分子动态模拟研究聚丙烯单链的玻璃化转变

用分子动态模拟法对单链高分子固体微粒的玻璃化转变与主链骨架键的构象态跃迁进行了模拟研究，发展了一种表征单链高分子固体微粒玻璃化转变过程的新方法．     

结合生活实际的《高分子物理》教学——以玻璃化转变为例

针对高分子物理内容较为抽象的特点,结合作者的教学实践,提出将高分子物理理论与生活中的高分子现象结合,以提高教学效果。以玻璃化转变的讲解为例,通过生活中的玻璃化现象引入玻璃化转变的概念,实现物理现象与理论知识的结合;进而在对玻璃化转变机理的学习中,引入自由体积理论在生活中的真实应用,消除学生对理论知识的陌生感。在这样的教学过程中,既可以克服学生对高分子物理的抽象感又可以培养学生的观察能力和思考能力,达到学以致用的目的。     

高分子玻璃化转变过程的分子动力学模拟本科教学实验

玻璃化转变是高分子专业教学中的重要内容,转变过程中自由体积的变化和分子链段的运动是理解玻璃化转变的难点。在计算机上使用分子模拟的方法得到三维的可视化图形,可以更直观地观察到自由体积和分子链的变化。提炼近年分子模拟技术在玻璃化转变研究中的最新科研成果,得到既有理论基础又适合本科教学的课程素材。分子模拟可以实现了仪器测试无法达到的超快速升降温,并与仪器测试得到的实验结果相对照,验证了普通实验很难验证的理论观点,从而拓展了本科实验的范围,达到了很好的教学效果。     

非晶物理框架下开展聚合物玻璃化转变教学的思考与探索

玻璃化转变是高分子物理教学中的重要内容.一些高分子物理教科书将聚合物玻璃化转变简单描述为玻璃到高弹态的力学转变,可能局限了学生对玻璃化转变重要科学问题认识的深度、广度和想象空间,甚至造成一些学生认为只有聚合物才存在玻璃化转变.笔者注意到了这一问题,考虑将聚合物玻璃化转变放在非晶物理框架下进行讲授;强调玻璃化转变是非晶物...     

玻璃化转变的热力学理论错在哪里？

现行国内高分子物理学教科书上都介绍玻璃化转变的热力学理论,其把玻璃化转变描述成为一个二级相转变.但是现在人们已经普遍接受玻璃化转变的本质是一个动力学过程的观点.我们通过讨论玻璃化转变热力学理论的来历,试图弄清这一理论到底错在哪里.首先,该理论所基于的Kauzmann佯谬可能不是一个真正的佯谬;其次,半柔顺链高分子溶液的经典格子统计理论结果中所谓的熵灾难可能是出于对构型熵的错误理解所致.因此,把以上二者联系起来构成玻璃化转变热力学理论的基本假定就从根本上来说是不可靠的.     

“高分子物理学”中的基础理论介绍

"高分子物理学"是高分子科学中重要的组成部分,为高分子的合成工艺提供理论基础,其中包含的基础理论部分内容抽象。本文从高分子长链结构出发,依次介绍橡胶弹性理论、高分子溶液理论、玻璃化转变理论、聚合物黏弹性理论和聚合物材料的断裂理论五方面的主要观点;同时分析理解高分子溶液、玻璃-橡胶转变、黏弹特性及聚合物材料断裂的相关内容,目的是使学生掌握"高分子物理学"中关键基础理论的指导思想内涵,系统化学习这门课程。     

不同构型聚丙烯的玻璃化转变温度的分子模拟

应用分子力学和分子动力学的方法对3种不同构型聚丙烯高分子的玻璃化转变温度进行了模拟.用NPT(等温等压)分子动力学模拟获得聚丙烯(PP)在不同温度下的特征体积,通过对模拟得到的V-T做图,求得玻璃化转变温度,其结果与实验值吻合较好.并分析了聚丙烯主链柔顺性和立构规整度对高分子玻璃化转变的影响.     

