Abstract
Molecular doping of a polythiophene with oligoethylene glycol side chains is found to strongly modulate not only the electrical but also the mechanical properties of the polymer. An oxidation level of up to 18% results in an electrical conductivity of more than 52 S cm-1 and at the same time significantly enhances the elastic modulus from 8 to more than 200 MPa and toughness from 0.5 to 5.1 MJ m-3. These changes arise because molecular doping strongly influences the glass transition temperature Tg and the degree of π-stacking of the polymer, as indicated by both X-ray diffraction and molecular dynamics simulations. Surprisingly, a comparison of doped materials containing mono- or dianions reveals that - for a comparable oxidation level - the presence of multivalent counterions has little effect on the stiffness. Evidently, molecular doping is a powerful tool that can be used for the design of mechanically robust conducting materials, which may find use within the field of flexible and stretchable electronics.
Highlights
Molecular doping is widely used to control the electrical properties of conjugated polymers, for applications ranging from flexible and wearable electronics to bioelectronics
To compare the doping-induced changes in elastic modulus that have been observed for different polymers we here define a figure of merit Z = log(Edoped/Eneat), which considers the ratio of the elastic modulus of the doped material Edoped and the neat
The doped material had a uniform appearance, which is in stark contrast to the granular texture of bulk samples of P3HT co-processed with F4TCNQ.[8]
Summary
Molecular doping is widely used to control the electrical properties of conjugated polymers, for applications ranging from flexible and wearable electronics to bioelectronics. A different perspective is established: molecular doping can be used as a tool to control the electrical and mechanical properties of conjugated polymers. The structural changes that occur as a result of doping can be used to tune the stiffness and toughness by a large amount, resulting in electrically conducting materials that feature an elastic modulus comparable to that of commodity polymers such as polyethylene. Molecular doping of stiff conjugated polymers does not tend to strongly alter their mechanical properties (Table 1).[7,8,9] As a result, doping is typically not considered as a tool that allows to adjust the elastic modulus of conjugated polymers. A comparison of the few existing reports allows us to predict what type of changes in stiffness can be expected upon doping, as discussed in more detail below: doping of stiff conjugated polymers can lead to a slight decrease in modulus while doping of soft materials tends to increase the modulus
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