Amorphous phase of polyethylene

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. 

Table I I. I Energies constants of van der Waals forces between gas molecules, and solubility in the amorphous phase of polyethylene...

Direct evidence of nucleation during the induction period will also solve a recent argument within the field of polymer science as to whether the mechanism of the induction of polymers is related to the nucleation process or to the phase separation process (including spinodal decomposition). The latter was proposed by Imai et al. based on SAXS observation of so-called cold crystallization from the quenched glass (amorphous state) of polyethylene terephthalate) (PET) [19]. They supposed that the latter mechanism could be expanded to the usual melt crystallization, but there is no experimental support for the supposition. Our results will confirm that the nucleation mechanism is correct, in the case of melt crystallization. 

Sometimes, small structural differences in morphology of polymer samples can be isolated by using a double subtraction technique. For example, with polyethylene terephthalate) PET, differences in the amorphous phase of the melt-quenched polymer and solution-cast polymer can be isolated by first subtracting out the contribution due to the trans isomer and then subtracting the two difference spectra from each other 214). (Fig. 16) shows the resultingdifference spectrum obtained after the second subtraction. Obviously the two amorphous structures are different from each other. 

Because diffusion is limited to the amorphous phase of semicrystalline polymers, and the crystalline phase can additionally restrict chain motion in the amorphous phase, the value of D is dependent on the degree of crystallinity of the polymer. To a first approximation, this effect may be expressed by equation 7, where x is the crystalline volume fraction and D is the diffusion coefficient of the totally amorphous polymer. For example, diffusion coefficients for high density polyethylene are lower than for low density polyethylene (3). 

Other. As a result, the difference between the crystalline and amorphous phases appears in the relaxation times and chemical shifts. Fyfe et al. [1] and Earl and VanderHart [2] independently observed the chemical shift difference between the crystalline and amorphous phases for polyethylene. Figure 7.1 shows a spectrum of polyethylene measured by the CPMAS method. If all of the methylene units in polyethylene are identical, the NMR spectrum gives only one peak. However, a strong peak and shoulder are observed in the real spectrum, which means that there exists two inequivalent methylene units in the solid polyethylene. From measurements on polyethylenes with various crystalline/amorphous ratios, peaks at about 33 and 31 ppm are attributed to the crystalline and amorphous phases, respectively [3]. Figure... 

The partition of end groups and side branches between the crystalline and amorphous phases in polyethylene is determined by isolating the backbone resonances corresponding to the pure crystalline and amorphous phases by assuming as follows (1) the backbone methylene resonance profile may be used to separate the contribution from the crystalline and amorphous phases ... 

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

Foams (cellular structures) made by expanding a material by growing bubbles in it [11]. A foam has at least two components. At a macroscopic scale, there are the solid and liquid phases. The solid phase can be a polymer, ceramic or metal. The fluid phase is a gas in most synthetic foams, and a liquid in most natural foams. At a microscopic scale, the solid phase may itself consist of several components. For example, the solid phase of an amorphous polystyrene foam has only one component. On the other hand, the solid phase of a polyethylene foam or a flexible polyurethane foam typically has two components. These components are the crystalline and amorphous phases in polyethylene foams, and the hard and soft phases formed by the phase separation of the hard and soft segment blocks in flexible polyurethane foams. The solid phase of a polyurethane foam may, in fact, have even more than two components, since additional reinforcing components such as styrene-acrylonitrile copolymer or polyurea particles are often incorporated [12,13]. The solid is always a continuous phase in a foam. Foams can generally be classified as follows, based on whether the fluid phase is co-continuous with the solid phase ... 

In this section, the metal-cationic salts of copoly(ethylene-methacrylic acid) are called the ethylene ionomers. This ethylene ionomer is one of the well-known commercial ionomers, marketed under the trade name Surlyn by DuPont. Many ethylene ionomers have crystalline and amorphous phases of ethylene chain units as well as polyethylene. Therefore, there is a three-phase structure, with crystalline, amorphous, and ionic aggregate phases this is a unique characteristic of ethylene ionomers compared with other ionomers. Although the ionic aggregate structure of the ethylene ionomer has not been fully established, its structural model is represented5 as shown in Fig. 1. In ethylene ionomers, therefore, it is necessary that some physical properties should be considered by correlating to not only the ionic aggregates but also the crystalline phases. 

