Crystallinity bands

Figure 5 Raman spectra of orthorhombic ethylene 1-hexene copolymer with band fitting. The crystalline band at 1,416 cm-1, and amorphous bands at 1,303 cm- and 1,080 cm- are used to compute the crystallinity content ac = 0.52, and the amorphous content aa = 0.42. (See Color Plate Section at the end of this book.)...

A quenched low-crystalline PTT often cold-crystallizes when it is heated to above its Tt (see Figure 5, curve (b)). Bulkin et al. [57, 58] found that PTT cold-crystallizes at a much faster rate than PET by following the increase in the PTT crystalline band at 1220 cm-1 using rapid-scanning Raman spectroscopy. 

In some cases crystalline polymers show additional absorption bands in the infrared spectrum, as in polyethylene ( crystalline band at 730 cm amorphous band at 1300 cm" ) and polystyrene (bands at 982,1318, and 1368 cm" ). By determining the intensity of these bands it is possible to follow in a simple way the changes of degree of crystallinity caused, for example, by heating or by changes in the conditions of preparation. 

In the high trans-1,4 polymers (Figure 2) an additional change is the destruction of the crystallinity bands at 775 and 1055 cm."1. Since the absorbance showed a different dependence on concentration in the crystalline and amorphous (emulsion and sodium) polymers, this reduction in crystallinity precluded determinations of radiation yields in the high trans-1,4 specimens. However, maximum and minimum limits could be set on the yields by using the molar absorptivity of the crystalline polymer... 

Rubberlike Properties. Figure 2 depicts the changes in the 13.7-micron infrared crystallinity band with increasing ethyl acrylate content. At 25-30% acrylate content in the copolymer, this band disappears, indicating that this polymer is essentially amorphous. This fact, plus the absence of carbon-carbon unsaturation, good filler compatibility, and... 

Figure 2. Effect of acrylate content on 13.7 p (crystallinity) band intensity...

The ir spectrum of all these copolymers in the solid state shows the disappearance of the crystallinity bands for molar concentrations of the predominant comonomer below 70%. This is due to the marked lack of sequential order along each chain. The melting temperatures vary almost linearly with the molar content of one of the comonomers in the case of complete isomorphism. Otherwise, a linear relationship still holds provided that one supposes to consider the melting temperature of a metastable crystalline phase. 

I. R. spectra of polymers of optically active and racemic monomers (12) having similar stereoregularity are identical in the case of poly-5-methyl-l-heptene, but slightly different in the case of poly-3-methyl-l-pentene and poly-4-methyl- 1-hexene. A very characteristic crystallinity band has been found in the I. R. spectrum of poly-5-methyl-l-heptene at 12.06 fi bands which seem connected with stereoregularity have been found in the I. R, spectrum of poly-4-methyl-l-hexene at 10.06 fi the nature of these bands has been proved when preparing a practically atactic sample by hydrogenation of poly-4-methyl-l-hexyne (24). 

Investigations were made on the I. R. spectra of poly-[(S)-l-methyl-propylj-vinyl-ether and poly-[(S)-2-methyl-butyl]-vinyl-ether (65) a band at 911 cm-1 related to stereoregularity was detected in the spectrum of the former while a crystallinity band at 827 cm-1 was found in the latter. The spectra of polymers obtained from an optically active or a racemic monomer did not reveal any remarkable difference. 

Saegusa, et al. (98) report the infrared spectrum of crystalline and amorphous PTHF (Fig. 34). The absorption band at 1000 cm-1 was designated as one of the crystalline bands of PTHF. An NMR spectrum of PTHF in THF is shown by Kuntz (44). 

Raman spectroscopy has been widely used to study the composition and molecular structure of polymers [100, 101, 102, 103, 104]. Assessment of conformation, tacticity, orientation, chain bonds and crystallinity bands are quite well established. However, some difficulties have been found when analysing Raman data since the band intensities depend upon several factors, such as laser power and sample and instrument alignment, which are not dependent on the sample chemical properties. Raman spectra may show a non-linear base line to fluorescence (or incandescence in near infrared excited Raman spectra). Fluorescence is a strong light emission, which interferes with or totally swaps the weak Raman signal. It is therefore necessary to remove the effects of these variables. Several methods and mathematical artefacts have been used in order to remove the effects of fluorescence on the spectra [105, 106, 107]. 

