Effect of Hydrophilic and Hydrophobic Interactions on the Rheological

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Effect of Hydrophilic and Hydrophobic Interactions on the Rheological Behavior and Microstructure of a Ternary Cellulose Acetate System John F. Kadla* and Reza Korehei Advanced Biomaterials Chemistry, Faculty of Forestry, University of British Columbia, Vancouver, BC V6T 1Z4 Canada Received January 11, 2010; Revised Manuscript Received March 1, 2010

The effect of hydrophilic and hydrophobic interactions on the rheological and microstructural behavior of cellulose acetate (CA) in a ternary CA, N,N-dimethylacetamide (DMA), nonsolvent (alcohol) system was examined. Increasing nonsolvent concentration increased the viscosity and dynamic viscoelastic properties of the system. At a critical nonsolvent concentration, a sol-gel transition was observed, which was dependent on nonsolvent structure. Increasing the available hydrogen bonding groups within the nonsolvent led to higher modulus (stronger gels) and a sol-gel transition at lower nonsolvent concentration. Likewise, increasing the alkyl chain length (hydrophobicity) of the nonsolvent also enhanced the viscoelastic properties; however, hydrogen bonding, specifically the ability to hydrogen bond donate was critical for gel formation. For all gels studied, the elastic modulus shifts to higher values with increasing hydrophilicity and hydrophobicity of the nonsolvent and exhibits a power-law behavior with nonsolvent content. All of the gels exhibit similar fractal dimensions; however, confocal images of the different systems reveal distinct differences. Increasing the hydrophilicity of the nonsolvent led to a more uniform denser gel microstructure, whereas increasing the hydrophobicity resulted in a larger more heterogeneous network structure despite the increase in moduli.

Introduction Recent advances in biotechnology are fuelling the development of separation and purification techniques for biological molecules. The sensitive nature of biological compounds to thermal and chemical environments limits conventional separation techniques. Polymeric membranes are an effective technology for biomolecule separation, particularly for large-scale separations owing to their simple and easy operation.1 As a result, many polymers have been investigated as separation membranes.2,3 Of these, cellulose acetate (CA) has been widely utilized as a separation and filtration medium. CA is biocompatible and has low protein adsorptivity.4 The utilization of CA in filtration and separation applications requires complex network structures with controlled and variable pore sizes. Such structures can be produced through phaseseparation-induced gelation.5 Depending on the polymer and the combination of solvent/nonsolvent, phase separation can occur with the formation of polymeric asymmetric porous membranes.6,7 Under specific conditions, phase separation is accompanied by a sol-gel transition with network formation dependent on the properties of the ternary system (polymer/ solvent/nonsolvent). Specifically, the viscoelastic behavior of the gel/solid network is highly dependent on the chemical/ physical structure of the polymer and the functional groups available on the solvent/nonsolvent; polymer-polymer, polymer-solvent, and solvent-nonsolvent interactions all play a role in network formation. For various polymeric systems, dipole moments and donor-acceptor properties of each component as well as the strength of hydrogen bonding have been used to explain the process of gel formation.5,8,9 * Corresponding author. Tel: 604 827-5254. Fax: 604-822-9104. E-mail: [email protected].

Recently, we reported that the addition of water to a CA/ dimethylacetamide (DMA) solution led to phase-separationinduced gelation and the formation of 3D network structures.5 In this system, specific interactions, which arise upon heating the CA solution,10 further intensify on the addition of water. Macromolecular dynamical fluctuations and competition between hydrogen-bonding and hydrophobic interactions dramatically change the conformation of the CA molecular chains. In the present article, we further investigate the role of hydrogen bonding and hydrophobic interactions on the sol-gel process of a CA ternary system. Using steady shear and dynamic rheology, infrared (FTIR) spectroscopy, and confocal microscopy (CLSM) we assess the impact of a series of alcohols with (i) increasing number of hydroxyl groups (e.g., mono-, di-, and trihydric alcohols) and (ii) increasing alkyl chain length (i.e., 1-propanol, 1-hexanol, 1-octanol, and 1-decanol) on the resulting viscoelastic properties and gel microstructure.

