Effect of chain entanglements on plastic deformation behavior of ultra‐high molecular weight polyethylene

Samples of ultra‐high molecular weight polyethylene, in which the chain topology within the amorphous component was altered using two‐stage processing, including crystallization at high pressure in the first step, were produced and their deformation behavior in the plane‐strain compression was studied. Deformation and recovery experiments demonstrated that the state of the molecular network governed by entanglement density is one of the primary parameters controlling the response of the material on the imposed strain, especially at moderate and high strains. Any change in the concentration of entanglements markedly influences the shape of the true stress–true strain curve. The strain hardening modulus decreases while the onset of strain hardening increases with a decrease of the entanglement density within the amorphous component. Density of entanglements also influences the amount of rubber‐like recoverable deformation and permanent plastic flow. In material of the reduced concentration of entanglements permanent flow appears easier and sets in earlier than in the material with a higher entanglement density, becoming a favorable deformation mechanism at moderate strains. As a result, strong strain hardening is postponed to higher strain when compared with the samples of equilibrium entanglement density. In the samples of an increased entanglement density the molecular network becomes stiffer, with a reduced ability of strain induced disentangling of chains. Consequently, there is a less permanent flow and strain hardening begins earlier than in the reference material of an unaltered chain topology. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 276–285, 2010     

Structure transfer from a polymeric melt to the solid state. Part II: Dependence on molecular weight

Melt spinning experiments of polyethylene, using a high quenching rate have been carried out. Molecular weight has been varied. From measurements of the mechanical properties of the monofilaments produced it is concluded that melt history influences the solid state behavior. This is reflected in the hypothesis of a transference of knots, preexisting in the melt into the solid state. Measurements of the elastic recovery allow to offer an interpretation, in which this network of knots does not percolate, until a critical value of the molecular weightM _{c knot}10⁵ is surpassed. The possible influence of these knots on the mobile entanglements is discussed.On leave from the Institut für Technische und Makromolekulare Chemie, Universität Hamburg, Hamburg, Germany.     

Critical review of the molecular topology of semicrystalline polymers: The origin and assessment of intercrystalline tie molecules and chain entanglements

Intercrystalline molecular connections in semicrystalline polymers have been the subject of numerous discussions and controversies. Nevertheless, there is one point of agreement: such intercrystalline tie molecules have a prime role in the mechanical and use properties of the materials, notably the resistance to slow crack growth. This article is a critical review of the mechanisms of generation of the tie molecules during the stage of crystallization and of the experimental and theoretical assessment of their concentration. Polyethylene and related materials are mainly studied. The contribution of chain entanglements is also discussed in parallel with tie molecules. Particular attention is paid to Huang and Brown's statistical approach, which appears to be the most appropriate one for predictive purposes and has aroused much interest from various authors. Attempts are made to provide solutions to the shortcomings of this model. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1729–1748, 2005     

Morphology and properties of globular polymeric materials in the solid state: A composite material of DNA with a cationic surfactant

The aim of the present study was to control entanglements in order to regulate the properties of polymeric solids. Initially, fabrication of polymeric solids with few entanglements was attempted. Films of the DNA–cationic surfactant, cetyltrimethylammonium bromide (CTAB) (DNA–CTA), were cast from ethanol solution at room temperature. Morphological examination of DNA–CTA complex films using atomic force microscopy (AFM) revealed that these films were constructed by particle‐like substances. Geometrical analysis of AFM images showed that the particle‐like substances were the aggregates of several DNA–CTA globules. Mechanical characterization suggested that there were fewer entanglements than with normal plastic films. Small angle X‐ray scattering experiments during annealing indicated that molecular motions were highly excited in the surface region of each particle. In conclusion, a globular polymeric film with fewer entanglements was fabricated. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 730–738     

Structure transfer from a polymeric melt to the solid state Part III: Influence of knots on structure and mechanical properties of semicrystalline polymers

Investigations of the influence of crystallization conditions and molecular weight on mechanical properties of melt spum monofilaments have been carried out in previous studies [1, 2]. The present results emphasize the influence of crystallization upon two classes of molecular entanglements present in the melt:

1. mobile entanglements with short life times;
2. long living knots as a superstructure of the entanglement network.

On the other hand, it is shown that the former number of entanglements controls the thickness of the crystalline lamellae. It is pointed out that knots also have a specific influence on morphology. Knots form the amorphous phase of a semicrystalline polymer and are mechanically active. The present model is comparatively discussed in light of Peterlin's plastic deformation model and Kilian's van der Waals Network Theory.     

