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Papers by Steven Armes

Journal of the American Chemical Society, Jan 24, 2015

Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can... more Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can be prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). In the present study, these syntheses are conducted in the presence of varying amounts of silica nanoparticles of approximately 18 nm diameter. This approach leads to encapsulation of up to hundreds of silica nanoparticles per vesicle. Silica has high electron contrast compared to the copolymer and its thermal stability enables quantification of the loading efficiency via thermogravimetric analysis. Encapsulation efficiencies can be obtained using disk centrifuge photosedimentometry, since the vesicle density increases at higher silica loadings while the mean vesicle diameter remains essentially unchanged. Small angle X-ray scattering (SAXS) is used to confirm silica encapsulation, since a structure factor is observed at q ~ 0.25 nm-1. A new two-population model provides satisfac...

Reactive and Functional Polymers, 2006

Recent progress in the synthesis of so-called ÔschizophrenicÕ water-soluble block copolymers is r... more Recent progress in the synthesis of so-called ÔschizophrenicÕ water-soluble block copolymers is reviewed. The original report in this new sub-field involved a tertiary amine methacrylate-based AB diblock copolymer synthesized by group transfer polymerization that was both pH-and salt-responsive, allowing the formation of either A-core or B-core micelles in aqueous solution. A second example involved a poly(propylene oxide)-tertiary amine methacrylate diblock copolymer synthesized via atom transfer radical polymerization (ATRP) that exhibited both pH-and thermo-responsive behavior. More recently, several examples of wholly pH-responsive zwitterionic diblock copolymers (prepared via ATRP, usually using protecting group chemistry) have been reported, with their aqueous solution behavior being characterized with varying degrees of precision. The synthesis and characterization of purely thermo-responsive diblock copolymers is also discussed, along with the first example of an ABC triblock copolymer that is capable of forming a ÔtrinityÕ of micelles in aqueous solution at 20°C simply by adjusting the solution pH. In this remarkable final example, the cores of the three types of micelles are formed by hydrophobic forces, polyion complexation and hydrogen bonding, respectively.

Macromolecules, 2002

A series of well-defined poly[(ethylene oxide)-block-2-(dimethylamino)ethyl methacrylateblock-2-(... more A series of well-defined poly[(ethylene oxide)-block-2-(dimethylamino)ethyl methacrylateblock-2-(diethylamino) methacrylate] (PEO-DMA-DEA) triblocks were synthesized by successive ATRP polymerization of DMA and DEA monomers using PEO-based macroinitiators of different molecular weights. These triblock copolymers dissolved molecularly in aqueous solution at low pH; on addition of NaOH, micellization occurred at pH 7.1 to form three-layer "onionlike" micelles comprising DEA cores, DMA inner shells, and PEO coronas. Above pH 7.3, dynamic light scattering studies indicated unimodal, near-monodisperse populations, with mean micelle diameters of 27-84 nm depending on block compositions (for PEO 113 triblock copolymers) and polydispersities typically less than 0.10. The average hydrodynamic diameter 〈Dh〉 of the micelles decreased as the solution pH was increased from pH 7.3 to pH 9.0, indicating that the micelles become more compact due to deprotonation of the tertiary amine residues in the DMA and DEA blocks. 1 H NMR studies supported a three-layer micelle structure and also revealed changes in the hydrophilicity of the DMA chains in the inner shell during cross-linking, which was achieved by adding the bifunctional alkyl iodide, 1,2-bis(2-iodoethoxy)ethane (BIEE). Selective quaternization of the DMA residues by the BIEE leads to increased hydrophilicity and colloid stability for the shell cross-linked (SCL) micelles. The minimum amount of BIEE required to "lock-in" the micellar structure depended on the thickness of the PEO corona: shorter PEO chains led to enhanced crosslinking efficiency. At pH 8.5, the hydrodynamic diameter of un-cross-linked micelles increased rapidly above 40-50°C due to the LCST behavior of the neutral DMA chains in the inner shell. In contrast, the dimensions of the SCL micelles in dilute aqueous solution are independent of temperature. These SCL micelles exhibit reversible swelling on varying the solution pH. At low pH, the DEA cores become protonated and hence hydrophilic. The effect of varying the block composition and the target degree of cross-linking on the structural stability and pH-dependent (de)swelling of the SCL micelles was systematically studied. Longer DEA blocks and lower target degrees of cross-linking led to increased swellability, as expected.

