Showing papers in "Journal of Applied Polymer Science in 1999"

A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy

Abstract: Hard and softwood and wood constituent polymers (cellulose and lignin) were studied using Fourier transform infrared (FTIR) spectroscopy. The hollocellulose-to-lignin ratio was estimated for some of the timber species. The structural difference between Klason lignin isolated from softwood (Pinus roxberghii and cupressus lusitanica) and hard wood (Acacia auriculaeformis and Eucalyptus tereticornis) species was studied. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1969–1975, 1999

Studies on nylon 6/clay nanocomposites by melt-intercalation process

Abstract: The preparation of nylon 6/clay nanocomposites by a melt-intercalation process is proposed. X-ray diffraction and DSC results show that the crystal structure and crystallization behaviors of the nanocomposites are different from those of nylon 6. Mechanical and thermal testing shows that the properties of the nanocomposites are superior to nylon 6 in terms of the heat-distortion temperature, strength, and modulus without sacrificing their impact strength. This is due to the nanoscale effects and the strong interaction between the nylon 6 matrix and the clay interface, as revealed by X-ray diffraction, transmission electron microscopy, and Molau testing. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1133–1138, 1999

Thermal conductivity, elastic modulus, and coefficient of thermal expansion of polymer composites filled with ceramic particles for electronic packaging

Abstract: The effective thermal conductivity, elastic modulus, and coefficient of thermal expansion of epoxy resins filled with ceramic fillers like silica, alumina, and aluminum nitride were determined. The data obtained was compared with theoretical and semitheoretical equations in the literature that are used to predict the properties of two phase mixtures. It was found that Agari's model provided a good estimate of the composite thermal conductivity. The Hashin-Shtrikman lower bound for composite modulus fits the modulus data fairly well at low concentrations of the filler. Also, it was found that the coefficients of thermal expansion of the filled composites lie in between Schapery's upper and lower bounds. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3396–3403, 1999

Crystallization, properties, and crystal and nanoscale morphology of PET–clay nanocomposites

Abstract: The crystallization process and crystal morphology of poly(ethylene terephathalate) (PET)–clay nanoscale composites prepared by intercalation, followed by in-situ polymerization, have been investigated by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM), dynamic scanning calorimetry (DSC), and X-ray techniques, together with mechanical methods. Results of the nonisothermal crystallization dynamics show that the nanocomposites of PET (Nano-PET) have 3 times greater crystallization rate than that of pure PET. The thermal properties of Nano-PET showed heat distortion temperature (HDT) 20–50°C higher than the pure PET, while with a clay content of 5%, the modulus of Nano-PET is as much as 3 times that of pure PET. Statistical results of particle distribution show that the average nanoscale size ranges from 10 to 100 nm. The particles are homogenously distributed with their size percentages in normal distribution. The agglomerated particles are 4% or so with some particles size in the micrometer scale. The morphology of exfoliated clay particles are in a diordered state, in which the morphology of the PET spherulitics are not easy to detect in most of microdomains compared with the pure PET. The molecular chains intercalated in the interlamellae of clay are confined to some extent, which will explain the narrow distribution of the Nano-PET molecular weight. The stripe-belt morphology of the intercalated clay show that polymer PET molecular chains are intercalated into the enlarged interlamellar space. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1139–1146, 1999

Alkali treatment of jute fibers: Relationship between structure and mechanical properties

Abstract: The mechanical properties of tossa jute fibers were improved by using NaOH treatment process to improve the mechanical properties of composites materials. Shrinkage of fibers during this process has significant effects to the fiber structure, as well as to the mechanical fiber properties, such as tensile strength and modulus. Isometric NaOH-treated jute yarns (20 min at 20°C in 25% NaOH solution) lead to an increase in yarn tensile strength and modulus of ∼ 120% and 150%, respectively. These changes in mechanical properties are affected by modifying the fiber structure, basically via the crystallinity ratio, degree of polymerization, and orientation (Hermans factor). Structure–property relationships, developed for cellulosic man-made fibers, were used with a high correlation factor to describe the behavior of the jute fiber yarns. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 623–629, 1999

Synthesis and characteristics of interpenetrating polymer network hydrogel composed of chitosan and poly(acrylic acid)

