Preparation and properties of thermoresponsive and conductive composite fibers with core‐sheath structure

In this research, thermoresponsive and conductive fibers with core‐sheath structure were fabricated by coaxial electrospinning. For preparing the spinning sheath solution, poly‐(N‐isopropylacrylamide‐co‐N‐methylolacrylamide) (PNN) copolymer having thermoresponsive and cross‐linkable properties was synthesized by free‐radical polymerization using redox initiators; it was then mixed with the conductive poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) at different weight ratios in water. On the other hand, poly(butyl acrylate‐co‐styrene) (PBS) copolymer synthesized by emulsion polymerization was dissolved in chloroform and used as the spinning core solution. After electrospinning, the fibers were treated at 110 °C for 1 h to cross‐link the PNN portion in the sheath for strengthening the fibers. Well‐defined core‐sheath fibers were observed from SEM pictures; the outside and inside (core) diameters were 568 ± 24 and 290 ± 40 nm, respectively, as determined from TEM pictures. The fiber mats were further doped by DMSO to enhance their conductivity. For the fiber mat with the weight ratio of PEDOT:PSS/PNN at 0.20 in the sheath, its surface conductivity could reach 29.4 S/cm. In addition, the fiber mats exhibited thermoresponsive properties that both swelling ratio and electric resistance decreased with temperature. Furthermore, the fiber mats exhibited improved flexibility as evaluated via bending test. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1299–1307     

Electrokinetic and contact angle measurements of grafted carbon fibers

 The grafting method that has successfully been applied to methacrylic acid and liquid crystalline monomers was expanded to prepare amphoteric carbon fiber surfaces using 2-(N,N-dimethylamino)ethyl methacrylate as monomer. The obtained carbon fiber surfaces were characterized by contact angle and ζ-potential measurements. The expected basic behavior was not observed, instead an amphoteric character of the modified carbon fiber surface was found. The fiber surfaces display a basic character in the acidic pH-range, while they are acidic in the alkaline part of the pH-scale. An important influence is derived from the amount of initiator used to graft the monomers onto the fibers. The smaller the initiator concen-tration used during polymerization, the larger the amount of amino functionalities introduced to the carbon fiber surface. The wetting behavior versus water depends on the overall conformation of the immobilized polymer. During immersion into water the polymer acts hydrophobic, while during emersion, a hydrophilic character is observed, probably derived from conformational changes and swelling during the contact angle measure-ments in water. Received: 9 June 1998 Accepted: 13 August 1998     

Swelling and Dissolution of Cellulose Part 1: Free Floating Cotton and Wood Fibres in N-Methylmorpholine-N-oxide–Water Mixtures

Summary: Five modes describing the behaviour of cellulose fibres dipped in a chemical have been identified:

• Mode 1: Fast dissolution by disintegration into fragments
• Mode 2: Large swelling by ballooning, and dissolution
• Mode 3: Large swelling by ballooning, and no dissolution
• Mode 4: Homogeneous swelling, and no dissolution
• Mode 5: No swelling and no dissolution

In the case of the behaviour of wood and cotton cellulose fibres in N-methylmorpholine-N-oxide (NMMO) and water mixtures, four domains of water content have been identified. Below 17% of water up to monohydrate (13%), the fibres are disintegrated into rod-like fragments and dissolve (mode 1). In NMMO – water mixtures containing 19–24% water, the cellulose fibres exhibit a heterogeneous swelling by forming balloons (composed of dissolved cellulose holds inside a membrane) separated with non-swollen sections. The whole fibre will completely dissolve (mode 2) in four successive steps (growth of the balloons, burst of the balloons, dissolution of the non-swollen sections and finally dissolution of the membrane). With still greater water contents (25–30%), only the ballooning phenomenon is observed, with a partial dissolution inside the balloon (mode 3). Above 35% of water, the fibres swell homogeneously and are not dissolving (mode 4).     

