Analysis of structural development during superdrawing of poly(ethylene terephthalate) fibers

A comprehensive experimental study was conducted to determine the limitations in processing conditions for superdrawing. Experimental studies were carried out by uniaxial drawing tests at temperatures from 90 to 120°C and at strain rates ranging from 0.008/s to 0.425/s. Crystallinity and orientation of the drawn samples were evaluated using differential scanning calorimetry and birefringence measurements. This study revealed that increasing temperature from 110°C to 120°C leads to more crystallization at low strain rates (0.001/s), and less crystallization at high strain rates (0.1/s). Furthermore, it was shown for the first time that the mechanism of crystallinity development in PET undergoes a transition at draw temperature of 113°C and strain rate of 0.17/s. A new one-dimensional constitutive model was developed to predict the stress-strain behavior of PET fibers as they are drawn to very large draw ratios (up to 10) over a wide range of temperature (90-120°C) and strain rate (0.008-0.425/s). The model was based on the rubber elasticity theory and non-linear viscoelasticity.Ph.D.Committee Chair: Wang, Youjiang; Committee Co-Chair: Jacob, K.I.; Committee Member: Aneja, A.P.; Committee Member: Garmestani, Hamid; Committee Member: Thio, Yonathan S.; Committee Member: Yao, Donggan

