Novel aspects in the crystallization of polybutene-1

Semicrystalline polymers cover over two thirds of commercially produced polymeric materials, and have been widely applied to many areas of the modern society, including building and construction, electronics, packaging, etc. Understanding the behavior of polymer crystallization is of critical importance due to the significant impact of the crystallization process on the properties of materials. Polymorphism is the ability of a polymer, in analogy with low molecular mass substances, to crystallize in different modifications, characterized by different crystal structures (polymorphic forms). Much effort has been made to find proper methods to develop different crystal modifications for polymorphic polymers. However, it is still a challenge to control the polymorphic outcome of the crystallization process. Furthermore, semicrystalline polymers are composed of stacked crystalline lamellae and entangled amorphous polymeric chain segments in between them. The interaction between amorphous and crystalline phases plays an important role in determining final mechanical and transport properties. Despite this importance, the effect of polymorphism on the amorphous phase is not well clarified because of its complexity. In this thesis, one typical polymorphic polymer, polybutene-1 (PB-1), was selected for a detailed crystallization study. Polybutene-1 is one of the most investigated polymorphic polymers. It has applications in pipes and films with a service life up to 50~100 years due to its excellent mechanical properties. In practice, among all crystalline polymorphisms within PB-1, Forms I and II are the most relevant modifications from the processing perspective. With the goal to establish a comprehensive understanding of the heterogeneous nucleation between these two modifications (cross-nucleation), we monitor the crystallization process of Form II induced by Form I crystals with different type of substrate (spherulitic, hedritic, fiber-like) using a direct investigation technique of optical microscopy. The different cross-nucleation efficiencies of Form II are tentatively attributed to differences in the Form I lamellar thickness, on the basis of an epitaxial crystallization and secondary nucleation mechanism. A quantitative analysis of the induction time for nucleation determined the cross-nucleation energy barrier, which could be reasonably described by classical models. The results revealed that the rate determining step for nucleation is the growth of the nucleus to critical sizes. Furthermore, the hypothesized epitaxy in PB-1 Form II-on-Form I cross-nucleation is probed by employing in-situ nanofocused synchrotron X-ray diffraction. Comparing the two-dimensional diffraction patterns at the interface between the two modifications, a preferred mutual orientation of the two structure, with the (200)II plane aligned ~8.5\uba apart from the (110)I plane, is revealed. This demonstrates a parallel (110) plane between the two polymorphs. Then, both mismatches between the inter-chain distances and along the chain axes within (110) plane were considered, and found to lay well-below the accepted mismatch criterium for epitaxy. This confirms that the cross-nucleation of Form II on Form I occurs at the (110) contact planes through epitaxial nucleation. Next, an in-depth study of fiber-induced nucleation ability and crystalline morphology in polybutene-1/single fiber composites is presented. Using different fibers as substrates, we could unveil the difference of Form II crystalline morphology: a transcrystalline layer (TCL) induced by PB-1 Form I fiber and hybrid shish\u2013calabash structure (HSC) induced by other fibers, namely carbon, glass, PP, PLLA homocrystal and stereocomplex. Based on a quantitative analysis of the nucleation kinetics, it was found that the nucleation free energy barrier is affected both by surface roughness and surface chemistry or specific surface-polymer interactions (such as epitaxy). In view of the number of nucleation sites correlating with the fiber surface roughness, it was demonstrated that transcrystallinity can be obtained only when a sufficient amount of nucleation sites is available, notwithstanding the height of the nucleation barrier. Besides the phenomenon of heterogeneous nucleation, the three-phase structure is also influenced by crystal polymorphism. Therefore, in the last part of the thesis, we focused on the study of the rigid amorphous fraction in both polymorphs, i.e., the part of amorphous chains constrained by direct coupling to the crystalline lamellae. Isochronous aging experiments with differential scanning calorimetry (DSC) on both crystalline phases are performed, in a wide temperature range between the glass transition of the mobile (bulk) amorphous fraction and the onset of crystal melting. An endothermic peak above the aging temperature is typically observed. The trend of the enthalpy of this annealing peak with temperature can be described by a bell-shaped curve, approaching zero recovered enthalpy at temperatures of 100-110 \ub0C, and 40-50 \ub0C for Form I and Form II, respectively. These temperatures are thus identified as the upper limit of the glass transition of rigid amorphous fraction for the two polymorphs. Overall, our results demonstrate that for PB-1, at least within the investigated temperature range, the annealing peaks can be related to the RAF strive to attain thermodynamic equilibrium in its glassy state

