Ordering at two length scales in comb-coil diblock copolymers consisting of only two different monomers

The microphase separated morphology of a melt of a specific class of comb-coil diblock copolymers, consisting of an AB comb block and a linear homopolymer A block, is analyzed in the weak segregation limit. On increasing the length of the homopolymer A block, the systems go through a characteristic series of structural transitions. Starting from the pure comb copolymer the first series of structures involve the short length scale followed by structures involving the large length scale. A maximum of two critical points exists. Furthermore, in the two parameter space, characterizing the comb-coil diblock copolymer molecules considered, a non-trivial bifurcation point exists beyond which the structure factor can have two maxima (two correlation hole peaks).Comment: 22 pages, 12 Postscript figures (revtex

Atomic Force Microscopy and Real Atomic Resolution. Simple Computer Simulations.

Using a simple computer simulation for AFM imaging in the contact mode, pictures with true and false atomic resolution are demonstrated. The surface probed consists of two f.c.c. (111) planes and an atomic vacancy is introduced in the upper layer. Changing the size of the effective tip and its registry with respect to the atoms of the crystal probed, images with completely different qualitative features are obtained. If the effective tip is a single atom the vacancy is clearly imaged. However, if the tip consists of several atoms and is in registry with the sample, a virtual atom appears instead of the vacancy and the crystal lattice is perfectly reproduced. If the tip is out of registry with respect to the sample, artifacts having the size of the effective tip are reported.

Stick and Slip Behaviour of Confined Oligomer Melts under Shear. A Molecular-Dynamics Study.

The flow behaviour of melts of short chains, confined in molecularly thin Couette flow geometries, is studied with molecular-dynamics simulations. The effect of wall attraction and confinement on the density and velocity profiles is analysed. In these highly inhomogeneous films, a strong correlation between the density and velocity profile is found. Sticking of the interfacial layer on the wall and slip on the wall and inside the film is manifested by changes in the velocity profile. The location of the slip is determined by the strength of the wall attraction.

A comparison between the morphology of semicrystalline polymer blends of poly(ε-caprolactone)/poly(vinyl methyl ether) and poly(ε-caprolactone)/(styrene-acrylonitrile)

The morphology of polymer blends of poly(ε-caprolactone) (PCL) and poly(vinyl methyl ether) (PVME) is compared with that of PCL and a random copolymer of styrene and acrylonitrile (SAN). The main objective is to determine the influence of the glass transition temperature of the amorphous component (Tg,a) on the morphology of the semicrystalline polymer blends. These blends represent the two extreme cases corresponding to Tc < Tg,a and Tc > Tg,a, where Tc is the crystallization temperature. The morphology of these blends, with PVME and SAN representing the amorphous components, have been studied by small angle X-ray scattering. For both blends the long period increases with the addition of amorphous polymer, which is a strong indication for an interlamellar morphology. D.s.c. experiments, including enthalpy relaxation, are used to investigate the crystallinity and the interphases. The overall amount of crystallinity in both blends decreases with increasing content of amorphous polymer. However, the fraction of PCL that crystallizes decreases in PCL/SAN and increases slightly in PCL/PVME. Apparently, the addition of the low Tg,a PVME improves the crystallization of PCL in accordance with a simple Gamblers Ruin Model type argument. The high Tg,a of SAN means this does not occur in PCL/SAN blends. Conventional d.s.c. experiments show an interphase of pure amorphous PCL in PCL/SAN blends and enthalpy relaxation experiments demonstrate its presence in PCL/PVME blends as well.

