Ordering Transitions of Fluoroalkyl-Ended Poly(ethylene glycol): Rheology and SANS

Synthesis and solution properties of telechelic poly(2-isopropyl-2-oxazoline) bearing perfluoro end groups

Telechelic FPIPOZ and its precursor N3PIPOZ films reassembled into discs and short fibers, respectively, when exposed to THF vapor.

Mucoadhesive and Rheological Studies on the Co-Hydrogel Systems of Poly(Ethylene Glycol) Copolymers with Fluoroalkyl and Poly(Acrylic Acid)

A self-assembled co-hydrogel system with sol-gel two-phase coexistence and mucoadhesive properties was developed based on the combined properties of fluoroalkyl double-ended poly(ethylene glycol) (R_f-PEG-R_f) and poly(acrylic acid) (PAA), respectively. We have synthesized an R_f-PEG-g-PAA (where g denotes grafted) copolymer and integrated it into the R_f-PEG-R_f physically cross-linked micellar network to form a co-hydrogel system. Tensile strengths between the co-hydrogel surfaces and two different sets of mucosal surfaces were acquired. One mucosal surface was made of porcine stomach mucin Type II, while the other one is a pig small intestine. The experimental results show that the largest maximum detachment stresses (MDSs) were obtained when the R_f-PEG-g-PAA’s weight percent in the dehydrated polymer mixture is ~15%. Tensile experiments also found that MDSs are greater in acidic conditions (pH = 4–5) (123.3 g/cm² for the artificial mucus, and 43.0 g/cm² for pig small intestine) and basic conditions (pH = 10.6) (126.9 g/cm², and 44.6 g.cm², respectively) than in neutral pH (45.4 g/cm², and 30.7 g.cm², respectively). Results of the rheological analyses using shear strain amplitude sweep and frequency sweep reveal that the R_f-PEG-g-PAA was physically integrated into the R_f-PEG-R_f micellar network, and the co-hydrogels remain physically cross-linked in three-dimensional micellar networks with long-term physical dispersion stability. Therefore, the co-hydrogel system is promising for drug delivery applications on mucosal surfaces.

Gel phase formation in dilute triblock copolyelectrolyte complexes

Assembly of oppositely charged triblock copolyelectrolytes into phase-separated gels at low polymer concentrations (<1% by mass) has been observed in scattering experiments and molecular dynamics simulations. Here we show that in contrast to uncharged, amphiphilic block copolymers that form discrete micelles at low concentrations and enter a phase of strongly interacting micelles in a gradual manner with increasing concentration, the formation of a dilute phase of individual micelles is prevented in polyelectrolyte complexation-driven assembly of triblock copolyelectrolytes. Gel phases form and phase separate almost instantaneously on solvation of the copolymers. Furthermore, molecular models of self-assembly demonstrate the presence of oligo-chain aggregates in early stages of copolyelectrolyte assembly, at experimentally unobservable polymer concentrations. Our discoveries contribute to the fundamental understanding of the structure and pathways of complexation-driven assemblies, and raise intriguing prospects for gel formation at extraordinarily low concentrations, with applications in tissue engineering, agriculture, water purification and theranostics.

Supramolecular polymer assembly in aqueous solution arising from cyclodextrin host-guest complexation

The employment of cyclodextrin host–guest complexation to construct supramolecular assemblies with an emphasis on polymer networks is reviewed. The main driving force for this supramolecular assembly is host–guest complexation between cyclodextrin hosts and guest groups either of which may be discrete molecular species or substituents on a polymer backbone. The effects of such complexation on properties at the molecular and macroscopic levels are discussed. It is shown that cyclodextrin complexation may be used to design functional polymer materials with tailorable properties, especially for photo-, pH-, thermo- and redox-responsiveness and self-healing.