Macromolecules, 2006

The solution properties and the micellization behavior of double hydrophilic diblock copolymers d... more The solution properties and the micellization behavior of double hydrophilic diblock copolymers depend on the degree of ionization of the pH-tunable block. This is investigated by dynamic light scattering and 1 H NMR spectroscopy in aqueous solutions of diblock copolymers comprising a neutral hydrophilic poly(hexa-(ethylene glycol) methacrylate), PHEGMA, block and an ionizable poly(2-(diethylamino)ethyl methacrylate), PDEAEMA, block. At low pH the copolymer is in its unimer state due to the hydrophilicity of the protonated tertiary amine units, while an increase of the solution pH results in the deprotonation of the amine residues, which become hydrophobic, and leads to the formation of micelles consisting of a PDEAEMA core and a PHEGMA corona. The critical degree of ionization for chain aggregation and micelle formation is determined. It is found that the polymer exists as unimers in the aqueous solution for as low as 30% protonated amine groups, whereas for 20% degree of ionization a slight increase in polymer aggregation is observed. When the fraction of ionized amines decreases further to 10%, hydrated micelles are initially formed, followed by the formation of equilibrium micellar structures only upon the complete deprotonation of the PDEAEMA block which thus becomes fully hydrophobic. This unimer to aggregate to micelle transition is observed in the quantitative analysis of the NMR data at similar degrees of ionization to those obtained by DLS, signifying that NMR spectroscopy can be used to follow the micellization process in block copolymer systems.

Macromolecules, 2002

We characterize the structures of various polyelectrolyte block copolymer micelles in dilute aque... more We characterize the structures of various polyelectrolyte block copolymer micelles in dilute aqueous solution as a function of pH and ionic strength. The block copolymers carry a common core block 2-(diethylamino) ethyl methacrylate (DEAEMA) and one of three coronal blocks: 2-(dimethylamino) ethyl methacrylate (DMAEMA), polyethylene oxide (PEO), and DMAEMA whose side-chain amine groups are selectively quaternized with benzyl chloride (Q-DMAEMA). The PEO-DEAEMA, DMAEMA-DEAEMA, and Q-DMAEMA-DEAEMA copolymers form micelles with electrostatically neutral, weakly charged, and highly charged coronae, respectively. We adjust the fractional charge α on the DEAEMA and DMAEMA blocks by adjusting the solution pH. For DMAEMA-DEAEMA micelles increasing the fractional charge α swells the micelle corona while decreasing the aggregation number due to electrostatic repulsions. The decrease in aggregation number is also observed with increasing α for the PEO-DEAEMA and Q-DMAEMA-DEAEMA micelles, due to electrostatic repulsions between the hydrophobic DEAEMA blocks. Increasing the ionic strength causes the DMAEMA-DEAEMA micelle corona to shrink as the salt screens electrostatic repulsions within the corona. In all three copolymers increases in the ionic strength causes the micelle aggregation number to increase by screening the electrostatic repulsions between chains. Trends in the corona thickness with varying fractional charge and ionic strength are compared with a number of theoretical models providing additional insight into the micelle structure.

Macromolecules, 2003

Binary mixtures of a poly(ethylene oxide-block-2-(diethylamino)ethyl methacrylate) (PEO-PDEA) cop... more Binary mixtures of a poly(ethylene oxide-block-2-(diethylamino)ethyl methacrylate) (PEO-PDEA) copolymer and poly(methacrylic acid) (PMAA) can form three types of micelles/aggregates in aqueous solution at ambient temperature, depending solely on the solution pH. Above pH 8.5, simple micelles are formed with hydrophobic PDEA cores and with the PEO chains located in the corona; the PMAA does not participate in micelle formation under these conditions. However, at pH 6-8.5, polyion complex micelles with charge-compensated PDEA/PMAA cores and PEO coronas are obtained. A third colloidal structure is formed below pH 3 due to hydrogen bonding between the PEO and PMAA chains. This remarkable reversible self-assembly behavior is unprecedented.

