Showing papers by "Nicholas A. Peppas published in 1993"

Use of ATR-FTIR to study interdiffusion in polystyrene and poly(vinyl methyl ether)

Abstract: ATR-FTIR was used to measure interdiffusion in a polystyrene (PS) and poly(vinyl methyl ether) (PVME) compatible pair below and above the glass transition of PS. At 105°C, corresponding to 5°C above the glass transition of PS, the interdiffusion coefficient was of the order of 1.1x10 -12 cm 2 /s. Interdiffusion was not dominated by either component and it was controlled by the rate of swelling of PS by PVME. At 85°C, the interdiffusion was not-Fickian and time-dependent. A combination of the Fickian and case II models was used to fit the data at 85°C as well as the data at 105°C

Release behavior of bioactive agents from pH-sensitive hydrogels.

Abstract: Controlled release systems of theophylline, proxyphylline and oxprenolol.HCl exhibiting modulated drug delivery were prepared by using pH-sensitive anionic copolymers of 2-hydroxyethyl methacrylate with acrylic acid or methacrylic acid. Drug release studies were carried out in simulated biological fluids. The initial drug release rates and the drug release mechanisms were dependent upon the pH and ionic strength of the buffer solution as well as its salt composition. Initial drug diffusion coefficients in these swelling-controlled release systems were calculated from the release curves; they were of the order of 10(-7) cm2/s and were dependent upon the degree of swelling. The drug release mechanism was non-Fickian in all the dissolution media studied. Lowest release rates were observed for drug release from nonionized polymer networks in agreement with the relationship between ionization, swelling and drug release.

Modeling of penetrant diffusion in glassy polymers with an integral sorption deborah number

Abstract: A mathematical model was developed to explain the anomalous penetrant diffusion behavior in glassy polymers The model equations were derived by using the linear irreversible thermodynamics theory and the kinematic relations in continuum mechanics, showing the coupling between the polymer mechanical behavior and penetrant transport The Maxwell model was used as the stress–strain constitutive equation, from which the polymer relaxation time was defined An integral sorption Deborah number was proposed as the ratio of the characteristic relaxation time in the glassy region to the characteristic diffusion time in the swollen region With this definition, an integral sorption process was characterized by a single Deborah number and the controlling mechanism was identified in terms of the value of the Deborah number The model equations were two coupled nonlinear differential equations A finite difference method was developed for solving the model equations Numerical simulation of integral sorption of penetrants in glassy polymers was performed The simulation results show that (1) the present model can predict Case II transport behavior as well as the transition from Case II to Fickian diffusion and (2) the integral sorption Deborah number is a major parameter affecting the transition © 1993 John Wiley & Sons, Inc

Models of Moisture Transport and Moisture-Induced Stresses in Epoxy Composites

Abstract: Mathematical models are presented to describe moisture transport in epoxy composites using concentration and stress dependent diffusion coefficients. These models take into consideration the stresses induced due to moisture and their influence on the transport mechanism. All governing differential equations are solved with appropriate boundary conditions to obtain concentration and stress profiles. It is concluded that non- Fickian and anomalous transport of moisture in epoxy composites can occur under certain conditions, leading to stress accumulation close to the fibers of the composite.

Water transport in graphite/epoxy composites

Abstract: The diffusive and mechanical behavior of tetraglycidyl diaminodiphenyl methane (TGDDM) resin-based composites and diglycidyl ether of bisphenol-A (DGEBA) resin-based graphite/epoxy composites were investigated during water sorption at different temperatures. The water-absorption kinetics in both systems at 50, 70, 90, and 100°C were fitted by a Fickian diffusion model. However, a Langmuir-type, two-step sorption behavior was observed for water transport in DGEBA-based systems at 50 and 70°C. Using scanning electron microscopy, internal cracks due to water absorption were found in the DGEBA-based samples after conditioning at 90 and 100°C in water, whereas no cracks were detected in TGDDM-based samples conditioned in water at 100°C. Ultrasonic testing did not show significant modulus or density change of the TGDDM-based samples conditioned in water at 100°C. No significant changes of dynamic modulus or damping factor were observed for the TGDDM-based samples redried after immersion in 100°C water, whereas slight changes were observed above 120°C for the samples containing absorbed water. However, both water-containing and redried DGEBA-based samples showed a decrease of dynamic modulus and an ω-transition around 120°C. A single-fiber fragment test revealed that the absorbed water at 80°C reduced significantly the interfacial shear strength of DGEBA/DDA resin-AS4 fiber samples and DGEBA/DDA resin-AU4 fiber samples. © 1993 John Wiley & Sons, Inc.

