Laura Sisti

Effect of carboxyl end groups content on the thermal and electrical properties of poly(propylene terephthalate)

Abstract Poly(propylene terephthalate) (PPT) samples with different carboxyl terminal groups content were synthesized in bulk and characterized in terms of chemical structure and molecular weight. The thermal behavior was examined by thermogravimetric analysis and differential scanning calorimetry. All the polymers under investigation show a good thermal stability, however decreasing with increasing carboxyl terminal groups content. No significant change of the glass transition temperature as well as melting temperature values was found in the samples under investigation; on the contrary, the crystallization rate of PPT was found to be affected by carboxyl terminal groups content, regularly decreasing as the amount of –COOH terminal groups is increased. This trend was interpreted on the basis of the interactions among the terminal groups of the polymeric chains, which determine a decrease in the chain mobility. Direct current (dc) electrical behavior was also investigated. The dc charging/discharging currents and electrical conductivity are studied as a function of temperature, time of applied voltage and amount of –COOH terminal groups. The conductivity values were found to increase as the content of –COOH end groups was increased, due to an increment of amount of ionic charge carriers.

Poly(butylene succinate) bionanocomposites: a novel bio-organo-modified layered double hydroxide for superior mechanical properties

Bionanocomposites based on poly(butylene succinate) and a novel organo-modified layered double hydroxide have been prepared by in situ polymerization. In order to enhance the compatibilization of the inorganic filler with the polymer matrix, an oligomer of PBS was intercalated between the layers of the clay. Composites with different percentages of filler (1, 3, 5, 10 wt%) and two different divalent cations (Zn or Mg) were prepared. The thermal, rheological and mechanical properties of the samples were investigated. The results showed that the materials feature a high thermal stability, more specifically so for the composites containing Zn2+ cations. Rheological investigations highlighted a significant chain extender effect of the filler toward the matrix, thus revealing a huge reinforcing effect imparted by the clays, especially for composites with Mg2+ cations. Such findings were also supported by an increase up to 30% of the tensile and flexural strength for a PBS composite loaded with 3 wt% of Mg/Al-LDH, thus suggesting a uniform level of dispersion and a pronounced interfacial interaction between the filler and polymer.

Hydrolytic stability of sulfonated poly(butylene terephthalate)

The hydrolysis of sulfonated poly(butylene terephthalate) copolymers was studied. Sulfonated poly(butylene terephthalate) copolymers, referred to as PBT-ionomers (PBTIs), were shown to hydrolyze faster than poly(butylene terephthalate) (PBT). An experiment designed to isolate the effect of the sulfonated isophthalate (SIP) moieties on hydrolysis rate showed that the SIP moieties were responsible for the faster hydrolysis. Experiments aimed at identifying the mechanism of influence of the SIP moieties on hydrolytic stability indicated that hydrolysis was enhanced by the presence of ionic multiplets which increase amorphous content, imbibe water, and perhaps exert a medium effect on the hydrolysis of esters associated with the ionic groups.

Electrical properties of resin monomers used in restorative dentistry

OBJECTIVES The application of an electric field has been shown to positively influence the impregnation of the resin monomers currently used in dentin bonding systems during hybrid layer formation. This study presents an experimental characterization of the electrical properties of these monomers with the aim of both correlating them to their chemical structures and seeking an insight into the mechanisms of the monomer migration under an applied electric field. METHODS Some common monomers examined were TEGDMA (triethyleneglycoldimethacrylate), HEMA (2-hydroxyethyl methacrylate), UDMA (urethane dimethacrylate), 2-MP (bis[2-(methacryloyloxy)ethyl] phosphate, TCDM di(hydroxyethyl methacrylate) ester of 5-(2,5-dioxotetrahydrofurfuryl)-3-methyl-3-cyclohexenyl-1,2-dicarboxylic anhydride) and Bis-GMA [2,2-bis(4-2-hydroxy-3-methacryloyloxypropoxyphenyl)propane]. A customized cell produced for the measurement of the electrical properties of monomers was manufactured and electrical conductivity and permittivity of resin monomers were measured. RESULTS The permittivity of the tested monomers is largely affected by electrical frequency. The large values of permittivity and dielectric losses observed as frequency decreased, indicate a dominant effect of ionic polarization, particularly evident in materials showing the highest conductivity. Permittivity and conductivity of the tested monomers showed a similar behavior, i.e. materials with the lowest permittivity also show small values of conductivity and vice versa. SIGNIFICANCE The results of the present study revealed a good correlation between electrical properties and Hoy solubility parameters and, in particular, the higher the polar contribution (polar forces plus hydrogen bonding) the higher the permittivity and conductivity. The most relevant outcome of this study is that the electrophoretic mechanism prevails on the electroendoosmotic effect in determining the monomer migration under the application of electric fields.

