The effect of cluster plasticisation on the self healing behaviour of ionomers

This paper explores the effect of cluster plasticisation on the autonomous damage initiated self healing behaviour of a partially neutralised poly(ethylene-co-methacrylic acid) ionomer copolymer, Surlyn 8940® (DuPont). Aliphatic di- and tri-carboxylic acid based modifiers and their analogues were blended with the ionomer at 10 wt% concentration using twin screw extrusion methods to study their effect upon self healing. In-situ mechanical evaluation of the healing process showed that carboxylic acid modifiers reduced the elastic properties of the blend, increased elastomeric behaviour and enhanced elastic healing. The sodium neutralised carboxylic acids analogues and succinamide modifiers displayed the opposite behaviour, increasing elastic properties, decreasing elastomeric behaviour and reducing elastic healing. A proposed relationship which accounted for both the elastic and elastomeric behaviour during impact was found to correlate well with independent measurements of elastic healing or hole closure. Thermal analysis suggested that the carboxylic acid modifiers plasticised the ionic clusters, reducing their strength and increasing their rate of recovery. The sodium neutralised analogues and succinamide modifiers, again conversely, produced negligible plasticisation but reduced the rate of reformation of the ionic clusters after annealing. Examination of the impact zone using scanning electron microscopy complemented the mechanical and thermal results revealing increased elastomeric and viscous behaviour for the carboxylic acid modified samples but increased brittle failure for the sodium neutralised analogues and succinamide modifiers. Systematic modification of the ionomer therefore has clearly illustrated the importance of the ionic clusters in determining static and dynamic properties. In addition to this, controlling their strength and mobility is shown to have a profound effect upon the efficiency of the damage initiated healing event.