Influence of Side Chain Length on the Self-Assembly of Hairy-Rod Poly(9,9-dialkylfluorene)s in the Poor Solvent Methylcyclohexane

Dynamics of Polyalkylfluorene Conjugated Polymers: Insights from Neutron Spectroscopy and Molecular Dynamics Simulations

The dynamics of the conjugated polymers poly(9,9-dioctylfluorene) (PF8) and poly(9,9-didodecylfluorene) (PF12), differing by the length of their side chains, is investigated in the amorphous phase using the temperature-dependent quasielastic neutron scattering (QENS) technique. The neutron spectroscopy measurements are synergistically underpinned by molecular dynamics (MD) simulations. The probe is focused on the picosecond time scale, where the structural dynamics of both PF8 and PF12 would mainly be dominated by the motions of their side chains. The measurements highlighted temperature-induced dynamics, reflected in the broadening of the QENS spectra upon heating. The MD simulations reproduced well the observations; hence, the neutron measurements validate the MD force fields, the adopted amorphous model structures, and the numerical procedure. As the QENS spectra are dominated by the signal from the hydrogens on the backbones and side chains of PF8 and PF12, extensive analysis of the MD simulations allowed the following: (i) tagging these hydrogens, (ii) estimating their contributions to the self-part of the van Hove functions and hence to the QENS spectra, and (iii) determining the activation energies of the different motions involving the tagged hydrogens. PF12 is found to exhibit QENS spectra broader than those of PF8, indicating a more pronounced motion of the didodecyl chains of PF12 as compared to dioctyl chains of PF8. This is in agreement with the outcome of our MD analysis: (i) confirming a lower glass transition temperature of PF12 compared to PF8, (ii) showing PF12 having a lower density than PF8, and (iii) highlighting lower activation energies of the motions of PF12 in comparison with PF8. This study helped to gain insights into the temperature-induced side-chain dynamics of the PF8 and PF12 conjugated polymers, influencing their stability, which could potentially impact, on the practical side, the performance of the associated optoelectronic active layer.

Buffer Chain Model for Understanding Crystallization Competition in Conjugated Polymers

It remains challenging to comprehensively understand the packing models of conjugated polymers, in which side chains play extremely critical roles. The side chains are typically flexible and non-conductive and are widely used to improve the polymer solubility in organic solutions. Herein, a buffer chain model is proposed to describe link between conjugated backbone and side chains for understanding the relationship of crystallization competition of conductive conjugated backbones and non-conductive side chains. A longer buffer chain is beneficial for alleviating such crystallization competition and further promoting the spontaneous packing of conjugated backbones, resulting in enhanced charge transport properties. Our results provide a novel concept for designing conjugated polymers towards ordered organization and enhanced electronic properties and highlight the importance of balancing the competitive interactions between different parts of conjugated polymers.

Pendant Group Modifications Provide Graft Copolymer Silicones with Exceptionally Broad Thermomechanical Properties

Graft copolymers offer a versatile platform for the design of self-assembling materials; however, simple strategies for precisely and independently controlling the thermomechanical and morphological properties of graft copolymers remain elusive. Here, using a library of 92 polynorbornene-graft-polydimethylsiloxane (PDMS) copolymers, we discover a versatile backbone-pendant sequence-control strategy that addresses this challenge. Small structural variations of pendant groups, e.g., cyclohexyl versus n-hexyl, of small-molecule comonomers have dramatic impacts on order-to-disorder transitions, glass transitions, mechanical properties, and morphologies of statistical and block silicone-based graft copolymers, providing an exceptionally broad palette of designable materials properties. For example, statistical graft copolymers with high PDMS volume fractions yielded unbridged body-centered cubic morphologies that behaved as soft plastic crystals. By contrast, lamellae-forming graft copolymers provided robust, yet reprocessable silicone thermoplastics (TPs) with transition temperatures spanning over 160 °C and elastic moduli as high as 150 MPa despite being both unentangled and un-cross-linked. Altogether, this study reveals a new pendant-group-mediated self-assembly strategy that simplifies graft copolymer synthesis and enables access to a diverse family of silicone-based materials, setting the stage for the broader development of self-assembling materials with tailored performance specifications.

Unraveling the Unconventional Order of a High-Mobility Indacenodithiophene-Benzothiadiazole Copolymer

A new class of donor-acceptor (D-A) copolymers found to produce high charge carrier mobilities competitive with amorphous silicon (>1 cm² V^-1 s^-1) exhibit the puzzling microstructure of substantial local order, however lacking long-range order and crystallinity previously deemed necessary for achieving high mobility. Here, we demonstrate the application of low-dose transmission electron microscopy to image and quantify the nanoscale and mesoscale organization of an archetypal D-A copolymer across areas comparable to electronic devices (≈9 μm²). The local structure is spatially resolved by mapping the backbone (001) spacing reflection, revealing nanocrystallites of aligned polymer chains throughout nearly the entire film. Analysis of the nanoscale structure of its ordered domains suggests significant short- and medium-range order and preferential grain boundary orientations. Moreover, we provide insights into the rich, interconnected mesoscale organization of this new family of D-A copolymers by analysis of the local orientational spatial autocorrelations.

