MANIPULATING SINGLE POLYMER MOLECULES FOR APPLICATIONS IN NANOMATERIALS

Polymeric nanoparticles have been utilized in an increasing number of fields over the past two decades due to their unique properties such as design flexibility and good biocompatibility. Despite various techniques available to produce polymer nanoparticles, the preparation of small nanoparticles with customized functions in the sub 20 nm dimension remains challenging. Inspired by the self-organizing behavior of natural biomacromolecules, a class of single-chain nanoparticles (SCNP) are synthesized featuring biomimicry and ultrafine size. These nanoparticles are prepared from self-folding of polymer precursors bearing reactive pendant groups by intramolecular cross-linking reactions. A variety of cross-linking chemistries are available including covalent, dynamic covalent and non-covalent chemistries. Among these methods intramolecular polymerization is of particular importance as it allows for easy control of an SCNP’s degree of cross-linking, and lead to SCNP with tunable level of compaction. The aim of this dissertation is to 1) provide a comprehensive overview of recent advances in the field of single-chain folding; 2) investigate the synthesis of SCNP by intramolecular polymerizations, and 3) study the synthetic variations relating to the efficiency of a polymer’s self-folding by intramolecular polymerization. Chapter 2 of this work discusses the synthesis of poly(oxanorbornene imide) single-chain nanoparticles by intrachain radical polymerization of pendant methacryloyl units. Structure/ property relationships related to methacryloyl pendant length and percent incorporation were studied. Chapter 3 investigates the synthesis of an epoxide-maleimide bifunctional monomer, and its ring-opening polymerization to afford polyethyleneglycol based polymer precursor. The polymer precursor could undergo intramolecular radical polymerization to afford SCNP, and the cross-linked moiety could potentially be isolated for the study of degree of intrachain polymerization. Chapter 4 expands the scope of intrachain polymerization and explores the synthesis of SCNP by intramolecular ring-opening metathesis polymerization (ROMP). A series of poly(pentafluoro-methacrylate)s containing pendant norbornene imide groups was synthesized and subjected to intrachain ROMP. The efficiency of chain folding was explored relating to norbornene content on the polymer precursor, species and feed ratio of Grubbs catalysts, as well as doping effects of fluorinated aromatic comonomer

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