HyperMacs - long chain hyperbranched polymers: A dramatically improved synthesis and qualitative rheological analysis

We have previously reported a novel (albeit modestly successful) strategy for the synthesis of polystyrene HyperMacs - long chain branched analogues of hyperbranched polymers. The building blocks for HyperMacs, AB₂ macromonomers are synthesized by living anionic polymerization and as such are well-defined in terms of molecular weight and polydispersity but the nature of the coupling reaction used to generate the highly branched HyperMacs results in branched polymers with a distribution of molecular weights and architectures. In our previously reported studies the extent of the coupling reaction was significantly hampered by side reactions, however, we report here dramatic improvements to the coupling chemistry which overcome the previously experienced limitations resulting in a fourfold increase in the extent of the coupling reactions. Furthermore we report the effect of the addition of varying amounts of a B₃ core molecule to the coupling reaction and the resultant ‘control’ of the final molecular weight of the HyperMac. Melt rheology showed polystyrene HyperMacs to be thermorheologically simple, obeying William-Landel-Ferry (WLF) behaviour. HyperMacs showed little evidence for relaxation by reptation and when the molecular weight of the macromonomer was ?M_e for polystyrene, HyperMacs resemble unentangled polymers below the gel point, despite being well above the entanglement molecular weight for linear polystyrene. Increasing the molecular weight of the macromonomer to substantially above M_e seems to introduce some entangled nature to the HyperMac as evidenced by the emergence of a near horizontal plateau in G″ - the loss modulus.     

Extent of branching from linear viscoelasticity of long‐chain‐branched polymers

We present new results and examine literature data concerning the linear viscoelastic behavior of polyethylene with sparse to intermediate levels of long‐chain branching (LCB). These branched polymers displayed a common rheological signature, namely, a region of frequency‐independent loss tangent along with the corequisite scaling of the storage and loss moduli to the same frequency exponent. This apparent power‐law response occurred within a finite frequency window and bore resemblance to the behavior of physical gels. The appearance of this region, however, was the consequence of the presence of two distinct, yet partially overlapping, terminal relaxation processes. After considering the analogous relaxation behavior of wholly linear polymers with bimodal molecular weight distributions, we considered the polymers with LCB as blends of linear and branched species to develop a simple method of quantifying the extent of LCB. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1671–1684, 2004     

Facile synthesis and characterization of hyperbranched poly(ether amide)s generated from Michael addition polymerization of in situ created AB2 monomers

A new straightforward strategy for synthesis of novel hyperbranched poly (ether amide)s from readily available monomers has been developed. By optimizing the reaction conditions, the AB₂‐type monomers were formed dominantly during the initial reaction stage. Without any purification, the AB₂ intermediate was subjected to further polymerization in the presence (or absence) of an initiator, to prepare the hyperbranched polymer‐bearing multihydroxyl end‐groups. The influence of monomer, initiator, and solvent on polymerization and the molecular weight (MW) of the resultant polymers was studied thoroughly. The MALDI–TOF MS of the polymers indicated that the polymerization proceeded in the proposed way. Analyses of ¹H NMR and ¹³C NMR spectra revealed the branched structures of the polymers obtained. These polymers exhibit high‐moderate MWs and broad MW distributions determined by gel permeation chromatography (GPC) in combination with triple detectors, including refractive index, light scattering, and viscosity detectors. In addition, the examination of the solution behavior of these polymers showed that the values of intrinsic viscosity [η] and the Mark–Houwink exponent α were remarkably lower compared with their linear analogs, because of their branched nature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4309–4321, 2007     

The thermal polymerization of amino acids revisited; Synthesis and structural characterization of hyperbranched polymers from L‐lysine

This contribution reports on the synthesis of hyperbranched polylysines via thermal polymerization of L ‐lysine hydrochloride. Polymerization of L ‐lysine hydrochloride in the presence of one equivalent KOH at 150 °C resulted in polymers with a number‐average molecular weight of 4600 g/mol and a polydispersity of 2.6 after 48 h. The rate of polymerization could be significantly enhanced and the polymer molecular weight improved by carrying out the polymerization with 3 mol % of an amidation catalyst. Among the different catalysts that were investigated Zr(OⁿBu)₄ was found to be the most effective. Unequivocal support for the branched architecture of the polymers was obtained from ¹H NMR spectroscopy, which allowed the identification and quantification of the four different structural units that constitute the polymer, viz. N^α and N^ε linked linear units, dendritic units and terminal units. The structure of the polymers was found to be relatively independent of the reaction conditions. The degree of branching and the average number of branches varied between 0.35–0.45 and 0.15–0.25, respectively. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5494–5508, 2007     

Relationship between the α‐ and β‐relaxation processes in amorphous polymers: Insight from atomistic molecular dynamics simulations of 1,4‐polybutadiene melts and blends

