Tack of Entangled Molecular Bottlebrushes.

Polymer softness (i.e., low elastic modulus) is a known requirement for good tack in adhesives.

Local and Global Stretching of Polymer Chains during Startup of Extensional Flow.

The nonlinear rheological response to extensional flows in entangled polymers is related to the segmental chain stretching and to the chemical identity of the monomeric units. The latter has a strong effect on the drag coefficients, and therefore, quantification of molecular conformation changes in the subnanometer scale (at the monomer level) are crucial to fully understand nonlinear viscoelastic behavior in polymer melts. We report in situ time-resolved extensional rheo-small-angle neutron scattering (tEr-SANS) and wide-angle X-ray scattering (tEr-WAXS) during startup of uniaxial flow on a monodisperse polystyrene melt.

Enhanced Foamability with Shrinking Microfibers in Linear Polymer.

Strain hardening has important roles in understanding material structures and polymer processing methods, such as foaming, film forming, and fiber extruding. A common method to improve strain hardening behavior is to chemically branch polymer structures, which is costly, thus preventing users from controlling the degree of behavior. A smart microfiber blending technology, however, would allow cost-efficient tuning of the degree of strain hardening.

Extensional Strain Hardening in Highly Entangled Molecular Bottlebrushes.

The highly strained conformation in dense molecular bottlebrushes (DB) has profound effects on the dynamics of these type of macromolecules. Understanding of such effects in both their linear and nonlinear viscoelastic responses is crucial for their design and processing. The nonlinear response of poly(1-octadecene), a highly entangled α-olefin DB with linear side chains sixteen carbons long, is studied here and compared to the nonlinear response of a linear polyolefin (polypropylene) with equivalent linear viscoelastic response.

Iono-Elastomer-Based Wearable Strain Sensor with Real-Time Thermomechanical Dual Response.

An ultrastretchable iono-elastomer with resistance sensitive to both elongation strain and temperature has been developed by hierarchical self-assembly of an end functionalized triblock copolymer in a protic ionic liquid (ethylammonium nitrate) followed by cross-linking. Small-angle X-ray scattering experiments in situ with uniaxial elongation reveal a nanoscale microstructural transition of the hierarchically self-assembled cross-linked micelles that is responsible for the material's remarkable mechanical and ionic conductivity responses. The results show that the intermicelle distance extends along the deformation direction while the micelles organize into a long-range ordered face-centered-cubic structure during the uniaxial elongation.

Extensional Strain Hardening Induced by π-π Interactions in Barely Entangled Polymer Chains: The Curious Case of Poly(4-vinylbiphenyl).

Aromatic π-π interactions between phenyl groups of adjacent chains in poly(4-vinylbiphenyl) (PVBP) have profound effects on the dynamics of this polymer. We report two unexpected nonlinear viscoelastic responses of PVBP when subjected to uniaxial flow. One is the unprecedented observation of extensional strain hardening (SH) in a barely entangled polymer melt.

Ultrastretchable Iono-Elastomers with Mechanoelectrical Response.

The emerging technologies involving wearable electronics require new materials with high stretchability, resistance to high loads, and high conductivities. We report a facile synthetic strategy based on self-assembly of concentrated solutions of end-functionalized PEO-PPO-PEO triblock copolymer in ethylammonium nitrate into face-centered cubic micellar crystals, followed by micelle corona cross-linking to generate elastomeric ion gels (iono-elastomers). These materials exhibit an unprecedented combination of high stretchability, high ionic conductivity, and mechanoelectrical response.

Microstructural evolution of a model, shear-banding micellar solution during shear startup and cessation.

We present direct measurements of the evolution of the segmental-level microstructure of a stable shear-banding polymerlike micelle solution during flow startup and cessation in the plane of flow. These measurements provide a definitive, quantitative microstructural understanding of the stages observed during flow startup: an initial elastic response with limited alignment that yields with a large stress overshoot to a homogeneous flow with associated micellar alignment that persists for approximately three relaxation times. This transient is followed by a shear (kink) band formation with a flow-aligned low-viscosity band that exhibits shear-induced concentration fluctuations and coexists with a nearly isotropic band of homogenous, highly viscoelastic micellar solution.

Spatiotemporal stress and structure evolution in dynamically sheared polymer-like micellar solutions.

The complex, nonlinear flow behavior of soft materials transcends industrial applications, smart material design and non-equilibrium thermodynamics. A long-standing, fundamental challenge in soft-matter science is establishing a quantitative connection between the deformation field, local microstructure and macroscopic dynamic flow properties i.e.

Spatially Resolved Concentration and Segmental Flow Alignment in a Shear-Banding Solution of Polymer-Like Micelles.

We measure the spatially resolved microstructure and concentration in the plane of flow for a viscoelastic solution of polymer-like micelles comprised of mass fraction 6.0% (volume fraction 6.6%) solution of 2:1 molar ratio cetylpyridinium chloride/sodium salicylate in 0.

Spontaneous thermoreversible formation of cationic vesicles in a protic ionic liquid.

