Dimethylsiloxane glass transition temperature

Whilst the Tg of poly(dimethylsiloxane) rubbers is reported to be as low as -123°C they do become stiff at about -60 to -80°C due to some crystallisation. Copolymerisation of the dimethyl intermediate with a small amount of a dichlorodiphenylsilane or, preferably, phenylmethyldichlorosilane, leads to an irregular structure and hence amorphous polymer which thus remains a rubber down to its Tg. Although this is higher than the Tg of the dimethylsiloxane it is lower than the so that the polymer remains rubbery down to a lower temperature (in some cases down to -100°C). The Tg does, however, increase steadily with the fraction of phenylsiloxane and eventually rises above that of the of the dimethylsilicone rubber. In practice the use of about 10% of phenyldichlorosilane is sufficient to inhibit crystallisation without causing an excess rise in the glass transition temperature. As with the polydimethylsilox-anes, most methylphenyl silicone rubbers also contain a small amount of vinyl groups. 

Thermogravimetric studies have displayed that the cyclic fragment causes a considerable effect on carbosiloxane copolymer at n= 2 only, and at n=23 no effect of cyclic fragment on the glass transition temperature of the copolymer is observed. Figure 5 shows dependence of Tg on the length of dimethylsiloxane unit for cyclolinear carbosiloxane copolymers. 

Thermomechanical studies of synthesized copolymers indicate that the glass transition temperature of copolymers is decreased with an increase linear dimethylsiloxane backbone length, n (Figure 8). Since n=12 carbotricyclodecasiloxane fragments in copolymers cause no effect on the dimethylsiloxane backbone and Tg of copolymer 6 (Table 6) remains equal -123°C. 

Figure 15 shows dependence of the glass transition temperature of copolymers on cyclic carbosilo-xane fragment content in dimethylsiloxane backbone. Similar to copolymers from poly(dimethyl-phenylsiloxane) sequence [53], the effect of non-dimethylsiloxane units on 7g of copolymers with cyclic carbosiloxane fragments in the backbone is primarily observed at 3% concentration of the latter. 

A HE SEARCH FOR PLASTIC, solvent-free electrolytes for use in solid-state batteries is being actively pursued in several laboratories (1-4). A number of reports have stressed the need for facile motion of the macromolecular chain in order to promote the ion conduction process in the polymer matrix, because this process occurs primarily via a free-volume mechanism (1-4). Comblike polymers with oligooxyethylene side chains constitute effective media for ion conduction of solubilized alkali salts (5-8). The low glass transition temperature (Tg) of poly(dimethylsiloxane) suggests that polysi-loxane could serve as a suitable backbone for such a comb polymer, and recent studies (9-J2) indicate this to be the case indeed. 

Attenuated total reflection infrared critical micelle concentration electron spectroscopy for chemical analysis hydrophilic-lipophilic balance poly(chlorotrifluoroethylene) poly(dimethylsiloxane) poly(tetrafluoroethylene) poly(trifluoropropylmethylsiloxane) glass transition temperature critical surface tension of wetting Owens-Wendt solid surface tension surface tension of aqueous solution surface tension of liquid... 

FIGURE 12.6 Plot of the glass transition temperature as a function of log r, where x is the number of chain atoms or bonds in the backbone. Data for (-0-) poly(a-methyl styrene), (-A-) poly(methyl methacrylate) (- -) polystyrene, (- -) poly(vinylchloride), (-A-) isotactic poly-propylene, (- -) atactic polypropylene, and (- -) poly(dimethylsiloxane). (From Cowie, J.M.G., Eur. Polym. J., 11, 297, 1975. With permission of Pergamon Press.)... 

Donth, E., The size of cooperatively rearranging regions in polystyrene and styrene— dimethylsiloxane diblock copolymers at the glass transition temperature, Acta Polym., 35, 120-123 (1984). 

Poly(dimethylsiloxane) (PDMS) is a well-known hydrophobic polymer with higher repellency for water than PS crosslinked siUcone elastomers (WCA = 112° for a smooth film) are commonly used for fabricating microfluidic devices. But forming solid fibers comprised solely of linear PDMS is not possible, due to its low glass transition temperature. Instead of using linear homopolymer PDMS, Ma et al. [21] electrospun fibers of poly(styrene-b-dimethylsiloxane) block copolymers blended with 23.4 wt% homopolymer polystyrene (PS-PDMS/PS) from a solution in a mixed solvent of THF and DMF. The resultant fiber mat, with fiber diameters in the range of 150-400 nm, exhibited a WCA of 163° and a hysteresis of 15°. An illustration of water droplets beaded up on such a mat is provided in Fig. 3. A PS mat of similar fiber diameter and porosity exhibited a WCA of only 138°. The difference was attributed to the lower surface tension of the PDMS component, combined with its spontaneous segregation to the fiber surface. X-ray photoelec-... 

Lonsdale, D. A. Monteiro, M. J., Kinetic Simulations for Cyclization of a,(0-Telechelic Polymers. J. Polym. Sci., Part A Polym. Chem. 2010,48,4496-4503. Clarson, S. J. Dodgson, K. Semiyen, J. A., Studies of Cyclic and Linear Poly(dimethylsiloxanes) 19. Glass Transition Temperatures and Crystallization Behaviour. Polymer 1985,26, 930-934. 

The most well-studied polysiloxane viz., [Mc2SiO]n has a Tg of -123 °C [1]. This is one of the lowest values for any polymer. As mentioned earlier in Chapter 2 the glass-transition temperature of a polymer corresponds to a description of its amorphous state. The Tg of a polymer can be taken as a measure of the torsional freedom of polymer chain segments. Above its Tg a polymer has reorientational freedom of motion of its chain segments, while below its Tg this is frozen. Usually elastomeric materials have low glass-transition temperatures. For example, natural rubber has a glass-transition temperature of -72 °C. Similarly, polyisobutylene has a ass-transition temperature of -70 °C. Thus, poly(dimethylsiloxane) has consid-... 

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