Abstract: Interpenetrating polymer network (IPN) hydrogels composed of chitosan and poly(acrylic acid) (PAAc) were synthesized by UV irradiation method, and their structure, crystallinity, swelling behavior, thermal property, and mechanical property were investigated. Chitosan/PAAc IPNs exhibited relatively high equilibrium water content and also showed reasonable sensitivity to pH. From the swelling behaviors at various pH's, Fourier transform infrared spectra at high temperature and thermal analysis confirmed the formation of polyelectrolyte complex due to the reaction between amino groups in chitosan and carboxyl groups in PAAc. For this reason, even at a swollen state, the present chitosan/PAAc IPNs possess good mechanical properties. Particularly, the CA-2 sample (with a weight ratio of chitosan/PAAc = 50/50, molar ratio [NH2]/[COOH] = 25/75) showed the lowest equilibrium water content and free water content, attributed to the more compact structure of the polyelectrolyte than CA-1 or CA-3 due to the high amount of interchain bond within the IPN. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 113–120, 1999

Moisture diffusion in epoxy systems

Abstract: The moisture diffusion process of an epoxy system is studied as a function of epoxy-amine stoichiometry and the resulting microstructure. Differences in diffusion behavior are related to the relative importance of diffusion through the low-density and high-density microstructural phases for different stoichiometries. Also, changes in saturation level with stoichiometry are explained by competing effects of free volume versus the content of the low-density phase. Increasing the humidity level causes a corresponding increase in saturation level, while increasing the temperature causes more pronounced non-Fickian behavior. The effects of absorbed moisture on the thermomechanical properties of the epoxies are also investigated. Reductions in the glass transition temperature, Tg, and moisture-induced swelling strains are measured after exposure of samples to the three conditioning environments. Moisture-induced swelling strains increase with increasing moisture content. The reductions in Tg range from 5 to 20°C and are generally larger for amine-rich samples than for epoxy-rich and stoichiometric samples. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 787–798, 1999

Highly active thermally stable β-nucleating agents for isotactic polypropylene

Abstract: Calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids were synthesized and implemented as in different grades of isotactic polypropylene (iPP). Propylene homopolymer, as well as random and block copolymers containing these additives, crystallized iPP into pure or nearly pure β modification in the isothermal and nonisothermal crystallization experiments. Recently, Ca-Sub proved to be the most effective β-nucleating agent of iPP. The Ca-Sub nucleating agent widens the upper crystallization temperature range of pure β-iPP formation up to 140°C. In this study the effect of the these additives on the crystallization, melting characteristics, and structure of the PP were studied. The degree of crystallinity of β-iPP was markedly higher than that of α-iPP. A widening in the melting peak of the samples crystallized in a high temperature range was first observed and discussed in regard to literature results of the same phenomenon for α-iPP. The morphology of the β-iPP samples was revealed by scanning electron microscopy. Independent of the type of polymer or nucleating agent, hedritic structures were found in the early stages of growth of the β-spherulites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2357–2368, 1999

Maleation of Polylactide (PLA) by Reactive Extrusion

Abstract: Free-radical-initiated grafting of maleic anhydride (MA) onto a polylactide (PLA) backbone was performed by reactive extrusion. A concentration of 2 wt % MA in the presence of 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (Lupersol 101) as the free- radical initiator was used for all experiments. Two reaction temperatures were studied (180 and 200°C) with a peroxide initiator concentration between 0.0 and 0.5 wt %. Under these conditions, between 0.066 and 0.672 wt % MA was grafted onto the PLA chains. Triple-detector size-exclusion chromatography (TriSEC), melt flow index (MFI), and thermal gravimetric analysis (TGA) were used to characterize the maleated PLA polymers. Increasing the initiator concentration resulted in an increasc in the grafting of MA, as well as a decrease in the molecular weight of the polymer. The maleation of PLA proved to be very efficient in promoting strong interfacial adhesion with corn native starch in composites as obtained by melt blending. 0 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 477-485, 1999

Properties and structure of poly(vinyl alcohol)/silica composites

Abstract: The relationships between the properties and structure are discussed for poly(vinyl alcohol)(PVA)/silica composites prepared through the sol-gel process. The composites became stiff and brittle with increasing the silica content. The properties of the composites were changed drastically at around the composition of PVA/silica = 70/30 wt %. For example, there was no large change in the Young's modulus above 30 wt % of silica content (Pure PVA: 31.8 MPa, silica 30%: 52.6 MPa, silica 50%: 55.2 MPa). Consequently, it was considered that the three-dimensional network structure of silica could be formed in the composite with more than 30 wt % of silica in PVA. From this behavior, it could be considered that the crystal growth of PVA was remarkably inhibited by silica network. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 133–138, 1999

