Deuterated cyclic PDMS

Blends of protonated and deuterated cyclic PDMS and also blends of protonated and deuterated linear PDMS were studied. It was found that the radius of gyration Rg of the linear and cyclic chains varies with molar mass according to different power laws. The exponent 0.53 for linears is dose to the value of 0.50 predicted for flexible chains, obeying a Gaussian statistics in solution. A lower value of 0.42 forcydic indicates a more compact conformation in the melt. Another consequence of these data concerns the ratio of the mean square radii of gyration (Rg) /(Rg)i, which is a constant in solution (0.5), but decreases with the molar mass for PDMS in the molten state. 

The polydisperse per-deuterated cyclic PDMS recovered from the catalytic reaction was fractionated in to twelve sharp fractions using the preparative GPC instrument detailed earlier [5]. A section of the preparative trace is shown in Figure 35. The fractions were an ysed using analytical GPC calibrated with cyclic PDMS standards and the results of the analysis are shown in Table IV. 

The purity of the deuterated cyclic PDMS fractions were verified using infrared absorption, mass spectrometry and and Si nuclear magnetic resonance spectroscopies. 

Figure 35 Preparative GPC chromatogram recorded during the fractionation of the deuterated cyclic PDMS.

In this chapter we show how hydrogenated cyclic PDMS is prepared, characterised and investigated. These studies form the basis of our preparation and characterisation of the first deuterated cyclic polymers [12] [13] [14]. These are per-deuterated PDMS [(CD3)2SiO], and they were obtained by a modification of the method described by Beltzimg and coworkers [15]. Preparative GPC was used to prepare sharp fractions in a similar way to the hydrogenated materials. 

The toher work carried out investigating die effects of dilution and triflic acid concentration on the reaction with bis(trideuteomcthyl)diphenylsilane has optimised a catalytic route to both linear and cyclic po -deuterated PDMS of high purity in good yields enabling the first narrow fractions of deuterated cyclic polymers to be prepared. 

The conformational and dynamic properties of cyclic polymers have been the subject of considerable interest over a number of years [73] [74]. The new synthetic procedure developed for the preparation of per-deuterated cyclic and linear poly(dimethylsiloxane) (PDMS) polymers [12] [13] [14] now offers unique possibilities for neutron scattering studies. For the first time, it has allowed sufficient deuterated materials to be available for fractionation using our preparative GPC instrument. The resulting narrow fractions are ideally suited for both static and dynamic studies of cyclic and linear polymers. This range incorporates the critical molar mass for entanglement in PDMS. 

The work summarized in Table 26.1 has been conducted for isolated rings in dilute solution, either at theta conditions or in good solvents. Recent small-angle neutron-scattering studies on bulk rings have shown that they adopt conformations more compact than those in solution (Arrighi et al, 2004). The polymers examined were hydrogenous and deuterated linear and cyclic PDMS. For linear PDMS, Rg oc For cyclic PDMS, Rg oc thus, R c) ... 

The H spectrum of the deuterated product shows the almost total deuteration due to the absence of peaks at s proximately 0 ppm. All of the phenyl groups have been completely removed from the silane and the hydrolysis has produced deuterated PDMS of high purity. The minute peaks observed at 7.27 and 7.37 ppm are due to residual solvents (CHCI3 and CgH respectively). The large peak at 5.31 ppm is due to the dichloromethane used as a reference for calculating the percentage deuteration in comparison with a similar spectrum recorded with the hydrogenated cyclic tetramer (D4). 

To try and confirm foe presence of any linear hydroxyl- terminated deuterated siloxanes, which are not easily resolved from the cyclic siloxanes by GLC, foe technique of GC/CI-MS was applied to the sample. This technique has been reported to clearly show foe presence of hydroxyl end-groups in the analogous hydrogenated PDMS [70]. 

Gas Chromatography/Ammonia Chemical Ionisation Mass Spectroscopic Analysis Of The Small Per-Deuterated Dimethylsiioxane Cyclics. The Cl spectra were also recorded of the small perdeuterated dimethylsiioxane cyclics using foe same instrument as previously with ammonia Three competing processes are known to occur with ammonia in foe case of PDMS [70]. Quasimolecular ions are formed either by hydrogen or ammonium ion addition as well as subsequent trideuteriomefoyl loss. Ions can be observed at M +18, M + 2(18 - 19), M + 1, and M -18. Each of these processes are observed for foe cyclics upto foe decamer and spectra were obtained for foe pentamer up to foe decamer inclusive (Figures 33 and 34 show those of the hexamer and foe nonamer respectively). 

Catalytic Preparation Of Both Linear And Cyclic Per-Deuterated PDMS. As... 

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