Linear Polyisoprene Research Articles

Synthesis of Mechanically Robust Very High Molecular Weight Polyisoprene Particle Brushes by Atom Transfer Radical Polymerization.

Linear polyisoprene (PI) and SiO2-g-PI particle brushes were synthesized by both conventional and activators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP). The morphology and solution state study on the particle brushes by transmission electron microscopy (TEM) and dynamic light scattering (DLS) confirmed the successful grafting of PI ligands on the silica surface. The presence of nanoparticle clusters suggests low grafting density (associated with the limited initiation efficiency of ARGET for PI). Nevertheless, particle brushes with very high molecular weights, Mn > 300,000, were prepared, which significantly improved the dispersion of silica nanoparticles and also contributed to excellent mechanical performance. The reinforcing effects of SiO2 nanofillers and very high molecular weight PI ligands were investigated by dynamic mechanical analysis (DMA) as well as computational simulation for the cured linear PI homopolymer/SiO2-g-PI particle brush bulk films.

Tube Survival Fraction in Primitive Chain Network Simulations

In the framework of the tube model, the survival fraction of dilated tube, φʹ(t), is essential since the relaxation of the entire system is described according to this quantity. However, a method to define it in multi-chain simulations has yet to be considered. In this study, primitive chain network simulations were performed, and a few structural quantities were discussed as possible candidates for use as φʹ(t). Before analysis, the simulation was evaluated against literature data for linear polyisoprene melts, and quantitative agreement was obtained for viscoelastic and dielectric relaxations. Afterward, the end-to-end relaxation function was compared to survival probability of the slip link φs(t) and the ratio of monomers between the most distant surviving slip link pair to the total number of monomers ns(t)/nt. The result implies that ns(t)/nt is comparable to φʹ(t). Comparison to atomistic molecular dynamics (MD) simulations for polyethylene showed similar results, implying that φʹ(t) can be obtained even in MD simulations if the surviving entanglement points can be specified.

Slip-spring simulations of different constraint release environments for linear polymer chains.

The constraint release (CR) mechanism has important effects on polymer relaxation and the chains will show different relaxation behaviour in conditions of monodisperse, bidisperse and other topological environments. By comparing relaxation data of linear polyisoprene (PI) chains dissolved in very long matrix and monodisperse melts, Matsumiya et al. showed that CR mechanism accelerates both dielectric and viscoelastic relaxation (Matsumiya et al. 2013 Macromolecules 46, 6067. (doi:10.1021/ma400606n)). In this work, the experimental data reported by Matsumiya et al. are reproduced using the single slip-spring (SSp) model and the CR accelerating effects on both dielectric and viscoelastic relaxation are validated by simulations. This effect on viscoelastic relaxation is more pronounced. The coincidence for end-to-end relaxation and the viscoelastic relaxation has also been checked using probe version SSp model. A variant of SSp with each entanglement assigning a characteristic lifetime is also proposed to simulate various CR environment flexibly. Using this lifetime version SSp model, the correct relaxation function can be obtained with equal numbers of entanglement destructions by CR and reptation/contour length fluctuation (CLF) for monodisperse melts. Good agreement with published experiment data is also obtained for bidisperse melts, which validates the ability to correctly describe the CR environment of the lifetime version model.

Synthesis and Characterization of Linear Polyisoprene Supramolecular Elastomers Based on Quadruple Hydrogen Bonding.

Supramolecular elastomers based on quaternary hydrogen bonding of ureido-pyrimidinone (UPy) groups own special properties such as reversibility, self-healing, and good processability, which can be used in many special fields. In this paper, a novel type of linear polyisoprene supramolecular elastomer (LPSE) was prepared via anionic polymerization by deliberately introducing hydroxyl, isocyanate, and UPy groups into the ends. The formation of supramolecular structure showed significant effects on the microphase structures of LPSE, which was characterized by Fourier-transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), hydrogen nuclear magnetic resonance (1H-NMR), and dynamic mechanical analysis (DMA). Results showed that the introduction of UPy groups played a certain role in the improvement of the thermal stability, toughness, and tensile strength of the elastomer. Moreover, from self-healing tests, the hydrogen bonds of UPy showed dynamic characteristics which were different from covalent sacrificial bonds and exhibited the reassociation phenomenon. This study can not only extend our understanding of the toughening effect of strong hydrogen bonds, but also help us to rationally design new and tough elastomers.

