Poly(HIPE) morphology, crosslink density, and mechanical properties influenced by surfactant concentration and composition

Colloids and Surfaces A 583 (2019) 123913

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Poly(HIPE) morphology, crosslink density, and mechanical properties influenced by surfactant concentration and composition

T

Kristen Rohma,b, Ica Manas-Zloczowera,b, Donald Fekea,

⁎

a b

Department of Chemical and Biomolecular Engineering, Cleveland 44106, United States Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland 44106, United States

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: PolyHIPEs Emulsion stabilization Emulsion templating Cellular mechanics Porous polymers

Porous polymer monoliths were prepared by emulsion templating. The relationship between emulsion behavior and cellular morphology and mechanical properties after polymerization of the monomeric continuous phase was investigated. Porous poly(High Internal Phase Emulsion)s were synthesized with the same strut material and relative density but different morphology and mechanical properties, achieved by varying the surfactant system and concentration. The range of morphologies spanned from cellular solids with hierarchical porosity and millimeter-scale voids to highly interconnected, micron-scale voids resembling Kelvin cell structures. The difference in mechanical properties was evaluated as a function of morphology and polymer crosslinking density. According to the Gibson-Ashby relative modulus – density relationship, morphology was not the sole indicator of mechanical properties. Variance in crosslinking density was inferred from swelling and thermal degradation, indicating that the interfacial film in the emulsion stage influences the polymerization reaction and the degree of crosslinking. A mixed surfactant system of cationic and nonionic surfactant showed the best ability to support small droplets and limit coalescence. However, the magnitude of the interfacial tension alone is not an indicator of the final properties.

1. Introduction Emulsion templating as a route to porous polymers is an area of

⁎

research that has seen increasing interest in the past twenty years [1,2]. Emulsions where the internal phase volume fraction is above 74%, the packing limit of spherical particles in a face-centered cubic array [3–5],

Corresponding author. E-mail addresses: [email protected] (K. Rohm), [email protected] (I. Manas-Zloczower), [email protected] (D. Feke).

https://doi.org/10.1016/j.colsurfa.2019.123913 Received 8 May 2019; Received in revised form 29 August 2019; Accepted 31 August 2019 Available online 07 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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as the only surfactant or a combination of Span80 and cationic CTAB. The ratio of nonionic to cationic surfactant was kept constant at 6:1 for all samples. Porous polymers prepared with HIPE stabilized by one emulsion are signified with the Roman numeral I followed by the total wt% of surfactant relative to the oil phase. The same system is used for the two-surfactant system (II). For example, II-12 refers to emulsion prepared with Span80 and CTAB at 12 wt% for the surfactant mixture. The water to oil ratio was 19:1 for all samples, and each batch of emulsion was 160 g. When preparing an emulsion, the oil phase and aqueous phase were first prepared separately. The surfactant system was added to the monomer mixture with 20 wt% crosslinker and mechanically stirred until dissolved. The same was done for the aqueous phase by mixing the requisite amount of deionized water and NaCl (2 wt%). Ten grams of the salt solution was taken aside and further mixed with the initiator NaPS (0.33 wt% of total aqueous phase). The oil phase was placed in a heated jacket kept at 65 C by circulating water. Then the aqueous phase including the 10 g of solution with NaPS was added dropwise under constant mixing by an overhead mixer at 300 rpm for 3 min followed by 7 min additional mixing. After mixing, the HIPE was poured into polyethylene centrifuge tubes and cured at 70 C for 24 h. The porous monoliths were washed with DI water to remove salt, replacing the water over the course of 36 h. Then the samples were washed with IPA for 24 h in a Soxhlet chamber to remove surfactant and un-polymerized monomers. After washing, the IPA was replaced with DI water and freeze dried.

are termed high internal phase emulsions (HIPE) [2,6]. When monomers make up the continuous phase surrounding aqueous droplets, the resulting porous materials are termed polyHIPE. These cellular solids have been useful in applications benefitting from high surface area and low density including reaction supports, separation and filtration media, tissue engineering scaffold, absorbent materials, microelectronics, controlled drug delivery, and chromatography [1,2,7,8]. The wide range of applications for porous polymers stems from their broad functionality and range of properties, which can be easily tuned via formula and preparation methods [2,9]. The benefits of low density usually come at the cost of mechanical properties. There is a need for a comprehensive study of how high internal phase emulsions can be tuned to create low density polymers while maximizing mechanical strength. The morphology and pore architecture of the monoliths are fixed by the state of the emulsion before the continuous phase is polymerized [10]. Therefore, the stability of the emulsion has a lasting influence on the polymer structure and is determined by surfactant type and concentration, temperature, processing conditions, and miscibility of the phases. Although studies exist on how emulsion stability effects the cellular morphology [11–15], a comprehensive study of how emulsion stability effects poly(HIPE) mechanical properties through its influence on morphology and the polymer network is needed. Interesting studies on the effect of non-reactive diluents on the polymerization reaction have underscored the connection between the solubility of the components and the morphology [11,12,16]. Cameron et al. have investigated the effect of adding porogenic solvents to a divinylbenzene-based continuous phase on the resulting poly(HIPE) structure. The authors found that the pore architecture was influenced by the interaction of the solvent and the polymer network, the solvent polarity, and the degree of solvent adsorption at the O/W interface. In this work, no organic solvent is added to the continuous phase. However, the surfactant (in particular surfactant of the oil-soluble nonionic type) may be considered a non-reactive diluent during the polymerization reaction. In this way, the surfactant can influence the polymerization reaction in two ways: affecting the strength of the interfacial film and as a diluent in the continuous phase. The effect on the flux at the droplet interface and the growing polymer chain is seen in the cellular morphology. Ultimately, the mechanical performance of the porous polymer is influenced by these mechanisms as well. By synthesizing cellular solids from the same materials and preparation conditions, but changing the surfactant system stabilizing the emulsion, this study investigates how dynamic interfacial behavior affects the morphology, crosslink density, and mechanical properties. The HIPE to poly(HIPE) transition not only determines the morphology, but the local environment for the polymerization reaction. For poly(HIPE), investigating the link between morphology and mechanical properties must also include a look into how interfacial behavior affects the constitutive material.

