Amorphous bands

Intermediate methods include the earliest procedure based on Stein s equation [33] and one based on Samuels equation [34]. Among the direct methods is an IR spectroscopic method based on the measurement of the dichroic ratio (R), of amorphous absorption bands. In the investigations [35], the amorphous bands 898 cm" and 1368 cm", for which the angles of transition moment are a898 = 39 and aneg = 80 , respectively, were used. Other methods are spectroscopy of polarized fluorescent radiation [35,36], measurement of color di-... 

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.)...

As another extracellular component in the cornea, the Bowman s layer is an acellular and amorphous band between the corneal epithelium and stroma. The layer is about 8-12 [im thick and consists of randomly arranged collagen fibers (types I and III) and proteoglycans. The physiological function of Bowman s layer is not yet completely understood, since not all animal species exhibit this membrane in the corneal structures, but an important role in the maintenance of the corneal epithelial structure is expected or probable, since a damaged Bowman s membrane usually results in scarring during wound repair [16],... 

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. 

Crystallinity In crystallization of polymers, the polymer forms crystalline and amorphous regions [2,4,25]. The formation of crystalline regions is accompanied by an increase in new vibrational modes caused by their crystal lattice interactions [2]. The IR spectrum of a given polymer differs by various absorption bands, depending on whether it is in the amorphous or crystalline state [2]. The IR spectrum exhibits regularity bands, splitting, and frequency shifts. Other absorption bands are not affected by crystallization and remain the same in both cases. Crystalline and amorphous bands can be used in the determination of the degree of crystallinity independent bands are useful for the determination of sample thickness [2],... 

Figure 20-3 Proposed structure of a molecule of amylopec-tin in a starch granule. The highly branched molecule lies within 9 nm thick layers, about 2 / 3 of which contains parallel double helices of the kind shown in Fig. 4-8 in a semicrystalline array. The branches are concentrated in the amorphous region.113 114 121 Some starch granules contain no amylose, hut it may constitute up to 30% by weight of the starch. It may he found in part in the amorphous bands and in part intertwined with the amylopectin.122...

A routine method for determining relative crystallinity based on the amorphous bands in the spectrum has proved more rapid and precise than the x-ray method. In practice, the ratio of the 778 cm-1 (12.85 ft) and 2367 cm-1 (4.22 ft) band intensities is measured. Use of a ratio eliminates the thickness measurement and increases precision to about 1% at 50% crystallinity and considerably better at higher levels. A density measurement and an infrared crystallinity determination when combined give an estimate of the fraction of microvoids which can occur in molded specimens of polytetrafluoroethylene. The density of a sample is predicted on the basis of its crystallinity as measured by the infrared method and the difference between this density and the actual density measured by displacement in water is a measure of the microvoid content. This determination is precise to about 0,2% voids by volume. By the use of confirmatory infrared measurements, it is possible to check the possibility that the presence of a substantial percentage of voids may have led to erroneous indications of the molecular weight in the standard specific gravity test discussed earlier. 

Specific interactions between PCL and PVC are clearly indicated. In the solid state (Figure 5.9a) the spectrum of neat PCL indicates the presence of crystalline (1724 cm 1) and amorphous (1737 cm"1) bands. At mole ratios up to 2 1 of PVC to PCL, the spectra indicate that in the solid state the blends consist of crystalline and amorphous phases. As the PVC concentration increases, a parallel increase of the intensity of the amorphous band is observed. Moreover, the frequency shifts observed for both the crystalline and amorphous bands as a function of the composition of the blend suggests that specific interactions between the two polymers occur. No shift is observed in the carbonyl stretching vibration of PPL/PVC blends, in the molten state or in the solid state over the entire range of compositions and the two polymers are incompatible [28]. 

Electron diffraction, which is a somewhat more sensitive method of detecting the presence of crystalline substances, likewise fails to show any evidence of a crystalline phase in fresh silica-alumina catalyst (354). The electron-diffraction patterns do reveal amorphous bands similar to those obtained with evaporated silica films. 

