Crystallization,Structure And Morphologies As Well As Mechanical Properties Of CIS-1,4-Polybutadiene/Polyethylene Blends

Blending of polymers is an effective and low-cost method to obtain new polymer materials with desirable properties. Phase structures and multiscale morphologies are the key factors to determine the performance of the polymer blends. Especially the crystalline morphologies of the crystalline component have important effects on the ultimate properties of the blends with semicrystalline components. The multiscale structures of the polymer blends including phase structure and the morphologies of each individual component are known to be strongly influenced by the preparation conditions of the blends, such as degree of supercooling, cooling rate, pressure, shear or tensile, confined space and so on. This has led numerous studies on polymer blends.Strengthening soft raw rubber with rigid materials or semicrystalline plastics has been described in many reports. It was well documented that polyethylene(PE) has excellent performance due to its characteristic molecular structure. Here, we choose the cis-1,4-polybutadiene (BR) and PE as example to study the crystallization behavior, multiscale structure and related mechanliacl properties in attempt to strengthen the soft BR with PE.In this thesis, we focused first on the crystallization behaviorof both PE and BR to illustrate their mutual influence on the crystallization. The multiscale structures obtained under different conditions were followed by optical, scanning electron and atomic force micrscopies. Finally, the mechanical properties of the related blends were studied per tensile and tear tests.1. Misibility and phase structure of the BR/PE blendsThe phase structures and morphologies of polymer blends are dependent on the miscibility of components.The results show that BR and PE are immisible. Therefore, phase segregation takes place during blending. The phase structures of the blends depend strongly on the PE content. It was found that, in the blends with PE less than 20 wt%, the PE is finely dispersed in the BR matrix and forms circular microdomains.With increasing PE content, some of the PE domains impinged each other leading to an increase in domain size as well as the number of the PE domains.2. Crystallization behavior of PE and BR in BR/PE blendsThe crystallization behavior of both the PE and BR component in the BR/PE blends were studied by optical microscopy under cross-polarizers and DSC measurement. It was found that the crystallization of PE is not affected by BR rather than by its domain size. If PE content is less than 10 wt%, the PE domains are very small. As a result, confined crystallization of PE in these small domains at relatively low temperatures, e.g. ca.80℃, happened. With the increase of PE content, some bigger PE domains formed, resulting in the coexistence of big and small PE domains. Therefore, the PE crystallizes stepwise with the PE in big domains crystallizes at higher temperature, e.g. about 116℃, while those in small domains still at low temperature. When content of PE is over 70 wt%, the crystallization of PE in the blends has a close resemblance with its bulk crystallization. On the other hand, the crystallization behavior of the BR in the blends is strongly affected by the PE. Considering the long chain feature of the BR and the existence of very small PE domains, the chain mobility of the BR has been reduced by the PE. This leads to a slower crystallization rate due to decrease chain segments diffusion on one hand and lower crystallinity due to some of the BR chain loss the ability to pack into the crystal lattice on the other hand.3. Crystal structure and morphology of PE in BR/PE blendsCrystal structure characterization of PE in the blends via X-ray diffraction show that the PE crystallized either at high temperature or at low temperature always in its orthorhombic unit cell, confirming that BR does not influence the crystallization of PE. Morphological studies via SEM and AFM show that the domain size of the PE increases with increasing PE content, while the connecting strips between the PE domains get wider. It was found that the PE in the microdomains are generally composed of fold chain lamellae or lamellar aggregates but the connecting strips are composed of Shish-Kebab like structures with center shish fibrils and their induced kebab lamellae. Of course, the PE morphology is sample preparation condition dependent. At low temperature and cooled under pressure encouraged the formation of PE lath like structure composed of ordered PE lamellae or lamellar aggregates.4. Mechanical properties of the vulcanizated BR/PE blendsThe performances of polymer blends are morphology and structure dependence.The blends of BR with PE were prepared and sulfur-vulcanized according to the standard mixing and curing procedure. The mechanical properties of the cured blends were characterized through tensile and tear tests. The results show that the tensile strength, tear strength, modulus as well as the elongation at break of BR increases gradually with the increase of PE component. While an increase of a factor of magnitude in tensile strength is achieved for the 40/60 PE/BR blends, the tear strength, modulus as well as the elongation at break are enhanced by 5-10 folds as compared to the pure BR treated under same conditions. These results confirmed that the PE exhibits an effective reinforcing capability towards BR.Morphological studies of the fractured samples show that PE as separated phase well-dispersed in the BR matrix in forms of microdomains. The domain size increases with increasing PE content. At the same time, the PE domains become elongated at high PE loading, e.g., over 30 wt%. Moreover, the PE in the cured BR matrix is identified to be in the crystalline state. It is these elongated PE domains that take the response of the improved mechanical performance.