This article is cited by 5 publications. Yingying Zhao, Seth S.
Journal of the European Ceramic Society , 39 4 , DOI: Synthesis of polypyrrole nanoparticles and their applications in electrically conductive adhesives for improving conductivity. RSC Advances , 7 84 , Synthesis, helical conformation, and infrared emissivity property study of optically active substituted polyacetylenes derived from serine.
Journal of Materials Science , 49 , Synthesis and surfactochromicity of 1,4-diketopyrrolo[3,4-c]pyrrole DPP -based anionic conjugated polyelectrolytes. Would you like to change to the United States site? Yoshiki Chujo Editor. Undetected location. NO YES. Conjugated Polymer Synthesis: Methods and Reactions. Selected type: Hardcover. Added to Your Shopping Cart. However, better methods are needed to characterize the distribution of molecular lengths between cross-links in networks produced under polymer-processing conditions typical of industrial practice.
Through use of methods such as neutron spin-echo spectrometry Mesei, , experiments have provided the most thorough test to date of whether the details of the reptation model are correct Richter et al. Quasi-elastic neutron scattering has extended the range of dynamic studies achievable by light scattering to smaller dimensions, utilizing the advantages of isotopic labeling.
These advantages are somewhat offset by the low neutron fluxes available, leading to long data acquisition times. The neutron scattering from polymer mixtures depends on fluctuations from uniformity. Such concentration fluctuations require work against osmotic forces, and so their size relates to the osmotic compressibility. Thus, neutron-scattering measurements can provide thermodynamic information. While such techniques have long been applied to the molecular characterization of dilute polymer solutions, recent developments in neutron scattering using deuterium-labeled molecules have permitted their extension to concentrated solutions, amorphous blends, and melts.
The excess scattering occurring as phase separation is approached serves to characterize critical phenomena and has proved useful for studying spinodal decomposition of polymer mixtures. Scattering techniques have grown in use because of developments of intense radiation sources lasers, synchrotrons , of efficient area detectors optical multi-channel analyzers, CCDs , and the employment of computers for rapid data analysis.
Faster X-ray detectors are needed for efficient use of the high fluxes available with synchrotrons. Currently, available neutron fluxes are too low to permit dynamic studies, so that the prospect of a high-flux advanced neutron source is a welcome possibility. The case for an advanced neutron source is reinforced by the emergence of neutron reflectometry as a primary tool for characterizing polymer surfaces and interfaces see below , because our aging reactor neutron sources are nearing the end of the time when they can be operated safely to give even-current fluxes of neutrons.
Synchrotron X-ray and neutron-scattering techniques.
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Structural characterization of the polymer solid state has advanced rapidly, driven partly by the need to understand the complicated structures present in semicrystalline polymers and partly by the desire to characterize the morphology of multiphase polymer blends. Traditional thermal analysis techniques, such as differential scanning calorimetry, have been combined with powerful synchrotron X-ray sources to allow simultaneous structure and calorimetric measurements in a time-resolved fashion. The structural transformation giving rise to a thermal effect thus can be unambiguously determined.
Many new microscopy techniques, developed in the last decade, are just beginning to be exploited by polymer scientists. For example, it is difficult to observe thick specimens owing to the limited depth of field of conventional optical microscopes. By focusing a laser beam on a particular point on the sample and scanning, an "in focus" three-dimensional image can be constructed and an image of a section at a given depth can be generated.
Resolution can be enhanced by using fluorescent dyes that require a two-photon process for emission. Near-field scanning optical microscopy NSOM is another new technique that has found important applications in biology and that could be very useful for high-resolution optical microscopy of polymers.
An evanescent optical wave field from the tip of an optical fiber, as small as 12 nm in diameter and much smaller than the wavelength of the light, is coupled to the surface of the sample, exciting fluorescence or scattering. As the tip is scanned over the surface of the sample, the image thus produced has a resolution much better than that of conventional optical microscopes, but without the problems such as radiation damage and vacuum associated with electron microscopies. New environmental scanning electron microscopes SEMs , which can operate in a partial pressure of water vapor, can eliminate the necessity for coating samples with metal to prevent charging artifacts, thus achieving superior resolution.
