含酰胺结构超低膨胀聚酰亚胺薄膜的热膨胀行为

采用联苯二酐与3种含酰胺结构二胺制备了具有不同取代基团的聚酰胺-酰亚胺薄膜,考察了酰胺结构对薄膜力学、耐热及尺寸稳定性的影响,研究了聚集态结构与薄膜热膨胀行为的关系和规律.该系列薄膜具有超高强度和优异的耐热性能,拉伸强度高达280. 5 MPa,玻璃化转变温度在389～409℃,并在30～300℃温度范围内表现出超低负膨胀,热膨胀系数(CTE,ppm/℃,即106cm·cm-1·℃-1)在-3. 05～-1. 74ppm/℃之间.聚集态分析结果表明,酰胺结构使分子链间形成了强氢键相互作用,分子链在薄膜面内方向高度有序取向,并在膜厚方向堆积更为紧密,使薄膜表现出热收缩现象.通过不同体积大小的取代基团进一步调控分子链间相互作用及排列堆积,可实现薄膜在高温下近乎零尺寸形变,为设计制备超低膨胀聚合物基板材料提供了新思路.     

X-ray diffraction study on the thermal expansion behavior of cellulose Iβ and its high-temperature phase

Oriented films of cellulose prepared from algal cellulose were hydrothermally treated to convert them into highly crystalline cellulose Iβ. The lateral thermal expansion behavior of the prepared cellulose Iβ films was investigated using X-ray diffraction at temperatures from 20 to 300 °C. Cellulose Iβ was transformed into the high-temperature phase when the temperature was above 230 °C, allowing the lateral thermal expansion coefficient of cellulose Iβ and its high-temperature phase to be measured. For cellulose Iβ, the thermal expansion coefficients (TECs) of the a- and b-axes were α_a = 9.8 × 10⁻⁵ °C⁻¹ and α_b = 1.2 × 10⁻⁵ °C⁻¹, respectively. This anisotropic thermal expansion behavior in the lateral direction is ascribed to the crystal structure and to the hydrogen-bonding system of cellulose Iβ. For the high-temperature phase, the anisotropy was more conspicuous, and the TECs of the a- and b-axes were α_a = 19.8 × 10⁻⁵ °C⁻¹ and α_b = −1.6 × 10⁻⁵ °C⁻¹, respectively. Synchrotron X-ray fiber diffraction diagrams of the high-temperature phase were also recorded at 250 °C. The cellulose high-temperature phase is composed of a two-chain monoclinic unit cell, a = 0.819 nm, b = 0.818 nm, c (fiber repeat) = 1.037 nm, and γ = 96.4°, with space group = P2₁. The volume of this cell is 4.6% larger than that of cellulose Iβ at 30 °C.     

The thermal expansion of cellulose II and IIIII crystals

Highly crystalline samples of cellulose II and III_II have been prepared from repeated mercerization of ramie fibers and supercritical ammonia treatment of the mercerized ramie fibers, respectively. The thermal expansion behavior of cellulose II and III_II was investigated using X-ray diffraction at temperatures ranging from room temperature to 250 °C. With increasing temperature, the unit cell of cellulose II expanded in the lateral directions and contracted in the longitudinal direction, with the a and b axes increasing by 0.54 and 3.4%, respectively, and the c axis decreasing by 0.09%. The anisotropic thermal expansion in these three directions was closely related to the crystal structure and the hydrogen bonding in cellulose II. A similar anisotropic thermal expansion was also observed in cellulose III_II. Cellulose III_II expanded in the lateral direction but contracted in the longitudinal direction.     

纤维素的功能化

综述近年来纤维素功能化和功能材料的发展情况,介绍了通过化学改性、特殊物理加工、表面改性等获得纤维素功能的功能的各种功能化的主最新的功能设计思路,并介绍了纤维素功能材料的应用,未来潜在的应用领域和发展前景。     

Cellulose Esters Prepared by Wood Esterification

Trends in the yields and properties of cellulose esters are studied as a function of wood esterification conditions. The yield, total extent of cellulose ester substitution, and degree of conversion depend on the duration and acylation temperature     

