L. Nicolais

Isothermal crystallization and chain mobility of poly(L-lactide)

Isothermal melt crystallization of poly(L-lactide) (PLLA) has been studied in the temperature range of 90 to 135°C. A maximum in crystallization kinetic was observed around 105°C. A transition from regime II to regime III is present around 115°C. The crystal morphology is a function of the degree of undercooling. At crystallization temperatures (Tc) below 105°C, further crystallization occurs upon heating; this behavior is not detected for Tc above 110°C. The analysis of the heat capacity increment at glass transition temperature (Tg) and of dielectric properties of PLLA indicates the presence of a fraction of the amorphous phase which does not relax at the Tg, and the amount of this so-called rigid amorphous phase is a function of Tc.

Poly(Ethylene oxide) (PEO) and different molecular weight PEO blends monolithic devices for drug release

An interpretation of the drug release from monolithic water-swellable and soluble polymer tablets is presented. A convenient parameter, alpha, which compares the drug-diffusive conductance in the gel layer with the swelling and dissolving characteristics of the unpenetrated polymer was used to describe the release behaviour of beta-hydroxyethyl-theophylline (etofylline) from compression-moulded tablets of hydrophilic pure semicrystalline poly(ethylene oxides) of mol wt 600,000 and 4,000,000 and of two blends of the two molecular weights of poly(ethylene oxides). The water swelling and dissolution characteristics of two polymers and two blends were analysed, monitoring the thickness increase of the surface-dissolving layer and the rates of water swelling and penetration in the tablets. The drug diffusivities in the water-penetrated polymer gels were measured by carrying out permeation tests. Finally, drug release tests were performed to investigate the release kinetics of the different systems in an aqueous environment at 37 degrees C. The drug release from the high molecular weight poly(ethylene oxide) is principally related to the material swelling rather than polymer dissolution, leading to a progressive decrease of the drugs diffusive conductance in the growing swollen layer, and hence to a non-constant release induced by the prevailing diffusive control. Conversely, drug release from the low molecular weight poly(ethylene oxide) is strictly related to the polymer dissolution mechanism. The achievement of stationary conditions, in which the rate of swelling equals the rate of dissolution, ensures a constant release rate, even in the case of very low drug-diffusive conductance in the external gel layer. Intermediate behaviours were detected in the case of the two blends.

A general approach to describe the antimicrobial agent release from highly swellable films intended for food packaging applications.

A mathematical model able to describe the release kinetics of antimicrobial agents from crosslinked polyvinylalcohol (PVOH) into water is presented. The model was developed by taking into account the diffusion of water molecules into the polymeric film, the counter-diffusion of the incorporated antimicrobial agent from the film into water, and the polymeric matrix swelling kinetic. To validate the model the water sorption kinetics as well as the release kinetics of three antimicrobial agents (i.e., lysozyme, nisin and sodium benzoate, all approved to be used in contact with food) were determined at ambient temperature (25 degrees C). The three investigated active agents were entrapped in four films of PVOH with a different degree of crosslink. The model was successfully used to fit all the above sets of data, corroborating the validity of the hypothesis made to derive it.

Effect of thermal history on water sorption, elastic properties and the glass transition of epoxy resins

Abstract The DGEBA/TETA (diglycidyl ether of bisphenol-A/triethylene tetramine) system was examined in postcuring conditions at 100°C for 3 and 6 days. Sorption kinetics and equilibria at different temperatures and mechanical tests have been performed on the two sets of samples. Thermal treatment of the epoxy resin containing sorbed water affects its subsequent water sorption characteristics, elastic modulus and glass transition temperature. Attention has also been given to the effects on solubility on thermal history in the presence of water. It was observed that the saturation values in such systems are determined once the higher temperature of the thermal cycle is defined. The differences in solubility of samples with different hygrothermal history are explained in terms of microcavities that can be formed by effect of crazing in the plasticized system exposed to high temperatures.