The thermo-oxidative degradation of metallocene polyethylenes: Part 2: Thermal oxidation in the melt state

The thermo-oxidative melt degradation of different metallocene polyethylenes (mPEs) was investigated in a torque rheometer open to air at 225 °C and 10 rpm. The mPEs differed essentially according to their initial melt index, molar mass distribution, density and ash content, but one characteristic was changed at a time in order to assess the influence of each specific property in the thermo-oxidative degradation of the PEs investigated. Crosslinking was found to dominate at the early stages of degradation during mastication for most polymers where reactions of alkyl radicals to vinyl groups were considered to be the dominant reaction. Furthermore, discolouration was attributed to both excessive levels of catalyst residues and extensive formation of conjugated systems. Finally, it was concluded that the polymer melt viscosity, i.e., molar mass and shape of molar mass distribution, appeared to govern the processing stability of the mPE. These results confirm the importance of shear as the major source for initiation of free radicals formed by homolytic fission caused via mechanical cleavage of polymer chains.     

Thermolysis of polyethylene hydroperoxides in the melt. Part 9: Re-examination of the experimental kinetics of hydroperoxide decomposition

The experimental kinetics of decomposition of polyethylene hydroperoxides in the melt is re-examined. It is found that the rates determined are more accurate if only the “free” hydroperoxides are taken into account instead of the total hydroperoxides that include also the “associated” hydroperoxides. Then, decomposition of polyethylene hydroperoxides in the melt can be attributed unambiguously to a first-order reaction that is valid in the whole time range of the thermolysis experiments. Nevertheless, the first-order rate constant determined this way increases with the initial hydroperoxide concentration. This constitutes a significant difference with the first-order rate constants that are valid in low molecular mass chemistry and are independent of the initial concentration of the reacting species. It has already been concluded previously that this experimental first-order rate cannot be attributed to true monomolecular hydroperoxide decomposition. Hence, another or other reactions must be envisaged for the interpretation of the specific first-order decomposition of the hydroperoxides in polyethylene melts.     

Radiolytic unsaturation decay in polyethylene. Part III—the effect of certain chain transfer agents

Small amounts of certain halogenated compounds are found to have, at most, only a slight enhancing effect on the radiolytic decay rates of added poly-unsaturated compounds in polyethylene, but significantly increase the elastic modulus at 433 K (melt modulus) obtained thereby. Experiments with model chlorine-containing additives suggest that this increase is due to a more random distribution of polymer and monomer mediated crosslinks in the polymer, that it does not result from a significant increase in crosslinking and that it is mediated by chlorine atoms, in a similar manner to radiolytic hydrogen atoms, through facilitation of long range free radical migration. Although low molecular weight chloro-paraffins inhibit radiolytically induced growth of melt modulus in monomer containing polyethylenes, even very small additions of chlorinated polyethylenes, which form a separate phase, increase the melt modulus. This again indicates that the active species is the chlorine radical.     

Effect of a cocatalyst modifier in the synthesis of ultrahigh molecular weight polyethylene having reduced number of entanglements

The use of a hindered phenol to trap free trimethylaluminum (TMA) in methylaluminoxane (MAO) solutions has been reported to improve the performance of single‐site, homogeneous catalysts for olefin polymerization. In the present study, with the help of rheological analyses, we have investigated and compared the molecular weight, molecular weight distribution and entanglement density of ultrahigh molecular weight polyethylene synthesized with a single‐site catalyst activated by MAO and phenol‐modified MAO. While the number average molecular weight (M_n) of the obtained polymers remains the same for both activations, a higher yield and a higher entanglement density are found in the initial stages of polymerization on using phenol‐modified MAO as the cocatalyst. These results suggest that on using the phenol‐modified MAO as activator, a higher number of active sites are obtained. Surprisingly in the presence of untreated MAO, a tail in the higher molecular mass region is produced. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013     

Calorimetry in nonstandard conditions: The noncrystalline phases of linear polyethylene

A linear Union Carbide PE (UC) has been analyzed by nonstandard calorimetry with a common DSC calorimeter and a Setaram C80 calorimeter. Nonstandard calorimetry entails using a low rate of heating (0.5–1 K/min), a small mass (0.5–1.5 mg), and an open cell (O‐cell) instead of the standard C‐cell. Events in O‐cells overlap less and occur with a faster kinetics than in C‐cells. PE crystals are nascent, solution‐grown (S‐grown), press‐grown (P‐grown), and strained by extrusion. In Part A, the traces show that the phase‐changes in the melt, previously observed in a C80 calorimeter (slow T‐ramp) and characterized by ΔH_network, can be observed with a common DSC in nonstandard conditions. In Part B, the difference between the C‐ and O‐cells and the changes in the main peak enthalpy (ΔH_ortho) are of interest. The main result is that, in O‐cells, the value of ΔH_ortho around T_ortho, exceeds unambiguously in certain conditions ΔH_ortho found for perfect orthorhombic crystals. The main endotherm contains then another contribution, namely ΔH_network. Crystal reorganization during the slow T‐ramp is followed in the C‐ and O‐cells on S‐grown crystals. In O‐cells, lamellar thickening observed in the slow‐ramp is more extensive. The ease of phase‐change depends on the sample history. It is as follows: strained‐part extruded > nascent > S‐grown > P‐grown. Co‐operative chain motions are more hindered in the standard C‐cells than in the O‐cells. In Part C, lower values of m succeed in bringing phase‐changes in P‐grown (O‐cells) samples. The origin of the events is discussed: three processes are thought to contribute to the phase‐changes namely, melting of strained short‐range order, activation of vibrations in the CH₂ groups, and fast decay of chain orientation which occurs simultaneously with melting. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1932–1949, 2007     

