publications

Innovation in Emulsion Polymerization process opens window to Janus and patchy particles

Emulsion polymerization is of pivotal importance as a route to the fabrication of water-based synthetic polymer colloids. The product is often referred to as a polymer latex and plays a crucial role in a wide variety of applications spanning coatings (protective/decorative/automotive), adhesives (pressure sensitive/laminating/construction), paper and inks, gloves and condoms, carpets, non-wovens, leather, asphalt paving, redispersible powders, and as plastic material modifiers.

Since its discovery in the 1920s the emulsion polymerization process and its mechanistic understanding has evolved. Our most noticeable past contributions include the first reversible-deactivation nitroxide-mediated radical emulsion polymerization (Macromolecules 1997: DOI 10.1021/ma961003s), and the development and mechanistic understanding of Pickering mini-emulsion (Macromolecules 2005: DOI 10.1021/ma051070z) and emulsion polymerization processes (J. Am. Chem. Soc. 2008: DOI 10.1021/ja807242k). The latest on nano-silica stabilized Pickering Emulsion Polymerization from our lab can be found here.

One quest in emulsion polymerization technology that remains challenging and intriguing is control of the particle morphology. It is of importance as the architecture of the polymer colloid influences its behavioural properties when used in applications. We now report in ACS Nano an elegant innovation in the emulsion polymerization process which makes use of nanogels as stabilizers and allows us to fabricate Janus and patchy polymer colloids.

False coloured SEM images of emulsion polymerizations using nanogels as stabilizers (N1) at 2.8 wt% wrt monomer in which the pH was adjusted to 8.8 (A), 5.5 (B), 5.0 (C) and 4.5 (D) prior to polymerization. Scale bars: 100 nm.

False coloured SEM images of emulsion polymerizations using nanogels as stabilizers (N1) at 2.8 wt% wrt monomer in which the pH was adjusted to 8.8 (A), 5.5 (B), 5.0 (C) and 4.5 (D) prior to polymerization. Scale bars: 100 nm.

The use of the nanogels in the emulsion polymerization leads to anisotropic Janus and patchy colloids, where a latex particle is decorated with a number of patches on its surface. In the paper we show that control of particle size and patch density can be achieved by tailoring the reaction conditions.

Proposed mechanism for the formation of Janus and patchy particles in the emulsion polymerization of styrene carried out in presence of nanogel particles.

Proposed mechanism for the formation of Janus and patchy particles in the emulsion polymerization of styrene carried out in presence of nanogel particles.

The work was carried out by a team of talented scientists from the BonLab, Andrea Lotierzo, Brooke Longbottom, and Wai Hin Lee. Prof.dr.ir. Stefan Bon says: “ I am absolutely delighted that our work is published in the internationally leading journal ACS Nano. It is a great achievement of the team who have worked tremendously hard in the realisation of this new innovative technology. It shows that the area of emulsion polymerization is very much alive and kicking!”

The link to the paper is here: DOI: 10.1021/acsnano.8b06557





Structure and behaviour of vesicles in presence of colloidal particles

We in the BonLab have occasionally played with vesicles, microscopic sacs dispersed in a liquid medium that enclose finite volumes of the liquid, thereby compartmentalising it from the bulk phase by a thin membrane. 

In our 2011 JACS paper we showed that we could decorate polymer vesicles with a layer of colloidal particles, effectively creating an armour. These colloidal particles could be silica nanoparticles, or polymer latex particles. The latter even had the ability to film-form, or to transform into a hydrogel.  When monodisperse latexes were used the 2D layer organised itself as a colloidal crystal.

In our 2016 Materials Horizons cover paper we showed that we spatially and temporally could regulate the membrane permeability of vesicular structures by embedding catalytic colloidal particles in a trans-membrane fashion.

Fascinating work has been done in this area by many others. We thought it was a nice idea to highlight these discoveries and bring them together in a review. It features recent studies that investigate the structural changes and behaviour of synthetic vesicles when they are exposed to colloidal particles. We will show examples to demonstrate the power of combining particles and vesicles in generating exciting supracolloidal structures. These suprastructures have a wide range of often responsive behaviours that take advantage of both the mechanical and morphological support provided by the vesicles and the associated particles with preset functionality. Since our review includes applications spanning a variety of disciplines, including chemistry, biology, physics and medicine, we felt that Soft Matter was the right place to publish. 

We hope you enjoy reading it.

