Arm Molecular Weight Research Articles

Fast and Scalable Synthetic Route to Densely Grafted, Branched Polystyrenes and Polydienes via Anionic Polymerization Utilizing P2VP as Branching Point.

Defined, branched polymer architectures with low dispersity and architectural purity are of great interest to polymer science but are challenging to synthesize. Besides star and comb, especially the pom-pom topology is of interest as it is the simplest topology with exactly two branching points. Most synthetic approaches to a pom-pom topology reported a lack of full control and variability over one of the three topological parameters, the backbone or arm molecular weight and arm number. A new, elegant, fast, and scalable synthetic route without the need for post-polymerization modification (PPM) or purification steps during the synthesis to a pom-pom and a broad variety of topologies made from styrene and dienes is reported, with potential application to barbwire, bottlebrush, miktoarm star, Janus type polymers, or multi-graft copolymers. The key is to inset short poly(2-vinyl-pyridine) blocks (<2 mol% in the branched product) into the backbone as branching points. Carb anions can react at the C6 carbon of the pyridine ring, grafting the arms onto the backbone. Since the synthetic route to polystyrene pom-poms has only two steps and is free of PPM or purification, large amounts of up to 300g of defined pom-pom structures can be synthesized in one batch.

Influence of side chain lengths in mechanophore‐containing polyisobutylene‐graft‐polystyrene

AbstractPolymer architecture plays an important role in morphology and mechanochemical response. Here, graft copolymers containing mechanophores at the backbone‐arm junction are synthesized through a combination of post‐polymerization modification of butyl rubber and controlled radical polymerization. A series of graft copolymers with 70:30 or 50:50 polyisobutylene:polystyrene (PIB:PS) volume fractions were synthesized by changing the grafting density and arm length to keep the volume fractions constant. The morphologies for these polymers are disordered but show a decreasing domain size with shorter arm lengths, and the samples with a greater number of PS side chains show greater domain mixing. Using pulsed ultrasound experiments, we found greater backbone scission occurs for polymers with a greater number of short grafts per chain than the long grafts with fewer grafting points. Finally, mechanophore activation is greater for samples with arm molecular weights ≥13 kg/mol, attributed to exceeding the cutoff molecular weight for PS.

Study of the phase-transition behavior of (AB)3 type star polystyrene-block-poly(n-butylacrylate) copolymers by the combination of rheology and SAXS

Abstract A series of three-armed star polystyrene-block-poly(n-butylacrylate) copolymers (PS-b-PBA)3 were synthesized to study the phase-transition behavior of the copolymers. The order-to-disorder transition temperature has been determined by oscillatory at different temperatures and dynamic temperature sweep at a fixed frequency. Moreover, the micro-phase separation in the block copolymers has been evaluated by time–temperature superposition, while the free volume and the active energy of the copolymers have been calculated. Interestingly, active energy decreased with the increase in the molecular weight of the PBA components. To further determine the order-to-disorder transition temperature precisely, small angle X-ray scattering was performed at different temperatures. These results confirm that the chain mobility of the star-shaped copolymers is strongly dependent on the arm molecular weight of the star polymers, which will be beneficial for the processing and material preparation of the block copolymers.

Molecular weight effect on the structural detail and chain characteristics of 33-armed star polystyrene

In this study, synchrotron X-ray scattering and quantitative data analysis were performed on a series of 33-armed polystyrenes with four different arm molecular weights in cyclohexane (CHX, Θ solvent) and tetrahydrofuran (THF, good solvent). The quantitative scattering analysis was successfully done, providing molecular structure details and their responses to the solvent change. Each star system is confirmed to be prepared in a very narrow unimodal size distribution; the distribution is slightly broadened with increasing the arm molecular weight. It has an oblate ellipsoid shape, which is composed of two phases, a denser and smaller core and a less dense and thicker fuzzy shell; the ellipsoidicity ranges in 0.56–0.62. The core and shell phases are enlarged and thickened respectively by the arm molecular weight increases. Both the phases are further expanded in THF. Interestingly, the THF-induced expansion is predominant in the core against the fuzzy shell with a Gaussian like density gradient. Overall, the 33-armed star polystyrenes in various molecular weights are quite unique in the structural details and performance in solvent changes, which would be beneficial for advanced applications in various technological areas including smart deliveries of desired molecules (drug, gene, imaging agent, etc.).

