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Properties of Single-Walled Aluminosilicate Nanotube/Poly(vinyl alcohol) Aqueous Dispersions Chien-You Su, An-Chih Yang, Jung-Shiun Jiang, Zhi-Huei Yang, Yan-Shu Huang, Dun-Yen Kang, and Chi-Chung Hua J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10079 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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The Journal of Physical Chemistry

Properties of Single-Walled Aluminosilicate Nanotube/Poly(vinyl alcohol) Aqueous Dispersions

Chien-You Su,a An-Chih Yang,b Jung-Shiun Jiang,a Zhi-Huei Yang,b Yan-Shu Huang,b Dun-Yen Kang*b and Chi-Chung Hua*a a

Department of Chemical Engineering, National Chung Cheng University, Chiayi 62102, Taiwan. b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.

ABSTRACT: The properties of (synthesized) single-walled aluminosilicate nanotube (AlSiNT; light-scattering characterized length ~2000 ± 230 nm and diameter ~35 ± 4 nm) dispersed in an aqueous poly(vinyl alcohol) (PVA) solution (10 wt%) are systematically explored using a comprehensive combination of (polarized/depolarized) dynamic light scattering, rheological, rheo-optical, and scanning electron microscopy analysis schemes. The nanotube/polymer dispersions under investigation are promising for their fair nanotube dispersion in pristine aqueous media (e.g., without salt or acid addition), as well as for the optical transparency that greatly facilitates systematic exploration of structural features and dispersion state that are practically inaccessible for many of their (opaque) companions such as carbon nanotube dispersions. We provide the first in-depth analysis revealing excellent dispersion state of (un-modified) AlSiNT in the PVA matrix, giving rise to (critical) gel-like features and substantially promoted elasticity that can be utilized, as a practical assessment, to produce uniform and defect-free electrospun nanofibers. Additionally, there is 1

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unambiguous evidence of nematic liquid crystal-like “wagging” (strain-invariant, periodic oscillation) under steady shear flow, a phenomenon previously unreported for nanotube composite materials. Overall, the present findings suggest that AlSiNT/PVA dispersions possess promising rheological, optical, and electrospinning properties that are highly desirable for current nanotechnological applications, and may serve as an ideal model system for establishing structure-performance relationships for like nanotube/polymer composite materials.

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1. INTRODUCTION Imogolite nanotubes, a naturally formed non-toxic clay material with single-walled structure and high aspect ratio, represent a promising inorganic material for fabricating composite thin films and membranes in current green-energy and biotechnological applications.1-4 While the synthesis schemes5-7 and some intrinsic properties8-13 of aluminosilicate nanotubes (AlSiNTs, also known as synthetic imogolites) have been fairly revealed, their potential in making ideal nanotube/polymer composites has been demonstrated recently for applications with low-k materials,14 water desalination15 and nanofiltration,16 as well as scaffolds or templates for biological utility.1,4 When compared with their well-known counterpart, i.e., carbon nanotubes (CNTs), un-modified AlSiNTs generally exhibit better dispersion in aqueous solutions17 (especially under acidic conditions, i.e., pH G' at all PVA concentrations and for the

whole range of frequencies, where the typical terminal-zone behaviors (G' ∝ ω2 and G" ∝ ω1 ) for entangled polymer solutions can be clearly observed. On the other hand, the anomalous low-frequency responses, especially for G' at higher PVA concentrations, might be ascribed to the

presence of a small partition (< 1 wt%) of micrometer-sized aggregate clusters, as revealed by the DLS analysis in Figure 3a. In early work, similar aggregate species have been reported for PVA aqueous systems, and were believed to arise from strong intra- and interchain hydrogen bonds of PVA chains,36 especially during the aging process.37 Due to the smallest degree of aggregate formation, the typical viscoelastic responses for entangled polymer solutions, and the capability to produce nanofiber in electrospinning, the 10 wt% PVA solution is selected as the “matrix system” to harbor AlSiNT in the dispersion samples. With a good understanding of the structural and rheological properties of PVA solutions, we next proceed to the characterizations of the dispersion state of AlSiNT in aqueous solution as well as in the AlSiNT/PVA system. 12


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Figure 2. (a) Steady shear viscosity as a function of shear rate; (b) G′ (closed symbols) and G″ (open symbols) as a function of angular frequency for PVA solutions with four different concentrations.

