Effect of Selected Parameters on Crystallization of Copper Sulphate Pentahydrate

Different batch cooling modes (quick and slow cooling with constant cooling rate, program-med cooling with nearly constant supersaturation) of copper sulphate aqueous solutions have been studied in order to find best conditions for the investigation of the effect of additives on crystallization. Three types of additives (solvents, ionic substances and surfactants) have been used and their effect on crystal size, habit and yield was studied.     

Crystal Growth and Structural Refinements of the Y2SiO5, Y2Si2O7 and LaBSiO5 Single Crystals

The laser crystals of chromium doped yttrium oxyorthosilicate (YSO) were grown from the melt by Czochralski technique. The crystals of YSO, yttrium pyrosilicate and stillwellite-like lanthanum borosilicate were obtained from high temperature solutions. Lithium and potassium di- and trimolybdates were used as fluxes in the growth experiments on silicate crystals. In the case of borosilicate crystals, the choice of fluxes was based on the potassium trimolybdate with an excess of potassium fluoride. The composition of grown crystals was studied by electron microprobe analysis, and structural characteristics were determined for all the single crystals.     

The Characteristics of the Mixed Crystals of the K2SO4-(NH4)2SO4-H2O System at 298 K

Equilibrium studies of the K₂SO₄-(NH₄)₂SO₄-H₂O system at 298 K have been conducted. The isotherm of solubility and the curve of distribution have been mathematically described. The parameters of the crystal lattice and the enthalpy of crystallization of the [K_x(NH₄)_1-x]₂SO₄ solid solutions as a function of their composition have been presented.     

Effect of self-seed crystal structure on growth of polymorphs in poly(butylene 2,6-naphthalate): A cross-nucleation study

Cross-nucleation between different crystal polymorphs is a particular, self-seed assisted type of heterogeneous nucleation, where a fast-growing polymorph nucleates at a pre-existing crystal surface of another polymorph. Here, we present a study on cross-nucleation between different crystalline phases of poly(butylene 2,6-naphthalate) (PBN), employing hotstage polarized-light optical microscopy and temperature-resolved wide-angle X-ray scattering as analysis tools. PBN forms α-crystals at relatively low temperature and β′-crystals at rather high temperature, with cross-nucleation experiments designed such to first obtain few α- or β′-seed crystals (mother phase) which then are transferred to higher or lower temperature, respectively, to monitor the continuation of the crystallization process and possible growth of the other polymorph. In case of cooling β′-crystals to lower temperature where typically α-crystals form in the non-seeded isotropic melt, β′-crystals nucleate growth of α-crystals, following many examples of cross-nucleation in the literature. In contrast, if low-temperature-generated α-crystals are heated to a temperature where β′-crystals form in a non-seeded melt, the cross-nucleation efficacy is reduced as, beside growth of cross-nucleated β′-crystals, also growth of the mother phase is observed. This unexpected result demonstrates the importance of the structure of the nucleating substrate and the interplay between kinetic and thermodynamic aspects of crystal growth.     

Crystallization of side chains in copolymers of ethylene and 1-alkenes

The phase structure of random copolymers of ethylene and ethylene-d₄ with 1-octadecene and other 1-alkenes has been investigated. CPMAS ¹³C NMR spectra show that a fraction of the central sections of C₁₆H₃₃ side chains in ethylene-d₄ copolymers are in ordered environments at 298 K. They give rise to resonances from 32.9 ppm to 33.8 ppm, which show that they are in trans conformations; T_1C values for this group of resonances range from 1 s to 7 s. The remaining side chains are in an amorphous environment, the internal methylenes having a chemical shift of 30.8 ppm and a T_1C close to 0.4 s. A Raman band at 1062 cm⁻¹ in the spectrum of an ethylene-d₄-1-octadecene copolymer is consistent with partial crystallization of side chains. Some side-chain crystallization also occurs in a 1-tetradecene copolymer. X-ray diffraction studies suggest that smaller side chains do not crystallize to a significant extent at 298 K. © 1996 John Wiley & Sons, Inc.     

