Preparation of thiol–ene porous polymers by emulsion templating

Reactive & Functional Polymers 72 (2012) 962–966

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Preparation of thiol–ene porous polymers by emulsion templating Benoit Sergent, Marc Birot, Hervé Deleuze ⇑ University of Bordeaux, Institut des Sciences Moléculaires, CNRS-UMR 5255, 351 cours de la Libération, F-33405 Talence, France

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Article history: Available online 21 February 2012 Keywords: polyHIPE Thiol–ene polymerization Monolith

a b s t r a c t Emulsion-templated porous polymeric materials are prepared by thermal thiol–ene reaction. The right choice of a non-methacrylate trivinyl monomer allows obtaining rigid monoliths with a total porosity of 80% and having the expected polyHIPE morphology. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Emulsion templating is a simple and versatile method for the preparation of microcellular materials (cell size range 2–100 lm) by polymerizing the continuous phase of a high internal phase emulsion (HIPE). The obtained materials have been called polyHIPEs by Unilever researchers [1]. Theoretically, the final solid polymer is expected to have an open-cell morphology only for a sufficiently concentrated HIPE (dispersed phase volume fraction >74% of the total emulsion) [2]. Nevertheless, such a structure has been observed for lower values [3]. The historical polyHIPE preparation involves the formation of a stable, water-in-oil concentrated emulsion using hydrophobic monomers as part of the continuous phase (most generally a mixture of styrene and divinylbenzene with, optionally, the addition of a functionalized styrene such as 4-vinylbenzyl chloride), and an aqueous phase as the dispersed phase. A great deal of work by a continuously increasing number of researchers has been devoted to the study of this particular system [4–8]. The main topics studied have been the control of the porous morphology (size dispersion of cells and interconnecting windows) [9–13], and attempts to increase the mechanical strength of the material that, in its native formulation, is generally considered insufficiently tough for practical applications [14–19]. Organic–inorganic hybrid polyHIPE materials have also been investigated [20]. The synthesis of polyHIPEs using step-growth polymerization has also been reported. However, usually, the resulting porous structures are not as highly interconnected and the resulting materials have relatively high densities. The reactions studied include base-catalyzed polycondensation of 2-nitroresorcinol with cyanuric chloride [21], resorcinol–formaldehyde, urea–formaldehyde, ⇑ Corresponding author. Tel.: +33 (0)5 4000 6444; fax: +33 (0)5 4000 6994. E-mail address: [email protected] (H. Deleuze). 1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2012.02.011

phenol–formaldehyde, melamine–formaldehyde, isocyanate–hydroxyl [22], and a system based on a polysiloxane elastomer [23]. Ring opening metathesis polymerization has also been used for polymerization within a HIPE [24,25]. A more recent work, using dicyclopentadiene as sole monomer, reports an access to polyHIPE monoliths with outstanding Young modulus [26]. The synthesis of polyHIPE containing biodegradable poly(e-caprolactone) (PCL) groups through the step-growth reaction of a diisocyanate with a flexible PCL triol to form a crosslinked polyurethane has also been reported [27]. The thiol–ene reaction is another kind of step-growth mechanism that has extensively been employed for polymer preparation [28]. Since Posner first discovered the thiol–ene addition reaction in 1905 [29], thiol–ene polymerization mechanism, kinetics, and material properties have been explored extensively [30–32]. In contrast to the free-radical chain-propagation mechanism associated with styrene/divinylbenzene systems, thiol–ene polymerization is unique in that it proceeds by a step-growth addition mechanism that is facilitated by a free-radical chain-transfer process [33,34]. When the average functionality of both the thiol and ene components is two, a linear polymer is formed. By increasing the average functionality of one or both components beyond two, a crosslinked polymer network is produced. Unlike typical chain-growth free-radical polymerizations or step-growth condensation polymerizations, thiol–ene polymers form in a stepwise manner, but their formation is facilitated by a rapid, highly efficient free-radical chain-transfer reaction. Thus, crosslinked thiol– ene polymerizations proceed very rapidly, but they will not reach the gel-point until relatively high functional group conversions. Recently, the use of thiol–ene reaction has been successfully reported for the preparation of polyHIPE materials [35]. This disclosure has encouraged us to present our preliminary results obtained in the application of thiol–ene polymerization to the preparation of polyHIPE macroporous polymers.

