Cyclic PDMS Research Articles

Synthesis of photoresponsive cyclic poly(dimethyl siloxane)s from monodisperse linear precursors

Polysiloxanes are industrially important silicon-based polymeric materials which cannot be replaced by carbon-based ones. In this area, synthesis of well-defined functional poly(dimethyl siloxane) (PDMS) with nonlinear macromolecular architectures is indispensable for finding next-generation applications. Here we report the synthesis of cyclic PDMS (C) with photocleavable and reformable hexaarylbiimidazole (HABI)-derived covalent bonds in the main chain from triphenylimidadole (lophine) end-functionalized monodisperse linear precursors (Ls). Interestingly, an end-to-end cyclization reaction of L with the concentration (10 mg/mL) much higher than that of a typical concentration (~0.1 mg/mL) dominantly produced unimolecularly cyclized product. Moreover, photoirradiation to C under a solvent-free condition, molecular weight increased despite the physical state of C was solid.

Effect of Chain Length and Topological Constraints on Segmental Relaxation in Cyclic PDMS

We present a detailed investigation of local dynamics of linear and cyclic poly(dimethylsiloxane) (PDMS) covering a wide range of molar masses. To aid interpretation of the experimental data, QENS measurements in the time scale from 2 to 200 ps and at Q = 0.3 to 1.8 A–1 are complemented by theoretical calculations. These make use of a methodology developed by us elsewhere applicable to both simple chain models and real chains and applied here, for the first time, to cyclic PDMS. Analysis of the incoherent dynamic structure factor at T < Tm shows that the rotational motion of the methyl groups is unaffected by polymer topology. At higher temperatures, the QENS data are described by a model that consists of two dynamic contributions: methyl group rotation and segmental motion, the latter described by a stretched exponential function. Relaxation times of both linear and cyclic PDMS increase with increasing molar mass. Several features predicted by theory are also reproduced by the experimental data. We show,...

Covalent functionalization of silica surface using "inert" poly(dimethylsiloxanes).

Methyl-terminated poly(dimethylsiloxanes) (PDMSs) are typically considered to be inert and not suitable for surface functionalization reactions because of the absence of readily hydrolyzable groups. Nevertheless, these siloxanes do react with silica and other oxides, producing chemically grafted organic surfaces. Known since the 1970s and then forgotten and recently rediscovered, this reaction provides a versatile yet simple method for the covalent functionalization of inorganic surfaces. In this work, we have explored the reactions of linear methyl-terminated and cyclic PDMS and bis-fluoroalkyl disiloxanes for the surface functionalization of mesoporous silica (Dpore ≈ 30-35 nm). The optimal reaction conditions included 24 h of contact of neat siloxane liquids and silica at 120-250 °C (depending on the siloxane). A study of the reactions of silicas with different extents of hydration demonstrated the critical role of water in facilitating the grafting of the siloxanes. The proposed reaction mechanism involved the hydrolysis of the adsorbed siloxanes by the Lewis acidic centers (presumably formed by water adsorbed onto surface defects) followed by the coupling of silanols to the surface to produce grafted siloxanes. For rigorously dehydrated silicas (calcination ∼1000 °C), an alternative pathway that did not require water and involved the reaction of the siloxanes with the strained siloxane rings was also plausible. According to FTIR and chemical analysis, the reactions of bis-fluoroalkyl disiloxanes and cyclic PDMS (D3-D5) produced covalently-attached monolayer surfaces, and the reactions of high-MM methyl-terminated PDMS produced polymeric grafted silicas with a PDMS mass content of up to 50%. As evidenced by the high contact angles of ∼130°/100° (adv/rec) and the negligible amount of water adsorption over the entire range of relative pressures, including saturation (p/p0 → 1), the siloxane-grafted porous silicas show uniform, high-quality hydrophobic surfaces. An overall comparison of siloxanes with classical silane coupling agents (i.e., silanes with readily hydrolyzable functionalities such as chloro, amino, etc.) demonstrated that the reactions of siloxanes produced surfaces of similar quality and, although requiring higher temperatures, used noncorrosive, less hazardous reagents, thereby providing an environmentally benign alternative to the chemical functionalization of metal oxide surfaces.