Multi-chain slip-spring simulations for polyisoprene melts

The multi-chain slip-spring (MCSS) model is a coarse-grained molecular model developed for efficient simulations of the dynamics of entangled polymers. In this study, we examined the model for the viscoelasticity of polyisoprene (PI) melts, for which the data are available in the literature. We determined the conversion factor for the molecular weight from the fitting of the molecular weight dependence of zero-shear viscosity. According to the obtained value, we calculated the linear viscoelasticity of several linear PI melts to determine the units of time and modulus. Based on the conversion factors thus determined, we predicted linear viscoelasticity of 6-arm star PI melts, and viscosity growth under high shear for linear PI melts. The predictions were in good agreement with the data, demonstrating the validity of the method. The conversion factors determined were consistent with those reported for polystyrene melts earlier, whereas the relations between the conversion factors are still unknown.

UNIVERSAL SCALING BEHAVIOR OF THE TACKINESS OF POLYMER MELTS

ABSTRACTTack properties of nine linear polyisoprenes (PIPs) with molecular weights ranging from 84 to 476 kg/mol and two star-branched PIPs with molecular weights ranging from 609 to 1130 kg/mol were investigated on various surfaces, such as stainless steel, aluminum, copper, quartz, and plastics. In the investigation, a finger-sized cylindrical rod having a flat end face was brought into contact with a PIP polymer. After equilibrium was reached, the cylindrical rod was removed from the substrate at a constant pull-off velocity, Vs. We found that when the pulling speed (Vs) is greater than a critical speed (Vc), the maximum tack force (Fmax) can be generally described by the following scaling relationships: Fmax ∼ Vs1/3 and Fmax ∼ tmax−1/2, where tmax is the time when the maximum force is reached in the force–time profile. Remarkably, this scaling behavior of the tackiness appears to be universal, as it is independent of the adhered surface preparation, the molecular weight distribution, and the linear or branched chain structure of a polymer melt.

Direct analysis of terpenes from biological buffer systems using SESI and IR-MALDESI

Terpenes are the largest class of natural products with a wide range of applications including use as pharmaceuticals, fragrances, flavorings, and agricultural products. Terpenes are biosynthesized by the condensation of a variable number of isoprene units resulting in linear polyisoprene diphosphate units, which can then be cyclized by terpene synthases into a range of complex structures. While these cyclic structures have immense diversity and potential in different applications, their direct analysis in biological buffer systems requires intensive sample preparation steps such as salt cleanup, extraction with organic solvents, and chromatographic separations. Electrospray post-ionization can be used to circumvent many sample cleanup and desalting steps. SESI and IR-MALDESI are two examples of ionization methods that employ electrospray post-ionization at atmospheric pressure and temperature. By coupling the two techniques and doping the electrospray solvent with silver ions, olefinic terpenes of different classes and varying degrees of volatility were directly analyzed from a biological buffer system with no sample workup steps.

Investigation of nonlinear rheological behavior of linear and 3-arm star 1,4-cis-polyisoprene (PI) under medium amplitude oscillatory shear (MAOS) flow via FT-rheology

