Showing papers in "British Polymer Journal in 1985"

A sulphonated poly(aryl ether ketone)

Abstract: Sulfonation et neutralisation controlees de PEEK, caracterisation de PEEK sulfone et effet de cette modification sur la cristallinite, les transitions thermiques et la stabilite thermique

Glass transition of poly(oxymethylene)

Abstract: Heat capacity data of semicrystalline poly(oxymethylene) samples. Delrin and Celcon, are analysed in order to discuss the glass transition behaviour of this polymer. There are two types of non-crystalline poly(oxymetheylene). the mobile and rigid amorphous parts. The glass transition of the former occurs in a rather wider range of temperature: it starts at 180 K and could end at 265 K. The latter, under restraint due to the crystallites, remains frozen up to the melting temperature.

The concentration dependence of biopolymer gel modulus

Abstract: Gel formation from aqueous biopolymer solutions is a common phenomenon and one of practical importance, yet it is much less well understood and characterised than the condensation of synthetic polymer precursors to form networks. There are many reasons for this, including the complexity of the aggregating species, the diversity of crosslinking mechanisms, the physical nature of the attractive forces, and the complexities introduced by having water as a solvent. In the present paper, however, an attempt is made to adapt classical theories of gelation and of rubber elasticity to treat the biopolymer case and to describe the shear modulus-versus-concentration behaviour of such non-ideal materials. Modulus data for cold-set gels formed from the polysaccharide agar and the protein gelatin are presented and analysed, as are similar data for substantially different gels formed by heating solutions of globular proteins. It is demonstrated how, by assuming a crosslinking equilibrium, such diverse sets of data can be reduced to a dimensionless master curve form, and the limiting concentration dependence of the modulus at high concentration is discussed in the light of the present work and of a recent theoretical description based on the osmotic scaling relationship.

Kinetics of gelation of aqueous gelatin solutions

Abstract: The kinetics of gelation of gelatin + water solutions have been investigated by several experimental methods, in particular optical rotation, 1H nuclear magnetic resonance and rheological measurements. During quenching and isothermal annealing of the solutions, the gelatin chains undergo a conformational coil→helix transition which is responsible for the network growth. The amount of helix (derived from optical rotation measurements) shows a time evolution which could be analysed as the combination of two processes, the first obeying an Avrami equation, the second proceeding logarithmically in time. The water 1H n.m.r. measurements reveal that the spin-spin relaxation time T2 is affected by these processes, thus implying that a fraction of water contributes to the network structure. Finally, the sol-gel transition could be followed with a Weissenberg rheogoniometer in oscillatory shear, for a suitable range of temperatures, when the system changes from a purely viscous liquid (the shear modulus G′=0), to an almost purely elastic solid (the loss modulus G″=0).

Rheological changes during crosslinking

Abstract: Huge rheological changes occur during thermoset polymerisation. Methods for measuring these changes and relating them to molecular structure through branching theory and reaction kinetic data are reviewed. Applications to thermoset processing are discussed.

Network topology and the theory of rubber elasticity

Abstract: The earliest investigations on rubber elasticity, commencing in the 19th century, were necessarily limited to phenomenological interpretations. The realisation that polymers consist of very long molecular chains. commencing c. 1930, gave impetus to the molecular theory of rubber elasticity (1932-). according to which the high deformability of an elastomer, and the elastic force generated by deformation, stem from the configurations accessible to long molecular chains. Theories of rubber elasticity put forward from 1934-1946 relied on the assumption that the junctions of the rubber network undergo displacements that are affine in macroscopic strain. The theory of James and Guth (1947) dispensed with this premise, and demonstrated instead that the mean positions of the junctions of a ‘phantom’ network consisting of Gaussian chains devoid of material properties are affine in the strain. The vital significance of the distinction between the actual distribution of chain vectors in a network and their distribution if the junctions would be fixed at their mean positions went unnoticed for nearly 30 years. Experimental investigations, commencing with the incisive work of Gee in 1946. revealed large departures from the relationship of stress to strain predicted by the theories cited. This discrepancy prompted extensive studies, theoretical and experimental, during succeeding years. Inquiry into the fundamentals of polymer networks, formed for example by interlinking very long polymer molecules, exposed the need to take account of network imperfections, typically consisting of chains attached at only one end to a network junction. Various means were advocated to make corrections for these imperfections. The cycle rank ζ of the network has been shown (1976) to be the fundamental measure of its connectivity, regardless of the junction functionality and pattern of imperfections. Often overlooked is the copious interpenetration of the chains comprising typical elastomeric networks. Theories that attempt to represent such networks on a lattice are incompatible with this universal feature. Moreover, the dense interpenetration of chains may limit the ability of junctions in real networks to accommodate the fluctuations envisaged in the theory of phantom networks. It was suggested in 1975 that departures from the form predicted for the elastic equation of state are due to constraints on the fluctuations of junctions whose effect diminishes with deformation and with dilation. Formulation of a self-consistent theory based on this suggestion required recognition of the non-affine connection between the chain vector distribution function and the macroscopic strain in a real network, which may partake of characteristics of a phantom network in some degree. Implementation of the idea was achieved through postulation of domains of constraint affecting the equilibrium distribution of fluctuations of network junctions from their mean positions. This led in due course to a theory that accounts for the relationship of stress to strain virtually throughout the ranges of strain accessible to measurement. The theory establishes connections between structure and elastic properties. This is achieved with utmost frugality in arbitrary parameters.

