Long‐time relaxations in rubber‐modified polymer systems

Linear and nonlinear viscoelastic properties were measured in the molten state for several model ABS polymers with different rubber particle contents. Linear viscoelastic functions for ABS polymers can be separated in two parts. One is a relaxation associated with the entanglement of matrix SAN chains and the other comes from the particle‐particle interactions of rubber particles. This relaxation depends strongly on the degree of dispersion of rubber particles. The second‐plateau modulus appeared at low frequency with samples in which rubber particles agglomerate. While, the second‐plateau modulus was not observed with samples in which rubber particles are finely dispersed. Matching of AN content between grafted and matrix SAN and optimum graft density form a finely dispersed system. Large deformation relaxation measurements revealed that the damping of ABS polymers having a good dispersion of particles become stronger with an increase in rubber content. This strong damping can be explained by a layered structure. The very long relaxation was found for higher rubber content, when the neighboring grafted SAN chains contact with each other.     

Recovery stress and work output in hyperbranched poly(ethyleneimine)‐modified shape‐memory epoxy polymers

In this study a series of hyperbranched modified shape‐memory polymers were subjected to constrained shape recoveries in order to determine their potential use as thermomechanical actuators. Materials were synthesized from a diglycidyl ether of bisphenol A as base epoxy and a polyetheramine and a commercial hyperbranched poly(ethyleneimine) as crosslinker agents. Hyperbranched polymers within the structure of the shape‐memory epoxy polymers led to a more heterogeneous network that can substantially modify mechanical properties. Thermomechanical and mechanical properties were analyzed and discussed in terms of the content of hyperbranched polymer. Shape‐memory effect was analyzed under fully and partially constrained conditions. When shape recovery was carried out with fixed strain a recovery stress was obtained whereas when it was carried out with a constraining stress the material performs mechanical work. Tensile tests at Tg^E′ showed excellent values of stress and strain at break (up to 15 MPa and almost 60%, respectively). Constrained recovery performances revealed rapid recovery stress generation and unusually high recovery stresses (up to 7 MPa) and extremely high work densities (up to 750 kJ/m³). The network structure of shape‐memory polymers was found to be a key factor for actuator‐like applications. Results confirm that hyperbranched modified‐epoxy shape memory polymers are good candidates for actuator‐like shape‐memory applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1002–1013     

Moisture‐mediated intrinsic self‐healing of modified polyurethane urea polymers

Functional materials having the ability to self‐heal cracks or scratches after damage are of great interest for a huge scope of applications. Herein, we report a self‐healing polyurethane urea‐based material with implemented 1‐(2‐aminoethyl) imidazolidone (UDETA) as a chain terminating molecule and for hydrogen bond network formation. Both, UDETA content and moisture affected the self‐healing process. The reversible change in the materials properties was proven by detailed analyses of hardness and thermomechanical behavior in dependence of the water uptake of the samples. FT‐IR analysis revealed that water is able to act as a plasticizer interrupting hydrogen bonding interactions within the polymer network and thus, influencing glass transition temperature and hardness of the samples. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 537–548.     

Compatibilization level effects on the structure and mechanical properties of rubber‐modified polyamide‐6/clay nanocomposites

Exfoliated polyamide‐6 (PA6)/organically modified montmorillonite clay (OMMT) nanocomposites (PNs) were modified with partially maleinized styrene–ethylene/butadiene–styrene triblock copolymers (SEBS) at three maleinization levels in an attempt to link in these materials high toughness with appropriate small‐strain and fracture tensile properties. OMMT stayed only in the PA6 matrix, and no preferential location in the matrix/rubber interphase was observed. The increased dispersed phase size upon the addition of OMMT was attributed to interactions between maleic anhydride (MA) functionalized SEBS and the surfactant of OMMT. The rubber particle size generally decreased when the MA content of SEBS increased, and this indicated compatibilization. The subsequent good adhesion led to tough nanocomposites across a wide range of both strain rates and fracture modes. As the critical interparticle distance (τ_c) decreased with the MA content, and the other parameters that could influence the surface‐to‐surface mean interparticle distance did not change, it is proposed that in these PNs higher adhesion leads to a smaller τ_c value. Finally, the presence in the matrix of a nanostructured clay makes the rubber content necessary for the toughness jump to increase and τ_c to decrease. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3611–3620, 2005     