Solvent as well as water effects on the mechanical properties can be studied in situ. For the ultimate in resolution, the transmission electron microscope TEM is necessary, but until recently TEM images of polymers were restricted to relatively low magnification because of radiation damage effects. Recently, however, imaging of molecular structures in radiation-resistant polymers has been demonstrated by using TEM image-processing methods.
Biologists have made use of electron tomography the electron equivalent of the X-ray CAT scan to reconstruct. A number of polymer problems would benefit from a similar approach. The ability to generate high-resolution chemical maps of the polymer structure in blends would be a breakthrough. Current scanning transmission electron microscopic methods, which employ energy-dispersive X-ray analysis or electron energy loss spectroscopy, are not suitable for most polymers. New chemical imaging electron microscopes, which permit recording of images created with electrons with a well-defined energy loss, may revolutionize this field.
By digitally subtracting two images, one just below and one just above the absorption edge of a particular element, one can generate a chemical map of that element, while minimizing radiation damage. A related technique for chemical mapping is soft X-ray microscopy. Tunable soft X-ray sources are available at synchrotron radiation facilities, and the necessary focusing X-ray optics are now becoming available; already, spatial resolutions of approximately 50 nm have been demonstrated.
For most of the applications of solid polymers, mechanical properties are of primary importance. While there have been advances in characterizing and understanding these properties, there are many areas where improvements can be made. Techniques to characterize the mechanical properties of small samples of polymers are needed. Most conventional mechanical testing requires very large samples, for example, compact tension samples for fracture toughness measurements, tensile "dogbone" bars, and so on. To evaluate a new polymer or polymer blend, one would like to use gram g batches synthesized by a polymer chemist rather than scaling up to produce 10 kilograms kg in a pilot plant.
Nevertheless, with experience it is possible to extrapolate from measurements on films to bulk samples. Thin film testing has the natural advantage that samples can be examined by both optical microscopy and transmission electron microscopy. Another area where improvements can be made rapidly is in fracture testing.
The fracture behavior of interfaces and thin polymer layers embedded between two tough, transparent polymer slabs may be measured by using nothing more than a razor blade and an optical microscope. Further development to allow opaque polymers to be tested would be of great benefit. While there has been considerable progress in the past several decades in our fundamental understanding of the deformation and fracture of solid polymers, particularly in the area of crazing and fracture, much more improvement is possible.
The microscopic mechanisms of shear deformation are little known,. The addition of rubber particles is the usual way of producing tough polymeric materials, yet the microscopic fundamentals of rubber toughening are not yet fully understood. Experimental techniques to probe the microscopic nature of the deformation and fracture of such systems, preferably in a time-resolved fashion at typical impact strain rates, are needed.
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Surfaces and interfaces present challenges that are distinctly different from those presented by bulk, three-dimensional polymeric materials. The surface or interface introduces new characteristics unique to its quasi-two-dimensional character. They require special techniques that sample only the layer of interest near the top surface or at the buried interface.
New depth-profiling techniques, which measure some attribute of the polymer as a function of depth from the outer surface, are advancing this field rapidly. While some well-established techniques, such as angle-resolved X-ray photoelectron spectrometry XPS , can sense the local chemistry near the surface, others, such as the ion spectrometries, forward recoil spectrometry FRES , nuclear reaction analysis NRA , and secondary ion mass spectrometry SIMS , as well as neutron reflectivity NR and attenuated total reflection FTIR Fourier transform infrared spectroscopy, can find the depth profile of deuterium-labeled polymers, with depth resolutions usually well below nm.
