Siloxane Chain Research Articles

Comparative analysis of gelatin scaffolds crosslinked by genipin and silane coupling agent

Scaffolds based on gelatin (G) are considered promising for tissue engineering, able to mimic the natural extracellular matrix. G drawback is its poor structural consistency in wet conditions. Therefore, crosslinking is necessary to fabricate stable G scaffolds. In this work, a comparative study between the performance of two different crosslinkers, genipin (GP) and γ-glycidoxypropyltrimethoxysilane (GPTMS), is presented. Flat membranes by solvent casting and porous crosslinked scaffolds by freeze-drying were prepared. Infrared spectroscopy and thermal analysis were applied to confirm G chain crosslinking. Moreover, GP and GPTMS increased the stability of G in aqueous media and improved the mechanical properties. Crosslinking reduced the wettability, especially in the case of G_GPTMS samples, due to the introduction of hydrophobic siloxane chains. Both G_GP and G_GPTMS scaffolds supported MG-63 osteoblast-like cell adhesion and proliferation.

Poly(dimethylsiloxane-urethane) membranes: effect of linear siloxane chain and caged silsesquioxane on gas transport properties

The effect of poly(dimethylsiloxane) (PDMS) or polypropylene glycol (PPG) linear chain and polyoctahedral oligosilsesquioxanes (POSS) cubic nanoparticles on surface and gas transport properties of poly(dimethylsiloxane-urethane) PDMS-PU or poly(propylene glycol-urethane) (PPG-PU) hybrid membranes were studied. PDMS-PU or PPG-PU hybrid membranes were prepared using PDMS-diol or PPG-diol as a chain extender and diisocyanate with POSS-amine macromonomer as a crosslinker. The macromer synthesized was characterized using FT-IR, 1H-, 13C- and 29Si-NMR spectroscopic methods. The hybrid membranes were characterized by CP-MAS 29Si-NMR, DSC, contact angle, WAXD, AFM and density measurements. The glass transition temperature (Tg) of the hybrid membranes were determined by differential scanning calorimetry (DSC) and were found to be in the range of 176–189°C. The surface free energy was reduced by increasing the POSS-amine crosslinker content of the membranes. The AFM measurement showed phase separation of POSS-amine molecule and PDMS with the urethane matrix on the surfaces. The XRD profiles confirm that the membranes were highly amorphous in nature. The decrease in permeability was observed by increasing the concentration of POSS-amine incorporated hybrid membranes. The selectivities of O2/N2 and CO2/N2 gas pairs increased with an increase in the POSS concentration. This suggests that the selectivities were dependent mainly on the presence of urethane and ester functional groups in the crosslinker.

Ultrafast broadband laser spectroscopy reveals energy and charge transfer in novel donor-acceptor triads for photovoltaic applications

Triggered by the quest for new organic materials and micro-structures for photovoltaic applications, a novel class of donor-acceptor-donor (DAD) triads extended with siloxane chains has been synthesized in our labs. Because of the siloxane chains, the molecules self-organize into a smectic liquid crystal phase, resulting in a stacking of the DAD cores.We report here a preliminary study of the ultrafast dynamics of energy and charge transfer studied by femtosecond broadband transient absorption experiments on isolated triads in chloroform.

New Semifluorinated Siloxane-Grafted Copolyimides: Synthesis and Comparison with Their Linear Analogs

A siloxane-grafted new diamino monomer DBPDMS has been prepared and used as a co-monomer in combination with the fluorinated diamine monomer TFBB to prepare siloxane-grafted polyimides. The polymers have been characterized by means of GPC, IR, and NMR. Their thermal, mechanical, and surface properties have been evaluated and compared with the homopolyimide and with polyimides where polysiloxane is incorporated in the main chain. DSC revealed melting of the grafted siloxane chain at sub-ambient temperature and a glass transition corresponding to the main polymer chain at high temperature. Isothermal gravimetric analysis at 350 °C indicated that grafted siloxane moiety can be removed thermally from the polymer chain without affecting the polymer backbone.

Thermal degradation of polysiloxane and polyetherester containing benzoxazine moieties in the main chain

In this study investigation of thermal characteristics of polysiloxane and polyetherester containing benzoxazine moieties in the main chain was performed via pyrolysis mass spectrometry. Pyrolysis mass spectrometry analyses revealed that the thermal stability and the extent of cross-linking enhanced when the benzoxazine moieties were separated by thermally more stable units such as siloxanes. However, when the siloxane chain units were long, possibility of polybenzoxazine growth decreased significantly and benzoxazine moieties were evolved in the temperature range where polysiloxane degradation took place.