The preparation and characterization of silver‐loading cellulose acetate hollow fiber membrane for water treatment

Silver‐loading asymmetric cellulose acetate (CA) hollow fiber membrane was spun via the dry jet‐wet spinning technique. The spinning solution was prepared by dissolving AgNO₃ and CA in N,N‐dimethylformamide (DMF). The silver ions were reduced in the spinning dope into silver nano‐particles. The morphology of the resulting hollow fibers was examined using a scanning electron microscope and the silver content in the fiber was measured using an inductively coupled plasma atomic emission spectrometer. The antibacterial activities were evaluated. These hollow fibers had a sponge‐like structure and dense inner and outer surfaces. At a 50 k magnification, the pore on the skin layer was not observable, while the nodule size was smaller than 10 nm. The residual silver content of as‐spun hollow fiber was about 60% of the original silver added in the polymer solution. After immersing in water bath for 180 days, the silver content in the bulk of the hollow fibers decreased to 60% and the silver content on the surface reduced to 10%, yet still showed antibacterial activity against Escherichia coli and Staphylococcus aureus. After permeating with water for 5 days, the silver content in the hollow fibers decreased, and did not show antibacterial activity against E. coli and S. aureus. Thus, silver content must be periodically replenished after permeation. The proper range of AgNO₃ in the spinning solution for CA hollow fiber should be about 100–1000 ppm. Copyright © 2005 John Wiley & Sons, Ltd.     

Thermoresponsive conductive polymer composite thin film and fiber mat: Crosslinked PEDOT:PSS and P(NIPAAm‐co‐NMA) composite

The thermoresponsive conductive composite (TCC) thin films and fiber mats, whose electrical property changed with temperature, were fabricated successfully. The thermocrosslinkable and thermoresponsive copolymer, poly(N‐isopropyl acrylamide‐co‐N‐methylolacrylamide) (PNN), was synthesized. The TCC thin film and fiber mat were fabricated by spin coating and electrospinning process of PEDOT:PSS/PNN solutions, respectively. After thermocrosslinking and doping by DMSO, the composite thin films and fiber mats were obtained. Fibrous structures of TCC fiber mats were observed by SEM. The surface resistance and conductivity of composites were measured. The thermoresponsivity and swelling ratio of TCCs were also studied. The thermoresponsive conductive property was analyzed by measuring the surface resistance of TCCs in water bath under various temperatures from 20 to 50 °C. With the increase of temperature, the TCCs shrank to be dense structure and showed lower surface resistance. The TCC fibers mat exhibited greater sensitivity to temperature than thin film owing to its fibrous structure. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1078–1087     

Fabrication and characterization of UV‐crosslinkable thermoresponsive composite fibers with magnetic properties

Crosslinking magnetic thermoresponsive composite (MTC) fiber mats were fabricated by electrospinning process and followed by UV curing. Thermoresponsive poly‐(N‐isopropylacrylamide) (PNIPAAm) and magnetic Fe₃O₄ were firstly synthesized by redox‐initiated polymerization and co‐precipitation, respectively. A crosslinking agent (dipentaerythritol hexylacrylate) and photoinitiator for providing crosslinking ability were then mixed with PNIPAAm and Fe₃O₄ in ethanol as the electrospinning solution. After electrospinning and subsequent UV irradiation, the MTC fiber mats were thus obtained. Thermoresponsivity of the MTC fibers was measured by both DSC and swelling test. MTC fiber mat exhibited better water‐absorption capability and thermoresponsivity than corresponding film. Morphological analysis was observed by SEM and TEM, and the magnetic property was measured by SQUID. The thermoresponsive magnetic behavior of MTC fiber mat in water was observed under various temperatures and magnetic fields. Vitamin B₁₂ used as a model drug was loaded in the MTC fiber mats and the drug‐release behavior was then studied. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2152–2162     

Preparation of submicron‐scale,electrospun cellulose fibers via direct dissolution

Cellulose nonwoven mats of submicron‐sized fibers (150 nm–500 nm in diameter) were obtained by electrospinning cellulose solutions. A solvent system based on lithium chloride (LiCl) and N,N‐dimethylacetamide (DMAc) was used, and the effects of (i) temperature of the collector, (ii) type of collector (aluminum mesh and cellulose filter media), and (iii) postspinning treatment, such as coagulation with water, on the morphology of electrospun fibers were investigated. The scanning electron microscopy (SEM) and X‐ray diffraction studies of as‐spun fibers at room temperature reveal that the morphology of cellulose fibers evolves with time due to moisture absorption and swelling caused by the residual salt and solvent. Although heating the collector greatly enhances the stability of the fiber morphology, the removal of salt by coagulation and DMAc by heating the collector was necessary for the fabrication of dry and stable cellulose fibers with limited moisture absorption and swelling. The presence and removal of the salt before and after coagulation have been identified by electron microprobe and X‐ray diffraction studies. When cellulose filter media is used as a collector, dry and stable fibers were obtained without the coagulation step, and the resulting electrospun fibers exhibit good adhesion to the filter media. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1673–1683, 2005     

