Advances in Elastomers II

Free volume theories of diffusion developed in the literature successfully treat penetrant transport behavior in rubbery polymers. The potential of such a free volume theory to predict the gas transport behavior in glassy polymers is explored here, duly taking into account the excess hole free volume present in the glassy state. The calculations require values for three model parameters appearing in the theory of diffusion which are obtained by fitting experimental gas permeability data in rubbery polymers. The calculations also require an estimate for the hole free volume in the gas-polymer mixture. This is obtained by fitting experimental gas sorption data in rubbery and glassy polymers to a free volume based theory of gas sorption developed here. Thus, the calculation of gas permeability in glassy polymers is strictly predictive in the sense that no use is made of experimental permeation data in the glassy state. I. Introduction Free volume concepts have been successfully used for the quantitative description of equilibrium and transport properties of polymeric systems. The equation of state models of Flory (1965, 1970), Prigogine (Prigogine et al., 1953a-c; Prigogine, 1957), Patterson (Patterson, 1969; Patterson and Delmas, 1970), and Sanchez and Lacombe (1976,1978) are examples of how the free volume concept has been employed in the quantitative treatment of equilibrium behavior. The treatment of viscosity of polymer melts by Williams et al. (1955) (see also Ferry, 1970) and of diffusion by Cohen and Turnbull (1959), Fujita (1961), and Vrentas and Duda (1977a,b, 1979,1986) are some examples of free volume based treatments of transport behavior. Given the success of free volume models in describing diffusion behavior in rubbery polymers, it is natural to explore whether they can be employed also to describe the permeability behavior of gases in glassy polymers. Specifically, we show in this paper that, from our knowledge of sorption behavior in rubbery and glassy polymers and of diffusion behavior in rubbery polymers, we can reasonably predict the permeability of gases through glassy polymer membranes without using any experimental permeability data in the glassy state.

Comparison of transport properties of rubbery and glassy polymers and the relevance to the upper bound relationship

: An investigation was conducted into the development of a self-healing fiber reinforced polymer (FRP) composite based on a nanostructured epoxy matrix resin, recently developed by EIC Laboratories. This novel material comprises a blend of epoxy and amphiphilic block copolymer, which separates during cure to form three separate phases of high strength epoxy, rubbery polar polymer and rubbery nonpolar polymer. The high strength epoxy and polar rubbery polymer are arranged in cocontinuous networks, while the third nonpolar rubbery polymer is arranged as discrete domains evenly distributed throughout the material. Reactive monomers, added to the uncured mixtures, will segregate into one of the rubbery phases depending on compatibility and will be free to diffuse to and repair, by polymerization, any voids formed in the matrix through microcracking or other accumulated damage. Segregated from the epoxy phase, these monomers do not plasticize or otherwise compromise the thermal mechanical properties of the matrix system. This study verifies the structure of the EIC material and provides evidence for the incorporation of active monomers within this material. The effect of these monomers on the durability and mechanical properties of composites is also assessed.

In 1967, Lee published two papers on adhesion of high polymers(1,2) on the basis of the Buche—Cashin—Debye equation(3) $$[tex]D\eta = (A\rho kT/36)({R^2}/M)[/tex] $$ (1) where D is the molecular diffusion constant, η the bulk viscosity, A Avogadro’s number, ρ the density, k Boltzmann’s constant, T the absolute temperature, M the molecular weight, and R 2 the mean-square end-to-end distance of a single polymer chain. It was concluded that the physical state of the polymer determines the major adhesion mechanism involved. Polymer adhesion can be subdivided into rubbery polymer-rubbery polymer adhesion (R—R adhesion), rubbery polymer—glassy polymer adhesion (R—G adhesion), and rubbery polymer—nonpolymer—solid adhesion (R—S adhesion). Diffusion, which depends to a great extent on the physical state of a polymer, is actually a limited selective process. Thus, diffusion of rubbery polymers can take place at the interface, but diffusion of a glassy polymer at a viscosity of 1013 poise or a diffusion constant of 10-21 cm2/sec appears to be nearly impossible. On the other hand, physical adsorption is common to all three types of the above adhesion systems.

