Synthesis and characterization of well-defined hydrophilic block copolymer brushes by aqueous ATRP

Homopolymer brushes of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA) and poly(N-isopropylacrylamide)(PNIPAM) grown on atom transfer radical polymerization (ATRP) initiator functionalized latex particles were used as macroinitiators for the synthesis of PDMA-b-PNIPAM/PMEA, PMEA-b-PDMA/PNIPAM and PNIPAM-b-PDMA block copolymer brushes by surface initiated aqueous ATRP. The grafted homopolymer and block copolymer brushes were analyzed for molecular weight, molecular weight distribution, chain grafting density, composition and hydrodynamic thickness (HT) using gel permeation chromatography-multi-angle laser light scattering, ¹H NMR, particle size analysis and atomic force microscopy (AFM) techniques. The measured graft molecular weight increased following the second ATRP reaction in all cases, indicating the second block had been added. Chain growth depended on the nature of the monomer used for block copolymerization and its concentration. Unimodal distribution of polymer chains in GPC with non-overlap of molar mass-elution volume curves implied an efficient block copolymerization. This was supported by the increase in HT measured by particle size analysis, equilibrium thickness observed by AFM and the composition of the block copolymer layer by ¹H NMR analysis, both in situ and on cleaved chains in solution. ¹H NMR analysis of the grafted latex and cleaved polymers from the surface demonstrated that accurate determination of the copolymer composition by this method is possible without detaching polymer chains from surface. Block copolymer brushes obey the same power law dependence of HT on molecular weight as homopolymer brushes in good solvent conditions. The NIPAM-containing block copolymer brushes were sensitive to changes in the environment as shown by a decrease in HT with increase in the temperature of the medium.     

Synthesis, characterization and application of well-defined environmentally responsive polymer brushes on the surface of colloid particles

Well-defined poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) brushes with high density were synthesized on the surface of polystyrene latex by atom transfer radical polymerization (ATRP) using acetone/water as the solvent and CuCl/CuCl₂/bpy as the catalyst. It was found that the polydispersity of PDMAEMA brushes decreased with the increasing external CuCl₂ concentration. The polymer brushes showed their lower critical solution temperature (LCST) at 31 and 33 °C under pH values of 10.0 and 8.0, respectively. Dynamic light scattering studies demonstrate that PDMAEMA brushes were pH- and salt-responsive. PDMAEMA domains were used as the nanoreactors to generate gold nanoparticles on the surface of colloid particles. TEM results indicate that monodispersed gold nanoparticles were obtained. These gold composite nanoparticles displayed effective catalytic activity in the reduction of 4-nitrophenol by NaBH₄.     

Obtaining polyacrylonitrile fiber material by spinneretless electrospinning

We have studied the basic characteristics of spinneretless electrospinning of polyacrylonitrile fibers. We show the effect of the viscosity and the type of solvent on the fiber drawability of the solutions.     

Effect of different salts on electrospinning of polyacrylonitrile (PAN) polymer solution

Electrospinning is a relatively simple method to produce submicron fibers from solutions of different polymers and polymer blends. If the solution is absolutely insulating, or the applied voltage is not high enough that electrostatic force cannot overcome the surface tension, then no fiber can be produced by electrospinning; however, if some salt is added in the solution, the problem can be overcome. The effect of different salts on electrospinning of polyacrlonitrile (PAN) polymer solution was investigated in this article. The various inorganic salts used in this work include LiCl, NaNO₃, NaCl, and CaCl₂.The results show that when the salts were added, respectively, into different concentrations of PAN solution, the order of conductant was LiCl > NaNO₃ > CaCl₂ > NaCl > no salt added. Viscosity and shearing strength of electrospinning solutions are slightly affected by the adding of salts and mainly affected by the changes in concentration of PAN electrospinning solutions. The diameter of nanofibers electrospun by solutions with different salts size down as follows: LiCl > NaNO₃ > CaCl₂ > NaCl. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3865–3870, 2007     

Synthesis and characterization of polymer brushes containing metal nanoparticles

Summary Poly(2-hydroxyethyl methacrylate) (PHEMA) brushes were grown from flat silicon wafers with surface-tethered α-bromoester initiator via atom transfer radical polymerization (ATRP) in aqueous media at ambient temperature first. Kinetics studies revealed an approximate linear increase in thickness with reaction time, indicating that the polymerization process owned some “living” character. Then a kind of polymer-metal complexes (PMCs) were obtained after the introduction of Cu²⁺ cations into the PHEMA brushes by the complexing bond between Cu²⁺ cations and the hydroxyl groups in PHEMA brushes. Finally, the Cu²⁺ cations in PMCs were reduced by NaBH₄ and a kind of PHEMA brushes containing metal nanoparticles of Cu⁰ was formed.     

