Stabilization, Aggregation, and Gelation of Microsphere Induced by

Article pubs.acs.org/Macromolecules

Stabilization, Aggregation, and Gelation of Microsphere Induced by Thermosensitive Microgel Chuanzhuang Zhao, Guangcui Yuan,* and Charles C. Han* State Key Laboratory of Polymer Physics and Chemistry, Joint Laboratory of Polymer Science and Materials, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, CAS, Beijing 100190, China ABSTRACT: Poly(N-isopropylacrylamide) (PNIPAM) microgels are adsorbable to the surface of polystyrene (PS) microspheres in the experimental temperature range (25−40 °C) and can induce aggregation of microspheres through the bridging mechanism. The bridging mechanism proceeds in two elementary steps: the first is an adsorption (negligible desorption relatively) of free microgel to one microsphere surface, and the next is a reversible connection of the adsorbed microgel to another microsphere surface. The surface coverage is found as an important factor in triggering the stabilization, aggregation, and gelation of the mixtures. And most importantly, the nonnegligible dynamic disconnection of a bridge is the essential prerequisite for structure or state transition. For mixtures with the same concentration of microsphere, depending on the concentration of microgel, various transitions can be induced by changing temperature, including the transition from a weaker gel to a stronger gel, from a fluid to a bridging gel and from a depletion gel to a bridging gel.

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INTRODUCTION Aggregation of colloidal particles and liquid−solid transition in colloidal systems are important to both fundamental researches and technological applications. On the one hand, using colloidal particles as models of “big atoms”, the liquid−solid transition of colloids can mimic many phase transition phenomena in condensed-matter physics.1 On the other hand, controllable aggregation and liquid−solid transition are helpful in matching different performance of industrial products such as coatings, cosmetics,2 and foods.3 For hard colloids without interaction, the suspension will transform into a crystalline or glassy state when the volume fraction (Φ) of particles is increased.4 When short-ranged attractive interaction is introduced, the particles would form a space-filling network, which is a solidlike material called colloidal gel.5 Adding polymer to the colloidal suspension is an effective way to adjust the attractive force between colloidal particles. If the added polymer is nonadsorptive to the colloid surface, the osmotic pressure between the “depletion layer” and the bulk solution will drive the colloid particles to form aggregates.6 If the polymer is adsorptive, the colloidal particles will either aggregate through the “bridging” mechanism or become stabilized by the steric repulsion of the adsorbed layer, depending on the concentration of added polymer.7 Thermosensitive polymers have been used as additives of the colloidal suspension. The properties of the mixed systems can be easily tuned by adjusting temperatures.8 When the conformation or stability of the thermosensitive polymer changes with temperatures, the mixed system will transform from a fluid state to a gel state in some cases9 or the gel will collapse in some other cases.10 As for using thermosensitive polymer as bridging flocculants, it has been reported that the © 2012 American Chemical Society

rate of sediment was faster at high temperatures than that at low temperatures.11,12 The origin of these temperaturetriggered phenomena is complicated because it involves the competition between adsorption and desorption of polymers as well as the competition between connection and disconnection of colloidal particles.13 Microgel is a kind of soft colloidal particle whose chemical and physical properties (e.g., rheological property) can be readily modified by changing the ambient conditions.14 Recently, microgels have been used to control the interaction between colloidal particles.15−19 The behaviors of aggregation and gelation in aqueous mixture of hard polystyrene (PS) microsphere and soft poly(N-isopropylacrylamide) (PNIPAM) microgel have been reported in our previous paper.20 It was found that the microspheres would be stabilized by the adsorbed microgels if the surface of microspheres was totally covered. At low volume fraction of microsphere (ΦMS), with increasing the concentration of microgel, a liquid−“bridging cluster”−liquid−“depletion cluster” transition is exhibited. The largest bridging cluster forms when half of the microsphere surface is covered by microgels. At high ΦMS, with increasing the concentration of microgel, a liquid−“bridging gel”− liquid−“depletion gel” transition is shown. The maximum modulus of the bridging gel was also achieved at half-coverage. It has been concluded from the experimental work conducted at a constant temperature that the surface coverage was the key factor to control the aggregation and gelation of aqueous mixtures of PNIPAM microgel and PS microsphere.20 Received: August 20, 2012 Revised: November 9, 2012 Published: November 16, 2012 9468

