Dynamics of Water Intercalated in Graphite Oxide

Intercalated water in graphite oxide (GO) was investigated by using different techniques: broadband dielectric spectroscopy (BDS) (10−2−109 Hz; 140−300 K), differential scanning calorimetry, X-ray diffraction, and attenuated total reflection geometry in Fourier transform infrared spectroscopy (ATR-FTIR). We have studied the water concentration (cw) region from 0 wt % (anhydrous GO) up to 25 wt % since in this range the water crystallization is avoidable. The interlayer distance during hydration increases from 5.67 to 8 Å which corresponds to the uptake of a water monolayer in the interlayer space of GO. A clear relaxation due to water molecule reorientation is seen by BDS. The rotational water dynamics is dependent on the hydration level. At high water concentration (cw > 15 wt %), water−water interactions seem to dominate the dielectric response. This result is also compatible with those from FTIR and X-ray measurements. In this water concentration region, a slight dynamical crossover in the temperature dependence of the relaxation times is observed. We show that the crossover temperature on this system (Tcross) depends on the confinement size

Self-Concentration and Interfacial Fluctuation Effects on the Local Segmental Dynamics of Nanostructured Diblock Copolymer Melts

Self-Concentration and Interfacial Fluctuation Effects on the Local Segmental Dynamics of Nanostructured Diblock Copolymer Melt

Dynamics of Water in Supercooled Aqueous Solutions of Poly(propylene glycol) As Studied by Broadband Dielectric Spectroscopy and Low-Temperature FTIR-ATR Spectroscopy

Binary water mixtures usually display a water relaxation (process II) which can be studied by broadband dielectric spectroscopy (BDS) at subzero temperatures. In a large collection of binary water mixtures, a slight increase of the relaxation strength is observed for low water concentration, whereas a faster increase is seen above a critical concentration. The assumption behind this result is that at high water concentration self-associations of water molecules are present in the solutions. In this work, we have studied poly(propylene glycol) water solutions by means of broadband dielectric spectroscopy and Fourier transform infrared spectroscopy (FTIR) using the attenuated total reflectance method (ATR) in the temperature range of 120–300 K. By combining both techniques, we found a critical water concentration <i>x</i>_w = 0.20 above which the relaxation strength of the water relaxation (process II) increases more rapidly than at low water concentration indicating the self-association of water molecules

Chain Length Effects on the Dynamics of Poly(ethylene oxide) Confined in Graphite Oxide: A Broadband Dielectric Spectroscopy Study

The dynamics of poly­(ethylene oxide) (PEO) intercalated in the subnanometer-spaced graphite oxide (GO) layers is investigated by using broadband dielectric spectroscopy (BDS). To this end, we compare BDS data obtained for PEO chains of increasing lengths, from three monomeric units to several thousand repetitive ethylene oxide units (<i>n</i> = 3–2135). Two relaxations were clearly identified for the confined PEO. The slowest one is proposed to originate from interfacial polarization. It is dependent on the chain length and exhibits a change in activation energy at 247 K, a temperature at which the GO exhibits an interlayer expansion when subjected to an increase in temperature. The fastest relaxation is nearly independent of the chain length, in contrast to the behavior that we found for the β-relaxation of bulk PEO. These results strengthen a previous hypothesis suggesting the emergence of a new set of chain length scales primarily dictated by the presence of anchoring points on the GO substrate upon intercalation. Additionally, the fastest relaxation exhibits a crossover at 175 K, which indicates the coexistence of two distinct processes, one occurring with the same activation energy as in the bulk polymer, and the other with lower activation energy. The latter is probably associated with the planar zigzag conformation in the confined PEO as previously determined by high-resolution inelastic neutron scattering

Thermal Stability of Polymers Confined in Graphite Oxide

In this study, polymer–graphite oxide (GO) interactions are demonstrated to induce thermal instability in both the polymer and GO phase in conditions of maximum polymer uptake, in which the polymer chains are either intercalated into the GO interlayer or adsorbed on the GO sheets. We investigate the thermal stability of poly­(ethylene oxide) (PEO) intercalated in GO (PEO/GO) in detail, using a combination of techniques including X-ray diffraction (XRD), thermogravimetry (TGA) and TGA–mass spectroscopy (TGA–MS) in dynamic (nonisothermal) and static (isothermal) modes. Our results show that intercalated PEO decomposes at 160 °C lower than neat PEO, and that GO decomposes at 50 °C lower than pristine GO, due to a synergistic instability between GO and intercalated PEO upon heating. Other hydrophilic polymerssuch as poly­(vinyl methyl ether) (PVME), poly­(vinyl alcohol) (PAA), poly­(vinylpyrrolidone) (PVP) and poly­(acrylic acid) (PAA)forming polymer-adsorbed GO structures are also observed to decompose at noticeably lower temperatures than either pristine GO or their own corresponding neat polymers. Unlike PEO/GO intercalation compounds, the excess of PEO phase in PEO–GO composites decomposes at temperatures close to that of neat PEO, which demonstrates that those polymer chains far from the adsorbing GO surface are not significantly affected by the presence of GO sheets. Furthermore, isothermal TGA data for PEO/GO intercalation compounds with PEO chains from 5 to 2135 monomeric units are well described by an autocatalytic model similar to that we found for pristine GO in a previous study, which suggests that the overall decomposition of PEO/GO is dominated by the reduction and exfoliation of GO. However, when PEO is adsorbed on graphene substrate, which is thermally stable at the studied temperatures, the kinetic mechanisms follow a first-order reaction similar to neat PEO, but the decomposition occurs with a considerably lower activation energy

