Showing papers in "British Polymer Journal in 1978"

Control of particle size in the formation of polymer latices

Abstract: A brief survey is given of the methods which can be used to produce polymer latices with a narrow size distribution. The factors which control both the particle size and the nature of the chemical groupings on the latex particle surface are discussed for latices prepared without the addition of an emulsifier. In particular, it is demonstrated experimentally that the ionic strength of the aqueous phase plays a major role in controlling particle size and that the free radicals produced by thermal decomposition of the initiator become incorporated as end-groups on the polymer chains and hence form the surface groups on the particle. A theoretical explanation is given which indicates the reasons for the importance of ionic strength in controlling particle size.

Relationship between structure and mechanical properties in rubber‐toughened epoxy resins

Abstract: Toughened polymers were prepared by adding CTBN rubbers to DGEBA-type epoxy resins. Structure was varied by altering the type and concentration of hardener, the initial molecular weight of the resin, the amount of Bisphenol A added, and the conditions of cure. Electron microscopy showed that these factors affected both particle size and degree of phase separation: rapid curing inhibited phase separation, and produced small particles. Increasing the molecular weight of the resin, either directly or by reaction with Bisphenol A, improved phase separation. Dynamic mechanical measurements of rubber phase volume proved possible, although Tg of the CTBN rubber coincided with a β process in the epoxy resin. Fracture resistance, measured by GIC, increased linearly with rubber phase volume. Creep and yield behaviour were also affected by the degree of phase separation.

Polymeric Inserts and Implants for the Controlled Release of Drugs

Abstract: Because of the ability of polymers to exist in glassy, rubbery or solvent-swollen forms which have very different diffusion coefficients they are able to be used very effectively to control the release of incorporated drugs. As the polymers can vary from inert in vivo to biodegradable an even greater scope for versatile formulation is provided. The various parameters governing the design of controlled release implants are discussed.

Fracture of particulate filled polymers

Abstract: The addition of particulate mineral fillers to polymers confers certain mechanical property improvements automatically. Stiffness increases, creep diminishes and distortion at elevated temperatures is often reduced. However, the fracture energy of a polymer, as measured in impact, cracking or tearing tests, may vary quite unpredictably when filler is incorporated. In some special cases the fracture energy increases when small amounts of filler are added although it falls away again at higher volume loadings. This enhancement of polymer toughness by filler is an example of reinforcement. More generally the addition of filler causes a continuous and drastic reduction in fracture energy, resulting in a brittle, weak product. This paper seeks to explain the common degrading effect of filler on polymer fracture energy by considering the progress of a crack through the composite material. The crack travels through regions of polymer and also along the interfaces between polymer and filler. Experiment demonstrates that, although fracture of the polymer regions absorbs considerable energy, fracture of the interfaces usually requires very little. These weak interfaces do not resist cracking and are the cause of brittleness in particulate filled systems. This idea was quantified for thermoplastics such as low density polyethylene and poly (methylmethacrylate) filled with colloidal silica by twin-roll milling. Where the interfacial adhesive energy was much smaller than the polymer fracture energy, the composite toughness dropped as predicted when filler was added. The particle size, the nature or dispersion of the filler, and the crystallinity of the polymer used, had little influence on this phenomenon, as pointed out theoretically. The crucial parameters influencing the fracture energy of the filled polymer were found to be the volume fraction of filler and the interfacial adhesion between polymer and filler. By chemical treatments the adhesive energy between filler and polymer was raised until the interface was almost as tough as the polymer itself. In this case the filled polymer showed good fracture toughness, lending further support to the theory.

Water Soluble Polymers for Medicine

Abstract: A number of general reviews have already been published on this topic and so the authors will present information on their approach and some results obtained. Polymers for medical uses are a traditional field at the Institute of Macromolecular Chemistry. However, crosslinked hydrophilic polymers - so called hydrogels - were the prevailing object in the past. Either as soft highly swollen materials for prothesis - like soft contact lenses - or heavily crosslinked macroporous beads designed as special sorbents or carriers of biologically active groups. Only a few years ago a research programme on soluble polymers was focussed mainly on general problems. The overall view about the main directions of the authors research is schematically presented on Fig. 1. It also shows the two types of polymers the authors are using in their experiments. The first one poly[N(2-hydroxypropyl)methacrylamide]3a was chosen from the set of N-substituted acryl-and methacrylamides4 because of simple preparation and purification of the monomer. The other is a polyamide (polypeptide) described by Neri et al. in 1973.5 It is more hydrophilic and provides a good possibility to prepare a whole set of derivatives from a common polymeric intermediate - polysuccinimide.

