Influence Of Molecular Architecture Research Articles

Influence of Molecular Architecture on the Viscoelastic Properties of Polymers with Phase-Separated Dynamic Bonds

Polymers with dynamic bonds attract significant attention due to their many unique properties, such as self-healing and recycling. However, understanding microscopic mechanisms controlling their dynamics and viscoelastic properties, especially in the case of phase-separated dynamic bonds, remains rather limited. We present analysis of model polymer systems with phase-separating dynamic bonds (stickers) placed along the chain (pendant stickers) or at the end of the chain (telechelic systems). We demonstrate that the bond rearrangement process in these systems is controlled by the pullout of a sticker from a cluster, with the characteristic time defined by structural relaxation in clusters and miscibility of stickers in the polymer matrix. This enables decoupling of terminal relaxation from segmental dynamics of the polymer. The analysis revealed the critical role of the total concentration of stickers that controls dynamics of the polymer in the broad time range from segmental relaxation up to the time of sticker pullout from a cluster, regardless of the sticker placement on the chain and the length of the chain. Only at longer time scales, the molecular length and sticker placement influence viscoelastic properties. Based on these results, we suggest a general scenario for mechanisms controlling the dynamics and viscoelasticity of polymers with phase-separated dynamic bonds. This scenario can be helpful in guiding the design of dynamic polymers with desired viscoelastic properties.

Influence of molecular architecture, hydrogen bonding, chemical moieties and flexible chain for smectic polymorphism in (4)EoBD(3)AmBA:nOBAs

A novel series of calamitic, complementary hydrogen-bonded (HB), meta-extended Schiff base core containing liquid crystalline (LC) complexes with ethoxy chain at one end and varied flexible chain at other end, viz., (4)EoBD(3)AmBA:nOBAs for n = 3–12 is reported. 1H, 13C NMR and FTIR spectroscopy confirm complex formation. LC phases, phase transition temperatures (T C) and enthalpy are characterized by polarizing optical microscopy and differential scanning calorimetry(DSC). SmA, SmC, SmBcryst,SmF, SmI, SmE, SmG and SmH phases are exhibited. Clearing and melting temperatures exhibit odd–even effect. Schiff base bridged by oxygen atom promoted of smectic phase occurrence. Complementary HB-induced and -enhanced tilted smectic phase range. Large LC phase variants, i.e., tetra, penta and hexa are observed. Hexa LC phase variant for n = 6 and 12. Unlike in p-n-alkoxy benzoic acids (nOBAs), Nematic phase is quenched. Quasi two-dimensional LC SmF and SmI phases identified. Lower (for n = 3,5), intermediate (for n = 6,7) and higher (n = 9) homologues exhibit 3D SmBcryst phase. Monotropic LC phase occurrence is also noticed. Apart from other LC phase transitions, SmA–SmC transition is mostly found to be second order or very weakly first order nature. Phase diagram by DSC infers existence of multi-critical points in the vicinity of SmF and SmI phase structures with psuedo hexagonal packing. Influence of meta-extended core, complementary HB and flexibility component are discussed for LC phase abundance. Role of orientational disorder induced by flexible component discussed. In comparison with LC phase occurrence in (4)MeoBD(3)AmBA:nOBAs, shielded inductomeric effect is argued for large thermal range of tilted smectic phases. Integrated and differential heat energies are estimated in successive heating and cooling scans to investigate optimized realization of LC phases. Results are discussed in the wake of contemporary reports in other LCs.

Influence of molecular architecture on behavior of thermoresponsive poly-2-ethyl-2-oxazine in saline media

Linear and calix[4]arene based star-shaped poly-2-ethyl-2-oxazines have been explored in saline solutions at NaCl concentration of 0.0050 g cm−3. The calix[4]arene core promotes an aggregation in solutions of the star-shaped polymer and a decrease in its phase separation temperature. Poly-2-ethyl-2-oxazine is more sensitive to the presence of NaCl in solution than the known poly-2-isopropyl-2-oxazolines.

Influence of Molecular Architecture and Chain Flexibility on the Phase Map of Polystyrene-block-poly(dimethylsiloxane) Brush Block Copolymers

We report the microphase-separated morphologies of model bottlebrush block copolymers (BBCPs) over a wide range of architectural design parameters. Densely grafted polystyrene-block-poly(dimethylsi...

