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Nanostructures of functional block copolymers
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Nanostructures of functional block copolymers Guojun Liu Block copolymers were long known to self-assemble and form various mesophasic structures under appropriate conditions. Other than a few scattered studies, advantage was not taken of these mesophasic structures for the preparation of 'permanent' or crosslinked nanostructures. Recently diblocks containing photocrosslinkable and sometimes degradable blocks have been synthesized. The different domains of the mesophases of these diblocks were crosslinked and sometimes degraded to yield nanostructures including nanospheres, nanofibers and nanochannels in polymer thin films. Some of the advances that have taken place of late are in the preparation, characterization and application of these three types of nanostructured materials.
Addresses Department of Chemistry,The University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4; e-mail: [email protected] Current Opinion in Colloid & Interface Science 1998, 3:200-208 Electronic identifier: 1359-0294'003'00200
© Current Chemistry Ltd ISSN 1359-0294 Abbreviations DMSO dimethol sulfoxide PAA poly(acrylic acid) PCEMA poly(2-cinnamoylethylmethacrylate) PS polystyrene PtBA poly(t-butyl acrylate)
Introduction A nanostructure has its smallest dimension in the 1-100nm size range [1]. The preparation and study of nanostructures have attracted much attention in the past decade mainly to meet the industrial demand for ever smaller electronic devices [1,Zoo]. Semiconductor nanostructures are also interesting for their unique properties derived from the quantum size effect. Further interest is derived from the expectation that composites made from nanoparticles may be useful as high performance construction materials and it is thought [1] that some nano-engineered materials may imitate the function of proteins and enzymes in molecular recognition. Nanostructures are typically prepared from inorganic or organic precursors. Polymers and, in particular,' block copolymers form various mesophasic structures including spherical, vesicular, and cylindrical micelles under appropriate conditions. These mesophasic structures have their smallest dimensions in the nanometer range. By cross-linking or locking in these mesophasic structures, my co-workers and I prepared an array of permanent mesophasic structures which we refer to as polymeric nanostruc-
trures, The structures we prepared included star polymers [3°,4], nanospheres [4,5°°], tadpole molecules [6°], crosslinked polymer brushes (monolayers) [7-10,11°,1Z], and nanofibers [13°°]. The nanospheres can be further divided into hairy nanospheres [4,5°°] (nanospheres with polymer chains on their surfaces), nanospheres with cross-linked shells [7-10,11°,1Z], and hollow nanospheres [14°°]. More sophisticated nanostructures such as nanochannels in thin films [15°°], shaved nanospheres [16°), and semishaved and fully shaved hollow nanospheres [14°°] were obtained by combining crosslinking and degradation treatment to these mesophasic structures. In this article, I will briefly review mesophase formation from block copolymers. The preparation of nanospheres, nanofibers, and nanochannels in polymer thin films from block copolymers will be described; and the potential applications of the three types of nanostructured materials will also be discussed.
Mesophase formation from block copolymers A copolymer is a macromolecule containing two or more types of basic units or monomers. A block copolymer is a linear copolymer in which the different monomers occur in long sequences or blocks. The simplest type of block copolymers are diblock copolymers, (AMB)m' which consists of two linear polymer chains with 11 units of A and m units of B joined together head to tail. The two functional diblock copolymers we first synthesized and used in nanostructure fabrication are polystyrene (PS)b/ock-poly(Z-cinnamoylethyl methacrylate) (PCEMA) [17] and polytz-buryl acrylate) (P/BA)-b/ock-PCEMA [soo,1Soo] (Figure 1).
Figure 1
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Current Opinion in Colloid & Interface Science
Structure of PS-b-PCEMA and PtBA-b-PCEMA
Nanosuuctures of functional block copolymers Liu
These block copolymers are functional because PCEMA is photo-crosslinkable and the r-butyl groups of P/BA are easily hydrolyzable. Other functional diblock copolymers we synthesized and used include PI-b-PCEMA [14") and PCEMA-b-PAA [5··1, where PI and PAA denote polyisoprene and poly(acrylic acid), respectively. In all cases, the precursors to the diblocks were prepared by anionic polymerization and thus the polymers. had low polydispersity. Block copolymers self-assemble and form mesophasie structures both in bulk and in solution. In a block-selective solvent.; a diblock copolymer may, for example, form spherical micelles with the insoluble block making up the core and the soluble block forming the corona stretching into the solution phase [18-28) . If the thermodynamic equilibrium sizes of the micelles are achieved, the micellar sizes should scale following theoretical relations derived by de Gennes [29) and Halperin [30); this was recently verified experimentally by my colleagues and I [3·). The diameters of the micelles are typically -100 nm. Block copolymers can also form vesicular [31,32,33··,34·), cylindrical [3·,32,34·,35], and donut-shaped micelles [34·) in a block-selective solvent. The exact morphology of the micelles formed depends on the relative lengths of the tWO blocks of a diblock and the compos ition of a binary mixture used as the solvent [32,33··,34·). In general, diblocks with a short soluble and a long insoluble block tend to form vesicles, cylinders, or donuts. Increasing the content of the block-selective solvent in a binary solvent mixture relative to that of the mutual solvent, which dissolves both blocks of a diblock, favors vesicle, cylinder, or donut formation. A diblock copolymer, (A)n(B)m, forms mesophases in bulk because the constitutent blocks segregate from one another. The size of the segregated A or B domains is similar to that of the individual A or B chains in solution. The domain shape of A or B varies with the relative Ii and m values and the temperature [36··,37,38·,39·). As the content of B is increased gradually to .- 50%, the shape of the B domain normally changes from spheres to cylinders, gyroids, and lamella (with the cylinders most preferred, and the lamellae least preferred). Further increase in the B volume fraction will make B the continuous phase and A will have the various morphologies as mentioned above for B (i.e. spherical, cylindrical and gyroidal). Domains of different blocks, such as cylinders of B dispersed in the continuous matrix of A, are normally ordered within grains in the micrometer range. Long-range or macroscopic scale ordering of the mesophasie domains can be achieved by mechanically shearing [40·) or electrically pooling the samples at elevated temperatures [41).
