Article

Effects of Functional Groups on Surface Pressure−Area Isotherms of Hydrophilic Silicone Polymers

Authors:
  • ELKAY Chemicals Pvt Ltd.
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Abstract

Organic/inorganic hybrid silicone polymers are increasingly used in cosmetics, inks and paints, and fabric care applications owing to their special Si-O bond characteristics. Because of the presence of organic as well as inorganic groups, they show the properties of both, and the presence of hydrophobic as well as hydrophilic character makes them behave like a hybrid polymer. Though they are widely used, the utilization of hydrophilically modified silicones on a large scale has mainly been empirical due to lack of fundamental knowledge about variation in their properties with systematic change in their structure. The choice of moieties for hydrophilic modification of silicones in most of the earlier works has been nonionic based on ethylene oxide and propylene oxide groups, however, very little is known about their ionic counterparts. The current work focuses on understanding the behavior of functionally grafted silicone polymers with respect to the variation in the hydrophilic part of the grafting chain. Hydrophilically grafted silicone polymers form monolayers at the air-water interface, which are stabilized by interactions of functional groups with water. The present work examined the effects of functional group modifications on the conformational behavior of chains at the interface. It was observed that the shape of the chain depends on the available area at the interface (or surface pressure), and there are conformational changes with an increase in the number of molecules per unit area. While a poly(dimethylsiloxanes) (PDMS) chain may undergo stretched to helix transition as predicted earlier, this may not be the case for hydrophilically grafted chains. On the basis of the shape of the surface pressure-area isotherm and correlation with the scaling theory, a gradation in hydrophilicity of functional groups and hence modified silicone chains at the air-water interface is predicted.

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... Conformational changes arising from the compression of a PDMS monolayer were analyzed in [4,16,30]. Four typical regions can be discerned on the PDMS π-А isotherm with methyl end groups (Fig. 2a) [21,31]. The modified model of conformational changes (see Fig. 2b) is based on investigation performed by means of sum-frequency vibrational spectroscopy. ...
... In region A (Fig. 2a), individual polymer chains from the conformation are separated from each other [4,21,31]. Oxygen atoms are hydrated, and methyl groups are disordered and arranged in a random way [32]. In region B (Fig. 2a), isolated PDMS chains approach each other to form a monolayer, and the methyl groups of PDMS macromolecules are ordered [4,16,21,31,32]. ...
... Oxygen atoms are hydrated, and methyl groups are disordered and arranged in a random way [32]. In region B (Fig. 2a), isolated PDMS chains approach each other to form a monolayer, and the methyl groups of PDMS macromolecules are ordered [4,16,21,31,32]. In region C, a plateau is observed at π-А isotherm at a surface pressure π of 8.7 mN/m, indicating conformational reconstruction or phase transition in a monolayer [21]. ...
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... The authors of [10][11][12] developed a model that describes the behavior of organosilicon surfactants in Langmuir layer under compression, in particular, as a function of a change in the surface pressure (Fig. 1). ...
... Upon further compression (step C), the conformation of the siloxane chain changes and forms, in accordance with some authors, helical structures or a bilayer at the interface (Fig. 2). At step D, deformation of the formed surfactant layer takes place; according to one hypothesis, helices undergo deformation, while according to the other one folded layers are formed [10][11][12]. Figure 3 shows surface pressure and surface potential isotherms for the studied dimeric organosilicon compounds with end carboxyl and amino groups (PDMS(СООН) and PDMS(NH 2 )). Regions A, В, C, and D may be distinguished on the surface pressure isotherms by analogy with the published data [10]. ...
... Figure 3 shows surface pressure and surface potential isotherms for the studied dimeric organosilicon compounds with end carboxyl and amino groups (PDMS(СООН) and PDMS(NH 2 )). Regions A, В, C, and D may be distinguished on the surface pressure isotherms by analogy with the published data [10]. ...
