Multiarm Star Research Articles

Advanced Computational Strategies for Modelling the Evolution of Full Molecular Weight Distributions Formed During Multiarmed (Star) Polymerisations

AbstractSummary: A novel computational strategy is described for the simulation of star polymerisations, allowing for the computation of full molecular weight distributions (MWDs). Whilst the strategy is applicable to a broad range of techniques for the synthesis of star polymers, the focus of the current study is the simulation of MWDs arising from a reversible addition fragmentation chain transfer (RAFT), R‐group approach star polymerisation. In this synthetic methodology, the arms of the star grow from a central, polyfunctional moiety, which is formed initially as the refragmenting R‐group of a polyfunctional RAFT agent. This synthetic methodology produces polymers with complex MWDs and the current simulation strategy is able to account for the features of such complex MWDs. The strategy involves a kinetic model which describes the reactions of a single arm of a star, the kinetics of which are implemented and simulated using the PREDICI® program package. The MWDs resulting from this simulation of single arms are then processed with an algorithm we describe, to generate a full MWD of stars. The algorithm is applicable to stars with an arbitrary number of arms. The kinetic model and subsequent algorithmic processing techniques are described in detail. A simulation has been parameterised using rate coefficients and densities for a 2,2′‐azoisobutyronitrile (AIBN) initiated, bulk polymerisation of styrene at 60 °C. A number of kinetic parameters have been varied over large ranges. Conversion normalised simulations were performed, leading to information regarding star arm length, polydispersity index (PDI) and the fraction of living arms. These screening processes provided a rigorous test for the kinetic model and also insight into the conditions, which lead to optimal star formation. Finally, full MWDs are simulated for several RAFT agent/initiator ratios as well as for stars with a varying number of arms.Full MWDs from a star with 1, 2, 4, 6 and 8 arms.imageFull MWDs from a star with 1, 2, 4, 6 and 8 arms.

Dynamic mechanical properties of suspensions of micellar casein particles

Small micellar casein particles, so-called submicelles, were obtained by removing colloidal calcium phosphate from native casein by adding sodium polyphosphate. Aqueous submicelle suspensions were characterized using light scattering and rheology as a function of concentration and temperature. The casein submicelles behave like soft spheres that jam at a critical concentration ( C c ) of about 100 g L −1. The viscosity does not diverge at C c , but increases sharply, similarly to that of multiarm star polymers. C c increases weakly with increasing temperature, which leads to a strong decrease of the viscosity close to and above C c . Concentrated submicelle suspensions show strong shear-thinning above a critical shear rate and the shear stress becomes independent of the shear rate. The critical shear rates at different temperatures and concentrations are inversely proportional to the zero-shear viscosity. At much higher shear rates, the shear stress fluctuates strongly in time indicating inhomogeneous flow. The frequency dependence of casein submicelle suspensions is characterized by elastic behavior at high frequencies (concentrations) and viscous behavior at low frequencies (concentrations).

Synthesis and Viscoelastic Behavior of Multiarm Star Polyelectrolytes

Star-shaped poly(tert-butyl acrylate)s were synthesized by the atom transfer radical polymerization (ATRP) method. Subsequently, star-shaped poly(acrylic acid)s were induced by hydrolysis of tert-butyl units. The structural ordering of these stars was investigated through small-angle X-ray scattering (SAXS) in aqueous solution. Star with f = 30 and f = 97 formed a bcc structure above the overlap threshold (C*). The viscoelastic behavior of the stars was also investigated. This system showed the transition from a Maxwellian fluid of a weak liquid ordering to a solid of macrolattice structure with increasing the polymer concentration. This result reflected the stronger interaction among star polyelectrolytes at higher polymer concentration.

Diffusional behavior of multi-arm star polymers by 1H pulsed field gradient spin-echo NMR method

The diffusional behavior of multi-arm star-shaped p( tBMA) was investigated in a concentration range from dilute to semidilute region with 1H pulsed field gradient spin-echo NMR (PFGSE-NMR). An 142-arm star-shaped p( tBMA) showed two diffusional mode, which reflected the coexistence of liquid ordering phase and liquid phase near the ordering transition. On the other hand, for star-shaped p( tBMA) with 55 arms showed a single diffusional relaxation in all concentration ranges during observation time. In the semidilute region, the relationship between the diffusion coefficient against the polymer concentration was affected strongly not only the arm number but also liquid ordering structure.

