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Measuring the anisotropy of the field-effect mobility provides insight into the correlation between molecular packing and charge transport in organic semiconductor materials. Single-crystal field-effect transistors are ideal tools to study intrinsic charge transport because of their high crystalline order and chemical purity. The anisotropy of the field effect mobility in organic single crystals has previously been studied by lamination of macroscopically large single crystals onto device substrates. Here, a technique is presented that allows probing of the mobility anisotropy even though only small crystals are available. Crystals of a soluble oligothiophene derivative are grown in bromobenzene and drop-cast onto substrates containing arrays of bottom-contact gold electrodes. Mobility anisotropy curves are recorded by measuring numerous single crystal transistor devices. Surprisingly, two mobility maxima occur at azimuths corresponding to both axes of the rectangular cyclohexyl-substituted quaterthiophene (CH4T) in-plane unit cell, in contrast to the expected tensorial behavior of the field effect mobility.

Organic field-effect transistors (FETs) may be cost-effective alternatives to amorphous-silicon transistors in certain applications. Vacuum-deposited polycrystalline organic semiconductor films exhibit equivalent mobilities to amorphous silicon and often outperform polymeric semiconductors. Here we discuss a lamination-based method for manipulating such films, featuring a thermoplastic “receiver” polymer layer that captures semiconductor films from “donor” substrates, and also acts as a gate insulator. The compound 5,5‘-bis(4-isopropylphenyl)-2,2‘-bithiophene (diPr-PTTP) is shown to have favorable solid-state properties for this process. Its X-ray structure is determined, and its performance and those of pentacene and copper phthalocyanine are evaluated. The influence of surface properties of the donor and receiver is examined. diPr-PTTP retained a mobility after transfer of one-third the value of the same material vapor-deposited directly onto the receiver layer.

The polymer field has experienced enormous growth in the last decade. One of the most exciting aspects of this growth is that it is not simply an extrapolation of pre‐existing lines of research, bu...

Sectorization is frequently observed in elongated rectangular single crystals of syndiotactic polypropylene (s-PP) grown from the melt in thin films. The crystals are bound laterally by {100} and {010} growth planes. The constituent sectors are readily observed because of a difference in thicknesses; specifically, the sectors bound by the {100} planes [the (100) growth sectors] are thicker than those bounded by the {010} planes [the (010) growth sectors]. Dark field (DF) transmission electron microscopy (TEM) was utilized to examine the chain orientation and lattice defects in the different growth sectors of s-PP single crystals. The (020) DF images exhibited pairs of bright streaks that are more or less perpendicular to the (100) planes and cross over the whole width of the (100) sectors. In the (200) DF images, the (100) sectors also exhibited similar but dimmer streaks than those in the (020) DF images. This suggests that the crystal c-axis orientation in the (100) crystal sectors undergoes a periodic change in inclination along both the longitudinal (parallel to the b-axis) and the transverse axis (parallel to the a-axis) directions of the single crystal. The ripples in the (100) sectors, previously observed in TEM, were also seen with atomic force microscopy as sinusoidal-like periodic height changes along the longitudinal axis direction at both room temperature and high temperatures. This periodic height change accounted for the pairs of bright streaks in the (020) DF images. The ripple formation was explained by lamellar thickening in the (010) sectors during crystal growth. This thickening process causes lateral contractions, which accumulate mainly along the longitudinal axis direction of the single crystal. On the other hand, the (020) DF images exhibited a relatively uniform brightness in the (010) sectors, while in the (200) DF images, several dark zones in the (010) sectors were more or less along the diagonal directions of the single crystal. This observation indicates that the crystal c-axis in these zones is slightly deviated from the (200) planarity due possibly to the lateral contraction in the (010) sectors. A regular Moiré pattern in the (010) sectors was observed in the (020) DF images, and no Moiré patterns were found in the (100) sectors. Again, this was presumably due to sinusoidal-like ripples, which substantially affect the crystal plane orientation with respect to the lamellar crystal normal. In the (200) DF images, only random Moiré fringes could be found and, in particular, when the (010) and the (100) sectors overlapped. However, regular Moiré fringes were observed continuously over both sectors in (220) DF images.