2.3. HIPE characterization 2.3.1. Interfacial tension Interfacial tension (IFT) measurements were performed with a Kruss Force Tensiometer – K100. The Wilhelmy plate method was used with a roughened platinum plate at room temperature. A 50 mL beaker with the oil phase is placed under the Wilhelmy plate, and the surface tension of the oil phase alone is measured (Fig. 2). After cleaning the plate by flame, the aqueous phase in a clean beaker is placed under the plate, sitting just at the air/water interface. At this point, the oil phase was pipetted carefully over the aqueous phase until the plate is completely covered. Data was collected through the entire process of adding the oil phase (approximately 25 s) and was collected for a total of 1800s. It was found that 8 wt% surfactant is much greater than the critical micelle concentration (CMC) of the interface. Therefore, comparing the onesurfactant system and the two-surfactant system at this concentration and higher showed no difference in interfacial behavior. In order to see the different effects of the systems, lower surfactant concentrations for each system were employed (0 wt%, 0.01 wt%, 1 wt%, 4 wt%) and each concentration was tested twice. 2.3.2. Droplet size distribution Optical micrographs were taken with OLYMPUS microscope at magnification of 50 × . Three images for each sample were further analyzed using the ImageJ software to determine droplet area. Each diameter was calculated assuming circular droplets. The droplet size distribution was evaluated by calculating the average diameter (D10), the Sauter mean diameter (D32), and the polydispersity index (PDI) according to the following:

2. Materials and experimental methods 2.1. Materials All material used as received. 2-ethylhexyl acrylate (EHA), 2ethylhexyl methacrylate (EHMA), and ethylene glycol dimethacrylate (EGDMA), Span80, and hexadecyltrimethlammonium bromide (CTAB) were purchased from Sigma-Aldrich. Sodium chloride (NaCl), sodium persulfate (NaPS), toluene, isopropyl alcohol (IPA), and cyclohexane were purchased from Fisher Scientific. 2.2. HIPE and poly(HIPE) preparation

D10 =

n n D1 i=1 i i n n i=1 i

(1)

D32 =

n n D3 i=1 i i n n D2 i=1 i i

(2)

PDI = D32 /D10

Poly(HIPE)s were made by HIPE templating, shown schematically in Fig. 1. HIPEs were prepared with 8 wt%, 12 wt%, and 16 wt% total surfactant relative to the oil phase consisting of either nonionic Span80

(3)

where ni is the number and Di is the diameter of each droplet. The Sauter mean diameter, also called the surface-volume diameter, represents a monodisperse droplet collection having the same surface 2

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Fig. 1. a) Schematic of HIPE and poly(HIPE) preparation. b) Photograph of I-8 and II-8 poly(HIPE) monoliths showing the effect of coalescence on I-8 morphology. Monoliths pictured have approximate diameter of 25 mm.

d void =

4

dSEM

(4)

Likewise, a correction is needed for the windows. The windows surround the cell walls and their shape can be distorted by the angle they are viewed. The correction takes into account the ellipsoid window captured by the 2D SEM micrograph and projects the area as a circle, according to [18]

d window =

2

dSEM

(5)

2.4.2. Oil uptake The absorption kinetics and capillary action of the poly(HIPE)s were studied using spontaneous imbibition of a completely wetting liquid (DOW Corning 200 Fluid - 100 cST PDMS). The general set-up for the oil imbibition test has been previously reported and the schematic is reprinted in Fig. 3 [19]. A poly(HIPE) monolith of approximately 25 mm in diameter and 1 cm height was placed at the bottom of a tube open at both ends, such that oil could only rise through the bottom face of the poly(HIPE) disk. The tube diameter was just larger than the polyHIPE diameter, allowing the polyHIPE to be fixed in place without being compressed. The tube was attached to a fixed support resting on a microbalance which recorded the change in mass over time at an interval of 1 s. A pool containing the

Fig. 2. Schematic of Wilhelmy plate experimental setup.

area/volume ratio of the studied polydisperse droplet collection. [17] Optical micrographs can be found in Fig. S1. 2.4. Poly(HIPE) characterization 2.4.1. Scanning electron microscopy Washed and dried monoliths were sputter coated with platinum before SEM (JEOL JSM-6510LV). Voltage was adjusted between 10 and 30 kV. The voids of the I-8 sample were on the scale of mm and visible to the naked eye. For this sample, an image was taken with a smart phone and the void size was analyzed in the same manner as other samples. ImageJ was again used to analyze void size and strut thickness and length. The strut thickness was measured as an average of three measurements along the length of one strut. The strut length was defined as the length along the straight portion of the struts. Typical of open cell cellular solids, the struts have the shape of plateau borders. At least three images were analyzed for each sample; the number of measurements differs between samples due to the morphology differences. More than 100 voids were measured for each sample. The plane of the SEM image cuts through the voids at a random distance from the center, therefore a correction factor was applied to the void size measurement calculated from SEM analysis. [18] The statistical correction yields the most likely diameter of a void seen in the SEM micrograph:

