The directions of the dipole moments in esters RCOOR' in which R and R' are alkyl or aryl groups have been deduced by analysis of experimental values of dipole moments of six compounds containing two polar groups, one or both of which are esters. New determinations of molecular dipole moments are reported for dimethyl truns-1,4-cyclohexanedicarboxylate and for truns-l,4-cyclohexanediol diacetate. The moment of the ester group is directed (in the positive sense) at an angle 73 = 123 f 3" from the R-CO axis in both aromatic and aliphatic esters, formates excepted.

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This work reports the permeation of argon through membranes prepared from poly(pentaerythritoltribenzoate acrylate) (PPTBA) (IUPAC name poly[1-(3-benzoyloxy-2,2-dibenzoyloxymethylpropyloxy)-2-propen-1-one]). The permeation measurements were performed in the vicinity of the glass transition temperature of the membranes (48 °C). The permeation coefficient only shows a slight dependence on temperature in the glassy state, but it undergoes a sharp increase with temperature in the rubbery state. The results for the diffusion coefficient do not show a definite temperature dependence in the glass−rubber transition. As expected, the solubility of the gas in the membrane is higher in the rubbery state than in the glassy state. The diffusion coefficient was calculated theoretically by using the Transition-State Approach which assumes that the diffusant path is independent of the structural relaxations in the polymer matrix. Reasonably good agreement between the simulated and the experimental values of the diffusion coefficient was obtained in the range of temperatures from 40 to 60 °C in which the measurements were performed.

Acknowledgment. We thank Prof. P.-G. de Gennes for a useful discussion. This study was performed with financial support from the "Piano Finalizzato per la Chimica Fine e Secondaria", CNR, Italy.