Fluids, Gels and Crystals : Phase behavior of binary thermoresponsive microgel mixtures

Abstract

Thermoresponsive colloidal microgels expel solvent from their interior upon crossing a threshold temperature, resulting in a significantly reduced size. The collapse of microgels can reversibly switch the interparticle interactions from soft repulsive to attractive, and the accompanying size reduction allows for precise control over volume fraction. Mixing two microgels that collapse at different threshold temperatures provides a system where size ratio, volume fraction and specific interparticle interactions can all be controlled externally with temperature. In this thesis, we investigate various transitions exhibited by these highly tunable systems, and analyze the resulting structural properties. Taking advantage of the two-step collapse behavior, it is shown how different gel structures can be formed depending on the temperature profile used: a fast heating profile yields a gel that has a random distribution of the two particles, while a slower heating profile yields a sequentially gelated structure where one particle type decorates a scaffold network of the other type. The sequential network possesses stronger mechanical responses than its homo-gelated counterpart, and its final mechanical properties are determined by the properties of the scaffold at the moment of secondary particle deposition. We furthermore propose a new model based on particles with patchy interactions that is used to simulate such colloidal gels. In addition to the formation of binary gels, we show how binary mixtures can be driven to self-assemble into binary colloidal crystals. Using a newly designed sample cell that allows in-situ salinity manipulation, we observe a binary soft particle crystal-to-crystal transition. Finally, a microgel synthesis method is proposed that allows for enhanced CLSM imaging compared to traditional synthesis methods. We thus demonstrate and investigate a number of systems that all allow extensive control over phase behavior and structure formation using externally controllable parameters.

Abstract (Swedish)

Particles that are a hundred- to ten thousand times smaller than the width of a hair are commonly referred to as colloidal particles. Colloidal particles occur in a variety of products, like foodstuffs (marmelade and dairy products), construction materials (cement) and stained glass. In water, these particles are strongly affected by the Brownian force: an force that was discovered in 1827 by the botanist Robert Brown, when he was using a microscope to look at pollen in water. He discovered that the pollen were vibrating randomly throughout the sample, which turned out to be caused by thermal energy. It became clear that, the smaller the particle, the stronger these vibrations, and we know now that atoms experience this vibration very strongly. Currently, scientists try to understand and describe the behavior of atoms, which is made difficult by the minute size of atoms. Therefore, scientists often study colloids, that exhibit behavior similar to atoms, but are much larger. This larger size allows us to image them with for instance microscopy.

In this thesis we study microgels, a type of colloid that consists of a polymer network. If these networks are placed in water, they swell up into a soft network depicted in Figure I.1. They experience a Brownian force, which is strong enough to overcome gravity. In other words, microgels distribute through the volume and will not deposit to the bottom. Our type of microgels have the remarkable property of being swollen at room temperature, but then collapse above 32 ◦C. This versatility allows us to use one colloid type for mimicking and studying a wide array of atomic behavior.

This work studies the collective behavior of microgels. At room temperatures, the microgels behave repulsively, and particles repel each other when at close distances. Collapse causes a particle attraction, and two microgels at high temperatures form a strong bond. At room temperature, the concentration of microgels decides their behavior, and a low concentration allows microgels to freely move through the water. A higher con- centration forces microgels to organize into a crystal, to give each particle the largest amount of free space. Heating a microgel system causes them to collapse and form bonds with neighboring particles. Given that the concentration is high enough, the particles form long elastic strands and the system at large will start behaving elastically.

In this work, we study mixtures of two different microgels: one type that collapses at 32 ◦C and one that collapses at 45 ◦C. This collapsing behavior yields interesting network behavior: if a mixture is heated to 40 ◦C, the first particle will form an elastic network, while the second particle still behaves repulsively. Heating the system further to 50 ◦C makes the second particle to deposit on the already present first particle network, decorating the first strand. Raising the temperature quickly causes the first particle to not have the time to form an network before the second particle collapses, and all particles bind other particles randomly, a process we call homo-gelation. Microscopy pictures of these two types of gels can be found in Figure I.2. The decorated gel networks are stronger than the networks obtained with homo-gelation, allowing us to understand the rigidity of gel networks.

We also study binary crystals, which are able to transform into another crystal by adding or removing salt. We use positively charged red particles and negatively charged green particles. This causes an attraction between red and green particles, with red-red and green-green combinations repelling. Salt weakens these charge-based forces, and the addition of salt can form a crystal where red and green are distributed randomly, due to there being no preference as to which neighbor a particle has. Removing salt causes the red-green attraction, and particles organize themselves to have as many red-green neighbors as possible.

• author: Immink, Jasper
• supervisor: Peter Schurtenberger; Joakim Stenhammar
• organization: Physical Chemistry; NanoLund: Centre for Nanoscience
• publishing date: 2019-03
• type: Thesis
• publication status: published
• subject: Physical Chemistry
• keywords: colloidal phase behavior, microgel, binary mixture, colloidal gel, sequential gelation, patchy particle, binary crystal, crystal-to-crystal transition
• pages: 168 pages
• ISBN: 978-91-7422-635-5
• ISBN: 978-91-7422-634-8
• language: English
• LU publication?: yes
• id: 821d206c-802c-44b2-b8ce-b4dcdaa52ea0
• date added to LUP: 2019-03-01 12:12:15
• date last changed: 2020-04-15 12:05:58