Polyurethane (PU) with lactate structures in its conformation can be used as a biological and biodegradable polymer. Polyurethane lactate (PUL) was doped with small quantities of an erbium (Er3+) complex, which hindered the N=N group. 2,2'-Dihydroxyazobenzene was used as a ligand for the Er3+ complex. PUL in the presence of the used Er3+ complex caused water from the complex to be replaced by the polar structures from the polymer. These structures acted as crystallization germs for the structuration of the PU matrix. As a consequence, both the mechanical and thermal properties of the polymer were improved. The sample with the highest mechanical properties contained only 0.064 mu mol of Er3+ complex/g of sample. (c) 2006 Wiley Periodicals, Inc.