Exciton trapping at heterojunctions in polymer blends

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency.     

Phase ordering in blend films of semi‐crystalline and amorphous polymers

A combination of optical and atomic force microscopy (AFM) is used for probing changes in the morphology of polymer blend films that accompany phase ordering processes (phase separation and crystallization). The phase separation morphology of a “model” semi‐crystalline (polyethyleneoxide or PEO) and amorphous (polymethylmethacrylate or PMMA) polymer blend film is compared to previous observations on binary amorphous polymer blend films of polystyrene (PS) and polyvinylmethylether (PVME). The phase separation patterns are found to be similar except that crystallization of the film at high PEO concentrations obscures the observation of phase separation. The influence of film defects (e.g., scratches) and clay filler particles on the structure of the semi‐crystalline and amorphous polymer films is also investigated.     

A chlorinated polymer promoted analogue co-donors for efficient ternary all-polymer solar cells

The efficient ternary all-polymer solar cells(PSCs) are designed and fabricated, using a polymer acceptor of NDP-V-C7 and analogue co-donors containing a chlorinated polymer PBCl T and classical PTB7-Th. PBCl T and PTB7-Th possess very similar chemical structure and matched energy levels to form the cascade of the co-donors. Meanwhile, benefiting from those analogous polymer structures, there is little influence of the morphology in blend film compared to their pristine polymer films. The binary PBCl T:NDP-V-C7 devices exhibit a high open-circuit voltage(V_(oc)) due to the deep HOMO level of PBCl T. The V_(oc)of all-PSCs could be finely manipulated by adjusting the content of PBCl T in blend film. The ternary all-PSCs have the more balanced charge mobility and prolonged carrier lifetime compared to the binary devices. The PBCl T also help improve the miscibility of ternary blend and suppress crystallization in films, bringing about favorable morphology with appropriate orientation and surface roughness in blend film. With the optimal processing, the champion ternary all-PSCs obtain a high PCE of 9.03%, which is about 10% enhancement compared to that of binary device. The results indicate that the ternary approach using analogue co-donors is a practical method to enhance the performance of all-PSCs.     

Simulation of the specific interactions between polyamide-6 and a thermoplastic polyurethane

Polyamides have many desirable properties such as high melting temperatures, chemical resistance and superior mechanical properties. However, its crystalline morphology can limit its applications. It is the specific interaction, hydrogen bonding that gives rise to the crystalline structure of polyamides. This interaction is strong and important when blending on the final morphology and mechanical properties. Polyurethane contains polar functionality that can also interact with the polar component of polyamide. Hence, it is important to study the interaction between such a blend as polyurethane can enhance the toughness of polyamide due to its elastic properties. This study is an insight into the specific interaction between two polar polymers in a simulation whereby the interaction is maximised. Hydrogen bonding has been observed between molecules of either polyamide–polyurethane and polyamide–polyamide, and it is sufficiently strong to cause the polymer chains to distort rather than disrupt the hydrogen bonds. When groups of like polarity, such as carbonyl groups, come into proximity, the polymer chains again distort from their regular conformation because of mutual repulsion.     

Mechanical properties and structure relationships in drawn fibers of elastomer–polyoxymethylene blends

Superdrawn fibers of an elastomer–poly(oxymethylene) (POM) blend have been prepared and investigated in terms of the structure and mechanical properties. The development of the mechanical properties along the fiber axis and the formation of a higher order structure during drawing were slightly retarded by blending, but the loop tenacity increased greatly with the elastomer content. The blend microtextures had an immiscible and phase-separated morphology in which the elastomer was dispersed in the form of streaks between the oriented POM layers, which allowed the fiber to split into smaller filaments on bending. The high loop tenacity of the blend fibers is due to an increase in the radius of curvature resulting from the filament splitting on bending, because the shear stress at the bending corner becomes higher as the radius of curvature increases. © 1997 John Wiley & Sons, Inc.     

聚合物二元体系动态力学性能的估算

动态热机械分析是多相聚合物体系的一个重要研究手段.分析动态力学性能可以研究共混高聚物的相容性、复合材料的界面特性以及高分子运动机理等.本文综述了聚合物二元体系,即填充、纤维增强、共混体系动态力学性能的估算方法.在填充体系中,分别概述有无界面作用两种情况,当存在界面作用时,界面作用越强,模量越大,阻尼越小.对纤维增强体系,讨论了玻璃纤维有无取向的情况下模量和阻尼的估算.特别对于聚合物二元共混体系,分"海-岛"结构和双连续相两种情况,分别讨论了模量与阻尼的估算.     

