Chain orientation in natural rubber, Part II: 2H-NMR study

Stress-induced crystallisation (SIC) and stress-induced melting (SIM) in natural rubbers (NR), unfilled and filled with carbon black (CB) have been studied by ²H-NMR measurements. Various materials have been swollen with small amount (< 2%) of deuterated alkane chains. The orientation of the amorphous chains, then the local deformation of the amorphous chains during deformation cycles and during stress relaxation, permits to clarify the SIC and SIM processes during hardening and recovery. By mechanical, WAXS and NMR measurements one determines the same critical draw ratio for appearance λ_A and disappearance λ_E of the crystallites. It is demonstrated that the hysteresis observed by the different techniques (stress σ, crystallinity χ, NMR splitting Δν) are due to the supercooling effect ( λ_A > λ_E, at constant temperature). During hardening at constant strain rate it is found that the local draw ratio remains constant and equal to λ_A, whereas the crystallinity increases linearly with the macroscopic draw ratio λ. The hardening σ ∼ (λ - λ_A)² is then interpreted as a reinforcement effect due to the crystallites, which act as new crosslinks. This confirms the prediction of Flory. In filled rubber the same effects are observed, and the stress amplification factor is determined as a function of the CB content. It is found that the fillers act as nucleation centres for the NR crystallites. The reinforcement of such materials is due principally to this nucleation effect and to the presence of a super network formed by both the NR crystallites and the CB fillers.     

Influence of Carbon Black on Crosslink Density of Natural Rubber

Vulcanization and reinforcement are two important factors contributing to the properties of vulcanized rubber. In order to investigate the influence of carbon black (CB) on chemical crosslinking, three groups of samples with different crosslink densities were prepared. In each group with the same crosslink density, different amounts of CB were introduced. Data fitting showed that delta torque (ΔM = M _H–M _L, the difference between the highest and lowest torques during curing) in the cure curves of each group had a good linear relationship with CB load and extrapolation of the fitting lines almost intercepted the x coordinate at the same value, which indicated that CB had no influence on the chemical crosslinking of the rubber. To verify the above result, a series of nonfilled natural rubber (NR) vulcanizates with different crosslink densities were studied using equilibrium swelling and the swelling ratios were compared with those of corresponding CB filled rubbers with the same sulfur and accelerator amount. The results of both the equilibrium swelling and NMR relaxation parameter measurements showed that CB filled vulcanizates had higher apparent crosslink densities than those of unfilled ones due to the strong interaction between rubber molecules and the surface of the CB particles. The swelling ratios of filled rubbers had a parallel relationship with those of the unfilled ones which indicated that CB had little influence on chemical crosslink density introduced by chemical vulcanization.     

Stress-Strain Behaviors and Crosslinked Networks Studies of Natural Rubber-Zinc Dimethacrylate Composites

The stress-strain behaviors of natural rubber (NR)-zinc methacrylate (ZDMA) composite have been studied by uniaxial tension. The results indicated that there was a large reinforcement by ZDMA and the NR/ZDMA composites exhibited a high stress-softening effect. Meanwhile, the recovery stretch curve was close to the second stretch curve; thus a weak stress recovery of the composites was shown. The analysis of crosslink density indicated that the damage to the crosslink network was mainly due to the breakage of ionic crosslinks at low strain (100%). A more developed ionic crosslink network was formed at a higher content of ZDMA. When the vulcanizate is subjected to loading in tension, the ionic crosslink network will suffer the force first. Next, the slippage of ionic bonds will take place under the stress. A new ionic crosslink network might be formed rapidly after the ionic bonds were broken during the stretching. Therefore, it could not return to the initial state. The analysis of crosslink density and stress recovery indicated that the rubber chains could be adsorbed to the ZDMA aggregates due to the formation of poly-zinc methacrylate (PZDMA). A molecular analysis of NR/ZDMA composites is proposed in the last part of this article.     