Hairy nanospheres Preparation
The preparation of hairy. nanospheres from a diblock copolymer involves two steps: preparation of spherical
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micelles in a block-selective solvent and the locking in of the micellar structure. Several groups ach ieved spherical diblock micelle crosslinking using thermal [42,43J or photo initiators [44,45). Such strategies worked well in the solid state, in which the rate of domain fixation was much faster than that of micelle chain exchange [46,47J. In solution, insoluble products were sometimes obtained due to micelle fusion during core crosslinking [42,43J. My co-workers and I prepared hairy nanospheres from PI-b-PCEMA [16·), PS-b·PCEMA [3·,4), and -PCEMA-bPAA [5··) containing the self-crosslinkable PCEMA block. Using these diblocks, soluble hairy nanospheres were always obtained [3·,4;5··,16·). Water-soluble nanospheres were prepared from PCEMAb-PAA with 3.6x th 10z units ofCEMA and S.6x lOz units of AA ([48··,49). Micelles with PCEMA as the core and PAA as the corona were obtained by stirring the polymer in hot water. UV photolysis of the micellar solutions locked in the micellar structure, as was verified by the stability of the nanospheres in dimethyl sulfoxide (DMSO), a mutual solvent for PCEMA and PAA. Illustrated in Figure 2 is a TEM image of some of these nanospheres. Potential applications
PCEMA-b-PAA nanospheres have many very interesting characteristics which make them potentially useful in the oil, gas and pharmaceutical industries. One interesting property, for example, is that PCEMA·b-PM nanospheres possess hydrophobic PCEMA cores and hydrophilic PAA coronae. The cores can take up many organic compounds from water. A typical hydrodynamic radius, Rh. of 48 nm for a PCEMA-b-PAA nanosphere sample in water was, for example, increased to 62 nm in water-DMSO solution with 2.5% DMSO by volume [5··). This represents a nanosphere volume increase of 116% due to DMSO uptake. The nanospheres can also take up solid organic compounds such as perylene from water [48·· 49). The partition equilibrium between the nanospheres and perylene was established with in one day. The coefficient of perylene partition between water and the nanosphere core was determined to be large, -6x 106 [48··]. The capacity of perylene uptake by the nanospheres was, however, low, -3 mg/g. This capacity was increased fourfold by the addition of 1% DMSO to water and by a factor of 100 in water-acetone mixtures with 33% acetone by volume [49). Thus, the capacity increased by swelling the nanosphere cores. More interesting is the fact that the nanospheres precipitated out, dragging perylene with them, when a 1.0 M solution of CaClz was added to the nanosphere solution so that the final calcium concentration was -2.0 x 10-3 M. The precipitated nanospheres could then be extracted with an organic solvent such as THF, which
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Materials aspects
Figure 2
200 nm TEM image of PCEMA·b-PAA nanospheres, magnification x135,000. The sample was aspirated as an aqueous solution on Formvar·covered copper grids and stained with'Os0 4 before viewing. The cross-linked micelles are spherical with a mean diameter of -35 nm.
removes the perylene. Once the perylene is removed, the nanospheres can be redispersed in water by adding sodium carbonate to precipitate out Ca Z+ or by adding EDTA, which complexes with Ca 2+. On the basis of these observations (0 Liu, unpublished data) I believe that the nanospheres should be useful in cleaning up tailings ponds left behind by the oil and gas industry. I In such ponds, hydro carbons with relatively low mol ar mas s should function to plasticize the core of the nanospheres. These plasticized nanospheres could then take up a large amount of high molar mass components such as ashphaltenes. An alternative use may be in the pharmaceutical industry where the nanospheres, [50,51] like water-soluble block copolymer micelles, may be employed as drug carriers in controlled drug release.
Block copolymer nanofibers Preparation
A nanofiber is a fiber with a nanometer-sized diameter. Nanofibers have been prepared previously mostly from inorganic or organic precursors through the template method. One method of preparing nanofibers [52-55] is, for example, by filling the nanometer-sized carbon or boron carbide rubes [56·,57] with metal oxides at elevated temperatures. Other templates used include the cylindrical pores in a polymer membrane or in anodized alumina [58]. More recently, 'soft templates' such as reverse micelles have been used for inorganic nanofiber or nanorod preparation [59]. In addition to the template method, carbon nanofibers have been prepared at particular crystal faces as a result of the decomposition of
Nanostructures of functional block copolymers Uu
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Figure 3
"
,
"
I urn Transmission electron microscopy image of PS·b-PCEMA nanofibers. Reproduced with permission from [13··}.
selected hydrocarbons [60]. Using nickel nanoparticles as the catalyst, the decomposition of trichloromethyl silane at high temperatures en abled the preparation of SiC nanofibers [61]. My co-workers and I prepared the first polymeric nanofiber sample from PS-b-PCEMA with 1.25x 103 units of styrene and 158 units of CEMA. This polymer, in which'PCEMA existed as cylinders dispersed in the PS matrix, has a PCEMA weight fraction of 24%. Photolysis of a solid sample crosslinked the PCEMA cylinders. Separating different cylindrical dom ains by dissolving the PS phase yielded nanofibers as shown in Figure 3. The nanofibers have a diameter of -50 nm and a length of -20 urn, At higher magnifications, the nanofibers were shown to have PCEMA cores and PS shells.
