The thermal properties of polysiloxanes poly(dimethyl siloxane) and poly(diethyl siloxane)

New measurements and literature data on polysiloxanes covering heat capacities, transition parameters, enthalpies, entropies and Gibbs energies are presented and critically reviewed. TheATHAS computation method is used to bring heat capacities into agreement with an approximate frequency spectrum. The various crystal and mesophases are discussed. TheATHAS (1990) recommended data are as follows: For poly(dimethyl siloxane) the glass transition is at 146 K with an increase in heat capacity of 29.24 J/(K mol). The completely crystalline sample melts at about 219 K with a heat of fusion of 2.75 kJ/mol. For poly(diethyl siloxane) the glass transition is at 135 K with an increase in heat capacity of 34.48 J/(K mol). The completely crystalline sample changes to a condis crystal at 206.7 K with a heat of disordering of 2.72 kJ/mol. The transition to a poorly characterized viscous crystal with thermodynamic properties close to the melt occurs at 282.7 K with an enthalpy of transition of 1.84 kJ/mol. Final fusion occurs at 308.5 K and a small endotherm of about 231 J/mol. Tables of heat capacities, enthalpies, entropies and Gibbs energies are given from 0 K to 550 K.

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Zusammenfassung Neue Messungen und Literaturangaben von Polysiloxanen über Wärmekapazität, Umwandlungsparameter, Enthalpien, Entropien und Gibbssche Energien werden vorgestellt und kritisch betrachtet. Das Rechenverfahren ATHAS wurde benutzt, um die Wärmekapazitäten mit einem annähernden Frequenzspektrum in Einklang zu bringen. Es wurden die verschiedenen Kristall- und Mesophasen diskutiert. Die von ATHAS (1990) empfohlenen Werte sind wie folgt: Für Poly(dimethylsiloxan) beträgt der Glasumwandlungspunkt 146 K bei Zunahme der Wärmekapazität um 29.24 J/(K.mol). Die vollständing kristalline Probe schmilzt bei etwa 219 K mit einer Schmelzwärme von 2.75 kJ/mol. Für Poly(diethylsiloxan) beträgt der Glasumwandlungspunkt 135 K bei Zunahme der Wärmekapazität um 34.48 J/(K.mol). Die vollständig kristalline Probe wandelt sich bei 206.7 K um, die Fehlordnungswärme beträgt 2.72 kJ/mol. Die Umwandlung in einen wenig verstandenen viskosen Kristall, dessen thermodynamische Eigenschaften denen der Schmelze gleichen, erfolgt bei 282.7 K mit einer Umwandlungsenthalpie von 1.84 kJ/mol. Letztendlich verläuft das Schmelzen bei 308.5 K mit einem kleinen endothermen Effekt von etwa 231 J/mol. Wärmekapazitäten, Enthalpien, Entropien und Gibbssche Energien sind für den Bereich 0 K–550 K tabellarisch angegeben.

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, , , , . ATHAS , - . . ATHAS (1990) : 146 29,24 / ·. 219 2,75 /. 135 34,48 /·. 206,7 2,72 /. « » 282,7 1,84 /. 308,5 231 /. , , 0–550 .

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This work was supported by the National Science Foundation, Polymers Program, Grant #DMR 83-17097 and early work of J.P.W. was supported by the Am. Chem. Soc. Petroleum Research Found, Grant 12431-AC7. In addition at Oak Ridge National Laboratory the work was sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems Inc.     

Fibrinolytic poly(dimethyl siloxane) surfaces

PDMS surfaces have been modified to confer both resistance to non-specific protein adsorption and clot lyzing properties. The properties and chemical compositions of the surfaces have been investigated using water contact angle measurements, ATR FT-IR spectroscopy, and XPS. The ability of the PEG component to suppress non-specific protein adsorption was assessed by measurement of radiolabeled fibrinogen uptake from buffer. The adsorption of plasminogen from human plasma to the various surfaces was studied. In vitro experiments demonstrated that lysine-immobilized surfaces with free epsilon-amino groups were able to dissolve fibrin clots, following exposure to plasma and tissue plasminogen activator. [Figure: see text].     

