Deformation behavior, amorphous polymers

The high temperature deformation behavior of polymer precursor derived amorphous Si-B-C-N ceramics was investigated in controlled atmosphere to understand the mechanisms of deformation... 

If the ordered, crystalline regions are cross sections of bundles of chains and the chains go from one bundle to the next (although not necessarily in the same plane), this is the older fringe-micelle model. If the emerging chains repeatedly fold buck and reenter the same bundle in this or a different plane, this is the folded-chain model. In either case the mechanical deformation behavior of such complex structures is varied and difficult to unravel unambiguously on a molecular or microscopic scale. In many respects the behavior of crystalline polymers is like that of two-ph ise systems as predicted by the fringed-micelle- model illustrated in Figure 7, in which there is a distinct crystalline phase embedded in an amorphous phase (134). 

In terms of the mechanical behavior that has already been described in Sections 5.1 and Section 5.2, stress-strain diagrams for polymers can exhibit many of the same characteristics as brittle materials (Figure 5.58, curve A) and ductile materials (Figure 5.58, curve B). In general, highly crystalline polymers (curve A) behave in a brittle manner, whereas amorphous polymers can exhibit plastic deformation, as in... 

Amorphous polymers exhibit two mechanisms of localized plasticity crazing and shear yielding. These are generally thought of separately, with crazing corresponding to a brittle response while shear yielding is associated with ductile behavior and the development of noticeable plastic deformation prior... 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

In view of the multitude of observed deformation mechanisms it is useful at this point to examine the effects of external variables, especially that of ambient temperature, on the deformation behavior of semi-crystalline thermoplastics. At room temperature many of these polymers are above their glass transition point and owe their strength and stiffness to the crystalline phases. The first displacements start in the relatively soft amorphous layers, but the stress-strain curve is largely determined by the presence and arrangement of the crystals. Interlamellar slip has been identified as an important mechanism, but, in addition, crystalline deformation mechanisms occur at moderate strains The corresponding stress-strain curve shows an... 

It is well known that the mechanical behavior of glassy amorphous polymers is strongly influenced by hydrostatic pressure. A pronounced change is that polymers, which fracture in a brittle manner, can be made to yield by the application of hydrostatic pressure Additional experimental evidence for the role of a dilatational stress component in crazing in semicrystalline thermoplastics is obtainai by the tests in which hydrostatic pressure suppresses craze nucleation as a result, above a certain critical hydrostatic pressure the material can be plastically deformed. 

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

There are significant differences between the small-strain (i.e., small deformation) and the large-strain behavior of polymers. The small-strain behavior is discussed in Section 1 l.B. It is mainly described by the moduli (or compliances) and Poisson s ratio. An amorphous polymer typically softens drastically as its temperature is raised above Tg, so that its structural rigidity is lost. At the typical time scale of a practical observation, the key indicators of stiffness (the tensile and shear moduli), which decrease very slowly with increasing temperature below Tg, decrease rapidly over a narrow temperature range with further increase in temperature by... 

The four-parameter model provides a crude quahtative representation of the phenomena generally observed with viscoelastie materials instantaneous elastie strain, retarded elastic strain, viscous flow, instantaneous elastie reeovery, retarded elastie reeovery, and plastic deformation (permanent set). Also, the model parameters ean be assoeiated with various molecular mechanisms responsible for the viscoelastic behavior of linear amorphous polymers under creep conditions. The analogies to the moleeular mechanism can be made as follows. 

For amorphous polymers, the situation is more complex. We can find low temperatures where the polymer acts like a solid, and high temperatures where the polymer flows like a liquid. However we are not able to find any region of a few degrees where we clearly have a transition from one type of behavior to another. Instead, like glass, the polymer gradually gets less and less resistant to deformation as the temperature increases, and more and more resistant as it decreases. Therefore the concept of melting temperature is not defined for amorphous polymers. 

Whether the polymer is totally amorphous or partially crystalline, the material will be glassy (hrittle) or ruhher-like (soft) depending on its temperature with respect to Tg. If an amorphous polymer is at a temperature helow Tg, it will be brittle and will show properties of a glassy material for example, it will fracture more easily. As the temperature of the sample increases and approaches Tg, it adopts a leathery behavior and its elastic modulus decreases. When the sample has reached several degrees above Tg, it shows a clear rubbery behavior and is easily deformable. If the temperature is increased even more, the polymer reaches liquid flow behavior. If the polymer is semicrystalline, it exhibits similar behavior, but when it reaches the melting temperature the crystals will break up, and the polymer will then reach the melted liquid state. This behavior is illustrated in Fig. 3.45 where the elastic modulus is plotted versus temperature. 

As with all other forms of plastic deformation in amorphous solids, we expect that in glassy polymers too the plastic deformation will consist of a series of discrete thermally assisted unit relaxation events on the atomic scale. Features of such discrete behavior have, e.g., been observed in deformation calorimetry by Oleinik (1991), who detected discrete unit inelastic events already in the pre-yield region in some glassy polymers. 

In the literature, the constitutive equation for both the amorphous polymer and crystalline polymer has been well established. Therefore, we can direcdy use these relations to model the amorphous phase and crystalline phase of the SMPFs. We then need to consider the cychc texture change of both subphases because the mechanical behaviors of the individual microconstituents may vary when they are packed in a multiphase material system and a certain deviation in their mechanical responses may exist between the individual and their assembled configurations. Since this is a shape memory material, we also need to model the shape recovery behavior. After that, we can use the above micromechanics relation to assemble the macroscopic constitutive relation. In order to determine the parameters used in the constitutive model, we need to consider the kinematic relations under large deformation. Finally, we will discuss the numerical scheme to solve the coupled equations. 

Thermoplasts are linear or weakly branched polymers. Their application temperature lies below the melting temperature in the case of crystalline polymers and below the glass transition temperature in the case of amorphous polymers. They are converted to an easily deformable plastic state on heating above these characteristic temperatures. This plastic state can be termed liquid with respect to the molecular order, or viscoelastic with respect to the rheological behavior. On cooling below the characteristic temperatures,... 

Based on the facts presented above, the plastic deformation behavior of semicrystalline polymer materials and the structural changes accompanying the defor-matimi of such materials are craitroUed by the properties of both crystalline and amorphous phases. 

---