160 Matching Results

Search Results

Advanced search parameters have been applied.

Polymethylsilsesquioxanes through base-catalyzed redistribution of oligomethylhydridosiloxanes

Description: There has been an increasing amount of interest in silsesquioxanes and polysilsesquioxanes. They have been used as models for silica surfaces and have been shown to have great potential for several industrial applications. Typical synthesis of polysilsesquioxanes involves the hydrolysis of organotricholorosilanes and/or organotrialkoxysilanes in the presence of acid or base catalysts, usually in the presence of organic solvents.
Date: April 4, 2000
Partner: UNT Libraries Government Documents Department

Conjugation of DNA to Silanized Colloidal Semiconductor Nanocrystalline Quantum Dots

Description: Water-soluble, highly fluorescent, silanized semiconductor nanocrystals with different surface charges were synthesized. To covalently attach the nanocrystals to biological macromolecules with a variety of mild coupling chemistries, the outermost siloxane shells were derivatized with thiol, amino, or carboxyl functional groups. Single- or double-stranded DNA was coupled to the nanocrystal surfaces by using commercially available bifunctional cross-linker. Conjugation had little effect on the optical properties of the nanocrystals, and the resulting conjugates were more stable than previously reported systems. By using the strategies developed in this study, most biomolecules can be covalently coupled to semiconductor nanocrystals. These nanocrystal-DNA conjugates promise to be a versatile tool for fluorescence imaging and probing of biological systems.
Date: September 10, 2001
Creator: Parak, Wolfgang J.; Gerion, Daniele; Zanchet, Daniela; Waerz, Anke S.; Micheel, Christine; Williams, Shara C. et al.
Partner: UNT Libraries Government Documents Department

Intramolecular condensation reactions of {alpha}, {omega}- bis(triethoxy-silyl)alkanes. Formation of cyclic disilsesquioxanes

Description: Under acidic sol-gel polymerization conditions, 1,3-bis(triethoxysilyl)-propane (1) and 1,4-bis(triethoxysilyl)butane (2) were shown to preferentially form cyclic disilsesquioxanes 3 and 4 rather than the expected 1,3-propylene- and 1,4-butylene-bridged polysilsesquioxane gels. Formation of 3 and 4 is driven by a combination of an intramolecular cyclization to six and seven membered rings, and a pronounced reduction in reactivity under acidic conditions as a function of increasing degree of condensation. The ease with which these relatively unreactive cyclic monomers and dimers are formed (under acidic conditions) helps to explain the difficulties in forming gels from 1 and 2. The stability of cyclic disilsesquioxanes was confirmed withe the synthesis of 3 and 4 in gram quantities; the cyclic disilsesquioxanes react slowly to give tricyclic dimers containing a thermodynamically stable eight membered siloxane ring. Continued reactions were shown to perserve the cyclic structure, opening up the possibility of utilizing cyclic disilsesquioxanes as sol-gel monomers. Preliminary polymerization studies with these new, carbohydrate-like monomers revealed the formation of network poly(cyclic disilsesquioxanes) under acidic conditions and polymerization with ring-opening under basic conditions.
Date: August 1, 1996
Creator: Loy, D.A.; Carpenter, J.P.; Myers, S.A.; Assink, R.A.; Small, J.H.; Greaves, J. et al.
Partner: UNT Libraries Government Documents Department

The in-situ generation of silica reinforcement in modified polydimethylsiloxane elastomers

Description: Structure and properties of a series of modified polydimethylsiloxane (PDMS) elastomers reinforced by {ital in situ} generated silic particles were investigated. The PDMS elastomer was modified by systematically varying the molecular weight between reactive groups incorporated into the backbone. Tetraethoxysilane (TEOS) and partial hydrolyzate of TEOS were used to generate silic particles. Chemistry and phase structure of the materials were investigated by {sup 29}Si magic angle spinning nuclear magnetic resonance spectroscopy and swelling experiments.
Date: June 1, 1996
Creator: Prabakar, S; Bates, S.E.; Black, E.P. & Ulibarri, T.A.
Partner: UNT Libraries Government Documents Department

Arylene-bridged 2,2,5,5-tetramethyl2,5-disila-1-oxacyclopentanes as precursors to non-shrinking polysiloxanes. A new route to sol-gel type polymers

