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Star polymers are prototype systems both for tethered chains as well as for branched polymers. In this study we present a thorough investigation of the single star form factors in dilute solutions spanning a range of functionalities from 8 to 128. Using the zero average contrast method the form factor determination was also extended into the semidilute regime. As a consequence of their very inhomogeneous monomer density distribution star polymers exhibit osmotic repulsion leading to ordering phenomena in the neighborhood of the overlap concentration. We present a detailed investigation of such ordering phenomena in star polymer solutions with varying functionality and molecular weight. In all cases we observe an interparticle structure factor which is most pronounced at the overlap concentration Φ*. The ordering tendency increases with increasing functionality. These results are compared with current theoretical predictions.

We present a detailed, systematic study of the linear rheological response of model comb homopolymers consisting of linear backbone chains on which branches of the same polymer are grafted. By using polymers of different molecular weights of backbone and branches, different number of branches, and different chemistries, we explore the relaxation mechanisms of these polymers and find their universal features. We apply a tube model theoretical analysis, originally developed by McLeish and co-workers, which has been appropriately modified in order to account for the effects of the fluctuations of the free ends of the backbone (due to the grafting procedure during synthesis) and the polydispersity on the rheology. The satisfactory fitting of the data with the model indicates that the latter is capable of providing a quantitative understanding of the rheology of branched polymers. More specifically, comb polymers exhibit two distinct relaxation processes, assigned to the branches and the backbone. Relaxation proceeds hierarchically with the grafted branches moving first. The free backbone ends contribute as extra asymmetric branches moving via fluctuations. Finally, important issues relating to the number of grafted branches and the possible tube dilation breakdown (which may explain the physics of the only adjustable parameter p²) are discussed.

In the linear viscoelastic regime, multiarm star chains having more than 24 arms bound by a soft core exhibit fast relaxation due to motion of individual arms and slow relaxation related to motion of the soft cores (structural rearrangement in a liquidlike order). In this regard, the multiarm stars are qualitatively similar to randomly dispersed polystyrene−polyisoprene (PS−PI) and polybutadiene−polystyrene (PB−PS) copolymer micelles, the former having rigid (glassy) PS cores and the latter with soft (rubbery) PB cores. However, the multiarm star PB chains and the copolymer micelles were found to exhibit significant rheological differences in the nonlinear regime possibly because of the differences in their core−arm (core−corona) compatibility and core softness. In long time scales, the nonlinear damping of the relaxation modulus measured for the multiarm star PB was much weaker than that for the micelles and somewhat similar to the damping of a critical gel. In contrast, the damping in short time scales was moderately stronger for the multiarm stars. A corresponding difference was observed for the viscosity under fast flow. These differences may be attributed to a strain-induced elastic coupling of the multiarm star chains:  The soft core of the multiarm star, chemically identical to the arms, may become scarcer on application of large deformation to suck in some segments of neighboring star arms so as to preserve the segment density therein. Then, the multiarm star chains are elastically coupled/connected through these cores to form a huge transient network under large deformation, thereby increasing and decreasing the relaxation intensities in long and short time scales, respectively. This mechanism, being absent for the PS−PI and PB−PS micelles because of lack of the core−corona compatibility (as well as the core rigidity for the PS−PI micelles), would have led to the observed weakening and enhancement of the damping for the slow and fast processes of the multiarm stars.

A star polymer composed of amphiphilic block copolymer arms has been synthesized and characterized. The core of the star polymer is polyamidoamine (PAMAM) dendrimer, the inner block in the arm is lipophilic poly(ε-caprolactone) (PCL), and the outer block in the arm is hydrophilic poly(ethylene glycol) (PEG). The star-PCL polymer was synthesized first by ring-opening polymerization of ε-caprolactone with a PAMAM-OH dendrimer as initiator. The PEG polymer was then attached to the PCL terminus by an ester-forming reaction. Characterization with SEC, ¹H NMR, FTIR, TGA, and DSC confirmed the star structure of the polymers. The micelle formation of the star copolymer (star-PCL−PEG) was studied by fluorescence spectroscopy. Hydrophobic dyes and drugs can be encapsulated in the micelles. A loading capacity of up to 22% (w/w) was achieved with etoposide, a hydrophobic anticancer drug. A cytotoxicity assay demonstrated that the star-PCL−PEG copolymer is nontoxic in cell culture. This type of block copolymer can be used as a drug delivery carrier.

