Search

The Dissipative Particle Dynamics (DPD) simulation technique was used to elucidate the composition-dependent equilibrium morphological behavior of three different symmetric ABA triblock copolymers, which were; poly(e-caprolactone)-poly(dimethylsiloxane)-poly(e-caprolactone) (PCL-PDMS-PCL), poly(e-caprolactone)-poly(ethylene oxide)-poly(e-caprolactone) (PCL-PEO-PCL) and poly(L-lactide)-poly(dimethylsiloxane)-poly(L-lactide) (PLLA-PDMS-PLLA). These polymers were chosen due to their biomedical and biotechnological importance. Polymeric A and B blocks were modeled as connected chain of beads with varying incompatibility. The impact of the block incompatibilities on the microphase separation as well as on the equilibrium phase behaviors were investigated at the mesoscopic scale. A detailed visual analysis of the DPD images and constructed phase diagram showed that quite different equilibrium morphologies were attainable by controlling the molecular weights of the blocks and the strength of the intermolecular interaction between them. More compatible A and B blocks underwent lamellar to cylindrical and cylindrical to spherical phase transitions at lower B block concentrations. Our results clearly showed that, Flory–Huggins interaction parameter (χ) and degree of polymerization (N) were the only control parameters, which determined the shape and size of the phase domains, as well as the extent of equilibrium nanophase separation. Our DPD simulated morphologies were compared with experimental images obtained by Atomic Force Microscopy (AFM).

Degumming of Bombyx mori silk cocoons by a novel and mild process using aqueous ammonia and fabrication of electrospun polycaprolactone/silk fibroin (PCL/SF) nanofibrous scaffolds is reported. Cocoons were degummed in 0.3% w/w solutions of boiling ammonia (28–30%) for 45 min. Degummed SF fibers were dissolved in phosphoric and formic acid (7/3 v/v) mixture, coagulated in methanol, filtered and dried. PCL solutions containing different amounts of SF were electrospun in formic acid, a green solvent. Scaffolds were characterized to confirm the successful incorporation of SF and to demonstrate formation of nanofibrous webs with good biomechanical properties. Cell viability assay was performed by seeding Human BJ fibroblast cells on scaffolds. In vitro analysis showed that the scaffolds produced were non-toxic and incorporation of SF resulted in enhanced cell proliferation. Nanofibrous PCL/SF scaffolds with good biomechanical properties developed through dialysis free processing of silk fibroin can be promising substrates for tissue engineering applications.

There has been a considerable interest in developing stiff, strong, tough, and highly stretchable hydrogels in various fields of science and technology including biomedical and sensing applications. However, simultaneous optimization of stiffness, strength, toughness, and extensibility is a challenge for any material, and hydrogels are well-known to be mechanically weak materials. Here, we demonstrate that poly(ethylene oxide)-based dual stimuli-responsive semicrystalline poly(urethane-urea) (PU) copolymers with high hard segment contents (30 and 40%) can be utilized as stiff, strong, tough, and highly stretchable hydrogels with an elastic modulus (4-10 MPa) tens to hundreds of times higher than that of conventional hydrogels (0.01-0.1 MPa), strength (7-13 MPa) and toughness (53-74 m(-3) MJ. m(-3)) fairly comparable to those of the toughest hydrogels reported in the literature, and stretchability beyond 10 times their initial length (1000-1250%). In addition, the shape-memory program has been used to tune the room temperature stiffness and strength of the studied PU copolymers. Finally, the materials show fast shape recovery (less than 10 s) during both heat- and water-activated shape memory cycles, which can be adjusted by a simple modulation of the hard segment content and/or soft segment molecular weight. Our findings may be of interest in emerging biomedical and sensing applications.

