Crystallinity polyethylene, dependence

The ease with which a polymer will form into crystalline regions depends on the structure of the molecular chain. It can be seen, for example, that if the polyethylene molecule has a high degree of branching then it makes it difficult to form into the ordered fashion shown in Fig. A.9. Also, if the side... 

In all cases filaments (ribbons) of crystalline polyethylene have been observed similar to the pol)mierization with soluble catalysts in high concentration (117). The perfection of these crystals must be dependent as before on polymerization rate, polymerization site density, solvent power, molecular weights, and in the case of heterogenous catalysis also on the presence of a support surface. The best perfection was obtained by the room temperature polymerization of polyethylene on glass sup-... 

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 ratios of diffusivities fora series of penetrants in ihe 50% crystalline polyethylene (Das) and in the hypothetical amotphous sample (D,.0) are shown in Fig. 20.4-2. In addition to the substantial nondis-criminaling reductions in mobility, there is an additional size-dependent reduclioo in mobility due to chain restriction effects. Note that si ace the solubilities of all the components are proponional to the amotphous fraction (Fig. 20.4-1), selectivity enhencements resulting from the introduction of crystallinity rely solely on the chain restriction effect,... 

Order and dense paeking are relative in the context of these systems and depend on the point of view. Usually the term order is used in eonneetion with translational symmetry in molecular structures, i.e. in a two-dimensional monolayer with a erystal strueture. Dense packing in organic layers is conneeted with the density of crystalline polyethylene. 

The degree and orientation of crystalline regions within a polyethylene depends on a variety of factors, including its molecular weight, processing conditions, and environmental conditions (such as loading), and will be discussed in later chapters. 

Figure 2.46 illustrates the completed analysis. A number of other polymers are described in the ATHAS Data Bank, described in the next section. Most data are available for polyethylene. The heat capacity of the crystalline polyethylene is characterized by a T dependence to 10 K. This is followed by a change to a linear temperature dependence up to about 200 K. This second temperature dependence of the heat capacity fits a one-dimensional Debye function. Then, one notices a slowing of the increase of the crystalline heat capacity with temperature at about 200 to 250 K, to show a renewed increase above 300 K, to reach values equal to and higher than the heat capacity of melted polyethylene (close to the melting temperature). The heat capacity of the glassy polyethylene shows large deviations from the heat capacity of the crystal below 50 K (see Fig. 2.45). At these temperatures the absolute value of the heat capacity is, however, so small that it does not show up in Fig. 2.46. After...

After the crystallinity dependence has been established, the heat capacity of the solid at constant pressure, Cp, must be changed to the heat capacity at constant volume C, as described in Fig. 2.31. It helps in the analysis of the crystalline state that the vibration spectrum of crystalline polyethylene is known in detail fromnonnal mode calculations using force constants derived from infrared and Raman spectroscopy. Such a spectrum is shown in Fig. 2.47 [22]. Using an Einstein function for each vibration as described in Fig. 2.35, one can compute the heat capacity by adding the contributions of all the various frequencies. The heat capacity of the crystalline polyethylene shown in Fig. 2.46 can be reproduced above 50 K by these data within experimental error. Below 50 K, the experimental data show increasing deviations, an indication that the computation of the low-frequency, skeletal vibrations cannot be carried out correctly using such an analysis. 

It is somewhat arbitrary to select the compliances rather than the moduli for decomposition, without more knowledge of the origin of the viscoelasticity. In fact, a similar analysis on the basis of the moduli E and E" has been performed on data for crystalline polyethylene terephthalate by Kawai and associates. The two procedures correspond to assuming that the stress or the strain, respectively, is homogeneous throughout the sample. Actually, depending on the structure and microscopic features of the viscoelastic response, neither may be strictly homogeneous cf. Chapter 14, Section F). 

In crystalline solid polymers, not only do the chains vibrate in the normal manner which we associate with infrared or Raman modes, but it is also possible for the all-tew.s section of a chain such as polyethylene to undergo an accordion type of motion. The element which is involved in the motion is constrained by the size of the crystal lamellae described in Chapter 6. The motion is not a relaxation and it gives rise to a resonance observed in the far infrared or Raman spectrum. Typically, crystalline polyethylene will possess a vibration band at about 120 cm" which is associated with this accordion motion. The precise position depends on the length of the lamellae, the shorter lamellae having a higher frequency of resonance. This collective vibration of the chains is quantised, and is a p/jowow. These well defined acoustic vibrations are very important in understanding the temperature dependent dynamic behaviour of crystalline solid polymers. 

Motion in polyethylene. I. Temperature and crystallinity dependence of the specific heat. J. Chem. Phys. 37,1203 (1962). II. Vibrations in crystalline polyethylene. Ibid. 1207. III. The amorplmus polymer. Ibid. 2429. 

The melting point of low-density polyethylene is about 117°C, and its degree of crystallinity will depend on its thermal history. When copolymerized with vinyl acetate molecular regularity is diminished and there is a large reduction in degree of crystallinity. EVA random copolymers containing up to 30% vinyl acetate are used. Further information has been provided by Eastman and Fullhart (1990). The thermal properties of an EVA hot-melt adhesive with an aromatic hydrocarbon tackifier have been examined in detail by Park and Kim (2003). 

The heat (enthalpy) of fusion (A///) of a sample is a measure of the amoimt of heat that must be introduced to convert its crystalline fraction to the disordered state. It is thus uniquely dependent upon the degree of crystallinity of the sample and the theoretical heat of fusion of a 100% crystalline sample. The heat of fusion of 100% crystalline polyethylene has been calculated to be 69 cal/g [31]. hr the case of commercial polyethylene samples, heats of fusion range from essentially zero up to values approaching the theoretical maximum. 

Fig. 22. Nomialized pull-off energy measured for polyethylene-polyethylene contact measured using the SFA. (a) P versus rate of crack propagation for PE-PE contact. Change in the rate of separation does not seem to affect the measured pull-off force, (b) Normalized pull-off energy, Pn as a function of contact time for PE-PE contact. At shorter contact times, P does not significantly depend on contact time. However, as the surfaces remain in contact for long times, the pull-off energy increases with time. In seinicrystalline PE, the crystalline domains act as physical crosslinks for the relatively mobile amorphous domains. These amorphous domains can interdiffuse across the interface and thereby increase the adhesion of the interface. This time dependence of the adhesion strength is different from viscoelastic behavior in the sense that it is independent of rate of crack propagation.

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. 

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