Reversible Structure Formation of Aluminum Trihydroxide (ATH

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Reversible Structure Formation of Aluminum Trihydroxide (ATH) Dispersions in Polydimetlylsiloxane (PDMS) Christopher J. Cox, Brentley Hovey, Timothy D. Fornes, and Saad A Khan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03577 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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Reversible Structure Formation of Aluminum Trihydroxide (ATH) Dispersions in Polydimetlylsiloxane (PDMS) Christopher J. Cox*#, Brentley Hovey*#, Timothy D. Fornes# and Saad A. Khan*1 *Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695-7905 #Lord Corporation, 111 Lord Dr, Cary, NC 27511 ABSTRACT ATH/PDMS systems are often used as potting compounds in electronic assemblies to guard the electronics from shock, vibration, corrosive agents and moisture. In this study, we use dynamic rheology and confocal/optical microscopy to understand the dramatic effects miniscule levels of water have on the microstructure and corresponding rheological behavior of PDMS filled with ATH. In the absence of water, PDMS containing 20 wt.% ATH readily flows, exhibiting viscoelastic behavior with some weak particle flocculation. However, the addition of only 0.045 wt.% water to the system results in the formation of a sample-spanning, self-supporting physical gel that exhibits an elastic modulus (G′) five orders of magnitude higher than the water-free system. A structure formation mechanism consisting of hydration layer formation followed by inter-particle water bridging has been proposed to explain the observed behavior. Recovery of the original viscoelastic fluid is demonstrated by adding molecular sieves or zeolites to the fully flocculated system. The recovery can likely be attributed to the adsorption of water by the sieves and the corresponding breakup of water bridges between the ATH particles. Based on the proposed mechanism, a variety of other polar and non-polar solvents have been found to induce physical gelation in ATH/PDMS dispersions with gel modulus being related to the Hildebrand solubility parameter mismatch between the solvent and PDMS fluid.

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Corresponding author: [email protected]; 919-515-4519

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INTRODUCTION Filled polymer systems are commonly employed as potting compounds within electronic assemblies to greatly improve the efficiency and reliability of electronic devices by protecting the components from stresses, vibration and external chemical and moisture intrusion.1-4 In several applications, a combination of thermal conductivity, heat stability, and low flammability is also required. These property attributes makes polydimethylsiloxane resins filled with ATH particles highly desirable for use in such applications. Alumina trihydroxide (gibbsite, alumina trihydrate, ATH) is unique in its ability to conduct heat as well as impart flame retardant properties to potting compounds because of its strongly endothermic (470 cal/g) decomposition to alumina and water when heated to approximately 180°C under atmospheric conditions.5-9 Despite ubiquitous use of ATH-filled silicones in potting compounds, significant concerns exist regarding its high sensitivity to moisture post-manufacturing. Specifically, small levels of water can turn a compound that has fluid-like consistency into a paste-like material over time. Gibbsite (ATH) occurs naturally and is one of the three minerals that make up bauxite, an aluminum ore that serves as a major source of aluminum.10,11 Commercially, aluminum trihydroxide is manufactured as an intermediate in the extraction of aluminum from bauxite via the Bayer process. The Bayer process is essentially a three step process consisting of extraction, precipitation, and calcination. Aluminum minerals from bauxite are first selectively solubilized in a strong caustic solution before being recovered as aluminum trihydrate crystals in a precipitation step and finally calcined at elevated temperatures.11-13 Aluminum trihydroxide is obtained directly during precipitation and has tabular habit with monoclinic crystal structure when nucleated from sodium aluminate solution.14 The crystal structure consists of layers of aluminum atoms, each coordinated by six oxygen atoms to form a distorted octahedron. The


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octahedron share edges and form layers of pseudohexagonal rings that are hydrogen bonded together.10,15 Oxygen coordination results in highly functional crystal surfaces containing ~12 OH groups per nm2 in the basal plane and ~4 OH groups per nm2 in the edge plane.16,17 The polar interactions existing between hydrophilic particles dispersed in a hydrophobic media are known to lead to attractions among the dispersed phase.18 In the presence of these interactions, small amounts of water can preferentially absorb to the dispersed phase and link surface active molecules, such as sugar in oil, leading to flocculation.19 Similar rapid aggregation of hydrophilic dispersed phases in hydrophilic media have been reported for suspensions of magnesium hydroxide near the isoelectric point and for aluminum oxide suspensions in the presence of sulfate ions.20,21 Though water-induced flocculation of aluminum oxide has been reported, the effect of water on the network structure of aluminum trihydroxide in non-polar polymer matrices remains to be explored.22 Specifically, studies to elucidate the effect of water on the rheological properties of commercially significant ATH dispersions are necessary to understand manufacturability and processability. The purpose of this work is to better understand how water influences the rheological behavior of a model ATH-silicone compound. Dynamic oscillatory rheology is used to characterize the linear and non-linear viscoelastic behavior of the compound with varying levels of water post-manufacture. Resulting data in conjunction with microscopic visualization provides insight into the mechanism of microstructural rearrangement responsible for the shift from readily flowable to paste-like consistency upon exposure of the materials to moisture. The effects of resin chemistry, ATH surface modification, and solvent solubility on rheological response are also explored in support of the proposed mechanism for microstructural


