Reversible addition fragmentation chain transfer copolymerization: influence of the RAFT process on the copolymer composition

Reversible addition fragmentation chain transfer (RAFT) mediated and conventional copolymerizations at low monomer conversions have been carried out for the systems methyl methacrylate (MMA)-styrene, methyl acrylate (MA)-styrene and methyl methacrylate-butyl acrylate (BA). The polymer samples have been analyzed via ¹H-NMR spectroscopy to obtain the copolymer composition and the terminal model reactivity ratios. In the RAFT mediated copolymerizations, the polymer mole fraction of the monomer with the larger reactivity ratio is increased compared to the conventional copolymerization. Simulations have been carried out using the program package PREDICI^® to examine possible explanations for the experimental findings. The simulations demonstrate that the RAFT process itself may alter the macroradical populations and the copolymer composition by offering additional reaction pathways. Further, the rate coefficients for the initiation reaction and the pre-equilibrium play an important role in determining the copolymer composition. The rate coefficients governing the main equilibrium of the RAFT process have only a minor impact on the copolymer composition.     

The effect of various fire‐exposed surface dimensions on the spalling of concrete specimens

Concrete spalling due to fire exposure is often defined as the sudden detachment of fragments from a concrete surface. It can be quantified by various parameters of which weight loss and spalling depth are the most common ones. The risk of spalling is influenced by many factors such as concrete composition, heating rate and applied testing methods. A reduced scale testing method should be developed to analyse the spalling behaviour and to understand its effectiveness in more detail. As a subsection of this development, this study aimed to analyse the effect of different‐sized, circular heated areas in semi full‐scale fire tests. Therefore, vermiculite slabs with varying cut‐outs in their centre were placed between a specimen made of a spalling‐sensitive concrete and the combustion chamber. The combustion chamber was heated following a standard fire curve. Our experimental results show that the thermal expansion inside of equal‐sized specimens is strongly dependent on the size of the heated area. In addition, this area also affects thermal stresses. They decrease as a result of lower temperature gradients for tests with smaller unheated boundary areas. Apart from this, the analysis of fragments shows no correlation between their relative volume distribution and the heated area. Copyright © 2014 John Wiley & Sons, Ltd.     

桦甸油页岩热解过程中的破碎粉化特性

提出了描述破碎粉化程度的多粒径表征指标,利用自行设计的转鼓式热解反应器系统地研究了桦甸油页岩在不同热解条件下的破碎粉化特性,并对破碎机理进行了分析。结果表明,热力-机械力耦合加载的方式下,粉化率是两种作用力单独加载方式之和的3.96倍,说明两者产生了协同作用,其中热力作用是煤颗粒发生破碎的主要影响因素,其实质是颗粒内部孔隙结构的演变,机械外力作为外在因素,其作用是增强宏观破碎现象;相对破碎率和粉化率与孔隙结构的变化趋势相一致;入料粒度越小,整体破碎程度越小;含油率越高,破碎程度越大,中矿油页岩因方解石含量较高在热解过程中不易分解为细小颗粒,因此粉化率最低;破碎和粉化是两个相对独立的概念,两者之间不存在相关性。     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part I. The Effects of Loading Rate,Test Temperature,Fiber Sizing and Global Strain Level

The effects of loading rate, fiber sizing, test temperature and global strain level on the adhesion strength between carbon fibers and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Analytical methodology describing the viscoelastic behavior observed is also presented. The possibility of rate-temperature-interphase thickness superposition for the interfacial strength function is illustrated based on the analytical models discussed. Experimental data are discussed using Weibull statistics and also presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. The use of histograms allows for interpretation of the skewness in the data population.     

Influence of reaction parameters on the synthesis of hyperbranched polymers via reversible addition fragmentation chain transfer (RAFT) polymerization

The synthesis of hyperbranched poly(methyl methacrylate) (PMMA) via reversible addition fragmentation chain transfer (RAFT) polymerization was investigated by varying the ratio chain transfer agent (CTA): monomer (methyl methacrylate, MMA): brancher (ethylene glycol dimethyl methacrylate, EGDMA): free radical initiator (AIBN) at various temperatures (50, 55, 60, 65, 70 °C). The rate of polymerization was observed to increase with temperature and concentration in brancher, whilst it was lowered by an increase in chain transfer agent concentration. The molecular weight of the samples increased with the ratios brancher: CTA and monomer: CTA. The polydispersity of the samples increase with conversion, as the level of branching increases. At fixed concentration in brancher, an increase of CTA concentration led to polymers with lower PDI. The variation of enthalpy and entropy relative to the monomer reaction were calculated, and it was observed that an increase in the brancher concentration induced an increase in both and , whilst lower CTA concentrations led to an increase in . The variation in Gibbs energy for the monomer reaction was calculated at 60 °C, and results confirmed the presence of a retardation effect when increasing CTA concentration generally observed in RAFT polymerization.     

