Thermo-mechanical characterization of polyurea variants

Abstract

The aim of this study is to investigate the role of chain length and hard segment dispersion within the soft segment on thermo-mechanical properties of a block copolymer by formulating and comparing three different polyurea compositions. Polyurea (PU) was synthesized by reacting oligomeric poly (tetramethylene oxide) (PTMO) diamines (Versalink, Air Products) with aromatic 4,4’-methylenediphenyldiisocyanate (Isonate 143L Modified MDI, Dow Chemicals). Three Versalink prepolymers were used: VP-1000 (long chain length), VP-650 (medium chain length), and VP-250 (short chain length), where 1000, 650, and 250 refer to the molecular weight of the PTMO repeating units. PU-105 and PU-605 were synthesized by mixing VP-1000 and VP-650 with Isonate 143L. A hybrid polyurea specimen, PU-HB05, was synthesized by reacting a blend of VP-1000:VP-250 at 76:24wt% with Isonate 143L. This weight ratio was calculated in order to ensure the total proportion of hard to soft segments of PU-HB05 match that of PU-605. A comparative study on PU-605 and PU-105 was conducted to determine the effect of diamine chain length physical, morphological, and mechanical properties. Concurrently, a blend of two diamines with significantly different chain lengths (short chain length Versalink P-250 and long chain length Versalink P-1000)was synthesized with the same diisocyanate prepolymer to create a hybrid variant of polyurea, PU-HB05. The blend was created in a proportion such that the hybrid polyurea PU-HB05 has the same hard to soft domain ratio as PU-605. It was shown that inclusion of short chain length diamine can increase the soft segment Tg by almost 15 ∘C in PU-HB05 compared to PU-105. Furthermore, the modulus in the hybrid blend formulation is also increased compared to PU-105, to the levels of PU-605. However, PU-HB05 demonstrates slightly better stability against high temperatures compared to PU-605. The high strain-rate measurements (including yield-like behavior and hardening plateau) using Split Hopkinson Pressure bar (SHPB) setup, indicate that in spite of close behavior of PU-HB05 and PU-605 (due to matching hard domain proportion), PU-HB05 appears to show less hardening post-yield and generally softer response at lower temperatures due to the presence of longer diamine chains. Master curves, relaxation spectra and relaxation modulus are reported for all three variants. Finally thermal properties of the three materials are reported indicating a more amorphous structure for PU-HB05 based on its lowest thermal conductivity and low heat capacity.