Interplay between Caging and Bonding in Binary Concentrated

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Interplay between Caging and Bonding in Binary Concentrated Colloidal Suspensions Di Jia,†,‡ He Cheng,*,†,‡ and Charles C. Han*,§ †

China Spallation Neutron Source (CSNS), Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), Dongguan 523803, China ‡ Dongguan Institute of Neutron Science (DINS), Dongguan 523808, China § Institute for Advanced Study, Shenzhen University, Shenzhen 518060, China S Supporting Information *

ABSTRACT: When a liquid becomes dynamically arrested, a gel, a repulsive glass, or an attractive glass state will form. Bonding and caging mechanisms decide their static structures and dynamic properties. To better understand their interplay, the competition between bonding and caging in a binary mixture of polystyrene core/poly(N-isopropylacrylamide) shell (CS) microgels and sulfonated polystyrene (PSS) particles is studied. CS microgels have short-range attraction above the volume phase transition temperature, whereas PSS species experiences relatively long-range electrostatic repulsion. Adding more PSS into the binary mixture will, of course, increase the total effective volume fraction but lead to different properties in gel or glass states. For instance, in gels, it increases the localization length and weakens the gel, whereas in glass, it decreases the localization length and strengthens the glass. This thus implies that the static and dynamic properties of gels are mainly controlled by bonding and those of both repulsive and attractive glasses are governed by caging. On the other hand, increasing the temperature will decrease the effective volume fraction because of the volume phase transition of the CS microgels. A discontinuous repulsive glass-to-liquid-to-gel transition can be observed when the PSS concentration is low, but a continuous repulsive glass-to-gel transition can also be observed with the increase of the PSS concentration. This may hint that glass transition and physical gelation share a similar mechanism, whereas the former has a longer relaxation time. occurring at the nearest-neighbor peak due to attraction.9 Moreover, AG exhibits less spatial heterogeneity than RG.9 Although numerous efforts11−15 have been made to distinguish the difference of these arrested states based on their static and dynamic properties, their structures are not clear yet. The mechanism of caging and bonding in the nonergodic states has been investigated.16,17 For example, because cages and bonds are in different length scales, they exhibit different yielding behaviors.14,18−20 Mode coupling theory (MCT) predicts that upon across the RG-to-AG transition line, there should be an abrupt increase of the nonergodicity parameter value f(q, ∞) from zero to some (q-dependent) finite value, reflecting the higher degree of confinement in interparticle bonds than in a topological cage.21,22 Zaccarelli and Poon have used simulations to propose that the arrested state of AG, in the long run, is still due to the topological cage.23 The competition between bonding and caging may lead to complex structures and dynamics on both a microscale and a macroscale.

1. INTRODUCTION When a liquid is cooled down and unable to access the thermodynamic equilibrium, it may undergo dynamical arrest, which broadly occurs in two forms, glass and gel.1 The nonequilibrium solid states (gel and glass) of colloidal suspensions are important in both fundamental and application fields.2,3 Cages and bonds are used to describe the origin of the nonergodic state in colloidal systems.4,5 Cage effect is usually created by increasing particle volume fraction. For example, in hard sphere suspensions, when the volume fraction reaches 0.58 or above, they become a nonergodic repulsive glass (RG), in which one particle will be trapped in the topological cage formed by its neighbors.6 On the other hand, bond effect is governed by the attraction between particles. Colloidal particles with short-range attraction can bond with each other to construct a fractal network at a low volume fraction, thus forming a physical gel.7,8 Further increasing the volume fraction of the attractive system will lead to the attractive glass (AG), in which both bond and cage exist.9,10 van de Laar et al. have recently studied the discontinuous nature of the RG-to-AG transition in a colloidal system and found that RG and AG have a similar global pair correlation function with no sign of longor medium-ranged orders and AG only has a subtle change © XXXX American Chemical Society

Received: November 18, 2017 Revised: February 8, 2018 Published: February 9, 2018 A

DOI: 10.1021/acs.langmuir.7b03965 Langmuir XXXX, XXX, XXX−XXX

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unary system different from that in a binary system? How does the localization length, which reflects the local structures, change when introducing repulsive species in an attractive system? Our main goal of this work is to find these answers. This work may shed light on the nature of these dynamically arrested structures and also open a new perspective on the metastable material science.37