These depth-profiling techniques have been used recently to characterize short-range diffusion of polymers across interfaces, the enrichment at the surface of one component in a miscible polymer blend, the depletion of polymers near a solution-solid interface, and the segregation of block copolymer surfactants to the interface between two immiscible polymers, to name only a few applications. It should be realized that the techniques have complementary capabilities, with some, such as NR, providing the best resolution of sharp features of the profile and others, such as FRES, enabling a better picture to be drawn of their long-range and integrated features.
While depth profiling is discovering aspects of the structure of polymer surfaces and interfaces never before accessible, it is also likely to have a major impact on our understanding of polymer fracture. Depth profiling the two surfaces produced by fracture has made it possible to determine the locus of fracture precisely with respect to particular deuterium-labeled polymers a deuterium-labeled block of a diblock copolymer and to derive great insight with regard to mechanism of fracture from this information.
Such depth-profiling measurements, however, usually require flat samples, which are stratified in depth; making such measurements on samples with geometries such as cylinders fibers and spheres particles is normally not possible. In addition, all of these techniques have much more limited resolution laterally,. These lateral resolutions range from centimeters to, at best, micrometers.
There is a need for interface analysis techniques that have both good depth and good lateral resolution and that can analyze curved interfaces. Because the different depth-profiling techniques have complementary capabilities and no one technique is usually adequate to provide the level of resolution, sensitivity, and quantitation desired, it is important that researchers in this field have access to a wide variety of depth-profiling instrumentation.
Facilities for SIMS and ion beam analysis, while common in semiconductor research, are not widely available to researchers interested in polymers. Some effort, perhaps on a regional basis and involving both universities and industry, should be made to ensure that this instrumentation is accessible to polymer scientists. The characterization of interfacial properties has also advanced dramatically in the past decade. The surface forces apparatus SFA has become a standard method for measuring the normal forces between layers of polymer adsorbed in solution on solid substrates.
Such measurements are important for understanding the steric stabilization of colloidal suspensions by polymers as well as for determining the thermodynamics of polymer adsorption from solution. For example, such measurements have led to an understanding of the stretching of polymer chains normal to interfaces between a solid and the solution as a function of their areal density.
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While the original SFA required the use of cleaved mica surfaces, recent workers have lifted this restriction, and a much wider variety of solid surfaces is now available. Shear forces between surfaces, as well as normal forces, can now be measured by some SFA devices, enabling the study of the fundamentals of polymer effects on lubrication.
The SFA remains an apparatus that demands considerable skill and experience on the part of the experimenter, however; efforts to simplify the apparatus by improving the drive and mechanisms for detecting distance are under way and should be encouraged. The SFA has also been used in the last few years to measure interfacial energies and adhesion; by measuring the area of contact between the polymer-coated mica cylinders as a function of the applied normal force, one can determine the reversible work of adhesion.
An elegant variant of this method has been developed recently using a elastomer sphere pressed into contact with a elastomer flat. The surface of the elastomer can be modified—for example, one can form a thin silicon dioxide layer on a siloxane elastomer by treatment with an oxygen plasma—and highly organized organic films can be self-assembled on such modified surfaces. The work of adhesion between a wide variety of surfaces can thus be measured with little more than an optical microscope and an analytical balance.
More conventional ways of measuring surface properties include the determination of contact angles of fluids on polymer surfaces and the measurement of the shapes of polymer melt and solution droplets, under conditions where these shapes are modified by the action of gravity or centrifugal force. Here the major. It is particularly difficult to measure the interfacial tension between phases in a polymer melt. One promising approach involves measuring the retraction and break-up into droplets of a fiber of one phase in a matrix of another, but more attention should be devoted to this problem because the interfacial tension helps determine the morphology of melt-processed polymer multiphase blends.
The atomic force microscope AFM has already had an impact on the measurement of the surface topology of polymers. It seems likely, however, that the AFM can be suitably modified to allow measurement of very local surface properties as well as mechanical properties of polymers. One example is the lateral force microscope LFM , an AFM modified so it can measure lateral forces as well as normal forces.
