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Poly(lactic acid) (PLA) toughening is often associated with significant modulus and/or strength losses making it unsuitable for many consumer and biomedical applications. The major objective of this research was to toughen PLA without significant loss in modulus and strength and to introduce reactive acid groups using reactive blending of PLA with a combination of polymers. PLA was reactive blended with poly(acrylic acid) (PAA) followed by physical blending with poly(ethylene glycol) (PEG) in solution. The modified PLA was extruded into films using a co-rotating twin-screw extruder and characterized using tensile testing, differential scanning calorimetry (DSC), and dynamic mechanical analyses (DMA). This technology resulted in films with a ten-fold increase in toughness compared to neat PLA with little or no decrease in strength and modulus.

A new approach is introduced to create submicrometer patterned surfaces using multilayer polymer films that contain alternating layers of two polymers, linear low-density polyethylene (LLDPE) and ethylene-co-(acrylic acid) copolymer (EAA). Patterned templates have been prepared by microtoming the multilayer molded sheets. Regionally confined chemical functionality is confirmed by grafting an amine-terminated biotin and adsorbing streptavidin specifically on the alternating layers of EAA.

Monte Carlo simulations are conducted in two and three dimensions to compute the diffusion coefficients of gas molecules permeating membranes containing oriented platelets. The effects of platelet aspect ratio, relative separation, loading and spatial dimension are examined. The model of Aris [R. Aris, On a problem in hindered diffusion, Arch. Ration. Mech Anal. 95 (2) (1986) 83–91] is shown to correlate well with a 2D Monte Carlo simulation. However, the Aris model, when compared to a 3D simulation, over-predicts the barrier-effect of filler, by 50% in some cases. Additionally, flaws in idealized geometries for 3D models are addressed and a new 3D geometry is presented. The results have particular value for nanocomposite films at low platelet loadings.

Many polymer films are produced by extruding the polymer in a cast- or blown-film process. Often an additive is preblended into the polymer and, once the film is formed, the additive migrates to the film surface(s). Erucamide is a migratory additive that is commonly placed in polyolefin films to reduce their coefficient of friction (COF). The aim of this study was to examine the relationship between erucamide surface concentration and COF of LLDPE films. The erucamide surface concentration was varied in two ways. In one set of experiments, a film containing 5,000 ppm of erucamide was surface-washed with solvent and/or aged at room temperature for specific time periods. In another set, films with different erucamide bulk loadings were aged for 7 days to achieve different equilibrium surface concentrations. The surface concentration was measured by using surface washing, and the bulk loading was quantified by using microwave extraction. A plot of COF as a function of surface concentration showed that all of the results fell on a single curve, regardless of the method used to regulate the erucamide surface concentration. At a surface concentration of ∼0.5 μg/cm2, the kinetic COF began to plateau to a value less than or equal to ∼0.2 for this erucamide-LLDPE system. J. VINYL. ADDIT. TECHNOL. 11:9–12, 2005. © 2005 Society of Plastics Engineers.

Erucamide is incorporated into polymer films to reduce their coefficient of friction (COF). Such COF reduction is important in packaging lines where the performance of the film in contact with rollers can be governed by the frictional characteristics of the film. This research explores the COF behavior of multilayer films with either polyolefin plastomer (POP) or linear low-density polyethylene (LLDPE) as the skin layer. Film-on-metal COF testing was performed repetitively with the same piece of film to investigate the extent of COF change with the number of runs for cast and blown films. Results showed that the COF of the film increased with the number of runs and plateaued at a higher steady state value. Complementary analysis conducted with atomic force microscopy (AFM) revealed that initially, erucamide was removed from the film surface, exposing the bare polymer film to the metal plate, but as more runs were performed the erucamide crystals were smeared over the film surface, re-covering the previously exposed film surface. Several combinations of slip and antiblock (AB) loadings were used to study their effects on the steady state COF obtained from repetitive testing on blown films. Results showed that the steady state COF decreased with the addition of both slip and AB, except for the lowest chosen loadings of 1250 ppm slip and 3000 ppm AB.

