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Use of macromonomers as reactive stabilizers in mini-emulsion polymerization
A mini-emulsion polymerization is a variation on the more conventional emulsion polymerization process in that in the ideal scenario latex particles are formed by monomer droplet nucleation. The monomer droplets are turned into polymer particles. The trick to achieve this is to shrink monomer emulsion droplets to sub-micrometer diameters. For this two ingredients are key, one is a lyophobe, a compound that dissolves in the monomer droplet but does not like to partition into the continuous phase, here water. Typically n-hexadecane is used. This compound suppresses coarsening, also called Ostwald ripening, of the droplets by providing an Osmotic counter pressure. The other essential ingredient is a surfactant which aids to stabilize the large combined surface area of the droplets and keeps then from colliding and fusing (colloidal stability).
The use of molecular surfactants, however, can have negative impacts when the polymer latex is used in formulations and applications as the surfactant can migrate. For example in a clear coating it could lead to uptake of water, causing the transparent coating to become opaque, a phenomenon known as water whitening.
A mini-emulsion polymerization is a variation on the more conventional emulsion polymerization process in that in the ideal scenario latex particles are formed by monomer droplet nucleation. The monomer droplets are turned into polymer particles. The trick to achieve this is to shrink monomer emulsion droplets to sub-micrometer diameters. For this two ingredients are key, one is a lyophobe, a compound that dissolves in the monomer droplet but does not like to partition into the continuous phase, here water. Typically n-hexadecane is used. This compound suppresses coarsening, also called Ostwald ripening, of the droplets by providing an Osmotic counter pressure. The other essential ingredient is a surfactant which aids to stabilize the large combined surface area of the droplets and keeps then from colliding and fusing (colloidal stability).
The use of molecular surfactants, however, can have negative impacts when the polymer latex is used in formulations and applications as the surfactant can migrate. For example in a clear coating it could lead to uptake of water, causing the transparent coating to become opaque, a phenomenon known as water whitening.
Already in the 1990s one came up with the solution to use surfactants that could become part of the polymer. Buzzwords were inisurfs, surfmers, and transsurfs.
In our paper we look at macromonomers that are surfactants, which have been made by a technique called catalytic chain transfer emulsion polymerization (see image for chemical structure), and use these as reactive stabilizers in mini-emulsion polymerization.
ω-Unsaturated methacrylate macromonomers as reactive polymeric stabilizers in mini-emulsion polymerization
These macromonomers are of renewed interest, and indeed we have used them in bonlab before to demonstrate polymerization induced self-assembly (PISA), their modification into reactive nanogels as Pickering stabilizers to make Janus and patchy polymer colloids, and as stabilizers in conventional emulsion polymerization.
The paper focusses on a mechanistic study on how they operate as colloidal stabilizers and what the impact of their use is on the mini-emulsion polymerization process with a focus on particle size distribution and morphology. PhD researchers Josh Booth and Josh Davies (an undergraduate student at the time) carried out the experimental work which formed the basis of this study.
We believe there is evidence that polymerization triggers a destabilization of the balance between Ostwald ripening and the added hexadecane hydrophobe, evident from particle shape analysis. Also we show that too much surfactant is bad, as it can trigger depletion flocculation on the mini-emulsion droplets during high-shear emulsification.
The paper is published in the scientific journal Polymer Chemistry from the Royal Society of Chemistry.
More on the paper:
Stefan Bon receives Warwick Award for Teaching Excellence (WATE)
The Warwick Awards for Teaching Excellence (WATE) are handed out annually by the University of Warwick (UK) and aim to recognise those who have made a difference to the student learning experience through their work and their teaching practice. This year there were over 400 nominations by staff and students.
After an extraordinary academic year (2020-2021), WATE will be celebrating stories of ’everyday excellence’ in challenging times. Award winners are recognised for their exceptional commitment, impact and innovation to higher eduction by supporting students and colleagues, planting the seeds for future approaches to teaching and learning as we emerge from the global COVID pandemic.
One of the recipients this year of an individual Warwick Award for Teaching Excellence (WATE) is prof. dr. ir. Stefan Bon, a chemical engineer with expertise in polymer and colloid science, who works in the Department of Chemistry within the Faculty of Science, Engineering, and Medicine.
Prof. Bon is passionate about and dedicated to education, has an excellent relationship with the students, and has a track record to innovate in and to deliver great learning environments and experiences for students at Warwick. Throughout the years he has delivered original and exciting modules in polymer and colloid science, thermodynamics, kinetics, mathematics, key skills, and chemistry practical lab classes.
His creativity, drive, and passion to teach science and his engagement with the student community have led to progressive educational ideas being implemented at Warwick University, amongst which Chemistry Café, a highly successful student led social informal learning space, a concept now rolled out to multiple departments.