The search for stable vesicular structures is a long-standing topic of research because of the usefulness of these structures and the scarcity of surfactant systems that spontaneously form vesicles in true thermodynamic equilibrium. We report the first experimental evidence of spontaneous formation of vesicles for a pure cationic double tail surfactant (didodecyldimethylammonium bromide, DDAB) in a protic ionic liquid (ethylammonium nitrate, EAN). Using small and ultra-small angle neutron scattering, rheology and bright field microscopy, we identify the coexistence of two vesicle containing phases in compositions ranging from 2 to 68 wt %.

Dynamics of melting and recrystallization in a polymeric micellar crystal subjected to large amplitude oscillatory shear flow.

Shear-induced structural transitions of a micellar cubic phase during large amplitude oscillatory shear flow is studied with time-resolved oscillatory rheological small angle neutron scattering. This technique allows us to resolve the structural changes within a cycle of oscillation. By applying a strain rate near the critical melting shear rate, melting and recrystallization occurs in a cyclic mode.

Structural transitions of CTAB micelles in a protic ionic liquid.

Micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in a protic ionic liquid, ethylammonium nitrate (EAN), are studied by shear rheology, polarizing optical microscopy (POM), conductivity measurements, and small angle neutron scattering (SANS). Three concentration regimes are examined: A dilute regime (with concentrations [CTAB] < 5 wt %) consisting of noninteracting spherical micelles, a semidilute regime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interact via electrostatic repulsions, and a concentrated regime (45 wt % < [CTAB] ≤ 62 wt %) where a reversible, temperature-dependent isotropic (L(1)) to hexatic (Hex) phase transition is observed. The L(1)-Hex transition, which has been predicted but not previously observed, is characterized by (1) a sharp increase in the shear viscosity, (2) the formation of focal conical birefringence textures (observed by POM), and (3) enhancement of the crystalline order, evidenced by the appearance of Bragg reflections in the SANS profiles.

Sponge-to-lamellar transition in a double-tail cationic surfactant/protic ionic liquid system: structural and rheological analysis.

The self-assembly of didodecyldimethylammonium bromide (DDAB) in a protic ionic liquid, ethylammonium nitrate (EAN), in the high surfactant concentration regime is studied using five different experimental techniques. A thermoreversible first-order sponge (L(3)) to lamellar (L(α)) transition occurring at [DDAB] > 80 wt % was identified by (1) a sharp increase in the elastic and viscous moduli, (2) a transition peak recorded by differential scanning calorimetry, (3) formation of Maltese cross birefringence textures observed via polarizing optical microscopy, (4) a decrease in the interbilayer mean distance measured by small angle neutron scattering, and (5) an abrupt increase in the conductivity obstruction factor. In contrast to aqueous DDAB solutions, this surfactant forms a stable L(3) phase in EAN in a wide window of compositions and temperatures, which is potentially useful for the synthesis of nanoporous materials.

Measurement of geometrical parameters in cocontinuous polymer blends: 3D versus 2D image analysis.

Formulae of stereology are used to estimate 3D geometrical parameters of cocontinuous structures measured from 2D micrographs of polymer blends. 3D images of symmetric and nonsymmetric polymer blends made of fluorescently labelled polystyrene and styrene-ran-acrylonitrile copolymer were obtained with laser scanning confocal microscopy. Geometrical parameters of the blend interface, specifically volume fraction, surface area per unit volume (S (V) ) and average of local mean curvature were measured directly from the 3D images and compared to the values estimated from analysis of a number of 2D slices combined with stereological relations.

Direct observation of flow-concentration coupling in a shear-banding fluid.

We present the first direct evidence of flow-concentration coupling in a shear-banding fluid using the first measurements of scanning narrow-aperture flow ultrasmall-angle neutron scattering. Transmission measurements in the flow-gradient shear plane determine the spatial concentration profile of the material. Shear banding in a model fluid composed of wormlike micelles near an equilibrium isotropic-nematic transition is shown to be concomitant with shear-induced demixing, providing new insight into the nonequilibrium phase behavior of the fluid.

Direct measurement of interface anisotropy of bicontinuous structures via 3D image analysis.

A very important morphological parameter in two-phase fluids is the interface anisotropy, which can be quantified using the interface tensor, q(ij). However, the computation of this tensor for complex interfaces is not straightforward. A novel method (the local cross product method, LCPM) to compute the interface tensor of two-phase fluids using 3D imaging coupled with differential geometry is presented here.

Characterizing interface shape evolution in immiscible polymer blends via 3D image analysis.

The coordinate transformation (CT) method was applied to measure the local curvature of the interface of an immiscible polymer blend made of fluorescently labeled polystyrene (FLPS) and styrene-ran-acrylonitrile copolymer (SAN). The CT method involves the local parametrization of the interface by a quadratic polynomial to compute the local values of the mean (H) and the Gaussian (K) curvatures. Distributions of the curvatures at different annealing times were obtained by measuring H and K at many (typically 10(7)) points on the interface.