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Showing papers by "Nicholas A. Peppas published in 1994"


Journal ArticleDOI
25 Mar 1994-Science
TL;DR: Approaches for controlling the interface between tissue and biomaterials and ways in which the engineered materials may contribute to medicine are considered.
Abstract: Significant opportunities and challenges exist in the creation and characterization of biomaterials. Materials have been designed for contact with blood, as replacements for soft and hard tissues, as adhesives, and as dental materials. Current methods of synthesis and characterization of these materials are outlined. Approaches for controlling the interface between tissue and biomaterials and ways in which the engineered materials may contribute to medicine are considered.

1,064 citations



Journal ArticleDOI
TL;DR: The mechanisms of water sorption and diffusion in amorphous polymers is of particular importance in relation to our ability to interpret related transport phenomena in foods in foods as discussed by the authors, and the mechanisms can be determined by a variety of experimental techniques.

164 citations


Journal ArticleDOI
TL;DR: In this paper, the volume shrinkage behavior and reaction kinetics were studied under various reaction conditions, and the maximum functional group conversion was dependent on the number of functional groups, type of functional group and the curing conditions.
Abstract: Multifunctional monomers (trimethylol propane trimethacrylate, trimethylol propane triacrylate, pentaerythritol tetraacrylate, and dipentaerythritol monohydroxy pentaacrylate) were photopolymerized with 2,2-dimethoxy-2-phenylacetophenone as the photoinitiator to produce highly crosslinked networks. The volume shrinkage behavior and the reaction kinetics were studied under various reaction conditions. The volume shrinkage and maximum functional group conversion were dependent on the number of functional groups, type of functional group, and the curing conditions. The maximum functional group conversion was also dependent on the reaction temperature. All the polymerized systems exhibited a strong coupling between the volume relaxation and the reaction kinetics. The kinetic constants were also determined as a function of conversion, and the termination mechanism was found to be reaction diffusion dominated even at low conversions. The importance of these results on the prediction of the reaction behavior for multifunctional monomers producing highly crosslinked polymers is discussed. © 1994 John Wiley & Sons, Inc.

110 citations


Journal ArticleDOI
TL;DR: In this article, a new polymer dissolution model was developed by incorporating the polymer chain disentanglement mechanism into the relevant transport equations, which was used as a dissolution characteristic time controlling the moving position of the solvent-polymer boundary.
Abstract: A new polymer dissolution model was developed by incorporating the polymer chain disentanglement mechanism into the relevant transport equations. The disentanglement time was used as a dissolution characteristic time controlling the moving position of the solvent-polymer boundary. A dimensionless dissolution number was defined as the ratio of the characteristic polymer disentanglement time to the characteristic solvent diffusion time. The dissolution number was shown to be proportional to the square of the gel layer thickness. Scaling law expressions for the dependence of the gel layer thickness and the polymer dissolution rate on polymer molecular weight were also derived

109 citations


Journal ArticleDOI
01 Mar 1994-Polymer
TL;DR: In this paper, the effect of monomer chain length on reaction kinetics was investigated and the termination step behavior was studied by performing dark reactions in which the initiation was halted and the reaction rate was followed.

65 citations


Journal ArticleDOI
TL;DR: In this article, a mean-field kinetic model was developed to describe a diffusion-controlled bulk cross-linking photopolymerization, incorporating all the physical transport limitations that directly or indirectly affect polymerization parameters and rate constants in such reactions.
Abstract: A mean-field kinetic model was developed to describe a diffusion-controlled bulk cross-linking photopolymerization. This model incorporates all the physical transport limitations that directly or indirectly affect polymerization parameters and rate constants in such reactions. Events such as the continuous decrease in initiator efficiency and the presence of a temporary excess free volume were accounted for in the model development. Rate equations were written for each of the species present in the reacting system. The initiator efficiency was calculated throughout the reaction using a model previously developed by us

57 citations


Journal ArticleDOI
TL;DR: In this article, hydrophilic copolymers of methacrylic acid (MAA) and methyl methacrylate (MMA) were prepared by suspension polymerization in an aqueous medium using 2,2'-azobisisobutyronitrile as the initiator and hydroxyapatite as the suspending agent.