An amorphous poly(urethane-urea) copolymer composed of 70 wt% poly(ethylene oxide) (PEO) soft segments (SS) (Mw = 2000 g mol-1) and 30 wt% cycloaliphatic hard segments (HS) was subjected to in-situ X-Rays during tensile deformation. Mechanical hardening at room temperature was attributed to strain induced crystallization (SIC) of the PEO SS through the multiplication of aligned crystallites. The permanent nature of these crystals after stress removal indicates a certain mechanical stability, which we related here to the concomitant effect of superstraining of the SS crystallites and the HS reorganization upon deformation. This is in marked contrast to previous studies which reported the crystalline phase to be temporary upon unloading. A manifestation of such enhanced stability is the memory effect evidenced by an increased crystallizability of PEO segments during incremental cyclic loading. These results offer a way to (i) tune the mechanical properties of TPUs via the formation of mechanically stable pre-oriented SS crystals and to (ii) tune the thermally/water activated shape memory properties of TPUs (shape fixity, kinetics of shape memory recovery).

Achieving a unique combination of stiffness, strength, extensibility, and toughness in sol-cast poly(urethane-urea) (PU) copolymer films is a challenge since these properties are-in general- mutually exclusive. Here we demonstrate that geometric confinement of the basic building blocks controls stiffness, strength, extensibility, and toughness in PU films. Our results suggest that the severity of geometric confinement can be tuned by adjusting (i) soft segment molecular weight (SSMW) and (ii) drying temperature (DT) thanks to their effects on the structure formation via microphase separation and/or (confined and/or bulk) crystallization. It is therefore possible to produce (i) soft (no notable confinement) and (ii) stiff, strong, extensible, and tough (severe confinement) materials without changing any other parameter except SSMW and DT. The former has a typical physically cross-linked network and shows a welldefined elastomeric behavior with an elastic modulus (E) of 5-20 MPa, a tensile strength (sigma(max)) of 30-35 MPa, an extensibility (epsilon) of 1000-1300%, and a toughness (W) of 90-180 MJ m(-3). The latter, on the other hand, possesses an elegant hierarchical structure containing tightly packed secondary structures (7(2)-helix, 4(1)-helix, and antiparallel beta-sheets) and displays an elastoplastic behavior with an E of 400-700 MPa, a sigma(max) of 45-55 MPa, an epsilon of 650-850%, and a W of 200-250 MJ m(-3). Hence, our findings may be of interest in designing advanced materials containing synthetic replica of the secondary structures found in protein-based materials. The structure formation in the materials with this structural hierarchy is driven by the confined crystallization of helical poly(ethylene oxide) (PEO) chains in subnanometer urea channels, which-to the best of our knowledge-is a phenomenon wellknown in host-guest systems but has not yet demonstrated in PU copolymers, and complemented by the "bulk" crystallization of PEO and/or the microphase separation.

Superhydrophobic PMMA surfaces were prepared by spin-coating and doctor blade coating of PMAA/hydrophobic silica (1/10 by weight) dispersions in toluene onto glass substrates. Influence of the number of coating layers applied and gauge thickness of the doctor blade used on surface properties were investigated. Formation of dual scale, micro/nano surface topographies were demonstrated by scanning electron microscopy, atomic force microscopy and white light interferometry studies. Roughness factor (r) and average surface roughness (Ra) values of the surfaces were determined. Wetting behavior of superhydrophobic PMMA surfaces obtained by introducing micro-nano, hierarchical roughness to inherently hydrophilic smooth PMMA films cannot be explained by Wenzel model. Therefore, wetting behavior of these surfaces were analyzed using Cassie-Baxter model and area fraction of surface protrusions were estimated.

Microporous poly(lactic acid) (PLA) coatings with fairly homogeneous pore size and distribution were obtained through spin-coating process. Critical effects of the thermal conductivity of the substrate (copper, glass, insulated glass), solvent type (dichloromethane, chloroform and tetrahydrofuran), polymer concentration (3–15% by weight) and relative humidity of the environment (0–85% relative humidity) on the formation and topography of microporous surfaces were investigated. Characterization of the surfaces was achieved via scanning electron microscopy, white light interferometry and static water contact angle measurements. By tuning the production parameters, PLA coatings displaying microporous surfaces with uniform pore sizes and distribution were obtained. Pore sizes varied from 1 to 7 μm depending on the experimental conditions.

