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P a t t e r n F o r m a t i o n i n U l t r a t h i n F i l m s o f a

N o v e l D i m e t h y l s il o x a n e-Acrylate Triblock

C o po l y m e r

J o o n g T a r k H a n a n d K i l w o n C h o *

Department of Chemical Engineering, Polymer Research Institute, Pohang University of Science and Technology,Pohang 790-784, Korea

Received July 3, 2003

Revised Manuscript Received August 14, 2003

I n t r o d u c t i o n

T he m ol ecul ar s t r uct ur e and t her m odynam i cs of polymer surfaces an d solid interface differ from thecorresponding bulk properties. This difference leads toa notable change in phase behavior and a lowering ofthe glass transit ion temperature (Tg) on moving fromthe bulk t o the interfacial region.1 Numerous studieshave investigated surface pattern formation in blockcopolymer films 2 on account of i ts importance in thedesign of highly functionalized surfaces. These studieshave shown that surface pattern formation in blockcopolymer thin films is strongly influenced by the filmth ickness, film properties, and th e chara cteristics of th e

substrate. However, many questions remain unsolvedon the surface pattern formation in polymer thin film.In the present study, we systematically investigated thesur face patt ern form at ion of a novel triblock copolymerwith h ighly repulsive blocks: poly(tert-butyl acrylate)-b-poly(dimethylsiloxane)-b-poly(tert-butyl acrylate) (PtBA-b-PDMS-b-PtBA). Highly repulsive interactions betweenchain segments of PDMS and PtBA blocks can give ahi gh s elect i vi t y t o t he s ubst r at e/pol ym er and ai r / polymer interfaces. The distinctive behavior of thetriblock copolymer system developed in t he presentstudy is due to the difference between the Tg of themat rix PtBA block in t he bulk an d tha t in th e thin film;t he Tg of this block in the bulk is slightly a bove roomtempera tur e (RT), whereas it is expected t o be slightlybel ow R T i n t hi n fi lm s due t o t he l ow i nt er act i oninterface between PtBA block an d P DMS block. Thus,if the Pt BA block is the continuous phase in t he thinfilm, chain segments of the triblock copolymer shouldbe able to rearra nge over time a t RT, thereby changingthe sur face pattern.

In the present study we examined the dependence ofsurface patterning on the thickness (t) of the thin filmin two differen t t hickness r egimes: t< 2R g an d t> 2R g,where R g is the unperturbed radius of gyration of thetriblock copolymer. In thin films with t> 2R g, Tg wa slower than in the bulk, and despite the high order-disorder t ransition temperatur e (TODT) of these system s,the film displayed a hiera rchical surface patter n du e to

the destabilization caused by the decrease in Tg. Inultrathin films with t< 2R g (two-dimen sional films), th efilm was stable, and a chemically heterogeneous patt ernwithout dewetting was formed in films of thickness t 2R g. Figure 1 shows optical

micrograph s recorded in reflection mode of the PtBA-b-PDMS-b-PtBA triblock copolymer thin film (t ) 45nm). In th e case of the as-cast film, no sur face patt ernis observed. However, th e sur face pat tern becomes clearover t ime, with a spinodal or impinged rims patternappearing at RT after ca. 7 days, despite t he fact thatthe bulk Tg (37 C) of the PtBA block is above ambienttemperature.

Films with thicknesses in the range 30 nm < t< 60nm formed an impinged rims structure at RT after ca.7 days, as sh own in Figure 1b. This ma croscopic pat terndid not change even after annealing at 70 C for 24 h(Figure 2b). Another hierarchical pattern, namely aspinodal patt ern, was observed in this triblock copoly-mer when the init ial f i lm thickness was in the range

1 4 n m ()2R g) < t < 30 nm. Immediately after spin-coating (i.e., for the as-cast film), no surface pattern wasobserved. However, a spinodal pattern evolved at RTafter ca. 7 days (Figure 2a, inset A). This patt ern becamefully developed after annealing at 70 C for 24 h. Theevol ut ion of t h e s ur face pat t er n i s at t r i but ed t o a nenhancement of the mobili ty of chain segments as aresult of the ma trix Pt BA block in the thin film (18 nm< t < 80 nm) having a Tg that is lower than the bulkvalue. These results constitu te a distinct evidence thatt h e Tg of the PtBA block (matrix polymer) in thin filmsis lower than that in the bulk. We also confirmed that

* To whom all correspondence should be addressed: e-mailkwcho@post ech.a c.kr.