Ordering of α , ω -dihexyl- α -quaterthiophene (DH α 4T) thin films on substrates including friction-transferred polytetrafluoroethylene (PTFE) is explored. DH α 4T exhibits a high-temperature mesophase from 81 to 181 °C. Textural analysis reveals this phase to be a tilted, highly ordered smectic phase. Upon cooling from the isotropic melt, the smectic phase grows layer by layer on many surfaces. Smectic thin films grown on friction-transferred PTFE exhibit a high degree of alignment with edge dislocations being the primary defect. Upon crystallization from the mesophase, the material retains for the most part the order of the mesophase. Growth of the smectic phase on directional surfaces such as friction-transferred PTFE appears to be a route to control of surface ordering of DH α 4T.

A series of thiophene-containing oligomers that are less electron rich than alpha-6T were synthesized, and the thin film morphologies and field-effect transistor characteristics of the oligomers were evaluated. Phenyl and thiazole rings were included in many of the oligomers, and a new synthesis of perfluoroalkylmethyl-terminated oligomers was developed. Desirably low off currents were associated with calculated highest occupied molecular orbital energies above ca. 5.0 eV relative to vacuum. Some of the oligomers displayed mobilities above 0.01 cm2 (Vs)-1, but there was no correlation of mobility with calculated orbital energies and some compounds with seemingly continuous morphologies had low mobilities nonetheless. One compound, 1,4-bis(5‘-hexyl-2,2‘-bithiophen-5-yl)benzene, showed promising behavior as a solution-deposited semiconductor, with mobility up to 0.02 cm2 (Vs)-1 and an on/off ratio up to 20 000.

Polymer science has in recent years become one of the most dynamic components of materials science [1, 2, 3], which in turn is a powerful bridge between basic science and advanced technology. The National Science Foundation (NSF) has periodically reviewed the status of polymer science and engineering to ensure that this important field continues to develop in an appropriate fashion. Thus, the NSF asked the U.S. National Academy of Sciences on two previous occasions, in 1981 and again in 1994, to assess progress in polymer science and engineering and to make recommendations for the future. The resulting reports [4, 5] received wide circulation in many countries and helped to focus attention on the changing nature of the polymer field. A clear trend that was identified in these reports is the greater commonality with other materials-related disciplines [1-3] and with the biological sciences [1-5]. Increasingly, these changes are being reflected in the programs of government agencies funding polymer research which are tending to be broader and more interdisciplinary than in the past.

Oriented thin films of organic semiconducting small molecules were prepared by crystallization on rubbed alignment layers. Polarized absorption spectra showed that the long axis of the conjugated backbones was highly oriented along the rubbing direction and parallel to the substrates. Transmission electron microscopy and diffraction confirmed that the molecules and in many cases the resulting crystals are aligned. Using the above aligned films as semiconducting layers, we fabricated field-effect transistors having anisotropic mobilities with ratios greater than 15. Several common organic semiconductors have been investigated, and the results indicate that this growth method is generally successful for achieving macroscopic alignment of these semiconducting molecules (and frequently their crystals, as well).

The synthesis and field-effect transistor (FET) electron mobility of ten N-substituted naphthalene 1,4,5,8-tetracarboxylic diimide (NTCDI) derivatives deposited at ambient and elevated temperatures are reported. Mobilities >0.01 cm2/(V s) were measured in air for three NTCDIs with partially fluorinated substituents, and >0.001 cm2/(V s) for a hydroxy-terminated compound. Mobilities 0.001−0.1 cm2/(V s) were also found for three n-alkyl NTCDIs, but only under vacuum; FET operation with gold bottom contacts was enabled by specific thiol coatings of the contacts. The highest mobility in air, >0.1 cm2/(V s), was conferred by 4-trifluoromethylbenzyl substitution, while 1H,1H-perfluorooctyl substitution resulted in an on/off ratio in air >105. Solution electrochemistry and solid-state X-ray and electron diffraction were employed to partially explain the results, and applications of the materials to complementary circuits are considered.

