The colloid-structure of semicrystalline low-density polyethylene

The length-distribution of stereoregular sequences in low-density polyethylene (LDPE) is determined by a computer-aided analysis of cp-melting curves within the framework of thermodynamics of eutectoid copolymers. Small-angle x-ray scattering patterns are then described by using the structure data derived. One of the results is that fluctuations of the mean electron-density are increasingly reduced with an increasing degree of crystallinity.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday     

Effect of bilayer thickness on membrane bending rigidity

The bending rigidity k(c) of bilayer vesicles self-assembled from amphiphilic diblock copolymers has been measured using single- and dual-micropipet techniques. These copolymers are nearly a factor of 5 greater in hydrophobic membrane thickness d than their lipid counterparts and an order of magnitude larger in molecular weight M(n). The macromolecular structure of these amphiphiles lends insight into and extends relationships for traditional surfactant behavior. We find the scaling of k(c) with thickness to be nearly quadratic, in good agreement with existing theories for bilayer membranes. The results here are key to understanding and designing soft interfaces such as biomembrane mimetics.     

Analyses of eutectoid phase transformations in Nb-silicide in situ composites.

Nb-silicide in situ composites have great potential for high-temperature turbine applications. Nb-silicide composites consist of a ductile Nb-based solid solution together with high-strength silicides, such as Nb5Si3 and Nb3Si. With the appropriate addition of alloying elements, such as Ti, Hf, Cr, and Al, it is possible to achieve a promising balance of room-temperature fracture toughness, high-temperature creep performance, and oxidation resistance. In Nb-silicide composites generated from metal-rich binary Nb-Si alloys, Nb3Si is unstable and experiences eutectoid decomposition to Nb and Nb5Si3. At high Ti concentrations, Nb3Si is stabilized to room temperature, and the eutectoid decomposition is suppressed. However, the effect of both Ti and Hf additions in quaternary alloys has not been investigated previously. The present article describes the discovery of a low-temperature eutectoid phase transformation during which (Nb)3Si decomposes into (Nb) and (Nb)5Si3, where the (Nb)5Si3 possesses the hP16 crystal structure, as opposed to the tI32 crystal structure observed in binary Nb5Si3. The Ti and Hf concentrations were adjusted over the ranges of 21 to 33 (at.%) and 7.5 to 33 (at.%) to understand the effect of bulk composition on the phases present and the eutectoid phase transformation.     

Self-assembly of star ABC triblock copolymer thin films: self-consistent field theory

Microphase separation and morphology of star ABC triblock copolymers confined between two identical parallel walls (symmetric wetting or dewetting) are investigated with self-consistent field theory (SCFT) combined with the "masking" technique to describe the geometric confinement of the films. In particular, we examine the morphology of confined near-symmetric star triblock copolymers under symmetric and asymmetric interactions as a function of the film thickness and the surface field. Under the interplay between the degree of spatial confinement, characterized by the ratio of the film thickness to bulk period, and surface field, the confined star ABC triblock copolymers are found to exhibit a rich phase behavior. In the parameter space we have explored, the thin film morphologies are described by four primary classes including cylinders, perforated lamellae, lamellae, and other complex hybrid structures. Some of them involve novel structures, such as spheres in a continuous matrix and cylinders with alternating helices structure, which are observed to be stable with suitable film thickness and surface field. In particular, complex hybrid network structures in thin films of bulk cylinder-forming star triblock copolymers are found when the natural domain period is not commensurate with the film thickness. Furthermore, a strong surface field is found to be more significant than the spatial confinement on changing the morphology of star triblock copolymers in bulk. These findings provide a guide to designing novel microstructures involving star triblock copolymers via geometric confinement and surface fields.     

Small-strain elastic moduli of quasi-isotropic semicrystalline eutectoid copolymers and aspects of universality

The small-strain elastic moduli of eutectoid random copolymers of ethylene are described in the total melting range. The approach is based upon the model of a cluster network constituted by the crystals which operate as active solid fillers. Number and average size of these fillers can be computed with the aid of a thermodynamic melting theory. Universal aspects of elastic small-strain behaviour in semicrystalline system will be discussed.     

