Properties of polymers as materials

Polymeric materials used in medical devices are selected in order to meet different requirements depending on the specific in vivo application. The reactions of living tissues in contact with a material are exquisitely sensitive to the material s surface properties, and so these and the mechanical properties given by the bulk of the material have to be characterised. 

Important information can be drawn from such stress/strain plots  

Other mechanical properties can be tested, depending on the specific application. Mechanical properties can be strongly affected by implantation in a living tissue, which can be very aggressive for some types of polymeric material that are otherwise usually stable in vitro. 

The biocompatibility of a material strongly depends on its surface properties, as living tissues are in contact with the surface of the material (see Section 4.3.3). The surface composition of a material can be very different from the bulk composition. This is well known for metals, e.g. the surface of titanium and titanium alloys is generally covered with titanium oxide. This can also be true for polymers, depending on the history of the material and even in the absence of contamination (an example of contamination is given in Section 4.3.5). 

For instance, PS has been widely used for in vitro biomedical applications. Storage boxes, Petri dishes for cell or tissue culture, and latexes for diagnosis purposes are frequently made of PS. However, although the bulk composition of these materials is similar, the surface compositions are different the surface of storage boxes made of pure PS is hydrophobic, whereas that of Petri dishes and latexes for diagnosis purposes is more hydrophilic. Pure PS is not suitable 

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Guyot, A. Bartholin, M. Design and properties of polymers as materials for fine chemistry. Prog. Polym. Sci. 1982, 8, 277-331. 

Table 5.1. Carbon nanotubes have a high potential to improve the mechanical, physical and electrical properties of polymers, as stated by Thostenson et al. (4). They exhibit an exceptionally high aspect ratio in combination with low density, as well as high strength and stiffness (Coleman et al. (5)), which make them a potential candidate for the reinforcement of polymeric materials.

We have approached the subject in such a way that the book will meet the requirements of the beginner in the study of viscoelastic properties of polymers as well as those of the experienced worker in other type of materials. With this in mind. Chapters 1 and 2 are introductory and discuss aspects related to chemical diversity, topology, molecular heterodispersity, and states of aggregation of polymers (glassy, crystalline, and rubbery states) to familiarize those who are not acquainted with polymers with molecular parameters that condition the marked viscoelastic behavior of these materials. Chapters 1 and 2 also discuss melting processes and glass transition, and factors affecting them. 

The morphological domain structure of polymers is determined during processing and often has a significant influence on the properties of polymers as construction materials. By mechanical load, electrical load, or by exposure to elevated temperatures, the morphological structures can be modified. Thus it is important to acquire information of... 

The quantitative structure-property relationships described earlier in this chapter necessarily treat the mechanical properties of polymers as "derived" properties, in the sense defined in Section l.B.2. In other words, the mechanical properties are expressed by equations in terms of material parameters of a more "fundamental" nature, instead of being correlated directly with either group contributions or with connectivity indices. This necessity to treat the mechanical properties as "derived" properties is a direct consequence of their great complexity. 

Attempts have been made to improve the mechanical properties of polymer-based materials, by adding a percentage of selected filler particles. There has been considerable improvement of properties such as elastic modulus, fracture toughness, flexural strength and hardness with the increase of the filler volume. 

Viscoeiasticity. As already noted, the time-dependent properties of polymer-based materials are due to the phenomenon of viscoelasticity (qv), a combination of solid-like elastic behavior with liquid-like flow behavior. During deformation, equations 3 and 6 above applied to an isotropic, perfectly elastic solid. The work done on such a solid is stored as the energy of deformation that energy is released completely when the stresses are removed and the original shape is restored. A metal spring approximates this behavior. 

This book deals with the most important substances used as additives in the plastics industry to improve the properties of polymer-based materials. 

Principally there are two classes of materials which can be used as a sol-gel electrolyte The first one is based on the fabrication of an inorganic oxide material (e.g., Ta20s, Nb20s) and the other on the preparation of organic-inorganic hybrids which combine the better conductive properties of polymer type material with the better mechanical strength of inorganic material. 

Sonochemistry is also proving to have important applications with polymeric materials. Substantial work has been accomplished in the sonochemical initiation of polymerisation and in the modification of polymers after synthesis (3,5). The use of sonolysis to create radicals which function as radical initiators has been well explored. Similarly the use of sonochemicaHy prepared radicals and other reactive species to modify the surface properties of polymers is being developed, particularly by G. Price. Other effects of ultrasound on long chain polymers tend to be mechanical cleavage, which produces relatively uniform size distributions of shorter chain lengths. 

Photoconductive polymers are widely used in the imaging industry as either photosensitive receptors or carrier (electron or hole) transporting materials in copy machines and laser printers. This is still the only area in which the photoelectronic properties of polymers are exploited on a large-scale industrial basis. It is also one electronic appHcation where polymers are superior to inorganic semiconductors. 

The properties of polymers formed by the step growth esterification (1) of glycols and dibasic acids can be manipulated widely by the choice of coreactant raw materials (Table 1) (2). The reactivity fundamental to the majority of commercial resins is derived from maleic anhydride [108-31-6] (MAN) as the unsaturated component in the polymer, and styrene as the coreactant monomer. Propylene glycol [57-55-6] (PG) is the principal glycol used in most compositions, and (i9f2v (9)-phthahc anhydride (PA) is the principal dibasic acid incorporated to moderate the reactivity and performance of the final resins. 

Antioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the mechanical, aesthetic, and electrical properties of polymers loss of flavor and development of rancidity ia foods and an iacrease ia the viscosity, acidity, and formation of iasolubles ia lubricants. The need for antioxidants depends upon the chemical composition of the substrate and the conditions of exposure. Relatively high concentrations of antioxidants are used to stabilize polymers such as natural mbber and polyunsaturated oils. Saturated polymers have greater oxidative stabiUty and require relatively low concentrations of stabilizers. Specialized antioxidants which have been commercialized meet the needs of the iadustry by extending the useflil Hves of the many substrates produced under anticipated conditions of exposure. The sales of antioxidants ia the United States were approximately 730 million ia 1990 (1,2). 

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