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rearrangement. The use of molecular sieves to scavenge water and disrupt the undesirable internal structure formed post-manufacture has also been investigated. EXPERIMENTAL SECTION Materials. Silicone fluid, trimethylsiloxy terminated polydimethylsiloxane (DMS T-21, 100 cSt, MW = 5,970 g/mol) was purchased from Gelest, Inc (Morrisville, PA) and was used as received. Untreated aluminum trihydroxide particles, Apyral 60D (d50 = 1.0 µm, surface area = 6 m2/g,   2.4 g/cm3) and SB-432 (d50 = 9.0 µm, surface area = 2 m2/g,   2.4 g/cm3), were obtained from Nabaltec AG (Schwandorf, Germany) and Huber Engineered Materials (Atlanta, GA). Alkyl surface treated aluminum trihydroxide, Hymod SB-432 SH2, was also obtained from Huber Engineered Materials (Atlanta, GA). Sodium aluminosilicate of the zeolite A type, UOP T Powder, was purchased from A.B. Colby, Inc (McMurray, PA). Prior to preparing dispersions, all aluminum trihydroxide particles and sodium aluminosilicate were dried at 110°C for at least 24 hours. Sample Preparation. Polymer dispersions (60 grams) were prepared by mixing different concentrations of dried ATH (i.e., 0, 5, 10, 15 or 20 Vol%) in PDMS fluid at 3500 rpm for 4 minutes using a Hauschild DAC-150LV speed mixer (Flacktek Inc, Landrum, SC). Samples were immediately stored under vacuum (32 in Hg) to prevent absorption of atmospheric moisture. The vacuum chamber was purged with nitrogen prior to use. Prior to measuring, “wet samples” were prepared by adding water to 20 grams of the dry samples using a micropipette to give a total water content of between 0 and 0.314 wt.%. These samples were mixed at 2500 rpm for 4 minutes using a Hauschild DAC-150LV and subsequently capped with nitrogen. Rheological measurements were taken immediately after mixing to minimize exposure to atmospheric humidity. Time-dependent dynamic rheological experiments conducted on various


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samples (Figure S1) under a nitrogen environment showed them to remain unchanged for an extended period, suggesting that the samples were at an equilibrated state. In addition, visualization experiments showed no change for three days. Samples containing other hydrogen bonding solvents were also prepared as described above. UOP T Powder molecular sieves were incorporated in selected samples between 0 and 6.5 wt.% by again mixing in a Hauschild DAC150LV speed mixer at 2500 rpm for nominally 4 minutes. Experimental Measurements. An ARES-G2 strain controlled rheometer (TA instruments) with 25 and 40 mm disposable parallel plate geometries was used to conduct dynamic rheological experiments. All measurements were made at room temperature (25°C). Parallel plate geometries were covered with 3M Hookit Gold sandpaper, 80 grit, to prevent wall slip. Previous work has shown that sand paper is an effective alternative to serrated plates for eliminating wall slip in filled systems.23,24 All samples were subject to a 2 minute equilibration step before measuring rheological properties. Dynamic strain and frequency sweeps were conducted on each sample using a 0.5 mm gap. All frequency-dependent experiments were performed within the linear viscoelastic regime for each sample as determined from strain sweep measurements. To ensure reproducibility, measurements were repeated for each sample and found to be within 10%. An upright laser scanning confocal microscope (Leica TCS SP5 continuous wave lasers, Leica, Germany) was used to visualize the interaction of water with the ATH particle surfaces. Dehydrated ATH/PDMS samples were prepared as previously described with an ATH loading of 5 vol.%. Water, containing the fluorescent marker pyranine, was subsequently added to these samples at a concentration of 0.153 wt.%. To prepare the water/pyranine solution, 0.05g of pyranine was added to 2 mL of water and dissolved using a vortex mixer. Pyranine was chosen


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as the fluorescent marker due to its hydrophilic nature and high water solubility (300 g/L at 25C) meaning that it would remain with the water phase as opposed to diffusing into the PDMS matrix. The samples were then mounted on a glass microscope slide, covered with a glass cover slide, and sealed with a UV cure acrylic. The samples were sequentially scanned at 510 nm and 633 nm with two photomultiplier tube detectors defined at 500 - 550 nm (for probing water molecule locations) and 600 - 650 nm (for examining ATH locations) to collect the emitted fluorescent signal. Images of each scan, collected on separate channels, were examined along with an overlay of the channels to determine the distribution of water in reference to the ATH location in the system. The images also provided microstructural characteristics of the gelled network. RESULTS & DISCUSSION Effect of water addition. We begin by examining the frequency spectrum of the elastic (G′) and viscous (G″) moduli of a representative sample, 20 vol.% ATH/PDMS, containing different amounts of water to elucidate the nature of the microstructure formed. Figure 1 shows the behavior of this sample containing 0, 0.045 and 0.153 wt.% water. For the sample with no water (Figure 1a), we observe frequency dependent G′ and G″ with G′ higher than G″ at low frequencies and G″ dominating G′ at high frequencies. The shape of the G′, G″ curves and their crossover observed for the ATH/PDMS dispersion is characteristic of a “somewhat” flocculated or weak gel structure as observed in other colloidal systems25,26 or during chemical crosslinking of polymers.27,28 In the latter case, such a feature is observed for samples that are past their gel point but not a fully crosslinked network. For our sample, since there is no water present, we attribute the aggregation behavior to be likely occurring via filler-


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filler interactions as opposed to water-filler interactions. More discussion of mechanism is provided later in the study.