Size effects on miscibility and glass transition temperature of binary polymer blend films

After determining the size dependent miscibility of binary polymer blend films using molecular dynamics simulation and thermodynamics, the size dependent glass transition temperatures Tg(w,D) of several polymer blend films in miscible ranges are determined by computer simulation and the Fox equation where w is the weight fraction of the second component and D denotes thickness of films. The Tg(w,D) function of a thin film can decrease or increase as D decreases depending on their surface or interface states. The computer simulation results are consistent with available experimental results and theoretical results for polymer blend films of PPO/PS [poly(2,6-dimethyl-1,4-phenylene oxide)/polystyrene] and stereoregular PMMA/PEO [poly(methyl methacrylate)/poly(ethylene oxide)]. The physical background of the above results is related to the root of mean square displacement of thin films in their different regions.     

嵌段共聚物相容剂对聚合物共混体系力学性能影响的连续介观模拟

结合介观动力学方法和三维弹簧格子模型,研究了嵌段共聚物相容剂对相容性较差的聚合物二元共混体系力学性能的影响.在适当范围内不断增加嵌段共聚物相容剂的用量,研究了相容剂含量对体系杨氏模数及拉伸强度的影响,同时也对不同体系材料的破碎位点进行了分析.结果表明,未加入相容剂的二元共混体系在拉伸模拟中表现出较低的拉伸强度,而适量添加相容剂可以显著提升材料的拉伸强度,随着相容剂含量的增加,共混体系的破碎位点会发生转移并最终改善材料的整体性能.而相容剂的加入对体系杨氏模数的影响较小.该连续模拟方法为关联聚合物复合体系的微观结构和宏观力学性能提供了一条高效的途径.     

Structure and mechanical properties for binary blends of polypropylene and ethylene‐1‐hexene copolymer

The structure and mechanical properties of the injection‐molded products for the binary blends composed of an isotactic polypropylene (PP) and a rubbery ethylene‐1‐hexene copolymer (EHR) were studied. The following two types of blends were employed: one is the incompatible blend of PP and ethylene‐rich EHR; the other is the compatible blend of PP and 1‐hexene‐rich EHR. The incompatible blend shows a phase‐separated morphology, in which EHR domains in the skin layer highly orient to the flow direction. On the other hand, the compatible blend shows fairly homogeneous morphology in the skin and core regions, in which EHR molecules are dissolved into the amorphous PP region. The measurements of birefringence and infrared dichroism revealed that the magnitude of molecular orientation along the flow direction for the compatible blend is larger than that for the incompatible blend. Nevertheless, it was also found that anisotropy of the mechanical properties for the compatible blend is less prominent, which is attributed to lack of the mechanical connection between neighbor crystalline fragments aligned perpendicular to the flow direction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 701–713, 1999     

嵌段共聚物相容剂对聚合物共混体系力学性能影响的连续介观模拟

结合介观动力学方法和三维弹簧格子模型, 研究了嵌段共聚物相容剂对相容性较差的聚合物二元共混体系力学性能的影响. 在适当范围内不断增加嵌段共聚物相容剂的用量, 研究了相容剂含量对体系杨氏模数及拉伸强度的影响, 同时也对不同体系材料的破碎位点进行了分析. 结果表明, 未加入相容剂的二元共混体系在拉伸模拟中表现出较低的拉伸强度, 而适量添加相容剂可以显著提升材料的拉伸强度, 随着相容剂含量的增加, 共混体系的破碎位点会发生转移并最终改善材料的整体性能. 而相容剂的加入对体系杨氏模数的影响较小. 该连续模拟方法为关联聚合物复合体系的微观结构和宏观力学性能提供了一条高效的途径.     

Selective protein adsorption on a phase-separated solvent-cast polymer blend

Polymer-based biomedical devices are growing increasingly sophisticated as compositions evolve toward copolymers and blends in order to satisfy complex design criteria. Such polymers afford opportunities for both micro- and macrophase separation at nano- and micro-length scales and raise questions concerning the role of heterogeneous surface morphology on protein adsorption. Adsorbed protein layers play a critical role in mediating the interaction of cells with polymer surfaces, and both understanding and controlling protein adsorption is assuming greater significance in the development of surfaces with enhanced physiological compatibility. Here we study the short-time adsorption of ferritin, a model protein highly resistant to denaturation and easily imaged in the transmission electron microscope (TEM), onto a phase-separated homopolymer blend of polycaprolactone (PCL) and a polycarbonate derived from desaminotyrosyl-tyrosine dodecyl ester (PDTD). At physiological pH, ferritin selectively adsorbs onto the PDTD phase at a surface density approximately three times greater than that on the PCL phase. By decreasing the pH below ferritin's isoelectric point so its average charge becomes positive, the selective adsorption disappears and the surface density of adsorbed ferritin becomes independent of the phase separation. We attribute the selectivity to the electrostatic repulsion between ferritin and hydrolytically charged PCL, both of which will have a net negative charge at physiological pH. To perform these experiments, we solvent-cast ultrathin polymer films onto dissolvable salt substrates, and we characterize the morphology by TEM imaging and quantitative spatially resolved electron energy-loss spectroscopy (EELS). We find that the film morphology depends strongly on such processing-related variables as the solvent evaporation rate and the nature of the surface in contact with the polymer film during casting. The adsorption of ferritin depends on whether the film is phase-separated as well as to which surface of the film the protein solution is exposed, and these findings suggest that seemingly small variations in polymer processing that influence both the bulk and surface morphology can have a profound effect on the short-time protein adsorption.     