Amorphous polymers exhibit a primary (glass, or α‐) relaxation process and a low‐temperature relaxation process associated with polymer backbone motion usually referred to as the β‐relaxation process. The latter process can be observed below the glass transition temperature of the polymer and usually merges with the α‐relaxation process at temperatures somewhat above the glass transition temperature. While it is widely held that both the α‐relaxation and β‐relaxation processes are engendered by localized (segmental) motions of the polymer backbone, and that there is a strong mechanistic connection between them, the molecular mechanisms of the α‐relaxation and β‐relaxation processes in amorphous polymers are not well understood. Recently, atomistic molecular dynamics simulations of melts and blends of 1,4‐polybutadiene have provided insight into the relationship between the α‐ and β‐relaxation processes in glass‐forming polymers and an improved understanding of their molecular origins. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 627–643, 2007     

The linear rheological responses of wedge‐type polymers

The linear rheological responses of a series of specially designed wedge‐type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, T_g. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at T_g ?60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 899–906     

HyperMacs. Long Chain Branched Analogues of Hyperbranched Polymers Prepared by the Polycondensation of AB2 Macromonomers

We describe here a new strategy for the synthesis of polymers with highly branched architectures. The strategy involves the synthesis by anionic polymerization of well-defined AB₂ polystyrene macromonomers with molecular weights from 3,600 to 94,000 gmol⁻¹, which are then converted via a one-pot polycondensation reaction into high molecular weight, long-chain (hyper)branched architectures. Since the Hyperbranched structures are built up from condensation Macromonomers we have coined the term ‘HyperMac’ to describe these branched polymers. In this paper we report the synthesis of the HyperMacs, the optimal conditions for the polycondensation reaction and some preliminary characterization studies.     

Fibrillar structure of self‐assemblies formed from heterocomplementary monomers linked through sextuple hydrogen‐bonding arrays

The nanostructure of the fibrillar supramolecular aggregates generated in decane solutions of homoditopic heterocomplementary monomers forming sextuple hydrogen‐bond‐mediated self‐assemblies was investigated by small‐angle neutron scattering and cryogenic‐temperature transmission electron microscopy. The persistence length (L_p) of the fibrillar aggregates was found to be ～18 nm, as inferred from combined measurements of the radius of gyration and of the contour length. The values of both the weight‐average molecular weight and the mass per unit length of the fibers suggest that the latter consist of few aggregated monomolecular wires. At T = 25 °C, the formation of branched aggregates occurs around the crossover concentration, C*, between the dilute and semidilute regimes, whereas the classical behavior of equilibrium polymers is observed at T = 65 °C. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 103–115, 2007     

Nonlinear viscoelastic behavior of styrene‐[ethylene‐(ethylene‐propylene)]‐styrene block copolymer

Studies on the nonlinear viscoelastic behavior of styrene‐[ethylene‐(ethylene‐propylene)]‐styrene block copolymer (SEEPS) were carried out. The nonlinear viscoelastic region was determined through dynamic strain sweep test, and the critical shear strain (γ_c) of transition from linear viscoelastic region to nonlinear viscoealstic region was obtained. The relaxation time and modulus corresponding to the characteristic relaxation modes were also acquired through simulating the linear relaxation modulus curves using Maxwell model, and the damping functions were evaluated. Meanwhile, it is found that the nonlinear relaxation modulus obtained at relatively low shear strains follows the strain–time separation principle, and the damping function of SEEPS can be fit to Laun double exponential model well. Moreover, the successive start‐up of shear behavior, the steady shear behavior, and the relaxation behavior after steady shear were investigated, respectively. The results showed that Wagner model, derived from the K‐BKZ (Kearsley‐Bernstein, Kearsley, Zapas) constitutive equation, could simulate the experiment data well, and in addition, experiment data under the lower shear rates are almost identical with the fitting data, but there exists some deviation for data under considerable high shear rates. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1309–1319, 2006     

Polymers from a levopimaric acid–acrylic acid Diels–Alder adduct: Synthesis and characterization

The Diels–Alder adduct of levopimaric acid with acrylic acid was efficiently prepared from resin acids. When the adduct was subjected to a dehydrodecarboxylation reaction, a ketone diacid derivative was obtained. New ketone type linear polymers were synthesized by the advanced dehyrodecarboxylation, a nonconventional polycondensation reaction, of both the above in presence of sulfonic catalysts. The polyketones turned out to be excellent tackifiers in adhesive formulae. The ketone polymers were condensed with diamines to give crosslinked polyazomethines. The structures of the monomers and polymers were established by means of elemental analysis, IR and NMR spectroscopy, and molecular weight determinations. Both the polyketones and polyazomethines were low‐molecular‐weight polymers, soluble in some polar and nonpolar solvents. The thermal behavior of the monomers and polymers was evaluated by thermogravimetric analysis. The thermal studies showed that the polymers were substances with good thermal stability, except the polyazomethine synthesized by the condensation of polyketone with an aromatic diamine, which appeared to be a substance with high thermal stability. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5979–5990, 2007     

