Showing papers in "Advances in Polymer Science in 1999"

Polymer-silicate nanocomposites : Model systems for confined polymers and polymer brushes

Abstract: The static and dynamic properties of polymer-layered silicate nanocomposites are discussed, in the context of polymers in confined spaces and polymer brushes. A wide range of experimental techniques as applied to these systems are reviewed, and the salient results from these are compared with a mean field thermodynamic model and non-equilibrium molecular dynamics simulations.

Solution Properties of Branched Macromolecules

Abstract: Dilute and semi-dilute solution properties of several classes of branched macromolecules are outlined and discussed. The dilute solution properties are needed for a control of the chemical synthesis. The molecular parameters also determine the overlap concentration which is an essential quantity for description of the semi-dilute state. This state is represented by a multi-particle, highly entangled ensemble that exhibits certain similarities to the corresponding bulk systems. Because of the rich versatility in branching the present contribution made a selection and deals specifically with the two extremes of regularly branched polymers, on the one hand, and the randomly branched macromolecules on the other. Some properties of hyperbranched chains are included, whereas the many examples of slight deviations from regularity are mentioned only in passing. The treatment of the two extremes demonstrates the complexity to be expected in the general case of less organized but non-randomly branched systems. However, it also discloses certain common features.

Novel macromolecular architectures based on aliphatic polyesters : Relevance of the coordination-insertion ring-opening polymerization

Abstract: Recent developments in the macromolecular engineering of aliphatic polyesters have been overviewed. First, aluminum alkoxides mediated living ring opening polymerization (ROP) of cyclic (di)esters, i.e., lactones, lactides, glycolide, is introduced. An insight into this so-called “coordination-insertion” mechanism and the ability of this living polymerization process to prepare well-defined homopolymers, telechelic polymers, random and block copolymers is then discussed. In the second part, the combination of the living ROP of (di)lactones with other well-controlled polymerization mechanisms such as anionic, cationic, free radical, and metathesis polyadditions of unsaturated comonomers, as well as polycondensations, is reported with special emphasis on the design of new and well-tailored macromolecular architectures. As a result of the above synthetic breakthrough, a variety of novel materials have been developed with versatile applications in very different fields such as biomedical and microelectronics.

Crystallization in Block Copolymers

Abstract: Crystallization in block copolymers has a profound effect on their structure. This review article focusses on the morphology of semicrystalline block copolymers, and those containing two crystallizable blocks. The effect of crystallization on mechanical properties is briefly considered. The extent of chain folding upon crystallization is discussed, as is the orientation of crystal stems with respect to the microstructure. The effect of selective solvent on solution crystallization is also highlighted. Recent work on crystallization kinetics is summarized and finally the theories for crystallization in block copolymers are outlined.

Interpolymer Complexation and Miscibility Enhancement by Hydrogen Bonding

Abstract: The main development in macromolecular complexation in aqueous media due to hydrogen bonding over the last decade has been a better understanding of the process of complex formation. Meanwhile an ever growing interest has occurred in introducing hydrogen bonding into the polymers which originally lack donor or acceptor groups. For such polymer solutions it has been demonstrated that the transition from separated coils to complex aggregates takes place when the content of introduced interaction sites reaches a certain level. For the blends with controllable hydrogen bonding in the solid state, the relevant experiments have shown that immiscibility-miscibility-complexation transitions occur upon progressive increase in the density of hydrogen bonding.

Surface-Anchored Polymer Chains: Their Role in Adhesion and Friction

Abstract: Polymer surfaces and interfaces have many technological applications. In the present article we review some recent experiments conducted on model systems with the aim of understanding the role played by surface-anchored polymer layers in adhesion and friction. We also discuss some of the related theoretical models. The key parameter for both situations is the degree of interdigitation between the surface layer and the bulk polymer system (an elastomer in the case of adhesion, a molten polymer in the case of friction). We analyze how this degree of interdigitation governs the optimum enhancement in the adhesion energy between the solid wall and an elastomer, and how it is at the origin of the various wall slip regimes observed experimentally.