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Instrumentation. Phase contrast microscopy was performed on a Nikon E600POL. A drop of sample was withdrawn and placed between a microscope slide and a coverslip. Images were taken within 5 min after the sample was prepared, such that the evaporation of water is not significant. Dynamic light scattering was performed on a commercial laser light scattering spectrometer (ALV/DLS/SLS5022F) equipped with a multi-τ digital time correlator (ALV5000) and a cylindrical 22 mW UNIPHASE He−Ne laser (λ = 632.8 nm). The scattering angle is 20°. The baseline-normalized intensity− intensity time correlation function g(2)(t) in the self-beating mode was measured. The CONTIN program supplied with the correlator was used to calculate the hydrodynamic radius (Rh).25 Small-amplitude oscillation sweeps measurement was performed on a stress-controlled rheometer (Haake MARS). Dynamic oscillatory tests were performed in a 35 mm cone−plate geometry to measure the storage and loss modulus, G′ and G″. Before each measurement, the sample was loaded and equilibrated at frequency of 1 Hz and amplitude of 0.1 Pa for 20 min.

It is well-known that PNIPAM microgel in aqueous solution will shrink when the temperature is raised above the volume phase transition temperature (∼32 °C) due to dehydration. However, for the three-component system containing PS microspheres and PNIPAM microgels in water, the selfassembly of particles involves interactions in two different scales: in the scale of 10 nm where hydrophobic interaction between PNIPAM chain segments and PS surface takes effect21 and in the scale of several micrometers where the collision between microspheres driven by Brownian motion occurs. Although the temperature effects on the interaction between PNIPAM chains and PS latexes have been studied extensively by Wu and co-workers,22,23 the temperature effects on the interaction between PNIPAM microgels and PS particles (latexes or microspheres) which involves bridging mechanism are still not very clear. For an aqueous mixture of PNIPAM microgels and PS particles, when the effect of temperature is considered, new questions come up naturally: As the temperature is raised, is the PS microsphere stable? Is the PNIPAM microgel stable? Is the shrunken PNIPAM microgel adsorbable to the PS microsphere? Can the shrunken PNIPAM microgels which are absorbed on the surface of microsphere stabilize the microspheres? Is the bridging mechanism working? Most importantly, is the cluster a dynamic cluster? This is an essential prerequisite for structure or state transition. If yes, how does temperature trigger the stabilization, aggregation, and gelation of the mixtures? The main aim of current work is to answer the above questions, that is, to investigate the bridging mechanism in aqueous mixture of PNIPAM microgels and PS microspheres and to illustrate how the temperature influences the stabilization, aggregation, and gelation of PS microspheres with addition of PNIPAM microgels.

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RESULTS AND DISCUSSION Stabilization of Mixed Suspension. Dynamic light scattering measurements were first performed to characterize the stability of PS microsphere and the reversibility of shrinking−swelling transition of PNIPAM microgel as a function of temperature before these two components were mixed in water. Figure 1 shows the Rh of PS microsphere at

EXPERIMENTS

Materials. N-Isopropylacrylamide (NIPAM, 98%, Aladdin Chemistry) was purified by recrystallization in hexane. 2,2′-Azobis(isobutyronitrile) (AIBN, 98%, Tianjin Yuming) was purified by recrystallization in ethanol. Styrene (98%, Sinopharm Chemical) was purified by passing through a basic alumina column to remove the inhibitor before use. Potassium peroxydisulfate (KPS, 99%, Beijing Chemical), sodium dodecyl sulfate (SDS, 99%, Beijing Chemical), N,N′-methylenebis(acrylamide) (BIS, 99%, Alfa Aesar), ethanol (95%, Beijing Chemical), and polyvinylpyrrolidone (PVP, Mw = 30 000 g/ mol, Xilong Chemical) were used without further purification. Water was obtained from a Milli-Q water purification system (Millipore). Synthesis and Characterization of PS Microspheres. PS microspheres were synthesized through a one-stage suspension polymerization. The following procedure was used: 90 mL of ethanol and 1.54 g of PVP were added into a 250 mL three-necked reaction flask equipped with a condenser and a gas inlet. After PVP was dissolved into the ethanol at room temperature, the flask was transferred into a 70 °C oil bath, and the solution was deoxygenated by bubbling argon gas for 30 min. Then 5.0 g of styrene dissolved with 0.054 g of AIBN was added to the reaction flask. The reaction was continued for 24 h with agitating of a magnetic stirrer. The product was collected by centrifugation and washed by ethanol and water. Powder of microsphere was obtained by lyophilization. Synthesis and Characterization of PNIPAM Microgel. The synthesis of PNIPAM microgel was followed the procedure of Senff and Richtering.24 1.6 g of NIPAM, 0.30 g of BIS, and 0.3 mg of SDS were dissolved in 90 mL of water in a reaction flask. Argon was bubbled through the reaction mixture for 30 min. 0.6 g of KPS was dissolved in 10 mL of water and added into the flask. The reaction was continued at 60 °C for 6 h with agitating of a magnetic stirrer. The resulting suspension was extensively dialyzed against water until its conductivity was less than 1 mS/cm.