Multi-level aggregation of conjugated small molecules and polymers: from morphology control to physical insights

Aggregation of molecules is a multi-molecular phenomenon occurring when two or more molecules behave differently from discrete molecules due to their intermolecular interactions. Moving beyond single molecules, aggregation usually demonstrates evolutive or wholly emerging new functionalities relative to the molecular components. Conjugated small molecules and polymers interact with each other, resulting in complex solution-state aggregates and solid-state microstructures. Optoelectronic properties of conjugated small molecules and polymers are sensitively determined by their aggregation states across a broad range of spatial scales. This review focused on the aggregation ranging from molecular structure, intermolecular interactions, solution-state assemblies, and solid-state microstructures of conjugated small molecules and polymers. We addressed the importance of such aggregation in filling the gaps from the molecular level to device functions and highlighted the multi-scale structures and properties at different scales. From the view of multi-level aggregation behaviors, we divided the whole process from the molecule to devices into several parts: molecular design, solvation, solution-state aggregation, crystal engineering, and solid-state microstructures. We summarized the progress and challenges of relationships between optoelectronic properties and multi-level aggregation. We believe aggregation science will become an interdisciplinary research field and serves as a general platform to develop future materials with the desired functions.

Discovery and structure characteristics of the intermediate-state conformation of poly(9,9-dioctylfluorene) (PFO) in the dynamic process of conformation transformation and its effects on carrier mobility

Good solution processability is a prerequisite for fabricating polymer optoelectronic devices. In this research, a new PFO chain conformation called "intermediate-state conformation" was found through UV-vis absorption spectroscopy, photoluminescence spectroscopy (PL) and Raman spectroscopy in the transition process of α conformation towards β conformation. The intermediate-state conformation not only remedies the defect of film-forming caused by large aggregation of β-conformation but also gains an equivalent carrier mobility similar to that of β conformation. Simultaneously, it was found that the film with the intermediate-state conformation had a smooth surface morphology compared to the film with β-conformation, which indicates that the intermediate-state conformation has good solution processability; thus, it is more suitable for the fabrication of photoelectric films with high carrier mobility. The results of high-resolution transmission electron microscopy (HR-TEM) measurements showed that there were obvious lattice fringes in the films with the intermediate-state conformation and β conformation; this reveals that the intermediate-state conformation has a more planar conformation with extended conjugation length than the β conformation, which is very beneficial to enhance carrier mobility. The research significantly reveals the dynamic evolution of polymer structures based on conjugated polymer physics. The conclusions enrich the understanding of the structure evolution and dynamic process of conjugated polymers and present broad application prospects for photoelectric and other functional devices due to the good film-forming properties of the intermediate-state conformation.

Unlocking the action mechanisms of molecular nonlinear optical absorption for optical conjugated polymers under aggregation states

Controlling the molecular microstructure and the molecular aggregation state under different conditions to improve the MNOA performance of OCPs.

Investigation of Substituent Effect in Modified Nature-Sourced Polymers: Rational Side Chain Engineering to Control Yield, Design, and Properties

"Side chain engineering" research has yielded many promising and beneficial results, with applications in various fields. However, this research did not receive sufficient focus when nature-sourced polymers are concerned. In this study, we have performed side chain engineering on chitosan, a nature-sourced polysaccharide, by coupling it with a number of aliphatic aldehydes of varying chain lengths. The side chains' length and the pursuing effect on the modified products' properties were studied in great detail. In terms of coupling yields, it was found that some substituents have displayed more favorable results than others by a factor of over 35 times. When studying the modified polymers' physical and mechanical properties, some of them were found to exhibit more rigid mechanical properties by a factor of 3.5 times than others. The effect was also expressed through self-assembly concentrations and encapsulation capabilities of the modified polymers. Remarkably, the combined experimental and calculated kinetic studies showed the results do not necessarily follow a linear progression relating to substituent chain length, but rather a parabolic pattern with a specific extremum point. This study has assisted in shedding light on the inspected phenomenon, explaining that not only steric and electronic factors but also interfacial solubility related factors govern the coupling reaction and the resulting modified polymers' properties. As chemical protocols in various academic, clinical, and industrial studies around the world slowly shift their norms toward finding safer ways for the production of novel materials and technologies, nature-sourced polymers hold great promise as virtually inexhaustible raw materials. The perfection of their chemical modification is therefore relevant now more than ever, with far-reaching and diverse applicative prospects.

