Yvette Tran

Wettability Patterning by UV-Initiated Graft Polymerization of Poly(acrylic acid) in Closed Microfluidic Systems of Complex Geometry

Many microfluidic applications require modified surface wettability of the microchannels. Patterning of wettability within enclosed microfluidic structures at high spatial resolution has been challenging in the past. In this paper, we report an improved method for altering the surface wettability in poly(dimethylsiloxane) (PDMS) microchannels by UV-induced graft polymerization of poly(acrylic acid). Our method presents significant improvements in terms of wettability contrast and spatial resolution of the patterned structures as compared to recent literature and is in particular applicable to complex microfluidic structures with a broad range of channel sizes and aspect ratios. A key part of our work is the clear description of the surface treatment process with the identification of key parameters, some of which have been overlooked, neglected, or misinterpreted in previous works. We have studied these key parameters in detail and provide recommended values for each parameter supported by experimental results. This detailed understanding of the treatment process and the effects of the critical parameters on it allowed us to significantly improve quality and reliability of the treatment process.

Benzophenone absorption and diffusion in poly(dimethylsiloxane) and its role in graft photo-polymerization for surface modification.

Following the great success of traditional microfluidic devices across many disciplines, a new class of microfluidic systems emerged in recent years, which features finely tuned, localized surface modifications within the microstructures in order to keep up with the demand for devices of ever increasing complexity (lab on chip, assay on chip, etc.). Graft photopolymerization has become a powerful tool for such localized surface modifications particularly in combination with poly(dimethylsiloxane) (PDMS) devices, as it is compatible with many functional monomers and allows for high spatial resolution. However, application within enclosed PDMS microstructures and in particular well-controlled surface-directed polymerization remains challenging. Detailed understanding of the interaction between photoinitiator, benzophenone (BP), and polymer matrix is needed. We have developed a visualization technique, which allows for observation of reacted BP in situ within the PDMS matrix. We present a detailed study on solvent-driven BP diffusion providing results essential to successful surface treatment. We also identified and investigated photoinitiator inhibition by oxygen and provide appropriate mitigation strategies.

Reversible adhesion between a hydrogel and a polymer brush

We have developed a new experimental methodology to investigate the adhesive properties of hydrogels on solid surfaces under fully immersed conditions. The method, based on contact mechanics, provides time-resolved reproducible and quantitative data on the work of adhesion between a hydrogel at swelling equilibrium and a planar surface grafted with responsive brushes. We used poly(N,N-dimethylacrylamide) (PDMA) and polyacrylamide (PAM) as model gels and poly(acrylic acid) (PAA) as pH dependent polymer brush. The effect of pH, contact time and debonding velocity on adhesive interactions was specifically investigated. As expected from molecular interactions, we found that adhesion increased as the pH decreased and this was attributed to the formation of hydrogen bonds at the interface. Surprisingly, however, the buildup of adhesion increased slowly with the time of contact up to one hour and depended markedly on debonding velocity despite the very elastic nature of the hydrogels. Furthermore, the maximum pH where adhesion was observed was significantly higher for the couple PAM–PAA than for the couple PDMA–PAA, in contrast with the onset of molecular interactions in dilute solutions.

Multiscale Surface-Attached Hydrogel Thin Films with Tailored Architecture

A facile route for the fabrication of surface-attached hydrogel thin films with well-controlled chemistry and tailored architecture on wide range of thickness from nanometers to micrometers is reported. The synthesis, which consists in cross-linking and grafting the preformed and ene-reactive polymer chains through thiol-ene click chemistry, has the main advantage of being well-controlled without the addition of initiators. As thiol-ene click reaction can be selectively activated by UV-irradiation (in addition to thermal heating), micropatterned hydrogel films are easily synthesized. The versatility of our approach is illustrated by the possibility to fabricate various chemical polymer networks, like stimuli-responsive hydrogels, on various solid substrates, such as silicon wafers, glass, and gold surfaces. Another attractive feature is the development of new complex hydrogel films with targeted architecture. The fabrication of various architectures for polymer films is demonstrated: multilayer hydrogel films in which single-networks are stacked one onto the other, interpenetrating networks films with mixture of two networks in the same layer, and nanocomposite hydrogel films where nanoparticles are stably trapped inside the mesh of the network. Thanks to its simplicity and its versatility this novel approach to surface-attached hydrogel films should have a strong impact in the area of polymer coatings.

