Search

Polyurethane materials have a huge role in everyday materials, for example, as foams they are used in car and household furnishings; the polymer is used in the construction industry, in the manufacture of footwear, and for coatings and adhesives, as well as in textiles. The demand for polyurethane materials is high (Biesman, 2002). Thus in a number of everyday guises and in specialist devices, polyurethanes play a vital role in medicine and are involved at all scales, from the construction materials used to build the wards through to bedding, surgical instruments, medical implants and ultimately microscale encapsulation devices. In this article, however, we shall focus largely on those structural devices which come into contact with the patient, for which the property of biocompatibility must be added to the other properties required such as flexibility, strength, and of course processibility. This places substantial additional demands on a material and a restricted range of synthetic organic polymers have been used in a wide variety of medical applications. Of these many materials, polyurethanes have attracted particular interest; firstly because the diisocyanate and diol monomers can be altered to provide materials with widely differing properties, such as melting or softening point andmechanical strength; secondly, other modifications such as the introduction of diamines to provide thermoplastic elastomers, or the combined use of multifunctional diols and foaming agents to produce rigid or flexible foams allow for the production of materials with the widest possible range of morphologies. In addition to the widely tunable physical properties, it has been found that thesematerials can also bemade to be substantially biocompatible, but where required can be made to biodegrade. Thus almost uniquely polyurethanes have the potential to be used both for permanent medical implants and to be used in systems where degradation is required such as scaffolds for tissue regeneration.

In general thermoplastic polymers are made up of long linear chain molecules which exhibit large scale chain mobility and deformation under shear forces above their softening temperature. This change is reversible. Above this temperature the thermal motions of the chain segments are sufficient to overcome inter- and intra-molecular forces. At room temperature the material is a viscoelastic solid. Their behaviour is dependent on chain morphology, structure, crystallinity and the types of additives added (often to aid processing). The materials can easily be processed into different type of products and are considered to be the most important class of plastic materials commercially available. The processability of this class of plastics is a key characteristic for developing biomedical applications.

Thermal methods of analysis predominate in the measurement of this characteristic of polymers. Thermal methods of analysis of polymers are important in that these techniques can provide information about the thermal and oxidative stability of polymers, their lifetime or shelf-life under particular conditions, phases and phase changes occurring in polymers, and information on the effect of incorporating additives in polymers. As will be seen in Chapters 12 and 13, these methods have also been applied to the measurement amongst others of melting points, percent crystallinity, crystallization temperature, glass transition temperature, melting temperature, cure rate, degree of cure, composition, thermal stability, expansion coefficient, and softening temperature.

Hot melt adhesives have been known for centuries. Historically, mixtures of natural waxes, rosin, pitch, and other naturally occurring substances were used alone or in mixtures to produce sealing compounds for a variety of applications. It was not, however, until the early 1950s that hot melts based on synthetic polymers appeared in the marketplace.

For assessing the mechanical properties of asphalts, a variety* of empirical tests are currently used: penetration, softening point, ductility, Fraass breaking point, etc... Although these tests provide useful comparative information, their interest is very limited since the ranges of temperature and loading time where they yield results are very small compared to service conditions. Further, due to the complexity of the loading patterns in such tests, it is difficult to analyze their rheological significance. Therefore, starting from these tests, the field behaviour of asphalts is difficult to analyze or to predict. Consequently, there has been a considerable interest for several years in determining asphalt rheological properties through more fundamental parameters. The mechanical behaviour of asphalt is complex and can be generally described adequately through the viscoelasticity pattern. The range of loading times to which asphalts may be subjected is very wide, depending on the type of application envisaged.

Softening temperatures Ts and melting temperatures Tm of a great number of aromatic polyimides and their chemical derivatives have been studied. The results obtained are generalized. The analysis is based on the chemical classification of polyimides taking into account the presence and location of “hinge” atoms and atom groups making polymer chain flexible. If there are such “hinges” in both components of a monomer unit then Ts and T are distinctly revealed. For these polyimides the dependence of Ts and Tm on the number of “hinges” in a monomer unit cân be successfully explained by the well-known physical theories of transition temperatures of polymers. Other groups of polyimides have no distinct Ts and Tm and are usually considered as “infusible”. Calculations and experiments have shown that it is because in these groups the values of Ts and Tm are close to or higher than the value of the polyimide thermal degradation temperati e. The maximum values of Ts and Tm can be as high as ≃ 1000 and ≃ 1300°C, respectively. The influence of chemical structure on Ts and Tm for the whole class of aromatic polyimides can be understood if strong intermolecular interaction of benzimide rings and internal degrees of freedom in monomer units are taken into consideration. Simple models are used in this work for this purpose. The conclusions made here are thought to be valid both for aromatic polyimides and other related classes of polyheteroarylenes.