Effects of different ionic liquids on the electrospinning of a polyacrylonitrile polymer solution

Additives have been proven to be useful in improving electrospinnability and controlling fiber morphology through the modification of solution properties, including the conductivity, viscosity, and surface tension. In this study, the effects of adding small amounts of four different types of ionic liquids [i.e., 1‐butyl‐3‐methylimidazolium chloride (C₄MIMCl), 1‐dodecyl‐3‐methylimidazolium chloride (C₁₂MIMCl), 1‐ethyl‐3‐methylimidazolium bromide (C₂MIMBr), and 1‐ethyl‐3‐methylimidazolium phosphate (C₂MIM)₃PO₄] on the solution properties, electrospinning process, and characteristics of polyacrylonitrile (PAN) were investigated. The results show that the solution conductivities significantly increased with the addition of different ionic liquids with concentrations varying from 0.1 to 1.0 wt %, and the tendency depended on the structures of the ionic liquids. (C₂MIM)₃PO₄ showed the highest conductivity value; this was followed by C₂MIMBr, C₄MIMCl, and C₁₂MIMCl. The ionic liquids formed visible crystals; this made the fiber surfaces rough, and some fiber segments underwent partial aggregation. A regular varying tendency between the minimum mean diameter of the PAN/ionic liquid fibers and the structure of the ionic liquid was found. The PAN/N,N‐dimethylformamide (DMF)/(C₂MIM)₃PO₄ solution showed the highest conductivity among the four systems with different ionic liquids added, and the thinnest minimum diameter of the PAN/(C₂MIM)₃PO₄ fibers appeared with a relatively low ionic liquid concentration of 0.25 wt %, whereas the PAN/DMF/C₁₂MIMCl solution had the lowest conductivity, and the minimum mean diameter of PAN/C₁₂MIMCl fibers appeared at a relatively high ionic liquid concentration of 0.8 wt %. Although the conductivity of the PAN/DMF/C₂MIMBr solution was higher than that of the PAN/DMF/C₄MIMCl solution, the minimum mean diameters of the PAN/C₂MIMBr and PAN/C₄MIMCl fibers appeared at the same ionic liquid concentration of 0.5 wt % because of the similar ionic activities of C₂MIMBr and C₄MIMCl. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2359–2368, 2013     

静电纺丝制备聚丙烯腈纳米纤维及其预氧化

利用聚丙烯腈／二甲基甲酰胺纺丝溶液由静电纺丝制备了聚丙烯腈纳米纤维,纳米纤维的直径在220～760nm。随着聚合物溶液浓度和纺丝施加电压的升高,纳米纤维的直径变大。采用热分析和热重分析研究了纳米纤维的热性能,还用红外光谱对纳米纤维预氧化过程分子化学结构的变化进行了表征,结果表明,纳米纤维有一个很尖锐的放热峰,是聚丙烯腈均聚物典型的放热峰。随着预氧化温度的升高,纤维的内部分子结构发生了变化,表现在红外光谱上最突出的是C≡N在2243～2241cm^-1峰的降低,以及C—H在1684cm^-1峰的降低。     

电纺法制备聚丙烯腈基纳米碳纤维

用电纺法制备了聚丙烯腈（PAN）纳米纤维,用场发射扫描电镜（FESEM）对其形态进行了研究,讨论了不同工艺参数对纤维直径和分散形态的影响。结果发现,纤维直径随着浓度的增加而增大,随着电压升高而减小,接收距离和溶剂类型对纤维直径的影响不大。将形态最好的纤维在240℃下进行活化处理,然后将活化处理过的纤维在氮气氛中煅烧,用FESEM观察了煅烧的纤维直径及形态的变化,红外（IR）分析了纤维化学结构的变化,证实了经900℃煅烧后的纤维为碳纳米纤维。     

Synthesis of flake-like graphene from nickel-coated polyacrylonitrile polymer

Graphene can be synthesized from polyacrylonitrile (PAN) polymer through pyrolysis. A metal catalyst such as nickel (Ni) is required for the conversion of the polymer to graphene. The metal catalysts can be placed either atop or underneath the polymer precursor. We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene. Formation of the flake-like graphene is attributed to the dewetting of the Ni layer coated on the PAN film. Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared. The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.     

