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I have used computer simulations to investigate the behavior of polymeric materials at the molecular level. The simulations were performed using the molecular dynamics method with Lennard-Jones potentials defining the interactions between particles in the system. Significant effort was put into the creation of realistic materials on the computer. For this purpose, an algorithm was developed based on the step-wise polymerization process. The resulting computer-generated materials (CGMs) exhibit several features of real materials, such as molecular weight distribution and presence of chain entanglements. The effect of the addition of a liquid crystalline (LC) phase to the flexible matrix was also studied. The concentration and distribution of the second phase (2P) were found to influence the mechanical and tribological properties of the CGMs. The size of the 2P agglomerates was found to have negligible influence on the properties within the studied range. Moreover, although the 2P reinforcement increases the modulus, it favors crack formation and propagation. Regions of high LC concentration exhibit high probability of becoming part of the crack propagation path. Simulations of the tensile deformation under a uniaxial force have shown that the molecular deformation mechanisms developing in the material depend on several variables, such as the magnitude of the force, the force increase rate, and the level of orientation of the chains. Three-dimensional (3D) graphical visualization tools were developed for representation and analysis of the simulation results. These also present interesting educational possibilities. Computer simulations provide us information which is inaccessible experimentally. From the concomitant use of simulations and experiments, a better understanding of the molecular phenomena that take place during deformation of polymers has been established.

L'objectif de cette thèse est de créer de nouveaux mélanges thermoplastiques présentant une processabilité accrue comparée à des matrices thermoplastiques hautes performances utilisées dans l'industrie aéronautique. Ces matériaux ont vocation à être intégrés en atmosphère avionique pressurisée. Afin de combler le cahier des charges imposé par le domaine aéronautique, le choix des matériaux s'est porté sur un mélange incompatible de polyétheréthercétone PEEK et de polymères à cristaux liquides LCP présentant une morphologie fibrillaire développée grâce à des conditions particulières d'écoulement lors de la mise en oeuvre.Dans un premier temps, les propriétés rhéologiques des matériaux sont caractérisées de manière à confirmer le respect des spécifications relatives à leur processabilité. La compréhension des phénomènes régissant la baisse de viscosité du mélange permet d'assurer la répétabilité et la reproductibilité des performances rhéologiques en vue d'un transfert industriel. L'étude se concentre ensuite sur les propriétés de cristallinité des mélanges, qui permettent de déterminer que la présence de deux matériaux semi-cristallins au sein d'une même structure ne perturbe pas leurs propriétés de cristallinité garantes de leurs performances thermomécaniques.Enfin, une campagne de caractérisation complète est effectuée afin de dresser une fiche matière et de la comparer aux spécifications exigées par l'industrie aéronautique. / The aim of this thesis is to create new thermoplastic blends exhibiting improved processability incomparison with high performance aeronautical thermoplastics. These materials are dedicated tointegrating pressurized avionic structures.To reach the specific aeronautical specifications, an incompatible polymer blend made ofpolyetheretherketone PEEK and liquid crystalline polymers LCP is chosen. The particular flow conditionsset for the compounding ensure the blend a fibrillar morphology.First, the material rheological properties are characterized so as to confirm that the processabilityspecifications are met. The viscosity drop causes are explained and grant the maintaining of therheological performances in any processing configuration.The study then focuses on the blend crystallinity properties in order to determine whether two interlinkedsemi-crystalline structures do not impede the thermomechanical performances.At last, a characterization campaign is led to compare the blend performances with the demandedaeronautical specifications.