Bimodal networks and networks reinforced by the in situ precipitation of silica

Abstract: The goal of primary interest in these investigations was the development of novel methods for preparing elastomeric networks having unusually good ultimate properties. The first technique employed involves endlinking mixtures of very short and relatively long functionally-terminated chains to give bimodal networks. Such (unfilled) elastomers show very large increases in reduced stress or modulus at high elongations because of the very limited extensibility of the short chains present in the networks. The second technique employs the in situ precipitation of reinforcing silica either after, during, or before network formation. The reaction involves hydrolysis of tetraethylorthosilicate, using a variety of catalysts and precipitation conditions, and the effectiveness of the technique is gauged by stress-strain measurements carried out to yield values of the maximum extensibility, ultimate strength, and energy of rupture of the filled networks. Information on the filler particles thus introduced is obtained from density determinations, light scattering measurements, and electron microscopy.

The elastic behaviour of hydrogels

Abstract: The mechanical properties and the swelling behaviour of aqueous poly(acryl amide) and poly(acryl amide-co-Na acrylate) gels were investigated. The ionic networks were prepared by hydrolysis of the nonionic ones, in this way preserving the network topology. By measurement of the modulus it was detected that networks prepared in solution contain a rather large portion of elastically ineffective rings. Hydrolysing a part of the amide groups resulted in a considerably increased swelling ratio. The modulus of such ionic gels did not show the predicted decrease with rising swelling ratio, but on the contrary increased up to a fivefold value. The reason for this behaviour is seen in the limited extensibility (non-Gaussian behaviour) of the network chains. At constant swelling ratio, the modulus of gels having identical topology decreased with rising degree of hydrolysis. This reflects the influence of the ionic groups on the mean-square end-to-end distance of the free polymer chains.

Inhomogeneity during the bulk polymerisation of divinyl compounds: Differential scanning calorimetry experiments and percolation theory

Abstract: Experiments are presented that indicate inhomogeneity in bulk divinyl polymerisation which persists far beyond the gel point and a percolation model is given by which this inhomogeneity is simulated. Isothermal differential scanning calorimetry experiments indicate trapping of radicals in regions of low mobility long before overall vitrification occurs. The results on pendant double bonds in divinyl polymerisation are similar to those obtained by Malinsky et al. for styrene/divinyl benzene copolymerisation. The percolation model used shows inhomogeneity in snapshots and provides a qualitatively correct description of the high extent of cyclisation at low conversion, in contrast to classical chemical kinetics. The model overestimates the crosslinking at high conversion.

Formation‐structure relationships in polymer networks

Abstract: The current state of the network formation theory in the pregel and postgel stages of network formation is reviewed. Attention is paid to stochastic and spatial correlations as well as diffusion control of the reaction.

Langmuir‐blodgett films from preformed polymers

Abstract: Recent work on Langmuir films and Langmuir-Blodgett films prepared from derivatives of styrene-maleic anhydride and octadec-1-ene-maleic anhydride copolymers is reviewed.

Interpenetrating polymer networks

Abstract: Interpenetrating polymer networks (IPNs) composed of two or more chemically distinct networks are not only intrinsically interesting as examples of macromolecular chemical topological isomerism but are in practice useful means of controlling mutual miscibility and phase morphology in crosslinked polymers. We will first review briefly the synthesis and properties of such IPN systems. This will be followed by an outline of a phenomenological theory of the phase stability and linearised theory of spinodal decomposition of binary, chemically quenched, low crosslink density IPNs recently developed by K. Binder and the author. Finally, we will discuss some aspects of the synthesis of ternary IPNs and the thermochromic properties of IPNs containing a crosslinked polydiacetylene.

