Polymer Mobility Research Articles

Role of Molecular Thermodynamical Processes at Functionalized Polymer/Metaloxide Interfaces for Photovoltaics

We present a combined theoretical and experimental investigation of the role of pyridine-based functionalizers (IMs) at the hybrid titania (TiO2)/poly(3-hexylthiophene) (P3HT) interface. We first used density functional theory to study the electronic structure and adsorption energy of several IMs, isolated and self-assembled, on a TiO2 anatase (101) surface. Small details in the molecular structure are found to induce strong differences in the morphology of the corresponding self-assembled monolayer. We then used model potential molecular dynamics simulations to study the translational and rotational diffusion of a P3HT oligomer on the naked and functionalized TiO2 surfaces. We correlate the photovoltaic performance measured on TiO2/IM/P3HT with the degree of order of the interface. In particular, we find that the thiol group of 4-mercaptopyridine is able to stabilize the corresponding self-assembled interlayer. This, in turn, increases the polymer mobility on the inorganic surface, yielding a larger poly...

Dynamics of two-dimensional and quasi-two-dimensional polymers

The dynamic properties of dense two-dimensional (2D) polymer melts are studied using discontinuous molecular dynamics simulations. Both strictly 2D and quasi-2D systems are investigated. The strictly 2D model system consists of a fluid of freely jointed tangent hard disc chains. The translational diffusion coefficient, D, is strongly system size dependent with D ∼ ln L where L is the linear dimension of the square simulation cell. The rotational correlation time, τrot, is, however, independent of system size. The dynamics is consistent with Rouse behavior with D∕ln L ∼ N(-1) and τrot ∼ N(2) for all area fractions. Analysis of the intermediate scattering function, Fs(k, t), shows that the dynamics becomes slow for N = 256 and the area fraction of 0.454 and that there might be a glass transition for long polymers at sufficiently high area fractions. The polymer mobility is not correlated with the conformation of the molecules. In the quasi-2D system hard sphere chains are confined between corrugated surfaces so that chains cannot go over each other or into the surfaces. The conformational properties are identical to the 2D case, but D and τrot are independent of system size. The scaling of D and τrot with N is similar to that of strictly 2D systems. The simulations suggest that 2D polymers are never entangled and follow Rouse dynamics at all densities.

Local demixion in plasticized polylactide probed by electron spin resonance

Improving the barrier properties to gas and organic compounds of biosourced polyesters, such as polylactides (PLAs), by increasing their crystallinity has been suggested by several authors. This paper investigates the risk of microphase separation for a technological approach that would involve a plasticization of PLA, to further its crystallization kinetics, with common plasticizers: Acetyl tributyl citrate (ATBC) and Poly(ethylene glycol) (PEG). Overplasticization effects following microphase separation were monitored along the film thickness by exposing dynamically thermo-compressed films to nitroxide spin-probes. The method enabled a scan of the local polymer mobility for different concentration profiles in spin-probes, with in particular a maximum moving continuously in time towards the geometric center. The results were interpreted as excess local temperatures that would give similar ESR spectra motion in the bulk. It was shown that measured excess temperatures could be related to local shifts in the glass transition temperature along the film thickness.

Design principle for increasing charge mobility of π-conjugated polymers using regularly localized molecular orbitals

The feasibility of using π-conjugated polymers as next-generation electronic materials is extensively studied; however, their charge mobilities are lower than those of inorganic materials. Here we demonstrate a new design principle for increasing the intramolecular charge mobility of π-conjugated polymers by covering the π-conjugated chain with macrocycles and regularly localizing π-molecular orbitals to realize an ideal orbital alignment for charge hopping. Based on theoretical predictions, insulated wires containing meta-junctioned poly(phenylene–ethynylene) as the backbone units were designed and synthesized. The zigzag wires exhibited higher intramolecular charge mobility than the corresponding linear wires. When the length of the linear region of the zigzag wires was increased to 10 phenylene–ethynylene units, the intramolecular charge mobility increased to 8.5 cm2 V−1 s−1. Theoretical analysis confirmed that this design principle is suitable for obtaining ideal charge mobilities in π-conjugated polymer chains and that it provides the most effective pathways for inter-site hopping processes.