Experimental Section Materials. CA (Mn ca. 30 000 g/mol), degree of acetylation (DA) ) 2.45 (39.7 wt % acetyl content), HPLC grade DMA, 1-propanol, 2-propanol, 1-hexanol, 1-octanol, 1-decanol, 1,2-propanediol, 1,3propanediol, and glycerol were purchased from Sigma Aldrich and used as received. Sample Preparation. All samples were prepared from a bulk 28 wt % CA in DMA solution. To ensure a homogeneous solution and facilitate adequate dissolution of the CA in the DMA, the mixture was heated to 100 °C for 10 min under a blanket of nitrogen and allowed to cool to room temperature prior to use. The specific CA/DMA/ nonsolvent concentrations used in this study were obtained by adding appropriate amounts of nonsolvent and DMA to the 28 wt % CA/DMA stock solution. The samples were mechanically mixed, blanketed with nitrogen, and heated to 100 °C for 10 min to ensure complete miscibility

10.1021/bm100034t  2010 American Chemical Society Published on Web 03/17/2010

Effect of Hydrophilic and Hydrophobic Interactions and left at ∼25 °C for exactly 1 week in a desiccator prior to analysis. All solutions and dilutions were prepared on a weight basis. Rheological Characterization. Steady-state and dynamic rheological experiments were conducted with a TA Instruments AR 2000 rheometer using either a cone and plate (60 mm diameter 2° cone angle) or parallel plate (20 mm diameter) geometry.5 For gel samples, a variable gap (1000-2000 µm) was used on the basis of an applied force, which was kept below 2 kPa. All experiments were performed at 25 °C. In the dynamic experiments, the samples were subjected to a sinusoidal deformation as both a function of increasing strain amplitude or frequency of oscillation, and the corresponding elastic (G′) and viscous (G′′) moduli were measured. Dynamic stress sweep experiments were performed to determine the linear viscoelastic (LVE) regime prior to the frequency sweep experiments. Fourier Transform Infrared (FTIR) Spectroscopy. FTIR spectra were measured on a Perkin-Elmer 16PG FTIR spectrometer. A total of 16 scans were collected at resolution of 4 cm-1 over the range of 400 to 4000 cm-1. FTIR spectra were collected using 10 mg samples positioned between ZnSe salt plates. Care was taken not to disrupt sample microstructure during the transfer and positioning between plates. Confocal Laser Scanning Microscopy (CLSM). A confocal laser scanning system (Chameleon, compact ultra fast Ti) connected to an inverted microscope (Zeiss Axiovert) was used to analyze the gels. Calcofluor white (0.01 wt %), a fluorescent dye, was used to tag the CA for CLSM.5 All samples were prepared as per the rheology experiments. A few drops of the hot bubble-free solution was placed onto a single concavity microscopy slide, covered with a coverslip of 0.5 mm thickness and then conditioned for 1 week in a desiccator at ambient temperature. Optical sectioning was performed as a function of the depth along the z axis. The IR laser excitation source was set to 705 nm with two channel spectra detection (Ch1 filter: 544-704 nm, Ch2 filter: 435-485 nm). Scans were collected as stack images with a stack size of X: 142.86 µm, Y: 142.86 µm, and Z: 27.87 µm. Data Analysis. All rheological analyses were within an error of glycerol. Figure 2 illustrates the progressive increase in shear viscosity (data obtained at a shear rate of 1 s-1) with increasing nonsolvent content for the various nonsolvents. Increasing the number of hydroxyl groups (OH) on the nonsolvent from 1 to 3 (1propanol, 1,3-propanediol, to glycerol) led to an increase in viscosity (Figure 2a). At a relatively high nonsolvent concentration (30 wt %), the samples, which are still clear solutions, vary in viscosity by almost an order of magnitude: 1-propanol ≈ 2 Pa · s, 1,3-propanediol ≈ 9 Pa · s, and glycerol ≈ 36 Pa · s. In all of the systems, a change in the slope of the viscosity versus nonsolvent content plot was observed at high nonsolvent contents. Moreover, the change in slope was greatest for the glycerol such that glycerol > 1,3-propanediol > 1-propanol. This may represent microstructure development as a result of increased intermolecular interactions between components. The same phenomenon was observed when the results were normalized to equivalent hydroxyl groups (data not shown), indicating that the change in viscosity and intermolecular interactions were not solely the result of more hydrogen bonding groups.