Chirality induction of achiral polydialkylfluorenes by chiral solvation: odd–even and side chain length dependence

An “odd–even” effect for the chiral β-phase of polydialkylfluorene/limonene aggregates was first observed, depending on the odd–even alkyl side chain length.

Towards the Development of a Low-Cost Device for the Detection of Explosives Vapors by Fluorescence Quenching of Conjugated Polymers in Solid Matrices

Conjugated polymers (CPs) have proved to be promising chemosensory materials for detecting nitroaromatic explosives vapors, as they quickly convert a chemical interaction into an easily-measured high-sensitivity optical output. The nitroaromatic analytes are strongly electron-deficient, whereas the conjugated polymer sensing materials are electron-rich. As a result, the photoexcitation of the CP is followed by electron transfer to the nitroaromatic analyte, resulting in a quenching of the light-emission from the conjugated polymer. The best CP in our studies was found to be poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2). It is photostable, has a good absorption between 400 and 450 nm, and a strong and structured fluorescence around 550 nm. Our studies indicate up to 96% quenching of light-emission, accompanied by a marked decrease in the fluorescence lifetime, upon exposure of the films of F8T2 in ethyl cellulose to nitrobenzene (NB) and 1,3-dinitrobenzene (DNB) vapors at room temperature. The effects of the polymeric matrix, plasticizer, and temperature have been studied, and the morphology of films determined by scanning electron microscopy (SEM) and confocal fluorescence microscopy. We have used ink jet printing to produce sensor films containing both sensor element and a fluorescence reference. In addition, a high dynamic range, intensity-based fluorometer, using a laser diode and a filtered photodiode was developed for use with this system.

Observation of aggregation triggered by Resonance Energy Transfer (RET) induced intermolecular pairing force

In this report, we showed the existence of RET induced intermolecular pairing force by comparing their fluorescence behaviors under room illumination vs standing in dark area for either PFluAnt solution or PFluAnt&PFOBT mixture. Their prominent emission attenuation under room illumination brought out the critical role of photo, i.e. RET induced intermolecular pairing force in induction of polymer aggregation. Constant UV-Vis absorption and fluorescence spectra in terms of both peak shapes and maximum wavelengths implied no chemical decomposition was involved. Recoverable fluorescence intensity, fluorescence lifetime as well as NMR spectra further exclude photo induced decomposition. The controllable on/off state of RET induced intermolecular pairing force was verified by the masking effect of outside PFluAnt solution which function as filter to block the excitation of inside PFluAnt and thus off the RET induced intermolecular pairing force. Theoretical calculation suggest that magnitude of RET induced intermolecular pairing force is on the same scale as that of van der Waals interaction. Although the absolute magnitude of RET induced intermolecular pairing force was not tunable, its effect can be magnified by intentionally turn it "on", which was achieved by irradiance with 5 W desk lamp in this report.

Molecular packing and morphological stability of dihydro-indeno[1,2-b]fluorenes in the context of their substitution pattern

The synthesis and structural characterization of a series of dihydroindeno[1,2-b]fluorene (IF) derivatives with various side chain substituents is reported.

Solution-Crystallization and Related Phenomena in 9,9-Dialkyl-Fluorene Polymers. I. Crystalline Polymer-Solvent Compound Formation for Poly(9,9-dioctylfluorene)

Polymer-solvent compound formation, occurring via co-crystallization of polymer chains and selected small-molecular species, is demonstrated for the conjugated polymer poly(9,9-dioctylfluorene) (PFO) and a range of organic solvents. The resulting crystallization and gelation processes in PFO solutions are studied by differential scanning calorimetry, with X-ray diffraction providing additional information on the resulting microstructure. It is shown that PFO-solvent compounds comprise an ultra-regular molecular-level arrangement of the semiconducting polymer host and small-molecular solvent guest. Crystals form following adoption of the planar-zigzag β-phase chain conformation, which, due to its geometry, creates periodic cavities that accommodate the ordered inclusion of solvent molecules of matching volume. The findings are formalized in terms of nonequilibrium temperature–composition phase diagrams. The potential applications of these compounds and the new functionalities that they might enable are also discussed. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1481–1491

Solution-crystallization and related phenomena in 9,9-dialkyl-fluorene polymers. II. Influence of side-chain structure

Solution-crystallization is studied for two polyfluorene polymers possessing different side-chain structures. Thermal analysis and temperature-dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X-ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar-zigzag chain conformation termed the β-phase, which is observed for certain linear-side-chain polyfluorenes, is necessary for the formation of so-called polymer-solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side-chains prevents formation of the β-phase conformation and results in non-solvated, i.e. melt-crystallization-type, polymer crystals. Unlike non-solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β-phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter-chain interactions. The presented results clarify the fundamental differences between the β-phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 1492-1506.

