Possibilities of formation or modification of polymers by means of chemical reactions during processing (polymer processing)

The trends for polymeric engineering materials are marked by the search for defined thermomechanical, plastic-elastic stability of dimension, and surface properties. This trend is caused by the demands for higher usage values of the polymers and more efficient processing technologies. The greatest chances are granted to the chemical modification and reactive compounding of known polymers to reach the aims of this development. Thermodynamically, most of the polymers are incompatible with each other and no sufficient disperse distribution is reached by only physical mixing. During the last 10 years a good compatibility at the interfaces has been found as one supposition and the influence of the viscosity of the polymer components and of the shear rate in the mixing machine as a further supposition to reach a fine-dispersed distribution. Polymer composites with fibres and fillers are a further importent group of materials. The properties of composites are also determined by the interactions in the interfaces, by the dispersity of the fibres and fillers, and by the physical properties of the interfaces. Polymer-analogous reactions, polymer blends, and polymer composites with and without low molecular reactants in the melt are decisively influenced by the parameters of the compounding machine. The quality of mixing, distribution of the residence time and of the shear rate, and the temperature profile of the reaction mixture are essential criterions for the course of the reaction. Continuous and quasi continuous technologies as, e. g., extruders and internal mixers are preferred. The lecture will deal with the present state and technical possibilities. Polymer syntheses in screw extruders are a new trend for the future. All these possibilities offer prospective chances of development.     

Evolution of gel structure during thermal processing of Na-geopolymer gels

The present work examines how the gel structure and phase composition of Na-geopolymers derived from metakaolin with varied Si/Al ratio evolve with exposure to temperatures up to 1000 degrees C. Gels were thermally treated and characterized using quantitative XRD, DTA, and FTIR to elucidate the changes in gel structure, phase composition, and porosity at each stage of heating. It is found that the phase stability, defined by the amount and onset temperature of crystallization, is improved at higher Si/Al ratios. Two different mechanisms of densification have been isolated by FTIR, related to viscous flow and collapse of the highly distributed pore network in the gel. Gels with low Si/Al ratio only experience viscous flow that correlates with low thermal shrinkage. Gels at a higher Si/Al ratio, which have a homogeneous microstructure composed of a highly distributed porosity, undergo both densification processes corresponding to a large extent of thermal shrinkage during densification. This work elucidates the intimate relationship between gel microstructure, chemistry, and thermal evolution of Na-geopolymer gels.     

Modeling heat transfer and transcrystallization kinetics during processing of polymer-based composites materials

Transcrystallization phenomena is a key issue to master for better understanding the role on the fiber-matrix interface in composites materials behavior during and after processing. In this paper, a non-isothermal kinetics model is presented to consider crystallization in fiber-based composite with thermoplastic matrix. The model extends Schneider's formulation to describe the development of transcrystalline layers around fibers. It also mentions the possibility to easily account for shear and flow conditions in addition to the transcrystallinity. Another approach, based on the volume fractions, giving the same results, is also introduced for comparison. Then, a parametric study is proposed in order to demonstrate the relevance of the developed model by comparing its results to well-known expecting behavior from the literature. Finally, an attempt is made to compare this model to the one proposed previously in the literature by Durin and al. after correcting some unclear points in the latter.     

New trends in polymer processing

Recent developments in polymer processing technology for the enhancement of physical properties by more informed control over composition and micromorphology are reviewed. This is followed by a more detailed review of the multi live-feed moulding process which can impart high levels of anisotropy, by design, in complex shaped articles produced by moulding routes.     

General principles of polymer processing modeling

Specific rheological and thermal properties of molten polymers and their consequences on the numerical simulation of forming processes are first reviewed. The strong coupling between flow kinematics and material temperature is pointed out. According to geometrical specificities, to the type of deformation the material is submitted to and to thermal conditions, continuum mechanics and energy balance equations can be simplified and more or less sophisticated rheological equations can be used. Finally, these considerations are applied to the modeling of steady extrusion and fiber spinning and of unsteady molding processes.     

