Swelling-Induced Deformation of Spherical Latex Particles

Article pubs.acs.org/Macromolecules

Swelling-Induced Deformation of Spherical Latex Particles Chunxiang Wei,† Alexander Plucinski,† Sukanya Nuasaen,† Amit Tripathi,† Pramuan Tangboriboonrat,‡ and Klaus Tauer*,† †

Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany P. Tangboriboonrat Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Phyathai, Bangkok 10400, Thailand

‡

ABSTRACT: Experimental evidence is presented showing that the direct formation of anisotropic colloidal polymer particles via aqueous heterophase polymerization is essentially controlled by the entropy gain of the linear fraction in the semi-interpenetrating network of the seed particles during swelling. Anisotropic particles are produced via photoinitiated polymerization, allowing swelling and polymerization to take place at the same temperature. These experiments prove that the temperature effect on rubber elasticity is, if at all, only of minor importance for the formation of anisotropic polymer particles. The major significance of swelling of the seed particles for the whole process is underlined by additional studies with optical microscopy and model simulations.

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INTRODUCTION General Remarks. Heterophase polymerization sets up an exciting connection between applied colloid and polymer chemistry. The colloidal state influences the polymerization kinetics and causes, compared to homogeneous conditions, either advantages or disadvantages with respect to polymerization kinetics and product properties. From the physical or physicochemical point of view, polymer swelling and the associated swelling pressure, in general, strongly interconnect both disciplines.1−4 Additionally, there is the well-established assumption for a tight relation between the temperature effect on rubber elasticity and the possibility to produce directly nonspherical or anisotropic latex particles via heterophase polymerization.5,6 For many mass applications of heterophase polymerization products (synthetic latexes or polymer dispersions), the shape of the particles has received comparatively limited interest. This is basically an expression of two facts. First, the typical shape of latex particles is that of a sphere and second, for most applications of latexes a specific particle shape is not required but film formation.7 Any deviation from spherical shape is rather untypical, to achieve or even to maintain nonsphericity needs special care with respect to polymerization strategy. The sphere is the equilibrium shape because it represents at given interfacial tension the minimum excess free energy, provided the mobility of the material inside the latex particles is high enough to level out possible fluctuations. However, sometimes unintentionally generated nonspherical particles have been observed in various heterophase polymerizations.8 In the majority of cases, these are the result of limited particle coagulation leading to dimers or trimers which are still small enough not to settle down quickly but stay dispersed within the latex. For statistical reasons and provided the coagulation rate is © XXXX American Chemical Society

not too high, the number concentration of the dimers is greater than that of the trimers but the sum of both is much smaller than that of the single spheres. The mass production of nonspherical latex particles directly via aqueous heterophase polymerization is quite a tedious process and requires more than one subsequent polymerization step, typically three with additionally repeated purification stages in between. Moreover, the seed particles for the last polymerization step must be composed of semi-interpenetrating network (SIPN). The experiments described so far in the public scientific literature seemingly are in agreement with the mechanistic idea based essentially on elastic network relaxation by increasing temperature. Quite a huge amount of experimental and modeling evidence is presented apparently supporting the idea that the temperature effect of the rubber elastic network in the SIPN particles is responsible for the formation of anisotropic shape. Accordingly, the development of a temperature-driven force (F) by the rubbery network fraction in the particles which according to the Maxwell relations, (1) leads to a decrease in entropy (S) with mesh length (l) of the network and hence, results in a positive restoring force during heating (2). ⎛ ∂F ⎞ ⎛ ∂S ⎞ ⟨F ⟩≈⎜ ⎟ = −⎜ ⎟ ⎝ ∂T ⎠V , l ⎝ ∂l ⎠T , V

(1)

∂F >0 ∂T

(2)

Received: November 4, 2016 Revised: December 12, 2016

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DOI: 10.1021/acs.macromol.6b02379 Macromolecules XXXX, XXX, XXX−XXX

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poly(vinyl alcohol)16−18 and polydimethylsiloxane19 matrix have been communicated by groups from the University of Bristol (U.K.) and the Purdue University (USA), respectively. Later the mechanical stretching was applied to produce also nanometer-sized polymer ellipsoids.20 Both methods require several preparation steps, are quite time-consuming, and need skilled experimenters. Typically, the work quoted above deals with micrometer-sized anisotropic particles which have the advantage of easy characterization with optical microscopy. There is of course no reason which prevents the production of nanometer-sized anisotropic particles by this technique which was successfully proven by Mock et al.13 Another direct synthetic route to control the shape of polymeric particles is the use of microfluidic devices for monomer droplet formation followed by fast photopolymerization in the capillary channels.21,22 This technique can be considered as a combination of mechanical shaping (here the monomer droplets) and polymerization. It is, however, beyond the scope of this contribution. For the sake of completeness in this brief summary, the great contributions of Okubo’s group should be mentioned.23−27 They developed various control strategies to shape polymer particles to different final morphologies utilizing the thermodynamics of swelling of colloidal latex particles and tailored polymerization conditions. Aim and Experimental Approach of this Study. Here we communicate experimental results proving that the swelling of SIPN seed particles is the crucial step for the synthesis of anisotropic polymer particles via seeded heterophase polymerization but not the increase of temperature. We adopted the experimental procedure used in Weitz’ group,9 however applied decisive alteration with respect to initiator and polymerization temperature. Most of our experiments comprise three successive polymerizations with alternately changing monomer mixture in each step as outlined in Figure 1. Special emphasis is

However, swelling of a polymer network is inevitably accompanied by changes in pressure (P) and volume (VdP or PdV), and hence, a mechanical work is acting in any case and possibly should not be neglected either. This mechanical work can contribute to deformation of the swollen, soft rubber-like particles. The swelling pressure adapted for colloidal SIPN particles plays an important role in this context. Swelling of SIPN particles with a monomer and subsequent polymerization cause the stabilization of the deformed structure provided the glass transition temperature of the resulting particle is high enough to withstand thermally induced shape equilibration. Brief Review of the State of the Art. As indicated, the intentional synthesis of nonspherical particles requires a welldesigned strategy and is everything but easy when focusing on predetermined form with uniformity of shape and size. Nevertheless, several ways to synthesize nonspherical particles are known which can be subdivided into two basic approaches: (I) the direct route via polymerization and (II) the modification of preformed particles without polymerization. Only a few recently published papers covering this topic should be mentioned here.9−11 Potential application possibilities for nonspherical or anisotropic polymeric particles are also mentioned in these papers and include biomedical and electronic areas. Here, the crucial point is that for certain applications of colloidal particles not only size but also shape matters. The first paper describing a direct polymerization route leading to well-defined nonspherical particles appeared 1986.12 Ugelstad et al. describe a synthetic procedure based on phase separation during seeded heterophase polymerization with a different second stage monomer or mixture where the seed particles are expelled from the final particles due to the incompatibility between both polymers. Unfortunately, a detailed experimental section is missing in this communication. Shortly after that, the Lehigh group published a comprehensive experimental study elucidating details of the separation mechanism.5 These results convincingly proved, that both the entropy-driven elastic recovery of the network during the temperature increase after swelling of the seed particles at room temperature and the incompatibility of the first and second stage polymer, contribute to anisotropic shape transformation of the spherical seed particles. In a subsequent study with toluene as swelling agent the authors showed that spherical swollen polystyrene (PS) SIPN particles develop even at room temperature bulges but only after maturation of 24 days. In contrast, bulge formation was observed already after a few minutes at temperature of 70 °C.6 Obviously, the anisotropic form of swollen SIPN particles is the equilibrium shape. However, to what extent, or even if so ever, the viscoelastic properties of the rubbery network (contraction upon temperature increase) contribute to the deformation is difficult to assess. Nevertheless, this strategy was later adopted by other groups and is still quite popular, cf. refs 9, 13, and 14. A clever strategy to check the influence of the network density (or the degree of cross-linking) was published by Kim et al.14 The authors show, that by manipulating the cross-linking density the directionality of the phase separation during the seeded polymerization can be controlled. Also in 1986, a paper appeared describing the electronmicroscopic characterization of ellipsoidal polymer particles made via mechanical stretching.15 But only a few years later, in the early 1990-ies, details of the mechanical stretching of micrometer-sized spherical polymer particles embedded in a

