SCIENCE & TECHNOLOGY ANALYSIS Boyes (sitting) and Brittain examine polymer brushes prepared by living radical polymerization techniques.
to low grafting density and, Brittain noted, is not commonly used. Grafting-from is more common, he said. With this technique, initiator molecules are immobilized on a surface and exposed to a monomer under appropriate polymerization conditions. Advances in the polymer brush field are closely allied to developments in nanotechnology because the brushes require control of polymer structure on the nanometer scale. The research is driven partly by the use of increasingly sophisticated techniques for polymer-brush synthesis and partly by advances in thin-film characterization techniques such as atomic force microscopy
F R O M T H E ACS
MEETING
BRUSHING UP ON POLYMERS Applications for polymers anchored to flat and nanoparticle surfaces are beginning to emerge MICHAEL FREEMANTLE, C&EN
A
LONDON
NOVEL CLASS OF MATERIALS
with "smart surfaces" that can adapt to their environment and be tailored for a wide range of uses—including adhesives, microfluidics, lithography, and chromatography—is emerging from the application of state-of-the-art polymerization techniques. The materials, known as polymer brushes, consist of polymer chains tethered at one end, usually by covalent bonds, to a surface or interface. "The surface is commonly an inorganic substrate such as gold or silicate, but can also be a polymer surface," according to William J. Brittain, professor of polymer science at the University of Akron, in Ohio. A symposium on the topic was held at the ACS national meeting in New Orleans last month. It was cosponsored by the Divisions of Polymer Chemistry and Polymeric Materials: Science & Engineering. The symposium was organized by Brittain; University of Houston associate chemistry professor Rigoberto C Advincula; Duke University senior research sciHTTP://WWW.CEN-ONLINE.ORG
entist Kenneth C Caster; and Jurgen Ruhe, professor of chemistry and physics of interfaces at the University of Freiburg, in Germany Polymer brushes covalently attached to the surface of a substrate are prepared by "grafting-to" or "grafting-from" techniques. The grafting-to technique involves the reaction of a preformed polymer with a surface. The technique leads
A WIDE RANGE of polymerization techniques—including living radical, conventional radical, carbocationic, anionic, ringopening-metathesis, and condensation techniques—has been employed to synthesize polymer brushes. "In addition to simple homopolymer brushes, avast array of complicated architectures has been reported including hyperbranched, cross-linked, highly functionalized, and block copolymer brushes, and densely grafted nanoparticles," Brittain said. "Recent advances have led to methods for systematically controlling grafting density "Because the surface properties of a substrate can be tailored by the brush structure, polymer brushes have been used to control micro-phase-separated morphology of adjacent polymerfilmsand to create highly hydrophobic surfaces," he explained. "Patterning techniques using lithography and microcontact printing have created brush patterns that can restrict cell
P0LYELEC1 Brush produces silver nanoparticles Ag s Ag+0^ Ag + 0
+
Ag 0: O-Ag Ag + 0"-
"0~Ag +
\q 0-
Ag+C 0"Ag
HOO-Ag
HO· AcfO
Hydrogénation
HO•OH HO
HOOH HO
HOOH HO
•OH
Diblock copolymer
-Silicon
C&EN / APRIL U ,
2003
41
SCIENCE & TECHNOLOGY ity for open-tubular capillary separations." In recent work, for exam Brush formed by ring-opening polymerization ple, the group employed two of glycidol A T R P steps to synthesize 0" 0" 0densely double-grafted brush copolymers. In the first step, Silicon wafer A T R P is used to prepare poly(2-bromopropionyloxyOH ethylmethacrylate) (PBPEM), which is used as a macroinitiator. Poly(ethylene glycol) methyl ether with a methacry late end group (PEO-MA) is then grafted by ATRP from the macroinitiator. T h e product is P[BPEM-gra/i(PEO-MA)}. Cross-linked, double-graft ed brushes behave as supersoft rubbers that mechani cally resemble hydrogels, Matyjaszewski observed. "They may be potentially used in many applications growth or adhesion to controlled areas." from tissue engineering and wound heal Living polymerization techniques ing to cosmetics and specialty coatings," (C&EN, Sept. 9,2002, page 36) have been he said. particularly useful in generating diverse ATRP is also being used to graft poly and controlled polymer brush structures. mer brush layers from the surfaces of in One such technique is atom-transfer rad organic nanoparticles. ical polymerization (ATRP). The process "This approach provides the means to typically employs an alkyl halide as an ini control not only the final material's surface tiator and a transition-metal complex as a functionality, as one can also do with small catalyst to create a polymer radical on a sil organic ligands, but also its mechanical, icon, gold, carbon black, or silica surface. optical, or charge carrier properties," not The radical grows until it is deactivated to ed Timothy E. Patten, associate professor form a dormant species. Reactivation of of chemistry at the University of Califor this species allows the polymer chain to nia, Davis. "The development of hybrid resume its growth. polymer/inorganic nanoparticles should At Michigan State University, a poly facilitate the preparation of an entire new merization group led by chemistry pro category of materials with structures that fessor Gregory L. Baker and a surface analy- are controllable on sis group led by associate chemistry the nanometer scale." professor Merlin L. Bruening have been working together to fabricate novel poly PATTEN'S GROUP mer films using ATRP. has prepared com 'ATRP from a surface yields polymer posite nanoparticles brushes with relatively uniform molecular using surface-initiat weights and affords control over brush ed ATRP of organic thickness through