Confinement effects of polymers in porous glasses

Abstract

Recently, confinement effects on dynamical properties of liquids inside restricted volumes have been extensively studied, either from a theoretical or technological point of view, thanks to the large possibility of industrial applications (building of optical switches, membrane separation, catalysis). We performed depolarized light scattering measurements on propylene glycol (PG) and its oligomers poly(propylene glycols) (PPG) having different molecular weights ( M w, 425, 725 and 4000 Da), in the bulk state and confined in a silica glass having 25 Å pores. Mainly, two relevant effects are responsible for the dynamical response of liquids that diffuse and reorient in a confined geometry: (a) the ‘physical traps’, related to both dead-end groups and the tortuosity of the percolated channels for diffusion; and (b) the ‘chemical traps’, related to the degree of the absorption of molecules on the active surface sites. Hence, by comparing the behaviour of bulk polymers with confined polymers we were able to analyse the confinement influence on the molecular mobility of hydrogen-bonded liquids with different steric hindrance. The experimental results showed a frustration of molecular mobility in the confined samples owing to chemical and physical traps whose main role was highlighted thanks to the opportunity to substitute the active silanol groups (SiOH) in the inner surfaces with the non-active groups in the surfaces (treatment with methanol). In particular, we found that, in the case of low molecular weight samples, the relevant retardation process is connected to the chemical traps while for long chain polymers the physical traps play the main role. Further, the fitting procedure provided a distribution of relaxation times in the bulk systems and in short chain systems when confined in modified glass showing, in particular in the first case, that the distribution of relaxation times increases with polymers weights, occurrence related to a variety of molecular motions triggered by the more or less entangled unites.