With the LFM, it is possible to reveal the morphology of a phase-separated blend of hydrocarbon elastomers simply by scanning a microtomed surface. Another, more speculative, possibility would be the chemically sensitive AFM. In such an instrument, the tip would be modified to expose an outer surface of hydrogen bonding groups. By operating such an AFM in a "tapping" mode, it might be possible to distinguish hydrogen bonding regions of the surface from nonhydrogen bonding ones. Such further developments of the AFM and related instruments should have a large payoff in polymer surface research.
Biopolymers require many techniques other than those used for synthetic polymers. To characterize a biopolymer, the first steps are to purify and produce the material in quantity and learn its molecular sequence. The main methods for sequencing and purification include chromatography, gel electrophoresis, centrifugation, and dialysis. They will continue to play important roles.
There are also new methods that promise to revolutionize how we obtain, purify, and understand the message encoded in biopolymer sequences. Capillary zone electrophoresis separates materials with very high resolution and works with extremely small volumes of material in some cases, even the volume of a single biological cell! Mass spectrometry, particularly in conjunction with electrospray and matrix-assisted laser desorption methods, can determine the sequences of informational polymers very rapidly and can detect subtle aspects of biologically important sequences.
At present, there are limitations on the sizes of molecules that can be studied in this way, but the maximum size is growing as the technology evolves. Because most molecules in the cell are in limited supply, this technology now opens the possibility of fishing out even the rarest of molecules and producing the appropriate DNA, RNA, or protein sequence in. Major advances in identifying and characterizing new proteins and nucleic acids will result from new electrophoretic techniques with enhanced resolution and reproducibility. Having determined the sequence of a biopolymer, computer database search methods now make it possible to search through the enormous number of known sequences and structures to make educated guesses about the biopolymer's possible structure and function.
Because sequencing and purification methods are developing rapidly, the sizes of the databases are growing exponentially. The sophistication of computer search methods is keeping pace with this information explosion. Once a sufficient quantity of a pure biopolymer is available, the next step is to learn its atomic structure. The main methods for determining biopolymer structures are X-ray crystallography, NMR spectroscopic methods, electron microscopy, and scanning tunneling microscopy.
The highest-resolution structures have been obtained by X-ray crystallography. Because crystallography requires that the molecules be crystallized, which is a poorly understood process, obtaining new structures by crystallography can be slow. A further bottleneck in obtaining crystal structures is the so-called "phase problem," which results from the unavoidable loss of information in recording X-ray intensities rather than amplitudes and makes it difficult to reconstruct the molecular structure from the diffraction data.
While this problem is most severe for the large molecules typical of biopolymers, new approaches and solutions continue to emerge. Multiple wavelength anomalous diffraction, a technique requiring a synchrotron source, provides one solution to the phase problem. A most exciting new technology is the Laue diffraction method for time-resolved X-ray crystallography. Preliminary successes indicate that this technique may allow us to watch the dynamic processes, step-by-step, in chemical reactions and physical changes in biomolecules.
Another major development has been the emergence of multidimensional NMR for obtaining high-resolution structures of biomolecules in solution. An advantage over crystallography is that the NMR experiment does not involve crystallization or the phase problem. However, NMR is currently limited to studying smaller molecules than X-ray crystallography, although this situation is rapidly improving.
NMR imaging of supermolecular biopolymer structures would offer a tool with almost limitless possibilities. Major advances could result from the ability to label biomolecules cheaply. Carbon NMR spectroscopy of proteins is extremely powerful. However, its use is limited owing to the present expense of preparing the required carbonenriched samples. Scanning tunneling microscopy is another new technology with preliminary successes and much promise for biomolecular structures.
How do biopolymer structures arise from interatomic forces? A major goal in characterization is to explain biomolecular structure using fundamental chemical principles. The "folding problem" is to understand how the monomer sequence encodes the conformation of the biopolymer. The fact that molecular "chaperones" are needed to obtain the correct folded structure for some proteins complicates the issue even further. Major efforts are going into developing theoretical models and methods of computational chemistry and statistical mechanics to allow one to predict the folding.