57 citations


Journal ArticleDOI
TL;DR: The most important theories of adhesion for polymer-polymer interfaces are the wetting and diffusion theories as mentioned in this paper, which describe the initial stage of the adhesion in bringing the two polymers into intimate contact.
Abstract: Adhesion is a phenomenon related to microscopic and macroscopic interactions between two polymer surfaces brought into intimate contact. This includes microscopic as well as macroscopic interactions. The most important theories of adhesion for polymer-polymer interfaces are the wetting and diffusion theories. The wetting theory describes the initial stage of adhesion in bringing the two polymers into intimate contact. After molecular contact is established, segments of the two polymers diffuse across the interface and the interface heals as a function of time [1, 2]. Therefore, the extent of adhesion at polymer-polymer interfaces is determined by the extent of diffusion and the interfacial thickness between the two polymers.

47 citations


Journal ArticleDOI
TL;DR: In this article, the authors used 2,2-dimethory-2-phenylacetophenone as a model initiator and performed an atheoretical investigation of the initiator efficiency.
Abstract: Using 2,2-dimethory-2-phenylacetophenone as a model initiator, atheoretical investigation of the initiator efficiency was undertaken. Taking into consideration all the significant reactions that the radicals undergo, erpressions were developed that allow for the prediction of the initiator efficiency at the onset and during the course of the polymerization. All the parameters involved in these expressions have physical and fundamental significance. A comparison of the predicted value for the initiator efficiency with an available experimental value showed excellent agreement. The effect of various model parameters on the efficiency prediction is discussed

45 citations


Journal ArticleDOI
TL;DR: In this article, the effect of polystyrene (PS) and polyvinyl methyl ether (PVME) molecular weight and polydispersity on interdiffusion at the interface of PS and PVME were investigated with attenuated total reflection infrared spectroscopy (ATR-FTIR).
Abstract: Time dependence and the effect of polystyrene (PS) and poly(vinyl methyl ether) (PVME) molecular weight and polydispersity on interdiffusion at the interface of PS and PVME were investigated with attenuated total reflection infrared spectroscopy (ATR-FTIR). The time dependence of interdiffusion was studied by varying the thickness of the slow-diffusing component, PS, and the results were analysed using a combination of Fickian and Case II models. PS samples with molecular weights ranging from 1.0 × 105 to 3.0 × 106, above the entanglement molecular weight, were used to study the effect of molecular weight on interdiffusion. PS samples with controlled polydispersity ranging from 1.1–3.0 were prepared using a trimodal distribution constructed from monodisperse PS samples. The polydispersity of the PS samples was controlled by varying the number average molecular weight of the distributions while keeping the weight average molecular weight constant. To study the effect of PVME molecular weight and molecular weight distribution on interdiffusion, PVME was fractionated three times from an aqueous solution to increase its molecular weight from 9.9 × 104 to 1.7 × 105 and reduce its polydispersity index from 2.1 to 1.4, respectivley.

Journal ArticleDOI
TL;DR: Depending on the restriction provided to the tablets by the coating, Fickian or non-Fickian transport can be observed in hydrophilic tablets coated with impermeable coatings.

Journal ArticleDOI
TL;DR: In this article, water and ion transport in thin sheets of initially dry, ionic, hydrophilic crosslinked polymers was modelled throughout the dynamic swelling process, and a new dimensionless number, the Stress Swelling number, A, was defined to quantify the relative importance of stress in the overall swelling process.
Abstract: Water and ion transport in thin sheets of initially dry, ionic, hydrophilic crosslinked polymers was modelled throughout the dynamic swelling process. The water transport was expressed in terms of a non-Fickian equation with a diffusion term containing a Fujitatype concentration-dependent diffusion coefficient coupled with a pseudoconvective term arising from the reasonable assumption that the stress in ionic polymers is proportional to the total number of ionized pendant groups in the polymer. Ion transport was expressed in terms of generalized Fickian equations with water concentration-dependent diffusion coefficients. These equations were solved with appropriate boundary conditions to establish the water uptake as a function of time, pH and ionic strength in a citrate-phosphate-borate buffer solution. A new dimensionless number, the Stress Swelling number, A, was defined to quantify the relative importance of stress in the overall swelling process. Water uptake was a strong function of A. © 1994 John Wiley & Sons, Inc.