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Figure 3. Angular dependences of the field autocorrelation function and the associated intensity-weighted (denoted as A(q,t)) decay time distributions extracted from CONTIN: (a) 10 wt% PVA solution, where the decay time t has been rescaled with q2. Note, in particular, that the second (long-time) mode represents the diffusional motion of micrometer-sized aggregate species. (b) 3 wt% AlSiNT dispersion, where the decay time t has been rescaled with q so that the long-time mode (indicated by the arrow) can be clearly seen to show superposition.

For anisotropic particles in gel38 and polymer matrix,39-41 DLS and DDLS may be used to reveal polymer-nanoparticle interactions and the resulting dispersion state. In Figure 3a, the 10 wt% PVA solution exhibits two distinct DLS relaxation modes, attributable to the translational diffusions of single chain and aggregate cluster, respectively. In contrast, the 3 wt% AlSiNT dispersion shows a dominant long-time relaxation that is q1-dependent in nature (see Figure 3b). This peculiar q-dependence might be ascribed to the effect of mutual confinement between different AlSiNT species when exceeding the overlapping concentration (or volume fraction), i.e., ν ≫ ν*.42,43 Given that the microstructures of colloidal rod suspensions were known to be determined, to a large extent, by the rod concentration and aspect ratio,44 it is instrumental to reveal the geometrical features of AlSiNT using combined DLS and DDLS analyses, as detailed below.

A dilute AlSiNT dispersion (0.01 wt%) was utilized for this purpose to minimize the effect of nanotube aggregation or overlapping. Figure 4a reveals the existence of two distinct diffusion modes for the VV relaxation function; similar behavior is found in Figure 4b for the 14


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VH relaxation function, with a slightly broader distribution for both modes. Accordingly, the translational and rotational diffusivities, DT and DR, may be determined by plotting the mean decay rate versus q2, wherein the slope gives the translational diffusivity and the intercept represents six times the rotational diffusivity (6DR), as shown in Figure 4c,d. Although the individual mode bears a similar slope in the VV and VH relaxation functions, suggestive of the same species being probed, only the fast mode leads to non-zero intercepts upon extrapolation to q = 0. Thus, the fast mode reflects the translational as well as the rotational motion of AlSiNT in both DLS and DDLS experiments. Purely diffusive in nature, the slow mode is suggested to reflect the collective motions of the transient network of AlSiNT (in near semidilute condition; see a later analysis). With the diffusion coefficients so determined, the following expressions may be adopted to simultaneously fit the full curves of g(1) VV(q, t) and g(1) VH(q, t):45 |g1 VV q,t| = Aexp−*+DT, fast q2 + 6DR,fast ,t- + 1 − Aexp .−*+DT, slow q2 ,t- / (7) α

β

and

|g1 VH q,t| = Aexp−*+DT, fast q2 + 6DR,fast ,t- + 1 − Aexp .−*+DT, slow q2 ,t- / (8) α

β

To determine the geometrical features of AlSiNT that are of primary interest here, the Broersma relations46 for DT and DR are utilized. For the translational diffusivity, one has

0T = kB T⁄3πηL[δ − 0.5γ∥ + γ2 ]

= ln2L⁄3

γ∥ = 0.807 + 0.15⁄δ + 13.5⁄δ2 − 37⁄δ3 + 22⁄δ4 15


(9)


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γ2 = −0.193 + 0.15⁄δ + 8.1⁄δ2 − 18⁄δ3 + 9⁄δ4 where L and b denote the nanotube length and diameter, respectively. For the rotational diffusivity, the formulas are 0R = 3kB T⁄πη43 [δ − ξ]

(10)

ξ = 0.807 + 0.15⁄δ + 13.5⁄δ2 − 37⁄δ3 + 22⁄δ4 Because of relatively weak signals associated with the VH relaxation function, the fitting is performed here mainly for the data on the VV relaxation function at a small scattering angle of 30°. The results and fitted parameters are shown in Figure 4a (the inset figure) and Table 1.

