高分子结晶的新进展、新模型

回顾了高分子结晶经典模型－成核与生长模型，讨论了近年来高分子结晶研究的新结果，新进展，特别介绍了德国著名高分子物理学家Strobl提出的高分子结晶是从熔体先形成中间相，中间相转变为小晶块，最后小晶块融合为均匀为片晶的全新模型。     

序列嵌段共聚酯液晶高分子相转变动力学及机理研究

用DSC、PLM、WAXD等方法研究了序列嵌段共聚酯液晶高分子的结晶动力学及相转变机理.结果表明,从各向同性相温度(T_i)结晶经历了中介相阶段,从中介相温度(T_N)和从T_i温度结晶的结晶机理均为取向链段的聚集成核及低维生长,Avrami指数都接近2.两种结晶途径均形成了不同完善程度的结晶形态.柔性嵌段PTHF含量对试样的液晶行为和结晶行为有很大影响,PTHF含量在20％左右向列型中介相表现出高取向,易成核的特性.     

高分子结晶的链内成核模型

高分子的微观晶体生长动力学理论是高分子科学的重要研究课题之一。本文系统地介绍了近年来发展起来的高分子晶体生长动力学模型—链内成核模型。该模型能够对高分子结晶过程中的链折叠现象、分子分凝现象、二元链长共结晶现象、Regime转变现象和半晶织构现象给出较为合理的解释。这些现象为高分子结晶所特有,揭示了高分子有序化相转变行为的特殊性。在结语部分,本文还对比描述了以链间成核模式为主的一些情景,并对链内成核模型的进一步发展做了展望。     

超快扫描量热技术表征高分子结晶动力学

高分子结晶行为是高分子材料加工过程研究的热点,因为高分子组分和加工工艺控制着高分子结晶及其产物性能。差示扫描量热仪(DSC)是研究高分子结晶动力学常规手段。但是,普通DSC所能达到的最快降温速率一般无法抑制较快的样品结晶,结晶行为将在等温结晶动力学测试之前发生,因此可进行等温结晶的研究温度范围局限于较低结晶过冷度的高温区域。近年来,具有超快速升降温扫描速率和精准控温的快速扫描芯片量热仪(FSC,其商业化版本Flash DSC 1)得到了广泛应用。FSC可以抑制高分子样品在升降温过程中的结晶成核,避免对之后的结晶动力学测试产生影响。因此FSC技术将高分子结晶动力学的研究温度区间延伸至具有较大过冷度的低温区,加深了我们对高分子结晶成核机理以及高分子工业加工过程的理解。本文首先介绍了由初级成核方程描述的高分子结晶动力学原理,初级成核自由能位垒(?G~*)和扩散活化能位垒(?U)分别控制了高低温区的结晶动力学。我们还总结了FSC技术的发展,包括氮化硅薄膜芯片技术、快速扫描量热仪、商业化Flash DSC 1在不同高分子结晶熔融行为研究中的应用。然后介绍表征高分子等温结晶动力学的方法,其中包括样品制备、质量估算、消除热历史、临界扫描速率的确定等,并举例介绍FSC在高分子结晶动力学研究中的应用,涵盖高分子总结晶动力学、结晶成核动力学、高分子焓松弛对结晶成核的影响、FSC联用技术等方面。应用举例中对应形貌和结晶信息,分析了通过FSC测试得到的结晶成核动力学特点。另外通过比较不同结构特点的高分子,总结了我们对结晶动力学行为的基本理解。总之,FSC技术是一种能够提供相转变动力学和热力学信息的高效工具,特别是应用于分析只能在快速扫描中得到的样品结构变化信息。同时我们希望本文能够帮助读者考虑超快扫描量热技术在其他材料研究上的应用,包括合金、药物、生物大分子等。     

高分子在氧化铝纳米孔道中受限结晶的研究进展

高分子材料在微纳米尺度常常表现出不同于本体的物理性质.对结晶性高分子来说,在纳米受限空间的成核机理、结晶结构和动力学特征都与本体材料有所不同.本文总结了近年来基于多孔氧化铝纳米模板(AAO)开展的高分子受限结晶的研究进展,重点介绍了本课题组的工作.研究发现,在AAO模板中,高分子结晶的过冷度大大增加,成核机理从本体的异相成核转变为均相成核或表面成核;高分子结晶结构通常表现为各向异性,动力学因素、热力学因素和界面性质均对取向结构有重要影响;受限情况下高分子结晶速率大大降低,表现出"成核控制"的动力学特征;空间受限使高分子结晶度降低,倾向于形成亚稳态晶型.最后,对该领域尚待解决的问题进行了展望.     

