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___________________________________________________________________________

CHAP III – MATERIALS AND METHODS

Trois méthodes expérimentales ont été utilisées lors de cette étude. Dans un premier cas, l'échantillon est indenté en laboratoire en utilisant (ou non) le microindenteur développé et décrit dans le chapitre II. L'analyse aux rayons X se fait donc a posteriori (ex-situ) et le plus souvent sur le microdiffractomètre de la ligne de microfocus (ID13) décrit au paragraphe I.4.4.2. Pour ces raisons, ces expériences sont appelées "lab. ex-situ". Il est important que le délai entre l'indentation et la fin de l'analyse soit le plus court possible dans le cas de polymères afin de minimiser tout phénomène de relaxation viscoélastique. Les deux autres types d'expériences nécessitent que le microindenteur soit couplé au système de balayage tel que précisé dans le chapitre précédent. Le contrôle des opération se fait donc totalement depuis le poste de commande de la ligne ("online"). L'échantillon peut alors être indenté puis immédiatement après analysé ("online ex-situ") ou indenté et balayé simultanément sous le microfaisceau ("online in-situ"). Selon la taille de l'échantillon, le faisceau peut être choisi de telle manière à englober la totalité ou une partie seulement de la zone d'indentation. Dans tous les cas, le temps d'exposition est limité par la sensibilité de l'échantillon aux rayons X. Un autre paramètre d'importance à prendre en compte est le délai d'acquisition et de stockage des données de la caméra CCD utilisée. Ces deux éléments ajoutés au temps de réponse des moteurs dans le cas d'un balayage ou d'une rotation déterminent ainsi la résolution temporelle de l'expérience (nombre de cliché de diffraction qu'il est possible d'effectuer durant le cycle d'indentation). Les différentes expériences sont détaillées avec précision dans la deuxième partie de ce chapitre. Le lecteur y est donc renvoyé lors de l'analyse des résultats décrits dans le chapitre suivant.

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___________________________________________________________________________ This chapter is dedicated in a first part to a general description of experimental possibilities using the indenter set-up developed for the microfocus beamline (see chapter II). Those can be undertaken either in-situ (during deformation) or ex-situ (a posteriori, after indentation) and require specific sample preparation methods. Different synthetic polymers were used throughout this work as demonstration samples, mainly in the form of fibres. These will be described in a second part along with corresponding experimental parameters. All samples are commercially available and were used without further treatment. The advantages of using fibres resides in the axial symmetry imposed by the fibre axis, which results in a fibre texture pattern. (see chapter I, section I.2 and I.4). Although only organic samples will be described, it is evident that the experimental techniques discussed below could be applied to other classes of materials such as metallic alloys or biopolymers. In particular, the study of the deformation mechanisms in biological materials could also be of interest. In this respect preliminary results of tests conducted on an Amyloid fibre are presented in the next chapter. This material will be described in the latter part of this section.

III.1.

METHODOLOGY

Three kinds of experiments were undertaken and will be described in this section : (i)

the sample is indented in the laboratory and then transferred to the beamline. Both the scanning set-up or the microdiffractometer (see section I.4.4) were used for data collection : (lab. ex-situ)

(ii)

the sample is indented online using the indentation set-up adapted to the beamline (described in chapter II), which is mounted on the scanning set-up. The sample is, however, only scanned after completion of the full indentation cycle : (online ex-situ).

(iii)

indentation and X-ray scanning diffractometry are done simultaneously in real time using the scanning set-up : (online in-situ).

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___________________________________________________________________________ III.1.1.

