The viscoelastic and mechanical behaviors of physically cross-linked copolymer hydrogels synthesized from N,N-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctane sulfonamido)ethyl acrylate (FOSA) ...with varying FOSA concentration were studied by rheological and static tensile tests. The strong hydrophobic association of the FOSA moieties in an aqueous environment produced core–shell nanodomains that provided the physical cross-links. These PDMA–FOSA hydrogels exhibited excellent mechanical properties, including a modulus of ∼130–190 kPa, elongation at break of 1000–1600%, and ∼500 kPa tensile strength, depending on the FOSA concentration. The physical gels were more viscous than comparable chemical gels and were much more efficient at dissipating stress. The latter characteristic produced relatively high tensile toughness, ∼4–6 MPa, because of the extra energy dissipation mechanism provided by the reversible, hydrophobic cross-links. The PDMA–FOSA hydrogel exhibited peculiar dynamic behavior which was greatly dependent on temperature. At 25 °C, the hydrogel was highly elastic, but as the temperature increased, its viscous behavior increased and a crossover of the dynamic moduli (i.e., G″ > G′) occurred at 55 °C, as the rheological characteristics of the material went from a viscoelastic solid to a viscoelastic liquid. That behavior is a consequence of the physical nature of the structure of the physical cross-links and the dynamic nature of hydrophobic associations, which are influenced by composition, temperature, and time.
The shape memory behavior of a series of strong, tough hybrid hydrogels prepared by covalently cross-linking quad-polymers of N,N-dimethylacrylamide (DMA), 2-(N-ethylperfluoro-octanesulfonamido) ...ethyl methacrylate (FOSM), hydroxyethyl acrylate, and 2-cinnamoyloxyethyl acrylate was investigated. The hybrid hydrogels, which had physical and covalent cross-links, contained ∼60–70% water, were relatively soft and elastic, and exhibited high mechanical strength, extensibility, and fracture toughness. The temporary network was derived from glassy nanodomains due to microphase separation of the FOSM species. The switching temperature for shape memory was the glass transition temperature of the nanodomains. Some creep relaxation occurred in the fixed shape due to viscoelastic effects of the nanodomain cross-links, but shape fixing efficiencies of 84–88% were achieved for the fixed shape after 24 h at 10 °C. Shape recovery to the permanent shape was achieved by reheating the hydrogel to 65 °C and was essentially quantitative.
Secreted proteins of eukaryotes are decorated with branched carbohydrate oligomers called glycans. This fact is only starting to be considered for
investigations of protein dynamics. Using all-atom ...molecular dynamics (MD) simulations and Markov state modeling (MSM), we unveil the influence of glycans on the conformational flexibility of the multidomain protein disulfide isomerase (PDI), which is a ubiquitous chaperone in the endoplasmic reticulum (ER). Yeast PDI (yPDI) from
is glycosylated at asparagine side chains and the knowledge of its five modified sites enables a realistic computational modeling. We compare simulations of glycosylated and unglycosylated yPDI and find that the presence of glycan-glycan and glycan-protein interactions influences the flexibility of PDI in different ways. For example, glycosylation reduces interdomain interactions, shifting the conformational ensemble toward more open, extended structures. In addition, we compare our results on yPDI with structural information of homologous proteins such as human PDI (hPDI), which is natively unglycosylated. Interestingly, hPDI lacks a surface recess that is present in yPDI. We find that glycosylation of yPDI facilitates its catalytic site to reach close to this surface recess. Hence, this might point to a possible functional relevance of glycosylation in yeast to act on substrates, while glycosylation seems redundant for the human homologous protein. We conclude that glycosylation is fundamental for protein dynamics, making it a necessity for a truthful representation of the flexibility and function in
studies of glycoproteins.
Adverse Health Effects of Marijuana Use Volkow, Nora D; Baler, Ruben D; Compton, Wilson M ...
The New England journal of medicine,
06/2014, Letnik:
370, Številka:
23
Journal Article
Recenzirano
Odprti dostop
As marijuana use becomes legal in some states, the dominant public opinion is that marijuana is a harmless source of mood alteration. Although the harms associated with marijuana use have not been ...well studied, enough information is available to cause concern.
In light of the rapidly shifting landscape regarding the legalization of marijuana for medical and recreational purposes, patients may be more likely to ask physicians about its potential adverse and beneficial effects on health. The popular notion seems to be that marijuana is a harmless pleasure, access to which should not be regulated or considered illegal. Currently, marijuana is the most commonly used “illicit” drug in the United States, with about 12% of people 12 years of age or older reporting use in the past year and particularly high rates of use among young people.
1
The most common route of . . .
