Stephen J. Clarson

Studies of cyclic and linear poly(dimethylsiloxanes): 19. Glass transition temperatures and crystallization behaviour

Differential scanning calorimetry (d.s.c.) was used to investigate the thermal behaviour of cyclic and linear poly(dimethylsiloxanes) over the temperature range 103–298 K. Fractions of the polymers studied had number-average molar masses in the range 160 < Mn < 25 500 g mol−1 and heterogeneity indices MwMn < 1.1 in most cases. D.s.c. was applied to measure the glass transition temperatures Tg cold crystallization temperatures Tc and polymer crystalline melting temperatures Tm of the oligomer and polymer fractions. Cyclic siloxanes [(CH3)2SiO]x with number-average numbers of skeletal bonds nn in the range 24 ≦ nn ≦ 79 and linear siloxanes (CH3)SiO[(CH3)2SiO]ySi(CH3)3 with nn in the range 10 ≦ nn ≦ 40 were found not to crystallize. The Tg values of the linear siloxanes were found to be in agreement with values in the literature and they increased with increasing Mn. By contrast, the Tg values of the cyclics were found to decrease with increasing Mn.

Silicification and biosilicification part 3

Biosilicification takes place at or very close pH 7.0 and under ambient conditions of temperature and pressure in vivo. The silicic acid transporters and the proteins facilitating biosilicification in diatoms have been identified. Silica synthesis under mild conditions in vitro has been demonstrated using synthetic polymers with control over the resulting silica morphology. The results presented herein show that the silica synthesis in vitro is not specific to particular enzymes/polypeptides due to their particular chemical structure and activity but that many other synthetic macromolecules are also capable of facilitating silica formation at neutral pH. We also report the synthesis of organic-inorganic hybrid materials that have potential in optoelectronic applications.

Synthesis, structure, and properties of hybrid organic-inorganic composites based on polysiloxanes. II. Comparisons between poly(methylphenylsiloxane) and poly(dimethylsiloxane), and between titania and silica

The work reported in the preceding article in this series is extended by consideration of polysiloxane–ceramic composites based on atactic poly(methylphenylsiloxane) (PMPS) elastomers instead of poly(dimethylsiloxane). The former is noncrystallizable because of its stereochemically irregular structure, while the latter is crystallizable. In addition, some composites were prepared by the in situ precipitation of titania instead of silica. The resulting materials were characterized using differential scanning calorimetry, equilibrium stress–strain measurements in elongation, small-angle neutron scattering, and transmission electron microscopy. The moduli of the PMPS elastomers were found to increase significantly with increase in amount of either type of filler, with reinforcing upturns at high elongation in the case of the silica. Because the PMPS elastomers were amorphous, it is obvious that strain-induced crystallization is not required for these upturns in modulus. Titania did not give as good reinforcement as did silica, at least in the case of PMPS. Differences in interactions between the polymer and the two fillers are obviously important in this regard, but differences in particle morphology probably also contribute. Specifically, the titania “particles” were significantly larger than the silica particles when observed in TEM, and appeared to be much more porous. The actual domain size as measured by scattering, however, was only approximately 5% larger.

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

The z-average radii of gyration 〈s2〉z of both cyclic (r) and linear (l) poly(phenylmethylsiloxanes) (PPMS) in dilute solution in benzene-d6 were measured by small-angle neutron scattering. The PPMS samples studied consisted of fractions with heterogeneity indices in the range 1.04 1 were also examined.

The use of poly-L-lysine to form novel silica morphologies and the role of polypeptides in biosilicification

Silicification at neutral pH and under ambient conditions in vitro is of great interest due to its relationship with silicification in vivo as well as for the benign conditions of the process. As it is important to know the exact group(s) or a particular site in the macromolecules that are responsible for the silicification under these conditions in vivo, poly-L-lysine (PLL) was chosen for this investigation in vitro. Here we report the use of tetramethoxysilane (TMOS) as a silica precursor and the utilization of poly-L-lysine (PLL) for silicification at neutral pH and under ambient conditions. We describe (1) the use of PLL to precipitate silica, (2) the effect of mixing of macromolecules PLL and poly(allylamine hydrochloride) (PAH) to control morphologies of the product, and (3) the formation of novel silica morphologies.

