DNA block copolymers: functional materials for nanoscience and biomedicine

We live in a world full of synthetic materials, and the development of new technologies builds on the design and synthesis of new chemical structures, such as polymers. Synthetic macromolecules have changed the world and currently play a major role in all aspects of daily life. Due to their tailorable properties, these materials have fueled the invention of new techniques and goods, from the yogurt cup to the car seat belts. To fulfill the requirements of modern life, polymers and their composites have become increasingly complex. One strategy for altering polymer properties is to combine different polymer segments within one polymer, known as block copolymers. The microphase separation of the individual polymer components and the resulting formation of well defined nanosized domains provide a broad range of new materials with various properties. Block copolymers facilitated the development of innovative concepts in the fields of drug delivery, nanomedicine, organic electronics, and nanoscience. Block copolymers consist exclusively of organic polymers, but researchers are increasingly interested in materials that combine synthetic materials and biomacromolecules. Although many researchers have explored the combination of proteins with organic polymers, far fewer investigations have explored nucleic acid/polymer hybrids, known as DNA block copolymers (DBCs). DNA as a polymer block provides several advantages over other biopolymers. The availability of automated synthesis offers DNA segments with nucleotide precision, which facilitates the fabrication of hybrid materials with monodisperse biopolymer blocks. The directed functionalization of modified single-stranded DNA by Watson-Crick base-pairing is another key feature of DNA block copolymers. Furthermore, the appropriate selection of DNA sequence and organic polymer gives control over the material properties and their self-assembly into supramolecular structures. The introduction of a hydrophobic polymer into DBCs in aqueous solution leads to amphiphilic micellar structures with a hydrophobic polymer core and a DNA corona. In this Account, we discuss selected examples of recent developments in the synthesis, structure manipulation and applications of DBCs. We present achievements in synthesis of DBCs and their amplification based on molecular biology techniques. We also focus on concepts involving supramolecular assemblies and the change of morphological properties by mild stimuli. Finally, we discuss future applications of DBCs. DBC micelles have served as drug-delivery vehicles, as scaffolds for chemical reactions, and as templates for the self-assembly of virus capsids. In nanoelectronics, DNA polymer hybrids can facilitate size selection and directed deposition of single-walled carbon nanotubes in field effect transistor (FET) devices.     

基于聚合物的一维光子晶体研究进展

从一维光子晶体组装材料角度出发,总结分析了基于聚合物的有机/有机型和有机/无机杂化型一维光子晶体的结构特点、组装原理和方法、性能及应用。在有机/有机型一维光子晶体中,主要介绍了含有不同亲疏水链段的嵌段共聚物和含有可聚合双键的表面活性剂自组装形成的一维光子晶体。在有机/无机杂化型一维光子晶体中,既论述了基于聚合物、无机材料直接交替组装形成的多层膜,也讨论了基于两种无机材料组装,然后再填充柔性聚合物的一维光子晶体。通过对以上两类光子晶体材料的总结分析可知,基于聚合物材料制备的一维光子晶体可以实现多种功能,在柔性传感器、柔性光电器件、光子晶体纸、电子皮肤、3D打印等方面具有良好的应用前景。但目前基于聚合物的一维光子晶体存在组装均匀性有待提高、组装面积较小等问题,如何大规模制备均匀的功能性一维光子晶体是重要的研究方向,也是影响其实际应用的关键。     

Theoretical modeling and simulations of self-assembly of copolymers in solution

Self-assembly of copolymers in solution is a promising way to prepare novel materials. An accurate control over the self-assembly of copolymers in solution requires a profound understanding about the related thermodynamic rules and kinetic mechanisms. Theoretical modeling and simulation play an increasingly important role in characterizing the structure details and the formation process of polymer assemblies. In this review, we first introduce theoretical modeling and simulation methods that have been applied to investigate the self-assembly of copolymers in solution, including particle-based methods, field-theoretical methods and hybrid modeling methods Then, the application of these methods for the self-assembly of linear block copolymers in solution is highlighted, including the thermodynamic rules and kinetic mechanisms underlying the formation of self-assembled structures. Furthermore, the simulation works of the self-assembly of branched copolymer systems, including graft copolymers, star-like copolymers, dendritic copolymers and bottle-brush copolymers, are addressed. In addition to the one-component polymer systems, simulation investigations of polymer mixture systems are discussed, both the polymer/polymer systems and polymer/nanoparticle systems are considered. Finally, perspectives on the theoretical modeling and simulation in the field of self-assembly of copolymers in solution are presented in the section of concluding remarks and outlook.     

