In the end: Alkyl end groups can induce a thermoreversible sol–gel transition in an otherwise sol‐like suspension by affecting the macroscopic self‐assembly behavior. The picture shows PLGA‐PEG‐PLGA triblock copolymer/water mixtures in the sol (polymer a), gel (polymers b and c), and precipitation states (polymer d). PLGA=poly(lactic acid‐co‐glycolic acid), PEG=poly(ethylene glycol).

The spontaneous hydrogel formation of a sort of biocompatible and biodegradable amphiphilic block copolymer in water was observed, and the underlying gelling mechanism was assumed. A series of ABA-type triblock copolymers [poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(D,L-lactic acid-co-glycolic acid)] and different derivatives end-capped by small alkyl groups were synthesized, and the aqueous phase behaviors of these samples were studied. The virgin triblock copolymers and most of the derivatives exhibited a temperature-dependent reversible sol-gel transition in water. Both the poly(D,L-lactic acid-co-glycolic acid) length and end group were found to significantly tune the gel windows in the phase diagrams, but with different behaviors. The critical micelle concentrations were much lower than the associated critical gel concentrations, and an intact micellar structure remained after gelation. A combination of various measurement techniques confirmed that the sol-gel transition with an increase in the temperature was induced not simply via the self-assembly of amphiphilic polymer chains but also via the further hydrophobic aggregation of micelles resulting in a micelle network due to a large-scale self-assembly. The coarsening of the micelle network was further suggested to account for the transition from a transparent gel to an opaque gel.

Incorporation of small reactive moieties, the reactivity of which depends on externally imposed load (so-called mechanophores) into polymer chains offers access to a broad range of stress-responsive materials. Here, we report that polymers incorporating spirothiopyran (STP) manifest both green mechanochromism and load-induced addition reactions in solution and solid. Stretching a macromolecule containing colorless STP converts it into green thiomerocyanine (TMC), the mechanically activated thiolate moiety of which undergoes rapid thiol-ene click reactions with certain reactive C=C bonds to form a graft or a cross-link. The unique dual mechanochemical response of STP makes it of potentially great utility both for the design of new stress-responsive materials and for fundamental studies in polymer physics, for example, the dynamics of physical and mechanochemical remodeling of loaded materials.

Large-scale synthesis of monodisperse ultrasmall metal ferrite nanoparticles as well as understanding the correlations between chemical composition and MR signal enhancement is critical for developing next-generation, ultrasensitive T magnetic resonance imaging (MRI) nanoprobes. Herein, taking ultrasmall MnFeO nanoparticles (UMFNPs) as a model system, we report a general dynamic simultaneous thermal decomposition (DSTD) strategy for controllable synthesis of monodisperse ultrasmall metal ferrite nanoparticles with sizes smaller than 4 nm. The comparison study revealed that the DSTD using the iron-eruciate paired with a metal-oleate precursor enabled a nucleation-doping process, which is crucial for particle size and distribution control of ultrasmall metal ferrite nanoparticles. The principle of DSTD synthesis has been further confirmed by synthesizing NiFeO and CoFeO nanoparticles with well-controlled sizes of ∼3 nm. More significantly, the success in DSTD synthesis allows us to tune both MR and biochemical properties of magnetic iron oxide nanoprobes by adjusting their chemical composition. Beneficial from the Mn dopant, the synthesized UMFNPs exhibited the highest r relaxivity (up to 8.43 mM s) among the ferrite nanoparticles with similar sizes reported so far and demonstrated a multifunctional T MR nanoprobe for in vivo high-resolution blood pool and liver-specific MRI simultaneously. Our study provides a general strategy to synthesize ultrasmall multicomponent magnetic nanoparticles, which offers possibilities for the chemical design of a highly sensitive ultrasmall magnetic nanoparticle based T MRI probe for various clinical diagnosis applications.

