Showing papers on "Polystyrene published in 2007"

Orientation-controlled self-assembled nanolithography using a polystyrene-polydimethylsiloxane block copolymer.

Abstract: Templated self-assembly of a cylinder-forming poly(styrene-b-dimethylsiloxane) (PS−PDMS) diblock copolymer has been investigated for nanolithography applications. The large χ-parameter of the blocks and the use of a PDMS−brush substrate surface treatment are especially advantageous for achieving long-range ordering and minimizing defect densities, and the high Si content in PDMS leaves a robust oxide etch mask after two-step reactive ion etching. By adjusting mesa width and solvent-annealing vapor pressure and time, the cylinders can be intentionally oriented parallel or perpendicular to the trench walls. Pattern transfer into thin silica is also demonstrated. This block copolymer system has excellent characteristics for self-assembled nanolithography applications.

Cell and protein compatibility of parylene-C surfaces.

Abstract: Parylene-C, which is traditionally used to coat implantable devices, has emerged as a promising material to generate miniaturized devices due to its unique mechanical properties and inertness. In this paper we compared the surface properties and cell and protein compatibility of parylene-C relative to other commonly used BioMEMS materials. We evaluated the surface hydrophobicity and roughness of parylene-C and compared these results to those of tissue culture-treated polystyrene, poly(dimethylsiloxane) (PDMS), and glass. We also treated parylene-C and PDMS with air plasma, and coated the surfaces with fibronectin to demonstrate that biochemical treatments modify the surface properties of parylene-C. Although plasma treatment caused both parylene-C and PDMS to become hydrophilic, only parylene-C substrates retained their hydrophilic properties over time. Furthermore, parylene-C substrates display a higher degree of nanoscale surface roughness (>20 nm) than the other substrates. We also examined the level of BSA and IgG protein adsorption on various surfaces and found that surface plasma treatment decreased the degree of protein adsorption on both PDMS and parylene-C substrates. After testing the degree of cell adhesion and spreading of two mammalian cell types, NIH-3T3 fibroblasts and AML-12 hepatocytes, we found that the adhesion of both cell types to surface-treated parylene-C variants were comparable to standard tissue culture substrates, such as polystyrene. Overall, these results indicate that parylene-C, along with its surface-treated variants, could potentially be a useful material for fabricating cell-based microdevices.

