Dynamic Molecular Phenomena in Polyimides Investigated by Dynamic Mechanical Analysis

ABSTRACT

The formation of polyimides by the combination of dianhydrides with diamines results in a large diversity of structures having various mobilities, inter/intra-chain interactions, and topologies. Generally, the processing of polyimides in the imidized form is complicated even because of molecular rigidity and strong inter-chain interactions that entails poor organosolu- bility. The molecular dynamics of polyimides is the result of the balance between molecular packing and chain motions. The phenomenon has effects also on the application of polyimide, for example, in the membrane field. Dynamic mechanical analysis (DMA) is a convenient method to investigate molecular dynamic in polyimides. The secondary relaxations of polyimides can be accurately defined by DMA, along with their influence on the whole behavior of the polymer. As a polyimide is prepared by thermal imidiza- tion of its precursor, poly(amic acid) (PAA), the result of a temperature sweep DMA gives also information on the evolution of the system during heating, hi addition, the accurate assessment of the glass transition region cannot be established without considering the occurrence of imidization by increasing temperature. Even more challenging is the situation when, during iinidization, the analyst deals with cross-linking, thermal rearrangement processes, or solvent evaporation. The chapter will frame all the aspects above by considering a couple of examples for illustration.

INTRODUCTION

A closer look to the history of polymers reveals that the appearance and development of a specific characterization method keeps the pace with the need to find answer for a specific polymeric behavior.¹’² In the early 1920s, Staudinger introduced to the scientific world his theory about the high macromolecular compounds, covalently bonded, that were named macromolecules. Few years later, in 1929, rheology was established as a new branch of science when scientists became aware of the special nature of polymers. They are viscoelastic materials having simultaneously solidlike and liquid-like properties.^3-5 Since then, the face of the world has changed as polymers invaded literally our lives.⁶ For example, we are surrounded by gadgets that help us in our work and keep us informed and connected to each other more than never before. One of the polymeric components of these electronic devices is a polyimide that usually functions as isolator.⁷

Polyimides belong to the class of high-performance polymers.⁸-⁹ Herein high-performance refers to an outstanding thermal behavior combined with excellent mechanical and electrical properties that come from the attribute of the stable aromatic ring (imide) present in the polymer backbone. Unfortunately, these rings confer rigidity to the backbone and induce high intennolecular associations. This is why it is a troublesome task to process the polymer from the melt or to find solvents for casting. In this sense, the effort was concentrated to the manufacture of stable and processable polyimides, without sacrificing the desired properties. The major parameters that govern the processes involved in the preparative steps are temperature and time profile. They decisively influence the properties of the final polyimide material. The trade-off between flexibility and thennomechanical properties is established by investigation of the main relaxational phenomena that characterize the polymer. Dynamic mechanical analysis (DMA) is one of the most reliable methods for ascertaining transitions in polymers.¹⁰¹¹ It is part of rheology and is based on the application of an oscillating stress (or strain) to a sample and on the recording of the material response to this stimulus. DMA tests the viscoelastic properties of a material (modulus and damping) and it can also be used to follow the temperature/ffequency (and therefore time) dependence of these transitions.

This chapter will start with a general view on polyimides: how they are obtained, why the preparative step is challenging, what are the factors that determine the process, and how can they be controlled. Some representative structures will be presented. The second part will describe succinctly the DMA method and how it can describe the relation structure-property in polymers. Then, the next two parts will intend to put forward the utility of the method as a research instrument in the field of polyimides, either for the characterization of the polyimide materials, or for understanding and optimization of the synthesis steps. DMA exploits the concept of time-temperature equivalence, that is, the analogy between the transformations determined by temperature and frequency changes. The last parts will overview when this principle was applied to polyimides and with what purposes.