Mechanical response of three semi crystalline polymers under different stress states: Experimental investigation and modelling

The mechanical responses of high‐density polyethylene (HDPE), polypropylene (PP) and polyamide 6 (PA 6) were experimentally investigated for a wide range of stress states and strain rates. This was accomplished by testing numerous specimens with different geometries. The uniaxial compression of cylindrical unnotched specimens and the uniaxial tensile behaviour of dumbbell specimens at different strain rates, was determined. A series of biaxial loading tests (combined shear and tension/compression, pure shear, pure tension/compression) using a designed Arcan testing apparatus were also performed. Flat and cylindrical notched specimens with different curvature radii were additionally tested in order to explore a wider range of stress states. The Drucker‐Prager yield criterion was calibrated with a set of experimental data, for which analytical formulae for stresses are available, and then applied to predict the deformation behaviour under different stress states, prior to strain localization. The results of the numerical simulations show that the Drucker‐Prager model can capture the initial elastic range and the post‐elastic response very satisfactorily. For triaxial and biaxial stress states there is a good agreement, however some load‐displacement responses are only satisfactorily described. Deviations observed in the predicted and experimental results are very likely attributed to the third invariant stress tensor, which was not explored in the model calibration. The evolution of stress triaxiality and Lode angle parameters with equivalent plastic strain were extracted and analysed for several specimens. The results show a plastic yielding behaviour sensitive to the stress state, which can be attributed to different combinations of stress triaxialities and Lode angle parameters.     

Relation between stress and segmental orientation during mechanical cycling of a natural rubber‐based compound

X‐ray diffraction technique is used to evaluate the evolution of the segmental orientation in a natural rubber sample during mechanical uniaxial stretching and in the Gaussian regime condition. The method proves to be sensitive enough for testing the validity of the stress‐optical law. Measurements are performed at different temperatures and show that the dependence of the orientation parameter upon elongation is very close to the prediction of classical phantom theories. On the contrary, a significant Mooney–Rivlin correction is needed to account for the stress–strain relation. Consequently, systematic deviations from the stress‐optical law are observed, in particular for elongations below 2. These deviations are adverse from predictions of both the constrained and the diffuse junction theories. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 943–950     

Brownian dynamics simulation of two‐dimensional nanosheets under biaxial extensional flow

The morphology dynamics of two‐dimensional nanosheets under extensional flow are investigated using a coarse‐grained model. Nanosheets (graphene, BNNS, MX₂) are promising materials for a variety of materials and electronics applications. Extensional flow fields are often present during dispersion processing, such as spin coating. Both nanosheet properties (e.g., moduli, size) and processing parameters (e.g., extension rate) can have a significant impact on the nanosheet morphology and thus, the structure and properties of the bulk material. Our previously developed dimensionless Brownian dynamics methodology is used to explore biaxial extensional flow. Nanosheets exhibit a flat conformation under extensional flow for high bending moduli and an extended “washrag” conformation for low bending moduli. Intrinsic extensional viscosity increases with strain before reaching a plateau. The intrinsic viscosity exhibits a weak power law with nanosheet molecular weight. These simulation results allow for experimental control over morphology as a function of nanosheet properties and flow type and strength. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1247–1253     

Thermomechanical and fractographic behavior of poly (HDDA‐co‐MMA): a study for its application in microcantilever sensors

