Characterizing electroactive polymer films : from fundamentals to fingerprints

Visualisation of latent fingerprints present on metallic surfaces has been demonstrated by means of spatially selective deposition of conducting copolymers. This novel technique utilises the inhibition of electrochemical processes on areas that have been masked by the fingerprint. This results in electropolymerisation between the ridges, generating a negative image of the fingerprint. The efficiency of recovery in forensically challenging scenarios of the copolymers was compared with the corresponding homopolymers. An extension of this electrochromic enhancement to include fluorescence has been developed using a novel synthetic pathway aiming to create more free volume to aid the inclusion of bulky fluorophore moieties. The unique selectivity of neutron reflectivity (NR), with isotopic contrast variation, permitted the diagnosis of chemical and structural changes within the depth profile of the polymer during the incorporation of the fluorophore moieties. The properties of conducting polymer films are determined by film composition and structure which, this thesis will show, can lead to different routes to electroneutrality maintenance during electrochemically controlled redox switching (doping/undoping). NR was used to quantify the diverse permeation characteristics of conventional solvents and ionic liquids into an electroactive copolymer. It revealed how the availabilities of these mechanisms are dictated by anion and cation sources and sinks in the film and liquid phases where molecular solvent is/is not present. In multi-layered systems, the nature of the polymer/polymer interface is central to the rectifying (segregated) or capacitive (interdiffused) characteristics of the films, such that the spatial distribution of the different polymer components determines the optical and electronic properties of the films. These properties are important for the potential applications of these multi-component systems in energy storage. NR was used to probe the extent of segregation and solvation properties of multi-layer films and revealed the order the components are deposited has a great effect on the final film properties

Electrochromic enhancement of latent fingerprints by poly(3,4-ethylenedioxythiophene)

Spatially selective electrodeposition of poly-3,4-ethylenedioxythiophene (PEDOT) thin films on metallic surfaces is shown to be an effective means of visualizing latent fingerprints. The technique exploits the fingerprint deposit as an insulating mask, such that electrochemical processes (here, polymer deposition) may only take place on deposit-free areas of the surface between the ridges of the fingerprint deposit; the end result is a negative image of the fingermark. Use of a surfactant (sodium dodecylsulphate, SDS) to solubilise the EDOT monomer allows the use of an aqueous electrolyte. Electrochemical (coulometric) data provide a total assay of deposited material, yielding spatially averaged film thicknesses, which are commensurate with substantive filling of the trenches between fingerprint deposit ridges, but not overfilling to the extent that the ridge detail is covered. This is confirmed by optical microscopy and AFM images, which show continuous polymer deposition within the trenches and good definition at the ridge edges. Stainless steel substrates treated in this manner and transferred to background electrolyte (aqueous sulphuric acid) showed enhanced fingerprints when the contrast between the polymer background and fingerprint deposit was optimised using the electrochromic properties of the PEDOT films. The facility of the method to reveal fingerprints of various ages and subjected to plausible environmental histories was demonstrated. Comparison of this enhancement methodology with commonly used fingerprint enhancement methods (dusting with powder, application of wet powder suspensions and cyanoacrylate fuming) showed promising performance in selected scenarios of practical interest

Nanoscale control of interfacial processes for latent fingerprint enhancement

Latent fingerprints on metal surfaces may be visualized by exploiting the insulating characteristics of the fingerprint deposit as a “mask” to direct electrodeposition of an electroactive polymer to the bare metal between the fingerprint ridges. This approach is complementary to most latent fingerprint enhancement methods, which involve physical or chemical interaction with the fingerprint residue. It has the advantages of sensitivity (a nanoscale residue can block electron transfer) and, using a suitable polymer, optimization of visual contrast. This study extends the concept in two significant respects. First, it explores the feasibility of combining observation based on optical absorption with observation based on fluorescence. Second, it extends the methodology to materials (here, polypyrrole) that may undergo post-deposition substitution chemistry, here binding of a fluorophore whose size and geometry preclude direct polymerization of the functionalised monomer. The scenario involves a lateral spatial image (the whole fingerprint, first level detail) at the centimetre scale, with identification features (minutiae, second level detail) at the 100–200 μm scale and finer features (third level detail) at the 10–50 μm scale. However, the strategy used requires vertical spatial control of the (electro)chemistry at the 10–100 nm scale. We show that this can be accomplished by polymerization of pyrrole functionalised with a good leaving group, ester-bound FMOC, which can be hydrolysed and eluted from the deposited polymer to generate solvent “voids”. Overall the “void” volume and the resulting effect on polymer dynamics facilitate entry and amide bonding of Dylight 649 NHS ester, a large fluorophore. FTIR spectra demonstrate the spatially integrated compositional changes. Both the hydrolysis and fluorophore functionalization were followed using neutron reflectivity to determine vertical spatial composition variations, which control image development in the lateral direction