Le lien entre la structure semi-cristalline et les propriétés d’usage (mécaniques, diélectriques et électro-actives) de films de copolymères fluorés électro-actifs développés pour des applications en électronique organique imprimée a été étudié. Les matériaux investigués sont des copolymères poly(VDF-co-TrFE) et des terpolymères poly(VDF-ter-TrFE-ter-CTFE) à base de fluorure de vinylidène (VDF), trifluoroéthylène (TrFE) et chlorotrifluoroéthylène (CTFE).Les films de polymères obtenus par évaporation du solvant sont étudiés par diffraction des rayons X (SAXS-WAXS), DSC, FTIR, DMA, spectroscopie diélectrique et cycles de polarisation afin de mettre en évidence l’impact de la composition et de la mise en forme (recuit, polarisation) sur la structure et les propriétés finales du matériau. Nous montrons ainsi qu’au sein des films de copolymères, la phase ferroélectrique (FE) coexiste avec une phase ferroélectrique défective (DFE). La fraction croissante de cette phase DFE avec la teneur en TrFE permet d’expliquer l’évolution des propriétés thermiques dont la transition de Curie. Une transition structurale continue, de la phase DFE vers la phase paraélectrique (PE), en température a été mise en évidence.La teneur en termonomère CTFE influence fortement la structure cristalline et les propriétés électro-actives des films de terpolymères (disparition du caractère ferroélectrique au profit du caractère ferroélectrique relaxeur (RFE)). Nous prouvons pour la première fois l’existence d’une transition structurale continue entre la phase RFE et la phase PE au voisinage de la température ambiante. Cette transition permet d’expliquer les propriétés exacerbées de ces matériaux (constante diélectrique et déformation sous champ électrique). Enfin, des analogies de comportement entre les copolymères et les terpolymères sont discutées, notamment l’évolution des phases cristallines sous champ électrique, afin de mieux comprendre le fonctionnement de ces polymères électro-actifs pour leur futur développement au niveau industriel. / The relationship between semi-crystalline structure and properties of use (mechanical, dielectric and electroactive) of fluorinated copolymer films was studied for applications in organic electronics. Investigated materials are poly(VDF-co-TrFE) copolymers and poly(VDF-ter-TrFE-ter-CTFE) terpolymers based on vinylidene fluoride (TrFE), trifluoroethylene (TrFE) and chlorotrifluoroethylene (CTFE). Polymer films, obtained after solvent evaporation, are studied by X-ray diffraction (SAXS-WAXS), DSC, FTIR, DMA, dielectric spectroscopy and polarization cycles in order to highlight the impact of composition and processing (annealing, poling) on structure and final properties of material. We showed that the ferroelectric (FE) phase coexists with the defective ferroelectric (DFE) phase in copolymer films. The increasing fraction of DFE phase with TrFE content allows explaining the evolution of thermal properties. A continuous structural transition, from DFE phase to paraelectric (PE) phase was highlighted. The CTFE termonomer content highly influences the crystalline structure and the electro-actives properties of terpolymer films (loss of ferroelectric behavior in favor of relaxor ferroelectric (RFE) behavior).We proved for the first time the existence of a continuous structural transition between RFE phase and PE phase around room temperature. This transition allows explaining exacerbated properties of these materials (dielectric constant and deformation under electric field).Finally, analogies of behavior between copolymers and terpolymers are discussed, especially the evolution of crystalline phases under electric field, in order to better understand how these electro-active materials work for their future development at industrial level.

The mechanical and physical properties of polymers are determined primarily by the underlying nano-scale structures and characteristics such as entanglements, crystallites, and molecular orientation. These structures evolve in complex manners during the processing of polymers into useful articles. Limitations of available and foreseeable computational capabilities prevent the direct determination of macroscopic properties directly from atomistic computations. As a result, computational tools and methods to bridge the length and time scale gaps between atomistic and continuum models are required. In this research, an internal state variable continuum model has been developed whose internal state variables (ISVs) and evolution equations are related to the nano-scale structures. Specifically, the ISVs represent entanglement number density, crystal number density, percent crystallinity, and crystalline and amorphous orientation distributions. Atomistic models and methods have been developed to investigate these structures, particularly the evolution of entanglements during thermo-mechanical deformations. A new method has been created to generate atomistic initial conformations of the polymer systems to be studied. The use of the hyperdynamics method to accelerate molecular dynamics simulations was found to not be able to investigate processes orders of magnitude slower that are typically measurable with traditional molecular dynamics simulations of polymer systems. Molecular dynamics simulations were performed on these polymer systems to determine the evolution of entanglements during uniaxial deformation at various strain rates, temperatures, and molecular weights. Two methods were evaluated. In the first method, the forces between bonded atoms along the backbone are used to qualitatively determine entanglement density. The second method utilizes rubber elasticity theory to quantitatively determine entanglement evolution. The results of the second method are used to gain a clearer understanding of the mechanisms involved to enhance the physical basis of the evolution equations in the continuum model and to derive the models material parameters. The end result is a continuum model that incorporates the atomistic structure and behavior of the polymer and accurately represents experimental evidence of mechanical behavior and the evolution of crystallinity and orientation.

