Shear Induced Irreversible Gelation through Physical Network Formation

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Shear Induced Irreversible Gelation through Physical Network Formation Tahmineh Mahmoudi,† Vahid Karimkhani,‡ Gwang Seok Song,† Dai Soo Lee,† and Florian J. Stadler*,† †

School of Semiconductor and Chemical Engineering, Chonbuk National University, 567 Baekjedaero, Deokjin-gu, Jeonju, Jeonbuk, 561-756, Republic of Korea ‡ Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, 424 Hafez Avenue, P.O. Box 15875-4413, Tehran, Iran S Supporting Information *

ABSTRACT: A resin mixture of vinyl ester and carboxyl terminated butadiene diluted with styrene shows an increase of viscosity when shearing the sample. Unlike previous reports on rheopexy and shear induced gelation of unfilled systems, this behavior leads to permanently elevated levels of viscosity. FTIR found no indication of a chemical reaction, and therefore, the changes must be physical in naturebelieved to be an increase in the number of hydrogen bonds. On the basis of the results, it is concluded that a physical network based on supramolecular attractions between the −COOH groups of the carboxyl terminated butadiene is formed, which leads to a time-dependent behavior and increase in viscosity at certain shear rate, at longer shearing times. This is a new effect, which we call “irreversible shear induced gelation by supramolecular bonding”.

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many suspensions7 and polymeric nanocomposites8 have aggregate induced behavior as well. Under certain conditions, rheopexy may lead to a liquid− solid (jamming) transition, which attracts significant attention for the design and production of new materials with unusual properties. Hence, these shear induced structures might lead the way to bio-inspired design for high performance functional materials. Spider silk formation, which is consequence of shear induced silk spinning causing large aggregate jamming and producing fibers with high strength and toughness, is one of the most significant examples of processes involving rheopexy.9,10 The microstructure of Nafion 3D scaffolds and membranes is significantly influenced by rheopectic behavior of its suspension in water, which can influence the final performance of fuel cells.11 On the other hand, the existing understanding of this phenomenon is not quantitative and largely built upon experimental support.1 The time-dependent change of the rheological behavior differs from system to system. In general, two cases can be distinguished: In most cases, viscosity increases sharply after an induction period, during which it is equal to the zero-time viscosity (the induction period, usually observed in systems with a repulsive barrier like charge stabilized particles, is an indication for existence of an activation mechanism for shear driven irreversible aggregation),7 and it reaches a plateau at

INTRODUCTION

The classical types of non-Newtonian behaviorshear thinning and shear thickeningare usually observed for polymer melts or solutions and for highly filled systems, respectively. Depending on the nature of and interactions in complex fluid microscopic constituents, these materials exhibit fascinating types of different rheological behavior. In contrast to high interest of researchers to time-independent shear thinning and shear thickening behavior of non-Newtonian fluids, less attention was focused on thixotropy and antithixotropy (also called rheopexy), which are viscosity decreases and increases with time at steady shear, respectively. So far, a lack of deep understanding of the interplay between structure-breaking and structure-building processes, from which they derive, is obvious.1 Especially, rheopexy (which in some literatures is referred to as shear thickening; this misuse of the word leads to confusion in the literature2) and aggregate related structures, which are a consequence of shear induced structure building and have significant effects in life cycle,3,4 remained unexplored. This phenomenon has significant effects in life cycle3,4 and industrial processes: for example, some polymer solutions,2,5 many biomaterials,6 biological fluids such as protein solutions, e.g., the synovial fluid, which lubricates mammalian freely moving joints and blood plasmas, irreversible fibrous assembly of proteins, which induces pathological conditions as found in a group of diseases, including Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease, and the transmissible spongiform encephalopathies, all of which include extremely stable, highly ordered fibrils termed amyloids.3 Furthermore, © 2013 American Chemical Society

Received: November 6, 2012 Revised: April 15, 2013 Published: May 2, 2013 4141

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become large enough to eject the sample is γ̇ = 2000 s−1. These tests were repeated 20 times to observe the effect of shear on the viscosity function. FT-IR. In order to determine the vibrational bonds of the samples, Fourier transform infrared (FTIR) spectroscopy (Nicolet Magna 750 IR series II spectrometer) was used in the range of 400−1200 cm−1 at room temperature, in transmission mode.

longer times. In other cases, a change of properties is obvious immediately. The term “gelation” refers to a viscosity increase and, furthermore, to a G′(ω) = G″(ω) in the case of a critical gel, i.e. a system, which is just at the point where it is started to be considered as a gel.12 For many gels, however, G′(ω) ≫ G″(ω), once the network is fully established. This also means that gels do not have a terminal regime and, thus, no zero shear-rate viscosity η0. Hence, a gelation is a solidification in the strict sense, although many gels still appear to have significant liquid like characteristics such as a flowability under mild stress. Typically, gels are more or less Newtonian liquids prior to gelation.13−17 The aim of the current research is to investigate the shearinduced irreversible gelation of a composite system, which can help to provide deeper understanding of rheopexy in polymeric solutions. In this respect, vinyl ester resin/CTBN (carboxyl terminated polybutadiene)/styrene has been chosen as the model system. This system is a resin mixture for many high performance applications like wind turbine blades, marine systems, and FRP (fiber reinforced plastics) tanks (because of high corrosion resistance) and airplanes. In this system, styrene acts as an efficient reactive diluent and CTBN is a toughener. Considering the practical importance of such systems, our objective is to show on which factors the shear induced gelation depends and how the irreversibility of the physical changes can affect the rheological behavior. Such understanding can help to get an insight into processing of resins to making composites and tune the microstructure and properties of the final product.