Processable Aromatic Polyimides

Abstract: This review article deals with aromatic polyimides that are processable from the melt or soluble in organic solvents. Conventional aromatic polyimides represent the most important family of heat resistant polymers, but they cannot be processed in the melt, and their application in the state of soluble intermediates always involves a hazardous step of cyclo-dehydration and elimination of a non-volatile polar solvent. A major effort has therefore been devoted to the development of novel soluble and/or melt-processable aromatic polyimides that can be applied in the state of full imidation. The structural factors conducive to better solubility and tractability are discussed, and representative examples of monomers showing favourable structural elements have been gathered and listed with the chemical criteria. Experimental and commercial aromatic polyimides are studied and evaluated by their solubility, transition temperatures and thermal resistance. An example is also given of the methods of computational chemistry applied to the study and design of polyimides with improved processability.

Mineral Fillers in Thermoplastics: Filler Manufacture and Characterisation

Abstract: The principal mineral fillers used in thermoplastics and the reasons for using them are identified, together with those features that have to be controlled in order to achieve the optimum results and to avoid associated deleterious effects. General methods of filler production are outlined in the light of these requirements and their application to the fillers in most use is described. Attention is given to the use of surface modification methods where these are part of the production process.

Diallyldimethylammonium chloride and its polymers

Abstract: The pyrrolidimium structure resulting from the cyclopolymerization of the water soluble monomer diallyldimethylammonium chloride is present in a variety of advanced polymeric materials. These materials range from water soluble polyelectrolytes to highly ordered solids. Applied research on diallyldimethylammonium chloride has been performed in order to optimize the monomer and polymer syntheses, characterize the polymers produced, improve the material properties as well as the applied technologies, and to develop new products. In addition, fundamental research has included the study of polyelectrolyte behavior of diallyldimethylammonium chloride polymers in solution. This article comprehensively reviews the work on diallyldimethylammonium chloride summarizing the current knowledge and recent progress in the field of kinetics and mechanism of homo- and copolymer syntheses, chemical structures, polyelectrolyte behavior in solution, molecular characterization, and interactions in solution and at interfaces. In particular, peculiarities of the syntheses and characterization resulting from polyelectrolyte influences are discussed in detail. A variety of current and emerging applications are presented.

Asymmetric Star Polymers: Synthesis and Properties

Abstract: The synthesis and the properties, both in bulk and in solution, of asymmetric star polymers are reviewed. Asymmetry is introduced when arms of different molecular weight, chemical nature or topology are incorporated into the same molecule. The phase separation, aggregation phenomena, dilute solution properties etc. are examined from a theoretical and experimental point of view. Recent applications of these materials show their importance in modern technologies.

Normal and Shear Forces Between Polymer Brushes

Abstract: Surface-polymer interactions are important in many technological applications, including colloidal stablization and adherence. Recently there has been considerable progress in understanding these interactions and the resulting forces between polymer-bearing surfaces. End-grafted polymers, commonly referred to as polymer brushes, are one example of a polymer-surface complex which has many interesting properties. In this article, recent progress in understanding the normal and shear forces between polymer brushes is reviewed with emphasis on the contributions from molecular simulations. These simulations show that under steady-state shear flow, some of the individual chains of a polymer brush stretch in the direction of flow while most are buried inside of the brush and are not affected by the shear flow. The height of the brush is only weakly dependent on the shear rate in contrast with several theoretical models. When two surfaces bearing end-grafted chains are brought into contact the normal force increases rapidly with decreasing plate separation, while the shear force is in most cases significantly smaller, particularly for large compressions. However, for weak compression, the range and the magnitude of the shear force depends on both the solvent quality and shear rate. These results, first observed experimentally using the surface force apparatus and recently confirmed in simulation, suggest a way to dramatically reduce the frictional force between two surfaces. For small relative velocity of the two surfaces, the surfaces slide pass each other with almost no change in the average radius of gyration of the chains or the amount of interpenetration of chains from the two surfaces. However, for large shear rates, there is significant stretching and some disentanglement of the chains.

