Semi-crystalline polymers fibers

Many industrial semi-crystalline polymer materials like polypropylene, polyamides, or polyesters contain nucleating agents or clarifiers which form needle-shaped aggregates already in the polymer melt. "For this purpose the pattern is desmeared using the measured primary beam. For a less involved treatment it may be sufficient to know the integral width of the primary beam profile in fiber direction. 

Poly(vinyl) alcohol (PVA) is a semi-crystalline polymer, which is already widely used for various applications, either under the form of films or fibers. Compared to other polymers, as it is water-soluble at high temperature, it is easy to process from aqueous solutions. Carbon nanotubes can also be dispersed or solubilized in water via different functionalization approaches. It was quite natural for researchers to try to mix carbon nanotubes and PVA to improve the properties of the neat polymer. In this chapter, we will first examine the different methods that have been used to process CNT/PVA composites. The structures and the particular interaction between the polymer and the nanotube surface have been characterized in several works. Then we will consider the composite mechanical properties, which have been extensively investigated in the literature. Despite the number of publications in the field, we will see that a lot of work is still to be done for achieving the most of the exceptional reinforcement potential of carbon nanotubes. 

This semi-crystalline polymer belongs to the family of polyether ketones, Tg= 145 C Tm = 235 C. Developed in 1980 by ICI, it is a superb engineering polymer, showing excellent mechanical properties that are retained at elevated temperatures. Due to its extremely high price, its utility is still limited to the field of aviation and space (reinforced with carbon fibers), electronics and machinery. Another advantage is its stability towards fire or chemicals, although it is sensitive to UV radiation. 

Also typical of fibers made from lyotropic polymers is the fact that the hysteresis observed during cyclic loading is very small compared with that of fibers of semi-crystalline polymers. The dissipated energy relative to the stored energy is 7% for PpPTA fibers with moduli up to 140 G Nm andl2% for the experimental PBO fiber with a modulus of 150 G Nm , while for a well-oriented poly (ethylene terephthalate) fiber (tire yarn) with a modulus of 18 GNm" this ratio is 47%. 

The morphology of PEEK has been described by Cogswell [98]. The crystalline structure in semi-crystalline polymers depends on the thermal history such as cooling rate, stress on the system, nucleation sites, as well as molecular structure and mobility of the molecule. Not all the nucleation sites will necessarily be within the resin phase. Heterogeneities can act as nucleation sites and these include fiber dust, gel particles, etc. Hobbs [99] believes that active sites account for most of the nucleation. 

Next section briefly reviews the main concepts and formulations for the analysis of SAXS patterns of semi-crystalline polymers with fiber symmetry. 

A number of well known methods are available for the determination of crystallinity in semi-crystalline polymers. However, most of these methods are not amenable for in-spin line crystallinity measurements. A vibrational spectroscopic technique like Raman offers several distinct advantages for crystallinity measurements in the spinline (1). A calibration curve for propylene cyrstallinity was developed offline, using fibers spun, under different processing conditions, from several homo-polypropylene (hPP) and propylene-ethylene copolymers (with 5-15%E). This calibration curve was subsequently used to predict the polypropylene crystallinity, in the spin line, as a function of distance from the spinneret. The calibration model correlates the normalized intensity of the 809 cm Raman band with the DSC measured crystallinity and covers a wide crystallinity range (15-67%) with an R value of 0.989. 

Friedrich, K. and Karger-Kocsis, J. (1989). Unfilled and short fiber reinforced semi-crystalline thermoplastics. In Fractography and Failure Mechanisms of Polymers and Composites, (A.C. Roulin-Moloney ed.), Elsevier Appl. Science, London, pp. 437-494. 

Hoeve, C. A. J.., and A. Ciferri Limitations of the application to semi-crystalline fibers of thermoelastic relations for high elastic materials A reply to W. Prins. J. Polymer Sci. 60, 68 (1962). 

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