Interplay of Phase Separation and Thermoreversible Gelation in

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Interplay of Phase Separation and Thermoreversible Gelation in Aqueous Methylcellulose Solutions S. A. Arvidson,† J. R. Lott,† J. W. McAllister,† J. Zhang,‡ F. S. Bates,*,‡ and T. P. Lodge*,†,‡ †

Department of Chemistry and ‡Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States

R. L. Sammler§ and Y. Li∥ §

Materials Science and Engineering Laboratory and ∥Analytical Sciences, The Dow Chemical Company, Midland, Michigan 48674, United States

Meinolf Brackhagen Products/Characterization R&D, Dow Wolff Cellulosics, Bomlitz, Germany 29699 S Supporting Information *

ABSTRACT: Rheology and turbidity measurements were performed under similar thermal histories to probe the relationship between thermoreversible gelation and phase separation for a set of three methylcellulose (MC) materials with similar degrees of substitution (DS) and contrasting molecular weights after hydration in cold water. Frequency-independent loss tangents were used to identify the gel point (Tgel) in MC solutions well over the chain overlap concentration (c ≥ 10c*). Transmittance of 633 nm laser light through the solutions revealed that all MC solutions cloud upon gelling, with a relative transmittance of 86% closely associated with the gel point. The gelation temperature of MC solutions was found to decrease with increasing MC concentration and the results for all molecular weights superposed. Using gel and cloud points, a phase diagram was constructed which reveals that clear MC solutions transition directly into turbid gels. Frequency-independent storage moduli of fully developed MC gels scaled with φ2.3, consistent with theory and experiment of entangled systems. Gelation of MC has strong dependence on heating rate while the melting of the gel has little dependence on cooling rate, suggesting that thermogelation of MC proceeded by a nucleation and growth mechanism rather than spinodal decomposition.

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INTRODUCTION

relationship between phase separation and gelation are still not clearly understood. In an early investigation of the sol−gel transition in methylcellulose, Heymann attributed gelation to dehydration upon heating.4 Kato et al. concluded that trisubstituted AGUs acted as cross-linking sites of the gel network.5 More recently, Haque and Morris6 described the gelation of MC in terms of a two-step process in which regions of residual cellulose (unsubstituted) dissociate upon heating to allow sections of the polymer chains to separate. With a further increase in temperature, water molecules, which are thought to form a cage-like structure around the methoxy substituents at low temperatures, are disrupted and lead to association of the hydrophobic methyl groups in these sections. Kobayashi et al. also described MC gelation in terms of a two-step process,

Methylcellulose (MC) is a hydrophobically modified cellulose formed by partial substitution of methyl moieties onto the C2, C3, and C6 positions of the anhydroglucose (AGU) repeat unit. The degree of substitution (DS, mol[−OCH3]/mol[AGU]) of these methyl groups is bounded by 0 (completely unsubstituted or native cellulose) and 3 (all three −OH groups per AGU unit converted to −OCH3). Heterogeneous preparation of MC with intermediate DS produces a watersoluble polymer at low temperatures, which reversibly transitions to a turbid hydrogel at elevated temperature.1 Methylcelluloses (MC and hydroxypropyl MC) are categorized by the U.S. Food and Drug Administration as generally recognized as safe2 and have found use in a wide variety of products as thickeners or binders in pharmaceuticals, foods, and cosmetics as well as in building materials, ceramics, and cements.3 However, despite widespread use of MC for nearly a century, the mechanism of thermoreversible gelation and the © 2012 American Chemical Society

Received: September 14, 2012 Revised: November 16, 2012 Published: December 14, 2012 300

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Table 1. Sample Characteristics

a

sample

DS31 (mol[−OCH3]/mol[AGU])

ηa (Pa·s)

Mw (kDa)

Đb

[η] (mL/g)

c* (g/mL)

c*c (wt %)

