Molecular dynamics (MD) simulations of melt films of poly(alkyl methacrylate)s (PAMAs) with methyl, ethyl, and n-butyl substituents, respectively, have been performed using an all-atom model to investigate their surface and thin film properties. The applied all-atom force fields predict the bulk densities of PAMAs in good agreement with experiments. Moreover, predictions of the surface tensions of PMMA, PEMA, and Pn-BMA melts are in reasonably good agreement with experiments. The density profiles and orientational-order parameters of chain segments show atomic-scale characteristics in the air/polymer interfacial region. In the surface region, the backbone segments of PAMAs form a well-defined layer structure with the chain vectors oriented parallel to the surface, while the ester side-chains strongly segregate to the surface region and show perpendicular orientation to the surface, with the most pronounced surface segregation noted for Pn-BMA. Such surface segregations of chain segments make it difficult to apply a simple relationship between the cohesive energy density and the surface tension of polymers, for example, and should be taken into account in relating the surface/thin film characteristics to the bulk properties of polymers in general.

Molecular dynamics simulations of free-standing thin films of neat melts of polyethylene (PE) chains up to C150H302 and their binary mixtures with n-C13H28 are performed employing a united atom model. We estimate the surface tension values of PE melts from the atomic virial tensor over a range of temperatures, which are in good agreement with experimental results. Compared with short n-alkane systems, there is an enhanced surface segregation of methyl chain ends in longer PE chains. Moreover, the methyl groups become more segregated in the surface region with decreasing temperature, leading to the conclusion that the surface-segregation of methyl chain ends mainly arises from the enthalpic origin attributed to the lower cohesive energy density of terminal methyl groups. In the mixtures of two different chain lengths, the shorter chains are more likely to be found in the surface region, and this molecular segregation in moderately asymmetric mixtures in the chain length (C13H28 + C44H90) is dominated by the enthalpic effect of methyl chain ends. Such molecular segregation is further enhanced and dominated by the entropic effect of conformational constraints in the surface for the highly asymmetric mixtures containing long polymer chains (C13H28 + C150H3020). The estimated surface tension values of the mixtures are consistent with the observed molecular segregation characteristics. Despite this molecular segregation, the normalized density of methyl chain ends of the longer chain is more strongly enhanced, as compared with the all-segment density of the longer chain itself, in the surface region of melt mixtures. In addition, the molecular segregation results in higher order parameter of the shorter-chain segments at the surface and deeper persistence of surface-induced segmental order into the film for the longer chains, as compared with those in neat melt films.

The structure and properties of segmented block copolymer films of aromatic polyimide (PI) and poly(ethylene glycol) (PEG) doped with an ionic liquid are studied for potential polymer electrolyte membrane applications for fuel cells. Poly(amic acid) precursors of PI-PEG copolymers of 4,4-(hexafluoroisopropylidene) diphthalic anhydride, 4,4-(1,3-phenylenedioxy) dianiline, and bis(3-aminopropyl) terminated PEG (M-n approximate to 1500) are synthesized and then thermally imidized in membrane films, followed by swelling in ethylammonium nitrate (EAN) ionic liquid. The small-angle X-ray scattering results from the EAN-doped PI-PEG copolymer films show disordered bicontinuous phase-separated nanostructures described by Teubner-Strey theory, with the interface fractal dimension determined from the Porod equation. Thermal annealing of the EAN-doped membranes at 100-140 degrees C results in increased correlation lengths and smoother interfaces of the bicontinuous nanostructures. Such improved nanostructures lead to the increased ionic conductivity by two to five times with the maximum conductivity of 210 mS cm(-1) at 60 degrees C and 70% RH, much greater (nearly fivefold) than that of Nafion films, while maintaining the mechanical stability possibly up to 140 degrees C. Moreover, the investigation of the disordered bicontinuous phase-separated nanostructure of EAN-doped PI-PEG copolymer membranes is highly relevant to understanding the nanostructures of hydrated Nafion membranes and segmented block copolymers in general.

United-atom molecular-dynamics computer simulations of atactic polystyrene (PS) were performed for the bulk and free-standing films of 2 nm-20 nm thickness, for both linear and cyclic polymers comprised of 80 monomers. Simulated volumetric glass-transition temperatures (Tg) show a strong dependence on the film thickness below 10 nm. The glass-transition temperature of linear PS is 13% lower than that of the bulk for 2.5 nm-thick films, as compared to less than 1% lower for 20 nm films. Our studies reveal that the fraction of the chain-end groups is larger in the interfacial layer with its outermost region approximately 1 nm below the surface than it is in the bulk. The enhanced population of the end groups is expected to result in a more mobile interfacial layer and the consequent dependence of Tg on the film thickness. In addition, the simulations show an enrichment of backbone aliphatic carbons and concomitant deficit of phenyl aromatic carbons in the interfacial film layer. This deficit would weaken the strong phenyl-phenyl aromatic (π-π) interactions and, hence, lead to a lower film-averaged Tg in thin films, as compared to the bulk sample. To investigate the relative importance of the two possible mechanisms (increased chain ends at the surface or weakened π-π interactions in the interfacial region), the data for linear PS are compared with those for cyclic PS. For the cyclic PS, the reduction of the glass-transition temperature is also significant in thin films, albeit not as much as for linear PS. Moreover, the deficit of phenyl carbons in the film interface is comparable to that observed for linear PS. Therefore, chain-end effects alone cannot explain the observed pronounced Tg dependence on the thickness of thin PS films; the weakened phenyl-phenyl interactions in the interfacial region seems to be an important cause as well.

