«A Study of Fundamentals in Emulsion Templating for the Preparation of Macroporous Polymer Foams By Dipl.-Chem. Nadine Graeber A dissertation ...»
Imperial College London
Department of Chemical Engineering
A Study of Fundamentals in Emulsion
Templating for the Preparation of
Macroporous Polymer Foams
By Dipl.-Chem. Nadine Graeber
A dissertation submitted to Imperial College London
in fulfilment of the requirements for the degree of
Doctor of Philosophy
and the Diploma of Membership of Imperial College London
I, hereby, certify that the work presented in this dissertation is the result of my own investigations, carried out at Imperial College London. Every work, which is not my own has been properly acknowledged.
Nadine Graeber September 2013 The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it.
For any reuse or redistribution, researchers must make clear to others the licence terms of this work.
-iv- Acknowledgements This work was carried out in the Polymer and Composite Engineering (PaCE) Group of the Department of Chemical Engineering, Imperial College London, from March 2009 to January 2012 supervised by Professor Dr. Alexander Bismarck.
First of all, I would like to acknowledge and to thank my supervisor, Professor Dr. Alexander Bismarck for giving me the opportunity and the freedom in my scientific research and Dr.
John Hodgkinson for all the discussions about the mechanical property tests. In particular, I would like to express my deepest gratitude and special thanks to Dr. Stefan Berendts for his uncountable and numerous inspiring conversations and discussions which without the present work would be completely different. Furthermore, I am very appreciative for the correction of my ‘run-on sentences’ in this thesis. If I could do so, I would pass ’Dr.h.c.’ to you.
I have an enormous list of people who have supported, trained and taught me during this study and many other people have contributed directly or indirectly to the completion of this work. Professor Dr. Michael Gradzielski and his group (Technische Universität Berlin, Germany) is kindly acknowledged for giving me the opportunity to do the rheological measurements in his laboratory and to benefit from his outstanding knowledge. Dorith Claes and Elisa Franzman (Justus-Liebig-Universität Gießen, Germany) are kindly acknowledged for giving me the opportunity to use their ESI-MS spectrometer, Dr. Dennis Blank (Justus-Liebig-Universität Gießen, Germany) for carrying out MALDI-TOF measurements and Dr. Jan-Dirk Epping (Technische Universität Berlin, Germany) for CP and CP-MAS-NMR measurements. Dr. Bjoern Luerßen (Justus-Liebig-Universität Gießen, Germany) for the enjoyable time and accurate artistic contributions to many of the drawings in this thesis. ‘My SEM images are still untouchable!’ Countless people around the Departments of Chemical Engineering, Mechanical Engineering, Chemistry and especially George Wang (SPEL Group, Imperial College London, U.K.), Dr. Natasha Shirshova (for pointed remarks) and Sarah Payne (language support) need to be acknowledged for their help and support. Also I want to thank my countless colleagues (about 42 new and former group members) and friends in the PaCE group for their help and support during my stay in the U.K.. Their kindness and humour has left me with wonderful memories and widened my view of the world. Thank you!
Additionally, I would like to acknowledge the Engineering and Physical Science Research Council (EPSRC) for the funding. My thanks and appreciation goes also to Professor Dr.
Cosima Stubenrauch (Universität Stuttgart, Germany) and Professor Dr. Paul Luckham (Imperial College London, U.K.) for serving as my examiners.
Finally, I would like to give unlimited thanks to my Dad, my little-big sister, my unborn nanobrother (now you are a year old!), my family and friends. Nothing could stop their never ending love and nothing could replace the support from them. Thank you!
-viiviii- ABSTRACT This thesis describes a series of styrene (ST) and divinylbenzene (DVB) emulsion templated polymer foams prepared via low, medium and high internal phase emulsion templates (L/M/HIPE templates). The emulsion templates were stabilized using different commercially available technical surfactants and surfactant mixtures. Since the chemical nature of the chosen technical surfactants is unknown, the surfactants where characterized by means of Fourier Transform Infrared (FT-IR) and Nuclear Magnetic Resonance (NMR) spectroscopy, Electro Spray Ionization Mass- (ESI-MS) and Matrix-Assisted-Laser-Desorption-IonizationTime-of-Flight-Mass Spectrometry (MALDI-TOF-MS). Additionally their adsorption at the water/ST:DVB interface was studied. The investigation regarding the preparation of surfactant stabilized emulsion templates and their polymerization products revealed that the most commonly used surfactant Span 80 is not the best suited surfactant to stabilize styrene/divinylbenzene emulsion templates which is why different surfactants were used in the thesis at hand. All successfully prepared poly(merized)HIPEs proved to have interconnected, open porous polymer foam structures. In contrast, the pore structure of polyMIPEs was open, closed or non-droplet shaped, depending on the surfactant used to stabilize the corresponding emulsion template. The mechanical compression properties of all prepared polyHIPEs were similar and independent of the HIPE formulation from which they were produced but the mechanical properties of polyMIPEs differed significantly. The influence of the surfactants on the morphology and mechanical properties of the resulting macroporous polymers will be discussed in detail.