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RESULTS Quiescent Cross-Linking over Long Times. As a reactive resin was used, the first obvious question is whether the system cross-links without any outer influences (e.g., by electron beam). Figure 1 shows the viscosity at low shear rate for 2

Figure 1. Viscosity of sample 5−40 (square) and 5−20 (circle) CTBN−styrene at different times.

MATERIALS AND METHODS

Materials. Vinyl ester resin (VE), Epovia-RF1001MV, was obtained from Cray Valley, Korea. Styrene monomer was purchased from Sigma-Aldrich. Carboxyl terminated butadiene−acrylonitrile (CTBN), Hycar 1300X13, was obtained from Emerald Performance Materials. All chemicals were used as received. Sample Preparation. The synthesis of CTBN/styrene/VE solutions was performed by mixing VE with CTBN in a mixer. This precursor is then put into an oven at 120 °C for 10 min. As a final step, different amounts of styrene are mixed with the mixture. The resulting paste was homogenized by intensively mixing the compound three times for 2 min each. The concentrations of all constituents were kept constant in the regime of a concentrated solution throughout the whole process. Rheology. All rheological measurements were performed on a stress-controlled KinexusPro rheometer (Malvern, UK) using a cone− plate geometry (2°/20 mm). All experiments were performed at T = 25 °C. Tests at constant shear rate γ̇ were performed in straincontrolled mode. After 0.1−2 s (depending on the sample viscosity), the set shear rate γ̇ was reached and kept constant within ±0.1%. Besides this, also so-called “thixotropic loop tests” were performed, in which the viscosity function is measured with increasing and decreasing shear rate. The viscosity was measured as the value obtained after 5 min in a creep test with a given shear stress. Following this, the shear stress was increased or decreased (depending on the direction, in which this subtest was taken) for measuring the viscosity at the next data point. For better comparison between different data sets, the data were plotted at the viscosity as a function of shear rate η(γ̇). While this setup provides a viscosity function, the values are not steady state but transient values, which is unavoidable considering the material behavior of these special mixtures. Depending on the viscosity of the samples, different ranges of shear stresses were chosen. For higher viscosity samples (η0 > 10 Pa·s), the applied shear stress was varied between 1 and 1000 Pa, and for lower viscosities only 1−500 Pa was applied, in order to avoid expulsion of the sample due to centrifugal forces. Typically, the limit at which these centrifugal forces

samples (5−40 and 5−20 CTBN−styrene) at different days during a period of 5 months. The samples were stored in the dark but without any refrigeration. It is obvious from these measurements that the sample viscosity at low shear rate does not change. Hence, neither vinyl ester nor the styrene tends to cross-link physically or chemically in the quiescent state. The standard deviations from for the 5−40 and 5−20 samples are 7.6% and 4.06%, respectively. A systematic increase or decrease of viscosity within in half a year was not observed. Therefore, chemical cross-linking can be safely excluded in the discussion to follow. Shear Induced Supramolecular Cross-Linking as a Function of Time, Styrene Content, and Shear Rate. In order to get a first overview of the rheological behavior of the samples, a startup shear experiment with a shear rate γ̇ = 1 s−1 was conducted for 1 h. The pure components, CTBN and styrene, were tested along the mixtures (the viscosity of pure vinyl ester could not be tested, as it is a solid at room temperature without the addition of a solvent, e.g. styrene). To compare the effect of dilution by styrene, solutions with content [wt %] of 5−20, 5−30, 5−40, and 5−50 of CTBN and styrene, respectively, were assessed. The remaining fraction (45−75 wt %) is the vinyl ester resin. Figure 2 clearly shows that the viscosity of styrene (leftward pointing triangles) and CTBN (diamonds) are constant as a function of time. Furthermore, the measured viscosity for styrene agrees with the literature value.18 The viscosity of styrene was measured in creep due to its low viscosity, and because of the inertia of the geometry, the data of the first 60 s are too high and, thus, omitted in Figure 2. Also, the viscosity of pure CTBN does not change as a function of time of shear, 4142

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Figure 2. Rheopectic behavior of samples, which have 5% CTBN with different amounts of styrene monomer, vs shear rate γ̇ = 1 s−1.

showing clearly that neither of the base components shows changes in viscosity during intensive shearing. These findings confirmed that the solvent trap setup, able to prevent evaporation of styrene, does not influence the measurement as well as that styrene does not undergo quiescent cross-linking. For the mixtures of the vinyl ester resin with the other components, however, it is clear that the viscosity increases as a function of time t and, thus, totally applied shear γ. Furthermore, it becomes obvious that the effect is the stronger the higher dilution with styrene, which also lowers the initial viscosity. While the viscosity of the samples with 20, 30, and 40% styrene increases with an approximately constant power law slope, the 5−50 sample with 50% styrene (tip to top triangle) first increases at a much higher slope than the other samples to decrease the slope around 1000 s and, by doing that, never significantly exceed the viscosity of the 5−40 sample. The origin of this behavior is not obvious, but the effect is reproducible. Problems due to asymmetric loading, air bearing damage, or a warped geometry can be safely excluded, as one full rotation of the 2° cone at γ̇ = 1 s−1 takes ∼30 s, and the “periodicity” of the data is in the order of 1000 s. Possible origins of this behavior will be discussed later. The magnitude of the effecta factor of about 7 for the 5−50 sampleclearly shows that this is not an experimental artifact due to e.g. a bad rheometer. These results already indicate that a structure is built up. When assuming that this structure is a temporary supramolecular network, the results indicate that shearing the sample makes the material form such a network, which does not form in quiescent state (cf. Figure 1). The relation τ = ηγ̇ clearly proves that the dependence on the styrene content cannot be explained by the higher shear stress τ, as the viscosity increase is higher for the samples experiencing a lower stress τ. As γ̇ is constant, it is also clear that this effect cannot be attributed to different shear rates γ̇ or different total deformations γ. The shear rate γ̇ also distinctly influences the shear induced gelation, which the effect of different shear rates on the 5−40 sample is shown in Figure 3, where different shear rates yield significantly different viscosity increases. A continuous constant shear rate was applied on the 5−40 sample for 1 h (3600 s). The experiment was performed at six different shear rates, γ̇ = 0.1, 1, 3, 10, 30, and 100 s−1, using a new sample for each experiment. As discussed before in Figure 2, the material shows an increase of viscosity as a function of time. However, for none of the shear rates was a steady state reached within a