Description: Sol-gel chemistry has been the focus of much attention in the design and preparation of highly crosslinked polysiloxane gels. Preparation of sol-gel processed silica or polysilesquioxane gels is carried out by the hydrolysis and condensation of alkoxysilyl monomers, usually in the presence of catalytic acid or base and an excess of water. Removal of the alcohol and water byproducts of the condensation reactions, in addition to the alcohol needed to co-dissolve the hydrophobic monomers with water, leads to substantial shrinkage during drying of the resulting gels. This limits the utility of sol-gel processing for applications requiring net-shape casting of artifacts, crack free coatings, or low vaporous organic contaminants (VOCs). It would be advantageous to have a sol-gel process based on an organosilicon monomer that would not require water as a reactant or produce water and alcohol condensation products and still result in siloxane network polymers capable of forming gels. Here, the authors show the synthesis and preparation of a novel sol-gel monomer which can easily be polymerized by ring opening polymerization to give highly crosslinked polysiloxane gels with no condensation byproducts.
Date: November 1, 1998
Creator: Rahimian, K. & Loy, D.A.
Partner: UNT Libraries Government Documents Department

Susceptibility of polysiloxane and colloidal silica to degradation by soil microorganisms

Description: This report is a description of the laboratory study undertaken to determine the biodegradability of Colloidal Silica (CS) and PolySiloXane (PSX), a new generation of barrier liquids employed by the Viscous Liquid Barrier (VLB) technology in the containment of subsurface contaminants. Susceptibility of either material to microbial degradation would suggest that the effectiveness of a barrier in the subsurface may deteriorate over time. Degradation may result from several different biological events. Organisms may consume the material as a carbon and/or energy source, organisms may chemically change the material as a detoxification mechanism, or organisms may erode the material by their physical penetration of the material during growth. To determine if degradation occurs, physical interactions between soil microbes and the barrier materials were analyzed, and the metabolic activity of individual organisms in the presence of CS and PSX was measured.
Date: November 1, 1997
Creator: Lundy, D.Z.; Hunter-Cevera, J.C. & Moridis, G.J.
Partner: UNT Libraries Government Documents Department

Polysilsesquioxanes through base-catalyzed redistribution of oligohydridosiloxanes

Description: Organopolysilsesquioxanes have recently gained much interest as materials for low-K dielectrics, ceramic precursors and photoresists. Typical sol-gel synthesis of polysilsesquioxanes involves the hydrolysis of organotricholorosilanes and/or organotrialkoxysilanes in the presence of acid or base catalysts and organic solvents. However, under sol-gel conditions most organotrialkoxysilanes do not afford silsesquioxane gels. This limits the range of organic functionalities that can be introduced into these hybrid organic-inorganic materials. An alternative route to polysilsesquioxanes is through oligohydridosiloxanes. Catalytic disproportionation, by titanium complexes, of linear or cyclic oligomers of methylhydridosiloxanes can lead to polymethylsilsesquioxanes. The authors have shown that disproportionation of oligomethylhydridosiloxanes can also be catalyzed by tetrabutylammonium hydroxide to yield polymethylsilsesquioxanes (scheme 1). This replaces the step-growth sol-gel polymerization process of organotrialkoxysilanes, which requires solvent, stoichiometric water and produces alcohol and water condensation by-products. Tetraalkylammonium hydroxides, as catalysts, are also attractive because they readily decompose by heating above 150 C; thus, they can be easily removed from the final materials. In this paper the authors report on both the catalytic and stoichiometric redistribution of organohydridosiloxanes to produce polysilsesquioxane foams and gels of the formula (RSiO{sub 1.5}){sub n} which otherwise cannot be obtained through traditional sol-gel means.
Date: May 1, 2000
Partner: UNT Libraries Government Documents Department

Polymerization of trialkoxysilanes. Effect of the organic substituent on the formation of gels

