The micellization of well-defined single graft copolymers in block copolymer/homopolymer blends

A series of well-defined (polyisoprene)2 (polystyrene), I2S, single graft copolymers with similar total molecular weights but different compositions, fPS, were blended with a low molecular weight polyisoprene homopolymer matrix at a constant concentration 2 wt%, and the micellar characteristics were studied by small-angle x-ray scattering. To investigate the effect of macromolecular architecture on the formation and characteristics of micelles, the results on the single graft copolymers were compared with those of the corresponding linear polystyrene-b-polyisoprene diblock copolymers, SI. The comparison reveals that the polystyrene core chains are more stretched in the case of graft copolymer micelles. Stretching turned out to be purely a result of the architecture due to the second polyisoprene block in the corona. The micellization of a (polystyrene)2 (polyisoprene), S2 I, graft copolymer was also studied, and the comparison with the results of the corresponding I2S and SI copolymers emphasizes the need for a critical core volume rather than a critical length of the core-forming block, in order to have stable micelles. Finally, the absence of micellization in the case of the I2S copolymer with the highest polystyrene volume fraction is discussed. For this sample, macrophase separation occurs, with polyisoprene cylinders formed in the copolymer-rich domains of the phase-separated blends. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Segmental dynamics of disordered styrene-isoprene tetrablock copolymers

The local segmental dynamics of four styrene-b-isoprene-b- styrene-b-isoprene (SISI) tetrablock copolymers with different styrene composition f(s) and constant total degree of polymerization Napproximate to120 has been studied in the disordered state in the nano-picosecond time scale, by incoherent quasielastic neutron (QENS), and Brillouin (BS) and depolarized Rayleigh (DRS) light scattering. Far above the glass transition temperature, all three techniques demonstrate the presence of two distinct time scales from which the fast segmental relaxation was quantitatively resolved. This process is associated with the mobility of the polyisoprene (PI) component, and is moderately slower and possesses a broader distribution of relaxation times than in bulk PI. The comparison between the correlation times of DRS and the characteristic times of QENS suggest that segment (hydrogen nucleus) diffusion over a distance of approximate to0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, approximate to0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here. (C) 2002 American Institute of Physics

Structure and dynamics in PEO nanocomposites

In this work, the effect of confinement on the local, the segmental and the ionic processes of poly(ethylene oxide)/montmorillonite (PEO/MMT) intercalates is investigated. X-ray diffraction and DSC data show that the intercalated PEO chains remain amorphous and it is only the excess PEO chains outside the galleries that crystallize. This is also evident in the behavior of the elastic intensity of quasi-elastic neutron scattering (QENS) experiments on the same hybrids. Dielectric relaxation spectroscopy reveals that confinement results in an acceleration of the PEO segmental dynamics that display an Arrhenius temperature dependence with low activation energy, whereas the local beta-process remains unaffected

Segmental dynamics of disordered styrene-isoprene tetrablock copolymers

The local segmental dynamics of four styrene-b-isoprene-b- styrene-b-isoprene (SISI) tetrablock copolymers with different styrene composition f(s) and constant total degree of polymerization Napproximate to120 has been studied in the disordered state in the nano-picosecond time scale, by incoherent quasielastic neutron (QENS), and Brillouin (BS) and depolarized Rayleigh (DRS) light scattering. Far above the glass transition temperature, all three techniques demonstrate the presence of two distinct time scales from which the fast segmental relaxation was quantitatively resolved. This process is associated with the mobility of the polyisoprene (PI) component, and is moderately slower and possesses a broader distribution of relaxation times than in bulk PI. The comparison between the correlation times of DRS and the characteristic times of QENS suggest that segment (hydrogen nucleus) diffusion over a distance of approximate to0.8 nm suffices for the loss of local orientation correlations. The faster times of the BS experiment correspond to shorter displacements, approximate to0.3 nm. These results demonstrate that the segmental dynamics of the PI are much faster than would be inferred from the monomeric friction factor of PI previously extracted by diffusion and viscosity measurements in the same tetrablock matrices. This, in turn, indicates a substantial local spatial heterogeneity in the segmental dynamics. The slow process is due to the PS segments, which do not relax, appreciably on the time scales accessible here. (C) 2002 American Institute of Physics

