Maria Rossella Nobile

Transient and steady rheological behavior of the thermotropic liquid crystal copolymer 73/27 HBA/HNA

Transient and steady rheological measurements of shear and normal stresses are reported for a liquid crystal copolymer of 73 hydroxybenzoic acid/27 hydroxynaphthoic acid. Dynamic time sweeps indicate a sensitiveness of the rheological parameters to thermal history, probably related to polymerization phenomena. In the shear rate range 0.1–1 s−1 at T=310 °C the normalized stress overshoots vs strain curves nearly superpose. The steady values of the viscosity seem to follow the three region scheme of Onogi and Asada. The steady values of the first normal stress difference are positive in all the shear rate range analyzed. Constant stress measurements are also reported to prove that the low shear rate data are not significantly affected by the polymerization phenomena. Moreover, the constant stress data show that a true yield stress cannot be detected in the LCP studied.

Shear‐induced crystallization of isotactic poly(1‐butene)

In the present work the kinetics of the isothermal quiescent crystallization of the isotactic Poly(1-butene) (i-PB) is analyzed by means of rheological measurements and is used as an internal reference to study the effects of the shear flow. In particular, the flow-induced crystallization of two i-PB samples with different molecular weight is investigated by step-shear experiments at the crystallization temperature, Tc, of 95°C. Indeed, with step-shear experiments the effects of the applied shear rate and shear strain on the crystallization kinetics can be separately analyzed. In all the cases the overall-crystallization rate constant, k, defined as the inverse of the half-time of crystallization (t 0.5 ) has been evaluated. The results show that a critical shear rate (at a constant shear strain) and a critical shear strain (at a constant shear rate) are necessary to obtain the enhancement of k. These results significantly depend on the molecular weight revealing a stronger sensitivity to flow for the higher molecular weight sample. On the contrary, in the quiescent experiments no evidence of relevant molecular weight effects is detected. The in-situ optical analysis has shown that faster growth rates and a strong enhancement of nucleation density are observed in the step-shear experiments compared to the quiescent case.

Influence of nanofiller morphology on the viscoelastic properties of CNF/epoxy resins

In this paper we discuss the viscoelastic properties of CNF/Epoxy-Amine Resins for structural applications. The use of reactive diluent inside the unfilled and nanofilled epoxy mixture as proven to be of benefit to strongly decrease the viscosity.

The linear viscoelastic behavior of a series of molecular weights of the thermotropic main-chain liquid crystal polymers HBA/HNA 73/27

In this work the rheological properties of the thermotropic liquid crystalline polymer 1,4 hydroxybenzoic acid (HBA)/2,6 hydroxynaphatoic acid (HNA) 73/27 produced by Ticona have been investigated over the molecular weight range 30 000– > 51 000. The thermotropic main-chain liquid crystal polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes, as detected in strain sweep experiments performed at a constant frequency of 1, 5, 10, 30, 50, and 100 rad/s. The onset for nonlinearity (γc) significantly decreases (from ∼35% to ∼5%) as the applied frequency (ω) increases, independently of the molecular weight. When the results are plotted in terms of G*/G0* (the complex modulus normalized to the corresponding linear viscoelastic value) as a function of the stress amplitude, the different curves do not seem to collapse upon each other in the large frequency range investigated in this paper. Indeed, the stress onset for nonlinearity increases as the applied frequency (ω) increases (σc∼700 Pa ...

Shear flow effects on polymer melts crystallization: kinetics features

In this work the kinetics of the isothermal crystallization from the melt of isotactic polyolefins in quiescent conditions as well as after the application of a step-shear flow is investigated by means of rheological measurements. It is shown that the kinetics of the crystallization, as measured by the increase of the storage modulus, is not affected by the strain amplitude and the frequency of the oscillation, once they are properly chosen. A strong enhancement of the crystallization kinetics has been obtained when the step-shear flow was applied at the crystallization temperature T C =92°C for two different molecular weight poly(1-butene) samples (i-PB400 and i-PB200) and at T C =137°C for a polypropylene (i-PP). In particular, the overall-crystallization rate constants of the i-PB400 increased with increasing the applied shear rate at a constant total strain of 60. At higher shear flow temperatures slower kinetics occurred in all the cases until the effect of the applied shear flow was lost. Moreover, the effect of the molecular weight on the flow induced crystallization phenomenon is investigated on the two i-PB samples and the results have clearly shown that the higher molecular weight i-PB200 polymer is much more sensitive than the i-PB400 to the flow history.

