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Ethylcellulose Colloids Incubated in Dilute Solution Han-Liou Yi, Liang-Je Lai, Jung-Shiun Jiang, and Chi-Chung Hua J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b09976 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 3, 2017

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The Journal of Physical Chemistry

Ethylcellulose Colloids Incubated in Dilute Solution

Han-Liou Yi, Liang-Je Lai, Jung-Shiun Jiang, and Chi-Chung Hua* Department of Chemical Engineering, National Chung Cheng University, Chiayi 62102, Taiwan, R.O.C.

ABSTRACT: This study revealed, for the first time, that dilute solutions made of a representative series of commercial ethylcellulose (EC; molecular weights 77~305 kDa, provided by the manufacturer) and four distinct organic solvents (α-terpineol, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (TPIB), tetrahydrofuran (THF), and benzene) can be used to foster stabilized, nearly monodisperse, nanoscale (pure) polymer colloids with no isolated chains present. Using combined light scattering (dynamic light scattering, static form factor, and Zimm/Berry plots) and intrinsic viscosity (Tanglertpaibul-Rao, Huggins, and Kraemer plots) analyses, the structural features of colloidal EC aggregates, ρ=〈 Rg 〉/ 〈 Rh 〉=0.67~0.83, were first shown to be described rather well by the theory on colloidal spheres (〈 Rg 〉 and 〈 Rh 〉 being the mean radius of gyration and hydrodynamic radius, respectively). An empirical scaling law relating the intrinsic viscosity to the mean colloid size can thus be established: ηH = (1.7±0.2)×10-3 〈Rh 〉(2.1±0.3) (ηH and 〈Rh 〉 in units of mL/g and nm, respectively), which may be contrasted with the Zimm model for isolated Gaussian coils, η]H ~ 〈Rh 〉 , and the Einstein equation for isolated solid spheres, η]H ~ 〈Rh 〉. Optical microscopy images of thin films cast from different EC solutions clearly revealed the abundance of micron EC agglomerates, contrary to the uniform thin-film morphology produced from dilute polystyrene solution which serves as a reference solution composed of isolated chains. These observations point to new features and applications of EC dispersions.

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1. INTRODUCTION Cellulose and its derivatives, such as ethylcellulose (EC), are conventionally important materials for a wide range of applications in cosmetics,1,2 pharmaceutics,3-6 and food industries.7-9 Recently, environmentally and economically favorable wet-processing technology makes almost ubiquitous use of EC as the polymer binder in a broad variety of functional pastes consisting of metal10-12 or ceramic13-16 powders for the fabrication of electronics and energy-storage devices. A deeper understanding of the inherent properties of dilute EC solutions is crucial to help guide the current and emerging applications with EC. As elucidated in this work, dilute solutions prepared using a representative series of commercial EC in four distinct solvent media revealed important new (colloidal) features not fully captured in prior research on dilute EC solutions, where extensive efforts had been devoted to the exploration of isolated-chain features in a wide range of aqueous and organic solvent media. Inherent properties of dilute polymer solution can be readily revealed by intrinsic viscosity [η] or light scattering analysis. While the former provides information about the specific hydrodynamic volume of single, isolated chain or colloid, the latter assesses the corresponding geometrical size, both under extremely dilute condition. Prior intrinsic viscosity analyses on EC solutions had unanimously reported an excluded-volume dominated, good-solvent condition through 13,17-20 the scaling relationship of [η] = KM a or the Huggins coefficient 0.25 < KH < 0.521 w , with a > 0.5

for a broad variety of solvent media (e.g., benzene, chloroform, α-terpineol, and toluene/ethanol 80:20 vol% etc.) at fixed or varying system temperature.22,23 These studies consistently implied that EC formed swollen chains in dilute solution, although the semiflexibility of EC chains seemed to be at work affecting their viscometric properties as well. Relatively few investigation had been dedicated to the light scattering analysis of dilute EC solutions, however, and existing ones using mainly the Zimm plot analysis revealed a zero (methanol)24 or negative (ethylacetate)25 second virial coefficient indicative of poor-solvent conditions. As Kamide26 pointed out lately that existing intrinsic viscosity and light scattering (Zimm plot) analyses on EC solutions were sometimes inconsistent in that the latter 2