Figure 3. (a) Viscosity measurements in six shear rates γ̇ = 0.1, 1, 3, 10, 30, and 100 s−1 for 1 h. (b) “Viscosity function” at three different times t = 60, 600, and 3600 s.

measurement time of 1 h. As will be shown later, also significantly longer measurement times do not lead to reaching a steady state. Depending on the shear rate, a different time dependence of the viscosity was observed. The two lowest shear rates (γ̇ = 0.1 and 1 s−1) show a steep increase of the viscosity at lower times with the slope leveling off somewhat at longer times. At γ̇ > 10 s−1, the viscosity increases quickly for a short time (until η ≈ 6 Pa·s is reached) followed by a lower increase, whose slope increases in an exponential-like fashion (in log scaling). The only shear rate with a significantly different behavior is γ̇ = 3 s−1, which up to around t = 1500 s exhibits a steep viscosity increase followed by a leveling off at longer times. In many ways, this data is very similar to the behavior of the 5−50 sample in Figure 2; hence, both conditions are equivalent to each other. When comparing the viscosity at a constant time as a function of shear rate γ̇ (Figure 3b), distinct differences become obvious for different times. For t = 60 s and t = 600 s, the lowest viscosity is found at γ̇ = 1 s−1, while for t = 3600 s, the minimum in the “viscosity function” is at γ̇ = 10 s−1. The highest viscosity, however, is observed for γ̇ = 0.1 s−1. Proposed Mechanism. These findings are interesting and puzzling at the same time. An attempt at an explanation can be drawn as follows. It was found earlier that a CTBN very similar to the one used for this article has a 6 times higher viscosity than the version without the carboxyl groups.19 Although this association between the carboxyl groups (−COOH) is significantly weaker than the associations when metal ions are added,19 it clearly demonstrates that supramolecular bonds, albeit weak ones, can be induced by carboxyl groups. These are based on hydrogen 4143

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3 The stronger the shear, the more likely a “streamlined” structure will be found. 4 The local entropy is not affected significantly by the chain ends having a network or a watermelon-like structure as long as the chain stretch is not too high. Hence, a rebuilding of the watermelon structure is unlikely, even when completely ceasing the shear. These thoughts would potentially lead to a complicated kinetic model with several constants and probabilities, which are rather hard to assess. Hence, we restrict this article to a qualitative description, which is also given in Figure 4 schematically.

bonds, which are in a dynamic equilibrium between open and closed states. Shearing will shift the equilibrium to open bonds, whichself-evidentlywill increase with the shear rate γ̇. At the same time, also chain orientation and stretching will increase proportional to γ̇. Based on the investigations, the Rouse time of the pure CTBN should be in the order of 10−6 s (for T0 = 25 °C, based on shifting measurements at lower temperatures) and, hence, significantly faster than the relaxation times accessed by the shear rate γ̇. However, the CTBN is entangled (based on Mw ≈ 8 kg/mol and Me of 1,4 PBd of ≈2 kg/mol)20 and shows a crossover frequency ωc around 100 s−1 at 25 °C. However, when assuming a good or θ-solution with the VE resin and styrene as solvent, and a maximum CTBN content, using Osaki et al.’s laws for semidilute materials,21 we can consider that the CTBN under the conditions used is on the threshold to being entangled. Under perfect mixing conditions it is unentangled, but when phase separation becomes more dominant, entanglements might be formed locally. Hence, stretching the chains is possible although the relaxation can also happen in the regime accessible in experiment. Based on this, stretching of CTBN is possible theoretically, but because of the nature of the mixtures with an unknown degree of phase separation, it is hard to measure of chain stretching. As the CTBN used contains two carboxyl functionalities per chain on the average, which are responsible for supramolecular bonding sites, it is clear that different conformations are imaginable from loops to 3D networks (previous investigations demonstrated that up to 80 carboxyl groups cluster together in such a supramolecular bonding site19,22,23). On the basis of Lo Verso and Likos,24 it is known that the equilibrium conformation of telechelic polymers in rather dilute solution (in our case 5% or 10%) can be approximated as a “watermelon”-like structure, i.e., several chains form a cluster by being attached end-to-end having an approximately spherical structure. A dynamic equilibrium between open and closed conformation of these “watermelons” means that as long as there is no deformation in the sample, reattachment of detached chain end to the same “watermelon” is more probable (p(csame)):

Figure 4. Scheme of the proposed network formation mechanism.

