doi: 10.1002/smll.200600544 ... · microfiber-arrayformation doi: 10.1002/smll.200600544...

Microfiber-array formation

DOI: 10.1002/smll.200600544

Nondestructive Mechanical Release of Ordered PolymerMicrofiber Arrays from Porous Templates**Silko Grimm, Kathrin Schwirn, Petra Gçring, Heiko Knoll, Paul T. Miclea,Andreas Greiner, Joachim H. Wendorff, Ralf B. Wehrspohn, Ulrich Gçsele, andMartin Steinhart*

The fabrication of one-dimensional (1D) nanostructures and microstruc-tures inside the pores of porous templates is intensively investigated. Therelease of these structures is commonly accomplished by etching and de-stroying the templates. The 1D nanostructures and microstructures tendto condense because of the occurrence of capillary forces during dryingof the specimens. It is shown that highly ordered arrays of polymer mi-crofibers can be easily detached from silanized porous templates by me-chanical lift-off. This procedure leaves the templates intact, thus allowingtheir recycling, and does not involve the use of solutions or solvents, thuscircumventing condensation. Therefore, mechanical lift-off may enablethe up-scaling of template-based approaches to the fabrication of highlyordered assemblies of 1D nanostructures and microstructures.

Keywords:· arrays· microstructures· polymers· templates· wetting

1. Introduction

One-dimensional (1D) nanostructures and microstruc-tures can be formed from almost any polymeric material.[1–7]

Owing to their intrinsic anisotropy and the possible genera-tion of potentially functional internal morphologies,[8–10]

these structures are considered to be promising componentsfor miniaturized building blocks. However, their assembly ina controlled manner and their integration into device archi-tectures remain challenges. Potential applications for arraysof polymeric nanofibers and microfibers include their use asartificial adhesive structures,[11–13] as surfaces exhibiting spe-cific wetting properties[13,14] or self-cleaning behavior,[15] andas an artificial matrix for protein folding.[16] The depositionof target materials or precursors into porous templates[2]

allows tailoring of the length and diameter of the 1D nano-structures and microstructures, as the structures are replicasof the pores. Common porous templates covering thepore diameter range from a few tens of nanometers up to afew microns are ordered porous alumina[17] and macropo-rous silicon (Si).[18] The use of ordered templates withpores arranged in a regular lattice potentially allows fabrica-tion of fiber arrays exhibiting a corresponding degree oforder.[5,6]

[*] S. Grimm, K. Schwirn, Dr. P. Gçring, Prof. U. Gçsele,Dr. M. SteinhartMax Planck Institute of Microstructure PhysicsWeinberg 2, 06120 Halle (Germany)Fax: (+49)345-5511223E-mail: [email protected]

Dr. M. SteinhartOak Ridge National LaboratoryOak Ridge, TN 37831-6493 (USA)

H. Knoll, Prof. R. B. WehrspohnFraunhofer Institute for Mechanics of MaterialsHeideallee 19, 06120 Halle (Germany)

Dr. P. T. Miclea, Prof. R. B. WehrspohnDepartment of PhysicsUniversity of Paderborn (Germany)

Prof. A. Greiner, Prof. J. H. WendorffDepartment of Chemistry and Center of Materials SciencePhilipps-UniversitBtHans-Meerwein-Strasse, 35032 Marburg (Germany)

[**] The authors gratefully acknowledge financial support from theVolkswagen Foundation within the framework of the thematicimpetuses “Complex Materials: Cooperative Projects of the Natu-ral, Engineering and Biosciences” and “Interplay between Molec-ular Conformations and Biological Function”, as well as technicalsupport by A. Langner, K. Sklarek, B. Boettge, and Dr. X. Chen.K.S. thanks the International Max Planck Research School for Sci-ence and Technology of Nanostructures for financial support.

