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Poly(lactic acid) or polylactide (PLA) is the most extensively researched and utilized biodegradable and renewable thermoplastic polyester, with potential to replace conventional petrochemical-based polymers. In recent times, several PLA-based technologies have emerged with an emphasis on achieving chemical, mechanical, and biological properties equivalent or superior to conventional polymers. The frequent need for a chemical or physical modification of PLA to achieve suitable properties for its intended consumer and biomedical applications, however, has demanded significant attention in the last decade. In the first part of this review, we briefly discuss the advantages, limitations, production methods, and applications of unmodified PLA. The second part, the major objective of this paper, focuses on the various bulk and surface-modification strategies used to date and their basic principles, drawbacks, and achievements. more...

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. more...

A two-step approach was used to micropattern covalently attached biotin on the surface of poly(lactic acid) (PLA) film. Poly(acrylic acid) (PAA) was micropatterned on PLA using photolithography in step 1, followed by carbodiimide wet chemistry to covalently attach biotin to acid groups in step 2. The PAA micropatterning reaction progression was monitored using attenuated total reflectance-Fourier transform infrared (ATR-FT-IR) spectroscopy, water contact angle goniometry, and atomic force microscopy (AFM). The PAA-biotin conjugation reaction characterization using XPS confirmed the carbodiimide mediated amidation reaction. The resultant PLA film was then immersed into a solution of fluorescence-conjugated streptavidin and examined under fluorescence microscopy to reveal various micropatterns. more...

The major objective of this research was to modify the surface properties of poly(lactic acid) (PLA) and poly(hydroxyalkanoate) (PHA) films by using a sequential two-step photografting approach. In step 1, benzophenone was photografted on the film surface and in step 2, hydrophilic monomers acrylamide and acrylic acid were photopolymerized from the film surfaces. Another objective was to study the effect of the reaction solvent in step 2 on surface and bulk properties of these films. ATR-FTIR spectroscopy and water contact angle goniometry were used to characterize the resultant film surfaces. When ethanol was used as the solvent in step 2, there was significant penetration of the monomers into the films, and bulk properties such as toughness and percent elongation at break were drastically affected. The penetration of these monomers into the bulk was characterized using transmission FTIR microspectroscopy. Microtomed sections of the surface-modified films were placed in a diamond compression cell to perform the FTIR microspectroscopic analyses. The observed monomer penetration into the films was successfully reduced by using water instead of ethanol in step 2, and resultant films showed higher toughness and percent elongation at break than films surface-modified using ethanol as a solvent in step 2. more...

Polyelectrolyte multilayers (PEMs) of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) were successfully deposited onto novel nano-striped surfaces, which were generated using the cross-section surfaces of multilayer polymer films made from poly(ethylene-co-acrylic acid) (EAA) and linear low-density polyethylene (LLDPE). The procedure of PEM patterning consisted of three steps: nano-striped pattern formation, PEM deposition, and chemical crosslinking. It was observed that the PEMs were selectively deposited on the EAA layers and linked through covalent bonds, but those on LLDPE layers were washed off during sonication in water and methanol. In addition, the reactive moieties introduced by PEMs were successfully used to bond covalently with other important molecules, such as amine-terminated fluorescein. This kind of nano-striped surface with alternating wettability could serve as a template for many applications, including biomedical, separations, and electronics. more...

This article describes the measurement of the diffusivity of a commercial hydrophilic additive (CHA), Irgasurf HL560, at concentrations of 5 and 10 wt% in polypropylene films at different temperatures. The model used was a standard 1-D diffusion model, in which the film weight was expressed as a function of time as the additive concentration at the film surface was kept at zero at all times. Experimentally, two cases were compared: (1) Washing additive off the film surface with limited contact with the solvent, while PP films were stored in air and tested at 25, 40, 55, and 70°C up to 20 days. (2) Immersing the film in solvent with continuous stirring at 25°C for 10 days, 44°C for 2 days, and 65°C for 10 h. Diffusivity measured by the surface-washing method was 10-14 to 10-12 cm2/s, and 10 -12 to 10-10 by the immersion method. TGA results showed no effect of solvent enhancement to additive diffusion at 25°C but slightly enhanced at 44 and 65°C. Results for both the surface-washing and immersion methods are shown for instructive purposes. It is clear that the diffusivity calculated from surface-washing is much lower than from continuous immersion, largely because the boundary condition of zero additive concentration of the film surfaces was invalid between washings as additive bloomed to the surfaces and accumulated there. The continuous immersion method more closely matched the zero boundary condition used in the model and thus gave more accurate diffusivity values than did the surface-washing method. more...

