Amorphous nylons development

With the growing demand for coextruded products, barrier plastics have shown significant growth in the last several years. Historically, the high barrier resins market has been dominated by three leading materials — vinylidene chloride (VDC) copolymers, ethylene vinyl alcohol (EVOH) copolymers, and nitrile resins. Since 1985, however, there has been a lot of interest worldwide in the development of moderate to intermediate barrier resins, as apparent from the introduction of a number of such resins, notably, aromatic nylon MXD-6 from Mitsubishi Gas Chemical Company, amorphous nylons SELAR PA by Du Pont and NovamidX21 by Mitsubishi Chemical Industries, polyacrylic-imide copolymer EXL (introduced earlier as XHTA) by Rohm and Haas and copolyester B010 by Mitsui/Owens-Illinois. 

Another amorphous nylon was developed at Emser Werke AG and is based on aliphatic, as well as cycloaliphatic, amines and terephthalic acid. It is marketed in Europe under the tradename Grilamid 55 and until recently was distributed in the United States by Union Carbide under the tradename Amidel. These amorphous nylons are not as tough as nylon 6 or 6,6, but they do offer transparency and good chemical resistance in some environments. 

Macosko et al. addressed this question in the case of model polystyrene (matrix)-amorphous nylon (dispersed phase) blends [45]. The morphology development at short mixing times can be summarized as follows ... 

Semicrystalline polymers, VDC copolymer and aromatic nylon MXD-6 (Table II) showed little if any reduction in permeability at these moderate orientation levels. In fact, recent unpublished work has shown that aromatic nylon MXD-6 exhibits an initial increase in permeability up to 3X orientation followed by a significant reduction in permeability at higher orientation levels. The VDC copolymer also showed higher permeability with moderate biaxial orientation — 1.5 times the permeability of the unoriented film. This is believed to be due to orientation of the polymer after crystallinity is fully developed. If the orientation of VDC copolymers is induced prior to full development of crystallinity in the material, one would not expect to see an increased oxygen permeability. In commercial practice, therefore, forming of VDC copolymer structures is normally done on rapidly quenched polymer to orient it while still in the amorphous state at temperatures near or above the Tm of VDC copolymer. 

Regardless of the water content or AK level, no DGB s have yet been observed in any nylon specimens (either Series A or Series B). By contrast, the discontinuous growth of fatigue cracks has been described in studies of both crystalline and amorphous polymers [for example, in polyacetal (29,30), polyethylene ( ), and a variety of poorly crystalline or amorphous polymers ( ) ]. While the process of DGB formation in poorly crystalline or amorphous polymers is fairly well understood ( 5, 12,29,32) the role of well developed crystallinity is not clear. Hence the reason why DGB bands are not observed in nylon 66 is as yet unknown. 

Blending even low percentages (20%) of Selar PA with nylon 6, nylon 66, and nylon copolymers will result in a product that behaves like an amorphous polymer. These blends retain all of the advantages of the Selar PA resin with some of the mechanical property advantages of semicrystalline nylon. DuPont has developed a special grade of Selar PA, known as 2072, which is specially designed for blending with ethylene vinyl alcohol copolymers... 

Kawai s (7) pioneering work almost thirty years ago in the area of piezoelectric polymers has led to the development of strong piezoelectric activity in polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene and tetrafluoroethylene. These semicrystalline fluoropolymers represent the state of the art in piezoelectric polymers. Research on the morphology (2-5), piezoelectric and pyroelectric properties (6-70), and applications of polyvinylidene fluoride 11-14) are widespread in the literature. More recently Scheinbeim et al. have demonstrated piezoelectric activity in a series of semicrystalline, odd numbered nylons (75-77). When examined relative to their glass transition tenq>erature, these nylons exhibit good piezoelectric properties (dai = 17 pCTN for Nylon 7) but have not been used commercially primarily due to the serious problem of moisture uptake. In order to render them piezoelectric, semicrystalline polymers must have a noncentrosynunetric crystalline phase. In the case of PVDF and nylon, these polar crystals cannot be grown from the melt. The polymer must be mechanically oriented to induce noncentrosynunetric crystals which are subsequently polarized by an electric field. In such systems the amorphous phase supports the crystalline orientation and polarization is stable up to the Curie temperature. 

It has been noted [11] that when unique vibrational bands can be associated with each phase, it is not necessary in principle to calibrate against other standards, although it is often desirable to do this. All that is required is a set of samples of widely varying composition for which the relative band intensities can be determined and extrapolated to the 100% and 0% crystallinity levels. Raman measurement of crystallinity in polyethylene illustrates this approach [229], as does the IR measurement of crystallinity of nylon [224]. Extrapolation of the spectroscopic calibration has sometimes been used to determine the amorphous and crystalline densities of a polymer [224, 225], thereby adding to the information available from the primary calibration standard. However, one must note that vibrational spectroscopy, DSC, X-ray and density actually measure different physical parameters and so a crystallinity determined solely by vibrational spectroscopy may well differ from that obtained by other techniques [230]. Also, with anisotropic materials, molecular orientation can alter band intensities and invalidate calibrations developed using isotropic standards [48, 227, 231]. 

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