F i g u r e 1 . Optical micrographs in reflection mode for PtBA-b-PDMS-b-PtBA triblock copolymer thin film with t) 45 nm:(a) as-cast film; (b) films at RT after 7 days.

8902 Macrom olecules 2003, 36 , 8902-8905

10.1021/ma 034930p CCC: $25.00 2003 American Chem ical SocietyPublished on Web 10/22/2003

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Tg of a thin film, 18 nm < t< 80 nm, is lower than that

of a 169 n m t hick film by the ellipsometry mea surem ent(Figure 2) even though the PtBA chain segment cani nt er act w it h a pol ar s ubs t r at e. O ur r es ult i s ver ysimilar to the Tg of a symmetric polystyrene-b-poly-(methyl methacrylate) diblock copolymer,1b in which thesurface Tg is much lower than that for its bulk sample.In our system, also, the reorganization of the ran domly

oriented microphase stru cture of block copolymer canbe att ained below the bu lkTg of PtBA, proceeding withmicro- and macrophase separation. Moreover, the mi-crostructure of a thin film of asymmetric diblock co-polymers is extremely sensitive to the film thickness andsurface-polymer interaction.2f In our case, as shown inFigure 2, the Tg of thin films, t < 2R g, is higher thant he bul k Tg since the carbonyl group in PtBA chainsegments can inter act with a polar substra te. AlthoughPtBA having carbonyl group can interact with a polarsubstrate, there is an interfacial layer affected by thesubstrate. In our case, the interfacial layer thicknessm i ght be com par able w it h 2R g because the surfacepattern in ultrathin fi lm (t< 2R g) was not changed atroom temperature.

Additionally, a PDMS microdomain structure wasobserved after an nealing at 70 C for 24 h, a s shown inp a r t s a (in s et B ) a n d b (in s et ) o f F i gu r e 3 . T h ismorphology could be obtained by the hard tappingmethod because the top layer is fully covered with arubbery PDMS, which has a very low surface energy.The ap peara nce of this m icrodomain stru cture impliestha t for t hin films of thickness t> 2R g t he TODT of thetriblock copolymer t hin film is above RT, a nd these

patterns were formed without dewetting.The surface morphology of PDMS-conta ining block

copolymers has been examined in various recent stud-ies.5 Those studies showed that the PDMS segments arethe most apt to segregate in the free surface region, andthis segregation behavior is dependent on the chainarchitecture of t he block copolymer. Green and co-

F i g u re 2 . Thickness dependence of the glass tran sitiontemperatu re for PtBA-b-PDMS-b-PtBA triblock copolymer thinfilm. Inset plot is the ellipsometric thickness with increasingtemperature for 58 nm thin film, in which the Tg could not bedetermined because the Tg of this film is lower than RT.

F i g u r e 3 . AFM images showing the stru ctura l evolution of (a) spinodal an d (b) impinged rims pat tern . Insets A and B in (a) arean evolution image at RT after ca. 7 days and a high-magnification image of the dark part in (a), respectively. Inset in (b) is ahigh-magnification image of the circular region in (b). (c) 3D AFM topographic image of 45 nm thin film and the height profilecorresponding to the line in Figure 2c. The initial thickness of (a) and (b) is 20 and 45 nm respectively.

