Stress relaxation and inverse stress relaxation in silk fibers

Stress‐relaxation experiments on four varieties of Indian silk fiber show that stress relaxation is significantly greater in non‐Mulberry silks than in the Mulberry silk and that the differences among non‐Mulberry silk fibers are relatively small. All the fibers studied also exhibit inverse stress relaxation. It has been shown that the Maxwell–Wiechert model, with two Maxwell elements in parallel, can be used to analyze and explain both the stress‐relaxation and inverse stress‐relaxation behaviors. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1147–1154, 2001     

Modeling the stress‐relaxation behavior of wool fibers

The stress‐relaxation behavior of wool fibers after a pretreatment with a chemical solution is particularly important for evaluating the efficiency of the pretreatment. In this study, three viscoelastic models, including the Maxwell, two Maxwell unit, and modified two Maxwell unit models, were established first. To verify the feasibility of the models, stress‐relaxation experiments for wool fibers were performed. The wool fibers were pretreated with a sodium bisulfite solution (1 and 3%) at various temperatures (293, 298, 303, 308, 313, and 318 K). Then, the experimental values were fitted to the three models to obtain the rate constants of relaxation. The activation energy of the wool fibers was calculated with the Arrhenius equation. The results showed that the modified two Maxwell unit model provided the best fit for the experimental data of the wool fibers. The stress‐relaxation process of the wool fibers could be divided into two stages, a rapid stage followed by a slow stage. The rapid relaxation of stress was attributed to the weak bonds in the wool fibers, and the following slow relaxation stage was attributed to strong bonds. The Arrhenius equation could describe the stress‐relaxation process of the wool fibers very well. Furthermore, the activation energy decreased in the presence of sodium bisulfite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Nonlinear viscoelastic behavior of wool fibers in a single step relaxation test

Experimental data and their analysis are presented on the nonlinear viscoelastic behavior of wool fibers in extensional stress–relaxation up to strains of 18.5%. The analysis, assuming a two-phase structure for the fiber, consists of superimposing the experimental curves onto the master curve for the nonaging material by multiplicative scaling and shifting on the logtime scale. The scaling and shift factors reflect the strain induced phase changes of the morphological components. Though different in their physical nature, these transitions show the same strain dependence.     

Stress relaxation in carbon nanotube-based fibers for load-bearing applications

Carbon nanotube (CNT) based continuous fiber, a CNT assembly that could retain the superb properties of individual CNTs on a macroscopic scale, has emerged as a promising candidate for reinforcement in multifunctional composites. While existing research has extensively examined their short-term mechanical properties based upon quasi-static measurements, the long-term durability of CNT fibers has been largely neglected. Here we report time-dependent behavior of CNT fibers, with a particular focus on tensile stress relaxation. Both the pure CNT fiber and the CNT/epoxy composite fiber exhibited significant stress decay during the relaxation process, and this time-dependent behavior became more significant at a higher initial strain level, a lower strain rate and a greater gauge length. The present approach signifies a fundamental difference in the load-bearing characteristics between CNT fibers and traditional advanced fibers, which has major implications for the long-term durability of CNT fibers in load-bearing multifunctional applications.     

Bending stress relaxation and recovery of wool,nylon 66, and terylene fibers

The bending stress relaxation and subsequent recovery behavior were determined for merino wool, nylon, and Terylene fibers. The effect of four experimental parameters were investigated, viz., the level of bending strain (0.5–4%), the time of stress relaxation before release (1–1000 min), the relative humidity (0–85%), and the temperature (20°–60°C). For small strains the merino and nylon fibers displayed behavior characteristic of linear viscoelastic materials, while Terylene exhibited a degree of nonrecoverable set. It was possible to construct master recovery curves for fibers held bent for different times before release. These curves can be used as a more convenient means of presenting the results. A relationship was found, for each fiber type, between the percentage stress relaxation and the time taken to recover to a given level of set. This relationship appeared to be independent of the experimental conditions employed. Although the fibers were not linear viscoelastic under all conditions, recovery could be roughly predicted from their stress relaxation behavior at the particular test conditions using the Boltzmann superposition principle.     

