Contribution à l'étude des mécanismes de déformation élastique et plastique du polyéthylène et polypropylène en fonction de la microstructure et de la température

The mechanical properties of semicrystalline polymers in relation to microstructure have been the subject of a large number of studies. However, there are still some unresolved issues, for instance, plastic deformation mechanisms in elevating temperature, intrinsic properties of interlamellar amorphous phase, local stress distribution in spherulites etc. The aim of this thesis is to address these issues in the case of PE and PP in different temperatures. A series of PE and PP samples with wide range of microstructures thanks to various thermal treatments were characterized by DSC, SAXS, WAXS and Raman spectroscopy. In elastic domain, the local strain εlocal in equator and polar regions of spherulites were measured by in situ SAXS. The ratio is a constant which only depends on drawing temperature. This ratio was used in a mechanical modelling as a transition factor from mesoscopic to macroscopic scale. Furthermore, the apparent modulus of the interlamellar amorphous phase Ma was estimated by the measured local stress and strain. The Ma of PE was found to be in the range 250 - 500 MPa which is surprisingly high comparing with the modulus of bulk rubbery PE. In the plastic domain, cavitation, martensitic transformation and crystal shear were observed by in situ SAXS and WAXS and their respective strain onsets were shown to be strongly dependent on crystallite thickness and temperature. It was found that competition exists between these plastic mechanisms. With increasing temperature, cavitation gradually disappears and martensitic transformation is delayed. A map for the onset of these plastic mechanisms was produced. In addition, the fibrillar structure induced by drawing at different temperature was studied by in situ SAXS. The long period and diameter of micro-fibrils proved to be dependent on the drawing temperature and also the initial structure via the melting-recrystallization and fragmentation-rearrangement mechanism. Similar investigations were performed with PP.Les propriétés mécaniques des polymères semi-cristallins en relation avec leur microstructure ont fait l'objet d'un grand nombre d'études. Cependant, il reste encore des questions non résolues, concernant les mécanismes de la déformation plastique à température élevée, les propriétés intrinsèques de la phase amorphe interlamellaire ou encore la distribution des contraintes locales dans les sphérolites… L'objectif de cette thèse est d‘adresses ces questions dans les cas du PE et du PP en fonction de la température. Pour atteindre cet objectif, une série d'échantillons de PE et PP des différents microstructures ont été préparées et caractérisés par DSC, SAXS, WAXS et spectroscopie Raman. Dans le domaine élastique, la déformation locale (εlocal) dans les régions équatoriales et polaires des sphérolites ont été mesurés par SAXS in situ. Le ratio εlocal / εmacro a été utilisé dans un modèle mécanique permettant le changement d‘échelle de mésoscopique macroscopique. En outre, le module apparent de la phase amorphe interlamellaire Ma a été estimé par la contrainte et la déformation locale. Les Ma valeurs du PE sont dans la gamme 250 - 500 MPa, ce qui est très élevé par rapport au module du PE amorphe caoutchoutique. Dans le domaine plastique, la cavitation, la transformation martensitique et le cisaillement du cristal ont été observées par SAXS et WAXS in situ. Une concurrence entre ces mécanismes plastiques a été mise en évidence. L‘augmentation de la température entraine une disparition progressive de la cavitation et un retard de la transformation martensitique vers de plus haute déformation. La structure fibrillaire, induite par étirage à différentes températures a été étudiée par SAXS in-situ. Il a été observé que la longue période et le diamètre des micro-fibrilles dépendent de la température d‘étirage de la structure initiale, via les mécanismes de « fusion-recristallisation » et « fragmentation-réarrangement ». Une étude similaire a été effectuée pour le PP

Micro/macro-stress relationship and local stress distribution in polyethylene spherulites upon uniaxial stretching in the small strain domain