Influence of Crosslink Density on Mechanical Properties of Natural Rubber Vulcanizates

Crosslink density is an important structural parameter for cured rubber. Natural rubber (NR) vulcanizates with different crosslink densities were obtained through using different sulfur and accelerator amounts and different accelerator types. The crosslink density was characterized by an ¹ H-NMR technique and its influence on mechanical properties, such as Shore A hardness, 300% modulus, tensile strength, and elongation at break, of NR vulcanizates was investigated. The results showed that both the sulfur amount and the accelerator type and amount had an influence on the crosslink density of the NR networks. The relationship between total crosslink density and mechanical properties was also studied. The results, by changing either the sulfur or the accelerator amount, showed that tensile strength of NR vulcanizates reached maximum value when the total crosslink density was around 13.5 × 10^?5 mol/cm³, equivalently the average molecular weight of the intercrosslink chains (Mc) was around 7000 g/mol. The maximum value of tensile strength came from the balance between contributions of crosslink joints and stretch-induced orientation and/or crystallization of intercrosslink chains. The study on influence of total crosslink density on Shore A hardness and 300% modulus of NR vulcanizates showed that they both increased linearly with the crosslink density, the slopes were 2.7 ～ 3.0 cm³/10^?5 mol and 0.27 ～ 0.31 MPa cm³/10^?5 mol for Shore A hardness and 300% modulus, respectively, whether the crosslink density was varied by sulfur or accelerator.     

A new analytical method for the optimization of thermomechanical conditions in zone drawing and its application to polyethylene

An original method of zone drawing of polymers at constant load and a procedure for the optimization of thermomechanical conditions (stress, temperature) are suggested, allowing high draw ratios and favorable strength values to be obtained. The temperature (or stress) range of necking has been determined in a nonisothermal and in an isothermal regime. The advantage of the method consists in that the increasing orientation in the neck starting from the initiation point and up to fracture allows the morphology and properties to be quickly examined, depending on the varying thermomechanical conditions in different regions of the neck. At a high temperature and low load the mechanism of oriented crystallization from melt becomes operative; in opposite cases, orientation of the solid state takes place. It is shown that in the nonisothermal regime an increased rate of heating allows extreme draw ratios (up to ca. 150) to be obtained, approximately twice as high as those obtained in the isothermal regime. This is interpreted as a gradual improvement of the oriented structure by recrystallization during extension and by a quick fixation on cooling below the zone.     

Biaxial Orientation Mechanism of Drawn Natural Rubber. I. Appearance of the Biaxial Orientation

Triple centrifuged natural rubber (NR) film was crystallized at room temperature by uniaxial drawing at draw ratio (λ) 6.0. Morphologies of the induced crystals were studied by wide angle X‐ray diffraction (WAXD). Using a special drawing apparatus for shifting the X‐ray beam positions, crystalline orientation was surveyed all over the drawn sample. WAXD results show the biaxial orientation (BO) with the c‐axis being parallel to the draw direction and the a‐axis parallel to the film surface. By means of the WAXD pattern with the incident beam perpendicular to the film, the drawn NR film was separated into three regions: (200+120) region indicating a low degree of BO, (200) region having a high degree of BO, and the intermediate region in between the above two regions. The degree of BO is enhanced from center to the clamping end depending on the increment of the sample width. At lower draw ratios (4.0<λ<6.0), BO was developed with increasing draw ratio. In the intermediate region, BO increases sharply with a small increment of the width. The degree of BO is lowered at the periphery of the film.     

The Effect of Reclaim Softener on Properties of Reclaimed Rubber

The reclaiming process of full tire rubber powder with no treatment was carried out with a torque rheometer. The effect of reclaim softener types and loads on reclaiming and the properties of the revulcanized reclaimed rubber were investigated. The sol fraction significantly increased and the crosslink density gradually decreased with the increase of reclaim softener loads, but the reclaimed rubber with wood tar reclaim softener had the lowest sol fraction and highest crosslink density at the same loads as other reclaim softeners. The types of reclaim softener had almost no influence on the types of scission, but the proportion of main-chain scission increased with the increase of reclaim softener loads. An improvement of processability was observed with the increase of reclaim softener, but it was accompanied by a decrease in tensile strength. Dynamic mechanical properties of the reclaimed rubber were also investigated. The uncured reclaimed rubbers loaded with reclaim softener showed much lower G′values and slightly higher tan δ values than the reclaimed rubber without reclaim softener and the cured reclaimed rubbers showed much lower G′values and slightly lower tan δ values than the blank.     

