Atomistic Investigation of the Solubility of 3-Alkylthiophene Polymers in Tetrahydrofuran Solvent

Competing single-chain folding and multi-chain aggregation pathways control solution-phase aggregate morphology of organic semiconducting polymers

Understanding the solution-phase behaviour of organic semiconducting polymers is important for systematically improving the performance of devices based on solution-processed thin films of these molecules. Conventional polymer theory predicts that polymer conformations become more compact as solvent quality decreases, but recent experiments have shown the high-performance organic-semiconducting polymer P(NDI2OD-T2) to form extended rod-like aggregates much larger than a single chain in poor solvents, with the formation of these extended aggregates correlated with enhanced electron mobility in films deposited from these solutions. We explain the unexpected formation of extended aggregates using a novel coarse-grained simulation model of P(NDI2OD-T2) that we have developed to study the effect of solvent quality on its solution-phase behaviour. In poor solvents, we find that aggregation through only a few monomers gives effectively inseparable chains, leading to the formation of extended structures of partially overlapping chains via non-equilibrium assembly. This behaviour requires that multi-chain aggregation occurs faster than chain folding, which we show is the case for the chain lengths and concentrations shown experimentally to form rod-like aggregates. This kinetically controlled process introduces a dependence of aggregate structure on concentration, chain length, and chain flexibility, which we show is able to reconcile experimental findings and is generalisable to the solution-phase assembly of other semiflexible polymers.

Effect of Branching on Mutual Solubility of Alkane-CO2 Systems by Molecular Simulations

Mutual solubilities of hydrocarbon-CO₂ systems are important in a broad range of applications. Experimental data and theoretical understanding of phase behavior of large hydrocarbon molecules and CO₂ are limited. This is especially true in relation to the molecular structure of hydrocarbons when the carbon number exceeds 12. In this work, the continuous fractional component Gibbs ensemble Monte Carlo simulations are used to investigate mutual solubility of different alkane and CO₂ systems and the molecular structure. We investigate the mutual solubility of n-decane, n-hexadecane, n-eicosane, and the corresponding structural isomers in the CO₂-rich and hydrocarbon-rich phase. The focus will be solubility of the heavy normal alkanes and their structural isomers in CO₂. The simulation results are verified by comparing the experimental data when measurements are available. The simulation of phase behavior of the n-decane-CO₂ system agrees with the experiments. We also present simulation results of n-hexadecane-CO₂ and n-eicosane-CO₂ systems away from the critical region partly due to the finite size effect. We establish that solubility of the hydrocarbons in CO₂ is improved by change of the molecular structure in heavier alkanes. The enhanced solubility is limited in decane isomers, but the isomers of hexadecane and eicosane show 2- to 3-time solubility enhancement. The molecular dynamics simulations suggest that the improvement is from a higher coordination number of CO₂ for methyl (CH₃) rather than for methylene (CH₂) groups. This study sets the stage for molecular engineering and synthesis of hydrocarbons that are soluble in CO₂ not only by considering functionality but also by changing the molecular structure. The solubility enhancement is the first step in viscosification of CO₂ which broadens the use of CO₂.

Effects of solvation shell relaxation on chain association mechanisms in poly(3-hexylthiophene) solutions

Using poly(3-hexylthiophene) (P3HT) as a model conjugated polymer and atomistic molecular dynamics simulations with carefully verified force fields, we performed in-depth investigations of solvation shell properties of P3HT chains (15 repeating units per chain) in two representative groups of non-polar (or aprotic) organic solvents (better solvents: ortho-dichlorobenzene, bromobenzene, and chlorobenzene; poorer solvents: chloroform, para-xylene, and toluene). We demonstrated that solvation shell relaxation properties in P3HT solutions dictate the formation of regular π-π associations and, hence, crystallinity through the initial chain association and subsequent chain sliding. In contrast, the mean features of polymer-solvent interactions, including solvation free energy and radial distribution function, present little or no difference for all solvent media investigated. Better-solvent media were revealed to bear relatively large values of the first solvation shell relaxation time (τ1 ≫ 100 ps) as well as larger ratios of relaxation times for the first two solvation shells (τ1/τ2 > 2), and vice versa for poorer-solvent media (τ1 ≪ 100 ps and τ1/τ2 < 2). The linear hexyl side-chain unit was noted to substantially enlarge both quantities while notably reducing the solvation free energy as well. As discussed herein, these findings shed new light on the mechanistic features by which solvent quality impacts the degree of π-π association crucial for modern applications with crystalline conjugated polymers.

