Fast self-repair mechanisms in plants: Biological latices as role models for the development of biomimetic self-healing, mechanically loaded polymers

Polymers are used in many fields of application with high mechanical loads. Due to micro-cracks components may fail spontaneously even (far) below the maximum critical load. Thereby these micro-cracks grow up to overcritical lengths due to cyclic loads, become unstable and finally lead to a catastrophic failure. Stopping or even healing the crack may inhibit or at least decelerate crack expansion. Intrinsic and autonomic healing processes of noncritical micro-cracks found in nature like by polymerization of latices as sealing agent of a fissure are investigated up to now only rudimentary and may serve as models for a transfer into technical applications. As known since several years, the latex of Hevea brasiliensis seals the lesion after injury by cross linking rubber particles with the protein hevein, which leads to coagulation of the latex. In our project the latices of H. brasiliensis and Ficus benjamina were compared by quantitatively analyzing structural and functional parameters, as particle size distribution, zeta potential, IR-spectra and rheology. Measurements of particle sizes were performed to characterize latex particles and the process of coagulation. Latices of both plant species showed a bimodal distribution whereas particles of F. benjamina latex are larger than those of H. brasiliensis latex. After initiation of the coagulation process a third peak of (much) larger particles was detected in the latex of H. brasiliensis but not in the latex of F. benjamina. The electric charge of particles in a suspension is specified by the zeta potential. The zeta potential of F. benjamina latex approaches an isoelectric point at about pH = 7. So in neutral media, such as the latex itself, the rejection of particles is minimized due to uncharged particles and thus the coagulation process of the latex is eased. IR-spectroscopic measurements were conducted to analyze the formation of new chemical bondings during the process of coagulation and revealed greatest changes in the spectra during 6 to 10 minutes after tapping in both latices of H. brasiliensis and F. benjamina. In this time span an increase in amid-, CH2- and CH3-bondings became apparent which supports the above mentioned theory of coagulation of H. brasiliensis latex. Observed changes in the transparency of the latices during this time span go along with these findings. The findings of rheologic measurements show an abrupt increase in the viscosity of both lattices. The point of time of these changes depends on the applied shear stress during the measurement and on the sample volume.