Penetrant transport in crosslinked polystyrene

Abstract: Swelling studies of cross-linked polystyrene were described, as performed in several penetrants including aromatic and aliphatic compounds at 50 and 70 o C. Aromatic penetrants such as toluene and dibutyl phthalate caused sample cracking. For aliphatic penetrants such as heptane, decane, and dodecane, the penetrant transport rate, equilibrium penetrant uptake, and degree of penetrant overshoot were affected by several parameters such as the sample cross-linking ratio, penetrant sise, and experimental temperature. The temperature dependence of the polystyrene/dodecane interaction parameter was estimated from the results of dodecane equilibrium uptake for polystyrene samples cross-linked at different levels. Dynamic swelling studies were analysed with a simple exponential expression

Numerical simulation of anomalous penetrant diffusion in polymers

Abstract: This work introduces a new numerical algorithm that can be used to analyze complex problems of penetrant transport. Penetrant transport in polymers often deviates from the predictions of Fick's law because of the coupling between penetrant diffusion and the polymer mechanical behavior. This phenomenon is particularly important in glassy polymers. This leads to a model consisting of two coupled differential equations for penetrant diffusion and polymer stress relaxation, respectively. If the polymer relaxation is the rate-limiting step, both the concentration and stress profiles are very steep. A new algorithm based on a finite difference method is proposed to solve the model equations. It features the development of a tridiagonal iterative method to solve the nonlinear finite difference equations obtained from the finite difference approximation of the differential equations. This method was found to be efficient and accurate. Numerical simulation of penetrant diffusion in glassy polymers was performed, showing that the integral sorption Deborah number is a major parameter affecting the transition from Fickian to anomalous diffusion behavior. © 1993 John Wiley & Sons, Inc.

Dynamic mechanical studies of the glass transition temperature of photopolymerized multifunctional acrylates

Abstract: The glass transition temperatures of the highly crosslinked networks formed from photopolymerizing-the multifunctional monomers trimethylol propane triacrylate (TrMP TrA), pentaerythritol tetraacrylate (PETeA), and dipentaerythritol monohydroxy pentaacrylate (DPEMHP eA) were determined using a dynamic viscoelastometer. All glass transition temperatures, Tg, were in excess of 150°C indicating the highly crosslinked structure of the resulting networks. The curves of the damping factor as a function of temperature were broadened peaks spanning up to 100°C due to the extreme heterogeneity of these structures. Evidence of trapped radicals was also observed as cured samples subjected to a thermal pretreatment had higher Tg than those without the thermal pretreatment.

Temperature- and pH- Sensitive Hydrogels for Controlled Release of Antithrombotic Agents

Abstract: Polymeric hydrogels capable of reversible swelling with changes in environmental temperature and pH were synthesized and studied as matrix systems capable of controlled release of antithrombotic drugs to the site of a blood clot. Monomers N-isopropylacrylamide (NIPAAm) and methacrylic acid (MAA) were chosen for the temperature- and pHdependent swelling characteristics of their polymers, respectively. Equilibrium swelling studies were performed as functions of pH and of temperature, and kinetic swelling studies were performed on the thermally responsive gels as a function of time after the gel was subjected to a step-change in temperature. Experimental results indicate that P(NIPAAmco- MAA) hydrogels can be synthesized so that they are either pH- or temperaturesensitive, depending upon the composition of the network. The pure PNIPAAm gels showed a lower critical solubility temperature (LCST) of around 32°C, below which the gel was in its swollen state, and above which the gel collapsed. As the amount of methacrylic acid in the gels was increased, the degree of swelling in deionized water decreased and the temperature sensitivity was lost. The hydrogels containing MAA displayed a transition in swelling behavior between pH 4 and 6. The mesh sizes of the hydrogel networks were calculated from the results of the swelling studies by Flory-Rehner theory. A kinetic experiment on the temperature-sensitive pure PNIPAAm hydrogel showed that the polymer network collapsed rapidly upon temperature changes across the LCST.

Preparation and characterization of novel biodegradable tri- and tetraacrylate intermediates

Abstract: Two biodegradable polymer intermediates, triacrylate 4 and tetraacrylate 7, were prepared by polycondensation reaction of ethyl β-hydroxybutyrate using glycerol and pentaerythritol as initiators and dibutyltin oxide as a catalyst, followed by functionalization of the hydroxyl end groups with acryloyl chloride in the presence of triethyl amine. These polymer intermediates were characterized using 1H-NMR and IR spectroscopic analysis and were employed as crosslinking agents during polymerization of partially neutralized acrylic acid to obtain the corresponding potentially biodegradable polyacrylates.