While mean-field approximations, such as the nuclear shell model, provide a good description of many bulk nuclear properties, they fail to capture the important effects of nucleon–nucleon ...correlations such as the short-distance and high-momentum components of the nuclear many-body wave function1. Here, we study these components using the effective pair-based generalized contact formalism2,3 and ab initio quantum Monte Carlo calculations of nuclei from deuteron to 40Ca (refs. 4–6). We observe a universal factorization of the many-body nuclear wave function at short distance into a strongly interacting pair and a weakly interacting residual system. The residual system distribution is consistent with that of an uncorrelated system, showing that short-distance correlation effects are predominantly embedded in two-body correlations. Spin- and isospin-dependent ‘nuclear contact terms’ are extracted in both coordinate and momentum space for different realistic nuclear potentials. The contact coefficient ratio between two different nuclei shows very little dependence on the nuclear interaction model. These findings thus allow extending the application of mean-field approximations to short-range correlated pair formation by showing that the relative abundance of short-range pairs in the nucleus is a long-range (that is, mean field) quantity that is insensitive to the short-distance nature of the nuclear force.Effects of nucleon–nucleon correlations are studied with the generalized contact formalism and ab initio quantum Monte Carlo calculations. For nuclei from deuteron to 40Ca, the many-body nuclear wave function is shown to factorize at short distances.
Shape memory behavior of a partially zinc-neutralized, poly(ethylene-co-methacrylic acid) ionomer was investigated. The ionomer was a semicrystalline ionomer with a broad melting transition in the ...range 60–100 °C. Physical crosslinks in the ionomer due to an ionic nanodomain structure provided a “permanent” crosslinked network, while polyethylene crystallinity provided a temporary network. The broad melting transition allowed one to tune the dual-shape memory behavior by choosing a switching temperature, T c, anywhere within the melting transition. Similarly, multiple-shape memory behavior was achieved by choosing two or more switching temperatures within the melting transition, though the effectiveness of fixing (F) depended on how much material was melted and recrystallized to support the specific temporary shape. Crosslinking improved the recovery efficiency (R), and the crosslinked ionomer exhibited nearly ideal shape memory behavior in the dual-shape memory cycle.
Flexible graphene polyimide nanocomposites (0.1–4 wt %) with superior mechanical properties over those of neat polyimide resin have been prepared by solution blending. Imide moieties were grafted to ...amine-functionalized graphene using a step-by-step condensation and thermal imidization method. The imide-functionalized graphene exhibited excellent compatibility with N-methyl-2-pyrrolidone. The dynamic storage moduli of the graphene polyimide nanocomposites increased linearly with increasing graphene content for both unmodified graphene and imidized graphene. Moduli of the imidized graphene nanocomposites were 25–30% higher than those of unmodified graphene nanocomposites. Both neat polyimide and polyimide nanocomposites exhibited shape memory effects with a triggering temperature of 230 °C. where addition of graphene improved the recovery rate. Addition of graphene improved thermal stability of the polyimide nanocomposites for both graphene and modified graphene.
Rheology studies were performed on tough PVP-in situ-PAAm hydrogels physically cross-linked by cooperative hydrogen bonding to understand their viscoelastic response and, hence, the interactions and ...microstructure. The viscoelasticity of the PVP-in situ-PAAm hydrogels was strongly affected by the monomer ratio (C AAm/C VP). Hydrogels prepared with a high monomer ratio exhibited weak time, temperature and frequency dependence of the viscoelastic properties, similar to those of chemically cross-linked hydrogels. The storage modulus (G′) of the gels was much greater than the loss moduli (G″) and low loss factor (tan δ < ∼ 0.1), which indicated that they were solid-like, and mostly elastic. These supramolecular gels exhibited a strain- and C AAm/C VP-dependent reversible gel (solid) to viscoelastic liquid transition due to the dynamic nature of the cooperative hydrogen bonds. That transition also coincided with the onset of nonlinear viscoelastic behavior. The addition of a low molecular weight compound, urea, that competes for hydrogen bonding sites weakens the gel by decreasing the effective cross-link density or weakening the intermolecular hydrogen bonding.
In the inertial limit, the resonance frequency of the quartz crystal microbalance (QCM) is related to the coupled mass on the quartz sensor through the Sauerbrey expression that relates the mass to ...the change in resonance frequency. However, when the thickness of the film is sufficiently large, the relationship becomes more complicated and both the frequency and damping of the crystal resonance must be considered. In this regime, a rheological model of the material must be used to accurately extract the adhered film’s thickness, shear modulus, and viscoelastic phase angle from the data. In the present work we examine the suitability of two viscoelastic models, a simple Voigt model (Physica Scripta 1999, 59, 391–396) and a more realistic power-law model (Langmuir 2015, 31, 4008−4017), to extract the rheological properties of a thermoresponsive hydrogel film. By changing temperature and initial dry film thickness of the gel, the operation of QCM was traversed from the Sauerbrey limit, where viscous losses do not impact the frequency, through the regime where the QCM response is sensitive to viscoelastic properties. The density-shear modulus and the viscoelastic phase angle from the two models are in good agreement when the shear wavelength ratio, d/λ n , is in the range of 0.05–0.20, where d is the film thickness and λ n is the wavelength of the mechanical shear wave at the n th harmonic. We further provide a framework for estimating the physical properties of soft materials in the megahertz regime by using the physical behavior of polyelectrolyte complexes. This provides the user with an approximate range of allowable film thicknesses for accurate viscoelastic analysis with either model, thus enabling better use of the QCM-D in soft materials research.
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