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. c .V C H V e r l a g s g e s e l l s c h a f tm b H , W e i n h e i m / B e r g s t r . Registered names. trademarks. etc. used in this journal. even without specific, indications thereof, are not to be considered unprotected by law. Printed in the Federal Republic of Germany

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/NNERIAtS 'let'hnol. 1 9 9 3/.1 . 6 1 5 . [9] J. P. ('anselier. J. Dispersion Sti. [ 1 0 ] J . M . D i d v m u s . P . O l i v e r . S . M a n n . A . L , Dc\ rics.P.V. Flauschka. P. Westbroek. J. Chent. Soc., Farttduv'l'rtrtts.1 9 9 3 . , 1 9 . 2 , 3 9 1 . [ 1 1 ] R . J . D a v e y , S . N . B l a c k . L . A . B r o m l c r. B . C o t t i e r .B Dobbs. J. E,. Rout. Nalrrre 1991.-15-1. 549. [ 1 2 ] S . M a n n . J . M . D i d y m u s . N . P . S a n d c r s o n .B . R . H e y w o o d . E . J . A . Samper,J. Chem. Soc., Furutlut''l'nttts / 1990.,96.1873. [ 1 3 ] L . A d d a d i . Z . B e r k o v i t c h - Y c l l i n . I . \ \ ' c i s s b u c h .J . r , , a nM i l . . L . J . S h i mcrn, M. Lahav, L. Liserorl'itz...lrrgt,rr.('ltt'nr.. lnt. f-,t1.Encl. 1985.24. 466. [14] J. Rudolph. J. Patsch.W. H. Nl*e r. Crtllttids Surt.A 1994.86.299. [15] J.Wang.S. K.Varshner'.R. Jcnrnrc. P lerssie. J. Polt,m. Sti. A 1992.30. 2251. [ 1 6 ] H . K a w a g u c h i , H . H i r a i . K . S a k a i . S . S c r a . ' l - .N a k a j i r n a . Y . E b i s a w a . K . K o v a m a .( o l l o i d I \ t l t n t . . t t l . 1 9 9 2 ). 7 0 . 1 1 1 6 . [ 7 ] A . B e r m a n . L . A d d a d i . S . \ \ ' c r n er . . \ ' u t t u ' (l 9 8 t l . . l . l / . 5 4 6 .

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Solvent-AssistedMicrocontact Molding: A Convenient Method for Fabricating Three-Dimensional Structures on Surfacesof Polymers**

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By Enoch Kint, Younun Xict, Xiao-Mei Zhao. and Georee M. Whitesides*

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"Soft lithography" is the name we use for a group of non-photolithographic methods for forming high-quality micro- and nanostructures.These strategies require remarkably little in capital investment. Thev are becoming attractive as routes to micro- and nanostructuresthat cannot be made (or cannot be made as easilr,)bi' photolithog r a p h y .M i c r o m a c h i n i n g . l rm l i c r o c o n t a c tp r i n i i n S ( p C p ) . 1 : l embossing (or imprinting;,t:1micromoldine in capillarics ( M I M I C ) . [ ] l m i c r o t r a n s f e rm o l d i n g ( p T M ) . 1 5 1a n c l r c p l i c a moldingl6a l r e a f e w e x a m p l e so f s u c h m e t h o d s t h a t h a v e been used to fabricate (in some cases.to manufacture) structures on the sub-micrometer scale. More recentlr'. embossinslTland replica moldinglslhzivebeen extencleclas proceduresto form patterned polymeric nanostructuresas small as 2-5nm in lateral dimension.These nanometer-sized structuresare beyond the capability of conventionalphotolithography: currently they are fabricated using advanced lithographic techniques[']-for example. deep UV photolithographv.X-ray lithography,electron-beamwritins. and scanningprobe lithography-that are still in development. Here we report a new method for fabricating structures and modifying surface morphologies of polymers. usins a method rve refer to as "solvent-assisted microcontactmoldi n g " ( S A M I M , s e e F i g . 1 ) . T h e o p e r a t i o n a lp r i n c i p l eo f t h i s technique shares characteristicswith both embossins and

