Microphase separation, stress relaxation and creep behavior of polyurethane nanocomposites

The microphase separation of polyurethane (PU) nanocomposite was studied. The result suggests that the addition of clay leads to a decrease in the size of hard domain and an increase in the degree of microphase separation. The stress relaxation and creep behavior of blank PU and PU/clay nanocomposites were investigated. The relaxation time spectrum and retardant time spectrum were derived according to the generalized Maxwell model and Voigt model with a Tikhonov regularization method. The characteristic relaxation time was identified with the corresponding relaxation process. At a small strain, the relaxation was mainly attributed to uncoiling/disentangling of soft segment chain network in the soft phase, with a single characteristic relaxation time in the range of 5~100s. The increase in the hard segment content leads to a decrease in the relaxation time, and the addition of clay leads to an increase in the relaxation time. At large strains, the multi-peak relaxations occurred, and they were attributed to the breakup of interconnected hard domains and pull-out of soft segment chains from hard domains, together with the disentangling of soft segment chain network in the soft phase. The creep results are in consistent with that of the stress relaxation. The relaxation and creep behavior were related to microphase separation of polyurethane. This study suggested that the relaxation spectrum H(ï´) can be used to examine the complicated relaxation processes for a multi-phase and multi-component polymer system

Poly(vinyl alcohol) Hydrogel Can Autonomously Self-Heal

It is discovered that poly­(vinyl alcohol) (PVA) hydrogel prepared using the freezing/thawing method can self-repair at room temperature without the need for any stimulus or healing agent. The autonomous self-healing process can be fast for mechanically strong PVA hydrogel yielding a high fracture stress. Investigation on the effect of the hydrogel preparation conditions points out that hydrogen bonding between PVA chains across the interface of the cut surfaces is at the origin of the phenomenon. The key for an effective self-healing is to have an appropriate balance between high concentration of free hydroxyl groups on PVA chains on the cut surfaces prior to contact and sufficient PVA chain mobility in the hydrogel

Preparation of Microporous Silicone Rubber Membrane with Tunable Pore Size via Solvent Evaporation-Induced Phase Separation

Silicone rubber membrane with ordered micropores in the surface was prepared by means of the solvent evaporation-induced phase separation. A ternary solution including liquid silicone rubber precursor, liquid paraffin, and hexane was cast to form a film with a two-phase structure after the hexane was evaporated. The micropores were generated by removing liquid paraffin phase in the cured silicone rubber film. The effects of the liquid paraffin concentration, casting temperature, initial casting solution thickness, air circulation, and addition of surfactant Span-80 on the pore structure in the membrane surface were investigated. The average pore size increases with increasing liquid paraffin concentration or the initial casting solution thickness. The formation of pore structure in the membrane surface is related to the phase separation and thus the phase separation process of the casting solution surface was in situ observed using the digital microscope. The formation mechanism of pore is attributed to a nucleation, growth, and coalescence process of liquid paraffin phase in the membrane surface

Dual-Stimuli-Responsive Micelle of an ABC Triblock Copolymer Bearing a Redox-Cleavable Unit and a Photocleavable Unit at Two Block Junctions

The design, synthesis, and study of a new dual-stimuli-responsible ABC-type triblock copolymer are reported. Using ATRP and click coupling reaction, the prepared copolymer is composed of poly­(ethylene oxide) (PEO), polystyrene (PS), and poly­[2-(dimethylamino)­ethylmethacrylate] (PDMAEMA) and features a redox-cleavable disulfide junction between the PEO and PS blocks as well as a photocleavable <i>o</i>-nitrobenzyl linkage between the PS and PDMAEMA blocks. This design allows the triblock copolymer to respond to both a reducing agent like dithiothreitol (DTT) and UV light, while having the minimum number of stimuli-reactive moieties in the copolymer structure (two per chain). The disruption of the triblock copolymer micelles in aqueous solution was examined under the action of either UV light or DTT alone or combined use of the two stimuli. It was found that the removal of one type of hydrophilic polymer chains from the water-soluble corona of the micelles with a hydrophobic PS core, that is, either redox-cleaved PEO or photocleaved PDMAEMA, could only result in a limited destabilization effect on the dispersion of the micelles. Severe aggregation of the polymer was observed only by applying the two stimuli converting the triblock copolymer onto three homopolymers. By monitoring the quenching by aqueous medium of the fluorescence of a hydrophobic dye (Nile Red) loaded in the triblock copolymer micelles, the effect on the payload release was also investigated of the different ways in which the micelles can be disrupted by the stimuli

Therapeutic-Ultrasound-Triggered Shape Memory of a Melamine-Enhanced Poly(vinyl alcohol) Physical Hydrogel

