Synthesis and properties of sulfonated poly(siloxane imide)s bearing dimethyl siloxane oligomers for fuel cell applications

A series of sulfonated poly(siloxane imide)s (SPSIs) were synthesized from 4,4′‐ketone dinaphthalene 1,1′,8,8′‐tetracarboxylic dianhydride (KDNTDA), a dimethyl siloxane oligomer‐based diamine, and a sulfonated diamine. The reduced viscosities ranged from 1.0 to 3.5 dL/g at 35°C in the triethylamine (TEA) salt form. The SPSIs showed anisotropic membrane swelling with larger swelling in thickness than in plane. They displayed reasonably high proton conductivity, thermal stability and good mechanical properties. The KDNTDA‐based SPSIs showed good solubility in common aprotic solvents not only in TEA salt form but also in proton form. The ¹H‐NMR results indicated that the molar content of the dimethyl siloxane oligomer in the SPSIs was 50–80% of that in the feed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Anomalous dielectric relaxation in poly(dimethyl siloxane) polymers

The dielectric relaxation in poly(dimethylsiloxane) of viscosity 10⁵ CSt at around the melting point is found to deviate from the existing theory. Although the position of maximum loss is insensitive to changes in temperature in the range ?40°C to ?54°C, the magnitude of the loss changes by about 50%.     

Electromagnetic wave absorption properties of graphene modified with carbon nanotube/poly(dimethyl siloxane) composites

By using a catalytic growth procedure, carbon nanotubes (CNTs) are in situ formed on reduced graphene oxide (RGO) sheet at 600 °C. CNTs growing on RGO planes through covalent C–C bond possess lower interfacial contact electrical resistance. As a hybrid structure, the CNTs/graphene (CNT/G) are well dispersed into poly (dimethyl siloxane). The hybrid combining electrically lossy CNTs and RGO, which disperses in electrically insulating matrix, constructs an electromagnetic wave (EM) absorbing material with ternary hierarchical architecture. The interfacial polarization in heterogeneous interface plays an important role in absorbing EM power. When the filler loading is 5 wt.% and thickness of absorber is 2.75 mm, the minimum value of reflection coefficient and the corresponding frequency are −55 dB and 10.1 GHz, and the effective absorption bandwidth reaches 3.5 GHz. Therefore, combining the CNTs and graphene sheet into three-dimensional structures produces CNT/G hybrids that can be considered as an effective route to design light weight and high-performance EM absorbing material, while the effective EM absorption frequency can be designed.     

Molecular motions in poly(dimethyl siloxane) oligomers and polymers

The glass transition temperatures of fifteen samples of poly(dimethyl siloxane) polymers and oligomers have been measured by differential scanning calorimetry. Polymers with M_n>2400 exhibit an asymptotic value of T_g(∞)=148K, but for shorter chains the T_g is lower. The thermomechanical behaviour has also been examined using a torsional braid analyser and this has served to confirm the chain length dependence of T_g. A sub-glass transition, located in the temperature range 80–120K for short chain samples, has been attributed to methyl group rotation.     

Synthesis and characterization of poly(dimethyl siloxane) containing poly(vinyl pyrrolidinone) block copolymers

ABA‐type block copolymers containing segments of poly(dimethyl siloxane) and poly(vinyl pyrrolidinone) were synthesized. Dihydroxyl‐terminated poly(dimethyl siloxane) was reacted with isophorone diisocyanate and then with t‐butyl hydroperoxide to obtain macroinitiators having siloxane units. The peroxidic diradical macroinitiators were used to polymerize vinyl pyrrolidinone monomer to synthesize ABA‐type block copolymers. By use of physicochemical methods, the structure was confirmed, and its characterization was accomplished. Mechanical and thermal characterizations of copolymers were made by stress–strain tests and differential scanning calorimetric measurements. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1915–1922, 1999     

Thermodynamics of polymer solutions,especially poly(dimethyl siloxane) solutions

An improved non-random free-volume theory of polymer solutions is presented. It is based on Flory's Equation-of-State theory which is modified to account for differences in the size of core volumes of segments in pure liquids and in solution. In addition, it is corrected for non-randomness through Guggenheim's Quasi-Chemical approach. The theory is tested against experimental results on heats of mixing, volumes of mixing, and χ interaction parameters of poly (dimethyl siloxane), polyisobutylene and natural rubber solutions with gratifying results. Relations with existing similar theories are discussed.     