Fig. 3. Schematic of experimental setup for spontaneous imbibition study. 3

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liquid to be imbibed, was placed under the poly(HIPE) monolith. The sample holder containing the poly(HIPE) monolith was tared and then the bottom was placed at the interface of the oil. Any buoyancy force was recorded as an initial negative mass and the recorded mass of imbibed fluid was adjusted. The uptake was continued until the rate of uptake stopped and the mass of imbibed oil was constant. The reported results are an average of three samples. Spontaneous imbibition kinetics are controlled by capillary rise. In his theoretical analysis of imbibition kinetics, Washburn represented porous media by a collection of parallel tubes of the same radius [20]. Since then there have been many models developed to account for the microstructure of different porous media [21]. Using Lucas-Washburn analysis, it has previously been found that the characteristic radius for the capillary rise process of open-cell porous solids correlates well with the measured window size, rather than the void size [18]. The modified Lucas-Washburn equation [22] can be used to determine a characteristic radius for the capillary rise process by

M2 =

2 A2

2 (S

wf )2 2µ 2

3r c

cos

t

volume, the monoliths were placed in a beaker containing enough solvent for completely immersion. Two monoliths were tested for their swelling behavior in cyclohexane, one was immersed in methanol. The monoliths instantly swelled (within a few seconds). After 3 h the swollen monolith’s weight and dimensions were recorded. Some samples were kept in swelling solvent for 2 wk, however, there was no significant difference in the measurements taken after 3 h of swelling and 2 wk. This may be attributed to the interconnected, porous structure of the monoliths. The swelling ratio, Q, was calculated by

Q=

(1

(7)

where Vswollen is the volume of the sample after swelling in solvent, and Vdry is the initial volume of the sample while dry. Volumes were calculated from dimensions of the cylindrical samples. To estimate the crosslinking density of poly(HIPE) samples, the Flory-Rehner theory of polymer network swelling was applied. 2.4.6. Determination of solubility parameters The Hansen Solubility Parameters in Practice (HSPiP) software was used to find the solubility parameters of the solvents (cyclohexane and methanol) and monomers (EHA, EHMA, and EGDMA) used in the swelling study (Table 1). These parameters were then used to estimate the polymer-solvent interaction parameter, χ. The software was also used to estimate the solubility parameters of a representative “poly(HIPE)” molecule. Fig. 4 shows the molecular structure of the representative molecule. The poly (HIPE) molecule is the product of a reaction with two EHA, two EHMA, and one EGDMA units, according to the ratio of the monomers to crosslinker in the formula. With the molecular structure, the HSPiP software uses group contribution theory to calculate the solubility parameters of the poly(HIPE) molecule. The total solubility parameter ( total ) for both the solvent and the polymer are functions of the dispersive (δd), polar (δp) and hydrogen (δh) Hildebrand solubility parameters according to

(6)

where the following parameters were determined experimentally: M is the mass of liquid imbibed in the monolith at time t, ρ is the uptake liquid density, A is the monolith cross-section area, Swf is the final monolith saturation by testing liquid (vol%), is the monolith porosity, μ is the liquid viscosity, σ is the liquid surface tension, and cos is the liquid and bulk polymer contact angle. Porosity was calculated acapparent density cording to = 1 . The remaining parameters were fitting material density parameters: α is a shape correction factor assumed to be one, [22] and τ is a correction factor for the degree of tortuosity calculated as a function of porosity = 1/3 [23]. For high porosity values, the tortuosity 1

Vswollen Vdry

)

value approaches unity. Therefore, in the highly porous solids studied, the degree of tortuosity spans a narrow range for all samples. The contact angle between the bulk polymer and the silicone oil was found to be less than 10°, ensuring good wettability between the polymer struts and the uptake liquid [18]. Plotting M by √t gives a linear relationship at small times. The slope of the linear region is the Washburn coefficient. The only remaining unknown, the characteristic diameter, rc, was then determined by Eq. (6).

2 total

=

2 d

+

2 p

+

2 h

(8)

2.4.7. Thermogravimetric analysis Thermal stability was analyzed with TGA Q500 (TA Instruments) under a nitrogen atmosphere (40 mL min−1). Approximately 6 g of crushed monolith samples were placed in a platinum pan and heated to 100 °C and held at 1 min to remove any moisture then the temperature was ramped at 10 °C/min to 600 °C.

2.4.3. Density Density of the poly(HIPE) monoliths was determined from mass and volume measurements on the disk samples. At least five disks were measured for each sample. Volume was calculated by measuring the diameter and height of the monoliths using calipers. An average of three measurements was recorded for each height and diameter. The density of the strut material was assumed to be 1 g/cm3 [24].