An extensive review of probabilistic techniques in fatigue analysis indicates that there is a need for new microstructure-sensitive methods in describing the effects of notches on the fatigue life reduction in cyclically loaded components. Of special interest are notched components made from polycrystalline nickel-base superalloys, which are used for high temperature applications in aircraft gas turbine engine disks. Microstructure-sensitive computational crystal plasticity is combined with novel probabilistic techniques to determine the probability of failure of notched components based on the distribution of slip within the notch root region and small crack initiation processes. The key microstructure features of two Ni-base superalloys, a fine and coarse grain IN100, are reviewed and the method in which these alloys are computationally modeled is presented. Next, the geometric model of the notched specimens and method of finite element polycrystalline reconstruction is demonstrated. Shear-based fatigue indicator parameters are used to characterize the shear-based, mode I formation and propagation of fatigue cracks. Finally, two different probabilistic approaches are described in this work including a grain-scale approach, which describes the probability of forming a crack on the order of grain size, and a transition crack length approach, which describes the probability of forming and propagating a crack to the transition crack length. These approaches are used to construct cumulative distribution functions for the probability of failure as a function of various notch root sizes and strain load amplitudes.

Controlling the micro-structure of organic materials is crucial for a variety of practical applications such as photonics, biomedicine or the rapidly growing field of organic electronics. Recent studies have shown a possibility of tailoring the polymer structure on the nanoscale using supramolecular self-assembly under spatial confinement. Despite extensive studies already performed in this field, many questions remain open. In particular, it will be important to understand how different structure formation processes such as crystallization, LC-phase formation, microphase separation, and others occur under confinement. In the present work, we address the effect of 1D- and 2D-confinement on the structure formation for a variety of systems including segmented poly(ether-ester-amide) (PEEA) copolymers, main-chain liquid-crystalline (LC) polymers belonging to the family of poly(di-n-alkylsiloxane)s and liquid-crystalline/semicrystalline block copolymers formed through complexation of poly (2-vinylpyridine-b-ethylene oxide) (P2VP-PEO) with a wedge-shaped ligand, 4'-(3'',4'',5''-tris(octyloxy) benzamido) propanoic acid. In order to reveal the morphological diversity of the studied systems under confinement, the work was carried out on bulk materials and on thin films employing a battery of experimental methods. The main experimental techniques operational in direct and reciprocal space applied in my work are described in chapter 2. [...]

Controlling conformation of macromolecules, both in solution and solid state, has remained an exciting challenge till date as it confronts the entropy driven random coil conformation. Folded forms of biomacromolecules, like proteins and nucleic acids, have served as role-models to the scientists in terms of designing synthetic foldamers. The folded functional forms of proteins and nucleic acids have been shown to rely heavily on various factors, like directional hydrogen bonding, intrinsic conformational preferences of the backbone, solvation (e.g. hydrophobic effects), coulombic interactions, charge-transfer interactions, metal-ion complexation, etc. Chapter-1 discusses various designs of synthetic polymers explored by research groups world-over to emulate the exquisite conformational control exercised by biomacromolecular systems. Our laboratory has been extensively involved since 2004 in designing charge-transfer complexation induced folding of flexible donor-acceptor (DA) polymeric systems, such as those shown in Scheme 1. It was observed that such polymers adopt a folded conformation in polar solvents, like methanol, in the presence of an excess of an appropriate alkali metal ion. To explore folding in the solid state, Jonas and co-workers recently showed that a polyethylene-like polyester with long alkylene segments containing periodically located pendant propyl group forms a semicrystalline morphology with alternating crystalline and amorphous regions primarily because of the periodic folding of the backbone due to the steric exclusion of the propyl branches from the crystalline domains. In order to explore immiscibility-driven folding of polyethylene-like polyesters, Roy et al. designed a periodically grafted amphiphilic copolymer (PGAC) containing long alkylene segments (mimicking polyethylene) and pendant oligoethyleneglycol chains at periodic intervals (Scheme 2). Scheme 2: Proposed folding of a periodically grafted amphiphilic copolymer It was demonstrated that immiscibility between the hydrocarbon backbone and pendant PEG segments drives the polymer to adopt a folded zigzag conformation as shown in Scheme 2. The above synthetic strategy, however, does