Surface modification of poly(ether sulfone) ultrafiltration membranes by low-temperature plasma-induced graft polymerization

Abstract: Low-temperature helium plasma treatment followed by grafting of N-vinyl-2-pyrrolidone (NVP) onto poly(ether sulfone) (PES) ultrafiltration (UF) membranes was used to modify commercial PES membranes. Helium plasma treatment alone and post-NVP grafting substantially increased the surface hydrophilicity compared with the unmodified virgin PES membranes. The degree of modification was adjusted by plasma treatment time and polymerization conditions (temperature, NVP concentration, and graft density). The NVP-grafted PES surfaces were characterized by Fourier transform infrared attenuated total reflection spectroscopy and electron spectroscopy for chemical analysis. Plasma treatment roughened the membrane as measured by atomic-force microscopy. Also, using a filtration protocol to simulate protein fouling and cleaning potential, the surface modified membranes were notably less susceptible to BSA fouling than the virgin PES membrane or a commercial low-protein binding PES membrane. In addition, the modified membranes were easier to clean and required little caustic to recover permeation flux. The absolute and relative permeation flux values were quite similar for the plasma-treated and NVP-grafted membranes and notably higher than the virgin membrane. The main difference being the expected long-term instability of the plasma treated as compared with the NVP-grafted membranes. These results provide a foundation for using low-temperature plasma-induced grafting on PES with a variety of other molecules, including other hydrophilic monomers besides NVP, charged or hydrophobic molecules, binding domains, and biologically active molecules such as enzymes and ribozymes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1699–1711, 1999

Viscoelastic properties of plasticized PVC reinforced with cellulose whiskers

Abstract: New nanocomposites are processed with a plasticized poly(vinyl chloride) matrix reinforced by cellulose whiskers whose characteristics are a high aspect ratio and a large interface area. Dynamic mechanical analysis performed on samples rein- forced with a filler fraction of up to 12.4 vol % gives the viscoelastic properties of the composite above and below its glass transition temperature. Different theoretical predictions are proposed to describe this behavior, but none of them is found wholly satisfactory for describing the reinforcing effect of these fillers. A model based on the Halpin-Kardos equation, with the assumption of an immobilized phase around the whiskers, is developed to account for significant decrease in the modulus drop, on passing above the glass transition temperature. The small discrepancy between this model and the experimental modulus measured in the rubber plateau is discussed as a possible effect of a percolating whisker network whose crosslinks are assured by chains adsorbed onto the whisker surface. Swelling experiments support this hypothesis. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1797-1808, 1999

Thermal and viscoelastic property of epoxy-clay and hybrid inorganic-organic epoxy nanocomposites

Abstract: The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface and chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. In this article, we investigated the thermal and viscoelastic property enhancement on crosslinked epoxy using two types of nanoreinforcement, namely, organoion exchange clay and polymerizable polyhedral oligomeric silsesquioxane (POSS) macromers. Glass transitions of these nanocomposites were studied using differential scanning calorimetry (DSC). Small-strain stress relaxation under uniaxial deformation was examined to provide insights into the time-dependent viscoelastic behavior of these nanocomposites. Since the size of the POSS macromer is comparable to the distance between molecular junctions, as we increase the amount of POSS macromers, the glass transition temperature Tg as observed by DSC, increases. However, for an epoxy network reinforced with clay, we did not observe any effect on the Tg due to the presence of clay reinforcements. In small-strain stress relaxation experiments, both types of reinforcement provided some enhancement in creep resistance, namely, the characteristic relaxation time, as determined using a stretched exponential relaxation function increased with the addition of reinforcements. However, due to different reinforcement mechanisms, enhancement in the instantaneous modulus was observed for clay-reinforced epoxies, while the instantaneous modulus was not effected in POSS–epoxy nanocomposites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1993–2001, 1999