[*l

[**]

Prot. G. M. WhitesidesD . r. E. Kim. Dr. Y. Xia. X.-M. Zhao D c p u r t m c n t o i C h e m i s t r ya n d C h e m i c a l B i o l o s r Harr ard L.'nive rsitl' l 2 O r l o r c l S t r e e t .C a m b r i d g e .M A 0 2 1 3 8( U S A ) This rescarch wns supported in part bv the Officc of Naval Research. t h e A d r a n c c d R e s e a r c hP r o j e c t sA g e n c v .a n d t h e A R O M u l t i - d i s c i p l i , nan L'nirersin Research Initiativcs (DAAH04-95-l-0102). It uscd M R S E C - S h a r e d F a c i l i t i e ss u p p o r t e db v t h e N a t i o n a l S c i e n c eF o u n d a tion (D\1R-9.1(X)-196). We thank Prof. L. Sohn at Princeton Universitv 1 o r k i n d h p r o r i d i n g t h e m a s t e ru s e df o r F i g u r e 4 .

I

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F i g . l . S c h e m a t i cp r o c e c l u r cl i r r S A M I I \ 1 .

r e p l i c am o l d i n g .S A M I M b e g i n sw i t h a t . v p eo f e l a s t o m e r i c m o l d ( u s u a l l vm a d e f r o m p o l v ( d i m e t h y l s i l o x a n e )P. D M S ) r'sl that has becn useclin much of our previous work.ll'1 In S A M I M . a l i q u i d t h a t i s a g o o d s o l v e n tf o r t h e p o l y m e r i c s u b s t r a t ei s a p p l i e c tl o t h e s u r f a c eo f t h e P D M S m o l d u s i n g Q-tips before it is bnrusht into contact with the polymer s u r f a c e .A s t h e m o l d i s p l a c e d o n t h e s u r f a c eo f t h e p o l y m e r . t h e s o l v e n t d i s s o l v e s( o r s w e l l s ) a t h i n l a v e r o f t h e p o l y m e r , a n d t h e r e s u l t i n g( p r o b a b l y g e l - l i k e ) f l u i d c o m prising polymer and solvent conforms to the surfacetopology of the mold. The polymer solidifiesas the solvent dissip a t e s . l l 0wl h i l e t h e m o l d m a i n t a i n sc o n f o r m a l c o n t a c t w i t h the substrate.The overall morpholoev of the patterned polymer representsa negativereplica of the relief patterns on the PDMS mold. The key element of this procedure is wetting of the PDMS mold by a solvcnt and conformal contact between t h e e l a s t o m e rm o l d a n d t h e s u b s t r a t e( F i g . l ) . T h e l i q u i d solvent fills the recesseclregions on the surface of the PDMS mold in order to minimize the area of the liquid/vapor interface and maximize that of the solid/liquid interface. For an illustration. we have concentratedon a comm e r c i a l l ya v a i l a b l ep h o t o r e s i s (t M i c r o p o s i t 1 8 1 3 .S h i p l e y .a positive-tonenovolak resin). but we have applied this procedure (with different solvcnts) to many other polymers. including polystvrene (PS), poly(methvlmethacrylate) ( P M M A ) , c e l l u l o s ea c e t a t e ( C A ) . a n d p o l y ( a c r y l o n i t r i l e b u t a d i e n e - s t y r e n e()A B S ) . A t h i n f i l m o f p h o t o r e s i s t( P R )

