EQUILIBRIUM SWELLING PROPERTIES OF LIGHTLY CROSSLINKED ISOTROPIC NETWORKS: NATURAL RUBBER CROSSLINKED IN DRY STATE

Lightly crosslinked natural rubber (NR) networks have been characterized by equilibrium swelling in toluene. A good agreement between the equilibrium swelling crosslink-junction densities (μ_E) and the values expected from the stoichiometry of dicumyl peroxide decomposition (μ_c) has been obtained using Flory's early equation of state. The applicable value of χ = 0.37 was found to compare equally well and is in tandem with the previously reported literature values of χ = 0.391 and χ = 0.35 from swelling and heat of mixing measurements, respectively.

At low crosslink-junction densities i.e., μ_E ≤ 4.2 mM.l^?1 corresponding on average to 4 crosslink-junctions per cis 1,4-poly-isoprene chain and below, network formation is found to be incomplete. The approach used also provides a simple way of differentiating lightly crosslinked networks from the state of gelation.     

HIGH INDEX OPTICAL MATERIALS PREPARED BY COPOLYMERIZATION OF NOVEL BIFUNCTIONAL THIOMETHACRYLATES

Optical materials, such as ophthalmic lens, are thermosetting resins, which require crosslinking agents. An optical resin, having a high refractive index, is usually produced by radical copolymerization in which high index crosslinking monomers are significantly important. We provide here the resins prepared by radical polymerization of novel bifunctional thiomethacrylates as high index crosslinkers.     

聚氨酯制备的多尺度模拟方案

针对聚氨酯材料特性设计了多尺度计算机模拟方案,并研究了不同原料及相同原料但不同官能度对所制备的聚氨酯材料力学性能和玻璃化转变温度的影响.基于原子级别的结构,建立了耦合聚合反应的粗粒化耗散粒子动力学模型来描述组分扩散及交联网络结构的形成过程.并反映射这个粗粒化结构到全原子级别来分析材料的力学性能和热力学性能.这个多尺度研究方案也可推广到研究多个竞争性因素同时主导的复杂体系中.     

Frontispiece: The Effect of the Axle End Structure and Number of Through-Space Bonds on the Properties of Rotaxane Crosslinked Polymers

A rotaxane crosslinker (RC) is known to toughen the resulting rotaxane crosslinked polymer (RCP) via a stress dispersion effect that is attributed to the movable nature of the crosslinking structure. To evaluate this toughening mechanism in detail, a series of structure-definite RCs equipped with different axle end structures or different numbers of wheel components were synthesized, and subjected to free radical polymerization with a vinyl monomer to obtain RCPs. Analyses of the obtained RCPs revealed that the size of the axle end structure should be well-balanced to produce a strong toughening effect, and a [3]rotaxane crosslinker works more effectively than [2]rotaxane to toughen RCPs. The mobility of the crosslinking points, in terms of rotational and flipping movements, was more crucial to toughening the RCP than that of translational movement along the axle. The first observation of the above crucial findings proved the utility of the systematic molecular design used in this study.     

Physical and chemical changes in feather keratin during pyrolysis

The structural changes of chicken feather fibers (CFF) were investigated during pyrolysis by thermal analysis techniques coupled with mass spectrometry, solubility tests and gel permeation chromatography. The experimental data showed simultaneous disulfide bond cleavage and peptide crosslinking reactions, and suggested the dependency of crystalline melting on disulfide bond cleavage. The variation in the kinetics of these reactions played an important role in the melting transition and stability. Thus, careful tuning of the pyrolysis thermal profile provided conditions to obtain useful fibrous material at high temperatures. For instance, long-time heat treatments below the melting point provided sufficient crosslinks in the protein matrix to keep the fibrous structure intact. The protein matrix went through a series of transformations including cyclization and aromatization reactions above the melting point. Degradation of the matrix and liberation of aromatic carbons and cyclic amines were observed during these transformations. These pyrolysis mechanisms can serve as a guide for producing materials with desired properties from CFF and other keratin fibers, particularly in textile, high performance composite and catalyst applications.     

Long term environmental effects on physical properties of vinylester composite pipes

Glass fiber reinforced vinylester pipes (GFRP) have been exposed to outdoor conditions for 60 months. The effect of the outdoor exposure on the glass transition temperature (Tg) and the crosslinking index is discussed. The Tg and the degree of crosslinking has been determined using differential scanning calorimetry (DSC). DSC has become a preferred technique for rapid determination of transition temperatures and crosslinking for pure polymers, however, the use of this technique for composites is quite complex. This complexity arises from the fact that, in addition to the temperature overlap between the Tg and that of the crosslinking range, the amount of glass fibers in the GFRP is not constant. For this reason, a special measuring technique has been developed for the measurement of the amount of crosslinking of GFRP material. In this paper, it is illustrated that the DSC results clearly show the glass transition temperature and the exothermic crosslinking reaction of the vinylester composites. The results show that the glass transition temperature of the vinylester rapidly increases with respect to exposure time up to 12 months and then slightly decreases for longer exposure times. Similar trends were also observed for the change of the degree of crosslinking. The outdoor environment has also affected the weight of the exposed GFRP samples. It is observed that the weight of GFRP has gradually decreased with the exposure time. These physical changes of Tg, crosslinking and weight have impact on the tensile strength of the GFRP. For the initial outdoor exposure up to 24 months there is a 12% increase in strength from 250 MPa to 280 MPa. This increase is attributed to about 12% increase in crosslinking of the GFRP. However, for longer exposure the tensile strength has decreased back to 250 MPa due to slight decrease in crosslinking and small reduction of weight. It is observed that most of the weight loss has taken place at the surface of the GFRP samples.     

Network characteristics of hydrogenated nitrile butadiene rubber networks obtained by radiation crosslinking by electron beam

The effects of high-energy radiation on hydrogenated nitrile butadiene rubber (HNBR) copolymer structure and properties were studied. Characterization by FTIR spectroscopy, swelling and mechanical measurements of irradiated and un-irradiated sample permit us to correlate the change in structure with properties. The modifications obtained are dependent on the radiation dose of the incident electron beam. FTIR spectroscopy in absorption mode shows that irradiation of HNBR first induces trans-vinylene bond formation and secondly small amounts of carbonyl (CO) groups. Moreover, more significant changes were observed with swelling method and mechanical behaviour showing the effect of crosslinking on the elastomer.     

Stability of biomimetic ferrofluids established by a systematic study using microwave irradiation at defined wattages

An established biomimetic process for the synthesis of aqueous ferrofluids using polymers has been subjected to systematic microwave irradiation at different wattages primarily to see if the magnetization could be increased by microwave irradiation and if so how would it affect the stability of the fluid. Care has been taken to maintain ambient conditions of synthesis even after three cycles of microwave irradiation before oxidation and ten cycles after it, so as not to violate the basic principles of the process. Detailed characterization using, x-ray diffractometry, transmission electron microscopy, fourier transform infra-red spectroscopy, dynamic light scattering, thermo-gravimetric analysis, differential thermal analysis and vibrating sample magnetometry showed that these fluids containing iron oxide nanoparticles-poly(vinyl) alcohol nanocomposites are highly stable in the proportions established by us. Measurements at five different wattages double the saturation magnetization but the stability remains unaffected compared to the one without microwave irradiation, forcing us to believe that the incubation of the iron salt solution and the polymer in the right proportion before oxidation is what contributes to the stability and that increasing the number of cycles of microwave irradiation at this stage, perhaps, would have led to a more pronounced effect.