Addition of water to an alcoholic RAFT PISA formulation leads to faster kinetics but limits the evolution of copolymer morphology

RAFT dispersion polymerization of benzyl methacrylate (BzMA) has been used previously (E. R. Jones, et al., Macromolecules, 2012, 45, 5091) to prepare poly(2-(dimethylamino)ethyl methacrylate)-poly- (benzyl methacrylate) (PDMA–PBzMA) diblock copolymer nanoparticles in ethanol via polymerizationinduced self-assembly (PISA). However, the rate of polymerization was relatively slow, with incomplete monomer conversions being obtained when targeting higher mean degrees of polymerization (DP) even after 24 h at 70 °C. Herein we examine the effect of the addition of up to 20% w/w water co-solvent on the kinetics of BzMA polymerization for this PISA formulation. Significantly faster polymerizations were observed: for a target DP of 200, 90% BzMA conversion was achieved within just 6 h in the presence of 20% w/w water, compared to only 35% conversion in anhydrous ethanol under the same conditions. This rate enhancement enables much higher mean DPs to be obtained for the core-forming PBzMA and is attributed to greater partitioning of the BzMA monomer within the particles, which increases the local monomer concentration. However, the presence of water adversely affected the evolution of copolymer morphology from spheres to worms to vesicles when employing a relatively short PDMA chain transfer agent, with only kinetically-trapped spheres being obtained at higher levels of added water. Aqueous electrophoresis studies indicate that the PDMA stabilizer chains acquired partial cationic charge in the presence of water. This leads to more efficient inter-particle repulsion, thus preventing the sphere-sphere fusion events required for an evolution in morphology. In summary, the addition of water to such PISA formulations allows the more efficient synthesis of spherical nanoparticles, but should be used with caution if either diblock copolymer worms or vesicles are desired

Block copolymer based novel magnetic mixed matrix membranes-magnetic modulation of water permeation by irreversible structural changes

This contribution focuses on understanding the effect of magnetic field intensity on the performance of novel hydrophilic and hydrophobic mixed matrix membranes (MMMs). The hydrophilic MMMs were made up of polymeric nanoparticles (PNPs) that were synthesized through polymerization-induced self-assembly (PISA) and iron oxide nanoparticles prepared in presence of poly (methacrylic acid)-b-poly quaternized (2-dimethylamino)ethyl methacrylate. The hydrophobic MMMs were prepared by the addition of iron oxide nanoparticles with different surface properties to a linear poly (methacrylic acid)-b-poly (methylmethacrylate) diblock copolymer dissolved in tetrahydrofuran (THF). Three different types of hydrophilic membranes were prepared with polymeric nanoparticles of different morphologies (spherical, vermicular and vesicular). In case of the hydrophobic membranes, six different membranes containing different iron oxide core coated with different stabilizers such as poly (methacrylic acid), quaternized poly(2-dimethylamino)ethyl methacrylate and meso-2, 3-dimercaptosuccinic acid were prepared. An external magnetic field with intensity values up to 1.15 T was used for the permeation studies and the results were compared with those obtained in the absence of magnetic field. The collected data indicate an increase in the water flux of up to 16% and 29% under the magnetic field for hydrophobic and hydrophilic membranes, respectively. The STEM analyses suggest that the magnetic nanoparticles move within the membrane structure during the application of the magnetic field. This displacement/rearrangement causes constant changes in the membrane structure (structure of the active layer) and consequently on the membrane permeability. These results suggest that the application of the magnetic field could be used as a pretreatment step to obtain high flux membranes

Block copolymer-based magnetic mixed matrix membranes-effect of magnetic field on protein permeation and membrane fouling

In this study, we report the impact of the magnetic field on protein permeability through magnetic-responsive, block copolymer, nanocomposite membranes with hydrophilic and hydrophobic characters. The hydrophilic nanocomposite membranes were composed of spherical polymeric nanoparticles (NPs) synthesized through polymerization-induced self-assembly (PISA) with iron oxide NPs coated with quaternized poly(2-dimethylamino)ethyl methacrylate. The hydrophobic nanocomposite membranes were prepared via nonsolvent-induced phase separation (NIPS) containing poly (methacrylic acid) and meso-2, 3-dimercaptosuccinic acid-coated superparamagnetic nanoparticles (SPNPs). The permeation experiments were carried out using bovine serum albumin (BSA) as the model solute, in the absence of the magnetic field and under permanent and cyclic magnetic field conditions OFF/ON (strategy 1) and ON/OFF (strategy 2). It was observed that the magnetic field led to a lower reduction in the permeate fluxes of magnetic-responsive membranes during BSA permeation, regardless of the magnetic field strategy used, than that obtained in the absence of the magnetic field. Nevertheless, a comparative analysis of the effect caused by the two cyclic magnetic field strategies showed that strategy 2 allowed for a lower reduction of the original permeate fluxes during BSA permeation and higher protein sieving coefficients. Overall, these novel magneto-responsive block copolymer nanocomposite membranes proved to be competent in mitigating biofouling phenomena in bioseparation processes

Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation

A range of cationic diblock copolymer nanoparticles are synthesised via polymerisation-induced self-assembly (PISA) using a RAFT aqueous dispersion polymerisation formulation. The cationic character of these nanoparticles can be systematically varied by utilising a binary mixture of two macro-CTAs, namely non-ionic poly(glycerol monomethacrylate) (PGMA) and cationic poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PQDMA), with poly(2-hydroxypropyl methacrylate) (PHPMA) being selected as the hydrophobic core-forming block. Thus a series of cationic diblock copolymer nano-objects with the general formula ([1 - n] PGMAx + [n] PQDMAy) - PHPMAz were prepared at 20% w/w solids, where n is the mol fraction of the cationic block and x, y and z are the mean degrees of polymerisation of the non-ionic, cationic and hydrophobic blocks, respectively. These cationic diblock copolymer nanoparticles were analysed in terms of their chemical composition, particle size, morphology and cationic character using 1H NMR spectroscopy, dynamic light scattering (DLS), transmission electron microscopy (TEM), and aqueous electrophoresis, respectively. Systematic variation of the above PISA formulation enabled the formation of spheres, worms or vesicles that remain cationic over a wide pH range. However, increasing the cationic character favors the formation of kinetically-trapped spheres, since it leads to more effective steric stabilisation which prevents sphere-sphere fusion. Furthermore, cationic worms form a soft free-standing gel at 25 °C that undergoes reversible degelation on cooling, as indicated by variable temperature oscillatory rheology studies. Finally, the antimicrobial activity of this thermo-responsive cationic worm gel towards the well-known pathogen Staphylococcus aureus is examined via direct contact assays

End-group ionisation enables the use of poly(N-(2-methacryloyloxy)ethyl pyrrolidone) as an electrosteric stabiliser block for polymerisation-induced self-assembly in aqueous media

A series of near-monodisperse poly(N-2-(methacryloyloxy)ethyl pyrrolidone) (PNMEP) homopolymers was prepared via reversible addition-fragmentation chain transfer (RAFT) solution polymerisation of NMEP in ethanol at 70 °C using a carboxylic acid-functional RAFT agent. The mean degree of polymerisation (DP) was varied from 19 to 89 and acid titration indicated end-group pK a values of 5.07-5.44. Turbidimetry studies indicated that homopolymer cloud points were significantly higher at pH 7 (anionic carboxylate) than at pH 3 (neutral carboxylic acid). Moreover, this enhanced hydrophilic character enabled PNMEP to be used as a steric stabiliser for aqueous polymerisation-induced self-assembly (PISA) syntheses. Thus, a PNMEP 42 precursor was chain-extended via RAFT aqueous dispersion polymerisation of 2-hydroxypropyl methacrylate (HPMA) at 44 °C. A series of PNMEP 42 -PHPMA x diblock copolymers were synthesised using this protocol, with target PHPMA DPs of 150 to 400. High conversions were achieved and a linear evolution in M n with increasing PHPMA DP was observed. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies confirmed a spherical morphology in all cases. The nanoparticles flocculated either below pH 4.5 (owing to protonation) or on addition of 60 mM KCl (as a result of charge screening). Thus the anionic end-groups on the PNMEP stabiliser chains make an important contribution to the overall colloidal stability. Similarly, a PNMEP 53 macro-CTA was chain-extended via RAFT aqueous emulsion polymerisation of 2-ethoxyethyl methacrylate (EEMA) at 44 °C. Again, a neutral solution pH was critical for the synthesis of colloidally stable nanoparticles. High conversions were achieved as the target PEEMA DP was varied between 100 and 600 and a linear evolution in molecular weight with PEEMA DP was confirmed by chloroform GPC studies. DLS experiments indicated a monotonic increase in nanoparticle diameter with PEEMA DP and TEM studies confirmed a spherical morphology in each case. In summary, PNMEP can be used as a water-soluble steric stabiliser for aqueous PISA syntheses provided that it contains an anionic carboxylate end-group to enhance its hydrophilic character

How Do Spherical Diblock Copolymer Nanoparticles Grow during RAFT Alcoholic Dispersion Polymerization?

