In situ polymerization of molecular macroclusters on a silica surface: poly(N-isopropylacrylamide) nanofilms

We have found that alcohols, carboxylic acids, and amides self-assemble into a unique molecular architecture, a hydrogen-bonded molecular macrocluster, when they are selectively adsorbed onto silica (glass and oxidized silicon) surfaces in nonpolar solvents such as cyclohexane. In our previous study, this phenomenon could be successfully applied to fabricate molecularly flat and defect-free nanofilms of several tens of nanometers thickness. In this study, we prepared a poly(N-isopropylacrylamide) [poly(NIPAAm)] film on the basis of in situ polymerization of a monomer macrocluster layer formed on silica surfaces and investigated how the molecular arrangement of the adsorbed NIPAAm monomers affects the efficiency of the polymerization of them. Poly(NIPAAm) films were prepared by the following two methods: (1) the one-solution method, the in situ photopolymerization of an NIPAAm monomer adsorption layer on silica in one solution (chloroform, cyclohexane, and toluene), and (2) the solution exchange method, adsorption of NIPAAm monomers onto a silica surface from NIPAAm (0.1 mol %) in chloroform, exhange of the solution to 0.005 mol % NIPAAm in cyclohexane, and then polymerization by UV irradiation. By the solution exchange method, molecularly flat, defect-free, and thermoresponsive films were obtained and the thickness could be controlled by the irradiation time, while only several nanometers thickness could be attained by the one-solution method. The structure of NIPAAm adsorption layers formed in each solution condition was characterized by attenuated total reflection Fourier transform infrared spectroscopy. It was revealed that only the solution exchange procedure induced the beta-sheet-like adsorbed structure of NIPAAm in which the double bonds of neighboring NIPAAm monomers were closely located, which should have resulted in effective polymerization.     

Elastomeric flexible free-standing hydrogen-bonded nanoscale assemblies

Poly(ethylene oxide) (PEO) is a key material in solid polymer electrolytes, biomaterials, drug delivery devices, and sensors. Through the use of hydrogen bonds, layer-by-layer (LBL) assemblies allow for the incorporation of PEO in a controllable tunable thin film, but little is known about the bulk properties of LBL thin films because they are often tightly bound to the substrate of assembly. The construction technique involves alternately exposing a substrate to a hydrogen-bond-donating polymer (poly(acrylic acid)) and a hydrogen-bond-accepting polymer (PEO) in solution, producing mechanically stable interdigitated layers of PEO and poly(acrylic acid) (PAA). Here, we introduce a new method of LBL film isolation using low-energy surfaces that facilitate the removal of substantial mass and area of the film, allowing, for the first time, the thermal and mechanical characterization that was previously difficult or impossible to perform. To further understand the morphology of the nanoscale blend, the glass transition is measured as a function of assembly pH via differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The resulting trends give clues as to how the morphology and composition of a hydrogen-bonded composite film evolve as a function of pH. We also demonstrate that LBL films of PEO and PAA behave as flexible elastomeric blends at ambient conditions and allow for nanoscale control of thickness and film composition. Furthermore, we show that the crystallization of PEO is fully suppressed in these composite assemblies, a fact that proves advantageous for applications such as ultrathin hydrogels, membranes, and solid-state polymer electrolytes.     

Catalyzed radical polymerization of styrene vapor on nanoparticle surfaces and the incorporation of metal and metal oxide nanoparticles within polystyrene polymers