MC-530 MC-300 MC-150

1.92 1.80 1.86

40 4 0.4

530 300 150

4.1 5.4 3.6

1093 ± 18 736 ± 36 473 ± 3

0.00092 0.0014 0.0021

0.092 0.14 0.21

Solution viscosity (2 wt %) in water at 20 °C. bMw/Mn from size exclusion chromatography. cBased on MC density of 1.39 g/mL.7

the phase boundary. However, the percent transmittance that is defined as the CP has been variously taken to be 50%, 95%, the onset of clouding (140 °C) was floated over the top of the solution. The dynamic shear storage and loss moduli, G′ and G″, were measured as functions of temperature and frequency by two methods. In the first, temperature sweeps were performed at a heating or cooling rate of 1 °C/min in strain-control mode with a strain amplitude, γ, of 5% and a frequency, ω, of 1 rad/s. Such tests are referred to as temperature ramps. In the second, frequency sweeps were performed isothermally from 0.05− 100 rad/s (strain-control mode, γ = 5%); a strain of 5% is well within the linear regime of the solutions studied. After a frequency sweep was completed, the temperature was increased by 1 °C, the solution was allowed to equilibrate for 15 min, and the frequency sweep procedure was repeated. Frequency sweeps lasted ∼15 min, so in this manner of testing, the effective heating rate was ∼2 °C/h in the vicinity of the gel point. Larger step changes in the temperature were made well outside of the gelation window, a strategy that did not significantly affect the measured moduli. This heating protocol is referred to as “stepwise” heating. All gel points are reported on heating of the solution and have been performed a minimum of three times at each reported concentration. The effect of instrument inertia was evaluated by monitoring the raw phase angle; in cases where inertial effects dominated, the data were excluded.32

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RESULTS AND DISCUSSION Rheological Determination of Gel Point. For many polymeric gels, the dynamic mechanical behavior at Tgel is characterized by a scaling relationship between the dynamic shear moduli and the frequency: G′ ∼ G″ ∼ ωn

(2)

with a frequency-independent loss tangent: tan δ(ω) =

⎛ nπ ⎞ G″(ω) = tan⎜ ⎟ ⎝ 2 ⎠ G′(ω)

(3)

where G′ and G″ are the elastic and viscous loss moduli, respectively, and n is the relaxation exponent.16 This scaling allows for facile identification of the gel point with a series of temperature or frequency sweeps. One strength of this method in identifying the gel point is in the interpolation of the time points before and after the critical gel point rather than by extrapolating pre-gel data to infinite viscosity or post-gel data to zero equilibrium modulus. The dynamic shear moduli of MC solutions were measured over the range of 0.1 ≤ c/c* ≤ 30. This range is selected to include solutions that contain insufficient polymer chains to form entanglements (c ≤ c*) as well as those that are expected to experience significant entanglement (the entanglement concentration, ce, is ca. 5−10c*).33 A representative heating ramp (1 °C/min) for MC-300 at 15c* is shown in Figure 1a for ω = 1 rad/s. The moduli decrease slightly with increasing temperature up to a point and then abruptly increase before reaching a near plateau at high temperatures. This sharp increase in moduli is associated with the gelation and clouding processes.17 Figure 1a also includes results obtained by a 302

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Figure 2. (a) tan(δ) as a function of temperature while heating MC-300 at c = 30c* (4.2 wt %) and (b) G′ and G″ at the approximate gel point for this solution. (c) tan(δ) as a function of temperature upon heating MC-300 at c = 8c* (1.12 wt %) and (d) G′ and G″ at the approximate gel point for this solution. Downward pointing arrow in (c) indicates power law region following W−C gelation, and upward point arrow indicates departure from power law and local maximum in tan(δ). tan(δ) as a function of temperature while heating MC-300 at (e) c = 3c* and (f) 0.5c*. Solutions were heated stepwise at an effective rate of ∼2 °C/h in the vicinity of the gel point.

monotonically increase with T. Despite the concentration being below ce, these solutions still exhibit orders of magnitude increases in the moduli upon heating, which suggests that increased interchain association contributes significantly to the rheological response even in the absence of entanglements. Figure 2 shows the result of stepwise heating plotted in the form of tan(δ) versus temperature. Figures 2a and Figure 2b illustrate identification of the W−C gel point at the temperature

slower, stepwise heating protocol for the same polymer and concentration, where frequency sweeps were performed at each temperature after an equilibration period. Reducing the heating rate decreases the temperature at which gelation is observed, which is in agreement with previous studies that indicate Tgel depends on the heating rate.8 Figures 1b and 1c show the effect of concentration on the dynamic moduli. For c/c* < 1, the moduli are roughly independent of concentration7 and 303