A VCH Verlagsgesellschof't mbH. D-69469 Weinheint, 1997

09-15-9618/q7/0806-0651 $ 17.50+.50/0

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/UATERIATS

was prepared on a Si wafcr by spin-coatingat 5500 rpm for 3 0 s . a n d t h e n b a k e d f o r 3 . 5 m i n a t l 0 - 5 " C tt h c r e s u l t i n s f i l m w a s - 1 . 2 p m t h i c k . P D M S ( S y l g a r d1 8 4 .D o w C o r n i n g ) m o l d s h a v i n g r c l i e f p a t t e r n s( - 1 . 2 p r m t h i c k ) o n t h c i r s u r faces were fabricated according to publishcd proceclures.ll'rt'l The PDMS mold was wetted with cthanol. w h i c h f i l l e d t h e r e c e s s c cr le g i o n so n t h e P D M S s u r f a c c . l l r l When the mold was placedon the film of PR. thc eltrstor.ner made conformal contact with the PR. forminq micrometers c a l ec h a n n e l s( o r c a v i t i e s )b e t w e e nt h e m . B c c a u s eP D M S i s c o m p l i a n t .t h e e l a s t o m c ra d h e r e d s p o n t a n e o u s l yt o t h c surfaceand squeezcdout the excessethanol front thosc reg i o n so f t h e m o l d t h a t w e r e i n c o n t a c tw i t h t h c P R . T h e r e m a i n i n g s o l v e n t d i s s o l v e d( o r s w e l l e d ) t h e P R p o l y , m e r . a n d a p a t t e r n c o m p l e m e n t a r yt o t h a t o n t h e m c t l d w a s formed in the resultingpolvrnericfluid bv a processsimilar t o e m b o s s i n g( o r i m p r i n t i n g ) .I ' h e m o l d w a s a l l o w e d t o r c main on the surfacefor --5 rnin at room tentperature until m o s t o f t h e s o l v e n t h a d d i s s i p a t e dT. h e s t a n t p w a s p c e l e c l away: a pertterncdmicrostructureof PR was lcl't behinclon the surfnccof the substrate.We have useclthe same PDMS m o l d f o r s e v e r a lc v c l e s( > 1 0 )o f t h i s p r o c e s sw i t h o u t o b s er ving a dccreasein pcrforntance. C h o o s i n gt h e a p p r o p r i a t es o l v e n tf o r a p o l v r n e r i cm a t c r i a l d e t e r m i n e st h e e f l ' c c t i v e n e sas n c l s u c c e s so f S A M I M . T h e s o l v c n ts h o u l dd i s s o l v eo r s w c l l t h e s u r f a c eo 1 ' t h cp o l v m c r : i t s h o u l d n o t . h o w e v c r .s w e l l t h c P D M S s t a m p a n c l therebv deslroy the conformzrlcontact between the polvm e r i i n d t h e s t a m p .M a n V n o n - p o l a rs o l v e n t s( f o r e x a m p l e . h e x a n e .t o l u e n e .a n c l m c t h v l e n e c h l o r i d e ) c a n n o t b e u s c d i n S A M I M b e c a u s ct h e v s w c l l t h e c r o s s - l i n k e cPl D M S . l r r l I n g e n e r a l .t h e s o h c n t s h o u l c lh a v c a r e l a t i v e l vh i g h v a p o r p r e s s u r ea n d a m o d e r a t el r , h i g h s u r f a c et e n s i o n( f o r e x a m p l e . m e t h z r n o lc. t h a n o l . a n c l a c e t o n e )t o e n s u r er a p i d e v a p o r a t i o n o 1 ' t h ee x c e s ss o l v e n ta n c lr n i n i n t a ls r v e l l i n go f t h e P D M S m o l d . D v e s a n d i n r t r g a n i cs a l t s c a n a l s o b c a c l c l e c l i n t o t h e s o l v e n t s .a n d s u t r s c q u c n t l rl r c r n c o r p o r a t e ciln t o t h e f o r m e d p o l y m r - - r i cn t i c r o s t r u c t L r r cS s .o l r c n t s u i t h l o u v a p o r p r e s s u r e s( f o r e x a n r p l e e. t h v l c n eg l r c t t l a n r l t l i n t et h r l s u l f o x i d e )a r e n o t w e l l s u i t e d f o r S A M I N I . S o l r c n t s n i t h h i g h s u r f a c et e n s i o n s( f o r e x a m p l e .w a t e r ) c l o n o t u o r k u t ' i r l l .h e c a u s et h e y o n l v p a r t i i r l l vw e t t h e P D M S s u r l ' i r c. 1 e1 F i g u r e 2 s h o w ss c a n n i n sc l e c t r o nm i c r o e r e r p h(sS E M s ) o 1 ' representative s t r u c t u r e st h a t w e r e f o r m e d b y S A M I M : a l l structureswere fabricatedfrom spin-coiitedfilms of PR with e t h a n o l a s t h e s o l v e n t .F i g u r e 2 A a n d C r e p r e s e n tp o s i t i v e a n d n e g a t i v es t r u c t u r e so f t h e s a m ep a t t e r n :e a c hw a s f a b r i c a t e du s i n gt h e a p p r o p r i u t cs t a m pc o n t a i n i n ga c o m p l e m e n tarv pattern. Figure 2B shor,vsan oblique view of the fract u r e d e d g eo f t h e p a t t e r ni n F i g u r e 2 , A :i t s h o w sw c l l d e l ' i n e d c o r n e r s a n d t h e p r e s e n c eo f a t h i n f i l m o n t h o s e r e s i o n s where the PDMS mold contacted the surftrceof PR. Figurc 2D shows a more complex pattern taken frclm a nticroe l e c t r o n i c sc i r c u i t : i t i l l u s t r a t e st h a t t h e t h r e e - d i m e n s i o n a l information on thc surfaceof the PDMS mold (Fie. 2E) can be faithfully transferredto the polymer layer in a singlestep.

