Attraction between Surfaces in a Polymer Melt Containing Telechelic Chains:  Guidelines for Controlling the Surface Separation in Intercalated Polymer−Clay Composites

Abstract

Using numerical and analytical self-consistent field (SCF) calculations, we investigate the interactions between two closely spaced surfaces and the surrounding polymer melt, which contains a volume fraction of end-functionalized polymers. The functionalized polymers contain two reactive “stickers”, one at each end of the chain, that are highly attracted to the surfaces. The surfaces model clay crystallites, or sheets. Through the calculations, we determine the free energy profiles as the surfaces are pried apart and the polymer fills the gap. The free energy vs distance plots reveal a distinctive minimum, even at very small volume fractions of the end-functionalized chains. Thus, the presence of these telechelic polymers promotes the formation of thermodynamically stable intercalated composites, where the polymers enhance the separation between the silicate sheets. However, the telechelic chains prohibit the creation of the more desirable exfoliated structures, where the sheets are uniformly dispersed throughout the polymer matrix. The results provide guidelines for significantly enhancing the separation between the sheets, thus possibly making properties of the intercalated composites more like the exfoliated material.

Numerical simulation of polymer nanocomposites using self-consistent mean-field model

“…[5][6][7][8][9][10] The assumption of infinitely large platelets means that the system is translationally invariant in the x-y directions and the 1D calculations are fully adequate. For the translation invariance of dependant variables, our computations virtually reproduced their published results.…”

“…[6][7][8][9] The initial SCF computations showed that at a constant grafting density, g ϭ0.04, and at neutral interactions between intercalant and polymer, oh ϭ0.0, the system was miscible and the miscibility increases with the intercalant chain length, N g ͓25-100͔. Furthermore, increasing the grafting density g was shown to reduce the free energy of mixing; thus, the system may become immiscible.…”

“…In their study of the effects of the statistical chain length and grafting density, Balazs et al [6][7][8][9] observed that the magnitude of the binary interaction parameter between two polymers with a short statistical chain length has a relatively small effect on exfoliation. By contrast, the effect is quite pronounced for relatively long polymer chains.…”

“…The adopted SCF simulation methodology is an extension of the work of Balazs et al [6][7][8][9] To bring the model closer to practical industrial strategies for the preparation of PNC, four components have been considered: solid clay lamellae, short-chain intercalant, long-chain end-functionalized compatibilizer, and host polymer. Accordingly, three types of interaction parameters were used in the simulations: solidsolid, solid-liquid, and liquid-liquid.…”

“…[6][7][8][9] The model used in the SCF calculations consists of two infinite parallel platelets immersed in a bath of molten polymer. To incorporate the concentration effects, Ginzburg et al hybridized the SCF one-dimensional ͑1D͒ model with the density functional theory ͑DFT͒ for intercalant-grafted clay platelets, 1-nm thick and with a 30-nm diameter, dispersed in a polymer.…”