The two main problems are the need for better interatomic and intermolecular potential functions, particularly for modeling aqueous solutions, and faster conformational search strategies, which will also rely on new developments in massively parallel computer hardware and software, in order to find conformations of low free energy. Experimental advances have been occurring rapidly in several areas. Mutation experiments replace one or more monomers, often through genetic engineering, and explore the consequences of the small changes on structure or function of a biopolymer.
Unnatural monomers are synthesized chemically to create mimics of biomolecules or monomer replacements. Hydrogen exchange NMR methods have recently begun to allow unprecedented access to information about the local motions and internal structure. Small-molecule models and high-resolution calorimetry are helping to understand the driving forces. Direct synthesis and redesign of proteins are now becoming feasible, for increased stability and new applications of proteins as catalysts, and in information and energy transduction and storage.
National facilities are important for polymer characterization. These facilities, producing neutron and synchrotron radiation, are employed for diverse purposes, and continuing support and improvement must be based on the sum of the benefits and costs. Improved characterization methods for structures between the monomeric and macroscopic levels are needed, for solutions, surfaces, solid-state polymers, and insoluble polymeric materials.
Methods and instrumentation need to be developed. Polymer characterization has prospered from the collaborative interaction of various disciplines with the common goal of developing characterization tools. While many techniques have been specially developed for polymers, many were imported into polymer science after being developed for other purposes.
Given the extent to which polymer characterization has been advanced by techniques developed first in other fields, the education of polymer scientists and engineers must be broad enough that they will be able to capitalize on similar. At the same time, increased contact between polymer workers and those in outside fields will ensure that new techniques developed within polymers will be rapidly exploited in those outside fields where they are useful. Enormous opportunities and new needs for polymeric materials exist in the rapidly expanding and internationally competitive high-technology areas, such as the information industry, aerospace industry, and biotechnology.
Realizing these opportunities and meeting the needs for novel materials, however, depend on a much deeper understanding of polymeric materials than has been necessary for developing commodity polymers in the traditional chemical industry. Theory and computations can help provide this deeper understanding. They reduce large collections of experimental observations to working knowledge, rules, patterns, models, and general understanding. They explain experimental observations; they correlate data from different materials and phenomena; and, in general, they quantify and unify our knowledge.
More importantly, theory provides new predictive capabilities to guide the development of new ideas and to direct experimental efforts in exploring new chemical structures, processes, and physical properties. In favorable cases, theory and simulation can reduce the amount of experimental work required or even eliminate it entirely.
For example, one of the classical theoretical problems is to predict ''phase diagrams," diagrams that describe the resulting states of matter and the conditions when polymers and solvents are mixed together. This is a central problem in the design of new materials. Modern polymeric materials often involve mixtures of several different types of polymers, of different molecular weights, and in complex solvents at different temperatures and pressures. They involve many variables. Such variables determine the difference between achieving a successful material with appropriate optical, thermal, mechanical, and chemical stability properties or producing a useless mixture.
To find optimal conditions could require exploring 5 or 10 variables in detail, which could require tens of person-years of experiments. Instead, theoretical or computational models often permit the exploration of many different variables quickly, and thus they reduce the amount of time to develop new materials by an enormous factor.
Another example of the use of theory and simulation is in the area of polymer processing. Many companies currently model 1 kinetics of polymerization reactions and 2 fabrication, combining phenomenological rheological and heat transfer characterization of the polymer, to assess a proposed design before cutting a mold or die. Extensive efforts have been devoted to modeling fabrication operations such as extrusion, mixing, and molding operations.
