Polymer Mobility Research Articles

Treatment of chenevotte, a co-product of industrial hemp fiber, by water or hydrochloric acid: impact on polymer mobility in the lignified cell walls

The impact of water- and of mild acid impregnation with hydrochloric acid (HCl) on the viscoelastic and breaking properties of woody hemp core (chenevotte) was studied from the polymer to the macroscale level. We have shown that transition temperature (T g) values of lignin and hemicelluloses were not affected by the acid treatment, whereas the relaxation mechanism of hemicelluloses, expressed by the apparent activation energy, was modified. The opposite impacts were observed in the water extraction. Changes in the sample strength were also noted, as the chenevotte treated with mild HCl became more brittle, as opposed to the samples extracted with water. It is suggested here that the moderate HCl treatment impacts chenevotte micro- and macro-properties through acid depolymerization of polysaccharides and extraction of low molecular mass entities, whereas water immersion contributes solely to the later mechanism.

Interfacial effects in polymer nanocomposites studied by dielectric and thermal techniques

Effects of interfaces and interphases in polydimethylsiloxane (PDMS) matrices with in situ synthesized titania (20 - 40 nm, in diameter) and silica nanoparticles (~5 nm), were studied employing dielectric and thermal techniques. The presence of the well dispersed inorganic particles and the hydrogen polymer - filler bonding result in a double effect on polymer mobility: suppression of crystallization and immobilization in a layer of a few nm around the particles. The effects were stronger in the case of titania nanoparticles, in consistency with stronger hydrogen bonding interactions, comparing to silica. Various contributions to the glass transition were recorded by both thermal and dielectric techniques, corresponding to bulk and modified polymer dynamics. The modified mobility originates from the restriction of polymer chains within the PDMS crystals and in an interafacial rigid amorphous PDMS layer around the nanoparticles. The thickness of the interfacial layer was estimated to 3-5 nm for titania and ~2 nm for silica. The mobile amorphous phase fraction giving rise to the glass transition was found to be nearly constant in the nanocomposites. The results were confirmed by employing various thermal (crystallization) treatments of the samples.

NMR relaxation study of the effect of molecular weight on the topological structure and molecular mobility of linear polymers

NMR relaxation study of the effect of molecular weight on the topological structure and molecular mobility of linear polymers

Mobility Gradient Induces Cross-Streamline Migration of Semiflexible Polymers.

Many aspects of modern material science and biology rely on the strategic manipulation and understanding of polymer dynamics in confining micro- and nanoflow. We directly observe and analyze nonequilibrium structural and dynamic properties of individual semiflexible actin filaments in pressure-driven microfluidic channel flow using fluorescence microscopy. Different conformational shapes, such as filaments fluctuating in an elongated manner, parabolically bent, as well as tumbling, are identified. With increasing flow velocity, a strong center-of-mass migration toward the channel walls is observed. This significant migration effect can be explained by a shear rate dependent spatial diffusivity due to a gradient in chain mobility of the semiflexible polymers.

Effect of electrization on molecular mobility in gold nanocomposites based on arabinogalactan

A correlation is found between the proton spin-spin relaxation times in gold nanocomposites based on arabinogalactan in aqueous solutions and the maximum conducting layer thicknesses of films cast from solutions of composites. The obtained correlation is considered from the viewpoint of electrization’s effect on the mobility of macromolecules of the investigated polymer nanocomposites. The dependence of arabinogalactan mobility on the type of solvent (H2O or D2O) is established, and a conclusion is drawn as to the effect of the hydrogen bonds of arabinogalactan with solvent on polymer mobility in solutions.

Study of the magnetic field effects on carriers’ mobility in polymer based light-emitting diodes

The magnetic field effects on the carriers’ mobility have been investigated for poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonat) (PFO) based light emitting diodes by the transient electroluminescence (EL) method. The EL pulses can be divided into the following six regions: (a) a delay region; (b) a fast initial rising edge; (c) a second slower rising edge; (d) a quasisteady state; (e) a fast falling edge; and (f) a slow decay region. The effect of the external magnetic field on the different regions can be obtained by measuring the EL pulses both with and without a magnetic field. It is found that the external magnetic field has no effect on both the electron and hole mobility in PFO based light-emitting diodes when the driving voltage is above the onset of the electroluminescence.

