John L. Stanford

A synchrotron X-ray study of melting and recrystallization in isotactic polypropylene

Abstract SAXS/WAXS/d.s.c. at the SRS has been used to follow the melting and recrystallization of isotactic polypropylene (iPP). The time correlation is perfect between the three techniques which show good agreement with respect to crystallinity and kinetics. The degree of crystallinity and the lamellar thickness of iPP have been calculated from SAXS and WAXS patterns, obtained simultaneously during melting and crystallization, by integrated intensity and correlation function techniques. The long spacing was obtained from the peak in the Lorentz-corrected SAXS pattern and was in good agreement with that calculated from the correlation fuction (γ1). The degree of crystallinity was obtained from a combination of the SAXS invariant and the integrated WAXS intensity (due to the crystals), and the local crystallinity was obtained from γ1. The data obtained during melting and recrystallization were interpreted in terms of a polydisperse lamellar model.

THERMAL, MECHANICAL AND FRACTURE PROPERTIES OF REACTION INJECTION-MOULDED POLY (URETHANE-UREA)S

Abstract A series of poly(urethane-urea)s similar to those used commercially (formed from a 4,4′-diphenylmethane diisocyanate-based polyisocyanate and a polyether triol in admixture with an aromatic diamine, 3,5-diethyltoluenediamine) were prepared, with appropriate catalysis over a wide range of composition, by reaction injection moulding (RIM). All reactants were thoroughly characterized prior to use and the polymers were formed on in-house RIM equipment under controlled processing conditions. The RIM materials formed ranged from translucent flexible elastomers to opaque rigid plastics depending on their composition and were characterized by dynamic mechanical thermal analysis (d.m.t.a.), tensile stress-strain and fracture mechanics studies. D.m.t.a. showed that (micro)phase separation had occurred during polymerization and the RIM materials had a non-equilibrium two-phase morphology. The modulus data from the tensile stress-strain studies were compared to the predictions of theories for two-phase materials. The comparative data showed good agreement, over a wide range of composition, to a model for materials comprising a co-continuous morphology. Fracture properties were determined using a single edge notch technique. Low hard-segment materials failed in a ductile manner accompanied by gross tearing, as observed on scanning electron microscopy fracture surfaces, with values of critical strain energy release rate, GIC > 6 kJ m−2. High hard-segment materials showed brittle failure with GIc

The conversion of polysaccharides into polyurethanes: A review

Abstract The conversion of polysaccharides into polyurethanes, which has commanded considerable academic and industrial interest over the past 30 years, is reviewed from chemical, structure-property and economic viewpoints. The basic chemistry of polyurethanes and the morphological structure of segmented copolyurethane materials are described. The processes used to fabricate such materials are outlined and the properties required in polysaccharides used as feedstocks for polyurethanes are defined. Several methods of incorporating polysaccharides in polyurethanes are available including their use (i) in unmodified form, (ii) after derivatisation, (iii) after depolymerisation and derivatisation, and (iv) by conversion to non-carbohydrate polyols. (Routes (ii)-(iv) yield a very wide range of glycosidic and non-glycosidic polyol structures.) Specific examples of each route and the advantages of the polyol products for use in polyurethane formation are detailed. Advantages include low cost and structural versatility of the polyols which provide increased cross-linking, and solvent and fire resistance in derived polyurethanes. Some developments in the understanding of the formation of glycosides and their industrial-scale production are presented, together with an indication of the increasing variety of polysaccharide structures used. Methodologies for the production of solid polyurethanes with high carbohydrate contents, compared to foams of relatively low carbohydrate contents, are described together with the structure-property relationships in both types of materials. Finally, the future potential of biotechnology in providing novel carbohydrate structures is briefly considered.

Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy

Raman spectroscopy has been used to monitor deformation micromechanics in a model discontinuous fibre composite comprising a single glass fibre in an epoxy resin. The glass fibre was coated with a diacetylene-containing urethane copolymer that was subsequently cross-polymerised thermally. During composite deformation, the stress-induced Raman band shifts of the polydiacetylene sequences in the cross-polymerised coating were used to map the distributions of strain along glass fibres inside the epoxy resin matrix. The fragmentation of the fibre has been followed in detail and the behaviour analysed using classical shear-lag analysis. Values of the interfacial shear stress along fibre fragments were determined from the measured fibre strain distributions and were shown to be limited by the shear yield stress of the matrix. The effect of adhesion between the coating and the fibre upon the strain distributions has been investigated in detail. The fibre strain distributions can only be determined accurately when the adhesion is good. However, in the case of poor adhesion, although the strain distribution in the coating follows that of the matrix, the fragmentation process can still be monitored.

Structure development in multi-block copolymerisation: comparison of experiments with cell dynamics simulations

Abstract A cell dynamics simulation of phase separation in block copolymers is compared with experimental observations for two related systems, polyurethane (poly(ether-urea)) foam and poly(ether-isocyanurate). Time resolved SAXS measurements on both systems suggest a spinodal-like mechanism with kinetics following a time-dependent Ginzburg–Landau (TDGL) model. TEM micrographs from a range of sources show reactively processed multi-block copolymers to have a bicontinuous morphology, which is discussed as a non-equilibrium relic of the phase separation process. A TGDL based cell-dynamics model gives predictions of the morphology, which can be compared to TEM images and SAXS patterns. The model does not contain any reactive aspects but captures the morphology of the systems which both showed pinning of the micro-structure at early stages of microphase separation in contrast to the equilibrium structures formed by block copolymers.

