Branched polymers flow properties

Whilst conventional polycarbonate based on bis-phenol A is essentially linear, branched polymers have recently been introduced. These materials have flow properties and a melt stability that makes them particularly suitable for large (20 litre) water and milk containers. Branched polymers have also been used in the manufacture of twin-walled sheet for the building industry. 

Solution viscosity measurements for Mn are calibrated from the flow characteristics of linear molecules of the equilibrium molecular weight distribution. Branched polymers have a lower radius of gyration for their molar mass than the corresponding linear molecule. One, therefore, expects different flow properties as branching increases, hence causing the viscosity numbers to become less and less accurate and so should only be used for trends - not exact calculations. 

A branched macromolecule forms a more compact coil than a linear polymer with the same molecular weight, and the flow properties of the two types can differ significantly in the melt as well as in solution. Controlled introduction of relatively long branches into diene rubbers increases the resistance of such materials to flow under low loads without impairing processability at commercial rates in calenders or extruders. The high-speed extrusion of linear polyethylene is similarly improved by the presence of a few long branches per average molecule. 

Problem 3.12 and Worked Example 3.13 illustrate the usefulness of the methods discussed here for calculating flow properties of entangled branched and polydisperse polymers. 

The following parameters and processing variables affect the electrospinning process (i) system parameters such as molecular weight, molecular weight distribution and architecture (branched, linear, etc.) of the polymer, and polymer solution properties (viscosity, conductivity, dielectric constant, and surface tension, charge carried by the spinning jet) and (ii) process parameters such as electric potential, flow rate and concentration, distance between the capillary and collection screen, ambient parameters... 

The effects of SCB and side groups are similar. They disrupt the ability of the polymer to crystallize. If the disruption is not complete, the added bulkiness will make the rate of crystallization slow down. SCB has little effect on the flow properties of a polymer, but LCB has a profound effect. We will discuss this more when we look at the differences in behavior between HDPE, LDPE, and LLDPE in Chapter 4. For now, we can illustrate the effects of branching by comparing LDPE with HDPE. The densities differ, the tensile properties differ, and the elastic character of the polymers differs greatly, even though both are made from the same monomer. 

Commercial grades of low-density polyethylene vary widely in the number of short and long branches, average molecular weights, and molecular weight distributions Mw/Mn is between 20 and 50 for commercial low-density materials. The short branches control the degree of crystallinity, stiffness, and polymer density. They also influences the flow properties of the molten material. 

The branched chain contains side subchains, mainly obtained by the chain transfer mechanism. Most branches are short but yet are able to decrease the crystallinity in the polymer, due to steric hindrance. Existence of longer branches affects the flow properties by decreasing viscosity. This... 

Rheology is a relatively young branch of natural science that deals with the relationships between forces (stresses) and deformations of material bodies. Hence, it is also connected to the flow properties of polymers both in solution and in the melt, as well as the reaction of materials in the solid state to mechanical stresses. Most polymeric materials exhibit the combined reactions of both liquid and solid states, called viscoelasticity, a combination of the viscosity of a liquid and the elasticity of a solid. 

The parameters involved in the electrospinning processes that affect the nanofiber geometry and structure can be divided into two groups (i) System parameters such as polymer molecular weight, molecular weight distribution, polymer architecture (branched, linear), concentration of the polymer solution and its properties, including viscosity, electrical conductivity, and surface tension and (ii) Process parameters such as applied electric voltage, polymer flow rate, distance between the needle tip and the collector, ambient parameters such as temperature, humidity, and air velocity in the chamber, and motion of the collector (Frenot and Chronakis 2003). 

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