聚碳硅烷纤维氧化交联机理的研究

对聚碳硅烷(PCS)原丝在不同氧化交联温度区间生成的逸出产物进行红外、核磁和GC-MAS分析,并结合交联丝的红外分析,推测氧化交联的机理。结果表明,PCS的氧化交联主要是其Si—H氧化生成Si—OH,后者进而彼此缩合生成Si—O—Si交联结构;氧化交联温度高于150℃时,其部分Si—CH3也开始氧化生成Si—OH并进而交联;同时,在氧化交联过程还发生PCS侧链的热裂解,所形成小分子也通过Si—OH彼此结合,形成较大分子,且其分子量随交联温度的提高而提高。因此,要及时排除氧化交联过程废气,以免逸出产物黏附在纤维表面而导致粘结。     

Synthesis of a polytitanocarbosilane and its conversion into inorganic compounds

The novel polytitanocarbosilane, formed by the cross-linking of polycarbosilane with titanium tetra-alkoxide, was synthesized to examine the process of converting a multielement organometallic polymer into an inorganic compound. The chemical structure of this polymer was investigated by the techniques of infra-red spectroscopy (IR), gel permeation chromatography (GPC), number average molecular weight measurements and²⁹Si nuclear magnetic resonance (NMR) measurements. The pyrolysis products in N₂ gas at 1400° C and 1700° C were the microcrystalline and crystalline states of silicon carbide and titanium carbide, respectively.     

Conversion of polycarbosilane (PCS) to SiC-based ceramic Part 1. Characterisation of PCS and curing products

A commercial polycarbosilane (PCS) preceramic polymer has been characterised as-received and following curing under a variety of conditions. Elemental analysis, gel permeation chromatography (GPC), infra-red spectroscopy (FT-IR), simultaneous thermogravimetric analysis-differential thermal analysis (TG-DTA) and solid state nuclear magnetic resonance (NMR) have been employed. A number average molar mass of 1200 was found with a broad molar mass distribution ( / = 2.97). Elemental analysis gave an empirical formula of SiC_2.2H_5.3O_0.3. IR and Solid state ²⁹Si and ¹³C NMR spectra showed the presence of Si-O-Si, SiC₄, SiC₃H, Si-Si, Si-CH₃ and Si-CH₂ groups. Simultaneous TG-DTA performed under an argon flow showed that there was a weight gain which started at approximately 240 °C. DTA showed an exotherm starting at this temperature showing that there was oxidation of the polymer even in an inert atmosphere. This is perhaps due to the oxygen in the PCS and there may also be some impurities in the inert atmosphere. Evidently the PCS is very sensitive to oxygen. Above 500 °C, weight loss dominated although the exotherm continued to approximately 700 °C. The effect of heating rate and dwell time at 200 °C on the changes in the chemical composition during curing have been explored using IR and solid state NMR spectroscopies, and elemental analysis. The longer the cure time the higher was the weight gain and greater was the extent of the oxidation reactions. Elemental analysis showed that the ratio of H and C to Si decreased with holding time at the cure-temperature while the amount of oxygen increased. Use of a higher heating rate resulted in a lower weight gain when the same holding time was used. From this it is clear that curing starts below the holding temperature.     

Effects of 400 keV electrons flux on two space grade silicone rubbers

Two different space grade silicone rubbers were irradiated by an electron flux of 400 keV. The irradiation impact strongly depends on the chemical structure of rubbers (one reinforced with MQ resins, and the other one functionalized with phenyl groups at the silicon atoms and reinforced with silica). The irradiated rubbers were studied by means of solvent swelling, solid-state ²⁹Si NMR, and ATR–FTIR spectroscopy. Physical properties were evaluated by thermal (differential scanning calorimetry), mechanical (dynamic mechanical analysis), and thermo-optical (ultraviolet–visible–near infrared spectroscopy) analyses. The formation of silicium T units and Si–CH₂–Si networks were evidenced by ²⁹Si NMR, and the increase of the glass transition temperature and of modulus reflect the substantial increase in the macromolecular chain rigidity of the irradiated material. Dramatic damages of mechanical properties were observed, depending on the reinforced materials used. Slight changes of thermo-optical properties were highlighted independently to the initial chemical structure.     