Characterization of polyethylene networks based on the jointed description of melting,swelling and deformation

Investigations of melting, equilibrium swelling and the stress-strain behavior of radiation crosslinked polyethylene are reported. The results are jointly described where by a novel phenomenological treatment of the swelling of van der Waals networks was needed implicating a discussion as to how the modulus is related to the concentration of the cross-linkages. This concentration is obtained from the analysis of the melting process by application of the thermodynamics of eutectoid copolymers, which is also shown to deliver structural parameters which are in good accord with X-ray data.     

Small-angle scattering analysis of hard-microdomain structure and microphase mixing in polyurethane elastomers

The microdomain structure of a series of segmented polyurethane block copolymers is characterized by small-angle x-ray and neutron scattering analyses. The materials contain hard segments formed from 4,4′- diphenylmethane diisocyanate (MDI) and butanediol (BD), and range in hard-segment content from 20 to 80% by weight. The results provide evidence for a transition from discrete to continuous hard-microdomain morphology as the hard-segment content is increased above ca. 50%. The measured concentration dependences of the interdomain spacing, specific interfacial area, diffuse microphase boundary thickness, and scattering invariants are used to examine the validity of present models for hard-microdomain structure. The observed behavior corresponds well with the general predictions of a lamellar model wherein partially coiled hard-segment sequence configurations are allowed. The thickness of the hard microdomains extracted from the model corresponds to approximately four hard-segment repeat units. Scattering invariant calculations are used together with determinations of the soft-microphase glass transition temperatures to examine possible models for microdomain mixing. These calculations suggest that both the hard and soft microphases are phase mixed.     

Mesoscopic patterns of block and graft copolymers in condensed systems

Equilibrium structures of two kinds of two‐component copolymers with equivalent chemical contents but with different chain architectures in bulk were compared. They are BAB triblock copolymers and AB₂ star‐branched graft copolymers. These copolymers have been confirmed to show quite different morphological change with composition. Deformation manner of B block chains of lamellar microphase‐separated BAB triblock copolymers depend on B contents, however, the volumes of the deformed coils are always kept constant to have those of the unperturbed chains irrespective of their architectures. The observed polystyrene/poly(2‐vinylpyridine) interfacial thickness is fairly thin though it is much thicker than the theoretically‐predicted one.     

Characterization of comonomer distributions in ethylene/diene copolymers by 13C-NMR,and using the segregation fractionation technique by DSC and DMTA

Ethylene and linear, nonconjugated dienes were copolymerized with the catalyst system Cp₂ZrCl₂/methylaluminoxane (MAO). The comonomer incorporation and the relationships between structure and properties were evaluated by NMR and by thermal techniques, especially the segregation fractionation technique (SFT) using DSC and dynamic mechanical thermal analysis (DMTA). The ethylene-1,5-hexadiene (HD) copolymers showed different behavior than the others and it was possible to incorporate as much as 7 mol % of the hexadiene comonomer into soluble polymer compared with only 2.4 mol % of the 1,7-octadiene (OD) and 7-methyl-1,6-octadiene (MOD). The melting endotherms of the HD copolymers obtained after segregation fractionation were very much like corresponding endotherms of high-density polyethylene (HDPE) and a population with nearly one lamellar thickness was postulated. This is in agreement with cyclic structure formation and absence of branching with crosslinking for these copolymers. The OD and MOD copolymers, on the other side, showed endotherms with several peaks indicating a distribution of the comonomers along the chain. Lamellar thickness distributions were calculated from the melting endotherms by using the Gibbs–Thomson equation. The DMTA measurements confirmed the absence of branches in the HD copolymers and the presence of branches in the OD and the MOD copolymers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2379–2389, 1999     

An Efficient Approach for Preparing Giant Polypeptide Triblock Copolymers by Protein Dimerization

An efficient and convenient approach for preparing a giant polypeptide–poly(ethylene oxide) triblock copolymer architecture of defined structure and composition is reported. This copolymer consists of two long polypeptide chains derived from bovine serum albumin of distinct lengths with stabilizing poly(ethylene oxide) side‐chains, a connecting poly(ethylene oxide) block, and the presence of secondary structure elements along the polypeptide backbone. It is synthesized from the abundant plasma protein serum albumin and the polypeptide backbone is fully biodegradable. This approach represents a convenient and efficient strategy for preparing giant polypeptide‐based block copolymers of defined structure via a semi‐synthetic strategy. Such high‐molecular‐weight, biodegradable copolymers are attractive for various biomedical applications     

