Crystallization in PEG networks: The importance of network topology and chain tilt in crystals
Crystallization in PEG networks: The importance of network topology and chain tilt in crystals
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DOI：
10.1016/j.polymer.2019.01.018
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发表时间：
2019-02-28
期刊：
影响因子：
4.6
通讯作者：
Reichert, Detlef
中科院分区：
文献类型：
--
作者：
Golitsyn, Yury;Pulst, Martin;Reichert, Detlef
我们使用具有叠氮端基的三臂星形-PEG和具有炔端基的不同摩尔质量的线性PEG低聚物，通过Cu(I)催化叠氮-炔环加成(CuAAC)合成了聚乙二醇(PEG)网络。通过 C-13 MAS 和 H-1 多量子 (MQ) NMR 光谱对网络进行广泛表征后，通过差示扫描量热法 (DSC)、温度依赖性小角和广角 X 射线散射（SAXS、WAXS）以及 l fl FID NMR 研究其结晶行为。由于 CuAAC 将 1,4-二取代的 1,2,3-三唑 (TR) 环作为链缺陷引入到网络中，因此我们将我们的结果与从线性 PEG 获得的结果进行比较，线性 PEG 在聚合物链中部特意引入了 TR 环。 PEG 网络链在冷却至零下 20 摄氏度时能够形成特征性的 7(2) 螺旋，并结晶成单斜晶胞，与均聚物完全类似。尽管与线性样品相比，网络的结晶度较低，但仍达到约30-50%的值。考虑到网络链的配置自由度受到高度限制，这是非常令人惊讶的。然而，交联和 TR 缺陷对晶体中 PEG 链的分子动力学影响很小，因为通过分析 H-1 FID 也可以在 PEG 网络中检测到特征 α(c)-弛豫过程（螺旋跳跃）。 SAXS数据显示，结晶时形成随机排列的片晶堆叠，并且晶体中的链以约40度-55度的大角度倾斜。在所有实验数据的支持下，我们提出了一个模型来解释网络中聚合物晶体的形成，并得出结论，将网络链延伸到线性前体长度之外的初级和次级环对于结晶起着至关重要的作用。
We synthesized poly(ethylene glycol) (PEG) networks by Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) using three-arm star-PEG with azide end groups and linear PEG oligomers of different molar masses with alkyne end groups. After extensive characterization of the networks by C-13 MAS and H-1 multiple-quantum (MQ) NMR spectroscopy, their crystallization behavior is studied by differential scanning calorimetry (DSC), temperature dependent small and wide angle X-ray scattering (SAXS, WAXS) as well as l fl FID NMR. Since CuAAC introduces 1,4-disubstituted 1,2,3-triazole (TR) rings as chain defects into the network, we compare our results with those obtained from linear PEG which contained a purposely introduced TR ring in the middle of the polymer chain. PEG network chains are able to form the characteristic 7(2) helices upon cooling to - 20 degrees C and crystallize into the monoclinic unit cell in complete analogy to the homopolymer. The crystallinity of the networks, though it is lower as compared to the linear samples, reached values of about 30-50%. Given the fact that the network chains are highly constrained in their configurational freedom, this is very surprising. However, cross-links and the TR defects have only a minor effect on the molecular dynamics of the PEG chains in the crystals since the characteristic alpha(c)-relaxation process (helix jumps) is also detected in the PEG networks by analysis of the H-1 FIDs. The SAXS data reveal that randomly arranged stacks of lamellae are formed upon crystallization and the chains are tilted in the crystals at large angles of about 40 degrees-55 degrees. Supported by all experimental data, we propose a model to explain the formation of polymer crystals in the networks and concluded that primary and secondary loops which extent the network chains beyond the length of the linear precursors play an essential role for the crystallization.