Oriented fibrillar structure

Crazing in anisotropic polymers has seldom been studied by means of optical interferometry. A block copolymer (Isoprene-Styrene) with a highly oriented fibrillar structure, and a PMMA oriented by means of drawing above Tg, have been studied so far. These results are reported in and The same subject has often been studied, however, using other experimental techniques... 

Dry spinning generates a fiber that initially appears different from typical wet-spun fibers as there is no opportunity for the spin bath to diffuse into the fiber. However, when the unoriented dry-spun fiber is stretched, an oriented fibrillar structure develops, indistinguishable from a stretched wet-spun fiber. Hence, all acrylic fibers, whether dry- or wet-spun are fundamentally similar. 

The liquid crystal polymerization method was modified to prepare a vertically aligned poly acetylene film [58]. As a result, a film with a very curious morphology was formed that was composed of two layers. The layer on the solvent side had a vertically oriented fibrillar structure, whereas the layer on the acetylene gas side had a randomly oriented one. The modified method is very promising for visualizing the polymer growth process and clearly demonstrates that PA chains grow in a liquid crystal solvent. 

During the drawing step, the blend components are oriented, and nano- or microfibrils are created fibrillation step). In the subsequent thermal treatment stage, when melting of the lower-melting component occurs isotropization step), the oriented fibrillar structure... 

The formation of a fibrillar structure in TLCP blends makes the mechanical properties of this kind of composites similar to those of conventional fiber reinforced thermoplastics [11,26]. However, because the molecular orientation and fibrillation of TLCPs are generally flow-induced, the formation, distribution, and alignment of these droplets and fibers are considerably more processing-dependent. We do not know ... 

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. ... 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

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. ... 

The neck is more or less fully developed and a fibrillar structure is obtained. This structure is less susceptible to degradation because of its high degree of orientation and high crystallinity. This explains the drop and then levelling off of the carbonyl content in this latter stage. 

In semicrystalline polymers such as polyethylene, yielding involves significant disruption of the crystal structure. Slip occurs between the crystal lamellae, which slide by each other, and within the individual lamellae by a process comparable to glide in metallic crystals. The slip within the individual lamellae is the dominant process, and leads to molecular orientation, since the slip direction within the crystal is along the axis of the polymer molecule. As plastic flow continues, the slip direction rotates toward the tensile axis. Ultimately, the slip direction (molecular axis) coincides with the tensile axis, and the polymer is then oriented and resists further flow. The two slip processes continue to occur during plastic flow, but the lamellae and spherullites increasingly lose their identity and a new fibrillar structure is formed (see Figure 5.69). 

Figure 5.69 Fibrillar structure of an oriented polymer crystal as the result of an applied tensile force. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 72. Copyright 1997 by Oxford University Press.

Crystallizable polymers tend to form randomly oriented crystallites which are oriented when the polymer is stretched or cold drawn at temperatures below the Tm. Crystallization under pressure may result in a fibrillar structure or extended chain structure. 

The fibrillar structure of crystalline polymers is determined by molecular characteristics, the initial morphology and orientation conditions. Recently, a complex investigation of the effect of molecular parameters (MW, MWD and degree of branching) and orientation parameters (temperature and draw ratio) on the morphology of PE and its thermomechanical behaviour has been reported 181 185). 

In stage four (not shown in Figure 7), a strong stress is present, which gives the polymer a fibrillar structure with a higher degree of orientation and... 

Fig. 9. Carbon-gold replica of the fibrillar structure of a low temperature craze in PP (due to the high tilting angle, the orientation of the fibrils is not perpendicular to the craze edges)...

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