Molecular packing and phase transitions of side-chain liquid crystalline polymethacrylates based on p-methoxyazobenzene
Molecular packing and phase transitions of side-chain liquid crystalline polymethacrylates based on p-methoxyazobenzene
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对甲氧基偶氮苯侧链液晶聚甲基丙烯酸酯的分子堆积和相变
DOI：
10.1016/j.polymer.2008.04.054
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发表时间：
2008-06
期刊：
影响因子：
4.6
通讯作者：
中科院分区：
文献类型：
--
作者：
采用差示扫描量热法、一维和二维广角X射线衍射和傅里叶变换红外光谱研究了一系列对甲氧基偶氮苯侧链液晶聚甲基丙烯酸酯（PMnAzs，n=6，8，10，12）的相结构和相变行为。PMnAz的LC相变遵循近晶A（SmA）参与（N）参与各向同性（I）的顺序。对于PM10 Az和PM 12 Az，加热时SmA-N转变不完全。在低温SmA相中，聚合物采用完全交叉的侧链堆积，近晶层周期几乎与侧链长度相同。对于所有四种样品，SmA结构的一级衍射仅在温度接近SmA ParticipN转变时呈现，其强度远低于二级衍射的强度。在低温下的一阶衍射的情况下归因于可能的匹配的电子密度之间的中心部分的侧链子层和主链子层的SmA结构。由于只有来自相同主链子层的介晶基元可以平行堆叠在一起，偶氮苯畴的分布可能导致近晶层内的某种密度波动。在所有的样品中，PM 6Az呈现出最强的波动，并具有一些额外的阶数。我们还研究了退火对PMnAzs的H-聚集的影响。结果表明，在稍高于PM 8Az和PM10 Az的Tg的温度下进行等温退火，可以显著增强326 nm处的紫外吸收，表明H-聚集进一步发展。然而，对于PM 6Az和PM 12 Az，退火样品的紫外-可见光谱与未退火样品的紫外-可见光谱几乎相同。
The phase structures and transition behaviors of a series of side-chain liquid crystalline (LC) polymethacrylates based on p-methoxyazobenzene (PMnAzs, n=6, 8, 10, 12) were studied using differential scanning calorimetry, one- and two-dimensional (1D and 2D) wide-angle X-ray diffraction, and Fourier transform infrared spectroscopic experiments. The LC phase transition of PMnAz follows the sequence of smectic A (SmA)↔nematic (N)↔isotropic (I). For PM10Az and PM12Az, the transition of SmA-to-N is not complete upon heating. In the low-temperature SmA phase, the polymers adopt a fully interdigitated side-chain packing with the smectic layer period almost identical to the side-chain length. For all the four samples, the first-order diffraction of the SmA structure only renders when the temperature approaches the transition of SmA ↔ N, with the intensity much lower than that of the second-order diffraction. The absence of the first-order diffraction at low temperatures is ascribed to the possible matching of the electron densities between the center portion of the side-chain sublayer and the main-chain sublayer of the SmA structure. Since only the mesogens from the same main-chain sublayer can stack parallel together, the distribution of the azobenzene domains may cause some sort of density undulation within the smectic layer. Among the samples, PM6Az presents the strongest undulation with some additional orders. We also examined the annealing effect on the H-aggregation of PMnAzs. It is found that isothermal annealing at a temperature slightly higher than the Tgof PM8Az and PM10Az can significantly enhance the UV absorption at 326nm, indicating a further development of H-aggregation. However, for PM6Az and PM12Az, the UV–vis spectrum of the annealed sample is nearly identical to that without annealing.
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