Planar Chain Conformation Research Articles

Facilitated phase transformation of PVDF in its composite with an ionic liquid

The phase transition of poly(vinylidene fluoride) (PVDF) from α-to γ′-phase is hardly accessible, especially for the low-temperature crystallized α-phase, owing to the diminished thermal energy available for transformation. In this study, the phase transition ability of α-PVDF is greatly improved in its composites with a typical imidazole ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). The enhanced phase transition is achieved by two thermal steps, including the crystallization of α-phase and subsequent annealing at a higher temperature. The transformed lamellar morphology is dependent on the crystallization temperature of α-phase, while the phase transition efficiency mainly rests with the annealing temperature. The phase transition is always accompanied by the thermal degradation with elimination of HF, which is significantly accelerated by [BMIM][PF6]. The introduction of molecular defects and carbon-carbon double bonds probably increase the fraction of planar chain conformation, therefore, enhancing the phase transition efficiency.

Thermally Stable Poly(vinylidene fluoride) for High-Performance Printable Piezoelectric Devices.

Piezoelectric polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymers, can achieve large strains and high work density under external electrical fields. These materials are highly desirable in the development of electronic devices and intelligent structures. Here, we demonstrate that dehydrofluorination (DHF) can provide a versatile chemical modification of the PVDF homopolymer that yields thermally stable ferroelectricity. The DHF process significantly increases the fraction of planar chain conformation in the PVDF and results in higher piezoelectric coupling with a wider processing temperature range, compared to traditionally processed PVDF. The efficacy of DHF in promoting planar chain conformation is demonstrated through molecular simulation and further proven by experimental characterization. The induced piezoelectric phases by DHF were able to be preserved through high temperature treatments up to 200 °C. The dehydrofluorinated PVDF exhibits improved electromechanical coupling with a high piezoelectric strain coefficient of d31 = 25.12 ± 1.13 pC/N, which can be further improved to 35.12 ± 0.69 pC/N by common mechanical drawing. This high piezoelectric voltage coefficient leads to an excellent actuation and energy harvesting behavior with a power density of 21.96 mW/cm3 in a flexible undrawn PVDF energy harvester, which is 3.13 times higher than conventionally drawn PVDF. The versatile and scalable method for preparing PVDF polymers with high piezoelectric coupling will enable new manufacturing processes not currently compatible with PVDF homopolymers.

Manipulation of Chain Conformation for Optimum Charge-Transport Pathways in Conjugated Polymers.

A pair of different diketopyrrolopyrrole-based conjugated polymers (CPs) were designed and synthesized to investigate the effect of chain conformation on their molecular assembly. Conformation management was achieved by the incorporation of different linkers during polymerization. Through the use of computational calculations and UV-vis absorption measurements, the resulting CPs (PDPP-T and PDPP-BT) were found to exhibit partly modulated chain geometry. Grazing incident X-ray diffraction experiments with a two-dimensional detector revealed that PDPP-T having a planar chain conformation exhibited an edge-on type molecular arrangement, which evolved to a face-on type chain assembly when the planar geometry was altered to a slightly twisted one as in PDPP-BT. In addition, it was verified that the directional electric carrier mobility of CPs was critically distinguished by the distinctive chain arrangement in spite of their similar chemical structure. Concentration-dependent absorption measurements could provide an improved understanding of the assembly mechanism of CP chains: the planar conformation of PDPP-T facilitates the formation of preassembled chains in a concentrated solution and further directs the edge-on stacking, while the twisted dihedral angle along the benzothiophene in PDPP-BT prevents chain assembly, resulting in the face-on stacking. Because CP chain conformation is inevitably connected with the generation of preassembled chains, manipulating CP geometry could be an efficient tool for extracting an optimum chain assembly that is connected with the principal charge-transport pathway in CPs.

