriety of heting nd conditioning tretments were pplied to thin polysulfone (PSF) memrnes (w15 nm in thickness) efore trcking their ging ehvior. The influences of nneling t vrious tempertures elow T g nd conditioning with high pressure CO for vrious times on the physicl ging ehvior of thin PSF films were studied using gs permeility mesurements. These tretments were pplied fter well-defined quench from ove the glss trnsition to ensure the histories were reproducile. The histories considered in this investigtion were not chosen to mimic ny prticulr ppliction conditions, ut rther to produce rnge of strting points for the ging studies. The nture of the ging response ws similr cross the rnge of histories studied; however, the mgnitude of ging-induced chnges ws sensitive to previous history. Additionlly, exposure to high pressure CO ccelerted physicl ging in comprison to unconditioned films. A previously developed model tht illustrtes ging ehvior in thin glssy films, sed on work y Struik, ws used to descrie the experimentl results. To the est of our knowledge, this report contins the first systemtic study on the influence of previous history on ging ehvior in thin glssy films.. Experimentl methods Polysulfone (PSF), sed on isphenol-a, from Solvy Advnced Polymers (UDEL PSF-3500 NT LCD) ws used s received in this study. PSF ws chosen ecuse it is widely used gs seprtion memrne mteril, nd its T g (186 C) is well ove the temperture of use, so the mteril is deep in the glssy stte during use [5,4]. Thin polymer films (w15 nm) were prepred y spin csting 3 wt.% PSF solutions in cyclopentnone onto silicon wfers t 1000 rpm for 60 s. A vrile ngle spectroscopic ellipsometer mnufctured y J.A. Woollm Co., Model 000D, ws used to mesure film thickness. The presence of microscopic pinhole defects, which form with incresing frequency s film thickness is reduced, cn led to difficulties in studying the gs trnsport properties of thin polymer films [5]. While smll frction of trns-memrne defects my hve little influence on studies using other techniques such s ellipsometry or fluorescence spectroscopy, these defects msk the ntive selectivity nd permeility chrcteristics of mteril under study y llowing gs to pss through the defects in the memrne vi convective trnsport rther thn y the solution-diffusion mechnism tht controls smll molecule trnsport through dense polymeric memrnes [6]. A defect frction s smll s 10 6 on n re sis is enough to prevent memrne from performing gs seprtion [7]. A previously developed coting technique ws pplied to circumvent this prolem [8]. After spin-coting thin glssy film of morphous PSF, lyer of highly permele, ruery poly(dimethylsiloxne) (PDMS) ws coted directly on top of the glssy film. The PDMS overcot lyer ws creted y spin csting PDMS solution in cyclohexne directly onto the glssy film, which ws supported on silicon wfer. The PDMS solution consisted of Dehesive 940A with proprietry crosslinker (V4) nd ctlyst (OL) system provided y Wcker Silicones Corportion, Adrin, MI; cyclohexne ws dded to crete 1 wt. % solution of polymer. The filmws then nneled on hot plte t 110 C for 15 min to crosslink the PDMS nd remove residul solvent. The thickness of the PDMS lyer (w3 mm) ws mesured using Dektk 6 M stylus profilometer. The PDMS lyer locks convective flow through ny pinhole defects of the glssy lyer nd, t the experimentl conditions, PDMS is w160 C ove its T g, so its properties do not chnge with time due to physicl ging [4]. According to the series resistnce model, the PDMS lyer ccounts for less thn 4% of the totl mss trnsfer resistnce though the films in this study [9]. Additionlly, the ging response of PSF ws previously shown to e unffected y the presence of the PDMS lyer [8]. A similr coting technique ws developed for hollow fier memrnes tht initilly enled the industril development of these mterils for gs seprtion pplictions [7,30]. The two lyer film ws then lifted from the silicon support using thin metl wire frme nd heted 15 C ove the ulk T g of PSF for 0 min to erse the therml history of the glssy film nd define strting point for the ging studies. More detils regrding the smple preprtion techniques were descried y Hung nd Pul [5]. After the quench from ove T g, smples were either tested immeditely (i.e., the se cse), nneled for prescried period of time t specific temperture under nitrogen purge, or conditioned with CO. All films used in this study were 15 nm thick PSF coted with PDMS. The nneling temperture rnged from 90 C to 170 C, nd the nneling time rnged from to 160 h. The smples conditioned with high pressure CO (800 psig (56. r)) t 35 C were ged for 500 h t 35 C in dry environment prior to the conditioning tretment. After the CO conditioning tretment, the CO ws removed slowly (over 40e50 min) through fine control needle vlve to prevent foming or cooling in the smple nd to prevent dmge to the delicte thin film. This procedure minimized the chnges due to physicl ging over the time scle of the conditioning tretment. If the smples were conditioned with CO immeditely fter the quench from ove T g, the influence of conditioning time on ging properties would e complicted y the rpid structurl relxtion tht occurs immeditely following vitrifiction. Scholes et l. recently reported tht the concentrtion of CO in PSF ws pproximtely 3 times higher t 15 C thn t 75 C t 58.8 psig (5.1 r) [31]. Although this pressure is considerly lower thn the pressure used in the conditioning tretment, the results illustrte the influence of temperture on soluility. Becuse soluility is temperture dependent nd influences the effects of conditioning, it is importnt to specify the temperture of the conditioning tretment, 35 C in this cse [11]. Gs permeility coefficients for O,N, nd CH 4 were determined using stndrd constnt volume, vrile pressure method [3]. Mesurements were conducted t 35 C, with n upstrem pressure of tm nd mximum downstrem pressure of 10 torr. When not eing tested, smples were ged in dry environment (i. e., <10% reltive humidity) t 35 C.

