异戊橡胶微观序列结构与结晶性能研究

天然橡胶特有的应变诱导结晶能力赋予其优异的力学性能.异戊橡胶作为唯一能够替代天然橡胶的合成橡胶品种,其应变诱导结晶能力受到关注.本文对3种异戊橡胶的微观序列结构进行了分析,并研究了其结晶性能.核磁分析结果表明:3种异戊橡胶的顺-1,4-结构含量差别不大;从1,4-结构单元的键接方式(序列结构)看,SKI-5和YS-IR分子链中顺-1,4-结构单元均以头-尾相接的方式沿分子链排列,不存在头-头键和尾-尾键接方式;SKI-3中约有0.4%~0.5%的1,4-单元采取头-头键接方式,约有0.3%~0.6%的1,4-单元采取尾-尾键接方式;根据定量计算结果,从分子链上1,4-结构单元的序列分布来看,SKI-3的规整性与SKI-5、YS-IR相近或略高.XRD研究结果表明:炭黑填充的天然橡胶硫化胶拉伸至400%以上时发生取向结晶;而炭黑填充的异戊橡胶硫化胶需拉伸至500%以上时才发生取向结晶.基本物理机械性能研究表明:3种异戊橡胶的性能相当,拉伸强度和撕裂强度明显低于天然橡胶;由于结构和组成上的差异,异戊橡胶的结晶能力较天然橡胶差.     

Halloysite nanotubes reinforced ultrahigh molecular weight polyethylene nanocomposite films with different filler concentration and modification

Halloysite nanotube/ultrahigh molecular weight polyethylene (HNT/UHMWPE) nanocomposite films were prepared by melt extrusion and thermal extension methods, and the morphology, microstructure, thermal properties, mechanical properties and wettability of these nanocomposite films were investigated with respect to the effects of HNT concentration and modification. HNTs were homogenously distributed and formed self entangled network structures in the UHMWPE matrix. The incorporation of HNT obviously accelerates the crystallization and enhances the thermal stability, mechanical strength and wettability of UHMWPE. However, after HNT concentration exceeds a critical value, the crystallinity and mechanical strength of UHMWPE decrease due to the restriction of HNT obstacles and the existence of large HNT agglomerates. Surface modification of HNT is a good method to further improve the crystallization, thermal stability and mechanical property of UHMWPE nanocomposites, with reduction of nanotube aggregation and strengthening of interfacial bonding between nanotubes and matrix.     

Inhomogeneous Natural Network Promoting Strain-induced Crystallization:A Mesoscale Model of Natural Rubber

Although synthetic rubbers show continuously improved mechanical properties,natural rubber (NR) remains irreplaceable in the rubber family due to its superior mechanical properties.A mainstream viewpoint regarding the superiority of NR is that NR possesses a natural network formed by linking the poly(cis-l,4-isoprene) chain terminals to protein and phospholipid aggregates;after vulcanization,the natural network additionally contributes to rubber mechanics by both increasing the network density and promoting the strain-induced crystallization (SIC) behavior.However,the reason why the natural network promotes SIC is still unclear;in particular,only using the increased network density cannot explain our finding that the NR shows smaller onset strain of SIC than Gel (the gel component of NR with higher network density) and even vulcanized NR.Herein,we point out that the inhomogeneous chain deformation is the alternative reason why SIC of NR takes place at smaller strain than that of Gel.More specifically,although the natural network is homogenous on the subchain length scale based on the proton double-quantum NMR results,it is essentially inhomogeneous on mesoscale (100 nm),as revealed by the small angle X-ray scattering analysis.This inhomogeneous network also leads to the mesoscale deformation inhomogeneity,as detected by the orientation of stearic acid (SA) probe,thus resulting in the smaller onset strain of SIC of NR.Based on the experimental results,a mesoscale model is proposed to qualitatively describe the crucial roles of inhomogeneous structure and deformation of natural network in NR?s mechanical properties,providing a clue from nature to guide the development of high-performance rubbers with controlled structures at mesoscale.     

