Polybutadiene Melts Research Articles

Controlling the self-assembly and dynamic response of star polymers by selective telechelic functionalization

We report on the structure and dynamics of model mono-, di-, and tri-ω-functionalized three-arm star polybutadiene melts. By using x-ray scattering and dynamic rheological measurements, we find that functionalization of the arm ends can lead to distinctly different supramolecular structures and material behavior. The monofunctionalized samples behave like multiarm nonionic star-like dendrimers, whereas the difunctional stars resemble a transient network consisting of highly branched structures with a very broad relaxation spectrum. On the other hand, the trifunctional stars seem to develop an unusually regular structure of dominant intramolecularly aggregated functional groups leading to collapsed star conformations, resembling soft spheres, and a well-defined terminal relaxation. These results suggest that by tailoring the telechelic functionalization of regular star polymers, a route to design and obtain a wealth of controlled supramolecular structures exhibiting a rich and variable dynamics could open.

Shear-driven diffusion in suspensions, studied via 1-d dynamic NMR projection

We have extended our earlier study of Couette flow using a dynamic 1-d NMR projection technique to suspensions of 50% polydisperse SiO 2 powder in a polybutadiene melt. The relatively narrow Couette gap and slow shaft rotation rate make the development of a concentration gradient from the migration of the solids to low-shear regions unobservable in static projections during and immediately following the experiment. Using dynamic projection during shaft rotation we detect pronounced departures from uniform tangential flow of the host liquid over spatial scales well below the positional resolution of the experiment. Using numerical and Monte Carlo simulations of the NMR interferograms based on a superposition of smooth flow and pseudo-Brownian motion, we successfully interpret host molecular motion in terms of an effective “dynamic” diffusion coefficient which is found to be proportional to the shaft rotation rate. This description of the motion of the host fluid complements recent observation of dynamic diffusion of the suspended particles in similar experiments.

Slip in Entangled Polymer Melts. 1. General Features

Apparent violations of the no-slip boundary condition are studied using a series of narrow molecular-weight distribution polybutadiene melts (67300 ≤ M̄n ≤ 650000), subjected to plane-Couette shearing over clean silica glass surfaces. Simultaneous measurements of slip velocity and shear stress reveal several new molecular characteristics of slip in entangled polymers. log−log plots of slip velocity versus shear stress display three distinct power-law regimes: (i) A weak slip regime at low shear stresses that is characterized by extrapolation/slip lengths b of the order of a few micrometers; (ii) A stick-slip regime at intermediate shear stresses marked by periodic oscillations in slip velocity and shear stress; (iii) A strong slip regime beyond a defined critical shear stress σ*. Slip violations in this last regime are characterized by large slip velocities and massive extrapolation lengths (b∞ ∼ 100−1500 μm). For all polymers studied the critical stress σ* for the weak-to-strong slip transition is found to be proportional to the plateau modulus Ge of the bulk polymer, σ* ≈ (0.2 ± 0.02) Ge. This finding is consistent with a shear-induced polymer disentanglement explanation for apparent slip violations in entangled polymers. Our experimental observations are also found to be in good agreement with a recently proposed scaling theory for friction and slip in entangled polymers, which assumes noninteracting surface chains. We rationalize this last result in terms of a polymer adsorption model in which a single macromolecule spontaneously attaches to numerous surface sites, yet offers a sufficiently long tail to resist relative motion of a chemically identical bulk polymer that attempts to slide over it.

Synthesis and Relaxation Dynamics of Multiarm Polybutadiene Melts

We describe synthesis and nonlinear relaxation dynamics of various multiarm entangled polybutadiene molecules of the general type A3-A-A3. In low-amplitude oscillatory shear, entangled multiarm polymers display broader relaxation spectra than linear polybutadienes of comparable molecular weight. Dramatic slowing down of cross-bar (A) relaxation by the entangled arms (A) is believed to be the source of this behavior. In nonlinear step strain experiments the A arms have a rather remarkable effect on polymer dynamics. At a critical shear strain γ of around 6.0, 〈|E·u|〉 = 3.2, the nonlinear relaxation modulus G(t;γ) abruptly decreases in value but retains similar time dependence to G(t;γ) at strains below the critical value. The sudden drop in G(t;γ) is reflected in the damping function and appears to be a consequence of arm withdrawal into the tube confining the cross-bar. This behavior is in near perfect agreement with a recent theoretical proposal for branched polymer dynamics. That this proposal is based on the notion of tensile forces on individual macromolecules due to tube confinement supports the existence of such forces and provides new circumstantial evidence for the existence of a mean-field tube. For all multiarm polymers studied we find time−strain separability at all strains with a separability time λk that appears insensitive to the arm withdrawal process. This last finding is not in agreement with current descriptions of multiarm polymer dynamics.