EX-SITU EXPERIMENTS

III.1.1.1. LABORATORY INDENTATION (LAB. EX-SITU)

Fibres are usually sold in yarns containing multiple filaments or fibres. One such fibre must first be isolated and cut in length under optical microscope using microscissors. Both ends of the fibre are glued (cyanoacrylate) on a glass tip, which is then fixed onto the microindentation set-up in place of the diamond sample holder shown in fig.II.15 (p.78). This allows indentation to be performed with controlled force, gradient and dwell time as described in section II.1.1. Once indented, the glass plate is removed and the fibre cut at both ends before the glue. One end of the fibre is then glued onto a sharp tip which is generally prepared from a glass capillary pulled under tensile stress into a microoven up to rupture. Fig.III.1 shows an indented Vectra fibre mounted on a capillary (in this case, the capillary has not been pulled under heat but simply broken with a razor blade). The two closest to the glue in fig.III.1-b and the last one on the left are used only for manipulation purposes. Also, one must ensure that the fibre axis is more-less parallel to the capillary in order to facilitate alignment on the goniometer. The glass tip is then glued on a brass pin, which is later mounted on the microdiffractometer (see fig.I.37, p.55).

(a)

(b)

Fig.III.1 : a – Indented Vectra fibre glued on a glass capillary and b – detail showing the indentations.

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___________________________________________________________________________ As explained in section I.4.4.2 (p.54), the microdiffractometer set-up allows rotating the fibre in the beam and record diffraction patterns at given rotation angles. The beam size is determined by the diameter of the collimator in the range 5 - 30 µm and can therefore be adapted to the experimental requirements (fig.III.2). Thus a beam size smaller than the fibre allows to scan the indented zone and a beam size larger than the fibre allows to average the whole indented zone. A scanning option allows recording rotation patterns at different positions along the fibre. ∆

φ

(a)

(b)

φ fibre axis

probing beam 25µm

probing beam Ø = 5-30 µm

Fig.III.2 : ex-situ microdiffractometer geometry of experiment with a – smaller and b – larger beam than fibre As will be seen in following sections, one can therefore extract information relative to texture, which is due to an anisotropy of the orientation of crystalline domains with respect to the indentation direction. Materials other than polymers can also be mounted in the same way providing sufficient care is taken when gluing the sample.

III.1.1.2. ONLINE INDENTATION (ONLINE EX-SITU)

In this case, the fibre sample is selected from the yarn and cut in length as in lab. exsitu experiments. However, instead of indenting and then gluing it on a capillary, it is directly glued on the diamond sample support described in section II.2.3.1, which is mounted on the indentation device coupled with the scanning set-up allowing online (remote) experiments. Gluing the fibre only at both ends has the advantage of avoiding constraints in the indentation

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___________________________________________________________________________ zone but has the disadvantage that the fibre can be displaced vertically by a few microns under the indenter. It was therefore generally preferred to deposit a very thin film of glue with a whisker on the diamond so that the full base of the fibre is in contact with the support (fig.III.3-a).

(a)

(b)

glue

glue

250µm

500µm

Fig.III.3 : fibre glued on sample support with a – full base in glue and b –glued at both ends only. Indentation experiments can then be done online with the scanning set-up as described in chapter II. In this case, the sample is indented and then scanned with the X-ray microbeam in the direction normal to indentation (fig.III.4). The major advantage of this technique compared to the previous is to avoid manipulating the fibre once indented. Also, the time delay between indentation and scanning is small enough to neglect relaxation phenomena.

fibre axis Diamond (sample support) probing beam

Fig.III.4 : scheme of online ex-situ experiment

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___________________________________________________________________________ III.1.2.

IN-SITU EXPERIMENTS

The sample is prepared in the same way as above and fixed on the indentation set-up mounted on the scanning stage. In this case, the scanning option is not used and the beam is placed at a fixed position with respect to the sample surface and the indentation direction. A sequence of diffraction patterns is recorded while the indentation process is running as described in fig.III.5 below. The temporal sequence is determined by the exposure time (∆e) and the time due to data transfer (∆ro) (b)

F

Load

(a)

Pattern recording ∆e ∆ro

fibre axis Diamond (sample support)

probing beam

time

Fig.III.5 : a – scheme of typical online in-situ experiment and b – corresponding recording scheme for the diffraction patterns ; the load curve is this applied on the diamond tip of the indenter ; ∆e and ∆ro respectively X-ray exposure and CCD camera readout time. The main limitation of this experiment lies in the minimum exposure time ∆e (fig.III.5-b) needed in order to collect sufficient statistics (in the best cases, in the submillisecond range, usually in the second range) and in the readout time ∆ro (fig.III.5-b) of the CCD camera. This depends on the X-ray optical system, sample characteristics and detection device. In any of the above experiments, particular care must be taken in order to avoid interferences between indentations when several deformations are produced on the sample fibre. In the case of polymers, as a general

rule of thumb,

distance between

indentation must

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___________________________________________________________________________ be at least 2 times the main diagonal of the larger one [1] eventhough variations to this rule can be expected depending on the type of polymer.