Monolithic poly(lactic acid) (PLA) foams were produced by thermally induced phase separation. PLA solutions with concentrations 8-22 wt % were prepared in tetrahydrofuran/methanol (THF/MeOH) solvent/nonsolvent mixtures at 55 degrees C. Homogenous solutions were quenched at -20 degrees C to induce phase separation and gelation. Resulting gels were mechanically stabilized by solvent exchange. Subsequent supercritical CO2 drying yielded monolithic PLA foams. Crystal structure and degree of crystallinity of the foams were obtained by x-ray diffractometry and differential scanning calorimetry. Morphologies were determined by scanning electron microscopy. Tuning the PLA concentration and THF/MeOH ratio enabled preparation of monolithic PLA foams. Depending on the experimental conditions various morphologies, such as: interconnected networks, thin platelets, lamellar stacks, axialites, and spherulites were formed. Monoliths obtained were highly crystalline. By changing the PLA concentration monoliths with controlled average pore sizes (170-1440 nm) and porosities (80-90%) were produced. (c) 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 98-108

Smart surfaces with preferably reconfigurable wetting properties can lead to key applications in labon- a-chip analytical and preparative systems. In this paper, we present our recent results obtained using polymer surfaces whose wetting properties are modified in a permanent manner using laser ablation and in a reconfigurable manner using plasma oxidation. Polydimethylsiloxane (PDMS) diluted in solvent is used as the polymeric material coated over microscope glass slides in our studies. In the first part, the tracks of ~ 70 μm width are defined over the surface by surface oxidation using cold plasma exposure through a microfluidic channel. In the second part, femtosecond laser micromachining is used for selective removal of polymer coating and uncovering the hydrophilic glass substrate. We experimentally demonstrate guiding of water in the form of filaments and droplets over the obtained hydrophilic tracks of ~ 110 μm. We also discuss preliminary experiments to coat light sensitive azobenzene over a glass substrate with the help of a silane in order to achieve reversible isomerization upon periodic exposure to UV/vis radiation. Furthermore, we elaborate advantages, challenges and the significant role of such patterned surfaces in future applications.

Polymer surfaces reversibly switchable from superhydrophobic to superhydrophilic by exposure to oxygen plasma and subsequent thermal treatment are demonstrated. Two inherently different polymers, hydrophobic segmented polydimethylsiloxane-urea copolymer (TPSC) and hydrophilic poly(methyl methacrylate) (PMMA) are modified with fumed silica nanoparticles to prepare superhydrophobic surfaces with roughness on nanometer to micrometer scale. Smooth TPSC and PMMA surfaces are also used as control samples. Regardless of their chemical structure and surface topography, all surfaces display completely reversible wetting behavior changing from hydrophobic to hydrophilic and back for many cycles upon plasma oxidation followed by thermal annealing. Influence of plasma power, plasma exposure time, annealing temperature and annealing time on the wetting behavior of polymeric surfaces are investigated. Surface compositions, textures and topographies are characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and white light interferometry (WLI), before and after oxidation and thermal annealing. Wetting properties of the surfaces are determined by measuring their static, advancing and receding water contact angle. We conclude that the chemical structure and surface topography of the polymers play a relatively minor role in reversible wetting behavior, where the essential factors are surface oxidation and migration of polymer molecules to the surface upon thermal annealing. Reconfigurable water channels on polymer surfaces are produced by plasma treatment using a mask and thermal annealing cycles. Such patterned reconfigurable hydrophilic regions can find use in surface microfluidics and optofluidics applications.

