Effect of Primary Particle Properties and Operating ... - ACS Publications

Oct 29, 2014 - Primary particle properties are found to be pivotal, namely, the ... degree of primary particles, waiting time before postpolymerizatio...
0 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/Langmuir

Macroporous Polymer Particles via Reactive Gelation under Shear: Effect of Primary Particle Properties and Operating Parameters Alexandros Lamprou, Itır Köse, Zoé Peña Aguirre, Giuseppe Storti, Massimo Morbidelli, and Miroslav Soos* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland S Supporting Information *

ABSTRACT: Reactive gelation under shear is a recently developed procedure toward rigid macroporous polymeric particles that does not require the use of any porogen. A comprehensive study of the effect of individual parameters on the resulting material characteristics is presented. Primary particle properties are found to be pivotal, namely, the primary particles size, cross-linking degree, and outer shell composition. Operating parameters also play a significant role; specifically, the effects of applied shear rate, salt feeding rate, swelling degree of primary particles, waiting time before postpolymerization, and postpolymerization temperature are investigated. By varying the operating conditions, the size, internal structure, as well as porosity of the fabricated microclusters may be controlled. The pores are invariably micrometer-sized, with pore size distributions exhibiting adjustable maxima. Thanks to the sequential character of the procedure, different parameters may be tuned individually at different stages along the preparation route, allowing thus for high versatility in the control of different properties of the final material.

■

INTRODUCTION Macroporous polymeric materials are conventionally prepared in the presence of foreign to the polymerization poregenerating systems, which comprise traditional porogens or more advanced templating approaches.1−11 The so-called porogens are responsible for the pore formation though an intricate interplay of thermodynamic and kinetic parameters, during a single preparation step.12−15 Reactive gelation under shear is a newly developed procedure for the preparation of polymeric macroporous microparticles that does not entail the use of any porogen. Instead, the pores of such microparticles are formed during the aggregation and breakage of colloidal aggregates, composed of primary particles. The proof-ofconcept of this alternative procedure has been previously discussed in detail.16 In brief, it consists of a series of welldefined, successive steps. First, a latex, that is, a colloidal dispersion of polymeric nanoparticles with specific properties, is prepared and then swollen by a monomeric mixture, containing a thermal free-radical initiator. The swollen, and thus softened, nanoparticles are fully destabilized inside a stirred reactor under turbulent conditions, by adding a salt amount exceeding their critical coagulation concentration (CCC). Hence, the primary particles are forced to form clusters, in the presence of shear, via multiple aggregation and breakage events.25,26 Such clusters are known to be fractal objects, exhibiting a fractal dimension that is characteristic of the compactness of their structure.17,18 After a steady-state has been established, the system is heated and thus the swelling monomers are postpolymerized, rigidifying the structure of the microclusters (Figure 1). Thorough characterization of the prepared microclusters16 revealed a very compact internal structure, as indicated by the fractal © XXXX American Chemical Society

dimension of df = 2.7. The microclusters are irregularly shaped, with an average diameter of tens of micrometers. Their size distribution is fairly broad (span ∼ 1.3), and this is expected to be the case even when different primary particles or operating parameters are used, due to local inhomogeneities of the flow field inside the reactor equipped with a paddle impeller.41 Internally, they are also highly porous, that is, ∼70%, with wellinterconnected19 and unprecedentedly large, micrometer-sized pores, exhibiting a fairly broad pore size distribution. Moreover, the presence of an atom-transfer radical polymerization (ATRP)20 initiator on the primary particles surface allows for subsequent surface modification, by grafting polymer brushes of variable composition. Thus, multiple functionalities can be realized, targeting several different applications. As the new method was promising, we decided to investigate how the properties of the resulting microclusters could be controlled. Regarding the porosity and internal porous structure, they should arise through a combination of four different densification mechanisms, which, qualitatively described, are as follows: cluster breakup, interparticle coalescence, polymerization-induced shrinkage, and surface energyinduced compaction. First, it has been demonstrated that breakage and restructuring of a fractal aggregate gives rise to denser structures of higher fractal dimension.21 Second, the initial latex nanoparticles are in a glassy state, that is, rigid, at room temperature; however, they are made to absorb a finite amount of water-insoluble monomers. Such swelling treatment6 Received: June 3, 2014 Revised: September 6, 2014

A

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Figure 1. Scheme of the macroporous polymeric microclusters synthesis.

Table 1. Characteristics of the Seven Polystyrene-Based Latexesa latex no.

DVB:MCA (mol %)

dp (nm)

dcore (nm)

rshell (nm)

Vζ (mV)

CCC (mM)

Scomp (m2/g)

xdry (%)

Tg (°C)

A C1 C15 AS I S A′

15:0/1:0/1:40 1:0/1:0/1:40 15:0/15:0/15:40 15:0/1:0/1:40 15:0/1:0/1:99 15:0/1:0/1:0 15:0/1:0/1:40

161 144 162 96 165 174 167

139 119 141 75 144 151 144

11.0 12.5 10.5 10.5 10.5 11.5 11.5

−42.3 −48.4 −49.1 −53.2 −43.3 −43.6 −49.0

33.0 22.5 22.5 35.0 150.0 20.0 25.5

35.3 39.5 35.1 59.9 34.6 32.8 34.2

12.0 11.7 11.4 13.3 12.2 11.2 11.8

108 103 112 n/a 110 92

DVB:MCA, DVB:MCA mol % in core/1st/2nd shell, respectively, dp, final nanoparticle diameter; dcore, core diameter; rshell, 1st and 2nd shell thickness; Vζ, ζ-potential, Scomp, computed primary particle specific surface area; xdry, final latex dry fraction.