We probe the reversible thermal solidification of crowded solutions of colloidal star polymers in a marginal solvent upon addition of linear homopolymer chains consisting of the same monomers, and for two different linear polymer molecular weights we find that they have a dramatic impact on this phenomenon. If the linear chain molecular weight is much smaller than the star arm molecular weight, penetration is favored, the stars swell (the linear polymer solution acts as a good solvent for the stars), and the jamming temperature drops for a given star concentration. On the other hand, if the linear chain molecular weight exceeds that of the star arm, the stars shrink and the mixture exhibits liquidlike behavior over a wide range of star concentrations and temperatures. These results offer new possibilities for molecular control of the flow properties of soft colloids.

Alternating copolymers of 9,9-dioctylfluorene and oxadiazole have been prepared by the tetrazole route or the Suzuki coupling reaction. In the polymers the oxadiazole units were evenly dispersed in the main chain at every one, P(F1-alt-Ox), three, P(F3-alt-Ox), or four, P(F4-alt-Ox), fluorene units. Another copolymer with an asymmetric repeat unit structure, P(F3−Ox−F1−Ox) has also been prepared for comparison. In this study, the tetrazole route has been demonstrated to offer several advantages for preparing polyoxadiazole with well-defined structures compared to other oxadiazole ring formation reactions. These advantages include:  clean and fast reactions, mild reaction conditions, high yields, and high molecular masses of product. The glass transition temperature of copolymers ranged from 98 to 150 °C, and the copolymers show high thermal stability with decomposition temperatures around 430 °C. The UV−vis absorption and photoluminescence properties of all the copolymers in solutions are similar to those of poly(9,9-dioctylfluorene). All copolymers fluoresce in the blue-light range with quantum yields of ~70% in CH2Cl2 solution.

Chromatographic retention of linear and ring polystyrene was investigated near the chromatographic critical condition. Reversed phase silica columns of four different pore sizes were employed to examine the pore size dependence. Adjusting the column temperature, the critical condition for linear polystyrene was searched for each column with a mixed mobile phase of CH2Cl2/CH3CN (57/43 v/v). It was practically impossible to establish an unambiguous critical condition with a single pore size column for a wide molecular weight range of polystyrenes, in particular with narrow pore size columns. At the best available condition, retentions of nine different molecular weight ring polystyrenes were measured relative to their linear precursors for each pore-sized column. As predicted theoretically, the partition coefficient (K) of ring polymers vs the size ratio of polymer chain to pore (R/d) shows a good linear relationship in the large pore regime (R/d ≪ 1). This linearity is found to be universal for all the pore sizes, which is consistent with the theoretical prediction. However, the K vs R/d dependency at the large pore regime did not follow the theoretical prediction quantitatively. In the narrow-pore regime (R/d ≫ 1) the experimental results did not follow the theory for ideal-chain ring macromolecules even qualitatively. To explain the observed chromatographic behavior at R/d ≫ 1, the scaling theory accounting for the polymer excluded volume was used, and the definition of the critical condition was revised. This analysis gave some keys for understanding the results at R/d ≫ 1 and revealed the possible nonequivalence of the conditions for theory and experiment as the most probable reason for observed discrepancies.

The segmental relaxation properties of a low molecular weight (4.6 kg/mol), cyclic polystyrene (PS) were characterized. The sample was obtained by fractionation using HPLC at the chromatographic critical condition, which yields a ring uncontaminated by its linear precursor. Both the glass temperature and the temperature dependence of the segmental relaxation times for the ring PS were equivalent to the high molecular weight limiting values for the linear polymer. These results are interpreted by considering the configurational mobility of a polymer lacking chain ends.

