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. 2021 Jan 27;37(5):1902–1912. doi: 10.1021/acs.langmuir.0c03386

Temperature-Dependent Nanomechanical Properties of Adsorbed Poly-NIPAm Microgel Particles Immersed in Water

Gen Li †, Imre Varga ‡,§, Attila Kardos ‡,§, Illia Dobryden †,∥, Per M Claesson †,⊥,*
PMCID: PMC7879429  PMID: 33502872

Abstract

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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.

1. Introduction

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) (pNIPAm) microgels.1 Poly(N-isopropylacrylamide) is a temperature-responsive polymer, which has a lower critical solution temperature (LCST) at ∼32 °C.2 Below this temperature the polymer is water soluble, but above the LCST the segment–segment interaction of the polymer becomes the dominant interaction leading to phase separation. pNIPAm microgels are cross-linked spherical pNIPAm networks with a typical diameter ranging from ∼50 nm to 1–2 μm. Their swelling is temperature dependent and pNIPAm microgels show an order of magnitude volume change as the LCST is crossed while they preserve their colloidal stability.3 Their synthesis,4−7 structure,8−12 swelling characteristics,13,14 as well as their interaction with metal ions, small organic molecules,15 surfactants,16,17 biomolecules,18−20 and nanofibers21 have been investigated in detail. This wide interest is not surprising in the light of the large number of suggested applications, including drug delivery,22−27 emulsion stabilization,28−31 sensing,32−35 cell encapsulation,36 lubrication,37 microgel-supported catalysis38,39 as well as their application in the fundamental studies of, e.g., flow behavior40,41 or glass formation,10,42 and as a building block in macroscopic hierarchical structures.43,44

While the bulk properties of pNIPAm microgels were in focus at the beginning, over the last two decades, the characteristics of surface-bound microgels have also gained widespread interest. Initially, the preparation of dense microgel monolayers and the effect of microgel/substrate interaction on the swelling of the surface-bound microgels were addressed.45 Usually pNIPAm microgels were copolymerized with acrylic acid and substrates functionalized with polycations45,46 or 3-aminopropyltriethoxysilane (APTS)47−49 to facilitate strong microgel/surface interactions. However, pNIPAm microgel monolayers on unmodified silica50 have also been prepared.

Local mechanical properties are of great importance for various applications of microgel particles, e.g., the elasticity may influence the targeting process when microgel particles are used as drug carriers.51 Thus, a need for characterizing the local mechanical properties of microgel particles has appeared and some efforts have been reported. For instance, Tagit52 et al. measured the topography and modulus below and above the LCST of one kind of stiff poly-NIPAm microgels (1.8 MPa below the LCST and 12.8 MPa above the LCST). Hashmi53 et al. followed the change of the elastic modulus of poly-NIPAm microgels as the temperature was increased, using a colloidal probe with a 1 μm PS bead attached to the cantilever. The nanomechanical property changes during electrostatic swelling of other types of microgel particles have also been investigated,54 and nanomechanical factors that influence interactions with peptides have been considered.55 Most studies have focused on the properties measured at the center of microgel particles and the heterogeneity on the particles’ surface was not considered. Later Aufderhorst-Roberts56 et al. took advantage of the development of atomic force microscopy (AFM) techniques and measured the nanomechanical properties and topography of poly-NIPAm microgels at room temperature with a high spatial resolution.

In this study, we aimed at investigating three additional aspects of the nanomechanical characteristics and topography of surface-bound pNIPAm microgel particles. It is expected that the strength of the microgel/surface interaction, the temperature, and the internal structure of the microgels have a profound effect on the microgel/substrate interaction, and thus also on the morphology and the mechanical properties of the adsorbed microgel particles. To address these factors, we used APTS-functionalized silica substrates to enhance the microgel/substrate interaction compared to the work of Aufderhorst-Roberts et al,56 where pNIPAm microgels were adsorbed on unmodified silica that provides only a weak microgel/substrate interaction. Further, we monitored the change in nanomechanical properties and topography with increasing temperature in situ with both high spatial resolution and accurate force control. Finally, 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 particles5 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. To achieve this goal, 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

2. Experimental Section

2.1. Materials

N-Isopropylacrylamide (NIPAm), methylenebisacrylamide (BA), ammonium persulfate (APS), and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich. N-Isopropylacrylamide was recrystallized from hexane, methylenebisacrylamide was recrystallized from acetone and kept in a freezer, usually for a few days, before being used for the synthesis of the microgel particles. All other materials were used as received. All solutions were prepared in ultraclean Milli-Q water (total organic content = 4 ppb; resistivity = 18 mΩ·cm, filtered through a 0.2 μm membrane filter to remove particulate impurities).

