Application of two phase model to linear viscoelasticity of reinforced rubbers

A two phase model proposed for accounting for linear viscoelasticity of polymer nanocomposite melts [14] is applied to rubbers filled with nanoclay and conventional fillers (carbon black and silica) to probe mechanisms of the fluid- and the solid-like behaviors beyond the terminal flow region. This model shows strong applicability in linear rheology beyond terminal region for a variation of filled rubbers. Characteristic moduli of the “filler phase” from different filled rubbers could collapse onto a master curve, which reveals a jamming transition with increasing filler concentration across the percolation threshold. The strain amplification effect and reduced characteristic moduli of the “filler phase” are discussed within the framework of the cluster-cluster aggregation model.     

Application of two phase model to linear dynamic rheology of filled polymer melts

The linear dynamic rheology for a series of filled polymer melts is investigated to take account for the respective contributions of the bulky polymer phase away from the filler inclusions and the “filler phase” composed of filler particles coated with polymer layer. Time-concentration superposition principles are introduced to the linear viscoelasticities of both the bulky polymer phase and the “filler phase”. The result highlights the importance of the polymer dynamics to both the “filler phase” and the composite melt. Polymer mediated filler jamming towards glass formation is revealed for the “filler phase”, which accounts for origin of the fluid-to-solid transition upon filling.     

Nanotubes as polymers

In this review, we show that the structure and behavior of single-walled nanotubes (SWNTs) are essentially polymeric; in fact, many have referred to SWNTs as “the ultimate polymer”. The classification of SWNTS as polymers is explored by comparing the structure, properties, phase behavior, rheology, processing, and applications of SWNTs with those of rigid-rod polymers. Special attention is given to research efforts focusing on the use of SWNTs as molecular composites (also termed nanocomposites) with SWNTs as the filler and flexible polymer chains as the host. This perspective of “SWNTs as polymers” allows the methods, applications, and theoretical framework of polymer science to be appropriated and applied to nanotubes.     

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.     

Emulsion-templated porous polymers: A retrospective perspective

PolyHIPEs are porous emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs). HIPEs are highly viscous, paste-like emulsions in which the major, “internal” phase, usually defined as constituting more than 74% of the volume, is dispersed as discrete droplets within the continuous, minor, “external” phase. The surge in polyHIPE research and development parallels that of porous polymers in general and reflects the increasing number of potential applications (reaction supports, separation membranes, tissue engineering scaffolds, controlled release matrices, responsive and smart materials, and templates for porous ceramics and porous carbons). This review focuses upon the research and development in polyHIPEs through the prism of the work in our laboratory. The review includes an overview of the developments in polymerization chemistry, in the types of monomers, in the types of stabilization, in the generation of more complex polyHIPE-based systems (e.g. interpenetrating polymer networks, hybrids, bicontinuous polymers), and in unusual materials systems such as water-retaining polyHIPEs and shape-memory polyHIPEs.     

Percolation transition and hydrostatic piezoresistance for carbon black filled poly(methylvinylsilioxane) vulcanizates

The percolation transition and the hydrostatic piezoresistance for carbon black (CB) filled poly(methylvinylsilioxane) vulcanizates were studied as a function of CB volume fraction. A revised tunneling-percolation model based on the method of “subcritical networks” was proposed, which can not only account for the apparent nonuniversal percolation, but also figure out the contribution of changing tunneling current to the hydrostatic piezoresistance. Although there is a general tendency that the relative contribution of tunneling current increases with increasing filler concentration, it is always the variation of effective filler volume fraction which dominates the hydrostatic piezoresistance for the present system, due to the rather limited mobility of the mediating polymer layer between neighboring CB aggregates. The pressure and concentration dependences of the hydrostatic piezoresistance were interpreted in terms of the connectivity and/or the fractal nature of the percolation network. The concentration dependence of hydrostatic piezoresistance could even be associated with the strength of filler-matrix interaction. The baseline drift and poor reproducibility of hydrostatic piezoresistance were ascribed to the residual compressive strain of the rubber matrix, which could not be completely eliminated but could be deducted from the piezoresistance by a novel resistance baseline removal method.     