3. Results and discussion 3.1. HIPE interfacial behavior

2.4.4. Mechanical testing Unconfined compression tests were done on dry, disk-shaped (25 mm diameter and 10 mm height) samples at room temperature with MTS Universal Tensile Tester (Model 2525-806) at a crosshead speed of 1 mm/min (10%/mm). Compressive modulus was calculated from the linear slope of the initial stress-strain curves and the reported values are averages of 3–6 samples. The compression modulus was reported as measured and normalized by the ratio of the measured density to the predicted density, ρpredicted. The ρpredicted of a cellular solid was based on the initial mass of monomer in a 19:1 W:O emulsion, accounting for the difference in polymer between samples with different surfactant concentration.

The average IFT after 30 min for each concentration compared to the oil/water interface with no surfactant is shown in Fig. 5a. At the lowest concentration tested, 0.01 wt%, the stabilizing effect of CTAB is evident. I-0.01 and II-0.01 have IFTs of 11.1 ± 1.6 and 1.4 ± 0.2 mN/ m, respectively, an almost 8-fold difference caused the CTAB. The oneTable 1 Summary of HIPE droplet size distribution.

2.4.5. Swelling and crosslinking density The crosslinking density was estimated using classical elastomer swelling theory. Dry poly(HIPE) monolith swelling behavior was studied using cyclohexane as the solvent. After recording dry mass and

Sample

D10 (μm)

D32 (μm)

PDI

Count

I-8 I-12 I-16 II-8 II-12 II-16

22.5 12.9 17.8 6.14 6.61 6.89

213 67.1 82.2 43.5 39.9 19.6

9.47 5.18 4.63 7.08 6.04 2.84

1722 2062 1102 8896 8151 12,860

± ± ± ± ± ±

38.3 17.1 22.5 7.93 8.67 5.80

Histograms of distributions found in Fig. S2. 4

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distributions as well as the length (L), width (t), and aspect ratio (L/t) of the struts found by image analysis is found in Table 2. Fig. 6 contains SEM micrographs displaying the void morphology for each sample, where magnification reveals stark structural differences. Micrographs with higher magnification, showing the window and strut morphology are located in Fig. S3, along with histograms of the droplet sizes Fig. S4. The morphology of the one-surfactant monoliths appeared to be very sensitive to the overall concentration of surfactant, while the two surfactant system showed less sensitivity. The large void sizes, a few millimeters in diameter, of the I-8 poly (HIPE) were the result of coalescence in the HIPE before the viscosity rise of the continuous phase locked-in the morphology during polymerization. The coalescence caused the hierarchical pore structure characterized by large, millimeter scale voids and a second scale of voids located in porous struts. I-8 and I-12 show evidence of coalescence during polymerization and the hierarchy of pore sizes from bimodal and multimodal void distributions (see Fig. S4). The I-8 sample has multiple scales of void sizes: the largest, visible voids are an average of 2.24 mm and the voids within the struts are an average of 58 μm. Increasing the surfactant concentration from I-8 to I-12, had the effect of reducing the degree of hierarchy in the void sizes. Despite the I-8 void size being one or more orders of magnitude larger than the other systems, the window size is the same order of magnitude. The hierarchical structure increases the average void size, yet the window size does not scale in the same way. This trend also holds for I-12 poly (HIPE), which also had a hierarchical structure with voids around one hundred microns. The degree of coalescence in the I-8 sample also effected the openness of the cell walls. As the small droplets coalesced into larger droplets, the walls formed by the continuous phase experienced less Laplace pressure as the droplet radius increased resulting in less windows and more cell wall material. For I-16, increasing the surfactant reduced the coalescence and prevented the formation of a hierarchical structure. Although, the HIPE droplet size distribution for I-16 and I-12 were similar, the emulsion behavior during polymerization as a result of different surfactant concentrations resulted in very different morphologies. The void and window structure was indistinguishable at any scale; therefore, no void sizes were analyzed. The structure appeared to be small spheres of polymer loosely connected by thin, fibrous “struts”. Pore throats can be seen around the edges of the windows. The struts measured for this sample are different from the struts that make up the cell walls in the other poly(HIPE)s. In this case, the thin connections were measured to be around 1 μm long and less than a half micron wide. The higher surfactant concentration was able to support smaller droplets during polymerization, but was unable to support the pressure of deformed droplets in the typical HIPE packing. The I-16 poly(HIPE) also shrunk during curing, as evidenced by a reduction in radius of about 12%. The shrinkage of the polymer during curing indicates significant emulsion destabilization, resulting in the uncharacteristic morphology. The mixed surfactant system samples do not resemble any onesurfactant system morphologies and there is less variation in

Fig. 4. Representative poly(HIPE) molecule from a reaction of two EHA, two EHMA, and one EGDMA units.

surfactant system shows the expected trend of lowered IFT with increasing surfactant concentration. The force reduction from 4 wt% to 8 wt% is small, the CMC of this system should be near this range of concentrations. Dynamic interfacial tension is shown in Fig. 5b. Most compositions immediately reach a stable value and remain there for the duration of the test, approximately 30 min. I-0.01 and I-1 reach stable values at a slower rate, steadily decreasing even after 500 s. This dynamic behavior emphasizes the stabilizing effect of the cationic surfactant at lower surfactant concentrations. Interestingly, at 8 wt% surfactant, the magnitude of the interfacial tension for both systems differed only slightly. However, a direct comparison between the interfacial tension found via the Wilhelmy plate method and the local interfacial tension in an emulsion is impossible due to the dynamic nature of the emulsion and the emulsification process. During emulsification, the surface area generated between the two phases by mixing is governed by the relation, W = γΔA, where W is the work input to the system to generate the change in interfacial area, ΔA, and γ is the interfacial tension between the two phases. [25] Therefore, for the same mixing energy, a lower surface tension should result in more interfacial area between the two phases. This mechanism supports the trends seen in the HIPE droplet size distribution shown in Table 2, specifically the decrease in PDI and D32 for the two surfactant system. Local surfactant concentration gradients naturally develop both during the emulsification process while new interface is being generated and after. The concentration gradients explain why CTAB has an influence on emulsion stabilization even at surfactant concentrations where Span80 alone lowers the interfacial tension by the same magnitude. 3.2. Poly(HIPE) morphology Poly(HIPE) morphology was found to be influenced by the surfactant system and concentration. At the lowest surfactant concentration, 8 wt%, the effect HIPE stability had on morphology can be seen by the eye (Fig. 1b). A summary of the void and window diameter