It is clear that shear increases the viscosity andconcluding from thisthe probability for destroying the “watermelon” structure. Within a shear time of 1 h, none of the experiments showed that an equilibrium of this process was reached, as the viscosity still increases for all conditions tested. At a low shear rate, we can assume that only the likeliness of an open chain end, which potentially can participate in H-bond formation, to bind to another site than the original “watermelon” is increased. This leads to the rather constant increase in viscosity and to the conclusion that the resulting 3D network is not very much disturbed by the shear. When shearing at a high rate, the destructuring of the “watermelons” is faster initially, leading to a fast increase for the first 100−2000 s (depending on γ̇); see Figure 3. The critical time, when this initial quick destructuring ends, is ∼100 s for γ̇ = 100 s−1, 700 s for γ̇ = 30 s−1, 1000 s for γ̇ = 10 s−1, and 2000 s for γ̇ = 3 s−1. However, also the formed 3D network cannot grow as much, as bonds perpendicular to the shear direction are very much under stress and often broken apart. As a consequence, a streamlined network is formed, which after initially quick buildup cannot grow as quickly after this initial phase. An increase in the shear rate leads to a higher and increasing slope in the next phase. While the level of viscosity at the critical time mentioned before does not vary very much for γ̇ = 100, 30, and 10 s−1, it is significantly higher for γ̇ = 3 s−1. This leads to the conclusion that the shear rate γ̇ = 3 s−1 is a transition between the behavior for low and high shear rates.

In a flow field, however, the open chain end is stretched and oriented, which leads to an increased probability that the carboxyl group will reattach to a cluster not belonging to the same “watermelon” (p(cother)). Furthermore, the flow field will increase the interaction rate between open chain ends and, thus, speed up the reattachment rate. The consequence is that in a shear field the equilibrium between a watermelon type and a 3D-network type structure shifts to the latter; i.e., the structure moves from a watermelon-like structure to a networklike structure, when sheared. Of course, several conditions should be considered:24 1 The stronger the shear, the more likely is that the open chain end meets chain ends not belonging to the same watermelon structure. 2 The stronger the shear, the lower is the likeliness that a bond is formed perpendicular to the shear direction, which is stable. 4144

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The minimum in the viscosity as a function of shear rate after a certain time (Figure 3b) can be interpreted as the balance between two processes. At low shear rates a slow but steady buildup occurs, while at high shear rates a faster buildup occurs, which is partially reversed by destructuring due to the higher shear stresses. As long as the structure buildup is not very strong (i.e., for η < 6 Pas), the viscosity increase and resulting from that the transient network formation at high γ̇ is very quick. At short times, the high shear rate structure buildup is, therefore, relatively strong, while the low shear rate structure buildup is weaker. Hence, the viscosity minimum is at a low shear rate (γ̇ ≈ 1 s−1). At short times, also η(γ̇ = 3 s−1) has the same shape as the lower shear rates. At long times (t ≈ 2000 s), a clear leveling off is found for the high shear rates, which corresponds to a strong competition between structural buildup and destructuring, which then speeds up toward longer times, a sign that the structure has been modified in a way by the large amounts of shear that can form a transient network at a growing rate. The measurements discussed above were done on the 5−40 CTBN−styrene sample. Because of the same composition (and, thus, an identical amount of hydroxyl groups relative to the CTBN content), the same kind of physical bonding is responsible for network formation for all composites in this article. Based on the above arguments, the finding of a higher viscosity at lower shear rates is reasonable. Reversibility of Built-up Structure. The above contemplations have proposed a mechanism for the increase of viscosity by the shear induced formation of a 3D network. This opens up the question whether this network will go back into the starting situation of a watermelon-dominated structure. To check this, tests were performed with a protocol, at which the sample was first sheared with γ̇ = 100 s−1 for 1 h and then left at rest for 2 h followed by a restart of the shearing for another 2 h at γ̇ = 100 s−1 (Figure 5a). For comparison, the sample was sheared at γ̇ = 100 s−1 for 3 h. In a variation of this test setup, the first shear (γ̇ = 100 s−1) period was 1 h followed by 24 h at rest, 1 h shear, 24 h at rest, and a final shear step of 1 h (Figure 5b). The first obvious finding is that even shearing for 3 h continuously at γ̇ = 100 s−1 will not lead to a constant viscosity. Instead, the viscosity increases from about η = 15 Pa·s at t = 3600 s to about η = 53 Pa·s at t = 10 800 s. Furthermore, the viscosity increase becomes lower and “bumpy” around 4500 s. These “bumps” have a periodicity not related to the time for a full rotation of the geometry or a too low torque and can only be interpreted as the consequence of an instability such as shear banding or slippage. These instabilities cease around 9000 s, which is also the time when the steep increase of the viscosity starts. The total increase of viscosity between 1 and 3 h is 38 Pa·s or 353%. The data of the stopped shear test agree very well with the before described test for the first 3600 s. When restarting the shear after 2 h waiting, however, an increase of only 2.7 Pa·s or 32% is found. Qualitatively, the increase of the viscosity fits the increase of the continuous shear test, but the total increase is lower, presumably because the instable, “bumpy” viscosity increase does not cease, whose origin will be discussed later. Figure 5c shows the same test setup as shown in Figure 5a but with γ̇ = 0.1 s−1. The first obvious point is that the viscosity increases smoothly, which is the consequence of the lower shear rate, having a lower tendency to form instabilities than γ̇ = 100 s−1. Second, the viscosity after the resting time decreases

Figure 5. Rheopectic behavior of 5−40 CTBN−styrene at shear rate γ̇ = 100 s−1 (a) in continuous and discontinuous mode in 3 h, (b) discontinuous in 3 days, (c) shear rate γ̇ = 0.1 s−1, and (d) magnification of the first 200 s in (c).

significantly (Figure 5d), which can be interpreted as a partial destructuring of the formed weak 3D network. Most likely, the 4145

