Giuseppe Ragosta

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.

The transport of water in a tetrafunctional epoxy resin by near-infrared Fourier transform spectroscopy

Abstract An epoxy resin formulation composed of tetraglycidyl-4,4′-diamino diphenylmethane (TGDDM) and 4,4′diamino diphenyl sulfone (DDS) was investigated by Fourier transform near infrared (FT-NIR) spectroscopy and dynamic-mechanical analysis. Both techniques have demonstrated the essentially complete cure state of the resin with the adopted curing schedule. A novel feature of this work is the possibility of monitoring the transport of water into the resin by using of an FT-NIR spectrometer as a detector in place of the currently employed gravimetric detectors. The data gathered at different temperatures have revealed a good agreement with conventional gravimetric measurements, which verifies the reliability and accuracy of the experimental approach presented herein.

An interpenetrated system based on a tetrafunctional epoxy resin and a thermosetting bismaleimide: Structure–properties correlation

The kinetics and mechanism of the curing process of a thermosetting blend formed by tetraglycidyl-4,4′-diaminodiphenyl methane and N,N′-bismaleimido-4,4′-diphenyl methane (BMI) cured in the presence of 4,4′-diaminodiphenyl sulfone, was investigated in detail by Fourier transform infrared spectroscopy. Information on the molecular structure of the network formed upon curing was derived. Dynamic-mechanical measurements on dry samples indicated an interpenetrated polymer network-like structure. Sorption measurements at 70°C showed a reduction of the water uptake at equilibrium in the presence of substantial amounts of BMI in the system (43.5% body weight). Finally, the dynamic-mechanical analysis of wet samples demonstrated a reduction of the plasticizing efficiency of the absorbed water in the presence of BMI.

Thermal-oxidative degradation of epoxy and epoxy-bismaleimide networks: Kinetics and mechanism

The thermal oxidative stability of an epoxide/diamine network (TGDDM/DDS) and of an epoxide/diamine/bismaleimide IPN system (TGDDM/DDS/BMI) have been investigated by thermogravimetric analysis (TGA), dynamic-mechanical thermal analysis (DMTA) and isothermal, time-resolved FTIR spectroscopy. Both TGA and DMTA data revealed a considerable enhancement of the stability of the material as the bismaleimide content in the system is increased. The FTIR measurements allowed us to monitor in-situ the degradation process and to obtain reliable kinetic data. These, together with the analysis of the time evolution of the infrared spectra, permitted to propose likely mechanisms to account for the experimental observations. It was found that in the IPN system the epoxide network degrades with the same mechanism occurring in the pure epoxy resin, albeit at an increased rate. No evidence has been found of alternative pathways introduced in the system by the presence of the bismaleimide component. The bismaleimide network was also found to undergo significant degradation in the IPN system, but to a considerably lesser extent than the more oxygen sensitive groups of the TGDDM/DDS network. The higher stability of the bismaleimide component represents the underlying reason for the enhanced thermal-oxidative resistance of the ternary system with respect to the binary TGDDM/DDS resin.

Tetrafunctional epoxy resins: Modeling the curing kinetics based on FTIR spectroscopy data

The curing kinetics of two thermosetting systems based on a tetrafunctional epoxy resin was investigated by Fourier transform infrared spectroscopy. Two formulations were studied, in which the hardener was an aromatic diamine and a carboxylic dianhydride, respectively. The quantitative evaluation of the epoxy conversion was based on spectra collected in the near-infrared range (8000–4000 cm−1) as well as in the medium infrared range (4000–400 cm−1). The kinetic parameters evaluated in the above frequency intervals were significantly different. The reasons for such a discrepancy are discussed critically. Several kinetic models, based on the widely employed Kamal approach, were applied to verify their predictive capability. Satisfactory results were obtained for the amine-cured system, particularly with a modified equation taking into account the autocatalytic nature of the process as well as a limiting diffusional effect. Less accurate results were achieved for the anhydride-cured system.

Isotactic polypropylenes of different molecular characteristics: influence of crystallization conditions and annealing on the fracture behaviour

Fracture mechanics analysis at high speed and low temperature has been carried out on polypropylene samples of various molecular weights, obtained under different crystallization conditions. In addition differential scanning calorimetry measurements and morphological observations by scanning electron microscopy have been also performed. It has been found that the fracture mechanics parametersGc andKc increase markedly with enhancing the molecular weight, whereas an opposite trend is observed with increasing the crystallization temperature. Furthermore it has been seen that an annealing procedure, on the already solidified samples, strongly enhances the fracture toughness parameters. The above findings have been confirmed by fracture surface observations showing that crazing is the dominant feature of deformation in such materials. Possible underlying molecular mechanisms, which can explain the overall properties and the morphological characteristics, are discussed as well.

Thermosetting bismaleimide/reactive rubber blends: Curing kinetics and mechanical behavior

A maleimido-terminated butadiene–acrylonitrile copolymer was employed as an impact modifier for a commercial thermosetting bismaleimide resin. FTIR spectroscopy was used to monitor in real time the kinetics of the curing process in this blend system at different temperatures. The toughening agent was found to produce strong effects on the kinetics and mechanism of the curing process. The fracture toughness parameters Kc and Gc showed a substantial enhancement as the rubber content in the blend was increased. Only a modest reduction of the elastic modulus and of the compressive yield stress was brought about by incorporation of the impact modifier.

The Curing Process and Moisture Transport in a Tetrafunctional Epoxy Resin as Investigated by FT-NIR Spectroscopy

An epoxy resin formulation composed of tetraglycidyl-4, 4′ diaminodiphenylmethane (TGDDM) and 4, 4′ diaminodiphenylsulfone (DDS) was investigated by Fourier transform near-infrared (FT-NIR) spectroscopy. In situ isothermal measurements allowed us to obtain the concentration versus time curves of the various reactive groups present in the system, thus affording a complete characterization of the curing process. The same experimental approach was used to monitor the transport of moisture into the resin at several temperatures. The data gathered in this way revealed a good agreement with conventional gravimetric measurements, which verifies the reliability and accuracy of FT-NIR spectroscopy for the study of these transport processes.

A novel reactive liquid rubber with maleimide end groups for the toughening of unsaturated polyester resins

An amino-terminated butadiene–acrylonitrile copolymer was chemically modified into a maleim-ido-terminated rubber and was used as a toughening agent for an unsaturated polyester resin. The reactive rubber was characterized by Fourier transform infrared spectroscopy. The mechanical and fracture properties of the blends containing the unmodified and the modified rubbers were investigated. Furthermore, a morphological analysis was carried out by scanning and transmission electron microscopy. A substantial enhancement of toughness was found when the modified rubber was used in place of the plain copolymer.

A polymer network of unsaturated polyester and bismaleimide resins: 1. Kinetics, mechanism and molecular structure

Abstract An unsaturated polyester resin has been modified by adding a thermosetting bismaleimide as a second co-reactive monomer. The bismaleimide resin was readily dissolved in the uncured polyester matrix up to a concentration of about 20 wt%. Fourier transform infra-red (FTi.r.) spectroscopy was employed to monitor the cure kinetics of this complex system. The available spectroscopic evidence indicated that both the mechanism and the kinetics of the crosslinking process were strongly affected by the presence of the bismaleimide in the system. The molecular structure of this intercrosslinked network is discussed. The dynamic mechanical behaviour of a typical blend composition was found to be in agreement with the proposed molecular structure.