Rings of Nanorods

Abstract: Finding new ways to assemble nano-objects into finite superstructures is an important task because their properties depend not only on their size and shape but also on the spatial arrangement and the degree of order among the individual building blocks. Ringlike assemblies are fairly common and have been documented for various polymers, small organic molecules, and spherical inorganic nanoparticles. 13] Several mechanisms have been proposed to explain the formation of rings, including the Marangoni effect, the hole-nucleation mechanism, “2D gas bubbles”, and the “breath figures” method (BF). The latter mechanism takes advantage of the condensation of water microdroplets from moist air and uses them as templates. Thus, the BF method typically produces a honeycomb structure, which can be viewed as an array of air bubbles embedded into a continuous 2D matrix formed by polymer chains or spherical nanoparticles. One can see that if the concentration of solute is reduced, the perforated film should transform into a set of isolated rings, as was recently demonstrated in the case of linear polymers. However, rings composed of anisometric nanocrystals, especially inorganic rods, have never been observed. Nearly all reports on the self-assembly of nanorods describe their packing in a parallel fashion. Theoretical models also predict a side-by-side packing of rods regardless of their overall morphology. In stark contrast to that, we describe herein a spontaneous assembly of hybrid gold/ polymer core–shell nanorods into ringlike arrays. A systematic investigation shows that the rings of rods described here are templated by water droplets that condense on the surface of nonpolar solvents from humid air, akin to the classical BF method. The high yield and the well-defined structure of rings are a result of the presence of polystyrene (PS) chains covalently attached to the surface of the gold nanorods. The polymer shell insures high solubility of the hybrid 1D nanostructures and keeps the AuNR(PS)n rods in solution during the evaporation until they are concentrated around the circumferences of water droplets. This method is surprisingly simple and can be used for organizing nanorods into circular arrays in a nearly quantitative yield. Our recent studies revealed that carboxyl-terminated polymers can be covalently attached to phenol-functionalized gold nanoparticles under mild esterification conditions. To test the versatility of such an approach, we attempted a synthesis of rodlike gold/polymer core–shell nanostructures. Unlike spherical nanoparticles, the functionalized nanorods cannot be directly prepared by reduction of gold ions in the presence of thiols. For that reason, we first prepared gold nanorods stabilized by cetyltrimethylammonium bromide (CTAB) surfactant using a modified seed-mediated growth method. However, a seemingly trivial step of exchanging CTAB for a functional thiol was found to be a serious challenge. For example, the removal of either water or CTAB surfactant causes an irreversible agglomeration of nanorods and the subsequent multiple attempts to attach thiols in organic media are completely unsuccessful. In addition, many functional thiols are not soluble in water and their dispersion in an aqueous solution of CTAB-stabilized nanorods does not result in ligand exchange. After many trials, we found that dropwise addition of a concentrated solution of 4-mercaptophenol in THF directly into an aqueous growth solution of rods led to CTAB–thiol exchange and a slow precipitation of phenol-functionalized nanorods. After centrifugal purification, AuNR(SC6H4OH)n rods could be dispersed in dichloromethane and covalently coupled with carboxybiphenyl-terminated polystyrene (Mw= 5000 gmol ; Figure 1). The reaction proceeded within several hours after the addition of 4-(N,N-dimethylamino)pyridinium-4-toluenesulfonate (DPTS) and 1,3-diisopropyl carbodiimide (DIPC). Remarkably, the reaction could be monitored by TLC as the starting material AuNR(SC6H4OH)n with a retention factor of Rf= 0 gradually transforms into AuNR(PS)n product, which moves on a TLC plate as a single dark-red spot (Rf= 0.6 in 15% THF in CH2Cl2). The product was isolated by centrifugal ultrafiltration of the THF solution using a regenerated cellulose membrane (MWCO=30 kDa). Importantly, the complete removal of linear polystyrene was confirmed by size-exclusion chromatography of the THF-soluble AuNR(PS)n product, whereas the presence of covalently attached PS arms was confirmed by H NMR spectroscopy (see the Supporting Information). The typical weight gain of such coupling reactions is about 40%, which allows us to estimate the grafting density of PS arms (5 kDa) on gold nanorods (10 nmC45 nm as determined by transmission electron microscopy (TEM)). The grafting density is about 2.1 chainsnm , which means that approximately 3000 PS chains are covalently attached to a given nanorod (see the Supporting Information). The value of the grafting density is comparable to that reported for spherical Au nanoparticles (2.9 chainsnm ). The presence of a dense polymer shell ensures high solubility and stability of the nanorods in organic solvents. [*] B. P. Khanal, Prof. E. R. Zubarev Department of Chemistry Rice University Houston, TX 77005 (USA) Fax: (+1)713-348-5155 E-mail: zubarev@rice.edu

In Situ Formation of Ag Nanoparticles in Spherical Polyacrylic Acid Brushes by UV Irradiation

Abstract: We reported on the in situ synthesis of silver nanoparticles onto polystyrene (PS) core−poly(acrylic acid) (PAA) polyelectrolyte brush particles. The synthesis of these composite particles proceeds...

