Microgel mechanics are central to the swelling of stimuli-responsive materials and furthermore have recently emerged as a novel design space for tuning the uptake of nanotherapeutics. Despite this importance, the techniques available to assess mechanics, at the sub-micron scale, remain limited. In this report, all mechanical moduli for a series of air-dried, polystyrene-co-poly(N-isopropylacrylamide) (pS-co-NIPAM) microgels of varying composition in monomer and crosslinker (N,N′-methylene-bisacrylamide (BIS)) mol% have been determined using Brillouin light scattering (BLS) and AFM nanoindentation. These techniques sample the material through distinct means and provide complementary nanomechanical data. An initial demonstration of this combined approach is used to evaluate size-dependent nanomechanics in pS particles of varying diameter. For the pS-co-NIPAM series, our BLS results demonstrate an increase in Young’s (E) and shear moduli with increasing NIPAM and/or BIS mol%, while the Poisson’s ratio decreased. The same rank order in E was observed from AFM and the two techniques correlate well. However, at low BIS crosslinking, an inverted particle structure persists and small increases in BIS yield a higher increase in E from AFM relative to BLS, consistent with a higher density at the particle surface. At higher BIS incorporation, the microgel reverts to a typical, dense-core structure and further increasing BIS yields changes to core-particle mechanics reflected in BLS. Lastly, at 75 mol% NIPAM, the microgels displayed a broad volume phase transition and increased crosslinking resulted in a minor, yet unexpected, increase in swelling ratio. This complementary approach offers new insight into nanomechanics critical for microgel design and application.
The
diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene,
undergoes a reversible photoisomerization between its ring-open and
ring-closed forms in the solid-state and has applications as a photomechanical
material. Mechanical properties of macrocrystals, nanowire single
crystals, and amorphous films as a function of multiple sequential
UV and visible light exposures have been quantified using atomic force
microscopy nanoindentation. The isomerization reaction has no effect
on the elastic modulus of each solid. But going from the macro- to
the nanowire crystal results in a remarkable over 3-fold decrease
in the elastic modulus. The macrocrystal and amorphous solids are
highly resistant to photomechanical fatigue, while nanowire crystals
show clear evidence of photomechanical fatigue attributed to a transition
from crystal to amorphous forms. This study provides first experimental
evidence of size-dependent photomechanical fatigue in photoreactive
molecular crystalline solids and suggests crystal morphology and size
must be considered for future photomechanical applications.
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