Entangled dynamics and melt flow of branched polymers

Abstract: One of the most puzzling properties of branched polymers is their unusual viscoelasticity in the melt state. We review the challenges set by both non-linear experiments in extension and shear of polydisperse branched melts, and by the growing corpus of data on well-characterised melts of star-, comb- and H-molecules. The remarkably successful extension of the de Gennes/Doi-Edwards tube model to branched polymers is treated in some detail in the case of star polymers for which it is quantitatively accurate. We then apply it to more complex architectures and to blends of star-star and star-linear composition. Treating linear polymers as “2-arm stars” for the early fluctuation-dominated stages of their stress-relaxation successfully accounts for the relaxation spectrum and “3.4-law” viscosity-molecular weight relationship. The model may be generalised to strong flows in the form of molecular constitutive equations of a structure not found in the phenomenological literature. A model case study, the “pom-pom” polymer, exhibits strong simultaneous extension hardening and shear softening, akin to commercial branched polymers. Computation with such a constitutive equation in a viscoelastic flow-solver reproduces the large corner vortices in contraction flows characteristic of branched melts and suggests possible future applications of the modelling tools developed to date.

Poly(macromonomers): Homo- and Copolymerization

Abstract: Syntheses and characterization of branched polymers prepared by homo- and copolymerization of macromonomers are reviewed. A number of macromonomers have so far been available as potential building blocks to design a variety of well-defined, branched homo- and copolymers including comb, star, brush, and graft types. Recent progress in macromonomer syntheses, macromonomers’ homo- and copolymerization, characterization of the branched polymers obtained, as well as application to design of polymeric microspheres are described. Macromonomers and their homo- and copolymerization appear to provide continuing interest in designing and characterizing a variety of branched polymers and in their unique applications.

Adhesion and Surface Modification

Abstract: This review emphasizes the role of interactions in particulate filled composites. In an introductory section a general view is given about the factors influencing composite properties. Two basic type of interactions must be considered: particle/particle and matrix/filler interaction. The effect of the former is detrimental to composite properties, it decreases strength and impact resistance. The occurrence and extent of aggregation is determined by the relative adhesion and shear forces during homogenization. Due to matrix/filler interaction an interphase forms spontaneously in the composite with properties different from those of both components. The amount and characteristics of the interphase strongly influence composite properties. The strength of adhesion between the components can be characterized by thermodynamic quantities, mainly by the reversible work of adhesion. The most important techniques used for the estimation of the strength of interaction and the properties of the interphase are briefly reviewed. The modification of interactions is achieved through the surface treatment of the filler. Surface treatments are divided into four arbitrary groups and are discussed accordingly, i.e. non-reactive and reactive treatment, application of functionalized polymers and introduction of a soft interlayer around the particles. The practical relevance of interactions and their modification is also mentioned in the last section.

Molecular Transitions and Dynamics at Polymer / Wall Interfaces: Origins of Flow Instabilities and Wall Slip

Abstract: This article reviews recent results on capillary melt flow anomalies. Long standing controversies and debates in this field are illustrated by summarizing previous results and clarified with an extensive discussion of the most recent results. Explicit molecular mechanisms for flow instabilities are presented in contrast to a background of 40 years’ continuous and far ranging research. New experiments show that the widely observed extrusion anomalies (including oscillating flow, discontinuous flow transition and sharkskin) of linear polyethylenes (LPE) originate from interfacial molecular transitions, which may or may not be stable depending the specific flow conditions. A global flow instability (commonly known as oscillating capillary flow) evidently arises from a time-dependent oscillation of the global hydrodynamic boundary condition (HBC) between no-slip and slip limits at the capillary die wall. Other convincing observations show that sharkskin originates from a local instability of HBC at the die exit wall. The global and local interfacial instabilities both originate from a reversible coil-stretch transition involving interfacial unbound chains that are entangled with the adsorbed chains. In other words, local and global stress oscillations result in the observed macroscopic sharkskin-like and bamboo-like extrudate distortions respectively. A second molecular mechanism for wall slip is also clearly identified, involving stress-induced chain desorption off low surface energy walls. An organic coating of capillary die walls produces massive chain desorption and a large magnitude wall slip at rather low stresses, whereas bare metallic and inorganic surfaces (e.g., steel, aluminum, and glass) usually retain sufficient chain adsorption and prevent catastrophic slip up to the critical stress for the coil-stretch transition. The intricate interfacial flow instabilities exhibited by LPE are also shared by other highly entangled melts such as polybutadienes. In contrast, monodisperse melts with high critical entanglement molecular weight (M e ) such as polystyrene of M w =106 show massive wall slip on low energy surfaces but no measurable interfacial stick-slip transition before reaching the plateau around 0.2 MPa. Tasks for future work include (i) direct molecular probe of melt chain adsorption and desorption processes at a melt/wall interface as a function of the surface condition, (ii) new theoretical studies of chain dynamics in an entangling melt/wall interfacial region as well as in bulk at high stresses, (iii) test of universality of the established physical laws governing melt/wall interfacial behavior and flow for all polymers, and (iv) development of tractable experimental and theoretical methods to study boundary discontinuities and stress singularities.