Eliminating the Enhanced Mobility at the Free Surface of Polystyrene: Fluorescence Studies of the Glass Transition Temperature in Thin Bilayer Films of Immiscible Polymers

Abstract: By selective placement of fluorescent dyes, we have measured the glass transition temperature (Tg) of individual layers within supported bilayer films of different polymers to determine the extent to which strong free-surface effects and substrate interactions are mediated by a narrow interface between immiscible polymers. We have discovered that the impact a free surface has on Tg within an ultrathin PS layer is extremely sensitive to the polymer species used in the underlayer. The large Tg reduction of ∼32 K relative to bulk Tg observed for a 14 nm thick surface layer of polystyrene (PS) supported on bulk PS is virtually eliminated when a 14 nm thick surface layer of PS is placed on an underlayer of poly(methyl methacrylate) or poly(2-vinylpyridine) (P2VP), even of bulk thickness. Thus, the cooperative segmental mobility associated with the Tg of the PS free-surface layer is greatly hindered by the narrow, several-nanometer-wide interfacial region formed with the dissimilar polymer underlayer. This indi...

Microencapsulation of PCMs with a polystyrene shell

Abstract: Microencapsulation of different phase change materials (PCMs) with a polymer shell of polystyrene by suspension polymerization has been carried out. This method based on a suspension polymerization allows the encapsulation of non-polar PCMs, while that it was not possible to encapsulate the polar PCMs (polyglycols). This study deals with preparation and characterization of encapsulated paraffin wax. Thermal properties, the morphology and the particle size distribution of the microcapsules obtained were determined by differential scanning calorimetry, scanning electron microscopy and laser diffraction, respectively. This encapsulated paraffin wax could be considered to have good potential for energy storage.

Rubber composition and pneumatic tire using the same

Abstract: This invention provides a rubber composition highly balancing flexibility at a low temperature and a low rolling resistance, which is formed by compounding a low-molecular weight conjugated diene-based polymer (B) having at least one functional group, an aromatic vinyl compound content of less than 5% by mass and a weight average molecular weight as measured without terminating by modification through a gel permeation chromatography and converted to polystyrene of not less than 2,000 but less than 150,000 and a filler (C) into a high-molecular weight rubber component (A) having a weight average molecular weight as measured without terminating by modification through a gel permeation chromatography and converted to polystyrene of not less than 150,000.

Synthesis of high molecular weight comb block copolymers and their assembly into ordered morphologies in the solid state.

Abstract: This Article describes a simple two-step synthesis of comb block copolymers with molecular weights exceeding 1 000 000 g mol-1 and their assembly into ordered morphologies in the solid state. This work is exciting because these polymers assembled into morphologies with domain sizes exceeding 100 nm and, in some examples, 200 nm without the use of additives. These materials reflected selected wavelengths of visible light, and these wavelengths could be affected by swelling with methylene chloride vapor. A comparison between the structures of comb block copolymers and linear block copolymers is presented with a discussion of important parameters affecting their assembly in the solid state. This Article will first describe the synthesis of comb block copolymers using ring opening metathesis polymerization and atom transfer radical polymerization and their subsequent characterization. The comb block copolymers, backbone polymers, and polystyrene arms were all characterized independent of each other and had lo...

