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Review Articles

In this short review, selected experimental approaches for probing the mechanism and kinetics of reversible addition-fragmentation chain transfer(RAFT) polymerisation are highlighted. Methods for studying RAFT polymerisation via varying reaction conditions, such as pressure, temperature, and solution properties, are reviewed. A technique for the measurement of the RAFT specific addition and fragmentation reaction rates via combination of pulsed-laser-initiated RAFT polymerisation and microsecond-time-resolved ESR spectroscopy is detailed. Mechanistic investigations using mass spectrometry are exemplified on dithiobenzoic-acid-mediated methyl methacrylate polymerisation. 33 refs. (IUPAC, International Symposium on 'Radical Polymerisation: Kinetics and Mechanism', Il Ciocco, Italy, Sept.2006)

With the recent development of new initiation techniques in atom transfer radical polymerisation(ATRP) that allow catalysts to be employed at unprecedented low concentrations (about 10 ppm), a thorough understanding of competitive equilibria that can affect catalyst performance is becoming increasingly important. Such mechanistic considerations are discussed including factors affecting the position of the ATRP equilibrium, dissociation of the ATRP catalyst at high dilution and loss of deactivator due to halide dissociation, conditional stability constants as related to competitive monomer, solvent, and reducing agent complexation as well as ligand selection with respect to protonation in acidic media, and competitive equilibria involving electron transfer reactions, including the radical oxidation to carbocations or reduction to carbanions, radical coordination to the metal catalyst, and disproportionation of the Cu I-based ATRP activator. 79 refs. (IUPAC, International Symposium on 'Radical Polymerisation: Kinetics and Mechanism', Il Ciocco, Italy, Sept.2006)

Nanoimprint lithography(NIL) is a non-conventional lithographic technique for high-throughput patterning of polymer nanostructures at great precision and at low costs. Unlike traditional lithographic approaches, which achieve pattern definition through the use of photons or electrons to modify the chemical and physical properties of the resist, NIL relies on direct mechanical deformation of the resist material and can therefore achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional techniques. This review covers the basic principles of nanoimprinting, with emphasis on the requirements on materials for the imprinting mould, surface properties, and resist materials for successful and reliable nanostructure replication. The main polymeric materials discussed are acrylic polymers and siloxane copolymers. 109 refs.

Recent developments in the area of dental adhesives and composites are reviewed. Examples of new monomers given include phosphonic acid ether acrylates and crosslinking bis(acrylamide)s for self-etching enamel-dentin adhesives with improved storage stability and free-radically polymerisable bicyclic cyclopropyl acrylates or cyclic allyl sulphides, which enable the preparation of low-shrinkage, storage-stable restorative composites. It is shown that designed methacrylates with tailor-made properties and sol-gel polycondensates can also contribute to the improvement of the currently used restorative composites. Radically-polymerisable dental materials initiated by light curing and by redox initiator systems are described. The use of discrete nanofiller particles for increasing the reinforcement and, in some cases, the X-ray opacity of composite filling materials is discussed. 181 refs.

Drawing of semicrystalline polymers generally involves a necking phenomenon, which is often referred to as plastic instability. The draw ratio in the stable neck is called natural draw ratio. A review is presented of papers dealing with the phenomenology of necking and attempts to identify its physical origin. The destruction of the spherulitic structure that consists of chain-folded lamellae involves a strain-softening accompanied by a localisation of the plastic deformation. In turn, the fibrillar transition that results from the lamellar fragmentation and subsequent rearrangement of the crystal blocks into microfibrils brings about a strain-hardening, which stabilises the plastic deformation. These competitive processes give rise to the natural draw ratio. The macromolecular network that consists of both chain entanglements and intercrystalline tie molecules appears to be a major factor of the neck stabilisation. Theoretical approaches of the phenomenon are critically reviewed. Finally, the practical usefulness of the natural draw ratio is discussed with regard to predicting the long-term mechanical behaviour of HDPE. 96 refs.

A review is presented of the state of the art in polymeric nanocomposites tribology and the need for more quantitative studies is highlighted. Examples of such quantitative measurements are given from recent studies, which mostly involve investigation of nanocomposites with PTFE matrices. Particular attention is paid to characterisation of dispersion, effects of internal interfaces, effects of phase and crystallinity, and quantification of transfer films. 90 refs.