Improved mechanical properties of chitosan fibers

Abstract: A highly deacetylated chitosan from shrimp with a degree of deacetylation of 95 ± 3% was prepared and spun into a monofilament fiber using a solution of 5% by weight chitosan in 5% by volume aqueous acetic acid. Samples of the spun fibers were immersed in separate solutions containing phosphate ions and phthalate ions, and subsequently washed and dried. The various solutions ranged in pH from 4.12 to 7.75. The highest dry mechanical properties resulted from solutions containing phthalate ions between 4.5–5.5 pH, and from solutions containing phosphate ions at pH 5.4. Immersion time was varied between 1 and 60 min at 25.8°C, and temperature was varied between 25.8 and 70.0°C, in the phosphate ion solutions at a pH of 5.8. Dry mechanical properties were highest at 25.8°C and after 1 h of treatment. Chitosan films were subjected to similar treatments in phosphate and phthalate ion solutions. Fourier transform infrared data (FTIR) on the films suggest that some interaction is occurring between the phosphate ions and the amine group on the chitosan backbone. An additional experiment was performed whereby the same chitosan was used to prepare a dope of 4% by weight chitosan in 4% by volume aqueous acetic acid with 30% by volume methanol. This solution was spun into fibers, but was subjected to a “final draw” by increasing the speed of the winder. With increasing the final draw, denier and elongation-at-break decreased, while the other mechanical properties showed a marked increase. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1721–1732, 1999

Preparation and mechanical properties of polystyrene–clay hybrids

Abstract: Polystyrene–clay hybrids (PSCHs) were prepared by melt blending a styrene vinyloxazoline copolymer with organophilic clay. In the PSCHs, the silicate layers of the clay were delaminated and dispersed homogeneously to the nanometer level. The moduli of the PSCHs were higher than that of the PS copolymer. For example, the tensile modulus of the PSCH with 5 wt % clay was 1.4 times higher compared to that of the PS copolymer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3359–3364, 1999

Study of jute fiber reinforced polyester composites by dynamic mechanical analysis

Abstract: Cyanoethylation of jute fibers in the form of nonwoven fabric was studied, and these chemically modified fibers were used to make jute–polyester composites. The dynamic mechanical thermal properties of unsaturated polyester resin (cured) and composites of unmodified and chemically modified jute–polyester were studied by using a dynamic mechanical analyzer over a wide temperature range. The data suggest that the storage modulus and thermal transition temperature of the composites increased enormously due to cyanoethylation of fiber. An increase of the storage modulus of composites, prepared from chemically modified fiber, indicates its higher stiffness as compared to a composite prepared from unmodified fiber. It is also observed that incorporation of jute fiber (both unmodified and modified) with the unsaturated resin reduced the tan δ peak height remarkably. Composites prepared from cyanoethylated jute show better creep resistance at comparatively lower temperatures. On the contrary, a reversed phenomenon is observed at higher temperatures (120°C and above). Scanning electron micrographs of tensile fracture surfaces of unmodified and modified jute–polyester composites clearly demonstrate better fiber–matrix bonding in the case of the latter. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1505–1513, 1999

Thermal study on the copolymers of phthalonitrile and phenylnitrile‐functional benzoxazines

Abstract: Ortho-, meta-, and para-phenylnitrile–functional benzoxazines are polymerized at different compositions with phthalonitrile-functional monomers providing copolybenzoxazines of high thermal stability and easy processability. The most positive effect on these properties is obtained on copolymerizing phthalonitrile- and ortho-phenylnitrile–functional benzoxazines. Thermal decomposition of these polybenzoxazines is studied by thermogravimetry (TGA). It is demonstrated that only 30 mol % of phthalonitrile-functional benzoxazine added to the ortho-phenylnitrile–substituted monomer improves char yield significantly from 59 to 77 wt %, which is the value of neat phthalonitrile-based polybenzoxazine. Glass transition temperature (Tg) also dramatically increases from 180°C for neat ortho-phenylnitrile polymer to 294°C for the copolymer with 30 mol % of phthalonitrile-functional monomer. Additionally, the high melt viscosity of phthalonitrile-functional benzoxazines is dramatically decreased upon blending with phenylnitrile-functional monomer. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2937–2949, 1999

Short jute fiber-reinforced polypropylene composites: Dynamic mechanical study

Abstract: Short jute fiber-reinforced polypropylene (PP) composites were prepared using a high-speed thermokinetic mixer. A compatibilizer was used to improve the molecular interaction between jute and PP. Both the percent weight fraction of the jute fiber and compatibilizer were varied to study the dynamic mechanical thermal (DMT) properties. Dynamic parameters such as storage flexural modulus (E′), loss flexural modulus (E″), storage shear modulus (G′), loss shear modulus (G″), and loss factor or damping efficiency (tan δ) were determined in a resonant frequency mode. The transition peak nature, amplitude, and temperature of E′, E″, G′, G″, and tan δ of different compositions were shown to indicate possible improvements of molecular interaction in the presence of a compatibilizer. The modulus retention term, a plot of the reduced modulus with the weight fraction of the jute fiber, also indicate its improvement. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 531–539, 1999