102

(a)

G’’

101

G’ 100

ɸ = 0.20, No water present

10-15 10

G' and G'' [Pa]

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(b) G’

104

103

G’’ ɸ = 0.20, [H2 O] = 0.045 wt.%

1026

(c)

G’

105

G’’ 104

103 10-1

ɸ = 0.20, [H2 O] = 0.153 wt.% 100

101

102

Frequency [rad/s]

Figure 1. Elastic (G′) and viscous (G″) moduli as a function of frequency for ATH/PDMS dispersion (20 vol.%) containing (a) 0.00, (b) 0.045 and (c) 0.153 wt.% water. As the water loading is increased to an intermediate concentration (0.045 wt.%), a large change in the linear viscoelastic response is observed, with G′ almost one order of magnitude higher than G″ over the entire range of frequencies explored. Both G′ and G″ are independent of


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frequency which is indicative of a sample-spanning network structure.29-31 The strength of the gel, which is reflected in the value of G′25,32, is found to be significantly higher than that observed with no water present with a four orders of magnitude increase in G′. As the water content is further increased to a concentration of 0.153 wt.% (Figure 1c), G′ increases but to a lesser degree (approximately one order of magnitude from its previous value). The response of G′ and G″ at intermediate and high water concentration is found to resemble that of other threedimensional flocculated systems (silica, clays, carbon black, etc.)33-35 and confirms that water plays a pivotal role in the formation of such a well-defined microstructure. Critical Water Concentration. In order to determine the critical water concentration that marks the onset of weak to strong gel transition,36 we conducted a series of strain amplitude sweeps at a constant frequency (1 Hz) for ATH/PDMS dispersions at various ATH loadings as a function of water content.

Figure 2 shows G′ from the LVE regime plotted as a function of water

concentration for various ATH loadings while supplementary Figure S2 shows strains sweeps for a representative 20 vol.% ATH/PDMS sample at various water concentrations. Focusing on this particular sample (Figure 2, Figure S2), we find that as we incrementally increase the water content from 0.000 to 0.036 wt.,%, the gel modulus of the dispersion remains low (1-10 Pa); however, between the water concentrations of 0.036 and 0.045 wt.%, G′ increases dramatically by approximately 4 orders of magnitude. Interestingly, beyond a concentration of 0.045 wt.% water, G′ increases more slowly with increasing water loading with an eventual plateau reached at concentrations above 0.153 wt.%. The drastic increase in G′ beyond 0.04 wt.% water is believed to be the critical water concentration at which a well-defined internal microstructure is formed. Above this concentration, the plateau in G′ implies a fortification of the internal microstructure.37


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107

ɸ = 0.20

106

105

ɸ = 0.15

104

ɸ = 0.10

103

102

ɸ = 0.05

102

10

1

~ɸ-1.4 101

100

100

ɸ = 0.00

10-1

10-2 0.00

H2O / nm2

G' [Pa]

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0.05

0.10

0.15

101

ATH Conc. [vol.%]

0.20

0.25

0.30

H2O Concentration [wt.%]

Figure 2. Evolution of elastic modulus within the region of linear viscoelasticity, as determined from strain amplitude sweeps, as a function of water concentration for various ATH volume fractions (φ = 0.00, 0.05, 0.10, 0.15 and 0.20). The inset illustrates the power law dependence of the water coverage per unit of ATH surface area required to induce the weak-strong gel transition as a function of ATH loading. An examination of the results for all loadings of ATH (0, 5, 10, 15 and 20 vol. %) shown in Figure 2, reveal several notable features First, in the absence of ATH, the addition of minute quantities of water has little to no impact on G′ thus indicating the previously mentioned effects are indeed attributable to water-filler interactions. Second, for ATH loadings of 5, 10, 15, and 20 vol.%, at least a three order of magnitude increase in G′ is observed with increased water addition as compared to the anhydrous state. Third, the critical water concentration decreases with increasing ATH loading. Interestingly, this critical water concentration (plotted in terms the amount of water per unit surface area of filler assuming spherical filler with a diameter of one


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micron), reveals a negative power-law dependence with ATH content (Figure 2 inset). Above the critical concentration, the gel strength as indicated by G′ is found to increase with increasing filler loading. The increase in gel strength is attributed to the formation of a more extensive network structure at higher ATH volume fractions. Finally, the G′ behaves as a logistic function (S-curve) with respect to water concentration at all ATH loadings investigated above 0 vol.%. From the S-curve, three distinct regions of dispersion behavior are identified: a lower plateau, a transition region, and an upper plateau. The lower plateau corresponds to the previously identified weak gel state and tracks the elastic response of the material before the development of a well-defined internal microstructure. The lower plateau is followed by the transition region where sample-spanning microstructure rapidly develops as indicated by the step growth in G′. Finally, the upper plateau corresponds to the previously mentioned strong gel state in which the internal network structure has fully developed. The frequency dependence of tan δ can also lend insight into changes in microstructure as shown in Figure S3. At low water concentration, the material displays a viscous response over most of the range of frequencies explored as indicated by tan δ > 1; however, as the water content approaches the critical water concentration, elastic response (tan δ < 1) becomes more dominate at low frequencies.. Above the critical water concentration, elastic behavior dominates over the entire frequency range explored and tan δ < 1 which is characteristic of gel-like materials.38,39 The critical water concentration identified by the elastic transition seen in Figure S3 is in good agreement with that observed in Figure 2. Network Structure Characterization. While it is clear that the addition of moisture leads to an internal structural rearrangement in ATH/PMDS dispersions, more discussion is warranted regarding the nature of the resultant microstructure. Figure S4 shows a plot of G′ as a function of