Understanding the Reduced Efficiencies of Organic Solar Cells Employing Fullerene Multiadducts as Acceptors

AbstractThe use of fullerenes with two or more adducts as acceptors has been recently shown to enhance the performance of bulk‐heterojunction solar cells using poly(3‐hexylthiophene) (P3HT) as the donor. The enhancement is caused by a substantial increase in the open‐circuit voltage due to a rise in the fullerene lowest unoccupied molecular orbital (LUMO) level when going from monoadducts to multiadducts. While the increase in the open‐circuit voltage is obtained with many different polymers, most polymers other than P3HT show a substantially reduced photocurrent when blended with fullerene multiadducts like bis‐PCBM (bis adduct of Phenyl‐C61‐butyric acid methyl ester) or the indene C60 bis‐adduct ICBA. Here we investigate the reasons for this decrease in photocurrent. We find that it can be attributed partly to a loss in charge generation efficiency that may be related to the LUMO‐LUMO and HOMO‐HOMO (highest occupied molecular orbital) offsets at the donor‐acceptor heterojunction, and partly to reduced charge carrier collection efficiencies. We show that the P3HT exhibits efficient collection due to high hole and electron mobilities with mono‐ and multiadduct fullerenes. In contrast the less crystalline polymer Poly[[9‐(1‐octylnonyl)‐9H‐carbazole‐2,7‐diyl]‐2,5‐thiophenediyl‐2,1,3‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl (PCDTBT) shows inefficient charge carrier collection, assigned to low hole mobility in the polymer and low electron mobility when blended with multiadduct fullerenes.

Preparation and characterization of poly(3-octyl thiophene)/polyvinyl chloride polymer blends

Different weight ratios of poly(3-octyl thiophene) / polyvinyl chloride blends were prepared using the casting method by dissolving poly(3-octyl thiophene) and polyvinyl chloride polymers in chloroform and Tetrahydrofuran, respectively. Fourier transform infrared spectroscopy indicates the existence of carbonyl group in both polyvinyl chloride neat and poly(3-octyl thiophene)–polyvinyl chloride blends. Electrical measurements reveal that the impedance values are increased with the increasing of both weight fraction of poly(3-octyl thiophene) and temperature. Optical results reveal that the increase of poly(3-octyl thiophene) weight fraction in blends enhances the UV-visible absorption of polyvinyl chloride and reduces the optical energy gap of blends. Differential scanning calorimetric measurements and thermal gravimetric analysis show that the glass transition temperature value and thermal stability of blend increases with increasing of poly(3-octyl thiophene) weight ratio. The obtained results, generally, indicate that the increase of poly(3-octyl thiophene) weight fraction enhances the interaction between poly(3-octyl thiophene)–polyvinyl chloride chains and as a result will restrict the chain mobility of polymers.

Selenophene vs. thiophene in benzothiadiazole-based low energy gap donor–acceptor polymers for photovoltaic applications

A series of low energy gap polymers comprising 2,7-linked carbazole or fluorene units flanked by thiophene or selenophene repeat units as alternating donor units and benzothiadiazole with or without alkoxy substituents as alternating acceptor repeat units is reported. The effects of replacing thiophene with selenophene in this series of polymers on their optical, electrochemical and photovoltaic device performance when fabricated into bulk heterojunction solar cells using PC70BM as an acceptor are investigated. Power conversion efficiencies (PCEs) ranging from 3.34 to 5.41% are obtained with these systems. Thiophene-based polymers are found to have higher efficiency compared to comparable selenophene-based polymers. We tentatively explain such differences on the basis of reduced molar absorbance and reduced charge-carrier mobility in the selenophene-based polymers.

