The crystallization and transition behavior of poly(vinylidene fluoride)/copoly(chlorotriofluorethylene-vinylidene fluoride) blends

Thermal analysis of solution precipitated blends of two crystallizable polymers, poly(vinylidene fluoride) (PVDF) and copoly(chlorotrifluorethylene-vinylidene fluoride) (copoly(CTFE-VDF)), has been carried out to study the transition temperatures, crystallinity, and crystallization rates. PVDF crystallizes over the whole blend composition either during precipitation from solution or upon cooling from the melt. The high degree of crystallinity attained, higher than in PVDF by itself, suggests the occurrence of partial PVDF-copolymer cocrystallization. The melt crystallization temperature, decreasing with cooling rate, is lower in PVDF-rich blends than for lean blends. However, the heat of crystallization increases with cooling rate, suggesting that the crystal composition depends on crystallization rate. No significant melting temperature depression due to blending was observed. However, the blends glass transition (T_g) changes linearly with composition, but less than expected by any mixing rule applicable to compatible systems. Annealing of the blends above T_g results in an additional crystalline phase consisting mainly of the copolymer. The amount of these crystals increases with PVDF content, due to partial cocrystallization and kinetic effects. The addition of the copolymer to PVDF results in a volume-filling spherulitic structure consisting of spherulites which decrease in size with increasing copolymer content.     

Influencing the crystallization behavior of PET-based segmented copoly(ether ester)

Segmented copoly(ether ester) based on poly(ethylene terephthalate) and poly(oxytetramethylene) suffers from having a rather low rate of crystallization. The crystallization behavior of this elastomer can be influenced by cocondensation of selected diols and addition of nucleating agents and plasticizers. Changes of the crystallinity and morphology are studied by differential scanning calorimetry and dynamic-mechanical analysis, respectively. Mechanical properties in relation to the different crystallinites obtained are reported.     

Effect of rod-like imide unit on crystallization of copoly(ethylene terephthalate-imide)

The effect of the imide unit on the isothermal and non-isothermal crystallization, kinetics crystallization of a new family of copoly(ethylene terephthalate-imides) (called copolyesterimides or PETIs) was investigated using differential scanning calorimetry. With a combined Avrami and Ozawa equation, one can describe the non-isothermal crystallization process of copolyesterimides, and the results show the same tendency as that in the isothermal crystallization process. These studies show that the processes of crystal nucleation and growth result in mainly three-dimensional growth with a thermal nucleation. In both isothermal and non-isothermal crystallization processes, the crystallization rate of PETIs, with imide content below 0.5%, is higher than that of neat PET, while PETI-3 (0.3 mol% imide) has the highest crystallization rate. This rate is significantly enhanced over PET homopolymer. It is proposed that imide units precipitate from the melt and act as nucleating agents during the crystallization process of these novel copolyesterimides.     

Synthesis of copoly(ethylene terephthalate /imide)s and crystallization kinetics

Copoly(ehtylene terephthalate/imide)s (PETI) were prepared by melt polycodensation of bis(2-hydroxyethyl)terephthalate (BHET) and imide containing comonomer, 4,4′-bis[(4-carbo-2-hydroxyethoxy)phthalimido]diphenylmethane (BHEI) with Sb₂O₃ as catalyst at 280°C under vacuum (～ 1 mm Hg). The change of T_m with an increase of the BHEI repeat unit in the PETI copolymer was analyzed by the Flory equation. On isothermal crystallization, a longer induction time and a lower activation energy than for the PET homopolymer were observed with an increasing amount of BHEI repeat unit. The Avrami exponent, n, increased from 1.5 to 2.3 as the content of BHEI or crystallization temperature was increased. The Avrami rate constant K decreased with the increase of the BHEL unit. On nonisothermal crystallization, the Ozawa equation and Lawton plot were used to investigate the effect of BHEI units on the crystallization kinetics of PETI copolymers. From the change of the cooling crystallization function and the result of the Lawton plot, it was found that the BHEI unit effectively decreases the rate of crystallization.     

