S. De Petris

Crosslink density and fracture toughness of epoxy resins

A series of networks of diverse crosslink density were prepared using bifunctional epoxide prepolymers of different molecular weight, crosslinked with diamine diphenyl sulphone, and their fracture behaviour investigated. The same set of resins was also modified with a reactive rubber. The fracture toughness regularly decreased on increasing the crosslink density for all formulations. The addition of the rubber gave rise to a marked increase in toughness, though it magnified the influence of the molecular weight of the prepolymer. Tests performed with blunt notches showed that the fracture toughness was maximum at medium crosslink densities. A dispersion of rubber particles caused a toughness increase through the formation of microcavities ahead of the crack tip. Failure was preceded by a rapid volume increase caused by void coalescence.

Study of blends of nylon 6 with EVOH and carboxyl-modified EVOH and a preliminary approach to films for packaging applications

Blends of nylon 6 (Ny6) with ethylene-co-vinyl alcohol (EVOH) and EVOH modified with the introduction of carboxyl groups (EVOH–COOH) have been studied by Fourier transform infrared spectroscopy, scanning electron microscopy, differential scanning calorimetry, and dynamic-mechanical thermal analysis. The thermal and thermomechanical analyses of the blends show that the melting, crystallization, and relaxational behavior are affected by the blend composition and the presence of carboxyl groups on the EVOH chains. Nevertheless, microscopic and thermal investigations demonstrate the biphasic nature of the two-blend systems. Selective solvent extraction of the EVOH or EVOH–COOH phase in their blends and Fourier transform infrared analysis of the residual products indicates the occurrence of ionic linkages between the amino groups of the polyamide and the carboxyl groups of the modified EVOH, whereas specific interactions are evidenced for Ny6/EVOH blends. Tests performed on extruded Ny6/EVOH films show that the addition of EVOH effectively reduces the gas permeability of Nylon, whereas the addition of small amounts of EVOH–COOH helps to control and stabilize melt rheology.

Transitions and Relaxations in Mesophase Polymers: Thermotropic Liquid Crystalline Polyesters with Mesogenic Groups in the Main Chain

Abstract The thermal and dynamic viscoelastic behavior of some thermotropic liquid crystalline polyesters based on α, ω-bis(4-hydroxybenzoyloxy)alkanes and terephthalic acid is reported. Original samples obtained by solution polymerization followed by precipitation showed physical and dynamic-mechanical properties typical of semicrystalline polymers. Quenching of the polymers from the LC state gave rise to glass-like mesomorphic modifications that retained a glass transition at considerably higher temperature T 8m than that of the corresponding amorphous material T 8a. However, the formation of a new crystal form with respect to the original sample could not be prevented and the polymers so obtained were in fact partially mesomorphic and partially crystalline materials. All the quenched samples exhibited by dynamic-mechanical and calorimetric analyses a first order solid-solid transition independent of the length of the flexible spacer. Such a phenomenon was interpreted as an order-disorder transition of ...

Synthesis and thermal properties of linear aromatic poly(amidehydrazides) and corresponding poly(amide-1,3,4-oxadiazoles) and poly(1,3,4-oxadiazolyl-benzoxazoles)

Abstract Some aromatic poly(amid-hydrazides) have been synthesized by solution polycondensation at low temperature of aromatic hydrazides with aroyl dichlorides. By heating these polymers at temperatures near 300, dehydrocyclization occurs to give polymers containing the oxadiazole ring in the main chain. Their excellent thermal stability has been tested by thermogravimetric analysis. Some physical properties of these aromatic heterocyclic polymers have been reported.

Kinetics of the phase transition of poly(p-biphenyllyl acrylate)

Abstract The kinetics of the isotropic-to-smectic A transition of poly(p-biphenylyl acrylate) has been investigated by differential scanning calorimetry. The parameter n of the Avrami equation has been found to be of ca. 3. The calculated formation energy of the nuclei is 40-50 KJ mole−1. From the thermal data, it is concluded that, in this polymer, a fraction of amorphous material is present together with the mesophase, and the relative amounts of the two phases varies with the thermal history.

Polycarbonate-linear low density polyethylene blends: Thermal and dynamic-mechanical properties

Blends of polycarbonate (PC) and linear low density polyethylene (LLDPE) of different compositions, in the form of slabs obtained by melt extrusion, have been examined by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA).DSC measurements show that the melting, crystallization and glass transition temperatures of the two polymeric components in the blends are slightly affected by the composition. On the contrary, large differences are observed in the melting behaviour of layers cut at various depths, parallel to the slab surfaces of samples. This supports the occurrence of different crystal morphologies and distribution of the two components within the samples. The study of the crystallization kinetics from the melt blends shows that the crystallization processes of LLDPE are affected by the presence of PC.The dynamic mechanical analysis indicates that modulus, transitions and relaxational behaviour of the polymer components are scarcely affected by the composition. Some variations of the damping factor have been interpreted as due to the phase heterogeneity of the system, arising from the processing conditions and rheological behaviour of the blends.

Mechanical Properties and Fracture of Hybrid Particulate Composites

A rigid epoxy resin was modified by acrylic core-shell particles and glass beads (with and without surface treatment). Glass transition temperature, elastic modulus and fracture toughness were determined for all formulations. The fracture toughness increased linearly with the rubber content (from 0.82 MPa√m for the neat resin to 2.6 MPa√m for 20% rubber). Toughness also increased on adding glass particles (for 50% glass up to 1.9 and 2.3 MPa√m depending on the surface treatment). The stress-strain behaviour in compression depended on glass content and surface sizing. The fracture behaviour markedly depended on the radius of curvature of the crack tip. The toughness of sharply cracked samples increased on increasing the glass content. On the contrary samples containing blunt cracks were embrittled by the hard particles. This was attributed to crack tip sharpening caused by detached beads.

Solid State Physics of Thermotropic Polyesters: Internal Friction of Mesomorphic Structures

There is currently considerable interest in the solid state physics of thermotropic liquid crystalline (LC) polymers and much emphasis is placed on studies regarding the solid-solid transitions and the chain dynamics of these systems. Recently, evidence for the occurrence of supercooled mesomorphic structures in the glassy state of polymers quenched from the anisotropic melt has been presented.1 – 3