Electronic transport and paramagnetic properties of acetylene–carbon monoxide copolymers

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Abstract
Transport and magnetic properties of acetylene–carbon monoxide copolymers have been studied. The copolymers AC-11, AC-21, and AC-41 have compositions [(C2H2)1(CO)0.15]x, [(C2H2)1(CO)0.1]x, and [(C2H2)1(CO)0.05]x, respectively, and the prefixes p and i designate the pristine and thermally isomerized copolymers. The undoped i-AC-copolymers have unpaired spin concentrations [S⋅] about 10−4 of that in trans-[CH]x, yet the amorphous copolymers have up to ten times the RT conductivity σRT of the crystalline polyacetylenes. These homo and copolymers have the same thermopower coefficient 𝒮 and comparable concentration of intrinsic positive carriers as judged from the log σRT vs log y dependence, where y is the mole fraction of dopant. Undoped p-AC copolymers have σRT lying between that of cis- and trans-[CH]x depending upon the trans content in the copolymers. The unpaired spins in the i-AC-copolymers showed motional narrowing and Curie dependence as in trans-[CH]x in spite of the fact that the average sequence lengths of CH segments between C=O units in i-AC-11-copolymer is less than the soliton domain width in trans-[CH]x. Heavily doped i-AC-copolymers have σRT within a factor of 2 of similarly doped trans-[CH]x. The σRT of heavily iodine doped p-AC-copolymer has about one-third to one-half of that of cis-[CHI0.1]x. Doping of both p-AC- and i-AC-copolymers with I2 is homogeneous. Some i-AC-copolymers can be doped homogeneously with AsF5 but doping is heterogeneous for the p-AC-copolymers. These doping behaviors are parallel to those reported earlier for polyacetylene. Abrupt increase of σRT of 106–107-fold occurred with about five-fold change of dopant concentration in the vicinity of y∼10−3. There was also a more sharp change in 𝒮 with y. The results cannot be reconciled with models invoking moving domain excitations; a possible mechanism for carrier migration may be vibronically coupled electron tunneling in undoped and lightly doped polymers. The low transport is attributed to carriers being strongly pinned by the Coulomb potential of the counterions. When this pinning potential is screened as dopant concentration increases, the carriers undergo a transition from a ‘‘glassy’’ state to a ‘‘melt’’ state with large increase in carrier mobility. This carrier glass transition or carrier mobility transition is distinct from that transition for the onset of Pauli susceptibility, which is observed at much higher dopant concentrations of few mole percent.