Organic and polymeric driving circuits may be a possible alternative to realize low cost flexible and large area flat panel displays. Recent efforts have been directed to the discovery of solution processable polymeric semiconductors for the ease of fabrication and low cost. In this paper, we report the investigation of various 3-substituted regioregular polythiophenes as active semiconductors for field-effect transistors. The best transistor performance has been found with regioregular poly(3-heaxylthiophene). The difference in their device performance was correlated to the structure and morphological behavior of these regioregular polythiophenes.

When outfitted with long alkyl chains, polycaps, capsules along a polymer chain, spontaneously organize themselves into a two-dimensional liquid crystalline phase. Further organization results from shearing or pulling the liquid crystalline samples, producing three-dimensional assemblies of micrometer-wide, infinitely long fibers (see schematic representation).

Soluble regioregular polythiophene derivatives with chiral alkyl side chains, bulky side chains, and carboxylic side chains have been synthesized. Their thin-film field-effect transistor performance and structural characteristics have been investigated. The above properties were found to be strongly affected by the nature of the side chains. Poor molecular ordering and low crystallinity were found for regioregular polythiophenes with bulky or carboxylic-substituted side chains, and their resulting transistor devices showed poor field-effect mobilities (less than 10-5−10-4 cm2/(V s)). A chiral alkyl-substituted regioregular polythiophene showed high crystallinity. However, its π−π overlap distance between the polythiophene backbones increase substantially (i.e., to ∼4.3 vs 3.8 A for regioregular poly(3-hexylthiophene) (PHT)) through the introduction of methyl branches in the side chains. The field-effect mobility of this polymer is reasonably high for solution-processable conjugated polymers (i.e., of the ...

Single layer polymer light emitting diodes (LEDs) are ideal candidates for practical applications due to their easy processing conditions. However, low quantum efficiency of light generation is often obtained due to imbalanced charge injection and transport of holes and electrons. In this paper we report new conjugated polymers with electron deficient oxadiazole moieties as side-chains. These polymers have shown at least an order of magnitude increase in electroluminescence efficiencies and better charge injection properties compared to their corresponding conjugated backbone polymers. In addition, novel patterned LED device structures which emit in geometries with features into the sub-micron range will be presented. Finally, issues regarding integration of polymer transistors with organic LEDs will be discussed.

Several new p-type semiconducting materials with lower electron-donating ability than the parent sexithiophene were synthesized and their thermal, morphological, and FET properties were investigated. The incorporation of thiazole rings into oligothiophenes was designed to lower the highest occupied molecular orbital (HOMO) level of the molecules and hence make them less susceptible to p-doping. FET devices based on a dihexylated six-ring compound with thiazoles as the central rings indeed showed enhanced stability to p-doping over those of typical sexithiophenes. Relatively high on/off current ratios (greater than 104 for gate voltages of −100 and 0 V) were routinely obtained from devices operating in air, eliminating the need for strict exclusion of oxygen. The threshold voltage of devices made from this compound showed no signs of shifting toward more positive gate voltages after more than one month's exposure to the air. The thiazole-containing oligomers generally had lower field-effect mobilities than...

Nonracemic helicene 1 forms high-quality Langmuir−Blodgett (LB) films, in which the molecules are organized in parallel columns with their edges exposed to the surface. Evidence for the edge-on exposure is the molecular area of a monolayer on water and, as determined by X-ray diffraction, the similarity of the interlayer spacings of multilayers to the diameters of the molecular columns that constitute nonracemic 1 in bulk. Evidence of the organization into parallel columns includes the linear dichroism of the films and the 180° periodicity with which rotating the films varies the amounts by which plane polarized light is rotated, the intensities of the second harmonics generated, and the films' degree of anisotropy. The circular dichroisms of a 10-layer LB film could be resolved from the effects of linear dichroism and linear birefringence. The CD and UV−vis absorption spectra of the films are similar to those of solutions in which the molecules are aggregated, not isolated. The measurement of a CD spectr...