Electrical properties of composites in the vicinity of the percolation threshold

Abstract: The electrical response of thermoplastic composites composed of carbon black and high-density polyethylene near the electrical percolation threshold ( pc) has been investigated through the study of the volume resistivity and complex permittivity. The change in conductivity beyond pc exhibited a critical exponent that was greater than predicted from percolation theory. Composites with carbon black contents slightly larger than pc exhibited the greatest sensitivity in volume resistivity with temperature variations under the melting point of polyethylene. In addition, percolating composites with low carbon black contents exhibited significant "negative temperature coefficient" (NTC) effects and improvements in conductivity with annealing. Maxwell-Wagner interfacial polarization resulted in moderate increases in both the permittivity (e9) and dielectric loss factor (e0) below pc, while at percolation, an abrupt and dramatic increase was observed for both components of the complex permittivity. © 1999 John Wiley & Sons,

Preparation of amphiphilic chitosan and their antimicrobial activities

Abstract: Two anionic soluble monomers, mono(2-methacryloyl oxyethyl)acid phosphate and vinylsulfonic acid sodium salt, were grafted onto chitosan to obtain copolymers with zwitterionic property. Graft reaction improved the antimicrobial activities of chitosan. Antimicrobial activities of chitosan and graft copolymers depended largely on the amount and type of grafted chains as well as changes of pH, against Candida albicans, Trichophyton rubrum, and Trichophyton violaceum. The most excellent antimicrobial activity among tested samples was shown at pH 5.75 with demonstrating strain selectivity against Candida albicans and Trichophyton violaceum due to the difference in affinity between cell wall of fungi and chitosan or its derivatives. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1713–1719, 1999

Shape memory effect of ethylene–vinyl acetate copolymers

Abstract: Crosslinked ethylene–vinyl acetate (EVA) copolymers with VA content of 28% by weight were prepared by a two-step method by evenly dispersing the crosslinking agent (dicumyl peroxide) into the EVA matrix and then crosslinking at elevated temperatures. The crosslinking features of the samples were analyzed by Soxhlet extraction with xylene and dynamic mechanical measurements. All the samples were crystalline at room temperature, and the chemical crosslinks seemed to have little effect on the melting behavior of polyethylene segment crystals in the EVA copolymers. The shape recovery results indicated that only those specimens that had a sufficiently high crosslinking degree (gel content higher than about 30%) were able to show the typical shape memory effect, a large recoverable strain, and a high final recovery rate. The degree of crosslinking can be influenced by the amount of the peroxide and the time and temperature of the reaction. The response temperature of the recovery effect (about 61°C) was related to the melting point of the samples. The EVA shape memory polymer was characterized by its low recovery speed that resulted from the wide melting range of the polyethylene segment crystals. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1063–1070, 1999

Synthesis, characterization, and biodegradability of fatty-acid esters of amylose and starch

Abstract: A series of starch and amylose esters with different degrees of substitution and side-chain length were prepared and studied. The esters were prepared by acyla- tion of the polysaccharide with the appropriate acid chlorides, such as octanoic, dode- canoic, and octadecanoic. The degrees of substitution were 0.54, 1.8, and 2.7. After preparation, the resulting esters were characterized by elemental analysis, 1 H nuclear magnetic resonance ( 1 H-NMR), Fourier transform infrared (FTIR), differential scan- ning (DSC), thermogravimetric analysis (TGA), contact angle, and water uptake mea- surements. Their mechanical properties and, in particular, the tensile strength and elongation at break depend on the side-chain length and on the degree of substitution. The extent of their biodegradability, after exposure to activated sludge, was assessed by weight loss measurements and scanning electron microscopy (SEM). It was found that these new materials are biodegradable, and the biodegradation rate decreases with increasing degree of esterification. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1440 -1451, 1999

Mechanical Properties and Viscoelastic Behavior of Basalt Fiber-Reinforced Polypropylene