Development of Materials Informatics Tools and Infrastructure to Enable High Throughput Materials Design

ABSTRACTPolymer nanocomposites (PNC) are complex material systems in which the dominant length scales converge. Our approach to understanding nanocomposite tradespace uses Materials Quantitative Structure-Property Relationships (MQSPRs) to relate molecular structures to the polar and dispersive components of corresponding surface tensions. If the polar and dispersive components of surface tensions in the nanofiller and polymer could be determined a priori, then the propensity to aggregate and the change in polymer mobility near the particle could be predicted. Derived energetic parameters such as work of adhesion, work of spreading and the equilibrium wetting angle may then used as input to continuum mechanics approaches that have been shown able to predict the thermomechanical response of nanocomposites and that have been validated by experiment. The informatics approach developed in this work thus enables future in silico nanocomposite design by enabling virtual experiments to be performed on proposed nanocomposite compositions prior to fabrication and testing.

Synthesis, photophysics, and photovoltaic properties of low-band gap conjugated polymers based on thieno[3,4-c]pyrrole-4,6-dione: a combined experimental and computational study

Four novel donor–acceptor (D–A) alternating copolymers were designed and successfully synthesized by the palladium-catalyzed Stille coupling and Suzuki coupling reactions. Utilizing thieno[3,4-c]pyrrole-4,6-dione (TPD) as an acceptor comonomer coupled with dialkoxybithiophene or cyclopentadithiophene as the donor gave polymers PTBT and PTCT. Employing carbazole as the donor and the dithiophene-substituted TPD serving as the acceptor monomers yielded polymers PTC1 and PTC2. Owing to the various strengths of electronic coupling between the donors and the acceptor unit, the band gaps of these polymers can be adjusted from 1.57 to 1.90 eV. Due to the different electron-donor ability of dialkoxybithiophene, cyclopentadithiophene, and carbazole, the HOMO energy levels of polymers were tuned from −5.34 to −5.67 eV, while LUMO levels remained relatively unchanged. The theoretical calculations provided insight to the observed photophysical properties of these polymers. Theoretically estimated band gaps and oxidation potentials correlate well with the experimental data. Carrier mobility and photovoltaic properties of TPD polymers were also investigated for which 1.3% power conversion efficiency was obtained from a blend of PTCT:PC71BM (1:2) bulk-heterojunction device.

Hybrid solar cells: basic principles and the role of ligands

For the last decade, researchers have attempted to construct photovoltaic (PV) devices using a mixture of inorganic nanoparticles and conjugated polymers. The goal is to construct layers that use the best properties of each material e.g., flexibility from the polymer and high charge mobility from the nanoparticles or blue absorbance from the polymer complementing red absorbance from the nanoparticles. This critical review discusses the main obstacles to efficient hybrid organic/inorganic PV device design in terms of contributions to the external and internal quantum efficiencies. We discuss in particular the role that ligands on the nanoparticles play for mutual solubility and electronic processes at the nanoscale. After a decade of work to control the separation distance between unlike domains and the connectivity between like domains at the nanoscale, hybrid PV device layers are gaining in efficiency, but the goal of using the best properties of two mixed materials is still elusive.

Solid particles in an elastomer matrix: impact of colloid dispersion and polymer mobility modification on the mechanical properties

The reinforcement of elastomers by inorganic fillers, a concept of very high technological importance, is commonly understood to result from the presence of a mechanical network of partially aggregated filler particles. The non-linear mechanical properties, in particular the decrease of the modulus at high strain (Payne effect), are further interpreted to be a consequence of the breakdown of this filler network. There are, however, many open questions concerning the actual nature of the interparticle connections, where a modified polymer layer forming “glassy bridges” constitutes one possibility. In this work, we address this issue with a suitable silica-filled model elastomer, where we characterize the silica dispersion by SANS in combination with reverse Monte-Carlo modeling, and the mobility modification of the polymer by low-field proton NMR spectroscopy. In our samples, we identify a glassy layer as well as a region of intermediate mobility (possibly modified Rouse modes). Based on the structural information from SANS, we are able to quantify the amount of interparticle connections, and correlate it with the magnitude of the Payne effect taken from shear rheology. This works only if we assume that these connections comprise both, the glassy layer as well as the region of intermediate mobility. The amount of glassy immobilized polymer only does not suffice to explain the mechanical properties.

Dynamics and Deformation Response of Rod-Containing Nanocomposites

Theoretical and computational studies of polymer nanocomposites have largely focused on spherical inclusions in a polymer matrix. In order to address the influence of particle shape on nanocomposite behavior, extensive Monte Carlo and molecular dynamics simulations are used to examine the structure and deformation behavior of a model polymer upon addition of rods of varying aspect ratios. It is found that, at constant temperature, nanorod length does not meaningfully affect the elastic properties of composites but does affect postyield properties, such as the strain hardening modulus. In contrast, at constant T/Tg, several trends with additive length arise. Examination of the polymer bond autocorrelation function during deformation reveals that longer, dispersed rods induce a broadening of the relaxation spectrum. Nanocomposites show longer bond orientation relaxation times than the pure polymer during all stages of deformation. For the truly nanoscale additives used in this study, polymer mobility is fou...

