R-3 Repository :: Browsing by Author "Armeniades, C. D."

Browsing by Author "Armeniades, C. D."

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A constitutive equation for creep in glassy polymers and composites
(1988) Kumar, Sandeep; Armeniades, C. D.
The creep of polymethyl methacrylate was investigated in four-point flexural loading mode. Measurements were taken at temperatures from 8$\sp\circ$C to 55$\sp\circ$C, time periods up to 450 hours and stresses ranging from 5 to 25 MN/m$\sp2$. The data obtained were successfully superposed vertically; the data reduction, in this way, was expressed in the form of a constitutive equation: e(t, T, S) = e$\sb0$ (ref). exp $\lbrack-(\Delta$H$\sb0$ $-$ $\beta$S)/R. (1/T $-$ 1/T$\sb{\rm ref}$)).exp ($\beta$/RT. (S $-$ S$\sb{\rm ref}$)). t$\sp{\rm n}$ which shows that the creep strain (e) may be obtained as a product of separable functions that express the effect of time (t), temperature (T) and stress (S). Subscript ref. indicates the chosen reference state. The creep behavior follows a power law time dependence with an exponent equal to 0.24. The apparent activation energy of the creep is independent of temperature (Arrhenius behavior), stress dependent and decreases with increasing stress.
Characterization of molecular motions in polymer gel networks using intensity-fluctuation spectroscopy
(1978) Schulken, Roger Moore; Armeniades, C. D.
This work seeks to investigate the intrinsic dynamics of synthetic polymer gel networks in terms of cooperative submolecular motions, which give rise to autocorrelation functions, observed by intensity fluctuation spectroscopy. Poly(dimethyl siloxane) samples were purified and chemically crosslinked to three levels of crosslink density. The resulting networks were swollen in toluene. Light scattering data were obtained over a range of swelling ratios from each sample as well as from the uncrosslinked polymer in solution. Due to a large amount of extraneous scattering, indicating the presence of a large component of static scattering, a "background" normalization technique was devised which allowed extraction of the desired correlation function. Also, a computer program utilizing the "method of cumulants" was developed to obtain diffusion coefficients from nonlinear autocorrelation decay data. The "diffusion coefficient" DT calculated for the uncrosslinked polymer in solution indicates scattering entities of about 6 A. This corresponds to a molecular weight of about 16,, considerably less than the molecular weight of the polymer (16,). The values for the diffusion coefficient were found to increase with increasing concentration, contrary to intuition. A similar increase in DT with polymer concentration was observed in swollen crosslinked gels. Also, there was evidence that the diffusion coefficient decreases with increasing degrees of crosslinking. A qualitative model was developed which describes a system of cooperative submolecular scatterers, consistent with the observed changes in D. The results of the light scattering measurements were also compared to mechanical measurements of the tensile elastic modulus for the different crosslinked systems. These tests indicate that not all of the available sites were utilized in the crosslinking reactions.
Cryogenic mechanical properties of polychlorotrifluoroethylene
(1974) Ross, Michael; Armeniades, C. D.; Roberts, John M.
Polychlorotrifluoroethylene (PCTFE), crystallized in two degrees of crystallinity, was subjected to tensile stress deformation experiments at several strain rates and temperatures in an attempt to relate the macroscopic deformation and failure characteristics of the polymer with its molecular organization and segmental mobility. At 3K, the higher crystallinity samples exhibited a higher tensile modulus, yield stress and a lower elongation to failure than lower crystallinity samples. At 78k and 4.2K, the lower crystallinity samples showed higher tensile moduli yield stress, stresses at failure and elongations to failure than did the higher crystalline samples. Sequential still photographs of the deforming sample were taken in the 3K experiments to determine the true-stress vs. true-strain curve for PCPFE. Fracture surfaces were examined with a scanning electron microscope to determine the mode of fracture. PCTFE appears to fail in a five-step process at 3K, in a three-step process at 78K and in a two-step process at 4.2K.
Cure shrinkage control in polymerization of multicomponent resin systems with phase separation
(1993) Becker, Christian Guy; Armeniades, C. D.
