Jacques Roovers

Dendrimers and Dendrimer-Polymer Hybrids

The synthesis and study of dendrimers has been truly dramatic in the last ten years. This review gives a brief introduction to some of the key concepts and main synthetic strategies in dendrimer chemistry. The focus of the chapter is a survey of modern analytical techniques and physical characterization of dendrimers. Results of model calculations and experiments probing the dimensions and conformation of dendrimers are reviewed. In the final sections the experimental work on dendrimer-polymer hybrids is highlighted. The dense spherical conformation of dendrimers has been combined with the loose random-coil conformation of ordinary polymers to form new hybrids with potentially interesting new properties.

Synthesis and dilute solution characterization of comb polystyrenes

Abstract Four new series of comb polystyrenes were prepared and characterized. The observed θ temperature depression of the combs can be described by Benoits three parameter theory with Ψ1 = 0.32 and ( 1 3 − χ 2 ) = 0.045 . Also, the dimensions of the combs at θ agree reasonably with the theory. Intrinsic viscosity measurements were used to investigate m in g′ = gmth. A decrease of m from 3 2 to about 0.6 is found on decreasing gth at θA2. At constant gth, m is slightly dependent on the comb structure. In a good solvent m varies less, an average m = 0.75 is found for the combs studied. The gel permeation chromatograms of the combs obey the universal calibration based on the hydrodynamic volume parameter. The glass transition temperatures of the combs depend on the number of end groups per unit volume, the branch points having a small Tg raising effect.

Synthesis and Properties of Regular Star Polybutadienes with 32 Arms

Abstract A dendrimer carbosilane containing 32 Si—Cl bonds in the perimeter has been prepared and has been used as a coupling agent to prepare 32-arm star polybutadienes. The dilute-solution properties 〈RG2〉, A2, [η], and D0 have been measured in one good solvent and in one ¸ -solvent. The dimensions of the 32-arm star polymers are compared with those of linear polymers at constant molecular weight. It is shown that the 32-arm star polybutadiene has the characteristic properties of a hard-sphere molecule in dilute solution. The equivalent hard-sphere radii calculated from A2, D0 and [η] are identical and 1.29 times larger than the root mean-square radius of gyration. The Daoud—Cotton scaling model for stars is also tested.

Segmental dynamics of miscible polymer blends: Comparison of the predictions of a concentration fluctuation model to experiment

We recently proposed a concentration fluctuation model to describe the segmental dynamics of miscible polymer blends [Kumar et al., J. Chem. Phys. 105, 3777 (1996)]. This model assumes the existence of a cooperative volume, similar to that in the Adam-Gibbs picture of the glass transition, over which segments have to reorganize in a concerted fashion to facilitate stress relaxation. No molecular theory exists for the cooperative volume. Consequently, here we critically compare two alternative functional dependences for this quantity in the context of the segmental dynamics of the most extensively studied miscible polymer blend, 1,4-polyisoprene (PI) and polyvinylethylene (PVE): (a) The Donth model, which assumes the Vogel form for the temperature dependence of relaxation processes, with a relaxation time that diverges at the Vogel temperature, roughly 50 K below the glass transition, and (b) a more recent dynamic scaling model that predicts the relaxation time diverges algebraically, only about 10 K below...

Shear banding phenomena in ultrasoft colloidal glasses

We explore the nonlinear rheological response of a soft gel formed by a crowded colloidal star polymer and focus on the occurrence of a stress plateau, marked hysteresis, and yield stress in its flow curve. With the aid of nuclear magnetic resonance velocimetry we find evidence for fluctuations in the flow behavior across the gap of the concentric cylindrical Couette device, in association with a degree of apparent slip at the inner wall. The time scale of the these fluctuations appears rapid (with respect to the measurement time per shear rate in the flow curve), on the order of tens to hundreds of milliseconds, with the speed of fluctuations associated with the flow history of the sample. Our velocity profile analysis suggests a qualitative model in which intermittent changes due to jamming/unjamming transitions occur, analogous to cage dynamics in colloidal glasses.

Tailoring the flow of soft glasses by soft additives.

We examine the vitrification and melting of asymmetric star polymer mixtures by combining rheological measurements with mode coupling theory. We identify two types of glassy states, a single glass, in which the small component is fluid in the glassy matrix of the big one, and a double glass, in which both components are vitrified. Addition of small-star polymers leads to melting of both glasses, and the melting curve has a nonmonotonic dependence on the star-star size ratio. The phenomenon opens new ways for externally steering the rheological behavior of soft matter systems.

