Coarsening of nanodomains by reorganization of polysiloxane segments at high temperature in polyurethane/α,ω-aminopropyl polydimethylsiloxane blends

The reaction in bulk at high temperature of α,ω-aminopropyl oligodimethylsiloxane and thermoplastic polyurethane (TPU) allowed observing interesting behavior. Mixing at 200 °C first involved dissociation of urethanes and splitting of polyurethane chains followed by reaction of the released isocyanates with amino end-groups of the oligosiloxane. At this stage, a copolymer was formed which morphology consisted of a very fine dispersion of the polysiloxane domains at the nanoscale (20 nm) with a narrow size dispersity. The polymer blend was perfectly transparent. Increasing the reaction time resulted in a significant coarsening of the morphology and a consequent loss of transparency. The reason for such a morphology evolution has been elucidated. The progressive formation of alkyl–alkyl urea linkages at the expense of aryl–alkyl bonds obtained earlier in the process caused an increase in the average number of successive polydimethylsiloxane (PDMS) blocks that organized in larger domains. The average number of consecutive polysiloxane segments was found to evolve from ∼1.5 in the first 10–15 min to a value of 3–5 at the end of the reactive process.

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► An amino-terminated oligodimethylsiloxane was reactively blended with a TPU at 200 °C.
► Some urethanes were split, followed by reaction of released NCOs with NH2 end-groups.
► A copolymer was formed leading to a nanometric dispersion of PDMS in the TPU matrix.
► For longer times a coarsening of morphology was observed, the material became opaque.
► This was due to a progressive increase in the length of PDMS blocks in the copolymer.