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implied the formation of EC aggregates in some nominally good solvents as inferred from the former. A similar situation, in fact, has been noted in a number of recent reports on dilute poly(ethylene oxide) (PEO) solutions.27-30 Polymer aggregates formed in dilute solution had been reported on a wide variety of solution systems including thermosensitive polymers,31,32 conjugated polymers,33-38 polysaccharides (e.g., chitosan,39-41 lentinan,42 protein,43-45 detran,46,47 cellulose48), dyestuff,49 and block-copolymers.50-53 Most of the literature revealed the coexistence of polymer aggregates with a certain fraction of isolated chains.31,37-39,41,42,47,49-51,53 It is, therefore, of both scientific and technological interest to understand how polymer aggregates (or colloids) with controllable partition and geometrical features may be fostered in dilute solution, through a proper combination of polymer-solvent pair, polymer concentration, and preparatory (or processing) scheme. This work represents the first that reported on the incubation of stabilized, nearly monodisperse, nanoscale EC colloids using dilute solution samples along with a standard preparatory procedure, for a series of commercial EC samples and four distinct solvent media. Similar to the early observation, comprehensive light scattering (dynamic and static) and intrinsic viscosity analyses revealed inconsistent trends regarding the apparent solvent quality and, consequently, the “isolated chain vs. aggregate” scenario of the dilute EC solutions under investigation. Leaving the causes of this apparent discrepancy to future investigations, though, we presently focused on the following perspectives: (1) To help corroborate the light scattering analyses on EC solutions, results on a representative, well-characterized polystyrene (PS) sample were employed as a standard reference on dilute, isolated-chain solution. (2) Confirming further that the present intrinsic viscosity measurements led to quantitative (for the absolute value) or qualitative (for the scaling constants and exponents) agreement with early reports on similar EC solutions, we proceeded to the establishment of an empirical scaling law relating the intrinsic viscosity to the mean colloidal size of EC, which may be contrasted with the Zimm model prediction on dilute polymer solution and, in practice, provides an important means to 3



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“measure” the colloidal size through easily accessible intrinsic viscosity data. (3) To shed light on the colloidal attributes of the present EC solutions, we presented optical microscopy (OM) images contrasting the thin-film morphologies of EC prepared from different solvent media and that produced from the reference PS solution. Accordingly, we remark on the central implications of this study as well as potential new applications with EC dispersions.

2. EXPERIMENTAL METHODS 2.1. Materials. Ethylcellulose (EC) with a degree of substitution about 2.41~2.46 (48.0~49.5 wt % of ethoxyl content) was provided by Dow Chemical Company (Shanghai, China), with five different molecular weights ranging from 77 to 305 kDa; see Table 1 for detailed specifications. Because the present study does not demand precise knowledge of the EC molecular weights, no attempts have been made to verify the EC molecular weights reported by the manufacturer. Four different solvent media were used to dissolve EC: α-terpineol (Taiwan Tekho Fine Chem Cooperation, Taiwan), 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (TPIB; Shiny Chemical Industrial Company, Taiwan), tetrahydrofuran (THF; Merck, Germany), and benzene (Sigma–Aldrich, USA). Toluene (Merck, Germany) was used to dissolve the reference PS sample (Sigma–Aldrich, USA), which bears a reported number-average molecular weight of 2,000,000 g/mol and polydispersity index (PDI) ~1.1. Polystyrene in toluene has a reported theta temperature of about 160 K.54 In all experiments, the solvents were filtered through 0.20 µm PTFE Millipore filter (Millipore Millex-GN) to remove dust and used without further purification. The four solvents used in this study possess Hansen solubility parameters as follows: α-terpineol (19.01 MPa1/2),55 TPIB (19.01 MPa1/2),56 THF (19.46 MPa1/2),56 and benzene (18.41 MPa1/2),56 which may be compared with EC (20.6 MPa1/2).56-58 The first two (α-terpineol and TPIB) have identical solubility parameter and represent two commonly used alcohol solvents, with a low volatility especially suitable for fabricating functional pastes or inks which utilize EC as a polymer binder; while THF is a representative aliphatic, polar solvent, benzene represents a standard non-polar, aromatic solvent. Past 4


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research on EC solutions had commonly used α-terpineol14,19 and benzene17,18,22,23,25 as a solvent, and therefore it is possible to make direct comparisons between the data reported herein and in the literature. Table 1. Information of Ethylcellulosea Used in This Study Molecular Weight Type PDI 〈 Mw, GPC 〉 (kDa)

a

EC10

77

3.5

EC45

135

2.4

EC100

180

3.0

EC200

187

2.3

EC300

305

3.5

Data provided by Dow Chemical Company.

2.2. Samples Preparation. The sample vials used in all experiments were washed with detergent and then treated with filtered deionized (DI) water. The EC and the filtered solvent were subsequently added to the same vial. A series of EC solutions of varying solvent and concentration was prepared, as detailed in Tables S1-S2 of the Supporting Information. The sample solutions were first sonicated for 20 h at 50.0 °C, and were allowed to equilibrate for at least 1 h at room temperature before being transferred to the sample carriers for subsequent characterization. Long-term sonication and elevated system temperature were noted to considerably expedite the dissolution process of EC, especially in the two alcohol solvents (α-terpineol and TPIB). Nevertheless, the effects of sonication time and system temperature have been scrutinized, and the results are shown in Supporting Information Figures S1-S4 indicating no appreciable changes in the DLS features. These observations, importantly, shed light on the stability of EC aggregates fostered in all four solvent media.