Dissolution of rayon fibers for size exclusion chromatography: a challenge

Application of size exclusion chromatography (SEC) for the analysis of cellulose samples is often limited due to poor solubility in the solvent system N,N-dimethylacetamide/lithium chloride (DMAc/LiCl). Hence different activation or derivatization methods have been developed and published. Most of these methods are laborious, influence the molar mass distribution or do not support dissolution of manmade fibers, such as viscose rayon. In this study, we have evaluated different activation methods for their applicability in viscose rayon dissolution and we present a novel method for activation. We found that an additional solvent exchange step with dimethyl sulfoxide (DMSO) increases and accelerates solubility of viscose fibers in DMAc/LiCl for subsequent SEC analysis. The improved dissolution by DMSO activation is mainly due to increased swelling and improved action towards the outer skin of the fiber. The novel approach has also been applied to the even more difficult dissolution of oxidized viscose fibers.     

How does the never-dried state influence the swelling and dissolution of cellulose fibres in aqueous solvent?

The swelling and dissolution capacity of dried and never-dried hardwood and softwood pulps and cotton linters was compared in two aqueous solvents, N-methylmorpholine-N-oxide (NMMO)-water at 90 °C with water contents ranging from 16 to 22% and NaOH—water at −6 °C with NaOH contents ranging from 5 to 8%. Swelling and dissolution mechanisms were observed by optical microscopy and dissolution efficiency was evaluated by recovering insoluble fractions. The results show a contrasted picture towards the effect of the never-dried state on the swelling and the dissolution capacity depending on the origin of the fibres and the type of aqueous solvent. In the case of NMMO—water, the presence of water within and around the fibre does not seem to favour dissolution initiation but after 2 h of mixing the dissolution yield appears to be similar for either dried or never-dried state. The limiting factor for dissolution in NMMO—water is not the penetration of the solvent inside the cellulose fibres, but only the local concentration of NMMO molecules around the fibre. For NaOH—water, both optical microscopy observations on individual fibres and dissolution yield measurements show that the never-dried state is more reactive for softwood pulps and cotton linters and has no significant effect on hardwood pulps. In this case, the local decrease of solvent strength is counteracted by the opening of the structure in the never-dried state which should enable the Na⁺ hydrated ions to penetrate easier.     

Swelling and dissolution of cellulose, Part V: cellulose derivatives fibres in aqueous systems and ionic liquids

The swelling and dissolution mechanisms of several cellulose derivatives (nitrocellulose, cyanoethylcellulose and xanthate fibres) are studied in aqueous systems (N-methylmorpholine-N-oxide—water with various contents of water, hydroxide sodium—water) and in ionic liquids. The results are compared with the five modes describing the swelling and dissolution mechanisms of cotton and wood cellulose fibres. The mechanisms observed for the cellulose derivatives are similar to the ones of cotton and wood fibres. Swelling by ballooning is also seen with cellulose derivatives, showing that this phenomenon is linked to the fibre morphology, which can be kept after undergoing a heterogeneous derivatisation. Patrick Navard and Thomas Heinze—Members of the European Polysaccharide Network of Excellence (EPNOE),     

Swelling and dissolution mechanism of lyocell fiber in aqueous alkaline solution containing ferric tartaric acid complex

Swelling properties of lyocell fibers in FeTNa (ferric tartaric acid complex) solutions were studied. Concentrations of Fe and free NaOH in FeTNa were varied, while the ratio between FeCl₃.6H₂O: tartaric acid was kept constant as 1:3.28. The concentration of Fe ion varied from 0.15 to 0.55 M. The free NaOH concentration in FeTNa solutions was chosen as 0.4; 0.8; 1.25; 2.5 and 5 M. Fiber diameter measurements following 2 min of swelling and swelling rate of lyocell fiber up to 60 min were studied. Depending on concentration of Fe and free NaOH in FeTNa solutions and fiber swelling time; swelling, dissolution, disintegration or dramatic swelling were observed. 0.4 and 5 M free NaOH containing FeTNa solutions could only swell the fiber but could not dissolve it. 2.5 M free NaOH containing FeTNa solutions dissolved the fiber in a few minutes. FeTNa solutions containing free NaOH concentration from 0.8 to 1.25 M resulted in either dissolution or limited swelling depending on Fe concentration.     

