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

Abstract

New measurements and literature data on polysiloxanes covering heat capacities, transition parameters, enthalpies, entropies and Gibbs energies are presented and critically reviewed. The ATHAS computation method is used to bring heat capacities into agreement with an approximate frequency spectrum. The various crystal and mesophases are discussed. The ATHAS (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/tool. Tables of heat capacities, enthalpies, entropies and Gibbs energies are given from 0 K to 550 K.Of the poiysiloxanes only poly(dimethyl siloxane), PDMS, and poly(diethyl siloxane), PDES, have found major attention. In the 1980 ATHAS data bank of heat capacities of macromolecules [1] only few data on semicrystalline PDMS and PDES could be presented [2]. The follow-up analyses and fitting of the data in the solid state to approximate vibrational frequency spectra done with many other polymers [3] could not be carried out before because of lack of data and understanding of the phase behaviour. This paper, largely based on the experimental work of one of us (J.P.W.) [4] and the continuing A THA S collection of thermal data, brings PDMS and PDES into the group of close to 100 well characterized polymers [5].

Structure and Phase Transitions of Poly(diethylsiloxane)

“…Δ S = Δ H / T ). The agreement with those collected in ref is good. The results presented in Table and Figure also show, however, that the “recommended average” values for the transition temperatures, enthalpies, and entropies given in ref are not meaningful.…”

Synthesis and characterization of poly(diethylsiloxane) and its copolymers with different diorganosiloxane units

“…In particular, their applications in severe environments, especially in low‐temperature regions, have become broader in recent years. Poly(diethylsiloxane) stands out among all other poly(diorganosiloxane)s in that it has, so far, the lowest glass‐transition temperature ( T g ): −138 °C,2, 3 although its low‐temperature use as an elastomer is limited by its crystallization at temperatures near −73 °C and a mesomorphic phase transition at approximately 20 °C 4–7. Previous studies on poly(diethylsiloxane) copolymers mainly focused on copolymers bearing two kinds of units, including the introduction of diphenylsiloxane (Ph 2 SiO), 3,3,3‐trifluoropropylmethylsiloxane, ethylphenylsiloxane, or methylphenylsiloxane (MePhSiO)8, 9 into the poly(diethylsiloxane) backbone to disrupt the crystallization.…”

Positron annihilation lifetime study of organic-inorganic hybrid materials prepared by irradiation

“…A low temperature region (bellow 155 K), where the samples are in a glassy state, a middle temperature region (155-220 K) and a high temperature region (above $220 K) -rubber state. According to DSC results of Varma-Nair et al the glass transition temperature, T g , and the melting point, T m , of PDMS are 146 K and 219 K, correspondingly [19]. The same temperature points for PDMS measured by PAL spectroscopy are given as 147 K and 238 K [20].…”