2.3. Light Scattering (DLS/SLS) Analyses. Light scattering measurements were performed on a laboratory-built apparatus as described elsewhere,33,59 with a 34 mW polarized He−Ne laser (λ0 = 632.8 nm; Lasos, LGK 7626 S) as the incident light. The vial containing the sample solution of 2 mL in 5



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volume is a cylindrical quartz cell with 1 cm in diameter (Hellma, 540.111, Germany), and a cap is utilized to prevent solvent evaporation during the entire experiment. All measurements were conducted at 25.0±0.1°C for a range of scattering angles θ = 30°-140°. In the SLS experiment, the angular dependence of the absolute excess time-averaging (for a duration of scanning of 15 s) scattering intensity, known as the Rayleigh ratio Rvv ( ),60 leads to the weight-average molar mass, 〈 Mw 〉, the mean square radius of gyration, 〈 R2g 〉, and the second virial coefficient, A2 , by using Zimm plot61,62 2 2 Kc 1  q Rg  1 +  + 2 A2c = 3  Rvv (θ ) Mw   

(1)

where q = (4πn0 / λ0)sin(θ / 2) is the scattering wave vector with λ0 (=632.8 nm) being the wavelength of the incident light in vacuum. In the case of vertically polarized incident light, the optical constant K is given by K = 4π2 n0 2 (dn / dc) /NAλ0 4 where n0 is the refractive index of the solvent, dn / dc is the specific refractive index increment of the polymer solution, and NA is the Avogadro’s number. In this work, dn / dc is evaluated by the following relation:63

dn ≅ ( n p o ly − n s o lv )ν s p dc

(2)

where npoly and nsolv denote the refractive indices of pure polymer and solvent, respectively, and νsp is the specific volume of the polymer in solution. If the volumes of the polymer and solvent are assumed to be additive, νsp can be taken from the reciprocal of the polymer density in bulk phase. The value of dn / dc for the reference PS/toluene solution was taken from the literature to be 0.110±0.001 cm3/g.64 In the DLS experiment, the normalized intensity autocorrelation function, g(2) (q, t), was collected in a homodyne mode, which is related to the normalized field autocorrelation function, |g(1) (q, t)|, through the Siegert relation:65 g(

2)

(q, t ) = 1 + β

g ( ) (q, t ) 1

2

(3) 6


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where t is the decay time of the autocorrelation function, and β (0 < β < 1) is the spatial coherence factor depending on the instrumental detection optics. For multiple-mode relaxation processes, the field autocorrelation function |g(1) (q, t)| can be related to the decay rate distribution function G(Γ) by the Laplace transformation: ∞

|g ( ) ( q, t ) | = ∫ G ( Γ ) exp ( − Γ t ) dΓ 1

(4)

0

The detailed distribution of G(Γ) may be retrieved by inverse Laplace transformation using the commercial software CONTIN.66 For a diffusive process, 〈 Γ 〉 can be further related to the mean translational diffusion coefficient as 〈 D 〉 = 〈 Γ 〉⁄q2 q→0 . Accordingly, the mean hydrodynamic radius 〈 Rh 〉 can be obtained using standard Stokes-Einstein relation 〈 Rh 〉 = kB T⁄6πη〈 D 〉, where kB is the Boltzmann’s constant, and η is the solvent viscosity measured at the absolute temperature T.

2.4. Intrinsic Viscosity Analysis. The viscosity of a polymer solution or solvent was measured using Cannon-Fenske Routine viscometers (size 200 for α-terpineol, size 150 for TPIB, and size 25 for THF; Cannon Instrument Co.), with the detailed procedure as follows: An appropriate amount of sample solution was transferred to a Cannon-Fenske Routine viscometer, the tube being then placed in a water bath with an external circulation system to help control the system temperature at 25.0±0.1°C. The system was allowed to equilibrate for 30 min prior to each individual measurement of the efflux time, which is required to be longer than 200 s so as to render the effect of flow kinetic energy negligible.67 The measurements were repeated at least five times to obtain the mean value, and the relative error was ensured to be within 5%. For EC of high molecular weights such as EC300, the number of independent measurements for a single solution system can be as many as 30 in some cases by which to warrant the accuracy as noted above. In this case, multiple sample batches were independently prepared and utilized for this purpose. A complete gathering of viscosity data was provided in Table S1 of the Supporting Information. The intrinsic viscosity [η] often serves as a measure of the hydrodynamic volume occupied by a 7



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polymer chain, and is closely related to both the size and conformation of the polymer chain dispersed in a solvent medium.68 According to its definition, the intrinsic viscosity of a dilute polymer solution can be obtained through the following power-law series: ηsp c

