Timothy S. Haddad

Viscoelastic and morphological behavior of hybrid styryl‐based polyhedral oligomeric silsesquioxane (POSS) copolymers

We report on the viscoelastic behavior of linear thermoplastic nonpolar hybrid inorganic-organic polymers. These materials have been synthesized through copolymerization of an oligomeric inorganic macromer with 4-methylstyrene where the inorganic portion of the material is a well-defined polyhedral oligosilsesquioxane (POSS), R7(Si8O12)(CH2CH2C6H4CH|CH2), with R A c-C6H11 or c-C5H9. A series of 4-methyl styrene copolymers with approximately 4, 8, and 16 mol % POSS macromer incorporation were investigated. Rheological measurements show that the polymer dynamics are profoundly affected as the percent of POSS increases. In particular, a high-temperature rubbery plateau develops (where a terminal zone is not observed), despite the fact that the parent poly 4-methylstyrene is unentangled. It is also observed that the thermal properties are influenced as the percent of POSS incorporation in- creases, with increases in the glass and decomposition temperatures. The results sug- gest that interchain interactions between the massive inorganic groups are responsible for the retardation of polymer chain motion, a mechanism similar to the sticky repta- tion model conceived for hydrogen-bonded elastomers and developed by Leibler et al. (Macromolecules, 24, 4701 (1991)).Control over the interchain interactions would also give rise to the observed increases in glass transition and the establishment of a rubbery plateau. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1857-1872, 1998

Superoleophobic surfaces through control of sprayed-on stochastic topography

The liquid repellency and surface topography characteristics of coatings comprising a sprayed-on mixture of fluoroalkyl-functional precipitated silica and a fluoropolymer binder were examined using contact and sliding angle analysis, electron microscopy, and image analysis for determination of fractal dimensionality. The coatings proved to be an especially useful class of liquid repellent materials due to their combination of simple and scalable deposition process, low surface energy, and the roughness characteristics of the aggregates. These characteristics interact in a unique way to prevent the buildup of binder in interstitial regions, preserving re-entrant curvature across multiple length scales, thereby enabling a wide range of liquid repellency, including superoleophobicity. In addition, rather than accumulating in the interstices, the binder becomes widely distributed across the surface of the aggregates, enabling a mechanism in which a simple shortage or excess of binder controls the extent of coating roughness at very small length scales, thereby controlling the extent of liquid repellence.

Fluoroalkyl-functionalized silica particles: synthesis, characterization, and wetting characteristics.

Fluoroalkyl-functionalized silica particles for use in nonwetting surfaces were prepared by treatment of silica particles with fluoroalkyl-functional chlorosilanes. Both fumed and precipitated silica were studied, as well as the efficiency of surface coverage using mono-, di-, and trifunctional chlorosilanes. The most effective surface treatment was accomplished via the surface grafting of monofunctional chlorosilanes in the presence of preadsorbed dimethylamine under anhydrous conditions at room temperature. Confirmation of covalent attachment was accomplished via Fourier transform infrared (FT-IR) spectroscopy, while elemental analysis, thermogravimetric analysis, and nitrogen adsorption isotherms were used to determine grafting densities and additional key geometric characteristics of the grafted layer. The effect of residual silanol content on the moisture uptake properties of the modified silica particles was determined by measuring the water uptake of unbound particles, while liquid wetting properties were determined by dynamic contact angle analysis of elastomeric composites. Although residual silanol content was shown to effect wetting properties, results suggest that surface geometry dominates the performance of liquid-repellent surfaces. The potential use of fluoroalkyl-functionalized silica particles for hydrophobic and oleophobic applications is discussed.