EFFECT OF MOLECULAR WEIGHT ON THE CRYSTALLIZATION BEHAVIOUR OF POLY(ARYL ETHER ETHER KETONE) : A DIFFERENTIAL SCANNING CALORIMETRY STUDY

Abstract The effect of molecular weight on the kinetics of the crystallization of poly(aryl ether ether ketone) has been investigated by means of a series of fractions of relatively narrow molecular-weight distribution. In this study the crystallization has been monitored by differential scanning calorimetry (d.s.c.). Analysis of the sigmoidal crystallization curves by means of the Avrami equation leads to n ≈ 2. In order to obtain a uniform set of rate constants (k), small adjustments in the zero time were made. Measurements have been made of the crystallization of amorphous samples just above the glass transition temperature. The temperature dependence of the observed rate constants follows a Vogel equation, log k = U∗ 2.3R(T c − T ∞ ), where U∗ ≈ 4500 cal mol −1 and T∞ = Tg − 55. These parameters are consistent with the universal WLF parameters. The experimental glass transition temperature of each sample has been used. It is found that the rate of crystallization considered under iso-free-volume conditions depends on M−2 as expected for a diffusion-controlled process that involves entangled linear polymers. This conclusion is supported by an analysis of the onset temperature of crystallization in dynamic d.s.c. scans of the different molecular-weight samples. In another series of experiments the crystallization rates have been measured below the melting point of the samples. Analysis of the temperature dependence of the rate constants reveals that high-molecular-weight polymers crystallize at lower temperature (Tc = 270–290°C) according to regime III. Low-molecular-weight samples require higher temperatures in order to measure the rates of crystallization (Tc > 300°C). The temperature dependence of the rate constants suggests that in that case crystallization occurs according to regime II. One sample (Mw = 32 000) showed a regime III-regime II transition at about 298°C. These conclusions are not drastically affected by reasonable variations in U∗ , T∞ and the equilibrium melting temperature Tom.

Properties of the plateau zone of starbranched polybutadienes and polystyrenes

Abstract The plateau zone of monodisperse star polymers has a maximum in G″(ω). The time associated with this relaxation process ( 1 ω max ) is found to depend on the square of the number of entanglements per arm. It depends also on the monomeric friction coefficient (ζ0). These are the dependencies expected for a Rouse type process. It is found that 1 ω max agrees well with Teq, the characteristic time for equilibration of the arm after deformation in the Doi and Pearson-Helfand theories. Teq is calculated from the zero-shear properties of the stars. The agreement of Teq and 1 ω max depends however on the choice of γ′. Reasonable agreement is also found when Teq is calculated from ζ0 and 〈S2〉0, molecular characteristics of the linear chain. This would indicate that ζ0 for the arm contraction is essentially the same as for other long-chain motions.

Synthesis and solution properties of comb polystyrenes

Abstract A new method for the synthesis of comb shaped polystyrenes of predetermined structure is described. Silicon-chlorine bonds are introduced into the backbone polystyrene by reaction of SiMe2Cl2 with hydrolysed styrene/vinyl acetate copolymers and coupled with polystyryl-lithium in benzene. From a common backbone polymer a series of comb polymers are prepared that have a constant number of branches but vary in branch length. The M w M n of the whole comb polymers is about 1.3. The comb polymers with high branch density show θ (A2) temperatures lower than that for linear polystyrene. The radius of gyration at θ (A2) [〈S2〉θ (A2)] is always larger than calculated from random flight statistics. For comb polymers with 20–30 branches 〈S 2 〉 θ (A 2 ) 〈S 2 〉 0, bb increases with λ−0.46 where λ is the fraction of polymer in the backbone. The intrinsic viscosities of the comb polystyrenes at θ (A2) are equal to that of the parent backbone polymer when λ > 0.25 and increase only little when λ becomes equal to 0.1. Similar behaviour is found in toluene. Intrinsic viscosities in cyclohexane at 35°C show a complex pattern because of the θ-temperature variation.

Viscoelastic response of hyperstar polymers in the linear regime

We compare the linear viscoelastic spectra of star polymer melts, with varying functionality (4-128) and chemistry (isoprenes, butadienes), with a recent parameter-free theory of arm relaxation [S. T. Milner and T. C. B. McLeish, Macromolecules 30, 2159 (1997)]. The theory, which considers this activated process within the framework of dynamic dilution and with appropriate account of the entanglement length scaling and the higher Rouse modes, is universal as it works remarkably well for a very wide range of star functionalities and arm molecular weights. However, for hyperstars consisting of 64 or 128 arms, the viscoelastic response is characterized by the presence of a slow relaxation process in addition to the faster arm relaxation. This additional process is due to the soft ordering of these systems because of their nonuniform monomer density distribution, and exhibits a very strong functionality and molecular weight dependence. It is accounted for by a mean field approach which considers the structura...