Germanium‐ and Silicon‐Substituted Donor–Acceptor Type Copolymers: Effect of the Bridging Heteroatom on Molecular Packing and Photovoltaic Device Performance

The effects of heteroatom substitution from a silicon atom to a germanium atom in donor‐acceptor type low band gap copolymers, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PSiBTBT) and poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PGeBTBT), are studied. The optoelectronic and charge transport properties of these polymers are investigated with a particular focus on their use for organic photovoltaic (OPV) devices in blends with phenyl‐C70‐butyric acid methyl ester (PC70BM). It is found that the longer C‐Ge bond length, in comparison to C‐Si, modifies the molecular conformation and leads to a more planar chain conformation in PGeBTBT than PSiBTBT. This increase in molecular planarity leads to enhanced crystallinity and an increased preference for a face‐on backbone orientation, thus leading to higher charge carrier mobility in the diode configuration. These results provide important insight into the impact of the heavy atom substitution on the molecular packing and device performance of polymers based on the poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole) (PCPDTBT) backbone.

Effects of a Heavy Atom on Molecular Order and Morphology in Conjugated Polymer:Fullerene Photovoltaic Blend Thin Films and Devices

We study the molecular order and morphology in poly(3-hexylthiophene) (P3HT) and poly(3-hexylselenophene) (P3HS) thin films and their blends with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). We find that substitution of the sulfur atoms in the thiophene rings of P3HT by heavy selenium atoms increases the tendency of the molecules to form better ordered phase; interestingly, their overall fraction of ordered phase is much lower than that of P3HT-based films. The higher tendency of P3HS molecules to order (aggregate) is consistent with more planar chain conformation simulated. The lower fraction of ordered phase (or the higher fraction of disordered phase) in P3HS-based films is clearly identified by in-plane skeleton Raman modes under resonant excitation conditions, such as a smaller ratio of the C═C modes associated with the ordered (∼1422 cm(-1)) and disordered (∼1446 cm(-1)) phases (I(1422 cm(-1))/I(1446 cm(-1)) = 1.4 for P3HS and 0.6 for P3HS:PCBM), compared with P3HT-based films (I(1449 cm(-1))/I(1470 cm(-1)) = 2.5 for P3HT and 1.0 for P3HT:PCBM) and a larger Raman dispersion of the C═C mode: P3HS (17 cm(-1)) versus P3HT (6 cm(-1)) and P3HS:PCBM (36 cm(-1)) versus P3HT:PCBM films (23 cm(-1)). The higher fraction of disordered phase in P3HS prevents the formation of micrometer-sized PCBM aggregates in blend films during thermal annealing. Importantly, this lower fraction but better quality of ordered phase in P3HS molecules strongly influences P3HS:PCBM photovoltaic performance, producing smaller short-circuit current (J(sc)) in pristine devices, but significantly larger increase in J(sc) after annealing compared to P3HT:PCBM devices. Our results clarify the effects of heavy atom substitution in low band gap polymers and their impact on blend morphology and device performance. Furthermore, our study clearly demonstrates resonant Raman spectroscopy as a simple, but powerful, structural probe which provides important information about "fraction/quantity of ordered phase" of molecules, not easily accessible using traditional X-ray-based techniques.

Surface Micelles of CF3(CF2)7(CH2)10COOH on Aqueous La3+ Subphase Investigated by Atomic Force Microscopy and Infrared Spectroscopy

