De Mattia cut growth, an important fatigue test, of a number of crosslinked elastomers based on terpolymers of isobutylene, p-methylstyrene, and p-bromomethylstyrene brominated isobutylene paramethylstyrene copolymers (BIMS), butadiene rubber (BR), and blends of BR and ~t u r a l rubber (NR) has been studied. In these compounds the carbon black level and the cure state are varied. We attempt to relate this fatigue performance to bulk mechanical properties (strain at break, eb, and strain energy density, U ) and network structul-al parameters (contour length, L,, and root-mean-square end-to-end distance, (r), of network molecular chains). INTRODUCTIONhe resistance to fatigue throughout its service life T is one of most important requirements for a crosslinked elastomer. One laboratory flex fatigue test, the De Mattia cut initiation or cut growth experiment (11, is widely employed for rubbers used in tires and engine mounts. Under limiting conditions with vanishlngly small contribution from viscous effects, the tear energy required to propagate a sharp cut in a molecular network as a function of its molecular structure was first calculated by Lake and Thomas (2): Go = (%pu/2m,) ( r ) (11 where N, is Avogadro's number, u is energy required to break a C-C bond, m, is the molecular weight associated with each C-C bond, p is density of the rubber and (r) is the root-mean-square end-to-end distance of the network chain containing n freely jointed subunits. For a hydrocarbon elastomer with a network chain consisting of 100 subunits, Go -20 J/m2. Thus far, to our knowledge, no systematic experimental study has been performed to verify the relationship between Go and (r).Measured at a normal pulling speed, the tensile strength of a vulcanized rubber goes through a maximum but the elongation at break drops with increasing network chain density, u, (3). Another fracture property, the tear strength measured under threshold conditions by Gent and Tobias (4), exhibits different behavior. It increases with decreasing crosslink density. Different elastomers such as polybutadiene, cis-~

Physical, mechanical and thermal properties of a new class of isobutylene based model block copolymers are studied and contrasted with those of traditional thermoplastic elastomers based on polydienes and saturated polydienes (polyolefins) such as Kraton (or Vector) and Kraton G block copolymers. Melt state rheological and dynamic mechanical measurements confirm that thermodynamic interactions between polystyrene (S) and polyisobutylene (iB) or poly‐p‐tert‐butylstyrene (tbS) and iB are comparable to, if not larger than, those between S and polybutadiene (B) or S and ethylene/butene‐1 copolymer (EB). Physical properties for the S‐iB‐S and tbS‐iB‐tbS block copolymers, particularly the injection‐moldability and cut‐growth characteristics, are found to be considerably different from those found for the S‐B‐S and S‐EB‐S systems. We attribute this behavior to the longer entanglement chain length of iB compared with those for B and EB.

The chemical reactivity of a new elastomer based on brominated poly(isobutylene-co-4-methylstyrene) in electrophilic additions to olefins has been investigated using model compounds as well as appropriate polymers. The reactions catalyzed by zinc salts are influenced by the solubility as well as the composition of the catalyst. While the reactivity of zinc bromide is limited by its low solubility in nonpolar medium, zinc oxide and zinc stearate can afford excellent results once an induction period has elapsed. The induction period likely corresponds to the formation of more reactive zinc based moieties through interchange reactions with the benzylic bromide groups. The mechanism of the addition process involves initial formation of carbocationic complexes with the zinc salts, followed by addition to the double bonds of the olefins. The products resulting from these additions have been characterized by NMR as well as gas chromatography-mass spectrometry. A comparison of potential crosslinking processes involving olefin addition or electrophilic aromatic substitution reactions shows that the former process is preferred. The findings of this study are directly applicable to the co-curing of elastomers based on brominated poly-(isobutylene-co-4-methylstyrene) with polyolefins.