Synthesis and properties of polymer brushes bearing ionic liquid moieties

Poly(1-ethyl 3-(2-methacryloyloxy ethyl) imidazolium chloride) (PEMEIm-Cl) brushes were grafted onto Au surface via surface initiated atom transfer radical polymerization (ATRP). The swelling/collapsed behavior of the brushes was characterized by AFM in different electrolyte solutions. These 15 nm ultrathin polyelectrolyte brushes can be used to modulate the interfacial resistance via conformational changes triggered by external electrolytes and solvent. The interfacial resistance was characterized using electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) using [Fe(CN)₆]^3−/4− as the redox probe. The effects of electrolytes, the concentration and type of electrolytes and temperature are investigated in more detail.     

Microscopic polymer cups by electrospinning

Polymer solution above a critical concentration is generally required for making a uniform diameter nano fiber by electrospinning. Below this critical concentration, nanofibers are accompanied with bead formation and, therefore, this concentration region has not received much attention. A systematic investigation reveals that unique particle shapes can be obtained by electroprocessing within this lower concentration range. Several micrometer diameter poly(methyl methacrylate) PMMA cups with wall thickness in the 200-800 nm range and 50-300 nm size pores have been obtained by electroprocessing in high dielectric constant solvents.     

Preparation of well-defined environmentally responsive polymer brushes on monolithic surface by two-step atom transfer radical polymerization method for HPLC

Well-defined poly (2-(dimethylamino) ethyl methacrylate) (PDMAEMA) brushes were successfully prepared on the monolithic surface via two-step atom transfer radical polymerization (ATRP). The polymer brushes synthesized by the second-step ATRP were based on the active bromic groups resulting from poly (ethylene glycol dimethacrylate) (pEDMA) monolith which was prepared at room temperature by the first-step ATRP. Element analysis was used to monitor the grafting process at different reaction times. Each step of preparation was characterized by scanning electron microscope, infrared spectrum and mercury intrusion porosimetry. Employment of PDMAEMA grafted monolith as the stationary phase for chromatographic analysis of steroids demonstrated that the PDMAEMA brushes possessed both pH- and salt-responsive properties. Noticeably, it has been found that the chain length of PDMAEMA brushes could influence the retention behavior of steroids due to the controllability of ATRP, which proposed an interesting alternative to modulate retention in HPLC. This is the first application of PDMAEMA brushes grafted monolith by two-step ATRP method for constructing responsive surface in HPLC and it might exploit a new path for widening the monolith application in various fields.     

Characterization on oxidative stabilization of polyacrylonitrile nanofibers prepared by electrospinning

Ultrafine polyacrylonitrile (PAN) fibers, as a precursor of carbon nanofibers, with diameters in the range of 220–760 nm were obtained by electrospinning of PAN solution using N,N-dimethyl formamide (DMF) as solvent. Morphology of the nanofibers for varying concentration and applied voltage was investigated by field emission scanning electron microscopy (FESEM). The thermal properties and structural changes during the oxidative stabilization process were primarily investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FT-IR) and Raman spectroscopy. The nanofiber diameters increase as the applied voltage is increased and they also increase with an increase in the concentration of the polymer solution. It was also concluded that the electrospun fibers displayed a very sharp exothermic peak at 297.34 °C. A transition temperature observed by FT-IR and Raman was approximately 300 °C, which was closely consistent with the results of DSC and TGA studies. It was also found that oxidative stabilization in air was accompanied by a change in color of nanofibers webs.     

