丙三醇浓溶液玻璃化转变与结构松弛参数DSC分析

利用差示扫描量热仪研究了5种高浓度丙三醇水溶液(60%、70%、80%、90%、100%)的玻璃化转变行为,以考察水分含量和升降温速率对其玻璃化转变行为和结构松弛参数的影响.采用4种线性升降温速率(10、15、20、25K/min)获得玻璃化转变的相关参数.利用GordonTaylor方程对玻璃化转变温度的分析结果表明,水对丙三醇增塑常数的计算结果与升降温速率和玻璃化转变温度的读取方法有很大关系.玻璃化转变过程的比热容变化不仅随水分含量的增加而增加,而且与升降温速率也有一定的依赖关系.结构松弛活化能的计算结果表明,随体系水分含量的增加,体系的结构松弛活化能和动力学脆度都逐渐降低.随水分含量的变化,热力学脆度和动力学脆度表现出相反的变化趋势.     

环氧树脂高温分子链松弛与玻璃化转变特性

环氧树脂是电力设备中广泛应用的一种绝缘材料, 其介电性能受到分子链运动特性的影响. 本文制备了直径为50 mm、厚度为1 mm的环氧树脂试样, 采用差示扫描量热仪和宽频介电谱仪测试了环氧树脂的玻璃化转变温度和介电特性. 实验结果表明, 环氧树脂的玻璃化转变温度为105 ℃, 在玻璃化转变温度以上, 高频段出现了由分子链段运动造成的松弛过程, 低频段出现了由载流子在材料中迁移造成的直流电导过程. 发现环氧树脂不同尺寸分子链段的松弛时间不同, 其松弛时间分布较宽, 计算得到了分子链段在不同温度下的松弛时间分布特性. 分子链松弛峰频率和直流电导随温度的变化关系服从Vogel-Tammann-Fulcher公式. 拟合实验结果得到分子链松弛峰频率和直流电导的Vogel温度和强度系数. 由Vogel温度计算得到了与差示扫描量热测试结果一致的玻璃化转变温度, 约为102 ℃. 结果表明玻璃化转变温度以上环氧树脂的自由体积增大, 分子链段有足够的空间来响应外电场从而产生分子链松弛极化, 载流子有足够的能量在材料中迁移形成电导.     

聚砜的加速老化效应分析

动态力学分析法基于高分子材料链段运动的活化能可用来表征材料老化速率的快慢，是一种研究高分子材料内部分子链段运动的有效方法，其参数能有效表征高分子材料分子链段的运动。研究表明：高分子材料的玻璃化温度Tg随着动态力学性能测试时的频率w的增加而升高，满足lnw=lnw0-(E/RTg)，即测试频率w自然对数和玻璃化温度Tg的倒数成线性关系。     

铂表面甘油脱除羟基的从头算分子动力学研究

随着生物质能源的开发,从生物柴油制备中大量获取的甘油成为热门的工业原材料. 甘油可以通过氢解生成1,2-丙二醇和1,3-丙二醇两种丙二醇,这两种丙二醇都具有十分广泛的用途. 实验上报道的众多相关金属催化剂中,铂具有性质稳定、不易失活、可活化氢分子提供氢原子等优点. 此外甘油在铂上氢解生成1,2-丙二醇的选择性高于1,3-丙二醇. 本文主要利用从头算分子动力学对甘油在Pt(111)和Pt(211)表面上发生的羟基解离过程进行了模拟计算,并对比分析了其自由能的变化和表面物种结构参数的变化,得出了以下结论：(i)密度泛函理论优化气相甘油分子结构的结果显示,氢键对于气相中分子的结构与能量有较大贡献,三个羟基形成三个分子内氢键结构时甘油分子能量最低;(ii)通过比较从头算分子动力学模拟得到的自由能能垒和反应自由能,可以得出,在Pt(111)和Pt(211)表面,末端碳上的羟基比中间碳上的羟基更容易发生解离. 这表明在类似的条件下,铂作为催化剂可以为1,2-丙二醇的生产提供更高的选择性,这与文献中报道的实验结果一致;(iii)通过对从头算分子动力学模拟得到的初始吸附态和过渡态结构参数的分析,发现在羟基解离的过程中,C-C键的键长没有明显变化,而氧原子的相对位置以及氢键的长度有明显变化,且氢键长度的变化更加剧烈;(iv)通过比较从头算分子动力学研究所得的自由能能垒和结构参数的相关趋势发现,自由能能垒与初始吸附态和过渡态的氧原子间位置的变化量之间存在线性关系,而分子内氢键对自由能能垒的贡献可以忽略不计. 氧原子间位置的变化越大,自由能能垒越高.     