This work investigates the volume changes effected during multicomponent resin polymerization/crosslinking (cure shrinkage) in an effort to reduce or eliminate this phenomenon (zero-shrinkage polymerization). The systems under study contained: a polyfunctional monomer; a second, difunctional low-boiling monomer; and a thermoplastic polymer additive. These components are miscible forming a single (transparent) phase, which remains stable and shows the expected cure shrinkage in slow polymerization. Upon rapid cure (within 2-10 minutes) these systems undergo phase separation and show significantly reduced cure shrinkage, attaining zero shrinkage within certain composition ranges. Experimental evidence based on Scanning Electron Microscopy, polymerization under pressure, photopolymerization, and solvent extraction indicates that the reduction in cure shrinkage is due to the nucleation and growth of vapor bubbles of the low-boiling monomer within separate microdomains formed during rapid polymerization and precipitated by the high temperatures attained at peak exotherm, as well as negative hydrostatic pressures arising from cure shrinkage of the crosslinked polymer phase.
Cure shrinkage control of polymerization systems
(1990) Liu, Chang-Feng; Armeniades, C. D.
Cure shrinkage is an inherent property of polymerizing systems due to the conversion of secondary bonds between monomer (or prepolymer) molecules to primary bonds, which have smaller interatomic distances. Cure shrinkage is highly undesirable: it impairs dimensional control and causes poor surface finish in molded polymers; it also generates setting stresses in highly filled systems. Previous methods for cure shrinkage control require special materials and conditions or entail the formation of voids. In this investigation, two processes were developed for producing polymer systems with zero shrinkage or slight expansion: (a) use of ammonia-modified montomorillonite as additive; and (b) microphase separation. The first method utilizes the dilatation of specially-modified montmorillonite (MMT) particles to counteract resin polymerization shrinkage. The MMT particles are first processed by replacing part of their hydration water with ammonia (which forms coordination bonds with SiO$\sb2$ in the mineral crystal) then dispersed into the resin. During cure at ambient temperatures the polymerization exotherm raises the temperature to 60-80$\sp\circ$C, breaking the SiO$\sb2$-NH$\sb3$ bonds; however the liberated gaseous ammonia cannot escape outside the resin-embedded MMT particles and forces them to dilate to more than twice their original size. By controlling the amount of ammonia-modified MMT added to the resin (in amounts of 6-10%) we obtain cured systems that show zero shrinkage and have no setting stresses. This can increase their strength by ca. 20-40%. The second method achieves cure shrinkage control by microphase separation. Certain multicomponent acrylic systems, when polymerized rapidly, separate into microdomains of different phases with a corresponding reduction in cure shrinkage. We attribute this phenomenon to lower efficiency in molecular chain packing at the interphase boundaries; the phase separation itself is attributed to local, diffusion-controlled composition changes during rapid cure. Accumulation of the local volume increase as the microphase boundaries gives rise to the observed reduction in cure shrinkage.
Development of a synthetic trileaflet aortic valve prosthesis
(1976) McKinley, James Robert; Armeniades, C. D.
The research reported herein has been an attempt to produce a trileaflet aortic valve prosthesis using a synthetic cusp material with mechanical properties similar to the natural tissue. Natural leaflets exhibit high compliance and pronounced anisotropic behavior. Using the stress-strain behavior of natural tissue as a guide, a synthetic composite material was formed which resembles the native tissue in terms of anisotropy and compliance. The composite is made of a thin knitted dacron fabric coated with a layer of a recently developed, non-thrombogenic polyurethane. The dacron knit simulates the collagenous region of the native material, providing the distinct mechanical properties. The polyurethane provides a continuous, non-permeable, blood compatible coating. A significant portion of the research involved selecting methods to describe the geometry of the aortic valve and to translate the geometry into techniques for forming valves. All the valves were formed in the open position. The cusps are described in the open, excised configuration as semi-ellipses with a ratio of semi major to semi minor axis of 1.3. A series of all-polyurethane valves were formed intact in mounting tubes, via a solvent casting process, for use in ventricular assist devices where the performance criteria are less stringent. One of these valves includes physiologically shaped simulated sinuses. Preliminary mock loop testing of this valve indicates excellent performance. The intact polyurethane valves are attractive candidates for incorporation into the left ventricular bypass pumps under development at Rice and Baylor. Trileaflet valves were constructed from the anisotropic composite material. In a pulsatile mock circulatory loop, the composite material failed. Separation occurred between the dacron and polyurethane fabric on portions of at least one cusp, rendering each valve incompetent. At the present state of development, the composite material made of these particular components is unsuitable for a prosthesis.