CF3(CF2)7(CH2)10COOH spontaneously assembles into monodispersed nanometer-sized micelles after spreading onto the aqueous lanthanum acetate subphase in a Langmuir trough at 283 K, according to the atomic force microscope (AFM) images. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and ordinary polarized infrared spectroscopy are applied to study the surface micelles in situ on the aqueous subphase and ex situ on the solid substrate, respectively. The long axes of the −(CH2)7− helix and −(CH2)10−chain are slightly inclined, with tilt angles of 25 ± 3°, on the solid substrate and basically so inclined on the aqueous subphase. The micelle formation comes from a subtle interplay of the steric hindrance imposed by the bulky −(CH2)7− helix and the van der Waals interaction between the underlying −(CH2)10− chains. The micelle has a CH2 antisymmetric stretching [νas(CH2)] frequency of 2918.0 ± 0.1 cm-1 on the solid substrate and 2916.4 ± 0.2 cm-1 on the aqueous subphase, corresponding to a predominantly trans zigzag planar chain conformation. This trans zigzag chain has a hexagonal packing in the micelle. Elevating the subphase temperature from 283 to 303 K and keeping the temperature at 303 K for some time (30−120 min) causes the micelles to fuse into each other, according to the AFM images. In the fused monolayer, the hydrocarbon segment has a higher νas(CH2) frequency of ∼2920 cm-1, implying that some gauche kinks appear during the thermal treatment. It is found that micelle fusion occurs preferentially at zero surface pressure in loosely packed monolayers rather than at 20 mN/m in tightly packed monolayers. Finally, the combined in situ and ex situ infrared characterization of the surface micelles eliminates transfer artifacts concerning the headgroup orientation.

Crystal Structure of the Stereoregular Ethylene-alt-styrene Copolymer Synthesized with a Zirconocene-Based Catalyst

A stereoregular alternating ethylene−styrene copolymer, also stretchable in fiber form, has been obtained with rac-isopropylidenebis(1-indenyl)zirconium dimethyldiamide activated by the methylalumoxane catalytic system. From X-ray diffraction fiber spectra, an isotactic structure has been assigned to this copolymer as well as the previously described stereoregular ethylene−styrene copolymers. The polymer has zigzag planar chain conformation with phenyl groups oriented perpendicularly to the chain axis. The crystallographic symmetry in B2/m. Lattice constants (monoclinic, unique axis c): a = 10.23, b = 15.53, c = 5.12 A; γ = 98.6°. The disagreement index for measured reflections is R1 = 0.10.

Crystal structure of poly(iε-caprolactone)

Combining the data from epitaxially oriented samples with those obtained from solution-grown crystals demonstrates that the chains pack in the nonplanar structure in space group P2 1 2 1 2 1 proposed by Chatani rather than an alternative planar chain conformation. The R-factor is 0.199

Study of the molecular mobility in swollen poly(epichlorohydrin) 1. Nonpolar swelling agent

The broad line n.m.r. spectra of the amorphous fraction of poly(epichlorohydrin) have been analysed. Two motional transitions were observed and the T g and a low temperature transition β assigned to the motion of the CH 2Cl groups. The study of the poly(epichlorohydrin) swollen by a low molecular weight liquid (deuterobenzene), has provided information about the influence of swelling on the molecular dynamics, and the participation of intermolecular interactions in the second moment of the resonance lines. The observed value of the second moment of the spectra is higher than that calculated for a planar chain conformation. This indicates that the chain conformation of the atactic, amorphous poly(epichlorohydrin) is different from the conformation observed in the crystalline, isotactic fraction of the polymer.

The crystal structure of polylauryllactam (nylon 12)

AbstractThe crystal structure of nylon 12 was determined from the X‐ray diffraction patterns of uniaxially oriented specimens. The unit cell is monoclinic (pseudo‐hexagonal) with the following dimensions: a = 4,79 Å, b = 31,9 Å (fiber axis), c = 9,58 Å, β = 120°. There are four monomeric units in the unit cell and the space group is P21/c. The structure presents a distortion of the amide group from the planar chain conformation and it is rather similar to γ‐nylon 6. The hydrogen bond length is about 2,80 Å and the molecular packing is characterized by alternate, “up and down”, highly disordered sheets of parallel chains, linked by hydrogen bonds. The agreement between observed and calculated intensities has been improved by introducing a statistical disorder.

Infrared spectra and rotational isomerism in 2-chloro, 2-bromo and 2-iodobutane

The i.r. spectra of 2-chloro, 2-bromo, 2-iodobutane in the crystalline and glassy states have been obtained. With the help of frequency calculations, an assignment of bands to the possible conformational isomers S HH, S HH', S CH has been made. In the crystal phase the zig-zag planar chain conformation S HH', is present.