The synthesis, thin-film morphology, and hole mobility in thin-film transistors (TFTs) of compounds based on the novel anthradithiophene (ADT) ring system are reported. The parent compound and its 2,8-dihexyl, didodecyl, and dioctadecyl derivatives (DHADT, DDADT, and DOADT, respectively), synthesized via alkylated thiophene dicarboxaldehyde acetals, were investigated. They all form highly ordered polycrystalline vacuum-evaporated films with mobilities as high as 0.15 cm2/(V s), as high as has ever been observed for a polycrystalline organic material. DOADT has a mobility of 0.06 cm2/(V s) even though 70% of its molecular volume is occupied by hydrocarbon chains. DHADT was cast from solution under atmospheric conditions onto a TFT giving a mobility of 0.01−0.02 cm2/(V s). Thus, the alkylated ADTs combine a pentacene-like intrinsic mobility with greater solubility and oxidative stability.

Our investigation of thiophene oligomers as organic thin film transistor (TFT) semiconductors is extended to the hexamer with dodecyl and octadecyl end-substituents and with side chains containing ethereal oxygens. Two thiophene tetramers are studied as well. All of the new compounds are prepared via polar, monosubstituted half-oligomers that are purified, further elaborated, and dimerized. Properties are reported in comparison with the previously reported dihexyl compounds. All of the compounds form ordered films with orientation perpendicular to the substrate. For the hexamers, the longer chains decrease the TFT mobility of evaporated films, while the oxygens have very little electronic effect, even though the oxygen does cause an approximate doubling of the solubility. A tetramer with an ether side chain has a mobility below 0.01 cm2/Vs. Films were also cast from dilute solution and showed mobilities at or above 0.01 cm2/Vs in several cases. This casting process may be useful in devising all-liquid-pha...

Nonracemic helicene 1 when cooled from the melt organizes itself into macroscopic liquid crystalline fibers. Transmission electron microscopic analysis shows that the fibers are comprised of lamellae 50−200 nm wide, each only ca. 10 nm high. Analyses by X-ray and electron diffraction demonstrate that the molecules are organized in hexagonally packed columns. Polarized light microscopy reveals that the columns are stacked so that their long axes parallel the long axes of the fibers. In ultrathin films, the alkyl side chains organize themselves further as in crystalline alkanes.

In previous work we have shown that the stable form of syndiotactic polypropylene (sPP) adopts a fully antichiral packing in its crystalline lattice. At the same time, we had also discovered a number of prohibited reflections from crystallographic planes with a c-axis component, indicating distortions or defects along the molecular-chain direction and possible departures from the ideal Ibca unit-cell structure. Three studies that appeared in 1996 have proposed a number of possible minimized structures to explore the energetics of departure from such an ideal Ibca symmetry and to account for the prohibited reflections. Two of the studies were based upon energy minimization of the unit cell using empirical or spectroscopically derived force fields. The third model was based upon Rietveld refinement of the bulk X-ray diffractogram combined with ad hoc choices of space groups selected to reproduce the prohibited reflections, and results in a structure with minimal departures from Ibca. In the present article we examine the structural implications of all these models on various reciprocal-lattice sections of sPP by comparing calculated electron-diffraction patterns for appropriate zones with those we obtained experimentally from single crystals. We find that none of the proposed models fully accounts for the experimental data. One of the energy-minimized models (involving translation along the molecular axis) and the Rietveld-refined structure reproduce some of the observed reflections but are not in agreement with observed intensities and introduce additional unobserved reflections or other artifacts. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2523–2533, 1997