Selective control over the lamellar thickness of one domain in thin binary blends of block copolymer films

Thin binary blends of poly(styrene‐b‐methyl methacrylate) (PS‐PMMA) block copolymers in films where the lamellar thickness of one domain is controlled while preserving the thickness of the other domain were demonstrated without microphase separation. One of the block copolymers used here was short and symmetric, and the other was long and asymmetric; the molecular weights of the PMMA block chains in the constituents were similar. A random copolymer brush was introduced and film thickness and composition of brush were adjusted to induce perpendicular orientation in thin film. As the blend composition of the long asymmetric block copolymer increased, the PS lamellar thickness increased from 15.8 to 25.1 nm, whereas the PMMA lamellar thickness remained constant at approximately 14 nm (the thickness decreased slightly from 14.0 to 13.3 nm). The domain spacing behavior in thin film was consistent in the bulk. These results were compared with the Birshtein, Zhulina, and Lyatskaya model and the theories for pure block copolymers in the strong segregation limit and in the intermediate segregation regime. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1393–1399     

Blends of propylene-ran-ethylene and propylene-ran-(1-butene) copolymers: Crystal superstructure and mechanical properties

Blends of isotactic propylene-ran-ethylene (EP) and propylene-ran-(1-butene) (BP) copolymers with various comonomer content (2-3.1 wt.% ethylene, 9.9 wt.% 1-butene), were prepared in Brabender internal mixer at various compositions (25/75, 50/50, 75/25). Static, impact and dynamic mechanical behavior of copolymers and their blends was investigated. The crystalline structure was studied by DSC and SAXS analysis. For all copolymers the lamellar thickness, crystallinity degree and glass transition temperature are lower than those of iPP homopolymer, depending on the comonomer content. It was found that the copolymers exhibit improved impact strength as compared to plain iPP, due to lower crystallinity and higher mobility of chains within amorphous component. Moreover, the elastic modulus as well as the yield behavior of the examined samples resulted to depend primarily on the amount of the crystalline phase and the thickness of the lamellar crystals, respectively. A linear dependence of yield stress on the logarithm of reciprocal lamellar thickness was observed for blends and copolymers, supporting the concept of thermal nucleation of dislocations which control the crystallographic slip processes initiated at the yield point. The blends of BPS with either EPS or EP2 display complete miscibility in the entire range of composition and their mechanical properties are intermediate between those of plain components, changing gradually with the composition.     

Recognition-induced polymersomes: structure and mechanism of formation

Random polystyrene copolymers grafted with complementary recognition elements were combined in chloroform producing vesicular aggregates, that is, recognition-induced polymersomes (RIPs). Reflection interference contrast microscopy (RICM) in solution, coupled with optical microscopy (OM) and atomic force microscopy (AFM) on solid substrates, were used to determine the wall thickness of the RIPs. Rather than a conventional mono- or bilayer structure (approximately 10 or approximately 20 nm, respectively) the RIP membrane was 43+/-7 nm thick. Structural arrangement of the polymer chains on the RIP wall were characterized by using angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The interior portion of the vesicle membrane was found to be more polar, containing more recognition units, than the exterior part. This gradient suggests that a rapid self-sorting of polymers takes place during the formation of RIPs, providing the likely mechanism for vesicle self-assembly.     

Photophysical processes and photochemical reactions involved in poly(N-vinylcarbazole) and in copolymers with carbazole units

This paper is devoted to the analysis of the photochemical behaviour of copolymers with carbazole units exposed to long-wavelength radiation. These copolymers are constituted of two types of carbazolylethyl methacrylate units (CEM) with octyl methacrylate moieties (OMA). The exposure of copolymers and PVK to UV light results in dramatic modifications of the physical and photophysical properties of the polymer. These modifications can be correlated with modifications of the chemical structure of the matrix. The photoageing of copolymers and PVK has been analysed by fluorescence, ESR, UV-vis and infrared spectroscopies. The effects of crosslinking and chain scissions were determined by gel fraction measurements and size exclusion chromatography.     