Description: Hydrolysis and condensation of trialkoxysilanes, R-Si(OR{prime}){sub 3}, generally leads to the formation of silsesquioxane oligomers and polymers. These polymers are composed of a monomer repeat unit, [R-SiO{sub 1.5}]{sub n}, with a single silicon atom attached to other repeat units in the polymer through one to three siloxane bonds. The remaining substituent is an organic group attached to the silicon through a silicon-carbon single bond. Silsesquioxanes have been the subject of intensive study in the past and are becoming important again as a vehicle for introducing organic functionalities into hybrid organic-inorganic materials through sol-gel processing. Despite all of this interest, there has not been a systematic study of the ability of trialkoxysilanes to form gels through the sol-gel process. In fact, it has been noted that silsesquioxanes are generally isolated as soluble resins rather than the highly crosslinked network polymers (gels) one would expect from a tri-functional monomer. In this study, the authors have examined the sol-gel chemistry of a variety of trialkoxysilanes with different organic substituents (R = H, Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, t-Bu, n-octadecyl, n-dodecyl, cyclohexyl, vinyl, phenyl, benzyl, phenethyl), with methoxide or ethoxide substituents on silicon, at varying monomer concentrations ranging up to neat monomer, and with different catalysts (HCl, NaOH, formic acid, fluoride). Gels were prepared from tetramethoxysilane and tetraethoxysilane at identical concentrations for purposes of comparison.
Date: September 1, 1998
Creator: Loy, D.A.; Baugher, B.M. & Schneider, D.A.
Partner: UNT Libraries Government Documents Department

Solid-state NMR studies for DC 745: April--May 2000

Description: Powles et al. reported in 1961 that pdydimethyl siloxane melts exhibit a transverse decay of nuclear spin magnetization (T{sub 2}) that is nonexponential. This observation was interpreted in terms of the proton NMR signal from the melt having characteristics of a broad line. Since then, similar findings were obtained for other polymer melts too. Accordingly to Powles et al. this peculiar proton line shape was the result of an intramolecular direct dipole-dipole interaction between a pair of protons. They also suggested that this dipolar interaction was weak because the dipolar interaction was motionally averaged since the orientation-dependent local dipolar field is modulated by molecular dynamics. In sum, the consequences and the origins of nuclear spin correlations in viscous polymer melts were recognized almost 4 decades ago. In the 1970s, Cohen-Addad suggested that these weak proton dipolar interactions could be used to characterize polymer melts and networks. In this work, we have used some of these ideas to probe the effects of thermal treatments on the proton spin-spin relaxation times for DC745.
Date: December 1, 2000
Creator: Stephens, T. & Labouriau, A.
Partner: UNT Libraries Government Documents Department

The Effect of Surface Contamination on Adhesive Forces as Measured by Contact Mechanics

Description: The contact adhesive forces between two surfaces, one being a soft hemisphere and the other being a hard plate, can readily be determined by applying an external compressive load to mate the two surfaces and subsequently applying a tensile load to peel the surfaces apart. The contact region is assumed the superposition of elastic Hertzian pressure and of the attractive surface forces that act only over the contact area. What are the effects of the degree of surface contamination on adhesive forces? Clean aluminum surfaces were coated with hexadecane as a controlled contaminant. The force required to pull an elastomeric hemisphere from a surface was determined by contact mechanics, via the JKR model, using a model siloxane network for the elastomeric contact sphere. Due to the dispersive nature of the elastomer surface, larger forces were required to pull the sphere from a contaminated surface than a clean aluminum oxide surface.
Date: December 18, 2000
Partner: UNT Libraries Government Documents Department

Reaction engineering of co-condensing (methyl)ethoxysilane mixtures: Kinetic characterization and modeling

Description: Molecular homogeneity frequently plays a decisive role in the effective application of organically modified silicate copolymers. However, methods of directly characterizing copolymerization extent in siloxanes generated from mixed alkoxysilanes are not always available or convenient. The authors present an alternative tool for determining kinetic parameters for models of alkoxysilane hydrolytic copolycondensation. Rather than restricting attention to single step batch reactors, they use a semibatch reactor with varying time of injection of one component. They describe the fitting method and show that all necessary kinetic parameters can be determined from a series of ordinary {sup 29}Si NMR data in a straightforward case study: copolymerization of dimethyldiethoxy silane and trimethylethoxysilane. Under conditions providing no direct {sup 29}Si NMR signature of copolymerization, they find kinetic trends consistent with those previously reported. As further validation, the results of a new series of experiments (varying the ratio of mono-functional to difunctional monomer) are predicted by the semibatch copolymerization model and measured parameters. Based on these results, they are able to calculate the molecular homogeneity in the copolymer products investigated. Even for this relatively simple system, the optimal injection time is a complex function of residence time, but early injection of the faster-condensing monomer gives the best homogeneity at long residence times.
Date: January 26, 2000
Partner: UNT Libraries Government Documents Department

