A process for producing an optically transparent peroxide cured article made from a peroxide curable butyl rubber ionomer, the process comprising: a. providing a halogenated butyl rubber polymer comprising repeating units derived from at least one isoolefin monomer and repeating units derived from one or more multiolefin monomers, the polymer comprising at least an allylic halogen moiety; and either as alternative 1 admixing a nitrogen or phosphorous based nucleophile comprising a pendant vinyl group and a peroxide curing agent with the halogenated butyl rubber polymer at a temperature of 25 to 80 °C; and forming the ionomer in situ during curing of the compound at a temperature of 100 to 200 °C or as alternative 2 admixing a nitrogen or phosphorous based nucleophile comprising a pendant vinyl group with the halogenated butyl rubber polymer and reacting the allylic halogen moiety with the nucleophile to form the peroxide curable butyl rubber ionomer and admixing a peroxide curing agent with the peroxide curable butyl rubber ionomer and curing the article at a temperature of 100 to 200 °C. whereby for each alternative 1 or 2 curing is effected by forming an article between two molding surfaces heated to a temperature of 100 to 200°C to obtain an optically transparent peroxide cured article having a root mean squared surface roughness of from 0.1 to 10 nm as measured by atomic force microscopy a. providing a halogenated butyl rubber polymer comprising repeating units derived from at least one isoolefin monomer and repeating units derived from one or more multiolefin monomers, the polymer comprising at least an allylic halogen moiety; and either as alternative 1 admixing a nitrogen or phosphorous based nucleophile comprising a pendant vinyl group and a peroxide curing agent with the halogenated butyl rubber polymer at a temperature of 25 to 80 °C; and forming the ionomer in situ during curing of the compound at a temperature of 100 to 200 °C or as alternative 2 admixing a nitrogen or phosphorous based nucleophile comprising a pendant vinyl group with the halogenated butyl rubber polymer and reacting the allylic halogen moiety with the nucleophile to form the peroxide curable butyl rubber ionomer and admixing a peroxide curing agent with the peroxide curable butyl rubber ionomer and curing the article at a temperature of 100 to 200 °C. whereby for each alternative 1 or 2 curing is effected by forming an article between two molding surfaces heated to a temperature of 100 to 200°C to obtain an optically transparent peroxide cured article having a root mean squared surface roughness of from 0.1 to 10 nm as measured by atomic force microscopy admixing a nitrogen or phosphorous based nucleophile comprising a pendant vinyl group and a peroxide curing agent with the halogenated butyl rubber polymer at a temperature of 25 to 80 °C; and forming the ionomer in situ during curing of the compound at a temperature of 100 to 200 °C admixing a nitrogen or phosphorous based nucleophile comprising a pendant vinyl group with the halogenated butyl rubber polymer and reacting the allylic halogen moiety with the nucleophile to form the peroxide curable butyl rubber ionomer and admixing a peroxide curing agent with the peroxide curable butyl rubber ionomer and curing the article at a temperature of 100 to 200 °C.