Structure–property relation in varieties of silk fibers

Crystallite shape ellipsoid in different varieties of silk fibers namely (i) Chinese (ii) Indian, and (iii) Japanese, has been computed using wide‐angle X‐ray data and Hosemann's one‐dimensional paracrystalline model. The estimated microcrystalline parameters are correlated with the observed physical property of the silk fibers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1979–1985, 2001     

Carbon fibers derived from UV‐assisted stabilization of wet‐spun polyacrylonitrile fibers

A rapid, dual‐stabilization route for the production of carbon fibers from polyacrylonitrile (PAN) precursor fibers is reported. A photoinitiator, 4,4′‐bis(diethylamino)benzophenone, was added to PAN solution before the fiber wet‐spinning step. After a short UV treatment that induced cyclization and crosslinking at a lower temperature, precursor fibers could be rapidly thermo‐oxidatively stabilized and successfully carbonized. Scanning electron microscopy micrographs show no deterioration of the microstructure or hollow‐core formation in the fibers due to UV treatment or presence of photoinitiator. Fast‐thermally stabilized pure PAN‐based carbon fibers show hollow‐core fiber defects due to inadequate thermal stabilization, but such defects were not observed in carbon fibers derived from fast‐thermally stabilized fibers that contained photoinitiator and were UV treated. Tensile testing results confirm that fibers containing 1 wt % photoinitiator and UV treated for 5 min display higher tensile modulus than all other sets of thermally stabilized and carbonized fibers. Wide‐angle X‐ray diffraction results show a higher development of the aromatic structure and molecular orientation in thermally stabilized fibers. No significant increase in interplanar spacing or decrease in crystals size were observed within the UV‐stabilized carbon fibers containing photoinitiator, but such fibers retain a higher extent of molecular orientation when compared with control fibers. These results establish for the first time, the positive effect of the external addition of photoinitiator and UV treatment on the properties of the PAN‐based fibers, and may be used to reduce the precursor stabilization time for faster carbon fiber production rate. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40623.     

Structure‐mechanical properties relationship of poly(ethylene terephthalate) fibers

The correlation between the fiber structure and mechanical properties of two different poly(ethylene terephthalate) fiber types, that is, wool and cotton types produced by three producers, was studied. Fiber structure was determined using different analytical methods. Significant differences in the suprastructure of both types of conventional textile fibers were observed, although some slight variations in the structure existed between those fibers of the same type provided by different producers. A better‐developed crystalline structure composed of bigger, more perfect, and more axially oriented crystallites was characterized for the cotton types of PET fibers. Crystallinity is higher, long periods are longer, and amorphous domains inside the long period cover bigger parts in this fiber type in comparison with the wool types of fibers. In addition, amorphous and average molecular orientation is higher. The better mechanical properties of cotton PET fiber types, as demonstrated by a higher breaking tenacity and modulus accompanied by a lower breaking elongation, are due to the observed structural characteristics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3383–3389, 2003     

Structure–property relationships in thermally aged cellulose fibers and paper

The research presented in this paper investigates the effect of thermally accelerated aging on the submicrostructure of cellulose and attempts to relate such changes to the well‐documented loss of mechanical strength in aged paper. Filter paper and ramie fibers samples were aged in vacuo at 160°C. Small angle X‐ray scattering (SAXS) was used to study void structure within the fibers and hydration used as a structural probe. On hydration, the void radius of gyration and area decrease, while the void aspect ratio and overall void fraction increase. After aging, the wet structure more closely resembles the dry, suggesting that water cannot expand the structure to the same extent. It is postulated that increases in local ordering on aging create a structure more resistant to disruption by water. The use of additional techniques, namely Fourier transform IR spectroscopy, wide angle X‐ray scattering, scanning electron microscopy (SEM), environmental SEM, and measurement of water retention value, provided additional indirect support for the postulated model. There is no direct evidence for significant crystallinity changes in aged material, suggesting that if structural rearrangements occur, they will be local in nature. There is also no evidence for the formation of covalent crosslinks or new chemical species on aging. Water retention values and wet SAXS results concur, highlighting the importance of water in the cellulose structure and the reduced capacity for water sorption in aged samples. SEM observations show that the failure mechanism in paper changes with age from fibers pull out (i.e., interfiber bond failure) to fiber failure, and wide and zero span tensile tests indicate a weakening of the fibers. These results are consistent with previous reports, and we attribute them primarily to chain scission, although the increased intrafiber bonding may have an influence on the values obtained. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1465–1477, 1999     