29Si MAS-NMR investigation of the conversion process of a polytitanocarbosilane into SiC-TiC ceramics

A polytitanocarbosilane has been prepared from polycarbosilane and titanium n-butoxide.²⁹Si MAS-NMR was used to characterize the various steps of the conversion process of the polymer into the final ceramic. The reaction of titanium butoxide with polycarbosilane introduces oxygen into the polymer that seems to play an important role in the pyrolysis process. In the first stage up to 1000 ° C, the study reveals the cleavage of Si-C bonds and the formation of SiC_4-xO_x units. In the second stage, above 1000 ° C, the number of Si-O bonds decreases, probably due to a carbothermal reduction process. At 1500 ° C, the product can be described as a mixture of crystalline SiC and TiC with no excess carbon.     

Preparation of silicon carbide fibers from the blend of solid and liquid polycarbosilanes

A highly branched liquid polycarbosilane (LPCS) was added into a solid polycarbosilane (PCS) to give a polymer blend. It was then melt-spun into precursor fibers, oxidation-cured in hot-air, and converted into ceramic fibers by pyrolysis under nitrogen. It was found that the addition of the LPCS resulted in a significant drop on the spinning temperature from 285 °C (without LPCS) to 225 °C (with 15% LPCS), while the spinning ability of the polymer blend was also markedly improved over the solid PCS. Furthermore, the LPCS enhanced the oxidation curing, reducing the curing temperature and hence the tendency for fiber partial melting and sticking. However, the strength of the silicon carbide fibers decreased owing to the presence of the LPCS. The effects of the LPCS addition and their mechanisms on the fiber processing and properties were studied using FTIR, NMR, GPC, XRD, SEM, and elemental analysis.     

Sol-gel processing of SiO2-B2O3 glasses

Glasses from the SiO₂-B₂O₃ binary system containing up to 25 mole percent B₂O₃ have been synthesized. These glasses have been characterized by x-ray diffraction, optical transmission measurement, and I.R. absorption spectroscopy. In order to obtain a homogeneous glass, it was determined that a certain degree of hydrolysis of Si(OC₂H₅)₄ was essential before B(OC₂H₅)₃ could be incorporated. Optical transmission data exhibit characteristic absorption bonds related to Si, B, and OH fundamental and combination frequencies. I.R. absorption spectra in the 620 – 1700 cm^?1 range revealed presence of absorption bands related to vibrations of O-Si-O, Si-O-Si, B-O, and Si-O-B bonds.     

Structural evolutions from polycarbosilane to SiC ceramic

The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state²⁹Si and¹³C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of -SiC and -SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.     

Reactions in polyethoxysilane coatings using solution and solid-state29Si NMR spectroscopy

Solution and solid-state²⁹Si nuclear magnetic resonance (NMR) spectroscopy was used to follow the prehydrolysis and subsequent film formation of a polyethoxysilane mixture. Significant differences in intensity of the Q₂ resonances in the cross-polarization and dipolar decoupled spectra suggest a mobile phase is present in the cured film. Incorporation of solvent in the film is observed directly by¹³C cross polarization magic-angle-spin (CP MAS) NMR. The NMR results support the conclusions reached from a parallel chemical study.[/p]     

Pyrolytic conversion of methyl- and vinylsilane polymers to Si-C ceramics

Poly(methylsilane) and poly(vinylsilane) were synthesized using a titanocene catalyst, and their pyrolytic conversion to ceramics was followed using a combination of thermal analysis and infrared spectroscopy. The two polymers have distinctly different backbone structures, as determined by²⁹Si NMR; methylsilane polymerizes to a polysilane, while vinylsilane polymers have a predominately polycarbosilane backbone, with some polysilane structure as well. The pyrolysis path and char yield were dependent primarily on backbone structure, with little influence of polymer molecular weight. The majority of the weight loss on conversion occurs below 650 °C, although bond rearrangement continues to 1400 °C. Poly(vinylsilane) produced a carbon-rich Si-C ceramic in which the carbon was dispersed on a sufficiently fine level to show resistance to oxidation on heating in air to 1400 °C. Copolymerization of methyl- and vinylsilane produced stoichiometric SiC; however, polymers of methylsilane were sensitive to oxygen incorporation and sometimes pyrophoric. Polymerization of vinylsilane with disilylethane permitted control of rheology and imparted thermoset behaviour.     