Some Properties of Polyelectrolyte-G rafted Cellulose

Anionic and cationic polyelectrolytes were grafted on a bleached kraft pulp. Grafting an anionic polyelectrolyte (sodium poly-acrylate-polyacrylamide copolymer) resulted in modified fibers possessing outstanding affinity for water and saline solutions in the pH range where the polymer is ionized. Swelling is the result of both the grafting operation itself and of the presence of the ionized polyelectrolyte.

The swollen grafted fibers could be disintegrated under intense shear to give a colloidal solution exhibiting pseudoplastic thixo-tropic behavior. Electron microscopic examination revealed that during the shearing process the fiber had been disintegrated into its constitutive elements, long rodlike protofibrils, which are believed to be mainly responsible for the high viscosities observed.

Grafting a cationic polyelectrolyte (polydimethylamino ethyl methacrylate hydrochloride) produced fibers with lower but significant water swelling. The influence of pH on swelling was similar, although reversed, to that observed with the anionic grafted fibers. The presence of a large number of cationic groups in the porous cellulose fiber gel points to applications in ion-exchange and adsorption processes.     

Swelling and dissolution of cellulose,Part III: plant fibres in aqueous systems

Raw and refined flax, hemp, abaca, sisal, jute and ramie fibres are dipped into N-methylmorpholine N-oxide (NMMO)–water with various contents of water and into hydroxide sodium (NaOH)–water. The swelling and dissolution mechanisms of these plant fibres are similar to those observed for cotton and wood fibres. Disintegration into rod-like fragments, ballooning followed or not by dissolution and homogeneous swelling are all observed as for wood and cotton fibres, depending on the quality of the solvent. Balloons are not typical of wood and cotton and they seem to be present in all plant fibres. Another interesting result is that the helical feature seen on the balloon membrane is not related to the microfibrillar angle. Plant fibres are easier to dissolve than wood and cotton. This is not related to the molar mass of the cellulose chain. Raw plant fibres keeping most its non-cellulosic components do not show the formation of balloons. Patrick Navard is a Member of the European Polysaccharide Network of Excellence (EPNOE)     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

A study of benzobisoxazole and benzobisthiazole compounds and polymers under hydrolytic conditions

The stability of benzobisoxazole and benzobisthiazole compounds and polymers under hydrolytic conditions was studied. 2,6-Bis(4-tert-butylphenyl)benzo[1,2-d;4,5-d′]bisoxazole (1) dissolved in acetonitrile containing sulfuric acid and water at 80°C is stable. A suspension of 2,6-bis[4-(2-benzoxazoyl)phenyl]benzo[1,2-d;5,4-d′]bisoxazole (2) in 0.2 N H₂SO₄ or 0.2 N NaOH solution at 100°C for 21 days is stable. The intrinsic viscosity of a poly(p-phenylene)benzobisoxazole (PBO) fiber sample soaked in 0.2 N H₂SO₄, water with 1 wt % polyphosphoric acid (PPA), or 0.2 N NaOH remained the same. Under very severe hydrolytic conditions such as dissolution of compound 2 or PBO in PPA or methanesulfonic acid with residual water followed by coagulation in water, benzobisoxazole underwent bond cleavage to generate carboxylic acid and o-aminophenol functional groups. This is in contrast to an earlier hypothesis that the decrease in intrinsic viscosity under these conditions was due to chain association. Poly(p-phenylene)benzobisthiazole (PBT) also underwent bond cleavage under these very severe conditions, which are unlikely to be encountered in normal applications. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2637–2643, 1999     

Highly absorbable lyocell fiber spun from celluloses/hydrolyzed starch-g-PAN solution in NMMO monohydrate

To improve the absorption properties of lyocell fiber, a hydrolyzed starch-grafted-polyacrylonitrile (HSPAN) superabsorbent whose powder size was less than 100 μm was added to the cellulose solution in N-methyl morpholine N-oxide (NMMO) monohydrate. The lyocell fiber was prepared by dry jet-wet spinning of the solution. To begin with HSPAN whose water-absorbency was 325 g/g (water/fiber) was prepared by hydrolysis of the copolymer in which acrylonitrile (AN) monomer was grafted onto the starch. The introduction of HSPAN led to a notable improvement of absorbency and absorption rate of lyocell fiber for water. The maximum water-absorbency of the fibers containing HSPAN 2 and 5 wt% was as high as 4.55 and 8.21 g/g, respectively. These values are comparable with 1.94 g/g for unmodified lyocell fiber. SEM and polarizing optical microscope results confirmed that HSPAN remained within the lyocell fiber at both wet and dry states. With increasing HSPAN content, however, the mechanical properties of lyocell fibers were decreased. Thus, the optimum HSPAN content may give a good combination of absorption and mechanical properties.     

