等规聚苯乙烯单链、寡链和多链聚集体的结晶、结构和形态

【作者】 辛燕；

【导师】 卜海山； 范仲勇；

【作者基本信息】 复旦大学 ， 材料化学与物理， 2005， 博士

【摘要】 本文以等规聚苯乙烯（i-Ps）为研究对象，在i-PS苯熔液的浓度范围(1.0×10^-3g·mL^-1～2.0×10^-5g·mL^-1)内，采用溶液雾化-冷冻-升华（SFD）法分别制备了非晶态的i-Ps单链、寡链和多链粒子的聚集体。观察了i-Ps单链、寡链和多链非晶态粒子的形态，计算了粒子的尺寸。研究了不同浓度制备的i-Ps样品的冷结晶行为和晶体形态，测试了粒子聚集体熔体结晶的线生长速度、并进行了晶体结构的研究。通过与i-Ps本体样品结晶行为的对比，揭示了链缠结对聚合物结晶、结构及其形态的影响。基于聚合物结晶理论，计算和分析了i-Ps单链、寡链或多链晶体的热力学参数和晶体尺寸等，旨在从分子水平上对聚合物结晶过程和机制有更深刻的新认识。用透射电镜（TEM）观察了分别由1.0×10^-3g·mL^-1、1.0×10^-4g·mL^-1和2.0×10^-5g·mL^-1浓度的溶液经雾化-冷冻-升华法制备的i-PS单链、寡链和多链非晶态聚集体的形态。结果表明，由溶液浓度为2.0×10^-5g·mL^-1所制备的i-Ps单链、寡链粒子呈非晶态的球体。其中一些小球体的直径两倍于θ状态下计算所得的均方回转半径，表明采用喷雾-冷冻-升华法可以得到单分子线团分散的样品。i-Ps单分子线团聚集体的电子选区衍射结果表明单分子小球中存在一定程度的近程有序结构。红外光谱（FTIR）中的吸收带500-600cm^-1经分峰处理后的结果也证实了雾化-冷冻-升华样品中存在局部近程有序结构。FTIR研究还表明用此技术制备的样品中i-Ps链段的堆砌较本体样品更松散。采用差热扫描量热法（DSC）研究了i-Ps单寡链、寡链和多链聚集体的冷结晶行为，结果表明，由上述三个溶液制得的试样的冷结晶峰分别比本体样品降低46.3℃、24.9℃和14.7℃。冷结晶峰温的降低，表明单链、寡链聚集体的结晶速度加快。这是因为随着溶液浓度的降低，雾化-冷冻-升华制备的样品中单链或寡链颗粒成了主要组成部分，这些颗粒间很少甚至没有链间缠结。另一方面，单链、寡链或多链生成的晶体的熔点也随制样溶液的浓度降低而逐渐降低，这是因为单链、寡链生成的晶粒体积逐渐减小的结果。对单链、寡链聚集体样品的多次DSC扫描实验表明，随着扫描次数的增加，冷冻升华样品中分子链的缠结程度增加，冷结晶峰和熔点向高温方向移动。i-Ps单链、寡链聚集体在243℃等温40min后，DSC扫描的结果表明其冷结晶行为已与本体相同。此时，单链、寡链分子线团因热扩散而完全贯穿为相互缠结的网络。