To read the review: DOI: 10.1039/C8SM01223G  

BonLab develops technology to program hydrogels

A hydrogel is a solid object predominantly composed of water. The water is held together by a cross-linked 3D mesh, which is formed from components such as polymer molecules or colloidal particles. Hydrogels can be found in a wide range of application areas, for example food (think of agar, gelatine, tapioca, alginate containing products), and health (wound dressing, contact lenses, hygiene products, tissue engineering scaffolds, and drug delivery systems).  

In Nature hydrogels can be found widely in soft organisms. Jellyfish spring to mind. These are intriguing creatures and form an inspiration for an area called soft robotics, a discipline seek to fabricate soft structures capable of adaptation, ultimately superseding mechanical hard-robots. Hydrogels are an ideal building block for the design of soft robots as their material characteristics can be tailored. It is however, challenging to introduce and program responsive autonomous behaviour and complex functions into man-made hydrogel objects.

 

Ross Jaggers and prof.dr.ir. Stefan Bon at BonLab have now developed technology that allows for temporal and spatial programming of hydrogel objects, which we made from the biopolymer sodium alginate. Key to its design was the combined use of enzyme and metal-chelation know-how.

This video shows a programmed hydrogel tree. The hydrogel is made from sodium alginate and cross-linked with Calcium ions. Two scenarios are shown. In a tree of generation 1 the leaves contain a pH sensitive dye and the enzyme urease. The enzyme is trapped into the hydrogel leaves. The tree is floating in water of acidic pH. The water contains urea as trigger/fuel. After a dormant time period the leaves change colour from yellow to blue. This happens as the enzyme decomposes urea into ammonia and carbon dioxide. The bell-shaped activity curve of the enzyme is key to program the time delay in colour change. In a tree of generation 2, the leaves contain emulsion droplets of oil which are coloured red. Again they contain the enzyme urease. In this case, however, the system also contains the compound EDTA, which is a great calcium chelator at higher pH values. After a certain dormant time period, the pH in the leaves rises sufficiently (as a result of the enzymatic reaction decomposing urea into ammonia and carbon dioxide) that EDTA does its job. This results in spatial disintegration of the hydrogel leaves.

This video shows a programmed hydrogel object containing the numbers 1, 2 and 3. The hydrogel is made from sodium alginate and cross-linked with Calcium ions. The numbers contain emulsion droplets of oil which are coloured blue, red and yellow respectively. Each hydrogel number is loaded with a different amount of the enzyme urease. The enzyme is trapped into the gel. Number 1 contains the highest amount, number 3 the lowest. The hydrogel object is floating in water of acidic pH. The water contains urea as trigger/fuel. The system also contains the compound EDTA, which is a great calcium chelator at higher pH values. After a certain programmed dormant time period (dependent on enzyme loading), the pH in each of the numbers rises sufficiently (as a result of the enzymatic reaction decomposing urea into ammonia and carbon dioxide) that EDTA does its job. This results in spatial and temporal disintegration of the numbers.

Details of the study can be found in our paper "temporal and spatial programming in soft composite hydrogel objects" which was published in the Journal of Materials Chemistry B (DOI: 10.1039/C7TB02011B).

The study forms part of a wider program to develop technology to design hydrogels that can be programmed and communicate. As part of this series we reported earlier in 2017 in Materials Horizons a study on the design of hydrogel fibres and beads with autonomous independent responsive behaviour and have the ability to communicate (DOI: 10.1039/C7MH00033B).

 

 

Roughening up polymer microspheres and their diffusion in a liquid

Spherical microparticles that are roughened up, so that their surfaces are no longer smooth, are intriguing. You can wonder that when we place a large number of these particles in a liquid, it may show interesting rheological behaviour. For example, would they behave like cornstarch in that when we apply a lot of shear it thickens? You can imagine that spiky spheres can interlock and jam. Biologists are interested in how microparticles interact with cells and organisms, and have started to show that the shape of the particle can play an important role. Similarly, these small particles of intricate shape may show fascinating behavior at deformable surfaces, for example is there a cheerio effect?, and may show unexpected motion. This sounds all fun, but how do we make rough microparticles, as for polymer ones this is not easy?

Fig. 1 Transmission electron microscopy (TEM) images of poly(styrene) microspheres deformed at 110 °C within a dried colloidal inorganic matrix for approximate time periods of 10, 30, 60 and 120 min (a–d), (e–h), (i–l). The inorganic particles utilized were cigar-shaped calcium carbonate (a–d), large, rod-shaped calcium carbonate (e–h) and small, spherical/oblong-shaped zinc oxide (i–l). Scale bars = 1.0 μm.

