Low Entanglement Density Research Articles

Effects of Polymerization Medium on Nascent UHMWPE Properties and Sintered Material Performance

High-performance and easy-processing UHMWPE products with less entanglement and small particle size are difficult to synthesize efficiently in the polymerization process. In this study, a mixture of heptane and light paraffin is proposed as the polymerization medium to control the concentration of ethylene in the slurry during polymerization. Influences of the mixture composition on the polymerization behavior of ethylene, the particle size and its distribution of nascent UHMWPE, and the initial entangled state of UHMWPE were explored. Experimental results showed that as the volume fraction of heptane decreased, the viscosity of the solvent mixture increased exponentially, and ethylene solubility as well as mass transfer coefficient of ethylene in solvent decreased, which greatly affected the ethylene polymerization rate. By adjusting the volume fraction of heptane in the polymerization medium, the particle size, initial entanglement state density, and molecular weight of nascent UHMWPE will be coordinately controlled. UHMWPE with smaller particle size, lower entanglement density, and higher molecular weight can be prepared using the conventional Ziegler–Natta catalyst and polymerization medium with a lower heptane content, and its sintered UHMWPE material shows good mechanical properties.

Investigating the disentanglement of long chain branched polypropylene under different shear fields

AbstractMany experimental results have revealed that entanglement networks of molecular chain would undergo the orientation, deformation, and breakdown under the condition with shear field, forming a new conformation with much reduced entanglement interactions with the surrounding medium. In this work, the role of shear mode on the disentanglement of chains was investigated to give us some guidance for designing a new polymer melts disentanglement machine to optimize the processability of polymers by reducing viscosity. Results showed that the intensity of the generated shear modification depends on shear procedure during treatment, ramping‐up shear has the most efficient influence on molecular chains disentanglement due to the absence of shear banding, resulting in a higher molecular weight between entanglements Me and much lower entanglement density. Besides, the reduced entanglement density enabled the higher chain mobility, which greatly enhanced crystallization kinetics during non‐isothermal and isothermal crystallization.

Nanocomposites of Au/Disentangled UHMWPE: A Combined Optical and Structural Study.

The term disentangled refers to polymers with fewer entanglements in the amorphous regions, a metastable condition that can significantly affect the material’s properties and processing behavior. The lower entanglement density in ultra-high molecular weight polyethylene (dis-UHMWPE) facilitates the solid-state processability into uniaxially-oriented specimens reaching very high draw ratios and crystallinities. In this study, Au/dis-UHMWPE nanocomposites were formulated and processed at variable draw ratios. Polarized light microscopy suggests gold nanoparticles are oriented in arrays following the drawing of polymer chains. The structural features, upon orientation, are studied by means of Raman spectroscopy, wide- and small-angle X-ray scattering, and near-infrared spectrophotometry. Crystallinity is found to increase by 15%, as calculated by wide-angle X-ray scattering. The change in optical absorbance in the visible spectrum indicates that, with orientation, the average size of gold aggregates increases, supported quantitatively by small-angle X-ray scattering. Since the gold nanoparticles are expected to be found within amorphous chain segments, the aforementioned findings are attributed to the increase of crystallinity and thus the decrease of available (amorphous) space.

New insights into the memory effect on the crystallization behavior of poly(phenylene sulfide)

In this article, the melt and crystallization kinetics of poly(phenylene sulfide) (PPS) powder was studied by differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). It is found that the folded-chain crystals of PPS (melting point at 281 °C) could be erased at about 350 °C, while the equilibrium melting point of extended-chain crystals (melting point at 295 °C) is 370 °C. Although the residual nuclei could be eliminated completely at 370 °C, the spherulite growth rate of PPS during crystallization keeps accelerating with the increasing melting relaxation time, resulting from the transformation of the lamellae and the amorphous cluster to an ordered structure with low entanglement density. Furthermore, due to the restricted chain mobility for the semi-flexible PPS, this unique memory effect would survive in the whole manufacturing process, resulting in the varying results from different researches and the significant influence on the processability and performance of PPS products.