2.2. Microgel Preparation

Microgel particles composed of a cross-linked poly(N-isopropylacrylamide) (pNIPAm) core and a similarly cross-linked pNIPAm shell were prepared using a semibatch precipitation polymerization technique. The cross-link density of the core and the shell was 30, which is one monomer in every 30 monomers was a cross-linker. In a typical polymerization reaction, a calculated amount of NIPAm (1.575 g), methylenebisacrylamide (BA, 0.074 g), and sodium dodecyl sulfate (0.030 g) were dissolved in 156 mL of Milli-Q water to yield 90 mM monomers and 0.65 mM SDS concentrations. The solution was introduced into a double-walled Pyrex glass reactor and it was stirred vigorously. To keep the temperature of the reaction mixture at a constant temperature of 80 °C, the outer shell of the reactor was connected to a temperature bath and controlled temperature water was circulated in it. The reaction mixture was degassed by purging with nitrogen for 60 min. The reaction was initiated by adding 4 cm3 aqueous APS solution into the reactor, which was prepared by dissolving 0.1097 g of APS in 10 mL of degassed Milli-Q water. After 25 min (at ∼90% conversion),7 a second batch of monomers (1.575 g of NIPAm and 0.074 g of BA dissolved in 10 mL of degassed Milli-Q water) was injected into the reaction mixture to form a cross-linked outer shell on the cross-linked pNIPAm core. To stop the polymerization, the reaction mixture was cooled to ∼15 °C by circulating tap water in the outer shell of the reactor and purging the reaction mixture with oxygen 4 h after the initiation of the reaction. As a reference, a microgel was also prepared with a cross-link density of 30 using the classical batch polymerization method. In this case, all monomers (3.150 g of NIPAm and 0.148 g of BA) as well as the SDS (0.03 g) were dissolved in 156 mL of Milli-Q water and polymerization was performed, as described above, for 4 h.

The final microgel products were purified from unreacted monomers and polymeric byproducts by ultracentrifugation (Beckman Optima XPN ultracentrifuge, 362 000g, at 25 °C), decantation, and redispersion. The centrifuged microgels were redispersed in Milli-Q water and the cycle was repeated up to three times. Finally, the purified microgels were freeze-dried and stored in a freezer before further use.

2.3. Silicon Surface Modification

Since the microgel particles are slightly negatively charged, due to the negatively charged APS and some carboxylic acid groups formed during the polymerization process,57 bare silicon wafers with a silica oxide layer were modified with 3-aminopropyltriethoxysilane (APTES, Sigma-Aldrich) to achieve a positively charged surface and strong adsorption of the microgel particles.58 The wafers were first thoroughly cleaned with 2% Hellmanex solution and Milli-Q water and dried with nitrogen gas. Then, 3 mL of APTES was put into a vial and placed together with a cleaned wafer inside a dry, sealed bottle at a temperature of 70 °C for 2 h. These modified wafers were then rinsed with ethanol and Milli-Q water alternatingly three times to remove physiosorbed silane. The surfaces were kept in a desiccator until use.