Polymer‐based materials have drawn significant attention lately for their application in design and fabrication of thermomechanically stable highly sensitive three‐dimensional (3D) micromechanical sensor structures. Among these materials, 1,6 Hexane diol diacrylate (HDDA) has been extensively explored in laser‐based 3D microfabrication by microstereolithography. The thermomechanical properties of poly HDDA are important for their end use application in microcantilever‐based sensors. The present work explores the optimization of these properties by copolymerization of HDDA and methyl methacrylate (MMA) for these specific applications. The dynamic mechanical analysis, thermal expansion and mechanical studies were carried out for different compositions of poly (HDDA‐co‐MMA). An increase in MMA content in the copolymer matrix resulted in the enhancement of the thermomechanical stability. The variations of thermal expansion coefficient (CTE) for different compositions were also studied. Microhardness, uniaxial tensile, and flexural tests along with Poisson's ratio were determined to understand the mechanical properties of these compositions. The scanning electron micrographs of fractured surfaces of all the compositions were analyzed to understand the fracture mechanism of various compositions of this copolymer. Copyright © 2012 John Wiley & Sons, Ltd.     

Mechanical,thermal, and UV‐shielding behavior of silane surface modified ZnO‐reinforced phthalonitrile nanocomposites

In the present work, zinc oxide nanoparticles were treated with aminopropyl trimethoxy silane‐coupling agent and used as a new kind of reinforcement for a typical high performance bisphenol‐A‐based phthalonitrile resin. The resulted nanocomposites were characterized for their mechanical, thermal, and optical properties. Results from the tensile test indicated that the tensile strength and modulus as well as the toughness state of the matrix were all enhanced with the increasing of the nanoparticles amount. Thermogravimetric analysis showed that the starting decomposition temperatures and the residual weight at 800°C were highly improved upon adding the nanofillers. At 6 wt% nanoloading, the glass transition temperature and the storage modulus were considerably enhanced reaching about 359°C and 3.7 GPa, respectively. The optical tests revealed that the neat resin possesses excellent UV‐shielding properties, which were further enhanced by adding the nanofillers. Furthermore, the fractured surfaces of the nanocomposites analyzed by scanning electron microscope exhibited homogeneous and rougher surfaces compared with that of the pristine resin. Finally, the good dispersion of the reinforcing phase into the matrix was confirmed by a high resolution transmission electron microscope. Copyright © 2015 John Wiley & Sons, Ltd.     

Mechanical and anti‐stabbing properties of modified thermoplastic polymers impregnated multiaxial p‐aramid fabrics

New forms of hybrid multiaxial nanocomposites with enhanced mechanical and stab resisting properties are presented. This study is motivated by the lack of knowledge in the study of the multiaxial fabric nanocomposites with two modified thermoplastic matrices for antiballistic protection. Introduction of 5 wt.% silica nanoparticles in the composite of polyurethane/p‐aramid/poly (vinyl butyral) leads to significant improvement in mechanical properties, and the addition of silane as a coupling agents and glutaraldehyde as a crosslinking agents yielded maximal values of storage modulus, tensile modulus and anti‐stabbing properties for hybrid nanocomposites. Ballistic resistance testing and penetration depth of the hybrid nanocomposites were visualized using image analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

Bound D‐states of helium atom under Debye screening

We have investigated the 1snd^1,3D (3 ≤ n ≤ 7) state energies of helium atom embedded in weakly coupled plasma environments using the Rayleigh–Ritz variational method. The effect of the plasma environment is taken care of using a Debye screening model. A correlated wave function involving exponential expansion has been used to represent correlation between the charge particles. The bound 1snd^1,3D (3 ≤ n ≤ 7) state energies of helium for various Debye lengths along with the excitation energies of few singlet and triplet states are reported. Our results are useful references to atomic physics, plasma physics, and astrophysics research communities. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

New analysis of the optimization of electromagnetic shielding properties using conducting polymers and a multi‐objective approach