Phase behavior in liquid crystallization for diblock copolymers consisting of rubbery amorphous and side-chain liquid crystalline components

Phase behavior in liquid crystallization was studied for a series of liquid crystalline (LC) diblock copolymers consisting of rubbery amorphous and side-chain liquid crystalline components, poly(n-butyl acrylate) (PBA) and poly[11-(4′-cyanophenyl-4″-phenoxy)undecyl acrylate] (PLC), respectively, using a time-resolved small-angle X-ray scattering (SAXS) techniques, DSC and polarized optical microscopy (POM). The block copolymers used had three kinds of copolymer compositions, 44, 20 and 15 wt% of PLC compositions (BLC44, BLC20 and BLC15, respectively). BLC44 showed a smectic liquid crystalline structure. In the process of liquid crystallization for BLC44, the SAXS peak due to the microphase separation structure existing before liquid crystallization was changed continuously to be at a smaller angular side, and at almost the same time, a new peak appeared at a further smaller angular side and developed. The former peak disappeared with the development of liquid crystallization. The behavior of these SAXS peaks suggests that the microphase separation structure was changed discretely at the transition from isotropic to smectic and that two phases coexist in the early stage of the liquid crystallization. The coexistence of two peaks in the early stage of the liquid crystallization corresponded to the POM observation. In the isotropization process, coexistence of two phases was not observed. For BLC20 exhibiting a cylindrical structure in both isotropic and liquid crystalline states, the liquid crystalline structure was not smectic but probably nematic, and the spacing was changed continuously in liquid crystallization. No liquid crystallization was observed in SAXS, POM and DSC for BLC15. The orientation of smectic layers within lamellar domains was investigated using 2D-SAXS images. The smectic layer was aligned perpendicularly to the lamellar interface.     

Crystallization behaviour of biodegradable poly(ethylene succinate) from the amorphous state

Nonisothermal and isothermal crystallization kinetics of biodegradable poly(ethylene succinate) (PES) from the amorphous state were studied by differential scanning calorimetry (DSC). For the nonisothermal crystallization, there were two crystallization exotherms upon heating from the amorphous state. One major crystallization exotherm located at low temperature corresponded to the real cold crystallization of PES, while the other minor one located at high temperature may correspond to the melt-recrystallization of the unstable crystals formed during the nonisothermal crystallization earlier. Several methods, such as Avrami equation, Tobin equation and Ozawa equation, were applied to describe the nonisothermal crystallization process of PES. Meanwhile, Avrami equation was also employed to study the isothermal crystallization of PES from the amorphous state. Similar to the nonisothermal crystallization the minor crystallization exotherm was also found in the DSC trace upon heating to the melt after the isothermal cold crystallization finished completely, and was attributed to the melt-recrystallization of the unstable crystals formed during the isothermal cold crystallization. Temperature modulated differential scanning calorimetry (TMDSC) was used in this work to investigate the origin of the minor crystallization exotherm located at high temperature, and the TMDSC experiments gave a direct evidence that the origin of the minor crystallization exotherm was from the melt-recrystallization of the originally existed unstable crystals formed through previous crystallization.     

The effect of fluoride ions on the crystallization of calcium hydrogen phosphate (DCP)

It was found that the average size of dicalcium phosphate (DCP) particles precipitated in the presence of fluoride ions was smaller than the particle size of DCP crystals precipitated from pure solutions. This special effect of fluoride ions on the precipitation of DCP was investigated by turbidimetric measurements, by Laser Doppler spectroscopy and by Scanning Electron Microscope inspection. The effect is ascribed to the formation of calcium fluoride particles at a very early stage of the reaction, which subsequently act as substrates for heterogeneous nucleation of DCP, thus increasing the number density of DCP crystals and reducing their average size.     

Highly oriented block copoly(ether ester) by solid state extrusion

Summary A segmented block copoly(ether ester) based on poly(butylene terephthalate) as “hard” segments and on poly(oxytetramethylene) as “soft” segments was highly oriented by cold flow extrusion. Upon further stretching of the high modulus material, the crystallites are tilted by 34∘ with respect to the stretching direction and a four point small angle X-ray pattern is obtained.     

New kinetic models for the crystallization of anatase nanoparticles from amorphous titania (TiO2)

In this article, amorphous Titania (TiO₂) nanoparticles were crystallized both in a solution environment and by a dry heat treatment using new kinetic models with the capabilities of more accurately predicting the polymorphic transformation behavior of TiO₂. In these models, both the nucleation and growth processes were simultaneously taken into account. The results were indicative of the Surface Nucleation (SN) of anatase occurring in a hydrothermal treatment process through the phase transformation. Also, both the surface and interface nucleation processes were found to play significant roles in the phase transformation kinetics when dealing with dry heat treatments at low temperatures. The proposed models were advantageous to any other published models, in which the nucleation mechanisms had been incorporated with suitable growth expressions. In other words, no experimental data of a particle size were required to investigate the phase transformation kinetics of TiO₂ nanoparticles in the models presented in this article.     

