The sulfonate-containing polyelectrolytes (SPE) from sulfonation of polystyrene
(PS) and copolyme... more The sulfonate-containing polyelectrolytes (SPE) from sulfonation of polystyrene (PS) and copolymerization of 3-sulfo-propyl methacrylate, potassium salt (SPMS) with styrene (S) were prepared. Photosensitive polyelectrolyte complexes (PECDR) derived from SPE and diazoresin (DR), which does not dissolve in water or organic solvent due to its ionic crosslinking structure, dissolves in aqueous solution of sodium dodecyl sulfate (SDS) due to the dissociation of PECDR and the hydrophobic interaction between SDS and the polymer chain. The photosensitive behavior and thermostability of the PECDR were investigated, and it was found that the thermostability of PECDR increases dramatically in SDS aqueous solution. It was proposed that the higher thermostability of PECDR in SDS aqueous solution is due to an aggregation of SDS molecules around the diazonium group of the PECDR, which protects the ON2 1 group of the DR from attack by the nucleophiles. The image-forming behavior of PECDR by ultraviolet (UV) light was examined and considered to be different from other PECs. It was concluded that the photoimaging behavior of PECDR is based on a reaction in which an ionic bond converts to a covalent bond.
The interaction of sulfonated polystyrene (SPS) with cetyl trimethylammonium
bromide (CTAB) and m... more The interaction of sulfonated polystyrene (SPS) with cetyl trimethylammonium bromide (CTAB) and myristyl trimethylammonium bromide (MTAB) in aqueous solution was investigated. It was found that the solution viscosity increases rapidly at the molar ratio of CTAB/-SO3Na = 0.5 or MTAB/-SO3Na = 0.6 (the first viscosity maximum) and CTAB/-SO3Na = ~1.5 or MTAB/-SO3Na = 2.0 (the second viscosity maximum). Both the first and the second viscosity maxima are related closely with the hydrophobic domains formed in solution, which can be detected using methyl orange as a spectral probe or N,Ndimethylamino flavone as fluorescent probe. In the higher concentration of [SPS], such as >0.05 M (-SO3Na unit), the SPS/CTAB solution converts to gel suddenly at CTAB/-SO3Na = ~1.5-1.7 region, but with a further addition of CTAB or SPS, the gel collapses quickly. A term “micelle crosslinking” was proposed to explain the conversion of solution/gel and the reversible nature of gel ↔solution.
Colloidal fouling behavior in forward osmosis (FO) was investigated, focusing on the role of reve... more Colloidal fouling behavior in forward osmosis (FO) was investigated, focusing on the role of reverse salt diffusion. Two suspensions of silica nanoparticles, with average particle diameters of 24 and 139 nm, were used as model colloidal foulants. To verify the effect of reverse salt diffusion on the colloidal fouling behavior, NaCl and LaCl3 were employed as draw solutions because they exhibit different reverse diffusion rates. Our results suggest that in colloidal fouling of FO, salts diffuse from the draw to the feed solution and accumulate within the colloidal fouling layer that forms on the membrane surface. The accumulated salts result in a marked acceleration of cake-enhanced osmotic pressure (CEOP), which reduces the net osmotic driving force for permeate water flux. Fouling was not observed with the small, 24-nm particles because of the lack of substantial cake formation, but was notable for the 139-nm particles and for a feed containing a mixture of the 24 and 139 nm particles. Our findings further indicate that colloidal fouling is enhanced under solution conditions (ionic strength and pH) within the colloidal cake layer that promote aggregation or destabilization of the silica particles. Colloidal fouling reversibility was also examined by varying the cross-flow velocity during the FO fouling runs. We showed that in the absence of colloidal particle destabilization/aggregation, the permeate flux during colloidal fouling in FO recovered almost completely when the cross-flow velocity was increased from 8.5 to 25.6 cm/s. Our results suggest that reverse salt diffusion in FO is a key mechanism that controls colloidal fouling behavior as well as fouling reversibility. Therefore, minimization of reverse salt diffusion through the selection of proper draw solutes and optimization of FO membrane selectivity are important for minimizing colloidal fouling as well as enhancing FO operation efficiency.