Synthesis and characterization of poly(multidimethylsiloxane-1,4-ethynylenephenyleneethynylene)s

A series of poly(multidimethylsiloxane-1,4-ethynylenephenyleneethynylene)s were synthesized by condensation reactions of 1,4-diethynylbenzene magnesium reagents with various α,ω-dichlorodimethylsiloxanes. The polymers obtained were solids or viscous liquids and are soluble in common organic solvents at room temperature. The structures and properties of the polymers were characterized by Fourier transform infrared, 1H nuclear magnetic resonance (NMR), 13C NMR, differential scanning calorimeter (DSC), dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) techniques. DSC analyses showed that these polymers can be thermally cured to form highly crosslinked structures. DMA studies revealed that the glass transition temperatures of the cured polymers decreased with increasing siloxane chain length. TGA measurements showed that the cured polymers were thermally stable up to almost 450 °C in both N2 and air. With increasing length of the siloxane units, the thermal stability of the cured polymers decreased. Silicon oxycarbide ceramics with high thermal stability were produced when the cured polymers were pyrolyzed at 1200 °C under argon. A series of poly(multidimethylsiloxane-1,4-ethynylenephenyleneethynylene)s were synthesized by condensation reactions of 1,4-diethynylbenzene magnesium reagents with various α,ω-dichlorodimethylsiloxanes. These polymers can be thermally cured to produce highly crosslinked structures. The glass transition temperatures of the cured polymers decrease with an increase in the siloxane chain length. The cured polymers are thermally stable up to almost 450 °C in both N2 and air. The silicon oxycarbide ceramics were obtained when the cured polymers were pyrolyzed at 1200 °C under argon.

Thermally Induced Changes in the Chemical and Mechanical Properties of Epoxy Foam

The influence of thermal treatment on the chemistry, physical, and mechanical properties of epoxy foam was evaluated. The foam was subjected to repeated thermal cycles (25—95°C), encompassing the temperature regime in which the forward and the retro Diels—Alder reaction occurs. Changes in the chemistry of the foam were evaluated using in situ FTIR spectroscopy during thermal exposure. In addition, the structural siloxane units within the epoxy foam were identified and evaluated using FTIR analysis. Thermal analysis was used to evaluate expansion, degradation, and mass loss during thermal exposure. Finally, the physical and mechanical properties were evaluated to determine how thermal cycling influences the density and modulus of the epoxy foam. Thermal exposure below the temperature required for the breakage of conjugated double bonds via Diels—Alder mechanism increases thermal expansion influencing the structural integrity and packing of siloxane chains. The data indicates that the chemical changes and the thermal expansion of the foam are irreversible. The combination of thermal expansion and the change in chemistry for the system strongly diminished the structural rigidity of the foam lowering the density and modulus of the material.

Bis(pyridyl)siloxane Oligomeric Ligands for Palladium(II) Acetate: Synthesis and Binding Properties

The complexation of a series of new bis(meta-pyridyl)methylsiloxane ligands with palladium(II) acetate in dilute toluene-d8 solution was studied by 1H NMR at 233−363 K, measuring the binding affinity and ring−chain distribution as a function of ligand structure, temperature, and concentration. Significant differences in the ring distribution and Pd binding affinity were observed as a function of siloxane chain length, with hexa- and heptasiloxane spacers binding Pd most effectively. The 1H NMR resonance of the singlet ortho aromatic proton was shifted unusually far downfield for the bidentate complex; the effect was strongest for the shortest chains and disappeared for chains with 10 or more siloxane units. Fitting the experimental data to a modified Jacobson−Stockmayer model demonstrated that the bidentate bis(pyridyl)siloxane complexes function as chelates in the concentration range 1−10 mM. Variable-temperature studies showed that all of the observed cyclic coordination oligomers are strainless. Theref...

Immobilization of Silanized DNA on Glass: Influence of the Silane Tether on the DNA Hybridization

Two trifunctional (trimethoxy and triethoxy) and one difunctional (methyldimethoxy) 3-mercaptopropyl-alkoxysilanes were covalently tethered to thiolated DNA oligonucleotides in solution. After deposition as microarrays onto glass, the immobilized DNA probes were tested for hybridization ability by a florescence-based method. The results demonstrate a large enhancement in the fluorescence signal when the functionality of the silane tether is reduced from three to two. An XPS analyses revealed that this is not due to a higher DNA surface density. FTIR spectra of the spin-coated silanes showed that the trifunctional silanes form branched and cyclic siloxane moieties, whereas the difunctional silane generates predominantly short straight siloxane chains. Therefore, the propensity of trifunctional silanes to form more complex networks leads to conformations of the bound DNA which are less favorable for the specific interaction with the complementary strand. The data implicate that further significant improvements in the DNA hybridization ability are possible by adroit choice of the silane system.

