Polyethylene preferred conformation

Considerations of minimum overlap of radii of nonbonded substituents on the polymer chain are useful in understanding the preferred conformations of macromolecules in crystallites. The simplest example for our purposes is the polyethylene (1-3) chain in which the energy barriers to rotation can be expected to be similar to those in /i-butane. Figure 4-2 shows sawhorse projections of the conformational isomers of two adjacent carbon atoms in the polyethylene chain and the corresponding rotational energy barriers (not to scale). The angle of rotation is that between the polymer chain substitutents and is taken here to be zero when the two chain segments are as far as possible from each other. 

This concept for the cycle- and nnsatnration-free control of molecular shapes is likely to find multiple applications. For example, fonr stereoisomeric vicinal diflnorinated analogues of the natural cyclic heptapeptide unguisin A were found to adopt dramatically different secondary strnctnres as a function of stereoelectronic and steric interactions. The preferred conformations of PEG (polyethylene glycol) groups (popular solubilizing substitnents for many biomedical applications) are likely to be affected by the gauche effect as well. 

The experiments were performed with a predominantly head-to-head, tail-to-tail chain that contained defects, arising from the 1,2 addition of 3 or 6% of the monomer units in the parent poly(2,3-dimethyl-1,3-butadiene). When short branches are present as defects in a polyethylene chain, they produce a decrease in Coo, by disrupting the preferred conformation consisting of short sequences of trans placements [23]. The defects present in the chains examined in the experiment, illustrated in Fig. 5, may play... 

The high axial elastic modulus of polyethylene and polyamide 6 is due to the fact that these polymers have a preferred conformation that is fully extended, i.e. all-trans. The elastic deformation is caused by the deformation of bond angles and by bond stretching, both showing high elastic constants. Isotactic polypropylene and polyoxymethylene crystallize in helical conformations and therefore exhibit a maximum stiffness which is only 20% of the maximum stiffness of the all-trans polymers. The elastic deformation of a helical chain involves, in addition to the deformation of bond angles and bond stretching, deformation by torsion about the G bonds. The latter... 

This means that the polyethylene chain will be expanded significantly compared with a freely-jointed chain. The freely-jointed chain is assumed to be made up of a series of vectors with continually variable bond angles and no preferred conformations. For a real polymer chain this is not the case and allowance for this can be made by taking into account the effect of restricted rotation. This is due to the preference of the segments of the molecule to adopt low energy trans and gauche conformations. Equation (3.12) can be further modified to allow for this effect and becomes... 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

The difference between the energy minima in the trans and gauche staggered conformations is labeled Ae in Fig. 4-2. When this energy is less than the thermal energy RT jL provided by collisions of segments, none of the three possible staggered forms will be preferred. If this occurs, the overall conformation of an isolated macromolecule will be a random coil. When Ac > RT/L, there will be a preference for the trans state. We have seen that this is the only form in the polyethylene crystallite. 

The t state is preferred in the melt at this temperature. The symmetry of the torsion potential energy function in equation (3) for the C—C hond in polyethylene, U(< ) = U(—< ), produces pg+ j = Pg -,i- Knowledge of all of the p y, is often usefiil in the interpretation of conformation-dependent spectral properties that have a local origin, such as the chemical shift and coupling constants in nmr spectroscopy (17). 

Fig. 1.15. Thermoelastic results on (amorphous) polyethylene networks and their interpretation in terms of the preferred, all-trani conformation of the chain [3, 6],...

Contrary to the case of polyethylene an all-gauche conformation is energetically preferred in polyoxymethylene. The first... 

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