a

Acros, 99%), 2,2′-azobisisobutyronitrile (AIBN, Aldrich, >98%), hexadecane (HD, ABCR, 99%), sodium dodecyl sulfate (SDS, Fluka, >96%), and magnesium chloride hexahydrate (MgCl2.6H2O, Fluka, >98%), were used as received. Deionized water was further treated by using a Millipore Simpack 2 purification device. Latex Preparation. A three-step, semibatch miniemulsion polymerization protocol was followed for the preparation of all primary latex particles. Table SI 1 (Supporting Information) contains the detailed recipes for all latexes considered in this study. In the first step, the particle core is prepared (“initial core batch” column in Table SI 1, Supporting Information). In the second and third steps, two shells are grown on the particles during two respective starved feeding loops (“1st and 2nd starved feeding loop” columns in Table SI 1, Supporting Information), allowing control of the cross-linking degree along the particle radius as well as surface functionality. The detailed procedure is reported elsewhere.16 Latex Swelling. A defined amount of monomeric mixture, typically 15% wt with respect to the latex dry content, was added dropwise over 20 min into a flat-bottom 30 mL vial containing the original latex under vigorous stirring, which was maintained for another 60 min. This mixture contained 20% wt DVB as well as defined amounts of MCA and/or St, depending on the composition of the outer shell of the particles; 1% wt AIBN was also added. Aggregation under Shear and Postpolymerization. The aggregation of primary particles was performed at a defined solid weight fraction, following dilution of the swollen latex inside a completely filled 150 mL jacketed reactor, equipped with a four-paddle impeller and two baffles. Details on the construction of this reactor have been reported previously.16 Aggregation was induced under defined stirring speed, that is, shear rate, by adding equal volumes of salt solutions during two successive gradients, that is, 2 × 10 mL, using a Lamda Vit-Fit programmable syringe pump operated at a defined flow rate. The concentration of the salt solution fed during the first gradient was selected such that the respective latex CCC was reached inside the reactor toward the end of the first gradient. The concentration of the solution fed during the second gradient was ∼10 times larger than the first. Hence, the total amount of added salt always brought the final salt concentration inside the reactor at least ∼10 times above the CCC. This ensured the complete destabilization of primary particles and their complete incorporation into clusters. After salt addition, the system remained under stirring at the defined rotation speed for 4 h before postpolymerization. Postpolymerization was typically conducted by heating the stirred reactor at 65 °C for 16 h. The dispersion medium of the obtained microclusters was removed by centrifugation at 4600 rpm for 15 min.

can lower their glass transition temperature, creating rubbery domains2 especially close to their surface. Rubbery particles are known to coalesce during aggregation,18 a process which is responsible for increasing the fractal dimension both under quiescent diffusion-limited cluster aggregation (DLCA)22 and during shear-induced aggregation.23 Further compaction is definitely occurring during postpolymerization, as an inherent effect of the vinyl groups’ polymerization.13,24 Lastly, highly probable is also a structural rearrangement, similar to the one taking place during sintering of inorganic materials. Such rearrangements are driven by the minimization of the curved interfacial area of the nanoparticles comprising the clusters, leading to more compact structures.25,26 Therefore, to investigate the effect of material properties and composition of primary particles, as well as the operating parameters on the final properties of prepared microlcusters, we prepared several different latexes that bear two successive shells, grown on the primary particle cores during starved feeding conditions. Thus, the radial composition of these shells is independent of monomers’ and radicals’ reactivities.27 In a first set of latexes, we chose to vary the cross-linking degree along the primary particle radius, while in a second set we chose to vary the amount of ATRP initiator in the outer shell. The last latex which was synthesized is composed of primary particles with smaller diameter compared to the other latexes. Using these latexes as starting materials, a series of polystyrene-based macroporous microparticles were prepared and the effects of several individual parameters were investigated in detail. The primary particle properties studied were the cross-linking degree along the particle radius, the ATRP initiator shell content, and primary particle size. The operating parameters studied were the applied shear rate, salt feeding rate, swelling degree of primary particles, waiting time before postpolymerization, and postpolymerization temperature. The obtained microclusters were characterized in terms of their particle size distribution, fractal dimension, surface morphology, specific surface area, pore volume, and pore size distribution.

■

EXPERIMENTAL SECTION

Materials. All chemicals, that is, styrene (St, Fluka, >99%), divinylbenzene (DVB, Fluka, ∼80%), methyl α-chloroacrylate (MCA, B