I.1.1.

DATA COLLECTION PARAMETERS

All experiments were done at a wavelength λ = 0.976 Å which corresponds to an energy of 12.72 keV. Most were done in 2/3 filling mode of the storage ring and test experiments were sometimes done in 16 bunch mode (more information concerning the characteristics of beam modes at the ESRF can be found in [14]). As mentioned in section I.4.4, all experiments carried out on the microdiffractometer (lab. ex-situ) involved the use of collimators. Accessible diameters were 5, 10 and 30µm. Experiments with the scanning set-up were done using collimators or tapered glass capillaries (described in appendix II) in combination with a condensing ellipsoidal mirror [2]. Two types of 2-D detectors were used in order to record the diffraction patterns. A slow-scan 16 bit-readout CCD detector (X-ray associates ; XRA) with a 130 mm diameter and 100 µm thick X-ray converter fluorescent screen was used for ex-situ experiments (both lab. and online). The resolution is 2048 × 2048 pixels where each pixels has a dimension of 64.45 × 64.45 µm2. The output data are in binary format and corrected for dark current. In this case, the read-out time is about 13 sec. For faster read-out (~ 1.5 s), in-situ experiments were done using a Gemstar detector (Photonic Science) with 12 bit-readout, 46 mm diameter and 1024 × 1024 pixels of 45 × 45 µm2. The output data are in tiff format without further corrections, which have to be applied during data reduction. Calibrations of beam centre on detector and distance sample-to-detector were done using either Al2O3 or Ag-Behanate [3] powder standards. For more details see Appendix A.3. Data reduction also involved some initial corrections of which most important is background subtraction using a specific file recorded during the experiment in the same condition and outside of the sample. This allows to account for X-ray scattering by air and for the specific experimental set-up (shading from objects etc...). Also important is the correction for intensity decrease due to the machine current and possible misalignments or intensity fluctuations. This was not straightforward until the recent installation of an ionisation chamber before the

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___________________________________________________________________________ sample, which allows to cope with intensity variations. Data analysis and corrections were done using the FIT2D software package [4] available at the ESRF.

III.2.

MATERIALS AND EXPERIMENTS

This section describes the test materials used and corresponding experimental conditions. Most experiments were performed on highly crystalline polymer fibres such as ultra-high molecular weight polyethylene (UHMW-PE) and Vectra liquid crystal copolyester (see section I.2.3 for detailed description). These materials are also called high performance polymer fibres [7]. Semicrystalline polymer fibres such as polyamide 6,6 (PA66), isotactic polypropylene (PP) and high-density polyethylene (HD-PE) were also tested. Their most relevant characteristics for our studies are given in table III.1. All samples were supplied by Goodfellow Cambridge Ltd. [6] except HD-PE (Esbjerg Thermoplast). Additionally, an amyloid fibre will be described as a representative biopolymer. Full experimental details, other than mentioned in the following can be found in previous section. Also, unless specified, the X-ray beam was 5 µm in diameter. Hardness values were calculated from eq.2 (p.16) with corrections due to fibre geometry [1,29] ± 1 MPa. Fibre diameter (µm)

Molecular weight Mw x 10-3 (g.mol-1)

Density ρ

Crystallinity χc

Glass transition Hardness (MPa) temperature Tg (oC)

UHMW-PE

12

3000-6000

0.94 [6-8]

0.85-0.95 [6-7]

Vectra

23

unknown

1.4

0.17-0.26 [12-13]

50-150 [16]

216

PA66

19

260

1.14

0.5

50

113

PP

28

280

0.9

0.45-0.6

-10

76

HD-PE

170

300-500 [5]

0.94-0.96

0.64-0.8 [17]

-125

153

Table III.1 : important parameters of polymer samples ; ref. other than mentioned [6]

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___________________________________________________________________________ III.2.1.