Miktoarm star polymers nanocarrier: synthesis, characterisation, and in-vitro drug release study

Conjugation of poly(ethylene glycol) (PEG) to poly(lactide-co-glycolide) (PLGA) renders the latter with enhanced biocompatibility and broader overall capability in biomedical application. Novel miktoarm star polymers comprising PLGA and PEG segments are of interest for their potential as drug carriers. Thus, a series of miktoarm star copolymers, PLGA-(mPEG)2, with different PLGA arm molecular weights and methoxy-PEG (mPEG) arm (2000 g/mol), were synthesised via a four-step reaction using carbodiimide chemistry with a low steric hindrance trifunctional linker aminoadipic acid (AAA) and characterised by proton nuclear magnetic resonance (1H NMR), fourier transform infrared (FTIR), gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). Results show that the miktoarm star polymers PLGA17.0-AAA(mPEG)2 and PLGA43.4-AAA(mPEG)2 formed stable nanoparticles and PLGA4.6-AAA(mPEG)2 self-assembled into stable nanomicelles. Fluorescence spectroscopy showed that the critical micelle concentration of PLGA4.6-AAA(mPEG)2 was very low at 6.03 × 10−7 g/mL. Model drug ibuprofen encapsulated nanoparticles and nanomicelles had good drug loading, high encapsulation efficiency, narrow size distribution, and spherical morphology with negative surface charges. The mean particle size increased with increasing PLGA molecular weights, from 37.28 ± 1.03 to 151.5 ± 0.86 nm. In-vitro release of model drug ibuprofen over 7 days from PLGA43.4-AAA(mPEG)2 nanoparticles (61.65 ± 3.04%) was higher than those of PLGA4.6-AAA(mPEG)2 nanomicelles (26.93 ± 1.49%) and PLGA17.0-AAA(mPEG)2 nanoparticles (10.57 ± 0.29%), with all demonstrating controlled release characteristics. In conclusion, the novel miktoarm star polymers PLGA43.4-AAA(mPEG)2 and PLGA17.0-AAA(mPEG)2 and their nanoparticles, and PLGA4.6-AAA(mPEG)2 and its nanomicelles have a great potential as a nanocarrier for controlled delivery of hydrophobic drugs.

Arm-First Synthesis of Star Polymers with Polywedge Arms Using Ring-Opening Metathesis Polymerization and Bifunctional Crosslinkers.

This work presents a two-step, one-pot process to make star polymers with polywedge arms. In a one-pot reaction, after the polywedge arms are synthesized, crosslinker species are added to the reaction, rapidly forming star polymers. Crosslinker species with different degrees of conformational freedom were designed and synthesized and their capacity to generate star polymers was evaluated. Mass conversions up to 92% and stars with up to 17 arms were synthesized with the most rigid crosslinker. The effects of arm molecular weight and molar ratio of crosslinker to arm on mass conversion and arms per star were explored further. Finally, the size-molecular weight scaling relationship for polywedges with linear and star architectures was compared, corroborating theoretical results regarding star polymers with arms much larger than their core.

Self-healing hydrogels formed by complexation between calcium ions and bisphosphonate-functionalized star-shaped polymers.

