Fracture Behaviour of Styrene/Butadiene Block Copolymers having Different Molecular Architecture

The crack toughness behaviour of styrene/butadiene block copolymers of triblock and star architectures was investigated using instrumented Charpy impact testing. In order to evaluate adequately the toughness behaviour of the investigated materials, different concepts of elastic‐plastic mechanics (J‐integral and crack‐tip opening displacement, CTOD concepts) were used. Although the lamellar block copolymers showed a remarkably enhanced ductility in the tensile test than the neat block copolymer having hexagonal PB cylinders in PS matrix, no pronounced difference in crack toughness was found. This behaviour implies that the tensile strain cannot be regarded as the only parameter defining the toughness value. A brittle/tough transition was observed in a lamellar star block copolymer on blending with a linear thermoplastic elastomeric SBS triblock copolymer.

SEM micrograph showing the details of the stable crack propagation region in a binary block copolymer blend.     

The Role of Filler Networking in Fatigue Crack Propagation of Elastomers under High‐Severity Conditions

Structural parameters of the filler network have been evaluated by fitting quasi‐static stress/strain cycles to the dynamic flocculation model. It is found that the size of filler clusters as well as the strength of filler–filler bonds increase with filler loading and carbon black activity (specific surface). This correlates with the behavior of the tear resistance obtained for pulsed loading under high‐severity conditions, implying that the characteristics of the filler network govern the fracture properties of filled elastomers. The behavior of the power law exponent of fatigue crack propagation versus tearing energy can be explained by flash temperature effects in the crack tip area.

     

1,8‐Naphthalimides as Blue Emitting Fluorophores for Polymer Materials

The copolymerization of 1,8‐naphthalimide derivatives (as fluorophore) with acrylonitrile has been investigated. The photophysical characteristics of monomeric and polymeric fluorophores in N,N‐dimethylformamide solution have been determined and discussed. During copolymerization, no changes in the chromophoric systems of the fluorophore occur. The influence of the studied monomeric 1,8‐naphthalimide fluorophores upon the structurally bleached polyacrylonitrile has been determined. Infrared absorption characteristics of the polymerizable 4‐alkoxy‐ and 4‐allyloxy‐N‐substituted‐1,8‐naphthalimides have been measured and discussed. The effect of the substituents upon the vibration frequencies of the carbonyl and allylic groups has been established.

Blue fluorescent polyacrylonitrile copolymers with 1,8‐naphthalimides side‐group.     

Layered Silicate‐Induced Enhancement of Fracture Toughness of Epoxy Molding Compounds over a Wide Temperature Range

Summary: The fracture toughness of EMC was dramatically increased over a wide temperature range by the addition of a very low volume fraction of layered silicates to EMC filled with micro‐silica particles. Layered silicate‐EMC nanocomposites containing intercalated and the exfoliated silicates were fabricated by using o‐cresol and biphenyl type epoxy resins, respectively. It was found that exfoliated silicates were more effective than intercalated silicates at toughening EMC at temperatures above T_g of the epoxy resin. Enhanced fracture toughness of EMC over a wide temperature range, from ambient to 230 °C has been attributed to the presence of layered silicates, which induces macroscopic crack deflection and severe plastic deformation in front of the crack tip.

     

Cover Picture: Macromol. Mater. Eng. 8/2005

Summary: Poly(butylene succinate‐co‐adipate) (PBSA) and organically modified montmorillonite (OMMT) nanocomposites of three different compositions were prepared by melt‐extrusion in a batch mixer. The structure of the nanocomposites was studied using X‐ray diffraction (XRD) and transmission electron microscopy (TEM) that revealed a coexistence of exfoliated and intercalated silicate layers dispersed in the PBSA matrix, regardless of the silicate loading. The degree of crystallinity of PBSA decreases with the addition of OMMT platelets. Dynamic mechanical analysis revealed remarkable increase in flexural storage modulus when compared with that of neat PBSA. Tensile property measurements exhibit substantial increase in stiffness with simultaneous increase in elongation at break of nanocomposites as compared to that of neat PBSA. The effect of clay loading on the melt‐state linear viscoelastic behavior of mixed intercalated/exfoliated nanocomposites was also investigated.