A continuum model for the dynamics of flow-induced crystallization

A model for flow-induced crystallization is developed which is based on ideas from the theory of strain-induced crystallization, coupled with an irreversible thermodynamic formalism based on the continuum Hamiltonian Poisson Brackets. The latter allows accounting for the changing energetics during simultaneous flow deformation and extended-chain crystallization. Input parameters to the model include the molecular relaxation time, a crystallization parameter, and the molecular weight. Calculations of the crystallization rate, chain elongation, stress, and birefringence are presented for a variety of flow kinematics and flow histories, including transient processes following cessation of flow. Induction times based on a reasonable choice for the induction crystallinity follow experimentally observed trends reported in the literature. © 1996 John Wiley & Sons, Inc.     

The competition between crystallization and demixing in polymer blends. IV. Detection of composition inhomogeneities around growing spherulites

In polymer blends of an amorphous and a semicrystalline component, the crystallization kinetics and the resulting morphology are heavily determined by the diffusion ability of the whole chains and by the dwelling site of the amorphous polymer. Depending on the relative rates of spherulite growth and chain diffusion, radial composition profiles around the growing spherulites and a gradual increase of the melt bulk composition can develop. The resulting change in composition, particularly at the crystallization front, causes a corresponding temporal variation of the spherulite growth rate. In the present article, two experimental techniques are introduced to prove the existence and to determine the course of these concentration profiles. They are based on the composition dependences of the spherulite growth rate and the number density of primary nuclei. Their efficiency is demonstrated by measurements on PVDF/PEA blends. The blend composition at the crystal growth front was found to change by absolute 25%, and the width of the profile can amount to up to 70 μm. © 1996 John Wiley & Sons, Inc.     

Isothermal crystallization kinetics of poly(ethylene terephthalate) in blends with a liquid crystalline polyester (Vectra A)

The isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) in blends with a fully aromatic liquid crystalline copolyester (Vectra A) were studied with differential scanning calorimetry. PET crystallization rates decreases with increasing Vectra fractions in the blends, and the percentage of PET that is crystalline also decreases with increasing Vectra. The equilibrium PET melting temperature for blends containing 40% or more Vectra is unambiguously below that of pure PET. Attenuated total reflectance Fourier-transform infrared spectroscopy measurements indicate that PET/Vectra transesterification does not take place. The results are consistent with a scenario based on prior NMR data in which there is some interphase mixing between the liquid crystalline and flexible polymers and an increase in the fraction of gauche conformers in the PET.     

含间位聚醚酮醚酮酮的合成与结晶

聚芳醚酮类聚合物因其综合性能优异而在高技术领域得到广泛应用．这些高聚物的主链大都为全对位连接,使其熔点较高以至加工难度增大．如果在聚合物主链中引入间位结构,则可在对玻璃化转变温度影响较小的情况下降低熔点来改善加工条件［１］．含间位聚醚酮醚酮酮（ＰＥＫＥＫｍＫ）也是其中一种．本文主要研究ＰＥＫＥＫｍＫ的合成、基本物性与结晶行为．样品的合成与制备：单体４,４′双（对苯氧基）二苯甲酮按文献［２］方法合成．聚合物参照文献［３］合成．将粉末样品在油压机上熔融后快速取出投入冰水中淬火得无定形样品,或将熔融…     

烷基取代聚噻吩的非等温结晶动力学

根据ＤＳＣ测得的数值,采用Ｊｅｚｉｏｒｎｙ ,Вороховский和由作者实验室提出的一种新方法研究了十二烷基取代聚噻吩（Ｐ３ＤＤＴ） 和十八烷基取代聚噻吩（Ｐ３ＯＤＴ） 的非等温结晶过程,并应用Ｋｉｓｓｉｎｇｅｒ 法求取其结晶表观活化能ΔＥ,探讨了不同烷基取代基团对结晶过程的影响．Ｐ３ＤＤＴ 应用Ｊｅｚｉｏｒｎｙ 和Вороховский法描述时在结晶后期均发生偏离现象,而作者提出的新方法描述时则得到较好的线性关系．求得Ｐ３ＤＤＴ 的ΔＥ 为１８４－７９ｋＪ／ｍｏｌ,Ｐ３ＯＤＴ 的ΔＥ 为２４６－９３ｋＪ／ｍｏｌ,比较结晶表观活化能数值可知,Ｐ３ＤＤＴ 比Ｐ３ＯＤＴ 更易结晶．