The diffusion of additives in thick (∼500 μm) single layer and multilayer films has been characterized using FT-IR microspectroscopy.1,2 The objective of this research was to investigate additive migration and concentration profiles in coextruded multilayer films of industrially relevant thicknesses. In particular, the investigation focused on the migration of an erucamide slip agent in 50-μm-thick coextruded bilayer films of linear low-density polyethylene (LLDPE) and a polyolefin plastomer (POP). Erucamide concentration profiles were successfully mapped using synchrotron-based FT-IR microspectroscopy. The synchrotron radiation helped to achieve a higher spatial resolution for the thin films. Meticulous sample preparation was needed to map the thin film samples. Results with FT-IR microspectroscopy showed that the additive-concentration profiles were relatively uniform across the multilayer-film thickness irrespective of the intended initial additive distribution. For example, a bilayer planned for 1 wt % erucamide in an LLDPE layer and no erucamide in a POP layer showed significant additive migration into the POP layer at the extrusion rates used. FT-IR microspectroscopy results also showed that more erucamide migrated to the surface of a POP layer than an LLDPE layer. Attenuated total reflectance (ATR) FT-IR spectroscopy was used to confirm the time-dependent increase of erucamide surface concentration and that the increase was more pronounced at the surface of the POP layers.

Additives are often blended into polymers to improve specific performance features of the finished film. Erucamide is a common slip agent used to reduce a film's coefficient of friction (COF), which is a good attribute for high speed packaging operations. Because such additives must migrate to the surface to be effective, this study focuses on erucamide blooming to the surface of linear low-density polyethylene (LLDPE) films. Both the surface and bulk concentrations of erucamide were measured over time to determine the equilibrium additive partitioning. The surface concentration was determined by surface washing for different time intervals and extrapolated back to a “zero” time to offset the solvent extraction of additive from the bulk film. The bulk concentration was determined by a deep microwave extraction. Initial experiments with forced aging at 60°C showed significant evaporative losses of erucamide. The final work focused on room temperature, at whcih additive evaporation was negligible. The room temperature blooming results showed that the erucamide surface and bulk concentrations reached equilibrium within 24 hours. The data also showed that erucamide surface concentration increased with initial additive loading for a fixed film thickness and with increased film thickness for a fixed additive concentration.

Barrier properties of polymer films can be improved in various ways, such as formation of multilayer structures by coextrusion, surface treatment, and coatings. This work explores the use of thousands of alternating layers of polymer (xyxy...) to modify the resistance to permeation. A model is presented to predict the number of layers needed in a laminate to change the flux of a permeant by a given amount. An important feature of the model is the species transport across the interdiffusion regions at the polymer-polymer interfaces where diffusivity of the penetrant, D i , is assumed to be a constant or a function of the volume fractions (Φ) of the interdiffusing polymers. For constant D i , the modeling results show that increasing the number of interfacial regions decreases the flux for a given condition, and a large number of layers are required to achieve appreciable flux reduction. For Φ-dependent D i , a balanced interdiffusion region was modeled in which the interdiffusing polymers were assumed to interpenetrate one another equally. In this particular case, the flux was predicted to always increase with the number of layers.

Coefficient of friction (COF) measurements and AFM of LLDPE films containing erucamide and behenamide indicated that COF reduction is not necessarily dependent on additive coverage of the film surf...

Part I provided a detailed description of a model developed to predict the mechanical properties of yarns and cords from single-filament properties. The objective in Part II is to assess the accuracy of the model by comparing predicted results with experimental data. Multifilament PET yams were obtained from various manufacturing facilities. For all but one case, the bundles were formed into twisted yams and two-ply cords using laboratory- scale equipment. The model shows good agreement with experimental force-elongation data, although there is some deviation at higher twist levels. This discrepancy is likely due to slight non-uniformities present in the laboratory-produced yarns and cords. A cord with a high level of twist and uniformity produced at a manufacturing site demonstrates excellent agreement between model and experimental results. As an example of its predictive capabilities, the model calculates the elongation to break and tenacity at break within 5% of the experimental value for the vast majority of twisted yarns and cords studied.