Figure 4. Angular dependences of the field autocorrelation function and the associated decay time distributions extracted from CONTIN for (a) DLS and (b) DDLS measurements on the 0.01 wt% AlSiNT dispersion, where the decay time t has been rescaled with q2. The inset in (a) demonstrates the fitted curve from eq 7 along with eqs 9 and10 at a scattering angle of 30°. The mean decay rates, 〈Γ〉, as a function of q2 retrieved from (c) VV and (d) VH relaxation functions, where the solid lines 16


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represent the results of a linear regression.

Table 1. Fitted Parameters Extracted from DLS Data on the 0.01 wt% AlSiNT dispersion. A 0.21

stretched exponent nanotube length nanotube diameter (α) (L) [nm] (b) [nm] 0.90

2000 ± 230

35 ± 4

1-A

stretched exponent (β)

0.78

1.00

The nanotube length, L, and diameter, b, are determined to be ~2000 ± 230 nm and ~35 ± 4 nm, respectively. Note that due to the uncertainties in the above fittings, the AlSiNT diameter so determined might be subjected to some error and, therefore, the possibility of some slight degree of nanotube aggregation cannot be precluded. Using the information above, the values of ν2 (~1/bL2, which marks the transition from semidilute to concentrated (isotropic) condition) and ν* ( ~4.2 ν2 ,47-49 which marks the transition from concentrated (isotropic) to liquid crystalline (anisotropic) state) for the present AlSiNT dispersions can be estimated to correspond to c2 ≅ 0.013 wt% and c* ≅ 0.06 wt% using the properties reported earlier by Hoshino et al.50 Accordingly, the 3 wt% AlSiNT dispersion under investigation bears the possibility of forming a nematic phase, as confirmed by the flow-birefringence measurements discussed later. Insightful information may be obtained by further comparing the DLS and DDLS results on representative PVA solution (10 wt%), AlSiNT (3 wt%) and PVA/AlSiNT (10:2.7) dispersions, as shown in Figure 5. In both experiments, the results for the PVA/AlSiNT dispersion fall somewhere in between, as might be expected; it is also evident (Figure 5b) that the PVA solution displays no discernible DDLS signal that would be indicative of dynamic anisotropy for isolated chains or 17



aggregates. An interesting feature is, however, that the DDLS curves for AlSiNT and PVA/AlSiNT dispersions are nearly identical. This feature is unexpected, because it would imply that the nanotubes display a substantially expedited relaxation in the PVA/AlSiNT dispersion when compared with that in the AlSiNT dispersion, considering a much larger viscosity of the PVA medium (~190 cP; cf. the measured viscosity is ~0.89 cP for the DI water). A possible explanation of this phenomenon is that good AlSiNT-PVA interactions lead to an enhanced “collective” movement of the nanotube with the PVA matrix, as supported by the analyses below.

Figure 5. Field autocorrelation functions retrieved from (a) DLS and (b) DDLS measurements at a representative (i.e., the smallest in this study) scattering angle of 30°. The “PVA+NT” denotes the 10:2.7 PVA/AlSiNT dispersion.

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3.2.

Rheological Features of AlSiNT/PVA Dispersions. In addition to the dispersion state

discussed above, the nanotube concentration was known to be among the factors determining the (nonlinear and linear) rheological properties of a nanotube/polymer dispesion.51 Figure 6 presents the steady shear viscosity for the AlSiNT/PVA dispersions with varying AlSiNT content. A salient feature is that the Newtonian plateau underlying the 10 wt% PVA solution is progressively replaced by a pronounced shear thinning with increased AlSiNT concentration. Similar phenomena have been observed for a variety of polymer solutions blended with nanofillers, such as carbon nanotubes,52-55 nanoparticles,56-58 and clays.59 As usually performed for systems that produce pronounced shear-thinning behavior, the so-called flow index and yield stress may be determined from the results shown in Figure 6 using the Herschel-Bulkley model as well as its modified form:59 σ=

σ0 + mγ% n (11) γ%

G∗ = G∗0 +

k γ ωl (12) γ0 0

In eq 11, σ denotes the shear stress, σ0 the yield stress, γ% the shear rate, m a constant, and n the flow index; in eq 12, G0∗ denotes the magnitude of complex modulus at the lowest frequency (ω = 0.1 rad⁄s in this study), γ0 the strain amplitude, k a constant, and l the flow index for small-amplitude oscillatory flows. The fitted parameters are listed in Table 2. Observing that the flow index for both models decreases (and the yield stress increases) notably with increased AlSiNT concentration, one might ascribe the prominent change in rheological features for the AlSiNT/PVA dispersions to good nanotube-polymer interactions.55,60 In fact, this central implication is consistent

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with the results of prior DLS/DDLS analyses, suggesting fair nanotube dispersion in the polymer matrix.

