Glassy polymer, deformation

Srnani, A. B., Stepanov, V A. (1981). Prediction of Glassy Polymer Deformation Properties with the Aid of Dislocation Analogues. Mekbanika Kompozitnykh Materi-... 

Kozlov, G. V, Sanditov, D. S. (1992). The Activation Parameters of Glassy Polymers Deformation in Impact Loading Conditions. sokomolek. Soed. B, 34(11), 67-72. Kozlov, G. V, Shustov, G. B., Zaikov, G. E., Burmistr, M. V, Korenyako, V. A. (2003> Polymers Yielding Description within the Franeworks of Thermodynamical Hierarchical Model. Voprosy Khimii I Khimicheskoi Tekhnologii, 1,68-72. 

Kozlov, G. V, Seidyuk, V. D., Dolbin, 1. V. (2000). Chain Fractal Geometry and Amorphous Glassy Polymers Deformability. Materialovedenie, 12, 2-5. 

Pa, would deform appreciably under the action of loads comparable to the pull-off force given by Eq. 16. It is for this reason that the JKR type measurements are usually done on soft elastic materials such as crosslinked PI rubber [45,46] or crosslinked PDMS [42-44,47-50]. However glassy polymers such as polystyrene (PS) and PMMA are relatively hard, with bulk moduli of the order of 10 Pa. It can be seen from Eq. 11 that a varies as Thus, increasing K a factor of... 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

The properties of glassy polymers such as density, thermal expansion, and small-strain deformation are mainly determined by the van der Waals interaction of adjacent molecular segments. On the other hand, crack growth depends on the length of the molecular strands in the network as is deduced from the fracture experiments. 

Reinforcing fillers can be deformed from their usual approximately spherical shapes in a number of ways. For example, if the particles are a glassy polymer such as polystyrene (PS), then deforming the matrix in which they reside at a... 

Amorphous adsorbents, 1 587-589 for gas separation, 1 631 properties and applications, l 587t Amorphous aluminum hydroxide, 23 76 Amorphous carbohydrates, material science of, 11 530-536 Amorphous carbon, 4 735 Amorphous cellulose, 5 372-373 Amorphous films, in OLEDs, 22 215 Amorphous germanium (a-Ge), 22 128 Amorphous glassy polymers, localized deformation mechanisms in, 20 350-351... 

Attempts have been made in the past to explain the plasticity of glassy polymers based on different theories (a review of which can be found in (1)). Most of the earlier approaches are based on models non-specific about the molecular deformation process. The theory by Argon... 

Strain rate, test temperature and the thermal history of the specimen all affect the appearance of shear bands in a particular glassy polymer [119]. The differences in morphology of shear bands was proposed to be due to different rates of strain softening and the rate sensitivity of the yield stress. Microshear bands tend to develop in polymers with a small deformation rate sensitivity of Oy and when relatively large inhomogeneities exist in the specimen before loading. This is sometimes characterized by a factor e j, introduced by Bowden in the form [119] ... 

Since at temperatures below the Tg the chains of an amorphous polymer are randomly distributed and immobile, the polymers are typically transparent. These glassy polymers behave like a spring and when subjected to stress, can store energy in a reversible process. However, when the polymers are at temperatures slightly above the Tg, i.e., in the leathery region, unless crosslinks are present, stress produces an irreversible deformation. 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

The existence of a wedge-shaped cavitated or fibrillar deformation zone or craze, ahead of the crack-tip in mode I crack opening, has led to widespread use of models based on a planar cohesive zone in the crack plane [39, 40, 41, 42]. The applicability of such models to time-dependent failure in PE is the focus of considerable attention at present [43, 44, 45, 46, 47]. However, given the parallels with glassy polymers, a recent static model for craze breakdown developed for these latter, but which may to some extent be generalised to polyolefins [19, 48, 49], will first be introduced. This helps establish important links between microscopic quantities and macroscopic fracture, to be referred to later. 

The second equation appears to be applicable to a number of glassy polymers, and also to other materials the exponent m is always about 3, so that creep can be described by two parameters, Do and to, while the immediate elastic deformation is also taken into account (Do). As a matter of fact, Do and to are temperature dependent. When the experimentally found creep curves are shifted along the horizontal axis and (slightly) along the vertical axis, they can be made to coincide... 

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