Fig. 1 Transmission electron microscopy (TEM) images of poly(styrene) microspheres deformed at 110 °C within a dried colloidal inorganic matrix for approximate time periods of 10, 30, 60 and 120 min (a–d), (e–h), (i–l). The inorganic particles utilized were cigar-shaped calcium carbonate (a–d), large, rod-shaped calcium carbonate (e–h) and small, spherical/oblong-shaped zinc oxide (i–l). Scale bars = 1.0 μm.

In our paper published in Soft Matter we report an easy and versatile method to morph spherical microparticles into their rough surface textured analogues. For this, we embed the particles into an inorganic matrix of intricate shape and heat them up above the temperature at which the particles become a polymer melt. Capillary imbibition imprints the inorganic texture into the particles, turning them rough. We also look at how the particles move through a liquid, by tracking their motional behavior. Rough particles appear bigger, than their smooth precursors.  

You can read the paper here: DOI:10.1039/C7SM00589J

A mechanistic investigation of Pickering emulsion polymerization

Emulsion polymerization is an important industrial production method to prepare latexes. Polymer latex particles are typically 40-1000 nm and dispersed in water. The polymer dispersions find application in wide ranges of products, such as coatings and adhesives, gloves and condoms, paper textiles and carpets, concrete reinforcement, and so on.

Conventional emulsion polymerization processes make use of molecular surfactants, which aids the polymerization reaction during which the particles are made and keeps the polymer colloids dispersed in water.  We, and others, introduced Pickering emulsion polymerization a decade ago in which we replace common surfactants with inorganic nanoparticles.

In Pickering emulsion polymerization the polymer particles made are covered with an armor of the inorganic nanoparticles.  This offers a nanocomposite colloid which may have intriguing properties and features not present in conventional "naked" polymer latexes.

To fully exploit this innovation in emulsion polymers, a mechanistic understanding of the polymerization process is essential. Current understanding is limited which restricts the use of the technique in the fabrication of more complex, multilayered colloids.

In our paper, recently published in Polymer Chemistry, clarity is provided through an in-depth investigation into the Pickering emulsion polymerization of methyl methacrylate (MMA) in the presence of nano-sized colloidal silica (Ludox TM-40). Mechanistic insights are discussed by studying both the adsorption of the stabiliser to the surface of the latex particles and polymerization kinetics. The adhesion of the Pickering nanoparticles was found not to be spontaneous, as confirmed by cryo-TEM analysis of MMA droplets in water and monomer-swollen PMMA latexes. This supports the theory that the inorganic particles are driven towards the interface as a result of a heterocoagulation event in the water phase with a growing oligoradical. The emulsion polymerizations were monitored by reaction calorimetry in order to establish accurate values for monomer conversion and the overall rate of polymerizations (Rp). Rp increased for higher initial silica concentrations and the polymerizations were found to follow pseudo-bulk kinetics.

The paper can be read here: http://dx.doi.org/10.1039/C7PY00308K

Independent responsive behaviour and communication in hydrogel objects

Autonomous response mechanisms are vital to the survival of living organisms and play a key role in both biological function and independent behaviour. The design of artificial life, such as neural networks that model the human brain and robotic devices that can perform complex tasks, relies on programmed intelligence so that responses to stimuli are possible. Responsive synthetic materials can translate environmental stimuli into a direct material response, for example thermo-responsive shape change in polymer gels or light-triggered drug release from capsules. Materials that have the ability to moderate their own behaviour over time and selectively respond to their environment, however, display autonomy and more closely resemble those found in nature.

In our recent paper, published in Materials Horizons, we present soft hydrogel objects that possess an individually programmed time delay in their response to a shared environmental stimulus. We utilize the enzyme urease to programme a self-regulated change in pH, which in turn activates the designed response of gel disintegration. This design allows for independent response behaviour of a collection of hydrogel fibers which contain coloured oil droplets in a single closed system. In addition, we show that hydrogel beads can communicate with one another, hereby influencing their pre-programmed individual behaviour. 

The incorporation of responsive time control directly into soft matter objects demonstrates an advance in the field of autonomous materials.

The paper can be read at http://dx.doi.org/10.1039/C7MH00033B

BonLab features on the cover of Materials Horizons

Our manuscript entitled Control of vesicle membrane permeability with catalytic particles has been selected for the jan-feb 2016 cover of Materials Horizons, a premier scientific journal published by the Royal Society of Chemistry which features first reports of exceptional significance across the breadth of materials research at the cutting-edge interface with chemistry, physics, biology and engineering.