Fibre porosity development of dissolving pulp during mechanical and enzymatic processing

Dissolving grade pulps are used as raw material for manufacture of regenerated cellulose fibres and their use is constantly growing. Despite intensive research, there is still a need to develop cellulose dissolution-regeneration processes that would be economically viable, fulfil the pre-conditions of sustainability and would be able to meet the strict product quality requirements. The basis for creation of such a process is in deep understanding of the biomass structure and factors affecting the cellulose modification and dissolution. In this paper, the effects of the mechanical and enzymatic pre-treatments on the pore structure and alkaline solubility of dissolving grade pulp are discussed. Formation of micro- and macropores in the pulp fibres during mechanical shredding was found to correlate with the susceptibility of the fibres to enzymatic hydrolysis. The fibre porosity development during the processing was studied by a modified solute exclusion approach, which revealed differences between the effect of mild enzyme or acid hydrolysis on the pore structure of fibres. The dissolution of the modified fibres in NaOH/ZnO was evaluated and found to correlate with overall pore volume and accessible surface area analysed by the modified solute exclusion method.     

Modeling of multilayer drying of polymer films

A model was developed to predict the drying behavior of multilayer polymer films on inert substrates. The model considers simultaneous heat and mass transfer controlled by complex thermodynamic and transport properties of polymer solutions. Key components of the model are the incorporation of the free volume theory to predict diffusivities in each polymer layer, the use of heat and mass transfer coefficients to describe complex transport phenomena in the gas phase, the incorporation of exact equilibrium boundary conditions at polymer–polymer interface, and the use of the Flory–Huggins theory to describe both liquid–liquid and vapor–liquid equilibria. The model can be applied to guide processing, product formulation, scale‐up, and oven design. As an example, the model is applied to simulate the drying of a two‐layer coating of poly vinyl acetate (in toluene) over polystyrene (also in toluene) on a polyester substrate. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1665–1675, 1999     

Crystallization kinetics in relation to polymer processing

Phase distribution of quenched samples has been determined by a deconvolution procedure of WAXS spectra in a wide range of cooling rates. The informations collected together with isothermal and DSC results provide a very wide set of data on the crystallization kinetics of polymers relevant which covers conditions encountered in most polymer processing operations. They have been compared with predictions of a non-isothermal crystallization model assuming two independent and parallel crystallization processes competing during solidification.     

Analysis of mixing efficiency in polymer processing equipment

Flow patterns In various processing equipment were obtained based on 3D isothermal flow simulations. These flow patterns were further processed for mixing efficiency analysis. The dynamics of the mixing process was studied numerically by means of tracers and their trajectories In the processing equipment. Dispersive mixing efficiency was studied in terms of shear stresses and elongational flow components distributions. Distributive mixing efficiency was characterized using length stretch distributions and average values. Features of chaotic flow were also analyzed. The framework developed to analyze mixing efficiency opens the way for an unambiguous evaluation of equipment performance and for process optimization.     

Modeling diffusion in miscible polymer blend films

Recent experiments designed to probe polymer transport in the bulk and in the vicinity of surfaces have examined the interdiffusion of multilayer sandwiches of isotopically labeled polymers. The measured time dependent concentration profiles normal to the surface are typically fit to Fick's law, with a single fitting parameter, the mutual binary diffusion coefficient (MBDC). The resulting MBDCs are found to vary over a broad range of film thicknesses and time, with the time dependence being viewed as a unique signature of the reptation mechanism of long chain motion, and the thickness dependence being attributed to the slowing down of chain dynamics near surfaces. Since the experiments are conducted at finite concentration, the MBDC, which is a product of the bare mobility and the concentration derivative of the chemical potential, could be dominated by the time and thickness dependence of this second term (which is ignored in Fick's law). To quantify this conjecture we consider the more rigorous Cahn formulation of the diffusion problem in terms of chemical potential gradients. We use square gradient theory to evaluate chemical potentials, and fit the resulting time dependent concentration profiles to the analytical solution of Fick's law. By thus mimicking the experimental analysis we find that the apparent MBDCs vary with time as t(-1/2) at short times, in good agreement with existing experiments. We show that this time dependence reflects the system's desire to minimize concentration gradients, a fact ignored in Fick's law. Since these arguments make no reference to the mechanism of chain motion, we argue that the time dependence of MBDC derived from interdiffusion experiments does not provide unequivocal support for the reptation mechanism of long chain transport. The MBDC values, which also vary with the degree of confinement, are predicted to increase with decreasing thickness for model parameters corresponding to experimental systems. In contrast, since the experimental fits yield an opposite trend, we suggest that the bare mobility of the chains decreases strongly with decreasing thickness. These findings strongly support the idea that the chains are "pinned" irreversibly to the surfaces, in good agreement with other, independent experiments.     