Antimicrobial properties of Ag + loaded zeolite polyester polyurethane and silicone rubber and long-term properties after exposure to in- vitro ageing

In biomedical applications, tubes (e.g. catheters etc.) are commonly produced from polyurethane (PU) and silicone rubber which are known to be biocompatible materials. Several studies have shown that tubes, which are connected to the body (invasive) (especially urinary, tracheotomy and central venous catheters) are associated with infections. The present study reports the development of a new method aiming at obtaining antibacterial properties for PU and silicone rubber by mixing respective material with a natural antibacterial agent (Ag+ loaded zeolite) in different weight fractions. The influence of the zeolite content on the antimicrobial properties were analysed by exposure to bacteria (ISO 22196) and mixtures of fungi (ISO 846). The materials were also subject to artificial body fluids (Artificial Lysosomal Fluid (ALF) and Gamble’s solution) for periods up to three months and the subsequent changes in the chemical properties after in-vitro exposure were determined by Matrix Assisted Laser Deposition/Ionization Time Of Flight Mass Spectrometry (MALDI-TOF MS) and Attenuated Total Reflection Fourier Transform Infra Red spectroscopy (ATR-FTIR). It was established that the antimicrobial effect of the materials increased with the increase of the zeolite content. The wettability of the materials was found to decrease significantly during the in-vitro exposure, but this could not be correlated to the zeolite content. In the PU samples, the formation of free carbonyl and –OH groups was observed, which corresponds to oxidative degradation. In case of the silicone rubber the ratio of cyclic PDMS to linear PDMS (H, CH3 and dimethyl terminated) decreased, which indicates a change in the concentration of the compounds. The formation and increase of the O–H bond during the exposure was also confirmed by the infrared spectra of the material which corresponds to hydrolysis of the silicone rubber.

Cyclic poly(dimethylsiloxane) via ring-closing dehydrocoupling of α,ω-dihydroxy-PDMS with α,ω-dihydrido-PDMS in dilute solution

Cyclic poly(dimethylsiloxane) (PDMS) was prepared from commercially available linear α,ω-dihydroxy-PDMS by a platinum-catalyzed dehydrocoupling of its chain ends with α,ω-dihydrido-PDMS in dilute solution. Ring closing was verified by MALDI-ToF mass spectrometry along with GPC, IR, 1H and 29Si NMR spectroscopy. Purification of the cyclic PDMS was achieved by inclusion complexation of linear byproducts with γ-cyclodextrin. Cyclic yields are≥50% and the average molecular weight (M) closely reflects that of the linear starting material. Yields of large cycles, [(CH3)2SiO]x where x>6, are greater than those achieved by the traditional ring-chain equilibration route beginning with [(CH3)2SiO]4 and [(CH3)2SiO]5[1], and the M is greater than that obtained for the cyclic product obtained from base-catalyzed cyclodepolymerization of the same α,ω-dihydroxy-PDMS precursor [2].

Preparation of three‐dimensional poly(dimethylsiloxane) (PDMS) with movable cross‐linking

AbstractA pentamethylcyclotrisiloxane moiety was introduced into cyclic polystyrene (cPSt) and cyclic PDMS (cPDMS) to obtain noncovalent cross‐linking agents, D3‐cPSt and D3‐cPDMS, respectively. Anionic ring‐opening polymerization of octamethylcyclotetrasiloxane (D4) in nitrobenzene was carried out in the presence of D3‐cPSt to obtain a cloudy white PDMS gel as a precipitation. On the other hand, bulk copolymerization of D3‐cPDMS with D4 proceeded in a homogeneous state to give a colorless transparent PDMS gel in high yield. The formation of mechanically linked PDMS with movable cross‐linking was indicated by control experiment. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5882–5890, 2009

Stability of Various Silicone Oil/Water Emulsion Films as a Function of Surfactant and Salt Concentration