Figure 1. Illustration of the procedure for the synthesis of anisotropic particles via the direct polymerization route comprising three consecutive polymerizations (P1−P3) and two swelling steps; typically but not necessarily the monomers for P1 and P2 are the same, however, P2 is carried out in the presence of a cross-linker and leads to the formation of a semi-interpenetrating network (SIPN), and for P3 a different monomer can be chosen; the different shades of gray sketched in the anisotropic particles represent different polymers assumed to be in the final particles: hatched area, SIPN portion; dark gray, portion of the polymer generated during P3; light gray, mixed portion of polymer (mainly due to chain transfer to polymer).

placed on the changes of the morphology during the whole synthesis process. The so far published results are exclusively based on experiments where the final polymerization was carried out at elevated temperatures and the swelling of the seed particles at room temperature. The idea behind this procedure is to freeze the anisotropic shape generated by temperature driven phase separation due to the elastic network B


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and carried out for 22 h. The linear PS particles were further used as seed in the preparation of SIPN particles in P2. Synthesis of SIPN Polystyrene Seed Particles (P2). A 5.0 mL aliquot of linear PS seed particles (20 w/v %) dispersed in a 1 w/v % poly(vinyl alcohol) (PVA) aqueous solution was mixed with a 20 vol % monomer emulsion which was made in a 1 w/v % PVA aqueous solution by homogenizing the mixture at 8000 rpm for 1 min. The monomer phase (styrene and DVB) contained the initiator (V-65 or Irgacure819, 0.5 wt %). The DVB content was varied between 0, 1, 5, and 10 vol % based on total amount of monomer. The volume ratio of the monomer solution to the seed particles was 4:1. The mixture was swollen with PS seed particles at room temperature for 20 h. If applied, then the aqueous phase inhibitor (10−4 mol/L Fremy’s salt) was added immediately before starting the polymerization which was performed at 70 °C for 8 h in an oil bath. Photopolymerizations were carried out at room temperature (20 °C) for 20 h by placing the reaction vials in front of normal fluorescence tubes used also for laboratory illumination. Preparation of Anisotropic (ANI) Particles (P3). Anisotropic particles were synthesized via seeded polymerization with SIPN seed particles. A 5.0 mL dispersion of SIPN particles (20 w/v %) was prepared in a 1 w/v % poly(vinyl alcohol) (PVA) aqueous solution. The SIPN seed particle dispersions was mixed with a 20 vol % monomer emulsion which was prepared in a 1% w/v PVA aqueous solution by homogenizing at 8,000 rpm for 1 min. The monomer emulsion (either MMA or styrene) contained the initiator (either V-65 or Irgacure819, 0.5 wt %). The volume ratio of the monomer solution to the SIPN particles was 4:1. Swelling was allowed at room temperature for 20 h. If applied, then the aqueous phase inhibitor (10−4 mol/L Fremy’s salt) was added immediately before starting the polymerization which was performed at 70 °C for 8 h in an oil bath. Photopolymerizations were carried out at room temperature (20 °C) for 20 h by placing the reaction vials in front of normal fluorescence tubes used also for laboratory illumination. Isolation of Target Particles. After each polymerization (P1−P3) the following procedure was applied to isolate the target particles. The purification was carried out in order to isolate the desired reaction product from the reaction mixture. This is an important step necessary after each polymerization! Isolation of the target particles was performed via repeated centrifugation and redisperion of the sediment. Particularly, the residual monomers, electrolytes, water-soluble oligomers, and byproducts were removed by centrifugation (5000 rpm, 20 min) and washed thrice with water. Identification of Anisotropic Phase: Treatment with Selective Solvents. ANI particles were dispersed in glacial acetic acid (1 mg/mL) which is a selective solvent for PMMA at room temperature for 24 h. The dispersed ANI particles were allowed to sedimentation for 24 h, dissolved and undissolved components were separated. After that the remains were treated with THF to dissolve the linear polystyrene. The leftovers after each solvent treatment were characterized by SEM. Synthesis of Large PS−SIPN Particles for Optical Microscopy. Large monodisperse seed particles have been synthesized by dispersion polymerization in butanol. Then 12 g of poly(vinylpyrrolidone) (K-29-32) from Sigma-Aldrich dissolved in 120 g of butanol, 5 g of toluene, 0.25 g of Aerosol OT dissolved in 15 g of butanol, 0.3 g of resorcinol dissolved in 12 g of butanol, and 20 g of styrene were mixed under nitrogen atmosphere. After styrene addition, the temperature was raised to 80 °C and 0.2 g of 2,2′-azobis(isobutyronitrile) dissolved in 8 g of butanol were added to start the polymerization. After 20 h, the dispersion was repeatedly centrifuged, the supernatant discarded, and the sediment redispersed with butanol, then two times with methanol, and finally three times with water. The average particle size as determined by optical microscopy and SEM is about 5.5 μm. These particles were subjected to a seeded polymerization according to the procedure described above for P2 followed by repeated washing procedure. Microscopy. Scanning electron microscopy (SEM) was performed according to standard procedures with a high-resolution scanning

recovery of swollen, partly cross-linked seed particles during thermally initiated radical polymerization. Consequently, the temperature increase acts in a 2-fold manner: causing the network retraction and the start of the radical polymerization. With this study we mainly pursued two goals. First, we wanted to achieve clarity over the prerequisites for and the behavior of the seed particles during the second seeded polymerization step (P3) leading to anisotropic particles. For this we varied the cross-linker content applied during P2 between zero and ten percent relative to the amount of styrene. In addition some seeded polymerizations were carried out not with SIPN particles but with only cross-linked seed particles (meaning P2 was skipped and P1 was carried out in the presence of cross-linker) containing no (or at least a very much reduced content of) linear polymer chains. The second main goal of our investigation was to get an idea to what extent the entropy-elastic recovery of the rubbery network at elevated temperature contributes to the formation of the particles’ shape. In order to do this we avoided the increase in temperature between swelling of the seed particles and subsequent polymerization. For the analysis of our results, we focused on the characterization of shape and morphology of the particles obtained after each polymerization step. To facilitate the morphological investigation, we have chosen another monomer than styrene for some experiments during P2 and P3. Particularly, the combination of styrene and methyl methacrylate allows the easy application of glacial acetic acid as selective solvent for the poly(methyl methacrylate) (PMMA) portions in order to study shape and morphology of the remnants. In addition we want to draw attention at and emphasize another peculiarity of any heterophase polymerization, the formation of byproducts, i.e. particles with size and morphology different from the envisaged target. To get rid of these unwanted products, special measures must be applied. In order to limit the polymerization to the seed particles, water-soluble inhibitors such as potassium nitrosodisulfonate (Fremy’s salt) and/or cleaning procedures for fractionation can be applied.28 The former strategy needs a well-balanced recipe with respect to the concentration ratio of radicals in the aqueous phase and inhibitor molecules which is, however, hard to meet. It is a matter of experimental experience that the second strategy has to be applied in many (almost all) cases.

