Witold Brostow

Epoxy+liquid crystalline epoxy coreacted networks: I. Synthesis and curing kinetics

Abstract In situ copolymerization of diglycidyl ether of 4,4′-dihydroxybiphenol (DGE-DHBP) with diglycidyl ether of bisphenol F (DGEBP-F) networks using an anhydride curing agent has been investigated. DGEBP-F is a commercial epoxy while cured DGE-DHBP shows liquid crystal transitions. Curing kinetics are determined using differential scanning calorimetry (DSC). The data were fitted using an autocatalytic curing model for both pure and mixed components. Isothermal and non-isothermal methods were compared. The glass transition ( T g ) was evaluated as a function of composition using DSC. The results show that the DGE-DHBP constituent affects the curing kinetics of the epoxy resin and that the network exhibits one T g .

Fluoropolymer addition to an epoxy : phase inversion and tribological properties

Abstract Morphology, elastic modulus, static and dynamic friction of a commercial epoxy resin+fluorinated poly(aryl ether ketone) (12F-PEK) system have been studied. The system was cured at 24 and 70°C. We have obtained significant friction lowering, namely 30% less than the value for the plain epoxy at the 12F-PEK concentration of only 10xa0wt% after curing at 24°C. By contrast, after curing at 70°C, increases in both static and dynamic friction are observed. At 24°C, the SEM analysis shows that a phase separation starts at 10% 12F-PEK; completely phase-inverted morphology was found above 30% of the fluoropolymer. The system cured at 70°C has more compact structure and the phase separation process is shifted towards higher fluoropolymer concentrations. We explain the phase inversion at low fluoropolymer concentrations by the low free surface energy of 12F-PEK; as a consequence, we observe its accumulation on the surfaces of the samples. The samples with high 12F-PEK concentrations exhibit elevated elastic modulae and are highly ductile.

Hydroxyapatite spheres with controlled porosity for eye ball prosthesis: processing and characterization.

Porous hydroxyapatite spheres were prepared by a modified gelcasting method producing a ceramic prosthesis with controled porosity. The spheres are approximately 2.2 cm in diameter with a relatively homogeneous pore size distribution from 10 to 40 μm in diameter. The samples were characterized by X-ray powder diffractometry (XRD) and Fourier transform infra-red spectroscopy (FTIR) to identify the phases both prior to and after the gelcasting process. Surface morphology analysis and porosity evaluations were performed with scanning electron microscopy (SEM), while surface area measurements were carried out by the BET technique.

Long-term service performance of polymeric materials from short-term tests: prediction of the stress shift factor from a minimum of data

We use the time‐stress correspondence (TSC) principle and consider creep compliance D data for a polymer liquid crystal (PLC). A master compliance plot is created from two stress s levels only; the lower stress level is the reference while the other set is shifted on the logarithmic time axis. The master plot so created is quite close to that constructed after a significant amount of experimentation at several s levels. A formula for stress shift factor as obtained by one of us is applied for the prediction of as values for the other s levels. Again, a low s level is used as the reference while D values for two higher s levels are used in calculations. The predicted as values agree with the experimental ones within the limits of the experimental accuracy. Very close values of the Doolittle constant B are obtained from different subsets of minimum amounts of results and also from the full set of experiments for nine stress levels. The accuracy of the results is affected by the stress range covered (a wider range gives better results) but not by the amount of data used. q 2001 Published by Elsevier Science Ltd.

Graphical modeling and computer animation of tensile deformation in polymer liquid crystals (PLCs)

Abstractu2002We create polymer liquid crystals (PLCs) on a computer and subject them to constant-force tensile deformations. Molecular dynamics simulation procedure is used and graphical models and animations of crack formation and propagation as a function of time are generated. Special attention is given to realistic rigid LC island geometry and the island spatial distribution in the material. Internal fracture processes can now be easily studied. As a result of the numerous animations recorded, clear patterns in PLC crack initiation and propagation emerge. With the help of the animations, the structure of PLCs may be understood without having to resort to costly laboratory experimentation.

Orientations and phase transitions in liquid crystals consisting of short linear polymer chains

The system of semiflexible linear polymer liquid crystal (PLC) macromolecules is studied. Each macromolecule constitutes an alternating chain of flexible (F) and rigid (LC) sequences. The distribution function of the chain conformations is factorized in three terms. The Gibbs distribution is used for anisotropically interacting LC sequences; products of the Dirac delta functions represent F sequences modeled by linear chains of freely jointed statistical segments; connections of LC and F sequences in a linear chain are controlled by the Dirac delta functions with a proper argument. The general formula for the Helmholtz function A for arbitrary types of anisotropic interactions between LC sequences and for an arbitrary number of statistical segments per flexible part of linear chain obtained by the present authors [J. Chem. Phys. 105, 4367 (1996)] is applied in numerical calculations performed for some special cases. The cases selected here are (a) the Maier and Saupe mean-field limit formula for LC+LC int...

Theory of thermotropic polymer liquid crystals

The thermal nematic system consisting of semiflexible polymer liquid crystal chains is considered. Orientational order for liquid crystal particles and for chain as a whole in addition to the chain anisotropy parameters at the nematic-to-isotropic transition and in the stable nematic phase are considered. Results are obtained by extension of the Maier and Saupe theory of thermotropic liquid crystals on systems of semiflexible macromolecules.

Instruction in materials science and engineering : modern technology and the new role of the teacher

Materials Science and Engineering (MSE) has been created by combining disciplines that are several thousands years old (Metallurgical Engineering) with quite recent ones (Solid State Physics). It includes atomic and molecular interactions and structures, synthetic chemistry, phase equilibria and phase diagrams, a variety of characterization methods, mechanical testing and fracture mechanics, visco-elasticity, rheology, materials processing from forging through sintering to injection molding, as well as prepreg making and composite manufacturing. Different sub-disciplines of MSE often require different instructional approaches. Teaching a single basic MSE course requires a combination of approaches. Fortunately, we have an increasing number of instructional tools: computer modeling (watching a known process); computer simulations (learning about an insufficiently understood process); interactive computer programs; TV broadcasting of lectures followed by discussions by phone, both video-recorded; the world wide web information accessible via search engines; the Journal of Materials Education; and traditional textbooks and class lecture notes. Moreover, MSE also exhibits certain unifying features (not limited to just, say, metals or semiconductors); these features are also discussed. The most important consequence of the existence of new technologies is the change in the role of the instructor. Instead of mostly dispensing knowledge, the instructor now has to make decisions. Particularly important in the new role of the teacher is his:her capability to make the student manage his:her time in a much more efficient way.

The Influence of External Electric Field on Local Orientations and Phase Transitions in Polymer Liquid Crystals (PLCs)

Full Paper: The starting point is our previous study of influence of the internal molecular mean field of dipoledipole interactions on local orientation and phase transitions in polymer liquid crystal (PLC) systems of longitudinal chains. [1, 2] Electric dipoles are created by LC mesogen moieties. The longitudinal PLC is a macromolecule of consecutively copolymerized LC and flexible polymer sequences. We now amplify the model by inclusion of dipole-external electric field interactions. We find that the external fields can seriously modify the local orientational order of the system and affect phase transition parameters dependent on that order. In particular, the external fields induce the formation of disoriented nematic phases with negative values of the second order orientation parameter pP2P for LC sequences in the longitudinal PLCs while the first order parameter pP1P is positive. However, some rapid decreases in pP1P are observed at points of positiveto-negative transitions of pP2P; thus the LC disorientation manifests itself. The limiting case of the monomer liquid crystal (MLC) systems is included also.