Oligomeric Dimethylsiloxane Research Articles

Structure-property interplay of asymmetric membranes comprising of soft polydimethylsiloxane chains and hard silica nanomaterials

Silica nanomaterials modified with monomolecular C18-alkyl silane or oligomeric dimethylsiloxane units were loaded in-situ during the formation of PDMS membrane. The small-angle neutron scattering (SANS) study was performed to probe structural compatibility between the modified silica and PDMS. The Ornstein-Zernike (OZ) and Debye-Anderson-Brumberger (DAB) models were excellently fit to the SANS data describing nanoscale structure morphology in terms of correlation length scales of the soft and hard segmental units. The polymeric chain was stretched or contracted depending upon the type and amount of silica incorporation. This indicated limiting amount of silica incorporation in the PDMS membrane structure depending upon structural compatibility between the silica and PDMS. The C18-alkyl silylated silica was found to be the best compatible with the membrane structure. The membrane with 30% (w/w) C18-alkyl silylated -silica loading exhibited the best performance in pervaporation separation of alcohol/water.

Characterisation and migration properties of silicone materials during typical long-term commercial and household use applications: a combined case study

In consequent continuation of previous described studies, pre-characterised silicone materials were assessed for chemical and physical parameters during long-term usage. In a particular case study silicone moulds were used in a commercial pizza bakery on a daily basis up to 1700 times. Migration behaviour, uptake of fat, the amount of volatiles and extractables, as well as physical properties (elongation, tensile strength) were monitored for the whole period. The main question was whether a significant degradation or even breakdown of the silicone elastomer could take place yielding enhanced migration of dimethyl siloxanes. Oligomeric dimethyl siloxanes are reaction side-products of the polymerisation process and despite their origin as so-called non-intentionally added substances (NIAS) were found to be the by far most dominating constituents of the overall migration. Furthermore, the influence of long-term thermal stress on the functionality of the elastomer was proven. Migration into food was determined by 1H-NMR and was found to decrease during the experiment from values between 11 and 18 mg kg−1 to levels below the limit of detection (LOD < 1 mg kg−1). No formation of migrating siloxanes beside the initial amount in the new, unused moulds could be observed. The loss of extractable siloxanes of the used compared with the new moulds was compensated by an uptake of fat and other lipophilic food constituents. The release of volatile organic compounds (VOC) decreased from 0.44% for the new moulds to 0.14% for the longest used ones (about 1700 individual uses; the corresponding summarised baking time was approximately 400 h at 180°C). GC-MS analysis of evaporating volatile compounds showed only cyclic oligomers for the new moulds but exclusively incorporated food components for the heavily used moulds. The physical properties of the silicone moulds remained almost constant during the experiment; no limitations in function due to the repeated thermal stress were observed. Similar results were obtained for baby teats under household conditions of use: a 100 times repeated simulated use in contact with milk followed by subsequent microwave sterilisation did not influence the function or mechanical properties. Because milk is only a weak extracting agent no significant changes in the amount of extractable siloxanes between new and used teats could was seen. Again an uptake of fat was seen and the amount of VOC decreased from 0.26% to 0.17%.

On the Role of Tin Catalysts in Step-Growth Polymerization Reactions

Organotin compounds have been used to catalyze the condensation of various α,ω-dihydroxyl terminated organosiloxane diols. This tin catalyzed reaction extends the length of siloxane chains and it also produces narrow molar mass distribution polyorganodisilanols (Mw/Mn = 1.4). The reaction occurs between a variety of siloxanes that are terminated with hydroxyl groups and it does not depend on the organic side groups connected to silicon for the systems studied here. These systems include silicon atoms bearing dimethyl, methylphenyl or methyl 1,1,1 trifluoropropyl substituents. The organotin catalyst in the reaction facilitates the organosilanol condensation releasing water as a byproduct. However it does not appear to facilitate the opening of siloxane bonds nor the redistribution of siloxane bonds under the conditions employed here. Copolymerization of linear oligomeric dimethylsiloxane diol and linear oligomeric methylphenylsiloxanediol was found to give a relatively equal reactivity of homo polymerization and hetro polymerization in the condensation and randomly alternating segmented block copolymers that were formed. The reaction kinetics of the polymerization was used to experimentally verify the fact that there is a chain length dependence of the reacting silanol end-groups. The molar mass values during the polymerizations were determined using gel permeation chromatography. The chromatography, viscosity and FTIR results demonstrate that the reactivity of the hydroxyl end-groups in the polycondensation reaction decreases upon increasing the chain length of the siloxane. It therefore appears that these tin catalyzed siloxane systems deviate from the widely demonstrated hypothesis of Paul Flory on the “equal reactivity of functional groups” for step-growth polymerizations.

