Kathryn L. Beers

Bio: Kathryn L. Beers is an academic researcher from National Institute of Standards and Technology. The author has contributed to research in topics: Polymerization & Radical polymerization. The author has an hindex of 37, co-authored 93 publications receiving 6327 citations. Previous affiliations of Kathryn L. Beers include University of North Carolina at Chapel Hill & Carnegie Mellon University.

A buckling-based metrology for measuring the elastic moduli of polymeric thin films

Abstract: As technology continues towards smaller, thinner and lighter devices, more stringent demands are placed on thin polymer films as diffusion barriers, dielectric coatings, electronic packaging and so on. Therefore, there is a growing need for testing platforms to rapidly determine the mechanical properties of thin polymer films and coatings. We introduce here an elegant, efficient measurement method that yields the elastic moduli of nanoscale polymer films in a rapid and quantitative manner without the need for expensive equipment or material-specific modelling. The technique exploits a buckling instability that occurs in bilayers consisting of a stiff, thin film coated onto a relatively soft, thick substrate. Using the spacing of these highly periodic wrinkles, we calculate the film's elastic modulus by applying well-established buckling mechanics. We successfully apply this new measurement platform to several systems displaying a wide range of thicknessess (nanometre to micrometre) and moduli (MPa to GPa).

The Synthesis of Densely Grafted Copolymers by Atom Transfer Radical Polymerization

Abstract: Recent progress in the field of densely grafted, or “brush” (co)polymers has prompted a need to develop efficient methods to synthesize a wider variety of materials with the same basic architectural design. These brushlike macromolecules have been prepared previously using the macromonomer method.1-6 Macromonomers, usually prepared by anionic polymerization, were homopolymerized using conventional radical methods to maximize the number of branches possible from a linear backbone based on vinyl monomers. Upon fractionation of these materials using size exclusion chromatography, samples of narrow polydispersities were obtained which could then be cast on surfaces to form highly ordered thin films. To avoid the rigorous methods necessary for ionic polymerizations and sample fractionation, and to extend the variety of compositional content of these types of materials, atom transfer radical polymerization (ATRP) has been used to prepare similar macromolecular architectures. The approach described here involves grafting from a macroinitiator and can offer greater versatility in terms of both the length and the composition of the backbone and/or the side chains than previous methods which employed the synthesis of high molecular weight macromonomers and their subsequent polymerization by uncontrolled radical techniques; to obtain welldefined polymeric brushes required their fractionation, generally by SEC. To our knowledge, there are no known examples of using a macroinitaitor with a grafting site at each repeat unit to make well-defined polymeric brushes. Combinations of nitroxide-mediated, conventional free radical polymerization and ATRP to prepare graft copolymers from macroinitiators have been used previously.7,8 ATRP has also been combined with conventional radical polymerization to prepare amphiphilic graft copolymers9 and thermoplastic elastomers,10 as well. In each of these cases, however, the materials are loosely grafted, having been prepared from a macroinitiator which is a copolymer containing both initiation/ branch sites and spacing repeat units. Controlled radical polymerization and ATRP in particular afford access to materials of controlled molecular weight, predicted by the ratio of consumed monomer to initiation sites.11,12 This method also yields polymer segments of narrow molecular weight distributions13 in addition to being applicable to a host of vinyl monomers such as styrene, (meth)acrylates, acrylonitrile, etc.14 Thus, there are many possibilities which make its application to the area of brush (co)polymers appealing. Included here are preliminary synthetic data and AFM images which show that it is possible to prepare densely grafted copolymers using ATRP. Two approaches were used to prepare the macroinitiators, Scheme 1. The first involved conventional free radical homopolymerization of 2-(2-bromopropionyloxy)ethyl acrylate (BPEA)15 using AIBN in the presence of carbon tetrabromide to attenuate the molecular weight (Mn ) 27 300, Mw/Mn ) 2.3). By use of AIBN as an intiator to prepare the ATRP macroinitiator, a polymer with a broad molecular weight distribution was obtained. Such a macroinitiator would consequently result in the formation of brush polymers with broad molecular weight distributions, no matter how well controlled the polymerization of the side chains. Thus, the preparation of a well-defined macroinitiator was undertaken. In the second approach, trimethylsilylprotected 2-hydroxyethyl methacrylate (HEMA-TMS)16 was polymerized via ATRP and subsequently esterified with 2-bromoisobutyryl bromide (BriBuBr) in the presence of a catalytic amount of tetrabutylammonium fluoride (TBAF) to yield a different macroinitiator, poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) (pBIEM)16 with controlled molecular weight and low polydispersity (Mn ) 55 500, Mw/Mn ) 1.3), Table 1. It should be noted that the macroinitiator prepared using ATRP was composed of a stiffer methacrylate structure and with a 2-bromoisobutyryl initiation site while the free radically prepared pBPEA contained an acrylate backbone and 2-bromopropionyl initiation sites. However, both types of initiating species have been shown to initiate styrene polymerization well.14 Both polymers were then used as macroinitiators for ATRP of styrene (S) and butyl acrylate (BA). Side chains with a degree of polymerization of about 40 from a macroinitiator of pBIEM with a Mn of approximately 50 000 (which contained about 200 initiation sites per Scheme 1 9413 Macromolecules 1998, 31, 9413-9415

