Plasma-based introduction of monosort functional groups of different type and density onto polymer surfaces. Part 1: Behaviour of polymers exposed to oxygen plasma

Several approaches were investigated to produce monosort functionalized polymer surfaces with a high density and homogeneity of functional groups: (i) Plasma oxidation followed by wet-chemical reduction, (ii) formation of radicals and grafting on of functional group carrying molecules, (iii) plasma bromination followed by (iv) Williamson or Gabriel-like synthesis of spacer molecules, and (v) a pulsed plasma polymerization of functional groups bearing monomers or (vi) their copolymerisation with other comonomers. The formation of hydroxyl (OH), primary amino (NH₂), and carboxyl (COOH) groups was studied in detail. The oxygen plasma treatment (i) in a low-pressure non-isothermal glow discharge results in the formation of a wide variety of O functional groups, polymer degradation and crosslinking. Low power densities and short exposure times (0.1 to 2 s) are required to functionalize a surface while preserving the original polymer structure. Carbonate, ester, and aromatic groups are rapidly degraded by an oxygen plasma treatment leading to scissions of polymer backbones and loss in molecular weight. Also the formation of macrocycles and C=C bonds was observed in a region of around 4 nm in depth. The investigated polymers could be classified by their degradation behaviour on exposure to the oxygen plasma. In order to maximize the process selectivity for OH groups, the variety of oxygen functionalities formed by the oxygen plasma was wet-chemically reduced by diborane, vitride? (Na complex), and LiAlH₄. Typical yields were 9 to 14 OH groups per 100 carbon atoms. Plasma bromination (iii) (40 Br per 100 C atoms) of polymers, followed by grafting of spacer molecules (iv), has been proved to be a highly selective reaction. Another way to produce high densities of monosort functionalities was the pulsed plasma polymerization of functional group bearing monomers such as allylamine, allylalcohol or acrylic acid (v). The retention of chemical structure and functional groups during plasma polymerization was achieved by using low power densities and the pulsed plasma technique. The maximum yields were 30 OH, 18 NH₂, and 24 COOH groups per 100 C atoms. To vary the density of functional groups a chemical copolymerization with 'chain-extending' comonomers such as butadiene and ethylene was initiated in the pulsed plasma (vi). Additionally, the often-observed post-plasma oxidations of such layers initiated by reaction of trapped radicals with oxygen from the air were successfully suppressed by using NO gas as radical quencher.     

Plasma-based introduction of monosort functional groups of different type and density onto polymer surfaces. Part 2: Pulsed plasma polymerization

This new functionalization method consists of the deposition of very thin plasma polymer layers (20 to 100 nm) of functional group bearing monomers in pulsed plasma. With allylalcohol, a maximum of 30 OH groups per 100 C atoms was measured with a selectivity of about 90% and a significant stability at long-time exposure to air. Allylamine was used to produce primary amino groups, with a maximum of 18 NH₂groups per 100 C atoms. Side reactions were observed during the storage in air, such as oxidation of the amino groups. Carboxylic groups could be produced using acrylic acid with a maximum concentration of 24 COOH groups per 100 C atoms. The most prominent side reaction was the decarbonylation/ decarboxylation of the acid group during plasma deposition. The variation of the density of functional groups using the pulsed-plasma polymerization of functional-group-bearing monomers was possible by the chemically-initiated radical copolymerization with either a chain-extending monomer, such as ethylene, or a cross linker, such as butadiene, in plasma. The density of functional groups could be adjusted continuously (0 to 30 OH, 0 to 18 NH₂ and 0 to 24 COOH groups per 100 C atoms). The successful application of these densely functionalized polymer surfaces for producing biocompatible surfaces and for use in metal–polymer composites is proposed.     

D abstraction by H at a D-saturated Ru(0 0 1) surface

D abstraction (ABS) by H at Ru(0 0 1) surfaces initially saturated with D adatoms has been investigated using in situ mass spectrometry. HD and D₂ desorption rates are measured at various surface temperatures T as a function of H exposure time. Yield of D₂ desorption increases with T, while that of HD is little affected. Analyzing the measured rate curves, HD and D₂ desorption orders are evaluated to be 1.7 ± 0.1 and 2.5 ± 0.1, respectively, with respect to D coverage θ_D. To pursue the origin of the derived non-integral reaction orders the rate curves are further analyzed with the rate equations constructed to involve several ABS channels. Consequently, we find that the HD desorption is mainly governed by a second-order rate law in θ_D rather than the conventional hot atom-mediated ABS reaction even when it is corrected to include an isotope effect on ABS. We argue that such second-order ABS kinetics becomes important when the H atoms in excited state of chemisorption have energetically relaxed to some extent, and thereby tend to reside at, e.g. hexagonal closed packed hollow sites, interacting with nearby adatoms. On the other hand, the D₂ rate curves can be fit with third-order kinetics, consistent with the Langmuir-Hinshelwood mechanism in a super-saturation state. The isotope effect plays an essential role in the ABS reaction of D abstraction by H which competes with H abstraction by H as D adatoms are replaced by H atoms.     