Design of Polymeric Immunomicrospheres for Cell Labelling and Cell Separation

Abstract: Synthesis of several classes of hydrophylic microspheres applied to cell labeling and cell separation is described. Five classes of cross-linked microspheres with functional groups such as carboxyl, hydroxyl, amide and/or pyridine groups were synthesized. These functional groups were used to bind covalently antibodies and other proteins to the surface of the microspheres. To optimize the derivatisation technique, polyglutaraldehyde immunomicrospheres were prepared and utilized. Specific populations of human and murine lymphocytes were labelled with microspheres synthesized by the emulsion of the ionizing radiation technique. The labelling of the cells by means of microspheres containing an iron core produced successful separation of B from T lymphocytes by means of a magnetic field.

Clotting Phenomena at the Blood‐Polymer Interface and Development of Blood Compatible Polymeric Surfaces

Abstract: In the past two decades many attempts have been made to relate surface and interfacial parameters with the blood compatibility of polymeric surfaces. It is however doubtful if by a single parameter the behaviour of blood on a surface can be predicted. Two major aspects of blood compatibility - the prevention of platelet adhesion and the deactivation of the intrinsic coagulation system are determined by the measure and nature of competitive blood protein adsorption on the foreign surface. The adhesion of blood platelets is promoted by adsorbed fibrinogen and gamma globulin, while adsorbed albumin inhibits platelet adhesion. Heparinised surfaces do not adsorb fibrin and consequently no adhesion of platelets takes place. Other surfaces with low platelet adhesion are the hydrogels, certain block copolyetherurethanes, polyelectrolyte complexes and biolised proteins. Heparinised surfaces of the cationically bonded type inhibit the intrinsic coagulation as well, however this may be due to unstable coatings and heparin leakage. In the authors laboratory a synthetic heparinoid was prepared with the structure - [CH2 - C(CH3 NHSO3 Na - C(H) COONa - CH2 -]x with Mw = (7.5 /pm 1.0) × 105 and an in vivo anticoagulant activity of 50% of heparin. Its coatings on PVC, using tridodecylmethyl-ammonium chloride as a coupling agent, are stable in plasma and salt solutions and provide surfaces which show negligible platelet adhesion and a strong inhibition of the intrinsic coagulation on contact with blood. Similar results were found with polydimethylsiloxane surfaces coated with this heparinoid.

Ultrahigh Molecular Weight Polyethylene (UHMW‐PE): Application in Artificlal Joints

Abstract: The molecular weight of UHMW-PE has an important influence on its physical properties. Different methods are applied to determine the molecular weight of the original powder and semi-finished products. The main processing methods such as compression moulding, extrusion, and injection moulding are described, also operations for the preparation of parts for medical use and sterilisation methods. Abrasion, chemical resistance, and its compatibility with body tissue are basic properties for the medical use of UHMW-PE. The bio-mechanical properties of the material in the form of a hip joint can be tested in a simulator.

Lower Critical Solution Temperatures, Part I Polymethylene in n‐alkanes

Abstract: Lower critical solution temperatures (LCST's) of three linear polyethylenes of differing molecular weights were measured visually in various n-alkanes ranging from n-hexane to n-tridecane. The LCST observed increases with decreasing molecular weight of polyethyline and with increasing the chain length of the n-alkanes. The limiting LCST's, θL for polyethylene/n-alkane systems were obtained by extrapolating to the infinite molecular weight of polyethylene using a Shultz-Flory plot. Flory's equation of state theory is discussed in conjunction with the conventional Flory-Huggins theory and the equation of state parameters for the pure solvents are obtained according to the Flory theory. The θL temperatures for polyethylene in n-hexane, n-heptane, and n-octane are calculated in accordance with Flory's and Patterson's equations and compared with the experimental values. The θL temperatures are found to be well predicted by one version of the Flory theory.

On the Applicability of Gardon's Equations for the Calculation of Particle Size in Emulsion Polymerisation†

Abstract: Gardon's revision of the Smith-Ewart theory of emulsion polymerisation makes it possible to calculate the average particle size of the latex produced from the conditions used for the polymerisation at least for monomers with a low monomer transfer constant. Good agreement has been found between the calculated particle sizes and the turbidity averages measured by light scattering for polystyrene latices prepared with various emulsifiers. However the theory neglects the effect of the ionic strength of the aqueous phase although Gardon's own results show this to be a significant parameter: increase of ionic strength increases the average size of the latex particles by reducing electrostatic stabilisation. The difference between the total and the micellar surfactant concentration is significant for surfactants with relatively high critical micelle concentrations and a considerable improvement in agreement between measured and calculated particle sizes is found when the concentration of micellar surfactant is used in the calculation. The theory requires the reaction rate to depend on the 3/5ths power of the concentration of micellar surfactant and although this would appear to have been satisfactorily verified experimentally, replotting of the extensive series of precise results published by Bartholome, Gerrens, Herbeck and Weitz shows that the double logarithmic plot is not actually linear. This is probably the result of a slow change of micelle size as the surfactant concentration is increased.

