Ronald J. Weagley

Manifestation of glass transition in electronic charge transport in Si and Ge Backbone polymers

Abstract Electronic transport of photoinjected holes in high molecular weight linear polymers with silicon and germanium backbone is electric field dependent and thermally activated. At fixed electric fields, during both rate and step heating, the mobility activation energies (ϵ) in all these polymers change abruptly and reversibly at critical temperatures (ifT c ) without concurrent abrupt changes in mobilities. These critical temperatures coincide with the respective glass transition temperatures. In all cases, ϵ at T > T g is lower than ϵ at T T g , typically by a factor of 2 to 3. In polymers with aliphatic side groups ϵ ( T > T g ) is extremely low, practically approaching zero. The change of activation near T g is similar to that observed in amorphous charge-transporting polymers and other glasses, including selenium and its alloys. The universality of this phenomenon is emphasized.

Chemical modification of charge transport in silicon backbone polymers

Abstract Charge transport in highly insulating Si backbone polymers has been studied by the Time Of Flight technique. Transit pulse shapes and the field and temperature dependence of the hole drift mobility (for many systems about 10 −4 cm 2 /Vs at 295K and E = 10 5 V/cm) have been characterized for a prototypical example, poly(methylphenylsilylene), PMPS. The effect of dopants, which differ in their relative oxidation potentials, on the drift mobility and the transit pulse shape have also been studied. Comparison of the overall results with other glassy polymeric media suggest that transport proceeds by hopping among chain backbone derived domain like segments.

Remnant Cis Bonds in “Trans”-Polyacetylene

Abstract We report the infrared spectra of “trans”-polyacetylene that has been thermally prepared from c/s-(CH)x. The minimum of 5-7 percent residual 740 cm−1 absorption observed for all cases studied is suggested to be due to isolated remnant c/s linkages. It is shown that these remnant cis linkages may confine the neutral soliton and hence affect the neutral soliton diffusion coefficient. Implications for the soliton generation mechanisms and for conjugation lengths derived from Raman spectra are pointed out.

Effect of Thermally Induced Transitions on Electronic Transport in Aliphatic Polysilylenes

Electronic transport of photoinjected holes in high molecular weight linear polymers with silicon and germanium backbone displays a familiar pattern of electric field and temperature dependence similar to behavior already reported in a very diverse collection of glassy molecular solids. At fixed electric fields, during both rate and step heating, the mobility activation energy e in all these polymers changes abruptly and reversibly at a temperature Tc, which is specific for each polymer, without concurrent abrupt changes in mobilities. These temperatures coincide with the respective glass transition temperatures Tg. In all cases, the below-Tg activation is higher than above Tg. In polymers with aliphatic side groups, e(t > t g) is extremely low, practically approaching zero. The change of activation near Tg is similar to that observed in amorphous charge-transporting polymers and other glasses including selenium and its alloys. Polymers such as poly(di-n-hexylsilylene) (PDHS) which undergo conformational transitions at temperatures TTr due to side chain crystallization and melting, also experience a change in mobility and its activation energy at TTr. The change in the transport behavior and the association with the bathochromic shift in the uv spectra is demonstrated.

Photoexcitation spectroscopy of iodine doped polyacetylene

Abstract We have measured the photomodulation and Raman spectra of iodine doped cis and trans polyacetylene [CH(I3−)y]x. The solitonic photoinduced absorption (PA) band gradually disappears from the spectra consistent with the reduction of neutral solitons density upon doping. The high-energy PA band and its associated interband bleaching shift towards higher energies with doping indicating a substantial increase of the system optical energy gap.

Thermal Isomerization of Polyacetylene Films

Abstract Isomerization of the as-formed (-78°C) cis-polyacetylene to the trans isomer is relevant to the electronic structure, the crystal structure and the electrical properties of this prototypical conductive polymer. We have studied the process using Fourier transform infrared (FTIR) and 13C magic angle nuclear magnetic resonance (NMR) spectroscopies at temperatures between 80 and 110°C. The observed kinetics will be discussed in terms of three basic processes: a second order process which randomly converts cis-cis-cis (CCC) sequences to trans-cis-trans (TCT) sequences via a pair of biradicals, a fourth order process that converts CTCC sequences to TCTT sequences via four biradicals, and two fourth order processes which allow sequential migration along the chain. The correlation of observed electron spin resonance and x-ray results, among others, with these mechanisms will be shown.