Molecular Weight Reduction During Processing

Molecular weight and crosslink modifications due to interference with curing processes. 

Adsorption of polar, low molecular weight species such as surfactants, plasticisers, stabilisers and antioxidants and oxidation products. 

Formation of an immobilised shell of polymer around the particles. Often this shell is rich in a certain molecular weight fraction, which is then depleted in the matrix. 

Effects on polymer conformation due to particle surfaces and inter-particle spacing. 

A brief description of the various effects together with examples of their importance is given in the next section. 

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Polyols may contain small amounts of peroxides formed during manufacture or by heating during processing operations. These can give rise to discoloration and perhaps to some reduction in physical properties due to molecular weight reduction by chain scission. 

The formation of poly(amic acid), which is the first step in this synthesis, is a fast exothermic reaction which is generally carried out at room temperature or slightly above in a solvent for the polymer. Under these conditions the extent of imidization is negligible. In a typical process, the diamine is dissolved in dimethylformamide, dimethylacetamide or dimethyl sulphoxide and the dianhydride is added portion-wise with cooling. A viscous solution of the poly(amic acid) is formed. The poly(amic acid) contains a mixture of m- and p- diamide units as shown above. The second step in the preparation of the polyimide is cyclization of the poly(amic acid). Aromatic polyimides are usually insoluble and infusible and cannot therefore be fabricated either in solution or in the melt. It is thus necessary to convert the poly(amic acid) to the polyimide in that physical form in which the final polymer is desired. For example, in the preparation of film the poly(amic acid) solution is cast and then heated firstly to about 150°C and finally to 300°C. During imidization, water is formed and may cause scission of the polymer chains, particularly in the casting of thick film. The addition of acetic anhydride and pyridine (catalyst) to the poly(amic acid) solution results in the effective removal of water from the reaction system and prevents molecular weight reduction. 

Van Den Einde RM, Van Der Goot AJ, Boom RM. 2003. Understanding molecular weight reduction of starch during heating-shearing processes. J Food Sci 68 2396-2404. 

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