Eric J. Amis

A SAXS study of the internal structure of dendritic polymer systems

Small-angle x-ray scattering was used to characterize the single-particle scattering factors produced by poly(amidoamine) dendrimers, poly(propleneimine) dendrimers, and polyol hyperbranched polymers in dilute solutions with methanol as solvent. Fits from electron density modeling reveal similar overall densities of the dendrimers as a function of dendrimer generation. The seventh through tenth generation poly(amidoamine) dendrimers exhibit higher order scattering features that require nearly monodisperse, spherical particles with essentially uniform internal segment densities. Dilute hyperbranched polymer solutions exhibit scattering more indicative of the inherent irregularity of internal segment densities and overall sizes to be expected within these systems. Radii of gyration estimated from electron density modeling agree reasonably well with those estimated by standard Guinier methods used in previous studies.

A rapid prototyping technique for the fabrication of solvent-resistant structures

We demonstrate a rapid prototyping technique for the fabrication of solvent-resistant channels up to and exceeding one millimeter in height. The fabrication of channels with such dimensions by conventional lithography would be both challenging and time consuming. Furthermore, we show that this technology can be used to fabricate channels with a depth that varies linearly with distance. This technique requires only a long-wavelength ultraviolet source, a mask made by a desktop printer and a commercially available optical adhesive. We demonstrate two lithographic methods: one that fabricates channels sealed between glass plates (close-faced) and one that fabricates structures on a single plate (open-faced). The latter is fully compatible with silicon replication techniques to make fluid handling devices.

Influence of counterion valency on the scattering properties of highly charged polyelectrolyte solutions

Light and neutron scattering measurements on highly charged polyelectrolyte solutions have recently provided firm evidence for the existence of “domain structures” containing many chains, even at rather low-polymer concentrations. In the present paper, we systematically investigate the influence of counterion charge valency Zc on the scattering properties of sulfonated polystyrene (PSS) solutions in water with monovalent and divalent counterions. This study is part of a larger effort to identify essential factors governing polyelectrolyte domain formation and the geometric properties of these transient structures. Neutron scattering measurements indicate that the interchain correlation length ξd within the domains becomes larger by a factor of 1.5–2 for divalent relative to monovalent counterions. This observation is consistent with the Manning model estimate of the change in effective polymer charge density Γ* with Zc and with previous observations linking ξd [from the peak position in the scattering int...

LCST Phase Separation in Biodegradable Polymer Blends: Poly(D,L-lactide) and Poly(ε -caprolactone)

A lower critical solution temperature (LCST) phase transition is reported for blends of the biodegradable polymers poly(D,L-lactide (PDLA) and poly(e-caprolactone) (PCL). From light scattering measurements the cloud point curve is determined to have a critical temperature of 86°C and a critical concentration of mass fraction 36 wt.-% PCL. Optical microscopy of phase-separated films indicates a spinodal morphology at the critical concentration, and droplet phases at off-critical concentrations. After quenching phase separated blends below the melting temperature of PCL (60°C), the crystallization of PCL is used to positively identify PCL-rich and PDLA-rich phases. When cystallization of PCL follows LCST phase separation, the separation, the size, shape, and distribution of crystalline regions can be adjusted by the degree of PCL/PDLA phase seperation. Thus, the LCST phase separation offers a novel method to control microphase structure in biodegradable materials. Applications to control of mechanical and physical properties in tissue engineering scaffolds are discussed in light of the results.

Rheology of Fibrin Clots. VI. Stress Relaxation, Creep, and Differential Dynamic Modulus of Fine Clots in Large Shearing Deformations

A fine, unligated clot of human or bovine fibrin prepared from purified fibrinogen is subjected to a large torsional deformation (maximum shear strain γ up to 1.37) with superposed small oscillating deformations (Δγ ca. 0.03) at frequencies from 0.2 to 1 Hz. The “secant modulus” is defined as Gi(γ)=σi/γ, where σ is stress and the subscript i refers to an initial measurement about 25 s after imposition of strain. The storage modulus G′(ω,γ) refers to a differential oscillating measurement at frequency ω superposed on a static strain γ. For γ up to about 0.1, Gi(γ) was independent of γ and equal to G′(ω,0) measured at about 1 Hz and zero static strain. At large static deformations, the differential storage modulus G′(ω,γ) could be used to monitor changes in structure. Since there is very little time dependence of the relaxation modulus in the range from 1 to 60 s, and the loss tangent is very small, Gi′(ω,γ) could be considered simply as the differential modulus Gi(γ)+γdGi/dγ, and agreed with the latter exp...