Molecular dynamics simulation of linear polyethylene blends: Effect of molar mass bimodality on topological characteristics and mechanical behavior

Blending different molar mass fractions of polyethylene (PE) in order to obtain materials with higher fracture toughness has previously proven beneficial. Our approach has been to use coarse-grained (CG) molecular dynamics (MD) simulations to obtain semicrystalline polyethylene systems on a nanoscale, and then draw them in order to mimic tensile testing. The CG potentials were derived, validated and utilized to simulate melt equilibration, cooling, crystallization and mechanical deformation. Crystallinity, tie chain and entanglement concentrations were continuously monitored. During crystallization, the low molar mass fraction disentangled to a greater degree and ended up with a lower entanglement density than the high molar mass fraction, although the tie chain concentration was higher for the low molar mass fraction. The deformation behavior of semicrystalline PE above its glass transition temperature was then assessed in a uniaxial tensile deformation simulation. The low-strain mechanical properties (i.e. elastic modulus, yield stress and strain) were in accordance with the literature. The high-strain mechanical features and toughness were improved in bimodal systems. The presence of a high molar mass fraction in bimodal systems was shown to affect the crystallinity and tie chain concentration during the strain hardening, leading to tougher model systems. Finally, the bimodal system with equal shares of the molar mass fractions showed the highest toughness and the best ultimate mechanical properties while having a concentration of tie chains and entanglements intermediate between the values for the other systems. This was a clear sign of the non-exclusive role of tie chains and entanglements in the mechanical behavior of bimodal PE and more generally of semicrystalline polymers at high strains.

Destruction mechanism of core–shell particles in impact polypropylene copolymer during short molten-state annealing

Morphology evolution of the dispersed phase with a multilayered core–shell structure in impact polypropylene copolymer (IPC) during molten-state annealing was systematically studied through scanning electron microscopy (SEM), phase contrast microscopy (PCM) and dynamic rheological test. To demonstrate the evolution path of the dispersed phase comprised of ethylene-propylene random copolymer (EPR) and ethylene–propylene block copolymer (EbP) during annealing, different binary blends comprised of different fractions were prepared and their diffusion behavior during liquid–liquid phase separation was investigated. Compared with EPR, EbP presented a higher diffusion rate in propylene homopolymer (hPP) matrix, owing to its lower molecular weight and lower entanglement density. The statistical results of EbP and EPR domain sizes reveal that the coalescence of EbP is faster than that of EPR. In addition, the interaction parameters of EbP/hPP and EbP/EPR estimated using the Nishi–Wang equation show that EbP has a stronger affinity for hPP than EPR. Based on the diffusion rates, entanglement densities of components and great disparity in viscosity between EPR and hPP, a potential mechanism was proposed for the morphology evolution of core–shell dispersed particles in IPC during molten-state annealing.

Nonlinear viscoelastic response of dendritic (arborescent) polyisobutylenes in single- and reversing double-step shearing flows

Single-step and three types of reversing double-step experiments were conducted on concentrated solutions of a linear and two dendritic (hyperbranched or arborescent) polyisobutylenes (PIBs). The results in reversing double-step experiments further confirm our previous findings on polyethylene melts that the K-BKZ theory provides better predictions for long chain branched polymers than linear ones. The short chain branched material looks more like the linear material in the present experiments. Isochronal derivatives of the strain potential function (W1 and W2) were calculated from torque (T) and normal force (N) responses in single-step parallel plate experiments. The damping function was extracted from W1 and W2 for both linear and branched PIBs. The damping function of the linear PIB falls between two versions of the Doi–Edwards model. The arborescent PIB with longer branches and lower branching frequency shows weak damping behavior, which is a specific behavior for long chain branched polymers. The arborescent PIB with shorter branches, higher branching frequency and low entanglement density, on the other hand, exhibits the same damping behavior as that of the linear PIB, which is consistent with the findings in the reversing double-step experiments in which it behaves more like a linear polymer.

Rheological properties of branched polystyrenes: linear viscoelastic behavior

Oscillatory shear measurements on a series of branched graft polystyrenes (PS) synthesized by the macromonomer technique are presented. The graft PS have similar molar masses (Mw between 1.3×105 g/mol and 2.4×105 g/mol) and a polydispersity Mw/Mn around 2. The molar masses of the grafted side chains Mw,br range from 6.8×103 g/mol to 5.8×104 g/mol, which are well below and above the critical entanglement molar mass Mc of linear polystyrene. The average number \({\ifmmode\expandafter\bar\else\expandafter\fi{p}}\) of side chains per molecule ranges from 0.6 to 6.7. The oscillatory measurements follow the time–temperature superposition principle. The shift factors do not depend on the number of branches. The zero-shear viscosities of all graft PS are lower than those of linear PS with the same molar mass, which can be attributed to the smaller coil size of the branched molecules. It is shown that the influence of branching on the frequency dependence of the dynamic moduli is weak for all graft PS that were investigated, which can be explained by the low entanglement density.