We review the device and materials science behind organic electroluminescent diodes made both using discrete evaporable molecules and spin-cast organic polymers. A great deal of progress has been made in improving the efficiencies and spectral properties of organic light-emitting diodes, and these are now adequate for many applications. More work is necessary to understand the stability and degradation of emissive and charge-transporting organics, but some systems have been shown to be stable for 104 hours at display brightness. Major challenges still face the community in terms of developing satisfactory systems design and processing techniques if organic electroluminescence is to realize either performance or economic advantages over technologies and significantly penetrate the display market. We present an analysis of the suitability of organic light-emitting diodes for various applications, and consider the materials and manufacturing obstacles that must be overcome.

The progress made in the understanding of the crystal polymorphism and morphology of isotactic and syndiotactic poly(propylene) (iPP and sPP) is reviewed. In 1960, the crystal structure of the stable form of iPP, the α phase, was already solved and the general principles of polyolefin chain conformation and crystallography established. The molecular analysis of the lamellar branching of αiPP—a case of homoepitaxy specific to this polymer and this crystal form—was given in 1986. The γ phase of iPP, solved in 1989, is the first and so far unique example of a polymer structure with non-parallel chain stems. The β phase is the first example of a frustrated structure in polymers, and rests on packing of isochiral helices in the unit-cell. The stable, high temperature form of sPP described in 1988 is based on full antichiral packing of helices, whereas the original structure corresponds to another limiting (but not experimentally observed in pure form) case based on packing of isochiral helices. Details of the molecular structure of iPP and sPP, such as the hand of individual helical stems and setting of chains, are accessible by crystallographic considerations which make use of molecular markers provided by the α-α and α-γ epitaxies in iPP or by direct visualization by atomic force microscopy.

Organic and polymeric materzials have seen a tremendous growth in research in the last five years as potential electroactive elements in thin-film-transistor (TFT) applications. These are driven by the increasing interest in flat-panel-display applications, for which organic and polymeric materials offer strong promise in terms of properties, processability, cost, and compatibility with eventual lightweight, flexible plastic displays. In this review we summarize the current status of our knowledge on the science of these organic and polymeric semiconducting materials. Most of these are based on linear thiophenes, especially a-hexathienyl, which has elicited by far the most attention. Mobility values in the 10−2–10−1 cm2/Vs and especially source-drain current on/off ratios of up to 106 make this a highly promising potential alternative to amorphous silicon. Other thienyl compounds are also discussed, as are polymeric analogues. A brief discussion of technological potential, limitations, and problems that need to be overcome is given at the end.

We have conducted a detailed electron-microscopic investigation of the structure and morphology of α-hexathienyl (α-6T), a hexameric homologue of polythiophene that shows great promise as a thin-film-transistor material. In ordinary films for device applications prepared by vacuum sublimation, α-6T crystals are very small (

We have studied the morphology of thin films of α-hexathienyl (α-6T), a hexamer of thiophene that is a very promising material for thin-film-transistor applications. Using electron- and atomic-force microscopies, we found that on both rigid (Si/SiO2 and glass) and flexible (polyimide) substrates, evaporated films show an apparently random, polycrystalline morphology. The crystals are lamellar, ca. 100-200 nm in lateral dimensions and 15-30 nm in thickness, and exhibit irregular boundaries. Nevertheless, electron-diffraction evidence from such films indicates that the constituent molecules are deposited preferentially end-on and assume a normal or nearly normal orientation with respect to their substrates. Rapid high-temperature annealing causes growth of much larger (μm-sized) crystalline grains and a partial transformation to a high-temperature polymorph; however, this process leads to formation of gaps in the film, which may cause deterioration of electronic performance.