Enzymatic digestion of the lens as an alternative to cataract surgery
(1988) Paul, Laura Lea; Armeniades, C. D.
The purpose of this research was to determine the effects of enzymatic digestion on the human lens, with the ultimate goal of rendering cataract surgery obsolete. A system for measuring the consistency of a lens, using an aspiration technique, was developed and was used to quantify the changes that occur in a lens after injection with one of four enzymes: pronase E, sphingomyelinase, phospholipase C, and carboxypeptidase B. Pronase was found to be the most effective enzyme with 20% more of the treated lens aspirating when compared to the untreated lens. The amount of pronase injected and the incubation time were varied and the response of the lens was measured. Pronase and sphingomyelinase were also used in conjunction with phacoemulsification to determine if one treatment would increase the effectiveness of the other. No synergism was found between the enzyme treatments and phacoemulsification. Other enzymes and techniques are suggested for further study.
Flexural creep in pure and filled acrylic resin systems
(1990) Nandi, Ranjan Kaushik; Armeniades, C. D.
The creep behavior of poly (methyl methacrylate) and acrylic polymer concrete (PC) systems based on methyl methacrylate was studied in flexure. Creep in both pure and filled systems followed a power law time dependence. A differential form of the power law equation was used to eliminate the irregularities associated with time-independent and transient strains in the experimental data. de$\sb{\rm tot}$/dt = ln nk + (n $-$ 1) ln t where e$\sb{\rm tot}$ is the total strain and n and k are constants in the power law equation. Plots of creep rates versus time yielded good linear fits to the data, the slopes providing values for 'n', the power exponent. Vertical superposition was then used to reduce data at different stresses and resin contents, for the PCs onto a single master curve using different shift-factors. This data reduction was expressed in the form of an empirical expression: e(t, S, v) = e$\sb0$(ref). EXP (K$\sb{\rm S}$. (S $-$ S$\sb{\rm ref}$)). EXP (K$\sb{\rm v}$. (v $-$ v$\sb{\rm ref}$)). t$\sp{\rm n}$ which expresses the total creep strain as a product of separable functions of time (t), stress (S) and the resin volume fraction (v).
Incorporation of carbon nanotubes in epoxy polymer composites
(2005) Kim, Jong Dae; Armeniades, C. D.
The goal of this research was to develop and explore methods for incorporating Single-Walled Carbon Nanotubes (SWNTs) into polymer matrices, to form composites, in which the presence of SWNTs would improve significantly their mechanical properties, electrical conductivity, and thermal conductivity. We have used the concept of interpenetrating polymer networks (IPNs) for the design of our composites. One (preformed) network would consist of tangled SWNTs. It would be penetrated and swollen by the prepolymer molecules, which would polymerize in place, forming the second (interpenetrating) network. For the polymer matrix we chose epoxy resin systems, because of their chemical versatility, low setting stresses, and good mechanical properties. The main problem to be overcome was the very pronounced tendency of SWNTs to aggregate and the inability of polymerizable organic molecules to penetrate the aggregates and disperse the SWNTs. Several attempts to disperse SWNTs into an epoxy-acrylate system, specially formulated for low viscosity, using sonication energy were not successful. Neither were attempts to infuse epoxy prepolymer into the pores of bucky paper. A novel process was developed, which uses solvent in conjunction with sonication to disperse SWNTs into epoxy prepolymers, then divides the system into droplets that are sprayed into a heated chamber and deposited on a heated substrate. Control of the relative rates of solvent evaporation and polymerization/cure resulted in the formation of composites with dispersed SWNTs. Such composites, containing 1 wt. % SWNTs showed a drop in electrical resistivity of ca. 14 orders of magnitude (5.0 Ohm•m, compared to 2.0 x 1014 Ohm•m for pure epoxy). An efficient method for making SWNTs/epoxy/fiberglass composites was developed by incorporating SWNTs on the surface of fiberglass using the incipient wetting method. Small amount of SWNTs (0.1 wt. % of SWNTs) increased the flexural strength of the composites by 13%. This method can be extended to other fiber reinforcements such as Kevlar, carbon fiber, and ceramic fibers. SWNTs were chemically bridged both glass fiber and Epoxy matrix in the glass fiber/SWNTs/Epoxy composites. Fluorinated SWNTs (F-SWNTs) were chemically reacted with amine group active silane agent coated glass fiber. Chemically coated SWNTs-glass fiber was further reacted with Epoxy matrix. Surface analysis of chemically coated SWNTs/glass fiber confirmed the introduction of chemical bonding between glass fiber and F-SWNTs. (Abstract shortened by UMI.)