Rheological properties of branched polystyrenes: linear viscoelastic behavior

Oscillatory shear measurements on a series of branched graft polystyrenes (PS) synthesized by the macromonomer technique are presented. The graft PS have similar molar masses (M w between 1.3×105 g/mol and 2.4×105 g/mol) and a polydispersity M w /M n around 2. The molar masses of the grafted side chains M w,br range from 6.8×103 g/mol to 5.8×104 g/mol, which are well below and above the critical entanglement molar mass M c of linear polystyrene. The average number \({\ifmmode\expandafter\bar\else\expandafter\fi{p}}\) of side chains per molecule ranges from 0.6 to 6.7. The oscillatory measurements follow the time–temperature superposition principle. The shift factors do not depend on the number of branches. The zero-shear viscosities of all graft PS are lower than those of linear PS with the same molar mass, which can be attributed to the smaller coil size of the branched molecules. It is shown that the influence of branching on the frequency dependence of the dynamic moduli is weak for all graft PS that were investigated, which can be explained by the low entanglement density.

Micromechanics of the growth of a craze fibril in glassy polymers

The primary objective of this work is to model the growth and eventual failure of a craze fibril in a glassy polymer, starting from a primitive fibril. Experimental investigations have shown that properties like the entanglement density of a polymer play a pivotal role in determining whether macroscopic failure of a polymer occurs through crazing or shear yielding. Failure is seen to be related to the formation of a soft ‘active zone’ at the craze-bulk interface, through disentanglement. The present work aims at explaining some of the experimental findings about fibril growth and failure in glassy polymers on the basis of a continuum model of a craze with a constitutive model that accounts for yield, network hardening and disentanglement. The results show that this approach is capable of providing explanations for experimentally observed facts such as the propensity to crazing in polymers with low entanglement density and the linearity between the stretch in a fibril and the maximum stretch of a molecular strand in the fibril.

On the entanglement density of differently N-substituted alternating styrene-maleimide copolymers

Abstract Differently N-substituted maleimides were copolymerized with styrene to yield alternating styrene-maleimide copolymers (SMI-R) with different chain diameters. The polymers were obtained by free radical polymerization and characterized by NMR and size exclusion chromatography / differential viscometry. Glass transition temperatures were measured by differential scanning calorimetry. An increase in chain diameter and chain stiffness is accompanied by a decrease in the entanglement density, reflected in lower values of the plateau modulus, which were corrected for the low molecular weight portion using the Wasserman/Graessley model. Increasing the chain diameter by a factor of two results in a decrease of the entanglement density to one third. SMI-Me showed a much lower entanglement density than polystyrene (PS) although they have the same chain diameter. SMIMe however is more rigid than PS because of the maleimide five-membered ring structure in the main chain. SMI-Me and SMI-PhOPh show the same glass transition temperature. However, because of the larger chain diameter of SMIPhOPh, it has a much lower entanglement density. Thus, both the chain flexibility and the chain diameter, two parameters that are strongly related, affect the entanglement density.

Nucleation and Morphology of Polyethylene Under Shear Flow

Effect of the shear flow (γ˙=0.5∼5 s−1) on the nucleation and on the morphology of polyethylene (PE) during crystallization from the melt was studied by means of polarizing optical microscopy and small angle X-ray scattering. In order to analyze the results by the nucleation theory, we observed the effect of shear flow on the equilibrium melting temperature The under shear flow and that under quiescent state are almost the same, Therefore, the shear flow does not affect the The “heterogeneous” primary nucleation rate (I) and the induction onset time of nucleation (τ onset) of the isolated crystals under shear flow are also almost the same as those under quiescent state. The heterogeneous primary nucleation means that isolated nuclei are sporadically generating from the melt with the aid of heterogeneities such as nucleating agents. After the generation of primary nuclei, the “shish” were generated independent of the primary nucleation. We found that the shish were formed by the chain elongation caused by the velocity difference between polymer chains and dust particles, etc. After that, we observed that the “kebabs” were formed on the shish. It was found that the nucleation from the melt after melting of the “shish-kebabs” (at melt annealing temperature T max=160°C for 5 min) was accelerated compared with the ordinary melt crystallization after melting of folded chain crystals. This indicates that the “solid memory effect” of the former solid is significant. This is because of the low entanglement density within the melt of shish kebabs.

