Unusual kinematics-driven chemistry: cleaving C-H but not COO-H bonds with hyperthermal protons to synthesize tailor-made molecular films

Unconventional reaction-design strategies have been developed to exploit the intriguing kinematics that occur when adsorbed organic molecules are bombarded by a beam of hyperthermal protons: kinematic energy transfer is only effective in H-->H collisions and thus only C-H bonds are cleaved. This process yields a cross-linked molecular film with its chemistry governed by the selection of appropriate precursor molecules. Unlike the conventional wet-chemistry synthesis of cross-linked polymeric films, this new route uses no chemical initiators, additives, nor catalysts, and only requires a proton beam with a kinetic energy of a few electron volts in a dry-process mode compatible with molecular-device fabrication. The reaction designs are expressed unconventionally: reaction energy is tuned by the kinetic energy of the proton beam and reactant supply is controlled precisely by the proton fluence. However, conventional considerations such as bond-strength effects on kinematic outcomes and branching-ratio statistics are also important and they can extend the reaction applicability of the kinematics concept. For example, taking advantage of the fact that COO-H bonds are stronger than C-H bonds, we show, with practical reaction conditions, synthesis results, and surface analysis using X-ray photoelectron spectroscopy and atomic force microscopy, that we can break C-H bonds without breaking COO-H and other bonds, in the production of cross-linked molecular layers with any desirable COOH concentration and with no ester nor other chemical contaminations. The new reaction-design strategies are also applicable to the synthesis of molecular layers with other functionalities such as OH, and to the synthesis of a mixture of functionalities, such as OH/COOH, with a controllable concentration ratio.     

Entanglement of polymer chains in ultrathin films

This investigation aimed to clarify the issue of whether polymer chains are entangled in ultrathin films spin-coated onto substrates. This was done using a fluorescence probe method to observe the behavior of two types of poly(methyl methacrylate) (PMMA), one having a carbazolyl (Cz) moiety (PMMA-Cz) and the other having an anthryl (At) moiety (PMMA-At). In both cases, the moiety fraction was 1 unit for 400 units of polymer. We prepared ultrathin films (thickness: 4-88 nm) on quartz substrates from PMMA-Cz, PMMA-At, and a mixture of the two using a spin-coating method. When the PMMA films prepared from the mixture of the two PMMAs were excited at 292 nm, which is preferentially absorbed by Cz rather than At, the Cz fluorescence was found to be quenched dramatically while the At fluorescence increased significantly. This effect is due to the proximity of the Cz to the At, which permits the transfer of excitation energy between them. The average distance between Cz and At can be calculated using the F?rster mechanism. When the ultrathin film thickness was between 12 and 88 nm, the average distance was found to be 2 nm. This is much shorter than the radii of gyration of the polymers. From this it is clear that two polymer molecules in an ultrathin film do experience entanglement, as has been hypothesized. Thus, we conclude that the difference between certain properties of ultrathin films and the properties of the same materials in bulk are not induced by a decrease in the level of polymer chain entanglement.     

A new cross-linking route via the unusual collision kinematics of hyperthermal protons in unsaturated hydrocarbons: the case of poly(trans-isoprene)

The bombardment of a approximately 18 nm film of poly(trans-isoprene) molecules (approximately 300,000 carbon atoms per molecule) with a mass-separated proton beam at 10 eV has been shown to induce highly efficient cross-linking of several macromolecules per incident proton. In this new method using physical means to conduct synthetic chemistry, the hyperthermal protons preferentially cleave C-H bonds because of their unusual kinematics in hydrocarbons, and the carbon radicals thus created initiate a polymerization chain reaction of the isoprene C[double bond, length as m-dash]C bonds. The method produces ultrathin polymeric films via cross-linking with a dry process having high chemical selectivity and reactivity but requiring no thermal cost and no chemical additives, conditions that are commonly required in the fabrication of microelectronics and photonics.     