Macromolecules, 1999

We use fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scatter... more We use fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) to characterize the structure of 2-(dimethylamino)ethyl methacrylate/2-(diethylamino)ethyl methacrylate (DMAEMA/DEAEMA) block copolymer micelles. The copolymers exhibit a strong pH dependence, where protonation of the tertiary amines along the side chains cause the blocks to be soluble in water. Fluorescence results show a critical degree of protonation below which single chains aggregate to form micelles. This critical degree of protonation depends on the copolymer concentration and solution ionic strength. Dynamic light scattering experiments provide unimer and micelle size distributions, and the measured critical degrees of protonation are consistent with the fluorescence data. The micelle hydrodynamic radius measured from DLS depends on the solution ionic strength, because of the polyelectrolyte nature of the protonated copolymers. Small-angle neutron scattering experiments in conjunction with a starlike micelle model provide additional insights into the micellar structures.

Langmuir, 2003

A novel zwitterionic poly[4-vinylbenzoic acid-block-2-N-(morpholino)ethyl methacrylate] (VBA63-b-... more A novel zwitterionic poly[4-vinylbenzoic acid-block-2-N-(morpholino)ethyl methacrylate] (VBA63-b-MEMA123) diblock copolymer was synthesized via atom transfer radical polymerization using protecting group chemistry for the acidic residues. The acidic VBA block has a pKa of 7.1, which is higher than that of the conjugate acid of the MEMA block (pKa) 4.9). This has important consequences for the aqueous solution properties of this zwitterionic diblock copolymer. For example, unlike other zwitterionic diblock copolymers such as poly[methacrylic acid-block-2-(dimethylamino)ethyl methacrylate], precipitation does not occur at the isoelectric point (IEP) of around pH 6.2. This is most likely due to the relatively low degree of charge density on both the VBA and MEMA blocks at this pH. The VBA63-MEMA123 copolymer exhibits interesting "schizophrenic" micellization behavior. Below pH 6, VBA-core micelles are formed, while above pH 6, the diblock copolymer can be dissolved as unimers. In the presence of sufficient Na2SO4 or at elevated temperature, well-defined MEMA-core micelles are formed in alkaline media. Thus, if dissolved in the presence of 0.80 M Na2SO4, the zwitterionic diblock copolymer can be switched from VBA-core micelles to MEMA-core micelles (and vice versa) simply by manipulating the solution pH. At intermediate pH around the IEP precipitation occurs, in contrast to the solution behavior in the absence of salt. The most likely explanation is that the weakly hydrophilic copolymer is simply "salted out" under these conditions. Alternatively, the added electrolyte leads to a higher charge density on the copolymer chains and precipitation occurs due to polyelectrolyte complexation. Both the VBA-core micelles and the "inverted" MEMA-core micelles have been characterized at different solution pHs, ionic strengths, and temperatures using potentiometric titration, aqueous electrophoresis, dynamic and static light scattering, and 1 H NMR spectroscopy measurements, respectively.

Langmuir, 2008

The kinetics of pH-induced formation and dissociation of vesicles self-assembled from a biocompat... more The kinetics of pH-induced formation and dissociation of vesicles self-assembled from a biocompatible zwitterionic diblock copolymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(2-(diisopropylamino)ethyl methacrylate) (PMPC-b-PDPA), was investigated in detail via a combination of stopped-flow light scattering and laser light scattering (LLS). Upon jumping from pH 2 to 10, stopped-flow light scattering reveals three distinct relaxation processes for the early stages of vesicle self-assembly (0-40 s). Kinetic sequences associated with the obtained three characteristic relaxation times have been tentatively proposed. Moreover, the kinetics of vesicle formation in the later stage (from 3 min onward) was investigated by dynamic LLS. It was found that both the intensity-averaged hydrodynamic radius, 〈R h 〉, and the polydispersity, µ 2 /Γ 2 , decrease exponentially, yielding a characteristic relaxation time of ∼350 s. To our knowledge, this is the first report on the kinetics of the unimer-to-vesicle transition of a stimulus-responsive diblock copolymer. The kinetics of vesicle dissociation for a pH jump from 12 to 2 was also investigated. The breakdown of polymeric vesicles is extremely fast and is independent of polymer concentration; it is complete within ∼5 ms and is in marked contrast to the much slower rate of vesicle formation.