A sequence of reaction steps was used to graft poly(tert-butyl acrylate) (PtBA) to silicon (Si) wafers and to modify the pendant groups of the tethered PtBA to create chemically tailored surface modifying layers. The Si wafers were first prepared for grafting by the deposition of a self-assembled monolayer (SAM) of N-aminopropyldimethylethoxysilane. The terminal primary amine groups on the SAM provided sites for reaction with carboxy-terminated PtBA. The choice of PtBA as the tethered polymer was due to the ability to substitute the tert-butyl ester groups with other functionalities. In this study, the tert-butyl esters were replaced with various diamines to create a series of polymers with tertiary amine pendant groups. The reactions and surface properties were monitored with external reflectance (ER) and transmission FTIR spectroscopies, ellipsometry, X-ray photoelectron spectroscopy (XPS), and static water contact angle goniometry. Hydrolysis and novel diamine conversion reactions were successfully per... more...

Linear and branched hydrophilic additives of various molecular weights (MWs) were extruded with polypropylene (PP) to make blend films. The surface-modifying additives included polyethylene glycol (PEG), hydroxyl-terminated four-arm polyethylene oxide (PEO), and a commercial hydrophilic additive, Irgasurf HL560. Films were extruded by using a twin-screw microcompounder at 200°C, and the resulting film thickness was 100 μm. Attenuated total reflectance (ATR)-FTIR spectrometry and water contact angle measurements were performed on the film surfaces over time to investigate the additive migration behavior. Although ATR-FTIR detected concentration increases for all additives in the subsurface region, there was no significant improvement in surface hydrophilicity for the PEGs and four-arm PEOs in the same period of time as water contact angles were measured on the surfaces. Among the linear additives, low MW PEG (1 kDa) was found to migrate faster than the high MW varieties. The linear PEG and four-arm PEO with MW higher than 2 kDa did not exhibit significant migration to the surface within a month. Irgasurf was found to change the surface wettability effectively in a relatively short time. J. VINYL ADDIT. TECHNOL., 13:57–64, 2007. © 2007 Society of Plastics Engineers. more...

The degradation of polymers under ultraviolet (UV) irradiation has been a great concern for biomaterial and agricultural applications. The major objective of this research was to study the effect of UV irradiation on the representative bulk and surface properties of poly (L-lactide) (PLA) films. Two UV sources with different spectral outputs and intensities were chosen so that one of them could be used for surface modification and the other could be used for UV sterilization of the PLA films. The results established that the molecular weight of PLA decreased significantly during irradiation from the photografting lamp under atmospheric conditions. Irradiation through a Pyrex container was shown to minimize polymer degradation during UV exposure from the photografting lamp. The PLA films UV-irradiated under the sterilization lamp for 12 h revealed a similar reduction in the molecular weight and no change in the surface hydrophilicity. However, significantly less photodegradation was observed under the sterilization lamp when the samples were held in a Pyrex container. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 more...

Poly(L-lactide) (PLL) has been used as a bioabsorbable material in the medical and pharmaceutical fields. The unmodified hydrophobic PLL surface generally has low cell affinity; thus, modification of PLL film surface properties is necessary to improve its use as a biomaterial. Our surface modification method involved the use of photografting and typical wet chemistry to create branched architectures containing amine functionalities on the periphery of the grafted layers. Amine (-NH2) groups were first introduced on the PLL film surface by photoinduced grafting of 4,4'-diaminobenzophenone and the grafted branched architectures were created by subsequent reactions with succinic acid and tris(2-aminoethyl) amine. The resulting film surface was analyzed using contact angle goniometry and X-ray photoelectron spectroscopy. MC3T3 fibroblasts were cultured on unmodified PLL film and PLL films grafted with the branched structures and the films were subsequently analyzed by optical microscopy. The contact angle goniometry results showed an initial decrease and subsequent plateau in the water contact angles for the PLL films with each successive generation of the branched architectures. The X-ray photoelectron spectroscopy data provided insight into the structure of the grafted layer and revealed an increase in the nitrogen content with each generation. Optical micrographs showed enhanced cell attachment and viability on the surface-modified PLL films. more...

