Molecular weight between physical entanglements in natural rubber: A critical parameter during strain-induced crystallization

The purpose of this paper is to clarify the role of crosslink density of natural rubber on its strain-induced crystallization. A series of new in situ synchrotron X-ray diffraction experiments were performed during the stretching process of weakly and highly vulcanized natural rubber samples. The experimental data have been analysed in terms of both crystallite size and crystallization rate. Moreover, a careful treatment of previously published data that might appear contradictory has been done. The comparison between all these data, coming from NR of different origins and with different crosslinking states, demonstrates that the molecular weight between physical entanglements in natural rubber appears as a key parameter for strain-induced crystallization (SIC).     

New insights into the relationship between network structure and strain-induced crystallization in un-vulcanized and vulcanized natural rubber by synchrotron X-ray diffraction

The relationship between the network structure and strain-induced crystallization in un-vulcanized as well as vulcanized natural rubbers (NR) and synthetic poly-isoprene rubbers (IR) was investigated via synchrotron wide-angle X-ray diffraction (WAXD) technique. It was found that the presence of a naturally occurring network structure formed by natural components in un-vulcanized NR significantly facilitates strain-induced crystallization and enhances modulus and tensile strength. The stress-strain relation in vulcanized NR is due to the combined effect of chemical and naturally occurring networks. The weakness of naturally occurring network against stress and temperature suggests that vulcanized NR has additional relaxation mechanism due to naturally occurring network. The superior mechanical properties in NR compared with IR are mainly due to the existence of naturally occurring network structure.     

The impact of strain-induced crystallization on strain during mechanical cycling of cross-linked natural rubber

The improvement of X-ray diffraction techniques since the last ten years allowed a renewal of the study of strain-induced crystallization in natural rubber. In particular real-time measurements are now commonplace. However, due to experimental difficulties, the exploitation of the X-ray data as far as the strain state of the remaining molten fraction is concerned has been left aside. Indeed the knowledge of the local extension of the molten chains is crucial for understanding the peculiar stress behavior observed during the crystallization process. This paper presents a systematic study of this parameter during mechanical cycling performed at various temperatures. It is shown that crystallization limits the amplitude of strain endured by the molten fraction during traction. This process of strain regulation may be one explanation for the protective effect of crystallization against tearing. A precise evaluation of the contribution of the molten fraction to the retraction force during mechanical cycling is made. The role of crystallites as nanofillers may be quantified this way.     

Role of stearic acid in the strain-induced crystallization of crosslinked natural rubber and synthetic cis-1,4-polyisoprene

Strain-induced crystallization of crosslinked natural rubber (NR) and its synthetic analogue, cis-1,4-polyisoprene (IR), both mixed with various amounts of stearic acid (SA), were investigated by time-resolved X-ray diffraction using a powerful synchrotron radiation source and simultaneous mechanical (tensile) measurement. No acceleration or retardation was observed on NR in spite of the increase of SA amount. Even the SA-free IR crystallized upon stretching, and the overall crystallization behavior of IR shifted to the larger strain ratio with increasing SA content. No difference due to the SA was detected in the deformation of crystal lattice by stress for both NR and IR. These results suggested that the extended network chains are effective for the initiation of crystallization upon stretching, while the role of SA is trivial. These behaviors are much different from their crystallization at low temperature by standing, where SA acts as a nucleating agent.     

Comprehensive study on temperature-induced crystallisation and strain-induced crystallisation behaviours of natural rubber/isoprene rubber blends

The temperature-induced crystallisation (TIC) and strain-induced crystallisation (SIC) of natural rubber (NR)/isoprene rubber (IR) blends varying in blend ratio were mainly studied by dilatometers and wide-angle X-ray diffraction measurements (WAXD). The dilatometers results showed that the real half-life of crystallisation (t_1/2) values of unvulcanised NR/IR blends were 50% less than the theoretical half-life of crystallisation values. From WAXD measurements, the degree of crystallisation of vulcanised NR/IR with IR content samples 30, 50 and 70% at strain 500% were larger than theoretical crystallinity values by 17.17, 19.99 and 29.69%. NR can significantly improve the property of crystallinity of IR during TIC and SIC processes. The results can be ascribed to the end-linked networks existing in NR which can simultaneously induce amorphous NR and IR chains into crystalline order in NR/IR blends and increase the crystallinity and crystallisation rate during TIC and SIC process.     