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Table 1. Concentration of Nonsolvent at Which Non-Newtonian Behavior Is Observed nonsolvent

critical concn (wt %) viscosity (Pa · s)* critical concna 20 wt %b

1-propanol

2-propanol

1,2-propanediol

48.3

46.6

43.3

1,3-propanediol 35

glycerol 30

4.4 1.4

4.3

14.3 2.4

21.6 3.0

24.2 5.3

1-hexanol 40 3.8 1.1

1-octanol

1-decanol

34.6

31.6

8.9 1.4

32.3 1.9

a Viscosity at highest concentration exhibiting Newtonian behavior. b Viscosity at 20 wt % nonsolvent content. * Viscosity values are within an error of 10%.

Figure 2. Effect of (a) hydrophilic and (b) hydrophobic interactions on the viscosity enhancement of the CA/DMA/nonsolvent solutions (values obtained at a shear rate of 1 s-1). 1-Propanol (1-Pro); 1,3propanediol (1,3-PD); glycerol (Gly); 1-hexanol (1-Hex); 1-octanol (1Oct); 1-decanol (1-Dec).

A similar behavior was observed for the monohydric alcohols (Figure 2b). At low nonsolvent contents, increasing the alkyl length or hydrophobicity of the nonsolvent also increased the solution viscosity, albeit not as dramatically as the hydrophilic nonsolvents (Figure 2a). At 30 wt % nonsolvent concentration, the viscosities are ∼1.5, ∼2, and ∼14 Pa · s for 1-hexanol, 1-octanol, and 1-decanol, respectively. Therefore, it appears that in dilute solutions hydrophilic interactions such as hydrogen bonding have a greater effect on viscosity than hydrophobic interactions. The presence of more hydroxyl groups on the nonsolvent enables increased hydrogen bonding between constituents, which leads to an increase in viscosity. However, at higher nonsolvent contents, viscosity enhancement occurred very fast in the hydrophobic nonsolvent system. From Figure 2, it is clear that the change in viscosity (slope) with increasing nonsolvent content was greater for the longer alkyl chain monohydric alcohols as compared with glycerol, 1,3-propanediol, or 1-propanol at high nonsolvent contents (>∼30 wt

Figure 3. FTIR spectra of the hydroxyl stretching region of (a) top spectra: CA/DMA/1-propanol (1-Pro), CA/DMA/1,3-propanediol (1,3PD), and CA/DMA/glycerol (Gly) solutions (33 wt % nonsolvent content and 15 wt % CA concentration) and bottom spectra: DMA/1propanol (1-Pro), DMA/1,3-propanediol (1,3-PD), and DMA/glycerol (Gly) solutions (33 wt % nonsolvent content) and of (b) CA/DMA/1propanol (1-Pro), CA/DMA/1-hexanol, CA/DMA/1-octanol, and CA/ DMA/1-decanol (1-Dec) solutions (33 wt % nonsolvent content and 15 wt % CA concentration).