Spatially‐confined crystallization of poly(vinylidene fluoride)

Nanometre‐sized poly(vinylidene fluoride) (PVDF) particle domains in a confined space were obtained by blending PVDF with excess poly(methyl methacrylate) (PMMA). When these particles were small enough they showed β‐form structure, which was different from the structure of bigger particles or PVDF bulk. However, the β‐form was thermodynamically metastable because it could eventually be transformed to a more stable phase by annealing at a certain temperature. Larger particle domains were of identical phase to the bulk, indicating that small size favours the formation of the β‐form. © 2000 Society of Chemical Industry     

Influence of molecular weight on the crystallization of poly(vinylidene fluoride)

Five samples of Poly (vinylidene fluoride) with a molecular weight from 45,000 to 260,000 were analyzed by means of FT-IR spectroscopy in order to study the influence of molecular weight on the crystallization process. The spectra of films obtained from methylethylketone solutions revealed the prevalence of form II () in the case of lower and of form III () in the higher molecular weight samples. The amount of form III was found to increase as the Mv decreased and as annealing temperatures increased. The deconvolution technique of vibrational spectra allowed detection of small amounts of other forms accompanying the prevailing one.     

Molecular weight dependent of the crystallization kinetics of poly(vinylidene fluoride)

Samples of poly(vinylidene fluoride) with weight average molecular weights between 100,000 to 500,000 were analyzed using differential scanning calorimetry to study the influence of molecular weight on crystallization. Isothermal crystallization from the melt was investigated between 140 and 145°C. Crystallization rates were analyzed in terms of the Avrami equation. Avrami exponents between 3 and 11 were observed depending upon the microstructure, molecular weight, and cooling rate. The midrange molecular weight samples were found to crystallize slower than the high and low molecular weight materials.     

Slow crystallization process from amorphous PbTiO3

The effect of isothermal annealing of amorphous PbTiO₃ at a temperature below the crystallization temperature, T_crys, has been investigated. Time dependent dielectric constant in the annealing process has been observed. From the experimental results, both the existence of the glass transition temperature, T_g, and the steadiness of dielectric constant at room temperature have been established.     

The effect of lanthanum oxide (La2O3) on the structure and crystallization of poly(vinylidene fluoride)

Rare earth polymers, due to their excellent luminescence, fluorescence, laser protective, optical and magnetic properties, have attracted much research attention in recent years. However, little attention has been paid to the effect of rare earths on the structure and crystallization of polymers, which is of important significance in the development of functional polymers. X‐ray diffraction and differential scanning calorimetry were used to investigate the structure and crystallization behavior of a poly(vinylidene fluoride) (PVDF)/lanthanum oxide (La₂O₃) composite. The results showed that the degree of perfection, crystal size, crystallization rate and isothermal crystallization activation energy of PVDF in the composite decreased, compared with pure PVDF. The spherulite nucleation and growth for PVDF and PVDF composite were analyzed in detail using the Lauritzen‐Hoffman equation. The modified Avrami equation and the Mo equation were used to study the non‐isothermal crystallization kinetics. The addition of La₂O₃ did not change the crystal structure and nucleation process for PVDF, but it decreased markedly the crystal growth rate and led to the formation of unstable crystals. This was attributed to the fact that too much La₂O₃ prevented PVDF molecular chains from moving and arranging in an orderly manner into crystals. Copyright © 2010 Society of Chemical Industry     

Helix sense of copoly(ethyl-l-aspartate-benzyl-l-aspartate)

Copoly(ethyl-L-aspartate-benzyl-L-aspartate)s with various compositions were prepared by ethylation of poly (β-benzyl-L-aspartate). Helix sense of copolyaspartates in chloroform solution was examined by optical rotatory dispersion and circular dichroism methods as a function of a degree of ethylation and temperature. Copolyaspartates of ethylation less than 35% exhibited the same left handed helix as that of poly (β-benzyl-L-aspartate), while copolymer of ethylation more than 70% assumed the right handed α-helical conformation like poly (β-ethyl-L-aspartate). Copolymers with the intermediate ethyl content of 35–70% indicated the reversal in helix sense from a right handed to a left handed helix with increasing temperature from ?20° to 60°C. The transition of reversal was reversibly observed and transition temperature rose with the increase of ethylation. In these intermediate copolymers, the mechanism of the reversal in helix sense was examined by circular dichroism and dielectric measurements. Three possibilities were postulated here for the conformation which should arise at the transition temperature of the reversal; First is the random conformation, second is the equimolar mixture of left and right handed α-helices in a chain, and the third is the equimolar mixture of left and right handed α helix chains. No observational circular dichroism spectra characteristic to random coil conformation and no remarkable change in residue dipole moment strongly suggested the third possibility.     

