La cristallisation statique et induite par écoulement du poly(acide lactique)

Le poly(acide lactique), PLA, est un polymère biocompatible et biodégradable, qui peut être produit à partir de ressources renouvelables. En conséquence, il a soulevé une attention toute particulière en tant que remplacement éventuel des polymères à base de pétrole. C’est un polyester aliphatique ayant des propriétés telles que module élevé, haute résistance, biocompatibilité et est donc un matériau prometteur pour diverses applications telles que les implants, l’encapsulation de médicaments et l'emballage. A cause de sa faible température de transition vitreuse, le PLA a une faible résistance thermique et les applications sont donc limitées à celles qui ne sont pas associées à des températures élevées. En outre, ce polymère souffre d'un faible degré de cristallinité. L'augmentation du taux de cristallinité dans de nombreuses techniques de mise en forme, telles que le moulage par injection, est nécessaire. Il y a plusieurs façons d'augmenter le niveau de cristallinité du PLA. Ces procédés comprennent l'utilisation d'agents nucléants, de plastifiants, ou de combinaisons d'agents plastifiants et de nucléation. La cristallisation du PLA à l'état fondu se présente sous deux formes cristallines légèrement différentes connues sous les noms α et α'. Cette étude compare la capacité d'auto-nucléation de ces deux formes cristallines par auto-nucléation. Ceci est réalisé en comparant les températures de cristallisation lors du refroidissement des échantillons préalablement cristallisés à diverses températures, puis de nouveau chauffé à une température dans la plage de fusion partielle du PLA. Dans la deuxième étape, l'effet des paramètres cinétiques et le poids moléculaire du PLA sur l'efficacité de nucléation des PLA phases cristallines a été étudié. Cette partie de l’étude ouvre une nouvelle voie pour comprendre le rôle des modifications cristallines du PLA qui mènent aux conditions optimales pour la cristallisation du PLA. La mise en forme des polymères implique des contraintes de cisaillement et d’élongation, ce qui implique une cristallisation induite par l’écoulement et la solidification qui s’en suit. Les propriétés mécaniques des produits finals dépendent du degré de cristallisation et de la nature des cristaux formés. Par conséquent, l'optimisation du procédé nécessite une bonne compréhension de la façon dont l’écoulement influence la cristallisation. Le type d'écoulement peut jouer un rôle important sur la cristallisation. Par exemple, l'écoulement élongationnel provoque l’orientation et l’étirement des molécules dans le sens de l'extension, comme dans le cas de la mise en forme de fibres et le soufflage de film, en aidant le processus de cristallisation induite par l'écoulement. Une littérature abondante existe sur la ii cristallisation des thermoplastiques classiques induite par l'écoulement. Cela dit, moins d'attention a été accordée à l'effet de l'écoulement de cisaillement et d'allongement sur la cristallisation du PLA. Comme étudié dans la dernière partie de ce document, l'effet du poids moléculaire sur la cristallisation induite par cisaillement du PLA est rapporté. Pour cela, trois différents PLA à faible, moyen et haut poids moléculaire ont été préparés par réaction d'hydrolyse. Ensuite, en utilisant un rhéomètre oscillatoire, l’effet du cisaillement sur la cinétique de cristallisation du PLA a été examiné.Abstract : Poly(lactic acid), PLA, is a biocompatible and biodegradable polymer that can be produced from renewable resources. As a result, it has raised particular attention as a potential replacement for petroleum-based polymers. It is an aliphatic polyester with properties such as high modulus, high strength, and biocompatibility and is thus a promising material for various applications such as implants, drug encapsulation, and packaging. In the wake of low glass transition temperature, PLA has a low heat resistance and its application is limited to those not associated with high temperatures. In addition, this polymer suffers from a low degree of crystalinity. Increasing the crystallization rate in many processing operations, such as injection molding, is required. So far, many routes have been found to improve the crystallinity of PLA. These methods include using nucleating agents, plasticizers, and combination of nucleating agents and plasticizers together. PLA crystallization in the melt state results in two slightly different crystalline forms known as α and α’forms. This thesis compares the self-nucleation ability of these two crystal forms by self-nucleation. This is achieved by comparing crystallization temperatures upon cooling for samples previously crystallized at various temperatures and then re-heated to a temperature in the partial melting range for PLA. In the second step, we study the effect of molecular weight of PLA on the nucleation efficiency of PLA crystalline phases. This part of the investigation opens a new pathway to understand the role of PLA crystalline phases on the optimal condition for its crystallization kinetics. Polymer processing operations involve mixed shear and elongational flows and cause polymer molecules to experience flow-induced crystallization during flow and subsequent solidification. The mechanical properties of the final products are significantly dependent upon the degree of crystallization and types of formed crystals. Therefore, optimization of any polymer process requires a good understanding of how flow influences crystallization. The type of flow can play a significant role in affecting crystallization. For example, elongational flow causes molecules to orient and stretch in the direction of extension, as in the case of fiber spinning and film blowing, helping the process of flow-induced crystallization. An extensive body of literature exists on flow-induced crystallization of conventional thermoplastics. Having said that, less attention has been paid to the effect of shear and elongational flow on the PLA crystallization kinetics. As investigated in the final part of this thesis, the effect of iv molecular weight on the shear-induced crystallization of PLA is reported. For this, low, medium and high molecular-weight PLAs were prepared from a high molecular weight one by a hydrolysis reaction. Next, by means of a simple rotational rheometry, effect of the shear flow was examined on the crystallization kinetics of these three PLAs