Organic–inorganic interpenetrating polymer networks and hybrid polymer materials prepared by frontal polymerization

Novel polyacrylamide‐based hydrogels containing 3‐(trimethoxysilyl)propyl methacrylate and/or tetraethoxy silane were synthesized by means of frontal polymerization, using ammonium persulfate as initiator, N,N′‐methylene bisacrylamide as crosslinking agent and dimethyl sulfoxide as solvent. The obtained samples were treated at pH of 2 or 5 to induce the sol–gel reaction and evaluate their swelling behavior in the conditions. The occurrence of this reaction was assessed by solid‐state NMR. Moreover, the thermal properties of the dry materials were studied by differential scanning calorimetry and thermal gravimetric analysis, and their water‐contact angles were measured. It was found that the amount of Si affects the extent of swelling and the hydrophilicity of the resulting materials. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4618–4625     

Formation and Dissolution of Twin ZnO Nanostructures Promoted by Water and Control over Their Emitting Properties

By using ZnO as a model system, the formation of twinned nanostructures has been investigated under microwave irradiation, exploiting experimental conditions ranging from purely solvothermal when N,N‐dimethylformamide was used, to purely hydrothermal when water was the solvent. A progressive increase in size, elongation and roughness of the surface was observed with increasing water content in the solvent mixture. Particular attention was paid to the reactivity of the ZnO surfaces towards dissolution. Our results show that the formation of twinned nanorods is a dynamic process and that the coupling interphase itself is highly reactive. Consequently, the twinned rods undergo a number of complex dissolution processes that are responsible for the appearance of a wide distribution of defects either on the surface or inside the structure. Poly(N‐vinyl pyrrolidone) influences the photoluminescent properties of the as‐synthesised materials and allows control of the ratio of the intensity of the UV and visible emission.     

Ultrafine hydrogel fibers with dual temperature‐ and pH‐responsive swelling behaviors

Ultrafine hydrogel fibers that were responsive to both temperature and pH signals were prepared through the electrospinning of poly(N‐isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) mixtures in dimethylformamide. Both the diameters (700 nm to 1.2 μm) and packing of the fibers could be controlled through changes in the polymer compositions and PNIPAAm molecular weights. These fibers were rendered water‐insoluble by the addition of either Na₂HPO₄ or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na₂HPO₄ swelled to 30–120 times in water; this was significantly higher than the swelling of those crosslinked with PVA. The PVA‐crosslinked hydrogel fibers, however, exhibited faster swelling kinetics; that is, they reached equilibrium swelling in less than 5 min at 25 °C. They were also more stable after 1 week of water exposure; that is, they lost less mass and retained their fibrous form better. All the hydrogel fibers showed a drastic increase in the swelling between pH 4 and 5. The PVA‐crosslinked hydrogel fibers exhibited distinct temperature‐responsive phase‐transition behavior of PNIPAAm, whereas the Na₂HPO₄‐crosslinked hydrogel fibers showed altered two‐stage phase transitions that reflected side‐chain modification of PNIPAAm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6331–6339, 2004     

Fibrillation Tendency of Cellulosic Fibers. Part 2: Effects of Temperature

The influences of temperature, concentration of swelling agents and fiber materials on the fibrillation tendency in various cellulosic fibers in aqueous solutions were investigated in terms of fibrillation stability and fibrillation sensitivities to alkali and heat. The fibrillation stability and the fibrillation sensitivity to swelling agents were evaluated with a critical point of fibrillation (CPF_conc.) that is the concentration of the swelling agents where fibrillation begins, and the ratio of initial increase in fibril number to increase in concentration of swelling agent (I_i). The fibrillation sensitivity to heat was estimated with the increase in I_i against temperature. The CPF_conc. of lyocell fiber was 16.7 mol/l water in ethanol/water mixture at 25 °C and decreased to 0 mol/l at 80 °C, indicating acceleration of the fibrillation at higher temperatures. The I_i of lyocell was enhanced from 3.50 to 7.57 count l/mol. The CPF_conc. increased in the order of viscose > cross-linked lyocell > modal > lyocell while the I_i decreased in the order of viscose < modal < cross-linked lyocell < lyocell at 40 °C. The I_i of lyocell fiber increased to the greatest extent with increase in temperature as compared with the other cellulosic fibers. Lyocell fiber has the lowest fibrillation stability and the highest fibrillation sensitivities to alkali and to heat resulting in the highest fibrillation tendency.