Investigation of a polymer concrete composite material for runway applications
(1983) Zeilenga, Jay Howard; Armeniades, C. D.; Hightower, Joe W.; Roberts, John M.
The use of fiberglass cloth layers as flexural reinforcement for polymer concrete was investigated. The influences of resin systems, cloth, aggregate, fine fillers, moisture content of aggregate, and specimen depth on the properties of the composite material were studied. The laboratory investigation was oriented towards the composite’s considerable potential for panel applications and possible application as a pavement material. The concept of quickly installed runways made of this polymer concrete composite material is discussed. The composite with two bottom layers of a light-weight glass cloth exhibited twice the flexural strength and three times the elongation limit of polymer concrete alone. A silane coupling agent was shown to improve the retention of strength of polyester concrete made with wet aggregate for moisture contents below three percent. The thickness of the composite, and of the polymer concrete alone, only slightly affected the flexural strength. This lack of sensitivity to specimen depth is important when considering production of panels of varying thicknesses. The ability of the composite to deform considerably, and its higher strength in excess of that for polymer concrete and ordinary concrete was noted as being desirable for constructing thin, durable, fast-setting pavements over weak and non-uniform subgrades.
Investigation of a tracer diffusion model used in void volume determination in biologic tissue
(1978) Johnson, Gregory H.; Armeniades, C. D.
The morphology of load-bearing tissues suggests that their internal void spaces fall into two different size distributions: macrovoids, the spaces between large morphological entities of several y in size, and microvoids, the spaces between structural elements. A mathematical model proposed by Lake and Armeniades gives an estimate of the fraction (F) of macrovoids in a given tissue on the basis of the rate of out-diffusion of ionic species from the tissue under controlled conditions. Knowledge of F would provide information on the state of aggregation of fibers or other structural elements in the tissue. This work seeks to determine whether the proposed model gives a true representation of the out-diffusion behavior in real systems with different pore size distributions. An inert porous material (Mi11ipore filter material type AAWP) was selected which contained two pore size populations. The properties of the material which correspond to the parameters of the model were observed by scanning electron microscopy. All parameters were quantified except D1, the tracer diffusivity in the microvoid region, and D2, the tracer diffusivity in the macrovoid region. D1 and D2 were varied to provide an exact fit of the model to the out-diffusion data. Variations of the parameters of the mathematical model show that the out-diffusion curve can be matched by only one unique value of F, regardless of how well the other parameters of the model are known. Out-diffusion experiments were carried out on tissue. The qualitative out-diffusion behavior of bovine aorta and pericardium suggests that the out-diffusion model may be used to obtain F with confidence. Variation in F and the sample half-thickness (M) for both tissues produces qualitative variation in the out-diffusion behavior which is predicted from the model.
New polymeric compositions for nonflammable and thermally stable foams
(1992) Bravo, Fabio; Armeniades, C. D.
There is a great need to develop new polymer foam systems that combine the advantages of polyurethane foams (low cost, ease of processing, desirable physical properties) with nonflammability and thermal stability which polyurethanes lack. In this project we address this need by synthesizing novel polymer networks with a substantial content of nonflammable elements. Specifically most of the pendant hydrogens have been eliminated (or replaced with fluorine), and a substantial fraction of the carbon atoms are replaced with oxygen, phosphorus, and nitrogen. Other elements such as Ca, Mg, Al, Sb, V, and B are also incorporated in the polymer network to enhance nonflammability. The selected phosphorus compounds are in their highest oxidation state (+5) and the nitrogen is present in the form of five or six-member heterocyclic rings. Following these guidelines, several rigid, semi-rigid, and flexible, foamed-in-place foams with outstanding nonflammability and thermal stability have been developed.