Interferometric studies on polymer structure

A multiple-beam interferometric method is used to study the change of optical orientation function and molecular structure of nylon-6 fibres due to γ-irradiation under vacuum. It was found that γ-radiation causes alignment to the fibre chains in the direction of the fibre axis, this alignment gives an increase in the optical orientation function. A two-beam interferometric method is used to study the changes in optical orientation function on drawn polypropylene fibres. Empirical formulae are suggested to correlate these changes in the optical orientation function with the dose and with the draw ratio. This study aims to show that the multiple-beam and two-beam interferometric methods can be used to study the changes that take place in polymers and fibres by irradiation or drawing.     

Modeling a tethered polymer in Poiseuille flow

We investigate the behavior of a tethered polymer in Poiseuille flow using a multiscale algorithm. The polymer, treated using molecular dynamics, is coupled to a solvent modeled by the stochastic rotation algorithm, a particle-based Navier-Stokes integrator. The expected series of morphological transitions of the polymer: sphere to distorted sphere to trumpet to stem and flower to rod are recovered, and we discuss how the polymer extension depends on the flow velocity. Backflow effects cause an effective increase in viscosity, which appears to be primarily due to the fluctuations of the free end of the polymer.     

Morphology development of main‐chain liquid crystalline polymer fibers during solvent evaporation

A nonequilibrium thermodynamic approach has been developed for describing the emergence of fiber morphologies from a liquid crystalline polymer solution undergoing solvent evaporation, including fibrillar structures, concentric rings, and spiral structures. We utilized Matsuyama–Kato free energy for main‐chain liquid crystalline polymer (MCLCP) solutions, which is an extension of Maier–Saupe theory for nematic ordering and incorporates a chain‐stiffening, combined with Flory‐Huggins free energy of mixing. Temporal evolution of the concentration and nematic order parameters pertaining to the above free energy density of liquid crystalline polymer solution was simulated in the context of time‐dependent Ginzburg–Landau theory coupled with the solvent evaporation rate equation under the quasi‐steady state assumption. The emerged morphological patterns are discussed in relation to the phase diagram of the MCLCP solution and the rate of solvent evaporation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 429–435, 2007     

Equilibrium structure of hydrogen-bonded polymer blends

Deuterium-labeled polystyrene modified by random distributions of the comonomer p-(1,1,1,3,3,3-hexaflouro-2-hydroxyisopropyl)-α-methyl-styrene [DPS(OH)] has been blended with poly(butyl methacrylate) (PBMA) and studied with small-angle neutron scattering (SANS). Miscibility is induced via hydrogen bonding between the DPS(OH) hydroxyl group and PBMA carbonyl groups. The data suggest that the nature of the miscible-phase structure in these blends differs from that of the usual homopolymer blends at small scattering angles, which we attribute to the short-range site specific nature of the hydrogen bond interaction. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2745–2750, 1998     

Transfer of contaminant during the processing of thick bi-layer food packages made of recycled and virgin polymer layers