Structure development and mechanical performance of polypropylene

Polymers are known for their ease of processability via automated mass production technologies. The most important process is injection molding that, due to its freedom in material choice and product design, allows producing a wide variety of thermoplastic products. Mechanical failure of these products, either upon impact or after prolonged exposure to load, limits their ultimate useful lifetime. To predict and control lifetime, understanding of the route from production to failure, i.e. the processing-structure-property relation, is necessary. This is a complex issue; especially in the case of semi-crystalline polymers. These are heterogeneous systems comprised of amorphous and crystalline fractions, of which the latter can be highly anisotropic with size and orientation that are strongly dependent on the precise processing conditions. As a consequence, these structural features in the microstructure, and the associated mechanical properties, generally exhibit distributions containing different orientations throughout a single processed product. Understanding polymer solidification under realistic processing conditions is a prerequisite to predict final polymer properties, since only a complete characterization of the morphology distribution within a product can lead to a meaningful and interpretable mechanical characterization. In this thesis we study the relation between processing conditions, morphology and mechanical performance of a semi-crystalline polymer, isotactic polypropylene. Key issue is the accurate control over all relevant processing parameters. Therefore, different experimental techniques are used to obtain samples at different high cooling rates, at elevated pressures, and high shear rates. A custom designed dilatometer (PVT- ¿T -¿¿ -apparatus) proves to represent the most important and useful technique. First, a predictive, quantitative model is presented for the crystallization kinetics of the multiple crystal structures of polypropylene, under quiescent conditions. The approach is based on the nucleation rate and the individual growth rate of spherulites of each type of polymorphism (a-, ß-, ¿- and mesomorphic phase), during non-isothermal, isobaric solidification. Using Schneider’s rate equations, the degree of crystallinity and distribution of crystal structures and lamellar thickness is predicted. Next, the effect of flow is introduced. Flow strongly influences the kinetics of the crystallization process, especially that of nucleation. Three regimes are observed in the experiments; quiescent crystallization, flow enhanced point nucleation and flow-induced creation of oriented structures. To assess the structure development under flow, a molecular-based rheology model is used. Combining the models derived for quiescent and for flow-induced crystallization, yields the tool that is capable of predicting the volume distributions of both isotropic and oriented structures, under realistic processing conditions. The kinetics of mechanical deformations strongly depend on the anisotropy in the crystalline morphology, thus the local orientation. To study this, uniaxially oriented tapes with a well defined, and high, degree of anisotropy are used as well as injection molded rectangular plates. Yield and failure are described using an anisotropic viscoplastic model, applying a viscoplastic flow rule. It uses the equivalent stress in Hill’s anisotropic yield criterion, and combines the Eyring flow theory with a critical equivalent strain. Factorization is used and the model is capable to quantitatively predict the rate, the angle and the draw ratio dependence of the yield stress, as well as the time-tofailure in various off-axis tensile loading conditions. To use the model, also for other polymers, characterization of only the isotropic state is sufficient. Therefore, the influence of the cooling rate on the deformation kinetics is studied in-depth on isotropic systems. Different cooling rates induce different crystal phases, both the stable a-phase and the mesomorphic phase, while also the degree of crystallinity and lamellar thickness are influenced. The deformation kinetics prove to be the same for the different microstructures, which means that the activation volume and energy are independent of the thermodynamic state. Differences in thermal history are, consequently, solely captured by two rate constants which are a function of the microstructure

Emerging Trends in Polymer Matrix Composites .