Prof.dr.ir. Stefan Bon says: "We are absolutely delighted that our research has made the cover of Materials Horizons. Rong Chen and especially Ross Jaggers worked very hard in the BonLab to fabricate giant polymer vesicles which have membrane-embedded catalytically active manganese oxide particles, hereby using droplet-based microfluidics. We demonstrate that these colloidal particles can regulate the membrane permeability of the polymersomes upon their exposure to, and catalytic reaction with, small amounts of dissolved hydrogen peroxide. Not only can we trigger complete release whereby the vesicle gets destroyed through membrane rupture by the formed oxygen bubbles as illustrated on the cover, exposure to small amounts of dissolved hydrogen peroxide leads to temporary enhanced release until all hydrogen peroxide is consumed by the catalytic particles after which the membrane permeability restores itself to its passive characteristic value." 

More on this can be read on our blog.

The paper (open access) can be read here:  http://dx.doi.org/10.1039/C5MH00093A

Fibers made by assembly of emulsion droplets

Fibers are interesting. They are made by a spinning process in which a liquid based mixture, referred to as spinning dope, is extruded through an orifice hereby generating a jet, which subsequently is solidified through either coagulation/precipitation and/or gelation. Two extreme fibers found in Nature are spidersilk, a super strong and extensible liquid-crystalline fiber, and the soft hydrogel double-strings of toad eggs, as spawn by the common toad (Bufo bufo). The production of manmade fibers using dry and wet spinning techniques – both starting from a liquid mixture – goes back to the 19th century. An early example is the development of Rayon fibers initiated by the discovery of Schweizer in 1857, who found that cellulose could be dissolved in and re-precipitated from an aqueous solution of ammonia and copper (II) hydroxide (coined Schweizer’s reagent (dry or wet)). Examples of wet-spun high performance fibers include ultrahigh molecular weight poly(ethylene) fibers, and polyaramid fibers.

An emerging trend is to make soft, hydrogel-based, fibers wet spun into water. Applications for example are in the area of tissue engineering. Microfluidic technologies are often employed to manufacture these fibers.

 

We asked ourselves whether it would be possible to fabricate fibers through assembly of thousands of emulsion droplets? We call these HIPE (High Internal Phase Emulsion) fibers.

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     Fig.  (a) Schematic representation of the fabrication of microfluidically spun HIPE fiber. The diagram shows coaxial flow channels in the microfluidic device, where the inner and outer capillaries contain concentrated emulsion and acidic water, respectively, in a continuous flow. A continuous HIPE fiber is produced upon exposure of the emulsion to the acidic water. The acidic conditions allow the formation of multiple hydrogen bonds between emulsion droplets, initiating them to assemble into a macroscopic supracolloidal fiber. (b) Photograph of a HIPE fiber in acidic aqueous solution (top left), and disintegration into individual dispersed emulsion droplets upon addition of base (top right). Scale bars represent 1 cm. Light micrographs of the HIPE fiber in acidic condition (bottom left) and disintegrated HIPE fiber in basic condition (bottom right). Scale bars represent 50 µm and 25 µm, respectively. (c) Photograph of HIPE fibers with uniform length. The HIPE fibers with uniform length were fabricated by utilizing air bubbles as a cutting mechanism. Scale bar represents 0.5 cm. (d) Photograph of asymmetric Janus HIPE fiber. Scale bars represent 0.1 cm (top) and 300 µm (bottom). (e) Photograph of asymmetric HIPE fiber consisting of three different sections ‘toothpaste’. Scale bars represent 0.1 cm (top) and 300 µm (bottom). (f) Photograph of magnetic HIPE fiber attracted by an external magnet (fiber with a slight yellow color - left hand side), and no magnetic response with the non-magnetic HIPE fiber (white color fiber - right hand side). Scale bar represents 1 cm.

