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Nanostructured Materials

  • Nanocomposites
    • Review Articles
    • Linear Hybrid Copolymers
  • Hybrid Brush Copolymers
    • Spherical particles
    • Flat surfaces
  • Carbon Nanostructures

Nanocomposites

Review Articles In the broader sense any biphasic material where each phase has a well defined structure, i.e. tens of nanometers in size, is a nanocomposite. In this section of our web page we are concerned with the synthesis of organic/inorganic hybrid materials which is an area of growing interest as the useful properties of disparate components can be combined into a single material.

Controlled/"Living" Radical Polymerization (CRP) has been shown to be suitable for the preparation of organic/inorganic hybrid materials with varying structural complexity on nano-, meso- and micro-scopic dimensions. Atom transfer radical polymerization (ATRP) has been particularly successful for the synthesis of nano-composite structures since inorganic particles and substrates can be easily functionalized with either polymerizable groups, or initiating alkyl halides and the resulting functional inorganic material is suitable for use in the CRP of organic vinyl monomers. (1-6)

Target applications include: surfactants, elastomers, opto/magnetic materials, sensors, reinforced ultra-thin films and patterned surfaces.

(1) "Synthesis of Nanocomposite Organic/Inorganic Hybrid Materials Using Controlled/\"Living\" Radical Polymerization." Pyun, J. and Matyjaszewski K., Chem Mater 13(10): 2001, 3436-3448.

(2) "Synthesis of polymer brushes using atom transfer radical polymerization;" Pyun, J.; Kowalewski, T.; Matyjaszewski, K. Macromolecular Rapid Communications 2003, 24, 1043-1059.

(3) "Organic-inorganic hybrid materials from polysiloxanes and polysilsesquioxanes using controlled/living radical polymerization;" Pyun, J.; Xia, J.; Matyjaszewski, K. ACS Symposium Series 2003, 838, 273-284.

(4) "Polymer brushes by atom transfer radical polymerization"; Pyun, J.; Kowalewski, T.; Matyjaszewski, K. Polymer Brushes 2004, 51-68.

(5) "Preparation of nanocomposites with well-defined organic (co)polymers, Matyjaszewski, K. NATO Science Series, II: Mathematics, Physics and Chemistry 2004, 175, 123-134.

(6) "Grafting from Surfaces for \"Everyone\": ARGET ATRP in the Presence of Air," Matyjaszewski, K.; Dong, H.; Jakubowski, W.; Pietrasik, J.; Kusumo, A. Langmuir 2007, 23, 4528-4531

Linear Hybrid Copolymers Hybrid segmented copolymers have been prepared by incorporation of inorganic oligomers/polymeric initiators or macromonomers into the ATRP of styrenic and (meth)acrylate monomers. Block and graft copolymers containing poly(dimethylsiloxane (PDMS) segments have been synthesized by sequential living anionic ring-opening of D3 and quenching the living PDMS chain with either a compound comprising either vinyl, or alkyl halide groups and subsequent use of the functionalized PDMS as a macromonomer, (A) or macroinitiator, (B) in an ATRP. (7-9)

In a similar approach, methacrylate monomers possessing bulky polyhedral oligomeric silsesquioxane (POSS) side groups have been used in ATRP systems to prepare well-defined random and block copolymers. (10, 11)

(7) "Development of novel attachable initiators for atom transfer radical polymerization. Synthesis of block and graft copolymers from poly(dimethylsiloxane) macroinitiators;" Nakagawa, Y.; Miller, P. J.; Matyjaszewski, K. Polymer 1998, 39, 5163-5170.

(8) "Atom Transfer Radical Polymerization of (Meth)acrylates from Poly(dimethylsiloxane) Macroinitiators." Miller, P. J.; Matyjaszewski, K. Macromolecules 1999, 32(26), 8760.

(9) "Improving the Structural Control of Graft Copolymers by Combining ATRP with the Macromonomer Method." Shinoda, H.; Matyjaszewski, K. Macromolecules 2001, 34, 3186.