ULTRA-HIGH MOLECULAR WEIGHT POLYETHYLENE

UHMW-PE fibres (Dyneema®, Goodfellow Cambridge Ltd. [6]) are known to exhibit extremely high impact strength (~ 1.6 KJ.m-1 [9]) and elastic modulus along the fibre axis (~ 0.2-1.2 GPa at 23oC [6]) due to their extended chain morphology (see section I.3.1). These technologically very interesting mechanical properties have led to extensive scientific examinations (review on materials properties : [7] ; crystallographic data on PE : [10]). It is therefore a very useful model system for our purposes. EXPERIMENTS : Exp. UPE 1 : online ex-situ experiment : a 10 µm (hor) x 5 µm (vert) mesh-scan was performed (fig.III.6). Prior to scanning, the fibre was indented under 5, 10 and 20 mN loads at respectively 1, 1 and 2 mN.s-1 and with dwell times of 10 s. The scans was performed immediately after indentation and took less than 15 min in order to minimize viscoelastic relaxation effects.

F 10µm 12µm

5µm fibres axis

Diamond (sample support) probing beam : Ø = 5µm

Fig.III.6 : experimental scheme of exp. UPE 1 Exp. UPE 2 : in situ experiment : X-ray exposure and readout time were respectively of 0.5 s/frame and 1.5 s/frame. The beam was positioned approximately at 8 µm from the top of the fibre as determined by a vertical scan prior to indentation of 5 mN at 1 mN.s-1 and with a dwell time of 10 s (fig.III.7).

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___________________________________________________________________________

Load (mN) 5

Pattern recording

0 ∆e = 0.5s ∆ro = 1.5s

F 8µm

12µm

fibre axis

10s

Diamond (sample support) probing beam : Ø = 5µm

time

Fig.III.7 : experimental scheme of exp. UPE 2 Exp. UPE 3 : lab. ex-situ experiment : indentations of 20 and 10 mN at respectively 2 and 1 mN.s-1 for 10 s were investigated with 5 and 30 µm beams (fig.III.8).

φ 12µm fibres axis

probing beam = 30µm

Fig.III.8 : experimental scheme of exp. UPE 3

III.2.2.

VECTRA® COPOLYESTER

As seen in section I.2.3, (Vectra®, Goodfellow Cambridge Ltd. [6]) is a thermotropic main chain liquid crystal polymer (LCP) composed of 4-hydroxybenzoic (HBA) and 2-hydroxy-6-naphtoic acid (HNA) (fig.I.18-b, p.34) in a molar ratio of 73/27 HBA/HNA [11]. The rigid rod nature of this copolyester favours a higher elastic modulus along the fibre axis

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___________________________________________________________________________ (2-10 GPa at 23oC [6]) than UHMW-PE but a lower impact strength (0.52 KJ.m-1 [6]) and this material is therefore recommended as reinforcing agents in composites. Fig.III.9 shows two optical micrographs of indented Vectra fibres with typical indentation loads used for the experiments described below. In all cases, the dwell time was 10 s and the indentation gradient was load / 10 mN.s-1 within the limit of 1 mN.s1 (limit of the Anton-Paar indenter). Note that the indentations at 500 mN are only used to define the zone where the indentations to be analysed are located and which cannot be seen under a low magnification microscope used for laboratory preparation.

A

40 mN

B

5 mN 20 mN 40 mN 500 mN

500 mN

10 mN

50 mN

10 mN 30 mN

50 mN

100µm

30 mN

20 mN

25µm

Fig.III.9 : typical micrographs of an indented Vectra fibre (Ø = 23µm) EXPERIMENTS : Exp. VEC 1 : ex-situ experiment : performed with the sample fixed to a goniometer mounted on scanning stage (this can be considered as a modified online experiment; see section III.1 for more details). A beam of approximately 3 µm was obtained using a tapered glass capillary [2]. The fibre was indented under a 50 mN load at 5 mN.s-1 and with a dwell time of 10 s. Scans were taken both along and normal to the direction of indentation with steps of 20 µm along the fibre axis and 2 µm across (fig.III.10).

