Random coil dimensions of poly(diethyl siloxane)

A sample of high molecular weight poly(diethyl siloxane), [Si(C₂H₅)₂O]_x, (PDES) was separated into a series of fractions by means of liquid-liquid precipitations. Four of the fractions were studied by osmometry and viscometry in toluene at 25°C. The resulting values of the number-average molecular weight, second virial coefficient, and intrinsic viscosity indicate that the unperturbed dimensions of PDES are essentially the same as those of poly(dimethyl siloxane) (PDMS) of the same chain length. Thus, differences in intramolecular interactions between these two chains have little effect on their unperturbed dimensions. Most pertinent in this regard are interactions between the side chains, which are favourable in trans conformations in both PDES and PDMS. These favourable interactions in trans conformations would be enhanced in the case of the ethyl groups in PDES, but this effect is apparently offset by a comparable increase in favourable interactions between ethyl groups and oxygen atoms in the competing gauche conformations.     

Influence of chain microstructure on the conformational behavior of ethylene-1-olefin copolymers. Impact of the comonomeric mole content and the catalytic inversion ratio

The rotational isomeric state model was employed to provide a better understanding of the role of chain microstructure on the conformational behavior of homogeneous ethylene-1-olefin copolymers. The chain microstructure was assembled in accordance with the copolymerization theory using a set of conditional probabilities in direct relation to the reactivity ratios and the feed compositions of the comonomers. The catalytic inversion influence on the tacticity of the polymeric microstructure was also taken into account by considering the replication probability during the Monte Carlo simulation. Statistical weight factors of the rotational isomeric states were evaluated using molecular dynamics runs of the various homopolymers according to the earlier work of Mattice et al. Probability distribution surfaces constructed by the integration of the molecular dynamics trajectories of sufficient length to sample all of the conformational space indicated the increase of the probability of g^±t joint states at the expense of g^±g^± pairs with the increase in the side chain length of the 1-olefin comonomers. It was also indicated that this behavior had a maximum around poly(1-butene)/poly(1-hexene) with an apparent reversal in case of poly(1-octene) due to the side chain crowding, which forces the chain to favor more of the g^±g^± joint states. The characteristic ratios calculated for the copolymers on the basis of the rotational isomeric state model also indicated the increased extension of the polymer backbone with the increase in the side chain length. The lower characteristic ratio calculated for the octene polymers may, in fact, explain the experimental observation that poly(1-octene) has a lower melting point than those of other poly(1-olefin)s of shorter side chains. A complete account of the role of tacticity on the characteristic ratio and the radial distribution function is also given.     

Thermally initiated cationic polymerization and properties of epoxy siloxane

Difunctional epoxy siloxane monomers containing disiloxane, trisiloxane, and tetrasiloxane were prepared by hydrosilylation of an α,ω‐difunctional Si? H‐terminated siloxane with a vinyl‐functional epoxide. Cationic polymerization of these monomers using 3‐methyl‐2‐butenyltetramethylenesulfonium hexafluoroantimonate and their reactivities were examined. The reactivity order was disiloxane > trisiloxane > tetrasiloxane. Thermal discoloration of these polymers increased with catalyst concentration and also with the length of dimethyl siloxane. UV discoloration was also accelerated by catalyst. From the thermo gravimetric analysis, it was found that the thermal stabilities of polymers increased with increasing the length of dimethyl siloxane chain. Mechanical properties of polymers were also tested by thermal mechanical analysis and dynamic mechanical analysis, and it was found that the flexibility of polymers was increased with increasing siloxane chain length. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2010–2019, 2006     

Fast and eco‐friendly synthesis of novel soluble thermally stable poly(amide‐imide)s modified with siloxane linkage with reduced dielectric constant under microwave irradiation in TBAB,TBPB and MeBuImCl via isocyanate method