Strain-induced crystallization of peroxide-crosslinked natural rubber

Effect of network-chain density (ν) on the strain-induced crystallization of peroxide-crosslinked natural rubber was studied by fast time-resolved synchrotron wide-angle X-ray diffraction. It was observed that the elongation ratio at the onset of strain-induced crystallization (α_c) became smaller with the increase of ν. The difference of the entropy between the undeformed and deformed states at each α_c was nearly equal in spite of the variation of ν. The calculated melting temperature at α_c of the samples was also almost the same regardless of their ν. These observations mean that the strain-induced crystallization occurred when the deformation brought about a definite entropic state for peroxide-crosslinked natural rubber. They agreed with the prediction by Flory and were consistent with the classical theory of rubber elasticity.     

A wide-angle X-ray study of the development of molecular orientation in crosslinked natural rubber

Molecular orientation parameters have been measured for the non-crystalline component of crosslinked natural rubber samples deformed in uniaxial tension as a function of the extension ratio and of temperature. The orientation parapeters 〈P₂(cosα)〉 and 〈P₄(cosα)〉 were obtained by an analysis of the anisotropy of the wide-angle X-ray scattering functions. For the measurements made at high temperatures the level of crystallinity detected was negligible and the orientation-strain behaviour could be compared directly with the predictions of molecular models of rubber elasticity. The molecular orientation behaviour with strain was found to be at variance with the estimates of the affine model particularly at low and moderate strains. Extension of the crosslinked rubber at room temperature led to strain-crystallization and measurements of both the molecular orientation of the non-crystalline chains and the degree of crystallinity during extension and relaxation enabled the role of the crystallites in the deformation process to be considered in detail. The intrinsic birefringence of the non-crystalline component was estimated, through the use of the 〈P₂(cosα)〉 values obtained from X-ray scattering measurements, to be 0.20±0.02.     

New insights into thermodynamic description of strain-induced crystallization of peroxide cross-linked natural rubber filled with clay by tube model

Clay changes the strain-induced crystallization behavior of natural rubber and induces a dual crystallization mechanism due to the orientation of clay layers during deformation. The structure evolution was probed by in-situ synchrotron wide-angle X-ray diffraction, while the thermodynamics of the onset of crystallization was analyzed by the tube model. The entropy change required for the onset of the strain-induced crystallization of the clay filled rubber is composed of the entropy reduction due to the rubber-filler interactions and also the stretching. The summation of the two kinds of the entropy reduction is nearly equal to that of the neat rubber. The thermodynamic analysis reveals that the orientation of clay layers along the direction of stretching reduces the chain conformational entropy and changes the crystallization mechanism. These results give some new insights into the strain-induced crystallization process and the reinforcement mechanism for the clay filled rubber.     

Changes in crosslinking structure of thermo-oxidative aging chloroprene rubber and its effect on strain-induced crystallization

Chloroprene rubber (CR) is frequently quoted as an example of strain-induced crystallizing elastomer. Many researchers have focused on studying the aging mechanism and mechanical properties of aged CR, little has been done on the effect of crosslinking structure on strain-induced crystallization (SIC). The present paper related systematic investigations combining the changes of crosslinking structure and its influence on SIC. It was firstly investigated based on FTIR, crosslinking density test and GPC. The results showed that the molecular chains were oxidized in the early stage of aging, but the crosslinking reaction still dominated. At the same time, sol content and polydispersity decreased. In the later stage of aging, CR was oxidized, resulted in severe molecular chains fracture and an increase in the free sol content and polydispersity. The impact of aging on SIC under tensile condition was further investigated by the tensile tests. An obvious stress-hardening was found for unaged CR, but not in aged CR. The crystallinity and microcrystalline region orientation tests showed that oxidation, crosslinking by double bonds and dechlorination during aging were not conducive to the orientation of molecular chains during stretching. It is the aging process that has an adverse effect on the SIC of CR.     

The use of model polyisobutylene networks to study strain-induced crystallization

Summary Model elastomeric networks were prepared by trifunctionally end linking hydroxyl-terminated chains of polyisobutylene having number-average molecular weights in the range 10^–3M_n = 2.4 – 10.7 g mol^–1. Their stress-strain isotherms in elongation at 25°C in the unswollen state showed significant increases or upturns in modulus at high elongations, due to strain-induced crystallization. Increase in degree of cross-linking (decrease in M_n) was found to decrease the elongation required to initiate crystallization and the maximum extensibility, but to increase the magnitude of the upturn in modulus.     