(10) "Synthesis of hybrid polymers Using atom transfer radical polymerization: Homopolymers and Block Copolymers from polyhedral oligomeric silsesquioxane monomers." Pyun, J.; Matyjaszewski, K. Macromolecules 2000, 33(1), 217.

(11) "Organic-inorganic hybrid materials from polysiloxanes and polysilsesquioxanes using controlled/living radical polymerization." Pyun, J., Xia, J., Matyjaszewski, K.: ACS Symposium Series 838: 2003, 273-284.

Hybrid Brush Copolymers

As noted above one advantage of ATRP is the ease with which targeted substrates can be functionalized using commercially available, or easily synthesized functional "-haloesters or benzyl halides. Functional ATRP initiators have been successfully tethered to both organic and inorganic materials with either flat (2 dimensional) or curved (3 dimensional) surfaces. As a result well-defined flat surface and spherical polymer brushes of varying composition and dimensions have been synthesized by the ATRP of organic vinyl monomers from various surfaces and colloidal particles. (12-17) Control over the degree of polymerization (DP) of each tethered segment, as well as of the functionality of the selected monomers enabled precise engineering of both surface properties and colloidal composite structures and the properties of the resulting hybrid nano-structures

One of the problems associated with use of multifunctional initiators in an ATRP is the impact of termination reactions on the properties of the final material. Termination can occur either inter-molecularly resulting in crosslinking or intra-molecularly which affects functionality and distribution of tethered chains. (13) Accordingly reaction conditions should be selected to minimize termination reactions including:

  • addition of the redox conjugate to minimize initial termination reactions that normally build up the persistent radical and to slow down propagation
  • stopping the reactions at low conversion
  • conducting the reactions in dilute solution or
  • conducting the reaction in a miniemulsion system

Tethered brush molecules have even been prepared by individual manipulation of molecules. (18) Scanning probe microscopy-based techniques were used to manipulate single molecules and deliver them in a precisely controlled manner to a specific target. The ultimate physical limit in the design and fabrication of organic surfaces can be reached using this approach. This article showed that the atomic force microscope (AFM), which has been used extensively to investigate the stretching of individual molecules, can deliver and immobilize single molecules, one at a time, on a surface. Reactive polymer molecules, attached at one end to an AFM tip, are brought into contact with a modified silicon substrate to which they become linked by a chemical reaction. When the AFM tip is pulled away from the surface, the resulting mechanical force causes the weakest bond - the one between the tip and polymer - to break. This process transfers the polymer molecule by molecule to the substrate where it can be tethered by further chemical reactions.

Spherical Particles The functionalization of the surfaces of many solids; including silica (SiO2), gold, silver, germanium, PbS, carbon black, iron oxides and other metal oxide systems has been achieved, (many references are provided in reference 14 below) allowing for subsequent attachment of initiators for the ATRP of many monomers forming organic/inorganic hybrid nanoparticles containing an inorganic core and tethered glassy or rubbery homopolymers or copolymers. The consequence of radical-radical termination is more serious during the preparation of colloidal particles than in a normal ATRP. In traditional ATRP reactions termination leads to coupling whereas with a particle with 1000's of initiation sites this leads to crosslinking; as a result

Pc= 1/ {r(fa -1)(fb -1)}1/2 -> 2/f -> 1/1000.

So gelation is predicted at 0.1% intermolecular coupling.

Gelation was initially avoided by running the reaction under high dilution conditions to low conversion, i.e. under conditions where there is a low concentration of active radicals and consequently slow propagation. (14)

However the first example of the successful synthesis of hybrid nanoparticles using multifunctional silica initiators in a miniemulsion ATRP was recently disclosed. The reaction was driven to higher conversion in a shorter time. (19)