The time-evolutions of nanoparticle hydrodynamic radius and aggregate fractal dimension during th... more The time-evolutions of nanoparticle hydrodynamic radius and aggregate fractal dimension during the aggregation of fullerene (C60) nanoparticles (FNPs) were measured via simultaneous multiangle static and dynamic light scattering. The FNP aggregation behavior was determined as a function of monovalent (NaCl) and divalent (CaCl2) electrolyte concentration, and the impact of addition of dissolved natural organic matter (humic acid) to the solution was also investigated. In the absence of humic acid, the fractal dimension decreased over time with monovalent and divalent salts, suggesting that aggregates become slightly more open and less compact as they grow. Although the aggregates become slightly more open, the magnitude of the fractal dimension suggests intermediate aggregation between the diffusion- and reaction-limited regimes. We observed different aggregation behavior with monovalent and divalent salts upon the addition of humic acid to the solution. For NaCl-induced aggregation, the introduction of humic acid significantly suppressed the aggregation rate of FNPs at NaCl concentrations lower than 150 mM. In this case, the aggregation was intermediate or reaction-limited even at NaCl concentrations as high as 500 mM, giving rise to aggregates with a fractal dimension of 2.0. For CaCl2-induced aggregation, the introduction of humic acid enhanced the aggregation of FNPs at CaCl2 concentrations greater than about 5 mM due to calcium complexation and bridging effects. Humic acid also had an impact on the FNP aggregate structure in the presence of CaCl2, resulting in a fractal dimension of 1.6 for the diffusion- limited aggregation regime. Our results with CaCl2 indicate that in the presence of humic acid, FNP aggregates have a more open and loose structure than in the absence of humic acid. The aggregation results presented in this paper have important implications for the transport, chemical reactivity, and toxicity of engineered nanoparticles in aquatic environments.
Microgels are colloidally stable, hydrogel microparticles that have previously been used in a ran... more Microgels are colloidally stable, hydrogel microparticles that have previously been used in a range of (soft) material applications due to their tunable mechanical and chemical properties. Most commonly, thermo and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels are synthesized in the presence of a crosslinking agent, such as N,N0-methylenebisacrylamide (BIS), however, microgels can also be synthesized under ‘crosslinker free’ conditions. The resulting particles have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to crosslinked microgels, with a Young's modulus of 10 kPa. Furthermore, ULC microgels are highly deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer, surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base material for a wide variety of biomedical applications including biomaterials for drug delivery and regenerative medicine.
Wereport direct measurements of the pairwise interparticle potential
between poly(N-isopropylacry... more Wereport direct measurements of the pairwise interparticle potential between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co- AAc) colloidal microgels as a function of pH, as determined through Ornstein–Zernike analysis of the pair distribution function of quasi- 2Ddilute colloidal suspensions.The interaction potential ranges from purely repulsive at high pHdue to electrosteric interactions toweakly attractive at low pH due to hydrogen bonding, which explains previous observations on the unique phase behavior of these particles in concentrated suspensions.
With the continued development of thermoresponsive colloidal hydrogel particles, a number of
grou... more With the continued development of thermoresponsive colloidal hydrogel particles, a number of groups have begun to exploit their properties to create dynamic materials self-assembled from those components. The fundamental details of how those building blocks are assembled, the component functionality, and the geometry or length-scales present in the assemblies contribute to the behavior of the resultant material. In this tutorial review, we examine recent progress in the assembly of responsive hydrogel colloids in two and three dimensions, highlighting their potential applications, especially in the domain of biotechnology.
We describe the straightforward synthesis of “clickable” multiresponsive microgels containing
bot... more We describe the straightforward synthesis of “clickable” multiresponsive microgels containing both carboxylic acid groups and azidohydrin or terminal alkyne groups, via a one-pot multistage polymerization approach. The clickable functional groups on the microgels were confirmed by FTIR. Additionally, we simultaneously performed “click” and acid-amine coupling reactions on microgels with fluorescent dyes containing complementary functional groups. Epifluorescence microscopy was employed to confirm the coupling of those dyes to the microgels. The orthogonality of the click reaction to other functional groups such as hydroxyl, carboxylic acid, and amino groups was confirmed, suggesting the potential utility of such microgels in applications where multifunctional colloidal particles are required.
Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid... more Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following ∼1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pKa, the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pKa, repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.
The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spheric... more The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N¢-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH ) 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.
