Siloxane copolymers microstructure

Knowledge of XA, XB and R thus permits one to calculate a set of microstructure parameters for a given copolymer. The Si-29 NMR analysis of the end-groups will provide a description of the 3 last siloxane units of the copolymer chain and the copolymer microstructure is fully revealed at the molecular level. The microstructure parameters for a polydimethyl co-methylphenylsiloxane are collected in Table 7. 

In the case of copolymerization using catalysts that do not rearrange the siloxane chain and under non-equilibrium conditions a theoretical model can be developed to describe the microstructure. This same microstructure can be analysed by NMR of proton or silicon-29 nuclei. The concordance between theory and experiment enables confirmation of the correctness of either the theoretical mathematical model or the assignment of NMR signals. Quantitative information on the microstructure of copolymers as a function of time or at any conversion or for any comonomer composition can be obtained. For copolymerization under less precise conditions, the microstructure can be derived from the siloxane copolymer itself by direct observation of the magnetic resonance signals of the silicon-29 nuclei. 

Spontak RJ, Williams MC (1988) Microstructural response of siloxane-imide and SBS block copolymers to heat treatment. Polym J (Tokyo) 20(8) 649... 

The diameters of the particles, prepared from dispersion, are approximately 120 nm each. The powder morphology of spray-dried PMMA grafted siloxane particles is illustrated in Fig. 3a the spherical agglomerates are approximately 1-20 pm in size. The microstructure of the graft copolymer after processing is shown in Fig. 3b. 

A copolymer of D2i-DiSiAn, BPADA, and mPD was prepared and examined by DMA. The DMA indicated a distinct transition at 120 corresponding to the siloxane block and a weaker transition at 75 corresponding to the imide hard block. This behavior is consistent with a phase-separated material with short, well-dispersed hard blocks. The physical properties of this copolymer are shown in the following list. The elastoplastic properties of the material are also consistent with this type of microstructure. 

The process of synthesizing high-molecular-weight copolymers by the polymerization of mixed cyclics is well established and widely used in the silicone industry. However, the microstructure which depends on several reaction parameters is not easily predictable. The way in which the sequences of the siloxane units are built up is directed by the relative reactivities of the monomers and the active chain-ends. In this process the different cyclics are mixed together and copolymerized. The reaction is initiated by basic or acidic catalysts and a stepwise addition polymerization kinetic scheme is followed. Cyclotrisiloxanes are most frequently used in these copolymerizations since the chain growth mechanism dominates the kinetics and redistribution reactions involving the polymer chain are of negligible importance. Several different copolymers may be obtained by this process. They will be monodisperse and free from cyclics and their microstructure can be varied from pure block to pure random copolymers. 

Schrotter I.C., Smaihi M., Guizard C. Polyimide-siloxane hybrid materials influence of coupling agent addition on microstructure and properties. J. Appl. Polym. Sci. 1996 61 2137-2149 Soler-Dlia G.I., de A.A., Crepaldi E.L., Grosso D., Sanchez C. Block copolymer-templated meso-porous oxides. Cnrr. Opin. Colloid Interface Sci. 2003 8(1) 109-126 Steele B. Ceramic Ion Conducting Membranes and Their Technological Apphcation. C.R. Acad. Sci. Paris 1998, Series lie. Title 1, pp. 533-543... 

---