Furthermore, the relationship between the relative density (porosity) of the polymer foams and the mechanical response under compression was investigated. The semi-empirical models developed by Gibson and Ashby were applied and additionally modified to provide a more accurate description of the mechanical behaviour over a larger relative density range of polymer foams prepared via emulsion templating (polyL/M/HIPEs). This allows a prediction of the mechanical properties as a function of the relative density of the respective polymer foams and vice versa for the specified emulsion template formulation. It is obvious that the surfactant type and the internal phase volume ratio of the emulsion template used to produce macroporous polymer foams significantly determine their resulting mechanical properties, as clear transition states for polyH/M/LIPEs were identified
-ixin which the mechanical properties of these materials changed dramatically. The effect of the surfactant on the mechanical properties and the polymer foam morphology is discussed in terms of the surfactant’s solubility in the polymer and thus in terms of its role as plasticizer.
Finally, the influence of the pore size on the mechanical properties was investigated. It was found that the preparation process (emulsification and polymerization) of the emulsion templates is very crucial for the mechanical properties of the resulting polymer foams (reproducibility). More precisely, it was found out that the emulsion templates need to ‘equilibrate’ after emulsification. It was only for these emulsions that average pore sizes and mechanical properties could be reproduced.
-xList of Communications
1. Graeber, Nadine; Hodgkinson, John M.; Bismarck, Alexander; Mechanical properties of macroporous polymer foams prepared via emulsion templating (polyH/M/LIPEs) and the impact of the internal phase ratio, Macro2012, 24th to 29th June 2012, Blacksburg, Virgina, U.S.A..
2. Graeber, Nadine; Bismarck Alexander; Impact of surfactants and Internal phase ratios on the mechanical properties of polyL/M/HIPEs, Eupoc2012, 3rd to 7th June 2012, Gargnano, Italy.
3. Graeber, Nadine; Lee, Koon-Yang; Menner Angelika; Bismarck Alexander;
Renewable Open Macroporous Nanocomposites, 2011 ICMAT, 26th June to 1st July 2011, Suntec, Singapore.
4. Graeber, Nadine; Menner Angelika; Bismarck Alexander, Polymer and Surfactant Interaction, 57th SEPAWA Congress, 2010, 13th to 15th October 2010, Fulda, Germany.
5. Graeber, Nadine; Menner, Angelika; Bismarck, Alexander; Renewable Macroporous Nanocomposites via Emulsion Templating, 2009 AIChE Annual Meeting, 2009, 8th to 13th November 2009, Nashville TN, U.S.A. (Invited talk).
6. Menner, Angelika; Graeber, Nadine; Bismarck, Alexander; Ultra permeable macroporous polymers from Pickering-high internal phase emulsions, 14th U.K.
Polymer and Colloids Forum, 2009, 14th to 16th September 2009, Hull, U.K..
I. Graeber Nadine; Menner, Angelika; Bismarck, Alexander; New perspectives in the synthesis of macroporous polymers via emulsion templating, Macro2010: 43rd IUPAC World Polymer Congress, 11th to 16th July 2010, Glasgow, U.K..
II. Graeber Nadine; Menner, Angelika; Bismarck, Alexander; Water in oil emulsions stabilized by hydrophilic particles to produce porous materials via emulsion templating, 14th U.K. Polymer and Colloids Forum, 14th to 16th September 2009, Hull, U.K..
III. Graeber, Nadine; Manley, Shu San; Menner, Angelika; Bismarck, Alexander, Turning liquid into solid: Materials based on polyHIPEs, JCF-Spring Symposium 2009, 11th to 14th March 2009, Essen, Germany.