652

I r s I \ I ) ) S I r \ l s ( ) l ' t c s ls l r u c l u r c sl o r n r c c il n t h i n f i l n r so f n o v o l l k p h o t o r r ' \ i \ tu \ i n s S ' \ \ l l l \ , l . r r i t h c t h a r t oa l s l h c s o l ' n c n tt.: ) S I : N , lo f t h e P D M S n t o l r l L r \ . r l ( ) g c n c r i r t ct h c s a r n n l cs h o r r ' ni n ( D ) .

A c o m m o n c h a r a c t c r i s t i co f p a t t e r n e d m i c r o s t r u c t u r e s src s c n c r a t c du s i n gS A M I M i s t l r a t t h c r c s u l t i n gs t r u c t u r e a l c l j o i n c c l b v a t h i n l ' i l n r o 1 'p o l l ' n r c r . F o r e x a m p l e .s t u r s h a P c csl t r u c t u r e si n F i q u r c 2 ( ' a r e c o n n c c t c c lt o o n c a r . r o t l r c r b v a t h i n I ' i l m o l ' P R t h a l i s . u s u a l l r 't.h i n n c r t h a n l ( X )n r l . W e h a v e b e e n a b l e t o r c n r o v c t h c s c t h i n f i l n t s u s i n s r c a c t i v e i o n e t c h i n g ( R I E ) . F i g l r r ci A s h o u s t h c

51tm I ; i g . . 3 .' \ ) S [ : \ l o l r l c s t p i r t l c n ro l P R f o r n r e du s i n c S ' \ \ l l \ l o n r r r r ] i e o r r n ' a f e r c o r c l c t l l r r t h e r n r a l l r 'l i r r n r c c lS i ( ) r ( 0 . . 1; r r l t h i c k ) . I l ) \ l \ l r , 1 t h i s -I'hc s a r n p l cl l t c r ' [ { l F - l i r r 5 r n i n . ( ' ) s a n t es a n t p l cl r : i n { [ ] ) r r l t e ' rl u r t h c r c t c h i n gi r . tl c l u c o u sI I F l o r - l n r i n . a n c li n a n a q u c o l l sK ( ) l l ] l . r , , l . r r n oslo l u t i o n l i r r ' . ' 1 0r l i n a t 6 5 ( ' . T l t c P R s t r u c l u r c sr l i s s o l r c t ol r . l i l t t t l r r l l i n t h c -fhc K C ) H / l - p r o p i t n o l s o l u t i o t ta t t h i s t c n r p c r a t u r c . S i ( ) - r r n i . k l r i r sl r c c n r c nroveca l ftcr Si ctching.