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Release of Polymer Microfiber Arrays from Templates

The release of the 1D nanostructures and microstruc-tures requires the destruction of the templates by wet-chem-ical etching with acids and bases. Several drawbacks associ-ated with this procedure prevent the upscaling of template-based methods. The consumption of the templates repre-sents a problem. The use of bases and acids requires specificprecautions regarding work safety and environmental com-patibility. Also, the corrosive etching solutions must be dis-posed of in compliance with environmental and safetyguidelines. Moreover, capillary forces occurring between ad-jacent 1D nanostructures or microstructures upon dryingcause their condensation, which may limit the usability ofthe arrays thus obtained. Strategies to overcome the latterproblem commonly avoid the occurrence of liquid/gas inter-faces, for example, by supercritical drying.[19] However,these techniques are tedious and require considerable exper-imental efforts.

In the following report,we will evaluate to whatextent the nondestructive re-lease of arrays of polymer mi-crofibers from porous tem-plates by a mechanical lift-offprocess can replace wet-chemical etching. Mechanicalrelease, recently applied toarrays of mesoporous silicamicrorods,[20] leaves the tem-plates intact and potentiallyenables their recycling. Nohazardous solvents or corro-sive solutions containing acidsor bases are required. More-over, this procedure may cir-cumvent condensation as themicrofibers are not exposedto liquids at any stage. Wewill particularly investigatehow the aspect ratios (Tp/Dp;Tp: pore depth; Dp: pore di-ameter) of the templatepores, the nature of theirwalls, and the mechanicalproperties of the polymers in-fluence the lift-off procedure.To this end, we employedmacroporous Si with a Dp

value of 1 mm, a lattice con-stant of 1.5 mm, and variousTp values as a templatesystem. The pore walls eitherconsisted of silica or were rendered nonpolar by silaniza-tion. Polystyrene (PS) was selected as a stiff model polymerand polyvinylidene difluoride (PVDF) as a ductile modelpolymer. We prepared the microfibers by wetting the mac-roporous Si templates with PS and PVDF melts.[5,6, 21, 22]

2. Results

2.1. Release of PVDF and PS Microfibers from Templateswith Pore Walls Covered with Silica

In a first series of experiments, we used macroporous sil-icon with pore walls covered with a native silica layer.Therefore, they had a high surface energy compared to thatof organic liquids such as polymer melts.[23] Under theseconditions and provided that the wetting temperature is suf-ficiently high, a mesoscopic polymer film with a thickness ofa few tens of nanometers rapidly covers the pore walls; thesolidification of this film yields tubular microfibers with alength corresponding to the depth of the templatepores.[5,6, 22] It is to be expected that their aspect ratio influ-ences the lift-off process (Figure 1) to a large extent. There-

fore, we varied the Tp values of the templates and, corre-spondingly, the length of the microfibers, with values of 1.5,2.5, and 10 mm. Figure 2 shows the surfaces of PVDF filmsdetached from macroporous Si with Tp values of 1.5, 2.5,and 10 mm (Figure 2a–c, respectively). We could pull thePVDF microfibers out of the pores with a Tp value of1.5 mm. The array of PVDF microfibers thus released re-mained connected to the PVDF film detached from the

Figure 1. Schematic diagram of the lift-off process. a) A polymer melt wets a macroporous Si template.b) Polymer microfibers form as replicas of the template pores. A bulk film of the polymer is located onthe surface of the template and remains connected to the microfibers inside the pores. c) Studs areglued onto the underside of the macroporous Si template and onto the polymer film covering the tem-plate surface. The samples are mounted on a tensile testing machine for pulling out the microfibers. Theposition of one stud is fixed, while the second stud is moved away from the fixed one. d) The microfi-ACHTUNGTRENNUNGbers can be detached from the template along with the polymer film on the surface. The fibers arepulled out of the pores and remain connected to the detached polymer film, therefore forming a highlyordered array. e) Owing to lateral forces occurring in the course of the lift-off, the microfibers may breakoff and remain located inside the pores. The detached polymer film only exhibits short fiber stumps.

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full papers M. Steinhart et al.

template surface and shows a long-range order correspond-ing to that of the pore array in the template. The caps ofthe PVDF microtubes, which are replicas of the pore bot-toms, are still intact, and only a small proportion of thePVDF microtubes are elongated due to stretching. Essen-tially, this result corresponds to the scenario sketched in Fig-ure 1d.