Memory effect in melting behaviour, crystallization kinetics and morphology of poly(propylene terephthalate)

Abstract Crystallization kinetics and melting behaviour of poly(propylene terephthalate) (PPT) were investigated by means of differential scanning calorimetry and hot-stage optical microscopy. Isothermal crystallization kinetics was analysed according to the Avrami treatment. The effects of temperature and duration of melting on the overall rate of isothermal crystallization were studied: the rate was found to decrease with increasing melting temperature and melting time. This result was discussed on the basis of the gradual destruction of predetermined athermal nuclei. Values of the Avrami exponent close to 3 were obtained, regardless of the adopted thermal treatment and the crystallization temperature, Tc, in agreement with a crystallization process originating from predetermined nuclei and characterized by three-dimensional spherulitic growth. As a matter of fact, spacefilling spherulites were observed by optical microscopy at all Tc’s, independent of the applied thermal treatments. For each of them, the rate of crystallization became lower as Tc increased, as usual at low undercooling where the crystallization process is controlled by nucleation. The observed multiple endotherms, which are commonly displayed by polyesters, were influenced by Tc and ascribed to melting and recrystallization processes. Linear and non-linear treatments were applied in order to estimate the equilibrium melting temperature for PPT, by using the corrected melting temperatures. The non-linear estimation yielded an about 33°C higher value with respect to the one obtained by means of the linear approach. Through the analysis of secondary nucleation theory, the classical II→III transition was found to occur at a temperature of 194°C. The average work of chain folding for nucleation was determined to be c. 5.2 kcal/mol. The heat of fusion was correlated to the specific heat increment for samples with different degree of crystallinity and the results were interpreted on the basis of the existence of an interphase, whose amount was found to depend on the thermal treatment the polymer was subjected to.

New Route to Poly(alkylene terephthalate)s by Reaction of Dimethyl Terephthalate with Cyclic Carbonates

A new route to poly(alkylene terephthalate)s by melt reaction of dimethyl terephthalate (DMT) with cyclic aliphatic carbonates such as ethylene carbonate and propylene carbonate is described. Poly(butylene carbonate) was used instead of the cyclic monomer for the synthesis of poly(butylene terephthalate). The polymers prepared by this alternative way presented a reduced amount of carboxyl end groups. These end groups are responsible for the poor thermal and hydrolytic stability of poly(alkylene terephthalate)s industrially prepared from DMT or phthalic acids and the corresponding diols. The thermal studies conducted on poly(propylene terephthalate) samples, obtained by this innovative procedure, confirmed the expected higher thermal stability. A detailed study of the mechanism, using a model compound, of this new route for polyesters synthesis together with a complete characterisation of the polymers prepared, is described.

Retting Process as a Pretreatment of Natural Fibers for the Development of Polymer Composites

The development of high-performance materials made from natural resources is increasing worldwide. Within this framework, natural fiber reinforced polymeric composites now experience great expansion and applications in many fields, ranging from the automotive to the construction sector. The great challenge in producing composites containing natural fibers and with controlled features is connected to the great variation in properties and characteristics of fibers. The quality of the natural fibers is largely determined by the efficiency of the treatment process and can dramatically influence the properties of the final composites. The overall fiber extraction processes, applied to vegetable fibers, is called retting and consists in the separation of fiber bundles from the cuticularized epidermis and the woody core cells. Today, many efforts are being made to optimize the retting methods in terms of fiber quality production, reduction of environmental issues and production costs. This chapter aims to provide a classification and an overview of the retting procedures that have been developed during years and are applied to extract mainly bast fibers.

Bio-Based PA11/Graphene Nanocomposites Prepared by In Situ Polymerization

Bio-based polyamide 11 (PA11)-graphene nanocomposites with different filler concentrations (0.25, 0.5, 0.75, 1.5 and 3 wt%) were prepared by In Situ polymerization starting from a water dispersed suspension of graphene nanoplatelets. The effects of the incorporation of the filler were studied in terms of molecular, morphological, thermal and dynamic mechanical properties of the final materials. During the crystallization process from the melt, the filler induces a notable nucleating effect even if the crystal growth rate tends to decrease. The glass transition temperature tends to shift to higher temperatures indicating a decrement of the molecular mobility. Thermal stability is enhanced confirming a good filler dispersion into the matrix. Mechanical reinforcement, investigated by means of a dynamic mechanical thermal analyzer was also highlighted. It was observed that a graphene concentration of 0.75 wt% induces the highest final performances.

Thermal and electrical characterization of poly(propylene terephthalate)

The thermal and electrical properties of poly(propylene terephthalate) samples with different carboxyl terminal groups content are investigated. The thermal behavior is studied through the rate of crystallization. As regards the electrical properties, dc conductivity and low-frequency relaxation processes are studied as a function of temperature. A transition in the electrical behavior is detected and related to the glass transition of the polymer.