Poly(n‐alkylsilsesquioxane)s: Synthesis, characterization, and modification with poly(dimethylsiloxane)

AbstractSelf‐supported translucent films constituted of poly(n‐octylsilsesquioxane) or poly(n‐dodecylsilsesquioxane) were obtained from the hydrolysis and condensation of n‐octyltriethoxysilane (OTES) or n‐dodecyltriethoxysilane (DTES), respectively. Dense films were obtained in the absence of organic solvents, with dibutyltin diacetate as catalyst. These films exhibited good optical transparency and thermal stability. The incorporation of oligomeric dimethylsiloxane units (DMe,Me) in these materials, derived from silanol‐terminated poly(dimethylsiloxane) (PDMS) or 1,1,3,3‐tetramethyl‐1,3‐diethoxydisiloxane (TMDES), was carried out during the hydrolysis and condensation of OTES and DTES and was confirmed by solid‐state 29Si NMR. Poly(n‐octylsilsesquioxane) showed a glass‐transition temperature at −65 °C, due to the increase in the free volume, promoted by the bulky n‐octyl groups. The differential scanning calorimetric (DSC) curves of the polymer derived from DTES were characterized by first‐order transitions at temperatures ranging from −15.8 to −0.7 °C. Further studies of these networks by low‐temperature XRD evidenced narrowing of the diffraction halos suggesting a partial order–disorder transition for these materials at lower temperatures. Good thermal stability up to 350 °C and the solvent‐free production process make these polymers potential candidates for the development of self‐supported hydrophobic protective coatings. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1220–1229, 2010

2D and 3D DOSY methods for studying mixtures of oligomeric dimethylsiloxanes

Diffusion-ordered spectroscopy (DOSY) is a powerful method for the NMR analysis of many types of mixture without the need for physical separation, and requires only relatively standard spectrometer hardware. The principal requirements for high resolution analysis using DOSY, that the basic NMR spectrum be well-resolved and that it have good signal-to-noise ratio, pose a dilemma where multiple chemically similar species with NMR-active heteronuclei are involved. Generally the 1H spectrum of such a mixture has good sensitivity but relatively poor chemical shift resolution, while for heteronuclei the situation is reversed. The dilemma is illustrated for the case of a mixture of cyclic dimethylsiloxanes, and the results of a range of 1H 2D DOSY, 29Si 2D DOSY and 1H {29Si} 3D DOSY–HMQC experiments are compared. In selecting the most appropriate technique to use for a given sample it is necessary to balance conflicting requirements for speed, resolution and accuracy, and to consider the balance between systematic and random errors. Results are presented for three different concentrations of a mixture of cyclic siloxanes containing between 4 and 20 monomer units. The diffusion coefficients measured show an inverse half power law relationship with molecular mass.

Synthesis and characterization of polyamideimide-branched siloxane and its gas-separation

Polyamideimide-branched siloxanes (PAIBrSs) were synthesized from 4, 4′-(hexafluoroisopropyllidene)diphthalic anhydride (6FDA), p,p′-oxydianiline (ODA), and aminopropyl-terminated oligomeric dimethylsiloxane (ODMS). In this investigation, a mixture of N-methylpyrrolidinone (NMP) and tetrahydrofuran (THF) was used as a cosolvent for the homogeneous mixing of poly(amic acid) (PA) and the ODMS solution. Thionyl chloride (SOCl2) was used for the acylation of PA to activate the reaction between PA and ODMS. FTIR spectra showed an increase in the intensity of characteristic absorption peaks of dimethylsiloxane units with the amount of ODMS reacted. From thermogravimetric analysis (TGA), PAIBrSs showed good thermal stability, but relatively low thermal stability when compared with that of block poly(imide siloxane) (PIBlS) or poly(amideimide siloxane) (PAIBlS). A solubility test showed that dipolar aprotic solvents were relatively good solvents for the polymer. Gas separation was performed by single gas-permeation equipment. The permeabilities of PAIBrS membranes are higher than those of PIBlS membranes and the selectivities of PAIBrS membranes are lower than those of PIBlS membranes. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 965–973, 1999