2.4. Dynamic Light Scattering (DLS)

The particle size and polydispersity were determined by DLS. The measurements were performed with a Brookhaven Instruments device, which consists of a BI-200SM goniometer and a BI-9000AT digital autocorrelator. A Coherent Genesis MX488-1000 STM laser was used as the light source. The laser was used at a wavelength of 488.0 nm and it emitted vertically polarized light. The autocorrelator was set in “multi τ” mode; i.e., the time axis was logarithmically spaced to span the required correlation time range. The autocorrelation functions were measured at a detection angle of 90° with a 100 μm pinhole size. The obtained autocorrelation functions were analyzed by a second-order cumulant and the CONTIN methods.59 In agreement with the literature data, both methods indicated the formation of highly monodispersed samples (PDI ∼0.01) at all investigated temperatures. The extracted diffusion coefficients (D) were converted into hydrodynamic diameters (dh) using the Stokes–Einstein equation.60

2.5. Atomic Force Microscopy

All AFM measurements were carried out in Milli-Q water and performed with a JPK Ultraspeed 3 instrument. The remarkably soft nature of the microgel particles, particularly below the phase transition temperature, combined with the viscosity of water, makes the experiments challenging and requires considerations of the measurement parameters and cantilever types. In this study, we take advantage of the Quantitative Image (QI) mode with high-speed data collection to make topographical and nanomechanical images, and force–volume (FV) mode for its highly regulated movement of the AFM head to measure accurate force vs distance curves.

For measurements in the QI mode a relatively sharp probe, MLCT-BIO-DC-D (Bruker), was used for high lateral resolution mapping. The nominal spring constant of this soft cantilever is 0.03 N/m and the nominal radius of the tip end is less than 20 nm. The approach and retract speed in different experiments were set to 5 or 100 μm/s. The faster speed was used for minimizing the instrumental drift for accurate measurements of topography, and the lower speed was utilized to minimize the hydrodynamic drag to allow accurate measurements of nanomechanical properties. The set point used was 0.6 nN, which achieved satisfying indentation at all temperatures studied, 26–35 °C.

In the FV mode, we utilized biosphere B-100-cont (Nanotools) probes to achieve higher force sensitivity. This probe has a nominal tip radius of around 95 nm and a nominal spring constant of 0.2 N/m. The piezo expansion/retraction speed for measuring mechanical properties was set to 2 μm/s to minimize hydrodynamic forces, while the influence of different approach speeds was also determined. The set point in these measurements was also 0.6 nN.

The real spring constant of each cantilever was determined using the Sader method61 in air, and the optical sensitivity was determined by pushing the tip against a clean sapphire surface in water.

A measuring cell with a temperature control and feedback was employed to maintain the aqueous environment and control the temperature. Before the AFM experiment, the surface-modified silica substrate was incubated in a microgel particle suspension for 5 min. Next, this wafer was carefully rinsed with Milli-Q water to remove nonadsorbed microgel particles. The incubated surface was submerged in water at the selected temperature for 20 min before imaging. After finishing the measurements at one temperature, the surface with adsorbed microgel particles was left at the next temperature for 20 min to reach the new equilibrium before continuing the measurements.

3. Results and Discussion

In this section, we first consider how the temperature affects the size of the microgel particles in bulk solution. 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.

3.1. Bulk Characterization of the Prepared pNIPAm Microgels

The hydrodynamic diameter of the prepared microgel particles as a function of temperature is reported in Figure 1. At a low temperature, the hydrodynamic diameter is ∼290 nm for the microgel prepared by the classical batch polymerization process and ∼340 nm for the microgel prepared by the semibatch method. The particle size gradually decreases with increasing temperature in both cases. We define the collapse temperature, CT, as the inflexion point of the curves, and this is located at around 32 °C for both samples. At even higher temperatures, the diameter of the collapsed microgel particles reaches a value of approximately 150 nm and it does not change much above 35 °C regardless of the preparation method. This is in agreement with the fact that the total monomer (180 mM) and SDS (0.65 mM) concentrations were identical in the two synthesis protocols. However, as noted above, the microgels prepared by the semibatch method have a much larger swollen size at room temperature, which reveals ∼1.6 times larger volume swelling (V25C/V40C). This implies that the semibatch method indeed gives rise to a less-cross-linked particle core, and presumably a more homogeneous cross-link density distribution, which in turn facilitates the larger swelling of the microgel.

Figure 1.

Figure 1

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Hydrodynamic diameter of the prepared microgels (blue squares—one-pot batch polymerization, red circles—two-pot semibatch polymerization) as a function of temperature as measured by DLS.

The observed temperature dependence of the microgel particle size implies that the mechanical properties will vary primarily in the interval of about 26–35 °C. Based on this, we explored the nanomechanical properties with AFM in this temperature range with 3 °C intervals (26, 29, 32, and 35 °C).

