Introduction

An Introduction to Polymer Chemistry by D East, G C Margerison

By D East, G C Margerison

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1. A typical number distribution of molecular weight 120 X1018 molecules containing more than 20 monomer units. A graph of this number distribution is shown in Fig. 1. We can now calculate the weight of each species in the sample. For example, we have 81 xlO 18 molecules each containing 3 monomer units in the chain. Since each monomer unit weighs 1-66 X10 ~22 g, the total weight of the 300 mole- DETERMINATION OF MOLECULAR WEIGHT 43 cular weight material in the sample is 81 X 1 0 1 8 x 3 x 1-66 X10" 2 2 g - 0-0403 g.

Flexible linear molecules in the pure liquid and solid states Polymer molecules in the pure liquid state can easily be pictured in the terms used to describe concentrated solutions. If we consider the effect of removal of solvent from solutions of polymer at temperatures above the polymer melting point, it is clear that the polymer molecules become more and more entangled as the amount of solvent in the solution decreases. The pure liquid state in which the molecular entanglement is at a maximum thus represents the limiting case of the very concentrated solution.

Flexible linear molecules in more concentrated solution The previous picture of a solution of a polymer is only useful for very dilute solutions (usually less than 1 % by weight of polymer—see Chapter 2). Because of the large dimensions of polymer molecules with high molecular weights, increasing the concentration of polymer from these low values quickly results in entanglement of the polymer chains. That is, the INTRODUCTION 21 solution changes from that shown in Fig. 9 to that shown in Fig. 10 on adding more polymer.

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