Once a process configuration has been selected, computer-based models are generally able to. More sophisticated versions are able to predict certain product qualities, such as molecular weight averages, copolymer composition, and branching frequencies. They can sometimes also evaluate questions of process safety. Additional modeling information is given in Chapter 3 in the section "Polymer Processing. While even rudimentary theories are often important sources of guidance, the ultimate power of theoretical and computational models depends on reducing their assumptions and approximations by building deeper and more fundamental models.
Theory and computer modeling will play increasingly valuable roles in the development of new advanced materials. The strong interdependence of experiment and theory in polymer science is well illustrated by the Nobel Prizes in polymer theory awarded in to Paul J. Flory in chemistry and in to Pierre-Gilles de Gennes in physics.
In certain areas of polymer theory, there has been enormous progress in the s and s. Some topics have been so thoroughly explored that they are currently receiving little attention, while other areas have moved to the forefront of theoretical activity. Further shifts in the centers of interest will undoubtedly be propelled by advances in theoretical methods, experimental developments, societal needs, and the phenomenal increase in computer performance and algorithm development.
Theory and computation have developed into two separate disciplines, although they are often intertwined to varying degrees. Theory is associated with the construction of physical and mathematical models of the system, with attempts to solve basic equations describing the properties of the system, and with predictions for the outcomes of experiments, including those that have not yet been done. Computation represents an application of theory, often with the aid of large-scale computers, either to compute the desired properties for a system of interest or to simulate the behavior of the system at a level of detail unavailable experimentally.
These simulations are designed for furthering our understanding to test current theories or to provide data necessary for developing new and improved theories. Great advances in theory and in computational resources have resulted in the development of a scientific computation industry, which constructs large-scale computer codes that are increasingly being used by scientists and engineers in industry and universities, in addition to the home-grown codes, which are in a constant state of renewal and development.
It must be stressed, however, that these computer packages are no better than their underlying theories and the input data employed. Thus, advances in theoretical methodologies, computer technology, and the acquisition and codification of relevant experimental data will play essential roles in improving the capabilities of these computer packages.
Theory and computation are applied on several levels, ranging from the microscopic level, involving a detailed description of the molecular constituents, to a macroscopic level, involving continuum mechanics or thermodynamics, to. Intermediate between the microscopic and macroscopic descriptions lies the mesoscopic world characterized by lengths long on a microscopic scale but short on a macroscopic one.
Each level generates its own conceptual, theoretical, and computational challenges, but perhaps the most significant challenge lies in providing a bridge between the different levels. Thus, a goal of the complete molecular modeling of materials involves use of molecular theories to compute the property information necessary to describe the processing or performance behavior of the new bulk materials. Because many theories of polymers in concentrated solutions and bulk rely on single-chain concepts developed and tested in dilute solutions, the understanding of dilute polymer solutions has repercussions throughout polymer science.
Hence, dilute solutions of flexible polymers have received much attention in the past 50 years, and as a result many aspects of the average conformation of an isolated polymer molecule are now well understood. Nevertheless, many problems remain, especially for solutions of stiff polymers and their hydrodynamic properties. As the solution concentration increases, these theoretical problems become even more complex, and these properties are less well understood. Flexible polymer molecules in the undiluted amorphous state generally assume unperturbed random-coil configurations. This fundamental conclusion is derived from both theoretical and experimental studies on very flexible and non-polar polymers.
However, many technologically important polymers normally contain considerable numbers of polar atoms or groups and exhibit considerably reduced chain flexibility. Simulations and theoretical studies are needed, therefore, of the local correlations among such molecules in the amorphous state to guide and interpret suitable experimental studies. An amorphous polymer is a glass below its glass transition temperature, T g. Such glasses are never in thermodynamic equilibrium, and consequently their properties depend on their thermal and mechanical histories.
Although significant efforts have been made recently in both theoretical and simulation studies of the glass transition, and new computational advances have opened the way to simulation of the properties of the glassy state, much more effort is needed to explain the underlying cause of the glass transition,. Above T g , thermal energy produces long-range molecular motions whose rate increases rapidly with the temperature. At sufficiently high temperatures, a polymer is either an elastic fluid or an elastomer, depending on whether or not it is cross-linked into a three-dimensional network structure.