Figure 1. Temperature dependence of hydrodynamic radii (Rh) of microsphere and microgel.

different temperatures and the Rh of PNIPAM microgel during a heating and cooling cycle. For PS microsphere, the Rh is about 880 nm and remains unchanged in the temperature ranging from 25 to 40 °C. The stabilization of microspheres in aqueous suspension is ascribed to the steric barrier of a water-soluble PVP layer,26 which is chemically bonded to the PS microspheres and has a thickness of about 18 nm (the thickness was determined by a rheological method).27 For PNIPAM microgel, the Rh reduces from 135 to 54 nm with temperature increasing from 25 to 40 °C. The shrinking of microgel is due to the dehydration of PNIPAM segments, and the transition is reversible with temperature decreasing. PNIPAM microgel is adsorbable to the surface of PS microsphere when they are mixed in water. The driving force is hydrophobic interaction, which is a long-range force.21 Meanwhile, the PNIPAM microgel can work as a surfactant to stabilize the hydrophobic interface in aqueous solution because it can be treated as a typical amphiphilic colloidal particle which bears both hydrophilic (amide) and hydrophobic (isopropyl) groups.28 Therefore, the addition of PNIPAM microgel into PS microsphere suspension will induce bridging or stabilizing effect depending on the concentration of microgel, and these effects have been observed when PNIPAM 9469

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microgel and PS microsphere were mixed at 25 °C.20 The PNIPAM microgel shrinks when the temperature is raised above its volume phase transition temperature (∼32 °C), but it remains the amphiphilicity.29,30 So the similar bridging and stabilizing effects were expected when PNIPAM microgel and PS microsphere were mixed at different temperatures. Figure 2

Figure 3. Minimum weight concentration of microgel (W*MG) to stabilize the microspheres at 25 and 40 °C as a function of ΦMS. The solid line is a linear regression for the experimental data. The error bars were obtained from at least 3 times independent measurements.

microsphere should be a single-layer adsorption. It was reported that proteins with a globular shape could form a single-layered film on the surface of PS latex in a hexagonal close packing,31 and the same phenomenon occurred for PNIPAM microgels packing on the oil/water interface.30 According to the linear relationship obtained in Figure 3, for a given concentration of microsphere ΦMS, W*MG,40 is 20 times of W*MG,25. That is, at 40 °C, the amount of microgel needed to cover the given surface is 20 times of that at 25 °C. Further, at 40 °C, the surface area of microsphere occupied by a microgel is 1/20th of that at 25 °C. It can be seen from Figure 1 that the Rh of microgel at 40 °C is about two-fifths of that at 25 °C. If calculated from the hard sphere model, when the radius reduces to two-fifths of its original size, the volume will reduce to 1/ 16th of its original value. Actually, PNIPAM microgel is a soft sphere, and it was reported that the microgel would adopt a “fried-egg-like” structure on the interface.32 The results are consistent with the fact that a highly swelled microgel at 25 °C is softer and has better deformability,33,34 while a contracted microgel at 40 °C cannot easily change its configuration to obtain more hydrophobic interaction anchoring points on the surface of a microsphere. Aggregation and Deaggregation in Mixed Suspension. On the basis of La Mer and Smellie’s theory on polymerbridged flocculation of colloidal particles,35 we proposed a bridging mechanism involving the aggregation process in aqueous mixture of PNIPAM microgel and PS microsphere which proceeds in two elementary steps. First, a microgel (represented by a “⊖”) adsorbs on one microsphere A (1) A + ⊖ → A⊖

Figure 2. Microscopic images of aqueous mixtures of PS microsphere and PNIPAM microgel. (a)ΦMS = 0.010, WMG = 0.0015 g/mL, 40 °C; (b) ΦMS = 0.010, WMG = 0.0030 g/mL, 40 °C. The inset figures are schematic illustration of self-assembled structures of microsphere (blue sphere) and microgel (red sphere).