B. Sergent et al. / Reactive & Functional Polymers 72 (2012) 962–966

2. Experimental 2.1. Materials Pentaerythritol tetrakis(3-mercaptopropionate) (TT1), tris[4(vinyloxy)butyl]-1,2,4-benzenetricarboxylate (TE1), 1,3,5-triallyl-1, 3,5-triazine-2,4,6(1H,3H,5H)-trione (TE2), 1,3,5-trivinyl-1,3,5-trimethylcyclotrisilazane (TE3), azobis(isobutyronitrile) (AIBN), and toluene were purchased from Sigma–Aldrich Chimie S.a.r.l. (Saint–Quentin Fallavier, France) and were used as received. Hypermer B246 was a gift from Croda France SAS (Trappes, France). 2.2. HIPE preparation In a typical experiment (sample 11), TE3 (1.3 mL), TT1 (1.4 mL), Hypermer B246 (0.3 mL), AIBN (0.1 g), and toluene (3.0 mL) were placed in a round-bottomed reactor. The mixture was stirred with a rod fitted with a D-shaped paddle, connected to an overhead stirrer motor, at approximately 300 rpm. The aqueous phase was prepared separately by dissolving calcium chloride (1.5% wt of the aqueous phase) in distilled water. This solution was added dropwise using a peristaltic pump at a speed of 0.8 mL min1, under constant mechanical stirring, to the organic solution until no more water insertion was visually observed (24.2 mL). The resulting thick, white homogeneous emulsion contained about 80% vol of dispersed phase. In order to increase this insertion level, the emulsification was further pursued using a laboratory-made system already described elsewhere [36]. Briefly, this device is composed of two polypropylene syringes (50 mL, internal diameter (ID) = 28 mm) connected with a small-section tube (ID = 4 mm, L = 20 mm). The emulsion obtained using the overhead stirrer (about 24 mL) was transferred into one of the syringes and a supplementary aqueous calcium chloride solution portion (10 mL) was added. The second syringe was connected to the first one using the small connecting tube. This system was then adjusted in the ‘‘twosyringe’’ emulsification device and the emulsion was formed by successive passages through the tube produced by the backwards and forwards motion of the syringe plungers. The rate of passage of the emulsion through the connecting tube was adjusted to 7 min1. The emulsification time was 60 min. 2.3. polyHIPE preparation The obtained thick, white emulsion was placed in tightly closed PTFE cylindrical moulds of varying sizes and was polymerized for 24 h at 60 °C in an oven, then at 80 °C for another 24 h. The resulting polyHIPE monoliths were extracted by successively washing with large quantity of ethanol (78 h, room temperature) then diethyl ether (78 h, room temperature). Drying was then performed at room temperature until constant weighing. 2.4. Characterization of the monoliths 2.4.1. Porosity determination The effective porosity and the connection size distribution were determined by mercury intrusion porosimetry using a Micromeritics Autopore IV 9500 porosimeter. The reported average connection size corresponds to the average pore diameter (4V/A). 2.4.2. Specific surface area determination The specific surface area was determined by N2 adsorption measurements performed on a Micromeritics ASAP 2010. The collected data were subjected to the Brunauer, Emmett and Teller (BET) method [37].

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2.4.3. Electron microscopy investigations The morphology of the monoliths was observed by scanning electron microscopy (SEM) in a Hitachi TM-1000 microscope. Micrographs were taken at several magnifications between 1000 and 10,000. Pieces of polyHIPEs (sections of about 0.5 cm2) cut from the corresponding monoliths were mounted on a carbon tab, which ensured a good conductivity. A thin layer of gold was sputtered on the polyHIPE fragments prior to analysis. Two-dimensional (2D) circular cross sections cell diameter was estimated for samples from SEM micrographs after image processing with Image J freeware (http://imagej.nih.gov/ij/, NIH, USA). The mean and the standard deviation were drawn by manual measurements of diameters from a population of 100 cells. Several methods have been devised to find a simple factor to convert the mean of such 2D size distribution to the actual 3D mean size of the spheres without a consensus. A standard assumption in the stereology literature [38], assumes that the distance y between the centre of a given sphere and a random plane AB that intersect it has the uniform distribution on [x,x] where x is the radius of the sphere. An approximate solution from this entirely theoretical approach lead to the results that the ratio of the mean diameter of a set of spheres (d3D) to that of its 2D intercept (d2D) is: d3D/d2D = 4/p  1.27, irrespective of the particular distribution of the 3D sizes [39]. We will use this correction factor in this work to estimate the mean diameter. The mean diameter (dm), the mean volume–surface diameter (the Sauter diameter, d32), and the uniformity factor (U) were calculated from the following relations:

.X ni di ni .X X 3 2 d32 ¼ ni di ni di X .X    3 3 U ¼ ð1=d Þ  jd  di j  ni di ni di

dm ¼

X

ð1Þ ð2Þ ð3Þ

where ni is the number of voids of diameter di and d⁄ is the median diameter (the diameter for which the cumulative undersize volume fraction is equal to 0.5). By analogy with emulsions, we will consider the void distribution of polyHIPEs to be monodisperse if U is smaller than 0.25 [40]. 2.4.4. Differential scanning calorimetry (DSC) Glass transition temperatures (Tg), were determined using a Perkin–Elmer Pyris 1 DSC apparatus equipped with a liquid nitrogen cryostatic cooling. The sample (5–10 mg) was sealed in an aluminium pan and quenched initially to 100 °C, and then heated at a rate of 5 °C min1. 2.4.5. Mechanical analysis Compression tests were carried out by the Pôle Européen de Plasturgie (Oyonnax, France), at room temperature on a Zwick 1455 dynamometer with a loading cell of 200 N. Cylindrical samples (diameter = 7 mm, thickness = 10 mm) were compressed at a constant rate (1 mm min1) on their flat surfaces. The mean value and the standard deviation of the Young modulus E were calculated from the data obtained for five samples of the same composition. 3. Results and discussion The thiol–ene reaction is generally conducted using photopolymerization. However, thermal initiation using a radical initiator is also possible [41]. That was our choice in this work, using AIBN as thermal radical initiator. Our initial attempts to prepare polyHIPE materials employed a tetrathiol–pentaerythritol tetrakis(3-mercaptopropionate), TT1 – and a trivinylether–tris[4-(vinyloxy)butyl]-1,2,4-benzenetricarboxylate, TE1 – in a 1/1 M ratio of ASH to C@C groups (Scheme 1).

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Scheme 1. Long-chain monomers used to prepare polyHIPE materials by thiol–ene polymerization.

Scheme 2. Schematic representation of the PHS–PEO–PHS blocks copolymer (Hypermer B246).

The chosen surfactant for stabilizing the HIPE with TT1 and TE1 monomers was Hypermer B246 [35]. Hypermer B246 is an ABA triblock copolymer of polyhydroxystearic acid (PHS) and polyethylene glycol (PEO). This PHS–PEO–PHS block copolymer meets all the conditions for an efficient stabilization of waterin-oil concentrated emulsions: the poly(ethylene oxide) B chain is highly soluble in water and electrolytes, whereas the poly(12-hydroxystearic acid) A chain is soluble in the continuous phase of hydrophobic monomers (Scheme 2). The molecular weights of the A and B chains have been chosen so that they will produce a hydrophilic–lipophilic balance (HLB) suitable for making a stable w/o emulsion. (Mw  6800 g mol1, Mn  4000 g mol1, d = 0.94, HLB = 5–6) [42]. Firstly, we established the best HIPE formulation in order to insert the highest aqueous volume, as this is directly connected to the final total porosity of the polyHIPE. For doing so, the monomers couple TT1–TE1 was employed. A preliminary experiment indicated that the use of monomers and surfactant in the bulk in the continuous phase failed to obtain a stable emulsion, even with a relatively low ratio of added water (70%). This behaviour can be attributed to the too high viscosity of the organic phase, strongly reducing its ability to insert water droplets. Therefore, toluene (46% v/v), was added to the continuous phase mixture to reduce its viscosity. The amount of added water was then gradually increased using the overhead stirrer device usually employed for polyHIPE preparation. In each experiment, the water was slowly added until no more insertion in the emulsion was visually observable i.e., until an excess of water remained on top of the emulsion. Results of the different successive experiments conducted are reported in Table 1.