An addition-type thermosetting polyimide, PMR-15, has a relatively high glass-transition temperature, T g , of approximately 340°C and attractive high-temperature composite mechanical properties after postcuring. PMR-15 prepreg is typically produced by pulling carbon fibers through a methanol solution that contains the PMR-15 monomers. One of the monomers, methylene dianiline, is highly toxic, thereby making the solution-coating technique undesirable. To alleviate the toxicity problem in the prepregging step, preimidized PMR-15 is now available in powder form, and this material can be used to produce towpreg using a powder-coating method. The powder-coating method that has been used in this work involves the use of an aqueous foam as a carrier medium to continuously deposit the PMR-15 powder onto carbon fibers. The powder is slurried in a surfactant solution into which nitrogen is dispersed using a foam-generating device. A predetermined amount of this foam is then applied to a moving carbon fiber low. The foam is collapsed and the powder fused to the fiber in a set of tubular ovens. The powder appeared to be particularly sensitive to the heat treatment, but towpreg was produced and composites were consolidated successfully. The mechanical properties of the composites formed from powder-coated towpreg compare favorably with the properties of composites fabricated from prepreg that was drum wound from a methanol solution.

In this study, the microbond technique has been simulated by the use of macrodroplets. These larger scale specimens consisted of cone-shaped epoxy droplets of length 6 mm, on 1 mm diameter steel wire. The specimens were formed reproducibly in a mold to provide 30 ° and 45 ° contact angles between fiber and resin. The model geometry was used to examine the effects of contact angle, loading position, and loading type (point versus axisymmetric loading) on the debonding strength. The results indicated that a higher debonding strength was achieved at the higher contact angle and at loading positions farther from the tip of the cone-shaped drop. When the load was applied near the tip of the drop, which is typical in microbond experiments, there was no statistical difference in debonding strength for point loading and axisymmetric loading. Finally, with the macrodroplets, it was possible to visually observe a three-stage debonding process.

Many techniques have been developed to coat carbon fibers with thermoplastic resins to form a prepreg. These techniques include melt impregnation and solution/slurry coating. The applicability of these techniques, however, may be restricted by high melt viscosities or significant drying times. Recent emphasis has been towards a dry powder coating technique using a fluidized bed. This technique may be limited by difficulties in fluidization of the polymer powders. To overcome these difficulties, a technique has been developed to continuously coat carbon fibers with thermoplastic resins using aqueous foam as a carrier medium. The polymer is slurried in a surfactant solution, into which air is dispersed using a foam-generating device. A predetermined amount of this foam is then applied to a moving carbon fiber tow using a foam application unit. The tow is then pulled through ovens where the foam is collapsed and the power fused to the fibers. Other aspects that are addressed include the stability of foams in the presence of powders, the mechanical properties of the composites formed using foam prepregging, and the effect of surfactant on composite properties.

A new experimental technique and method of analysis have been developed to study the flow of liquid foams in fiber networks. Foam is injected into the center hole of a compressed mat and the resulting radial spreading pattern is observed through the top transparent plate. The existence of a minimum critical pressure gradient for foam flow in porous media is demonstrated as the foam front spreads, slows down, and eventually stops. The radial position at which it stops is a function of the inlet pressure. A two-parameter fluid model is proposed that incorporates "critical pressure gradient" and fluid mobility parameters to describe the interaction of the foam with the porous material. From the model, radial pressure profiles are generated that predict a qualitative difference between the pressure distribution and spreading rate of a fluid with yield behavior (that is, a minimum critical pressure gradient) compared to the flow of a Newtonian fluid. From the position at which the foam front stops, the critical pressure gradient for foam mobilization is quantified. By numerically integrating the equation for the frontal advance rate and using experimental data, a value is obtained for the foam mobility. The mobility is observed to increase with increasing foam quality and remain relatively independent of the bubble size of the foam injected into the mat. The effects of dynamic surface tension on foam flow are also shown. Foams were generated from aqueous surfactant solutions at different concentrations, each with the same equilibrium surface tension but vastly different dynamic surface tensions. As the dynamic surface tension increased, foam breakdown occurred, resulting in a significant region of liquid saturation within the mat. This effect was attributed to the inability of the solutions with a higher dynamic surface tension to prevent film rupture and foam collapse.