Thermal vitrification in suspensions of soft colloids: Molecular dynamics simulations and comparison with experiments

Dense suspensions of multiarm star polymers are known to develop liquidlike microstructure, which has been attributed to the similarities between high functionality stars and colloidal particles interacting via soft, long ranged potentials. Recent experimental studies reported a counterintuitive solidification of suspensions with f=128 , upon increase of the temperature in marginal solvents. We present our results from molecular dynamics simulations of dense suspensions of multiarm star polymers. Star polymers are modeled as "soft spheres" interacting via a theoretically developed potential of mean field. Our results show a transition towards a "glassy" state at a temperature very close to the one reported experimentally. The features of the transition are consistent with those of ideal glass transitions, as described by ideal mode coupling theory. Furthermore, our findings illustrate the road to vitrification for these soft-colloidal suspensions. Higher temperatures result in arm expansion that causes jamming and more than compensates for faster short time, temperature induced kinetics.

Synthesis of Amphiphilic (ABC)n Multiarm Star Triblock Terpolymers

Synthesis of Amphiphilic (ABC)<i>_n</i> Multiarm Star Triblock Terpolymers

Ionic conductivity of multiarm star polymer/LiClO4 electrolytes

AbstractA series of polymer electrolytes based on multiarm polymers and lithium salt complexes were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and impedance measurement. The relationships of conductivity with salt concentration, temperature, and arm numbers are discussed. It is suggested that the star polymer has a higher solvency and ion transfer ability on lithium salts than on linear polymers. The conductivity maximum appeared at a higher salt concentration ([EO]/[Li] = 4). Impedance measurement suggested that the optimum conductivity was 2 × 10−4 s · cm−1. The conductivity increased with temperature and the dependence of ionic conductivity on temperature fits the Arrhenius equation. Among the studied systems, the star polymer with a five arm number performs better than other structures. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4195–4198, 2004

Structure, Dynamics and Properties of Materials with Polymers Having Complex Architectures

AbstractVarious non‐linear highly branched polymers such as multiarm stars, block copolymer micelles and bottlebrush‐like polymers have been studied in order to analyze their intramolecular structure and effects of spatial ordering resulting from their specific macromolecular architecture. These polymers constitute a class of compact macromolecules which, due to the high intramolecular density, interact strongly, excluding each other in space. Investigations of the structure and dynamics in such systems, using various experimental methods as well as computer simulations, have been performed. Small angle X‐ray scattering is used to characterize the structure and mechanical spectroscopy is used for the detection of the dynamic behavior of the systems. Simulations have been performed using the cooperative motion algorithm with lattice polymers equivalent to the considered macromolecules. Results of both experiments and the simulation seem to support the concept of slow structural cooperative rearrangements controlling the flow in such systems. The effects are analogous to those related to flow in low molecular liquids but take place on another size scale. The new slow relaxation processes creates new super soft‐states which are characterized by shear modulus plateau lower than 104 Pa.

Synthesis of novel multi-arm star azobenzene side-chain liquid crystalline copolymers with a hyperbranched core

A series of novel multi-arm star side-chain liquid crystalline (LC) copolymers with hyperbranched core moieties were synthesized by atom transfer radical polymerization (ATRP) using a multi-functional hyperbranched polyether as the initiator and chlorobenzene as the solvent. The multi-functional hyperbranched polyether initiator was prepared from poly(3-ethyl-3-(hydroxymethyl)oxetane) (PEHO) and 2-bromo-2-methylpropionyl bromide. The azobenzene side-chain liquid crystalline arms were designed to have an LC conformation of poly[6-(4-methoxy-4 ′-oxy-azobenzene)hexyl methacrylate] with different molecular weights. Their characterization was performed with 1H NMR, size exclusion chromatograph (SEC), differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). The multi-arm star side-chain liquid crystalline copolymers exhibited a smectic and a nematic phase, and the phase transition temperatures from the smectic to the nematic phase and from the nematic to isotropic phase increased with increasing the molecular weight of the multi-arm star side-chain liquid crystalline copolymers from 1.78 × 10 4 to 9.07 × 10 4.