Thermal expansion of structure-H clathrate hydrates

The temperature dependence of the unit cell parameters of two newly identified hexagonal structure clathrate hydrates of hexamethylethane (HME) and 2,2-dimethylbutane (DMB) have been measured by X-ray powder diffraction. The thermal expansion of the two distinct crystallographic axes was found to be inequivalent. However, the coefficients of cubic expansion are comparable to that in the cubic structure I and II hydrates. The larger thermal expansivity in the clathrate hydrates relative to ice is attributed to the weakening of the host lattice due to the internal pressure generated by the rattling motions of the encaged guests.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Thermal Expansion in the Zr and 1-, 2-valent Complex Phosphates of NaZr2(PO4)3 (NZP) Structure

A_5–4xZr_xZr(PO₄)₃ (A=Na, K;0≤x≤1.25), Na_1-xCd_0.5xZr₂(PO₄)₃ (0≤x≤1), Na_5–xCd_0.5xZr(PO₄)₃ (0≤x≤4) compositions which belong to the NZP structural family were synthesized using the sol-gel method. The lattice thermal expansion of members of these rows were determined up to 600°C by high-temperature X-ray diffractometry. The axial thermal expansion coefficients change from -5.8·10^-6to 7.5·10^-6 °C^-1 (α_a) and from 2.6·10^–6 to 22·10^–6 °C^-1 (α_c). These results, in addition to those for other NZP compounds allow us to explain their low thermal expansion. The mechanism can be attributed to strongly bonded three-dimensional network structure, the existence of structural holes capable to damp some of the thermal vibrations and anisotropyin the thermal expansion of the lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal Expansion Behaviors of Li3AsW7O25: A Case Study for Comparative Debye Temperature for a Large Polyatomic Unit Cell

The thermal expansion behavior of Li₃AsW₇O₂₅ has been studied. The temperature‐dependent development of crystal structural parameters was obtained from Rietveld refinement using neutron time of flight powder diffraction data. Modeling of the lattice thermal expansion was carried out using a Grüneisen first‐order approximation for the zero‐pressure equation of state, where the temperature‐dependent vibrational energy was calculated taking the Debye‐Einstein‐Anharmonicity approach. Temperature‐dependent Raman spectra shed light on some selective modes with unusual anharmonicity. Debye temperatures were calculated using three different theoretical approaches, namely, thermal expansion, mean‐squared isotropic atomic displacement parameter and heat capacity. Similarities as well as discrepancies between the numerical values obtained from different theoretical approaches are discussed.     

Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules

We investigated for the first time the abnormal thermal expansion induced by an asymmetric guest structure using high‐resolution neutron powder diffraction. Three dihydrogen molecules (H₂, D₂, and HD) were tested to explore the guest dynamics and thermal behavior of hydrogen‐doped clathrate hydrates. We confirmed the restricted spatial distribution and doughnut‐like motion of the HD guest in the center of anisotropic sII‐S (sII‐S=small cages of structure II hydrates). However, we failed to observe a mass‐dependent relationship when comparing D₂ with HD. The use of asymmetric guest molecules can significantly contribute to tuning the cage dimension and thus can improve the stable inclusion of small gaseous molecules in confined cages.     

低热膨胀聚酰亚胺薄膜的研究进展与展望

聚酰亚胺是一种很有发展前途的高分子材料,热膨胀系数高的问题限制了聚酰亚胺的应用,降低热膨胀系数已成为聚酰亚胺研究热点之一。本文概述了国内外关于降低聚酰亚胺薄膜热膨胀系数的主要方法:分子结构设计法、共聚法、树脂共混法、添加纳米粒子法。阐述了工艺因素(如涂膜方式、牵伸条件等)对聚酰亚胺热膨胀系数的影响,并对未来低热膨胀系数聚酰亚胺薄膜的发展方向进行了展望。     

Structural Changes of Cellulose Crystallites Induced by Mercerisation in Different Solvent Systems; Determined by Powder X-ray Diffraction Method