Mobile amorphous phase fragility in semi-crystalline polymers: Comparison of PET and PLLA

PET and PLLA were cold crystallised at various times and the two polymers were studied by differential scanning calorimetry (DSC), dielectric spectroscopy (DS) and thermally stimulated depolarisation currents (TSDC). The crystalline, the amorphous and the rigid amorphous fraction were quantified. The percentage of rigid amorphous fraction is very large in semi-crystalline PET and very low in semi-crystalline PLLA. From DSC, DS and TSDC data, the values of the relaxation times of four samples were obtained above and below the glass transition. The “strong-fragile” glass former liquid concept was used and the fragility of polymers was obtained. The presence of the crystalline phase and of a rigid amorphous fraction does not significantly modify PLLA fragility parameters and the polymer remains “fragile”, while for PET the semi-crystalline material goes towards a “strong character”. The coupling between phases is much weaker in PLLA than in PET.     

Plasticity/damage coupling in semi-crystalline polymers prior to yielding: Micromechanisms and damage law identification

This work addresses the question of the intimate coupling of plastic and damaging processes during the deformation of semi-crystalline polymers at small strains. The evolution of the spherulitic structure in the pre-yield strain range under tensile testing is investigated by atomic force microscopy for three semi-crystalline polymers, namely polycaprolactone, poly(1-butene) and polyamide 6. These materials have different spherulite size, crystallinity index and lamella thickness, and different glass transition temperature of the amorphous phase. Strain-induced damage is clearly evidenced through the gradual loss of elastic properties upon cyclic tensile tests, since the early stage of stretching. In parallel, volume strain appears to be about nil up to the yield point for the three polymers. AFM reveals that fragmentation of the crystalline lamellae occurs well before the yield strain at room temperature, starting about the core region of the spherulites and extending towards the periphery, for all polymers. This is claimed as evidence that lamella fragmentation is a basic mechanism of damage without significant cavitation at low strain. An approach of damage modeling is carried out via preliminary assessment of the viscoelastic contribution from low strain dynamic mechanical analysis using a generalized Maxwell model. It is shown that computing the viscoelastic contribution in the strain range up to yielding, in the assumption of linearity, fairly account for the loading-unloading hysteresis of the tensile cycles. A phenomenological plasticity/damage coupling law is established from the elastic modulus drop with increasing plastic strain, both assessed from the “relaxed” tensile cycles. The same kind of law is shown to apply for the three polymers. A physical meaning to the phenomenological law is proposed via a simple model of fiber rupture in single-fiber-reinforced composite.     

PolyHIPEs: Recent advances in emulsion-templated porous polymers

Porous polymers with well-defined porosities and high specific surface areas in the form of monoliths, films, and beads are being used in a wide range of applications (reaction supports, separation membranes, tissue engineering scaffolds, controlled release matrices, responsive and smart materials) and are being used as templates for porous ceramics and porous carbons. The surge in the research and development of porous polymer systems is a rather recent phenomenon. PolyHIPEs are porous emulsion-templated polymers synthesized within high internal phase emulsions (HIPEs). HIPEs are highly viscous, paste-like emulsions in which the major, “internal” phase, usually defined as constituting more than 74% of the volume, is dispersed within the continuous, minor, “external” phase. This review focuses upon the recent advances in polyHIPEs involving innovations in polymer chemistry, macromolecular structure, multiphase architecture, surface functionalization, and nanoparticle stabilization. The effects of these innovations upon the natures of the resulting polyHIPE-based materials (including bicontinuous polymers, nanocomposites, hybrids, porous ceramics, and porous carbons) and upon the applications involving polyHIPEs are discussed. The advances in polyHIPEs described in this review are now being used to generate new families of porous materials with novel porous architectures and unique properties.     

Synthesis and characterization of well-defined poly(l-lactide) functionalized graphene oxide sheets with high grafting ratio prepared through click chemistry and supramolecular interactions

Two simple and effective methods, “click” chemistry and supramolecular interactions, are demonstrated here to synthesize well-defined poly(l-lactide) (PLLA) functionalized graphene oxide (GO) sheets. We provide a simple method to introduce azide groups on GO sheets by the ring opening reaction of sodium azide with the epoxide groups of GO. The GO-N₃ sheets can easily undergo “click” reaction with alkyne-terminated PLLA by “grafting onto” method to produce GO/PLLA composites with high grafting ratio and exfoliated structure. Interestingly, GO-N₃ can be grafted with oxygen-containing polymers such as PLLA, polymethyl methacrylate (PMMA) or polyethylene oxide (PEO) via supramolecular interactions between the azide groups and these oxygen atoms on polymers, producing GO/polymer composites with low grafting ratio and intercalated structure. These “grafting onto” methods are useful to produce a variety of GO/polymer composites with different structure via “click” reaction or supramolecular interactions, which have potential applications in material science.     

