Molecular-Weight-Dependent Infiltration and Adsorption of Polymers into Nanochannels

Abstract

The mesoporous silica shell coating hydrogenolysis nano-catalysts alters the molecular weight distributions of cleaved polymer chains compared to catalysts without a shell. The shell, composed of radially aligned narrow cylindrical nanopores, reduces the formation of low-valued gaseous products and increases the median molecular weight of the product, thus enhancing the value of the products for polymer upcycling. To understand the role of the mesoporous shell, we have studied the spatial distribution of polystyrene chains, used as a model polymer, in the nanochannels in both the melt phase and solution phase. In the melt, we observed from small-angle X-ray scattering experiments that the infiltration rate of the polymer into the nanochannels is inversely proportional to the molecular weight, which is consistent with theory. In theta solution experiments using UV−vis spectroscopy, we found that the shell significantly enhances polymer adsorption compared to nanoparticles without pores. In addition, the degree of polymer adsorption is not a monotonic function of molecular weight but initially increases with the molecular weight before eventually decreasing. The molecular weight for the peak adsorption increases with the pore diameter. This adsorption behavior is rationalized as resulting from a balance between the mixing entropy gain by surface adsorption and the conformational entropy penalty incurred by chains confined in the nanochannels. The spatial distribution of polymer chains in the nanochannels is visualized by energy-dispersive X-ray spectroscopy (EDX), and inverse Abel-transformed data reveals a less uniform polymer distribution along the primary pore axis for longer chains.

Chemical Upcycling of Polyolefin Plastics Using Structurally Well-defined Catalysts

“…A microkinetic model coupled with population balance models was proposed to predict the transformation of molecular weight distribution during deconstruction and simultaneously validate the processivity in the catalytic reactions, consistent with the results of UV–vis-based experiments that the adsorption behavior of hydrocarbon chains is molecular-weight dependent. 32 , 33 Compared with long polymer chains, shorter segments would not have a longer residence time in the pore, which prevents them from converting to gases through further C–C bond cleavages and enables the processivity of this mesoporous catalyst. 32 These results reveal that mSiO 2 /Pt/SiO 2 can effectively mimic the catalytic behavior of natural enzymes and realize the processive cleavage of the C–C bonds in PO, as depicted in Figure 1 a.…”

“… 32 , 33 Compared with long polymer chains, shorter segments would not have a longer residence time in the pore, which prevents them from converting to gases through further C–C bond cleavages and enables the processivity of this mesoporous catalyst. 32 These results reveal that mSiO 2 /Pt/SiO 2 can effectively mimic the catalytic behavior of natural enzymes and realize the processive cleavage of the C–C bonds in PO, as depicted in Figure 1 a. This processive PE hydrogenolysis yields narrowly distributed alkane products.…”