J. A. Odell

The extensibility of macromolecules in solution; a new focus for macromolecular science

The paper is a summary of ongoing work in this laboratory laid on foundations of about 10 years standing. It concerns the extensional and aligning effect which appropriately designed elongational flow-fields have on linear macromolecules in solution. In the case of flexible molecules the chains can be fully stretched out, and the corresponding conformational relaxation time, thus determined, provides information on the molecular weight (amongst others providing a new method for determination of the molecular weight distribution), on the coil dimension as relevant to most recent theories, on the draining characteristics of the coil, and on the difference in extension and retraction characteristics. Further, it can provide information on chain flexibility, utilised here in the case of polyelectrolytes where this flexibility can be systematically varied and thus followed. It also signals the onset of associations, geometric entanglements in particular, opening a new window on entanglement behaviour. Also it offers a uniquely definitive method for the study of flow induced chain scission. In addition, these studies reveal how chain extension reacts back and modifies the flow-field producing it with relevance to rheology and fluid transport. Beyond this all, the work shows explicit connections with gelation and absorption phenomena, to the study of which it promises to contribute. In the realm of rigid rod molecules it indicates how elongational flow can promote liquid crystal formation and, more comprehensively, it provides a definitive method for the determination of rotational diffusion as a function of a number of variables. The most salient conclusion in the latter area is the realisation that rigid rods are incomparably less restricted by their neighbours in their rotational frreedom, and are thus correspondingly more orientable than predicted by theory.

Preparation of monodisperse ellipsoidal polystyrene particles

A method is described for the preparation of monodisperse ellipsoidal particles of polystyrene in the colloidal size range. Monodisperse polystyrene particles were dispersed in a solution of polyvinyl alcohol. This dispersion was then allowed to form, by evaporation, a thin film of polyvinyl alcohol containing spherical polystyrene particles. Strips of this film were clamped into a metal frame, heated rapidly in an oil bath to 200°C and stretched to a predetermined extent in order to convert the spherical particles into ellipsoids; the film was then cooled. A wide range of axial ratios for a variety of initial particle sizes was obtained by this method.

A new route to high modulus polyethylene by lamellar structures nucleated onto fibrous substrates with general implications for crystallization behaviour

Abstract In the case of recent experiments with polyethylene involving nucleation of lamellar crystals along flow induced fibrous substrates unexpectedly high modulus (up to 10 11 N/m 2 ) filaments were obtained in spite of the fact that the samples consisted predominantly of lamellar material. This finding has led to reconsideration of the requirements for high moduli in general providing explanation for our observation and indicating new routes for the attainment of high moduli. In brief, the principle is that high modulus need not require complete chain extension and can be generated by lamellar crystals. We identify the following conditions as necessary: (1) the lamellae are all parallel; (2) the plane normals are along the tensile direction; (3) the straight chain segments within the lamellae are also parallel to the tensile direction (hence perpendicular to the lamellar surfaces); (4) the lateral extension of the lamellae in all directions is large compared to the lamellar thickness; (5) there is intimate and strong contact between consecutive lamellae piled onto each other; (6) there is interlocking between lamellae in lateral contact. It is shown how and why these conditions are fulfilled by our lamellar samples of ultra-high modulus while additional reasons for the observed stiffness are being looked for. It is noticeable that samples based on lamellar morphology have advantages over the more usual high modulus material of purely fibrous nature as obtained along more conventional routes. The issue of modulus apart, the lamellae of tapering thickness in our samples (the morphological feature meeting requirement 6) imply the presence of folds of unprecedently short stem lengths which raise new issues in crystallization studies, notably relating to crystallization of chains in confined spaces, and opens up avenues towards the visualization of space filling by lamellae in bulk samples in general.

Study of crystallization and isothermal thickening in polyethylene using SAXD, low frequency Raman spectroscopy and electron microscopy

Abstract This study contains a combined application of three different techniques for the study of polyethy-lenes crystallized from the melt under different circumstances, small-angle X-ray diffraction (SAXD), low frequency Raman spectroscopy to examine the longitudinal acoustic ( LA ) mode, and electron microscopy. In particular, the combination of SAXD and Raman methods enables the separation of the situation where there is only one lamellar structure which displays several orders in the SAXD pattern, from that where there is more than one type of lamellar thickness present. The superior power of the Raman method, which does not depend on the regularity in the lamellar stacking, becomes apparent. The multiplicity of the lamellar population could be associated with lamellae formed isothermally at the preselected crystallization temperature and with lamellae which originated from material which has remained uncrystallized at this temperature and formed subsequently with smaller lamellar thickness during cooling of the sample. The existence of the corresponding double lamellar population could be made directly visible using electron microscopy on freeze-cut and stained sections. The thinner lamellae in the double population could be extracted by solvents, removing the corresponding SAXD and Raman peaks, and leaving blank image areas in place of the thin lamellae in the electron-micrographs. These extracted thinner lamellae correspond to lower molecular weights as assessed by g.p.c. Thus molecular segregation during crystallization is involved. Furthermore the segregated texture units and their arrangement within the full morphology could now be identified. Pronounced changes in lamellar thickness with crystallization time were observed throughout and were associated in the early stages of crystallization with molecular fractionation and in the later stages with thickening of lamellae already present. An unexpected interrelation between nucleation density and the final lamellar thickness through the agency of isothermal lamellar thickening has been established. Examples are quoted which are contrary to the expected trend of lamellar thickness with crystallization temperature, but which are interpretable nevertheless in the light of the effect of isothermal lamellar thickening. The potential significance of all these findings and of this kind of approach for the characterization of crystalline bulk polymers is made throughout.