not permit easy structural variation of the side chain segments because the side-chain segment is covalently linked to the malonate monomer. In Chapter-2, a more general strategy to prepare periodically grafted copolymers has been described. In an effort to do so, we designed a series of clickable polyesters carrying propargyl/allyl functionality at regular intervals along the polymer backbone, as shown in Scheme 3. Scheme 3: Periodically clickable polyesters for the preparation of periodically grafted copolymers The polyesters were prepared by reacting either 2-propargyl-1,3-propanediol, 2,2-dipropargyl-1,3-propanediol or 2-allyl-2-propargyl-1,3-propanediol with an alkylene diacid chloride, namely 1,20-eicosanedioic acid chloride, under solution polycondensation conditions. Since these polyesters carry either, one propargyl, two propargyls or one propargyl and one allyl group on every repeat unit, it provides us an opportunity to synthesise exact graft copolymers with one side chain, two side chains or even two dissimilar side chains per repeat unit. In Chapter-3, the periodically clickable polyesters were reacted with MPEG-350 (PEG 350 monomethyl ether) azides using Cu(I) catalyzed azide-yne click reaction to generate periodically grafted amphiphilic copolymers (PGAC) carrying crystallizable hydrophobic backbone and pendant hydrophilic MPEG-350 side-chains (Scheme 4). Since the PGACs carry either one or two pendant MPEG-350 chains on every repeat unit, it allowed us to examine the effect of steric crowding on the crystallization propensity of the central alkylene segment. Scheme 4: Functionalization of periodically clickable polyesters with MPEG 350 azide by azide-yne click reaction From DSC studies, it was observed that increase in steric crowding at junctions resulting from increased side-chain volume hinders effective packing of the hydrocarbon backbone. As a result, both transition temperatures and the enthalpies associated with these transitions decreases. SAXS and AFM studies revealed the formation of lamellar morphology with alternate domains of PEG and hydrocarbon. Based on these observations, we proposed that self-segregation between hydrophobic backbone and hydrophilic side-chains induce the backbone to adopt a folded zigzag conformation (Scheme 5). Scheme 5: Schematic depiction of self-segregation induced folding of PGAC and their assembly on mica surface (AFM image) In order to study the effect of solvent polarity on conformational evolution of the periodically grafted amphiphilic copolymers, we randomly incorporated pyrene in the backbone of the polymer by reacting a small fraction (~ 5 mole %) of the propargyl groups with pyrene azide. Fluorescence study of the pyrene labelled polymer showed that increase in solvent polarity increases the intensity of the excimer band dramatically; this suggests the possible collapse of the polymer chain to the folded zigzag form. In an extension of this work, the PGAC was further used as template to synthesise layered silicates that appears to replicate the lamellar periodicity seen in the polymer. In order to study the effect of reversing the amphiphilicity on self-segregation, in Chapter-4, we synthesised a series of clickable polyesters carrying PEG segments of varying lengths, namely PEG 300, PEG 600 and PEG 1000, along the polymer backbone. The polymers were prepared by trans-esterification of 2-propargyl dihexylmalonate with different PEG-diols. These polyesters were then clicked with docosyl (C22) azide using Cu(I) catalyzed azide-yne click reaction to generate the desired periodically grafted amphiphilic polymers carrying crystallizable hydrophobic pendant chains at periodic intervals; the periodicity in this case was governed by the length of the PEG diols (Scheme 6). Scheme 6: PGACs carrying hydrophilic PEG backbone and crystallizable hydrophobic pendant docosyl chains Varying the average periodicity of grafting provided an opportunity to examine its consequences on the self-segregation behavior. Given the strong tendency of the pendant docosyl segments to crystallize, DSC studies proved useful to analyse the self-segregation; DOCOPEG 300 clearly exhibited the most effective self-segregation, whereas both DOCOPEG 600 and DOCOPEG 1000 showed weaker segregation. Based on the observations from DSC studies, we proposed that the PEG backbone adopts a hairpin like conformation (Scheme 7). Scheme 7: Proposed self-segregation through hairpin like conformation of backbone PEG segments In order to confirm the bulk morphology, we carried out small angle X-ray scattering (SAXS) and atomic force microscopic (AFM) studies. The SAXS profiles confirmed the observations from DSC studies, and only DOCOPEG 300 exhibited well-defined lamellar ordering. Thus, it is clear that the length of the backbone PEG segment (volume-fraction) strongly influences the morphology of the PGACs. Based on the inter-lamellar spacing from SAXS and the height measurements from AFM studies (Scheme 8), we proposed that these polymers form lamellar morphology through inter-digitation of the pendant docosyl side-chains. The observations from Chapters 3 and 4 suggested that the crystallization of the backbone has a dramatic effect on the conformation of the polymer