Recent Advances in the Study of Synthetic Polyampholytes in Solutions

Abstract: This article reviews the theoretical and experimental material accumulated in recent years on water-soluble and water-swelling polyampholytes of integral and pendant types. The theory of polyampholytes in comparison with experimental results, acid-base equilibrium, hydrodynamic, conformational, and the molecular and colloidal properties of amphoteric copolymers are considered. The responsibility of Coulomb forces, hydrogen bonds and hydrophobic interactions in the formation of compact structures is shown. Basic attention is paid to the specific structure and behaviour of polyampholytes at or near the isoelectric point (IEP), in particular to the “forcing out” phenomenon found at the IEP. The ability of polyampholytes to associate with various low- and high-molecular-weight substances as well as to be absorbed on dispersed particles is discussed. An attempt to show the closeness of structural organization or molecular recognition of synthetic polyampholytes to natural ones is undertaken.

Phase Transitions of Polymer Blends and Block Copolymer Melts in Thin Films

Abstract: 2.1 Mean-Field Theory of Phase Separation in Thin Films. . . . . . . . 8 2.2 Block Copolymers in the Weak Segregation Limit . . . . . . . . . . 22 2.3 Block Copolymers in the Strong Segregation Limit. . . . . . . . . . 37 2.4 Survey of Results Obtained with the Self-Consistent Field Theory . . 41 2.5 Concepts on Interfaces in Confined Geometry . . . . . . . . . . . . 47 2.6 Computer Simulation of Polymer Blends in Thin Films . . . . . . . 51 2.7 Computer Simulation of Confined Block Copolymers . . . . . . . . 60 2.8 Dynamics of Phase Separation in Films: an Introduction . . . . . . 65

Dendritic and Hyperbranched Macromolecules — Precisely Controlled Macromolecular Architectures

Abstract: The development of procedures for the synthesis of dendritic macromolecules by either the convergent or divergent growth approaches is outlined with emphasis placed on the controlled manipulation of three-dimensional structure. The utility of these techniques to prepare a wide variety of different dendritic structures is then discussed in terms of the three distinct regions associated with these novel macromolecular systems — the central core, the interior building blocks, and the chain ends. Control of these regions, coupled with changes in the synthetic approach, gives tailor-made dendritic macromolecules with predetermined physical properties and/or function. The application of current spectroscopic methods to the structural elucidation of dendritic macromolecules is then detailed along with an examination of the influence of dendritic structure on the physical properties of these novel materials. Finally, a comparison is made with the related class of highly branched polymers, hyperbranched macromolecules, and the manipulation of structure/function for these materials is examined.

Conformational Properties of Branched Polymers: Theory and Simulations

Abstract: The prediction and interpretation of conformational properties of branched polymers is difficult, due to the complexity and variety of these structures. Numerical simulations are, consequently, very useful in the investigation of these systems. This review describes the application of numerical simulation techniques to relevant theoretical problems concerning branched polymer systems, taking also into account the related experimental data. Monte Carlo, Molecular Dynamics and Brownian Dynamics methods are employed to simulate the equilibrium and dynamic behavior, and also to reproduce hydrodynamic properties. The simulations are performed on several polymer models. Thus, different Monte Carlo algorithms have been devised for lattice and off-lattice models. Moreover, Molecular Dynamics and Brownian Dynamics can be carried out for detailed atomic or coarse-grained chains. A great amount of investigation has been engaged in the understanding of uniform homopolymer stars as single chains, or in non-diluted solutions and melts, employing this variety of techniques, models and properties. However, other important structures, such as stars with different types of monomer units, combs, brushes, dendrimers and absorbed branched polymers have also been the subject of specific simulation studies.