The evidence from wide angle x-ray scattering (WAXS), differential scanning calorimetry (DSC), and IR spectroscopy (IR) shows that both polymers crystallize separately according to their own unit cell structure. The WAXS diffraction lines of each component are present in the blends no new bands appear (12,13,14). By DSC one observes the melting peaks corresponding to each polymer (Figure 1), and IR shows the typical characteristic crystalline bands of the pure polymers in the blends. The IR spectra of the blend can essentially be accounted for as the sum of the spectra of the components. 

The intensity of the crystalline bands was monitored simultaneously during the crystallization. To correct for changes in density or thickness in the different samples, the intensities were normalized by the reference band. These normalized intensities were plotted vs. log time for each of the blends at the different crystallization temperatures. The curves obtained are sigmoidal in nature and they level off when the final crystallinity is achieved. A typical curve for the normalized intensity of the 848-cm-1 band vs. log time is plotted in Figure 7 for PET. 

The intensity for the PET and PBT crystalline bands in the blends must be corrected to take into account the contribution to these bands of the other component and the dilution effect also caused by it. The corrected intensity reflects the change in crystallinity of the PET and PBT phases individually, based upon the weight of that phase. In other words, the corrected intensity would be the observed intensity if the sample had only one component, and it would have crystallized at the same rate and manner as it did in the blend. 

For the pure polymers, since the values of the IR crystalline bands and the density are known, a linear correlation can be established between the intensity of the crystalline band for each crystallizing polymer and its degree of crystallinity obtained from the density measurements. By assuming that the same relationship exists in the blends between the IR intensity and the degree of crystallinity, the partial degrees of crystallinity of each component in the blends are obtained. 

In conclusion, the deformation behavior of poly(hexamethylene sebacate), HMS, can be altered from ductile to brittle by variation of crystallization conditions without significant variation of percent crystallinity. Banded and nonbanded spherulitic morphology samples crystallized at 52°C and 60°C fail at a strain of 0.01 in./in. whereas ice-water-quenched HMS does not fail at a strain of 1.40 in./in. The change in deformation behavior is attributed primarily to an increased population of tie molecules and/or tie fibrils with decreasing crystallization temperature which is related to variation of lamellar and spherulitic dimensions. This ductile-brittle transformation is not caused by volume or enthalpy relaxation as reported for glassy amorphous polymers. Nor is a series of molecular weights, temperatures, strain rates, etc. required to observe this transition. Also, the quenched HMS is transformed from the normal creamy white opaque appearance of HMS to a translucent appearance after deformation. 

The model in Fig. 3.2 is sufficient to predict the general features of N E), but much more detailed calculations are needed to obtain an accurate density of states distribution. Present theories are not yet as accurate as the corresponding results for the crystalline band structure. The lack of structural periodicity complicates the calculations, which are instead based on specific structural models containing a cluster of atoms. A small cluster gives a tractable numerical computation, but a large fraction of the atoms are at the edge of the cluster and so are not properly representative of the real structure. Large clusters reduce the problem of surface atoms, but rapidly become intractable to calculate. There are various ways to terminate a cluster which ease the problem. For example, a periodic array of clusters can be constructed or a cluster can be terminated with a Bethe lattice. Both approaches are chosen for their ease of calculation, but correspond to structures which deviate from the actual a-Si H network. 

The Raman spectra For 1-10% 205/, -Al203 are shown in Figure 5. Crystalline bands appear at 13% V205/, -... 

Polymerization of Cyclic Ethers and Formats by Poly-THF Dioxolenium Salt. The polymerization of cyclic ethers and formals by PTHF-dioxolenium salt was carried out to clarify the presence of termination or transfer reactions. The results are shown in Table IV. In the polymerization of 3,3-bischloromethyloxetane (BCMO), block copolymer soluble in chloroform and having the expected molecular weight was formed the homopolymer of BCMO insoluble in chloroform was not observed. The block copolymer showed crystalline bands of BCMO at 700, 860, and 890 cm 1, suggesting the formation of ABA block. 

Figure 5.1 (a) DSC thermograms and (b) the temperature dependence of infrared absorbance estimated for the D and H infrared crystalline bands in the cooling process from the melt measured for a series of DHDPE/LLDPE(2) blend samples. The starting temperature of crystallization is slightly different between these two curves probably because of the difference in the cooling rate, the monitoring point of temperature, and so on. But essential behavior is the same. 

The intensities of the vinyl absorption bands increase as the temperature is lowered. The crystalline bands at 1050 and 1176 cm exhibit a much narrower bandwidth at lower temperatures. Only slight changes in the amorphous bands are observed with temperatme. However, differences between slow-crystallized and quenched samples are apparent... 

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