Development of High Barrier Nylon Based Multilayer Films

Les films multicouches sont composés d’une couche de coeur possédant de bonnes propriétés barrières et mécaniques, prise en sandwich entre deux couches de polyoléfines. Cette composition est couramment utilisée dans l’industrie des emballages alimentaires afin d’améliorer les propriétés mécaniques et barrières à l’oxygène et à l’humidité des films. Au cours de cette étude, des films multicouches à base de nylon aromatique (MXD6), aliphatique (PA6) et leurs nanocomposites, à hautes propriétés barrières ont été développés. Les performances thermiques, barrières (oxygène et vapeur d’eau) et mécaniques des films multicouches ont été comparées entre elles, en faisant varier la couche de coeur (PA6, MXD6 ou leurs nanocomposites). Dans la première partie de ce travail, des films de nylon aliphatique (PA6), de nylon aromatique (MXD6) ainsi que leurs nanocomposites, préparé par polymérisation in-situ avec 4wt% d’argile, ont été extrudés par calandrage à l’aide d’une extrudeuse de laboratoire et refroidis rapidement à l’aide de couteaux d’air. Les propriétés rhéologiques, cristallines, thermiques, barrières et mécaniques des résines pures et des films monocouches extrudés ont été étudiées et comparées. Dans la seconde partie de ce travail, les films monocouches produits ont été étirés uniaxialement à 110 ºC avec un rapport d’étirage variant de 1.5 à 5. L’alignement de l’argile généré par l’étirement des films de nanocomposites ont été mesurés à l’aide de trois techniques différentes : déconvolution des pics en FTIR, soustraction spectrale interactives en FTIR, et diffraction aux Rayons X. Il a été déterminé que les particules d’argile sont principalement orientées dans la direction machine (MD) et que leur orientation est améliorée sous l’effet de l’étirement uniaxial. L’effet des changements d’orientation des cristaux pour toutes les phases III cristallines et amorphes a été examiné à l’aide de la diffraction aux rayons X et de l’analyse trichroique des spectres FTIR. Basé sur les modèles WAXD et les résultats en FTIR, il a été possible de proposer un model schématique afin de décrire le mécanisme de cristallisation du nylon en présence d’argile. Dans la troisième partie de cette étude, l’effet de l’étirement uniaxial sur la structure cristalline, les propriétés thermiques, mécaniques et barrières à l’oxygène des nylons aromatiques et aliphatiques ainsi que de leur nanocomposites ont été étudiés et comparées. Finalement, les films multicouches contenant en couche de coeur, le PA6, ou le MXD6, ou leur nanocomposites ont été produits en utilisant une unité de coextrusion calandrage de laboratoire. Les couches sandwitch de peau sont en LLDPE, et agissent comme des couches barrières à l’humidité. Des films multicouches à 5 couches (une couche supplémentaire a été ajouté de chaque côté entre la couche de coeur et de peau afin d’en améliorer la compatibilité) ont également été produits. Durant le procédé, les paramètres de production ont été optimisés afin d’éliminer les instabilités interfaciales et d’améliorer l’uniformité des films multicouches. Les films produits ont été caractérisés et comparés. ----------- Insufficient barrier properties of commercial thermoplastics to the permeation of atmospheric gases such as oxygen and water vapor are a major problem in the packaging industry. In particular, a high oxygen permeation rate reduces the shelf life of packaged food products, which results in higher costs for food processors and retail customers. Multilayer films having a core layer with good barrier and mechanical properties coextruded between two polyolefin layers have been used in food packaging industry to improve the mechanical performance and the barrier properties. Nylon is an engineering thermoplastic used in flexible packaging due to its high stiffness, toughness, tensile strength, flex crack and puncture resistance as well as low oxygen transmission rate. The objective of this research was fundamental understanding on the differences between properties of monolayer and multilayer aromatic and aliphatic nylon films and their nanocomposites. In the selection of the aromatic and aliphatic nylons, particular attention paid to the oxygen barrier properties of the films as the targeted application is for food packaging and this property plays a critical role in determining shelf life of packed product. Resin characteristics particularly the rheological and thermal properties, morphology, molecular orientation, ability to crystallize (i.e. fast or slow crystallization rate), type of crystalline structure are the key factors for the production of the precursor films with appropriate crystallinity and orientation, which in turn control the final film properties. The extent of nanoclay intercalation and exfoliation, crystal structure, crystallinity, thermal, rheological, barrier and mechanical properties of polyamide 6 (PA6), poly (m-xylene adipamide) (MXD6) and their in-situ polymerized nanocomposites with 4 wt% clay were studied and compared. Dynamic rheological measurements confirmed a strong interfacial interaction between the silicate platelets and the MXD6 chains. A longer relaxation V time for the MXD6, which was related to its higher intermolecular interactions compared to the PA6, resulted in a slower rate of crystallization and lower crystallinity in the former. It was found that due to the stronger polymer chain interaction of the MXD6, there was a lower free volume and gas diffusion path for the MXD6 nanocomposite film compared to the PA6 nanocomposite. In the second part of this project, the precursor monolayer films were uniaxially stretched at 110 ºC with draw ratios varying from 1.5 to 5. The clay alignment was measured with three different techniques: FTIR peak deconvolution, FTIR interactive spectral subtraction and X-ray diffraction. It was found that the clay platelets are mainly oriented in the machine direction (MD) and their orientation improved upon uniaxial stretching. The changes in orientation of crystal axes of all the crystalline phases and amorphous region of the aromatic and the aliphatic nylons and their nanocomposites were examined using X-ray diffraction and Trichroic Infrared analyses. Based on the WAXD patterns and FTIR results, schematic models were proposed to describe the crystallization mechanism of the nylon in the presence of the clay platelets. The crystalline and amorphous orientations as well as the clay alignment significantly affect the performance of the stretched films. In the third step of this study, the structural development of the aliphatic and aromatic nylons and their nanocomposite films during uniaxial stretching was investigated. The effect of uniaxial drawing on the morphology, crystallinity, thermal, mechanical and oxygen barrier properties of the polyamide 6 and the MXD6 as well as their insitu polymerized nanocomposites were studied. A significant enhancement in the Young’s modulus and tensile strength of the uniaxially stretched aliphatic and aromatic nylons was observed. The oxygen permeability and oxygen diffusion through the nylon nanocomposite films VI were predicted with theoretical models and with incorporating structural parameters such as the crystalline phase orientation, clay aspect ratio and clay orientation. In the last phase, coextruded multilayer films with the PA6 and MXD6 nylons as well as their in-situ polymerized nanocomposites, as an oxygen barrier layer (core), and a linear low-density polyethylene (LLDPE) as the moisture barrier layers (skin) with the adjacent tie were produced and characterized. The effect of core layer material on the thermal, optical, barrier and mechanical properties of the coextruded multilayer films has been investigated

