Heterogeneous Structure Of Solid Polymer And Polymer Composites Investigated By Nuclear Magnetic Resonance Techniques

Heterogeneous structure of solid polymer and polymer composites were investigated in this thesis employing mainly nuclear magnetic resonance(NMR) techniques,with the aid of some conventional analysis methods such as DSC,XRD and DMA.Polyethylene for pipe applications,polyethylene/clay nanocomposites and carbon black filled natural rubber were selected as the representative systems.Proton wideline spectra seperation,relaxation time determination,proton spin diffusion, double quantum buildup,pulsed field gradient NMR and field cyling NMR were adopted to quantitatively determine the structural parameters.The goal of this research focused on 1)constructing a platform for characterizing the phase structure of solid polymer and polymer composites by using standard proton NMR techniques. 2)setting up structure and property relationships with the emphasis on the supermolecular structures rather than on chain structures.3)feeding back to the real producing and application processes of the polymer materials with the obtained knowledge.On the topic of polyethylene pipe materials,series of basic resins including Cr-catalyzed ethylene-butene and ethylene-hexene copolymers, Ziegler-Natta-catalyzed bimodal polyethylene produced by tandem reactor techniques were analyzed and compared with respect to the three-fraction model of semicrystalline polymers.Different structural parameters including fraction composition,molecular mobility and domain sizes were extracted from solid state proton NMR analysis.Furthermore,the obtained results were correlated with the long-term mechanical properties of the pipe materials.It was found that the efficieny of chain entanglements in the noncrystalline region could be directly related to 1)the composition of interface component and 2)the decay of M_I/M_A versus testing temperature,where M_I and M_A indicated the compostion of the interface component and amorphous component respectively.These two parameters played an important role in determining the long-term mechanical properties of the pipe materials. Additionally,the long-term mechanical property were improved by either increasing the comonomer content or introducing long-chain comonomer,which could be seen from the increased amount of interface component and the slower decay of M_I/M_A versus temperature.However,one should take care the composition of crystalline component since it would decrease at the same time,which would impose negative effects on the long-term use property and duration grades.On the topic of polyethylene/clay nanocomposites,montmorillonite and palygorskite clay filled polyethylene nancomposites were prepared through in situ polymerization technique.The selected naturally occurring clay material presented either a nanofibre or nanolayer microstructure.Cp₂TiCl₂ was used as the main catalyst and MAO as the cocatalyst.It was found that the catalyst activity was significantly depressed with the feeding of MMT,and was further decreased once the filler load increased.In comparison,the palygorskite supported catalyst gave a comparable activity to the solution counterpart,which was explained from the strong Lewis acidity of the thermally treated clay material.In the next step,the phase structure of the prepared nanocomposites were analyzed by solid state NMR techniques and further correlated with the dynamic mechanical testing results.It was shown that all aspects of the phase structure including phase composition,molecular mobility and domain sizes had been affected due to the existence of nanoscale filler.Firstly,the exfoliated nanofiller depressed the chain folding process of melt-crystallized PE and thus decreased the crystallinity compared to the one without filler.Secondly,the effects of the filler on the noncrystalline region were more pronouced.Thirdly,there were motion-hindered PE chains located on the surface of the filler,which contributed to the NMR crystallinity and increased with the filler loading.Finally,when using MMT as the filler,intercalated or exfoliated nanocomposites could be prepared depending on the filler loading,which in turn resulted in the different ways of PE crystallization and phase structure characteristics.The interplay between the filler load and its effect on the PE matrix determined the final mechanical properties of the nanocomposites.In the last step,pulsed field gradient NMR technique was applied to measure the self-diffusion coefficents of organic solvents in the nanocomposites. Tortuosities inside the nanocomposites were calculated based on the obtained diffusion coefficients.It was found that the tortuosity was significantly increased due to the introduction of nanofiller.The calculated tortuosities of different solvents inside the same nanocomposite material differed obviously to each other,which illustrated that the barrier property presented by the filler was not only physical effect but also connected to the interaction between the filler and the small molecules.The last topic of this thesis focused on the quantitative characterization of the rubber-carbon black interaction by using NMR techniques and evaluating the possibility for pyrolytic carbon black to replace the commerical carbon black when used as the filler for natural rubber.The crude pyrolytic carbon was firstly washed with nitric acid and then chemically modified with a titanate coupling agent.Two types of commerical carbon black with different particle size and specific area were tested for comparison.Proton transverse relaxation time measurement,double quantum buildup and field cycling NMR were employed to characterize the NR-CB interaction in the composite materials.It was shown by the NMR results that washing with nitric acids alone could not improve the interaction but tend to weaken it,while the chemical modification with the titanate coupling agent had greatly favored the interaction and improved both the rheological and tensile properties.According to the NMR results,the NR-CB interaction with the chemically modified pyrolytic CB could even be comparable to the commerical N330.However,the residual ashes in the pyrolytic carbon black limited the further improvement of the tensile property,which made the replacement for commerical N330 not possbile in the present stage but the replacement for commerical semireinforced carbon black were technically feasible.