= [η ] + K1 [η ] c + K2 [η ] c2 + K3 [η ] c3 +L 2

3

4

(5)

where c is the polymer concentration, ηsp is the specific viscosity (ηsp = ηrel - 1), ηrel is the relative viscosity (ηrel = ηsolu / ηsolv ) with ηsolv and ηsolu being the solvent and solution viscosities, respectively, and Ki is a dimensionless constant. Under dilute conditions, c →0, the so-called reduced viscosity ηsp /c (= ηred ) can be related to the intrinsic viscosity [η] in the following linear approximation, best known as the Huggins equation:69 ηsp c

= [η]H + KH [η]H c 2

(6)

where KH is the Huggins coefficient, which is closely related to the size and shape of the polymer chain and, in particular, is influenced by the hydrodynamic interactions between different segments of the same chain.70 Note that the applicability of eq 6 is restricted to the condition [η]c ≪ 1 (or ηsp≪ 1). By making an additional assumption, the Huggins equation can be cast into an alternative form, usually referred to as the Kraemer equation:71 ln η rel 2 = [η ]K + K K [η ]K c c

(7)

where KK is the Kraemer coefficient, and the prior assumption requires that KH - KK = 0.5. In practice, it is often required that the extrapolations based on these two equations must coincide to a single point, which serves to validate the above assumption. The Huggins and Kraemer equations both apply to the range of relative viscosity: 1.3 < ηrel < 2.0, or the corresponding specific viscosity between 0.3 and 1.0.68,72 For extreme dilute solutions, eq 5 can be further reduced to retaining only the zeroth-order term, and [η] can be directly determined from the slope of a plot of ηrel versus c:73 8


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ηrel =1+[η]T c

(8)

Determinations of the intrinsic viscosity based on all three formulas, eqs 6-8, will be scrutinized for EC solutions in this study.

2.5. Optical Microscopy (OM). OM images were obtained using a conventional microscope (Olympus BX51, Japan) equipped with a high resolution digital camera (Olympus DP22, Japan). Samples for OM characterization were prepared using a drop-casting method on a glass substrate, followed by natural drying at room temperature. The particle size distribution from the OM images was evaluated using the ImageJ program (version 1.46r, National Institutes of Health, Bethesda, MD, USA).

3. RESULTS AND DISCUSSION 3.1. General Features of Solvent Aggregate. Given that the intrinsic properties of a solvent medium can significantly affect its solvation power for a polymer species, we first examine the scattering features of the solvent media used in this study. Figure 1 shows the angular dependence of light scattering intensity for various solvent media. Two of them (i.e., α-terpineol and TPIB) exhibited a pronounced angular dependence, indicative of the formation of solvent aggregates of tens of nanometers in size. This observation, consistent with later light scattering analyses on EC solutions, suggested that these two alcohol solvents represent relatively poor solvent quality for EC. For a comprehensive discussion on solvent aggregates, the reader is referred to the literature on solutions of low molar mass electrolytes, nonelectrolytes, and mixtures of liquids.74-77

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Figure 1. Angular dependence of light scattering intensity for four different solvent media, where the scattering intensity, I, was normalized by the corresponding scattering intensity of the toluene medium, Itol. 3.2. Light Scattering (DLS/SLS) Analyses of EC solutions. A plausible way to characterize isolated chains or colloids is to capitalize on combined DLS/SLS analyses, as described below. Figure 2a shows the angular dependence of the field autocorrelation function, |g(1) (q, t)|, for a representative EC100/α-terpineol solution at 1.0 mg/mL, along with the decay time distribution function G(τ) (= G(1/Γ)) extracted from CONTIN. It can be seen that the DLS curves at four different scattering angles all collapse on the same curve as the decay time is rescaled with q2 , indicative of a single diffusive mode and, accordingly, EC chains or aggregates with a narrow size distribution. In this case, the mean decay rate, 〈 Γ 〉, can be plotted as a function of q2 to obtain the mean hydrodynamic radius, 〈 Rh 〉, as shown in the inset figure. For all the EC solutions in this study, the DLS analysis ubiquitously revealed a mean (aggregate) size 〈 Rh 〉 of one to a few hundred nanometers, as shown in Supporting Information Figure S5. Formation of bulky polymer aggregates, usually in coexistence with a certain fraction of isolated chains, revealed by DLS had previously been reported for a variety of polymer solutions.31-36,39-48,50-53 Yet, it is important to note that Figure 2a revealed no other short-time modes indicative of the presence of isolated chains. Repeated measurements confirmed that the results presented here were quite 10


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reproducible, suggesting that EC forms stabilized colloids in dilute solution with no isolated chains coexistent. For comparison, Figure 2b shows the results on a reference PS solution. A similar feature was observed as with EC solutions, except for a much smaller coil size for 〈 Rh 〉 (ca. 42 nm) typical of an isolated chain. Note that the PS molecular weight is much greater than the reported EC molecular weights. Therefore, the pronounced size difference observed here is clearly attributable to the formation of aggregate clusters in EC solutions. Another interesting feature is that, despite a small polydispersity index (~1.1) of the PS sample, the DLS curves in Figure 2 indicated that the EC aggregates are even more uniform in size. This essential feature, for instance, could greatly facilitate the fabrication of uniform EC powders, as we discuss later.

