Synthesis and crystallization behavior of segmented copolyesters/silica composites

Copolyesters, namely, poly(butylene terephthalate)-poly(ethylene terephthalate-co-isophthalate-co-sebacate) (PBT-PETIS) were synthesized by means of melting transesterification. In the five-component copolycondensates system, average sequence length was calculated based on the analysis of 1H-NMR spectra and the equal probability of interchange reaction for three kinds of polyester units in PETIS with PBT. The results reveal that the average sequence length of both PBT and PETIS segments vary almost linearly with reaction time. The determination of degree of randomness that lies within the range of 0.1749–0.5282 suggests the formation of segmented copolyesters.

Morphology, melting behavior, and non-isothermal crystallization of poly(butylene terephthalate)/poly(ethylene- co-methacrylic acid) blends

The dependence of the kinetics of crystallization and melting behavior in isotactic polystyrene/poly‐o‐chlorostyrene‐co‐p‐chlorostyrene (iPS/Po‐CIS‐co‐p‐CIS) blends on temperature, thermal history, and blend composition has been investigated. The crystallization rate at a given temperature and copolymer composition decreases with increasing copolymer content in the blend when the samples are premelted. These effects can be ascribed to the reduction of mobility of the crystallizable chains due to the presence of the copolymer and to the decrease in the number of heterogenous iPS nuclei as a result of the premelting process. The Avrami exponent values and the analysis of the blend morphology indicate that the growth mechanism of the crystals is strongly influenced by thermal treatment. There is no measurable change in the melting temperature of iPS in the blends, with composition indicating that, on the basis of the Flory‐Huggins approximation of the thermodynamics of polymer mixing, the net interaction parameter at the melting temperature is close to zero. From the comparison of the phase diagram for the isotactic polystyrene‐containing blend with that of the atactic‐containing blend, it can be concluded that in the amorphous state polystyrene with a regular configuration is slightly less compatible with the P(o‐CIS‐co‐p‐CIS) than is polystyrene with random configuration.

The melting behavior and the isothermal crystallization kinetics of linear and randomly branched poly(butylene terephthalate)System. name: poly(tetramethylene terephthalate). (PBT) samples were investigated by differential scanning calorimetry. The multiple endotherms typical of linear PBT were observed also in branched samples and were found to be influenced both by temperature and degree of branching. The isothermal crystallization kinetic constants, obtained applying Avrami's treatment, were analysed on the basis of Hoffman's secondary nucleation theory. A decrease in the crystallization rate was observed as the content of branching units increased. This result was correlated to surface effects: the product of the surface free energies σσe was found to increase with the ramification content. As far as the nucleation process is concerned, the induction time was determined for all the samples at different crystallization temperatures, and the results suggested that in the presence of branching points the nucleation process is favoured.

The multiple melting behavior of many semicrystalline polymers is generally attributed to fusion of original crystals reorganized/recrystallized upon heating at conventional scanning rates. In the present study, the triple and double melting behavior of poly(butylene terephthalate) (PBT) and isotactic polystyrene (iPS), which is observed after isothermal crystallization at low and high T c s, respectively, is put into relation with the presence and absence of rigid amorphous/crystal interphase around the original primary crystals. Such constrained amorphous interphase has been recognized as decisive for the definition of the mechanical, thermal, and barrier properties of semicrystalline polymers. The method for the approximate determination of the limit temperature for the presence of this rigid amorphous fraction (RAF), already applied to poly[(R)-3-hydroxybutyrate] (PHB) and poly(ethylene terephthalate) (PET), is here utilized for PBT and iPS. The complex melting behavior displayed by all these polymers is explained by assuming that two distinct morphologies of primary crystals develop during crystallization at temperatures lower than the RAF limit temperature, in a percentage that is a function of the crystallization temperature. Simultaneously with the RAF devitrification, these distinct morphologies differently reorganize/recrystallize upon heating at low scanning rate, originating the triple melting behavior. On the contrary, after crystallization at high T c s, a single reorganization route is followed by all the primary crystalline structures, characterized by the same morphology made of tightly chain-folded lamellae and absence of vitrified RAF at the amorphous/crystal interphase.

