Memory effects in isothermal crystallization II. Isotactic polypropylene

The effects of temperature and duration of melting on the rate of isothermal crystallization of isotactic polypropylene were investigated by differential scanning calorimetry (DSC). Crystallization rates were found to decrease with increasing melt temperature and melting time. The results were discussed in the framework of the theoretical model of transient isothermal crystallization developed by the present authors [1]. The results suggest gradual destruction of predetermined nuclei with activation energyE _a=89±7 kJ/mole as a main mechanism of the observed effects.This work and Part I of this research [1] have been supported in part by Research Grant Number PB 1291/52/93/04 from State Committee for Scientific Research (KBN), Poland, and by 40% MURST founds, Italy.     

Determination of nucleation rate in polymers using isothermal crystallization experiments and computer simulation

Within the frame of overall kinetics of crystallization, polymer crystallization is characterized by only three parameters: the initial density of potential nucleiN ₀, their activation frequencyq and the growth rateG. The growth rateG is rather easy to measure, whereas the nucleation parameters are generally unknown. Our purpose is to determine bothN ₀ andq using experimental isothermal two-dimensional crystallizations and computer simulation. Both these parameters are deduced from the rate of appearance of spherulites expressed as a function of the transformed surface fraction. The activation times of the spherulites are deduced from the shape of the boundaries between spherulites at the end of the transformation. When the growth rateG is known, the evolution of the transformed surface fraction is rebuilt using a computer simulation, so that only one observation of the final stage of the crystallization is needed.     

Crystallization of polymers in variable external conditions

A new model of crystallization kinetics in variable external conditions has been developed. The model concerns situations when temperature, pressure, stress, change in time. Compared to earlier models, the present treatment includes transient and athermal effects, proportional to the rate of change of the external conditions. The model can be used for simulation of crystallization in industrial processes (injection molding, fiber spinning, film blowing). The present paper offers general theoretical fundamentals of the model. Applications concerning more specific cases will be published separately.     

Molecular fractionation in isotactic polypropylene during isothermal crystallization

The nucleation frequency of isotactic polypropylene shows for certain molecular weight distributions during isothermal crystallization a “stepwise” development of the nuclei as a function of time. The resulting curve can be fitted by a superposition of exponential functions assuming a separate nucleation density for each nucleation step. The multimodal nucleation is discussed as a consequence of molecular fractionation effects during crystallization. Received: 23 October 1997 Accepted: 12 May 1998     

Crystallization of polymers in variable external conditions IV. Isothermal crystallization in the presence of variable tensile stress or hydrostatic pressure

General equations of crystallization in variable conditions have been applied to isothermal crystallization affected by variable tensile stress or hydrostatic pressure. In a system under stress, the crystallization rate involves relaxational and athermal effects, and is controlled by the normal stress difference and the stress rate. Similarily, pressure effects include the hydrostatic pressure and its rate of change. Received: 1 December 1998 Accepted in revised form: 23 March 1999     

Enthalpy relaxation in the cooling/heating cycles of polypropylene/organosilica nanocomposites: I: Non-isothermal crystallization

Non-isothermal crystallization of the neat isotactic polypropylene homopolymer (PP-0) and of a series of nanocomposites (PNC) containing up to 4.68 vol.% of organosilica was studied in the standard DSC mode during constant-rate cooling from the melt state.Analysis of the nucleation parameters derived from cooling rate dependencies of the temperatures for the onset of crystallization exotherms revealed a slight but systematic increase of the nucleation barrier for lamellar crystallization of PP in the PNC concomitant to stronger restrictions to transport of PP segments across the melt/lamellar crystal interface. The overall crystallization rate data for the PNC were consistent with the assumption of two separate contributions from the initial (unconstrained), and the subsequent (constrained) growth mechanisms, respectively.The obtained results were considered as evidence for the coexistence in undercooled PP melts of the PNC of initial crystal nucleation and growth sites characteristic for the neat PP-0, and the basically different sites (presumably, PP chains anchored by both ends to the surfaces of two adjacent nanoparticles).     

Guaranteed Behavior in Shape Memory Alloys. Short- and long-time effects related to temperature and phase coexistence

A phenomenological approach, in the parent phase of Cu-Zn-Al shape memory alloy, establishes a predictable model (or mathematical equations) relating the dependence of Ms with the temperature over a long period of time (i.e. seasonal or yearly room temperature). High-resolution resistance and temperature measurements vs. time are used. The long time Ms tracks the external room temperature via two temperature dependent time constants. In steady state, the changes in Ms approach17 per cent of the ‘room’ temperature change. The detailed analysis shows the puzzling disappearance of the after quench effects. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of the order of polymer melt on the crystallization behavior: I. Double melting endotherms of isotactic polypropylene

The influence of the order of polymer melt on the subsequent crystallization and melting has been carefully studied. The experimental data show that the order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature. For an iPP sample isothermally crystallized at 130 °C for half an hour, the degree of order of melt is higher when the fusion temperature is lower than about 170.5 °C, hence the lamellae formed in a rapid cooling process are perfect. If the fusion temperature is not higher than 167 °C, some thicker lamellae can exist in the melt. The melting of these unmelted lamellae and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when the fusion temperature is higher than 170.5 °C, the order of the iPP melt decreases greatly; thus, the lamellae formed in the following cooling process are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae also leads to double melting endotherms. Received: June 16, 2000 Accepted: October 16, 2000     

Crystallization of polymers in variable external conditions. III: Experimental determination of kinetic characteristics

In two recent papers [1,?2] a new model of non-isothermal crystallization kinetics has been proposed. We are discussing now the possibility of experimental determi-nation of material characteristics appearing in the model. These include relaxation time, τ, and athermal nucleation function, B_ath. Two kinds of experiments are discussed: isothermal and non-isothermal, with constant cooling or heating rate. The following approach should be valid for polymers characterized by not too high crystallization rates, like polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT, Corterra^®), polypropylene and others. Preliminary experiments on isotactic polypropylene and polyvinylidene fluoride will illustrate possibilities of the suggested experimental procedures.     