Therapeutic-ultrasound-triggered shape memory was demonstrated for the first time with a melamine-enhanced poly­(vinyl alcohol) (PVA) physical hydrogel. The addition of a small amount of melamine (up to 1.5 wt %) in PVA results in a strong hydrogel due to the multiple H-bonding between the two constituents. A temporary shape of the hydrogel can be obtained by deformation of the hydrogel (∼65 wt % water) at room temperature, followed by fixation of the deformation by freezing/thawing the hydrogel under strain, which induces crystallization of PVA. We show that the ultrasound delivered by a commercially available device designed for the patient’s pain relief could trigger the shape recovery process as a result of ultrasound-induced local heating in the hydrogel that melts the crystallized PVA cross-linking. This hydrogel is thus interesting for potential applications because it combines many desirable properties, being mechanically strong, biocompatible, and self-healable and displaying the shape memory capability triggered by a physiological stimulus

High Intensity Focused Ultrasound Responsive Metallo-supramolecular Block Copolymer Micelles

The metal–supramolecular diblock copolymer containing mechano-labile bis­(terpyridine)–Cu­(II) complex linkage in the junction point was synthesized. These metal–ligand containing amphiphilic copolymers are able to self-assemble in aqueous solution to form spherical micelles with poly­(propylene glycol) block forming the hydrophobic core. It is found that high intensity focused ultrasound can open the copolymer micelles and trigger the release of the payload in the micelle. The micellar properties and release kinetics of encapsulated guest molecule in response to ultrasound stimuli were investigated. The weak Cu­(II)–terpyridine dynamic bond in the copolymer chain can be cleaved under ultrasound and thus leads to the disruption of the copolymer micelle and the release of loaded cargo. This study will open up a new way for the molecular design of ultrasound modulated drug delivery systems

Gas-Barrier Hybrid Coatings by the Assembly of Novel Poly(vinyl alcohol) and Reduced Graphene Oxide Layers through Cross-Linking with Zirconium Adducts

Gas-barrier materials obtained by coating poly­(ethylene terephthalate) (PET) substrates have already been studied in the recent literature. However, because of the benefits of using cheaper, biodegradable, and nonpolar polymers, multilayered hybrid coatings consisting of alternate layers of reduced graphene oxide (rGO) nanosheets and a novel high amorphous vinyl alcohol (HAVOH) with zirconium (Zr) adducts as binders were successfully fabricated through a layer-by-layer (LbL) assembly approach. Atomic force microscopy analysis showed that rGO nanoplatelets were uniformly dispersed over the HAVOH polymer substrate. Scanning and transmission electron microscopies revealed that multilayer (HAVOH/Zr/rGO)_<i>n</i> hybrid coatings exhibited a brick-wall structure with HAVOH and rGO as buildings blocks. It has been shown that 40 layers of HAVOH/Zr/rGO ultrathin films deposited on PET substrates lead to a decrease of 1 order of magnitude of oxygen permeability with respect to the pristine PET substrate. This is attributed to the effect of zirconium polymeric adducts, which enhance the assembling efficiency of rGO and compact the layers, as confirmed by NMR characterization, resulting in a significant increment of the oxygen-transport pathways. Because of their high barrier properties and high flexibility, these films are promising candidates in a variety of applications such as packaging, selective gas films, and protection of flexible electronics

Enhancing Mechanically Induced ATRP by Promoting Interfacial Electron Transfer from Piezoelectric Nanoparticles to Cu Catalysts

A robust mechanically controlled atom transfer radical polymerization (mechano-ATRP) was developed by enhancing the interaction between piezoelectric nanoparticles and ATRP Cu catalysts. The interactions favor a mechano-induced electron transfer from the surface of the nanoparticles to the deactivator Cu^II/L complex under ultrasonic agitation, promoting the formation of the activator Cu^I/L complex, thereby increasing the rate of the polymerization. This mechano-ATRP was carried out with a low loading of zinc oxide nanoparticles, providing a polymer with high end-group fidelity, predetermined molecular weight, and low dispersity. Propagation of the polymer chains was switched on/off in response to the ultrasound. The effects of the nature of the nanoparticle, nanoparticle loading, and targeted degrees of polymerization were investigated to evaluate the mechanism of mechano-ATRP

Ultrasonication-Induced Aqueous Atom Transfer Radical Polymerization

A new procedure for ultrasonication-induced atom transfer radical polymerization (sono-ATRP) in aqueous media was developed. Polymerizations of oligo­(ethylene oxide) methyl ether methacrylate (OEOMA) and 2-hydroxyethyl acrylate (HEA) in water were successfully carried out in the presence of ppm amounts of CuBr₂ catalyst and tris­(2-pyridylmethyl)­amine ligand when exposed to ultrasonication (40 kHz, 110 W) at room temperature. Aqueous sono-ATRP enabled polymerization of water-soluble monomers with excellent control over the molecular weight, dispersity, and high retention of chain-end functionality. Temporal control over the polymer chain growth was demonstrated by switching the ultrasound on/off due to the regeneration of activators by hydroxyl radicals formed by ultrasonication. The synthesis of a well-defined block copolymer and DNA–polymer biohybrid was also successful using this process