SYNTHESIS AND MECHANICAL ANALYSIS OF
TRANSIENT-COVALENT DOUBLE NETWORKS
C. H. Hövelmann1, B. Gold1, A. Brás1, J. Allgaier1, W. Pyckhout-Hintzen1, A. Wischnewski1 and D. Richter1
1 JCNS : Jülich Centre for Neutron Science, Forschungszentrum Jülich,
Wilhelm-Johnen-Straße, 52428 Jülich, Germany – email : c.hoevelmann@fz-juelich.de; b.gold@fz-juelich.de; a.bras@fz-juelich.de; j.allgaier@fz-juelich.de; w.pyckhout@fz-juelich.de; a.wischnewski@fz-juelich.de, d.richter@fz-juelich.de
Keywords: self-healing, network, elastomer, hydrogen bond ABSTRACT
Nature uses a combination of dynamic hydrogen bonds and static covalent bonds in e.g. the muscle sarcomer to achieve toughness in otherwise elastic materials. The dynamic bonds can open when a force is applied, thereby providing a stress-relieve mechanism to prevent rupturing of the covalent links. Upon release of external force the hydrogen bond network can heal to restore the former properties. To mimic these astonishing effects we envisioned a similar combination in elastomeric compounds by creating a dual network of hydrogen bonded and covalently linked polyisoprene. We synthesized a number of dual networks by first modifying polyisoprene with hydrogen bond forming urazole groups to form supramolecular networks with a cross-linking density between 0 and 15 mol%. In a second step covalent crosslinks were added by hydrosilylation with a bissilane linker.
Linear rheological analysis of the transient network showed a dissipation mechanism extrapolated to be on the order of 0.01 - 0.10 s at ambient temperature. In combination with neutron scattering, which provides unrivalled and first insights on the chain level, the self-healing properties of these novel semi-transient networks are studied on the molecular level in static and dynamic deformation through a selective labelling of chains. Understanding of the self-healing mechanism in these mixed covalent and transient systems will allow the development of new polymeric materials with advanced functionality.
1. INTRODUCTION
The combination of dynamic and static bonds can lead to improved material properties. This unique design is credited with the unusual combination of strength, toughness and elasticity that is found in muscle proteins [1]. We envisioned to mimic this properties by constructing a twice crosslinked elastomeric network consisting of transient links by H-bonding urazole connections and permanent links via a covalent connection by hydrosilylation crosslinking.
Figure 1: Elastomeric double networks with transient and covalent crosslinks. 2. MATERIALS
Polyisoprene and polybutadiene were synthesized by standard anionic polymerization techniques using t-butyl lithium as initiator and cyclohexane as a solvent with a molecular mass of M=100K and a PDI of 1.02. The urazole functionalization was performed according to known literature procedures [2] by reacting a THF solution of the polydiene with 4-Phenyl-1,2,4-triazoline-3,5-dione to give a number of functionalized polyisoprenes with 1,2,4,8 and 15% of the monomer units functionalized. These transient networks were further crosslinked by hydrosilylation with a 4,4´-bis(dimethylsilane)biphenyl linker using 0.1mol% of Karstedt´s catalyst at 95°C. This final crosslinking was performed in bulk state in a teflon mold to obtain 15x60x1 mm elastomer films that were used in all further investigations.
Figure 2: Teflon mold (left) and polyisoprene double network (right) 3. METHODS
The rheological and mechanical experiments on the uncrosslinked and crosslinked networks respectively were conducted using an Advanced Rheometric System
(ARES, Rheom. Sci. Instrum.), equipped with a 2K-FRT transducer using a parallel plate geometry for the dynamical modulus G*(w) in the temperature range of -40 to +25°C and frequency range between 0.01 and 100 rad/s, and a tensile testing tool for the measurement of the normal stress as a function of the travelled distance at ambient temperature. The normal force was recorded as a function of time during the extension to +/- 100%.
4. RESULTS
We synthesized a number of polydiene transient-covalent double networks by combination of an ene-reaction for the urazole functionalization and hydrosilylation for the covalent crosslinking. Rheological experiments were performed both on the transient and the double networks. To investigate the dissipation of the H-bonds step-shear as well as oscillatory shear experiments were performed on the urazole functionalized polyisoprene. The transient network component is evidenced from a pronounced new loss peak in G"(w) and corroborated from the time dependence in a separate step-shear and subsequent relaxation at the temperature of -20°C as is shown to avoid the onset of thermorheological complex behavior. The increase of the storage modulus as well as the slowing down of the dynamics compared to the unmodified polymer points at an additional elastic contribution from the H-bonds.
Figure 3: Shear modulus of polyisoprene with 2% urazole functionalization. The evaluation of the stress-strain data on the crosslinked networks with and without supramolecular functionalization was limited to the lowest strains where Hookean behavior i.e. 𝜎 = E ε should prevail. The increase of the Young modulus E reflects the additional networking and compares well to the increase in the high frequency plateau of the storage modulus G'(w).
Figure 4: Stress-strain experiment on a double network compared to the corresponding covalent crosslinked network.
5. CONCLUSIONS
We synthesized a new elastomeric material that combines transient and covalent crosslinks. Preliminary rheological experiments show a dissipation of the H-bonds in the transient networks. Stress-strain experiments show the contribution of the transient bonds to the Young modulus of the compound. Both can be combined to achieve new adaptive systems for dynamical applications.
REFERENCES
[1] R. J. Wojtecki, M. A. Meador, S. J. Rowan, Using the dynamic bond to access macroscopically responsive structurally dynamic polymers, Nature Mater. 10 (2011) 14-27
[2] R. Stadler, J. Burgert, Influence of hydrogen bonding on the properties of elastomers and elastomeric blends, Makromol. Chem. 187 (1986) 1681-1690