Non-Shrinking Sol-Gel Type Polymers by Ring Opening Polymerizations

Description: We have designed a new class of cyclic siloxane compounds that behave as sol-gel systems when ring open polymerized using a hydroxide base. These monomers polymerize through chain growth polymerization. unlike conventional alkoxysilane sol-gel precursors, to form sol-gel polymers. They do not require solvent or water for polymerization, show no visible shrinkage or cracking during polymerization and are thermally stable. We have successfully utilized these materials in encapsulation of microelectronics. Current efforts are focused toward expanding this family of ROP monomers and optimization of their mechanical properties.
Date: April 5, 1999
Creator: Loy, D.A. & Rahimian, K.
Partner: UNT Libraries Government Documents Department

Structure-property relationships in silica-siloxane nanocomposite materials

Description: The simultaneous formation of a filler phase and a polymer matrix via in situ sol-gel techniques provides silica-siloxane nanocomposite materials of high strength. This study concentrates on the effects of temperature and relative humidity on a trimodal polymer system in an attempt to accelerate the reaction as well as evaluate subtle process- structure-property relations. It was found that successful process acceleration is only viable for high humidity systems when using the tin(IV) catalyst dibutyltin dilaurate. Processes involving low humidity were found to be very temperature and time dependent. Bimodal systems were investigated and demonstrated that the presence of a short-chain component led to enhanced material strength. This part of the study also revealed a link between the particle size and population density and the optimization of material properties.
Date: March 1, 1997
Creator: Ulibarri, T.A.; Derzon, D.K. & Wang, L.C.
Partner: UNT Libraries Government Documents Department

TEOS reaction rates on SiO{sub 2} at 1000K: Zero-order dependence on hydroxyl coverage and implications for reactions with three-membered siloxane rings

Description: We have determined key kinetic parameters for the reaction of TEOS (tetraethylorthosilicate) on SiO{sub 2}. This was accomplished under conditions (20 to 500 mTorr at 1000 K) that pertain directly to TEOS-based CVD (chemical vapor deposition) processes. TEOS reactions were carried out using deuterated silanols (SiOD) on the initial SiO{sub 2} surface. This allowed FTIR (Fourier transform infrared spectroscopy) measurements to distinguish the consumption of SiOD by TEOS from the concurrent formation of SiOH which results from TEOS decomposition at 1000 K. While SiOD consumption did exhibit a first-order dependence on SiOD coverage, SiOH formation exhibited a zero-order dependence on the total coverage of hydroxyl groups. This suggests that reactions with hydroxyl groups alone can not account for all of the TEOS decomposition reactions at 1000 K. Since the low coverage of two-membered siloxane ((Si-O){sub 2}) rings was consumed during the initial TEOS exposure, siloxane (Si-O-Si) bridges in three-membered siloxane ((Si-O){sub 3}) rings may be the additional species responsible for the constant rate of TEOS decomposition. However, it is not conclusive that this type of site-specific mechanism controls the chemistry. The data may also be explained with a site-independent mechanism in which intramolecular decomposition of TEOS on the surface provides a common rate-determining step for subsequent consumption of hydroxyls and siloxane bridges on SiO{sub 2}. Regardless of the specific mechanism, our results predict that deposition rates will be insensitive to the relative coverages of siloxane bridges and hydroxyls on SiO{sub 2}. Therefore, a precise knowledge of the coverages of these species on SiO{sub 2} is not essential for modeling thermal TEOS decomposition rates.
Date: December 1, 1995
Creator: Bartram, M.E. & Moffat, H.K.
Partner: UNT Libraries Government Documents Department

Crystallization Behavior of M97 Series Silicone Cushions

Description: M97 series siloxanes are poly(dimethyl-diphenyl) siloxanes that are reinforced through a mixture of precipitated and fumed silica fillers which are blended in through the addition of a short chain polydimethylsiloxane processing aid. M97 silicones exhibit crystallization at -80.25 C by thermal (modulated differential scanning calorimetry) and mechanical (dynamic mechanical analysis) techniques. Isothermal dynamic mechanical analysis experiments illustrated that crystallization occurred over a 1.8 hour period in silica-filled systems and 2.8 hours in unfilled systems. The onset of crystallization typically occurred after a 30 minute incubation/nucleation period. {gamma}-radiation caused the crystallization rate to decrease proportionally with dosage, but did not decrease the amount of crystallization that ultimately occurred. Irradiation in vacuum resulted in slower overall crystallization rates compared to air irradiation due to increased crosslinking of the polymer matrix under vacuum. Modulated differential scanning calorimetry contrasted the crystallization and melting behavior of pure PDMS versus the M97 base polymer and helped determine which component of the composite was the origin of the crystallization phenomena.
Date: September 7, 2000
Creator: Chien, A.; DeTeresa, S.; Cohenour, R.; Schnieder, J.; LeMay, J. & Balazs, B.
Partner: UNT Libraries Government Documents Department