Vinylidene fluoride- and trifluoroethylene-containing fluorinated electroactive copolymers. How does chemistry impact properties?

Fluoropolymers are attractive niche polymers used in high added value materials for high-tech applications in aerospace, electronics, coatings, membranes, cables, and the automotive industries. Among them, VDF- and TrFE-based copolymers exhibit remarkable electroactive properties allowing their incorporation into a wide range of devices such as printed memories, sensors, actuators, artificial muscles, and energy storage devices. In a first section, a detailed overview of semi-crystalline poly(VDF-co-TrFE) copolymers and of their ferroelectric (FE) properties from the point of view of polymer chemists is supplied. In addition to the polymer microstructure that may sometimes be controlled or influenced by the synthesis strategies, physical properties such as the phase transitions, and electroactivity are also affected by processing, such as annealing for example, and film thickness for example. Building on the conclusions and understanding obtained from the first section, the effect of the introduction of a termonomer (leading to poly(VDF-ter-TrFE-ter-M) terpolymers) is detailed in a second section of this review. Modifying the terpolymer chain microstructure has a major impact on the crystalline phase of the terpolymers that may result in a relaxor-ferroelectric behavior (RFE). The distribution of the termonomer along the polymer chain, the capacity of the termonomer units to enter the crystal lattice, as well as its dipole moment govern in large part the terpolymer electroactive properties. Poly(VDF-ter-TrFE-ter-CFE) and poly(VDF-ter-TrFE-ter-CTFE) terpolymers appeared to be the best candidates for RFE properties and were thus the most studied. In two following sections, the block or graft architectures of VDF- and TrFE- based copolymers, and the various crosslinking strategies used so far for such copolymers are described. Chemical modification is indeed a very powerful tool to tune electroactive properties of copolymers or to impart additional properties. Finally, in the last section, a few examples of emerging applications for these fluorinated electroactive polymers (EAPs) are briefly discussed. This review aims to provide a comprehensive report on the use of polymer chemistry as a tool to produce better electroactive fluorinated polymers, and highlights possible opportunities and perspectives for future progress in this field. Research in this interdisciplinary field requires different kinds of expertise, ranging from organic and polymer chemistries, polymer films engineering, physics of semi-crystalline polymers and electroactivity, to the design and fabrication of electronic devices.     

Directed self-assembly of block copolymers by chemical or topographical guiding patterns: Optimizing molecular architecture,thin-film properties,and kinetics

Patterning strategies based on directed self-assembly (DSA) of block copolymers, as one of the most appealing next-generation lithography techniques, have attracted abiding interest. DSA aims at fabricating defect-free geometrically simple patterns on large scales or irregular device-oriented structures. Successful application of DSA requires to control and optimize multiple process parameters related to the bulk morphology of the block copolymer, its interaction with the chemical or topographical guiding pattern, and the kinetics of structure formation. Most studies have focused on validating DSA patterning techniques using PS-b-PMMA block copolymers as a prototypical material. As the development of DSA techniques advances, recent efforts have been devoted to extending the materials selection in order to fabricate more complex geometric patterns or patterns with smaller characteristic dimensions. How to select appropriate polymer materials in a vast parameter space is a critical but also challenging step. In this review, we discuss recent progress in the research of DSA of block copolymers focusing on three aspects: (i) screening the block copolymer materials, (ii) controlling the film properties, and (iii) tailoring the phase separation kinetics.     