%). This is probably due to hydrophobic interactions leading to phase separation and eventually solvent phase partitioning of the system (discussed below). Intermolecular interactions (physical entanglement and hydrogen bonding) are important parameters for viscosity enhancement in CA/DMA solutions.5 The addition of a hydrogen bond accepting/donating nonsolvent can lead to the formation of new hydrogen bonds between the nonsolvent and DMA as well as the nonsolvent and CA and potentially compete against those between CA and DMA. Using FTIR spectroscopy, distinct differences in the hydrogen bonding patterns were observed between the various nonsolvent systems. Figure 3a,b shows the FTIR spectra of the hydroxyl stretching region of the (a) hydrophilic (1-propanol, 1,3-propanediol, and glycerol) and (b) hydrophobic (1-propanol, 1-hexanol, 1-octanol, and 1-decanol)

Effect of Hydrophilic and Hydrophobic Interactions

CA/DMA/nonsolvent ternary systems at the same nonsolvent contents. The FTIR spectra of the 1-propanol, 1,3-propanediol, and glycerol (Figure 3a) shows a dramatic difference between the nonsolvents. Increasing the number of the hydroxyl groups on the nonsolvent led not only to more hydrogen bonding but also to the formation of stronger intermolecular hydrogen bonds between components. The FTIR hydroxyl stretching band of the CA/DMA/glycerol ternary system indicates a more complex hydrogen bonding system (increased size of the band envelope) with stronger hydrogen bonds (hydroxyl stretching band shifts to lower wavenumber) as compared with that involving 1,3propanediol, which is more than that involving 1-propanol (Figure 3a: top spectra). In addition, the change in the hydroxyl stretching band envelope between the DMA/nonsolvent (Figure 3a: bottom spectra) and CA/DMA/nonsolvent (Figure 3a: top spectra) is greatest for glycerol. This is consistent with the observed change in viscosity (Figure 2a) and suggests that the hydrogen bonds formed between CA and glycerol are stronger than those between CA and either 1,3-propanediol or 1-propanol. Interestingly, the difference between the spectra for DMA/1,3propanediol and CA/DMA/1,3-propanediol and that of DMA/ 1-propanol and CA/DMA/1-propanol is quite similar, and the change in wavenumber was quite small. In contrast, the FTIR spectra of the hydrophobic (1-propanol, 1-hexanol, 1-octanol and 1-decanol) CA/DMA/nonsolvent ternary systems showed very little change in the hydroxyl stretching band envelopes (Figure 3b). Increasing the alkyl chain length of the nonsolvent monohydric alcohols led to a slight shift of the hydroxyl stretching band envelope to higher wavenumber, indicating slightly weaker hydrogen bonding interactions. However, this change in band profile was very slight. Therefore, the change in viscosity (Figure 2b) observed in changing the nonsolvent structure from 1-hexanol to 1-octanol to 1-decanol implies that hydrophobic, nonbonding interactions also played a role in the development of microstructure and viscoelastic properties. Further information regarding hydrophilic and hydrophobic interactions can be seen from the stress sweep data for the respective ternary systems. Figure 4a shows the stress sweep spectra of the hydrophilic (1-propanol, 1,3-propanediol, glycerol) ternary systems at the same nonsolvent content (33 wt %) and CA concentration (15 wt %). At 33 wt % nonsolvent, the CA/ DMA/1-propanol system (clear solution) displays the lowest elastic modulus of the three nonsolvents. As expected, the CA/ DMA/1,3-propanediol system exhibits a substantially higher modulus, and the solutions appeared cloudy/turbid, whereas the CA/DMA/glycerol system appears as a self-supporting network, with the highest modulus of the three, G′ ≈ 80 000 Pa. Moreover, in the CA/DMA/glycerol system, the elastic modulus (G′) is greater than the viscous moduli (G′′) indicative of a gel network. The other two systems exhibit viscous solution behavior with G′′ > G′. Similar results were found for the hydrophobic (1-propanol, 1-hexanol, 1-octanol, 1-decanol) ternary systems at the same nonsolvent content (33 wt %) and CA concentration (15 wt %). Increasing the alkyl chain length of the nonsolvent from 1-hexanol to 1-octanol and finally to 1-decanol enhanced the elastic modulus by almost four orders of magnitude (Figure 4b). For all of the ternary systems, physical gels are observed at high nonsolvent contents, wherein G′ > G′′ (Figure S1 of the Supporting Information). Figure 5 illustrates the effect of nonsolvent on G′ for the various CA/DMA/nonsolvent systems. The elastic modulus, G′, increases by over four orders of magnitude as the nonsolvent content increases from 0 to


