Solubility of Homopolymers and Copolymers in Carbon Dioxide

Fabrication of Mechanically Strong Silica Aerogels with the Thermally Induced Phase Separation (TIPS) Method of Poly(methyl methacrylate)

Constructing and maintaining a three-dimensional network structure with high porosity is critical to the preparation of silica aerogel materials because this structure provides excellent properties. However, due to the pearl-necklace-like structure and narrow interparticle necks, aerogels have poor mechanical strength and a brittle nature. Developing and designing lightweight silica aerogels with distinct mechanical properties is significant to extend their practical applications. In this work, thermally induced phase separation (TIPS) of poly(methyl methacrylate) (PMMA) from a mixture of ethanol and water was used to strengthen the skeletal network of aerogels. Strong and lightweight PMMA-modified silica aerogels were synthesized via the TIPS method and supercritically dried with carbon dioxide. The cloud point temperature of PMMA solutions, physical characteristics, morphological properties, microstructure, thermal conductivities, and mechanical properties were investigated. The resultant composited aerogels not only exhibit a homogenous mesoporous structure but also achieve a significant improvement in mechanical properties. The addition of PMMA increased the flexural strength and compressive strength by as much as 120% and 1400%, respectively, with the greatest amount of PMMA (M_w = 35,000 g/mole), while the density just increased by 28%. Overall, this research suggests that the TIPS method has great efficiency in reinforcing silica aerogels with less sacrifice of low density and large porosity.

A Polysiloxane Delivery Vehicle of Cyclic N-Halamine for Biocidal Coating of Cellulose in Supercritical CO2

Cyclic N-halamines are highly antimicrobial, very stable, and not susceptible to bacterial resistance. A polysiloxane delivery vehicle was synthesized to deliver cyclic imide N-halamine onto cellulose via a benign and universal procedure that does not require a harmful solvent or chemical bonding. In brief, Knoevenagel condensation between barbituric acid and 4-hydroxybenzaldehyde furnished 5-(4-hydroxybenzylidene)pyrimidine-2,4,6-trione, whose phenolic O-H was subsequently reacted with the Si-H of poly(methylhydrosiloxane) (PMHS) via silane alcoholysis. The product of silane alcoholysis was interpenetrated into cellulose in supercritical CO₂ (scCO₂) at 50 °C, to form a continuous modification layer. The thickness of the modification layer positively correlated with interpenetration pressure in the experimental range of 10 to 28 MPa and reached a maximum value of 76.5 nm, which demonstrates the ability for tunable delivery, to control the loading of the imide N-H bond originating from barbituric acid unit. The imide N-H bonds on cellulose with the thickest modifier were then chlorinated into N-Cl counterparts using tert-butyl hypochlorite, to exert a powerful biocidability, providing ~7 log reductions of both S. aureus and E. coli in 20 min. The stability and rechargeability of the biocidability were both very promising, suggesting that the polysiloxane modifier has a satisfactory chemical structure and interlocks firmly with cellulose via scCO₂ interpenetration.

First Approach to Measure Interfacial Rheology at High-Pressure Conditions by the Oscillating Drop Technique

An oscillating drop rheometer capable of operating under conditions of high pressure and high temperature has been built. The oscillating drop mechanism was able to support pressures as high as 1300 bar and successfully performed oscillations at constant pressure. Apparent elastic and viscous complex moduli were measured for a system of CO2 and synthetic seawater containing 100 ppm of a linear alkyl ethoxylate surfactant for different pressures and temperatures. The moduli had strong dependencies on both pressure and temperature. At temperatures of 40 and 80 °C, the apparent elastic modulus passed through a maximum for pressures between 100 and 300 bar. The harmonic distortion of the oscillations was calculated for all measurements, and it was found that drop oscillations below ca. 2.6 µL caused distortions above 10% due to a mechanical backlash of the motor.

Investigation on modified polyether as an efficient CO2 thickener

Based on the CO₂-philic silicon group and the CO₂-phobic aryl group, silicon modified polyether exhibits both high solubility in carbon dioxide and a high thickening ability toward CO₂.

Supercritical carbon dioxide application using hydantoin acrylamide for biocidal functionalization of polyester

Biocidal functionalization in polyester fibers is a really tough challenge because of the lack of tethering groups. This study indicated supercritical carbon dioxide application using N-halamine would be an alternative solution for obtaining antibacterial function on the polyester surface. Firstly, N-(2-methyl-1-(4-methyl-2,5-dioxo-imidazolidin-4 yl)propan-2 yl)acrylamide was synthesized and applied to the polyester in supercritical carbon dioxide medium, at 120 °C, 30 MPa for different processing times. The addition of N-halamine on the surface significantly brought antibacterial activity against E. coli. The chlorine loadings showed that 6 -h exposure time was critical to obtain sufficient antibacterial activity. This treatment caused a reasonable and tolerable loss in color and mechanical properties. But, the durability to abrasion, stability, and rechargeability of oxidative chlorine, and the durability of N-halamine on the surface were remarkably good. Conclusively, it can be available to work on polyester surfaces with resource-efficient and eco-friendly supercritical carbon dioxide technique for getting more functionalization and modification.

Supercritical Fluid Applications in the Design of Novel Antimicrobial Materials

Bacterial resistance to antibiotics is one of the biggest problems in the modern world. The prevention of bacterial spreading from hospitals to the community and vice versa is an issue we have to deal with. This review presents a vast potential of contemporary high-pressure techniques in the design of materials with antimicrobial activity. Scientists from all over the world came up with ideas on how to exploit extraordinary properties of supercritical fluids in the production of advantageous materials in an environmentally friendly way. The review summarizes reported methods and results.

Microencapsulation and Nanoencapsulation Using Supercritical Fluid (SCF) Techniques

The unique properties of supercritical fluids, in particular supercritical carbon dioxide (CO₂), provide numerous opportunities for the development of processes for pharmaceutical applications. One of the potential applications for pharmaceuticals includes microencapsulation and nanoencapsulation for drug delivery purposes. Supercritical CO₂ processes allow the design and control of particle size, as well as drug loading by utilizing the tunable properties of supercritical CO₂ at different operating conditions (flow ratio, temperature, pressures, etc.). This review aims to provide a comprehensive overview of the processes and techniques using supercritical fluid processing based on the supercritical properties, the role of supercritical carbon dioxide during the process, and the mechanism of formulation production for each process discussed. The considerations for equipment configurations to achieve the various processes described and the mechanisms behind the representative processes such as RESS (rapid expansion of supercritical solutions), SAS (supercritical antisolvent), SFEE (supercritical fluid extraction of emulsions), PGSS (particles from gas-saturated solutions), drying, and polymer foaming will be explained via schematic representation. More recent developments such as fluidized bed coating using supercritical CO₂ as the fluidizing and drying medium, the supercritical CO₂ spray drying of aqueous solutions, as well as the production of microporous drug releasing devices via foaming, will be highlighted in this review. Development and strategies to control and optimize the particle morphology, drug loading, and yield from the major processes will also be discussed.

New trends in cellular silicone: Innovations and applications

The purpose of this article is to provide an overview of manufacturing processes used in the development of cellular silicone for a wide variety of applications. The combination of intrinsic properties of silicone and foam is considered as an attractive solution in many applications. With regard to the long-standing interest of the industry in silicone chemistry, foaming is very common from hydrosilylation/condensation reactions. This well-known technology leads to homogeneous, elastic, low density and biocompatible foams. However, the size of the cells remains large, the reactions are sensitive to humidity and the dangerousness of the hydrogen could be an industrial concern. Many researches are moving towards alternatives to the manufacture of silicone cellular materials such as gas foaming, phase separation, emulsion and sacrificial models, and syntactic charges. In addition, the theories of sorption, diffusion, nucleation and cell growth are detailed to explain the formation of gaseous foam. CO2 is commonly used to physically foam silicone because of its good solubility. However, the diffusive behavior of CO2 is high in silicone as explained by the free volume theory. Silicone–CO2 foaming is essentially triggered by rapid depressurization leading to a cell density around 1 × 109 cells/cm3 in the best case. In addition, templated foams are divided into emulsion polymerization (polyHIPE), sacrificial foams and syntactic foams. These methods are simple because they do not need specific foaming equipments. Pore sizes are also tunable as function of template sizes.

Solvent-based separation and recycling of waste plastics: A review

Since the creation of first man-made plastic, the global production and consumption of plastics have been continuously increasing. However, because plastic materials are durable and very slow to degrade, they become waste with high staying power. The over-consumption, disposal, and littering of plastics result in pollution, thus causing serious environmental consequences. To date, only a fraction of waste plastics is reused and recycled. In fact, recycling plastics remains a great challenge because of technical challenges and relatively insufficient profits, especially in mixed plastics. This review focuses on an environmentally friendly and potentially profitable method for plastics separation and recovery and solvents extraction. It includes the dissolution/reprecipitation method and supercritical fluid extraction, which produce high-quality recovered plastics comparable to virgin materials. These methods are summarized and discussed taking mass-produced plastics (PS, PC, Polyolefins, PET, ABS, and PVC) as examples. To exploit the method, the quality and efficiency of solvent extraction are elaborated. By eliminating these technical challenges, the solvent extraction method is becoming more promising and sustainable for plastic issues and polymer markets.

Thickening Supercritical CO₂ with π-Stacked Co-Polymers: Molecular Insights into the Role of Intermolecular Interaction

Vinyl Benzoate/Heptadecafluorodecyl acrylate (VBe/HFDA) co-polymers were synthesized and characterized as thickening agents for supercritical carbon dioxide (SC-CO₂). The solubility and thickening capability of the co-polymer samples in SC-CO₂ were evaluated by measuring cloud point pressure and relative viscosity. The molecular dynamics (MD) simulation for all atoms was employed to simulate the microscopic molecular behavior and the intermolecular interaction of co-polymer⁻CO₂ systems. We found that the introduction of VBe group decreased the polymer⁻CO₂ interaction and increased the polymer⁻polymer interaction, leading to a reduction in solubility of the co-polymers in SC-CO₂. However, the co-polymer could generate more effective inter-chain interaction and generate more viscosity enhancement compared to the Poly(Heptadecafluorodecyl) (PHFDA) homopolymer due to the driving force provided by π-π stacking of the VBe groups. The optimum molar ratio value for VBe in co-polymers for the viscosity enhancement of SC-CO₂ was found to be 0.33 in this work. The P(HFDA_0.67-co-VBe_0.33) was able to enhance the viscosity of SC-CO₂ by 438 times at 5 wt. %. Less VBe content would result in a lack of intermolecular interaction, although excessive VBe content would generate more intramolecular π-π stacking and less intermolecular π-π stacking. Both conditions reduce the thickening capability of the P(HFDA-co-VBe) co-polymer. This work presented the relationship between structure and performance of the co-polymers in SC-CO₂ by combining experiment and molecular simulations.

Incorporating a silicon unit into a polyether backbone—an effective approach to enhance polyether solubility in CO2

A series of poly(silyl ether)s were prepared by condensation polymerization and hydrosilation polymerization through incorporating a silicon unit into a polyether backbone.

Synthesis of fluorinated nanoparticles via RAFT dispersion polymerization-induced self-assembly using fluorinated macro-RAFT agents in supercritical carbon dioxide

Nano-sized fluorinated block copolymer particles are prepared by RAFT dispersion polymerization with polymerization-induced self-assembly proceeding in supercritical carbon dioxide.

Fabrication of Free-Standing, Self-Aligned, High-Aspect-Ratio Synthetic Ommatidia

Free-standing, self-aligned, high-aspect-ratio (length to cross-section, up to 15.5) waveguides that mimic insects' ommatidia are fabricated. Self-aligned waveguides under the lenses are created after exposing photoresist SU-8 film through the negative polydimethylsiloxane (PDMS) lens array. Instead of drying from the developer, the waveguides are coated with poly(vinyl alcohol) and then immersed into a mixture of PDMS precursor and diethyl ether. The slow drying of diethyl ether, followed by curing and peeling off PDMS, allows for the fabrication of free-standing waveguides without collapse. We show that the synthetic ommatidia can confine light and propagate it all the way to the tips.

Laboratory evaluation of a novel anaesthesia delivery device

Here, we describe proof of concept of a novel method for delivering volatile anaesthetics, where the liquid anaesthetic (sevoflurane or isoflurane) is formulated into an emulsion that is contained in a compact, lightweight device through which carrier gas flows. Release of anaesthetic is achieved by stirring of the formulation, allowing controlled and responsive release of anaesthetic at a variety of fixed flow rates between 0.5 l.min^-1 and 5 l.min^-1 , with ventilated, non-ventilated and draw-over breathing systems. Anaesthetic release was evaluated using target anaesthetic concentrations ranging from 0.5% v/v to 8% v/v to mimic those typically required for induction and maintenance of anaesthesia, and lower concentrations suitable for sedation. Under all conditions, output could be maintained within 0.1% v/v of the intended setting, and the device could deliver a controlled level of anaesthetic for at least 60 min, with compensation for different ambient temperatures (10-30 °C) and carrier gas flow rates. This device offers a simple, inexpensive method of delivering safe concentrations of volatile anaesthetics for a wide range of applications.

Structure-Property Relationships in CO2-philic (Co)polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions

This Review provides comprehensive guidelines for the design of CO2-philic copolymers through an exhaustive and precise coverage of factors governing the solubility of different classes of polymers. Starting from computational calculations describing the interactions of CO2 with various functionalities, we describe the phase behavior in sc-CO2 of the main families of polymers reported in literature. The self-assembly of amphiphilic copolymers of controlled architecture in supercritical carbon dioxide and their use as stabilizers for water/carbon dioxide emulsions then are covered. The relationships between the structure of such materials and their behavior in solutions and at interfaces are systematically underlined throughout these sections.

Block copolymer synthesis by controlled/living radical polymerisation in heterogeneous systems

We review the range of CLRP-controlled syntheses of block copolymer particles in dispersed systems, which are being exploited to create new opportunities for the design of nanostructured soft materials.

Exploration of CO2-Philicity of Poly(vinyl acetate-co-alkyl vinyl ether) through Molecular Modeling and Dissolution Behavior Measurement

Hydrocarbon CO2-philes are of great interest for use in expanding CO2 applications as a green solvent. In this work, multiscale molecular modeling and dissolution behavior measurement were both applied to explore CO2-philicity of the poly(vinyl acetate) (PVAc)-based copolymer. Introduction of a favorable comonomer, i.e., vinyl ethyl ether (VEE), could significantly reduce the polymer-polymer interaction on the premise that the polymer-CO2 interaction was not weakened but enhanced. The ab initio calculated interaction of the model molecules with CO2 demonstrated that the ether group in VEE or VBE was the suitable CO2-philic segment. From the molecular dynamics (MD) simulations of polymer/CO2 systems, the interaction energy and Flory-Huggins parameter (χ12) of poly(VAc-alt-VEE)/CO2 supported that poly(VAc-alt-VEE) possessed better CO2-philicity than PVAc. The dissolution behaviors of the synthesized poly(VAc-co-alkyl vinyl ether) copolymers in CO2 showed the best CO2-phile had the VEE content of about 34 mol %. The MD simulations also indicated that the interaction of random poly(VAc-co-VEE) containing about 30 mol % VEE with CO2 was the strongest and the χ12 was the smallest in these polymer/CO2 systems. Not only could the VEE monomer reduce the polymer-polymer interaction, but it could also enhance the polymer-CO2 interaction with an optimized composition. Introducing a suitable comonomer with a certain composition might be a promising strategy to form the synergistic effect of polymer-polymer interaction and polymer-CO2 interaction for screening the hydrocarbon CO2-philes.