Physicochemical driving forces behind exfoliation process of a synthetic montmorillonite in PDMS polymers

The conditions under which exfoliation of organo-montmorillonite in Poly(dimethylsiloxane) elastomers may occur were investigated via a three approaches: determination of the inter-platelet distance (as measured by WAXS), the surface energy evaluation (via inverse gas chromatography) of the polymer matrix and the clays, and measurement of heat of interaction (using a flow microcalorimeter in heptane) between polymers and the clay. The exfoliation efficiency is estimated by performing dynamic mechanical measurements. The results indicate that compatibilization and geometrical considerations are not sufficient requirements to transform clay particles into platelets. It evidences the determinant role of specific interactions between the reactive polymer end-groups and the filler surface. Polymer conformation on the clay surface and heat of adsorption associating “dispersive or London” forces and hydrogen bonding of respectively trimethyl- and hydroxyl-terminated polymer are evaluated.     

Computer simulation study on the shear-induced phase separation in semi-dilute polymer solutions by using Ianniruberto-Marrucci model

In our previous study, a computer simulation scheme based on Doi-Onuki theory is proposed to simulate the dynamics of the shear-induced phase separation in semi-dilute polymer solutions [KOBUNSHI RONBUNSHU 2007, 64, 324]. The scheme employs Ianniruberto-Marrucci model as a constitutive equation to express the viscoelastic behavior of the solution explicitly. The scheme enables us to simulate the time-evolution of stress as well as that of shear-induced structure upon shear-jump. In this study, we focus on the conformation of polymer chains. The dynamics of the polymer chains agrees with those in the “solvent squeeze” model which interprets the shear-induced phase separation phenomena in semi-dilute polymer solutions by Saito et al. [Macromolecules 1999, 32, 4879].     

Precise synthesis of polymers containing functional end groups by living ring-opening metathesis polymerization (ROMP): Efficient tools for synthesis of block/graft copolymers

This article summarizes recent examples for precise synthesis of (co)polymers containing functional end groups prepared by living ring-opening metathesis polymerization (ROMP) using molybdenum, ruthenium complex catalysts. In particular, this article reviews recent examples for synthesis of amphiphilic block/graft copolymers by adopting transition metal-catalyzed living ROMP technique. Unique characteristics of the living ROMP initiated by the molybdenum alkylidene complexes (so-called Schrock type catalyst), which accomplish precise control of the block segment (hydrophilic and hydrophobic) as well as exclusive introduction of functionalities at the polymer chain end, enable us to provide the synthesis of block copolymers varying different backbones by adopting the “grafting to” or the “grafting from” approach as well as “soluble” star shape polymers with controlled manner. The “grafting through” approach (polymerization of macromonomers) by the repetitive ROMP technique offers precise control of the amphiphilic block segments.     

Surface functionalization by smart coatings: Stimuli-responsive binary polymer brushes

Heterogeneous binary polymer brushes consist of an assembly of polymer chains of two incompatible polymers that are attached by one end to the surface with sufficient grafting density. They have been investigated experimentally only for a short time. Those brushes can be used in the form of ultrathin polymeric layers as a versatile tool for surface engineering to tune physico-chemical surface characteristics as wettability, surface charge, chemical composition or morphology, and furthermore to create switchable and responsive surface properties. For the fabrication of these layers “grafting-from” (as radical polymerization at the interface) and “grafting-to” (as tethering of the polymer chains from solution) methods were developed and investigated in detail.     

Ductility of filled polymers

The ductility of a calcium carbonate-filled amorphous copolyester PETG in a uniaxial tensite test was examined as a fiction other filler volume fraction. A ductile-to-quasibrittle transition occurred as the volume fraction of filler increased. This transition was from propragation of a stable neck through the entire gauge length of the specimen to fracture in the neck without propagation. The draw stress (lower yield stress) did not depend on the filler content and was equal to the draw stress of the unfilled polymer. It was therefore possible to use a simply model to predict the dependence of the fracture strain on the filler volume fraction. It was proposed that when the fracture strain decreases to the draw strain of the polymer the fracture mechanism changes and the fracture strain drops sharply. The critical filler content at which the fracture mode changes is determined primarily by the degree of strain-hardening of the polymer. © 1994 John Wiley & Sons, Inc.     

