ABSTRACT
The aggregation behavior of amphiphilic block copolymers at the air-water interface has been extensively studied, but less attention was given to that of star copolymers. In this work, we studied the interfacial aggregation behavior of two double hydrophilic pH- and temperature-responsive miktoarm star copolymers of poly[di(ethylene glycol) methyl ether methacrylate]-poly[2-(dimethylamino)ethyl methacrylate] (PDEGMA3-PDMAEMA3 and PDEGMA4-PDMAEMA7, the subscripts denote arm numbers) with different molecular weights. The effects of subphase pH and temperature on the monolayer isotherms and hysteresis curves of the two star copolymers and the morphologies of their Langmuir-Blodgett (LB) films were studied by the Langmuir film balance technique and atomic force microscopy, respectively. At the air-water interface, the two star copolymers tend to form closely packed micelles. These micelles exhibit a core-shell structure, where the small hydrophobic core consists of cross-linker of ethylene glycol dimethacrylate (EGDMA) and the carbon backbones of PDEGMA and PDMAEMA arms and the short hydrophilic shell is composed of di(ethylene glycol) and tertiary amine side groups. With increasing subphase pH, the surface pressure versus molecular area isotherms shift toward larger mean molecular areas as a result of the enhanced interface adsorption of nonprotonated tertiary amine groups. The isotherm shift of PDEGMA3-PDMAEMA3 monolayers is primarily attributed to high density of tertiary amine groups in the shells, while that of PDEGMA4-PDMAEMA7 is mainly attributed to high density of di(ethylene glycol) groups in the shells. The hysteresis degrees in the monolayers of the two copolymers under alkaline and neutral conditions are greater than those under acidic conditions due to the decreased protonation degree of the tertiary amine groups. At 10 °C, the mobility of the shells is poor and the isotherms are located on the right. Above the lower critical solution temperature, di(ethylene glycol) groups contract, which causes a slight shift of the isotherms toward smaller mean molecular areas.
ABSTRACT
This review article aims to cover the most recent advances regarding the synthesis of linear ABC-type triblock terpolymers and star-shaped polymers by RAFT polymerization, as well as their self-assembly properties in aqueous solutions. RAFT polymerization has received extensive attention, as it is a versatile technique, compatible with a great variety of functional monomers and reaction conditions, while providing exceptional and precise control over the final structure, with well-defined side-groups and post-polymerization engineering potential. Linear triblock terpolymers synthesis can lead to very interesting novel ideas, since there are countless combinations of stimuli/non-stimuli and hydrophilic/hydrophobic monomers that someone can use. One of their most interesting features is their ubiquitous ability to self-assemble in different nanostructures depending on their degree of polymerization (DP), block composition, solubilization protocol, internal and external stimuli. On the other hand, star-shaped polymers exhibit a more stable nanostructure, with a distinct crosslinked core and arm blocks that can also incorporate stimuli-responsive blocks for "smart" applications.
ABSTRACT
In this work, the synthesis, selective chemical modifications, and self-assembly behavior in aqueous media of a novel poly(2-(dimethylamino)ethyl acrylate)20-b-poly(N-isopropylacrylamide)11-b-poly(oligo ethylene glycol methyl ether acrylate)18 (PDMAEA20-b-PNIPAM11-b-POEGA18) dual-responsive (pH and temperature) and triply hydrophilic amino-based triblock terpolymer are reported. The amine functional triblock terpolymer was synthesized by sequential reversible addition fragmentation chain transfer polymerization (RAFT) polymerization and molecularly characterized by size exclusion chromatography (SEC) and 1H-NMR spectroscopy that evidenced the success of the three-step polymerization scheme. The tertiary amine pendant groups of the PDMAEA block were chemically modified in order to produce the Q1PDMAEA20-b-PNIPAM11-b-POEGA18 as well as the Q6PDMAEA20-b-PNIPAM11-b-POEGA18 quaternized triblock terpolymers (Q1 and Q6 prefixes show the number of carbon atoms (C1 and C6) attached on the PDMAEA groups) using methyl iodide (CH3I) and 1-iodohexane (C6H13I) as the quaternizing agents and the SPDMAEA20-b-PNIPAM11-b-POEGA18 sulfobetainized triblock terpolymer using 1,3 propanesultone (C3H6O3S) as the sulfobetainization agent. The self-assembly properties of the triblock terpolymers in aqueous solutions upon varying temperature and solution pH were studied by light scattering and fluorescence spectroscopy experiments. The novel triblock terpolymers self-assemble into nanosized aggregates upon solution temperature rise above the nominal lower critical solution temperature (LCST) of the temperature-responsive PNIPAM block. The remarkable stimuli-responsive self-assembly behavior of the novel triblock terpolymers in aqueous media make them interesting candidates for biomedical applications in the fields of drug and gene delivery.
