Synthesis, properties and structure development of variable modulus polyisocyanurate materials for damping applications.

Damping vibrations in mechanical parts by the transformation into heat can be readily achieved by making use of the viscoelastic properties of polymer materials. The damping capability, tan δ, can be described as the ratio of the loss modulus, G", to the storage modulus, G'. This has a maximum around the glass transition temperature, Tg, for polymeric materials. Varying the composition of a polymer material is therefore a very important method of controlling the Tg. The use of polyisocyanurates gives good control of the Tg and the resultant materials have high thermal stability.
A series of poly(ether-isocyanurates) was synthesised by the end capping of hydroxyl-terminated polyoxypropylenes with methylene diphenyldiisocyanate (MDI), forming urethane links and leaving terminal isocyanate groups. The subsequent heat-activated, catalysed trimerisation of the isocyanate end groups gave the isocyanurate group which effectively cross-linked the material. Varying both the polyether (soft segment) chain length and the MDI (hard segment) content gave a range of samples with varied moduli. Morphology and subsequent mechanical properties are determined by the microphase separation of the hard and soft segments.
Bulk, layered and graded poly(ether-isocyanurates) were prepared and analysed by fourier transform infra-red microscopy and laser ablation mass spectroscopy. The formation of the gradient in modulus was attributed to a separation of the polymers due to density differences.
The morphologies of the poly(ether-isocyanurates) were investigated using synchrotron small angle x-ray scattering (SAXS). Development of this morphology was also studied by performing in situ time resolved experiments. Phase separation domain size varied with polyether chain length and with hard segment content. Microphase separation was observed to occur via spinodal decomposition (Cahn-Hilliard theory). The Teubner-Strey micro emulsion model, based on a Ginzburg-Landau free-energy expansion, was applied to interpret the development of a bicontinuous morphology. The bicontinuity was preserved due to the vitrification of the polyisocyanurate domains.
Mechanical measurements were conducted using a nanoindenter. Static measurements gave values of indentation modulus from 5 MPa to 3 GPa. These results were modelled using a modified version of the Kerner equation. Phase inversion of glassy particles in a rubber matrix to rubber particles in a glassy matrix was observed for materials with a polyether soft segment number average molecular weight of 2200 with increasing hard segment content. The modulus variation of the gradient was studied by the positionability of the tip. Dynamic nanoindentation data were combined using time-temperature-superposition, and values of tan δ were evaluated over a range of frequencies for a number of samples.
Blends of polystyrene-polyisoprene-polystyrene triblock copolymer and homopolystyrene with well documented properties were also investigated to compare with the poly( ether-isocyanurate) elastomers, and similar changes in modulus and domain size with hard segment content were observed. Good correlations were found between microstructures from SAXS and the mechanical properties from nanoindentation measurements for both systems.