We present a novel approach to polymerize olefin vapors on the surfaces of metallic and semiconductor nanoparticles. In this approach, a free radical initiator such as AIBN is dissolved in a volatile solvent such as acetone. Selected nanoparticles (prepared separately using the laser vaporization-controlled condensation method) are used to form initiator-coated nanoparticles placed on a glass substrate. The olefin (styrene) vapor is polymerized by the thermally activated initiator on the nanoparticle surfaces. Our approach also provides structural and mechanistic information on the early stages of catalyzed gas-phase polymerization, which can be used to correlate the gas-phase structural properties with the bulk properties and the performance of the polymer nanocomposites. This correlation is the key step in controlling the properties of the polymer nanocomposites. Our results clearly demonstrate the success of this method in preparing polymer coated nanoparticles for a variety of interesting applications. The precise control of the chemical functionality, thickness, and morphology of the polymer film and the size, size distribution, and properties of the core nanoparticles (photoluminescence, magnetic) may lead to major technological breakthroughs in a variety of applications including drug delivery, ultrasensitive detectors, and chemical and biological sensors.     

Molecular recognition and self-assembled polymer films for vapor phase detection of explosives

Selective and sensitive polymer films for detecting explosives were studied and fabricated onto surface acoustic wave (SAW) devices. Polymers and molecular host species were self-assembled on functionalized silicon oxide surfaces through a catalytic hydrosilylation reaction. The covalently attached thin polymer films are stable, continuous and uniform with film thickness ranging from 15 to 30 nm. The microsensors coated with the polymer films show high sensitivity towards 2,4-dinitrotoluene (DNT) and o-nitrotoluene, an explosive simulant. Sensor responses to possible common interferents in land mine detection were studied and the sensor sensitivities to them were found to be much lower than that to DNT and explosives simulants. Response patterns for interferants and o-nitrotoluene were constructed and the sensors coated with functionalized cyclodextrins were able to detect 2,4-DNT and 2,4,6-trinitrotoluene (TNT) under ambient laboratory conditions.     

Reversibility of pH-induced dewetting of poly(vinyl pyridine) thin films on silicon oxide substrate

Thin PVP films deposited on a silicon oxide surface have been found to form a dewetting pattern when treated with basic solutions (pH > or = 10). We studied the dependence of pattern morphology on the polymer's molecular weight and thickness of the polymer layer, and observed the formation of three distinctive structures. The structure formed by large drops of polymer is characteristic of a polymer with low molecular weight and the thinnest polymer layer, whereas other samples form holes or a weblike pattern upon dewetting. These experiments have demonstrated for the first time the reversibility of the dewetting process in a liquid environment. The polymer layer has revealed reversible behavior toward flat film when exposed to a pH 4 buffer solution. More complex structures can be obtained by consecutive treatments with acidic (pH 4) and basic (pH 10) solutions. We used atomic force microscopy (AFM) to study both the morphology and elastic properties of polymers in media with different acidity, in order to determine the mechanism behind the dewetting process.     

Direct writing of a conducting polymer with molecular-level control of physical dimensions and orientation

Polymer nanostructures composed of poly(3-dodecylthiophene) (PDDT) have been directly written with control of polymer strand alignment and monolayer-by-monolayer thickness down to a single molecular monolayer (2.6 nm). The molecularly ordered nanostructures were written on silicon oxide surfaces using thermal dip-pen nanolithography, where an atomic force microscope cantilever with integrated tip heater was precoated with solid PDDT. The PDDT was precisely deposited onto the surface when the tip temperature was set close to PDDT's melting temperature.     

Patterning molecularly thin films of polymers—new methods for photolithographic structuring of surfaces

Three novel photolithographic processes for patterning of molecularly thin polymer films are described. The polymer monolayers are prepared by immobilization of initiator molecules to surfaces of solid substrates followed by thermally or photochemically activated radical chain polymerization. Thus polymer chains which are covalently linked to the surfaces of the substrates are obtained. The films can be patterned using appropriate masks and deep or near UV irradiation before, during or after polymer formation. The procedures described in this paper allow the chemical tailoring of surfaces with high spatial resolution. Step-and-repeat procedures, which take advantage of the covalent linking of the polymers to the surfaces, permit the preparation of multifunctional polymer patterns.     

