Characterization of Polyester−Polyurethane Soft and Hard Blocks by a Combination of MALDI, SEC, and Chemical Degradation

Abstract

Size exclusion chromatography and matrix-assisted laser desorption ionization mass spectrometry (SEC/MALDI) coupled with selective degradation reactions have been applied for characterization of polyurethane soft and hard blocks. A series of model PUR's were prepared from 4,4‘-diphenylmethane diisocyanate (MDI) and poly(butylene adipate) (pBA)−polyols with molecular weights of 1000 and 4000 Da. The weight ratio of the pBA polyols was varied: 1:3, 1:1, and 3:1; the amount of MDI was adjusted accordingly. In these model PUR systems no additional chain extender was added in addition to that in the polyester soft segments (butanediol), as a consequence their Flory distribution was used. Therefore, the model systems only have a minimum of so-called hard segments (oligo urethanes consisting of MDI and butanediol). Molecular weights of soft blocks, liberated by isocyanatolysis using phenyl isocyanate and measured by SEC/MALDI, showed reasonable agreement with those estimated from tandem light scattering and SEC (MALS/SEC). The increase in molecular weights observed with increasing amounts of pBA4000 indicated that selective degradation combined with SEC/MALDI is sensitive to the polymer soft block composition. Polydispersity indices (PDs), determined for the soft blocks recovered from phenyl isocyanate degradation, were lower than those expected on the basis of reaction theory. Partial acid-catalyzed hydrolysis was applied to determine the hard block chain length distribution for polyester-based PUR samples having different amounts of MDI. MALDI spectra of the degraded products provided proof for a degradation mechanism proposed in the literature. The results presented here demonstrate that applying partial acid hydrolysis to polyester−polyurethane generates exclusively a series of hydroxy-terminated oligomers, which can be identified as former hard segments of the polyester−polyurethane elastomer. The methodology hydrolyzes selectively all ester bonds while leaving the urethane groups containing hard segments completely intact, thus providing an additional tool for the complete characterization of polyurethanes.