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Structure, Dynamics and Phase Behaviour of Charged Soft Colloidal Dispersions

Nöjd, Sofi LU (2016)
Abstract (Swedish)
Inom fysiken används ofta hårda sfärer som modellsystem för att beskriva beteenden och strukturer hos grundläggande byggstenar såsom atomer och molekyler. Dessa sfärer kan ses som en samling biljardbollar och kan därav bara organiseras på ett begränsat antal sätt. Med den ökade förståelsen för många komplexa fysikaliska fenomen och den snabbt växande utvecklingen av nya material behövs modellsystem som kan interagera och organiseras på mer komplicerade sätt än vad de hårda sfärerna kan. För att öka komplexiteten används i denna avhandling "mjuka" sfärer som ett alternativt modellsystem. Dessa sfärer, även kallade mikrogeler, har en kompakt kärna omgiven av ett löst, fluffigt skal, och tack vare en inre laddning svarar de på förändringar i... (More)
Inom fysiken används ofta hårda sfärer som modellsystem för att beskriva beteenden och strukturer hos grundläggande byggstenar såsom atomer och molekyler. Dessa sfärer kan ses som en samling biljardbollar och kan därav bara organiseras på ett begränsat antal sätt. Med den ökade förståelsen för många komplexa fysikaliska fenomen och den snabbt växande utvecklingen av nya material behövs modellsystem som kan interagera och organiseras på mer komplicerade sätt än vad de hårda sfärerna kan. För att öka komplexiteten används i denna avhandling "mjuka" sfärer som ett alternativt modellsystem. Dessa sfärer, även kallade mikrogeler, har en kompakt kärna omgiven av ett löst, fluffigt skal, och tack vare en inre laddning svarar de på förändringar i sin miljö. Vid låga koncentrationer sväller mikrogelerna avsevärt på grund av att de interna laddningarna vill sprida ut sig. När mikrogelkoncentrationen ökar, ökar även antalet laddningar i lösningen och de interna laddningarna har inte längre samma drivkraft att sprida ut sig. Detta kommer ifrån att skillnaden i antalet laddningar inuti och utanför mikrogelerna blir mindre. Sfärstorleken minskar därför tills laddningarnas drivkraften att sprida ut sig helt avtagit. Vid mycket höga koncentrationer börjar de fluffiga skalen att överlappa vilket leder till att partiklarna komprimeras.

På grund av att dessa mjuka mikrogeler innehåller laddningar svarar de även på elektriska fält. Då de interna och externa laddningarna förflyttas av fältet uppstår en attraktiv kraft mellan mikrogelerna. Mikrogelerna arrangerar sig då i långa ”pärlband” i samma riktning som det elektriska fältet. Dessa pärlband bildar kristallstrukturer när fältstyrkan eller koncentrationen av sfärerna ökas. Vid mycket höga koncentrationer har vi visat att sfärernas fluffiga skal överlappar signifikant och trasslar ihop sig, vilket leder till att de bildar nya kristallstrukturer då fältet stängs av. Partiklarnas mjuka skal gör även att de lätt deformeras, men vi har visat att de bara krymper en aning när de bildar ”pärlband”.

Genom denna avhandling har vi samlat in mycket av den information som behövs för att i framtiden kunna sätta upp en teoretisk modell för att beräkna hur starkt dessa mjuka sfärer interagerar, både i och utanför elektriska fält. Med en sådan modell kan vi få en djupare förståelse för interaktioner och strukturer som involverar mjuka sfärer. (Less)
Abstract
Soft and deformable ionic microgels such as poly(N-isopropylacrylamide-co-acrylic acid), PNIPAM-co-AA, microgels have shown to be a versatile alternative to the already well-established hard sphere model systems. Their tuneable interaction potential makes them well suited as model systems to study the phase behaviour found for particles interacting via a soft isotropic potential. This stems from the microgels ability to respond to changes in the environment, such as changes in temperature, pH or particle concentration. Additionally, subjecting these particles to an alternating electric field induces a dipolar contribution to the interaction potential, which strongly depends on the amplitude and frequency of the applied field. This... (More)
Soft and deformable ionic microgels such as poly(N-isopropylacrylamide-co-acrylic acid), PNIPAM-co-AA, microgels have shown to be a versatile alternative to the already well-established hard sphere model systems. Their tuneable interaction potential makes them well suited as model systems to study the phase behaviour found for particles interacting via a soft isotropic potential. This stems from the microgels ability to respond to changes in the environment, such as changes in temperature, pH or particle concentration. Additionally, subjecting these particles to an alternating electric field induces a dipolar contribution to the interaction potential, which strongly depends on the amplitude and frequency of the applied field. This additional, tuneable and directional attraction allows us to explore and deepen our understanding of systems interacting via an even more complex and anisotropic potential.