Caging and gelation of binary mixtures of soft colloids exhibit rich phase behaviors.24−26 For example, asymmetric star polymer mixtures can form both single glass and double glass, in which both components are vitrified, depending on the star−star size ratio.27,28 Weitz et al.29 used dense poly(Nisopropylacrylamide) microgel suspensions to observe RG-toliquid-to-gel transition by changing the temperature. They showed that the three states exhibit different relaxation dynamics. Moreover, gelation and glass transition have some similarities. For example, Segrè et al.30 proposed that both gelation and glass transition are manifestations of a more general jamming transition based on the static and dynamic light scattering (DLS) properties. Bergenholtz et al.31 suggested that gelation should be caused by a low-temperature extension of the glass transition based on idealized MCT. Simulations also demonstrated that when gelation interferes with glass transition, a three-step relaxation of time correlation function shows up, corresponding to two distinct length scales and wellseparated time scales for cage/bond relaxation,32 and the gel has properties that are clearly distinct from those of both AG and RG.33 Here, a binary system, which includes both attractive and repulsive species, is used to investigate the interplay between caging and bonding. One species is a thermal-sensitive microgel composed of polystyrene (PS) core and poly(N-isopropylacrylamide) shell (CS). It is repulsive at room temperature. As the temperature increases, CS particles will gradually shrink, so they have a temperature-dependent size. When the temperature is above the volume phase transition temperature (VPTT ≈ 33 °C), CS particles will interact via short-range attraction because of the van der Waals attraction. On the other hand, the other species is sulfonated polystyrene (PSS) particles, which experience long-range electrostatic repulsion. The interaction between CS microgels and PSS particles can be neglected compared to the CS−CS and PSS−PSS interactions. Both species can contribute to caging, but only attractive species can form bonds. In our previous study of the binary system, we found a gel-to-defective gel-to-glass transition at 35 °C when changing the ratio of the two species at a fixed total volume fraction by their yielding behaviors. When small amounts of repulsive species are introduced into the gel network composed of attractive species, some of them are kinetically trapped in the gel network and act as defects to weaken the gel strength; thus, a so-called “defective gel” is formed.34 Therefore, the repulsive species can not only contribute to caging but also weaken the bonds in a binary system. Mohraz et al. also studied the dynamics of binary colloidal mixtures composed of both hydrophobic and hydrophilic species by confocal microscopy and found that repulsive species makes attractive species form more spatially homogeneous structures, whereas attractive species makes repulsive particles exhibit dynamical heterogeneity.35 Naive mode coupling theory (NMCT) and the nonlinear stochastic Langevin equation also have predicted the complex behavior of the localization lengths and shear moduli in the binary mixture due to the competition between excluded volume caging and attraction-induced physical bonding between sticky particles. It forecasts that the localization length of the attractive species is much smaller than that of the repulsive species.36 However, the structure differences among gel, AG, and RG still remain unanswered. For instance, both gel and AG have attractive interaction. How does the introduction of repulsive species influence the attractive bonds in gel and AG? How is the interplay between caging and bonding in a