When the elastomer chains are sufficiently long, its elastic behavior is explained on a single-chain molecular level. However, our understanding of the role played by trapped entanglements and the swelling behavior of networks due to solvent remains limited. Crystallinity reinforces and stiffens an otherwise compliant amorphous polymer so that when oriented in fibers and films, semicrystalline polymers can exhibit outstanding physical properties.
For example, highly oriented crystalline polyethylene fibers exhibit tensile properties nearly equivalent to those of carbon fibers, whereas the noncrystalline ethylene-propylene copolymers are soft, rubbery materials. Over the past 40 years, the semicrystalline state of flexible polymers has been the subject of many investigations, and some theoretical understanding has been generated concerning the thermodynamics and kinetics of crystallization. However, this area still has many unanswered questions.
For example, the structures and properties of the crystal-amorphous interphase and their dependence on chemical structures need more attention in light of their technological importance for high-performance materials. Also needed is a better understanding of chain topology, such as entanglements and tie-chains interconnecting crystals, in the intercrystalline region, its variation with crystallization conditions, and its influence on bulk properties. Theoretical and simulation studies will aid greatly in further improving the properties of numerous synthetic fibers and films.
Rodlike polymers, and many semiflexible polymers with limited flexibility, are now known to exhibit liquid crystalline, or mesomorphic, order in solutions, in melts the liquid state , and in the solid state. The spontaneous ordering of these polymers provides a unique means of aligning polymer chains to obtain materials having exceptional strength, rigidity, and toughness. Therefore, the fabrication of these polymers into fibers, films, and molded parts promises many new opportunities that have not been fully exploited because of a lack of understanding of the relationship of molecular structure to the properties of the liquid crystalline state.
Theoretical advances will be key to establishing this badly needed understanding. They include describing the basic thermodynamics and kinetics of formation of the mesomorphic phase, the nature of defects disclinations , the chain conformations and intermolecular packing in mesomorphic states, the dependence on processing history, and heterogeneities at large spatial scales.
In particular, molecular descriptions are required that can explain and predict the influence of monomer structure, copolymer sequence, and interactions between chains. Multicomponent polymer systems, such as polymer mixtures blends , provide a new approach to the development of novel materials. These materials enable properties to be tailored, without resorting to costly new synthetic routes and also without the problems of proof of environmental and health safety entailed in new syntheses.
Many important polymer properties, such as toughness, impact strength, heat and solvent resistance, and fatigue, have thus been improved significantly by blending polymers with different properties, and this has enabled many new commercial applications. Despite these advances, questions remain unanswered concerning predictions of miscibility and phase separation, control of morphology, and the structures of the interfaces between domains of different phases. Unlike small molecules, the mixing of two polymers is accompanied by only a small gain in entropy, and hence miscibility is more the exception than the rule.
Because many of these materials must be processed in the liquid state, it is important to understand those features governing the phase diagrams of polymer liquid mixtures, that is, those factors determining whether the system is homogeneous or phase separated at a given temperature, pressure, composition, and so on. Traditionally, theories of this phase behavior have been based on simple models, but such models have not been fully satisfactory in relating the thermodynamic behavior of liquid polymer mixtures to the detailed chemical structures and interactions of their constituents; this is necessary for the molecular design of novel composite materials.
Newly emerging theories are establishing the much-needed link between phase behavior and molecular driving forces, through generalization of the classic lattice model of polymer solutions and through generalization, to polymers, of integral equation methods that have traditionally been applied to small-molecule liquids. A growing body of theoretical and experimental information suggests an alteration of chain dimensions in polymer blends, and further investigation is desirable. More difficult problems are those dealing with composite systems with mixtures of different phases, such as mixtures of crystalline with amorphous, liquid crystalline stiff chains with amorphous flexible chains , and so on.