On the definition of thermoreversible gels: the case of syndiotactic polystyrene

High Modulus, Strut-like poly(ether ether ketone) Aerogels Produced from a Benign Solvent

Poly(ether ether ketone) (PEEK) was found to form gels in the benign solvent 1,3-diphenylacetone (DPA). Gelation of PEEK in DPA was found to form an interconnected, strut-like morphology composed of polymer axialites. To our knowledge, this is the first report of a strut-like morphology for PEEK aerogels. PEEK/DPA gels were prepared by first dissolving PEEK in DPA at 320 °C. Upon cooling to 50 °C, PEEK crystallizes and forms a gel in DPA. The PEEK/DPA phase diagram indicated that phase separation occurs by solid-liquid phase separation, implying that DPA is a good solvent for PEEK. The Flory-Huggins interaction parameter, calculated as χ12 = 0.093 for the PEEK/DPA system, confirmed that DPA is a good solvent for PEEK. PEEK aerogels were prepared by solvent exchanging DPA to water then freeze-drying. PEEK aerogels were found to have densities between 0.09 and 0.25 g/cm3, porosities between 80 and 93%, and surface areas between 200 and 225 m2/g, depending on the initial gel concentration. Using nitrogen adsorption analyses, PEEK aerogels were found to be mesoporous adsorbents, with mesopore sizes of about 8 nm, which formed between stacks of platelike crystalline lamellae. Scanning electron microscopy and X-ray scattering were utilized to elucidate the hierarchical structure of the PEEK aerogels. Morphological analysis found that the PEEK/DPA gels were composed of a highly nucleated network of PEEK axialites (i.e., aggregates of stacked crystalline lamellae). The highly connected axialite network imparted robust mechanical properties on PEEK aerogels, which were found to densify less upon freeze-drying than globular PEEK aerogel counterparts gelled from dichloroacetic acid (DCA) or 4-chlorphenol (4CP). PEEK aerogels formed from DPA were also found to have a modulus-density scaling that was far more efficient in supporting loads than the poorly connected aerogels formed from PEEK/DCA or PEEK/4CP solutions. The strut-like morphology in these new PEEK aerogels also significantly improved the modulus to a degree that is comparable to high-performance crosslinked aerogels based on polyimide and polyurea of comparable densities.

Poly(vinylidene fluoride) Aerogels with α, β, and γ Crystalline Forms: Correlating Physicochemical Properties with Polymorphic Structures

Strategic customization of crystalline forms of poly(vinylidene fluoride) (PVDF) aerogels is of great importance for a variety of applications, from energy harvesters to thermal and acoustic insulation. Here, we report sustainable strategies to prepare crystalline pure α, β, and γ forms of PVDF aerogels from their respective gels using a solvent exchange strategy with green solvents, followed by a freeze-drying technique. The crucial aspect of this process was the meticulous choice of appropriate solvents to enable the formation of thermoreversible gels of PVDF by crystallization-induced gelation. Depending on the polymer-solvent interactions, the chain conformation of PVDF can be modulated to obtain gels and aerogels with specific crystalline structures. The crystalline pure α-form and piezoelectric β-form aerogels were readily obtained by using cyclohexanone and γ-butyrolactone as gelation solvents. On the other hand, the γ-form aerogel was obtained using a binary solvent system consisting of dimethylacetamide and water. These aerogels with distinct crystalline structures exhibit different morphologies, mechanical properties, hydrophobicities, acoustic properties, and electrical properties. Measurement of thermal conductivity for these aerogels showed exceptionally low thermal conductivity values of ∼0.040 ± 0.003 W m-1 K-1 irrespective of their crystal structures. Our results showcase the fabrication approaches that enable PVDF aerogels with varied physicochemical properties for multifunctional applications.