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same is also found for γ̇ = 100 s−1, but the 1000 times higher shear rate makes it impossible to observe it due to the much slower transient behaviorconsidering that the destructuring takes 200 s at γ̇ = 0.1 s−1, one can assume that the same process would take only 0.2 s at γ̇ = 100 s−1, which is too fast for the experimental setup. It is also visible that the quiescent increase of η is significantly larger than for the higher shear rate γ̇ = 100 s−1 (80% vs 32%), which is the consequence of the smaller tendency of the low shear rate to destruct the network formed quiescently. Several things from these measurements can be concluded. First, the increase in viscosity does not decrease when waiting. Second, it increases somewhat, presumably by repairing shear induced changes, coalescence of network fragments, and/or mending of the shear bands and surface slippage zones. Third, the evaporation of the styrene solvent plays a minor role. Increasing the waiting time to 1 day confirms these results (Figure 5b). The increase of viscosity for 1 day of waiting is slightly larger than for a waiting time of 2 h, which is logical considering the previously mentioned arguments of a network perfectioning. It is important to mention that this is not in agreement with common antithixotropy, where flow structures and rest destructures the material.25 The second shear leads to a stronger “bumpiness” in η(t), which leads to the strong suspicion that the “bumps” are caused by shear banding or slippage, as the perfectioning of the network will make a single larger slip plane more likely than a continuous shear because the continuous shear does not allow the network to perfect itself in a way that most parts of the network are so rigid that they induce a slip plane. The final shear step shows a smaller increase in viscosity than the first one, which indicates that the network is already rather well perfected. Furthermore, it seems to approach a steady state around 130 Pa·s, which is about 20 times the original viscosity of the sample. As even after more than 2 days of measurement time, the styrene evaporation is not a significant factor (which was confirmed by the observation of the gap filling), we can conclude that the effects described above are the consequence of changes in the material, the kind of which has to be discussed. Dependence on the CTBN Content. In Figure 6, the viscosity as a function of time for three samples containing 40% styrene but different 0, 5, and 10% CTBN is discussed. In that way the effect of CTBN on the rheological behavior at constant shear rate can be assessed. In Figure 6, the time-dependent viscosity η(t) can be divided into two parts with the first part showing a high slope, while the second part only has a lower and nonconstant slope. The slope of increasing viscosity in first 1 h (3600 s) is directly related to the concentration of CTBN. For sample 5−40 CTBN−styrene the slope is 5 times larger than 0−40 CTBN−styrene. For the sample with 10% CTBN the highest slopes are found. It is also evident that the higher the CTBN content, the more “wavy” is the second part of the viscosity as a function of time η(t). This and the lower slope lead to the conclusion that the flow behavior becomes unstable in the second regime. Most probably some kind of shear banding or incomplete adhesion occurs. An experimental artifact due to too low torque or insufficient shear rate control can be excluded, as the first 3600 s does not show this effect, while a lower viscosity (and, thus, a lower torque) is observed. The shear rate is within an error margin of ±0.1 s−1 (±0.1%) throughout the experiment.

Figure 6. Viscosity of the 0−40, 5−40, and 10−40 CTBN−styrene samples at γ̇ = 100 s−1 at 25 °C for the measurement time of 3 h.

The mechanism behind these fluctuations is most likely related to the finding in Figure 5a,b that a transient network is formed. When imagining this network as a spongelike structure, it becomes obvious that the interactions in a shear band fluctuate depending on how many associative groups are present above and below the shear band at a given point in space and time. As the viscosity measures the average over the whole area of the geometry, many such stick−slip phenomena are averaged, leading to a “wavy” viscosity as a function of time. “Thixotropic Loop” Tests. Figure 7 displays the viscosity function determined in a shear stress range between 1 and 1000 Pa. The measurements consist of increasing the shear stress in increments of 1/5 of a decade and determining the viscosity for 1 min before going up in shear stress to the next level, at which this procedure is repeated. Self-evidently, the viscosity at constant shear rate is time-dependent itself (Figures 2−6), and thus, the measured valuestrictly speakingis not a steadystate value. The viscosity function obtained this way is measured for 20 full cycles. Figures 7a and 7b show the measurements for the sample 5−40 and 10−40, respectively. For the sake of clarity, only the rising branches (γ̇ increasing) of the measurements are given and only the 1st, 2nd, 12th, 13th, 19th, and 20th cycle have their own color. Several additional measurements are given in the Supporting Information. The viscosity as a function of shear-rate and cycle number can be divided into three distinct regimes, which can be described in the following way: before inducing any large shear stress on the sample, the viscosity is almost perfectly Newtonian (black squares). When reaching approximately γ̇ = 30 s−1, the viscosity starts to increase somewhat, which is very similar to shear-thickening in aggregating polymer solutions (I).26 In this regime, the CTBN molecules are in their low energetic level connected by H-bonding among their carboxyl groups (Figure 7a,b), which theoretically was described as a watermelon structure.27 Measuring the same shear stress range downward already shows a significant difference to the measurement with increasing shear stress. The viscosity increases with a slope of ∼0.07 in double-logarithmic scaling (II). The first cycle shows a distinctly different characteristic for the rising and decreasing branch of the viscosity function (black squares (I) and red circles (II) in Figure 7c for the 5−40 sample; the number on the left side of the curves denotes the 4146