Biaxial Orientation in Triple Centrifuged Natural Rubber

Natural rubber (NR) latex was purified through triple centrifugation process and film samples were prepared. The strain-induced crystallization was studied during the uniaxial elongation of the NR samples at room temperature. Crystalline orientation was detected by wide angle x-ray diffraction (WAXD) measuring the intensity of 200 and 120 reflections. The WAXD patterns were compared with the aspect ratio (width/length) of the original NR films. The results indicate that the induced crystals have a biaxial orientation (BO), where the c-axis is parallel to the draw direction and the a-axis is parallel to the film surface. Using the WAXD through patterns, we distinguish the highly oriented BO region in the samples. In order to analyze the structure and properties of the NR, the effect of BO is an essential factor.     

Impacts of filler covalent and non‐covalent modification on the network structure and mechanical properties of carbon–silica dual phase filler/natural rubber

The carbon–silica dual phase filler (CSDPF) was modified by bis (3‐triethoxy‐silylpropyl) tetrasulphane (Si69) and 1‐allyl‐3‐methyl‐imidazolium chloride (AMI), respectively. The natural rubber (NR) vulcanizates filled with modified CSDPF were fabricated through mechanical mixing followed by a high‐temperature cure process. The impacts of filler surface modification on the curing characters, crosslinked junctions, network structure, and mechanical properties of NR vulcanizates were investigated. The results showed that the Si69 interacted with CSDPF through covalent bond, while the interaction between AMI and CSDPF was hydrogen bond. Both modifications increased the cure rate of CSDPF/NR compounds as well as the crosslinked degree, compared with those of pristine CSDPF/NR compound. The modifications improved the dispersion of CSDPF in NR matrix. The covalent modification by Si69 caused a limited movement of NR chains in the CSDPF surface, which contributed to a greater tensile modulus of Si69‐modified CSDPF/NR. However, the higher content of mono‐sulfidic crosslink and the poorer content of strain‐induced crystallization in the NR matrix led to a slight increase of tensile strength and tear strength of Si69‐modified CSDPF/NR, compared with those of CSDPF/NR. The tensile modulus of AMI‐modified CSDPF/NR had a lower value due to a faster polymer chain motion on the CSDPF surface. However, the tensile and tear strength of AMI‐modified CSDPF/NR increased significantly because of the increase of mono‐sulfidic crosslink, strain‐induced crystallization, and the existed hydrogen bond between CSDPF and NR. Copyright © 2015 John Wiley & Sons, Ltd.     

Stepwise strain-induced crystallization of soft composites prepared from natural rubber latex and silica generated in situ

Novel biphasic structured in situ silica filled natural rubber composites were focused on their strain-induced crystallization (SIC) behavior from the viewpoint of morphology. The composites were prepared by in situ silica filling in natural rubber (NR) latex using a sol–gel reaction of tetraethoxysilane. Simultaneous time-resolved wide-angle X-ray diffraction and tensile measurements revealed a relationship between the characteristic morphology and tensile stress–strain properties of the composites associating with the SIC. Results showed stepwise SIC behaviors of NR-based composites for the first time. Pure rubber phases in the biphasic structure were found to afford highly oriented amorphous segments and oriented crystallites. The generated crystallites worked as reinforcing fillers together with the in situ silica to result in high tensile stresses of the composites. The observed characteristics are useful for understanding a role of filler network in the reinforcement of rubber.     