Heterogeneous Nucleation in Semicrystalline Polymers

Nucleation of polymer crystals is a key issue in polymer science and technology. Indeed, it is of outmost importance for industrial application of semicrystalline polymers, since the nucleation rate often dictates the processing time of the products, and strongly affects the resulting mechanical or optical properties of the material. Despite this a comprehensive understanding of the phenomenon is still lacking, as testified for instance by the fact that the scouting of new heterogeneous nucleating agents for polymers is still mostly driven by empiricism. With this thesis, we aim to provide novel quantitative approaches to quantify the heterogeneous nucleation efficiency of different surfaces in various, industrially relevant, systems, such as polymer/fiber composites or nucleated polymers. The presented results are a contribution towards the clarification of the mechanism of heterogeneous nucleation in semicrystalline polymers.The nucleation process of biodegradable poly(lactide) on a series of fibers, including commercially available fibers, natural fibers and a custom-spun fiber of stereocomplex enantiomeric PLA blend, was studied by polarized optical microscope during crystallization. The nucleation ability of different fiber substrates was derived and compared in the light of classical heterogeneous nucleation theory, by considering the interfacial free energy difference parameter, \u394\u3c3, directly related to the nucleation barrier. The role of fiber surface topography and chemical interactions between the fiber substrate and the crystallizing polymer in promoting the nucleation was investigated and discussed in detail. While a general effect of surface roughness on lowering the heterogeneous nucleation energy barrier can be deduced, the role of chemical interactions between the fiber substrate and the crystallizing polymer cannot be neglected.Furthermore, a novel approach was proposed to quantitatively evaluate the nucleation efficiency of several additives for isotactic polypropylene (i-PP), using droplets containing nucleating agents (i.e., sodium benzoate, NA11, quinacridone quinone) dispersed in an immiscible polystyrene matrix. The crystallization was investigated by isothermal step crystallization and melting with Differential Scanning Calorimetry (DSC). Moreover, self-nucleation of neat i-PP droplets is also studied in detail, enabling to derive an \u201cintrinsic\u201d nucleation efficiency scale based on the ratio of the free energy barrier, \u394G*, of heterogeneous nucleation on different substrates with respect to that of self-nucleation, which is found equivalent to the secondary nucleation barrier for crystal growth. Having established the interfacial free energy difference parameter, governing the heterogeneous nucleation kinetics of i-PP onto different substrates, a simple correlation curve useful to describe non-isothermal fractionated crystallization of i-PP/PS blends with droplet morphology was constructed.Finally, we propose a Differential Scanning Calorimetry (DSC) approach for the quantitative investigation of isotactic poly(1-butene) Form II-on-Form I cross-nucleation. Seeds of trigonal Form I crystal were produced in PB1 samples, and their amount and characteristic size were varied by using different crystallization conditions. DSC isothermal and non-isothermal crystallization measurements of Form I seeded samples were performed, highlighting a clear nucleation effect of the stable polymorph on Form II. Moreover, the nucleating efficiency is highly dependent on the content of Form I seeds, more specifically, on the area of Form I spherulites per unit of sample volume. Depending on the seeding and crystallization conditions, Form II crystallization is controlled either by nucleation on foreign heterogeneous surfaces or on Form I crystals

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

The role of nucleating agents on flow-induced crystallisation of polymers

Isotactic polypropylene (iPP) is one of the widely used commercial thermoplastics. Physical properties of iPP can be tailored to the requirements with respect to structure, microstructure and processing, thus research continues in the development and modification of the polymer. With the advancement of chemistry, as our understanding in tailoring of the molecular structure has enhanced, iPP has become more of a generic name. [Continues.