The effect of fluorine-substituted acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystals

The effects of fluorinated acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystal (PDLC) films are reported. The partial fluorination of host polymer matrices resulted in improved optical properties and better defined morphologies. An enhancement in contrast ratio was observed for fluorinated systems containing trifluoroethyl acrylate (TFEA) and hexafluoroisopropyl acrylate (HFIPA). Conversely, the incorporation of methyl acrylate (MA), a chemically similar non-fluorinated acrylate, resulted in no appreciable change in contrast ratio and an increase in relaxation time. Scanning electron microscopy morphological studies were conducted to understand further the influence of fluorinated monomers in PDLC systems.

Effect of process parameters on the polymer mediated synthesis of silica at neutral pH

We report herein the synthesis of well-defined silica structures atneutral pH and ambient conditions using poly(allylamine hydrochloride)(PAH), a cationically charged synthetic polymer, as a catalyst/template.Tetramethoxysilane (TMOS) was used as the precursor and the synthesisprocess parameters varied include TMOS pre-hydrolysis time(tP), reaction time (tR), buffer, molecular weightof the polymer, TMOS concentration, polymer concentration andperturbation of the reaction mixture. It was found that the TMOSpre-hydrolysis time was an important parameter governing the resultingsilica morphology along with the reaction time and the TMOSconcentration. Characterization of the silica was performed using SEM,FTIR, EDS and XRD. The poly(allylamine hydrochloride), which was thecatalyst/template, was found to be incorporated into the silicaparticles. These findings are of importance for understanding the roleof polypeptides, in nature, and macromolecules, in general, that arecapable of forming similar silica structures.

Cyclic polysiloxanes: 1. Preparation and characterization of poly(phenylmethylsiloxane)

Abstract A ring-chain equilibration reaction of poly(phenylmethylsiloxane) (PPMS) was carried out on a kilogram scale in toluene at 383 K. The cyclic oligomeric and polymeric phenylmethylsiloxanes formed were isolated and then fractionated using preparative gel permeation chromatography (g.p.c.). Fifteen sharp fractions were obtained, containing cyclics [C 6 H 5 (CH 3 )SiO] x with (an average) from nine to over three hundred skeletal bonds. Linear PPMS was also prepared and fractionated using preparative g.p.c. The cyclic and linear oligomers and polymers were characterized using gas-liquid chromatography (g.l.c.), high performance liquid chromatography (h.p.l.c.) and low-angle laser light scattering (LALS).

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

Abstract 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 the mechanical properties of free films prepared using an ethylcellulose pseudolatex coating system

Free films produced using a commercially available ethylcellulose (EC) pseudolatex coating system (Aquacoat®) were prepared by a spraying method. The effect of either of 10 different plasticizers representing three chemical classes (citrate esters, di-acid esters and fatty acids/alcohols) on free film mechanical properties was determined using an Instron tensile testing apparatus. The plasticizers were each added to the EC pseudolatex formula at levels of 25, 30 or 35% (w/w), respectively. Increasing the amount of plasticizer led to an increase in free film elongation and a decrease in modulus and stress. Differences among the individual plasticizers and classes of plasticizers evaluated were correlated with their molecular structure and a rank order could be identified. Selected films containing 30% plasticizer were evaluated under conditions of elevated temperature and humidity for storage times of up to 1 month. The results indicated that plasticizer type and amount and elevated storage temperature and humidity influence free film mechanical behavior, particularly free film elongation and toughness. These changes probably arise from further gradual coalescence of EC pseudolatex particles.