Novel associative nanoparticles grafted with hydrophobically modified zwitterionic polymer brushes for the rheological control of aqueous polymer gel fluids

Novel associative nanoparticles (ANPs) are synthesized by grafting poly(2-methacryloyloxyethyl phosphorylcholine-co-stearyl methacrylate) polymer brushes on silica nanoparticles by using surface-mediated living radical polymerization.

Preparation and application of micro/nanoparticles based on natural polysaccharides

Polysaccharides have attracted more and more attentions and been recognized to be the most promising materials in recent years because of their outstanding merits such as easily available, non-toxic, biocompatible, biodegradable, and easily modified. Considerable research efforts have been directed toward developing polysaccharides-based micro/nanoparticles (PM/NPs). The new major studies of PM/NPs over the past few years are outlined in this review. Methods of preparation, including self-assembly, ionic-gelation, complex coacervation, emulsification, and desolvation method and some others, are summarized. Different applications of PM/NPs in the field of drug-delivery system are highlighted. Besides, another novel application of PM/NPs that are used as emulsifiers to stabilize Pickering emulsion is also introduced. These environmental-friendly particle emulsifiers have received reasonable attention due to their novel applications, especially in food, cosmetics, and pharmaceutics. From literature surveys, we realized that studies on PM/NP systems for different applications have increased rapidly. Hence, the present review is timely.

Synthesis of recyclable, chemically cross-linked, high toughness, high conductivity ion gels by sequential triblock copolymer self-assembly and disulfide bond cross-linking

We reported the synthesis of a high toughness, high conductivity ion gels by a sequential triblock copolymer self-assembly and disulfide bond cross-linking, combining the high toughness of chemical with recyclability of physical cross-linking ones.

A recyclable disulfide bond chemically cross-linking, high toughness, high conductivity ion gel based on re-shaping and restructuring in the gel state

We report the recyclability of a disulfide bond cross-linking ion gel with high toughness and ionic conductivity based on re-shaping and restructuring in the gel state.

Concentration dependence of rheological properties of telechelic associative polymer solutions

We consider concentration dependence of rheological properties of associative telechelic polymer solutions. Experimental results for model telechelic polymer solutions show rather strong concentration dependence of rheological properties. For solutions with relatively high concentrations, linear viscoelasticity deviates from the single Maxwell behavior. The concentration dependence of characteristic relaxation time and moduli is different in high- and low-concentration cases. These results suggest that there are two different concentration regimes. We expect that densely connected (well percolated) networks are formed in high-concentration solutions, whereas sparsely connected (weakly percolated) networks are formed in low-concentration solutions. We propose single chain type transient network models to explain experimental results. Our models incorporate the spatial correlation effect of micellar cores and average number of elastically active chains per micellar core (the network functionality). Our models can reproduce nonsingle Maxwellian relaxation and nonlinear rheological behavior such as the shear thickening and thinning. They are qualitatively consistent with experimental results. In our models, the linear rheological behavior is mainly attributable to the difference of network structures (functionalities). The nonlinear rheological behavior is attributable to the nonlinear flow rate dependence of the spatial correlation of micellar core positions.

Energetics of association in poly(lactic acid)-based hydrogels with crystalline and nanoparticle-polymer junctions

We report the energetics of association in polymeric gels with two types of junction points: crystalline hydrophobic junctions and polymer-nanoparticle junctions. Time-temperature superposition (TTS) of small-amplitude oscillatory rheological measurements was used to probe crystalline poly(L-lactide) (PLLA)-based gels with and without added laponite nanoparticles. For associative polymer gels, the activation energy derived from the TTS shift factors is generally accepted as the associative strength or energy needed to break a junction point. Our systems were found to obey TTS over a wide temperature range of 15-70 °C. For systems with no added nanoparticles, two distinct behaviors were seen, with a transition occurring at a temperature close to the glass transition temperature of PLLA, T(g). Above T(g), the activation energy was similar to the PLLA crystallization enthalpy, suggesting that the activation energy is related to the energy needed to pull a PLLA chain out of the crystalline domain. Below T(g), the activation energy is expected to be the energy required to increase mobility of the polymer chains and soften the glassy regions of the PLLA core. Similar behavior was seen in the nanocomposite gels with added laponite; however, the added clay appears to reduce the average value of the activation enthalpy. This confirms our SAXS results and suggests that laponite particles are participating in the network structure.

