**4.2. Copolymerisation**

Copolymerisation of two or more polymers together is a strategy to produce a new materials of tailored dielectric properties. In copolymersation, two or more different monomer units were covalently bound thus producing a synergesic effect of respective constituents. Copolymerisation of polyimide with polysiloxane is popularly performed due to the complementary chemical and mechanical properties between the two. The polyimide has superior thermal and mechanical properties but too intractable to normal processing methods. For example the modulus of polyimides are in the range of 109 to 1012 Pa but their Tg are above 260 oC. On the other hand the polysiloxane is flexible and easily processable beside having a stable thermal degradation (> 400 oC). Copolymers of these materials produce an optimized dielectric material of practical application for several electronic packagings. Attempt was made with the following structures. [29]

Polymeric Dielectric Materials 21

volume. Several recent studies have demonstrated a similar trend of a decreasing dielectric

The traditionally used inorganic material as a dielectric possesses several superior qualities such as excellent thermal, dielectric and magnetic properties. However they are brittle and consume high energy for processing. [32] On the other hand polymers are more flexible, strong resistivity and offer a tractable prosessibility. The disadvantages of polymeric materials are that they possesses lower thermal and dielectric properties. Combining the two materials in the form of nanocomposites offer an alternative in fabricating material of synergesic properties which displayed a tremendous improvement in dielectric properties with high flexibility and ease of processing. Their combination could readily geared towards

Of late several attempts were made towards this strategy. Incorporation of alumina (Al2O3), barium titanate (BaTiO3), titania (TiO2) and zirconia (ZrO2) into PI matrix were attempted.. [33,34] Several methods were employed in preparing these nanocomposites. It has been established that method of preparation affect the dielectric properties of these materials. A nanocomposite of PI/Al2O3 was prepared by mechanical stirring of prepolymer polyamic acid with the inorganic filler followed by thermal curing. [35] The nanocomposite showed an improved dielectric constant compared to a neat polymer material from about 3.0 to 3.4 at 1 MHz. This values increases correspondingly with the amount of filler loading. A further increase in dielectric constant was achieved when mixing was performed using ultrasonication. It has been shown from SEM result that this improvement was due to a better mixing during the latter treatment. Under these processes, the crystal structure of the inorganic fillers remains intact as shown by XRD data. The effect of good miscibility in improving the dielectric constant was proven when using a 3-Aminopropyltrimethoxysilanetreated (APS) ultrasonication. The APS served as an interface layer between the two immiscible organic PI with inorganic filler which reduced any agglomeration between the different phases. This is brought about possibly through the formation of hydrogen bond between the amine moeity of APS with the polar group of polyimide while the inorganic part of the methoxysilane of APS form secondary interaction with the inorganic fillers. Figure 13

reveals the SEM images of PI/ Al2O3 composites doped by the treated Al2O3 powder.

PA0 demonstrated a neat and clean morphology. The Al2O3 particles were homogeneously dispersed into PI matrix in all PA10, PA20 and PA30. The inset images revealed the average size of Al2O3 was around 2µm - 4µm. There was no obvious aggregation observed suggesting the improved compatibility between PI matrix and Al2O3 attributed to the APS coupling agent. The bahaviour of PI-nanocomposites for BaTiO3, TiO2 and ZrO2 displayed similar trend with that of PI-Al2O3 nanocomposites. They can be summarized as in the following Figures 14:

constant, with an increasing siloxane content into polyimide structures [30,31]

miniaturization of electronic devices fabrication.

**5.1. Polyimide-ceramic composites** 

**5. Composites** 

**Scheme 5.** Series of PI-polysiloxane copolymers

Their dielectric constant are shown in the following Table 5:


**Table 5.** Effect of silicone content in silicon-polyimide copolymers on dielectric constant.

The table above shows there is a decreasing trend in dielectric constant with the increase in siloxane units. Silicon is comparatively larger than a carbon atom and the Si - O bond is more flexible than the C - C bond. Thus, the bulky silicone units would be less mobile. Its presence affects the bulk movement of the whole polyimide network which reduces the efficiency of the dipole in reacting to polarity change during treatment with an alternating frequency. Furthermore, the molar polarization significantly decreases as the result of an increase in free volume. Several recent studies have demonstrated a similar trend of a decreasing dielectric constant, with an increasing siloxane content into polyimide structures [30,31]