Solution properties of poly(dimethyl siloxane)

The solution properties of poly(dimethyl siloxane) (PDMS) were studied with light scattering (LS), gel permeation chromatography/light scattering (GPC/LS), and viscometry methods. PDMS samples were fractionated, and the weight‐average molecular weights, second virial coefficient, and the z‐average radius of gyration of each fraction were found according to the Zimm method with the LS technique. In this work, the molecular weight range studied was 7.5 × 10⁴ to 8.0 × 10⁵. Molecular weights and molecular weight distributions were determined by GPC/LS. The intrinsic viscosities of these fractions were studied in toluene at 30 °C, in methyl ethyl ketone (MEK) at 20 °C, and in bromocyclohexane (BCH) at 26 °C and 28 °C. The Mark–Houwink–Sakurada relationship showed that toluene was a good solvent, and MEK at 20 °C and BCH at 28 °C were θ solvents for PDMS. The unperturbed dimensions were calculated with LS and intrinsic viscosity data. The unperturbed dimensions, expressed in terms of the characteristic ratio, were found to be 6.66 with different extrapolation methods in toluene at 30 °C. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2678–2686, 2000     

Stress relaxation and dynamic viscoelastic properties of end-linked poly(dimethyl siloxane) networks containing unattached poly(dimethyl siloxane)

Stress relaxation in uniaxial extension and dynamic shear moduli G′ and G″ have been studied in networks of vinyl-terminated poly(dimethyl siloxane) (PDMS) of five different molecular weights (M _n from 1800 to 29,200) crosslinked with cis-dichlorobis (diethyl sulfide) platinum (II) and containing 10 and 15 wt % of two samples of high-molecular-weight unattached linear hydroxyl-terminated PDMS (M _w 700,000 and 950,000). The M _w/M _n ratio of both the network prepolymers and the unattached linear species was approximately 2. In stress relaxation the stretch ratio was 1.25 or less and the shear relaxation modulus was calculated from the neo-Hookean stress-strain relation. In the dynamic measurements, the strain amplitude was 15% or less; after conversion to the timedependent shear relaxation modulus G(t) the two sets of measurements were combined and the contribution of the unattached species G₁(t) was calculated by difference. After multiplication by (1 − v)⁻¹G/G_e, where v₂ is the volume fraction of network, G is the plateau modulus of the uncrosslinked polymer, and G_e is the equilibrium modulus of the network containing unattached molecules, G₁(t) was compared with G₁₁(t), the relaxation modulus was essentially the same in both environments. The relaxation was slower in the networks than in the uncrosslinked polymer by 1 to 2 orders of magnitude, and it increased gradually with increasing G_e, which is a measure of total to pological obstacles represented by crosslinks plus trapped entanglements. A similar but less striking difference between relaxation in a network and in the homologous environment of a linear polymer was previously observed in end-linked polybutadiene networks and the butadiene phase of a styrene-butadiene-styrene block copolymer. It appears that, in these systems where the topology of the obstacles is fixed, the reptation is severely restricted or else alternative modes of configurational rearrangement which contribute to relaxation in the uncrosslinked polymer are suppressed.     

Oxygen diffusion in poly(dimethyl siloxane) using fluorescence quenching. II. Filled samples

The method of fluorescence quenching has been used to measure the transport properties of oxygen in poly(dimethyl siloxane) (PDMS) filled with small weight fractions of fumed silica. The experimental results show that the diffusion coefficient of oxygen and the pre-exponential term in the Arrhenius relationship are reduced, but the activation energy is not affected. In the pressure and temperature ranges used in these experiments, the diffusion coefficient was found to be independent of both the oxygen and fluorophor concentrations. A discussion concerning the effect of fillers on the pre-exponential term is presented.     

Oxygen diffusion in poly(dimethyl siloxane) using fluorescence quenching. I. Measurement technique and analysis

The transport properties of oxygen in poly(dimethyl siloxane) have been measured using the method of quenching of fluorescence. This paper discusses the uniqueness of this method and its use in measuring the diffusion coefficient of oxygen in unfilled PDMS. The results show (1) a large value for the diffusion coefficient of oxygen in pure PDMS at 25°C, D = 3.55 × 10^?5 cm²/s, (2) a low value of the acitivation energy, E_D = 4.77 kcal/mole, which was not temperature dependent in the ranges evaluated, and (3) a large value of the preexponential term, D₀ = 0.115 cm²/s. The diffusion coefficient was found to be independent of both the oxygen concentration and fluorophor concentration in the pressure and temperature ranges used in these experiments. The import of these experiments lies in their application to a unique biomedical oxygen sensor which is fast, sensitive, and does not consume oxygen.     

