P. J. Barham

Crystallization and morphology of a bacterial thermoplastic: poly-3-hydroxybutyrate

This paper presents a number of interesting results on the physical properties of poly-3-hydroxybutyrate (PHB). Data are presented on crystallization kinetics, morphology of melt- and solution-crystallized PHB, the variation of lamellar thickness with crystallization temperature, and the assessment of some thermodynamic quantities. These properties include surface free energies, heat of fusion and melting, and glass transition temperatures. It is shown that the special properties of PHB such as the large spherulite size, which is probably due to its exceptional purity, make it an ideal material for model studies of polymer crystallization and morphology. For example, we show that the variation of growth rate with crystallization temperature is consistent with the very latest theories; and that the single crystal morphology has important implications for the understanding of crystal growth in other polymer systems.

An approach to the formation and growth of new phases with application to polymer crystallization: effect of finite size, metastability, and Ostwald's rule of stages

This article aims to link the mainstream subject of chain-folded polymer crystallization with the rather speciality field of extended-chain crystallization, the latter typified by the crystallization of polyethylene (PE) under pressure. Issues of wider generality are also raised for crystal growth, and beyond for phase transformations. The underlying new experimental material comprises the prominent role of metastable phases, specifically the mobile hexagonal phase in polyethylene which can arise in preference to the orthorhombic phase in the phase regime where the latter is the stable regime, and the recognition of “thickening growth” as a primary growth process, as opposed to the traditionally considered secondary process of thickening. The scheme relies on considerations of crystal size as a thermodynamic variable, namely on melting-point depression, which is, in general, different for different polymorphs. It is shown that under specifiable conditions phase stabilities can invert with size; that is a phase which is metastable for infinite size can become the stable phase when the crystal is sufficiently small. As applied to crystal growth, it follows that a crystal can appear and grow in a phase that is different from that in its state of ultimate stability, maintaining this in a metastable form when it may or may not transform into the ultimate stable state in the course of growth according to circumstances. For polymers this intermediate initial state is one with high-chain mobility capable of “thickening growth” which in turn ceases (or slows down) upon transformation, when and if such occurs, thus “locking in” a finite lamellar thickness. The complete situation can be represented by a P, T, 1/l (l ≡ crystal thickness) phase-stability diagram which, coupled with kinetic considerations, embodies all recognized modes of crystallization including chain-folded and extended-chain type ones. The task that remains is to assess which applies under given conditions of P and T. A numerical assessment of the most widely explored case of crystallization of PE under atmospheric pressure indicates that there is a strong likelihood (critically dependent on the choice of input parameters) that crystallization may proceed via a metastable, mobile, hexagonal phase, which is transiently stable at the smallest size where the crystal first appears, with potentially profound consequences for the current picture of such crystallization. Crystallization of PE from solution, however, would, by such computations, proceed directly into the final stage of stability, upholding the validity of the existing treatments of chain-folded crystallization. The above treatment, in its wider applicability, provides a previously unsuspected thermodynamic foundation of Ostwalds rule of stages by stating that phase transformation will always start with the phase (polymorph) which is stable down to the smallest size, irrespective of whether this is stable or metastable when fully grown. In the case where the phase transformation is nucleation controlled, a ready connection between the kinetic and thermodynamic considerations presents itself, including previously invoked kinetic explanations of the stage rule. To justify the statement that the crystal size can control the transformation between two polymorphs, a recent result on 1 -4-poly-trans-butadiene is invoked. Furthermore, phase-stability conditions for wedge-shaped geometries are considered, as raised by current experimental material on PE. It is found that inversion of phase stabilities (as compared to the conditions pertaining for parallel-sided systems) can arise, with consequences for our scheme of polymer crystallization and with wider implications for phase transformations in tapering spaces in general. In addition, in two of the Appendices two themes of overall generality (arising from present considerations for polymers) are developed analytically; namely, the competition of nucleation-controlled phase growth of polymorphs as a function of input parameters, and the effect of phase size on the triple point in phase diagrams. The latter case leads, inter alia to the recognition of previously unsuspected singularities, with consequences which are yet to be assessed.

High-strength polyethylene fibres from solution and gel spinning

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.