SiO2-g-pSt hybrid nanoparticles with tethered polystyrene possessing molar masses in the range of Mn = 5,000 to 33,000 g/mol were prepared using commercially available silica nanoparticles as colloidal initiators, which greatly facilitated scale-up synthesis. The hybrid particles were characterized both in the solid state and in solution using transmission electron microscopy (TEM) and dynamic light scattering (DLS) respectively. TEM images of the SiO2-g-pSt colloids revealed the formation of (sub)monolayer patches with interparticle spacing that increased with an increase in the molar mass of the tethered polystyrene. Comparison of the hydrodynamic radii (Rh) of hybrid nanoparticles of varying size determined by DLS in toluene, versus the molar mass (Mn) of the polystyrene chains cleaved from colloids, determined by SEC, revealed a linear relationship. Such a linear dependence of Rh vs. Mn is a strong indication that when the particles are dispersed in toluene, the tethered chains adopt highly chain extended conformations, presumably due to steric interactions caused by the high grafting density. (16)

AFM examination of spherical brushes formed by grafting n-butyl acrylate from silica particles deposited on mica surfaces show that the swollen brush has collapsed and that the hard silica core is surrounded by the soft grafted chains.

Since the polymerization from the surface was being conducted by ATRP, it is simple task to isolate the particles and add them to a fresh monomer solution and form tethered block copolymers.

Tri-phasic separation was clearly observed by AFM examination of a particle with tethered poly(St-b-nBA) block copolymer chains. A hard polystyrene core surrounding the silica particle and the soft acrylate shell strongly adsorbed on the mica substrate are observed forming three distinct observable phase separations.

As noted above, conducting the "grafting from" reaction in miniemulsion systems provide some additional benefits. The reactions follow first order kinetics and can be driven at a higher rate to higher conversion without excessive production of coupled particles. Equivalent polymerization rates were observed for miniemulsion reactions initiated by alkyl halides, regardless of whether the initiators were attached to particle surfaces or free in solution. Therefore, the procedure provides a viable commercial approach to novel, functionally-designable materials with properties that can be pre-selected to target many specific applications. (20)

This success with miniemulsion relies on the compartmentalization of the reaction media which segregates the reaction medium and therefore minimizes the effect of radical termination and macroscopic gelation. Since termination reactions should be limited to individual droplets the proportion of terminated chains should be relatively small and the degree of crosslinking is less than in bulk conditions. The confinement effect increases the deactivation rate and termination rate so control is better but reaction rate is also decreased.

As shown in the AFM image below each silica particle is evenly coated with polymer and the individual particles are not crosslinked even though conversion was driven to greater than 60%. The particles were prepared with a 200:1 monomer to initiator ratio and each n-BA tethered chain has an average Mn=15,900 g/mol.

This approach is currently being applied to other multifunctional initiators, including multi-arm star molecules, molecular brushes, and other well-defined polymers with complex architectures. Indeed molecular brushes were successfully synthesized in a miniemulsion system via AGET ATRP. (21) Macroscopic gelation was observed for bulk ATRP but was not detected in miniemulsion. The side-chain polymers grew from backbones rapidly in miniemulsion droplets and high monomer conversion was reached in relatively short time. Molecular visualization by AFM proved that some cross-linking did occur in miniemulsion droplets when the conversion was high (84%). However, this cross-linking showed no effect to the miniemulsion stability and fluidity, and therefore, the synthesized materials can be easily processed for further uses/applications.

Flast Surfaces. There are several factors determining structure of the polymer layertethered to a flat surface. One is anchor distance, or distribution of tethering sites on the surface and another is polymer molecular weight. There is a direct dependence of polymer conformation on grafting density.

D = Distance between attachment points and RF = Flory radius = aNv

(RF = Flory radius, a = monomer dimension, N = degree of polymerization, v = Flory exponent, n = 0.6 (PEO in good solvent)

Four different topologies have been described depending on relationship between density of grafting sites and Flory radius

(12) "Polymers at Interfaces: Using Atom Transfer Radical Polymerization in the Controlled Growth of Homopolymers and Block Copolymers from Silicon Surfaces in the Absence of Untethered Sacrificial Initiator." Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32(26), 8716.

(13) "Synthesis of Well-Defined Block Copolymers Tethered to Polysilsesquioxane Nanoparticles and Their Nanoscale Morphology on Surfaces." Pyun, J, K Matyjaszewski, T Kowalewski, D Savin, G Patterson, G Kickelbick and N Huesing. J Am Chem Soc 2001, 123(38), 9445-9446.

