Crystallization of polyethylene terephthalate in injection moulding : experiments, modelling and numerical simulation.

Crystallization of polyethylene terephthalate in injection moulding : experiments, modelling and numerical simulation.

This thesis provides a novel view on the early stage of crystallization utilizing calcium carbonate as a model system. Calcium carbonate is of great economical, scientific and ecological importance, because it is a major part of water hardness, the most abundant Biomineral and forms huge amounts of geological sediments thus binding large amounts of carbon dioxide. The primary experiments base on the evolution of supersaturation via slow addition of dilute calcium chloride solution into dilute carbonate buffer. The time-dependent measurement of the Ca2+ potential and concurrent pH = constant titration facilitate the calculation of the amount of calcium and carbonate ions bound in pre-nucleation stage clusters, which have never been detected experimentally so far, and in the new phase after nucleation, respectively. Analytical Ultracentrifugation independently proves the existence of pre-nucleation stage clusters, and shows that the clusters forming at pH = 9.00 have a proximately time-averaged size of altogether 70 calcium and carbonate ions. Both experiments show that pre-nucleation stage cluster formation can be described by means of equilibrium thermodynamics. Effectively, the cluster formation equilibrium is physico-chemically characterized by means of a multiple-binding equilibrium of calcium ions to a ‘lattice’ of carbonate ions. The evaluation gives GIBBS standard energy for the formation of calcium/carbonate ion pairs in clusters, which exhibits a maximal value of approximately 17.2 kJ mol^-1 at pH = 9.75 and relates to a minimal binding strength in clusters at this pH-value. Nucleated calcium carbonate particles are amorphous at first and subsequently become crystalline. At high binding strength in clusters, only calcite (the thermodynamically stable polymorph) is finally obtained, while with decreasing binding strength in clusters, vaterite (the thermodynamically least stable polymorph) and presumably aragonite (the thermodynamically intermediate stable polymorph) are obtained additionally. Concurrently, two different solubility products of nucleated amorphous calcium carbonate (ACC) are detected at low binding strength and high binding strength in clusters (ACC I 3.1EE-8 M^2, ACC II 3.8EE-8 M^2), respectively, indicating the precipitation of at least two different ACC species, while the clusters provide the precursor species of ACC. It is proximate that ACC I may relate to calcitic ACC –i.e. ACC exhibiting short range order similar to the long range order of calcite and that ACC II may relate to vateritic ACC, which will subsequently transform into the particular crystalline polymorph as discussed in the literature, respectively. Detailed analysis of nucleated particles forming at minimal binding strength in clusters (pH = 9.75) by means of SEM, TEM, WAXS and light microscopy shows that predominantly vaterite with traces of calcite forms. The crystalline particles of early stages are composed of nano-crystallites of approximately 5 to 10 nm size, respectively, which are aligned in high mutual order as in mesocrystals. The analyses of precipitation at pH = 9.75 in presence of additives –polyacrylic acid (pAA) as a model compound for scale inhibitors and peptides exhibiting calcium carbonate binding affinity as model compounds for crystal modifiers- shows that ACC I and ACC II are precipitated in parallel: pAA stabilizes ACC II particles against crystallization leading to their dissolution for the benefit of crystals that form from ACC I and exclusively calcite is finally obtained. Concurrently, the peptide additives analogously inhibit the formation of calcite and exclusively vaterite is finally obtained in case of one of the peptide additives. These findings show that classical nucleation theory is hardly applicable for the nucleation of calcium carbonate. The metastable system is stabilized remarkably due to cluster formation, while clusters forming by means of equilibrium thermodynamics are the nucleation relevant species and not ions. Most likely, the concept of cluster formation is a common phenomenon occurring during the precipitation of hardly soluble compounds as qualitatively shown for calcium oxalate and calcium phosphate. This finding is important for the fundamental understanding of crystallization and nucleation-inhibition and modification by additives with impact on materials of huge scientific and industrial importance as well as for better understanding of the mass transport in crystallization. It can provide a novel basis for simulation and