A new series of poly(amide‐imide)s (PAI) modified with a siloxane linkage was synthesized under microwave radiation in ionic liquids and organic salts via the isocyanate method. The polymerization reactions of a novel siloxanic diacid monomer with 4,4′‐methylene‐bis(4‐phenylisocyanate) MDI were studied in ammonium, phosphonium, and imidazolium‐type organic salts. These poly(amide‐imide‐siloxane)s (PAI‐Si)s were obtained with high yields and good inherent viscosities ranging from 0.30 to 0.55 dL/g. The normally high softening temperatures and poor solubility of PAIs in organic solvents were improved via the incorporation of the flexible siloxane segments into the polymer backbone. The PAI‐Sis showed glass transition temperatures around 100°C and their 10% mass loss was about 300°C. They have a char yield in the range of 30–40% at 800°C. Calculated limiting oxygen index values of the polymers were about 30; therefore, they can be considered as self‐extinguishing. The dielectric constants of these silane‐containing PAIs (2.5) are lower than common siloxane‐free polyimides (～ 3). Their good thermal stability, enhanced solubility, and low dielectric constants suggest they may function as electrical insulators. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Equilibrium ring concentrations and the statistical conformations of polymer chains: 14. Calculation of the concentrations of medium sized cyclics in poly(dimethyl siloxane) equilibrates

Theoretical molar cyclization equilibrium constants K_x for dimethyl siloxane cyclics [(CH₃)₂SiO]_x in undiluted poly(dimethyl siloxane) equilibrates are calculated for values of x in the range 7 < x < 13. The Jacobson and Stockmayer theory is used for the calculations and K_x values are computed by finding the statistically weighted fraction of the 3^2x?3 conformations [defined by the Flory, Crescenzi and Mark (FCM) rotational isomeric state model of poly(dimethyl siloxane)] that have terminal atoms in close proximity for ring formation. The FCM model gives theoretical K_x values in good agreement with experiment for x = 8, 9, 10 but yields a K₁₁ value that is nine times greater than that found experimentally. This result is discussed in terms of the molecular structure of dimethyl siloxane chains. The effect on the calculated K_x values of modifying the FCM model by assigning a smaller value to the SiOSi bond angle is explored.     

Adhesion of PDMS Elastomers to Functional Substrates

The JKR technique was applied to study the influence of interfacial reactions on the adhesion between functional elastomer gels and functional solid substrates. The gelation chemistry of poly(dimethyl siloxane) (PDMS) gels cured by hydrosilylation reactions was purposely adjusted to produce an excess of either silane or vinyl functionality. Hemispherical lenses of these materials were then contacted under load with a variety of functionalized solid substrates: poly(styrene-b-butadiene) copolymers with vinyl functionality, vinyl-terminated trimethoxysilane self-assembled monolayers, and α,ω-functional PDMS brushes terminated with either monomethoxy or hydroxyl groups. To rule out chain interpenetration effects, the molecular weights were kept below the entanglement molecular weight or immiscible polymers were employed on opposite sides of the interface. Significant adhesion enhancement was observed for most systems, indicating that a variety of different interfacial reactions can occur across the interface between PDMS elastomers and solid polymeric substrates. The overall nature of the adhesion enhancement found is consistent with the predictions of the Lake–Thomas theory for failure of elastomers, increasing linearly with the length and areal density of covalent linker chains that span the interface.     

Packing mode and conformational transition of alkyl side chains in N-alkylated poly(p-benzamide) comb-like polymer

A series of rigid-rod poly(p-benzamide) polymers (PBA) with different length of flexible alkyl side chains, denoted as PBA(n)Cs, have been prepared by N-alkylation method. The alkyl side chain lengths varied from decyl (n=10) to octadecyl (n=18), with an interval of two carbon atoms. The packing mode and conformational transition of the alkyl side chains of the prepared PBA derivatives were characterized by wide-angle X-ray diffraction (WAXD), differential scanning calorimertry (DSC), and Fourier transform infrared spectroscopy (FTIR). WAXD results revealed that the derivatives form layered structure in which the distance between the backbones depends on the length of alkyl side chains. DSC studies indicated that melting transition temperature of PBA(n)Cs increases accordingly with increasing the alkyl chain length of the substituents. Meanwhile, DSC results proved that as the carbon atom number of the side group exceeds 14, alkyl side chains in PBA(n)Cs tend to crystallize FTIR spectroscopic investigation showed that the all-trans zigzag conformation is the most stable state for the alkyl side chain in PBA18C comb-like polymer. Temperature variation caused the reversible transition between trans and gauche conformational states of the side octadecyl group, and in turn made the molecular chain packing mode of PBA18C comb-like polymer undergo an reversible transformation from ordered packing to disordered packing.     

