The sol-gel process opens the possibility of combining inorganic and organic units to new hybrid polymers. Organic units can be used for structural modification of the inorganic backbone, for creating new functions within an inorganic network and for building up organic polymeric chains. The materials show interesting perspectives with respect to structural (surface hardness, strength) and functional properties (e. g. diffusion, photocuring, incorporation of dyes, optical properties). A review over structural and functional properties of sol-gel derived inorganic-organic polymers (ORMOCERs = organically modified ceramics) is given.

The synthesis, structure/property behavior of inorganic-organic hybrid network materials prepared by the sol gel process have been chronologically reviewed with emphasis on those systems prepared in the authors laboratory. Specific features of reactions as well as the nature of reactants are included. The morphological features of these “ceramer” systems formed have many features in common even though the reactants may be quite different. The mechanical properties of the final materials in conjunction with SAXS have proven beneficial in establishing the basics of their morphological texture. Finally, it is demonstrated how microwave radiation in some cases, can serve as an efficient way in processing the ceramer systems.

Polysilsesquioxanes,-[RSi(O)1.5 ]x-, exhibit many properties that are potentially quite useful for industrial applications. These properties include high temperature stability (−600°C in O2); good adhesion and, liquid crystal-like behavior for some derivatives. Moreover, [MeSi(O)l.5]x, polymethylsilsesquioxane has been used successfully as a precursor for the fabrication of carbon fiber/“black glass” (SiO2/SiC/C) composites and “black glass” fibers.

Current methods of preparation depend on hydrolysis of RSiCl3 or RSi(OR)3. Unfortunately, this approach leads to several products that are difficult to purify because polysilsesquioxanes exhibit a great propensity for forming gels. We describe here a simple catalytic approach to the synthesis of polymethylsilsesquioxane copolymers of the type -[MeRSiO].3[MeSi(O)1.5].7- where R - H, OMe, OEt, OnPr and OnBu. The R - H copolymer is produced by catalytic redistribution of -[MeHSiO]xoligomers using dimethyltitanocene, Cp2TiMe2 as the catalyst precursor.

Following catalytic redistribution, the resulting copolymer, -[MeHSiO].3[MeSi(O)1.5].7− , is reacted in situ with alcohols to produce -[Me(R'O)SiO].3[MeSi(0)1.5].7− (where R' - Me, Et, nPr and nBu) which serve as masked forms of the polymethylsilsesquioxane. These new copolymers have been characterized by 1H, 13C and 29Si NMR TGA and DTA. The NMR studies allow us to assign structures for the copolymer.

These new copolymers exhibit improved tractability. Their high temperature properties are all quite similar; although, the MeO-, EtO- and especially the nPrO- derivatives give much higher ceramic yields than expected.

Polyaniline has been synthesized in the galleries of fluorohectorite, a two-dimensional mica-type layered silicate. Intercalation of aniline in the intracrystalline region of Cu-exchanged fluorohectorite results in oxidative polymerization to polyaniline (emeraldine base form) as demonstrated by electronic, infrared and Raman spectroscopy and x-ray diffraction data. The intercalated insulating form of polyaniline becomes conducting on exposure to HCl. In-plane electrical conductivity data measured in the temperature range 274 to 573 K show a complex thermally activated behavior with room temperature conductivity 0.05 Ohm−1cm−1. The polyaniline/layered silicate hybrids represent a new class of nanocomposites consisting of synthetic conductors with molecular dimensions contained in a quasi twodimensional environment of a crystalline host.

ζ-Caprolactam was polymerized in the interlayer spacing of montmorillonite, a clay mineral, yielding a nylon 6-clay hybrid (NCH) 1).

X-ray and TEM measurements revealed that each template of the silicate, which is 10 Å thick, was dispersed in the nylon 6 matrix and that the repeat unit increased from 12 Å in unintercalated material to 21 Å in the intercalated material.

Thus NCH, is a “polymer based molecular composite” or “nanometer composite”. NCH, when injection-molded, shows excellent properties as compared to nylon 6 in terms of tensile strength, tensile modulus and heat resistance. Heat distortion temperature increased from 65 °c for nylon 6 to 152 °c for NCH, containing 4 wt% (1.6 vol%) of clay mineral.

