A. G. Andreopoulos

Synthesis and properties

Poly(D,L lactic acid) was prepared by bulk polymerization of D,L lactide, both under atmospheric pressure and in vacuum. The obtained polymeric products were characterized in terms of molecular weight, Mw, melting point, calorimetric response and swelling behaviour. All products were amorphous. Their molecular weights were determined by viscosimetry and ranged from 2×103 to 9×104. Similarly, the melting points ranged from 90 to 210°C. Swelling experiments, with specimens immersed in buffer solutions, showed that hydrolytic degradation started in a few days for the low Mw material, whereas for the higher molecular weight products it took much longer and probably followed a two-stage mechanism. This study suggests that the high molecular weight material could be an interesting carrier for the preparation of controlled release products, in cases where prolonged delivery is necessary.

Physical properties of a silicone prosthetic elastomer stored in simulated skin secretions

STATEMENT OF PROBLEM Facial prostheses worn over an extended time are exposed to various environmental factors, including sebaceous oils (sebum) and perspiration. PURPOSE This study investigated the physical properties of tensile strength and modulus, elongation, tear strength, hardness, weight, and color change, of a silicone facial elastomer after immersion for 6 months in simulated sebum and perspiration at 37 degrees C. MATERIAL AND METHODS Specimens made of Episil silicone elastomer were immersed in simulated alkaline or acidic perspiration as well as in sebum. Tensile and tear tests were conducted according to ISO specifications no. 37 and 39, respectively, in a Monsanto testing machine. Shore A hardness measurements were run according to ASTM D 2240. Weight changes were followed at 5, 15, 30, and 180 days, and color changes were determined in the CIE LAB system using a tristimulus colorimeter. RESULTS An improvement of mechanical properties for specimens immersed in acidic perspiration was attributed to facilitation of the propagation of cross-linking reaction during aging of the silicone samples. Some weight increase was observed for the specimens immersed into the aqueous solution, whereas for those immersed in sebum, weight loss was recorded, probably because of extraction of some compounds. In this latter case, the color change was lower than that corresponding to simulated perspiration. CONCLUSION The silicone specimens aged for a period, which simulates 1.5 years of clinical service, showed minimal changes with respect to the properties studied.

Acrylic resin denture repair with adhesive resin and metal wires: Effects on strength parameters

The fracture of acrylic resin dentures is an unresolved problem in prosthodontics. In this study one brand of denture base acrylic resin was used to make specimens in the form of strips and maxillary denture bases. The specimens were cut and repaired with one type of an autopolymerizing adhesive resin and metal wires. The mechanical properties of the repaired specimens were measured, and the efficiency of each method was evaluated. The statistical results of this study revealed that geometric characteristics of a maxillary denture combined with the shape and pretreatment of reinforcement were the controlling factors for the overall mechanical behavior. Furthermore this study revealed that data with clinical significance can only be obtained by testing specimens similar to the original items used in dental practice.

Mechanical properties of epoxies reinforced with chloride-treated aramid fibers

Surface treatment of aramid fibers by immersion in a solution of methacryloyl chloride in carbon tetrachloride was carried out, and the resulting material was examined by means of electron microscopy and chemical analysis in an attempt to record any changes in the morphology and nature of the surface. Mechanical testing of tensile, flexural, and interlaminar shear strength, as well as dynamic mechanical analysis (DMA), were performed in an attempt to explore the effect of this treatment on the strength of the fiber. In a subsequent stage, the performance of those fibers as reinforcement in composites of epoxy matrix was assessed. The aim of this study was to provide more information about the interactions between the chloride-treated aramid fibers and the epoxy resin and, more specifically, to compare the behavior of the epoxy matrix composites with those composed of unsaturated polyester, polyethylene, and polyurethane matrix, which were studied in the past. It was found that specimens containing chloride-treated aramids display better flexural properties, whereas their tensile strength is drastically reduced. Improved performance was also identified by the DMA experiments.

Polymers for use in controlled release systems: the effect of surfactants on their swelling properties.

The effect of an ampholytic surfactant on the swelling properties of polymeric materials was studied, using various swelling liquids. Tablets were prepared consisting of hydroxypropyl methylcellulose, poly(oxyethylene) and sodium alginate. Tego betain was the non-ionic surfactant used as an additive in a series of samples made of the above polymers. Those tablets were immersed in distilled water, phosphate buffer and 0.1 N HCl, and their weight uptake was recorded as a function of time, in order to assess the swelling process. Measurements of the contact angle of the above systems were also carried out for estimating their wetting properties. The results of this study showed a selectivity among polymers, surfactant and surrounding liquid. Clearly, an enhancement of the swelling capacity of hydroxypropyl methylcellulose tablets due to the surfactant was recorded. An unclear effect was observed in the case of poly(oxyethylene), whereas for sodium alginate, the dominant factor is its water solubility that controls swelling behaviour.