Nano- and Micromechanics of Crystalline Polymers

It is currently thought that crystalline polymers consists of lamellar crystals which are separated from each other by a layer of amorphous polymer and are held together by tie molecules through the amorphous phase [e.g. 1]. The lamellae are formed from mostly folded chains. The thickness of lamellae is determined by the parameters such as interfacial energies, glass transition temperature and melting temperature, undercooling, segmental diffusivity, etc. The thickness reported lies usually in a narrow range between 3 and 20 nm as obtained from observations in various types of microscopes or calculated from the degree of crystallinity and long period. It has been recognized that chain folding is not so regular as it was thought and molecular packing in lamellae is subject to considerable and irregularly distributed disorder depending on undercooling- regimes of crystallization. It has been demonstrated in various ways that the planar growth front will always break up into fibrous or cellular growth. Also crystallization of polymers leads to interface instability. More sophisticated treatment of the instabilities involve perturbation analyses of planar interfaces, correlating diffusion, temperature gradients along the interface, and interfacial energy with the size of the growing crystals

Optimization of polypropylene cellular films for piezoelectric applications

Cette thèse comporte deux objectifs principaux: la production en continu de films de polypropylène (PP) moussés ayant une structure cellulaire de forme oculaire, suivie par la préparation de films PP ferroélectrets par décharge corona pour des applications piézoélectriques. Dans la première partie de ce travail, une production en continu par extrusion-calandrage a été développée pour produire des films de PP moussés pour des applications piézoélectriques. Le système est basé sur un moussage physique en utilisant de l'azote supercritique (SC-N2) et le carbonate de calcium (CaCO3) comme agent de nucléation. Les paramètres de mise en œuvre (conception de vis, profil de température, agent gonflant et de nucléation ainsi que leur contenu, et la vitesse d'étirement) ont été optimisés pour obtenir une forme spécifique (oculaire) comme structure cellulaire avec une distribution uniforme de la taille des cellules. Les résultats ont montré qu'une structure cellulaire avec un plus grand rapport d'aspect (AR) des cellules possède un plus faible module de Young, ce qui est approprié pour les films cellulaires piézoélectriques. Dans la deuxième partie, des films PP ferroélectrets ont été produits. Suite à l'optimisation du procédé de décharge corona (tension de charge, distance de l'aiguille, temps de charge), les propriétés piézoélectriques des films obtenus ont été caractérisées et le coefficient piézoélectrique quasi-statique d33 a produit une valeur de 550 pC/N. Afin de mieux caractériser le comportement du film, l’analyse mécanique dynamique (DMA) a été proposée comme une méthode simple pour relier les propriétés piézoélectriques des films PP cellulaires à leur morphologie (taille, géométrie et densité des cellules). Finalement, grâce à un post-traitement basé sur la saturation du film PP moussé avec le SC-N2, une procédure en température et pression a été développée afin d’améliorer la structure cellulaire (cellules plus allongées). Ce traitement a permis d’augmenter de 45% le coefficient d33 (800 pC/N).This thesis is composed of two main objectives: the continuous production of thin foamed polypropylene (PP) films having an eye-like cellular structure, followed by the preparation of ferroelectret PP films through corona discharge for piezoelectric applications. In the first part of this work, a continuous extrusion-calendaring setup was developed to produce PP foamed films for piezoelectric applications. The setup is based on physical foaming using supercritical nitrogen (SC-N2) and calcium carbonate (CaCO3) as nucleating agent. The processing parameters (screw design, temperature profile, blowing agent and nucleating agent content, and stretching speed) were optimized to achieve a specific stretched eye-like cellular structure with a uniform cell size distribution. The results showed that a cellular structure with higher cell aspect ratio (AR) has lower Young’s modulus, which is appropriate for piezoelectric cellular films. In the second part, ferroelectret PP films were produced. After optimization of the corona discharge process (charging voltage, needle distance, charging time), the piezoelectric properties of the resulting films were characterized and the optimum quasi-static piezoelectric d33 coefficient value was 550 pC/N. To better characterize the film behavior, dynamic mechanical analysis (DMA) was proposed as a simple method to relate the piezoelectric properties of the cellular PP films to their morphology (cell size, geometry and density). Finally, through a post-processing treatment based on the saturation of the foamed PP film with SC-N2, a temperature-pressure procedure was developed to improve the cellular structure (more stretched eye-like cells). This treatment was shown to increase by 45% the d33 coefficient (800 pC/N)