Elongation at break of compression molded annealed samples of neat PBSA and various PBSACNs.     

Bio‐Based Rubbers by Concurrent Cationic and Ring‐Opening Metathesis Polymerization of a Modified Linseed Oil

Bio‐based rubbers prepared by tandem cationic polymerization and ROMP using a norbornenyl‐modified linseed oil, Dilulin?, and a norbornene diester, NBDC, have been prepared and characterized. Increasing the concentration of the NBDC in the mixture results in a decrease in the glass transition temperature. The new bio‐based rubbers exhibit tensile test behavior ranging from relatively brittle (18% elongation) to moderately flexible (52% elongation) and with decreasing values of tensile stress with increasing NBDC content. Thermogravimetric analysis reveals that the bio‐based rubbers have maximum decomposition temperatures of over 450 °C with their thermal stability decreasing with increasing loadings of NBDC.

     

Time‐ and Temperature‐Dependent Fracture Mechanics of Polymers: General Aspects at Monotonic Quasi‐Static and Impact Loading Conditions

Well‐defined correlations exist between the maxima in mechanical loss factor and the local maxima in temperature‐ or loading‐speed‐dependent fracture toughness. The non‐linear viscoelastic fracture processes and small‐strain deformations are characterised by the same Arrhenius‐type activation enthalpies. The local increase in toughness is linearly correlated with the relaxation strength of molecular relaxation processes. Stable crack propagation can be understood as a three‐phase process resulting in steady‐state stable crack growth. The normalised steady‐state crack‐tip‐opening displacement is independent of matrix material, temperature and loading speed.

     

Direct Melt Extrusion Processing of Nylon 6/Clay Nanocomposites Based on Pristine Montmorillonite

Summary: An original direct melt extrusion processing of nylon 6/clay nanocomposites was reported based on pristine (Na⁺‐based) montmorillonite as well as a simple approach using a typical two‐screw extruder. By the application of intercalation agents as the thermodynamic assistants, this method is as an appropriate procedure for industrialized manufacture together with much lowered production cost. Interestingly enough, the synergistic effects of montmorillonite with other inorganic particulates was observed for the first time here.

X‐ray diffraction patterns of pristine MMT and nylon 6/MMT composites with grouped intercalation agents.     

The Effect of Particle Matrix Adhesion on the Mechanical Properties of Silica Filled Cyanate Ester Composites

Summary: The effect of silica and its surface treatment on the mechanical properties of composites was studied as part of the evaluation of cyanate ester matrices as potential electronic encapsulants. Three filler surface treatments were used, as a qualitative interfacial adhesion scale, in an attempt to gauge the magnitude of interfacial adhesion between untreated filler and the cyanate ester matrix. There was strong interfacial adhesion between matrix and untreated filler. The level of silica content most affected composite modulus and fracture toughness. Filler surface treatment most affected composite strength and fracture toughness/energy. Composite fracture was found to occur via crack pinning and/or crack blunting depending on the strength of adhesion. The composites evaluated were found to possess suitable mechanical properties for potential use as electronic encapsulants.

     

Environmental Stress‐Cracking Resistance of LDPE/EVA Blends

Blends of low‐density polyethylene with random copolymers of ethylene and vinyl acetate (PE/EVA) are studied with respect to their environmental stress‐cracking resistance (ESCR) using the Bell‐telephone test. This system shows the shortest time to failure in the ESCR test after annealing at 50 °C in a stress‐cracking agent (Igepal solution) compared with that in the tests conducted at 30 and 70 °C. The increase of the time to failure at 70 °C as compared with that at 50 °C is probably the result of the semicrystalline proportion of EVA melting. Transmission electron microscopy images (see Figure) reveal that EVA particles are molten and deformed in bending direction of the sample at 70 °C in contrast to samples annealed at 50 and 30 °C. TEM pictures of a failed sample during the test conducted at 50 °C indicate that EVA particles can stop crack propagation.