Abstract: In the present article, a series of commercial-grade polypropylenes (PP) filled with different contents of short basalt fibers were studied. This composite mate- rial presented deterioration of both mechanical characteristics, for example, stress and strain at yield with increasing of the fiber content. On the other hand, the impact strength was fourfold higher than that of unfilled PP. A poor adhesion between the PP matrix and the basalt fibers was detected. This is why interfacial interactions were promoted by the adding of poly(propylene-g-maleic anhydride) (PP-g-MA). It was ob- served that the tensile properties of the obtained materials and their impact strengths increased significantly with increasing of the amount of PP-g-MA in the blend. The adhesion improvement was confirmed by scanning electron microscopy as well. Fourier transform infrared spectroscopy was applied to assess if any chemical interactions in the system PP/PP-g-MA/basalt fibers exist. Dynamic mechanical thermal analysis data showed an increase of the storage modulus with increasing fiber content. The conclu- sion was made that the modification of the PP matrix led to a higher stiffness but its value remained constant, irrespective of the PP-g-MA content. With increasing fiber content, damping in the b-region decreased, but increase of the coupling agent content restored its value back to that of PP. The loss modulus spectra presented a strong influence of fiber content on the a-relaxation process of PP. The position of the peaks of the above-mentioned relaxation processes are discussed as well. © 1999 John Wiley &

Preparation and properties of Organosoluble polyimide/ silica hybrid materials by sol-gel process

Abstract: Organosoluble polyimide/silica hybrid materials were prepared using the sol–gel process. The organosoluble polyimide was based on pyromellitic anhydride (PMDA) and 4,4′-diamino-3,3′-dimethyldiphenylmethane (MMDA). The silica particle size in the hybrid is increased from 100–200 nm for the hybrid containing 5 wt % silica to 1–2 µm for the hybrid containing 20 wt % silica. The strength and the toughness of the hybrids are improved simultaneously when the silica content is below 10 wt %. As the silica content is increased, the glass transition temperature (Tg) of the hybrids is increased slightly. The thermal stability of the hybrids is improved obviously and their coefficients of thermal expansion are reduced. The hybrids are soluble in strong polar aprotic organic solvents when the silica content is below 5 wt %. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2977–2984, 1999

Properties of electroresponsive poly(vinyl alcohol)/poly(acrylic acid) IPN hydrogels under an electric stimulus

Abstract: Interpenetrating polymer networks (IPNs) composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAAc) exhibited electrical-sensitive behavior. PAAc as an initial network was prepared inside a PVA solution using UV irradiation; then, PVA networks as a secondary network were formed by a repetitive freeze–thawing process. Their mechanical properties were influenced by the swelling ratio, crosslinking by UV radiation and a freeze–thawing process, and intermolecular force by hydrogen bonding. When a swollen PVA/PAAc IPN was placed between a pair of electrodes, the IPN exhibited bending behavior upon the applied electric field. The equilibrium bending angle (EBA) and the bending speed of the PVA/PAAc IPN increased with the applied voltage and the content of the PAAc network having negatively charged ionic groups within the IPN. The electroresponsive behavior of the present IPN was also affected by the electrolyte concentration of the external solution. Particularly, IPN37 showed a maximum EBA when the critical ionic strength was 0.1. Anisotropic deswelling of the IPN was observed in a direct contact with a pair of electrodes under aerobic conditions. The PVA/PAAc IPN also showed stepwise bending behavior depending on the electric stimulus. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1675–1683, 1999

Fiber‐reinforced cellulosic thermoplastic composites

Abstract: Steam-exploded fibers from Yellow poplar (Liriodendron tulipifera) wood were assessed in terms of their thermal stability characteristics, their impact on torque during melt processing of a thermoplastic cellulose ester (plasticized CAB) matrix, their fiber–matrix adhesion and dispersion in composites, and their mechanical properties under tension. Fibers included water-extracted steam-exploded fibers (WEF), alkali extracted fibers (AEF), acetylated fibers (AAEF), and a commercial milled oat fiber sample (COF) (i.e., untreated control). The results indicate that the thermal stability of steam-exploded fibers increases progressively as the fibers are extracted with water and alkali and following acetylation. The greatest improvement resulted from the removal of water-soluble hemicelluloses. The modification by acetylation contributed to improved interfacial wetting that was revealed by increased torque during melt processing. Whereas modulus increased by between 0 and 100% with the incorporation of 40% fibers by weight, tensile strength either declined by ⅓ to ½ or it increased by a maximum of 10%, depending on fiber type. AAEF composites produced the best mechanical properties. Fiber–aspect ratio was reduced to an average of 25–50 from ≫ 200 during compounding. The superior reinforcing characteristics of AAEF fibers were also reflected by SEM, which revealed better fiber–matrix adhesion and failure by fiber fibrillation rather than by fiber pullout. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1329–1340, 1999