Monodisperse linear and 3-arm star polyisoprene (PI) melts were investigated under dynamic oscillatory shear flow. Master curves of linear rheological properties, G′ (ω) and G″ (ω), obtained by small amplitude oscillatory shear (SAOS) testing and nonlinear rheological properties, Q0 (ω), obtained by medium amplitude oscillatory shear (MAOS) testing, were created using the same shift factors by applying the time–temperature superposition (TTS) principle. Linear master curves of 3-arm star PIs were broader than those of linear PIs because of the contribution made by an additional relaxation mode due to arm retraction. This difference was clearer in nonlinear master curves. In nonlinear master curves (Q0 (ω)), linear PIs showed only one peak corresponding to relaxation of the backbone chain, whereas two weak local peaks were observed for 3-arm star PIs. One peak at lower frequency reflected backbone relaxation and the other at higher frequency reflected branch relaxation. These findings confirm that Q0 is a highly sensitive parameter for characterizing the effect of one branch by detecting backbone and branch relaxation modes. Based on the hierarchical relaxation concept of branched polymers, three characteristic relaxation times of 3-arm star PIs were determined from a nonlinear master curve: backbone relaxation time (τb) from local maximum Q0 at lower frequency, backbone Rouse time (τR,b) from local minimum Q0, and arm relaxation time (τa) from local maximum Q0 at higher frequency. Backbone relaxation times (τb) from nonlinear master curves almost coincided with linear viscoelastic terminal relaxation times (τw) from linear master curves within the limits imposed by experimental error. These relaxation times, especially τR,b, were used for molecular stress function (MSF) model predictions of 3-arm star PIs as a criterion parameter to determine the terminal relaxation mode of the backbone chain, and τR,b was found to be a reasonable fitting parameter for the MSF model. Fitting results showed that the weak peak arising from the one branch could be quantified using the MSF model. However, both linear and 3-arm star PIs had the same β value of 1, meaning a linear topology without a branch. It was concluded that the effect of only one branch could not be detected using β values. Our results suggest the possibility of using the nonlinear parameter Q0 for determining model parameters for constitutive model equations.

A multichain polymer slip-spring model with fluctuating number of entanglements for linear and nonlinear rheology.

A theoretically informed entangled polymer simulation approach is presented for description of the linear and non-linear rheology of entangled polymer melts. The approach relies on a many-chain representation and introduces the topological effects that arise from the non-crossability of molecules through effective fluctuating interactions, mediated by slip-springs, between neighboring pairs of macromolecules. The total number of slip-springs is not preserved but, instead, it is controlled through a chemical potential that determines the average molecular weight between entanglements. The behavior of the model is discussed in the context of a recent theory for description of homogeneous materials, and its relevance is established by comparing its predictions to experimental linear and non-linear rheology data for a series of well-characterized linear polyisoprene melts. The results are shown to be in quantitative agreement with experiment and suggest that the proposed formalism may also be used to describe the dynamics of inhomogeneous systems, such as composites and copolymers. Importantly, the fundamental connection made here between our many-chain model and the well-established, thermodynamically consistent single-chain mean-field models provides a path to systematic coarse-graining for prediction of polymer rheology in structurally homogeneous and heterogeneous materials.

Dielectric and Viscoelastic Behavior of Low-M Linear Polyisoprene Blended in Long Matrix

Cis-polyisoprene (PI) has the type-A dipole parallel along the chain backbone thereby exhibiting slow dielectric relaxation that reflects the global chain motion (end-to-end vector fluctuation). For linear PI probes having the molecular weights M1 in a range between 21.4× 103 and 179 × 103 (M1/Me = 4-36) and being blended with a longer PI matrix (M2 = 1.12 × 106), a recent dielectric experiment revealed that the probe relaxation is moderately retarded on blending and this retardation is enhanced with decreasing M1 in this range; cf. Matsumiya et al., Macromolecules, 46, 6067 (2013). Those dielectric data were analyzed within the context of the tube model to suggest that the probe relaxation in monodisperse bulk occurs through reptation along a partially dilated tube that wriggles in a fully dilated tube. However, this mechanism of relaxation is expected to vanish on a further decrease of M1 because the tube itself loses its sound meaning for small M1. This expectation was examined in this study. Dielectric and viscoelastic tests revealed that the retardation of the probe relaxation on blending becomes minor (though not negligible) on a decrease of M1 to 7.8× 103 (≈ 1.6Me), as expected.