Poly(Methacrylic Acid-g-Ethylene Glycol) Hydrogels as pH Responsive Biomedical Materials

Abstract: Novel graft copolymeric, pH responsive hydrogels have been synthesized by solution polymerization. Poly(methacrylic acid-g-ethylene glycol) hydrogels exhibit reversible complexation as a function of pH because of hydrogen bonding between the PEG grafts and the PMAA main chains. These gels are being developed as pH responsive membranes for use in biomedical applications. ATR-FTIR results indicate that there is hydrogen bonding occurring in membranes swollen under conditions favorable for complexation. Equilibrium swelling studies have shown that the pH of the surrounding environment has a significant effect on the swelling behavior of these membranes. The swelling behavior has also been shown to respond rapidly to changing pH conditions. P(MAA-g-EG) membranes also show pH sensitive permeability toward solutes as a result of this environmentally sensitive swelling.

On the accurate experimental determination of drug diffusion coefficients in polymers

Abstract: La determination precise du coefficient de diffusion des principes actifs dans les polymeres necessite l'utilisation de cellules de diffusion parfaitement adaptees de facon a reduire l'erreur experimentale. Dans le travail presente ici, les auteurs analysent le transport du principe actif du compartiment donneur au compartiment receveur d'une cellulle de diffusion, en fonction des caracteristiques precises de flux des compartiments. Il est montre que le coefficient de transfert global du principe actif peut conduire au coefficient de diffusion du principe actif a travers la membrane de polymere lorsque des calculs exacts des coefficients de transfert partiels des compartiments donneur et receveur peuvent etre effectues

Temperature Effects on Interdiffusion at Glassy/Rubbery Interfaces

Abstract: Attenuated total reflection infrared spectroscopy was used to measure interdiffusion at a model glassy/ rubbery interface consisting of polystyrene (PS) below its glass transition temperature in contact with poly(vinyl methyl ether) (PVME). PVME swelled PS at temperatures ranging from 60 to 95°C, corresponding to 41 to 6°C below the glass transition of PS. This swelling was confirmed with dynamic mechanical analysis using glassy crosslinked PS in contact with PVME. The swelling process was characterized by an interfacial velocity as the PVME swelled PS. The relaxation time for the swelling process was determined from the interface velocity as a function of temperature and the results indicated that the swelling process is controlled by the relaxation time of the slowly diffusing component (PS). The relaxation time ranged from 141 min at 60°C to 16.8 s at 95°C. The activation energy for the relaxation of the PS matrix was determined as 40.9/pm 2.6 kcal/mol using the Arrhenius expression, in good a...

Novel Preparation of Poly(Vinyl Alcohol) Microparticles without Crosslinking Agent for Controlled Drug Delivery

Abstract: Poly(vinyl alcohol) microparticles were prepared by a novel freezing-thawing process in the absence of a crosslinking agent. An aqueous PVA solution to which 1.25 wt% sodium lauryl sulfate was added was dispersed in corn oil. The system was agitated and the ensuing suspended droplets of PVA solution were solidified by a cyclic freezing-thawing process. Key parameters of the process were the PVA to corn oil ratio, the amount of surfactant added, agitation speed, number of freeze cycles, temperatures of freezing and thawing, and presence of additional components. Crystallization was observed during the freezing-thawing process. The degree of crystallinity was measured with differential scanning calorimetry. Bovine serum albumin was incorporated into the particles by an absorption technique. Studies of BSA release from the microparticles in vitro showed that the release could be prolonged for up to 7 days. BSA diffusion coefficients were calculated from these data and the release mechanism was identified.

Controlled Release of Trimaterene from Poly(DL-Lactide-Co-Glycolide) Microspheres

Abstract: Release of triamterene from 150–300 gm poly(DL-lactide-co-glycolide) (PLGA) microspheres was investigated in vitro as a function of lactic acid/glycolic acid (LA/GA) copolymer ratio and drug loading both with free microspheres and with microspheres embedded in a silicone matrix. Biphasic release consisting of diffusion controlled release followed by erosion controlled release corresponding to polymer degradation was observed in all samples. Drug release from PLGA 50:50 copolymer microspheres was three times faster than the release from PLGA 75:25 microspheres for the higher drug loading (20 wt%) and slightly faster for the lower drug loading (10 wt%). Release rates from spheres containing the higher drug loading were approximately one order of magnitude faster than release from spheres containing the lower drug loading for the same PLGA copolymer. The same qualitative results were observed for the spheres embedded in silicone matrices; however, the overall release was much slower. The results demonstrate that release behavior may be altered by changing LA/GA copolymer ratio, drug loading, and microsphere environment to obtain the desired release characteristics.