Role of molecular architecture in interfacial self-adhesion of polyethylene films

The influence of molecular architecture on interfacial self-adhesion above polyethylene film melt temperature was examined in this study. The investigated molecular structures include molecular weight (Mw), molecular weight distribution, long chain branch amount and distribution and short chain branch among and along polyethylene chains. The long chain branches concentration was quantified using gel permeation chromatography and short branches concentration using nuclear magnetic resonance techniques. The adhesion strength was measured immediately after melt bonding using a T-Peel test. The results showed that increasing Mw resulted in higher adhesion strength in linear metallocene ethylene α-olefins. Low long chain branch concentrations hinder reptation motion and diffusion, and result in lower adhesion strength. Low density polyethylene with highly branched chains yielded very low self-adhesion. A drastic difference in adhesion strength between metallocene and conventional linear low density polyethylene is attributed to homogeneity versus heterogeneity of composition distribution. The low interfacial self-adhesion in conventional polyethylene was concluded to be due to enrichment of highly branched low molecular weight chains at the film surface. These segregated chains at the interface diffuse before the high molecular weight chains located in the bulk.

Influence of molecular architecture on the entanglement network: topological analysis of linear, long- and short-chain branched polyethylene melts via Monte Carlo simulations.

We present detailed results on the effect of chain branching on the topological properties of entangled polymer melts via an advanced connectivity-altering Monte Carlo (MC) algorithm. Eleven representative model linear, short-chain branched (SCB), and long-chain branched (LCB) polyethylene (PE) melts were employed, based on the total chain length and/or the longest linear chain dimension. Directly analyzing the entanglement [or the primitive path (PP)] network of the system via the Z-code, we quantified several important topological measures: (a) the PP contour length Lpp, (b) the number of entanglements Zes per chain, (c) the end-to-end length of an entanglement strand des, (d) the number of carbon atoms per entanglement strand Nes, and (e) the probability distribution for each of these quantities. The results show that the SCB polymer melts have significantly more compact overall chain conformations compared to the linear polymers, exhibiting, relative to the corresponding linear analogues, (a) ∼20% smaller values of 〈Lpp〉 (the statistical average of Lpp), (b) ∼30% smaller values of 〈Zes〉, (c) ∼20% larger values of 〈des〉, and (d) ∼50% larger values of 〈Nes〉. In contrast, despite the intrinsically smaller overall chain dimensions than those of the linear analogues, the LCB (H-shaped and A3AA3 multiarm) PE melts exhibit relatively (a) 7-8% larger values of 〈Lpp〉, (b) 6-11% larger values of 〈Zes〉 for the H-shaped melt and ∼2% smaller values of 〈Zes〉 for the A3AA3 multiarm, (c) 2-5% smaller values of 〈des〉, and (d) 7-11% smaller values of 〈Nes〉. Several interesting features were also found in the results of the probability distribution functions P for each topological measure.

Biocompatible multiblock aliphatic polyesters containing ether-linkages: influence of molecular architecture on solid-state properties and hydrolysis rate

A new class of biodegradable and biocompatible multiblock copolyesters containing butylene 1,4- cyclohexanedicarboxylate sequences is presented.

Influence of molecular architecture on the isothermal time-dependent response of amorphous shape memory polyurethanes

The thermomechanical response of a series of thermally activated shape memory polyurethanes (SMPUs), determined by dynamic mechanical analysis (DMA), has been adjusted by systematic modification of the molecular architecture. It is argued that the free recovery behavior of these SMPUs at temperatures in the vicinity of the calorimetric glass transition temperature is dependent not only on the recovery temperature, but also on the form of the corresponding peak in tan δ in DMA temperature scans at constant frequency. On the basis of simple correlations between recovery rates and the width and shape of the tan δ peak, it is suggested that DMA may provide a relatively simple and rapid means of assessing the potential of the SMPUs with respect both to recovery and shape fixity at a given storage temperature. This in turn allows establishment of a direct link between the shape memory performance and molecular architecture.

Influence of molecular architecture and processing on properties of semiconducting arylacetylene: Insulating poly(vinylidene fluoride) blends

Blends of chemically readily accessible, small-molecular arylacetylene derivatives with poly(vinylidene fluoride) (PVDF) are presented that allow reliable solution processing of field-effect transistor (FET) architectures with electronic characteristics comparable to those of the neat semiconductors. We demonstrate that having the chemical means and corresponding processing protocols to control solid-state microstructures by either adjusting the chemical nature of the organic semiconductor, blend composition or deposition temperature, permit straight-forward comparison between materials and allow probing if electronic characteristics are affected by the chemical structure of the organic semiconductor and/or selected processing protocols.

Thin Film Assembly of Spider Silk-like Block Copolymers

We report the self-assembly of monolayers of spider silk-like block copolymers. Langmuir isotherms were obtained for a series of bioengineered variants of the spider silks, and stable monolayers were generated. Langmuir-Blodgett films were prepared by transferring the monolayers onto silica substrates and were subsequently analyzed by atomic force microscopy (AFM). Static contact angle measurements were performed to characterize interactions across the interface (thin film, water, air), and molecular modeling was used to predict 3D conformation of spider silk-like block copolymers. The influence of molecular architecture and volume fraction of the proteins on the self-assembly process was assessed. At high surface pressure, spider silk-like block copolymers with minimal hydrophobic block (f(A) = 12%) formed oblate structures, whereas block copolymer with a 6-fold larger hydrophobic domain (f(A) = 46%) formed prolate structures. The varied morphologies obtained with increased hydrophobicity offer new options for biomaterials for coatings and related options. The design and use of bioengineered protein block copolymers assembled at air-water interfaces provides a promising approach to compare 2D microstructures and molecular architectures of these amphiphiles, leading to more rationale designs for a range of nanoengineered biomaterial needs as well as providing a basis of comparison to more traditional synthetic block copolymer systems.