Poly(tert-butyl acrylate) (PtBA) was grafted to the surface of poly(ethylene-co-acrylic acid) (EAA) film and the pendant groups of the tethered PtBA were modified to create chemically tailored surface modifying layers. The carboxylic acid groups in the copolymer film served as the grafting sites for the covalent tethering of end-functionalized PtBA. The progression of these reactions was monitored using attenuated total reflectance (ATR)-FTIR and X-ray photoelectron (XPS) spectroscopies along with static contact angle measurements. By controlling the reaction conditions, the chemical functionality of the grafted layer ranged from tert-butyl ester (EAA-g-PtBA) to carboxylic acid (EAA-g-PAA) and was demonstrated by corresponding changes in wettability. The choice of PtBA as the tethered polymer allows for the subsequent substitution of the tert-butyl ester groups. To demonstrate, a novel procedure was used to replace the tert-butyl ester with N,N-dimethylethylenediamine (DMEDA) to form EAA-g-PDMEDA. These reaction schemes can be used to create tunable surface-grafted layers with various pendant group chemistries. more...

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. more...

The major objective of this research was to modify the surface characteristics of poly(L-lactide) (PLA) by grafting a combination of hydrophilic polymers to produce a continuum of hydrophilicity. The PLA film was solvent cast, and the film surfaces were activated by ultra violet (UV) irradiation. A single monomer or combination of two monomers, selected from vinyl acetate (VAc), acrylic acid (AA), and acrylamide (AAm), were then grafted to the PLA film surface using a UV induced photopolymerization process. The film surfaces resulting from each reaction step were analyzed using ATR-FTIR spectroscopy and contact angle goniometry. Results showed that AAm dominated the hydrophilicity of the film surface when copolymerized with VAc or AA, while the water contact angles for PLA films grafted with poly(vinyl acetate-co-acrylic acid) varied more gradually with feed composition. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6534-6543, 2006 more...

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. more...

Dansyl cadaverine and polyethylene glycol (PEG) derivatives were grafted on the surface of EAA film and in its subsurface region through formation of amides and esters, respectively. A two-step reaction was conducted. First, EAA film was activated with PCl 5 at room temperature. Second, the acid chloride was reacted with dansyl cadaverine or a PEG derivative to form a modified film. ATR-FTIR spectroscopy and fluorometry were employed to analyze the modified films after each step. It was found that dichloromethane yielded the highest grafting efficiency, with the dansyl cadaverine penetrating throughout the ATR-FTIR analysis region (∼400 nm) in a few minutes. As the grafting depth increased with time, so did the amount of fluorescence intensity of grafted dansyl cadaverine. ATR-FTIR spectra for PEG grafting indicated that the acid chloride peak decreased with time, while the ester peak increased. However, hydrolysis occurred at later times, consuming the acid chloride groups within the film. A marked decrease of static water contact angle was observed for EAA grafted with PEG99 (PEG that contains 99 ethylene glycol repeat units), almost 40° lower than that of neat EAA (∼99°). For other PEG-grafted films, the surface hydrophilicity was also improved. more...

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. more...

The effect of covalently bound 12-aminododecanamide on the surface coefficient of friction (COF) of ethylene–acrylic acid copolymer (EAA) films was investigated. The reaction involved grafting 12-aminododecanoic acid to the inherent carboxylic acid groups on the film, followed by amidation of the grafted amino acid. Conversion of film carboxylic acid groups to primary amide groups was also conducted to compare the impact of direct surface amidation. Subsequent measurements showed that both surface amidation schemes reduced the kinetic COF from 0.30 to 0.15 ∼ 0.18. Repetitive COF testing revealed that amide-modified EAA films maintained low COF values that were independent of the number of COF test runs. However, control experiments showed that COF values also depended greatly on simply exposing film to the various reaction solvents, which increased surface roughness. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2242–2248, 2005 more...