modelling approaches. New mechanisms of scale formation in Bio- and Geomineralization and also in scale inhibition on the basis of the newly reported reaction channel need to be considered. / Die vorliegende Arbeit zeichnet ein neuartiges Bild der frühen Kristallisationsphase von Calciumcarbonat. Calciumcarbonat hat als Hauptbestandteil der Wasserhärte und als weit verbreitetes Biomineral und Geomineral, das als Sediment in den Ozeanen große Mengen Kohlendioxid bindet, große Bedeutung. Die grundlegenden Experimente basieren auf der sehr langsamen Einstellung von Übersättigung, die durch langsame Zugabe verdünnter Calciumlösung in verdünnten Carbonatpuffer erreicht wird. Zeitabhängige Messung des Ca2+ Potentials bei gleichzeitiger pH = konstant Titration zeigt, dass zeitgemittelt vor der Nukleation gleiche Stoffmengen von Calcium- und Carbonat Ionen in Clustern gebunden sind, die bis jetzt noch nicht experimentell nachgewiesen werden konnten. Analytische Ultrazentrifugation belegt unabhängig die Existenz der Cluster, und es zeigt sich, dass sich die bei pH = 9,00 bildenden Cluster zeitgemittelt aus insgesamt etwa 70 Calcium und Carbonat Ionen bestehen. Die Experimente weisen darauf hin, dass sich die Clusterbildung auf der Grundlage von Gleichgewichtsthermodynamik beschreiben lässt. Ein multiples Bindungsgleichgewichtsmodell ermöglicht die Bestimmung der freien Standard Reaktionsenthalpie für die Bildung von Calcium/Carbonat Ionenpaaren in den Clustern, die ein Maß für die Bindungsstärke in Clustern darstellt. Die Bindungsstärke weist ein Minimum bei pH = 9,75 auf, und es zeigt sich, dass außerhalb dieses Minimums amorphes Calciumcarbonat ausfällt, das sich letztendlich in Calcit (das thermodynamisch stabile Calciumcarbonat Polymorph) umwandelt, während im Minimum und in der Nähe des Minimums amorphes Calciumcarbonat ausfällt, das sich letztendlich hauptsächlich in Vaterit (das thermodynamisch am wenigsten stabile Polymorph), Calcit und möglicherweise Spuren von Aragonit (das Polymorph mittlerer Stabilität) umwandelt. Gleichzeitig treten zwei unterschiedliche Löslichkeitsprodukte für das bei hoher und niedriger Bindungsstärke in Clustern ausgefällte, amorphe Calciumcarbonat auf (ACC I 3,1EE-8 M^2, ACC II 3,8EE-8 M^2). Das zeigt, dass die sich vor der Nukleation bildenden Cluster Vorläuferspezies (Precursor) des ausgefällten, amorphen Calciumcarbonats darstellen, wobei ACC I in der Literatur diskutiertem, calcitischem ACC entsprechen und ACC II vateritischem Calcit entsprechen kann. Eine detaillierte SEM, TEM, WAXS und Lichtmikroskopie Untersuchung der bei minimaler Bindungsstärke in Clustern (pH = 9,75) ausgefällten Partikel zeigt, dass sich hauptsächlich Vaterit mit Spuren von Calcit und möglicherweise Aragonit bildet. Die sich früh bildenden, kristallinen Partikel sind jeweils aus nano-Kristalliten von etwa 5 bis 10 nm Größe aufgebaut, die wie in Mesokristallen eine hohe wechselseitige Ordnung aufweisen. Die Untersuchung der frühen Kristallisation in Gegenwart von Additiven wurde ebenfalls bei minimaler Bindungsstärke in Clustern durchgeführt. Als Additive wurden Polyacrylsäure (PAA) als Beispiel für einen Hemmstoff gegen die Bildung von Verkalkungen und drei Peptide, die Bindungsaffinität zu Calciumcarbonat zeigen, als Beispiel für Kristallisations-Modifikatoren untersucht. Die Analyse zeigt, dass ACC I und ACC II parallel ausfallen; pAA stabilisiert ACC II gegenüber Kristallisation und führt dazu, dass es sich zugunsten von Kristallen, die sich aus ACC I bilden, auflöst, wobei letztendlich reines Calcit erhalten wird. Die Peptide hingegen hemmen die Bildung von Calcit in analoger Weise, wobei in einem Fall letztendlich reines Vaterit entsteht. Die Ergebnisse zeigen, dass die klassische Nukleationstheorie auf die Nukleation von Calciumcarbonat kaum anwendbar ist. Das metastabile System wird durch die Clusterbildung deutlich stabilisiert, und nicht Ionen, sondern Cluster sind die relevanten Spezies in der Nukleation. Wahrscheinlich ist das gefundene Konzept der Clusterbildung ein allgemeines Phänomen, das während der Kristallisation aller schwer löslichen Substanzen auftritt, da es auch für Calciumoxalat und Calciumphosphat qualitativ gezeigt werden konnte. Das Ergebnis ist wichtig für das fundamentale Verständnis der Nukleation, von Nukleationshemmung und der Modifikation von Kristallen mit Auswirkungen auf Materialen von großer industrieller und auch wissenschaftlicher Bedeutung. Ferner gibt es einen Hinweis, wie Masse während der Kristallisation –auch in Lebewesen transportiert werden kann und es kann einen neuen Ansatz für Kristallisationssimulationen liefern. Auf der Basis dieses neuartigen Reaktionskanals müssen neue Kristallisations-Mechanismen in Bio- und Geomineralization in Betracht gezogen werden.