Thermal mechanical and dynamic mechanical property of biphenyl polyimide fibers

High‐performance polyimide fibers possess many excellent properties, e.g., outstanding thermal stability and mechanical properties and excellent radiation resistant and electrical properties. However, the preparation of fibers with good mechanical properties is very difficult. In this report, a biphenyl polyimide from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride and 4,4′‐oxydianiline is synthesized in p‐chlorophenol by one‐step polymerization. The solution is spun into a coagulation bath of water and alcohol via dry‐jet wet‐spinning technology. Then, the fibers are drawn in two heating tubes. Thermal gravimetric analysis, thermal mechanical analysis, and dynamic mechanical analysis (DMA) are performed to study the properties of the fibers. The results show that the fibers have a good thermal stability at a temperature of more than 400°C. The linear coefficient of thermal expansion is negative in the solid state and the glass transition temperature is about 265°C. DMA spectra indicate that the tanδ of the fibers has three transition peaks, namely, α, β, and γ transition. The α and γ transition temperature, corresponding to the end‐group motion and glass transition, respectively, extensively depends on the applied frequency, while the β transition does not. The activation energy of α and γ transition is calculated using the Arrhenius equation and is 38.7 and 853 kJ/mol, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1653–1657, 2004     

Polyimide–silica hybrid films made from polyamic acids containing phenolic hydroxyl groups

A diamine and polyamic acid containing phenolic hydroxyl group was synthesized. A series of polyimide/silica hybrid films with strong interacton between organic and inorgamic components was prepared via sol–gel reaction. The morphology of the hybrid films was investigated by scanning electron microscopy and atomic force microscopy. The thermal stability and mechanical properties of the films were detected. The results indicated that the introduction of phenolic hydroxyl groups remarkably attributed to the improvement of tensile strength. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1198–1202, 2004     

Highly crystalline and oriented high‐strength poly(ethylene terephthalate) fibers by using low molecular weight polymer

High‐strength poly(ethylene terephthalate) (PET) fibers were obtained using low molecular weight (LMW) polymervia horizontal isothermal bath (hIB), followed by postdrawing process. We investigated the unique formations of different precursors, which differentiated in its molecular orientation and crystalline structures from traditional high‐speed spinning PET fibers. Sharp increase in crystallinity was observed after drawing process even though the fibers showed almost no any crystallinity before the drawing. Properties of as‐spun and drawn hIB and control filaments at different process conditions were compared. As would be expected, performances of resulted treated undrawn and drawn fibers have dramatically improved with developing unique morphologies. Tenacities more than 8 g/d for as‐spun and 10 g/d for drawn treated fibers after just drawn at 1.279 draw ratio were observed. These performances are considerably higher than that of control fibers. An explanation of structural development of high‐strength fibers using LMW polymer spun with hIB is proposed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42747.     

Structure–property relationship of starch‐filled chain‐extended polyurethanes

Chain‐extended polyurethane (PU) elastomers were prepared using castor oil with 4,4′‐methylene bis (phenyl isocyanate) (MDI) as a crosslinker and 4,4′‐diamino diphenyl sulphone (DDS) as an aromatic diamine chain extender. A series of starch‐filled (from 5 to 25% wt/wt) diamines chain‐extended PUs have been prepared. The starch‐filled PU composites were characterized for physico‐mechanical properties viz, density, surface hardness, tensile strength, and percentage elongation at break. Thermal stability of PU/starch have been carried out by using thermogravimetric analyzer (TGA). Thermal degradation process of PU/starch were found to proceed in three steps. TGA thermograms of PU/starch shows that all systems were stable upto 235°C, and maximum weight loss occur at temperature 558°C. The microcrystalline parameters such as crystal size (〈N〉) and lattice strain (g in %) of PU/starch have been established using wide‐angle X‐ray scattering (WAXS) method. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2945–2954, 2003     

Compatibilizing polyimide/silica hybrids by alkoxisilane‐terminated oligoimides: Morphology–Properties relationships