Polymers containing metal ions complexed by nitrogen-containing ligands
(1979) Bell, David Allen; Armeniades, C. D.; Leland, Thomas W.; Sass, Ronald L.
The feasibility of synthesizing the coordination compound [equation] and using it, as well as the complex [equation] to create polymer systems incorporating these complexes was investigated. These metal-organic complexes were used in epoxy-amine and formaldehyde-amine type polymerizations. The[equation] complex was synthesized according to Maerker's (1957) technique. This produced an impure product that could not be purified by recrystallization. Attempts to synthesize a pure complex by isolating the ligand and then reforming the complex failed because the instability of the complex in weak acids prevented the coordination reaction from occurring in suitable solvents, since the metal salts formed weak acids when dissolved. An unsuccessful attempt was made to synthesize a polymer from the impure[equation] complex and formaldehyde using techniques similar to those employed in the commercial manufacture of amine formaldehyde polymers. An epoxy crosslinking reaction was not attempted because no solvent could be found that would be compatible with both the complex and the epoxy resin. A molding powder synthesized from [equation] and formaldehyde failed to produce a polymer during compression molding because the complex decomposed on heating. An epoxy crosslinking reaction was not attempted. A polymer was synthesized from formaldehyde and 4,4' methylenedianiline using techniques similar to those commercially used in amineformaldehyde polymer manufacture. The polymer was brittle and inhomogeneous. A commercially available instrument used to measure the dynamic mechanical properties of polymers, the Rheovibron manufactured by the Toyo Measuring Instruments Co., was modified. These modifications extended the low temperature measurement limit from slightly below °C to liquid nitrogen temperatures. This instrument was then used to measure the dynamic mechanical properties of the 4,4' methylenedianilineformaldehyde polymer mentioned above and a polymer synthesized from 4,4' methylenedianiline and Epon 828, an epoxy resin made by the Shell Chemical Co. Measurements were taken over a temperature range of -17°C to +4°C.
Rheometric studies on the in vitro polymerization of microtubules
(1984) Nelson, Rickey L.; McIntire, Larry V.; Hellums, Jesse D.; Armeniades, C. D.
Microtubules form weakly associated networks with many properties similar to networks of rigid rods. To study the in vitro polymerization of bovine brain tubulin, a Fluids Rheometer was used in the oscillatory mode. The maximum elastic modulus for microtubule networks in the standard PIPES buffer was observed to be proportional to the tubulin concentration over much of the investigated range. A maximum elastic modulus of 2 dynes/cm2 occurred at a tubulin concentration of 8 mg/ml. At even higher concentrations, a decrease in the maximum elastic modulus was observed. Other additions to the standard buffer primarily affected the interactions between microtubules as opposed to shifting the overall polymer-heterodimer equilibrium. 9% deuterium oxide increased the maximum elastic modulus by a factor of 4, while 25% glycerol led to approximately a doubling in the maximum elastic modulus when compared to microtubule networks in the standard buffer. Microtubule associated proteins (MAPs) were shown to inititate the formation of microtubules when added back to purified tubulin solutions. Even though more microtubules were formed as the MAPs were added, the elasticity decreased with increasing MAPs additions because shorter, more numerous microtubules were present. The interactions between bovine brain tubulin and rabbit muscle actin were also investigated. The resulting networks were found to be extremely strain sensitive but had elastic moduli considerably greater than the sum of the individual components. The MAPs fraction was found to cross-link actln filaments with the maximum interaction occurring at .3 mg MAPs/mg actln, corresponding to 1 mole of MAPs per 23 moles of actin. The extent of phosphorylation state of the MAPs also affected the interaction. At an actin concentration of 1 mg/ml, MAPs with a lower phosphate content would form a network with a maximum elastic modulus of 4 dynes/cm. However, at approximately the same actin and MAPs concentrations the maximum elastic modulus from MAPs with a higher phosphate content was 25% lower at 3 dynes/cm.
Shear induced tissue factor activity in human leukocytes
(1983) Pankowsky, Dan A.; McIntire, Larry V.; Hellums, Jesse D.; Armeniades, C. D.