Morphology and mechanical properties of poly(dimethyl siloxane-b-styrene-b-dimethyl siloxane) block copolymers

The films of poly(dimethyl siloxane-b-styrene-b-dimethyl siloxane) block copolymers cast from different solvents showed significant changes in both the phase morphology and the tensile behaviour. Methyl ethyl ketone and tetrahydrofuran gave hard films and appear to have a continuous polystyrene phase. Conversely cyclohexane, a good solvent for polydimethyl siloxane segment gave softer more elastic films. Intrinsic viscosity data of block copolymers of varying siloxane content showed highest value in toluene and least in cyclohexane which is a theta solvent for polystyrene segment. The tensile properties are also influenced by thermal ageing of films at 100 and 150°C.     

Electrospinning of poly(dimethyl siloxane) by sol–gel method

Crosslinked poly(dimethyl siloxane) (PDMS) fibers were fabricated by electrospinning in combination with a sol–gel process followed by heat treatment. Before and after heat treatment, the changes in the chemical and thermal properties of the electrospun PDMS fibers were examined by differential scanning calorimetry (DSC), equilibrium swelling tests, and contact angle measurements. There was no significant change in morphology and average diameter of the as‐spun PDMS fibers after heat treatment. The tensile properties of the as‐spun PDMS fibers mat increased with increasing heat treatment temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Surface modification of poly(vinyl chloride) with poly(methyl methacrylate)/poly(dimethyl siloxane) graft copolymers

X-ray photoelectron spectroscopy is used to study the surface segregation of siloxane in dilute blends of poly(methyl methacrylate)/poly(dimethyl siloxane) graft copolymers in poly(vinyl chloride)(PVC). The graft copolymers are found to be extremely efficient surface modifiers, which form, when added in amounts of 0.5% or more, a continuous siloxane overlayer on the surface of PVC. © 1995 John Wiley & Sons, Inc.     

Compounding of poly(dimethyl siloxane) to reduce tack in natural rubber

Tack in natural rubber latex was reduced by compounding poly(dimethyl siloxane) (PDMS) emulsion in concentrated latex. Sheet and dipped film surfaces were examined with Fourier transform infrared spectroscopy using attenuated total reflection (FTIR–ATR) and by contact angle measurements. Autohesive tack and tensile properties were also determined. For both sheet and dipped film, FTIR–ATR showed that the PDMS concentration was higher at the glass surface than at the air surface. The contact angle of ethylene glycol on the rubber decreased with increasing PDMS content. Autohesive tack for sheet and dipped film also decreased with increasing PDMS amount; however, annealing for 1 week at 70°C in air did cause tack to rise in the sheets. The rubber surface could be made nonadhesive by addition of sufficient PDMS. PDMS caused a decrease in tensile strength for the sheet, especially after annealing; however, PDMS did not cause a substantial decrease in percentage elongation for the sheets, except at relatively high PDMS contents. The tensile strength and percentage elongation for dipped film was not affected by PDMS over the much more limited PDMS concentration range studied. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 519–526, 2001     

Formation of bubbles during ultrasonic treatment of cured poly(dimethyl siloxane)

A static ultrasonic treatment device was used to investigate the effect of ultrasound on degradation of unfilled poly(dimethyl siloxane) (PDMS) vulcanizate in the absence of shearing effect. The effects of pressure, ultrasound intensity and thickness of the disks upon ultrasonic treatment were investigated. The power consumption was measured as a function of sample thickness, ultrasonic amplitude, and applied pressure. The dynamics of bubble (nucleation, growth and coalescence) was also studied with respect to applied pressure and amplitude. An increase in thickness of the sample was observed during ultrasonic treatment. The unique correlation between gel fraction and crosslink density obtained in the present static experiments was compared with those of continuous devulcanization studied earlier.     

Synthesis and properties of polyurethane modified with aminoethylaminopropyl poly(dimethyl siloxane)

A series of polyurethanes with different siloxane contents were synthesized, which were based on 4,4′‐methylene diphenyl diisocyanate (MDI), poly(tetramethylene oxide) (PTMO), aminoethylaminopropyl poly(dimethyl siloxane) (AEAPS), and butanediol (BD). The chemical compositions, structures, and bulk and surface properties were investigated using an infrared surface quantitative analysis technique (FTIR‐ATR), surface contact angle, electron spectroscopy for chemical analysis (ESCA), stress–strain analysis, and dynamic mechanical thermal analysis (DMTA). It was shown that siloxane concentration on the surface region of the elastormers was higher than that in the bulk for a resulting surface enrichment of the siloxane, and the tensile properties of these elastomers were not changed significantly with the AEAPS modification. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2552–2558, 1999     

Synthesis and characterization of multiblock copolymers containing poly(dimethyl siloxane) blocks

Multiblock copolymers of polystyrene and poly(dimethyl siloxane) were obtained by a hydrosilylation reaction between a,w dihydrogeno poly(dimethyl siloxane) and a,w-di(vinyl silane) polystyrene. Under well chosen experimental conditions the polycondensation is free of site reactions and the macromolecules formed are linear with up to 10 blocks, which corresponds to reaction of 90% of the functions initially present. The block copolymers obtained have been characterized by g.p.c. viscosimetry and light scattering.     