Synthesis of Molecular Brushes with Block Copolymer Side Chains Using Atom Transfer Radical Polymerization

Abstract: Brush macromolecules having poly(n-butyl acrylate-block-styrene) and poly(styrene-block-n-butyl acrylate) side chains have been synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection. The block copolymer side chains were cleaved from the backbone and analyzed by GPC, confirming the synthesis of well-defined copolymer brushes. Visualization of individual molecules by atomic force microscopy (AFM) enabled analysis of the conformation and microstructure of the brush macromolecules on mica surface. The brushes with the pnBuA core were almost fully stretched, while the inverted structure with the pS core exhibited longitudinal contraction compared to the contour length of the main chain. In addition, the poly(n-butyl acrylate-block-styrene) brushes demonstrated a characteristic necklace morphology which...

Atom transfer radical polymerization of 2-hydroxyethyl methacrylate

Abstract: Atom transfer radical polymerization (ATRP) has been used to directly prepare linear pHEMA of controlled molecular weight and low polydispersity. Reaction conditions were adjusted to successfully polymerize the functional, polar methacrylate monomer. These adjustments included the use of a mixed solvent system consisting of methyl ethyl ketone and 1-propanol, lowering the temperature to 50 °C or less and using an alkyl bromide initiator with a copper chloride catalyst. The Cu/2,2‘-bipyridyl (bpy) complex, normally a heterogeneous mixture in nonpolar, organic solvents, was completely soluble in the above solvent system. A block copolymer was prepared with a poly(methyl methacrylate) macroinitiator. The monomer can also be protected and polymerized under conditions very similar to those used for the ATRP of methyl methacrylate.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Emerging applications of stimuli-responsive polymer materials

Abstract: Responsive polymer materials can adapt to surrounding environments, regulate transport of ions and molecules, change wettability and adhesion of different species on external stimuli, or convert chemical and biochemical signals into optical, electrical, thermal and mechanical signals, and vice versa. These materials are playing an increasingly important part in a diverse range of applications, such as drug delivery, diagnostics, tissue engineering and 'smart' optical systems, as well as biosensors, microelectromechanical systems, coatings and textiles. We review recent advances and challenges in the developments towards applications of stimuli-responsive polymeric materials that are self-assembled from nanostructured building blocks. We also provide a critical outline of emerging developments.

Thiol–Ene Click Chemistry

Abstract: Following Sharpless' visionary characterization of several idealized reactions as click reactions, the materials science and synthetic chemistry communities have pursued numerous routes toward the identification and implementation of these click reactions. Herein, we review the radical-mediated thiol-ene reaction as one such click reaction. This reaction has all the desirable features of a click reaction, being highly efficient, simple to execute with no side products and proceeding rapidly to high yield. Further, the thiol-ene reaction is most frequently photoinitiated, particularly for photopolymerizations resulting in highly uniform polymer networks, promoting unique capabilities related to spatial and temporal control of the click reaction. The reaction mechanism and its implementation in various synthetic methodologies, biofunctionalization, surface and polymer modification, and polymerization are all reviewed.