NANO LETTERS
Electrospun Polymer Nanofibers with Internal Periodic Structure Obtained by Microphase Separation of Cylindrically Confined Block Copolymers
2006 Vol. 6, No. 12 2969-2972
Minglin Ma,† Vahik Krikorian,‡ Jian H. Yu,† Edwin L. Thomas,‡ and Gregory C. Rutledge*,† Department of Chemical Engineering and Department of Material Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts AVenue, Cambridge, Massachusetts 02139 Received October 1, 2006; Revised Manuscript Received October 27, 2006
ABSTRACT Continuous fibers are described having concentric layer or aligned sphere microphase-separated, styrene-isoprene block copolymer morphologies. The fibers are obtained by a two-fluid coaxial electrospinning technique in which the desired block copolymer is encapsulated as the core component within a polymer shell having a high glass transition temperature (Tg). The fibers range in diameter from 300 to 800 nm, and the block copolymer core ranges from 50 to 500 nm. Subsequent annealing of the fibers above the upper Tg of the block copolymer but below the Tg of the shell polymer results in microphase separation of the block copolymer under cylindrical confinement. The resulting fibers exhibit improved long-range order. This two-step strategy creates the opportunity to fabricate continuous nanofibers with periodic internal structure.
Electrospinning has emerged in recent years as a relatively easy, efficient, and robust method for making ultrafine (∼10-1000 nm diameter) continuous fibers from a variety of materials.1-4 As a result, numerous applications for electrospun fibers have already been proposed in diverse areas ranging from membranes and sensors to nanocomposites, nanodevices, and tissue engineering.4 The development of internal structures in electrospun fibers could significantly expand their applications; examples include sustained drug release, photonic fibers, and multifunctional textiles. In a few instances, fibers with surface or internal heterogeneities have been reported, for example porous and particle-decorated fibers that exhibit interesting surface active properties such as superhydrophobicity.5-10 Improved thermal and mechanical properties have been demonstrated for aluminosilicate- and carbon nanotube-filled composite fibers, respectively.11-13 Additional properties such as magnetic and reactive properties can also be incorporated into electrospun fibers by introducing functional nanoparticles.14-15 Despite these developments, the internal structures reported so far are highly irregular. Electrospinning of block copolymers (BCP’s) offers another way to form internally structured fibers.16-19 However, due to the rapid * Corresponding author. E-mail
[email protected]; tel: (617) 253-0171; fax: (617) 258-8992. † Department of Chemical Engineering. ‡ Department of Material Science and Engineering. 10.1021/nl062311z CCC: $33.50 Published on Web 11/16/2006
© 2006 American Chemical Society
solidification typical of electrospinning, the fabrication of fibers with well-formed internal structures having long-range order through microphase separation of BCP’s has been limited to date, with nothing approaching the periodic equilibrium morphologies exhibited by such materials.16-19 In the bulk, BCP’s are known to form various periodic microphase-separated morphologies, such as bcc packed spheres, hexagonally packed cylinders, bicontinuous cubic double gyroid, and lamellae, depending on the copolymer molecular weight, the volume fractions of the blocks, and the interaction parameter between the respective monomers.20 Long-range order can sometimes be achieved when the bottom-up self-assembly of the microphase-separated morphology is combined with a top-down fabrication technology that helps to direct the self-assembly process.21 In addition, it has been shown both theoretically22-29 and experimentally22-23,30-38 that the sample geometry, size, and imposed curvature can dramatically alter the BCP morphology from that in bulk, giving rise to novel morphologies when the sample dimension is reduced to the order of ten or fewer periods of the BCP. Microphase separation under cylindrical confinement, in particular, has been reported for BCP’s absorbed by capillary forces into porous alumina matrices, followed by extensive annealing and subsequent removal of the matrix phase.31-34 Various morphologies such as concentric cylinders, stacked disks, and helices have been