Controlling the miscibility of polyethylene/layered silicate nanocomposites by altering the polymer/surface interactions

Polyethylene/layered silicate nanocomposites are synthesized utilizing three types of polymeric surfactants/compatibilizers in order to influence the miscibility of polyethylene with the nanoparticle surface. The additives are designed so that they can play the role of a polymeric surfactant modifying the hydrophilic clay or of a compatibilizer with the organoclay. Model additives, especially synthesized for this study, included: polyethylene chains, which possess either a single functional end-group or multiple functional groups along the chain, as well as functional diblock copolymers. Maleic anhydrite grafted polyethylene with a low degree of functionalization was used as well. The structure of the resulting micro- or nanocomposites was investigated by X-ray diffraction and transmission electron microscopy. Immiscible hybrids as well as intercalated and/or exfoliated nanocomposites are obtained in a controlled way, depending on the kind of additive and its concentration in the mixture. The most important factor controlling the structure and the properties is the ratio of additive to nanoparticles. The rheological properties of the hybrids correlate well with the final micro- or nanostructure. © 2005 Elsevier Ltd. All rights reserved

Dynamics of the most probable composition fluctuations of "real" diblock copolymers near the ordering transition

Semidilute solutions of ultrahigh molecular weight diblocks in a nonselective good solvent allow investigation of the dynamic structure factor S(q,t) by photon correlation spectroscopy for wavevectors q in the vicinity (and on both sides) of the maximum of the static structure factor q* (0.2 ≤ q/q* ≤ 2.1) as a function of copolymer concentration in the ordering regime. The relaxation rate of the short-range composition fluctuations at q*, Γ(q*), shows a significant slowing down relative to the respective long-range ones at low wavevectors; as the ordering transition is approached, this slowing down becomes very pronounced. Γ(q*) has been anticipated to influence the low-frequency rheological behavior of disordered diblocks. Additionally, a general theory for the S(q,t) of entangled polydisperse diblock copolymers is developed in the framework of the random phase approximation assuming reptation dynamics. Although both internal diffusion and ordinary interdiffusion contribute to the dynamics of long-range composition fluctuations, it is the internal diffusion at finite wavevectors near q*, which is affected by approaching the ordering transition from the disordered state. Nevertheless, composition polydispersity causes a coupling of these relaxation modes, which hinders their identification over the whole q range. The theoretical results are favorably compared with the experimental data

Self-diffusivity in block copolymer solutions. 2. A2B simple grafts

The composition and concentration dependencies of the self-diffusion coefficients for a series of miktoarm star block copolymers of the A2B type in solutions in a common good solvent have been investigated by pulsed-field-gradient nuclear magnetic resonance; the graft copolymers are composed of A = polyisoprene and B = polystyrene, where the B block was attached either in the middle of the A block (τ = 0.5) or at a position with τ = 0.25. The dependence of the diffusion coefficients on concentration, in both A2B graft copolymers and AB diblock copolymers, can be collapsed onto a master curve with a proposed scaling procedure which takes into account the difference in the entanglement characteristics of the parent homopolymers and the different radii of gyration of stars vs linear polymers. The exponential slowing down of the diffusivities expected for star molecules was not observed due to the only moderately high molecular weights and concentrations

UMacromolecular architecture effects on block copolymer dynamics: Linear tetrablocks and inverse starblocks

Photon correlation spectroscopy and pulsed-field-gradient NMR have been used to investigate the role of different macromolecular architecture on the dynamics of block copolymer solutions in the low wavevector (q) limit using linear tetrablock (TB) and inverse starblock (SB) copolymers composed of two jointed TB's (NSB = 2NTB). The three relaxation processes observed for diblocks are also evident in the present case: the cooperative diffusion, the internal copolymer chain relaxation (q2-dependent intensity and qindependent rate), and the polydispersity diffusive mode, controlled by the self-diffusion (due to the finite composition polydispersity). The internal mode characteristics are identical for both systems at a given concentration, in agreement with theoretical estimates for Rouse chains, whereas the selfdiffusivities show the effect of the total molecular weight and the architecture. At high concentrations of the SB copolymers, there is evidence for an extra relaxation process with a strong-concentration-dependent intensity apparently due to the proximity to the ODT and due to the macromolecular architecture. © 1999 American Chemical Society