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where tan(δ) becomes independent of frequency (Figure 2a), which coincides with G′ and G″ following power law scaling described by eq 2 (Figure 2b). Power law scaling at Tgel is apparent over the full measured range spanning more than 3 decades of frequency and results in a relaxation exponent around 0.55 ± 0.1 at a concentration of 4.2 wt % for MC-300. Relaxation exponents extracted from the moduli at Tgel and plotted versus both the measured and calculated tan(δ) via eq 3 are available in the Supporting Information (Figure S1). Relaxation exponents are discussed in more detail below. Similar W−C gelation was observed at the highest concentration tested (30c*) down to 10c* (the entanglement concentration, ce, is ∼10c*). From about 8−10c* for all three Ms, the ω-independence of tan(δ) is apparent only over 1−2 decades of frequency (for example, Figure 2c, 8c* = 1.12 wt % MC-300). From this range of frequencies, a Tgel can still be clearly identified, though this should not be considered a true W−C gel point because it does not span the entire frequency range. At the lowest frequencies there is a local maximum in tan(δ) near the same temperature as the Tgel identified at higher frequencies, but there is no intersection of the constantfrequency traces in this range of frequency. This local maximum in tan(δ) is present at low frequencies for c* < c ≤ 10c* (Figure 2e), concentrations that are on the verge of molecular entanglement, and is clearly related to the gelation phenomenon, though the conditions of W−C gelation are not strictly satisfied. We attribute the departure from power law behavior at low frequencies to intermolecular (hydrophobic) association. The strength and/or number of interactions increase with temperature (see increase in modulus for c ≤ c* in Figure 1) and contribute to the overall network structure, which accounts for the quasi-W−C gelation at concentrations below ∼10c*. Below c*, tan(δ) does not provide any evidence of gelation (Figure 2f). The presence of W−C gelation only for c ≥ 10c* indicates that the molecular associations that dominate the rheological response at low concentrations in the absence of molecular entanglements are too sparse to function as crosslinking loci. Solutions at 0.5c* and 0.1c* show some evidence of the sol−gel transition at elevated temperatures in terms of the frequency independence of the moduli for the small range of frequencies that do not result in instrument inertial effects (data not shown), but they do not satisfy the “vial inversion” test and are not classified as gels in the ensuing discussion. Figure 3 shows the gel point of MC for a range of concentrations and Ms. Included are concentrations which satisfy W−C gelation for the full range of measured frequencies as well as those which exhibit frequency independent of tan(δ) for a smaller range of frequencies. Tgel decreases with increasing concentration, though it is virtually independent of the molecular weight. Figure 4a shows the relaxation exponent n (defined at the gel point, i.e., for a critical gel) as a function of M and concentration. Also shown in Figure 4a are solid lines indicating a linear best fit of each M. The relaxation exponent varies smoothly with concentration and M. Critical gels that are relatively soft and fragile result in n → 1, while for stiffer gels n → 0, where n is known to depend on factors such as the degree of cross-linking and bulkiness of the cross-links in addition to concentration of polymer or cross-linker.34 By plotting the relaxation exponent versus the product of the concentration and molecular weight, as in Figure 4b, the data collapse reasonably well onto a single curve. Because n depends not just on the concentration of polymer in the gel, but also on the chain length, n reflects the degree of cross-linking per chain, not

Figure 3. Gel point for several Ms. Filled circles indicate solutions that satisfy W−C gelation for the full frequency range. Hollow circles with horizontal bar indicate tan(δ) converges for a portion of the tested frequency range, while hollow circles with a vertical bar indicate gel point determined from local maximum in tan(δ). Gel points are reported upon heating stepwise at an effective rate of ∼2 °C/h in the vicinity of the gel point.