Self-assembly of non-linear polymers at the air/water interface: the effect of molecular architecture

In comparison with conventional linear polymers, due to the presence of joints, branches, end groups, etc, non-linear polymers possess more variables that allow for the tailoring of steric constraints, stacking interactions, and hydrogen bonding of these polymers at the air/water interface to precisely control the self-assembly process to yield functional Langmuir–Blodgett (LB) films for various applications. This Review summarizes recent developments in the field of self-assembly of non-linear polymers at the air/water interface, focusing on the influence of molecular architecture. Four classes of non-linear systems (polymer brushes, star-like polymers, dendritic polymers, and linear–dendritic polymers) with representative examples are highlighted to elaborate their interfacial behaviors originating from different molecular architectures.

Influence of Macromolecular Architecture on the Crystallization of (PCL2)-b-(PS2) 4-Miktoarm Star Block Copolymers in Comparison to Linear PCL-b-PS Diblock Copolymer Analogues

Miktoarm block copolymers (A2B2) composed of two poly(ε-caprolactone) (PCL) arms and two polystyrene arms (PS) were synthesized by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Linear analogue PCL-b-PS diblock copolymer samples were also synthesized in almost identical composition regarding the content of each component. Almost all samples were found to be weakly segregated in the melt according to small angle X-ray scattering (SAXS) experiments. While the expected morphology (revealed by transmission electron microscopy, TEM) was found for the linear diblock copolymers, the miktoarm block copolymer samples exhibited different morphologies that indicated more entropic restrictions for chain stretching. For example, when a linear diblock copolymer with 41% PCL formed lamellae, the analogue miktoarm star copolymer with 39% PCL formed hexagonally packed PCL cylinders in a PS matrix. These results may imply changes in the phase diagram between miktoarm and linear block copolymers that have been previously predicted theoretically, however a larger number of samples should be used to corroborate this hypothesis. Additionally, the effect of polydispersity of the samples as a possible source of phase boundary variations should also be considered. The enhanced topological restrictions in the miktoarm star copolymers were also strongly reflected in the overall crystallization kinetics of the PCL component within the copolymers, as determined by differential scanning calorimetry (DSC). The supercooling needed for crystallization of the PCL component was much larger for the miktoarm star copolymers than for the linear analogue block copolymer samples of similar composition or even of similar morphology, while the crystallization rate was also depressed. The degree of crystallinity of the micro- or nanodomains was also a function of composition (decrease with PS content in the copolymers) or molecular architecture (lower in the stars than in linear copolymers) and as a general rule decreased as the level of confinement for the PCL component increased. Several kinetic theories of crystallization were applied to the overall isothermal crystallization data and regardless of the theory employed, the parameters proportional to the energy barriers for overall crystallization also increased with the confinement of the PCL component. Both the confinement degree and the influence of molecular architecture on the nucleation and crystallization of the PCL component generally increased with the content of covalently bonded glassy PS.

Surface Relief Gratings in Azobenzene‐Containing Polymers with Linear and Star‐Branched Architectures: A Comparison

AbstractThe influence of molecular architecture on light‐induced SRG formation was investigated. Polymers with different degree of branching were synthesized by ATRP and functionalized with azobenzene chromophores. The polymers differ only in their architecture – linear, 4‐, 6‐, or 12‐arms stars. The photo‐induced dichroism as well as the efficiency of SRG formation was similar for all polymers of this series. New consideration for the origin of the driving force was used to explain this behavior. The comparable SRG inscription rate in differently branched polymers can be rationalized by assuming that azobenzene acts as an internal molecular motor and can cause a non‐turbulent motion on a scale smaller than that on which normal entanglement restriction forces act.magnified image

Assembly of triblock copolymer brush at glass fiber/polystyrene interface and its effect on interfacial shear strength

To control the interfacial structure of glass fiber-reinforced composites, triblock copolymer coupling agents polystyrene-block-poly( n-butylacrylate)-block-poly(γ-methacryloxypropyltrimethoxysilane), (PS-b-PnBA-b-PMPS) were synthesized by atom transfer radical polymerization, then were chemically grafted on the glass fiber surfaces with a sufficiently high grafting density. When the grafted glass fibers are embedded in polystyrene (PS) matrix melt, copolymer brushes assemble at the interface of glass fiber/PS system to form the hemispherical domain morphology, where PnBA blocks form the inner core collapsing on fiber surface, while the outer layer PS blocks shield the PnBA blocks from PS homopolymer melt and entangle with the molecular chain of PS homopolymer. The influence of molecular architecture and organization of triblock copolymer coupling agents on the interfacial shear strength of glass fiber/PS system was investigated by means of Microbond Test. It is found that an increase in the PS block lengths is of advantage to the interfacial shear strength. A PnBA block length about the degree of polymerization (DPn) of 50 is desirable for the interfacial shear strength.