Table 1 Formulation of TT1–TE1 HIPEs. Sample

H2O (mL)

Aqueous phase content (%)

Water non-inserted in emulsion

1 2 3 4 5 6

26.3a 34.0a 45.5a 64.5a 45.5 + 19.0b 45.5 + 30.0b

70 75 80 85 85 87

No No No Yes No Yes

a

Maximum water volume added using the overhead stirrer. Maximum water volume added using the overhead stirrer plus supplementary volume added using the ‘‘two syringes’’ device. Composition of the continuous phase: TE1: 2.2 mL, 4 mmol; TT1: 1.4 mL, 3 mmol; surfactant: 2.5 mL; toluene: 5.2 mL. b

From the results presented in Table 1 it appears that the possible maximum of water insertion using the overhead stirrer device is about 80% v/v in the case of the TT1–TE1 monomer system (Sample 3). To increase further the insertion value of the water phase, we decided to use our laboratory-built emulsification device that, generally, generates higher concentrated emulsions, due to a better shearing [36]. Starting from the 80% v/v HIPE obtained using the classical emulsification device (sample 3), it was thus possible to produce a 85% v/v concentrated emulsion using the ‘‘double syringe’’ device (sample 4). However, further attempts to increase this value failed (sample 5). After having determined the highest aqueous phase content possible for the system studied, we decided to determine the

B. Sergent et al. / Reactive & Functional Polymers 72 (2012) 962–966 Table 2 Optimized formulation of TT1–TE1 HIPEs. Sample

Surfactant (mL)

H2O (mL)a

Aqueous phase content (%)

Emulsion stability

7 8 9

1.25 0.6 0.3

40.3 + 17.0 37.8 + 16.0 36.0 + 16.0

85 85 85

Good Good Good

a Maximum water volume added using the overhead stirrer plus supplementary volume added using the ‘‘two syringes’’ device. Composition of the continuous phase: TE1: 2.2 mL, 4 mmol; TT1: 1.4 mL, 3 mmol; surfactant: various amounts; toluene: 5.2 mL; AIBN: 0.1 g.

Scheme 3. Short-chain polyene monomers used to prepare polyHIPE materials by thiol–ene polymerization.

Table 3 Formulation of TT1–TE2 and TT1–TE3 HIPEs. Sample Polyene H2O (mL)a monomer

Aqueous phase Emulsion polyHIPE content (%) stability behaviour

10 11 12 13

85 85 80 80

TE2 TE3 TE2 TE3

24.2 + 10.0 24.2 + 10.0 24.2 + 0.0 24.2 + 0.0

Good Good Good Good

Partial shrinking Partial shrinking Partial shrinking No shrinking

a Maximum water volume added using the overhead stirrer plus supplementary volume added using the ‘‘two syringes’’ device. Composition of the continuous phase: TE2 or TE3: 1.3 mL, 4 mmol; TT1: 1.4 mL, 3 mmol; surfactant: 0.3 mL; toluene: 3.0 mL; AIBN: 0.1 g.

lowest amount of surfactant needed in order to obtain a stable HIPE. The emulsification procedure was that employed previously for sample 5 i.e., preparation of a 80% v/v HIPE using the overhead stirrer followed by its concentration to 85% v/v by transfer into the ‘‘two syringe’’ device containing a supplementary volume of water. Addition of AIBN in the continuous phase allowed the thiol–ene polymerization to proceed by heating of the HIPE (60 °C for 24 h, then 80 °C for 24 h). The results of the experiments conducted are reported in Table 2.

