Lewis J. Fetters

Anionic Polymerization of Vinyl Monomers

Abstract 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 th...

Chain dimensions and entanglement spacings in dense macromolecular systems

In this article, we reexamine and extend a relationship proposed earlier between entanglement density and chain dimensions in polymer melts. The power-law equation presented in the earlier work, relating the entanglement molecular weight Me, melt chain density ρ, and the packing length p is tested with additional polymer species. Now included are additional polydienes and their hydrogenated derivatives, the isotactic forms of polypropylene and polystyrene, the essentially syndiotactic form of poly(methyl methacrylate), along with poly(tetrafluoroethylene), poly(vinylmethyl ether), various poly(methacrylates), and polymeric sulfur. We find that within experimental uncertainties, Me/ρ and p are related through an equation (Me/ρ = 218p3) that is insensitive to temperature (25°C ≤ T ≤ 380°C) and which seems to be universal for flexible Gaussian chains in the melt state.

Tunable Block Copolymer/Homopolymer Photonic Crystals

Interest in the photonic properties of dielectrically struc-tured materials is growing rapidly. This is fueled by the potential application of such materials in a broad range of fields. Much recent work has focused on lithographically fabricated photonic materials, [1,2] and on self-assembled colloidal crystals [3,4] and inverse opal structures. [5±7] The former need to be fabricated on small-area flat surfaces, the latter tend to have long timescales inherent in the fabrication and both have limited tunability with respect to the precursor materials. Our recent work [8,9] has shown that self-assembled block copolymer systems can exhibit a one-dimensionally periodic lamellar structure with component domains having widths large enough (l/4n) to act as visible light photonic materials. Their use in photonics would bring many advantages in terms of materials properties, process-ability, as well as cost, and would enable the fabrication of large-area conformable photonic materials. In this communication we show that multilayer photonic crystals active within a broad range of wavelengths in the visible spectrum can easily be produced from a simple system comprised of a block copolymer and two homopolymers. Ternary blends of a poly(styrene-b-isoprene) (S/I) di-block copolymer, polystyrene, and polyisoprene were solution cast from cumene. As the samples begin to dry they selectively reflect light, giving the appearance of green or red color. After drying, each sample showed a single well-defined peak in reflectivity in the visible wavelength range (350±600 nm) and a corresponding drop in the transmission profile. This implies that the reflection was fairly efficient and that the color observed in the samples was not due to significant absorption. In Figure 1a, we show typical reflec-tivity curves for several blend samples. The relative width of the reflectivity peaks (or transmission dips), Dl/l, varies approximately from 0.15 to 0.25, widening as the fraction of homopolymer in the blend increases. The wavelength of the reflectivity peak versus the homopolymer composition is plotted in Figure 1b. There is a monotonic increase in the peak reflective wavelength with the fraction of homopoly-mer. Scanning electron microscopy (SEM) of the samples confirmed that a lamellar morphology was present in the samples , with typical grain sizes of the order of several micro-meters. The backscattered electron imaging (BEI) image of a sample containing 40 % homopolymer is shown in Figure 2a. The bright regions correspond to the OsO 4 stained polyisoprene domains and the dark regions are the un-stained polystyrene. The lamellar repeat from the …

Chain Dimensions and Entanglement Spacings

25.1 Chain Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 25.2 Chain Entanglement and Tube Diameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 25.3 Critical Molecular Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 446 25.4 Temperature Dependence of Chain Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

Molecular Weight Dependence of Hydrodynamic and Thermodynamic Properties for Well‐Defined Linear Polymers in Solution

Data on hydrodynamic and thermodynamic properties of several well‐defined, linear flexible polymer chains in thermodynamically good solvents and theta solvents are tabulated and critically evaluated. The polymers considered are 1,4‐polybutadiene, 1,4‐polyisoprene, polyisobutylene, polystyrene, and poly(α‐methylstyrene). Extensive data, obtained as a function of molecular weight, are evaluated for the intrinsic viscosity, radius of gyration, hydrodynamic radius, and second virial coefficient. The resulting power law relationships between the measured properties and polymer molecular weight are reported.