An increase in the Tp value to 2.5 mm yields ensemblesof released PVDF fibers connected to the detached PVDFfilm with a length of �6 mm and a diameter of �400 nm;these fibers tend to condense. Apparently, the microfiberswere stretched in the course of the lift-off procedure (Fig-ure 2b). A further increase in the Tp value up to 10 mm ledto consistent breakage of the PVDF microfibers at the poreopenings, and only stumps of the PVDF microfibers, as wellas some stretched microfiber segments, remained connectedto the detached polymer film (Figure 2c).

No PS microfibers could be pulled out of the pores, in-dependent of the Tp value of the template pores. The PS mi-crofibers consistently broke off at the pore openings. Thisfinding is representative of all samples investigated contain-ing PS microfibers in pores with walls covered with a silicalayer.

2.2. Release of PVDF Microfibers from Templates withSilanized Pore Walls

It is to be expected that minimization of adhesion be-tween the microfibers inside the pores and the pore walls fa-cilitates the lift-off process. To this end, we reduced the sur-face energy of the latter by silanization by using the hydro-phobic silane 1H,1H,2H,2H-perfluorodecyltrichlorosiliane,thus converting the oxidic high-energy surface of the porewalls into a low-energy surface.[23] However, changes in thenature of the pore walls might result in a different wettingmechanism. Indeed, SEM investigations on several samplessuggest that solid microfibers form inside silanized pores in-stead of microtubes. This is obvious from the bird@s eyeview of an ensemble of truncated PVDF microfibers, whichwe released from macroporous Si with a Tp value of 15 mm(Figure 3).

Detachment of the PVDF from silanized macroporousSi templates with a Tp value of 6 mm yielded highly orderedarrays of PVDF microfibers located on the detached PVDFfilm; these arrays extended over several 100 mm2 and exhib-

Figure 2. Scanning electron microscopy (SEM) images of PVDF filmsdetached from macroporous Si templates with different Tp valuesand silica-covered pore walls. a) Tp=1.5 mm; b) Tp=2.5 mm; c)Tp=10 mm.

Figure 3. SEM image of truncated PVDF microfibers released frommacroporous Si (Tp=15 mm) with silanized nonpolar pore walls. Themicrofibers are apparently solid.

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ited hexagonal long-range order, as the pore arrays in thetemplates did. Large portions of the PVDF microfiberarrays were apparently free of defects (Figure 4 a,b). TheSEM image of a deformed area of the PVDF film, seen inFigure 4c, shows side views of individual PVDF microtubes,whose length of 6 mm corresponds to the Tp value of thetemplate pores. Therefore, the PVDF melt completely filled

the pores, and the hemispherical caps of the PVDF micro-fibers are apparently replicas of the pore bottoms (Fig-ACHTUNGTRENNUNGure 4b, c). The PVDF microfibers remained intact upon lift-off. None of them were elongated.

An increase in the Tp value of the template pores to15 mm yields disordered arrays of PVDF microfibers. The in-itial hexagonal arrangement of the pores in the templates isnot conserved upon lift-off (Figure 5a). The PVDF micro-

fibers located on the detached PVDF film are bent, andsome of them broke off so that only stumps were present attheir former positions. Figure 5b shows a cross section of aPVDF microfiber array. The length of the PVDF microfi-ACHTUNGTRENNUNGbers amounts to �15 mm and corresponds again to the Tp

value of the template pores. The hemispherical shape of thecaps indicates that they are replicas of the pore bottoms, afact that in turn requires that the pores were completelyfilled with the PVDF melt during the wetting. A closer lookat Figure 5b reveals the presence of some stretched PVDFmicrofiber segments with a diameter of �400 nm.

Figure 4. SEM images of PVDF microfibers released from macropo-rous Si (Tp=6 mm) with silanized nonpolar pore walls. a) Bird’s eyeview of a highly ordered array. b) Detail showing the hemisphericalcaps of the PVDF microfibers. c) Side view of some PVDF microfibersproviding evidence that their aspect ratio is the same as that of thetemplate pores.