Dynamic Heterogeneity in Random and Gradient Copolymers: A Computational Investigation

By means of molecular dynamics simulations, we investigate the structural relaxation in disordered random copolymers and lamellar phases of gradient copolymers, containing chemical species of very different mobilities. Two models have been investigated: a generic bead–spring system and a MARTINI coarse-grained model of a polyester resin. The lamellar phase of the gradient copolymer is formed by domains rich in one species and poor in the other one, which are separated by broad interfaces. Unlike in strongly segregated block copolymers, there is a finite probability of finding monomers of a given species at any position within the domains rich in the other species. A direct consequence of this feature is that monomers can probe very different chemical environments, and because of the strong dynamic asymmetry between the two components, their relaxation are characterized by an extreme dynamic heterogeneity. This is confirmed by an analysis of dynamic correlators as a function of the distance to the interface. In the case of random copolymers long-range ordering is not possible, and local microsegregation results in a much weaker dynamic heterogeneity. The former features are consistent with the experimental observation of narrow glass transitions in random copolymers but extremely broad ones in lamellar gradient copolymers

Metallo-Folded Single-Chain Nanoparticles with Catalytic Selectivity

Mimicking the substrate specificity and catalytic activity of enzymes is of great interest for different fields (e.g., chemistry, biology, nanomedicine). Enhanced reaction rates using artificial, enzyme-mimic catalysts based on a variety of molecular structures and nanoentities (e.g., macrocyclic compounds, star and helical polymers, dendrimers) have been previously reported. However, examples of enzyme-sized soft entities displaying substrate specificity are certainly scarce. Herein, we report the synthesis and characterization of single-chain nanoparticles based on metallo-folded polymer chains containing complexed Cu­(II) ions showing catalytic specificity during the oxidative coupling of mixtures of chemically related terminal acetylene substrates. This work paves the way for the easy and efficient construction of other Pd-, Ni-, Co-, Fe-, Mn-, or Mo-containing soft nanoentities approaching the substrate specificity of natural enzymes for a variety of organic reactions

Microscopic Evidence for the Topological Transition in Model Vitrimers

In addition to the glass transition, vitrimers undergo a topological transition from viscoelastic liquid to viscoelastic solid behavior when the network rearrangements facilitated by dynamic bond exchange reactions freeze. The microscopic observation of this transition is elusive. Model polyisoprene vitrimers based on imine dynamic covalent bonds were synthesized by reaction of α,ω-dialdehyde-functionalized polyisoprenes and a tris(2-aminoethyl)amine. In these dynamic networks nanophase separation of polymer and reactive groups leads to the emergence of a relevant length scale characteristic for the network structure. We exploited the scattering sensitivity to structural features at different length scales to determine how dynamical and topological arrests affect correlations at segmental and network levels. Chains expand obeying the same expansion coefficient throughout the entire viscoelastic region, i.e., both in the elastomeric regime and in the liquid regime. The onset of liquid-like behavior is only apparent at the mesoscale, where the scattering reveals the reorganization of the network triggered by bond exchange events. The such determined “microscopic” topological transition temperature is compared with the outcome of “conventional” methods, namely viscosimetry and differential scanning calorimetry. We show that using proper thermal (aging-like) protocols, this transition is also nicely revealed by the latter

Dielectric Study of Hydration Water in Silica Nanoparticles

The effect of water content on silica nanoparticles was examined by thermogravimetry analysis (TGA), broadband dielectric spectroscopy (from 10^–2 to 10⁷ Hz), and differential scanning calorimetry for a wide temperature range (110–250 K). Silica nanoparticles were dried and rehydrated at different water levels to determine the critical factors affecting the dielectric response. The dynamics of both hydration water and hydrated silanol groups were addressed. Whereas hydration water dynamics depend on the water content, the dynamics corresponding to hydrated silanol groups are almost water independent once the maximum hydroxylation level is reached. In addition, we determined that during hydration water molecules prefer to form clusters instead of filling a complete layer around the particles. Finally, we observed that contrary to other water containing systems, the corresponding relaxation times of water molecules do not show any crossover (from high-T super-Arrhenius to low-T Arrhenius behavior)

Concentrated Solutions of Single-Chain Nanoparticles: A Simple Model for Intrinsically Disordered Proteins under Crowding Conditions

By means of large-scale computer simulations and small-angle neutron scattering (SANS), we investigate solutions of single-chain nanoparticles (SCNPs), covering the whole concentration range from infinite dilution to melt density. The analysis of the conformational properties of the SCNPs reveals that these synthetic nano-objects share basic ingredients with intrinsically disordered proteins (IDPs), as topological polydispersity, generally sparse conformations, and locally compact domains. We investigate the role of the architecture of the SCNPs in their collapse behavior under macromolecular crowding. Unlike in the case of linear macromolecules, which experience the usual transition from self-avoiding to Gaussian random-walk conformations, crowding leads to collapsed conformations of SCNPs resembling those of crumpled globules. This behavior is already found at volume fractions (about 30%) that are characteristic of crowding in cellular environments. The simulation results are confirmed by the SANS experiments. Our results for SCNPsa model system free of specific interactionspropose a general scenario for the effect of steric crowding on IDPs: collapse from sparse conformations at high dilution to crumpled globular conformations in cell environments