天然橡胶硫化动力学研究

采用硫化仪测定了2种不同凝固工艺的天然橡胶（NR）的硫化过程,探讨了制胶工艺、配方和温度对硫化动力学参数的影响。研究结果发现,随着温度的上升,硫化速率常数迅速上升。在纯胶体系中,微生物凝固胶的活化能高于酸凝固胶。当加入炭黑后,酸凝固胶的活化能升高,而微生物凝固胶的活化能下降。     

Parameters governing strain induced crystallization in filled natural rubber

A series of in situ synchrotron X-ray diffraction experiments are performed during the stretching of weakly and highly vulcanized carbon black (CB), silica and grafted silica filled natural rubber sample (NR). Conversely to literature, Mullins effect observed after one stretching cycle modifies the strain induced crystallization (SIC) behaviour of the sample. The onset of crystallization is ruled by the strain amplification induced by the filler presence. Moreover, fillers (CB and silica) behave as additional crosslinks into NR network, through filler-rubber interactions that either accelerate or slow down the crystallization rate depending on NR matrix chemical crosslink density. This is consistent with the assumption that effective network density, which is due to chemical crosslinks, entanglements, and filler-rubber interactions, controls the crystallization rate.     

High‐density polyethylene/natural rubber blends filled with thermoplastic tapioca starch: Physical and isothermal crystallization kinetics study

High‐density polyethylene/natural rubber (HDPE/NR) blends filled thermoplastic tapioca starch was studied. In this blend system, thermoplastic starch (TPS) acted as an inert component, and the influence of TPS incorporation was studied in terms of tensile properties and crystallization kinetics. Tensile properties of the blends were affected by the addition of TPS particles, which reflects the incompatibility and lack of adhesion at the interface. The effect of TPS incorporation on the crystallization behavior of the HDPE/NR blends was also evaluated at different predetermined crystallization temperatures. The isothermal crystallization data obtained in this study were analyzed by using the Avrami equation. The Avrami exponent for HDPE/NR and HDPE/NR‐10% TPS blends varied around 2.0 and slightly decreased around 1.8 for HDPE/NR‐30% TPS, implying the nucleation process is heterogeneous and the crystal growth is 2D. J. VINYL ADDIT. TECHNOL., 22:191–199, 2016. © 2014 Society of Plastics Engineers     

Characteristic time of strain induced crystallization of crosslinked natural rubber

Real time Wide-Angle X-ray Scattering (WAXS) measurements during cyclic tensile tests at high strain rates (from 8 s^?1–280 s^?1) and at room temperature on crosslinked Natural Rubber (NR) are performed thanks to a specific homemade device. From the observed influence of the frequency on the crystallization index at the maximum sample elongation, a characteristic crystallization time is deduced. This is done taking into account the material self-heating during such unusually high strain rates. Two regimes for the dynamic process of strain induced crystallization are evidenced. For the NR tested, the obtained characteristic time is around 20 ms when the material average elongation during the cyclic test is above a critical elongation value λ_c. λ_c is the minimum elongation needed to induce crystallization during low strain rate tensile tests. Moreover, a rapid increase of this characteristic time is found when the average elongation decreases below this critical value.     

Control the strain-induced crystallization of polyethylene terephthalate by temporally varying deformation rates: A mechano-optical study

The mechano-optical behavior of PET is, for the first time, investigated under temporally varying rates to influence the basic mechanisms of structural organization leading to strain-induced crystallization. For this purpose, four rate profiles, Linear, Sigmoidal, Logarithmic and Exponential, were chosen and films were stretched in Uniaxial Constrained Width mode using newly developed biaxial stretching machine. This machine allows real time direct measure of true stresses, strains, in-plane and out-of-plane birefringences during the deformation. Substantial differences in the mechano-optical behavior and resulting structural mechanism were observed in all of the chosen rate profiles. Linear profile, taken as a standard, yields three stress-optical regimes (SOR) during deformation. At early stages of deformation the birefringence remains linear with stress and material remains amorphous. This is designated as Regime I representing classical stress-optical behavior observed in large number of non-crystallizable polymers. In Regime II, a fast increase of birefringence accompanies formation of crystalline structure with establishment of long-range connected network. In the final Regime III birefringence levels off as the chains approach their finite extensibilities.All three regimes observed in Linear profile are also observed in Logarithmic and Exponential cases. However, Sigmoidal deformation shows only the first two regimes even though the film was stretched to the same total engineering strain as applied to all profiles. Logarithmic profile was found to induce early strain crystallization leading to early development of strain hardening. Exponential profile on the other hand retards the formation of “potentially constraining” long-range physical networks. This allowed the development of higher birefringence and crystallinity levels using this mode. A logarithmic birefringence-work relationship with two distinct stages was found to apply to all temporally varying profiles.     