Figure 5. SEM images of arrays of PVDF microfibers released frommacroporous Si (Tp=15 mm) with silanized nonpolar pore walls.a) Bent PVDF microfibers connected to a detached PVDF film. Somemicrofibers broke off so that only stumps are seen. b) Side view ofthe PVDF microfibers, some of which have stretched, neck-like seg-ments.



2.3. Release of PS Microfibers from Templates withSilanized Pore Walls

As in the case of PVDF, we observed the formation ofsolid PS microfibers inside silanized pores. This is obviousfrom the bird@s eye views of truncated PS microfibers pulledout of silanized pores with a Tp value of 10 mm (Figure 6).

PS films detached from silanized macroporous Si with a Tp

value of 2 mm exhibit highly ordered arrays of PS microrodswith an aspect ratio corresponding to that of the templatepores (Figure 7a). An increase in the Tp value to 6 mmyields arrays of released PS microfibers that partially showhexagonal order. However, we also found disordered areasconsisting of stretched PS microfibers (Figure 7b). The PSmicrofibers released from templates with a Tp value of10 mm possess caps with a hemispherical shape, thereby indi-cating that the PS melt filled the entire pore volume. How-ever, many PS microfibers broke off or had neck-like seg-ments with a thickness of �500 nm, as is obvious from Fig-ure 7c.

3. Discussion

We will limit the discussion to a qualitative evaluation ofthe SEM images of the specimens, because several problemsprevent a quantitative interpretation of the correspondingstress–strain curves. In several experiments, cleavage oc-ACHTUNGTRENNUNGcurred either in the macroporous Si or at the interface be-tween the polymer film on the template surface and theglue. Shortcomings of the experimental set-up, such as mis-alignment of the studs and lateral offset between the pointsof application of the forces acting during the lift-off, causethe inevitable occurrence of lateral forces and, in turn, ofshear stress. Therefore, the polymer film might tear off inan irregular manner during the detachment from the tem-plate surface. We can nevertheless draw some importantconclusions from our observations. Two parameters appearto be crucial to controlling the mechanical release of poly-

meric microfibers from macroporous Si templates: thestrength of adhesion between the microfibers and the porewalls and the magnitude of the shear forces exerted on themicrofibers. Ductile polymers can dissipate shear stressduring the lift-off to a greater extent than stiffer polymers.This is obvious from the results obtained with templateshaving pore walls covered with silica. The release of theductile PVDF microfibers without stretching was possiblefor pores with small Tp values of 1.5 mm (Figure 2a). If thefibers have aspect ratios of 6:1 or 10:1, they show a muchmore pronounced tendency towards stretching and, eventu-

Figure 6. SEM image of truncated PS microfibers released from mac-roporous Si (Tp=10 mm) with silanized nonpolar pore walls. Themicrofibers are solid.

Figure 7. SEM images of arrays of PS microfibers released from mac-roporous Si with silanized nonpolar pore walls: a) Tp=2 mm;b) Tp=6 mm; c) Tp=10 mm.



ally, fracture. The stiffer PS microfibers were prone to frac-ture, even for aspect ratios of 2:1 and below. None of themremained connected to the detached PS films.

A general conclusion that can be drawn from the set ofexperiments described above is that the contact area be-tween the pore walls and the microfibers, and in turn theadhesive resistance to the removal of the latter, increasesalong with the aspect ratio of the microfibers. However,minimization of the surface energy of the pore walls by sila-nization significantly facilitates lift-off because the strengthof adhesion between the microfibers and the templates is re-duced.[24] Arrays of microfibers with aspect ratios of 10:1can easily be detached. This is in line with results recentlyreported for silica microrods prepared inside silanized mac-roporous Si templates.[20] Extended microfiber arrays exhib-iting hexagonal long-range order, such as that of the latticeformed by the template pores, were obtained for aspectratios up to 6:1. In cases with larger aspect ratios, the lateralforces exerted on the microfibers in the course of the lift-offcaused bending, stretching, and fracture. Slight misalign-ment of the studs has dramatic consequences. The longerthe distance the microfibers have to be pulled, the largerthe lateral offset between the point of application of thetraction force and the location of the pore. Owing to thisoffset, the long axes of the released microfiber segments areinclined with respect to those of the segments still residinginside the pores. The ductile PVDF microfibers may be de-formed nondestructively, and the stress is still transmitted tothe fiber segments inside the pores. As a result, PVDF mi-crofibers with an aspect ratio of �10:1 that are connectedto the detached PVDF film are bent, as seen in Figure 5a.

