Modification of polystyrene glass transition by high pressure methane
Introduction
Compressed fluids, particularly in their supercritical state, are widely used in the polymer industry, typically in foam processing. The major advantage using supercritical fluids is the possibility to modify their characteristics (density, viscosity and diffusivity) by only changing the experimental parameters; by controlling these parameters it is then possible to tailor foams having well-defined structures.
Up to now chlorofluorocarbons (CFCs) were widely used as blowing agents in the foaming industry. Presently, international regulation has lead to a ban on CFCs, to be replaced by blowing agents which are less harmful to the ozone layer. Currently, hydrofluorocarbons (HFCs) or hydrochlorofluorocarbons (HCFCs) are being considered instead. However, it is expected that by year 2004 most of these substituted hydrocarbons will eventually be prohibited if they do not have a zero ozone depletion potential (ODP). There is hence a tense competition to select new blowing agents. For this, it is necessary not only to know the thermodynamic properties of these fluids [1] but also to characterize the “interaction” under pressure between such fluids and polymers [2] in order to obtain the product of interest, having the appropriate thermophysical properties and the required structures for specific applications, while keeping similar performance properties.
The present work forms a study in a very active field of research. We report here preliminary results, using the technique of temperature modulated differential scanning calorimetry (TMDSC), of the study of polystyrene modified by scanning transitiometry (ST). Polystyrene (PS) samples were first treated under specific conditions of pressure, induced either by mercury (as a neutral fluid) or a supercritical gas, (more “active” fluid) like methane. Scanning transitiometry is a new technique [2], [3] which is used to perfectly control these modifications under pressure. As a matter of fact, methane is an interesting “weakly active” fluid since its interaction with PS is rather weak due to the symmetric geometry and the non-polarizability nature of the molecule of methane.
We compare, in what follows, the results obtained when investigating the initial PS (that is to say, “untreated” PS) with PS “treated” by high pressure transmitted either by mercury or by methane.
Section snippets
Scanning transitiometry
Scanning transitiometry is a relatively new technique (making use of a transitiometer, actually, a p–V–T calorimeter) which is based on inducing a change in the thermodynamic state of a sample under study by scanning at a low rate (or stepwise) one of the independent variables (p,V,T) and keeping automatically constant the other independent variable. From the output signals recorded simultaneously (rate of heat exchange and variation of the mechanical variable, volume or pressure) a respective
Experimental
The experiments were carried out with a polystyrene (PS) of the atactic type provided by Fibran SA (Thessaloniki, Greece), in the form of pellets.
Different samples of polystyrene were submitted to different “treatments” to induce modifications under well-defined conditions. The controlled conditions were obtained by the transitiometric method. Two series of samples were “treated” on heating and then on cooling in the range 303–453 K with a fixed scanning rate of 0.16 K min−1 under isobaric
Results and discussion
There is not much information available in the literature on calorimetric study of plasticization of polymers at high pressures, above say 50 MPa, induced by gases. O'Neill and Handa [10] have reported data concerning the plasticization of PS using methane (CH4), ethylene (C2H4) and carbon dioxide (CO2), under relatively small pressures up to 6.0, 8.8 and 35.9 MPa for CO2, C2H4 and CH4, respectively. For our part, we deliberately worked at much higher pressures. Plasticization is well
Conclusions
When PS is submitted to high pressure, its glass transition is consequently modified. Methane, regarded as a non-plasticizing gas, is able to diminish the Tg when the pressure is significantly increased. One can observe that the contribution of the hydrostatic pressure effect and of the plasticization effect is largely dependent on the pressure. At high pressure plasticization seems to overtake the hydrostatic effect, but the interactions between CH4 and PS are not strong enough to modify in an
Acknowledgements
Financial support through the Brite Euram Program No. 97-4154 “Interactions between gases and polymers at high pressures. Polymer foaming process” is highly appreciated.
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