shows microscopic observation at 40 °C of the aggregation of microspheres at low concentration of microgel (Figure 2a) and the deaggregation of microspheres at relative high concentration of microgel (Figure 2b). Here, the weight concentration of microgel (WMG, g/mL) instead of volume fraction of microgel (ΦMG) was used because the size of microgel varies with temperatures. The minimum weight concentration of microgel (W*MG, g/ mL) to stabilize given concentration of microsphere was determined by gradually increasing the amount of microgel until the clusters disappeared as observed by optical microscopy. The values of W*MG at 25 and 40 °C (denoted as W*MG,25 and W*MG,40, respectitively) are plotted as a function of ΦMS in Figure 3. The relation between W*MG and ΦMS could be fit quite well with a straight line through original point. Linear regressions to the data showed that W*MG,40 = 0.28ΦMS and W*MG,25 = 0.014ΦMS. Here, an assumption was made that the equilibrium concentration of microgel remaining in the suspension after the completion of the adsorption process was very small and could be neglected. So W*MG could be taken as the amount of microgel needed to fully cover the surface of microsphere or the saturated absorption concentration. The linear relationship between W*MG and ΦMS indicates that the adsorption of microgel on the surface of

Then the microsphere A⊖ connects with another microsphere B through the bridging of microgel when they collide with each other (2) A⊖ + B → A⊖B Three points should be noted here. First, there is a probability between step 1 and step 2 which is proportional to θ(1 − θ) (θ is the surface coverage).35 Second, the reverse process of step 2, that is, the disconnection of a bridge from B, is nonnegligible. This is very important, and it is the essential prerequisite for structure or state transition in this specific system. Last, the absorption of free microgel in step 1 is much faster than the diffusion and connection of microspheres in step 2, and the desorption (back reaction of step 1) could be very small and neglected. 9470

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isolated particles, started to show up and became more and more prominent with temperature increasing. It should be noted that the slow peak at about 106 nm is not necessarily related to the actual size of clusters which may be out of the range of the maximum correlation time, but it is an automatic mathematical solution of the Laplacian inversion in the dynamic light scattering analysis due to distorted baseline, implying the existence of large structures. Meanwhile, when the mixed suspension was heated above 32.5 °C, the peak for isolated particles shifted to smaller value (about 920 nm) due to the shrinking of the microgel on the microsphere surface. This shrinking of microgels will give rise to some bare surface on microspheres. In the study of a mixed suspension of PNIPAM coils and PS latexes, Gao and Wu have reported that the collapsing of the adsorbed PNIPAM chains at high temperature leads to an additional adsorption of free PNIPAM coils onto the PS latex surface.22 Since there was no free PNIPAM microgel in the solution, the exposed microsphere surface would have posibilities to connect with the microgel that has already been adsorbed on other microspheres, which means the microspheres might aggeregate at high temperature. To avoid multiple scattering, a very dilute sample was kept at each set temperature for 0.5 h. The longer the mixed suspension stays at high temperatures (above 32.5 °C), the larger the clusters can grow. The curve (40 °C) in Figure 4a was obtained after the sample was kept at 40 °C for 0.5 h, and the curve (40 °C) in Figure 4b was obtained after the same sample was kept at 40 °C for 1 h where the clusters have grown to macroscopic sizes. Whether the bridging cluster is a dynamic (temporary) cluster or a static (permanent) cluster can be determined from the studies on deaggregation process. The first requirement of deaggregation is that all the anchoring points of a bridge can be departed from the surface of microsphere simultaneously. For the sample kept at 40 °C, bridging clusters formed because the concentration of microgel added is less than the amount needed to fully cover the surface of microsphere (WMG = 0.07W*MG,40). Part of the suspension was taken out but kept at 40 °C. Then excessive microgel was added into the suspension until WMG > W*MG,40. After waiting for 1 h without agitation, it was found that the clusters disappeared. As shown in Figure 5 (curve c), the peak of large clusters disappeared and another peak of extra free microgels (about 50 nm) could be seen. This result confirmed that the bridging clusters formed in mixed