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EXPERIMENTAL DETAILS

Materials. Styrene (St) (Sigma-Aldrich, purum) and methyl methacrylate (MMA) (Fluka, purum) were distilled under reduced pressure to remove inhibitor and stored in refrigerator until use. Poly(vinyl alcohol) (PVA) type M05/140 saponification number 140 with 86−89 mol % OH group was a gift from Wacker Chemie, Germany. Water was taken from a SG purification system with a conductivity of 0.055 μS cm-1. Divinylbenzene (DVB) (Merck or Alfa Aesar), ammonium persulfate (APS, Sigma-Aldrich), 2,2′-azobis(2,4dimethylvaleronitrile) (V-65, Wako), photoinitiator Irgacure819 [bis(2,4,6-trimethylbenzoyl)- phenylphosphineoxide] from Ciba, potassium nitrosodisulfonate (Fremy’s salt) (Santa-Cruz Biotechnology), tetrahydrofuran (THF) and glacial acetic acid (gAA), both from Sigma-Aldrich, were used as received. Synthesis of Linear Polystyrene Seed Particles (P1). Linear PS latex particles were synthesized by the surfactant-free emulsion polymerization.29 The deionized water (DI water, 190 g) was degassed with nitrogen gas for 30 min before adding styrene monomer (STY, 20 g). 0.2 g ammonium persulfate (APS) was dissolved in 10 g water, and the polymerization was started with addition of APS solution at 70 °C C


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polymerization sequence P1 → P2 → P3 (cf. Figure 1) proving the above statement.

electron microscope operating at an acceleration voltage of 3 kV (LEO1550 Gemini, Carl Zeiss AG, Germany). Optical light microscopy was carried out with a Keyence VH-X digital microscope (Keyence, Osaka, Japan) either with an objective VH-Z100 or VH-Z500 allowing magnifications up to 1,000 and 5,000fold, respectively.

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RESULTS AND DISCUSSION

Prior to creation of anisotropic particles using SIPN seed particles, we will briefly discuss the challenges in producing uniform particles. Despite all precautions, batch ab initio seeded heterophase polymerizations results frequently in creating small amount of secondary particles (or fines). This aspect along with removal procedure of such secondary particles is discussed first. Afterward the results from polymerization of SIPN particles at room temperature and high temperature (70 °C) are compared. Particularly to obtain further insight in the swelling process and anisotropy development, results of swelling studies using nonpolymerizable swelling agent, obtained with optical microscopy, are discussed afterward. This discussion is succeeded by thermodynamic modeling of the swelling process which generates protrusion in SIPN particles. In the end, the impact of surfactant and stirring is discussed. Uniformity of the Reaction Product. This is, according to our understanding, an important general issue of heterophase polymerization. The nonuniformity of the polymer chains is widely accepted and holds for both homogeneous and heterogeneous polymerizations. One should always bear in mind that a polydispersity index (defined as ratio of weight to number-average value) as low as 1.05, assuming Gaussian distribution, means that less than 2% of all chains have the length corresponding to the peak value.30 During heterophase polymerization the particle size distribution superimposes that of the chain length and the kinetics of chain growth depends on the particle size. This fact is key for understanding that during heterophase polymerization the nonuniformity of the polymeric reaction product is even greater. Another issue is, as frequently observed particularly during seeded polymerization, the formation of a second particle generation in a very different size range (much smaller or bigger as the main fraction, socalled particle fines or coagulum, respectively). Interestingly, this can also happen when the generation of growing radicals is restricted entirely to the organic phase. Experimental results and a possible explanation have recently been published.1 Multiple Products of Seeded Heterophase Polymerization. In an ab initio batch aqueous heterophase polymerization (HetP) with initially present monomer droplets a major reason for the heterogeneity of the reaction products is the fact that reactions take place in various loci (water phase, monomer droplets, and latex particles after their generation) under very different conditions with respect to the concentration of the reactants. The quantities of the reactants in the loci of HetP are controlled by thermodynamic, dynamic, and kinetic parameters such as partition coefficients, diffusion coefficients, and reaction rate constants. The consequences of the diversity of reaction loci and products during HetP are significant.1 Typically the reaction product of HetP is not uniform with respect to chain length and particle size. To develop a single step polymerization leading to almost monodisperse latex particles is already quite challenging.31,32 However, for multistep procedures comprising alternating polymerization and swelling steps the experimental challenge is even greater. Figure 2 shows an example for the

Figure 2. SEM micrographs of latexes as obtained after the polymerization (without purification) illustrating the formation of small sized particles during the polymerization sequence P1 (a) → P2 (b) → P3 (c). The scale bar indicates 1 μm for all images; P1 was carried out with APS at 70 °C, and P2 and P3 were both carried out with Irgacure819 at room temperature.

An essential step, to obtain the desired product in the highest possible purity, is the fractionation or separation of the reaction product after polymerization, in which one gets rid of all unwanted byproducts. For typical polymerizations considered in this study an example is given in Figure 3. The degree of diversity of the final

Figure 3. SEM micrographs illustrating the effectiveness of the sedimentation procedure used for fractionation of final latexes exemplarily for a latex made with 1% cross-linker in P2 and V-65 in P3: (a) original latex as obtained after polymerization; (b) particles in redispersed sediment; (c) particles in supernatant after sedimentation with an average diameter of 166 nm. The scale bar marks 2 μm for images a and c and 1 μm for image b.

product depends on polymerization conditions, particularly on the content of cross-linker for the polymerizations considered here. In addition, the solids content of the latex used in the sedimentation−redispersion cycle and the number of repeats determines the level of fractionation. Typically, at least three repeats are necessary to achieve an almost complete separation. D