Microfluidic interfacial tensiometry

A microfluidic approach to measure interfacial tension σ of immiscible fluids rapidly is reported. This method rests upon quantitative real-time analysis of two-phase flow and drop-shape dynamics. Drops of prescribed dimension and spacing are produced, accelerated, and deformed under extensional flow. These measurements compare well with existing published data and demonstrate a wide range of accessible interfacial tension (e.g., from 2.5 to 60mN∕m).

Microfluidic analog of the four-roll mill

We describe a microfluidic trap, for analysis of fluids and suspensions, that simulates the function of a four-roll mill, a rheological tool with adjustable flow type and rate. These flow characteristics were designed with the assistance of flow simulations and are measured here by micro-particle-image-velocimetry. This miniature device permits microscopic manipulations and measurements (e.g., of cells, particles, and drops) and it is capable of a range of flow types, including simple shear.

Cooperative and self-diffusion of polymers in semidilute solutions by dynamic light scattering

Abstract The quasielastic light scattering from semidilute solutions of polystyrene in tetrahydrofuran has been measured and we observe two distinct exponential decays separated by several orders of magnitude. The angular dependence of the decay constants is indicative of diffusive processes which we identify with the cooperative diffusion coefficient, Dc and the self diffusion coefficient Ds. It is found that Dc, identified with the fast decay, increases with polystyrene concentration and is independent of molecular weight. However, Ds decreases sharply with concentration and molecular weight. An explanation is given for the light scattering detection of these two diffusion coefficients which is based only on the assumptions inherent in the reptation model. In a limited region of molecular weight and concentration the experimental results appear to be consistent with the predictions of scaling theory.

Modification of the phase stability of polymer blends by fillers

Abstract We investigate the influence of filler particles on the phase stability of a model blend of polystyrene (PS) and polybutadiene (PB). The upper critical solution temperature cloud point curve of a PS:PB blend is “destabilized” (upward shift of cloud point temperature) by the addition of untreated fumed silica filler particles. Preliminary cloud point measurements on surface-functionalized fumed silica particles were also performed. A small downward shift of the cloud point temperature was observed by functionalizing the filler particles with grafted polystyrene chains, while grafting with a silane coupling agent resulted in a large apparent “stabilization” effect or a decrease of the cloud point temperature. (Further measurements on surface-functionalized filler particles over a range of polymer compositions are required to determine the filler concentration dependence of the shift of the blend critical temperature.) Surface treatments were selected to demonstrate that modifying the filler particle surface chemistry can alter the blend phase boundary. These phase boundary shifts have significance in commercial blends containing dispersions of filler particles.

Combinatorial investigation of dewetting: polystyrene thin films on gradient hydrophilic surfaces

Film stability and dewetting is important to control for applications in coatings such as photoresists, paints, adhesives, lubricants, and biomaterials. We demonstrate the use of 2D combinatorial libraries to investigate thin film dewetting. Substrate libraries with gradients in contact angle ðuÞ were prepared by immersing Si ‐ H passivated Si in a Piranha solution (H2SO4/H2O2/H2O) at a controlled rate. Libraries of thin films of polystyrene on gradient etched silicon substrates containing orthogonal continuous variation of thickness were screened for dewetting behavior using automated optical microscopy. After comparing the high-throughput screening method to conventional studies of thickness effect on dewetting, a detailed morphological phase-map of the effects of contact angle on dewetting of polystyrene film was generated. Dewetting trends were visibly apparent. The number of polygons of dewetted polymer is sensitive to surface hydrophilicity as characterized by contact angle studies. q 2002 Elsevier Science Ltd. All rights reserved.