the number of cross-links per volume of gel. A similar dependence of n on M was noted for chemically cross-linked polycaprolactone prepolymers with Mw ranging from 4 to 40 kDa.35 Note that the best fit of the data in Figure 4 results in a y-intercept of approximately n = 1 (calculated value 0.91), which is consistent with a very weak gel network and the number of cross-links per molecule vanishing as the concentration approaches zero. It has been suggested that an entangled gelling system provides a concentration-dependent n, such as seen here for MC, whereas in a nonentangled incipient gel such as the polymerization of low-M prepolymers or condensation of small molecules, n will not depend on the concentration of the gelling species.36,37 Cloud Point. Figure 5 shows characteristic relative transmittance curves for MC-300 over a range of concentrations. Upon heating, there is a monotonic decrease in the transmittance. At low concentrations, the solutions are clear (that is, exhibit 100% transmittance compared to that of water) at 1 °C, while more concentrated solutions exhibit reduced transmittance over the full measured range of temperatures compared to water. All solutions exhibit a plateau in transmittance at low temperatures and decrease sharply in a similar manner, regardless of concentration. Similar results are found for MC-530 and MC-150. The gelation and LCST phenomena in MC are known to be related, but the exact relation is controversial because the locations of the cloud (Tcloud) and gel points in previous studies do not necessarily coincide. For example, Fairclough et al. found that the temperature at the onset of clouding (obtained by extrapolation) was close to the onset of the G′ increase11 while Sarkar found no correlation between incipient cloud and gelation temperatures.3 We, too, find that the onset of clouding is not associated with a distinct rheological response, such as the onset of G′ increase or the W−C Tgel, over the full range of concentrations. This is not surprising based on the sensitivities and length scales associated with rheology versus cloud point; rheology probes molecular interactions via macroscopic solution behavior while cloud point probes existence of structures with sizes that are on the order of the wavelength of light. Using a 500 nm laser source and a 50% transmittance 304

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Figure 4. (a) Relaxation exponent n versus concentration for several Ms. Solid lines indicate linear best fit. (b) n versus product of concentration and M (kDa) with linear fit.

accompanied by the presence of a volume-spanning network. These gel and cloud point data are consistent with a gelation scenario in which phase separation and gelation occur together; the association of different chains arrests macroscopic liquid− liquid phase separation. While we would not necessarily expect this same relationship (i.e., Tgel equals the temperature at which relative transmission equals 86%) to hold for all other polymers that gel via phase separation, the fact that such a close correspondence exists argues strongly against the view that phase separation and gelation are distinct events in MC solutions. Phase Diagram. Figure 7 shows the phase diagram constructed from optical cloud points and rheological gel points. The most important result is that, at concentrations high enough to gel, MC transforms directly from a clear solution to a turbid gel over the entire accessible volume fraction (φ) range for all three Ms. (We use volume fraction as the composition variable here to facilitate comparison with prior literature.) Previous reports variously indicate sol−gel transitions below the cloud point for some or all portions of the phase diagram,14,17,21,22,38 the cloud point below the gel point for the entire evaluated phase space,28 and most recently the cloud point and gel point at approximately the same temperature for a single evaluated concentration.11 Some of these identifications emerge from performing the gel and clouding experiments at different heating rates or from interpretation of the low-ω region of ω-sweep curves (such as in Figure 2d) as indicative of the presence of a distinct gel phase.17,21 Two representative phase diagrams are reproduced schematically in Figure 8a,b and are rescaled to match Figure 7. (Note that the “fuzzy” bands in these figures are intended to reflect the scatter in the original data.) Figure 8a, based upon the results of Takahashi et al.,22 displays phase boundaries for a MC with Mw = 936 kDa and DS = 1.78. Gel and phase separation transitions were obtained by DSC, whereby the onset and end of an endothermic event in the heat flow traces (upon heating) were assigned to represent the gel point and phase separation, respectively. Small-angle X-ray scattering (SAXS) showed that the appearance of a characteristic dimension ∼23 Å that increased with T, and a subsequent change in the slope of this characteristic dimension with respect to temperature, correspond to the thermal events measured by DSC. Consequently, assignment of the gel phase in Figure 8a