At low number densities particles are shown to be aligning into strings along the direction of the field. These strings assemble into crystal structures as the field strength is further increased. At concentrations above freezing the parent face-centred cubic, FCC, structure is found to melt and diffusively transforms into a BCT crystal via nucleation and growth but in the reverse direction, the BCT phase transforms cooperatively into a metastable body-centred orthorhombic, BCO, phase, which only relaxes back to the parent FCC phase as the temperature is increased. The kinetics is consequently either diffusive or martensitic depending on the path and is believed to be due to the interpenetrable nature of the microgel particles.

In order to learn more about the origin of this puzzling path dependence, we studied the shape and size of the particles as a function of packing fraction and field strength by performing a combination of scattering experiments, both in the absence and presence of the electric field. We found that the particle size is dramatically altered by a small increase in particle concentration to reach a plateau value at intermediate concentration. In the over-packed state the particle size is again seen to decrease due to shell overlap. The applied electric field however was shown to only slightly alter the particle size, thus confirming the interpenetration of particles in field-induced structures. As a last step we performed dielectric spectroscopy measurements to obtain information about the polarisation mechanisms present in the system.

In the future the collected information will be used to derive a theoretical model that will provide us with the intrinsic and field-induced interaction potential at the relevant concentrations and field-strengths. This potential will be compared to obtained data in the absence and presence of the alternating electric field.
(Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Prof. Dr. Egelhaaf, Stefan U., Heinrich Heine University Düsseldorf, Germany
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Microgels, directed self-assembly, interaction potentials, scattering methods, confocal laser scanning microscopy, dielectric spectroscopy
pages
148 pages
publisher
Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry
defense location
Center for chemistry and chemical engineering, lecture hall B, Naturvetarvägen 14, Lund
defense date
2016-11-24 10:15
ISBN
978-91-7422-482-5
language
English
LU publication?
yes
id
a076f8cd-972c-4edc-a87f-3213e77c4e87
date added to LUP
2016-10-31 10:05:10
date last changed
2016-11-22 14:41:19
@phdthesis{a076f8cd-972c-4edc-a87f-3213e77c4e87,
  abstract     = {Soft and deformable ionic microgels such as poly(N-isopropylacrylamide-co-acrylic acid), PNIPAM-co-AA, microgels have shown to be a versatile alternative to the already well-established hard sphere model systems. Their tuneable interaction potential makes them well suited as model systems to study the phase behaviour found for particles interacting via a soft isotropic potential. This stems from the microgels ability to respond to changes in the environment, such as changes in temperature, pH or particle concentration. Additionally, subjecting these particles to an alternating electric field induces a dipolar contribution to the interaction potential, which strongly depends on the amplitude and frequency of the applied field. This additional, tuneable and directional attraction allows us to explore and deepen our understanding of systems interacting via an even more complex and anisotropic potential.<br/><br/>At low number densities particles are shown to be aligning into strings along the direction of the field. These strings assemble into crystal structures as the field strength is further increased. At concentrations above freezing the parent face-centred cubic, FCC, structure is found to melt and diffusively transforms into a BCT crystal via nucleation and growth but in the reverse direction, the BCT phase transforms cooperatively into a metastable body-centred orthorhombic, BCO, phase, which only relaxes back to the parent FCC phase as the temperature is increased. The kinetics is consequently either diffusive or martensitic depending on the path and is believed to be due to the interpenetrable nature of the microgel particles. <br/><br/>In order to learn more about the origin of this puzzling path dependence, we studied the shape and size of the particles as a function of packing fraction and field strength by performing a combination of scattering experiments, both in the absence and presence of the electric field. We found that the particle size is dramatically altered by a small increase in particle concentration to reach a plateau value at intermediate concentration. In the over-packed state the particle size is again seen to decrease due to shell overlap. The applied electric field however was shown to only slightly alter the particle size, thus confirming the interpenetration of particles in field-induced structures. As a last step we performed dielectric spectroscopy measurements to obtain information about the polarisation mechanisms present in the system.<br/><br/>In the future the collected information will be used to derive a theoretical model that will provide us with the intrinsic and field-induced interaction potential at the relevant concentrations and field-strengths. This potential will be compared to obtained data in the absence and presence of the alternating electric field.<br/>},
  author       = {Nöjd, Sofi},
  isbn         = { 978-91-7422-482-5},
  keyword      = {Microgels,directed self-assembly,interaction potentials,scattering methods,confocal laser scanning microscopy,dielectric spectroscopy},
  language     = {eng},
  pages        = {148},
  publisher    = {Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry},
  school       = {Lund University},
  title        = {Structure, Dynamics and Phase Behaviour of Charged Soft Colloidal Dispersions},
  year         = {2016},
}