TEM image of PE/EVA‐5.4 after 1 000 h in ESCR test conditions at 70 °C.     

Dual‐Curing of Waterborne Urethane‐Acrylate Coatings by UV and Thermal Processing

A waterbased dual‐cure urethane‐acrylate oligomer has been synthesized by polycondensation of monomers bearing hydroxyl, isocyanate and acrylate groups. To obtain a stable aqueous dispersion, carboxylate groups were grafted on the oligomer chain and the isocyanate groups were protected by a blocking agent. After water release by a brief heating, the dry films were cured either by a short UV exposure in the presence of a photoinitiator to induce the polymerization of the acrylate double bonds, or by heating up to 150 °C to release the isocyanates and promote the polycondensation by reaction with the hydroxyl groups, but mainly by a combination of UV and thermal cure. Both processes have been followed quantitatively by infrared spectroscopy to evaluate the influence of the temperature on the reaction rate and on the cure extent. The newly developed waterbased dual‐cure coatings were found to be quite resistant to accelerated weathering because of their aliphatic structure and their high crosslink density. Their light stability was substantially improved by the addition of a hydroxyphenyltriazine UV absorber and a hindered amine radical scavenger.

Thermal curing of the dual‐cure waterborne PUA formulation at a temperature of 150 °C.     

In‐Line Monitoring and Control of Conversion and Weight‐Average Molecular Weight of Polyurethanes in Solution Step‐Growth Polymerization Based on Near Infrared Spectroscopy and Torquemetry

Summary: It is well known that the weight‐average molecular weight ( ) is strictly dependent on conversion in step‐growth polymerizations performed in batch and that the is very sensitive to impurities and molar imbalance. This makes the work of controlling a non trivial job. In this paper a new methodology is introduced for in‐line monitoring and control of conversion and of polyurethanes produced in solution step‐growth polymerizations, based on near‐infrared spectroscopy (NIRS) and torquemetry. A calibration model based on the PLS method is obtained and validated for monomer conversion, while the weight‐average molecular weight is monitored indirectly with the relative shear signal provided by the agitator. Control procedures are then proposed and implemented experimentally to avoid gelation and allow for maximization of . The proposed monitoring and control procedures can also be applied to other step growth polymerizations.

Proposed control scheme.     

An Approach to Calculating Wear on Annular Non‐Return Valves

The serviceability of non‐return valves has a major influence on the productivity of the injection molding process. During a meeting of experts held at our Institute, it was seen that closing behavior and wear are the key problems encountered in practice. The conducted investigations to tackle these questions have shown that both an improved closing behavior and a lower level of wear can be achieved by reducing the inside radius of the locking ring.

Pressure profile over the length of a non‐return valve (n = 0.4; = 25 000 mm³/s).     

Improvement of Flex‐Fatigue Life of Carbon‐Black‐Filled Styrene‐Butadiene Rubber by Addition of Nanodispersed Clay

Summary: The flex‐fatigue life of carbon‐black‐filled SBR was dramatically improved by incorporation of 4–5 phr nanodispersed clay. Addition of clay did not decrease the degree of crosslinking of the composite but improved the hysteresis and tearing energy. ESEM observation of the flexing‐fracture morphology indicated that nanodispersed clay layers had the advantage over carbon black in that they could blunt the crack.

Effect of the clay amount on the flex fatigue life of the composites.     

Extrusion Monitoring of Polymer Melts Using a High‐Temperature Surface‐NMR Probe

Summary: Extrusion is one of the major ways of polymer processing, and extrusion monitoring is important to control or improve the extrusion process and the product quality. Especially methods for in‐line monitoring are desired because those enable the fastest possible response and a localization of chemical reactions in reactive processing. Methods of optical and acoustic spectroscopy have successfully been implemented so far. NMR provides a wealth of information. Surface NMR mostly gives access to molecular mobility via relaxation times. These can be correlated to temperature, composition and homogeneity which are substantial indicators for the product quality. NMR is a non‐destructive method, which does not require direct contact with the sample and it is not restricted to optical transparent materials and is therefore suitable for extrusion monitoring. The problem for the adaptation is the hostile environment in and in front of an extruder for polymer melts. Here, a NMR surface probe is presented sustaining this environment in front of an extruder. First measurements with this device are shown.