Fabrication and characterization of biodegradable poly(butylene succinate-co-adipate) nanocomposites with halloysite nanotube and organo-montmorillonite as nanofillers

Biodegradable poly(butylene succinate-co-adipate) (PBSA)-based nanocomposites were successfully prepared. A commercial halloysite nanotube (HNT) and an organo-montmorillonite (denoted as 15A) served as reinforcing fillers. Scanning electron microscopy and transmission electron microscopy results confirmed the nano-scale dispersion of HNT and 15A in the composites. Differential scanning calorimetry results showed that 15A served as nucleating agent for PBSA crystallization, but HNT hardly affected the nucleation of PBSA. Both nanofillers assisted the isothermal crystallization of PBSA, with 15A demonstrating superior efficiency. Melting behavior study suggests that the presence of HNT or 15A hampered the melting-recrystallization process of the originally less stable crystals during heating scans. Thermogravimetric analyses revealed that 15A enhanced the thermal stability of PBSA in air environment, but HNT caused a decline at high loadings. The rigidity of PBSA, including Young’s/flexural moduli, evidently increased after the addition of HNT or 15A, with 15A showing higher enhancing efficiency than HNT at similar loadings. The flexural modulus increased up to 94% with 20 wt% in HNT and up to 48% with 5 wt% 15A loading. The rheological property measurements confirmed the achievement of pseudo-network structure at 5 wt% 15A loading, whereas the HNT-included system did not develop a network structure.     

Strain‐induced crystallization and mechanical properties of functionalized graphene sheet‐filled natural rubber

The effects of functionalized graphene sheets (FGSs) on the mechanical properties and strain‐induced crystallization of natural rubber (NR) are investigated. FGSs are predominantly single sheets of graphene with a lateral size of several hundreds of nanometers and a thickness of 1.5 nm. The effect of FGS and that of carbon black (CB) on the strain‐induced crystallization of NR is compared by coupled tensile tests and X‐ray diffraction experiments. Synchrotron X‐ray scattering enables simultaneous measurements of stress and crystallization of NR in real time during sample stretching. The onset of crystallization occurs at significantly lower strains for FGS‐filled NR samples compared with CB‐filled NR, even at low loadings. Neat‐NR exhibits strain‐induced crystallization around a strain of 2.25, while incorporation of 1 and 4 wt % FGS shifts the crystallization to strains of 1.25 and 0.75, respectively. In contrast, loadings of 16 wt % CB do not significantly shift the critical strain for crystallization. Two‐dimensional (2D) wide angle X‐ray scattering patterns show minor polymer chain alignment during stretching, in accord with previous results for NR. Small angle X‐ray scattering shows that FGS is aligned in the stretching direction, whereas CB does not show alignment or anisotropy. The mechanical properties of filled NR samples are investigated using cyclic tensile and dynamic mechanical measurements above and below the glass transition of NR. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

The effect of halloysite nanotubes and N,N'- ethylenebis (stearamide) on the properties of polylactide nanocomposites with amorphous matrix

Amorphous polylactide/halloysite nanotube (PLA/HNT) nanocomposites were prepared and examined. Neat HNT and HNT treated with N,N'- ethylenebis(stearamide) (EBS) were used as nanofillers. The role of HNT and/or EBS content on the cold crystallization of amorphous PLA matrix, HNT dispersion, as well as on the dynamic mechanical and optical properties of the materials was determined.The PLA/HNT-based nanocomposites contained well-distributed nanotubes and occasionally micron-sized aggregates, especially at high loading. HNT, EBS treated HNT and EBS influenced the cold crystallization of PLA, therefore the formation of the disorder α′ and the order α crystallographic forms of PLA.The nanocomposites exhibited increased stiffness and decreased transparency compared to the neat PLA. Due to the reinforcing effect and additional specific features of HNT, the addition of the nanofiller allows tuning of the properties of the nanocomposites with amorphous PLA matrix.     

Kinetics of strain-induced crystallization of a trans-polypentenamer

The kinetics of the strain-induced crystallization of a crosslinked 89% trans-polypentenamer was studied as a function of temperature and strain by using a combined birefringence—stress relaxation technique. The crystallization rate was found to be extremely sensitive to both variables and was, within experimental error, a function of the degree of undercooling alone, the increase in crystallization rate with strain being a direct consequence of the melting point elevation. From the isothermal, isometric crystallization rates, the development of crystallinity in constant-rate-of-strain experiments was calculated and related to characteristics of the stress--strain curve and of the temperature-strain rate dependence of tensile strength. These calculations allow one to estimate under what conditions sufficient crystallinity may be expected for the attainment of high strength in absence of reinforcing fillers. Crystallinities measured in this work were of the order of 10% or less. A limited x-ray diffraction study confirmed the low order of crystallinity indicated by the stress-birefringence measurements. The relatively high precision of the latter makes this technique attractive for the estimation of small straininduced crystallinities in rubbers.     