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

better the resistance of the final resins’ network to “good” organic solvents will be.2,8 The effect of primary particle cross-linking was investigated using latexes A, C1, and C15 as starting materials (see Table 1). Quantification of the primary particles’ Tg by DSC confirmed that their Tg is increasing with increasing cross-linker content from 1% DVB over the whole particle for latex C1, to 15% in the core for latex A and then to 15% over the whole particle for latex C15 (see Table 1 and Supporting Information Figure SI 2). Upon examination by SEM of the microclusters produced from latex containing 1% cross-linker, their constituting primary particles appear substantially fused together and barely identifiable (see Figure 2a). Conversely, in the microclusters produced from latex containing 15% cross-linker, the primary particles are apparently less interpenetrated and retain their individuality to a large extent (see Figure 2b). In microclusters produced from the latex where only the core contained 15% DVB, there is clearly interparticle fusion due to the presence of the shell with just 1% cross-linker content (Figure 2c). These structural observations from SEM pictures are confirmed by the corresponding surface area values summarized in Table 2, which increase with increasing DVB content. The surface area reduction from the computed specific surface area of primary particles in Table 1, down to the measured Brunauer− Emmett−Teller (BET) area of microclusters in Table 2, is indicative of the extent of fusion of primary particles. As a matter of fact, this surface area reduction ranges from 63% for microclusters originating from primary particles with low crosslinker content, up to an astonishing 81% for high cross-linker content, which suggests that all four compaction mechanisms mentioned above are operative. The fractal dimension values decrease from 3.0 for 1% cross-linker-containing particles, to 2.7 for core−shell particles, and further down to 2.3 for 15% cross-linker content (see Figure 3a). This trend further corroborates the evidence from SEM and surface area measurements; that is, primary particles with lower cross-linker content yield more compact microclusters. Owing to the more extensive interpenetration between primary particles containing 1% cross-linker, the resulting microclusters have better mechanical properties and they are consequently able to withstand higher stress and grow larger in size30,31 (see Table 2). It is worth noting that a similar trend in df as a function of primary particle rigidity (decreasing df with increasing rigidity) has been also observed for clusters prepared by DLCA under quiescent conditions22 and under shear in the absence of additional electrolytes.23 This indicates that the primary particle rigidity has a similar effect to the final cluster morphology, irrespective of the applied aggregation mechanism. The structure factor plot for 1% cross-linker content, shown in Figure 3a by open circles, exhibits a change in slope for large wave vectors q. This may be attributed to relatively open structures which result from the ductile rupture of the clusters, while still in softened state, under the presence of shear. In fact, the morphology of the deformed regions observable on SEM pictures (see Figure 2a) resembles strongly fractured surfaces of ductile materials, such as rubbery polymers that have undergone extensive plastic deformation upon fracture.32−34 In line with the observations from SEM, SLS, and BET, the porosity measured by Hg intrusion for the microclusters originating from the less cross-linked particles is significantly smaller than the corresponding porosities measured for the more cross-linked and the core−shell particles (see Table 2), as

The resins were further washed by repeating the centrifugation six times with water and then dried slowly at ambient conditions. Analytical Tools. The latex was characterized by determining the polymerization conversion, final dry fraction, average size of primary particles, and their ζ potential, as well as the latex CCC (see Figure SI 1, Supporting Information). The glass transition temperature (Tg) of dried primary particles was also quantified (see Figure SI 2, Supporting Information). The characteristics of all latexes used in this work are summarized in Table 1. Characterization of the microclusters includes determining their mean square radius of gyration ⟨Rg⟩ and fractal dimension df by static light scattering (SLS), scanning electron microscopy (SEM) imaging, as well as nitrogen sorption and mercury intrusion porosimetry. Inverse size-exclusion chromatography (ISEC) measurements were also performed on chromatographic columns packed with microclusters. A description of the analytical tools employed is provided in the Supporting Information, while further details can be found in Lamprou et al.16 Field Flow Characterization. In order to obtain a quantitative relation between the microclusters size and the hydrodynamic conditions present during cluster synthesis28 the fluid flow inside the stirred reactor was characterized by Computational Fluid Dynamics (CFD) simulations using the commercial software ANSYS Fluent v12.1. Thus, the average shear rate ⟨γ⟩ applied at any defined stirring speed was evaluated. Further details are provided in the SI.

■

RESULTS AND DISCUSSION In order to investigate the effect of primary particle properties and operating parameters, seven latexes with different compositions and/or sizes have been prepared; their characteristics are summarized in Table 1. All latexes have similar dry fraction of ∼12 wt % and high colloidal stability, indicated by comparable ζ-potential values above |−40 mV|. They all bear two shells surrounding their core, with a combined shell thickness of about 11 nm. For latexes C1 and C15, the cross-linker content is uniform along the whole particle radius and corresponds to 1 and 15 mol %, respectively. On the other hand, latexes A and AS bear both a 15% cross-linker-containing core and two 1% crosslinker-containing shells. However, the AS particles are significantly smaller, namely, 96 nm in diameter, as opposed to those of latexes C1, C15, and A, whose diameters are between 144 and 162 nm. The above four latexes have the same surface composition, that is, ∼40 mol % MCA on their outer shell, while latexes I and AS contain 99 mol % MCA and 99 mol % St on their outer shells, respectively. The cross-linker content of these two latexes is the same as that of A, that is, 15% in the core and 1% in the two shells, while their size is also comparable, that is, 165 and 174 nm, respectively. Effect of Primary Particle Properties. Cross-Linking Degree of Primary Particles. The glass transition temperature of a polymer network depends not only on the solvent it may contain, but also on its cross-linking degree.2 Therefore, particles that are swollen with the same amount of monomer but have different cross-linking degrees are expected to behave differently during aggregation and postpolymerization, owing to the different flexibility of their polymer chains. The less crosslinked a particle is, the more it is softened by monomer swelling and hence the more prone it is to coalescence during aggregation.13,18,29 Moreover, clusters comprising particles with less cross-linked and thus more mobile chains should rearrange easier during postpolymerization. The opposite effects are expected with more densely cross-linked particles, which are more rigid after swelling. It also follows that, after postpolymerization, the higher the cross-linking degree, the C

