Polyimides group contribution methods

The chapter is divided into the following sections. First, a brief introduction to group contribution methods is given with a major emphasis on the concept and limitations of this technique. An introduction to the use of chemical graph theory and how it applies to polymers and in particular to the dielectric constant is given next. Application of the method to a number of polyimides is then demonstrated and predictions are compared to experimental results. 

Robeson et al. have also reported a group contribution approach to predict permeability and permselectivity for O2/N2 He/N2 gas pairs of aromatic polymers by determining the contribution of each structural unit of the polymer to the overall permeation behavior(- 2). Their ability to predict permeability/permselectivity values for polymers outside the database is also good. However, we could not apply their new group contribution method to our experimental data for polyimides because their group contribution units did not include all of the necessary structural units. 

We have investigated relations between apparent gas diffusion coefficients in glassy polymers and both calculated and measured physical parameters. Cohesive forces and polymer mobility influence gas diffusivities in our glassy polyimides. The correlation between D and common parameters (i.e. Vf, CED, E ) of polyimides having side chain substituted polar groups differs from those of nonpolar polymers and weakly polar polymers. Gas diffusivity values were not well-correlated with free volume. Relative to free volume, CED and E are more sensitive to polymer mobility. Estimation of gas diffusivities based on group contribution methods is possible by using factors, such as CED, which can be calculated without any measurements. 

The author worked for many years at BP Research on the synthesis of LCPs and devised an empirical method called the Mesogenic Index, which employs functional group contributions on an additive score basis to predict whether a particular random copolymer is likely to exhibit a mesophase (subject to the polymer being soluble or fusible). This chapter explores the general features and theoretical aspects of the chemical structures of main chain LCPs and describes the Mesogenic Index and how it was successfully applied to polyesters, polyamides and polycarbonates. The final section describes the extension of the MI empirical method to the various types of LC polyimides reported in recent years. 

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