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... The col loid-chemical properties of these surfactants (namely, a low interfacial tension and the presence of liquid crystalline phases) provide the formation of the strong interfacial adsorption layer on the surfaces of mono mer droplets and polymer-monomer particles (PMPs) and ensure their high stability even at a high concentration of the surfactant. The behavior of sur factants at the interface is determined by both the properties of the main chain and the kinds of func tional substituents at silicon atoms [17]. ...
... During compression, the Langmuir films of PDMS molecules transfer from the planar trans zigzag con formation to the maximally dense helical packing [17][18][19][20][21][22][23][24][25][26]. The second step, corresponding to the detachment of hydrophilic end groups from the sur face of water, appears on the π-A isotherms of such oligomers [17,18]. ...
... During compression, the Langmuir films of PDMS molecules transfer from the planar trans zigzag con formation to the maximally dense helical packing [17][18][19][20][21][22][23][24][25][26]. The second step, corresponding to the detachment of hydrophilic end groups from the sur face of water, appears on the π-A isotherms of such oligomers [17,18]. This circumstance may be respon sible for a decrease in interfacial tension and for stabi lization of polymer dispersions at the boundary with the water phase. ...
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... The expected behavior for the osmotic pressure of polymer chains in θ -solvent in 3D is π c 3 , [ 1 ] which is not supported by the data shown in Figure 3 . Moreover, using Equation ( 2) and y = 8 from Figure 3 one fi nds that 4/7 ν = rather than the value expected for 3D chains in good solvent, 3/5 ν = , [ 15,16 ] in agreement with experiments [ 30,31 ] and confi rming the essential correctness of Equation ( 1) . ...
... We fi nd that ν is in good agreement with experiments with large N -poly mer chains. [ 22,30,31 ] The reason why quasi-2D scaling behavior is obtained in these 3D systems regardless of solvent quality can be found in the "blob" argument of de Gennes. [ 1 ] As the separation between the planes in Figure 1 is reduced (increasing concentration), the chains begin to overlap and the characteristic length is no longer R G 3 but instead the size of the blobs, ξ , which scales in 2D once h h* < . ...
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... In the case of ionic silicone surfactants, cationic [17,18] and anionic [19] trisiloxanes have been found to behave as excellent foaming agents [20,21]. There has been some work done on the solution properties of cationic [22], anionic [23] and zwitterionic [21] silicone surfactants. However, very little is known about the emulsifying properties of these ionic compounds. ...
... The approximate molecular weight of all the compounds as measured from viscosity was 5000 and a volatile content below 1%. The details on the synthesis of each polymer has been reported earlier [23]. The general structures of the above mentioned silicone surfactants are illustrated in Fig. 1. ...
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... The expected behavior for the osmotic pressure of polymer chains in θ -solvent in 3D is π c 3 , [ 1 ] which is not supported by the data shown in Figure 3 . Moreover, using Equation ( 2) and y = 8 from Figure 3 one fi nds that 4/7 ν = rather than the value expected for 3D chains in good solvent, 3/5 ν = , [ 15,16 ] in agreement with experiments [ 30,31 ] and confi rming the essential correctness of Equation ( 1) . ...
... We fi nd that ν is in good agreement with experiments with large N -poly mer chains. [ 22,30,31 ] The reason why quasi-2D scaling behavior is obtained in these 3D systems regardless of solvent quality can be found in the "blob" argument of de Gennes. [ 1 ] As the separation between the planes in Figure 1 is reduced (increasing concentration), the chains begin to overlap and the characteristic length is no longer R G 3 but instead the size of the blobs, ξ , which scales in 2D once h h* < . ...
... Moreover, using eq. 2 and y = 8 from Fig. 3 one finds that = 4 7 ⁄ rather than the value expected for 3d chains in good solvent, = 3 5 ⁄ [15,16], in agreement with experiments [30,31] and confirming the essential correctness of eq. 1. 1) and (2), and reference [24]. For all cases the surfacemonomer interaction was set to aw = 120.0, ...