尼龙1218的等温及非等温结晶动力学研究

采用示差扫描量热计DSC考察了一种新型长烷基链偶偶尼龙 尼龙 12 18 自熔体的结晶过程 ,分别利用Avrami方程和Ozawa方程对等温及非等温结晶动力学进行了描述与研究 ,计算了相关的结晶动力学参数 ,得出相应的结晶机理 .最后计算了等温结晶活化能和非等温结晶活化能 ,依此得到烷基链段长度与尼龙结晶过程有密切关系     

一个判别液晶聚合物状态的新方法

伴随液晶相转变而产生的转变焓、转变熵可提供有关各相的结构、状态及有序度等信息.本文从动力学、热力学角度分析了液晶聚合物从各向同性熔体冷却时系统的热熵变化,建立了液晶聚合物升降温过程中的热熵图,提出了定量判断液晶聚合物从各向同性熔体冷却后所处状态的新方法.     

间规1,2-聚丁二烯结晶结构研究

以间规1,2-聚丁二烯(s-PB)为研究对象,通过原位同步辐射小角X射线散射(SR-SAXS)和广角X射线衍射(WAXD)研究其结晶结构的变化过程.SR-SAXS曲线中存在明显的散射峰,表明在等温结晶过程中形成有序结晶结构;在等温结晶后间规1,2-聚丁二烯的片晶厚度、微晶尺寸均正比于1/Tc∞-T,根据高分子结晶中介相机理可以做出合理的解释.     

中介相态丙烯-乙烯无规共聚物在热处理过程中的结构演变

通过熔体淬冷方式制备了中介相态丙烯-乙烯无规共聚物(PPR),综合使用原位红外光谱、原位X-射线散射(WAXD/SAXS)、示差扫描量热分析(DSC)和动态机械热分析(DMA)等方法系统研究了中介相态PPR在升温过程中的微观结构演变.红外光谱研究结果表明,在连续升温过程中,中介相态PPR在30~50?C之间分子链构象发生了变化,其可能源于刚性无定形区(RAF)中部分链段构象的无序化转变,并发现在RAF中存在长度为n≤13(n为31螺旋序列中丙烯单元的个数)的螺旋序列.中介相态PPR在连续升温过程中经历了RAF中链段构象的无序化转变、中介相向α晶的转变、不完善α晶的熔融和α晶的完善化,以及α晶熔融4个转变过程.中介相向α晶的转变是一个异相成核生长过程,其结晶活化能ΔE=67.94 k J/mol.     

PZT/P(VDF-TrFE)7030 0-3型复合材料的非等温结晶和居里相变行为

采用DSC研究了PZT P(VDF TrFE) 70 30复合材料的非等温结晶和居里相变过程 .结果表明 ,PZT粒子对共聚物基体的结晶行为 ,包括结晶温度、结晶速率和结晶度等影响不大 ;然而共聚物的居里相变过程则受到PZT粒子的影响 ,随PZT粒子含量的增大 ,共聚物基体的居里相变热焓与熔体结晶热焓的比值减小 ,而且其中对应从顺电性的高温相 (HT)转变为铁电性的过冷相 (CL)的相变热焓含量减小 ,意味着PZT粒子的存在能够抑制共聚物铁电晶体中旁式构象缺陷的形成 ,从而提高铁电晶体的完善度     

Kinetics of mesophase formation and crystallization in poly(diethylsiloxane)

Optical and calorimetric studies of the kinetics of mesophase formation and crystallization in poly(diethylsiloxane) have been conducted. The mesomorphic phase is found to grow from the isotropic melt in the form of lamellar domains about 2 μm thick in the temperature range 293–307 K. According to birefringence data, macromolecules in the mesomorphic lamellae are perpendicular to the end faces. The kinetics of mesophase formation obey the Avrami equation with the morphological parameter n close to 2 (it is equal to 1.75 ± 0.05), which corresponds to the two-dimensional growth of the mesomorphic phase from athermal nuclei. The limiting conversion of the isotropic melt was shown to be temperature-dependent. This is likely to be connected with a change in the number of nuclei with temperature. The crystallization of polymer from the mesomorphic state occurs with retention of the optical texture of the sample. The process proceeds not as a sporadic crystallization of individual mesomorphic lamellae but as a growth of the nucleated crystalline regions via a consecutive incorporation of adjacent crystallizing lamellae.     