Flow‐Induced Birefringence of Concentrated Polyisoprene Solutions

Experiments measuring the birefringence following the inception of a steady shearing flow were conducted on a series of concentrated polyisoprene solutions. The steady‐state shear stress was also measured in a mechanical rheometer. The data obtained from the two experiments confirmed that the birefringence and stress were linearly related (stress‐optical law) over a range of shear rates that extended far into the non‐Newtonian region. Interesting nonlinear effects were also observed during the transient response at high shear rates. Both the shear stress and the first normal stress pass through maximums before reaching their steady‐state values. A maximum in the normal stress is not predicted by the original Doi‐Edwards model but a modification of their theory which includes chain stretching is in qualitative agreement with our results.

The effect of surface constraints on the ordering of block copolymer domains

The effect of surfaces on the ordering of block copolymer microdomains has been studied by TEM observation of thin-film droplets. Poly(styrene butadiene) block copolymers exhibiting spherical, cylindrical and lamellar microdomain morphologies were formed by solvent evaporation on a carbon substrate, annealed aboveTg, and stained with OsO4 prior to TEM observation. Samples with lamellar morphologies exhibited orientations of the lamellae in which the domains were either perpendicular to the substrate surface, or parallel to the substrate surface in which a layered structure resulted. Specimens in which cylinders were formed revealed sheets in which the cylinder axes lie parallel to the substrate, as well as parallel to each other. Stacking of such layers was observed, with exceptional registration between layers such that the cylinders in one layer were located over the inter-domain regions of the cylinders in adjacent layers. Specimens with spherical microdomain morphologies revealed a layer structure that consisted of hexagonally packed arrays of the spherical micelles. The stacking of layers of discrete thickness of all three types of microdomain morphology to produce the observed geometries indicates that the exterior of bulk samples may also exhibit such surface steps that reflect the ordering of the microdomains within the sample. Regardless of the microdomain type, all samples revealed a peripheral region in the thinnest area of the droplets in which the formation of the normal microdomain morphology (i.e. that of spheres, cylinders or lamellae) was inhibited. The uniform thickness of this region provides evidence that preferential segregation of the block component with the lowest surface energy takes place regardless of the sample composition.

Suppression of rupture in thin, nonwetting liquid films

Stabilization against the rupture and breakup of thin, nonwetting liquid films spread on surfaces is generally sought by modification of equilibrium interfacial properties. A mechanism for suppressing rupture in such films that uses surface-attached polymers togetherwithfree chains in the bulk of the film is reported. Films of an oligostyrene liquid, which rupture within several minutes when spread on a silicon wafer, may be stabilized for many months by a polystyrene brush attached to the substrate, together with some free polystyrene in the liquid. The effect may arise from entanglements of the free chains with the immobilized brush.

Complete wetting from polymer mixtures.

Coexisting polymer phases are characterized by very small interfacial energies, even well below their critical solution temperature. This situation should readily lead to the exclusion of one of the phases from any interface that favors the other. Such complete wetting behavior from a binary mixture of statistical olefinic copolymers is reported. By means of a self-regulating geometry, it is found that the thickness of a wetting layer of one of the phases at the polymer-air interface, growing from the other coexisting phase, attains macroscopic dimensions, increasing logarithmically with time. These results indicate that binary polymer mixtures could be attractive models for the study of wetting phenomena.

Ordered morphologies in binary blends of diblock copolymer and homopolymer and characterization of their intermaterial dividing surfaces

Binary blends of a lamellar poly(styrene‐b‐isoprene) diblock copolymer and homopolystyrenes of various molecular weights self‐assembled into highly ordered morphologies upon solvent evaporation. The equilibrium blend morphologies were studied by transmission electron microscopy and small angle x‐ray scattering. The type of ordered morphology (lamellae, the ordered bicontinuous double diamond morphology, cylinders on a hexagonal lattice, and spheres on a cubic lattice) observed in each blend was generally dictated by the overall polystyrene volume fraction in the blend, i.e., the homopolymer concentration. The blend morphology also depended on the homopolymer molecular weight at a single fixed composition. The intermaterial dividing surface between the polystyrene and the polyisoprene microdomains was characterized by the interfacial mean curvature (H) and the average area per copolymer junction (σj). H and σj increased as the homopolymer concentration increased and/or as the homopolymer molecular weight d...