H Henning Winter

Shear induced Long-Range Alignment of BCC ordered Block-Copolymers

H Henning Winter et al. — Thu, 10 Nov 2011 09:09:00 PST

Effect of large shear on an asymmetric block copolymer with nanospherical domains has been studied using rheology and small angle X-ray scattering. The material investigated was a triblock copolymer poly[styrene-b-(ethylene-co-butylene)-b-styrene] swollen in a midblock-selective solvent. When cooled below the order–disorder transition temperature (T ODT), the system forms a locally ordered structure of grains with body-centered cubic (BCC) lattice. Isothermal shearing, either at constant rate or with large amplitude oscillatory shear (LAOS) at low frequencies and strain amplitude greater than or equal to 2.0, leads to the destruction of the BCC lattice (isothermal “shear melting”). Upon cessation of the shear, the BCC structure recovers with kinetics similar to the one after thermal quench from above T ODT. Under certain experimental conditions, LAOS leads to alignment of the BCC lattice. The lattice orientation depends primarily on shearing frequency. At low frequencies, there exists an upper and lower bound on strain amplitude where monodomain textures can be obtained. Upon alignment, the modulus drops by about 30% of that of the polycrystalline structure. Measurement of rheological properties offers an indirect method for distinguishing between polycrystalline structure (grains) and monodomain texture.

Modulus-Switching Viscoelasticity of Electrorheological Networks

H Henning Winter et al. — Thu, 10 Nov 2011 09:07:11 PST

To form an electrorheological network (ERN), semiconducting nanoparticles were embedded in a polymer that was cross-linked to restrict particle motion. The microstructure ranged from random to aligned, depending on the degree of field-induced particle alignment during chemical network formation. We investigated in detail the softness effects of the matrix, having a relatively low storage modulus, on the dynamic rheological behavior of the ERN and analyzed its anisotropy. The anisotropy of the microstructure was probed rheologically with the modes of small-amplitude oscillatory shear (loading perpendicular to the field direction) and compression (loading in the field direction). The storage shear modulus was found to be a function of the applied electric field, particle volume fraction, and the pre-alignment electric field strength during the cross-linking reaction of the matrix, which governs the thickness of particle columns and intercolumn distance. Nonlinear behavior at small strain (below 0.1%) was conspicuous; this nonlinear viscoelasticity was accompanied by only a limited deformation of ordered connectivity. Throughout this study, we fabricated the ERN with the highly controllable modulus-switching effect acting in a shear-mode operation. Managing this anisotropy of an ERN by the electrical and chemical process is important in the design of smart materials that will provide improved stability and mechanical strength compared with fluid-type electrorheological materials and faster response time compared with that of conventional charged polymer gel.

Bringing Eureka into Rheology at ARC08. Applied Rheology

H Henning Winter — Thu, 10 Nov 2011 09:05:11 PST

Experiencing Synergy Between Experiments and Theory in Rheology

H Henning Winter — Thu, 10 Nov 2011 09:02:11 PST

Relaxation Patterns of Long, Linear, Flexiable, Monodisperse Polymers: BSW Spectrum Revisited

H Henning Winter et al. — Thu, 10 Nov 2011 09:00:16 PST

Theoretical predictions for the dynamic moduli of long, linear, flexible, monodisperse polymers are summarized and compared with experimental observations. Surprisingly, the predicted 1/2 power scaling of the long-time modes of the relaxation spectrum is not found in the experiments. Instead, scaling with a power of about 1/4 extends all the way up to the longest relaxation times near τ/τ max = 1. This is expressed in the empirical relaxation time spectrum of Baumgaertel-Schausberger-Winter, denoted as “BSW spectrum,” and justifies a closer look at the properties of the BSW spectrum. Working with the BSW spectrum, however, is made difficult by the fact that hypergeometric functions occur naturally in BSW-based rheological material functions. BSW provides no explicit solutions for the dynamic moduli, G ′(ω), G ″(ω), or the relaxation modulus G(t). To overcome this problem, close approximations of simple analytical form are shown for these moduli. With these approximations, analysis of linear viscoelastic data allows the direct determination of BSW parameters.

Three views of Viscolelasticity

H Henning Winter — Thu, 10 Nov 2011 08:56:56 PST

A slight rearrangement of the classical Cox and Merz rule suggests that the shear stress value of steady shear flow, , and complex modulus value of small amplitude oscillatory shear, G ∗ (ω) = (G′2 + G″2)1/2, are equivalent in many respects. Small changes of material structure, which express themselves most sensitively in the steady shear stress, τ, show equally pronounced in linear viscoelastic data when plotting these with G ∗ as one of the variables. An example is given to demonstrate this phenomenon: viscosity data that cover about three decades in frequency get stretched out over about nine decades in G ∗ while maintaining steep gradients in a transition region. This suggests a more effective way of exploiting the Cox–Merz rule when it is valid and exploring reasons for lack of validity when it is not. The τ −G ∗ equivalence could also further the understanding of the steady shear normal stress function as proposed by Laun.