Effect of Molecular Architecture on Phase Behavior of Graft Copolymers

Influence of molecular architecture on phase behavior of graft copolymer melts was studied by using a reciprocal-space self-consistent filed theory (SCFT). The phase diagrams were examined as functions of the architectural parameters describing the graft copolymers (i.e., the number of grafts and the position of first junction). In comparison with the well-known phase diagram of diblock copolymers, the phase diagrams of the graft copolymers are asymmetric. When the number of grafts or the position of first junction varies, the boundaries of order-order transitions have shifts due to the variation in the chain stretching energy. The change in molecular architecture also significantly alters the domain spacing of ordered structures but has weak impact on the density distributions of graft copolymers. For comparison of the theoretical predictions with the existing experimental results, the phase diagrams of graft copolymers were also calculated at strong segregation. The SCFT calculations can accurately capture the characteristics of the phase behavior of graft copolymer melts.

Influence of molecular architecture and melt rheological characteristic on the optical properties of LDPE blown films

A systematic investigation on the origin of the haze of LDPE blown films was conducted, aiming to correlate the film haze with the molecular architecture and melt rheological properties. First of all, the haze measurement indicated that the surface haze, rather than the bulk haze, is the dominating factor for the total haze of the investigated films. No spherulitic crystals or other superstructures were observed for the LDPE blown films, implying that the crystallites formed in the film-blowing process are too small to be responsible for the optical haze. Rheological study revealed that the surface roughness was originated from the irregular flow of LDPE melt during the extrusion process. NMR, GPC and parallel-plate rheology were applied to study the molecular architecture of the LDPE resins. It was found that the LDPE sample with higher haze value exhibits distinctly larger portion of higher molecular weight component, broader molar mass distribution, significantly higher side chain branch density.

Rapid crack propagation failures in HDPE pipes: Structure–property investigations

The influence of molecular architecture on the rapid crack propagation (RCP) resistance of a wide variety of high-density polyethylene pipes was investigated. It was concluded that high molecular weight, high crystallinity and a relatively narrow molecular weight distribution are important architectural attributes for RCP resistance. The ductile-brittle transition temperature, as measured on compression-molded specimens using the razor-notched Charpy impact test, appears to be a reasonably good indicator of the RCP resistance of the resultant pipes. POLYM. ENG. SCI., 46:1358–1362, 2006. © 2006 Society of Plastics Engineers

Influence of Molecular Architecture on the Dewetting of Thin Polystyrene Films

The control of dewetting for thin polymer films is a technical challenge and of significant academic interest. We have used polystyrene nanoparticles to inhibit dewetting of high molecular weight, linear polystyrene, demonstrating that molecular architecture has a unique effect on surface properties. Neutron reflectivity measurements were used to demonstrate that the nanoparticles were uniformly distributed in the thin (ca. 40 nm) film prior to high temperature annealing, yet after annealing, they were found to separate to the solid substrate, a silanized silicon wafer. Dewetting was eliminated when the nanoparticles separated to form a monolayer or above while below this surface coverage the dewetting dynamics was severely retarded. Blending linear polystyrene of similar molecular weight to the polystyrene nanoparticle with the high molecular weight polystyrene did not eliminate dewetting.

Viscoelastic properties of dendrimers in the melt from nonequlibrium molecular dynamics

The viscoelastic properties of dendrimers of generation 1-4 are studied using nonequilibrium molecular dynamics. Flow properties of dendrimer melts under shear are compared to systems composed of linear chain polymers of the same molecular weight, and the influence of molecular architecture is discussed. Rheological material properties, such as the shear viscosity and normal stress coefficients, are calculated and compared for both systems. We also calculate and compare the microscopic properties of both linear chain and dendrimer molecules, such as their molecular alignment, order parameters and rotational velocities. We find that the highly symmetric shape of dendrimers and their highly constrained geometry allows for substantial differences in their material properties compared to traditional linear polymers of equivalent molecular weight.

Influence of molecular architecture and intermixing on the photovoltaic, morphological and spectroscopic properties of CuPc?C60 heterojunctions

Influence of molecular architecture and intermixing on the photovoltaic, morphological and spectroscopic properties of CuPc?C60 heterojunctions