Semi-crystalline materials

These materials have the same state diagram as the amorphous materials, but with addition of the melting phenomenon (see Fig. 7.15). Melting point increases with molecular weight. 

Waxy state for M Me- This is a very fluid amorphous phase containing crystalline domains, which increase its modulus. As the chains are short, very few of them belong to more than one crystalline domain, and the material is easily deformed. Paraffin waxes are polyethylene chains falling into this range of molecular weights. 

Flexible state for M Me- Now the chains are long and entangled in the amorphous regions. Furthermore, the same chain may belong to several crystalline and amorphous regions. Cohesion is therefore strong between 

This is useful for many applications, but in the freezer compartment, it loses its flexibility and becomes sensitive to impact. 

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The use of Equation (22) is very general, but it is also possible, with accurate measurements and data treatment, to perform the quantitative phase analysis in semi-crystalline materials without using any internal standard. This procedure is possible only if the chemical compositions of all the phases, including the amorphous one, are known. If the composition of the amorphous phase is unknown, the quantitative analysis without using any internal standard can still be used provided that the chemical composition of the whole sample is available [51]. This approach, until now, has been developed only for the XRD with Bragg-Brentano geometry that is one of the most diffused techniques in powder diffraction laboratories. 

Esterification of at least 45% of the hydroxyl groups with long chain fatty acids, e.g., stearic or behenic acid, results in a semi crystalline material (side chain crystallization). The obtained materials are characterized by melting point ranges which are approximately 10 °C lower than the comparable methyl esters. 

Escaig B, G Sell C, Plastic Deformation of Amorphous and Semi-Crystalline Materials, Les Editions de Physique, Les Ulis (France), 1982. 

All oligomers are semi-crystalline materials, although they have low melting enthalpies. Their absolute degree of crystallinity is obviously unknown, but in the case of SPC 1, which has the lowest content of cyclic units, it approaches... 

Filled rubbers behave in the same way as semi-crystalline materials. 

The following part of this report will be limited entirely to cases where craze-like features are observed in semi-crystalline materials. It is planned to show some of the phenomena and to discuss possible mechanisms. 

In a final chapter a closely related phenomenon, the formation of shear bands in semi-crystalline polymers under compressive load will be described. It is attempted to discuss under which conditions shear bands are formed in semi-crystalline materials and how they interact with each other or with certain microstructural features, finally leading to crack initiation and shear fracture of the bulk polymer. 

Post-failure studies of the fracture surface morphology of bulk semi-crystalline polymers are more difficult than those of amorphous materials due to the more complex multiphase structure associated with semi-crystalline materials However, it was shown that some fractographic details point to the formation of a stress whitened region ahead of a notch prior to final fracture of the material. In particular, stress-whitened regions were easily visible in semi-crystalline polymers such as LDPE and HDPE The resulting, macroscopically apparently brittle fracture... 

As already mentioned there are only very few studies dealing with the formation of shear bands in semi-crystalline materials. Some recent papers on oriented and non-oriented semi-crystalhne polymers have indicated that under... 

For pharmaceutical materials moisture is known to affect a wide range of properties such as powder flow compactibility and stability (physical chemical and microbiological) (8 46-53). The interaction between moisture and a solid is complex and can occur in a variety of ways. For example water can be stoichiometrically incorporated into a solid s crystal structure in the form of a hydrate (pseudo-hydrate) as discussed previously in this section. In addition moisture can have non-stroichiometrical i.e., nonspecific interactions with a solid by adsorbing on the surface or being absorbed into the material and acting as a plasticizer. These non-specific interactions are more common in amorphous or semi crystalline materials and are the subject of this section. 

The ionomer which was isolated from the neutralization of sample SBD-2 was a brown-colored elastic network of moderate strength. Ionomer samples SBD-1 and SBD-2, neutralized to the stoichiometric end point using KOH, were compression molded at 140°C and examined for tensile properties. The results, as shown in Figure 16, illustrate the profound influence of crystallinity on the elastomeric inner block. The semi-crystalline material (SBD-1) behaves much like a rigid plastic, while the amorphous sample (SBD-2) is an elastomer of moderate strength. 

Owing to the method and the type of plasticizer used for TPS production, the process leads to the destruction of the starch granule lamellar structure, giving rise to a quasi-amorphous or semi-crystalline material. 

Finally, chemical alteration of surfaces can improve adhesion. Semi-crystalline materials have low polarity and low surface tension that must be increased to enable... 

PVC is a semi-crystalline material containing small, not very well developed crystallites. Size of crystallites seems to depend on the conditions of polymerization and on the method of processing. It varies in a quite narrow range of 0.7 to 15 nm, with spacing between crystallites from 0.5 to 20 nm. There is almost firll agreement that PVC does not form spherulites because of the small size of its crystals. 

PPS is a semi-crystalline material. It exhibits an excellent balance of properties, including high-temperature resistance, chemical resistance, flowa-bility, dimensional stability, and electrical characteristics. PPS is brittle. Therefore, it must be filled with fibers and fillers. 

The deformation behaviour of semi-crystalline materials is mainly determined by the behaviour of the two components - the crystalline and the amorphous phase with their characteristic temperature-dependent mechanical behaviour and sometimes their anisotropy. So the crystalline phase is elastically with a rather high modulus. Above a certain stress the crystallites break down into smaller fragments. Aligned chains enable recrystallisation. The mobility in the amorphous phase depends on the difference between the ambient temperature and the temperature characteristic of the glass transition, which is the dominant relaxation process in the temperature range under investigation. On the other side the amorphous phase is constrained within the crystalline one. So it shows to some extent stress relaxation or frozen stress. Both phases are connected via anchor molecules, bridging the phase boundaries. Those molecules are mainly responsible for stress transfer between the phases. 

To describe the temperature-dependent stress-strain behaviour of semi-crystalline materials we established a model on the basis of the described structural elements. It describes the interaction of the temperature-dependent mobility of the amorphous phase, the initially relative stable crystalline phase, the step>-wise re-arrangement of the crystalline phase with respect to orientation and transformation via crystallite fragments and extended chains into fibrils. It will be soon published separately. 

On the other side by re-melting, stretching and re-crystallizing semi-crystalline material it is possible to get materials with highly improved mechanical behaviour due to well-defined and strongly oriented re-ciystallisation. Of cause for using this general technique the parameters of treatment must be optimized. 

The detailed understanding of the multiple deformation and structure-establishing processes is an essential base for the development of semi-crystalline materials with well-defined properties. 

Storbeck and Ballauff prepared polyesters from isohexides and terephthaloyl dichloride by solution polymerization in pyridine, giving colourless, fibrous materials [13]. Isomannide and isoidide yielded semi-crystalline materials however, crystallinity could not be recovered after annealing since their glass transition temperature and melting temperature are too close to each other for them to be crystallized from the melt. Thermogravimetric analysis (TGA) of poly(isosorbide terephthalate) (PIT) showed thermal stability up to 360 °C under a nitrogen atmosphere. 

Figure 10.58 indicates that temperature is an essential factor in creation of a crystalline stiucture. Figure 10.59 shows the importance of the crystalhzation time on the stiuc-ture of plasticized material. Three major parameters are involved here concentration of plasticizer, temperature, and time. On the one hand these parameters influence materials stracture, on the other the time-temperature regime is one of the factors complicating the determination of the degree of crystallinity. In spite of matty methods used for crystallinity determination (IR, WAXS, DSC) or perhaps because of many methods and variability in conditions of sample preparation and treatment, only rough estimates of crystallinity may be obtained for semi-crystalline materials. " ... 

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