Spin-echo small-angle neutron scattering (SESANS) studies of diblock copolymer nanoparticles

Poly(glycerol monomethacrylate)–poly(benzyl methacrylate) (PGMA–PBzMA) diblock copolymer nanoparticles were synthesized via polymerization-induced self-assembly (PISA) using reversible addition–fragmentation chain-transfer (RAFT) aqueous emulsion polymerization in D2O. Such PISA syntheses produce sterically-stabilized nanoparticles in situ and can be performed at relatively high copolymer concentrations (up to 50 wt%). This PGMA–PBzMA formulation is known to form only spherical nanoparticles in water using aqueous emulsion polymerization (Macromolecules, 2014, 47, 5613–5623), which makes it an ideal model system for exploring new characterization methods. The polymer micelles were characterized using small-angle X-ray scattering (SAXS) and a recently developed form of neutron scattering, spin-echo small-angle neutron scattering (SESANS). As far as we are aware, this is the first report of a study of polymer micelles by SESANS, and the data agree well with reciprocal-space scattering. Using this technique enables characterization of the concentrated, as synthesized dispersions directly without dilution, and this will provide a method to study self-assembled polymer systems that have concentration dependent morphologies, while still maintaining the advantages of scattering techniques

Synthesis and electrokinetics of cationic spherical nanoparticles in salt-free non-polar media

Cationic diblock copolymer nanoparticles have been prepared in n-dodecane via polymerization-induced self-assembly (PISA). A previously reported poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) PISA formulation (Chem. Sci. 2016, 7, 5078-5090) was modified by statistically copolymerizing an oil-soluble cationic methacrylic monomer, (2-(methacryloyloxy)ethyl)trimethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, with either SMA or BzMA, to produce either charged shell or charged core nanoparticles. The electrokinetics were studied as a function of many variables (function of volume function, particle size, solvent viscosity, and number of ions per chain). These data are consistent with electrophoresis controlled by counterion condensation, which is typically observed in salt-free media. However, there are several interesting and unexpected features of interest. In particular, charged shell nanoparticles have a lower electrophoretic mobility than the equivalent charged core nanoparticles, and the magnitude of the electrophoretic mobility increases as the fraction of cationic stabilizer chains in the shell layer is reduced. These results show that cationic PSMA-PBzMA spheres provide an interesting new example of electrophoretic nanoparticles in non-polar solvents. Moreover, they should provide an ideal model system to evaluate new electrokinetic theories

Influence of an ionic comonomer on polymerization-induced self-assembly of diblock copolymers in non-polar media

A series of poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) diblock copolymer nano-objects has been synthesized via reversible addition-fragmentation chain-transfer (RAFT) dispersion polymerization in n-dodecane at 20 wt%. This polymerization-induced self-assembly (PISA) formulation was modified by the incorporation of an anionic monomer, tetradodecylammonium 3-sulfopropyl methacrylate ([NDod 4] +[SPMA] -) into the oil-insoluble PBzMA block. According to the literature (M. J. Derry, et al., Chem. Sci., 2016, 7, 5078-5090), PSMA 18-PBzMA diblock copolymers only form spheres using this formulation for any core degree of polymerization. Unexpectedly, incorporating just a small fraction (<6 mol%) of [NDod 4] +[SPMA] - comonomer into the structure-directing block resulted in the formation of non-spherical diblock copolymer nano-objects, including pure worm-like and vesicular morphologies. However, only spherical micelles could be formed using a longer PSMA 34 stabilizer. These diblock copolymer nano-objects were characterized by transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. The bulky nature of the ionic comonomer appears to make it possible to avoid the kinetically-trapped sphere morphology. This study reveals a new approach for tuning the morphology of diblock copolymer nano-objects in non-polar media

The Relationship Between Mental Toughness, Job Loss, and Mental Health Issues During the COVID-19 Pandemic

Concerns toward public well-being and mental health are increasing considering the COVID-19 pandemic's global societal and individual impact. The present study builds on the current body of COVID-19 literature by examining the role of mental toughness (MT) in predicting negative affective states (depression, anxiety and stress) during the pandemic. The study also examined the effects of changes in employment on mental health and MT. Participants (N = 723) completed a battery of questionnaires including the Mental Toughness Questionnaire 48-item, The State-Trait Anxiety Inventory, and the Depression, Anxiety and Stress Scale – 21 items. Participants reported relatively higher levels of depression, stress and anxiety in comparison to pre-COVID-19 samples from previous research, with respondents who had lost their jobs during the pandemic reporting higher levels of negative affective states. Despite this, mentally tough individuals appeared to report lower levels of depression, anxiety and stress. Moreover, moderation analyses identified some interaction between MT and employment status when predicting depression, anxiety and stress. Our findings suggest that MT may have some utility in reducing the adverse mental health effects of the pandemic on individuals, however, further longitudinal research is needed to support these implications