Some studies have examined the effect of solvent on the structure of EB films cast from solutions. For example, Ou and Samuels193 found that EB films cast from NMP solution were partially crystalline, as evidenced by shoulders appearing above the amorphous band in x-ray diffraction (XRD) studies. However, when the EB was cast from a solution in A, A-dimethyl propylene urea (DMPU), no evidence of crystallinity was observed. Whereas the NMP solutions were unstable (undergoing gelation owing to the formation of H-bonded networks between the PAn chains), the... 

Figure 5.8 Temperature dependence of infrared spectra measured for DHDPE/LLDPE(2) 75/25 blend sample during the cooling process from the melt. In these profiles, the contribution of the amorphous band was subtracted. (From Reference 36 with permission from the American Chemical Society.)...

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... 

Additional information is available by subtracting spectra taken at different temperatures for the same sample. Since the position of the film was not altered as the temperatures varied, a 1 1 subtraction is a systematic method to illustrate the thermal eflFects. The spectra at the two temperature extremes for a slow-crystallized sample are subtracted in Figure 20. The 909 and 990 cm vinyl bands narrow in width, increase in intensity, and shift to slightly higher frequency as the temperature is decreased. The crystalline absorptions at 1050 and 1176 cm shift to lower frequency and sharpen considerably. The 1303 cm amorphous absorption shifts its maximum to 1300 cm and possibly increases in intensity. The 1353 cm" band remains fixed in position. The most intense amorphous band, 1369 cmmoves to 1371 cm at 78 K and has an intensity increase. These results are shown clearly in the difference spectrum. Similar results are obtained for the isopentane-quenched sample before and after annealing. 

A common method of determining the crystallinity from ir measurements relates the measured absorbance (previously extinction) Acr of a crystalline band to the absorbance of a 100% crystalline sample via the degree of crystallinity a. For amorphous bands it follows analogously that Aam = (1 ir) Atm. This asscssmcnt is also based on a two-phase model. Both /lam and Act are naturally also proportional to the total quantity of the sample, but since only the ratio D = Acr I Aamis measured, the influence of the amount is eliminated. By inserting the expressions for /Icr and /lam into that for D, the following is obtained ... 

Marcilla and Beltr studied the evolution of PVC-plastidzer mixtores by FTtK dnring heating. When the plasticizer spectnun was subtracted fiom the plastisol one, the resultant spectral difference was nearly the same as the spectrum of pure PVC (some modifications observed depended on the plasticizer type). Figure 9.7 shows the results for DBP, DOP and DIDP. The more compatible plasticizer (DBP > DOP > DIDP) cansed greater modifications in the PVC spectrum (DIDP difference spectrum nearly matches that of pure resin). Moreover, it was observed that the relative intensity of the crystalline bands of PVC (1427 and 637 cm ) decreases as compared to the amorphous bands (at 1435 and 616 cm, respectively) with increasing compatibility between the resin and the plasticizer. 

For semicrystalhne polymers the same workers identified relevant absorptions for crystalhne and amorphous bands and concluded that, for blends containing 70 wt % or more phenoxy, there was no PCL crystalhnity and blends were totally amorphous [85]. This limitation is consistent with the data of Harris et al. [83]. 

Fig. 2. Amorphous bands found for polyethylene (a) polyethylene film quenched from melt (b) same film after annealing.

In some chain segments, the CH2—CH2 groups may no longer be trans to each other, which may result in observation of trans-gauche amorphous bands. 

An infrared study of oxidative crystallisation of PE was made from examination of the 5.28 pm crystallinity band and 7.67 pm amorphous band and carbonyl absorption at 5.83 pm [21]. Miller and co-workers [22] used FT-IR infrared to study the effect of irradiation on PE. Aldehydic carbonyl and vinyl groups decreased and the ketonic carbonyl and r ns-vinylene double bonds increased on irradiation. 

Figure 14.3 Calibration specific volume versus infrared absorption of polyethylene terephthalate. (a) Crystalline bands at 632 cm- = 0.66 cm /g Amorphous bands at...

---