| While “silicone rubbers” formed from poly(dimethyl siloxane) have been exhaustively studied, there are still many unanswered questions. Are there inhomogenous regions? How are inhomogeneous networks formed? What happens when there is a distribution of network strand lengths? What happens if one tries to deform the network (obviously, this would be important for a fly trapped in the web!); We will show that most PDMS networks have spatially inhomogeneous regions. We can develop micron scale phases that exactly emulate phase separated polymer blends. The crosslinking chemistry, molecular weight, catalyst, and curing conditions are varied, and prove that the phase separation is an intrinsic mechanism of forming regions of high and low crosslink density. We characterize these phase separated networks with light microscopy, atomic force microscopy, mechanical properties, and a variety of scattering techniques. We also show that the scattering response seems to indicate that some model networks can exhibit entanglement interactions. The deformation of several networks appears to occur by unfolding on large length scales.; We have also prepared several bimodal networks by endlinking very short oligomeric chains with much longer chains. Some of these networks were formed from monodisperse dimer (M = 166 g/mol) and trimer (M = 240 g/mol) chains, having 6 and 8 skeletal bonds between junctions respectively. We also used mechanical and swelling techniques to show that these networks appeared to have high functionality crosslinks, which likely arised from “clustering” of the short chains into high crosslink density regions. The birefringence of these networks suggests that the “classical” rubber elasticity theories based on the Gaussian distribution can apply in certain circumstances. Furthermore, we note that several networks described in the literature as having significant “entanglement” effects were likely “clusterlike.” We also describe a multimodal network, formed by endlinking a variety of functionalized chains in molecular weights ranging from 300 g/mol to well over 50,000. The chain length polydispersity, Mw/Mn of this network was 10, but similar networks could have polydispersities over 50!... |
