Tack and Related Properties of Isopropyl Azodicarboxylate Modified Polybutadiene

Abstract Polybutadiene has been modified by reaction with isopropyl azodicarboxylate (IAD). The reaction is quite efficient, resulting in a structure in which there are pendant hydrazoester groups dangling from the BR backbone. The Tg's of the modified polymers increase with IAD content. Eventually, at high levels of modification, two Tg's are observed, indicating the presence of two phases. Infrared analysis indicates the presence of H-bonding within the modified polymers. The green strength of these polymers increases with IAD content, whereas, tack passes through a maximum with increasing IAD level. At low IAD levels, the increase in green strength is attributed to increased polar interactions between chains, whereas, at higher IAD levels, there appears to be an additional contribution from strain-induced phase separation (see Reference 1). This type of behavior is favorable for high tack. The initial increase in tack as IAD is incorporated is due, in part, to the enhanced green strength. A more important feature is that the green strength is enhanced without great loss of the molecular mobility needed to form a tack bond. This occurs because of 1) the slow rise in Tg and 2) a lowering of the entanglement density with modification. Blocky additions of the IAD contributes to the former effect as does the presence of the alkyl group (isopropyl) in the polar IAD moiety. Additionally, it is inferred that the IAD-BR molecules do not have to interdiffuse as far across the interface to develop a strong tack bond as do purely hydrocarbon BR chains. Evidence for this is the rapid initial increase in relative tack for the IAD modified elastomers. Finally, the loss of tack at high IAD levels is due to a severe decrease in chain mobility, thereby prohibiting the formation of an extensive tack bond.