Effect of chain extender dichlorodimethylsilane (DDS) on the mechanical properties of hydroxyl terminated polydimethylsiloxane (HOPDMS)

The effect of the chain extender dichlorodimethylsilane (DDS) on the mechanical properties of hydroxyl terminated polydimethylsiloxane (PDMS) was investigated. Different amounts of DDS ranges from 0.5 to 5.0 phr were added to the base polymer. After degassing (degasification) the prepared solution of chain extender (DDS) for 5h, viscosity was measured. It was observed that viscosities were increased from 1189 to 212246 cst (almost 180 times), while their molecular weights increased from 13899 to 206019 g/mol (almost 15 times) at 3.5 phr DDS concentration. Crosslinking agents such as tetraethoxysilane [tetraethyl orthosilicate (TEOS)] and tin catalyst were utilized for fabricating the sheet. Mechanical properties were evaluated by using universal testing machine. Tear strength was increased from 0.057 to 0.352 N/mm while Tensile strength was decreased from 0.40 to 0.23 MPa % and elongation at break was increased from 38.87 to 552.20%. Durometer shore A was utilized to measure hardness of the thin films which was decreased from 25 to 10 shore A. Swelling measurement indicated that crosslinking density was decreased from 3.6x10 ⁻⁴ to 1.1x10 ⁻⁵ mol/cm ³, while molecular weight between crosslinks was increased from 2979 to 89597 g/mol. Increase in viscosity, molecular weight and average molecular weight between junctions and decrease in crosslinking density with the addition of dichlorodimethylsilane as chain extender confirmed the chain extension of low molecular weight PDMS. Results showed that maximum outcome in physical and mechanical properties of cured PDMS were obtained at 3.5 phr DDS concentration. Relation between tear strength and average molecular weight between junctions obeyed the lake and Thomas equation.