随星形聚氧化乙烯的分子拓扑形状演变的单层片晶图案

以不同臂(Arm)数的星形聚氧化乙烯(PEO)为对象,系统地研究了其在不同温度下结晶的晶体冰花图案.实验中采用的星形PEO样品,其臂数分别为3,4和8(3-arm-PEO,4-arm-PEO和8-arm-PEO)且每臂的分子量均为5000,线形PEO其分子量为5000.显然,随着星形PEO分子臂数的增加,分子拓扑形状的各向异性不断减小.在单层片晶冰花图案研究中发现,随着结晶温度逐渐向平衡熔点靠近,这些PEO样品的结晶冰花图案具有从树枝状晶体转变为海藻状晶体,然后转变为非规整的紧凑形晶体,最后变成多面晶体的变化规律.对细节的分析进一步表明,随着星形PEO分子臂数的增加,由于分子的各向异性减小,导致冰花状晶体的各向异性程度不断减弱,因此从树枝状晶体到海藻状晶体和从树枝状晶体到多面晶体的转变温度也都逐渐降低.将这些转变温度对臂数作图获得的一张形态相图(morphology diagram)说明了星形PEO结晶图案的分子形状依赖性,也阐明了冰花图案形成的大分子拓扑形状效应.

EVOLUTION OF CRYSTAL PATTERNS OF SINGLE-LAYER LAMELLAE OF STAR-SHAPED PEO SAMPLES

Studies on evolution of crystal patterns of single-layer lamellae of three star-shaped poly(ethylene oxide)s(PEO) samples with 3,4 and 8 arms and a 5000 molecular weight for each arm and a 5000 linear PEO sample were reported.With increasing crystallization temperature the dendritic-to-seaweed and seaweed-to-faceted crystal pattern transitions in these samples were observed.Plotting the transition temperatures with the arm number generated a morphology diagram that clearly reflected the effect of macromolecular topology on the crystal pattern formation and selection.Increasing the arm number of the star-shaped PEOs results in a topological change from an "anisotropic line" to an "isotropic sphere",so the molecular motion also alters from a sliding motion of linear macromolecules to a coordination motion of star-shaped macromolecules.These changes further affect the two key issues of crystal pattern formation:the diffusion and crystallization of macromolecules,so the crystal pattern formation and selection of star-shaped macromolecules depend on their molecular topology.