Relation between molecular structure and flow instability for ethylene/α-olefin copolymers

Surface instabilities in a capillary extrusion have been studied for various ethylene/α-olefin copolymers. It is found that the onset stress of shark-skin failure for ethylene/1-hexene copolymer (EHR) decreases rapidly with increasing 1-hexene content, whereas that of ethylene/propylene copolymer (EPR) is independent of propylene content in the experimental region. Consequently, EHR with high 1-hexene content exhibits shark-skin at low stress level compared to EPR. Lower rubbery plateau modulus, leading to higher Deborah number at the same stress level, is attributed to the lower onset stress. Further, the low entanglement density will cause cracks at lower stress level like glassy polymers, which is also responsible for the low onset stress for shark-skin.

Fluctuation-entanglement mechanism for director anchoring at nematic polymer surfaces.

The existence of polymeric entanglements in a polymer nematic liquid crystal modifies the Frank elastic description governing director deformation. We present a calculation of the director fluctuation contribution to polymer nematic surface tension, in which we supplement the usual Frank terms with a damping term due to entanglements. We infer an effective surface anchoring potential, going to zero as n ln n in the limit of low entanglement density n. The anchoring easy axis depends on the relative magnitudes of the Frank constants governing director elasticity. We comment on generalizing the approach to take into account polydomain structure.

Toward understanding the stress oscillation phenomenon in polymers due to tensile impact loading

Though stress oscillation phenomena can be triggered by static and dynamic tensile loading in amorphous (co)polyesters, they have nothing in common. Static loading-caused stress oscillation appears to be controlled by strain-induced crystallization and related heat release and thus can be observed only in crystallizable polymers. On the other hand, dynamic loading-induced stress oscillation is a more universal phenomenon as related to the natural frequency of the test set-up. The latter was established by determining the natural frequency of the instrumented anvil via fast Fourier transformation using a crystallized poly(ethylene terephthalate) (cPET) specimen failing brittlely. When the instrumented tensile impact response of amorphous PET (aPET) and commercial copolyesters (Eastar® PCTG and A150, respectively) has been filtered by considering the natural frequency of the test configuration, the related fractograms no longer showed stress oscillation. It was concluded that samples undergoing adiabatic-type multiple shear banding and prone to extensive cold-drawing are the best candidates to demonstrate the onset of dynamic stress oscillation. The latter is favored by low entanglement density of copolyesters.

Chain entanglement, mechanical properties and drawability of poly(lactide)

The observed brittle fracture behavior of amorphous polylactides seems to be contradicted by the low value ofC ∞ =2 determined for poly(L-lactide) by Flory and coworkers. Such very flexible polymer chains deform by shear yielding, and fracture in a ductile manner. In this study,C ∞ was estimated in a number of ways, resulting in much higher values ofC ∞ =11.7 andC ∞=9.1 for poly(L-lactide) and L- and D-lactide copolymers, respectively. These high values ofC ∞ and the low entanglement density account for the brittle fracture behavior of amorphous poly(lactide), as well as for the maximum attainable draw ratios of poly(L-lactide) networks and melt spun fibers. Bulk polymerized poly(L-lactide) networks, where crystallization during polymerization impedes severe entangling, could be hot-drawn most effectively to draw ratios of 8–16, resulting in very strong materials with tensile strengths of 550–805 MPa. By comparison, amorphous, non-crystallizable L/D lactide networks, which do not crystallize during polymerization, could be drawn less, to λ=7. These materials with strengths up to 460 MPa could, nevertheless, be oriented much more effectively than linear, amorphous L/D lactide copolymers.