Temperature and rate dependent finite strain behavior of poly(ethylene terephthalate) and poly(ethylene terephthalate)-glycol above the glass transition temperature

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003.Includes bibliographical references (p. 333-348).Poly(ethylene terephthalate) is widely used for consumer products such as drawn fibers, stretched films, and soda bottles. Much of its commercial success lies in the fact that it crystallizes at large strains during warm deformation processing. The imparted crystallinity increases its stiffness and strength, improves its dimensional stability, and increases its density. The crystallization process and the stress-strain behavior above the glass transition depend strongly on temperature, strain rate, strain magnitude, and strain state. A robust constitutive model to accurately account for this stress-strain behavior in the processing regime is highly desirable in order to predict and computationally design warm deformation processes to achieve desired end product geometries and properties. This thesis aims to better understand the material behavior above the glass transition temperature in the processing regime. It examines the strain rate, strain state, and temperature dependent mechanical behavior of two polymers: PET and PETG, an amorphous non-crystallizing copolymer of PET, in order to isolate the effects of crystallization on the stress-strain behavior. Experiments over a wide range of temperatures and strain rates were performed in uniaxial and plane strain compression. A constitutive model of the observed rate and temperature dependent stress-strain behavior was then developed. The model represents the material's resistance to deformation with two parallel elements: an intermolecular resistance to flow and a resistance due to molecular network interactions.(cont.) The model predicts the temperature and rate dependence of many stress-strain features of PET and PETG very well, including the initial modulus, flow stress, initial hardening modulus, and dramatic strain hardening. The modeling results indicate that the large strain hardening behavior of both materials can only be captured by including a critical orientation parameter to halt the molecular relaxation process once the network achieves a specific level of molecular orientation. This suggests that much of the strain hardening in PET is due to molecular orientation and not to strain-induced crystallization. An example blow molding process is simulated to demonstrate the industrial applicability of the proposed model.by Rebecca B. Dupaix.Ph.D

NBS monograph

From Abstract: "This compilation abstracts original experimental data on the mechanical, thermal, and electrical properties of six commercially available polymers. The data are also summarized and a brief description of each polymer is included.

Rotary Jet Spinning of Polymer Fibres

Polymeric nanofibres can be produced from a variety of methods such as electrospinning and melt blowing, with fibres being produced having applications in many sectors such as biomedicine, composites and filtration. Existing methods are not however capable of producing nanofibres to commercial volumes in an energy efficient way. In this research we investigate a new method of producing nanofibres, namely Rotary Jet Spinning (RJS), which is a relatively new method of fibre production similar to candyfloss production, where centrifugal forces are used to expel jets of polymer from a state of melt or solution in order to produce polymeric fibres. We investigate this method in detail, initially concentrating on the comparison between electrospinning and RJS. Firstly, it was found that electrospinning produced slightly smaller fibre diameters compared to RJS over a broader range of solution concentrations. Secondly, the ability to produce high modulus fibres was investigated by means of an imidization technique, where polyamic acid solution was produced and spun into fibres before conversion to a co-polyimide fibre with an elastic modulus of around 40 GPa. In the third experimental chapter, the viscosity reliability of the RJS process was evaluated by means of computational fluid dynamics simulations, where it was shown that low viscosity (1-10 Pa.s) Newtonian fluids are required to establish fibre production. For fluids with lower viscosities, beading occurred in solution spinning and droplets were produced from melt spinning. Viscosities higher than the recommended value resulted in blockage, with no fibres being produced from either method. Lastly, the production of ceramic fibres was evaluated to establish the ability of the RJS process to produce a ceramic nanofibre. Fibres on the nanoscale were not achieved, however a variation in solvent volatility and crosslinking time were factors in fibre diameter reduction, with solvent variations highlighting the potential of this process to achieve the required fibre size from RJS and thereby demonstrating this technology as a viable option for high volume fibre production.EPSRC grant number 150219