Total Electron Detachment and Induced Cationic Fragmentation Cross Sections for Superoxide Anion (O2−) Collisions with Benzene (C6H6) Molecules

In this study, novel experimental total electron detachment cross sections for O₂⁻ collisions with benzene molecules are reported for the impact energy range (10–1000 eV), as measured with a transmission beam apparatus. By analysing the positively charged species produced during the collision events, relative total ionisation cross sections were derived in the incident energy range of 160–900 eV. Relative partial ionisation cross sections for fragments with m/z ≤ 78 u were also given in this energy range. We also confirmed that heavier compounds (m/z > 78 u) formed for impact energies between 550 and 800 eV. In order to further our knowledge about the collision dynamics governing the fragmentation of such heavier molecular compounds, we performed molecular dynamics calculations within the framework of the Density Functional Theory (DFT). These results demonstrated that the fragmentation of these heavier compounds strongly supports the experimental evidence of m/z = 39–42, 50, 60 (u) cations formation, which contributed to the broad local maximum in the total ionisation observed from 550 to 800 eV. This work reveals the reactivity induced by molecular anions colliding with hydrocarbons at high energies, processes that can take place in the interstellar medium under various local conditions.     

Fragmentation and granular transition of ceramics for high rate loading

The paper presents a numerical model to study the transition of brittle materials from a cracked solid to a granular medium under impact loading. The model addresses competitive crack coalescence in the transition regime and provides insight into the onset of comminution and the initial conditions for subsequent granular flow. Crack statistics obtained from initial flaws using a wing crack growth-based damage model have been used to discretely model elliptical cracks in three dimensions. These discrete cracks are either generated randomly in space or with a constraint that minimizes the intersection between neighboring cracks. These cracks are then allowed to coalesce with nearby cracks along with favorable directions and the output fragment statistics are predicted. A simple statistical model is proposed that suggests a transition criterion resembling the one obtained from the numerical model. Initial fragments are power-law distributed similarly to experimental observations and particle-based models. A generalized form of a microstructure-dependent granular transition criterion based on a threshold measure of crack lengths has been proposed. This model can be implemented in numerical codes to activate granular physics and calibrate the initial conditions of granular flow, such as fragment size and morphology.     

Model for the Atmospheric Fragmentation and Scattering of a Small Celestial Body

This paper considers the interaction of a small celestial body with a planetary atmosphere, which is treated as a two-stage process: the first stage is the fragmentation of the parental body (a model for this stage was developed by the authors) and the second stage involves breakup and scattering of the fragments. A model for the second stage is proposed in which the breakup is treated as a two-phase process of macrodisplacement of the fragments resulting from the fracture of the parental body. In the first phase there is accelerated rotation of the fragments around their centers of mass with preservation of contact between them. By the moment the contact ceases, they acquire a transverse velocity and there comes the second phase — the dispersion of the fragments, which ends with their scattering on the ground. A feature of this model is that the breakup occurs by an aerodynamic mechanism. In an analysis of the first phase of breakup, a system of differential equations for the kinetostatics of the fragments is formulated. Calculations are performed using as an example the atmospheric breakup of the Sikhote Alin meteoroid and the scattering of its fragments on the ground. It is shown that along with the numerical method for solving the system, an approximate analytical method is also possible. Calculation results for both methods are close and are in good agreement with observations of the indicated phenomenon.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 3, pp. 121–132, May–June, 2005.     

活性自由基聚合法合成偶氮聚合物研究进展

偶氮聚合物具有的光致顺反异构和光学各向异性使之在光电信息技术领域具有重要的潜在应用前景。利用活性自由基聚合的方法可以在温和的条件下合成到特定结构与预定相对分子质量的偶氮聚合物,本文综述了该技术领域的最新研究进展,并对近年来出现的聚合体系与方法作了简要的评述。     

The effect of natural products combination on MCF-7 cells exceeds tamoxifen therapeutic dose effects <i>in vitro</i>

Cancer remains to be one of the most severe sicknesses globally. Cases have kept rising over the years. Breast cancer (BC), which is among the leading types of cancers and predominantly affects women, is the second leading cause of cancer mortality. Researchers have developed interventions over the years; however, the BC survival rate has not improved since the 1980s. This has created the need for novel drug interventions that would manage and treat BC more effectively. This study focused on using a combination of natural product extracts such as phytoestrogen (Ziziphus jujube) and Tannin nanoparticles (NP99) together, which we have referred to as (Z.NP99) and tamoxifen (Tam) as one of the leading BC drugs since the 70s, in treating BC. The effectiveness of Tam if used alone in the treatment and if combined with Z.NP99 was evaluated using MCF-7 cells in vitro. The findings showed that the combination treatment of Z.NP99 affected the proliferation and viability of MCF-7 cells more than Tam at a 10 μg/mL dose. Moreover, Z.NP99 with Tam stimulated the maximum reduction of MCF-7 proliferation and viability in a time-dependent manner. Furthermore, Tam and Z.NP99 augmented the DNA fragmentation percentage combined with the upregulation of the apoptotic genes. Additionally, the results showed that the apoptotic impact of Z.NP99 and Tam on MCF-7 cells may be intermediated by down-regulating some genes such as Claudin-1 followed by down-regulating mRNA expression of MMP-9, VEGF, and BCL-2 genes of treated cells. Combining Tam with Z.NP99 considerably enhanced the effectiveness of conventional therapy. As a result, this study suggested that the Z.NP99 was ideal for developing effective natural treatments that would improve BC outcomes.