Submicrometric Films of Surface-Attached Polymer Network with Temperature-Responsive Properties.

Temperature-responsive properties of surface-attached poly(N-isopropylacrylamide) (PNIPAM) network films with well-controlled chemistry are investigated. The synthesis consists of cross-linking and grafting preformed ene-reactive polymer chains through thiol-ene click chemistry. The formation of surface-attached and cross-linked polymer films has the advantage of being well-controlled without any caution of no-oxygen atmosphere or addition of initiators. PNIPAM hydrogel films with same cross-link density are synthesized on a wide range of thickness, from nanometers to micrometers. The swelling-collapse transition with temperature is studied by using ellipsometry, neutron reflectivity, and atomic force microscopy as complementary surface-probing techniques. Sharp and high amplitude temperature-induced phase transition is observed for all submicrometric PNIPAM hydrogel films. For temperature above LCST, surface-attached PNIPAM hydrogels collapse similarly but without complete expulsion of water. For temperature below LCST, the swelling of PNIPAM hydrogels depends on the film thickness. It is shown that the swelling is strongly affected by the surface attachment for ultrathin films below ∼150 nm. For thicker films above 150 nm (to micrometers), surface-attached polymer networks with the same cross-link density swell equally. The density profile of the hydrogel films in the direction normal to the substrate is confronted with in-plane topography of the free surface. It results that the free interface width is much larger than the roughness of the hydrogel film, suggesting pendant chains at the free surface.

Structure of surfaces and interfaces of poly(N,N-dimethylacrylamide) hydrogels.

We investigated the surface structure of hydrogels of poly(N,N-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds. While the macroscopic tests proved the existence of a relatively thick under-cross-linked layer, we also demonstrated by neutron reflectivity that the gel surface at the submicrometric scale (500 nm) was also affected by the surface treatment of the mold. These results should have important implications for the measurement of macroscopic surface properties of these hydrogels such as friction or adhesion.

Probing pH-responsive interactions between polymer brushes and hydrogels by neutron reflectivity.

We investigated the effect of specific interactions on the structure of interfaces between a brush and a hydrogel on the polymer chain length scale. We used a model system for which the interactions between the brush and the gel are switchable. We synthesized weak polyelectrolyte brushes of poly(acrylic acid) and hydrogels of polyacrylamide and poly(N,N-dimethylacrylamide) which interact solely when the poly(acrylic acid) is mainly in its acidic form. The monomer density profiles of the poly(acrylic acid) brush immersed in pure deuterium oxide (D2O) or in contact with a D2O-swollen gel were determined by neutron reflectivity. At pH 2 when the brush is in its neutral form, it interacts with the gel by hydrogen bonds while at pH 9 when the brush is a polyelectrolyte it is not interacting with the gel. Our results show that the presence of interactions with the gel at pH 2 increases the swelling ratio of the brush relative to that in pure D2O, meaning that the brushes exhibit conformations which are more extended from the surface than in the absence of interactions.

pH- and Thermo-responsive Polymer Assemblies in Aqueous Solution

Responsive polymers were prepared by copolymerizing a small amount of ionizable monomers, acrylic acid or N,N-dimethylaminopropylmethacrylamide, with N-isopropylacrylamide (NIPA) and the solubility in aqueous solution of these PNIPA derivatives were quantitatively studied. From differential scanning calorimetry experiments, it was shown that the pH strongly influences the phase separation of these copolymers (temperature and enthalpy), which totally disappears when ionizable groups are fully charged. At pH 7, all PNIPA copolymers remain soluble in water at all temperatures but their mixtures show a phase separation above a critical temperature due to the formation of a reversible inter-polyelectrolyte complex. When the responsive stickers are grafted on a poly(acrylamide) backbone, pH and temperature are still able to drive the association process at a local scale, giving rise to a sol/gel transition of semi-dilute solutions. The structure and the viscoelastic properties of these macromolecular assemblies are investigated by small angle neutron scattering and rheology and their responsivity is discussed as a function of pH and temperature.