Progress in Understanding of Polymer Crystallization

Abstract: 1. Shifting Paradigms in Polymer Crystallization.-2. Theoretical aspects of the Equilibrium State of Chain Crystals.-3. Intramolecular Crystal Nucleation.-4. Kinetic Theory of Crystal Nucleation Under Transient Molecular Orientation.-5. Precursor of Primary Nucleation in Isotactic Polystyrene Induced by Shear Flow.-6. Structure Formation and Glass Transition in Oriented Poly(ethylene terephthalate).-7. How Do Orientation Fluctuations Evolve to Crystals?.-8. Role of Chain Entanglement Network on Formation of Flow-Induced Crystallization Precursor Structure.-9. Full Dissolution and Crystallization of Polyamide 6 and Polyamide 4.6 in Water and Ethanol.-10. Small Angle Scattering Study of Polyethylene Crystallization from Solutions.-11. Morphologies of Polymer Crystals in Thin Films.-12. Crystallization of Frustrated Alkyl Groups in Polymeric Systems Containing Octadecylmethacrylate.-13. Crystallization in Block Copolymers with More than one Crystallizable Block.-14. Monte Carlo Simulations of Semicrystalline Polyethylene:Interlamellar Domain and Crystal-Melt Interfcace.-15. The Role of the Interphase on the Chain Mobility and Melting of Semi-Crystalline Polymers a Study on Polyethylenes.-16. Polymer Crystallization under High Cooling Rate and Pressure: a Step Towards Polymer Processing Conditions.-17. Stress-Induced Phase Transitions in Metallocene-Made Isotactic Polypropylene.-18. Insights into Polymer Crystallization from In-Situ Atomic Force Microscopy.-19. Temperature and Molecular Weight Deppendencies of Polymer Crystallization.-20. Step-Scan Alternating Differential Scanning Calorimetry Studies on the Crystallisation Behaviour of Low Molecular Weight Polyethylene.-21.Order and Segmental Mobility in Crystallizing Polymers.-22. Atomistic Simulation of Polymer Melt Crystallization by Molecular Dynamics.-23. A Multiphase Model Describing Polymer Crystallization and Melting.

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles in solution.

Abstract: Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles with degrees of polymerization of 962 for the PS and 227 for the PEO blocks (PS962-b-PEO227) in N,N-dimethylformamide (DMF)/water, in which water is a selective solvent for the PEO block, were observed. For a system with 0.2 wt % copolymer concentration and 4.5 wt % water concentration in DMF/water, the micelle morphology observed in transmission electron microscopy changed from vesicles at room temperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologies were also formed in the intermediate temperature regions. Cooling the system back to room temperature regenerated the vesicle morphology, indicating that the morphological changes were reversible. No hysteresis was observed in the morphological changes during heating and cooling. Dynamic light scattering revealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combined static...

Investigation of Molecular Interactions between SWNT and Polyethylene/Polypropylene/Polystyrene/Polyaniline Molecules

Abstract: Single-walled nanotubes (SWNTs), which have a unique electronic structure, nanoscale diameter, high curvature, and extra-large surface area, are considered promising reinforcement materials for the next generation of high-performance structural and multifunctional composites. In the present study, force-field-based molecular dynamics simulations are performed to study the interaction between polymers and SWNTs. The “wrapping” of nanotubes by polymer chains was computed. The influence of temperature, nanotube radius, and chirality on polymer adhesion was investigated. Furthermore, the “filling” of nanotubes by polymer chains was examined. The results show that the interaction between the SWNT and the polymer is strongly influenced by the specific monomer structure such as aromatic rings, which affect polymers' affinities for SWNTs significantly. The attractive interaction between the simulated polymers and the SWNTs monotonically increases when the SWNT radius is increased. The temperature influence is neg...

Continuous hydrophoretic separation and sizing of microparticles using slanted obstacles in a microchannel

Abstract: We report a microfluidic separation and sizing method of microparticles with hydrophoresis—the movement of suspended particles under the influence of a microstructure-induced pressure field. By exploiting slanted obstacles in a microchannel, we can generate a lateral pressure gradient so that microparticles can be deflected and arranged along the lateral flows induced by the gradient. Using such movements of particles, we completely separated polystyrene microbeads with 9 and 12 µm diameters. Also, we discriminated polystyrene microbeads with diameter differences of ∼7.3%. Additionally, we measured the diameter of 10.4 µm beads with high coefficient of variation and compared the result with a conventional laser diffraction method. The slanted obstacle as a microfluidic control element in a microchannel is analogous to the electric, magnetic, optical, or acoustic counterparts in that their function is to generate a field gradient. Since our method is based on intrinsic pressure fields, we could eliminate the need for external potential fields to induce the movement of particles. Therefore, our hydrophoretic method will offer a new opportunity for power-free and biocompatible particle control within integrated microfluidic devices.