Nucleation behaviour of poly-3-hydroxy-butyrate

Poly-3-hydroxy-butyrate (PHB) is a thermoplastic polyester produced by bacterial fermentation. Because of this bacterial origin PHB is a very pure polymer. This high purity in turn leads to very few (if any) heterogeneous nuclei, which gives a much wider scope for a systematic study of nucleation behaviour and the effect of nucleating agents than was possible before. It is shown that in pure PHB nucleation is sporadic. The nucleation rate may be measured over a temperature range of some 100° C. The nucleation rate data are recorded in the range where homogeneous nucleation is possible and are at least consistent with it. When foreign particles are added the nucleation rate is modified. Two distinct types of behaviour are observed. One may be interpreted quantitatively as the local raising of the crystal melting point due to the constraints of the actual presence of a surface; and the other as being due to epitaxial growth on the foreign surface. Detailed kinetic data are presented to support these conclusions.

Molecular Gastronomy: A New Emerging Scientific Discipline

The science of domestic and restaurant cooking has recently moved from the playground of a few interested amateurs into the realm of serious scientific endeavor. A number of restaurants around the world have started to adopt a more scientific approach in their kitchens,1–3 and perhaps partly as a result, several of these have become acclaimed as being among the best in the world.4,5 Today, many food writers and chefs, as well as most gourmets, agree that chemistry lies at the heart of the very finest food available in some of the world’s finest restaurants. At least in the world of gourmet food, chemistry has managed to replace its often tarnished image with a growing respect as the application of basic chemistry in the kitchen has provided the starting point for a whole new cuisine. The application of chemistry and other sciences to restaurant and domestic cooking is thus making a positive impact in a very public arena which inevitably gives credence to the subject as a whole. As yet, however, this activity has been largely in the form of small collaborations between scientists and chefs. To date, little “new science” has emerged, but many novel applications of existing science have been made, assisting chefs to produce new dishes and extend the range of techniques available in their kitchens. Little of this work has appeared in the scientific literature,2,3,6–9 but the work has received an enormous amount of media attention. A quick Google search will reveal thousands of news articles over the past few years; a very few recent examples can be found in China,(10) the United States,11,12 and Australia.(13) In this review we bring together the many strands of chemistry that have been and are increasingly being used in the kitchen to provide a sound basis for further developments in the area. We also attempt throughout to show using relevant illustrative examples how knowledge and understanding of chemistry can be applied to good effect in the domestic and restaurant kitchen. Our basic premise is that the application of chemical and physical techniques in some restaurant kitchens to produce novel textures and flavor combinations has not only revolutionized the restaurant experience but also led to new enjoyment and appreciation of food. Examples include El Bulli (in Spain) and the Fat Duck (in the United Kingdom), two restaurants that since adopting a scientific approach to cooking have become widely regarded as among the finest in the world. All this begs the fundamental question: why should these novel textures and flavors provide so much real pleasure for the diners? Such questions are at the heart of the new science of Molecular Gastronomy. The term Molecular Gastronomy has gained a lot of publicity over the past few years, largely because some chefs have started to label their cooking style as Molecular Gastronomy (MG) and claimed to be bringing the use of scientific principles into the kitchen. However, we should note that three of the first chefs whose food was “labeled” as MG have recently written a new manifesto protesting against this label.(14) They rightly contend that what is important is the finest food prepared using the best available ingredients and using the most appropriate methods (which naturally includes the use of “new” ingredients, for example, gelling agents such as gellan or carageenan, and processes, such as vacuum distillation, etc.). We take a broad view of Molecular Gastronomy and argue it should be considered as the scientific study of why some food tastes terrible, some is mediocre, some good, and occasionally some absolutely delicious. We want to understand what it is that makes one dish delicious and another not, whether it be the choice of ingredients and how they were grown, the manner in which the food was cooked and presented, or the environment in which it was served. All will play their own roles, and there are valid scientific enquiries to be made to elucidate the extent to which they each affect the final result, but chemistry lies at the heart of all these diverse disciplines. The judgment of the quality of a dish is a highly personal matter as is the extent to which a particular meal is enjoyed or not. Nevertheless, we hypothesize that there are a number of conditions that must be met before food becomes truly enjoyable. These include many aspects of the flavor. Clearly, the food should have flavor; but what conditions are truly important? Does it matter, for example, how much flavor a dish has; is the concentration of the flavor molecules important? How important is the order in which the flavor molecules are released? How does the texture affect the flavor? The long-term aims of the science of MG are not only to provide chefs with tools to assist them in producing the finest dishes but also to elucidate the minimum set of conditions that are required for a dish to be described by a representative group of individuals as enjoyable or delicious, to find ways in which these conditions can be met (through the production of raw materials, in the cooking process, and in the way in which the food is presented), and hence to be able to predict reasonably well whether a particular dish or meal would be delicious. It may even become possible to give some quantitative measure of just how delicious a particular dish will be to a particular individual. Clearly, this is an immense task involving many different aspects of the chemical sciences: from the way in which food is produced through the harvesting, packaging, and transport to market via the processing and cooking to the presentation on the plate and how the body and brain react to the various stimuli presented. MG is distinct from traditional Food Science as it is concerned principally with the science behind any conceivable food preparation technique that may be used in a restaurant environment or even in domestic cooking from readily available ingredients to produce the best possible result. Conversely, Food Science is concerned, in large measure, with food production on an industrial scale and nutrition and food safety. A further distinction is that although Molecular Gastronomy includes the science behind gastronomic food, to understand gastronomy it is sometimes also necessary to appreciate its wider background. Thus, investigations of food history and culture may be subjects for investigation within the overall umbrella of Molecular Gastronomy. Further, gastronomy is characterized by the fact that strong, even passionate feelings can be involved. Leading chefs express their own emotions and visions through the dishes they produce. Some chefs stick closely to tradition, while others can be highly innovative and even provocative. In this sense gastronomy can be considered as an art form similar to painting and music. In this review we begin with a short description of our senses of taste and aroma and how we use these and other senses to provide the sensation of flavor. We will show that flavor is not simply the sum of the individual stimuli from the receptors in the tongue and nose but far more complex. In fact, the best we can say is that flavor is constructed in the mind using cues taken from all the senses including, but not limited to, the chemical senses of taste and smell. It is necessary to bear this background in mind throughout the whole review so we do not forget that even if we fully understand the complete chemical composition, physical state, and morphological complexity of a dish, this alone will not tell us whether it will provide an enjoyable eating experience. In subsequent sections we will take a walk through the preparation of a meal, starting with the raw ingredients to see how the chemical make up of even the apparently simplest ingredients such as carrots or tomatoes is greatly affected by all the different agricultural processes they may be subjected to before arriving in the kitchen. Once we have ingredients in the kitchen and start to cut, mix, and cook them, a vast range of chemical reactions come into play, destroying some and creating new flavor compounds. We devote a considerable portion of the review to the summary of some of these reactions. However, we must note that complete textbooks have failed to capture the complexity of many of these, so all we can do here is to provide a general overview of some important aspects that commonly affect flavor in domestic and restaurant kitchens. In nearly all cooking, the texture of the food is as important as its flavor: the flavor of roast chicken is pretty constant, but the texture varies from the wonderfully tender meat that melts in the mouth to the awful rubber chicken of so many conference dinners. Understanding and controlling texture not only of meats but also of sauces, souffles, breads, cakes, and pastries, etc., will take us on a tour through a range of chemical and physical disciplines as we look, for example, at the spinning of glassy sugars to produce candy-floss. Finally, after a discussion of those factors in our food that seem to contribute to making it delicious, we enter the world of brain chemistry, and much of that is speculative. We will end up with a list of areas of potential new research offering all chemists the opportunity to join us in the exciting new adventures of Molecular Gastronomy and the possibility of collaborating with chefs to create new and better food in their own local neighborhoods. Who ever said there is no such thing as a free lunch?