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ATH volume fraction above the critical water concentration. A power law dependency, n  4.6, of G′ on volume fraction is observed which is similar to prior reports on fumed silica and clays dispersions. This scaling behavior suggests that interconnected flocs are responsible for gelation of ATH/PDMS dispersions in the presence of water.40-42 Interestingly, the n value observed here is similar to those found for other strong-linked flocculated systems such as nanodiamonds and bohemite alumina.43,44 Yield stress for the present system is another means of assessing the nature of internal microstructure. Walls et al. have shown that the elastic stress (G′γ), when plotted as a function of strain, can be used for yield stress determination in colloidal dispersions. Using this approach, the yield stress corresponds to the maximum in elastic stress.45,46 In Fig 3(a), the dependence of the elastic stress on percent strain for various ATH loadings is shown. From the plot, we find that the maximum in elastic stress shifts to lower percent strains as the ATH loading is increased at constant water concentration (0.314 wt.%). We also observe an increase in the yield stress with increasing ATH loadings. Plots of yield stress as well as yield strain as a function of ATH concentration are shown in Fig 3(b).


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104

(a)

Elastic Stress [Pa]

103

102

[ATH] 101

           

100

10-1 0.01

0.1

1

10

100

Strain [%] 102

104

101

103

100

102

10-1

10

Yield Stress [Pa]

(b) Yield Strain [%]

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101

ATH Concentration [vol.%]

Figure 3. (a) Elastic stress (G′γ) plotted as a function of percent strain for various ATH loadings. Data obtained from dynamic oscillatory strain sweeps at 0.314 wt.% water. (b) Dependence of yield stress and strain, as determined from the maximum in elastic stress, as a function of ATH loading. A power-law dependence for both yield stress and strain is observed with a strong, steady increase and decrease occurring in yield stress and strain, respectively, with increasing ATH loading. A positive power law index of n  3 is observed for the yield stress which is similar in magnitude, though slightly lower, than that found for G′. A negative power law index of n  -2


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is observed for the yield strain. The observed sign and magnitude of the power law indices for yield stress and yield strain on ATH loading is in good agreement with behavior encountered for other gel networks consisting of sample-spanning flocs.44,47,48 Moreover, Shih et al43 have shown that the power law dependence of the yield strain can be used to classify flocculated systems as either strong- or weak-linked. Strong-linked flocculated networks are those for which the breaking of bonds occurs first within flocs during structural breakdown while weak-linked flocculated networks are those for which breakdown will first occur between flocs. For the case of ATH/PDMS dispersions above the critical water concentration, strongly-linked networks are present as indicated by the negative power law dependence of yield strain with respect to filler loading. Figure S5 shows the elastic stress versus strain plots as a function of water concentration for a sample containing 10 vol.% ATH. We observe the appearance of a yield stress above 0.045 wt.% water. The need for a critical water content to induce yield stress is consistent with visual observation which reveals a transition from a readily-flowable sample in the absence of water to a paste-like consistency with increasing water concentration. The inset in Figure S4 shows that at higher water concentrations, the yield stress increases significantly, but ultimately plateaus, analogous to what is seen in G′ as a function of water content (Figure 2). Structure Formation Mechanism. Figure 4 provides an illustration of the proposed mechanism of structural rearrangement that helps explain the observed viscoelastic behavior of ATH/PDMS dispersions with varying levels of water. In the initial state (Figure 4, left), where there is no water, van der Waals forces and hydrogen bonding are the primary forces impacting particle arrangement within the PDMS matrix. The van der Waals forces, consisting of Keesom interactions, Debye forces, and London dispersion forces, may be better represented through


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consideration of the Hamaker constants of the two component dispersion where the difference in Hamaker constants can be used to estimate the net van der Waals forces present (i.e., ΔA = ((APDMS)1/2 - (AATH)1/2)2 ).49,50 In the case of ATH and PDMS (AATH ~ 9.9x10-20 and APDMS ~ 4.4x10-20), attractive Van der Waals forces are found to dominate (A > 0) which implies that the ATH particles will have a tendency to agglomerate within the PDMS matrix.51-55 Additionally, given that the aluminum trihydroxide surface functionality is primarily comprised of hydroxyl groups and the low hydrogen bonding ability of the PDMS matrix (Ԑr ~ 2.3),56 one would not expect the matrix fluid to interact with the ATH particles to form a stable dispersion. As such, ATH particles are expected to interact directly with one another via hydrogen bonding between hydroxyl groups on neighboring surfaces to form isolated, network-spanning agglomerates. Such an initial microstructure results in a weak gel that is easily disrupted as indicated by the absence of any measurable yield stress. Initial State

Aluminum Trihydroxide (ATH) Polydimethylsiloxane (PDMS)

Aluminum Trihydroxide (ATH)

= 15.1 = 47.8 Intermediate State

Aluminum Trihydroxide (ATH)

Increasing [H2O]

Water

Polydimethylsiloxane (PDMS)

Final State

Aluminum Trihydroxide (ATH) Water

Water

Figure 4. Schematic representation of the proposed mechanism for internal microstructural rearrangement observed in ATH / PDMS dispersions upon addition of water. The addition of small levels of water to the ATH/PDMS system results in a new intermediate state of the internal microstructure consisting of partially-bridged ATH