Synthesis and characterization of a multiarm star polymer

AbstractA multiarm star polymer was synthesized through the grafting of oligo polyglycol with urethane chain end units onto the core of hyperbranched polyglycerol (HPG), which was obtained through the cationic ring‐opening polymerization of glycidol. Samples were characterized with 13C NMR, liquid chromatography/mass spectrometry, vapor pressure osmometry, and Raman spectroscopy. The degree of branching of HPG was 0.54, and the number of arms grafting onto HPG was 4. The urethane of the arms mainly reacted with the terminal hydroxy groups of HPG. The differences between the spin–spin relaxation times indicated that the terminal segments of the star were more flexible than those of the core. Grafting polyglycol polyurethane (soft segments of polyurethane is polyglycol) onto HPG improved its dimensional stability. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2356–2364, 2004

Investigation of the effect of reaction conditions on the synthesis of multiarm-star polyisobutylene–polystyrene block copolymers

New multi-arm star block copolymers comprising of rubbery polyisobutylene (PIB) midsegment and glassy polystyrene (PS) end blocks have been synthesized by carbocationic polymerization using a new multifunctional initiator, hexaepoxy squalene (HES), with TiCl 4 coinitiator, di- t-butylpyridine (D tBP) as a proton trap and N, N-dimethylacetamide (DMA) as an electron pair donor in methylcyclohexane (MeCHx)/methyl chloride (MeCl) solvent mixtures at −80 °C. It was found that reaction conditions, such as solvent composition, HES/isobutylene (IB) ratio and TiCl 4 concentration, have profound influence on initiator efficiency and functionality. Living conditions were achieved in the presence of DMA in MeCHx/MeCl 60/40 v/v, while in the absence of DMA, the M n–conversion plot showed a considerable intercept. Depending on the reaction conditions, the PIB midblocks had 3–10 arms. Reaction rates increased with increasing solvent polarity and TiCl 4 concentration. Living narrow molecular weight distribution PIBs ( M w/ M n=1.1–1.2) were reacted with styrene (St) solution containing D tBP and DMA to yield multiarm-star PIB–PS block copolymers. Blocking was evidenced by SEC analysis and copolymers with 8.9–28.6 wt.% PS, M n∼164,000–609,000 g/mol and M w/ M n=1.32–1.88 were successfully synthesized.

Living Free Radical Polymerization with Reversible Addition−Fragmentation Chain Transfer (RAFT Polymerization): Approaches to Star Polymers

The synthesis of multiarm star polymers by radical polymerization with reversible addition−fragmentation chain transfer (RAFT) is described. When the precursor RAFT agents to star polymers are trithiocarbonate derived, fragmentation of the radical intermediate can lead to different products depending on the leaving abilities of the two groups attached to sulfur. To demonstrate this, two types of RAFT agents, ones that allow growth of arms away from the core and the other attached to the core during propagation, were designed, and an example of each was synthesized. While both star RAFT agents gave excellent molecular weight control and low polydispersities (typically <1.2), the one that grows its arms away from the core offers polymers free from star−star coupled products. Thus, the direction of fragmentation should be considered when designing efficient star RAFT agents. The living nature of the arms of these polymers was demonstrated by extending the arms with a different monomer to afford a star block ...

Linear rheology of multiarm star polymers diluted with short linear chains

We present experimental results on the linear rheology of multiarm star/linear polymer mixtures, the latter having molecular weight much smaller than the star arm molecular weight. In such a case the linear chains act as ideal macromolecular solvents, which dilute entanglements of the arms. Using different star polymers we show that it is possible to account for this dilution and describe the linear rheology of the mixtures using the Milner–McLeish theory for arm relaxation, complemented by the longitudinal modes of stress relaxation and high frequency Rouse modes. A universal description of the isofrictional arm relaxation time as a function of the number of entanglements is obtained for stars of any functionality and degree of dilution. The slow structural mode, related to the diluted star’s colloidal core, also depends on the number of entanglements, but in a more complex way.

Linear Viscoelastic Properties of Concentrated Solutions of Multi-Arm Star Polystyrenes

The dynamic viscoelasticity of solutions of multi-arm star polystyrenes is investigated. The relaxation spectrum is calculated from the master curves of the dynamic storage and loss moduli. The relaxation spectra for the solutions as well as the melts exhibit a shoulder at long times. The relaxation intensity and the relaxation time for the long time relaxation are estimated from the relaxation spectra. The relaxation intensity strongly depends on the number density of the star molecules, suggesting that the number density dependence of the relaxation intensity is closely related with the degree of interpenetration between the star polymers, while the relaxation time of the long time relaxation is almost independent of the number density of the star molecules.

A modular approach toward functionalized three-dimensional macromolecules: from synthetic concepts to practical applications.