蛋白质对天然橡胶硫化动力学的影响

选用与天然胶乳中的d球朊结构类似的牛白蛋白,将其加入到经Alcalase蛋白酶处理得到的低蛋白天然橡胶中,通过硫化仪法研究了蛋白质对胶料硫化动力学的影响。结果表明：各胶样的硫化反应可按一级反应处理;在相同温度条件下,随着蛋白质用量的增加,焦烧时间（t10）和正硫化时间（t90）缩短,硫化反应速率提高,说明牛白蛋白能够提高天然橡胶的硫化速度;硫化速率随着温度的升高而显著提高,且都能很好地符合阿伦尼乌斯方程;反应活化能随着蛋白质用量的增加而提高。     

Effect of non‐rubber substances on vulcanization kinetics of natural rubber

Effect of non‐rubber components on vulcanization kinetics of natural rubber was studied with the use of a Rheometer MDR‐2000. The results show that the rate constants of induction period and curing period of natural rubber (NR) are greater than that of natural rubber extracted with acetone (NR_E), and the activation energies of induction period and curing period of NR are lower than that of NR_E. The activation energy of induction period of NR is reduced by16.9% and the activation energy of curing period of NR is reduced by 3.2% compared to the activation energies of NR_E. The time t_dis of NR is shorter than that of NR_E at the same temperature. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

PET/PP共混合金结晶动力学研究

用GJY-3光学解偏振仪测定了PET、PP与PET/PP合金的结晶速度,并且在不同的结晶温度下比较了它们结晶速度的快慢。最后通过Avrami方程对PET/PP(3:1)合金进行了结晶动力学的研究。结果表明,其Avrmi指数为3。     

A differential scanning calorimetry study of the crystallization kinetics of tristearin-tripalmitin mixtures

A comprehensive study of the isothermal crystallization kinetics of tripalmitin-tristearin mixtures was carried out using DSC, with data fitted to the Avrami equation. Polymorphs were identified by subsequent melting of samples in the differential scanning calorimeter, with additional confirmatory information obtained from wide-angle X-ray diffraction. It was found that α-, β′-, and β-forms require small (<1.0°C), moderate (3.5–8.5°C), and large (9.0–13.0°C) amounts of subcooling below their respective polymorph melting temperatures for nucleation to occur. Concurrent crystallization of β and β′ polymorphs was not observed. The β polymorphs exhibited sharper heat flow exotherms than β′, due to the higher crystallization driving forces experienced. Analysis of apparent induction times shows that the activation free energy of nucleation for the β-form is significantly higher than for the β′-form. Samples rich in either species crystallized faster (both shorter apparent induction times and sharper peaks) than samples with equivalent compositions. Driving-force arguments do not fully explain this behavior, strongly suggesting that mass transfer resistances (greatest for equivalent compositions) have a significant effect on kinetics. Multiple crystallization events were observed for 50–80% tristearin samples between 56 and 60°C and were attributed to a demixing of tripalmitin-rich and tristearin-rich β phases, in line with established phase diagrams.     

A 13C NMR study of hydroxylated natural rubber

Two methods have been used to introduce hydroxyl groups into natural rubber. In the first method, epoxidized natural rubber was reduced with lithium aluminium hydride or aluminium hydride with the production of a tertiary alcohol. The ¹³C NMR spectrum of this product was assigned in terms of shift factors from model compounds and quantitative analysis of the spectra confirmed that the hydroxylation process of partially epoxidized rubber was a random process. At higher levels of epoxidation, evidence was obtained for the formation of larger rings from adjacent epoxy groups. The second method involved hydroboration of natural rubber followed by oxidation, which was expected to produce secondary alcohol groups, but gave a mixture of secondary and tertiary alcohol groups as shown by the ¹³C NMR spectrum. A reaction scheme was proposed to explain this result.