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agglomerates (see Figure 4, middle). In this state, water molecules will migrate to the surface of the ATH particles upon mixing. The preferential migration of water to the ATH particle surfaces is to be expected due to its large hydrogen bonding ability with the polar ATH compared to the non-polar PDMS matrix. Interestingly, the polar and hydrogen bonding contributions to the Hildebrand solubility parameter for PDMS (δP PDMS ~ 1.9, δH PDMS ~ 8.0) and water (δP H2O ~ 16.0, δP H2O ~ 42.0), provide quantitative evidence of immiscibility between the two fluids and thus a strong driving force for preferential water-ATH and water-water interactions.57,58 As the water concentration within the system is incrementally increased, hydrogen bonding of water molecules to the polar surface of the ATH particles and to themselves is believed to form a water rich, hydration layer at the ATH particle/PDMS matrix interface. The hydration layers initiate bridging between ATH agglomerates present in the initial, water-free state, thus creating flocs of agglomerates.59 Lastly, we will consider the final state of the internal microstructure above the critical water concentration. At and above the critical water concentration, overlap of neighboring hydration layers is believed to lead to inter-floc bridging which gives rise to a sample-spanning 3D network structure (see Figure 4, right). The strength of the 3D network is a result of the strong hydrogen bonding ability of water, δH H2O = 42, across the aggregated system. This behavior is in accord with previous reports on the effects of water in alumina and aluminum hydroxide in alcohol dispersions.60 Similarly, Kandori et al.61,62 have reported water induced flocculation for iron oxide dispersion in 2-butanone and cyclohexane while Malbrel and Samosundaran22 have reported water induced flocculation for suspensions of alumina in cyclohexane. The schematic explanation of the formation of a network upon addition of water is borne out by direct visualization of the microstructure using optical and confocal microscopy


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respectively (Figures S6 and S7). For instance, digital optical microscopy on a low ATH content (0.5 vol.%) sample (Figure S6) shows initial dispersion of ATH that forms aggregates and finally networks upon addition of water. For a higher content ATH (5 vol.%) sample (Figure S7), we observe presence of slight agglomeration initially in the absence of water; this microstructure transforms into an interconnected network upon water addition, reminiscent of the depiction in Figure 4 (left and right).

Figure 5: Confocal scanned images of an ATH/PDMS sample containing a small amount of water (0.153 wt.%) with pyranine die dissolved in it(scale bar corresponds to 10 m). The ATH structure is captured by scanning in the wavelengths of Channel 1, the location of the dye in the water is captured by its fluorescence in the wavelengths of Channel 2. The overlay of the two channels shows no green fluorescence (i.e., no water present) in the matrix. Further, the green fluorescence of the water-soluble die seen in Channel 2 suggests water present only on the ATH surface. We have also attempted to map out the location of the water molecules in reference to the ATH network using direct visualization. Figure 5 shows confocal images of a representative ATH/PDMS containing 5% ATH and 0.153% wt. water. As described in the experimental section, the water contains a highly hydrophilic fluorescent marker pyranine that fluoresces in the 500-500 nm range. From the above images, we clearly see that the water has migrated to the ATH/PDMS interface and possibly hydrogen bonded to the particle surface. The water, containing dissolved pyranine, is indicated by the green fluorescence in the above images. The


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ATH particles are identified as red via reflection of the 633 nm laser line. By overlaying the two channels, we can visualize the water in relation to our ATH particles (yellow overlays areas). We observe presence of water at the particle surface, and not within the PDMS matrix, upon addition to the system. The flocculated network structure, resulting from water bridges formed between neighboring particles, is also observed. Mechanistic Support. Given the preferential migration of water to the hydrophilic surface of ATH particles, it was of interest to investigate the effect of the matrix resin and filler surface polarity on viscoelastic behavior. Figure 6 shows the effect of replacing non-polar PDMS with polar diglycidal ether of bisphenol A (DGEBA) on G′ and G″. Interestingly, in DGEBA a flocculated network structure is no longer observed. It is important to note the concentration of water was maintained well above the critical water concentration observed in PDMS. Unlike with PDMS, G′ and G″ for the DGEBA-based system depend strongly on frequency with G″ dominating G′ for the entire frequency range investigated. Such behavior is characteristic of a non-flocculated dispersion comprised of discrete units and correspondingly implies the absence of any internal structure.63


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G' and G'' [Pa]

107

(a)

PDMS Matrix G’

106

105

G’’ 104

1043 10 103

G' and G'' [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

(b)

DGEBA Matrix

Frequency [rad/s]

G’’

102 101

G’

100 10-1 10-2 10-1

ɸ = 0.20, [H2O] = 0.314 wt.% 100

101

102

Frequency [rad/s]

Figure 6. The elastic (G′) and viscous (G″) moduli response as a function of frequency for (a) ATH/PDMS and (b) ATH/DGEBA dispersions at a volume fraction of 0.20. The absence of structure formation in DGEBA can once again be explained in terms of the relative affinity of the water to the ATH particle interface. In this regard, let us compare the total (δTotal) and polar Hansen (δPolar) solubility parameters of water (47.8, 16), PDMS (15.1, 1.9) and DGEBA (27.8, 14.8)57, where the first and second numbers in each parenthesis correspond to δTotal and δPolar, respectively. We find the difference in total and polar solubility parameter

between water and PDMS to be ΔδTotal=32.7 and ΔδPolar=14.1 whereas that between water and DGEBA to be ΔδTotal=20 and ΔδPolar=1.4. The improved compatibility of DGEBA with water observed in the smaller delta points toward a smaller driving force for the water to migrate from the matrix to the particle surface. The smaller driving force prevents the formation of hydration layers which are necessary for water-bridged structure as indicated in the structure formation