A study on electrospinning of polyacrylonitrile nanofibers

Aligned and molecularly oriented Polyacrylonitrile (PAN) nanofibers were prepared using a non-conventional approach. Various take up velocity of rotating drum was examined for increasing of productivity, alignment and mechanical properties of nanofibers. Impressive techniques like Raman spectroscopy were utilized for characterization of nanofibers.     

Preparation of mesoporous polyacrylonitrile and carbon fibers by electrospinning and supercritical drying

Mesoporous polyacrylonitrile and carbon fibers have been prepared by electrospinning and subsequent supercritical drying and carbonization. Polyvinylpyrrolidone was used as a template. Ambient drying, oxidation, and supercritical drying were conducted to investigate the effects of treatment methods on the structure of the fibers. Interesting surface morphologies of the fibers, including nanoconvexities and nanorods, were found when the different drying methods were used. The surface area of the mesoporous carbon fibers was estimated as 602.0 m² g⁻¹, with an average pore size of 3.6 nm.     

Hydrophilic modification of polyacrylonitrile membranes by oxyfluorination

The surface of polyacrylonitrile (PAN) membranes was modified by oxyfluorination with various conditions to improve its wettability. The membranes were characterized in terms of morphology, structure, hydrophilicity, and membrane performance. The properties and functional groups on the surface of PAN membranes were investigated by contact angle, SEM, ATR-IR and XPS. And permeability of PAN membranes was compared by permeating pure water flux through membrane surface under 100, 150 and 200 kPa pressure. Oxyfluorination introduced oxygen contained functional groups such as the carboxylic acid groups that help increment of wettability on the surface of PAN membrane. Water flux of oxyfluorinated PAN UF membrane increased 20% at pure water permeation pressure 200 kPa compared to that of untreated PAN UF membrane.     

Synthesis of isopropenylbenzyl‐terminated macromonomers and preparation of polymer brushes by anionic homopolymerization

Isopropenylbenzyl‐terminated polystyrene (PS) macromonomers were synthesized by anionic addition in a two‐stage process using styrene and 1,4‐diisopropenylbenzene (DIPB) in benzene. The reaction products of polystyryl anions with DIPB provided PS macromonomer possessing less than two isopropenylbenzyl groups at the propagating end under the condition of being in hydrocarbon solvent at 25°C (ceiling temperature) because of the anionic equilibrium nature. Subsequently, anionic homopolymerization of such macromonomers was carried out in tetrahydrofuran (THF) at ?78°C using anionic initiators to prepare the polymer brushes. The conversion of polymer brushes was very low (ca. 5%). Moreover, the degree of polymerization (DP) was less than 50. The low concentration of propagating chain ends seemed to affect the formation of polymer brushes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87:1790–1793,2003     

Effect of electrospinning processing variables on polyacrylonitrile nanoyarns

With recent developments in the field of smart textiles, researchers have been working toward fabricating architectures of nanofibers, known as nanoyarns, which mimic the geometry of a conventional yarn. In doing so, one can leverage the unique properties of nanoscale fibers, including high surface‐to‐volume ratio and tunable porosity, for the development of smart garments. In the last 5 years, researchers have produced nanoyarns from a limited number of polymers, including polyacrylonitrile (PAN) and poly(vinylidene fluoride) and its co‐polymers. However, to our knowledge, there has been little research on the solution properties and electrospinning parameters needed to fabricate these higher‐order architectures from nonwoven mats. In this work, a modified electrospinning setup, enclosed in a humidity‐controlled chamber, was developed to fabricate nanoyarns for integration into knitted textiles. We fabricated nanofibers and nanoyarns from PAN/DMF solutions and conducted a systematic study to analyze the effect of solution conductivity, viscosity, and electrospinning parameters (applied voltage, collector distance, and humidity) on fiber and yarn fabrication and morphology. Polymer concentration had a significant effect on fibrous cone and yarn fabrication. Low polymer concentrations resulted in poor cone formation, whereas high concentration resulted in dense cones that were difficult to draw into nanoyarns. Overall, the matrix of electrospinning parameters that resulted in the formation of homogenous nanofiber mats was larger than that of nanoyarn formation. Nanoyarn formation required higher polymer concentration and/or applied voltage than nonwoven mat formation. The influence of these parameters on nanoyarn formation and fiber diameter can be used to expand the library of spinnable nanoyarns and optimize their properties for specific applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46404.