Inhomogeneities in sheared ultrathin lubricating films

Nonequilibrium molecular dynamics computer simulations have been used to study nanoscopically confined oligomer films under shear. Beyond the well-known density layering across such films, other structural and dynamical inhomogeneities exist across such films and are discussed here. When these films are subjected to strong shear flows, slip appears at the confining surfaces or inside the pore, depending on the wall interactions. For strong wall affinities interlayer slip develops between the adsorbed layer and the free chains, resulting in a structural discontinuity; a molecular mechanism, involving shear induced conformational changes of the adsorbed chains, is associated with this interlayer slip. Moreover, the resistance to flow (quantified through an effective viscosity) changes considerably across the film, with a dramatic viscosity increase of the adsorbed layer near attractive surfaces. Shear thinning is mainly taking place inside this more viscous interfacial layer, whereas the dynamics in the middle of the film remain bulklike; thus, there also exists strong inhomogeneity in the dynamics of the system. A comparison with SFA experimental and theoretical studies is also made

Microphase Separation within a Comb Copolymer with Attractive Side Chains: A Computer Simulation Study

Computer simulation modelling of a flexible comb copolymer with attractive interactions between the monomer units of the side chains is performed. The conditions for the coil-globule transition, induced by the increase of attractive interaction, ε, between side chain monomer units, are analysed for different values of the number of monomer units in the backbone, N, in the side chains, n, and between successive grafting points, m. It is shown that the coil-globule transition of such a copolymer corresponds to a first-order phase transition. The energy of attraction (ε) required for the realisation of the coil-globule transition decreases with increasing n and decreasing m. The coil-globule transition is accompanied by significant aggregation of side chain units. The resulting globule has a complex structure. In the case of a relatively short backbone (small value of N), the globule consists of a spherical core formed by side chains and an enveloping shell formed by the monomer units of the backbone. In the case of long copolymers (large value of N), the side chains form several spherical micelles while the backbone is wrapped on the surfaces of these micelles and between them.

Effect of shear on the desorption of oligomers in nanoscopically confined films

Bitsanis et al. J. Chem. Phys. 99, 5520 (1993) found that in nanoscopically confined films between strongly physisorbing surfaces chains with many contacts with the walls are irreversibly adsorbed. When shear is imposed to these systems molecular dynamics (MD) simulations show that the majority of the adsorbed oligomers adopts flat conformations on top of the walls. Although these conformations are characterized by high molecular adsorption energies, the same MD simulations show that desorption is strongly promoted by shear. The underlying mechanism is discussed

DSC experiments on gel-spun polyethylene fibers

The tensile strength of gel-spun polyethylene fibers obtained after hot-drawing depends on spinning conditions such as spinning speed, spinning temperature, spinline stretching, polymer concentration, and molecular weight/molecular weight distribution. High deformation rates in the spinline result in shish-kebab structures which after hot-drawing lead to fibers with poor properties. This is in contrast to hot-drawn fibers obtained from gel-spun fibers with a lamellar structure. Lamellar or shish-kebab structures in the gel-spun fibers can be distinguished by means of DSC experiments on strained fibers. On the basis of these experiments a qualitative prediction of the final tensile properties can be made. DSC experiments on (un)strained hot-drawn fibers show that in the case of shish-kebab structures an incomplete transformation into a fibrillar structure takes place which partly explains the low tensile strength. Chain slippage which becomes possible after the orthorhombic-hexagonal phase transition is involved in the fracture mechanism. The shift of the orthorhombic-hexagonal phase transition to higher temperatures with increasing tensile strength indicates that the increase in strength corresponds to an increase in length of the crystal blocks. Consequently, creep failure also occurs at higher stresses. The melting behavior of cold-drawn and hot-drawn fibers is qualitatively similar, but the transformation into a fibrillar structure is more complete in the latter case

Ordering Lamellar-Forming Copolymer Thin Films in 3D Bicontinuous Morphologies via Lamellar Patterned Substrate

The formation of ordered morphologies in thin films of symmetric diblock copolymer melts is considered theoretically. Somewhat surprisingly, under proper boundary conditions the presence of a lamellar chemical pattern on the substrate, being sufficiently pronounced and with the right period, is found to induce the formation of diamond-like morphologies. The phase diagram of the most stable phases on the plane (the substrate period Lx the film width H) is built within the self-consistent field theory numerical procedure. We also discuss the behavior of the order parameter Fourier spectrum at the transitions between the various morphologies