Photoresponsive liquid crystalline block copolymers: From photonics to nanotechnology

Being one of the most fascinating multi-functional materials, photoresponsive liquid crystalline block copolymers (PLCBCs) have attracted much attention because of their light controllable properties of supramolecularly self-assembled structures. These originate from their unique features combining the advanced function of photoresponsive liquid crystalline polymers (PLCPs) with the inherent property of microphase separation of block copolymers (BCs). Benefiting from recent progresses in materials chemistry, diverse PLCBCs have been designed and synthesized by controlled polymerization using different synthetic routes and strategies. Generally, PLCBCs show different performance depending on their self-organization and molecular composition, with the PLCP blocks in the minority phase or in the majority phase. One of the most important properties of PLCBCs is supramolecular cooperative motion, resulted from the interactions between liquid crystalline elastic deformation and microphase separation, which enables them to self-assemble into regularly ordered nanostructures in bulk films with high reliability. These nanostructures contribute to improving the optical performance of polymer films by eliminating the scattering of visible light, in favor of their photonic applications. With the help of liquid crystal alignment techniques, both parallel and perpendicular patterning of nanostructures has been fabricated in macroscopic scale with excellent reproducibility and mass production, which provides nanotemplates and nanofabrication processes for preparing varieties of nanomaterials. Recent findings about PLCBCs including their synthesis, diagram of microphase separation, structure-property relationship, precise control of nanostructure as well as their applications in photonics to nanotechnology are reviewed.     

Recent advances in stereocomplexation of enantiomeric PLA-based copolymers and applications

Poly(lactic acid) or poly(lactide) (PLA) is a biodegradable and biocompatible thermoplastic polymer, being derived from renewable resources such as corn and sugar cane. The building block of PLA, lactic acid is chiral and the polymerization of lactic acids (or lactides) leads to isotatic, syndiotatic and atactic/heterotactic PLA primary structures. The stereoselective interaction between two complementary enantiomeric PLLA and PDLA has led to enhanced physical properties such as mechanical properties, thermal resistance and hydrolytic stability compared with the parent polymers. Progress in controlled and/or living polymerization techniques combined with other new synthetic methodologies has facilitated the preparation of PLA-based copolymers with complex architectures such as diblock, triblock, multiblock, star-shape block, comb-shape block and various PLA-grafted structures. The utilization of stereocomplexation strategy to these newly developed copolymers has opened avenues to access a variety of new materials with unique characteristics, including novel chemical functionalities, bioactivities, and smart (responsive to external stimulus) properties tailored for specific applications. This review presents recent advancements in the synthesis of PLA-based block/graft copolymers having complex architectures, with emphasis on the enhanced material performances induced by PLA stereocomplex formation. The origin of the enhanced thermal mechanical property observed in PLA stereocomplex is first discussed. The strong interaction resulted from stereocomplexation in PLA based copolymers could be exploited not only for fabrication of advanced therapeutic delivery carriers and tissue engineering devices, but also for stabilizing colloidal systems in microparticles, micelles and hydrogels, that further broaden the applications of PLA that could not have been attained with single PLLA or its copolymers. The stereocomplexation could also be used to tailor the interface interactions between fillers and PLA matrix that lead to higher strength and toughness of PLA.     

含聚乙二醇的嵌段共聚物的合成及自组装研究进展

综述了通过活性自由基聚合如原子转移自由基聚合（ATRP）、氮氧稳定自由基聚合（NMP）、可逆加成断裂链转移聚合（RAFT）等方法合成含聚乙二醇（PEG）的嵌段共聚物的研究进展,并对含PEG类嵌段共聚物在溶液中的自组装技术和在药物载体、介孔材料以及碳纳米管中的应用进行了归纳,指出含PEG的嵌段共聚物可以自组装成多种形貌,直接影响材料的性能和应用,所以这些结构有潜在的应用价值和应用前景,并且合成新的含PEG的嵌段共聚物和开发具有新型结构、形貌可控的自组装体以及新的应用领域是今后的一个热点问题,具有重要的科学研究意义和实际应用价值。     

Synthesis and characterization of phenylene-thiophene all-conjugated diblock copolymers