observed. The porous alumina membrane was also used to study the co-assembly of silica and BCP’s under cylindrical confinement.23 Electrospun BCP fibers represent another sample geometry in which BCP microphase separation occurs under the influence of cylindrical confinement. Unlike the short “nanorods” having lengths of ∼5 micrometers produced by sorption into porous alumina, electrospun BCP fibers are of comparable diameter, but continuous, and can be produced at a much higher rate (on the order of 0.1 gram, or 106 meters, of fiber per hour per spinneret). We report here preliminary results on the formation of novel structures comprised of BCP’s under cylindrical confinement and a practical route to their manufacture in the form of ultrathin continuous fibers. We used a two-fluid coaxial electrospinning technique3,39-40 to encapsulate the desired BCP as the core with another, protective polymer as the shell. The two-fluid electrospinning provides at least three advantages for making internally structured BCP fibers. First, being electrospinnable itself the shell fluid serves as a process aid for BCP’s that are otherwise difficult or impossible to electrospin; subsequent removal of the shell yields ultrafine BCP fibers.3 Second, independent control of the two fluid flow rates permits wider variation in the core fiber diameter, in particular enabling the production of much smaller diameter BCP fibers. Third, the shell polymer defines a cylindrical confining geometry and may have a glass or melt transition temperature well in excess of either component of the BCP, thereby creating a temperature window in which the confined BCP may be annealed without compromising the integrity of the fiber. To demonstrate this concept, we chose as the shell material a thermally stable poly(methyl methacrylate-co-methacrylic acid) random copolymer (P(MMA-ran-MAA)) with a molecular weight of about 300 kg/ mol and MAA weight percent of 80%. The glass transition temperature of this polymer is 167.5 °C as determined by differential scanning calorimetry (DSC). Two different block copolymers comprised of styrene and isoprene that form lamellar and spherical morphologies, respectively, in bulk were chosen as the core material. After electrospinning to form core-shell fibers with a range of diameters, the resulting as-spun fibers were annealed to obtain various welldefined microphase-separated BCP structures under different degrees of confinement, that is, different core diameters. In the first demonstration, a poly(styrene-block-isopreneblock-styrene) (SIS) BCP was chosen as the core material. This SIS copolymer (V4411, Dexco Polymers) contains about 41% styrene by volume and forms a lamellar microphase-separated morphology in the bulk with an equilibrium long period of 25 nm.41 Figure 1a shows a TEM image of a section microtomed from a thick (∼1 mm) film cast from a dilute solution in toluene. The isoprene domains were selectively stained with osmium tetraoxide (OsO4) and thus appear dark, whereas the unstained styrene domains appear light in the TEM images. Figure 1b shows scanning electron microscopy (SEM) images of the as-spun core-shell fibers that were obtained by electrospinning a 21 wt % P(MMAran-MAA) solution in dimethylformamide (DMF) as the shell fluid and an 18 wt % copolymer solution in a mixed 2970
Figure 1. (a) TEM image of solution-cast film from the SIS triblock copolymer, showing lamellar morphology; (dark, isoprene block; light, styrene block). (b) Typical SEM images of the asspun core-shell fibers with SIS core and P(MMA-ran-MAA) shell; the scale bar in the inset is 1 µm. (c) A representative TEM image (longitudinal view) of the core-shell fibers with SIS core; the dark cores of the fibers are OsO4-stained SIS BCP, and the light parts of the fibers are P(MMA-ran-MAA) shells. (d) TEM images (axial view) of the fibers; the inset is a magnified image of a single BCP core; the scale bar is 100 nm.