(Co)polymers of L‐lactide, 2. Mechanical properties

AbstractThe effect of copolymerization of L‐lactide with several lactones and cyclic carbonates and the processing variables in compression moulding of poly(L‐lactide) homopolymers on the mechanical properties was investigated. Sharp maxima in the impact strength versus comonomer composition and heat of fusion were obtained, reflecting the important role of the presence of a crystalline fraction in the material. Crystalline domains increase the interconnectivity of a physical polylactide network and decrease the molecular weight between these physical crosslinks. The brittle strength concomitantly increases, and as the yield stress remains more or less constant, an increase in impact strength is observed. Very high crystalline as‐polymerized poly(L‐lactide) prepared with less active catalysts on the other hand has an extremely low entanglement density and therefore a low brittle tensile strength.

Ultimate toughness of amorphous polymers

AbstractThe deformation and toughness of amorphous glassy polymers is discussed in terms of both the molecular network structure and the microscopic structure at length scales of 50–300 nm. Two model systems were used: polystyrene‐poly(2,6‐dimethyl‐1,4‐phenylene ether) blends (PS‐PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non‐adhering core‐shell‐rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50–80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non‐adhering rubbery particles (‘holes’). The value of the critical matrix ligament thickness (IDc) uniquely depends on the molecular structure: at an increasing network density, IDc increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain‐rate and temperature dependency.

Strength of incompatible amorphous polymer interfaces

The mechanical strength of incompatible polymer interfaces welded above T[sub g] was investigated as a function of time, temperature, and composition. Three pairs of polymers were used: polystyrene-poly(methyl methacrylate), poly(styrene-co-acrylonitrile)-poly(methyl methacrylate), and poly(styrene-coacrylonitrile)-polycarbonate. For each pair, the weld strength G[sub 1c], measured by wedge cleavage, attained a constant value which increased with welding temperature. While the plateau strengths are typically only about 5-10% of bulk G[sub 1c] values, they are orders of magnitude greater than the work of adhesion calculated using intermolecular forces. For the copolymer-homopolymer pairs, the maximum plateau strength was reached when the Flory-Huggins interaction parameter, [chi], was a minimum. These results are in agreement with a model based on Helfand's molecular theories of the structure of incompatible interfaces, coupled with a microscopic deformation mechanism, which predicts that G[sub 1c] [approximately] 1/[chi]. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis of the fracture surfaces revealed dissimilar fracture surfaces for a given pair, with evidence of stick-slip crack growth. XPS revealed residues of one polymer on the other's surface, indicating cohesive fracture occurred to some extent. In each case, the cohesive fracture occurred in the polymer with the lower entanglement density and lower craze stress.

Deformation and toughness of polymeric systems: 2. Influence of entanglement density

In part 1 of this series, the concept of a critical material thickness was introduced and demonstrated experimentally using polystyrene (PS) as a test material. Below the critical thickness, brittle polymers become ductile. The value of the critical thickness is material-dependent and related to the entanglement density. The dependence of the critical thickness on the entanglement density was investigated using the miscible system polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE). PS possesses a low entanglement density and PPE a high entanglement density, and the system's entanglement density can be varied depending on the relative ratio of PS and PPE in the mixture. Equivalent to the experimental procedure developed in our previous paper, the thickness was set by either changing the PS-PPE layer thickness in stratified PS-PPE/PE tapes (polyethylene (PE) is present to separate the PS-PPE layers) or by adjusting the volume fraction of non-adhering core-shell rubbery particles in the PS-PPE blend, i.e. the ligament thickness. The experimentally determined critical thickness ( ID c) proved to increase continuously from 0.06 μm for PS-PPE 80–20 to 0.18 μm for PS-PPE 40–60 blends. This compares well with the value of 0.05 μm found for pure PS. Under the (moderate) testing conditions used, the PS-PPE 20–80 blend was always tough. The maximum macroscopic strain to break ( λ macr) of the PS-PPE blends correlated with the theoretical value ( λ max) based on stretching the entanglement network to its full extension. The transition from a macroscopic, brittle-to-ductile deformation behaviour is associated with a change in type of deformation mechanism from void formation (e.g. crazing) to shearing, except for the PS-PPE 20–80 blend, which always deforms by shear deformation. A simple model based on an energy criterion could explain the occurrence of a critical material ligament thickness as well as its dependence on the entanglement density.