The Nanostructured Self-Assembly and Thermoresponsiveness in Water of Amphiphilic Copolymers Carrying Oligoethylene Glycol and Polysiloxane Side Chains

Amphiphilic copolymer self-assembly is a straightforward approach to obtain responsive micelles, nanoparticles, and vesicles that are particularly attractive for biomedicine, i.e., for the delivery of functional molecules. Here, amphiphilic copolymers of hydrophobic polysiloxane methacrylate and hydrophilic oligo (ethylene glycol) methyl ether methacrylate with different lengths of oxyethylenic side chains were synthesized via controlled RAFT radical polymerization and characterized both thermally and in solution. In particular, the thermoresponsive and self-assembling behavior of the water-soluble copolymers in water was investigated via complementary techniques such as light transmittance, dynamic light scattering (DLS), and small-angle X-ray scattering (SAXS) measurements. All the copolymers synthesized were thermoresponsive, displaying a cloud point temperature (T_cp) strongly dependent on macromolecular parameters such as the length of the oligo(ethylene glycol) side chains and the content of the SiMA counits, as well as the concentration of the copolymer in water, which is consistent with a lower critical solution temperature (LCST)-type behavior. SAXS analysis revealed that the copolymers formed nanostructures in water below T_cp, whose dimension and shape depended on the content of the hydrophobic components in the copolymer. The hydrodynamic diameter (D_h) determined by DLS increased with the amount of SiMA and the associated morphology at higher SiMA contents was found to be pearl-necklace-micelle-like, composed of connected hydrophobic cores. These novel amphiphilic copolymers were able to modulate thermoresponsiveness in water in a wide range of temperatures, including the physiological temperature, as well as the dimension and shape of their nanostructured assemblies, simply by varying their chemical composition and the length of the hydrophilic side chains.

Self-Assembled Amphiphilic Fluorinated Random Copolymers for the Encapsulation and Release of the Hydrophobic Combretastatin A-4 Drug

Water-soluble amphiphilic random copolymers composed of tri(ethylene glycol) methacrylate (TEGMA) or poly(ethylene glycol) methyl ether methacrylate (PEGMA) and perfluorohexylethyl acrylate (FA) were synthesized by ARGET-ATRP, and their self-assembling and thermoresponsive behavior in water was studied by dynamic light scattering (DLS) and UV-vis spectroscopy. The copolymer ability to self-fold in single-chain nano-sized structures (unimer micelles) in aqueous solutions was exploited to encapsulate Combretastatin A-4 (CA-4), which is a very hydrophobic anticancer drug. The cloud point temperature (T_cp) was found to linearly decrease with increasing drug concentration in the drug/copolymer system. Moreover, while CA-4 was preferentially incorporated into the unimer micelles of TEGMA-ran-FA, the drug was found to induce multi-chain, submicro-sized aggregation of PEGMA-ran-FA. Anyway, the encapsulation efficiency was very high (≥81%) for both copolymers. The drug release was evaluated in PBS aqueous solutions both below and above T_cp for TEGMA-ran-FA copolymer and below T_cp, but at two different drug loadings, for PEGMA-ran-FA copolymer. In any case, the release kinetics presented similar profiles, characterized by linear trends up to ≈10–13 h and ≈7 h for TEGMA-ran-FA and PEGMA-ran-FA, respectively. Then, the release rate decreased, reaching a plateau. The release from TEGMA-ran-FA was moderately faster above T_cp than below T_cp, suggesting that copolymer thermoresponsiveness increased the release rate, which occurred anyway by diffusion below T_cp. Cytotoxicity tests were carried out on copolymer solutions in a wide concentration range (5–60 mg/mL) at 37 °C by using Balb/3T3 clone A31 cells. Interestingly, it was found that the concentration-dependent micro-sized aggregation of the amphiphilic random copolymers above T_cp caused a sort of “cellular asphyxiation” with a loss of cell viability clearly visible for TEGMA-ran-FA solutions (T_cp below 37 °C) with higher copolymer concentrations. On the other hand, cells in contact with the analogous PEGMA-ran-FA (T_cp above 37 °C) presented a very good viability (≥75%) with respect to the control at any given concentration.

Water dynamics within nanostructured amphiphilic statistical copolymers from quasielastic neutron scattering

Understanding the properties of water under either soft or hard confinement has been an area of great interest, but nanostructured amphiphilic polymers that provide a secondary confinement have garnered significantly less attention. Here, a series of statistical copolymers of 2-hydroxyethyl acrylate (HEA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl methacrylate (FOSM) are swollen to equilibrium in water to form nanostructured physically cross-linked hydrogels to probe the effect of soft confinement on the dynamics of water. Changing the composition of the copolymer from 10 to 21 mol. % FOSM decreases the average size of the assembled FOSM cross-link, but also the spacing between the cross-links in the hydrogels with the mean distance between the FOSM aggregates decreasing from 3.9 to 2.7 nm. The dynamics of water within the hydrogels were assessed with quasielastic neutron scattering. These hydrogels exhibit superior performance for inhibition of water crystallization on supercooling in comparison to analogous hydrogels with different hydrophilic copolymer chemistries. Despite the lower water crystallinity, the self-diffusion coefficient for these hydrogels from the copolymers of HEA and FOSM decreases precipitously below 260 K, which is a counter to the nearly temperature invariant water dynamics reported previously with an analogous hydrogel [Wiener et al., J. Phys. Chem. B 120, 5543 (2016)] that exhibits nearly temperature invariant dynamics to 220 K. These results point to chemistry dependent dynamics of water that is confined within amphiphilic hydrogels, where the interactions of water with the hydrophilic segments can qualitatively alter the temperature dependent dynamics of water in the supercooled state.

Investigation of the LCST-Thermoresponsive Behavior of Novel Oligo(Ethylene Glycol)-Modified Pentafluorostyrene Homopolymers

Amphiphilic tetrafluorostyrene monomers (EFS8) carrying in the para position an oligoethylene glycol chain containing 8 oxyethylenic units on average were synthesized and used for preparation via activator regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) of the corresponding amphiphilic homopolymers (pEFS8-x) with different degrees of polymerization (x = 26 and 46). Combining light transmittance and nano-differential scanning calorimetry (n-DSC) measurements revealed that pEFS8-x homopolymers displayed a lower critical solution temperature (LCST) thermoresponsive behavior in water solutions. Moreover, n-DSC measurements revealed the presence in heating scans of a broad endothermic peak ascribable to the dehydration process of the polymer single chains (unimers) and their collapse into aggregates. Consistently, dynamic light scattering (DLS) measurements showed below the LCST the presence of small nanostructures with a hydrodynamic diameter size Dh of 6–7 nm, which collapsed into concentration-dependent larger multichain aggregates (Dh = 300–3000 nm) above LCST. Interestingly, n-DSC data showed that the unimer-aggregate transition was reversible up to a specific temperature (Trev) of each homopolymer, which in any case was higher than Tmax. When heating above Trev the transition was no longer reversible, causing the shift of Tonset and Tmax at lower values, thus suggesting an increase in hydrophobicity of the polymer systems associated with a temperature-dependent dehydration process.