One-Pot In-Situ Synthesis of Conductive Polymer/Metal Oxide Composites

Composites containing an inorganic material and conductive polymer have a wide range of material applications, specifically in electronics, sensors, membranes, and batteries. With increasing demand for these applications there is a pressing need for improved materials achieved by synthetic methods capable of creating a homogeneous and tunable product. Creating a composite can improve the properties of the individual constituents while enabling the selection and tunability of specific properties of the system such as conductivity, thermal stability, morphology and processability. These properties can be altered by the selection of composite components, the ratio of matrix to filler, and the method of composite synthesis. The most prevalent method to make composites is using a multistep process in which the polymer and inorganic components are synthesized or modified separately and then combined (ex-situ). Although this method allows for careful optimization of each individual component, this process is not economical or efficient, and it can decrease the homogeneity of the composite. Another common method for composite synthesis is an in-situ method in which one material is preformed and the secondary material is formed in the presence of this preformed component. This method can improve some of the shortcomings of an ex-situ method but may still encounter fundamental limitations with respect to homogeneity, cost, efficiency, and performance. To overcome these deficiencies, a synthetic approach that possesses high atom efficiency, low cost, and creates a homogeneous product is required. The work in this dissertation develops a one-pot in-situ method to form composites. It is vital to the success of this method that the formation of both phases can be achieved under compatible conditions, therefore, significant study on the individual formation of each component was carried out first. These phases will be combined under compatible conditions to form a homogeneous product.

The specific conductive polymers investigated in this research for use in composites are poly(3-hexylthiophene) (P3HT) and polypyrrole (ppy). P3HT is widely considered as a benchmark material to study the effect of various fillers or synthetic methods in electronic and photovoltaic applications. Ppy has become a primary conductive polymer of interest due to its high electrical conductivity, chemical and thermal stability, cost, and environmental friendliness and is used in a wide variety of applications, including membranes, sensors, coatings, wires, and polymeric batteries. Although there is an overlap in applications between the two polymers, the necessary synthetic methods are very different, as P3HT requires a non-aqueous environment and ppy synthesis can be completely performed in aqueous media.

To compensate for the shortcomings of the polymer system and to improve the properties of the material, transition metal oxides can be used as the inorganic filler. Vanadium oxide synthesis was independently investigated for use in P3HT composites using the non-hydrolytic sol-gel method (NHSG). This method allows low temperature crystallization of metal oxides under compatible conditions with the P3HT polymerization process. V₂O₅ was successfully crystallized using NHSG synthesis after heat treatment to 200 to 250 °C. This process involved the optimization of precursor, oxygen source, concentration, reaction time, and ratio of O:V to obtain consistent results.

Based on previous P3HT syntheses in the Lind lab and comprehensive synthetic conditions established for V₂O₅, conditions for a one-pot in-situ synthesis were selected. Two series of composites were synthesized with different solvent systems (chloroform and an 18:2 v:v mixture of dichloromethane:nitrobenzene) as these systems showed improved molecular weights and high percent yields compared to other solvent systems for P3HT formation. Composites were formed by polymerizing 3HT using FeCl₃ as the oxidizing agent while simultaneously reacting vanadium oxide precursors (VCl₃ and VOCl₃) with diisopropyl ether. P3HT/V₂O₅ composites were successfully synthesized by this method when VCl₃ was the precursor. Most composites resulted in a product with improved conductivity and thermal stability compared to pure P3HT.

Molybdenum and tungsten oxides have drawn particular interest for similar reasons to ppy. These metal oxides appeal since they are inexpensive, electrochemically active, easy to synthesize, relatively non-toxic, and environmentally friendly. In addition, both metal oxides can be crystallized at low temperatures, making them a good target for use in aqueous synthesis for polypyrrole composites. By studying these metal oxides, a range of amenable reaction conditions were developed for use in composites via a polycondensation reaction. It was determined that the synthesis of WO₃ would have a synergistic relationship with polypyrrole formation without any additional adjustments to pH, while MoO₃ required slightly lower pH values to crystallize.

Pure polypyrrole synthesis was optimized prior to use in a composite. Ammonium persulfate served as the oxidizing agent to polymerize the pyrrole. Initial studies established that a 1:1 ratio of monomer:oxidizing agent at a concentration of 0.175 M produced high conductivities and optimal yields, therefore these conditions were applied to composite synthesis. Composites with ppy/MoO₃ and ppy/WO₃ were successfully synthesized in aqueous media using a one-pot in-situ method with various weight loadings between 1 and 50%. These composites were designed to improve or maintain the electrical and thermal properties of the individual components, while increasing or improving the potential use in various applications. For ppy/MoO₃ composites most weight loadings gave conductivities similar to pure polypyrrole synthesized by the previously developed method. Compared to ppy/MoO₃ composites in literature, the majority of composites developed by this method possessed superior conductivities. Furthermore, the presence of MoO₃ increased thermal stability of the polymer. For ppy/WO₃ composites conductivities were equal to pure polypyrrole at low weight loadings but thermal stability did not improve until weight loadings above 33.7%. The conductivities at low weight loadings were similar to literature results for higher WO₃ loadings and were achievable through simpler synthetic means. By using chemical methods that are highly scalable and have markedly improved or similar performance, future work involving the implementation of these materials in devices is possible.