Star-shaped poly(ethylene glycol) (PEG) chain termini were functionalized with alendronate to create transient networks with reversible crosslinks upon addition of calcium ions. The gelation ability of alendronate-functionalized PEG was greatly dependent on the number of arms and arm molecular weight. After mixing polymer and calcium solutions, the formed hydrogels could be cut and then brought back together without any visible interface. After 2 minutes of contact, their connection was strong enough to allow for stretching without tearing through the previous fracture surface. Oscillatory rheology showed that the hydrogels recovered between 70 and 100% of the original storage and loss modulus after rupture. Frequency sweep measurements revealed a liquid-like behavior at lower frequencies and solid-like at high frequencies. Shifting frequency curves obtained at different calcium and polymer concentrations, all data collapsed in a single common master curve. This time-concentration superposition reveals a common relaxation mechanism intrinsically connected to the calcium-bisphosphonate complexation equilibrium.

Glassy Dynamics of Polymers with Star-Shaped Topologies: Roles of Molecular Functionality, Arm Length, and Film Thickness

Structural relaxations of a substance quenched to a temperature Tage below its glass transition temperature Tg enable the structure of the substance to approach equilibrium. This phenomenon, also known as physical aging, has been studied for many decades in bulk linear-chain polymer systems, where the aging rates are generally, to first order, independent of chain length. More recently, the phenomenon has been of keen interest in thin films, where the aging rate is shown to be film thickness H dependent, for films in the thickness range of nanometers to a few hundred nanometers. We show here, based on a study of polystyrene star-shaped polymers of a wide range of functionalities 2 ≤ f ≤ 64 and arm molecular weights Marm, that in the limit of sufficiently large values of Marm the aging behavior is similar to that of linear chains—independent of Marm and f. More importantly, in the limit of sufficiently small Marm and large f, the aging rate is independent of film thickness. Otherwise, the rate is a nonmono...

Amphiphilic Polymer Conetworks Based on Interconnected Hydrophobic Star Block Copolymers: Synthesis and Characterization

SummaryHerein we report on the preparation and structural characterization of six amphiphilic polymer conetworks (APCN) based on end‐linked amphiphilic “core‐first” star block copolymers, comprising methyl methacrylate and 2‐(dimethylamino)ethyl methacrylate as the monomer repeating units in the hydrophobic and hydrophilic blocks, respectively. The various APCNs differed either in the arm composition or in the arm molecular weight. APCN synthesis was accomplished via the one‐pot, sequential group transfer polymerization (GTP) of cross‐linker, hydrophobic monomer, hydrophilic monomer, and cross‐linker again. The soluble precursors to the APCNs, i.e., the star homopolymers, the star block copolymers and the initial cross‐linker cores, were characterized in terms of their composition, size and size dispersity. The degrees of swelling in tetrahydrofuran were found to increase with the molecular weight of the arms of the stars of the APCNs, whereas the degrees of swelling in water increased with the content in hydrophilic units in the arms of the constituting stars. Small‐angle neutron scattering (SANS) indicated that most APCNs nanophase separated in D2O. The structure and size of the hydrophobic domains determined by fitting the SANS data to an appropriate model could be correlated with the molecular architecture of the APCNs. Finally, polarized light microscopy showed that all APCNs were birefringent both in water and in the dried state.

Effect of chain architecture on the phase transition of star and cyclic poly(N-isopropylacrylamide) in water

The heat-induced phase transition of aqueous solutions of Poly(N-isopropylacrylamide) (PNIPAM) in water is examined for a four-arm PNIPAM star (s-PNIPAM), a cyclic PNIPAM (c-PNIPAM), and their linear counterparts (l-PNIPAM) in the case of polymers (1.0 g L−1) of 12,700 g mol−1 < Mn < 14,700 g mol−1. Investigations by turbidity, high-sensitivity differential scanning calorimetry (HS-DSC), and light scattering (LS) indicate that the polymer architecture has a strong effect on the cloud point (Tc: decrease for s-PNIPAM; increase for c-PNIPAM), the phase transition enthalpy change (ΔH decrease for s-PNIPAM and c-PNIPAM), and the hydrodynamic radius of the aggregates formed above Tc (RH: c-PNIPAM < s-PNIPAM < l-PNIPAM). The properties of s-PNIPAM are compared with those of previously reported PNIPAM star polymers (3 to 52 arms). The overall observations are described in terms of the arm molecular weight and the local chain density in the vicinity of the core of the star, by analogy with the model developed for PNIPAM brushes on nanoparticles or planar surfaces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 2059–2068.