Crystallization kinetics were used to develop a spherulite growth model, which can determine local crystalline distributions through an optimization algorithm. Kinetics were used to simulate spherulite homogeneous nucleation, growth, and heterogeneous nucleation in a domain discretized into voxels. From this, an overall crystallinity was found, and an algorithm was used to find crystallinities of individual spherulites based on volume. Then, local crystallinities within the spherulites were found based on distance relative to the nucleus. Results show validation of this model to differential scanning calorimetry data for polyether ether ketone at different cooldown rates, and to experimental microscopic images of spherulite morphologies. Application of this model to various cooldown rates and the effect on crystalline distributions are also shown. This model serves as a tool for predicting the resulting semi-crystalline microstructures of polymers for different manufacturing methods. These can then be directly converted into a multiscale thermomechanical model.

The adverse effect of knit‐line formation on the mechanical properties of thermoplastics has been well documented for unfilled materials. A majority of these investigations concern amorphous polymers, whereas a number of engineering polymers are semicrystalline and also contain reinforcing fillers and fibers. The present investigation deals with the effect of knit‐lines on the mechanical behavior of unfilled and glass fiber‐reinforced semicrystalline polymers, such as polypropylene, poly(butylene terephthalate), poly(ethylene terephthalate), and poly(phenylene sulfide). Unfilled and fiber‐reinforced polystyrene was also investigated for reference. Tensile specimens with knit‐lines were produced by impinging melt fronts from two gates located at opposite ends of the dumbbell‐shaped mold. The effect of process conditions on the knit‐line strength was studied by varying primarily the melt and mold temperatures. The presence of glass fibers significantly reduced the knit‐line factor based on strength and strain to failure for the reinforced grades of all polymers relative to their respective unfilled grades. This is attributed to the lack of fiber flow across the knit‐line, which makes the material in the knit‐line region act as if it is not reinforced. The knit‐line strength could be changed through process modifications. For the unfilled grades of semicrystalline polymers, the presence of knit‐lines did not affect the yield strength but reduced the elongation at onset of necking, indicating that little spherulitic growth takes place across the knit‐line.

Experiments have been conducted to examine the effect of the following agents: TiO2, CaO, MgO, BaSO4, SiO2, and Al2O3 as modifiers on the poly(ethylene terephthalate) crystallization kinetics. Modifier concentration in the polymer varied from 0.5% to 3%. Crystallization rate measurements at a temperature of 237°C were performed by dilatometric method and measurements of spherulitic growth rate, by microscopic method. It has been stated that the addition of a modifier affects the crystallization rate, dimension, and homogeneity of spherulitic sizes. The spherulitic growth rate, with the exception of that modified by CaO, does not depend on the kind and the concentration of a modifier. Heterogeneous nucleating agents exert an effect, then, mainly on the heterogeneous nucleation process. In the presence of the agents, the nucleation in the poly(ethylene terephthalate) is of athermal character. The kind of a modifier applied exerts also an effect on the final crystallization degree.

In order to overcome the defect of slow crystallization rate of Poly(L-lactic acid) (PLLA) during manufacturing, a new organic nucleating agent, N,N'-succinic bis(hydrocinnamic acid) dihydrazide (BHSH), was synthesized to evaluate its influence on the crystallization and other physical properties of PLLA in this work. For this purpose, the non-isothermal crystallization, isothermal crystallization of PLLA modified using BHSH were investigated using differential scanning calorimetry and depolarized-light intensity measurement. Additionally, the melting behavior, thermal stability, and optical property of BHSHnucleated PLLA were further studied. The result of the non-isothermal crystallization behavior indicated that BHSH had a distinct accelerating effect for the melt crystallization process of PLLA and the non-isothermal crystallization peak shifted to the higher temperature and became sharper. Meantime, non-isothermal crystallization result further revealed that both the cooling rate and the final set melting temperature affected the non-isothermal crystallization behavior of PLLA. The addition of BHSH could cause the crystallization halftime of PLLA to become shorter, meaning that PLLA/BHSH samples had a faster overall crystallization rate comparing with the pure PLLA. Upon the addition of 2 wt% BHSH, the crystallization half-time of PLLA/BHSH decreased from 575.7 s to the minimum value 48.6 s at 100°C. Melting behavior after crystallization further confirmed the crystallization promoting effect of BHSH for PLLA, and the double-melting peaks were attributed to the melting-recrystallization. Although that the BHSH could improve the crystallization of PLLA, unfortunately, the presence of BHSH decreased the thermal stability and light transmittance of PLLA.

