Crystalline Fibre

Relation of Structure to Thermal and Mechanical Properties AMORPHOUS,GLASS-LIKE CRYSTALLINE, FIBRE-FORMING... 

The regular syndiotactic and isotactic structures are capable of crystallisation whereas the atactic polymer carmot normally do so. In the case of polypropylene the isotactic material is a crystalline fibre-forming material. It is also an important thermoplastic which can withstand boiling water for prolonged periods. Atactic polypropylene is a dead amorphous material. Polystyrene as commonly encountered is atactic and glass-like but the syndiotactic material... 

Whilst the crystalline fibres and their thermoplastic counterparts are no longer of importance, elastic polyurethane fibres, commonly known as spandex fibres, are of significance. These will be considered further in Section 27.4.1. 

If the orientation process in semi-crystalline fibres is carried out well below the melting point (Tm), the thread does not become thinner gradually, but rather suddenly, over a short distance the neck. The so-called draw ratio (A) is the ratio of the length of the drawn to that of the undrawn filament it is about 4-5 for many polymers, but may be as high as 40 for linear polyolefins and as low as 2 in the case of regenerated cellulose. 

Although initially the model has been developed for well-oriented para-crystalline fibres, experimental results have shown that it can also be applied to well-oriented semicrystalline fibres. 

TABLE 19.11 Physical properties of para-crystalline fibres in comparison with other reinforcing materials... 

Molecular anchorages are of two main types crystalline regions and diemical cross-links (to which physical entanglements may also contribute in polym networks). Not surprisingly, then, most ESR studies of polymers under tensile stress have been carried out using crystalline fibres (nylon, PET, PE, PP for example) or cross-linked polymers ... 

The most extensive ESR studies of polymers under tensile load have undoubtedly been carried out on drawn crystalline fibres, and this work has been reviewed recently by Kausch and De Vries It is clear that the morpholines of oriented crystalline fibres, with extensive tie-chain populations and hn d rees of molecular uncoiling, strongly favour the incidence of molecular hracture under tensile stress. 

Highly drawn crystalline fibres produce abundant radicals at strains exceeding about 8% and rising to the breaking strain of, say, 15 to 20%. Such fibres of course have already undergone draw ratios of 500% or more from the isotropic condition. 

It is not at all certain that the void formation and crazing discussed in the previous paragra di is directly associated with molecular fracture in the manner proposed by Zhurkov for crystalline fibres and films. ESR s als are certainly produced simultaneously with visible crazing in cross-linked glasses, and the chemical nature of the radicals observed is affected by the penetration of environmental gasses. Further-... 

More work is required in order to clarify the molecular structure of these fascinating molecular assemblies which seem to be on the borderline between fluid and solid micellar rods. Their formation develops through a certain type of precipitation, also typical for solid micellar fibres. However, the binding forces between the head group molecules (tetraalkylammonium and phenol) are weak, meaning that the fibres are not as stiff and uniform as the crystalline fibres described later. Aqueous suspensions of the described fibres dissolve massive amounts of small hydrocarbon molecules, e.g. 20 mol % of hexane, but the dissolving of hydrophobic porphyrins in them has not yet been achieved. 

These fibres are solid-like and should not be confused with the fluid myelin figures and their helical precursors obtained upon the swelling of lecithin crystals (see Figure 5.1). The fluid structures flow and change their shape and width constantly, whereas the solid types simply widen after addition of more material. Once a crystalline fibre is formed it adds material to the highly curved edges, much less to the more planar bilayer surfaces (Figure 5.7). 

The properties of elastomeric materials are greatly influenced by the strong inter-chain, i.e., intermolecular forces which can result in the formation of crystalline domain. Thus the elastomeric properties are those of an amorphous material having weak inter-chain interaction and hence no crystallisation. At the other extreme of polymer properties are fibre-forming polymers, such as Nylon, which when properly oriented lead to the formation of permanent crystalline fibres. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonate, etc. 

