Core and surface microgel mechanics are differentially sensitive to alternative crosslinking concentrations

Area of Science:

• Materials Science
• Polymer Science
• Nanotechnology

Background:

• Microgel mechanics are crucial for stimuli-responsive materials and nanotherapeutic delivery.
• Assessing sub-micron scale mechanics of microgels is challenging due to limited techniques.

Purpose of the Study:

• To determine mechanical moduli of polystyrene-co-poly(N-isopropylacrylamide) (pS-co-NIPAM) microgels.
• To evaluate the combined use of Brillouin light scattering (BLS) and AFM nanoindentation for microgel nanomechanics.
• To investigate the influence of composition (monomer and crosslinker) on microgel mechanical properties.

Main Methods:

• Synthesized pS-co-NIPAM microgels with varying monomer and crosslinker (N,N'-methylene-bisacrylamide - BIS) concentrations.
• Employed Brillouin light scattering (BLS) to measure bulk mechanical moduli.
• Utilized Atomic Force Microscopy (AFM) nanoindentation for localized mechanical measurements.

Main Results:

• Young's (E) and shear moduli increased with higher NIPAM and BIS content, while Poisson's ratio decreased.
• AFM and BLS showed good correlation in the rank order of E, but differed at low BIS concentrations, indicating surface density variations.
• Microgel structure transitioned from inverted to dense-core with increasing BIS, affecting mechanics differently based on the technique used.

Conclusions:

• Complementary BLS and AFM nanoindentation provide comprehensive nanomechanical data for microgels.
• Microgel composition significantly dictates mechanical properties, offering a design space for tuning nanotherapeutic uptake.
• Understanding these mechanics is vital for designing advanced microgel-based applications.