A review is presented on the preparation of aqueous polymer dispersions via catalytic polymerisation and their colloidal properties, particle formation mechanisms, morphologies and microstructures. Sections are included on dispersions of polyolefins, polybutadiene, polyalkenamers, polynorbornenes, polyketones and conjugated polymers. 135 refs.

The structural applications of polymer matrix composites (PMC) demand lifetimes of 15, 25 and 50 years. However, the mechanical properties of these composites have a time dependent nature, i.e. strength and stiffness are time-dependent due to the hereditary nature (viscoelasticity) of polymers. In this context lifetime models for viscoelastic materials, i.e. energy-based criteria and fracture mechanics extended to viscoelastic media, are revised. These models are applied to predict the lifetime of composite materials under special cases of constant load (creep rupture) and constant stress rate to failure. It is verified that these lifetime theories predict similar relationship between creep failure and constant stress rate failure strength. Alternative approaches based on Strength Evolution Integral [Reifsnider KL, Stinchcomb WW. A critical element model of the residual strength and life of fatigue-loaded composite coupons. In: Hahn HT, editor. Composite materials: fatigue and fracture (ASTM STP 907). Philadelphia (PA): American Society for Testing and Materials; 1986. p. 298-313; Reifsnider KK, Case SC, Duthoi J. The mechanics of composite strength evolution. Compos Sci Technol 2000; 60:2539-46; Reifsnider KK, Case SC. Damage tolerance and durability in material systems. Wiley-Interscience; 2002] and on Linear Damage Accumulation (LCD) law confirm these results. In addition the LCD law was found to be generally unsatisfactory except for the special case of constant stress rate to failure. Accordingly this result validates the accelerated methodology proposed by [Miyano Y, McMurray M, Enyama J, Nakada M. Loading rate and temperature dependence on flexural fatigue behaviour of a satin woven CFRP laminate. J Compos Mater 1994; 28(13):1250-60; Miyano Y, Nakada M, McMurray MK, Muki R. Prediction of flexural fatigue strength of CRFP composites under arbitrary frequency, stress ratio and temperature. J Compos Mater 1997; 31(6):619-38; Miyano Y, Nakada M, Kudoh H, Muki R. Prediction of tensile fatigue life for unidirectional CFRP. J Compos Mater 2000; 34(7):538-50; Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of GFRP laminates for marine use. Compos: Part B 2004; 35:497-502; Miyano Y, Nakada M, Sekine N. Accelerated testing for long-term durability of FRP laminates for marine use. J Compos Mater 2005; 39(1):5-20], which is based on LCD law, to characterize long-term creep failure of polymer composites based on the constant stress rate failure strength curves. Finally a new formulation is proposed, based on Strength Evolution Integral, to predict of fatigue failure load for an arbitrary load ratio. 47 refs. Copyright (c) 2007 Elsevier Ltd.

This review describes the polymer electrolyte membranes (PEM) that are both under development and commercialised for direct methanol fuel cells (DMFC). Unlike the membranes for hydrogen fuelled PEM fuel cells, among which perfluorosulphonic acid based membranes show complete domination, the membranes for DMFC have numerous variations, each has its advantages and disadvantages. No single membrane is emerging as absolutely superior to others. This review outlines the prospects of the currently known membranes for DMFC. The membranes are evaluated according to various properties, including: methanol crossover, proton conductivity, durability, thermal stability and maximum power density. Hydrocarbon and composite fluorinated membranes currently show the most potential for low cost membranes with low methanol permeability and high durability. Some of these membranes are already beginning to impact the portable fuel cell market. 83 refs. Copyright (c) 2007 Elsevier B.V.

The applications of microwave methodology to chemical synthesis are reviewed with emphasis on nanomaterials, biochemical reactions and chemicals for medical and pharmaceutical uses. The advantages of microwave irradiation are discussed in terms of the use of environmentally-friendly solvents or solvent-free conditions, higher product yields and the development of novel synthetic routes. 96 refs.

The synthesis and thermal properties of poly(thioester)s, including aliphatic and aromatic polythiocarbonates, aliphatic polythiourethanes and poly(thioester)s derived from alpha- and omega-mercapto carboxylic acids and alpha,omega-dimercaptoalkanes, are reviewed. Synthetic strategies discussed include anionic and cationic ring-opening polymerisations, polycondensations in bulk or in solution, interfacial polycondensation and in vitro enzymatic polycondensations. 210 refs.

