Influence of two-step process and of different stoichiometric ratios on morphologies generated in castor oil epoxy resins

Phase separation during polymerization was studied in a model system consisting of a diepoxide based on diglycidyl ether of bisphenol A (DGEBA), variable amounts of ethylenediamine (EDA) and the mass of castor oil (CO) necessary to obtain a mass fraction equal to 0-15 in a final system where the stoichiometric ratio of amine to epoxy equivalents, r, was equal to 1. A two-step polymerization process was performed by curing first a system with r = 0-5, during variable times before phase separation, and then carrying the system to r = 1. Thermodynamic analysis of samples with different r values led to a linear relationship between the Flory-Huggins interaction parameter and r. The concentration (P) and average size (D?) of dispersed-phase particles followed opposite trends, i.e. P increased while D? decreased, when either r was increased or the time of curing in the first step of a two-step process was decreased. This was explained by assuming that the competition between nucleation and growth was determined by the viscosity at the cloud point, η_cp. Low values of η_cp favoured growth over nucleation and led to fewer but larger particles.     

Surface tailoring of an ethylene propylene diene elastomeric terpolymer via plasma‐polymerized coating of tetramethyldisiloxane

In the research presented here, we explore the use of a low‐energy plasma to deposit thin silicone polymer films using tetramethyldisiloxane (TMDSO) (H(CH₃)₂? Si? O? Si? (CH₃)₂H) on the surface of an ethylene propylene diene elastomeric terpolymer (EPDM) in order to enhance the surface hydrophobicity, lower the surface energy and improve the degradation/wear characteristics. The processing conditions were varied over a wide range of treatment times and discharge powers to control the physical characteristics, thickness, morphology and chemical structure of the plasma polymer films. Scanning electron microscopy (SEM) shows that pore‐free homogeneous plasma polymer thin films of granular microstructure composed of small grains are formed and that the morphology of the granular structure depends on the plasma processing conditions, such as plasma power and time of deposition. The thicknesses of the coatings were determined using SEM, which confirmed that the thicknesses of the deposited plasma‐polymer films could be precisely controlled by the plasma parameters. The kinetics of plasma‐polymer film deposition were also evaluated. Contact angle measurements of different solvent droplets on the coatings were used to calculate the surface energies of the coatings. These coatings appeared to be hydrophobic and had low surface energies. X‐ray photoelectron spectroscopy (XPS) and photoacoustic Fourier‐transform infrared (PA‐FT‐IR) spectroscopy were used to investigate the detailed chemical structures of the deposited films. The optimum plasma processing conditions to achieve the desired thin plasma polymer coatings are discussed in the light of the chemistry that takes place at the interfaces. Copyright © 2004 Society of Chemical Industry     

Cationic ring-opening polymerization of hexamethylcyclodisilazane: General aspects and tentative mechanisms

The ring-opening polymerization of hexamethylcyclodisilazane (D_2NMe), initiated by methyl triflate in 1,2-dichloroethane, forms in the first, fast kinetic step both linear polymer and cyclic dimer (D_4NMe). Subsequently a slow depolymerization process occurs leading to exclusively cyclotrisilazane (D_3NMe). On the basis of kinetic measurements carried out using an adiabatic calorimetric technique and complementary experiments, mechanisms of both propagation, cyclization and back-biting reactions are proposed. The influence of the reaction temperature was also studied; it is demonstrated that, at low temperature, polymerization does not lead to cyclic oligomer formation. In such conditions, the polymerization presents a ‘living’ character.     

γ 射线引发 VAc 乳液聚合以制备白乳胶的动力学及工艺条件研究

研究了ＶＡｃ的辐射乳液聚合动力学，并对其聚合工艺条件作了初步探索。得出ＶＡｃ乳液在进行辐射聚合时，成核期结束较早（转化率在１０％以下），有相当长的恒速期存在。乳胶粒子直径在０．５～３μｍ之间，比一般的乳液聚合乳胶粒子要大。同时从动力学数据还可以看出几种成核机理（胶束成核，水相成核和液滴成核）同时存在，由此对试验条件提出了相应的要求。     

一种新的无线电高度表原位检测的高度模拟方法

针对目前对高度表进行原位检测的需求,设计了一种新的高度模拟方法;该方法的实现以ARM为核心,通过高度模拟电路产生各种需要的高度电压信号,从而达到高度模拟的目的,并且无需对机载高度表进行拆装,操作简单、快捷、可靠性较高;实验结果表明,这种方法的高度模拟精度较高,具有一定的实用价值和经济效益.     