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the data disclose quite a strong influence of the cross-linker content (the strength of the polymeric network) on the particular shape of the anisotropic particles. The graphs of Figure 5 show how the ratio of the long to short axis of the particles changes in dependence on the crosslinker content for various polymerization conditions during P3 with respect to monomer (styrene and MMA) and initiator (V65 and Irgacure819). The experimental data confirm a significant influence of the network strength of the SIPN particles (after P2) on the anisotropic deformation of the particles after P3. The higher the cross-linker content during P2 the smaller the ratio long to short axis of the final anisotropic particles. Also, the influence of the compatibility of the third stage polymer with the polymer of the seed particles diminishes with increasing cross-linker content. Finally, a similar trend is observed for the influence of the polymerization temperature. At a cross-linker content of only 1% in the monomer mixture for P2, the polymerization temperature during P3 has quite a strong influence whereas at 10% cross-linker it is, within the experimental error, not significant anymore. The linear correlation suggests that at a cross-linker content of 69.2% the anisotropy disappears completely and aR again equals 1, as also experimentally observed for zero cross-linker content. Another extreme situation in this context is the use of interpenetrating network or microgel particles, that is, without any linear polymer portion in the seed particles subjected to P3. The IPN particles were prepared by adding the corresponding amount of DVB directly to P1 and skipping P2. In contrast to SIPN particles, the use of IPN particles as seed for P3 leads with respect to the morphology to very different final particles. In this case, the resulting morphology depends quite strongly on the content of cross-linker (network strength). Typical features as observed for lower and higher network strength are illustrated by the SEM micrographs put together in Figure 6. Micrograph b of Figure 6 reveals at low cross-linking density of 1% at least two events. The first observation is the occurrence of particle coagulation as beside singlet also duplets and triplets of particles can be seen. Obviously, after encounters the particles occasionally cannot depart again and stick together. In a sample of 415 particles 345 (83.1%) of single, 57 (13.7%) of double, and 13 (3.1%) of triple particles have been counted. Admittedly, the sample size is not very large but the result seems nevertheless reasonable. The second observation is that the single particles possess a certain degree of anisotropy because they are deformed with an axes ratio aR of about 1.2. If the cross-linking density in the IPN seed particles is increased (10%) the situation after the final polymerization changes completely (cf. micrograph d of Figure 6). Interestingly, the size and shape of both IPN seed particles (cf. micrographs a and c of Figure 6) is very similar but is drastically different for the final particles (cf. micrographs b and d of Figure 6). Surprisingly, in the case of the seed particles with the high cross-linking density (micrograph d of Figure 6), the particle size distribution of the final dispersion is extremely broad and does not resemble at all that of the seed particles. The micrograph shows particles which are both about an order magnitude larger and smaller than the seed particles. Moreover, the surface of the larger particles has a golf ball−like appearance which resembles budding of cell membranes.35 The noticeable influence of the content of oil-soluble crosslinker in aqueous emulsion polymerization is well-known since the early days.36 Interestingly, it has also been observed for oil

Despite the different level of fractionation the examples put together in Figure 3 clearly prove the benefit of the sedimentation technique. As expected, the small-sized particles show an almost spherical shape in any case (Figure 3 c) and are composed of the polymer corresponding to the third stage monomer (MMA in the example shown in Figure 3 as easily proven by FT-IR spectroscopy). The interesting question to be answered in this context is that regarding the formation mechanism of the tiny particles when the seeded polymerization is initiated with oil-soluble initiators (V-65 and Irgacure819). The solubility of both initiators in water is extremely low, much lower than that of 2,2azobis(isobutyronitrile) (AIBN) which can be considered as kind of standard oil-soluble initiator for emulsion polymerization.33 Particularly, Irgacure819 has an extremely low solubility in water of below 10−7 M,34 which makes desorption and reactions in the aqueous phase and subsequent particle nucleation rather unlikely. Also, it should be mentioned that the application of Fremy’s salt could not prevent the formation of the secondary particles. Another explanation is needed and has been recently proposed1 and considers, based on experimental data, the swelling pressure as possible cause. However, the discussion of this idea is beyond the scope of the present contribution. Formation of Anisotropic Particles. The micrographs a and b of Figure 3 show already the successful generation of anisotropic particles during the last polymerization stage for the “standard” procedure utilizing SIPN seed particles and swelling at lower temperature than subsequent polymerization. However, the micrographs put together in Figure 4 reveal that the temperature gap between both steps is not a necessary condition for the synthesis of anisotropic particles. In addition,

Figure 4. SEM micrographs of anisotropic particles obtained with V65 at 70 °C (images a, c, e) and with Irgacure819 at room temperature after P3 for various amount of cross-linker during P2 (a, b. 1 vol %; c, d, 5 vol %; e, f, 10 vol % DVB in the overall monomer mixture), with styrene as monomer during P1 and P2 and MMA as monomer during P3. E


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Figure 5. Correlation between the ratio long to short axis (aR) of the particles after P3 and the cross-linker content applied during P2 (XL) for making the seed particles: (a) individual data points for the various conditions during P3 as given in the inset; (b) averaged data points at a given XL over all polymerization conditions, the dashed line is a linear regression of the experimental data points for XL > 0.

Figure 6. SEM micrographs of IPN seed particles (a, c) and the resulting final particles (b, d) with V-65 as initiator and MMA as monomer during the seeded polymerization: a (IPN seed particles with 1% DVB) → b; c (IPN seed particles with 10% DVB) → d. The scale bar marks 1 μm for parts a, b, and c and 2 μm for part d.

Figure 7. SEM micrographs showing the decomposition of PS− PMMA composite particles synthesized in the absence of cross-linker during P2 by application of gAA: (a, b) particles subjected to decomposition (a, prepared with V-65 at 70 °C; and b, prepared with Irgacure819 at room temperature during P3); (c, d) remnants after gAA treatment a → c and b → d. The he scale bar indicates 1 μm for parts a, b, and d and 200 nm for part c; the sketch on the left illustrates the decomposition procedure.

in water magnetic droplets as seed in styrene−DVB HetP where the morphology of the final magnetic particles depends on the DVB content.37,38 Treatment of Anisotropic Particles with Selective Solvents. The composite particles can be decomposed again by the treatment with selective solvents. Subsequent treatment of the anisotropic PMMA−PS particles with gAA and THF left the cross-linked PS portion. Again, we focus on the influence of the content of cross-linker during P2. In the absence of crosslinker, the overall shape of the final particles is almost perfectly spherical (cf. Figure 5) but nevertheless the morphology exhibits an interesting feature (cf. Figure 7). Both the morphology of the final PS−PMMA composite particles, prepared in the absence of cross-linker during P2, and that of the PS fragments after treatment with gAA show a distinct influence of the polymerization temperature during P3 (micrographs a, b of Figure 7). Reasonably, the phase separation between both incompatible polymers is facilitated at higher polymerization temperature. Consequently, the particles of micrograph Figure 7a exhibit a Janus-like morphology with a smooth PS and a rather rugged PMMA part. This difference is due to the limited stability of PMMA in the electron beam compared with PS. 39 The PMMA polymerized at room temperature is much more evenly

distributed around the PS seed particles. This could have been expected considering the different polarity (hydrophilicity) of both polymers. The most surprising result, however, is the different morphology of the PS residuals in both cases. Polymerization at 70 °C leads to PS leftovers with smooth surfaces whereas in the other case particles exhibiting rough surfaces and perforations are observed. The easier phase separation at higher temperature during stage 3 might also contribute to this difference. Increasing the cross-linker content during P2 from 1% (Figure 8) to 10% (Figure 9) has a distinct influence on the morphology of the remnants after each defragmentation step. The SEM micrographs in Figure 8 and 9 show that the anisotropic PS−PMMA composite particles possess a Janus− like morphology with one part appears quite smooth and the other shows quite a few wrinkles. The shriveled portion of the particles is richer in PMMA than the smoother one. The morphology of the composite particles looks very similar to that obtained during irradiation of PS−PMMA core−shell particles with an electron beam.40 The anisotropy is greater of the final composite particles made with 1% than that of the ones made with 10% cross-linker seed particles (cf. also Figure 5). Taking away the PMMA fraction of the composite particles with gAA left the PS−SIPN. As expected, also the PS−SIPN F