Fluoroalkylated Silicon-Containing Surfaces−Estimation of Solid-Surface Energy

The design of robust omniphobic surfaces, which are not wetted by low-surface-tension liquids such as octane (γlv=21.6 mN/m) and methanol (γlv=22.7 mN/m), requires an appropriately chosen surface micro/nanotexture in addition to a low solid-surface energy (γsv). 1H,1H,2H,2H-Heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) offers one of the lowest solid-surface energy values ever reported (γsv≈10 mN/m) and has become the molecule of choice for coating textured surfaces. In this work, we synthesize and evaluate a series of related molecules that either retain the POSS cage and differ in fluoroalkyl chain length or that retain the fluorodecyl chains surrounding a linear or cyclic molecular structure. The solid-surface energy (γsv) of these molecules was estimated using contact angle measurements on flat spin-coated silicon wafer surfaces. Zisman analysis was performed using a homologous series of n-alkanes (15.5≤γlv≤27.5 mN/m), whereas Girifalco-Good analysis was performed using a set of polar and nonpolar liquids with a wider range of liquid surface tension (15.5≤γlv≤72.1 mN/m). The hydrogen-bond-donating, hydrogen-bond-accepting, polar, and nonpolar (dispersion) contributions to the solid-surface energy of each compound were determined by probing the surfaces using a set of three liquid droplets of either acetone, chloroform, and dodecane or diiodomethane, dimethyl sulfoxide, and water.

Incompletely Condensed Fluoroalkyl Silsesquioxanes and Derivatives: Precursors for Low Surface Energy Materials

A novel synthetic method was developed for the controlled functionalization of fluorinated polyhedral oligomeric silsesquioxanes (F-POSS), which are useful as low surface energy materials for superhydrophobic and superoleophobic materials. Utilizing triflic acid, open-cage compounds were created and then reacted with a variety of dichlorosilanes to produce functional F-POSS structures possessing alkyl-, aryl-, and acrylate-based moieties. The crystal structure for an endo,endo-disilanol F-POSS compound was determined by single-crystal X-ray diffraction. The chemical structures were confirmed using multinuclear NMR spectroscopy ((1)H, (13)C, (19)F, and (29)Si), FT-IR, and combustion analysis. Dynamic contact angle measurements of these compounds were taken with water and hexadecane. These novel structures were found to possess excellent wetting-resistant behavior, similar to that of the parent F-POSS compound. They are the first well-defined fluorinated nano-building blocks with a controlled level of reactive functionality for the development of new superhydrophobic and superoleophobic materials.

Polyhedral Oligomeric Silsesquioxane (POSS) Styrene Macromers

Four polyhedral oligomeric silsesquioxane (POSS) macromers, R7Si8O12styrene (R=isobutyl, cyclopentyl, cyclohexyl, or phenyl), containing a single polymerizable styrene unit were synthesized from the POSS-trisilanols R7Si7O9(OH)3 in a high yield and purity. The base-assisted reaction to produce these macromers appears to be general for POSS-trisilanols of this type. However, in some cases it may be necessary to control the rate at which the base is introduced during reaction to prevent unwanted side reactions that decompose the trisilanol. Cyclohexyl-, cyclopentyl-, and isobutyl-substituted POSS-stryenes (at 30 wt% or approximately 4 mol% loadings) undergo free radical bulk polymerizations with styrene to produce polymers that show variation in the modulus above the glass transition temperature. The phenylPOSS derivative is too insoluble in styrene to undergo this polymerization. The effect on the modulus is more pronounced for the copolymers containing 30 wt% cycloalkylPOSS than that seen with isobutylPOSS, which is similar to that for bulk polymerized polystyrene. The effect of the group at the POSS cage on bulk polymer properties has been noted before and is presumed to arise from differences in polymer microstructure.