冷冻速率对蚕豆热物性变化规律影响的实验研究

研究了不同冷冻速率下蚕豆热物性的变化规律。利用Hot Disk得到蚕豆在-30～30℃下的热导率变化曲线;通过DSC和低温冷冻实验台,冷冻速率在5～80℃/min条件下将蚕豆切片进行冷冻实验,得到了不同冷冻条件下冻结点、结晶点及比热容的变化规律。测试结果表明:蚕豆热导率随着温度的降低先减小再增大;冻结点随着降温速率的增大而增大;蚕豆的结晶点在降温速率小于20℃/min时出现浮动,但总体上,结晶点随着降温速率的增大而降低。在不同降温速率下,比热容随温度的变化规律基本相同,但在冷冻相变过程中发生剧烈变化。通过对冷冻过程中蚕豆热物性的实验研究,为冷冻干燥种子的传热传质分析提供了实验数据的支撑。     

纳米微粒对低温保护剂溶液玻璃化性质的影响

利用差示扫描量热仪(DSC)研究了加入羟基磷灰石(HA)纳米微粒对低温保护剂溶液玻璃化的影响,实验得到了不同粒径和不同质量浓度的HA纳米微粒加入PVP溶液的玻璃化转变温度与反玻璃化温度.实验结果表明加入纳米微粒能显著的影响低温保护剂溶液的玻璃化性质.且随着纳米微粒质量分数的增加,溶液的玻璃化转变温度与反玻璃化温度均显著...     

三元溶液共晶DSC实验研究

抑制共晶产生对低温保存非常重要。本文利用差示扫描量热法研究了加低温保护剂(DMSO、乙二醇、 1,2丙二醇、甘油和1,3丁二醇)的NaCl水溶液的共晶行为。得到以5％、10％、15％NaCl水溶液为母液的五种保护剂溶液热流曲线图。研究发现,溶液共晶是过冷、随机过程。低温保护剂有抑制NaCl水溶液共晶的作用。低温保护剂浓度越高, 共晶焓越小,对共晶的抑制作用越大。不同种类保护剂的抑制共晶的能力从强到弱依次是甘油、乙二醇、 DMSO、 1,2 丙二醇和1,3丁二醇。升温过程中,溶液发生共晶反玻璃化现象和玻璃化现象。     

降温速率对La0.7Ca0.3Mn1-xCrxO3(x=0.00,0.08)电磁性质的影响

我们在不同的降温速率下得到了若干La0．7Ca0．3Mn1-xCrxO3-δ(x=0．00,0．08)多晶样品,对其进行了XRD、电阻．温度关系和磁化强度．温度关系的测量．实验发现,随降温速率的增大,LCM系列样品的电阻变大,电阻峰温度移向低温端,磁化强度值变小,金属．绝缘体转变依然很陡峭,而LGMC系列样品也兼有这些现象,前两的程度更大些,后较为平缓,另外还出现了电阻的双峰．归结原因,我们认为在制备过程中随降温速率的不同,样品中会有不同程度的氧空位存在,利用双交换的减弱和样品中氧的分布不均匀产生的相分离解释了有关现象．通过本实验可知,改变降温速率也是调节样品中氧含量的一种有效的方法．     

快速冷却条件下银的比热及玻璃化过程的分子动力学模拟

本文采用以嵌入原子模型为基础的分子动力学方法对液态银快速冷却条件下的比热及玻璃化过程进行了模拟，得到了银的比热及其与温度的关系．同时，对快速冷却条件下银的结构特性以及动力学特性进行模拟和分析，结果表明在所模拟的冷却速率下，银在从 １２００ Ｋ冷却到 １０００ Ｋ的过程中发生了玻璃化转变，其玻璃化温度在 １０００ Ｋ左右．     

SMMA/SMA共聚物共混物的自由体积的热动态特性与相分离行为的PALS研究

基于自由体积理论，利用正电子湮灭寿命谱仪(PALS)分别研究在不同升温速率条件下，聚苯乙烯-甲基丙烯酸甲酯共聚物(SMMA)和聚苯乙烯-马来酸酐共聚物(SMA)混合物(30/70)的自由体积参数的温度依赖性，探索相容共混物相行为的热动态特性.在PALS实验中，在玻璃化转变温度T_g以上，当结构松弛的弛豫时间与等温停留时间相当，发现在某一段温度范围内，共混物的自由体积参数随着温度的变化明显地偏离线性关系.从自由体积孔的浓度I₃值在该段的变化趋势，初步推断共混物在该温 关键词： 正电子湮没 正电子素 聚合物共混物 相分离     

