G. Mensitieri

A novel spectroscopic approach to investigate transport processes in polymers: the case of water–epoxy system

Abstract A novel experimental approach, based on in situ FTIR spectroscopy in the transmission mode, has been developed to monitor sorption–desorption behaviour of small molecules in polymer films. This technique, along with classical gravimetric analysis, has been used to investigate water vapour transport in an epoxy resin. Aim of the investigation was to elucidate the different types of interaction that water molecules form with the macromolecular network and their change as function of the amount of sorbed water. This analysis has been performed at several water vapour activities (0.08, 0.2, 0.4, 0.6, 0.8) at 24°C by using a FTIR cell specifically designed for the in situ monitoring of water sorption in the epoxy film. Particular attention has been paid to realise a very accurate control of water vapour pressure and temperature in the cell. Water sorption kinetics has been also investigated in the same conditions by using an electronic microbalance. Information gathered from the results of both experimental approaches is potentially useful to clarify the plasticising action of sorbed water. To this aim, the amount of water sorbed by itself, is not a reliable predictor of possible physical effects on the matrix, since the plasticising efficiency of water molecules is expected to change with the level of interaction they establish with the polymeric matrix. FTIR based analysis supplies a very useful experimental tool to discriminate among different types of penetrant–polymer molecular interaction. In fact, several spectroscopically distinguishable ‘types’ of sorbed water molecules have been detected in the case of the analysed water–epoxy system and their evolution as a function of the penetrant concentration has been followed.

Vapor sorption in emptied clathrate samples of syndiotactic polystyrene

The analysis of chloroform vapor sorption at 35°C in semicrystalline syndiotactic polystyrene samples shows remarkably different sorption isotherms, depending on the crystalline form of the samples. In particular, “emptied” clathrate (“emptied” δ form) samples are characterized by higher equilibrium sorption levels and the differences are particularly relevant for low vapor activities. Moreover, sorption kinetics detected at a vapor activity equal to 0.5 show that in the case of “emptied” δ form samples the sorption rate is much higher than for the other semicrystalline samples. The larger sorption equilibrium uptakes and sorption rates of the “emptied” δ form samples are essentially due to their ability to absorb chloroform, already for low activities, by clathration in the crystalline phase. The measured equilibrium uptakes and sorption kinetics suggest that “emptied” δ form samples of syndiotactic polystyrene could be suitable for removing polluting chlorinated compounds from vapor and liquid streams.

Polymeric sensing films absorbing organic guests into a nanoporous host crystalline phase

Abstract Syndiotactic polystyrene (s-PS) semicrystalline films, including the nanoporous crystalline δ phase, have been tested as sensing elements of resonant sensors for vapors of volatile organic compounds (VOCs). In particular, the response to chloroform vapor of quartz crystal microbalance (QCM) sensors, coated with films of semicrystalline δ form of s-PS have been analyzed and compared to analogous systems coated with films of amorphous atactic polystyrene (a-PS). The sensitivity of sensors based on s-PS films was found markedly higher than those based on a-PS, particularly for low chloroform pressures. The higher sensitivity of the semicrystalline s-PS films is associated with a peculiar sorption mechanism: in fact, the organic compound, rather than being dissolved only into the amorphous phase, as it is generally the case for semicrystalline polymers, is mainly absorbed into the nanoporous crystalline phase, each molecule being confined into regularly spaced crystalline nanocavities, thus leading to a clathrate structure.

Anomalous diffusion in poly-ether-ether-ketone

Poly-ether-ether-ketone (PEEK) is a thermoplastic tough polymer used as a matrix for advanced composite materials in aeronautic applications. The investigation of its resistance to humid environment and to exposure to organic and chlorinated solvents is extremely important. A series of integral sorption experiments were performed on PEEK with three different kinds of penetrant, namely water, methylene chloride and methylene chloride—n-heptane mixtures. Coupled diffusion and relaxation phenomena were observed to occur in most of the adopted experimental conditions. Moreover, for the case of methylene chloride sorption at medium and high activities a solvent-induced crystallization phenomenon was also detected. The sorption behavior was found to range from ideal Fickian diffusion (where the molecular mobility is not affected by the penetrant concentration) to so-called anomalous diffusion. In the latter case the penetrant mobility is a complex function of penetrant concentration, time and temperature. An overview of theoretical models reported in the literature to describe such phenomena is also presented with the aim of interpreting the experimental results obtained.

Solvent induced crystallization in poly(aryl-ether-ether-ketone)

Sorptions of methylene chloride and methylene chloride/n-heptane liquid solutions in poly(aryl-ether-ether-ketone) (PEEK) are analysed and interpreted assuming multiple transport mechanisms. The presence of the highly interacting methylene chloride is responsible for anomalous sorption behaviours. PEEK films immersed in methylene chloride poor n-heptane solutions sorb by non-ideal Fickian diffusion, while limiting Case II and diffusion controlled swelling accompany equilibration in progressively richer methylene chloride solutions. The optical microscopy observation of cryogenically fractured samples conditioned in pure liquid methylene chloride at 5, 20 and 36 ° C for different times, revealed the presence of a sharp moving front. Differential scanning calorimetry (DSC) indicates that the glass transition temperature of the penetrated shells was always lower than the treatment temperature. The high levels of solvent swelling and plasticization were probably responsible for the crystallinity detected in DSC thermograms and wide angle X-ray scattering (WAXS) diffractograms of the solvent treated and initially amorphous samples. The differences of WAXS diffractograms of samples crystallized by immersion in methylene chloride at 5 and 36 ° C for various times and by thermal treatment are discussed. Increasing levels of crystallinity were induced in the samples equilibrated in progressively richer methylene chloride solutions.