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whereas for the fragments with the lower content of crosslinker two phases are only hardly to distinguish (micrographs c, d of Figure 8). Independent of the polymerization temperature during P3 the PS−SIPN fragments show, most clearly for the samples with the higher content of cross-linker, a smooth or compact component and another perforated or riddled component. Looking at the morphology of the remnants after THF treatment which takes away the linear PS chains, particularly for micrographs of Figure 9, the smoother fragment represent obviously the cross-linked portions. However, the most important results with respect to the changes during P3 based on the decomposition studies are the following (cf. micrograph b of Figure 2 which shows the morphology of the particles after P2). First, the shape of the PS−SIPN particles deforms and changes from spherical to anisotropic. Second, the linear and cross-linked portion of the SIPN seed particles which are homogeneous after P2 undergo phase separation. These conclusions rely on the fact that gAA is a selective solvent for PMMA and unable to swell polystyrene. In contrast, THF easily dissolves the linear PS and swells the cross-linked parts up to a certain level which depends on the cross-linking density. Obviously, the degree of swelling of the network made with 1% cross-linker is so high that the molecules in the swollen network are mobile enough allowing the particles to return to its original spherical shape (cf. micrographs e, f of Figure 8). Increasing the content of crosslinker to 10% the network strength is so high that the network even in the swollen state does not relax to the spherical shape (cf. micrographs e, f of Figure 9). Direct Observation of Swelling-Induced Deformation of SIPN−Latex in Contact with Swelling Agent. Knowing now that the temperature increase after swelling is not a necessary requirement, it is quite straightforward to use optical microscopy to study online the morphology development during swelling of SIPN seed particles. In this investigation, we are observing the step before starting P3, however, not in the presence of a monomer but a nonpolymerizable solvent (ethylbenzene, EB). The technical details and procedure have been described previously for swelling studies of ordinary latex particles.41 It is important to note that the solvent is placed on top of the latex inside a glass cuvette (with a thickness of 1 or 2 mm) which is positioned either perpendicular standing or horizontally lying, slightly inclined, with respect to the objective of the microscope. Recently published experimental as well as simulation data show that the accumulation of swelling agent at the particle− water interface is crucial for fast swelling.42,43 Consequently, the intensity of mixing the insoluble swelling agent and latex particles, in essence the collision frequency between swelling agent drops and polymer particles, is critical.55 This fact immediately reveals the challenge of a direct microscopic observation of the shape transition because it requires resting particles but for intense mixing a corresponding hydrodynamic force field is needed causing the particles and droplets similarly to move. The issue can be tackled in a suitable way by controlling the pressure while plunging the swelling agent on top of the latex in the cuvette. Applying a higher pressure during the addition of the swelling agent with a syringe leads to formation of more drops and vice versa. Figure 10 summarizes the outcome by comparing the swelling behavior of ordinary PS particles containing only linear chains and PS-SIPN particles of the same origin.

Figure 8. SEM micrographs showing the decomposition of PS− PMMA composite particles synthesized in the presence of 1% crosslinker during P2 by subsequently applying gAA and THF: (a, b) particles subjected to stepwise decomposition (a, prepared with V-65 at 70 °C; and b, prepared with Irgacure819 at room temperature during P3); (c, d) remnants after gAA treatment a → c and b → d; (e, f) remnants after THF treatment c → e and d → f. The scale bar indicates 1 μm for parts a, b, c, d, and f and 300 nm for part e; the sketch on the left illustrates the decomposition procedure.

Figure 9. SEM micrographs showing the decomposition of PS− PMMA composite particles synthesized in the presence of 10% crosslinker during P2 by subsequently applying gAA and THF: (a, b) particles subjected to stepwise decomposition (a, prepared with V-65 at 70 °C; and b, prepared with Irgacure819 at room temperature during P3); (c, d) remnants after gAA treatment a → c and b → d; (e, f) remnants after THF treatment c → e and d → f. The scale bar indicates 1 μm for parts a, b, c, and d, 300 nm for part e, and 200 nm for part f; the sketch on the left illustrates the decomposition procedure.

remnants show almost no variation of their morphology in dependence on temperature during P3 but change considerably with the network strength. In the case of the PS−SIPN fragments made with 10% of cross-linker the SEM micrographs show two different structures (micrographs c, d of Figure 9) G


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diameter of about 9.5 μm swell after 12.5 h to anisotropic structures exhibiting two bulbs with diameters of about 15−16 μm and 9−11 μm. The shape of the swollen PS−SIPN particles (micrograph d) differs distinctly from that of the swollen PS particles containing only linear chains (micrograph b). It should be mentioned that our results, put together in Figure 10, contradict earlier communicated findings that PS− SIPN particles (initial particle diameter of 5.2 μm and 0.2% DVB) develop at room temperature anisotropic structure only after swelling for a very long period of time (24 days).6 Surely, the question regarding the transformation rate is an important one and deserves particular consideration as it is strongly connected with the swelling kinetics of latex particles which in turn is important for emulsion polymerization kinetics.44 Only recently,42,43 we have presented both experimental evidence and theoretical support by means of simulations using Fick’s diffusion laws that in order to achieve reasonably high rates of swelling, allowing the instantaneous replenishment of the consumed monomer during emulsion polymerization requires close contact between the monomer and the polymer particles. Simple diffusion from a reservoir through the aqueous phase into the particles is very slow and inconsistent with the extraordinary high polymerization rate of emulsion polymerization. It turned out that the collision between swelling agent drops and latex particles is important for fast swelling kinetics. The time-dependent sequence of optical micrographs depicted in Figure 11 shows that transformation of spherical PS−SIPN particles into anisotropic starts almost immediately after pouring the swelling agent in the cuvette on top of the PS−SIPN dispersion. The micrographs of Figure 11 prove that the transformation is indeed much faster than previously reported.6 Already after a couple of minutes the deformation of particles is clearly visible. Moreover, the micrographs show a variety of shapes coexisting particularly during the inception phase as sketched in the panel of Figure 11. In coherence with our previous results on latex particle swelling42,43 and the processes that happen at liquid− liquid interfaces45,46 many of these shapes are structures

Figure 10. Optical micrographs illustrating the difference in the swelling behavior of polystyrene particles containing only linear chains (made with no cross-linker in P2, micrograph a and b) and semiinterpenetrating network (made with 1% DVB in P2, micrograph b and c); micrographs a and c showing the particles before addition of EB and b and d after 12.5 h (a → b and c → d). The scale bar marks 50 μm for micrograph a, 10 μm for micrograph b, and 20 μm for both micrographs c and d.

The largest starting particles of the linear PS particles with a size of about 10.7 μm (micrograph a of Figure 10) swell, provided they are close enough to the swelling agent, to about 18.3 μm in diameter (cf. micrograph b). The slight deviation from the spherical shape of the most swollen, largest particles might be due to lateral pressure acting in the assembled layer of densely packed particles. The accumulation of micrometersized latex particles at the water−swelling agent interface even against gravity has been already frequently observed and can be explained with the thermodynamic force.41−43 Similarly, also the PS−SIPN particles assemble during swelling at the interface (cf. micrographs c and d). Most importantly, however, the PS− SIPN particles clearly change their shape. The unswollen particles are spherical whereas the swollen particles are anisotropic and consisting of noticeably two distinguishable parts. The initial PS−SIPN particles of micrograph c with a

Figure 11. Optical micrographs illustrating the development of PS-SIPN particles’ morphology during swelling with EB over time: (a) before the addition of EB to the cuvette; (b) 12, (c) 44, and (d) 59 min after EB addition taken at a spot close to the interface. The bar indicates 20 μm; the right panel sketches various shapes recognizable on micrographs b−d. H