Process–Structure–Properties in Polymer Additive Manufacturing

Additive manufacturing (AM) methods have grown and evolved rapidly in recent years. AM for polymers is an exciting field and has great potential in transformative and translational research in many fields, such as biomedical, aerospace, and even electronics. Current methods for polymer AM include material extrusion, material jetting, vat polymerisation, and powder bed fusion. With the promise of more applications, detailed understanding of AM—from the processability of the feedstock to the relationship between the process–structure–properties of AM parts—has become more critical. More research work is needed in material development to widen the choice of materials for polymer additive manufacturing. Modelling and simulations of the process will allow the prediction of microstructures and mechanical properties of the fabricated parts while complementing the understanding of the physical phenomena that occurs during the AM processes. In this book, state-of-the-art reviews and current research are collated, which focus on the process–structure–properties relationships in polymer additive manufacturing

Liquid crystalline polymers

The remarkable mechanical properties and thermal stability of fibers fabricated from liquid crystalline polymers (LCPs) have led to the use of these materials in structural applications where weight savings are critical. Advances in processing of LCPs could permit the incorporation of these polymers into other than uniaxial designs and extend their utility into new areas such as nonlinear optical devices. However, the unique feature of LCPs (intrinsic orientation order) is itself problematic, and current understanding of processing with control of orientation falls short of allowing manipulation of macroscopic orientation (except for the case of uniaxial fibers). The current and desirable characteristics of LCPs are reviewed and specific problems are identified along with issues that must be addressed so that advances in the use of these unique polymers can be expedited

Modified poly(lactic acid) sheets manufactured by one-step reactive extrusion-calendering : thermal, rheological, mechanical and fracture behaviours