A poly(2-(dimethylamino)ethyl methacrylate) (PDMA) chain transfer agent (CTA) is used for the reversible addition–fragmentation chain transfer (RAFT) alcoholic dispersion polymerization of benzyl methacrylate (BzMA) in ethanol at 70 °C. THF GPC analysis indicated a well-controlled polymerization with molecular weight increasing linearly with conversion. GPC traces also showed high blocking efficiency with no homopolymer contamination apparent and Mw/Mn values below 1.35 in all cases. 1H NMR studies confirmed greater than 98% BzMA conversion for a target PBzMA degree of polymerization (DP) of up to 600. The PBzMA block becomes insoluble as it grows, leading to the in situ formation of sterically stabilized diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA). Fixing the mean DP of the PDMA stabilizer block at 94 units and systematically varying the DP of the PBzMA block enabled a series of spherical nanoparticles of tunable diameter to be obtained. These nanoparticles were characterized by TEM, DLS, MALLS, and SAXS, with mean diameters ranging from 35 to 100 nm. The latter technique was particularly informative: data fits to a spherical micelle model enabled calculation of the core diameter, surface area occupied per copolymer chain, and the mean aggregation number (Nagg). The scaling exponent derived from a double-logarithmic plot of core diameter vs PBzMA DP suggests that the conformation of the PBzMA chains is intermediate between the collapsed and fully extended state. This is in good agreement with 1H NMR studies, which suggest that only 5−13% of the BzMA residues of the core-forming chains are solvated. The Nagg values calculated from SAXS and MALLS are in good agreement and scale approximately linearly with PBzMA DP. This suggests that spherical micelles grow in size not only as a result of the increase in copolymer molecular weight during the PISA synthesis but also by exchange of individual copolymer chains between micelles and/or by sphere–sphere fusion events

Thermo-responsive Diblock Copolymer Worm Gels in Non-polar Solvents

Benzyl methacrylate (BzMA) is polymerized using a poly(lauryl methacrylate) macromolecular chain transfer agent (PLMA macro-CTA) using reversible addition–fragmentation chain transfer (RAFT) polymerization at 70 °C in n-dodecane. This choice of solvent leads to an efficient dispersion polymerization, with polymerization-induced self-assembly (PISA) occurring via the growing PBzMA block to produce a range of PLMA–PBzMA diblock copolymer nano-objects, including spheres, worms, and vesicles. In the present study, particular attention is paid to the worm phase, which forms soft free-standing gels at 20 °C due to multiple inter-worm contacts. Such worm gels exhibit thermo-responsive behavior: heating above 50 °C causes degelation due to the onset of a worm-to-sphere transition. Degelation occurs because isotropic spheres interact with each other much less efficiently than the highly anisotropic worms. This worm-to-sphere thermal transition is essentially irreversible on heating a dilute solution (0.10% w/w) but is more or less reversible on heating a more concentrated dispersion (20% w/w). The relatively low volatility of n-dodecane facilitates variable-temperature rheological studies, which are consistent with eventual reconstitution of the worm phase on cooling to 20 °C. Variable-temperature 1H NMR studies conducted in d26-dodecane confirm partial solvation of the PBzMA block at elevated temperature: surface plasticization of the worm cores is invoked to account for the observed change in morphology, because this is sufficient to increase the copolymer curvature and hence induce a worm-to-sphere transition. Small-angle X-ray scattering and TEM are used to investigate the structural changes that occur during the worm-to-sphere-to-worm thermal cycle; experiments conducted at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)reversibility of these morphological transitions

Cationic and reactive primary amine-stabilised nanoparticles via RAFT aqueous dispersion polymerisation

The synthesis of primary amine-functionalised diblock copolymer nanoparticles via polymerisation-induced self-assembly (PISA) using a RAFT aqueous dispersion polymerisation formulation is reported. The primary amine steric stabiliser is a macromolecular chain transfer agent (macro-CTA) based on 2-aminoethyl methacrylate AMA, which can be readily polymerised in its hydrochloride salt form with good control (Mw/Mn < 1.30) using RAFT aqueous solution polymerisation. Subsequent chain extension of this macro-CTA with 2-hydroxypropyl methacrylate (HPMA) leads to the formation of relatively monodisperse spherical nanoparticles (68 to 288 nm) at pH 6. However, worms or vesicles could not be obtained, because strong lateral repulsion between the highly cationic PAMA stabiliser chains impedes the formation of these higher order copolymer morphologies. Deprotonation of the primary amine stabiliser chains at or above pH 9 results in flocculation of these spherical nanoparticles as the PAMA block becomes uncharged. Diblock copolymer spheres, worms or vesicles can be synthesised that remain stable at pH 9 by supplementing the PAMA macro-CTA with a poly(glycerol monomethacrylate) (PGMA) macro-CTA, since this non-ionic block confers effective steric stabilisation in alkaline media. A series of diblock copolymer nanoparticles with the general formula ([1 − n]PGMAx + nPAMAy)–PHPMAz can be synthesised by optimising: (i) the mean degree of polymerisation (DP, or x) of the PGMA block, (ii) the PHPMA core-forming DP (or z); (iii) the mol fraction (n) of the PAMA stabiliser; and (iv) the copolymer concentration. These spheres, worms and vesicles are both cationic at low pH and colloidally stable at high pH. Furthermore, deprotonation of the protonated primary amine groups on the PAMA stabiliser chains at high pH renders these particles susceptible to epoxy-amine conjugation. This is demonstrated by the reaction between the primary amine groups on (0.8PGMA101 + 0.2PAMA96)–PHPMA1000 diblock copolymer spheres, and epoxide-functionalised diblock copolymer nanoparticles in aqueous solution at pH 8