The stiffer PS microfibers may be canted inside thepores, and they fold at the pore openings. As the segmentsstill located within the pores cannot move outwards any-more, the PS microfibers are stretched, undergo neck for-mation, and eventually fracture. We wetted a silanized mac-roporous Si template with a Tp value of 10 mm with PS andcleaved it. Figure 8 a shows a cross-sectional SEM image ofthe specimen. A bulk PS film at the top covers the macro-pore array in the center. At the bottom, the underlying Siwafer can be seen. The lateral forces occurring during thecleavage led to consistent breakage of the PS microfibers atthe pore openings. Similarly to many detached PS films, thebulk PS film on the cleaved template only exhibits stumps.Occasionally, some of the PS microfibers located in front ofthe edge of the specimen remained intact and connectedwith the bulk PS film, as seen in the foreground of Fig-ure 8a. Their long axes are inclined by �208 with respect tothe pore axes. In the center of the image, a fiber segmentinside a pore and above a stump attached to the surfacefilm can be seen. The long axes are inclined, and the PS mi-crofiber broke at the pore opening. It is reasonable toassume the lift-off has a similar impact to cleavage on themicrofibers. Figure 8 b shows the surface of a macroporousSi template (Tp=10 mm) wetted with PS after the lift-off.Most pores appear to be empty, but several pores still ac-commodate PS microfibers that apparently broke off uponlift-off. Some PS microfibers are partially pulled out of thepores and show either smooth fractured surfaces or necks,

thereby indicating that either brittle or ductile fracture canoccur. Much better results might be obtained when thesetup used is optimized with regard to the minimization oflateral forces. In particular, the surface of the polymer filmcovering the template, which acts as a substrate for the re-leased microfibers, might not be smooth and parallel to theunderside of the macroporous Si template. We consider thisas the main cause of misalignment between the studs. Nev-ertheless, even under these conditions, a standard tensiletesting machine is sufficient to release highly ordered arraysof polymeric microfibers with aspect ratios up to 10:1. Nocondensation of the microfibers occurred when silanizedmacroporous Si templates were used.

An issue to be addressed in the future is the influence ofthe wetting mechanism on the lift-off process and the prop-erties of the fiber arrays thus obtained. Wetting of poroustemplates with pores having high-energy walls[23] at suffi-ciently high temperatures leads to the rapid formation of amesoscopic wetting layer on the pore walls,[5, 6] similar to theformation of precursor films on smooth substrates.[25] Nocomplete filling of the pore volume takes place on an exper-

Figure 8. SEM images of broken PS microfibers inside macroporousSi with silanized nonpolar pore walls. a) PS microfibers inside acleaved macroporous Si template (Tp=10 mm). At the top, the bulkPS film is seen, in the middle is the pore array as well as the micro-fibers, and at the bottom is the underlying Si wafer. b) Bird’s eyeview of a macroporous Si template (Tp=10 mm) after wetting and lift-off, containing some broken PS microfibers.



imentally accessible time scale, and solidification of thepolyACHTUNGTRENNUNGmer yields tubular fibers. Zhang et al. reported anothermechanism where a solid–liquid thread of the melt precededby a meniscus infiltrates the pores at lower temperatures,thereby resulting in the formation of solid rods.[22] Render-ing of the high-energy pore walls into low-energy pore wallsalso leads to a change of the wetting mechanism. Pore wallswith a high surface energy are converted into a low-energysurface when covered with a polymeric wetting film, and theoverall energy of the system decreases. If the pore surfacealready exhibits a low-energy character, no driving force forthe rapid formation of a wetting film exists, and the infiltra-tion of the polymer melt occurs similarly to classical capilla-ry rise,[26] even at 220 8C, well above the melting tempera-ture of the PVDF and the glass transition temperature ofthe PS. Therefore, the surface energy of the pore walls de-termines whether tubes or solid rods form.