Study of the biaxiality in the nematic phase of liquid crystals in terms of orientational order parameters by infrared spectroscopy

The observation and quantification of the biaxiality in the nematic phase of thermotropic liquid crystals is one of the topical problems in the science of liquid crystals. The biaxiality in the refractive index and/or relative permittivity, both constituting tensors of the second rank, are expressed in terms of the four scalar order parameters S, D, P and C and the relevant molecular quantities. In this paper we review the theory of determining these order parameters using polarised infrared spectroscopy and present the method that we have developed in obtaining results for the order parameters of tetrapodes (with symmetrical and asymmetrical mesogens) and tripodes with symmetrical mesogens. In the tetrapodes, four mesogens are linked through the siloxane chains to the central Si atom. In the tripode, three mesogens are linked to a benzene ring via the oxygen atom through the siloxane spacers. In the first case a platelet-like structure is formed where the major director corresponds to the highest refractive index. A disc-like structure is formed for a tetrapode with symmetrical mesogens and for a tripode. The order parameters are calculated in the framework of the quasi-flat platelets. For the discotic-like structure, the major director corresponds to the lowest of the three refractive indices. For the three cases, the compounds are shown to exhibit a biaxial nematic phase. Biaxiality in the refractive indices is expressed in terms of the biaxial order parameters. A comparison of the results on the I–NU and NU–NB transitions for tetrapodes compare favourably with the predictions from the mean field models of de Gennes, Virga and his co-workers.

Controlled stepwise growth of siloxane chains using bivalent building units with different functionalities

Organic/inorganic hybrids of silicon and their subsequent chemical modification are of interest for tailoring and structuring surfaces on the nanoscale. The formation of organic (sub)monolayers on hydroxylated silicon surfaces was employed to synthesize molecular siloxane chains by stepwise wet-chemical condensation reactions. Functionalizations with small alkyl-monochlorosilanes, yielding dimethylsilyl, diisopropylsilyl, di- tert-butylsilyl terminations, and with an aryl-monochlorosilane, providing diphenylsilyl groups, were studied. The SiH moiety of these end groups could be selectively oxidized to the corresponding silanol. The reactivity of this group opened the possibility of bonding additional species step by step in bottom-up growth reactions. In particular, dimethylsiloxane units were studied as possible molecular building blocks. By repeating the oxidation and condensation reactions the stepwise growth of one-dimensional siloxane chains was demonstrated. The ongoing chain growth was characterized by attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry (NIR–UV), and contact-angle experiments.

Telechelic siloxanes with hydrogen‐bonded polymerizable end groups. I. Liquid rubbers and elastomers

AbstractFunctional siloxanes are widely used in the synthesis of model networks used in rubber elasticity studies, and various functional siloxanes are also used in the synthesis of silicone rubbers. We present here the results of studies of two classes of telechelic, free‐radically polymerizable siloxanes carrying either amide or urea groups attached at both ends of siloxane chain. The presence of polar, hydrogen‐bonding end‐groups favors the formation of aggregates which is reflected in the drastically increased viscosity of the resulting “liquid rubbers.” In addition, such aggregation accounts for high local concentrations of reactive groups thus making it possible to have high molecular weight precursors undergo facile room temperature polymerization. In this article, we study the effect of the nature of these functional groups on the viscosity of polymerizable “liquid rubbers,” finding that the different endgroups lead to dramatically different viscosities. We also study the silicone rubber films obtained by a UV‐initiated free‐radical polymerization of the liquid rubbers, using dynamic mechanical analysis and large‐strain uniaxial deformation. We find here that important properties such as plateau modulus and Young's modulus are highly dependent on both endgroup type and precursor molecular weight. The crosslinking of telechelic siloxanes in dilution was also studied to further explore the effect of crosslink density of the mechanical properties of the model networks. Finally, we evaluated the role of dangling ends within the networks through the incorporation of monofunctional macromers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Phase structure and molecular dynamics of liquid-crystalline side-on organosiloxane tetrapodes