3 Results nd discussion 3.1. Influence of nneling time B.W. Rowe et l. / Polymer 51 (010) 3784e379 of lrger molecules more thn for smller molecules. Consistent with the lower permeility in the nneled films s compred to the se cse, the initil selectivities re higher in the nneled films. Fig. 1ec present the O,N, nd CH 4 permeility coefficients s function of ging time for thin PSF films tht hve een nneled t 170 C for up to 160 h following quenching from ove T g. After nneling, the films were stored t 35 C in dry environment etween mesurements. All films disply decresing permeility with ging time, consistent with the loss of frctionl free volume during physicl ging. The initil permeility nd ging ehvior of the films nneled for h t 170 C nerly mtches tht of the freshly quenched se cse. Further nneling time t 170 C decreses the initil permeility nd slows the ging process s compred to the se cse, s expected due to the reduced free volume stte of the nneled mteril, which lowers permeility nd decreses the driving force to rech equilirium. After w1000 h of ging, the O permeility of the films nneled t 170 C for 160 h prior to ging decresed y 4% while the O permeility decresed nerly 40% in the se cse. The O /N nd N /CH 4 pure gs selectivities re shown s function of ging time in Fig. nd, respectively, for the films nneled t 170 C. Selectivity increses with physicl ging due to the loss of excess free volume, which cuses the mteril to ecome more size selective, primrily y reducing the diffusion coefficients 3.. Influence of nneling temperture The influence of ging time on the O,N, nd CH 4 permeility coefficients of thin PSF films fter nneling for 160 h t 90 C, 150 C, nd 170 C re presented in Fig. 3ec. The ging ehvior is similr in ll films, showing decresing permeility with ging time. As might e expected, the initil permeility nd extent of permeility decline decreses with incresing nneling temperture prior to ging. The chnges seen fter nneling t 90 C nd 150 C re similr to one nother, while nneling t 170 C cuses more significnt chnges in the permeility nd ging response s compred to the se cse. Fig. 4 nd show the O /N nd N /CH 4 pure gs selectivities of the nneled films s function of ging time, respectively. Selectivity increses with physicl ging in ll cses, indicting the smples ehve s defect free films. Agin, consistent with the lower permeilities of the films nneled t higher tempertures, the initil selectivities re higher thn in the se cse. After long enough periods of time, the permeility dt seem to e converging to common permeility versus time ehvior. These results show tht the nture of the ging response is similr Oxygen permeility (Brrer) Anneled t Nitrogen permeility (Brrer) Anneled t c Methne permeility (Brrer) 0.6 Anneled t Fig. 1. Influence of ging time t 35 C on () O, () N, nd (c) CH 4 permeility coefficients in thin PSF films nneled for specified time t 170 C fter quench from ove T g.

4 B.W. Rowe et l. / Polymer 51 (010) 3784e Anneled t Anneled t O /N Selectivity N /CH 4 Selectivity Aging time fer nneling (hr) 0.7 Aging time fer nneling (hr) Fig.. Influence of ging time t 35 C on () O /N, nd () N /CH 4 idel selectivity in thin PSF films nneled for specified time t 170 C fter quench from ove T g. in ll films considered; however, the extent of ging-induced chnges depends on the nneling tretments pplied prior to ging t 35 C. The reduced ging rtes t 35 C for the nneled films, s compred to the se cse, re likely relted to the decresed driving force for physicl ging in these mterils. These findings re consistent with Berens nd Hodge s oservtion tht while the development of endothermic peks in DSC scns (i.e., evidence of enthlpy relxtion) fter ging ws completely Anneled for 160 hours Anneled for 160 hours Oxygen permeility (Brrer) Nitrogen permeility (Brrer) c 0.6 Anneled for 160 hours Methne permeility (Brrer) Fig. 3. Influence of ging time t 35 C on () O, () N, nd (c) CH 4 permeility coefficients in thin PSF films nneled for 160 h t specified temperture fter quench from ove T g.