Fig. 5. a) IFT of surfactant systems at the water/oil interface. The IFT of the water/oil interface with no surfactant is γ = 14.58 ± 0.082 mN/m. b) Dynamic IFT of surfactant systems at the water/oil interface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

5

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Table 2 Summary of poly(HIPE) features including, void, window, and strut from analysis of SEM images. Void

I-8 I-12 I-16 II-8 II-12 II-16

Window

D10

D32

PDI

D10

823 ± 1120 107 ± 59.7 n/a 20.6 ± 8.3 11.0 ± 4.6 10.3 ± 3.7

2640 189 n/a 27.8 14.5 12.9

3.21 1.77 n/a 1.35 1.32 1.25

6.04 3.24 2.38 1.02 1.35 1.30

± ± ± ± ± ±

Strut

2.42 2.09 1.63 0.94 1.01 0.84

D32

PDI

L

t

L/t

7.80 6.13 4.84 2.71 2.99 2.39

1.29 1.89 2.03 2.65 2.22 1.83

129 ± 49 108 ± 22 1.39 ± 1.15 8.95 ± 3.97 5.67 ± 2.58 2.84 ± 1.02

345 ± 210 43.8 ± 25.2 0.203 ± 0.153 0.989 ± 0.271 0.696 ± 0.130 0.697 ± 0.167

0.374 2.47 7.15 10.9 8.25 4.20

All features measured in μm.

Fig. 6. SEM showing void morphology of poly(HIPE) cured from HIPE with different concentrations of Span80 a) 8 wt%, b) 12 wt%, c) 16 wt% and different total concentrations of Span80 and CTAB d) 8 wt%, e) 12 wt%, f) 16 wt%. Scale bars differ (50x to 1,000x).

morphology between samples. All samples demonstrated distinguishable voids, windows, and struts with no hierarchical structure. The II-8 sample had slightly larger void sizes than both II-12 and II-16 poly (HIPE)s, from approximately 20 to 10 μm. Overall, the strut aspect ratio decreased from 10.9 to 4.2 with increasing surfactant concentration. Comparing the HIPE droplet size distribution to the void morphology of the mixed-surfactant samples shows the effect of the cationic surfactant. The PDI of the droplet sizes decreased with increasing surfactant concentration as a result of emulsion stabilization. For the void sizes, the PDI hovered around 1.3 for all samples. The mixed surfactant system was more effective at limiting these emulsion destabilizing processes than only nonionic surfactant, evidenced by the lack of hierarchical structure, the relatively small void and window sizes, and the integrity of the struts. Furthermore, the similarity of all the two-

surfactant poly(HIPE) morphologies corroborated the picture of strong, stable interfacial films that preserve the HIPE morphology during polymerization. This interfacial behavior seems to be more strongly influenced by the surfactant composition (nonionic and cationic) rather than overall concentration. Fig. 7 shows representative imbibition curves of the mass of oil imbibed over the square root of time. The imbibition curve typically followed a linear uptake region followed by a plateau where the poly (HIPE) reached its equilibrium saturation. The connectivity, openness, and window size all have some influence on the imbibition behavior. The average porosity of II-8 was 0.95 and all other samples had an average porosity of 0.94. For I-8 and I-16, the section of the curve prior to the plateau was linearized into three and two sections, respectively. These sections were considered to represent the presence of more than 6

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Fig. 7. Representative oil imbibition curves for (a) one-surfactant system and (b) two-surfactant system. Uptake liquid was 100 cST PDMS. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Because the deformation mechanism of open and closed cell cellular solids differ, two general models are needed to describe them. For open cell cellular solids, the Young’s modulus scales by its relative density following a power law expression [28–31]:

Table 3 Summary of results from oil imbibition study. Sample

Characteristic Diameter μm

Washburn Coefficient mg/s1/2

Saturation

I

35.3 2.66 0.39 3.04 8.15 0.72 3.95 2.66 3.14

69.4 18.0 7.67 39.3 79.8 25.3 97.8 84.3 98.3

0.231 ± 0.015

8 12 16

II

8 12 16

± ± ± ± ± ± ± ± ±

15.5 1.20 0.01 0.92 3.01 0.02 0.75 1.03 1.05

± ± ± ± ± ± ± ± ±

5.4 0.8 1.41 8.6 36.5 6.41 7.1 27.3 19.9

Ef Es

0.411 ± 0.025 0.585 ± 0.141

m

f

=C

(9)

s

where E is the Young’s modulus, C is a material constant containing any geometric constants of proportionality, ρ is the density, m is related to the deformation of the struts, and the subscripts f and s refer to properties of the porous structure and solid, respectively. Gibson and Ashby found m = 2 and C = 1 best described the relative density – modulus relationship for both a cubic and tetrakaidecahedral array of cells. [29] For closed cell porous solids, explored in the modeling section to follow, the relative modulus – density relationship must take into account the deformation mechanisms of cell-edge bending, air compression, and membrane stretching. The relation takes the following form, when assuming a Poisson’s ratio of zero [29,31]:

0.798 ± 0.106 0.847 ± 0.088 0.923 ± 0.024

one characteristic diameter, due to their hierarchical structure. Table 3 shows a summary of the Washburn coefficient(s), final saturation, and characteristic diameter(s) for each sample. Overall, the two-surfactant poly(HIPE)s absorbed more oil, evidenced by a saturation increase of 21.3% from the best performing poly (HIPE) from one-surfactant group, I-16, with the least saturated twosurfactant poly(HIPE), II-8. The I-8 poly(HIPE)s had the lowest saturation, caused by low interconnectivity and hierarchical structure. The rise of an absorbing liquid is limited by the diameter of the voids, seen in the case of the large voids in I-8. The height a fluid can climb in 2 cos a capillary is described by Jurin’s law [26], h = gr , where γ is the liquid surface tension, θ is the contact angle, ρ is the density of the liquid, and r is the radius. The context of Jurin’s law is for fluid rising in capillaries of uniform diameter, however the law gives the expectation of an inverse relationship between feature radius and fluid height: as the radius increases, the height of the fluid meniscus decreases. When liquid is rising by capillary action through a hierarchical structure, it will tend to bypass larger channels, travelling faster up channels with smaller diameters. This leads to trapped air pockets and decreased saturation. Therefore, the saturation is also an indication of the connectivity of the poly(HIPE).

Ef Es

= C1

f

2 s

s

m

+ C2 (1

s)

f s

+

p0 Es (1

f s

)

(10)

where s is the volume fraction of the polymer in the cell walls, p0 is the atmospheric pressure, and C1 and C2 are geometric constants for cell edges and faces. The value of s is hard to determine experimentally, however, Mills and Zhu [32] found that the mechanical behavior was insensitive to changes in s for values greater than 0.6. Gibson and Ashby related the relative density to the morphology through strut features, thickness and length [29]. The relative density of open cell cellular solids can be related to struts of length l and thickness t by f

=C

s

t l

2

(11)

Substituting this relation into Eq. (9) for m = 2, the modulus would be expected to have a fourth power dependence on the strut aspect ratio:

3.3. Poly(HIPE) mechanical properties All samples, except I-16, followed typical cellular solid compression behavior with three distinct regions of linear deformation, plateau, and densification [27]. Fig. 8 depicts representative stress-strain curves for each of the poly(HIPE)s. The I-16 curves lacked the shoulder representing the transition from the linear to the plateau region, indicative of their weak mechanical strength. Fig. 9 shows the modulus and normalized modulus of all samples and Table 4 summarizes the measured density and modulus for each sample. The trend in mechanical properties remained unchanged when accounting for differences in density and initial amount of polymer. It is well-known that the linear elastic behavior of cellular solids is related to intrinsic material properties and cell geometry [28,29].

Ef = Es

t L

4

(12)

Whether the changes in modulus are due to differences in pore architecture was analyzed by applying the data from the SEM analysis and compression testing and assuming the same strut modulus for all poly(HIPE)s. The increase in modulus from I-8 to I-12 contradicts what would be predicted by Eq. (12). The strut aspect ratio was 0.347 and 2.47 for I-8 and I-12, respectively. According to Eq. (12), I-8 should have a modulus 51.07 kPa higher than I-12, but the measured modulus showed the opposite of this prediction. For the two-surfactant system, modulus decreases with increasing surfactant concentration. This trend also conflicts with the predicted modulus using Eq. (12), which predicts 7

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Fig. 8. Representative stress-strain curves from foam compression for (a) one-surfactant system and (b) two-surfactant system. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

the lowest modulus for II-8 and the highest for II-16. The prediction also fails when accounting for poly(HIPE) geometry better described by the Kelvin cell array, where the geometrical constant C was found to equal 0.593 [33]. The error in this model likely comes from the polydispersity of the poly(HIPE)s and the assumption that all struts have the same modulus. 3.4. Evaluating crosslink density The degree of swelling is inversely related to the degree of crosslinking; the more a polymer network swells, the less crosslinked points exist in the network. The polymer volume fraction vp is found from the equilibrium swelling ratio of the polymer in solvent, Q, according to 1 vp = Q [34]. The Flory-Rehner theory of polymer network swelling can be used to estimate the crosslink density [35–37]: 1

vp) + vp + vp2 ) = Nvs (vp3

(ln(1

vp 2

)

(13) 3

where vs is the molar volume of solvent (mol/cm ), χ is the FloryHuggins interaction parameter between polymer and solvent, and N is the crosslinking density (mol/cm3). The degree of crosslinking can be estimated from intrinsic solvent properties, determination of the χ parameter between the polymer and solvent, and calculation of vp from experiment. It should be noted that the cross-link density determined by the Flory-Rehner theory can only yield a qualitative analysis of the polymer network due to the approximations made in the model’s derivation [38]. In order to apply Eq. (13), determining the interaction parameter χ for the polymer-solvent interaction is necessary. The following approach was used to estimate an effective χ. The entropic portion is usually estimated as entropic = 0.34 for polymer-solvent systems [39]. The enthalpic contribution can be calculated by a relation to the solubility parameters of the polymer and solvent [40]: enthalpic