Sorption and partial molar volume of gases in poly (dimethyl siloxane)

Sorption of N₂, O₂, Ar, CH₄, CO₂, C₂H₄, and C₂H₆ in poly (dimethyl siloxane) liquid and rubber and the dilation of the polymers due to sorption of the gases are studied at 25°C under pressures up to 50 atm. In the liquid, the sorption isotherms for low-solubility and high-solubility gases are described by Henry's law and the Flory–Huggins equation, respectively. Gas sorption in the rubber, which contains a 29 wt % silica filler, follows the dual-mode sorption model, though marked hysteresis is observed in the sorption of O₂ and CH₄. The dilation isotherms increase linearly or exponentially in both polymers with increasing pressure. Considering that gas molecules adsorbed into micropores of the filler particles do not participate in the dilation, partial molar volumes of the dissolved gases in the rubber are determined from data of sorption and dilation. The values are nearly equal to the partial molar volumes in the liquid (48–60 cm³/mol).     

Electrochromatography in poly(dimethyl)siloxane microchips using organic monolithic stationary phases

This paper shows the in situ synthesis of an hexyl acrylate monolith in PDMS microfluidic devices and its subsequent use as stationary phase for electrochromatography on chip. To overcome the ability of PDMS material to absorb organic monomers, surface modification of the enclosed channels was realized by UV-mediated graft polymerization. This grafting procedure is based on the preliminary adsorption of a photoinitiator onto the PDMS surface and polymerization of charged monomers. Next, hexyl acrylate monoliths were cast in situ using photopolymerization process. The chromatographic behavior of the monolithic column was confirmed by the successful separation of derivatized catecholamines in the PDMS device using a 30 mm effective separation length (100 microm x 100 microm section). Efficiencies reached up to 200,000 plates per meter.     

Influence of interfaces on the rates of crosslinking in poly(dimethyl siloxane) coatings

We have determined with infrared spectroscopic ellipsometry how the nature of the interface between a thin poly(dimethyl siloxane) (PDMS) coating and its substrate affects the rate of PDMS crosslinking reactions. Reactions between vinyl (? CH?CH₂) end groups on PDMS and silyl (SiH) groups in a crosslinker (hydrosilylation) and between SiH groups and silanol (SiOH) groups, during the so‐called postcure crosslinking stage, have been probed in situ. The overall consumption of SiH follows first‐order reaction kinetics. The first‐order reaction coefficient (k₁) for the hydrosilylation crosslinking reaction is the same for coatings on three different substrates: native oxide on silicon (SiO₂/Si), polystyrene (PS), and poly(ethylene terephthalate). For the slower postcure reactions, however, the rate of SiH consumption depends on the substrate. In 2.5‐μm PDMS coatings on PS, k₁ is about seven times greater than k₁ in the same coating on SiO₂/Si. In PDMS coatings on a PDMS substrate, when the effect of the interface is thus minimal, k₁ is 16 times higher than on SiO₂/Si. The dependence of k₁ on the type of interface is probably the result of the interfacial segregation and complexation of the Pt catalyst for the postcure reactions. We propose that polar surfaces more strongly attract Pt and form complexes that inhibit the postcure reactions. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1421–1431, 2004     

Polarized Raman scattering of concentrated solutions of poly(dimethyl siloxane) in benzene

Raman depolarization (ρ) measurements have been made over the temperature range 20 > T > 60°C for solutions of poly(dimethyl siloxane) (mol wt 7.7 × 10⁴ and 2.0 × 10⁴) for several concentrations up to 100%. The band studied was the highly polarized methyl stretch at 2907 cm^?1. Computer calculations of the probability p_t of a rotational isomer being trans allow the ρ values to be related to ΔG, the free energy of mixing. ΔG is plotted as a function of concentration and minima are observed at 60 ± 3% (mol wt = 7.7 × 10⁴) and 70 ± 3% (mol wt = 2 × 10⁴).     

Interpretation of differences in temperature and pressure dependences of density and concentration fluctuations in amorphous poly(phenylmethyl siloxane)

Differences in the temperature and pressure dependences of the relaxation times of a slow diffusional process and the α structural relaxation pose an interesting problem. This feature, observed by dynamic light scattering in amorphous poly(phenylmethyl siloxane), is related to another basic feature of lack of thermorheological simplicity discovered by Plazek in polystyrene, poly(vinyl acetate), and amorphous polypropylene. A quantitative explanation based on the predictions of a general coupling theory of relaxations has been found. The coupling theory also predicts the Kohlrausch fractional exponential time correlation function exp[?(tτ^*)^1?n] at long times, as observed by photon correlation spectroscopy, and crossover to an exponential time dependence exp–(t/τ₀) at short times, as frequently assumed in Brillouin scattering. An additional relation between τ^* and τ₀ predicted by the theory is confirmed also by the experimental data.     