Figure 6. Steady shear viscosity (open symbols) and complex viscosity (solid lines) as a function of shear rate and angular frequency, respectively, for the 10 wt% PVA solution and the AlSiNT/PVA dispersions with varying AlSiNT content.

Table 2. Herschel-Bulkley Parameters for the AlSiNT/PVA Dispersions Investigated Methods Steady-shear

Parameters

oscillatory shear a

10:0.18 10:0.36 10:0.81 10:1.26 10:1.45 10:1.98

10:2.7

σ0a [Pa]

0.031

0.035

0.084

0.133

0.168

0.233

0.343

m

0.32

0.44

0.73

1.05

1.34

1.83

2.84

n

0.956

0.928

0.890

0.854

0.842

0.795

0.752

[Pa] k

0.033

0.049

0.087

0.147

0.189

0.233

0.500

0.28

0.39

0.57

0.69

0.99

1.00

1.54

l

0.986

0.903

0.895

0.866

0.842

0.821

0.774

measurement Small-amplitude

Samples

G*0

the yield stress, σ0, is taken from the shear stress at the lowest shear rate (0.1 s-1 ) due to the lack of the

Newtonian plateau as well as weak signals in the low-shear-rate region (< 0.1 s-1 ).

Another noteworthy rheological feature of the AlSiNT/PVA dispersions is concerned with the

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(excellent) applicability of the Cox-Merz rule,61 as can be seen from Figure 6 as well as from the results gathered in Table 2 (wherein similar values of σ0 and G*0 as well as n and l for each sample can be noticed), especially in the shear-thinning region at high shear rates. The Cox-Merz rule states that the steady shear viscosity, 7γ% , may be equated with the corresponding complex viscosity,

7 ∗ ω, as 7 γ% = |7∗ ω|ω8γ% , whose validity has been observed for a wide range of complex fluids including entangled polymer solutions,62,63 polymer melts,63-65 micellar solutions,66 and concentrated

suspensions (for an extended version of the Cox-Merz rule).67,68 For polymer/filler blends, however, prior studies have revealed numerous ways of deviations that show either 7 γ% < |7 ∗ ω|ω8γ% in the shear-thinning region due to the breakage of supramolecular structures51,57 and the flow-induced alignment of nanotubes/nanofibers,69,70 or 7γ% > |7 ∗ ω|ω8γ% at relatively low shear rates for yet unknown reasons.52,54 In Figure 6, and in a subsequent analysis employing flow birefringence, it seems clear that the pronounced shear-thinning observed with the AlSiNT/PVA dispersions arises primarily from the effect of flow-induced alignment of the nanotube species, and the direct contribution of PVA becomes insignificant in this case. Accordingly, the apparent validity of the Cox-Merz rule for the AlSiNT/PVA dispersions may be rationalized as follows: As discussed above, good AlSiNT-PVA interactions contribute to an excellent nanotube dispersion in the PVA matrix. Under this circumstance, the shear-thinning behavior that results principally from flow-induced nanotube alignment may be likened to that of entangled polymer chains, both contributing to the additional stress by way of “linear responses” (see, for example, discussions in prior work).62,71 Namely, the Cox-Merz rule is expected to hold true when nonlinear responses (such as those arising 21