Gelation of pectinic acids in the presence of calcium counterions

Abstract: Pectins are polysaccharides which are able to gel in the presence of calcium ions if their degree of esterification is lower than 40%. From a purified pectin, a de-esterification was carried out by alkali and by enzymes in order to obtain samples with a different charge density and esterification pattern (random or blockwise distribution of carboxylic acid sites). The pectin-calcium interactions were studied in a dilute solution of calcium pectinates by conductimetry. If the concentration of highly charged polymer is increased, a multichain association can occur: the same process is observed with dilute solutions (cp≤10-3 eq. 1−1) but in the presence of an excess of calcium ions. This first step of the gelation was followed by light scattering and by viscometry. The gelling system can be described by a power law including a critical ratio (equivalent concentration of calcium ions/pectin carboxylic sites) and quasi-critical exponents. The values of these critical parameters have been discussed as a function of charge density of the pectins, their esterification pattern, their concentration, and the nature of the divalent counterion.

A preliminary study of blends of bisphenol a polycarbonate and poly(ethylene terephthalate)

Abstract: The microstructure of blends of bisphenol A polycarbonate (PC), and poly(ethylene terepthalate) (PETP) has been studied by solvent extraction, infrared spectrophotometry, differential scanning calorimetry and dynamic mechanical thermal analysis. The blends appear to contain two amorphous phases over the whole composition range. The tensile behaviour and the Charpy impact strength of some of the blends have been determined, before and after heat treatment at 125°C for 18 hours. Improved performance of the blends, compared with that of the homopolymers PC and PETP, has been demonstrated.

Deposition mechanisms in langmuir-blodgett films

Abstract: As part of a study of the possible application of polymerisable Langmuir-Blodgett (LB) films as ultra-fine-line e-beam resists, an investigation of the variation of film structure of 22-tricosenoic acid with differing deposition conditions has been made. Unexpected effects with significant implications for deposition speed and resist sensitivity have been observed, and the new techniques for film characterisation developed during the investigation have resulted in a revised model of deposition explaining the observed independence of the disorder causing optical scattering and the macroscopic features observed by polarised microscopy.

Molecular weights of poly(butyl 2‐cyanoacrylate) produced during nanoparticle formation

Abstract: The molecular weights and distributions of poly(buty1 2-cyanoacrylate) produced during the formation of nanoparticles under a range of polymerisation conditions have been determined by gel permeation chromatography. The data indicate that the type of steric stabiliser and concentration of the monomer added to the polymerisation medium influences the resulting molecular weight of the polymer. In all systems the bulk of the polymer had a low molecular weight ( 20000). The results are interpreted with regard to the possible effects on the degradation of nanoparticles and their drug release characteristics. Tentative proposals are made concerning the mechanisms of nanoparticle formation.

Networks in nature

Abstract: Synthetic networks, their structure and mechanical properties have been studied since the beginning of polymer science, while the considerable variety of networks in nature has been largely neglected, though use has been made of them as long as human culture has existed. In principle synthetic and natural networks are equivalent, but there are significant differences. (1) The main difference consists in the high polarity of the polymeric components and the related capability of binding large quantities of water. (2) The biosynthesis of covalently linked, macroscopic networks is rare and often too slow a process. In many cases the networks are formed via physical forces between various polymeric components. (3) Often the network formation can be reversed by changing the temperature, or pH, or by adding urea or detergents. (4) The viscoelastic behaviour is often governed by the rigidity of the macromolecular substructures. Examples are given for reversible gels with some charged polysaccharides and with fibrin. Special interactions are demonstrated with the network of connective tissue. The important role of comb-like macromolecules with protein backbone and polysaccharide side chains is outlined. Special interactions are crucial also for immunologic response, i.e. for cell recognition and antibody-antigen complex formation. The latter is one of the best examples of a successful application of the Flory-Stockmayer theory of branching and gelation. Most of the reversible gels are not yet fully understood in their properties, and theory is lacking. This is particularly true for products like gelatin and starch.

The radiation crosslinking and scission of ethylene‐propylene copolymers studied by solid‐state nuclear magnetic resonance

Abstract: Methyl groups from chain scisson and H-crosslinks have been identified by solid-state nuclear magnetic resonance in amorphous ethylene-propylene copolymers containing 23 and 36 mole % propylene after gamma -irradiation to 10 MGy at 30 degree C. G (scission) and G (crosslink) values determined from the n. m. r. spectra and by extraction are in agreement, which suggests that the crosslinks are not clustered. This may differ from the situation in polyethylene where there is a substantial crystalline content. G(S), G(X) and the ratio G(S)/G(X) increase with increasing propylene contant of the popolymers.

