Polymeric materials based on reversible supramolecular assemblies

Reversible polymers are formed by quadruple hydrogen bonding between two 2-ureido-4-pyrimidone units of difunctional compounds. Due to the high association constant (~ 108 l/mol) a very high molecular weight linear polymer is obtained. The properties are determined in solution and in the bulk. Difunctional compounds (molecular weight 800 g/mol) form viscous solutions in chloroform and the viscosity is highly concentration and temperature dependent. Like covalently linked polymers, reversible polymers exhibit shear thinning in the melt. Further a master plot of the storage (G') and the loss (G") modulus is constructed. From the master plot a rubber plateau around 5·105 Pa at 30 °C is determined, which corresponds to a molecular weight of around 5·103 g/mol between entanglements (Me). In solution and in the bulk a strong temperature dependency is observed. It is noticeable that at low frequency another plateau in the storage modulus is reached, indicating the presence of a network at low frequencies. The compounds exhibit elastic properties at room temperature. When a small test bar is released after stretching, it almost completely relaxates back to its initial length. We extended our study to reversible networks by using a trifunctional low molecular weight compound. This compound also exhibit shear thinning in the melt and viscoelastic behavior. The plateau modulus is at a higher level when compared with the plateau modulus of the difunctional compound, at 90 °C the plateau is above 106 Pa. The high plateau modulus indicates that indeed a tightly cross-linked network is formed. Due to the reversibility of the hydrogen bonds, self-healing leads to this densely cross-linked network. In the plot of the storage modulus also a second plateau is discernable at low frequencies. This trifunctional compound can act as a cross linker when an amount is added to a difunctional compound. The properties of these materials are currently investigated. In conclusion, we have shown that low moleculair weight compounds with ureidopyrimidone units form reversible polymers and exhibit properties ordinarily only observed for high molecular weight compounds.