Fluorescence Studies on a Thermoresponsive PNIPAM-Polyfluorene Graft Copolymer
Fluorescence Studies on a Thermoresponsive PNIPAM-Polyfluorene Graft Copolymer
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DOI：
10.1021/acs.macromol.1c00827
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发表时间：
2021-08-13
期刊：
影响因子：
5.5
通讯作者：
Seixas de Melo, J. Sergio
中科院分区：
文献类型：
--
作者：
Cunha, Carla;Klein, Patrick;Seixas de Melo, J. Sergio
通过紫外-可见光谱和稳态和时间分辨荧光技术，在 5 至 70 摄氏度的温度范围内，在 DMF 和丙酮：水 (1:2) 混合物中研究了 PNIPAM 接枝聚芴共聚物 (PF-PNIPAM) 以及相应的前体聚合物（9,9-双(.-羧戊基)-取代的聚芴，PF-COOEt）的荧光行为。温度依赖性图显示出明显不同的行为在丙酮:水 (1:2) 混合物中，根据总体数据，这归因于 PF-PNIPAM 出现较低的临界溶解温度 (LCST)，该温度类似于 45 摄氏度，与 PNIPAM 链在水中类似于 32 摄氏度的值不同。在丙酮:水 (1:2) 混合物中，荧光衰减时间分量在 20 到 70 摄氏度之间的弱温度依赖性记录了占主导地位的内部和链间跳跃型失活机制。然而，此外，热响应 PNIPAM 侧链会导致在 LCST 温度下形成聚集体，并伴随荧光强度的突然变化。在 20-55 摄氏度的温度范围内，对丙酮:水 (1:2) 混合物中的 PF-PNIPAM 进行的动态光散射 (DLS) 研究表明，当 T > 40 摄氏度时，对应于聚集体的直径 >200 nm 的单峰尺寸分布达到峰值，与较低温度下具有较低流体动力学半径的双峰分布形成鲜明对比。该研究为 PNIPAM 基（共）聚合物令人费解的 LCST 行为提供了更多信息，该行为在 PF-PNIPAM 中影响聚芴 PF 主链和 PNIPAM 侧链的存在。
The fluorescence behavior of a PNIPAM-grafted polyfluorene copolymer (PF-PNIPAM) together with the corresponding precursor polymer (9,9-bis(.-carboxypentyl)-substituted polyfluorene, PF-COOEt) was investigated by UV-Vis spectroscopy and steady-state and time-resolved fluorescence techniques in DMF and in an acetone:water (1:2) mixture over the temperature range 5 to 70 degrees C. Temperature dependence plots show a clearly different behavior in the acetone:water (1:2) mixture, which, based on the overall data, is attributed to the occurrence of a lower critical solution temperature (LCST) for PF-PNIPAM, which is found at similar to 45 degrees C, different from the value for PNIPAM chains in water of similar to 32 degrees C. In the acetone:water (1:2) mixture, the weak temperature dependence of the fluorescence decay time components between 20 and 70 degrees C documents a dominant intra- and interchain hopping-type deactivation mechanism. However, additionally, the thermoresponsive PNIPAM side chains cause aggregate formation at the LCST temperature coupled to abrupt changes of the fluorescence intensity. Dynamic light scattering (DLS) studies for PF-PNIPAM also in the acetone:water (1:2) mixture in the temperature range of 20-55 degrees C show for T > 40 degrees C a unimodal size distribution peaking for diameters of >200 nm corresponding to aggregates, contrasting to a bimodal distribution with lower hydrodynamic radii at lower temperatures. The study delivers additional information to the puzzling LCST behavior of PNIPAM-based (co)polymers, which in PF-PNIPAM impacts the presence of both polyfluorene PF main chains and PNIPAM side chains.