Collapse of South Africa's penguins in the early 21st century

The number of African penguins Spheniscus demersus breeding in South Africa collapsed from about 56 000 pairs in 2001 to some 21 000 pairs in 2009, a loss of 35 000 pairs (>60%) in eight years. This reduced the global population to 26 000 pairs, when including Namibian breeders, and led to classification of the species as Endangered. In South Africa, penguins breed in two regions, the Western Cape and Algoa Bay (Eastern Cape), their breeding localities in these regions being separated by c. 600 km. Their main food is anchovy Engraulis encrasicolus and sardine Sardinops sagax, which are also the target of purse-seine fisheries. In Algoa Bay, numbers of African penguins halved from 21 000 pairs in 2001 to 10 000 pairs in 2003. In the Western Cape, numbers decreased from a mean of 35 000 pairs in 2001–2005 to 11 000 pairs in 2009. At Dassen Island, the annual survival rate of adult penguins decreased from 0.70 in 2002/2003 to 0.46 in 2006/2007; at Robben Island it decreased from 0.77 to 0.55 in the same period. In both the Western and Eastern Cape provinces, long-term trends in numbers of penguins breeding were significantly related to the combined biomass of anchovy and sardine off South Africa. However, recent decreases in the Western Cape were greater than expected given a continuing high abundance of anchovy. In this province, there was a south-east displacement of prey around 2000, which led to a mismatch in the distributions of prey and the western breeding localities of penguins.