Verification of Controlled Grafting of Styrene from Cellulose via Radiation-Induced RAFT Polymerization

Abstract: Reversible addition - fragmentation chain transfer (RAFT) polymerization was applied to radiation-induced graft polymerization of styrene from cellulose. The grafting of styrene from cellulose substrates using the chain transfer agent cumyl phenyldithioacetate was confirmed by Raman and X-ray photoelectron spectroscopy, differential scanning calorimetery, thermogravimetric analysis, scanning electron microscopy, and contact angle analysis. Grafted polystyrene chains were cleaved from the cellulose surface by acidic hydrolysis of the cellulose. The number-average molecular weight and polydispersity index of the grafted and the free (nongrafted) polystyrenes obtained under identical conditions were determined by size exclusion chromatography. Grafted and nongrafted polystyrenes have almost the same (near theoretical) molecular weight and narrow polydispersity, thus proving for the first time the control of the grafting process mediated via RAFT without any prior functionalization of the surface.

Adsorption of trypsin on hydrophilic and hydrophobic surfaces.

Abstract: The adsorption of trypsin onto polystyrene and silica surfaces was investigated by reflectometry, spectroscopic methods, and atomic force microscopy (AFM). The affinity of trypsin for the hydrophobic polystyrene surface was higher than that for the hydrophilic silica surface, but steady-state adsorbed amounts were about the same at both surfaces. The conformational characteristics of trypsin immobilized on silica and polystyrene nanospheres were analyzed in situ by circular dichroism and fluorescence spectroscopy. Upon adsorption the trypsin molecules underwent structural changes at the secondary and tertiary level, although the nature of the structural alterations was different for silica and polystyrene surfaces. AFM imaging of trypsin adsorbed on silica showed clustering of enzyme molecules. Rinsing the silica surface resulted in 20% desorption of the originally adsorbed enzyme molecules. Adsorption of trypsin on the surface of polystyrene was almost irreversible with respect to dilution. After adsorption on silica the enzymatic activity of trypsin was 10 times lower, and adsorbed on polystyrene the activity was completely suppressed. The trypsin molecules that were desorbed from the sorbent surfaces by dilution with buffer regained full enzymatic activity.

Production of controlled polymeric foams by supercritical CO2

Abstract: The foaming of polystyrene and cellulose acetate has been studied using a technique based on the saturation of the polymer by supercritical CO2 and the rapid decompression of samples. The diameter of resulting microcellular structures has been controlled manipulating the process conditions. The experiments performed at various foaming temperatures (from 55 to 125 °C for polystyrene and from 215 to 245 °C for cellulose acetate) and at 230 bar for polystyrene and at 250 bar for cellulose acetate, showed that higher temperatures produce larger cells (with mean diameters from 5 to 65 μm for polystyrene and from 7 to 70 μm for cellulose acetate). The effect of foaming pressure has been also tested, at T = 75 °C for polystyrene and at T = 230 °C for cellulose acetate, from 80 to 230 bar for polystyrene and from 100 to 250 bar for cellulose acetate. Larger pressures generate smaller cells with mean diameters from 60 to 15 μm for polystyrene and from 80 to 20 μm for cellulose acetate. The influence of the contact time has also been studied: from 60 to 240 min for polystyrene and from 180 to 480 min for cellulose acetate; larger contact times assure homogeneous CO2 diffusion inside the sample and, consequently, a symmetric microcellular structure.

Successful Chain Extension of Polyacrylate and Polystyrene Macroinitiators with Methacrylates in an ARGET and ICAR ATRP

Abstract: Well-defined block copolymers poly(n-butyl acrylate)-b-poly(methyl methacrylate-co-styrene) and polystyrene-b-poly(methyl methacrylate-co-styrene) were synthesized using activators regenerated by electron transfer (ARGET) and initiators for continuous activator regeneration (ICAR) for atom transfer radical polymerization (ATRP). In order to overcome the poor initiation efficiency of polystyrene and poly(n-butyl acrylate) macroinitiators during chain extension with methacrylates, a small amount of styrene was used as a comonomer in the second block. To determine the percentage of styrene necessary for efficient chain extension, studies with low molecular weight alkyl halide initiators that model the poly(n-butyl acrylate) and polystyrene macroinitiators, respectively, were performed. Extension from the macroinitiators was then conducted using the appropriate methyl methacrylate/styrene comonomer mixture to obtain well-defined block copolymers, for example, starting from poly(n-butyl acrylate) macroinitiato...

