Structure semi-crystalline polyethylene

At room temperature, the structure of polyethylene is semi-crystalline. Polyethylene samples whose origin is the same polymerization product may have various morphologies depending on the manner in which they are obtained. 

The results of such calculations for semi-crystalline polyethylene have been reviewed elsewhere [37]. A rather wide range of predicted values is obtained, due to the choice of force constants and also to sensitivity to detailed assumptions on the unit cell structure. In spite of these limitations the principal predictions for the elastic anisotropy are clear. These include the anticipated high values for C33 and the very low values for the shear stiffnesses C44, C55 and cee, which reflect the major differences between bond stretching and bond bending forces that control C33 and the intermolecular dispersion forces that determine the shear stiffnesses. It is therefore of value to compare such theoretical results with those obtained experimentally. Table 7.3 shows results for polyethylene where data for the orthorhombic unit cell at 300 K are used to calculate these constants for an equivalent fibre (Voigt averaging procedure see Section 7.5.2 below) compared with ultrasonic data for a solid sheet made by hot compaction. It can be seen that... 

An important subdivision within the thermoplastic group of materials is related to whether they have a crystalline (ordered) or an amorphous (random) structure. In practice, of course, it is not possible for a moulded plastic to have a completely crystalline structure due to the complex physical nature of the molecular chains (see Appendix A). Some plastics, such as polyethylene and nylon, can achieve a high degree of crystallinity but they are probably more accurately described as partially crystalline or semi-crystalline. Other plastics such as acrylic and polystyrene are always amorphous. The presence of crystallinity in those plastics capable of crystallising is very dependent on their thermal history and hence on the processing conditions used to produce the moulded article. In turn, the mechanical properties of the moulding are very sensitive to whether or not the plastic possesses crystallinity. 

Here m is the mode order (m — 1,3,5. .., usually 1 for polyethylenes), c the velocity of light, p the density of the vibrating sequence (density of pure crystal) and E the Young s modulus in the chain direction. The LAM band has been observed in many polymers and has been widely used in structural studies of polyethylenes [94—99,266], as well as other semi-crystalline polymers, such as poly (ethylene oxide) [267], poly(methylene oxide) [268,269] and isotactic poly(propylene) [270,271], The distribution of crystalline thickness can be obtained from the width of the LAM mode, corrected by temperature and frequency factors [272,273] as ... 

Early on, before the existence of macromolecules had been recognized, the presence of highly crystalline structures had been suspected. Such structures were discovered when undercooling or when stretching cellulose and natural rubber. Later, it was found that a crystalline order also existed in synthetic macromolecular materials such as polyamides, polyethylenes, and polyvinyls. Because of the polymolecularity of macromolecular materials, a 100% degree of crystallization cannot be achieved. Hence, these polymers are referred to as semi-crystalline. It is common to assume that the semi-crystalline structures are formed by small regions of alignment or crystallites connected by random or amorphous polymer molecules. 

Information from a molecular probe experiment is usually presented in the form of a retention diagram, that is, a plot of log Vg against 1/T(K). A sample curve for a semi-crystalline polymer is shown in Figure 2. The slope reversals are indicative of phase transitions. Such transitions had been noted (28) as early as 1965 for polyethylene (PE) and polypropylene (PP), but the first comprehensive study of polymer structure using IGC was done in 1968 by Smidsrod and Guillet (1) on poly(N-isopropyl acrylamide) (poly-(NIPAM)). 

We also note that certain polymer melts crystallize partially upon cooling. The transition occurs at a well-defined temperature rf. Crystallization takes place only if the polymer has a perfect linear structure for instance, it must not contain any asymmetric carbon. However, this tacticity condition is not sufficient, since polydimethylsiloxane, which is perfectly periodical, does not crystallize under normal conditions. On the contrary, polyethylene and isotactic polyethylene crystallize easily. In general, these polymers contain a large amorphous fraction. This is why they are called semi-crystalline. In certain conditions, it is possible to prepare polymer samples that are perfect crystals, in particular by polymerization in situ of a crystal made of monomers (polydiacetylene and polyoxyethylene). 

The polymer s wear and impact resistance is due to the extremely long molecular chains of the material and its semi-crystalline solid state conformation. The surface and subsurface structure of the polymer is highly ordered and the crystal sites offer resistance to deformation and conformance to a wear mating surface and to sections being tom loose. (3). UHMWPE does not easily flow when high shear forces are applied due to the excessive entanglements of the long polyethylene chains. 

Load-deformation curves for polyethylene and pofysQrrene are shown in Figure 5.2. Polyethylene necks and cold draws in the manner described above (5.N.1). As it is a semi-crystalline pofymer, yielding involves considerable disruption of the crystal structure. Slip occurs... 

Information on the local structure of polymers is not limited to proton-proton spin diffusion experiments. Spin diffusion among the carbons can also be used [97], However, because of the low gyromagnetic ratio and natural abundance of the carbon-13 nuclei, it is less efficient than proton-proton spin diffusion. At natural abundance, the rate of spin diffusion is usually too low to compete with the rate of spin-lattice relaxation in most polymers. However, an interesting exception is that of semi-crystalline polymers such as linear polyethylene and cellulose which have very long longitudinal relaxation times and for which natural-abundance spin... 

In this method an extruded film or foil made from a partially crystalline polymeric material (polytetrafluoroethylene, polypropylene, polyethylene) is stretched perpendicular to-the direction of the extrusion, so that the crystalline regions are located parallel to the extrusion direction. When a mechanical stress is applied small ruptures occur and a porous structure is obtained with pore sizes of about 0.1 pm minimum to a maximum of about 3 pm maximum. Only (semi) crystalline polymeric materials can be used for this technique. The porosity of these membranes is much higher than that of the membranes obtained by sintering, and values up to 90% can be obtained. 

Crystallinity is a state of molecular structure referring to a long-range periodic geometric pattern of atomic spacings. In semi-crystalline polymers, such as polyethylene (PE), the degree of crystallinity (% crystallinity) influences the degree of stiffness, hardness, and... 

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