Boron–Oxygen Precursor Engineering Enhances Shear Stiffening and High-Strain-Rate Mechanical Response of Polyborosiloxane Gels

Polyborosiloxane-based shear-stiffening materials are promising candidates for flexible impact-protection systems because they combine a strong high-strain-rate mechanical response with long-time scale deformability. However, in conventional dehydration–condensation synthesis, improving the high-strain-rate mechanical response generally requires a higher boric acid feed to enhance the contribution of dynamic boron-containing linkages. At higher feed levels, direct incorporation of boric acid can lead to macroscopic heterogeneity and loss of gel deformability, thereby limiting the accessible high-boron composition range. Herein, a boron–oxygen precursor strategy was developed by reacting boric acid with dichlorodimethylsilane to regulate the formation of dynamic boron-containing networks in polyborosiloxane gels. This precursor route converts boric acid into borosiloxane oligomeric structures bearing multiple reactive B–OH functionalities, which alter how boron-containing species are incorporated into the network and thereby enable higher boron incorporation while retaining a macroscopically homogeneous, deformable gel state. The resulting Si–O–B linkages can act as transient load-bearing junctions. Compared with conventionally synthesized gels, the precursor-derived polyborosiloxane gels exhibit stronger frequency-dependent shear stiffening and higher storage modulus. The resulting high-boron-content gel achieves a dynamic compressive modulus of 123.43 MPa at a strain rate of 12 000 s–1 while maintaining pronounced cold-flow behavior. Moreover, under high-strain-rate loading, its mechanical response falls between those of conventional soft elastomers and higher-modulus polyurethane materials under high-strain-rate loading, while it also exhibits relatively high dynamic wave impedance and favorable self-healing capability. This precursor-based network design strategy provides a pathway for developing flexible polymeric protective materials that reconcile high-strain-rate mechanical response with long-time scale deformability.