Model polyimides

The ultimate goal here was to tailor the dielectric material to the particular requirements of this circuit. This paper describes the initial work done on a model polyimide resin toward this goal. 

The model polyimide chosen for this study was the polymer derived from 3,3 ,4,4-benzophenone tetracarboxylic dianhydride and 2,3,5,6-tetramethyl-l,4-phenylene... 

Imidization is achieved by thermal treatment of the poly(amic ester) precursor in the usual way, with elimination of alcohol. This method, because of its relative complexity, has not got practical significance for conventional polyimides. However it has been of great importance in the development of photocurable condensation polyimides [8], and to study the behaviour of different isomers as starting materials for model polyimides [67]. 

Figure 7. Comparison of various models for the F650 polyimide cure at 3° F/min.

The first section, Chemical Reactions on Polymers, deals with aspects of chemical reactions occurring on polymers—aspects relating to polymer size, shape, and composition are described in detail. One of the timely fields of applications comprises the use of modified polymers as catalysts (such as the immobilization of centers for homogeneous catalysis). This topic is considered in detail in Chapters 2, 3, 8, 9, and 11 and dealt with to a lesser extent in other chapters. The use of models and neighboring group effect(s) is described in detail. The modification of polymers for chemical and physical change is also described in detail in Chapters 2 (polystyrene) 4 (polyvinyl chloride) 5 (polyacrylic acid, polyvinyl alcohol, polyethyleneimine, and polyacrylamide) 6 (polyimides) 7 (polyvinyl alcohol) 8 (polystyrene sulfonate and polyvinylphosphonate) 10 (polyacrylamide) and 12 (organotin carboxylates). 

The thermal polymerization of reactive polyimide oligomers is a critical part of a number of currently important polymers. Both the system in which we are interested, PMR-15, and others like it (LARC-13, HR-600), are useful high temperature resins. They also share the feature that, while the basic structure and chemistry of their imide portions is well defined, the mode of reaction and ultimately the structures that result from their thermally activated end-groups is not clear. Since an understanding of this thermal cure would be an important step towards the improvement of both the cure process and the properties of such systems, we have approached our study of PMR-15 with a focus only on this higher temperature thermal curing process. To this end, we have used small molecule model compounds with pre-formed imide moieties and have concentrated on the chemistry of the norbornenyl end-cap (1). 

In comparison, no structural modification of model B was seen before 120 h of aging (80 °C). However, after 120 h two small doublets appeared in the NMR spectrum and several additional peaks became noticeable in the NMR spectrum. It was determined by NMR and IR spectroscopy that the hydrolysis products were an imide/carboxylic acid and an imide/anhydride. Model B was then aged for 1200 h at 80 °C to quantitatively determine the amount of hydrolysis products as a function of time. The relative intensity of the peaks due to carboxylic acid is constant after some time. The authors suggest that an equilibrium occurs between model B and the products formed during hydrolysis, and therefore, the conversion to hydrolysis products is limited to about 12%. This critical fraction is probably enough to cause some degradation of polymeric materials, but research on six-membered polyimides has remained active. 

Three different dry etch techniques were investigated isotropic O2 plasma etching in a Tegal 200 reactor, R.I.E. in a parallel-plate in-house modified Tegal AOO reactor and R.I.M. in a Veeco, Model RG-830. The conditions of operation for each system were as follows where time is the time to etch 1.2 p of fully cured polyimide. 

The effective bulk polyimide resistivity can be extracted from comparison of the measured curves in Figure 6 with this model, and values at 100°C and 158°C are shown in Figure 7, which is a reproduction of the Arrhenius plots of Figure 4. It is seen that both the values of the resistivity and the temperature dependence are in good agreement with those obtained from the dc conduction measurements. In addition, the saturated value of the measured threshold instability is well predicted by the model. 

Initial investigations of base-catalyzed imidization of polymeric systems, in particular PMDA/ODA based polyfamic alkyl esters), have been difficult due to the insolubility of the polyimide precursor at imidization levels exceeding 40%. Nevertheless, preliminary studies indicate that the base-catalyzed polymer imidization reaction appears to be significantly slower at ambient temperatures as compared to the phthalamide model compounds. It is yet unclear whether this is a direct result of the conformational aspects associated with the polymer chain or solubility considerations arising from the less soluble, partially imidized polymer chain. Since much of the initial work involved IR studies of supported... 

Model polyamicdialkylamides were prepared by the reaction of the secondary amines (XX) and (XXI) with the polyisoimide (XVII) in DMAC at room temperature. Unfortunately, the resultant polyamicdialkylamides only exhibited limited softening behavior for processing into bulk specimens. The inability to provide molded specimens prevented the validation of the In Situ Molecular Composite concept. Research is currently being carried out at a number of academic and industrial institutions on new thermoset chemistry, polyamices-ters, and extended chain polyimides. 

The origin of this absorption may be water molecules with one OH group bonded to the membrane and the second OH group non-H-bonded or water molecules with H-bonds to weak acceptors of the membranes. In both cases the water transport through these membranes may be related to weak H-bonds water-membrane. In addition the spectroscopic observations show in celluloseacetate — or polyimide — membranes are less water molecules of the type of liquid water. In agreement with this observation we have found in model glass membranes with low salt rejection (about 70%) at high relative humidity water spectra not far from the spectra of liquid water. As result of these first experiments we may discuss two possible mechanism of membranes for desalination processes ... 

The polyimide-base PR system [79,80] was designed on the premise that porphyrin-electron acceptor (quinones or imide moieties) systems are well-known model compounds for photosynthetic processes and exhibit very interesting charge transfer properties [81], A high quantum yield of charge separation can be achieved in these systems. Polyimides are found to be photoconductive and allow charge transport [82], Furthermore, polyimides possess high Tg and therefore, the electric field-induced dipole orientation can be fixed after imidization [83],... 

Figure 2.3 Motion of a carbon dioxide molecule in a 6FDA-4PDA polymer matrix [8]. Reprinted from J. Membr. Sci. 73, E. Smit, M.H.V. Mulder, C.A. Smolders, H. Kar-renbeld, J. van Eerden and D. Feil, Modeling of the Diffusion of Carbon Dioxide in Polyimide Matrices by Computer Simulation, p. 247, Copyright 1992, with permission from Elsevier...

Fig. 3.10. Experimental (dotted lines) and best-fit model (solid lines) IRSE spectra of a PLD-grown (0001) ZnO thin film on (001) silicon (panel (a), film thickness d 670 nm), and magnetron-sputtered ZnO thin films on metallized polyimide foil (panel (b), d 500nm) and on metallized glass (panel (c), d 30nm) [43]. ZnO phonon-mode frequencies, as obtained by best-model analysis, are marked by vertical arrows...

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