Pervaporation has been regarded as a promising separation technology in separating azeotropic mixtures, solutions with similar boiling points, thermally sensitive compounds, organic–organic mixtures as well as in removing dilute organics from aqueous solutions. As the pervaporation membrane is one of the crucial factors in determining the overall efficiency of the separation process, this article reviews the research and development (R&D) of polymeric pervaporation membranes from the perspective of membrane fabrication procedures and materials.
As an abundant and attractive element, the emergence of new carbon-based materials brings revolutionary development in material science and technology. Carbon-based materials have spawned considerable interest for fabricating polymer composites/nanocomposites with greatly improved mechanical, thermal, gas barrier, conductivity, and flame retardant performance. In this review, the importance of carbon-based materials and the necessity of fire resistance for polymeric materials are initially introduced. Then, the fundamental flame retardant mechanisms and experimental analytical techniques are described to understand the relationship between structures and flame retardant properties. The main section is dedicated to the preparation and properties of multifunctional polymer composites/nanocomposites with carbon-based materials, with special emphasis on the flame retardant properties of these materials. A wide variety of carbon-based materials are discussed for use in flame retardant polymer nanocomposite, including graphite, graphene, carbon nanotubes, fullerenes as well as some new emerging carbon forms (carbon nitride, carbon aerogels, etc). Finally, a brief outlook at the developments in carbon-based materials for flame retardant polymeric composites is given by discussing the major progress, opportunities, and challenges.
Reversible covalent polymers are able to change their bond arrangement and structure via reversible reaction triggered by external stimuli including heating, light and pH, while retaining the stability of irreversible covalent polymers in the absence of the stimuli. In recent years, more and more research has been devoted to utilization of reversible covalent bonds in synthesizing new materials, which not only overcomes disadvantages of permanent covalent polymers, but also brings in new functionalities. More importantly, a series of novel techniques dedicated to polymerized products with features such as properties regulation, self-healing, reprocessing, solid state recycling, and controllable degradation are developed, heralding the opportunity of upgrading of traditional polymer engineering. Although the exploration of this emerging topic is still in its infancy, the advances so far are encouraging and clearly directed to large scale applications. This review systematically outlines this promising trend, following a bottom-up strategy, taking into account both theoretical and experimental achievements. It mainly consists of four parts, involving design and preparation: (i) the basis of reversible covalent chemistry, (ii) rheology of reversible covalent polymers, (iii) methods of construction of reversible covalent polymers, and (iv) smart, adaptive properties offered by reversible covalent chemistry. The key elements for realizing reorganization of polymers containing reversible covalent bonds are covered. The advantages and weaknesses of representative reaction systems are analyzed, while the challenges and opportunities to engineering application of the equilibrium control based on reversible covalent chemistry for producing end-use polymers are summarized. In this way, the readers may grasp both the overall situation as well as insight into future work.
Contemporary environmental problems in industrial gas production and purification have driven researchers to search for green, elegant and sustainable technologies to resolve these issues. To this end, membrane technology is a promising and environmentally friendly alternative separation technique for mitigation of carbon dioxide (CO2) emissions in addition to gas purification for energy development (particularly from syngas, natural gas or flue gas streams). Nevertheless, traditional polymeric membranes have demonstrated insufficient capability for CO2 removal because the performance of these membranes is primarily controlled by the diffusion of various gases based on their molecular sizes. Most recently, poly(ethylene oxide) (PEO) membranes have garnered growing interest because their performance for CO2 removal can be elegantly controlled by the solubility of the different gases in the membranes. The PEO membranes have a high affinity towards CO2 and have demonstrated simplicity in membrane fabrication. However, drawbacks such as a high crystallization tendency and a weak mechanical strength have curtailed its industrial application. Various strategies have been considered to overcome these drawbacks using structural design of polymers via copolymerization with additional rigid repeating segments, crosslinking and physical blending with other polymers to produce ultra-permeable PEO-based membranes for CO2 separation. In this review, the state-of-the-art for PEO-containing membranes is evaluated alongside the benefits and shortcomings of various related methodologies. In addition, recent developments are reviewed in the fabrication of PEO-containing asymmetric and composite membranes with a thin separation layer. An assessment of the benefits and drawbacks of various approaches for fabrication of advanced PEO-containing membranes is highlighted, and future research directions in this field are also proposed.