Strain‐induced crystallization and strength of elastomers. I. cis‐1,4‐polybutadiene

Crosslinked samples of cis‐1,4‐polybutadiene (BR) were crystallized at low temperatures and then slowly melted. From volume changes and differential scanning calorimetry measurements, the degree of crystallization in the unstrained state was estimated to be about 20%, much lower than for natural rubber (NR). Crystallization and melting were followed in stretched samples by corresponding changes in tensile stress. Crystallization was faster at higher strains, and the melting temperature was raised significantly on stretching but less than for NR, and the decrease in stress on crystallizing was smaller. Measurements of tensile strength were made over a wide temperature range and showed a marked drop with heating to temperatures of 40–60 °C, falling to values of only 1–2 MPa. A similar drop in strength occurred in NR vulcanizates at high temperatures and was attributed to failure to crystallize on stretching (A. G. Thomas & J. M. Whittle, Rubber Chem Technol 1970, 43, 222; A. N. Gent, S. Kawahara & J. Zhao, Rubber Chem Technol 1998, 71, 668). At ambient temperatures, where strain‐induced crystallization occurred, the strength of BR samples was only about one‐half of that of similar NR materials. This was attributed to less strain‐induced crystallinity in BR (verified by X‐ray studies), paralleling the lower amount developed at low temperatures. We speculate that the higher density of molecular entanglements in BR than in NR prevents BR from crystallizing to the same degree as NR. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 811–817, 2001     

Cellulose–halloysite nanotube composite hydrogels for curcumin delivery

Halloysite nanotubes (HNTs) were added to cellulose NaOH/urea solution to prepare composite hydrogels using epichlorhydrine crosslinking at an elevated temperature. The shear viscosity, mechanical properties, microstructure, swelling properties, cytocompatibility, and drug delivery behavior of the cellulose/HNT composite hydrogels were investigated. The viscosity of the composite solution increases with the addition of HNT. The compressive mechanical properties of composite hydrogels are significantly improved compared with pure cellulose hydrogel. The compressive strength of the composite hydrogels with 66.7% HNTs is 128 kPa, while that of pure cellulose hydrogel is only 29.8 kPa in compressive strength. Rheological measurement suggests the resistance to deformation is improved for composite hydrogels. X-ray diffraction and Fourier transform infrared spectroscopy show that the crystal structure and chemical structure of HNT are not changed in the composite hydrogels. Hydrogen bonding interactions between HNT and cellulose exist in the composites. A porous structure of the composite hydrogels with pore size of 200–400 μm was found by scanning electron microscopy. The addition of HNT leads to decreased swelling ratios in NaCl solution and pure water for the composite hydrogels. Cytotoxicity assays show that the cellulose/HNT composite hydrogels have a good biocompatibility with MC3T3-E1 cells and MCF-7 cells. Curcumin is further loaded into the composite hydrogel via physical adsorption. The curcumin-loaded composite hydrogels show a strong inhibition effect on the cancer cells. All the results illustrate that the cellulose/HNT composite hydrogels have promising applications such as anticancer drug delivery systems and anti-inflammatory wound dressings.     

Theory of deformation and strain-induced crystallization of an elastomeric network polymer

A new theory of deformation and strain induced crystallization of network polymers has been developed. The effects of lattice vacancies, variation in distribution of trans and gauche bond conformations in stretched amorphous polymers, and the crystallite orientation in the partially crystalline stretched vulcanizate were considered in the evaluation of their partition functions. Stress-extension ratio relationships were evaluated for the amorphous and semicrystalline polymers. The rise in melting temperature due to strain induced crystallization is discussed. The new theory seems to be in closer agreement with the actual strain-induced crystallization process than earlier research.     