The performance characteristics of PMC products are determined by the microstructure developed during the processing of composite materials. The structure development in processing is the result of integration of process parameters and inherent material characteristics. The properties of PMCs can thus be manipulated through both changes in the materials composition and process conditions. The present article illustrates the scientific approach followed in engineering of matrix materials and optimization of the processing conditions with specific reference to case studies on toughening of thermosetting resins and structure development in injection molding of thermoplastic composites. A novel approach is demonstrated for toughening of unsaturated polyester resins that involves the use of reactive liquid polymers chemically bonded to the matrix. The use of processing science is demonstrated by the significant effect of the mold temperature on the crystallinity and properties of molded poly (phenylene sulfide), a high performance engineering thermoplastic. An interactive approach is proposed for specific product and applications development

Relationship between the micromorphology and mechanical properties of semicrystalline polypropylene

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The objective of this research project was to carry out the investigation of the relationship between processing conditions, micromorphology and mechanical properties of isotactic polypropylene homopolymer using conventional and shear controlled orientation injection moulding (SCORIM) techniques by systematically changing carefully controlled processing conditions, mould geometry and compound additives. Both SCORIM and conventional techniques were employed for iPP injection moulding using three moulds of different shapes by varying the processing conditions, including nozzle temperature, mould temperature, injection speed, hold pressure and oscillating patterns of pistons. The results obtained were compared so as to indicate the differences in microstructure and physical properties resulting from the two moulding techniques. A range of analytical methods were employed. Optical transmitted light microscopy was used to reveal the skin-core morphology and preferentially oriented fibrous textures. Transmitted Electron Microscopy represented the enlargement of the fibrous alignment. Micro hardness analysed the hardness and isotropy characteristics by measuring the diagonal lengths of the indentations. Mechanical testing determined Young's modulus, the strength and toughness of the mouldings. X-ray diffraction exhibited the distribution of the cc, 6 and 7 crystalline phases of the iPP mouldings. The WAXS Debye patterns confirmed the existence of the preferred orientation through the thickness of the moulding. Differential Scanning Calorimetry analysed the thermal behaviour from the endothermal and exothermal curves. In the initial stage of the study, the polypropylene was moulded in the form of a standard tensile bar on a conventional Sandretto injection machine in order to obtain the basic characteristics of the polypropylene study material, which could then be used to compare with those properties to be gained using the SCORIM technique. A ring mould was then used in a Negri Bossi twin injection machine to investigate improvements in uniformity of micromorphology and dimensional reproducibility of mouldings made possible by four live-feed injection moulding. Later, a study was carried out on injection moulding of polypropylene by varying processing conditions, including three hold pressures, two mould temperatures and two nozzle temperatures for both conventional and SCORIM injection processes by using a rectangular bar mould in a Demag injection moulding machine. In the finial stage, the study explores the influences of composition, in essence a limited range of nucleating agents, and processing methods, and aspects of the micromorphology, dimensional control and the mechanical properties of polypropylene. Polypropylene, as a sernicrystalline polymer, represents a class of materials in which mechanical properties are strongly influenced by processing conditions and micromorphology.Financial support was obtained from the UK Ministry of Defence

Crystallization behaviour of recycled polyolefins blends

A novel tailor-made thermal fractionation protocol, based on the Successive Self-nucleation and Annealing (SSA) method, was developed to investigate the complex chemical composition of PE/PP blends derived from recycling. The temperature regions where co-crystallization among the blend components do not occur were assessed, enabling the development of the quantitative method. Furthermore, a set-up for achieving Continuous Cooling Curve diagrams was designed, and allowed to study the crystallization kinetics at processing-relevant cooling conditions of the phases in the blends. An \u201cinversion point\u201d in the crystallization order of the two polymers arises from the difference in crystallization rates between PP and PE with increasing cooling rate. Mutual nucleating effects, found at the interface between the phases, correlate with the inversion point. Moreover, the order of crystallization of the two polymers at low cooling rates, i.e., before the inversion point, can be tuned by employing neat or nucleated PP. This demonstrates the importance of knowing and controlling the type of components in recycled blends. Finally, the nature of such nucleating effects was revealed by a novel approach for studying surface-induced crystallization in the blends. The method consists of detecting variations in the crystallization kinetics of the dispersed phase (PE) with changing the crystalline state of the matrix (PP) through self-nucleation. The enhancement of crystallization kinetics of PE that was achieved when increasing the lamellar thickness of PP, together with the very low value found for the interfacial free energy difference, are evidence that such nucleating effects occur through epitaxial growth