Grignard metathesis (GRIM) polymerization for all-conjugated diblock copolymers comprising poly(2,5-dihexyloxy-1,4-phenylene) (PPP) and poly(3-hexylthiophene) (P3HT) blocks were systematically studied with LiCl as additive and 1,2-bis (diphenylphosphino) ethane nickel dichloride (Ni(dppe)Cl₂) or 1,3-bis(diphenylphosphino) propane nickel dichloride (Ni(dppp)Cl₂) as catalyst. It was found that the addition order of the monomers was crucial for the success of copolymerization. With the monomer addition in the order of phenyl and then thienyl Grignard reagents, all-conjugated PPP-b-P3HT diblock copolymers with different block ratios were successfully synthesized. In contrast, the inverted addition order only afforded a mixture containing both block copolymers and deactivated or end-capped homopolymers. Mass spectroscopic analysis indicates that the effect of the addition order of the monomers on copolymerization is attributed to the low efficiency of intramolecular Ni transfer from thiophene to phenylene units. The resulting PPP-b-P3HT diblock copolymers were characterized by differential scanning calorimetry (DSC) and atomic force microscopy (AFM). It was found that both PPP and P3HT blocks in the copolymers were crystalline, and microphase separation between them took place, as indicated by two endothermal transitions corresponding to the melting of PPP and P3HT blocks, respectively. These unique properties may render PPP-b-P3HT diblock copolymers potential applications in optoelectronics.     

Directed self-assembly of block copolymers on chemical patterns: A platform for nanofabrication

Directed self-assembly (DSA) of block copolymers (BCPs) on lithographically defined chemically nanopatterned surfaces (or chemical patterns) combines advantages of conventional photolithography and polymeric materials and shows promise for meeting a sufficiently inclusive set of manufacturing constraints for applications in semiconductors and data storage. DSA attracts attention from both academia and industry and tremendous progress has been achieved in the past decade. This review highlights the development of DSA with an emphasis on efforts toward the integration of block copolymer lithography into the current lithographic process for the fabrication of devices for integrated circuits and bit-patterned media.     

Thermal conductivity of polymer-based composites: Fundamentals and applications

Thermal management is critical to the performance, lifetime, and reliability of electronic devices. With the miniaturization, integration and functionalization of electronics and the emergence of new applications such as light emitting diodes, thermal dissipation becomes a challenging problem. Addressing this challenge requires the development of novel polymer-based composite materials with enhanced thermal conductivity. In this review, the fundamental design principles of highly thermally conductive composites were discussed. The key factors influencing the thermal conductivity of polymers, such as chain structure, crystallinity, crystal form, orientation of polymer chains, and orientation of ordered domains in both thermoplastics and thermosets were addressed. The properties of thermally conductive fillers (carbon nanotubes, metal particles, and ceramic particles such as boron nitride or aluminum oxide) are summarized at length. The dependence of thermal conductivity of composites on the filler loading, filler aggregate morphology and overall composite structure is also discussed. Special attention is paid to recent advances in controlling the microstructure of polymer composites to achieve high thermal conductivity (novel approaches to control filler orientation, special design of filler agglomerates, formation of continuous filler network by self-assembly process, double percolation approach, etc.). The review also summarizes some emerging applications of thermally conductive polymer composites. Finally, we outline the challenges and outlook for thermally conductive polymer composites.     

Nanostructured poly(3-hexylthiophene-2,5-diyl) films with tunable dimensions through self-assembly with polystyrene

Poly(3-hexylthiophene-2,5-diyl) is among the most widely used conjugated polymers for opto-electronic applications. To enhance its properties, researchers have attempted to nanostructure this polymer using various processes including breath figure arrays, nanolithography and elaborated organic synthesis. We here demonstrate a simple process to nanostructure the conjugated polymer using self-assembly with polystyrene and selective removal of one of the phases. The influence of the molecular weight of each polymer on the thin film morphology was systematically studied by atomic force microscopy. Using this approach, we observe two types of nanostructure, namely, nanoporous and nanoisland structures, of which the dimensions can be tuned by modifying the molecular weight of each polymer in the blend. This simple process introduces a cost-effective alternative to produce thin films of conjugated polymer with average nano-features from 100 nm up to 500 nm which could be used in a wide range of applications.     