Reinforcement and Polymer Mobility in Silica–Latex Nanocomposites with Controlled Aggregation

The tunable structure of silica–latex nanocomposites made of silica nanoparticles (radius ≈ 80 A) and a copolymer of methyl methacrylate and butyl acrylate–latex beads (radius ≈ 210 A) has been studied by small-angle neutron scattering and transmission electron microscopy. An aggregation diagram as a function of the control parameters—silica volume fraction and precursor solution pH—has been established. In this aggregation diagram, isoaggregation lines have been identified. It was used to express the small-strain reinforcement factor measured with stress–strain isotherms as a function of volume fraction at fixed aggregation number in the range from 50 to 100. The large-strain properties have been rationalized using the energy needed to rupture samples, and this quantity has been found to present an optimum at intermediate volume fractions (15%). In order to understand the striking rheology of the system, a neutron contrast-matching study has been undertaken by adding deuterated polymer beads. The bead de...

Nanocomposites Based on Hyperbranched Polymers and Montmorillonite

Nanocomposites based on hyperbranched polymers and sodium montmorillonite were prepared over the full range of compositions. The XRD analysis showed the full exfoliation of silicate layers at lower silicate content (up to 9.1 wt%). With the further increase of silicate loading, an intercalated structure was developed with a constant d-spacing due to the unique structure of hyperbranched polymers. The heat capacity jump at the glass transition of the nanocomposites was found to deviate from the two-phase model prediction, indicating the formation of a rigid amorphous fraction. The glass transition temperature and heat capacity jump behaviors suggested that the molecular mobility of hyperbranched polymers were restricted by the introduction of silicate layers. The mechanical properties of the nanocomposites were also investigated.

Reduced Crystallinity and Mobility of Nylon-6 Confined near the Organic–Inorganic Interface in a Phosphate Glass-Rich Nanocomposite Detected by 1H–13C NMR

The effects of confinement of solid polyamide 6 in a nanocomposite with a rigid inorganic glass, a hybrid organic–inorganic material with potential applications in optoelectronics, gas/liquid barrier membranes, and heterogeneous catalysis, have been investigated by 13C, 1H–13C, and 1H–1H solid-state nuclear magnetic resonance (NMR) spectroscopy. The material is synthesized by melt blending 90 vol % of a low-Tg tin fluorophosphate glass (Pglass) with 10 vol % polyamide 6 (PA6). 13C NMR with selective spectroscopy of rigid and mobile segments shows a 2-fold suppression of the average crystallinity of PA6 in the composite. The reduction in crystallinity, to ca. 5%, is even more pronounced for PA6 near the interface with Pglass, as seen in selective 13C spectra of PA6 near the organic–inorganic interface obtained by two-dimensional 1H–13C correlation NMR. The spectrum after a 13C T1 filter indicates a reduced mobility of the interfacial PA6 polymer, which is in ∼5 nm thick domains according to 1H spin-diffusion experiments. The spectra prove that about half of the PA6 is in the nanocomposite, while the rest is bulklike in large domains. The presence of large domains is also deduced from incomplete 1H spin exchange between PA6 and Pglass in the 1H spin-diffusion experiments.

A multi-step simulation of electron mobility in fluorene–benzothiadiazole conjugated polymer – Case study

Recently, organic solar cells have attracted considerable attention in the field of photovoltaic devices. Charge mobility of conjugated polymer materials is one of the important factors that determine the efficiency of organic solar cells. We investigate charge transport characteristics of conjugated polymers using computational means. We employ a multi-step approach that involves the use of the density functional theory (DFT), semiempirical (ZINDO) and Monte Carlo (MC) theoretical methods within the context of a hopping (Marcus–Hush) model to simulate the electron mobility of poly(9,9′-di-n-octylfluorene-alt-benzothiadiazole) (F8BT). Taking into account the three dimensional (3D) structure of the annealed F8BT as obtained from the X-ray diffraction experiments we illustrate that the multi-step computational approach can give electron mobility values that are in good agreement with the experimental data for the ordered (crystalline) system provided the planes of the side-by-side polymer chains are tilted relative to the bottom plane (which is parallel to the substrate) of the unit cell and the π-chains stacked in a direction perpendicular to the substrate have an “alternating” structure. We also find that the inclusion of an orientational disorder in the crystal, in general, tends to reduce the electron mobility of F8BT. The proposed research is a case study. It illustrates that the use of computations/simulations based on a chemical structure of a monomer and known crystal structure of an organic semiconductor is one possible approach to studying charge mobility in organic polymers.