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the outcome quite clear but nonetheless, it is an important experimental proof for the mechanism of the formation of anisotropic particles as discussed below. Figure 13 illustrates the changes in the appearance of the SIPN particles in the dry and swollen state. Besides the changes connected with the alteration in the difference of the refractive indexes, the optical micrographs show isotropically swelling particles with no deviations from the spherical shape. Notably, the swelling of the SIPN particles with EB is fast. When the EB front reaches the particles complete swelling (as judged by the change in the refractive index) of the micrometer-sized particles happens within a few seconds. Both the shape of the swollen particles and the rate of swelling are important differences compared with the swelling behavior of SIPN particles when dispersed in water. The average ratio of the particles diameter in the wet and dry state is about 1.5. Swelling with EB and drying can be carried out repeatedly without significantly changing the arrangement of the particles. However, after the first cycle when the EB has evaporated, irregularly formed spots appear between the spherical particles which are obviously remnants of the linear polymer portion in the SIPN. Simulation of the Shape Transition: Model Development. The evolution of shape transition during swelling of semi-interpenetrating network particles is simulated by balancing the contributions of the elastic and interfacial energy. Before swelling, the SIPN particle of radius r0 contains both linear and cross-linked polymer chains where the linear polymer chain fraction is ϕl0. From this, the volume of linear (Vlin) and cross-linked (VXlink) polymer can be estimated via eqs 3−5 where Vp,0 is the volume of the particle prior to swelling.

resulting from droplet−particle collisions. Interestingly, the protrusions are mobile and diffusing along the interface until they finally combine, resulting in the characteristic structures with two bulges. Another peculiarity, always found during this kind of swelling experiments in the absence of mechanically forced mixing, is the observation that the particles arrange in layers close the interface to the swelling agent and that the actual size of the swollen particles decreases from layer to layer toward the bottom of the cuvette (cf. also micrographs b and d of Figure 10).56 This size difference means that there is accordingly a different amount of swelling agent from row to row. The micrographs of Figure 12 compare the particles found close to

Figure 12. Optical micrographs of swollen PS-SIPN particles close to the interface (a) and at the bottom of the cuvette (b) 160 h after placing EB on top of the latex; the bar indicates 5 μm for both micrographs.

the swelling interface with that at the bottom of the cuvette. The difference is very clear and confirms that the transition from spherical to anisotropic shape is directly related to the degree of swelling. Another interesting fact revealed by the micrographs of Figure 12 (particularly micrograph a) is the independence of the shape transition of the particle size. Regardless the size, all particles of Figure 12 (micrograph a) possess anisotropic shape. SIPN Particles in Direct Contact with Swelling Agent. The three component system as investigated in the previous chapter leads to the formation of anisotropic particles. The shape transition starts almost immediately after the addition of the swelling agent to the latex. Now, it appeared interesting to study the behavior of isolated dried SIPN particles in contact with the swelling agent by optical microscopy. For this purpose a drop of diluted SIPN latex was spread on a microscopy slide and after complete evaporation of the water, swelling agent was placed so close to the particles to ensure that it reaches the particles via spreading. This experiment might appear trivial and

Vp,0 =

4 3 πr0 3

(3)

Vlin = Vp ,0 × ϕl 0

(4)

VXlink = Vp0(1 − ϕl 0)

(5)

Swollen particles are characterized by the swelling ratio SW which is the ratio of volume of swelling agent to volume of the unswollen particles (6). The swollen particles deform and are characterized as sketched in Figure 14.

VSA = Vp0S W

(6)

The protrusion or bud or lobe which develops upon swelling consists mainly of linear polymer chains leaving the SIPN driven by quite a huge gain of entropy. During the swelling process the bud is attached to the spherical parent particle, both are denoted as L and X phases, respectively. The radius of the

Figure 13. Optical micrographs illustrating the swelling of dry SIPN particles with EB: (a) dry particles after the first swelling, (b) swollen (wet) particles during the second swelling cycle. The bar indicates, for both micrographs, 10 μm. I


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π (d 2 + 2drL − 3rL 2 + 2drX + 6rXrL − 3rX 2) 12d

Vlens =

(rX + rL − d)2

(12)

The center to center distance d can be represented in terms of, rL, rX, and θ by eq 13. d = rX cos θ ± (rL 2 − (rX sin θ)2 )1/2

(13)

Now, from eqs 8 and 11−13, rL can be estimated. The total Gibbs free energy of the swollen anisotropic particle (ΔGani) is accessible with eq 14 where the subscript “Int” means interfacial free energy and “Elastic” the elastic free energy due to cross-linked network. X/W, L/W and X/L refers to the interface between the X-phase and water, the L-phase and water, and the X- and L-phases, respectively. ΔGani = ΔG Int,X/W + ΔG Int,L/W + ΔG Int,X/L + ΔG Elastic

Figure 14. Illustration of the particle after shape transition generated from spherical SIPN particles.

(14)

The interfacial free energy across all interfaces can be calculated by eqs 15−17 with γi/j denoting the corresponding interfacial tension.

bud and the spherical particle is rL and rX, respectively. The center to center distance between them is d. As shown in Figure 14, θ describes the angle between a line from the center of the X-phase to the contact point of both phases and the line joining the center of both phases (d). θ is a measure how large is the contact area between the bud and the cross-linked parent particles in a way that θ = 0° and θ = 180° means complete separation and full engulfment of the X-phase by the L-phase, respectively. The first step is to estimate the volume of swollen anisotropic structure. At equilibrium, the bud contains f L fraction of total linear polymer chains. From swelling experiment, it is evident that the amount of swelling agent in both phases is different. In order to take in account this difference, the distribution of the swelling agent between the X- and L-section is represented as a distribution coefficient (Kd) as defined by eq 7a or 7b. ϕSA,L = KdϕSA,XL VSA × ϕSA,L VL

= Kd

ΔG Int,X/L = 2πrX 2(1 − cos θ )γX/L

(16)

ΔG Int,L/W = 2πrLhLγL/W

(17)

VSC,L =

1 πhL(3(rX sin θ)2 + hL 2) 6

VSC,L = VL +

ϕSA,L is the fraction of total swelling agent in the protrusion (L-phase). VL and VSph are the total volumes of the swollen Lphase and X-phase given by eq 8 and 9, respectively. (8)

VSph = VXLink + Vlin(1 − fL ) + VSA(1 − ϕSA,L)

(9)

1 πrX(1 − cos θ)(3(rX sin θ)2 6

+ (rX(1 − cos θ ))2 )

(7b)

VL = VlinfL + VSAϕSA,L

(18)

Also, VSC,L can be estimated as the sum of the volume of the L-phase and the spherical cap formed by the sphere X (shape P1P5P3P4P1) as given by eq 19.

VSA(1 − ϕSA,L)

(19)

VL is accessible from eq 11 and hL then from eqs 18 and 19. For the elastic free energy (ΔGElastic) the Flory−Rehner equation, eq 20,49,50 can be used where R is the gas constant, T the temperature, υe the effective moles of chains in the network, and αS3 the swelling ratio of the X-phase (αS = (VSph/ VXLink)1/3). ΔG Elastic = (RTυe /2)(3αS2 − 3 − ln αS3)

Assuming that during bud formation, the region X remains almost spherical, the radius of the portion rich in cross-linked polymer (X-phase) is given by 10. rX = r0(VSph /Vp0)1/3

(15)

The value for hL (cf. Figure 14) needs to be estimated. The volume of the spherical cap (VSC,LT) formed by the L-phase (shape P1P2P3P4P1) is48 given by eq 18.

