@article{d1f86a7227864ebeac19a2c9de3f1c6b,
title = "Organic-inorganic telechelic molecules: Solution properties from simulations",
abstract = "We report molecular dynamics simulations for telechelic molecules composed of two polyhedral oligomeric silsesquioxane (POSS) cages connected by one hydrocarbon backbone dissolved in liquid normal hexane. Silsesquioxanes are novel hybrid organic-inorganic molecules that are useful as building blocks for the synthesis of nanostructured materials. By including POSS molecules within a polymeric material it is possible to modify mechanical properties such as resistance to heat and glass transition temperatures. Unfortunately, the molecular mechanisms responsible for these enhancements are at present not completely understood. In an effort to elucidate the molecular phenomena responsible for these effects, we have studied the conformation of telechelic POSS molecules in solution, as well as their self-diffusion coefficients, as a function of the length of the hydrocarbon backbone. We focus on molecules in which the radius of gyration of the alkane backbone is comparable to the size of the silsesquioxane cages. Our results indicate that the backbone has a significant influence on both the equilibrium and the transport properties of dissolved telechelic hybrid molecules. These observations are useful for developing strategies to direct the self-assembly of nanostructured materials.",
author = "Alberto Striolo and Clare McCabe and Cummings, {Peter T.}",
note = "Funding Information: The authors acknowledge financial support from the US National Science Foundation under Contract No. DMR-0103399. One of the authors (A.S.) thankfully acknowledges partial financial support from the Vice President for Research at the University of Oklahoma through a Junior Faculty Research Program Award. Calculations were performed on the VAMPIRE cluster at Vanderbilt University. Table I. Potential parameters for the molecular models employed in this work. Bonds r 0 ({\AA}) k b [ kcal ∕ ( mol {\AA} 2 ) ] Si–O 1.64 350.12 Si – C H 3 1.90 189.65 Si – C H 2 1.90 189.65 Angles θ 0 (deg) k θ [ kcal ∕ ( mol rad 2 ) ] Si–O–Si 146.46 14.14 O–Si–O 107.82 94.50 O – Si – C H 3 110.69 49.97 O – Si – C H 2 110.69 49.97 C H 3 – C H 2 – C H 2 114.00 62.09 C H 2 – C H 2 – C H 2 114.00 62.09 Angles θ 0 (deg) k θ I [ kcal ∕ ( mol rad 2 ) ] k θ II [ kcal ∕ ( mol rad 3 ) ] k θ III [ kcal ∕ ( mol rad 4 ) ] Si – C H 2 – C H 2 112.67 39.52 − 7.44 0.00 Dihedrals c 1 (kcal/mol) c 2 (kcal/mol) c 3 (kcal/mol) Si–O–Si–O 0.2250 0.0000 0.0000 Si – O – Si – C H 3 0.0000 0.0000 0.0100 Si – O – Si – C H 2 0.0000 0.0000 0.0100 O – Si – C H 2 – C H 2 0.3527 − 0.0677 0.7862 Si – C H 2 – C H 2 – C H 2 0.7054 − 0.1355 1.5724 C H 3 – C H 2 – C H 2 – C H 2 0.7054 − 0.1355 1.5724 C H 2 – C H 2 – C H 2 – C H 2 0.7054 − 0.1355 1.5724 Nonbonded interactions σ i j ({\AA}) ε i j (kcal/mol) Si–Si 4.29 0.1310 O–O 3.30 0.0800 Si–O 3.94 0.0772 C H 3 – C H 3 3.75 0.1947 C H 2 – C H 2 3.95 0.0974 C H 3 – C H 2 3.85 0.1377 Si – C H 3 3.83 0.1596 Si – C H 2 3.93 0.1093 O – C H 3 3.38 0.1247 O – C H 2 3.48 0.0854 FIG. 1. Schematic representation of one POSS-alkane telechelic hybrid monomer [ ( Si O 1.5 ) 8 ( C H 3 ) 7 B ( Si O 1.5 ) 8 ( C H 3 ) 7 ] considered in this work. The backbone B is composed of 20 C H 2 united-atom groups. In each POSS monomer seven silicon atoms are bound to a methyl group. The eighth silicon atom in each cage is bound to the hydrocarbon backbone. FIG. 2. Average distance between the centers of the two POSS monomers in a POSS-alkane telechelic hybrid molecule when the hydrocarbon backbone is 20 carbon atoms in length. The system temperature is 300 K . The gray dotted line represents the results obtained in a simulation box with size of 4.0 nm . The black