Effect of mastication time on the low strain properties of short pineapple leaf fiber reinforced natural rubber composites

Pineapple leaf fiber (PALF), used as a reinforcing agent, does not have good adhesion to natural rubber (NR) due to the difference in their polarities. As a result, the degree of reinforcement of NR imparted by PALF remains low compared to that in a polar rubber like acrylonitrile butadiene (NBR). One of the factors that determines the adhesion between the rubber and the reinforcement is the rubber molecular weight. Thus, the aim of this paper is to demonstrate that the stress at very low strains of short pineapple leaf fiber (PALF) reinforced natural rubber (NR) can be significantly increased by lowering the matrix molecular weight. This can be achieved by increasing the matrix mastication time. The composites studied here contain a fixed amount of PALF at 10 part (by weight) per hundred rubber (phr). The PALF fibers were both untreated (UPALF) and sodium hydroxide treated (TPALF). Mastication times of 2, 4, 8 and 16 min were used. Stress-strain curves of PALF reinforced NR prepared with different mastication times were then compared. The most affected region of the curve is in the low strain region. The slopes of the stress-strain curves (moduli) increase with increasing mastication time, indicating better fiber-rubber interaction. The maximum stress achieved at 10% strain is almost 370% that obtained with the usual short mastication time (2 min). The effect remains up to very high strains, although becoming smaller as the strain is increased. Hence, we demonstrate that, by using long enough mastication time, stress-strain curves and stress at low strain of PALF reinforced NR can be improved without the need of any other adhesion promoters.     

Polymerization of styrene in presence of masticated rubber

The influence of the concentration of hydroperoxide (peroxide) groups in rubber (formed during mastication) and the influence of concentration of rubber on the polymerization of styrene were studied at 95, 105, 115 and 130°. The retardation effect of rubber (or non-rubber ingredients) is also due to the participation of less reactive (allylic) radicals from rubber on the termination. The derived kinetic relations allows calculation of a complex constant B* and the rate constant of decomposition of -OOH groups of rubber (k_d) at various temperatures; the latter are essentially smaller than those in masticated rubber. From kinetic analysis of experimental results, it follows that, during the polymerization of styrene in the presence of rubber, two types of rubber radicals can be formed, viz. a less reactive allylic radical and a more reactive alkyl radical.     

Physically persistent stabilizers by functionalization of polymers

The durability of polymers exposed to harsh application conditions is reduced by physical losses of stabilizers. Their physical persistence can be improved by increasing their molecular weight, without changing the action mechanism. Funtionalization of conventional polymers by grafting with monomers bearing stabilizing moieties during reactive processing, grafting during mastication, and phototriggered grafting are particularly suitable methods for synthesis of polymer‐bound stabilizers. Properties and examples of application of stabilizer‐functionalized polymers are given.     

Repeatability and reproducibility of thermal field-flow fractionation in molecular weight determination of processed natural rubber

The purpose of this study is (1) to determine the repeatability and reproducibility of thermal field-flow fractionation (ThFFF) in measuring the molecular weight of compounded natural rubber, and (2) to examine the correlation between the molecular weights obtained from ThFFF and the rheological data. 8 batches of compounded natural rubber were obtained from a thermo-mechanical mastication process, and were analyzed by ThFFF in a designed testing sequence. ThFFF analysis showed the compounded natural rubbers range in weight-average molecular weight (M(w)) from 143,000 to 360,000. By taking into account both the short term variability (repeatability) as well as the long term variability (reproducibility) of the instrument, ThFFF was shown to be able to distinguish between samples differing by as little as 21,000 in M(w) and 15,600 in number-average molecular weight, M(n) (based on cis-polyisoprene calibration); and thus is a useful tool for the molecular weight analysis of natural rubber-related materials. It was also found that the rheological data (G' and tan delta) measured on both the virgin and the compounded natural rubber correlated well with the molecular weights obtained from ThFFF when normalized.     

高密度聚乙烯／天然橡胶共混体系的流变性能研究

高密度聚乙烯／天然橡胶共混体系的流变性能研究方征平，许承威（杭州大学化学系杭州３１００２８）关键词高密度聚乙烯，天然橡胶，共混物，流变，改性用橡胶与聚乙烯共混是提高后者的物理机械性能，尤其是耐环境应力开裂性能（ＥＳＣＲ）的主要方法之一“－“．然而，由...     