α-hexathienyl (α–6T) is a highly promising material for application in thin film transistor devices. Recently, record high mobilities, together with record high current on/off ratios, have been reported.1 Thus far, structural information on this exciting material is sketchy. The crystal structures of several such hexamers have been investigated, but only with powder samples, since the crystal growth has proven exceedingly difficult.2-5 Powder Rietveld refinements on these materials are severely hampered by the large number of overlapping reflections, preferred orientation, ambiguities in symmetry, etc. Here, we present a crystal structure of the high-temperature polymorph of α–6T (α–6T/HT), as determined from a single-crystal structure analysis. In this polymorph, the hexamer crystallizes in the smallest unit cell so far reported for this material, but the molecule is flat. Extended Hückel theory (EHT) band structure calculations show that α–6T/HT is an indirect gap semiconductor, with the conduction band minimum at Y and the valence band maximum at Γ. The conduction and valence bands both show a remarkable degree of dispersion along X and Y for a molecular crystal. The electronic band structure of this material is strikingly similar to that of the two-dimensional organic superconductors based on bis(ethylenedithio)tetrathiafulvalene (ET), such as κ−(ET)2 Cu(NCS)2.

The epitaxial interactions between syndiotactic polypropylene (sPP) and polyethylene (PE) are analyzed on the basis of electron diffraction patterns of composite bilayers. They rest on the near-parallelism of PE chains with short pseudo n-pentane molecular segments of sPP exposed on the (100) plane and inclined by ca. 31 o to the helical molecular axis. A related homoepitaxy of sPP, with more limited impact on bulk crystallization, is also described and analyzed on a similar structural basis. It rests on the parallelism of the same n-pentane segments for isochiral helices which face each other across the (110) contact plane and results in chain axes mutually rotated by 14 o

The morphology of syndiotactic polypropylene (sPP) containing >99% racemic diads has been investigated following controlled crystallization from the melt in thin films. At high growth temperatures, large rectangular single crystals are obtained with preferred growth along the crystallographic b-axis, while at lower temperatures these branch out to yield dendritic, axialitic, and eventually spherulitic aggregates. These crystals exhibit two very unusual properties: transverse fractures and ripples. Both are quasi-periodic, the fractures with a spacing of 3-7 μm and the ripples generally 1 μm or less

The molecular orientation and disclination defects in injection molded bulk samples of liquid crystal polymers have been revealed by transmission electron microscopy (TEM). Using a recently optimized lamellar decoration and etch technique for use with poly(benzoate-co-naphthoate), we have studied in detail the skin-core morphology produced by realistic injection molding processing. In the skin, the director is observed to be highly aligned along the flow direction, and the defect density is very low (10 cm−2). In the core, however, the defect density is six orders of magnitude greater, and the director lies predominantly in the plane defined by the flow and neutral directions. The flow during processing is turbulent, giving rise to distinct morphological features and disclination clusters. When the material is filled at 20% volume fraction with long glass fibers, the molecular and filler orientations are nearly parallel. Dimensional precision also correlates with the morphology and improves with increasing core fraction.

The semicrystalline morphology of the semiflexible smectic polyester [OCOPhOCOPhCOOPhCOO(CH 2 ) 7 ] n is investigated by high-resolution electron microscopy. Images combining diffraction and phase contrast allow simultaneous visualization of the mesomorphic and semicrystalline phases. A smectic C 2 mesophase is confirmed by electron diffraction of quenched (noncrystalline) specimens. A new disordered-uniaxial model is developed to evaluate the molecular tilt in smectic mesophases in which the layers rather than the molecules are aligned. Biaxial symmetry for small selected areas (

The structure of β-isotactic polypropylene (β-iPP) has been investigated taking into consideration X-ray and electron-diffraction data supplemented by packing energy calculations. The structure is characterized by extensive disorder, and two structural models can be envisaged: both are based on domains of helices all of the same chirality, arranged on a pseudohexagonal lattice. The simplest satisfactory model is trigonal (P3 1 21 and the enantiomorphic P3 3 21) with a= b = 11.09 A and c= 6.49 A. Three monomers form the asymmetric unit, and six chains, each with a 0.5 occupancy factor, coexist in the unit cell

