A. Keller

The role of metastability in polymer phase transitions

Abstract Polymer phases can be described in the same way as phases in other condensed matter using a number density operator and its correlation functions. This description requires the understanding of both symmetry operations and order at different atomic and molecular levels. Statistical mechanics provides a link between the microscopic description of structure and motion and the macroscopic thermodynamic properties. Within the limits of the laws of thermodynamics, polymers exhibit a rich variety of phase transition behaviours. By definition, a first-order phase transition in a temperature–pressure ensemble describes a transformation which involves a discontinuous change of all the thermodynamic functions but the Gibbs free energy at the transition temperature. Higher-ordered phase transitions are classified as critical phenomena. Of special interest is the role of metastability in phase and phase transition behaviours. A classical metastable state possesses a local free energy minimum, but it is not at the global stable equilibrium. Further, the existence of circumstantial metastability need to be invoked based on the constraints of size, dimensionality, order and symmetry; examples include polymorphism, mesophase concepts, crystal size, and thin film effects. Metastable behaviour is also observed in phase transformations that are impeded by kinetic limitations along the pathway to thermodynamic equilibrium. This is illustrated in structural and morphological investigations of crystallization and mesophase transitions, liquid–liquid phase separation, vitrification and gel formation, as well as combinations of all such transformation processes. In these cases, the metastable state often becomes the dominant state for the entire system, and is observed over a range of time and size scales. This review will describe the general principles of metastability in polymer phases and phase transitions and will provide illustrations from current experimental works in selected areas together with raising so far unaddressed conceptual issues of wider applicability to phase transformations in general.

Direct evidence for distinctive, stress-induced nucleus crystals in the crystallization of oriented polymer melts

Abstract A previous paper proposed a model for polymer crystallization under stress [1]. This paper suggested that there could be distinctive crystals which formed under stress at unusually high temperatures to act later as nuclei for the bulk of the crystals. The present paper details definitive evidence for these nucleating crystals. Electron microscope observations are fully consistent with the expected two-phase structure for crystallization under stress, although they are not conclusive. However, X-ray diffraction patterns recorded while the samples crystallized under stress, at the appropriate elevated temperature, conclusively demonstrated the two-stage nature of the crystallization. First, a small number of crystals (type I) form which are highly c-axis oriented, followed by a second group of crystals (type II) having a more complex stress dependence of texture patterns. This is in complete agreement with the model [1] that c-axis-oriented fibers induce transversely growing lamellar crystals. Type...

Flow Induced Polymer Chain Extension and Its Relation to Fibrous Crystallization

This work examines the effect that appreciable molecular extension has on the crystallization of long chain molecules. Elementary theoretical considerations presented indicate that to achieve high molecular extensions in solution a longitudinal velocity gradient of strain rate about 103s-1 is required. A method of generating such a velocity gradient, involving flow between opposed jets, is reported and the nature of this flow pattern is examined and quantitatively analysed. The behaviour of polyethylene-xylene solutions in the flow field is presented, notably birefringence observations and measurements indicate that a high degree of molecular alinement can be achieved in specific localized areas of the flow field; also concentration effects are observed which are discussed in terms of entanglement concepts. The effect chain alinement has on crystallization is examined in detail, in particular the ‘shish kebab ’ morphology of the crystals so produced is examined in relation to the hydrodynamic conditions in which they were grown.

Time-resolved synchrotron X-ray study of chain-folded crystallization of long paraffins

Abstract The examination of the crystallization of strictly uniform ultra-long n-alkanes was resumed with the view of exploring the onset of chain-folding, as laid out previously in ref. 1. The present study, largely on C 246 H 494 , centred on the initial stages of crystallization in the melt, registered in situ by time-resolved small-angle X-ray scattering using a synchrotron X-ray source. The salient new feature was the identification of transient initial fold lengths which were non-integer fractions (NIF) of the chain length. This NIF structure transforms subsequently into forms with integer fraction (IF) fold lengths. In the present study the latter have the extended chain (E) and once-folded (F2) configurations, while NIF has a fold length between the two. The NIF → IF transformation occurs either by lamellar thickening or thinning, or by both. It was found that the NIF state had a more disordered layer surface as compared to the final E and F2 structures, the latter being the states on which the conclusions in ref. 1 had been drawn, which accordingly should apply to the transformed material. Implications of these and several other findings for some central issues in polymer crystallization are briefly discussed. The existence of an initial NIF phase focusses attention to the importance of the fastest kinetic pathway as the determining factor for chain-folded crystal growth with particular attention to the initial chain deposition probability, a line made accessible by the present alkanes.