... Notice that eq. 2 becomes c singular for  = 0.5, so the poor solvent limit can never truly be achieved. We find that  is in good agreement with experiments with large Npolymer chains [22,30,31]. The reason why quasi -2d scaling behavior is obtained in these 3d systems regardless of solvent quality can be found in the "blob" argument of de Gennes [1]. ...
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... It is fundamentally important that this was achieved at concentrations an order of magnitude lower than those normally used for hydrocarbon surfactants (0.5 ÷ 1.0 wt.% and 3.0 ÷ 5.0 wt.% per monomer, respectively). It was shown in [10,11] that the ability of organosilicon surfactants to provide stability of latex particles at low concentrations in the synthesis of polymer microspheres, ranging from monomer droplets to polymer particles at full conversion of monomer, is due to unusual conditions of formation of interphase adsorption layers on the surface of the particles, while the strength of former determines the stability of the latter. ...
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... They then adopted more ordered conformations with the more hydrophilic oxygen atoms immersed in the subphase and hydrophobic silicone-methyl groups sticking into the air.While most researchers agree on the chain conformation model of regions I and II, more controversy surrounds regions III and IV. Earlier studies using reflected infrared spectroscopy proposed the helix model, which states that upon further compression from region III to IV, the helices slide on each other, which leads to the second increase in surface pressure149,151 . The helical structures of PDMS chains in regions III and IV are similar to the structures found by X-ray diffraction and NMR of PDMS crystals 152,153 . ...
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... Their distinctive chemistry imparts a range of characteristics, including improved softness, dimensional stability, fabric physical properties, wrinkle recovery, and stretch (Jang & Yeh, 1993). Silicones can also be used to provide hydrophilicity (Mehta, Somasundaran, Maldarelli, & Kulkarni, 2006) or hydrophobicity, static control, lubrication, antimicrobial treatments and anti-slip properties (Abidi, Hequet, & Cabrales, 2009;Gao, Zhu, Guo, & Yang, 2009;Xue, Jia, Zhang, & Ma, 2010). A variety of silicone technologies have different applications in the textile industry (Abidi et al., 2009). ...
... Note that in all cases the expansion of the compressed samples is not accompanied by a hysteresis, which indicates that no stable structures are formed during the compression. According to the literature data, the isotherms for linear PDMS have two distinctive regions [32]. The first one corresponds to the formation of a monolayer with the surface area per unit equal to 20Å 2 ; the second one is usually considered as a formation of a monolayer consisting of helical PDMS macromolecules, and its further collapse [33]. ...
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A new family of zwitterionic organofunctional siloxanes of the general formula Me3SiO(SiMe2O)x-(SiMeR(1)O) ySiMe3 (R(1) = (CH2)3NR(2)2+-(CH2) zSO3-; R(2) = CH3, CH2CH3, CH2CH2OH; x = 0-3, y = 1, 2, z = 3, 4) have been prepared and characterized both structurally and as aqueous surfactants. They are prepared by the quaternization of the precursor amino functional siloxane with either cyclic 1,3-propanesultone or cyclic 1,4-butanesultone. They were structurally characterized by elemental analysis, proton and silicon-29 NMR, and IR spectroscopies. Members of this family reduced the surface tension of water to approximately 21 mN/m at concentration levels of 0.005-0.5 wt %. The species where x = 0 and y = 1 displayed very rapid lowering of the surface tension of water. These silicone surfactants demonstrated adequate shelf life stability as the isolated compound or in aqueous solution between pH 5 and 8. Aqueous instability of some of these solutions could be attributed to hydrolysis and subsequent rearrangement of the Si-O-Si bonds in these molecules, yielding hexamethyldisiloxane as a byproduct.