Biodegradation of Blown Films Based on Poly(lactic acid) under Natural Conditions

Summary: Commercially available polymer Bioflex® 219F, blend of polylactic acid and biodegradable co-polyester, was used for film preparation, performed on mono-extrusion blown moulding machine. Resulting thin film was investigated on biodegradability in composting conditions for 6 weeks. The influence of microbial attack on mechanical, physico-chemical properties, weight loss and surface morphology was tested weekly. The results obtained during 6 weeks of composting indicate relatively good accessibility to biological degradation. Moreover, the time course of studied properties was observed through the test period.     

Computer Simulation of Thin Film Wrinkling on Elastic Substrate

Numerous theoretical and experimental efforts have been made to explain the dependence of the static wrinkling morphology on the materials' physical properties, whereas the dynamic wrinkling process remains elusive. In the present work, we design a wrinkling model consisting of a soft substrate and a graphene-like rigid thin film to investigate this dynamic process. The simulation shows that the whole wrinkling process includes three stages. At the incubation and wrinkling stages, the stress along the horizon direction of the soft substrate transfers to the stiff film. However, at the equilibrium stage, the stress of the rigid film slowly transfers back to the substrate although the total energy still decreases. It is found that the stress of the substrate concentrates at the top surface, especially at the trough, whereas the stress distribution of the film depends on direction. In the perpendicular direction, the stress at the wave's equilibrium position surpasses that at the crest and trough and, oppositely, the stress concentrates at the crest and trough in the horizon direction. Present model reproduces both wrinkling and delamination patterns and can be a powerful tool to deeply understand the structure deformation of material induced by stress release.     

Surface aggregation structure and surface mechanical properties of binary polymer blend thin films

During preparation of very thin polymer belnd films from a solution of polymers, the phase‐separated structures which are quite different from that observed for the bulk blend film was observed. From atomic force microscopic(AFM) observation, it is concluded that the surface undulation, which reflects the phase separated morphology of the blend system, is present. In the case of (polystyrene(PS)/poly(methyl methacrylate)(PMMA)) blend system, a large influence of end‐group chemistry on the surface morphology was observed. The phase identification of the (rubbery polymer/glassy polymer) binary blend thin films was successfully achieved by scanning vioscoelasticity microsopy(SVM).     

Tubular or subsurface morphology of octabutoxyphthalocyanine upon self-assembly in polymer matrices: effect of the casting solvent

Spontaneous phase-separated, controlled aggregate structures of photo- and electroactive molecules in polymer matrices are of interest for device fabrication. We show that the self-assembly of octabutoxyphthalocyanine (Pc) in polymer matrices leads to tubular morphology of Pc when the film is prepared with tetrachloroethane (TCE) and subsurface droplet morphology with chloroform. The same morphology is seen with both bisphenol A polycarbonate (BPAPC) and poly(methyl methacrylate) (PMMA) as the matrix. The subsurface morphology results from the rapid association of Pc in the polymer matrix, as the film forms. With the tubular morphology in the films prepared with TCE, percolation threshold is reached with a concentration of Pc as low as 3% (wt) in the polymer. Such phase-separated self-assembly occurs, without any annealing of the films. Even in the absence of the polymer, Pc crystallized from TCE also shows tubular morphology, whereas it exhibits a columnar morphology with chloroform. X-ray diffraction of Pc crystallized from either solvent shows the columnar stacking of the Pc molecules. However, the morphology is tubular when TCE is used. We attribute the difference in the morphology to the higher viscosity of TCE and the diffusion-limited growth, which causes the tubular morphology, whereas the instantaneous self-assembly in less-viscous chloroform leads to droplets. The solvent effect observed here could be used to tailor the morphology of such photoconductive molecules in polymer matrices.     