Aqueous polyurethane dispersions (PUDs) have recently emerged as important alternatives to their solvent-based counterparts for various applications due to increasing health and environmental awareness. There are a number of important variables in the preparation of aqueous PUDs such as carboxylic acid content, solid content, degree of pre-/post-neutralization of the carboxylic acids and chain extension that all impact the dispersion particle sizes and distributions, viscosity, molecular weights, and glass transition temperatures of the PUDs and thin films made from them. This article reviews some new insights into the synthesis, characterization, structure evolution and kinetics, and rheological properties of representative examples of polyurethanes and POSS/polyurethane nanocomposites dispersions and films with prescribed rheological properties, macromolecular structure dynamics and function with the aim of understanding the complex relationships amongst the polymer structure, rheological properties, and performance of the PUDs and nanocomposite films under conditions that they are likely to encounter during use. It will be demonstrated that incorporation of small amounts of POSS into PU films can significantly enhance the thermal stability and mechanical properties, and present a new class of materials for special industrial applications. The unanswered questions are discussed to guide future research directions, and facilitate progress in this area so that the materials can be rationally engineered during synthesis and processing to yield new materials with enhanced properties for a number of applications. Overall, the present review article will provide a quantitative experimental basis for any future theory development of the relatively new waterborne PUDs and hybrid PU/POSS nanocomposites, and their structural dynamics, phase behavior, molecular relaxation, and rheological properties, increasing our level of understanding of the behavior of this important class of polymeric materials and other similar water soluble polymers.
This review provides a survey of nonlinear optical (NLO) chromophores and materials incorporating them which have been demonstrated over the last several years. Conventional polymeric materials, dendrimers, and other material design approaches are reviewed. Macroscopic nonlinear optical properties are introduced mainly in terms of second-harmonic generation (SHG) and the electro-optic (EO) effect. The temporal and thermal stability of nonlinear optical properties are also discussed. The review begins with a brief introduction explaining overall principles relating to the origin of second-order nonlinear optical properties. The structure of NLO materials, methods for their characterization, and structure–property relationships are also introduced. Much current research is aimed at optimizing microscopic nonlinearity in well-defined heterocyclic NLO chromophores which are then embedded in materials such as polymers, dendrimers, etc. Strong electron density donor and acceptor groups are connected through an efficient π-electron conjugative bridge to yield a highly electronically asymmetric and hyperpolarizable NLO chromophore. This review highlights the design and synthesis of recent chromophores that simultaneously exhibit large molecular hyperpolarizability, and low optical absorption at an operating wavelength, in addition to good processability and thermal/photochemical stability. Although NLO chromophores have been designed to exploit both dipolar and octupolar symmetry, the former has exhibited the strongest potential for the development of practical NLO devices. Organic materials based on polymers and dendrimers containing dipolar chromophores are mainly demonstrated for nonlinear optics. Experimental results corresponding to such dipolar compounds are mainly considered in this review. In addition, new strategies to improve thermal stabilities are also discussed herein.
Display omitted] 2,2,6,6-Tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation is a unique reaction to native and regenerated celluloses, and has advantages in terms of position-selective reaction at room temperature under aqueous conditions. When the TEMPO/NaBr/NaClO oxidation is applied to native celluloses in water at pH 10 under suitable conditions, the C6-primary hydroxy groups present on crystalline cellulose microfibril surfaces are mostly converted to sodium C6-carboxylate groups. Anionic sodium glucuronosyl units are densely, regularly, and position-selectively formed on crystalline cellulose microfibril surfaces, while maintaining the original cellulose morphology, cellulose I crystal structure, crystallinity, and crystal width. When TEMPO-oxidized celluloses (TOCs) prepared from, for example, wood cellulose have sodium C6-carboxylate contents >1 mmol/g, transparent highly viscous gels consisting of TEMPO-oxidized cellulose nanofibrils (TOCNs) with homogeneous widths of ≈3 nm and lengths >0.5 μm, dispersed at the individual nanofiber level, are obtained by gentle mechanical disintegration of TOCs in water. Alternative systems are as follows: TEMPO/NaClO/NaClO2 system, TEMPO electro-mediated oxidation, etc. TOCNs are promising new plant-based renewable nanofibers applicable to high-tech material fields.
