Highly branched, segmented polyurea elastomers through oligomeric a(2) + b-3 approach

Abstract

Oligomeric A2 + B3 approach was used for the preparation of highly branched, elastomeric, segmented polyurea, polyurethane and poly(urethane urea) copolymers where A2 was a difunctional monomer or oligomer and B3 was a trifunctional monomer or oligomer. In most cases, oligomeric A2 had isocyanate functional ends, whereas B3 was a triamine or triol. Three different synthetic methods were investigated; where (i) A2 was slowly added over B3, (ii) B3 was slowly added over A2 or (ii) both reactants were mixed together at the beginning and reacted. A2 type oligomeric soft segments included PTMO, PEO, PPO and PDMS with number average molecular weights between 1000 and 10800 g/mol. A PPO based B3 oligomer with average branch length of 1000 g/mol and low molecular weight triamines with different structures were also used. The diisocyanates utilized were bis(4isocyanatocyclohexyl)methane (HMDI), 1,6-hexamethylene diisocyanate (HDI) and trans1,4-cyclohexyl diisocyanate (CHDI). Gel points were determined experimentally and with Monte Carlo simulations, for reactions where A2 was slowly added onto B3 or vice versa at different solution concentrations. When B3 was added onto A2 gelation took place at lower conversions when compared with the reaction where A2 was added on B3. A number of highly branched polymers with different compositions were synthesized. Materials obtained were characterized by FTIR spectroscopy and thermal and mechanical tests. DMA and stress-strain tests indicated that for polymers with similar chemical compositions those prepared by the addition of A2 over B3 possessed better mechanical properties than those prepared by B3 over A2 addition. As expected the end capped polymers showed better mechanical strength than their uncapped counterparts like the chain-extended ones. Modeling studies were also done to determine the influence of reaction procedure on polymer topology for; (i) A2 over B3 addition, (ii) B3 over A2 addition, and (iii) mixing A2 and B3 together at the beginning. Structures formed by the addition of B3 over A2 displayed different topologies and possessed much higher degree of branching than other two polymerization methods, which interestingly displayed very similar topologies.