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cycle number). The viscosity is roughly constant up to approximately γ̇ = 20 s−1, followed by a shear-thinning-like behavior. The viscosity is always larger than for the decreasing branch of the first cycle. It is visible in Figure 7c that the decreasing branches (filled symbols) are similar to each other in shape, just shifted in viscosity, while increasing branches (open symbols) change from a shear thickening behavior at high γ̇ to an increasingly clearly pronounced double relaxation behavior, which has a shear thickening peak around γ̇ = 0.3 s−1 and a clear shoulder around γ̇ = 100 s−1. From cycle 7 onward, the two branches cross each other several times with this behavior becoming more pronounced with increasing number of cycles, indicating that the structures dominating at low shear rate γ̇ are different from the ones at high γ̇ (III). When increasing γ̇ beyond ≈1 s−1, one slowly destructures these low γ̇ structures, while they cannot form until much lower shear rates γ̇ when decreasing γ̇. This behavior becoming increasingly obvious at high cycle numbers again resembles similar findings on associative polymer solutions.28,29 However, unlike these wellknown associative systems, this behavior develops over time and even after 20 cycles (with a total deformation γ > 10 000% for each cycle), an equilibrium for the structure is not encountered. The double relaxation found suggests that there are two levels of dynamic hierarchical order developing. At γ̇ ≈ 100 s−1, a rather short-range order seems to present, which is found for the rising and decreasing γ̇ branches, while the longer range order, being responsible for the peak or shoulder around γ̇ = 0.3 s−1, develops the more the more cycles are imposed on the system and, thus, indicates a shear-induced ordering, which requires some low or no shear conditions to establish the longer range order, required for transient shear thickening. For γ̇ > 10 s−1, both branches are roughly identical, indicating that the labile long-range order does not exist under these conditions anymore. Figure 7d shows the viscosities of the 5−40 and 10−40 samples as a function of cycle number, determined at σ = 6 Pa (one of the lowest shear stresses applied) and σ = 1000 Pa (highest shear stress applied). While the slope of the viscosity increase as a function of cycle number is roughly constant at σ = 1000 Pa, the slope at σ = 6 Pa is not constant, corresponding to the jumps mentioned previously (III). This clearly proves that the structures causing the low viscosity increase are not built up linearly. Furthermore, it is clear that the increase as a whole is related to the CTBN content, as the 10−40 power law slope in viscosity vs cycle number is about twice as high as the 5−40 sample. The increase of the viscosity as a function of cycle number can be explained in the following way: After starting shearing and by increasing the shear stress, the watermelon structures break down (Figure 4). This opens up the possibility for the formation of three-dimensional transient networks, which are very sensitive to shearing, that causes the shear thinning behavior especially observed after several cycles. The fact that the effect continues to increase suggests that only a fraction of the “watermelons” are torn apart and consequently used to form a transient network. On one hand, the shear stress destroys the H-bonding interactions, and the logical consequence is the molecules disperse in the medium with the maximum possible entropy. But a high shear stress cannot stop the carboxyl group from forming hydrogen bonds in a “streamlined fashion”. Hence, the CTBN molecules form layered structures that are more stable

Figure 7. Shear-stress and shear-rate dependence of the viscosity for (a) 5−40 CTBN−styrene sample (γ̇ ↑ only) and (b) 10−40 CTBN− styrene sample (γ̇ ↑ only), (c) cycle 1, 7, 14, and 20 (γ̇ ↑ and ↓), and (d) preshear behavior of 10−40 and 5−40 CTBN−styrene samples. 4147

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Figure 9. FT-IR spectra of sample 5−40 CTBN−styrene after different shearing times, (a, b, c) comparing the pure styrene, vinyl ester, and CTBN with 5−40 CTBN−styrene sample (unsheared) and 5−40 CTBN−styrene sample sheared at constant shear rate γ̇ = 100 s−1 for 1 h per day for 3 days, at room temperature, (d) 5−40 CTBN−styrene sample before and after measurement, constant shear rate γ̇ = 100 s−1 for 4 h at room temperature, and (e) 5−40 CTBN−styrene sample before and after measurement, constant shear rate γ̇ = 100 s−1 for 1 h per day for 3 days, at room temperature.

However, in comparison to normal polymeric systems, shear thickening is found for some condition as well as an increase of viscosity with each additional viscosity function measurement. The shear thickening at certain shear rates is observed for some but not all supramolecular systems,28,29 while so far no report of shear induced gelation of such systems is known to the authors. But in our samples the noncovalent hydrogen bonding between the molecules is responsible for higher viscosity. Unlike the definition of the term “gelation” just via a viscosity increase, the “gelation” for these samples also led to an intermediate region of constant power law slope in the viscosity function ≫0, which indicates the buildup of significant elasticity in an initially more or less Newtonian liquid. These rheological properties can be considered to be gel-like, although

under shear stress, probably involving shear bands. This is the result of the competition between hydrogen bonding and the entropy changes of the system. The third regime, obvious in Figure 7b, is a jumping in viscosity function after 14 cycles at low γ̇ (this number is different for each material). In this regime, it seems that the presheared sample creates temporary 3-dimensional networks found only in the low shear rate regime, which causes a significant increase in viscosity of the sample. As can be seen, such networks are not very stable against high shear rates and high shear deformations destruct these assemblies. Similar behavior was reported by Benchabane et al.26 in carboxymethyl cellulose (CMC) solutions. By increasing the concentration of the solution, the viscosity of the sample increases. Qualitatively, the same behavior in can be observed. 4148