Stiff and strong polyethylene with shish kebab morphology by continuous melt extrusion

In a previous work, it was shown that highly oriented fibres with 10 GPa modulus could be obtained by continuous single-stage melt extrusion of a medium molecular weight polyethylene to which 3% ultra-high molecular weight (Mw ∼ 3 to 5 × 106) material had been added by solution blending. It was demonstrated that a special interlocking shish kebab structure was responsible for the favourable mechanical properties. In the present work, we succeeded in achieving the same effect from an unblended polyethylene by choosing starting materials with an inherently suitable molecular weight distribution. Both the high and low molecular weight tails of the distribution are very influential: the high tail contributes to the formation of extended-chain fibrils (which constitute the backbones of the shish kebabs), while the low tail affects melt extrudability and strength. Melt strength is important because unusually high tensile stresses are required during wind-up. The wind-up stress was measured and found to be an order of magnitude greater than that encountered in conventional melt spinning — where no shish kebabs are formed. The implications of the above findings for polymer processing, crystal morphology and melt rheology are discussed.

High modulus filaments of polyethylene with lamellar structure by melt processing; the role of the high molecular weight component

In a previous work it was shown that, by appropriate melt processing, oriented filament polyethylene plugs could be produced such as possessed ultra-high modulus in spite of containing predominantly lamellar structures, the advantageous properties being due to the parallel and interlocking arrangement of the lamellae. In the present work it is demonstrated how the extreme high end of the molecular weight distribution is instrumental in the attainment of such structures. The longest chains, even if present in very small amounts, produce fibrous crystals during extrusion which are inadequate to influence the properties in themselves but by serving as nuclei for lamellar crystallization determine the detailed arrangements and hence the mechanical effectiveness of the lamellar texture. We show examples of how sensitively these structures and the resulting properties can be influenced by slight variations in the high molecular weight content of the material. In fact, materials which otherwise would not produce the desired effect can be made to do so through prior blending with a few percent of ultra high molecular weight material. By judicious creation of bimodal distributions, however, the previous batch production of high modulus plugs with lamellar structures can be turned into a continuous spinning process, thus for the first time achieving the production of oriented, high modulus filaments in the course of a single-step continuous extrusion.

Assessment of molecular connectedness in semi-dilute polymer solutions by elongational flow

Abstract The study of the response of polymer solutions to purely elongational flow-fields, as assessed by birefringence, has been extended to the semi-dilute region. As the concentration was increased the optical effects seen gave direct indication of the onset of network behaviour above a critical strain-rate. The concentration at which such chain interactions first occur was found to be significantly lower than identified by the conventional c ∗ criterion. At any given concentration a time scale could be identified below which the system responds as a network and above which, as an assembly of isolated chains. This critical disentanglement time decreased with concentration, consistent with the time needed for overlapping chains to diffuse apart. On a time-scale longer than this disentanglement time, the chains display the same coil-stretch transition with increasing strain-rate as in dilute solutions, with allowance for the increased solution viscosity. On this longer time-scale the chains can slip out of each others environment, in spite of their geometric overlap. Atactic polystyrene and poly(ethylene oxide) were compared, polystyrene showing the greater entanglement effects by the present criterion. Some reference is made to more strongly interacting systems (H bonds, ionic forces) where chains can only extend in a mutually interacting fashion.

Non-Newtonian behaviour of hydrolysed polyacrylamide in strong elongational flows: a transient network approach

Abstract In this paper we report a new technique that links molecular behaviour and macrorheology in idealized elongational flow systems. The effective elongational viscosity of aqueous solutions of hydrolysed polyacrylamide (widely used and studied in applied hydrodynamics) is determined and correlated with the various stages of chain stretching and transient network formation. Beyond a critical strain rate, strong non-Newtonian dilatant effects are observed. These are unmistakably due to the existence of transient networks, which arise as a consequence of entanglements becoming mechanically effective at timescales shorter than the disentanglement time. We report strong parallels between observations in our idealized experiments and dilatant effects commonly observed in other flow systems that contain appreciable elongational components. These effects had been previously generally attributed to the viscosity enhancement due to the stretching of isolated molecules. On the basis of our observations, which include the coil-stretch transition, we are forced to reinterpret such effects as also due to the development of transient entanglement networks.

Structural studies of poly(p-phenylene benzbisthiazole) films

Poly(p-phenylene benzbisthiazole) (PBT) is one member of a new class of highly-rigid, linear, thermally-stable aromatic heterocyclic polymers. The role of heat-treatment in the improvement of the perfection of crystallinity and mechanical properties of oriented films is discussed. Part of the heat-treatment process seems to be to increase the conjugation length of the polymer chain by increasing the planarity of the molecule, as revealed by visual colour changes and by differential scanning calorimetry. This may in turn account for the improved quality of crystallinity. Considerable detail can be seen in the electron diffraction patterns of heat-treated films. With the exception of the equatorial diffraction peaks this scatter can be accounted for by the detailed molecular transform of the PBT polymer, suitably cylindrically averaged, indicating that the crystal structure is essentially two-dimensional, that is the chains while closely and regularly packed lack longitudinal register. A two-dimensional unit cell with the corresponding molecular packing is proposed which can satisfactorily account for the observed density and for the equatorial diffraction peaks.

Thermomechanical Degradation of Macromolecules

Extensional flow techniques are used to investigate thermomechanical scission of polymer solutions from ambient temperatures up to 150°C. We report precise central scission of chains beyond a critical fracture strain-rate. These results can be well accounted for by a Thermally Activated Barrier to Scission (TABS) model. We speculate upon the origin of degradation in simple shear flows and report novel results on degradation in porous media and ultrasonic sound fields, which contain dominant extensional components. Finally, we show how the nature and degree of degradation is affected by concentration and polydispersity. In semi-dilute entangled solutions, the degradation rates increase, are much higher for polydisperse solutions and the scission becomes progressively more random along the chain.