(7a)

VSph

ΔG Int,X/W = 2πrX 2(1 + cos θ )γX/W

(20)

The Gibbs free energy for the anisotropically deformed particle is compared with the spherical case of an isotropically swollen particle. The ratio of these two values (RΔG) is a measure for the tendency of shape transformation. Simulation of the Shape Transition: Comparison with Experimental Data. The capability of the simulation as outlined above is illustrated by the following examples which allow comparison with experimental data, at least to a certain degree. One should always bear in mind that the thermodynamics included in the model is based on equilibrium condition. Making the assumption that equilibrium conditions can be applied after a certain period of time anyway, experimental data can be used to validate the model.

(10)

The second objective is to obtain the radius of the L-phase (rL). The volume of the bud will be the volume for the sphere with radius rL with the exclusion of the lens formed by intersection of sphere X and the L-phase (11). 4 VL = πrL 3 − Vlens (11) 3 The volume of the lens formed at the X and L intersection can be estimated by eq 12.47 J


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Macromolecules The anisotropic morphology as depicted in Figure 14, particularly the ratio X- to L-phase results from the assumption that SW is independent of the nature of the phase (linear or cross-linked does not matter), and thus, it basically reflects the volume ratio X-linked to linear portion as used during P2. However, the experiments show that SW is different for both phases and the model assumptions should therefore, be adjusted accordingly. Only for the simulation it is possible to assume a certain value for SW and look at the resulting morphology with respect to the connecting angle (θ) between the X- and L-phases. However, it is much closer to reality to feed the simulation with experimental data, particularly for the swelling ratio which is accessible form optical micrographs. The experimental data depicted in Figure 15, obtained for a single particle which could be traced during the swelling

Figure 16. Temporal evolvement of the swelling ratio (SW) for the overall anisotropic particle (gray circles), the L-phase (open circles), and the X-phase (patterned circles); the time is relative and started with t = 0 when the tracing of that particular particle was started; the arrows and numbers on the right-hand side of the graph refer to the values of SW in the apparent steady state.

that the increase in size is entirely due to the uptake of swelling agent. The most surprising result is the very high swelling ratio of the L-phase with SW of about 130 as an apparent equilibrium value. This means that the swollen bud contains less than 1 volume percent of polymer. Indeed an extraordinary high value since the precursors of the SIPN particles swell by about an order of magnitude less (SW of about 10).41 A possible explanation might be that predominantly shorter chains escape the network and subsequently causing such high entropy-driven swelling of the bud. Feeding the simulation model with the experimental data for SW (cf. Figure 16) and the corresponding distribution coefficient (Kd), allows to calculating the morphology exhibiting the lowest RΔG value in dependence on θ for selected times. The simulations qualitatively confirming the experimental trend that the θ values increase with increasing degree of swelling or time (cf. Figure 15). Evaluating the data, one should bear in mind that the experimental estimation of the angle θ from optical micrographs is afflicted with great uncertainty (Figure 17). Nevertheless, the simulation data of RΔG show that the high swelling of the bud is thermodynamically favored. Influence of Process Parameters during Swelling. The transition from spherical to anisotropic shape happens during the swelling step and the morphology is “only” fixed during subsequent polymerization. Close contact between the swelling agent and the particles establishes the concentration gradient which is necessary for fast diffusion of the swelling agent into the polymer particle.42 This experimental fact suggests that attention should be paid to stirring which is an effective tool for droplet generation and applied during the swelling step before P2 and P3. In addition, colloid chemistry demands an effective stabilization during the swelling process because the increase in the volume of the particles is accompanied by an increase in the free energy that needs to be countered by proper stabilizers to prevent coagulation/coalescence. However, the stabilizer layer around the particles, which causes stability, acts also as quite an effective mass transport barrier for the swelling agent and delays its uptake significantly.43,51−53 On the basis of these general considerations at least four different scenarios with respect to stabilizer and stirring conditions can be envisaged (C1−C4, cf. Figure 18). The experimental results are clear and show significant differences in

Figure 15. Characteristic changes during swelling of SIPN particles with EB in water as observed with optical microscopy: rL/rX, size ratio of both portions (filled circles); θ, the angle between the L-phase and the spherical contact perimeter and the line joining the center−center between both spherical portions (open triangles, cf. Figure 14). The optical micrographs show the initial and final state of the evaluated particle; note that the time is relative, and t = 0 is when the particle appeared in the focus and the evaluation was started.

process for about 10 h, show that both SW and θ increase with time. However, the swelling ratio in both phases is apparently not identical; one phase swells much stronger than the other. The stronger and faster swelling phase is assumed to be the Lphase, completely based on logic arguments of polymer chemistry. Following the theory of polymer swelling it is absolutely reasonable to assume that the portion of the particles containing the chemically cross-linked network swells considerably lesser.49,50 This behavior is quite an interesting scenario proving a certain tendency of both separation and cohesion of the components in the swelling SIPN particles. Consequently, the bud as part of the anisotropic particles that grows larger contains the linear chains which were able to escape from the SIPN. To find direct experimental proof for this disintegration of the SIPN during swelling is not easy; however, the swelling of SIPN particles in pure EB sheds some light onto the process by proving the release of polymer. The simulation model can be used to extract from experimental swelling data (Figure 15) the development of the swelling ratio (SW) of the overall particles and both phases separately. The data are depicted in Figure 16 and reveal extreme differences in the swelling ability between the X- and L-phases of an initially homogeneous and spherical SIPN particle. The estimation of SW is based on the size of each phase as extracted from the optical micrographs with the assumption K


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Figure 17. Graph on the left-hand side shows the correlation between RΔG (the ratio of Gibbs free energy for an anisotropically deformed particle and an isotropically swollen spherical particle) and the angle θ (between a line from the center of the X-phase to the contact point of both phases and the line joining the center of both phases) for the time evolution of swelling SIPN particles as indicated. The simulation snapshots on the righthand side display the particles shape at the angle corresponding to the minimum in RΔG, L and X mark the linear and cross-linked phase in the anisotropic particles, the axes labels relate to the relative distance where the origin (x = y = 0) is put at the center of the X-phase sphere.