The in situ characterization and structuring of electrografted polyphenylene films on silicon surfaces. An AFM and XPS study

An atomic force microscope was used so as to structure by nanofriction films of polynitrophenylene electrografted on substrates of n-type silicon (100) with the native oxide on the top of the surface. AFM measurements of thin films thickness have been carried out in the electrolytic solution for different applied potentials during the electrografting. This investigation allows (i) to determine the relationship between the applied potential and the final thickness of electrografted polyphenylene films and (ii) to specify how the thin layers grow. XPS analysis confirmed the AFM observations on (i) the effective shaving of the grafted polymer chains under mechanical stress and (ii) the existence of a potential threshold for electrografting a polyphenylene film on silicon oxide surfaces. The presence of a residual film in the rubbed zone was attributed to stronger interactions between the first electrografted layer and the native oxide of silicon (through Si-C or/and Si-O-C bonds) than those insuring the cohesion of the multilayer (C-C and C-N bonds).     

X-ray and neutron reflectometry study of glow-discharge plasma polymer films

Radio-frequency glow-discharge plasma polymer thin films of allylamine (AA) and hexamethyldisiloxane (HMDSO) were prepared on silicon wafers and analyzed by a combination of X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray reflectometry (XRR), and neutron reflectometry (NR). AFM and XRR measurements revealed uniform, smooth, defect-free films of 20-30 nm thickness. XPS measurements gave compositional data on all elements in the films with the exception of hydrogen. In combination with XRR and NR, the film composition and mass densities (1.46 and 1.09 g cm(-)(3) for AA and HMDSO, respectively) were estimated. Further NR measurements were conducted with the AA and HMDSO films in contact with water at neutral pH. Three different H(2)O/D(2)O mixtures were used to vary the contrast between the aqueous phase and the polymer. The amount of water penetrating the film, as well as the number of labile protons present, was determined. The AA film in contact with water was found to swell by approximately 5%, contain approximately 3% water, and have approximately 24% labile protons. The HDMSO polymer was found to have approximately 6% labile protons, no thickness increase when in contact with water, and essentially no solvent penetration into the film. The difference in the degree of proton exchange within the films was attributed to the substantially different surface and bulk chemistries of the two films.     

Formation of calcite thin films by cooperation of polyacrylic acid and self-generating electric field due to aligned dipoles of polarized substrates

We demonstrated the formation of calcite thin films on positively and negatively charged surfaces of a hydroxyapatite (HAp) electret coexisting with polyacrylic acid (PAA) and self-generating surface electric fields due to HAp electrets with electrically aligned dipoles. The cooperation of PAA and the self-generating surface electric field due to the electrets favored the formation of calcite thin films and acted remarkably on the negatively charged surface. Calcite thin films, 4–10 μm thick, with a shell-like microstructure were produced on the negatively charged surfaces with a small amount of PAA. In contrast, under other reaction conditions, calcite thin films with a fan-like structure in the cross section formed on the polarized substrates, and their thickness ranged from 2 to 7 μm. The films were composed of hemispheric- or flat island-shaped aggregates that were made of the calcite crystals that elongated along the c-axis. The morphology of the PAA–Ca²⁺ complex assembly, which adsorbed onto the polarized HAp substrates, was controlled by the balance of the spatial charge distribution in its structure and the properties of the self-generating surface electric field, which led to the different morphologies of the calcite thin films. We proposed that the formation mechanism of the films formed coexisting with PAA and the self-generating electric fields.     