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___________________________________________________________________________

23µm

2µm

2µm

fibres axis

20µm

20µm probing beam

probing beam

Fig.III.10 : experimental scheme of exp. VEC 1 Exp. VEC 2 : lab. ex-situ experiment : Indentations of 50, 30 and 10 mN at respectively 5, 3 and 1 mN.s-1 were investigated. Additional patterns were taken at 15 µm steps from the indentations (fig.III.11)

15µm

φ

Ø ~ 5µm

25µm fibres probing beam

Fig.III.11 : experimental scheme of exp. VEC 2 Exp. VEC 3 : lab. ex-situ experiment : using a 30 µm beam. X-ray exposure and readout time were respectively of 15 s/frame and 4 s/frame. Indentations of 50, 30 and 10 mN at respectively 5, 3 and 1 mN.s-1 were performed with a dwell time was 10 s (fig.III.12).

φ 23µm fibres axis

probing beam = 30µm

Fig.III.12 : experimental scheme of exp. VEC 3

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___________________________________________________________________________ III.2.3.

OTHER SYNTHETIC POLYMERS

III.2.3.1. POLYAMIDE 6,6

Commercial PA66 fibres of 19 µm in diameter were obtained from Goodfellow Cambridge Ltd [6] under the trade name Nylon66. This thermoplastic has a large number of applications and is most widely used in the form of fibres in the textile industry. Fig.III.13 shows a series of optical micrographs of indented PA66 fibres with typical indentation loads used for the experiments described below. In all cases, the dwell time was 10 s and the indentation gradient was load/10 mN.s-1 within the limit of 1 mN.s1 (limit of the Anton Paar indenter).

50µm

10mN

15mN

5mN 40mN

25mN

30mN

35mN

20mN 100mN

30mN

50mN

Fig.III.13 : optical micrographs of indented PA66 19µm fibres EXPERIMENTS : Exp. PA 1 : online ex-situ experiment : a 10 µm (hor) x 5 µm (vert) mesh-scan was performed (see fig.III.6, p.91 ; in this case the diameter is 19µm). Prior to scanning, the fibre was indented under 100 mN load at 10 mN.s-1 and with a dwell time of 10 s.

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___________________________________________________________________________ III.2.3.2. POLYPROPYLENE

Polypropylene is a rather versatile polymer. In the form of fibres, it is mainly used for textile, automotive, furnishing (etc...) applications. Fig.III.14 shows an optical micrograph of a sample indented under 50 mN, at 5 mN.s-1 and with a dwell time of 10 s.

25µm Fig.III.14 : optical micrograph of 28 µm PP fibres indented under 50 mN (5 mN.s-1, 10 s) EXPERIMENTS : Exp. PP 1 : online ex-situ experiment : similar to UPE 1, the fibre was scanned in steps of 10 µm along the fibre axis and 5 µm across (fig.III.6 p.91; in this case the fibre diameter is 28 µm). Prior to scanning, the fibre was indented under 40 mN load at 1 mN.s-1 and with a dwell time of 10 s. Exp. PP 2 : in situ experiment : X-ray exposure and readout time were respectively of 0.5 s/frame (∆ro) and 0.8 s/frame (∆e). The beam position was approximately 8 µm vertically from the indentation obtained under 30 mN at 1 mN.s-1 and with a dwell time of 10 s (fig.III.15). This experiment was repeated with the beam displaced horizontally (along the fibre axis) by steps of 7 µm.