Inhomogeneity of gel and microsyneresis in porous styrene-divinylbenzene copolymers

Abstract: The porosity of styrene-divinylbenzene copolymers prepared by suspension copolymerisation of the monomers carried out in the presence of inert diluent changes due to solvent pretreatment is investigated. The changes seem to reflect the inhomogeneity of copolymer gel. Various portions of it are transferred from the rubber-like into the glassy state at different stages of either swelling solvent exchange or the drying process. A microsyneresis, i.e. a local collapse of loosely crosslinked gel portions not followed by a shrinkage of the whole system, leads to the porosity changes.

Computer simulation of urethane formation.

Abstract: Computer model calculations have been used to simulate the reaction of polyoxypropylene triol with hexamethylene diisocyanate. Various molecular weight distributions and degrees of polymerisation are used in the calculation. The effect of side reactions is investigated and compared with experiment. Good agreement for both cyclisation and gel points is found for bulk and concentrated solutions. The model can be used to simulate random polycondensation reactions with a reliability approaching that of experiments, and it gives crucial results that cannot be obtained in any other way.

Cure and properties of thermosetting systems

Abstract: A generalised time-temperature-transformation cure diagram for thermosetting systems is discussed from the point of view of understanding relationships between cure, properties, and thermal degradation.

Deuterium magnetic resonance studies on strained selectively deuterated elastomers

Abstract: The lineshape analysis of the 2H nuclear magnetic resonance of uniaxially strained poly(1,4-butadiene) networks deuterated homogeneously and selectively at network junctions shows that the orientation of chain segments attached to crosslinks is larger than the average segmental orientation. In addition, it is shown that the molecular effective extension ratio of all elastically effective chains is smaller than the macroscopic extension ratio, and short chains are stretched to a higher extent than long chains. The deviation from the affine deformation behaviour and the onset of inhomogeneous deformation of individual chains takes place at the transition from the Gaussian to the non-Gaussian network. In the Gaussian regime the segmental orientation varies with the inverse of the molecular weight of elastically effective chains in agreement with molecular theories of rubber elasticity. 2H n. m. r. spectra of a thermoplastic poly(styrene-b-butadiene-b-styrene) block copolymer carrying selectively deuterated butadiene segments at the block boundaries reveal an enhanced mobility of these segments under deformation. 2H n. m. r. experiments on amorphous segmented polyurethanes selectively deuteratured in hard and soft segments, respectively, show that the elasticity in these samples is achieved by physical crosslinks formed by very small highly mobile aggregates of hard segments dispersed in the soft phase. Within the hard domains two processes for motion due to phenyl groups in 4. 4'-methylenebis(phenylisocyanate) units and CH2 groups in 1. 4-butanediol units can be discriminated.

Epoxy mechanical properties: Function of crosslink architecture

Abstract: A thermoset resin's molecular architecture is characterised by extent of reaction, crosslink average molecular weight and the average size of constituent macromolecules. Molecular analyses of extracts from cured resins via gel permeation chromatography yield microscopic estimates of this infrastructure. The number-average molecular weight within the soluble oligomeric fraction correlates with crosslink average molecular weight: the extent of monomer conversion assists in developing a description of the molecular weight of constituent marcomolecules within the network structure. Macroscopic conformation couples observations by dynamic mechanical spectroscopy, compressive stress-strain phenomena and impact resistance.

Studies on chitin 3—effect of coagulants and annealing on the preparation and the properties of chitin membrane

Abstract: Chitin membrane was prepared by casting a N,N-dimethyl acetamide, N-methyl 2-pyrrolidone and lithium chloride (DMA-NMP-LiCl)solution of chitin and coagulating with several media. The effect of the coagulants on membrane formation was studied. 2-Propanol was found to be more favourable than methanol, ethanol, acetone and mixtures of 2-propanol and water. The membrane obtained in 2-propanol was subjected to annealing. Annealing made the membrane dense and strong. The tensile strength of the membrane annealed at 145°C for 2hr was about twice that of an unannealed membrane. The solute permeability of the annealed membranes was lower than that of the original one. These phenomena could be clearly interpreted in terms of crystallinity.