Gas and water vapour transport in a polyketone terpolymer

Abstract Gas and water vapour transport properties of a polyketone terpolymer (0.93/0.07/1 ethylene/propylene/carbon monoxide) have been investigated and related to the polymer structure. Permeability tests have been performed at several temperatures (from 25 to about 65°C) with five different gases (oxygen, nitrogen, methane, ethane and carbon dioxide), evaluating permeabilities, diffusivities and solubilities. Their dependence on temperature was interpreted on the basis of apparent activation energies of permeation and diffusion (EP and ED) and of heats of solution (ΔHS). The investigated polymer was found to be rubbery at the test temperatures (glass transition temperature is about 17°C), but the detected permeabilities are comparable to those of the glassy polymers widely used for packaging applications. Data obtained in this investigation on samples exposed to moulding temperatures (240°C) for 3 min were compared to gas permeation data (presented in a previous paper) obtained for samples exposed at that temperature for 33 min in order to assess possible effects on gas transport properties. Water vapour transport was analysed by performing both sorption (35, 34, 55 and 65°C) and permeation (35°C) experiments at several activities. The analysis of sorption isotherms revealed the occurrence of water clustering, which was confirmed by a reduction of water diffusivity as a function of water concentration in the polymer.

Crystalline orientation and molecular transport properties in nanoporous syndiotactic polystyrene films

The orientation of the crystalline δ nanoporous phase in syndiotactic polystyrene films, obtained by different procedures, have been characterized. For both solution cast and biaxially stretched films a high degree of uniplanar orientation, corresponding to the tendency of the ac crystallographic planes, to be parallel to the film plane has been observed and rationalized. According to molecular dynamics simulations of diffusion of small molecules into the δ nanoporous phase, this uniplanar orientation would minimize the molecular diffusivity through the nanoporous crystalline phase.

Moisture transport properties of a degradable nylon for food packaging applications

Water transport properties of a degradable polyamide to be used for food packaging applications have been analyzed. The polymer under investigation, poly((4,9)-dioxa-1,12-dodecamethylenesebacamide) (sample A), can be envisaged as modified nylon 12/10 (sample B) in which two of the C–C linkages of the backbone have been replaced with two ether linkages. The introduction of ether linkages should enhance the degradability of the polymeric material compared with nylon 12/10, but, in turn, is also expected to increase both macromolecular mobility and hydrophilicity, promoting an increase in water diffusivity and water sorbance compared with nylon 12/10. Water vapour sorption tests were performed at three temperatures (30, 40 and 50°C) and at several water activities on both samples A and B. Water diffusivity and solubility as well as mixing enthalpies and activation energies were evaluated as a function of water concentration and their values related to macromolecular chain modification deriving from the introduction of ether linkages.

Designing microporous macromolecular hydrogels for biomedical applications: a comparison between two techniques

A comparison between two different techniques, namely gel leaching technique (GLT) and a phase inversion technique (PIT),to induce a connected micro-porosity in polymer hydrogels is presented. The gel leaching technique (GLT), applied to an acrylamide hydrogel, consists of carrying the crosslinking reaction within the interstitial space of an agarose gel which, after the setting of the acrylamide hydrogel, is removed by increasing the temperature above its melting point. Following this procedure, an ordered interconnected porous structure is scripted within the acrylamide hydrogel. The phase inversion technique (PIT), applied to a cellulose hydrogel, consists of a quick desiccation of the swollen hydrogel by phase inversion. This is obtained by increasing the swollen sample in a bath of acetone that has a low affinity for the hydrogel but a very high affinity for water. In this case, due to the rapid water removal, a connected but non homogeneous microporosity is achieved. Swelling studies performed on samples obtained by both techniques demonstrated an enhancement of the equilibrium water sorption properties induced by the presence of porosity. However, by comparing the sorption data, the GLT led to a higher enhancement in sorption properties compared to the PIT indicating that beside the degree of porosity also the degree of interconnectivity and the pattern of porosity may control the sorption properties of polymer hydrogels.

Transport of water dissolved oxygen in polymers via electrochemical technique

Abstract Transport properties of polymer membranes for dissolved oxygen in a liquid-membrane-liquid configuration were investigated by using an electrochemical technique based on the use of the Clark oxygen electrode. The oxygen permeability and the diffusivity coefficient in a polymer-water system can be determined accurately by appropriate choice of the experimental conditions. Polymers that do not absorb water such as polyethyleneterephtalate (PET) and bioriented polypropylene (OPP) were tested for permeation experiments in both gas-membrane-gas and liquid-membrane-liquid configuration. Data obtained in the liquid-membrane-liquid configuration with the electrochemical technique show excellent agreement with those obtained with the gas-membrane-gas configuration. The permeability of water saturated Kapton® polyimide (PI) for dissolved oxygen was also evaluated and compared with data obtained in the gas-membrane-gas configuration.