Preparation of siloxane-like films by glow discharge polymerization

Glow discharge polymerizations of tetramethoxysilane (TMOS), tetramethylsilane/oxygen mixture (½ molar ratio) (TMS/O₂), hexamethyldisiloxane (HMDSO), and tetramethyldisiloxane (TMDSO) were carried out for the preparation of thin, polymeric films with siloxane structures. The chemical composition of the formed polymers was examined by elemental analysis, infrared spectroscopy, and electron spectroscopy for chemical analysis (ESCA) in connection with polymerization conditions, especially, a level of the radiofrequency (rf) input power per mass of the monomers (W/FM value). The polymers prepared from HMDSO at fairly low W/FM values resembled in chemical composition the conventionally polymerized polydimethylsiloxane. The surface properties of the formed polymers also were evaluated by the measurement of surface energy.     

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 (M_w/M_n = 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.     

Quantification of silane grafting efficacy,weak IR vibration bands and percentage crystallinity in post e-beam irradiated UHMWPE

Vinyl-tri-methoxy silane (VTMS) and vinyl-tri-ethoxy silane (VTES) were grafted onto ultra-high molecular weight polyethylene (UHMWPE) by irradiating the UHMWPE/silane hybrids with e-beam. The samples were irradiated under high moisture contents for total dose values of 30, 65 and 100 kGy, respectively. The synergistic effect of silane and irradiation on the grafting efficacy, concentration of weak bonds like trans-vinylene (–CH=CH–) and vinyl (–CH=CH₂) and percentage values of crystallinity were studied using FTIR spectroscopy. For the estimation of grafting reactions efficiency, absorption due to characteristic infrared absorption bands of –Si–CH– in the region ~800 cm^?1 was monitored and found that grafting efficacy of VTMS on UHMWPE was higher as compared to VTES and increased with irradiation. The relative amounts of grafting extension (R) for 100 kGy irradiated UHMWPE/VTMS and UHMWPE/VTES hybrids were found to increase 20 and 15 %, respectively. The concentration of trans-vinylene in UHMWPE was found to increase from 0.015 to 0.035 mmol/l due to synergistic effects of silane and irradiation. Moreover, crystallinity of UHWMPE was found to decrease from 65 to 55 % due to the abovementioned synergistic effects which was also confirmed with DSC tests. Furthermore, oxidation index values were measured to confirm the efficacy of silane as free radical quencher via silane grafting extension reactions.     

Substituted poly(silylenemethylene)s with short range interactions that induce a preference for the same local conformation in unperturbed atactic, isotactic, and syndiotactic chains

A rotational isomeric state model has been developed for the poly(silylenemethylenes) with repeating sequence [Si(CH₃)R-CH₂]_x, R=-O(CH₂)_NOC₆H₄C₆H₅. The model incorporates all first- and second-order interactions, as well as higher order interactions that are mandatory in some of the conformations. Chains with all possible stereochemical sequences prefer a local conformation that is a run of trans states at the C-Si bonds in the backbone. This preference arises from an attractive second-order interaction of the first methylene group in the side-chain bonded to chain atom i with the silicon atoms indexed i±2. Unperturbed chains have larger dimensions than the simpler chain in which R is merely a methyl group. The temperature coefficients of the unperturbed dimensions are large and negative. The preference of unperturbed atactic, isotactic, and syndiotactic chains for the same local conformation may contribute to the facile formation of smectic phases by the presumably atactic chain, as reported by Park et al. [Macromolecules 35 (2002) 2776].     