Fluoroalkylation of poly(cyclopentadienyl)siloxane and the surface properties of poly(fluorocyclopentenyl)siloxane films

In this work, poly(fluorocyclopentenyl)siloxane (FPCS) was obtained via a single electron transfer addition reaction of poly(cyclopentadienyl)siloxane (PCS) and perfluoroalkyl iodides, and reduction reaction of the intermediates. PCS was prepared by substitution and hydrolysis reactions using methyltrichlorosilane and sodium cyclopentadienide (NaCp) as raw materials. Fourier transform infrared (FTIR), ¹H NMR, and ¹⁹F NMR indicated the structures of the target polymers. The XPS results showed that the thin films prepared by dip-coating were fluorine enriched at the surface. Atomic force microscopy (AFM) image showed that on the rough surface of films, there were many pinnacles which were generated through the migration of side chain fluoroalkyl groups. The relative static contact angles of water and n-hexadecane on FPCS and PCS indicated that sodium dithionite initiated the reaction of perfluoroalkyl iodides and PCS so that FPCS was successfully synthesized. The measured surface energy of PCS was 2.57 × 10^?2 N/m; while FPCS was 2.14 × 10^?2 N/m, which represented better liquid repellent property compared to PCS.     

Molecular mobility of ultrasonically devulcanized silica‐filled poly(dimethyl siloxane)

We used NMR relaxation and pulsed‐gradient diffusion measurements at 70 °C in precipitated silica‐filled poly(dimethylsiloxane) (PDMS; silicone rubber) after crosslinking, after subsequent devulcanization by intense ultrasound, and after subsequent revulcanization. As in unfilled PDMS, transverse relaxation displays three distinct rate components attributed to an entangled and crosslinked network (similar in T₂), light sol plus dangling network fragments, and unreactive trace oligomers. Ultrasound produces copious amounts of extractable sol. The T₂ relaxation times decreased modestly with increasing filler content, but they and the components' proportions correlated mainly with the sol fraction, that is, the network degradation. In rupturing the network, devulcanization produces large diffusing fragments and dangling ends; revulcanization largely reverses these effects. The rates and amplitudes of the bimodal diffusivity distribution confirmed this conclusion. The weakness of the effects of filler suggests that ultrasound devulcanization is easily adaptable to the recycling of the preponderantly particulate‐filled industrial rubbers. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 454–465, 2003     

Conformation of poly(dimethyl siloxane) and polystyrene in solution measured by polarized Raman scattering

Depolarization ratios ρ have been measured over ranges of temperature T and molecular weight M for polystyrene (PS) dissolved in cyclohexane (1002 cm^?1 Raman band) and for poly(dimethyl siloxane) (PDMS) dissolved in benzene (2907 cm^?1 Raman band). The ranges in the case of PS are 15 < T < 65°C and 2 × 10³ < M < 4 × 10⁵ and in the case of PDMS are ?3 < T < 60°C and M = 10⁴. Measurements were also made of PDMS radii of gyration using conventional light scattering. The results are interpreted in terms of a theory connecting rotational isomeric populations with polymer extension. In the case of PDMS, an experimental value of the proportionality constant for trans isomers (D₂ = ?3.9 ± 0.9) is deduced. This is closer to the theoretical value than previous estimates but there is still some discrepancy. In the case of PS the isomeric changes resulting from extension are independent of M for M > 10⁴. Deviations are observed for lower M.     

Anomalous melting behavior of cyclohexane and cyclooctane in poly(dimethyl siloxane) precursors and model networks

Building on previous observations of anomalous melting behavior of solvents in polyisoprene, we have expanded our insight into the melting behavior of organic solvents in polymers and polymer networks through a calorimetric investigation of cylcohexane and cyclooctane in poly(dimethyl siloxane) (PDMS) precursors and model networks. The results are contrary to general expectations. Besides deviations between the predictions of the Flory-Huggins model and observed melting point depression of the small molecule organics, it is found that the melting point depression of cyclohexane in model networks is lower than that in the uncrosslinked precursors and unaffected by the molecular weight between crosslinks, which is not consistent with the general observation that higher crosslinking density leads to greater melting point depression. We interpret the observed phenomenon in terms of phase separation. In the case of cyclooctane, it exhibits a double melting peak in the model networks with high molecular weight between crosslinks. Furthermore, the heats of fusion of both cyclohexane and cyclooctane decrease with increasing polymer volume fraction which violates the underlying assumption that the heat of fusion of solvent in the polymer is the same as that in the bulk for both the Flory-Huggins model and the Gibbs-Thomson equation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2779–2791, 2008     