International audienceLocal stress of polyethylene spherulites under uniaxial stretching in small strain domain has been investigated by in-situ wide angle X-ray diffraction (WAXD) in comparison with macroscopic stress. Particular attention is paid to the elastic strain range. The local stress in equatorial and polar regions both increase in absolute value and follow a linear relation versus macroscopic stress. However, local stress turns out to be heterogeneous over the whole spherulites volume. The polar region shows higher local stress than equatorial region. Moreover, local stress along the a and b crystallographic axes in the crystalline lamellae enable determining the local stress triaxiality which is heterogenous. The increase of crystallinity promotes stress distribution heterogeneity throughout the spherulites. The origin of these phenomenon are discussed in terms of crystalline network percolation and effect of amorphous layer that modifies the local mechanical behavior. The present study provides a useful means for achieving the scale transition between the micro and the macro structural levels that is necessary for the modeling of the mechanical behavior of semi-crystalline polymers via micromechanical approaches. (C) 2018 Elsevier Ltd. All rights reserved

Scalable production of structurally colored composite films by shearing supramolecular composites of polymers and colloids

Abstract Structurally colored composite films, composed of orderly arranged colloids in polymeric matrix, are emerging flexible optical materials, but their production is bottlenecked by time-consuming procedures and limited material choices. Here, we present a mild approach to producing large-scale structurally colored composite films by shearing supramolecular composites composed of polymers and colloids with supramolecular interactions. Leveraging dynamic connection and dissociation of supramolecular interactions, shearing force stretches the polymer chains and drags colloids to migrate directionally within the polymeric matrix with reduced viscous resistance. We show that meter-scale structurally colored composite films with iridescence color can be produced within several minutes at room temperature. Significantly, the tunability and diversity of supramolecular interactions allow this shearing approach extendable to various commonly-used polymers. This study overcomes the traditional material limitations of manufacturing structurally colored composite films by shearing method and opens an avenue for mildly producing ordered composites with commonly-available materials via supramolecular strategies

Structure-Tunable Construction of Colloidal Photonic Composites via Kinetically Controlled Supramolecular Crosslinking

Colloidal photonic composites (CPCs) combine a unique array of colloidal particles (CPs) with a polymer matrix and exhibit intriguing optical and mechanical properties strongly depending on their structures. One-step construction of CPCs with tunable structures is crucial for enriching their properties and matching application requirements, which is highly desirable yet challenging. Here, we present a general strategy for CPC construction with tunable structures from short-range to long-range order by one-step kinetically controlling the supramolecular crosslinking between CPs and supramolecular oligomers. Importantly, the assembly process is monitored in situ and the key factors for structural regulation, i.e., the critical volume fraction of CPs and the structural transition from crystal growth to lattice compression, are disclosed, which play critical roles in obtaining CPCs with a wide range of controllable structures. The as-obtained CPCs exhibit structural colors with different angle dependencies, versatile mechanical strengths, and appealing mechanochromic and self-healing capabilities. This work provides insights into the one-step construction of structure-tunable photonic materials, opening up exciting avenues for novel solution-processable photonics. © 2022 American Chemical Society. All rights reserved.11Nsciescopu

Preparation of Poly(butylene succinate) Crystals with Exceptionally High Melting Point and Crystallinity from Its Inclusion Complex

We successfully developed an effective strategy for preparing poly­(butylene succinate) (PBS) material with exceptionally high crystallinity (94.4% from density data) and melting point (∼136 °C) from its urea inclusion complex under normal air pressure for the first time. The strategy consists of three steps: co-electrospinning PBS with urea, thermal treatment, and coalescence of PBS in water. Electrospinning provides well physical mixing between PBS and urea. Thermal treatment causes the assembly of PBS and urea and leads to formation of their inclusion complex, in which PBS chains are separately isolated in urea channels. During the coalescing process the isolated and ordered PBS chains formed crystals with lamellar thickness larger than 26 nmlarger than 5-fold of common PBS crystalafter removing host urea frames. PBS crystals obtained with such high crystallinity and melting point are proved to be the extended-chain crystals (ECCs). Comparative study reveals that the electrospinning and thermal treatment process are necessary in this strategy for preparation of ECC although many other methods can be also used to obtain inclusion complex. Further study shows that the strategy is also feasible for obtaining ECCs of other aliphatic polyesters