Water sorption characteristics of epoxy resin-UHMPE fibers composites

Specimens of epoxy resin reinforced with ultrahigh-modulus polyethylene (UHMPE) fibers were immersed in water, and their swelling characteristics were recorded at various temperatures. In addition to an estimation of the response of those composite, the above study aimed at the exploration of the role of the fiber–matrix interface on the water sorption. Therefore, specimens containing original, calendered, and corona-treated fibers were tested. UHMPE fibers were found to limit the extent of sorption due to the nonhydrophilic character of polyethylene. However, specimens with poor interfacial properties, such as those with the original, untreated fibers, showed enhanced sorption since their surface area is drastically increased. As expected, the raise of temperature has a positive contribution to water sorption, and, furthermore, it seemed to affect the interface between epoxy and calendered fibers. On the other hand, the increase of filler volume fraction leads to a decrease in the amount of water uptake. The water transport in the neat epoxy resin specimens is rather diffusion-controlled, and this behavior was also recorded for the composite specimens reinforced with original UHMPE, which presented the maximum absorption.

Synthesis and Characterization of Low Molecular Weight Polylactic Acid

The polymerization of (D, L)-lactic acid in the absence of catalysts was studied. Azeotropic distillation using xylene, gave poly(lactic acid) with very low molecular weight, which however, was further increased by post-curing in an air oven. Moreover degradation phenomena and residual xylene were observed, with this procedure. Polycondensation of lactic acid at temperatures up to 180 C under vacuum, resulted in fast reaction with poor control and the products are still characterized by low molecular weight. When the reaction was run at 220 C in inert atmosphere, amorphous poly(lactic acid) was obtained with molecular weight exceeding 3000. The product was found pure, since it is free from solvents, catalysts and monomer. Also, no evidence of thermal degradation was observed. The above characteristics are well acceptable design aspects for biomedical uses.

Repair of denture base resins using visible light-cured materials

Specimens of denture base resins were repaired with autopolymerizing and visible light-cured (VLC) repair materials. Flexural properties were measured and revealed that the highest strength and toughness of joint are obtained by the autopolymerizing repair material and are independent on the base resin. The VLC materials exhibited a lower repair strength (22% to 58%) and toughness (9% to 33%) than those of the autopolymerizing resin. No interaction between base and repair material was detected, which was attributed to poor adhesive bonding created at the interface.

Controlled release systems based on poly(lactic acid). An in vitro and in vivo study

A new biodegradable delivery system based on poly(lactic acid) has been formulated, with potential applications in sustained antibiotic release against bone infection. The in vitro release of a new quinolone (pefloxacin) from low molecular weight poly(D,L-lactic acid) Mw = 2×103 lasted for 56 d whereas the in vivo delivery lasted 33 d. In both cases, the release rate is controlled by the drug diffusion and the polymer degradation, which seems to be the predominant factor. For the release experiments, discs were prepared from poly (D,L-lactide) Mw = 2×104 with drug loadings of 2% and 10% w/w. It was concluded that pefloxacin concentration remains higher than the Minimum Inhibitory Concentration (MIC) against the major causative bacteria of bone infection. The results indicate that the two different types of poly(lactic acid) can be used effectively in an implantable antibiotic release system. ©2000 Kluwer Academic Publishers

Swelling properties of various polymers used in controlled release systems.

The effect of powder packing and porosity of specimens on the swelling properties of polymeric materials was studied, in various swelling liquids, such as distilled water and 0.1 N hydrochloric acid solution. Capsules, tablets and films of hydroxypropyl methylcellulose, poly(ethylene oxide) and sodium alginate were prepared and their weight uptake after immersion into the above solutions was recorded as a function of time, in order to assess the swelling process. Measurements of some characteristics of the as received powders were also performed as an attempt to classify the specimens prepared according to their porosity. Within the experimental conditions of this work, it was shown that the porosity of polymeric specimens is a dominant factor that controls their swelling behaviour. Increased porosity leads to fast initial rates of weight uptake and high extent of equilibrium swelling. On the other hand, dissolution and possible degradation of polymers susceptible to acid hydrolysis, results in some variations from the above-mentioned behaviour. With respect to the application in controlled release systems, the overall delivery rate from a polymeric specimen is expected to be a function of both swelling and disintegration characteristics of a specimen and, therefore, the weight uptake can be considered a measure of the release only in the case of polymers with low water solubility and increased stability to hydrolysis.