**5. Composites**

20 Dielectric Material

**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]

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

Their dielectric constant are shown in the following Table 5:

Sample Dielectric constant at 1 kHz 293 oC n (Si-O repeat unit) S1 2.90 0 S2 2.57 1 S3 2.43 34

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

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

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 miniaturization of electronic devices fabrication.