3.2. Effect of Temperature on the Topography and Height

The topography of one selected microgel particle measured in QI mode is shown in Figure 2 using a driving speed of 100 μm/s. We also performed measurements at other driving speeds, and topography images obtained at a driving speed of 5 μm/s are shown in the Supporting Information, Figure S1.

Figure 2.

Figure 2

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Topography of one PNIPAM microgel particle at different temperatures of (a) 26 °C, (b) 29 °C, (c) 32 °C, and (d) 35 °C measured using a driving speed of 100 μm/s. Note the different scales on the horizontal and vertical axes.

The height and base diameter of the microgel particle are also listed in Table 1. The base diameter is the mean value of three random diameters measured from the topographic map recorded with a driving speed of 100 μm/s.

Table 1. Height and Diameter at Different Temperatures.

  height (nm)
 diameter (nm)
  5 μm/s 100 μm/s 100 μm/s
26 °C 118 121 691
29 °C 101 96 624
32 °C 62 63 575
35 °C 52 49 557
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From Figures 2 and S1 as well as Table 1, we notice that the driving speed has a rather limited effect on the observed topography. Importantly, both the lateral dimension and the particle height decrease with increasing temperature, as expected due to the decreased solvent quality of water for poly-NIPAm at higher temperatures. However, if we compare the lateral dimension with the particle height, we find that the absorbed microgel particles are highly flattened, i.e., the lateral diameter is 500–600 nm, while the height is at the most slightly more than 100 nm at 26 °C. This is due to the strong interaction between the negatively charged particle and the positively charged surface. The strong surface affinity of the microgel particles also results in a marginal contraction along the surface, compared to the strong contraction normal to the surface, with increasing temperature.

3.3. Nanomechanical Mapping with QI Mode

The QI mode is a nanoindentation-based method that measures a force–distance curve at each image pixel while simultaneously recording the topography. By postprocessing the force–distance curves (FDC), one can reconstruct the mechanical property maps without compromising the lateral resolution. The procedure has been reported in detail in previous studies, e.g., in ref (62).62 In this section, we first discuss the FDCs at the center of the microgel particles, and then turn our attention to the FDCs at the microparticle edge. In the final section, we report the nanomechanical properties extracted from these FDCs.

3.3.1. FDCs at the Microgel Particle Center

Typical FDCs recorded at the center of the particle at different temperatures are presented in Figure 3.

Figure 3.

Figure 3

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Typical force–distance curves measured at the center of the particle at different temperatures of (a) 26 °C, (b) 29 °C, (c) 32 °C, and (d) 35 °C. The force was measured on compression (black line) and on decompression (red line), and data is shown after subtraction of the optical interference.

All FDCs show an oscillatory baseline at large separations, which is due to optical interference.63 This effect was largely canceled by fitting a sinus function to the data at large separations and subtracting it from the raw data (see the Supporting Information for details). In addition, all FDCs display hysteresis between the compression and decompression curve, which is partly due to tip-sample adhesion and also results from the viscoelastic nature of the hydrogel material.

The FDCs measured in the temperature range of 26–32 °C share similar features, including a soft contact upon compression, and stepwise polymer chains stretching upon decompression. The latter feature results in sawtooth features in the attractive regime.64 However, the FDCs obtained at 35 °C display a significantly harder contact during compression and significantly lower adhesion force. In addition, a repulsive region appeared before the contact between the tip and sample. This may be attributed to the higher charge density on the microgel surface at higher temperatures, which is due to the fact that the volume of the particle shrinks but the total charge is kept constant. Due to the different ranges of the short-range repulsion observed during compression, we fitted the stiffness from the first 20 nm of the FDCs in the temperature range of 26–32 °C and from the first 5 nm at 35 °C.

3.3.2. FDCs at the Microgel Particle Edge

Another interesting phenomenon was observed close to the edge of the microgel particles. Figure 4 presents a typical FDC against silanized silica (Figure 4a) and four FDCs at four consecutive pixels along the same scan line at the edge of the particle (Figure 4b,c). Here, the data for curve e is the farthest from the particle center and the data for curve b is closest to it. All of these force curves were measured at 26 °C.

