Fibrillar crystallization

Kavesh et al. [226] reported the production of polyethylene filaments having a tenacity of at least 30 g/den by pulling a filament from a curved Teflon surface submerged in a solution of polyethylene. The rate of production of the filament was a least 2 m/min. 

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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. 

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 ... 

The row nucleated structure contains two types of crystals a small fraction of fibrillar crystals (row nuclei) with partially or even fully extended chains and the normal type folded chain lamellae. The existence of two types of crystals is detectable by calorimetry and the resistance to filmic nitric acid attack, high in the row nuclei and low in the surface layers of lamellae. The number of tie molecules between consecutive... 

Cellulose esters and ethers also give fibrillar crystal structures of the same type. The cohesive forces between the chains are, however, weaker than in cellulose. Cellulose triacetate and triethyl-cellulose, for instance, show a melting point, but melting can not be accomplished without a marked breakdown of the chains. According as the substituents themselves are larger and their polarity decreases, the melting point of the derivatives becomes lower, and their plasticity increases. This becomes clearly apparent if the properties of the triesters of cellulose of the homologous series of the normal fatty acids are compared. ... 

The remaining fiber runs diagonal through the figure. The small, fibrillar crystals of about 100 nm length were sufficiently crystalline not to be fully etched and have been removed from the fiber. 

Spheralites are spherical crystal patterns, considered as a centrally nucleated and grown radially into fibrillar crystal organizations. Based on the chemical components (molecular geometry and molecular weight) and physical properties (crystallization conditions and crystalline lattice), the spheralites exhibit variety of shapes. The material systems under specific conditions can form spherulitic crystal... 

Fig. 12. Schematic representation of networks formed by extended-chain crystals with cross-linking by (a) bifurcation of the fibrillar crystals or (b) common folded-chain crystals.

It is very likely that the lamellar crystals give the lower-angle component (filled circles in Fig. 2.7) in the X-ray diffraction profiles, and the fibrillar crystals the higher-angle component (open circles in Fig. 2.7). 

Since it was reportedthat the diffuse equatorial scattering in SAXS is caused by the microvoids between the fibrillar crystals, the lamellar crystal formed from the partial melt may fill up the microvoids so that the diffuse scattering in the equatorial direction decreases. 

The reductions in fractional amoimts of the as-polymerized main and sub-crystallites during irradiation is in the order of 25% of the original main- and sub-crystallite from Table 5.1. The layer-like voids as well as local defects are formed by the irradiation of PTOX crystals, suggesting the periodical (ca. 1,000 A) diarac-teristics of the fibrillar crystal of PTOX. For example, the exist ice of a s ctive region accessible to radiolysis is assumed along the c-axis of PTOX crystals. 

Radiolysis reveals the characteristic features of polymers, where chain scission by irradiation occurs at a selective point. Poly(trioxane) forms a layer-like void, suggesting periodical characteristics of about 1000 A along the c-axis of the polymer crystals. In the case of poly(tetroxocane) similar results are observed. The mean crystallite lengths of fibrillar crystals are obtained as about 500 A in poly(trioxane) and 300 A in poly(tetroxocane). 

It was therefore suggested that there is a possible aggregation of the domains into superdomains , similarly with the growth of spherulites in crystalline polymers, inducing formation of lamellar and fibrillar crystals (a model proposed by Kabanov [694]). Such a quasi-crystalline structure may have a characteristic period of ordering of 1 pm. 

The ordering increase facilitates the developing of a fibrillar crystallization process, after which the paralleled macromolecule can fuse into a fibrillar crystal. 

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