Ring‐opening polymerization of lactones using zirconocene catalytic systems: Block copolymerization with methyl methacrylate

The ring‐opening polymerization of ε‐caprolactone (ε‐CL) and δ‐valerolactone (δ‐VL) using nine catalytic systems consisting of a combination of three C_2v zirconocene complexes and three borate cocatalysts is discussed. The polymerizations proceed in a well‐controlled manner, producing polymers with relatively high molecular weights and narrow molecular weight distributions. Kinetic experiments of the polymerization of ε‐CL with the catalytic system Cp₂ZrMe₂/B(C₆F₅)₃ (1) showed a linear dependence between polymerization yield and molecular weight with time, as well as between the molecular weight with the molar ratio of the monomer over the catalyst [ε‐CL]/[Zr], indicating sufficient control of the polymerization reaction. The catalytic system (1) was utilized for the synthesis of well‐defined block copolymers of MMA with ε‐CL and δ‐VL. All samples were characterized by size exclusion chromatography, nuclear magnetic resonance, and differential scanning calorimetry. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3524–3537, 2007     

Pressure relaxation of polystyrene and its comparison to the shear response

Isothermal pressure relaxation as a function of temperature in two pressure ranges has been measured for polystyrene using a self-built pressurizable dilatometer. A master curve for pressure relaxation in each pressure regime is obtained based on the time–temperature superposition principle, and time–pressure superposition of the two master curves is found to be applicable when the master curves are referenced to their pressure-dependent T_g. The pressure relaxation master curves, the shift factors, and retardation spectra obtained from these curves are compared with those obtained from shear creep compliance measurements for the same material. The shift factors for the bulk and shear responses have the same temperature dependence, and the retardation spectra overlap at short times. Our results suggest that the bulk and shear response have similar molecular origin, but that long-time chain mechanisms available to shear are lost in the bulk response. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3375–3385, 2007     

Melt rheology of hyperbranched‐polystyrene synthesized with multisite macromonomer

Melt rheological behaviors of hyperbranched‐polystyrene (PS) copolymerized by dendric macromonomer technique are presented. The time–temperature superposition principle was applicable to the hyperbranched‐PS. The branched‐PS showed slightly lower zero‐shear viscosity in comparison with linear PS regardless of a presence of a number of branches expected from the dendric macromonomer technique. Although the influence of use of multimethacryloyl macromonomer in the polymerization process was marginal for linear viscoelastic regime, nonlinear shear and uniaxial elongational flows showed distinct differences between linear and branched‐PS. The strain dependence of the damping function became weak as increase of macromonomer content. The branched‐PS exhibited the growing elongational viscosity function comparing with linear PS. This prominent effect on the elongational flow behavior can be explained by the molecular architecture of the branched‐PS. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2226–2237, 2009     

Monte Carlo simulation of size exclusion chromatography for randomly branched and crosslinked polymers

The elution curves of size exclusion chromatography for nonlinear polymers formed through random branching and crosslinking of long polymer chains were simulated with a Monte Carlo method. We considered two types of measured molecular weight distributions (MWDs): (1) the MWD calibrated relative to standard linear polymers and (2) the MWD obtained with a light scattering (LS) photometer in which the weight‐average molecular weight of polymers within the elution volume is determined directly. The calibrated MWDs clearly underestimate the molecular weights for both randomly branched and crosslinked polymers, and this technique can be used to assess the degree of deviation from the true MWD. When the primary chains conform to the most probable distribution, the calibrated MWD can be estimated reasonably well with the Zimm–Stockmayer equation for the g factor with the help of the relationship between the average number of branch points per molecule and the degree of polymerization. However, the LS method gives good estimates of the true MWD for both randomly branched and crosslinked polymers, although the agreement is better for the branched ones. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2009–2018, 2000     

Synthesis and polymerization of first‐generation dendritic methacrylate macromonomers

First‐generation dendritic macromonomers with a methacryloyl end group on one side, long alkyl chains on the other side, and a biuret system with two urethane groups in the core have been synthesized. The synthesis comprises three steps with hexamethylene diisocyanate uretdione as the starting material. The branching points were introduced via biuret groups and the prepared macromonomers were polymerized by free and controlled radical polymerization. Depending on the reaction conditions linear dendronized polymers as well as branched dendronized polymers and microgels with long alkyl chains were obtained. Scanning force microscopy was used to visualize high molecular weight molecules spincoated on highly oriented pyrolytic graphite. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 614–628, 2007     