Polymeric materials with multiple crystalline phases: structure, morphology and crystallization

303 p.El objetivo de esta tesis doctoral es el análisis y la comparación de sistemas poliméricos multifásicos con al menos dos fases cristalizables, para diseñar y desarrollar nuevos materiales con propiedades mejoradas. La primera parte se centra en el estudio de mezclas PET/HDPE doblemente cristalinas con nanopartículas de titanio dióxido (tres tipos) y agentes compatibilizantes. La reducción de tamaño de partícula como consecuencia de la adición de las nanopartículas mejora las propiedades mecánicas.La segunda parte del trabajo consiste en el estudio de copolímeros de bloques multicristalinos. La adición de un tercer y cuarto bloque cristalino hace el análisis mucho más desafiante, pero se han identificado las cristalizaciones de todos los bloques en: terpolímeros PE-b-PEO-b-PLLA y PE-b-PCL-b-PLLA, y tetrapolímeros PE-b-PEO-b-PCL-b-PLLA y sus correspondientes terpolímeros (PE-b-PEO-b-PCL) y demás precursores (PE-b-PEO y PE). El efecto de la velocidad de enfriamiento (20 vs. 1 ºC/min) en los terpolímeros PE-b-PEO-b-PLLA y PE-b-PCL-b-PLLA hace variar la secuencia de cristalización, siendo a20 ºC/min el bloque de PE el primero en cristalizar, mientras que a 1 ºC/min la cristalización comienza con el bloque de PLLA, finalizando en todos los casos con los bloques de PEO o PCL, lo que dará lugar a distintas morfologías y propiedades mecánicas. El estudio de los tetrapolímeros (PE-b-PEO-b-PCL-b-PLLA) y sus terpolímeros precursores (PE-b-PEO-b-PCL) mediante DSC, SAXS/WAXS y PLOM demuestra su naturaleza triple- y tetra- cristalina, siguiendo las secuencias de cristalización e identificando esferulitas triple- y tetracristalinas

Shear controlled orientation effects with injection mouldings produced by the SCORIM process

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Injection moulding using the process of Shear Controlled Orientation Injection Moulding (SCORIM) to enhance the aesthetic characteristics of plastics was investigated. Unsightly surface weld lines were successfully removed from highly reflective aluminium flake pigmented plastics by the application of a single macroscopic SCORIM shear when used in series with Bright Surface Moulding (BSM). A gonio spectrophotometer (GSP) was used for the quantitative characterisation of the Al flake pigmented mouldings as a measure of surface reflectivity and preferred angle of reflection. The different directional properties of surface reflectivities to either side of a conventional weld line are unacceptable, but were successfully reoriented approximately uniformly with the use of SCORIM and BSM moulding (i. e. SBM) used in series. SBM therefore provided an acceptable quality of surface finish for mouldings originally containing a weld line, without deterioration of mechanical properties. Indeed, some improvements in mechanical properties were observed. Translucent two-colour mouldings were used to successfully demonstrate the flow paths taken by sheared material during the application of macroscopic shears. The use of intermittent shearing to encapsulate shear oriented material in the solidifying layers, manifested original and profound aesthetic effects. This resulted from mixing the two colours and was reproducible and widely variable. The morphology of isotactic polypropylene (iPP) processed in this way and examined by light and electron microscopy revealed how only one or two intermittent shears were required to orient a large volume of the moulding in the shear direction. Moreover, U-shaped flow paths demonstrated that the easiest shear route was close to the mouldings edges, an observation supported by x-ray analysis. The addition of Al flake pigment was found to act as a heterogeneous nucleant for ß-spherulites. This acted as a suitable marker for the clear identification of the displaced weld interface using polarised light microscopy, of filled and unfilled iPP. y-phase was identified with the use of only one or two intermittent shears which reflects an increase in molecular alignment and consequent improved mechanical properties. The intensity of the y-phase increased with the volume of material sheared. Strong evidence was also obtained of a linear relationship between the logarithm of the time lapse between two intermittent shears and the corresponding values of a-phase index, crystallinity index and percentage crystallinity. The values of each increasing proportionally with the length of time used. Microhardness characterisation revealed anisotropy within SCORIM samples consistent with preferred orientation and increased modulus in the shear direction. The skin layers were characterised as the softest region through the thickness of SCORIM mouldings. The results of this work were used to provide the basis of a computer simulation of the SCORIM process under development at the University of Wales Swansea

Sequential Crystallization and Multicrystalline Morphology in PE‑b‑PEO‑b‑PCL‑b‑PLLA Tetrablock Quarterpolymers

