No abstract

Polyurethanes represent extraordinarily versatile polymeric materials which can be tailored to meet the highly diversified demands of modern technologies such as coatings, adhesives, reaction injection molding, fibers, foams, rubbers, thermoplastic elastomers, and composites. [1±3] It is an important objective in polyurethane development to improve resistance against mechanical deformation without sacrificing high elongation at break. Since the early pioneering advances by Otto Bayer, [4] polyurethane properties are modified either by varying polyurethane microstructure, resulting from step-growth polymerization of isocyanate resins with polyols, or by dispersing inorganic and organic fillers within the polyurethane continuous matrix. A wide variety of fillers, including clay and wollastonites, are being applied in polyurethane formulations to reduce costs and to reinforce the polyurethane matrix. [5] Properties of filled polyurethanes are dependent upon filler shape, average diameter and interfacial coupling. Anisotropic particles with a large length/diameter ratio (aspect ratio), e.g., whiskers and fibers, were found to be especially effective in matrix reinforcement. Frequently, filler addition is accompanied by increased strength and stiffness at the expense of substantially reduced elongation at break.Although nanoscale anisotropic particles are expected to offer attractive potential for polyurethane reinforcement, application of anisotropic nanofillers is restricted by the limited availability of nanowhiskers, poor dispersion, and handling problems associated with potential health hazards resulting from inhalation of such particles. Therefore, several attempts have been made to produce nanowhisker dispersions in polyols which are much easier to handle. For example, N-(4-aminobenzoyl)-caprolactam was polymerized at 200 C in dihydroxy-terminated poly(tetrahydrofuran) to produce dispersions of rigid polybenzamide nanowhisker of approximately 2000 nm length and 200 nm diameter. Interfacial coupling and steric stabilization were achieved by reacting hydroxy end groups of the polyol with N-acyl-lactam end groups located at the polybenzamide whisker surface. Upon curing with diisocyanate, the resulting polyurethane nanocomposites exhibited the unusual combination of increased tensile strength and Young's modulus without sacrificing high elongation at break. [6,7] Particle formation via sol/gel technology leads to dispersions of mainly isotropic nanoparticles in polyurethane. [8] Recently, intercalation of organophilic layered silicates with polymers has been introduced as an attractive route leading to versatile polymer nanocomposites which contain nanoscale layered silicates with high aspect ratio. Layered silicates, e.g., clay, consist of anionically charged layers of aluminum/magnesium or magnesium/lithium silicates where cations such as sodium, potassium, magnesium or calcium are located in the interlayer galleries. They are rendered organophilic when gallery cations are exchanged by quaternary alkyl ammonium...

High performance epoxy‐layered silicate nanocomposites based on tetra‐glycidyl4,4'‐diamino‐dipheny1 methane (TGDDM) resin cured with 4,4'‐diaminodipheny1 sulfone (DDS) have been successfully synthesized. Fluorohectorites modified by means of interlayer cation exchange of sodium cations for protonated dihydro‐imidazolines and octadecylamine were used. Fluorohectorite exchanged with 1‐methy12‐norsteary1‐3‐stearinoacid‐amidoethy1‐dihydro‐imidazolinium ions was immiscible with the epoxy matrix. In contrast, fluorohectorites exchanged with hydroxyethy1‐dihydro‐imidazolinium (HEODI) and riciny1‐dihydro‐imidazolinium ions (RDI) favored the formation of a nanocomposite structure. This is most likely due to the presence of ‐OH groups in their molecular structure, which has a catalytic effect on the polymerization occurring between the silicate layers. The diffusion of epoxy and curing agent molecules between the silicate layers is also promoted. Microscopy observations revealed that the dispersion of the silicate aggregates on a microscale was proportional to the degree of separation of the silicate layers on a nanoscale. Decreased apparent glass transition temperature was observed in all the nanocomposites. Finally, mechanical property studies showed that epoxy‐layered silicate nanocomposite formation could simultaneously improve fracture toughness and Young's modulus, without adversely affecting tensile strength.

Fluorohectorites were rendered organophilic through the cation exchange of sodium intergallery cations for protonated monoamine, diamine, and triamine oligopropyleneoxides and octadecylamine, benzylamine, and adducts of octadecylamine and benzylamine with diglycidyl ether of bisphenol A (DGEBA). The influence of the silicate surface modification and compatibility on the morphology and thermal and mechanical properties was examined. Surface modification with protonated octadecylamine and its adduct with DGEBA promoted the formation of microscale domains of silicate layers separated by more than 50 Å, as evidenced by transmission electron microscopy and wideangle X-ray scattering. Young's modulus of these two nanocomposites increased parabolically with the true silicate content, whereas conventionally filled composites exhibited a linear relationship. The highest fracture toughness was observed for conventionally filled composites.

SUMMARY: Basic correlations between polymer morphology, silicate superstructures, glass temperature, stiffness and toughness of thermoset nanocomposites were investigated as a function of layered silicate type and content. The nanocomposites were based upon hexahydrophthalic anhydride-cured bisphenol A diglycidyl ether and layered silicates such as synthetic fluoromica (Fmica), purified sodium bentonite and synthetic hectorite, all of which were rendered organophilic by means of ion-exchange with various mono-and difunctional alkyl ammonium ions. Enhanced toughness was associated with the formation of dispersed anisotropic laminated nanoparticles consisting of intercalated layered silicates. Nanocomposite superstructures were imaged by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM).