Thermodynamic Model of Liquid−Liquid Phase Equilibrium in Solutions of Alkanethiol-Coated Nanoparticles (original) (raw)
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Two-Stage Collapse of Unimolecular Micelles with Double Thermoresponsive Coronas
Langmuir, 2006
Phase transition behavior of unimolecular dendritic three-layer nanostructures with dual thermoresponsive coronas is studied. Successive reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide (NIPAM) and 2-(dimethylamino)ethyl methacrylate (DMA) were conducted using fractionated fourth-generation hyperbranched polyester (Bolton H40) based macroRAFT agent. At lower temperatures (<20°C), dendritic macromolecules H40-poly(N-isopropylacrylamide)-poly(2-(dimethylamino)ethyl methacrylate) (H40-PNIPAM-PDMA) exist as unimolcular core-shell-corona nanostructures with hydrophobic H40 as the core, swollen PNIPAM as the inner shell, and swollen PDMA as the corona. PNIPAM and PDMA homopolymers undergo phase transitions at their lower critical solution temperatures (LCST), which are found to be 32°C for PNIPAM and 40-50°C for PDMA, respectively. Upon continuously heating through the LCSTs of PNIPAM and PDMA, such dendritic unimolecular micelles exhibit two-stage thermally induced collapse. This process is reversible with a two-stage reswelling upon cooling. Laser light scattering, micro-differential scanning calorimetry, and excimer fluorescence measurements are used to investigate the double phase transitions.
Langmuir, 2019
The importance of thermodynamics does not need to be emphasized. Indeed, elevated temperature processes govern not only industrial scale production but also self-assembly, chemical reaction, interaction between molecules, etc. Not surprisingly, biological processes typically take place at a specific temperature. Here, we look at possibilities to raise the localized temperature by a laser around noble-metal nanoparticles incorporated into shells of layer-by-layer polyelectrolyte microcapsules freely suspended delivery vehicles in an aqueous solution, developed in the Department of Interfaces, Max Planck Institute of Colloids and Interfaces, headed by Helmuth Mö hwald. Understanding the mechanisms of localized temperature rise is essential, that is why we analyze the influence of incident intensity, nanoparticle size, their distribution and aggregation state, as well as thermodynamics at the nanoscale. This leads us to scrutinize "global" (used for thermal encapsulation) versus "local" (used for release of encapsulated materials) temperature rise. Similar analysis is extended to planar polymeric coatings, the lipid membrane system of vesicles and cells, on which nanoparticles are adsorbed. Insights are provided into the mechanisms of physicochemical and biological effects, the nature of which has always been profoundly, interactively, and engagingly discussed in the Department of Interfaces. This analysis is combined with recent developments providing outlook and highlighting a broad range of emerging applications. ■ INTRODUCTION The importance of thermodynamics is difficult to overestimate. 1 With contributions and understanding of thermal heat exchange processes provided and inspired by Boltzmann, Carnot, Boyle, Kelvin, Clausius, Thompson, Joule, Gibbs, and others, essential developments have been made. One of the most tangible "products" of these developments is a refrigeratornowadays used in many households. In early days, it was important to bring understanding of microprocesses to a macro level, and gas was one of the most important subjects of research, not least because of the significance of the industrial revolution and the heat engine. The goals reflected in research subjects were big and grandiose then. Several hundred years fast forward and we are "back to future" looking at nanoscale processes, and the significance of interconnected processes in biology brings water (and not gas) and molecules dissolved in water/solvents at the forefront of research subjects. Here, we look into nanoscale thermal processes associated with temperature rise associated with nanoparticle immobilized in the shell of polyelectrolyte multilayer (PEM) capsules, which are freely suspended in an aqueous solution. First, we introduce polyelectrolyte multilayer capsules highlighting further the nature of absorption of nanoparticles and heat generation. We consider also spectral responsiveness, determined by the aggregation state of nanoparticles due to the dipole−dipole interaction of neighboring nanoparticles. Subsequently, we analyze thermodynamics and heat propagation around nano-particles. Thermodynamics processes of this system are discussed in light of nanoscale phenomena outlining phenomena and elaborating such fundamental processes associated with nanoscale heat conduction as encapsulation from and release into microcapsules. Nanoscale processes associated with temperature rise are analyzed on nano-, micro-and macroscale scales. On the one hand, a temperature increase leads to encapsulation of molecules (thermal-based encapsulation method), while on the other hand, and in sharp contrast, an increase of temperature to similar values (albeit locally) induces release of the molecules from similar capsules. Then, we transfer to such captivating applications as localized permeability change of a hybrid system composed of polymers, lipids (an organic phase), and nanoparticles (an inorganic phase). We conclude by looking at a wide range of applications of microcapsules, many of
Thermodynamic parameters controlling nanoparticle spatial packing in polymer solutions
Despite their unprecedented potential, polymer nanocomposites (PNCs) have not reached their forecasted industrial utilization, yet. Insufficient control of nanoparticle (NP) spatial organization in the polymer matrix was recognized as the bottleneck of further PNC applications. Therefore, thermodynamic parameters enabling a general estimate of the nanocomposite (NC) structure in any polymer solution were investigated in this study. The effect of polymer−particle−solvent interactions on the final NP dispersion in PNCs was examined in depth. Our approach was based on assessing the surface charge (ζpotential) of NPs and specifying the difference in solubility parameters between the polymer, nanoparticles, and the solvent used during the preparation. To generalize our findings, four different polymer matrixes, poly(methyl methacrylate) (PMMA), poly(vinyl acetate) (PVAc), polycarbonate (PC), and polystyrene (PS), and three types of NPs, spherical colloidal and fumed nanosilica and functional ZnO 2 doped with Al 2 O 3 NPs blended in various solvents, were investigated. The overall interaction balance present in the PNC solution was estimated using solubility parameters and ζ-potential (represented by polarity index), and the influence on final NP dispersion after NC solidification was described. This approach offers a valuable tool that only requires several readily accessible physicochemical parameters (solubility parameters and ζ-potential) as an input for the structural prediction of the final PNCs. Hydrogen bonds play an important role in the formation of the PNC structure due to the absorption of polymer chains onto the NP surface. Generalized features described on a wide range of composition and preparation conditions will help to advance the fundamental understanding of NP self-assembly in polymer liquids. Moreover, the presented relation between the solvent− polymer−particle interaction strength, NP spatial organization, chain stiffness, and relaxation properties, which was evaluated by comparing PNCs with various matrixes, will contribute new evidence to the general description of the PNC's structure-property function. As an addition, we present anisotropic microstructures composed by the self-assembly process of spherical NPs prepared in dioxane.
Soft Matter, 2011
The structure of diblock copolymers micelles depends on a delicate balance of thermodynamic forces driving the system towards equilibrium and kinetic factors which limit the systems' exploration of the phase space. The factors governing the morphological transition between cylindrical and spherical micelles are related to a fine balance between entropic forces from chains within the micellar core and corona. In order to understand and control these structures, it is important to gain insight into the fundamental thermodynamic driving forces governing the structure and answer fundamental questions concerning its equilibrium nature. In this work we aim to understand the relationship between thermodynamics and morphological transitions by investigating the detailed structure of a system undergoing a cylinder-to-sphere transition. We focus on the structural properties of micelles constituted of poly(ethylene-alt-propylene)-poly(ethylene oxide) (PEP1-PEO1, the numbers indicate the molar mass in kg/mole) diblock copolymers in dimethylformamide (DMF)/water solvent mixtures. This system is ideal for fundamental studies as it represents a classical well-segregated block copolymer micelle system where the interfacial tension can be controlled in detail without significantly changing other thermodynamic properties. Using small-angle neutron scattering (SANS) it is shown that the system undergoes a cylinder-to-sphere transition upon addition of DMF which lowers the interfacial tension. By applying a detailed thermodynamic model we show that both the dependence of the structural parameters with the interfacial tension as well as the morphological transition can be quantitatively understood. The transition itself is governed by the interfacial tension which dictates the stretching of chains within both corona and core. At high interfacial tensions (in water-rich solutions) discrepancies between structural data and predictions from the thermodynamic model are observed. A qualitative comparison with some preliminary results on the chain exchange kinetics in the system show that these deviations coincide with the region where this equilibration mechanism is not active, i.e. when the kinetics are frozen at high interfacial tensions.