Unformatted post-print version of the accepted articleWe investigate for the first time the morphology and crystallization of two novel tetrablock quarterpolymers of polyethylene (PE), poly(ethylene oxide) (PEO), poly(ε-caprolactone) (PCL), and poly(L-lactide) (PLLA) with four potentially crystallizable blocks: PE18 7.1-b-PEO37 15.1-b-PCL26 10.4-b-PLLA19 7.6 (Q1) and PE29 9.5-b-PEO26 8.8-b-PCL23 7.6-b-PLLA22 7.3 (Q2) (superscripts give number average molecular weights in kg/mol, and subscripts give the composition in wt %). Their synthesis was performed by a combination of polyhomologation (C1 polymerization) and ring-opening polymerization techniques using a ″catalyst-switch″ strategy, either ″organocatalyst/metal catalyst switch″ (Q1 sample, 96% isotactic tetrads) or ″organocatalyst/ organocatalyst switch″ (Q2 sample, 84% isotactic tetrads). Their corresponding precursorstriblock terpolymers PE-b-PEO-b-PCL, diblock copolymers PE-b-PEO, and PE homopolymerswere also studied. Cooling and heating rates from the melt at 20 °C/min were employed for most experiments: differential scanning calorimetry (DSC), polarized light optical microscopy (PLOM), in situ small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS), and atomic force microscopy (AFM). The direct comparison of the results obtained with these different techniques allows the precise identification of the crystallization sequence of the blocks upon cooling from the melt. SAXS indicated that Q1 is melt miscible, while Q2 is weakly segregated in the melt but breaks out during crystallization. According to WAXS and DSC results, the blocks follow a sequence as they crystallize: PLLA first, then PE, then PCL, and finally PEO in the case of the Q1 quarterpolymer; in Q2, the PLLA block is not able to crystallize due to its low isotacticity. Although the temperatures at which the PEO and PCL blocks and the PE and PLLA blocks crystallize overlap, the analysis of the intensity changes measured by WAXS and PLOM experiments allows identifying each of the crystallization processes. The quarterpolymer Q1 remarkably self-assembles during crystallization into tetracrystalline banded spherulites, where four types of different lamellae coexist. Nanostructural features arising upon sequential crystallization are found to have a relevant impact on the mechanical properties. Nanoindentation measurements show that storage modulus and hardness of the Q1 quarterpolymer significantly deviate from those of the stiff PE and PLLA blocks, approaching typical values of compliant PEO and PCL. Results are mainly attributed to the low crystallinity of the PE and PLLA blocks. Moreover, the Q2 copolymer exhibits inferior mechanical properties than Q1, and this can be related to the PE block within Q1 that has thinner crystal lamellae according to its much lower melting point.This work has received funding from MINECO through projects MAT2017-83014-C2-1-P and MAT2017-88382-P, from the Basque Government through grant IT1309-19, and from the ALBA synchrotron facility through granted proposal u2020084441 (March 2020). We would like to thank the financial support provided by the BIODEST project; this project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 778092

Dynamics and structure of hydrophilic polymers at the interface between two surfaces: Smart Sealing in Short Time

The comprehension of mechanisms that drive the dynamics of hydrophilic polymer chains at the interface between two surfaces is relevant for fundamental research and, at the same time, can bring useful applicative implications in industrial field. In particular, understanding all the involved processes at molecular level is useful to clarify and optimize the welding mechanism between hydrophilic polymers, identifying the key parameters which control an efficient sealing in a short time. In this framework, Polyvinil-alcohol (PVA) is selected as hydrophilic polymer and welding between polymer layers is carried out using the water as diffusivity enhancer. As water is a good solvent for PVA, water acts as an adhesive, triggering different processes at the interface. In our approach, we focused on the molecular comprehension of polymer-solvent interactions and their diffusion mechanisms, as well as the role of crystallization at interface. We observed that the welding process of PVA films is likely to involve not only the dynamics of the chains, and polymer dissolution by effect of water, but also the crystallization taking place at the interface including melting/recrystallization phenomena, nucleation and/or grown rate of the crystals, location of nuclei, etc.. By the individuation of the parameters affecting the final welding, this work lays the basis for implementing a more efficient welding process, tuning the sealing strength, and opens the way for the design of different films suitable for specific commercial needs