Thermal and mechanical properties of polypropylene in the thermoplastic elastomeric state

One result of the discovery of homogeneous metallocene stereospecific catalysts is the ability to prepare polypropylene in a stereoblock form in which the isotactic stretches give crystallites acting as temporary crosslinks in an elastomeric network structure. The fact that these elastomers are thermoplastic and thus reprocessible increases the importance of establishing their structure-property relationships. In this report, the dependence of their physical properties on isotactic pentad content, molecular weight, and possible strain-induced crystallization are described. Thermal evaluations and mechanical tests of these materials under oscillatory strain, continuous extension and near-equilibrium uniaxial and biaxial elongation showed that they were multiphase, tough elastomeric materials. Their moduli and tensile strengths increased with increase in % isotactic pentad content and with increase in molecular weight. Equilibrium stress-strain measurements showed the occurrence of strain-induced crystallization in uniaxial, but not in biaxial, deformations.     

MULTIPLE MELTING TRANSITIONS IN POLYURETHANE-UREA ELASTOMERS UNDER EXTENSION

Two melting transitions were observed in linear segmented polyurethane-urea elastomers underextension using thermal, mechanical and X-ray diffraction techniques, and the results are compared.These data indicate both strain-induced and temperature-induced crystallization in the stretchedclastomers, which may result from two different types of crystallites with different melting tempera-tures. These have been assigned as type 1 appearing around 60℃, and type 2 around 30℃. Thetype 1 crystallization can be induced by stretching at room temperature to large strain, and is mechani-cally reversible, but the type 2 crystallization is mainly induced by cooling below its crystallizationtemperature. These two crystalline structures are interchangeable under suitable conditions. Atelongations greater than 300%, the low temperature peak observed on fusion thermograms disappearsor combines with the high temperature peak. When the temperature of the sample is over the meltingpoint of the type 1 crystal, irreversible melting occurs and only the type 2 crystal develops on cooling.The results of stress-strain and stress hysteresis experiments at different temperatures indicate therelative importance of strain-induced and temperature-induced crystallization on the mechanicalproperties of these materials.     

Mechanical characteristics of hydrogenated natural rubber vulcanizates

Mechanical properties of partially hydrogenated natural rubber (HNR) vulcanizates were evaluated regarding their chemical structure and crystallizable nature of HNR, and are reported here, to the best of our knowledge, for the first time. HNRs of three levels of hydrogenation (20.6, 29.0, and 40.6 mol%) were successfully prepared by the chemical modification of natural rubber (NR) latex using N₂H₄ and H₂O₂ as reagents, in a sufficient amount for preparing sulfur‐crosslinked samples to be subjected to mechanical and structural measurements. The three HNR vulcanizates were found to be crystallizable upon stretching; it is noted that even 40.6 mol% hydrogenation did not prevent HNR vulcanizates from crystallization upon stretching, while their onset strain of crystallization was higher than that of NR vulcanizate. The hysteresis loss and residual strain up to a stretching ratio of 2 for the HNR vulcanizates tended to become larger with the increase in the degree of the hydrogenation. Tensile and dynamic mechanical properties of 20.6 mol% hydrogenated HNR vulcanizate were comparable to those of NR vulcanizate. From differential scanning calorimetry and temperature dispersion of dynamic modulus or loss, the glass transition temperatures of HNR vulcanizates were found to be almost the same as that of NR vulcanizate, which is also notable. The thermal stability of HNR vulcanizates was better than that of NR vulcanizate. Thus, this chemical modification seems to give a promising NR derivative whose properties can be equivalent or even better than the mother polymer. Copyright © 2008 John Wiley & Sons, Ltd.     