2. EXPERIMENTAL SECTION 2.1. Materials. N-Isopropylacrylamide (97%, Aldrich) was purified by recrystallization in hexane. Styrene (St, 98%, Sinopharm Chemical) was purified by passing through a basic alumina column to remove the inhibitor. Sodium dodecyl sulfate (99%, Beijing Chemical), potassium peroxodisulfate (≥99%, Sigma-Aldrich), N,N′-methylenebisacrylamide (99%, Alfa Aesar), potassium chloride (KCl, 99%, Amethyst Chemicals), ethanol (95%, Beijing Chemical), and concentrated sulfuric acid (95−98%, Beijing Chemical) were used as received. Water was obtained from a Milli-Q water purification system (Millipore, Bedford, MA, USA). The sub-micrometer PS particles were kindly provided by BASF Company. The synthesis of PS core and poly(N-isopropylacrylamide) shell (CS) microgels followed the procedure of Ballauff et al.,38 and the synthesis of PSS particles was the same as our previous work.34 2.2. Preparation of a Binary Mixture. To fully screen the electrostatic interaction due to residual charges affixed to the PS core in CS microgels, CS and PSS particles were mixed in 0.05 M KCl solution. Therefore, above the VPTT (33 °C),39,40 the CS suspensions become gels because of the hydrophobic interaction, whereas PSS particles still remain stable because they are negatively charged with a zeta potential of −45 mV in 0.05 M KCl solution. The mixed suspension was then homogenized by ultrasonic waves for 20 min. The lid of the vial was sealed with Parafilm to avoid water evaporation. To fully swell the CS particles, the samples were placed in a refrigerator at 4 °C for 2 days before use. 2.3. Instrumentation. Rheological measurements were performed with a stress-controlled rheometer (Anton Paar MCR 502), using a 25 mm roughened cone−plate geometry. Silicone oil was coated on the edge of the cone−plate to prevent water evaporation. Yielding measurements were carried out in the strain control mode with a fixed frequency (ω = 10 rad/s) and strain amplitude. The oscillatory preshear (see the Supporting Information) was carried out before each measurement. DLS was performed on a modified laser light scattering spectrometer (ALV/DLS/SLS-5022F) equipped with a multi-τ digital time correlator (ALV5000) and four lasers with different wavelengths.41 A cylindrical 22 mW Uniphase He−Ne laser (λ0 = 632.8 nm) was used to do the test, and the scattering angle was 30°. The baseline-normalized intensity−intensity time correlation function g(2)(t) − 1 in the self-beating mode was measured, and CONTIN program was used to calculate the hydrodynamic radius distribution.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Binary Mixture. DLS was used to characterize the temperature dependence of hydrodynamic radius Rh for both species. As temperature increases from 15 to 40 °C, Rh decreases from 128 to 82 nm for CS microgels (Rh of the PS core is 54 nm) and the VPTT is about 33 °C. Above the VPTT, the microgel remains stable because of the electrostatic repulsion without salt but flocculates with salt. For PSS particles, Rh is about 117 nm and remains unchanged in the observed temperature range, as shown in Figure 1. The volume fraction will also change with temperature, so in all of the measurements, the mass fraction was fixed. Several methods have been used to estimate the effective volume fraction of soft particles;42,43 here, we followed Ballauff’s B


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As known, the formation of RG or physical gel is due to the caging or bonding mechanism, respectively. Increasing the mass fraction (or the volume fraction) will increase the cage effect for RG and also the number of bonds for gels, but the bond strength stays constant at the same temperature. For CS 0.107, the elastic modulus of the gel at 40 °C is almost 20 times larger than that of the RG at 20 °C, indicating that bonding effect is much stronger, whereas for CS 0.24, G′ of the RG at 20 °C is already higher than that of the gel at 40 °C, exhibiting that caging is stronger than bonding here. There is a competition between caging and bonding. For gels, increasing the mass fraction will increase the number of bonds, but the bond strength still remains constant, whereas for glass, the free volume will be reduced significantly so that the caging effect will be strengthened by increasing the mass fraction. Therefore, increasing the mass fraction contributes to the moduli of both RG and gels but contributes more to the RG. Finally, for sample CS 0.24, the elastic modulus of cagedominated glass at 20 °C is larger than that of bond-dominated gel at 40 °C, as shown in Figure 2.

Figure 1. Temperature dependence of hydrodynamic radius Rh of both CS and PSS particles.

method44 to estimate the effective volume fraction of CS microgels: ⎛ R ⎞3 ΦCS = ΦC⎜ h ⎟ ⎝ RC ⎠

(1)

where ΦC is the volume fraction of core particles, RC is the core radius (RC = 54 nm determined by DLS), and Rh is the hydrodynamic radius of CS microgels. PSS volume fraction was estimated by converting the weight fraction using a density of 1.05 g/cm3 for PS because the precise effective volume fraction of charged PSS is difficult to obtain because of its concentration dependence.45,46 3.2. Temperature Sweeps of Moduli for Both Unary and Binary Systems. Temperature sweeps of both elastic and loss moduli were measured first in the CS unary system. The sample information is listed in Table 1 (the name “CS 0.107” Figure 2. Temperature sweeps for the CS samples at different weight fractions. Storage moduli G′ (filled symbols) and loss moduli G″ (open symbols) were measured at γ = 0.1% and ω = 1 rad/s. At each temperature, the data were obtained after the sample reached equilibrium (wait until the modulus does not change).