Temperature-Induced Gelation in Dilute Nanofluids
Langmuir, 2011
Colloidal self-assembly is an elegant and simple approach to produce optical, electronic, and biosensing devices. 1,2 Colloidal suspensions of nanoparticles (NPs), popularly known as nanofluids (NFs), are fascinating materials owing to their interesting properties and flow behavior. 3À8 Nanofluids containing electrically or magnetically polarizable particles come under the category of smart materials and have several fascinating technological applications, besides being a model system for fundamental studies. 9À12 These soft matter systems exhibit interesting phase behavior under different conditions. 13À26 The aggregation mechanisms of nanoparticles in base fluids 27 and polymer melts 26,28,29 have been another topic that have captured the attention of scientific community recently. The sol to gel transition is mostly seen in polymeric materials or polymers incorporated with particles. Gelling materials exhibit critical behavior that is manifested by the divergence of several physical properties, including the zero shear viscosity, equilibrium modulus, and long relaxation time. 30 At the solÀgel transition point, a samplespanning critical network cluster is formed first. The actual gel point appears to be determined by the formation of a particle cluster near the wall. Multifold increase in viscosity can be either due to the formation of clusters leading to an increase in the effective volume fraction (ϕ) of disperse phase due to occlusion of liquid within the clusters or due to shape of clusters (e.g., elongated nonspherical structures). In general, structural transition studies are observed in dispersion of high ϕ, with relatively large particle size, where the surface area effects are not so predominant. For example, Trappe et al. elegantly demonstrated the composite jamming phase diagram for attractive colloidal particles where the ratio of thermal energy (k B T) to the strength of the attractive interparticle interaction (U) is less than unity. 24,31 They used relatively high ϕ of submicrometer-sized particle of carbon black, polymethyl methacrylate, and polystyrene to demonstrate that the fluid-to-solid transition undergoes markedly similar gelation behavior with increasing concentration and decreasing
Aqueous Dispersions of Extraordinarily Small Polyethylene Nanoparticles
Angewandte Chemie International Edition, 2005
General methods and materials. Ethylene of 3.5 grade was supplied by Praxair. Demineralised water was distilled under argon. 2-propanol (p.a.) was degassed by four repeated freeze-thaw cycles. Chloranil (Aldrich) und sodium dodecyl sulfate (Fluka) were used without further purification. Potassium 4-(diphenylphosphino)benzenesulfonate (TPPMS) was synthesized as described in [1]. bis(1,5-cyclooctadiene) nickel was synthesized according to [2] (the compound is also available commercially, e.g. at Aldrich or Strem). TEM investigations were carried out on a LEO 912 Omega apparatus using an acceleration voltage of 120 kV. Samples were stained with RuO 4. For microtome cutting, the latex particles were embedded in nanoplast ® (a hydrophilic melamine resin). Microtome cuts of ca. 50 nm thickness were prepared with a Reichert & Jung Ultracut E microtome equipped with a 45° diamond knife supplied by Diatome. AFM experiments were performed with a Nanoscope III scanning probe microscope. The height and phase images were obtained simultaneously while operating the instrument in the tapping mode under ambient conditions. Images were taken at the fundamental resonance frequency of the Si cantilevers which was typically around 180 kHz. Typical scan speeds during recording were 0.3-1 line/s using scan heads with a maximum range of 16 × 16 µm. The phase images represent the variations of relative phase shifts (i. e. the phase angle of the interacting cantilever relative to the phase angle of the freely oscillating cantilever at the resonance frequency) and are thus able to distinguish materials by their material properties (e.g. amorphous and crystalline polymers). DSC was performed on a Perkin Elmer DSC 7 instrument or on a Pyris 1 DSC at a heating and cooling rate of 10 K min-1. T m data reported are local maxima of the second heats. DSC traces of polymer dispersions were obtained on 20 to 30 mg of dispersion with ca. 5 % by weight polymer content. NMR spectra were recorded on a Bruker ARX 300 instrument (1 H: 300 MHz; 13 C: 75 MHz). 1 H and 13 C NMR spectra were performed in 1,1,2,2-tetrachloroethane-d 2 at 122 °C. GPC analyses were carried out by Basell GmbH, Ludwigshafen on a Waters150 or GPC2000 instrument equipped with Shodex columns at 140 °C in 1,2,4-trichlorobenzene. Data is referenced to linear polyethylene standards. Dynamic light scattering on dispersions was performed on a Malvern particle sizer. Catalyst Preparation. The preparation of catalyst was performed by standard Schlenk techniques under argon. Equal molar amounts of chloranil and TPPMS were dissolved in a given amount of 2-propanol (2 to 10 mL). The obtained yellow solution was transferred to a 1.1-fold molar excess of bis(1,5-cyclooctadiene)nickel.