由聚合物乳液的发展看高分子物理与化学知识的综合运用

以乳胶漆用聚合物乳液的发展为对象,采用专题讲座的形式,分析所涉及的高分子物理与化学的理论和概念,尤其是高分子物理中的结晶性及光学性能、玻璃化转变及高分子链的柔顺性、高分子的极性、聚电解质的黏度等方面的知识点。     

聚甲基丙烯酸胆甾醇酯共聚物的合成、结构与性能研究 Ⅳ.PMACE的相态转变及光学性质

Finkelmann和等人对侧链胆甾型高分子液晶的研究表明,将具有液晶功能的低分子基团,经过一个软段连接到柔性高分子主链上的梳型高分子在一定的温度下可以形成液晶态,调节侧链高分子液晶的分子结构、软段长度,可以改变其相态转变温度及微区形态。前已报导具有不同侧链结构的聚甲基丙烯酸胆甾醇酯共聚物的合成、相态转变及光学性质,本文通过对聚甲基丙烯酸胆甾醇乙烯酯共聚物(PMACE)的液晶态及结晶态的微细结构及相态转变与胆甾侧链含量关系的研究,给出了液晶态的形成条件及结构特征。     

聚N-异丙基丙烯酰胺在十二烷基硫酸钠水溶液中的线团球体转变

高分子链在溶液中,当溶剂由良变劣时,将发生线团-球体转变,而同时由于链间聚集造成的相分离,使得很难真正观察到热力学意义上稳定的高分子单链紧缩球体。表面活性剂的加入有助于阻止高分子链间发生聚集,在不同的外部条件下,表面活性剂同高分子链将形成形态迥异且又复杂有趣的复合结构。本文主要讨论了聚N-异丙基丙烯酰胺在十二烷基硫酸钠水溶液体系中的线团-球体转变行为,以及聚N-异丙基丙烯酰胺与十二烷基硫酸钠分子间的相互作用及结构模型。     

Molecular dynamics simulation of the glass transition temperature of fullerene filled cis-1,4-polybutadiene nanocomposites

t In this work,the effect of the fullerene (C60) weight fraction and PB-C60 interaction on the glass transition temperature (Tg) of polymer chains has been systemically investigated by adopting the united atom model of cis-1,4-poly(butadiene) (cis-PB).Various chain dynamics properties,such as atom translational mobility,bond/segment reorientation dynamics,torsional dynamics,conformational transition rate and dynamic heterogeneity of the cis-PB chains,are analyzed in detail.It is found that Tg could be affected by the C60 weight fraction due to its inhibition effect on the mobility of the cis-PB chains.However,Tg is different,which depends on different dynamics scales.Among the chain dynamics properties,Tg is the lowest from atom translational mobility,while it is the highest from the dynamic heterogeneity.In addition,Tg can be more clearly distinguished from the dynamic heterogeneity;however,the conformational transition rate seems to be not very sensitive to the C60 weight fraction compared with others.For pure cis-PB chains,Tg and the activation energy in this work can be compared with those of other polymers.In addition,the temperature dependence of the dynamic properties has different Arrhenius behaviors above and below Tg.The activation energy below Tg is lower than that above Tg.This work can help to understand the effect of the C60 on the dynamic properties and glass transition temperature of the cis-PB chains from different scales.     

Thermal analysis of PMMA/gel silica glass composites

Thermal analysis of poly-methylmethacrylate (PMMA) impregnated porous gel silica glasses confirms that the PMMA chains form hydrogen bonds with the pore surface silanol groups. The adopted conditions for the insitu polymerisation result in about 4% of residual monomers trapped in the polymer, most of them in the amorphous structure. The polymer and monomer mixture takes up the whole of the free pore volume. Most of the residual monomer polymerises during the DSC scans above the glass transition temperature providing an excellent probe for the weak glass transition. Polymerisation in the gel silica glass medium affects the glass transition temperature, the length of polymer chains, and the degree of polymerisation.     