(14) "Synthesis and Characterization of Organic/Inorganic Hybrid Nanoparticles: Kinetics of Surface-Initiated Atom Transfer Radical Polymerization and Morphology of Hybrid Nanoparticle Ultrathin Films." Pyun, J.; Jia, S.; Kowalewski, T.; Patterson, G. D.; Matyjaszewski, K. Macromolecules 2003, 36, 5094-5104.

(15) "Grafting Poly(n-butyl acrylate) from a Functionalized Carbon Black Surface by Atom Transfer Radical Polymerization." Liu, T.; Jia, S.; Kowalewski, T.; Matyjaszewski, K.; Casado-Portilla, R.; Belmont, J.; Langmuir 2003, 19, 6342-6345.

(16) "Synthesis and surface attachment of ABC triblock copolymers containing glassy and rubbery segments." Pyun, J.; Jia, S.; Kowalewski, T.; Matyjaszewski, K. Macromolecular Chemistry and Physics 2004, 205, 411-417.

(17) "Synthesis and characterization of silica-graft-polystyrene hybrid nanoparticles: Effect of constraint on the glass-transition temperature of spherical polymer brushes." Savin, D.A.; Pyun, J; Patterson, G.D.; Kowalewski, T.; Matyjaszewski, K. " J. Polym. Sci., Polym. Phys. Ed. 2002, 40, 2667.

(18) "Mechanochemistry: targeted delivery of single molecules," Anne-Sophie Duwez, StÈphane Cuenot, Christine JÈrÙme, Sabine Gabriel, Robert JÈrÙme, Stefania Rapino and Francesco Zerbetto. Nature Nanotechnology (2006) 1, 122 " 125.

(19) "Preparation of silica hybrids by ATRP in miniemulsion" Bombalski, L.; Min, K.; Tang, C.; Matyjaszewski, K; Polymer Preprint; 2005, 46(2), 359.

(20) "Atom transfer radical polymerization in the presence of a reducing agent;" Matyjaszewski, K.; Bombalski, L.; Jakubowski, W.; Min, K.; Spanswick, J.; Tsarevsky, N. In PCT Int. Appl.; (Carnegie Mellon University, USA). WO 2005087819, p 96.

(21) "High Yield Synthesis of Molecular Brushes via ATRP in Miniemulsion," Min, K.; Yu, S.; Lee, H.-i.; Mueller, L.; Sheiko, S. S.; Matyjaszewski, K. Macromolecules (Washington, DC, United States), ACS ASAP 10-Aug. 2007.

Carbon Nanostructures

The field of carbon structures was reenergized by the discovery of fullerenes in 1985 (22) and carbon nanotubes in 1991. (23) In addition to generating tremendous interest in the fundamental properties of discrete carbon molecules and nano-objects these nano-structured carbons have found, or are expected to have, numerous commercial applications such as advanced fillers, materials for energy and gas storage, sensors and elements for molecular electronics devices. Two different strategies have been developed for the preparation of engineering carbon materials; one includes pyrolysis of organic precursors (mostly polymeric) under an inert atmosphere to yield large-scale engineering carbons and the other involves physical/chemical vapor deposition techniques that produce well-defined nanostructured carbons. Whereas techniques from the first group are applicable to large scale production, they offer very limited control over carbon (nano)structure, techniques from the second group do allow atomic scale precision in control of the nanostructure but they are relatively expensive, have limited yield and require complex equipment.

Recently, we have developed a novel, low-cost route to well-defined nanostructured carbon materials based on the pyrolysis of polyacrylonitrile (PAN) block copolymer precursors containing a sacrificial block (e.g., poly(n-butyl acrylate). (24, 25) The structure of the final carbon nanostructure is templated by the initial structure of the PAN domains in the phase separated block copolymer. The PAN domains are stabilized by heating to 230 &Mac176;C in the presence of air and subsequent pyrolyized by heating to 800&Mac176;C in an inert environment. This converts the PAN domains into partially graphitic carbon, whereas the sacrificial phase is volatilized. Stabilization therefore allows the PAN domains to retain their phase separated nanostructure throughout the thermal treatment.