Prof. dr. ir. Stefan Bon says: “ This award came as a total surprise. I am absolutely delighted to have been nominated by our students and I am thrilled that the WATE committee decided to make me one of this year’s WATE recipients. I truly feel touched by this, it means a lot.
I would like to say a massive ‘thank you’ to all the students for their enthusiasm and dedication to their studies, standing up for themselves and each other, and forming together the Warwick community, especially in these unusual and difficult COVID times. I am proud of what they have achieved, especially in this academic year.
Teaching science is a key reason why I became an academic. The opportunity to mesmerise people with scientific concepts and see them use and apply these with enthusiasm to discover new science and develop innovative strategies for a more sustainable society, makes me very happy.
The combined learnings from teaching over one academic year remotely/blended, together with the STEM-grand challenge plans Warwick University has for the sciences, is a not to be missed opportunity to provide a step-change in higher education delivery.”
Prof. Bon is contributing actively to this with colleagues from across the University and is leading a task group under the Education and Students Experience Work Group, to look at new and exciting learning opportunities and curriculum design and how these can be implemented for Warwick science.
Replacing titanium dioxide as opacifier: consider a shape change
A fresh lick of paint breathes new life into a tired looking place. Ever wondered how a thin layer of paint is so effective in hiding what lies underneath from vision? Beside colour pigments, and a binder that makes it stick, paints contain microscopic particles that are great at scattering light and turning that thin layer of paint opaque. The golden standard for these opacifiers is small titanium dioxide particles, of dimensions considerably smaller than one micron. Their use is not without controversy, as they pose a significant environmental burden, with a substantial carbon footprint and a questionable impact on human health. Ideally, though, titanium dioxide should be replaced, but the list of safe, high refractive materials is very limited. Here we provide a potential solution.
A fresh lick of paint breathes new life into a tired looking place. Ever wondered how a thin layer of paint is so effective in hiding what lies underneath from vision? Beside colour pigments, and a binder that makes it stick, paints contain microscopic particles that are great at scattering light and turning that thin layer of paint opaque. The golden standard for these opacifiers are small titanium dioxide particles, of dimensions considerably smaller than one micron. Their use is not without controversy, as they are a big environmental burden, with a large carbon footprint and a questionable impact on human health. The reason why titanium dioxide particles are great at scattering light is that they have a high refractive index compared to the other paint ingredients, so when distributed throughout the dried paint film their hiding power of the underlying surface is fantastic. When no coloured pigments are used, the coated surface appears then whiter than white.
Ideally though, titanium dioxide should be replaced, but the list of safe high refractive materials is very limited. This makes you wonder if there is another handle, beside refractive index? Can we design efficient scattering enhancers from materials of lower refractive index?. Inspiration came from the white Cyphochilus beetle, native to southeast Asia. The scales of the beetle are not made of high refractive index materials, but they thank their white appearance to an intricate anisotropic porous microstructure, resembling the bare branches of a dense bush.
We at BonLab formed a team where researchers dr. Brooke Longbottom and dr. Chris Parkins together with dr. Gianni Jacucci and prof. Silvia Vignolini at the University of Cambridge (UK) designed a simplified mimic in the form of tiny rodlike silica particles and compared their scattering performance with spherical silica particles.
Our work published in the Journal of Materials Chemistry C from the Royal Society of Chemistry is part of their HOT paper collection and shows that the anisotropic silica particle outperform their spherical counterparts, and show excellent scattering performances across the visible electromagnetic spectrum when casted as a film.
SEM images and optical characterization of white silica supraparticles. a) low magnification SEM image of supracolloidal balls, b) higher magnification image of single supracolloidal balls, c) supracolloidal ball assembled in the presence of 0.01 M calcium chloride. Scale bars: a) = 15μm b&c) = 10μm. d) Reflectance spectra comparing the scattering properties of supraparticles with films of silica rod particles of similar size (thickness of 25μm). Supraparticles show performance comparable to the corresponding films. Increasing the disorder reduces the scattering efficiency. The reflectance spectra for the supraparticles were measured using a microscope, while for the film they were retrieved from the total transmission data.
We did not stop there, and went a step further to develop a prototype of a new class of micron-sized hiding pigment. We took these rodlike silica particles and assembled and sintered them into stable porous supracolloidal microspheres, as can be seen in the image above.
Prof. dr. ir. Stefan Bon says: “This work has been a number of years in the making. It was an absolute pleasure to work with prof. Silvia Vignolini and her team. We are very happy with the end result. We hope that this new type of hiding pigment provides inspiration to those who wish to replace titanium dioxide. After all, there is more to opacifiers than refractive index.”
The paper can be accessed from here:
https://doi.org/10.1039/D1TC00072A