Journal of the American Chemical Society, 2014

In this Perspective, we discuss the recent development of polymerization-induced self-assembly me... more In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition−fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells.

Biomacromolecules, 2015

Surface patterning in three dimensions is of great importance in biomaterials design for controll... more Surface patterning in three dimensions is of great importance in biomaterials design for controlling cell behavior. A facile one-step functionalization of biodegradable PDLLA fibers using amphiphilic diblock copolymers is demonstrated here to systematically vary the fiber surface composition. The copolymers comprise a hydrophilic poly-[oligo(ethylene glycol) methacrylate] (POEGMA), poly[(2methacryloyloxy)ethyl phosphorylcholine] (PMPC), or poly-[2-(dimethylamino)ethyl methacrylate)] (PDMAEMA) block and a hydrophobic poly(L-lactide) (PLA) block. The block copolymer-modified fibers have increased surface hydrophilicity compared to that of PDLLA fibers. Mixtures of PLA−PMPC and PLA−POEGMA copolymers are utilized to exploit microphase separation of the incompatible hydrophilic PMPC and POEGMA blocks at the fiber surface. Conjugation of an RGD cell-adhesive peptide to one hydrophilic block (POEGMA) using thiol-ene chemistry produces fibers with domains of celladhesive (POEGMA) and cell-inert (PMPC) sites, mimicking the adhesive properties of the extracellular matrix (ECM). Human mesenchymal progenitor cells (hES-MPs) showed much better adhesion to the fibers with surface-adhesive heterogeneity compared to that to fibers with only adhesive or only inert surface chemistries.

Journal of the American Chemical Society, Jan 24, 2015

Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can... more Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can be prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). In the present study, these syntheses are conducted in the presence of varying amounts of silica nanoparticles of approximately 18 nm diameter. This approach leads to encapsulation of up to hundreds of silica nanoparticles per vesicle. Silica has high electron contrast compared to the copolymer and its thermal stability enables quantification of the loading efficiency via thermogravimetric analysis. Encapsulation efficiencies can be obtained using disk centrifuge photosedimentometry, since the vesicle density increases at higher silica loadings while the mean vesicle diameter remains essentially unchanged. Small angle X-ray scattering (SAXS) is used to confirm silica encapsulation, since a structure factor is observed at q ~ 0.25 nm-1. A new two-population model provides satisfac...

Reactive and Functional Polymers, 2006

Recent progress in the synthesis of so-called ÔschizophrenicÕ water-soluble block copolymers is r... more Recent progress in the synthesis of so-called ÔschizophrenicÕ water-soluble block copolymers is reviewed. The original report in this new sub-field involved a tertiary amine methacrylate-based AB diblock copolymer synthesized by group transfer polymerization that was both pH-and salt-responsive, allowing the formation of either A-core or B-core micelles in aqueous solution. A second example involved a poly(propylene oxide)-tertiary amine methacrylate diblock copolymer synthesized via atom transfer radical polymerization (ATRP) that exhibited both pH-and thermo-responsive behavior. More recently, several examples of wholly pH-responsive zwitterionic diblock copolymers (prepared via ATRP, usually using protecting group chemistry) have been reported, with their aqueous solution behavior being characterized with varying degrees of precision. The synthesis and characterization of purely thermo-responsive diblock copolymers is also discussed, along with the first example of an ABC triblock copolymer that is capable of forming a ÔtrinityÕ of micelles in aqueous solution at 20°C simply by adjusting the solution pH. In this remarkable final example, the cores of the three types of micelles are formed by hydrophobic forces, polyion complexation and hydrogen bonding, respectively.