There have been reports, originally by the Bristol group, and subsequently by others, of the preparation and properties of emulsions of stable, nearly monodisperse droplets of poly(dimethylsiloxane) (PDMS) in water, where no added surfactant is used. It has been assumed that their stability is due to the high density of surface-ionized hydroxyl groups, similar in fact to the closely related Stöber silica particles. In this study we confirm, from droplet lifetime studies, that droplets, prepared from such synthesized PDMS, are significantly more stable to coalescence than similar-sized droplets prepared from three types of commercially available PDMS, containing HO-, MeO-, or Me3-terminated chains, respectively. It is shown, however, that the zeta potentials of the synthesized PDMS and of the various commercial oils are all very similar (as indeed are their Hamaker constants). So some other explanation must be inferred for the enhanced stability to coalescence of the synthesized PDMS droplets compared to the commercial PDMS droplets. It is shown, for droplets formed from n-hexane and the synthesized oil, that stability to coalescence is conferred at PDMS volume fractions (phiPDMS) around 0.2 in the mixture. The synthesized PDMS is known to consist of mixtures of cyclic PDMS and short-chain linear species, with terminal -OH groups. There is some (indirect) evidence that in the interval 0.25 < phiPDMS < 0.35, the linear PDMS chains may be adsorbed close to a monolayer at the mixed oil/water interface, possibly conferring some enhanced Gibbs elasticity to the interface. This underpins the possibility that, in the synthesized oil droplets themselves, there is also preferential adsorption of the linear chains at the PDMS/water interface, and this leads to a value of the Gibbs elasticity, sufficient to significantly reduce coalescence. Unfortunately, the Gibbs elasticity could not be measured in this case. However, such preferential adsorption is unlikely to occur with the commercial PDMS oils, which are not so heterogeneous. Finally, it is shown that droplets of the three commercial PDMS oils could be stabilized against coalescence, if a sufficient, minimum amount of sodium dodecyl sulfate (SDS) is added. Gibbs elasticity values have been estimated in these cases, from plots of interfacial tension against ln(SDS concentration).

Studies of cyclic and linear poly(dimethylsiloxanes): 34: Preparation, fractionation and characterisation of the first per-deuterated macrocyclic poly(dimethylsiloxanes)

A range of per-deuterated cyclic poly(dimethylsiloxanes) (PDMS) have been conveniently prepared in a high dilution reaction from bis(trideuteriomethyl)diphenylsilane using a catalytic amount of trifluoromethanesulphonic (triflic) acid in the presence of water as proposed. The cyclic and linear per-deuterated polymers produced have been separated and the cyclics fractionated by preparative gel permeation chromatography (GPC). The resulting sharp fractions of per-deuterated cyclic PDMS have been obtained in up to 0.6 gram quantities ranging in size from 45 to 605 number–average number of skeletal bonds, all with dispersities less than 1.2. The purity of the samples obtained has been verified by various analytical techniques and these materials will allow the first studies of the conformations and properties of cyclic polymers in chemically identical blends using neutron scattering.

Studies of cyclic and linear poly(dimethylsiloxane)s: 31. Effect of molecular architecture/topology on blends of poly(methylphenylsiloxane) and poly(dimethylsiloxane)

The phase behaviour of binary mixtures of cyclic poly(dimethylsiloxane) (PDMS) and linear poly(methyl-phenylsioxane) (PMPS) was investigated using a static wide-angle light-scattering technique. Evidence for the effect of component molecular architecture/topology (cyclic or linear) on the phase separation of blends was sought through cloud point measurements and by examining the composition dependence of the resulting phase diagram. The cyclic-linear blend exhibited an upper critical solution temperature ( UCST) and showed a dependence on the molecular architecture/topology in that the critical temperature T c for the cyclic-linear blend was lower than expected when compared with PDMS-PMPS blends in which both components had a linear polymeric structure. The interaction energy density parameter A 12 for the polymers was evaluated via the Flory-Huggins treatment for the thermodynamics of mixing of two polymers. The blending of cyclic PDMS led to a lower A 12 when compared with the corresponding linear-linear binary blend, again because of the chain architecture/topology effect. For this UCST system, it thus appears that PDMS cyclics are more soluble/compatible than their linear analogues of the same degree of polymerization. These findings are consistent with the current, but very limited, experimental data in the literature for large ring systems.

Cyclic polymers: past, present and future

AbstractMethods for characterising cyclic polymers are illustrated by reference first to dilute solution methods for cyclic poly(dimethylsiloxane) (PDMS) and then to the entrapment of cyclic polymers in networks. Preparative routes to cyclic polymers are then reviewed, including ring‐chain equilibration reactions, coupling and condensation reactions and new methods using polymer‐supported reagents. Some of the properties of cyclic PDMS are discussed, including differences between ring and chain polymer properties such as their melt viscosities and glass transition temperatures. Methods for preparing the first polymeric catenanes are described, using polymer‐supported reagents. Future directions for cyclic polymer chemistry are indicated, including topological polymer chemistry.