The completeness of mercerisation can be evaluated by investigating the changes in the crystalline regions of cellulose from cellulose I (C-I) to cellulose II (C-II) by the X-ray powder diffraction method. Mercerisation experiments in four different solution systems: ethanol/water, acetone, DMSO and xylene, are reported. Also the effect of some additives, external pressure, treatment time and alkalisation temperature were studied. In two-phase solvent systems, structural changes of cellulose crystallites depended primarily on the distribution and solubility of sodium hydroxide in the solvent phases. The sodium hydroxide concentration in the hydrophilic phase must exceed 7–8 w/w-% before complete crystal change from C-I to C-II can occur. The precipitation of sodium hydroxide due to high concentration prevents the successful use of one-phase ethanol/water system in slurry process. On the contrary, the 2-propanol/water/sodium hydroxide system separates into two layers; to the water-rich lower layer and the 2-propanol-rich upper layer, where the sodium hydroxide remains mainly in the water-rich lower layer. This prevents the precipitation of sodium hydroxide and promotes the alkalisation of cellulose. Ammonium chloride and ammonium hydroxide clearly had a positive effect by promoting the crystal changes, however, the urea concentration used in this study was obviously too small. In the advantageous two-phase 2-propanol/water systems, the alkalisation time was only 15 min when the treatment temperature was kept between 0 and 10 °C. Reduced external pressure was found to have a small but still detectable positive effect on cellulose alkalisation while over-pressure prevented crystal changes.     

Extraction and Characterization of Natural Cellulose Fibers from Maize Tassel

This article reports on the extraction and characterization of novel natural cellulose fibers obtained from the maize (tassel) plant. Cellulose was extracted from the agricultural residue (waste biomaterial) of maize tassel. The maize tassel fibers were obtained after treatment with NaOH and were carefully characterized while the chemical composition was determined. The chemical composition of the maize tassel fibers showed that the cellulose content increased from 41% to 56%, following alkali treatment. FT-IR spectroscopic analysis of maize tassel fibers confirmed that this chemical treatment also shows the way to partial elimination of hemicelluloses and lignin from the structure of the maize tassel fibers. X-ray diffraction results indicated that this process resulted in enhanced crystallinity of the maize tassel fibers. The thermal properties of the maize tassel fibers were studied by the TGA technique and were found to have improved significantly. The degradation temperature of the alkali-treated maize tassel fiber is higher than that of the untreated maize tassel fibers. This value convincingly showed the potential of maize tassel fibers for use in reinforced biocomposites and waste water treatment.     

Thermal Behavior and Thermal Safety of Nitrate Glycerol Ether Cellulose

The thermal behavior, nonisothermal decomposition reaction kinetics and specific heat capacity of nitrate glycerol ether cellulose(NGEC) were determined by thermogravimetric analysis(TGA), differential scanning calorimetry(DSC) and microcalorimetry. The apparent activity energy(E_a), reaction mechanism function, quadratic equation of specific heat capacity(C_p) with temperature were obtained. The kinetic parameters of the decomposition reaction are E_a=170.2 kJ/mol and lg(A/s^–1)=16.3. The kinetic equation is f(α)＝(4/3)(1–α)[–ln(1–α)]^1/4. The specific heat capacity equation is C_p=1.285–6.276×10^–3T+1.581×10^–5T²(283 KSADT), critical temperature of thermal explosion(T_b) and adiabatic time-to-explosion(t_Tlad). The results of the thermal safety evaluation of NGEC are: T_SADT=459.6 K, T_b=492.8 K, t_Tlad=0.8 s.     

Effect of Thermal Cycles on the Thermal Expansion Behavior of T700 Carbon Fiber Bundles

The relationships between the coefficient of thermal expansion(CTE) of T700 carbon fiber bundles(CFBs) and the thermal cycles were investigated. The microstructure of T700 CFBs was analyzed with Raman spectra and XRD before and after the thermomechanical test. The results indicated that the T700 CFBs exhibited negative expansion in the direction of parallel fibers in the temperature range of -150-150℃. The thermal strain that occurred during the heating and the cooling thermal cycle had an unclosed curve that served as the loop. When the experimental load was the same, the position of strain loop tended to move upward, and the length of the specimen increased continuously with the thermal cycles increasing. The microstructural analysis suggested that the degree of structural order and the degree of orientation along the fiber axis were improved with the increase of thermal cycles. The change of microstructure parameters could be the primary cause of the negative CTE's variation within the T700 CFBs.     

