NMR Study on the Effects of Sodium n-Dodecyl Sulfate on the Coil-to-Globule Transition of Poly(N-isopropylacrylamide) in Aqueous Solutions

Comparing polymer-surfactant complexes to polyelectrolytes

HYPOTHESIS

Understanding the complex interactions between polymers and surfactants is required to optimise commercially relevant systems such as paint, toothpaste and detergent. Neutral polymers complex with surfactants, forming 'pearl necklace' structures that are often conceptualised as pseudo-polyelectrolytes. Here we pose two questions to test the limits of this analogy: Firstly, in the presence of salt, do these polymer-surfactant systems behave like polyelectrolytes? Secondly, do polymer-surfactant complexes resist geometric confinement like polyelectrolytes?

EXPERIMENTS

We test the limits of the pseudo-polyelectrolyte analogy through studying a poly(N-isopropylacrylamide) (PNIPAM) brush in the presence of sodium dodecylsulfate (SDS). Brushes are ideal for interrogating pseudo-polyelectrolytes, as neutral and polyelectrolyte brushes exhibit distinct and well understood behaviours. Spectroscopic ellipsometry, quartz crystal microbalance with dissipation monitoring (QCM-D), and neutron reflectometry (NR) were used to monitor the behaviour and structure of the PNIPAM-SDS system as a function of NaCl concentration. The ability of the PNIPAM-SDS complex to resist geometric confinement was probed with NR.

FINDINGS

At a fixed SDS concentration below the zero-salt CMC, increasing NaCl concentration <100 mM promoted brush swelling due to an increase in osmotic pressure, not dissimilar to a weak polyelectrolyte. At these salt concentrations, the swelling of the brush could be described by a single parameter: the effective CMC. However, at high NaCl concentrations (e.g., 500 mM) no brush collapse was observed at all (non-zero) concentrations of SDS studied, contrary to what is seen for many polyelectrolytes. Study of the polymer-surfactant system under confinement revealed that the physical volume of surfactant dominates the structure of the strongly confined system, which further differentiates it from the polyelectrolyte case.

Competitive Adsorption of Interfacially Active Nanoparticles and Anionic Surfactant at the Crude Oil-Water Interface

The interfacial activity of poly(N-isopropylacrylamide) (pNIPAM) nanoparticles in the absence and presence of an anionic surfactant (sodium dodecyl sulfate, SDS) was studied at a crude oil-water interface. Both species are interfacially active and can lower the interfacial tension, but when mixed together, the interfacial composition was found to depend on the aging time and total component concentration. With the total component concentration less than 0.005 wt %, the reduced interfacial tension by pNIPAM was greater than SDS; thus, pNIPAM has a greater affinity to partition at the crude oil-water interface. However, the lower molecular weight (smaller molecule) of SDS compared to pNIPAM meant that it rapidly partitioned at the oil-water interface. When mixed, the interfacial composition was more SDS-like for low total component concentrations (≤ 0.001 wt %), while above, the interfacial composition was more pNIPAM-like, similar to the single component response. Applying a weighted arithmetic mean approach, the surface-active contribution (%) could be approximated for each component, pNIPAM and SDS. Even though SDS rapidly partitioned at the oil-water interface, it was shown to be displaced by the pNIPAM nanoparticles, and for the highest total component concentration, pNIPAM nanoparticles were predominantly contributing to the reduced oil-water interfacial tension. These findings have implications for the design and performance of fluids that are used to enhance crude oil production from reservoirs, particularly highlighting the aging time and component concentration effects to modify interfacial tensions.

Effect of heterogeneous and homogeneous polymerisation on the structure of pNIPAm nanogels

The choice of the polymerisation temperature and initiator in the synthesis of poly(N-isopropylacrylamide)-based nanogels can significantly influence their structure, morphology and thermoresponsive properties.

Bottom-Up Approach to Assess the Molecular Structure of Aqueous Poly(N-Isopropylacrylamide) at Room Temperature via Infrared Spectroscopy