Fabricating a reactive surface on the fibroin film by a room-temperature plasma jet array for biomolecule immobilization

A simple dielectric barrier discharge(DBD) jet array was designed with a liquid electrode and helium gas.The characteristics of the jet array discharge and the preliminary polymerization with acrylic acid(AA) monomer were presented.The plasma reactor can produce a cold jet array with a gas temperature lower than 315 K,using an applied discharge power between 6 W and 30 W(V dis × I dis).A silk fibroin film(SFF) was modified using the jet array and AA monomer,and the treated SFF samples were characterized by atomic force microscopy(AFM),scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FTIR),and contact angle(CA).The deposition rate of the poly acrylic acid(PAA) was able to reach 300 nm/min,and the surface roughness and energy increased with the AA flow rate.The FTIR results indicate that the modified SFF had more carboxyl groups(-COOH) than the original SFF.This latter characteristic allowed the modified SFF to immobilize more quantities of antimicrobial peptide(AP,LL-37) which inhibited the Escherichia coli(E.Coli) effectively.     

Pressure- and temperature-induced transformations in crystalline polymers of C60

The great advantage of the C₆₀ molecule is its potential for polymerization, due to which the molecule can be the building block of new all carbon materials. In addition, it contains, both sp ² and sp ³ hybridized carbon atoms, which allows synthesizing new carbon materials with desired physicochemical properties using both types of carbon bonding. The one- and two-dimensional polymeric phases of C₆₀ are prototype materials of this sort. Their properties, especially polymerization under pressure and room temperature via covalent bonding between molecules belonging to adjacent polymeric chains or polymeric layers, can be used for further development of new materials. The present review focuses on the study of the pressure-induced polymerization and thermodynamic stability of these materials and their recovered new phases by in-situ high-pressure Raman and X-ray diffraction studies. The phonon spectra show that the fullerene molecular cage in the high-pressure phases is preserved, while these polymers decompose under heat treatment into the initial fullerene C₆₀ monomer.     

Thin-film deposition by combining plasma jet with spark discharge source at atmospheric pressure

This study demonstrates a method for the deposition of CuO_x thin films by combining atmospheric pressure plasma jet with spark discharge. In this type of discharge source, the bulk copper material of spark discharge electrodes plays the role of a precursor. Copper atoms and particles go through the physical processes of sputtering, evaporation, and further agglomeration and condensation in the plasma jet and on the substrate. The experiments were carried out with and without a combination of discharges. The material coated on the substrate was studied using a scanning electron microscope, Raman spectroscopy, and energy-dispersive X-ray spectroscopy. The characteristics of the set-up and plasma, such as I-V curves, optical emission spectra, and substrate temperature, were also measured. Copper electrodes were examined for erosion by a scanning electron microscope. The results demonstrate that deposits coated by combined discharge show denser and thicker films.     

Investigations of Pulse‐Plasma‐Polymer Coating on TiO2 Nanoparticles and Their Dispersion

An amorphous acrylic acid (AA) polymer coating was generated on TiO₂ nanoparticles through pulse radio frequency (RF) plasma polymerization. The AA plasma synthesis mechanism was studied by its optical emission spectrum. The chemical structures of AA–plasma‐polymer were carefully investigated by Fourier transform infrared spectroscopy (FTIR). The dispersion behaviors of AA‐coated and uncoated TiO₂ nanoparticles in glycol solution were characterized by ultraviolet absorbency and particle size distribution measurements. The results showed that the aggregation of TiO₂ nanoparticles in glycol solution was effectively lowered and the dispersion was improved a lot after AA–plasma‐polymer coating. The pulse plasma coating parameters played an important role in the dispersion enhancement of TiO₂ nanoparticles. By properly regulating the pulse discharge parameters, the system could gain the highest radical–monomer reactions rate, the most compatible functional groups on the nanoparticles, and the best dispersion in the background media.     