Table 1. Mass Fraction and Effective Volume Fraction at 40 °C of the One-Component CS Samples Measured in Figure 2 sample name

m (CS)/g

mass fraction (%)

Φeffective (at 40 °C)

CS 0.107 CS 0.195 CS 0.24

0.107 0.195 0.240

15.0 24.5 28.6

0.21 0.35 0.40

On the basis of the NMCT and the nonlinear stochastic Langevin equation, Schweizer et al. predicted the elastic modulus:36,48 G′ ≈ Φ/rL 2

indicates the sample contains 0.107 g CS microgels). For sample CS 0.107, the particle−particle interaction is repulsive at 20 °C. As the temperature increases, both G′ and G″ decrease because of the volume shrinkage. When it is above 30 °C, G″ starts to be larger than G′ and the sample becomes a liquid. Further increasing the temperature above the VPTT, the attraction between CS microgels will turn on and finally lead to the formation of the gel state. G′ increases by three orders in a very narrow temperature range (33−35 °C), indicating that it is a discontinuous transition. When the temperature increases from 35 to 40 °C, the moduli increase slightly and the gel becomes more robust because of the increase of attraction strength between the microgels.47 Therefore, there is a RG-toviscous liquid-to-gel transition during the temperature sweeps. When increasing the mass fraction of the CS microgels in the unary system, higher moduli are obtained, but the transition temperature remains constant and the sol−gel transitions are all sharp and discontinuous, which is very different from what is observed in a binary system. This will be discussed later.

(2)

where Φ is the volume fraction and rL is the localization length. rL of RG is similar to the cage size, whereas rL of gels is much smaller because of the physical bonding length. From the above equation, we can estimate rL/D in Figure 2 (see Figure S2a in the Supporting Information). For gels at 40 °C, as Φ increases from 0.21 (sample CS 0.107) to 0.4 (sample CS 0.24), rL decreases roughly from ∼0.023D to ∼0.013D (D is the average particle diameter). This indicates that the gel is densified and becomes more localized as Φ increases in the unary system. For RG, rL also decreases as Φ increases. To verify the competition between caging and bonding, repulsive species PSS is introduced in the CS system. PSS particles are negatively charged so that there is electrostatic repulsion between them, whereas the CS−CS interaction is attractive above the VPTT. The CS content is fixed when PSS is gradually added (the name of the binary system “CS 0.107 PSS 0.05” indicates that the sample contains 0.107 g of CS microgels and 0.05 g of PSS particles). The corresponding C


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particles are added at a fixed CS content. rL increases from ∼0.02D to 0.07D as the PSS concentration increases from PSS 0 to PSS 0.08, which is the opposite to the trend of unary gels (Figure 2). In unary gels, increasing the total volume fraction will make the gel become more localized and lead to smaller rL, whereas in binary gels, increasing the total volume fraction by adding more PSS will lead to larger rL. It originates from the fact that PSS particles are kinetically trapped inside the gel network composed of CS attractive species during the gelation process and thus weaken and loosen the bonds between CS; the gel thereby becomes less localized and rL becomes larger as more PSS particles are added, although the total volume fraction increases. Overall, the opposite trends of rL in unary and binary gels indicate that bonding plays an important role in physical gels. Although the VPTT remains constant for samples with different PSS concentrations (Figure 3), the gelation transition becomes more and more continuous as the PSS concentration increases. For sample PSS 0, G′ increases by 3 orders of magnitudes in a very narrow temperature range from 33 to 35 °C, indicating a sharp and discontinuous sol−gel transition, which looks like a first-order transition. Royall et al. found that gelation is discontinuous and first-order-like through the structural relaxation time and the first minimum of the radial distribution function g(r).49 However, with the addition of only 0.01 g of PSS (7%), G′ increases only from 2.5 to 31.6 Pa in the same temperature range (33−35 °C) and the crossing between G′ and G″ disappears. When the PSS content is above 0.05 g (PSS 0.05), G′ first decreases linearly and then increases linearly with temperature within the whole temperature range. The continuous RG-to-gel transition indicates that there may be no further phase separation between CS and PSS species. Above the VPTT, CS−CS interaction is attractive and PSS− PSS interaction is repulsive. If phase separation of the two species happened during the gelation process, the CS-rich phase would respond to the temperature similarly to the CS unary system, whereas the PSS-rich phase would have no temperature response, and then the gelation transition would always remain sharp and discontinuous, which was the opposite to our observation, so there should be no phase separation in the binary system. When gelation happens, PSS will sterically hinder the CS to attract each other to form the gel network, and part of the PSS is finally jammed inside the CS gel network, acting as defects in the gel, and thus makes the gelation process more continuous. Figure 4a shows the frequency sweeps of sample CS 0.107 PSS 0.01 in the three states in Figure 3, i.e., RG (at 20 °C), liquidlike (weak solid) (at 33 °C), and defective gel (at 40 °C). G′ of the RG has nearly no frequency dependence, and the minimum of G″ at a lower frequency is a sign of in-cage to outof-cage transition, whereas G′ of the gel has a power law relationship with frequency for over 2 orders of magnitude, G′ ≈ ωn, where n = 0.15, which is a characteristic of physical gels.50,51 The modulus of the liquidlike (weak solid) is so weak that it is not stable within the detection range. Figure 4b is the photograph of the solid−liquidlike−solid transition of sample CS 0.107 PSS 0.01 (the same sample in Figure 3) at different temperatures. From 20 to 40 °C, the solid−liquidlike−solid reentrance is corresponding to the RG−liquidlike−gel transition. The solid at 20 °C is the cage-dominated glass, and the solid at 40 °C is the bond-dominated gel, whereas at 33 °C, the volume fraction is far below the threshold of glass transition, so the cage effect disappears and the bonds cannot be formed, and RG then

