Designer poly(urea-siloxane) microspheres with controlled modulus and size: Synthesis, morphology, and nanoscale stiffness by AFM

Crosslinked poly(urea-siloxane) (PUS) copolymer microspheres with diameters ranging from 0.8 to 1.8 μm with diameter polydispersity indices between 1.15 and 1.60 were synthesized in one-step precipitation polymerization in water-acetone solvent mixtures at room temperature. The spheres were obtained using aminopropyl-terminated siloxanes, isophorone diisocyanate and a tetrafunctional isocyanate crosslinker, employing a systematically varied molar ratio. The length of the siloxanes was controlled to obtain spheres with pre-determined Young's modulus values. Phase separation between soft disiloxane/poly(dimethylsiloxane) segments and hard isophorone bisurea units was observed with an excellent spatial resolution of ∼5 nm using atomic force microscopy (AFM). AFM mapping of the Young's modulus was achieved utilizing the PeakForce Quantitative Nanomechanical Mapping (QNM) mode. This AFM method allowed us to also measure the values of elasticity moduli of individual microspheres, ranging from 200 to 900 MPa. The microspheres can be used as filler to fine-tune the properties of composite materials, particularly with regard to elasticity. The hydrophobicity was also varied as indicated by water contact angle values between 122° - 132°. These features open the possibility of preparing designer composites for a range of applications from coatings to the biomedical field.