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

The overall particle diameter of latex AS is 96 with respect to 161 nm for A, while the other latex properties, including the thickness of the shell with low cross-linker content, as well as the preparation procedures were the same (Table 2). The internal cluster morphology from SEM imaging (see Figure SI 3, Supporting Information) appears similar to the one of clusters from latex A produced at 200 rpm, which can be explained by the core−shell structure of the primary particles. However, the relative amount of material with low cross-linker content per particle is 52 vol % for latex AS with respect to 35% for latex A. Hence, the relative particle interpenetration for latex AS is larger and so the resulting cluster porosity is smaller. Indeed, for microclusters generated from smaller primary particles, the porosity measured by Hg intrusion porosimetry decreases from 0.70 to 0.55 (see Table 2) and the df value measured by SLS increases from 2.7 to 2.85 (see Figure 4a), respectively. This increase of compactness translates into increased toughness, so the microclusters ultimately reach larger sizes,30 namely, 53 μm with respect to 40 μm. From the pore size distribution of Figure 4c, it is evident that, contrary to the microclusters composed of large primary particles, the very large pores between 1 and 10 μm have been to a large extent eliminated. Actually, the distribution has shifted to smaller pore sizes by roughly 1 order of magnitude and is clearly skewed toward its left part, exhibiting a very clear maximum at 60 nm. This can explain the fact that despite their lower porosity, the specific surface area of clusters produced from smaller primary particles is approximately 3 times larger compared to the microclusters produced from larger particles (see Table 2). Such behavior is in agreement with results observed during the processing of inorganic materials like ceramics where smaller grain sizes result in smaller pore sizes.26,35 It has also been shown that during the preparation of macroporous copolymer resins by the porogen method the agglomeration of smaller globules, albeit produced by a completely different mechanism than our primary particles, yields materials with smaller pores.9,13,14,29 The primary particle size thus emerges as a key and definitively tunable parameter for controlling the pore sizes of our resins. ATRP initiator Shell Content. The amount of MCA comonomer in the outer shell of the primary particles is obviously crucial for the extent that the surface of the final material is covered with ATRP functionalzation sites. In order to investigate whether it could also play a role during the microclusters preparation, we used latexes S, A, and I. At the one end, the microclusters originating from latex S, with 0% mol MCA in the outer shell, were millimeter-sized, which prevented their analysis by SLS due to suspension difficulties. Latex A, with the intermediate shell composition of 40% mol MCA, yielded substantially smaller microclusters, with radius of 40 μm and df = 2.7. The characterization of this material is detailed in ref 16 and also briefly outlined above. At the other end, latex I, with 99% mol MCA, yielded very small microclusters, that is, 21 μm, with df = 2.25 (see Table 2). This value is very close to the df obtained for the microclusters originating from latex C15, with 15% cross-linker content, and indicates minimal coalescence among primary particles. Moreover, these microclusters were so fragile that we were unable to pack them into a chromatographic column. This decreasing trend in size and df with increasing MCA content is due to increasing swollen primary particle rigidity, similarly to the case of increasing cross-linker content discussed above. As a matter of fact, the Tg of dried primary particles

Figure 2. Illustrative SEM pictures of clusters obtained from primary particles containing (a) 1% DVB in both shells and core, (b) 1% DVB in both shells and 15% in core, and (c) 15% DVB in both shells and core.

indicated by the plateau height of the intrusion curves in Figure 3b. The span of the extracted pore size distribution is similarly broad for all three cross-linking degrees, although for low crosslinking the distribution maximum is around 1.5 μm, while for high cross-linking is about 500 nm (Figure 3c). These results indicate that primary particles with low cross-linker content actually coalesce and rearrange to a greater extent during aggregation, breakage, and postpolymerization, so that the resulting fractal structures are more compact and less porous. Size of Primary Particles. The effect of primary particle size was investigated using latexes A and AS as starting materials. D

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Table 2. Effect of Cross-Linking Degree, Primary Particle Size, ATRP Initiator Shell Content, and Rotation Speed on the Produced Microclusters Propertiesa latex no.

DVB:MCA (mol %)

rotation speed (rpm)

⟨Rg⟩ (μm)

df (−)

SBET (m2/g)

SHg (m2/g)

ε (−)

A C1 C15 AS I A A A

15:0/1:0/1:40 1:0/1:0/1:40 15:0/15:0/15:40 15:0/1:0/1:40 15:0/1:0/1:99 15:0/1:0/1:40 15:0/1:0/1:40 15:0/1:0/1:40

200 200 200 200 200 200 400 800

40 85 40 53 21 40 22 16

2.7 3.0 2.3 2.85 2.25 2.7 2.7 2.7

11. 2 7.5 13.1 30.0 n/a 11.2 14.1 16.7

9.7 7.1 12.3 35.2 n/a 9.7 15.0 22.9

0.70 0.48 0.66 0.55 n/a 0.70 0.61 0.60

a

Other conditions: Monomeric content of swelling mixture, DVB:MCA:St = 20:40:20 for all latexes except DVB:MCA:St = 20:80:0 for I; swelling degree, xswell = 15% wt; primary particles concentration inside the reactor, φ = 1% wt; concentration of salt solutions fed during 1st and 2nd salt gradient, respectively, Csalt1 = 0.5 M and Csalt2 = 4.9 M; volumetric flow rate of salt feeding, Qsalt = 4.23 mL/min; waiting time before postpolymerization, twait = 4 h; postpolymerization temperature, Tpp= 65 °C and time tpp= 16 h.

Figure 4. Effect of primary particles size on internal cluster structure (a), mercury intruded volume (b), and pore size distribution (c). (■) dp = 161 nm, (○) dp = 95 nm. Line in (a) indicates power law scaling of S(q) vs q⟨Rg⟩ with slope equal to −2.85.