from significant chain stretch for entangled polymers) and certain formation/destruction in microstructures are absent or play only minor roles. Of course, the above arguments should be left open to further assessments. When compared with the steady-shear viscosity, the (linear) dynamic modulus responses are often more versatile in providing insightful information on the peculiarities of a material system under investigation. Figure 7 presents the results from frequency sweep for the 10 wt% PVA solution and a series of AlSiNT/PVA dispersions. The frequency dependences of G' and G" clearly demonstrate the trend G" > G' for all samples and frequencies investigated, indicating fair fluidity that is crucial for the electrospinning applications discussed later. With increasing AlSiNT content, however, the scaling behavior of the dynamic modulus starts to deviate substantially from what underlies Maxwell-like fluids, an indication of the dominant effect of the nanotube species. In particular, a universal scaling law G' ~ G"~ ωn with n = 0.75 seems to be reached at the highest AlSiNT content in this study. This characteristic feature should be compared with what has been known for critical gels with G' ~ G"~ ωn (0 < n < 1) near the gel point (i.e., tanG" /G' ≅ 1).72 Although the general significance of this rheological trend of AlSiNT/PVA dispersions remains to be deciphered, the gel-like feature at high AlSiNT contents could be responsible for the failure noted in the electrospinning application.

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Figure 7. Dynamic moduli G' and G" as a function of angular frequency at a strain magnitude of γ = 0.1.

For nanotubes dispersed in a polymer melt matrix, the “percolation threshold” usually referred to can be clearly identified in rheological characterizations, when the physical network driven by strong particle-particle interactions is formed at above a critical nanotube concentration.48,55,73-76 On this occasion, most systems exhibit a marked transition from fluid-like to solid-like behavior (i.e., tanG" /G' 9 1),72 or the appearance of a G′ plateau at low frequencies.52,53 Herein, none of these features seem to be observable, possibly because the AlSiNT concentrations still fall below the percolation threshold. There could be other reasons, though. The variation of percolation threshold, in general, might be correlated with the dispersion quality,77 as evidenced by polymer/nanotube78 and 23



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microfibril79 systems which demonstrated de-agglomeration of fillers in the presence of specific polymer that shows good interactions with the functional groups on the outer surface of fillers. Especially, the formation of H-bonds, as has been revealed in imogolite-related systems,24,26,80 is likely to play an important role for the dispersion systems investigated. Shikinaka et al.,24 for instance, reported for an imogolite/maleic acid system the formation and effect of intermolecular H-bonding between imogolite nanotubes. Kang et al.26 observed that the crystallinity of the PVA matrix reduces with the addition of AlSiNT, possibly due to the formation of H-bonds between PVA and AlSiNT. Thus, it is curious whether the excellent dispersion of AlSiNT in the PVA matrix is aided by intermolecular H-bonding between polymer and nanotube species, and, in particular, if similar mechanisms might be responsible for a general delay in AlSiNT percolation that notably relies on strong nanotube interactions.

3.3.

Flow-Birefringence

Features

of

AlSiNT/PVA

Dispersions.

Rheo-optical

characterizations are especially instrumental to connect rheological responses with the underlying structures induced by flow.30,49,81,82 Figure 8 presents the transient viscosity growth and the in-situ birefringence over a range of shear rates for the 10:0.9 PVA/AlSiNT dispersion. The synchronization of the two quantities is evident. As discussed earlier, the rheological responses in this (shear-thinning) regime are primarily contributed by the flow-induced alignment of AlSiNT. Thus, the flow birefringence may also be attributed to a similar effect, as found previously for imogolite hydrogels.83,84 Still, to discern the sole effect of PVA matrix,85,86 the measurements for the 10 and 20 24


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wt% PVA solutions have also been carried out, indicating little or no birefringence signals up to a shear rate as high as ~800 1/s.

Figure 8. (a) Transient viscosity growth and (b) in situ birefringence during start-up of steady shear flow at various shear rates for the 10:0.9 PVA/AlSiNT dispersion; the symbol Θ denotes the magnitude of retardation angle, |∆δ|, divided by the imposed shear rate, i.e., |∆δ|/γ% .

