The synthesis of linear and non-linear (star, cyclic) triblock terpolymers through living, controlled/living polymerization methods and their combinations are reviewed. The theoretical predictions ...and the experimental results concerning the self-assembly of these materials in selective solvents and in bulk are also discussed. A plethora of self-assembled structures have been detected either in solution or in bulk, potentially leading to a rich variety of high-tech applications. First results regarding the synthesis and morphology of multicomponent multiblock (tetrablock quarter and pentablock quinto) polymers are also given.
In this paper, we exploit the stochastic slip-spring model to quantitatively predict the stress relaxation dynamics of star/linear blends with well-separated longest relaxation times and we analyze ...the results to assess the validity limits of the two main models describing the corresponding relaxation mechanisms within the framework of the tube picture (Doi’s tube dilation and Viovy’s constraint release by Rouse motions of the tube). Our main objective is to understand and model the stress relaxation function of the star component in the blend. To this end, we divide its relaxation function into three zones, each of them corresponding to a different dominating relaxation mechanism. After the initial fast Rouse motions, relaxation of the star is dominated at intermediate times by the “skinny” tube (made by all topological constraints) followed by exploration of the “fat” tube (made by long-lived obstacles only). At longer times, the tube dilation picture provides the right shape for the relaxation of the stars. However, the effect of short linear chains results in time-shift factors that have never been described before. On the basis of the analysis of the different friction coefficients involved in the relaxation of the star chains, we propose an equation predicting these time-shift factors. This allows us to develop an analytical equation combining all relaxation zones, which is verified by comparison with simulation results.
We present a small angle neutron scattering (SANS) investigation of a blend composed of a dendritic polymer and a linear matrix with comparable viscosity in start-up of an elongational flow at T g + ...50. The two-generation dendritic polymer is diluted to 10% by weight in a matrix of a long well-entangled linear chains. Both components consist of mainly 1,4-cis-polyisoprene but differ in isotopic composition. The resulting scattering contrast is sufficiently high to permit time-resolved measurements of the system structure factor during the start-up phase and to follow the retraction processes involving the inner sections of the branched polymer in the nonlinear deformation response. The outer branches and the linear matrix, on the contrary, are in the linear deformation regime. The linear matrix dominates the rheological signature of the blend and the influence of the branched component can barely be detected. However, the neutron scattering intensity is predominantly that of the (branched) minority component so that its dynamics is clearly evident. In the present paper, we use the neutron scattering data to validate the branch point withdrawal process, which could not be unambiguously discerned from rheological measurements in this blend. The maximal tube stretch that the inner branches experience, before the relaxed outer arm material is incorporated into the tube is determined. The in situ scattering experiments demonstrate for the first time the leveling-off of the strain as the result of branch point withdrawal and chain retraction directly on the molecular level. We conclude that branch point motion in the mixture of architecturally complex polymers occurs earlier than would be expected in a purely branched system, presumably due to the different topological environment that the linear matrix presents to the hierarchically deep-buried tube sections.
Polystyrene-block-poly(1,4-isoprene)-block-poly(dimethyl siloxane)-block-poly(tert-butyl methacrylate)-block-poly(2-vinyl pyridine), PS-b-PI-b-PDMS-b-PtBMA-b-P2VP, self-assembles in acetone into ...polymersomes with asymmetric (directional) PI-b-PDMS membranes. The polymersomes, in turn, self-assemble into superstructures. Analogically to supravesicular structures at a smaller length scale, we refer to them as suprapolymersome structures. Electron tomograms are shown to be invaluable in the structural assessment of such complex self-assemblies.
We investigate the linear viscoelastic response of model telechelic linear and star (of varying functionality) polyisoprene melts with different molar masses above the entanglement limit in relation ...to their structure. We find that these systems self-assemble as a result of the strong dipolar interactions and form clusters that seem to depend primarily on the number of dipolar groups per star. The dynamics is rather complex, but some pertinent features are observed: the segmental dynamics is affected by the telechelic functionalization, especially for short arm lengths; this reflects the change of microstructure (and thus glass-transition temperature) with functionalization. The terminal relaxation is much slower compared to similar nonionic stars, reflecting the relaxation of clusters. Linear semitelechelic polymers (with only one end functionalized) aggregate in a star-like fashion. We further develop a tube model based on the time-marching algorithm for stars and linear chains, where we incorporate the association status of the chains via the dipolar interactions at each time step. The agreement of the predictions with the data, using two adjustable parameters (the average times when two dipolar pair remain associated or free, respectively), is remarkable and suggests design criteria for forming desired supramolecular assemblies.
Complex comblike block copolymers of various architectures containing polystyrene, PS, and polyisoprene, PI, or polybutadiene, PB, components were synthesized by anionic polymerization high-vacuum ...techniques and the macromonomer strategy. Their micellization behavior was studied in n-decane, a selective solvent for the PI or PB blocks, and in N,N‘-dimethylacetamide, DMA, selective for the PS blocks. Utilizing static and dynamic light scattering techniques, parameters such as the aggregation number and the hydrodynamic radius were determined. On the basis of these results, structural parameters of the micelles, i.e., core and corona radii as well as core−corona interfacial area, were calculated. The thermal stability of the micelles was also examined in both selective solvents. The macromolecular architecture was found to have a considerable effect on the micellization behavior of the block copolymers.