The major objective of this research was to modify PLA film surfaces with the ultimate aim of making a bioactive surface that will show faster degradation. The PLA film was solvent-cast, and the film surfaces were grafted with poly(acrylic acid) (PAA) and poly(acrylamide) (PAAm) using a UV-induced photopolymerization process. The film surface resulting from each reaction step was analyzed using ATR−FTIR spectroscopy and contact angle measurements. Results showed that PAA was grafted from PLA film surfaces in 2 or 3 h, while PAAm was grafted in 3 or 5 h depending on the method of activation. Films grafted with PAA and PAAm exhibited improved wettability. The neat and surface-modified films were incubated in different pH solutions, viz., pH = 4, 7, and 10, for specified time periods. The films resulting from each treatment were analyzed using atomic force microscopy (AFM). The molecular weights of the incubated films were measured using chloroform-based GPC. Results established that faster degradation of th... more...

Erucamide is incorporated into polymer films to reduce their coefficient of friction (COF). At elevated temperatures, erucamide may be lost from the film surface to the surroundings, undergo chemical change or decomposition, and/or migrate back into the film. The major objective of this work was to investigate the fate of erucamide upon exposure to a higher temperature (55°C). Trilayer films containing a polyolefin plastomer (POP) as one skin layer and LLDPE as the other two layers with erucamide incorporated in the POP layer were used. Based on ATR-FTIR spectroscopy, solvent washing, and GC-MS results it was concluded that, upon exposure to 55°C, the major reduction in erucamide surface concentration was due to migration of erucamide back into the film and a slight reduction was due to the decomposition of erucamide present on the surface of the polymer film. Polym. Eng. Sci. 44:2247–2253, 2004. © 2004 Society of Plastics Engineers. more...

Atom transfer radical polymerization (ATRP) was used to grow polyacrylamide from the surface of ethylene–acrylic acid copolymer (EAA) film. The surface functionalization constituted initiator immobilization and surface graft polymerization. All reaction steps were conducted at 24 ± 3°C; polymerization was done in aqueous solution. For initiator immobilization, the carboxylic acid groups on EAA film were converted to acid chloride groups; further reaction with ethanolamines gave hydroxyl groups onto which 2-bromoisobutyryl bromide initiator was attached. ATR-FTIR data indicated that 1.64 ± 0.09 times higher initiator density was achieved by using diethanolamine, relative to ethanolamine. Acrylamide monomer was polymerized from the initiator by ATRP to yield nondistorted, transparent films with polymerization times of up to 1 h. For films prepared using diethanolamine, 1 h polymerization time reduced the static water contact angle by more than 50°, significantly increasing the hydrophilicity of the film surface. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1589–1595, 2004 more...

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. more...

Amide groups were anchored covalently on the surface of ethylene-co-acrylic acid (EAA) copolymer film by surface grafting of amino acid intermediates. The process consisted of four steps: conversion of carboxylic acid groups on the EAA surface to acid chloride groups, amino acid attachment, conversion of amino acid carboxyl groups to acid chloride groups, and amidation. All steps were carried out at room temperature. ATR-FTIR spectroscopy was used to characterize the film after each step and to measure the kinetics of amino acid attachment. Three amino acids were studied: 12-aminododecanoic acid (12-ADDA), 5-aminophthalic acid (5-APA), and L-aspartic acid (AA). The longer-chain 12-ADDA compound was selected for its chemical similarity to migratory fatty amides that are commonly used to alter the frictional behavior of polyolefin films. The 5-APA and AA compounds were selected because each has two carboxylic acid groups that can be converted to amide groups. After amidation, the modified EAA films were characterized by static water contact angle measurements and scanning probe microscopy. Results showed that the 12-ADDA reacted to the surface much faster than the 5-APA or AA. Several steps of aggressive rinsing confirmed that the 12-aminododecanamide was chemically anchored onto the EAA surface. As a result, both hydrophilicity and surface roughness were increased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1688–1694, 2004 more...

