R. J. J. Visser

Time-resolved determination of volume shrinkage and refractive index change of thin polymer films during photopolymerization

Abstract Polymerization shrinkage and change of refractive index have been studied on thin sample layers during photopolymerization. By using laser interferometry and special substrates it was possible to determine both parameters simultaneously. The monomer studied was bis(2-hydroxyethyl)bisphenol A dimethacrylate (HEBDM). Once the refractive index and the shrinkage were known, the double-bond conversion could be calculated using the Lorentz-Lorenz equation. It has been found that shrinkage proceeds linearly with conversion until vitrification of the system sets in. From that moment shrinkage cannot keep up with conversion and lags behind. The described model can also be used to predict changes of thickness and refractive index upon further relaxation.

Crystal Field Effects, Interchain Charge Transfer and the Phase Transition of DMTM(TCNQ)2

Abstract Preliminary structural data for the low temperature phase of the pseudo one-dimensional conductor DMTM(TCNQ)2 strongly indicate, that the peculiar converse changes in electrical behaviour of this compound at Tc = 272K should be attributed to crystal field effects. The loss of monoclinic symmetry at T c most probably accompanied by an ordering of the DMTM cations, affords an inequivalent crystal potential at successive sheets of TCNQ ions. The potential difference reduces the energy gap for electron transfer in a lateral direction between the successive sheets.

The structure of the low-temperature modification of N,N-dimethylmorpholinium di-7,7,8,8-tetracyano-p-quinodimethanide; DMM(TCNQ)2 (Iα) at 95 K

Monoclinic DMM(TCNQ)2 (I), space group P21/m at 294 K, has two low-temperature modifications, (Ia) and (I/3). The crystal structure of (Ia) is described: T = 95 K, C6H14NO+.2C12H4N41/2-, monoclinic, P21/c, a = 13.280 (4), b =26.497 (4), c= 14.782 (4)/~, /3 =92.35 (3) °, V= 5197/~3, Z=8, Mr = 524.6, Dx = 1.341 Mg m -a, MoK~, A = 0.71069 A,/x = 0.095 mm -l, F(000) = 2184, wR(F) = 0.062 for 12 232 independent reflections. At 294 K, the DMM molecules in (I) are randomly disordered, occupying two positions related by the mirror plane in the structure. At the phase transition from (I) to (Ia) at Tc=260 K, this disorder disappears. The DMM ordering is accompanied by slight shifts of the TCNQ molecules, which change the TCNQ stacks from dimeric to octameric. The physical significance of the structural changes is discussed.