Rate theory of random polymerisation: RA3 polymerisation

Abstract: The limiting form of rate theory using the first generation subset of states of monomer units is applied to the self-polymerisation of RA3 monomers. It affords a description of the polymerisation process which specifies how species interconvert. The case of no intramolecular reaction is considered first and it is shown how characteristics of the polymerisation, unit fractions and number fractions of molecular and continuing species, the gel point, the sol fraction and the network at complete reaction, may be evaluated. The gel point is defined in terms of a quantity called the critical total. Intramolecular reaction is then introduced and analytical expressions for the unit fraction of ring structures and for the gel point are derived. They are valid provided the ring-forming parameter (λ) is not too large. However, in general, numerical evaluation of the characteristics of the polymerisation is required when intramolecular reaction is present. In the paper, emphasis is placed on correlations between the extent of reaction at gelation (Pc) and the structure of the network at complete reaction formed in the presence of pregel and postgel intramolecular reaction. The correlations are compared in general terms with those obtained experimentally for RA2 + RB3 polymerisations.

Further studies of polycarbonate-poly(butylene terephthalate) blends

Abstract: The mechanical properties (tensile strength, low temperature falling weight impact strength) of XENOY CL-100 (a blend of polycarbonate and polybutylene terephthalate) have been studied. This blend is comprised of tough components and the property combinations that result may be useful for certain applications. The hydrolysis resistance of the blend in deionised water at 40°C has been studied. After 30 weeks it did not lose its tensile strength, but subsequently it did lose 50% of the low temperature (−70°C) falling weight impact strength. If the −70°C low temperature toughness is an important quality, this should be taken into account. However, the low temperature (−70°C) falling weight impact strength of pure polycarbonate and of pure poly(butylene terephthalate) is very low, whilst XENOY CL-100 is still tough material, even after 30 weeks immersion at 40°C. Further study of the composition of XENOY CL-100 showed that this blend is composed of polycarbonate. poly(butylene terephthalate) and crosslinked acrylic rubber; traces of titanium-catalyst residue for the poly(butylene terephthalate) polymeristaion and black dye have also been found.

The continuous-time random walk description of the non-equilibrium mechanical response of crosslinked elastomers

Abstract: The application of the continuous-time random walk model to the description of the non-equilibrium stress response of a crosslinked network is discussed. Different versions of the continuous-time random walk model lead to the prediction of a stress relaxation function having either a fractional-power law form or a fractional-exponential law form. Both forms are tested against experimental data on the stress relaxation and loss modulus behaviours of crosslinked natural rubber.

Electron irradiation of polymerisable langmuir‐blodgett multilayers as model resists for electron‐beam lithography

Abstract: Amphiphilic molecules containing a polymerisable oxiran (epoxide) group have been assembled into Langmuir-Blodgett multilayers. Lines were drawn on the multilayers by exposure to an electron beam and subsequent development by rinsing in toluene. Line widths and heights were examined for various doses of electrons of energies 10, 5 and 2 kV. Generally, for the higher energies and shorter exposures, narrow well defined lines were obtained of height similar to the thickness of the original multilayers. Conversely, at lower energies and exposure times, troughs rather than lines were observed following incomplete development. The implications of these results are compared with theoretical predictions of bbnd forming and bond scission processes during electron irradiation of organic molecules.

Resonance raman spectroscopy of polydiacetylene langmuir‐blodgett films

Abstract: Resonance Raman excitation profiles of two vibrational modes have been measured for Langmuir-Blodgett films of a polydiacetylene. The polymer films were formed by exposing multilayer samples of the Cd salt of 10,12-pentacosadiynoic acid to ultraviolet radiation. Raman spectra were measured for incident laser photon energies between 1.85 and 2.20 eV (670-564 nm), the region over which the films exhibited optical absorption associated with an electronic transition of the π-electrons on the conjugated backbone. The excitation profiles have been used to determine both the strength of the vibronic coupling and the differences in vibrational frequency between the ground and excited states. The results are in reasonable agreement with those obtained previously for polydiacetylene single crystals.

Intermolecular interactions in blends of functionalised propylene polymers with poly(vinylchloride) or polystyrene

Abstract: Atactic and isotactic polypropylene (APP and IPP), and ethylene-propylene copolymers (EP). functionalised by reacting the molten polymer with diethylmaleate (DEM) in the presence of dicumylperoxide (DCP). have been blended with poly(vinylchloride) (PVC) and polystyrene (PS). These mixtures have been investigated by means of infrared spectroscopy at different compositions in the region of carbonyl absorption. The features of this last band in the mixture suggest the occurrence of intermolecular interactions involving mainly the carbonyl groups of the functionalised polymer and the methine hydrogens of PVC.