The goal of primary interest in these investigations was the development of novel methods for filling elastomeric networks. The techniques developed employ the in-situ generation of reinforcing fillers such as silica or a glassy polymer such as polystyrene either after, during, or before network formation. The reaction involves decomposition of organometallic compounds, using a variety of catalysts and precipitation conditions, or freeradical polymerization of a suitable monomer. The effectiveness of the technique is gauged by stress-strain measurements carried out on these elastomeric composites to yield values of the maximum extensibility, ultimate strength, and energy of rupture. Also of interest are calorimetric studies of the networks, to determine their crystallizability. Information on the filler particles themselves is obtained from density determinations, electron microscopy, and scattering measurements.

The structure of several classes of silica/siloxane molecular composites is investigated using small-angle x-ray and neutron scattering. These filled elastomers can be prepared through different synthethic protocols leading to a range of fillers including particulates with both rough and smooth surfaces, particulates with dispersed interfaces, and polymeric networks. We also find examples of bicontinuous filler phases that we attribute to phase separation via spinodal decomposition. In-situ kinetic studies of particulate fillers show that the precipitate does not develop by conventional nucleation-and-growth. We see no evidence of growth by ripening whereby large particles grow by consumption of small particles. Rather, there appears to be a limiting size set by the elastomer network itself. Phase separation develops by continuous nucleation of particles and subsequent growth to the limiting size. We also briefly report studies of polymer-toughened glasses. In this case, we find no obvious correlation between organic content and structure.

Polydimethylsiloxane (PDMS) model networks reinforced by in situ precipitated SiO2, and polymer-modified silica glasses were obtained following the usual sol-gel methods. The conditions were chosen to increase the probability of observing inhomogeneities: (i) bulky samples, and (ii) limited reaction times. These composites were characterized by measuring bulk spin-lattice (T1) and spin-spin (T2) relaxation times and using 1H NMR two-dimensional Fourier transform (2DFT) spin echo imaging techniques. The T1 and T2 maps show clear and significant variations of NMR signal intensity throughout the sample due to nonuniform hydrolysis of the tetraethylorthosilicate (TEOS) in the specimens.

High molecular weight water-soluble polymers are usually supplied in the form of water-in-oil emulsions which have advantages of low viscosity and easy storage and dissolution. Most uses in water treatment, flocculation, paper manufacture or mining fields require polymer latexes formed of finely dipersed particles. Polymerization in reverse micelles or microemulsions appears to be an attractive technique because it can lead, under appropriate formulations, to high molecular weight polymers entrapped within small-sized stable particles. The main characteristics and properties of the latexes and polymers formed by this process are described.

Partially crystalline silicon nitride, with a specific surface area greater than 200 m2/g, is obtained by the pyrolysis of an organometallic, polymeric precursor under NH3 to 1000 °C. Additional heating to 1400 °C under N2 produces alpha-Si3N4. The addition of up to 15% h-BN was found to affect the coarsening characteristics of amorphous silicon nitride by promoting surface area reduction and suppressing crystallinity. By combining Si3N4 and BN molecular and polymeric precursors prior to ceramic conversion, or incorporating Si, N, and B into a single preceramic polymer, the relative proportion and crystallinity of the ceramic phases can be controlled in the resulting Si3N4/BN composites.

The structural characteristics and the formation of monomeric and stabilized polymeric micelles and vesicles are reviewed. Characterization of these nanoparticles involved stability studies, molecular weight determination, permeability and fluorescence investigations, as well as electron microscopy and DSC studies.

Recently we reported on a method of preparing microcellular composite foams. In this procedure an open-celled polystyrene foam is prepared by the polymerization of a high-internal-phase water-in-oil emulsion containing styrene, divinylbenzene, surfactant, free-radical initiator and water. After drying, the cells of the polystyrene foam are then filled with other materials such as aerogel or resoles. The physical properties of these materials e.g., surface area, density, thermal conductivity, and compressive strength will be presented.