Figure 5. Percent relative transmittance of MC solutions while heating (MC-300). Concentrations are listed as wt % and as a ratio of c/c*. Cloud points are reported upon heating stepwise at an effective rate of ∼2 °C/h in the vicinity of the gel point.

cutoff for the cloud point, Zheng et al. found that the appearance of finite-sized aggregates and an infinite network occurred at similar temperatures.28 To evaluate whether a correspondence exists between Tcloud and Tgel and whether it is independent of M, a suitable percent transmission cutoff must be selected. Initially, solutions which exhibited high transmittance (approximately that of water) at 1 °C and a W−C Tgel (such as that shown in Figure 3A) are considered. The average percent transmittance of these solutions at the rheological Tgel corresponds to ∼86% and is independent of M. (A plot of these data can be found in the Supporting Information.) Figure 6 compares the gel point of MC solutions with the cloud point defined as 86% transmittance as well as two more commonly employed cutoffs, 95% and 50%. For the whole range of Ms, the 86% transmittance cloud point corresponds very closely to the rheological gelation temperature, while 95% and 50% tend to under- and overestimate, respectively, the gel point. Included in Figure 6 are all gel points from Figure 3 (both W−C gels and those obtained by the local maximum in tan(δ)). Clearly, the local maximum in tan(δ) is related to the cloud point in the same manner as a W−C gel point. From Figure 6, with thoughtful selection of the cloud point cutoff, it is also apparent that the formation of nanoscale structure is 305

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Figure 6. Comparison of gelation temperatures (solid circles) to cloud point (open triangles) for three definitions of the relative transmittance (TR) at the cloud point: 95, 86, and 50%. Gel points and cloud points are both reported upon heating stepwise at an effective rate of ∼2 °C/h in the vicinity of the gel point.

using conventional oscillatory shear approaches, where it was not evident; rather, the authors assumed that the sol−gel transition could be given by the onset of non-Newtonian flow behavior. However, despite seemingly large differences in the phase diagram we present in Figure 7 and those previously published, there is an overarching similarity for an equivalent range of φ, in the dependence of gelation and phase separation on MC concentration. A number of experimental and theoretical studies14,17,38 indicate the presence of a “clear gel” phase that is absent in Figure 7, although these and other more subtle differences can be attributed more to differences in analytical methods than to truly different gelling or phase separation behavior of these altogether similar MC systems. Mechanistic Aspects of Physical Gelation of MC. The simplest mode of physical gelation might be termed vulcanization, by analogy with chemical cross-linking of preformed long chains. In this scenario, an entangled solution of individual polymers is converted to an elastic solid by introduction of physical cross-linking points. The resulting modulus reflects the density of cross-links for weakly entangled precursor chains and by the entanglement density for higher M chains. In the latter case the frequency-independent storage modulus will be independent of M but vary with concentration as

Figure 7. Phase diagram of aqueous MC. Cloud point is defined here as 86% transmittance. NC, marked with ▲, indicates a solution that was nonclouding (did not fall below TR = 86%) by 80 °C. Solid line indicates sol−gel transition determined by W−C gelation and cloud point, while the dashed line indicates sol−gel transition determined by local maximum in tan(δ) and cloud point. Lines are to guide the eye.

was not based on rheological measurements at all. Takahashi et al. also noted that molecular weights of 124, 357, and 783 kDa result in a shift of the sol−gel and phase separation boundaries to higher temperature and concentration with decreasing M.22 In contrast, we find that for 150 kDa ≤ Mw ≤ 530 kDa the sol− gel phase transition does not depend on M and that there is no distinction between the sol−gel transition and phase separation. Figure 8b is based on the results of Chevillard et al.21 for a MC with Mw = 140 kDa and DS = 1.8. The transition from clear to turbid was determined visually, while the sol−gel transition was determined using the rheological G″(ω)−G′(ω) crossover temperature. The upturn in the turbidity curve at high φ is the result of a single data point, at a concentration that we found to be well above the practical solubility limit of MC using the preparation procedure described herein for a similar polymer. The “clear gel” phase in the low φ region was not obtained