The chemical-interaction energy in binary and ternary chalcogenides decreases when the sulfides are replaced by selenides, and particularly by tellurides. Compounds of tellurium with elements of groups IV and V of the periodic system are, as a rule, unstable. The possibility of formation of such compounds depends to a considerable degree on the correct choice of the synthesis conditions and of the regime of subsequent heat treatment of the alloys. Therefore when the classical method of physicochemical analysis is used, it is not always possible to determine unambiguously the character of the interaction of the components in telluride systems.

It is possible to introduce into chalcogenide glasses considerable amounts of potassium (and other alkali metals), copper, silver, and thallium.

For a number of years we have been seeking heat stable polymers which were relatively cheaply made, processable either by solution or melt techniques, stable in air to temperatures of at least 300°C, curable by heating to give insoluble high melting and infusible products without gas formation, and stable to oxidation and hydrolysis, which would maintain their strength to at least 200°C. They should also have excellent adhesive properties toward metals, fibers, and glass cloth, and thus be useful as matrix resins for composites and for protective coatings for metals.

Organic polymers provide one of the most versatile groups of materials and find widespread use as plastics, rubbers, fibres, adhesives and coatings. The advantages of these materials, especially their ease of fabrication and low density, for use in an ever widening range of products have slowly become more and more appreciated by designers and engineers. However, some inherent deficiencies in these materials, which have been apparent even from the earliest days of their use, have prevented their employment in certain areas of application. Compared to traditional materials, especially metals, organic polymers show extreme sensitivity to temperature. Most importantly, owing to the intrinsic flexibility of their molecular chains, they exhibit very low softening points. Thus, whereas most metals do not soften appreciably below their melting points (which may be 1000 °C or higher) many common polymers used as plastics, including polystyrene, polyethylene and polyvinyl chloride (PVC), soften sufficiently by about 100 °C to be of no use in any loadbearing applications. As explained in Chapter 1, a measured softening point is usually just below the T g for an amorphous polymer or between T g and T m for a partially crystalline polymer. Therefore polymers will have high softening points if they have a high T g and/or T m .

The objective of this paper is to review the published literature concerning chemical structure-physical property-bond performance relationships in synthetic polymer adhesives. The structure factors that have been identified as affecting the physical properties of the adhesive include molecular weight, composition of the polymer chain, functionality, polarity, chain structure, crystallinity, crosslinking, and additives. The physical properties of the adhesive polymer or formulation which are controlled by the chemical structure include glass transition temperature, melting or softening point, surface energy, bulk and rheological properties, and stability. The relationship of bond strength, locus of failure, bond performance as a function of temperature, and pressure-sensitive tack to those physical properties of the adhesives is reviewed.

The softening point of polybenzimidazole involving aromatic ether linkage in the main chain is not measurable below degradation of the polymer. The various oligobenzimidazoles and benzothiazole and benzoxazole derivatives with analogous structures were synthesized and their melting point Tm and heat of fusion ΔH were measured by differential scanning calorimetry. The entropy of fusion ΔS was calculated by ΔS = ΔH/Tm. The accuracy of the method was examined and the effect of imidazole ring on the melting behavior was investigated.

The use of low softening point, coloured glasses as applied decoration on different types of glass articles is popular and widespread. Enamel colours which are permanently fused to the glass surface have the advantage of being uniformly glossy and durable.

When group V and IV elements are added to group VI elements, glasses are formed in which structural groups V VI3/2 and IV VI4/2 modify and replace their chain and ring structures. In the resulting binary and polynary systems atomic structure and microstructure can be correlated with properties of increasing technological significance. Threshold and memory switchingdiode, photocopy, acousto-optical and infrared optical devices are based on these properties. The increase in softening temperature attainable by the incorporation of V-VI structures and micro structures is particularly promising for the design of temperature-stable infrared-transmitting materials.