An emerging challenge in polymer physics is the quantitative understanding of the influence of a macromolecular architecture (
i.e.
, branching) on the rheological response of entangled complex ...polymers. Recent investigations of the rheology of well-defined architecturally complex polymers have determined the composition in the molecular structure and identified the role of side-products in the measured samples. The combination of different characterization techniques, experimental and/or theoretical, represents the current state-of-the-art. Here we review this interdisciplinary approach to molecular rheology of complex polymers, and show the importance of confronting these different tools for ensuring an accurate characterization of a given polymeric sample. We use statistical tools in order to relate the information available from the synthesis protocols of a sample and its experimental molar mass distribution (typically obtained from size exclusion chromatography), and hence obtain precise information about its structural composition,
i.e.
enhance the existing sensitivity limit. We critically discuss the use of linear rheology as a reliable quantitative characterization tool, along with the recently developed temperature gradient interaction chromatography. The latter, which has emerged as an indispensable characterization tool for branched architectures, offers unprecedented sensitivity in detecting the presence of different molecular structures in a sample. Combining these techniques is imperative in order to quantify the molecular composition of a polymer and its consequences on the macroscopic properties. We validate this approach by means of a new model asymmetric comb polymer which was synthesized anionically. It was thoroughly characterized and its rheology was carefully analyzed. The main result is that the rheological signal reveals fine molecular details, which must be taken into account to fully elucidate the viscoelastic response of entangled branched polymers. It is important to appreciate that, even optimal model systems,
i.e.
, those synthesized with high-vacuum anionic methods, need thorough characterization
via
a combination of techniques. Besides helping to improve synthetic techniques, this methodology will be significant in fine-tuning mesoscopic tube-based models and addressing outstanding issues such as the quantitative description of the constraint release mechanism.
By coupling and confronting results obtained with different characterization techniques, a detailed description of the sample architectural dispersity is obtained.
The long-standing observations that different amorphous materials exhibit a pronounced enhancement of viscosity and eventually vitrify on compression or cooling continue to fascinate and challenge ...scientists, on the ground of their physical origin and practical implications. Glass formation is a generic phenomenon, observed in physically quite distinct systems that encompass hard and soft particles. It is believed that a common underlying scenario, namely cage formation, drives dynamical arrest, especially at high concentrations. Here, we identify a novel, asymmetric glassy state in soft colloidal mixtures, which is characterized by strongly anisotropically distorted cages, bearing similarities to those of hard-sphere glasses under shear. The anisotropy is induced by the presence of soft additives. This phenomenon seems to be generic to soft colloids and its origins lie in the penetrability of the constituent particles. The resulting phase diagram for mixtures of soft particles is clearly distinct from that of hard-sphere mixtures and brings forward a rich variety of vitrified states that delineate an ergodic lake in the parameter space spanned by the size ratio between the two components and by the concentration of the additives. Thus, a new route opens for the rational design of soft particles with desired tunable rheological properties.
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.
For an entangled Cayley-tree-type cis-polyisoprene (CT-PI) composed of two classes of arms, trifunctionally branched inner arms each carrying two outer arms (M i = 24.7 × 103 and M o = 12.3 × 103 for ...each inner and outer arm, respectively), the molecular picture of dynamic tube dilation (DTD) was examined through viscoelastic and dielectric data. The global motion of the arms was dielectrically active because they had the so-called type A dipole parallel along their backbone. The dielectric relaxation function Φ(t) thus measured for CT-PI enabled us to experimentally evaluate the survival fraction of the dilated tube φ′(t), a key quantity in the DTD picture. In the conventional picture of full-DTD, relaxed portions of the arms are regarded as a simple solvent, thereby allowing the number of mutually equilibrated entanglement segments per dilated segment, β(t), to fully increase to that in a corresponding solution, βf-DTD(t) = {φ′(t)}−d with d = 1.3 for PI. The corresponding full-DTD relationship between φ′(t) and the normalized viscoelastic relaxation function μ(t), μf-DTD(t) = {φ′(t)}1+d , did not hold for the φ′(t) and μ(t) data of CT-PI. In contrast, in the molecular picture of partial-DTD, β(t) is determined through competition of the constraint release (CR) motion of the focused chain and the motion of surrounding chains, the former determining the maximum possible βCR(t) value in a given time scale, t. In this picture, the length and time scales are consistently coarse-grained on the basis of the CR mechanism. The partial-DTD relationship, μp-DTD(t) = φ′(t)/β*(t) with β*(t) = minβf-DTD(t), βCR(t), was found to hold for the φ′(t) and μ(t) data satisfactorily. In addition, analysis of the φ′(t) data unequivocally indicated that the inner arm hardly relaxed until the outer arm relaxation completed; i.e., the hierarchical relaxation occurred through partial-DTD. These results suggested the importance of the consistent coarse graining in molecular description of the dynamics of branched chains in general.
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