A new strategy for the preparation of functional, multiarm star polymers via nitroxide-mediated "living" radical polymerization has been explored. The generality of this approach to the synthesis of three-dimensional macromolecular architectures allows for the construction of nanoscopically defined materials from a wide range of different homo, block, and random copolymers combining both apolar and polar vinylic repeat units. Functional groups can also be included along the backbone or as peripheral/chain end groups, thereby modulating the reactivity and polarity of defined portions of the stars. This modular approach to the synthesis of three-dimensional macromolecules permits the application of these tailored materials as multifunctional hosts for hydrogen bonding, nanoparticle formation, and as scaffolds for catalytic groups. Examples of applications of the functional stars in catalysis include their use in a Heck-type coupling as well as an enantioselective addition reaction.

Kinetic arrest of crowded soft spheres in solvents of varying quality.

Crowded solutions of multiarm star polymers, representing model colloidal spheres with ultrasoft repulsive interactions, undergo a reversible gelation transition upon heating in solvents of intermediate quality (between good and Theta). This unusual phenomenon is due to the kinetic arrest of the swollen interpenetrating spheres at high temperatures, forming clusters, in analogy to the colloidal glass transition. In this work we demonstrate that the choice of the solvent has a dramatic effect on the gelation transition, because of the different degree of star swelling (at the same temperature) associated with the solvent quality. We construct a generic kinetic phase diagram for the gelation of different stars in different solvents (gelation temperature against effective volume fraction, phi) and propose a critical "soft sphere close packing" volume fraction phi(c) distinguishing the temperature-induced (for phi<phi(c)) from the concentration-induced (for phi>phi(c)) glass-like gelation. We conclude that appropriate selection of the solvent allows for manipulation of the sol-gel transition in such ultrasoft colloids.

Stress Relaxation of Multi-Arm Star Polystyrenes in the Molten State.

Stress relaxation of multi-arm star polystyrenes were investigated in the molten state. The relaxation modulus was measured over a wide range of strain. The plots of relaxation modulus against time for the multi-arm stars showed a new mechanical relaxation at long times. For entangled systems, although the strain dependence of the damping function for entanglement relaxation was almost identical to that for linear and regular star polymers, the strain dependence for the long-time relaxation was weak. On the other hand, the damping function for the non-entangled systems showed the stronger dependence than the entangled systems. The damping at large strains became weaker with increasing weight-average molecular weight of the arm but appeared to be independent of number of arms.

A common route to A 2B and A 3B multiarm star block copolymers

We have previously described a new route to A 2B star block copolymers by a double addition–elimination reaction of a polystyryl carbanion with a particular 1,1-diphenylethylene derivative. By using the same reagent, we are now able to prepare A 2 or A 3 homopolymers by changing the solvent, i.e. tetrahydrofuran or toluene/tetrahydrofuran, respectively. We propose a mechanism involving nucleophilic substitution reactions to explain this peculiar result and use this opportunity to prepare A 2B or A 3B star block copolymers based on styrene and ethylene oxide or tert-butyl methacrylate.

Poly(amidoamine-organosilicon) (PAMAMOS) multi-arm star polymers

A new class of hydrophilic core – hydrophobic arms multi-arm star polymers is described: the first such materials to have silicon located in the side arms. They belong to the broad family of radially layered copolymeric amidoamine-organosilicon (PAMAMOS) dendritic macromolecules and may be viewed as nanostructured, covalently bonded, inverse micelles. Two types of hydrophobic, silicon-containing arms have been used, one based on polydimethylsiloxane (PDMS) and another based on predominantly alkyl chains attached by organosilicon chemistry to the hydrophilic polyamidoamine (PAMAM) core. The former polymers are synthesized by a reaction of amine-terminated PAMAMs by mono-functional epoxy PDMS, while the latter are obtained by haloalkylation of the same PAMAMs with a suitable unsaturated silane followed by alkylthiol addition. The Langmuir film behavior of the multi-arm star polymers with PDMS arms is described and rationalized in comparison with that of other hydrophobically modified PAMAM dendrimers reported in the literature. Their uptake of Cu2+ from aqueous solutions is also described.

A practical approach to the living polymerization of functionalized monomers: application to block copolymers and 3-dimensional macromolecular architectures

The tolerance of living free radical procedures to reactive functional groups, coupled with their ability to prepare well-defined random and block copolymers is demonstrated by the use of novel α-hydrogen alkoxyamine derivatives instead of the traditional TEMPO-based systems. This refinement in the nitroxide structure overcomes many limitations typically associated with TEMPO and has permitted a dramatic increase in the range of monomers, which can be polymerized under controlled conditions. The ability to prepare well-defined multi-arm star polymers from a variety of alkoxyamine terminated vinyl and non-vinyl linear polymers are major benefits when compared to traditional living procedures, such as anionic polymerizations.