Polyethylene is a semicrystalline polymer. It means that at ambient temperatures the polymer consists of two rather distinct fractions, or phases—crystalline and amorphous. The amorphous part of polyethylene, which is a sort of rubbery at ambient temperatures, becomes a glass-like at a certain transition temperature, the so-called glass transition point. For polyethylene the glass transition point varies from very low to low (from -130 to 20°C), thus making the plastic ductile at common temperatures. The lower glass transition point (y-transition) is always present in the range of -130 to -100°C, the higher one (P-transition, at —20°C) is manifested not in all PE materials. To complicate the picture even more, we can notice that there is one more transition in polyethylenes, called a-transition, commonly found between 10 and 70°C, and it is associated with crystallinity of PE. For WPC the last two transitions (a- and P-) are of little importance. 

The concept of percentage crystallinity implies the existence of separate crystalline and amorphous phases of constant structure, whereas there may be defects inside, or folds on the surfaces of, lamellar crystals. Crystallinity is usually measured indirectly, via measurements of density or enthalpy of fusion, although these give slightly different results than X-ray diffraction methods. For polyethylene, the density of the crystal unit cell... 

For the calculation of the rate constants of olefin polymerization as well as the constants of copolymerization, it is necessary to know the actual concentration of monomer near the active centers [56]. According to the known schemes [57-59], polyolefin is formed on the surface of the catalyst particles as a polymer shell, and monomer access to the active centers is by diffusion through this polymer shell. As shown [60], the crystallites in polyethylene are impenetrable and are randomly distributed on a macroscopic scale with respect to the diffusion and dissolution processes the amorphous phase of polymer behaves as a homogeneous liquid. That is, monomer access to the active centers occurs by monomer dissolution in... 

Men Men, Y. F., Rieger, J., Enderle, H. F., Lilge, D. The mobility of the amorphous phase in polyethylene as a determining factor for slow crack growth. European Phys. J. E - Soft Matter 15 (2004) 421 25. 

While these materials do contain two crystalline phases, it appears that small quantities of amorphous polyethylene are important in the mechanical property improvements noted above. It should be noted that the amorphous portions of polyethylene have a glass temperature near — 80°C, compared to — 10°C (Nielsen, 1962, p. 16) for polypropylene. 

Semicrystalline polymers, such as polyethylene [45-47] and polypropylene [5, 48], may also be studied by using the 2D IR technique. By taking advantage of the enhanced spectral resolution of 2D IR, overlapped IR bands assigned to the coexisting crystalline and amorphous phases of semicrystalline polymers can be easily differentiated. Sueh differentiation has become especially useful, for example, in the study of blends of high-density polyethylene and low-density polyethylene [47], Here it was found that blends of polyethylenes are mixed at the molecular scale only in the amorphous phase, while each component crystallizes separately. In this section, an example of a 2D IR analysis applied to a film of linear low-density polyethylene is discussed [46]. 

In order to show the differences between the low energy excitations of amorphous and crystalline phases, we focus on the results of polyethylenes (PE) with different degrees of crystallinity [29,34] (see Fig. 3gdi). Assuming that the observed S(Q,(0) is describedby a linear combination of those of the crystalline and amorphous phases, the dynamic scattering laws S(Q,m) of the crystalline and amorphous phases of PE were evaluated and shown in Fig. 4a.In the spectriun of the amorphous phase, a broad peak is observed at about 3 meV. This broad peak is absent in the spectrum of the crystalline phase. It is then directly concluded that the boson peak at around 3 meV is characteristic of the amorphous phase. 

Malamatov, A. Kh., Serdyuk, V. D., Kozlov, G. V. (1998). A Clusters Formation in Amorphous Phase of Modified High Density Polyethylene. Doklady Adygsk. (Cherkessk.) Intemat. AN, 3(2), 74-77. 

---