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An ellipsoid-chain model for conjugated polymer solutions
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Image of FIG. 1.
FIG. 1.

Sketches illustrating (from top to bottom) AMD, CGMD, CGLD, and CGMC realizations of a single MEH-PPV (poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene)) chain in various model descriptions.

Image of FIG. 2.
FIG. 2.

Pictorial representations of a pair of 10-mer MEH-PPV in (a) vacuum, (b) toluene bath, and (c) chloroform bath (note that the solvent molecules in cases (b) and (c) were not plotted for clarity), where the PMFs have been computed using AMD simulations (symbols). Also, a comparison is shown with the predictions of the parameterized GB potential (lines) for (d) toluene or (e) chloroform medium. For each of the pair arrangements, the mean separation distance varies in the range of 0–20 Å with a uniform increment of 0.1 Å. At a given distance, AMD simulations (with DREIDING force fields) were performed using Nosé–Hoover thermostat at T = 298 K (with a coupling constant of 0.1 ps) for 1 ns, with an integration time step of 1 fs and a cutoff distance of 15 Å for all nonbonding interactions. This amounts to a total of 1000 nearly independent atomistic configurations later used to compute the PMFs at a given oligomer separation. The density of the solution was estimated using Nosé–Hoover NPT ensemble (1 atm and 298 K) to be 0.9 and 1.4 g/cm3 for toluene and chloroform, respectively, with the total numbers of solvent particles being 5800 and 7500, respectively.

Image of FIG. 3.
FIG. 3.

Comparison of the intrachain RDFs (unnormalized) for two single-solvent media, where M/C and M/T denote the MEH-PPV/chloroform and MEH-PPV/toluene, respectively. The CGMD and CGLD simulations were both executed for a rescaled (AMD) time about 10 ns, with a time step 2 fs for CGMD and 5 fs for CGLD. The CGMC simulation involves totally 300 000 steps.31

Image of FIG. 4.
FIG. 4.

The predicted scaling laws for mean end-to-end distance (ETE) of MEH-PPV in (a) chloroform, (b) toluene, and (c) chloroform/toluene = 1:1 in number density. In all cases, an ensemble average based on 100 independent chains was utilized to create the plot, and the symbols represent CGMD (triangles), CGLD (squares), and CGMC (circles). For clarity, the curves have been shifted in the x-axis direction by varying degree. The error bar given for the slope of each curve, which yields the solvent quality exponent, was evaluated using the mean values of ETE for the three chain lengths investigated. In this way, it is evident that only the result shown in (c) for the CGMD simulation exhibited appreciable deviation from the other two cases.

Image of FIG. 5.
FIG. 5.

Snapshots of single-chain (300-mer MEH-PPV) conformation with various amounts of defect: (a) 10%, (b) 5%, or (c) 0% in toluene; (d) 10%, (e) 5%, or (f) 0% in chloroform. The CGMC simulations were performed using NVT (298 K) ensemble for 300 000 steps. Understanding the real impact of defects requires a more detailed analysis; see discussion in the main text and results gathered in Table IV.


Generic image for table
Table I.

Intramolecular CG potentials and parameters used for defect locations for the ellipsoid-chain model. The results for the other two CG models were different from what given here due to a different coarse-graining level, and some information may be found in Ref. 4.

Generic image for table
Table II.

Parameter values for the intermolecular GB potential determined for three solvent media, each ellipsoid segment representing ten monomer units of MEH-PPV. The GB potential utilized is given by , where u i and u j are the unit vectors describing the orientations of the ellipsoid pairs i and j under consideration, r ij is a vector connecting their geometric centers, σ0 is related to the ellipsoidal dimension, and ξ is a dimensionless parameter which helps control the width of the potential well independent of its depth or location of the potential minimum. The parameters χ and (ɛ0, χ, ν, μ) appear in the functionals of σ(u i , u j , r ij ) and ɛ(u i , u j , r ij ), respectively; see detailed specifications in Ref. 27.

Generic image for table
Table III.

Comparison of computing times for three CG models (based on a single CPU for an 300-mer MEH-PPV).

Generic image for table
Table IV.

Single-chain structures predicted by CGMC simulation of the ellipsoid-chain model with varying degree of defect.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: An ellipsoid-chain model for conjugated polymer solutions