David J. Lohse

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.

Morphology of miktoarm star block copolymers of styrene and isoprene

The morphologies of several styrene–isoprene miktoarm star copolymers have been examined by small angle x‐ray scattering and electron microscopy. Three of these polymers were of the three‐armed A2B type, and six were four‐armed A3B miktoarms. The ordered microphase‐separated morphologies displayed by these polymers were seen to be highly dependent on the architecture of the chains, and were quite different from those that occur in the corresponding linear block copolymers. These results can be quite well explained by a theory of Milner which is based on the bending energy of the microphase interface and the ability of the chains to stretch away from the interface between the microphases. We speculate that the small differences between the observations and theoretical predictions are due to the effects of the compression of chains in small gaps between adjacent domains.

Microphase separation in block copolymers

Block copolymers have been the focus of intense scientific and commercial development because of their ability to organize into precise structures on the scale of ten to one hundred nanometers. The wide variety of morphologies exhibited by linear block copolymers made from two chemically distinct monomers has been extensively studied over many years and is now fairly well understood. Three extensions of these studies have uncovered new classes of structures and new relations between copolymer composition and morphology. These include block copolymers with a nonlinear chain architecture, linear terpolymers with three chemically different blocks, and mixtures of linear diblocks with molecular weight or compositional differences. These results have greatly expanded the range of domain sizes and types of morphology that can be found in block copolymer materials.

The compositional dependence of thermodynamic interactions in blends of model polyolefins

We have investigated the thermodynamic interactions as functions of component volume fraction φ and temperature T in six binary polymer blend systems. The components in all cases were model polyolefins, made by saturating the double bonds of nearly monodisperse polydienes. Small‐angle neutron scattering measurements for single‐phase melts, analyzed with the incompressible random phase approximation, were expressed in terms of the Flory–Huggins interaction parameter χ for 27 °C≤T≤167 °C and 0.1≤φ≤0.9. In most systems we found a characteristic upturn in χ at the composition extremes that diminished with increasing temperature. We also found that the component chain dimensions were unaffected by the strength of the interactions. Several theoretical attempts to explain the dependence of χ on component volume fraction were examined. Most were qualitatively inconsistent with the results, and none were fully satisfactory.

Small‐angle neutron scattering by partially deuterated polymers and their blends

Partially deuterated polymers, made by saturating the double bonds of polydienes with deuterium, have been found to produce weak but significant coherent patterns in small‐angle neutron scattering (SANS) experiments. The results suggest the presence of slight differences in deuteration levels among the chains. The scattering profiles for such materials and for blends containing them were calculated with the multicomponent random phase approximation (RPA). The coherent intensity for the deuterated component alone is predicted to be proportional to the variance of its scattering length distribution and to have a q dependence corresponding to the pure component structure factor. The SANS data for a variety of partially deuterated polyolefins are shown to be consistent with these predictions. The theory also predicts that the scattering from their blends with an hydrogenous component is enhanced by this scattering length inhomogeneity, that the enhancement is additive, and that the conventional two component ...

The Influence of Chemical Structure on Polyolefin Melt Rheology and Miscibility

The ability to predict polymer properties from a knowledge of the chemical architecture of the chains is one of the main goals of polymer science, and achievement of this can be very useful in the development of new polymeric materials. In this paper the connection between dimensions of polyolefin chains and some of their most fundamental properties, such as the degree of entanglement and miscibility, are described. The experimental and theoretical justifications of these relations are outlined in the first sections, and then the ways these can be used to predict the performance of polyolefins are demonstrated. Finally, other areas where such connections may be found in the future are suggested.