Functionalization and patterning of reactive polymer brushes based on surface reversible addition and fragmentation chain transfer polymerization

We present the synthesis of reactive polymer brushes prepared by surface reversible addition–fragmentation chain transfer polymerization of pentafluorophenyl acrylate. The reactive ester moieties can be used to functionalize the polymer brush film with virtually any functionality by simple post‐polymerization modification with amines. Dithiobenzoic acid benzyl‐(4‐ethyltrimethoxylsilyl) ester was used as the surface chain transfer agent (S‐CTA) and the anchoring group onto the silicon substrates. Reactive polymer brushes with adjustable molecular weight, high grafting density, and conformal coverage through the grafting‐from approach were obtained. Subsequently, the reactive polymer brushes were converted with amino‐spiropyrans resulting in reversible light‐responsive polymer brush films. The wetting behavior could be altered by irradiation with ultraviolet (UV) or visible light. Furthermore, a patterned surface of polymer brushes was obtained using a lithography technique. UV irradiation of the S‐CTA‐modified substrates leads to a selective degradation of S‐CTA in the exposed areas and gives patterned activated polymer brushes after a subsequent RAFT polymerization step. Conversion of the patterned polymer brushes with 5‐((2‐aminoethyl)amino)naphthalene‐1‐sulfonic acid resulted in patterned fluorescent polymer brush films. The utilization of reactive polymer brushes offers an easy approach in the fabrication of highly functional brushes, even for functionalities whose introduction is limited by other strategies. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Three-dimensional Pt-coated Au nanoparticle arrays: applications for electrocatalysis and surface-enhanced Raman scattering

This study demonstrates a novel approach to synthesis methods for core-shell nanoparticle assembly using nanoparticle trapping at an interface and subsequent transfer onto a substrate for electrochemical ultrathin layer coating. The transferred nanoparticle array can have a tunable surface area depending on the number of transferred layers. Subsequently coating the surface with Pt-group metals that behave as an ultrathin film provides electrocatalytic activities with respect to a variety of chemical reactions, depending on the properties of the selected coating materials. The transferred 3D Au nanoparticle arrays act as a high-surface-area platform for the diversity of overlayer materials. The resulting 3D core-shell nanoparticle films could be utilized as a highly active electrocatalysis and Raman scattering substrate. The approach provides a versatile and convenient synthesis route to new nanoporous material with tailorable pore structure and material properties through bottom-up assembly.     

DNA-templated organic synthesis: nature's strategy for controlling chemical reactivity applied to synthetic molecules

In contrast to the approach commonly taken by chemists, nature controls chemical reactivity by modulating the effective molarity of highly dilute reactants through macromolecule-templated synthesis. Nature's approach enables complex mixtures in a single solution to react with efficiencies and selectivities that cannot be achieved in conventional laboratory synthesis. DNA-templated organic synthesis (DTS) is emerging as a surprisingly general way to control the reactivity of synthetic molecules by using nature's effective-molarity-based approach. Recent developments have expanded the scope and capabilities of DTS from its origins as a model of prebiotic nucleic acid replication to its current ability to translate DNA sequences into complex small-molecule and polymer products of multistep organic synthesis. An understanding of fundamental principles underlying DTS has played an important role in these developments. Early applications of DTS include nucleic acid sensing, small-molecule discovery, and reaction discovery with the help of translation, selection, and amplification methods previously available only to biological molecules.     

Molecular modeling of the short-side-chain perfluorosulfonic acid membrane

Presented here is a first principles based molecular modeling investigation of the possible role of the side chain in effecting proton transfer in the short-side-chain perfluorosulfonic acid fuel cell membrane under minimal hydration conditions. Extensive searches for the global minimum energy structures of fragments of the polymer having two pendant side chains of distinct separation (with chemical formula: CF(3)CF(O(CF(2))(2)SO(3)H)(CF(2))(n)CF(O(CF(2))(2)SO(3)H)CF(3), where n = 5, 7, and 9) with and without explicit water molecules have shown that the side chain separation influences both the extent and nature of the hydrogen bonding between the terminal sulfonic acid groups and the number of water molecules required to transfer the proton to the water molecules of the first hydration shell. Specifically, we have found that fully optimized structures at the B3LYP/6-311G** level revealed that the number of water molecules needed to connect the sulfonic acid groups scaled as a function of the number of fluoromethylene groups in the backbone, with one, two, and three water molecules required to connect the sulfonic acid groups in fragments with n = 5, 7, and 9, respectively. With the addition of explicit water molecules to each of the polymeric fragments, we found that the minimum number of water molecules required to effect proton transfer also increases as the number of separating tetrafluoroethylene units in the backbone is increased. Furthermore, calculation of water binding energies on CP-corrected potential energy surfaces showed that the water molecules bound more strongly after proton dissociation had occurred from the terminal sulfonic acid groups independent of the degree of separation of the side chains. Our calculations provide a baseline for molecular results that can be used to assess the impact of changes of polymer chemistry on proton conduction, including the side chain length and acidic functional group.     