Figure 4.

Figure 4

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FDCs at (a) APS-modified substrate and (b, c) at the edge of a microgel particle. The tip moves toward the particle center (but still at the particle edge) as we go from FDC (e) (green) to FDC (b) (black). The distance moved laterally by the tip between the two force curves (b–e) is 8 nm. The FDCs are shifted horizontally for clarity.

The force curves reported in Figure 4b,c show some features similar to the FDC measured against the silica surface, i.e., a small jump-in due to attractive tip-surface forces before the steep short-range repulsion is encountered. The attractive force can be due to a combination of attractive electrostatic forces between the oppositely charged surface and tip and van der Waals forces. However, before the jump-in, there is a region that also shows a slowly increasing repulsion with decreasing separation, which suggests contact between the tip and the soft microgel particle.

In the decompression curve, there are also two significant features present. The first attractive minimum has a similar value as that measured against the silanized silica surface, while the second one appears at larger distances and is similar to the typical adhesion between the tip and microgel particle. The data suggests that the tip first comes into contact with a thin layer of microgel, giving a repulsion on compression, and then penetrates through this layer to reach the silica surface where a steep repulsion sets in. On separation, the first attractive peak is associated with the removal of the tip from the silanized silica surface and the second one from tip detachment from the microgel particle.

FDCs b and c in Figure 4b, show no jump-in during compression, suggesting no significant long-range attraction between the tip and the silanized silica surface. However, there is still a small region of hard wall repulsion at the very end of the compression curve, suggesting that the tip eventually comes into contact with the substrate surface. Even at the maximum force the contact between the tip and the surface is limited, giving rise to a less attractive first force minimum in the decompression curve. The outer force minimum in the decompression curve is always attributed to the adhesion between the tip and the microgel particle.

3.3.3. Nanomechanical Properties

The stiffness of the microgel particle determined in the QI mode is presented in Figures 5,8. We note that the stiffness of the silanized silica surface in the image of Figure 5 is inaccurate. One reason is that the chosen tip is too soft for measuring the stiffness of hard surfaces, and the other is that in the batch process function, the stiffness was fitted over a distance range suitable for the particle, but too large for the substrate. This means that the stiffness of the silanized silica surface is largely underestimated. However, the stiffness of the microgel particles can be measured accurately. From the property maps and the line profile across the particle, we note that the microgel particles are not completely homogeneous, though they do not show any core–shell structure. The line profiles will differ depending on how the line is drawn but are used as an easy way to illustrate the variations that are fully shown in the images.

Figure 5.

Figure 5

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Stiffness maps and line profiles across the microgel particle evaluated at different temperatures of (a) 26, (b) 29, (c) 32, and (d) 35 using a driving speed of 5 μm/s. The blue dotted lines show where the line profiles are extracted. The unit of the y-axis of the line profiles is pN/nm.

Figure 8.

Figure 8

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Top: temperature dependence of the average value of stiffness, adhesion, and apparent elastic modulus at the microgel particle center measured in QI mode with a tip of 20 nm nominal end radius. The y-axis has a logarithmic scale. Bottom: stiffness, adhesion, apparent elastic modulus, and deformation measured in FV mode using a spherical tip with a 95 nm radius. The y-axis has a logarithmic scale.

The stiffness increases with increasing temperature as water becomes a poor solvent for poly-NIPAm and thus to a large extent leaves the microgel. A similar trend is seen for the size of the particles in bulk solution as evaluated by DLS (Figure 1).

Adhesion forces determined in the QI mode are shown in Figures 6, 8. The data demonstrate that the adhesion force between the tip and the microgel particle decreases with increasing temperature, and also in the adhesion maps the microgel particles appear somewhat inhomogeneous. The magnitude of the adhesion force is decided by three competing factors. First, the contact area between the tip and the microgel particles decreases with increasing temperature due to the increasing microgel particle stiffness at higher temperatures. Second, due to the shrinking of the microgel, the negative surface charge density of the particle increases and the electrostatic repulsion between the negatively charged tip and surface of the particle increases. These two factors contribute to a decreasing adhesion. Finally, as the temperature increases, water changes from being a good solvent to becoming a poor solvent. This is expected to increase the tendency of the polymer to adhere to the tip and leads to a higher adhesion force. Evidently, the first two effects dominate in our case.