High‐temperature surface‐NMR probe.     

Photoinduced Polymerization in the Wall of Hollow Capsules

Summary: Novel light‐sensitive hollow capsules were fabricated from the small molecule 3‐sulfopropylacrylate potassium (SPA) and poly(allylamine hydrochloride) (PAH). With UV irradiation, SPA could be photopolymerized in the wall of hollow capsules. After photopolymerization the capsule size and surfaces showed pronounced differences. The capsules became much more rigid as indicated by an increase in the modulus of more than a factor of 5.

CLSM image of SPA/PAH hollow capsule emission at 554 nm, from rhodamine B after photopolymerization.     

A Model for the Elastoplastic Behavior of Isotactic Poly(propylene) Below the Yield Point

Observations are reported on isotactic poly(propylene) (iPP) in a series of tensile loading‐unloading tests with a constant strain rate at room temperature. A constitutive model is developed for the elastoplastic behavior of a semicrystalline polymer at isothermal uniaxial deformations with small strains. The stress‐strain relations are determined by 5 adjustable parameters which are found by fitting the experimental data.

The stress σ (MPa) versus strain ε in a tensile loading‐unloading test with the maximum strain ε_max = 0.09. Circles: experimental data. Solid line: results of numerical simulation.     

Design of a Microfluidic System to Investigate the Mechanical Properties of Layer‐by‐Layer Fabricated Capsules

A microfluidic system was designed, fabricated and implemented to study the behavior of polyelectrolyte capsules flowing in microscale channels. The device contains microchannels that lead into constrictions intended to capture polyelectrolyte microcapsules which were fabricated with the well‐known layer‐by‐layer (LbL) assembly technique. The behavior of hollow capsules at the constrictions was visualized and the properties of the capsules were investigated before and after introduction into the device.

Time series of video frames showing capsules being compressed into a constriction.     

New Insights into Fatigue Crack Growth in Graphene‐Filled Natural Rubber Composites by Microfocus Hard‐X‐Ray Beamline Radiation

Natural rubber (NR) composites containing graphene (GE) are prepared by ultrasound‐assisted latex mixing and in situ reduction. The fatigue crack propagation of the composite is examined. It is observed that GE has an opposite effect on crack growth resistance of NR, i.e., at lower fatigue strains, the inclusion of GE accelerates the crack growth, whereas at higher strains, the crack growth is retarded. It is suggested that the reason for this behavior is a competition between strain‐induced crystallization and cavitation at crack tip. Through microfocus hard‐X‐ray diffraction beamline with high spatial resolution, fatigue crack resistance is correlated to strain‐induced crystallization and new insights into the mechanism of fatigue crack growth are obtained.

     

Development of a 3D‐Finite Element Module for the Prediction of the Fibre Orientation in Injection Moulded Parts

This article presents a 3D‐finite element simulation package for the prediction of the fibre orientation in injection moulding. The fibre orientation is calculated by using a mathematical model based on orientation tensors, which not only takes into account the 3D‐flow field, but also the shape of the fibres and the fibre‐fibre interaction. The tensor model has been verified in many scientific investigations and has proven to yield satisfying results. Therefore we decided to use this model for implementation into a 3D‐finite element simulation package, which is currently being developed at the Institute of Plastics Processing (IKV) at RWTH Aachen University, Germany. The implementation of this model using both the finite element method and the finite volume method is described and the different parameters used in this model are analysed and discussed separately, as are the parameters used for the injection moulding process. The results obtained are compared to similar results from literature.

Model of the flow channel (10 mm × 10 mm × 100 mm) used for the simulations.