Crystallization and melting of polyethylene copolymers: Differential scanning calorimetry and atomic force microscopy studies

The Hoffman–Lauritzen theory of secondary, surface nucleation and growth was primarily relied upon for about 40 years after its introduction in about 1960 to rationalize the crystallization of flexible chain polymers into lamellar crystals. However, in about 1998, Strobl and coworkers introduced a different model for crystallization, based on the stage‐wise formation of lamellae. Two major components of this model were as follows: (1) the concept of the formation of a mesomorphic melt as a precursor to crystallization and (2) the control of the melting temperature range of lamellar crystals of homogeneous polyolefin copolymers by an inner degree of order or perfection rather than on the crystal thickness. The first concept is in disagreement with the HL theory and the second with the Gibbs‐Thomson theory, which associates melting temperature with lamella thickness. In the present study, differential scanning calorimetry and atomic force microscopy were successfully employed to monitor the in situ quiescent crystallization of polyethylene homopolymer and copolymer. In the present study, evidence was not found to support the concept of lamellae with equal thickness melting over a broad temperature range. Some evidence was found that might be interpreted to support the concept of a mesomorphic melt as a precursor to crystallization. At present, the model promoted by Strobl and coworkers appears to be at an uncertain stage at which strong proof or disproof are not available. However, this alternative model has injected a new vitality into the study of crystallization of flexible chain polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2369–2388, 2006     

Characterization of polyethylene crystallization from an oriented melt by molecular dynamics simulation

Molecular dynamics is used to characterize the process of crystallization for a united atom model of polyethylene. An oriented melt is produced by uniaxial deformation under constant load, followed by quenching below the melting temperature at zero load. The development of crystallinity is monitored simultaneously using molecular-based order parameters for density, energy, and orientation. For crystallization temperatures ranging from 325 to 375 K, these simulations clearly show the hallmarks of crystal nucleation and growth. We can identify multiple nucleation events, lamellar growth up to the limit imposed by periodic boundaries of the simulation cell, and lamellar thickening. We observe a competition between the rate of nucleation, which results in multiple crystallites, the rate of chain extension, which results in thicker lamellae, and the rate of chain conformational relaxation, which is manifested in lower degrees of residual order in the noncrystalline portion of the simulation. The temperature dependence of lamellar thickness is in accord with experimental data. At the higher temperatures, tilted chain lamellae are observed to form with lamellar interfaces corresponding approximately to the [201] facet, indicative of the influence of interfacial energy.     

剪切对iPP/PEcO共混物结晶行为的影响

对剪切场作用下的全同聚丙烯/弹性体乙烯-辛烯共聚物(iPP/PEcO)的共混物结晶行为进行研究, 结果表明, 剪切使得iPP球晶密度增加, 微晶和片晶均发生取向, 且片晶取向明显; 片晶取向度随共混物中PEcO含量的增加而增大, 而微晶取向度随PEcO含量的增加而减小; 强剪切诱导出现纤维状结晶形态. 利用同步辐射(SAXS)技术对共混物在剪切场下的等温结晶行为进行研究, 结果表明, 随结晶的进行长周期呈现先减少而后固定的趋势; 高剪切速率缩短了结晶诱导时间, 加快了共混物中结晶部分的结晶动力学过程.     

