Thermoresponsive associations in aqueous solution of poly(N-vinylcaprolactam) chains grafted with ω-methoxy poly(ethylene oxide) undecyl α-methacrylate (PVCL-g-C11EO42) at different copolymer concentrations have been investigated with the aid of various experimental methods. Turbidity measurements reveal that the cloud point (CP) decreases with increasing polymer concentration, and this effect is more pronounced in the semidilute concentration regime. Small-angle neutron scattering (SANS) results at large scattering wavevectors suggest a gradual conformational transition from random coils to contracted coils in the dilute concentration regime as the temperature rises. The rheological experiments disclose a strong temperature-induced contraction of the molecules in the dilute concentration regime, and the results in the semidilute regime indicate that the polymer chains are unentangled even at high concentrations. Dynamic light scattering results on solutions at temperatures up to the CP show the existence of a bimodal relaxation process; one mode is associated with the diffusion of individual polymer coils, or small clusters of molecules, and the other one is ascribed to interchain aggregation and the formation of aggregates at higher concentrations. At low concentrations, the intrachain species exhibit a temperature-induced compression, whereas at higher concentrations a temperature rise generates intermolecular aggregates. At a low constant temperature, an increase in polymer concentration leads to poorer thermodynamic conditions of the system, and shrinkage of the polymer coils is observed. At temperatures slightly above CP, the decay of the correlation function is characterized by a narrow unimodal relaxation mode. At this stage, most of the intrachain structures have been consumed in the formation of more compact particles.
Novel information about the effects of beta-cyclodextrin (beta-CD) addition and temperature on structural and rheological features of semidilute solutions of alginate and its hydrophobically modified analogue (HM-alginate) is given. Enhanced turbidity is observed for the HM-alginate solutions at high levels of beta-CD addition and low temperatures. The viscosity results revealed cross-linking of the alginate chains at high beta-CD concentrations and low temperatures. Rheological results for the HM-alginate solutions demonstrated that high levels of beta-CD addition and elevated temperatures promoted decoupling of the hydrophobic polymer-polymer associations via inclusion complex formation between beta-CD cavities and the hydrophobic side chains of the polymer. Analysis of small-angle neutron scattering (SANS) results from HM-alginate solutions in the presence of beta-CD suggested that the polymer chains are locally stretched at all of the considered levels of beta-CD and temperatures. The SANS results revealed association structures. The general picture that emerges is that beta-CD addition and temperature can be combined to tune the intensity of the hydrophobic interactions and to cross-link the unmodified alginate.
We study the self-assembly of mixtures of n-alkyl mono- and difunctionalized poly(ethylene oxide) (PEO) chains in the dilute concentration regime. The monofunctional PEOs were prepared by living anionic polymerization with varying n-alkyl length (n = 14, 16, 22, 28) and constant PEO molecular weight of 5 kg/mol. The difunctional materials were obtained through end-to-end coupling of two of the monofunctionalized PEOs via their terminal hydroxyl groups. The chosen synthetic pathway yields well-defined model compounds with narrow molecular weight distribution and complete end-group functionalization. By using both small-angle neutron scattering (SANS) and dynamic light scattering (DLS) combined with theoretical data modeling, we have systematically investigated both the global and inner structure of the self-assembled micellar structures. For short n-alkyl chain-ends, we find a formation of clustered micelles with a finite size whereas, intriguingly, at longer n-alkyls, we observe a crossover to flowerlike micelles. This was confirmed both by DLS, which is very sensitive to formation of larger clusters, as well as with SANS, which also showed a clear transition from attractive to repulsive intermicellar interactions upon increasing n-alkyl length. We attribute this to the balance between the hydrophobic enthalpic terms that favor anchoring of both chain-ends to the core and the entropic cost associated with the bending of the polymer chains. For short n-alkyls, exposure of the chain-ends in the corona structure leads to net dominance of the attractive interactions while for longer hydrophobic chains it leads to a stabilization of loops and consequently flowerlike micellar morphology. Using contrast-variation SANS, the contribution of mono- and difunctional chains could be separated, confirming the flowerlike micellar structure.
Severe poisoning with potassium pills is rare but patients may present with serious cardiovascular symptoms requiring immediate and effective treatment. A 30-year-old healthy woman presented to the emergency department after ingestion of 300 slow-release pills of potassium-chloride with serum potassium of 9.5 mmol/l, and poor cardiovascular function. Gastric lavage was performed with poor outcome. Despite intensive medical treatment serum potassium remained very high around 9 mmol/l. Haemodialysis was initiated but despite ongoing dialysis, potassium increased to 10.3 mmol/l. Hence, a parallel dialysis was started and after 4 h of parallel dialysis, serum potassium decreased to 6.4 mmol/l. An x-ray revealed large amounts of pills remaining in the stomach and the surgeon was able to remove about 200 pills through an acute laparotomy. The patient recovered slowly thereafter, but later developed a gastric stricture and pulmonary embolism.
In this work we report on the synthesis and self-assembly of a thermo-sensitive block copolymer system of n-octadecyl-poly(ethylene glycol)-block-poly(N-isopropylacrylamide), abbreviated as C18-PEGn-b-PNIPAAMm. We present a facile synthetic strategy for obtaining highly tunable thermo-responsive block copolymers starting from commercial PEG-based surfactants (Brij®) or a C18 precursor and conjugating with PNIPAAM via an Atom Transfer Radical Polymerization (ATRP) protocol. The self-assembly and detailed nanostructure were thoroughly investigated in aqueous solutions using both small-angle X-ray and neutron scattering (SAXS/SANS) combined with turbidity measurements. The results show that the system forms rather well defined classical micellar structures at room temperature that first undergo a collapse, followed by inter-micellar aggregation upon increasing the temperature. For the pure C18-PNIPAAM system, however, rather ill-defined micelles were formed, demonstrating the important role of PEG in regulating the nanostructure and the stability. It is found that the PEG content can be used as a convenient parameter to regulate the thermoresponse, i.e., the onset of collapse and aggregation. A detailed theoretical modeling analysis of the SAXS/SANS data shows that the system forms typical core-shell micellar structures. Interestingly, no evidence of back folding, where PEG allows PNIPAAM to form part of the C18 core, can be found upon crossing the lower critical solution temperature (LCST). This might be attributed to the entropic penalty of folding a polymer chain and/or enthalpic incompatibility between the blocks. The results show that by appropriately varying the balance between the hydrophobic and hydrophilic content, i.e. the amphiphilicity, tunable thermoresponsive micellar structures can be effectively designed. By means of SAXS/SANS we are able to follow the response on the nanoscale. These results thus give considerable insight into thermo-responsive micellar systems and provide guidelines as to how these systems can be tailor-made and designed. This is expected to be of considerable interest for potential applications such as in nanomedicine where an accurate and tunable thermoresponse is required.