Rotational Relaxation of Linear π-Systems with Flexible Side Chains in Solution

The classical hydrodynamic theory for Brownian rotational motion is applied to model compounds of conjugated polymers with alkoxy side chains of variable length. Theory predicts two rotational relaxation times for these types of molecules with the dipole transition moment parallel to the longest axis whereas experiments reveal only one. The rotational relaxation times and their relative amplitudes were calculated for a wide span of axial ratios of a general ellipsoid. In this way, the range in the axial ratios is obtained such that there is a chance to detect both rates experimentally. Rotational relaxation times of five particular molecules were measured in liquid n-butane. Theoretical calculations using ellipsoid parameters obtained from molecular dynamics calculations compare well with experimental results. Calculation of the rotational relaxation times from the autocorrelation function of the transition dipole moment vector requires significantly greater computational effort.     

A simple capacitive cell for the measurement of liquids dielectric constant under transient thermal conditions

A simple device for the measurement of the complex dielectric permittivity of liquids in various thermodynamic states has been developed. It uses a cylindrical aluminium capacitor of a type currently applied in tuning antenna circuits. The capacitor is filled with the liquid solution under study. A comparison of its capacity is made with that of the nitrogen filled capacitor tested under the same thermal conditions. This comparison allows the determination of the real and imaginary part of the solutions permittivity as a function of temperature (between 150 and 300 K) and frequency (between 100 Hz to 2 MHz). After validating the technique with pure glycerol and pure 1,2-propanediol, spectroscopic measurements have been undertaken on pure and diluted 1,2-propanediol in water. Due to the low heat capacity and the high thermal conductivity of the capacitor, cooling rates of 40 K/min have been achieved inside the solution, allowing measurements in the supercooled liquid and vitreous states. Results are presented and discussed in terms of relaxation and the physical states of the sample. By selecting the required thermal conditions, this device permits the observation of thermal transitions, such as ice crystallisation, and measurements to be conducted in the unstable supercooled liquid state. These measurements are necessary in the development of an effective electromagnetic warming device for vitrified cryoprotective solutions.     

Integrity of endothelium in cryopreserved human cornea

The aim of the present study was to elaborate an optimal method for cryopreservation of human donor cornea for transplantation and to follow the morphological changes in the structure of the endothelial cell layer using scanning electron microscopy (SEM). Sixteen groups, with four donor cornea each, were cryopreserved at cooling rates of 1 degree C per min and 5 degree C per min. Four cryoprotectants (glycerol, dimethyl sulfoxide, 1,2-propanediol, polyethylene glycol-400) in two concentrations (5% and 10% v/v) were prepared on the bases of medium Optisol GS supplied with 20% v/v human serum albumin. Four additional human cornea were used as controls. Endothelial cell recovery of the cornea after thawing and 24 hours culture, was calculated as a percent of the preserved recovered cells. Sufficient recovery of the endothelial cell layer, making the cornea suitable for transplantation was obtained using the cryoprotectants dimethyl sulfoxide and especially polyethylene glycol-400.     

Friction and Wear of Polyphenylene Sulfide (PPS), Polyethersulfone (PES) and Polysulfone (PSU) Under Different Cooling Conditions

The friction and wear properties of polyphenylene sulfide (PPS), polyethersulfone (PES) and polysulfone (PSU), which have similar molecular structure, were investigated using an end-face contact tribometer in three different cooling ways: sliding without air cooling, sliding with air cooling, and sliding in water. The worn surface and wear debris were observed using a scanning electron microscope (SEM). The effect of frictional heat on the tribological properties of the polymers was comparatively studied. When sliding in air, with increasing applied load, the wear rate of PPS decreased slightly initially then increased later while the wear rate of PES and PSU increased through out. The results suggested that the friction coefficient was mainly affected by the temperature of the worn polymer that was controlled by the balance of heat flow of the whole sliding contact system. When sliding in water, the friction coefficients of the three polymers decreased compared to that sliding in air and remained relatively steady through the whole process under different load. The wear rates of the three polymers had a close value and, remarkably, increased compared to that sliding in air. The water cooling and lubrication role decreased the tribological properties difference between the polymers.     