Macromolecules, 2002

A series of well-defined poly[(ethylene oxide)-block-2-(dimethylamino)ethyl methacrylateblock-2-(... more A series of well-defined poly[(ethylene oxide)-block-2-(dimethylamino)ethyl methacrylateblock-2-(diethylamino) methacrylate] (PEO-DMA-DEA) triblocks were synthesized by successive ATRP polymerization of DMA and DEA monomers using PEO-based macroinitiators of different molecular weights. These triblock copolymers dissolved molecularly in aqueous solution at low pH; on addition of NaOH, micellization occurred at pH 7.1 to form three-layer "onionlike" micelles comprising DEA cores, DMA inner shells, and PEO coronas. Above pH 7.3, dynamic light scattering studies indicated unimodal, near-monodisperse populations, with mean micelle diameters of 27-84 nm depending on block compositions (for PEO 113 triblock copolymers) and polydispersities typically less than 0.10. The average hydrodynamic diameter 〈Dh〉 of the micelles decreased as the solution pH was increased from pH 7.3 to pH 9.0, indicating that the micelles become more compact due to deprotonation of the tertiary amine residues in the DMA and DEA blocks. 1 H NMR studies supported a three-layer micelle structure and also revealed changes in the hydrophilicity of the DMA chains in the inner shell during cross-linking, which was achieved by adding the bifunctional alkyl iodide, 1,2-bis(2-iodoethoxy)ethane (BIEE). Selective quaternization of the DMA residues by the BIEE leads to increased hydrophilicity and colloid stability for the shell cross-linked (SCL) micelles. The minimum amount of BIEE required to "lock-in" the micellar structure depended on the thickness of the PEO corona: shorter PEO chains led to enhanced crosslinking efficiency. At pH 8.5, the hydrodynamic diameter of un-cross-linked micelles increased rapidly above 40-50°C due to the LCST behavior of the neutral DMA chains in the inner shell. In contrast, the dimensions of the SCL micelles in dilute aqueous solution are independent of temperature. These SCL micelles exhibit reversible swelling on varying the solution pH. At low pH, the DEA cores become protonated and hence hydrophilic. The effect of varying the block composition and the target degree of cross-linking on the structural stability and pH-dependent (de)swelling of the SCL micelles was systematically studied. Longer DEA blocks and lower target degrees of cross-linking led to increased swellability, as expected.

Macromolecules, 2006

The solution properties and the micellization behavior of double hydrophilic diblock copolymers d... more The solution properties and the micellization behavior of double hydrophilic diblock copolymers depend on the degree of ionization of the pH-tunable block. This is investigated by dynamic light scattering and 1 H NMR spectroscopy in aqueous solutions of diblock copolymers comprising a neutral hydrophilic poly(hexa-(ethylene glycol) methacrylate), PHEGMA, block and an ionizable poly(2-(diethylamino)ethyl methacrylate), PDEAEMA, block. At low pH the copolymer is in its unimer state due to the hydrophilicity of the protonated tertiary amine units, while an increase of the solution pH results in the deprotonation of the amine residues, which become hydrophobic, and leads to the formation of micelles consisting of a PDEAEMA core and a PHEGMA corona. The critical degree of ionization for chain aggregation and micelle formation is determined. It is found that the polymer exists as unimers in the aqueous solution for as low as 30% protonated amine groups, whereas for 20% degree of ionization a slight increase in polymer aggregation is observed. When the fraction of ionized amines decreases further to 10%, hydrated micelles are initially formed, followed by the formation of equilibrium micellar structures only upon the complete deprotonation of the PDEAEMA block which thus becomes fully hydrophobic. This unimer to aggregate to micelle transition is observed in the quantitative analysis of the NMR data at similar degrees of ionization to those obtained by DLS, signifying that NMR spectroscopy can be used to follow the micellization process in block copolymer systems.