A new route of recycling old polymers consists of using them as new food packages. Because of the potential contamination of the old polymer resulting from its previous use, the new packages are made of the old polymer layer bound to a virgin polymer layer. As it takes some time for the contaminant to diffuse through the bi-layer packaging, the food is protected over a period of time. The process of binding the two polymer layers is studied, when the two layers are heated at around the melted temperature and then cooled down at room temperature. Heat transfer by conduction through the polymer and the mould during the heating stage is considered as well as the cooling period in air. During this heating and cooling period, the process of contaminant diffusion through the two polymer layers is evaluated by considering a temperature-dependent diffusivity. The results are expressed in terms of profiles of temperature and of profiles of concentration of contaminant developed through the polymer layers. The kinetics of contaminant transfer from the recycled layer to the virgin polymer layer are also determined, for various thicknesses of the package. Dimensionless numbers are used, as well as various thicknesses of the package.     

Sandwich structure containing liquid crystal polymer grafted on polymer support

Graft post-polymerization of mesogenic monomers onto fluorine-containing polymer support was initiated by the simultaneous action of vacuum ultraviolet radiation and atomic oxygen. The resultant two-layer structure possesses the combined physical–mechanical properties of the polymer-supporting film and the optical characteristics of the anisotropic liquid crystal layer.     

Towards realistic rheological models for polymer melt processing

Greatly needed improvements to rheological models of polymer fluids can best be made by understanding and modifying a model on the molecular level, where the physics governing real fluids are created. For this purpose, stochastic simulations are an indispensible tool. The usefulness of stochastic simulations for kinetic theory models are well-known. In this paper, we show that the class of Rivlin-Sawyers, or K-BKZ, integral models is also subject to a molecular interpretation and can be simulated using stochastic techniques. We also illustrate how an existing molecular model can be modified, resulting in better predictions of viscoelastic behavior. For this we formulate a stochastic simulation of a reptation model modified to account for double reptation and tube-length fluctuations.     

Synthesis, processing and properties of conjugated polymer networks

Despite the diverse research activities focused on the chemistry, materials science and physics of conjugated polymers, the feature of conjugated cross-links, which can provide electronic communication between chains, has received little attention. This situation may be a direct consequence of the challenge to introduce such links while retaining adequate processability. Focusing on recent studies of materials for which charge transport or electrical conductivity data are available, this feature article attempts to present an overview of the synthesis, processing and electronic properties of conjugated polymer networks. For the purpose of this discussion, two distinctly separate architectures-featuring covalent cross-links on the one hand and non-covalent organometallic bridges on the other-are treated in separate sections. The available data indicate that cross-linking can have significant benefits for intermolecular charge transfer if the polymers are carefully designed.     

Controlled processing of polymer nanoribbons: Enabling microhelix transformations

Flow‐coated, two‐dimensional polymer ribbon structures undergo a shape‐transformation into a three‐dimensional helix upon their release into a solution. Driven by surface forces and due to geometric asymmetry, the helix radius and spring constant depend upon the ribbon cross‐section dimensions, surface energy, and material elastic modulus. Such spring‐like microhelices offer multiple functionalities combined with mechanical stretching and shape recovery. Fabricating such microhelices requires a sequence of processing steps, beginning with flow‐coating of ribbons on a substrate, followed by etching of a “scum layer” to allow for an independent release into a solution, upon which shape‐transformation occurs. During the deposition‐etch‐release sequence, various control parameters influence the nanoribbon size and geometry, hence the helix properties. The experimental study presented here focuses on the influence of meniscus height, substrate velocity, substrate surface energy, and etch time on nanoribbon size (height and width), scum layer thickness, and helix radius. The results show that meniscus height and contact angle dictate flux toward the meniscus edge and volume available for spatial assembly, allowing control over the aspect ratio of ribbons. We vary the aspect ratio by two orders of magnitude, while maintaining geometric asymmetry needed for helix shape‐transformation. We provide robust scaling for the nanoribbon size and geometry and report the advantages and disadvantages of different parameters, in the control of polymer nanoribbon and helix fabrication. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1270–1278