volume fractions of these samples are shown in Table 2. Figure 3 clearly shows that G′ of RG at 20 °C increases with PSS Table 2. Mass Fraction and Effective Volume Fraction of the Samples Measured in Figure 3 sample name CS 0.107 PSS 0 CS 0.107 PSS 0.01 CS 0.107 PSS 0.02 CS 0.107 PSS 0.05 CS 0.107 PSS 0.08

m (CS)/g

m (PSS)/g

mass fraction (%)



0.107

0

15.1

0.68

0.21

0.107

0.01

16.3

0.69

0.22

0.107

0.02

17.5

0.70

0.24

0.107

0.05

20.7

0.71

0.26

0.107

0.08

23.8

0.72

0.29

Figure 3. Temperature sweeps for the binary samples of 0.107 g of CS with 0, 0.01, 0.02, 0.05, and 0.08 g of PSS. Storage moduli (filled symbols) and loss moduli (open symbols) were measured at γ = 0.1% and ω = 1 rad/s. At each temperature, the data were obtained after the samples reached equilibrium.

because the addition of PSS will increase the total volume fraction and contribute to the cage effect. However, for gels at 40 °C, G′ first decreases with PSS concentration because PSS particles are trapped inside the gel to reduce the number of bonds and weaken the bond strength34 and then increases slightly with PSS because of the larger volume fraction. Note that G′ of all binary gels is always smaller than that of unary gel (PSS 0). For sample PSS 0, G′ of the gel at 40 °C is 388 Pa, and G′ of the RG at 20 °C is only 18 Pa, whereas for PSS 0.05, G′ of the RG at 20 °C is 100 Pa, which is already double that of the gel at 40 °C. Further increasing PSS concentration makes caging stronger than bonding so that G′ of the RG is always larger than that of the gel. Above all, increasing PSS content will increase the total volume fraction and contribute to the cage effect while weakening the bond effect so that the strength of the RG will gradually become larger than that of the gel with the addition of more PSS. From eq 2, we can also estimate rL in the binary systems (see Figure S2b in the Supporting Information). For all RGs at 20 °C in Figure 3, rL of unary glass PSS 0 is ∼0.2D, and for binary system PSS 0.01, rL is ∼0.13D; when increasing the volume fraction to PSS 0.08, rL decreases to ∼0.08D. Therefore, for binary RG, rL also decreases with volume fraction because, at 20 °C, the interaction is repulsive and cage effect dominates. However, for binary gels at 40 °C, rL increases as more PSS D


