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We investigate the response of a well-characterized colloidal star glass to large-amplitude oscillatory stress and strain fields. By combining these measurements with dynamic time sweeps we demonstrate the importance of probing both strain- and stress-induced nonlinear rheology of such complex fluids in order to elucidate the yielding and fluidization behavior. We also show that, due to the strong time dependence, it is essential to perform dynamic time sweeps at different strain and stress amplitudes, which result in different departures of the glass cage from its quiescent quasiequilibrium structure. This allows for steady-state responses to be reached and for nonlinear oscillatory responses to be treated properly while also suggesting that yielding is a gradual process. Further, we use a recently published framework for analyzing nonlinear responses to large-amplitude oscillatory shear [Rogers et al., J. Rheol. 55, 435 (2011)], based on the analysis of the whole stress waveforms as a sequence of physic...

We investigate the complex flow behavior of a well-characterized colloidal star glass using linear and nonlinear rheology. The results are integrated into a generic state diagram which specifies the states of the glass for different initial conditions and mechanical histories: viscoelastic liquid and solid, homogeneous shear-thinned solution, and shear-banded material. Aging takes different forms which can be interpreted as kinetic pathways through the state diagram. Shear-banding appears to be an intrinsic mechanical instability which occurs when the solid state of the glass is shear-melted. Our results provide a straightforward methodology to fingerprint the material behavior of soft glassy materials undergoing rheological transitions which can be used as predictive tool to design systems with a desired rheological response.

A strain-rate frequency superposition (SRFS) technique, recently proposed to analyze the low-frequency behavior of soft materials, is evaluated. The application of SRFS to an emulsion and multiarm star polymer solution produces master curves that, while promising in appearance, do not match the anticipated linear response as they seem inconsistent with dynamic frequency sweep data and the Kramers–Kronig relation. While the raw unscaled frequency sweep data are well described using the generalized Maxwell model, strong discrepancies appear when the same procedure is applied to SRFS data. These inconsistencies appear to be generic and are observed in other materials as well. An examination of Lissajous curves obtained from large amplitude oscillatory strain shows that nonlinear contributions, such as solvent-mediated convective flow, strongly affect SRFS master curves. Based on these findings, SRFS should be approached with caution when applied to soft materials.

We use multi-arm star polymers as model soft colloids with tuneable interactions and explore their behaviour in the glassy state. In particular, we perform a systematic rheological study with a well-defined protocol and address aspects of ageing and shear melting of star glasses. Ageing proceeds in two distinct steps: a fast step of O (10 3 s) and a slow step of O (10 4 s). We focus on creep and recovery tests, which reveal a rich, albeit complex response. Although the waiting time, the time between pre-shear (rejuvenation) of the glassy sample and measurement, affects the material’s response, it does not play the same role as in other soft glasses. For stresses below the yield value, the creep curve is divided into three regimes with increasing time: viscoplastic, intermediate steady flow (associated with the first ageing step) and long-time evolving elastic solid. This behaviour reflects the interplay between ageing and shear rejuvenation. The yield behaviour, as investigated with the stress-dependent recoverable strain, indicates a highly nonlinear elastic response intermediate between a low-stress Hookean solid and a high-stress viscoelastic liquid, and exemplifies the distinct characteristics of this class of hairy colloids. It appears that a phenomenological classification of different colloidal glasses based on yielding performance may be possible.

We report dynamics data on a family of indane glass-forming liquids. Although structural variations make the glass transition vary over a 77 K range, the fragility and width of the relaxation time distribution from oscillatory shear measurements are quite similar for all 14 indanes studied. In contrast, the measured width of the primary relaxation time distribution from dielectric measurements varies considerably for different indanes. The temperature dependence of relaxation time can be described using either Vogel–Fulcher or dynamic scaling, but only the latter allows both mechanical and dielectric data sets to have the same divergence temperature. The consequences of this observation for the thermodynamic significance of such a divergence temperature are discussed.

When considered in the framework of a dynamic scaling model, the length scale of cooperative motion of all glass-forming liquids appears to have a universal temperature dependence. This model also predicts relaxation times with a system-specific temperature dependence, as the product of the universal cooperative length scale raised to the sixth power and a non-universal thermally activated process. In the glassy state, the length scale for cooperative motion can become temperature-independent, so the model predicts relaxation times to have an Arrhenius temperature dependence. As expected by the model, the same activation energy is observed both above and below the glass transition. At sufficiently high temperatures the model ceases to apply to relaxation times, and the crossover to a high-T Arrhenius temperature dependence provides an experimental estimate of the caging temperature. We also report an approximate power law relation between the fragility index and the cooperative length scale at the glass transition.

A robust strain gage based on alloys of indium–tin–oxide (ITO) has been developed, which is capable of measuring strain at temperatures up to 1450 °C. These thin film sensors are ideally suited for in-situ strain measurement in harsh environments since they are non-intrusive, have minimal impact on vibration patterns due to their negligible mass and are robust enough to withstand the high ‘g’ loading associated with rotating components. Thus, this ITO strain gage is well suited to meet instrumentation requirements in advanced propulsion systems. Static strain tests performed at temperatures as high as 1400 °C have resulted in a relatively large and repeatable piezoresistive response. However, in the vicinity of 950 °C, a change in sign of the piezoresistive response from −G to +G was observed, suggesting that the active ITO strain element had been converted from a net ‘n-carrier’ to a net ‘p-carrier’ semiconductor. The ‘n’ to ‘p’ transition has been shown to be reversible over many temperature cycles from room temperature to 1400 °C. This repeatability implies that the carrier species is the predominate factor controlling the observed changes in the resistance and gage factor. Consistent with this change in sign of the gage factor was a change in the sign of the slope of emf vs. temperature (dV/dT) for hot probe measurements made on the same ITO films that were thermally cycled over the temperature range (600 °C to 1300 °C). This finding supports the premise that change in sign of the gage factor from −G to +G occurred within the same temperature range (∼950 °C) and that a change in the charge carrier type was responsible for observed transition in both cases.