Thermolysis of polyethylene hydroperoxides in the melt, Part 11: Relative yields of thermolysis products

The experimental ratios of the main products from polyethylene hydroperoxide thermolysis are examined. Comparison with the corresponding theoretical ratios calculated for different hydroperoxide decomposition reactions allows discriminating between the main hydroperoxide decomposition reactions. The experimental values can usually be explained best by the true bimolecular reaction involving two hydroperoxide groups. Mostly these values are significantly different from the theoretical ratios calculated for the bimolecular reaction with an alcohol group and for the pseudo-monomolecular reaction with a segment of the polymer. The bulk of the results points unequivocally to true bimolecular hydroperoxide decomposition for explaining thermolysis of polyethylene hydroperoxides.     

The influence of short chain branch on formation of shish‐kebab crystals in bimodal polyethylene under shear at high temperatures

Formation of shish‐kebab crystals due to the coil–stretch transition under shear in the molten state using a bimodal polyethylene system with high molecular weight (HMW) fraction having different branch content was investigated. In specific, in situ small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques were used to study the structure evolution of shish‐kebab crystals at high temperatures under simple shear. The SAXS results revealed that with the increase of branch content, shish‐kebab crystals became more stable at high temperatures (e.g., 139 °C). However, the shish length of the bimodal PE containing 0.11% branch was shorter than that with no branch. The WAXD results showed that the degree of crystallization for bimodal PE with HMW fraction having 0.11% branch increased with time but reached a plateau value of 1%, while that with no branch increased continuously till 11%. Furthermore, the crystal orientation of bimodal PE with HMW fraction having 0.11% branch was above 0.9 and maintained at a constant value, while that with no branch decreased from 0.9 to 0.1 upon relaxation. This study indicates that even though the crystallizability of the HMW fraction with branch content decreased, they could effectively stabilize the shear‐induced crystalline structure with shorter shish‐kebab crystals. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 786–794     

Composition-Dependent Properties of Polyethylene/Kaolin Composites: VI. Thermoelastic behavior in the melt state

The equilibrium pressure-volume-temperature properties and thermoelastic behavior of kaolin-filled composites of the injection molding grade of high density polyethylene (HDPE) were studied in the temperature interval 423–473 K and in thepressure range 30–100 MPa. It was established that the HDPE melts in filled composites existed in a somewhat expanded, more compressible state. This effect was quantitatively accounted for by the increased number of external degrees of freedom derived from the Simha-Somcynsky equation of state. The effect of the coupling agent on thermal diffusivities and specific heat capacities of filled samples turned out negligibly small.This revised version was published online in November 2005 with corrections to the Cover Date.     

The influence of vinyl activating groups on β‐allyl sulfone‐based chain transfer agents for tough methacrylate networks

In radical polymerization of monofunctional monomers, addition fragmentation chain transfer (AFCT) agents are well known to regulate polymerization and yield polymers with lower molecular weights and narrower molecular weight distributions. Papers concerning bulk photopolymerization of monomer mixtures with AFCT agents are rarely found in literature. In this article, AFCT reagents based on β‐allyl sulfones with different vinyl activating groups were synthesized and compared. The compounds were tested in mono‐ and difunctional monomer systems providing information about the influence on photoreactivity, molecular weight, as well as thermal and mechanical properties of the resultant polymers. Where more potent activating groups (‐Ph, ‐CN) markedly influenced polymerization at lower concentrations, the AFCT reagent with an ester activating group reacted at a similar rate to the methacrylate monomer (C_T ≈ 1) and provided the best overall performance. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1417‐1427     

The side chain effects on TPD-based copolymers: the linear chain leads to a higher jsc