The synthesis of coordination polymers based on various transition metals and the Schiff bases, 1,1'-dihydroxy-2,2'-(1-isonitriloethylidyne-4- nitrilo-methylidenephenyl)diphenyl or 1-thio-1'-hydroxy-2,2'-(1- isonitriloethylidyne-4-nitrilomethylidenephenyl)diphenyl, is described. The products were characterised by elemental analysis, proton and carbon-13 NMR, magnetic susceptibility, FTIR, TGA and UV-visible spectroscopy, and their molecular structure is discussed. 40 refs.

The importance and implications of the definition of a solid as a condensed phase at a temperature below its glass transition are reviewed. Although liquid crystals, plastic crystals, and conformationally disordered crystals are still ordered above the glass transition, it has recently been shown that even monoclinic crystals of poly(oxyethylene) have an additional glass transition below the melting temperature. This supports the inference that in defining the solid state, glass transitions take preference over melting transitions. The main tools for recognizing the various equilibrium and non-equilibrium phases are differential scanning calorimetry (DSC) and temperature-modulated DSC. These techniques yield quantitative heat capacities that can be interpreted in terms of molecular motions and latent heats. When separated into reversible and irreversible contributions, the entropy changes related to ordering and disordering can be evaluated. The value of the recognising phase transitions lies in the fact that materials must be pliable for manufacture into final shapes, but are often required to have a high modulus and strength. Near the glass transition, forming is easy, whereas below the glass-transition temperature, a high modulus is attained for properly oriented polymer molecules. A better understanding of the solid state of soft materials is attained by identifying the glass transitions of materials with various states of order. 41 refs.

A series of liquid crystalline copolymers of (S)-2-methyl-1-butyl methacrylate with 6-[4-(4-cyanophenylazo)phenoxy]hexyl methacrylate with contents of chiral units of 17, 36, 54 and 78 mol% were prepared and their structures and properties of the copolymers were characterised by a range of techniques. Circular dichroism (CD) measurements suggested that the absorption by azobenzene chromophores was observed in films of the copolymers containing 17, 36 or 54 mol% of chiral units, but not in that of the copolymer containing of 78 mol% chiral units. The CD of the copolymeric films decreased with increasing chiral content. After irradiation with linear polarised light at 442 nm, the CD values of all the copolymer films changed. The CD values increased nonlinearly with decreasing chiral content, whereas the photoinduced change in chirality of the copolymers increased linearly with decreasing chiral content. 37 refs.

A synthetic absorbable bicomponent monofilament suture (MonoFlex), composed of poly(p-dioxanone) and its copolymer, was prepared by a conjugate spinning method, and its degradation behaviour was investigated in vitro and in vivo. MonoFlex degraded by hydrolysis, and retained approximately 55% of its original strength after four weeks of incubation in PBS at 37 deg.C. About 70% of the original strength was maintained after four weeks of implantation in rats, and the suture material was completely absorbed after 180-210 days post-implantation in rats. No remarkable tissue reactions were observed during degradation, and foreign body reactions were similar to those of commercially available suture materials composed of poly(p-dioxanone). This study to monitor the degradation behaviour of monofilament sutures in vitro as well as in vivo may be useful in the development of novel suture materials for extended wound support. 30 refs. Copyright (c) 2007 Elsevier Ltd.

In this work, black rice hull ash (BRHA) and natural rubber-grafted- polystyrene (NR-g-PS) were both used as fillers for preparing unsaturated polyester (UPE) composite materials. It was expected that both would produce an increase in the physical and thermal properties of the resin. The study revealed the effects of incorporating the fillers in various proportions. The impact strength was improved, with the composite of 100 UPE/30BRHA/5 NR-g-PS exhibited the highest flexural and impact strength, and the lowest water absorption. SEM micrographs indicated that the dispersion of NT-g-PS was more uniform than in the case of the ungrafted natural rubber in the UPE matrix. 11 refs.

A review is presented of improvements and developments in rheological testing equipment and procedures used to characterise the processing and processability of polymers and compounds. For many years, the Mooney viscometer, the oscillating-disc rheometer, and the durometer hardness tester have been practical and simple solutions for understanding the rheological properties. The introduction of modern dynamic mechanical rheological testers (DMRTs) allows for full characterisation of a material's viscoelastic properties. The use of DMRT is examined with respect to variable temperature testing to better define a material's scorch characteristics; to differentiate between batches of differently aged materials; and for quality control optimisation. 13 refs.

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