Crowded solutions of densely branched polymers with starlike architecture undergo a reversible gelation upon heating. This phenomenon is characterized by slow kinetics and is attributed to the formation of clusters causing a partial dynamic arrest of the swollen interpenetrating spheres at high temperatures. A kinetic pseudo-phase diagram of gelation temperature against the effective volume fraction is constructed. Stars with different functionalities exhibit a different sol−gel boundary; small differences in the internal structure of the stars (regular with spherical dense core vs irregular with nonspherical core but spherical overall shape) are presumably responsible for these differences. Thermal gelation is proposed as an alternative route to jamming of soft materials.

A supramolecular assembly with eight peripheral ruthenium(II) tris(bipyridine), [Ru(bpy)3]²⁺, units covalently linked to a carbosilane dendrimer platform has been synthesized. ¹H NMR and MALDI-TOF mass spectrometry confirm the target structure. Spectroscopic and electrochemical studies disclose that the identical [Ru(bpy)3]²⁺ units in this system interact neither in the ground state nor in the excited state. In acetonitrile solutions of identical molar concentration of [Ru(bpy)3]²⁺ units, both the dendrimer and reference monomeric [Ru(bpy)2(4-octoxy-2,2‘-bipyridine)]²⁺ exhibit identical absorption and emission spectra. Cyclic voltammetry reveals that the dendrimer and monomer possess the same redox potentials of the metal-centered oxidation process [Ru(bpy)3]^2+/3+ and the first ligand-centered reduction process [Ru(bpy)3]^1+/2+. The abnormal redox peaks of the [Ru(bpy)3]^0/1+ and [Ru(bpy)3]^1-/0 transitions of the dendrimer are attributed to the accumulation of neutral dendrimer on the electrode. A preliminary study of the electrochemiluminescence (ECL) in tripropylamine−acetonitrile solution indicates that the ECL intensity of the dendrimer with eight [Ru(bpy)3]²⁺ units is 5 times higher than that of the reference monomeric species. Therefore, the molar emission, generated either by photoexcitation or by electrochemical excitation, can be amplified by incorporating multiple luminophores into a multibranch platform without significantly changing the redox and photophysical properties. The possible use of supramolecular assemblies as labels for biodiagnostics is discussed.

A new regioregular polythiophene derivative bearing side chains with exactly six oxyethylene units has been synthesized. From the thermochromic and ionochromic behavior of this polymer, it is concluded that the hexa(oxyethylene) side chains induce a more planar conformation of the polythiophene backbone than random-length oxyethylene side chains with on average seven oxyethylene units. It is further observed that the ionochromic transition from the planar to the nonplanar conformation is [K⁺]² dependent, suggesting that solvation of K ions by adjacent side chains is required. The polymer is ionically as well as electronically conductive. The ionic conductivity is only measurable at 80 °C, i.e., above Tg and the order−disorder temperature of the polymer.

Several amorphous narrow molecular weight fractions of poly(aryl ether ether ketone) or PEEK have been heated from below their glass transition temperature to above their final melting point. The semicrystalline morphology induced by cold crystallization and its thermal evolution are studied by time-resolved simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction, and differential scanning calorimetry. The long period and the crystalline and the amorphous thicknesses are computed from the correlation function of the SAXS curves. The evolution of the structural parameters of the various semicrystalline samples can be reconciled with the existence of a single distribution of lamellae which undergoes reorganization during the continuous heating. The reorganization process as well as the semicrystalline morphology depends on the molecular weight of the fractions. The amorphous thickness strongly increases with the molecular weight while the thermal evolution of the thickness of the crystalline layers is similar for all fractions. These experimental results support the view that the larger degree of entanglements of the high molecular weight samples impedes the reorganization mechanism with the consequence that the apparent melting temperature decreases with increasing average molecular weight.

A series of 4-arm, 8-arm, and 16-arm poly(ethylene oxide)s (PEO) have been synthesized from hydroxy functionalized carbosilane dendrimers of generation zero, one, and two, respectively. The PEO arms are grown anionically from the multifunctional cores. The polymers have narrow molecular weight distributions. Analysis of the molecular weight, intrinsic viscosity, and translational diffusion coefficient in methanol confirms the star structure of the polymers. The aqueous solutions of the star PEOs appear normal. Low molecular weight star polymers, however, show abnormally low intrinsic viscosities and are adsorbed on the size exclusion column hydrogel material.