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mechanism. One may argue that the interaction between fillers occurs through ‘crosslinking’ of Al(OH)4- formed via reaction of ATH with water. We believe Al(OH)4- formation does not occur because of the poor solubility of ATH with water, the limited amount of water present in our system and the observance of bridging behavior in other solvents besides water as discussed subsequently (cf. Figure 7). Further amplification on this topic is presented in the supplementary section (ATH Interaction). To further probe the hydrogen bonding effect, water was replaced with various solvents having significantly different hydrogen bonding capability, Figure 7, with the premise that flocculation in ATH/PDMS dispersions could be induced with solvents other than water. The chart in Figure 7 shows a variety of polar protic and non-polar solvents that were investigated as replacements for water in ATH/PDMS dispersions. Note all solvents were added at an equimolar amount to that of water and at a concentration well above the previously observed critical water concentration. All solvent containing systems were tested using the same procedure used for water containing samples. Qualitatively, the selected solvents were found to induce gel-like features upon addition to ATH/PDMS dispersion warranting further characterization. Figure 7 shows the strength of the network formed is directly dependent on the mismatch in the total Hansen solubility parameter of the solvent and matrix resin. This strong dependency suggests the better the compatibility between solvent and matrix the lower the driving force will be for the solvent to preferentially migrate from the matrix to the particles surface. A similar dependence of solvent solubility in PDMS matrix resin has been reported by Jessamine et al.58 in terms of PDMS resin swell.


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Label Solvent 1 Water 2 Glycerol 3 Ethylene Glycol 4 Methanol 5 n-Propanol 6 Aniline 7 Cyclohexanone 8 Methyl Methacrylate 9 Ethylbenzene 10 Butyl Acetate 11 Methyl isobutyl Ketone

(δs-δm)2 1069.8 443.6 318.8 204.6 90.2 54.8 19.9 7.9 7.6 5.3 3.5

δT 47.8 36.2 33.0 29.4 24.6 22.5 19.6 17.9 17.9 17.4 17.0

107

[Solvent] = 3.5 mmoles

δH-1 = 42.3 MPa1/2

106

1 4

105

2 3

5

G' [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

6

104

~ ( - )4 R2 = 0.96

103

7 10

2

10

1

10 11

8 9

δH-11 = 4.1 MPa1/2 100 100

101

102

103

104

2

s-m) [MPa]

Figure 7. Power law scaling relationship between G’ and the mismatch in Hildebrand solubility parameter between various solvents and the matrix resin. The table shows the solubility parameter for each of the solvents investigated as well as the scaling parameter for reference. The Hansen hydrogen bonding solubility parameter can be used as an indicator of gel strength. In addition to the observed dependency of G′ on solubility parameter mismatch between the PDMS matrix and solvent, we also observed a dependence of network strength on the hydrogen bonding potential of the added solvent. Closer examination of the data in Figure 7 and prior work by Raghavan et al.25 lend deeper insight to the strength of interaction between small,


20

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solvent-like molecules with ATH in PDMS. Raghavan et al. have shown that for particles with non-polar grafted alkyl chains, steric interactions are governed by the solvency of the grafted alkyl chains with the matrix resin. By correlating G′ to (δs – δm)2, where δs is the total solubility parameter for the grafted alkyl chains and δm is the total solubility parameter for the dispersion media, they concluded that contact dissimilarity between the grafted alkyl chains and dispersion media as well as the “stickiness” of grafted chains were responsible for gelation in the systems studied. The results in the present work for hydrogen bonded solvation layers suggest analogous behavior. From Figure 7, a clear correlation between the affinity of the solvent for the matrix and gel strength is observed and can be explained in terms of the stickiness of the solvation layers to one another (i.e., higher affinity between solvation layers surrounding particles than for the dispersion matrix); however, unlike Raghavan et al, a stronger scaling relationship between the mismatch in solubility parameters is observed, i.e. (δs – δm) scales to the 4th power, n ~ 4, as opposed to n ~ 2. The stronger scaling is a reflection of the presence of strong hydrogen bonding between hydration layers as opposed to alkyl chains interacting primarily by the weak van der Waals interactions. In addition to matrix/solvent effects, the effect of ATH surface treatment was examined. Figure 8 shows the effect of water concentration on the storage modulus of PDMS containing ATH with and without surface functionalization, Hymod SB-432 SH2 and Hymod SB-432 respectively. The untreated ATH gives the same step increase and overall sigmoidal response as previously seen for Apryal 60D with the critical water concentration found to be between 0.080.10 wt.% (Figure 8a). We note that the amount of water needed, i.e., the critical water concentration, for the SB-432 filler to make the sol to gel transition is slightly higher than that of Apyral 60D observed in Figure 2. An understanding of the underlying mechanism of this


21

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observation taking into account particle size and concentration would be interesting, but remains beyond the scope of this work. Surface functionalization of the filler with an alkyl hydrophobic group (SB-432 SH2) causes the transition from a weak to strong gel to completely disappear, i.e., G′ remains approximately constant over the water concentration range explored.