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the CS content is further increased to CS 0.41 (Figure 5e), which is already above the volume fraction threshold for AG (Φ ≥ 0.58), the defective effect of PSS disappears. As shown in Figure 5e, as more PSS particles are introduced in the fixed CS content, G′ increases with total volume fraction, similarly to RG. PSS can act as a probe to explore the structure of both gel and glass. The gel strength is weakened as more PSS particles are trapped inside the fractal gel network, although the total volume fraction increases.34 The gel has a heterogeneous structure, whose heterogeneity can be characterized by its cluster length scale,35 whereas AG has not only bonding but also configurational caging, so AG has a more homogenous structure. Although PSS particles weaken part of the bonding in AG, they can also strengthen the caging in AG. The modulus increasing with PSS in AG indicates that caging dominates. Besides, the localization lengths rL of AG in Figure 5e are ∼0.014D, 0.012D, and 0.011D for samples PSS 0, PSS 0.02, and PSS 0.05, respectively. Therefore, rL decreases slightly when increasing the volume fraction by adding more PSS, implying that PSS just densifies the AG structure and has little influence on the bonding between attractive species. Such an rL trend of AG is consistent with that of RG but is opposite to that of binary gels, which further verifies that caging dominates in both AG and RG, whereas bonding dominates in gels. Although both AG and gels have attractive interaction, we can distinguish the two arrested states by their dynamical properties with the inspection of the PSS probe. Moreover, once the attraction turns on, G′ has a frequency dependence, which is different from that of RG. G″ of gels has a minimum at a much lower frequency than that of RG, indicating a long-time α relaxation inside the gel phase,52 whereas for AG, the minimum of G″ indicates cage relaxation with a characteristic time of 1/ω at ″ .10 Gmin To further explore the structure differences of gels, RG, and AG, their yielding behaviors are also investigated. A two-step yielding behavior often occurs in systems with two competing length scales and/or time scale interactions.53−55 For gels (CS 0.107), there is two-step yielding: the first step yielding (γ1) is due to weaker intercluster bond breakage and the second one (γ2) is corresponding to intracluster bond breaking and fragmentation of dense clusters.14,20,56 Figure 6a,b shows two representations of the yielding behaviors, one is shear stress and the other is modulus. We should note that the two-step yielding behavior does not depend on frequency (see Figure S1 in the Supporting Information). Figure 6a shows that as more PSS particles are gradually added at a fixed CS content, γ1 moves to higher strain, whereas γ2 shifts to lower strain because PSS particles act as defects, are trapped in the gel network, and thus weaken and loosen the bond, making the bonds more fragile. Meanwhile, these kinetically trapped PSS particles make the gel network more homogeneous, so the cluster length scale decreases with PSS, and γ2 thus becomes smaller. Finally, for CS 0.107, PSS 0.08, and CS 0.107, PSS 0.1, the two-step yielding merges into a broad one-step yielding because the intercluster and intracluster bond breakages tend to superimpose.34 For RG (Figure 6c), the yielding behavior is dominated by caging. More PSS will increase the total volume fraction and make a smaller cage; therefore, the peak of G″, which is corresponding to the cage breakage,57 shifts to smaller strain with PSS. The peak of G″ shows the energy dissipation during cage breaking, which coincides with the crossover point

Figure 4. (a) Dynamic frequency sweeps for sample PSS 0.01 (CS 0.107) at different states: RG (at 20 °C), liquidlike (weak solid) (at 33 °C), and defective gel (at 40 °C). Storage moduli (filled symbols) and loss moduli (open symbols) were measured at γ = 1%. The green line for the gel state is the best fit. (b) Photograph of the solid−liquidlike− solid transition of sample CS 0.107 PSS 0.01: RG (at 20 °C), liquidlike (at 33 °C), and binary gel (at 40 °C).

melts. We should note that the samples investigated here look opaque because of high concentrations. The competition between caging and bonding in RG and gels has already been shown. Next, the competition in the AG is also investigated. Although the structure of AG is not clear yet,9 both bonds and cages exist in AG at the same time. We first increase the CS content to reach the glass region (Φ ≥ 0.58) and then introduce PSS species to probe the structure differences. The sample information is listed in Table 3. From Figure 5a,c, we can see that for RG at 20 °C, increasing either CS or PSS content will increase both the total volume fraction and G′, thus leading to a stronger glass. Table 3. Mass Fraction and Effective Volume Fraction of the Samples Measured in Figure 5 sample name CS CS CS CS CS CS CS CS CS

0.107 PSS 0 0.107 PSS 0.02 0.107 PSS 0.05 0.107 PSS 0.1 0.24 PSS 0 0.24 PSS 0.02 0.41 PSS 0 0.41 PSS 0.02 0.41 PSS 0.05

m (CS)/g

M (PSS)/g


0.107 0.107 0.107 0.107 0.24 0.24 0.41 0.41 0.41

0 0.02 0.05 0.1 0 0.02 0 0.02 0.05

0.21 0.24 0.26 0.31 0.40 0.42 0.58 0.59 0.60

For the gel state at 40 °C, we also increase the CS content from CS 0.107 (Figure 5b) to CS 0.24 (Figure 5d). Increasing the CS content would increase the number of bonds so that the gel strength will be higher. However, adding PSS inside the gel network can weaken the gel strength because PSS particles act as defects in the gel network to reduce the number of bonds and weakens the bond strength; therefore, G′ values of all binary gels are thus smaller than that of the unary gel, although the total volume fraction of the binary gel is higher. However, if E


