The influence of a highly anisotropic layered silicate ͑organically modified montmorillonite͒ in directing the mesoscopic self-assembly of a block copolymer blend is studied as a model for the development and tailoring of templated inorganic-organic hybrid materials. The potential for nanometer thick layers to induce large-scale mesoscopic ordering of cylindrical and spherical microdomains in asymmetric block copolymers is studied using a combination of rheology, electron microscopy, and small angle neutron scattering. Spherical microdomains arranged on a bcc lattice are templated by the anisotropic layered silicate and the kinetics of their growth are dramatically accelerated by the presence of even 0.1 wt.% ͑0.04 vol.%͒ of the filler. However, for cylindrical microdomain ordering, the kinetics are essentially unaffected by the addition of layered silicates and the development of three-dimensional mesoscopic order is possibly even disrupted. These results suggest that for the development of three-dimensional well-ordered nanostructures, the surface defining the pattern has to be significantly larger than the leading dimension of the structure being templated.

The pathway and kinetics for the cylinder-to-sphere order−order transition in a mixture of a matched diblock and triblock copolymer of styrene and ethylene−butene-1 is reported. The microstructure transformation was monitored by viscoelastic measurements, and the structural assignments of the intermediate states were performed by electron microscopy. The kinetics of transformation from macroscopically unaligned wormlike cylindrical microdomains to spherical microdomains arranged on a bcc lattice were extremely slow. The wormlike cylinder-to-sphere transition slowed with decreasing quench depth from the order−disorder transition. Additionally, these order−order kinetics were in quantitative agreement with those for the development of spherical microdomains from an initial disordered state. Further, during the initial induction time following a temperature jump from a wormlike cylindrical order to a spherical state, the sample exhibited liquidlike viscoelastic characteristics and structurally showed the absence of long-range order. In contrast, shear-aligned cylinders rapidly transformed to spheres, adopting a viscoelastic pathway distinct from that of the unaligned samples. The cylinder-to-sphere transition is thermotropically reversible, with the viscoelasticity-based kinetics of the sphere-to-cylinder transition being slower than the cylinder-to-sphere transition.

The phase behavior and linear viscoelasticity of a mixture of a matched diblock and triblock copolymer of styrene and ethylene−butene-1 are reported. The mixture undergoes reversible thermotropic transitions from a cylindrically ordered state at low temperatures to a spherically ordered state at intermediate temperatures to a disordered state at high temperatures. The morphological assignments were based on TEM and SANS measurements, which also revealed a decrease in the domain size in the spherically ordered state as compared to the cylindrically ordered state. The kinetics of the order−order transition are extremely slow, with the sample exhibiting rheological characteristics similar to a disordered material immediately after heating from the cylindrical to the spherical state. The transition exhibits a long induction period wherein the sample continues to behave like a disordered material followed by a rapid increase in the storage modulus and a final plateau value. The kinetics and rheological characteristics of a material quenched from the disordered state to the spherical state are remarkably similar to those obtained after heating from the cylindrical to the spherical state, suggesting that the transition from cylindrically ordered to spherically ordered microdomains proceeds via a mesoscopically disordered state.

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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-~