Studies of the oxidative photoinduced degradation of silicones in the aquatic environment

During irradiation of aqueous solutions of a mixture of oligomeric dimethylsiloxanes with artificial sunlight in the presence of nitrate ions, a reduction in the concentration of the siloxanes was demonstrated. This was verified by atomic absorption spectroscopy and proton nuclear magnetic resonance spectroscopy. In two separate tests, average halflives of 9 and 4 days, respectively, were determined after extraction of the aqueous solutions and analysis by atomic absorption spectroscopy. Silicate was demonstrated to be the final photodegradation product of these water soluble siloxanes. Photometric determination of silicate after 35 days showed nearly complete transformation, based on the original quantity of siloxanes. When an aqueous emulsion of high molecular-weight poly(dimethylsiloxane) was irradiated in the presence of nitrate ions under similar experimental conditions, a level of silicate corresponding to 1.7% of the original quantity of poly(dimethylsiloxane) was found after 5 weeks.

Polymeric and oligomeric dimethylsiloxanes with alcoholic groups and their conversions into polymers with dimethylsiloxane and urethane moieties

AbstractAddition of allyloxytrimethylsilane (2a) or allyl 2,3‐epoxypropyl ether (2c) to dihydrosiloxanes (1a and 1b) leads to the protected organoalkylsiloxanes 3a, 4a, 5a, 3c and 4c, which can be hydrolyzed to the corresponding hydroxyl compounds 3b, 4b, 5b and 4d. Addition of isocyanates 6a—c to these hydroxyl compounds results the model compounds 7a—c and 8a—c. Polymers 10a, b and 11a, b were prepared under similar conditions by addition of diisocyanates 9a, b to the hydroxyl compounds.

Thermodynamic affinity of oligomeric dimethylsiloxane for toluene as revealed by rayleigh light scatter and static sorption

Raleight light scatter has been used to study the general scatter and degree of depolarization of scattered light of solution of oligo-dimethylsiloxane in toluene over the whole concentration range. Static isothermal sorption of toluene vapours on the oligomer at 298 K was investigated. The concentration functions of the differences in the chemical potentials of the components Δμ i and the Gibbs mixing energy Δg m of the oligomer with toluene have been calculated in the whole range of compositions.

Polydimethylsiloxanes with methyl and vinyl silsesquioxane units in the chains

It has been found that oligomeric dimethylsiloxanes with methyl and vinyl silsesquioxane units in the chain prepared with use of organoacetoxysilanes condense at a lower temperature than oligomers based on organochlorosilanes. As the amount of silsesquioxane fragments in the chain increases, the rate at which the specific viscosity of the polymers rises is reduced. It has been found for polymers obtained using organochlorosilanes that the degree of branching occurring in the thermal polycondensation reaction increases as the amount of silsesquioxane fragments in the chain rises. There is a linear relationship between the degree of branching and the specific viscosity of the polymers obtained.

Equilibrium ring concentrations and the statistical conformations of polymer chains I—Oligomeric dimethylsiloxanes

The molar cyclization equilibrium constants Kx of dimethylsiloxanes [(CH3)2SiO]x have been measured in the bulk polymer to within ±5 per cent at 110°C (x=4–40) and 145°C (x=4–30). Values at 110°C are compared with those of Brown and Slusarczuk for an equilibrate containing 0.22 g. ml−1 siloxane in toluene, and it is concluded that K4–K10 increase markedly with solvent dilution whereas K11–K40 remain constant. Using the Jacobson and Stockmayer equilibrium theory of macrocyclization, the dimensions of dimethylsiloxane chains with 40–80 chemical bonds in the bulk polymer at 110°C are deduced. They are found to be similar to those in Brown and Slusarczuk's equilibrate and in two θ-solvents of different cohesive energy density. Dilution effects in dimethylsiloxane systems are contrasted with predictions of the Jacobson-Stockmayer theory, and the experimental molar cyclization equilibrium constants of the smallest siloxane rings are discussed in terms of the statistical properties of the corresponding oligomeric chains using Flory, Crescenzi and Mark's rotational isomeric state model of poly(dimethylsiloxane).