NMR,XRD, IR and synchrotron NEXAFS spectroscopic studies of OPC and OPC/slag cement paste hydrates

This work aims to determine the fundamental similarities and/or differences between OPC and OPC/slag paste hydrates. OPC and 35% slag pastes are investigated using five techniques: ²⁹Si NMR, ²⁷Al NMR, X-ray diffraction (XRD), infrared (IR) and synchrotron near edge X-ray absorption fine structure (NEXAFS) spectroscopy. ²⁹Si NMR provides valuable information related to the formation of the C–S–H gel, the main hydrated phase of the cement paste. ²⁷Al NMR is a useful tool to characterize calcium aluminates and aluminate hydrates such as ettringite and monosulphate hydrate. XRD identifies polycrystalline phases of the hardened cement paste, including ettringite, monosulphate and CaOH₂. Vibrational frequencies in IR assist in identifying the silicate, sulphate and carbonate phases of the cement paste. As far as we are aware, Si K-edge NEXAFS has never been applied in cement research and its advantages and disadvantages are discussed. Using these techniques, a comparison between OPC and 35% slag paste hydrates is made, shedding light on differences in the amount and form of hydrated phases present, especially the absence of ettringite in the 35% slag paste.     

Preparation and characterization of mesoporous lithium borosilicate material via the sol–gel process

The lithium borosilicate gels were prepared from the cohydrolysis of the tetraehtylorthosilicate (Si(C₂H₅O)₄) and triethylborate (B(C₂H₅O)₃) by using an ethanolic solution of tetradecyltrimethylammonium bromide (TTAB) as surfactant. The Li⁺ ions were introduced from an acidic solution of lithium carbonate (Li₂CO₃). Depending on the B/Si, Li/Si and TTAB/Si molar ratios at pH equal to 1 and at room temperature, monolithic and transparent colourless gels were obtained. The structure of the gel was investigated by infrared spectroscopy (IR), ²⁹Si, ¹¹B and ⁷Li solid-state magnetic resonance (MAS NMR) and by thermal analysis (DTA–TG). The results show the possibility of obtaining a borosilicate network via B–O–Si bonds in which Li⁺ ions were dispersed. The adsorption–desorption isotherms of the xerogel were characteristic of mesoporous materials. These materials may provide a greater free volume through which conducting ions can move.     

Polymer–ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and ¹H, ¹³C, ²⁹Si nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 °C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si–H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 °C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si–CH₂–Si chains, the backbone of original polymer, can be retained up to 1,200 °C. At temperatures higher than 1,200 °C, the Si–CH₂–Si chains broke down and crystalline SiC began to form. The final crystalline products were β-SiC and a small amount of carbon.     

Heterogeneously catalysed crosslinking of polycarbosilane with divinylbenzene

Crosslinking of polycarbosilane (PCS) with divinylbenzene (DVB) is readily accomplished using heterogeneous catalysis with platinum chloride in heptane to provide a silicon carbide precursor that produces a ceramic with significantly reduced oxygen content. The ceramic yield after crosslinking increased from 47% to between 72% and 78%; however, crosslinking may be influenced by dehydrogenative silylation of hydroxyl groups. Solid-state ¹³C NMR spectroscopy of the crosslinked PCS showed peaks assignable to the aromatic group at 144 and 126.7 ppm. Monitoring of the crosslinking reaction by ¹H NMR spectroscopy indicated 40% consumption of the vinyl bonds of DVB within 10 min and complete consumption within 16 h. Infrared spectroscopy showed no increase in the peak at 3,650 cm⁻¹ due to O–H stretching in Si–OH, demonstrating that hydrosilylation crosslinking is a highly effective non-oxidative crosslinking technique. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Structural studies of geopolymers by <Superscript>29</Superscript>Si and <Superscript>27</Superscript>Al MAS-NMR

A systematic study of geopolymers by ²⁹Si and ²⁷Al MAS NMR has been carried out in an attempt to understand polymer structural details. ²⁷Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of ²⁹Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. ²⁹Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na²O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na²O content], a clear ²⁹Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na²O content (20 mol%) and varied SiO²/Al²O³ ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed ²⁹Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by ²⁹Si NMR as the signal from metakaolin is indistinguishable from a broad ²⁹Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO²/Al²O³ molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved ²⁹Si NMR lines. The ²⁹Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.     

29Si and 17O NMR investigation of the structure of some crystalline calcium silicate hydrates

This paper presents the results of a systematic investigation of the structure of ¹⁷O-enriched, hydrothermally synthesized 1.1-nm tobermorite, 1.4-nm tobermorite, jennite, calciochondrodite, xonotlite, and hillebrandite, using ²⁹Si magic angle spinning (MAS) NMR, ¹H-²⁹Si cross-polarization magic angle spinning (CPMAS) NMR, and ¹⁷O MAS NMR. The ¹⁷O and most of the ¹H-²⁹Si CPMAS results are the first reported for these phases. Six types of oxygen sites were observed in tobermorite and jennite, including both SiOH CaOH linkages. The structure of 1.4-nm tobermorite is similar to that of 1.1-nm tobermorite with about 26% of the Ca²⁺′s in the interlayers. The results support the proposed jennite structure in which silicate chains and rows of OH⁻ groups alternately occur along the CaO layers [1]. Jennite contains long, single silicate chains similar to those in 1.4-nm tobermorite, with SiOH sites primarily occurring on bridging tetrahedra, and there seems to be no interlayer Ca²⁺′s. Although the Si sites in xonotlite and calciochondrodite cross-polarize well, neither contains SiOH linkages.     