Effects of partial replacement of silicone rubber with flurorubber on properties of dynamically cured poly(vinylidene fluoride)/silicone rubber/flurorubber ternary blends

Blends of poly(vinylidene fluoride) (PVDF), silicone rubber (SR) and flurorubber (FKM) were prepared via peroxide dynamic vulcanization. The effect of FKM loading on the morphology, mechanical properties, crystallization behavior, rheology and dynamic mechanical properties of the PVDF/SR/FKM ternary blends was investigated. A “network” was observed in the PVDF/SR binary blends, which disappeared in the ternary blends, but a core-shell-like structure was formed. The mechanical properties were significantly improved. The Izod impact strength of PVDF/SR/FKM blend with 19 wt% FKM was 18.3 kJ/m², which was 3–4 times higher than the PVDF/SR binary blend. The complex viscosity and storage modulus of the PVDF/SR/FKM blends decreased with increasing FKM content, hence the processability was improved. The increase of FKM content seemed to show a favorable effect on the crystallization of the PVDF component. It promoted the nucleation process of PVDF, leading to increased polymer crystallization rate and higher crystallization temperature. The glass-rubber transition temperature of the PVDF phase moved to a lower temperature.     

聚氯乙烯—低分子量聚异丁烯共混物的阻尼性质

聚氯乙烯(PVC)用作阻尼材料时多作成与高聚物的共混物,也有采用IPN的方法改善PVC的阻尼性能,前巳报导PVC-丁腈羟低聚物共混物有较佳的阻尼性质。本工作考察了PVC-PIB(聚异丁烯)组成及添加剂对共混物力学性能和动态力学性质的影响。     

Novel biodegradable poly(butylene succinate)/poly(ethylene oxide) blend film with compositional and spherulite‐size gradients

Biodegradable poly(butylene succinate) (PBS)/poly(ethylene oxide) (PEO) polymer blend film with compositional gradient in the film thickness direction was prepared using a method of interdiffusion across the interface between the PBS and PEO layers at a temperature above the melting points of both the component polymers. The miscibility between PBS and PEO was confirmed by observation of the glass transition temperature by differential scanning calorimetry. The compositional gradient structure of PBS/PEO was characterized by microscopic mapping measurement of Fourier transform infrared spectra and dynamic mechanical thermal analysis. Furthermore, a new method for confirming the crystalline/crystalline compositional gradient structure through observing the crystallization behavior by POM (polarized optical microscopy) was put forward. A continuous gradient of the spherulite size along the film thickness direction was succeessfully generated in the PBS/PEO blend film. The compositional gradient blend was found to have significantly improved physical properties that cannot be realized for pure PBS, pure PEO, and even their homogeneous miscible blend system. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 368–377, 2005     

Compatibilizing effect of functionalized polystyrene blends: a study of morphology,thermal, and mechanical properties

Polystyrene (PS), being an amorphous polymer is immiscible with other polymers. To engender miscible blends, PS has been functionalized with an active amino‐functional group on the molecular chains of PS to yield amino‐substituted polystyrene (APS), which serves as a reactive compatibilizer. The compatibilization effect of amino functionalized polystyrene on the rubber toughening was explored and results were compared in terms of morphology, thermal, and mechanical properties of PS/SEBS‐g‐MA versus APS/SEBS‐g‐MA blends. In addition, the effect of rubber content on the blend morphology and mechanical properties were investigated. An appreciable change in the thermal stability of APS blends in comparison with PS blend has been probed. A marked correlation has been observed between phase morphology and thermal stability. Use of APS produced the compatibilized blends which render improved blend morphology, enhanced thermal and mechanical properties. Optimal thermal, morphological and mechanical profiles were depicted by 20‐wt% APS blend. Copyright © 2008 John Wiley & Sons, Ltd.     

Thermodynamic characterization of polymer‐polymer interfaces

The properties of multiphase polymer blends are determined in part by the nature of the polymer‐polymer interface. The interfacial tension, γ, influences morphology development during melt mixing while interfacial thickness, λ, is related to the adhesion between the phases in the solid blend. A quantitative relation between the thermodynamic interaction energy and these interfacial properties was first proposed in the theory of Helfand and Tagami and has since been correlated with experimental measurements with varying degrees of success. This paper demonstrates that the theory and experiment can be unified for polymer pairs of some technological importance: copolymers of styrene and acrylonitrile (SAN) with poly (2, 6‐dimethyl‐1, 4‐phenylene oxide) (PPO) and with bisphenol‐A polycarbonate (PC). For each pair, the overall interaction energy was calculated using a mean‐field binary interaction model expressed in terms of the interactions between repeat unit pairs extracted from blend phase behavior. Predictions of γ and λ as a function of copolymer composition made by combining the binary interaction model with the Helfand‐Tagami theory compare favorably with experimental measurements.