Article
Relationships of film pressure (F) vs. area (A), surface potential (ΔV) vs. A, and the vertical component of the surface dipole moment (μp) vs. A were studied of two groups of liquid polysiloxanes, (CH3)3Si[OSi-(CH3)(R)]nOSi(CH 3)3, spread on water as adsorbed monolayers. In the first group R is either CH3, C2H5, or n-C4H9 and n is 14 or 12; in the second group R is (CH2)2CF3, and n varies as the 25° viscosities of the polymers range from 72 to 1000 cSt. For any given area in the first group, the three surface-physical properties measured decreased in value as the carbon number increased. The F vs. A isotherms of the second group of polymers are similar to one another but are different from the members of the first group. The electrical properties (ΔV vs. A and μp vs. A) show that the net dipole moment per monomer of the (CH2)2CF3 substituted polysiloxanes is oriented with the negative end away from the water, i.e., in the opposite direction from that of the n-alkyl substituted polymers. It was possible to compute, under a given set of conditions, μp of the terminal CF3 group only, and the values are in good agreement with those found by us in an earlier investigation of progressively fluorinated n-alkanoic carboxylic acids adsorbed closely packed on water. All results are discussed in terms of chain length and fluorine subsitution for hydrogen in the alkyl side chains, tactic configuration, effects of steric hindrances of the substituents, and possible molecular arrangements of the various adsorbed polymer chains. It is concluded that the difference in magnitude and sign of μp per close-packed, well-oriented monomer is highly indicative of a major difference in the biochemical and medical properties of these two classes of poly(organosiloxanes).
Article
Detailed surface pressure measurements have been performed in the dilute regime for Langmuir monolayers of atactic poly(methyl methacrylate) chains spread at the air-water interface. Marked deviations from the ideal gas law give clear evidence for attractive interactions between the two-dimensional polymer chains. The second virial coefficient is negative and scales with the chain molecular weight as A22Γ ∝ M2v when the concentration is measured in units of the number of chains Γ = c/M. For chains with molecular weights between 3250 and 18600, we obtain v = 0.57 ± 0.03 for the Flory exponent describing the single-chain conformation RG ∝ Nv. This new, and independent, derivation of the v exponent in two dimensions is in complete agreement with the value based on the concentration dependence of the surface pressure in the semidilute regime. This is the first time that the molecular weight dependence of the second virial coefficient is clearly established for polymer chains in two dimensions.
Article
A method is developed for the detailed atomistic modeling of well-relaxed amorphous glassy polymers. Atactic polypropylene at -40°C is used as an example. The model system is a cube with periodic boundaries, filled with segments from a single "parent" chain. An initial structure is generated by using a modified Markov process, based on rotational isomeric state theory and incorporating long-range interactions. This structure is then "relaxed" by potential energy minimization, using analytical derivatives. Computing time is kept relatively small by stagewise minimization, employing a technique of "blowing up" the atomic radii. Model estimates of the cohesive energy density and the Hildebrand solubility parameter agree very well with experiment. The conformation of the single chains in the relaxed model system closely resembles that of unperturbed chains. Pair distribution functions and bond direction correlation functions show that the predominant structural features are intramolecular and that long-range orientational order is completely absent.
Article
Diffusion coefficients, radii of gyration, and second virial coefficients of short polystyrene chains have been measured by dynamic light scattering, small-angle neutron scattering, and static light scattering. Deviations from the established exponential laws at high molecular weights are observed in all cases. The hydrodynamic behavior can equally well be described by Kirkwood's diffusion equation, if the free draining term is properly taken into account, and by the Yamakawa-Fujii theory of cylindrical wormlike chains. The enhanced increase of A2 for short chains is caused by the inherent chain stiffness. Satisfactory agreement of the Stockmayer-Yamakawa theory with experiment is obtained if a 45% higher Kuhn length is assumed in toluene at 20°C than in cyclohexane at 34.5°C.