Large strain and toughness enhancement of poly(dimethyl siloxane) composite films filled with electrospun polyacrylonitrile-graft-poly(dimethyl siloxane) fibres and multi-walled carbon nanotubes

Unfilled cross-linked poly(dimethyl siloxane) (PDMS) is a weak material and is generally filled with high levels of particulate fillers such as silica, calcium carbonate and carbon black to improve its mechanical properties. The use of fibrous fillers such as electrospun nanofibres and multi-walled carbon nanotubes as fillers for PDMS has not been widely studied. In this study anew copolymer, polyacrylonitrile-graft-poly(dimethyl siloxane) (PAN-g-PDMS), is used as fibrous filler for PDMS. The graft copolymer is electrospun to produce the fibre filler material. It is shown how the PDMS content of the graft copolymer provides increased compatibility with silicone matrices and excellent dispersion of the fibre fillers throughout a silicone matrix. It is also shown that it is possible to include multi-walled carbon nanotubes in the electrospun fibres which are subsequently dispersed in the PDMS matrix. Fibre mats were used in the non-woven and the aligned forms. The differently prepared fibre composites have significantly different mechanical properties. Conventional composites using fibrous fillers usually show increased strength and stiffness but usually with a resultant loss of strain. In the case of the composites produced in this study there is a dramatic improvement in the extensibility of the non-woven PAN-g-PDMS fibre mat filled silicone films of up to 470%.     

Effect of ultraviolet/ozone treatment on the surface and bulk properties of poly(dimethyl siloxane) and poly(vinylmethyl siloxane) networks

We present a comparative study aiming at comprehending the effect of ultraviolet/ozone treatment on the modification of poly(dimethyl siloxane) (PDMS) and poly(vinylmethyl siloxane) (PVMS) silicone elastomers networks (SENs). Both PDMS and PVMS SENs undergo dramatic changes in their properties when exposed to UVO. The surface chemical composition of both PDMS and PVMS at long UVO treatment times changes substantially and features a high density of hydrophilic groups. There are two major differences in behavior in the two classes of materials. First, relative to PDMS, the PVMS-based SENs get modified throughout the entire bulk. Second, the physico-chemical changes detected in PVMS take place on much shorter time scale relative to PDMS. These results are in accord with our earlier reports that indicated that when exposed to UVO, the topmost ≈5 nm of PDMS gets converted into a silica-like material, which then acts as a barrier for diffusion of atomic oxygen. In this case, the bulk of PDMS maintains its elasticity. In contrast, both the surface and bulk of PVMS films undergo substantial changes in properties when exposed to UVO. First, the surface modification of PVMS SENs takes place after only a few seconds of the UVO treatment. In addition, we register substantial modification of bulk properties, including the complete densification accompanied with increased bulk modulus. Likely, the susceptibility of the vinyl bonds to radical reactions is responsible for this effect.     

Polystyrene crosslinked with oligomeric and polymeric poly(dimethyl siloxane) derivatives. Thermal and dynamic mechanical studies

Styrene has been copolymerized with various bifunctional poly(dimethyl siloxane) cross-linkers having molar masses in the range 200–35,000. Calorimetric and rheological measurements show that the crosslinkers change T_g, T_ll, and T_exo, as well as melt viscosity and elasticity of polystyrene. These changes are strongly dependent on the molar mass of the crosslinker. Increasing the length of the siloxane crosslinker lowers T_g and melt viscosity, if the molar mass of the crosslinker is in a range 200–2000. Copolymerizing styrene with a bifunctional poly(dimethyl siloxane) having molar mass near 35,000 leads to a phase-separated polymer where polystyrene phase largely retains its original properties.     

Comparison of the initial reactant structure and crosslinked network of poly(dimethyl siloxane) membranes from different macromonomers

Some of the best entities for the removal of volatile organic pollutants from water, poly(dimethyl siloxane) (PDMS) membranes from two types of initial PDMS precursors, were thoroughly investigated. In both the cases, the sizes of the initial macromonomers increased because of the crosslinking in the liquid state, and they exhibited Gaussian chain statistics, which were condensed by further crosslinking to the formation of the membranes. The structures of both the membrane types exhibited large interchain spacing on swelling; this implied a high degree of chain mobility. Their structural properties were corroborated by their separation performances. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41461.