On line interferometric investigation of the neck propagation phenomena of stretched polypropylene fibre

On line interferometric investigation of the neck propagation phenomenon of stretched fibre is carried out using an automatic multiple-beam Fizeau fringes technique. It was observed that under such a deformation condition, a neck deformation is formed and propagated steadily towards the gripped ends of the stretched sample. The neck propagation was recorded carefully during the course of the tests by means of a CCD camera. The fringe shift profile due to the neck formation was analyzed during the propagation stage. A polypropylene (PP) sample was stretched until a neck deformation is formed at room temperature. The 3D time-refractive index profile is investigated to clarify the fixed profile of propagation. The obtained microinterferograms are clarifying the fixed neck profile during the propagation of necking phenomena. The speed of neck propagation was calculated. The dependence of the propagation on the drawing speed and draw ratio was investigated.     

Biaxial Orientation Mechanism of Drawn Natural Rubber. IV. The Effect of Aspect Ratio

Biaxial orientation (BO) of crystallites was observed for uniaxially elongated natural rubber (NR) films as a function of an aspect ratio (AR=width/length of the original sample). The wide angle X‐ray diffraction results were analyzed, showing that the crystalline c‐axis was parallel to the draw‐direction, while the a‐axis became parallel to the film surface. The degree of BO was quantitatively evaluated by an orientation function of the (200) plane, F ₂₀₀. The value of F ₂₀₀ increased with increasing AR, showing that the mechanism of microscopic crystallization is controlled by the macroscopic AR. The observed BO is consistently explained by the shish kabab‐like secondary crystallization in the ellipsoidal free space surrounding the primary fibril. The shape of free space becomes highly ellipsoidal in the film having high AR.     

Characterization of commercial polybutylenes. part II. Crystallinity and tensile properties

A series of previously characterized polybutylenes were compression molded using various cooling conditions. Crystallinity decreased as cooling rate increased. The samples exhibited three types of tensile behavior. The crystallinity of samples containing around 5% ethylene was about 40%. These behaved as elastomers, exhibiting typical entropic elasticity. For the remaining samples, crystallinities between 55% and 70% were observed, depending on grade, and more particularly on cooling conditions. For these, lower crystallinity levels favor sample necking and stress-induced orientation, producing high elongation at break and high tensile strength values, while the higher crystallinity samples show high moduli and elasticity values coupled with uniform deformation. In the former case, significant crystalline deformation—causing alignment of the c axes of the crystallites towards the stretching direction, and thereby reinforcing the sample—is responsible for the high elongations and tensile strength encountered. In the higher-crystallinity case much of the deformation is taken up by interlamellar void generation, with little or no c-axis alignment, this elastic mechanism having been termed “hard elasticity.”     

三种测量橡胶交联密度的核磁共振方法比较

核磁共振（NMR）技术已经成为测量橡胶交联密度的一种常用技术手段．该文探讨了用于测量橡胶交联密度的三种NMR方法：¹H双量子（DQ）NMR、¹H Hahn回波和¹H CPMG回波,讨论了这些方法测量值差异的来源．在此基础上进一步研究了¹H CPMG回波方法中回波时间对测量值的影响,探讨了改善¹H CPMG回波方法的方案．     

Infrared studies of drawn polyethylene part I. Changes in orientation and conformation of highly drawn linear polyethylene