01 Jan 1994
TL;DR: In this article, N-isopropylacrylamide (NIPAAm) and methacrylic acid (MAA) were chosen for the temperature and pH-dependent swelling characteristics of their polymers, respectively.
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.


01 Jan 1994
TL;DR: In this article, polyvinyl alcohol (PVA) microparticles were prepared by a novel freezing-thawing process in the absence of a crosslinking agent and the degree of crystallinity was measured with differential scanning calorimetry.
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.

Journal ArticleDOI
TL;DR: The self-diffusion coefficient of dodecane in cross-linked polystyrene was measured using pulsed gradient spin echo nuclear magnetic resonance (PGSE-NMR) spectroscopy as discussed by the authors.
Abstract: The self-diffusion coefficient of dodecane in cross-linked polystyrene was measured using pulsed gradient spin echo nuclear magnetic resonance (PGSE–NMR) spectroscopy. The concentration and temperature dependence of the diffusion coefficient was analyzed by the Fujita and Vrentas–Duda models. Parameters describing the Fujita model were determined from fitting of diffusion data to the PVT behavior of the system. Parameters describing the Vrentas–Duda model were determined from the analysis of the viscosity of dodecane, the viscoelastic relaxation properties, and the glass transition temperature of polystyrene as well as from the diffusion coefficient of the system, measured from independent experiments. Both the Fujita and Vrentas–Duda models described well the concentration and temperature dependence of the diffusion coefficient. Mutual diffusion coefficients were determined from these results. © 1994 John Wiley & Sons, Inc.

Journal ArticleDOI
TL;DR: In this article, the Tait equation was used to describe the PVT behavior of polystyrene systems in both the glassy and rubbery regions, and the volume of the swollen polymer, Vm, was determined by simple addition of the volumes of the pure components at the appropriate temperature and pressure.
Abstract: The PVT properties of crosslinked polystyrene samples containing various amounts of dodecane were measured. The Tait equation was used to describe the PVT behavior of each system in both the glassy and rubbery regions. The glass transition temperature was determined from the abrupt change of the thermal expansion coefficient. Increase in the dodecane content in the samples resulted in a significant decrease of the difference between the expansion coefficients in the glassy and rubbery regions. Addition of dodecane lowered the glass transition temperature linearly. However, the dependence of the glass transition temperature on pressure was not affected by the presence of dodecane in the polymer samples. Above the glass transition temperature, the volume of the swollen polymer, Vm, could be determined by simple addition of the volumes of the pure components at the appropriate temperature and pressure; the volume change of mixing, δVm, was independent of temperature and pressure. Below the glass transition temperature, volume additivity of the two components was also applicable after appropriate adjustment of the glass transition temperature of the polymer to that of the dodecane/polymer samples. © 1994 John Wiley & Sons, Inc.

Journal ArticleDOI
TL;DR: In this article, the swelling behavior of crosslinked cationic copolymers of 2-hydroxyethyl methacrylate with trimethylammonium chloride was investigated in various buffered solutions as a function of ionic strength and pH.
Abstract: The swelling behavior of crosslinked cationic copolymers of 2-hydroxyethyl methacrylate with (3-methacryloylamino propyl) trimethylammonium chloride was investigated in various buffered solutions as a function of ionic strength and pH. The equilibrium degree of swelling increased as the pH decreased or as the cationic group-containing comonomer in the polymer increased. The water transport in such copolymers was controlled by a non-Fickian mechanism. These results can be used in the development of physiologically sensitive controlled drug delivery systems. © 1994 John Wiley & Sons, Inc.