Detailed structural characterization of the grafting of [Ta(=CHtBu)(CH2tBu)3] and [Cp*TaMe4] on silica partially dehydroxylated at 700 C and the activity of the grafted complexes toward alkane metathesis

Description: The reaction of [Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 3}] or [Cp*Ta(CH{sub 3}){sub 4}] with a silica partially dehydroxylated at 700 C gives the corresponding monosiloxy surface complexes [({triple_bond}SiO)Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 2}] and [({triple_bond}SiO)Ta(CH{sub 3}){sub 3}Cp*] by eliminating a {sigma}-bonded ligand as the corresponding alkane (H-CH{sub 2}tBu or H-CH{sub 3}). EXAFS data show that an adjacent siloxane bridge of the surface plays the role of an extra surface ligand, which most likely stabilizes these complexes as in [({triple_bond}SiO)Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 2}({triple_bond}SiOSi{triple_bond})] (1a') and [({triple_bond}SiO)Ta(CH{sub 3}){sub 3}Cp*({triple_bond}SiOSi{triple_bond})] (2a'). In the case of [({triple_bond}SiO)Ta({double_bond}CHtBu)(CH{sub 2}tBu){sub 2}({triple_bond}SiOSi{triple_bond})], the structure is further stabilized by an additional interaction: a C-H agostic bond as evidenced by the small J coupling constant for the carbenic C-H (H{sub C-H} = 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [({triple_bond}SiO)Ta({double_bond}CHtBu)-(CH{sub 2}tBu){sub 2}({triple_bond}SiOSi{triple_bond})] (1a') as a catalyst precursor and the inactivity of the surface complex [({triple_bond}SiO)Ta-(CH{sub 3}){sub 3}Cp*({triple_bond}SiOSi{triple_bond})] (2a') show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.
Date: August 30, 2004
Creator: Le Roux, Erwan; Chabanas, Mathieu; Baudouin, Anne; de Mallmann, Aimery; Coperet, Christophe; Quadrelli, E. Allesandra et al.
Partner: UNT Libraries Government Documents Department

Solvent effects on silica domain growth in silica/siloxane composite materials

Description: The effect of solvent addition on the phase separation, mechanical Properties and thermal stability of silica/siloxane composite materials prepared by in situ reinforcement was examined. The addition of a solvent enhances the miscibility of the reinforcement precursor, a partial hydrolyzate of tetraethoxysilane (TEOS-PH), with the polydimethylsiloxane (PDMS) polymer. As a result, the phase separation at the micron level, termed the large-scale structure, diminished in size. This decrease in particle size resulting from the addition of moderate amounts of solvent was accompanied by an improvement in the mechanical properties. However, solvent addition in the excess of 50 weight percent led to a decrease in mechanical properties even though the large-scale structure continued to diminish in size. Small Angle X-Ray Scattering (SAXS) was used to examine the Angstrom level or small-scale structure. This small-scale structure was only affected by the presence of solvent, not the amount. The silica/siloxane composite materials showed the same thermal transition temperatures as the original PDMS material.
Date: July 1, 1995
Creator: Ulibarri, T.A.; Bates, S.E.; Black, E.P.; Schaefer, D.W.; Beaucage, W.G.; Lee, M.K. et al.
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of vinyl-bridged polysilsesquioxane sol-gel materials

Description: Vinyl-bridged polysilsesquioxane gels were formed through the use of sol-gel polymerization methods. Acid- and base-catalyzed samples were prepared from both the pure cis-(l) and pure trans-(2) isomers of 1, 2-bis(triethoxysilyl)ethylene. Gelation times of the two isomers were compared. The trans monomer 2 formed gels within a week while the cis monomer I failed to gel-even after several months. Gelation of 1 could be promoted by the addition of a coordinating metal such as palladium. The resulting cis- and trans- vinyl-bridged polysilsesquioxane gels were then processed either by vacuum drying to afford xerogels or by extracting with supercritical carbon dioxide to afford aerogels. These vinylbridged polysilsesquioxanes were characterized by SEM, nitrogen sorption porosimetry, solid State {sub 29}Si and {sub 13}C NMR and x-ray powder diffraction.
Date: August 1, 1995
Creator: Yamanaka, S.A.; Carpenter, J.P.; McClain, M.D. & Loy, D.A.
Partner: UNT Libraries Government Documents Department