Admixtures like reactants and byproducts are solublenon-crystallizing compounds that can be present in industrialsolutions and affect crystallization of the main substance.This thesis provides experimental and molecular modellingresults on the influence of admixtures on crystal nucleation ofvanillin (VAN). Seven admixtures: acetovanillone (AVA),ethylvanillin (EVA), guaiacol (GUA), guaethol (GUE), 4-hydroxy-acetophenone (HAP), 4-hydroxy-benzaldehyde (HBA), andvanillic acid (VAC) have been used in this study. Classicalnucleation theory is used as the basis to establish arelationship between experimental induction time andsupersaturation, nucleation temperature, and interfacialenergy. A novel multicell device is designed, constructed, andused to increase the experimental efficiency in thedetermination of induction times by using 15 nucleation cellsof small volumes simultaneously. In spite of the largevariation observed in the experiments, the solid-liquidinterfacial energy for each VAN-admixture system can beestimated with an acceptable statistical confidence. At 1 mole% admixture concentration, the interfacial energy is increasedin the presence of GUA, GUE, and HBA, while it becomes lower inthe presence of the other admixtures. As the admixtureconcentration increases from 1 to 10 mole %, the interfacialenergy also increases. The interfacial energies obtained are inthe range 7-10 mJ m-2. Influence of admixtures on metastable zone widthand crystal aspect ratio of VAN is also presented. Theexperimental results show that the admixtures studied arepotential modifiers of the nucleation of VAN. Molecularmodelling by the program Cerius2 is used to identify the likelycrystal growth faces. Two approaches, the surface adsorptionand the lattice integration method, are applied to estimatequantitatively the admixture-crystal interaction energy on thedominating crystal faces of VAN,i.e., {0 0 1} and {1 0 0}. However, a simple and clearcorrelation between the experimental values of interfacialenergy and the calculated interaction energies cannot beidentified. A qualitative structural analysis reveals a certainrelationship between the molecular structure of admixtures andtheir effect on nucleation. The determination of the influenceof admixtures on nucleation is still a challenge. However, themolecular and crystal structural approach used in this thesiscan lead to an improved fundamental understanding ofcrystallization processes. Keywords: Crystallization,nucleation, vanillin, admixtures, additives, impurities,induction time, interfacial energy, molecular modelling,interaction energy.