Interactions of incident H atoms with metal surfaces

Atomic hydrogen is a highly reactive species of interest because of its role in a wide range of applications and technologies. Knowledge about the interactions of incident H atoms on metal surfaces is important for our understanding of many processes such as those occurring in plasma-enhanced catalysis and nuclear fusion in tokamak reactors. Herein we review some of the numerous experimental surface science studies that have focused on the interactions of H atoms that are incident on low-Miller index metal single-crystal surfaces. We briefly summarize the different incident H atom reaction mechanisms and several of the available methods to create H atoms in UHV environments before addressing the key thermodynamic and kinetic data available on metal and modified metal surfaces. Generally, H atoms are very reactive and exhibit high sticking coefficients even on metals where H₂ molecules do not dissociate under UHV conditions. This reactivity is often reduced by adsorbates on the surface, which also create new reaction pathways. Abstraction of surface-bound D(H) adatoms by incident H(D) atoms often occurs by an Eley-Rideal mechanism, while a hot atom mechanism produces structural effects in the abstraction rates and forms homonuclear products. Additionally, incident H atoms can often induce surface reconstructions and populate subsurface and bulk absorption sites. The absorbed H atoms recombine to desorb H₂ at lower temperature and can also exhibit higher subsequent reactivity with adsorbates than surface-bound H adatoms. Incident H atoms, either directly or via sorbed hydrogen species, hydrogenate adsorbed hydrocarbons, sulfur, alkali metals, oxygen, halogens, and other adatoms and small molecules. Thus, H atoms from the gas phase incident on surfaces and adsorbed layers create new reaction channels and products beyond those found from interactions of H₂ molecules. Detailed aspects of the dynamics and energy transfer associated with these interactions and the important applications of hydrogen in plasma processing of semiconductors are beyond the scope of this review.     

A model of gravity-induced distribution of material in plasma polymerized aerosols and films

A mathematical model of the volumetric part of plasma polymerization influenced by gravity is presented. Plasma-activated adhesion of monomer molecules to a surface of a germinal particle is assumed as a basic mechanism of particulate growth. The continuity equation for the flow of matter through the discharge has been formulated and solved in two extreme asymptotic approximations --for small and major duration of the process. Several non-equilibrium distribution functions of the polymer were obtained, for instance, an amount of the particles as a function of their size or time of fall. Within the adopted model this function demonstrates a sharp downward increase inside a discharge. In addition it contains such parameters as the free fall acceleration or reaction rate coefficients, variations of which enable control of the discharge and properties of the disperse medium.     

Variable morphology of PTFE-like polymer nanocrystals fabricated by oriented plasma polymerization at atmospheric pressure

Rapid formation of PTFE-like polymer nanocrystals was achieved by a novel synthesis method—oriented plasma polymerization (OPP) at atmospheric pressure. The entire process was completed within a short period of time ranging between a few seconds to several minutes through dielectric barrier discharge (DBD) at atmospheric pressure. The surface morphology of the coated organic crystal film was observed through scanning electronic microscope (SEM) and different morphology nanocrystals were found such as nanorods and nanotubes. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the single-crystalline phase of these nanocrystals. The sizes of the nanosingle crystals were from 10 nm to 1 μm. The effects of discharge conditions such as discharge time, ratio of the monomer to carrier gas and power on the nanocrystalline morphology and crystallinity were investigated. As a result, the physical morphology and structures could be controlled through the conditions of the oriented plasma polymerization to some extent. This novel polymerization method opened a new way to nanofabricate polymeric crystallines fast and effectively.     

Discharge Development in ac Plasma Displays

The discharge development in ac plasma displays is quantitatively assessed on the basis of comparatively great data quantities under well defined parameter conditions by means of computer controlled equipment. Investigated are ac plasma displays of three different sizes: PAF 58, 90 and PAF 128. The development of an individual discharge in a stationary pulse sequence is described by analyzing the light emitted by the discharge and by comparing the data obtained with theoretical calculations. Essential characteristics of the pulsed light emission of an ac plasma display discharge are the build-up time t_A, i.e. the period from cell voltage switch-on to the discharge maximum, and the characteristic time τ, which is the time constant describing the exponential growth of the discharge in its amplification range. The theoretical model shows the transition, during discharge build up, from directionization of neon atoms, mainly at low τ values, to ionization essentially via the Penning effect at high τ values. Therefore the effective Townsend ionization coefficient is dependent on τ. The measured time constants of the discharge build up are in good agreement with the calculated values. Adaption between measurement and calculation is achieved by variation of the secondary emission coefficient γ of the cathode.     

