Synthesis and Characterization of Biopolymer and Poly (3.4-ethylenedioxythiophene) 3-Dimensionally Ordered Structures via Templating with Synthetic Polystyrene Opal

Abstract

Photonic crystals are excellent candidates for a wide range of applications due to their optical and structural properties and responsivity to different media. In particular colloidal crystals can be utilized as a template to make inverse structures with high levels of well ordered porosity. Polystyrene is widely used to make colloids. Acid-terminated polystyrene colloids with diameters of 190.3 nm, 210.6 nm, 233.9 nm, 263.5 nm, 278.6 nm and 293.9 nm have been produced. Evaporation-based deposition was used to produce synthetic opal due to self assembly of the colloids into a crystal lattice. The six colloid sizes formed photonic crystals, known as synthetic opal, with FCC lattice packing and well defined band-gaps, at 387.4 nm, 493.4 nm, 556.8 nm, 579.4 nm, 637.2 nm and 695.6 nm respectively. The polystyrene synthetic opal was used as a template for the infiltration of the void spaces by sol-gel infiltration and electropolymerisation methods. The removal of the polystyrene template via solvent etching typically gives a highly ordered 3D arrangement of pores, inverse to the original opal structure, known as inverse opal. The synthesis of inverse opal with chitosan, and chitosan-polyethelene glycol (PEG) mixtures where attempted via sol-gel infiltration. While this was largely unsuccessful, relevant results were still obtained. A chitosan photonic crystal structure was achieved that was colourless in a dry state, but turned red upon the exposure to water. Scanning electron microscopy (SEM) revealed a regular stacking of rigid structure filled with polymer network. Samples produced with a chemically cross-linked solution of chitosan and PEG, via dip-coating the polystyrene template in the infiltrate solution, produced blue-green films after solvent etching. The structure was shown to have a mixture of collapsed, and semi-ordered pore although not quite achieving the high order of inverse opal. Poly(3,4-ethylenedioxythiophene)doped with polystyrene sulfonate (PEDOT:PSS) was electropolymerized, using a galvanostatic current of 600 uA. To examine the optimal polymer amount to form a robust structure, on removal of the template, deposition times two ranges of films were produced: 1 set using current density of 300 uA cm-2, (area of 2 cm 2) with a range of 4 total currents and PS templates 387, 493 and 580 nm; and 1 set using 375 uA cm-2, (area of 1.6 cm 2) and 1 total current (225 mC), using 4 PS templates 387, 493, 580 and 695. The first set showed a lack of PEDOT:PSS film coverage upon solvent etching with dimethylformamide (DMF). The second set produced with higher current density and total charge showed improved film uniformity and inverse opal structure was apparent, after solvent etching, with the use of SEM. SEM revealed inverse opal, average pore diameters of 133.8 nm, 169.9 nm, 219.9 nm and 212.2 nm produced using the polystyrene templates: 387 nm, 493 nm, 580 nm and 695 nm respectively. There was only a 3.1 - 4.6% reduction of charge storage capacity CSC) with the introduction of pores into the structure, in comparison to bulk PEDOT:PSS, except for one sample produced with a 695 nm polystyrene template which saw a 15% reduction in CSC correlating with an increased pore wall thickness. Electrochemical impedance spectroscopy did not reveal a large difference between samples in terms of their electrical impedance compared to bulk films. The benefits of this structure would be found in the increased physical surface area, increasing the capacity for adsorption and drug loading.