Potential applications of block copolymer nanofibers Since microdomain formation occurs in most block copolymers, this repre sents a general method for preparing nanofibers with un iform sizes. I expect th is method to be useful in produ cing precursor fibers which can be pyrolyzed coyield carbon or metal carbide nanofibers. This method can also be modified for making nanowires by replacing the core block with a conductive polymer. In th at case, the outer block would function as an insulating plastic layer. Recently, the PS-b-PCEl\IA nanofibers were found to be soluble in bromoform at all mixing ratios (8 Dymov, ] Ding, ] Gleeson, G Liu, unpublished data). Illustrated in Figure 4 are polarized optical microscopic images of a
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Materials aspects
track left in a 42.5% by weight PS-b-PCEMA nanofiber solution in bromoform, after a sp atula stroke through the solution (B Dymov, J Ding, J Gleeson, G Liu, unpublished dat a). The birefringence suggests the alignment of the fibers along the shearing direction. As the relative position of the polarizers on the incident and transmitted beam side changes, the bright regions in the image turn dark and vice versa, as expected. While such nanofibers, particularly water-soluble nanofibers, may be' useful as 'environmentally friendly' liquid crystals, a further study on the response rate . of such nanofibers to changes in electric fields is required. The liquid crystalline properties ofthe nanofibers would certainly facilitate the preparation of macroscopic fibers or ropes consisting o'r well-aligned nanofibers or nanofilarnents.
diblock solid [76]. No evaluation of thin films containing hexagonally packed nanochannels as - membranes has, however, been reported. No evaluation was performed of the porous diblock solid sample prepared by Lee el of. [76], probably because the objective of Lee el 01. was to create some porous particulate materials rather than porous thin films. The zeolite films were not evaluated because the nanochannels lie in the place of the zeolite films [73,75··] and these films in this form are not applicable as membranes. Metals with nanochannels were not evaluated as membranes, because metals are normally for the more common metals such as iron and aluminum and the high cost for the rare metals. Preparation
The procedure for membrane preparation from a diblock copolymer involves firstly the synthesis of a diblock copolymer (A)n(B)m with a degradable A block and a crosslinkable B block, followed by the preparation of (A)n(B)m solid with A forming the .regularly packed cylinders dispersed in the continuous B matrix. Thin films of the diblock are then obtained using microtomy and are crosslinked with the continuous B I1h ase until the A cylinders are fully or partially degraded [IS··J. The first polymer we used . for this purpose was P/BA-b-PCEMA with 11=3.8xlO Z and m=6.4xlO z [15··].
Figure 4 (a)
When the njm value equaled 0.59, P/BA formed cylinders dispersed in the PCEMA matrix. Slices with thicknesses between 0.050 and 2 urn were obtained by ultrarnicrotorny and irradiated with UV light which had passed a 310 nm cutoff filter to obtain a PCEMA conversion of -38%. The films were then supported on gold TEM grids and soaked in a 0.050 M (CH3>JSiI solution in CHzCl z for two weeks for the following reaction to occur in the P/BA cylinders.
( b)
Current Op inion in Colloid & Interlace Science
Optical microscopic images, obtained at (a) parallel and (b) crossed polarizer positions, of a trace left after a spatula stroke across a PS·b·PCEMA nanofiber solution (42.5% by weight in bromoform).
Nanochannels in thin films Individual nanotubes have been prepared from carbon [56·,62] metal carbides [57], pep tides [63], and silica gel ' [64]. When the tubes are dispersed in a solid matrix, they are referred to as nanochanncls. Nanochannels with a narrow size distribution but no regular packing were prepared in polymer films by the track-etch method [65,66··,67]. Methods have also been developed for the formation of hexagonally packed nanochannels with narrow size distributions in metals [68,69], glass [70], zeolite particles [71-73], zeolite films [74,75"J, and in a
The trimethylsilyl groups were subsequently cleaved by hydrolysis in a water/methanol (v/v a 5 / 9 5 ) mixture. Upon drying, the PAA chains should collapse to the PCEMA walls to yield thin films containing nanochannels partially filled with PAA chains. Illustrated in Figure 5 is a TEM image of a small area of a SO nm thick film, where the light circles represent channels normal to the picture. The hexagonally packed channels, in this case, have a diameter of -17 nm and -the thin film has a density of -5 x 1010/cmZ• Chemical valving effect
A 2 urn thick PS-b-PAA film was mounted between two arms of a Ll-tube with a ground interface and a circular opening of -1.0 mm [77··]. Water at a height difference of h was added into the two arms. The variation in h with time, I, for aqueous solutions at different pHs was monitored. The data were fitted with:
Nanostructures of functional block copolymers Liu
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Figure 5
Transmission electron microscopy image of a small area of a SOnm thick PAA·b-PCEMA film. The PCEMA region of the film has been crosslinked and the I-butyl groups of the PtBA cylinders have been removed. Reproduced with permission from [1S··] .