4. Conclusions

The mechanical removal of microfibers from poroustemplates has great potential to replace methods of releasebased on wet-chemical etching. This is particularly the casefor fibers consisting of ductile polymers, as they can dissi-pate the inevitable shear stress occurring during lift-off to acertain extent and can deform nondestructively when the di-rection of the stress does not coincide with that of the poreaxes. Microfibers consisting of stiff materials, however, tendto form necks and to undergo brittle or ductile fractureunder these conditions. Therefore, the necessary optimiza-tion of the set-up used for the lift-off will aim at the minimi-zation of the lateral forces causing the shear stress. The lift-off can be significantly facilitated by reducing adhesion be-tween the pore walls and the fibers. To this end, the mostpromising strategy is the conversion of pore walls into sur-ACHTUNGTRENNUNGfaces with low surface energy, for example, by silanization.However, as a result, the wetting mechanism changes andsolid instead of tubular microfibers form upon infiltration ofpolymer melts. The easy detachment of the polymer fromthe templates yields highly ordered arrays of released mi-crofibers, and the templates can be recycled. Mechanicallift-off also circumvents condensation of the microfibers,which commonly occurs when arrays are released by wet-chemical etching. The future optimization of this processmight pave the way for up-scaling of the template-basedproduction of nanofibers and microfibers. The concept pre-sented here might also be extended to complex functionalmaterials such as organic/inorganic nanocomposites.[27]

5. Experimental Section

Macroporous Si was prepared according to the proceduresdescribed in the literature.[18] The macroporous array was at-ACHTUNGTRENNUNGtached to an underlying Si substrate so that the pores wereopen only at one end, while the pore bottoms had a hemispheri-

cal shape. The templates were heated for 6 h at 900 8C in air toproduce an �160-nm-thick SiO2 layer on the pore walls; thiswas subsequently removed by etching with a 5 wt% aqueous so-lution of hydrofluoric acid (HF) for 2 h at room temperature. Thetemplates thus prepared had a Dp value of 1 mm and the porewalls were covered with a thin native silica layer due to the oxi-dation of Si that takes place under ambient conditions. Sometemplates were modified by silanization. To this end,1H,1H,2H,2H-perfluorodecyltrichlorosilane (C10H4Cl3F17Si) wasgrafted onto the pore walls according to procedures reportedelsewhere.[20] Masks with circular holes were located on tem-plate pieces with an edge length of �0.5 cm and the area of0.07 cm2 thus uncovered was exposed to PS (BASF 1440;weight-average molecular weight Mw=159490 gmol

�1; number-average molecular weight Mn=56500 gmol

�1; glass-transitiontemperature Tg=100 8C) and PVDF (Solvay; Mw=

100000 gmol�1; Mn=38000 gmol�1; melting temperature Tm=

178 8C) melts heated to 220 8C for 5 min (Figure 1a). In the caseof silanized templates, a weight of 717.5 g was placed onto thepolymer. Subsequently, the samples were quenched to roomtemperature. A 1–2-mm-thick polymer film covered the surfaceof the templates after the wetting; this was polished in order togenerate a smooth film surface parallel to the template surface(Figure 1b). The samples were prepared for the lift-off experi-ments by gluing studs onto the undersides of the Si substratesand onto the polymer films on the surfaces of the templates byusing epoxy adhesive (UHU plus endfest 300; two-componentepoxy adhesive; load capacity as specified by the manufacturer:3000 Ncm�2; binder: epoxy resin; curing agent: N,N-dimethyldi-propylentriamine). Subsequently, the samples were mounted ona tensile testing machine (ZWICK 1445; force range of 0–10 kN)and one side of the sample was pulled with a velocity of2 mms�1 and an increment of 60 nm (Figure 1c). The desiredresult would be that the released microfibers remain connectedwith the polymer film detached from the template surface andthat they form an ordered array (Figure 1d). However, the forceexerted on the sample might have a lateral component perpen-dicular to the long axes of the pores and the microfibers. Shearstress acting on the microfibers might then lead to fracture and,as a result, they might remain located inside the template pores.The detached surface film would only exhibit stumps (Figure 1e).

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Received: October 6, 2006Published online on March 13, 2007



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