The aim of the present investigation is to increase theunderstanding of the role played by the solvent in inhibitingor enhancing crystal growth. Drowning-out crystallizationexperiments has been performed by the controlled addition ofwater or ethanol water mixtures to a saturated solution ofbenzoic acid in ethanol-water mixtures. Crystal habitcontrolling factors have been identified.Seededcrystallization experiments have been carried out to evaluatethe effect of solvent composition on crystal habit at constantsupersaturation. The solubility of benzoic acid inethanol-water mixtures at the working temperatures has beendetermined. Electro-zone sensing determinations and microscopicmeasurements are used to characterize the final crystallineproduct. It has been found that the shape of the benzoic acidcrystals grown from ethanol-water solutions ranges from needlesto platelets. Platy particles possess a predominant basal plane(001), bound by (010) and (100) faces, while needles aredeveloped along the b-axis. Long needle-shaped particles havebeen produced at low initial bulk concentration and highethanol concentration in the feed. Small platelets are obtainedat high initial bulk concentrations and high waterconcentration in the feed. The effect of solvent composition on the growth rate hasbeen evaluated at constant supersaturation. Seed crystals arecharacterized by image analysis measurement both before andafter each experiment. Length and width dimensions have beenmeasured on the particle silhouette. The growth rate, thesolid-liquid interfacial energy and the surface entropy factorfor the (010) faces (length dimension) and (100) faces (widthdimension) have been estimated. The interfacial energy andsurface entropy factor decreases in the direction of increasingethanol concentration due to increasing solubility. The results suggest that at low ethanol concentration(xEtOH&lt;60%) growth proceeds by screw dislocation mechanism,and adsorption of ethanol molecules may reduce the growth rate.As the ethanol concentration increases above a critical value(xEtOH ≥60%), the growth mechanism shifts to surfacenucleation and the growth rate increases with increasingethanol concentration. It has been suggested that the observedeffect of the solvent composition on crystal habit is theresult of two conflicting effects here referred as the kineticand interfacial energy effects. High interactions of the pairethanol-benzoic acid seem to be responsible of the growthretardation (kinetic effect) exerted by the solvent. On theother hand, increased ethanol concentration leads to reduceinterfacial energy and increasing surface nucleation whichmight contribute to enhance growth kinetics. <b>Keywords:</b>drowning-out crystallisation, solventcomposition, benzoic acid, solubility, crystal growth,interfacial energy, surface entropy factor, growth mechanism,crystal shape distribution.

The knowledge regarding certain issues in polymer crystallization e.g. the possible existence of short–lived mesophases remains inconclusive due to experimental limitations. Polymers undergo chain folding upon crystallization, which introduces some complications that are not found in crystallization of low molar mass materials. Chain–folded crystals are far from their equilibrium shape and they rearrange rapidly at the crystallization temperature. This, together with the slow experimental techniques traditionally used, impedes the observation of the originally formed structures. To approach this problem, molecularly constrained polymer structures (in which the crystallizing chains are fixed at one end whereas the other end is free to move) have been studied by X–ray diffraction, differential scanning calorimetry, polarized optical microscopy, transmission electron microscopy and atomic force microscopy. The crystallization studies performed in star–branched polyesters showed that the dendritic cores have a pronounced effect on the crystallization of the linear poly(ε–caprolactone) (PCL) arms attached to them. The star–branched polymers showed slower crystal rearrangement, higher equilibrium melting point, higher fold surface free energy, moderately lower crystallinity, and a greater tendency to form spherulites in comparison with linear PCL. The crystal unit cell was the same in both linear and star–branched PCL. Single crystals of the star–branched polymers were more irregular and showed smoother fold surfaces than linear PCL crystals. No sectorial preference was observed in the crystals of the star–branched polymers upon melting while the single crystals of linear PCL showed earlier melting in the {100} sectors than in the {110} sectors. Some of the differences observed can be attributed to the dendritic cores, which must be placed in the vicinity of the fold surface and thus influence the fold surface structure, the possibility of major crystal rearrangement and the presence of a significant cilia phase during crystal growth causing diverging crystal lamellae and consequent spherulite formation. The attachment of the many crystallizable chains to a single core reduces the melt entropy, which explains the higher equilibrium melting point of star–branched PCL. The crystallization behavior of a series of poly(ethylene oxybenzoate)s was also studied. The polymers showed a profound tendency for crystal rearrangement during melting even at high heating rates. The Hoffman–Weeks extrapolation method was found to be unsuitable to calculate the equilibrium melting point of the samples studied because the melting point vs. crystallization temperature data were sensitive to the variations in crystallisation time, which led to significant variations in the equilibrium melting points obtained. / QC 20100914

Membrane proteins are involved in a number of disease pathologies and thus comprise a large number of drug targets. Determination of the high-resolution three-dimensional structure is essential for rational drug design, but several hurdles need to be overcome, primarily the over-expression and purification of said membrane proteins. Human leukotriene C4 synthase (hLTC4S), an 18 kDa integral membrane protein localized in the outer nuclear membrane of eosinophils and basophils, catalyzes the conjugation of LTA4 and reduced glutathione to produce LTC4. LTC4 and its metabolites LTD4 and LTE4 are the cysteinyl leukotrienes implicated in bronchoconstriction and inflammation pathways. The focus of my project involves optimizing the over-expression and purification of hLTC4S, which was heterologously expressed in Schizosaccharomyces pombe, purified by immobilized affinity chromatography, and finally "polished" with a buffer exchange step to remove excess co-purified lipids. The optimized protocol yielded ~1 mg of ~90% homogenous, pure protein per liter of cell culture. The finalized purified protein can then be used for further investigation of two-dimensional crystals by electron crystallography with the overall goal of structure determination.