104

(a)

104

103

(b)

Untreated; SB-432 Allyl Functional, SB-432 SH2

G' and G'' [Pa]

105

Al(OH)3

10

102

G’’ 101

3 10 100

101

100

10

-1

10-2 0.00

0.05

0.10

0.15

0.20

0.25

0.30

G’

SB-432

3

102

G' and G'' [Pa]

G' [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

10

2

10

1

ɸ = 0.20, [H2O] = 0.314 wt.%

(c)

Frequency [rad/s]

G’’ SB-432 SH2 G’

100

10-1 10-1

H2O Concentration [wt.%]

ɸ = 0.20, [H2O] = 0.314 wt.% 100

101

102

Frequency [rad/s]

Figure 8. Evolution of the elastic modulus as a function of water concentration (a) for untreated, SB-432, and treated, SB-432 SH2, ATH at constant volume fraction (φ = 0.20). The inset schematically illustrates the effect of surface functionalization on hydration layer formation, where red represents the water molecule and blue the surface group. Elastic (G′) and viscous (G″) modulus response as a function of frequency for SB-432 (b) and SB-432 SH2 (c) above the critical water concentration ([H2O] = 0.314 wt.%). The absence of the weak to strong gel transition with surface functionalization is further supported by the frequency spectrum data of G′ and G″ at high water concentration (0.314 wt.%) shown in Figures 8b & 8c. For the surface functionalized filler SB-432 SH2, we observe a weak gel or flocculated system akin to that observed for Apyral 60D ATH without water (Figure 1a) where G′ dominates at lower frequencies and a crossover of G′ and G″ occurs at intermediate


22

Page 23 of 31

frequencies. In contrast the untreated SB-432 exhibits strong gel behavior with G′ > G″ over the frequency range explored (Figure 8c).The weak gel behavior for the surface functionalized ATH is likely attributed to its hydrophobic nature and steric stabilization in PDMS imparted by the alkyl chains on the particle surfaces. The inability of water molecules to hydrogen bond at the particle surface prevents the formation of the water-rich hydration layer thought to ultimately lead to particle flocculation and network growth. These results are consistent with microstructural visualization obtained via confocal microcopy. In the case of the unmodified SB432 or Apyral 60D ATH, we observe a network structure with water molecules located on the surface of the particles (Figures 5 and S8). In contrast for the modified SB-432 SH2 filler, we observe no distinct three dimensional network or any water molecules on the filler surface.

102

(a)

G’

106

G' and G'' [Pa]

107

G' and G'' [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

105

G’’ 104

[H2O] = 0.314 wt.%, [LTA] = 0.0 wt.%

103 10-1

100 101 Frequency [rad/s]

102

(b)

G’’

101

G’ 100 [H2O] = 0.314 wt.%, [LTA] = 6.5 wt.%

10-1 10-1

100 101 Frequency [rad/s]

102

Figure 9. The elastic (G′) and viscous (G″) modulus response of a gelled ATH/PDMS dispersion ([H2O] = 0.314 wt.%) as a function of frequency for (a) 0.0 wt.% LTA zeolite and (b) 6.5 wt.%. For 20% vol. dispersions, viscoelastic response was recovered at 6.5 wt.% zeolites. Structure Reversibility. Given the dramatic effect of water on the rheology of the ATH-PDMS system, it was of interest to determine if the 3D network caused by the water could be disrupted by introducing a water scavenging species. Linde Type A (LTA) zeolites based on sodium aluminosilicate were incorporated at various concentrations to PDMS-ATH-water systems above


23

Langmuir

the critical water concentration. Figure 89 shows the effect of LTA content on viscoelastic behavior at 0.314 wt.% water. A dramatic decrease in G′ is observed with increasing LTA loading. The reduction in G′ is accompanied by a transition from a purely elastic, Figure 9a, to viscoelastic response, Figure 9b, as indicated by the frequency spectrum of G′ and G″. The transition is also visibly observed by the return of the material to a readily flowable state with the addition of LTA. Please note that zeolite has no direct interactions with the ATH in the samples, except for scavenging water when that is present. This has been explored by adding zeolite to an ATH/PDMS sample containing no water and finding the rheology (dynamic frequency) to be essentially unaffected. 107

ɸ = 0.20, [H2O] = 0.314 wt.%

Low [LTA]

106 105

G' [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

Strong Gel

104 103

Weak Network

102 101

High [LTA] 100

0

1

2 3 4 5 Zeolite Concentration [wt.%]

6

Figure 10. Changes in the elastic modulus (G′) upon addition of zeolites to an ATH-PDMSwater gelled system (φ = 0.20, [H2O] = 0.314 wt.%) Interestingly, plotting G′ as a function of LTA concentration (Figure 10), three distinct regions corresponding to changes in internal microstructure: an upper plateau (strong gel), a


