Conformational Characteristics of Short Poly(ethylene oxide) Chains Terminally Attached to a Wall and Free in Aqueous Solution

SARMORIA, C; BLANKSCHTEIN, D

JOURNAL OF PHYSICAL CHEMISTRY

American Chemical Society

Año: 1992 vol. 96 p. 1978 - 1983

0022-3654

Many systems of practical importance contain short polymer chains terminally attached at interfaces. Such systems include sterically stabilized colloids, coated surfaces, compatibility enhancers, micelles, and biological membranes. Although it is generally accepted that for very short chains the well-known polymer scaling laws do not apply, the lower limit of applicability of such laws cannot be easily established a priori. For an aqueous solution of poly(ethylene oxide) (PEO), a system of considerable fundamental and practical importance, we have studied the lower limit of validity of polymer scaling laws for isolated PEO chains free in solution and attached at one end to an inert and impenetrable planar interface (a wall). For this purpose, we have utilized the rotational isomeric state (RIS) model to describe the conformations of PEO chains in water over the temperature range 0-100 °C. The root-mean-square end-to-end distance of free PEO chains ranging in length from 3 to 18 000 bonds was calculated exactly using Flory’s serial matrix multiplication method. For the terminally attached PEO chains, this exact method cannot be utilized due to the spatial symmetry breaking induced by the wall. Consequently, we have used a Monte Carlo approach to the RIS model to calculate the root-mean-square end-to-end distance of attached chains ranging in length from 3 to 900 bonds. We find that polymer scaling laws are applicable within an uncertainty of a few percent for fairly short PEO chains. Specifically, the shortest free PEO chain that satisfies scaling with a 2% accuracy has 12 bonds, while the shortest attached chain has 30 bonds. It is also interesting to note that we find a weak temperature variation of the root-mean-square end-to-end distance over the temperature range studied for both free and attached chains. Our results are valid under Theta solvent conditions, as well as over a range of good-solvent conditions. A discusion of the implications of these new findings for modeling micelles containing alkyl poly(ethylene oxide) (CiEj) nonionic surfactants in aqueous solution, as well as for interpreting results of force measurements between two surfaces coated with CiEj surfactants and immersed in water, is also presented.