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in the precise sense of the word “gel”, the property of having no terminal regime could not be proven.17 FT-IR. The rheological indicators of the buildup of a network are rather clear. The question, however, is whether the proposed mechanism can be confirmed by other means. The most suitable method for this purpose is to check the functional groups by Fourier transform infrared spectroscopy (FT-IR), which gives information on whether chemical changes occurred in the sample during the measurement. To do that, FT-IR measurements were performed before and after the rheological test. In Figure 9, the FT-IR spectra of 5−40 CTBN−styrene sample is presented in comparison to pure styrene, vinyl ester, and CTBN. In general, there are two regions in the spectrum of 5−40; the first one starts from 400 to 2500 cm−1, and the second one is between 2500 and 4000 cm−1. The first part is a mixture of all chemical bonds of functional groups of three pure samples (styrene, vinyl ester, and CTBN), which are compared in Figure 9a−c. The significant changes are found in the second part, the region of hydroxyl group band. In the Figure 9d, the peak due to carboxyl group of the resin at 3459 cm−1 undergoes a shift to 3434 cm−1 after the rheological measurement. Weaker bond dissociations require lower energy which in FTIR a peak appears in lower wavenumbers (red-shift).30 As chain stretching and orientation can happen during shearing, but being entropically not favored, H bonds in the network are weaker than watermelon structures (because of chain’s tendency for going back to the thermodynamically equilibrium state). Therefore, the red-shift was observed in sheared samples. In some cases bimodal peaks indicate that the sample contains two different types of Hbonded structures. Considering the unsteady flow conditions observed for this sample in the later shear steps (Figure 5b), it can be supposed that the intermittent shearing has caused microscopic inhomogeneities consisting supposedly of watermelons and networks, whose interactions and local distribution cause the instable flow conditions. After 3 days, with 1 h of shear applied per day, the results are slightly different. In Figure 9e, with comparing the FT-IR spectra before and after measurement, there are two peaks for the −OH bond: one is at 3333 cm−1 and close to −OH bond peak in CTBN molecules, and the second one is at 3512 cm−1, which is close to CTNB−styrene compound −OH bond before shearing. This is an indicator that after shearing for a long time free molecules both attach to each other molecules in the same was as without in both (watermelons) as well as to form in shear induced structures discussed above (network). The explanation discussed previously suggests that the topography changes happens for transition from “watermelons” to a 3D network when shearing the sample. The mechanism, however, also clearly states that the overall number of H-bonds in the system increases with shearing of the resin. Hence, the FT-IR results confirm this conclusion. The following explanation can be used to understand the findings: The rheological measurements show that a permanent physical network is formed based on hydrogen bonding. The dynamic equilibrium in the H-bonding responsible for the rheological properties depends on the shear history of the material, which in general leads to an increase in viscosity. However, clearly no reversibility was observed, as the difference between the initial watermelon-like structure and the transient network formed by the shear is small in terms of entropy and enthalpy.

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CONCLUSIONS

In this article, the rheological behavior of vinyl ester/carboxyl terminated butadiene (CTBN)/styrene compounds are investigated. By applying shear in different fashions and by controlling the temperature in a cone−plate rheometer, significant changes of the rheological behavior can be obtained. By inducing the shear stress, a competition between the CTBN and vinyl ester molecules for instruction and destructuring of the physical network started via hydrogen bonding. Such reactions does not allow the system to reaching a steady state, even at long times. Increasing the styrene monomer content can also decrease the total viscosity of the system in the range of applied shear rates, but the point is more dilution cannot stop the CTBN molecules from formation of a network. For CTBN-free samples this effect is significantly lower than the samples, which contain CTBN, because vinyl ester molecules have a small but non-negligible contribution in network formation. The viscosity increase effect is stronger when applying thixotropic loops, as the variation of the shear rate γ̇ on one hand significantly destructures the sample (as γ̇max ≈ 1000 s−1) and on the other hand allows for reordering of the structure in the phases of low γ̇. This variation is the key to a more rapid buildup of the structure. Performing repeated “thixotropic loops” leads to the conclusion that the system indeed is also transiently shear thickening, but only when increasing shear rate γ̇, as such a structure needs low or no shear conditions to build up. Just like in continuous shear, this material does not reach an equilibrium value within the time of the experiment. The increase of the viscosity by shear is concluded to be the consequence of the destructuring of watermelon-like structures and the formation of a three-dimensional network, which is sensitive to shear itself and adapts by forming in a stream-lined fashion, i.e., by formation of shear bands or incomplete plate adhesion. Stopping and continuing the shearing leads to a rather small continuation of the gelation, which is significantly smaller than the viscosity increase when shearing. Continuing the shear after an interruption of several hours leads to a very qualitatively similar viscosity increase as if the shear interruption would not have taken place. Hence, it can be concluded that these mixtures show a permanent physical shear induced gelation based on H-bonding by a transition from a watermelon-like structure. This effect is the stronger the higher the styrene content. This is most likely the consequence of the increased mobility of the chains, which make the association with chains not belonging to the same “watermelon” more likely. Moreover, unlike the common rheopectic systems, in this case there is an increment from beginning for all shear rates, and it is not possible to reach the plateau, within the time scale of experiments. We presented a model that can explain the behavior based on association, disassociation, chain stretching, and orientation as well as shear-rate-dependent interactions between functional groups. The central point of the model is that the balance between association and disassociation and to which groups it bonds depends on the shearing state of the system. While in most cases thixotropy and antithixotropy behavior are reversible,31 our system exhibits irreversible changes (within the range of applied stresses and experimental time scale), related to network formation via hydrogen bonding. As hydrogen bonding exists in the system before shearing, and there is no considerable difference in its amount before and 4149