Poly(dithiotriethylene adipate): Melting behavior, crystallization kinetics and morphology

Spherulitic growth of poly(ethylene oxide) from 30 wt% tripropionin (glycerol tri-n-propionate) solution was investigated in the supercooling range ΔT=11–37 K. The solution was pre-annealed at 200°C before isothermal crystallization. The estimated total crystallization rate decreases with increasing pre-annealing time, and becomes almost constant with a pre-annealing time longer than ca. 200 h. This suggests that traces of crystalline nucleus remaining in the solution dissolve during the pre-annealing process. The isothermal crystallization rate G with respect to the crystallization temperature was measured using isothermal calorimetric measurements and direct observation of the spherulitic growth using a CCD camera. The Avrami exponent estimated from the isothermal crystallization data ranges from ca. 2.7 to 3.2 depending on the crystallization temperature. The profile of G vs. T was also analyzed to estimate the parameters for the diffusion factor, and the result shows that the diffusion process belongs to the low friction limit, i.e., frictional interaction between the polymer and solvent is extremely weak.

This study focuses on the effect of various processing and cooling conditions upon the subsequent melting and solidification behavior of blends of poly(butylene terephthalate) (PBT) and high density polyethylene (HDPE). Differential scanning calorimetry (DSC) measurements have shown that the crystallization of reprocessed PBT is different from the crystallization behavior of virgin samples. Two melting endotherms for PBT were observed for reprocessed PBT and for PBT‐PE blends. The nature of each PBT peak is discussed in relation to processing history, solidification conditions, and composition. The presence of two crystallization peaks for the PE component in blends of PE and PBT are thought to be associated with the restriction of molecular motion of PE in the presence of the second component. The relative magnitudes of the two exotherms of PE vary with composition and cooling rate during solidification.

Silver (Ag) nanoparticles were synthesized by a facile route in the presence of oleic acid and n-propylamine. It was shown that the average primary size of the as-synthesized Ag nanoparticles was approximately 10 nm and the surface of as-synthesized Ag nanoparticles was capped with monolayer surfactants with the content of 19.6%. Based on as-synthesized Ag nanoparticles, polyoxymethylene (POM)/Ag nanocomposites were prepared. The influence of Ag nanoparticles on non-isothermal crystallization behavior of POM was investigated by differential scanning calorimetry (DSC). The Jeziorny, Jeziorny-modified Avrami, Ozawa, Liu and Mo, Ziabicki and Kissinger models were applied to analyze the non-isothermal melt crystallization data of POM/Ag nanocomposites. Results of half time (t1/2), crystallization rate parameter (CRP), crystallization rate function (K(T)), kinetic parameter (F(T)), the kinetic crystallizability at unit cooling rate (GZ) and the crystallization activation energy (∆E) were determined. Small amounts of Ag nanoparticles dispersed into POM matrix were shown to act as heterogeneous nuclei, which could enhance the crystallization rate of POM, increase the number of POM spherulites and reduce POM spherulites size. However, the higher loading of Ag nanoparticles were easily aggregated, which restrained POM crystallization to some degree. Furthermore, the POM/Ag nanocomposites showed robust antibacterial activity against Escherichia coli and Staphylococcus aureus.

Correlation between interactions, miscibility, and spherulite growth in crystalline/crystalline blends of poly(ethylene oxide) and polyesters

Crystallization of poly(ethylene oxide) with acetaminophen – A study on solubility, spherulitic growth, and morphology