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Figure 5. Dynamic frequency sweeps for samples with CS = 0.107 g fixed and PSS 0, 0.02, 0.05, and 0.1 g at 20 °C (a) and 40 °C (b); samples with CS = 0.24 g fixed and PSS 0 and 0.02 g at 20 (c) and 40 °C (d); and samples with CS = 0.41 g fixed and PSS 0, 0.02, and 0.05 g at 40 °C (e). γ = 1% was fixed, and the closed and open symbols represent the storage and loss moduli, respectively.

γC of G′ and G″, indicating that the cage breakage can make the whole system flow (G′ < G″). For AG CS 0.41 (Figure 6d), there is also two-step yielding due to bond and cage breakages.10,57 γ1 remains constant, whereas γ2 shifts to smaller strain with more PSS. Increasing the total volume fraction by adding PSS will make the cage smaller, so γ2 becomes smaller, whereas γ1, which is due to bond breakage, almost remains unchanged with PSS, further verifying that PSS has little influence on bonding in AG. We should also note that only with the addition of 2−3% PSS probes can greatly influence the cage effect, implying that caging dominates in AG. In Figure 7, the binary AG and gel are schematically illustrated. The gel is dominated by bonding, so when PSS particles are introduced in the gel composed of CS attractive species, some PSS particles are trapped inside the gel during the gelation process; these trapped PSS particles act as defects to reduce the number of attractive bonds and weaken the bond

strength. On the other hand, AG is cage-dominated and it has a more homogeneous structure than gel, so the addition of PSS has little influence on bonding; instead, the PSS strengthens the caging effect in AG, as shown in Figure 7a. Therefore, PSS will make stronger glass and weaker gel. Pham et al.58 studied the intermediate scattering function by echo DLS for both RG and AG. They found that AG shows a much higher plateau than RG, indicating that AG has much more restricted motion of the bonded particles. Particles caged in a stressed AG can be viewed as “stretched clusters”, and they can still hold together by particle attractions even though the bonds are “making and breaking” between different neighbors when the shear strain is not large enough to break the cage. Only when the shear strain is large enough, bonds are broken almost as soon as they form and the attraction becomes ineffective; then, the AG appears to flow with much the same microstructure as RG. Besides, Segrè et al.30 showed that gelation and glass transition exhibit similar behaviors in both F


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Figure 6. Dynamic strain sweeps of different samples at different states with a frequency ω = 1 rad/s. (a) Shear stress as a function of strain. The arrows are used to guide the eye to indicate the first and second yielding points. The data of PSS 0.1 in (a) are shifted by 6/. In (b−d), the closed symbols represent the storage modulus (G′) and the open symbols represent the loss modulus (G″). The data of PSS 0.02 and PSS 0.1 in (b) are shifted by 1.4/ and 10/, respectively; the data of PSS 0.02 and PSS 0.05 in (d) are shifted by 3/ and 7/, respectively.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03965. Frequency dependence of two-step yielding, oscillatory preshear procedure, and localization length as a function of temperature (PDF)

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Figure 7. Schematic illustration of binary mixtures: (a) AG and (b) binary gel.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-010-82618089. Fax: 86-010-62521519 (H.C.). *E-mail: [email protected] (C.C.H.).

static light scattering and DLS, indicating that both gelation and glass transition are manifestations of a more general jamming transition.

ORCID

He Cheng: 0000-0001-8718-4110 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS In summary, the interplay between caging and bonding is studied in a binary system. Addition of PSS can weaken the binary gels, whereas it can strengthen the glass (both AG and RG). On the basis of the localization length calculation, we found that more PSS can densify the glass structure, whereas it loosens the bonds of the gels. Moreover, the yielding behaviors further confirm that PSS particles have little influence on bonding in AG. All of these indicate that bonding determines the final property of the gel, whereas caging dominates in both AG and RG. Finally, the introduction of repulsive species PSS makes the gelation process more and more continuous, implying that glass transition and physical gelation might share a similar mechanism.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (nos. 21174152 and 21674020) is gratefully acknowledged.

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