Free-standing mesoporous carbon thin films with highly ordered pore architectures for nanodevices

We report for the first time the synthesis of free-standing mesoporous carbon films with highly ordered pore architecture by a simple coating-etching approach, which have an intact morphology with variable sizes as large as several square centimeters and a controllable thickness of 90 nm to ～3 μm. The mesoporous carbon films were first synthesized by coating a resol precursors/Pluronic copolymer solution on a preoxidized silicon wafer and forming highly ordered polymeric mesostructures based on organic-organic self-assembly, followed by carbonizing at 600 °C and finally etching of the native oxide layer between the carbon film and the silicon substrate. The mesostructure of this free-standing carbon film is confirmed to be an ordered face-centered orthorhombic Fmmm structure, distorted from the (110) oriented body-centered cubic Im3?m symmetry. The mesoporosity of the carbon films has been evaluated by nitrogen sorption, which shows a high specific BET surface area of 700 m(2)/g and large uniform mesopores of ～4.3 nm. Both mesostructures and pore sizes can be tuned by changing the block copolymer templates or the ratio of resol to template. These free-standing mesoporous carbon films with cracking-free uniform morphology can be transferred or bent on different surfaces, especially with the aid of the soft polymer layer transfer technique, thus allowing for a variety of potential applications in electrochemistry and biomolecule separation. As a proof of concept, an electrochemical supercapacitor device directly made by the mesoporous carbon thin films shows a capacitance of 136 F/g at 0.5 A/g. Moreover, a nanofilter based on the carbon films has shown an excellent size-selective filtration of cytochrome c and bovine serum albumin.     

Dispersion of nano-carbon filled polyimide composites using self-degradated low molecular poly(amic acid) as impurity-free dispersant

Polyimide (PI)-based composite films incorporated with carbon black (CB), carbon nanotube (CNT) and carbon nanofiber (CNF), respectively, were prepared using low-molecular-weight poly(amic acid) (PAA), a precursor of PI, as an impurity-free dispersant. High-energy ball mill was employed not only to downsize the nano-carbon agglomerates, but also to cut off the PAA chains for in-situ stabilizing the dispersion. Effect of the ball milling time, procedure, and filler species on the filler dispersion was investigated by means of electrical resistivity reproducibility, morphology observation, and mechanical testing. Comparing with direct dispersion of the nano-carbon in PAA, the composite films fabricated by a two-step approach, that is dispersion from the in-situ degradated low-molecular-weight PAA stabilized nano-carbon slurry, presented a uniform electrical conductivity with a lower percolation concentration and excellent reproducibility in the percolation region. A significant improvement in the Young’s modulus for the CNT loaded PI film was achieved, which was much more effective than those filled with CB or CNF.     

N,N-二乙基丙烯酰胺与丙烯酸在聚丙烯薄膜表面光接枝制备二元接枝膜及其温度、pH值敏感特性研究

以二苯甲酮为引发剂,聚丙烯薄膜(CPP)为基材,通过紫外光接枝的方法制备了具有温度和pH值双重敏感特性的聚N,N-二乙基丙烯酰胺(PNDEA)与聚丙烯酸(PAA)二元接枝膜.在PNDEA一次接枝膜的制备过程中,引发剂与单体配比相同时,本体接枝方法的接枝速率在反应初期明显高于溶液接枝方法;采用溶液法时,增大引发剂与单体配比等可提高接枝率.用本体法所制得的PNDEA一次膜光活化接枝PAA时接枝速率较溶液法高,并且能够实现较高的PAA接枝率.用红外光谱(FTIR)、X射线光电子能谱化学分析(ESCA)对接枝层组成的表征结果证实了二元接枝层的存在.在不同温度下,PNDEA一次接枝膜的FTIR谱图中酰胺Ⅰ带特征吸收峰发生位移表明它具有温度敏感特性.用扫描电子显微镜(SEM)对PNDEA接枝层表征结果表明,用不同接枝手段所制备的接枝膜具有不同的表面形貌.通过吸水率测定研究了二元接枝膜的温度及pH值敏感特性.     