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Figure 2. Angular dependence of the field autocorrelation function for (a) EC100/α-terpineol and (b) PS/toluene solution at 1.0 mg/mL and T = 25.0 °C, where the decay time (τ) has been rescaled with q2; the inset shows the mean decay rate 〈 Γ 〉 as a function of q2.

While the DLS analysis yields the mean hydrodynamic radius, 〈 Rh 〉, the corresponding SLS data may be utilized along with known form factors to retrieve the mean radius of gyration, 〈 Rg 〉, for dilute, noninteracting systems. In Figure 3a, the SLS data were analyzed using two different form factors, P(q), that accounted for a slight polydispersity in aggregate size for a representative EC100/α-terpineol solution at 1.0 mg/mL:

12


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I ( q) ∫0∞Gn ( R) α2 ( R) P( qR) dR P( q) ≡ = ∞ 2 I ( 0) ∫0 Gn ( R) α ( R) dR

(9)

where Gn (R) is the number-weighted distribution of the characteristic radius R, α ( R) the polarizability, and P(qR) the form factor. The polarizability α ( R) is proportional to the power-law of R as α ( R) ∝ R γ.78 For Gaussian coils, γ = 2 and P(x) = 2(ex 1 + x) / x2, with x = (qR)2 ; for solid spheres, γ = 3 and P(qR) = [3(sin qR qR cos qR )]2 / (qR)3 . The results shown in Figure 3a clearly indicated that the SLS data can be well described by the form factor of solid spheres (solid line), but not Gaussian coils (dashed line). Figure 3b shows the result for a reference PS solution. As might be expected, a small coil size of PS, as will be further revealed in Zimm plot analysis, prevents a pronounced q dependence necessary for the form-factor analysis. Figure 4 presents the results on mean radius of gyration 〈 Rg 〉 (from SLS analysis) and mean hydrodynamic radius 〈 Rh 〉 (from DLS analysis) over a range of EC concentrations. Both quantities are basically independent of the EC concentration, suggesting that individual aggregates were being probed and, hence, the solutions investigated may be classified as dilute dispersions. A complete gathering of data for other EC solution systems can be found in Supporting Information Figure S6.

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Figure 3. Experimental form-factor analysis of (a) EC100/α-terpineol and (b) PS/toluene solution at 1.0 mg/mL and T = 25.0 °C, where the solid line in (a) is the fit of data using eq 9 along with the form factor of solid spheres and the dashed line is for Gaussian coils; the inset shows the number-weighted size distribution used in the fitting.

Figure 4. Concentration independence of the mean radius of gyration 〈 Rg 〉 and mean hydrodynamic radius 〈 Rh 〉 for EC100/α-terpineol solutions at T = 25.0 °C.

The results on 〈 Rh 〉 and 〈 Rg 〉 together can be utilized to infer the structural compactness through the ratio ρ= 〈 Rg 〉 / 〈 Rh 〉, and a smaller value of ρ may be assigned to a more compact (interior) structure. The results for all sample solutions are reported in Table 2. It can be seen that all of the EC solutions investigated yield ρ=〈 Rg 〉/〈 Rh 〉=0.67~0.83 in close agreement with the theoretical 14


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prediction for colloidal spheres, i.e., ρ = 0.775, while deviating substantially from that for Gaussian coils, ρ = 1.505, which was found to describe excellently the reference PS solution (ρ = 1.5). The distinct contrast noted here serves as clear evidence that, while the PS solution forms an ideal dilute polymer solution, the EC solutions under investigation should be characterized as dilute colloidal dispersion. It is worth noting that the ρ-ratio remains nearly constant for the same solvent medium. When the (constant) ρ-ratio is used to infer the solvent quality of EC solutions, we have the sequence of THF > benzene > TPIB > α-terpineol with decreasing solvent quality, which differs notably from the prediction based on the Hansen solubility parameters: THF > TPIB = α-terpineol > benzene. The effect of solvent quality can also be seen from the thin-film morphologies discussed later. Because the Zimm plot analysis requires 〈 Rg 〉 ≪ 1, which is only marginally satisfied for the EC solutions investigated, we briefly discuss the essential features from this conventional analysis. In Zimm

plot

analysis,

the

excess

inverse

Rayleigh

ratio Kc/Rvv (θ)