Molecular Engineering of π-Conjugated Polymers

Abstract: An extensive review of the synthesis of π-conjugated polymers is presented using a tutorial approach to provide an introduction to the field intended for the undergraduate student and the experienced chemist alike. The many synthetic methodologies that have been used for the synthesis of conjugated polymers are outlined for each class of polymers with a focus on research from the 1990s. The effect of structure on electrical properties is detailed. Specific systems reviewed include the polyacetylenes, polyanilines, polypyrroles, polythiophenes, poly(arylene vinylenes), and polyphenylenes.

Flexible Polymers in Nanopores

Abstract: Polymer mixtures and block copolymers in thin film geometry find much recent attention, both theoretically [1-63] and experimentally [5,7,8,29,64-122]. This interest arises because of various applications of thin polymer films in materials science (adhesive properties, lubrication, coatings, etc.) [123,124], but also from the point of view of basic science: e.g., from suitable measurements of thin films of polymer mixtures one can extract information on bulk phase behavior (e.g. [70,81]), interfacial widths (e.g. [84,125]), and surface properties controlling surface enrichment or the formation of wetting layers (e.g. [5,8,65,69,74-82,126]). In such thin films, one may also observe very interesting kinetic phenomena and associated structure formation, e.g. growth of surface enrichment layers and adjacent depletion layers [66,69,83], dynamics of phase inversion in unmixed films [77], and — last but not least — surface-directed spinodal decomposition [5,127-158]. Of course, also the dynamics of ordering in block copolymer films is a topic of great current interest (e.g. [101]), but will not be considered further in this article.

Dendrimers and Dendrimer-Polymer Hybrids

Abstract: The synthesis and study of dendrimers has been truly dramatic in the last ten years. This review gives a brief introduction to some of the key concepts and main synthetic strategies in dendrimer chemistry. The focus of the chapter is a survey of modern analytical techniques and physical characterization of dendrimers. Results of model calculations and experiments probing the dimensions and conformation of dendrimers are reviewed. In the final sections the experimental work on dendrimer-polymer hybrids is highlighted. The dense spherical conformation of dendrimers has been combined with the loose random-coil conformation of ordinary polymers to form new hybrids with potentially interesting new properties.

Rheology, Compounding and Processing of Filled Thermoplastics

Abstract: The influence of fillers on the rheology of polymer melts is reviewed, together with an account of mechanisms involved during their combination in melt-mixing procedures. The application of these principles to the design and operation of industrial compounding technologies is then discussed. Means for inducing further microstructural changes during secondary melt processing are described, leading to the achievement of enhanced composite performance.

Synthesis and characterization of segmented polyimide-polyorganosiloxane copolymers

Abstract: Polyimide-polydimethylsiloxane block or segmented copolymers are reviewed with respect to synthesis, characterization, structure-property relationships and other special characteristics. The siloxane-modified imides have developed into important materials over the past 10 years. Although some thermal and thermooxidative stability is lost through introduction of the siloxane segment, a number of improvements are also observed, in processibility, toughness, flexibility, adhesion and membrane performance, to mention a few. The chemistry involves first preparing an organo-functional siloxane oligomer of a few thousand molecular weight, by ring-opening equilibration processes in the presence of an end blocker to produce either aminopropyl or anhydride end-groups. The resulting oligomers are then usually reacted with the two imide-forming monomers to generate “randomly segmented copolymers”. If the siloxane block lengths are designed to exceed even 1000\( \overline M n, \) a microphase separation allows for the generation of two glass-transition temperatures and other desirable multiphase bulk properties, which are compositionally dependent. In addition, the hydrophobic nature of the siloxane microphase influences the surface properties, often in desirable ways, useful for aerospace and microelectronics, and these features are discussed in this review.

