Ion-beam-induced aggregation in polystyrene: The influence of the molecular parameters
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Lucia Calcagno and Gaetano Foti DipartimentodiFisica, corso Italia 57, 95100 Catania, Italy (Received 12May 1988;accepted lOAugust 1988) The formation of an insoluble gel under ion-beam bombardment is governed by ion-beam parameters and target parameters. Here reported is a study of the influence of the target molecular parameters on the sol-gel transition of ion-bombarded polystyrene with particular emphasis for the number-average molecular weight Mn. It is shown that the main parameter is the number of macromolecules of the film so that by adopting a "corrected" fluence F/n (ions per macromolecule), the different curves of the various polymers collapse in only one universal curve. The importance of the "corrected" fluence is shown also at molecular level and the MWD of the various polymers is similar at equal F/n values. An experimental model is outlined which explains the sol-gel transition on the basis of transition from an isolated-track regime to an overlap regime where the formation of insoluble giant macromolecules occurs.
I. INTRODUCTION The formation of new molecular species under ionbeam bombardment and the corresponding change of macroscopic properties of the bombarded target is a well-known phenomenon.1'2 This chemical phenomenology is initiated by the energy loss of the penetrating ion along the track so that most of the new molecular species formed as a consequence of the bombardment do not contain the projectile atom, even for bombardment with reactive ions.3'4 The most interesting fundamental aspects of this kind of phenomenon are related to peculiar features of this "high-energy-density chemistry," which seems to generate final products structurally different than those generated by low-energy-loss particles, electrons, for example.5 The technological implications (and expectations) also, are numerous owing to the possibility of exploiting this phenomenology for microlithography purposes.6 However, the phenomenology is often unpredictable even with the simplest molecular solids,2 so that, when possible, the studies have to be carried out at very low fluence (ions/cm 2 ) in order to avoid the complications arising from the overlap of the ion tracks.7 This can be done with nearly monodisperse polymers (i.e., polymers composed of macromolecules of nearly uniform length), but also in this case the phenomenology appears to depend on too many parameters for the quantitative aspects being elucidated. The formation of a three-dimensional macromolecular network is probably one of the most simple chemical-physical phenomenon induced by the ion bombardment. This phenomenon, often referred to as "gelation," J. Mater. Res. 3 (6), Nov/Dec 1988
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yields a macromolecular solid that is insoluble in all the solvents that do not destroy the polymer. The parameters that play some role on this sol-gel transition can be divided into two groups: (i) ion-beam parameters, i.e., energy, mass, fluence, etc., with particular emphasis on the energy loss parameters and (ii) target parameters such
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