Infrared spectroscopy is applied to discuss the orientation, the crystallinity, and the conformation of chain segments in the amorphous regions in drawn high-density polyethylene. The orientation of the crystals as well as the crystallinity are derived from the dichroism and the absorbance, respectively, of the band at 1894 cm-¹. The orientation and some aspects about the conformation of the chain segments in the amorphous regions can be obtained from the bands in the 1400-1300 cm-¹ region (gauche) and at 1078 cm-1 (gauche and trans). The dichroic studies show a high degree of orientation increasing with draw ratio λ for the chain segments in the crystals, but a low orientation reaching saturation at λ between 5 and 10 for those in the amorphous regions. The experiments indicate a change in crystallinity during the drawing process which depends on the thermal treatment of the undrawn sample. In the amorphous regions the number of CH₂ groups in gauche conformations decreases up to λ between 10 and 15 and remains nearly constant with further drawing. Since the sum of gauche and trans conformations remains unchanged, it can be deduced that the number of loops decreases and that of tie molecules increases with draw ratio.     

Solid-state 13C NMR and synchrotron SAXS/WAXS studies of uniaxially-oriented polyethylene

We report solid-state ¹³C NMR and synchrotron wide-and small-angle X-ray scattering experiments (WAXS, SAXS) on metallocene linear low density polyethylene films (e.g., Exceed™ 1018 mLLDPE; nominally 1 MI, 0.918 density ethylene-hexene metallocene copolymer) as a function of uniaxial draw ratio, λ. Combined, these experiments provide an unambiguous, quantitative molecular view of the orientation of both the crystalline and amorphous phases in the samples as a function of draw. Together with previously reported differential scanning calorimetry (DSC), gas transport measurements, transmission electron microscopy (TEM), optical birefringence, small angle X-ray scattering (SAXS) as well as other characterization techniques, this study of the state of orientation in both phases provides insight concerning the development of unusually high barrier properties of the most oriented samples (λ=10). In this work, static (non-spinning) solid-state NMR measurements indicate that in the drawn Exceed^TM films both the crystalline and amorphous regions are highly oriented. In particular, chemical shift data show the amorphous phase is comprised increasingly of so-called “taut tie chains” (or tie chains under any state of tautness) in the mLLDPE with increasing draw ratio – the resonance lines associated with the amorphous phase shift to where the crystalline peaks are observed. In the sample with highest total draw (λ=10), virtually all of the chains in the non-crystalline region have responded and aligned in the machine (draw) direction. Both monoclinic and orthorhombic crystalline peaks are observed in high-resolution, solid-state magic-angle spinning (MAS) NMR measurements of the oriented PE films. The orientation is comparable to that obtained for ultra-high molecular weight HDPE fibers described as “ultra-oriented” in the literature. Furthermore, the presence of a monoclinic peak in cold-drawn samples suggests that there is an appreciable internal stress associated with the LLDPE. The results are confirmed and independently quantified by Herman's Orientation Function values derived from the WAXS measurements. The degree of orientation approaches theoretically perfect alignment of chains along the draw direction. We deduce from this observation that a high fraction of the non-crystalline chains are either tie chains that directly connect adjacent lamellae or are interlocking loops from adjacent lamellae. In either case, the chains are load-bearing and are consistent with the idea of “taut tie chains”. We note that transmission electron micrographs recorded for the ultra-oriented Exceed showed the lamellae are often appreciably thinner and shorter than they are for cast or blown Exceed 1018. Combined with higher crystallinity, the thinner lamellae statistically favor more tie chains. Finally, the remarkably large decrease in permeability of the λ=10 film is primarily attributed to the high degree of orientation (and loss of entropy) of the amorphous phase.     

Mean stress effect in fatigue crack propagation

Crack propagation rates in three different grades of mild steel and two types of age hardening aluminium alloys have been measured for different stress ratios. The results show a pronounced stress ratio effect for all these materials. A model of fatigue crack propagation is formulated in terms of the size of the cyclic plastic instability zone at the crack tip rather than the zone of plastic yielding. The micro-plastic instability zone is measured by a parameter involving the ratio of the maximum stress intensity and the stress level at which macro-plastic instability occurs in the $\text{S}\text{N}$ curve of plain fatigue test pieces. Such a parameter provides a means of normalizing crack propagation results obtained for various stress ratios.     