基于实验结果，提出了分子链在结晶驱动力下从过冷熔体向结晶生长面扩散的过程需要克服缠结的阻滞作用机制。在165℃～200℃温度范围，采用热台偏光显微镜（POM）测量了i-Ps本体和单链、寡链和多链聚集体的熔体结晶球晶的线生长速率。基于Hoffmann的表面成核、生长的结晶理论，讨论了线生长速率方程中成核项和扩散项的不同简化条件。根据实验结果，计算得i-Ps本体样品结晶的成核因子K_g和扩散活化能B^*，与文献报道数值一致。同理，对雾化-冷冻-升华样品进行了计算、分析，结果表明晶体生长动力学方程中的指前因子G₀和成核因子K_g均随聚合物链间缠结的减少而减小；反之，扩散活化能B^*则增大。利用Ewards-Philips法讨论了K_g的物理含义。在晶体生长过程中，由于缠结的减少，单链、寡链近邻折叠的比例增加，使得片晶的表面粗糙度降低，因而折叠面上分子链的拥挤程度下降，生成片晶的端表面自由能降低。对实验数据的计算得到了一组新的i-Ps结晶热力学参数，包括G₀，K_g，B^*，σ_e和σ等。该结果表明链间缠结对聚合物结晶和结构有很大的影响。采用由线生长速率数据得到的新σ_e值，将i-Ps单链、寡链聚集体结晶的实验数据进行了重新计算，计算结果与WAXD测试得到的（220）晶面的测定尺寸相吻合。这些研究成果均未见文献报道。采用偏光显微镜（POM）观察了i-Ps本体和冷冻升华样品熔体结晶的球晶形态，生成的球晶均为正球晶。利用扫描电镜（SEM）观察了单链、寡链聚集体的冷结晶形态，首次观察到i-Ps单链和寡链聚集体结晶时生成大量的leaflike形状的晶体，讨论了leaflike晶形的生长机制。i-Ps单链、寡链和多链熔体结晶生成的球晶，经高锰酸钾蚀刻后，SEM观察发现其球晶均为捆束状“sheaflike”形态。实验观察表明，寡链和多链熔体结晶时，随着链缠结的减少，球晶的填空生长机理由“branching”转变为“spawn”模式。而单链聚集体熔体结晶的球晶是由非扭转的、无分枝片晶组成。这些实验事实是由我们首次观察到的，证明了链间缠结对聚合物球晶生长机制及形态能产生很大影响。同时在单链聚集体的熔体结晶时也观察到密集海草型（Compact Seaweed CS）晶体形态，说明了晶体生长受扩散控制。i-Ps熔体结晶时CS晶体与球晶共存也是首次观察到实验现象。此外，i-Ps多链聚集体熔体结晶时，观察到多种六边形片晶聚集体，包括：规整、圆弧化或严重退化的六边形形态。该实验现象与Bassett报道的研究结果一致。还观察到有些六边形的片晶聚集体的中心部分为明显的螺旋生长模式，随着螺旋的发展，片晶排列方式由“flat-on”转变为“edge-on”，当用POM观察时，螺旋中心呈负光性。更多还原