Methods for controlling structure and photophysical properties in polyfluorene solutions and gels

Knowledge of the phase behavior of polyfluorene solutions and gels has expanded tremendously in recent years. The relationship between the structure formation and photophysics is known at the quantitative level. The factors which we understand control these relationships include virtually all important materials parameters such as solvent quality, side chain branching, side chain length, molecular weight, thermal history and myriad functionalizations. This review describes advances in controlling structure and photophysical properties in polyfluorene solutions and gels. It discusses the demarcation lines between solutions, gels, and macrophase separation in conjugated polymers and reviews essential solid state properties needed for understanding of solutions. It gives an insight into polyfluorene and polyfluorene beta phase in solutions and gels and describes all the structural levels in solvent matrices, ranging from intramolecular structures to the diverse aggregate classes and network structures and agglomerates of these units. It goes on to describe the kinetics and thermodynamics of these structures. It details the manifold molecular parameters used in their control and continues with the molecular confinement and touches on permanently cross-linked networks. Particular focus is placed on the experimental results of archetypical polyfluorenes and solvent matrices and connection between structure and photonics. A connection is also made to the mean field type theories of hairy-rod like polymers. This altogether allows generalizations and provides a guideline for materials scientists, synthetic chemists and device engineers as well, for this important class of semiconductor, luminescent polymers.

Network structure of polyfluorene sheets as a function of alkyl side chain length

The formation of self-organized structures in poly(9,9-di-n-alkylfluorene)s ∼1 vol % methylcyclohexane (MCH) and deuterated MCH (MCH-d(14)) solutions was studied at room temperature using neutron and x-ray scattering (with the overall q range of 0.00058-4.29 Å(-1)) and optical spectroscopy. The number of side chain carbons (N) ranged from 6 to 10. The phase behavior was rationalized in terms of polymer overlap, cross-link density, and blending rules. For N=6-9, the system contains isotropic areas and lyotropic areas where sheetlike assemblies (lateral size of >400 Å) and free polymer chains form ribbonlike agglomerates (characteristic dimension of >1500 Å) leading to a gel-like appearance of the solutions. The ribbons are largely packed together with surface fractal characteristics for N=6-7 but become open networklike structures with mass fractal characteristics for N=8-9, until the system goes through a transition to an isotropic phase of overlapping rodlike polymers for N=10. The polymer order within sheets varies allowing classification for loose membranes and ordered sheets, including the so-called β phase. The polymers within the ordered sheets have restricted motion for N=6-7 but more freedom to vibrate for N=8-9. The nodes in the ribbon network are suggested to contain ordered sheets cross-linking the ribbons together, while the nodes in the isotropic phase appear as weak density fluctuations cross-linking individual chains together. The tendencies for macrophase separation and the formation of non beta sheets decrease while the proportion of free chains increases with increasing N. The fraction of β phase varies nonlinearly, reaching its maximum at N = 8.

Ambidextrous circular dichroism and circularly polarised luminescence from poly(9,9-di-n-decylfluorene) by terpene chirality transfer

Solvent chirality transfer using enantiomeric pairs of limonene and α-pinene allowed for the successful production of ambidextrous CD-/CPL-active polyfluorene aggregates from achiral polyfluorenes at 25 °C in a few minutes.

Scattering study of the conformational structure and aggregation behavior of a conjugated polymer solution

The conformational structure and the interchain aggregation behavior of a semirigid conjugated polymer bearing a decyl side chain, poly(2,3-diphenyl-5-decyl-1,4-phenylenevinylene) (DP10-PPV), in solutions with chloroform and toluene have been investigated by means of small-angle neutron scattering (SANS), static light scattering (SLS) and dynamic light scattering (DLS). The radius of gyration, persistence length, and the second virial coefficient of the polymer in dilute solution as determined by SLS were higher in chloroform than in toluene; consequently, the polymer assumed a more extended wormlike chain conformation in the former. The difference in the strength of interaction in the two solvents gave rise to contrasting aggregation behavior of the polymer in the semidilute regime. While only a minor fraction of the polymer underwent segmental association in chloroform, a considerable fraction of it formed clusters (microgels) with several micrometers in size in toluene. These clusters were further found to consist of sheetlike nanodomains. Compared with the DP-PPV bearing a shorter hexyl side chain, DP6-PPV, the aggregates of DP10-PPV in toluene were weaker as they could be easily disrupted by moderate heating. This was attributed to a lack of strong pi-pi interaction between the DP10-PPV segments due to the greater steric hindrance imposed by the longer decyl side chains.