Dielectric elastomers (DEs) are a promising material for use in robotic, biomedical, energy, aerospace and automotive technologies. However, currently available DEs are limited by weak electromechanical coupling and our general understanding of DEs could improve. In this work, a multiscale model of dielectric elastomers is developed. At the molecular scale, an electrostatic response of a single DE monomer is assumed and, using statistical mechanics, the thermodynamics of a DE chain is investigated. This chain scale model leads to an important insight: the role of electrostatic torque on polymer chains in the electromechanical coupling of dielectric elastomers. This chain torque occurs because there is a connection between a chain’s end-to-end vector and its polarization. At the continuum-scale, this macromolecular phenomena manifests itself in the form of a deformation dependent susceptibility. Not only are novel modes of electromechanical coupling discovered, but also lessons learned from (standard) isotropic dielectric elastomers are then used to guide an in-depth analysis of the implicationsof designing and manufacturing anisotropic dielectric elastomers. The work in theoretical design reveals how the deformation and usable work derived from (anisotropic) dielectric elastomer actuators may be increased by as much as 75 - 100% relative to standard, isotropic dielectric elastomers.

Elastomer mechanical characterization and modeling are subjects that attract the scientific community since more than 50 years ago. However, despite the ample literature and the related scientific and industrial activity in this field, many different phenomena are not yet consolidated. With this Thesis, then, the Author wants to try to fill in some of the gaps while trying to use a 'hybrid' approach, for what concerns modeling and multi-axial characterization of filled elastomers by trying to use forefront techniques like Digital Image Correlation (DIC) together with mechanical testing machines able to perform non-standard biaxial tests, in static and dynamic loading conditions. With this purpose, a significant part of the research activities focused on the use and the partial redesign of some components of the BiaxTester, a planar biaxial testing device used to perform mechanical testing on polymeric materials. The PhD Thesis is then divided in two different sections: the first is composed by a brief introduction on elastomers and on active reinforcing fillers, together with some background about mechanical properties of filled rubbers and about the evolution of modeling of this kind of materials. Afterwards, some remarks of Continuum Mechanics in a nonlinear framework are included, with the purpose of underlining some of the aspects that will be encountered through the Thesis. The second part of the Thesis is instead based on the different research activities developed during the PhD: in Chapter 2 there is a first introduction to biaxial testing and about the aforementioned BiaxTester, followed by a section regarding the sample and clamping system optimization and results comparison obtained by using the optimized shape with some more conventional tests. In this section, the need to redesign the measurement process and control of the boundary conditions is shown to be fundamental in order to obtain reliable results in terms of stress-strain curves; the results obtained gave then the opportunity to choose the optimal configuration terms of homogeneity of deformations, with the aim of performing different types of tests on such materials. Successively, a novel approach to characterize mechanical behavior of elastomers through by imposing a planar biaxial loading state that follows certain paths of deformation, showing that different phenomena, such as relaxation and connected dissipative phenomena characteristic of the tires are not negligible in the case of loads in complex combinations. With this objective, it is shown how, by imposing displacements of the different clamps in two orthogonal directions (the two directions in which it is possible to impose deformations on these samples) different resulting forces will be registered, even in limit cases in which such complex loading combinations are reduced to equibiaxial loading. Moreover, it is shown how these phenomena are gradually more pronounced when increasing the reinforcement fraction and for bigger 'distances' of the biaxial path from the one imposed in the two orthogonal directions during the test. Chapter 3 focuses instead on the description of the Mullins effect in filled rubbers: for this reason, some basic knowledge about pseudoelasticity theory is given, in order to then focus on a model able to describe different phenomena found in filled elastomers, such as stress softening, hysteresis and residual strain after loading application. This is done by following some other approaches found in literature, and by revising some parts of the Dorfmann-Ogden model, based on internal variables which, together with the hyperelastic modeling, allows to describe the aforementioned phenomena. On the basis of the previously obtained results, Chapter 4 is focused on the crack propagation characterization in filled elastomers; this problem is of great interest due to the crucial importance of generalizing the concepts of stress intensification and of crack growth in complex loading conditions, with the objective of foresee and extend the fatigue life of filled elastomers undergoing cyclic solicitations also in case of complex geometries, such as car tires. Within this framework, the BiaxTester can be an interesting instrument thanks to its flexibility of applicable conditions; this will be shown in this work by comparing crack propagation data obtained through conventional testing setup and through the BiaxTester.

1. The chemical structure of spongy butadiene polymer, obtained at 15–20° was examined by the ozonolysis method. 2. The per cent of chains with external double bonds in the spongy polymer was found to be 22.8%. 3. The spongy polymer of butadiene is composed, like the rubbery polymers, of chains with external and internal double bonds. 4. It was established that parts of the molecule of spongy polymer have the same structure as the rubbery butadiene polymers: -1,4-1,4-; -1,4-1,2-1,4-; -1,4-1,2-1,2-1,4-. 5. A chloroform-soluble rubberlike polymer was separated from the butadiene autopolymer. The degree of unsaturation of the chloroform-soluble polymer was determined (86.7%), also the relative content of internal and external double bonds.

Unexpected brittleness: Does the major component in binary polymer blends always make sense?