Chain transfer to polymer in emulsion polymerization

Chain transfer to polymer in emulsion polymerizations of acrylate monomers and vinyl acetate has been studied using ¹³C NMR spectroscopy to elucidate the chemistry by which chain transfer occurs and to quantify the mol% branches resulting from the reaction. In emulsion polymerizations of n-butyl acrylate, ethyl acrylate and methyl acrylate, chain transfer to polymer proceeds via abstraction of hydrogen atoms from backbone tertiary C-H bonds and typically gives rise to 2-4 mol% branches in the polymers obtained at complete conversion, the level of branching increasing with reaction temperature. For these acrylates, there is no evidence for a significant difference between the extent of chain transfer to polymer. In emulsion polymerizations of vinyl acetate, chain transfer to polymer proceeds mainly via H-abstraction from methyl side-groups, though there is a small contribution from abstraction at backbone tertiary C-H bonds. The levels of branching that result are substantially lower than in acrylate emulsion polymerizations, typically being in the range 0.6-0.8 mol% in the polymers obtained at complete conversion. The level of branching increases with temperature and as the degree of monomer starving (and hence instantaneous conversion) increases. Emulsion copolymerization of vinyl acetate with a small amount (5-20 wt%) of n-butyl acrylate gives rise to a significant increase in the level of branching (to values around 1.3-1.6 mol%), which results predominantly from H-abstraction of backbone tertiary C-H bonds in n-butyl acrylate repeat units by propagating radicals with vinyl acetate end units.     

Preparation of polyaniline conductive composites with diene‐rubber or polyphenylacetylene

We describe the preparation of polyaniline (PANI‐EB) by aniline oxidation with KIO₃ and the purification of the resulting dedoped polymer by an acetone extraction step to eliminate undesired by‐products from polyaniline, which could generate some safety concerns in the application and use of PANI. Excellent homogeneous and electrically conducting composite films can be prepared from chloroform solutions of purified PANI doped with camphorsulfonic acid in presence of cis‐1,4‐polybutadiene as the film‐forming agent. These films have been characterized by FT‐IR and UV‐VIS‐NIR spectroscopy. A method to synthesisze PANI directly doped with dodecylbenzenesulfonic acid (DBSA) is also reported. DBSA‐doped‐PANI was then used to prepare composites with polyphenylacetylene (PPA) by growing homogeneous films from chloroform solution. These films were conductive and were studied by FT‐IR and UV‐VIS‐NIR spectroscopy. In view of the application of these composites as gas sensors or in “electronic noses”, a short discussion is presented about the criteria used in the selection of the chemical nature of the host polymer where doped PANI is included to confer electrical conductivity. The interaction between the molecules to be detected and the polymeric sensing surface is discussed in terms of physisorption, chemisorption and charge‐transfer‐complex formation. Copyright © 2001 John Wiley & Sons, Ltd.     

Thickness-dependent molecular chain and lamellar crystal orientation in ultrathin poly(di-n-hexylsilane) films