Ionic and Nonspherical Polymer Nanoparticles in Nonpolar Solvents

A series of ionic diblock copolymer nanoparticles was prepared in a typical nonpolar solvent (n-dodecane) via polymerization-induced self-assembly (PISA). A single cationic repeat unit was incorporated into the poly(stearyl methacrylate) (PSMA) stabilizer of otherwise uncharged poly(stearyl methacrylate)–poly(benzyl methacrylate) (PSMA–PBzMA) diblock copolymer nanoparticles. By using short PSMA stabilizer blocks, it was possible to obtain nanoparticles with the range of morphologies expected (spheres, worms, and vesicles). For nanoparticles where all stabilizer chains possessed an ionic group, higher-order morphologies were obtained at lower PBzMA degrees of polymerization than corresponding uncharged particles, and the particles were electrophoretic. For nanoparticles where only a fraction of the stabilizer chains contained an ionic group, higher-order morphologies were obtained at precisely the same PBzMA degrees of polymerization, and the electrophoretic response was greater than when the shell was fully ionic. These particles with a partially ionic shell are a fascinating system, providing morphologies that can be predicted from the existing knowledge of the diblock copolymer morphology yet with the highest possible electrophoretic mobility

Thermoreversible crystallization-driven aggregation of diblock copolymer nanoparticles in mineral oil

A poly(behenyl methacrylate)37 (PBeMA37) macromolecular chain transfer agent is utilized for the reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate (BzMA) directly in mineral oil at 90 °C. Polymerization-induced self-assembly (PISA) occurs under these conditions, yielding a series of sterically-stabilized PBeMA37–PBzMAx diblock copolymer spheres of tunable diameter as confirmed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies. Rheological studies indicate that a relatively transparent, free-flowing, concentrated dispersion of non-interacting 32 nm PBeMA37–PBzMA100 spheres at 50 °C forms a turbid, paste-like dispersion on cooling to 20 °C. Turbidimetry and differential scanning calorimetry (DSC) studies conducted on solutions of PBeMA37 homopolymer in mineral oil suggest that this switchable colloidal stability is linked to crystallization-induced phase separation exhibited by this stabilizer block. Indeed, variable-temperature small-angle X-ray scattering (SAXS) indicates that a loose mass fractal network of strongly interacting spheres is formed on cooling to 20 °C, which accounts for this thermoreversible sol–gel transition. Moreover, SAXS, DSC and wide-angle X-ray scattering (WAXS) analyses indicate that the behenyl (C22H45) side-chains first form crystalline domains comprising adjacent stabilizer chains within individual spherical nanoparticles, with subsequent crystallization between neighboring nanoparticles leading to the formation of the mass fractal aggregates

Poly(N-2-(methacryloyloxy)ethyl pyrrolidone)-poly(benzyl methacrylate) diblock copolymer nano-objects via RAFT alcoholic dispersion polymerisation in ethanol

N-2-(methacryloyloxy)ethyl pyrrolidone (NMEP) is a highly polar monomer that is much more readily copolymerised with methacrylic monomers than its structural analogue, N-vinyl pyrrolidone. The RAFT solution polymerisation of NMEP has been conducted in ethanol at 70 °C while varying the degree of polymerisation (DP) from 70 to 400. Reducing the CTA/initiator molar ratio from 10.0 to 3.0 when targeting PNMEP100 led to a three-fold rate enhancement and increased the final monomer conversion from 72% to 98%. Targeting DPs of 200 or greater led to lower monomer conversions. DMF GPC analysis of a series of PNMEPx homopolymers confirmed a linear increase in molecular weight with conversion and relatively low dispersities (Mw/Mn 90%) were obtained and electron microscopy studies indicated that a range of diblock copolymer morphologies, e.g. spheres, worms and vesicles, were produced by polymerisation-induced self-assembly (PISA). Finally, a PNMEP47-PBzMA243 diblock copolymer was synthesised via a convenient 'one-pot' protocol whereby a PNMEP47 macro-CTA was first prepared at 97% conversion, followed by in situ RAFT dispersion polymerisation of BzMA to produce diblock copolymer nano-objects. TEM analysis of aliquots taken during the diblock copolymer synthesis indicated a gradual evolution in copolymer morphology from spherical micelles after 1 h to a pure vesicle phase after 8 h via intermediate mixed phases (including worms)