a9 VC.II VerIu,q.s.qcseIlschuI't tttbI{. D-69169 WeirrIteitrr.I997

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/UATERIATS c r o s s - s e c t i o n aSl E M o f a t e s t p a t t e r n o f P R t h a t w a s f o r m c c l o n S i / S i O l u s i n g S A M I M : F i . e u r e3 8 s h o w s t h e c r o s s - s e c t i o n aSlE N , Io f t h i s s a n t p l ea f t e r R I E i n a n o x v g e n p l a s m a( l f i O n t t o r r ' )f o r - 5 r n i n . F i g u r e 3 C s h o w st h e c r o s s s e c t i o n aS l E \ l o l . t h e s a m es a n t p l e a . f t e r i t h a d b e e nc t c h e d i n H F ( - 1 " , , ) f ' o r - 2 m i n . f o l l o w c db v a n i s o t r o p i cc t c h i n g in an ilqLleous KOH/2-propanol solution att 6-5"C' for - 1 0 n r i n . ' l - h c s cS E M s c l e a r l y d e m o n s t r a t et h a t t h e t h i n f i l n t sc l r r tl ' r cr c n t o v c c bl v h o n t o g e n e o u tsh i n n i n g w i t h R I E . F i c L l r e- { s h o n s a t o n t i c f o r c e m i c r o s c o p y( A F M ) i m a g c s o 1 ' t h e: n r u l l c s tf ' c a t u r e st h a t h a v e b e e n g e n e r a t e di n a t h i n l - i l n ro t P R ( \ l i c r o p o s i t 1 8 0 - 5S. h i p l e y :t h e t h i c k n e s so f t h c f i l n r u r r : - - 0 . Jp r n t ) u s i n g S A M I M : p a r a l l e l l i n c s t h a t a r c - 6 0 n r n i r t u i c l t h a n c l- - 5 0n n r i n h e i e h t .

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F i g . . 5 .S E N I so l p o l v n r c r i cs l r u c t u r c sl i r r n c c l u s i n g S A N 4 I \ 1 ( u , i t h l c e t o n e i l s t h c s o l v c n t ) i n t h i n l ' i l n r so l ' A ) A B S . l 3 ) p o l v s t r r c n c .a n c lC - ) c c l l u l o s ci t c e t atc.

F i g .l . . \ l ' \ l i n l r g c s o l t h c s n t a l l c s l l - r c c nl i r l r r i e i r t . ' .u1s i ng S A \ ' l I N l .

S A \ l l \ l i s a n " a p p l y a n c lw i c k " p r o c c s s i:t i n v o l v c sa p p l r i n e u s o l r c r . rot n t h e P D M S m o l c l a n c lr i i c k i n g t h e s o l v c n t i n t o t h c c l t a n n e l s( o r c a v i t i e s )f o r n r c c lb c t w e e n t h e n r o l c la n d t h c s u b s t r a t e .S A M I M c a n a l s o b e c i t r r i c d o u t u s i n g p r c t r c l t l l c n t o f t h e s u r f a c eo f t h e p o l v n t e r . I f t h c p o l l ' n r c r i c\ L l l ) \ t r i l t c( l i r r e x a m p l e .a P R I ' i l m ) i s e x p o s c dt c - r a s o l r c n t o r t h c \ u p o r o f a s o l v e n t .t h e s u b s t r a t eb e c o m e s " s o f t e n er l " . \ \ ' h c n t l t c n t o l c li s b r o u g h t i n t o c o n t a c tw i t h t h e s u r f a c eo l ' t h r sp r c t r c a t c csl u b s t r a t ct,h e s u r f a c em o r p h o l o u v o f t h e p o l r l l e r c h a n g c st o a r e l i e f p a t t e r n c o m p l e m e n t a r v t o t h a t o n t h c s u r l ' u c co l ' t h e n t o l d . S A M I M c a n . t h e r e f o r e . b e a p p l i c c lt o p i l t t e r n a l a r g e n u m b e r o f p o l y m e r s w h o s c surfacescun bc rnodified using appropriate solvents.Figure 5,A shou'sthc SEM of a patterned surfaceof ABS (0.t15ntm thick. Gooclfellow).The surfacemorpholoev was g e n e r a t e crl i a S A M I M u s i n g a r c e t o n ea s t h e s c l l v c n t .T h c p a t t e r n e c tl ' c a t u r c sh. a v i n g t h r e e d i f f e r c n t l e v e l s o f h e i g h t ( 0 . 8 . 1 . 3 .a n c l 1 . - lp r m .r e s p c c t i v e l y )s. h o w s m o o t h s u r f a c c s and goocl structural integritv. Figure -5Bshows a patterncd ',