h(t} = ho exp(-tlt)
where l/t is proportional to the membrane permeability of the nanochannels. Plotted in Figure 6 is the variation in 't with pH for the membrane. The enormous variation in 't with pH demonstrates the tremendous potential of the nanochannels as chemical valves [77--]. The water permeation rate was the lowest at pH = 3.0, because hydrogen bonding between the AA groups of different chains induced PAA network formation in the channels [78]. The network was destroyed at lower and higher pHs. Alternatively, a network can form with the addition of Ca Z+. In the presence of a 1.0 M CaClz solution, no water flow was observed during the course of 28hours despite a water height difference of 7.6cm
between the two arms. Ca Z+ induced nanochannel closure probably because one Ca z+ ion can bind to two AA units of different PAA chains. Potential applications
The methodology used for PCEMA-b-PAA membrane preparation is general and can be used to prepare membranes from other diblocks. Like traditional membranes, membranes produced this way should have a wide range of applications. The potential advantage of membranes produced from this method is the expected impro ved selectivity towards different permeates as a result of the uniform pore sizes. These membranes should also work well as the template for further metal or semiconductor nanostructurc fabrication [58]. The particular PCEMA-b-PAA films prepared should be useful in sensing devices and controlled drug delivery because
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Materials aspects
Park M, Harrison C. Chaikin PM, Register RA, Adamson DH: Block copolymer lithography: periodic arrays of 10 11 holes in 1 square centimeter. Science 1997, 276:1401-1404. The domain patterns of diblock copolymers in thin films were transferred to the underlying silicon nitride layer by lithography. This yielded patterns with sizes on scales that are difficult to obtain by traditional semiconductor lithography techniques. . 2. ••
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Tao J, Stewart S, Liu G, Yang M: Star and cylindrical micelles, of polystyrene-blocK-poly(2-cinnamoylethyl methacrylate)• Macromolecules 1997, 30:2738-2745. Scaling equations relating the aggregation number of PS-b-PCEMA star micelles and the PS and PCEMA block lengths were verified experimentally. A total of nine polymers were examined in this study. The data represent the best agreement with theory so far,
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• 4
6
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Guo A, Liu G, Tao J: Star polymers and nanospheres from crosslinkable diblock copolymers. Macromolecules 1996, 29:2487-2493.
Henselwood F, Liu G: Water-soluble nanospheres of poly(2cinnamoylethyl methacrylate)-block-poly(acrylic acid). Macromolecules 1997, 30:488-493. The preparation of water-soluble nanospheres from PCEMA-b-PM is reported. The nanospheres are shown to take up much DMSO from water. 5. ••
6.
Variation in r as a function of pH. Reproduced with permission from [77"].
the PAA channels close and open depending on their chemical environments. Also, the membranes should be useful in studying polymer chain reptation across the nanochannels.
Tao J, Liu G: Polystyrene-blocK-poly(2-cinnamoylethyl methacrylate) tadpole molecules. Macromolecules 1997, 30:2408-2411. In THF-cyclopentane mixtures with sufficiently.high cyclopentane content, some PS·b-PCEMA chains exist as unimolecular micelles in which the PCEMA block clusters together and the PS block assumes the normal random coil conformation. The structure of the PCEMA globule of such unimolecular micelles is locked in by intramolecular crosslinking to yield tadpole molecules. The tadpoles are separated from the nanospheres and the structure of the tadpoles is confirmed by light scattering, gas permeation chromatography, and NMR experiments. 7.
Liu G: Crosslinked Polymer Brushes. In The Polymeric Materials Encyclopedia - Synthesis,' Properties, and Applications. Edited by Salamone JC. New York: CRC Press; 1996:1548-1552.
8.
Ding J, Tao J, Guo A, Stewart S, Hu N, Birss VI, Liu G: Polystyrene-block-poly(2-cinnamoylethyl methacrylate) adsorption in the van der Waals-Buoy regime. Macromolecules 1996, 29:5398-5405.
9.
Tao J, Guo A, Stewart S, Birss VI, Liu G: Polystyrene-blockpoly(2-cinnamoylethyl methacrylate) adsorption in the Buoydominated regime. Macromolecules 1998, 31:172-175.
10.
Tao J, Guo A, Liu G: Adsorption of polystyrene-blocK-poly(2' cinnamoylethyl methacrylate) by silica from block-selective solvent mixtures. Macromolecules 1996, 29:1618-1624.
Conclusions The range of nanostructures that have been prepared are diverse. The underlying principle behind preparation of nanostructures is, however, straightforward and involves diblock self-assembly, locking in the mesophasic structure by crosslinking one block, and sometimes the degradation of the other block. .The block copolymer nanostructures prepared may 'have applications not only in the general area of nanosized electronic device manufacturing but also in fields such as catalysis, controlled drug delivery, water reclamation, separation science, and nanocomposite preparation etc. While 01;lr work has, so far, been restricted .to the use of diblock copolymers, the use of triblock copolymers in the future will further expand the repertoire of functional polymeric nanostructures.
11.
Ding J, Birss VI, Liu G: Formation and properties of polystyrene· block'poly(2-cinnamoylethyl methacrylate) brushes studied by surface-enhanced Raman scattering and transmission electron microscopy. Macromolecules 1997, 30:1442-1448. The uniform thickness and the layered structure of a polymer brush (diblock monolayers) has been observed directly for the first time with an electron microscope. Surface-enhanced Raman scattering was used to establish brush formation conditions. 12.
Acknowledgements J Ding,
L Qiao, and B Dyrnov and F Henselwood contributed to the work reviewed. My collaborators on some aspects of these projects include G Wang of Nankai University, China and J Gleeson of Kent State University. The Natural Sciences and Engineering Research Council of Canada is thanked for its Research Grants, Equipment Grants, and an Industrial Oriented Research Grant to myself as well as a Strategic Grant to VI Birss and myself. The generous' support provided by the Environmental Sciences and Technology Alliance of Canada and VX Optronics is also gratefully acknowledged.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • •• 1.
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14. ••
Ding J, Liu G: Hairy, semi-shaved, and fully-shaved hollow nanospheres from polyisoprene-blocK-Poly(2-cinnamoylethyl methacrylate). Chem Mater 1998,10:537-541. PI-b-PCEMA.formed vesicles with PCEMA as the solid shell. PI chains stretched into the solution phase from both the outer and inner surfaces of the shell. Crosslinking the PCEMA shell yielded hairy hollow nanospheres. Superb control in nanoengineering was demonstrated by selectively cutting off the outer PI chains, while maintaining the integrity of the PI chains in the inner cavity. 15. ••
Liu G, Ding J, Guo A, Herfort M, Bazett-JonesD: Potential skin layers for membranes with tunable nanochannels. Macromolecules 1997, 30:1851·1853. Hexagonally-packed nanochannels partially filled with PM chains were prepared in crosslinked PCEMA thin films. The procedure for PtBA-b-PCEMA in the PCEMA synthesis and nanochannel preparation were reported. The formation of nanochannelsin the PCEMA matrixwas verified by TEM studies.
