Theo A. Tervoort

A multi‐mode approach to finite, three‐dimensional, nonlinear viscoelastic behavior of polymer glasses

In this study a phenomenological constitutive model is proposed to describe the finite, nonlinear, viscoelastic behavior of glassy polymers up to the yield point. It is assumed that the deformation behavior of a glassy polymer up to the yield point is completely determined by the linear relaxation time spectrum and that the nonlinear effect of stress is to alter the intrinsic time scale of the material. A quantitative three‐dimensional constitutive equation for polycarbonate as a model polymer was obtained by approximating the linear relaxation time spectrum by eighteen Leonov modes, all exhibiting the same stress dependence. A single Leonov mode is a Maxwell model employing a relaxation time that is dependent on an equivalent stress proportional to the Von Mises stress. Furthermore, a Leonov mode separates the (elastic) hydrostatic and (viscoelastic) deviatoric stress response and accounts for the geometrical complexities associated with simultaneous elastic and plastic deformation. Using a single set of...

A Constitutive Equation for the Elasto-Viscoplastic Deformation of Glassy Polymers

Constitutive equations for finite elastic-plastic deformation of polymers and metals are usually formulated by assuming an isotropic relation between the Jaumann rate of the Cauchy-stress tensor and the strain-ratetensor. However, the Jaumann-stress rate is known to display spuriousnon-physical behaviour in the elastic region. Replacing the Jaumann-stress rate by a Truesdell-stress rate results in an adequate description in the elastic region, but gives rise to a volume decrease during plastic flow intensile deformation. In this paper a ’’compressible-Leonov model‘‘ is introduced, in which the elastic volume response is rigorously separated from the elasto-viscoplastic isochoric deformation. This has the advantage that the model can be extended in a straightforward way to include aspectrum of relaxation times. It is shown that in the limit of small elastic strains, the compressible Leonov model reduces to the Jaumann-stress rate model, but diverges from the Truesdell-stress rate model. Finally, a comparison is made of the above mentioned models in ahomogeneous uniaxial tensile test and a homogeneous plane-stress sheartest, using polycarbonate (PC) as a model system. All models considered in this paper are ’’single mode‘‘ models (i.e. one relaxation time), and, therefore, cannot describe the full (non)linear viscoelastic region, northe strain-hardening or strain-softening response.

Strain-hardening behavior of polycarbonate in the glassy state

This paper presents an experimental characterization of the three-dimensional strain-hardening response of polycarbonate in the glassy state. Using a special mechanical conditioning technique, large homogeneous deformations were obtained in tension, compression, and shear. The experimental results are compared to a number of existing network models. It was found that the state-of deformation dependence of the strain-hardening response was adequately described by neo-Hookean behavior with a shear modulus G=26 MPa. Up to the deformations applied in this study, no sign of a finite extensibility of the entanglement network was observed.

Collagen nerve conduits releasing the neurotrophic factors GDNF and NGF

Artificial nerve conduits (NC) can clinically be instrumental for facilitating the surgery of damaged peripheral nerves. To improve axonal regeneration of injured peripheral nerves, we have developed collagen nerve conduits (NC) releasing glial cell line-derived neurotrophic factor (GDNF) alone or in combination with nerve growth factor (NGF), which exert synergistic action on axonal growth. Degradation of the NC and their mechanical and drug release properties were controlled by two means: (i) cross-linking the collagen tubes by physical means, through a dehydro-thermal treatment (DHT), before loading with the neurotrophic factors (NTFs) GDNF or GDNF/NGF; and (ii) coating the drug-loaded collagen tubes with layers of poly(lactide-co-glycolide) (PLGA). Non-cross-linked collagen NC (C-NC) released high amounts of NTFs during the initial 2-3 days of incubation, whereas the DHT-treated collagen NC (C(dht)-NC) did not show a prominent burst effect. The release kinetics was similar for GDNF alone and GDNF co-delivered with NGF. Within 30 days, the C-NC released 78% and 83% of the total doses of GDNF and NGF, respectively, whereas the C(dht)-NC released only 68% of GDNF and 56% of NGF. The bioactivity of the NTFs released up to 30 days was confirmed by an in vitro bioassay using chicken embryonic dorsal root ganglion (DRG) explants. The C(dht)-NC also possessed adequate mechanical resistance against radial compression, the pull-out of a suture thread, and loss of patency upon bending. Modulus and pull-out strength increased in the order of C-NC, C(dht)-NC approximately Neuragen, and Neurolac, with the latter two products being commercially available collagen and polyester NC, respectively. In vitro degradation time upon incubation with collagenase increased in the same order for the collagen-based NC. In conclusion, co-delivery of synergistically acting GDNF and NGF from structurally improved NC may be a promising tool for the successful repair of peripheral nerve defects.

