Function Of Polymer Chain Length Research Articles

Photothermal and Reorientational Contributions to the Photomechanical Response of DR1 Azo Dye-Doped PMMA Fibers

This work is a comprehensive experimental and theoretical study aimed at understanding the photothermal and molecular shape-change contributions to the photomechanical effect of polymers doped with azo dyes. Our prototypical system is the azobenzene dye Disperse Red 1 (DR1) doped into poly (methyl methacrylate) (PMMA) polymer formed into optical fibers. We start by determining the thermo-mechanical properties of the materials with a temperature-dependent stress measurement. The material parameters, so determined, are used in a photothermal heating model—with no adjustable parameters—to predict its contribution. The photothermal heating model predicts the observations, ruling out mechanisms originating in light-induced shape changes of the dopant molecules. The photomechanical tensor response along the two principle axes in the uniaxial approximation is measured and compared with another independent theory of photothermal heating and angular hole burning/reorientation. Again, the results are consistent only with a purely thermal response, showing that effects due to light-induced shape changes of the azo dyes are negligible. The measurements are repeated as a function of polymer chain length and the photomechanical efficiencies determined. We find the results to be mostly chain-length independent.

Adsorption and Desorption of Polymers on Bioinspired Chemically Structured Substrates.

Natural biological surfaces exhibit interesting properties due to their inhomogeneous chemical and physical structure at the micro- and nanoscale. In the case of hair or skin, this also influences how waterborne macromolecules ingredients will adsorb and form cosmetically performing deposits (i.e., shampoos, cleansers, etc.). Here, we study the adsorption of hydrophilic flexible homopolymers on heterogeneous, chemically patterned substrates that represent the surface of the hair by employing coarse-grained molecular dynamics simulations. We develop a method in which the experimental images of the substrate are used to obtain information about the surface properties. We investigate the polymer adsorption as a function of polymer chain length and polymer concentration spanning both dilute and semidilute regimes. Adsorbed structures are quantified in terms of trains, loops, and tails. We show that upon increasing polymer concentration, the length of tails and loops increases at the cost of monomers belonging to trains. Furthermore, using an effective description, we probe the stability of the resulting adsorbed structures under a linear shear flow. Our work is a first step toward developing models of complex macromolecules interacting with realistic biological surfaces, as needed for the development of more ecofriendly industrial products.

Terraced spreading of nanometer-thin lubricant using molecular dynamics

Ultra-thin lubricant films are essential in the design of nanoscale systems and devices as surface effects become increasingly important on the nanoscale. We have used molecular dynamics simulations to quantify terraced spreading of perfluoropolyether lubricant on a flat substrate as a function of polymer chain length, lubricant thickness, and functional end groups of the lubricant and the substrate. In addition, we have investigated the physical mechanisms that drive terraced lubricant spreading on a flat substrate. The results show that terraced lubricant spreading follows a process of diffusion and instability, where functional lubricant end groups are attracted to other functional end groups to form clusters that organize into layers. These distinct layers of functional end groups cause the lubricant thickness profile to take on a terraced shape, where layers correspond to the locations at which terraced formations occur. The presence of functional end groups determines the locations of both layer and terrace formations, and greatly affects lubricant spreading.

Length-dependent thermal conductivity of single extended polymer chains

The low thermal conductivity of polymers will be one of the major roadblocks for the polymer-based microelectronics and macroelectronics due to the limited heat spreading capability. Despite that the thermal conductivity of bulk polymers is usually low, a single extended polymer chain could have very high thermal conductivity. In this paper we present atomistic simulation studies on the phonon transport in single extended polymer chains of various polymers as a function of polymer chain length. The thermal conductivity of single extended polymer chains can be 1--2 orders of magnitude higher than their bulk counterparts. The thermal conductivity of single extended polymer chains is a strong function of monomer type. For example, the thermal conductivity of the extended polymer chains with aromatic backbone can be up to 5 times as that of a polyethylene chain, while the thermal conductivity of the extended polymer chains with bond-strength or mass disorder can be only 1/25 as that of a polyethylene chain. We analyze the phonon transport mechanisms in single extended polymer chains of various polymers and find that the competition between ballistic phonon transport and diffusive phonon transport in a polymer chain leads to a diverging length-dependent thermal conductivity.