A fluorinated polyimide (PI), synthesized from 4,4′‐(hexafluoroisopropylidene) diphthalic anhydride and 4,4′‐diaminodiphenyl ether, was used as matrix for the preparation of PI/silica hybrids. The inorganic phase was obtained in situ by a sol–gel route with tetraethoxysilane as precursor. Both micron‐ and nano‐scale hybrids were obtained depending on the interfacial interaction between the organic and inorganic phases. To promote such interaction a compatibilizing agent was synthesized, in the form of an alkoxisilane‐terminated oligoimide. Both the PI and the coupling agent were characterized by FTIR and Raman spectroscopies and by GPC measurements. The effect of the coupling agent on the morphology of the hybrids and on the size of the silica particles was investigated by scanning electron microscopy. The viscoelastic, mechanical, and thermal properties of hybrid composites were studied. It was shown that by adding appropriate amounts of the compatibilizer it was possible to control the morphology and to obtain homogeneous nanostructured systems. A general improvement of the mechanical performances and of the thermal stability was demonstrated, together with an increase of T_g, which was found more pronounced for the nanocomposites than for the microcomposites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structure–property correlations of heat‐shrinkable polymer blends based on ethylene vinyl acetate/carboxylated nitrile rubber in the presence of different curatives

Elastic memory was introduced into heat‐shrinkable polymer blends in the form of an elastomeric phase and through subsequent crosslinking. Blends of ethylene vinyl acetate and carboxylated nitrile rubber with different curative systems were studied with respect to their shrinkability. With an increase in the cure time (the crosslinking density, or memory point), shrinkage increased for the blends with all the curative systems except dicumyl peroxide (DCP). Increasing the elastomer content increased shrinkability because of the increasing driving retraction force of the oriented elastomer phase. A sample stretched at a high temperature (HT) showed greater shrinkage than a sample stretched at room temperature (RT) because of the greater concentration and degree of orientation of the extended chains. Generally, the crystallinity of the stretched (RT and HT) samples was higher than that of ordinary unstretched and shrunk samples, and this increased the effectiveness of intermolecular interactions in the former. For all systems except DCP, RT‐stretched samples showed higher crystallinity than corresponding HT‐stretched samples. With RT stretching, rapid extension and subsequent recrystallization occurred in samples molten at high local values of the stored elastic energy. An increase in the crosslinking density and orientation of the blends increased the thermal stability because of the formation of strong networks and compact structures. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1414–1420, 2003     

Simulation of dry‐spinning process of polyimide fibers

As one type of high‐performance fibers, the polyimide fibers can be prepared from the precursor polyamic acid via dry‐spinning technology. Unlike the dry‐spinning process of cellulose acetate fiber or polyurethane fiber, thermal cyclization reaction of the precursor in spinline with high temperature results in the relative complex in the dry‐spinning process. However, the spinning process is considered as a steady state due to a slight degree of the imidization reaction from polyamic acid to polyimide, and therefore a one‐dimensional model based on White‐Metzer viscoelastic constitutive equation is adopted to simulate the formation of the fibers. The changes of solvent mass fraction, temperature, axial velocity, tensile stress, imidization degree, and glass transition temperature of the filament along the spinline were predicted. The effects of spinning parameters on glass transition temperature and imidization degree were thus discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Structure–property relationships of medium‐density polypropylene foams

This paper is focused on the production and characterization of a collection of polypropylene (PP) foams with relative densities ranging from 0.3 to 0.6. Samples were foamed using the improved compression moulding method. The process allows controlling density and cellular structure independently as well as obtaining PP foams without fillers, crosslinking or using special PP grades. The influence of blowing agent content, density, cellular structure and foaming conditions on the mechanical response measured in compression, tensile, bending and Charpy impact tests was determined. Results show that density, open cell content and blowing agent concentration have a significant influence on the mechanical performance of medium‐density PP foams. © 2013 Society of Chemical Industry     

X‐Ray reflectivity study of polyimide thin films swollen by 1,3‐butadiene and n‐butane

X‐ray reflectivity measurements were performed on two different polyimide thin films synthesized from 2,2‐bis(3,4‐carboxyphenyl)hexafluoropropane dianhydride (6FDA) in 1,3‐butadiene and n‐butane. In 1,3‐butadiene at 2.3 atm, the film thickness increased by 24–30%. However, the film thickness increased by only 10% in n‐butane at 2.3 atm. Excessive increases in film thickness were shown in 1,3‐butadiene, but the decreases in film density were minor. The probability of the condensation of 1,3‐butadiene in the films is indicated. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1818–1825, 2000     