Rational synthesis of epoxy-functional spheres, worms and vesicles by RAFT aqueous emulsion polymerisation of glycidyl methacrylate

The rational synthesis of epoxy-functional diblock copolymer nano-objects has been achieved via RAFT aqueous emulsion polymerisation of glycidyl methacrylate (GlyMA; aqueous solubility ∼22 g dm-3 at 50 °C) by utilising relatively mild conditions (pH 7, 50 °C) to preserve the epoxy groups. High monomer conversions were achieved within 1 h when using a poly(glycerol monomethacrylate) chain transfer agent with a mean degree of polymerisation (DP) of 28, with GPC analysis indicating relatively narrow molecular weight distributions (Mw/Mn < 1.40) when targeting PGlyMA DPs up to 80. A phase diagram was constructed to identify the synthesis conditions required to access pure spheres, worms or vesicles. Transmission electron microscopy, dynamic light scattering and small-angle X-ray scattering (SAXS) studies indicated the formation of well-defined worms and vesicles when targeting relatively long PGlyMA blocks. These epoxy-functional nano-objects were derivatised via epoxy-thiol chemistry by reaction with l-cysteine in aqueous solution. Finally, an in situ SAXS study was conducted during the RAFT aqueous emulsion polymerisation of GlyMA at 50 °C to examine the nucleation and size evolution of PGMA48-PGlyMA100 diblock copolymer spheres using a bespoke stirrable reaction cell

Block copolymer microparticles comprising inverse bicontinuous phases prepared via polymerization-induced self-assembly

Traditionally, post-polymerization processing routes have been used to obtain a wide range of block copolymer morphologies. However, this self-assembly approach is normally performed at rather low copolymer concentration, which precludes many potential applications. Herein, we report a facile method for the preparation of block copolymer particles exhibiting complex internal morphology via polymerization-induced self-assembly (PISA). More specifically, a series of diblock copolymers were synthesized by reversible addition–fragmentation chain transfer (RAFT) alternating copolymerization of styrene (St) with N-phenylmaleimide (NMI) using a poly(N,N-dimethylacrylamide) (PDMAC) stabilizer as a soluble precursor. Conducting such PISA syntheses in a 50 : 50 w/w ethanol/methyl ethyl ketone (MEK) mixture leads directly to the formation of micrometer-sized PDMAC-P(St-alt-NMI) diblock copolymer particles at 20% w/w solids. Adjusting the degree of polymerization (DP) of the core-forming P(St-alt-NMI) block to target highly asymmetric copolymer compositions provides convenient access to an inverse bicontinuous phase. TEM studies of intermediate structures provide useful insights regarding the mechanism of formation of this phase. SEM studies indicate that the final copolymer particles comprise perforated surface layers and possess nanostructured interiors. In addition, control experiments using 1,4-dioxane suggest that the high chain mobility conferred by the MEK co-solvent is essential for the formation of such inverse bicontinuous structures. One-pot PISA formulations are reproducible and involve only cheap, commercially available starting materials, so they should be readily amenable to scale-up. This augurs well for the potential use of such nanostructured micrometer-sized particles as new organic opacifiers for paints and coatings

In situ SAXS studies of a prototypical RAFT aqueous dispersion polymerization formulation: monitoring the evolution in copolymer morphology during polymerization-induced self-assembly

Small-angle X-ray scattering (SAXS) is used to characterize the in situ formation of diblock copolymer spheres, worms and vesicles during reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate at 70 °C using a poly(glycerol monomethacrylate) steric stabilizer. 1H NMR spectroscopy indicates more than 99% HPMA conversion within 80 min, while transmission electron microscopy and dynamic light scattering studies are consistent with the final morphology being pure vesicles. Analysis of time-resolved SAXS patterns for this prototypical polymerization-induced self-assembly (PISA) formulation enables the evolution in copolymer morphology, particle diameter, mean aggregation number, solvent volume fraction, surface density of copolymer chains and their mean inter-chain separation distance at the nanoparticle surface to be monitored. Furthermore, the change in vesicle diameter and membrane thickness during the final stages of polymerization supports an ‘inward growth’ mechanism