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workers6 reported that spinodal and rims structuressimilar to those found in t he present study can form inth in films of a P S-b-PMMA diblock copolymer a bove th e

TODT . In addition, Spontak and co-workers7

observedmultiscale dewetting in low molecular weight styrene-isoprene diblock copolymer thin films at RT, which theyattributed to the depression of the upper Tg a n d TODT .The major difference between the present tr iblockcopolymer and other systems is that for our system,even though the TODT is far above RT, a spinodal-likeand impinged rims pattern without dewetting is ob-served at RT after ca. 7 days. In fact, the rims str uctureobserved in our system, which is generated withoutdewetting, is rema rka bly different from tha t which ha sbeen generally documented for systems that dewet bya nucleation and growth mechanism. Both Green 6 a n dSpontak7 f ound t hat t he dew et t ed r egi on i s a dens ebrush structure due to the surface-induced orderingof block copolymer. Although the pat tern in F igure 1bis very similar t o a typical dewetting region, h owever,in our system th e inner region of the r im structure isnot flat from the height profile as shown in Figure 3c,and ther e is no dense brush region; instead, a micro-domain str ucture of block copolymer a s can be seen inF ig u re 3 b. I t is t e mp t in g t o s pe cu la t e a b ou t t h isbehavior th at ther e is an inter facial layer ( 2R g. I n a f i l m of thickness 12 nm (R g< t< 2R g), only a dropletlike su rfacepattern was observed (Figure 4a). Similar dropletlikediscrete m ounds h ave been observed previously in otherthin films such a s poly(styrene-b-isoprene) block copoly-mer thin films,7 although in our system the droplets arelarger and show a distribution of sizes. We found thatth e top layer of th e 12 nm th ick film wa s mostly coveredwith PDMS, as determined from the observation thatthe contact angle of water on this film was similar tothat of the thick film (ca. 110), which corresponds tothe chara cteristic contact angle of water on PDMS

F i g u r e 4 . AFM images after annealing at 70 C for 24 h. Images of as-cast films do not differ from that after annealing. Initialthickness: (a) 12, (b) 6.5, (c) 3, and (d) 2 nm. Inset images ar e pha se images.

8904 Notes Macromolecules, Vol. 36, No. 23, 2003

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(Figure 5). However, when t was below R g (t< 7 nm), amicroscale islandlike and dropletlike surface patternwithout dewetting were formed. In particular, i t isnoticeable that an interconnected island structure was

generated in the 6 nm ultrathin f i lm (Figure 4b). Onfur t her decr eas e of t he t hi ckness t o 3 nm , i sol at edislands were observed (Figure 4c), and only dropletswere observed at t) 2 nm (Figure 4d). The substrat ewas fully covered with block copolymer even in a 2 nmultrathin film. These results might be attributed to thegood affinity of PtBA for th e silicon substra te an d t helarge repulsive force between PtBA an d PDMS. Bym eas ur ing t he cont act angle of w at er on t he fi lmsurfaces (Figure 5), we confirmed that the surfacepatt erns of thin films of thickness t R g. Fr omthe phase images ( inset in Figure 4a,c) in the AFMimages of ultra thin films (t) 12 and 3 nm), we speculatetha t th e islandlike phase is a lamellar ph ase tha t formsas a result of surface-induced ordering of the triblockblock copolymer and that the dropletl ike phase is aPDMS-rich region because PDMS blocks tend to belocated at the air/polymer interface due to t he very lowsurface energy of the PDMS block.

C o n c l u s i o n

We systematically examined the effect of film thick-ness on surface pattern in ultrathin films of an asym-metric ABA triblock copolymer in which the A blockinteracts preferentially with the polar substrate a nd t hei nt er act i on bet w een t he A a nd B blocks i s h i ghl yrepulsive.

In thin films (t> 2R g) of the P tBA-b-PDMS-b-PtBAtriblock copolymer, surprisingly, impinged rims struc-tures without dewetting were observed below TODT atRT. The formation of these stru ctures was attr ibutedto the depression ofTg of PtBA block, and the interplay

between th e highly repulsive intera ctions between cha insegments and the high selectivity of the chain segmentsto the air or substrate interface. This result provides adistinct evidence tha t th e Tg of the polymer thin film islower tha n that of the bulk due to th e low interactioninterface between P tBA block a nd PDMS block. H ow-ever, films of thickness t < 2R g w er e s t able at R Tbecause the chain segments of the PtBA block interactsand is confined with a substrate. Films in the range R g