This research focuses on mapping the concentration profile of erucamide in LLDPE film. Attenuated Total Reflectance (ATR) FTIR and FTIR microspectroscopy (FTIR-mS) have both been used previously to map concentration profiles in polymer films. However, ATR-FTIR spectroscopy is restricted to the near-surface region of a film while FTIR-mS works well in the film bulk but has deficiencies near film surfaces. Two other techniques, Raman microspectroscopy (R-mS) and FTIR photoacoustic spectroscopy (FTIR-PAS), reportedly work well for depth-profiling in polymers with micron or submicron resolution. Experiments were conducted with R-mS and FTIR-PAS to attempt to quantify the spatial distribution of erucamide in LLDPE film. The amide I carbonyl peak was identified from neat erucamide powder for both R-mS and FTIR-PAS. That peak was not observed with R-mS for film containing erucamide, even for films with a relatively high erucamide loading (1 wt.%). However, FTIR-PAS could detect statistically significant differences in erucamide concentration as a function of penetration depth, indicating that FTIR-PAS may be used as an effective depth-profiling tool for the erucamide-LLDPE system. more...

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. more...

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. more...

Glycerol monostearate (GMS) can serve as an anti-fogging agent by increasing the hydrophilic nature of a film surface. In this study, blends of GMS and LLDPE were extruded into film and the GMS was allowed to migrate to the surface over time. The surface was characterized by measuring the static water contact angle, which was then used to calculate the surface free energy of the film. Results showed that the equilibrium wettability of the film deviated dramatically from that of neat LLDPE when the GMS concentrations were greater than about 1900 ppm. Time-dependent studies demonstrated that the rate of surface-energy change was significantly influenced by the GMS concentration. more...

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. more...

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... more...

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. more...

The objective of this research is to construct a computer model that can predict the properties of flat yams, twisted yams, and twisted cords from single-filament data. In this model, a modulus versus strain curve is obtained by taking the derivative of the average stress-strain curve from single-filament tensile tests. The distribution of elongation to break is then fit to a Gaussian curve and a relationship is derived between filament elongation and bundle elongation in a twisted structure. Using the principle of conserva tion of energy, the forces from the surviving filaments at a specific strain are summed to yield the resulting force on a ply, which is then used to calculate the tensile force on the cord. The model also predicts elongation to break, tenacity at break, and initial modulus of yams and cords. more...

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. more...

A preliminary investigation into an experimental technique, using surfactants, for producing organized thin polystyrene films on glass fibers has been performed. The thin-film polymerization process occurs in three steps, namely, the formation of surfactant admicelles, the partitioning of the styrene monomer into the admicelles, and the in situ polymerization of the absorbed monomer. Treated fibers have been observed using scanning electron microscopy to evaluate the film formation. The micrographs showed a nonuniform coating on the fiber surface. Fibers were also subjected to a THF extraction process that confirmed the formation of polystyrene. Experiments conducted also revealed that polymerization was not restricted to the admicelles and that some polymerization occurred in the supernatant as well. more...

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. more...

The maximum bubble pressure method has been used recently in a continuous-bubbling mode to measure the dynamic surface tension of surfactant solutions at varying bubble frequencies. A modification of this method, which utilizes a syringe pump supplying gas at controlled rates, has been developed to examine bubble growth by providing an initially stationary gas-liquid interface. Videomicroscopy results indicate that the bubble-growth rate is not constant, and the bubble-growth time from a stationary interface at the tip of a Teflon capillary to a hemispherical shape may be only a small fraction of the "peak-to-peak" time for some concentrations of surfactant solution. In these cases, bubble generation occurs in three steps: growth at the capillary tip, rapid expansion at the end of the tip, followed by repressurization of the system. Of particular significance is the observation that the first of a series of bubbles, generated in surfactant solutions at and below the critical micelle concentration (CMC), has a lower dynamic surface tension than subsequent bubbles. This result is attributed to the initial equilibrium condition of the interface. more...

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. more...

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. more...

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. more...

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 more...