Fig. (a) Schematic representation of the fabrication of microfluidically spun HIPE fiber. The diagram shows coaxial flow channels in the microfluidic device, where the inner and outer capillaries contain concentrated emulsion and acidic water, respectively, in a continuous flow. A continuous HIPE fiber is produced upon exposure of the emulsion to the acidic water. The acidic conditions allow the formation of multiple hydrogen bonds between emulsion droplets, initiating them to assemble into a macroscopic supracolloidal fiber. (b) Photograph of a HIPE fiber in acidic aqueous solution (top left), and disintegration into individual dispersed emulsion droplets upon addition of base (top right). Scale bars represent 1 cm. Light micrographs of the HIPE fiber in acidic condition (bottom left) and disintegrated HIPE fiber in basic condition (bottom right). Scale bars represent 50 µm and 25 µm, respectively. (c) Photograph of HIPE fibers with uniform length. The HIPE fibers with uniform length were fabricated by utilizing air bubbles as a cutting mechanism. Scale bar represents 0.5 cm. (d) Photograph of asymmetric Janus HIPE fiber. Scale bars represent 0.1 cm (top) and 300 µm (bottom). (e) Photograph of asymmetric HIPE fiber consisting of three different sections ‘toothpaste’. Scale bars represent 0.1 cm (top) and 300 µm (bottom). (f) Photograph of magnetic HIPE fiber attracted by an external magnet (fiber with a slight yellow color - left hand side), and no magnetic response with the non-magnetic HIPE fiber (white color fiber - right hand side). Scale bar represents 1 cm.

In our paper published in the Journal of Materials Chemistry A we show the fabrication of fibers from emulsion droplets. We use flow-focussing microfluidic set-up whereby a generated jet of emulsion droplets stabilized by amphiphilic pH-responsive branched copolymers (pH-BCP) and reinforced by Laponite clay discs is exposed to an external surrounding liquid flow of lower pH. Proton diffusion into the stream of emulsion droplets induces the self-assembly process leading to the formation of a continuous supracolloidal fiber. We demonstrate that the fiber can disintegrate back into an oil-in-water emulsion. We discuss control of fiber composition hereby using two and three combined streams of emulsion droplets to generate Janus fibers, and using ferrofluids to produce magnetic fibers. We show control of fiber length by employing air bubbles as a mean to produce short fibers of discrete length.  Looking towards applications we demonstrate the use of our supracolloidal emulsion droplet fibers as a material to control the delivery of volatile compounds through evaporation, and we show that the dried fibers are a nanocomposite highly porous and light material.

 

You can read the paper here: http://dx.doi.org/10.1039/C5TA08917D

Fabrication of calcium phosphate microcapsules using emulsion droplets stabilized with branched copolymers as templates

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	mso-hansi-theme-font:minor-latin;}     SEM micrographs illustrating the mineralization of CaP at the surface of oil droplets stabilized with BCP. (A) Incubation periods of 0 hours (scale bar = 37 µm), (B) 48 hours (scale bar = 16 µm), (C) 60 hours (scale bar = 7 µm), (D and E) 72 hours (scale bars = 23 µm and 7 µm, respectively), (F) surface morphology of CaP capsule (scale bar = 704 nm), (G) CaP capsules annealed at 600 oC (scale bar = 2 µm), (H) surface morphology of CaP capsule after annealing at 600 oC (scale bar = 648 nm), and (I and J) shell thickness of the CaP capsules before annealing (scale bars = 1 µm and 540 nm, respectively).

SEM micrographs illustrating the mineralization of CaP at the surface of oil droplets stabilized with BCP. (A) Incubation periods of 0 hours (scale bar = 37 µm), (B) 48 hours (scale bar = 16 µm), (C) 60 hours (scale bar = 7 µm), (D and E) 72 hours (scale bars = 23 µm and 7 µm, respectively), (F) surface morphology of CaP capsule (scale bar = 704 nm), (G) CaP capsules annealed at 600 oC (scale bar = 2 µm), (H) surface morphology of CaP capsule after annealing at 600 oC (scale bar = 648 nm), and (I and J) shell thickness of the CaP capsules before annealing (scale bars = 1 µm and 540 nm, respectively).

Calcium phosphate based hybrid materials are of great interest for bio-related science, for example our bones and teeth contain mineral components made from calcium phosphate. One class of materials of great interest are microcapsules, as these can store and release active ingredients. Calcium phosphate microcapsules have been made before via a number of synthetic pathways. Key drawbacks however are tedious and long (up to a month) fabrication methods. In our paper published recently in the Journal of Materials Chemistry B we report on a versatile and time-efficient method to fabricate calcium phosphate (CaP) microcapsules by utilizing oil-in-water emulsion droplets stabilized with synthetic branched copolymer (BCP) as templates. The BCP was designed to provide a suitable architecture and functionality to produce stable emulsion droplets, and to permit the mineralization of CaP at the surface of the oil droplet when incubated in a solution containing calcium and phosphate ions. The CaP shells of the microcapsules were established to be calcium deficient hydroxyapatite with incorporated chlorine and carbonate species. These capsule walls were made fluorescent by decoration with a fluorescein-bisphosphonate conjugate.

To read the paper: DOI: 10.1039/C5TB00893J