Manley, Shu San; Graeber, Nadine; Grof, Zdenek; Menner, Angelika; Hewitt, Geoffrey F.;
Stepanek, Frantisek; Bismarck, Alexander; New insights into the relationship between internal phase level of emulsion templates and gas-liquid permeability of interconnected macroporous polymers., Soft Matter (2009), 5(23), 4780-4787.
-xvxvi- List of Figures Figure 1: Schematic presentation of water-in-oil-surfactant stabilized emulsion templating................1 Figure 2: Two-dimensional schematic formulation−composition map, showing the so-called optimum formulation inversion (Adapted from Ref. )
Figure 3: Mechanisms of emulsion destabilization which results finally in two distinct phases (oil and water).
Figure 4: Schematic potential energy according to the DLVO-theory. The potential energy (U) results from the sum of van der Waals attraction (UA) and electrostatic repulsion (UE) during the interactions between two spherical droplets or particles depending on the droplet distance (x). U0 contains all other non-DLVO interactions, which need to be considered in case of adsorbing polymers on the surface (U0). Additionally, a shortrange Born repulsion (UB) exist which arise from the fact that each atom, molecule, particle etc. occupies a certain amount of space.
Figure 5: Various mechanisms of emulsion stabilization imparted by different emulsifiers. For example; a) adsorbed Ca2+-ions in alginates (gelation), b) short molecules emulsifier such as soaps, c) solid particles, so called Pickering or Ramsden stabilization and d) high molecular weight polymeric stabilizers.
Figure 6: Some physico-chemical properties, such as electrical conductivity, surface/interfacial tension and turbidity as a function of surfactant concentration. Additionally, the adsorption behaviour of the surfactant molecules at the interface as a function of their concentration is shown at the bottom of the figure. Surfactant concentrations higher than the critical micelle concentration (CMC) result in the formation of surfactant micelles
Figure 7: Schematic representation of an emulsion and the polymerized products if one polymerizes only the dispersed phase, the continuous phase and both phases for the preparation of colloids/beads, porous materials and composites, respectively (Adapted from Ref. ).
Figure 8: Morphologies of polyHIPEs: a) definition of pores and pore throats of polyHIPEs, b) the pore throats are partially covered with a thin solid film, c) closed porous structure and
d) non-droplet shaped (bicontinuous) foam morphology.
Figure 9: SEM image of a polyMIPE (left, contained an internal phase of 50 vol.-%) and a polyLIPE (right, contained an internal phase of 25vol.-% ) (Reprinted from Ref. ).
Figure 10: Schematic representation of particles on a planar (top) and curved (middle) interface in order to define the contact angle (θOW) of the particle in water. Hydrophilic particles (θOW 90°) being adsorbed at the liquid-liquid interface (left) curve the interface to such an extent that o/w-emulsions are obtained. Hydrophobic particles (θOW 90°) curve the interface in the direction of w/o-emulsions (right) (Adapted from Ref. ).....36 Figure 11: Decomposition of 2,2’-azobis(2-methylpropionitrile) (AIBN) for free radical polymerization reaction.
Figure 12: The one electron step involved in the redox reaction between cumene hydroperoxide (CHP) and Fe2+.
Figure 13: Initiator decay of peroxodisulphate (APS) in a combination with tetramethylenediamin (TMEDA) to form two radical species.
Figure 14: Basic schematic reaction for a free radical polymerization reaction of vinyl monomers, where kd, ki, kp, and kt are the rate constants of dissociation, initiation, propagation and termination by combination, respectively.
Figure 15: ST-DVB-polyHIPE with metal organic framework (MOF) crystals inside the pores (Adapted from Ref. ).
Figure 16: Proposed chemical structure of the surfactants used. In the case of Hypermer 2296 it is assumed that this is a mixture of sorbitan oleate (Span 80) and PIBSA.
-xviiFigure 17: Photograph of emulsification set-up used (left) and dimensions of the set-up (right);
diameter of stirrer arm (b), outer diameter of stirrer (d2), vessel height (h1), blade height (h3), inner diameter of vessel (d1) and stirrer pitch (h2) 5 mm. Fill volume of 50 mL water = 43 mm height within the vessel.
Figure 18: Definition of sample specimens from the resulting polymer foam monolith used for various characterizations throughout this thesis.
Figure 19: Comparison of pore size counting via SEM and log-normal distribution function (Equation 10) of a polyHIPE, at least 200 pores were counted. The sum of all counts is equal to 1