Study of crystallization behavior of neat poly(vinylidene fluoride) and transcrystallization in carbon fiber/poly(vinylidene fluoride) composite under a temperature gradient

The crystallization behavior of poly(vinylidene fluoride) (PVDF) and transcrystallization in carbon fiber (CF)/PVDF composite were investigated under a temperature gradient. The crystallization temperature (T_c) was controlled in the range of 110–180 °C. For neat PVDF, the results showed that exclusive γ phase formed at T_c above 164 °C, but coexisted with α phase at T_c ranging from 137 to 160 °C. The promotion of γ phase to nucleation of α phase at low T_c was observed for the first time. For CF/PVDF composite, a cylindrical transcrystalline (TC) layer formed on the surface of CF when T_c was between 137 and 172 °C. The TC layer was exclusively composed of γ phase at T_c above 164 °C. The hybrid nucleation was dominated by γ phase though some α phase nuclei emerged on the surface of CF when T_c was in the range of 144–160 °C. As T_c decreased, competition between the hybrid nucleation of α and γ phase became more intense. The γ phase nuclei was soon circumscribed by the rapidly developed α phase when T_c was below 144 °C. Furthermore, some α phase nuclei were induced at the surface of the γ phase TC layer, and developed into α phase TC layer when T_c was in the range of 146–156 °C, which resulted in a doubled TC layer of α and γ phase at the interface of the composite. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43605.     

Review of the hydrolysis of iron(III) and the crystallization of amorphous iron(III) hydroxide hydrate

Hydrolysis of ferric solutions leads initially to mono- and dinuclear species which interact to produce further species of higher nuclearity. These polynuclear species age eventually to either crystalline compounds or to an amorphous precipitate (amorphous iron(III) hydroxide hydrate). Amorphous iron(III) hydroxide hydrate is thermodynamically unstable and gradually transforms to α-FeO(OH) and α-Fe₂O₃. These crystalline products form by competing mechanisms and the proportion of each in the final product depends on the relative rates of formation. The master variable governing the rates at which these compounds form is pH. Other important factors are temperature and the presence of additives. Most additives retard the transformation and by suppressing formation of α-FeO(OH) lead to an increase in the amount of α-Fe₂O₃ in the product; some additives also directly promote formation of the latter compound. Metal ions can oftxen replace a proportion of Fe in the α-FeO(OH) and α-Fe₂O₃ lattices. At high enough concentrations they can induce formation of additional phases. Additives may also modify the morphology of the crystalline products.     

Effect of poly(vinylidene fluoride) polymorphic structures on the crystallization behavior of poly(butylene succinate)

The detail information of both α and β form poly(vinylidene fluoride) (PVDF) crystal effect on the crystallization behavior of poly(butylene succinate) (PBS) were systematically studied. The results show that β form PVDF can obviously improve the melt‐crystallization temperature of PBS during the nonisothermal crystallization process. Both crystallization time span and spherulitic size of PBS decrease with the increasing amount of β form PVDF, which enhances the primary nucleation of PBS. But α form PVDF shows no nucleating effect on PBS crystallization, exhibiting as almost unchanged T_c values for α form PVDF‐blended PBS samples. The intrinsic mechanism for the nucleating effect of β form PVDF on PBS was proposed to be the epitaxial crystallization. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40991.     

Solid state properties of thin films of new copoly(azomethine‐sulfone)s

A series of four copoly(azomethine‐sulfone)s have been synthesized and extensively characterized. The chemical design has been thought to obtain semiconducting properties and thermotropic behavior matching the processing of the materials from the ordered melted state, suitable for the spontaneous alignment of molecules in the active layer on a substrate. Prototypes of thin films based on this series of polymers have been electrically and structurally characterized. It was found that the respective polymers show typical semiconducting properties. The thermal properties with particular reference to their mesomorphic state were checked by using differential scanning calorimetry, polarized light microscopy, and thermogravimetric analysis techniques. Morphological characterization was realized by atomic force microscopy and X‐ray diffraction measurements. Some correlations between electrical properties and packing closeness were established. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Melting and crystallization behaviour of the poly(vinylidene fluoride)/poly(monobenzyl itaconate) blends

Summary Melting and crystallization behaviour of blends of poly(vinylidene fluoride)(PVF₂) and poly(monobenzyl itaconate) (PMBzI) have been analyzed as a function of the composition in the range 100-40% PVF₂. Using the Hoffman-Week plot we have not found an equilibrium melting point depression in all the blends studied. However, the addition of PMBzI to pure PVF₂ leads to an increase in its crystallization rate. The results suggest that both polymers are incompatible and that PMBzI acts as nucleating agent in the PVF₂ crystallization.