G′ ∝ φ 2.3

(4)

This scaling of G′ with an exponent of ca. 2.3 results for polymers in both theta and good solvents, although values near 2.0 have also been reported.32,39−42 As concentration increases, G′ approaches the plateau modulus of the entangled melt, GN°. Figure 9 displays the modulus (taken at 1 rad/s) as a function of φ at 80 °C, well into the gel state. Consistent with the above picture, the moduli scale with φ2.27 and do not depend on M. The plateau modulus can also be used to estimate the average mesh size of the gel, ξ, by

G≈ 306

kT ξ3

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Figure 8. Schematically reproduced MC phase diagrams from (a) Takahashi et al.22 and (b) Chevillard et al.21 with the sol−gel transition in gray and the phase separation (or clear−turbid transition) in black. Lines represent a qualitative smearing of the data points to reflect the scatter in the original figures.

insufficient to capture several important features that we observe, including the high modulus of MC gels and clouding, and does not account for the semiflexible nature of the MC chains. Viscoelastic phase separation (VPS) is a term that has been used to describe transient gelation in polymer blends and solutions; VPS occurs when the mobility of one phase is locked in (due to crystallinity, glass transition temperature, or other condition) while the other phase is mobile. The resulting stress imbalances are relieved by phase separation of the components.43 MC gelation can be evaluated in terms of the critical features of VPS:12 phase inversion of a polymer in solution, where the polymer-lean phase becomes the matrix, is a unique feature of VPS. With time (on the order of minutes), the polymer-rich domains are stretched and broken by the large elastic deformations leaving “holes” of solvent.44 This is clearly absent in optical micrographs of gelled MC11 and has not been observed in the course of this work. Instead, syneresis, which is a well-known phenomenon in MC gelation, occurs when gels are stored for long periods of time (hours to days) well above their gel points. With syneresis, the polymer-rich phase excludes water slowly at increasing temperature but remains intact; in VPS, the polymer-rich phase does not necessarily remain connected. Further, in a viscoelastically phase-separating solution, the polymer-rich phase volume shrinks as the overall solution transitions from a bicontinuous phase-separated state (where both polymer-rich and polymer-lean phases span the sample) to a state where only the polymer-lean phase spans the sample. Therefore, the gel-like behavior of the cocontinuous phase is transient, and liquid-like rheological behavior is recovered. As can be seen from Figure 1a, MC shows no drop in modulus (and maintains a distinct gel-like rheological response where G′ and G″ are ω-independent, data not shown) well above the gelation temperature for samples that have remained heated for durations on the order of 12 h, with no indication of macroscopic phase separation and a return to liquid-like behavior. The close correlation between the rheological gel point and the optical cloud point, over a range of M and concentration, supports a picture of gelation due to arrested liquid−liquid phase separation (though not liquid−liquid phase separation as described by VPS). The network structure is thus formed by

Figure 9. Modulus of MC versus volume fraction. Solid line corresponds to combined best fit of three MC Ms. Dashed line represents extrapolation of PE melt modulus in the (φ = 1).

where k is the Boltzmann constant. Moduli ranging from 101 to 104 Pa imply values of ξ on the order of 8−80 nm. Despite this agreement with the scaling properties of swollen flexible polymer networks, the vulcanization picture cannot fully account for the behavior of MC gels. First, the appearance of turbidity concomitant with gelation requires development of heterogeneity on a much larger length scale than the inferred values of ξ. Thus, it is hard to picture the resulting gel modulus as being determined solely by the entanglement network that existed prior to the cloud point. While the entanglement network clearly is responsible for inhibiting macroscopic phase separation, it cannot confine it below the micrometer scale. Second, if we extrapolate the modulus to the undiluted limit in Figure 9, we obtain a theoretical modulus of ∼40 MPa, which is remarkably high; for example, polyethylene (PE), poly(ethylene oxide), polybutadiene, or other flexible, linear polymer melts exhibit entanglement moduli ∼2 MPa or less.33 The behavior of PE, one of the most highly entangled polymers, is also shown for comparison in Figure 9. Interpreted literally, the extrapolation for MC implies a melt entanglement molecular weight on the order of one repeat unit, which seems unlikely. Thus, despite the apparent consistency of MC gelation with a scenario involving vulcanization of an entangled solution, it is 307