An acoustic-emission characterization of the failure modes in polymer-composite materials

Abstract A new methodology for the analysis of failure modes in composite materials by means of acoustic emission techniques has been developed. A single-carbon-fiber composite based on a polyester matrix, has been used as a simple model. The occurrence of fiber-breakage during tensile loading tests has been observed by a polarized light microscope and concurrently detected by a resonant acoustic probe. The resonant probe has been used as a trigger for the reading of fiber failure events. Single acoustic emission events from a wide-band probe has been recorded for FFT Analysis. The single-fiber specimen, having a unique failure mode, has advantages for the standardization of AE techniques for the quantitative analysis of failures in polymer-composite materials. The same procedure can be exploited to investigate other failure modes namely, fiber matrix solidus debonding and matrix cracking.

Alcohol detection using carbon nanotubes acoustic and optical sensors

We demonstrate the integration of single-walled carbon nanotubes (SWCNTs) onto quartz crystal microbalance (QCM) and standard silica optical fiber (SOF) sensor for alcohol detection at room temperature. Different transducing mechanisms have been used in order to outline the sensing properties of this class of nanomaterials, in particular the attention has been focused on two key parameters in sensing applications: mass and refractive index changes due to gas absorption. Here, Langmuir–Blodgett (LB) films consisting of tangled bundles of SWCNTs without surfactant molecules have been successfully transferred onto QCM and SOF. Mass-sensitive 10MHz QCM SWCNTs sensor exhibited a resonant frequency decreasing upon tested alcohols exposure; also the normalized optoelectronic signal (λ=1310nm) of the refractive index-sensitive SOF SWCNTs sensor was found to decrease upon alcohols ambient. Highly sensitive, repeatable and reversible responses of the QCM and SOF SWCNTs sensors indicate that the detection, at room t...

The water ageing of unsaturated polyester-based composites: influence of resin chemical structure

Abstract Mechanical and calorimetric analyses have been carried out on four glass fibre laminated polyester resins (isophthalic, vinyl ester and bisphenol A and B) aged in liquid water at 20°C and 90°C, and the water uptake kinetics are described. Weight losses, disc shaped crack formation, embrittlement, fibre debonding and delamination are the principal morphological changes observed. The autocatalytic hydrolysis of the ester groups may be responsible for the chemical degradation. The hydrolytic stability of the systems investigated was improved by the inclusion of fewer ester groups in the repeating units of the initial polyester prepolymer.

Cure kinetics of neat and carbon-fiber-reinforced TGDDM/DDS epoxy systems

The cure kinetics of neat and carbon fiber-reinforced commercial epoxy systems, based on Tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and 4,4′-diaminodiphenylsulfone (DDS) were studied by means of differential scanning calorimetry (DSC). Analysis of DSC data indicated that the presence of the carbon fibers has a very small effect on the kinetics of cure. A kinetic model, arising from an autocatalyzed reaction mechanism, was applied to isothermal DSC data. The effect of diffusion control was incorporated into the reaction kinetics by modifying the overall rate constant, which is assumed to be a combination of the chemical rate constant and the diffusion rate constant. The chemical rate constant has the usual Arrhenius form, while the diffusion rate constant is described by a type of the Williams-Landel-Ferry (WLF) equation. The kinetic model, with parameters determined from isothermal DSC data, was successfully applied to dynamic DSC data over a broad temperature range that covers usual processing conditions.

A universal method for the synthesis of metal and metal sulfide clusters embedded in polymer matrices

Alkanethiolates of transition metals undergo thermolysis reactions at moderately low temperatures (120–200 °C), producing metal atoms or metal sulfide molecules and an organic by-product (disulfide or a thioether). Metal n-alkanethiolates (i.e., Mex(SCnH2n+1)y, with n = 12, 16 and 18) dissolve in polymers and the resulting solid solutions can be annealed to give polymer-embedded metal or metal sulfide clusters (i.e., metal/polymer nanocomposites). As an example, the preparation of silver, gold, cobalt and palladium clusters dispersed in polystyrene has been described, but the method is very general and a variety of metal/polymer and metal sulfide/polymer combinations can be obtained.

Water sorption and mechanical properties of a glass-reinforced polyester resin

Abstract A mechanical and calorimetric analysis was performed on a glass fibre-reinforced polyester resin aged in water at different temperatures. Data were compared for the dry and aged matrix and composites. The exposure of the resin and composite to water at different temperatures induced modifications in the mechanical properties and the morphology of the polymeric samples. The loss of low molecular weight components initially present in the resin, plays an important role in the embrittlement of the samples.