Glassy Thermoplastics

More successful attempts to interpret yielding on a molecular level were based on an extension of the Eyring phenomenological flow theory by incorporating molecular characteristics [132,133]. The modification is based on changes in distribution of rotational conformation states of segments upon stress action and the effect of temperature on them. 

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The cured polymers are hard, clear, and glassy thermoplastic resins with high tensile strengths. The polymers, because of their highly polar stmcture, exhibit excellent adhesion to a wide variety of substrate combinations. They tend to be somewhat britde and have only low to moderate impact and peel strengths. The addition of fillers such as poly (methyl methacrylate) (PMMA) reduces the brittleness somewhat. Newer formulations are now available that contain dissolved elastomeric materials of various types. These mbber-modifted products have been found to offer adhesive bonds of considerably improved toughness (3,4). 

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Plasticisers are added to a polymer to reduce the glass rubber transition temperature drastically (e.g. with PVC), so that the polymer behaves as a rubber at ambient temperature rather than as hard glassy thermoplast. 

Fig. 6. Dependence of an arbitrary mechanical property on the molar weight of linear glassy thermoplastic...

The major contributing dissipative processes at the crack tip are shear yielding in semicrystaUine thermoplastics and crazing in glassy thermoplastics. 

R. N. Haward and G. Thackray, The Use of a Mathematical Model to Describe Isothermal Stress-Strain Curves in Glassy Thermoplastics, Proc. R. Soc. London, Series A, 302 (1471), 453 (1967). 

In glassy thermoplastics the process of fracture is preceded by the formation of a characteristic zone of heavily deformed material, the craze zone. Just ahead of the crack tip, where the stresses are particularly concentrated, molecular chains of the... 

The aim of this review is to concentrate mainly on these fundamental aspects of the fracture behavior of glassy thermoplastics. In the first Section, following an outline of the relevant fracture mechanics theory, the optical interference method is described and the nature of the results obtainable from it is discussed. The next Section then considers the behavior of cracks and crazes in specimens subjected to quasistatic loading, whilst the final Section examines the role of crazing associated with fatigue crack growth. 

The classical methods of light optical interference are well known in the determination of small dimensions which are of the order of the wave length of light. A new scientific field has been opened up by their application to investigations of the craze behavior at crack tips in transparent glassy thermoplastics... 

At present there is only very little and indirect information available on the crazing behavior at the tips of fast running cracks in glassy thermoplastics (e.g. s.se.wi) Hence this Section will confine itself to a discussion of the behavior of single crazes at the tips of stationary or slowly moving cracks. 

Thermal history such as annealing or quenching significantly affects the mechanical behavior of thermoplastics. As a typical example in amorphous glassy thermoplastics. 

A new model [109] for the deformation of glassy thermoplastics captures the behavior as a function of strain rate and temperature up to the yield point, based on a physical picture of a polymeric glass as a mosaic of nanoscale clusters of differing viscoelastic characteristics. It does not require computationally demanding simulations. It does, however, require a limited set of experimental stress-strain data to obtain values for its fitting parameters for a given material and to then allow both interpolations and extrapolations to be made to other testing conditions. 

The synthesis of ABA blocks from a glassy thermoplastic A and an elastomeric B produces other elastoplastics with attractive properties. Polyester chains can be extended with di-isoeyanate, which is then treated with cumene hydroperoxide to leave a peroxide group at both ends of the chain. By heating this in the presence of styrene, a vinyl polymerization is initiated and an ABA block created. The modulus-temperature curves show how the mechanical properties can be modified in this way (Figure 15.7). These block copolymers are known as thermoplastic elastomers. 

Adapted with permission of lOP Publishing Limited from Schinker, M.G. and Doell, W. Interference optical measurements of large deformations at the tip of a running crack in a glassy thermoplastic in lOP Conf. Series No.47, Chap 2, p.224, lOP, 1979. 

Somewhat fibrous Viscous, glassy Thermoplastic injection molding compounds... 

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