The rheological behavior of a polyetherimide and of its blends with a thermotropic copolyester

A commercial engineering polymer of polyetherimide (ULTEM), a liquid crystal copolyester (TLCP), and blends with different percentages of TLCP in ULTEM have been rheologically analyzed in this work. Rheological measurements have been performed both at low and high shear rates. In particular, by means of a cone and plate rheometer, the transient shear flow behavior has been studied. The time necessary to reach the steady stress value was much longer in the case of the TLCP than for the amorphous ULTEM. Steady state rate sweeps as well as dynamic sweeps have been also carried out and discussed. Moreover, data obtained at high shear rates revealed that ULTEM viscosity is possibly pressure dependent. A procedure is shown for correlating the results obtained with the different techniques. The possibility of improving the processability of ULTEM has been also studied. In this regard the effect of the addition of low percentages of TLCP on the rheological properties of the thermoplastic matrix has been analyzed.

Viscoelastic Properties of Graphene-Based Epoxy Resins

In this paper the viscoelastic properties of an epoxy resin filled with graphene-based nanoparticles have been investigated in the liquid state, before curing, by means of a rotational rheometer equipped with a parallel plate geometry. Exfoliated graphite was prepared using traditional acid intercalation followed by a sudden treatment at high temperature (900°C). The percentage of exfoliated graphite was found to be 56%. The epoxy matrix was prepared by mixing a tetrafunctional precursor with a reactive diluent which produces a significant decrease in the viscosity of the epoxy precursor so that the dispersion step of nanofillers in the matrix can easily occur. The hardener agent, the 4,4-diaminodiphenyl sulfone (DDS), was added at a stoichiometric concentration with respect to all the epoxy rings. The inclusion of the partially exfoliated graphite (pEG) in the formulated epoxy mixture significantly modifies the rheological behaviour of the mixture itself. The epoxy mixture, indeed, shows a Newtonian beha...

Influence of the nanotube oxidation on the rheological and electrical properties of CNT/HDPE composites

Rheological and electrical properties of nanocomposites based on multi-walled carbon nanotubes (MWNTs) and high density polyethylene (HDPE), prepared by melt mixing in a micro-twin screw extruder, have been investigated. The effect of MWNT concentration (0.5 and 2.5 wt %) and nanotube surface treatment (oxidative treatment in a tubular furnace at 500°C for 1 hr in a 95% nitrogen, 5% oxygen atmosphere) has been analyzed. It has been found that the sample conductivity with oxidation of the nanotubes decreases more than 2 orders of magnitude.Scanning electron microscopy showed good adhesion and dispersion of nanotubes in the matrix, independently of the surface treatment. Electrical and rheological measurements revealed that the oxidative treatment, causing some reduction of the MWNT quality, decreases the efficiency of the nanotube matrix interaction.

Biodegradable compounds: Rheological, mechanical and thermal properties

Recently great attention from industry has been focused on biodegradable polyesters derived from renewable resources. In particular, PLA has attracted great interest due to its high strength and high modulus and a good biocompatibility, however its brittleness and low heat distortion temperature (HDT) restrict its wide application. On the other hand, Poly(butylene succinate) (PBS) is a biodegradable polymer with a low tensile modulus but characterized by a high flexibility, excellent impact strength, good thermal and chemical resistance. In this work the two aliphatic biodegradable polyesters PBS and PLA were selected with the aim to obtain a biodegradable material for the industry of plastic cups and plates. PBS was also blended with a thermoplastic starch. Talc was also added to the compounds because of its low cost and its effectiveness in increasing the modulus and the HDT of polymers. The compounds were obtained by melt compounding in a single screw extruder and the rheological, mechanical and therma...

Rheological and morphological properties of graphene-epoxy nanocomposites

In this paper the rheological and morphological properties of an epoxy resin filled with graphene-based nanoparticles have been investigated. Two samples of partially exfoliated graphite (pEG) and carboxylated partially exfoliated graphite (CpEG), differing essentially for the content of carboxylated groups, are used. The percentage of exfoliated graphite is slightly different for the two samples: 56% for pEG and and 60% for CpEG. Exfoliated graphite is prepared using traditional acid intercalation followed by a sudden treatment at high temperature (900°C). The epoxy matrix is prepared by mixing a tetrafunctional precursor with a reactive diluent which produces a significant decrease in the viscosity of the epoxy precursor so that the dispersion step of nanofillers in the matrix can easily occur. The hardener agent, the 4,4-diaminodiphenyl sulfone (DDS), is added at a stoichiometric concentration with respect to all the epoxy rings. The inclusion of the pEG and CpEG samples in the formulated epoxy mixture...