macromolecule

4 reasons cross linking ::
- restricts freedom of motion ( prevents chain from sliding )
- raises tensile strength and generate elastic properties of material
- prevent dissolution of polymers
- improve thermal stability

PDI :: a measure how widely distributed the range of mw are in a mixture and value is more than 1

cross-linking :: bonds are form between different chains

PDMS why flexible ::
- all trans conformation
- Si-O bond shorter than Si-C
- Si-O-Si bond angle 145 ---> much longer than average C-C bond so less steric hindrance and intramolecular congestion in PDMS )
                                       ----> Si-O-Si bonds wider than tetrahedral bonding in C ( undergo free internal rotation )
                                        ----> only silicon has 2 substituents while O has none ( less steric congestion along backbone )
                                        ----> lack intramolecular interactions between methyl group ( pairs of CH3 groups separated by 4 bonds )