Nanoparticle Dispersion and Glass Transition Behavior of Polyimide-grafted Silica Nanocomposites

How to control the spatial distribution of nanoparticles to meet different performance requirements is a constant challenge in the field of polymer nanocomposites.Current studies have been focused on the flexible polymer chain systems.In this study,the rigid polyimide(PI) chain grafted silica particles with different grafting chain lengths and grafting densities were prepared by "grafting to" method,and the influence of polymerization degree of grafted chains(N),matrix chains(P),and grafting density(a) on the spatial distribution of nanoparticles in the PI matrix was explored.The glass transition temperature(Tg) of PI composites was systematically investigated as well.The results show that silica particles are well dispersed in polyamic acid composite systems,while aggregation and small clusters appear in PI nanocomposites after thermal imidization.Besides,the particle size has no impact on the spatial distribution of nanoparticles.When σ·N0.5<<(N/P)2,the grafted and matrix chains interpenetrate,and the frictional resistance of the segment increases,resulting in restricted relaxation kinetics and Tg increase of the PI composite system.In addition,smaller particle size and longer grafted chains are beneficial to improving Tg of composites These results are all propitious to complete the microstructure control theory of nanocomposites and make a theoretical foundation for the high performance and multi-function of PI nanocomposites.     

Influence of substrate temperature on the stability of glasses prepared by vapor deposition

Physical vapor deposition of indomethacin (IMC) was used to prepare glasses with unusual thermodynamic and kinetic stability. By varying the substrate temperature during the deposition from 190 K to the glass transition temperature (Tg=315 K), it was determined that depositions near 0.85Tg (265 K) resulted in the most stable IMC glasses regardless of substrate. Differential scanning calorimetry of samples deposited at 265 K indicated that the enthalpy was 8 J/g less than the ordinary glass prepared by cooling the liquid, corresponding to a 20 K reduction in the fictive temperature. Deposition at 265 K also resulted in the greatest kinetic stability, as indicated by the highest onset temperature. The most stable vapor-deposited IMC glasses had thermodynamic stabilities equivalent to ordinary glasses aged at 295 K for 7 months. We attribute the creation of stable IMC glasses via vapor deposition to enhanced surface mobility. At substrate temperatures near 0.6Tg, this mobility is diminished or absent, resulting in low stability, vapor-deposited glasses.     

Thermoreversible gelation strongly coupled to coil-to-helix transition of polymers

This paper theoretically studies thermoreversible gelation driven by aggregation of helices formed on the polymer chains. Two fundamentally different cases of (i) multiple association of single helices and (ii) association by multiple helices with multiplicity k (such as double helices (k=2), triple helices (k=3), etc.) are treated on the basis of different equations. The helix length distribution on a polymer chain (or assemble of chains for multiple helices) is derived as a function of polymer concentration and temperature. Theoretical calculation of the total helix content in the solution is compared with experimental data of optical rotation in iota-carrageenan solutions at different polymer concentrations. It is shown that at low temperature there is a sharp transition from network to bundle state (pair, triplet, etc.). To confirm such a network/pairing transition, we carried out Monte Carlo simulation of polymer solution in which hydrogen-bonded zipper-like cross-links are formed.     