Kinetics of polymer crystallization from the melt (calorimetric approach)

Crystallization kinetics for 12 polymers including polyolefins, polyesters, polyurethanes, polysiloxanes was measured by the evolution of heat in a modified Calvet-type calorimeter over wide temperature ranges. The results are analyzed in terms of the Avrami equation and a comparison between calorimetric and dilatometric results is carried out. It is concluded that, although in the majority of cases experimental results do not obey the Avrami equation, for some polymers the agreement is rather good. The Avrami parameter obtained, however, depends on the experimental technique. Possible reasons for this disagreement are discussed. Analysis of the calorimetric crystallization rate in the vicinity of the melting point by using the kinetic theory of crystallization shows that the growth is controlled by surface (two-dimentional) nucleation. Energy parameters for the crystallites were determined and it is shown that the surface energy of the crystallites depends on the molecular structure of the polymer. Temperature dependence of the calorimetric crystallization rate of the polymers for which crystallization rates could be determined above and below the maximum rate are analyzed using a kinetic equation with common approximations for the transport term. The influence of melting conditions on the crystallization rate was studied. The results indicate heterogeneous nucleation in the polymer melt. It is concluded that this may be due both to impurities and to high regularity of macromolecules in the polymer melt.     

Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene

In this study, memory effect of mesomorphic isotactic polypropylene (iPP) was investigated using polarized optical microscope and small‐angle X‐ray scattering. Differing from classical memory effect, mesomorphic iPP melt had a higher growth rate and a higher memory temperature. The relative growth rate increased with increasing crystallization temperature. Lauritzen–Hoffman plots indicated that the increased growth rate arose from reduced surface nucleation barrier. The highest memory temperature was estimated to be 185 °C, which was close to the equilibrium melting point of iPP crystal. Additionally, Small‐angle X‐ray scattering measurements showed that a liquid crystal layer might exist between lamellar and amorphous layers. Based on above results, a crystallization model was proposed. In the mesomorphic iPP melt, there exist aggregates structurally similar to β phase except α‐phase crystal residuals, which cannot act as nucleation sites or transform to β crystal through surface nucleation. The only way for the aggregate is to transform to α crystal during crystal growth. The aggregate decreases the surface nucleation barrier and promotes the helical growth, leading to higher growth rate. Only when the aggregate relaxes to polymer coils through thickening at a higher temperature, can the memory effect be erased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1573–1580     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

原子力显微镜研究高分子超薄膜结晶机理及功能化调控

近十年来,随着功能高分子单晶(含单层或寡层片晶)工程及应用研究的不断深入,除了纳米尺度结晶形貌的表征以外,多功能原子力显微镜还被用于研究分子结构、结晶条件和后处理条件对功能高分子晶体性能(电、热、光、磁等)的影响,进一步还可采用扫描探针加工技术(机械刻蚀、电致刻蚀和热致刻蚀等)对其性能进行调控以构筑功能化聚集态结构和微图案.另一方面,超薄膜中单层或寡层片晶可为研究高分子结晶提供合适的模型体系,与原子力显微镜相结合,不但可以原位、实空间、高分辨地研究高分子的成核与生长过程(生长形态演变和生长动力学),还可以用于研究亚稳态折叠链片晶厚度和形态随热处理温度与时间的演化,从而加深对片晶内有序差异、片晶增厚与熔融行为和自诱导成核的认识.     

Fractionated crystallization of α‐ and β‐nucleated polypropylene droplets

A layer multiplying coextrusion process was used to produce multilayered polypropylene/polystyrene (PP/PS) films with various nucleating agents. When heated into the melt, the thin PP layers broke up into submicron PP droplets that exhibited fractionated crystallization. If the initial PP layers were 20 nm or less, the resulting droplets exhibited exclusively homogeneous nucleation. If a nucleating agent was added, the systematic departure from homogeneous nucleation provided insight into the nature of the heterogeneous nucleation. In this study, we used thermal analysis, atomic force microscopy (AFM), and wide angle X‐Ray scattering (WAXS) to examine the effect of two nucleating agents. We confirmed with WAXS and AFM that a soluble sorbitol nucleating agent for the PP α‐form operates in three concentration regimes as proposed in a previous study. Morphologically, homogeneous nucleation of the submicron droplets produced a granular texture. The correlation length from small‐angle X‐Ray scattering (SAXS) suggested that the grains contained 1–3 mesophase domains. Drawing on classical nucleation theory, the critical size nucleus of an individual mesophase domain was estimated to be about 2 nm³, which was considerably smaller than the mesophase domain. This pointed to mesophase crystallization that included the processes of nucleation and growth. Additional experiments were performed with nucleating agents for the PP β‐form. However, they were not effective in nucleating crystallization of the droplets, presumably because they were essentially insoluble in PP and the nucleating particles were too large to be accommodated in the PP droplets. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011