Molecular mobility in wool fibers as determined from nuclear magnetic resonance studies

The mobility of protons in wool samples of Lincoln, Chokla, and Merino, equilibrated to various moisture contents in the range of 0–98% relative humidity (RH), have been studied by pulsed nuclear magnetic resonance (NMR) technique. Peak height and peak width were determined from absorption and derivative curves and the relaxation times T₁ and T₂ were determined from relaxation curves. The mobility increases with increase in moisture content. Among the three wools, the mobility was high in Merino as compared to the other two wools. The differences in the measured mobilities were related to structural and morphological differences in the three wools. The present analysis suggests that water in wool has at least three different associations, each with a different binding energy.     

Stress relaxation studies of model silicone RTV networks

The stress relaxation behavior of model silicone room temperature vulcanizing (RTV) elastomers has been used to examine the chemistry of the cured silicone network. It has been shown that the tin crosslinking catalyst, together with water, produces siloxane bond rearrangement which results in chemical stress relaxation. The measurement of the rate of stress relaxation of unfilled model elastomers at various temperatures gave an apparent activation energy of 10.3 kcal/mole^?1 for the relaxation process. In addition, the effects of some of the constituents of the RTV on relaxation behavior have been examined.     

Sonochemical deposition of silver nanoparticles on wool fibers

Silver nanoparticles were deposited on the surface of natural wool with the aid of powered ultrasound. The average particle size was 5–10 nm, but larger aggregates of 50–100 nm were also observed. The sonochemical irradiation of a slurry containing wool fibers, silver nitrate, and ammonia in an aqueous medium for 120 min under an argon atmosphere yielded a silver–wool nanocomposite. By varying the gas and reaction conditions, we could achieve control over the deposition of the metallic silver particles on the surface of the wool fibers. The resulting silver‐deposited wool samples were characterized with X‐ray diffraction, transmission electron microscopy, high‐resolution transmission electron microscopy, high‐resolution scanning electron microscopy, electron‐dispersive X‐ray analysis, Brunauer, Emmett, and Teller physical adsorption method, X‐ray photoelectron spectroscopy, and Raman and diffused reflection optical spectroscopy. The results showed that the strong adhesion of the silver to the wool was a result of the adsorption and interaction of silver with sulfur moieties related to the cysteine group. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1732–1737, 2007     

Studies on disulfide-crosslinked nylon. II. Stress relaxation of disulfide-crosslinked polycaprolactam fibers in water

The stress relaxation at 100% elongation in water was investigated for disulfide-crosslinked polycaprolactam (DSPC) fibers prepared from N-mercaptomethyl polycaprolactam. The stress decreased faster with increasing temperature and with increasing mercaptan content of the fiber. Little stress decay took place when the fiber was treated with a mercaptan-blocking reagent. It was concluded that the controlling mechanism of the stress relaxation was the mercaptan/disulfied (SH/SS) interchange reaction. The remanent stress observed for the stress relaxation was fairly high and increased with decreasing mercaptan content of the fiber. The stress decay curve was not Maxwellian. It has been suggested that the concentration of mercaptan that could not participate in the interchange with stressed disulfide bonds increased with increasing cycles of the interchange reaction. A kinetic equation is presented and the activation energy of the SH/SS interchange reaction was evaluated as 22.3–23.9 kcal/mole. The stress relaxation of DSPC fibers in dilute β-mercaptoethanol was also studied. The stress decreased more rapidly to almost zero and the decay curve was Maxwellian. The activation energy of the reaction was 17.1 kcal/mole. These results were compared with the stress relaxation of wool fibers.     