Quantitative Analysis of Microstructure in Polysiloxanes Using High Resolution Si29 NMR Spectroscopy: Investigation of Lot Variability in the LVM97 and HVM97 PDMS/PDPS Copolymers

Description: The quantitative analysis of microstructure and sequence distribution in polysiloxane copolymers using high-resolution solution {sup 29}Si NMR is reported. Copolymers containing dimethylsiloxane (DMS) and diphenysiloxane (DPS) monomer units prepared with either high vinyl content (HVM) or low vinyl content (LVM) were analyzed. The average run length (R{sub exp}), the number average sequence length (l{sub A}, l{sub B}), along with the various linkage probabilities (p{sub AA}, p{sub AB}, p{sub BA}, and p{sub BB}) were determined for different production lots of the LVM97 and HVM97 samples to address the lot variability of microstructure in these materials.
Date: November 1, 2002
Creator: ALAM, TODD M.
Partner: UNT Libraries Government Documents Department

Commercial synthesis of M97KVB gum, a precursor to cellular silicone cushions. Part I

Description: The technology for producing an LLNL-developed polymer, L97KVB, has been transferred to a commercial speciality silicones manufacturer, McGhan-NuSil Corporation. Workers there have demonstrated both on a small scale and on a 200 lb. scale that they can produce a polymer which meets our analytical specifications and which will also perform satisfactorily in our load deflection and compression set tests.
Date: May 14, 1982
Creator: Riley, M.O.; Kolb, J.R. & Jessop, E.S.
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of a new silicone multiblock polymer

Description: The Lawrence Livermore National Laboratory (LLNL) has an active interest in the synthesis of new polysiloxanes as base polymers for cellular silicone materials. These elastomers have properties uniquely suited to very specific engineering requirements. While the polymers which we have prepared via random equilibrium of various cyclic tetrasiloxanes have adequate properties for certain applications, there is evidence to suggest that alternating block polysiloxanes prepared via condensation-polymerization techniques have properties more suited to our end uses as flexible foam materials (cushions). The synthetic sequence developed to prepare these materials involves reactions of functionally terminated (silylamino and silanol) polysiloxane oligomers to produce alternating multiblock (ABAB...) materials of high molecular weight. Dialkylamines are condensation byproducts in this reaction. The analysis and characterization of these multiblock polymers is reported.
Date: May 10, 1982
Creator: Riley, M.O.; Kolb, J.R. & Jessop, E.S.
Partner: UNT Libraries Government Documents Department

Tethered Chains in Poor Solvent Conditions: An Experimental Study Involving Langmuir Diblock Copolymer Monolayers

Description: We have employed Langmuir monolayer of highly asymmetric polydimethylsiloxane- polystyrene (PDMS-PS) diblock copolymers on dioctyl phthalate (DOP) at temperatures ranging from 22 "C to -35 `C as a model system for tethered chains in poor solvent conditions. The thicknesses of the tethered PS layers extending into the DOP subphase, measured by neutron reflection, decrease with decreasing temperature (T) over this entire r~ge. However, the v~iation with T becomes weak below -20 "C. At the ]owest T, the layer thicknesses are contracted 55 % -75 `% of their values at the theta condition (T8 = 22 "C), but are still quite swollen compared to the fully collapsed, nonsolvent limit. The contraction of the layer with decreasing T is determined as a function of surface density and molecular weight. These data are compared to universal scaling forms. The PS segments are depleted from the air surface over the entire T range, the thickness of the depletion layer increasing slightly with decreasing T. The free energy of the surface layer is probed by surface tension measurements. Negative surface pressures are observed at low coverages for both PDMS-PS and PDMS monolayer, indicating metastability toward lateral phase separation. Evidence for a trruisition from a dispersed phase to a condensed phase with decreasing T was observed in the reflectivity at very low PDMS-PS coverage.
Date: October 13, 1998
Creator: Kent, M.S.; Lee, L.T.; Majewski, J.; Satija, S. & Smith, G.S.
Partner: UNT Libraries Government Documents Department