Figure 3. Effect of primary particle DVB content on the size and internal structure of formed clusters (a), Hg intruded volume (b), and corresponding pore size distribution (c). Primary particles with various DVB content: 1% in both shells and 15% in core (■), 1% (○), and 15% (▲) in both shells and core, respectively.

determined by DSC is increasing from 92 to 110 °C with increasing MCA comonomer content in the outer shell (see E

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Table 1 and Supporting Information Figure SI 2). This is also in agreement with reported literature values for the Tg of poly(methyl α-chloroacrylate) homopolymer prepared by freeradical polymerization,36 which is ∼50 °C higher than the corresponding Tg reported for polystyrene.37 while that of poly(methyl α-chloroacrylate) ∼150 °C.36 Therefore, the larger the MCA content in the shell of primary particles, the less such particles should be softened after absorbing a certain amount of monomer.2 Effect of Operating Parameters. Shear Rate. The effect of shear rate was studied by running the aggregation, breakage, and postpolymerization experiments of swollen A latex at different impeller rotation speeds, namely, 200, 400, and 800 rpm, as reported in Table 2. As can be seen, with increasing stirring speed, that is, shear rate, the aggregate size obviously decreases. The dependency of measured ⟨Rg⟩ as a function of applied shear rate obtained through CFD simulations and summarized in Table SI 2 is presented in Figure SI 4 (Supporting Information). However, the internal cluster morphology appears similar for all three rotation speeds, with df values about 2.7, essentially unaffected by the change in shear rate, as shown in Figure 5a. These results are in line with previous studies, where apart from the size reduction with shear rate, it has been demonstrated that df is practically independent of the applied shear rate not only for electrolyte-induced turbulent aggregation,28,38 but also for shear-induced aggregation in the absence of additional electrolytes.31 It is worth pointing out that the solid straight line in Figure SI 4 (Supporting Information) represents the theoretical scaling for the steady-state cluster size ⟨Rg⟩, as suggested by Zaccone et al.30 and calculated with df = 2.7, in fact exhibiting remarkable agreement with the experimental data. Similar scaling has been experimentally obtained in the past for turbulent aggregation of rigid, fully destabilized polystyrene latexes.28,38 From a practical viewpoint this is very useful as, for given latex, once the cluster size corresponding to certain shear rate is known the sizes for all other shear rates may be predicted. This again can be used as design value for preparing microclusters of desired size. As expected for fractal aggregates,17,30,31 the porosity values in Table 2 and the plateau heights of the Hg intrusion isotherms in Figure 5b, indicate that the void volume fraction of the microclusters decreases when their size decreases at higher rotation. Similarly to the case of microclusters originating from smaller primary particles, the surface areas increase with increasing shear rate despite the porosity decrease (see Table 2). This is presumably due to the presence of smaller pores, evidenced in Figure 5c by a systematic shift of the pore size distributions to the left. At higher shear rates the recombining cluster fragments that build up the final microclusters are certainly smaller, therefore the resulting pores are again smaller.9,13,14,29 The applied shear rate is consequently one more parameter for tuning the final pore sizes. Furthermore, ISEC measurements were performed on chromatographic beds packed with microclusters prepared under different rotation speeds or from primary particles with different cross-linking degree. No exclusion from the total pore space of even the largest molecular probes used, with diameter ∼ 120 nm, was detected. This is in agreement with the large pore sizes determined by Hg porosimetry. The corresponding chromatographic peaks are also rather symmetric, as indicated by the low39 values of their calculated asymmetry factors As, which are presented in Figure SI 5 (Supporting Information).

Figure 5. Comparison of structure factors of microclusters prepared at different rotation speeds (a), volume of intruded mercury (b), and pore size distribution (c). Applied stirring speed was equal to (■) 200 rpm, (○) 400 rpm, and (△) 800 rpm. In (a), line indicates power law scaling used to determine cluster fractal dimension of 2.7 ± 0.1.

These observations are indicative of good pore interconnectivity.40,41 Swelling Degree of Primary Particles. The effect of the primary particles swelling degree was studied using latex I. Increasing the swelling degree from 15% (see entry for latex I in Table 2), to 30%, to 50% wt (see Table 3) yields microlclusters that are both larger and significantly more compact. The dependency of the fractal dimension on the swelling degree is so pronounced that df increases from 2.25, to 2.6, to 2.95, respectively (see Figure SI 6, Supporting Information), indicating progressively more extensive coalescence. In fact, the extent to which primary particles with a given cross-linking degree are softened by the swelling monomers depends on the amount of monomer mixture that they are made to absorb.2 This explains why increasing the swelling degree of primary F

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

Table 3. Effect of Swelling Degree, Primary Particles Concentration, Salt Feeding Rate, Waiting Time, and Postpolymerization Temperature on the Produced Microclusters Propertiesa latex no.

xswell (wt %)

φ (wt %)

Csalt1/2 (M)

Qsalt (mL/min)

⟨Rg⟩ (μm)

df (−)

twait (h)

Tpp/tpp (°C/h)

I I I A′ A′ A′ A′ A′

30 50 50 15 15 15 15 15

1 1 2 1 1 1 1 1

4.9/4.9 4.9/4.9 4.9/4.9 0.38/3.8 0.38/3.8 0.49/4.9 0.38/3.8 0.38/3.8

4.23 4.23 4.23 1.45 4.23 4.23 1.45 1.45

27 55 80 37 49 53 34 74

2.6 2.95 2.95 2.7 2.7 2.7 2.75 2.75

4 4 4 4 4 4 16 4

65/16 65/16 65/16 65/16 65/16 65/16 65/16 90/0.6

a

Other conditions: rotation speed, 200 rpm; monomeric content of swelling mixture, DVB:MCA:St = 20:80:0 for latex I and DVB:MCA:St = 20:40:40 for latex A′.

