Polyimide (PI) composite films were synthesized incorporating amino modified silicon nitride (Si

3

N

4

) nanoparticles into PI matrix

via in situ

polymerization technique. The mechanical and thermal performances as well as the hydrophobic properties of the as prepared composite films were investigated with respect to the dosage of the filler in the PI matrix. According to Thermogravimetric (TGA) analysis

meaningful improvements were achieved in

T

5

(5% weight loss temperature) and

T

10

(10% weight loss temperature) up to 54.1℃ and 52.4℃

respectively when amino functionalized nano-Si

3

N

4

particles were introduced into the PI matrix. The differential scanning calorimetry (DSC) results revealed that the glass transition temperature (

T

g

) of the composites was considerably enhanced up to 49.7℃ when amino functionalized Si

3

N

4

nanoparticles were incorporated in the PI matrix. Compared to the neat PI

the PI/Si

3

N

4

nanocomposites exhibited very high improvement in the tensile strength as well as Young's modulus up to 105.

4% and 138.3%

respectively. Compared to the neat PI

the composites demonstrated highly decreased water absorption behavior which showed about 68.1% enhancement as the content of the nanoparticles was increased to 10 wt%. The SEM (Scanning electron microscope) images confirmed that the enhanced thermal

mechanical and water proof properties are essentially attributed to the improved compatibility of the filler with the matrix and hence

enhanced distribution inside the matrix because of the amino groups on the surface of Si

3

N

4

nanoparticles obtained from surface functionalization.