Patterning of polymer-supported metal films by microcutting

The ability to micropattern materials is of great importance for manufacturing advanced electronic, optical and mechanical devices ranging from displays to biosensors. For this purpose a variety of methods have been developed, including X-ray, electron-beam and photo-lithography, microcontact printing, embossing, micromoulding and cold welding. But these techniques are often of restricted applicability, involve a multitude of elaborate and cumbersome processing steps, or require aggressive chemistry. Here we describe a simple and versatile way to create well resolved metallic structures on polymer substrates, which is based on solid-state embossing of metal-coated polymer films. Ductility of both the metal layer and the polymer substrate permits the metal to be cut into surprisingly regular, micrometre-size structures. We illustrate the method by preparing patterned electrically conducting structures, highly efficient infrared polarizers and polarization-dependent colour filters.

Solid-state replication of relief structures in semicrystalline polymers

The feasibility of structuring the surfaces of semicryst. polymers is explored using poly(tetrafluoroethylene-co-hexafluoropropylene) as a model material. Hot embossing was performed in the polymer melt at 330 Deg after which the samples were quenched at room temp. [on SciFinder (R)]

Deformation and toughness of polymeric systems: 2. Influence of entanglement density

Abstract In part 1 of this series, the concept of a critical material thickness was introduced and demonstrated experimentally using polystyrene (PS) as a test material. Below the critical thickness, brittle polymers become ductile. The value of the critical thickness is material-dependent and related to the entanglement density. The dependence of the critical thickness on the entanglement density was investigated using the miscible system polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE). PS possesses a low entanglement density and PPE a high entanglement density, and the systems entanglement density can be varied depending on the relative ratio of PS and PPE in the mixture. Equivalent to the experimental procedure developed in our previous paper, the thickness was set by either changing the PS-PPE layer thickness in stratified PS-PPE/PE tapes (polyethylene (PE) is present to separate the PS-PPE layers) or by adjusting the volume fraction of non-adhering core-shell rubbery particles in the PS-PPE blend, i.e. the ligament thickness. The experimentally determined critical thickness (IDc) proved to increase continuously from 0.06 μm for PS-PPE 80–20 to 0.18 μm for PS-PPE 40–60 blends. This compares well with the value of 0.05 μm found for pure PS. Under the (moderate) testing conditions used, the PS-PPE 20–80 blend was always tough. The maximum macroscopic strain to break (λmacr) of the PS-PPE blends correlated with the theoretical value (λmax) based on stretching the entanglement network to its full extension. The transition from a macroscopic, brittle-to-ductile deformation behaviour is associated with a change in type of deformation mechanism from void formation (e.g. crazing) to shearing, except for the PS-PPE 20–80 blend, which always deforms by shear deformation. A simple model based on an energy criterion could explain the occurrence of a critical material ligament thickness as well as its dependence on the entanglement density.

Synthesis and Characterization of Surface-modified Rutile Nanoparticles And Transparent Polymer Composites Thereof

Nanoparticles of rutile, a crystal modification of titanium dioxide, were synthesized in strongly acidic solutions by hydrolysis of titanium tetrachloride. The particles of average diameter 2 nm were coated in situ with a layer of dodecylbenzenesulfonic acid (DBSA) and isolated as a powder. Remarkably, dispersions of this powder in toluene were essentially transparent at the visible wavelengths but absorbed UV radiation over a broad wavelength range. The DBSA-coated rutile was also embedded in poly(styrene) and a poly(carbonate), resulting in polymer nanocomposites acting as visually transparent UV filters.

Ultra-high performance photoluminescent polarizers based on melt-processed polymer blends

Photoluminescent polarizers that comprise uniaxially oriented photoluminescent species which absorb and emit light in highly linearly polarized fashion, can efficiently combine the polarization of light and the generation of bright colors. We here report the preparation and characterization of such polarizers by simple melt-processing and solid-state deformation of blends of a photoluminescent guest and a thermoplastic matrix polymer. The orientation behavior of a poly(2,5-dialkoxy-p-phenyleneethynylene) derivative (EHO-OPPE), 1,4-bis(phenylethynyl)benzene, and 1,4-bis(4-dodecyloxyphenylethynyl)benzene was systematically compared in different polyethylene grades. Experiments suggest that if phase-separation between the photoluminescent guest and the matrix polymer is reduced during the preparation of the pristine (i.e. unstretched) blend films, photoluminescent polarizers can be produced which exhibit unusually high dichroic properties at minimal draw ratios. In connection with this finding, an optimized, melt-processed blend based on 1,4-bis(4-dodecyloxyphenylethynyl)benzene and linear low-density polyethylene was developed that allows efficient manufacturing of photoluminescent polarizers which at draw ratios of only 10 exhibit dichroic ratios exceeding 50.

Thermo-reversible Liquid-Crystal Gels: Towards a New Processing Route for Twisted Nematic Displays

Thermo-reversible gels of the liquid-crystal system LC-TL213 with 12-hydroxyoctadecanoic acid (HOA) were cast in a twisted nematic (TN) configuration that is switchable by AC electric fields. Critical in this process was to choose a processing temperature higher than the sol–gel temperature but lower than the nematic-isotropic transition of the system. The latter condition originates in the phase diagram of the system which displays a monotectic-type point at which a liquid is in equilibrium with a solid and a mesophase. The solid-like nature of the gels produced, combined with their electro-optical activity, opens the possibility to manufacture flexible TN cells via continuous processes as an alternative to the currently employed batch-type methods.