Investigation of the glass transition and viscoelastic properties of polycarbonate/multi-walled carbon nanotube composites by positron annihilation lifetime spectroscopy

The influences of free volume and temperature on the viscoelastic properties of polycarbonate (PC)/multi-walled carbon nanotube (MWNT) composites were investigated by positron annihilation lifetime spectroscopy (PALS) and dynamic mechanical analysis (DMA). Three methods, including PALS, DMA and differential scanning calorimetry were used to determine the glass transition temperature (Tg) of PC/MWNT composites. The experimental results indicated that the higher the MWNT contents, the lower the Tg, which attributes to the large free volume hole and the enhanced polymer mobility in PC/MWNT composites with higher MWNT contents. The effect of MWNTs on viscoelastic property has been investigated in detail. A direct linear relationship between fractional free volume and viscoelastic property has been obtained using the Williams–Landel–Ferry equation based on free volume theory, which indicates that the free volume plays an important role in determining the viscoelastic property.

Mobility control requirement in multiphase displacement processes in porous media

ABSTRACTAccording to the conventional mobility control requirement, the displacing phase mobility (e.g. polymer) should be equal to or less than the sum of the mobilities of several displaced phases (e.g. water and oil). When the oil mobility is much lower than the water mobility, the sum of the displaced phase mobilities will be almost the same as the water mobility. Then, according to the requirement, the displacing polymer mobility is almost the same as the water mobility. As a result, the polymer solution will flow preferentially along water channels, leaving oil undisplaced.In this paper, we propose that the displacing phase mobility (polymer) should be equal to or less than the lowest mobility of displaced phases (generally, oil phase mobility) multiplied by its normalized saturation. This mobility requirement is validated by extensive simulation results at different conditions. It is also justified by field practices. Some published experimental data are analyzed to justify the proposed requirement. The proposed mobility requirement provides a criterion for mobility control design in multiphase displacement processes in porous media. © 2012 Curtin University of Technology and John Wiley & Sons, Ltd.

Impact of nZVI stability on mobility in porous media

Nano-scale zero valent iron (nZVI) has received significant attention because of its potential to rapidly reduce a number of priority source zone contaminants. In order to effectively deliver nZVI to the source zone the nZVI particles must be stable. Previous laboratory studies have demonstrated the mobility of polymer modified suspensions of low concentration nZVI. More recently studies have shown potential for higher concentration nZVI suspensions to be transmitted through porous media. However, with increasing nZVI concentration aggregation is accelerated, reducing the available time for injection before nZVI settles. In this study the colloidal stability and mobility of nZVI concurrently synthesized and stabilized in the presence of carboxy-methyl-cellulose (CMC) are evaluated in one-dimensional column experiments. Low pore water velocity nZVI injections (4, 2, and 0.25 m/day) conducted over periods as long as 80 h with no mixing of the influent reservoir were used to investigate the effects of prolonged aggregation and settling of colloids on transport. A numerical simulator, based on colloid filtration theory, but accounting for particle aggregation and settling was used to evaluate the contributions of aggregation and settling on nZVI mobility. Results suggest that the prediction of nZVI sticking efficiency in column experiments becomes increasingly influenced by aggregation and settling in the influent reservoir as the period of injection increases. Consideration of nZVI stability is required for the prediction of nZVI mobility at the field scale and for the design of successful nZVI remediation plans.

Effects of intracellular poly(3-hydroxybutyrate) reserves on physiological–biochemical properties and growth of Ralstonia eutropha

Microbial polyhydroxyalkanoates (PHAs), because of their well studied complex physiology and commercial potential, are vehicles for carbon and potential storage reduction for many microbial species. Even with the wealth of studies about microbial PHAs in the scientific literature, polymer accumulation and degradation are still not comprehensively understood. Poly(3-hydroxybutyrate) (P3HB) granule formation and polymer mobility were studied here in the bacterium Ralstonia eutropha strain B5786 in autotrophic cultures. Electron microscopy studies revealed decreasing cell size concomitant with enlargement of size and number of intracellular granules, and inhibition of cell division during intracellular polymer production. Activities of key P3HB biosynthetic enzymes demonstrated correlations with each other during polymer accumulation, suggesting an intricately regulated P3HB cycle in autotrophically grown R. eutropha cells.