Properties of small molecular drug loading and diffusion in a fluorinated PEG hydrogel studied by H molecular diffusion NMR and F spin diffusion NMR

R(f)-PEG (fluoroalkyl double-ended poly(ethylene glycol)) hydrogel is potentially useful as a drug delivery depot due to its advanced properties of sol-gel two-phase coexistence and low surface erosion. In this study, (1)H molecular diffusion nuclear magnetic resonance (NMR) and (19)F spin diffusion NMR were used to probe the drug loading and diffusion properties of the R(f)-PEG hydrogel for small anticancer drugs, 5-fluorouracil (FU) and its hydrophobic analog, 1,3-dimethyl-5-fluorouracil (DMFU). It was found that FU has a larger apparent diffusion coefficient than that of DMFU, and the diffusion of the latter was more hindered. The result of (19)F spin diffusion NMR for the corresponding freeze-dried samples indicates that a larger portion of DMFU resided in the R(f) core/IPDU intermediate-layer region (where IPDU refers to isophorone diurethane, as a linker to interconnect the R(f) group and the PEG chain) than that of FU while the opposite is true in the PEG-water phase. To understand the experimental data, a diffusion model was proposed to include: (1) hindered diffusion of the drug molecules in the R(f) core/IPDU-intermediate-layer region; (2) relatively free diffusion of the drug molecules in the PEG-water phase (or region); and (3) diffusive exchange of the probe molecules between the above two regions. This study also shows that molecular diffusion NMR combined with spin diffusion NMR is useful in studying the drug loading and diffusion properties in hydrogels for the purpose of drug delivery applications.

Morphology of Photopolymerized End-linked Poly(ethylene glycol) Hydrogels by Small Angle X-ray Scattering

Due to the biocompatibility of poly(ethylene glycol) (PEG), PEG-based hydrogels have attracted considerable interest for use as biomaterials in tissue engineering applications. In this work, we show that PEG-based hydrogels prepared by photopolymerization of PEG macromonomers functionalized with either acrylate or acrylamide end-groups generate networks with crosslink junctions of high functionality. Although the crosslink functionality is not well controlled, the resultant networks are sufficiently well ordered to generate a distinct correlation peak in the small angle x-ray scattering (SAXS) related to the distance between crosslink junctions within the PEG network. The crosslink spacing is a useful probe of the PEG chain conformation within the hydrogel and ranges from approximately 6 to 16 nm, dependent upon both the volume fraction of polymer and the molecular weight of the PEG macromonomers. The presence of a peak in the scattering of photopolymerized PEG networks is also correlated with an enhanced compressive modulus in comparison to PEG networks reported in the literature with much lower crosslink functionality that exhibit no scattering peak. This comparison demonstrates that the method used to link together PEG macromonomers has a critical impact on both the nanoscale structure and the macroscopic properties of the resultant hydrogel network.

Nanoparticle-reinforced associative network hydrogels

ABA triblock copolymers in solvents selective for the midblock are known to form associative micellar gels. We have modified the structure and rheology of ABA triblock copolymer gels comprising poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) through addition of a clay nanoparticle, laponite. Addition of laponite particles resulted in additional junction points in the gel via adsorption of the PEO corona chains onto the clay surfaces. Rheological measurements showed that this strategy led to a significant enhancement of the gel elastic modulus with small amounts of nanoparticles. Further characterization using small-angle X-ray scattering and dynamic light scattering confirmed that nanoparticles increase the intermicellar attraction and result in aggregation of PLA-PEO-PLA micelles.