The image of carbon nanostructures shown above (similar to the images in reference 24) is a result of the self-assembly of phase separated PAN-b-PBA block copolymer with spherical PAN domains that was prepared by ATRP and was then converted into discrete carbon nanoclusters (D~30 nm) by fixing the PAN domains through crosslinking in air and then pyrolyzing under nitrogen. The carbon structures exhibited high surface area, high thermal conductivity and have potential for catalytic activity.

When the mole fraction of the PAN is increased and a poly(BA)251-b-poly(AN)320 block copolymer is prepared and the block copolymer is cast from solution it forms cylinders or lamellae as shown below in the image on the left. When a similar block copolymer was stabilized and pyrolyzed it provided the carbon nanowires shown in the image on the right. The structures are similar to those seen in multiwalled carbon nanotubes. The carbon nanostructures are expected to find application in photovoltaics, field emitters and supercapacitors. (25)


The spectrum of carbon structures that can be prepared from PAN segmented copolymers can span the continuum from carbon nanoparticles, prepared from water soluble shell crosslinked micelles with a PAN core, (26) through lamellar structures (27) to porous carbon structures. (28-30) The later materials were prepared by using PEO-b-PAN diblock copolymers as templates for the synthesis of mesoporous silicas, since PEO is known to promote the condensation of silica under acidic conditions. The templated PAN was stabilized then pyrolyzed prior to dissolution of the silica in aqueous sodium hydroxide forming porous carbons with high surface areas, ~900 m2 g-1.

In the images shown above, the block copolymers were drop cast onto the substrate and the phase separated domains did not exhibit any long range order. However when precursor films were prepared by zone-casting (31) they can display long-range order. The motivation was the preparation of macroscopically aligned anisotropic carbon structures suitable for electron transport or magnetic data storage (32)

Zone-casting of PBA-b-PAN block copolymers and subsequently thermal conversion into anisotropic carbon.

Zone casting involves deposition of a polymer solution at a controlled temperature on a moving substrate whose temperature is also controlled. The task was performed using a custom-built apparatus, equipped with two computer controlled linear stages and independently-controlled solution and substrate heaters. The upper image above shows the morphology attained when a diblock copolymer, PBA-b-PAN block copolymer, is cast from dimethylformamide solution at 90 oC. The lower images show the morphology attained when a triblock copolymer with a crystallizable segment, such a poly(t-butyl acrylate)-b-poly(octadecyl methacrylate-b-poly(t-butyl acrylate) PtBA-b-PODMA-b-PtBA) is cast onto the moving substrate at two different temperatures, one below the Tm and the other above the Tm.

Zone-casting a PtBA-b-PODMA-b-PtBA block copolymers with crystalline segments at different casting temperature (Left: T>Tm; Right: T<Tm)

In the first case, a block copolymer with a number molecular weight of 37,500 g/mol (with the structural formula (BA)240(AN)124) and polydispersity index of 1.22 (from gel permeation chromatography (GPC)) was prepared and subjected to zone casting. The long-range ordered lamellae were perpendicular to the substrate and casting direction. Upon thermal stabilization and carbonization, the precursors were converted into anisotropic carbon with long-range order. (27) In the second case, the orientation of the nano-morphology is governed by the process of nucleation and crystallization of the crystallizable phase. The lamellar structure is aligned along the casting direction through the crystallization of the pODMA domains if the casting temperature is below melting point (Tm) of crystallizable segments (32).

Other materials containing PAN with different architectures can be used as the sacrificial phase to prepare nano-structured carbon. Nano-porous carbon with high surface area was prepared by conducting a grafting from polymerization of acrylonitrile from the surface of silica nanoparticles by ATRP. The (PAN)-grafted silica nanoparticles were cast into a film, stabilized, carbonized and then etched to form highly nanoporous carbon films. The motivation was the preparation of a carbon membrane suitable for gas adsorption, filtration and separation or use in photovoltaic cells. The TEM image clearly shows that a well structured material was prepared. (28-30) The structure had the capacity to adsorb a significant volume of nitrogen.

