Oriented polyamide fibers

Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. 

Applications. The polyamides have important appHcations. The very high degree of polymer orientation that is achieved when Hquid crystalline solutions are extmded imparts exceptionally high strengths and moduli to polyamide fibers and films. Du Pont markets such polymers, eg, Kevlar, and Monsanto has a similar product, eg, X-500, which consists of polyamide and hydra2ide-type polymers (31) (see High performance fibers Polyamides, fibers). 

High performance polymer fibers (HPPF) have excellent mechanical properties compared to traditional textile fibers such as nylon. The typical HPPFs are aramid and polyethylene fibers (6). Aramid is a generic name for a class of aromatic polyamide fibers, most of which are varieties of poly(p-pheny-lene terephthalamide). Kevlar is the trade name of the varieties of aramid polymers introduced conunercially by Dupont. The molecules in the fibers of these materials are oriented in the axial direction. Poly(p-phenylene terephthalamide) is a rigid molecule with the following structure ... 

The peaks A and B in both spectra clearly originate from the noncrystalline domain in the samples, indicating that the noncrystalline domain reported from X-ray diffraction has a relatively ordered structure rather than a randomly distributed one. This conclusion derived from NMR is also supported by the conclusion that the noncrystalline domain is highly oriented in aromatic polyamides on the basis of X-ray diffraction studies [30, 32], The relatively broad peaks show a wider distribution for each 0nh value compared with peak C, which is also reasonable. The chemical shieldings of the peaks A and B are almost the same between PMIA and P4M-MPTA. This indicates that the local structure in the noncrystalline domain is similar for these polyamide fibers. It has been reported that the fraction of noncrystalline domain in the P4M-MPTA sample is higher than in the PMIA sample. The increase in the fraction of the noncrystalline domain comes predominantly from the contribution of peak A, i.e., the structure with 0nh = 31-42°, which is derived from the solid-state NMR experiment. 

The structure of polyamide fibers is defined by both chemical and physical parameters. The chemical parameters are related mainly to the constitution of the polyamide molecule and are concerned primarily with its monomeric units, end-groups, and molecular weight. The physical parameters are related essentially to chain conformation, orientation of both polymer molecule segments and aggregates, and to crystallinity. 

The degree of crystallinity of polyamide fibers may be estimated from density determinations, calorimetric measurements, and infrared and x-ray data. Although not an absolute method, assessment of the degree of crystallinity from the density is a very facile, rapid, and precise procedure. It is independent of orientation or geometry of the sample, but requires dry samples that are free of voids and pigments. This method is based on the assumption that the density p or its reciprocal value and the specific volume V are represented by Equation 2.85 and Equation 2.86, respectively. 

This mechanical model is depicted in Fig. 21. The Eqs. (15) and (17) have been confirmed for aromatic polyamide fibers by a variety of experiments. Figure 22 shows the dynamic compliance versus the orientation parameter measured during extension of medium and high-modulus PpPTA fibers. It confirms the linear relation (15) and yields e = 240 G Nm and go = 2 G Nm . It has been shown that the tensile curves of the second and higher extensions of an aramid fiber are well described by Eq. (17) [143]. A relation between the strain and the dynamic... 

As discussed previously para-aromatic polyamide fibers show a pleated sheet, which can also be described as a sinusoidal undulation of the fibrils. The effective orientation parameter determining the modulus of the fiber may then be approximated by... 

Crystallization in step-growth polymers such as polyesters and nylons is known to assist their subsequent solid-state polymerization because exclusion of reactive end-groups from crystalline domains enhances their effective concentration in the amorphous domains [14,15]. However, the condensation reaction between the last fraction of end-groups may be hindered by crystallization [16, 17]. The possibility and rate of crystallization can also be enhanced by processes that enhance orientation, such as shearing and fiber drawing [18]. For example, partial replacement of terephthalic units with isophthalic units in PET reduces crystallinity, so that no crystallization in seen in 70 30 random poly(ethylene terephthalate-co-ethylene iso-phthalate) under quiescent conditions. However, heating its amorphous fiber above its Tg under a moderate tensile force results in rapid stress-induced crystallization [19]. The reduction in crystallization by copolymerization has been employed to enhance drawability of melt-spun polyester and polyamide fibers [20]. 

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