The purpose of this PhD thesis was to investigate the sensitivity of the rheological, thermal, mechanical and fracture behaviours on the topological changes induced in two commercial PLA grades. PLA 2002D and PLA 4032D (NatureWorks, Belgium) with a D-lactide content of 4.25 and 2 %, respectively, were reactively modified in the melt with predetermined concentrations of a multi-functional epoxide agent. In a preliminary study, the evolution of the coupling reactions was followed by monitoring the torque versus time in an internal mixer device. Once all parameters were optimized, reactive extrusion was performed in a co-rotating twin screw extruder with a screw diameter of 25 mm (L/D=36). The main emphasize of this work was on the production of modified PLA sheets (thickness: 1 mm, width: 100 mm) through a one-step reactive extrusion-calendering process in pilot plant. Under similar processing conditions, the higher the D-lactide content, the lower was the reactivity of the PLA type towards the reactive agent. During processing, a competition between degradation and chain extension/branching reactions took place which led to a stabilization of the melt properties and an increase in the molecular weight (MW). Classical spectroscopic (FT-IR, NMR) and chromatographic (SEC-multiple detectors) methods failed to highlight structural differences between unmodified (PLA) and modified (PLA-REX) samples. On the other hand, rheological properties were significantly modified and suggest the formation of non-uniform branched structures, which include sparsely long chain branching (LCB) macromolecules. Both the melt elasticity and the melt response time increased. Bi-modal molecular weight distribution (MWD) spectra were inferred from the complex viscosity functions of PLA-REX samples which exhibit a double curvature in the shear-thinning regime. Based on these MWDs, a procedure is suggested to quantitatively estimate the amount of modified PLA chains from solely melt measurements. Thus, rheological measurements turned out to be a valuable tool for the detection and the quantification of small contents of topological changes in the work at hand. Thermal properties were slightly modified with the modification of the chain architecture. Upon constant heating, the maximum rate of conversion declined for PLA-REX samples; thus leading to a decrease in the degree of cold-crystallization. Thermal fractionation, according to the successive self-nucleation and annealing methodology, suggests a modification of the length distribution of crystallisable PLLA chain segments. In the present work, These trends mainly accounted from the increase in MW coupled with the enhanced concentration of defects into the polymeric chains (e.g. branch point). In the Tg region, the rate of enthalpy relaxation was found to decrease with increasing the entanglement network density under controlled aging periods at 30 °C. For a given thermal history, PLA-REX yielded comparative mechanical properties as those of PLA samples under uniaxial tensile loading. This is an important finding, because rheological properties (i.e. melt elasticity, viscosity, etc) may be controlled independently of the mechanical properties according to both the processing conditions and the concentrations of reactive agent used in the current study. While de-aged samples behaved in a ductile manner, aged samples were brittle. This behaviour was attributed to the effect of the physical aging process on the mechanical behaviour which commonly promotes the ductile/brittle transition. Regarding de-aged samples, the essential work of fracture analysis revealed no changes in the typical fracture parameters. Regarding aged PLA-REX samples, the energy consumed up to the onset of crack propagation was found to decrease due to an apparently decreased network extensibility, promoting a premature craze-crack transition.El propósito de esta tesis doctoral ha sido investigar los efectos inducidos por cambios topológicos en dos grados comerciales de PLA en sus comportamientos reológicos, térmicos, mecánicos y a fractura. PLA 2002D y PLA 4032D con un contenido de enantiomero D igual a 4,25 y 2%, respectivamente, fueron modificados a través de un proceso de extrusión reactiva con concentraciones predeterminadas de un agente multifuncional con grupos epóxidos reactivos. En un estudio preliminar, la evolución de las reacciones de acoplamiento fue registrada a través del seguimiento del par de fuerzas en función del tiempo de mezcla en un dispositivo de mezclador interno. Una vez todos los parámetros optimizados, el proceso de extrusión reactiva se realizó en una extrusora doble husillo co-giratorio con un diámetro de usillo igual a 25 mm (L/D = 36). El principal objetivo de este trabajo, fue la producción de láminas de PLA modificadas estructuralmente a través de un proceso simultáneo de extrusión reactiva-calandrado en un solo paso. Bajo condiciones similares de procesamiento, cuando el contenido de enantiomero D sea mayor, la reactividad del tipo de PLA será menor hacia el agente reactivo. Durante el procesamiento, una competición entre degradación, extensión de cadenas y reacciones de ramificación conlleva a una estabilización de las propiedades del fundido y a un aumento en el peso molecular. Mientras que los métodos clásicos de espectroscopia (FT-IR, NMR) y cromatográfico (SEC-múltiples detectores) fallaron en resaltar cambios arquitecturales, las propiedades reológicas confirmaron la formación de estructuras ramificadas no uniformes, que incluyen escasamente ramificaciones largas. Tanto la elasticidad como la respuesta del fundido en el tiempo aumentaron. Los espectros de distribución de peso moleculares (MWD) bi-modal se infirieron de las funciones de viscosidad compleja de las muestras de PLA-REX las cuales presentan una doble curvatura en el régimen pseudo-plástico. En base a estos MWD, se sugiere un procedimiento, para estimar cuantitativamente la cantidad de cadenas de PLA modificadas a partir de mediciones realizadas únicamente en el fundido. Las propiedades térmicas se vieron ligeramente alteradas con la modificación de la arquitectura molecular. Tras un calentamiento constante, la tasa máxima de conversión para las muestras de PLA-REX se redujo; lo que conlleva a una disminución en el grado de cristalización en frío alcanzable. El fraccionamiento térmico, de acuerdo con la metodología de auto-nucleación y recocidos sucesivos (SSA), sugiere una modificación de la distribución de la longitud de los segmentos cristalizables de cadena de PLLA. En la región de Tg, los resultados sugieren una reducción de la cinética de envejecimiento físico con un aumento en la densidad de enredos físicos bajo períodos de envejecimiento controlados a 30 ° C. Para una historia térmica dada, las muestras de PLA-REX exhiben propiedades mecánicas comparativas a las muestras de PLA. Este podría ser considerado un hallazgo importante, puesto que las propiedades reológicas (elasticidad del fundido, viscosidad, etc.) pueden ser controladas de forma independiente a las propiedades mecánicas de acuerdo a las condiciones de procesamiento y a las concentraciones de agente reactivo utilizadas en el estudio actual. Mientras que las muestras rejuvenecidas presentaron un comportamiento dúctil, las muestras envejecidas resultaron frágiles. Este comportamiento, se atribuyó al efecto del proceso de envejecimiento físico en el comportamiento mecánico que promueve la transición dúctil/frágil. En las muestras rejuvenecidas, el trabajo esencial de fractura no reveló ningún cambio en los parámetros típicos de fractura. Por otra parte las muestras envejecidas de PLA-REX, la energía consumida hasta el inicio de la propagación de la grieta disminuyó debido a una disminución de la extensibilidad aparentemente de la red molecular, promoviendo una transición prematuraPostprint (published version