Figure 6.

Figure 6

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Adhesion force maps and line profiles at different temperatures of (a) 26 °C, (b) 29 °C, (c) 32 °C, and (d) 35 °C measured in QI mode with a driving speed of 5 μm/s. The blue dotted lines show where the line profiles are extracted. The unit of the y-axis of the line profiles is nN.

The apparent elastic modulus of the particle as evaluated from QI-mode data using the Hertz–Sneddon model is shown in Figures S2 and 8. The values reported here should not be taken literally since the microgel deforms viscoelastically instead of purely elastically. Thus, we emphasize that it is an apparent elastic modulus. Just as for the stiffness, the apparent modulus increased slowly between 26 and 32 °C, but surged from the kPa range to the MPa range as the temperature reached 35 °C.

3.4. Nanomechanical Mapping in Force–Volume Mode Using Tips with a 100 nm Radius

By employing QI mode, we have quantitatively mapped the nanomechanical properties of the microgel particles with a relatively high lateral resolution. In this part, we instead investigate the microgel particles using a tip radius of around 95 nm (Figure S3), using force–volume mode (FV). This gives better force sensitivity and allows us to compress a large portion of the entire microgel particle at one and the same time. Again, we first consider FDCs measured at the microgel particle center, followed by FDCs measured at its edge, and finally the nanomechanical properties are discussed.

3.4.1. FDCs at the Microgel Particle Center

Typical FDCs measured at different temperatures in the FV mode are shown in Figure 7. In these measurements there is no problem with optical interference, which is due to the larger width of the cantilever. Thus, the contact point can be defined with high accuracy, which makes it easier to determine the deformation and the apparent elastic modulus.

Figure 7.

Figure 7

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Typical FDCs measured at the center of a microgel particle at different temperatures of (a) 26 °C, (b) 29 °C, (c) 32 °C, and (d) 35 °C measured in FV mode using a spherical probe of radius 95 nm.

Similar to what was observed in QI mode, FDCs recorded in the temperature range of 26–32 °C have similar features, while those at 35 °C demonstrate a substantially more rigid microgel particle. We have extracted the stiffness from the first 10 nm from the compression curve in the temperature interval of 26–32 °C and from the first 5 nm at 35 °C. This allowed us to extract the stiffness from the (close to) linear part of the compression curve.

A significant new feature observed in these FDCs, compared to those reported in the QI mode, is the presence of a long-range attraction in the compression curve measured in the temperature range of 26–32 °C. We attribute this to the bridging attraction caused by dangling polymer chains from the particle attached to the tip. This feature is facilitated by both the large tip radius (95 nm) that allows bridging of many chains, and the slower approach speed (2 μm/s) that allows more time for the polymer chains to change their conformations.

3.4.2. FDCs at the Microgel Particle Edge

At the edge of the particle we observed similar force curves with the large tips used in FV mode, see Figure S4, as reported above for QI mode in Figure 4. However, with the larger probe a net attraction is observed in the compression curve, due to more extensive polymer bridging before the hard wall contact is reached, than for the smaller probe. Again, two adhesion minima are observed in all decompression curves at the particle edge. The combination of these features suggests that when the large spherical tip approached the surface, it first interacted with the soft microgel. As the tip was further pushed down, the particle deforms and may partly move away from the tip. This leads to the tip eventually contacting the silanized silica surface. The larger the distance from the particle center, the lower the interaction between the tip and the microgel, as suggested by the steeper short-range repulsion on the compression curve and the larger first adhesion minimum on decompression.

3.4.3. Nanomechanical Properties

The material properties measured with the small probe in QI mode and the large spherical tip in FV mode are summarized in Figure 8. We find that the stiffness measured by the larger spherical probe (radius ≈95 nm) is always larger than that measured by the smaller probe (nominal radius ≈ 20 nm). This is due to the larger contact area in the former case, which results in a more global compression compared to the more local compression exerted by the smaller tip. The apparent modulus is also higher when a larger tip radius is used, particularly when the microgel particles collapse at higher temperatures. The higher adhesion observed with the larger tip is also due to the larger contact area.