Evaluation of CO2-philicity of poly(vinyl acetate) and poly(vinyl acetate-alt-maleate) copolymers through molecular modeling and dissolution behavior measurement

Multiscale molecular modeling and dissolution behavior measurement were both used to evaluate the factors conclusive on the CO2-philicity of poly(vinyl acetate) (PVAc) homopolymer and poly(vinyl acetate-alt-maleate) copolymers. The ab initio calculated interaction energies of the candidate CO2-philic molecule models with CO2, including vinyl acetate dimer (VAc), dimethyl maleate (DMM), diethyl maleate (DEM), and dibutyl maleate (DBM), showed that VAc was the most CO2-philc segment. However, the cohesive energy density, solubility parameter, Flory-Huggins parameter, and radial distribution functions calculated by using the molecular dynamics simulations for the four polymer and polymer-CO2 systems indicated that poly(VAc-alt-DBM) had the most CO2-philicity. The corresponding polymers were synthesized by using free radical polymerization. The measurement of cloud point pressures of the four polymers in CO2 also demonstrated that poly(VAc-alt-DBM) had the most CO2-philicity. Although copolymerization of maleate, such as DEM or DBM, with PVAc reduced the polymer-CO2 interactions, the weakened polymer-polymer interaction increased the CO2-philicity of the copolymers. The polymer-polymer interaction had a significant influence on the CO2-philicity of the polymer. Reduction of the polymer-polymer interaction might be a promising strategy to prepare the high CO2-philic polymers on the premise that the strong polymer-CO2 interaction could be maintained.

Melt crystallization/dewetting of ultrathin PEO films via carbon dioxide annealing: the effects of polymer adsorbed layers

The effects of CO2 annealing on the melting and subsequent melt crystallization processes of spin-cast poly(ethylene oxide) (PEO) ultrathin films (20-100 nm in thickness) prepared on Si substrates were investigated. By using in situ neutron reflectivity, we found that all the PEO thin films show melting at a pressure as low as P = 2.9 MPa and at T = 48 °C which is below the bulk melting temperature (Tm). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium swollen state, and the melt crystallization (and/or dewetting) process was carried out in air via subsequent annealing at given temperatures below Tm. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO2-treated PEO films: (i) a flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the original film thickness and the annealing temperature; and (ii) the dewetting kinetics for the 20 nm thick film is much slower than that for the thicker films. The key to these phenomena is the formation of irreversibly adsorbed layers on the substrates during the CO2 annealing: the limited plasticization effect of CO2 at the polymer-substrate interface promotes polymer adsorption rather than melting. Here we explain the mechanisms of the melt crystallization and dewetting processes where the adsorbed layers play vital roles.

Continuous polymer nanocoating on silica nanoparticles

Continuous polymer coating of nanoparticles is of interest in many industries such as pharmaceuticals, cosmetics, food, and electronics. Here we introduce a polymer coating/precipitation technique to achieve a uniform and controllable nanosize polymer coating on nanoparticles in a continuous manner. The utility of this technique is demonstrated by coating Aerosil silica nanoparticles (SNPs) of diameter 12 nm with the polymer Eudragit RL 100. Both hydrophilic and hydrophobic SNPs were successfully coated. After determining the cloud point of an acetone solution of the polymer containing a controlled amount of the nonsolvent water, the solid hollow fiber cooling crystallization (SHFCC) technique was employed to continuously coat SNPs with the polymer. A suspension of the SNPs in an acetone-water solution of the polymer containing a surfactant was pumped through the lumen of solid polypropylene hollow fibers in a SHFCC device; cold liquid was circulated on the shell side. Because of rapid cooling-induced supersaturation and heterogeneous nucleation, precipitated polymers will coat the nanoparticles. The thickness and morphology of the nanocoating and the particle size distribution of the coated SNPs were analyzed by scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS), thermogravimetric analysis (TGA), and dynamic light scattering (DLS). Results indicate that uniformly polymer-coated SNPs can be obtained from the SHFCC device after suitable post-treatments. The technique is also easily scalable by increasing the number of hollow fibers in the SHFCC device.