continuous line is for the data obtained in a simulation box with size of 6.0 nm . The insets schematically illustrate the face-to-face (left) and corner-to-corner configurations (right). FIG. 3. Average distance between the centers of the two POSS monomers in POSS-alkane hybrid molecules at 300 K . Results are for different hydrocarbon backbones. The gray continuous line is for backbones of length 20 carbon atoms, the dotted gray line is for backbones of length 16 carbon atoms, the broken gray line is for backbones of length 12 carbon atoms, and the black continuous line is for backbones of 8 carbon atoms. In all cases the simulation box is of size 4.0 nm . FIG. 4. Average density distribution for the n C 6 monomers (either C H 3 or C H 2 groups) around the POSS monomers. The top figures are for POSS monomers maintained at a fixed center-to-center separation of 0.99 nm , whereas the bottom figures are for POSS monomers maintained at a fixed separation of 1.29 nm . White indicates 0 segments ∕ nm 3 , and black ∼ 45 segments ∕ nm 3 . From left to right results are for POSS-alkane hybrid molecules with backbone of lengths 12 and 20 carbon atoms. Results are obtained at 500 K . In all cases, the simulation box size is 4.0 nm . FIG. 5. Average POSS-POSS center-to-center distance for POSS-alkane hybrid molecules with backbones of 8 (panel A), 12 (panel B), 16 (panel C), and 20 carbon atoms (panel D). Results are obtained at 300 K (light dotted gray lines), 400 K (continuous gray lines), 500 K (dark dotted gray lines), and 600 K (black continuous lines). The simulation box size is 4.0 nm for the results shown in panels A, B, and C; and 6.0 nm for the results shown in panel D. FIG. 6. Average distance between the centers of the two POSS monomers at 400 K . Results are for different hydrocarbon backbones. The gray continuous line is for backbones of length 20 carbon atoms, the dotted gray line is for backbones of length 16 carbon atoms, and the black continuous line is for backbones of 12 carbon atoms. The simulation box is of size 4.0 nm when the backbone is of length 12 or 16 carbon atoms, and 6.0 nm when the backbone is 20 carbon atoms in length. FIG. 7. Representative mean-square displacement for the POSS monomers of POSS-alkane hybrid molecules dissolved in n C 6 at 300 K . Different panels represent the results obtained for hybrid molecules with backbones of different lengths. Panel A is for backbone of length 8 carbon atoms, panel B is for 12 carbon atoms, panel C is for 16 carbon atoms, and panel D is for backbone of length 20 carbon atoms. In all cases the simulation box is of size 4.0 nm . The arrow in panel C indicates the change in slope observed for the mean-square displacement as a function of time. FIG. 8. Representative displacements for the two POSS monomers of POSS-alkane hybrid molecules dissolved in n C 6 at 300 K . Panel A is for backbone of length 8 carbon atoms, panel B is for 12 carbon atoms, panel C is for 16 carbon atoms, and panel D is for backbone of length 20 carbon atoms. In all cases the simulation box is of size 4.0 nm . The circles in panels B and C highlight the events in which one POSS monomer travels more than 0.5 nm in a very short period of time. FIG. 9. Self-diffusion coefficient for the POSS monomers in POSS-alkane hybrid molecules dissolved in n C 6 at 300 K as a function of the number of carbon atoms in the hydrocarbon backbone (C). In all cases the simulation box is of size 4.0 nm . The error bars are calculated as 1 standard deviation from the mean value. ",
year = "2006",
month = sep,
day = "14",
doi = "10.1063/1.2348641",
language = "English",
volume = "125",
journal = "The Journal of Chemical Physics",
issn = "0021-9606",
publisher = "American Institute of Physics Publising LLC",
number = "10",
}