Direct Correlation of Nanoscale Morphology and Device Performance to Study Photocurrent Generation in Donor-Enriched Phases of Polymer Solar Cells

The nanoscale morphology of polymer blends is a key parameter to reach high efficiency in bulk heterojunction solar cells. Thereby, research typically focusing on optimal blend morphologies while studying nonoptimized blends may give insight into blend designs that can prove more robust against morphology defects. Here, we focus on the direct correlation of morphology and device performance of thieno[3,4-b]-thiophene-alt-benzodithiophene (PTB7):[6,6]phenyl C₇₁ butyric acid methyl ester (PC₇₁BM) bulk heterojunction (BHJ) blends processed without additives in different donor/acceptor weight ratios. We show that while blends of a 1:1.5 ratio are composed of large donor-enriched and fullerene domains beyond the exciton diffusion length, reducing the ratio below 1:0.5 leads to blends composed purely of polymer-enriched domains. Importantly, the photocurrent density in such blends can reach values between 45 and 60% of those reached for fully optimized blends using additives. We provide here direct visual evidence that fullerenes in the donor-enriched domains are not distributed homogeneously but fluctuate locally. To this end, we performed compositional nanoscale morphology analysis of the blend using spectroscopic imaging of low-energy-loss electrons using a transmission electron microscope. Charge transport measurement in combination with molecular dynamics simulations shows that the fullerene substructures inside the polymer phase generate efficient electron transport in the polymer-enriched phase. Furthermore, we show that the formation of densely packed regions of fullerene inside the polymer phase is driven by the PTB7:PC₇₁BM enthalpy of mixing. The occurrence of such a nanoscale network of fullerene clusters leads to a reduction of electron trap states and thus efficient extraction of photocurrent inside the polymer domain. Suitable tuning of the polymer-acceptor interaction can thus introduce acceptor subnetworks in polymer-enriched phases, improving the tolerance for high-efficiency BHJ toward morphological defects such as donor-enriched domains exceeding the exciton diffusion length.

Thermal Stability of π-Conjugated n-Ethylene-Glycol-Terminated Quaterthiophene Oligomers: A Computational and Experimental Study

This work represents a joint computational and experimental study on a series of n-ethylene glycol (PEOn)-terminated quaterthiophene (4T) oligomers for 1 < n < 10 to elucidate their self-assembly behavior into a smectic-like lamellar phase. This study builds on an earlier study for n = 4 that showed that our model predictions were consistent with experimental data on the melting behavior and structure of the lamellar phase, with the latter consisting of crystal-like 4T domains and liquid-like PEO4 domains. The present study aims to understand how the length of the terminal PEOn chains modulates the disordering temperature of the lamellar phase and hence the relative stability of the ordered structure. A simplified bilayer model, where the 4T domains are not explicitly described, is put forward to efficiently estimate the disordering effect of the PEO domains with increasing n; this method is first validated by correctly predicting that layers of alkyl (PE)-capped 4T oligomers (for 1 < n < 10) stay ordered at room temperature. Both 4T-domain implicit and explicit model simulations reveal that the order-disorder temperature decreases with the length of the PEO capping chains, as the associated increase in conformational entropy drives a tendency toward disorder that overtakes the cohesive energy, keeping the ordered packing of the 4T domains.

Deconstructing the behavior of donor–acceptor copolymers in solution & the melt: the case of PTB7

Molecular dynamics simulations of the donor–acceptor copolymer PTB7 at near experimental scale reveal structure–dynamics correlations in the condensed phase.