Polycaprolactone (PCL) foams were produced by thermally induced phase separation. Tetrahydrofuran/methanol (THF/MeOH) (solvent/non-solvent) mixture was used for the induction of liquid-liquid phase separation of PCL solutions at three different temperatures. Subsequent solvent exchange followed by vacuum drying yielded polymeric foams with different morphologies. Characterization of foams was obtained by scanning electron microscopy, x-ray diffractometry, mercury intrusion porosimetry and compression tests. Influence of polymer concentration (8, 10 and 12 wt%), quench temperature (4, -20 and -80 degrees C), and THF/MeOH ratio from (42/58) to (54/46) (wt/wt) on the foam formation, morphology and properties were investigated systematically. Lower PCL concentration, lower THF content and higher quench temperature lead to larger pore sizes in the foams obtained. Detailed discussions of the influence of processing parameters on foam structure and porosity, foam density, percent crystallinity and compressive properties are provided. By selectively tuning the process parameters, foams with controlled pore sizes (10-450 mu m), porosity (83-91%) and morphology (cellular, bead-like, micro-spherical) were obtained. (C) 2018 Elsevier Ltd. All rights reserved.

Preparation and characterization of chitosan/hydroxyapatite (CS/HA) nanocomposites displaying an intercalated structure is reported. Hydroxyapatite was synthesized through sol-gel process. Formic acid was introduced as a new solvent to obtain stable dispersions of nano-sized HA particles in polymer solution. CS/HA dispersions with HA contents of 5, 10 and 20% by weight were prepared. Self-assembling of HA nanoparticles during the drying of the solvent cast films led to the formation of homogeneous CS/HA nanocomposites. Composite films were analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-rays (EDX) analysis, Fourier transform infrared (FTIR) spectroscopy, X-rays diffraction (XRD) analysis and thermogravimetric analysis (TGA). SEM and AFM confirmed the presence of uniformly distributed HA nanoparticles on the chitosan matrix surface. XRD patterns and cross-sectional SEM images showed the formation of layered nanocomposites. Complete degradation of chitosan matrix in TGA experiments, led to the formation of nanoporous 3D scaffolds containing hydroxyapatite, β-tricalcium phosphate and calcium pyrophosphate. CS/HA composites can be considered as promising materials for bone tissue engineering applications.

Hydrogen bonding between hard segments has a critical effect on the morphology and properties of polyurethanes. Influence of temperature on hydrogen bonded urethane network and melting behavior of a model semicrystalline segmented polyurethane was investigated by experiments and simulations. Polyurethane was synthesized by the stoichiometric reaction between p-phenylene diisocyanate and poly(tetramethylene oxide) (PTMO) with a molecular weight of 1000 g/mol. Simulations were carried out using dissipative particle dynamics (DPD) and molecular dynamics (MD) approaches. Experimental melting behavior obtained by various techniques was compared with simulations. DPD simulations showed a room temperature microphase morphology consisting of a three-dimensional hydrogen-bonded urethane hard segment network in a continuous and amorphous PTMO matrix. The first-order melting transitions of crystalline urethane hard segments observed during the continuous isobaric heating in DPD and MD simulations (340–360 K) were in reasonably good agreement with those observed experimentally, such as AFM (320–340 K), WAXS (330–360 K), and FTIR (320–350 K) measurements. Quantitative verification of the melting of urethane hard segments was demonstrated by sharp discontinuities in energy versus temperature plots obtained by MD simulations due to substantial decrease in the number of hydrogen bonds above 340 K. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017.

Poly(ethylene oxide) (PEO) based polyurethaneurea-silica nanocomposites were prepared by solution blending and characterized by Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Differential Scanning Calorimetry and tensile testing. The colloidal silica nanoparticles with an average size of 50 nm were synthesized by modified Stober method in isopropanol. Silica particles were incorporated into three cycloaliphatic polyurethaneurea (PUs) copolymers based on PEO oligomers with molecular weights of 2,000, 4,600, and 8,000 g/mol. Hard segment content of PUs was constant at 30% by weight. Silica content of the PU nanocomposites varied between 1 and 20% by weight. Soft segment (SS) glass transition and melting temperatures slightly increased with increasing filler content for all the copolymers. Degree of SS crystallinity first increased with 1% silica incorporation and subsequently decreased by further silica addition. Elastic modulus and tensile strengths of PU copolymers gradually increased with increasing amount of the silica filler. Elongation at break values gradually decreased in PEO-2000 based PU copolymer with increasing silica content, whereas no significant change was observed in PUs based on PEO-4600 and PEO-8000. Enhancement in tensile properties of the materials was mainly attributed to the homogeneous distribution of silica filler in polymer matrices and strong polymer-filler interactions.