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Figure 4. Stress sweep spectra for ternary systems consisting of (a) 1-propanol (1-Pro), 1,3-propanediol (1,3-PD), and glycerol (Gly) and (b) 1-hexanol (1-Hex), 1-octanol (1-Oct), and 1-decanol (1-Dec) at 33 wt % nonsolvent content and 15 wt % CA concentration.

∼30-50 wt %. As previously observed,5 a sigmoidal shape with three distinct phases is seen in terms of G′ enhancement with increasing nonsolvent content. The elastic modulus initially increases slowly with increasing nonsolvent content (homogeneous solution); then, an intermediate phase of sharp increase in G′ and gel formation occurs, followed once again by a slow increase in G′. Although all of the ternary systems exhibit the same sigmoidal shape, the phase of sharp increase in G′ happened faster or at a lower nonsolvent content with increasing number of OH groups in the nonsolvent structure (hydrophilic system) or increasing length of alkyl chain (hydrophobic system). As discussed above, increasing the number of OH groups in the nonsolvent leads to faster phase separation because of hydrophilic intermolecular interactions; that is, hydrogen bonding is stronger and more pronounced. In the case of the hydrophobic nonsolvents, increasing the alkyl chain length or hydrophobicity of the nonsolvent further intensified nonbonding interactions, affecting solvent properties and resulting in phase separation. However, unlike the hydrophilic system, at higher nonsolvent contents the hydrophobic system gels underwent solution partitioning, wherein a very strong gel network formed under a clear liquid solution (top picture in Figure 5b). This occurred in all of the hydrophobic nonsolvents but was most pronounced for the longer chain alcohols. Phase separation is a phenomenon that often occurs when different materials are mixed and results in the formation of regions of materials rich in one of the components dispersed in a region that is rich in the other component. There are two mechanisms of phase separation: (i) nucleation and growth and

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Figure 6. Digital images showing the effect of (a) hexane, (b) dibutyl ether, and (c) 1-octanol on the phase behavior of the CA/DMA/ nonsolvent ternary systems at 15 wt % CA concentration. (wt % concentrations of nonsolvent are shown below each vial).

Figure 5. Effect of increasing nonsolvent content on the elastic modulus (G′ at 1 rad s-1) for the (a) hydrophilic and (b) hydrophobic ternary systems (15 wt % CA concentration). 1-Propanol (1-Pro); 1,3propanediol (1,3-PD); glycerol (Gly); 1-hexanol (1-Hex); 1-octanol (1Oct); and 1-decanol (1-Dec).

(ii) spinoidal decomposition, which lead to different initial morphologies. Nucleation and growth gives a random polydisperse distribution of droplets with sharp interfaces, whereas spinoidal decomposition leads to either droplets or a bicontinuous morphology.13 In the later stages of phase separation, coarsening occurs via Ostwald ripening and liquid droplet coalescence, eventually leading to bulk-phase separation.13,14 To investigate the role of intermolecular interactions on phase separation and gel formation further, we used a series of nonsolvents with varying functional groups: hexane, dibutyl ether, and 1-octanol. Hexane can only participate in nonbonding interactions, whereas dibutyl ether and 1-octanol can also form hydrogen bonds; dibutyl ether as a hydrogen bond acceptor and 1-octanol as a hydrogen bond donor and acceptor. Figure 6 shows the solution behavior of CA/DMA with increasing amounts of hexane, dibutyl ether, and 1-octanol, respectively. Figure 6a shows that the CA/DMA/hexane ternary systems phase separate with increasing hexane content into two clear liquid layers but did not exhibit gel formation. Likewise, the CA/DMA/dibutyl ether system also underwent phase separation but again did not lead to gel formation (Figure 6b). Only the 1-octanol system (Figure 6c) formed a self-supporting gel network at high nonsolvent contents and ultimately a “twophase” liquid layer on top of a stiff gel system. These results support that intermolecular interactions based on hydrogen-bond donor-acceptor groups appear critical for gel formation. In the case of phase-separation-induced gelation, phase separation is commonly triggered via solvent evaporation, changes in temperature, or the addition of a nonsolvent.15 In the development of microstructure, it is important to control