Thermodynamics, morphology and molecular structure of molecular compounds in trisamide triarylamine organogels and pseudo-organogels

In this paper, potentially-gelling binary systems are investigated by DSC, X-ray and Electron microscopy in order to assess their gel status and the role of the Hansen solubility parameter. The low molecular weight organogelator is a Triarylamine Trisamide (TATA) while the solvents consist of a series of halogeno-ethanes and of toluene. Temperature-concentration phase diagrams are mapped out from DSC traces. They reveal the existence of one or more TATA/solvent molecular compounds. The X-ray data, that display different diffraction patterns depending on the solvent and the temperature, show the existence of different molecular structures, and thus confirm the outcome of the T-C phase diagram. Tentative molecular organizations are also discussed in light of previous results obtained in the solid state. The morphology by TEM on dilute systems, and TEM on more concentrated systems highlight the degree of physical cross-links, which leads one to regard some systems as pseudo-gels.

Meso- and Macroporous Polymer Gels for Efficient Adsorption of Block Copolymer Surfactants

An understanding of surfactant adsorption at solid-liquid interfaces is important for solving many technological problems. This work evaluates surfactant adsorption abilities of high surface area (200-600 m²/g), high porosity (>90%), hierarchically structured open pore polymer gels. Specifically, the interactions of a nonionic block copolymer surfactant, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with three polymer gels, namely, syndiotactic polystyrene (sPS), polyimide (PI), and polyurea (PUA) offering different surface energy values, are evaluated at surfactant concentrations below and well above the critical micelle concentration (CMC). Two distinct surfactant adsorption behaviors are identified from the surface tension and nuclear magnetic resonance data. At concentrations below CMC, the surfactant molecules adsorb as a monolayer on polymer strands, inferred from the Langmuir-type adsorption isotherm, with the adsorbed amount increasing with the specific surface area of the polymer gel. The study reports for the first time that the gels show a strong surfactant adsorption above CMC, with the effective surfactant concentration in the gel reaching several folds of the CMC values. The effective surfactant concentration in the gel is analyzed using surfactant micelle size, polymer surface energy, and pore size of the gel. The findings of this study may have strong implications in liquid-liquid separation problems and in the removal of small dye molecules, heavy metal ions, and living organisms from aqueous streams.

Self-assembled morphologies of polyelectrolyte-grafted nanoparticles directed by oppositely charged polymer matrices

Self-assembled structure of polymer grafted nanoparticles is an interesting and growing subject in the field of hybrid electronics and high energy density materials. In light of this, the self-assembled morphologies of polyelectrolyte (PE) sparsely grafted nanoparticles tuned by oppositely charged matrix chains are studied using molecular dynamics simulations. Our focus is to elucidate the effect of matrix chain polymerization on modulating the stretching properties of tethered PE layers, on the self-assembled structuring of nanoparticles. Through varying the matrix chain length and stiffness as well as electrostatic interaction strength, rich phase behaviors of PE coated nanoparticles are predicted, including spherical micelle-like structures being preferred with short matrix chains and percolating network morphologies favored with long matrix chains, which is more pronounced with an enhanced matrix chain rigidness. To pinpoint the mechanisms of self-assembled structure formation, the thickness of grafted layers, the gyration radius of tethered chains, and pair correlation functions between nanoparticles are analyzed carefully. Additionally, electrostatic correlations, manifested as the bridging via matrix chains, are examined by identifying three states of matrix PE chains. Our simulation results may be useful for designing smart polymer nanocomposites based on PE coated nanoparticles.

Self-assembled structure of polymer grafted nanoparticles is an interesting and growing subject in the field of hybrid electronics and high energy density materials.

A supramolecular host-guest interaction-mediated injectable hydrogel system with enhanced stability and sustained protein release

Injectable hydrogels have been studied as drug delivery systems because of their minimal invasiveness and sustained drug release properties. Pluronic F127, consisting of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymers, exhibits thermo-responsive properties and hence is injectable due to its rapid sol-gel transition. Unmodified Pluronic F127-based hydrogels, however, have limited long-term stability and controllable release of drugs entrapped within them. In this study, host-guest interactions between adamantane-conjugated Pluronic F127 (F127-Ad) and polymerized β-cyclodextrin (CDP) were employed to develop a hydrogel-based protein delivery system. Single or multiple adamantane units were successfully introduced at the termini of Pluronic F127 with a 100% conversion yield, and the synthesized F127-Ad polymer produced a physically crosslinked micelle-packing structure when mixed with CDP. As the number of adamantanes at the terminal ends of Pluronic F127 increased, the critical gelation concentration of F127-Ad/CDP hydrogel decreased from 15 to 6% (w/v). The F127/CDP hydrogel was able to maintain its structure even with lower polymer content, and its injectability improved with a reduction of the hydrogel viscosity. The long-term stability of F127/CDP hydrogels was evaluated in vitro and in vivo, and it was demonstrated that the subcutaneously injected hydrogel did not disintegrate for up to 30 d. Throughout the drug release test using gelatin and insulin as model drugs, it was demonstrated that their release rates could be regulated via complexation between the protein drugs and the β-cyclodextrin molecules inside the hydrogel. In conclusion, the F127-Ad/CDP hydrogel is expected to be a versatile protein delivery system with controllable durability and drug release characteristics. STATEMENT OF SIGNIFICANCE: Pluronic F127 is one of the widely studied polymeric materials for thermo-sensitive injectable hydrogels due to its high biocompatibility and rapid sol-gel transition. Since the Pluronic F127-based hydrogel has some limitations in its long-term stability and mechanical property, it is inevitable to modify its structure for the application to drug delivery. In this study, mono- or multi- adamantane-conjugated Pluronic F127s were synthesized and mixed with β-cyclodextrin polymers to form hydrogels with host-guest interaction-mediated micelle-packing structures. The host-guest interaction introduced into the hydrogel system endowed it a sustained protein drug release behavior as well as high durability in vitro and in vivo. By increasing the number of adamantane molecules at the end of the Pluronic F127, both the stability and injectability of the hydrogel could be also modulated.

Physical Aspects of Organogelation: A Point of View

The physics side of organogelation is broached through three main aspects, thermodynamics (formation and melting), structure (morphology and molecular organization), and rheology. A definition of a gel is first discussed so as to delimit the field of investigation; namely, systems constituted of fibril-like entities. It is again highlighted that gel formation occurs through first-order transitions, chiefly by homogeneous nucleation. A deeper knowledge of the system is thus achieved by mapping out the temperature–concentration phase diagram. Some experimental diagrams are shown, while diagrams likely to pertain to these systems are presented. The molecular arrangement is basically crystallization that occurs in a preferred direction, hence the formation of fibrils. The effects of the solvent type, the quenching process of the solution are discussed with respect to the morphology and the crystal structure. Finally, the rheological properties are tackled. Notions of critical gelation concentration and percolation are debated. The interest of mapping out the temperature–concentration phase diagram is emphasized, particularly for understanding the variation of the gel modulus with temperature.

Solid-State Thermal Memory of Temperature-Responsive Polymer Induced by Hydrogen Bonds

Memory is an essential element for a computer to process information, which is integrated by logical circuits. Like electronic computing, thermal information can also be stored and read out by a thermal memory. Here, we show that a phase-changing polymer with hysteretic thermal transport properties can be experimentally processed into thermal memories at room temperature. We used a temperature-responsive and reversible polymer synthesized with melamine (M) and 6,7-dimethoxy-2,4[1H,3H]-quinazolinedione (Q) as a model system to demonstrate the manipulation of thermal transport at a molecular level. Fourier transform infrared spectroscopy and differential scanning calorimetry measurements indicate that this hysteretic behavior is based on the interaction of hydrogen bonds at high (317 K) and low (297 K) temperatures. This work demonstrates a controllable phonon transport process through the manipulation of hydrogen bonds, and thus it has potential applications in thermal memories.

Binary mixtures of semicrystalline polymers with low-molecular-mass compounds: thermal behaviour and phase structure

The results of phase equilibrium studies in mixtures of semicrystalline polymers with low-molecular-mass compounds are summarized and analyzed. A new classification of phase diagrams for such mixtures is proposed. Alternative points of view on the phase composition of semicrystalline polymers are presented. The phase structure evolution during the thermally induced phase separation of mixtures is monitored and the morphology of the forming capillary porous bodies as precursors of polymeric membranes is described. The general regularities concerning the influence of the nature of mixture components, polymer molecular mass, temperature scanning rate and other factors on the topology of phase diagrams are considered. Experimental methods used to construct the phase diagrams of mixtures and to study features of their phase structure are compared.

The bibliography includes 203 references.

Strong and Flexible Composite Solid Polymer Electrolyte Membranes for Li-Ion Batteries

A composite solid polymer electrolyte (CSPE) is studied in this work to alleviate the concerns associated with poor mechanical strength of a solid polymer electrolyte (SPE) system composed of poly(ethyleneglycol)diacrylate, an electrolyte lithium bis(trifluoromethane)sulfonamide, and a plasticizer succinonitrile. CSPE is fabricated by incorporating the ingredients of SPE in the macroporous membranes of syndiotactic polystyrene to render flexibility and mechanical robustness with a 6-fold increase in tensile strength over SPE. The data from differential scanning calorimetry and wide-angle X-ray diffraction confirm the amorphous nature of the polymeric domains of SPE that produce high room-temperature ionic conductivity of ∼0.43 mS/cm. The flexible CSPE membranes are used as the electrolyte in Li-ion battery (LIB) half cells in conjunction with lithium iron phosphate as the counter electrode. The use of CSPE helps expand the electrochemical window of the cell to 5 V, indicating strong potential in the fabrication of flexible rechargeable LIBs.

Electrically Conductive Gels Prepared from Syndiotactic Polystyrene and an Ionic Liquid

Syndiotactic polystyrene (SPS) physical gels containing a large amount of the ionic liquid 1-butylpyridinium bromide ([C₄py]Br) were systematically prepared and their physical properties were examined in detail. The gels had stable forms for a long time, having storage elastic modulus values of normal gels. They showed nearly the same values of the electrical conductivity (∼7 mS/cm) as those of the mixed solutions of [C₄py]Br, suggesting that the distribution of [C₄py]Br was uniform in these gels and that the charge transportation in these SPS gels was not interrupted by a three-dimensional network of SPS fibrils consisting of the SPS δ crystalline phases.

Hierarchical Morphology of Poly(ether ether ketone) Aerogels

The phase diagram for the thermoreversible gelation of poly(ether ether ketone) (PEEK) in 4-chlorophenol (4CP) was constructed over broad temperature and concentration ranges, revealing that PEEK is capable of dissolving and forming gels in both 4CP and dichloroacetic acid (DCA) up to a concentration of 25 wt %. Highly porous aerogels of PEEK were prepared through simple solvent exchange followed by one of two drying methods of solvent removal from the wet gel: freeze-drying or supercritical CO₂ fluid extraction (SC-drying). The field-emission scanning electron microscopy analysis showed that gelation of PEEK in 4CP, followed by SC-drying, produced aerogels with well-defined lamellar aggregates as compared to less ordered aggregates formed from DCA. Mechanical properties (in compression) were shown to improve with increasing density, resulting in equivalent compressive moduli at comparable density, regardless of the preparation method (gelation solvent selection, concentration variation, or drying method). Nitrogen adsorption-desorption isotherms indicate that PEEK aerogels are comprised of mesopores (2-50 nm diameter pores) formed from stacked crystalline lamella. PEEK aerogels prepared using SC-drying exhibit higher Brunauer-Emmett-Teller surface areas than freeze-dried aerogels of comparable density. The ultra-small-angle X-ray scattering/small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering analysis revealed a hierarchical morphology of the PEEK aerogels with structural features from PEEK crystallites to agglomerates of stacked lamella that spanned a wide range of length scales. SANS contrast-matching confirmed that the morphological origin of the principle scattering feature in PEEK aerogels is stacked crystalline lamella. Nitrogen sorption measurements of porosity and the specific surface area of the PEEK aerogels were correlated with the SAXS analysis to reveal a remarkably high surface area attributed to the platelet-like, lamellar morphology. Contact angle and contact angle hysteresis (CAH) revealed that low-density PEEK aerogels (ρ < 0.15 g/cm³) have water contact angles above the superhydrophobicity cutoff angle (>150°) and a very low CAH near 1°.

Solvent Effects on Tuning Pore Structures in Polyimide Aerogels

This work evaluates the effects of solvents and a block copolymer surfactant on pore structures in polyimide aerogels synthesized via sol-gel reaction process. Specifically, cross-linked polyimide gel networks are synthesized in single or mixed solvents from a combination of dimethylformamide, N-methylpyrrolidone, and dimethylacetamide and supercritically dried to obtain aerogels. The bulk density, pore size, and mechanical properties of aerogels are determined. The results show that gel times are strongly dependent on the electron acceptance ability of the solvent system and concentration of the surfactant. At longer gel times, the polyimide strands coarsen and the pores in aerogel shift from predominantly mesoporous to macroporous state with corresponding reduction in compressive modulus. The block copolymer surfactant also slows down gelation and coarsens the polyimide strands but only weakly affects the compressive modulus of the aerogels.

Open Cell Aerogel Foams via Emulsion Templating

The water-in-oil emulsion-templating method is used in this work for fabrication of open cell aerogel foams from syndiotactic polystyrene (sPS). A surfactant-stabilized emulsion is prepared at 60-100 °C by dispersing water in a solution of sPS in toluene. sPS gel, formed upon cooling of the emulsion to room temperature, locks the water droplets inside the gel. The gel is solvent exchanged in ethanol and then dried under supercritical condition of carbon dioxide to yield the aerogel foams. The aerogel foams show a significant fraction of macropores with a diameter of a few tens of micrometers, defined as macrovoids that originated from the emulsified water droplets. In conjunction, customary macropores of diameter 50-200 nm are derived from sPS gels. The macrovoids add additional openness to the aerogel structures. This paper evaluates the structural characteristics of the macrovoids, such as diameter distribution, macrovoid interconnect density, and skin layer density, in conjunction with the final aerogel foam properties.

Syndiotactic Polystyrene-Based Ionogel Membranes for High Temperature Electrochemical Applications

This work focuses on ionogel membranes for use in Li-ion batteries fabricated from syndiotactic polystyrene (sPS) gels filled with ionic liquids (ILs). The aim is to increase the operating temperature of Li-ion batteries. Thermal stability and safe operation of Li-ion batteries are two key attributes for their success in hybrid vehicles and other high-temperature applications. The volatility of the liquid electrolytes in current lithium-ion battery technology causes thermal runaway leading to fire, explosion, and swelling of the cell. The approach followed in this work combines the thermal stability and ruggedness of sPS and the extremely low volatility of ILs. The performances of lithium metal/graphite half-cells fabricated with ionogel membranes and those with Celgard-3501 membranes are evaluated at both room temperature and at elevated temperatures of 100 °C. Our data show that the cells with ionogel membranes can be operated continuously at 100 °C without failure. In addition, better charge-discharge capacity is obtained due to high ionic conductivity and high electrolyte retention both derived from high porosity of sPS gels and better wetting of sPS by the ILs.

Artifact Correction in Temperature-Dependent Attenuated Total Reflection Infrared (ATR-IR) Spectra

A spectral processing method was developed and tested for analyzing temperature-dependent attenuated total reflection infrared (ATR-IR) spectra of aliphatic polyesters. Spectra of a bio-based, biodegradable polymer, 3.9 mol% 3HHx poly[(R)-3-hydroxybutyrate- co-(R)-3-hydroxyhexanoate] (PHBHx), were analyzed and corrected prior to analysis using two-dimensional correlation spectroscopy (2D-COS). Removal of the temperature variation of diamond absorbance, correction of the baseline, ATR correction, and appropriate normalization were key to generating more reliable data. Both the processing steps and order were important. A comparison to differential scanning calorimetry (DSC) analysis indicated that the normalization method should be chosen with caution to avoid unintentional trends and distortions of the crystalline sensitive bands.

Thermoreversible Gelation of Poly(ether ether ketone)

Solutions of poly(ether ether ketone) in dichloroacetic acid have been shown to form monolithic, thermoreversible gels at temperatures ranging from 10 to 140 °C. A phase diagram was constructed over broad concentration and temperature ranges, and the phase boundary suggests an upper critical solution temperature (UCST) behavior. Furthermore, poly(ether ether ketone) (PEEK) gels were solvent-exchanged with water to form hydrogels and subsequently lyophilized to form PEEK aerogels. The PEEK aerogels of density 0.2 g/mL were found to be highly porous and composed of uniform 200 nm morphological features. The crystal structure of the PEEK hydrogels and aerogels was found to be identical to that of melt-crystallized PEEK. The mechanical properties of the PEEK aerogels (in compression) were found to be superior to conventional silicate aerogels of comparable density. This report is the first example of a monolithic, thermoreversible gel of PEEK and the first demonstration of PEEK hydrogels and aerogels.

Hybrid supramolecular gels of Fmoc-F/halloysite nanotubes: systems for sustained release of camptothecin

Novel supramolecular gel hybrids were prepared by self-assembly of Fmoc-l-phenylalanine in the presence of functionalized halloysite nanotubes and employed as carriers for the delivery of camptothecin molecules.

A vegetable oil-based organogel for use in pH-mediated drug delivery

Organogels prepared with vegetable oils as the liquid organic phase present an excellent platform for the controlled delivery of hydrophobic guest molecules. We disclose a graft copolymer comprised of a poly(L-serine) backbone linked to alkane side-chains by hydrolytically susceptible ester bonds, that is capable of gelating edible safflower oil. The thermoresponsive organogel formed, which is non-cytotoxic, is capable of withholding guest molecules before undergoing targeted disassembly upon incubation in solutions of acidic pH, permitting the directed release of payload molecules. The presented material offers an extremely promising candidate for the controlled delivery of hydrophobic agents within acidic environments, such as cancer tumour sites.

Polybenzoxazine aerogels with controllable pore structures

Polybenzoxazine gelation is expedited in the presence of p-toluenesulfonic acid and the solid networks show strong dependence on the solvent.

Light induced E–Z isomerization in a multi-responsive organogel: elucidation from 1H NMR spectroscopy

Light induced E–Z isomerization along imine bond in a multiresponsive organogel of anthracene attached 3,4,5-tris(dodecyloxy)benzohydrazide gelator altering morphology, fluorescence and mechanical properties is elucidated from ¹H NMR spectra.

A thixotropic supramolecular hydrogel of adenine and riboflavin-5'-phosphate sodium salt showing enhanced fluorescence properties