A series of syndiotactic polypropylene (s-PP) fractions with constant syndiotacticities and different molecular weights have been studied through differential scanning calorimetry (d.s.c.), wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering experiments. The molecular weights, molecular weight distributions, syndiotacticities and sequence distributions of this series of fractions have been characterized by gel permeation chromatography, solution nuclear magnetic resonance and Fourier transform infra-red spectroscopy. The equilibrium melting temperature of sufficiently high molecular weight (above 40 000) s-PP with about 94% racemic dyads is 160±1°C, and the heat of fusion is 8.0±0.3 kJ mol −1 . Overall crystallization rates exhibit a molecular weight dependence and a discontinuity with respect to crystallization temperature for the fractions. The temperature at which this discontinuity happens is at an undercooling of ca. 50°C. Based on nucleation theory, this discontinuity may be recognized as a regime III to regime 11 transition. With decreasing undercooling (increasing crystallization temperature) and molecular weight, a doubled crystal unit cell along the b axis becomes increasingly dominant during the crystallization. In this unit cell, opposite handedness of the helical chains exists along both the a and b axes (antichiral packing). Double melting peaks can be observed for all fractions in the high to middle undercooling region (ΔT> 50°C), while only one melting peak can be found in the relatively low undercooling region. Different heating rate experiments after isothermal crystallization in d.s.c. and WAXD indicate that the low-melting crystal may undergo reorganization and melt-recrystallization processes to form the high-melting crystal. During this transformation, doubling of the crystal unit cell along the b axis with an antichiral packing of the chain molecules is obtained.

We have investigated the channel-length dependence of responses to a vapor analyte with a series of α,ω-dihexylquarterthiophene (DHα4T) field-effect transistors (FETs). Single-crystalline DHα4T devices deposited by vacuum sublimation at substrate temperatures of 70 °C are compared with polycrystalline DHα4T films deposited at room temperature. By changing the length of FET channels and/or the size of polymer grains, the number of grain boundaries per device is changed systematically. A larger response to vapor analyte is obtained by increasing the number of grain boundaries per device, showing that vapor sensing occurs mainly at grain boundaries.

Building upon the previous results on silicon-based homopolymers, the authors have now synthesized and characterized copolymers of these novel materials. The copolymers poly(di-n-butylsilylene-co-di-n- pentylsilylene) (PB-co-PS) and poly(di-n-pentylsilylene-co-di-n-hexylsilylene) (PP-co-HS) were synthesized from solutions containing equal molar concentrations of the two respective monomers. The materials were examined by solution- and solid-state NMR, X-ray diffraction, DSC, and UV spectroscopy to obtain a description of the solid-state structures, phase transitions, and thermochromism of these copolymers. The conformational properties, crystal packing, and UV absorption characteristics of PB-co-PS are found to be the same as those of the two corresponding homopolymers poly(di-n-butylsilylene) (PDBS) and poly(di-n-pentylsilylene) (PDPS). In contrast, the authors find that while the silicon chain conformation of PP-co-HS (either a 7/3 or 9/4 helix) appears similar to that of the PDPS homopolymer, the UV absorption characteristics and the crystal packing differ from those of either of the corresponding homopolymers, PDPS and poly(di-n-hexylsilylene) (PDHS). The results of this study demonstrate that the properties of the polysilylene copolymers are complex and not directly predictable solely on the basis of the corresponding homopolymers.