There is a growing amount of evidence a significant link exists between the body's immune system and blood coagulation pathways. Specifically, the monocyte, a peripheral blood leukocyte, has been found to be the source of leukocyte tissue factor activity (TFA). This TFA activates the extrinsic blood clotting pathway. It is becoming apparent that there are two pathways for monocyte TFA production a lymphocyte dependent pathway and a lymphocyte independent pathway. Problems of blood clotting and thrombus formation in clinical situations include the use of extra-corporeal circulation and artificial heart valves. In addition disease states such as atherosclerosis and rheumatic heart disease are often associated with (or in some instances possibly caused by) coagulation abnormalities. These disease states also have in common non-physiologic levels of shear stress applied to the blood. The possibility exists that there is a significant link between the applied shear stress in the blood and the development of pathological coagulation. Since monocytes are known to be a source of TFA in response to various stimuli, this study was undertaken to measure monocyte TFA in response to shear stress. It was found that mechanical trauma could induce tissue factor production and that shear-induced TFA required the presence of lymphocytes. That the functional capability to produce TFA was there after shear stress in cells not producing TFA was demonstrated by subsequent treatment with endotoxin. Therefore, in clinical situations where mechanical trauma to leukocytes is a problem, the possibility of leukocyte TFA needs to be considered.
Shear stress effects on epithelium cell cultures in vitro
(1983) Stathopoulos, Nikos A.; Hellums, Jesse D.; Armeniades, C. D.; McIntire, Larry V.
This study aims to determine the influence of shear stress on cell cultures under carefully controlled conditions. Human embryonic kidney cells grow^as an attached, confluent monolayer on a flat substrate were subjected to steady, uniform laminar flow in a specially designed flow chamber, in which shear stress was precisely defined and controlled. Experiments performed for shear stresses ranging from .2 to 6. N/m, with a varied shear stress exposure time from 2 to 24 hours. The influence of the shear field was slight at low shear stress (.26 N/m) but it had a dramatic effect on cell morphology at higher shear stresses (above 2.6 N/m ) and long exposure times (24 hours). It was also found that shear stress and time stimulated the release of urokinase enzyme. The significance of such flow studies is presented in relation to their possible use for biosynthesis of urokinase enzyme by kidney epithelial cells/
Structure-property relations in epoxy metal-ion polymers
(1977) Oma, Kenton H.; Armeniades, C. D.
The feasibility of crosslinking a diepoxy (EPON 828 Shell Chemical Company) with paraaminopyridine (PAP) coordinated to various metal-ions was investigated by means of wide angle x-ray diffraction, infrared spectroscopy, and metallographic examination. Insolubility of the metal-ion ligands in EPON 828, resulting from their ionic nature, necessitated the development of a solution polymerization based on acetone. The effect of molecular structure on the internal friction of these polymers was studied using dynamic mechanical measurements at temperatures from 8°K to Tg with a free oscillating inverted torsional pendulum at 1 Hz. Dynamic mechanical properties of a model system were established by studying the chemically and sterically similar, EPON 828-methylene dianaline (MDA) polymer. The y relaxation temperature, intensity, and the modulus defect decreased with stoichiometrically excessive epoxy and the y relaxation intensity increased with excess MDA. As expected, Tg was maximized in a stoichiometric system. A new mechanical relaxation appeared near 275°K for all specimens prepared from acetone solution. The peak may be due to a relaxation of hydroxy ether links which are bound by secondary forces to residual acetone. Attempts to react EPON 828 with Ni(PAP)x:Cl2 directly and from solution were unsuccessful in forming a completely amorphous highly crosslinked polymer system. However, a solution polymerization technique, using acetone as the solvent was developed. This technique resulted in complete reaction of EPON 828 with Co(PAP)2:CI2. The success in this case is attributed to the relatively high solubility of Co(PAP)2 : CI2 in acetone. The metal-ion polymers showed higher glass transition temperatures than EPON 828-MDA, but had lower tensile strengths at room temperature.
Study of prothrombin activation by taipan snake venom using light beating spectroscopy
(1976) Agarwal, G. P. (Gopal Prasad); Armeniades, C. D.
