Subnanoscopic inhomogeneities in model end-linked PDMS networks probed by positron annihilation lifetime spectroscopy and their effects on thermomechanical properties

- Investigation of PDMS network structure at elementary mesh size level by PALS.
- Correspondence between segmental spacing and free volume hole sizes.
- Crystallinity influences sub-room temperature properties and thermal transitions.
- Subnanoscopic inhomogeneities influence tensile properties of networks at RT.

The subnanoscopic free volume hole properties of model poly(dimethylsiloxane) (PDMS) networks with unimodal and bimodal distribution of chain lengths between the cross-links were determined using positron annihilation life time spectroscopy (PALS). Wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) studies were carried out to further understand the structural inhomogeneities of the networks at different length scales. The networks were prepared by end-linking combinations of sufficiently longer (Mn = 18,952) and shorter (Mn = 4323) precursor PDMS chains relative to entanglement spacing (Me). The macroscopic properties of the networks were studied by tensile testing, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), as well as equilibrium swelling measurements. Through PALS studies it was possible to delineate the subtle variations in the elementary mesh sizes, a measure of the segmental spacing, of the unimodal short and long chain networks influenced by cross-links or entanglements. The intersegmental spacing of the unimodal PDMS networks at a length scale of few angstroms obtained from WAXD measurements were found to be almost in quantitative agreement with the free volume hole sizes obtained from PALS. SAXS measurements on solvent swollen PDMS networks from short and long chain precursors were used to obtain correlation length of the networks at a higher length scale of a few nanometers. Further, the studies revealed that the enhanced mechanical properties seen in certain bimodal elastomer networks are connected with topological features of the networks. A noteworthy finding from PALS studies was that the bimodal composition with 20 wt% short chain, which showed the best combination of mechanical properties with a feature of stress-upturn in tensile testing, exhibited the least free volume size and relatively lower dispersion in its size distribution among the networks. These fundamental studies are expected to provide new insights into the network structure of model elastomers at a molecular scale and its possible connection with macroscopic properties.

Schematic of the free volume hole sizes in the unimodal PDMS networks (a) bigger size free volume holes in cross-link dominated short chain network (b) the network structure from long chain network with the free volume hole size influenced by trapped entanglements.336