Synthesis of Chlorambucil-Tempol Adduct and its Delivery using Fluoroalkyl Double-Ended Poly (Ethylene Glycol) Micelles

In our pursuit to find potent anticancer drugs, we have covalently bonded free radical tempol to chlorambucil giving a chlorambucil-tempol (CT) adduct in which both of the anticancer active sites in tempol and chlorambucil were left intact. Analysis using NMR, Maldi-TOF, and EPR verified the designed chemical structure. Because the CT adduct is more hydrophobic than chlorambucil, its delivery also was investigated using fluoroalkyl double-ended poly (ethylene glycol) (Rf-PEG) micelles. Results from EPR spectra and(19) F and(1) H NMR spin lattice relaxation times show that the Rf-PEG micelles are able to encapsulate CT into the Rf cores of the micelles.

New properties from PLA-PEO-PLA hydrogels

Polymeric materials are important in many medical applications. Regenerative medicine offers the potential to repair or replace damaged tissue and polymers are an essential component of many tissue engineering approaches. Hydrogels have many advantageous properties but, generally, lack robust mechanical properties. At the same time, mounting evidence points to the importance of the matrix modulus when constructing devices. In this context, triblock copolymers made from poly(-lactide)-poly(ethylene glycol)-poly(-lactide) have been prepared and formulated into hydrogels. Investigations into their mechanical properties found the elastic modulus to be greater than 10 kPa which is at least one order of magnitude stiffer than previously reported from macromolecules composed of similar monomers. Part of the reason is the presence of crystalline lactide domains. Creating hydrogels with tailored modulus across the kPa range will likely have important ramifications in regenerative medicine.

Structure and rheology of aqueous micellar solutions and gels formed from an associative poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) triblock copolymer

The structure and shear flow behaviour of aqueous micellar solutions and gels formed by an amphiphilic poly(oxybutylene)-poly(oxyethylene)-poly(oxybutylene) triblock copolymer with a lengthy hydrophilic poly(oxyethylene) block has been investigated by rheology, small angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS). SANS revealed that bridging of chains between micelles introduces, in the micellar solution, an attractive long-range component which can be described through a potential of interaction corresponding to sticky soft spheres. The strength of the attractive interaction increases with increasing concentration. Rheology showed that the dependence of the storage modulus with temperature can be explained as a function of the micellar bridging, micellisation and phase morphology. SAXS studies showed that the orientation adopted by the system in the gel phase under shear is similar to that previously observed by us for the gel phase of a poly(oxyethylene)-poly(oxybutylene) diblock copolymer with a long poly(oxyethylene) chain, suggesting that the micellar corona/core length ratio and not the architecture of the block copolymer influences the alignment of the gel phase under shear.

Chemically tuned amphiphilic diblock copolymers dispersed in water: from colloids to soluble macromolecules

We investigate by small-angle scattering the structural behavior in water of a family of asymmetric poly(styrene-stat-(acrylic acid))-block-poly(acrylic acid), i.e., P(S-stat-AA)-b-PAA, diblock copolymers. These diblocks are of constant block ratio and increasing molar fraction, phi(AA), ranging from 0 to 1, of acrylic acid in the first P(S-stat-AA) statistical block. We identify three types of structural behavior in water: (i) for phi(AA) </= 0.25, the structures found in water are out-of-equilibrium micelle-like objects, reminiscent of the macrophase separation in the solid state, with no reorganization upon dispersion; (ii) for phi(AA) >/= 0.50, the diblocks dispersions in water are at equilibrium. For high phi(AA), the diblocks are soluble in water, demonstrating that a transition from colloid-like objects to soluble macromolecules is achieved. Close to the transition, (phi(AA) approximately 0.50), the diblocks form objects interpreted as comprising a water-swollen core formed by the P(S-stat-AA) block, surrounded by a swollen brush composed of the majority PAA block, above a apparent critical micelle concentration. However, these diblocks do not behave as macrosurfactants, and their self-association behavior is rather interpreted as a microphase separation which can arise from the incompatibility between two polymer blocks P(S-stat-AA) and PAA placed in a common solvent water.