Molecular Weight and size Measurement of Colloidal, Polymer and Macromolecular Materials by Light Scattering Techniques

Abstract

Polymer and colloidal systems which have been characterized making use of light scattering techniques are sample of cellulose acetate obtained from Ravi Rayon Chemical complex, kala shah kaku, Lahore, Pakistan. The method of light scattering has particular advantage for determining parameters like molecular weight and size of the material with in a very broad range. A further advantage being that moderately concentrated solution can be studied to a great accuracy as completed to the other methods employed for this purpose. The data obtained by light scattering techniques involves several different methods for these measurement and depending upon the methodology employed the data is interpreted in several different ways. During this work, we have used the following three methods: i. Turbidity Method ii. Dissymetry Method iii. Zimm Plot Method These methods, yield, with the help of lengthy mathematical manipulation, parameters like weight average molecular weight, second varial co-efficient, sizes, shapes, specific volume and excluded volume of the samples being studied. The problem has been divided into two parts. Part 1 consists of the characterization of polymer sample obtained from Ravi Rayon complex, in which the major polymer of interest was cellulose acetate. The work done so for on this polymer which is of a very different nature, for example, Ermolenko E.N.F and levina S.A. of U.S.S.R. in 1950 studied the effect of non-solvent in acetone solution, while Subramanian, N.M. and Sivarajan, S.R (INDIA) in 1961 used the solution of blank and dyed samples cellulose acetate molecules. Lately Kanji Kamida etal (1975) studied the elution gel permeation chromatography of cellulose diacetate dissolved in acetone. Very little work has been done on molecular weight measurement of this polymer, which is extremely vital for the quality control of the final product of this polymer. Stein (USA) studied this property by making use of different techniques. During the course of present investigation we made use of the above noted three methods and several different concentration at two different wave lengths of 436nm and 546nm, a small part of work has also been done at wave length of 632nm. The range of the results obtained are apparent molecular weight 1,00,000-7, 75,194 g/mole, second varial co-efficient 7.08x10-6 to 2.06x10-2, excluded volume 1.5x10-18 to 2.26x10-15 ml/g. Specific volume 936 to 1.012 ml/g and radii of gyration 1095o – 1398 Ao. These parameters if properly applied will definitely improve the quality of the final product obtained from the cellulose acetate. The second part comprised the studies of size of the cellulose acetate particles for different fractions determined by the dissymmetry method. The fractions of cellulose acetate were obtained by fractional Precipitation method. These fractions were dissolved in acetone and the solutions of different concentrations were prepared. The concentration interval used was 1x10-3 g.ml-1. The dissymmetry of these fractions was measured at 450 and 1350 angles at two wavelengths 436 nm and 546 nm of incident light. These values of intrinsic dissymmetry were obtained for all the fractions. From these values the diameter of these fractions were calculated and were compared with standard data of rod like coil like shapes. Both of these studies have been carried out with the aim in view of developing the newly up-coming polymer industry in Pakistan. In the next phase of the project a more Closer co-operation and collaboration with the polymer industry in sought.