Synthesis and characterization of an exact comb polyisoprene with three branches having the middle branch twice the molecular weight of the other two identical external branches

An exact comb polyisoprene (PI) with three branches, with the middle branch having twice the molecular weight of the two other identical external branches, was synthesized by using anionic polymerization high vacuum techniques and appropriate chlorosilane chemistry. The synthetic approach involves (a) the selective replacement of the two chlorines of 4-(dichloromethylsilyl)diphenylethylene (DCMSDPE, key molecule) with identical PI chains by titration with PILi, (b) the addition of sec-BuLi to the double bond of DPE followed by the polymerization of isoprene from the newly created anionic site to form a 3-arm living star PI, (c) the selective replacement of the two chlorines of trichloromethylsilane with 3-arm star PI to form an H-shape intermediate, and (d) the replacement of the remaining chlorine of trichloromethylsilane by linear PI chains with double the molecular weight. All intermediate and final products were characterized via size exclusion chromatography, temperature gradient interaction chromatography and 1H-NMR spectroscopy. As expected, due to the inability to control the exact stoichiometry of the linking reactants, the main product (exact comb PI) is contaminated by a few by-products, despite the fact that anionic polymerization is the most efficient way to produce well-defined polymers.

Experimental evaluation of the pure configurational stress assumption in the flow dynamics of entangled polymer melts

A filament stretching rheometer was used for measuring the startup of uni-axial elongational flow followed by reversed bi-axial flow, both with a constant elongational strain rate. A narrow molecular mass distribution linear polyisoprene with a molecular weight of 483 kg/mole was subjected to the flow in the non-linear flow regime. This has allowed highly elastic measurements within the limit of pure orientational stress, as the time of the flow was considerably smaller than the Rouse time. A Doi–Edwards [J. Chem. Soc., Faraday Trans. 2 74, 1818–1832 (1978)] type of constitutive model with the assumption of pure configurational stress was accurately able to predict the startup as well as the reversed flow behavior. This confirms that this commonly used theoretical picture for the flow of polymeric liquids is a correct physical principle to apply.

Fluorine-containing arborescent polystyrene-graft-polyisoprene copolymers as polymer processing additives

Linear low density polyethylene (LLDPE) can suffer from melt extrusion defects including sharkskin, cyclic melt fracture, and gross melt fracture during processing. Arborescent polymers are dendritic macromolecules with characteristics, such as a compact structure and a rigid spherical topology, making them potentially useful as polymer processing additives (PPA) to alleviate melt extrusion defects. Arborescent polystyrene-graft-polyisoprene copolymer samples were synthesized from polystyrene substrates of linear and branched architectures functionalized with acetyl groups, and coupled with polyisoprene macroanions. A linear polyisoprene sample was also investigated for comparison. The polymers were hydrosilylated with (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylsilane on 17–52% of the isoprene units and blended with LLDPE at 0.1 and 0.5% w/w to evaluate their performance as PPA by extrusion at different shear rates. All the samples led to some degree of improvement in the extrusion of LLDPE, albeit the performance of the branched additives was inferior to a commercial fluoroelastomer PPA. The lower molecular weight and more compact (G0 or comb-branched) PPA generally performed better than those with a high molecular weight. Several PPA samples induced the early onset of cyclic melt fracture but glossy, defect-free surfaces were obtained at higher shear rates. This suggests that a minimum shear rate is required for these additives to coat the extrusion die under the experimental conditions used.