was

plotted

against

sin2 (θ/2) + const. × c. A typical result on EC45/TPIB solutions is shown in Figure 5a, and a complete gathering of data can be found in Supporting Information Figure S7. The extrapolations of both c→0 and →0 lead to the weight-average molar mass of the EC aggregates. From the plot of Kc/Rvv(θ)→0 (by extrapolation to zero scattering angle) versus c, the second virial coefficient, A2 , can be determined; from the plot of Kc/Rvv (θ)c→0 (by extrapolation to zero concentration) versus sin2 (θ/2), the mean radius of gyration, 〈 Rg 〉, is estimated. Prior work79-81 had suggested that, for structural features with q〈 Rg 〉 > 1, the Berry plot82 should be used instead, and the results are provided in Supporting Information Figure S8. Comparing the results shown in Table 2 and Table S3 for 〈 Rg 〉 determined from Zimm plot and Berry plot analyses, respectively, revealed no major differences pertinent to the following discussion. Like the trend in Figure 5a, we have generally observed negative values of A2 , and the 〈 Rg 〉 so obtained is in fair agreement with prior SLS (form-factor) analysis. The disparity between these two 15



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analysis schemes consists in that Zimm plot makes use of the Guinier regime to determine the size, whereas form-factor analysis involves fitting the entire q range pertinent to the determinations of both size and shape. For the reason mentioned earlier, we expect that the latter should yield more precise estimate of EC colloid sizes. For comparison, Figure 5b presents the results on the reference PS solution, which yielded a positive A2 as well as an average molecular weight in excellent agreement with what was provided by the manufacturer. The average molar mass obtained in either Zimm or Berry analysis of EC solutions, however, was of poor quality and therefore was not shown, although the results generally confirmed the expectation that the molar mass so extracted for EC (aggregates) falls substantially above the (single-chain) EC molecular weights reported in Table 1. For dilute solution consisting of smaller aggregate species (e.g., tens of nanometers in size and narrow size distribution), the Zimm plot had previously been shown to yield credible results on mean aggregate size and molar mass.83,84 As discussed above, however, the form-factor analysis should yield more credible results for the EC solutions under investigation. For colloidal dispersions, prior reports suggested that negative values of the second virial coefficient could be indicative of a reaction-limited aggregation mechanism.85 In summary, combined dynamic and static light scattering analyses have unambiguously revealed that the presently prepared EC solutions foster stabilized, nearly monodisperse, nanoscale colloids without isolated chains coexistent.

16


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Figure 5. Zimm plot analyses of (a) EC45/TPIB and (b) PS/toluene solutions at T = 25.0 °C, where the concentration c ranges from 0.6 to 1.0 mg/mL.

17



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Table 2. Summary of Mean Aggregate Properties of EC in Four Different Solvent Media Zimm Plot

〈 Rg, Zimm 〉

SLS/DLS

A2 ×106

〈 Rg 〉

(molcm3/g2)

〈 Rh 〉

(nm)

(nm)

(nm)

EC10

-

-

135

206

0.658

EC45

123

-39.5

144

219

0.661

139

-32.4

168

252

0.668

EC200

146

-49.6

193

286

0.675

EC300

130

-20.6

170

252

0.675

EC10

104

-21.1

129

176

0.732

EC45

128

-137.5

137

188

0.730

158

-20.9

182

247

0.739

EC200

134

-45.0

214

291

0.736

EC300

302

-9.3

225

308

0.732

EC10

203

-39.3

151

183

0.822

EC45

184

-214.4

173

209

0.825

294

-70.3

208

250

0.831

EC200

260

-145.3

224

273

0.823

EC300

504

-34.6

235

284

0.826

EC10

285

-31.3

187

244

0.767

EC45

255

-52.4

196

266

0.737

273

-58.3

214

272

0.785

EC200

212

-240.3

209

274

0.763

EC300

295

-53.0

235

320

0.734

Type

EC100

EC100

EC100

EC100

Solvent

α-terpineol

TPIB

THF

Benzene

18


ρ

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3.3. Intrinsic Viscosity Analyses. Figure 6 presents the Tanglertpaibul-Rao, Huggins, and Kraemer plots, respectively, for a representative EC10/α-terpineol solution system; a complete gathering of data on other EC solution systems can be found in Supporting Information Figure S9. For the Tanglertpaibul-Rao plot, [η] can be directly determined from the slope of ηrel versus c. In the other cases, [η] and the associated (Huggins or Kraemer) constant are obtained simultaneously by extrapolation to the zero concentration. The results from these three fitting schemes are gathered in Table 3 for comparison. In addition, we have utilized the VISFIT computer program86 to fit the dual Huggins-Kraemer plot with a common intercept, [η], and the results are summarized in Supporting Information Table S4. It can be seen that the intrinsic viscosities obtained using various schemes are in close agreement.