Mechanical,surface, and thermal properties of polyamideimide–polydimethylsiloxane nanocomposites fabricated by sol–gel process

Polyamideimide (PAI)–epoxysilane (coupling agent) composites were reacted with oligomeric polydimethylsiloxane (PDMS), a condensation product of difunctional silane, by using the sol–gel process, and were then dried into films. After this procedure, the surface, mechanical, and thermal properties were measured. The study showed that PDMS existed in the PAI matrix by the use of FTIR. With respect to mechanical properties, the maximum elongation and toughness were increased in the PAI with silane groups, although the maximum tensile strength was slightly decreased. In this experiment, PAI–30 wt % epoxysilane composite had the best mechanical properties. The intensive dispersion of the silane groups on the surface of PAI was confirmed through XPS measurement. As a function of the siloxane contents, the TGA curve shows less thermal stability in terms of their initial weight loss. However, in an oxygen atmosphere at about 700°C, the series of PAI–siloxane composites indicated a significant increase in char concentration. In the end, PAI with a relevant amount of silane groups was improved in both toughness and surface properties. This experiment showed that PDMS added to PAI had better properties than those of classical materials. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1774–1783, 2004     

An application of Monte Carlo method to simulate disorders in polymer crystals

The Monte Carlo method is applied to polymer crystals of idealized linear chain molecules of 30 carbon atoms, and the unharmonic, large-amplitude, oscillations and the subsequent conformational disorders of the chains are investigated. A crystalline field that confines the chain is treated by the molecular field approximation, and assumed to be cylindrical in this work. A production type simulation is adopted taking into account rigorous statistical weights for each sample conformation. Both the rotational isomeric model and the continuous rotation model of chain conformation are considered. By averaging over 10⁴–8 × 10⁴ chains, mean-square end-to-end distance, fractions of gauche and trans states and a detailed distribution of internal rotation angle are obtained. The effects of temperature and pressure on the conformation of the chain in the crystals are also simulated.     

Atomistic molecular dynamics simulations of gas diffusion and solubility in rubbery amorphous hydrocarbon polymers

Diffusion coefficients D of H₂, He, O₂, N₂, and CO₂ in different rubbery amorphous polymeric matrices were estimated by atomistic molecular dynamics simulations at 298 K using the Einstein relationship, and compared with the relevant experimental values, where available. The simulated diffusion coefficients D of all the gases in all polymers considered almost regularly decreased with increasing molecular gas volumes and increasing polymer glass transition temperature. Further, solubility coefficients and heats of solution were obtained for all gases from Grand Canonical Monte Carlo simulations, which were also used to calculate sorption isotherms. In general, there is a good agreement between experimental and simulated values of diffusion and solubility coefficients for all gases considered.     

Effect of silane structure on the properties of silanized multiwalled carbon nanotube-epoxy nanocomposites

Multiwalled carbon nanotubes (MWCNTs) were functionalized with aminosilanes via an aqueous deposition route. The size and morphology of siloxane oligomers grafted to the MWCNTs was tuned by varying the silane functionality and concentration and their effect on the properties of a filled epoxy system was investigated. The siloxane structure was found to profoundly affect the thermo-mechanical behavior of composites reinforced with the silanized MWCNTs. Well-defined siloxane brushes increased the epoxy T_g by up to 19 °C and significantly altered the network relaxation dynamics, while irregular, siloxane networks grafted to the MWCNTs had little effect. The addition of both types of silanized MWCNTs elicited improvements in the strength of the nanocomposites, but only the well-defined siloxane brushes engendered dramatic improvements in toughness. Because the silanization reaction is simple, rapid, and performed under aqueous conditions, it is also an industrially attractive functionalization route.     

Surface Activities, Foam Properties, HLB, and Krafft Point of Some n-Alkanesulfonates (C14–C18) with Different Isomeric Distributions

A homologue series of sodium secondary n-alkanesulfonates (C₁₄, C₁₆ and C₁₈) were obtained by photosulfochlorination process with two different reaction conditions. Different length chains with different isomeric distributions of n-alkanesulfonates are expected to present variations in physicochemical properties. In this investigation, the relationships between their isomeric distribution and their chain length and micellar behaviors were thoroughly explored. Their CMC at different temperatures were determined using specific conductivity and surface tension measurements. Through surface tension isotherms, the surface activities (??_CMC) were obtained. The surface absorption amounts (??_max) and the molecular areas (A _min) were calculated using Gibb??s equation. As expected, these surfactants exhibit good surface properties. It was shown that the CMC values increase with increasing the percentage of secondary isomers, with a surface tension decrease. It was also shown that the CMC values decrease with increasing chain length. The HLB values were calculated for each surfactant and the results obtained suggest that they are O/W emulsifiers. The foam properties of synthesized surfactants were evaluated and compared to those obtained for commercial samples. It was shown that the foamability is influenced both by the length of the hydrophobic moiety and the percentage of secondary isomers. It can be easily concluded that the C₁₄ sulfonates show the best foaming properties independently of their isomeric distribution. The Krafft point values obtained indicate that the micellization and the surfactant solubility mainly depend on the proportion of secondary isomers and the length of hydrophobic moiety.     