Plasticization of poly(dimethyl siloxane) by high-pressure gases as studied by NMR relaxation

The molecular relaxation behavior of poly(dimethyl siloxane) (PDMS) exposed to various gases pressurized to 207 megapascals (MPa) was investigated by pulsed nuclear magnetic resonance spectroscopy. For a gas of low solubility, such as helium, the gas acts only as a pressurizing medium allowing the effect of pressure on the glass transition to be determined. For gases of high solubility the gas acts not only as a pressurizing medium but also as a plasticizing agent, expanding the polymer lattice and increasing the frequency of molecular motions. The plasticizing effect of argon was found to increase the temperature dependence of the molecular correlation frequency.     

Improvement of thermal conductivity of poly(dimethyl siloxane) using silica-coated multi-walled carbon nanotube

In order to enhance the thermal conductivity of MWCNT filled poly(dimethyl siloxane) (PDMS) composites, the MWCNT was coated with silica layer by three step reactions. The composites filled with raw and silica-coated MWCNTs were prepared and the properties were investigated in terms of the curing characteristics, mechanical properties, and thermal conductivity. Due to the poor compatibility between raw MWCNT and PDMS, raw MWCNT showed poor dispersion uniformity and wettability in PDMS. On the other hand, due to the chemical affinity between silica/MWCNT and PDMS throughout the hydrogen bonding, the silica-coated MWCNT filled PDMS showed improved mechanical properties in terms of tensile strength and 100% modulus, and good interfacial compatibility than raw MWCNT incorporated PDMS. Finally, the good wettability of silica/MWCNT in PDMS resulted in higher thermal conductivity caused from the facile phonon movement at the interface even with the smaller MWCNT contents.     

Local composition effect in poly(dimethyl siloxane) solutions: A comparison of two models

Two polymer solution models have been used to analyze poly(dimethyl siloxane) (PDMS) + alkane and PDMS + ketone systems. The free-volume and nonrandom interaction contributions are studied separately in order to compare the expressions for the local site fractions used in the two models. Predictions of the reduced residual chemical potential and excess volumes are discussed and compared with those found in the literature for these systems.     

Flocculation studies of non-aqueous poly(methyl methacrylate) dispersions stabilized by AB block copolymers of polystyrene and poly(dimethyl siloxane)

Summary Well defined non-aqueous dispersions of poly(methyl methacrylate) stabilized by AB block copolymers of polystyrene and poly(dimethyl siloxane) were prepared with a dispersion medium consisting of a binary mixture of n-heptane and ethanol. Flocculation was induced by cooling, and the lower critical flocculation temperature was studied as a function of the molecular weight and composition of the stabilizing copolymer, the particle size of the dispersion, and the surface coverage of the particles. The theta temperature for poly(dimethyl siloxane) homopolymer in the same binary liquid mixture was determined by two procedures. The lower critical flocculation temperature was found to be close to the theta temperature and was independent of the length of the stabilizing poly(dimethyl siloxane) chains for the molecular weight range 3200-48000.

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Zusammenfassung Gut definierte, durch AB-Block-Copolymere von Polystyrol und Polydimethylsiloxan stabilisierte nichtwässerige Dispersionen von Polymethylmethacrylat wurden mit binären Mischungen von Heptan und Äthylalkohol als Dispersionsmittel hergestellt. Die Flockung wurde durch Abkühlung herbeigeführt und die untere kritische Flockungstemperatur wurde als Funktion des Molekulargewichts und der Zusammensetzung des stabilisierenden Copolymeren, der Größe der dispergierten Teilchen und dem Bedeckungsgrad der Teilchenoberfläche studiert. Die Theta-Temperatur des Polydimethylsiloxan-Homopolymers in derselben binären flüssigen Mischung wurde durch zwei Verfahren bestimmt. Es wurde gefunden, daß die untere kritische Flockungstemperatur nahe an der ThetaTemperatur liegt und im Molekulargewichtsbereich 3200–48000 von der Länge der stabilisierenden Polydimethylsiloxan-Ketten unabhängig ist.

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With 2 figures and 2 tables