Waiting Time before Postpolymerization and Postpolymerization Temperature. The aggregated system typically remained under stirring for 4 h before postpolymerization. Prolonging the waiting time to 16 h resulted in microclusters without any noticeable differences in either size or internal structure (see Table 3 and Figure 6). Indeed, 4 h is sufficient time for the system to reach dynamic equilibrium between aggregation and breakage.43

particles has a similar effect with decreasing their cross-linking degree. Consequently, the above discussion on the effect of cross-linking degree applies also to the current case. Actually, the shape of the structure factor plot is similar for microclusters prepared from primary particles swollen to a high degree (30% wt and above) and from primary particles with low cross-linker content (1% mol), exhibiting a change in slope for larger q values. That is also indicative of the similarity between these two materials, therefore no further characterization was deemed necessary. Concentration of Primary Particles Inside the Reactor. Increasing the concentration of primary particles of latex I inside the reactor from φ= 1 to 2 wt.%, increases the microclustrers size from 55 to 80 μm, while the df remains 2.95 (see Table 3 and Supporting Information Figure SI 7). This is an expectable result, as the size of aggregates of fully destabilized particles under turbulent conditions has been found to increase monotonically with the solid weight fraction, while their df is not affected, provided that steady state has been reached.27 Higher throughput can therefore be achieved by increasing the primary particles concentration, while the microclusters size may be controlled by properly adjusting the shear rate, as explained above. Salt Feeding Rate. The effect of salt feeding rate was examined using latex A′, by varying either the flow rate of the syringe pump or the concentration of the salt solutions fed. The feeding rate of the first salt gradient was increased from 0.55 to 1.61 mmol MgCl2/min, by increasing the flow rate, and then further to 2.07 mmol MgCl 2 /min, by increasing the concentration of the salt solution. This resulted in an increase of the microcluster size from 37, to 49, to 53 μm, respectively, while the respective df values remained the same and equal to 2.7 (see Table 3 and Supporting Information Figure SI 8). For an ideal DLCA aggregation under shear, the rate at which the CCC is approached should not affect the final steady state. Hence, the observed size increase, although not dramatic, indicates some nonideality in the system, particularly in the mixing of the salt. This could be associated with large pieces of colloidal gel forming locally at the point of salt injection, due to the inhomogeneity of the shear field above the impeller, combined with the large concentration of primary particles.42 Such large objects are subsequently not able to break effectively and cause a small increase in the measured average aggregate size. As the internal structure of the actual microclustes is not affected by this nonideality, the products of the feeding rate experiments were not further characterized. Nevertheless, these experiments indicate that the salt feeding rate is a parameter that should be carefully controlled.

Figure 6. Comparison of structure factors of microclusters prepared employing different waiting times before postpolymerization or different postpolymerization temperatures: (●) twait = 4 h, Tpp= 65 °C, tpp = 16 h; (△) twait = 16 h, Tpp = 65 °C, tpp = 16 h; (◇) twait = 4 h, Tpp = 90 °C, tpp = 0.6 h.

The increase of temperature for postpolymerization actually disrupts the steady state established during waiting. On one hand, the surfaces of polymers in a rubbery state become substantially more adhesive at higher temperatures.44,45 Thus, the bonds between the swollen primary particles should become stronger, increasing the fracture toughness of the aggregates and inducing an increase in aggregate size.30 On the other hand, the increased polymer chain mobility at higher temperatures may induce additional coalescence and compaction of the aggregates structure.22 A particularity of our system is that as the swelling monomer is consumed during postpolymerization, the aggregates are progressively rigidified and therefore become less prone to additional size growth or coalescence. Nevertheless, the net effect on the microclusters’ size should be a balance between these two opposing trends. In order to investigate the effect of postpolymerization temperature on the size and structure of our microclusters, the postpolymerization of latex A′ was conducted at 90 °C for 35 min, instead of the typical 65 °C for 16 h. The microclusters G

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

■

prepared at 90 °C were significantly larger than those prepared at 65 °C, with a radius of 74 vs 37 μm, respectively, yet their df remained the same (see Table 3 and Figure 6). It seems therefore that, with respect to the effects of postpolymerization at 65 °C, the time scale for postpolymerization at 90 °C is sufficient for additional growth to occur, but insufficient for any observable additional coalescence. As the internal structure was unaffected, further structural characterization was not pursued. Conclusively, elevated postpolymerization temperatures can be used as another increased throughput route for controlling the microclusters size.

■

ABBREVIATIONS ATRP, atom-transfer radical polymerization; CCC, critical coagulation concentration; DLCA, diffusion-limited cluster aggregation; DVB, divinylbenzene; ISEC, inverse size-exclusion chromatography; MCA, methyl α-chloroacrylate; SLS, static light scattering; St, styrene

CONCLUSION Following the introduction of reactive gelation under shear,16 which is a new procedure for preparing macroporous microparticles, without the use of any pore-generating agent, a comprehensive study of the effects of individual parameters has been presented. We have demonstrated that, thanks to the sequential character of this new method, the properties of the resulting microclusters, such as size and internal porous structure, can be independently controlled along the procedure. Such tuning flexibility is a big advantage over the conventional single-step approach for producing macroporous resins. In particular, the properties of the primary latex particles are pivotal to the final material. By progressively decreasing the rigidity of swollen primary particles, either by decreasing their cross-linking degree or by decreasing their MCA content, more compact, larger clusters, with smaller porosities and smaller surface areas are obtained. Decreasing the primary particle size brings about the same effect in compactness, size, and porosity, albeit larger surface areas are obtained. The pore size distribution again appears narrower, with pronounced small pore sizes by roughly one order of magnitude. The operating parameters during the procedure are also of significant importance. As expected, increasing the applied shear rate does not affect the internal cluster structure. Yet, it decreases the cluster size and porosity in a trend consistent with a previously established scaling, while the pore size distributions shift to smaller pore sizes. Increasing the amount of swelling monomer essentially decreases the rigidity of swollen primary particles and therefore has the same effect as decreasing the cross-linker or MCA content. Larger concentration of primary particles results in larger microclusters with the same internal structure. The waiting time before postpolymerization is not relevant, provided that a steady state between aggregation and breakage has been established. Last, performing the postpolymerization at a higher temperature yields larger microclusters without affecting their internal structure.

