| Popis: |
Light scattering determinations of molecular weight and coil size made at room temperature with three different cells at two wavelengths of incident light and in two solvents (acetone and acetonitrile) on fourteen fractions and six whole polymers of polymethyl methacrylate ranging in molecular weight from 2.7 × 103 to 2.5 × 106 provide a basis for estimation of the validity and reliability of such measurements. It is concluded that the absolute scale for Mw may be high by a factor of 1.05 ± 0.05. Values of Φ × 10−21 of 2.3 ± 0.2 and 2.6 ± 0.4 obtained in acetone and acetonitrile, respectively, agree with the ‘best value’ of 2.1 suggested earlier, within the uncertainty (ca. 15 to 20 per cent) arising from the experimental uncertainty in [η], M and ( r 2 ) 3 2 . The close agreement between [η] M w relationships established herein for conventional polymethyl methacrylate fractions at 30° in benzene, for M from 200 to 2.5 × 106, and in eight other solvents, with similar results from other laboratories demonstrates that reliable and reproducible dilute solution measurements of [η] and of Mw are indeed possible. |
