Polymer Design and Processing for Liquid-Core waveguides

Block copolymers are known for their self-assembling ability utilized for bottom-up nanostructure fabrication. It is particularly capitalized in the context of present work where nanoporous scaffolds are created from a 1,2-polybutadiene-b-polydimethylsiloxane (1,2-PB-b-PDMS) block copolymer precursor material. Upon attaining thermodynamically stable gyroid phase segregation, nanoporosity is induced by chemically removing PDMS, the so-called sacrificial block. The isotropic nanoporosity in the polymer is utilized in fabricating a novel type of waveguides for opto-fluidic applications, which we call solid-liquid core waveguides, shortly SLCW. The high refractive index core of a SLCW consists of nanoporous polymer (solid) rendered hydrophilic and filled with water (liquid), while the low refractive index cladding consists of air-filled hydrophobic nanoporous polymer. Under conditions of total internal reflection, light is confined within the solid-liquid core. Controlled regions of the originally hydrophobic nanoporous 1,2-PB are
rendered hydrophilic by photochemical modification of the polymer in the presence of photolithographic masks. In contact with water the hydrophilic regions are spontaneously filled with water by capillary suction, forming the core, while the unmodified hydrophobic regions remain dry, forming the clad. Two types of photo-modification reactions are presented in this thesis: photo-oxidation and thiol-ene photo-clicking.
The hydrophilicity is firstly induced by surface photochemistry via UV photo-oxidation of nanoporous 1,2-PB. Detailed quantitative and qualitative analysis of photo-oxidation in the presence of air is carried out by gravimetry, titrimetry and spectrometry. Distribution study of the hydrophilic photo-products relative to the polymer-air interface shows high concentration at the nanoporous interface. Thus, the majority of cross-linked PB matrix remains unmodified.
Distribution of the hydrophilic groups along the depth is carried out by energy dispersive x-ray spectroscopy. It shows a highly heterogeneous photo-oxidation reaction with most of the oxygen fixed close to the surface facing the UV source. Optical characterization of UV photo-oxidation based devices is performed to report various losses occurring during light guiding experiments.
Thiol-ene click reactions are also used for the hydrophilization of the internal surface area of nanoporous polymer. This is done by photochemical reaction of the vinyl unsaturations of 1,2-PB at the interface (ene functionality) with hydrophilic thiols in the presence of a photoinitiator. The reaction is monitored by UV-Vis, FT-IR and contact angle measurements. Quantum yields of the photochemical reactions are estimated. Kinetic aspects of the photo grafting reaction on the
nanoporous wall are studied using gravimetry. The fabrication of solid-liquid core waveguides is done by adapting the know-how on thiol-ene photochemistry to standard microfabrication cleanroom setup and UV lithography. Contrast curves for thiol-ene systems are reported to comment on the homogeneity of the polymer modification by the reaction. Finally optical characterization of devices is carried out to report propagation loss values.
The UV photo-oxidation of nanoporous 1,2-PB is found to be a simple but heterogeneous surface modification technique compared to thiol-ene photo-modofication. Reaction times for photooxidation are longer by factors of 50-300 than thiol-ene reaction times. Compared to the oxidation, the thiol-ene reaction imparts better control, homogeneity and results in about half propagation loss in the fabricated waveguides.
The fabricated waveguides are also tested in few preliminary biosensing experiments. Antibody fragments, Fab that quench fluorescence from the fluorescein dye are introduced into the nanopores. The distribution of Fab fragments in the hydrophilic core is mapped by confocal microscopy. The study of their ability to quench the fluorescence from the dye is work under progress.