Properties of the Amorphous Phase

For the representation of the stmcture-sensitive properties, it is necessary to develop an even more detailed model on the molecular level. A key example treated in Sect. 5.3 is the description of the strength of semicrystaUine fibers. The properties 

Important structure-sensitive properties of glasses limit the ultimate strength. A description must describe the crack-propagation, crazing, and fibrillation on fracture and plastic deformation. All of these create new external or internal surfaces, which are critically dependent on the conformation and large-amplitude motion of the macromolecules in relation to the new interfaces. Little more is discussed about this topic. The continually changing special literature on this rather empirical topic must be checked for further information. 

Liquids are condensed phases that respond lo shear stress by flow rather ilian exhibiting a fixed strain 

Total energy dissipated per second (frictional force, f vj (velocity, v) = fcVxv (AW/At)j = fcrj wS = (l/4)fcrj2(dv/dy)2 

---

In the 1970s a model for semi-crystalline polymers was presented by Struik (1978) it is reproduced here as Fig. 2.13. The main feature of this model is that the crystalline regions disturb the amorphous phase and reduce its segmental mobility. This reduction is at its maximum in the immediate vicinity of the crystallites at large distances from the crystallites will the properties of the amorphous phase become equal to those of the bulk amorphous material. This model is similar to that of filled rubbers in which the carbon black particles restrict the mobility of parts of the rubbery phase (Smith, 1966). 

No change of the sorption properties of the amorphous phase is observed by thermal treatment. A low gas permeability measured at the biaxially stretched films is related to both a change of the free volume sizes distribution and a tortuosity effect. The barrier properties of biaxially stretched films are kept even after annealing the film at 250°C. 

Table 5.2 summarizes the values of the obtained so far except those shown in Figure 5.26. The sign represents that of the Hall coefficient. A positive Hall coefficient was reported by Komfeld and Sochava (1959). These measurements were recently verified by Nagels etal. (1970) who took special care to ascertain that the Hall coefficient was a genuine property of the amorphous phase and not caused by crystalline inclusions. This exception is remarkable because of the close chemical similarity of all the systems investigated. It wiU be remembered that this small gap material had a thermopower which was difficult to interpret. 

The mechanisms of plastic deformation at microscopic level of amorphous polymers are mainly crazing and shear yielding [3-5]. In semicrystalline polymers, although the glass transition temperature, density, infrared spectrum and other properties of the amorphous phase interdispersed between the crystalline lamellae are close to those of bulk amorphous polymers, the mechanisms of plastic deformation are very different from those of the amorphous materials, since also the crystalline phase plays a key role [Ij. However, because of the presence of the entangled amorphous phase, the mechanisms of plastic deformation of semicrystalline polymers are also different from those of other crystalline materials (for instance metals). 

Zheng and Keimedy (2004) followed the approach of a two-phase model, but in their model, the effect of crystallinity is not buUt-into the amorphous parameters. They view the system as a suspension of crystalhne phase in a matrix of amorphous phase. The physical properties of the amorphous phase are independent of a, while the viscosity of the whole system is a function of the relative crystalhnity. 

It appeared that a successful modeling of mechanical properties of crystalline polymers within the elastic range requires a consideration of lamellae thickness and crystallinity but also the lamellae width and length. Varying elastic properties of the amorphous phase are to be considered when the constraints between lamellar crystals change due to differentiated solidification conditions. 

We have provided examples of three extreme cases of the effect of stereo-regularity on thermodynamic properties. In PS, tacticity seemingly has no effect on T nor on any other property of the amorphous phases of the respective isomers. In contrast, the T s as well as many of the other thermodynamic... 

It is worth emphasizing that the physical, chemical, mechanical, and so on, properties of amorphous and crystalline phases are very diff nt In most cases, there is a proportional additivity of the specific properties. If Pa, Pc, and P represent, respectively, the specific property of the amorphous phases, the crystalline phases, and all the phases, one can write... 

Most polymeric adhesives are amorphous rather than crystalline, and the glass transition is a property of the amorphous phase. It is caused by changes in molecular motion, such that in... 

It is known that mechanical and physical properties of the amorphous and crystalline phases differ significantly [80T01]. For this reason, it is anticipated that properties of the mechanically and electrically treated film will depend explicitly on its history. Shock-compression measurements such as those carried out on amorphous materials in a thick form [80M01] will not prove characteristic of thin, treated films. 

The amorphous orientation is considered a very important parameter of the microstructure of the fiber. It has a quantitative and qualitative effect on the fiber de-formability when mechanical forces are involved. It significantly influences the fatigue strength and sorptive properties (water, dyes), as well as transport phenomena inside the fiber (migration of electric charge carriers, diffusion of liquid). The importance of the amorphous phase makes its quantification essential. Indirect and direct methods currently are used for the quantitative assessment of the amorphous phase. 

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

Many crystalline solids can undergo chemical transformations induced, for example, by incident radiation or by heat. An important aspect of such solid-state reactions is to understand the structural properties of the product phase obtained directly from the reaction, and in particular to rationalize the relationships between the structural properties of the product and reactant phases. In many cases, however, the product phase is amorphous, but for cases in which the product phase is crystalline, it is usually obtained as a microcrystalline powder that does not contain single crystals of suitable size and quality to allow structure determination by single-crystal XRD. In such cases, there is a clear opportunity to apply structure determination from powder XRD data in order to characterize the structural properties of product phases. 

Low polarisation ratios (<2 1) for absorption had also been found for amorphous PPV (1) deposited from solution by spin-coating on rubbed poly(tetrafluoroethylene) [PTFE]. It is evident that this could be improved on by making use of the high order parameter and self-organising properties of the nematic phase of liquid crystalline electroluminescent polymers such as those (16, 28 and 78-82) shown in Table 6.16. - 2 ° This was then found subsequently to be the case using thermotropic liquid crystalline polyfluorenes, such as 28 and 80 shown in Table 6.6 and segmented PPV derivatives, such as 81. The nematic phase exhibits the lowest viscosity of... 

The viscoelastic properties of the crystalline zones are significantly different from those of the amorphous phase, and consequently semicrystalline polymers may be considered to be made up of two phases each with its own viscoelastic properties. The best known model to study the viscoelastic behavior of polymers was developed for copolymers as ABS (acrylonitrile-butadiene-styrene triblock copolymer). In this system, spheres of rubber are immersed in a glassy matrix. Two cases can be considered. If the stress is uniform in a polyphase, the contribution of the phases to the complex tensile compliance should be additive. However, if the strain is uniform, then the contribution of the polyphases to the complex modulus is additive. The... 

Particularly for freeze-dried products, formulation and process are interrelated. Properties of the formulation, in particular the collapse temperature, will have a significant impact on the ease of processing. An efficient process is one that runs a high product temperature. However, the temperature cannot be too high or product quality will be compromised. As the glass transition temperature depends on chemical composition of the amorphous phase, Tg and collapse temperature are strongly formulation dependent. Collapse temperatures for common excipient systems vary from less than —50°C to around —10°C (Table 2). 

---