Glass transition broadening via nanofiller-contiguous polymer network in aromatic thermosetting copolyester nanocomposites

The glass transition is a genuine imprint of temperature-dependent structural relaxation dynamics of backbone chains in amorphous polymers, which can also reflect features of chemical transformations induced in macromolecular architectures. Optimization of thermophysical properties of polymer nanocomposites beyond the state of the art is contingent on strong interfacial bonding between nanofiller particles and host polymer matrix chains that accordingly modifies glass transition characteristics. Contemporary polymer nanocomposite configurations have demonstrated only marginal glass transition temperature shifts utilizing conventional polymer matrix and functionalized nanofiller combinations. We present nanofiller-contiguous polymer network with aromatic thermosetting copolyester nanocomposites in which carbon nanofillers covalently conjugate with cure advancing crosslinked backbone chains through functional end-groups of constituent precursor oligomers upon an in situ polymerization reaction. Via thoroughly transformed backbone chain configuration, the polymer nanocomposites demonstrate unprecedented glass transition peak broadening by about 100 °C along with significant temperature upshift of around 80 °C. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1595–1603     

Relations between Glass Transition Temperatures in Miscible Polymer Blends and Composition: From Volume to Mass Fractions

The use of volume fractions in the empirical mixing laws to predict the glass transition temperatures (Tg) of polymer blends provides good agreement with experimental values, even for polymer systems with different densities. No adjustment parameter is therefore required whereas Gordon-Taylor and Kwei equations based on weight fractions need the use of a fitting parameter which has to be determined from experimental data. This assumption was validated from Tg measurements through DSC experiments conducted on PMMA /PVDF blends which have significantly different densities.     

POLYMER CHAIN DIFFUSION AT A TEMPERATURE BELOW ITS BULK GLASS TRANSITION TEMPERATURE

In this study, it was examined whether the dynamics of polymer chains at a surface is different from that in thebulk, and if so, to what extent they differ in terms of surface glass transition temperature and diffusion coefficient. Obtainedresults clearly indicate that surface chains can travel for a relatively large distance in comparison with the characteristiclength scale of usual segmental motion even at a temperature below its bulk glass transition temperature, T_g~b. This isconsistent with our previous results that the surface glass transition temperature is much lower than the corresponding T_g~b.Also, it was experimentally revealed that there was a gradient of molecular motion in the surface region.     

HTPB/增塑剂玻璃化转变温度及力学性能的分子动力学模拟

为了预测高分子粘结剂端羟基聚丁二烯(HTPB)与增塑剂癸二酸二辛酯(DOS)、硝化甘油(NG)的相容性及HTPB/增塑剂共混物的玻璃化转变温度(Tg)和力学性能,在COMPASS力场条件下采用分子动力学(MD)模拟方法对相容体系(HTPB-DOS)和不相容体系(HTPB-NG)进行了研究.结果表明,通过比较溶度参数差值(Δδ)的大小可以预测HTPB与增塑剂的相容性,即HTPB与DOS属于相容体系,而HTPB与NG不相容.通过温度-比容曲线可以得到HTPB、HTPB/DOS与HTPB/NG的Tg分别为197.54,176.30和200.03K.力学性能分析结果表明,添加DOS增塑剂后使HTPB的弹性模量(E),体积模量(K)和剪切模量(G)下降,材料刚性减弱,柔性增强,力学性能得到改善.本模拟方法可以作为预测聚合物/增塑剂共混物性能的有利工具,也可以为固体推进剂和高聚物粘结炸药的配方设计提供理论指导.     

Synthesis and Characterization of New Polymer Systems Containing 4‐Silylmethylstyrene Units

A series of terpolymers containing silyl pendant groups were prepared by free radical cross‐linking copolymerization. Et₃Si and HMe₂Si were covalently linked with 4‐vinylbenzyl and abbreviated as TESiMSt and DMSiMSt, respectively. Et₃Si was covalently linked with 2‐hydroxyethyl methacrylate (HEMA). The silyl‐linked HEMA are abbreviated as TESiEMA. Free radical terpolymerization of the methacrylic acid (MAA) with different molar ratios of organosilyl monomers was carried out at 60–70 °C. The compositions of the polymers were determined by FT‐IR spectroscopy and ¹H‐NMR. The glass transition temperature (Tg) of the polymers was determined calorimetrically. The study of DSC curves showed that incorporation of monomers with cyclic units in polymer chains increases the rigidity of terpolymers and the Tg value is subsequently increased.