backbone. In order to explore the possibility of independent crystallization of both backbone and pendant side-chains, the periodically clickable polyesters, described in Chapter-2, were quantitatively reacted with a fluoroalkyl azide, namely CF3(CF2)7CH2CH2N3 using Cu(I) catalyzed azide-yne click reaction; Chapter-5 describes these polyesters carrying long chain alkylene segments along the backbone and either one or two perfluoroalkyl segments located at periodic intervals along the polymer chain (Scheme 9). DSC thermograms of two of the samples showed two distinct endotherms associated with the melting of the individual domains, while the WAXS patterns confirm the existence of two separate peaks corresponding to the inter-chain distances within the crystalline lattices of the hydrocarbon (HC) and fluorocarbon (FC) domains; this confirmed the occurrence of independent crystallization of both the backbone and side chains. Scheme 10: Left-variation of SAXS profile of all three polymers as a function of temperature, Right- molecular modelling of representative FC-HC-FC triblock structures. Interestingly, a smectic-type liquid crystalline phase was observed at temperatures between the two melting transitions. SAXS data, on the other hand, revealed the formation of an extended lamellar morphology with alternating domains of HC and FC (Scheme 10). The inter-lamellar spacing calculated from SAXS matches reasonably well with those estimated from TEM images. Based on these observations, we proposed that the FC modified polymers adopt a folded zigzag conformation whereby the backbone alkylene (HC) segment becomes colocated at the center and is flanked by the perfluoroalkyl (FC) groups on either side, as depicted in Scheme 11. Melting of alternate HC domains first leads to the formation of a smectic-type liquid crystalline mesophase, wherein the crystalline FC domains retain the smectic ordering; this was confirmed by polarizing light microscopic observations. Scheme 11: Schematic presentation of self-segregation induced folding of polymer chains; and hence crystallization assisted assembly of these singly folded chains to form lamellar structure One interesting challenge would be to create unsymmetrical folded structures, wherein the top and bottom segments of the zigzag folded form would be occupied by two different segments, such as PEG and FC, whereas the backbone alkylene segment would form the central domain; this would lead to the possible formation of consecutive domains of PEG, HC and FC through immiscibility driven self-segregation process. In Chapter-6, several approaches to access such systems have been described; one such design that could have resulted in the successful synthesis of a periodically clickable polymer carrying orthogonally clickable propargyl and allyl groups along the backbone in an alternating fashion is depicted in (Scheme 12). The parent polyester was successfully synthesized and the propargyl group was first clicked with the FC-azide to yield the FC-clicked polyester; however, several attempts to click MPEG-SH onto the allyl groups using thiol-ene click reaction failed. Scheme 12: Scheme for the synthesis of alternating orthogonally clickable polymer In order to accomplish our final objective, we chose to first prepare the FC-clicked diacid chloride and polymerize it with an azide-alkyne clickable macro-diol, as depicted in Scheme 13; this approach was successful and yielded the desired clickable polyester bearing the FC segments at every alternate location. This polymer was then clicked with PEG-750 azide to yield the final targeted polymer that carries mutually immiscible FC and PEG-750 segments at alternating positions along the polymer backbone. The occurrence of self-segregation of FC, PEG-750 and the alkylene backbone (HC) was first examined by DSC studies, which appeared to suggest the presence of three peaks, although these were not very well-resolved. Scheme 13: Schematic for the synthesis of the polymer carrying FC and PEG 750 alternatingly along the backbone A schematic depiction of the anticipated organization of such unsymmetric folded macromolecules is shown in Scheme 15; it is evident that because of mutual immiscibility, the layers will be organized such that the FC domains of adjacent layers will be together and similarly the PEG domains of adjacent layers will also be together. Such an organization would lead to an estimated spacing that would correspond to a bilayer of the folded structures. Interestingly, SAXS study (Scheme 14) reveals the formation of lamellar morphology with a d-spacing of 14.6 nm. Scheme 14: Figure 6.10: SAXS profile of the polymer PE-FC-PEG 750 In order to gain an estimate of the expected inter-lamellar spacing, the end-to-end distance of a model repeat-unit was computed to be ~ 9.4 nm. It is, therefore, evident that the inter-lamellar spacing of 14.6 nm seen in the SAXS is significantly larger and must represent a bilayer type organization (Scheme 15). In this regard it is important to say that the organization of these alternatingly functionalized folded chains should give a variety of d-spacings. Because of highest electron density contrast of FC among PEG, HC and FC, we proposed that the d-spacing calculated from the SAXS profile corresponds to ‘d4’ in Scheme 15. This first demonstration of the formation of zigzag folded unsymmetric entities bearing dissimilar segments on either side of the folded chain holds exciting potential for a variety of different applications and beckons further investigations. Scheme 15: Schematic for the proposed self-assembly of the singly folded polymer chains