Figure 6. Intrinsic viscosity analysis of EC10/α-terpineol solutions at T = 25.0 °C, where EC concentration c ranges from 0.0001 to 0.007 g/mL: (a) plot of ηrel vs. c, with the dashed line marking the boundary set by the usual criterion ηrel =1.3, (b) the Tanglertpaibul-Rao plot, and (c) the Huggins and Kraemer plots.

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Both Huggins and Kraemer coefficients had been considered adequate criteria to evaluate the solvent quality of a polymer solution. Existing data, for instance, seemed to indicate that a higher affinity between polymer and solvent leads to a lower value of KH . Specifically, values in the range of 0.25~0.5 are attributed to good-solvent conditions, whereas values in the range of 0.5~1.0 are regarded as poor solvents. Alternatively, negative values of the Kraemer constant, KK , are an indication of good solvents, and positive values represent poor solvents.68 Accordingly, the results shown in Table 3 appear to suggest that most of the EC solutions under investigation fall in the category of good solvents. When the scaling law [η]H = K M a w was constructed using the molecular weights reported in Table 1, as typically performed in prior work, we observed that the exponent falls in the range of α=0.8~1.0, as previously found for many EC solutions wherein the semiflexiblility of EC chains was believed to be at work.13,17-23 Importantly, comparisons between the intrinsic viscosity reported herein and in the literature on a series of EC/α-terpineol solutions14,19 revealed excellent quantitative agreement. Thus, despite an apparent discrepancy in the implied solvent quality between intrinsic viscosity and light scattering analyses, both sets of data should be credible for a further analysis of fundamental scaling law that helps reveal the true nature of the EC colloids under investigation. To this end, it seems evident that while the classical polymer theory and scaling laws work fairly well for dilute solution made of isolated chains, related theories for polymer colloids are still lacking and, therefore, cautions should be taken in interpreting the intrinsic viscosity data on dilute systems where the underlying molecular state (i.e., isolated chains or isolated aggregates) remains unidentified.

20


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Table 3. Summary of Measured Intrinsic Viscosity and Fitted Parameters for Various EC Species in Three Different Solvent Mediaa Type

Solvent

EC10

η]T (mL/g)

η]H

η]K

(mL/g) (mL/g)

KH

KK

KH -KK

115±4

94±2

94±2

0.49±0.05 -0.09±0.03

0.58

159±6

152±3

151±2

0.33±0.03 -0.15±0.04

0.48

190±12

180±4

180±2

0.37±0.04 -0.14±0.02

0.51

220±16

216±5

217±4

0.38±0.03 -0.14±0.05

0.52

157±7

160±6

166±4

0.35±0.06 -0.16±0.04

0.51

EC10

103±9

95±4

96±2

0.40±0.08 -0.13±0.05

0.65

EC45

172±8

167±5

168±3

0.57±0.06 -0.08±0.03

0.65

192±8

185±5

188±3

0.59±0.05 -0.05±0.03

0.64

EC200

224±14

213±5

215±3

0.46±0.05 -0.12±0.03

0.58

EC300

223±16

221±3

219±2

0.37±0.02 -0.12±0.03

0.49

96±8

89±5

93±3

0.73±0.10 -0.02±0.06

0.75

145±7

145±5

150±4

0.67±0.09 -0.02±0.05

0.69

191±6

190±3

191±2

0.67±0.04 -0.04±0.03

0.71

236±14

244±8

244±5

0.44±0.06

-0.11±0.03

0.55

241±12

241±7

246±5

0.55±0.06 -0.07±0.04

0.62

EC45 EC100

α-terpineol

EC200 EC300

EC100

TPIB

EC10 EC45 EC100 EC200 EC300 a

THF

Data were unavailable for the EC/benzene solution due in part to hazard considerations.

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3.4. Scaling Laws. For dilute solution consisting of Gaussian coils, the Zimm model predicts a renowned scaling law of [η]H ~ 〈Rh〉1 .87 For dilute solid spheres, the Einstein equation, ηsolu / ηsolv = 1 ! (! being the volume fraction of the spheres), yields an intrinsic viscosity that scales linearly

with the specific volume of the spheres, independent of the geometrical size. For dilute polymer colloid, the relationships were generally unknown, however. Saricay and co-workers,88 for instance, had recently reported a scaling exponent of 0.51 for apo-α-lactalbumin aggregate solutions. Figure 7a-c shows the results on three different EC solution systems, where scaling exponents of 2.1±0.4, 1.2±0.3, and 2.2±0.1 were found for α-terpineol, TPIB, and THF media, respectively. A useful empirical relationship provided in the figure caption, along with the ρ-ratios listed in Table 2, can be used to predict the geometrical features of colloidal aggregates in dilute EC solution. Notably, the scaling exponent (~2) presented in Figure 7d differs from the known ones for dilute Gaussian coils (1), solid spheres (0), and biological polymer aggregates (0.51), suggesting that polymer colloids could represent a unique class of material systems worthy of a systematic exploration.

