In this paper, the synergistic effects that carbon nanotubes (CNTs) produce on the basic rheological properties and crystallization of polyethylenes with different branch contents and molecular weights was investigated. Multiwalled carbon nanotubes coated with polyethylene (as produced by in situ polymerization) were blended in the melt (in a 1% wt. ratio) with three polyethylene matrices of different molecular weights and branch contents. Transmission electron micrographs demonstrated excellent carbon nanotube dispersion in all samples and the existence of a geometrical percolation network. The rheological and calorimetric properties of the nanocomposites were determined and the results compared to those obtained for neat polyethylene resins. Both Newtonian viscosity and steady-state shear recoverable compliance increased with the addition of CNTs in all cases. However, the increase was strongly dependent on the molecular weight (and dispersity index) of the matrices regardless of the branch content. A novel screening effect of the CNTs network due to the high relaxation times of the matrix with the highest molecular weight was detected. This important result demonstrates that viscoelasticity can hinder the measurement of the rheological percolation threshold of CNTs network depending on the scale of relaxation times involved. Additionally, it was found that in relative terms (comparing each nanocomposite with its neat polyethylene matrix), the M w values also play a vital role in CNT nucleation besides chain branching content. Both nonisothermal and isothermal nucleation effects caused by CNTs increased as the M w of the polyethylene matrix decreased in spite of the role played by short chain branches in decelerating their overall crystallization kinetics. The capability for producing more stable lamellae through successive annealing of the nanocomposites as compared to their neat matrices also followed a decreasing trend with molecular weight increases, as indicated by SSA thermal fractionation results. Nevertheless, the presence of branches played a major role, since fractionation quality improved greatly as the branch content increased in the samples, as expected on the basis of the sensitivity of thermal fractionation to the presence of defects along crystallizable sequences.

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The stabilization of human papillomavirus type 16 virus-like particles has been examined by means of different techniques including dynamic and static light scattering, transmission electron microscopy and electrophoretic mobility. All these techniques provide different and often complementary perspectives about the aggregation process and generation of stabilized virus-like particles after a period of time of 48 hours at a temperature of 298 K. Interestingly, static light scattering results point towards a clear colloidal instability in the initial systems, as suggested by a negative value of the second virial coefficient. This is likely related to small repulsive electrostatic interactions among the particles, and in agreement with relatively small absolute values of the electrophoretic mobility and, hence, of the net surface charges. At this initial stage the small repulsive interactions are not able to compensate binding interactions, which tend to aggregate the particles. As time proceeds, an increase of the size of the particles is accompanied by strong increases, in absolute values, of the electrophoretic mobility and net surface charge, suggesting enhanced repulsive electrostatic interactions and, consequently, a stabilized colloidal system. These results show that electrophoretic mobility is a useful methodology that can be applied to screen the stabilization factors for virus-like particles during vaccine development.

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