A closer examination of the signals of flow birefringence and orientation angle reveals interesting new insight into the AlSiNT/PVA dispersions investigated, as shown in Figure 9a,b for two representative shear rates. Both quantities display periodic oscillations under steady shear flow. Figure 9c presents the results of a detailed Fourier analysis for the flow-birefringence signals over a range of shear rates. By identifying the characteristic frequency—or the reciprocal periodic time 25



(T)—and plotting it against the shear rate, as shown in the inset, an inverse proportionality between the two is confirmed, as found in early rheo-optical observations on some nanorod suspensions87-89 as well as in computer simulations for the so-called “wagging” phenomenon.90,91 To our knowledge, however, no prior studies have reported on a similar feature for nanotube/polymer composite systems. The results clearly suggest that the overlapping nanotubes of AlSiNT in the PVA matrix are capable of forming nematic liquid crystal-like phase, without the aid of external electrical field which has previously been utilized to bring in a similar phase for dilute imogolite dispersions.12 Despite the fact that the role played by the PVA matrix remains unclear, the consequently enlarged background viscosity can be expected to greatly facilitate the revelation of such phenomenon for AlSiNT at relatively low shear rates that become accessible in standard rheometry system (cf. pure AlSiNT dispersions of a similar concentration display no discernible flow birefringence). In short, the excellent optical transparency associated with the AlSiNT/PVA dispersions along with the high aspect ratio of the presently synthesized AlSiNT (~60) has rendered it possible to systematically reveal the static (geometry and dispersion state) and dynamic (translational/rotational diffusions, rheology, and flow-induced alignment/wagging) features that are, however, practically inaccessible for many of the counterpart (opaque) nanotube/polymer dispersions studied in early research.

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Figure 9. Time-dependent signals of flow birefringence and orientation angle under steady shear flow at shear rates of (a) 5 and (b) 20 1/s for the 10:0.9 PVA/AlSiNT dispersion. (c) Fourier analysis of flow-birefringence signals for a range of shear rates 5–100 s-1, where the characteristic frequency at each individual shear rate has been extracted and shown as the peak (indicated by the arrow); the inset shows the inversely proportional relationship between the periodic time and shear rate.

3.4. Electrospinnability and Fiber Morphology of PVA Solutions and AlSiNT/PVA Dispersions. As a practical assessment of the impact of AlSiNT dispersed in the PVA matrix as well as the substantially promoted elasticity for the resulting dispersions, the results of electrospinning for PVA solutions and AlSiNT/PVA dispersions are compared. Figure 10 presents the nanofiber morphologies corresponding to the bead-on-string structure, defect-free fiber, and flat fiber, as produced from 10, 15, and 17 wt% PVA solutions, respectively. An existing (semi-empirical) theory predicts, on the basis of the average number of entanglements in solution (ne), that fiber formation 27



initiates at ne ≅2 and bead-free fibers may be produced with ne : 3.5.92 We note that the values of ne for the present PVA solutions fall in the range of 2.1–3.6. Thus, there is a good agreement between theory and the present data. Another theory utilized the Berry number, c[η] (c being the polymer concentration and [η] the intrinsic viscosity), to predict the fiber morphology, and suggested three major regimes of the Berry number that produce bead-on-string structure (c[η] < 5), circular fibers (5 < c[η] < 9), and flat fibers (c[η] > 9) on PVA aqueous solutions.93 The Berry numbers of the present PVA solutions, as estimated with the scaling law of [η] given by Tacx et al.,94 fall in the range of 9–17, in good agreement with the predictions above.

Figure 10. SEM images of electrospun nanofibers produced from (a) 10, (b) 15, and (c) 17 wt% PVA solutions (scale bar = 5 µm).

Figure 11 shows the SEM morphologies of electrospun nanofibers produced from four representative AlSiNT/PVA dispersions. Notably, these dispersion samples basically produce uniform and defect-free fibers, although a close inspection reveals a trace amount of bead-on-string structure for the 10:0.18 sample (see the location guided by the arrow in Figure 11a), which bears the 28


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lowest AlSiNT content of the four. The results clearly suggest that the additional elasticity arising from good nanotube-polymer interactions helps suppress the jet break-up phenomenon during electrospinning process. Moreover, a certain degree of aggregation frequently observed with electrospun CNT nanofibers at high nanotube concentrations (~ 2 wt%)76,95,96 is not seen here, confirming the excellent dispersion of AlSiNT as inferred in previous discussions. Finally, the nanofibers fabricated from AlSiNT/PVA dispersions possess the optical transparency as for the pristine solutions, which may find important applications in optics-oriented fields.