Thermodynamics of the lower critical threshold line in solutions of cyclic and linear poly(dimethylsiloxane)s in tetramethylsilane

The main features of the critical line in presure-temperature-composition space for mixtures containing either a linear or a cyclic poly(dimethylsiloxane) (PDMS) in tetramethylsilane (TMS) are predicted through the use of the Patterson modification of Prigogine's corresponding-states theory, which incorporates the cell model of a liquid together with a simple van der Waals dependence of the configurational energy on volume. Two main kinds of critical lines are obtained: one is a continuous line for those mixtures with a small chain-length difference between the components, leading to a fairly good prediction of the liquid-vapour critical temperatures of the pure substances; and the other is a broken line for mixtures with a large chain-length difference between the polymer and solvent, indicating the occurrence of a lower critical threshold temperature ( LCTT) at low polymer concentrations. A full set of temperature and pressure reduction parameters for both linear and cyclic oligomers and PDMS are reported, and are used as the basis for all the calculations with no adjustment whatsoever. It is predicted that the system of minimum chain length to show an LCTT is that containing a PDMS of 24 skeletal bonds, compared to the methane-hexane system in the n-alkanes, thus giving an insight into the more flexible and more expanded nature of the PDMS series. Results for the interaction parametert χ 1 confirm that any difference in chemical nature between TMS and the PDMS series may be largely ignored since the structural term c 1 τ alone can account for the LCTT behaviour.

Entanglements in linear and cyclic poly(dimethyl siloxane) and other polymers

Linear and cyclic PDMS have a common value of M c, the molar mass between entanglements. This phenomenon can be understood by recasting the Graessley-Edwards approach to entanglements in linear polymers in terms of binary interactions. The resulting equations provide a correction to the Graessley-Edwards equations for M c and M e, the entanglement molar mass from the plateau modulus, and define the conditions for common values of M c and M e for linear and cyclic polymers of the same chain structure.

Modelling of polymeric threading processes

The threading of linear molecules through cyclic polymers was studied using rotational isomeric state and molecular modelling methods. The rotational isomeric state model calculated the proportions of cyclic poly(dimethylsiloxane) (PDMS) molecules with different ring sizes that could be threaded onto chain molecules of different diameters. It was found that the proportions threaded agreed quite well with topological trapping experiments. Molecular modelling was used to study the threading of cyclic PDMS molecules and the formation of rotaxanes. For each system studied the minimum energy of the molecules was measured as the linear molecule was translated through the cyclic molecule. The potential energy changes and the potential energy barriers during threading were found to be dependent on the type and size of the molecules studied. The minimum ring sizes for threading for two of the systems were found to agree with values determined experimentally.

Studies of cyclic and linear poly(dimethylsiloxanes): 28. Viscosities and densities of ring and chain poly(dimethylsiloxane) blends

Two physical properties, density and low-shear viscosity, were studied in order to investigate the effects of blending chemically identical but topologically different poly(dimethylsiloxane) (PDMS) fractions (i.e. rings and chains). Deviations from a purely additive model were obtained for 50:50 (by volume) blends of cyclic and linear PDMS samples. Such investigations may be able to probe packing capabilities, the amount of free volume in a fraction and the extent and effects of chain threading.

Cyclic polysiloxanes: 2. Neutron scattering from poly(phenylmethylsiloxane)

The z-average radii of gyration 〈 s 2〉 z of both cyclic (r) and linear (l) poly(phenylmethylsiloxanes) (PPMS) in dilute solution in benzene-d 6 were measured by small-angle neutron scattering. The PPMS samples studied consisted of fractions with heterogeneity indices in the range 1.04 < M w M n < 1.15 and z-average molar masses in the range 3500 < M z < 21 200 g mol −1. The neutron scattering results were found to be in agreement with the theoretically predicted ratio 〈s 2〉 l 〈s 2〉 r = 2 for ‘flexible’ linear and cyclic polymers unperturbed by excluded volume effects. The neutron scattering functions of cyclic and linear PDMS with scattering vectors Q in the region Q〈s 2〉 1 2 > 1 were also examined.