FTIR investigation of ethylene coordination and polymerization on reduced Cr/SiO2 catalyst

FTIR spectroscopy was applied to study the initial steps of ethylene polymerization on reduced chromia-silica (0.5 wt% Cr/SiO₂). To decrease the speed of the reaction small doses of gas were introduced to the catalyst in each run and C₂D₄ was used to confirm band assignments. At the initial steps of the reaction only ethylene molecules coordinated to probably Cr _A ²⁺ cations were observed. The concentration of such complexes was estimated to be about 50% of the total amount of Cr atoms in the sample. The FTIR spectrum of the polymer formed at the initial doses of C₂H₄ (when [C₂H₄] [Cr]) was found to be slightly different from that formed after excess ethylene was introduced onto the catalyst ([C₂H₄] > [Cr]).     

Properties of poly(n‐butyl methacrylate) prepared by reverse atom transfer radical polymerization in an aqueous dispersed system

Reverse atom transfer radical polymerization (ATRP) of n‐butyl methacrylate (BMA) in waterborne media using Cu(II) complexes with azo initiators (i.e., reverse ATRP) was conducted. The influence of several factors, such as surfactant, catalyst, and reaction time, on the stability of the emulsion, the particle size, the morphology of the emulsion particles, and the control of the polymerization was investigated. The results showed great differences between ATRP and conventional emulsion polymerization, especially the nucleation mechanism and the kinetics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1542–1547, 2003     

High solids content emulsions. III. Synthesis of concentrated latices by classic emulsion polymerization

Two different methods of producing bi‐ and trimodal latices of a mixture of methyl methacrylate, butyl acrylate, and small amounts of acrylic acid were tested. It is shown that a combination of concentrating blends of seed particles by semibatch reaction, followed by a nucleation of small particles plus a second semibatch phase allowed us to obtain stable latices with solids contents over 65% and viscosities of below 2500 mPa s^?1 with little coagulum formation. The key parameter in determining latex stability, coagulum formation, and viscosity appears to be the the particle size distribution, and especially its modification attributed to secondary nucleation. Because it is not possible to eliminate water‐soluble monomers from the polymerization recipe, secondary (homogeneous) nucleation must be minimized by careful addition of the free‐radical initiator and choice of monomer feed flow rates. The nucleation of the third population in the trimodal latices is best accomplished with a mixed surfactant system because renucleation by anionic surfactant alone leads to detrimental changes in the particle size distribution (PSD) resulting from excessive flocculation of particles. In addition, it was found that the viscosity of the final products was not sensitive to small changes in the ionic strength of the latex, although neutralization to a pH of 6 effectively doubles the final latex viscosity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1916–1934, 2002; DOI 10.1002/app.10513     

Poly(butylene terephthalate)/organoclay nanocomposites prepared by in situ interlayer polymerization and its fiber (II)

Intercalated nanocomposites with poly(butylene terephthalate) (PBT) incorporated between the montmorillonite layers were synthesized from dimethyl terephthalate and 1,4-butane diol by using an in situ interlayer polymerization. The PBT nanocomposites were melt-spun at different organoclay contents to produce monofilaments. The samples were characterized by using wide angle X-ray diffraction, electron microscopy, thermal analysis, and tensile testing. The extent of the clay layer in the PBT was confirmed by using X-ray diffraction and electron microscopy, and the clay layer was found to be highly dispersed on a nanometer scale. The addition of only a small amount of organoclay was enough to improve the thermo-mechanical properties of the PBT hybrid fibers. The hybrids were extruded with various draw ratios (DRs) to examine the tensile mechanical property of the fibers. At DR=1, the ultimate tensile strength of the hybrid fibers increased with the addition of clay up to a critical content and then decreased. However, the initial modulus monotonically increased with increasing amount of organoclay in the PBT matrix. When the DR was increased from 1 to 6, for example, the strength and the initial modulus values of the hybrids containing 3 wt% organoclay decreased linearly.     

悬浮法合成聚丙烯腈高吸水性树脂

以偶氮二异丁腈为引发剂，用悬浮聚合法合成了交联聚丙烯腈，然后对其进行水解，得到了高吸水性聚丙烯腈。交联剂用量和水解条件对树脂吸水倍数的影响很大。树脂吸去离子水倍数为420倍，平均粒径为250μm。