Article
Surface pressure π has been measured as a function of surface concentration c for monolayers of linear and cyclic poly(dimethylsiloxane) (PDMS) of molecular weight 730-14 800, spread on water and tricresyl phosphate at 26.0°C. In the transition region where the surface pressure rises much more rapidly than proportional to the surface concentration, the findings for linear and cyclic PDMS were indistinguishable and independent of molecular weight. The findings in this region could be described as power laws with powers corresponding to scaling predictions for the semidilute region of concentrations (coil overlap accompanied by low overall polymer concentration) on near-θ and fairly good surface solvents, respectively. However the second virial coefficient of the surface pressure appeared to be negative for both liquid supports. These observations, the fact that the transition region occurred at quite high fractional surface coverage, and the instability of films from small oligomers suggest that recent interpretations of behavior in the transition region in terms of predictions for a semidilute surface solution are invalid in this case. The existence of a semidilute region of concentrations for polymer monolayers is uncertain in principle. In addition the comparisons of linear and cyclic polymer above the overlap concentration c* lead to the surprising conclusion that even for three-dimensional semidilute solutions, the ratio c/c* is not a universal reduced concentration.
Article
The phase behavior of long hydrophobic A−B type silicone surfactants, Me3SiO−(Me2SiO)m-2−Me2SiCH2CH2CH2−O−(CH2CH2O)nH (SimC3EOn), in water and water + octamethylcyclotetrasiloxane (D4) was investigated by studying phase behavior and small-angle X-ray scattering. Si25C3EO15.8 forms a reverse micellar cubic phase (I2) in water and water + D4 systems. This cubic phase is highly thermally stable in a surfactant−water binary system. The thermal stability decreases monotonically with addition of silicone oil. Although the solubilization of water in the reverse cubic phase is low, a very large amount of excess water can be incorporated in a so-called reverse cubic phase based concentrated emulsion. The emulsion stability is enhanced upon addition of silicone oil. D4 molecules penetrate into the surfactant palisade layer in the reverse micelles forming the I2 phase and expand the effective cross-sectional area per surfactant, aS (penetration). The continuous penetration of oil destabilizes the I2 phase structure, and therefore the melting temperature of the phase decreases. The incorporation of D4 into the I2 phase in the aqueous mixtures of Si14C3EO7.8, Si25C3EO7.8, Si25C3EO12.2, and Si25C3EO15.8 varies with both the hydrophilic and lipophilic chain lengths of silicone surfactants.
Article
The pseudophase separation model is used to describe pH and concentration effects on dimethyldodecylamine oxide (DDAO) solutions. If the protonated and neutral species are treated as separate surfactants, an amine oxide surfactant can be modeled thermodynamically as a binary mixture, the composition of which is varied by adjusting the solution pH. With the Gibbs-Duhem equation written for the micelle pseudophase, activities of the surfactant species at concentrations greater than the critical micelle concentration (cmc) can be calculated directly from experimental titration curves. It is not necessary to introduce an "apparent" pKa for the surfactant in micellar form.
Article
The formation of Langmuir monolayers from poly(dimethylsiloxane) oligomers (molecular weights of 900-4000) terminated with methyl, hydroxyl, epoxide, carboxyl, and amine groups is described. The isotherms (except for oligomers with molecular weights below 1500) show the characteristic transitions commonly observed for methyl-terminated PDMS. In addition, the functionally-terminated materials show a transition associated with orientation of the PDMS chains normal to the surface to form a close-packed monolayer. For oligomers with molecular weights below 1000 this transition overlaps the standard configurational transitions, and the overall shape of the isotherm is determined by the oligomer molecular weight, the functional group, and the nature of the subphase. Most of the materials have cross-sectional areas at collapse of about 100 Å2/chain, consistent with a structure where the molecules form helices oriented perpendicular to the surface of the subphase. Shorter amine-terminated materials have areas as low as 50-60 Å2/chain, consistent with the formation of extended cis-trans caterpillar structures oriented normal to the subphase surface.