Enhanced Computational Sampling of Perylene and Perylothiophene Packing with Rigid-Body Models

Molecular simulations have the potential to advance the understanding of how the structure of organic materials can be engineered through the choice of chemical components but are limited by computational costs. The computational costs can be significantly lowered through the use of modeling approximations that capture the relevant features of a system, while lowering algorithmic complexity or by decreasing the degrees of freedom that must be integrated. Such methods include coarse-graining techniques, approximating long-range electrostatics with short-range potentials, and the use of rigid bodies to replace flexible bonded constraints between atoms. To understand whether and to what degree these techniques can be leveraged to enhance the understanding of planar organic molecules, we investigate the morphologies predicted by molecular dynamic simulations using simplified molecular models of perylene and perylothiophene. Approximately, 10 000 wall-clock hours of graphics processing unit-accelerated simulations are performed using both rigid and flexible models to test their efficiency and predictive capability with the two chemistries. We characterize the 1191 resulting morphologies using simulated X-ray diffraction and cluster analysis to distinguish structural transitions, summarized by four phase diagrams. We find that the morphologies generated by the rigid model of perylene and perylothiophene match with those generated by the flexible model. We find that ordered, hexagonally packed columnar phases are thermodynamically favored over a wide range of densities and temperatures for both molecules, in qualitative agreement with experiments. Furthermore, we find the rigid model to be more computationally efficient for both molecules, providing more samples per second and shorter times to equilibrium. Owing to the structural accuracy and improved computational efficiency of modeling polyaromatic groups as rigid bodies, we recommend this modeling choice for enhancing the sampling in polyaromatic molecular simulations.

Thermodynamic compatibility of actives encapsulated into PEG-PLA nanoparticles: In Silico predictions and experimental verification

Achieving optimal solubility of active substances in polymeric carriers is of fundamental importance for a number of industrial applications, including targeted drug delivery within the growing field of nanomedicine. However, its experimental optimization using a trial-and-error approach is cumbersome and time-consuming. Here, an approach based on molecular dynamics (MD) simulations and the Flory-Huggins theory is proposed for rapid prediction of thermodynamic compatibility between active species and copolymers comprising hydrophilic and hydrophobic segments. In contrast to similar methods, our approach offers high computational efficiency by employing MD simulations that avoid explicit consideration of the actual copolymer chains. The accuracy of the method is demonstrated for compatibility predictions between pyrene and nile red as model dyes as well as indomethacin as model drug and copolymers containing blocks of poly(ethylene glycol) and poly(lactic acid) in different ratios. The results of the simulations are directly verified by comparison with the observed encapsulation efficiency of nanoparticles prepared by nanoprecipitation. © 2016 Wiley Periodicals, Inc.

Solvent effects on polymer sorting of carbon nanotubes with applications in printed electronics

Regioregular poly(3-alkylthiophene) (P3AT) polymers have been previously reported for the selective, high-yield dispersion of semiconducting single-walled carbon nanotubes (SWCNTs) in toluene. Here, five alternative solvents are investigated, namely, tetrahydrofuran, decalin, tetralin, m-xylene, and o-xylene, for the dispersion of SWCNTs by poly(3-dodecylthiophene) P3DDT. The dispersion yield could be increased to over 40% using decalin or o-xylene as the solvents while maintaining high selectivity towards semiconducting SWCNTs. Molecular dynamics (MD) simulations in explicit solvents are used to explain the improved sorting yield. In addition, a general mechanism is proposed to explain the selective dispersion of semiconducting SWCNTs by conjugated polymers. The possibility to perform selective sorting of semiconducting SWCNTs using various solvents provides a greater diversity of semiconducting SWCNT ink properties, such as boiling point, viscosity, and surface tension as well as toxicity. The efficacy of these new semiconducting SWCNT inks is demonstrated by using the high boiling point and high viscosity solvent tetralin for inkjet-printed transistors, where solvent properties are more compatible with the inkjet printing head and improved droplet formation.

Multi-length-scale relationships between the polymer molecular structure and charge transport: the case of poly-naphthalene diimide bithiophene

Charge transport in organic polymer semiconductors is a complex phenomenon affected by structural and electronic properties ranging over different length scales, from the molecular one up to the macro-scale.

The study of polythiophene/water interfaces by sum-frequency generation spectroscopy and molecular dynamics simulations

Polythiophene/water interfaces are investigated by sum frequency generation spectroscopy and molecular dynamics simulations, showing a preferential edge-on molecular orientation.