■

LIST OF SYMBOLS Csalt1/2 salt concentration using during first and second aggregation step (M) dp primary particle diameter (nm) dcore diameter of the core (nm) df fractal dimension (−) ⟨Rg⟩ mean radius of gyration (microns) rshell thickness of the shell (nm) CCC critical coagulation concentration (mM) Scomp computed primary particle specific surface area SBET surface area measured by BET (m2/g) SHg surface area measured by Hg intrusion porosimetry (m2/g) S(q) structure factor (−) q wave vector (1/nm) Qsalt salt flow rate using during aggregation (mL/min) Tg glass transition temperature twait waiting time for aggregation (h) Tpp temperature for postpolymerization (°C) tpp time of the postpolymerization (h) xswell weigth fraction of swelling mixture compare to polymer mass xdry final latex dry fraction (wt %) Greek symbols

ε intraparticle porosity (%) Vζ zeta potential (mV) ⟨γ⟩ average shear rate (1/s)

■

REFERENCES

(1) Hentze, H.-P.; Antonietti, M. Porous Polymers and Resins; WileyVCH Verlag GmbH: Amsterdam, 2008; pp 1964−2013. (2) Okay, O. Macroporous copolymer networks. Prog. Polym. Sci. 2000, 25, 711−779. (3) Sherrington, D. C. Preparation, structure and morphology of polymer supports. Chem. Commun. 1998, 21, 2275−2286. (4) Millar, J. R.; Marr, W. E.; Kressman, T. R.; Smith, D. G. Solventmodified polymer networks .3. Cation-exchange equilibria with some univalent inorganic + organic ions. J. Chem. Soc. 1964, 2740. (5) Winter, J.; Seidl, J.; Malinský, J.; Dušek, K.; Heitz, W. Makroporöse Styrol-Divinylbenzol-Copolymere und ihre Verwendung in der Chromatographie und zur Darstellung von Ionenaustauschern. In Fortschritte der Hochpolymeren-Forschung; Cantow, P. D. H.-J., Ed.; Springer: Berlin/Heidelberg, 1967; Vol. 5, pp 113−213. (6) Ugelstad, J.; Kaggerud, K. H.; Hansen, F. K.; Berge, A. Absorption of low-molecular weight compounds in aqueous dispersions of polymer-oligomer particles. 2. Step swelling process of polymer particles giving an enormous increase in absorption capacity. Makromole. Chem. 1979, 180, 737−744.

ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

■

ACKNOWLEDGMENTS

This work was financially supported by the Swiss National Science Foundation (Grant No. 200020-147137/1) and ETH Research Grant ETH-21 09-2. The authors acknowledge M. Hinca, Slovak Technical University, for performing the Hg porosimetry measurements.

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: e-mail:[email protected]. Phone: +41 44 6334659. Fax: +41 44 6321082. Notes

The authors declare no competing financial interest. H

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX

Langmuir

Article

(7) Cheng, C. M.; Micale, F. J.; Vanderhoff, J. W.; Elaasser, M. S. Pore structural studies of monodisperse porous polymer particles. J. Colloid Interface Sci. 1992, 150, 549−558. (8) Cheng, C. M.; Vanderhoff, J. W.; Elaasser, M. S. Monodisperse porous polymer particles - Formation of the porous structure. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 245−256. (9) Wang, Q. C.; Hosoya, K.; Svec, F.; Frechet, J. M. J. Polymeric porogens used in the preparation of novel monodispersed macroporous polymeric separation media for high-performance liquid chromatography. Anal. Chem. 1992, 64, 1232−1238. (10) Zhou, W. Q.; Gu, T. Y.; Su, Z. G.; Ma, G. H. Synthesis of macroporous poly(styrene-divinyl benzene) microspheres by surfactant reverse micelles swelling method. Polymer 2007, 48, 1981−1988. (11) Zhang, H. F.; Cooper, A. I. Synthesis and applications of emulsion-templated porous materials. Soft Matter 2005, 1, 107−113. (12) Svec, F. Porous polymer monoliths: Amazingly wide variety of techniques enabling their preparation. J. Chromatogr. A 2010, 1217, 902−924. (13) Svec, F.; Frechet, J. M. J. Kinetic control of pore formation in macroporous polymers - Formation of molded porous materials with high-flow characteristics for separations or catalysis. Chem. Mater. 1995, 7, 707−715. (14) Svec, F.; Frechet, J. M. J. Temperature, a simple and efficient tool for the control of pore-size distribution in macroporous polymers. Macromolecules 1995, 28, 7580−7582. (15) Dubinsky, S.; Il Park, J.; Gourevich, I.; Chan, C.; Deetz, M.; Kumacheva, E. Toward controlling, the surface morphology of macroporous copolymer particles. Macromolecules 2009, 42, 1990− 1994. (16) Lamprou, A.; Köse, I.; Storti, G.; Morbidelli, M.; Soos, M. Synthesis of macroporous polymer particles using reactive gelation under shear. Langmuir 2014, 30, 6946−6953. (17) Meakin, P. Fractal aggregates. Adv. Colloid Interface Sci. 1988, 28, 249−331. (18) Fitch, R. M. Polymer colloids: A comprehensive introduction; Academic Press: San Diego, 1997. (19) Trtik, P.; Soos, M.; Muench, B.; Lamprou, A.; Mokso, R.; Stampanoni, M. Quantification of a single aggregate inner porosity and pore accessibility using hard X-ray phase-contrast nanotomography. Langmuir 2011, 27, 12788−12791. (20) Matyjaszewski, K.; Tsarevsky, N. V. Nanostructured functional materials prepared by atom transfer radical polymerization. Nat. Chem. 2009, 1, 276−288. (21) Harshe, Y. M.; Lattuada, M.; Soos, M. Experimental and modeling study of breakage and restructuring of open and dense colloidal aggregates. Langmuir 2011, 27, 5739−5752. (22) Gauer, C.; Wu, H.; Morbidelli, M. Control of coalescence in clusters of elastomer colloids through manipulation of polymer composition. Macromolecules 2009, 42, 9103−9110. (23) Arosio, P.; Xie, D. L.; Wu, H.; Braun, L.; Morbidelli, M. Effect of primary particle morphology on the structure of gels formed in intense turbulent shear. Langmuir 2010, 26, 6643−6649. (24) Takata, T.; Endo, T. Recent advances in the development of expanding monomers - Synthesis, polymerization and volume change. Prog. Polym. Sci. 1993, 18, 839−870. (25) Kunin, R.; Meitzner, E.; Bortnick, N. Macroreticular ion exchange resins. J. Am. Chem. Soc. 1962, 84, 305−&. (26) Mayo, M. J.; Hague, D. C.; Chen, D. J. Processing nanocrystalline ceramics for applications in superplasticity. Mater. Sci. Eng., A 1993, 166, 145−159. (27) Guyot, A.; Bartholin, M. Design and properties of polymers as materials for fine chemistry. Prog. Polym. Sci. 1982, 8, 277−331. (28) Soos, M.; Moussa, A. S.; Ehrl, L.; Sefcik, J.; Wu, H.; Morbidelli, M. Effect of shear rate on aggregate size and morphology investigated under turbulent conditions in stirred tank. J. Colloid Interface Sci. 2008, 319, 577−589. (29) Viklund, C.; Svec, F.; Frechet, J. M. J.; Irgum, K. Monolithic, “molded”, porous materials with high flow characteristics for

