The optoelectronic properties of conjugated polymers are dictated by their chain conformations, which depend on the interplay of delocalization of electrons along the π-conjugated backbone and the ...intrachain interactions of pendant side chains. Here, we leverage small-angle neutron scattering to measure the chain shapes of several classes of commonly used, high mobility donor–acceptor conjugated polymers in dilute solution. We find that these model conjugated polymers are semiflexible with persistence lengths ranging from several to hundreds of nanometers, dependent on the molecular structure of the polymer, indicating the importance of repeat unit geometry, particularly side-chain size and branching, on the overall chain conformations. The measured persistence lengths show good agreement with those calculated according to dihedral distributions predicted from density functional theory. Larger persistence lengths are shown to correlate with increased charge-carrier mobility, signifying the importance of rational molecular design to obtain high persistence length organic semiconductors and thus advantageous optoelectronic properties.
The interactions between counterions and electronic carriers in electrically doped semiconducting polymers are important for delocalization of charge carriers, electronic conductivity, and thermal ...stability. The introduction of a dianions in semiconducting polymers leads to double doping where there is one counterion for two charge carriers. Double doping minimizes structural distortions, but changes the electrostatic interactions between the carriers and counterions. Polymeric ionic liquids (PIL) with croconate dianions are helpful to investigate the role of the counterion in p‐type semiconducting polymers. PILs prevent diffusion of the cation into the semiconducting polymers during ion exchange. The redox‐active croconate dianions undergo ion exchange with doped semiconducting polymers depending on their ionization energy. Croconate dianions are found to reduce doped films of poly(3‐hexyl thiophene), but undergo ion exchange with a polythiophene with tetraethylene glycol side chains, P(g42T‐T), that has a lower ionization energy. The croconate dianion maintains crystalline order in P(g42T‐T) and leads to a lower activation energy for the electrical conductivity than PF6− counterions. The control of the doping level with croconate allows optimization of the thermoelectric performance of the semiconducting polymer. The thermal stability of the doped films of P(g42T‐T) is found to depend strongly on the nature of the counterion.
Polymeric ionic liquids with croconate dianions are used to investigate the role of the charge state of the counterion in p‐type semiconducting polymers. By precisely matching energy levels of the redox‐active croconate dianion and the semiconducting polymer, efficient ion‐exchange is found, providing a route to tune the doping level while minimizing structural perturbations for optimization of thermoelectric performance.
Electrical doping of organic semiconductors is critical for their use in electrical devices. However, many high-performance semiconductors exhibit drastically reduced solubilities when doped in ...solution, making it difficult to deconvolute the roles of doping and morphological changes on their electrical properties. Here, we report the synthesis of a semiconducting polymer based on poly(3,4-propylenedioxythiophene) (ProDOT) substituted with oligo(ethylene glycol) (EG) side chains that is designed to solvate dopant molecules. When doped with F4TCNQ in solution, the polymer P(ProDOT-EG) undergoes efficient charge transfer while remaining soluble with no indication of aggregation. The electrical conductivity of thin films cast from heavily doped solutions is ∼1 S/cm; optical spectroscopy reveals that the doping efficiency is reduced upon formation of the solid film. Diffusion of F4TCNQ from the vapor phase into films of neutral P(ProDOT-EG) yields comparable conductivities to films cast from doped solutions. Absorption spectroscopy and X-ray diffraction of doped films reveal that the doping efficiency and nature of charge carriers (polaron or bipolaron) are strongly affected by the processing route. The lack of the localized polaron signature for films doped by infiltration of the dopant from the vapor phase indicates that morphology has a large role in determining the nature of the charge carriers in semiconducting polymers.
Decoupling bulk mechanics and ion conduction in conventional ion conducting polymers is challenging due to their mutual dependence on segmental chain dynamics. Polymers based on dynamic metal–ligand ...coordination are promising materials toward this aim. This work examines the effect of the nature and concentration of metal bis(trifluoromethylsulfonyl)imide (MTFSI) salts on the mechanical properties and ionic conductivity of poly(ethylene oxide)-stat-(allyl glycidyl ether) functionalized with tethered imidazole ligands (PIGE). Varying the cation identity of metal salts mixed in PIGE enables dramatic tunability of the zero-frequency viscosity from 0.3 to 100 kPa s. The ionic conductivity remains comparable at approximately 16 μS cm–1 among mono-, di-, and trivalent salts at constant metal-to-ligand molar ratios due to negligible changes in glass transition temperatures at low ion concentrations. Thus, polymers based on metal–ligand coordination enable decoupling of polymer zero-frequency viscosity from ion conduction. Pulsed-field-gradient NMR on PIGE containing Li+ or Zn2+ salts complement electrochemical impedance spectroscopy to demonstrate that both the anion and cation contribute to ionic conductivity.
Poly(dimethylsiloxane) (PDMS)- and poly(ethylene oxide) (PEO)-based block copolymer coatings functionalized with amphiphilic, surface-active, and sequence-controlled oligomer side chains were ...studied to directly compare the effects of hydrophilicity, hydrogen bonding, and monomer sequence on antifouling performance. Utilizing a modular coating architecture, structurally similar copolymers were used to make direct and meaningful comparisons. Amphiphilic character was imparted with non-natural oligopeptide and oligopeptoid pendant chains made from oligo-PEO and surface-segregating fluoroalkyl monomer units. Surface analysis revealed rearrangement for all surfaces when moved from vacuum to wet environments. X-ray photoelectron spectroscopy (XPS) spectra indicated that the polymer backbone and oligomer interactions play key roles in the surface presentation. Biofouling assays using the macroalga Ulva linza showed that the presence of peptoid side chains facilitated the removal of sporelings from the PDMS block copolymer, with removal matching that of a PDMS elastomer standard. The lack of a hydrogen bond donor in the peptoid backbone likely contributed to the lower adhesion strength of sporelings to these surfaces. Both the initial attachment and adhesion strength of the diatom Navicula incerta were lower on the coatings based on PEO than on those based on PDMS. On the PEO coating bearing the blocky peptoid sequence, initial attachment of N. incerta showed no measurable cell density.