Development of Natural Fiber Reinforced Polylactide-Based Biocomposites

Le polylactide ou PLA est un polymère biodégradable qui peut être produit à partir de ressources renouvelables. Ce polyester aliphatique présente de bonnes propriétés mécaniques similaires à celles du polyéthylène téréphtalate (PET). Depuis 2003, du PLA à haut poids moléculaire est produit à l'échelle industrielle et commercialisé sous forme de grades amorphes et semicristallins pour diverses applications. L’amélioration de la cinétique de cristallisation du PLA est cruciale pour que ce biopolymère devienne compétitif face aux plastiques issus du pétrole et puisse les remplacer. D'autre part, la combinaison de fibres naturelles avec des matrices polymériques issues de ressources renouvelables, afin de produire des polymères composites entièrement biosourcés et biodégradables, a été une tendance marquée dans les activités de recherche au cours de la dernière décennie. Néanmoins les différences liées à la structure chimique, notamment observées dans la nature fortement hydrophile des fibres et le caractère hydrophobe des matrices thermoplastiques, représentent un inconvénient majeur pour les interactions fibre/matrice. Le but de la présente recherche a été d'étudier les interactions intrinsèques fibre/matrice dans des polymères composites issus de fibres naturelles et du PLA, préparés par un mélange à l’état fondu. Des fibres de lin courtes présentant une longueur nominale d’environ 1 mm ont été sélectionnées comme renfort; par la suite, des biocomposites contenant une charge de fibres faible à modérée ont été préparés par mélange direct des fibres avec la matrice à l’état fondu. La rupture des faisceaux de fibres pendant la mise en forme a conduit à d’importantes réductions dans la longueur et le diamètre des fibres. Le rapport de forme moyen a été diminué d'environ 50%. La cinétique de cristallisation au repos du PLA et des systèmes biocomposites a été examinée sous conditions isothermes et non isothermes. La capacité de nucléation des fibres de lin a été démontrée et la cristallisation du PLA a été effectivement accélérée en présence du renfort. Cette amélioration a été de plus contrôlée par la température à laquelle la cristallisation a eu lieu, la transition de la phase liquide à la phase solide étant thermodynamiquement favorisée par le degré de surfusion. Lors de la cristallisation, les propriétés viscoélastiques devraient être fortement influencées par le développement des cristallites et leur empiètement. La rhéométrie a été choisie comme technique appropriée pour étudier l'évolution de la viscosité complexe et des modules de conservation et de perte lors de la cristallisation du PLA et de ses biocomposites. L'optimisation des conditions expérimentales a été nécessaire afin de compenser adéquatement le vi rétrécissement du polymère, qui est un souci majeur pour la reproductibilité des mesures, en particulier à niveaux élevés de surfusion. Cette étude a démontré que la vitesse de cristallisation était accrue en présence des fibres de lin, et ce dans une large gamme de températures de cristallisation. Étant donné que le développement de la cristallisation dans les procédés à échelle industrielle peut différer grandement des études sous conditions au repos, une investigation préliminaire de l'effet de l'écoulement en cisaillement sur la cristallisation du PLA a été entreprise. En utilisant le même intervalle de vitesses de cisaillement, deux ensembles de conditions ont été étudiés, à savoir (1) déformation totale constante, et (2) temps de cisaillement constant. Dans les deux cas, l'accroissement de la cinétique de cristallisation a été démontré par une diminution du temps d'induction, laquelle est devenue plus importante aux vitesses de cisaillement les plus élevés. Environ 75% de réduction du temps d’induction a été observée à 4 s-1, le taux de cisaillement maximal atteint dans cette recherche. La réduction de la taille des particules de cellulose de l’échelle micro à l'échelle nanométrique a également attiré une attention croissante lors de la dernière décennie. Les réseaux bien dispersés de fibres nanométriques dans des matrices polymères peuvent apporter des améliorations extraordinaires dans la résistance du matériau et modifier les interactions particule/polymère au niveau moléculaire. En conséquence, la cristallisation peut être favorisée à très basses concentrations de renfort. Il est bien connu que la dispersion des nanocristaux de cellulose (NCC) dans les systèmes non aqueux est un défi majeur pour développer davantage ces promesses nanométriques. Dans ce travail, un nouveau procédé composé de deux étapes: le mélange en solution suivi par le malaxage à l'état fondu, a montré d’excellents résultats dans la dispersion de faibles charges de NCC dans le PLA. L'oxyde de polyéthylène (PEO) à bas et haut poids moléculaires a été proposé comme un polymère porteur de nanocristaux et l'encapsulation des NCC dans le PEO a été réussie. La réduction de la taille des agglomérats a été contrôlée par l'augmentation