Fig. 9. Compression modulus for poly(HIPE)s (a) measured from linear region of stress-strain curve and (b) modulus normalized by the ratio ρmeas/ρpredicted. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

I II

8 12 16 8 12 16

ρmeas (g/cm3)

Modulus (kPa)

0.059 0.062 0.067 0.056 0.058 0.057

98.7 ± 21.6 151 ± 23.1 16.7 ± 0.1 148 ± 21 112 ± 17 66.8 ± 6.8

± ± ± ± ± ±

0.002 0.003 0.010 0.007 0.006 0.004

vseg RT

(

p, d

s, d )2

+ 0.25(

p, p

s, p )

2

+ 0.25(

p, h

s, h )

2

(14)

where vseg is the molar density of the polymer segment, α is taken to be 1, δ is the solubility parameter with the first subscript for the polymer (p) or solvent (s) and the second subscript for the dispersive forces (d), polar forces (p), and hydrogen-bonding forces (f). Fig. 10 shows the swelling behavior of the poly(HIPE) monoliths in cyclohexane and methanol. Cyclohexane was a good solvent for the poly(HIPE)s and methanol was a poor solvent. The difference in swelling ratio between the two solvents shows that cyclohexane did penetrate into the polymer network whereas methanol merely wet the monolith. Table 5 shows the solubility parameters of the solvents cyclohexane and methanol with the monomers, and Fig. 4 showed the representative molecule of reacted poly(HIPE). I-8 swelled the most, with all other samples close to the same degree of swelling.

Table 4 Measured density and compression modulus of poly(HIPE)s. Sample

=

8

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Table 6 Summary of results from TGA analysis.

Table 5 Solubility parameters of solvents and poly(HIPE) monomers.

Methanol Cyclohexane poly(HIPE)a EHA EHMA EGDMA

δp

δh

δtotal

14.7 16.8 16.9 16.8 18 17.8

12.3 0 0.7 0 1.4 3.1

22.3 0.2 0.6 0.2 2 5.7

29.61 16.8 16.93 16.8 18.16 18.95

T5%

Tmax1

dW/dT1

Tmax2

dW/dT2

I-8 I-12 I-16 II-8 II-12 II-16

258.7 286.5 254.9 320.1 318.9 293.3

246.2 222.1 235.2 237.1 238.9 228.4

0.10 0.05 0.08 0.04 0.05 0.06

391.6 422.2 400.0 412.0 420.3 410.7

1.05 1.25 1.12 1.20 1.22 1.30

The correlation between crosslink density and thermal stability has been studied by others for a variety of polymer systems and so far has shown that the correlation is dependent on the polymer system and its preparation [41–45]. In one study, Levchik et al. examined the thermal properties of crosslinked polystyrenes and methacrylates and found contradicting results: crosslinked polystyrene degrades at higher temperatures than linear polystyrene while crosslinked methacrylates degrade earlier than the uncrosslinked system [43]. They concluded that the chemical nature of the crosslinks, preparation of the polymer, and degradation mechanism of the polymer chain influences the correlation between crosslink density and thermal stability. Within this study, the thermal degradation behavior correlates well with the swelling ratio of the poly(HIPE)s. The swelling ratio was greatest for I-8 and I-16, indicating the lowest crosslink density. These samples also had the lowest degradation temperatures at 5% mass loss and Tmax2 and the fastest rate of the first degradation phenomena. The lower onset temperatures (T5%) of I-8 and I-16 may be due to a higher concentration of unsaturated double bonds from unreacted crosslinker or pendant chains. These unsaturated double bonds are the initiation points for the unzipping of the polymer backbone in poly(alkyl acrylates) [42,43,46]. These correlations point to the conclusion that the thermal stability is dependent on the preparation and local polymerization environment, not only the ratio of monomers to crosslinker.

Fig. 10. Swelling ratio of poly(HIPE)s swollen in cyclohexane (red) and methanol (black) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

δd

Sample

Values for δd, δp, δh from HSPiP in MPa1/2. a Representative poly(HIPE) molecule.

Interestingly, the II samples had more variability within the sample. This behavior may indicate a more homogenous network in the onesurfactant system. Further evaluation of crosslink density was done by TGA; curves for the poly(HIPE) samples are shown in Fig. 11. All samples show twostage degradation behavior, but differences in the rates and temperatures of the two degradation phenomena are evident. Table 6 summarizes the data from the TGA analysis. Looking at the one-surfactant samples, I-8 and I-16 behave in the same way, having the highest dW/ dT1 and the lowest Tmax2 compared to all samples. I-12 degrades more in line with the two-surfactant samples, with higher Tmax2 and dW/dT2 values and shorter, broader peaks around 232 °C. The difference in the thermal degradation behavior of the poly(HIPE)s are further evidence for the creation of different polymer networks by different emulsion environments.