An equimolar mixture of riboflavin-5′-phosphate sodium salt (RP) and adenine (AD) dissolved in a phosphate buffer (pH 4.0, 1.0% w/v) produces a red coloured transparent thixotropic hydrogel at 30 °C. The gelation of the RPAD system occurs in the pH range of 2–5. FTIR spectra and WAXS patterns indicate self-assembly via H-bonding between the >C=O group of RP and the amino/imino group of AD followed by supramolecular organization through a π-stacking process producing a fibrillar network structure. FESEM images clearly indicate that the nanofibres are produced from the intertwining of helical fibrils. The dynamic frequency sweep experiment of the supramolecular gel at a constant strain of 1% exhibits a wide linear viscoelastic region and a considerably higher G′ value (460 Pa) than that of G′′ (21 Pa) confirming the gel nature of the RPAD system. The hydrogel shows high stiffness (G′/G′′ = 3.3), a high yield stress (σ*) (79.5 Pa) and a moderate critical strain (γ = 17.5%). Time sweep experiments at both low (0.1%) and high strain (100%) indicate the thixotropic property of the gel. The RPAD hydrogel shows non-Newtonian viscosity in the shear rate region (0.1–158 s(−1)) and after that there is a sudden fall of viscosity. The gel melting point obtained by the falling ball method is 6° higher than that obtained by the DSC method probably due to the presence of the thixotropic property of the gel. The UV-vis spectra indicate a red shift of the π–π* transition band of RP in the RPAD xerogel. On excitation of the RPAD hydrogel at 373 nm it shows twelve times enhancement of emission intensity with a 7 nm red shift of the emission peak. This has been attributed to the enhancement of lifetime from 2.2 ns in RP to 3.4 ns in the RPAD hydrogel. With increase of temperature, the fluorescence intensity of the RPAD hydrogel at first increases till 40 °C, then decreases up to 55 °C and it again increases after 60 °C.

Monolithic aerogels based on poly(2,6-diphenyl-1,4-phenylene oxide) and syndiotactic polystyrene

Molecular sorption behavior of amorphous and semicrystalline samples based on poly(2,6-diphenyl-1,4-phenylene oxide) (PPPO) has been compared. Fully amorphous PPPO powders, as obtained by supercritical carbon dioxide (scCO2) extraction of concentrated solutions, present uptake of pollutants much higher than for commercial sorbent materials based on semicrystalline PPPO (Tenax TA). Robust monolithic aerogels with good handling characteristics can be easily obtained by solvent extraction by scCO2 from gels including PPPO blends with syndiotactic polystyrene (s-PS). These monolithic PPPO/s-PS aerogels present many advantages as sorbent materials with respect to both amorphous and semicrystalline PPPO powders. In fact, besides the obvious advantages in terms of easier and safer handling, the new monolithic aerogels present higher surface areas and equilibrium guest uptakes.

Tailoring of morphology and surface properties of syndiotactic polystyrene aerogels

This study evaluates a method for rendering syndiotactic polystyrene (sPS) aerogels hydrophilic using polyethylene oxide (PEO) of different molecular weights. The highly porous sPS aerogels are inherently hydrophobic although applications involving absorption of moisture and removal of particulate solids may benefit from the high surface area of sPS aerogels provided some degree of hydrophilicity is induced in these materials. In this work, sPS gels are prepared by thermo-reversible gelation in tetrahydrofuran in the presence of PEO. The gels are dried under supercritical conditions to obtain aerogels. The aerogels are characterized by scanning electron microscopy, nitrogen-adsorption porosimetry, helium pycnometry, and contact angle measurements. The data reveal that the pore structures and surface energy can be controlled by varying the concentration and molecular weight of PEO and using different cooling rates during thermo-reversible gelation. In the first case, sPS aerogels, aerogels containing PEO of a low molecular weight or low concentration show superhydrophobic surface presenting the "lotus effect". In the second case, PEO at a higher concentration or with higher molecular weight forms phase-separated domains yielding new hydrophilic macropores (>10 μm) in the aerogel structures. These macropores contribute to the superhydrophobic surface with the "petal effect". The cooling rate during gelation shows a strong influence on these two cases.

Structural transitions of solvent-free oligomer-grafted nanoparticles

Novel structural transitions of solvent-free oligomer-grafted nanoparticles are investigated by using molecular dynamics simulations of a coarse-grained bead-spring model. Variations in core size and grafting density lead to self-assembly of the nanoparticles into a variety of distinct structures. At the boundaries between different structures, the nanoparticle systems undergo thermoreversible transitions. This structural behavior, which has not been previously reported, deviates significantly from that of simple liquids. The reversible nature of these transitions in solvent-free conditions offers new ways to control self-assembly of nanoparticles at experimentally accessible conditions.