Effect of the Molecular Structure Change of a Matrix Polymer (Nylon 6) on the Deformation of Dispersed Phase (a Thermotropic Liquid Crystalline Polymer) Droplets in Shear Flow.

In this work, we investigated the effect of a change in the molecular structure and ensuing molar mass change of a matrix polymer (polyamide 6, Ny 6) on droplet deformation of a dispersed thermotropic liquid crystalline polymer (TLCP, a poly(ester amide)) in shear flow. This study focuses on a total capillary number (the sum of the shear capillary number and the elasticity capillary number) and the viscosity ratio between the TLCP and Ny 6, for the morphological development and mechanical performance of TLCP/Ny 6 blends. In contrast to Ny 6, which has a lower melt viscosity than the TLCP melt, a modified Ny 6 (m-Ny 6) with ca. 2 orders higher melt viscosity than that of Ny 6 at a shear rate of 1 s-1 was found to facilitate the deformation of the TLCP phase. A total capillary number was defined to characterize the viscoelasticity effect on droplet deformation in the blend system. The first normal stress difference obtained from the viscosity curve using Steller's method was used for the evaluation of the elasticity capillary number. The total capillary number for the Ny 6 blend was far less than the critical capillary number and was insufficient for the dispersed TLCP droplets to be deformed. The shear capillary number of the m-Ny 6 blend was greater than the critical capillary number but was still insufficient for droplet deformation into fibril shapes. The total capillary number, including the elastic capillary number, was sufficiently greater than the critical capillary number for deformation of the dispersed TLCP droplets. Morphological observations and a comparison with the theoretical work confirmed the importance of the viscoelasticity of the melt in the immiscible Ny 6/TLCP blends for in situ composite fabrication in shear flow. Both the high viscosity and the first normal stress difference of m-Ny 6 promote the deformation and fibrillation of the dispersed TLCP droplets.

Nonisothermal Crystallization Behaviors of Structure-Modified Polyamides (Nylon 6s).

The morphological development and thermal properties of different polyamides with long-chain branches without forming a network structure were characterized by differential scanning calorimetry, polarized optical microscopy, and nonisothermal crystallization kinetics. The crystallization characteristics were analyzed using the nonisothermal kinetic equation proposed by Seo. Polarized optical microscopy and the Avrami exponent show the effect of the structural changes on the molecular ordering during the crystallization and early morphological development. The Avrami exponent, n, determined from the analysis of the nonisothermal crystallization kinetics, indicates a reduced heterogeneous nucleation for the modified polyamides. Structural changes (branching) of the polyamides impede crystallization, as indicated by the shift of the crystallization peaks to lower temperatures.

Searching for a Stable High-Performance Magnetorheological Suspension.

Magnetorheological (MR) fluids are a type of smart material with rheological properties that may be controlled through mesostructural transformations. MR fluids form solid-like fibril structures along the magnetic field direction upon application of a magnetic field due to magnetopolarization of soft-magnetic particles when suspended in an inert medium. A reverse structural transition occurs upon removal of the applied field. The structural changes are very fast on the order of milliseconds. The rheological properties of MR fluids vary with the application of a magnetic field, resulting in non-Newtonian viscoplastic flow behaviors. Recent applications have increased the demand for MR materials with better performance and good long-term stability. A variety of industrial MR materials have been developed and tested in numerous experimental and theoretical studies. Because modeling and analysis are essential to optimize material design, a new macroscale structural model has been developed to distinguish between static yield stress and dynamic yield stress and describe the flow behavior over a wide range of shear rates. Herein, this recent progress in the search for advanced MR fluid materials with good stability is described, along with new approaches to MR flow behavior analysis. Several ways to improve the stability and efficiency of the MR fluids are also summarized.

High-Performance Magnetorheological Suspensions of Pickering-Emulsion-Polymerized Polystyrene/Fe3O4 Particles with Enhanced Stability.

The magnetorheological (MR) performance of suspensions based on core-shell-structured foamed polystyrene (PSF)/Fe3O4 particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Core-shell-structured polystyrene (PS)/Fe3O4 was synthesized by using the Pickering-emulsion polymerization method in which Fe3O4 nanoparticles were added as a solid surfactant. Foaming the PS core in PS/Fe3O4 particles was carried out by using a supercritical carbon dioxide (scCO2) fluid. The density was measured by a pycnometer. The densities of PS/Fe3O4 and PSF/Fe3O4 particles were significantly lowered from that of the pure Fe3O4 particle after Pickering-emulsion polymerization and foaming treatment. All tested suspensions displayed similar MR behaviors but different yield strengths. The important parameter that determined the MR performance was not the particle density but rather the surface density of Fe3O4 on the PS core surface. The morphology was observed by scanning electron microscopy and transmission electron microscopy. Most Fe3O4 particles stayed on the surface of PS/Fe3O4 particles, making the surface topology bumpy and rough, which decreased the particle sedimentation velocity. Finally, Turbiscan apparatus was used to examine the sedimentation properties of different particle suspensions. The suspensions of PS/Fe3O4 and PSF/Fe3O4 showed remarkably improved stability against sedimentation, much better than the bare Fe3O4 particle suspension because of the reduced density mismatch between the nanoparticles and the carrier medium as well as the surface topology change.

Effect of Molecular Structure Change on the Melt Rheological Properties of a Polyamide (Nylon 6).