X-ray diffraction and proton NMR relaxation measurements were carried out on two liquid-crystalline organosiloxane tetrapodes with side-on mesogenic groups, exhibiting nematic and smectic- C phases, and on a monomeric analog. Packing models for the mesophases exhibited by these systems are proposed on the basis of x-ray diffraction data. As a consequence of microsegregation, the aromatic cores are packed in between two sublayers formed by a mixture of interdigitated aliphatic and siloxane chains. The mixed sublayers are characteristic for the tetrapodes with side-on mesogenic groups presented in this work and have not been observed in tetrapodes with terminally attached mesogens. The tilt angle in the smectic- C phase is found very large, i.e., approximately 61 degrees -62 degrees . Notably, smectic- C clusters are present also in the whole temperature range of the nematic phase. NMR relaxometry yields T(1)-1 dispersions clearly different from those of conventional calamitics. The influence of molecular tendency to form interdigitated structures is evidenced by frequency-dependent relaxation rate in the isotropic phase-indicating the presence of ordered clusters far above the phase transition-and by the diminished role of molecular self-diffusion in ordered phases. Nematiclike director fluctuations are the dominating relaxation mechanism whereas the translational displacements are strongly hindered by the interdigitation of dendrimer arms.

Controlled Stepwise Growth of Siloxane Chains Using Bivalent Building Units With Different Functionalities

AbstractOrganic/inorganic hybrids of silicon and their subsequent chemical modification are of interest for tailoring and structuring surfaces on the nanoscale. The formation of monolayers on hydroxylated silicon surfaces was employed to synthesize molecular dimethylsiloxane chains by wet-chemical condensation reactions, using dimethylmonochlorosilane as the precursor. The SiH group of the resulting dimethylsilyl termination could be selectively oxidized to the SiOH group, which opened the possibility of bonding another species. By repeating the condensation and oxidation cycle the stepwise growth of one-dimensional dimethylsiloxane chains was achieved. The ongoing chain growth was characterized by attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and determination of the surface energy by contact-angle experiments.

Chain mobility in bulky carbosilane modified polymeric siloxane systems

AbstractHydrosilylation of bulky carbosilanes [CH2=CHRC(SiMe2R')3, R=CH2, (CH2)3 or SiMe2; R'=Me or Ph] with oligosiloxane precursors [M(D2DH)7D2M, M(D2DH 2)11D2M, PDMSx%-PMHS100-x% (x=85, 50, 30, 0)] was carried out. The relationship between the mobility of siloxane chain and the type of carbosilane moiety, as well as the type of distribution of carbosilane group along the polymer chain, was consecutively studied. Carbosilane substituents, due to their exceptional steric requirements, have been shown to strongly affect thermal properties of siloxane oligomers, implying a substantial change of the chain flexibility and free volume of modified oligomers.

Properties of Multiple-Functional Cationic Silk Fabrics

The silk fabric was chemically modified with 3-(trimethoxysilyl) propyl dodecyl dimethyl ammonium chloride (HSQA), a cationic modifying agent, to promote antibacterial property against S. aureus and E. coli. The dyeing and colorfastness properties of modified fabrics with commercial synthetic dyes, that is, acid and reactive dyes were also improved. The modified silk fabrics also exhibited some improved resiliency. Therefore, the wear properties of fabrics were expected to be improved by the technique of modification. However, the modification caused a slight decrease in the tensile strength of silk fabrics, as well as the whiteness and wettability because of introducing hydrophobic siloxane chains into amorphous regions of silk fibroin. The surface structure of modified silk fiber was studied by X-ray photoelectron spectra (XPS) and FT-IR spectra. Key words: Silk fabric; multiple-function; Wear property; antibacterial property; X-ray photoelectron spectrum; FT-IR spectrum

Nanostructured hybrid networks based on highly fluorinated acrylates

A dual-curing process is described for obtaining highly fluorinated acrylic networks containing nanosilica. The new materials showed improved hardness, while maintaining the hydrophobicity of the fluorinated polymer. They were obtained by UV-curing a suitable mixture of a perfluoropolyetherurethanediacrylate and alkoxysilanes, followed by a thermal treatment during which the sol–gel process took place. The obtained crosslinked networks had a complex morphology: nanometric silica particles were formed, as observed by TEM, and embedded in a biphasic crosslinked matrix made of perfluoropolyether domains and of copolymeric crosslinked domains containing the polyurethane acrylic units and siloxane chains formed by the alkoxides condensation.