Effects of discharge current and voltage on the high density of metastable helium atoms

Both hollow-cathode and Penning-type discharges were adopted to excite helium atoms to a metastable state. Experimental data indicate that Penning discharge is more suitable for generating high fractions of metastables in a low-density helium beam for laser-induced fluorescence technique in measuring electric fields at the edge of a plasma. The metastable density increases with increasing helium gas pressure in the range of 1.33×10^{-2}－66.7Pa. The highest metastable density of 3.8×10^{16}m^{-3} is observed at a static gas pressure of 66.7Pa. An approximately linear relationship between the density of metastable helium atoms and the plasma discharge current is observed. Magnetic field plays a very important role in producing a high density of metastable atoms in Penning discharge.     

Rodlike polymer gelatination as studied by PFG NMR: Fibrin polymerization in human plasma

The structure formation in rodlike polymers was investigated by the example of polymerization of fibrin rodlike, molecules in native plasma. Translational mobility of fibrin, molecules in anticoagulated plasma and native plasma during fibrin polymerization was studied by pulse field gradient nuclear magnetic resonance. It was shown that the diffusion decay of anticoagulated plasma can be fitted by the sum of exponents and fibrin molecules have the self-diffusion coefficientD _f of 1.53·10^?11 m²/s. The diffusion decay of native plasma during fibrin polymerization becomes nonexponential and is described by the lognormal distribution of fibrin self-diffusion coefficients. Qualitative and quantitative changes of the spectrum of fibrin self-diffusion coefficients (SDCs) during polymerization were investigated and analyzed. A symmetrical broadening of the spectrum at the beginning of polymerization and symmetrical narrowing at its final stages with conservation of the most probable SDC was explained on the basis of the hypothesis about the simultaneous action of fibrin polymerization and lyses.     

Kinetic characteristics of the photoreduction of benzophenone in solid polymers in the presence of 4-halophenols

Benzophenone photoreduction in the presence of 4-halophenols (RC₆H₄OH; R = Cl, Br, and I) in a polymer glass is studied in terms of steady-state and nanosecond laser photolysis. The experimental data on the kinetics of the decay of the ketone triplet state are treated using a polychronous kinetic model in the assumption that two concurrent processes (hydrogen atom abstraction from the polymer and from the phenol) occur. The proton transfer rate constants averaged over the distribution and the parameter n that characterizes the distribution width were determined. The value of k _av for the halophenols is shown to be more than an order of magnitude higher. No heavy atom effect is observed. The reaction product composition is demonstrated to change upon addition of a halophenol. The photoreduction in glass polymers is controlled by hydrogen atom abstraction from the respective donor by triplet ketone molecules. The reaction occurs predominantly in a polymer cage, a kind of polymer nanoreactor.     

Stability study of polyacrylic acid films plasma-polymerized on polypropylene substrates at medium pressure

Plasma polymerization of acrylic acid has become an interesting research subject, since these coatings are expected to be beneficial for biomedical applications due to their high surface density of carboxylic acid functional groups. However, the application of these monomers is counteracted by their low stability in humid environments, since a high stability is a required characteristic for almost any biological application. The present work investigates whether it is possible to obtain stable deposits with a high retention of carboxylic acid functions by performing plasma polymerization on polypropylene substrates with a dielectric barrier discharge operating at medium pressure. In order to obtain coatings with the desired properties, the plasma parameters need to be optimized. Therefore, in this paper, the influence of discharge power and location of the substrate in the discharge chamber is examined in detail. The properties of the deposited films are studied using contact angle measurements, X-ray photoelectron spectroscopy, atomic force microscopy and Fourier transform infrared spectroscopy. Moreover, to determine whether the obtained deposits are soluble in water, the coatings are once again analyzed after rinsing in water. This paper will clearly show that stable COOH-rich surfaces can be obtained at high discharge power and close to the monomer inlet, which might open perspectives for future biomedical applications.     

Relationship Between the Molecular Dynamics of Polystyrene and Its Modifications and Parameters of Poley Absorption in Far-infrared Spectra

The far-infrared spectra of polystyrene (PS) and its modifications, obtained by methyl group, chlorine and/or bromine atom substitutions for hydrogen atoms in the benzene ring, was measured in the frequency range from 20 to 450 cm^?1. All these spectra exhibited abnormally broad asymmetric absorption bands with the position of maximum in the range from 40 to 80 cm^?1, depending on the nature of the substituent, which can be assigned to the absorption associated with libration (rotational vibrations) of the phenyl ring of the monomer units, i.e., to the absorption occurring through the Poley mechanism. The relationship between the spectral parameters of the Poley absorption and the molecular characteristics of the studied polymers make it possible to directly analyze the role of molecular structure and intermolecular forces in the dynamics of macromolecules. The heights of the potential barriers of the librating monomer units, Q_L , estimated based on analysis of this absorption, were found to be close to the activation energies of the low-temperature δ-relaxation in amorphous polymers. A comparison of the barriers to libration in the monomer units of the macromolecules of PS and its modifications with the activation barriers of the local segmental mobility in the same PSs confirms that the β-process, as an elementary act of segmental dynamics in this polymers, is associated with the correlational librational motion of part of the chain, which is statistically independent of its neighboring chain parts. In this sense, a universal δ-process is a high-frequency predecessor of the β-process in PS-based polymers.     