Electronic structure and properties of alternating donor-acceptor conjugated copolymers: 3,4-Ethylenedioxythiophene (EDOT) copolymers and model compounds

The electronic structure and properties of 3,4-ethylenedioxythiophene (EDOT) based alternating donor-acceptor conjugated copolymers and their model compounds were studied by the density functional theory (DFT) at the B3LYP level with 6-31G or 6-31G** basis set. The acceptors investigated include thiazole (Z), thiadiazole (D), thienopyrazine (TP), thienothiadiazole (TD), thiadiazolothienopyrazine (TPD), quinoxaline (BP), benzothiadiazole (BD), pyrazinoquinoxaline (BPP), benzobisthiadiazole (BDD), and thiadiazoloquinoxaline (BDP). The torsional angle, intramolecular charge transfer, bridge bond length, and bond length alternation were analyzed and correlated with the electronic properties. It was found that the geometries of the donor-acceptor materials were significantly affected by the ring size and intramolecular charge transfer. The HOMO level, LUMO level, and band gap of the model compounds were well correlated with the acceptor strength. However, the electronic properties of the copolymers did not vary systematically with the acceptor strength due to the change in geometry from model compound to polymer. The aromatic geometry of EDOT-TP model compound is transformed to quinoid in the corresponding copolymer and results in a small band gap (E_g) of 0.97 eV. Large intramolecular charge transfer and the small bond length alternation in the EDOT-BDP copolymer resulted in an E_g of 0.7 eV. Hence, these two polymers could have potential applications for transparent conductors or photovoltaic devices. The small effective masses and large HOMO and LUMO bandwidths of PEDOT-TP and PEDOT-BDP make them potential materials for ambipolar thin film transistors. The theoretical results suggest that both the acceptor strength and the stable geometry contribute significantly to the electronic properties of alternating donor-acceptor conjugated copolymers.     

梯度共聚物的可控制备及其性能

梯度共聚物是近年来伴随着活性聚合方法而发展起来的一种新型共聚物,其特点在于单体单元组成沿着分子链方向逐渐变化,链结构界于常见的无规共聚物和嵌段共聚物之间。本文从梯度共聚物的结构特点入手,总结了其可控制备方法、表征手段、物化性质以及应用前景。基于共聚动力学模型控制单体加料速率的半连续活性/可控自由基聚合可实现梯度共聚物的结构定制,基于多步单体进料方式的RAFT乳液聚合则由于其简单和高效将成为梯度共聚物可控制备的重要方法。梯度共聚物的自组装行为和微观聚集态不同于嵌段共聚物,表现出独特的界面活性、热学特性和力学性能,组成梯度结构有望成为调控高分子材料性能的新参数,梯度共聚物有望在乳化剂、相相容剂、阻尼材料、多形状记忆材料等领域得到应用。     

有序介孔聚合物的研究进展

综述了以自组装法、硬模板法和软模板法合成有序介孔聚合物及介孔碳的研究进展。对上述3种制备方法及原理进行了比较,指出目前以嵌段共聚物进行自组装以及采用软模板法制备介孔聚合物的途径更有利于制备有序的介孔聚合物及介孔碳。讨论了采用自组装法及软模板法时,嵌段共聚物的种类、模板剂的类型、聚合物前躯体的结构等对所制备的介孔聚合物以及介孔碳的形貌、介孔结构、骨架结构以及介孔材料的物理化学性能的影响。指出目前在介孔聚合物以及介孔碳的研究中,主要问题是如何提高介孔聚合物的有序性以及其介孔结构的稳定性。最后对有序介孔聚合物及介孔碳的发展方向及应用领域进行了展望。     

Optimization of opto‐electronic property and device efficiency of polyfluorenes by tuning structure and morphology