Influence of KH-550 Silane Treatment on the Corrosion Resistance of AZ31 Magnesium Alloy

Silane treatment is an effective method of chemical conversion as a replacement for chromium treatments. In this paper, KH-550 silane treatment was adopted to improve the corrosion resistance AZ31 magnesium alloy. Hydrolysis process of KH-550 in the silane solutions was detected by the conductivity detection method. It was found that KH-550 silane could be hydrolyzed easily and its hydrolysis time was about 10 minutes. Polarization curve and salt immersion were carried out to investigate the corrosion behavior of the coated magnesium alloy treated by five kinds of silane solutions with different silane concentrations. When the concentration of KH-550 in the silane solution was 8%, the coated specimens by heating at 100°C for half an hour after the surfaces dry had more excellent corrosion resistance than others. It was shown that the KH-550 silane treatment could form a -Si-O-M covalent bonded film/metal interface and -Si-O-Si siloxane chains to improve the corrosion resistance of coated magnesium alloy.

New polysilsesquioxane materials of ladder-like structure

Alkoxy derivatives of linear oligomethylsiloxanes were applied as substrates for preparation of novel type, ladder-like polymeric silsesquioxane–siloxane materials. A systematic study of the effect of diverging the structure of a polysiloxane chain (distribution of alkoxysilyl groups along the polymer chain) on the properties of silsesquioxane materials obtained by condensation of ethoxy groups, has been undertaken. A series of polysiloxane materials cross-linked with –SiOSi– bonds, forming ladder-like linkages between main siloxane chains was prepared. There were used siloxanes of a regular structure with formula [M(D 2D OR) 10D 2M] and [M(D 2D OR 2) 10D 2M] ] as well as siloxane oligomers of random distribution of MeSi-OR (D OR) units along the main siloxane chain [MD (1− x )D OR x M ( x = 0.3, 0.5, 1.0)]. Alkoxy-functionalized oligosiloxane precursors were cross-linked under hydrolytic condensation conditions catalyzed by HCl/NH 4OH system or nucleophilic catalysts (TBAF and TBAH). The relationship between the structure of siloxane chain (the amount of Si-OR units and their distribution along the polymer backbone) and the properties of obtained preceramic materials was studied with FTIR (as a main method) and NMR, XRD, DTA. The surface area and pores distributions of the studied samples were measured using nitrogen adsorption methods (BET).

Amphiphilic Siloxane Phosphonate Macromolecule Monolayers at the Air/Water Interface: Effects of Structure and Temperature

A comprehensive study is reported of Langmuir-Blodgett (LB) films (spread at the air/water interface using the Langmuir balance technique) composed of surface active, nonionic, and OH-free amphiphilic siloxane phosphonate ester macromolecules. Analysis is made on three molecular structures in the form of linear polymer poly(diethylphosphono-benzyl-alphabeta-ethyl methylsiloxane) (PPEMS), cyclic oligomer methylphosphonobenzyl-alphabeta-ethyl cyclosiloxane (MPECS), and copolymer poly(PEMS-co-DMS). The surface pressure-surface area (pi -A) isotherms of homopolymer at 3-40 degrees C show a clear temperature-induced phase transition (plateaus at pit approximately 17-19 mN/m) below 10 degrees C. The magnitude of the transition substantially increases upon lowering the temperature (partial differential DeltaAt/ partial differential T approximately -0.1 nm2 unit(-1) deg(-1) and partial differential pi t / partial differential T approximately -0.25 mN m(-1) deg(-1)). The positive entropy and enthalpy gain infers that strong coupling with the subphase and excess hydration attributed to hydrogen bonding between the P=O bond and the subphase prevails at low temperatures. The cyclic oligomer MPECS forms a condensed monolayer at the air/water interface that does not display a similar transition in the experimental temperature range. The temperature sensitivity of MPECS film is observed only in the collapsed region. The nature of the interaction with the subphase is similar for MPECS and PPEMS, indicating that the size and thermal mobility are the controlling factors in these processes. The elasticity plot reveals two distinct states (above and below transition). This observation is supported by BAM images that show irregular spiral structures below 10 degrees C. The transition occurring in the copolymer at 20 degrees C is due to relaxation of the PDMS component. The two maxima shown in the elasticity plot indicate additive fractions of PPEMS and PDMS. The surface areas of these macromolecules in the relaxed (1.48 nm2/unit) and packed (0.45 nm2/unit) forms obtained by PM3 modeling agree well with the experimental data and seem to indicate that the siloxane chain is being lifted off the subphase by the hydrophobic phenylic part of the molecule.