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The results obtained indicates that the level of surfactant can be reduced to a value as low as about 3% v/v of the continuous phase, still giving a stable 85% v/v HIPE (sample 9). A polymerized material is obtained in each case with good yield (>90%). However, a strong shrinking of the monoliths is observed, even after soft washing and drying at room temperature. Therefore, these materials show no porosity. This behaviour may be explained considering the low glass transition temperature observed for sample 9 (Tg = 26 °C), which is in accord with values previously reported for thiol–ene polymers prepared from monomers of similar structure [43]. To resolve this problem, we decided to prepare thiol–ene polyHIPE using more rigid ene monomers such as 1,3,5-triallyl1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TE2), and 2,4,6-trimethyl2,4,6-trivinylcyclotrisilazane (TE3) whose structures are reported in Scheme 3. In both experiments, the tetrathiol was TT1. The HIPEs were prepared using the procedure previously described for sample 9. The results of the experiments conducted are reported in Table 3. The results in Table 3 indicates that stable 85% v/v HIPE can be obtained using about 3% v/v of surfactant in the continuous phase with either TE2 or TE3 as ene monomer. Concerning the polyHIPE monoliths behaviour after washing and drying, in any case, the observed shrinking is much less intense than for TE1. For 85% v/v HIPE, a partial shrinkage is still observed in both cases, whereas for 80% v/v HIPE, the use of TE3 yields a material with a very low shrinking and, consequently, having a potential porosity (sample 13). The morphology of sample 13 was therefore more thoroughly examined. SEM analysis revealed that this sample possessed the characteristic interconnected macroporous structure of polyHIPE materials, that is: micron-sized spherical voids corresponding to the internal phase droplets imprint interconnected by small circular pores (or interconnections) (Fig. 1) [4]. Porosity characteristics and specific surface area determined for sample 13 are reported in Table 4. The interconnected pore size distribution curve as given by mercury intrusion penetrometry is reported in Fig. 2. The distribution obtained is rather narrow and close to that estimated by image analysis of SEM micrographs. Experimental porosity (/exp) of sample 13 is close to the expected value (/total), confirming the low volume shrinkage of the monoliths during the washing and drying steps. The average voids and connections diameters are in the range of those already reported for polyHIPE materials [4]. The value obtained for the BET specific surface area attests the porogenic behaviour of toluene, inducing the apparition of some mesoporosity in the material walls [12].

Fig. 1. SEM micrographs of the TT1–TE3 sample 13.

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B. Sergent et al. / Reactive & Functional Polymers 72 (2012) 962–966 Table 4 Porosity characteristics and stress–strain analysis of TT1–TE3 polyHIPE monolith (sample 13). /totala (%) /expb (%) Specific surface area (BET) (m2 g1) Average connection sizec (lm): d Average connection sized (lm): d Average void sized (lm): D d/D U factore Young’s Modulus (MPa) Bulk densityc (g mL1) Apparent (skeletal) densityc (g mL1)

80 79.4 ± 0.1 125.7 ± 0.1 1.18 ± 0.01 1.20 ± 0.04 8.6 ± 0.5 0.14 1.14 10.6 ± 4.0 0.28 1.38

a Porosity expected from the volume fraction of dispersed phase in the emulsion. b Experimental porosity estimated by mercury porosimetry. c Estimated by mercury porosimetry. d Estimated from SEM micrographs. e See text for determination.

Fig. 2. Interconnected pore size distribution of sample 13.

Voids are the solid imprint of the dispersed phase droplets of the concentrated emulsion used as template. Therefore, their size distribution reflects directly the droplets size distribution of the original emulsion [44]. The U value reported in Table 4 indicates that the voids distribution of sample 13 is rather broad, far from the value generally accepted for a monodisperse emulsion (U < 0.25) [40]. The d/D ratio provides information about the structure of the emulsion before gelling [13]. The low value obtained reflects the good stability of the emulsion and the rather low permeability of the material prepared. The Young modulus is somewhat lower than that reported for similar styrene/divinylbenzene polyHIPE materials. For example, S/DVB polyHIPE with an experimental porosity of about 83%, an average connection size of 1.8 lm, and an average cell size of 6.5 lm ± 0.5 lm possesses a compressive Young’s modulus of approximately 20 MPa [45]. It is however much higher than that of thiol–ene polyHIPE materials previously reported [35]. 4. Conclusions In this work, we have confirmed the possibility to prepare polyHIPE macroporous monolithic materials using the thiol–ene reaction.

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