When a liquid droplet (e.g., coffee, wine, etc.) is splattered on a surface, the droplet dries in a ring-shaped stain. This widely observed pattern in everyday life occurs due to the phenomenon known as a coffee stain (or coffee ring) effect. While the droplet dries, the capillary flow moves and deposits the particles toward the pinned edges, which shows a 2D ring-like structure. Here we demonstrate the transition from a 2D to a 3D coffee stain that has a well-defined and hollow sphere-like structure, when the substrate surface is switched from hydrophilic to superhydrophobic. The 3D stain formation starts with the evaporation of the pinned aqueous colloidal droplet placed on a superhydrophobic surface that facilitates the particle flow towards the liquid-air interface. This leads to spherical skin formation and a cavity in the droplet. Afterwards the water loss in the cavity due to pervaporation leads to bubble nucleation and growth, until complete evaporation of the solvent. In addition to the superhydrophobicity of the surface, the concentration of the solution also has a significant effect on 3D coffee stain formation. Advantageously, 3D coffee stain formation in a pendant droplet configuration enables the construction of all-protein lasers by integrating silk fibroin with fluorescent proteins. No tools, components and/or human intervention are needed after the construction process is initiated; therefore, 3D coffee-stains hold promise for building self-assembled and functional 3D constructs and devices from colloidal solutions.

Rigid poly(lactic acid) (PLA) foams were prepared by thermally induced phase separation followed by solvent exchange and vacuum drying. A novel tetrahydrofuran (THF)/water solvent system was used for the induction of liquid-liquid phase separation of PLA solution at three different temperatures; 24 degrees C, 4 degrees C and -20 degrees C. PLA gels obtained were mechanically stabilized by replacing THF/water solvent mixture with ethanol nonsolvent. Characterization of rigid PLA foams was obtained by scanning electron microscopy, mercury intrusion porosimetry, x-ray diffractometry, infrared spectroscopy and differential scanning calorimetry analyses. Effects of fabrication parameters on the morphology and pore structure were systematically examined. The parameters investigated included; (i) polymer concentration, (ii) THF/water ratio and (iii) quench temperature. Results showed that degree of porosity and the morphology of the pores, such as the pore size and shape could be controlled by tuning the fabrication parameters. By controlling the degree of phase separation of PLA solution, foams with dual micro and nano structures were obtained. (C) 2016 Elsevier Ltd. All rights reserved.

Two simple processes; (i) spin-coating, and (ii) doctor blade coating of silica/polymer dispersions are described for the preparation of superhydrophobic polymer surfaces. To demonstrate the versatility and broad applicability of the processes, polymeric surfaces modified included a thermoplastic resin, polystyrene (PS) and a thermoset, crosslinked epoxy resin (ER). Micro/nano hierarchical nature of the surface topographies obtained were demonstrated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and white light interferometry (WLI) studies. Roughness factor (r) and average surface roughness (Ra) values, which are critical in obtaining superhydrophobic surfaces were determined for each polymeric system. It was clearly demonstrated that increased (r) and (Ra) values resulted in superhydrophobic behavior with very high static, advancing and receding water contact angles, well above 150° and contact angle hysteresis values of less than 10°. Incorporation of small amounts (1.0% by weight) of a silicone copolymer or a perfluoroether glycol oligomer reduced the contact angle hysteresis in the epoxy resin system well below 10° and produced truly superhydrophobic surfaces.

