Preparation and characterization of surfactant‐free stimuli‐sensitive microgel dispersions

A surfactant‐free method to produce responsive polymer microgels is introduced. As an example, poly(methacrylic acid) hydrogels with varying crosslinking density have been synthesized in bulk and then chopped using a high shear mechanical cutter to form microgel particles dispersed in water. The mechanical cutting technique enables the concentration and particle size distribution of the microgel suspensions to be easily controlled, therefore making the rheology of the suspensions tuneable. The particle size distribution of the dispersions, characterized using light scattering, was dependent on the speed and duration of mechanical cutting. The particle size distribution also depended on the degree of crosslinking of the hydrogel. The higher the crosslinking density, the lower the average mean diameter of the resulting microgel particles. The lower the crosslinking density of the hydrogel, the larger the difference between the maximum and minimum particle size. The time to complete swelling of the particles upon change in pH was measured to be up to 45 s, depending on the particle size. The rheology of the resulting suspensions as a function of pH was investigated. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 104: 1912–1919, 2007     

HDPE/LLDPE reactor blends with bimodal microstructures—Part II: rheological properties

Reactor blends of polyethylene/poly(ethylene-co-1-octene) resins with bimodal molecular weight and bimodal short chain branching distributions were synthesized in a two-step polymerization process. The compositions of these blends range from low molecular weight (LMW) homopolymer to high molecular weight (HMW) copolymer and vice versa HMW homopolymer to LMW copolymer. The shear flow characteristics of these polymers in the typical processing range mostly depend on the molecular weight and MWD of the polymer and are independent of the short chain branch content. From oscillatory shear measurements, it was observed that the viscosity of HMW polymers was reduced with the addition of LMW material. For the polymers produced with this two-step polymerization process, the LMW homopolymer and HMW copolymer blends and HMW homopolymer and LMW copolymer blends were melt miscible, despite the large viscosity differences of the pure components.     

Equi-affine Invariant Geometry for Shape Analysis

Traditional models of bendable surfaces are based on the exact or approximate invariance to deformations that do not tear or stretch the shape, leaving intact an intrinsic geometry associated with it. These geometries are typically defined using either the shortest path length (geodesic distance), or properties of heat diffusion (diffusion distance) on the surface. Both measures are implicitly derived from the metric induced by the ambient Euclidean space. In this paper, we depart from this restrictive assumption by observing that a different choice of the metric results in a richer set of geometric invariants. We apply equi-affine geometry for analyzing arbitrary shapes with positive Gaussian curvature. The potential of the proposed framework is explored in a range of applications such as shape matching and retrieval, symmetry detection, and computation of Voroni tessellation. We show that in some shape analysis tasks, equi-affine-invariant intrinsic geometries often outperform their Euclidean-based counterparts. We further explore the potential of this metric in facial anthropometry of newborns. We show that intrinsic properties of this homogeneous group are better captured using the equi-affine metric.     

Geschäftsmodelle

The business model concept, although a relatively new topic for research, has garnered growing attention over the past decade. Whilst it has been robustly defined, the concept has so far attracted very little substantive research. In the context of the wide-spread digitization of businesses and society at large, the logic inherent in a business model has become critical for business success and, hence, a focus for academic inquiry. The business model concept is identified as the missing link between business strategy, processes, and Information Technology (IT). The authors argue that the BISE community offers distinct and unique competencies (e.g., translating business strategies into IT systems, managing business and IT processes, etc.) that can be harnessed for significant research contributions to this field. Within this research gap three distinct streams are delineated, namely, business models in IT industries, IT enabled or digital business models, and IT support for developing and managing business models. For these streams, the current state of the art, suggest critical research questions, and suitable research methodologies are outlined.     

Hydrolytic degradation of ionically cross-linked polyphosphazene microspheres

The hydrolytic degradation of gel microspheres based on calcium cross-linked phosphazene polyelectrolytes, poly[di(carboxylatophenoxy)phosphazene] (PCPP) and poly[(carboxylatophenoxy) (glycinato)phosphazene] (PCGPP), was investigated. These microspheres are of importance as carriers in protein and cell encapsulation. Both PCPP and PCGPP ionotropic polyphosphazene hydrogels are degradable in an aqueous environment (pH 7.4, 37°C). The degradation rates can be increased by incorporation of hydrolysis sensitive glycinato groups as the pendant structures in the polymer (PCGPP). Hydrolysis of these polymer hydrogels led to low molecular weight (<1,000 Da) products. The erosion and molecular weight profiles varied also according to the molecular weight of the polyphosphazene constituting the gel beads. Another approach to affect the degradation rates consists of coating microspheres with poly-L -lysine. Ionotropic polyphosphazene hydrogels have potential as biodegradable devices for controlled drug delivery systems. © 1994 John Wiley & Sons, Inc.     