Laser plasma plume kinetic spectroscopy of the nitrogen and carbon species

The formation and decay of carbon and nitrogen atoms, CN radicals and C₂ molecules were monitored using spatial‐ and time‐resolved emission spectroscopy in a plasma plume formed during laser ablation of a graphite target in nitrogen atmosphere. A simple exponential model was used to explain the effect of the individual chemical reactions and plasma dynamics on the measured kinetic characteristics. The succession of emissions C → N → CN was observed in the time‐resolved spectra, supporting the suggestion that the CN radical is formed mainly by the direct reaction C + N → CN or C₂ + N₂ → 2CN. The formation of CN radical was enhanced by the additional generation of atomic nitrogen through the RF discharge. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of carboxylic functionalization of polypropylene surface using the underwater plasma technique

Non-equilibrium solution plasma treatment of polymer surfaces in water offers the possibility of more dense and selective polymer surface functionalization in comparison to the well-known and frequently used low-pressure oxygen plasma. Functional groups are introduced when the polymer surface contacts the plasma moderated solution especially water solutions. The emission of ions, electrons, energy-rich neutrals and complexes, produced by the ion avalanche are limited by quenching, with the aid of the ambient water phase. The UV-radiation produced in plasma formation also helps to moderate the reaction solution further by producing additional excited, ionized/dissociated molecules. Thus, monotype functional groups equipped polymer surfaces, preferably OH groups, originating from the dissociated water molecules, could be produced more selectively. An interesting feature of the technique is its flexibility to use a wide variety of additives in the water phase. Another way to modify polymer surfaces is the deposition of plasma polymers carrying functional groups as carboxylic groups used in this work. Acetic acid, acrylic acid, maleic and itaconic acid were used as additive monomers. Acetic acid is not a chemically polymerizing monomer but it could polymerize by monomer/molecular fragmentation and recombination to a cross linked layer. The other monomers form preferably water-soluble polymers on a chemical way. Only the fragmented fraction of these monomers could form an insoluble coating by cross linking to substrate. The XPS analysis was used to track the alterations in –O-CO- bond percentage on the PP surface. To identify the -COOH groups on substrate surface unambiguously, which have survived the plasma polymerization process, the derivatization with trifluoroethanol was performed.     

富勒烯C60的固态聚合

综述了近年来固体C₆₀聚合理论和实验研究的最新结果，分析几种聚合途径各自的特点和共同之处.重点讨论了聚合的物理机制，指出由于聚合反应需要C₆₀分子处于特定的相对位置，自旋指向无序是发生聚合过程的重要前提. 关键词：     

丙烯酰胺聚合物与氧化铝晶体相互作用的分子动力学模拟

将丙烯酰胺单体分别与丙烯酸(AA)、阳离子单体丙烯酰氧乙基二甲基乙基溴化铵(DMB)、疏水性单体丙烯酸十八酯(OA)共聚,分别得到阴离子聚丙烯酰胺P(AA-co-AM)、阳离子聚丙烯酰胺P(DMB-co-AM)、非离子聚丙烯酰胺P(AM)和疏水性聚丙烯酰 胺P(OA-co-AM)等四种丙烯酰胺高分子絮凝剂. 用分子动力学方法,模拟计算了四种聚丙烯酰胺絮凝剂与氧化铝晶体(012)晶面的相互作用,以获得相应的形变能,结合能及理论排序,为阐释四种高分子絮凝剂的絮凝作用机理提供理论依据. 研究结果表明：四种聚合物分子相对Al₂O₃(012)面的初始位置均已贴近Al₂O₃(012)面,且絮凝剂分子中的O原子与Al₂O₃(012)面的Al原子之间存在强烈的相互作用;与Al₂O₃(012)晶面结合的高分子絮凝剂发生扭曲变形,但形变能远小于相应的非键作用能. Al₂O₃(012)晶面结合能的大小排序为P(DMB-co-AM)>P(OA-co-AM)>P(AA-co-AM)>P(AM), 显示四种絮凝剂中P(DMB-co-AM)的絮凝性能最佳,PAM絮凝效果最差.