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Exploring the Upper Size Limit for Sterically Stabilized Diblock Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly in Non-Polar Media.

ABSTRACT: Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate is used to prepare a series of well-defined poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) diblock copolymer nanoparticles in mineral oil at 90 °C. A relatively long PSMA54 precursor acts as a steric stabilizer block and also ensures that only kinetically trapped spheres are obtained, regardless of the target degree of polymerization (DP) for the core-forming PBzMA block. This polymerization-induced self-assembly (PISA) formulation provides good control over the particle size distribution over a wide size range (24-459 nm diameter). 1H NMR spectroscopy studies confirm that high monomer conversions (?96%) are obtained for all PISA syntheses while transmission electron microscopy and dynamic light scattering analyses show well-defined spheres with a power-law relationship between the target PBzMA DP and the mean particle diameter. Gel permeation chromatography studies indicate a gradual loss of control over the molecular weight distribution as higher DPs are targeted, but well-defined morphologies and narrow particle size distributions can be obtained for PBzMA DPs up to 3500, which corresponds to an upper particle size limit of 459 nm. Thus, these are among the largest well-defined spheres with reasonably narrow size distributions (standard deviation ?20%) produced by any PISA formulation. Such large spheres serve as model sterically stabilized particles for analytical centrifugation studies.

SUBMITTER: Parker BR

PROVIDER: S-EPMC7161081 | biostudies-literature | 2020 Apr

REPOSITORIES: biostudies-literature

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Exploring the Upper Size Limit for Sterically Stabilized Diblock Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly in Non-Polar Media.

Parker Bryony R BR Derry Matthew J MJ Ning Yin Y Armes Steven P SP

Langmuir : the ACS journal of surfaces and colloids 20200402 14

Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate is used to prepare a series of well-defined poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) diblock copolymer nanoparticles in mineral oil at 90 °C. A relatively long PSMA<sub>54</sub> precursor acts as a steric stabilizer block and also ensures that only kinetically trapped spheres are obtained, regardless of the target degree of polymerization (DP) for the core-forming PBz ...[more]

PMID: 32216260

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Project description:Reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate (BzMA) is utilized to prepare a series of poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) diblock copolymer nano-objects at 90 °C directly in mineral oil. Polymerization-induced self-assembly (PISA) occurs under these conditions, with the resulting nanoparticles exhibiting spherical, worm-like or vesicular morphologies when using a relatively short PSMA13 macromolecular chain transfer agent (macro-CTA), as confirmed by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies. Only kinetically-trapped spherical nanoparticles are obtained when using longer macro-CTAs (e.g. PSMA18 or PSMA31), with higher mean degrees of polymerization (DPs) for the PBzMA core-forming block simply producing progressively larger spheres. SAXS is used for the first time to monitor the various morphological transitions that occur in situ during the RAFT dispersion polymerization of BzMA when targeting either spheres or vesicles as the final copolymer morphology. This powerful characterization technique enables the evolution of particle diameter, mean aggregation number, number of copolymer chains per unit surface area (Sagg) and the distance between adjacent copolymer chains at the core-shell interface (dint) to be monitored as a function of monomer conversion for kinetically-trapped spheres. Moreover, the gradual evolution of copolymer morphology during PISA is confirmed unequivocally, with approximate 'lifetimes' assigned to the intermediate pure sphere and worm morphologies when targeting PSMA13-PBzMA150 vesicles. Within vesicle phase space, the membrane thickness (Tm) increases monotonically with PBzMA DP. Furthermore, a combination of dynamic light scattering (DLS), TEM and post mortem SAXS studies indicate that the lumen volume is reduced while the overall vesicle dimensions remain essentially constant. Thus the constrained vesicles grow inwards, as recently reported for an aqueous PISA formulation. This suggests a universal vesicle growth mechanism for all PISA formulations.

Project description: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.

Dataset Information

Exploring the Upper Size Limit for Sterically Stabilized Diblock Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly in Non-Polar Media.

Publications

Exploring the Upper Size Limit for Sterically Stabilized Diblock Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly in Non-Polar Media.

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