A study on Xe excilamp treatment of wool fibers

A Xe excilamp (λ = 172 nm) was applied to photo irradiate and surface modify wool fibers. The scanning electron microscopy (SEM) showed that the outer surface of the fibers was etched after excilamp treatment, and some microcracks emerged on the surface scales. X‐ray photoelectron spectroscopy (XPS) analysis indicated that the excilamp‐treated fibers possessed high concentration of oxygen, sulfur, and nitrogen, as well as increased hydrophilic groups, such as hydroxyl group, carboxyl group, and sulphur oxide species on the surface. Fourier transform infrared spectroscopy with attenuated total internal reflectance (FTIR‐ATR) mode measurement showed that the sulphur oxide species was mainly composed of cysteic acid and S‐sulphonate together with a small amount of cystine monoxide and cystine dioxide on the outer surface of excilamp‐treated wool fiber. The contact angle of water on the excilamp‐treated fiber decreased about 110°. Using an acid dye, deeper hue was achieved in dyeing the treated fibers, evidenced by improved relative color strength (K/S) and brightness (L^*) values. And the directional friction effect (DFE) of the wool fibers in wet obviously decreased after the excilamp treatment. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Deposition of acrylate polymers in wool fibers

The factors influencing the kinetics of formation of acrylate polymers in wool fibers are described. Allyl methacrylate and N,N′-methylenebisacrylamide were found to copolymerize satisfactorily with methyl or ethyl acrylate to form crosslinked polymers. Conditions were found for almost complete conversion of the available monomer to polymer in a batch-type process.     

Graft copolymerization of methacrylates onto wool fibers

Grafting of butyl (BuMA), decyl (DeMA), and octadecyl methacrylate (ODeMA) onto reduced wool was carried out by using K₂S₂O₈ or K₂S₂O₈–LiBr redox system as initiator. The influence of the monomer and monomer concentration, temperature, and duration of the reaction on the percentage of grafting was studied. Evidence of grafting was provided by scanning electron microscopy, size exclusion chromatography, and infrared spectroscopy.     

Immobilization of Candida rugosa lipase on modified natural wool fibers

A method has been developed to immobilize lipase from Candida rugosa on modified natural wool fibers by means of graft copolymerization of poly ethylacrylate in presence of potassium persulphate and Mohr’s salt redox initiator. The activities of free and immobilized lipase have been studied. FTIR spectroscopy, scanning electron microscopy, and the Bradford method were used to characterize lipase immobilization. The efficiency of the immobilization was evaluated by examining the relative enzymatic activity of free enzyme before and after the immobilization of lipase. The results showed that the optimum temperature of immobilized lipase was 40 °C, which was identical to that of the free enzyme, and the immobilized lipase exhibited a higher relative activity than that of free lipase over 40 °C. The optimal pH for immobilized lipase was 8.0, which was higher than that of the free lipase (pH 7.5), and the immobilization resulted in stabilization of enzyme over a broader pH range. The kinetic constant value (km) of immobilized lipase was higher than that of the free lipase. However, the thermal and operational stabilities of immobilized lipase have been improved greatly.     

Graft copolymerization of vinyl monomers in wool fibers

Graft copolymerization of vinyl monomers, mainly methyl methacrylate, in reduced, successively alkylated, or KCN-Treated wool fibers was performed in the redox LiBr–persulfate system without homopolymer. The reduction gives a striking effect in promoting the graft copolymerization. Methylation or ethylene recrosslinking of the reduced wool, especially the former, decreases the graft-on remarkably. By the KCN treatment in which the conversion of disulfide to lanthionine bonds occurs, the grafting is decreased in the bromide–persulfate system but promoted in the system with persulfate alone. Methylation or KCN treatment of wool as well as reduction brings about a great increase in the absorption of persulfate. The grafting of the lanthionine-containing wool in the redox system accompanied by the liberation of bromine might be retarded by the pronounced bromination of monomers over the inhibiting of homopolymerization, because the lanthionine bonds are more stable to bromine than the disulfide bonds. In general, disulfide bonds and the other easily oxidized components of wool may perhaps play an important role in regulating the bromination of monomers and in the graft copolymerization without homopolymer. The molecular weight of graft polymer is decreased distinctly with increasing extent of reduction of wool. From these results, the thiol groups on wool are considered to give predominantly graft centers by the radicalotropy from SO₄, OH·, and/or Br·.     