Molecular dynamics of natural rubber as revealed by dielectric spectroscopy: The role of natural cross–linking

In order to understand the molecular dynamics of natural rubber, the dielectric relaxation behavior of its different components were investigated. These components included: (1) the linear polyisoprene fraction, obtained after deproteinization and transesterification of natural rubber (TE–DPNR), (2) the gel (GEL) fraction, corresponding to pure natural chain–end cross–linked natural rubber, (3) deproteinized natural rubber (DPNR), in which the protein cross–links at the ω–end have been removed, and (4) natural rubber (CNR) purified (through centrifugation) but still containing proteins, phospholipids and the sol phases. The dielectric relaxation behaviour of natural rubber revealed a segmental mode (SM) which is not affected by natural chain-end cross-linking (so-called naturally occurring network) and a normal mode (NM) which depends on a naturally occurring network. The dynamics of the NM, which is associated to chain mobility, seems to be strongly affected by natural chain-end cross-linking. We propose a model based on a hybrid star polymer in which the low mobility core (phospholipids) controls the mobility of the polyisoprene arms.

Effects of Chain Topology on the Tracer Diffusion in Star Polyisoprenes

Fluorescence correlation spectroscopy (FCS) was employed to study the effect of chain topology on the small tracer diffusion in linear and star-shaped 1,4-polyisoprenes (PIs) at temperatures well above the glass transition. In the linear polyisoprene, a Fickian diffusion behavior was observed that is characterized by a single diffusion coefficient. In contrast, the diffusion of the same tracers in three-arm star PI was non-Fickian and could best be described assuming two diffusion modes. The fast mode diffusion has the same magnitude and temperature dependence as the tracer diffusion in linear PI. The slow mode present only in the star polymer is apparently related to topological restrictions that cause retention of the tracer. The temperature dependence of the retention time is very similar to that of the polymer arm relaxation, indicating a direct correlation between the two dynamic processes.

Cross-linked latex particles grafted with polyisoprene as model rubber-compatible fillers

Model filler particles were obtained by grafting polyisoprene (PIP) chains onto spherical latex particles of polystyrene cross-linked with 12 mol% divinylbenzene. These particles, with a narrow size distribution and a diameter of ca. 400 nm, were synthesized by emulsifier-free starved-feed emulsion polymerization. Acetyl coupling sites were introduced randomly at either low (5 mol%) or high (30 mol%) target substitution levels on the latex particles by Friedel–Crafts acylation with acetyl chloride and AlCl 3 in nitrobenzene. ‘Living’ polyisoprenyllithium chains, generated from isoprene and sec-butyllithium ( sec-BuLi), were then coupled with the acetylated particles. The PIP side chains had a high 1,4-polyisoprene microstructure content and a number-average molecular weight ( M n) of either 1.5 × 10 3 (1.5 K), 5 × 10 3 (5 K), or 3 × 10 4 (30 K). The PIP content of the grafted particles was determined from the yield of isolated particles and by 1H NMR spectroscopy analysis. The grafted latex particles were blended in solution with linear polyisoprene ( M n = 3.95 × 10 5, 395 K). The influence of the filler–matrix interactions on the rheological behavior of the blends was determined by dynamic mechanical analysis for the different filler blends. Increases in complex viscosity and storage modulus, and decreased damping factors were observed in all cases relatively to the pure matrix polymer. The enhancements, decreasing in the order 30 mol% > 5 mol% acetylation, and with the grafted PIP chain length as 30 K > 5 K ≈ 1.5 K, are deemed to reflect the extent of interactions between the filler particles and the polymer matrix.

Branched Versus Linear Polyisoprene: Flory–Huggins Interaction Parameters for their Solutions in Cyclohexane

AbstractFlory‐Huggins interaction parameters were determined as a function of composition for solutions of linear and of branched polyisoprene in cyclohexane (CH) at 25, 45, and 65 °C by means of vapor pressure measurements (moderate to concentrated solutions) and by vapor pressure osmometry (dilute solutions). The results demonstrate that CH is a considerably worse solvent for branched polyisoprene than for the linear analog at all temperatures and at all compositions. This observation corroborates the expectation based on a recent phenomenological approach, which accounts explicitly for the incapability of the segments of an individual polymer molecule to spread out over the entire volume of the system and for its ability to adjust its chain conformation to an altering molecular environment.magnified image

Linear and nonlinear shear flow behavior of monodisperse polyisoprene melts with a large range of molecular weights