The molecular structure, crystallization, solid-state morphology, thermal properties, and phase behavior of two copolymers consisting of a poly(dimethylsiloxane) (PDMS) mid-block coupled to polycaprolactone (PCL) end-blocks were investigated. Both copolymers (which differ only in the molecular lengths of the PCL end-blocks) were found to be lamellar systems, whose core consists of PCL chains having the same crystal structure as PCL homopolymer, and whose amorphous interlayers contain the PDMS blocks and the PCL noncrystalline segments. From x-ray and electron-microscopy results, it is expected that the PCL blocks may be folded once in the longer copolymer and not at all in the shorter. As a result of their differing PCL lengths, the former crystallizes as regular PCL spherulites (at a growth rate reduced with respect to PCL homopolymer), whereas the latter yields only defective, immature axialites of low overall crystallinity. Electron diffraction showed that these spherulites grow preferentially along b crystallographic axis and that the PCL crystalline stems are arranged perpendicularly to their lamellae. © 1993 John Wiley & Sons, Inc.

The semicrystalline morphology of a 73:27 mol% poly(hydroxybenzoate-co-hydroxynaphthoate) liquid crystal polymer is studied via transmission electron microscopy (TEM) and wide-angle X-ray diffraction. A permanganic etchant was used for TEM sample preparation and the etchant composition was optimized for selectivity of the non-crystalline phase. Crystalline lamellae, developed during cooling from the nematic phase, measure approximately 10 nm in thickness and 100 nm laterally. The lamellae are periodic, having a repeat of 34 nm. Because the molecules are long (∼200 nm) and extended, tie chains between lamellae are abundant. Crystallization must therefore occur via small axial shifts of approximately a monomer length. Based on our observations of disclination morphologies, the splay elastic constant is greater than that for bend, indicating considerable molecular semiflexibility. The interior morphology of bulk injectionmoulded specimens is also revealed following microtomy and selective etching. Molecular orientation and defect density can be accurately measured.

We show that organic thin-film transistors have suitable properties for use in gas sensors. Such sensors possess sensitivity and reproducibility in recognizing a range of gaseous analytes. A wealth of opportunities for chemical recognition arise from the variety of mechanisms associated with different semiconductor–analyte interactions, the ability to vary the chemical constitution of the semiconductor end/side groups, and also the nature of the thin-film morphology.

Through gate-modulated electrochemical doping, ambipolar operation in thin-film transistors (TFTs) can be realized in air with solution processable conjugated polymers. Unlike other typical organic TFTs, which rely on high crystallinity for better charge transport, these electrochemically-doped transistors operate under a different mechanism and show very high current output even with completely amorphous polymers.

The polysilylenes exhibit complex solid-state structures and electronic properties. In this report the solid-state conformational structures of symmetrically substituted poly(di-n-alkylsilylenes) are described. Comparisons between the different structures and their respective UV absorption characteristics demonstrate that the relationship is complex and aspects of molecular geometry beyond chain conformation must be considered. The thermochromic and piezochromic behavior of several of these polysilylenes is discussed. The all-trans conformation of the silicon backbone is associated with both phenomena. The properties of several polysilylene copolymers have also been investigated. We find that some of the copolymers form a well-ordered structure and exhibit absorption characteristics similar to the crystalline phases of the corresponding homopolymers.

A comparison is made between the structures and properties of the poly(dimethylsilylene-co-di-n-hexylsilylene) and those of the corresponding homopolymers. While the two homopolymers adopt the same chain conformation in the solid state, they differ significantly in their absorption characteristics and in the nature of their solid-state transitions. At room temperature the copolymer is found to be mostly disordered with a small amount of a well ordered crystalline phase in which the silicon backbone adopts an all-trans conformational arrangement as is observed for the poly(dihexylsilylene)

The polysilylenes exhibit complex solid-state structures and electronic properties. In this report the solid-state conformational structures of symmetrically substituted poly(di-n-alkylsilylenes) are described. Comparisons between the different structures and their respective UV absorption characteristics demonstrate that the relationship is complex and aspects of molecular geometry beyond chain conformation must be considered. The thermochromic and piezochromic behavior of several of these polysilylenes is discussed. The all-trans conformation of the silicon backbone is associated with both phenomena. The properties of several polysilylene copolymers have also been investigated. We find that some of the copolymers form a well-ordered structure and exhibit absorption characteristics similar to the crystalline phases of the corresponding homopolymers.