We describe the synthesis, characterization and properties of various types of siloxane polymers containing diphenylsiloxane (P) as a component. The polymer types include di-and tri-block copolymers with dimethylsiloxane (M) as the second component, and random and statistical copolymers with dimethylsiloxane or methylphenylsiloxane (P/M) as the second component. Such copolymers combine siloxane units whose polymers have very different properties. The polydiphenylsiloxane chain is rigid and inflexible, and the polymer is a highly crystalline solid with a liquid crystalline or condis crystalline state and a very high melting (clearing) temperature. In contrast, the polydimethylsiloxane or polymethylphenylsiloxane chains are very flexible and the polymers have very low glass transition temperatures.

Polymers of controlled molecular composition, size and architecture were prepared by anionic polymerization of the ‘cyclic trimers’, using lithium-based initiators.

The physical properties of the copolymers vary dramatically with composition and architecture. Two types of ‘random’ copolymers can be prepared. In one type, siloxane units of a given type are randomly placed in the chain in groups of three, i.e., the minimum sequence length of a given siloxane type is three siloxane units. In the other type of random copolymer, individual siloxane units are randomly distributed so that the minimum sequence length is a single siloxane unit. The properties of the two types are quite different, showing that subtle changes in sequence distribution can have major effects on physical properties. At molar ratios near 1/1 and with molecular weights of ∼105, the first type of ‘random’ copolymer is an elastic solid with appreciable mechanical properties, whereas the latter type is a sticky gum.

Diblock copolymer (P-M) with dimethylsiloxane as the major component are paste-like, whereas the triblock (P-M-P) and star-block copolymers of the same composition are tough elastomers. The block copolymers are molecular composites, in which the polydiphenylsiloxane component separates into crystalline microphases with very uniform fibrillar or lamellar morphologies, and with widths or thicknesses comparable to the length of the polydiphenylsiloxane block, i.e, typically of the order of 100 Å.

By probing the localized segmental motion of isotope-labeled block copolymers, the physical nature of the interphase region between microphaseseparated domains of block polymers was examined. Dynamic infrared linear dichroism (DIRLD) spectroscopy, which measures the reorientations of submolecular structures induced by a small-amplitude oscillatory strain, was combined with specific isotope-labeling using deuterium-substituted monomers. The latter technique enabled us to differentiate the dynamic responses of well-defined parts of block segments, e.g., near the segment junction, chain end, or middle of the block. The degree of segmental interactions near the interphase region of styrene-isoprene diblock copolymers were studied as a function of the segment location and temperature. The reorientational motion of the polystyrene segment, especially near the block junction, was monitored around the glass transition temperature of the polyisoprene matrix. From this result, the degree of segmental mixing in the interphase region which leads to local plasticization of the polystyrene segment was determined.

Polymer solutions comprising stiff-chain polyester and flexible polysulfone were examined via light scattering techniques. Results were analyzed using the Stein-Wilson extension of the Debye-Bueche theory, in which the correlation lengths due to orientation fluctuations and mean-squared fluctuations of the molecular anisotropy were obtained. For a molecular dispersion, the correlation length is small and a function of concentration; as the anisotropy is attributed to the rod molecules. Aggregation of rods is associated with an increase in the magnitude and size of the density fluctuations, and a change in anisotropy fluctuations is dependent on the degree of orientation correlation of the rods in the aggregate. Blends prepared by solution casting were studied by a small-angle light scattering method. Results thus far demonstrate that aggregates are present in most of the rod/coil composites prepared.