The structure of poly(N-isopropylacrylamide) (PNIPAM) in solution is still an unresolved topic. Here, the PNIPAM structure in water was investigated using a bottom-up approach, involving the monomer, dimer, and trimer, and a combination of infrared (IR) spectroscopies as well as molecular dynamics simulations. The experiments show that the monomer and oligomers exhibit a broad and asymmetric amide I band with two underlying transitions, while PNIPAM presents the same major transitions and a minor one. Analysis of the 2D IR spectra and theoretical modeling of the amide I band indicates that the two transitions of the monomer do not have the same molecular origin as the oligomers and the polymer. In the monomer, the two bands originate from the ultrafast rotation of its ethyl group, which leads to different solvation structures for the various rotational conformers. In the case of the oligomers, the asymmetry and splitting of the amide I band is caused by the vibrational coupling among adjacent amide side chains. Moreover, it is deduced from the simulations that the oligomers have three distinct backbone conformations for neighboring amides. In particular, two of the backbone conformations have a closed and compact structure, while in the third, the backbone is open and elongated. The bottom-up approach allowed us to infer that such backbone conformations exist in PNIPAM as well. Consequently, the two major amide I transitions of the polymer are also assigned to split amide I transitions resulting from the vibrationally coupled nearest-neighboring amides. In contrast, the additional minor transition observed in PNIPAM is assigned to unsolvated amide units of the polymer. The proposed molecular model successfully describes that PNIPAM amide I band changes with temperature in terms of its molecular structure. This new model strongly suggests that PNIPAM does not have a completely random backbone structure, but has distinct backbone conformers between neighboring amides.

Thermoresponsive Polymers of Poly(2-(N-alkylacrylamide)ethyl acetate)s

Thermoresponsive poly(2-(N-alkylacrylamide) ethyl acetate)s with different N-alkyl groups, including poly(2-(N-methylacrylamide) ethyl acetate) (PNMAAEA), poly(2-(N-ethylacrylamide) ethyl acetate) (PNEAAEA), and poly(2-(N-propylacrylamide) ethyl acetate) (PNPAAEA), as well as poly(N-acetoxylethylacrylamide) (PNAEAA), were synthesized by solution RAFT polymerization. Unexpectedly, it was found that there are induction periods in the RAFT polymerization of these monomers, and the induction time correlates with the length of the N-alkyl groups in the monomers and follows the order of NAEAA < NMAAEA < NEAAEA < NPAAEA. The solubility of poly(2-(N-alkylacrylamide) ethyl acetate)s in water is also firmly dependent on the length of the N-alkyl groups. PNPAAEA including the largest N-propyl group is insoluble in water, whereas PNMAAEA and PNEAAEA are thermoresponsive in water and undergo the reversible soluble-to-insoluble transition at a critical solution temperature. The cloud point temperature (Tcp) of the thermoresponsive polymers is in the order of PNEAAEA < PNAEAA < PNMAAEA. The parameters affecting the Tcp of thermoresponsive polymers, e.g., degree of polymerization (DP), polymer concentration, salt, urea, and phenol, are investigated. Thermoresponsive PNMAAEA-b-PNEAAEA block copolymer and PNMAAEA-co-PNEAAEA random copolymers with different PNMAAEA and/or PNEAAEA fractions are synthesized, and their thermoresponse is checked.

Interfacial Interaction Enhanced Rheological Behavior in PAM/CTAC/Salt Aqueous Solution-A Coarse-Grained Molecular Dynamics Study

Interfacial interactions within a multi-phase polymer solution play critical roles in processing control and mass transportation in chemical engineering. However, the understandings of these roles remain unexplored due to the complexity of the system. In this study, we used an efficient analytical method-a nonequilibrium molecular dynamics (NEMD) simulation-to unveil the molecular interactions and rheology of a multiphase solution containing cetyltrimethyl ammonium chloride (CTAC), polyacrylamide (PAM), and sodium salicylate (NaSal). The associated macroscopic rheological characteristics and shear viscosity of the polymer/surfactant solution were investigated, where the computational results agreed well with the experimental data. The relation between the characteristic time and shear rate was consistent with the power law. By simulating the shear viscosity of the polymer/surfactant solution, we found that the phase transition of micelles within the mixture led to a non-monotonic increase in the viscosity of the mixed solution with the increase in concentration of CTAC or PAM. We expect this optimized molecular dynamic approach to advance the current understanding on chemical-physical interactions within polymer/surfactant mixtures at the molecular level and enable emerging engineering solutions.

Overcoming Surfactant-Induced Morphology Instability of Noncrosslinked Diblock Copolymer Nano-Objects Obtained by RAFT Emulsion Polymerization

RAFT emulsion polymerization techniques including polymerization-induced self-assembly (PISA) and temperature-induced morphological transformation (TIMT) are widely used to produce noncrosslinked nano-objects with various morphologies. However, the worm, vesicle and lamellar morphologies produced by these techniques typically cannot tolerate the presence of added surfactants, thus limiting their potential applications. Herein we report the surfactant tolerance of noncrosslinked worms, vesicles, and lamellae prepared by RAFT emulsion polymerizations using poly(di(ethylene glycol) ethyl ether methacrylate-co-N-(2-hydroxypropyl) methacrylamide) (P(DEGMA-co-HPMA)) as a macromolecular chain transfer agent (macro-CTA). Significantly, these P(DEGMA-co-HPMA) nanoparticles are highly stable in concentrated solutions of surfactants (e.g., sodium dodecyl sulfate (SDS)). We also demonstrate that the surfactant tolerance is related to the limited binding of SDS to the main-chain of the P(DEGMA-co-HPMA) macro-CTA constituting the particle shell. This work provides new insight into the interactions between surfactants and thermoresponsive copolymers and expands the scope of RAFT emulsion polymerization techniques for the preparation of noncrosslinked and surfactant-tolerant nanomaterials.