Grafting vinyl monomers onto wool fibers: Graft copolymerization of methyl methacrylate onto wool fibers using vanadyl acetyl acetonate complex

The graft copolymerization of methyl methacrylate (MMA) onto native and reduced Indian Chokla wool fibers was studied in aqueous solution using the acetylacetonate oxovanadium (IV) complex. The rate of grafting was investigated by varying the concentration of the monomer and the complex, acidity of the medium, and the solvent composition of the reaction medium. The graft yield increases with increasing concentration of the initiator up to 8.75 × 10^?5 mol/L, of the monomer up to 0.5634 mol/L, and thereafter it decreases. MMA was found to be the most active monomer when compared to other vinyl monomers. Grafting increases with increasing concentration of HClO₄ and with increasing temperature. Reduced and oxidized wools were found to be better substrates than untreated, esterified, crosslinked, and trinitrophenylated wools. The extent of grafting was mostly dependent upon the concentration of ? SH groups in case of reduced wool. A suitable reaction scheme has been proposed and the activation energy was calculated from Arrhenius plot.     

Characterization of the weak link of wool fibers

The variance of fiber morphology along a fiber and the natural and artificial flaws in the fiber structure represent the primary reasons for the weak link of fibers. Accordingly, the fiber weak link can be divided into two types, that is, the geometrical thinnest part and the structural weak point. Scanning electron microscopic observation was used to characterize the morphological features of the fiber weak points whose forms are the normal thin sections, natural flaws, and artificial damage. Both the fiber profile morphology and the tensile behavior of wool fibers have been measured using a single‐fiber analyzer (SIFAN) and an optical microscope with a CCD camera plus an XQ‐1 fiber tensile tester (OM + XQ). The results from the SIFAN and OM+XQ methods indicate that the fibers breaking at their minimum diameters represent only one part of the broken fibers. The percentage of this kind of breakage is in the range of 40–60%. A new approach is presented to identify the weak‐point breakage relying on the fiber tensile behavior. The experimental results show that the probabilities of weak‐point, normal, and thinnest‐part breakage evaluated by these methods approximate 40, 60, and slightly more than 80%, respectively. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1206–1212, 2003     

Chemical modification of wool fibers with acid anhydrides

Wool fibers were chemically modified by reaction with succinic and glutaric anhydrides. The weight gain (and acyl content) increased with increasing the reaction temperature (65–80°C) and time (1–2 h), attaining 18.9% (158.9 mol/10⁵ g) and 23% (163.9 mol/10⁵ g) for succinylated and glutarylated wool, respectively. Changes in the amino acidic pattern of acylated wool, i.e., decrease of basic amino acid residues and formation of ornithine, were observed by acid hydrolysis. The X‐ray diffraction profiles of modified wool fibers remained essentially unchanged, suggesting that the crystalline structure was not affected by reaction with acid anhydrides. The degree of molecular orientation of acylated wool slightly decreased, especially at high weight gain. The viscoelastic response of wool modified with succinic and glutaric anhydrides was characterized by a shift to a lower temperature of both the drop of the storage modulus and the peak of the loss modulus. These features are indicative of a higher mobility of the keratin chians in the amorphous and crystalline domains. In fact, it is suggested that the chemical agent diffused into the accessible parts of α‐crystallites, reaching the available reactive sites. This did not cause changes in the crystalline pattern of wool, but resulted in a different thermal behavior of fibers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1573–1579, 1999