One of the main goals of polymer science has been to relate the structure of macromolecular chains to their macroscopic properties. In particular, it has been hoped that one could relate the sizes of polymer coils to the degree to which they entangle with one another and thus to their viscoelasticity in the melt. In recent years, the availability of model polymers with nearly monodisperse molecular weight distributions and precisely controlled chemical structures has allowed for improved data both on rheology and on the dimensions of the chains. This has now allowed us to determine the correlations between such properties as chain dimensions, density, and plateau modulus and to show that some quite simple relations exist between them. The main body of these data is on polymers that can be considered to be models for polyolefins. These have been made by the hydrogenation of polydienes synthesized by anionic polymerization techniques. In this way the molecular weight distribution can be made to be nearly monodisperse (Afw/M" < 1.1) and the chemical structure is well controlled. For example, models of a wide range of ethylene-butene copolymers have been made by the saturation of polybutadienes with a range of vinyl content. Such polymers can be made at many molecular weights as well. The viscoelastic properties of these polymers have been measured very precisely, and their chain dimensions have been determined by small-angle neutron scattering. To a high degree of correlation, we find that the mean-square unperturbed end-to-end distance,

We report the identification of a new equilibrium microdomain morphology in an intermediate to weakly segregated diblock copolymer melt. A polystyrene-polyisoprene (SI) diblock copolymer consisting of 37 wt % styrene and of total Afw = 27 400 was observed to transform from the lamellar morphology (in equilibrium at low annealing temperatures) to a new morphology at annealing temperatures approximately 50 °C below the order-disorder transition (ODT). The transformation was observed to be thermoreversible. Investigation of the new morphology via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed the new structure to have remarkable three-dimensional long-range order, to belong to the cubic space group Ia3d, and to possess a bicontinuous cubic microstructure. From computer simulations of model structures and comparison with microscopy results, we propose models for the new morphology based on the triply periodic G minimal surface (gyroid) discovered by Schoen;1 similar morphologies have been observed in a variety of microphase-separated surfactant-water systems. Blends of this diblock with various short-chain homopolymers were used to investigate the compositional extent of the region of IaZd stability on the phase diagram; the results indicate that the Ia3d phase is stable over a wide range of minority component volume fractions.ogy, the ordered bicontinuous double diamond (OBDD), in a strongly segregated melt;3 the new structure has since been observed in a variety of block copolymer systems.4-6Recently, Olmsted and Milner7 have developed methods for calculating the free energy of bicontinuous morphologies in the strong segregation limit. A variety of new structures not predicted by the early theories have been observed in the weak segregation regime, including the lamellar-catenoid,8'9 hexagonally modulated lamellae, and hexagonally packed lamellae.10 As part of a study on block copolymer thermal behavior, Gobran11 observed an unusual microphase-separated morphology in a polystyrene-polyisoprene (SI) diblock copolymer. After casting from toluene, the sample formed a lamellar phase; upon heating to temperatures above 120

We have shown in previous studies that the entanglement molecular weight for a polymer melt, M e, is related by a power law to p, the packing length of the polymer species. We now find that power laws also describe the molecular weights characterizing the melt viscosity, M c marking the onset of entanglement effects and M r the crossover to the reptation form. The packing length exponents for M e, M c, and M r differ significantly, however. The long-held notion that the ratio M c/M e has the same value for all species is therefore incorrect. Further, the observed and predicted values of M r for two species, 1,4-polybutadiene and polyisobutylene, have been found to agree, within the uncertainties, with the projected values. Finally, the variations with packing length are such that all three characteristic molecular weights would appear to converge on the same value near p = 9 Å. As yet, no species with such a large packing length has been completely studied rheologically. But the range is not outlandish and is clearly reachable by appropriate synthetic methods.

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Addition polymerizations involving soluble organometallic species have received intensive attention in recent years with special reference to the type of counterion and solvent. An anionic mechanism is proposed for those systems where there is good reason to assume that the metal is strongly electropositive relative to the carbon (or other) atom at the tip of the growing chain. Hence, the metal, e.g. lithium, becomes a cation either in the free state or coupled with the growing carbanion. Under the appropriate experimental conditions, spontaneous termination is avoidable in many of those systems when one of the metals of Group I is used as the counterion. The alkali metals sodium and potassium were revealed to be polymerization initiators of isoprene in the disclosures of Matthews and Strange in 1910 and Harries in 1911. The first unambiguous report of the use of lithium in reactions with diolefins appears to be that of Ziegler and coworkers in 1934. Their work consisted of an investigation of the reaction between the alkali metals (lithium, sodium) or alkyllithium species and butadiene, isoprene, 2,3-dimethylbutadiene, or piperylene.

We have measured the force-distance profiles between two curved mica sheets immersed in toluene and in xylene, both in the pure solvents and following addition of polystyrene (PS), end-functionalized polystyrene of different molecular weights Af, PS-X(AÍ), and polystyrene-poly (ethylene oxide) diblock copolymers (PS-PEO) with a short PEO block. Our results show that PS does not adsorb from the (good solvents) toluene and xylene but that once PS-X or PS-PEO are added to the solution the surface is rapidly covered, showing that the nonadsorbing PS tails are anchored at one end only. The force profiles following surface coverage are monotonically repulsive, and the range 2L0 for onset of interaction is roughly twice that of the corresponding adsorbed chains. There is no evidence of bridging attraction at low surface coverage or of finite relaxation times following strong compression, both of which are characteristic of adsorbed chains. The qualitative behavior with both PS-X and PS-PEO is very similar. For the PS-X(ilf), we find La Af0•6, and we are also able to estimate the mean interanchor spacing s and find it to increase markedly with M. These features are in accord with equilibrium expectations for a fixed anchoring energy of the polystyrene on the mica. We find that our data are well fitted quantitatively both by scaling and by mean-field models.