Appl. Phys. A 74 [Suppl.], S510–S512 (2002) / Digital Object Identifier (DOI) 10.1007/s003390201524

Applied Physics A Materials Science & Processing

Composites reinforcement by rods: a SAS study V. Urban1 , A. Botti2 , W. Pyckhout-Hintzen2,∗ , D. Richter2 , E. Straube3 1 ESRF, BP220, 38043 Grenoble Cedex, France 2 IFF-Forschungszentrum Jülich, 52425 Jülich, Germany 3 University of Halle, FB Physik, 06099 Halle, Germany

Received: 11 July 2001/Accepted: 13 November 2001 –  Springer-Verlag 2002

Abstract. The mechanical properties of composites are governed by size, shape and dispersion degree of so-called reinforcing particles. Polymeric fillers based on thermodynamically driven microphase separation of block copolymers offer the opportunity to study a model system of controlled rodlike filler particles. We chose a triblock copolymer (PBPSPB) and carried out SAS measurements with both X-rays and neutrons, in order to characterize separately the hard phase and the cross-linked PB matrix. The properties of the material depend strongly on the way that stress is carried and transferred between the soft matrix and the hard fibers. The failure of the strain-amplification concept and the change of topological contributions to the free energy and scattering factor have to be addressed. In this respect the composite shows a similarity to a two-network system, i.e. interpenetrating rubber and rod-like filler networks.

regular butadiene and in the blends a mixture of deuterated (DPB) and non-deuterated (HPB) polybutadiene was used. In both cases the molar ratio of the isotopes was chosen to give a net neutron scattering length density, which matches that of polystyrene. In this way, only the butadiene homopolymer should contribute to the SANS signal of the composite samples and the chain deformation in the PB matrix can directly be measured. On the other hand, in a SAXS experiment on the identical samples, only the electron density difference between PB and PS gives rise to scattering contrast, since the electron density differences of HPB and DPB can be neglected. Therefore, complementary information of the deformation dependence of the filler domain morphology is obtained. 1 Experimental

PACS: 61.12.Ex; 61.10.Eq; 61.41.+e Rubber-elastic polymer networks are an interesting class of materials due to their morphological “semi-order” over a length-scale of several decades. The relations between molecular structure of polymer networks and mechanical properties of elastomers are of great technical importance in rubber and tire industry. Reinforcement of the elastomeric polymer network through embedded filler particles is crucial for the optimization of mechanical properties [1]. However, the relationships between the details of the microscopic structure of the filled network and its mechanical properties are still poorly understood. Small-angle scattering provides a unique tool for the investigation of structural changes on the microscopic length-scale as a rubber sample undergoes macroscopic deformation. We investigated a model-system of filled rubber, consisting of cross-linked blends of poly-butadieneblock-polystyrene-block-polybutadiene triblock copolymer (short: PBPSPB) and butadiene homopolymer. The PB blocks of the PBPSPB were statistical copolymers of deuterated and ∗ Corresponding

author. (Fax: +49-2461/61-2610, E-mail: [email protected])

The samples in this experiment are ternary blends of PBPSPB copolymer, deuterated and non-deuterated butadiene homopolymer. PBPSPB and butadiene homopolymers were synthesized via anionic polymerization. The total molecular weight Mw of the triblock was 154 000 g/mol with a volume fraction of PS = 0.28, whereas the homopolymers had a Mw of 135 000 g/mol. The butadiene blocks of the PBPSPB consisted of statistical copolymers of deuterated and regular butadiene in the molar ratio 83.4/16.6. This ratio of hydrogen isotopes matches the average neutron scattering length density of the PS central block. Hence, no intra-chain scattering contribution is expected for the triblock. The polymers were characterized by membrane osmometry in toluene (Knauer), size-exclusion chromatography in THF (Waters 150C, Millipore) and static low-angle laser-light scattering (Chromatix). Cross-linked blends of PBPSPB, HPB and DPB were prepared by dissolving the appropiate amounts of polymers and DCP cross-linker in dried THF, casting the solutions into teflon molds, drying for three days in vacuum and subsequent tempering at 418 K under Ar atmosphere for 3 h to complete cross-linking. Two different, cross-linked blends with effectively 21 and 25% volume fraction of PS and homopolymers in the ratio above were prepared and are referred to as cyl21

