Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms

doi:10.1016/S0032-3861(01)00785-6

Polymer

Volume 43, Issue 7, March 2002, Pages 2011-2015

https://doi.org/10.1016/S0032-3861(01)00785-6 Get rights and content

Abstract

The products of the thermal degradation of polydimethylsiloxane (PDMS) are determined by the heating conditions, since two competing mechanisms are involved.

Cyclic oligomers are formed in the low degradation temperature range and during slow heating in programmed degradation. This involves molecular splitting of oligomers from loop conformations of the PDMS chain favoured by its flexibility, and assistance on the part of empty silicon d-orbitals.

Methane and oligomers are formed in the high temperature range and during fast heating. This shows that homolytic scission of Si–CH₃ also takes place and is followed by hydrogen abstraction.

Introduction

The mechanisms of the thermal degradation of polydimethylsiloxane (PDMS) end-blocked with (CH₃)Si-groups have been intensively studied [1], [2]. Degradation in an inert atmosphere (N₂) and under vacuum results in depolymerisation and the production of cyclic oligomers [3], [4]. The trimer is the most abundant product, with irregularly decreasing amounts of the tetramer, pentamer and hexamer and higher oligomers [3].

Examination of linear and cyclic PDMS has indicated that they share a common molecular depolymerisation mechanism [5]. Nielsen [6] has proposed that cationic reactions on a glass surface (borosilicate, quartz) contributes to the depolymerisation of linear PDMS, while Zeldin et al. [7] suggest that it may be catalysed by ionic impurities from the walls of Pyrex glass vessels. The small amounts of macro-cycles (>10 Si atoms) formed during the thermal degradation of PDMS have also been the subject of attention [8]. Their presence points to ionic ring-opening polymerisation reactions and formation of a distribution of cyclic species. Clarson and Semlyen [9], in fact, have demonstrated ring-opening polymerisation catalysed by ions of the surface of the glass, when the cyclic tetramer, octamethylcyclotetrasiloxane is heated at 420 °C under vacuum in a Pyrex vessel. The degradation products are consistent with subsequent formation of cyclic species from the linear high molecular mass PDMS [9].

It can be thus assumed that at temperatures below 500–600 °C, PDMS depolymerise completely in an inert atmosphere and do not form a solid residue, whereas in air their decomposition is accompanied by the formation of some white silica powder [10]. However, the products of the thermal PDMS degradation are essentially determined by the temperature and the heating rate. This question is examined in depth in the present paper.

Section snippets

Materials

Polydimethylsiloxane end-blocked with trimethyl-siloxy-groups (CH₃)₃Si-containing a vinyl-methyl-siloxane unit every 1400th –(CH₃)₂–Si–O unit (V1400) with a viscosity of 8×10⁶ mPa was supplied by Wacker–Chemie Gmbh. The effect of cross-linking involving –CC– bonds during the thermal degradation of PDMS can be ignored owing to its very low cross-linking density.

Thermal degradation

Degradation in He or air was carried in the glass apparatus of Fig. 1. In air, methanol cooled at −78 °C was used to condense volatiles

Slow heating rate

The thermal degradation of PDMS to cyclic oligomers has been illustrated in a well-known depolymerisation diagram [10]. It has been suggested that the formation of an intramolecular, cyclic transition state is the rate-determining step [10]. Silicon d-orbital participation was postulated with siloxane bond rearrangement leading to the elimination of cyclic oligomers and shortening of the chain. The mechanism for formation of the smallest cyclic product, hexamethylcyclotrisiloxane, is

Conclusions

Thermal degradation of PDMS occurs through two competing mechanisms. A molecular mechanism takes place with formation of cyclic oligomers. This implies Si–O bond scission, whose dissociation energy (108 kcal/mol) is lowered below that of Si–C (78 kcal/mol) by silicon d-orbitals involved in cyclic transition state favoured by chain flexibility.

A radical mechanism occurs through homolytic Si–CH₃ bonds scission. This prevails at high temperatures and leads to methane through hydrogen abstraction.

Acknowledgements

The authors would like to thank BRITE EURAM III, project no BRPR-CT098-0655 for their financial support.

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Thermal polydimethylsiloxane degradation. Part 2. The degradation mechanisms

Abstract

Introduction

Section snippets

Materials

Thermal degradation

Slow heating rate

Conclusions

Acknowledgements

Eur Polym J

Polymer

Polymer

J Am Chem Soc

J Polym Sci (A-2)

J Polym Sci