Excess entanglements

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. 

Like PEO-LiCl04, a 6 1 crystalline compound is formed but, in this instance, the weakened interactions between polymer chains [18] contributes to the lowest melting point for any PEO-salt crystalline complex. A eutectic with composition 0 Li = 11 1 forms, provided the PEO molecular chain length is beyond the entanglement threshold [31]. For lower molecular weights, the 6 1 compound dose not crystallize in the presence of excess PEO and a crystallinity gap exists over the range 6 l < 0 Li < 12 1 [26]. 

For imperfect epoxy-amine or polyoxypropylene-urethane networks (Mc=103-10 ), the front factor, A, in the rubber elasticity theories was always higher than the phantom value which may be due to a contribution by trapped entanglements. The crosslinking density of the networks was controlled by excess amine or hydroxyl groups, respectively, or by addition of monoepoxide. The reduced equilibrium moduli (equal to the concentration of elastically active network chains) of epoxy networks were the same in dry and swollen states and fitted equally well the theory with chemical contribution and A 1 or the phantom network value of A and a trapped entanglement contribution due to the similar shape of both contributions. For polyurethane networks from polyoxypro-pylene triol (M=2700), A 2 if only the chemical contribution was considered which could be explained by a trapped entanglement contribution. 

Tanaka et al. have studied the surface molecular motions of PS films coated on a solid substrate by lateral force microscopy and revealed that the Tg at the surface was much lower than the corresponding bulk one [148]. Possible reasons for this included an excess free volume induced by localized chain ends, a reduced cooperativity for of-relaxation process, a reduced entanglement, and a unique chain conformation at the surface. For comparison, they examined surface relaxation behavior of high-density PMMA brushes. 

Flow, on the contrary, is strongly hindered by the entanglements between the coils this is why fluid polymers show excessively high viscosities, which necessitate the use of heavy processing machines. 

Fig. 20. Schematic representation of a composite membrane (Figs. 1 and 7) at liquid saturation showing a single gelled particle enmeshed in PTFE microfibers as described in the text. The bold straight lines represent the PTFE fibers. The entangled network of curved lines represent the crosslinked polymer that supports the liquid saturated gel. Each empty circle (o), superimposed on the curvy lines, represents a set of molecules (

Branches in polymers play important roles in determining their properties, such as viscosity, density, and toughness. However, excessive branching is also known to cause deterioration of physical properties due to low probability of chain entanglement. Only recently, highly branched polymers have attracted increasing attention with the expectation that their unique structures will impart unusual properties creating novel applications. 

In an extrusion process, where shear forces and high pressures are applied, the entire process is obviously much more complicated. However, either with limited amounts or in excess of water, the main step associated with the melting of crystallites is the same, leading to an amorphous entangled mass of amylose and amylopectin macromolecules. 

---