Distinct Dynamics Pattern during the Ripening of Nanocomposite Gels

H Henning Winter et al. — Thu, 10 Nov 2011 08:55:29 PST

There is a large class of physical gels that develop their structure slowly. Most of the structural development occurs in the solid state after having passed through an early liquid-to-solid transition. Such physical gels are often called “soft glasses” because of their slow out-of-equilibrium dynamics. Here we examine the ripening of an out-of-equilibrium model colloidal solid that consists of clay particles that swell and exfoliate into randomly oriented clay sheets through the action of end-functionalized (“sticky”) polymer molecules. The nano-composite gel serves as model material in search of regular patterns in the non-equilibrium dynamics in the approach of equilibrium. Surprisingly, there exists a stunningly simple pattern in the time-resolved viscoelasticity: the product of equilibrium modulus and longest relaxation time, Ge(tr)lmax(tr), remains constant throughout the ripening process. Parameter is the duration of the isothermal ripening process (“ripening time”, tr). The product Gelmax has the dimension of a viscosity but belongs to the solid state of the gel. As a consequence of Gelmax =constant, a single scaling relation with two power law regions, a fast ripening process (~ tr-2) followed by slow ripening (~ tr-1/2), defines the state of ripening, e.g. the time necessary to reach equilibrium. Experiments on a wider group of physical gels is in progress with the objective of confirming or rejecting universality of the novel findings. The experimental protocol includes time-resolved rheometry (Rheol Acta 33:385-397, 1994) and rescaling of data (Rheol Acta 45:331-338, 2006). Acknowledgment: NSF support through CBET-0651888.

Late-State Ripening Dynamics of a Polymer/Clay Composite Gel

H Henning Winter et al. — Thu, 10 Nov 2011 08:52:44 PST

An Empirical Constitutive Law for Cencentrative Law for Concentrated Colloidal Suspension in the Approach of the Glass Transition

H Henning Winter et al. — Thu, 10 Nov 2011 08:50:05 PST

Steric Effects on the Rheology of Nanocomposite Genls of Organoclay in Dicarboxyl-Terminated Polybutadiene

H Henning Winter et al. — Thu, 10 Nov 2011 08:44:18 PST

Nanocomposite gels were formed by mixing organically modified clay into a linear, end-functionalized polymer (dicarboxyl-terminated polybutadiene). Two differently sized but otherwise similar counterions were chosen for preparing the organoclay. Hydrogen bonding between polymer and clay causes the polymer/clay interface to grow by splitting the clay aggregates into smaller clay particles, then swelling these particles, exfoliating the clay sheets, and eventually assuming a stable dispersion in the polymer matrix. The clay with the larger counterion exfoliates faster, but does not form the stronger network (lower modulus, lower yield stress), and it needs more clay to reach its gel point (percolation threshold [curly or open phi]c). These seemingly contradictory observations (fast exfoliation but weak gel and later gel point) are attributed to steric effects of the larger macro-counterion. Parameters of the study are clay concentration [curly or open phi] and distance from the gel point. The low frequency linear viscoelastic behavior was analyzed using a percolation model (near [curly or open phi]c) and a power law in concentration (far above [curly or open phi]c). The use of two different organoclays allows comparison of the observed phenomena. The extent of agreement between experimental data and known models was used to theorize that the particle–polymer interactions are the controlling factor in the increasing solid-like behavior with increasing clay content.

Steric Effects on the Rheology of Nanocomposite Gels of Organoclay

H Henning Winter et al. — Thu, 10 Nov 2011 08:40:29 PST

Late-State Ripening Dynamics of a Polymer/Clay Nanocomposite

H Henning Winter et al. — Thu, 10 Nov 2011 08:35:43 PST

A remarkably simple viscoelastic relaxation pattern was found for a physical gel that, beyond its gel point, slowly ripens toward a stable structural state. The material is a nanocomposite that consists of two components, an organoclay and an end-functionalized polymer, which get mixed at prescribed ratio. When freshly combined, the polymer intercalates into the clay galleries and eventually exfoliates the clay. The exfoliation occurs without applying flow. The nanocomposite quickly passes through the gel point and, with increasing ripening time, tr, its characteristic modulus Gc(tr) and relaxation time λc(tr) grow and decay by orders of magnitude, respectively. Surprisingly, their product, Gc(tr)λc(tr), is found to remain constant during the two vastly different structuring processes of intercalation and exfoliation. For the rheological experiments, Gc(tr)λc(tr) = constant means that dynamic mechanical data can be merged into each other by log/log shifting under 45°.

Viscoelasticity and Shear Flow of Concentrated, Non-Crystallizing Colloidal Suspensions: Comparison with Mode-Coupling Theory

H Henning Winter et al. — Thu, 10 Nov 2011 08:28:48 PST

We present a comprehensive rheological study of a suspension of thermosensitive particles dispersed in water. The volume fraction of these particles can be adjusted by the temperature of the system in a continuous fashion. Due to the finite polydispersity of the particles (standard deviation: 17%), crystallization is suppressed and no fluid-crystal transition intervenes. Hence, the moduli G′ and G″ in the linear viscoelastic regime as well as the flow curves (shear stress σ(math) as function of the shear rate math) could be measured in the fluid region up to the vicinity of the glass transition. Moreover, flow curves could be obtained over a range of shear rates of 8 orders of magnitude, while G′ and G″ could be measured spanning over 9 orders of magnitude. Special emphasis has been laid on precise measurements down to the smallest shear rates/frequencies. It is demonstrated that mode-coupling theory generalized in the integration through transients framework provides a full description of the flow curves as well as the viscoelastic behavior of concentrated suspensions with a single set of well-defined parameters.

Enhanced Exfoliation of Organo-Clay in Partially End-Functionalized, Non-Polar Polymers.

H Henning Winter et al. — Thu, 10 Nov 2011 08:20:15 PST

We found that enhanced exfoliation of clay up to 20 wt.-% in non-polar polybutadiene (PB) if the PB was blended with a relatively small fraction of hydroxyl-terminated PB (HTPB). The choice of an intermediate polymer composition to enhance exfoliation was motivated by theoretical predictions of end-functionalizing effects of Balazs, Farmer, and coworkers. A combination of X-ray diffraction and rheological measurements were used to optimize HTPB content for enhanced exfoliation. We also observed the competition of the kinetic and thermodynamic processes during the ripening of the exfoliated clay structure.