Polymers

Company or institute

Maastricht University/Aachen Maastricht Institute for Biobased Materials and Corbion Purac Biomaterials.

Challenge

Fused deposition modelled (3D printed) poly-lactide polymers have strong layer strength but poor interlayer strength. This study was to test to what length-scales interfacial diffusion of polymers occurs and whether stereocomplexation between polymers of opposite enantiomeric composition in adjacent layers could be used to strengthen interlayer bonding. Stereocomplexation is a phenomenon whereby thermodynamically more stable crystals are formed by the co-crystalisation of equal quantities of enantiomers (D and L forms). With respect to stereo-crystals, polymeric crystals of a single, pure enantiomeric composition (homo crystals) possess a low crystallisation rate and a melting temperature that is about 50°C lower, facilitating mild processing and molecular diffusion across inter-layer welds. This X-ray study aimed to show the degree of stereocomplexation at the interface between two layers of polymers of opposite enantiomeric composition.

Sample

Two stacked disks of opposite enantiomeric composition were pre-compacted and successively molten in a rheometer, using an optimised temperature above the melting point of the individual polymeric samples and mechanically controlling the degree of stereo-crystallinity between the two layers. Stereocrystallisation was followed for 60 minutes, after which the sample was cooled to room temperature inducing homo-crystallisation of the unmixed polymers.

Solution

Wide-angle X-ray diffraction computed tomography at beamline ID15A was used to study the cooled disks. Measurements were performed with a resolution of 20 × 100 × 100 µm³. The tomographic reconstruction based on voxel specific analysis revealed the contributions of the stereo-crystalline, homo-crystalline and non-crystalline phases to the diffraction pattern in three-dimensional space.

Tomographic reconstruction of a PLLA−PDLA rheometry disk based on wide-angle X-ray diffraction, visualising the spatial distribution of homo- and stereocrystals. a) Layer (z) averaged integrated diffraction patterns as a function of the z-axis. b) The spatial distribution in 3D. The reconstruction is based on the integration of the (110)/(200) and (110) diffraction signals of the homo- and stereocrystals, respectively (red = high intensity; blue = low intensity). c) The averaged population of homo- and stereocrystals as a function of sample thickness. (Image credit: V. Srinivas et al.)

Benefits

Diffraction contrast tomography provided unique information about the phases that could not have been obtained through other tomography methods because of the identical density of the phases. The 3D map revealed the extent of stereocrystallisation between phases of opposite enantiomers. The relationship discovered through this research between the time- and length-scales of stereo-crystallisation has been successfully applied to fused deposition modelling, using alternating layers of poly-D-lactide and poly-L-lactide polymers to create mechanically stronger interfacial bonds through controlled physical interlocking of polymer molecules originating from different layers.

Reference

Interfacial stereocomplexation to strengthen fused deposition modeled poly(lactide) welds, V. Srinivas, C.S.J. van Hooy-Corstjens, G.B.M. Vaughan, B. van Leeuwen, S. Rastogi, and J.A.W. Harings, ACS Applied Polymer Materials 1, 2131-2139 (2019); doi: 10.1021/acsapm.9b00421.

Company

Continental Reifen Deutschland GmbH

Challenge

The challenge was to accurately estimate the size distribution of the macro-dispersion of carbon black filler in a rubber matrix through a synchrotron experiment. This was to provide a high-accuracy 3D reference scan of the bulk material with which to compare the size distribution of the filler particles obtained from a new optical microscopy technique, radiometric stereo microscopy, used to examine the surface of freshly made planar sections, i.e. “fresh cuts”, of rubber, for future use in quality control.

Sample

The sample was a rubber in an early state of carbon black filler incorporation.

Solution

Tomographic data sets were acquired at beamline ID19 using propagation-based phase contrast to enhance contrast between the filler and the rubber matrix.
 

Tomographic image showing carbon black filler particles in rubber: volume rendering of a sub‐volume of 1100 × 1000 × 1300 pixels out of the complete 3D data set with 2560 × 2560 × 2160 pixels of size 0.65 μm. Filler particles (or particle agglomerates) of a volume larger than 100 μm3 are highlighted in red. Credit: J. Ohser et al., Journal of Microscopy 274, 32–44 (2019); doi: 10.1111/jmi.12782.

Benefits

Partially-coherent illumination and phase-retrieval techniques made it possible to exploit the full (complex-valued) refractive index of the materials. The contrast obtained was an order of magnitude better in sensitivity to material changes than the plain attenuation signal. This allowed the different material components to be identified directly in the grey-scaled tomographic image: the rubber matrix as well as the filler particles.

Macro-dispersion of globular filler particles (e.g. carbon black or silica) in a rubber matrix is an important quantity that depends on manufacturing parameters and influences various rubber properties. Therefore, it must be carefully adjusted during the incorporation process and investigated by industrial quality control. The synchrotron experiment provided a benchmark used to gauge the accuracy of radiometric stereo microscopy which would be used for manufacturing quality control of rubber products such as vehicle tyres.

Reference

Estimation of filler macro-dispersion in rubber matrix by radiometric stereo microscopy, J. Ohser, J. Lacayo-Pineda, M. Putman, A. Rack & D. Dobrovolskij, Journal of Microscopy 274, 32–44 (2019); doi: 10.1111/jmi.12782.

Company

University of Manchester and Rolls-Royce

Challenge

Crack propagation in metallic materials is well understood. But aircraft manufacturers are increasingly turning to more complicated composite materials that are lighter, stronger and can operate at higher temperatures. Lower weight reduces fuel consumption, while higher engine operating temperatures allow aeroengines to be more efficient. The challenge is to understand how cracks propagate in such materials.

Sample

Titanium reinforced with silicon carbide fibres. This composite material can operate at higher temperatures than titanium alone, making it a promising candidate for jet engine parts.

Solution

Electron microscopy reveals the surface features of micro-cracks, but synchrotron X-rays penetrate tens of millimetres into a sample where the behaviour of cracks can be very different. On beamline ID15, scientists can use imaging, to see how cracks grow, and diffraction, which tells them about the local stresses that the cracks grow under.

Benefits

The ability to monitor cracks under load at high temperatures allows researchers to evaluate the potential of these materials under realistic conditions. It also helps to make realistic estimates

of the lifetime of existing components and to design safer, more crack-resistant materials for the future.

Better knowledge of crack propagation transfers directly to other industries in which failure is unacceptable, notably the nuclear industry.

Proc. R. Soc. A 468 2722.

Acta Materialia 60 958.

3D crack-tip microscopy shows a crack (purple) growing in a composite material containing silicon carbide fibres.