The sulfonate-containing polyelectrolytes (SPE) from sulfonation of polystyrene
(PS) and copolyme... more The sulfonate-containing polyelectrolytes (SPE) from sulfonation of polystyrene (PS) and copolymerization of 3-sulfo-propyl methacrylate, potassium salt (SPMS) with styrene (S) were prepared. Photosensitive polyelectrolyte complexes (PECDR) derived from SPE and diazoresin (DR), which does not dissolve in water or organic solvent due to its ionic crosslinking structure, dissolves in aqueous solution of sodium dodecyl sulfate (SDS) due to the dissociation of PECDR and the hydrophobic interaction between SDS and the polymer chain. The photosensitive behavior and thermostability of the PECDR were investigated, and it was found that the thermostability of PECDR increases dramatically in SDS aqueous solution. It was proposed that the higher thermostability of PECDR in SDS aqueous solution is due to an aggregation of SDS molecules around the diazonium group of the PECDR, which protects the ON2 1 group of the DR from attack by the nucleophiles. The image-forming behavior of PECDR by ultraviolet (UV) light was examined and considered to be different from other PECs. It was concluded that the photoimaging behavior of PECDR is based on a reaction in which an ionic bond converts to a covalent bond.
The interaction of sulfonated polystyrene (SPS) with cetyl trimethylammonium
bromide (CTAB) and m... more The interaction of sulfonated polystyrene (SPS) with cetyl trimethylammonium bromide (CTAB) and myristyl trimethylammonium bromide (MTAB) in aqueous solution was investigated. It was found that the solution viscosity increases rapidly at the molar ratio of CTAB/-SO3Na = 0.5 or MTAB/-SO3Na = 0.6 (the first viscosity maximum) and CTAB/-SO3Na = ~1.5 or MTAB/-SO3Na = 2.0 (the second viscosity maximum). Both the first and the second viscosity maxima are related closely with the hydrophobic domains formed in solution, which can be detected using methyl orange as a spectral probe or N,Ndimethylamino flavone as fluorescent probe. In the higher concentration of [SPS], such as >0.05 M (-SO3Na unit), the SPS/CTAB solution converts to gel suddenly at CTAB/-SO3Na = ~1.5-1.7 region, but with a further addition of CTAB or SPS, the gel collapses quickly. A term “micelle crosslinking” was proposed to explain the conversion of solution/gel and the reversible nature of gel ↔solution.
Colloidal fouling behavior in forward osmosis (FO) was investigated, focusing on the role of reve... more Colloidal fouling behavior in forward osmosis (FO) was investigated, focusing on the role of reverse salt diffusion. Two suspensions of silica nanoparticles, with average particle diameters of 24 and 139 nm, were used as model colloidal foulants. To verify the effect of reverse salt diffusion on the colloidal fouling behavior, NaCl and LaCl3 were employed as draw solutions because they exhibit different reverse diffusion rates. Our results suggest that in colloidal fouling of FO, salts diffuse from the draw to the feed solution and accumulate within the colloidal fouling layer that forms on the membrane surface. The accumulated salts result in a marked acceleration of cake-enhanced osmotic pressure (CEOP), which reduces the net osmotic driving force for permeate water flux. Fouling was not observed with the small, 24-nm particles because of the lack of substantial cake formation, but was notable for the 139-nm particles and for a feed containing a mixture of the 24 and 139 nm particles. Our findings further indicate that colloidal fouling is enhanced under solution conditions (ionic strength and pH) within the colloidal cake layer that promote aggregation or destabilization of the silica particles. Colloidal fouling reversibility was also examined by varying the cross-flow velocity during the FO fouling runs. We showed that in the absence of colloidal particle destabilization/aggregation, the permeate flux during colloidal fouling in FO recovered almost completely when the cross-flow velocity was increased from 8.5 to 25.6 cm/s. Our results suggest that reverse salt diffusion in FO is a key mechanism that controls colloidal fouling behavior as well as fouling reversibility. Therefore, minimization of reverse salt diffusion through the selection of proper draw solutes and optimization of FO membrane selectivity are important for minimizing colloidal fouling as well as enhancing FO operation efficiency.