Microstructural investigation of butt‐fusion joint in high‐density polyethylene pipes using wide‐ and small‐angle X‐ray scattering

The melt fusion zone (MFZ) of polyethylene pipe was investigated employing synchrotron wide‐ and small‐angle X‐ray scattering at various locations in MFZ by changing X‐ray incidence angles to probe three‐dimensional structural features. It was determined that the crystals were oriented in two different modes. One is that the polymer chains are oriented parallel to the joint interface line consistently throughout the MFZ. The other is that the crystals are oriented in particular directions depending on the positions in MFZ. The combination of pressure and melt flow during joining process resulted in such a complex structure. It was notable that the boundary of MFZ against the base material was found to be very different depending on the structures involved such as crystallographic unit orientation, lamellae orientation, crystallinity, and spherulitic morphology. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45668.     

Structure and property development of aromatic copolysulfonamide fibers during wet spinning process

The structure and performance changes of aromatic copolysulfonamide (co‐PSA) fibers that occurred during wet spinning process have been studied. While using different length scale characterization, including scan electron microscopy (SEM), wide‐angle X‐ray scattering (WAXS), and small‐angle X‐ray scattering (SAXS), it was found that the molecular chains of co‐PSA formed an isotropic network during coagulation which further lead to extension and orientation of these chains during the subsequent stretching. As a result, only after heat stretching and heat setting the molecular chains tended to pack into crystal lattice in the fibrils. This gave rise to a much denser structure along the spinning line and the glass transition temperature of co‐PSA fibers increased a little after heat setting. Before heat stretching, the co‐PSA fibers were in amorphous state, and only the amorphous orientation was observed within the fibers. After heat stretching at the temperature higher than T_g, the fraction of amorphous region decreased, and the crystal structure formed in the fibers, which became more perfect during heat setting. The structure development during spinning process contributed toward the improvement of thermo‐mechanical stability, tenacity and modulus of the co‐PSA fibers. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42343.     

Structure–property relationship in PP/LDPE blend composites: The role of nanoclay localization

This article reports, for the first time, on how the kinetics and thermodynamics of the melt‐processing control the nano/micro‐structure development and properties of nanoclay‐filled polypropylene (PP)/low‐density polyethylene (LDPE) blend ternary composites. Morphological characterization suggests that the nano/micro‐structure of the PP/LDPE (80/20) blend can be controlled by incorporating nanoclay alone or by adding a mixture of organoclay and maleated compatibilizers. Simultaneous mixing of PP, LDPE, maleated compatibilizers, and organoclay results in homogeneous distribution of intercalated silicate layers in all the phases of the blend, a feature which profoundly affects the thermal stability and tensile and rheological properties of the blend composites. For example, the elongation‐at‐break for PP increases from 28.1 to 155.6% for composite containing both organoclay and maleated compatibilizers, whereas the thermal stability for PP increases from 269.8 to 303.3 °C for the same composite. However, the impact strength of the PP/LDPE blend decreases with incorporation of organoclay, regardless of the phase in which the nanoclay particles are localized. In summary, the obtained results show that regardless of the phase in which the nanoclay is localized, the morphology, and hence the properties, of the ternary composites are superior to those of the neat blend. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46193.     

Effect of the drawing process on the wet spinning of polyacrylonitrile fibers in a system of dimethyl sulfoxide and water

The effect of the drawing process on the structural characteristics and mechanical properties of polyacrylonitrile (PAN) fibers was comparatively studied. The protofibers extruded from the spinneret were the initial phase of stretching, which involved the deformation of the primitive fiber with the concurrent orientation of the fibrils. Wet‐spun PAN fibers observed by scanning electron microscopy exhibited different cross‐sectional shapes as the draw ratio was varied. X‐ray diffraction results revealed that the crystalline orientation of PAN fibers increased with increasing draw ratio; these differences in the orientation behaviors were attributed to the various drawing mechanisms involved. The crystalline and amorphous orientations of the PAN fibers showed different features; at the same time, the tensile properties were strongly dependent on the draw ratio. However, the stream stretch ratio had most influence on the tensile strength and the orientation of PAN fibers for the selected process parameters. Electron spin resonance proved that the local morphology and segmental dynamics of the protofibers were due to a more heterogeneous environment caused by the sequence structure. Differential scanning calorimetry indicated that the size and shape of the exotherm and exoenergic reaction were strongly dependent on the morphology and physical changes occurring during fiber formation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1026–1037, 2007     