Abstract

Alkyl substituents appended to polymers play the determining role on self-assembly and film-forming properties, and on device performance. In this work, we highlight the effects of the linear and branched flexible chains appended to the acceptor moiety (A) in D-A type copolymers. Two thieno[3,4-c]-pyrrole-4,6-dione (TPD) based copolymers PT1 and PT2 with different alkyl chains, were designed and synthesized. By comparison their UV-vis absorptions, HOMO/LUMO energy levels, as well as the characters in polymer solar cells, the influences of alkyl chains were investigated. Both copolymers showed molecular weights of 21?kDa and similar optical properties with a medium band gap of 1.93?eV, while PT2 with the branched chain exhibited a lower HOMO than that of PT1 (?5.43 vs???5.37?eV). In bulk heterojunction (BHJ) solar cells, PT1 with a linear chain presented a short circuit current (J_sc) of 6.76?mA cm^?2, open circuit voltage (V_oc) of 0.89?V and power conversion efficiency (PCE) of 2.92%. To the contrary, PT2 showed a J_sc of 3.53?mA cm^?2, V_oc of 0.99?V, delivering a relatively lower PCE of 2.05%. The result indicates that appending a linear alkyl chain to the TPD unit could sufficient enhance the J_sc value of the related polymer.     

Diffusion of linear aliphatic esters in polyethylene as a function of chain length of the esters,pressure and temperature

Diffusion of linear aliphatic mono- and diesters (C _N ) havingN main chain atoms (N=13–68) in bulk medium-density polyethylene (MDPE) has been studied under hydrostatic pressures up to 2500 bar at temperatures between 60°C and 125°C. Three triglycerides, phenyl stearate, and p-aminoazobenzene (pAAB, 80°C) as the diffusants and low-density (LDPE) and high-density (HDPE) polyethylenes as polyethylene substrate were used for comparison. Diffusion coefficientD was determined from concentration distribution of the diffusants through stacked PE sheets as substrate. Regarding the linear esters at 90°C, the relationshipD N holds at constant pressures. Under the atmospheric pressure, became –2.10 in accordance with de Gennes's proposal (1971)D N ^–2 as well as with the experimental results reported by Klein and Briscoe (1979) forN larger than 30.D's for the glycerides deviate from the relationshipD N ^–2 toward the smaller values by comparison at the sameN. The exponent is pressure-dependent. It decreases with increasing pressure according to =–2.10–0.000942P, whereP is measured by the unit of bar. Plots of lnD vsP for all the diffusants show linear relationships with negative slopes, from which activation volume for the diffusion V was calculated. At 90°C, V increases slowly with increasingN and increasingV _Ki, the intrinsic molecular volume of the diffusant, from 39.3 cm³/mol for ethyl caprate (C ₁₃,V _Ki=136 cm³/mol) to 76.8 cm³/mol for behenyl behenate (C₄₅,V _Ki=466 cm³/mol). Observed V s are explainable on the basis of the reptation mode of the chain molecule diffusion. V s for C₂₅ and C₄₅ are found to increase with increasing degree of crystallinity where MDPE, heat-treated MDPE, LDPE, and HDPE were used. The results obtained by varying temperature are as follows. V for C₄₅ was always found to be larger than C₂₅. Both decreased linearly with increasing temperature, giving two linear lines with different slopes whose extensions intersected at 132°C, the melting point of the MDPE, where the difference in V disappeared. The apparent activation energiesE _Ds for the diffusion of C₂₅ and C₄₅ increased linearly with increasing pressure, whose slopes are explainable according toE _D=E ₀+PV [1-(dln V /dlnT) _P ].     

Novel highly elastic magnetic materials for dampers and seals: Part I. Preparation and characterization of the elastic materials

The new generation of magnetic elastomers represents a new type of composites, consisting of small (mainly nano and micron‐sized) magnetic particles dispersed in a highly elastic polymeric matrix. The combination of polymers with magnetic materials displays novel and often enhanced properties. Highly elastic magnetic composites are quite new and understanding of the behavior of these materials depending on the composition, external conditions, and the synthesis processes is still missing. Thus, the aim of this work is the study of fundamental principles governing the preparation of these materials as well as their structure and elastic properties. Copyright © 2007 John Wiley & Sons, Ltd.     

The temperature window of minimum flow resistance in melt flow of polyethylene. Further studies on the effect of strain rate and branching

The existence of a narrow temperature window (150–153°C) of smooth extrudability coupled with a minimum in flow resistance (extrusion pressure) in high-molecular weight polyethylene (>4 × 10⁵ g mol^?1) was the subject of a previous article where it was associated with strain-induced formation of the mobile hexagonal mesophase. The new findings of this note show that this minimum in flow resistance only sets in above a critical strain rate; this is interpreted in terms of the requirement of a critical strain rate in order to stretch molecules to their fully extended configuration. Furthermore, this critical strain rate is shown to be higher for lower molecular weight materials, in agreement with a priori considerations. Additionally, the temperature at which the pressure minimum occurs in a polyethylene containing methyl branches shifts to a significantly lower value than that for the linear material. This is interpreted in terms of the ? CH₃ groups raising the crystal free energy, thereby lowering the temperature at which the transition to the hexagonal phase occurs.