Variation of Elastic Properties of Responsive Polymer Nanotubes

The mechanical properties of thermo-responsive nanoribbons made of poly(N-isopropylacrylamide)-block-poly(ethylene terephthalate) (PNIPAM-b-PET) have been investigated. The nanoribbons are produced by grafting PNIPAM chains from the walls of nanopores of PET track-etched membranes, by dissolving the membranes. The swelling ratio and elastic modulus of the nanoribbons are evaluated as a function of polymer chain length and temperature, from atomic force microscopy (AFM) images and using the thermodynamic theory of swelling. Whereas the elastic properties of nanoribbons are similar to those of PNIPAM gels and brushes when the PNIPAM chain length is low, they increase by 2 orders of magnitude when the chain length becomes similar to the diameter of the nanopores used to synthesize them. This indicates that PNIPAM brushes synthesized in severe conditions of confinement are strongly cross-linked, either due to irreversible trapping of entanglements, or chemical cross-linking occurring in the dense reaction medium. This provides a way to modulate mechanical properties of soft nanostructures, by playing with the degree of confinement of the chains during synthesis.

Chain-Length Dependent Nematic Ordering of Conjugated Polymers in a Liquid Crystal Solvent

In this communication we demonstrate the dependence of the solute order parameter on the solute molecular weight for polymer solutes dissolved in liquid crystalline solvents. Using ensemble absorption polarization spectroscopy together with single molecule fluorescence polarization measurements, we have determined the order parameter of the conjugated polymer MEH-PPV (poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene]) in the liquid crystal 5CB (4-cyano-4-n-pentylbiphenyl) as a function of polymer chain length. Ensemble absorption polarization measurements agree well with results obtained by single molecule fluorescence polarization spectroscopy, indicating a large-scale ordering of the MEH-PPV solute in 5CB. These results demonstrate that the increasing number of defects for larger polymer weights inherently limits the alignment of the polymer solute.

Chain Length Distributions of Polyolefins Made with Coordination Catalysts at Very Short Polymerization Times – Analytical Solution and Monte Carlo Simulation

AbstractWe developed an analytical solution to describe how the CLD of polymers made with coordination polymerization catalysts vary as a function of time for very short polymerization times before the CLD becomes completely developed. We compared the analytical solution with a dynamic Monte Carlo model for validation, obtaining excellent agreement. Our analytical solution can be used to determine when the steady‐state hypothesis, commonly used in polymerization models, becomes valid as a function of polymer chain length. We also extended our model to describe polymerization with multiple‐site‐type catalysts. Depending on the polymerization kinetic parameters of the different site types on the catalyst, the fully developed CLD is reached through very different intermediate CLDs. This modeling approach, although rather simplified, can be used to interpret results from short polymerization time experiments such as the ones done in stopped‐flow reactors.magnified image

Effects of Packing Structure on the Optoelectronic and Charge Transport Properties in Poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)

Spin-coated poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) films of different molecular weights (Mn= 9-255 kg/mol), both in the pristine and annealed state, were studied in an effort to elucidate changes in the polymer packing structure and the effects this structure has on the optoelectronic and charge transport properties of these films. A model based on quantum chemical calculations, wide-angle X-ray scattering, atomic force microscopy, Raman spectroscopy, photoluminescence, and electron mobility measurements was developed to describe the restructuring of the polymer film as a function of polymer chain length and annealing. In pristine high molecular weight films, the polymer chains exhibit a significant torsion angle between the F8 and BT units, and the BT units in neighboring chains are close to one another. Annealing films to sufficiently high transition temperatures allows the polymers to adopt a lower energy configuration in which the BT units in one polymer chain are adjacent to F8 units in a neighboring chain ("alternating structure"), and the torsion angle between F8 and BT units is reduced. This restructuring, dictated by the strong dipole on the BT unit, subsequently affects the efficiencies of interchain electron transfer and exciton migration. Films exhibiting the alternating structure show significantly lower electron mobilities than those of the pristine high molecular weight films, due to a decrease in the efficiency of interchain electron transport in this structure. In addition, interchain exciton migration to low energy weakly emissive states is also reduced for these alternating structure films, as observed in their photoluminescence spectra and efficiencies.