Pressure–Volume–Temperature properties of polyolefin liquids and their melt miscibility

The pressure–volume–temperature (P–V–T) properties of a number of metallocene-produced polyolefins were measured experimentally at 10 MPa ≤ P ≤ 200 MPa and 30°C ≤ T ≤ 220°C in a dilatometer-type P–V–T apparatus. These included ethylene copolymers typical of linear low density polyethylene, with several α-olefins as comonomers and a wide range of comonomer content. The experimental P–V–T data were correlated with the equations of state from the Sanchez–Lacombe and Flory–Orwoll–Vrij theories. The solubility parameter map of the polyolefins, at atmospheric pressure, was established on the basis of the thermodynamic data. As the temperature increases, the solubility parameter of the polyolefin decreases. The solubility parameters of copolymers of ethylene with propylene, butene, hexene, and octene under constant temperature are all more or less the same at equal weight percent of comonomer. As the incorporation of branches increases, the solubility parameter decreases. The melt miscibility of the polyolefin blends can be predicted to design various blend products for specific applications from this solubility parameter map.

Spherulite formation from microphase-separated lamellae in semi-crystalline diblock copolymer comprising polyethylene and atactic polypropylene blocks

Abstract The characteristics of crystallization in a semi-crystalline block copolymer have been experimentally examined using small-angle X-ray scattering and small-angle light scattering. For this purpose, a flow-oriented polyethylene- block -(atactic polypropylene) diblock copolymer with M w =113 kg / mol and a polyethylene volume fraction of 0.48 was used. First of all, the crystallization was found to be much suppressed as compared to that of polyethylene homopolymer (homo-PE), as evidenced by a crystallinity that was approximately one-third of homo-PE. Spherulite growth with rupturing the microphase-separated lamellae (micro-LAM) was found at crystallization temperatures in the range 95≤T c ≤101 ° C . That is, spherulites can grow even if the crystallization is initiated in the spatially confining micro-LAM structure. However, it was further found that the micro-LAM structure did not co-exist with a spherulite. On the other hand, at much lower crystallization temperatures (T c ° C ), the micro-LAM structure was retained whereas no spherulites formed. The suppression of spherulite formation can be accounted for by an auto-decelerating effect of nuclei in a confining microdomain space.

Blends of amorphous-crystalline block copolymers with amorphous homopolymers. Morphological studies by electron microscopy and small angle scattering

A morphological study was performed with symmetric diblock ethylene-propylene copolymer (DEP) and the binary blends made from DEP and atactic polypropylene (APP) by use of small angle X-ray, light and neutron scattering, and also scanning and transmission electron microscopy. DEP contains a crystallizable polyethylene block and an amorphous atactic polypropylene block. Quenching the blends in liquid nitrogen preserved the morphology in the melt state. This quenching technique revealed that DEP forms a lamellar microdomain structure and blending DEP and APP includes morphological changes in the microdomain structures as well as macrophase separation. When the APP chain was shorter than the APP block, the addition of APP changed the morphology from a lamellar to a bicontinuous cylindrical and then a discrete cylindrical and finally to a spherical structure. On the other hand, when the APP chain was longer than the APP block, macrophase separation was observed and only a transition from a lamella to a bicontinuous cylinder occurred. These morphological transitions in the melt state can be correlated to differences in the crystallization kinetics of the blends.

Morphology of semicrystalline block copolymers: polyethylene-b-atactic-polypropylene

Abstract Transmission electron microscopy (TEM) and electron diffraction (ED) are used to study the morphology of a semicrystalline polyolefin diblock copolymer polyethylene- b -atactic-polypropylene (PE- b -aPP) and its blend with polyethylene homopolymer. By using RuCl 3 /NaClO as the staining agent, both the contrast between amorphous PP and amorphous PE regions and the contrast between amorphous PE regions and crystalline PE regions can be obtained. As a result, both the larger lamellar structures due to microphase separation and the smaller crystalline PE lamellae can be resolved on TEM micrographs. Electron diffraction coupled with TEM imaging is used to elucidate the orientation of PE crystallites with respect to the interfaces of the microphase separated block copolymer lamellar domains. Fast quenching from a microphase separated melt-state was found to result in confinement of PE crystallization within the microphase separated PE domains of the block copolymer morphology. The orientation of the PE crystallites thus formed was found to be random.