Macromolecules, 2002

We characterize the structures of various polyelectrolyte block copolymer micelles in dilute aque... more We characterize the structures of various polyelectrolyte block copolymer micelles in dilute aqueous solution as a function of pH and ionic strength. The block copolymers carry a common core block 2-(diethylamino) ethyl methacrylate (DEAEMA) and one of three coronal blocks: 2-(dimethylamino) ethyl methacrylate (DMAEMA), polyethylene oxide (PEO), and DMAEMA whose side-chain amine groups are selectively quaternized with benzyl chloride (Q-DMAEMA). The PEO-DEAEMA, DMAEMA-DEAEMA, and Q-DMAEMA-DEAEMA copolymers form micelles with electrostatically neutral, weakly charged, and highly charged coronae, respectively. We adjust the fractional charge α on the DEAEMA and DMAEMA blocks by adjusting the solution pH. For DMAEMA-DEAEMA micelles increasing the fractional charge α swells the micelle corona while decreasing the aggregation number due to electrostatic repulsions. The decrease in aggregation number is also observed with increasing α for the PEO-DEAEMA and Q-DMAEMA-DEAEMA micelles, due to electrostatic repulsions between the hydrophobic DEAEMA blocks. Increasing the ionic strength causes the DMAEMA-DEAEMA micelle corona to shrink as the salt screens electrostatic repulsions within the corona. In all three copolymers increases in the ionic strength causes the micelle aggregation number to increase by screening the electrostatic repulsions between chains. Trends in the corona thickness with varying fractional charge and ionic strength are compared with a number of theoretical models providing additional insight into the micelle structure.

Macromolecules, 2003

Binary mixtures of a poly(ethylene oxide-block-2-(diethylamino)ethyl methacrylate) (PEO-PDEA) cop... more Binary mixtures of a poly(ethylene oxide-block-2-(diethylamino)ethyl methacrylate) (PEO-PDEA) copolymer and poly(methacrylic acid) (PMAA) can form three types of micelles/aggregates in aqueous solution at ambient temperature, depending solely on the solution pH. Above pH 8.5, simple micelles are formed with hydrophobic PDEA cores and with the PEO chains located in the corona; the PMAA does not participate in micelle formation under these conditions. However, at pH 6-8.5, polyion complex micelles with charge-compensated PDEA/PMAA cores and PEO coronas are obtained. A third colloidal structure is formed below pH 3 due to hydrogen bonding between the PEO and PMAA chains. This remarkable reversible self-assembly behavior is unprecedented.

Macromolecules, 1999

We use fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scatter... more We use fluorescence spectroscopy, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) to characterize the structure of 2-(dimethylamino)ethyl methacrylate/2-(diethylamino)ethyl methacrylate (DMAEMA/DEAEMA) block copolymer micelles. The copolymers exhibit a strong pH dependence, where protonation of the tertiary amines along the side chains cause the blocks to be soluble in water. Fluorescence results show a critical degree of protonation below which single chains aggregate to form micelles. This critical degree of protonation depends on the copolymer concentration and solution ionic strength. Dynamic light scattering experiments provide unimer and micelle size distributions, and the measured critical degrees of protonation are consistent with the fluorescence data. The micelle hydrodynamic radius measured from DLS depends on the solution ionic strength, because of the polyelectrolyte nature of the protonated copolymers. Small-angle neutron scattering experiments in conjunction with a starlike micelle model provide additional insights into the micellar structures.