Thermally Tunable Pickering Emulsions Stabilized by Carbon-Dot-Incorporated Core-Shell Nanospheres with Fluorescence "On-Off" Behavior

Lack of deep understanding of nanoparticle (NP) actions at oil/water interface set an obstacle to practical applications of Pickering emulsions. Fluorescence labels fabricated by incorporation of carbon dots (CDs) into poly(N-isopropylacrylamide) (PNIPAM) matrix can not only mark the action of PNIPAM-based NPs in the interface but also reflect the colloidal morphologies of PNIPAM. In this work, we employed coaxial electrospraying for fabricating core-shell nanospheres of cellulose acetate encapsulated by PNIPAM, and facile incorporation of CDs in PNIPAM shells was achieved simultaneously. The coaxial electrosprayed NPs (CENPs) with temperature-dependent wettability can stabilize heptane and toluene in water at 25 °C, respectively, and reversible emulsion break can be triggered by temperature adjustment around the low critical solution temperature (LCST). Remarkably, CENP/CD composites exhibited a fluorescence "on-off" behavior because of the volume phase transition of the PNIPAM shell. CENP/CD composites in Pickering emulsions clearly elucidated the motions of CENPs in response to temperature changes. At temperatures below the LCST, the CENP concentration played an important role in surface coverage of oil droplets. Specifically, the CENP concentration above the minimum concentration for complete emulsification of oil phase led to high surface coverage and two-domain adsorption of CENPs at the interface including primary monolayer anchoring of CENPs on droplets surrounded by interconnected CENP networks, which contributed to the superior stability of the emulsions. Moreover, CENP/CD composites can be recycled with well-preserved core-shell structure and stable fluorescent properties, which offers their great potential applications in sensors and imaging.

Post-synthetic modification of polyvinyl alcohol with a series of N-alkyl-substituted carbamates towards thermo and CO2-responsive polymers

Intensive efforts have been devoted to the synthesis of thermoresponsive polymers with terminal N-alkyl-substituted groups.

A new thermoresponsive polymer of poly(N-acryloylsarcosine methyl ester) with a tunable LCST

A thermoresponsive polymer of the tertiary amide-based polyacrylamide, PNASME, was synthesized and its tunable thermoresponse was investigated.

Weak interactions and their impact on cellulose dissolution in an alkali/urea aqueous system

This work exhibited the indispensability and significance of weak non-covalent interactions between urea and macromolecules in a sophisticated physical chemistry process.

N-Ester-substituted polyacrylamides with a tunable lower critical solution temperature (LCST): the N-ester-substitute dependent thermoresponse

New thermoresponsive polymers ofN-ester-substituted polyacrylamides were discovered, and theN-ester-substitute exerting a great influence on the solution property was demonstrated.

Interaction of poly(N-isopropylacrylamide) with sodium dodecyl sulfate below the critical aggregation concentration

Interaction between the thermoresponsive polymer poly(N-isopropylacrylamide) (P-NIP) and sodium dodecyl sulfate (SDS) both above and below its phase transition temperature was examined under dilute conditions. Above the lower critical solution temperature (LCST) of P-NIP (32 °C), 0.01 wt % P-NIP specifically interacted with 1.0 × 10(-5) mol/L SDS to form a precipitate. However, when SDS was added at concentrations above or below 1.0 × 10(-5) mol/L, the P-NIP solution remained clear above the LCST. A fluorometric probe, N-phenyl-naphthalene, indicated that the hydrophobicity of the aggregates composed of P-NIP and SDS changed at an SDS concentration of 1.0 × 10(-5) mol/L. Although the hydrophobicity of the precipitate was similar to that of P-NIP alone at less than 1.0 × 10(-5) mol/L, it approached that of SDS homomicelles as the SDS concentration increased above 1.0 × 10(-5) mol/L. Dynamic light scattering and turbidimetry studies showed no P-NIP phase transition above an SDS concentration of 1.0 × 10(-5) mol/L, which is much lower than the reported critical association concentration (CAC) of SDS with P-NIP. This indicates that P-NIP interacted with SDS above the LSCT at much lower SDS concentration than the reported CAC.

Trail of pore shape and temperature-sensitivity of poly(N-isopropylacrylamide) hydrogels before and after removing Brij-58 template and pore formation mechanism

Modulate porous morphology and properties of thermosensitive hydrogels by containing different contents of lyotropic liquid crystalline.