Recently, an approach was proposed to optimize multi‐layer shields of polyaniline–polyurethane (PAni/PU) conducting composites in the microwave band. Though by this method shields for different applications can be obtained which are light‐weight and offer a low percolation threshold, the full potential of the design process could not be tapped since the underlying optimization problem includes only one objective. In this work we go one step beyond and re‐formulate the design problem as a multi‐objective optimization problem (MOP). To be more precise, we involve simultaneously the shielding efficiency as well as the weight and the cost of the material—i.e. all the requirements for modern shielding materials—within the optimization process. After having stated the model we present two possible ways to approximate the solution set—the so‐called Pareto set—and address the related and important decision‐making problem. All steps are demonstrated on a particular three‐layered composite in order to show the applicability of the novel approach. Copyright © 2008 John Wiley & Sons, Ltd.     

Gas transport in vulcanized natural rubber‐cellulose. II. Composites

This work reports the transport of carbon dioxide, oxygen, and nitrogen in amorphous membranes of vulcanized natural rubber reinforced with regenerated cellulose. The values of the permeability coefficient of carbon dioxide, oxygen, and nitrogen in the composites with 25% of cellulose, measured at 25 °C and 15 cmHg of pressure, are roughly one‐third of those measured in the same conditions for these gases in natural rubber. The isotherms representing the variation of both the permeability and diffusion coefficients of the gases with pressure present a relatively sharp increase in the region of low pressures, attributed to changes in the free volume. The analysis of the permeability characteristics of the membranes in terms of the free‐volume theory suggests that gas transport is severely hindered in both the cellulose phase and the cellulose–rubber interphase of the composites. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 393–402, 2000     

Properties of acyl modified poly(glycerol‐adipate) comb‐like polymers and their self‐assembly into nanoparticles

There is an increasing need to develop bio‐compatible polymers with an increased range of different physicochemical properties. Poly(glycerol‐adipate) (PGA) is a biocompatible, biodegradable amphiphilic polyester routinely produced from divinyl adipate and unprotected glycerol by an enzymatic route, bearing a hydroxyl group that can be further functionalized. Polymers with an average Mn of ～13 kDa can be synthesized without any post‐polymerization deprotection reactions. Acylated polymers with fatty acid chain length of C₄, C₈, and C₁₈ (PGAB, PGAO, and PGAS, respectively) at different degrees of substitution were prepared. These modifications yield comb‐like polymers that modulate the amphiphilic characteristics of PGA. This novel class of biocompatible polymers has been characterized through various techniques such as FT‐IR, ¹H NMR, surface, thermal analysis, and their ability to self‐assemble into colloidal structures was evaluated by using DLS. The highly tunable properties of PGA reported herein demonstrate a biodegradable polymer platform, ideal for engineering solid dispersions, nanoemulsions, or nanoparticles for healthcare applications. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3267–3278     

Water‐mediated transport in ion‐containing polymers

Water‐mediated ion conduction enables high conductivity in hydrated polymer membranes commonly used in electrochemical devices. Understanding the coupling of the absorbed water with the polymer matrix and the dynamics of water inside the polymer network across the full range of length scales in the membrane is important for unraveling the structure–property relationships in these materials. By considering the water behavior in ion‐containing polymers, next‐generation fuel cell membranes are being designed that exceed the conductivity of the state‐of‐the‐art materials and have optimized conductivity and permeability that may be useful in other types of devices such as redox flow batteries. Water–polymer associations can be exploited to tune the transport and mechanical property tradeoffs in these polymers. Measurements of water motion provide important criteria for assessing the factors that control the performance of these types of materials. This review article discusses current understanding of water behavior in ion‐containing polymers and the relationship between water motion and ion and molecular transport. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Electrochemically‐controlled DNA Release under Physiological Conditions from a Monolayer‐modified Electrode