3.5. Relative modulus – density relationship Deviations from the Gibson-Ashby model can be explained by taking into account differences in crosslinking density and, thus, polymer strut modulus. The strut modulus can be estimated from the swelling behavior of the poly(HIPE)s using rubber elasticity theory where Es = 3NRT, where N is the crosslink density, R is the gas constant, and T is the temperature [47]. In poly(HIPE), the modulus of the struts themselves is unknown and difficult to determine experimentally. Preparation of bulk polymer samples would require an oil based initiator, completely changing the polymerization reaction kinetics from the emulsion system. Table 7 summarizes the experimental and theoretical results of the swelling study, calculated strut modulus, and relative modulus relationship. Looking at the one-surfactant poly(HIPE)s, the I-8 sample

Fig. 11. a) TGA weight loss curve and b) derivative weight loss of poly(HIPE) samples. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 9

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K. Rohm, et al.

open cell poly(HIPE)s studied in this work fall below the classical Gibson and Ashby model. The two-surfactant samples fall between two models: the red line described by C = 0.593 and m = 2 and the blue line described by C = 1.05 and m = 2.54. The placement of the samples between these lines may be the result of changing deformation modes from II-8 to II-16. I-16 falls on the green line described by C = 0.593 and m = 3. This indicates that the morphology may be described by the Kelvin cell model and the deformation may be characterized by a high degree of cell wall bending and twisting. The interesting behavior of the I-16 and II-16 samples may be due to the role of surfactant as a non-reactive diluent. This role becomes more apparent for the higher surfactant concentrations, as there is excess surfactant than what is needed to stabilize the interface. This excess surfactant may be present in the bulk of the continuous phase during polymerization, acting as a diluent and lowering the crosslinking density. The swelling behavior of the I-16 sample and its uncharacteristic morphology supports this conclusion. Despite being more dense than the other polyHIPEs, I-16 has the lowest modulus. The II-16 sample swells to the same degree as the other poly(HIPE)s in its group. The sensitivity of the swelling test may be too low of a resolution to see the effect for the two-surfactant poly(HIPE)s. Theoretically, the higher surfactant concentration would affect the degree of crosslinking in the II-16 sample, explaining the lower mechanical properties.

Table 7 Poly(HIPE) swelling behavior, crosslinking density, polymer modulus, and model type applied to determine relative modulus Ef/Es. Sample

Swelling Ratio

N (mol/m3)

Es x 104 (kPa)a

Model Typeb

Ef/Es

I

1.35 1.22 1.28 1.21 1.25 1.23

5800 ± 149 9460 ± 309 7630 ± 667 10600 ± 3600 9080 ± 3800 9620 ± 3070

4.31 7.03 5.67 7.89 6.75 7.15

closed closed open open open open

0.013 0.014 0.0003 0.0019 0.0017 0.00093

II

a b

8 12 16 8 12 16

± ± ± ± ± ±

0.01 0.01 0.02 0.07 0.10 0.07

Calculated from Es = 3NRT. For closed cell models, s = 0.8.

4. Conclusions Adding a cationic co-surfactant decreased the interfacial tension at very low concentrations compared to the system with only nonionic surfactant. This dynamic effect plays a role in lowering the interfacial tension during emulsification when new interface is being generated. However, the magnitude of the equilibrium interfacial tension is not the sole indicator of final poly(HIPE) properties. The ability of the HIPE interfacial film to prevent coalescence and Ostwald ripening not only has a significant impact on the poly(HIPE) morphology, but also affects the transport of reactants across the membrane, influencing the polymerization reaction. An application of the Gibson and Ashby model relating relative density to the strut aspect ratio revealed that morphology alone could not predict the mechanical performance of the poly(HIPE)s. This work attempted to decouple the influence of HIPE interfacial tension and porous polymer morphology. The monoliths prepared from HIPE which underwent the most coalescence (I-8) swelled the most, indicating a lower degree of crosslinking. Overall, the effect of interfacial behavior on the polymer network, in connection with the morphology, should be accounted for when predicting the mechanical behavior of poly(HIPE)s and can be used to tune poly(HIPE) properties for specific applications.

Fig. 12. Comparison of relative density – modulus relationship from experiment (circles) and models. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). Table 8 Summary of relative density – modulus models plotted in Fig. 12. Number

Model Type

C

m

Reference

1 2 3 4 5

open open open open closed

1 0.593 0.593 1.05 0.64

2 2 3 2.54 1.4

Gibson and Ashby [29] Warren and Kraynik [33] Warren and Kraynik [33] Liu et al. [49] Roberts and Garboczi [48]

swelled the most and, accordingly, has the lowest crosslinking density and polymer modulus. A lower material modulus can partially explain the difference in poly(HIPE) modulus from I-8 to I-12. For I-16, the low strength can be attributed to both a lower crosslinking density and the aspect ratio of the struts. Using the theoretical Es and measured aspect ratio in Table 2, the predicted modulus is 16.3 kPa, which is very comparable to the measured modulus of 16.7 kPa. For the two-surfactant system, all three samples demonstrated similar swelling behavior; consequently, the polymer modulus for all three are in the same range, with II-8 being slightly higher than II-12 and II-16. Fig. 12 shows the relative density – modulus relationship between the poly(HIPE) samples compared with models from literature. The poly(HIPE)s have around the same relative density, but the wide range of mechanical performance highlights the contribution of morphology and polymer properties. The models and fitting parameters are summarized in Table 8. I-8 and I-12 both have the highest relative modulus ratios, which were calculated from the closed cell model due to the low interconnectivity and cell walls. The closed cell model [48] sits just under these samples, suggesting that a model with parameters close to C1 = C2 = 0.64 and m = 1.4 would describe the hierarchy and low interconnectivity characteristic of the I-8 and I-12 morphology. All

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