solvent (chloroform/DMF ) 3/1 by volume) as the core fluid, using the two-fluid electrospinning method described previously.3 The continuous core-shell nature of these fibers is confirmed by the TEM images in Figure 1c and d. The fibers have diameters ranging from 300 to 800 nm with the core sizes varying from 50 to 500 nm. The microphase separation of the BCP in the as-spun fibers was observed (see the inset of Figure 1d) but lacked any long-range order, as reported in previous experiments by our lab and others.16-19 To induce long-range order, the as-spun fibers were annealed in a vacuum oven at 140 °C for 10 days. The annealing temperature was chosen to be well above the Tg of the styrene and isoprene blocks (∼105 °C and ∼ -70 °C, respectively) but not to exceed the order-disorder transition temperature of SIS or the onset of thermal degradation of the isoprene block. (Both temperatures are higher than 160 °C.41) The fibers remained intact due to the high Tg of the P(MMA-ran-MAA) shell material. The internal structure of the SIS core was significantly improved by annealing. Well-defined structures consisting of concentric layers are clearly seen from TEM images in Figure 2. The concentric layer structures result from the curving of the lamellar phase due to the fiber confinement. Structures similar to these were originally predicted theoretically by He et al.24 and Sevink et al.25 They were first observed experimentally by Xiang et al.31 in nanorods of lamellaforming diblock poly(styrene-block-butadiene) having diameters of ∼100-200 nm. In our electrospun fibers, the confinement effect is made evident by the decreasing number of concentric cylinders as the fiber core size decreases, from 21 in a fiber of core diameter 510 nm (Figure 2a) to only 3 in a fiber of core diameter 50 nm (Figure 2d). Compared to Nano Lett., Vol. 6, No. 12, 2006
Figure 3. TEM images of the SI block copolymer in the cores of annealed fibers with different diameters; (a-c) axial views; (d) longitudinal views. The spherical isoprene domains in a and d were distorted by the cutting during microtomy. All of the images are in the same magnification with scale bars of 100 nm.
Figure 2. TEM images of the SIS BCP in the fiber cores of different diameters after annealing. (a-d) axial views; (e-g) longitudinal views. Only BCP cores are shown. See the Supporting Information for representative TEM images showing both cores and shells (scale bars: 100 nm).
the equilibrium period (25 nm) of this BCP in bulk,41 the average periods under confinement in fibers of different diameter vary from 19 to 29 nm. The average period is defined as the core diameter (along the shortest dimension if the core is not circular) divided by the number of periods. The outermost isoprene domain in some cases appears to be approximately half the thickness of those closer to the fiber axis, consistent with the formation of a PI monolayer at the edge of the fiber core, rather than the bilayers typical of interior domains. PI is usually the outermost monolayer in BCP films and particles with free surfaces.30,43 This observation may be indicative of debonding between the core and shell components in some fibers during solidification because the isoprene block has a lower surface tension (∼31 mN/m at 20 °C) than the styrene block (∼40 mN/m at 20 °C)39 and would therefore tend to segregate to the free surface in such instances. Many fibers display an outermost PI layer that has the same thickness as the interior PI bilayers (Figure 1b), suggesting that in these fibers styrene actually comprises the outermost domain, but it cannot be resolved by TEM images because of its low contrast with the surrounding shell. This behavior is consistent with good adhesion between the core and shell regions of these coaxial two fluid electrospun fibers because estimates of the relevant χ parameters (χPS-PMMA≈ 0.105, χPS-PMAA≈ 0.154, χPI-PMMA≈ 0.382 and χPI-PMAA≈ 0.472)42 indicate that the styrene would have the preferred interaction with the P(MMA-ran-MAA) shell. Other variations of the period along the radial direction as well as under confinement in fibers of different core diameter may be due to the accommodation of the microdomains to Nano Lett., Vol. 6, No. 12, 2006