Molecular structure of synthetic rubber. I. Light-scattering properties of styrene–butadiene copolymers

The discrepancy between the values reported for the weight-average molecular weight and molecular weight distribution of cold-type styrene-butadiene rubber is examined. The results obtained indicate that aggregation of the rubber due to hydrogen bonding or cluster formation is not a contributing factor to the high weight-average molecular weights obtained. The very broad molecular weight distributions, the M?_w/M?_n of the order of 10–20, are attributable to the presence of a few per cent of very high molecular weight fraction microgel in samples polymerized to moderate conversions. This microgel has been removed to various degrees by several methods: (1) mastication, (2) treatment with CaSO₄, (3) ultracentrifugation, and (4) ultrafiltration. The nature of this microgel is examined in terms of its light-scattering property, intrinsic viscosity, and concentrated solution viscosity. The weight-average molecular weight obtained by light scattering on these samples after removal of microgel are lower by as much as an order of magnitude. The operational definition of the weight-average molecular weight, M?′_w, is therefore introduced, corresponding to the one obtained after removal of the microgel. It is suggested that the actual and the operational weight-average molecular weights be used in conjunction in the characterization of these copolymers.     

Hydrodynamic, molecular, and conformational characteristics of macromolecules of a random copolymer of N-Methyl-N-vinylacetamide and N-Methyl-N-vinylamine Hydrochloride

The development and optimization of processes involving polymeric molecules require determination and monitoring of key molecular characteristics of the polymer. Samples of poly-N-methyl-N-vinylacetamide were synthesized and fractionated. Partial hydrolysis of the homopolymer fractions was performed, and fractions of a random copolymer of N-methyl-N-vinylacetamide and N-methyl-N-vinylamine hydrochloride were obtained. The molecular weights and conformational parameters of the homo- and copolymers in 0.2 M aqueous NaCl were determined by methods of molecular hydrodynamics.     

Influences of molecular weight and crystalline structure on fracture behavior of controlled-rheology-polypropylene prepared by reactive extrusion

The molecular weight of ethylene-block-co-polypropylene (co-PP) was adjusted by reactive extrusion with the incorporation of dicumyl peroxide (DCP), and the effect of molecular weight on the crystallization behavior, crystal morphology, and fracture behavior was investigated. It was found that, with increasing DCP content, the molecular weight (MW) decreased and the polydispersity (M_w/M_n) slightly decreased. After modification, the number of spherulites with obscure boundaries increased, and the size of the spherulites was more even due to increasing amount of grafting and micro-cross-linking structures, generated in co-PP degradation, which were acting as nucleating agents. Evaluated by essential work of fracture method, the specific essential work of fracture, w_e, was found to be strongly dependent on the molecular weight, especially, on the number average molecular weight (M_n) linearly, while the specific non-essential work of fracture, βw_p, was enhanced with decreasing z-average molecular weight (M_z), probably owing to the reduction of ultra-high molecular weight component in degraded co-PP.     

Thermodynamic parameters of dilute solutions of a polymer in benzene at varying temperatures and concentrations

Dilatometric studies of solutions of four different mole fractions of poly(ethylene glycol) of average molecular mass 400 g mol^?1 dissolved in benzene have been performed to estimate molar volumes (V) and volume expansion coefficients (α) at different temperatures. From these, thermodynamic parameters appropriate to these solutions have been evaluated at different temperatures ranging from 299 to 328 K. These parameters have been employed to collect and discuss the information on molecular structure, molecular order, molecular packing, polymer segment extension, and molecular interactions.     

Bipartite molecular approach for species delimitation and resolving cryptic speciation of Exobasidium vexans within the Exobasidium genus

Exobasidium vexans, a basidiomycete pathogen, is the causal organism of blister blight disease in tea. The molecular identification of the pathogen remains a challenge due to the limited availability of genomic data in sequence repositories and cryptic speciation within its genus Exobasidium. In this study, the nuclear internal transcribed spacer rDNA region (ITS) based DNA barcode was developed for E. vexans, to address the problem of molecular identification within the background of cryptic speciation. The isolation of E. vexans strain was confirmed through morphological studies followed by molecular identification utilizing the developed ITS barcode. Phylogenetic analysis based on Maximum Parsimony (MP), Maximum Likelihood (ML) and Bayesian Inference (BI) confirmed the molecular identification of the pathogen as E. vexans strain. Further, BI analysis using BEAST mediated the estimation of the divergence time and evolutionary relationship of E. vexans within genus Exobasidium. The speciation process followed the Yule diversification model wherein the genus Exobasidium is approximated to have diverged in the Paleozoic era. The study thus sheds light on the molecular barcode-based species delimitation and evolutionary relationship of E. vexans within its genus Exobasidium.     