The Role of Architecture in the Melt-State Self-Assembly of (Polystyrene)star-b-(Polyisoprene)linear-b-(Polystyrene)star Pom-Pom Triblock Copolymers

Using a unique one-pot convergent anionic polymerization strategy, 18 (polystyrene)star-b-(polyisoprene)linear-b-(polystyrene)star (SnISn) pom-pom triblock copolymers were synthesized varying a range of architectural parameters including PS arm molecular weight (Mn,star), the number of arms contained in the star (n), and the PI midblock molecular weight (Mn,PI). A selected series of five of these 18, in which Mn,star was held approximately constant between 14.3 and 16.5 kDa, but with the numbers of arms in the star and PI midblock molecular weight varied, were selected for detailed characterization using rheology, AFM, and SAXS. The five selected all shared PS as the minority component, with star volume fractions (fPS) varying between 0.11 and 0.22. All samples showed clear phase separation, with three of the five adopting a highly ordered hexagonal packing of cylinders (HPC) confirmed through SAXS and AFM. The remaining two systems were limited to liquid-like packing of cylindrical domains (LLP). Longer midblock molecular weights and increased numbers of arms in the star both showed a propensity to hinder formation of a highly ordered hexagonal lattice. Increasing the number of arms in the star also favored transitions to a disordered phase at lower temperatures when overall SnISn molecular weight was held constant. The behavioral trends identified suggest interfacial packing frustration plays a prominent role in determining the ability of the system to develop highly ordered periodic structures. The chain crowding produced by the PS star architecture intrinsically favors interfacial curvature toward the majority PI component, contrary to that intrinsically favored by the block composition alone. In the two systems in which the frustration was architecturally most severe (largest n of 7.1, highest Mn,PI of 191 kDa), evolution of a hexagonal lattice could not be induced, even after significant thermal annealing. The pom-pom architecture itself also appears to have a significant impact on entanglement relaxation dynamics, with development of HPC morphologies only possible at elevated temperatures.

Particle-size dependent melt viscosity behavior and the properties of three-arm star polystyrene-Fe3O4 composites.

The melt viscosity of three-arm star polystyrene (S3PS)-Fe(3)O(4) nanoparticle composites was studied by means of rheological measurements. The arm molecular weight (M(a)) of S3PS (or radius gyration) and the particle size of Fe(3)O(4) (radius (R(p)): 3 nm and 44 nm) showed a strong influence on the melt viscosity behavior (at low shear frequencies) of S3PS-Fe(3)O(4) composites. The reinforcement (viscosity increase) was observed in the composites where the M(a) was higher than the M(c) of PS (M(c): the critical molecular weight for chain entanglement). For M(a) < M(c), when the size of Fe(3)O(4) nanoparticles was changed, the melt viscosity of the composites exhibited either plasticization (melt viscosity reduction) or reinforcement. When the content of Fe(3)O(4) was low (1 wt%), the transformation from plasticization to reinforcement behavior could be observed, which strongly depended on the size ratio of the radius of gyration (R(g)) of S3PS to the size of nanoparticles (R(p)). In addition, the magnetic properties and thermal stability of S3PS-Fe(3)O(4) composites were studied.