Polythiophenes have long played a major role in the field of conducting polymers due to their relative ease of synthesis, good thermal and oxidative stability, high charge carrier mobility and ease of processing and they have found widespread use in electronic applications such as field-effect transistors (FETs), organic photovoltaics (OPVs), light-emitting diodes (LEDs) and electrochromic displays (ECDs). In this review, we summarize the most important synthetic approaches to thiophene-, thienothiophene- and other fused thiophene-based polymers, highlight a number of significant findings relating to their properties with an emphasis on device performance in organic field-effect transistors and reflect on existing challenges and future opportunities in the field.
Layered nanohybrids are heterostructured materials composed of two-dimensional inorganic host and intercalating inorganic-, organic-, bio-, or polymer guests. Such materials have been extensively explored to create new multifunctional hybrid systems that integrate nanotechnology (NT), biotechnology (BT), information technology (IT), and even congnitive technology (CT). In this review, an attempt is made to classify and highlight recent advances in multifunctional nanohybrids based on layered materials and their related application systems; (i) red nanohybrid on life science and health-care sectors, (ii) white nanohybrid on energy and environmental ones, (iii) green nanohybrid on agriculture and food ones, and (iv) blue hybrid on aqua and marine ones. In details, the structural features and functions of the layered nanomaterials and their hybrid systems are discussed in each section.
Scattering methods based on spatial and temporal contrast fluctuations in polymer-network gels, which originate from polymer-segmental density fluctuations, reveal rich insight into different types and levels of nanostructural inhomogeneity in these soft materials. Complementary contrasting as provided by light, neutron, and X-ray scattering allows such information to be obtained on nano- to micrometer length scales. On top of that, complementary use of static and dynamic scattering methods allows the interplay and effect of these inhomogeneities to be unraveled. This article interrelates a multitude of studies on the application of scattering techniques for analytical assessment of structural inhomogeneity in polymer-network gels conducted since the 1970s.
Hydrogels are important materials for a variety of applications, particularly biomedical devices, but they generally have poor mechanical properties since they consist predominantly of water held in place by a relatively fragile polymer network. This brief review describes a few novel methods to control or improve the mechanical properties of hydrogels including slide-ring gels, double-network gels, nanocomposite gels, and photoactive gels. Our goal is to encourage more researchers to be aware of and to exploit these methods.
Over the last three decades, conductive polymers have been investigated extensively due to their wide application potentials. Among the conducting polymers, polyaniline (PANI) has been of great interest to many researchers because of its reasonably good conductivity, stability, easy preparation, affordability and redox properties. Even though they have potential application in several fields, their utility is restricted by poor processability. Efforts are being taken to improve the processability of PANI. Emulsion polymerization pathway is identified as one of the ways to improve the processability of the PANI. In this review, we discuss the synthesis of PANI, substituted PANI, copolymers of PANI by the emulsion/inverted emulsion polymerization pathway. In another part of this review, the synthesis of PANI blend and composite with other commodity polymers is described. Preparation of PANI films by solution and thermal processing methods is described. Chemical, physical, and structural aspects of the obtained material via emulsion/inverted emulsion polymerization pathway are also discussed. The present status and future work of PANI materials by emulsion polymerization pathway are brought out in this review.
Linear, flexible macromolecules in the semicrystalline state have long been recognized as being globally metastable and divided into microphases and nanophases with strong, covalent bonds crossing the phase boundaries. The different phases can be crystals, mesophases, liquids, and glasses. The glasses may have structures which correspond to liquids or mesophases and can exist even above the glass transition temperature of the mobile macrophase as rigid–amorphous fractions. This multilevel structure causes rather unique properties which vary with the thermal and mechanical history of the materials. Temperature-modulated calorimetry and related techniques which can separate equilibrium and nonequilibrium responses are ideal for the analysis of such structures. The techniques of thermal analysis needed to separate reversible from irreversible processes is described, and the transitions of the rigid–amorphous phase and the major reversible processes involving latent heats is discussed on hand of the literature. As baselines for this discussion, the vibrational heat capacity of crystals and glasses of the ATHAS Data Bank will be used.