Analysis of Polymer Latexes by Small-Angle X-Ray Scattering

Abstract: Small-angle X-ray scattering (SAXS) is a well-established tool of materials science and colloidal physics. It has been applied to a great number of polymeric and colloidal systems [1-5]. This method, however, has not often been applied to latexes. It is interesting to note on the other hand that the analysis of polystyrene latexes with narrow size distributions [6,7] by SAXS played an important role in the early history of this method. A number of early workers in the field of SAXS [8-13] had shown that the minima and maxima seen in the scattering curve of a polystyrene latex (see e.g. p. 54 of ref. [1]) are related to the form factor of a homogenous sphere. Small-angle neutron scattering (SANS) [14,], on the other hand, has been used quite often to analyze the radial structure [15-27] and the surface [28-30] of latex particles. In the case of SANS a high contrast between the particle and the medium or between the different constituents of the particle may be achieved through appropriate substitution of hydrogen atoms by deuterium atoms [14]. Moreover, a variable contrast between the latex particles and the surrounding medium (water) can be adjusted through mixtures of H2O and D2O [14]. This allows the detailed study of the radial structure and surface of the particles through contrast variation which had been earlier established as an investigative tool for polymeric systems [31-37], biological structures (see ref. [38,39] and further citations given there), and in general for colloidal systems [40,41]. In addition, very small scattering angles can be achieved by neutron spectrometer as, for instance, the D11 at the Institut Laue-Langevin [42]. Hence, the magnitude of the scattering vector q (q=(4π/λ)sin(θ/2); λ: wavelength of radiation, θ: scattering angle; [1, 2, 3, 4, 5]) can attain values much smaller than 0.lnm-1 which is necessary for a meaningful analysis of many colloidal svstems.

Interfacial Phenomena in Thin Polymer Films: Phase Coexistence and Segregation

Abstract: Interfaces formed or exposed by polymer mixtures are often employed in modern technological applications. This is especially true for thin polymer films used nowadays in photoresist lithography, electrooptical devices or nanometer-scale surface patterning. Apart from these technology-oriented aspects the interfacial phenomena in thin polymer films pose a fundamental scientific challenge to physics of polymers and thermodynamics of condensed matter. This work reviews experimental results on the equilibrium properties of interfaces created by polymer mixtures confined in thin films. It confronts experimental data with theoretical expectations based mainly on mean field models. Most of the data have been obtained by high resolution profiling techniques emerged in the last decade. These techniques allow us to trace concentration vs depth profiles across a thin film with a depth resolution better than the characteristic size of a polymer coil. The interfacial phenomena of phase coexistence and segregation are described as observed in thin polymer films. This work also considers related issues in the focus of current research such as wetting phenomena, finite size effects expected in very thin films, surface (and bulk) properties of mixtures with stiffness disparity and conformational properties of end-segregated macromolecules forming polymer brushes.

Synthesis of Branched Polymers by Cationic Polymerization

Abstract: The synthesis of branched polymers by cationic polymerization of vinyl monomers is reviewed. This includes star, graft, and hyperbranched (co)polymers. The description is essentially focused on the synthetic approach and characterization results are provided as a proof of the structure. When available, specific properties of the materials are also given.

Progress in polyimide chemistry I

Abstract: Nanoporous Polyimides.- Poly(ester-imide)s for Industrial Use.- Liquid-Crystalline Polyimides.- Calculation of a Mesogenic Index with Emphasis Upon LC-Polyimides.

Thermosetting Oligomers Containing Maleimides and Nadimides End-Groups

Abstract: In the field of high thermomechanical performance polymers, linear and thermosetting systems offer complementary properties. Among the thermosetting materials, BMIs and BNIs have been extensively studied and are now commercially available. In this chapter, firstly the main preparation and characterization methods are reviewed, and then the chemistry of the polymerization processes is discussed for both families. For the BMIs, due to the electrophilic character of their double bond, different polymerization pathways have been published, which is not the case for BNIs. Special attention has been paid to thermal polymerization which has already been used in industrial achievements; however, on the other hand, the structure of these materials has been considered for the purpose of establishing relationships between processability, stability and thermomechanical properties.

Poly(ester-imide)s for Industrial Use

Abstract: Poly(ester-imide)s are a class of polymers known for more than 35 years. They are used today in large tonnage as electrical insulating materials. The patent literature reviewed shows that predominant research activities in the past were focused on improving the electrical, thermal and mechanical properties. In recent times new applications for these polymers have been found, such as engineering thermoplastics, adhesives, printed circuit boards and membranes. Excellent properties and easy processing will probably lead to a continuous growth of poly(ester-imide) business.