Morphology, Structure And Dynamic Rheological Properties Of Particles Filled High-density Polyethylene Composites

The properties of particles filled polymer composites are related not only to the chemical structure and properties of polymer matrix, but also to the particle aggregated structure formed during the preparation and applications. In this thesis, the dynamic rheological properties of graphite (GP) and carbon (CB) filled high-density polyethylene (HDPE) composites were investigated systematically. The relationship between the dynamic rheological properties and the morphology of composites was also carried out in order to explore the influence of filler parameters such as filler kind, filler geometrical parameters and/or topological parameters and filler concentration Φ etc., on the evolution and development of filler agglomeration structure.Studies on the dynamic rheological properties of HDPE with or without antioxidant added were carried out. For HDPE without adding atioxidant, there exists characteristic storage modulus (G'), loss modulus (G" ), and loss tangent (tanδ). Within low frequency (ω) region, these moduli increase with the increase of the testing time while the tanδ obviously decreases. Also, the higher the temperature, the more notable is the relative change, which can be attributed to the oxidation-induced crosslinking of HDPE. And the degree of the oxidation-induced crosslinking of HDPE can be measured through the increase of moduli and the decrease of tanδ. No changes are found in the case of the protection of the antioxidant.The influence of temperature and stain on the dynamic rheological properties of HDPE with antioxidant was studied. The results reveal that the "payne effect" of virgin HDPE is unnoticeable even at a large strain level. In other word, there exists a wide linear scope for virgin HDPE system. For virgin HDPE system, the sensitivity of the rheological response to temperature is nearly the same within the whole observed co region.The influence of testing time on the dynamic rheological properties for particles filled HDPE systems at small shear strain was carried out. The results reveal that G'and G" increase with the increase of testing time. For HDPE/GP composites, such phenomenon can be attributed to the reconstruct of the particle aggregates and the decrease of structure defect. However, for HDPE/CB composites, the increase of moduli is related to the increase of the effective volume fraction of CB particles.The studies concerning the influence of filler concentration on the dynamic rheological properties of GP and CB particles filled (HDPE) composites indicate that the deviation of particles filled composites from the viscoelasticity of matrix increases with increasing 0. In high-loaded composites, the effect of filler is much more significant and the plateaus of G' and G" appearing at low cos expectantly occur, i.e. so-called "pseudo solid-like" behavior. It is believed that the maximum peak of tanS ~ co curves and the significant cy-dependence of//* are involved in this phenomenon. The ^-dependence of dynamic rheological properties behaves percolation behavior. The threshold (&c) corresponding to different rheological properties is in some degree different, i.e. the &c of G' percolation is smaller than that of G", rj* and torque.The results indicate that the filler parameters such as particle type, particle size and size distribution, particle shape and so on play an important role in the value of 0C. Generally, particles with large surface activity and large specific surface area result in the decrease of 0C. It should be noted that particle type is more important to determine 0C than those geometric parameters. The influence of filler parameters is listed and ranked as: particle type > particle shape > size distribution > particle size.By using Winter-Chambon method, the critical gel point for particles filled HDPE composites was accurately determined based on the data of tan& The characteristic parameters such as the critical gelation concentration 0cg, the critical relaxation exponent nc and the gel strength S were obtained. It is found that the values of S and nc for the composites investigated are comparable with those reported for polymer gels in literature. As compared with 0C, 0cg is more or less higher than 0C for all observed composites, and the filler parameters dependence of 0cg is close to that of 0C. It is clear that the gel points coincide with the percolation threshold of the rheological properties. The difference between 0cg and 0C are due to the different level of particle aggregation corresponding to them. Hence, we believe that the determination of gelpoint by Winter-Chambon method is useful as a characterization technique for the dispersion and aggregation of filler particles.It is well-accepted that the percolation behavior above the threshold should obey the scaling law. By fitting the experimental data to the scaling law, we obtained the pre-factor A and the scaling exponent t. It is also observed that the filler parameters affect the values of A and /. It seems that the composites in which particles with irregular shape, smaller size and larger specific surface dispersed easily form the percolated structure and have larger A and /.The temperature dependence of rheological properties of virgin HDPE and HDPE/GP composites is discussed. Within high co region, the addition of GP particles does not affect the thermo-sensitivity of rheological properties. However, within low co region, the filler concentration plays a key role in the thermo-sensitivity of rheological properties. In this co region, the thermo-sensitivity of rheological properties can be amplified by addition of a small amount of GP particles, but excessive GP amount enough to form particle aggregation result in the opposite effect. It is suggested that the particle aggregation structure remarkably limits the long-range molecular motions and minifies the thermal expansion of matrix, leading to the T-independence of G' in low co region.