Theory of neck development in tensile tests on thermoplastics

Abstract: A one-dimensional finite element model is used to relate the constitutive equation of a thermoplastic with the phenomena observed when it is tensile tested. Predictions are made of the temperature distribution in a propagating neck, and in particular PVC is shown to be liable to thermal fracture in fast tensile tests. The neck geometry is shown to be an important factor in stable, high speed, neck propagation.

Problems with Polymers in Dentistry

Abstract: Despite the important contributions made by polymers in dental treatment procedures, many shortcomings of these materials are apparent. Rigid type denture base polymers (commonly P.M.M.A.) have low impact and abrasion resistance and lack radiopacity. Resilient polymers used as denture liners may bond poorly to the denture base and suffer changes in physical properties in use. Early problems of colour stability, high polymerisation and thermal contraction, low surface hardness and poor abrasion resistance of the first polymeric filling materials have been only partly overcome by the introduction of polymer/ceramic composites. In dental cement applications, present bonding systems include unfilled dimethacrylates, which do not attach to dentine. A system based on polyacrylic acid, or copolymers thereof, is believed to bond chemically to the calcium in tooth structure, but the bond to dentine is poorer than that to enamel and also the hydrolytic stability is suspect.

The short and long term performance of polymers in different environments

Abstract: The behaviour of polymeric solids in contact with different environments is considered. A thermodynamic viewpoint is adopted and worked out in terms of chemical reactions, physical processes and the interaction between the two. It is shown that chemical attack alone does not normally cause serious loss of physical properties, but that a combination of chemical and physical (or physico-chemical) interactions between polymer and environment can lead to rapid deterioration. An understanding of these processes, and their synergistic effects, is essential in designing against environmental failure.

A comparison of the thermal stabilities of a siloxane substituted polycarbonate with a standard polycarbonate and a polydimethylsiloxane

Abstract: The thermal stability in inert and oxidising atmosphere of an aromatic polycarbonate with a siloxane side chain has been compared with those of a polycarbonate and a poly-(dimethylsiloxane) using thermogravimetry. The presence of the siloxane side chain is seen to marginally enhance the thermal stability compared to that of the unsubstituted polycarbonate.

The influence of liquid environmental media on the mechanical behaviour of thermoplastics

Abstract: Thermoplastic materials, having a high chemical stability, are used on a very large scale in corrosive environmental media. Nevertheless, in many cases unexpected failure occurs, since valid construction guide-lines are missing. For this reason, in the present work a deformation-related dimensioning criterion is presented, based on the appearance of first irreversible material-damage. Further, the sorption-, swelling-, short-term- and creep-behaviour of PMMA in the presence of distilled water are described and discussed. Finally, mathematical and graphical procedures for the determination of the creep-strain under different loading and environmental conditions are shown and explained.

Load Bearing Polymers used in Orthopaedic Surgery

Abstract: One of the major contributions to orthopaedic surgery has been the use of ultra-high-molecular-weight polyethylene in prosthetic joint replacements. The part played by this polymer is set in the context of biomechanical requirements. Work dealing with wear found in components of knee prostheses removed from patients is presented, the main mechanism being cold flow followed by abrasive wear. Surface delamination and the effect of inhomogeneous regions are described. Polyacetals, homo and copoly mers are being used as bearing surfaces and the current position is reported. Work in the author's laboratory to develop carbon fibre reinforced plastics is described briefly. The development of Standards is mentioned.

Mechanical properties of polystyrene/ethylene-propylene copolymer blends

Abstract: Some mechanical properties of blends of polystyrene (PS) and ethylene-propylene-rubber (EP) were derived from stress-strain and impact measurements. The strength and impact properties are improved by adding EP-g-PS graftcopolymer, prepared by reacting PS with EP, to the blends. It is assumed that the EP-g-PS graftcopolymer acts as an adhesive at the interface between the thermoplast and the rubber phases. The addition of the graftcopolymer reduces the dimensions of the dispersed rubber particles. High values of impact strength at reasonable values of tensile moduli could be reached by replacing EP for a smaller or larger part by EP-g-PS copolymer. These kinds of EP-modified PS blends had much higher impact values than those of comparable PS blends containing low density polyethylene (1 dPE) and 1dPE-g-PS graftcopolymer or this graftcopolymer only. It seems attractive to ascribe these results to the non-crystallinity of the PE-g-PS as compared with the crystallinity of 1dPE in 1dPE and in 1dPE-g-PS. However some caution seems recommendable as EP-modified PS fractures with microshear whereas, the PE-modified PS shows crazing.

