Microstructure And Segmental Dynamics Of Polybutene-1 By Means Of Low-field Solid-state Nuclear Magnetic Resonance

Polybutene-1(PB-1)is a high-performance polyolefin.Goods made from PB-1 have high flexibility,high environmental-stress-cracking and abrasive resistances,and some other high-performance characteristics.Properties of PB-1 are largely influenced by its complex polymorphic behavior.In this thesis,PB-1 homopolymers with different molecular weights and butene-1 copolymers were used to investigate the molecular segmental dynamics,the structural morphology of the crystalline phase and the entanglement network structure of the amorphous phase by low-field solid state nuclear magnetic resonance(LF-NMR),differential scanning calorimetry(DSC)and X-ray diffraction/scattering(WAXD/SAXS)techniques.The main results are summarized as follows:1.Molecular segmental dynamics of PB-1Phase composition and molecular mobility were studied using 1H NMR T2 relaxometry in PB-1 homopolymer with two polymorphs-form Ⅰ and Ⅱ crystals.Several types of NMR relaxation methods and data analysis were evaluated for determining the most reliable way for studying physical phases in PB-1.Three-phase model provided the most appropriate description of the phase composition in PB-1,i.e.,a crystal-amorphous interface separates the crystalline and the amorphous phases.Due to complex molecular mobility in PB-1,NMR crystallinity is the corrected value of the amount of crystalline fraction of form Ⅱ.Two types of chain segments are present in the amorphous phase:chain segments with anisotropic mobility due to chain anchoring to crystals and chain entanglements;and highly mobile chain end segments.The polymorphic phase Ⅱ to Ⅰ transition causes significant immobilization of polymer chains in the crystalline and the amorphous phases.Molecular weight of PB-1 largely influences phase composition and molecular mobility in crystalline and amorphous phases.Finally,the phase transformation behavior of butene-1/hexene copolymer(PB-1/C6)was studied,and the factors inhibiting phase transformation were briefly analyzed.2.Effect of phase transition on the morphology of PB-1 at lamellar levelMorphology and domain sizes in PB-1 homopolymer and butene-1/ethylene random copolymer(PB-1/C2)were studied by spin-diffusion time-domain 1H NMR and SAXS experiments.PB-1/C2 with form I’ crystals could be directly crystallized from a heterogeneous melt by controlling the crystallization and melting temperatures.The equilibrium melting temperature(Tm∞)of PB-1/C2 with form Ⅰ’crystals(PB-1/C2/fI’)is higher than Tm∞ of PB-1/C2 with form I crystals(PB-1/C2/fI),indicating that PB-1/C2/fI with infinite lamellar thickness has more crystal defects.The spin-diffusion curve shows that PB-1/C2/fI’ has typical lamellar morphology of polyolefins.Different lamellar structure is observed in PB-1 and PB-1/C2 with form Ⅰ crystals which were obtained from form Ⅱ.The phase transition causes fragmentation of crystal lamellae into small blocks with a wide size distribution.Two types of the interface are present in these samples:the inner interface that separates crystal blocks within fragmented crystal lamellae and the crystal-amorphous interface on the surface of fragmented lamellae.The molecular mechanism of polymorphic form Ⅱ to form Ⅰ phase transition is proposed.High chain mobility in form Ⅱ crystals and chain folding structure on the lamellar surface play significant role at the initial and final stages of the restructuring of crystal lamellae during the phase transition,respectively.3.Physical network structure of amorphous phase for PB-1Chain entangling is an intrinsic property of long-chain molecules influencing many macroscopic properties of polymers.Chain entanglements reside in the amorphous phase of semicrystalline polymers.A significant controversy exists about the role of crystallization and subsequent melting on the entanglement density.The effect of the phase transitions on the entanglement density in PB-1 is studied by proton NMR relaxometry.The polymers were crystallized at different temperatures to kinetically favored metastable form Ⅱ followed by the polymorphic phase transition to form Ⅰ with frozen chain mobility in crystals.Crystallization of PB-1 to conformationally disordered crystals leads to partial disentangling which is larger for slowly-crystallized polymer and for PB-1 with lower molecular weight.The entanglement density approaches its equilibrium value at the final stage of melting.In addition to the entanglement density,the relative density of interlamellar links,which interconnects adjacent crystals,was estimated.Its value increases with increasing molecular weight and is larger at faster crystallization rate due to thinner lamellae and smaller thickness of interlamellar amorphous layers.