Investigation of the Water-Induced Reorganization of Polycaprolactone−Poly(fluoroalkylene oxide)−Polycaprolactone Triblock Copolymer Films by Angle-Dependent X-ray Photoelectron Spectroscopy

The water-induced reorganization of a series of poly(caprolactone)−poly(fluoroalkylene oxide)−poly(caprolactone) (PCL−PFPE−PCL) thin film surfaces was studied by angle-dependent X-ray photoelectron spectroscopy (XPS). The reorganization was studied as a function of polymer chain length, water exposure time, and XPS sampling depth. The prepared films were exposed to water and then frozen in liquid nitrogen to preserve the surface composition during XPS analysis. The XPS results showed the PFPE block segment length influenced the extent and rate of segment reorganization at the surface. The detected changes were shown to be reversible upon re-exposure to air and the effect of liquid-nitrogen freezing was determined to be negligible.

Molecular Dynamics Simulation of Polymer Fine Particles. Physical and Mechanical Properties

Molecular dynamics simulations are used to study the atomistic details of nanometer scale polyethylene (PE) particles to gain insight into the properties and behavior of ultra fine polymer powders. From the nano-sized particles generated with up to 60000 atoms using an efficient MD method, structure and a variety of structural and physical characteristics were computed by averaging over sets of microstates at particular temperatures. The melting point, glass transition temperature, and heat capacity of the particles as a function of polymer chain length and particle size were obtained by monitoring the molecular volume and total energy. The results of our simulations predict an interesting reduction of the melting point and significantly smaller compressive modulus in comparison with the bulk system.

Competitive adsorption between non-ionic polymers and surfactants on silica

The competitive adsorption on silica between non-ionic surfactants of the alkylphenol poly(ethylene oxide) type and neutral polymers of the poly(ethylene oxide) type has been investigated as a function of polymer chain length by determining adsorption isotherms of 50:50 wt.% mixtures. For molecular weights of the polymer below 10 000 g mol −1, surfactant adsorption at saturation is not affected by the presence of the polymer. For higher molecular weights, polymer molecules are preferentially adsorbed at low concentrations, whereas at higher concentrations the behaviour of these systems can be characterized by a threshold molecular-weight value, above which the polymer displaces the surfactant. The results are interpreted by the competition between two types of macromolecules: the adsorbed polymer and the surfactant surface aggregate.

Functional silica supported polymer V ‘Onto’ versus ‘from’ grafting processes

Silanes are versatile coupling agents. They represent an easy way of chemically bonding polymers to silica. Two processes have been considered: polymerization from a coupling agent grafted on the surface; and reaction of a prefunctionalized polymer by condensation of its alkoxy head with silanols of the surface. Each of them has advantages and limitations. We focus here on the ‘onto’ grafting mechanism. The main advantages are the control of polymerization degree in the first step of polymer synthesis which occurs in a homogeneous medium and the easy polymer characterization of the soluble precursor. The semi-batch procedure avoids molecular weight shift and broadening with conversion. Grafting efficiency is measured in function of polymer chain length, which indicates a steric limitation by the silica pore size. Trialkoxy silanes are more reactive than monoalkoxy for silica coverage and exhibit a better chemical resistance.

Dislocation melting inn-paraffin homologs

A formula for the melting temperature of $n$-paraffin homologs as a function of polymer chain length is derived in a simple model of dislocation catastrophe. The salient features of the melting curve can be understood as a consequence of the gradual transition with increasing chain length from a melting process dominated by dislocations which bridge the paraffin chain lamellae, to one in which nonbridging loops play the principal role.

Gamma radiation‐initiated polymerization of styrene at high pressure. II. Chain termination

AbstractThe pressure dependence of the termination rate constant kt for the free radical polymerization of monomers such as styrene is a function of polymer chain length, chain stiffness, and monomer viscosity, all of which influence the rate of segmental diffusion of an active radical chain end out of the coiled polymer chain to a position in which it can react with a proximate radical. Although kt is not sensitive to changes in chain length, the large increase in molecular weight is responsible for a significant reduction in kt at high pressures. For most of the common vinyl polymers, which exhibit some degree of chain stiffness, kt is inversely proportional to a fractional power of the monomer viscosity because it depends in part on the resistance of chain segments to movement and in part on the influence of viscosity in controlling diffusion of the chain ends. The fractional exponent appears to increase with pressure and this is interpreted as evidence that the polymer chains become more flexible in a more viscous solvent. Because the fractional exponent is higher for more flexible chains, the value of the activation volume for chain termination is an indication of the degree of flexibility of the polymer chains, provided that the monomer is a good solvent for the polymer and that chain transfer is negligible.