separations, catalysis, or solid-phase chemistry: Control of porous properties during polymerization. Chem. Mater. 1996, 8, 744−750. (30) Zaccone, A.; Soos, M.; Lattuada, M.; Wu, H.; Babler, M. U.; Morbidelli, M. Breakup of dense colloidal aggregates under hydrodynamic stresses. Phys. Rev. E 2009, 79, 6. (31) Xie, D. L.; Wu, H.; Zaccone, A.; Braun, L.; Chen, H. Q.; Morbidelli, M. Criticality for shear-induced gelation of chargestabilized colloids. Soft Matter 2010, 6, 2692−2698. (32) Ford, W. T.; Lee, J.; Tomoi, M. Mechanisms of polymersupported catalysis. 3. Ion exchange limitations and macroporous polystyrene supports. Macromolecules 1982, 15, 1246−1251. (33) Loyens, W.; Groeninckx, G. Deformation mechanisms in rubber toughened semicrystalline polyethylene terephthalate. Polymer 2003, 44, 4929−4941. (34) Roulin-Moloney, A. C. Fractography and failure mechanisms of polymers and composites; Elsevier Applied Science: London, 1989. (35) Brook, R. J. Pore-grain boundary interactions and grain growth. J. Am. Ceram. Soc. 1969, 52, 56. (36) Dever, G. R.; Karasz, F. E.; Macknight, W. J.; Lenz, R. W. Poly(alkyl alpha-chloroacrylates) 5. Preparation and properties of methyl, ethyl and isopropyl polymers of varied tacticity. J. Polym. Sci., Part A: Polym. Chem. 1975, 13, 2151−2179. (37) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook; John Wiley & Sons, Inc.: New York, 1999. (38) Ehrl, L.; Soos, M.; Morbidelli, M. Dependence of aggregate strength, structure, and light scattering properties on primary particle size under turbulent conditions in stirred tank. Langmuir 2008, 24, 3070−3081. (39) Foley, J. P.; Dorsey, J. G. Equations for calculation of chromatographic figures of merit for ideal and skewed peaks. Anal. Chem. 1983, 55, 730−737. (40) Guiochon, G.; Felinger, A.; Shirazi, D.; Katti, A. Fundamentals of Preparative and Nonlinear Chromatography; Academic Press, Elsevier Inc.: San Diego, 2006. (41) Guiochon, G. Monolithic columns in high-performance liquid chromatography. J. Chromatogr. A 2007, 1168, 101−168. (42) Soos, M.; Kaufmann, R.; Winteler, R.; Kroupa, M.; Luthi, B. Determination of Maximum Turbulent Energy Dissipation Rate generated by a Rushton Impeller through Large Eddy Simulation. AIChE J. 2013, 59, 3642−3658. (43) Moussa, A. S.; Soos, M.; Sefcik, J.; Morbidelli, M. Effect of solid volume fraction on aggregation and breakage in colloidal suspensions in batch and continuous stirred tanks. Langmuir 2007, 23, 1664−1673. (44) Hammerschmidt, J. A.; Gladfelter, W. L.; Haugstad, G. Probing polymer viscoelastic relaxations with temperature-controlled friction force microscopy. Macromolecules 1999, 32, 3360−3367. (45) Tsui, O. K. C.; Wang, X. P.; Ho, J. Y. L.; Ng, T. K.; Xiao, X. D. Studying surface glass-to-rubber transition using atomic force microscopic adhesion measurements. Macromolecules 2000, 33, 4198−4204.

I

dx.doi.org/10.1021/la502153j | Langmuir XXXX, XXX, XXX−XXX