The role of ion placement was systematically investigated in imidazolium bis(trifluoromethane)sulfonimide (ImTFSI) polymerized ionic liquids (PILs) containing pendant charges and charges in the ...backbone (sometimes called ionenes). The backbone PILs were synthesized via a facile step growth route, and pendant PILs were synthesized via RAFT. Both PILs were designed to have nearly identical charge density, and the conductivity was found to be substantially enhanced in the backbone PIL systems even after accounting for differences in the glass transition temperature (T g). Wide-angle X-ray scattering (WAXS) revealed an invariance in the location of the amorphous halo between the two systems, while the anion–anion correlation peak was shifted to lower scattering wavevector (q) in the backbone PILs. This indicates an increase in the correlation length of ions and is consistent with charge transport along a more correlated pathway following the polymer backbone. Due to the linear nature of the backbone PILs, crystallization was observed and correlated with changes in conductivity. Upon crystallization, the conductivity dropped, and eventually, two populations of mobile ions were observed and attributed to ions in the amorphous and near-crystallite regions. The present work demonstrates the important role of ion placement on local structure and conductivity as well as the ability of backbone PILs to be used as controllable optical or dielectric materials based on crystallization or processing history.
The self-assembly of a series of monodisperse rod−coil block copolymers is studied in the weak segregation limit. This unusual weakly segregated system consists of polyisoprene (PI) coil blocks and ...poly(alkoxyphenylene vinylene) (PPV) rod blocks solubilized with alkoxy side groups. The order to microphase disorder transition (ODT) and nematic isotropic (NI) transition are experimentally investigated to produce a rod−coil block copolymer phase diagram in a system that follows polymeric scaling relationships. Small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), polarized optical microscopy, depolarized light scattering, and wide-angle X-ray scattering (WAXS) are used to map the phase diagram. As the symmetric diblock copolymer is heated, a series of transitions from lamellar to nematic to isotropic phases are observed. The NI transition temperature decreases with increasing coil fraction, and at high coil fractions only an isotropic phase is observable. The phase behavior is in qualitative agreement with weak segregation calculations based on Landau expansions reported by other groups. Transmission electron microscopy (TEM) reveals unusual grain structures in the low-temperature lamellar phase. The high bending energy of the rod microdomains results in lamellae with long persistence lengths and grain boundaries defined by broken lamellae. Changes in domain spacing with temperature suggest rod rearrangements within the lamellar phase.
Progress toward durable and energy-dense lithium-ion batteries has been hindered by instabilities at electrolyte–electrode interfaces, leading to poor cycling stability, and by safety concerns ...associated with energy-dense lithium metal anodes. Solid polymeric electrolytes (SPEs) can help mitigate these issues; however, the SPE conductivity is limited by sluggish polymer segmental dynamics. We overcome this limitation via zwitterionic SPEs that self-assemble into superionically conductive domains, permitting decoupling of ion motion and polymer segmental rearrangement. Although crystalline domains are conventionally detrimental to ion conduction in SPEs, we demonstrate that semicrystalline polymer electrolytes with labile ion–ion interactions and tailored ion sizes exhibit excellent lithium conductivity (1.6 mS/cm) and selectivity (t + ≈ 0.6–0.8). This new design paradigm for SPEs allows for simultaneous optimization of previously orthogonal properties, including conductivity, Li selectivity, mechanics, and processability.
A scarcity of stable n-type doping strategies compatible with facile processing has been a major impediment to the advancement of organic electronic devices. Localizing dopants near the cores of ...conductive molecules can lead to improved efficacy of doping. We and others recently showed the effectiveness of tethering dopants covalently to an electron-deficient aromatic molecule using trimethylammonium functionalization with hydroxide counterions linked to a perylene diimide core by alkyl spacers. In this work, we demonstrate that, contrary to previous hypotheses, the main driver responsible for the highly effective doping observed in thin films is the formation of tethered tertiary amine moieties during thin film processing. Furthermore, we demonstrate that tethered tertiary amine groups are powerful and general n-doping motifs for the successful generation of free electron carriers in the solid-state, not only when coupled to the perylene diimide molecular core, but also when linked with other small molecule systems including naphthalene diimide, diketopyrrolopyrrole, and fullerene derivatives. Our findings help expand a promising molecular design strategy for future enhancements of n-type organic electronic materials.
Proton-conducting polymer membranes frequently contain nanoscale structural and ion-conducting phases. In addition to enhancing the mechanical properties, this mesoscale structure often leads to a ...significant increase in ion dynamics; however, the molecular underpinnings behind this phenomenon are not well understood. Here, a model proton-conducting polymeric ionic liquid (PIL) block copolymer is shown to have conductivity up to an order of magnitude higher than an analogous homopolymer. Variable temperature 1H solid-state magic angle spinning (MAS) NMR spectroscopy reveals that confinement in the block copolymer PIL decreases the segmental motion of the PIL and increases the structural rigidity of the ionic functional groups. This increased structural rigidity is found to dramatically increase the connectivity of the hydrogen bonding network in the block copolymer PIL according to double quantum–single quantum 1H MAS NMR. This nanoscale restructuring leads to a significant increase in Grotthuss proton hopping dynamics in the block copolymer PIL compared to the homopolymer.
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