Fabrication and characterization of poly(vinyl alcohol)/chitosan hydrogel thin films via UV irradiation

A series of poly(vinyl alcohol)/chitosan (PVA/CTS) hydrogel thin films were prepared via ultraviolet (UV) irradiation, with acrylic acid (AA) monomer added as a crosslinker without the addition of any other photo-initiator. The swelling behaviors, intermolecular chemical bonds, molecular structures, thermal behaviors, degrees of crystallinity, morphologies of the surfaces and internal structure, and their relationship to the AA content were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Poly(acrylic acid) (PAA) and its chemical crosslinks formed in hydrogel films via free-radical reactions were confirmed using FTIR and DSC analyses. The XRD patterns indicated that the degree of crystallinity of the hydrogel films decreased as the PAA content was increased. SEM micrographs showed that a uniform interconnected pore structure was formed through the entire hydrogel structure, and a gradient in the crosslinking density through the film thickness was observed to result from extended irradiation times. The swelling behaviors revealed that the formation of PAA and its crosslinking in the hydrogel thin films improved the pH stability and controlled the degree of swelling while retaining a high swelling rate. The successful formation of chemical crosslinking without any specific photo-initiator improves the natural characteristics of CTS and PVA and imparts the resulting PVA/CTS hydrogel thin films with properties that make them very promising in biomedical applications.     

Combination of electrografting and atom-transfer radical polymerization for making the stainless steel surface antibacterial and protein antiadhesive

A two-step "grafting from" method has been successfully carried out, which is based on the electrografting of polyacrylate chains containing an initiator for the atom transfer radical polymerization (ATRP) of 2-(tert-butylamino)ethyl methacrylate (TBAEMA) or copolymerization of TBAEMA with either monomethyl ether of poly(ethylene oxide) methacrylate (PEOMA) or acrylic acid (AA) or styrene. The chemisorption of this type of polymer brushes onto stainless steel surfaces has potential in orthopaedic surgery. These films have been characterized by ATR-FTIR, Raman spectroscopy, atomic force microscopy (AFM), and measurement of contact angles of water. The polymer formed in solution by ATRP and that one detached on purpose from the surface have been analyzed by size exclusion chromathography (SEC) and (1)H NMR spectroscopy. The strong adherence of the films onto stainless steel has been assessed by peeling tests. AFM analysis has shown that addition of hydrophilic comonomers to the grafted chains decreases the surface roughness. According to dynamic quartz crystal microbalance experiments, proteins (e.g., fibrinogen) are more effectively repelled whenever copolymer brushes contain neutral hydrophilic (PEOMA) co-units rather than negatively charged groups (PAA salt). Moreover, a 2- to 3-fold decrease in the fibrinogen adsorption is observed when TBAEMA is copolymerized with either PEOMA or AA rather than homopolymerized or copolymerized with styrene. Compared to the bare stainless steel surface, brushes of polyTBAEMA, poly(TBAEMA-co-PEOMA) and poly(TBAEMA-co-AA) decrease the bacteria adhesion by 3 to 4 orders of magnitude as revealed by Gram-positive bacteria S. aureus adhesion tests.     

Preparation and Characterization of Broad-Spectrum Artificial Antibody for OPPs Based on Dummy Template Imprinting Technique

A molecularly imprinted polymer was prepared using diethylphosphonoacetic acid (DPA) as a dummy template for use as an artificial receptor for organophosphrous pesticides (OPPs). Scanning electron microscopy (SEM) was employed to observe the morphology of the prepared polymer. Ultraviolet spectroscopy and ¹H NMR analysis reveal that DPA and the monomer integrate by hydrogen bonding with a molar ratio of 1:2. A static adsorption experiment, Scatchard analysis, and dynamic adsorption and selective binding tests demonstrated that the obtained polymer can effectively bind a group of OPPs containing dimethoxyl or diethyoxyl.     

一种利用梯度光掩模制备梯度表面的简便方法

以自制杂化双向拉伸聚丙烯/氧化硅(BOPP/SiOx)有机/无机杂化膜为基材,由喷墨打印机直接在杂化膜表面打印色阶图案,制备出对紫外光强度呈梯度透过的梯度光掩模;通过此掩模控制,在双向拉伸聚丙烯(BOPP)和聚对苯二甲酸乙二醇酯(PET)表面实施受限光催化氧化(CPO)光感应羟基化反应、受限光接枝丙烯酸(AA)以及表面...     