This study deals with reactively compatibilized Polyamide 6 / High Density Polyethylene blends. More precisely, it focuses on the relationship between (1) the formulation, the processing parameters in corotating twin screw extrusion and (2) the morphologies and the microstructures of blends. Multi-scale morphologies were observed by Scanning and Transmission Electron Microscopy. At the micron scale, the following morphologies were developed: nodular dispersions, stretched nodules and co-continuous morphology. As the processing conditions did not influence the types of morphology, the different morphological regions were reported in ternary diagrams. In the case of compatibilized blends, two mechanisms for morphology development have been proposed: (1) the compatibilization reaction, being very fast, leads to the formation of nano-dispersions by interfacial instabilities and (2) the standard break-up/coalescence mechanism of domains poor in copolymer could lead to the formation of morphologies up to the micron scale. Both the evolution of the largest size as a function of the composition and the distribution of sizes were modeled using percolation concepts. The stability of the morphologies was then studied either during static annealing or controlled shear or in a second step processing. The copolymer formed at the interface allows stabilizing the size of the morphologies. Finally, crystallization at lower temperature was observed by Differential Scanning Calorimetry when the polymers are confined in submicron domains.

DNA polymerase binds to the double stranded DNA and extends the primer strand by adding deoxyribonucletide to the 3’-end. Several reactions in the polymerase active site have been reported by Kornberg in addition to the polymerization. We observed DNA polymerase I can act as a pyrophosphatase and hydrolyze deoxyribonucletide. In performing the pyrophosphatase activity, DNA polymerase I requires to interact with RNA. RNA in general, was found to activate the DNA polymerase I as pyrophosphatase. This hydrolysis causes depletion of dNTP and inhibits DNA polymeration synthesis in vitro. In this RNA-dependent catalysis, DNA polymerase I catalyzes only dNTP but not rNTP. We have also observed that many other DNA polymerases have this type of the RNA-dependent pyrophosphatase activity. Our experimental data suggest that the exonuclease active sites most likely play the critical role in this RNA-dependent dNTP hydrolysis, which might have a broader impact on biological systems. On the basis of the crystal structure of a ternary complex of RNase H (Bacillus halodurans), DNA, and RNA, we have introduced the selenium modification at the 6-position of guanine (G) by replacing the oxygen (SeG). The SeG has been incorporated into DNA (6 nt. - 6 nucleotides) by solid phase synthesis. The crystal structure and biochemical studies with the modified SeG-DNA indicate that the SeDNA can base-pair with the RNA substrate and serve as a template for the RNA hydrolysis. In the crystal structure, it has been observed that the selenium introduction causes shifting (or unwinding) of the G-C base pair by 0.3 Å. Furthermore, the Se-modification can significately enhance the phosphate backbone cleavage (over 1000 fold) of the RNA substrate, although the modifications are remotely located on the DNA bases. This enhancement in the catalytic step is probably attributed to the unwinding of the local duplex, which shifts scissile phosphate bond towards the enzyme active site. Our structural, kinetic and thermodynamic investigations suggest a novel mechanism of RNase H catalysis, which was revealed by the atom-specific selenium modification.

Human Immunodeficiency Virus (HIV) has become a global pandemic with at least 25 million deaths and no cure. One of the most important targets to inhibit this virus is HIV-1 protease (PR), which is required to cleave the viral proteins needed for maturation of the virus after it invades and replicates in the host cell. There are nine protease inhibitors that are used in AIDS treatment. The virus loses susceptibility to these inhibitors by drug resistance due to mutations. The goal of the project is to examine the highly drug resistant HIV PR P51 in its complex with inhibitors. In this experiment we expressed and purified HIV PR P51 protein. We performed protein crystallization with inhibitors Tipranavir, Amprenavir, Darunavir, and Saquinavir to obtain the structure of the protease and the inhibitors in their complexes. Future analysis of the crystal structures will help with the development of successful therapeutic inhibitors.