Crystalline state spherulites

The linewidth-temperature relation of the polyethylene oxide samples are given in Fig. 5. Despite the large differences in molecular weight, these samples have about the same linewidth, 300-350 Hz, in the crystalline state at 25°C. They all also possess a spherulitic type of morphology. The influence on the linewidth of the different types of supermolecular structures... 

The process of formation of the crystalline state is controlled by the kinetics of nucleation and this may arise in a number of ways. Primary nucleation in a quiescent state must be associated with foreign bodies such as deliberately added nucleating agents, such as fine talc particles, or residual impurities such as heterogeneous catalyst particles followed by spherulite growth. The plot of extent of crystallinity, (p, as a function of time is sigmoidal in nature and follows an Avrami equation of the form... 

Polymers possess condensed state mainly referred to as crystalline state, and the crystalline spherulite form is shown in Figure 2.11. The condensed state can be divided into four phases, namely, no fixed shape, the transition state, liquid crystalline state, and crystalline state. Due to the complex formation process and peculiar phenomenon, transition state and liquid crystalline state have attracted much attention. The crystalline state is interesting mainly due to its multicrystalline form. 

Blends of polymers show complicated spherulite patterns. For example, in the blend sample of PVDF with atactic poly (methyl methacrylate) (PMMA), PVDF is in the crystalline state and PMMA is in the amorphous state [32]. For polymer-diluent systems, the diluent molecules are expelled out of the lamellae and the spherulite because of their thermal mobility thus, the diluent concentration in the system changes in front of the growing spherulite. For polymer-polymer blend systems, the thermal mobility of the polymers is not very high thus, the amorphous polymers remain trapped between the lamellae of the crystalline polymer component. For example, in the crystallization of PVDF70/PMMA30... 

Amorphous stereotactic polymers can crystallise, in which condition neighbouring chains are parallel. Because of the unavoidable chain entanglement in the amorphous state, only modest alignment of amorphous polymer chains is usually feasible, and moreover complete crystallisation is impossible under most circumstances, and thus many polymers are semi-crystalline. It is this feature, semicrystallinity, which distinguished polymers most sharply from other kinds of materials. Crystallisation can be from solution or from the melt, to form spherulites, or alternatively (as in a rubber or in high-strength fibres) it can be induced by mechanical means. This last is another crucial difference between polymers and other materials. Unit cells in crystals are much smaller than polymer chain lengths, which leads to a unique structural feature which is further discussed below. 

PVDF is mainly obtained by radical polymerisation of 1,1-difluoroethylene head to tail is the preferred mode of linking between the monomer units, but according to the polymerisation conditions, head to head or tail to tail links may appear. The inversion percentage, which depends upon the polymerisation temperature (3.5% at 20°C, around 6% at 140°C), can be quantified by F or C NMR spectroscopy [30] or FTIR spectroscopy [31], and affects the crystallinity of the polymer and its physical properties. The latter have been extensively summarised by Lovinger [30]. Upon recrystallisation from the melted state, PVDF features a spherulitic structure with a crystalline phase representing 50% of the whole material [32]. Four different crystalline phases (a, jS, y, S) may be identified, but the a phase is the most common as it is the most stable from a thermodynamic point of view. Its helical structure is composed of two antiparallel chains. The other phases may be obtained, as shown by the conversion diagram (Fig. 7), by applying a mechanical or thermal stress or an electrical polarisation. The / phase owns ferroelectric, piezoelectric and pyroelectric properties. 

Just above the melting point the polymer is visually quite viscous and numerous observations have been made that the polymer exhibits a memory effect, that is to say, on recooling the melt crystallites will appear in the same sites where they had been before melting the polymer. Hartley, Lord and Morgan (1954) state It is reasonable to suppose that there will be a few localities in the crystalline polymer which have a very high degree of crystalline order, and therefore the melt can contain, even at considerable temperatures above the observed melting or collapse point, thermodynamically stable minute crystals of the polymer . Especially if the polymer has been irradiated so as to contain a few crosslinks as in irradiated polyethylene, then flow is inhibited and spherulites can be made to appear on recrystallization in the same sites that they had before the polymer was melted, Hammer, Brandt and Peticolas (1957). However, as mentioned above, the specific heat of irradiated polyethylene in the liquid state is identical with that of the unirradiated material, within the limits of experimental error. Dole and Howard (1957). 

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