A study of molecular mobility and relaxation properties of epoxy-allyl polymers during formation of interpenetrating polymer networks under conditions of the phase separation of the networks

Molecular mobility and relaxation properties of epoxy-allyl polymers (EAPs) and features of the formation of interpenetrating polymer networks under conditions of the phase separation of the network have been studied. The presence of the extensive interphase zone between the individual epoxy and allyl component networks has been established. The segregation degree has been calculated taking into account the volume fractions of the components of the polymer mixture. The interrelation between the features of the structure of the obtained EAP with a high level of adhesion and deformation-strength characteristics of polymers in glassy and highly elastic states has been demonstrated.

A study of the effect of molecular weight epoxy and allylic oligomers on the molecular mobility and properties of epoxy-allylic polymers

The influence of initial molecular weight oligomers on molecular mobility and the properties of epoxy-allylic polymers (EAPs) was considered. Obtaining of EAPs with no signs of phase separation due to the presence of general epoxy-diane structural fragments in epoxy and allylic oligomers was found. It is shown that the maximum values of adhesive and cohesive properties are observed in EAPs when combined in a grid structure formed of rigid and flexible fragments—relaxation oscillators.

Ion-Beam Induced Surface Roughening of Poly-(methyl methacrylate) (PMMA) Tuned by a Mixture of Ar and O2Ions

The evolution of ion-beam induced surface roughening of PMMA was investigated using an anode-layer ion beam source with O2, Ar, and their mixture. Use of the O2 ion beam created rough granular patterns on the surface of PMMA because O2 ions could break the CC backbone and subsequently combine with the broken C bonds, resulting in local cross-linking and aggregation. However, the Ar ion beam tended to reduce the surface roughness due to the decomposition of the oxygen-containing ester groups (OCH3 and OCO) and the increase of an overall cross-linking of the amorphous carbon layer, which played a role in etch resistance and suppression of polymer mobility as well as aggregation on the modified PMMA surface. By varying the ratio of Ar over O2, the surface morphology of PMMA was modified to exhibit features ranging from granular roughness to crater-like features.

Glass-Transition Temperature Profile Measured in a Wood Cell Wall Using Scanning Thermal Expansion Microscope (SThEM)

This study aims to assess the in situ spatial distribution of glass-transition temperatures (Tg) of the main lignocellulosic biopolymers of plant cell walls. Studies are conducted using scanning thermal expansion microscopy to analyze the cross-section of the cell wall of poplar. The surface topography is mapped over a range of probe-tip temperatures to capture the change of thermal expansion on the sample surface versus temperature. For different temperature values chosen between 20 °C and 250 °C, several quantitative mappings were made to show the spatial variation of the thermal expansion. As the glass transition affects the thermal expansion coefficient and elastic modulus considerably, the same data line of each topography image was extracted to identify specific thermal events in their topographic evolution as a function of temperature. In particular, it is shown that the thermal expansion of the contact surface is not uniform across the cell wall and a profile of the glass-transition temperature could thus be evidenced and quantified corresponding to the mobility of lignocellulosic polymers having a role in the organization of the cell wall structures.

Controlling band gap and hole mobility of photovoltaic donor polymers with terpolymer system

Energy level, band gap, and hole mobility of photovoltaic donor polymers are crucial factors in achieving high power conversion efficiency. Controlling these characteristics, however, requires complicated synthesis routes. This work reports an effective way to control these properties simultaneously by introducing a simple terpolymer system. Simulation of energy level (band gap), synthesis of donor polymers, and calculation of solubility were systematically coordinated to explain the advantages of the terpolymer system. Anthradithiophene was introduced to elucidate the effect of the extended conjugation length on band gap and hole mobility of donor polymers. Benzodithiophene and thienothiophene which have shown good properties in bulk heterojunction organic solar cells were used as comonomers. Based on the result of the energy level calculation, terpolymers with various mole ratios of the monomers were synthesized; as the ratio of anthradithiophene to benzodithiophene increased, band gap of the terpolymers decreased while hole mobility increased. In addition, we explained the morphology of the terpolymer:fullerene bulk heterojunction films using solubility parameters of materials. With this method, we investigated the relationship between the properties of donor polymers and device performances systematically. The terpolymer system can be widely utilized for tuning the material properties in a systematic way which is quite similar to the band gap engineering in the inorganic semiconductors.