Industrial processes frequently use surfactants to lower liquid surface tension and promote wetting. In many instances, it is incorrect to apply the equilibrium (static) surface tension to predict the behavior of these processes because they operate under time-dependent (dynamic) conditions. This paper describes the measurement of dynamic surface tension, that is, the surface tension under dynamic conditions where the limitation of surfactant diffusion to the growing interface results in tensions above the equilibrium surface tension. Experimental results, obtained using the maximum bubble pressure method, are presented for aqueous solutions of hydrocarbon and fluorocarbon surfactant systems and for dilute polymer solutions. In particular, the experiments have shown the combined effects of the rate of generation of a gas/liquid interface and the surfactant concentration on the surface tension. Noteworthy are results for fluorocarbon surfactant solutions which can exhibit dynamic surface tension behavior at exceedingly small rates of surface dilation. Synergistic effects of surfactant mixtures and the effect of temperature on dynamic surface tension are presented, as well as the effects of foaming and liquid viscosity on the measurement technique. more...

Two reaction schemes were developed to covalently graft poly(ethylene glycol) (PEG) chains on poly(ethylene-co-acrylic acid) (EAA) surfaces. The schemes involved surface grafting of linker molecules l-lysine or polypropyleneamine dendrimer (AM64), with subsequent covalent bonding of PEG chains to the linker molecules. NHS and EDC were used to activate the carboxylic acid groups of the EAA in the outermost region of the film, estimated to be 20 nm by ATR-FTIR spectroscopy. XPS demonstrated that the conversion of this activation step was almost 100% in the detected region. After activation, l-lysine or dendrimer was grafted onto the EAA surface, followed by PEG grafting. Combining the data from ATR-FTIR, XPS, and contact angle goniometry, it was found that the PEG chains were grafted on the surface of the EAA film and larger surface coverage was achieved when the dendrimer was used as the intermediate layer. This surface also had the lowest water contact angle. more...

Poly(ethylene-co-acrylic acid) (EAA) films were reacted with glycine, 12-aminododecanoic acid, aspartic acid, 5-aminoisophthalic acid, ethanolamine, diethylamine, dimethylamine, N-isopropylamine, and dimethylaminoethyleneamine to prepare EAA films with negatively charged, non-charged, hydrophilic, and hydrophobic functionalities. Attenuated total reflectance Fourier transform infrared spectroscopy, differential scanning calorimetry, and contact angle measurements were used to characterize the modified EAA films. Analyses revealed that the films were modified on the surfaces and also in the bulk; therefore, bulk properties such as cohesive energy density were changed even though the surfaces remained hydrophobic. Adsorption studies were performed for two fluorescently labeled protein residues, dansyl-L-phenylalanine (dansyl-F) and dansyl-L-glutamine (dansyl-Q), from pH 7.4 buffer solutions. The adsorption results revealed that dimethylaminoethyleneamine functionality gave the highest uptake among the functionalities studied, and adsorption was more favorable for dansyl-F than dansyl-Q. Adsorption behavior is discussed in terms of hydrophobic-hydrophobic (dispersion) interactions and Coulombic interactions. more...

Polymer films can be tailored for a specific application by modifying their surface properties. In this study, linear and branched architectures were grafted to ethylene-acrylic acid (EAA) copolymer films using the so-called grafting from approach. Dicyclohexylcarbodiimide was used to activate the carboxylic acid functionality on the surface of the EAA copolymer film before reacting it with selected di- and tri-amine compounds. The carboxylic acid functionality was subsequently regenerated by reacting the amine-grafted film with succinic anhydride. These reaction steps were then repeated to create the linear and branched architectures on the EAA film surface. The film surface resulting from each reaction step was analyzed using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and contact angle measurements. A systematic analysis of the ATR-FTIR results was performed to estimate the average conversion of the reaction schemes and to explain the observed contact angle results. A significant reduction in water contact angle for the EAA film grafted with a branched architecture was observed. The EAA film grafted with a linear architecture showed a marginal reduction in water contact angle when ethanol was used as a solvent for ethylenediamine. When the solvent for ethylenediamine was changed to water, the contact angle decreased noticeably. However, analysis of control films showed that the reduction in the contact angles was due to the solvent treatment. In the case of branched architectures, such reduction in contact angle due to the solvent treatment was not observed. Several control experiments were performed to ensure that the reduction in the contact angles was in fact due to the grafted species and not due to exposure to various solvents used in the reaction scheme. more...