Water Separation
4.3 Categories of Particles
4.3.3 Polymeric Particles (Synthetic) and Microgel Dispersions
polymeric particles are of great interest in pickering emulsions. this is because polymeric particles can be easily synthesized and modified to numer- ous characteristics. they can be hydrophilic particles such as poly(N-iso- propylacrylamide) (pnipaM). they can also be hydrophobic polystyrene particles (pS).33 Colloidal-sized polymeric particles are usually synthesized by emulsion polymerization or suspension polymerization.57–59 this is radical polymerization of unsaturated carbon compounds, which form colloidal- sized polymeric particles. however, surfactants are used in the syntheses.
the stability of the particles at the early stage of the reaction is not high.
Surfactants stabilize them from uncontrolled aggregation so that monodis- persed particles can be obtained. Consequently, the surfactants stay with the final particles. Complete removal of these surfactant molecules from the par- ticles is very difficult and time consuming, and yet these surfactants may sometimes be undesirable.60 therefore, surfactant-free emulsion polymer- ization (SFep), also known as precipitation polymerization, has been devel- oped. polymeric particles prepared by precipitation polymerization do not suffer from the problem of surfactant removal. one well-known example of particles prepared by precipitation polymerization is pnipaM microgel par- ticles.61 pnipaM microgels are cross-linked gel particles, where their diam- eters are in micron or submicron range. they were first prepared by pelton et al. in 1986.62 the reaction was very simple. only the monomer nipaM, cross-linker N,N′-methylenebisacrylamide (MBa) and initiator (potassium persulfate, KpS) were added to the reaction mixture and then the temperature was maintained at around 70 °C. the diameters of the resulting particles were around 1 µm in the swollen state. at that time, the diameters of the swollen
and dried particles were just compared under teM. indeed, pnipaM micro- gel dispersion systems have been so popular in the recent decades because pnipaM microgel particles undergo volume-phase transition at increasing temperature. Figure 4.13 shows a typical diameter dependence on tempera- ture of a pnipaM microgel sample.
when the temperature is at room temperature, which is lower than the lower critical solution temperature (LCSt), the pnipaM microgel is very hydrophilic. it is at its highly swollen state, which may have water content above 90%.63 then, the temperature is increased and the diameters of the microgel particles decrease gradually. however, when the temperature reaches the LCSt of pnipaM, the diameter is reduced sharply in a matter of a few degrees Celsius. this is because the particles become much less hydro- philic and they expel their water content.64 the microgel shrinks because of the formation of intra-molecular hydrogen bonds and the loss of hydration with the methyl groups in the polymeric network. Lai et al. studied the mech- anism of this volume-phase transition of the microgel by using infrared spec- troscopy.65 amazingly, this transition often introduces a ten-fold shrinkage in volume of the particle.62 Besides the changes in volume and surface area of the particles, the dangling chains also turn into globular conformation. Ste- ric repulsion given by them will be greatly reduced. the contact angle of pni- paM is also a function of temperature as it becomes much less hydrophilic at temperatures above its LCSt (Figure 4.14).66 therefore, the sharp changes in microgel properties at LCSt allow microgel dispersions to be a great choice of responsive particles, which can be used in preparing responsive pickering emulsions.
the thermo-responsive property is definitely an important advantage of microgel dispersions. nevertheless, the ph-responsive property of pni- paM-based microgel gels is even more interesting. originally, pnipaM
Figure 4.13 dependence of hydrodynamic diameter on temperature for a pnipaM microgel.
polymers possess amide groups, which are not ph responsive at all. But an olefin monomer with ionizable function groups can easily co-polymerize with nipaM. For examples, they are acrylic acid (aa), methacrylic acid (Maa),60,67 vinylacetic acid (Vaa),68 2-aminoethyl-methacrylate hydrochloride (aeM),69 etc. after the co-polymerization, these ionizable functional groups are linked to the polymeric networks of the microgels. they usually are carboxylic acid or amine groups. Just like ordinary organic acids (or bases), they are weak acids (or bases). the degree of dissociation α depends on the ph value. at low ph values, the carboxylic groups are protonated and they deprotonate to carboxylate at high ph values. when they are deprotonated, counterions are introduced to the polymeric network. therefore, the microgel particles become even more hydrophilic and the osmotic pressure increased by the ions draws in more water.
Figure 4.15 shows the ph dependence of ph-responsive microgel samples with different morphologies. the micron-sized microgels were prepared by semi-batched precipitation polymerization with temperature programmed technique.60 this demonstrated the flexibility of polymeric particles syn- theses. Figure 4.16 shows the ph and temperature dependence of a typical pnipaM-co-Maa microgel. From Figure 4.16, it can be discovered that the thermo-responsive property is also a function of ph if carboxylic groups are co-polymerized.70 other than pnipaM microgel particles, there are many more examples of responsive polymeric colloidal particles. they can be responsive polystyrene particles,71,72 poly(methyl methacrylate) (pMMa) particles,73 poly(vinylamine) microgels (which are highly swollen at low ph values rather than alkaline conditions).74 the diversity and speciality of poly- meric particles are just too many to be included in a few paragraphs.
Figure 4.14 the relationship between contact angle of water on pnipaM gel and temperature.66 reprinted (adapted) with permission from J. Jhang, r. pelton and Y. L. deng, Langmuir, 1995, 11, 2301–2302. Copyright 1995 american Chemical Society.
Figure 4.15 ph dependence of pnipaM-co-Maa microgel samples with different morphologies.60 reprinted (adapted) with permission from M. h.
Kwok, Z. F. Li and t. ngai, Langmuir, 2013, 29, 9581–9591. Copyright 2013 american Chemical Society.
Figure 4.16 the ph and temperature dependence of a typical pnipaM-co-Maa microgel. as the degree of ionization of the carboxylic groups increases, the LCSt of the particle also increases.