Properties and recyclability of thermoplastic elastomer prepared from natural rubber powder (NRP) and high density polyethylene (HDPE)

Preparation of thermoplastic natural rubber (TPNR) was carried out by blending high density polyethylene (HDPE) with natural rubber powder (NRP) obtained from spray drying of pre-vulcanized natural rubber latex. The blend ratio of NRP/HDPE was varied and the properties and recyclability of the TPNRs were investigated. The results reveal that, due to flow restriction of the crosslinked NRP, viscosity of the TPNR increases continuously with increasing NRP content. As expected, when NRP content is increased, properties of the TPNR are more rubber-like, as clearly observed from the stress–strain curve characteristics, the reductions in modulus and hardness and, most importantly, the significant improvement of permanent set. Although phase size of the dispersed NRP is relatively large, the tensile strength tends to increase gradually with increasing NRP content. The phenomenon of strain-induced crystallization is proposed to explain the results. It is also found that recycling has a noticeably detrimental influence on most properties of the TPNRs such as tensile strength, tear strength, hardness and tension set. This is thought to be a consequence of thermal degradation of the NRP during the recycling process.     

The effect of halloysite nanotubes and N,N′-ethylenebis (stearamide) on morphology and properties of polylactide nanocomposites with crystalline matrix

Poly(lactide)/halloysite nanotubes (PLA/HNT) nanocomposites with crystalline matrix were obtained by cold crystallization and examined. Neat HNT and HNT treated with N,N′- ethylenebis(stearamide) (EBS) were used as nanofillers. Reference materials, PLA and PLA/EBS blend, prepared in the same way, were also considered. The influence of HNT and/or EBS content on the crystallinity and morphology of PLA matrix, as well as on the dynamic mechanical and optical properties of the materials, was determined.The nanocomposites contained well-distributed HNT, with only occasional agglomerates. HNT, EBS-treated HNT and EBS influenced the morphology of the crystalline PLA matrix and the amounts of the disorder α’ (termed also δ) and order α crystallographic forms of PLA. Crystallinity increased stiffness of the materials compared to their counterparts with the amorphous matrix. Owing to the crystallinity and the presence of the nanofillers, the storage modulus at 20 °C and 60 °C increased by up to 30 and 60%, respectively, compared to neat amorphous PLA. Interestingly, at lower nanofiller content the crystalline nanocomposites with EBS were more transparent than neat crystalline PLA.     

Strain‐induced crystallization of natural rubber with high strain rates

Strain‐induced crystallization (SIC) of natural rubber (NR) samples with different strain rates at a fixed strain was investigated by synchrotron radiation X‐ray diffraction measurements, which provided the evolution trends of crystal sizes and crystallinity during the SIC process. It was found that the Avrami index was about 1 during the crystallization of NR after the cessation of stretch, which demonstrated that sporadic nucleation occurred during SIC process. The increase of the crystallinity was attributed to the increase of the number of new crystallites rather than the growth of the crystal size. An unexpected relationship between the final crystallinity and the strain rates was observed. The increase of physical crosslink points originated from either entanglement or crystallite was considered as the reason that leads to the nonmonotonic variation of the final crystallinity with strain rates. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

The use of zinc dimethacrylate functionalized graphene as a reinforcement in rubber composites

Zinc dimethacrylate functionalized graphene (Z‐GE), as reinforcing nanofiller for natural rubber (NR), was synthesized by liquid‐phase exfoliation and in situ method. The morphology and structure of Z‐GE were characterized to confirm the exfoliation and functionalization of GE. The NR/Z‐GE composites were prepared and investigated by mechanical analysis, crosslinked network analysis and the analysis of thermal conductivity. The results showed that the tensile strength, tear strength and modulus at 300% strain of NR/Z‐GE‐20 composites (contents 1.400 phr GE) were increased by 142%, 76% and 231% as compared with the pure NR, respectively. And the thermal conductivity of NR/Z‐GE‐30 composites is enhanced by 39% as that of the pure NR. This significant improvement is attributed to the formation of covalent crosslinked network and ionic crosslinked network and efficient interfacial interaction between GE and NR matrix. This method provides a new insight into the fabrication of multifunctional GE composites and enlarges its potential applications in high performance GE‐based rubber composites. Copyright © 2015 John Wiley & Sons, Ltd.