Hill Jacob A, Endres Kevin J, Mahmoudi Pendar, Matsen Mark W, Wesdemiotis Chrys, Foster Mark D
ACS Macro Lett. 2018 Apr 17;7(4):487-492. doi: 10.1021/acsmacrolett.7b00993. Epub 2018 Apr 2.
The preference for a shorter chain component at a polymer blend surface impacts surface properties key to application-specific performance. While such segregation is known for blends containing low molecular weight additives or systems with large polydispersity, it has not been reported for anionically polymerized polymers that are viewed, in practice, as monodisperse. Observations with surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS), which distinguishes surface species without labeling and provides the entire molecular weight distribution, demonstrate that entropically driven surface enrichment of shorter chains occurs even in low polydispersity materials. For 6 kDa polystyrene the number-average molecular weight () at the surface is ca. 300 Da (5%) lower than that in the bulk, and for 7 kDa poly(methyl methacryalate) the shift is ca. 500 Da. These observations are in qualitative agreement with results from a mean-field theory that considers a homopolymer melt with a molecular-weight distribution matched to the experiments.
聚合物共混物表面对较短链段组分的偏好会影响特定应用性能所关键的表面性质。虽然对于含有低分子量添加剂的共混物或具有大多分散性的体系，这种偏析现象是已知的，但对于在实际中被视为单分散的阴离子聚合聚合物，尚未有相关报道。利用表面层基质辅助激光解吸电离飞行时间质谱（SL-MALDI-ToF-MS）进行的观察，该方法无需标记就能区分表面物种并提供完整的分子量分布，结果表明，即使在低多分散性材料中，较短链段也会因熵驱动而在表面富集。对于6 kDa的聚苯乙烯，表面的数均分子量（）比本体中的低约300 Da（5%），对于7 kDa的聚甲基丙烯酸甲酯，分子量偏移约为500 Da。这些观察结果与一种平均场理论的结果定性一致，该理论考虑了分子量分布与实验匹配的均聚物熔体。