Dielectric and Viscoelastic Behavior of Star-Branched Polyisoprene: Two Coarse-Grained Length Scales in Dynamic Tube Dilation

cis-Polyisoprene (PI) chain has the type A dipole parallel along the backbone so that its large-scale (global) motion results in not only viscoelastic but also dielectric relaxation. Utilizing this feature of PI, this paper examined dielectric and viscoelastic behavior of star PI probe chains (arm molecular weight 10–3Ma = 9.5–23.5, volume fraction υ1 = 0.1) blended in a matrix of long linear PI (M = 1.12 × 106). The constraint release (CR)/dynamic tube dilation (DTD) mechanism was quenched for those dilute probes entangled with the much longer matrix, as evidenced from coincidence of the frequency dependence of the dielectric and viscoelastic losses of the probe in the blend. Comparison of the probe data in the blend and in monodisperse bulk revealed that the star probe relaxation is retarded and broadened on blending and the retardation/broadening is enhanced exponentially with Ma. This result in turn demonstrates significant CR/DTD contribution to the dynamics of star PI in bulk. The magnitude of retardation was quantitatively analyzed within the context of the tube model, with the aid of the dielectrically evaluated survival fraction of the dilated tube, φ′(t), and the literature data of CR time, τCR. In the conventional molecular picture of partial-DTD, the tube is assumed to dilate laterally, but not coherently along the chain backbone. The corresponding lateral partial-DTD relationship between φ′(t) and the normalized viscoelastic relaxation function μ(t), μ(t) = φ′(t)/β(t) with β(t) being the number of entanglement segments per laterally dilated segment (that was evaluated from the φ′(t) and τCR data), held for the μ(t) and φ′(t) data of star PI in bulk. Nevertheless, the observed retardation of the star probe relaxation on blending was less significant compared to the retardation expected for the arm motion (retraction) along the laterally dilated tube in bulk PI. This result suggests that the relaxation time of the probe in bulk is governed by the longitudinal partial-DTD that occurs coherently along the chain backbone. In fact, the magnitude of retardation evaluated from the φ′(t) and τCR data on the basis of this longitudinal partial-DTD picture was close to the observation. These results strongly suggest that the star PI chains in monodisperse bulk have two different coarse-grained length scales: the diameter of laterally dilated tube that determines the modulus level and the diameter of longitudinally dilated tube that reflects the path length for the arm retraction and determines the relaxation time. Thus, the star PI chains in bulk appear to move along the longitudinally dilated tube that wriggles in the laterally dilated tube. This molecular scenario is consistent with the previous finding for bulk linear PI [Matsumiya et al. Macromolecules 2013, 46, 6067].

A Comparison of Tube Model Predictions of the Linear Viscoelastic Behavior of Symmetric Star Polymer Melts

We present a modified version of the time-marching algorithm (TMA) [van Ruymbeke et al., 2005, 2010] for predicting the linear viscoelasticity of monodisperse symmetric star polymer melts. Several new elements were added to the original TMA model. In particular, the remaining fraction of the initial tube segments is considered as a function of time, while taking into account the past relaxation history of each molecular segment. We validate the TMA model and, for the first time, compare it with the so-called BoB model [Das et al., 2006]. The predictions obtained with the two models are compared to a large set of experimental data, which cover a broad range of arm molecular weights (from 1.5 to 55 entanglements per arm) of different chemistries (polystyrene, polybutadiene, and polyisoprene). We indicate the significant differences between the two approaches, which mainly affect the choice of material parameters of the models. We then point out a systematic deviation of the TMA model in accurately predicting the intermediate regime of relatively short star chains, while the BoB model fails in correctly describing the plateau modulus of these short chains. From our point of view, both disparities have the same origin and, in the case of the TMA, can be solved by removing the high-frequency mode contribution to the contour length fluctuation of the primitive path. An excellent agreement between data and theory is then obtained for all molecular weights of the arms, which indicates the capability of the TMA to provide a quantitative prediction of the rheology of monodisperse star-shaped polymers. A very good agreement is also obtained with the BoB model, despite the expected discrepancy observed with short star chains. This proves the fact that current tube-based models are successful in quantitative predictions of linear rheology of monodisperse star-shaped polymers.