Nanostruetures of functional block copolymers Liu
16. •
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48. ••
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Nanostructures of functional block copolymers Guojun Liu Block copolymers were long known to self-assemble and form various mesophasic structures under appropriate conditions. Other than a few scattered studies, advantage was not taken of these mesophasic structures for the preparation of 'permanent' or crosslinked nanostructures. Recently diblocks containing photocrosslinkable and sometimes degradable blocks have been synthesized. The different domains of the mesophases of these diblocks were crosslinked and sometimes degraded to yield nanostructures including nanospheres, nanofibers and nanochannels in polymer thin films. Some of the advances that have taken place of late are in the preparation, characterization and application of these three types of nanostructured materials.
Addresses Department of Chemistry,The University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1N4; e-mail: [email protected] Current Opinion in Colloid & Interface Science 1998, 3:200-208 Electronic identifier: 1359-0294'003'00200
© Current Chemistry Ltd ISSN 1359-0294 Abbreviations DMSO dimethol sulfoxide PAA poly(acrylic acid) PCEMA poly(2-cinnamoylethylmethacrylate) PS polystyrene PtBA poly(t-butyl acrylate)
Introduction A nanostructure has its smallest dimension in the 1-100nm size range [1]. The preparation and study of nanostructures have attracted much attention in the past decade mainly to meet the industrial demand for ever smaller electronic devices [1,Zoo]. Semiconductor nanostructures are also interesting for their unique properties derived from the quantum size effect. Further interest is derived from the expectation that composites made from nanoparticles may be useful as high performance construction materials and it is thought [1] that some nano-engineered materials may imitate the function of proteins and enzymes in molecular recognition. Nanostructures are typically prepared from inorganic or organic precursors. Polymers and, in particular,' block copolymers form various mesophasic structures including spherical, vesicular, and cylindrical micelles under appropriate conditions. These mesophasic structures have their smallest dimensions in the nanometer range. By cross-linking or locking in these mesophasic structures, my co-workers and I prepared an array of permanent mesophasic structures which we refer to as polymeric nanostruc-
trures, The structures we prepared included star polymers [3°,4], nanospheres [4,5°°], tadpole molecules [6°], crosslinked polymer brushes (monolayers) [7-10,11°,1Z], and nanofibers [13°°]. The nanospheres can be further divided into hairy nanospheres [4,5°°] (nanospheres with polymer chains on their surfaces), nanospheres with cross-linked shells [7-10,11°,1Z], and hollow nanospheres [14°°]. More sophisticated nanostructures such as nanochannels in thin films [15°°], shaved nanospheres [16°), and semishaved and fully shaved hollow nanospheres [14°°] were obtained by combining crosslinking and degradation treatment to these mesophasic structures. In this article, I will briefly review mesophase formation from block copolymers. The preparation of nanospheres, nanofibers, and nanochannels in polymer thin films from block copolymers will be described; and the potential applications of the three types of nanostructured materials will also be discussed.
Mesophase formation from block copolymers A copolymer is a macromolecule containing two or more types of basic units or monomers. A block copolymer is a linear copolymer in which the different monomers occur in long sequences or blocks. The simplest type of block copolymers are diblock copolymers, (AMB)m' which consists of two linear polymer chains with 11 units of A and m units of B joined together head to tail. The two functional diblock copolymers we first synthesized and used in nanostructure fabrication are polystyrene (PS)b/ock-poly(Z-cinnamoylethyl methacrylate) (PCEMA) [17] and polytz-buryl acrylate) (P/BA)-b/ock-PCEMA [soo,1Soo] (Figure 1).
Figure 1
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Current Opinion in Colloid & Interface Science
Structure of PS-b-PCEMA and PtBA-b-PCEMA
Nanosuuctures of functional block copolymers Liu
These block copolymers are functional because PCEMA is photo-crosslinkable and the r-butyl groups of P/BA are easily hydrolyzable. Other functional diblock copolymers we synthesized and used include PI-b-PCEMA [14") and PCEMA-b-PAA [5··1, where PI and PAA denote polyisoprene and poly(acrylic acid), respectively. In all cases, the precursors to the diblocks were prepared by anionic polymerization and thus the polymers. had low polydispersity. Block copolymers self-assemble and form mesophasie structures both in bulk and in solution. In a block-selective solvent.; a diblock copolymer may, for example, form spherical micelles with the insoluble block making up the core and the soluble block forming the corona stretching into the solution phase [18-28) . If the thermodynamic equilibrium sizes of the micelles are achieved, the micellar sizes should scale following theoretical relations derived by de Gennes [29) and Halperin [30); this was recently verified experimentally by my colleagues and I [3·). The diameters of the micelles are typically -100 nm. Block copolymers can also form vesicular [31,32,33··,34·), cylindrical [3·,32,34·,35], and donut-shaped micelles [34·) in a block-selective solvent. The exact morphology of the micelles formed depends on the relative lengths of the tWO blocks of a diblock and the compos ition of a binary mixture used as the solvent [32,33··,34·). In general, diblocks with a short soluble and a long insoluble block tend to form vesicles, cylinders, or donuts. Increasing the content of the block-selective solvent in a binary solvent mixture relative to that of the mutual solvent, which dissolves both blocks of a diblock, favors vesicle, cylinder, or donut formation. A diblock copolymer, (A)n(B)m, forms mesophases in bulk because the constitutent blocks segregate from one another. The size of the segregated A or B domains is similar to that of the individual A or B chains in solution. The domain shape of A or B varies with the relative Ii and m values and the temperature [36··,37,38·,39·). As the content of B is increased gradually to .- 50%, the shape of the B domain normally changes from spheres to cylinders, gyroids, and lamella (with the cylinders most preferred, and the lamellae least preferred). Further increase in the B volume fraction will make B the continuous phase and A will have the various morphologies as mentioned above for B (i.e. spherical, cylindrical and gyroidal). Domains of different blocks, such as cylinders of B dispersed in the continuous matrix of A, are normally ordered within grains in the micrometer range. Long-range or macroscopic scale ordering of the mesophasie domains can be achieved by mechanically shearing [40·) or electrically pooling the samples at elevated temperatures [41).