Figure 7. Scaling laws of intrinsic viscosity, [η]H , versus mean hydrodynamic radius, 〈 Rh 〉, for (a) α-terpineol, (b) TPIB, (c) THF media, respectively, at T = 25.0 °C. The results shown in (d) represent an

overall

fit

of

the

present

data

on

EC

solutions,

22


which

yields

the

relation:

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ηH = (1.7±0.2)×10-3 〈Rh 〉(2.1±0.3) . The statistical errors are smaller than the symbol sizes in all cases.

3.5. OM Images. An illuminating way to perceive the difference between solutions that incubate isolated chains and isolated aggregates, respectively, is to examine the thin-film morphologies prepared from the counterpart solutions. The results shown in Figure 8 clearly confirmed this notion. It can be seen that while PS solution produces a uniform and featureless thin film, those fabricated from EC solutions reveal the prevalence of micron “dots”— a clear manifestation of the agglomerates formed by polymer colloids during the drying process. In addition to a slight disparity in the mean particle size as well as in the size distribution, the droplets produced from the two more volatile solvents (i.e., THF and benzene) were noted to result in more extended thin film and, consequently, monolayers of the EC agglomerates that make the impression of a more densely populated particulate state. Although the causes of these disparities are yet to be further clarified, the observation is undoubtedly very important for those who are interested in fabricating (pure) polymer powders of controllable size, shape, and porosity. To our knowledge, there seem to be no similar applications with EC to date.

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Figure 8. OM images (left) and particle size distributions (right) of the thin-film morphology produced from (a) PS/Toluene, (b) EC300/α-terpineol, (c) EC300/TPIB, (d) EC300/THF, and (e) EC300/Benzene at a concentration of 1.0 mg/mL.

24


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4. CONCLUSION We showed that a representative series of commercial EC can be used to foster stabilized, nearly monodisperse, nanoscale colloids through dilute solutions made of four distinct organic solvents. When compared to prior research on dilute EC solutions, which had focused primarily on isolate-chain features and viscometric properties, the present findings should bear the following significances. First, we demonstrated that while the intrinsic viscosity measurements yield results basically no different from early reports, the light scattering analyses suggest, nonetheless, that the solvent qualities of the investigated EC solutions should be classified as “poor”, which consistently accounted for the formation of stabilized EC colloids revealed herein. Disregarding the possibility that different EC samples or preparatory schemes might result in isolated chains in some cases and isolated colloids in others, the present findings clearly suggest that intrinsic viscosity data alone cannot be used to unambiguously determine the solvent quality (i.e., good-solvent vs. poor-solvent condition) and molecular state (i.e., isolated-chain vs. isolated-aggregate scenario) of a dilute polymer solution. In view of colloid sciences, these EC dispersions bear the promise of producing (pure) polymer powders of controllable size, shape, and porosity. Furthermore, the empirical equation relating intrinsic viscosity to colloid size possesses a scaling exponent ~2, in distinct contrast with the known ones for dilute Gaussian coils (1), solid spheres (0), and biological polymer aggregates (0.51). The disparity implies that polymer colloids could represent a unique class of material systems worthy of a systematic exploration. The reported colloidal features should also prompt a rethinking in recent applications of EC as a polymer binder for fabricating a wide variety of functional pastes and inks, especially when considering the way the colloids are to interact with the functional particles and, moreover, the expected state of “entangled colloids” at moderate and high concentrations, and pave the way to more precisely engineer the structure and rheology in practical, undiluted systems.

25



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Original viscosity data on all EC solutions, effects of sonication time and system temperature on EC solution dynamics, specification of solution samples in light scattering analyses, results from Berry plot analysis, results from dual Huggins-Kraemer plot analysis, gathering of data on excess scattering intensity as a function of concentration, summary of mean aggregate sizes from light scattering analyses, gathering of Zimm/Berry plots, gathering of Tanglertpaibul-Rao, Huggins, and Kraemer plots.

AUTHOR INFORMATION Corresponding Author *

Email: [email protected] (C.C.H.).

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS The authors acknolwedge the support from the Ministry of Science and Technology of ROC (MOST 103-2221-E-194-059-MY3).

26


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For Table of Contents Graphic use only

Ethylcellulose Colloids Incubated in Dilute Solution Han-Liou Yi, Liang-Je Lai, Jung-Shiun Jiang, and Chi-Chung Hua*

Department of Chemical Engineering, National Chung Cheng University, Chiayi 62102, Taiwan, R.O.C.

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Ethylcellulose Colloids Incubated in Dilute Solution - The Journal of

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