Polyfluorene‐based oligomers and polymers (PFs) have been studied intensively as active materials for organic optoelectronic devices. In this review, the optimization of the opto‐electronic property and device efficiency of polyfluorenes in the field of light‐emitting diodes (LEDs) and photovoltaic cells (PVs) by tuning structure and morphology are summarized in terms of two typical modification techniques: copolymerization and blending. The relationships between molecular structures, thin film morphologies, opto‐electronic properties and device efficiencies are discussed, and some recent progress in LEDs and PVs is simultaneously reviewed. After the introduction, the basic knowledge of molecular structures and properties of polyfluorene homopolymers is presented as a background for a better understanding of their great potential for opto‐electronic applications. Immediately after this, three different opinions on the origin of low‐energy emission band at 520–540 nm in polyfluorene‐based LEDs are addressed. Rod–coil block copolymers and alternative copolymers are focused on in the next section, which are a vivid embodiment of controlling supramolecular structures and tailoring molecular structures, respectively. In particular, various supramolecular architectures induced by altering coil blocks are carefully discussed. Recent work that shows great improvement in opto‐electronic properties or device performance by blending or doping is also addressed. Additionally, the progress of understanding concerning the mechanisms of exciton dynamics is briefly referred to. Copyright © 2006 Society of Chemical Industry     

Conjugated rod-coil block copolymers: Synthesis, morphology, photophysical properties, and stimuli-responsive applications

Conjugated rod-coil block copolymers have been accorded great importance since they provide a powerful route towards supramolecular objects with novel architectures, functions and physical properties. This review summarizes recent progress on the synthesis, morphology, optoelectronic properties and applications of such block copolymers. The combination of the precise condensation and living polymerization through the grafting-from or grafting-onto methodology produce various architectures of conjugated rod-coil block copolymers, including rod-coil, coil-rod-coil, rod-coil-coil, and rod-coil-rod. In the following, the relationships between polymer morphologies and photophysical properties in different phases are reviewed, as classified by solution micelles, thin films or bulk samples, polymer brushes and electrospun nanofibers. The effects of the rod/coil ratio and polymer architecture on the morphology and optoelectronic properties are discussed. The control of nanosize domain and the aligned direction of conjugated rods are the key issues for enhancing the optoelectronic device performance. Moreover, novel multifunctional sensory materials based on combining the tunable photophysical properties of the π-conjugated rod and the stimuli-responsive coil are also highlighted. It is believed that conjugated rod-coil block copolymers could spark the future evolution of nanostructured polymers for multifunctional device applications.     

Poly(hydroxyether of bisphenol A) ‐alt‐polydimethylsiloxane: a novel thermally crosslinkable alternating block copolymer

BACKGROUND: An important strategy for making polymer materials with combined properties is to prepare block copolymers consisting of well‐defined blocks via facile approaches. RESULTS: Poly(hydroxyether of bisphenol A)‐block‐polydimethylsiloxane alternating block copolymers (PH‐alt‐PDMS) were synthesized via Mannich polycondensation involving phenolic hydroxyl‐terminated poly(hydroxyether of bisphenol A), diaminopropyl‐terminated polydimethylsiloxane and formaldehyde. The polymerization was carried out via the formation of benzoxazine ring linkages between poly(hydroxyether of bisphenol A) and polydimethylsiloxane blocks. Differential scanning calorimetry and small‐angle X‐ray scattering show that the alternating block copolymers are microphase‐separated. Compared to poly(hydroxyether of bisphenol A), the copolymers displayed enhanced surface hydrophobicity (dewettability). In addition, subsequent crosslinking can occur upon heating the copolymers to elevated temperatures owing to the existence of benzoxazine linkages in the microdomains of hard segments. CONCLUSION: PH‐alt‐PDMS alternating block copolymers were successfully obtained. The subsequent self‐crosslinking of the PH‐alt‐PDMS alternating block copolymers could lead to these polymer materials having potential applications. Copyright © 2008 Society of Chemical Industry     

两亲性嵌段共聚物的合成及其自组装行为

采用原子转移自由基聚合（ATRP）方法合成了两亲性嵌段共聚物PSt-b-PAA。用1H NMR和GPC等手段对活性聚合进行了确认,对嵌段共聚物的结构进行了表征。两亲性嵌段共聚物在离子液体1-丁基-3-甲基咪唑六氟磷酸盐（[BM IM][PF6]）中形成胶束溶液。用透射电子显微镜（TEM）观察聚合物在离子液体中形成胶束的纳米结构。当疏水链长固定时,胶束的自组装形状主要依赖于亲水链的长度。两亲性共聚物在离子液体中可自组装成可控制结构的纳米胶束,这种纳米胶束可应用在很多领域。