Characterization of polyurethane hard segment length distribution using soft hydrolysis/MALDI and Monte Carlo simulation

In this study, the distributions of hard segment lengths in flexible and viscoelastic polyurethane foams are investigated. To that extent, MALDI spectra of hard segment chains from polyurethane (“PU”) viscoelastic (“Visco”) and conventional flexible (“Flex”) foam series were obtained and analyzed. Results indicate that within each foam series, there was no significant difference at different water levels. However, comparing Visco and Flex shows that the distribution is shifted to lower urea count for the case of Visco. The experimentally determined peak in the distribution was 1–2 urea repeats, in contrast with what is reported in literature. To understand the fine impacts of formulation, Monte Carlo simulations were performed on the Visco and Flex foam formulations, with urethane-urea catalysis ratio and water levels being varied within each foam series. Experimentally obtained distributions agree very well with the simulation results. Values for number-average degree of polymerization from simulations agree with those predicted from the Schulz–Flory distribution. Furthermore, one finds that the while the calculated shape of the hard segment length distributions is predicted to be sensitive to the catalytic enhancement factor, the average hard segment length values are predicted to be essentially independent of catalysis. The fraction of diurethane linkages was calculated and is found to be sensitive to catalysis (more urethane catalysis = more diurethane linkages). Furthermore, the fraction of diurethane linkages as simulated from foam formulation is correlated well with the Tg of the Flex foam series as obtained from DMA. The same relationship for the Visco foam series is also discussed.