Article
We report the first direct measurements of the molecular weight distribution in transferred condensed monolayers of amphiphilic oligomers determined by time-of-flight secondary ion mass spectrometry (TOFSIMS). Two α,ω-functional oligomers of poly(dimethylsiloxane) are investigated, comprising pentylamine and propylcarboxy end groups. Measurements are performed directly on condensed monolayer films transferred to silver-coated substrates using the Langmuir-Blodgett-Kuhn (LBK) technique. These results are compared to the original distributions measured on submonolayer thin films of the original oligomers prepared by spin coating onto silver-coated substrates and to molecular weight determinations provided by end group titration and size exclusion chromatography analyses. Different families of ions are found for the two different thin film preparation methods. A number of tentative assignments are proposed for these masses, based upon consideration of the effects of the LBK film deposition process and the influence of interactions between the end groups and the substrate. The molecular weight distributions for LBK films of both oligomers is found to be narrower and shifted to higher molecular weights than are those for the corresponding spin-coated films. The changes in molecular weight distribution observed are attributed to dissolution of lower molecular weight species into the aqueous subphase during the LBK film deposition process.
Article
Surface activity at the interface of organic liquids with air has been studied by direct observation of the force-area properties of surface films. The all-Teflon film balance developed for this purpose and the special techniques required are discussed. Liquid substrates studied include n-hexadecane, white mineral oil and tricresyl phosphate. On n-hexadecane and tricresyl phosphate, a linear polymethylsiloxane of high molecular weight was spread from solution to form monolayers having limiting cross-sectional areas which agree well with each other and with those found by other methods. Spreading coefficients of four liquids on white mineral oil could be measured using the piston film technique. From these the liquid/liquid interfacial tensions were calculated. For two of these spreading liquids the interfacial tensions against white mineral oil were determined by the ring method and agreed to 0.1 dyne/cm. with the calculated values. Films of zein could be spread from solution on the surface of tricresyl phosphate. The results suggest that this protein can be spread on the organic liquid substrate without the denaturation that accompanies its spreading on water. Many other compounds were found to be surface active on organic liquids. Among these were other silicones, organic silicates, polyacrylates, polyalkylene ethers and fluorocarbon derivatives. A plausible explanation of the defoaming power of polymethylsiloxanes for organic liquids is offered.
Article
The absolute entropy of the chloride ion in six different concentrations of hydrochloric acid, ammonium chloride, sodium chloride, and potassium chloride at a mean temperature of 12.5°C. has been calculated from data obtained from thermocells, using silver-silver chloride electrodes.
Article
Linear organosiloxanes with various substituents differ in their spreading velocities and with respect to the film structure formed on aqueous substrates. The material investigated allows a distinction between siloxanes in which the molecules placed on the subphase in helical form become oriented on the surface of the water to form “spreading chains” and siloxanes in which this is not the case. It is believed that hydrogen bridging is responsible for the orientation effect. Within the first group, a distinction can be made between polymers which on strong compression are lifted off from the water and can be returned to the helical form, and those in which due to great hydrophilicity of the substituents release of “dry” spreading chains from the water can not be achieved. Methyl-substituted siloxanes, among them especially the dimethyl- and methyl-H-siloxanes, show quick spreading and good orientability. This may explain why these siloxanes play such an important role since the beginning of silicone technology up to the present day as interfacially active substances. But this behavior is not representative for organosiloxanes in general. The structure of the siloxane skeleton seems to have no decisive influence on the spreading phenomena. Branched or crosslinked methylsiloxanes behave similar to the linear ones and form also, by and large, monomolecular films. Hydrate phases, i.e., mixed phases of the spread substance and the subphase, play an important part in the interpretation of the force/area (F/A) isotherms.