Influence of Zinc Oxide Amount on Cure Kinetics of Sulfur-Vulcanized Natural Rubber

A kinetic cure model with induction, curing, and postcure periods, based on cure characteristics of Natural rubber/sulfur/N-t-butylbenzothiazole-2-sulfenamide (NR/S/TBBS) system, was chosen to simulate the cure reaction of this system. Cure-curves reflecting the evolution of crosslink density were recorded as a function of curing time for NR compounds using a rubber processing analyzer (RPA). The data of experimental cure curves were nonlinear fitted with the chosen model and the kinetic parameters were determined. The results showed that the simulated curves fit well with the experimental curves. The incorporation of zinc oxide changed the cure mechanism of this system. The concentration of activated sulfurating agents, A ₀, increased with increased zinc oxide amount. With the increase of zinc oxide amount, the activity of crosslink precursors decreased slightly (k ₂ decreased) which had little influence on its activation energy. The activity of crosslink degradation decreased with the increase of zinc oxide amount from 1 phr to 2 phr. Higher zinc oxide load had no further contribution to crosslink density and reversion resistance. Parameter ψ, representing the competition between reactions of activated crosslinking precursors to form crosslinks and to form dead side-products, decreased significantly when zinc oxide increased from 1 phr to 2 phr. Higher zinc oxide load (from 2 ～ 4 phr) induced a slight increase of ψ, especially at higher cure temperature.     

Quantitative characterization of the orientation of macromolecules in intercalated nanocomposites of polyolefins/layered silicates by Raman spectroscopy

Raman spectroscopy is used to study variations in the orientational order of macromolecules in the uniaxially drawn intercalated nanocomposites based on two polymer matrices (polyethylene (PE) and isotactic polypropylene (PP)) and a filler (modified clay (MC)). The orientation parameters of macromolecules measured using Raman spectroscopy are compared with the X-ray data. It is demonstrated that, for the uniaxially drawn PE-MC and PP-MC intercalated nanocomposites, the filler impedes the orientation along the draw direction for the macromolecules localized in the noncrystalline phase of the polymer matrix. The orientational ability of the PE and PP crystallites in nanocomposites is not affected by the filler.     

Biaxial Orientation Mechanism of Drawn Natural Rubber. II. Quantitative Analysis of the Biaxial Orientation

Natural rubber film was prepared from triple centrifuged latex. The film was uniaxially drawn at room temperature and the induced crystalline orientation was quantitatively studied by wide angle X‐ray diffraction. The intensity distribution of 200 and 120 reflections confirmed that the induced crystals have a biaxial orientation with the c‐axis parallel to the draw direction. The orientation of the a‐axis was evaluated by using an orientation function of the (200) plane. The function (F ₂₀₀) indicated that the crystalline a‐axis is parallel to the film surface depending on the draw ratio and on positions in the film. The experimental results are possible to explain by shish‐kebab‐like crystallization developed from a highly oriented fibril. The secondary crystallization grows perpendicularly to the draw direction along the a‐axis. The population of the secondary crystals is controlled by an ellipsoidal free space. The shape of the ellipsoid is changed by the fibril distribution depending on width and thickness of the sample film. In this study, the quantitative biaxial orientation is consistently explained by the shish‐kebab‐like crystallization and the ellipsoidal free space.     

Characterisation of crosslinked elastomeric materials by 1H NMR relaxation time distributions.

One of the most critical structural parameters in elastomeric materials is the density of cross-linking between the polymeric chains. This chemical feature greatly affects chain motions and is determinant in controlling mechanical properties of the final product. NMR techniques are widely and efficiently applied to investigation of such materials. In this study we have measured both transverse and longitudinal 1H relaxation times of a series of polybutadiene rubber samples with increasing crosslink density induced by chemical treatment. This approach allowed the observation of T(1) and T(2) decrease with the increase of crosslink density in the samples examined. The data obtained have been analyzed and compared to theoretical models.