Figure 18. Sketch of the various conditions (C1−C4) applied during the swelling of SIPN seed particles (left-hand side) and optical micrographs illustrating the outcome and hence, the influence of the various conditions: C1, no shear, no additional surfactant, snapshot taken 1 min after EB addition; C2, no shear plus additional PVA, image taken 4 min after EB addition; C3, stirred during swelling no additional surfactant after 20 h; C4, stirred during swelling plus additional surfactant after 20 h. The bar indicates 10 μm for C1−C3 and 50 μm for C4, and EB marks the swelling agent phase.

the swelling behavior of SIPN particles for C1 (no stirring, no additional stabilizer57), C2 (no stirring but in the presence of additional stabilizer), C3 (stirring on but no additional stabilizer), and C4 (stirring on in the presence of additional stabilizer). The snapshots of Figure 18 have been selected to illustrate the very specific behavior as observed many times for given condition C1−C4. The typical result for C1 is that the formation of anisotropic particles starts immediately after EB

addition, the particles accumulate at the EB−water interface, and the nearer to the interface the more anisotropic they are. Compared to C1, the scene for C2 changes in one key respect which is the time when the shape change starts after EB addition. In the presence of PVA, the stabilizer layer surrounding the particles quite drastically delays the deformation. For the particles seen on the picture close to the interface, the anisotropic shape change can be detected only after about 20 h. Applying stirring during swelling (C3, C4) requires a L


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immiscible liquids. As experimentally proven, this spontaneous emulsification leads to droplets with a broad size distribution spanning the range from micrometers down to nanometers and even smaller aggregates.45,46 Another interesting but also rather unfamiliar experimental result is connected with the observation that a lot of small particles are generated during the seeded polymerizations P2 and P3, even when the initiation is restricted to inside the particles. The most striking finding is that these small-sized particles, which are not present in the seed latex, also exhibit anisotropic shape change during swelling (cf. Figure 12). Considering the above proven experimental fact that anisotropic deformation necessarily requires the presence of SIPN particles, a reasonable explanation of the formation of these fine anisotropic particles is the decomposition of swollen SIPN particles, whether or not they are polymerizing, due to the action of the swelling pressure as recently shown in single droplet polymerization experiments.1

change in the experimental setup and allows the microscopic observation only after stopping the supply of shear. Also under these conditions the stabilizer has a strong influence. In the absence of additional stabilizer (C3 of Figure 18), phase separation happens immediately after stirring has been stopped. Surprisingly, almost all of the particles have been transferred into the EB phase. In the presence of PVA (C4 of Figure 18), no phase separation is observed within at least 2 h after stirring has been stopped, and the optical micrographs show the coexistence of spherical and anisotropic particles. The former ones are presumably EB droplets coexisting with anisotropically deformed SIPN particles. The experimental results of the present study are completely in line with recently reported data on swelling of latex particles.42,43 The shape transition flashes very fast after the encounter between drops of the swelling agent and SIPN particles. Remarkably, this happens also in the absence of any mechanically induced droplet formation (cf. Figure 11 and the corresponding discussion). The drops can be formed either by actively dispersing the swelling agent in the polymer dispersion or by spontaneous emulsification in an interfacial region, even in the absence of stirring.45,46 As soon as after an encounter between polymer particles and swelling agent droplets the contact time is long enough, swelling starts. In the case of SIPN particles, entropy drives the linear polymer chains, at the beginning preferentially the shorter ones, from the polymer particle into the adhering drop, stabilizing at the same time the particle−droplet connection. Stirring increases the frequency of encounters between particles and droplets. According to Mason,54 the frequency of shear induced collisions is proportional to the product of volume and concentration of the colliding objects and rate of shear. In addition, the data show that a stabilizer layer (here PVA) delays the transfer of the swelling agent into the particles and vice versa.58 On the other hand it is important to mention that spontaneous droplet formation is enhanced in the presence of any kind of stabilizer. Shear forces are effectively able to facilitate coalescence between droplets and particles in the absence of stabilizer (C3 of Figure 18). This process is very effective at lower stabilizer concentration.

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AUTHOR INFORMATION

Corresponding Author

*(K.T.) E-mail: [email protected]. ORCID

Klaus Tauer: 0000-0001-8641-9950 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.T. and S.N. acknowledge a scholarship from The Thailand Research Fund (TRF) through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0190/2553). Financial support of the MPI of Colloids and Interfaces is gratefully acknowledged. The authors are thankful to Mrs. Rona Pitschke and Heike Runge for support with electron microscopy and to Mrs. Ursula Lubahn for technical assistance.

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REFERENCES

(1) Krüger, K.; Wei, C. X.; Nuasaen, S.; Höhne, P.; Tangboriboonrat, P.; Tauer, K. Heterophase polymerization: pressures, polymers, particles. Colloid Polym. Sci. 2015, 293 (3), 761−776. (2) Morton, M.; Kaizerman, S.; Altier, M. W. Swelling of Latex Particles. J. Colloid Sci. 1954, 9 (4), 300−312. (3) Antonietti, M.; Kaspar, H.; Tauer, K. Swelling equilibrium of small polymer colloids: Influence of surface structure and a sizedependent depletion correction. Langmuir 1996, 12 (26), 6211−6217. (4) Tauer, K.; Kaspar, H.; Antonietti, M. Equilibrium swelling of colloidal polymeric particles with water-insoluble organic solvents. Colloid Polym. Sci. 2000, 278 (9), 814−820. (5) Sheu, H. R.; Elaasser, M. S.; Vanderhoff, J. W. Phase Domain Formation in Latex Interpenetrating Polystyrene Networks. Polym. Mater. Sci. Eng. 1987, 57, 911−915. (6) Sheu, H. R.; Elaasser, M. S.; Vanderhoff, J. W. Phase-Separation in Polystyrene Latex Interpenetrating Polymer Networks. J. Polym. Sci., Part A: Polym. Chem. 1990, 28 (3), 629−651. (7) Urban, D.; Takamura, K. Aqueous Polymer Dispersions. WileyVCH: Weinheim, Germany, 2002;. (8) Rupar, W.; Mitchell, J. M. A Study of Synthetic Rubber Latexes by the Electron Microscope. Rubber Chem. Technol. 1962, 35 (4), 1028−1040. (9) Kim, J. W.; Larsen, R. J.; Weitz, D. A. Synthesis of nonspherical colloidal particles with anisotropic properties. J. Am. Chem. Soc. 2006, 128 (44), 14374−14377.

CONCLUSIONS The experimental results obtained in this study clearly prove the necessary prerequisite for the direct synthesis of anisotropic polymer particles via heterophase polymerization. The crucial condition is the swelling of SIPN precursor particles with a monomer followed, after a certain period of time, by polymerization. Importantly, the temperature-induced recovery of the elastic network in the SIPN precursor particles during the final polymerization is not a major contribution in the process of the formation of anisotropic polymer particles. In summary, the experimental data reported here are all consistent with former results on experimental studies of particle nucleation in emulsion polymerization41 and swelling of polymer latexes.42,43 Likewise, the current data highlight the enormous importance of monomer droplets in emulsion polymerization, which goes way beyond their only passive role as monomer reservoir as assumed since more than 60 years in texts on emulsion polymerization. Probably the biggest problem to receive the crucial role of monomer drops is due to the fact that these are not only generated by the application of shear forces (stirring, sonication, or membrane emulsification) but also simply at the interface after quiescently contacting two M


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Macromolecules soluble swelling agent (ethylbenzene) mimicking the properties of typical monomers used for emulsion polymerization. (56) In contrast to our former studies,41 the latexes employed in this study are after P2 quite polydisperse, and hence, some smaller particles are between the swelling agent and the assembled larger particles. However, the observation that at given time the degree of swelling of the particles is the higher the closer the particles to the interface holds anyway. (57) Additional stabilizer means that during the swelling step PVAl was added (particles were suspended in 1% aqueous PVAl solution; otherwise, the SIPN latex was used as obtained after the washing steps as outlined in the Experimental Details. (58) The effect of a stabilizer layer is rather complex, and we restrict ourselves here to experimental data obtained with PVAl which was used also for the polymerization experiments. However, sodium dodecyl sulfate (SDS) acts in a similar way but shows due to its very different adsorption−desorption dynamics a quantitatively different behavior. Important here, SDS also delays the uptake of the swelling agent by the particles and hinders the transition of the particles into the oil phase.

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Swelling-Induced Deformation of Spherical Latex Particles

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