5 3788 B.W. Rowe et l. / Polymer 51 (010) 3784e379 O /N Selectivity Anneled t for 160 hours N /CH 4 Selectivity Anneled for 160 hours Fig. 4. Influence of ging time t 35 C on () O /N, nd () N /CH 4 idel selectivity in thin PSF films nneled for 160 h t specified temperture fter quench from ove T g. generl feture of the ging process, the mgnitude of the oserved peks were strongly dependent on previous history [9] Influence of conditioning with CO In ddition to therml nneling elow the glss trnsition, the stte of glssy polymer cn e modified y conditioning with high pressure CO. Fig. 5ec present the impct of ging on O,N, nd CH 4 permeility coefficients of thin PSF films fter conditioning with 800 psig (56. r) CO for, 40, nd 160 h t 35 C; the unconditioned se cse is included for comprison. Prior to the conditioning tretment, the films were ged for 500 h t 35 Cto minimize the influence of physicl ging during the tretment process. As in ll other films studied, permeility decreses with ging time. The lower permeility in the CO conditioned films, s compred to the se cse with no CO exposure, results from the decrese in permeility tht occurred during the 500 h of ging prior to the conditioning tretment. The initil permeility fter conditioning for or 40 h ws similr to tht of unconditioned films ged for 500 h; however, the initil permeility fter conditioning for 160 h ws slightly lower thn the ged se cse. Although the permeility decrese fter 160 h of conditioning is not fully understood, it could e cused y physicl ging during the conditioning tretment. Furthermore, this permeility decrese in thin films fter long-term exposure to CO is consistent with preliminry results from ongoing studies in our lortories (dt not shown). The ehvior of these films following the conditioning tretment my not mtch predictions sed on ulk ehvior; however, it is not well documented how these processes ffect thin film properties. While outside the immedite scope of this study, ny devitions from ulk ehvior indicte opportunities for dditionl investigtions to etter understnding thin film phenomen. The permeility coefficients were ll clculted ssuming constnt thickness during ging nd conditioning. Although film thickness increses slightly upon exposure to high pressure CO, the chnge is negligile in comprison to the permeility chnges mesured nd experimentl error. After exposure to 800 psig (56. r) CO for 40 h, film thickness incresed y w1% in thin PSF films, s mesured using ellipsometry. Fig. 6 nd show the O /N nd N /CH 4 pure gs selectivities, respectively, of the conditioned films s function of ging time. Selectivity increses with physicl ging in ll cses, s expected. The initil selectivities of the conditioned films re higher thn in the se cse, mtching their initilly lower permeility coefficients. To illustrte the similrity in the ging response of the freshly quenched nd conditioned films, Fig. 7 presents the reltive O permeility coefficients of these films over time, normlized y the permeility t 1 h of ging. Despite the initilly lower solute permeility coefficients of the conditioned films, the reltive permeility decline is nerly identicl to tht of the se cse. These results re consistent with vrile energy positron nnihiltion lifetime spectroscopy (PALS) study on the influence of ging nd conditioning on the free volume properties of thin PSF films [33]. In the PALS study, ging of conditioned nd unconditioned films ws shown to proceed similrly, s indicted y decresing free volume element size with ging. Additionlly, the o-ps intensity, which is relted to the numer of free volume elements in mteril, ws lower in the CO conditioned films thn in the se cse. This result is likely relted to the lower permeility of the conditioned films s compred to the unconditioned cse. While the reltive ging ehvior is similr in these films, the mgnitude of ging induced permeility chnges is greter fter the conditioning tretment thn in the unconditioned cse. Fig. 8 shows the ehvior of these films sed on the totl time elpsed fter the quench from ove T g nd illustrtes the more rpid ging of the films fter CO conditioning s compred to the unconditioned se cse. In the se cse, the chnge in permeility mesured fter 500 h of ging is quite smll for the next severl hundred hours; however, fter exposure to high pressure CO for time s short s h, permeility declines more rpidly with ging time thn would hve occurred without this CO tretment, e.g., the se cse. After initilly ging for 500 h, the O permeility coefficient in the se cse decresed y w Brrer over then next 500 h; upon conditioning with CO for h, the O permeility coefficient decresed y w Brrer over the sme 500 h period. Kim et l. hve previously shown tht permeility decreses more quickly with time in thin films fter exposure to CO thn ged films not exposed to CO [34]. The qulittive similrities nd quntittive differences of the oserved ging ehviors depending on previous history ws lso reported y Alcoutli et l for the ging response of epoxy resins with different previous histories [17] Modeling considertions A modified version of Struik s ging model ws used to mthemticlly chrcterize the influence of previous history on