Structural and spatially resolved studies on the hardening of a commercial resin-modified glass-ionomer cement

A commercial photopolymerizable resin-modified glass-ionomer (Fuji II LC) was studied using a variety of nuclear magnetic resonance (NMR) techniques. ¹H and ¹⁹F stray-field imaging (STRAFI) enabled to follow the acid–base reaction kinetics in self-cured (SC) samples. Gelation and maturation processes with 25 min and 40 h average time constants, respectively, were distinguished. In self- & photo-cured (SPC) samples, two processes were also observed, which occurred with 2 s and 47 s average time constants. ¹H, ²⁷Al and ²⁹Si magic angle spinning (MAS) NMR, ¹³C cross-polarization (CP)/MAS NMR and ²⁷Al multiple quanta (MQ)MAS NMR spectroscopy were used to obtain structural information on the glass and cements that were either SC or SPC. The presence of methacrylate groups was identified in the solid component. Unreacted hydroxyl ethylmethacrylate (HEMA) was detected in self-cured cement. ²⁷Al data showed that approximately 28% and 20% of Al is leached out from glass particles in SC and SPC samples, respectively. The upfield shift detected in ²⁹Si MAS NMR spectra of the cements is consistent with a decrease in the number of Al species in the second coordination sphere of the silicon structures. Scanning electron microscopy (SEM) showed existence of 3D shrinkage of the cement matrix in photo-cured cements.     

Silicon oxycarbide glasses derived from polymeric networks with different molecular architecture prepared by hydrosilylation reaction

We have prepared two polysiloxane networks with different molecular structures and similar compositions as suitable precursors of silicon oxycarbide glasses (SiOC), in order to understand the influence of the molecular architecture of the polymeric networks on the structure of these glasses. The structural evolution from the polymeric precursor to glasses was followed by solid-state ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR) and FTIR spectroscopies, X-ray diffraction, thermogravimetric analysis, and density measurements up to 1000 °C. The high- temperature behavior of SiOC glasses, up to 1600 °C, was studied by X-ray diffraction, and ²⁹Si MAS NMR spectroscopy. The formation of carbosilane bridges in the polymeric networks during the hydrosilylation reaction contributed to the generation of a larger amount of carbidic sites in the final products. An increase in the pyrolysis temperature led to a distribution of silicon sites and crystallization profiles that depended on the molecular architecture of the polymeric precursors.     

Effect of temperature on C–S–H gel nanostructure in white cement

Different ages of white cement pastes hydrated at 100 % RH and 25 or 65 °C were characterised with ²⁹Si MAS NMR spectroscopy. The findings showed that raising the curing temperature from 25 to 65 °C accelerated hydration of the belite phase considerably, inducing a sixfold rise in its one day degree of hydration, while alite phase hydration grew by a factor of only 1.5 in the first day. Moreover, the C–S–H gel formed at the higher temperature had a longer mean chain length and a higher initial uptake of Al³⁺. Lastly, curing at a higher temperature stabilised only one crystalline aluminate phases, calcium hemicarboaluminate.     

Ormosil coatings of high hardness

Organically modified silicates (ormosils) of two systems were coated on polyethyleneterephthalate (PET) substrates. One was the tetraethoxysilane (TEOS)-vinyltriethoxysilane (VTES) system and the other was the TEOS-tetraisopropyltitanate (TIPT)-VTES system. The reactions among the alkoxides were examined by liquid state ²⁹Si nuclear magnetic resonance (NMR) spectroscopy. The chemical bonds between TEOS and VTES, between TEOS and TIPT, and between VTES and TIPT are shown in the spectra and the reaction schemes are proposed. Vickers hardnesses of the ormosil coatings were obtained by using the models developed by Jönsson and Hogmark, and the theoretical models developed by the authors were valuable to predict the hardness values. The ormosil coatings obtained in the present study were much harder than the hardest transparent plastics, and thus are very useful for hard or protective coatings on organic polymers.