Avalanching flow of cohesive powders

The flow dynamics of cohesive powders is investigated in rotating cylinders with an L : R ratio of 3 : 1 using experiments and DEM simulations. Flow onset and steady-state behavior are compared for free-flowing (cohesionless) dry glass beads, wet glass beads, and “dry” cohesive powders (lactose, microcrystalline cellulose). The avalanching dynamics of powders is substantially different from those observed for free-flowing or wet-cohesive glass beads. Dry cohesive powders exhibit history-dependent flow dynamics, significant dilation, aperiodic avalanche frequencies, and variable avalanche size. These behaviors also provide a route for effective characterization of cohesive forces under dilated conditions characteristic of unconfined flows.     

Melting and reorganization of poly(ethylene terephthalate) on fast heating (1000 K/s)

For poly(ethylene terephthalate) (PET) and other polymers the origin of the multiple melting peaks observed in differential scanning calorimetry (DSC) curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. Developments in thin-film (chip) calorimetry allow scanning rates as high as several thousand Kelvin per second. We utilized a chip calorimeter based on a commercially available vacuum gauge, which is operated under non-adiabatic conditions. The calorimeter was used to study the melting of isothermally crystallized PET. Our results on melting at rates as high as 2700 K/s give clear evidence for the validity of a melting-recrystallization-remelting process for PET at low scanning rates (DSC). At isothermal conditions PET forms crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of different populations of crystals with significantly different stability (melting temperatures) under isothermal conditions. Superheating of the crystals is of the order of 10 K at 2700 K/s.     

Buckling of jets in electrospinning

Various buckling instabilities of electrospinning jets were observed and compared with the buckling instabilities of uncharged fluid jets. Buckling instability arises due to jet compression at impingement on a collector surface and occurs independently of the electrical bending instability. The velocity, diameter, density and viscosity of the electrospinning jets are the key factors that determine the buckling frequency. The electrically charged jets impinging onto grounded, horizontal or inclined (wedge-like) electrodes moving laterally at a constant velocity are studied experimentally. Straight and bending (electrospinning) jets emerge at short and sufficiently long inter-electrode distances, respectively. The experiments show that both straight segment and bending jets, when impinging onto a counter-electrode, buckled and produced patterns of meandering deposits. In the case of bending electrospun jets these short-length buckling patterns were superimposed on the bending loops found in the deposits. Buckling-related and bending-related morphologies are easily distinguishable. The buckling patterns have frequencies of the order of 10⁵-10⁶ Hz, whereas the bending loops are formed at the frequencies of the order of 10³ Hz. The deposited buckling patterns include sinuous, zigzag-like, figures-of-eight, recurring curves, coiled and other structures that resembled many patterns created by uncharged jets of highly viscous fluids impinging a hard flat surface. In addition, several new morphologies which were not observed before with uncharged jets were found. The experimentally measured frequencies of the buckling patterns were compared to the theoretical predictions and a reasonable agreement was found.     

HDPE/LLDPE reactor blends with bimodal microstructures—part I: mechanical properties

Reactor blends of polyethylene/poly(ethylene-co-1-octene) resins with bimodal molecular weight and bimodal short chain branching distributions were synthesized in a two-step polymerization process. The compositions of these blends range from low molecular weight (LMW) homopolymer to high molecular weight (HMW) copolymer and, vice versa, HMW homopolymer to LMW copolymer. The physical properties of the blends were found to be consistent with the nature of the individual components. For the tensile properties, the stiffness decreases with increasing the fraction of the copolymer, regardless of the molecular weight of the homopolymer fraction. For these blends with bimodal microstructures, it was confirmed that the degree of crystallinity governs the stiffness of the polymer. However, the energy dampening properties of the polymers benefit from the presence of the copolymer. A balance of stiffness and toughness can be obtained by altering the composition of the blends. For some blends, the presence of HMW homopolymer can dominate the tensile properties, showing little variation in the stiffness with increased addition of copolymer. It was also demonstrated that the testing conditions and thermal treatment of the polymer greatly influence the resulting elastic and energy dampening properties. Depending on the desired application, annealing these polymers (especially very low density copolymers) not only increases the crystallinity and stiffness, but also changes the frequency response of the dynamic mechanical properties.     

Parametric study on smoothed particle hydrodynamics for accurate determination of drag coefficient for a circular cylinder

Simulations of two-dimensional (2D) flow past a circular cylinder with the smoothed particle hydrodynamics (SPH) method were conducted in order to accurately determine the drag coefficient. The fluid was modeled as a viscous liquid with weak compressibility. Boundary conditions, such as a no-slip solid wall, inflow and outflow, and periodic boundaries, were employed to resemble the physical problem. A sensitivity analysis, which has been rarely addressed in previous studies, was conducted on several SPH parameters. Hence, the effects of distinct parameters, such as the kernel choices and the domain dimensions, were investigated with the goal of obtaining highly accurate results. A range of Reynolds numbers (1–500) was simulated, and the results were compared with existing experimental data. It was observed that the domain dimensions and the resolution of SPH particles, in comparison to the obstacle size, affected the obtained drag coefficient significantly. Other parameters, such as the background pressure, influenced the transient condition, but did not influence the steady state at which the drag coefficient was determined.