Phase segregation in melts of blends of linear and branched polyethylene

Abstract A linear polyethylene (LPE) has been blended with a branched polyethylene (BPE) in a range of concentrations. Using several experimental methods in combination, the full phase diagram for this polymer pair has been mapped out. In particular, the polymers are shown to be conditionally miscible in the melt; the melt segregated region occurs at low LPE content and is of a closed loop shape. It is, however, impossible to reach parts of the region due to crystallization. Considerable co-crystallization takes place whenever any of the blends are crystallized isothermally; crystals commonly include LPE and BPE in the ratio 6:4 and in an extreme case have been found to contain LPE to BPE in the ratio 1:3. Attention is drawn to the wide range of morphologies (and, by implication, properties) which can be obtained from this system.

A study on the achievement of high- modulus polyethylene fibres by drawing

Several methods of preparing spherulitic sheets of high-density polyethylene from which high draw ratios (∼ 30x) and high moduli (∼ 800 kbar) may be obtained are described. It is shown that, independent of the method of preparation of the initial sheet, and provided certain conditions are met, the modulus is a unique function of draw ratio. The maximum draw ratio (and hence modulus) achievable from a particular sheet is shown to depend on its morphology and its molecular weight distribution; in particular, the presence of some segregated low molecular weight material appears to be essential. When viewed in the polarizing microscope a “black” region often is observed bounding the spherulites, particularly in those sheets which give high draw ratios. This region is correlated with segregated low molecular weight material. In addition, the recovery properties of high modulus fibres are reported, both after isothermal strain and on annealing when near complete recovery is observed.

Direct observations of the growth of spherulites of poly (hydroxybutyrate-co-valerate) using atomic force microscopy

Abstract Atomic force microscopy (AFM) has been used to observe, in real time, the growth of two-dimensional poly(hydroxybutyrate-co-valerate) (PHB/V) ‘spherulites’ in thin films. The AFM permits us to image the growth over a wide range of magnifications, from the macroscopic spherulitic growth down to observations of growth of individual lamellae. The lamellar growth images are obtained using a special, high resolution, phase-imaging technique. Low magnification images show, in common with optical microscope techniques, sharp circular growth fronts which move at a constant growth rate. At higher magnifications the rough nature of the growth front on a lamellar scale is clearly revealed with dominant lamellae leading the growth. The most remarkable observation is that these dominant lamellae do not grow at a fixed, constant rate, as predicted by most growth theories, but rather they initially spurt forwards at a rate substantially faster than the macroscopic growth rate, and then slow down or stop. A new theory, in which the spherulite growth rate is controlled not by the growth rate of the individual lamellae, but rather by the rate at which new lamellae nucleate on existing, dormant lamellae, is suggested. It is believed that these observations, although only made on one system, may be more widely applicable.

Phase segregation in blends of linear with branched polyethylene: the effect of varying the molecular weight of the linear polymer

Abstract This paper reports investigations into the phase behaviour of six blend systems containing linear with branched polyethylenes. Five systems used linear polyethylenes of differing molecular weight blended with the same branched polyethylene. The sixth system involved a branched polyethylene of lower branch content and shorter branches. The main experimental methods used were differential scanning calorimetry and transmission electron microscopy. Very good agreement was found between results obtained from both experimental methods. The phase diagrams deduced are of the same general form, all but one showing liquid-liquid phase separation in the melt with both upper and lower critical temperatures. This phase separation is most extensive where the molecular weight of the linear polymer is highest. At the other extreme, for a linear component of molecular weight of 2155, no liquid-liquid phase separation is observed. Of the two branched polyethylenes, that with the higher branch content is more prone to phase separation. These results lead us to conclude that the phase diagram found previously for a similar blend pair is not unique since the various systems studied here all give similar thermal results, morphologies and phase diagrams.