Abstract
The temperature dependence of nanomechanical properties of adsorbed poly-NIPAm microgel particles prepared by a semibatch polymerization process was investigated in an aqueous environment via indentation-based atomic force microscopy (AFM) methods. Poly-NIPAm microgel particles prepared by the classical batch process were also characterized for comparison. The local mechanical properties were measured between 26 and 35 °C, i.e., in the temperature range of the volume transition. Two different AFM tips with different shapes and end radii were utilized. The nanomechanical properties measured by the two kinds of tips showed a similar temperature dependence of the nanomechanical properties, but the actual values were found to depend on the size of the tip. The results suggest that the semibatch synthesis process results in the formation of more homogeneous microgel particles than the classical batch method. The methodological approach reported in this work is generally applicable to soft surface characterization in situ.
Highlights
Soft responsive nanoparticles, often called smart microgels, have been the focus of many investigations since Pelton et al published their paper on the synthesis of monodisperse poly(N-isopropylacrylamide) microgels.[1]
After that we thoroughly consider the nanomechanical properties of the microgel particles prepared by the semibatch process, and we end this section by comparing these with the nanomechanical properties of microgel particles prepared by the batch process
This is in agreement with the fact that the total monomer (180 mM) and sodium dodecyl sulfate (SDS) (0.65 mM) concentrations were identical in the two synthesis protocols
Summary
Often called smart microgels, have been the focus of many investigations since Pelton et al published their paper on the synthesis of monodisperse poly(N-isopropylacrylamide) (pNIPAm) microgels.[1]. Instead of the microgel particles prepared in the classical batch polymerization, which have a highly cross-linked core and a barely cross-linked outer shell,[8] we used microgel particles prepared in a two-step semibatch process to gain soft but more homogeneous microgel particles[5] bearing a cross-linked outer shell Such particles are expected to have more uniform mechanical properties and exert larger forces on swelling due to the polymer network in the particle’s outer shell being interlocked by the cross-links. We used a semibatch method that is suitable for tuning the composition of the outer shell of the pNIPAm microgel particles, as described previously.[44]
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