Abstract"Nanocavitation was detected for the first time in carbon black filled styrene-butadiene rubber (CB-SBR) under uniaxial loading by real time small-angle X-ray scattering (SAXS) using synchrotron X-ray radiation. A three phase model was developed to calculate the void volume fraction from the scattering invariant Q determined from the observed SAXS patterns. The normalized scattering invariant Q/Q(0), where Q(0) is the invariant before deformation, greatly increased above a critical extension ratio lambda(onset) which we attribute to the formation of nanovoids. Analysis of the 2D scattering patterns show that voids formed are 20-40 nm in size and elongated along the tensile direction. Cavities formed beyond lambda(onset) are smaller as lambda increases. Results from the scattering experiments are strongly supported by macroscopic volume change measurements on the samples under similar uniaxial strain. A nearly constant nanocavitation stress sigma(onset) (25 MPa) was observed when the filler volume fraction phi(CB) was larger than 14%. This value is much higher than that predicted based on the elastic instability of small voids in an unfilled elastomer and shows only a weak dependence on the cross-linking density v(C) in heavily cross-linked samples. An energy based cavitation criterion stressing the importance of confined domains between particles or clusters of particles was adopted and found to be consistent with the observed results. The nanocavities are thought to alter the local stress state and promote local shear motion of filler particles." Keywords ray, x, angle, small, time, real, detected, scattering, tension, nanocavitation, under, rubber, butadiene, styrene, filled, black, carbon Publication DetailsZhang, H., Scholz, A. K., Crevoisier, J., Vion-Loisel, F., Besnard, G., Hexemer, A., Brown, H. R., Kramer, E. & Creton, C. (2012 ABSTRACT: Nanocavitation was detected for the first time in carbon black filled styrene−butadiene rubber (CB-SBR) under uniaxial loading by real time small-angle X-ray scattering (SAXS) using synchrotron X-ray radiation. A three phase model was developed to calculate the void volume fraction from the scattering invariant Q determined from the observed SAXS patterns. The normalized scattering invariant Q/Q 0 , where Q 0 is the invariant before deformation, greatly increased above a critical extension ratio λ onset which we attribute to the formation of nanovoids. Analysis of the 2D scattering patterns show that voids formed are 20−40 nm in size and elongated along the tensile direction. Cavities formed beyond λ onset are smaller as λ increases. Results from the scattering experiments are strongly supported by macroscopic volume change measurements on the samples under similar uniaxial strain. A nearly constant nanocavitation stress σ onset (25 MPa) was observed when the filler volume fraction ϕ CB was larger than 14%. This value is much higher than that predicted based on the elastic instability of small voids in an unfilled elastomer and shows only a weak dependence on t...

This study examines in vitro and in vivo biodegradation and biocompatibility of a thermogelling polymeric material, which we call a mixture hydrogel. The mixture contains two ABA-type triblock copolymers poly(d,l-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(d,l-lactic acid-co-glycolic acid) (PLGA-PEG-PLGA) with different block ratios, and one polymer is soluble in water, but the other is not. The aqueous solutions of some mixtures with appropriate mix ratios form hydrogels at the body temperature. The degradation of mixture hydrogels proceeded by hydrolysis of ester bonds followed by the erosion of gel in phosphate saline buffer solution at 37 degrees C for nearly one month. The mass loss and reduction of molecular weight were detected. The mix ratio was found to significantly influence the degradation profiles. The rapid in vivo gel formation was confirmed after subcutaneous injection of the thermogelling copolymer mixtures into Sprague-Dawley rats. The in vivo degradation was a bit accelerated than in vitro hydrolysis, and the persistence time of injected hydrogels in vivo was found to be tuned by mix ratio. MTT assay and histological observations were used to examine the copolymer mixtures. Both in vitro and in vivo results illustrate acceptable biocompatibility of our materials. As such, the thermosensitive hydrogel of copolymer mixture is confirmed to be a promising candidate of an injectable biomaterial for drug delivery and tissue engineering.

We study the mechanical activation of spiropyran (SP) in a doubly cross-linked polyurethane elastomer. Besides chemical cross-linking, the elastomer comprises polytetrahydrofuran as soft segments and hydrogenbonding 2-ureido-4-pyrimidone (UPy) as hard segments. The material shows two color changes because of the ring-opening reaction of SP to merocyanine (MC) at strained state and the isomerization about the methane bridge of MC at relaxed state. Increasing tensile strain rate leads to stiffer and stronger elastomer as well as earlier activation of SP. The activation point of SP to MC always coincides well with strain hardening of the stress−elongation curves. We further use the two-color transitions of SP to study the fracture of the elastomer during crack propagation.

A mechanically active spiropyran (SP) mechanophore is incorporated into the backbone of prepolymer which is further end-capped with ureidopyrimidinone (UPy) or urethane. Strong mechanochromic reaction of SP arises in the bulk films of UPy containing materials whereas much weaker activation occurs in urethane containing counterparts, coincident with their stress−strain responses. The difference in the magnitudes of supramolecular interactions leads to different degrees of chain orientation and strain induced crystallization (SIC) in the bulk and consequently distinct capabilities to transfer the load to the mechanophores. This study may aid the design of novel mechanoresponsive materials whose mechanoresponsiveness can be tailored by tuning supramolecular interactions.