The molecular chain and lamellar crystal orientation in ultrathin films (thickness < 100 nm) of poly-(di-n-hexylsilane) (PDHS) on silicon wafer substrates have been investigated by using transmission electronic microscopy, wide-angle X-ray diffraction, atomic force microscopy, and UV absorption spectroscopy. PDHS showed a film thickness-dependent molecular chain and lamellar crystal orientation. Lamellar crystals grew preferentially in flat-on orientation in the monolayer ultrathin films of PDHS, i.e., the silicon backbones were oriented along the surface-normal direction. By contrast, the orientation of lamellar crystals was preferentially edge-on in ultrathin films thicker than ca. 13 nm, i.e., the silicon backbones were oriented parallel to the substrate surface. We interpret the different orientations of molecular chain and lamellar crystal as due to the reduction of the entropy of the polymer chain near the substrate surface and the particularity of the crystallographic (001) plane of flat-on lamellae, respectively. A remarkable influence of the orientations of the silicon backbone on the UV absorption of these PDHS ultrathin films was observed due to the one-dimensional nature of sigma-electrons delocalized along the silicon backbone. With the silicon backbones perpendicular or parallel to the surface of the substrate, the UV absorbance increased or decreased with an increase of the angle between the incident UV beam direction and direction normal to the thin film, respectively.     

Thermal imidization and structural evolution of thin films of poly(4,4'-oxydiphenylene p-pyromellitamic diethyl ester)

The evolution of chemical composition and structure during the thermal imidization of an ester-type polyimide precursor, poly(4,4'-oxydiphenylene p-pyromellitamic diethyl ester), in micrometer scale films were studied for a heating rate of 2.0 degrees C/min with time-resolved synchrotron X-ray diffraction, in-situ infrared spectroscopy, and modulated differential scanning calorimetry. Our analyses show that the precursor polymer undergoes imidization in a two-step process. In the first step, the precursor polymer is decomplexed from the residual solvent molecules, and in the second step, it undergoes imide ring formation with the release of ethanol as a byproduct. The imidization reaction starts around 210 degrees C and continues up to 320 degrees C. The thermal imidization reaction induces the structural evolution of the film. As the imidization reaction proceeds, the coherent length along the polymer chain axis increases. This imidization-induced structural evolution was found to occur via three steps: (i) initiation, (ii) the first crystallization, and (iii) the second crystallization. The initiation step is necessary prior to the evolution of the crystalline structure to increase the chain mobility of the precursor polymer chains, and it requires thermal heating up to at least 238 degrees C at which point 22.5% of the imidization is complete. Thereafter, the first crystallization occurs up to 310 degrees C, at which point 98.3% of the imidization is complete. In the range 310-380 degrees C, the second crystallization occurs and produces almost complete imidization of the polymer chains.     

Nanoscale surface patterns from 10(3) single molecule helices of biodegradable poly(L-lactic acid)

Atomic force microscopy, reflection absorption infrared spectroscopy, and X-ray reflectivity studies reveal that poly(L-lactic acid) molecules in Langmuir-Blodgett (LB) films exist as 10(3) helices over nearly the entire length of the polymer chain. This feature gives rise to LB films with highly ordered nanoscale smectic liquid crystalline-like surface patterns with low surface roughness and lamellar spacings that scale with molar mass. These studies provide a new approach for controlling surface morphology with a biodegradable polymer commonly used for drug delivery and tissue engineering.     

Polymer chain organization and orientation in ultrathin films: A spectroscopic investigation

The effect of confinement on the crystallinity and chain orientation of ultrathin poly(di-n-hexylsilane) films has been investigated using UV absorption, fluorescence and IR spectroscopies. UV absorption measurements in a series of poly(di-n-hexylsilane) films having thicknesses between 50 and 3500 Å have shown that, for thicknesses less than 500 Å, the polymer backbone disorders and extensive crystallization of the films is hindered irrespective of molecular weight or surface hydrophobicity. Fluorescence studies showed that rapid energy transfer occurs from the disordered chain segments to the crystalline ones. The orientation of both the polymer backbone and side chains was probed with IR absorption and grazing incidence reflection measurements. The side chains are extended, although not completely in the all-trans conformation, with their carbon plane mostly perpendicular to the substrate. The backbone lies extended, with the polymer axis parallel to the plane of the film. The hexyl side-chains disorder in films less than 2000 Å thick and this disordering occurs through the introduction of gauche bonds. Our findings suggest the possibility of using thickness to control the chain organization and morphology of a polymer thin film. © 1996 John Wiley & Sons, Inc.     