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and cyl25 in following. Cross-link densities were measured by swelling in cyclohexane. The scattering experiments were carried out on the undeformed samples and under uniaxial stretching with extension ratios λ = /0 up to 1.5, where  and 0 are the actual and initial length of the sample, respectively. SANS data were recorded at the D11 (ILL) and KWS1 (FZJ) spectrometers at wavelengths 7–8 Å. SAXS experiments were carried out at the ID2 beamline at ESRF. Here, the beam size was 0.2 × 0.2 mm and the X-ray wavelength 0.995 Å. All SAS data were corrected and placed on absolute scale following standard procedures at both beamlines. 2 Results and discussion As described above, in the SAXS experiment the measured intensities arise solely from the contrast of the PS filler domains embedded in the PB matrix and are shown in Fig. 1. The two-dimensional data of the undeformed samples show an interference peak typical for liquid-like correlation of filler domain distances. From the absence of sharp diffraction rings or peaks on top of this, we conclude that no strong ordering in the filler domains exists (e.g. hexagonal packing of rods). However, a small contribution of anisotropic scattering intensity is observed and must be attributed to partial preferred alignment of the cylindrical phases. A texture scan across different positions on the sample showed that the degree and orientation of the alignment varies in a random fashion. Also, the order disappeared after a strain of 10% was applied which proofs the fragility of the structure. We found that this effect

of local texture does not interfere with our analysis of the total scattering pattern discussed below; it does, however, exhibit an interesting deformation dependence, which is not yet fully interpreted but will not be addressed further in this paper. The scattering curves from the PS phases were interpreted in terms of a product of a homogenous circular cylinder form factor [2] and a static structure factor of effective hard spheres in the Percus-Yevick approximation [3]. This model agrees well with the azimuthally averaged data in Fig. 2 and the parameters of the cylinder, i.e. Radius R and mean preferential cylinder distance D0 could be extracted. We find R = 104 Å and D0 ∼ 350 Å for the undeformed sample. This implies that the rods are rather short with low L/R ratio. Upon stretching the position of the correlation peak along the principal deformation axis shifts affinely with the macroscopic deformation tensor. Thus, the microscopic deformation at the probed length scale of about 300 Å corresponds to the macroscopic strain. The cylinder radius of the PS domains, on the other hand, does not change upon stretching, as can be seen from the perseverance of the form factor features in the high-q limit. For a detailed discussion of the decomposition of SANS intensities into contributions from polymer and domain scattering we refer to previous work [4, 5]. The concept has been applied already successfully by the authors [6]. The SANS signal of our samples should purely arise from the single chain form factor of the butadiene homopolymer in the system. The measured scattering curves, however, exhibit some residual PS domain scattering. Since the shape and deformation dependence of the domain scattering function is known from the SAXS experiment, this residual part could

Fig. 1. 2-dimensional SAXS data of sample cyl25 for different strains, given in the legend (from top left to bottom right). The strain direction is vertical. The q-range spans between − 0.06 A−1 and 0.06 A−1 . Indicated axes (0–300) are arbitrary

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Fig. 2. Principal axis data for sample cyl25 at different strains. The form factor minimum at q ∼ 0.04 Å−1 remains unchanged

be subtracted incoherently and the pure PB single chain scattering was obtained. The deformation dependence of the PB chain was then analyzed in terms of a tube-constrained form factor [7]:
1 P(q, λ) = 2


η dη

0



dη 0

 µ

    2 exp −Q µ λ2µ η − η

     η − η 2 − 1 − λµ γµ 1 − exp − γµ

(1)

Here, µ denotes the principal directions of the deformation tensor, Q = qRg is the reduced scattering vector and γ contains the localization parameter or tube diameter d0 . Figure 3 presents the best fit of the tube model to corrected twodimensional SANS data of the sample cyl25 at strains 1.2 and 1.25. The results for sample cyl21 are similar. The agreement of the presented model with the data is excellent. For the tube diameter we find d = d0 λν with d0 = 36 Å and ν = 0.5 in agreement with our previous studies and numerous other experimental evidence. However, we find zero over-strain in the matrix i.e. pure affine deformational behaviour. This is contrary to our results on microphase separated networks with

Fig. 3. 2d SANS fits for λ = 1.2 (top) and 1.25 (bottom) for cyl25. The strain direction is horizontal. Left: low q-data, Right: high q-data

spherical filler domain morphology [6]. For the higher strain of λ = 1.5 even a smaller d0 of 26 Å and ν = 0 are necessary to model adequately the SANS data. This result is new to our experience and needs further investigation. We may conclude that the concept of homogoneous local strain amplification fails as soon as anisometric particles, which give rise to anisotropic strain fields, are involved. Acknowledgements. We acknowledge the European Synchrotron Radiation Facility (ESRF, Grenoble) for provision of synchrotron-radiation facilities.

References 1. 2. 3. 4. 5. 6.

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