Interface characteristics of polystyrene (PS) melts in free-standing thin films and on a graphite surface were investigated by molecular dynamics simulations employing an explicit all-atom force field. The calculated surface tension is in good agreement with experiment, which provides good support for the force field parameters employed. In the polymer/vacuum free-surface region, the density profile exhibits an enrichment of phenyl groups relative to the backbone alkyl groups at the outermost low-density free surface, but this free surface is followed by a layer of relatively depleted phenyls and enriched alkyls of ca. 7 angstrom thickness. In the free surface, the phenyl-ring normal vectors and backbone chain vectors are both preferentially oriented along the film surface, in agreement with available experiments. At the polymer/graphite interface, the backbone chain vectors are strongly oriented along the graphite surface whereas the orientation distribution of phenyl-ring normal vectors exhibits two maxima along the nearly parallel (20 degrees) and the perpendicular direction to the graphite-surface normal. A densely packed structure is formed at the PS-graphite interface, which strongly decreases the segmental chain mobility, in contrast to the enhanced segmental mobility in the free-surface region. (C) 2017 Elsevier Ltd. All rights reserved.

A recent study has shown that the use of a semiconducting surfactant in hybrid bulk heterojunction solar cells based on poly[ 2-methoxy-5-(3',7'-dimethyloctyloxyl)-1,4-phenylene vinylene] and zinc oxide significantly increases their performance. To further study the mechanism underlying the improved performance resulting from the addition of a semiconducting surfactant, we compared the temperature and light intensity dependence of the current voltage characteristics and impedance spectra of devices that used no surfactant, an insulating oleic acid (OA) surfactant, or a semiconducting 2-(2-ethylhexyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de] isoquinoline-6,7-dicarboxylic acid (BQ) surfactant. The temperature and light intensity dependence data enabled us to extract the activation energy for different devices, and we measured the junction resistance through impedance spectroscopy. The device that used BQ had significantly lower activation energy (9-12 meV) than that without surfactant or oleic acid (17-21 meV), thus indicating improved charge transport and consequently an increased short-circuit current. Furthermore, the impedance characteristics showed that the injection resistance, when the BQ surfactant was used, was lower than that when no surfactant or oleic acid was used. These experiments provide further evidence that the use of semiconducting surfactants is essential to improvethe performance of hybrid bulk heterojunction solar cells.

We have investigated the surface reorganization characteristics of a novel amphiphilic diblock copolymer, poly(acetic acid-2-(2-(4-vinyl-phenoxy)-ethoxy)-ethylester)-block-polystyrene (PAEES-b-PS), in response to varying interfaces from air to water and vice-versa at various temperatures. The surface reorganization characteristics of the block copolymer films was monitored by scanning force microscopy, in order to delineate the kinetically controlled morphological process of surface reorganization of a diblock copolymer, with a particular emphasis on the phase contrast signal which allowed the determination of local composition patterns of PAEES-b-PS at the surface. Upon heating a water-annealed sample in air, the initially hydrophilic liquid-like surface exhibited a typical dewetting pattern comprising holes and elevations of different copolymer components. In contrast, air-annealed samples with glassy polystyrene surfaces exhibited a distinctly different reorganization pattern upon heating in water, possibly due to the swelling of the underlying liquid-like hydrophilic block by penetrated water. In both air and water environments, the major surface reorganization occurred around 70℃, well below the glass transition temperature (100℃) of the higher Tg block, polystyrene, in the copolymer.

The concentration dependence of the conformations of ring polymers is investigated by lattice Monte Carlo simulations and compared with that of linear polymers. The relative radii of gyration of linear polymers follow a universal master curve as a function of the scaled concentration for various chain lengths, with a scaling relationship < R-g(2)> similar to phi(-0.25), which is consistent with scaling theory and neutron scattering experiments. Ring polymers of different lengths also follow a universal behavior with a broad crossover to a scaling behavior < R-g(2)> similar to phi(-0.59) for long chains. The scaling relationship between the concentration dependence and the chain-length dependence of the radius of gyration implies < R-g(2)> similar to N-0.72, indicating highly collapsed conformations for long-chain ring polymers in the melt.