In this study, the recently developed technique of light beating spectroscopy has been used for the first time to measure the dynamic properties of prothrombin and its activation kinetics via Taipan Snake Venom, micellar phospholipid and Ca++. The correlation times fCc) on the samples of human prothrombin, which were prepared by membrance filtering techniques, were measured as a function of scattering angle to determine the particle size uniformity. P vs K plot was found to be linear for one sample at 1°C, but nonlinearity was seen for other two samples of human prothrombin at 2°C, indicating some aggregation. The translational diffusion coefficient was measured on more than 2 samples at 2 mg/ml. The average value of diffusion coefficient (D2,Tvf ) was measured to be 4. 72 x 1 -7c m2/sec with a standard deviation of . 23 x 1"-7c m2/sec. The diffusion coefficient value for a sample of 5 mg/ml human prothrombin was 4. 51 x 1"-7c m7^/sec. These values are in agreement with the reported values of 4. 8 x 1“-c7m / sec2and 4-. 67x 1 2cm /sec at the respective concentrations. Sedimentation coefficients determined from sedimentation velocity experiments were 5, 37S and 6. 7S at 2 mg/ml and 5 mg /ml. These were higher than the reported values of 4. 8S and 5. 2S for the corresponding concentrations, which could be attributed to aggregation at very low ionic strength and low pH. The molecular weights of human prothrombin calculated from sedimentation and diffusion coefficients data were found to be 92, and 12, at 2 mg/ml and 5 mg/ml concentrations, respectively. From a sedimentation equilibrium experiment on a sample of first lot human prothrombin diluted to . 2 mg/ml, the sample was found to be homogeneous species having 69, molecular weight. In solution, the conversion of prothrombin was observed by monitoring the change of translational diffusion coefficient during its activation process. A single clipped autocorrelation function at low counts per sample interval (<<1) and clipping level k = , is approximately equal to . . Consequently, the autocorrelation function is proportional to the second power of M and c and therefore very sensitive to the presence of big particles in the solution. To satisfy this requirement nonphysiological activator Taipan Snake Venom (oxyuranus Scutellatus) and micellar phospholipid, Dihexanoyllecithin were used which gave negligible contribution to the autocorrelation function. An unusual y long time of 3-5 days was observed for the complete conversion of first lot of human prothrombin. This slow rate could be due to possible conversion of prothrombin to Intermediate I and inhibitory fragment 1 prior to the activation reaction. These products retard the activation rate by an order of magnitude. Other factors such as the low surface area per unit mass of the micellar phospholipid (compared to the commonly used bilayer phospholipid), the low concentration of CaCl (.3mM) set by its precipitation threshold and the use of low temperatures to avoid aggregation may also contribute to the slow conversion. For a sample of second lot human prothrombin 18% change in diffusion coefficient was observed at 2°C in less than an hour. Complete conversion study could not be done as the solution aggregated. This supports our hypothesis that a prior conversion of prothrombin of first lot had taken place.
Three-dimensional, "in vitro", cell migration on an artificial substrate: the effects of a collagen coating
(1974) Shepherd, James Robert; Armeniades, C. D.
This study is part of a research project, which seeks to develop techniques for inducing histogenesis of differentiated tissue on three-dimensional porous substrates. It is concerned with developing a simple, dependable method of coating polyurethane foam with collagen and with defining the characteristics of the collagen coat. The collagen, extracted from bovine tendon, was applied as a one percent dispersion. The collagen coat was directly measured to be 7 to 1A thick by a scanning electron microscope. The coating consisted of intertwining fibrils running in the same general direction. About 8% of the sponge surface area was covered. Adherence tests utilizing the above dispersion, and a glutaraldehyde cross-linked dispersion revealed the collagen coat to be invulnerable to lysis by the tissue or culture medium. The coated sponges exhibited greater invasion by the cells than the uncoated. Elongated cells lying along the coated fibers of the sponge indicated that migration was not random, but rather guided by the collagen, little can be said about improved histological reconstruction of the tissue without a much more extensive sampling. Investigations into optimum sponge pore size are not completed; preliminary results suggest that a pore size of .4 millimeters or larger yields the best growth characteristics.

Browsing by Author "Armeniades, C. D."

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