Well defined carbon structures were also prepared by preparing densely grafted brushes molecules with polyacrylonitrile (PAN) di-(AB) and triblock-(ABC) copolymer side chains. (33) Thin films of the SC brushes were prepared by drop casting aqueous solutions onto clean silicon wafer substrates. The samples were then subjected to annealing at 250 &Mac176;C in the presence of air in order to stabilize PAN domains, and subsequently pyrolyzed at 600 &Mac176;C under N2 flow. After thermal treatment, SC brush precursors were converted into nanostructured carbon. AFM images of such films showed the surface with characteristic round protrusions and rms roughness of 4.0 nm. Such morphology was consistent with the film composed of well-defined, discrete nano-objects.


(22) "C60: buckminsterfullerene" H. W. Kroto, J. R. Heath, S. C. O"Brien, R. F. Curl, R. E. Smalley, Nature 1985, 318, 162-163.

(23) "Helical microtubules of graphitic carbon." S. Iijima, Nature 1991, 354, 56-58.

(24) "Nanostructured Carbon Arrays from Block Copolymers of Polyacrylonitrile." T. Kowalewski, N. V. Tsarevsky, K. Matyjaszewski, J. Am. Chem. Soc. 2002, 124, 10632-10633.

(25) "Complex nanostructured materials from segmented copolymers prepared by ATRP." T. Kowalewski, R. D. McCullough, K. Matyjaszewski, Eur. Phys. J. E. 2003, 10, 5.

(26) "Well-defined carbon nanoparticles prepared from water-soluble shell cross-linked micelles that contain polyacrylnitrile cores." Tang, C.; Qi, K.; Wooley, K. L.; Matyjaszewski, K.; Kowalewski, T. Angew. Chemie, 2004, 43, 2783-2787.

(27) "Long-Range Ordered Thin Films of Block Copolymers Prepared by Zone-Casting and Their Thermal Conversion into Ordered Nanostructured Carbon." Tang, C.; Tracz, A.; Kruk, M.; Zhang, R.; Smilgies, D.-M.; Matyjaszewski, K.; Kowalewski, T. J. Am. Chem. Soc., 2005, 127, 6918-6919.

(28) "Well-Defined Poly(ethylene oxide)-Polyacrylonitrile Diblock Copolymers as Templates for Mesoporous Silicas and Precursors for Mesoporous Carbons" Kruk, M.; Dufour, B.; Celer, E. B.; Kowalewski, T.; Jaroniec, M.; Matyjaszewski, K. Chemistry of Materials 2006, 18, 1417-1424.

(29) "Synthesis of Mesoporous Carbons Using Ordered and Disordered Mesoporous Silica Templates and Polyacrylonitrile as Carbon Precursor." Kruk, M.; Dufour, B.; Celer, E. B.; Kowalewski, T.; Jaroniec, M.; Matyjaszewski, K. J. Phys. Chem. B, 2005, 109, 9216-9225.

(30) "Partially graphitic, high-surface-area mesoporous carbons from polyacrylonitrile templated by ordered and disordered mesoporous silicas," Kruk, M.; Kohlhaas, K. M.; Dufour, B.; Celer, E. B.; Jaroniec, M.; Matyjaszewski, K.; Ruoff, R. S.; Kowalewski, T. Microporous and Mesoporous Materials 2007, 102, 178-187.

(31) "Highly anisotropic conductive materials: polymers doped with crystalline charge-transfer complexes." Burda, L.; Tracz, A.; Pakula, T.; Ulanski, J.; Kryszewski, M. Journal of Physics D: Applied Physics 1983, 16, 1737-1740.

(32) "Controlling long range ordered structures from ABA type block containing crystalline domains" Wu, W.; Matyjaszewski, K.; Kowalewski, T.; Polymer Preprints 2005, 46, 804-805.

(33) "Synthesis and Morphology of Molecular Brushes with Polyacrylonitrile Block Copolymer Side Chains and Their Conversion into Nanostructured Carbons," Tang, C.; Dufour, B.; Kowalewski, T.; Matyjaszewski, K. Macromolecules 2007, 40, 6199-6205.

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