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(4) Geerligs, M.; Peters, G. W. M.; Ackermans, P. A. J.; Oomens, C. W. J.; Baaijens, F. P. T. J. Biomech. 2010, 43 (6), 1153−1159. (5) Feng, Y. J.; Grassl, B.; Billon, L.; Khoukh, A.; Francois, J. Polym. Int. 2002, 51 (10), 939−947. (6) Wang, B.; Li, D.; Wang, L. J.; Ozkan, N. Carbohyd. Polym. 2010, 79 (4), 1130−1139. (7) Guery, J.; Bertrand, E.; Rouzeau, C.; Levitz, P.; Weitz, D. A.; Bibette, J. Phys. Rev. Lett. 2006, 96 (19), 198301. (8) Ray, S. S.; Okamoto, M. Prog. Polym. Sci. 2003, 28 (11), 1539− 1641. (9) Jin, H. J.; Kaplan, D. L. Nature 2003, 424 (6952), 1057−1061. (10) Vollrath, F.; Knight, D. P. Nature 2001, 410 (6828), 541−548. (11) Estevez, L.; Kelarakis, A.; Gong, Q. M.; Da’as, E. H.; Giannelis, E. P. J. Am. Chem. Soc. 2011, 133 (16), 6122−6125. (12) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30 (2), 367−382. (13) Ferry, J. D. Viscoelastic Properties of Polymers; John Wiley and Sons: New York, 1980. (14) Vatankhah-Varnoosfaderani, M.; Hashmi, S.; GhavamiNejad, A.; Stadler, F. J. Chem. Commun. 2013, DOI: 10.1039/C3CC41332B. (15) Hamley, I. W. Introduction to Soft Matter: Synthetic and Biological Self-Assembling Materials; John Wiley & Sons Ltd.: Chichester, UK, 2007. (16) Macosko, C. W. Rheology: Principles, Measurements and Applications; Wiley/VCH: Poughkeepsie, NY, 1994. (17) Zhao, X. J. Biomater. Sci., Polym. Ed. 2006, 17 (4), 419−433. (18) Wohlfarth, C. Viscosity of styrene. In Springer Materials - The Landolt-Börnstein Database; Lechner, M. D., Ed.; Springer-Verlag: Berlin, 2008; Vol. 25, pp 499−499. (19) Stadler, F. J.; Schumers, J.-M.; Fustin, C.-A.; Gohy, J.-F.; Pyckhout-Hintzen, W.; Bailly, C. Macromolecules 2009, 42, 6181− 6192. (20) Fetters, L. J.; Lohse, D. J.; Colby, R. H. Chain Dimensions and Entanglement Spacings. In Physical Properties of Polymers, 2nd ed.; Mark, J. E., Ed.; Springer: Heidelberg, 2007. (21) Osaki, K.; Nishimura, Y.; Kurata, M. Macromolecules 1985, 18 (6), 1153−1157. (22) van Ruymbeke, E.; Orfanou, K.; Kapnistos, M.; Iatrou, H.; Pitsikalis, M.; Hadjichristidis, N.; Lohse, D. J.; Vlassopoulos, D. Macromolecules 2007, 40 (16), 5941−5952. (23) Davidson, N. S.; Fetters, L. J.; Funk, W. G.; Graessley, W. W.; Hadjichristidis, N. Macromolecules 1988, 21 (1), 112−121. (24) Lo Verso, F.; Likos, C. N. Polymer 2008, 49 (6), 1425−1434. (25) Barnes, H. A. J. Non-Newtonian Fluid Mech. 1997, 70 (1−2), 1− 33. (26) Karaaslan, M. A.; Tshabalala, M. A.; Yelle, D. J.; Buschle-Diller, G. Carbohydr. Polym. 2011, 86 (1), 192−201. (27) Lo Verso, F.; Panagiotopoulos, A. Z.; Likos, C. N. Phys. Rev. E 2009, 79 (1), 010401. (28) Calvet, D.; Collet, A.; Viguier, M.; Berret, J.-F.; Séréro, Y. Macromolecules 2003, 36 (2), 449−457. (29) Annable, T.; Buscall, R.; Ettelaie, R.; Whittlestone, D. J. Rheol. 1993, 37 (4), 695−726. (30) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P. Pure Appl. Chem. 2011, 83 (8), 1637. (31) Mewis, J.; Wagner, N. J. Adv. Colloid Interface Sci. 2009, 147− 148, 214−27. (32) Münstedt, H. Colloid Polym. Sci. 1981, 259, 966−972.

after shearing, there is no tendency for chains to come back to watermelon structure enthalpically. However, as is obvious from FTIR data, entropy penalty due to chain stretching is weakening hydrogen bonding, but this is not enough to break the whole 3D built-up structure. Therefore, unlike the most cases, this system shows irreversible rheopexy. In summary, we report the formation of a 3D network by irreversible shear induced gelation. Unlike previous reports of shear induced gelation, the viscosity increase is permanent (within the measurement time of the experiment) despite no chemical changes, and furthermore, the sample does not exhibit shear thickening in the classical sense, i.e., an increase of viscosity with increasing shear rate. Instead, the viscosity increases as a function of time, while going from a Newtonianlike behavior to a shear thinning behavior. The effect can be seen as the opposite to the shear refining effect, observed for some highly entangled polymers.32 This type of behavior to the best of our knowledge has never been reported before, which we call “shear induced irreversible gelation by supramolecular bonding”. This behavior is important for understanding resin transfer molding (RTM) of resin mixtures being toughened by CTBN or similar other ionomers, microstructure, and final properties of composite. The resin transfer takes a lot of time, and the shear between the bundles of fibers will cause high shear rates γ̇ locally, which need to be taken into account when understanding resin transfer molding (RTM) fully. Failure to take these effects into account might increase the viscosity locally so much that the mold is not completely filled.

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AUTHOR INFORMATION

S Supporting Information *

Graphs showing the “thixotropic loop” tests. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail [email protected], Ph +82 63 270 4039 (F.J.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Korean Atomic Energy Research Institute, Daejeon, for the financial support (Studies on the rheological properties of vinyl ester resin/CTBN/styrene solutions), Starting Fund of Chonbuk National University, and “Human Resource Development (Advanced track for Sibased solar cell materials and devices, project number: 201040100660)” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy. V.K. acknowledges the Ministry of Science, Research and Technology of Iran for the financial support. Discussions with Prof. Dr. Helmut Münstedt and Prof. Dr. Manfred Wilhelm contributed to understanding the system.

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REFERENCES

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