The feasibility of utilizing stable free-radical polymerization (SFRP) in the synthesis of well-defined poly(2-vinylnaphthalene) homopolymers has been investigated. Efforts to control molecular weight by manipulating initiator concentration while maintaining a 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO):benzoyl peroxide (BPO) molar ratio of 1.2:1 proved unsuccessful. In addition, systematic variations of the TEMPO: BPO molar ratio did not result in narrow molecular weight distributions. In situ Fourier transform infrared spectroscopy (FTIR) indicated that the rate of monomer disappearance under SFRP and thermal conditions were identical. This observation indicated a lack of control in the presence of the stable free radical, TEMPO. The similarities in chemical structure between styrene and 2-vinylnaphthalene suggested thermally initiated polymerization occurred via the Mayo mechanism. A kinetic analysis of the thermal polymerization of styrene and 2-vinylnaphthalene suggested that the additional fused ring in 2-vinylnaphthalene increased the propensity for thermal polymerization. The observed rate constant for thermal polymerization of 2-vinylnaphthalene was determined using in situ FTIR spectroscopy and was one order of magnitude greater than styrene, assuming pseudo-first-order kinetics. Also, an Arrhenius analysis indicated that the activation energy for the thermal polymerization of 2-vinylnaphthalene was 30 kJ/mol less than styrene.

Using model-based infrared reflectometry (MBIR) technique [1] we have developed a method for in-line process monitoring of polyimide passivation films to support the fabrication of fine pitch flip chip devices. A permanent passivation layer is incorporated in semiconductor wafers before the addition of solder in flip chip interconnects to protect sensitive on-chip components from chip-package interconnection (CPI) stresses, the egress of moisture and chemicals, while providing dielectric isolation. Photosensitive Polyimide (PSPI) [2] is often selected for this application because of its well established track record coupled with thermal and chemical stability, and mechanical strength. With the advent of 3-D integration technologies, new attention has been focused on creating options for reducing controlled collapse chip connection (C4) pitch and solder volumes, a change which causes co-planarity of the interconnect and the passivation layer which supports it to play an increasingly important role. An accurate in-line characterization method is needed to monitor and reduce variability in polyimide passivation layer thickness. We have developed an in-line metrology process utilizing MBIR tools which provide valuable wafer level thickness characterization of PSPI films on bare and processed 300 mm Si wafers.

We use time-resolved rheology to elucidate the slow dynamics and aging in highly concentrated suspensions of multiarm star polymers. The linear and nonlinear rheological properties exhibit a terminal regime corresponding to a well-defined maximal relaxation time. Terminal relaxation is driven by arm relaxation which speeds up the escape of stars from their cages. The fact that the system fully relaxes and flows at long times has important consequences. The yield stress only exists in the limited range of frequencies or shear rates where solid-like behavior is observed. Aging is controlled by the total time elapsed after flow cessation and not by the time elapsed from flow cessation to the beginning of the measurement as in other glassy materials. Our results, which demonstrate the importance of particle architecture with respect to glassy dynamics, should be generic for long hairy particles.

We present experimental evidence confirming the recently established rich dynamic state diagram of asymmetric binary mixtures of soft colloidal spheres. These mixtures consist of glassy suspensions of large star polymers to which different small stars are added at varying concentrations. Using rheology and dynamic light scattering measurements along with a simple phenomenological analysis, we show the existence of re-entrance and multiple glassy states, which exhibit distinct features. Cooperative diffusion, as a probe for star arm interpenetration, is proven to be sensitive to the formation of the liquid pockets which signal the melting of the large-star-glass upon addition of small stars. These results provide ample opportunities for tailoring the properties of soft colloidal glasses.

We attempt to elucidate the dynamics of multiarm star polymer solutions, which are representative of a large class of soft hairy colloids, over a wide range of concentrations. In addition to the usual β-relaxation (in-cage rattling) and α-relaxation (terminal cage escape), the relaxation of stars in the glassy state involves a mode associated with arm interpenetration. From linear and nonlinear rheological measurements, we establish a set of criteria for identifying the colloidal glass transition, including aging, yielding and elastic properties. Linear viscoelasticity and steady-shear measurements merge at low frequencies, indicating that terminal behavior is accessible, albeit at very long times. The transition from linear to nonlinear response is controlled by star elasticity and a balance between Brownian diffusion and flow advection. In the nonlinear regime, the flow curves collapse on a universal flow curve using a scaling that expresses a competition between solvent-mediated interactions and elastic forces. While applied to stars, our framework appears to be a generic tool for fingerprinting liquid-solid transitions in colloidal suspensions.

Recently, large-amplitude oscillatory shear has been studied in great detail with emphasis on its impact on the material response. Here we present a conceptually different, robust methodology based on viewing the stress waveforms as representing a sequence of physical processes. This novel approach provides the viscous and elastic contributions while overcoming the problems with infinite series encountered by Fourier transformation. Application to a soft colloidal star glass leads to the unambiguous determination and quantification of rate-dependent static and dynamic yield stresses, the rationalization of the response to strain sweeps and the post-yield regime by introducing the apparent cage modulus, and a connection to the steady-shear stress, all from a single-amplitude experiment. We propose that this approach is generic, but focus in this contribution only on a yield stress material which exhibits repeating cycles of (i) elastic extension, (ii) yielding, (iii) flow, and (iv) reformation. We show tha...