Synthesis of Aromatic Polyhedral Oligomeric SilSesquioxane (POSS) Dianilines for Use in High-Temperature Polyimides

A series of novel aromatic Polyhedral Oligomeric SilSesquioxane (POSS) dianiline molecules has been synthesized for use in the preparation of high temperature aromatic polyimides. A general synthetic strategy was devised to improve the structure, yield, and utility of POSS dianilines over those currently available. Peripheral aromatic functionality was specifically incorporated in order to improve thermal properties and to increase compatibility with aromatic polymers. Silyloxy and/or aromatic functional groups were used to link the aniline groups to the POSS cage in order to ensure that this linkage was not a thermal weak point. Additionally, the stereochemistry of the aniline functionality has been varied to produce both meta- and para- isomers. The new dianiline monomers can be used to produce high molecular weight polyimide polymers. These dianilines have been incorporated both as pendants to the polymer backbone, and in a “bead-on-a-string” fashion by insertion into the polymer main-chain. In addition to producing POSS polyimides with superior thermal stability, these new monomers will allow full characterization and comparison of POSS-polyimides with differing structures in order to delineate the structure-property relationships of POSS and polymer architectures.

Thermoplastics Modified with Nanoscale Inorganic Macromers

Abstract : We have taken a unique approach to the synthesis and study of hybrid organic/inorganic materials. Our method involves synthesizing nano-size inorganic P(l0)R(7)Si(8)O(12) clusters which contain seven inert R groups for solubility and only one functional P group for polymerization. This strategy permits the synthesis of melt processable, linear hybrid polymers containing pendent inorganic clusters and allows us to study the effect these clusters have on chain motions and polymer properties. The synthesis of norbornenyl-based polyhedral oligomeric silsesquioxane (POSS) macromers, their ring opening metathesis copolymerizations with varying amounts of norbornene, and analysis of the effect of the pendent POSS group is presented. The mechanical relaxation behavior and microstructure of norbornyl-POSS hybrid copolymers have been examined for their dependencies on the mole fraction of POSS-norborny monomer, as well as for potential sensitivity to the seven inert R groups present in each POSS macromer. POSS copolymerization is observed to enhance the cc-relaxation temperature, T(alpha) in proportion to the mole fraction of POSS-norbornyl comonomer. Interestingly, however, the magnitude of this dependence is larger for POSS-norbornyl comonomer possessing cyclohexyl groups (CyPOSS) than for cyclopentyl groups (CpPOSS). While POSS copolymerization yields only slight enhancement of the tensile storage modulus measured near room temperature at temperatures lower than a strong mechanical relaxation ((Beta)-relaxation near T = -75 degrees C), there is a significant POSS-reinforcement of the storage modulus.

Hybrid Inorganic/Organic Diblock Copolymers. Nanostructure in Polyhedral Oligomeric Silsesquioxane Polynorbornenes.

Our main approach to the synthesis and study of hybrid organic/inorganic materials involves incorporating nano-size inorganic polyhedral oligomeric silsesquioxane (POSS) clusters into various polymeric resins. A typical POSS cluster is a discrete silicon and oxygen framework solubilized with organic groups and contains a single reactive site. This lone site of reactivity is used to covalently attach the inorganic macromers pendent to a polymer backbone without causing any crosslinking. This strategy permits the synthesis of melt processable, linear hybrid polymers containing pendent inorganic clusters, and allows us to study the effect these clusters have on chain motion, polymer properties and morphology. The synthesis of norbornenyl-based (POSS) macromers, their ring opening metathesis copolymerizations with varying amounts of norbornene, and analysis of the effect of the pendent POSS group is presented. Ring opening metathesis polymerization permits the easy synthesis of both random and diblock copolymers. Transmission electron microscopy (TEM) clearly images POSS-rich domains against the POSSfree regions. Major differences in polymer morphology are observed as the amount of inorganic POSS is varied, between random and diblock copolymers, as well as between polymers that differ only in the solubilizing cycloalkyl groups on the POSS cluster.

Bis-phenylethynyl polyhedral oligomeric silsesquioxanes: new high-temperature, processable thermosetting materials

Bis-phenylethynyl polyhedral oligomeric silsesquioxane (bis-PE-POSS) compounds were synthesized and thermally cured yielding crosslinked materials. After curing at 370 °C, thermal decomposition occurs near 600 °C under nitrogen. These materials were synthesized by condensation of a new phenylethynyl-functional dichlorosilane onto tetrasilanol phenyl POSS, yielding two geometric isomers.