phase separation to impart stability and alter resulting material texture and properties. The morphology of the final microstructure is controlled by the interplay between the kinetics of phase separation and the kinetics of gel formation.13,16 When these two time scales are comparable, the phase-separation process will become trapped in some intermediate state upon the onset of gelation, which preserves the phase-separated morphology.13,17 Depending on the system and the phase-separation process, there are hydrodynamic flows that allow for fast coarsening as well as the diffusion of individual chains. Upon gel formation, the former are completely suppressed, and the interconnected morphology is retained because the materials cannot change over to droplet morphology. The result is that in the presence of gelation, phase separation leads to a structure with a wide range of length scales. Figures 7 and 8 show the CLSM micrographs obtained for the gels formed using the various nonsolvent systems; hydrophilic nonsolvent systems (1-propanol, 1,3-propanediol, glycerol) and hydrophobic nonsolvent systems (1-hexanol, 1-octanol, 1-decanol) at the same G′ (∼60 kPa) and same CA concentration (15 wt %). The dark blue (top) images represent fluorescence mode images of the samples stained with calcofluor white (a cellulose selective dye). In the fluorescence images, the CA microstructure is shown by the bright-color segments. By contrast, in the reflection images (bottom images, seen in red), the dark areas correspond to the CA domains.11,18,19 There is clearly a difference in microstructure between the various gel networks. Increasing the number of hydroxyl groups on the nonsolvent structure appears to lead to a more uniform gel microstructure; a denser network with a more fine and uniform structure is observed as the nonsolvent is changed from 1-propanol to 1,3-propanediol to glycerol (Figure 7). This increased network uniformity is consistent with the stronger hydrophilic interactions (Figure 3a) and enhanced viscoelastic behavior (Figures 2 and 4) with the changing nonsolvent. However, comparison of these three


Figure 7. CLSM images of gels at the same elastic modulus (G′ ) 60 kPa) and at the same CA concentrations (15 wt %) for the (a) 1-propanol, (b) 1,3-propanediol, and (c) glycerol ternary systems. The top images are in fluorescence mode (bright areas represent CA domains), and the bottom images are in reflective mode (dark areas represent CA domains).

Figure 8. CLSM images of gels at the same elastic modulus (G′ ) 60 kPa) and at the same CA concentrations (15 wt %) for the (a) 1-hexanol, (b) 1-octanol, and (c) 1-decanol ternary systems. The top images are in fluorescence mode (bright areas represent CA domains), and the bottom images are in reflective mode (dark areas represent CA domains).

hydrophilic nonsolvents shows a substantially different microstructure between 1-propanol and the other two alcohols, 1,3-propanediol and glycerol. The 1-propanol LSCM image is very similar to that of the other monohydric alcohols (1-hexanol, 1-octanol, 1-decanol), which show a larger macrocluster network structure (Figure 8). One possible explanation for this observation may lie in the difference in phase behavior and gelation mechanism between the nonsolvents. In the case of 1,3-propanediol and glycerol, the phase-separated gel morphologies appear bicontinuous and spinoidal, which is consistent with that obtained via spinoidal decomposition of near-critical mixtures.10,13 In these systems, as the samples are cooled and spontaneous phase separation occurs, that is, a quench from the mixed region into the unstable region, gelation of the CA retards coarsening and traps the respective microstructure. On the basis of the gel morphology, the stronger hydrogen bonding in the glycerol system likely accelerates the gel formation, minimizing the coarsening, and resulting in the fine uniform morphology. In contrast, the phase-separated morphology for the monohydric alcohol gels appears as an array of droplets likely obtained via nucleation and growth and spinoidal decomposi-


