Structure-dependent magnetic susceptibility of sharp poly(ethylene oxide) fractions

Abstract

Nine sharp fractions of poly(ethylene oxide) (PEO) glycol with number-average molar masses (M n) in the range from 0.6 × 103 to 20 × 103 (PEO-0.6 to PEO-20) were characterized by magnetic susceptibility χ measured in the temperature interval 293 K to 378 K. In contrast to the liquidlike PEO-0.6 with temperature-invariant χ, the values of χ for each of the remaining solid samples, after the initial increase, exhibited two plateaus separated by a relatively narrow temperature interval of their second increase. The jumps of χ at lower and higher temperatures were attributed to a solid-state transition of unspecific nature and to the melting of the crystal fraction, respectively. The temperature-invariant values of χn in the melt state above T m pass through a minimum for the sample PEO-2.0 and then increase again with (Mn) to a limiting value χ∞ = −0.622 × 10−6. It is concluded that a considerable contribution of the molar-mass-dependent “paramagnetism” χP = χ − χd (where χd is the diamagnetic contribution estimated by Kirkwood's equation) to the total magnetic susceptibility of PEO fractions reflects distortions of the spherical symmetry of the electron shells around chain atoms resulting from the discontinuous change of both inter- and intrachain interactions as the (Mn) increases through and above the critical crossover molar mass (Mcr ) = 2 × 103.