Wet‐spun,photoinitiator‐modified polyacrylonitrile precursor fibers: UV‐assisted stabilization

The role of dilute concentrations (～1 wt %) of a photoinitiator, 4,4 ′ ‐bis(diethylamino)benzophenone, on the processability and properties of the resulting wet‐spun polyacrylonitrile (PAN) fibers are reported. Rheology measurements show no adverse effect on the viscosities of solutions by the addition of the photoinitiator. Fibers containing photoinitiator were successfully wet‐spun from PAN – DMSO solution. FTIR results prove that 4,4 ′ ‐bis(diethylamino)benzophenone was retained in the fibers after coagulation and post‐stretching. SEM micrographs show no deterioration of the post‐stretched fiber microstructure due to the presence of photoinitiator. Tensile testing results show a small reduction in the strain‐at‐break of post‐stretched fibers containing photoinitiator when compared with pure (control) PAN fibers. After UV treatment, fibers with 4,4 ′ ‐bis(diethylamino)benzophenone display a higher tensile modulus compared with the other sets. Wide‐angle X‐ray diffraction results show no significant decrease in interplanar spacing and size of the crystals within the fibers containing photoinitiator, but such fibers retain a higher extent of molecular orientation after being UV treated. Conversion indices were measured from the WAXD spectra and compared with conventional thermal stabilized fibers. This correlation confirms that the addition of 1 wt % photoinitiator to PAN followed by 5 min of UV treatment leads to a conversion index that is observed in control fibers after more than an hour, which could reduce the conventional thermo‐oxidative stabilization time significantly. These results indicate the potential of the dual stabilization route in generating precursor fibers with higher molecular orientation, and possibly reducing the thermo‐oxidation time during carbon fiber processing. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2494–2503, 2013     

Structure‐property relationship of substituted pyrrolidine functionalized CNT epoxy nanocomposite

Carbon nanotubes (CNTs) based polymer nanocomposites hold the promise of delivering exceptional mechanical properties and multifunctional characteristics. However, the realization of exceptional properties of CNT based nanocomposites is dependent on CNT dispersion and CNT‐matrix adhesion. To this end, we modified MWCNTs by Prato reaction to yield aromatic (phenyl and 2‐hydroxy‐4‐methoxyphenyl) substituted pyrrolidine functionalized CNTs (fCNT1 and fCNT2) and aliphatic (2‐ethylbutyl and n‐octyl) substituted pyrrolidine functionalized CNTs (fCNT3 and fCNT4). The functionalization of CNTs was established by Thermogravimetric analysis (TGA), Raman Spectroscopy, and XPS techniques. Optical micrographs of fCNT epoxy mixture showed smaller aggregates compared to pristine CNT epoxy mixture. A comparison of the tensile results and onset decomposition temperature of fCNT/epoxy nanocomposite showed that aliphatic substituted pyrrolidine fCNT epoxy nanocomposites have higher onset decomposition temperature and higher tensile toughness than aromatic substituted pyrrolidine fCNT epoxy nanocomposites, which is consistent with the dispersion results of fCNTs in the epoxy matrix. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42284.     

Structure–process–yield interrelations in nanocrystalline cellulose extraction

In order to improve the extraction of nanocrystalline cellulose (NCC) from sulfuric acid hydrolysis of chemical pulps, we have studied the effect of hydrolysis conditions on the degree of polymerization (DP), the extent of sulfation, morphological, and solid‐state characteristics of the extracted materials vis‐à‐vis yield. Our results demonstrate that sulfation plays a significant role in (i) determining the yield of, and (ii) imparting the unique solid‐state characteristics to, the extracted, H₂O‐insoluble cellulose nanomaterial from sulfuric acid hydrolysis. The hydrolysis process is itself proven to be highly reproducible, and NCC with high crystallinity (>80%) and a yield between 21% and 38% could be extracted from a fully bleached, commercial softwood kraft pulp using 64 wt.% sulfuric acid at 45–65°C after freeze drying. The NCC aggregates, with iridescent patterns typical of chiral nematic materials, are parallelepiped rod‐like structures which possess cross‐sections in the nanometer range and lengths orders of magnitude larger, resulting in high aspect ratios. The Ruland–Rietveld analysis was employed to precisely resolve X‐ray diffraction patterns and obtain information on crystallite size, crystalline and amorphous areas, and crystallinity of the extracted materials.