s u r l ' a c ef o r m c d o n a P S f i l m ( 2 . 0 m r n t h i c k . G o o d f c l l o w ) 'l-he r u s i n gS A M I M r i ' i t h l r c c t o l r cu s t h e s o l v c n t . f e a t u r e sa r e u c l l c l c l ' i n c culn r l c l c u r l r r c s o l r e c lF. i g u r c5 C s h o w sa p a t t c r n c r l s L r r l r r c oc l ( ' . \ ( 0 . 5 n t n t t h i c k . G o o d f c l l o w ) p r o r l u c c t lu s i n gS , \ \ l l \ l u i t h i r c et o l t c u s t h c s o l v c n t T . h c o v er a l l n r o r p h o l o g ro l t h c p a t t c r r rs h o r v su o o c lr c p l i c a t i o no f t h c l ' c a t u r c so n t h c s u r t ' a c eo t ' t h c P D M S n r o l c l .c x c c p t t h a t t h c s u r f a c eo f t h e p a t t c r n c c lp o l v m e r i s h i e h l r t c x t u r e c l .S u c h patterned and tcxturcd surfuccsrltAvbe uscl'ulin processcs w h c r c a h i g h s u r f a c c / v o l u m er a t i o i s i n t p o r t e r n ts. u c h a s i n a d h c s i o na n c lc a t a l v s i s . I n s u m m a r v .S A M I M i s a c o n v e n i e n t e c h n i q u ef o r s i n glc-stcp patterning. It can generate quasi-three-dimens i o n a l s t r u c t u r c so r m o r p h o l o g i c so n t h e s u r f a c co f a p o l v n r c r u s i n ga s o l v c n tt h a t c a n s o f t e nt h e p o l v m e r i cs u b s t r a t e u ' i t h o u t a f f c c t i n s t h e P D M S n r o l c l .S A M I M h a s i m p o r t a n t c l i f f c r c n c e sf r c m o t h c r e x i s t i n gt c c h n i q u e s .l ) I t d o e s n o t s u f f e r l r o m t h e c l i s a c l v a n t a goef s l o w r a t e so f c a p i l l a r yf i l l i n g t h a t l i n t i t s M I M I C t o r c l a t i v e l v s m a l l a r e a s .W e h a v e uscd SAMIM to pattcrn areas of severalsquare centim e t e r s .a n d c x t e n s i o nt o c v c n l a r g e r a r c a ss h o u l d b e p o s s i b l e .2 ) A p p l i c a t i o no f S A M I M i s n o t l i m i t c d t o f a b r i c i i t i o n o f h y d r a u l i c a l l l ,c o n n c c t c d s t r u c t u r c s :i s o l i i t e c ls t r u c t u r e s c a n e r l s ob e f a b r i c a t e d . 3 )S A M I M c a n b e u s e c w l ith a wide r a n g e o f p o l y ' m e r so r p r e p o l v m e r s :t h c o n l y r e q u i r e m e n t s e e m st o b e f o r a s o l v e n l t h a t d i s s o l v e st h e p o l v m e r o f t h e

l'( H VerIug.sta.saIl.stltuf't rtrbIl. D-69169 \I'aitthainr.I997

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/NAIERIATS

substrateand also wets the surfaceof the elastomericmold. 4) It uses an elastomer as the component in the mold. PDMS provides a surface that is low in interfacial free energy and inert in reactivity: polymers being molded do not adhere to or react with the surfaceof PDMS. The elastic characteristicof PDMS also allows it to be releasedeasily, even from complex and fragile structures.5) The basis of SAMIM is spontaneousformation of channels or cavities when an elastomercontaining three-dimensionalrelief structures comes in intimate contact with a substrate:it does not require external pressurein maintaining conformal contact. 6) The surface of the polymer is "softened" using a solvent (or vapor) instead of temperaturetthe processis rapid and does not require specializedequipment or systemsfor aligning mold and substrate. R e c c i v e d :J a n u a r v9 . l 9 9 l Final version: March 21. 1991