Experimental Studies on the Interfacial Shear-Transfer Mechanism in Discontinuous Glass-Fibre Composites

Interfacial stress/strain distributions in discontinuous glass-fibre/epoxy composites have been determined experimentally for the first time by the simultaneous use of tensile deformation measurements and Raman spectroscopy. Model single-fibre composites were used and the glass fibres were coated with a diacetylene-urethane copolymer which was thermally cross-polymerised. During composite deformation, the stress-induced band shifts of C≡C in the polydiacetylene phases of the coating were used to map the strain distribution along a fibre as a function of overall matrix strain up to fibre fracture. Measured strain distributions in fibres were analysed by using classical shear-lag theory to determine interfacial shear-stress distributions. Fibre fragmentation was monitored in detail up to saturation, and the limiting value of interfacial shear stress is shown to be determined by the shear yield stress of the matrix.

Effects of soft-segment prepolymer functionality on structure–property relations in RIM copolyurethanes

Segmented copolyurethanes comprising 40–60% by weight of polyurethane hard segments (HS) and polyether soft-segment (SS) with different functionalities (SS-fn), have been formed by reaction injection moulding (RIM). The HS were formed from 4,4′ diphenylmethane diisocyanate (MDI) reacted with ethane diol (ED). The three SS-prepolymers used were all hydroxyl-functionalised poly(oxypropylene-b-oxyethylene)s with different nominal functionalities (fn) of 2, 3 and 4 but with a constant molar mass per functional group of ∼2000 g mol−1. RIM materials were characterised using differential scanning calorimetry, dynamic mechanical thermal analysis, tensile stress–strain and single-edge notch fracture studies. Predictions using a statistical model of the RIM-copolymerisation showed that increasing SS-fn lead to more rapid development of copolymer molar mass with isocyanate conversion. Experimentally, the RIM-PU exhibited a wide range of mechanical behaviour resulting from differences in molecular and morphological structures. Increasing SS-fn produced materials with improved mould release behaviour and fracture resistance. However, increasing SS-fn also reduced the degree of phase separation developed in the copolyurethanes, resulting in increased modulus–temperature dependence and poorer tensile properties.

Morphology and properties of novel copoly(isocyanurate-urea)s formed by reaction injection moulding

Abstract Novel copoly(isocyanurate-urea)s have been produced by reaction injection moulding (RIM). The materials were formed from ∼2:1 weight ratio of polyisocyanate to polyoxypropylene polyamine in the presence of an organic trimerization catalyst. In some cases an aromatic diamine chain extender was also used. The effects of the polyether polyamine functionality and the incorporation of aromatic diamine chain extenders on the morphology and properties of the copoly(isocyanurate-urea)s were studied using the SAXS, TEM, d.m.t.a. and tensile stress-strain techniques. The RIM materials were stiff plastics with room-temperature Youngs moduli between 1.5 and 0.7 GPa, depending on the morphology. Development of morphology is a result of competition between polymerization kinetics, chemical gelation and (micro)phase separation. Materials with an isotropic, co-continuous morphology with a size-scale of ∼100 A had higher moduli than materials containing isolated glassy particles of ∼1 μm in size. Correlations between morphology and dynamic mechanical-thermal and tensile stress-strain properties of a systematic series of copoly(isocyanurate-urea)s have been established.

Formation and properties of urethane-diacetylene segmented block copolymers

Abstract Two types of urethane-diacetylene segmented block copolymers have been prepared using a one-step bulk polymerization process. By varying the length of the polyether soft segments using different polyols, copolymers with different physical properties have been produced. In one case a copolymer which is rubbery at room temperature has been prepared by using a polyoxypropylene diol with a molecular weight of 1000 g mol −1 . The other material obtained using a polyoxypropylene diol with a molecular weight of 400 g mol −1 is glassy at room temperature. It has been found that the phase-separated hard domains will undergo thermally induced, solid-state cross-polymerization of the diacetylene groups in the domains. This is found to lead to a modification of the physical properties of the materials involving a change in colour, and an increase in the thermal stability, glass transition temperatures and the Youngs moduli of the copolymers. The mechanical properties of the polymers have been interpreted using the Takayanagi mechanical models and the average modulus of the cross-polymerized hard segments has been determined to be about 5 GPa. Well-defined Raman spectra characteristic of conventional polymers have been obtained from the copolymers and it is found that the position of the Raman bands shifts to lower frequency during tensile deformation. This is shown to be due to deformation taking place within the hard segments and it is demonstrated that Raman spectroscopy offers a unique way of determining hard segment deformations in these materials.

The relationship between structure and properties in titanium dioxide filled polypropylene

SummaryThe effect of a pigment grade of titanium dioxide on the morphology, crystallisation and mechanical properties of a typical grade of isotactic polypropylene are described. Pigment particles are able to nucleate additional spherulites, and are incorporated within spherulitic and lamellar morphologies. Measurements of isothermal crystallisation rates using differential scanning calorimetry and hot-stage microscopy show that bulk crystallisation rate is increased by addition of the pigment due to the larger number of spherulites, while the growth rate of the spherulites is unchanged. Addition of the pigment increased tensile modulus, and reduced yield stress. The elongation at break and impact resistance were modulus, and reduced yield stress. The elongation at break and impact resistance were increased by the addition of pigment, showing that particle debonding processes can lead to toughening of the polypropylene matrix.