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tion of off-critical mixtures. Here the samples quench from the mixed region into the metastable region, and incipient droplets redissolve and grow in size. Because the extent of hydrogen bonding is less in these systems, the kinetics of phase separation is initially faster than gelation, and as the nonsolvent content is increased, the system initiates bulkphase separation. However, as the concentration of components in the CA-rich phase changes, so does the kinetics of phase separation and gelation. Depending on the system, as phase separation proceeds, the microstructure coarsens via droplet coalescence and Ostwald ripening, and ultimately, gel formation occurs. In the case of 1-propanol and to a slightly lesser extent 1-hexanol, the corresponding gels appear as a more uniform array of droplets (Figures 7a and 8a, respectively). However, increasing the alkyl chain length of the monohydric alcohol nonsolvent dramatically affects gel morphology (Figure 8a-c). In the longer alkyl chain length alcohols (1-octanol, 1-decanol), very inhomogeneous structures are observed, which is likely the result of extensive coarsening, wherein Ostwald ripening is likely the dominant mechanism.13 Interestingly, this decreasing uniformity is accompanied by an enhancement in viscoelastic behavior (Figures 2 and 4), perhaps due to enhanced intermolecular nonbonding interactions, which arise because of the phase separation and the resulting manipulation of component concentrations. Most biopolymer-based gels are either (entropic) polymer gels or (enthalpic) particle gels. In the latter, diffusion-limited aggregation of particle clusters leads to a percolating particle network with a characteristic fractal microstructure.20 The mechanism of gelation and whether or not the gels are fractal in nature can be determined by rheology.21 This is done by examining the relationship between G′ and volume fraction or content of the polymer or in our case nonsolvent. If G′ exhibits a power-law behavior with nonsolvent content (Φ), that is, G′ ≈ Φn, then the gels are fractal in nature. Both the hydrophilic and hydrophobic nonsolvent systems exhibit a power-law behavior between elastic modulus and nonsolvent content. The power-law exponents varied between the various systems, ranging from ∼134 to 28 (Figure 9). In both systems, the weaker modulus gels (1-propanol and 1-hexanol, respectively) exhibited larger power-law exponents, both significantly higher than the other nonsolvents. The similarity in morphology of the 1-propanol and 1-hexanol gels (Figures 7a and 8a) and their larger power-law exponent values suggest that their gelation mechanism is different from that of the other nonsolvents. Increasing both hydrophobicity and hydrophilicity in nonsolvent caused significant decreases in the power-law exponent values. Because the gels are fractal in nature, they can be viewed as being made up of flocs consisting of aggregated macromolecules, which interconnect to form a 3D gel network.22 As such, the gels can be classified as being either strong-linked or weaklinked systems. In strong-linked systems, the links between flocs are stronger than those within the flocs, and as a result, failure under deformation occurs via breaking of intrafloc interactions. This leads to a decrease in the LVE regime with increasing sample concentration. In contrast, weak-linked gels possess strong intrafloc links, more rigid than the interfloc linkages, and an increase in the limit of linearity occurs with increasing concentration. Figures 10 and 11 illustrate the effect of increasing strain on G′ for the different nonsolvent ternary systems. It can be seen that the onset of nonlinearity (regardless of how one defines it) shifts to lower values with increasing

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Figure 9. Elastic modulus of CA/DMA/nonsolvent gels as a function of nonsolvent content for (a) hydrophilic and (b) hydrophobic nonsolvent systems (G′ obtained at frequency of 1 rad s-1). 1-Propanol (Pro); 1,3-propanediol (PD); glycerol (Gly); 1-hexanol (Hex); 1-octanol (Oct); and 1-decanol (Dec).