The phenomenon that microspheres aggregate with small amount of microgel added and stably disperse with large amount of microgel added (Figure 2) indicates that the absorption of microgels on the microspheres is much faster than the diffusion of microspheres toward each other. Therefore, the concentration of microgel which determines the surface coverage of microsphere is of greatest importance for the formation of clusters. If the concentration of microgel added into the suspension of microsphere is high enough to cover the surface of microsphere, no clusters form during the subsequent collision of particles. However, the surface area occupied by a microgel varies with temperature, so the aggregation of microspheres can be triggered by changing temperatures. Figure 4 shows the temperatures effects on the

Figure 4. Hydrodynamic radius distribution of a mixed suspension of PS microsphere (ΦMS = 1 × 10−7) and PNIPAM microgel (WMG = 2 × 10−9 g/mL) in a heating (a) and cooling (b) cycle. The data were collected after the sample was equilibrated for 0.5 h at each temperature. The red dashed lines are guides for the eyes to see the temperature-dependent evolution of Rh of the microsphere.

aggregation process of a mixture which is stabilized at 25 °C. The volume fraction of microsphere is ΦMS = 1 × 10−7. The concentration of microgel is WMG = 2 × 10−9 g/mL, which lies between the saturated adsorption concentration of 40 °C and that of 25 °C, WMG = 1.4W*MG,25 = 0.07W*MG,40. After the mixture was prepared at 25 °C, as shown in Figure 4a, only one hydrodynamic radius peak at about 1150 nm was observed by dynamic light scattering. The peak value is larger than the Rh of bare microsphere (880 nm), and it is corresponding to the size of an isolated microsphere covered with a layer of microgel. The onset of microsphere aggregation was monitored by dynamic light scattering where the baseline of intensity− intensity time correlation function could be distorted due to rarely occurring scattering peaks. However, no sign of clustering was observed at 25 °C, indicating that the microsphere was completely stabilized by microgel. When the mixed suspension was heated above 32.5 °C, another peak, which was tens or hundreds of times larger than the size of

Figure 5. Hydrodynamic radius distribution of a mixed suspension of PS microsphere (ΦMS = 1 × 10−7) and PNIPAM microgel which was first heated to 40 °C (a) and then added with excessive microgel at 40 °C (c) or cooled to 25 °C before (b) and after ultrasonicating (d). The final WMG of each sample was labeled on the corresponding curve. 9471

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Figures 6a−e, which are explained in detail as follows. (a) At 25 °C, the amount of microgels in step 1 was enough to fully cover

suspension of PNIPAM microgel and PS microsphere are dynamic clusters. Only when the anchoring points of bridges can be disconnected and the adsorption of free microgels is faster than the collision and reconnection of microspheres can the clusters break up in this procedure where the bare surface is released due to the disconnection of bridges and is quickly occupied by the free microgels. Changing the interaction between microgel and microsphere by cooling down from 40 to 25 °C is another pathway to study the deaggregation of bridged clusters formed at 40 °C. As shown in Figure 4b, the peak of clusters is always present during the cooling process. However the sizes of the clusters shift to small value when the temperature is lowered to below 30 °C, indicating that the cluster is partly broken. The partly broken of clusters is also an evidence of which the anchoring points of the bridges are able to depart from the microspheres. In fact, each microgel bridge may have multiple anchoring points on the microsphere surface, and each microsphere inside the cluster may be linked with several neighboring microspheres. If all the anchoring points are not simultaneously released, or if the swelling of microgels is not quick enough, the dynamic cluster could be observed in the experimental time scale. Hu and co-workers have mentioned that the desorption of a long PNIPAM chain from PS latex surface requires a simultaneous release (desorption) of all the anchoring points and a fast diffusion of the chain away from the particle surface.23 The slow swelling phenomenon, named the “size hysteresis effect”, was attributed to the chain knotting introduced in the collapse-and-swelling process by Napper et al.36 or to the additional hydrophobic interaction occurred during the heating process by Gao and Wu.22 The principle of deaggregation by adsorption of free microgels which was added subsequently and that by swelling of microgels during decreasing temperatures are the same, that is, from the point of reducing the exposed surface before the transiently separated microspheres reconnect again. Agitating the suspension by external force will accelerate the diffusion of transiently separated microspheres away from the cluster and will accelerate the breaking of the bridges, which will result in the promotion of deaggregation. As can be seen from the curve d in Figure 5 that the peak of microsphere clusters almost disappeared after the mixed suspension was ultrasonificated for 20 min at 25 °C. The existence of an inconspicuous peak of small cluster is a result of “size hysteresis effect”.22,36 When the original sample was prepared at 25 °C, WMG = 1.4W*MG,25, the amount of microgels is enough to fully cover the surface of microspheres. After a heating−cooling cycle, although the shrinking−swelling of a free microgel is reversible (Figure 1), the relaxation of a microgel stick on the surface of microsphere is constrained. As indicated by the dashed line in Figure 4b, the size of the isolated particles during the cooling process is almost unchanged and smaller than the size of the as-prepared stabilized particles (Figure 4a, 25 °C). Although the weight concentration of microgel remains the same during the heating−cooling, the surface area occupied by the microgels is different due to the hysteresis in their size. After cooling back to 25 °C, the amount (in volume) of microgel may not be enough to fully cover the microsphere, so clusters may still form once the particles collide with each other. In summary, the aggregation process is controlled by the absorption−desorption equilibrium (step 1) as well as connection−disconnection equilibrium (step 2). The experimental processes described above are schematically showed in