the cylindrical confinement to minimize the total free energy. The total free energy in confinement consists of regular bulk free-energy terms, and additionally the enthalpic term due to interfacial wetting and the entropic term of chain deformation (stretching or compression) due to the size and curvature constraints. Detailed quantitative analysis in this aspect is the subject of ongoing studies. Last, removal of the P(MMAran-MAA) shell by dissolution in methanol can be used to expose the SIS core fibers (see Figure S1 in the Supporting Information). In a second demonstration, a diblock copolymer of styrene and isoprene, poly(styrene-block-isoprene) (SI) having total molecular weight of 85 kg/mol and a styrene volume fraction of 83%, was electrospun to form core-shell fibers and its internal morphology was examined. According to previous studies, this SI BCP forms a body-centered cubic (bcc) lattice of spheres in the bulk.44 Figure 3 shows typical TEM images of the SI core after annealing at 140 °C for 10 days. The images show clearly the spherical isoprene microdomains dispersed in the styrene matrix, in contrast to the incomplete microphase separation observed in as-spun fibers (see the Supporting Information). The number of spherical microdomains within a cross section of the fiber core decreases as the core diameter decreases, from approximately 20 domains in a fiber of core diameter 160 nm (Figure 3b) down to a single domain in a fiber of core diameter 60 nm (see Figure S3d in the Supporting Information). In small fibers (see Figure 3b and c), the spherical microdomains tend to line up in concentric shells parallel to the core-shell interface. This is in contrast to the bcc packing typical of spheres in the bulk state but is consistent with observations by Cheng et al.45 of spherical domain packing in parallelsided grooves etched in silicon. Similarly, Yokoyama et al. have shown that hexagonally packed spheres persist for many layers away from a flat substrate before the packing reverts to the bcc structure characteristic of the bulk.46 In some of these fibers, isoprene is observed to form a thin surface layer (e.g., Figures 3b and 3c), again suggesting debonding of the SI core from the surrounding shell in some fibers. In summary, we report various well-defined internal structures of core-shell fibers containing BCP’s and their 2971
fabrication using a two-fluid electrospinning technique. Independently, Joo and co-workers have demonstrated similar success in the formation of microphase-separated BCP (SI) fibers, again using the two-fluid electrospinning method but with a sol-gel precursor as the confining material.47 Compared to the capillary absorption technique method developed by Russell et al.31-34 and Stucky et al.23 to confine BCP’s, the process described here allows the study of BCP microphase separation under cylindrical confinement by an industrially practical route that leads to continuous, structured fibers. Combined with the recent results of Joo and coworkers, this report demonstrates the possibility of altering the shell material, thereby permitting greater freedom in the formation of fibers of different size and composition, as well as potential for exploration of surface and interfacial energy effects between the respective blocks and the shell material on the details of microphase-separated structures of cylindrically confined BCP’s. Theoretical work has shown that a variety of morphologies such as stacked-disk and helical structures may arise as the surface energy varies.25,27-29 From the technological point of view, making well-structured, tailored BCP fibers is the first step toward a number of potential applications. For example, decorating one of the blocks with nanoparticles or carbon nanotubes can be an effective way to disperse these nano-objects in electrospun fibers in a well-controlled manner. Selective functionalization or removal of one of the blocks may lead to highly functional or porous fibers. Acknowledgment. This research was supported in part by the U.S. Army through the Institute for Soldier Nanotechnologies (ISN), under Contract DAAD-19-02-D-0002 with the U.S. Army Research Office. The content does not necessarily reflect the position of the Government, and no official endorsement should be inferred. We are grateful to Dr. Alex J. Hsieh of (ISN) for providing us the P(MMAran-MAA) polymer and also to Dr. Pradipto K. Bhattacharyya of MIT and Dr. Randal M. Hill of Dow Corning for their helpful discussions and assistance during this project. Supporting Information Available: Operating parameters in electrospinning; experimental procedures; SEM images of the SIS core fibers after removal of the P(MMAran-MAA) shells; TEM images of P(MMA-ran-MAA)/SIS core-shell fibers after annealing; and TEM images of the sphere-forming SI BCP in as-spun and annealed fibers. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216. (2) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Polymer 2001, 42, 9955. (3) Yu, J. H.; Fridrikh, S. V.; Rutledge, G. C. AdV. Mater. 2004, 16, 1562. (4) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151. (5) Bognitzki, M.; Czado, W.; Frese, T.; Schaper, A.; Hellwig, M.; Steinhart, M.; Greiner, A.; Wendorff, J. H. AdV. Mater. 2001, 13, 70. (6) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Macromolecules 2002, 35, 8456. 2972
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NL062311Z Nano Lett., Vol. 6, No. 12, 2006
