Strain-induced crystallization in natural rubber

The basic concept, shared by all fields of research in the Mechanics and Structure Department, is the development of an understanding of the macroscopic mechanical behavior of polymers based on their microstructure.

The strain-induced crystallization in natural rubber presents a particularly intriguing case in this respect. Stretching a piece of natural rubber at room temperature turns the amorphous rubber into a semicrystalline material. These crystallites are highly oriented along the tensile direction and, acting like filler particles or crosslinks, tremendously elevate the tensile strength (fig. 1). The tire industry takes advantage of these outstanding mechanical properties, using compounds high in natural rubber for truck tires to extend the tire lifetime. Due to the local reinforcement of strain-induced crystallites at the highly strained crack tip, the crack growth resistance in natural rubber is orders of magnitude higher than in conventional synthetic non-crystallizing rubbers (fig. 2).

Fig. 1: Stress-strain curves of natural rubber (NR) and styrene-butadiene rubber (SBR). The strain-induced crystallization leads to an upturn at high strains at enhances the ultimate properties.
Fig. 2: Crack growth rate vs. tearing energy for natural rubber (NR) and styrene butadiene rubber (SBR), obtained from a tear fatigue test (TFA).

Despite the economical importance, the scientific community is far from a quantitative understanding of the strain-induced crystallization and its implications on the mechanical behavior. While the basic relationships between crystallization and strain, temperature and filler content have been established over the last two decades, the time-dependence of the strain-induced crystallization has only recently gained interest by researchers, despite its significance for the engineering of rubber parts. For instance, tires are dynamically loaded systems, with mechanical time scales typically being much shorter than the crystallization time scales.

To study the crystallization in natural rubber at short time scales and on small length scales, e.g. around a crack tip, we perform in-situ mechanical experiments at a synchrotron. In a collaboration with Stephan Roth, we set up various experiments at the MiNaXS beamline at DESY (Deutsches Elektronen-Synchrotron, Hamburg). Several miniature tensile machines, specifically designed by IPF Forschungstechnik, allow to study the microstructure evolution under complex loading scenarios (fig. 3). Crack tip scans by wide-angle X-ray diffraction take advantage of the microfocus beam and give insight into the reinforcement in the crack process zone. Strain-jump experiments, stretching the rubber sample in less than 10 ms to several hundred percent of strain, make use of the high photon flux of the MiNaXS beamline and allow to establish a kinetic law for the crystallization rate (fig. 4).

Fig. 3: Miniature tensile machine designed and built by IPF Forschungstechnik for in-situ tensile experiments at the synchrotron. Two actuators stretch the sample in a symmetric fashion. Electric motors rotate the sample with respect to the beam.
Fig. 4: Degree of crystallinity vs. time for natural rubber filled with 20 phr N234, after a strain jump at t = 0 s from 0 % to [240, 510] % (from yellow to black). After a fast initial increase, crystallization proceeds on a time scale of several seconds.

Performing dynamic cyclic experiments at a frequency of 1 Hz showed that under dynamic conditions, which are e.g. encountered in a tire under operation, the crystallinity is considerably suppressed as compared to the quasistatic level (fig. 5). This directly affects the mechanical behavior, in particular the tear properties under fatigue conditions. These were investigated in dynamic crack tip scans, combining time-resolved and spatially resolved setups to give a direct view at the crystallinity distribution around a crack tip under fatigue loading in real time (fig. 6).

The large amounts of scattering data (up to 50 GB per experiment) are processed in self-written code in the image processing language PV-Wave, based on the concept by Professor Norbert Stribeck (TU Hamburg) (fig. 6).

The work has been funded by the Deutsche Forschungsgemeinschaft in the DFG Forschergruppe FOR597 “Fracture Mechanics and Statistical Mechanics of Reinforced Elastomeric Blends”.

Fig. 5: Crystallinity vs. time in unfilled natural rubber under dynamic load with a frequency of approx. 1 Hz. Whereas the static degree of crystallinity is 6%, it does not exceed 2% at the same strain level under dynamic conditions.
Fig. 6: Crystallinity map of a crack tip in natural rubber filled with 40 phr N234 at 70% global strain. Each pixel corresponds to 100x100 µm².

Selected Publications:

K. Brüning, K. Schneider, S.V. Roth, G. Heinrich: Kinetics of strain-induced crystallization in natural rubber studied by WAXD: Dynamic and impact tensile experiments. Macromolecules 45 (2012), 7914-7919

K. Brüning, K. Schneider, K. Heinrich: Deformation and orientation in filled rubbers on the nano- and microscale studied by X-ray scattering. Journal of Polymer Science Part B: Polymer Physics 50 (2012), 1728-1732

K. Brüning, K. Schneider, S.V. Roth, G. Heinrich: Strain-induced crystallization around a crack tip in natural rubber under dynamic load. Polymer 54 (2013), 6200-6205

K. Brüning, K. Schneider, G. Heinrich: In-situ structural characterization of rubber during de-formation and fracture; in: M. Klüppel, W. Grellmann, G. Heinrich, K. Schneider, M. Kaliske, T. Vilgis (Editors): Fracture Mechanics and Statistical Mechanics of Reinforced Elastomeric Blends (Lecture Notes in Applied and Computational Mechanics), 43-80, Springer 2013