nonsolvent concentration, indicating that the CA/DMA/nonsolvent gels are strong-linked. Because the gels are strong-linked aggregated macromolecules, their fractal dimensions can be determined through confocal microscopy21-24 by the box-counting technique using Image J software.21 In this method, the scaling relation is N ∝ r-D, where N is the number of boxes filled with CA, r is the size of the square box, and D is the fractal dimension of the CA network. The fractal dimension of the gels obtained via confocal microscopy (Figures 7 and 8) show similar values between the various systems, D ≈ 1.90 ( 0.05, regardless of CA concentration and type of nonsolvent, and suggest a similar reaction-limited aggregation (RLA) mechanism.22 The similarity in fractal dimensions is quite surprising based on the dramatic differences between the LSCM images of the respective systems. However, a similar discrepancy was reported for a series of caseinate gels,25 which exhibit visually different structures, ranging from open to dense and more uniform, yet all having the same calculated fractal dimension from image analysis. The authors suggested that the fractal dimension alone may not always be sensitive enough to capture the differences observed in gel microstructure. This may also be the case for our system.

Conclusions The effect of nonsolvent structure on the phase behavior and viscoelastic properties of a CA/DMA/nonsolvent ternary system was investigated using steady-state and dynamic rheology. At low nonsolvent concentration, Newtonian behavior was observed, followed by shear thinning at high shear rates. Increasing nonsolvent concentration enhanced the intermolecular interactions, and the system exhibited non-

Figure 10. Elastic modulus of CA gels for the different nonsolvent, 1-propanol (1-Pro), 1,3-propanediol (1,3-PD), and glycerol (Gly) concentrations as a function of strain. The limit of linearity shifts to lower strain as concentration increases (wt % concentrations of nonsolvent are shown in parentheses).

Newtonian behavior at a low shear rate, representing the development of microstructure. Changing the hydrogen bonding properties of the nonsolvent by increasing the number of hydroxyl groups from 1 (1-propanol) to 3 (glycerol) increased the strength of intermolecular hydrogen bonding in the system, resulting in enhanced solution viscosity and modulus. A similar enhancement in viscoelastic behavior was also observed upon changing the hydrophobicity of the nonsolvent by increasing the alkyl chain length of the nonsolvent (1-hexanol , 1-decanol). At high nonsolvent concentrations, phase separation and gelation occurred, the mechanism of which depended on the nonsolvent. In the case of 1,3-propanediol and glycerol, the phase-separated gel morphologies are consistent with that obtained via spinoidal decomposition, whereas those of the monohydric alcohols likely arise via nucleation and growth mechanisms. Rheological and microscopic analyses revealed that the various gels are fractal in nature and can be classified as strong-linked gel networks. In these systems, gelation is initiated by, and in fact requires, hydrogen bonding interactions to occur. The gels produced from the hydrophilic ternary system showed increased uniformity and density with increasing hydrophilicity of the system, which is consistent with their increased moduli. However, the hydrophobic systems had a more open nonuniform network structure with


Figure 11. Elastic modulus of CA gels for the different nonsolvent, 1-hexanol (Hex), 1-octanol (Oct), and 1-decanol (Dec) concentrations as a function of strain (wt % concentrations of nonsolvent are shown in parentheses).

increasing hydrophobicity, despite the increase in moduli. In this system, phase separation occurred at high nonsolvent contents with the formation of a rigid gel network phase underneath a clear liquid phase. Acknowledgment. We greatly acknowledge the Natural Sciences and Engineering Research Council of Canada (STPGP 336241) for financial support of this research. Supporting Information Available. Steady-state and dynamic rheology data. This material is available free of charge via the Internet at http://pubs.acs.org.

References and Notes (1) Opong, W. S.; Zydney, A. L. Diffusive and convective protein-transport through asymmetric membranes. AIChE J. 1991, 37, 1497–1510. (2) Li, N. N.; Fane, A. G.; Ho, W. S. W.; Matsuura, T. AdVanced Membrane Technology and Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2008. (3) Charcosset, C. Membrane processes in biotechnology: An overview. Biotechnol. AdV. 2006, 24, 482–492.


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