Gas barrier of polystyrene montmorillonite clay nanocomposites: Effect of mineral layer aggregation

Abstract: Three polystyrene (PS)/clay hybrid systems have been prepared via in situ polymerization of styrene in the presence of unmodified sodium montmorillonite (Na-MMT) clay, MMT modified with zwitterionic cationic surfactant octadecyldimethyl betaine (C18DMB) and MMT modified with polymerizable cationic surfactant vinylbenzyldime-thyldodecylammonium chloride (VDAC). X-ray diffraction and TEM were used to probe mineral layer organization and to expose the morphology of these systems. The PS/Na-MMT composite was found to exhibit a conventional composite structure consisting of unintercalated micro and nanoclay particles homogeneously dispersed in the PS matrix. The PS/C18DMB-MMT system exhibited an intercalated layered silicate nanocomposite structure consisting of intercalated tactoids dispersed in the PS matrix. Finally, the PS/ VDAC-MMT system exhibited features of both intercalated and exfoliated nanocomposites. Systematic statistical analysis of aggregate orientation, characteristic width, length, aspect ratio, and number of layers using multiple TEM micrographs enabled the development of representative morphological models for each of the nanocomposite structures. Oxygen barrier properties of all three PS/clay hybrid systems were measured as a function of mineral composition and analyzed in terms of traditional Nielsen and Cussler approaches. A modification of the Nielsen model has been proposed, which considers the effect of layer aggregation (layer stacking) on gas barrier.

Phase behavior and temperature-responsive molecular filters based on self-assembly of polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene

Abstract: This work describes the synthesis of temperature-responsive polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene triblock copolymers, i.e., PS-b-PNIPAM-b-PS, their self-assembly and phase behavior in bulk, and demonstration of aqueous thermoresponsive membranes. A series of PS-b-PNIPAM-b-PS triblock copolymers were synthesized using reversible addition−fragmentation chain transfer (RAFT) polymerization. The hydrophobic PS end blocks were selected to form the minority component, whereas the temperature-responsive PNIPAM midblock accounted for the majority component. The self-assembly and phase behavior in bulk of PS-b-PNIPAM-b-PS as well as selected blends with low molecular weight PNIPAM homopolymers were studied using transmission electron microscopy (TEM). Classical lamellar, cylindrical, spherical, and bicontinuous double gyroid morphologies were observed in the dried state. In aqueous solutions, the glassy PS domains act as physical cross-links, and hydrogels were therefore formed. The bulk...

Nanoconfinement Revealed in Degradation and Relaxation Studies of Two Structurally Different Polystyrene−Clay Systems

Abstract: Degradation and relaxation studies have been performed on two polystyrene (PS)−montmorillonite clay nanocomposites, one of which has an intercalated PS−clay structure and the other an exfoliated PS−clay brush structure. Compared to virgin PS, both nanostructured materials have demonstrated the following similarities: (a) a high yield of α-methylstyrene in the degradation products as measured by infrared spectroscopy; (b) larger values of the activation energy of the thermal degradation as determined by isoconversional kinetic analysis of thermogravimetric data; and (c) larger values of the activation energy for the glass transition as found from the frequency dependence of the glass transition temperature measured by multifrequency temperature-modulated differential scanning calorimetry. These effects are taken as structure independent manifestations of nanoconfinement of the PS chains in the PS−clay materials. Heat capacity measurements have been employed to evaluate the size of the cooperatively rearra...