trl L2l

N. L. Abott. J. P. Folkers. G. M. Whitesidcs.Sr'lotcc 1994.l-t7. 1.1S0 M i c r o c o n t a c t p r i n t i n g o f a l k a n e t h i o l so n A u : A . K u n t r r . H . . - \ . I l i c b u y c k . G . M . W h i t e s i d e s ./ - a r i g r r r r i r1 9 9 4 . 1 r . l l 9 3 I ' r r n t i n g o l u l k r t n e t h i o l s c r n A g : Y . X i a . E . K i r n . C i . M . \ \ ' h i t e s i c l c s . . /l.. , l t ' c t r t t t l t t ' t r.r\ .t t t . 1 9 9 6 .1 1 . 1 . 1 0 7 0P. r i n t i n g o f a l k a n e t h i o l so n ( ' u : \ ' . \ i a . \ 1 . \ l r k s i c h . t : . K i m . G . M . W h i t e s i d e s .C l t e t r t .M a t e r . 1 9 9 6 . 8 . 6 0 1 . P r i n t i n s o t a l k r l s i l o x a n e s o n S i / S i O z :Y . X i a . M . M r k s i c h . E . K i m . G . M . W h i t e s i d c s ." / . A m . C h e m . S o c . 1 9 9 5 .1 1 7 . 3 2 7 4 . P r i n t i n g o f P d c o l l o i d s o n S i / S i O l : P. C. Hidber, W Helbig. E. Kim, G. M. Whitesides. Larrgrurir 1996. 12.1375. H. C. Haverkorn van Rijsewijk. P.E. J. Legierse.G. E. Thomas. Philips Tec'h. Rev'. 1982. 40. 287. M. Emmelius, G. Pawlowski. H. W. Vollmann. Angew. Chem., Int. Ed. En91.1989,28.1445. E . K i m . Y . X i a . G . M . W h i t e s i d e s .N n l r r r c 1 9 9 5 . 3 7 6 . 5 8 1 . E . K i m . Y . X i a . G . M . W h i t e s i d es . J . A n t . C l r e r t t .S o c .1 9 9 6 . 1 1 8 . 5 1 2 2 .E . K i m . Y . . . Xia. E. Kim. Ci.NI. X i a . G . M . W h i t c s i d e s .A d v . M u t t ' t : 1 9 9 6 . 8 .2 . 1 5 Y Whitesides.(-hent. Mater. 1996.,9.1.5.58. X.-M. Zhao. Y. Xia. G. M. Whitesides.z1rlr:,'Vlatcr1996.,\.fi37. M. C. Hutlev. Dillntttiort Grutitrgs,Acadcntic Press.Neu York 1982. Y . X i a . E . K i m . X . - M . Z h a o . J .A . R o g e r s . M . P r e n t i s s . G . M . Whitesides.Sc'iettce1996.27.1.347. S. Y. Chou. P. R. Krauss. P. J. Renstrom. Appl. Phvs. Lett. 1995.67. 3l14. S. Y. Chou. P. R. Krauss. P. J. Renstrom. St'ience1996.272.85. Y . X i a . J . J . M c C l e l l a n d . R . G u p t a . D . Q i n . X . - M . Z h a o . L . L . S o h n .R . Celotta. G. M. Whitesides.,4dvrMoter. 1997.9, 147. For a recent review, see Special Issue on Nanotechnolog\. Microelett r ( ) t t . E t r g .1 9 9 6 . . 1 2l -t 1 t . The solvent seems to evaporate from those regions of the polvmer film that are not cclvered bv the mold: the diffusion of solvent from the mold-covered regions to the uncovered regions is a relativelv fast proCCSS.

