Moduli of as-spun fibers

The liquid crystallinity and the moduli of as-spun fibers of polyarylates from substituted HQs and substituted PECs could be controlled by the substituents shown in Table 19.1. Polyarylates derived from /-butyl HQ (fBu-HQ) and PEC (fBu-HQ/PEC), and phenyl-HQ (Ph-HQ) and PEC (Ph-HQ/PEC), showed decreased liquid crystallinity when compared to chloro-HQ (Cl-HQ) and PEC (Cl-HQ/PEC), and methyl-HQ (Me-HQ) and PEC (Me-HQ/PEC), because of the bulky substituents on the HQ units. 

Therefore, the moduli of as-spun fibers of fBu-HQ/PEC and Ph-HQ/PEC were lower than those of Cl-HQ/PEC and Me-HQ/PEC [18, 22, 23], Tensile-fractured as-spun fibers of f Bu-HQ/PEC and Ph-HQ/PEC exhibited decreased orientation of fibrils when compared to Cl-HQ/PEC and Me-HQ/PEC. 

Polyarylates prepared from cyclohexyl-HQ (Ch-HQ) and PEC (Ch-HQ/PEC) did not show liquid crystallinity due to the more bulky substituent on the HQ unit compared to those on f Bu-HQ and Ph-HQ. As-spun fibers of Ch-HQ/PEC exhibited lower moduli than those of fBu-HQ/PEC and Ph-HQ/PEC. Therefore, in order to obtain high-modulus as-spun fibers, the stability of the liquid crystalline state (7j — 7j,) is an influential factor, as shown in Table 19.1. 

On the other hand, the moduli of as-spun fibers of polyarylates from Cl-HQ and l,2-bis(2-chlorophenoxy)ethane-4,4/-dicarboxylic acid (Cl-PEC) (Cl-HQ/ Cl-PEC), and Me-HQ and Cl-PEC (Me-HQ/Cl-PEC), were higher than those of Cl-HQ/PEC and Me-HQ/PEC. The reason for the higher moduli seemed to be the increased rigidity of the polymer chain caused by the restricted rotation of the ether linkage of Cl-PEC as a result of the steric hindrance of the Cl atoms. However, the moduli of polyarylates from fBu-HQ and Cl-PEC (fBu-HQ/Cl-PEC), and Ph-HQ and Cl-PEC (Ph-HQ/Cl-PEC), were lower than those of fBu-HQ/PEC and Ph-HQ/PEC, because the Cl atoms on the PEC units prevent 

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Figure 19.6 shows the relationship between the moduli of as-spun fibers and the F-values of substituted HQs/BB and substituted HQs/BB modified with... 

Table 19.4 Thermal properties and moduli of as-spun fibers and injection molded specimens of Ph-HQ/BB and Ph-HQ/HQ/BB (m/n = 50/50) polyarylates [19] From Inoue, T. and Tabata., N., Mol. Cryst. Liq. Cryst, 254, 417-428 (1994), and reproduced with permission of Gordon and Breach (Taylor and Francis) Publishers...

The moduli of as-spun fibers of polyarylates depend highly upon the stability of the liquid crystalline state, the rigidity of the polymer chain, and the degree of elongational flow orientation (F-value). On the other hand, the moduli of injection molded specimens of polyarylates depend upon the stability of the liquid crystalline state, and the rigidity and packing density of the polymer chain. 

The moduli of as-spun fibers of Me-HQ/Cl-PEC or Cl-HQ/Cl-PEC were 72 and 68 GPa,respectively which were higher than those of Me-HQ/PEC or Cl-HQ/PEC.The reason of higher modulus appeared to be the increased rigidity of polymer chain caused by the restricted rotation of the ether-linkage of Cl-PEC due to the steric hindrance as shown in Fig. 2(1,5).Ph-HQ/... 

Figure 19.6 Moduli and F-values of as-spun fibers O, Me-HQ/BB and copol-yarylates , copolyarylates of CI-HQ/BB <>, Ph-HQ/BB and copolyarylates x, CP-HQ/BB [31], From Inoue, T., Tabata, N. and Yamanaka, T., Polym. J., 28, 424-431 (1996), and reproduced with permission of The Society of Polymer Science, Japan...