CDDP supramolecular micelles fabricated from adamantine terminated mPEG and β-cyclodextrin based seven-armed poly (l-glutamic acid)/CDDP complexes

This research is aimed to develop a nano-sized supramolecular micelle delivery system of cis-dichlorodiammine platinum (II) (CDDP) in order to achieve the passive tumor targeting. Firstly, star-shaped poly (γ-benzyl-l-glutamate) was synthesized by the ring-opening polymerization of γ-benzyl-l-glutamate-N-carboxyanhydride initiated with per-6-amino-β-cyclodextrin. After removal of benzyl groups, β-cyclodextrin based seven-armed poly (l-glutamic acid) (β-CD-7PLGA) was obtained. β-CD-7PLGA/CDDP complexes were prepared by the complex reaction between the carboxylic groups of β-CD-7PLGA and CDDP. Further inclusion of β-CD-7PLGA/CDDP complexes with adamantine terminated mPEG (mPEG-Ad) gave CDDP supramolecular micelles (mPEG-Ad@β-CD-7PLGA/CDDP). The formation of mPEG-Ad@β-CD-7PLGA/CDDP supramolecular micelles was confirmed by fluorescence spectrophotoscopy and particle size measurements. All the micelles showed spherical shape, and their sizes increased from 100 to 135nm with the increase of PLGA arm molecular weight. mPEG-Ad@CD-7PLGA/CDDP micelles showed sustained drug release profiles over 50h in PBS. Compared with CDDP, mPEG-Ad@β-CD-7PLGA/CDDP supramolecular micelles showed essential decreased cytotoxicity to KB cells, suggesting their great potential as the delivery carriers of CDDP.

Aging and stiction dynamics in confined films of a star polymer melt

The stiction properties of a star polyisoprene (PIP) melt (having 22 arms and an arm molecular weight of around 5000, M(w) ≈ 110,000) confined between mica surfaces were investigated using the surface forces apparatus. Stop-start experiments were carried out and the stiction spike was measured as a function of surface stopping (aging) time t and applied pressure P; the time constants of the phase transitions in the stiction dynamics (freezing on stopping and melting on starting) were obtained from the force relaxation behaviors. The results were compared with those of a confined linear-PIP melt (M(w) ≈ 48,000) and other confined fluid systems; the effect of star architecture on the phase transitions in confinement during aging is discussed. Estimation of the molecular size gives that the confined star-PIP films consist of three molecular layers; a non-adsorbed layer sandwiched between two layers adsorbed on opposed mica surfaces. There are (at least) four time constants in the freezing transition of the confined star-PIP melt; fast (τ(1)) and slow (τ(2)) time constants for lateral force relaxation on stopping, critical aging time for freezing (τ(f)), and the logarithmic increase of the spike height against t. The three time constants on stopping, τ(1), τ(2), and τ(f), increase with the increase of P (decrease of the thickness D). As regards the melting transition on starting, spike force decay was fitted by a single exponential function and one time constant was obtained, which is insensitive to P (D). Comparison of the time constants between freezing and melting, and also with the results of linear-PIP reveals that the stiction dynamics of the star-PIP system involves the relaxation and rearrangement of segmental-level and whole molecular motions. Lateral force relaxation on stopping is governed by the individual and cooperative rearrangements of local PIP segments and chain ends of the star, which do not directly lead to the freezing of the system. Instead, geometrical rearrangements of the soft star-PIP spheres into dense packing between surfaces (analogous to the concept of a colloidal glass transition) are the major mechanism of the freezing transition (stiction) after aging. Interdigitation of PIP segments/chain ends between neighboring star molecules also contributes to the spike growth along with aging, and the melting transition on starting.

