Polymers with pendent functional groups. V. Thermooxidative and thermal behavior of chloromethylated polysulfones

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Abstract

The first step of the thermooxidative and thermal decomposition of some chloromethylated polysulfones (CMPSF) with various degrees of substitution has been studied by thermogravimetry (TG) and thermal volatilization analysis (TVA). A step occurring at lower temperatures 260–370°C is particular for halomethylated polysulfones because the polysulfone (PSF) is thermally stable in this temperature range (its first decomposition step occurs from 400 to 550°C). The higher the degree of substitution, the lower the degradation temperature and a better separation of the processes was found. The thermal decomposition products collected were analyzed by IR spectroscopy and elemental analysis. According to our TG and TVA results and literature data , it can be proposed that –CH2Cl groups react at low temperatures giving cross-linked polymer and gas or liquid products. The thermal characteristics of this process depend on the chlorine content of the sample and explain undesirable cross-linking that could take place during the synthesis of the chloromethylated polysulfones.

Introduction

Poly(arylene ether sulfone)s are amorphous polymers which possess excellent thermal stability [1]. This characteristic is not difficult to explain since highly thermally stable diphenylether and diphenylsulfone units are present in the polymer backbone.

It is well known that polysulfone gives outstanding polymer membranes with a high glass transition temperature (195°C), good thermal and oxidative stability (decomposition range between 400 and 550°C regardless of environment — under air, argon or vacuum), excellent strength and flexibility, resistance to extremes pH values and low creep [2], [3]. There is an interest in the chemical modification of polysulfones especially in the halomethylation reaction [4], [5], [6], [7] which leads to precursors for important functional membranes, coatings, ion exchange resins, ion exchange fibers, selectively permeable films, etc. In a previous paper [8], the kinetic aspects of the first step of thermooxidative decomposition under dynamic temperature conditions for bromomethylated and carboxylated polysulfones was studied and it has been established that the chemical modification of polysulfone changes the thermal behaviour. Chemically modified polysulfones decompose in two thermogravimetric steps. The first occurs at low temperatures and corresponds with the elimination and reaction of the functional groups [9].

This paper deals with the thermooxidative and thermal decomposition of the chloromethylated polysulfones, especially with the influence of the degree of substitution (DS) on the thermal characteristics.

Section snippets

Synthesis of the chloromethylated polysulfones and their characterisation

Methylene bridges between the macromolecular chains can appear during the chloromethylation reaction and the corresponding cross-linked chloromethylated polysulfones are insoluble. To prevent this phenomenon, the chloromethylation was performed at high dilution and low amount of catalyst and in the presence of paraformaldehyde.

The mixture of commercial paraformaldehyde/chlorotrimethylsilane (ClSiMe3) from Merck as chloromethylation agent and tin tetrachloride (SnCl4) from Fluka as catalyst were

Results and discussion

The characteristic TG DTG and TVA curves are given in Fig. 3, Fig. 4, respectively.

It can easily be remarked from all three types of curves that with increasing substitution degree, the chloromethylated polysulfones exhibit a particular behavior in decomposition in respect with that of polysulfone.

A new thermo-oxidative or thermal decomposition step is developing at low temperatures that becomes well defined with increasing substitution. This should mean that if for the CMPSF with low degree of

Conclusion

In order to prevent the cross-linking reaction and consequently insolubility of the chloromethylated polysulfones, the conditions of the chloromethylation reaction have been modified.

Chloromethylated polysulphones exhibit a supplementary thermal and thermo-oxidative decomposition step, occurring at low temperatures in comparison with unmodified polysulfone.

Weight losses, characteristic temperatures and overall kinetic parameters have a particular variation with the increase of the substitution

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    As shown in Fig. 9a, with the extension of the reaction time, the trend in DCM values was similar to that in different solvent volumes. According to previous reports, it required more than 72 h under harsh conditions for the preparation of CMPSF with trimethylchlorosilane in most studies, and DCM of between 0.3 and 1.5 were obtained [35–41]. In our current work, when the reaction time was controlled at 24 h, the DCM value could be as high as 1.95.

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