Spectrofluorimeters as Light-Scattering Apparatus Application to Polymers Molecular Weight Determination Manuel A. Mougan, Adela Coello, Aida Jover, Francisco Meijide, and Jose Vazquez ~ a t o ' Universidade de Santiago, Campus de Lugo, Facultade de Ciencias, Departamento d e Quimica Fisica, 27002,Lugo, Spain
The determination of the molecular weight is one of the most i m ~ o r t a nlaboraton, t methods for characterizing-anv. chemical compound. I t is not therefore surprising that a wide variety of methods are described in the educational literature. This area is particularly important for polymer characterization a s recoenized in a recent monomaoh (1) and papers hy Mathias 'i2) and Hardgrove et a c (3j who include one molecular weight determination for polymer and physical chemistry laboratory courses. Table 1 lists molecular weight polymers published hv - determinations of . . this Journal. Light-scattering techniques are very important in polymer characterization (1, 4, 5 ) in providing information about molecular weight, polydispersity, polymer size and shape, intermolecular interactions, and diffusion coefflcient-hydrodynamic radius. However, only two papers (6, 7) and one general physical chemistry laboratory textbook (8)describe the technique and propose systems to he studied. Such a low numher of descriptions may be related to the relatively high cost of commercial equipment. In his textbook, White (8) describes the construction of a lightscattering apparatus and proposes the measurement of oolv(vinvl acetate) in acetone solution. The e a u i ~ m e n t cite"d by Mathias (6) and Matthews (7) are commLrEia~instruments. More common than light-scattering equipment in chemistry laboratories are fluorescence spectrometers. In the present paper we suggest the use of the Rayleigh scattering band for an introduction to the light-scattering technique. The use of the same apparatus for obtainingdifferent physicochemical parameters is economical and has an important educational impact. Polyethylene glycols (PEG'S) of different molecular weights were chosen for study because they are readily available, inexpensive, easily soluble in water (avoiding the use of organic solvents), and innocuous.
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Theoretical Background Because a simple analysis of the interaction of the incident light with the sample was given by White (8)and Atkins (9). we will only summarize the eauations and their physicochemical meanings. Experimental results are usuallv in terms of the inverse of the increase of the . exoressed . Rayleigh scattering ratio of the solution with respect to the solvent, AR,, according to
is its mowhere C is the solute concentration in dmL: M... lecular weight;A2 is the second virial cGfficikn