The time-evolutions of nanoparticle hydrodynamic radius and aggregate fractal dimension during th... more The time-evolutions of nanoparticle hydrodynamic radius and aggregate fractal dimension during the aggregation of fullerene (C60) nanoparticles (FNPs) were measured via simultaneous multiangle static and dynamic light scattering. The FNP aggregation behavior was determined as a function of monovalent (NaCl) and divalent (CaCl2) electrolyte concentration, and the impact of addition of dissolved natural organic matter (humic acid) to the solution was also investigated. In the absence of humic acid, the fractal dimension decreased over time with monovalent and divalent salts, suggesting that aggregates become slightly more open and less compact as they grow. Although the aggregates become slightly more open, the magnitude of the fractal dimension suggests intermediate aggregation between the diffusion- and reaction-limited regimes. We observed different aggregation behavior with monovalent and divalent salts upon the addition of humic acid to the solution. For NaCl-induced aggregation, the introduction of humic acid significantly suppressed the aggregation rate of FNPs at NaCl concentrations lower than 150 mM. In this case, the aggregation was intermediate or reaction-limited even at NaCl concentrations as high as 500 mM, giving rise to aggregates with a fractal dimension of 2.0. For CaCl2-induced aggregation, the introduction of humic acid enhanced the aggregation of FNPs at CaCl2 concentrations greater than about 5 mM due to calcium complexation and bridging effects. Humic acid also had an impact on the FNP aggregate structure in the presence of CaCl2, resulting in a fractal dimension of 1.6 for the diffusion- limited aggregation regime. Our results with CaCl2 indicate that in the presence of humic acid, FNP aggregates have a more open and loose structure than in the absence of humic acid. The aggregation results presented in this paper have important implications for the transport, chemical reactivity, and toxicity of engineered nanoparticles in aquatic environments.
Microgels are colloidally stable, hydrogel microparticles that have previously been used in a ran... more Microgels are colloidally stable, hydrogel microparticles that have previously been used in a range of (soft) material applications due to their tunable mechanical and chemical properties. Most commonly, thermo and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels are synthesized in the presence of a crosslinking agent, such as N,N0-methylenebisacrylamide (BIS), however, microgels can also be synthesized under ‘crosslinker free’ conditions. The resulting particles have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to crosslinked microgels, with a Young's modulus of 10 kPa. Furthermore, ULC microgels are highly deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer, surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base material for a wide variety of biomedical applications including biomaterials for drug delivery and regenerative medicine.
Wereport direct measurements of the pairwise interparticle potential
between poly(N-isopropylacry... more Wereport direct measurements of the pairwise interparticle potential between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co- AAc) colloidal microgels as a function of pH, as determined through Ornstein–Zernike analysis of the pair distribution function of quasi- 2Ddilute colloidal suspensions.The interaction potential ranges from purely repulsive at high pHdue to electrosteric interactions toweakly attractive at low pH due to hydrogen bonding, which explains previous observations on the unique phase behavior of these particles in concentrated suspensions.
With the continued development of thermoresponsive colloidal hydrogel particles, a number of
grou... more With the continued development of thermoresponsive colloidal hydrogel particles, a number of groups have begun to exploit their properties to create dynamic materials self-assembled from those components. The fundamental details of how those building blocks are assembled, the component functionality, and the geometry or length-scales present in the assemblies contribute to the behavior of the resultant material. In this tutorial review, we examine recent progress in the assembly of responsive hydrogel colloids in two and three dimensions, highlighting their potential applications, especially in the domain of biotechnology.
We describe the straightforward synthesis of “clickable” multiresponsive microgels containing
bot... more We describe the straightforward synthesis of “clickable” multiresponsive microgels containing both carboxylic acid groups and azidohydrin or terminal alkyne groups, via a one-pot multistage polymerization approach. The clickable functional groups on the microgels were confirmed by FTIR. Additionally, we simultaneously performed “click” and acid-amine coupling reactions on microgels with fluorescent dyes containing complementary functional groups. Epifluorescence microscopy was employed to confirm the coupling of those dyes to the microgels. The orthogonality of the click reaction to other functional groups such as hydroxyl, carboxylic acid, and amino groups was confirmed, suggesting the potential utility of such microgels in applications where multifunctional colloidal particles are required.
Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid... more Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following ∼1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pKa, the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pKa, repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.
The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spheric... more The equilibrium phase behavior and the dynamics of colloidal assemblies composed of soft, spherical, colloidal particles with attractive pair potentials have been studied by digital video microscopy. The particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and N,N¢-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels) with temperature- and pH-tunable swelling properties. It is observed that in a pH ) 3.0 buffer with an ionic strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels, in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization for these microgels.