Slowing Down versus Acceleration in the Dynamics of Confined Polymer Films

We have performed molecular dynamics (MD) simulations on coarse-grained polymer films which are confined between two attractive crystalline Lennard-Jones substrates with three different substrate–substrate separations. Two different polymer–substrate interactions strengths have been studied. Detailed analysis of the structural properties of each film showed a layering of the monomers close to the polymer–substrate interface and a preferential orientation of the bonds parallel to the substrate surface; both depend on substrate attraction strength and temperature, but not on film thickness. The rotational and translational segmental dynamics were analyzed for each film thickness in different film layers, for a wide range of temperatures and for both substrate attraction strengths. For all simulated films, the segmental dynamics was found to be faster than that in the bulk. For relatively thick films and energetically neutral polymer–substrate interaction, a dramatic slowing down of the polymer mobility was found close to the polymer–substrate interface, when compared with the middle of the film, thus providing stiffness enhancement due to the presence of the attractive substrates. With decreasing substrate–substrate separation these gradients in stiffness became overlapping. However, this did not lead to an overall stiffness enhancement in the film, as expected; instead, a large shift toward lower overall mobility was found for the thinner films, which was attributed to finite-size scaling effects.

Electron and Hole Contributions to the Terahertz Photoconductivity of a Conjugated Polymer:Fullerene Blend Identified.

Time-resolved terahertz spectroscopy was employed for the investigation of charge-transport dynamics in benzothiadiazolo-dithiophene polyfluorene ([2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]) (APFO-3) polymers with various chain lengths and in its monomer form, all blended with an electron acceptor ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). Upon photoexcitation, charged polaron pairs are created, negative charges are transferred to fullerenes, while positive polarons remain on polymers/monomers. Vastly different hole mobility in polymer and monomer blends allows us to distinguish the hole and electron contributions to the carrier mobility.

Network formation of nanofibrillated cellulose in solution blended poly(methyl methacrylate) composites

Composites of poly(methyl methacrylate) (PMMA) and nanofibrillated cellulose (NFC) were prepared by solution blending and further processed by injection and compression molding. To improve adhesion at the PMMA/NFC interface, the nanofibrils were covalently grafted with PMMA. Formation of a percolating nanofibril network was observed between 1 and 5wt.% of NFC by dynamic rotational rheometry in molten state. This observation was further supported by the behavior of glass transition temperature which decreased at low NFC concentrations but recovered above the percolation threshold, indicating a decreased mobility of the matrix polymer. This effect was more pronounced with ungrafted NFC, possibly due to a stronger network. The unmodified NFC induced a minor degradation of the molar mass of PMMA. As thin plates, the composites were transparent at low NFC concentrations but became partially aggregated at the highest NFC concentrations. Despite the continuous NFC network, tensile testing showed no improvement of the mechanical properties.

Method for Simultaneously Improving the Thermal Stability and Mechanical Properties of Poly(lactic acid): Effect of High-Energy Electrons on the Morphological, Mechanical, and Thermal Properties of PLA/MMT Nanocomposites

Nanocomposites derived from poly(lactic acid) (PLA) and organically modified montmorillonite (oMMT) have been cross-linked by high-energy electrons in the presence of triallyl cyanurate (TAC). The morphology of untreated and cross-linked PLA/MMT nanocomposites was characterized by wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). This treatment can improve both the thermal stability and the glass-transition temperatures of the PLA nanocomposites (e.g., PLA-MMT-TAC 30kGy, 50kGy, and 70kGy) because of the formation of cross-linking structures in the nanocomposites that will considerably reduce the mobility of polymers. Interestingly, at relatively low irradiation doses (e.g., 30 and 50 kGy) a good balance between tensile strength and elongation at break for the PLA nanocomposites could be achieved. These mechanical properties are superior to those of pure PLA. Therefore, combining nanotechnology and electron beam cross-linking is a promising new method of simultaneously improving the mechanical properties (toughness and tensile strength) and thermal stability of PLA.