Molecular weight of methacrylate polymers

Abstract

Previous dilute solution work in these has involved use of the techniques of light scattering and viscosity. Recently some experimental information has been obtained on the polydispersity of some of the methacrylate polymer samples by additional measurements of number-average molecular weights.Stabin-Immerguts osmometers were used with duPont gel cellophane No. 300 membranes. Membrane pretreatment included immersion in a series of liquids, the last one being the solvent (toluene) in vr-hich measurements were to be made. A static method was used, equilibrium osmotic pressures usually being reached in 48 to 72 hr. Corrections were made for membrane asymmetry. The osmometers were maintained in a thermostat at 25 f 0.005°C.Over the concentration range studied the plots are satisfactorily linear. Number-average molecular weights are shown in Table I.From this table it is observed that Afw/Mn values for the fractions are in the range 1.5 t o 2, while the value for an unfractionated poly(n-docosyl methacrylate) is 10. The osmotic measnrements on fractions of poly(n-lauryl methacrylate) and poly(2-ethylbutyl methacrylate) in conjunction with previous light-scattering studies on the same ~amples,~,3 indicated that M J M , ratios were surprisingly high. Therefore, some of the poly(2-ethylbutyl methacrylate) fractions were re-examined by light scattering in methyl ethyl ketone a t 25°C. Since only dissymmetry measurements had been made earlier,' measurements were carried out at a series of angles between 32 and 135" for 4 or 5 concentrations of each fraction. Table I compares molecular weights obtained in the two cases. If there was any degradation of the samples during storage, it must have been small. It is of interest to obtain the relation between intrinsic viscosity and viscosity-average molecular weight once M , and M, are known. An exponential distribution was assmned.6 Using the viscosity data obtained earlier for poly(2-ethylbutyl methacrylate) in methyl ethyl ketone a t 25"C., the viscosity-niolecular weight relations are: Figure 1 shows the plots of T / C vs. c.[q] = 3.72 X 10-6M,P.77