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Papers by Zhiyong Meng
(PS) and copolymerization of 3-sulfo-propyl methacrylate, potassium salt
(SPMS) with styrene (S) were prepared. Photosensitive polyelectrolyte complexes
(PECDR) derived from SPE and diazoresin (DR), which does not dissolve in water or
organic solvent due to its ionic crosslinking structure, dissolves in aqueous solution of
sodium dodecyl sulfate (SDS) due to the dissociation of PECDR and the hydrophobic
interaction between SDS and the polymer chain. The photosensitive behavior and
thermostability of the PECDR were investigated, and it was found that the thermostability
of PECDR increases dramatically in SDS aqueous solution. It was proposed that
the higher thermostability of PECDR in SDS aqueous solution is due to an aggregation
of SDS molecules around the diazonium group of the PECDR, which protects the ON2
1
group of the DR from attack by the nucleophiles. The image-forming behavior of PECDR
by ultraviolet (UV) light was examined and considered to be different from other PECs.
It was concluded that the photoimaging behavior of PECDR is based on a reaction in
which an ionic bond converts to a covalent bond.
bromide (CTAB) and myristyl trimethylammonium bromide (MTAB) in aqueous
solution was investigated. It was found that the solution viscosity increases
rapidly at the molar ratio of CTAB/-SO3Na = 0.5 or MTAB/-SO3Na = 0.6 (the
first viscosity maximum) and CTAB/-SO3Na = ~1.5 or MTAB/-SO3Na = 2.0
(the second viscosity maximum). Both the first and the second viscosity maxima
are related closely with the hydrophobic domains formed in solution,
which can be detected using methyl orange as a spectral probe or N,Ndimethylamino
flavone as fluorescent probe. In the higher concentration of
[SPS], such as >0.05 M (-SO3Na unit), the SPS/CTAB solution converts to gel
suddenly at CTAB/-SO3Na = ~1.5-1.7 region, but with a further addition of
CTAB or SPS, the gel collapses quickly. A term “micelle crosslinking” was
proposed to explain the conversion of solution/gel and the reversible nature of
gel ↔solution.
diffusion. Two suspensions of silica nanoparticles, with average particle diameters of 24 and 139 nm,
were used as model colloidal foulants. To verify the effect of reverse salt diffusion on the colloidal fouling
behavior, NaCl and LaCl3 were employed as draw solutions because they exhibit different reverse diffusion
rates. Our results suggest that in colloidal fouling of FO, salts diffuse from the draw to the feed solution
and accumulate within the colloidal fouling layer that forms on the membrane surface. The accumulated
salts result in a marked acceleration of cake-enhanced osmotic pressure (CEOP), which reduces the net
osmotic driving force for permeate water flux. Fouling was not observed with the small, 24-nm particles
because of the lack of substantial cake formation, but was notable for the 139-nm particles and for a
feed containing a mixture of the 24 and 139 nm particles. Our findings further indicate that colloidal
fouling is enhanced under solution conditions (ionic strength and pH) within the colloidal cake layer
that promote aggregation or destabilization of the silica particles. Colloidal fouling reversibility was also
examined by varying the cross-flow velocity during the FO fouling runs. We showed that in the absence of
colloidal particle destabilization/aggregation, the permeate flux during colloidal fouling in FO recovered
almost completely when the cross-flow velocity was increased from 8.5 to 25.6 cm/s. Our results suggest
that reverse salt diffusion in FO is a key mechanism that controls colloidal fouling behavior as well
as fouling reversibility. Therefore, minimization of reverse salt diffusion through the selection of proper
draw solutes and optimization of FO membrane selectivity are important for minimizing colloidal fouling
as well as enhancing FO operation efficiency.
aggregation of fullerene (C60) nanoparticles (FNPs) were measured via simultaneous multiangle static
and dynamic light scattering. The FNP aggregation behavior was determined as a function of monovalent
(NaCl) and divalent (CaCl2) electrolyte concentration, and the impact of addition of dissolved natural
organic matter (humic acid) to the solution was also investigated. In the absence of humic acid, the fractal
dimension decreased over time with monovalent and divalent salts, suggesting that aggregates become
slightly more open and less compact as they grow. Although the aggregates become slightly more open,
the magnitude of the fractal dimension suggests intermediate aggregation between the diffusion- and
reaction-limited regimes. We observed different aggregation behavior with monovalent and divalent
salts upon the addition of humic acid to the solution. For NaCl-induced aggregation, the introduction
of humic acid significantly suppressed the aggregation rate of FNPs at NaCl concentrations lower than
150 mM. In this case, the aggregation was intermediate or reaction-limited even at NaCl concentrations
as high as 500 mM, giving rise to aggregates with a fractal dimension of 2.0. For CaCl2-induced aggregation,
the introduction of humic acid enhanced the aggregation of FNPs at CaCl2 concentrations greater
than about 5 mM due to calcium complexation and bridging effects. Humic acid also had an impact on
the FNP aggregate structure in the presence of CaCl2, resulting in a fractal dimension of 1.6 for the diffusion-
limited aggregation regime. Our results with CaCl2 indicate that in the presence of humic acid, FNP
aggregates have a more open and loose structure than in the absence of humic acid. The aggregation
results presented in this paper have important implications for the transport, chemical reactivity, and
toxicity of engineered nanoparticles in aquatic environments.
