Crystalline phase of polyethylene

LDPE in the amount of 15phr was blended with ethylene-propylene-diene terpolymer. The method of blend preparation, at the temperature of 145 °C, that is, well above melting point of the crystalline phase of polyethylene, was described in Ref [2]. To crosslink elastomer matrix 0.6phr... 

Polyethylene is a polymer with the simplest repeating unit stracture. The crystalline phase of polyethylene typically is composed of orthorhombic unit cells, with edge lengths of a = 0.740 nm, b = 0.493 mn, and c = 0.254 mu. In the crystal-fine phase, the chains of polyethylene are in dXL-trans conformation and have a linear zigzag conformation. Figure 3.5 shows the unit cell stmcture of polyethylene. It is seen that each unit cell contains two repeating units. 

Fig. 7.2. Recovery of peaks for the crystalline and amorphous phases of polyethylene measured by (180° - T - 90° - 100 s) with high-power decoupling.

The chemical shift is determined by the relatively local electronic structure. One of the most important parameters which affect chemical shift is conformation. As mentioned in the section about crystalline and amorphous phases, a typical example for the conformational effect on the chemical shift is the chemical shift difference between the crystalline and amorphous phases of polyethylene. In the crystalline phase, polyethylene takes the all trans-zigzag conformation, while, in the amorphous phase, a rapid transition between the trans and gauche conformations takes place. As a result, the chemical shift of the amorphous phase is the average of the trans and gauche conformations. 

Fig. 7.14. C MAS spectra corresponding to the pure amorphous (top) and pure crystalline (bottom) phases of polyethylene [25].

Fig. 7.25. 2DECSA spectra of the crystalline phase for polyethylene (a) normal spectrum (b... 

A liquid crystalline phase is a phase in which molecules or components of molecules take a parallel alignment with thermal fluctuation and with certain transitional disorder. Thus in principle, most linear polymers and even branched or slightly crosslinked polymers can fit in with a liquid crystalline phase. Taking polyethylene as an example, the lowest energy conformation of this polymer is the extended trans form. Suppose that the extended chains or chain segments of PE align in parallel fashion leaving... 

Other strategies aim to restrict formation of the crystalline phases of the polymeric electrolytes based on polyethylene oxide (PEO) ... 

Klepac extensively studied the effect of uniaxial deformation on the molecular chain segmental mobility of linear low density polyethylene (LLDPE) by ESR spin probe method. The simulated ESR spectra of undeformed and deformed (parallel and perpendicular) LLDPE samples at 0 °C are shown as dotted line in Figure 25.31. The results obtained by simulations indicate that the amount of slow components and corresponding Tr of LLDPE films deformed in both parallel and perpendicular directions are significantly larger than those of the undeformed LLDPE films. However, the amount of fast components decreased in the deformed sample. It was concluded from the results that uniaxial deformation reduces the molecular chain segmental mobility in the amorphous region and increased the amount of crystalline phase of the LLDPE films. 

Stmctural aspects of components constituting low-density polyethylene/ ethylene-propylene-diene rubber (LDPE/EPDM) blends are studied in bulk and compared to the surface layer of materials. Solvation of a crystalline phase of LDPE by EPDM takes place. The effect is more significant for systems of amorphous matrix, despite a considerable part of crystalline phase in systems of sequenced EPDM matrix seems to be of less perfect organization. Structural data correlate perfectly with mechanical properties of the blends. Addition of LDPE to EPDM strengthens the material. The effect is higher for sequenced EPDM blended with LDPE of linear structure. 

Low density polyethylene is the most frequently used because of its low branching degree. The radiation processing increases the stability by the continuous increasing of the crosslink density from zero in pristine material to 0.204, 1.022 and 4.807 mol dm at 100, 200 and 400 kGy, respectively [59]. Of a great importance for the effect of irradiation is the orientation of molecules that determines the differences in the repartition of crystalline phase in polyethylenes. The gel fraction in LDPE, LLDPE and HOPE depends not only on the received dose, but also on the drawing ratios [60]. The differences in the accumulation of insoluble fraction are explained by the unlike values of crystallinities. 

Polyesters containing ethylene moities, namely polyethylene-biphenylate (PEBB) and naphthalate (EN) copolsrmers, were analyzed by Wendling and co-workers (401) in an effort to comprehend their crystal/amorphous structures. They found that minority repeat imits were excluded from the crystalline phase of the majority comonomer when the concentrations of the two are very dissimilar, whereas the two comonomers are able to cocrystalUze by segregation when their compositions are more nearly equal. 

Figure 1-17 shows an asynchronous 2D IR spectrum correlating the backbone of polyethylene segments located in either crystalline and amorphous domains and deuterium-substituted branches. Asynchronous cross peaks develop between the crystalline component of the symmetric CHi-stretching band of the polyethylene segments and bands for octene side branches. Such asynchronicity suggests short side branches can move independently of the polyethylene chain located in the crystalline phase of the sample. No noticeable asynchronicity, however, is observed... 

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