Solution viscosity chain entanglements

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

The plateau region appears when the molecular weight exceeds Mc [(Mc)soln. for solutions], and is taken to be a direct indication of chain entanglement. Indeed the presence of a plateau may be a more reliable criterion than r 0 vs M behavior, especially in solutions of moderate concentration where viscosity may exhibit quite complex concentration and molecular weight behavior. It is postulated that when M greatly exceeds Mc, a temporary network structure exists due to rope-like interlooping of the chains. Rubber-like response to rapid deformations is obtained because the strands between coupling points can adjust rapidly, while considerably more time is required for entire molecules to slip around one another s contours and allow flow or the completion of stress relaxation. 

Worsfold found that the degree of association as measured from viscosity was less than that indicated by the light scattering and spectroscopic results. It was therefore concluded that the association dissociation rates were comparable to the chain entanglement lifetime. As a consequence, Worsfold concluded that viscosity measurements involving concentrated solutions of poly(dienyl)lithium in the entanglement regime could not detect the presence of, for example, star-shaped tetramers if the equilibrium... 

The viscosity of a solution of a given polymer will be dependent upon the concentration of the polymer in that solution. Viscosity depends also upon molar mass. Higher molar mass means greater chain entanglement and therefore higher viscosity. Thinking about this in reverse, we can use viscosity as a method to determine the molar mass of a polymer sample. 

Polymer conformation within the solvent system in question also has an impact on both the ink viscosity and the cleanliness of drop break up. Some polymer chains have a tendency to fold in upon themselves, thus not entangling as much with other polymer chains in solution. These tend to break off well and not form satellites, whereas other polymers can form very twisted and entangled networks in solution. These very entangled polymer chains more often lead to what is termed "stringy break-up" and usually form the unwanted satellites. 

Solution Properties. Typically, if a polymer is soluble in a solvent, it is soluble in all proportions. As solvent evaporates from the solution, no phase separation or precipitation occurs. Tire solution viscosity increases continually until a coherent film is formed. Tire film is held together by molecular entanglements and secondary bonding forces. The solubility of the acrylate polymers is affected by the nature of the side group. Polymers that contain short side chains are relatively polar and are soluble in polar solvents such as ketones, esters, or ether alcohols. As the side chain increases in length the polymers are less polar and dissolve in relatively nonpolar solvents, such as aromatic or aliphatic hydrocarbons. 

Viscosity is a measure of the resistance to flow. Flow, which is the result of cooperative movement of the polymer segments from hole to hole in a melt or solution, is impeded by chain entanglement, intermolecular forces, cross-links, and the presence of reinforcing agents. 

It is well known that rates of polymerizations can increase markedly w ith the degree of conversion or with the polymer concentration. Some workers have attributed this solely or partly to a template effect. It has been proposed that adventitious template polymerization occurs during polymerizations of AA, MAA and AN, and that the gel or Norrish-Trommsdorff effect observed during polymerizations of these monomers is linked to this phenomenon. However, it is difficult to separate possible template effects from the more generic effects of increasing solution viscosity and chain entanglement at high polymer concentrations on rates of termination and initiator efficiency (Section 5.2.1.4). 

Polymer concentration affects particle size and morphology because it influences polymer chain entanglements and solution viscosity. The size of the particle generally decreases as the polymer concentration decreases because of the decrease in the fraction of solid material for a defined volume of solution. A further decrease in concentration creates irregular or biconcave particle shapes. 

---