material applications due to their tunable mechanical and chemical properties. Most commonly, thermo
and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation
polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels
are synthesized in the presence of a crosslinking agent, such as N,N0-methylenebisacrylamide (BIS),
however, microgels can also be synthesized under ‘crosslinker free’ conditions. The resulting particles
have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM
nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to
crosslinked microgels, with a Young's modulus of 10 kPa. Furthermore, ULC microgels are highly
deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate
through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and
charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer,
surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on
the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under
crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base
material for a wide variety of biomedical applications including biomaterials for drug delivery and
regenerative medicine.
between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-
AAc) colloidal microgels as a function of pH, as determined through
Ornstein–Zernike analysis of the pair distribution function of quasi-
2Ddilute colloidal suspensions.The interaction potential ranges from
purely repulsive at high pHdue to electrosteric interactions toweakly
attractive at low pH due to hydrogen bonding, which explains
previous observations on the unique phase behavior of these particles
in concentrated suspensions.
groups have begun to exploit their properties to create dynamic materials self-assembled from
those components. The fundamental details of how those building blocks are assembled, the
component functionality, and the geometry or length-scales present in the assemblies contribute to
the behavior of the resultant material. In this tutorial review, we examine recent progress in the
assembly of responsive hydrogel colloids in two and three dimensions, highlighting their potential
applications, especially in the domain of biotechnology.
both carboxylic acid groups and azidohydrin or terminal alkyne groups, via a one-pot multistage
polymerization approach. The clickable functional groups on the microgels were confirmed by FTIR.
Additionally, we simultaneously performed “click” and acid-amine coupling reactions on microgels with
fluorescent dyes containing complementary functional groups. Epifluorescence microscopy was employed to
confirm the coupling of those dyes to the microgels. The orthogonality of the click reaction to other functional
groups such as hydroxyl, carboxylic acid, and amino groups was confirmed, suggesting the potential utility of
such microgels in applications where multifunctional colloidal particles are required.
(pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These
studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions;
this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular
scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles
(i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions
are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular
capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this
structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage
size) decreases with time, indicating that the physical aging process does not end following assembly, but
instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature
dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated
during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal
stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the
aging process that we associate with an evolution of attractive interparticle interactions during aging. These
attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion,
the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the
pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function
of pH and volume fraction following ∼1 month of aging. This diagram reveals highly complex dispersion
characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that,
at pH < pKa, the final dispersions behave in a manner that can be associated with attractive interparticle
interactions, whereas at pH > pKa, repulsive interactions appear to be dominant. These results are discussed
in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from
reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.
particles with attractive pair potentials have been studied by digital video microscopy. The particles were
synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and
N,N¢-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels)
with temperature- and pH-tunable swelling properties. It is observed that in a pH ) 3.0 buffer with an ionic
strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between
weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements
of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase
does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels,
in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The
unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization
for these microgels.
(PS) and copolymerization of 3-sulfo-propyl methacrylate, potassium salt
(SPMS) with styrene (S) were prepared. Photosensitive polyelectrolyte complexes
(PECDR) derived from SPE and diazoresin (DR), which does not dissolve in water or
organic solvent due to its ionic crosslinking structure, dissolves in aqueous solution of
sodium dodecyl sulfate (SDS) due to the dissociation of PECDR and the hydrophobic
interaction between SDS and the polymer chain. The photosensitive behavior and
thermostability of the PECDR were investigated, and it was found that the thermostability
of PECDR increases dramatically in SDS aqueous solution. It was proposed that
the higher thermostability of PECDR in SDS aqueous solution is due to an aggregation
of SDS molecules around the diazonium group of the PECDR, which protects the ON2
1
group of the DR from attack by the nucleophiles. The image-forming behavior of PECDR
by ultraviolet (UV) light was examined and considered to be different from other PECs.