Figure 6. Schematic illustration of the self-assembly processes of mixed suspension of PS microsphere and PNIPAM microgel.

the surface of microspheres, so step 2 was terminated because it is impossible to from a bridge and the microspheres are stabilized by the adsorbed microgels although they would still collide with each other. (b) Shrinking of microgels at high temperature (40 °C) gives rise to some exposed surface, thus making step 2 possible to form clusters. (c) Clusters formed at 40 °C are completely broken up with addition of excessive microgels because the absorption of free microgels is much faster than the diffusion and connection of microspheres. Once the anchoring points of the bridges depart from the surface, the bare surface released from the disconnection of bridges will be quickly occupied by free microgels and step 2 will terminate. (d) When the temperature is cooled down to 25 °C, clusters formed at 40 °C are partly broken up because the swelling of microgel is slow and not enough to inhibit the step 2. (e) Agitation the suspension by external force promotes the deaggregation of clusters because the diffusion of microspheres and disconnection of bridges are accelerated, but there are still inconspicuous dynamic clusters due to the “size hysteresis effect”. Gelation in Mixed Suspensions. The bridging of colloidal particles induced by temperature will lead to gelation if ΦMS is above the percolation threshold.37 As shown in Figure 7a, the

Figure 7. Photographs of a microsphere and microgel mixed suspension with ΦMS = 0.25 and WMG = 5 × 10−3 g/mL: (a) 25 °C; (b) heated from 25 to 40 °C; (c) cooled from (b) to 25 °C.

mixed suspension with ΦMS = 0.25 and WMG = 5 × 10−3 g/mL was a milky white fluid. Using the empirical equations obtained from the fitting in Figure 3, for ΦMS = 0.25, we knew W*MG,25 = 3.5 × 10−3 g/mL and W*MG,40 = 7.0 × 10−2 g/mL. So it can be judged that the microgel concentration WMG = 5 × 10−3 g/mL was high enough to stabilize the microspheres at 25 °C, but not enough at 40 °C. Therefore, the microspheres could aggregate and form a continuous network at 40 °C. This can be seen from Figure 7b; the mixed suspension did not flow at 40 °C in an inverted vial, indicating the gelation of mixed suspension. When the gel formed at 40 °C was cooled back to 25 °C, it became a 9472

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very viscous fluid containing some solid flocs because the aggregation process was partial reversible during the heating− cooling cycle as discussed above. To form a bridge, it requires one surface site of microsphere to be covered by the microgel and another surface site remaining uncovered; thus, the maximum of bridging extent can be achieved when the surface coverage is equal to 0.5. In our previous work, we have found that the mixed suspensions with a constant ΦMS underwent a liquid (ΦMG = 0) ∼ bridging gel (ΦMG < Φ*MG) ∼ liquid (ΦMG > Φ*MG) ∼ depletion gel (ΦMG ≫ Φ*MG) transition at 25 °C depending on ΦMG. The bridging gel has maximum modulus and yielding stress at ΦMG ≈ 0.5Φ*MG. Here, the value of WMG/W*MG could be taken as an approximate measure of surface coverage when the sample was prepared. Changing temperatures will result in the change of surface coverage, which will consequently affect the pathway of gelation transition depending on the initial state (that is, depending on the concentration of microgel). Four samples with the same concentration of microsphere ΦMS = 0.25 were studied by small-amplitude oscillation sweep measurements during the increasing temperatures, and the results are shown in Figure 8.