Hairy nanospheres Preparation
The preparation of hairy. nanospheres from a diblock copolymer involves two steps: preparation of spherical
201
micelles in a block-selective solvent and the locking in of the micellar structure. Several groups ach ieved spherical diblock micelle crosslinking using thermal [42,43J or photo initiators [44,45). Such strategies worked well in the solid state, in which the rate of domain fixation was much faster than that of micelle chain exchange [46,47J. In solution, insoluble products were sometimes obtained due to micelle fusion during core crosslinking [42,43J. My co-workers and I prepared hairy nanospheres from PI-b-PCEMA [16·), PS-b·PCEMA [3·,4), and -PCEMA-bPAA [5··) containing the self-crosslinkable PCEMA block. Using these diblocks, soluble hairy nanospheres were always obtained [3·,4;5··,16·). Water-soluble nanospheres were prepared from PCEMAb-PAA with 3.6x th 10z units ofCEMA and S.6x lOz units of AA ([48··,49). Micelles with PCEMA as the core and PAA as the corona were obtained by stirring the polymer in hot water. UV photolysis of the micellar solutions locked in the micellar structure, as was verified by the stability of the nanospheres in dimethyl sulfoxide (DMSO), a mutual solvent for PCEMA and PAA. Illustrated in Figure 2 is a TEM image of some of these nanospheres. Potential applications
PCEMA-b-PAA nanospheres have many very interesting characteristics which make them potentially useful in the oil, gas and pharmaceutical industries. One interesting property, for example, is that PCEMA·b-PM nanospheres possess hydrophobic PCEMA cores and hydrophilic PAA coronae. The cores can take up many organic compounds from water. A typical hydrodynamic radius, Rh. of 48 nm for a PCEMA-b-PAA nanosphere sample in water was, for example, increased to 62 nm in water-DMSO solution with 2.5% DMSO by volume [5··). This represents a nanosphere volume increase of 116% due to DMSO uptake. The nanospheres can also take up solid organic compounds such as perylene from water [48·· 49). The partition equilibrium between the nanospheres and perylene was established with in one day. The coefficient of perylene partition between water and the nanosphere core was determined to be large, -6x 106 [48··]. The capacity of perylene uptake by the nanospheres was, however, low, -3 mg/g. This capacity was increased fourfold by the addition of 1% DMSO to water and by a factor of 100 in water-acetone mixtures with 33% acetone by volume [49). Thus, the capacity increased by swelling the nanosphere cores. More interesting is the fact that the nanospheres precipitated out, dragging perylene with them, when a 1.0 M solution of CaClz was added to the nanosphere solution so that the final calcium concentration was -2.0 x 10-3 M. The precipitated nanospheres could then be extracted with an organic solvent such as THF, which
202
Materials aspects
Figure 2
200 nm TEM image of PCEMA·b-PAA nanospheres, magnification x135,000. The sample was aspirated as an aqueous solution on Formvar·covered copper grids and stained with'Os0 4 before viewing. The cross-linked micelles are spherical with a mean diameter of -35 nm.
removes the perylene. Once the perylene is removed, the nanospheres can be redispersed in water by adding sodium carbonate to precipitate out Ca Z+ or by adding EDTA, which complexes with Ca 2+. On the basis of these observations (0 Liu, unpublished data) I believe that the nanospheres should be useful in cleaning up tailings ponds left behind by the oil and gas industry. I In such ponds, hydro carbons with relatively low mol ar mas s should function to plasticize the core of the nanospheres. These plasticized nanospheres could then take up a large amount of high molar mass components such as ashphaltenes. An alternative use may be in the pharmaceutical industry where the nanospheres, [50,51] like water-soluble block copolymer micelles, may be employed as drug carriers in controlled drug release.
Block copolymer nanofibers Preparation
A nanofiber is a fiber with a nanometer-sized diameter. Nanofibers have been prepared previously mostly from inorganic or organic precursors through the template method. One method of preparing nanofibers [52-55] is, for example, by filling the nanometer-sized carbon or boron carbide rubes [56·,57] with metal oxides at elevated temperatures. Other templates used include the cylindrical pores in a polymer membrane or in anodized alumina [58]. More recently, 'soft templates' such as reverse micelles have been used for inorganic nanofiber or nanorod preparation [59]. In addition to the template method, carbon nanofibers have been prepared at particular crystal faces as a result of the decomposition of
Nanostructures of functional block copolymers Uu
203
Figure 3
"
,
"
I urn Transmission electron microscopy image of PS·b-PCEMA nanofibers. Reproduced with permission from [13··}.
selected hydrocarbons [60]. Using nickel nanoparticles as the catalyst, the decomposition of trichloromethyl silane at high temperatures en abled the preparation of SiC nanofibers [61]. My co-workers and I prepared the first polymeric nanofiber sample from PS-b-PCEMA with 1.25x 103 units of styrene and 158 units of CEMA. This polymer, in which'PCEMA existed as cylinders dispersed in the PS matrix, has a PCEMA weight fraction of 24%. Photolysis of a solid sample crosslinked the PCEMA cylinders. Separating different cylindrical dom ains by dissolving the PS phase yielded nanofibers as shown in Figure 3. The nanofibers have a diameter of -50 nm and a length of -20 urn, At higher magnifications, the nanofibers were shown to have PCEMA cores and PS shells.