It was concluded that the photoimaging behavior of PECDR is based on a reaction in
which an ionic bond converts to a covalent bond.
bromide (CTAB) and myristyl trimethylammonium bromide (MTAB) in aqueous
solution was investigated. It was found that the solution viscosity increases
rapidly at the molar ratio of CTAB/-SO3Na = 0.5 or MTAB/-SO3Na = 0.6 (the
first viscosity maximum) and CTAB/-SO3Na = ~1.5 or MTAB/-SO3Na = 2.0
(the second viscosity maximum). Both the first and the second viscosity maxima
are related closely with the hydrophobic domains formed in solution,
which can be detected using methyl orange as a spectral probe or N,Ndimethylamino
flavone as fluorescent probe. In the higher concentration of
[SPS], such as >0.05 M (-SO3Na unit), the SPS/CTAB solution converts to gel
suddenly at CTAB/-SO3Na = ~1.5-1.7 region, but with a further addition of
CTAB or SPS, the gel collapses quickly. A term “micelle crosslinking” was
proposed to explain the conversion of solution/gel and the reversible nature of
gel ↔solution.
diffusion. Two suspensions of silica nanoparticles, with average particle diameters of 24 and 139 nm,
were used as model colloidal foulants. To verify the effect of reverse salt diffusion on the colloidal fouling
behavior, NaCl and LaCl3 were employed as draw solutions because they exhibit different reverse diffusion
rates. Our results suggest that in colloidal fouling of FO, salts diffuse from the draw to the feed solution
and accumulate within the colloidal fouling layer that forms on the membrane surface. The accumulated
salts result in a marked acceleration of cake-enhanced osmotic pressure (CEOP), which reduces the net
osmotic driving force for permeate water flux. Fouling was not observed with the small, 24-nm particles
because of the lack of substantial cake formation, but was notable for the 139-nm particles and for a
feed containing a mixture of the 24 and 139 nm particles. Our findings further indicate that colloidal
fouling is enhanced under solution conditions (ionic strength and pH) within the colloidal cake layer
that promote aggregation or destabilization of the silica particles. Colloidal fouling reversibility was also
examined by varying the cross-flow velocity during the FO fouling runs. We showed that in the absence of
colloidal particle destabilization/aggregation, the permeate flux during colloidal fouling in FO recovered
almost completely when the cross-flow velocity was increased from 8.5 to 25.6 cm/s. Our results suggest
that reverse salt diffusion in FO is a key mechanism that controls colloidal fouling behavior as well
as fouling reversibility. Therefore, minimization of reverse salt diffusion through the selection of proper
draw solutes and optimization of FO membrane selectivity are important for minimizing colloidal fouling
as well as enhancing FO operation efficiency.
aggregation of fullerene (C60) nanoparticles (FNPs) were measured via simultaneous multiangle static
and dynamic light scattering. The FNP aggregation behavior was determined as a function of monovalent
(NaCl) and divalent (CaCl2) electrolyte concentration, and the impact of addition of dissolved natural
organic matter (humic acid) to the solution was also investigated. In the absence of humic acid, the fractal
dimension decreased over time with monovalent and divalent salts, suggesting that aggregates become
slightly more open and less compact as they grow. Although the aggregates become slightly more open,
the magnitude of the fractal dimension suggests intermediate aggregation between the diffusion- and
reaction-limited regimes. We observed different aggregation behavior with monovalent and divalent
salts upon the addition of humic acid to the solution. For NaCl-induced aggregation, the introduction
of humic acid significantly suppressed the aggregation rate of FNPs at NaCl concentrations lower than
150 mM. In this case, the aggregation was intermediate or reaction-limited even at NaCl concentrations
as high as 500 mM, giving rise to aggregates with a fractal dimension of 2.0. For CaCl2-induced aggregation,
the introduction of humic acid enhanced the aggregation of FNPs at CaCl2 concentrations greater
than about 5 mM due to calcium complexation and bridging effects. Humic acid also had an impact on
the FNP aggregate structure in the presence of CaCl2, resulting in a fractal dimension of 1.6 for the diffusion-
limited aggregation regime. Our results with CaCl2 indicate that in the presence of humic acid, FNP
aggregates have a more open and loose structure than in the absence of humic acid. The aggregation
results presented in this paper have important implications for the transport, chemical reactivity, and
toxicity of engineered nanoparticles in aquatic environments.
