Studies of the thermal expansion of solids in the temperature range 1–100 K are reviewed. Effects due to thermal vibrations of the lattice, electrons, magnetic interactions and defects are fully discussed. The survey of experimental data is intended to be comprehensive. From the large volume of theoretical work in recent years the authors have aimed to select for discussion the most significant contributions.

Thermal expansion of solids at low temperatures

Differential scanning calorimetry (DSC) is an effective analytical tool to characterize the physical properties of a polymer. DSC enables determination of melting, crystallization, and mesomorphic transition temperatures, and the corresponding enthalpy and entropy changes, and characterization of glass transition and other effects that show either changes in heat capacity or a latent heat. Calorimetry takes a special place among other methods. In addition to its simplicity and universality, the energy characteristics (heat capacity CP and its integral over temperature T—enthalpy H), measured via calorimetry, have a clear physical meaning even though sometimes interpretation may be difficult. With introduction of differential scanning calorimeters (DSC) in the early 1960s calorimetry became a standard tool in polymer science. The advantage of DSC compared with other calorimetric techniques lies in the broad dynamic range regarding heating and cooling rates, including isothermal and temperature-modulated operation. Today 12 orders of magnitude in scanning rate can be covered by combining different types of DSCs. Rates as low as 1 μK s−1 are possible and at the other extreme heating and cooling at 1 MK s−1 and higher is possible. The broad dynamic range is especially of interest for semicrystalline polymers because they are commonly far from equilibrium and phase transitions are strongly time (rate) dependent. Nevertheless, there are still several unsolved problems regarding calorimetry of polymers. I try to address a few of these, for example determination of baseline heat capacity, which is related to the problem of crystallinity determination by DSC, or the occurrence of multiple melting peaks. Possible solutions by using advanced calorimetric techniques, for example fast scanning and high frequency AC (temperature-modulated) calorimetry are discussed.

Differential scanning calorimetry (DSC) of semicrystalline polymers

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed.

Microhardness relating to crystalline polymers

There are many recent reports in the literature of high-strength, high-stiffness polyethylene fibres produced by a variety of techniques, all of which involve at some stage crystallizing the polymer (invariably a high molecular weight material) from solution. In this review we try to place these reports in their proper context and to show how and why the various techniques have been developed. To do this we present brief historical reviews of two distinct subjects: the drawing of single-crystal mats and the preparation of “shish kebabs”. Both of these have, when used in conjunction with very high molecular weight material, led to very strong and stiff fibres. We then describe the recent gel spinning techniques and arrive at the conclusion that there are essentially just two distinct processes involved: the solid state deformation of single crystals, and the crystallization of pre-extended chains to form shish kebabs. Either or both of these processes can occur in gel spinning. In addition to the scientific subject some technical aspects, including material from the patent literature, are also covered.

High-strength polyethylene fibres from solution and gel spinning

Chain-folding is a central feature of the self-organizational aspect of polymer in the solid state, yet the ability of a polymer chain to organize into a folded morphology depends upon its length. The shorter chains seem to fold with relative ease in dilute solution crystallization, but in the melt where topological constraints are encountered, spherulitic crystallization is less facile especially in the higher molecular weight fractions. Polymer morphology and properties demonstrate this point clearly according to the experimental evidence, obtained from several sources and presented in this article. For topological and statistical reasons, longer chains are responsible for more interfacial disorder if the degree of crystallinity, density, mechanical behavior, “fold” surface free energy, “transition” from brittleness to toughness, and so on, may be used as guidelines. Spherulites of homopolymers or copolymers are undoubtedly less ordered than crystals of comparable MW fractions, if measured properties are a manifestation of morphology. The imperfections in spherulites are mainly associated with the quasi-amorphous interlamellar regions within them. Polymer crystallinity decreases as the molecular weight of polymer fractions increase at comparable crystallization temperatures or undercooling conditions. This trend is true for all polymers that have been studied extensively.

Review Spherulites: A personal perspective

Mechanical properties of ultra‐oriented polyethylene

The general properties of a novel process for producing high modulus polyolefins are discussed. The technique is an extrusion drawing involving a crystal-crystal transformation. The principal tests have been made on polyethylene and the guidelines have been established for extending the technique to other polyolefins. The characterization of such materials is extensively discussed, particularly in the light of the concept of continuous crystals.

Polymer modulus and morphology: The tensile properties of polyethylene

Unusual morphologies have been shown to be produced in high density polyethylene near its melting point and under selective conditions of orientation and pressure existing in an Instron capillary extrusion rheometer. Points of study involve the remarkably high optical clarity and tensile moduli for the morphologies produced in these filaments. Details of the crystallization procedure are provided. The morphologies resulting from the Instron procedure have been analyzed by several different techniques. X-ray, calorimetry, microscopy and modulus data are discussed. The transparent filaments are composed of a large number of microfibers having a high degree of crystalline orientation parallel to the capillary axis. Electron diffraction and calorimetric studies indicate that a portion of the strands have an unusually high degree of order. Structural models for the strands are also proposed.



Es wurde gezeigt, das sich in Hochdruck-Polyathylen in der Nahe der Schmelztemperatur und unter bestimmten Bedingungen in bezug auf Orientierung und Druck, wie sie im Instron-Extrusions-Kapillar-Rheometer vorliegen, ungewohnliche morphologische Strukturen ausbilden. Die bemerkenswert hohe optische Transparenz und die Spannungsmoduln der in diesen Faden vorliegenden morphologischen Gebilde waren Gegenstande der Untersuchungen. Einzelheiten des Kristallisationsverfahrens werden beschrieben. Die morphologischen Strukturen, die sich bei dem „Instron-Verfahren” herausbilden, wurden mit Hilfe von rontgenographischen, kalorimetrischen, mikroskopischen und mechanischen Messungen untersucht. Die transparenten Fasern setzen sich aus einer grosen Zahl von Mikrofibrillen, die einen hohen kristallinen Orientierungsgrad parallel zur Kapillarachse bestizen, zusammen. Elektronenbeugung und kalorimetrische Untersuchungen zeigen, das ein Teil der Fasern einen ungewohnlich hohen Ordnungsgrad besitzt. Es werden Strukturmodelle fur die Fasern vorgeschlagen.

Unique polyethylene morphologies produced under extrusion conditions

The morphology of ultradrawn polyethylene. I. Nitric acid etching plus gel permeation chromatography

Thermal expansion coefficients have been measured on ultraoriented polyethylene fibers. Samples were prepared by a solid-state (crystalline) extrusion in an Instron Capillary Rheometer at 136°, 2100 atm. and at a draw ratio of 50. The expansion coefficients between −150 and +50°, perpendicular to the fiber axis, were positive and of the order of 10−4 °C−1. Conversely, parallel to that direction, the observed values were both negative and small, of the order −10−6 °C−1. Length changes were reversible and reproducible between −150 and < +50°. Negative coefficients were attributed to the presence of highly-extended and fully-aligned chains along the fiber. Valuable information about chain conformation, dynamics and lattice perfection can be obtained for the ultraoriented morphologies simply by measuring the linear thermal expansion.

Norman E. Weeks

Papers

Mechanical properties of ultra‐oriented polyethylene

Polymer modulus and morphology: The tensile properties of polyethylene

Unique polyethylene morphologies produced under extrusion conditions

The morphology of ultradrawn polyethylene. I. Nitric acid etching plus gel permeation chromatography

Concerning the Negative Thermal Expansion for Extended Chain Polyethylene.