Reactive μCP on ultrathin block copolymer films: Localized chemistry for micro- and nano-scale biomolecular patterning

Three different, complementary soft lithographic approaches for the fabrication of chemical patterns on ultrathin polystyrene-block-poly(tert-butyl acrylate) (PS₆₉₀-b-PtBA₁₂₁₀) films are discussed. Central to the methodology is the previously introduced reactive PS₆₉₀-b-PtBA₁₂₁₀ platform that allows one to covalently graft (bio)molecules via robust amide linkages in high densities on flat, as well as on structured, surfaces. As shown in this paper, the combination of the polymer-based platform and reactive microcontact printing (μCP) patterning approaches allows one to obtain patterns of (bio)molecules with (sub)micrometer feature sizes. The μCP approaches comprise: (A) the direct transfer of functional (bio)molecules from an oxidized elastomeric stamp to hydrolyzed and N-hydroxysuccinimide (NHS) activated PS₆₉₀-b-PtBA₁₂₁₀; (B) the transfer of a passivating poly(ethylene glycol) layer to hydrolyzed and NHS-activated PS₆₉₀-b-PtBA₁₂₁₀ followed by wet chemical grafting of functional moieties; (C) the local hydrolysis of the PtBA skin layer with trifluoroacetic acid (TFA), followed by NHS activation and wet chemical derivatization. The applicability and the versatility of the combination of the polymer thin film-based platform and soft lithographic methodologies for patterning biologically relevant molecules is demonstrated for polyamidoamine (PAMAM) dendrimers, different proteins, as well as probe DNA. The successful hybridization of target DNA and the immobilization of fibronectin in micropatterns show that ultrahigh density patterns for micro- and nano-arrays, as well as for studies of cell-surface interactions, can be conveniently fabricated based on these approaches and platforms.     

ATR-FTIR光谱技术在聚合物膜研究中的应用

红外光谱是聚合物研究中常用的一种表征手段,而衰减全反射红外光谱（ATR-FTIR）更是由于在研究聚合物薄膜方面具有显著的优势而被广泛使用。逐层组装（layer-by-layer Assembly）技术是一种常用的组装聚合物超薄膜的方法,ATR-FTIR光谱技术的引入可以在获取膜组装过程中相应信息的同时有效地避免表征过程中对样品的损害。另一方面,ATR-FTIR方法与二维相关光谱技术（two-dimensional correlation spectroscopy, 2D correlation spectroscopy）相结合也是研究小分子（主要是水分子）在聚合物薄膜中的渗透行为的有效手段。本文对ATR-FTIR的基本原理和显著特点作了介绍,并以实例阐述该方法在逐层组装技术和水分子在薄膜内渗透行为研究两方面的应用。     

Template Synthesis of Linear‐Chain Nanodiamonds Inside Carbon Nanotubes from Bridgehead‐Halogenated Diamantane Precursors

A simple method for the synthesis of linear‐chain diamond‐like nanomaterials, so‐called diamantane polymers, is described. This synthetic approach is primarily based on a template reaction of dihalogen‐substituted diamantane precursors in the hollow cavities of carbon nanotubes. Under high vacuum and in the presence of Fe nanocatalyst particles, the dehalogenated radical intermediates spontaneously form linear polymer chains within the carbon nanotubes. Transmission electron microscopy reveals the formation of well‐aligned linear polymers. We expect that the present template‐based approach will enable the synthesis of a diverse range of linear‐chain polymers by choosing various precursor molecules. The present technique may offer a new strategy for the design and synthesis of one‐dimensional nanomaterials.     

Fabrication of a covalently attached self-assembly multilayer film based on CdTe nanoparticles

A precursor film has been fabricated from TGA (thiolglycolic acid)-stabilized CdTe nanoparticles and NDR (nitro-containing diazoresin) using electrostatic interactions and the standard layer-by-layer assembly method; covalent bonds are formed under ultraviolet irradiation. XPS provided evidence for the presence of CdTe nanoparticles within the polymer ultrathin films. UV-visible spectroscopy and FTIR spectroscopy provide evidence for the formation of a covalent linkage. Moreover, the UV-visible spectroscopy and AFM measurement support the improvement of the stability of the hybrid organic/inorganic film toward polar solvents when the linkages between the nanoparticles and polymer changed from ionic bonds to covalent bonds.     