Les polymères semi-cristallins à chaînes semi-rigides, bien qu’abondamment utilisés dans la vie quotidienne, représentent des systèmes complexes qui ne sont pas encore parfaitement compris. Parmi les nombreux domaines de recherche sur cette famille de polymères, l’étude de la morphologie semi-cristalline et des processus de cristallisation et de fusion de ceux-ci restent des sujets très importants. L’investigation de la morphologie semi-cristalline est rendue difficile car elle présente une structure hiérarchique composée de plusieurs niveaux d’organisation, dont le plus petit est observable à une échelle très réduite de quelques nanomètres. De plus, les aspects liés à la cinétique des processus de cristallisation et de fusion n’ont pas toujours permis de bien les mettre en évidences, les rendant ainsi par très bien compris. Cependant, les nouvelles avancées technologiques dans le domaine de la physique expérimentales ont beaucoup profité à la science des polymères. <p>Dans ce travail, une contribution originale est apportée à cette étude, et cela en combinant diverses techniques expérimentales permettant des mesures calorifiques et structurales en températures et temps réels. L’intérêt c’est porté sur les polymères linéaires aromatiques tels que le polyéthylènes teréphthalate, PET, et le polytriméthylène téréphthalate, PTT, caractérisés par une température de transition vitreuse supérieure à l’ambiante ( Tg > 50°) et une température de fusion élevée (Tm>220°C), offrant ainsi une assez large gamme de température de cristallisation (Tm-Tg). L’étude de la structure semi-cristalline du PET à l’échelle du nanomètre et de la relaxation des phases amorphes présentes dans sa structure est facilitée par l’utilisation d’un diluant amorphe tel que le polyétherimide (PEI), qui forme un mélange miscible avec le PET. <p>L’utilisation de microscopie de force atomique AFM à haute température a permis d’observer la cristallisation isotherme de PET en temps réel et de décrire ainsi la cristallisation secondaire comme un processus d'épaississement des piles lamellaires. De plus, l’analyse de la structure semi-cristalline du PET et du PTT, dans l’espace direct, sont en faveur d’un modèle structural homogène, où l’épaisseur lamellaire moyenne est légèrement inférieure à l’épaisseur moyenne des régions amorphes interlamellaires. Ces résultats ont permis, d’une part, d’apporter une meilleure interprétation aux données obtenues par diffusion des rayons X aux petits angles (SAXS), et d’autre part, d’ interpréter le comportement de fusion multiple caractéristique des polymères semi-cristallin à chaînes semi-rigides par le seul processus de fusion-recristallisation. Dans l’étude investiguée sur les mélanges PET/PEI et sur le PTT pur, on montre que la cinétique d’un tel processus est particulièrement rapide comparée à la cristallisation. De plus, les observations par AFM et par microscopie optique de même que les mesures SAXS en temps réel ont montré la simultanéité et la compétition existant entre la fusion des cristaux et leur réorganisation durant la chauffe. Par ailleurs, la relaxation des régions amorphes interlamellaires, souvent considérées comme rigides, a pu être mise en évidence par les mesures AFM et SAXS réalisées à haute température sur des échantillons de PET/PEI semi-cristallins.<p> / Doctorat en sciences, Spécialisation physique / info:eu-repo/semantics/nonPublished

Small cantilevers with ultra-high resonant frequencies (1–3 MHz) have paved the way for high-speed atomic force microscopy. However, their potential for multi-frequency atomic force microscopy is unexplored. Because small cantilevers have small spring constants but large resonant frequencies, they are well-suited for the characterisation of delicate specimens with high imaging rates. We demonstrate their imaging capabilities in a bimodal frequency modulation mode in constant excitation on semi-crystalline polypropylene. The first two flexural modes of the cantilever were simultaneously excited. The detected frequency shift of the first eigenmode was held constant for topographical feedback, whereas the second eigenmode frequency shift was used to map the local properties of the specimen. High-resolution images were acquired depicting crystalline lamellae of approximately 12 nm in width. Additionally, dynamic force curves revealed that the contrast originated from different interaction forces between the tip and the distinct polymer regions. The technique uses gentle forces during scanning and quantified the elastic moduli Eam = 300 MPa and Ecr = 600 MPa on amorphous and crystalline regions, respectively. Thus, multimode measurements with small cantilevers allow one to map material properties on the nanoscale at high resolutions and increase the force sensitivity compared with standard cantilevers. / Dieser Beitrag ist aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.