Langmuir, 2003

A novel zwitterionic poly[4-vinylbenzoic acid-block-2-N-(morpholino)ethyl methacrylate] (VBA63-b-... more A novel zwitterionic poly[4-vinylbenzoic acid-block-2-N-(morpholino)ethyl methacrylate] (VBA63-b-MEMA123) diblock copolymer was synthesized via atom transfer radical polymerization using protecting group chemistry for the acidic residues. The acidic VBA block has a pKa of 7.1, which is higher than that of the conjugate acid of the MEMA block (pKa) 4.9). This has important consequences for the aqueous solution properties of this zwitterionic diblock copolymer. For example, unlike other zwitterionic diblock copolymers such as poly[methacrylic acid-block-2-(dimethylamino)ethyl methacrylate], precipitation does not occur at the isoelectric point (IEP) of around pH 6.2. This is most likely due to the relatively low degree of charge density on both the VBA and MEMA blocks at this pH. The VBA63-MEMA123 copolymer exhibits interesting "schizophrenic" micellization behavior. Below pH 6, VBA-core micelles are formed, while above pH 6, the diblock copolymer can be dissolved as unimers. In the presence of sufficient Na2SO4 or at elevated temperature, well-defined MEMA-core micelles are formed in alkaline media. Thus, if dissolved in the presence of 0.80 M Na2SO4, the zwitterionic diblock copolymer can be switched from VBA-core micelles to MEMA-core micelles (and vice versa) simply by manipulating the solution pH. At intermediate pH around the IEP precipitation occurs, in contrast to the solution behavior in the absence of salt. The most likely explanation is that the weakly hydrophilic copolymer is simply "salted out" under these conditions. Alternatively, the added electrolyte leads to a higher charge density on the copolymer chains and precipitation occurs due to polyelectrolyte complexation. Both the VBA-core micelles and the "inverted" MEMA-core micelles have been characterized at different solution pHs, ionic strengths, and temperatures using potentiometric titration, aqueous electrophoresis, dynamic and static light scattering, and 1 H NMR spectroscopy measurements, respectively.

Langmuir, 2008

The kinetics of pH-induced formation and dissociation of vesicles self-assembled from a biocompat... more The kinetics of pH-induced formation and dissociation of vesicles self-assembled from a biocompatible zwitterionic diblock copolymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(2-(diisopropylamino)ethyl methacrylate) (PMPC-b-PDPA), was investigated in detail via a combination of stopped-flow light scattering and laser light scattering (LLS). Upon jumping from pH 2 to 10, stopped-flow light scattering reveals three distinct relaxation processes for the early stages of vesicle self-assembly (0-40 s). Kinetic sequences associated with the obtained three characteristic relaxation times have been tentatively proposed. Moreover, the kinetics of vesicle formation in the later stage (from 3 min onward) was investigated by dynamic LLS. It was found that both the intensity-averaged hydrodynamic radius, 〈R h 〉, and the polydispersity, µ 2 /Γ 2 , decrease exponentially, yielding a characteristic relaxation time of ∼350 s. To our knowledge, this is the first report on the kinetics of the unimer-to-vesicle transition of a stimulus-responsive diblock copolymer. The kinetics of vesicle dissociation for a pH jump from 12 to 2 was also investigated. The breakdown of polymeric vesicles is extremely fast and is independent of polymer concentration; it is complete within ∼5 ms and is in marked contrast to the much slower rate of vesicle formation.

Journal of the American Chemical Society, 2014

In this Perspective, we discuss the recent development of polymerization-induced self-assembly me... more In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition−fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells.

Biomacromolecules, 2015

Surface patterning in three dimensions is of great importance in biomaterials design for controll... more Surface patterning in three dimensions is of great importance in biomaterials design for controlling cell behavior. A facile one-step functionalization of biodegradable PDLLA fibers using amphiphilic diblock copolymers is demonstrated here to systematically vary the fiber surface composition. The copolymers comprise a hydrophilic poly-[oligo(ethylene glycol) methacrylate] (POEGMA), poly[(2methacryloyloxy)ethyl phosphorylcholine] (PMPC), or poly-[2-(dimethylamino)ethyl methacrylate)] (PDMAEMA) block and a hydrophobic poly(L-lactide) (PLA) block. The block copolymer-modified fibers have increased surface hydrophilicity compared to that of PDLLA fibers. Mixtures of PLA−PMPC and PLA−POEGMA copolymers are utilized to exploit microphase separation of the incompatible hydrophilic PMPC and POEGMA blocks at the fiber surface. Conjugation of an RGD cell-adhesive peptide to one hydrophilic block (POEGMA) using thiol-ene chemistry produces fibers with domains of celladhesive (POEGMA) and cell-inert (PMPC) sites, mimicking the adhesive properties of the extracellular matrix (ECM). Human mesenchymal progenitor cells (hES-MPs) showed much better adhesion to the fibers with surface-adhesive heterogeneity compared to that to fibers with only adhesive or only inert surface chemistries.