6 B.W. Rowe et l. / Polymer 51 (010) 3784e Oxygen permeility (Brrer) Nitrogen permeility (Brrer) c Methne permeility (Brrer) 0.6 Fig. 5. Influence of ging time t 35 C on () O, () N, nd (c) CH 4 permeility coefficients in thin PSF films conditioned with 800 psig (56. r) CO fter quench from ove T g nd 500 h ging t 35 C. physicl ging in thin PSF films. The model descries the rte of physicl ging s function of the deprture from equilirium nd relxtion time, which depends on the system conditions nd the current stte of the mteril, given y: ddf dt ¼ Df s ¼ Df s N expð gdf Þ where Df is the excess frctionl free volume (i.e., the difference etween the frctionl free volume in the polymer, f, t time t nd (1) O /N selectivity N /CH 4 selectivity Conditioned with CO (800 psig) 5.0 Conditioned with CO (800 psig) 0.7 Fig. 6. Influence of ging time t 35 C on () O /N, nd () N /CH 4 idel selectivity in thin PSF films conditioned with 800 psig (56. r) CO fter quench from ove T g nd 500 h ging t 35 C.

7 3790 B.W. Rowe et l. / Polymer 51 (010) 3784e379 P ¼ Ae B=f () Reltive oxygen permeility (P/P 0 ) Fig. 7. Influence of ging time t 35 C on reltive O permeility coefficients in thin PSF films conditioned with 800 psig (56. r) CO fter quench from the ove T g nd 500 h ging t 35 C. The reltive oxygen permeility coefficients re sed on the permeility t 1 h of ging time. the frctionl free volume in the fully relxed, equilirium stte, f e ), Df ¼ f f e, s is the relxtion time t time t, s N is the equilirium relxtion time (i.e., t t / N), nd g is constnt chrcterizing the sensitivity of relxtion time to excess frctionl free volume. Frctionl free volume, f, is relted to the polymer s specific volume, v, yf ¼ (v v 0 )/v, where v 0 is the occupied volume of the polymer nd cn e estimted y the Bondi method (i.e., v 0 ¼ 1.3v w ), where v w is the vn der Wls volume clculted using group contriution methods [35]. The originl Struik model ws developed for ulk polymers, where physicl ging is independent of smple dimensions, so in the modified model, s N ws llowed to depend on film thickness to cpture thickness-dependent ging ehvior. The model ws solved numericlly using MATLAB softwre. Using the frctionl free volume, f, given y the model, gs permeility ws clculted s function of time from the following correltion: where A nd B re constnts sed upon oxygen permeility mesurements in smples of ulk thickness from the literture, 397 Brrer nd 0.839, respectively [36]. To cpture the influence of su-t g nneling on ging ehvior, model prmeters reported for the ging of 15 nm PSF films t 35 C were used, i.e., s N, g, nd f e [8]. The initil frctionl free volume, f i, ws llowed to vry with the films nneling tretments to mtch their initil permeility coefficients. Thus, the vlue of f i used in the model represents the mteril s stte fter the nneling tretment nd is considered the pprent initil frctionl free volume; differences in this vlue from the se cse re not ment to suggest tht the initil stte immeditely fter the quench from ove T g is different in these smples. The model prmeters, including the pprent initil frctionl free volumes, used to descrie the ging ehvior of the nneled films re shown in Tle 1. The equilirium frctionl free volume t 35 C, f e,ws clculted sed on extrpoltion of the experimentl pressurevolume-temperture dt in the melt stte to the ging temperture [37]. Fig. 9 compres the model predictions to the experimentl dt for films nneled t 170 C for vrious times. Anneling t 170 C for h did not significntly influence the initil permeility coefficients or ging ehvior of the thin PSF films, so this tretment is descried using the se cse prmeters. After nneling t 170 C for 40 h, the initil O permeility coefficient decresed y Brrer s compred to the se cse nd the consequent ging ehvior mtches the model predictions. Anneling for 160 h t 170 C lowered the initil O permeility coefficient y 0.7 Brrer s compred to the freshly quenched film. While susequent ging ehvior is similr to the model predictions, the model under-predicts permeility loss with time, suggesting tht some free volume chnges t the higher temperture does not influence permeility in the sme wy s ging t 35 C. The mteril s free volume stte t 170 C, which is 16 C elow T g, is very different thn t 35 C. Conceivly, relxtions fr elow T g influence permeility nd ging differently thn high temperture relxtions. While eyond the scope of this study, understnding this ehvior would require proing the free volume distriution s function of nneling tretments. Films nneled for 160 h t 90 C nd 150 C disply similr ehvior to the cse of nneling for 40 h t 170 C; therefore, these model predictions re not included for revity. CO exposure Aging t 35 C Aging t 35 C 6.8 CO exposure Aging t 35 C Aging t 35 C Oxygen permeility (Brrer) O /N selectivity Totl ging time (hr) 5. Totl ging time (hr) Fig. 8. Influence of totl elpsed ging time fter quench from ove T g on () O permeility coefficients nd () O /N pure gs selectivity in thin PSF films conditioned with 800 psig (56. r) CO. Line drwn to guide the eye.