The reason for the lower liquid crystalline (LC) temperature of the BB model compound seems to be that the biphenyl unit of this compound was adopting a twisted structure in the LC state [26], Therefore, we prepared various polyarylates containing the BB unit and determined their thermal properties and the moduli of the as-spun fibers, as shown in Table 19.3. 

Although as-spun fibers of Me-HQ/BB exhibited a lower modulus than those of Me-HQ/Cl-PEC, injection molded specimens of Me-HQ/BB exhibited a higher flexural modulus than those of Me-HQ/Cl-PEC due to the higher rigidity of the polymer chain, in spite of the lower / -value. Flexural-fractured injection molded specimens of Me-HQ/BB exhibited fewer fibrils than Me-HQ/Cl-PEC due to the lower / "-value, as shown in Figure 19.7. Thus, there seemed to be no relationship between the moduli of injection molded specimens and the E-values. 

Although as-spun fiber of Me-HQ/Cl-PEC showed high modulus of 72 GPa,the modulus of the injection molded test pieces of Me-HQ/Cl-PEC was only 26 GPa.Ph-HQ/Cl-PEC and tBu-HQ/Cl-PEC showed no liquid crystallinity,but the moduli of the injection molded test were 7.5 and 15.1 GPa,respectively,which were 2-5 times higher than that of isotropic polymers.tBu-HQ/Cl-PEC showed many fibrils on the cross section of the flexural fractured test pieces as shown in Fig. 6(2).We assumed that tBu-HQ/Cl-PEC was a quasi liquid crystalline polymer which could orient at the high shear rate.Although W.B.Black reported a lyotropic quasi liquid crystalline polymer which could orient at the high shear rate,tBu-HQ/Cl-PEC is the first example of the thermotropic quasi liquid crystalline polymer(6). 

Bianchi et al. (19) spun fibers from isotropic and anisotropic solutions of cellulose (D.P. 290) in LiCl (7.8%)-DMAC solutions. The fiber mechanical properties increased throimh the isotropic-anisotropic transition with elastic moduli as high as 22 GPa (161 g/d) being obtained. 

The additional chain orientation in the direction of the fiber long axis, obtained from the nematic self-ordering in the system, leads to a dramatic enhancement of the mechanical properties of the polymer. A number of aromatic polyamides have thus achieved commercial importance because of the very high tensile strengths and moduli of the fibers that can be spun from the nematic solutions. These have consequently become attractive alternatives to metal or carbon fiber for use in composites as reinforcing material. The most significant of these aramid fibers are ... 

Polyim-phenylene isophthalamide) forms lyotropic liquid crystals. Fibers can therefore be spun at lower concentration from isotropic solutions and at higher concentrations from nematic solutions. Because of their high degree of chain segment orientation, fibers spun from nematic solutions show, as expected, higher moduli of elasticity, tensile strengths and lower elongation at rupture values than those spun from isotropic solutions. 

The aminohydrazide residue in round brackets may be partly arranged in reverse, so that the polymers are referred to commercially as partly ordered poly(amide hydrazides). The polymers can be spun to fibers of exceptionally high moduli of elasticity, to what are known as high-modulus fibers, and these are used to reinforce plastics and as tire cord. 

As-spun PBT fibers reach static moduli of about 190G Nm" and a tenacity of 2.1 GNm [184], Heat-treatments at temperatures between 630 and 680 °C with tensions in the range of 0.1 to 0.2 GNm" produce fibers with moduli up to 335 G Nm and tenacities up to 3.9 G Nm" [184, 185]. The largest reported values for as-spun PBO fibers are 179GNm and 4.6GNm , whereas a heat-treatment yielded a modulus of 345 G Nm" and a tenacity of 5.8 G Nm [184]. Aramid fibers differ in this respect, since only the modulus can be substantially increased by a heat-treatment the maximum tenacity values for both as-spun and heat-treated filaments, measured for a testlength of 25 mm, are close to 5 G Nm ... 

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