Hydrogen silsesquioxane (HSQ) is an attractive electron-beam (e-beam) resist for sub-20 nm lithography owing to its high resolution, excellent line-edge roughness (LER) and good plasma etch resistance. However, the sensitivity and long-term stability of HSQ need to be significantly improved to have HSQ resists adopted for volume manufacturing. Here we develop novel organosilicate e-beam resists with improved e-beam sensitivity and stability as an alternative to HSQ resists. Copolymers of norbornene ethyltrimethoxysilane (NETMS) with 1,2-bis(triethoxysilyl)ethane, synthesized via acid-catalyzed sol–gel reactions, show excellent e-beam sensitivity with around a sixfold reduction in the critical dose as compared with HSQ but poor LER characteristics. Terpolymers were then synthesized using p-chloromethylphenyl trimethoxysilane (p-CMPTMS), NETMS and tetraethoxysilane, which exhibit significant improvement in sensitivity as compared with previously reported materials, together with high-resolution patterns and long-term stability. High-resolution patterns of features as small as 20 nm with excellent LER were successfully fabricated employing organosilicate terpolymers using a 100 keV e-beam. The dose for patterning 20 nm lines was reduced from 4000 μC cm−2 for HSQ to 900 μC cm−2 for an optimized terpolymer resist. FT-IR measurements suggest that the main reason for the increased e-beam sensitivity is chain transfer reaction between the norbornene moieties, which provide an efficient cross-linking mechanism by the e-beam generated radicals. Copyright © 2013 John Wiley & Sons, Ltd.

Various structures of poly(norbornene-imide)s and poly(oxanorbornene-imide)s were synthesized and an optimal hydrogenation method was established. The thermal, thermomechanical, mechanical, and optical properties of thus prepared polymers were investigated, together with the structural characteristics shown by wide-angle X-ray scattering patterns. Poly(oxanorbornene-imide)s exhibit local nanostructure caused by intermolecular interactions between the imide and backbone oxygen moiety, which appears to prevent a significant decrease in Tg when compared with poly(norbornene-imide)s. Overall, hydrogenated poly(oxanorbornene-imide)s, obtained with more readily prepared exo-monomer, exhibit similar physical properties when compared with the corresponding hydrogenated poly(norbornene-imide)s.

A new π-conjugated polymer (PFDTBT-BTBT) with planar core based on 4,7-dithiophen-2-yl-benzo(2,1,3)thiadiazole (BT), [1]benzothieno-[3,2-b][1]benzothiophene (BTBT) and 9,9-didecylfluorene was synthesized by the Suzuki coupling reaction. In order to improve the photovoltaic performance, we introduce planar and rigid shaped BTBT in the polymer backbone. We also synthesized alternating copolymer of 9,9-didecylfluorene and BT (PFDTBT) to compare with optical, electrochemical and photovoltaic properties. The HOMO and LUMO energy levels of PFDTBT-BTBT were −5.67 and −3.75eV, respectively, which were very similar to those of PFDTBT (−5.65 and −3.75eV). The power conversion efficiency (PCE) of the device with a structure of ITO/PEDOT:PSS/PFDTBT-BTBT:PCBM (1:3)/Al was 2.08%, which is higher than that of PFDTBT:PCBM (1:3) (1.66%). The root-mean-square (RMS) roughness of PFDTBT-BTBT:PCBM (1:3) blend was 0.820nm, which was smaller than that of PFDTBT:PCBM (1:3) blend (1.75nm). PFDTBT-BTBT:PCBM showed smaller domain size and more well-penetrated morphology than PFDTBT:PCBM. The average field-effect hole mobility of the PFDTBT-BTBT was 6.2×10−4cm2/Vs, much larger than that of PFDTBT (2.3×10−4cm2/Vs). The results strongly support that polymer solar cell based on PFDTBT-BTBT shows better performance than that of the device based on PFDTBT.

Novel organosilicate polymer e-beam resists are reported, which show significantly improved e-beam sensitivity and long-term stability over those of conventional hydrogen silsesquioxane (HSQ) while maintaining its high resolution capability and good line-edge roughness. The improvement is made possible by incorporating norbornene groups in the organic moiety of organosilicate polymers, and the responsible mechanism is proposed.

A novel series of poly(p-xylylene) homopolymer and copolymers containing thermally cross-linkable cyclohexenyl moiety are prepared via base-catalyzed Gilch route to yield high-molecular-weight polymers. The resulting polymers are highly soluble in a wide range of organic solvents and could be solution cast into flexible and transparent films. The polymers are thermally stable up to 350 degrees C and the glass transition temperature (Tg) is in the range of 136 - 250 degrees C. They undergo thermal cross-linking via the cyclohexenyl moiety. The cross-linked polymer exhibits a high Tg of 294 degrees C, a low coefficient of thermal expansion (CTE) of 45 ppm K-1. A low dielectric constant of 2.5 and a very low dielectric loss tan d of 0.0004 at 1 GHz are obtained, which are superior to conventional interconnect polymers.