8 B.W. Rowe et l. / Polymer 51 (010) 3784e Tle 1 Model prmeters. s N (s) Apprent f i f e g h t 170 C Y 56 Y Y 160 h t 170 C 498 e40 h t 800 psig CO h t 800 psig CO To cpture the ging ehvior of the CO conditioned films, the vlue of s N ws llowed to vry with the conditioning tretment. If the se cse equilirium relxtion time ws used, s in the nneled films, the model would predict very little chnge in permeility with ging time fter the conditioning tretment. To mtch the experimentl dt, the relxtion times were shortened fter the CO conditioning, consistent with the pprent restrting of the ging process in the conditioned films. This restrting of the ging process is consistent with the de-nneling effect of CO conditioning discussed y Chn nd Pul [11]. The model prmeters used re listed in Tle 1. Good greement etween the model nd the experimentl ging dt for the CO conditioned films is shown in Fig. 10. Similr ehvior is seen in the films conditioned for nd 40 h, nd the sme model prmeters were used to descrie their ehvior. According to this modeling scheme, the equilirium relxtion time decreses y e3 orders of mgnitude fter the CO conditioning tretment. This enhnced moility fter exposure to CO is consistent with n increse in the verge free volume element size upon conditioning with CO, s suggested y the results of the PALS study. The pplied model does not directly incorporte ny informtion regrding to the distriution of free volume element size; rther vlue tht represents the totl free volume in the mteril is used. Thus, if the free volume distriution is ltered, modifiction of other model prmeters my e needed to ccount for the chnged ehvior. In comprison with smll free volume elements, lrger free volume elements will ge more rpidly due to their greter devition from equilirium. If the free volume elements Oxygen permeility (Brrer) Anneled t Fig. 9. Influence of ging time t 35 ConO permeility coefficients in thin PSF films nneled for specified time t 170 C fter quench from ove T g. Lines generted from the model. Oxygen permeility (Brrer) sizes re incresed y CO conditioning, s suggested y the PALS results, the rte of ging is expected to increse. This ccelerted ging, which is relted to shift in the free volume distriution, is cptured y the current modeling tretment y decrese in the equilirium relxtion time, s N. The ility of the model to descrie the ging ehvior of thin glssy films with vriety of previous histories illustrtes the rod pplicility of the model nd the similrities in the ging responses of these smples. 4. Conclusions The influence of previous history on the physicl ging ehvior of thin PSF films ws trcked using gs permeility mesurements. Similr ging ehvior ws seen in ll films, regrdless of previous history, i.e., permeility decresed nd selectivity incresed with ging time. Anneling films elow T g reduces the initil permeility nd slows the rte of physicl ging y reducing the driving force to equilirium, s compred to film freshly quenched from ove T g. Exposure to high pressure CO fter ging for 500 h cuses the ging process to ccelerte in comprison to the se cse, consistent with previous results from vrile energy PALS study. The ging ehvior of these films with vrious previous histories is well descried y modified Struik model. The utility of this work is to shed light on the influence of initil properties on physicl ging ehvior, not to investigte the ging ehvior under the tretment conditions considered, where more informtion is needed for full understnding. Devitions from expected ging ehvior s function of experimentl conditions highlight the importnce of further investigtion regrding influences on deprture from ulk ehvior in these thin films. Acknowledgements Fig. 10. Influence of ging time t 35 ConO permeility coefficients in thin PSF films conditioned with 800 psig (56. r) CO fter quench from ove T g nd 500 h ging t 35 C. Lines generted from the model. This reserch ws supported y Air Liquide/MEDAL nd the Ntionl Science Foundtion Science nd Technology Center for Lyered Polymeric Systems (Grnt DMR ).

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