Shear rheological data for a wide range of nearly monodisperse linear polyisoprene melts are reported herein using the results of linear and nonlinear measurements. The number of entanglements per chain ranges from about 0.5 to 230. In order to extend the range of available frequencies and shear rates the procedure of time-temperature superposition is improved and validated in both the linear and the nonlinear regime. In the linear flow regime a detailed comparison with the tube theory by Likhtman and McLeish [Macromolecules 35, 6332–6343 (2002)] was performed. The overall agreement was found to be satisfactory but several minor disagreements, e.g., at the crossover regime and for the steady-state compliance, were observed and have been discussed. Until now, most nonlinear models for entangled polymers were compared with shear data on solutions. However, the equivalence between solutions and melts is not well established. In this paper the results of a series of nonlinear start-up shear experiments are compared with those predicted by the tube model [Graham et al., J. Rheol. 47, 1117–1200 (2003)] and with several literature datasets on entangled solutions having similar numbers of entanglements. The melts were found to behave in a very similar manner to the previously studied solutions and agree well with the tube theory. Disagreements are observed when the shear rate exceeds the Rouse time of one entanglement and also for chains with very small number of entanglements. For the high molecular weight samples possible instabilities and edge fracture can occur at high strains and the results are discussed in light of these effects. The extensive dataset presented here could serve as a test for future theories and models of entangled melts.

Branched versus linear polyisoprene: Fractionation and phase behavior

Branched polyisoprene (PI) was prepared from PI-macromonomers. Linear byproducts of the synthesized polymer were removed by means of inverse spin fractionation, using the solvent cyclohexane (CH) and the precipitant acetone (AC). A well-defined fraction (Mw = 17.5 kg/mol, Mw/Mn = 1.8) of the branched polyisoprene obtained in this manner was used to determine different phase diagrams with branched and/or linear PI in the mixed solvent CH/AC at 25 °C. For comparable molar masses of the polymers the two-phase area is smallest for the branched PI and slightly larger for the linear PI; in the case of the unfractionated original sample of the branched polymer one observes a pronounced peninsula of immiscibility extending into the region of high CH concentrations. This feature is attributed to a large miscibility gap between the branched and the linear polymer, which was studied in more detail for the ternary system CH/branched PI/linear PI.

Viscoelastic Properties of Arborescent Polystyrene-graft-polyisoprene Copolymers

Arborescent polymers are highly branched macromolecules obtained from successive grafting reactions according to a generation-based scheme. Grafting side chains onto a linear substrate randomly functionalized with coupling sites thus yields a generation zero (G0) polymer, with a comb-branched structure. Further cycles of substrate functionalization and grafting lead to arborescent polymers of generations G1, G2, etc., with a dendritic architecture. Arborescent copolymers can also be obtained when side chains with a chemical composition different from the substrate are used in the last grafting cycle. Copolymers of generations G0−G3 were thus obtained by grafting polyisoprene chains with a high cis-1,4-unit content onto polystyrene substrates. The dynamic mechanical behavior of the arborescent polystyrene-graft-polyisoprene copolymers was investigated as a function of side-chain length and generation number. The zero-shear viscosity of the copolymers was lower than for linear polyisoprenes of comparable molecular weight, but its scaling behavior as a function of molecular weight was consistent with entanglement formation. The zero-shear recoverable compliance increased with the overall molecular weight of the copolymers to reach values up to 10 times larger than for the linear analogues. Isoprene copolymers of generations G0 and G1 displayed viscous flow behavior at low shear rates, their terminal relaxation time increasing with the molecular weight of the side chains. A relaxation at intermediate frequencies, associated with the polyisoprene chains, appeared at similar frequencies for copolymers of different generations with side chains of comparable molecular weights. The frequency dependence of the modulus observed for highly branched (G2 and G3) isoprene copolymers was analogous to microgels and filled polymer systems, the modulus−frequency master curves displaying thermorheological complexity with breakdown of the time−temperature superposition principle for copolymers with short side chains.