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___________________________________________________________________________

Load (mN) 30

F

Pattern recording

0 ∆e = 0.5s ∆ro = 0.8s

8µm 28µm fibres axis

10s

Diamond (sample support) probing beam : Ø = 5µm time

Fig.III.15 : experimental scheme of exp. PP2

III.2.3.3. HIGH DENSITY POLYETHYLENE

Samples of high-density polyethylene (HD-PE) fibres were used to test the feasibility of online ex-situ and in-situ SAXS experiments as they are semicrystalline and show a meridional SAXS-pattern. In contrast, the highly crystalline UHMW-PE does not show a meridional SAXS pattern. (for further details, see section I.2 and I.4). EXPERIMENTS : Exp. HDPE 1 : in-situ SAXS experiment : for this experiment a Be refractive lens [18] replaced the condensing ellipsoid mirror (section I.4.4.1). The Be-lens optics was commissioned during the experimental period available for the indentation experiment. A 20 µm guard aperture was brought close to the sample to reduce the scattering from the defining aperture [19]. X-ray exposure and readout time were respectively of 1 s/frame and 1.75 s/frame. Samples were indented under 1 N at 10 mN.s-1 for 10 s (similar experimental scheme is described in fig.III.7, p.92). This experiment was repeated with beam displaced vertically (across fibre axis) by steps of 5 µm from the fibre surface within the range 0 (surface) – 20 µm.

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___________________________________________________________________________ III.2.4.

AMYLOID FIBRES

Emphasis was put on synthetic polymers in this work, but as already mentioned, other samples could also be tested using the microindenter set-up. Of particular interest are biological materials, both from a fundamental and practical point of view. They are often found to exhibit higher degrees of structural complexity and can in some cases also be of industrial interest [24-27]. In particular, many biopolymer fibres could be tested using the microindentation set-up without any further instrumental developments. In order to demonstrate these possibilities, an experiment was done using amyloid fibres supplied by A. Mitraki and K. Papanikolopoulou (Institut de Biologie Structurale, IBS, Grenoble, France [20]). These consist of amyloid microfibrils which form by self-assembly of a synthetic peptide (molecule composed of a sequence of amino-acids) corresponding to a portion of the adenovirus fibre protein (polypeptide). Amyloid-type fibres formed from a variety of proteins are also of medical importance since they are known to be involved in Alzheimer's disease, diabetes and spongiform encephalopathies [15,28]. Because of its stability and morphology, the adenovirus fibre has served as a model for synthetic fibre design [21]. In this respect, a short sequence of the adenovirus was bacterially expressed as inclusion bodies (produced inside a bacteria), refolded, purified (isolated) and spun into fibres [23]. In one case, the fibres obtained showed mechanical properties comparable to several commercial textile fibres. It is generally believed that a better understanding of the structure and related mechanical properties of amyloid-type fibres could lead to further design of synthetic fibres with improved or novel properties. The fibre stalks used in this study were formed by dissolving the peptide in ammonium acetate buffer. Droplets containing the amyloid fibrils formed by aggregation of the peptide were deposited between the ends of two glass rods and left to air-dry [15,22]. Samples were in the range of 80-150 µm in diameter and were indented under several loads. Indentation patterns could in some cases clearly be resolved by light microscopy but fibres were found to be very inhomogeneous and broke under forces in the range of 30-90 mN (fig.III.16) depending on the sample used. This can be explained from scanning electron microscopy (SEM) results (Leika Electron Optics) analysis (fig.III.17). According to the SEM data the sample consists of small blocks, which are loosely associated. Those are thought to

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___________________________________________________________________________ be entangled microfibrils observed in [15] covered with gold. Indentation results apparently in a breaking of the weak bonding between blocks, analogous to deformation of pressure compacted metallic powders before sintering. In such a case, the material is extremely brittle, which implies that there is little plastic deformation and above a critical stress the material breaks. It is therefore expected that the indentation will only be visible when the stress induced by the indenter tip is smaller than the critical stress, which has been found to be sample dependent. glue

60mN

70mN

200µm

30µm

Fig.III.16 : optical micrographs of an amyloid fibre a – indented under 60 and 70 mN loads at respectively 6 and 7 mN.s-1 for 10s and b – glued on diamond sample support of indenter setup before and after application of 90mN, 9mN.s-1, 10s (a)

(b)

(c)

Fig.III.17 : SEM pictures of an amyloid fibre sputtered with gold (~ 150 µm thickness) at magnifications of a – 1430 ×, b – 5230 × and c – 5920 × under 20kV; b – and c – are enlargements of red box in a – using different filtering methods

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___________________________________________________________________________ EXPERIMENTS : Exp. AMY 1 : online ex-situ experiment : a fibre of 120 µm in diameter was indented under 30 and 40 mN at respectively 3 and 4 mN.s-1 with a dwell time of 10 s and scanned in steps of 10 µm along the fibre axis and 5 µm across (fig.III.6, p.91 ; in this case, fibre diameter is 120 µm).