Extraction and Characterization of Cellulose Nanowhiskers from Balsa Wood

Summary: In this study cellulose nanowhiskers were obtained from balsa wood. For this purpose, fibers of balsa wood were exposed to hydrolysis reactions for lignin and hemicellulose digestion and acquisition of nano-scale cellulose. Transmission electron microscopy (TEM) results demonstrated that the obtained cellulose nanocrystals had average length and thickness of 176 (±68 nm) and 7.5 (±2.9 nm), respectively. Infrared spectroscopy (FTIR) and wide angle x-ray diffraction (WAXD) showed that the process for extracting the nanowhiskers digested nearly all the lignin and hemicellulose from the balsa fiber and still preserved the aspect ratio and crystallinity satisfactory enough for future application as nanofillers in polymer nanocomposites. The thermogravimetric analysis (TGA) showed that the onset temperature of thermal degradation of the cellulose nanocrystals (226 °C) was higher than the onset temperature of the balsa fiber (215 °C), allowing its use in molding processes with polymers melts.     

纤维素及其衍生物液晶研究近况

就纤维素及其衍生物溶性液晶和热致性液晶的液晶性及其影响因素,和流变性能等方面的研究作了较为详尽的综述。     

The effect of refluxing on the alkoxide-based sodium potassium niobate sol-gel system: Thermal and spectroscopic studies

A study on the effects of prolonged heating under reflux conditions of up to 70 h on alkoxides of sodium, potassium and niobium dissolved in 2-methoxyethanol for the synthesis of sols of composition Na_0.5K_0.5NbO₃ (NKN) has been carried out using combined thermogravimetric-Fourier transform infrared spectroscopic analyses. Extended refluxing increases the homogeneity of the Na_0.5K_0.5NbO₃ (NKN) system. Spectroscopic analyses on the non-refluxed and 70 h refluxed NKN gels reveal the existence of inorganic hydrated carbonates and bicarbonates, which we propose arise from the hydration and carbonation of the samples on standing in air. The X-ray diffraction patterns of these two types of gels show orthorhombic NKN phase evolutions at higher temperatures.     

Low-Barrier Hydrogen Bonds in Negative Thermal Expansion Material H3[Co(CN)6]

The covalent nature of the low-barrier N−H−N hydrogen bonds in the negative thermal expansion material H₃[Co(CN)₆] has been established by using a combination of X-ray and neutron diffraction electron density analysis and theoretical calculations. This finding explains why negative thermal expansion can occur in a material not commonly considered to be built from rigid linkers. The pertinent hydrogen atom is located symmetrically between two nitrogen atoms in a double-well potential with hydrogen above the barrier for proton transfer, thus forming a low-barrier hydrogen bond. Hydrogen is covalently bonded to the two nitrogen atoms, which is the first experimentally confirmed covalent hydrogen bond in a network structure. Source function calculations established that the present N−H−N hydrogen bond follows the trends observed for negatively charge-assisted hydrogen bonds and low-barrier hydrogen bonds previously established for O−H−O hydrogen bonds. The bonding between the cobalt and cyanide ligands was found to be a typical donor–acceptor bond involving a high-field ligand and a transition metal in a low-spin configuration.     

Thermal behaviour of lyocell fibres

Fourier-transform infrared (FTIR) spectroscopy has been applied in combination with wide-angle X-ray diffraction and measurements of strength, fluidity, yellowness, birefringence, and moisture regain to detect microstructural changes in lyocell fibres, a regenerated cellulose fibre, subjected to direct heat and annealing treatments. TMA, and SEM were used to show the effect of direct heat and annealing on lyocell fibres. The FTIR spectroscopy results show that a decrease in intermolecular hydrogen bonding occurs at 70 and 80 °C for annealed and directly heated samples, respectively. The results demonstrate increase of the intensity of O–H stretching vibrations, this associated with hydrogen bonds reforming around 130 °C. Lyocell fibres shrink with direct heating in the temperature range 130–160 °C. The crystallinity decreases gradually with increasing temperature. There is no significant change in colour of the samples annealed up to 150 °C. A continuous increase in the fluidity occurs for the annealed samples in the range 150–230 °C. The tenacity and breaking extension of heated samples decrease with increasing temperature. The lower annealing temperatures cause no observable change in the smooth and void-free surface, but in the annealing temperature range 170–230 °C, substantial non-uniformity is apparent on the surface of the fibres.