Cyclic polysiloxanes

AbstractThe preparation, fractionation and characterisation of synthetic cyclic polysiloxanes (R(CH3)SiO)x are reviewed. Particular emphasis is given to the cyclic poly(dimethylsiloxanes) (PDMS) (R is CH3) cyclic poly(phenylmethylsiloxanes)(PPMS) (R is C6H5) and cyclic poly(vinylmethylsiloxanes) (PVMS) (R is CH2=CH).The application of high performance liquid chromatography (HPLC) and analytical and preparative gel permeation chromatography (GPC) to the preparation and characterisation of cyclic polysiloxanes is discussed. The methods include preparation of sharp fractions of relatively high molar mass cyclic polymers (with heterogeneity indices typically ca. 1.05) and comparison of the GPC and retention volumes of cyclics with the corresponding linear polymers.Investigations of the cyclic polymers prepared have been made using a number of experimental methods and techniques. These include dilute solution viscometry and low angle neutron scattering. Comparisons are drawn between cyclic PDMS and other cyclic polysiloxanes and also between cyclic polysiloxanes and the corresponding commercially‐important linear polymers.

Studies of cyclic and linear poly(dimethyl-siloxanes): 21. High temperature thermal behaviour

The high temperature thermal behaviour of cyclic and linear poly(dimethylsiloxanes) (PDMS) have been investigated using differential scanning calorimetry and thermogravimetry. The cyclic and linear polymers were found to have similar thermal stabilities. They depolymerized at temperatures above 623 K, apparently by a common mechanism. The cyclic PDMS were sealed under vacuum in borosilicate glass or quartz ampoules and heated at temperatures greater than 623 K. Detailed chromatographic analyses of the products formed showed that these were consistent with ring opening polymerizations of the cyclic siloxanes (catalysed ionically by the surfaces of the vessels) and not with bond interchange reactions, as had previously been suggested.

Studies of cyclic and linear poly(dimethylsiloxanes): 18. Surface pressures of the monolayers in the plateau region

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 × 10 5 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.

Studies of cyclic and linear poly(dimethyl siloxanes): 14. Particle scattering functions

Particle scattering functions P( Q) (where Q represents the wave vector), have been calculated using a Monte Carlo method for cyclic and linear poly(dimethyl siloxanes) (PDMS) containing up to 100 skeletal bonds. A maximum is found in the Kratky plot at u (=Q〈s 2〉 1 2 )≈2.0 for cyclic PDMS (with root-mean-square radii of gyration 〈s 2〉 1 2 ) and this is in satisfactory agreement with the analytical calculations of Casassa and of Burchard and Schmidt. In addition, other clearly-defined maxima are found at u≈5.0 for PDMS ring molecules with less than ≈40 skeletal atoms. These maxima are believed to be characteristic of small cyclic molecules as they are also predicted for small polymethylene rings. Comparisons are made with experimental small-angle neutron scattering (SANS) data for cyclic and linear PDMS. A single maximum in the experimental Kratky plot at u≈2.0 is found for PDMS ring molecules with an average of 550 skeletal atoms. The experimental data for cyclic PDMS are in better agreement with the Monte Carlo calculations of P( u) than with analytical predictions up to u≈2.0.

Studies of cyclic and linear poly(dimethyl siloxanes):7. Diffusion behaviour in a poor solvent

The diffusion coefficients ( D) of cyclic and linear poly(dimethylsiloxanes) (PDMS) have been measured in bromocyclohexane at 288 K and 301 K. Bromocyclohexane has previously been reported to be a θ-solvent for high molar mass linear PDMS at 301 K, but the hydrodynamic radii reported here apparently show the effects of molecular expansion at both temperatures. In addition, the hydrodynamic radii of both linear and cyclic PDMS are found to be insensitive to whether the solvent is toluene or bromocyclohexane. The ratio of friction coefficients f r f l for the ring (r) and linear (l) molecules of the same number of segments ( x) is in good agreement with the theoretical value of 8 3π in the impermeable limit and with the experimental value found previously in toluene solution. As x decreases the ratio f r f l tends to unity, illustrating the increasing importance of free-draining at low molar mass.