Figure 11. SEM morphologies of electrospun nanofibers produced from (a) 10:0.18, (b) 10:0.36, (c) 10:1.45, and (d) 10:1.67 PVA/AlSiNT dispersions (scale bar = 1 µm). The mean diameters for nanofibers are estimated to be 148 ± 41, 146 ± 24, 158 ± 28, and 194 ± 32 nm, respectively. 29



We note, however, that electrospinning fails to produce nanofibers for the AlSiNT/PVA dispersions with AlSiNT content greater than about 2 wt%, despite the fact that the loss modulus G″ in these cases remains notably above the elastic modulus G′ (see, for example, Figure 7) indicating fair fluidity for these dispersion samples. Importantly, these observations suggest that there might exist certain upper limits in fluid elasticity—and not fluid viscosity, which was often considered as the major criterion for electrospinning—as evidenced by the fair electrospinnability of PVA solutions with similar and even higher viscosities.97,98 Possible roles of fluid elasticity in electrospinning applications with pure polymer97,99 and polymer blend100 solutions have been scrutinized in early work; no similar assessments seemed to exist, however, for nanotube/polymer dispersions. As the first attempt, the effects of fluid elasticity on the electrospinnability of AlSiNT/PVA dispersions are examined by capitalizing on the following two “elastic factors” (fe): G′/G″ and G′/G*, which characterize the importance of fluid elasticity with respect to the fluid viscosity or the total modulus. The results presented as a function of the AlSiNT content can be found in Figure 12. Interestingly, both factors exhibit a similar linear relationship with the AlSiNT concentration, as the Doi-Edwards model has predicted for the relationship between elastic modulus and rod concentration for overlapping rod systems (i.e., η∗ ω = 3νkB Tτr ⁄51 + iωτr , τr =1⁄6DR ) being the rotational relaxation time).42 Accordingly, the lower and upper bonds of fe within which defect-free nanofibers may be produced are determined, as indicated by the two horizontal dashed lines in the same figure. The results are of interest to be compared with future experimental data on similar composite materials. 30


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Figure 12. Elastic factors as a function of the AlSiNT concentration for the AlSiNT/PVA dispersions investigated herein. The shaded area represents the parameter range within which defect-free fibers may be produced.

4. CONCLUSIONS Synthesized single-walled (and un-modified) AlSiNT was shown to exhibit excellent dispersion properties in a pristine aqueous PVA solution (e.g., without salt or acid addition), fostering critical-gel and liquid crystal-like features along with pronounced fluid elasticity that are particularly favorable for a potential range of nanotechnological applications making use of nanotube/polymer composites. As a practical assessment, the results of electrospinning for PVA solutions and AlSiNT/PVA dispersions were compared, with the latter observed to produce uniform and defect-free electrospun nanofibers within a moderate range of elastic factors—proposed herein to help quantify the effect of fluid elasticity in the present study as well as in future applications with electrospinning. More generally, we demonstrated through detailed analyses that the optical transparency of AlSiNT/PVA dispersions greatly facilitates a systematic exploration of the static (geometry and 31



dispersion state) and dynamic (translational/rotational diffusions, rheology, and flow-induced alignment/wagging) features that are practically inaccessible for many of the counterpart (opaque) nanotube/polymer dispersions studied in early research. Therefore, the AlSiNT/PVA dispersions may serve as an ideal model system to help establish the structure-performance relationships of scientific and technological interest for like nanotube/polymer dispersions.

32


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AUTHOR INFORMATION Corresponding Authors *

Email: [email protected] (D.Y.K.); [email protected] (C.C.H.)

Notes The authors declare no competing ﬁnancial interest.

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

MOST

104-2628-E-002-009-MY3,

105-2221-E-002-056-MY2).

33


and

MOST


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For Table of Contents Graphic use only

Properties of Single-Walled Aluminosilicate Nanotube/Poly(vinyl alcohol) Aqueous Dispersions

Chien-You Su,a An-Chih Yang,b Jung-Shiun Jiang,a Zhi-Huei Yang,b Yan-Shu Huang,b Dun-Yen Kang*b and Chi-Chung Hua*a a

Department of Chemical Engineering, National Chung Cheng University, Chiayi 62102, Taiwan. b

Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.

TOC Graphic

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Properties of Single-Walled Aluminosilicate ... - ACS Publications

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