Subject spherulite

Fig. 1.9 AFM tapping mode images of a spherulitic texture in isotact polypropylene. The sample was crystallized to completion at 145°C and subjected to permanganic etching prior to examination. Image (a) shows topography while (b) contains phase information. Scale bars 5 iim.

The results of Figure 11 indicate that the polymers studied were subject to different microstructural deformation mechanisms. In this connection it must be borne in mind that the maximum nominal deformation of POM and PA66 was only 1% whereas PP and PTFE were deformed up to 1.8% and 3.3% respectively. Therefore it may be assumed that for POM and PA66 only an instantaneously reversible deformation of the amorphous matrix of the spherulitic microstructure occurred (18) whereas for PP and PTFE some irreversible effects, like interlamellar shearing or reorientation of the lamellae may have taken place. 

Some polymers, when they are suitably prepared in thin slices or as thin films, exhibit circular features when they are viewed in the optical microscope (fig. 3.13), whereas others show less regular patterns, depending on the polymer and the method of preparation of the sample. In order to see these features the polarising microscope with crossed polarisers (see section 2.8.1) is used. The circular features shown in fig. 3.13 are caused by spherical structures called spherulites which are a very important feature of polymer morphology, the subject of much of chapter 5, where the Maltese cross appearance seen in fig. 3.13 is explained. Each spherulite consists of an aggregate of crystallites arranged in a quite complicated but regular way. 

Such theories may be used to determine the degree of spherulite deformation from experimental patterns, and have been used for interpreting rates of deformation from light scattering motion pictures. They have been used more recently for explaining the results of dynamic light scattering measurements where samples are subjected to an oscillatory strain. ... 

The core of the book is devoted to subjects starting with anelastic behavior of polymers and rubber elasticity, but proceeds with greater emphasis in following chapters to mechanisms of plastic relaxations in glassy polymers and semicrystalline polymers with initial spherulitic morphology. Other chapters concentrate on craze plasticity in homo-polymers and block copolymers, culminating with a chapter on toughening mechanisms in brittle polymers. To make the... 

The subject of the crystallization of copolymers can be quite complex, dependent on the comonomer. It should also be recognized that the effects of variations in tacticity are very similar to the effects of comonomer inclusion, since both are effectively the insertion of defects into the polymer chain. The earliest treatment [26] recognized this fact, and is applicable to any defect, whether tactic, head-to-head link or comonomer, when measured as a defect content. This approach makes the assumption that all defects are excluded from the crystal. On this basis the probability of forming a critical secondary nucleus is dependent on the distribution of the defects throughout the polymer chain. The formulation of the probabilities leads to the logarithm of the rate of linear growth of a spherulite being dependent on the defect concentration. In practice, the behavior of most copolymers... 

In the following part, a discussion on the crystallization behavior in immiscible polymer blends is given, including the nucleation behavior, spherulite growth, overall crystallization kinetics, and final semicrystalline morphology. Each topic is illustrated with several examples from the literature to allow the reader to find enough references on the discussed subject for further information. 

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

Early studies on sphemlitic growth were sectioned in different subjects because they have investigated in numbers of material systems of various characteristics. Some well-established morphologies of sphemUte are presented in Fig. 1.15. Efforts have been made to propose a common feature of growth and mechanism of sphemlitic patterns. However, it observed that mechanism of spherulitic growth is not depends only on the molecular property and chemical stmctures of material systems. The morphologies and physical characteristics of a sphemlite from a system to other materials are varied drastically and found to be reliable on few key factors. 

Crystallisation processes in PEEK have been the subject of many academic papers [10-13]. However, the crystallisation of PEEK generally matches the classic behaviour of other polymers. The effect of time is described by Avrami kinetics ( 3) and secondary crystallisation occurs after the spherulites have impinged. This secondary crystallisation results from an increase in the crystallinity within the spherulites and is probably related to the existence of the low-temperature melting peaks (LTMP) described later. A number of non-isothermal crystallisation models have been developed. 

El Fray and Altstadt [12] used MTA to study the relationship between morphological features of semi-crystalline and multi-block polymeric materials and their thermal properties. Samples of semi-crystalline polybutylene terephthalate and its copolymer were crystallised from the melt showing a spherulitic morphology. The surface of the spherulitic shapes was subjected to L-TA at selected regions of different thermal conductivity (at the centre of the spherulite and at its outer surface). This reveals information, which cannot otherwise be obtained. 

The long-range influence of the surface on crystallization, which determines the thickness of the surface layers of crystaUine pol3rmers, is comparable with the spherulite sizes (5-10)xl0 m. At a rather high amount of filler, when the distance between filler particles is lower than the spherulite size, the polymer is subjected to surface effects. In this case, the structrue and properties of pol5Tner in the surface layer must depend on the distance from the surface. In this respect, one more level of microheterogeneity of the surface layers arises, due to the influence of the surface on crystalhne structrue. 

PCA was employed to classify the differences in the spectra for each local area spectrum of the spherulite. Consequently, 20 single spectra were extracted from 20 points of groups A-C, but for group D only five spectra were obtained as the amorphous area is limited (as shown in Figure 22.23c). All spectra in the region of 5500-3300 cm were subjected to a linear baseline correction and a second derivative pretreatment to highlight subtle differences in the spectral features among the spectra before the PCA calculations. 

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