Measurement and mechanical aspects of the microbond pull-out technique for measuring fiber/resin interfacial shear strength have been examined to determine the origins of the consistent variability in bond strength results. Improved instrumentation for measuring fiber diameters and debonding loads make it possible to rule out these measurements as a significant cause of bond strength variability. Bond failure under the microbond shearing conditions is catastrophic. Variations in droplet contour near its intersection with the fiber are a consequence of the variations of contact angle between the two surfaces. Since the contact angle is less than 90°, a compressive force which acts against the shearing process must be generated, but the variability of bond strength values cannot be accounted for by this contact angle-dependent compressive loading. Shear stress distributions along the fiber/droplet interface and contact angle variations make it vital that the shearing plates be as close as possible to the intersection of the fiber and resin surfaces. The results presented in this paper reinforce the idea that the principal cause of bond strength measurement variability lies in the inherent nonuniformity of fiber surfaces.

The present research investigates the effect of photoinduced grafting reaction on the bulk properties of melt processed poly(L-lactic acid) (PLA)-poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] (PHBHHx) blend films. PLA-PHBHHx blend films, comprising 10 wt % PHBHHx showed a remarkable toughness improvement. From dynamic mechanical analysis of melt processed PLA-PHBHHx blend films, the blend appears to be noncompatible. Unfortunately, PLA-PHBHHx blend films underwent rapid physical aging as characterized using differential scanning calorimetry, resulting in a significant toughness loss. Physically aged films regained the original toughness on annealing at 60°C for 30 min. Annealed PLA-PHBHHx blend films also underwent physical aging leading to a significant toughness loss. Hydrophilic monomers like acrylic acid and acrylamide were successfully photopolymerized from the film surface using a sequential, two-step photografting approach. The resultant films were characterized using water contact angle goniometry, ATR-FTIR spectroscopy, and mechanical testing. PLA-PHBHHx blend films lost their toughness significantly on surface modification and this was assigned to UV-assisted solvent induced crystallization as characterized using wide-angle X-ray diffraction analyses. © 2008 Wiley Periodicals, Inc. J Biomed Mater Res, 2009

The mean cell (bubble) diameter and foam quality for two-phase foams generated by an Oakes mixer were measured for foams having gas volume fractions from 23 to 81 percent. Factorial design analyses were used to study the effects of liquid and gas flow rates, mixing speed, and liquid (continuous) phase viscosity on the foam cell size. It was found that the mixing speed had the largest effect on the mean bubble diameter, with the diameter decreasing with increased mixing speed. Nonlinear, empirical models were developed to predict bubble diameters from known liquid and gas flow rates and mixing speeds. Finally, the mean bubble diameter was significantly lower for a foam generated frora a higher viscosity liquid (100 mPa-s) than from a lower viscosity liquid (1 mPa*s).

Several techmques employing fluid flow to characterize the structure of fibrous materials are presented. A radial flow cell is described in which a fibrous mat with a central hole is compressed between two transparent plates. A liquid is injected into the center of the cell and the radial-spreading pattern of the liquid front is momtored with a video camera By observing the flow patterns, in-plane permeabilities and amsotropy indices are calculated which characterize the fibrous materials. By varying the spacing between the transparent plates the effect of compression on permeability is measured. Two flow techmques for measuring pore volume and pore throat distri butions on compressed mats are also described. Models are developed that describe the radial flow of Newtonian and Bingham-like fluids in the plane of the fiber mats. In the case of the Bingham-like fluid (foam) model, the yield stress parameter and the mobility can be obtained from one experiment.

This work examines the flow of foams through porous materials by characterizing the foam bubble sizes before and after injection through a porous medium. Foam cell sizes are measured using an image analysis system and software developed in our laboratory. Glass mats are used as the porous medium, and foam is forced to flow through a mat at a specified porosity under a constant driving pressure. Results indicated that the size of the bubbles exiting the porous mat was independent of the bubble size of the foam entering the mat and dependent on the average pore size of the medium. Results are also presented which demonstrate the effect of foam quality on the bubble size.

L'appareil de mesure concu produit une diffusion radiale du front de mousse dans un reseau de fibres en fonction du temps avec une pression constante