An indium tin oxide (ITO) electrode prepared on a flexible polymeric support was modified with an amino‐silane and then functionalized with trigonelline and 4‐carboxyphenylboronic acid covalently bound to the amino groups. The trigonelline species containing quarterized ammonium group produced positive charge on the electrode surface regardless of the pH value, while the phenylboronic acid species were neutral below pH 8 and negatively charged above pH 9 (note that their pK_a=8.4). The total charge on the monolayer‐modified electrode was positive at the neutral pH and negative at pH>9 (note that 4‐carboxyphenylboronic acid was attached to the electrode surface in excess to trigonelline, thus allowing the negative charge to dominate on the electrode surface at basic pH). Single‐stranded DNA molecules were loaded on the modified electrode at pH 7.0 due to their electrostatic attraction to the positively charged surface. By applying electrolysis at −1.0 V (vs. Ag/AgCl reference) electrochemical oxygen reduction resulted in the consumption of hydrogen ions and local pH increase in the vicinity of the electrode surface. The process resulted in the transition to the total negative charge due to the negative charges formed on the phenylboronic acid species. This resulted in the electrostatic repulsion and release of the loaded DNA. The developed approach allowed the electrochemically‐triggered DNA release not only in the aqueous solutions, but also in human serum solution, thus giving promise for future biomedical applications.     

Processing‐induced properties in glassy polymers: Application of structural relaxation to yield stress development

A method is presented to predict the yield stress distribution throughout an injection‐molded product of an amorphous polymer as it results from processing conditions. The method employs the concept of structural relaxation combined with a fictive temperature, following the Tool–Narayanaswamy–Moynihan formalism. The thermal history, as it is experienced by the material during processing, is obtained by means of numerical simulation of the injection molding process. The resulting predictions of yield stress distributions are shown to be in excellent agreement with experimental findings, both for different mold temperatures and for different part thicknesses. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1212–1225, 2006     

Charge‐carrier transport in disordered polymers

The general properties of charge‐carrier transport in disordered organic materials are discussed. The spatial correlation between energies of transport sites determined the form of drift‐mobility field dependence. The type of spatial correlation in a disordered material depends on its nature. Mobility field dependences must be different in polar and nonpolar materials. Different methods of mobility calculation from the shape of photocurrent transient were analyzed. A widely used method is very sensitive to the variation of the shape of the transient and sometimes produces results that effectively masquerade the true dependence of the mobility on the electric field or trap concentration. Arguments in favor of the better, more reliable method are suggested. Charge transport in materials containing charged traps was considered without using the isolated trap approximation, and this led to qualitatively different results. The results indicated that the effect of charged traps can hardly be responsible for the experimentally observed transport properties of disordered organic materials. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2584–2594, 2003     

Cure kinetics modeling and thermomechanical properties of cycloaliphatic epoxy‐anhydride thermosets modified with hyperstar polymers

Hyperstar polymers (HSPs) with hyperbranched aromatic polyester core and arms consisting of block copolymers of poly(methyl methacrylate) and poly(hydroxyethyl methacrylate) have been used as polymeric modifiers in cycloaliphatic epoxy‐anhydride formulations catalyzed with tertiary amines, with the purpose of enhancing the impact strength of the resulting materials without compromising other thermal and mechanical properties.> In this work, the effect of these polymeric modifiers on the curing kinetics, processing, thermal‐mechanical properties and thermal stability has been studied using thermal analysis techniques such as DSC, TMA, DMA, and TGA. The morphology of the cured materials has been analyzed with SEM. The curing kinetics has been analyzed by isoconversional procedures and phenomenological kinetic models taking into account the vitrification during curing, and the degradation kinetics has been analyzed by means of isoconversional procedures, summarizing the results in a time‐temperature‐transformation (TTT) diagram. The results show that HSPs participate in the crosslinking process due to the presence of reactive groups, without compromising significantly their thermal‐mechanical properties. The modified materials show a potential toughness enhancement produced by the formation of a nano‐grained morphology. The TTT diagram is shown to be a useful tool for the optimization of the curing schedule in terms of curing completion and safe processing window, as well as for defining storage stability conditions. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1227–1242