Catalytic conversion of carbon dioxide to liquid fuels and basic chemicals by using solar-derived hydrogen at, or near, ambient pressure is a highly desirable goal in heterogeneous catalysis. If realized, this technology could lead to a more sustainable society together with decentralized power generation. A novel class of holmium-containing multi-metal oxide Cu catalysts, discovered through the application of high-throughput methods, is reported. In particular, ternary systems of Cu-GaOx-HoOy>Cu-CeOx-HoOy approximate to Cu-LaOx-HoOy supported on -Al2O3 exhibited superior methanol production (10x) with less CO formation than previously reported catalysts at 1atm pressure. Holmium was shown to be highly dispersed as few-atom clusters, suggesting that the formation of trimetallic sites could be the key for the promotion of methanol synthesis by Ho.

Poly(e-caprolactone)–polydimethylsiloxane–poly(e-caprolactone) (PCL-PDMS-PCL) triblock copolymers with a wide range of block lengths (1000–32,000 g/mol) were synthesized by the ring-opening polymerization of e-caprolactone using aminopropyl terminated PDMS oligomers as the initiator. Reactions were carried out in bulk or solution at 125 ± 5 °C under the catalytic action of tin octoate. Products obtained in high yields were characterized by FTIR spectroscopy, gel permeation chromatography, X-ray diffraction, temperature-dependent optical microscopy, differential scanning calorimetry, atomic force microscopy, scanning electron microscopy and static water contact angle measurements. Effect of the polymer composition and the molecular weight of the PDMS and PCL blocks on; (i) copolymer morphology and the extent of microphase separation, (ii) crystallization of PCL segments and (iii) surface properties of copolymers were investigated. Regardless of the block lengths, all PCL-PDMS-PCL copolymers displayed well microphase separated morphologies. The extent of microphase separation, resultant morphology and sizes of the microphases were strongly dependent on the copolymer composition and the block lengths of both PCL and PDMS segments. Crystalline PCL microphase was observed in all copolymers, which increased as a function of PCL content and molecular weight. Hydrophobicity of the copolymer surfaces was improved with increasing PDMS molecular weight, as determined by static water contact angle measurements.

Today the high demand for electronics leads to massive production of waste, thus green materials based electronic devices are becoming more important for environmental protection and sustainability. The biomaterial based hydrogels are widely used in tissue engineering, but their uses in photonics are limited. In this study, silk fibroin protein in hydrogel form is explored as a bio-friendly alternative to conventional polymers for lens applications in light-emitting diodes. The concentration of silk fibroin protein and crosslinking agent had direct effects on optical properties of silk hydrogel. The spatial radiation intensity distribution was controlled via dome- and crater-type silk-hydrogel lenses. The hydrogel lens showed a light extraction efficiency over 0.95 on a warm white LED. The stability of silk hydrogel lens is enhanced approximately three-folds by using a biocompatible/biodegradable poly(ester-urethane) coating and more than three orders of magnitude by using an edible paraffin wax coating. Therefore, biomaterial lenses show promise for green optoelectronic applications., Marie Curie Career Integration Grant (PROTEINLED); Turkish Academy of Sciences; Scientific and Technological Research Council of Turkey (TÜBİTAK)

Synthesis of hydroxyapatite (HA) through sol-gel process in different solvent systems is reported. Calcium nitrate tetrahydrate (CNTH) and diammonium hydrogen phosphate (DAHP) were used as calcium and phosphorus precursors, respectively. Three different synthesis reactions were carried out by changing the solvent media, while keeping all other process parameters constant. A measure of 0.5 M aqueous DAHP solution was used in all reactions while CNTH was dissolved in distilled water, tetrahydrofuran (THF) and N, N-dimethylformamide (DMF) at a concentration of 0.5 M. Ammonia solution (28-30%) was used to maintain the pH of the reaction mixtures in the 10-12 range. All reactions were carried out at 40 +/- 2 degrees C for 4 h. Upon completion of the reactions, products were filtered, washed and calcined at 500 degrees C for 2 h. It was clearly demonstrated through various techniques that the dielectric constant and polarity of the solvent mixture strongly influence the chemical structure and morphological properties of calcium phosphate synthesized. Water-based reaction medium, with highest dielectric constant, mainly produced beta-calcium pyrophosphate (beta-CPF) with a minor amount of HA. DMF/water system yielded HA as the major phase with a very minor amount of beta-CPF. THF/water solvent system with the lowest dielectric constant resulted in the formation of pure HA., Higher Education Commission, Pakistan under HRDI-UESTP programme

