CONFINEMENT EFFECTS OF POLYMERS IN POROUS GLASSES

Recently, confinement effects on dynamical properties of liquids insid e restricted volumes have been extensively studied, either from a theo retical or technological point of view, thanks to the large possibilit y of industrial applications (building of optical switches, membrane s eparation, catalysis). We performed depolarized light scattering measu rements on propylene glycol(PG) and its oligomers poly(propylene glyco ls) (PPG) having different molecular weights (M-W 425, 725 and 4000 Da ) in the bulk state and confined in a silica glass having 25 Angstrom 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 tort uosity of the percolated channels for diffusion; and (b) the 'chemical traps', related to the degree of the absorption of molecules on the a ctive surface sites. Hence, by comparing the behaviour of bulk polymer s with confined polymers we were able to analyse the confinement influ ence on the molecular mobility of hydrogen-bonded liquids with differe nt steric hindrance. The experimental results showed a frustration of molecular mobility in the confined samples owing to chemical and physi cal traps whose main role was highlighted thanks to the opportunity to substitute the active silanol groups (Si-OH) in the inner surfaces wi th the non-active groups in the surfaces (treatment with methanol). In particular, we found that, in the case of low molecular weight sample s, 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 t imes in the bulk systems and in short chain systems when confined in m odified glass showing, in particular in the first case, that the distr ibution of relaxation times increases with polymers weights, occurrenc e related to a variety of molecular motions triggered by the more or l ess entangled unites. (C) 1998 Elsevier Science B.V. All rights reserv ed.