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21.
采用液相多肽合成法制备得到窄分子量分布、结构可控的生物相容性聚乙二醇嵌段共聚树枝状聚赖氨酸阳离子功能大分子(PEG-b-Dendritic PLL). 运用1H NMR核磁共振、凝胶电泳以及荧光淬灭滴定手段对所得阳离子两嵌段大分子的化学结构及其与质粒DNA (pDNA)结合作用与复合行为进行了研究. 结果表明聚乙二醇嵌段树枝状聚赖氨酸与pDNA分子可以在缓冲溶液中形成稳定的胶束, pDNA与阳离子树枝赖氨酸嵌段通过静电相互作用形成胶束核, 其水溶性聚乙二醇嵌段形成水溶性胶束壳, 提高了阳离子大分子/pDNA复合胶束的稳定性. 同时发现随着阳离子嵌段树枝状赖氨酸代数的增加, 阳离子两嵌段大分子与pDNA的结合作用增强, 有利于其作为基因转染生物功能载体的应用. 相似文献
22.
Using 3D Langevin dynamics simulations,we investigate the effects of the shape of crowders on the dynamics of a polymer chain closure.The chain closure in spherical crowders is dominated by the increased medium viscosity so that it gets slower with the increasing volume fraction of crowders.By contrast,the dynamics of chain closure becomes very complicated with increasing volume fraction of crowders in spherocylindrical crowders.Notably,the mean closure time is found to have a dramatic decrease at a range of volume fraction of crowders 0.36-0.44.We then elucidate that an isotropic to nematic transition of spherocylindrical crowders at this range of volume fraction of crowders is responsible for the unexpected dramatic decrease in the mean closure time. 相似文献
23.
Alayna M. Johnson Jeremiah A. Johnson 《Angewandte Chemie (International ed. in English)》2023,62(51):e202315085
Polyethylene (PE) is the most widely produced synthetic polymer. By installing chemically cleavable bonds into the backbone of PE, it is possible to produce chemically deconstructable PE derivatives; to date, however, such designs have primarily relied on carbonyl- and olefin-related functional groups. Bifunctional silyl ethers (BSEs; SiR2(OR′2)) could expand the functional scope of PE mimics as they possess strong Si−O bonds and facile chemical tunability. Here, we report BSE-containing high-density polyethylene (HDPE)-like materials synthesized through a one-pot catalytic ring-opening metathesis polymerization (ROMP) and hydrogenation sequence. The crystallinity of these materials can be adjusted by varying the BSE concentration or the steric bulk of the Si-substituents, providing handles to control thermomechanical properties. Two methods for chemical recycling of HDPE mimics are introduced, including a circular approach that leverages acid-catalyzed Si−O bond exchange with 1-propanol. Additionally, despite the fact that the starting HDPE mimics were synthesized by chain-growth polymerization (ROMP), we show that it is possible to recover the molar mass and dispersity of recycled HDPE products using step-growth Si−O bond formation or exchange, generating high molecular weight recycled HDPE products with mechanical properties similar to commercial HDPE. 相似文献
24.
The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9-T superconducting magnet are described. The probehead contains a compact two-loop, one gap resonator, and is inside the variable temperature insert of the magnet enabling measurements in the 0-9T magnetic field and 1.5-400 K temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the g approximately 2 paramagnetic condition such as in antiferromagnets. The low quality factor of the resonator allows time resolved experiments such as, e.g., longitudinally detected ESR. We demonstrate the performance of the spectrometer on the NaNiO2 antiferromagnet, the MgB2 superconductor, and the RbC60 conducting alkaline fulleride polymer. 相似文献
25.
Stephen D. Theriault 《Transactions of the American Mathematical Society》2001,353(4):1551-1566
Several properties of Anick's spaces are established which give a retraction of Anick's off if and . The proof is alternate to and more immediate than the two proofs of Neisendorfer's.
26.
应用美国ADI公司生产的数字分频器与鉴相器ADF4113和压控振荡器HE714构成锁相环。通过单片机AT89C51产生控制信号,设计了一个稳定的2 GHz本振源电路模块,应用于高频宽带线性调频源系统中。详细介绍了系统中核心芯片的性能、结构以及应用方法,设计出了完整的硬件电路并对电路的各个参数进行了评估。最后对电路产生的2 GHz本振信号进行仿真测试,结果基本上符合要求。 相似文献
27.
28.
通过调制VCO实现FSK调制的锁相频率合成系统,由于环路误差传递函数的高通特性会使低频调制频偏(或相偏)降低甚至消失,针对这个缺点,提出了双调制的方法,即同时对基准源和环路的VCO进行调制。在一定条件下,双调制后的频偏不受环路的影响且又能保证调制载频的高稳定度,通过计算给出了满足这种条件的关系式和实际电路的表现特征及实现方法。实验证明该方法效果显著,可实现低频至直流任意信号的调制。 相似文献
29.
Zhang Yongjun 《电子科学学刊(英文版)》1996,13(1):11-16
This paper provides a new architecture of neural network, called loop architecture neural network(LANN), and its learning rules. One of its features distinguished from other network, such as Hopfield and bidirectional associative memories, is that it can perform the associative memory among multiple categories. Analysis and simulated results have proved that it is an effective network with excellent convergence. 相似文献
30.
遥测中载波同步的实现 总被引:5,自引:1,他引:5
同步是通信中的一个重要问题 ,载波同步的方法有很多 ,其中同相正交 (Costas)环尤为常用。文中阐述了同相正交环在遥测中的具体实现 ,并对照SystemView的仿真结果对实验结果进行了必要的分析 ,同时对环路滤波的数学模型进行了重点说明 相似文献