Potential applications of block copolymer nanofibers Since microdomain formation occurs in most block copolymers, this repre sents a general method for preparing nanofibers with un iform sizes. I expect th is method to be useful in produ cing precursor fibers which can be pyrolyzed coyield carbon or metal carbide nanofibers. This method can also be modified for making nanowires by replacing the core block with a conductive polymer. In th at case, the outer block would function as an insulating plastic layer. Recently, the PS-b-PCEl\IA nanofibers were found to be soluble in bromoform at all mixing ratios (8 Dymov, ] Ding, ] Gleeson, G Liu, unpublished data). Illustrated in Figure 4 are polarized optical microscopic images of a
204
Materials aspects
track left in a 42.5% by weight PS-b-PCEMA nanofiber solution in bromoform, after a sp atula stroke through the solution (B Dymov, J Ding, J Gleeson, G Liu, unpublished dat a). The birefringence suggests the alignment of the fibers along the shearing direction. As the relative position of the polarizers on the incident and transmitted beam side changes, the bright regions in the image turn dark and vice versa, as expected. While such nanofibers, particularly water-soluble nanofibers, may be' useful as 'environmentally friendly' liquid crystals, a further study on the response rate . of such nanofibers to changes in electric fields is required. The liquid crystalline properties ofthe nanofibers would certainly facilitate the preparation of macroscopic fibers or ropes consisting o'r well-aligned nanofibers or nanofilarnents.
diblock solid [76]. No evaluation of thin films containing hexagonally packed nanochannels as - membranes has, however, been reported. No evaluation was performed of the porous diblock solid sample prepared by Lee el of. [76], probably because the objective of Lee el 01. was to create some porous particulate materials rather than porous thin films. The zeolite films were not evaluated because the nanochannels lie in the place of the zeolite films [73,75··] and these films in this form are not applicable as membranes. Metals with nanochannels were not evaluated as membranes, because metals are normally for the more common metals such as iron and aluminum and the high cost for the rare metals. Preparation
The procedure for membrane preparation from a diblock copolymer involves firstly the synthesis of a diblock copolymer (A)n(B)m with a degradable A block and a crosslinkable B block, followed by the preparation of (A)n(B)m solid with A forming the .regularly packed cylinders dispersed in the continuous B matrix. Thin films of the diblock are then obtained using microtomy and are crosslinked with the continuous B I1h ase until the A cylinders are fully or partially degraded [IS··J. The first polymer we used . for this purpose was P/BA-b-PCEMA with 11=3.8xlO Z and m=6.4xlO z [15··].
Figure 4 (a)
When the njm value equaled 0.59, P/BA formed cylinders dispersed in the PCEMA matrix. Slices with thicknesses between 0.050 and 2 urn were obtained by ultrarnicrotorny and irradiated with UV light which had passed a 310 nm cutoff filter to obtain a PCEMA conversion of -38%. The films were then supported on gold TEM grids and soaked in a 0.050 M (CH3>JSiI solution in CHzCl z for two weeks for the following reaction to occur in the P/BA cylinders.
( b)
Current Op inion in Colloid & Interlace Science
Optical microscopic images, obtained at (a) parallel and (b) crossed polarizer positions, of a trace left after a spatula stroke across a PS·b·PCEMA nanofiber solution (42.5% by weight in bromoform).
Nanochannels in thin films Individual nanotubes have been prepared from carbon [56·,62] metal carbides [57], pep tides [63], and silica gel ' [64]. When the tubes are dispersed in a solid matrix, they are referred to as nanochanncls. Nanochannels with a narrow size distribution but no regular packing were prepared in polymer films by the track-etch method [65,66··,67]. Methods have also been developed for the formation of hexagonally packed nanochannels with narrow size distributions in metals [68,69], glass [70], zeolite particles [71-73], zeolite films [74,75"J, and in a
The trimethylsilyl groups were subsequently cleaved by hydrolysis in a water/methanol (v/v a 5 / 9 5 ) mixture. Upon drying, the PAA chains should collapse to the PCEMA walls to yield thin films containing nanochannels partially filled with PAA chains. Illustrated in Figure 5 is a TEM image of a small area of a SO nm thick film, where the light circles represent channels normal to the picture. The hexagonally packed channels, in this case, have a diameter of -17 nm and -the thin film has a density of -5 x 1010/cmZ• Chemical valving effect
A 2 urn thick PS-b-PAA film was mounted between two arms of a Ll-tube with a ground interface and a circular opening of -1.0 mm [77··]. Water at a height difference of h was added into the two arms. The variation in h with time, I, for aqueous solutions at different pHs was monitored. The data were fitted with:
Nanostructures of functional block copolymers Liu
205
Figure 5
Transmission electron microscopy image of a small area of a SOnm thick PAA·b-PCEMA film. The PCEMA region of the film has been crosslinked and the I-butyl groups of the PtBA cylinders have been removed. Reproduced with permission from [1S··] .