Poly(ɛ-caprolactone)-based ‘green’ plasticizers for poly(vinyl choride)

A series of proposed plasticizers for poly(vinyl chloride) (PVC), based on poly(ɛ-caprolactone) (PCL) with octanoate and benzoate-terminal groups, were synthesized with various microstructures and molecular weights (MW) and tested for biodegradability as well as for mechanical performance, and leaching resistance in blends with PVC. The plasticization efficiency of each was characterized by measuring the glass transition temperature ( T g) and tensile properties of PCL/PVC blends. The PCL-octanoate plasticizers demonstrated plasticization efficiency similar to di(ethylhexyl) phthalate (DEHP) with the same plasticizer loading. PCL-benzoate/PVC blends had much higher T gs (∼20 °C higher) compared to PCL-octanoate/PVC and DEHP/PVC blends. Yield stresses were about two times higher for PCL-benzoate/PVC blends compared to PCL-octanoate/PVC and DEHP/PVC blends, reflecting the stiffer nature of such blends. Biodegradation was rapid for all PCL-octanoates, with the exception of linear PCL-octanoates with arm molecular weights >10 3 g mol −1. Biodegradation rates of PCLs by Rhodococcus rhodocrous were not affected by microstructure for the range of PCL topologies studied (linear versus three or four arms) but were slower for PCLs made from commercial PCL-diols that had a central ether linkage due to the initiator used to make these compounds. Leaching resistance was higher as PCL molecular weight increased and, for pairs of comparable sized species, significantly less PCL-benzoate leached out compared to the PCL-octanoate. For the range of PCL topologies studied, the number of arms did not significantly affect leaching resistance. In summary, both the end group and the molecular weight influenced the leaching resistance of the PCL. PCL-octanoates were comparable plasticizers to DEHP in terms of the mechanical properties examined, and were rapidly degraded by a common soil microorganism.

Optimizing the generation of narrow polydispersity ‘arm-first’ star polymers made using RAFT polymerization

In this article, the synthesis of well-defined, narrow polydispersity (PDI < 1.2) star polymersviaRAFT polymerisation is detailed. An ‘arm-first’ method is described using a crosslinked nanogel core. The synthetic conditions and variables were thoroughly investigated and optimised so that nearly quantitative arm incorporation was found to occur. The parameters studied included polymerization time, the arm molecular weight, and the nature of the solvent and cross-linker.

Star‐shape poly(acrylic acid)‐composed glass‐ionomer cements: Effects of MW and arm number on mechanical properties

AbstractThis study reports the synthesis and characterization of the star‐shape poly(acrylic acid)s with different arm numbers and molecular weight (MW)s. The effects of arm number and initiator concentration on the atom‐transfer radical polymerization reaction kinetics and solution viscosity were studied. The effects of MW and arm number on mechanical properties were evaluated. The results showed that both arm number and MW had significant impacts on the polymerization kinetics, solution viscosity, mechanical strengths, and wear‐resistance. Decreasing arm number and increasing initiator concentration increased the reaction rate. Increasing arm number and initiator concentration decreased the solution viscosity. Decreasing arm number and increasing MW increased mechanical strengths and wear‐resistance. Within the limitations of this study, the experimental cement was 28% in compressive strength, 48% in compressive modulus, 39% in diametral tensile strength, 60% in flexural strength, and 62% in Knoop hardness higher but 19% in fracture toughness lower than commercial Fuji II LC cement. The abrasion and attrition of the experimental cement were only 1.3% and 9.5% of Fuji II LC. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Stress softening of multigraft copolymers

The hysteresis behaviour of multigraft (MG) copolymers, with a polyisoprene backbone and polystyrene (PS) side chains, was investigated by applying a modified softening model proposed by Elías-Zúñiga, which uses an approach of Ogden and Roxburgh. The model was combined with the non-affine tube model of rubber elasticity of Kaliske and Heinrich. Four parameters are obtained: chemical and physical cross-link moduli ( G c, G e), the number of statistical segments between two successive entanglements ( n e/ T e) and a softening parameter ( b). The model was proven to be valid by a comparison with other methods evaluating hysteresis behaviour. The characterization of the multigraft copolymers revealed a branch point and molecular architecture dependence of the softening parameter. b was low for tetrafunctional MG copolymers with cylindrical microdomains, and it was further reduced for a spherical morphology and for more complex molecular architectures. The magnitude of b also depends on the PS arm molecular weight for hexa- and tetrafunctional multigraft copolymers.