We probe the structure−dynamics relationship in melts of star polymers with high functionality using small-angle X-ray scattering, rheology, and Monte Carlo computer simulations. These materials represent model steric soft spheres, which order in a liquidlike structure on a macromolecular scale because of the impenetrability of the cores. This is a consequence of the complex star topology, which is characterized by an inhomogeneous intrastar monomer density distribution. The signature of the structure appeares in the dynamics of these systems as a slow terminal relaxation process, in addition to the classical faster arm retraction mechanism. The slow process is attributed to the structural rearrangements of the stars within their liquidlike order and can be thought of as a reflection of their colloidal nature. We identify the dependence of this dynamic mode on the molecular parameters, i.e., star functionality and arm size.

The morphology of several semicrystalline poly(aryl ether ether ketone) (PEEK) samples with narrow molecular weight distributions and of a commercial sample of PEEK has been investigated by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). The use of different molecular weights allows us to enlarge the macroscopic crystallinity range usually attainable for the commercial PEEK grades. WAXD and DSC data are in good agreement to estimate the weight degree of crystallinity values ranging from ~15% for the highest molecular weight sample to ~55% for the lowest molecular weight sample. Most of the investigated samples exhibit the well-known double melting behavior with a low and a high melting endotherm. This behavior is interpreted in the light of a melting−recrystallization mechanism which is strongly affected by the presence of entanglements in the amorphous zones. The analysis of the SAXS data shows that, under constant annealing conditions, the crystal thickness (given by the smallest length obtained from the correlation function) remains unaffected by a change of the molecular weight while the amorphous layer is found to strongly increase with increasing chain length due to the presence of entanglements. This leads to a decrease in the linear degree of crystallinity within the lamellar stacks which is consistent with the behavior of the macroscopic degree of crystallinity obtained from WAXD, DSC, and density measurements.

The apparent thermorheological simplicity of disordered diblocks is reconciled with the failure of time−temperature superposition of corresponding miscible blends by examining the relaxation of the constituent blocks using rheo-optical techniques. Diblocks of 1,4-polyisoprene (PIP) and polyvinylethylene (PVE) are examined over a range of temperatures for two compositions (φPIP = 0.25 and 0.75). Unlike blends of PIP and PVE, the block copolymers appear to obey time−temperature superposition on the basis of their viscoelastic properties. However, departure from thermorheological simplicity is exposed in their stress−optical behavior. In particular, the copolymer rich in the high Tg component (φPIP = 0.25) shows distinct temperature dependencies for the individual blocks, in accord with the behavior of PIP/PVE blends. The block copolymer rich in the low Tg component (φPIP = 0.75) is thermorheologically simple because both blocks have similar monomeric friction coefficients ζo,PVE ≈ ζo,PIP, again in accord with prior results on PIP/PVE blends. The failure of time−temperature superposition in these diblocks was not previously observed because the change in ζo,PVE/ζo,PIP with temperature produces subtle changes in the overall relaxation spectrum relative to a linear chain of uniform friction.

The dimensionless ratio σG, expressing quantitatively the effects of heterointeractions between unlike units on the conformational properties of copolymers, has been determined experimentally by intrinsic viscosity analysis for the A2B and A3B miktoarm stars in various solvent conditions. In the case of symmetric samples (where the lengths of A and B arms are the same) in a common good solvent, the values of the required Flory parameter Φ for homopolymer stars with three and four arms were used. For the other cases and solvent conditions, Φ was determined by extended Monte Carlo calculations which provide a lower bound of this quantity. The experimental values of σG are compared with the results obtained by Monte Carlo simulations and recent renormalization group predictions.

