Melt viscosity solutions

Moore, L. D. Relations among melt viscosity, solution viKosity, molecular weight, and long-chain branching in polyethylene. J. Polymer Sci. 36, 155 (1959). 

A lot of molar mass dependencies can be described by eq. (4.1.4). Most of the thermodynamical properties of polymers, e.g. T, Tg, depend on M . Often, the limiting value is attained aheady at molar masses below the real molar mass range of a polymer. Then, no dependence of the characteristics can be observed from the molar mass. Properties of bulky polymers connected with large deformations, e.g. melt viscosity, solution viscosity, are determined by M . Viscoelastic properties, e.g. melt elasticity, depend on M. ... 

Some plastics caimot be obtained in a low viscosity melt or solution that can be processed into a cellular state. For these cases two methods have been used to achieve the needed dispersion of gas in solid sintering of solid plastic particles and leaching of soluble inclusions from the solid plastic phase. 

Polymer solutions are often characterized by their high viscosities compared to solutions of nonpolymeric solutes at similar mass concentrations. This is due to the mechanical entanglements formed between polymer chains. In fact, where entanglements dominate flow, the (zero-shear) viscosity of polymer melts and solutions varies with the 3.4 power of weight-average molecular weight. 

Another major area of use is in the field of adhesives. The main attractions of the material are the absence of a need for mastication, easy solvation of the polymer, which is supplied in a crumb form, the production of low-viscosity solutions and high joint strength. In conjunction with aromatic resins they are used for contact adhesives whilst with aliphatic resin additives they are used for permanently tacky pressure-sensitive adhesives. In addition to being applied from solution they may be applied as a hot melt. 

There are several types of nozzle. The simplest is an open nozzle as shown in Fig. 4.34(a). This is used whenever possible because pressure drops can be minimised and there are no hold up points where the melt can stagnate and decompose. However, if the melt viscosity is low then leakage will occur from this type of nozzle particularly if the barrel/nozzle assembly retracts from the mould each cycle. The solution is to use a shut-off nozzle of which there are many types. Fig. 4.34(b) shows a nozzle which is shut off by external means. Fig. 4.34(c) shows a nozzle with a spring loaded needle valve which opens when the melt pressure exceeds a certain value or alternatively when the nozzle is pressed up against the mould. Most of the shut-off nozzles have the disadvantage that they restrict the flow of the material and provide undersirable stagnation sites. For this reason they should not be used with heat sensitive materials such as PVC. 

The effect of MW and MWD on the solid state properties have been extensively studied [11,12,82]. These studies have been made both on fractionated and whole polymer samples. Attempts have also been made to correlate the solution viscosity, melt viscosity, MFI and other related parameters, which represent the MW and MWD of the polymers, with the solid state properties. Table 6 summarizes the results of various studies on effect of MW and MWD on the properties of PEs. 

Conventionally fillers are divided into reinforcing, active, and inert ones. The reinforcing class includes mainly fibrous materials. Disperse fillers may also perform the reinforcing function, and then they are called active. For the criterion of activity of a filler it has been proposed to employ, for example, the extent of variation of the relative viscosity of the melt or solution caused by introduction of a filler [23] — the greater the variation the better the affinity between the polymer and the filler. 

Rheological methods of measuring the interphase thickness have become very popular in science [50, 62-71]. Usually they use the viscosity versus concentration relationships in the form proposed by Einstein for the purpose [62-66], The factor K0 in Einstein s equation typical of particles of a given shape is evaluated from measurements of dispersion of the filler in question in a low-molecular liquid [61, 62], e.g., in transformer oil [61], Then the viscosity of a suspension of the same filler in a polymer melt or solution is determined, the value of Keff is obtained, and the adsorbed layer thickness is calculated by this formula [61,63,64] ... 

An alternative reason why particle size can affect the polymer melt or solution viscosity consists of the agglomeration of the filler. The higher the dispersity of the filler the higher is its tendency to agglomeration [138-140,161]. Agglomeration, as will be shown below, affects the properties of filled polymers. 

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... 

Unlike the case with dilute solutions of polymer, the variation of the melt viscosity and molar mass is far from completely understood. However, the melt viscosity, has been found to vary uniformly with number of carbon atoms in the chain above about 300-500, according to the equation ... 

When the full distribution is needed, it is measured by size-exclusion chromatography (also called gel permeation chromatography). This is a solution technique that requires dissolution of the polymer in a reasonable solvent such as tetrahydrofuran or tetrachlorlobenzene. For polymers that require exotic solvents or solution temperatures above about 150°C, a simple measurement of solution viscosity can be a useful surrogate for the actual molecular weight. The viscosity of the pure polymer (i.e., a polymer melt viscosity) can also be used. Such simplified techniques are often satisfactory for routine quality control, particularly for condensation polymers such as PET that vary in average molecular weight but usually have a polydispersity of 2. 

For polymer melts or solutions, Graessley [40-42] has shown that for a random coil molecule with a Gaussian segment distribution and a uniform number of segments per unit volume, a shear rate dependent viscosity arises. This effect is attributed to shear-induced entanglement scission. 

Chain-growth polymerizations are diffusion controlled in bulk polymerizations. This is expected to occur rapidly, even prior to network development in step-growth mechanisms. Traditionally, rate constants are expressed in terms of viscosity. In dilute solutions, viscosity is proportional to molecular weight to a power that lies between 0.6 and 0.8 (22). Melt viscosity is more complex (23) Below a critical value for the number of atoms per chain, viscosity correlates to the 1.75 power. Above this critical value, the power is nearly 3 4 for a number of thermoplastics at low shear rates. In thermosets, as the extent of conversion reaches gellation, the viscosity asymptotically increases. However, if network formation is restricted to tightly crosslinked, localized regions, viscosity may not be appreciably affected. In the current study, an exponential function of degree of polymerization was selected as a first estimate of the rate dependency on viscosity. 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene... 

If the branching density is sufficiently high to hinder segmental flexibility and impose strong excluded volume and even steric interactions, molecular dimensions become rigid. Measurements of solution and melt viscosity showed that the properties of dendritic molecules approached that of solid spheres as the... 

We will now turn our attention from the viscosity of dilute solutions and look at the viscosity of melted polymers. The viscosity of melted polymers is important in transferring resins and in polymer processing such as determining the correct conditions to have a specific flow rate for injection processing and in determining the optimum conditions to get the necessary dimensions of extruded shapes. Fillers, plasticizers, temperature, solvents, and molecular weight are just some of the variables that influence the viscosity of polymer melts. Here we will look at the dependence of melt viscosity on polymer molecular weight. Polymer melts have viscosities on the order of 10,000 MPa (1 centipoise =0.001 Pa/sec). 

Star-branched polyesters exhibit unique properties such as lower melt viscosities, lower crystallinity, and smaller hydrodynamic volume in solution by comparison with their linear counterparts. Two general strategies are possible for their... 

Solution viscosity, melt viscosity, modulus of elasticity... 

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