Synthesis and characterization of high cis-polybutadiene: influence of monomer concentration and reaction temperature

This paper reports the study of the dependence of reaction conversion, catalyst activity, polymer microstructure, molecular weight, molecular weight distribution curves and Mooney viscosity on reaction temperature and monomer concentration in the reaction medium used in the synthesis of high cis-polybutadiene. A ternary catalyst system composed by neodymium versatate, trans-butyl chloride and diisobutylaluminum hydride was used in its synthesis. The highest molecular weights were obtained at polymerization temperatures in the range from 70 to 80 °C. The highest content of cis-1,4 repeating units (about 99%) was observed when the polymerization was carried out at the lowest initial monomer concentration (0.56 mol/l).     

Interaction of rotationally aligned and of oriented molecules in gas phase and at surfaces

Recent developments concerning the generation of molecular beams containing oriented/aligned molecules will be reviewed and applications of such tools to the study of elementary processes occurring both in homogeneous and heterogeneous phases will be presented. First we will discuss the case of symmetric top molecules oriented by hexapoles. Here the molecular polarization is obtained by the use of an external field and allows to control which end of the molecular projectile is going to collide with the target. Then we will review the so-called collisional alignment, a molecular polarization phenomenon occurring in supersonic expansions of gaseous mixtures. The key feature, in this case, is the velocity dependence of the alignment degree, which allows the use of mechanical devices to filter out of the beam the molecules having either a random (statistical) or a preferential (non-statistical) spatial distribution of their rotational angular momentum J with respect to the molecular beam axis. The physical mechanisms underlying the collisional alignment will be resumed and some relevant gas-phase experiments demonstrating its occurrence will be illustrated. Application of such methodologies to the investigation of the stereodynamics of elementary processes occurring in gas-surface interaction will be presented and discussed for both weakly and strongly interacting systems.     

Desulfonyloxyiodination of arenesulfonic acids with mCPBA and molecular iodine

Treatment of p-alkylbenzenesulfonic acids with mCPBA and molecular iodine gave p-alkyliodobenzenes in good to moderate yields via electrophilic ipso-substitution by the iodonium species (I⁺) formed. This desulfonyloxyiodination was promoted by the addition of a catalytic amount of iodoarenes, such as o-iodobenzoic acid. The same treatment of dimethylbenzenesulfonic acids and trimethylbenzenesulfonic acids with mCPBA and molecular iodine proceeded smoothly both in the absence and in the presence of o-iodobenzoic acid to provide the corresponding monoiodo-dimethylbenzene and diiodo-dimethylbenzene, and diiodo-trimethylbenzene and triiodo-trimethylbenzene, in good to moderate yields, respectively. On the other hand, the same desulfonyloxyiodination of benzenesulfonic acid and p-chlorobenzenesulfonic acid with mCPBA and molecular iodine proceeded only in the presence of o-iodobenzoic acid to generate iodobenzene and p-chloroiodobenzene, respectively, in moderate yields.     

Comparative analysis of molecular interactions in quaternary fluid system performed by classical and ab initio molecular dynamics

Molecular interactions in the quaternary fluid system acetic acid–n-propanol–n-propyl acetate–water were analyzed by classical and ab initio molecular dynamics methods. It was shown that ab initio molecular dynamics simulation can reproduce the molecular mobility tendency and structural features of a multicomponent system without empirical parameters.     