Sorption and transport of water vapor in nylon 6,6 film

Abstract: The sorption and transport of water in nylon 6,6 films as functions of the relative humidity (RH) and temperature were studied. Moisture-sorption isotherms determined gravimetrically at 25, 35, and 45°C were described accurately by the GAB equation. Water-vapor transmission rates were enhanced above ≈ 60–70% RH, primarily due to the transition of the polymer from glassy to rubbery states. The glass transition temperatures (Tg's) of nylon 6,6 were measured at various moisture contents using differential scanning calorimetry. The results showed that the sorbed water acted as an effective plasticizer in depressing the Tg of the polyamide. Fourier transform infrared spectroscopy (FTIR) was utilized to characterize the interaction of water and the nylon. Evidence from FTIR suggested that the interaction of water with nylon 6,6 took place at the amide groups. Based on the frequency shift of the peak maxima, moisture sorption appeared to reduce the average hydrogen-bond strength of the NH groups. However, an increase was seen for the CO groups. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 197–206, 1999

Relationships between stoichiometry, microstructure, and properties for amine-cured epoxies

Abstract: Changes in microstructure and mechanical properties are investigated as a function of epoxy–amine stoichiometry The epoxy–amine system studied exhibits a two-phase structure consisting of a hard microgel phase and a dispersed phase of soft, unreacted and/or partially reacted material The size distribution of the microgel regions tends to increase with increasing amine content Concurrently, the connectivity of the softer phase increases dramatically This two-phase structure is inherently fractal, exhibiting a single glass transition temperature, Tg The Tg and elevated-temperature properties of the epoxy are directly correlated with crosslink density and the percentage of microgel phase observed in microstructure studies The fracture toughness at room temperature increases with increasing amine content, most likely due to the increased presence of the soft phase, which absorbs more energy during crack growth Changes in modulus values at 30°C with stoichiometry are explained by considering the effective aspect ratio of the polymer structure in the determination of sample rigidity Relationships between microgel sizes and the sizes of interphase regions that form in composite and adhesive systems are also discussed in terms of interphase properties © 1999 John Wiley & Sons, Inc J Appl Polym Sci 71: 699–712, 1999

Synthesis, characterizations, and physical properties of carbon nanotubes coated by conducting polypyrrole

Abstract: A new type of carbon nanotube (CNT) (diameter of <100 nm) coated by conducting polypyrrole (PPY) was synthesized by in situ polymerization on CNTs. The structure of the resulting complex nanotubes (CNT-PPY) was characterized by elemental analysis, X-ray photoelectron spectroscopy, Raman spectra, and X-ray diffraction. These indicated no significant chemical interaction between PPY and the CNT. The electrical, magnetic, and thermal properties of the complex nanotubes were measured and showed the physical properties of the CNTs were modified by conducting PPY. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2605–2610, 1999

Sorption and diffusion of water, salt water, and concrete pore solution in composite matrices†

Abstract: In recent years, the use of fiber-reinforced polymer composites in civil infrastructure has been promoted as a solution to the deterioration of bridges, buildings, and other structures composed of traditional materials, such as steel, concrete, and wood. Any application of a polymer composite in an outdoor environment invariably involves exposure to moisture. There is also potential for exposure to saline conditions in waterfront or offshore structures, and alkaline environments, as would be encountered by a reinforcing bar in a cementitious material. This study characterizes the sorption and transport of distilled water, salt solution, and a simulated concrete pore solution in free films of vinyl ester, isophthalic polyester (isopolyester) and epoxy resins, all commercially important materials for use in structural composites. Diffusion of all three liquids in each of the three materials was observed to follow a Fickian process. Mass loss was observed for the isopolyester in salt water and concrete pore solution at 60°C, suggesting hydrolysis that was accelerated by the high temperature exposure. Both the rate of uptake, as well as the equilibrium uptake, were greater at 60°C, compared with ambient conditions. Diffusion coefficients calculated from the mass uptake data revealed that, although the epoxy resin had the highest equilibrium uptake, it had the lowest diffusion coefficient. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 483–492, 1999