1,2-聚丁二烯的固态13C-NMR线宽与自由体积

本工作测定了一系列1，2-聚丁二烯样品在不同温度下的固态¹³C-NMR线宽,研究了线宽对链结构和温度的依赖性;发现在一定的温度范围内,线宽与温度的关系符合WLF方程,这表明高聚物的NMR弛豫不仅取决于分子结构,也与自由体积有关。自由体积理论在NMR弛豫中也是适用的。     

Probing slow dynamics in supported thin polymer films

We have used variable cooling rate ellipsometric measurements to probe the slow dynamics in thin supported polystyrene films. For the slowest cooling rates (approximately 1 K/min) the measured Tg values are reduced below the bulk value with the measured Tg of 341 K for a 6 nm film. As the cooling rate is increased the Tg reductions become smaller until at cooling rates >90 K/min there is only slight evidence for a film-thickness-dependent Tg value. By relating the cooling rate to a relaxation time, we show that the relaxation dynamics of the thin films appears to become Arrhenius with an activation energy that decreases with decreasing film thickness. We discuss this in terms of a possible connection to a length scale for cooperative motion. Finally, the results can be used to resolve a number of outstanding contradictory reports in the literature.     

Macromolecular composites of extruded thermotropic polymer sheets

Using two types of thermotropic liquid-crystal polymers, experimental results reconfirmed previous reports that the relaxation time of extruded liquid-crystal polymers is much longer than that of conventional polymers. The initial modulus and elongation of sheets extruded with liquidcrystal polymers were found to be strongly dependent on the gauge length. In addition, it was observed, for the first time, that the mechanical properties of the extruded thermotropic copolyesters follow the general equations for fiber-reinforced composites such that the angular dependence of the tensile strength of the extruded liquid-crystal polymeric sheets obeys the Tsai-Hill theory, while the angular dependence of the initial modulus follows the Lees equation. These results suggest that the highly oriented liquid-crystal domains may be considered as reinforcing fibers in the extruded articles, and their mechanical properties can be predicted using existing composite theories.     

Anderson-grüneisen parameter δ, bulk modulus and the compression curves of rhenium

The third-order elastic (TOE) constants of rhenium obtained from a model based on Keating's approach have been used to calculate the Anderson-Grüneisen (AG) parameter δ for this metal following the procedure suggested by Ramji Rao. The temperature dependence of the bulk modulus of rhenium has been calculated using Anderson's theory. The agreement with the experimental results of Fisher and Dever is good. The AG parameter has also been used to calculate the second Grüneisen constant q for rhenium. The variation of the lattice parameters of rhenium with hydrostatic pressure upto 500 kbars has been calculated using the theoretical TOE constants and Thurston's extrapolation formula. There is very good agreement with the experimental results of Liu et al.     

Study on dipolar orientation and relaxation characteristics of guest-host polymers affected by corona poling parameters

The dipolar orientation and relaxation characteristics of guest-host polymers poled by corona poling have been studied in detail. The mechanisms of dipolar orientation affected by poling parameters (voltage, temperature, time and cooling velocity) in polymers have been analyzed by UV-Vis absorption spectra and microscope. The results show that with increasing poling voltage, the orientation order parameter increases to maximum and then drops. Also the same trend of orientation order parameter has been obtained with increasing poling temperature. While the orientation order parameter increases to saturation with increasing poling time. With the biexponential model analyzing, it is shown that the relaxation of dipolar orientation can be slowed by slowing the cooling velocity during the poling process.     

Small activation entropy bestows high-stability of nanoconfined D-mannitol

It has been a long-standing puzzling problem that some glasses exhibit higher glass transition temperatures(denoting high stability) but lower activation energy for relaxations(denoting low stability). In this paper, the relaxation kinetics of the nanoconfined D-mannitol(DM) glass was studied systematically using a high-precision and high-rate nanocalorimeter.The nanoconfined DM exhibits enhanced thermal stability compared to the free DM. For example, the critical cooling rate for glass formation decreases from 200 K/s to below 1 K/s; the Tg increases by about 20 K–50 K. The relaxation kinetics is analyzed based on the absolute reaction rate theory. It is found that, even though the activation energy E~*decreases,the activation entropy S~*decreases much more for the nanoconfined glass that yields a large activation free energy G~*and higher thermal stability. These results suggest that the activation entropy may provide new insights in understanding the abnormal kinetics of nanoconfined glassy systems.