material applications due to their tunable mechanical and chemical properties. Most commonly, thermo
and pH-responsive poly(N-isopropylacrylamide) (pNIPAm) microgels can be fabricated by precipitation
polymerization in the presence of the co-monomer acrylic acid (AAc). Traditionally pNIPAm microgels
are synthesized in the presence of a crosslinking agent, such as N,N0-methylenebisacrylamide (BIS),
however, microgels can also be synthesized under ‘crosslinker free’ conditions. The resulting particles
have extremely low (<0.5%), core-localized crosslinking resulting from rare chain transfer reactions. AFM
nanoindentation of these ultralow crosslinked (ULC) particles indicate that they are soft relative to
crosslinked microgels, with a Young's modulus of 10 kPa. Furthermore, ULC microgels are highly
deformable as indicated by a high degree of spreading on glass surfaces and the ability to translocate
through nanopores significantly smaller than the hydrodynamic diameter of the particles. The size and
charge of ULCs can be easily modulated by altering reaction conditions, such as temperature, monomer,
surfactant and initiator concentrations, and through the addition of co-monomers. Microgels based on
the widely utilized, biocompatible polymer polyethylene glycol (PEG) can also be synthesized under
crosslinker free conditions. Due to their softness and deformability, ULC microgels are a unique base
material for a wide variety of biomedical applications including biomaterials for drug delivery and
regenerative medicine.
between poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-co-
AAc) colloidal microgels as a function of pH, as determined through
Ornstein–Zernike analysis of the pair distribution function of quasi-
2Ddilute colloidal suspensions.The interaction potential ranges from
purely repulsive at high pHdue to electrosteric interactions toweakly
attractive at low pH due to hydrogen bonding, which explains
previous observations on the unique phase behavior of these particles
in concentrated suspensions.
groups have begun to exploit their properties to create dynamic materials self-assembled from
those components. The fundamental details of how those building blocks are assembled, the
component functionality, and the geometry or length-scales present in the assemblies contribute to
the behavior of the resultant material. In this tutorial review, we examine recent progress in the
assembly of responsive hydrogel colloids in two and three dimensions, highlighting their potential
applications, especially in the domain of biotechnology.
both carboxylic acid groups and azidohydrin or terminal alkyne groups, via a one-pot multistage
polymerization approach. The clickable functional groups on the microgels were confirmed by FTIR.
Additionally, we simultaneously performed “click” and acid-amine coupling reactions on microgels with
fluorescent dyes containing complementary functional groups. Epifluorescence microscopy was employed to
confirm the coupling of those dyes to the microgels. The orthogonality of the click reaction to other functional
groups such as hydroxyl, carboxylic acid, and amino groups was confirmed, suggesting the potential utility of
such microgels in applications where multifunctional colloidal particles are required.
(pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These
studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions;
this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular
scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles
(i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions
are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular
capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this
structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage
size) decreases with time, indicating that the physical aging process does not end following assembly, but
instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature
dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated
during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal
stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the
aging process that we associate with an evolution of attractive interparticle interactions during aging. These
attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion,
the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the
pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function
of pH and volume fraction following ∼1 month of aging. This diagram reveals highly complex dispersion
characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that,
at pH < pKa, the final dispersions behave in a manner that can be associated with attractive interparticle
interactions, whereas at pH > pKa, repulsive interactions appear to be dominant. These results are discussed
in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from
reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.
particles with attractive pair potentials have been studied by digital video microscopy. The particles were
synthesized by precipitation copolymerization of N-isopropylacrylamide (NIPAm), acrylic acid (AAc), and
N,N¢-methylene bis(acrylamide) (BIS), yielding highly water swollen hydrogel microparticles (microgels)
with temperature- and pH-tunable swelling properties. It is observed that in a pH ) 3.0 buffer with an ionic
strength of 10 mM, assemblies of pNIPAm-AAc microgels crystallize due to a delicate balance between
weak attractive and soft repulsive forces. The attractive interactions are further confirmed by measurements
of the crystal melting temperatures. As the temperature of colloidal crystals is increased, the crystalline phase
does not melt until the temperature is far above the lower critical solution temperature (LCST) of the microgels,
in stark contrast to what is typically observed for phases formed due to purely repulsive interactions. The
unusual thermal stability of pNIPAm-AAc colloidal crystals demonstrates an enthalpic origin of crystallization
for these microgels.