Crystal fibrillar

This argument was put forth In 1962 by Judge and Stein (1) to explain the behavior of polyethylene networks under a constant load. That same year Smith (4) showed that a folded-chain morphology was thermodynamically favored at lower temperatures (high crystallization) whereas at temperatures near the melting point (low crystallization) fibrillar morphology was stable a transformation of one morphology Into the other was thermodynamically... 

Hence, the main aim of the technological process in obtaining fibres from flexible-chain polymers is to extend flexible-chain molecules and to fix their oriented state by subsequent crystallization. The filaments obtained by this method exhibit a fibrillar structure and high tenacity, because the structure of the filament is similar to that of fibres prepared from rigid-chain polymers (for a detailed thermodynamic treatment of orientation processes in polymer solutions and the thermokinetic analysis of jet-fibre transition in longitudinal solution flow see monograph3. ... 

The formation of fibrillar structures during the crystallization of deformed solutions and melts under various conditions of mechanical treatment was observed by many authors22,24,25 who studied the crystallization in stirred or flowing solutions. In all cases... 

The most important feature of polymers obtained by these methods is their high mechanical strength, primarily their elastic moduli and tenacities that, in some cases, approach the theoretical values. It has been recognized that these mechanical properties are uniquely related to the existence of ECC with very perfect chain packing in fibrillar crystals. 

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

We will compare the change in the free energy per macromolecule in the formation of folded-chain (curve 1) and fibrillar (curve 2) crystals as a function of /J. Figure 8 b shows these curves calculated for the corresponding equilibrium values [a]. The increase... 

Fig. 8a-c. Dependence of the degree of crystallinity (a), free energy (b) and melting temperature (c) on fi. 1 folded-chain crystals, 2 fibrillar crystals the broken line corresponds tofi=fia... 

To avoid misunderstanding, it should be emphasized that if the transition from one type of crystallization to the other one is considered, this does not imply a transformation of crystals of one type into the other one during stretching. In contrast, if the molecule enters a folded-chain crystal, it is virtually impossible to extend it. In this case, we raise the question, which of the two crystallization mechanisms controls the process at each given value of molecular orientation in the melt (this value being kept constant in the crystallization process during subsequent cooling of the system). At /J < /3cr, only folded-chain crystals are formed whereas at / > only fibrillar crystals result at /8 /3cr, crystals of both types can be formed. 

It has recently been shown (7) that a transformation from fibrillar to lamellar morphology is not required to replicate the force-temperature profile of stretched networks in the crystallization region. This latest work shows that a close duplication of the behavior of gutta percha (8) can be predicted with a model (7) of fibrillar crystallization that Incorporates several new features omitted in earlier theories, specifically ... 

They therefore finally appear as polyhedra. Because of their radial growth the fibrillar or lamellar crystals have only little space-filling ability as they move away from the central nucleus. Typically for spherulitic structures, an irregular noncrystallo-graphic branching usually at small angles can be observed. 

The morphology of spin-cast film, thickness of 180 nm, from polycaprolactone shows many spherulitic structures with fibrillar nanostructures formed of lamellae lying edge on (about 10 nm thick) and areas with lamellar sheets lying flat on. Different crystalline structures are found when the sample is melted and crystallized as a function of temperature. These two studies reinforce the complex inner relationship between physical treatment and nanostructure. 

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