Hydrogen adsorption by δ and ε crystalline phases of syndiotactic polystyrene aerogels

The H₂ uptake from s-PS samples exhibiting different crystalline phases and different morphologies has been studied by gravimetric measurements at 77 K in the hydrogen pressure range from 0 up to 1.7 MPa and compared with molecular simulations relative to s-PS crystals. Gravimetric experiments show that the molecular hydrogen sorption is strongly dependent on the sample morphology and is maximum for low-density polymer aerogels. However, independently of the morphology, the H₂ uptake is minimum for the dense β and γ crystalline phases, intermediate for the channel-shaped nanoporous ε phase, and maximum for the cavity-shaped nanoporous δ phase. In particular, although the two nanoporous crystalline phases present essentially the same density (0.98 g/cm³), the hydrogen uptake from the δ phase is roughly double with respect to the uptake from the ε phase, both for powders and for aerogels. Infrared measurements and molecular simulations well agree with these quantitative sorption data and clearly indicate that, for both low and high pressure, the hydrogen molecules are preferentially adsorbed into the nanoporous crystalline phases. In particular, molecular simulations indicate that the maximum average hydrogen uptake is of nearly 3 molecules per cavity of the δ phase and of nearly 3.5 molecules per unit height of the channels of the ε phase.