Pulsed NMR of cis-polyisoprene solutions T1 and T2 relaxations,free volume,viscosity relationships

Previous pulsed NMR studies of polyisoprene have largely been concerned with entangled or crosslinked networks. This paper deals with (i) the relaxation of high molecular weight entangled; (ii) cross-linked; (iii) monodisperse low molecular weight; and (iv) high molecular weight polymer in the presence of tetrachloroethylene which, by increasing molecular mobility, can be expected to influence the NMR relaxation. For all four types of polyisoprene, the spin-lattice T₁, relaxation shows a minimum with position depending only on the free volume, as influenced by changes in temperature T and polymer concentration v₁,. For monodisperse polyisoprene of molecular weight 7200, insufficient to form an entangled network, the spin-spin relaxation decay constant T_2L is quantitatively related to the free volume ₁ by two parameters A′ and B″ when the free volume is altered by a change in temperature, or in polymer concentration (10–100/). This can also be expressed in the form where the parameter T_∞ at 100% concentration agrees with the value used to describe rheological properties. At other concentrations of polymer, T_∞ and B′ can be derived quantitatively from the coefficients of volume expansion of polymer and solvent. The variation of T_2L with molecular weight (T_2L ∝ M^?0.5) occurs via the A′ parameter. It is concluded that T_2L can be quantitatively related to the free volume available for molecular motion (as influenced by temperature and solvent concentration) as well as to molecular weight. Furthermore T_2L is simply related to viscosity n, over a wide range of temperatures and concentrations. T₂ can be used to analyse the molecular motions involved in theology.     

Branched polyacrylamides: Synthesis and effect of molecular architecture on solution rheology

Linear, star and comb-like polyacrylamides (PAM) have been prepared by atomic transfer radical polymerization (ATRP) in aqueous media at room temperature. The influence of the molecular architecture of PAM on the rheological properties in aqueous solution has been investigated. The well-known theory of increased entanglement density by branching for polymers in the melt can also be applied to polymers in the semi-dilute water solutions. We have demonstrated this by investigating the rheological properties of PAM of similar molecular weights with different molecular architectures. Interestingly, the solution viscosity of a comb PAM is higher compared to its linear and star analogues (both at equal span molecular weight, M_n,SPAN, and total molecular weight, M_n,tot). In addition to the pure viscosity, we also demonstrate that the visco-elastic properties of the polymeric solutions depend significantly on the molecular architecture of the employed PAM. The elastic response of water solutions containing comb PAM is more pronounced than for solutions containing either linear or star PAM at similar M_n,SPAN and M_n,tot. The obtained results pave the way for application of these polymeric materials in Enhanced Oil Recovery (EOR).     

Impact of regio-isomeric monochlorinated end groups on packing mode,miscibility, and photovoltaic performance of asymmetric selenophene-fused M-series acceptors

Optimal bulk-heterojunction (BHJ) morphology is crucial for efficient charge transport and good photovoltaic performance in organic solar cells (OSCs). Yet, the correlation between chemical structures of nonfullerene acceptors (NFAs) and molecular interaction in the BHJ blends remains opaque. Herein, we study three isomeric NFAs referred to as MQ1-x (x = β, γ, or δ) that shared an asymmetric selenophene-fused heteroheptacene backbone end-capped by two monochlorinated end groups. Remarkably, miscibility between the polymer donor of PM6 and MQ1-x successively elevates as the chlorine atoms move from β-, to γ-, to δ-position of terminals. Combined with the varied molecular crystallinity of these NFAs, diverse BHJ morphologies are observed in their blend films. As a result, the MQ1-δ-based devices present the highest PCE of 12.08% owing to the efficient charge dissociation and transport induced by the compact molecular packing and optimal BHJ morphology. Our investigation provides a new insight in the material design that has a good balance in molecular packing and film morphology for high-performance OSCs.     

Rheological and mechanical properties of ABS plastics prepared by bulk polymerization

A series of ABS plastics prepared by bulk polymerization was studied. The test samples contained almost equal amounts of PB but mostly differed in the molecular mass of a styrene-acrylonitrile copolymer. It was shown that the molecular mass of the copolymer strongly affects the rheological and mechanical properties of ABS plastics. An increase in molecular mass leads to a rise not only in the non-Newtonian viscosity of plastics but also in their yield point, storage modulus under periodic steady-state shear flow in the low-frequency plateau region, and impact strength. Quantitative correlations between these rheological and mechanical characteristics of the copolymers and their M _w values were established. As opposed to homophase polymer systems, a marked increase in the shear stress has no effect on viscosity in relation to the molecular mass of ABS plastics. In the case of melts, the influence of the M _w of the styrene-acrylonitrile copolymer on the rheological behavior of ABS plastics is apparently related to a change in the interaction of PB particles with the copolymer that controls the structural framework of the system. The relationship between the impact strength of the copolymer and its M _W may be explained by the fact that the latter parameter influences orientational effects in crazes that arise during steady-state shear flow of ABS plastics in the solid state.