The Reaction of Plumbonacrite (Basic Lead Carbonate) with Hot DOP, and the Stabilisation of Plasticised PVC

Abstract: The basic lead carbonate ‘plumbonacrite’, 6PbCO3.3Pb(OH)2.PbO, has been reacted with di-2-ethyl hexyl phthalate (dioctyl phthalate, DOP), C6H4(COOC8H17)2 at 180dC under nitrogen. Anhydrous monobasic lead orthophthalate, PbO.Pb6H4(COO)2, was the only product detected by X-ray powder diffraction (X.R.P.D.). These studies are related to previous work on reactions taking place in hot polyvinyl chloride (PVC) blends containing DOP plasticiser and basic lead stabilisers. It is concluded that plasticiser stabiliser interaction can play a significant role in stabilising PVC against thermal decomposition, and that efficient stabilisation takes place largely by a free-radical exchange mechanism. Characteristic X.R.P.D. data are tabulated for anhydrous monobasic lead orthophthalate, and for monobasic lead orthophthalate monohydrate, PbO. PbC6H4(COO)2- H2O, which can be made in aqueous suspension and which converts to the anhydrous salt at 180dC. X.R.P.D. data are also given for dibasic lead orthophthalate, 2PbO.PbC6H4(COO)2.1/2H2O, which is well known as a stabiliser for PVC, and which might have been expected to form in these experiments, (but did not in fact do so).

Light Scattering from Copolymer Solutions

Abstract: A new type of copolymer from para-aminobenzoic acid and 6-aminohexanoic acid was obtained by reacting the monomers in four different mole ratios. The polycondensation was carried out at 80dC with triphenyl phosphite as the initiator in N-methyl-2 pyrrolidone/pyridine mixture. The molecular weights of unfractionated and fractionated samples were determined by light scattering techniques in solvents of varying refractive index. From the values of second virial coefficient it is concluded that the solvent power for copolymer increases with increasing aminohexanoic acid mole fraction in the case of dichloroacetic acid and trifluoroacetic acetic acid while in dimethyl acetamide it decreases.

Polyester-Polyether Block Copolymers II. Thermal and Mechanical Properties of Poly(hexamethylene terephthalate)/Poly(oxytetramethylene) Block Copolymers

Abstract: The melting and crystallisation behaviour of crystalline phases in poly (hexamethylene terephthalate)/poly(oxytetramethylene) block copolymers have been investigated in relation to copolymer composition and polyether block molecular weight (m.w.). In contrast to that in corresponding homopolymer blends, the polyester crystallinity in the block polymers is greatly reduced by incorporation of polyether units, though some persists even at low polyester contents. Concomitant changes in the glass transition temperatures show part of the polyester component to form a homogeneous component of the amorphous phase. The mechanical properties change with composition in parallel with the changes in copolymer crystallinity and Tg. Copolymers with 20-60 w % of poly(oxytetramethylene) units of m.w. 2000 are highly extensible elastomers. Those with higher m. w. polyether blocks have higher modulus and strength but suffer a serious loss of properties at 60dC. The observations are interpreted in terms of a model in which polyester crystallites (and polyether crystallites also, for the higher m. w. polyether blocks) are supported within an amorphous matrix by tie-molecules whose nature changes with the copolymer compositions. The results are compared with those for analogous polyester-polyethers having different structural components.

Chemistry, Properties and Orthopaedic Applications of Acrylic Cements

Abstract: The early development and use of acrylic cement in orthopaedic surgery has been reviewed by Charnley. This paper is an attempt to describe the chemical background to the use of acrylics as bone cements with particular reference to the well recognised rise in temperature during the setting of the cement, a phenomenon of concern to some surgeons but easily explained if not overcome by classical reaction kinetics. The influence of the operating theatre temperature on the speed of setting is discussed. The fears that some surgeons may have that the incorporation of barium sulphate as a radio-opacifier into the cement could reduce the compressive strength of the latter are allayed. The mechanical properties of the set cement are discussed and some criticism is levied at the various International Standards proposed for some cements. The paper concludes by reviewing some of the work carried out at Bradford in an attempt to produce modified cements designed to overcome some of the shortcomings of the existing materials.