The alkaline hydrolysis of PET involves treating the polyester with an aqueous solution of sodium hydroxide (4-20 wt%) under pressure at temperatures between 200 and 250 °C for periods of several hours.53,54 Under these conditions the sodium salt of TPA is formed and by acidification TPA is recovered from the solution as a precipitate. It has been observed that the rate of the PET alkaline hydrolysis increases in the presence of quaternary ammonium compounds. Thus, Niu et al.55 have reported on the alkaline degradation of PET fibres with addition of dodecylbenzyldimethylammonium chloride (DBDMAC) into the reaction mixture. A sharp increase in the PET hydrolytic degradation at 80 °C was observed with DBDMAC concentrations in the range 0-1.0 g/1, especially for the least crystalline fibres. The authors concluded that the rate enhancement by quaternary ammonium compounds occurs preferentially on the amorphous regions of the PET fibres. 

In addition to this, as mentioned in the previous s tion, annual exposure measurements for certain hazardous substances including lead, cadmium, quartz, styrene, ethylene oxide, propylene oxide and synthetic inor mic crystalline fibres are stipulated by law and such measurements must be reported to Ihe Work Environment In ectorate. 

The molecular structure imparts cellulose with its characteristic properties such as hydrophilicity, chirality, degradability, and broad chemical variability initiated by the high donor reactivity of the OH groups. It is also the basis of the extensive hydrogen bond networks, which give cellulose a multitude of partially crystalline fibre structures and morphologies. 

In 1956 Thompson and Woods reported that dynamic experiments in extension indicated that orientation increased the temperature of the p transition, about 80°C, for oriented crystalline fibres, and reduced the drop in modulus occurring at higher temperatures. Subsequently nuclear magnetic resonance was used to demonstrate that orientation reduced molecular mobility above the glass transition temperature. Measurements of dynamic extensional and torsional moduli of hot stretched filaments and films were reported in 1963 by Pinnock and Ward, who found that the relations between measured compliances below the glass transition temperature were consistent with the deformation of an incompressible elastic solid. 

The mechanisms of formation of this carbon-rich layer at the fibre-matrix interfaces have been studied [36,71,75,77]. It has been proposed that the carbon layer is the result of a fortunate combination of silicate matrix chemistry and non-stoichiometric/non-crystalline fibre stmcture [71], The solid-state reaction between the SiC in the fibre and oxygen from the glass and the fibre surface can be written as ... 

The most utilized PAI congeners, namely B-PEI and L-PEI, have quite different solubility behaviour. B-PEI is soluble in water, independent of solution pH, and various organic solvents, while L-PEI in its free base form is insoluble in water and most organic solvents at room temperature, except lower alcohols, due to the formation of insoluble L-PEI crystals. Aqueous solutions of the L-PEI freebase also display temperature-dependent solubility behaviour as it becomes soluble in water above 64 °C. FTIR spectroscopy has confirmed that this phase transition is due to a melting transition from a crystalline zig-zag state to the hydrated random coil state.When cooled from the heated soluble state, the polymer forms a crystalline fibre-based hydrogel, which can be chemically crosslinked with glutaric anhydride. ... 

Virtually all the natural manifestations of cellulose are in the form of semi-crystalline fibres whose morphology and aspect ratio can vary greatly from species to species, as shown in Fig. 1.2. The subunits of each individual fibre are the microfibrils which in turn are made up of highly regular macromolecular strands bearing the cellobi-ose monomer unit, as shown in Fig. 1.3. 

Another approach has called upon the coagulation of amorphous cellulose around semi-crystalline fibres (i.e. the preparation of an all-cellulose composite) [19]. 

Northolt, M.G. and van der Hout, R. (1985) Elastic extension of an oriented crystalline fibre. Polymer, 26 310-316. 

As indicated earlier in this chapter, although diisocyanates are the intermediates responsible for chain extension and the formation of urethane links or a variety of crosslinks by further reaction, much of the ultimate polymer structure is dependent upon the nature of the components carrying the groups with which the isocyanates react initially. Such components can be simple acu-diols, such as were employed in early work on linear polyurethanes, giving polymers linked only by —NHCOO—. Examples of such polymers are listed in Table 1.2. Linear polyurethanes of this type are crystalline, fibre-forming polymers but are lower melting than the corresponding polyamides, and none has become of real importance either as a synthetic fibre or as a thermoplastic material. 

Carbon fibres Potassium titanate Silicon carbide whiskers Other inorganic crystalline fibres... 

---