Nylon melting peak

An initial run on a 76 ug sample of Nylon 6, 6 yarn (seen in fig. 14) shows a processing effect at about 210°C, the same temperature at which this material displayed a break in the TMA extension curve. After shock cooling, the processing mark has been erased and the cold recrystallization peak appears--an indication that the glass transition is about 25°C lower. Figure 15 shows the effect of program-cooling and the characteristic nylon double peak in the subsequent melt (9). 

Changes in the crystallization behavior of the thermoplastic phase of the IPN have been observed in DSC (differential scanning calorimetry) experiments (i3). In Figure 7, a series of DSC scans on silicone-nylon 6,6 composites of various silicone contents are presented. The characteristic melt peak of the nylon matrix is observed in all of the thermograms, but a second, slightly lower temperature melt peak becomes more pronounced as the concentration or the cross-link density of silicone polymer in the IPN is... 

DSC curves can also be used to identify individual polymers in a polymer mixture. This is a rather unique capability of DSC or DTA. Figure 10.17 shows DSC curves of plastic waste. The individual melting peaks reveal the waste s polymer content. By comparing these curves with DSC curves of pure polymers, we can assign the melting peaks as those of low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), Nylon-6, Nylon-66 and polytetrafluoroethylene (PIPE). 

Multiple melting peaks are observed when the polymer exhibits polymorphism like nylon 6, 6 and isotactic poly(propylene) (/-PP). 

Todoki M, Kawaguchi Y (1977) Origin of Double Melting Peaks in Drawn Nylon 6 Yams. J Polymer Sci, Polymer Phys Ed 15 1067-1075. 

Fig. 10.6 Thermograms of heating curves of Nylon 6 fabrics illustrating effect of fiber presentation on multiple melting peaks, (a) short cut fibers, (b) textile fabric disk,...

Thin films were obtiincd by melt-pressing at temperatures of 30 K above their melting points and then slowly cooled or quenched in cold water. Thble 5 lists the thermal properties and remanent polarization for nylon-MXDs. Nylon-MXD-13 showed no glass transition temperature, but peaks for both crystallization and melting. The quenched ny-lon-MXD-9 showed no exothermic crystalltzalion. but T, and a dispersed melting peak. 

The dispened melting peaks in nykm-MXO>7 and nylon MXD-9 indkaied vety low crystallinity. The quenched sample of nyloo-MXD-6. nykm-MXD, nylon-MXD-10. oyhm-MXD ll, and nylon-MXD>13 weie found to be either amorphous or with very 1 crystallinity. As slowly cooled samples of these nylons showed no crystallization peak, they may have higher crystallinity. 

The mixture of oligomers was heated in a flask to 200°C in a mineral bath (the DSC peak melting temperature of the oligomers was 230°C) while stirring and removing water formed with a vacuum pump to form high-molecular-weight nylon-6,6. 

White (1955) believes that differential thermal analysis curves obtained by him on heating drawn fibers of 6, 6-6 and 6-10 nylons as well as polyethylene terephthalate, indicate that at first crystallites disorient and then melt. He found two peaks in what would correspond to a specific heat-temperature curve. However, it is difficult to understand why the disorientation of oriented crystals would involve the absorption of energy. 

The double-melt endotherms of the silicone-nylon 6,6 IPN do not disappear with thermal cycling. Instead the peaks are further resolved in the DSC when samples are scanned to 50 °C above the melt point, cooled slowly to room temperature, and rescanned (Figures 7 and 8). The stability of the melt endotherm associated with the presence of IPN suggests the presence of an interpenetrated polymer phase of various crystalline structures. 

An additional major transition which is observed in DSC scans of stratum comeum is a doublet endotherm which peaks at 194 °C and 210°C in dry samples and at 120°-130°C in wet samples (Figure 14). These transitions are also characteristic of the more extensively investigated keratin-containing wool (49, 65). Polyamides such as the various nylons also show melting endotherms above 200 °C (63). 

Figure 12.3(a, b) shows expanded CP/MAS and PST/MAS NMR spectra as functions of the CH2 carbons in the nylon 4 melt-quenched sample, which contains a larger noncrystalline fraction compared with the single crystal samples. In the CP/MAS spectra, the noncrystalline jS-CH2 and W-CH2 peaks are observed with a weak intensity, but in the PST/MAS spectra their peak intensities increased drastically. This leads to the correct... 

CP/MAS and PST/MAS NMR spectra of nylon 6 single crystals sample, melt quenched sample, and drawn sample at room temperature (the carbonyl peak is not shown because a single sharp line appears without significant change for all samples). The lineshapes of the CHa peaks depend on the crystallization conditions, and, furthermore, those for the same sample obtained by CP/MAS and PST/MAS are markedly different. It is noted that the PST/MAS method, in contrast to the CP/MAS method, enhances the peak intensity for CHa carbons, such as those in the noncrystalline state, which undergo relatively rapid reorientation. Therefore, a comparison of C CP/MAS and PST/MAS spectra leads to a discussion of the structure and dynamics of the crystalline and noncrystalline states in nylon 6 sample crystallized under various conditions. The C chemical shifts are listed in Table... 

Figure 12.5(b) shows expanded PST/MAS NMR spectra of the CH2 carbons in nylon 66 melt-quenched sample as a function of temperature. The aN(n)CH2 peak is more intense than the nCH2 peak, which appears as a small shoulder on the aN(n)CH2 in the CP/MAS spectrum. At 20°C, a peak appears between the, yiM)CH2 and )8cCH2 peaks. This peak moves to low frequency as the temperature is increased and overlaps with the i3cCH2 peak. This peak is assigned to the noncrystalline 7nCH2 carbon. Figure 12.5(c) shows the expanded LD/MAS NMR spectra of the nylon 66 sample at 20, 60 and 100°C. In the spectrum no peak is observed at 20°C, and very broad peaks appear at 60°C. At 100°C, five CH2 peaks and a carbonyl peak appear clearly due to fast molecular motion (T = 50°C). The chemical shifts are listed in Table 12.3. The carbonyl chemical shift... 

Mathias et al. have studied the structure of polyamides, such as nylon 6, 11, 12, etc., in the solid state by solid-state NMR at the natural abundance level [17-19]. Figures 12.8-12.10 show typical CP/MAS NMR spectra of nylon samples in the solid state. In Fig. 12.8, the spectrum A was observed for a nylon 6, 10 sample that had been melt-pressed into a clear thin film and annealed to promote formation of the stable a-crystal form. The peak at 83.8 ppm is relatively sharp with a peak width at half-height of 3.2 ppm. The spectrum B is for a nylon 12 sample with the y-form. The peak is located at 88.7 ppm with a peak width of 4.1 ppm. 

A mat of nylon 6 crystals from 0.05% glycerol solution exhibits (42) loss maxima at 150°, 230°, and 410°K. (102 c.p.s.). The lower two maxima are similar to those found for melt-formed samples. The major maximum at 410°K. is 60° higher than the major loss peak for melt-formed samples and is therefore believed to be caused by a different mechanism. Other nylon 6 samples show (42) a loss modulus shoulder at 470°K. (102 c.p.s.) near the melting point, and therefore the motion responsible is believed to take place in the crystalline regions, while the 410°K. peak is ascribed to motion in a metastable crystalline phase made up of a mixture of the a and 7 forms (42). 

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