A homogeneous thin laver of solvent could be easily applied to the surface of PDMS mold using Q-tips. See. for example. a) L. C. DeBolt. J.E. Mark. Macrotrtolecules1987. 2 0 , 2 3 6 9 . b ) L . E . P i e r r e .H . A . D e w h u r s t . A . M . B u e c h e .J . R t l v m . S t i . 1 9 5 9 . . 1 6r. 0 - 5 . We can modify the surface properties of the PDMS mold by plasmatreatment or othcr chcmical processesto make its surface wettable bv a oolar scllvent.

Effect of Surface Fractality on the Permeability of Transparent Gas Barrier Coatings** By Genta Garcfa-Avuso,Roherto Sulv'arezza, Jost M. Murtinez-Duart, Olga Sdnchez..and Luis Vii:que:* In the last few decades inorganic barrier coatings on polvmers have been used as protective coatingsin several industrial applications,in particular in the flexible packaging industry.In this case the coating depositedon a polymer film should behave as a good barrier againstmoisture. oxygen. and exposureto light in order to avoid the degradation of the packed product (usually food). As the preservation requirements become increasingly more stringent. the development of coatingswith better barrier properties has attractedthe interestof groups in both researchand inclustrv.From the scientific point of view. different aspects of thc barrier properties have been studied. In particular ot' qascs.such as oxygen and water vapor. the clil't'usiorr t h n r u s h p o r o L l sb a r r i c r sh a s b c en i n v e s t i g a t e d . l l ' 2 1 D i l ' l ' u s i o no l s a s c st h r o u s h i i p o r o u s s o l i d i s a c o m p l e x p r o c c s s t h a t r c q u r r c s . t ' o r a c o m p l e t e u n d e r s t a n d i n g .a d e t a i l e d d c s c r i p t i o na t t h e m i c r o - a n d n a n o m e t e rl e v e l o f t h e s o l i d s t r u c t u r e . l ' lT h e r e a r e s e v e r a l m e c h a n i s m st h a t explain the diffusion of gasesand liquids through porous solids. In the case of solids with pore stze (d) in the 330 nm range so-calledKnudsen diffusion is one of them.t2l This mechanism.which involves diffusion in the gas phase, allows different diffusing speciesto be separated,as the permeabilitv (P) is inverselv proportional to the square roc-rtof thc molecular weight of these species.The degree of the pore network irresularitv is describedin terms of a tortuosity factor. tn. which is reflected in the Knudsen transport oSP - 1/to.[:l In addition to the Knudsen transport. there could be a flow of adsorbed gaseous species on the pore surface. which in a first approximation occurs parallel to and independently of the gas flow.lrl In many casesthe magnitude of this mechanismof transport can be 2-3 times that of the This surface diffusion Knudsen transport contribution.[3'a] bv the desree of internal be affected contribution should

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- r. J. M. Martinez-Duart. D r . L . V i i z q u e z .D r . R . S a l v a r e z z a . D Dr. O. Siinchez I n s t i t u t o d e C i e n c i a d e M a t e r i a l e sd e M a d r i d . C S I C Cantoblanco. E-28049 Madrid (Spain) D r . G . G a r c i a - A v u s o .D r . J . M . M a r t i n e z - D u a r t D e p a r t a m e n t od e F f s i c aA p l i c a d a . C - X l l [Jnivcrsidad Aut6noma de Madrid (IJAM) C'antoblanco.E-28049 Madrid (Spain)

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V i s i t i n g r e s e a r c h c rf r o m I N I F T A . [ - a P l a t a .A r g e n t i n a . T h i s w o r k w a s f i n a n c i a l l vs u p p o r t e dt h r o u g h B R I T E E U R A M I I p r o ject BRE 2-CT92-0237 and the CSIC/CONICET cooperation program. The authors are indebted to CE,.TE.V. (ltaly) for providing the samples and performing the water vapor diffusivity measurements. T h e y a r e a l s o i n d e b t e d t o O t t o N i e l s e nE m b a l l a g e ( D e n m a r k ) f o r p e r forming the oxygen permeability measurements.

O VCH Verlugsgesellschaf't mbH, D-69469 Weinheim, 1997

09-15-qU8/97/0806-0651$ 17.50+.50/0

Adv'. Marer.1997,9, No. I