The structure changes accompanying phase transitions in poly(diethylsiloxane) (PDES) have been studied by WAXS and SAXS techniques using oriented and isotropic samples. PDES may exist in two low-temperature modifications (the monoclinic α1-form and presumably the “tetragonal” β1-form) and two high-temperature modifications (the monoclinic α2-form and the “tetragonal” β2-form). In linear PDES the crystal - crystal transitions α1–α2 and β1–β2 occur near 214 and 206 K, respectively. At higher temperatures α2 (280 K) and β2 (290 K) forms transform into the mesomorphic phase αm that gradually melts at 280–300 K giving an amorphous phase. According to x-ray and density data, αm phase is also characterized by monoclinic structure slightly different from hexagonal packing.
Article
Polymer monolayers spread at the air/water interface were obtained for: poly(monooctyl itaconate) (PMOI), poly(monodecyl itaconate) (PMDI), poly(monododecyl itaconate) (PMDoI), poly(monobenzyl itaconate) (PMBzI), poly(methyldodecyl itaconate) (PMeDoI) and the alternating copolymer (monooctyl itaconate-alt-maleic anhydride) (MOI-alt-MA). By monolayer compression at constant temperature, the respective Langmuir isotherms for these polymers were obtained. For all polymers the zero-pressure limiting area per repeating unit (ru) Ao, and the collapse pressure πc were determined. At low surface polymer concentrations, the monolayers characterization was carried out according to the surface pressure expressed as a function of the surface concentration. The behavior observed was described by the virial expansion development. At the semidilute region, the surface pressure variation was expressed in terms of the scaling laws as a power function of the surface concentration.
Article
Measurements of the two-dimensional surface pressure π have been made as a function of average area per monomer A at areas too small to support a single monolayer, for films of cyclic and linear poly(dimethylsiloxane) (PDMS) spread on the surface of water at temperatures between 6°C and 31°C. The number-average numbers of monomer units were 10 to 196 for the rings and 10 to 1.2 × 105 for the linear chains. The surface pressures were stable with time for films formed from molecules with more than 20 monomer units. For these films, two plateaux of surface pressure linked by a rounded step were observed for both rings and chains, and the step occurred at the same average area per monomer. However, although the levels of the plateaux were the same for films formed from all linear species, increases in levels of the plateaux with decreasing number of monomer units were observed for cyclic species. The temperature coefficient of the surface pressure was negative in all instances, suggesting that surface entropy is gained upon compression. The unstable films formed from the linear and cyclic decamers were also studied. The former displayed a step in the plateau region, the latter did not appear to. The present findings suggest that cyclic and linear PDMS with more than 20 monomer units collapse by a common mechanism. If the long-standing hypothesis that PDMS collapses by adopting a helical configuration is correct, this implies that rings with as few as 20 repeat units on the average coil into helices.
Article
Surface pressure (π) of monolayers of two polyethers, i.e., poly(ethylene oxide) and poly(tetrahydrofuran) at the air-water interface have been measured as a function of surface polymer concentration (Γ) at 22°C using the surface balance with the sensitivity of 0.03 dyn cm−1. Poly(ethylene oxide) and poly(tetrahydrofuran) with sharp molecular weight distributions were used. Both polyether monolayers showed the condensed-type surface pressure-area isotherm. At the lower polymer concentration the surface pressure was interpreted by the virial expansion form and the number average molecular weight can be determined by extrapolation of π/ΓRT to Γ = 0, showing very good agreement with the weight average molecular weight. It was found that the air-water interface at 22°C corresponds to a good solvent condition for both polyethers. At the intermediate polymer concentration the exponent for the concentration dependence of surface pressure was in excellent agreement with that predicted from the scaling concepts.