Quantifying the effects of Processing and detaeriorating environments on polypropylene by infrared microscopy

Polypropylene (PP) is a highly versatile polyolefin, which is being used in a wide variety of applications. Depending on the final application, the chemical and morphological properties of PP have to be tailored. The current research work was performed with a particular focus towards the morphological changes in PP triggered by welding. Welding is a widely applied method for joining plastics which, however, generates a complex morphological polymer structure at the weld. The effect of processing on the morphology of PP was studied for welds between plates of PP homopolymer (PP-H). Due to the complex interplay of heating and cooling rates as well as the mechanical forces during welding, the PP chains rearrange themselves in different patterns and forms different zones in the weld, which were qualitatively analyzed by polarized light microscopy (PLM). Three distinct zones, which differ in their crystal structure and size, were observed in PLM, namely the injection molded plate with the original morphology, the weld seam and the weld core. The different cooling and heating rates cause the polymer to crystallize at different rates. Quantitative studies on these effects raised upon welding are a prerequisite to modulate the end use properties of a welded PP. Hence, the orientation of polymer chains was quantified by infrared microscopy (µFT-IR) across PP welds. Thus it could be shown that the polymer chains were oriented along machine direction (MD) at the plates and normal direction (ND) at the weld seams. One of the major applications of PP is the sector of pipes which are being used to transport various kinds of liquid media. Disinfection of water by strong oxidants due to health reasons is a routine process in the current situation. Yet, the transportation of this disinfected water can cause severe damages to PP. A detailed research on the deterioration rates and the mechanisms underlaying when PP is in contact with hot water and chlorinated water with different chlorine concentrations and at different temperatures was carried out. The pipes chosen for the durability studies were a random copolymer of propylene with ethylene (PP-R) nucleated with α- and β-nucleating agents (PP-R1 and PP-R2 respectively). The pipes were stabilized with 1,3,5-tris (3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-methylbenzene (AO-13), Pentaerythritol tetrakis (3-3,5-di-tert-butyl-4-hydroxyphenyl) propionate (AO-18) and the processing stabilizer tris(2,4-di-tert-butylphenyl) phosphite (PS-2). The characteristic ester carbonyl absorption in the infrared spectrum at 1740 cm-1 was used to quantify AO-18. However, the characteristic bands of AO-13 overlapped with the bands of AO-18 and PS-2. To overcome this problem a new method has been developed for µFT-IR to quantify AO-13 from a mixture with AO-18 and PS-2. The quantification results of AOs obtained from µFT-IR were well supported by Extraction→HPLC. PP is vulnerable to deterioration in the absence of stabilizers. Thus, monitoring the stabilizer loss is an alternative approach to determine the durability of polymers in reactive environments. Degradation/extraction of AOs is highly dependent on the morphology of the material. In that case, the extrusion of polymer melt into the form of hollow cylindrical pipes generates a distinct morphological profile across the pipe wall. Using µFT-IR it could be shown that the polymer chains in PP-R1 were orientated along the extrusion direction due to the fast crystallization of the injected melt triggered by α-nucleation. The diffusion of small molecules through these kinds of oriented networks is faster than randomly oriented ones. In PP-R2, the β- nucleation led to a random orientation of polymer chains at the inner surface which gave impermeability towards the small molecules. This difference in the penetration characteristics of PP-R1 and PP-R2 manifested in different deterioration rates of the pipes. The faster penetration of the media caused the formation of a higher amount of macroradicals in PP-R1. Hence a higher amount of AOs was required to stabilize these macroradicals to prevent the polymer chain scission. In order to understand the influence of temperature on the degradation of the polymer, ageing was carried out at two temperatures 95 and 110 °C. Out of these experiments, the degradative processes of the polymer, namely the loss of antioxidants and reduction in molar mass of the polymer, proceeded more rapidly at 110 °C. Experiments with different chlorine concentrations showed that deterioration was more pronounced at higher chlorine concentrations. The depletion of the two AOs, AO-18 and AO-13, differs in their rates. In detail it was found that AO-18 is lost faster to the inner medium than AO-13, irrespective of ageing condition and temperature. The degradation of AO-18 can be caused via two mechanisms: The hydrolysis of the ester bond and via the intended donation of phenolic hydrogen to the macroradicals. On the contrary AO-13 is susceptible towards degradation solely by the quenching of macroradicals. Therefore, an accelerated degradation of AO-18 was observed irrespective of the ageing temperature