Chain-Order Effects in Polymer Melts Probed byH1Double-Quantum NMR Spectroscopy

Double-quantum NMR experiments are presented, elucidating the nature of the polymer chain dynamics in a polybutadiene melt far above the glass transition. The obtained experimental information about the rotational dynamics of spin pairs allows the translational dynamics as well as the long-term stability of the topological constraints on a microscopic scale to be probed. The results are shown to support some but not all aspects of the reptation model. For short times, the local chain order reflecting the time averaged configurations is much larger than expected, indicating strong packing effects.

Dynamics of bimodal polymer melts in the crossover-region from Rouse-to reptation-like behaviour—A study with NSE-spectroscopy

The single chain dynamics of bimodal saturated polybutadiene melts is addressed by neutron spin echo spectroscopy. A mode analysis of the structure factor provides an access to different relaxation modes (different spatial extension) separately. For the generalized Rouse model formulated by Hess an extension from monomodal to bimodal melts is presented.

Enhancement of Orientational Coupling in Bidisperse Polybutadiene Melts through the Implementation of Directed Interactions

Enhancement of Orientational Coupling in Bidisperse Polybutadiene Melts through the Implementation of Directed Interactions

Study of Couette flow in polymer-based liquids via 1-D NMR imaging

Couette flow in or near the laminar regime has been studied with a 1-D nuclear magnetic resonance spin-echo projection technique, and compared with numerical simulations. Newtonian flow is confirmed for water (power law exponent n = 1.00 ± 0.05), shear thinning is measured for a polybutadiene melt ( n = 0.60 ± 0.04) and departures are detected from uniform tangential flow in slurry of polybutadiene +SiO 2 powder. The method does not require transparent samples or trace scatterers, operates with small specimens (∼ 0.1 ml) and can be extended to deal with complex and turbulent flows.

The effect of entanglements on the rheological behavior of polybutadiene critical gels

We investigated the stress relaxation behavior of critical gels originating from six nearly monodisperse, highly entangled polybutadiene melts of different molecular weight from 18000 to 97 000 g/mole. The polymers were vulcanized by a hydrosilation reaction which takes place nearly exclusively at the pendant 1,2-vinyl sites distributed randomly along the polybutadiene chain. The BSW spectrum represents the relaxation of the initial uncrosslinked precursor. A characteristic parameter is the longest relaxation time of the precursor. Crosslinking increases this longest time even further. Surprisingly, the relaxation spectrum of the precursor is not altered much by the crosslinking except for an additional long time behavior. At the gel point (critical gel), this long time behavior is self-similar. It follows the typical power law as described by the Chambon-Winter gel equation, G(t) = St −n , in the terminal zone. The critical relaxation exponent was found to be close to n = 0.5 over a range of stoichiometric ratios and for all precursor molecular weights analyzed. A new scaling relationship was found between the gel stiffness, S, and the precursor molecular weight of the form: S ∼ M , where exponent z from the zero shear viscosity-molecular weight relationship, η0 ∼ M , is commonly found to be z = 3.3 – 3.6.

Relaxation Dynamics of Bidisperse Temporary Networks

Simultaneous measurement of IR dichroism and birefringence is used to examine the component relaxation in bidisperse, fully entangled melts of polybutadienes with 1% of the repeating units modified with 4-phenyl-1,2,4-trixoline-3,5-dione (urazole) groups. Since the urazole groups are capable of forming interchain hydrogen bonds, the rheological properties of these materials differ significantly from those of the analogous polybutadiene melts without polar stickers. Samples with 30, 50, and 70% long chains are examined by step strain and oscillatory shear experiments. In addition, the step strain experiments are carried out at two temperatures