Viscoelastic properties of decrosslinked irradiation‐crosslinked polyethylenes in supercritical methanol

The viscoelastic properties of decrosslinked irradiation‐crosslinked polyethylenes using a supercritical methanol were investigated via oscillatory dynamic shear measurements. Decrosslinked polymers at a low reaction temperature exhibited solid‐like rheological properties, as evidenced by a small slope at G′ and G″, a long relaxation time, slow stress relaxation behavior, and considerable yield stress. In contrast, decrosslinked polymers at a high temperature exhibited liquid‐like rheological properties that included a large slope in G′ and G″, a short relaxation time, fast stress relaxation behavior, and nonyield stress. The difference in the viscoelastic properties of the decrosslinked polyethylenes was attributed to the difference in the gel content with the reaction temperature. A higher gel content induced stronger solid‐like viscoelastic properties. Hence, the rheological measurements were useful for analyzing the molecular structure of decrosslinked polymers using a supercritical fluid. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1265–1270, 2010     

Effects of hydroxyl‐group end capping and branching on the physical properties of tailored polyethylene terephthalates

Polyethylene terephthalates (PETs) with well‐defined chemical structures were prepared by molecular design, and the effect of the chemical structure on the physical properties of PET was investigated. Hydroxyl‐group end‐capped PETs with η_inh = 0.4–0.6 dL/g exhibited a viscosity behavior similar to Bingham fluids, although other PETs with similar molecular weights (MWs) showed Newtonian flow behavior. This rheological feature was more noticeable for hydroxyl‐group end‐capped branched PETs. In addition, hydroxyl‐group end‐capped branched PETs became solidlike from 80 rad/s as the frequency was increased. On the other hand, hydroxyl end‐capped linear PETs showed a noticeable viscoelastic transition peak around 20 rad/s. High MW linear and branched PETs with η_inh ≥ 0.9 prepared by multistep synthesis showed non‐Newtonian fluid behavior. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1027–1035, 2001     

Synthesis and characterization of hyperbranched polystyrene via click reaction of AB2 macromonomer

Well‐defined hyperbranched polystyrenes have been successfully prepared by polymerization of AB₂ macromonomer, polystyrene containing an azide group at its one end and two terminal propargyl groups at the other end via click reaction. For preparation of AB₂ macromonomers, an ATRP initiator, bispropargyl 2‐bromosuccinate (BPBS) with two propargyl groups and one bromine group was synthesized by the successive bromination and esterification reaction of L ‐aspartic acid. The resulting BPBS initiated the ATRP of St, and subsequently, the terminal bromine groups of (CH≡C)₂‐PS‐Brs were substituted by N₃ via the reaction with sodium azide resulting the AB₂ macromonomer, (CH≡C)₂‐PS‐N₃ with various molecular weights. All intermediates and the resultant polymers were characterized by GPC, ¹H NMR, FTIR, and MALLS methods. The polymerization kinetics study showed fast increase of DP at the initial stage of polymerization and then slow increase of their DP. The final “HyperMacs” have high‐molecular weight up to M_w,MALLS = 340,000 g/mol, their molecular weight distributions were moderately narrow (M_w/M_n = 1.47–1.65). The ratios of [η]_H/[η]_L of the HyperMacs formed in the polymerization system increased with evolution of polymerization. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 454–462, 2010     

Ring‐opening metathesis polymerization of functionalized cyclooctene by a ruthenium‐based catalyst in ionic liquid

This paper describes the synthesis of a novel monomer of 5‐substituted cyclooctene with the pendant of imidazolium salt (7) and the ring‐opening metathesis polymerization of the functionalized cyclooctenes ( 4 and 7 ) in CH₂Cl₂ and ionic liquid [bmim][PF₆] by a ruthenium‐based catalyst RuCl₂(PCy₃)(SIMes)(CHPh) (2). The polymerization, which was carried out in ionic liquid, afforded improved control over the molecular weight (M_n) and polydispersity of the resultant products (PDI <1.4). Furthermore, to facilitate the GPC measurement for molecular weight of polymers, the charged polymers (poly‐ 7 ) were hydrolyzed to give uncharged polymers (poly‐ 4 *) by removing the imidazolium pendant from the polymer chains. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3986–3993, 2007     

Electrospinning of ultrahigh‐molecular‐weight polyethylene nanofibers

The electrospinning method has been employed to fabricate ultrafine nanofibers of ultrahigh‐molecular‐weight polyethylene for the first time with a mixture of solvents of different dielectric constants and conductivities. The possibility of producing highly oriented nanofibers from ultrahigh‐molecular‐weight polymers suggests new ways of fabricating ultrastrong, porous, and single‐component nanocomposite fibers with improved properties. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 766–773, 2007