The effect of poly(ethylene oxide) (PEO) soft segment molecular weight (M-n = 2000, 4600 and 8000 g mol(-1)) on the glass transition, crystallinity, molecular mobility and segmental dynamics of a series of aliphatic polyurethaneurea copolymers (PUU) with a constant hard segment content of 30% by weight was investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The soft segment (PEO) glass transition temperature increased with increasing molecular weight. Furthermore, five different relaxations were observed in dielectric analyses of all copolymers. These included local glassy state motions (gamma) and (beta), segmental motion of the soft phase (alpha), conductivity relaxation, and interfacial Maxwell-Wagner-Sillars (MWS) polarization. Local relaxations follow Arrhenius behavior and their time scale is not affected by the soft segment molecular weight. alpha-Relaxation follows Vogel-Tammann-Fulcher (VTF) behavior and is slower for the copolymer based on PEO-4600. Conductivity relaxation and the interfacial MWS polarization also follow VTF behavior and have quite similar slopes since both are related to the same phenomena. Unexpectedly, the interfacial MWS polarization is not affected by the soft segment molecular weight. Although this result suggests that the soft segment molecular weight does not affect the microphase separation in these copolymers, we believe that no safe conclusions can be extracted for this system due to the high complexity and the presence of many phases with different conductivity. However, significant differences were observed in the conductivity relaxation, which is much faster for the copolymer based on PEO-2000, due to its lower crystallinity when compared with others.

Segmented thermoplastic polyurethanes, polyureas and polyurethaneureas (TPU) based on a given hard segment and two chemically different soft segments display interesting microphase morphologies and thermal, mechanical and surface properties. In these systems the final TPU morphology is mainly controlled by the structure, amount and molecular weight of the soft segment oligomers and the nature and extent of specific intermolecular interactions between the mixed soft segments themselves and with the urethane hard segments. These interactions lead to variable compatibilities between the soft and hard segments resulting in interesting TPU morphologies. The proper choice of the two chemically different soft blocks provides more flexibility in controlling the extent of microphase separation, size and shape of the microphase domains and offers new possibilities for controlling the properties of TPUs. In this study coarse grained computer simulations were carried out to better understand the nature of intermolecular interactions and to elucidate the equilibrium microphase morphologies of TPUs with two different soft segments at 300 K. Model TPU systems investigated are comprised of poly(tetramethylene oxide) (PTMO) or poly(hexylethyl carbonate) (PHEC) and polydimethylsiloxane (PDMS) or polyisobutylene (PIB) soft segments with molecular weights in the range of 500-2500 g/mol. Hard segments consisted, in all cases, of diphenylmethane diisocyanate (MDI) based urethane repeat units and ranged from 25 to 50% by weight. Through coarse grained Dissipative Particle Dynamics (DPD) simulations it was demonstrated that by varying the composition and the chain lengths of the soft and hard blocks, quite different morphologies from homogeneous (or mixed) to gradient and to completely microphase separated structures were attainable. As expected, fairly hydrophobic soft blocks such as PIB and PDMS favored strong microphase separation when compared with relatively hydrophilic PHEC and PTMO segments. For comparison, morphologies of the TPUs based on single soft segments (PTMO, PHEC, PDMS and PEO) with varying molecular weights and hard segment contents were also simulated. (C) 2016 Elsevier Ltd. All rights reserved.