variation of polymer soluble and hydroization time," Bruening told C&EN. "We philic m o n o m e r s have recently used ATRP in aqueous so from silica, germani lution to prepare giant poly(2-hyckoxyethyl um, and core/shell methacrylate) brushes with thicknesses cadmium sulfide/silica nanoparticles. approaching 1 μιη. "The properties of the final material can be varied by changing the inorganic core, "We are now trying to exploit this syn the interior polymer layer, and the exteri thesis to develop unique polymer brush or or chain-end layer," he said. coatings for capillary electrochromatogIn work soon to be published in Chem raphy " he said. "The length of these brush istry ofMatenals, Patten describes the syn es increases the ratio of stationary phase thesis of germanium nanoclusters termi to mobile phase in electrochromotogranated with organic groups containing phy and should provide increased capac
HYPERBRANCHED
i>—
acetals, alcohols, esters, and polymer chains grafted using ATRP. He carried out the work with Susan M. Kauzlarich, chemistry professor at UC Davis; assistant professor of chemistry Robin S. Tanke at the Uni versity of Wisconsin, Stevens Point; and coworkers. Brittain, at the University ofAkron, has been using ATRP to synthesize various types of polymer brushes on silicon sub strates, including electrically charged diblock copolymer brushes containing blocks ofpoly(acrylic acid) and polystyrene or poly(metnyl acrylate). He is currently working with postdoc toral research associate Stephen G. Boyes on the preparation and properties of polyelectrolyte brushes containing silver. "Starting from a diblock with the upper block composed of poly(acrylic acid), which is prepared by hydrolysis of a pre cursor polyCtert-butyl acrylate) block, the acid group can be ionized with silver ac etate," Brittain explained. "Subsequent hy drogénation reduces the silver ions to silver particles, which are observed in atomic force microscopy analysis. Our current work is aimed at creating ordered arrays of silver particles by starting with a patterned diblock copolymer brush." Agroup at the Institute of Microsystem Technology at the University of Freiburg led by Ruhe is investigating the use of surface-initiated polymerization to prepare tailor-made coatings on solid substrates. "We are currently working on the generation of functional polymer brushes— that is, brushes that carry a number of functional groups—for the preparation of functional coatings," Ruhe told C&EN. "Examples are brushes with liquid-crystalline side chains or brushes which can render surfaces uitrahydrophobic." Last year, Ruhe showed how surfaceinitiated radicalchain polymerization can be used to generate perfluorinated water- and oil-repellent surfaces on porous silica substrates [Langmuir, 18, 6133 (2002)}. He carried out the work in collaboration with a team of South Korean scientists that included a group at the Korea Research Institute of Chemical Technology, Taejon, led by Injun Park. The repellent surface consists of a film ofpoly(perfluoroalkylethyl acrylate) brushes that are grafted from self-assembled azochlorosilane initiators attached to the
In addition to simple homopolymer brushes, a vast array of complicated architectures have been reported."
42
C&EN / APRIL U ,
2003
HTTP://WWW.CEN-ONLINE.ORG
cidol to form hyperbranched brushes. The "micro-rough" silica surface. Rûhe and his dent Majad Khan have used anionic ring advantages of this approach are the rapid Korean coworkers showed that the peropening multibranching polymerization introduction ofhigh densities of functional fluorinated polymer films can be grown to prepare hyperbranched polyglycidol groups and the eUrnination of any surface with controlled thicknesses without changbrushes on silicon wafers. Most polymer prepreparation." ing the roughness of the surface. They brushes described to date consist of linear Huck andKhanuse anovel one-pot pro demonstrated the repellent nature of the chains, they note. There are only a few pa cedure to covalently link the polyglycidol surfaces by measuring the wetting of the pers reporting the formation of surfacebrushes to the silicon substrate, surface-modified materials by which involves treating the wafer water, oil, and other test liquids. BLOCK COPOLYMER with sodium methoxide solution T h e group is also using its Polystyrene-polyisoprene brush is prepared by living and the glycidol monomer. They grafting-from approach with anionic surface initiated polymerization show that the polymer brushes monolayers of azo-initiators to can easily be modified through generate high-sensitivity D N A esterification of the alcohol sensors consisting of oligonugroups. cleotide probes attached to polyII II Another form of anionic poly mer brushes. merization, known as living an H-S-lCH 2 )iiO "Brush-based D N A chips alPolyisoprene Gold ionic surface initiated polymer low more probe molecules per ization (LASIP), is being used to surface area to be immobilized Polystyrene grow homopolymer and block on the chip, and the wetting copolymer brushes from gold properties of the surfaces can surfaces, as well as silica and clay nanoparbe chemically tailored to give optimal initiated hyperbranched polymers. ticles. The work is being carried out by Adprinting performance," Ruhe said. "Such "Earlier work by our group exploited vincula's group at the University of Hous tailoring is not possible with convenaqueous ATRP to grow brushes in a fast, ton and coworkers at the University of tional chips based on self-assembled yet controlled way from a variety of sur Tennessee, Knoxville, and the University monolayers." faces," Huck said. "In our latest work, we of Athens, Greece [Langmuir, 18, 8672 use surface Si-OH groups to directly initi Cambridge University chemistry lec(2002)}. ate a ring-opening polymerization of glyturer Wilhelm Τ S. Huck and Ph.D. stu
I Organic Synthesis for Early Discovery
Χ,-'
έ
' ^Λ
rf^A