Terpene Resins are low molecular weight hydrocarbon resins prepared by cationic polymerization of certain terpenes. They are used as tackifiers in pressure-sensitive tapes, masking tapes, hot melt coatings and adhesives, laminating adhesives and rubber solution adhesives. Terpene resins can be considered as solid solvents for a rubber which function by solubilizing the smaller tack-bestowing molecules from their dispersion in the mass of a rubber.

As part of an ongoing program of study at Brunel University of the fundamental chemical and physical factors which influence the properties of cured epoxy resins, an examination of the effects of crosslinking has been undertaken. Two approaches to this part of the program have been made.

For the purposes of this discussion, “detergency” will be defined as the removal of an unwanted soil from a substrate by the physicochemical action of an aqueous bath. The bath could consist for reference purposes of pure water or of water of specified hardness, salt content, pH, etc. In the practical situation, however, the bath will contain one or more solutes that greatly increase both the rate and the degree of soil removal over that achievable by water alone. These solutes include surfactants, builders (materials that by various mechanisms promote the soil-removing action of surfactants), and antiredeposition agents (materials that inhibit the recombination of soil with previously separated substrate). Substrates can vary quite widely in both chemical composition and physical form. In general, they must be solid, with a melting or softening point well above the washing temperature, and they must be insoluble and resistant to attack by the bath; in short, the type of material that is normally cleaned with “soap and water.” Substrates are usually considered, for practical purposes, in two categories:—fibrous, comprising textile fabrics, and hard surface, comprising tile, painted surfaces, metal, glass, ceramic, and similar materials. Soil has broadly and jokingly been defined as “matter out of place.” In the study of detergency, however, certain types of “matter out of place” are usually excluded from consideration. Among these are substances easily removed by dissolution or rinsing in water alone, and at the other extreme, stains of dyelike materials that are molecularly dispersed below the surface of the substrate and can be removed in reasonably short time only by chemical action. Soils are generally grouped as solid, liquid (or “oily”), and mixed. They adhere to the substrate but do not exhibit strong chemical interaction with it. The detersive process consists essentially of a breaking or weakening of the soil-substrate adhesive bond so that the soil can be separated and carried away easily by the hydraulic action of the bath. The literature on surfactants and other detergent components and their behavior and properties is exceedingly extensive and will not be considered here. Theoretical discussions of the physical chemistry of detergency are also available elsewhere.(1)The present discussion will be concerned primarily with the methodology and techniques used in studying the detersive process.

Polyblends are physical mixtures of structurally different polymers which adhere together through the action of secondary bond forces and no covalent bonding exists between them.1 Blending of polymers is a common industrial practice to offer various grades of polymers to customers and to exhibit properties that are superior to any of the component polymers alone. The degree of compatibility, which is one of the major factors in determining the final properties of the system, can be described by the size and distribution of segregated phases within the mixture. Major controlling factors include chemical structure, polymer crystallinity and morphology, molecular weight and processing parameters. While only several important polymer pairs are apparently compatible, most pairs exhibit pronounced incompatability.2

Previous studies have demonstrated that nuclear waste forms coated with chemical vapor deposited pyrolytic carbon (PyC) at ~ 1273 K can provide ground water leach protection. To minimize the release during coating of volatile material from the waste forms and permit the coating of waste forms with a low softening point, a study was initiated to develop parameters for the catalytic deposition of PyC at low temperatures (LT). The parameters surveyed in a fluidized-bed coater were deposition temperatures, carbon precursor, catalyst, diluent gas, concentration, and pressure.

The increasing practical utilization of crystallized glasses makes necessary a precise knowledge of the course of their crystallization. There are few publications dealing with studies of the spontaneous crystallization of glasses over a wide temperature range (up to the softening temperature),with identification of the devitrification products by simultaneous use of x-ray phase analysis, microscopy, and molecular spectroscopy. To the best of our knowledge, no work of this kind has been published on the Li2O-SiO2 system, which is of special interest as a part of the Li2O-Al2O3-SiO2 system, which is one of practical importance.

The unusual properties of atactic poly(parabiphenyl acrylate reveal that some degree of order in this polymer exists. An x-ray study of this polymer and some related polymers confirm this and indicate that the order is one-dimensional. Some annealing studies show that the extent of this order can be increased by thermal treatments. A model of the molecular packing is suggested to account for these results.