Article
A homologous series of polydimethylsiloxanes were spread as insoluble monomolecular films on water and on a number of organic liquids. The organic liquids studied as substrates were tricresyl phosphate, propylene carbonate, diethylphthalate, bis(2-ethylhexyl)adipate, and hexadecane. Monolayer stability and insolubility were found to vary with the molecular weight of the polydimethylsiloxane and the polarity of the substrate liquid. Stable, reversible monomolecular films were formed on all substrates by the higher molecular weight polymers (molecular weight > 6,000); however, film stability decreased with decreasing polymer molecular weight and decreasing substrate polarity.The orientations of the polydimethylsiloxane films at the organic liquid/air interfaces, as inferred from these studies, appeared to be functions of the surface composition of the substrate liquids. On tricresyl phosphate and propylene carbonate the high molecular weight polymers adsorbed in a fully extended configuration at large areas/molecule, analogous to their behavior on water. Upon compression the film pressure vs. area/molecule curves suggested they assumed a helical configuration. On the less polar substrates the higher molecular weight polydimethylsiloxanes apparently remained in the helical configuration regardless of the state of compression. Owing to the instability and/or solubility of the lower molecular weight polydimethylsiloxane monolayers on the less polar substrates, no conclusions were made regarding changes in molecular orientation as a function of film pressure.
Article
Silicone surfactants are becoming increasingly important in the pharmaceutical and cosmetic industry, because of their versatility, low cost, and technological advantages. The present study was designed to measure the critical micellar concentration of three non-ionic silicone surfactants, one water-soluble and two lipid-soluble. We measured surface tension with a technique based on drop geometry. Solubility and dispersibility in water were tested in the two lipophilic surfactants with visible and UV light spectrophotometry. The data obtained with all techniques showed a characteristic behavior of lipophilic silicone surfactants, which did not entirely conform to the definition of critical micellar concentration.
Silicone Oil Systems
  • Surfactants
Surfactants/Silicone Oil Systems. Langmuir 2001, 17, 5169- 5175.
Concentration dependence of surface pressure of polyether monolayers at the air-water interface Exact tricritical exponents for polymers at the theta point in two dimensions
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Hydrodynamic and thermodynamic behavior of short chain polystyrene in toluene and cyclohexane at 34.5 °C Figure 6. Plot of ln π vs ln Γ in the semidilute region of the surface pressure-area isotherm to obtain the characteristic critical exponent of excluded volume. π ) Γ dν
  • K Huber
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Huber, K.; Bantle, S.; Lutz, P.; Burchard, W. Hydrodynamic and thermodynamic behavior of short chain polystyrene in toluene and cyclohexane at 34.5 °C. Macromolecules 1985, 18, 1461-1467. Figure 6. Plot of ln π vs ln Γ in the semidilute region of the surface pressure-area isotherm to obtain the characteristic critical exponent of excluded volume. π ) Γ dν/(dν -1)
X-ray investigation of the structure of silicone rubber
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Damaschun, G. X-ray investigation of the structure of silicone rubber.
Organosilicone having a carboxyl functional group thereon
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Synthesis and characterization of zwitterionic silicone sulfobetaine surfactants) Banks, W. H. Surface films of polydimethylsiloxanes on organic liquid substrates
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Owen, M. J. Synthesis and characterization of zwitterionic silicone sulfobetaine surfactants. Langmuir 1990, 6, 385-391. (11) Banks, W. H. Surface films of polydimethylsiloxanes on organic liquid substrates. Nature 1954, 174, 365-366.
Exact tricritical exponents for polymers at the theta point in two dimensions Hydrodynamic and thermodynamic behavior of short chain polystyrene in toluene and cyclohexane at 34.5 °C
  • B Duplantier
  • H Saleur
  • K Huber
  • S Bantle
  • P Lutz
  • W Burchard
(33) Duplantier, B.; Saleur, H. Exact tricritical exponents for polymers at the theta point in two dimensions. Phys. ReV. Lett. 1987, 59 (5), 539-542. (34) Huber, K.; Bantle, S.; Lutz, P.; Burchard, W. Hydrodynamic and thermodynamic behavior of short chain polystyrene in toluene and cyclohexane at 34.5 °C. Macromolecules 1985, 18, 1461-1467.