Rigid-rod molecular composites are a new class of high performance structural polymers which have high specific strength and modulus and also high thermal and environmental resistance. The concept of using a rigid-rod, extended chain polymer to reinforce a ductile polymer matrix at the molecular level has been demonstrated with morphological and mechanical property studies for aromatic heterocyclic systems, but new materials systems and processing techniques will be required to produce thermoplastic or thermoset molecular composites. Improved characterization and modeling will also be required. In this regard, new results on modeling of mechanical properties of molecular composites are presented and compared with experimental results. The Halpin-Tsai equations from ‘shear-lag’ theory of short fiber composites predict properties reasonably well when using the theoretical modulus of rigid-rod molecules in aromatic heterocyclic systems, but newer matrix systems will require consideration of matrix stiffness, desired rod aspect ratio, and rod orientation distribution. Application of traditional and newer morphological characterization techniques are discussed. The newer techniques include: Raman light scattering, high resolution and low voltage SEM, parallel EELS in TEM, synchrotron radiation in X-ray scattering, and ultrasound for integrity studies. The properties of molecular composites and macroscopic composites are compared and it is found that excellent potential exists for use of molecular composites in structural applications including engineering plastics, composite matrix resins, and as direct substitutes for fiber reinforced composites.

Rheological properties of isotropic solutions of rodlike poly(p-phenylene benzbisoxazole), PBO, flexible chain poly(2,5-benzoxazole), ABPBO, and their miscible blends in solution are described. Measurements include steady state properties (the viscosity and recoverable compliance as functions of shear rate), transient properties (the recoverable compliance), and dynamic mechanical properties (the loss and storage compliances as functions of frequency). The relaxation spectrum of the blends is broader than that for the rodlike chain, and tends to occur at longer times, reflecting a viscosity enhancement that occurs with the blends. The measured zero shear viscosities for rod and blend solutions are compared with predictions based on the model of Doi and Edwards.

Thermoplastic microcomposites offer the potential for better economics and improvements in composite processing, and possibly performance, over conventional ‘string-and-glue’ composites. The early development of molecular composite technology focused on polyamide matrix polymers; however, for many aerospace applications higher use temperatures and greater solvent resistance than that of conventional polyamide matrices will be required. This paper describes work performed under contract to the U.S. Air Force to develop PBZT (poly p-phenylene benzobisthiazole)/thermoplastic molecular composites with high performance matrix resins into a viable technology.

A scaleable process has been defined based on a novel technology developed by Du Pont. Advantages of this process include better economics, superior processing performance, and improved MC fiber tensile properties versus prior art. Using this process we have obtained rule-of-mixtures properties in our microcomposite fibers with matrix polymers offering use temperatures from 330 to 600°F. Consolidation of PBZT/PEKK fibrous preforms into uniaxial panels up to 1011 × 1511 has been demonstrated and material propperty evaluation and data base development are in progress. Uniaxial property levels achieved to date for all systems compare favorably with conventional ‘string-and-glue’ PBZT/epoxy composites although as with other organic fiber reinforcements, compressive and shear performance may be limiting factors in MC applications.

Molecular composites are dispersions of rigid-rod polymer molecules in a matrix of flexible coil polymers, formed by the coagulation of a solution containing these components. Where there is aggregation of the rigid-rod molecules, such composites are called micro-composites (MC's). These composites offer the potential for better economics and improvements in composite processing, and possibly performance, over conventional ‘string and glue’ composites. This paper describes work performed under contract to the U. S. Air Force to develop PBZT/thermoplastic molecular composites into a viable technology.

A commercially viable MC spinning and heat-treatment process has been defined based on a novel mixed solvent/quaternary solution technology developed by Du Pont. Advantages of this process include better economics, superior processing performance, and improved MC fiber tensile properties versus prior art. PBZT/polyamide MC fibers with strength/modulus of 332 ksi/29 Msi have been produced using this process. Adhesion equivalent to that obtained in conventional composites has been demonstrated. Uni-axial properties achieved to date compare favorably with conventional ‘string and glue’ PBZT/epoxy composites although compressive and shear strengths may be limiting factors in MC applications.

Electron microscopy studies showed that the porous structure of PBO fiber may contain fractal geometries; i.e., the void spaces are self-similar with variations in magnification. At the fiber surface, a dense skin which consists of fibrils was observed. In the matrix, the fibril size is about 5 to 50 nm with the voids distributed randomly among the fibrils. The fractal dimension of voids in PBO fiber as determined by microscopy and image analysis was found to be 2.44. The fibrillated fiber showed a continuous fibril size distribution with no evidence of fibril size hierarchy. These observations suggest a nucleation and growth mechanism for the formation of the fibrillar structure in PBO fiber.