Display omitted] Bioprinting offers a highly-automated and advanced manufacturing platform that facilitates the deposition of bio-inks (living cells, biomaterials and growth factors) in a scalable and reproducible manner, a process that is lacking in conventional tissue engineering approaches. Significant improvements in the field of bioprinting have occurred over the last two decades. This reviews provides an in-depth analysis of recent improvements in the bioprinting techniques, progress in bio-ink development, implementation of new bioprinting and tissue maturation strategies. Special attention is givent to the role of polymer science and how it complements 3D bioprinting to overcome some of the major impediments in the field of organ printing. A concise overview of the anatomy and physiology of different tissues/organs is provided, followed by important design considerations to better facilitate the fabrication of biomimetic tissues/organs for tissue engineering and regenerative medicine (TERM). Last, a realistic overview of current status in organ bioprinting is presented, including recent accomplishments in bioprinting tissue-engineered constructs, the limitations and challenges, as well as opportunities for future research. We strongly believe that with the advances in polymer sciences, it will be an impending reality for on-demand bioprinting of patient-specific tissues/organs.
Display omitted] Mixtures of neutral polymers and lithium salts have the potential to serve as electrolytes in next-generation rechargeable Li-ion batteries. The purpose of this review is to expose the delicate interplay between polymer-salt interactions at the segmental level and macroscopic ion transport at the battery level. Since complete characterization of this interplay has only been completed in one system: mixtures of poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI), we focus on data obtained from this system. We begin with a discussion of the activity coefficient, followed by a discussion of six different diffusion coefficients: the Rouse motion of polymer segments is quantified by Dseg, the self-diffusion of cations and anions is quantified by Dself,+ and Dself,−, and the build-up of concentration gradients in electrolytes under an applied potential is quantified by Stefan-Maxwell diffusion coefficients, D0+, D0-, and D+-. The Stefan-Maxwell diffusion coefficients can be used to predict the velocities of the ions at very early times after an electric field is applied across the electrolyte. The surprising result is that D0- is negative in certain concentration windows. A consequence of this finding is that at these concentrations, both cations and anions are predicted to migrate toward the positive electrode at early times. We describe the controversies that surround this result. Knowledge of the Stefan-Maxwell diffusion coefficients enable prediction of the limiting current. We argue that the limiting current is the most important characteristic of an electrolyte. Excellent agreement between theoretical and experimental limiting current is seen in PEO/LiTFSI mixtures. What sequence of monomers that, when polymerized, will lead to the highest limiting current remains an important unanswered question. It is our hope that the approach presented in this review will guide the development of such polymers.
Display omitted] Protein-polymer conjugates are complex molecules with major societal implications. Many advances in the fields of medicine, biotechnology and nanotechnology have been associated with the development of these bioconjugates. Synthetic polymers are usually attached to proteins in order to change or enhance their native properties. Polymer-enhanced biomacromolecular activity, specificity and stability are of particular interest. The most visible impact of coupling polymers to proteins has been on therapeutic proteins. There is a rich history and literature which describes how such polymers increase protein lifetimes in vivo and mask the protein from circulating antibodies and immune cells. Although the attachment of polymers to therapeutic proteins has these benefits, it often comes at an unpredictable cost of reduced functionality. Several studies have shown modification decreases the bioactivity of therapeutic proteins. In this review, we explore different synthetic approaches to protein-polymer conjugation and whether more rational and controlled attachment chemistries can reveal how to create molecular sieves around a protein without sacrificing activity. Research has begun to reveal the influence of polymer molar mass, number of attached polymers, synthetic approach, and polymer architecture on molecular sieving properties. Although rational approaches to polymer-based protein engineering are relatively new additions to the arsenal of approaches to the design of protein-displayed molecular sieves, general trends are emerging that will help guide future research in the field.
The design and synthesis of new materials are two key steps in the advancement of technology. One of the most promising approaches of development of new materials that combine advantages of organic polymers with those of inorganic solids is to devise polymers that have a backbone of inorganic atoms to which are attached organic side groups. Among the best developed examples of ‘inorganic–organic polymers’ are organosilicon polymers. The synthesis of new organosilicon polymers using silyl triflate intermediates is reviewed in this article. Protodephenylation of phenylated polysilanes as well as poly(silylenemethylenes) by triflic acid gave new functionalized compounds. Network polymers were obtained by reductive coupling of silyl triflates with potassium–graphite. Novel poly(silylenealkynes) and poly(silylenearylenes) containing a regular alternating arrangement of silylene groups and organic units were prepared from α,ω-bis[(trifluoromethyl)sulfonyloxysilyl]-substituted compounds and dinucleophiles. Some of the polymers are potential organic precursor for ceramic materials. The correlation between structure and thermolytic behavior is demonstrated on selected examples.