Multiarm star polymers represent a valuable model system for investigating the dynamics of tethered chains, spherical brushes, or grafted colloidal spheres. Because of their topology, the multiarm stars exhibit a nonuniform monomer density distribution leading to a core−shell morphology, which is responsible for their rich dynamic structure. When the stars interpenetrate, they exhibit liquidlike (macrocrystalline) order due to the enhanced osmotic pressure which balances the entropic stretching of the near-core segments and the excluded volume effects. Using dynamic light scattering, we probe three relaxation modes in the semidilute regime:  (i) the fast cooperative diffusion, which is characteristic of their polymeric nature (entangled shell arms); (ii) the self-diffusion of the stars (essentially cores), probed because of finite functionality polydispersity, as confirmed by independent pulsed-field gradient NMR measurements; and (iii) the structural mode, which corresponds to rearrangements of the ordered stars. We develop a mean-field scaling theory, which captures all features observed experimentally with good quantitative agreement. The two slow modes, ii and iii, are reminiscent of the behavior of interacting hard colloidal spheres and are governed essentially by the same physics. We propose these model soft spheres as appropriate vehicles for unifying the descriptions of the dynamics of polymers and soft colloidal dispersions.

The linear viscoelastic properties of a series of RADEL R polyethersulfone fractions have been studied. The molecular weight of the fractions ranged from 2.4 X 103to 7.2 X 104. The RADEL R polyethersulfone is a thermorheologically complex material. After correction to iso-free volume, it has been found that 1/0∝M3.4,win agreement with the relation expected for entangled flexible polymers. Below Mw= 10 000, 1/0∝ M2.0w. From integration under the loss peak of high molecular weight fractions it is found that the plateau modulus G°N=3.6x 107. It is calculated that at 282°C the molecular weight between entanglements is Me=1550. The high plateau modulus is related to the molecular parameters of RADEL R polyethersulfone. Both the Graessley-Edwards model and the Lin model are discussed for a number of polymers that contain phenylene groups in the main chain.

A series of aromatic polyethersulfones based on biphenol and dichlorodi phenylsulfone have been prepared and fractionated. The fractions have been character ized by light scattering and intrinsic viscosity measurements. The characteristic ratio C∝= 1.96 which is in agreement with the freely rotating nature of the polyethersulfone chain. The molecular weight dependence of the glass transition temperature is established. The low temperature β-transition is at -113°C when measured at 1 rad/s with a mechanical spectrometer.

The large scale architecture of polymers is recognized to create special properties in functional polymers. One of the more striking examples is the ability of certain diblock copolymers to associate into micelles in a selective solvent for one of the blocks.(1) The insoluble block forms a dense core that is kept in solution by a soluble corona. The diblock micelle has the overall conformation of a star polymer.(2,3) However, the self-association of block copolymers into micelles depends strongly on the chemical nature of the two blocks, on the respective sizes of the blocks and on the selective solvent. In general, it is observed that the molecular weight of the insoluble block must be kept low in order to form micelles. This is especially true when water or a polar solvent is used as the selective good solvent.(1,4) Otherwise, a cloudy unstable two phase system is produced. The association number of diblocks in micelles varies considerably and makes their systematic study difficult. The association number is known to depend on the preparation method and on the presence of traces of a cosolvent.(4) Star block copolymers whose structure is fixed by covalent bonds are obvious models for diblock micelles.(5,6) In this paper we describe the synthesis and some properties of umbrella-star block copolymers. This architecture allows us to influence the chain density in the corona and to study its effect on the solution properties of the polymers. These polymers are direct models for micelles prepared from umbrella polymers(7) or mixed arm star polymers.(8)

Barriers to rotation have been calculated by molecular mechanics, using the force field MM+, for a fragment of PEEK and its methyl, ethyl, isopropyl, tert-butyl and di-tert-butyl, substituted derivatives. The barrier for the nearer C—O bond increases from 2.0 kcal/mol in KEEK to 16 kcal/mol in dibutyl substituted KEEK as the bulkiness of the substituent increases. The rotation barrier of the further C—O bond is not affected by the substituent. Keywords: rotation barriers, PEEK, molecular mechanics.

The propagation reaction in the anionic polymerization of styrene has been studied in cyclohexane with the counter-ions: K, Rb, and Cs. Some association of the ion-pairs was found with K. Absolute propagation rate constants were determined in the three cases. Some experiments were carried out in benzene–cyclohexane mixtures to check that the results were consistent with those previously observed in benzene.