Electrophoretic deposition of polyacrylic acid and composite films containing nanotubes and oxide particles

Electrophoretic deposition (EPD) method has been developed for the deposition of thin films of polyacrylic acid (PAA). This method allowed the formation of uniform films of controlled thickness on conductive substrates. It was shown that PAA can be used as a common dispersing agent suitable for charging and EPD of various materials, such as multiwalled carbon nanotubes, halloysite nanotubes, MnO(2), NiO, TiO(2) and SiO(2). The feasibility of EPD of composite films containing the nanotubes and oxide particles in a PAA matrix has been demonstrated. The kinetics of deposition and deposition mechanisms were investigated and discussed. The films were studied by thermogravimetric analysis, differential thermal analysis, X-ray diffraction and scanning electron microscopy. The results indicated that film thickness and composition can be varied. Obtained results pave the way for the fabrication of PAA and composite films for biomedical, electrochemical and other applications.     

Effect of thickness on the thermal properties of hydrogen-bonded LbL assemblies

Layer-by-layer (LbL) assemblies have attracted much attention for their functional versatility and ease of fabrication. However, characterizing their thermal properties in relation to the film thickness has remained a challenging topic. We have investigated the role of film thickness on the glass transition temperature (T(g)) and coeffecient of thermal expansion for poly(ethylene oxide)/poly(acrylic acid) (PEO/PAA) and PEO/poly(methacrylic acid) (PEO/PMAA) hydrogen-bonded LbL assemblies in both bulk and ultrathin films using modulated differential scanning calorimetry (modulated DSC) and temperature-controlled ellipsometry. In PEO/PAA LbL films, a single, well-defined T(g) was observed regardless of film thickness. The T(g) increased by 9 °C relative to the bulk T(g) as film thickness decreased to 30 nm because of interactions between the film and its substrate. In contrast, PEO/PMAA LbL films show a single glass transition only after a thermal cross-linking step, which results in anhydride bonds between PMAA groups. The T(g), within error, was unaffected by film thickness, but PEO/PMAA LbL films of thicknesses below ~2.7 μm exhibited a small amount of PEO crystallization and phase separation for the thermally cross-linked films. The coefficients of thermal expansion of both types of film increased with decreasing film thickness.     

Hydrogen‐Bonded Complexes and Blends of Poly(acrylic acid) and Methylcellulose: Nanoparticles and Mucoadhesive Films for Ocular Delivery of Riboflavin

Poly(acrylic acid) (PAA) and methylcellulose (MC) are able to form hydrogen‐bonded interpolymer complexes (IPCs) in aqueous solutions. In this study, the complexation between PAA and MC is explored in dilute aqueous solutions under acidic conditions. The formation of stable nanoparticles is established, whose size and colloidal stability are greatly dependent on solution pH and polymers ratio in the mixture. Poly(acrylic acid) and methylcellulose are also used to prepare polymeric films by casting from aqueous solutions. It is established that uniform films can be prepared by casting from polymer mixture solutions at pH 3.4–4.5. At lower pHs (pH < 3.0) the films have inhomogeneous morphology resulting from strong interpolymer complexation and precipitation of polycomplexes, whereas at higher pHs (pH 8.3) the polymers form fully immiscible blends because of the lack of interpolymer hydrogen‐bonding. The PAA/MC films cast at pH 4 are shown to be non‐irritant to mucosal surfaces. These films provide a platform for ocular formulation of riboflavin, a drug used for corneal cross‐linking in the treatment of keratoconus. An in vitro release of riboflavin as well as an in vivo retention of the films on corneal surfaces can be controlled by adjusting PAA/MC ratio in the formulations.