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SUMMARY The thermoreversible gelation of commercial methylcelluloses with similar DS was investigated in detail for a range of Ms (150−530 kDa) and concentrations (0.1−30c*) by rheology and optical cloud points. Solutions with concentrations above the entanglement threshold (c ≥ ce ≈ 10c*) exhibit a welldefined gel point following the Winter−Chambon criteria. Solutions with concentrations between the entanglement concentration and the overlap concentration (ce ≥ c ≥ c*) gel upon heating but do not follow Winter−Chambon gelation. Nevertheless, they exhibit a distinct rheological feature associated with gelling (local maximum in tan(δ)) which indicates that, even in the absence of a classic W−C gelation, tan(δ) may still provide the sol−gel transition temperature in gelling materials. Most importantly, rheological and cloud point measurements performed at comparable heating rates indicate that the cloud and Winter−Chambon gel points occur at consistent temperatures for a range of Ms and are independent of M. Thus, the resulting phase diagram (T vs φ) exhibits only two regimes: clear solution at lower T and cloudy gel at higher T. This result differs from several previous reports, and possible sources of the discrepancies are discussed. Measurements with varied heating rates show that the gelation temperature is very sensitive to heating rate, providing strong evidence in favor of a nucleation and growth mechanisms of phase-separation induced gelation.

the polymer-rich phase, trapping a solvent comprising the polymer-lean phase. The detailed structure of this network remains to be elucidated; experimental studies utilizing a combination of small-angle neutron scattering and cryogenic electron microscopy techniques are currently in progress. However, we can consider the question of whether the phase separation process proceeds via spinodal decomposition or nucleation and growth. In principle, the early stage of spinodal decomposition should be signaled by a maximum in SANS scattering I(q) at a finite q.45 As some of us have shown for MC previously,7 with slow heating, there is no maximum in I(q) at any finite q for the measured range of 0.003 < q < 0.1 Å−1. Furthermore, preliminary SANS measurements on the samples described in this paper also show no hint of a scattering peak (data not shown). However, this evidence is not sufficient to rule out a spinodal contribution, as a putative peak could be at much lower q and shorter times than accessed by SANS. Stronger evidence for a nucleation and growth process is provided by the heating-rate dependence of the gelation process. Figure 10 shows G′ at 1 rad/s vs T for which a 2.1 wt

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.P.L.); [email protected] (F.S.B.). Notes

The authors declare no competing financial interest.

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Figure 10. G′ of 2.1 wt % MC-300 at several continuous (not stepwise) heating rates measured at 5% strain and 1 rad/s. The crossover of G′ and G″ at each heating rate is noted (★ on heating, ☆ on cooling) and serves to approximate the Tgel for each condition.

ACKNOWLEDGMENTS This work was supported by The Dow Chemical Company and Dow Wolff Cellulosics. We acknowledge Dr. David Giles for assistance with the rheological measurements.

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% solution of MC-300 was heated and subsequently cooled at several rates. For gelation by spinodal decomposition, Tgel should be independent of heating rate. In contrast, Tgel varies by over 23 °C as the heating rate is increased from 0.2 to 9 °C/ min (Figure 10). Then, upon cooling the melting of the gel is virtually independent of cooling rate. This is strong evidence for facile “superheating” of the solution state, typical of a firstorder phase transition that proceeds by nucleation and growth. While not explored further here, it is important to note that the hysteresis in modulus shown in Figure 10 has also been observed in the optical transmittance of aqueous MC.6,11 Thus, on the basis of the evidence so far, we conclude that gelation proceeds by a nucleation and growth mechanism over the range of solution conditions we have employed. Note that phase separation by spinodal decomposition mechanism has been observed, however, for MC solutions subjected to deep temperature quenches23,45 well above the cloud point/gel point curve depicted in Figure 7.

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dx.doi.org/10.1021/ma3019359 | Macromolecules 2013, 46, 300−309