h(t} = ho exp(-tlt)
where l/t is proportional to the membrane permeability of the nanochannels. Plotted in Figure 6 is the variation in 't with pH for the membrane. The enormous variation in 't with pH demonstrates the tremendous potential of the nanochannels as chemical valves [77--]. The water permeation rate was the lowest at pH = 3.0, because hydrogen bonding between the AA groups of different chains induced PAA network formation in the channels [78]. The network was destroyed at lower and higher pHs. Alternatively, a network can form with the addition of Ca Z+. In the presence of a 1.0 M CaClz solution, no water flow was observed during the course of 28hours despite a water height difference of 7.6cm
between the two arms. Ca Z+ induced nanochannel closure probably because one Ca z+ ion can bind to two AA units of different PAA chains. Potential applications
The methodology used for PCEMA-b-PAA membrane preparation is general and can be used to prepare membranes from other diblocks. Like traditional membranes, membranes produced this way should have a wide range of applications. The potential advantage of membranes produced from this method is the expected impro ved selectivity towards different permeates as a result of the uniform pore sizes. These membranes should also work well as the template for further metal or semiconductor nanostructurc fabrication [58]. The particular PCEMA-b-PAA films prepared should be useful in sensing devices and controlled drug delivery because
206
Materials aspects
Park M, Harrison C. Chaikin PM, Register RA, Adamson DH: Block copolymer lithography: periodic arrays of 10 11 holes in 1 square centimeter. Science 1997, 276:1401-1404. The domain patterns of diblock copolymers in thin films were transferred to the underlying silicon nitride layer by lithography. This yielded patterns with sizes on scales that are difficult to obtain by traditional semiconductor lithography techniques. . 2. ••
Figure 6
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•
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Tao J, Stewart S, Liu G, Yang M: Star and cylindrical micelles, of polystyrene-blocK-poly(2-cinnamoylethyl methacrylate)• Macromolecules 1997, 30:2738-2745. Scaling equations relating the aggregation number of PS-b-PCEMA star micelles and the PS and PCEMA block lengths were verified experimentally. A total of nine polymers were examined in this study. The data represent the best agreement with theory so far,
•
• • 2
4.
• 4
6
8
10
12
14
pH
Guo A, Liu G, Tao J: Star polymers and nanospheres from crosslinkable diblock copolymers. Macromolecules 1996, 29:2487-2493.
Henselwood F, Liu G: Water-soluble nanospheres of poly(2cinnamoylethyl methacrylate)-block-poly(acrylic acid). Macromolecules 1997, 30:488-493. The preparation of water-soluble nanospheres from PCEMA-b-PM is reported. The nanospheres are shown to take up much DMSO from water. 5. ••
6.
Variation in r as a function of pH. Reproduced with permission from [77"].
the PAA channels close and open depending on their chemical environments. Also, the membranes should be useful in studying polymer chain reptation across the nanochannels.
Tao J, Liu G: Polystyrene-blocK-poly(2-cinnamoylethyl methacrylate) tadpole molecules. Macromolecules 1997, 30:2408-2411. In THF-cyclopentane mixtures with sufficiently.high cyclopentane content, some PS·b-PCEMA chains exist as unimolecular micelles in which the PCEMA block clusters together and the PS block assumes the normal random coil conformation. The structure of the PCEMA globule of such unimolecular micelles is locked in by intramolecular crosslinking to yield tadpole molecules. The tadpoles are separated from the nanospheres and the structure of the tadpoles is confirmed by light scattering, gas permeation chromatography, and NMR experiments. 7.
Liu G: Crosslinked Polymer Brushes. In The Polymeric Materials Encyclopedia - Synthesis,' Properties, and Applications. Edited by Salamone JC. New York: CRC Press; 1996:1548-1552.
8.
Ding J, Tao J, Guo A, Stewart S, Hu N, Birss VI, Liu G: Polystyrene-block-poly(2-cinnamoylethyl methacrylate) adsorption in the van der Waals-Buoy regime. Macromolecules 1996, 29:5398-5405.
9.
Tao J, Guo A, Stewart S, Birss VI, Liu G: Polystyrene-blockpoly(2-cinnamoylethyl methacrylate) adsorption in the Buoydominated regime. Macromolecules 1998, 31:172-175.
10.
Tao J, Guo A, Liu G: Adsorption of polystyrene-blocK-poly(2' cinnamoylethyl methacrylate) by silica from block-selective solvent mixtures. Macromolecules 1996, 29:1618-1624.
Conclusions The range of nanostructures that have been prepared are diverse. The underlying principle behind preparation of nanostructures is, however, straightforward and involves diblock self-assembly, locking in the mesophasic structure by crosslinking one block, and sometimes the degradation of the other block. .The block copolymer nanostructures prepared may 'have applications not only in the general area of nanosized electronic device manufacturing but also in fields such as catalysis, controlled drug delivery, water reclamation, separation science, and nanocomposite preparation etc. While 01;lr work has, so far, been restricted .to the use of diblock copolymers, the use of triblock copolymers in the future will further expand the repertoire of functional polymeric nanostructures.
11.
Ding J, Birss VI, Liu G: Formation and properties of polystyrene· block'poly(2-cinnamoylethyl methacrylate) brushes studied by surface-enhanced Raman scattering and transmission electron microscopy. Macromolecules 1997, 30:1442-1448. The uniform thickness and the layered structure of a polymer brush (diblock monolayers) has been observed directly for the first time with an electron microscope. Surface-enhanced Raman scattering was used to establish brush formation conditions. 12.
Acknowledgements J Ding,
L Qiao, and B Dyrnov and F Henselwood contributed to the work reviewed. My collaborators on some aspects of these projects include G Wang of Nankai University, China and J Gleeson of Kent State University. The Natural Sciences and Engineering Research Council of Canada is thanked for its Research Grants, Equipment Grants, and an Industrial Oriented Research Grant to myself as well as a Strategic Grant to VI Birss and myself. The generous' support provided by the Environmental Sciences and Technology Alliance of Canada and VX Optronics is also gratefully acknowledged.
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