Tailoring the polymer melt rheology and the chain relaxation dynamics permits easy handling of polymer processing and enables broader range of applications. Novel strategy to control the polymer melt rheology and the chain relaxation dynamics was devised. A simple process for molecular structural change in a polyamide (nylon 6) to easily generate a long-chain branching in a controllable manner without forming a network structure led to unusually large enhancements in the relaxation dynamics. The zero shear viscosity of the polyamide has increased more than 200 folds of linear chains viscosity, whereas the molar mass change was ca. 1.6 times. Storage modulus and the loss modulus at low frequency increased more than 104 and 103 times to those of neat polyamide without forming a network structure. The rheological properties of the polymer (nylon 6) melts can be finely tailored by this simple process to cover a broad range of applications.

Static yield stress of a magnetorheological fluid containing Pickering emulsion polymerized Fe2O3/polystyrene composite particles.

The flow behaviors of magnetorheological (MR) suspensions containing Pickering emulsion polymerized Fe2O3/polystyrene (PS) composite particles were reanalyzed using the Seo-Seo model. The experimental shear stress data obtained experimentally from the magnetorheological fluid fit well to the Seo-Seo model, indicating that this model can describe the structural reformation process of the aligned fibers at various shear rates. Unlike the dynamic yield stress obtained from the Cho-Choi-Jhon (CCJ) model, the static yield stresses obtained from the Seo-Seo model exhibit the same quadratic dependence on the magnetic field strength for both pure Fe2O3 particle suspension and Fe2O3/PS particle suspensions, which is in agreement with the predictions of the polarization model. The static yield stress plausibly explains the difference in underlying mechanism of MR fluids.

Analysis of giant electrorheological fluids.

The yield stress dependence on electric field strength for giant electrorheological (GER) fluids over the full range of electric fields was examined using Seo's scaling function which incorporated both the polarization and the conductivity models. If a proper scaling was applied to the yield stress data to collapse them onto a single curve, the Seo's scaling function could correctly fit the yield stress behavior of GER suspensions, even at very high electric field strengths. The model predictions were also compared with recently proposed Choi et al.'s model to allow a consideration of the universal framework of ER fluids.

Dispersion and reaggregation of nanoparticles in the polypropylene copolymer foamed by supercritical carbon dioxide.

For the preparation of nanocomposites, we conducted environmentally benign foaming processing on polypropylene (PP) copolymer/clay nanocomposites via a batch process in an autoclave. We investigated the dispersion and the exfoliation of the nanoclay particles. Full exfoliation was achieved by the foamability of the matrix PP copolymer using supercritical carbon dioxide (sc CO2) and subcritical carbon dioxide (sub CO2). More and smaller cells were observed when the clay was blended as heterogeneous nuclei and sc CO2 was used. Small angle X-ray scattering showed that highly dispersed states (exfoliation) of the clay particles were obtained by the foaming process. Since the clay particles provided more nucleating sites for the foaming of the polymer, a well dispersed (or fully exfoliated) nanocomposite exhibited a higher cell density and a smaller cell size at the same clay particle concentration. Expansion of the adsorbed CO2 facilitated the exfoliation of the clay platelets; thus, sc CO2 at lower temperature was more efficient for uniform foaming-cell production. Fully dispersed clay platelets were, however, re-aggregated when subjected to a further melting processing. The reprocessed nanocomposites still had some exfoliated platelets as well as some aggregated intercalates. The dual role of the nanoclay particles as foaming nucleus and a crystallization nucleus was confirmed by cell growth observation and nonisothermal crystallization kinetics analysis. A low foaming temperature and a high saturation pressure were more favorable for obtaining a uniform foam. The PP copolymer was found to be foamed more easily than polypropylene. A small amount of other olefin moieties in the backbone of the polymer facilitated better foamability than the neat polypropylene.

Modeling and analysis of electrorheological suspensions in shear flow.

A model capable of describing the flow behavior of electrorheological (ER) suspensions under different electric field strengths and over the full range of shear rates is proposed. Structural reformation in the low shear rate region is investigated where parts of a material are in an undeformed state, while aligned structures reform under the shear force. The model's predictions were compared with the experimental data of some ER fluids as well as the CCJ (Cho-Choi-Jhon) model. This simple model's predictions of suspension flow behavior with subsequent aligned structure reformation agreed well with the experimental data, both quantitatively and qualitatively. The proposed model plausibly predicted the static yield stress, whereas the CCJ model and the Bingham model predicted only the dynamic yield stress. The master curve describing the apparent viscosity was obtained by appropriate scaling both axes, which showed that a combination of dimensional analysis and flow curve analysis using the proposed model yielded a quantitatively and qualitatively precise description of ER fluid rheological behavior based on relatively few experimental measurements.

Enhanced interfacial adhesion between an amorphous polymer (polystyrene) and a semicrystalline polymer [a polyamide (nylon 6)].

We studied enhanced interfacial adhesion between an amorphous polymer (polystyrene, PS) and a semicrystalline polymer (a polyamide, Ny6). The fracture mechanism for this system was investigated to elicit a universal description on the fracture mechanism. The surface modification of PS to provide functional groups that can react with the functional groups of Ny6 was carried out with ion-beam and/or plasma treatment. These surface modifications were found to alter the interfacial adhesion strength between PS and Ny6. A remarkable enhancement was found with the surface functionalization of PS. Though the fracture toughness was varied depending on the process, its overall behavior was quite similar to that of others; the fracture toughness increased with increasing bonding temperature and bonding time, passed through a peak, and then decreased with a further increase of the bonding time or temperature. The variation of the fracture toughness with the bonding time and temperature can be plausibly explained in terms of two different failure mechanisms of adhesive failure and cohesive failure. This change appears more evidently for the interface between an amorphous polymer and a semicrystalline polymer than the interface between semicrystalline polymer pairs. Surface functionalization could exclude the effect of diffusion, thus clarifying the failure mechanisms occurring at the interface.