du rapport en poids PEO:NCC. Un effet synergique entre la plastification et le renforcement de la matrice PLA a été clairement mis en évidence par le comportement cristallin des nanocomposites. La méthode de préparation de nanocomposites issus de PLA, NCC et PEO, présentée dans cette thèse, représente un pas en avant dans les applications potentielles de ces nanoparticules dans les matériaux composites verts. ----------- Polylactide or PLA is a biodegradable polymer that can be produced from renewable resources. This aliphatic polyester exhibits good mechanical properties similar to those of polyethylene terephthalate (PET). Since 2003, bio-based high molecular weight PLA is produced on an industrial scale and commercialized under amorphous and semicrystalline grades for various applications. Enhancement of PLA crystallization kinetics is crucial for the competitiveness of this biopolymer as a commodity material able to replace petroleum-based plastics. On the other hand, the combination of natural fibers with polymer matrices made from renewable resources, to produce fully biobased and biodegradable polymer composite materials, has been a strong trend in research activities during the last decade. Nevertheless, the differences related to the chemical structure, clearly observed in the marked hydrophilic/hydrophobic character of the fibers and the thermoplastic matrix, respectively, represent a major drawback for promoting strong fiber/matrix interactions. The aim of the present study was to investigate the intrinsic fiber/matrix interactions of PLA-based natural fiber composites prepared by melt-compounding. Short flax fibers presenting a nominal length of ~1 mm were selected as reinforcement and biocomposites containing low to moderate fiber loading were processed by melt-mixing. Fiber bundle breakage during processing led to important reductions in length and diameter. The mean aspect ratio was decreased by about 50%. Quiescent crystallization kinetics of PLA and biocomposite systems was examined under isothermal and non-isothermal conditions. The nucleating nature of the flax fibers was demonstrated and PLA crystallization was effectively accelerated as the natural reinforcement content increased. Such improvement was controlled by the temperature at which crystallization took place, the liquid-to-solid transition being thermodynamically promoted by the degree of supercooling. During crystallization, viscoelastic properties are expected to be strongly influenced by crystallite development and impingement. Rheometry was selected as a suitable technique to study the evolution of complex viscosity and storage and loss moduli during the crystallization of compounded PLA and PLA-biocomposites. Optimization of experimental conditions was needed for achieving the compensation of polymer shrinkage, which was a major concern for the reproducibility of measurements, particularly at high supercooling level. Fruitful information about the enhanced crystallization rate due to the presence of flax fibers in a wide viii range of crystallization temperatures was obtained from this study. Since development of crystallization in industrial processing may differ greatly from quiescent studies, a preliminary investigation of the effect of shear flow on the improvement of PLA crystallization was carried out. Using the same shear rate interval, two different sets of conditions were explored, namely (1) constant total deformation and (2) constant shearing time. In both cases, the crystallization enhancement was evidenced by a decrease in the induction time which became stronger as shear rate augmented. About 75% of reduction was observed at 4 s-1, the maximum shear rate reached in this research. The size reduction of cellulose particles from micro to the nanoscale has also drawn special attention over the last decade. Well-dispersed nanosized fiber networks into polymeric matrices may bring extraordinary strength enhancement and modify the particle/polymer interactions at the molecular level. As a consequence, crystallization may be promoted at considerably low concentrations of reinforcement. It is well-known that dispersion of cellulose nanocrystals (CNC) in non-aqueous systems is a major challenge for further developments. In this work, a novel two-step process involving solvent-mixing and melt-mixing was found to successfully dispersed cellulose nanocrystals at low weight loadings in the PLA matrix. Polyethylene oxide (PEO) of high and low molecular weight was proposed as a polymer carrier for nanocrystals, and the encapsulation of CNC in this polymer was achieved. Reduction of agglomerate size was controlled by the increase of PEO:CNC weight content ratio in the final nanocomposites. A synergistic effect between plasticization and reinforcement of the PLA matrix was clearly evidenced from the crystallization behavior of nanocomposites. The PLA nanocomposite preparation method presented in this dissertation represents a step forward in the potential applications of CNC in green composite materials