New approaches to mapping through-thickness variations in the degradation in poly (lactide-co-glycolide)

Biodegradable poly (lactide-co-glycolide) (PLGA) copolymers have been used for many years for biomedical applications such as soluble sutures, orthopaedic implants and more recently as potential tissue scaffold materials. The rate at which the copolymers degrade can be manipulated from a period of days to months by changing the lactide/glycolic acid ratio. Degradation of PLGA copolymers occurs by hydrolysis of the ester bonds in the polymer backbone. The hydrolysis reaction is autocatalytic and is accelerated by the build up of degradation products in the bulk of the material. As a consequence, material degradation is expected to be non-uniform through the specimen thickness with the material at the centre degrading at a faster rate than at the surface. Despite many studies of PLGA degradation, information on this local variance is sparse as the techniques used to track the process are usually bulk measures. In this study, two new approaches for monitoring degradation have been developed that enable local measurements of degradation to be made throughout the specimen over an extended period of time. Chemical and mechanical variations in the structure of the polymer have been mapped using attenuated total reflectance infrared spectroscopy (ATR-FTIR) and nanoindentation. These have produced comparable results and show that the degradation rate at the centre of the specimens is almost an order of magnitude higher than at the surface.     

Structure of some ethylene–phosphonic acid copolymers

An x-ray study has been made of the structure of a series of ethylene–phosphonic acid copolymers and the parent low-density polyethylene from which they were derived. The phosphonic acid contents (groups per 100 carbon atoms) of the copolymers were: A, 0.8; B, 1.8; C, 2.8; and D, 8.0. Small-angle x-ray scattering (SAXS) results show that the phosphonic acid substituents do not incorporate into the crystal lattice to any appreciable extent, that the substituents have the effect of decreasing the average thickness of the crystal lamellae and increasing the breadth of the size distribution of thicknesses, and that a two-phase model does not adequately account for the observed SAXS invariant. Wide-angle x-ray scattering (WAXS) results show that specimens, A, B, and C are partially crystalline with the polyethylene crystal structure and that D is amorphous. The observed broadenings of the 110 and 200 crystal reflections in the copolymers indicate that the substituents decrease the lateral dimensions of the crystalline lamellae and/or increase the deformation of the lattice due to external strain. Specimen D, completely amorphous according to the WAXS criterion, exhibits the largest value of the SAXS invariant of all the copolymers studied and must thus possess a multiphase structure consisting of small ordered regions and a disordered phase. The results of the study show the structure of the copolymers to be consistent with the fringe-micelle model but do not rule out the folded-chain model, although a regular fold surface is unlikely.     

Morphology and domain size of a model graft copolymer

We prepared well-defined styrene (S)-2-vinyl-pyridine (P) graft copolymers of the ABB type or SPP graft copolymers, in which as block chain is grafted at the center of a block chain, as a model graft copolymer by anionic polymerization and coupling reaction. The composition dependence of morphology of SPP graft copolymers is qualitatively the same as that of SP diblock copolymers, but the composition range of each structure is shifted to the higher volume fraction of S block chain. The molecular weight dependence of lamellar domain size of SPP graft copolymers is almost the same as that of SP diblock copolymers, but the magnitudes are smaller. These experimental results are well explained by the difference in chain architectures.     

Macromolecular self-assembly in dilute sequence-controlled block copolymer/homopolymer blends

Conventional block copolymers consist of two long contiguous monomer sequences (‘blocks’) that can, in the same fashion as low-molar-mass surfactants, self-assemble into various microstructural elements (e.g., micelles at low copolymer concentrations) to minimize repulsive contacts in the presence of a parent homopolymer. In this work, we explore the existence of segment-specific interactions, as well as the possibility of tailoring these blend morphologies (and producing altogether new ones), with novel sequence-controlled block copolymers. These copolymers are comprised of at least one block that is a random segment composed of both constituent monomer species. Transmission electron microscopy is employed here to examine the bilayered membranes and channel structures that form in two different series of such copolymers in dilute copolymer/homopolymer blends.