24

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transition region, and a lower plateau (weak gel) are once again observed, but in reverse order to that previously shown in Figure 2. Strain sweep data taken at different water contents (Figure S9) also illustrates this behavior with a distinct transition occurring at a water content of 0.045 wt.%. It is believed that when LTA is added to the “wet” system, water is scavenged from the hydration layer and can coordinate to the sodium ions and/or hydrogen bond to the oxygen atoms present in the cage structure of the zeolite,64 thereby disrupting water-bridges responsible for gelation. It should be noted that high shear mixing is required for the observed structure recovery to take place. CONCLUSIONS In this study, the viscoelastic properties of ATH/PDMS dispersions in the presence of minute quantities of water were investigated using dynamic rheology. In agreement with visual observation, a transition from viscoelastic to elastic response for the system was found to occur with increasing water concentration. The onset of elastic response was observed to occur at a critical water concentration, as indicated by at least a four order of magnitude increase in the elastic modulus, which decreased with increasing ATH loading. The evolution of the elastic response, culminating in strong gel formation, is consistent with the development of an internal network structure. Further characterization of the system above the critical water concentration found the yield stress, yield strain, and elastic modulus to scale with ATH concentration. The scaling relationships were found to be in good agreement with network structures consisting of strong-linked, sample-spanning flocs. Based on the results, the floc formation was attributed to hydration layer formation and inter-particle bridging via water molecules present at the ATH particle interfaces. Evidence of the latter was borne out by microstructural visualization using confocal microscopy. Gel, and thereby floc, formation was shown to be tunable based on the


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Page 26 of 31

solubility parameters of the matrix and solvent. Furthermore, the network structure development was shown to be reversible upon the addition of Linde Type A zeolites to the system and was attributed the preferential adsorption of water ‘bridges’ by the zeolite. These results, taken together, provide insight into the mechanism of water induced structure formation in ATH/PDMS dispersion and strategies to improve stability and dispensability post manufacture. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6bXXXX. Results are provided on effect of water on the elastic modulus versus strain behavior for a representative 20 vol.% ATH/PDMS sample; effect of water on loss tangent; power-law dependence of elastic modulus on ATH volume fraction; the role of water content to induce and affect yield stress; microstructural evolution upon water addition to ATH filler; effect of treated and untreated filler on microstructure; discussion on ATH interaction with water; and, effect of adding zeolite in reversing structure formation measured in terms of elastic modulus versus strain behavior for a representative ATH/PDMS sample. ACKNOWLEDGMENTS: The authors gratefully acknowledge the help of Prof. Lilian Hsiao and Yunhu Peng in obtaining and analyzing the microstructural results.

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(52) Koh, A.; Gillies, G.; Gore, J.; Saunders, B. R. Flocculation and Coalescence of Oil-in-Water Poly(dimethylsiloxane) Emulsions. Journal of Colloid and Interface Science 2000, 227 (2), 390–397. (53) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Direct Force Measurement between Silica and Alumina. Langmuir 1997, 13 (7), 2109–2112. (54) Bergström, L. Hamaker constants of inorganic materials. Advances in Colloid and Interface Science 1997, 70, 125–169. (55) Gan, Y.; Franks, G. V. Charging Behavior of the Gibbsite Basal (001) Surface in NaCl Solution Investigated by AFM Colloidal Probe Technique. Langmuir 2006, 22 (14), 6087– 6092. (56) Mark, J. E. Polymer data handbook; Oxford University Press: New York, 1999. (57) Hansen, C. M. Hansen solubility parameters: a user's handbook; CRC Press: Boca Raton, FL, 2000. (58) Lee, J. N.; Park, C.; Whitesides, G. M. Solvent Compatibility of Poly(dimethylsiloxane)Based Microfluidic Devices. Analytical Chemistry 2003, 75 (23), 6544–6554. (59) Yucel, U. Ultrasonic characterization of crystal dispersions, M.S. Thesis, Pennsylvania State University, University Park, PA, 2010. (60) Romo, L. A. Effect of C3, C4 and C5 Alcohols and Water on the Stability of Dispersions with Alumina and Aluminum Hydroxide. Discussions of the Faraday Society 1966, 42, 232. (61) Kandori, K.; Kitahara, A.; Kon-No, K. Effect of Water on the Stability of Magnetic and Nonmagnetic Iron(III) Oxides Dispersed in 2-Butanone. Journal of Colloid and Interface Science 1984, 99 (2), 455–462. (62) Kandori, K.; Kazama, A.; Kon-No, K.; Kitahara, A. Dispersibility of Magnetic and Nonmagnetic Iron(III) Oxides in Cyclohexane and Effect of Water on the Stability. Bulletin of the Chemical Society of Japan 1984, 57 (7), 1777–1783. (63) Raghavan, S. R.; Walls, H. J.; Khan, S. A. Rheology of Silica Dispersions in Organic Liquids: New Evidence for Solvation Forces Dictated by Hydrogen Bonding. Langmuir 2000, 16 (21), 7920–7930. (64) Shirazian, S.; Parto, S. G.; Ashrafizadeh, S. N. Effect of Water Content of Synthetic Hydrogel on Dehydration Performance of Nanoporous LTA Zeolite Membranes. International Journal of Applied Ceramic Technology 2013, 11 (5), 793–803.


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TOC GRAPHIC

106

OH OH OH OH Al(OH)3 OH OH OH OH

OH OH OH OH OH OH OH OH

Al(OH)3

G' and G'' [Pa]

H2O ‘Bridge’

G’ 105

G’’ Gel-like

LTA Zeolite

10 1024

Al(OH)3


Disruption of H2O ‘Bridge’


Al(OH)3

G' and G'' [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

101

100

10-1 10-1

G’ G’’

Loss of network 100

101 Frequency [rad/s]


102

31

Reversible Structure Formation of Aluminum Trihydroxide (ATH

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