Solid-state NMR Study On Polymer Entanglements

The chain entanglement in polymeric material has very important influences on the viscoelasticity,mechanical properties and crystallization ability of polymer materials.Establishing the correlation between the chain entanglement and the macroscopic properties of polymeric materials has always been an important direction and long-term difficulty in the field of polymer physics.However,due to the complex structure of polymer and the varied molecular motions,there has been a lack of effective methods for the study of polymer chain entanglement for a long time.In this thesis,taking advantage of solid-state nuclear magnetic resonance(NMR)in the study of polymer structure and molecular motion,a new NMR method to probe polymer chain entanglement was developed.By using the developed method,the content and distribution of the chain entanglements in a series of poly(ethylene oxide)(PEO)melts were studied systematically.On this basis,the influence mechanism of molecular size and molecular weight polydispersity on the chain entanglements in the PEO melts were studied.The effects of the chain entanglements on the rheological properties and crystallization properties were studied and discussed afterwards.The main contents of this thesis are as follows:1)In the second chapter,according to the characteristic properties of PEO melts,we developed a solid-state NMR method for detecting the chain entanglements in PEO melts.The work in this chapter first systematically compares the adaptability and advantages/disadvantages of the three solid-state NMR methods,1H Hahn echo,1H double-quantum(DQ),and the CPMG method with variable echos,in the detection of chain entanglement.Then,aiming at the instability problem of traditional 1H DQ NMR in the data regularization,a new 1H DQ signal processing method based on Tikhonov regularization was developed to analyze the 1H-1H residual dipole interactions in polymer melts.Accurate measurement and the characterization of molecular chain entanglement content and distribution in polymer melts can be obtained afterwards.The work in this chapter is the foundation for solid-state NMR methods for the subsequent chapters.2)In the third chapter,the study mainly focused on the dynamic characteristics and distribution of the chain entanglements in "homogeneous" polymer melts,showing that there may be some heterogeneity in polymer chain entanglement in traditional "homogeneous" melts.The novel Tikhonov-regularized 1H DQ NMR was used to systematically investigate the chain entanglement in PEO melts.The results show that the chain entanglement in PEO melts has a significant dynamic non-uniform distribution.The results from the rheological measurements of the same sample also showed the presence of non-uniformly distributed chain entanglements.This discovery breaks through the prediction and description of the entanglement distribution in polymer melts by traditional polymer theories,and will play an important role on the present polymer physics theories.3)In the fourth chapter,the study mainly focused on the effect of molecular weight polydispersity on the chain entanglements in polymer melts.We prepared a series of PEO samples having similar molecular weights but significantly different molecular weight distributions.The results of NMR and rheological measurements showed that the small molecular weight PEO could have a significant disentanglement effect on the entangled network.We found for the first time that the size of the small molecule PEO can have a more significant disentanglement effect on the entangled network than the chain ends.The disentanglement effect of low molecular weight PEO on PEO melts can even lead to inhomogeneous distribution of the chain entanglements in PEO melts.The above findings proposed a novel mechanism of polymer disentanglement which has not been predicted by conventional polymer theory.4)In chapter 5,the influence of entanglements on crystallization were discussed.It is controversial as to whether entanglement affects crystallization.Based on the study of entanglements in the above chapters,the influence of entanglements on crystallization were studied via ss-NMR.A series of disentangled PEO samples were prepared by freeze-drying method.The disentangled PEO samples were analyzed after isothermal crystallization,and the results were compared with reentangled PEO samples.The results evidence that the disentangled PEO samples shows higher crystallinity than raw material and reentangled PEO samples,which indicates that entanglements limites the crystallizaton process.The work in this chapter preliminarily explores the effect of entanglement on crystallinity,and attempts to explain the mechanism of the effect of entanglement on crystallinity from the molecular level.