Effect of Temperature on Environmental Stress Cracking Resistance and Crystal Structure of Polyethylene

Studies were conducted on the crystalline properties of different polyethylene resins to identify their influence on phase interconnectivity between amorphous and crystalline regions. This work offers a thorough investigation on the potential correlation between environmental stress cracking resistance (ESCR) and crystalline structure characteristics, namely, crystallinity, mean lamella thickness and its distribution, and lamella surface area (LSA). The initial objective of this work was to investigate an existing ambiguity in the literature with respect to the effect of the crystalline phase on ESCR. In addition, research was conducted to evaluate the degree of variability in the lamella surface area, as a measure of phase interconnectivity and ESCR, with processing temperature and post-process annealing. Annealing at various conditions, along with cooling at different rates, were employed to investigate the effect of temperature on LSA. It was observed that a clear correlation exists between crystalline phase properties and ESCR, given that the comparison is made between polymers with similar molecular weights. Annealing temperature and time were found to cause a general reduction in LSA, however, to varying degrees, according to the type of PE molecular structure (significant interactions exist between annealing conditions and polymer type). LSA showed a significant dependence on cooling rate, however, no interaction was found between cooling rate and type of PE molecular structure. Additionally, lamella surface areas obtained from quenching and air cooling were found to be almost the same.     

Nonlinear imaging microscopy techniques as diagnostic tools for art conservation studies

We present results of the implementation of three-photon excitation fluorescence (3PEF) and third harmonic generation imaging measurements for the precise and nondestructive detection of natural and synthetic varnish layers, which are used for the surface protection of painted artifacts. For this purpose, we employ as an excitation source a compact femtosecond laser operating at 1028 nm. Two-dimensional images of the multilayer structures from different samples are depicted. The third harmonic signals show the interface between the different materials, when its refractive index mismatch is high enough. The depths of different layers of varnishes, presenting similar refractive index, are distinguishable with an axial resolution of approximately 1 microm by employing 3PEF measurements.     

Characterization of hydrophilic networks synthesized by group transfer polymerization

Group transfer polymerization was used to synthesize several series of hydrophilic random and model networks. Cationic random networks were prepared both in bulk and in tetrahydrofuran (THF) using a monofunctional initiator and simultaneous polymerization of monomer and branch units, while a bifanctional initiator was employed in THF for the synthesis of model networks comprising basic or acidic chains. Upon polymerization of the monomer, the latter initiator gives linear polymer chains with two “living” ends, which are subsequently interconnected to a polymer network by the addition of a branch unit. Homopolymer network star polymers were also synthesized in THF by a one‐pot procedure. The synthesis involved the use of a monofunctional initiator and the four‐step addition of the following reagents: (i) monomer, to give linear homopolymers; (ii) branch unit, to form “arm‐first” star polymers; (iii) monomer, to form secondary arms and give “in‐out” star polymers; and, finally (iv) branch unit again, to interconnect the “in‐out” stars to networks. Different networks were prepared for which the degree of polymerization (DP) of the linear chains between junction points was varied systematically. For all networks synthesized, the linear segments, the “arm‐first” and the “in‐out” stars were characterized in terms of their molecular weight (MW) and molecular weight distribution (MWD) using gel permeation chromatography (GPC). The degrees of swelling of both the random and model networks in water were measured and the effects of DP, pH, and monomer type were investigated.