Molecular Dynamics of Linear and Crosslinked Polyester Urethanes Studied by Dielectric Spectroscopy

The molecular dynamics of crosslinked polyurethanes have been studied by dielectric spectroscopy and compared with the dynamics of linear polyurethanes. Two local relaxations, γ and β, and a primary relaxation, α, were observed for all the samples, appearing in relation with the increase in temperature. The conductivity was studied at temperatures higher than the glass transition temperature and it was found to decrease with the increasing content of crosslinker. The presence of crosslinks strongly influences their dielectric properties, especially in the elastic state. The linear polyurethane exhibits the lowest α-relaxation temperature. For the crosslinked samples, α-relaxation temperature increases with increase in the amount of crosslinkers; β- and γ-relaxations are less affected by chemical crosslinking and their activation energies are in the ranges 40–55 and 34–37 kJ/mol, respectively. For the linear polyurethane, there is an exception in the case of the γ-relaxation, which has a higher activation energy determined by its physical crosslinks. Conductivity analysis reveals a high mobility of charge carriers and low barriers for the transport of the charged particles. Also, the conductivity process is dependent on the segmental mobility of polymers.

Markov chain modeling of polymer translocation through pores

We solve the Chapman-Kolmogorov equation and study the exact splitting probabilities of the general stochastic process which describes polymer translocation through membrane pores within the broad class of Markov chains. Transition probabilities, which satisfy a specific balance constraint, provide a refinement of the Chuang-Kantor-Kardar relaxation picture of translocation, allowing us to investigate finite size effects in the evaluation of dynamical scaling exponents. We find that (i) previous Langevin simulation results can be recovered only if corrections to the polymer mobility exponent are taken into account and (ii) the dynamical scaling exponents have a slow approach to their predicted asymptotic values as the polymer's length increases. We also address, along with strong support from additional numerical simulations, a critical discussion which points in a clear way the viability of the Markov chain approach put forward in this work.

ChemInform Abstract: Synthetic Glycosylphosphatidylinositol (GPI) Anchors: How these Complex Molecules Have Been Made

AbstractReview: 116 refs.

Quantification of total iodine in intact granular starches of different botanical origin exposed to iodine vapor at various water activities

Iodine has been used as an effective tool for studying both the structure and composition of dispersed starch and starch granules. In addition to being employed to assess relative amylose contents for starch samples, it has been used to look at the molecular mobility of the glucose polymers within intact starch granules based on exposure to iodine vapor equilibrated at different water activities. Starches of different botanical origin including corn, high amylose corn, waxy corn, potato, waxy potato, tapioca, wheat, rice, waxy rice, chick pea and mung bean were equilibrated to 0.33, 0.75, 0.97 water activities, exposed to iodine vapor and then absorbance spectra and LAB color were determined. In addition, a new iodine quantification method sensitive to <0.1% iodine (w/w) was employed to measure bound iodine within intact granular starch. Amylose content, particle size distribution of granules, and the density of the starch were also determined to explore whether high levels of long linear glucose chains and the surface area-to-volume ratio were important factors relating to the granular iodine binding. Results showed, in all cases, starches complexed more iodine as water content increased and waxy starches bound less iodine than their normal starch counterparts. However, much more bound iodine could be measured chemically with waxy starches than was expected based on colorimetric determination. Surface area appeared to be a factor as smaller rice and waxy rice starch granules complexed more iodine, while the larger potato and waxy potato granules complexed less than would be expected based on measured amylose contents. Corn, high amylose corn, and wheat, known to have starch granules with extensive surface pores, bound higher levels of iodine suggesting pores and channels may be an important factor giving iodine vapor greater access to bind within the granules. Exposing iodine vapor to moisture-equilibrated native starches is an effective tool to explore starch granule architecture.

Poly(3-hexylthiophene) Adhesion on Zinc Oxide Nanoneedles

We study the interface between poly(3-hexylthiophene) and wurtzite ZnO nanoneedles by molecular dynamics simulation. We provide evidence that the polymer is easily adsorbed on the nanostructured surface with the largest binding energy found when the chain is aligned to the needle axis. Helically wrapped polymer configurations are nevertheless possible, and they are found to be metastable with long lifetimes and small polymer mobility on the nanoneedle surface. The wrapped configuration lifetime has been compared with the case of carbon nanotube-based systems at finite temperatures and calculated to be much longer. The dependence of the adhesion energy on the polymer orientation is discussed and explained by a model, including strain, adhesion anisotropy, and nanoneedle shape effects (i.e., the presence of edges between facets).