Viscosity bulk polymers

In this chapter we examine the flow behavior of bulk polymers in the liquid state. Such substances are characterized by very high viscosities, a property which is directly traceable to the chain structure of the molecules. All substances are viscous, even low molecular weight gases. The enhancement of this property due to the molecular structure of polymers is one of the most striking features of these materials. 

Before we are in a position to discuss the viscosity of polymer melts, we must first give a quantitative definition of what is meant by viscosity and then say something about how this property is measured. This will not be our only exposure to experimental viscosity in this volume—other methods for determining bulk viscosity will be taken up in the next chapter and the viscosity of solutions will be discussed in Chap. 9—so the discussion of viscometry will only be introductory. Throughout we shall be concerned with constant temperature experiments conducted under nonturbulent flow conditions. 

There are three major aspects of polymer viscosity discussed in this chapter. First, we shall consider the fact that most bulk polymers display shear-dependent viscosity that is, this property does not have a single value but varies with the shearing forces responsible for the flow. Second, the molecular weight dependence of polymer viscosity is examined. We may correctly expect a... 

In the last three chapters we have examined the mechanical properties of bulk polymers. Although the structure of individual molecules has not been our primary concern, we have sought to understand the influence of molecular properties on the mechanical behavior of polymeric materials. We have seen, for example, how the viscosity of a liquid polymer depends on the substituents along the chain backbone, how the elasticity depends on crosslinking, and how the crystallinity depends on the stereoregularity of the polymer. In the preceding chapters we took the existence of these polymers for granted and focused attention on their bulk behavior. In the next three chapters these priorities are reversed Our main concern is some of the reactions which produce polymers and the structures of the products formed. 

We saw in Sec. 2.9 that the viscosity of a bulk polymer is proportional to M, where a is either 1.0 or 3.4, depending on whether the polymer is below or above, respectively, the critical chain length for entanglement. For solutions, a similar result is obtained, only it is [r ] rather than r itself which is proportional to M ... 

Nobile et al. [3] reported that viscosity of a polycar-bonate-TLCP blend can increase or decrease in the same system at the same temperature, depending on the shear condition. At very low shear rates the viscosity was found to increase with TLCP loading, whereas at high shear rates a significant drop was observed. But in all of these cases, the way in which the TLCPs alter the bulk polymer flow is not yet well understood. 

The bulk viscosity referred to here (pg) should not be confused with the so-called bulk viscosity of polymers which refers to the steady flow shear viscosity of the bulk undiluted polymer. Here it represents all the causes of sound absorption other than those produced by shear viscosity or thermal conductivity. Typically these may be ... 

Miyata and Nakashio [77] studied the effect of frequency and intensity on the thermally initiated (AIBN) bulk polymerisation of styrene and found that whilst the mechanism of polymerisation was not affected by the presence of ultrasound, the overall rate constant, k, decreased linearly with increase in the intensity whilst the average R.M.M. increased slightly. The decrease in the overall value of k they interpreted as being caused by either an increase in the termination reaction, specifically the termination rate constant, k, or a decrease in the initiator efficiency. The increase in kj(= kj /ri is the more reasonable in that ultrasound is known to reduce the viscosity of polymer solutions. This reduction in viscosity and consequent increase in Iq could account for our observed reductions [78] in initial rate of polymerisation of N-vinyl-pyrrolidone in water. However this explanation does not account for the large rate increase observed for the pure monomer system. 

A continuous capillary viscosity detector has been developed for use in High Performance Gel Permeation Chromatography (HPGPC). This detector has been used in conjunction with a concentration detector (DRI) to provide information on the absolute molecular weight, Mark-Houwink parameters and bulk intrinsic viscosity of polymers down to a molecular weight of about 4000. The detector was tested and used with a Waters Associates Model 150 C ALC/GPC. The combined GPC/Viscometer instrumentation was automated by means of a micro/mini-computer system which permits data acquisition/reduction for each analysis. 

Andrews,R.D., Tobolsky.A. V. Elastoviscous properties of pdyisobutylene. IV. Relaxation time spectrum and calculation of bulk viscosity. J. Polymer Sci 7, 221-242 (1951). 

Chikahisa,Y. A theory on the relationship between viscosity and molecular weight in bulk polymers. J. Phys. Soc. Japan 19,92-100 (1964). 

Bueche,F. Influence of rate of shear on the apparent viscosity of a -dilute polymer solutions and b-bulk polymers. J. Chem. Phys. 22,1570-1576 (1954)... 

III) In dilute solution, the friction factor / is equal to tj0a where rj0 is the viscosity of the solvent in which the polymer molecules are dissolved. If rj0 is 0.01 poises, / is 9.4 x 10 9 dyne sec/cm. For bulk polymer or intermediate concentrations, r)0 is usually replaced by the sample total viscosity. [The Stokes law form / was not specifically introduced by Rouse (/) and Bueche (2). It is however, a component of one of the simplest expressions of their basic idea (5,23)]. 

Degradation in bulk. Davis and Golden (85) have studied the degradation of PTHF in bulk at various temperatures. The polymers that they studied were prepared using a THF/PF5 complex either in an open flask (polymer A) or in vacuum with exposure to air during the work up (polymer V). The intrinsic viscosity of polymer A. heated at fixed temperatures up to 150° C in a sealed system, fell rapidly to a constant value. Polymer V behaved similarly but the decrease was considerably smaller. When heated in air at a fixed temperature the viscosity of both polymers decreased continuously with eventual destruction of the polymer. Temperatures well in excess of 150° C were required for complete degradation of polymer A or V in vacuum. 

Schreiber HP, Bagley EB, West DC (1963) Viscosity/molecular weight relation in bulk polymers-I. Polymer 4 355-374... 

For concentrated solutions of polyisobutylene in decalin ( 0.1), estimate the viscosity as a function of the volume fraction of polymer at a temperature of 20 °C. The viscosity of the bulk polymer at this temperature is r/P = 6.5 xlO9 Ns/m2. 

Smets and Evens assumed that the pre-exponential term of the Arrhenius equation of the rate constant is proportional to the jump frequency of a molecular segment from one position to another, i.e. proportional to the reciprocal of the internal viscosity of the bulk polymer at a given temperature. 

The bulk viscosity of polymer solution is an important parameter also when polymers are being used as suspending agents to maintain solid particles in suspension by prevention of settling (see Chapter 7) and when they are used to modify the properties of liquid medicines for oral and topical use. 

---