The results of an experimental investigation of the deformation behavior of rubbery amorphous polymers (polybutadiene SKD) at low strain rates and large deformation times are described. It is shown for the first time by a viscometric method that the process of strain development in rubbery polymers has a stepwise character. A model that takes the breakdown of the structure into account is proposed for describing the stepwise nature of the strain development.

Polymer dense membranes have gas permselective properties. In general, the gas permeability of glassy polymers decreases with increasing gas size. The permselectivity of a large gas molecule to a small gas molecule is always smaller than 1. In contrast, the gas permeability of rubbery polymers increases with increasing gas size. The permselectivity of a large molecule to a small molecule is then always greater than 1. The transport of gases in polymer dense membranes is thought to obey a solution-diffusion mechanism. The total permeability is a product of the solubility and the diffusivity. Hence the permselectivity is a product of the solubility selectivity and the diffusivity selectivity. The solubility selectivity of a large molecule to a small molecule is larger than 1. In addition, the solubility selectivity of glassy polymers is the same as that of rubbery polymers. On the other hand, the diffusivity selectivity of a large molecule to a small molecule is always smaller than 1. Glassy polymers have stronger size-sieving ability compared to rubbery polymers. Therefore the diffusivity selectivity of a large molecule to a small molecule in glassy polymers is much smaller than that in rubbery polymers. The dominant factor to determine the permselectivity is the diffusivity selectivity for glassy polymers and the solubility selectivity for rubbery polymers. Interestingly some highly permeable glassy substituted polyacetylenes show the permeation properties opposite to other glassy polymers. Most glassy substituted polyacetylenes obey the behavior of common glassy polymers. However, some of them show the transport behavior like rubbery polymers. Their gas permeability increases with increasing gas size. Because these polymers have much larger fractional free volume compared to common glassy polymers, they show weak size-sieving ability like rubbery polymers. As a result, the solubility selectivity is dominant relative to the diffusivity selectivity unlike common glassy polymers.

Transformation of a Kurome natural lacquer film from glassy to rubbery polymer by the presence of moisture

A theoretical approach has been developed to describe the processes of gases diffusion and sorption in rubbery and glassy polymers. Various models (Flory-Huggins, dual-mode sorption, gas-polymer-matrix) used for interpreting the sorption-diffusion experiments are discussed within this approach framework. Experimental data on carbon dioxide sorption in glassy and rubbery polymers have been considered using the proposed approach. The comparison of the experimental and theoretical data has permitted to make the conclusion on the developed concepts adequacy. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1339–1348, 1997

Water sorption thermodynamics has been effectively investigated in rubbery and glassy polymers using, respectively, an equilibrium lattice fluid model, originally introduced by Panayiotou et al. [Panayiotou, C.; Tsivintzelis, I.; Economou, I.G. Ind. Eng. Chem. Res.2007, 46, 2628], accounting for hydrogen bond (HB) interactions in the system (i.e., the nonrandom hydrogen bonding, NRHB, model), and an extension of this model to a nonequilibrium glassy state (i.e., nonequilibrium thermodynamics for glassy polymers (NETGP)) that follows the same line of thought adopted originally by Doghieri and Sarti [Doghieri, F.; Sarti, G.C. Macromolecules1996, 29, 7885] to develop the NETGP approach. NRHB and NETGP-NRHB models have been used to interpret water sorption thermodynamics respectively in polycaprolactone and in polyimides. Model predictions in terms of self- and cross-HB established in the system are compared with quantitative information gathered from in situ infrared spectroscopy experiments, exploiting the wealth of information provided by proper elaboration of spectroscopy data by means of 2D correlation techniques.

Towards a predictive thermodynamic description of sorption processes in polymers: The synergy between theoretical EoS models and vibrational spectroscopy

The rheological behavior of the rubbery amorphous polymers SKB-35 (sodium-butadiene rubber), SKN-26M(butadiene-nitrile rubber), and PIB-85 (polyisobutylene) has been investigated in relation to the creep process. The tests were conducted at low shear stresses, in the constant shear stress regime, on the temperature interval from 0 to 100°C using a parallel-plate viscometer. We have shown, for the first time, by a viscometric method that in the high-elastic state rubbery polymers have a particular equilibrium structure corresponding to each temperature. A reduction in temperature leads to molecular ordering processes associated with an increase in the viscosity of the polymer. A temperature rise leads to molecular disordering and a gradual fall in viscosity. At a given temperature the two processes converge to the same value of the equilibrium viscosity. The rate of the process of equilibrium structure formation is determined not only by the temperature but also by the nature of the polymer. The higher the polarity, the more slowly the equilibrium structure is established.