Although the trends of the property variations with temperature are similar for the two types of probes, the data obtained differ significantly just at the collapse point at 32 °C, where a more global compression with the larger radius probe reports a significantly stiffer particle compared to the results of a more local deformation by the sharper probe. Our interpretation of this effect of the probe size is as follows. When the temperature is low, the particle is highly hydrated with up to 80–90% water,65 due to the high water content a global or more local compression provides a similar mechanical response. However, dynamic cross-links are formed between the poly-NIPAm chains as water becomes a poorer solvent and now a larger tip compresses a larger portion of this network, while a smaller tip reports more on the local response. This interpretation is similar to that invoked for describing the properties of the poly-NIPAm core of block co-polyelectrolyte micelles.66

To illustrate the difference between poly-NIPAm microgel particles prepared in a batch process and a semibatch process, we show stiffness maps of the two microgel particles at 26 °C in Figure 9. We note the clear core–shell structure of the particles prepared using the conventional batch process, which is lacking for the particles prepared using the semibatch protocol. The data for the particle prepared using the batch process is similar to a previous study of nanomechanical properties of poly-NIPAm microgel particles, where a core–shell structure was observed in deformation maps measured by the PeakForce QNM method.56 The difference between the particles prepared by the two synthesis methods is that in the traditional batch polymerization, the cross-linker accumulates in the core and the shell is depleted in the cross-linker due to its higher polymerization rate. In the case of the applied semibatch polymerization, the outer shell is formed by a freshly injected monomer mixture that contains the same amount of cross-linkers as the already formed particle core. As a consequence, relatively homogeneous particles are produced instead of the traditional highly cross-linked core and a less-cross-linked shell structure.

Figure 9.

Figure 9

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Stiffness maps of (a) poly-NIPAm microgel particle prepared by the semibatch process and (b) similar microgel particle prepared by the conventional batch process. In both cases, the monomer-to-cross-linker ratio was 30, and DLS data for these two particle types are shown in Figure 1. The biosphere B-100-CONT probe with a spherical tip end of radius 95 nm was used for these measurements. The temperature was 26 °C.

4. Conclusions

The temperature dependence of the nanomechanical properties of poly-NIPAm microgel particles was characterized by means of AFM-based indentation methods. The surface stiffness and apparent modulus increased, while the deformation decreased with increasing temperature. Similar trends were observed with both probes used in this investigation, and these temperature effects are expected when water is expelled as the volume of the microgel particles shrinks. In addition, due to the increased surface charge density due to the shrinkage of the particle size, together with the reduced contact area between the tip and the particle surface, the tip-microgel adhesion decreased with increasing temperature.

The scanning speed was found to have a limited effect on the measured topography. However, the use of a larger probe results in a larger stiffness, larger apparent modulus, and larger adhesion compared to what is determined by a smaller probe. This difference was particularly important just above the collapse temperaure, CT, and it is attributed to the formation of a dynamic physically cross-linked network above the CT due to attractive interactions between the poly-NIPAm chains. The presence of this network was more clearly sensed by the larger probe, as it compressed the poly-NIPAm microgel particles globally.

Two different preparation methods were also compared. Our results demonstrate that while the classical batch synthesis method gives rise to a “core–shell” structure, the semibatch procedure greatly improved the homogeneity of the poly-NIPAm microgel particles. This difference is due to the more rapid reaction rate of the cross-linker compared to that of the monomer.

Acknowledgments

G.L. acknowledges financial support from the China Scholarship Council for pursuing a Ph.D. degree. I.V. acknowledges the financial support from the Hungarian National Research, Development and Innovation Office (NKFIH K116629) and from the ELTE Thematic Excellence Programme (FIKP DT - NKFIH-1157-8/2019-DT) supported by the Hungarian Ministry for Innovation and by the Hungarian National Research, Development and Innovation Office.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.0c03386.

  • Additional topography; nanomechanical and FDC data; scanning electron microscopy (SEM) image of the biosphere probe; and the procedure for correcting for optical interference (PDF)

The authors declare no competing financial interest.

Supplementary Material

la0c03386_si_001.pdf (1MB, pdf)

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