Porous wollastonite-hydroxyapatite bioceramics from a preceramic polymer and micro- or nano-sized fillers

Wollastonite-hydroxyapatite ceramics have been successfully prepared by a novel method, corresponding to the thermal treatment in air of a silicone embedding micro- and nano-sized fillers. CaCO₃ nano-sized particles, providing CaO upon decomposition, acted as “active” filler, whereas different commercially available or synthesised hydroxyapatite particles were used as “passive” filler. The homogeneous distribution of CaO, at a quasi-molecular level, favoured the reaction with silica derived from the polymer, at only 900 °C, preventing extensive decomposition of hydroxyapatite. Open-celled porous ceramics suitable for scaffolds for bone-tissue engineering applications were easily prepared from filler-containing silicone resin mixed with sacrificial PMMA microbeads as templates. The pore size (in the range of 80-400 μm) and the open porosity percentage (40-50%) were evaluated by means of micro-computerized tomographic analysis. A preliminary assessment of the biocompatibility and cell activity of the produced ceramics was performed successfully by in vitro tests using human osteoblast cells.     

Evaluation of different catalyst systems for bulk polymerization through “click” chemistry

“Click” chemistry is a frequently used technique in polymer chemistry for coupling polymer end groups to construct novel copolymer architectures and as a step-growth polymerization technique. In this study, the bulk polymerization of a diyne and a bisazide were achieved through the copper-catalyzed alkyne-azide 1,3-cycloaddition (CuAAC) with different catalysts, including the homogeneous catalyst Cu(PPh₃)₃Br and a heterogeneous Cu/C catalyst. The effects of different catalyst systems on the kinetics of the “click” polymerization were evaluated by differential scanning calorimetry (DSC) and oscillatory rheology. Additionally, the thermomechanical properties of resulting polymers were evaluated by temperature-ramp oscillatory rheology and thermal stabilities by thermogravimetry.     

Examination of the fundamental relation between ionic transport and segmental relaxation in polymer electrolytes

Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes is strongly limited by their poor ionic conductivity. The classical theory predicts that the ionic transport is dictated by the segmental motion of the polymer matrix. As a result, the low mobility of polymer segments is often regarded as the limiting factor for development of polymers with sufficiently high ionic conductivity. Here, we show that the ionic conductivity in many polymers can be strongly decoupled from their segmental dynamics, in terms of both temperature dependence and relative transport rate. Based on this principle, we developed several polymers with “superionic” conductivity. The observed fast ion transport suggests a fundamental difference between the ionic transport mechanisms in polymers and small molecules and provides a new paradigm for design of highly conductive polymer electrolytes.     

The role of clay network on macromolecular chain mobility and relaxation in isotactic polypropylene/organoclay nanocomposites

It is well known that a so-called “three-dimensional filler network structure” will be constructed in the polymer/layered silicate nanocomposites when the content of layered clay reaches a threshold value, at which the silicate sheets are incapable of freely rotating, due to physical jamming and connecting of the nanodispersed layered silicate. In this article, the effect of such clay network on the mobility and relaxation of macromolecular chains in isotactic polypropylene(iPP)/organoclay nanocomposites was investigated in detail with a combination of DMTA, DSC, TGA, TEM, rheometry and melt flow index measurements. The main aim is to establish a relationship between the mesoscopic filler network structure and the macroscopic properties of the polymer nanocomposites, particularly to explore the role of the clay network on the mobility and relaxation of macromolecular chains. It was found that the nanodispersed clay tactoids and layers play less important or dominant roles on the mobility of iPP chains depending on the formation of percolating filler network. The turning point of macroscopic properties appeared at 1 wt% organoclay content. Before this point, the effect of organoclay can be negligible, and the increase of chain mobility was ascribed to the decrease of molecular weight of polymer chains, as commonly occurs during dynamic melt processing; after this point, however, a reduced mobility of chains and a retarded chain relaxation were observed and attributed to the formation of a mesoscopic filler network. The essential features of such a mesoscopic organoclay network were estimated and discussed on the basis of stress relaxation and structural reversion measurements. A schematic model was proposed to describe the different relaxation and motion behaviors of macromolecular chains in the unfilled polymer and the filled hybrids with partial and percolated organoclay networks, respectively.