surface are exposed and available for the formation of additional bridges. For WMG/W*MG,25 = 1.2 (Figure 8c), the microspheres are stabilized by the adsorbed microgel and the mixture is a fluidlike liquid at 25 °C indicated by G′ < G″. With increasesing temperatures, G′ increases faster than G″, leading to a liquid− gel transition at about 38 °C which depends on the heating rate. This transition is reminiscent of the visual observation of liquid−gel transition in Figure 7, which is a result of the temperature-induced aggregation of microsphere particles and the percolation of the microsphere clusters. For WMG/W*MG,25 = 2.2 (Figure 8d), according to our previous report, the volume fraction of the microgel is 0.20 and the mixed suspension forms a “depletion gel” at 25 °C.20 The depletion potential in a system with large sphere and small sphere mixed together is proportional to the volume fraction of the small particles and the ratio of the radius of large particles over that of small particles.38 With temperature increasing, the depletion potential was reduced because the size of microgel decreased. Therefore, the modulus of the depletion gel decreases at the beginning. This phenomenon was similar to Fernandes et al.’s observation,15 in which they found that the clusters of silica microsphere particles disaggregate in the presence of PNIPAM microgel depletant during the heating process. With further increasing of temperature (or longer time above volume phase transition temperature of microgel), the subsequent shrinking of microgel will result in the formation of bridging bonds because the concentration of microgel is not enough to fully cover the surface of microspheres at 40 °C (WMG/W*MG,40 = 0.1). Because shrinking and bridging occur simultaneously, there is a minimum of modulus (G′ > G″) but not an observable intermediate liquid state (G′ < G″) between the transition from a “depletion gel” to a “bridging gel”.

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CONCLUSION The PNIPAM microgel is adsorbable to the PS microsphere surface in the temperature range from 25 to 40 °C. The elementary aggregation process in mixture of PS microsphere and PNIPAM microgel which involves the bridging mechanism proceeds in two steps. First, a microgel adsorbs onto one microsphere (step 1), and then this microsphere connects with another microsphere when they collide with each other (step 2). The deaggregation in step 2 cannot be neglected which makes a static cluster into a dynamic cluster and plays a crucial role in controlling the stabilization and aggregation of microspheres. The probability of step 2 which is determined by the surface coverage is another important factor in triggering the aggregation and gelation of mixtures by changing temperatures. The pathways of gelation transition during increasing temperatures vary with the initial states. The mixture may experience the transition from a weak gel to a strong gel, or from a fluid to a bridging gel, or from a depletion gel to a bridging gel, depending on the concentration of microgel. Our results can bring a new perspective in designing soft materials with thermosensitive microgels.

Figure 8. Small-amplitude oscillatory measurements during heating (1.0 °C/min) for mixtures with ΦMS = 0.25 and different WMG: (a) WMG = 7 × 10−4 g/mL; (b) WMG = 2.8 × 10−3 g/mL; (c) WMG = 4.2 × 10−3 g/mL; (d) WMG = 7.7 × 10−3 g/mL.

For WMG/W*MG,25 = 0.2 (Figure 8a), the storage modulus (G′) is larger than the loss modulus (G″) at 25 °C, indicating the mixture is in a gel state. With increasing temperature, both G′ and G″ increase slightly, indicating that the network is slightly strengthened. The strengthening is related to the enhancement of the hydrophobic interaction between microgels and microspheres. The bridges are more compact due to shrinking. For WMG/W*MG,25 = 0.8 (Figure 8b), the surface coverage is over 0.5, so a part of the absorbed microgel do not participate in bridging of microspheres and the mixed mixture is a weak gel (G′ > G″) at 25 °C. The G′ and G″ increase sharply at about 32 °C because some former covered microsphere

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.Y.), [email protected] (C.C.H.); Tel +86 10 82618089; Fax +86 10 62521519. Notes

The authors declare no competing financial interest. 9473

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(37) Eberle, A. P. R.; Castaneda-Priego, R.; Kim, J. M.; Wagner, N. J. Langmuir 2012, 28, 1866−1878. (38) Kaplan, P. D.; Rouke, J. L.; Yodh, A. G.; Pine, D. J. Phys. Rev. Lett. 1994, 72, 582−585.

ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (973 Program, 2012CB821503). C. Zhao appreciates the China Postdoctoral Science Foundation for financial support (Grant 20110490606).

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dx.doi.org/10.1021/ma301747s | Macromolecules 2012, 45, 9468−9474