In situ Synthesis of Oligonucleotide Arrays on Surfaces Coated with Crosslinked Polymer Multilayers

We report an approach to the in situ synthesis of oligonucleotide arrays on surfaces coated with crosslinked polymer multilayers. Our approach makes use of methods for the 'reactive' layer-by-layer assembly of thin, amine-reactive multilayers using branched polyethyleneimine (PEI) and the azlactone-functionalized polymer poly(2-vinyl-4,4'-dimethylazlactone) (PVDMA). Post-fabrication treatment of film-coated glass substrates with d-glucamine or 4-amino-1-butanol yielded hydroxyl-functionalized films suitable for the Maskless Array Synthesis (MAS) of oligonucleotide arrays. Glucamine-functionalized films yielded arrays of oligonucleotides with fluorescence intensities and signal-to-noise ratios (after hybridization with fluorescently labeled complementary strands) comparable to those of arrays fabricated on conventional silanized glass substrates. These arrays could be exposed to multiple hybridization-dehybridization cycles with only moderate loss of hybridization density. The versatility of the layer-by-layer approach also permitted synthesis directly on thin sheets of film-coated poly(ethylene terephthalate) (PET) to yield flexible oligonucleotide arrays that could be readily manipulated (e.g., bent) and cut into smaller arrays. To our knowledge, this work presents the first use of polymer multilayers as a substrate for the multi-step synthesis of complex molecules. Our results demonstrate that these films are robust and able to withstand the ~450 individual chemical processing steps associated with MAS (as well as manipulations required to hybridize, image, and dehybridize the arrays) without large-scale cracking, peeling, or delamination of the thin films. The combination of layer-by-layer assembly and MAS provides a means of fabricating functional oligonucleotide arrays on a range of different materials and substrates. This approach may also prove useful for the fabrication of supports for the solid-phase synthesis and screening of other macromolecular or small-molecule agents.     

Synthesis of nanostructured metal-organic films: surface X-ray radiolysis of silver ions using a langmuir monolayer as a template

An application of the radiolysis method using an X-ray synchrotron beam is developed as a novel approach to the synthesis of metal-organic films with controlled shapes and thickness. We demonstrate that a Langmuir monolayer deposited onto a silver ion containing subphase, irradiated by an incident beam impinging below the critical angle for total reflection, induces the synthesis of a stable nanostructured silver-organic ultrathin film at the air-water interface. The X-ray scattering is also used to monitor in situ the structure of the silver layer during the synthesis process. The layer is observed by atomic force microscopy after its transfer onto a silicon substrate. One observes a film thickness of 4.6 nm, in good agreement with the X-ray penetration depth, about 4.5 nm. The silver structure is oriented by the initial organic film phase. This experiment demonstrates the considerable potential of this approach to produce various controlled metal-organic films with a surfactant self-assembly as a template.     

Stimuli-responsive mixed grafted polymer films with gradually changing properties: direct determination of chemical composition

Infrared spectroscopic ellipsometry (IRSE) and visible monochromatic ellipsometry (VISE) approaches were applied to investigate the chemical structure and thickness of ultrathin polymer films. Mixed polystyrene-poly(2-vinylpyridine) and polystyrene-poly(tert-butyl acrylate) polymer grafted films (mixed brushes) with gradually changing composition (1D gradient mixed brush) along the sample were prepared on a temperature gradient stage via two subsequent "grafting to" reactions. The films were characterized by high-precision mapping VISE at a single wavelength (632.8 nm) and IRSE. The set of 1D IRSE spectra of the polymer brush films obtained by mapping the 1D gradient brush were used to estimate the thickness and the local composition of the film and to construct the 1D map of the film in terms of the chemical composition of the brush. The results were compared with the data obtained using monochromatic ellipsometry where the brush composition was estimated from the results of two subsequent measurements followed each grafting step. The measurements of the brush thickness and composition with both methods were found to be in gratifying agreement. The results demonstrate the high potential of IRSE methods for the one-step characterization (by thickness and chemical composition) of ultrathin polymer films of complex composition.