Stereoregularity in poly[methyl(3,3,3-trifluoropropyl)siloxane]

Anionic ring-opening polymerization of the cis and trans isomers of 1,3,5-trimethyl-1,3,5-tris(3′,3′,3′-trifluoropropyl)cyclotrisiloxane (cis- and trans-F₃) and their mixtures was conducted under conditions that suppressed siloxane bond redistribution in order to preserve stereoregularity formed in the resulting poly[methyl(3,3,3-trifluoropropyl)siloxane] (PMTFPS). Proton decoupled ¹⁹F NMR of the resulting polymers was interpreted by assuming that the original stereoconfiguration of the monomer was preserved and that the monomer inserted with a roughly equal probability of forming meso or racemic configurations relative to the configuration of the reacting chain end. Polymers from cis-F₃ were solid and crystalline at room temperature while the more typical polymers from mixtures of isomers containing greater than 50% trans-F₃ were liquid and amorphous.     

N.m.r. spectroscopy of polyesters from bridged bicyclic lactones

The ¹³C n.m.r. spectra of some polyesters produced by anionic ring-opening polymerization of 2-oxabicyclo[2,2,2] octan-3-one are reported and discussed. From a comparison with the spectrum of the monomer and by hydrolysis of the polymer, it was possible to assign to the polymer, obtained using n-butyllithium as a catalyst, a microstructure in which the ester groups on the 1 and 4 positions of the cyclohexane unit are cis to each other. The polymers obtained both by sodium tert-butoxide and sodium-potassium alloy as initiators, showed spectral differences associated with the presence of another isomeric structural unit. Decreasing of the relaxation times (T₁) of cis unit by increasing the concentration of this new unit was reasonably explained by assuming for this latter a trans structure linked with the cis unit to form a stereocopolymer.     

The use of UNIFAC for the estimation of adhesion enhancement between polymers and mineral surfaces treated with silane coupling agents

The group contribution method of UNIFAC is used to investigate the influence on adhesion of thermodynamic compatibility between the filler surface and the polymeric matrix in filled polymeric composites. Compatibility is enhanced between polymers and mineral surfaces through the use of silane coupling agents of varying chemistry. In this study, glass beads were treated with ten different organofunctional silanes intended to induce differences in interfacial strength. Interfacial strength measurements were obtained from tests in which single, silane-treated glass beads were embedded in rectangular poly(vinyl butyral) specimens subjected to uni-axial stress until interfacial failure occurred at one of the poles of the sphere. The UNIFAC method was used to estimate the Gibbs free energies of mixing using the chemical structure of the polymer repeat unit and each of the silane organofunctional groups, and these values were correlated with the measured interfacial strengths. The results indicate that enhanced interfacial strength corresponds to systems with more favorable thermodynamic mixing.     

ATOMISTIC MOLECULAR DYNAMICS SIMULATIONS OF GAS DIFFUSION AND SOLUBILITY IN RUBBERY AMORPHOUS HYDROCARBON POLYMERS

Diffusion coefficients D of H 2 , He, O 2 , N 2 , and CO 2 in different rubbery amorphous polymeric matrices were estimated by atomistic molecular dynamics simulations at 298 K using the Einstein relationship, and compared with the relevant experimental values, where available. The simulated diffusion coefficients D of all the gases in all polymers considered almost regularly decreased with increasing molecular gas volumes and increasing polymer glass transition temperature. Further, solubility coefficients and heats of solution were obtained for all gases from Grand Canonical Monte Carlo simulations, which were also used to calculate sorption isotherms. In general, there is a good agreement between experimental and simulated values of diffusion and solubility coefficients for all gases considered.