【Abstract】 The isotactic polystyrene （i-Ps） was chosen as a sample, and amorphous aggregates of single-, pauci- and multi-chain particles were prepared by spraying-freeze-drying （SFD） procedure from dilute solution of i-Ps in benzene with concentration ranging from 1.0×10^-3g·mL^-1 to 2.0×10^-5g·mL^-1. The morphology of i-Ps single-, pauci- and multi-chain particles was observed, and the particle dimension was calculated. The investigation of cold crystallization and crystal morphology was carried out for the samples prepared from solutions of various concentrations, and also linear growth rate of spherulites crystallized from the melt of aggregate was measured and the crystal structure was characterized. The influence of entanglements on the crystallization process, structure and morphology was revealed by a parallel study of the properties of these single-, pauci- and multi-chain particles with those of bulk sample,. According to the crystallization theory of macromolecules, the thermodynamics parameters and structure dimension were calculated and analyzed in order to get a better understanding of crystallization process of polymers on a molecular level.By means of TEM, we observed the morphologies of amorphous single-, pauci- and multi-chain aggregates prepared by spraying-freeze-drying from solutions with various concentrations. It was found that the i-Ps aggregates prepared from the concentration of 2.0×10^-5g·mL^-1 is composed of isolated globules and most of themhas a diameter about twice times of the <R|₀²>^1/2 of i-Ps random coil under θ state.This leads to an convincible conclusion that isolated single-chain coil can be prepared by spraying-freeze-drying method from extremely dilute solution. Electron diffraction pattern of i-Ps single-chain globules showed that a certain short-range order is maintained during the freeze-drying process, and it is further confirmed by fourier transform infrared spectrograph （FTIR） spectrum of single-chain aggregates after separation of bands in the region 500-600cm^-1. The FTIR spectrum also showed that the packing of segments in the aggregates prepared by SFD method is in a more dilated state than that of bulk sample.The cold crystallization behaviors of the single-, pauci- and multi-chain aggregates were investigated by DSC. It was found that as the solution concentration decrease, the cold crystallization temperature of the samples prepared from solutions was greatly decreased. For the samples prepared from three solutions mentioned above, the cold crystallization temperature is about 46.3°C, 24.9°C or 14.7°C lower than that of bulk sample, respectively. This depression in cold crystallization temperatureindicates an acceleration of crystallization rate, which could be ascribed to fewer and even no interchain entanglements in i-Ps aggregates prepared from more dilute solution. On the other hand, the melting point of i-Ps crystals formed in aggregates decreases with decreasing solution concentration, most likely due to the more smaller crystals were obtained in the case of solution with more dilute solution. The sample was heated in DSC cell to the end point of melting peak, followed by quenching to room temperature, and this procedure was repeated several times. It was observed that with the increase of DSC scan times, the cold-crystallization peak and melting peak shift to the higher temperature. Clearly, this is because the increase of entanglement density. While the aggregates were heated at 243°C for 40min, the single- and pauci-chain coils interpenetrated each other by thermal diffusion to form an entangled network with the same entanglement density as the normal bulk, therefore, the recovery of crystallization behavior characterized by bulk i-PS was observed in DSC scan. On the basis of the experimental observation, it could be put forward that during the crystallization the macromolecules should overcome the retardation of entanglement to approach the growth surface by self-diffusion.The growth rate of i-Ps bulk and aggregates prepared from various solution concentrations were measured under polarized optical microscope （POM） equipped with a hot stage in the range of 165°C-200°C. Different simplifications and treatments of nucleation term and transport term were discussed based on the LH theory of Surface-Nucleation and Growth. It is found that the nucleation factor K_g and activation energy of transport B^* calculated according to the growth rate are consistent with the classic literature. For i-Ps aggregates prepared by SFD procedure, the prefactor Go and nucleation factor K_g were also obtained, and they are found to decrease with decreasing entanglements, but the transport factor B^* is showed to increase with decreasing entanglements. The physical meaning of K_g were carefully discussed based on the Edwards-Philips method, and it is found that with decreasing entanglements, tight folding is more preferential during the substrate completion in the sample prepared by SFD method, and the end surface become less rough. Thus, it is reasonable to assume that the depression in the congestion of segments at melt-crystal interfacial would lead to a smaller value of end surface free energy. A new series of values were attained for thermodynamics parameters G₀, K_g, B^* , σ_e and σ according to the measurement of growth rate of crystals formed in aggregates melt. A more satisfied coincidence was achieved for the crystal dimension along chain direction from both approaches: one obtained from （220） reflection in WAXD pattern and another calculated by using melting point and new thermodynamics parameter σ_e.To our knowledge, all these results mentioned here are firstly reported until now.The morphologies of spherulites crystallized from the melt of bulk sample and aggregates prepared from various concentrations were observed by POM, and all spherulites are optically positive. The morphologies of crystals grown from the amorphous state of single- and pauci-chain aggregates at 195°C were investigated by scanning electron microscopy （SEM）, abundant leaflike crystals were observed, which have not seen before. Such kind of crystals can be ascribed to the coexistence of low index growth face and high index ones, and they may be formed owing to the lack of available stems. Spherulites grown from the melt of the particle aggregates was observed by SEM after treating with Permanganic etchant, and they are found to be of sheaflike. Observation on the fine texture of spherulites shows that the space-filling mechanism of spherulite growth changes from "branching" to "spawn". While the spherulites grown in single-chain sample is composed of untwisted and unbranched lamellae. These observations are reported for the first time and they indicate that entanglements play a very important role in morphology and growth mechanism of crystals. Meanwhile, the Compact Seaweed （CS） crystal was observed to coexist with sheaflike spherulites in the melt-crystallized single-chain samples, indicating a strong diffusion-controlled characteristic of crystal growth. Various morphologies of hexagonal lamellae aggregates were observed, such as regular, rounded, and polyhedral with a sheaflike interior fibrosity; their formation is dependent on the crystallization condition. Such kind of crystal structure and morphology is consistent with Bassett’s observation. What’s more, the screw growth of some hexagonal lamellae aggregates is observed at the center of hexagonal lamellae aggregates, and with development of the screw, the lamellae change its arrangement from "flat-on" to "edge-on". The central parts exhibit a negative birefringence while they were observed by using POM.更多还原