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___________________________________________________________________________

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Baltà-Calleja, F.J., Fakirov, S., Microhardness of polymers, Cambridge University Press, 2000

[2]

Riekel, C., Rep. Prog. Phys., 63, 233, 2000

[3]

Blanton, T.N., Huang, T.C., Toraya, H., Hubbard, C.R., Robie, S.B., Louer, D., Goebel, H.E., Will, G., Gilles, R., Raftery, t., Powder Diffr., 10, 91, 1995

[4]

Hammersly, A.P., ESRF internal report, ESRF97 HA02T, 1997

[5]

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[6]

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[7]

Jiang, H., Adams, W.W., Eby, R.K., High performance fibres in Cahn, R.W., Haasen, P., Kramer, E.J., Structure and Properties of Polymers, Materials Science and Technology, vol. 12, vol. ed. Thomas, E.L., VCH: Weinheim, 597, 1993

[8]

Lemstra, P.J., Kirschbaum, R., et al., High-Strength/High-Modulus Structures Based on Flexible Macromolecules: Gel-Spinning and Related Processes, in Developments in Oriented Polymers, vol. ed. I. M. Ward, London, Elsevier. 39, 1987

[9]

Web page : http://www.ticona-us.com/

[10]

Tadokoro, H., Structure of crystalline polymers, Krieger Publishing, 1990

[11]

Calundann, G.W., Jaffee, M., Proc. Robert A. Welch Found. Conf. Chem. Res., 26, 247, 1982

[12]

Wilson, D. J., Vonk, C. G. and Windle, A.H., Polymer, 34, 225, 1993

[13]

Hanna, S., Lemmon, T.J., Spontak, R.J. and Windle, A.H., Polymer, 33, 3, 1992

[14]

Web page : http://www.esrf.fr/Accelerators/Status/Operation/Modes

[15]

Mitraki, A., Forsyth, T., et al., FEBS Letters, 468, 23, 2000

[16]

Sauer, B., Beckerbauer, R., Wang, L., J. Poly. Sci., Poly. Phys. Ed., 31, 1861, 1993

[17]

Web page : www.cheric.org/ippage/d/ipdata/2001/01/file/d20010102.doc

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___________________________________________________________________________ [18]

Schroer, C.G., et al., SPIE, 2002

[19]

Riekel, C., Burghammer, M., Müller, M., J. Appl. Cryst., 33, 421, 2000

[20]

Web page : www.ibs.fr

[21]

Van Raaij, M.J., Mitraki, A., Lavigne, G., Cusack, S., Letters to Nature, 401, 935, 1999

[22]

Papanikolopoulou et al., to be published

[23]

O'Brian, J.P., et al., ACS Symposium, Charlottesville, Virgina, 544, 104, 1993

[24]

Mc Grath, K., Kaplan, D., eds., Protein Based Materials, Birkhauser, Boston, 1997

[25]

Heslot, H., Biochimie, 80, 19, 1998

[26]

Zhang, S., Altman, M., Reactive and functional Polymers, 41, 91, 1999

[27]

Zhang, S., The Encyclopedia of Materials, Elsevier, Oxford, 5822, 2001

[28]

Benson, M.D., Wallace, M.R., Amyloidosis, in The metabolic basis of disease, Scriver, C., et al., Eds., McGraw-Hill, N.Y., 2439, 1989

[29]

Rueda, D.R., Ania, F., Balta-Calleja, F.J., J. Mater. Sci., 17, 3427, 1982