Radiation effects and crystallinity in polyethylene

Abstract A survey is presented of a series of works on the influence of crystallinity on the radiation induced effects, cross-linking in particular, in polyethylene and paraffins. The principal theme is that the usual conception of random introduction of cross-links into a random assembly of chains needs to be modified in the presence of crystallinity in general and chain folding in particular. A long series of varied investigations on polyethylene have indeed demonstrated through a series of conspicuous effects that not only the ordering intrinsic to crystals and the increased intrachain contacts due to chain folding, but the higher level morphology, the nature and mutual arrangement in particular, have a major influence on the effectivity of the radiation leading to networks. Extension of the works to paraffins identified unsuspected mobility of both the radiation precursor species and the paraffin molecule itself (cross-linked and uncross-linked) within the crystal lattice, leading to phase segregation of the cross-linked species into microscopically identifiable domains together with identifying a trend for the cross-links themselves to form non-randomly in groups. The latter phenomenon, observed also in the molten state, indicates that the departure from randomness in the cross-linking process is much more deep-rooted than originally anticipated, and calls for a general reassessment of our knowledge of cross-linking. Other topics included as part of the general enquiry are the destruction of crystallinity, the promotion of hexagonal phase through radiation, the effect of morphology on chain scission and the general, still unsolved issue of how to assess cross-links by a direct analytical method (involving NMR). The hope is expressed that the bringing together of these varied pieces of work will serve the unification of presently widely diverse areas of experience and might influence developments in the radiation studies of paraffinoid substances.

Investigations on the crystallization of polyethylene under high pressure: role of mobile phases, lamellar thickening growth, phase transformations, and morphology

Abstract The outlines of a comprehensive study on crystallization of polyethylene under elevated pressure (P) are being reported. Through in situ viewing of formation and growth of the lamellar crystals, complemented by x-ray diffraction information, it could be established that within the range of experiments (P = 1.8–5.5 kbar and up to supercoolings (ΔT) of about 10°C) formation and growth of identifiable single crystals only proceeded in the mobile hexagonal (h) phase, irrespective whether this was the stable or metastable phase in the appropriate portion of the pressure-temperature (T) phase diagram. In the latter case (when h metastable) all growth stopped on transformation into the stable orthorhombic (o) phase. All this applies both to lateral growth and to growth into the thickness direction which proceeded from chain folded to extended chain thickness and to thicknesses considerably beyond. The foregoing identifies a primary thickening growth mechanism which, as here recognized, is distinct from ...

Nature of self-seeding polyethylene crystal nuclei

Abstract In a limited temperature range above the clearing point of crystal suspensions, submicroscopic entities representing unmeasurably small portions of the original crystal population may survive whose existence is revealed by crystals nucleated on subsequent cooling [1]. Factors influencing the number of these nuclei and the nature of these nuclei themselves were explored in the case of polyethylene.

Experiments on the location of chain ends in monolayer single crystals of polyethylene

Abstract The location of vinyl end groups in monolayer polyethylene crystals (specially prepared to have surfaces highly accessible to gaseous reactants) has been investigated by exposing the crystals to ozone and determining the extent of reaction by infrared spectroscopy. It was found that, for the type of crystals used, about nine-tenths of the chain ends reacted very rapidly and it is deduced that these are excluded from the crystal lattice and should lie on the crystal surfaces. The remaining fraction of ends were, by comparison, highly resistant to oxidation. The consequences of this deduction are discussed; it is suggested that the chain segment ejected together with the chain ends could account for a large part of the amorphous content associated with this type of crystal. In addition, the results imply that the crystal lattice is inaccessible to ozone even when it contains defects such as those caused by end groups, which is of consequence for work on oxidative degradation both for its own sake a...

Some macroscopic properties of stirring-induced crystals of polyethylene

Abstract Fibrous crystals of polyethylene are produced by crystallization from agitated xylene solutions. In this paper, the following properties of macroscopic aggregates of such crystals, grown under carefully specified conditions, and having well-defined morphologies, are investigated: (a) regularity and magnitude of long periods, (b) crystal densities, (c) melting and annealing behavior, and (d) response of crystals to attack with fuming nitric acid. Electron microscopy reveals two distinct fiber types-a compact, high-temperature form, comprising both extended-chain and folded-chain regions (designated micro shish kebabs in this paper) and a low-temperature (crystallized below 97[ddot]C) form, in which lamellar crystals are deposited upon micro-shish-kebab fiber backbones (referred to in this paper as shish kebabs). Shish kebabs display properties closely similar to those of single crystals, as would be expected from the electron microscopy results, except for some novel features in their annealing be...

Controlled crystal-growing procedures in polyethylene involving self-seeding: Some novel twinning habits

Abstract The paper demonstrates the potentiality of the self-seeding technique [3] for the growing of single crystals of polyethylene. It is shown that with appropriate measures, uniform single-layer crystals of strictly controlled habit can be obtained. Concrete recipes are given for the growing technique. The habit features are systematized as functions of crystallization temperature and concentration. Novel twin formations with coincident centers arising from this work are described and analyzed. Their origin is correlated with the result suggested by an associated investigation that the nuclei have a multicomponent structure [4]. The advantages of the new crystal-growing technique for other lines of work are pointed out.