Abstract
A range of amphiphilic statistical copolymers is synthesized where the hydrophilic component is either methacrylic acid (MAA) or 2-(dimethylamino)ethyl methacrylate (DMAEMA) and the hydrophobic component comprises methyl, ethyl, butyl, hexyl, or 2-ethylhexyl methacrylate, which provide a broad range of partition coefficients (log P). Small-angle X-ray scattering studies confirm that these amphiphilic copolymers self-assemble to form well-defined spherical nanoparticles in an aqueous solution, with more hydrophobic copolymers forming larger nanoparticles. Varying the nature of the alkyl substituent also influenced self-assembly with more hydrophobic comonomers producing larger nanoparticles at a given copolymer composition. A model based on particle surface charge density (PSC model) is used to describe the relationship between copolymer composition and nanoparticle size. This model assumes that the hydrophilic monomer is preferentially located at the particle surface and provides a good fit to all of the experimental data. More specifically, a linear relationship is observed between the surface area fraction covered by the hydrophilic comonomer required to achieve stabilization and the log P value for the hydrophobic comonomer. Contrast variation small-angle neutron scattering is used to study the internal structure of these nanoparticles. This technique indicates partial phase separation within the nanoparticles, with about half of the available hydrophilic comonomer repeat units being located at the surface and hydrophobic comonomer-rich cores. This information enables a refined PSC model to be developed, which indicates the same relationship between the surface area fraction of the hydrophilic comonomer and the log P of the hydrophobic comonomer repeat units for the anionic (MAA) and cationic (DMAEMA) comonomer systems. This study demonstrates how nanoparticle size can be readily controlled and predicted using relatively ill-defined statistical copolymers, making such systems a viable attractive alternative to diblock copolymer nanoparticles for a range of industrial applications.
Highlights
Self-assembled copolymers have applications in a wide range of diverse fields, including healthcare,[1−7] energy,[8,9] and coatings.[10−12] The assembly of diblock copolymers in solution has been studied extensively and is driven by minimization of the energetically unfavorable interactions between the solvent and the solvophobic block.[13]
The morphology of diblock copolymer nano-objects depends on the relative volume fractions of solvophilic and solvophobic blocks and can be rationalized in terms of the fractional packing parameter.[14−16] For a fixed diblock composition, the nano-object dimensions depend on both the overall copolymer molecular weight and the aggregation number, with the latter parameter depending on the processing conditions.[17,18]
Scheme 1. (a) Representative Diagram of a Statistical Copolymerization; (b) reversible addition− fragmentation chain transfer (RAFT) Solution Copolymerization of Either methacrylic acid (MAA) or DMAEMA (B) with EHMA, HMA, BMA, EMA, or MMA (A) to Form a Range of P(A-stat-B) amphiphilic statistical copolymers (ASC); and (c) Standard FreeRadical Copolymerization of MAA and a Hydrophobic Methacrylate Monomera aCopolymerization of BMA with MAA was performed in 1,4-dioxane at 50% w/w, whereas all other copolymerizations were performed in IPA at the same concentration
Summary
Self-assembled copolymers have applications in a wide range of diverse fields, including healthcare,[1−7] energy,[8,9] and coatings.[10−12] The assembly of diblock copolymers in solution has been studied extensively and is driven by minimization of the energetically unfavorable interactions between the solvent and the solvophobic block.[13]. The scattering patterns can be satisfactorily fitted using an intensity equation (see the Supporting Information, eqs S1, S34, or S35) incorporating a spherical core−shell form factor (eqs S8−S11), which accounts for a “shell” of cations (protonated TEA molecules) that are associated with the anionic nanoparticles (Figure 1b) This structural feature cannot be ignored in the scattering analysis because the scattering length density (SLD) of TEA is substantially greater than that of water (ξTEA = 10.54 × 1010 cm−2 and ξwater = 9.42 × 1010 cm−2), which produces significant contrast. A high q plateau (q > 0.1 Å−1) is observed in the scattering patterns recorded for most of the copolymer dispersions (Figures 1a, 2a, S6, and S9) This structural feature has been observed for similar ASC systems[34,46] and is possibly associated with electron density fluctuations within the nanoparticles owing to the statistical distribution of comonomer repeat units and/or thermal motion of the copolymer chains. The particle size increases from 37 to 105 Å in the EHM series as the acid content is reduced from 70 to 30 mol %, while the particle size increases from 20 to 68 Å in the EM series as the acid content is lowered from 20
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