Crystalline state polyethylene

This effect is also observed with some polymers. The trans form of a hydrocarbon chain requires an energy about 0.8 kcal/mole less than the gauche. The trans form leads to an extended molecule and in hydrocarbons this becomes more favoured as the temperature is lowered. Linear polyethylenes take up this conformation in the crystalline state. 

Many high molecular weight synthetic polymers, such as polyethylene and polypropylene, have a large percentage of their molecules in the crystalline state. Prior to dissolution, these polymers must usually be heated almost to their melting points to break up the crystalline forces. Orthodichlorobenzene (ODCB) is a typical mobile phase for these polymers at 150°C. The accuracy and stability of the Zorbax PSM columns under such harsh conditions make them ideal for these analyses (Fig. 3.8). 

The linewidth-temperature relation of the polyethylene oxide samples are given in Fig. 5. Despite the large differences in molecular weight, these samples have about the same linewidth, 300-350 Hz, in the crystalline state at 25°C. They all also possess a spherulitic type of morphology. The influence on the linewidth of the different types of supermolecular structures... 

In the present work the limiting value of the linewidths for polyethylene oxide increases from 135 Hz in the melt above 70°C, to the range 300-350 Hz in the crystalline state at room temperature. As is indicated in Table I, the resonant linewidths for linear polyethylene increase substantially upon crystallization and attain values in the range 500-900 Hz at 45°C and 57.9 MHz. 

LLDPE is made by a catalytic process very similar to that for HDPE, but it is a softer polyethylene than HDPE with properties similar to those of LDPE. Its properties are achieved by inclusion of comonomers such as butene or hexene. A relatively disordered crystalline state is obtained by introducing many short branches into an otherwise highly linear molecule. Thus, the less expensive equipment of the HDPE process can be used to make a product having the greater flexibility and impact strength characteristic of LDPE. 

Commercially available polyethylene fiber has a degree of crystallinity between 70 and 80% and a density 0.97 gcm . There is a linear relationship between density and crystallinity for polyethylene. A 100% crystalline polyethylene will have a theoretical density, based upon an orthorhombic unit cell, of about lgcm . A totally amorphous polyethylene (0% crystallinity) will have a density of about 0.85gcm . Khosravi et al. (1995) used nitric acid attack on gel-spun polyethylene fibers to observe structural imperfections such as fold, molecular kinks and uncrystallized regions. Raman spectroscopy has been used to study the deformation behavior of polyethylene fiber. This technique gives peaks for the crystalline and amorphous states of polyethylene (see Chapter 9). 

It must be amorphous in its unstressed state. (Polyethylene is not an elastomer, but copolymerization of ethylene with suflicient propylene reduces chain regularity sufficiently to eliminate crystallinity and produce a useful elastomer.)... 

In these connections, it is useful to consider the behavior of the chemical shift of the CH2 carbons of paraffins and polyethylene in the crystalline and noncrystalline components. It is known that the CH2 carbons in paraffinic chains appear at lower frequency by 4-6 ppm if a carbon atom three bonds away is in a gaMc/ie-conformation rather than in a frans-confor-mation (y-effect) [7]. In fact, cyclic paraffins which crystallize in a conformations are characterized by two parallel all-fran5-planar zigzag strands connected by two GGTGG loops, it is found that the CH2 carbons with a y-effect resonates at a lower frequency by about 6.5 ppm as compared with those with no y-effect [8, 9]. Furthermore, it is found that the CH2 carbons of cyclic paraffins, and n-paraffins in the noncrystalline state, appear at lower frequency by 2-3 ppm more than those in the crystalline state. In the crystalline state, the CH2 carbons assume the all-trans-zigzag conformation, which is fixed because motion is frozen, but in the noncrystalline state a rapid transition between the trans- and gawc/ie-conformations occurs [11]. Weeding et al. [12] obtained the same results on the noncrystalline state. 

The relative susceptibilities of the crystalline and amorphous portions of polyethylene to high energy radiation has been a moot question. Present evidence indicates that for linear polyethylenes without vinyl end groups the G-values for the production of hydrogen, cross-links, and trans-vinylene groups are identical for the melt at 133 °C and the crystalline state at 130°C (I). (Scission is now believed to be essentially nil at least, its occurrence is not demonstrated). 

In Sect. 3 a set of experimental data on cyclic molecules is described supporting the basic discussions of Sects. I and 2. Central are the cycloalkanes that ultimately serve as a model for adjacent reentry, sharply folded polyethylene crystals. Chain-folded polyethylene was shown in the early 1960 s to thicken in the crystalline state by straightening as many as 100 to 1000 folds when brought to elevated pressure and temperature. This surprising observation found its explanation in the fast reptation possible in the condis-crystal state. 

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