Hold-up time in gas chromatography I. New approach to its estimation

A new procedure to reach a good estimate of the hold-up time value (t_M) in gas chromatography is presented. The value of t_M obtained lies close to the experimental retention time of neon, the gas which has shown the shortest retention time in the columns studied. The new method, based on the retention of n-alkanes, is easily applied with any personal computer and may be used with detectors which do not respond to permanent gases. The procedure is reliable, produces very reproducible hold-up times and the value of t_M obtained may be safely used to calculate both chromatographic and thermodynamic parameters.     

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.     

Role of relativistic effects in the electronic structure and chemical bond of lead hexacyanoferrate

The electronic structure of a large fragment of the Pb₂Fe(CN)₆ crystal lattice with the trigonal structure is studied in the framework of a fully relativistic discrete variational cluster method. On the basis of comparing the results obtained with the data of previous non-relativistic calculations we have considered the relativistic effects on the electronic energy spectrum, the charge density distribution, and Pb-N, N-C, and C-Fe chemical bonding.     

Efficiency for unretained solutes in packed column supercritical fluid chromatography. I. Theory for isothermal conditions and correction factors for carbon dioxide

A general theory for efficiency of nonuniform columns with compressible mobile phase fluids is applied to the elution of an unretained solute in packed-column supercritical fluid chromatography (pSFC). The theoretical apparent plate height under isothermal conditions is given by the Knox equation multiplied by a compressibility correction factor f1, which is equal to the ratio of the temporal-to-spatial average densities of the mobile phase. If isothermal conditions are maintained, large pressure drops in pSFC should not result in excessive efficiency losses for elution of unretained solutes.     

Nonempirical Description of the Atmospherically Important Anionic Species. I. Water Cluster Anions

Water cluster anions, comprising up to 20 molecules, are simulated at the second-order unrestricted Møller–Plesset perturbation theory level with 6-31++G** basis set augmented with a floating center of 8 s diffuse functions. Interface structures composed of two to four chainlike or cyclic subclusters found to be most stable among anions of the same molecular size are shown to serve as a reliable restricted model of the hydrated electron. The calculated values of the adiabatic electron affinity of neutral clusters and the vertical energies of electron detachment from anions fit in with n ^–1/3 dependences that provide the corresponding estimates of the bulk water or ice specimens. The radius of a circumsphere containing about 85% of the excess electron density is treated as an effective radius of the excess electron and found to approach 2.5 Å as the size of cluster increases to infinity.     

Calculation of relaxation effects in electrolyte conductance by numerical analysis

The principles employed for calculating relaxation effects by numerical analysis with a computer are outlined. Results are expressed as tables of ratios which can be incorporated into a general computer program for application to experimental conductivity measurements. As an example of such applications, data for aqueous solutions of MgSO₄ have been analyzed.     

Ageing effects and internal stresses in quenched unfilled and clay-filled high density polyethylene

The stress relaxation and creep behaviour of high density polyethylene (HDPE), unfilled or filled with clay particles, were measured after different ageing times after quenching from 120 °C. The measurements were performed at room temperature in the uniaxial extension mode. Ageing time had a pronounced influence on the viscoelastic properties, e.g. the creep curves shifted to longer times with increasing ageing time. The internal stresses, as evaluated from stress relaxation data, were found to increase markedly when the ageing period was extended, and it was suggested that this behaviour may be associated with relaxation of thermal residual stresses. It was furthermore suggested that the change in residual stresses, in addition to the physical ageing process, could also affect the ageing behaviour of HDPE, i.e. the change in viscoelastic properties with ageing time. Clay addition changes the ageing behaviour of HDPE, which could be attributed to a change in the internal stress dependence of the ageing time and/or to a difference in the physical ageing process. The effects of a surface treatment on the ageing of HDPE-clay composites are also evaluated and discussed.     

Stereoelectronic effects on type I 1,2-dyotropic rearrangements in vicinal dibromides

The effects of different substituents on type I-dyotropic rearrangements of open-chain and cyclic 1,2-dibromo hydrocarbons have been studied by means of DFT calculations. The activation energy (E(a)) of this transformation decreases with the pi-donor ability of the substituent attached to the reacting ethylenic system. This is due to donation of electronic density by conjugation or hyperconjugation. This donation leads to longer C--C and C--Br bond lengths in the corresponding four-membered transition states (TSs). Linear relationships between the E(a) and either the sigma(p) Hammett substituent constants and the C--C bond length of the TSs were also found. In all cases, the processes have a high value of synchronicity, which is mainly independent on the substituents. A model based on the second-order perturbational analysis for one ethylene unit with two apical bromine radicals accounts for all the computed results.     

Metastable effects on martensitic transformation in SMA (I) recoverable effects by the action of thermodynamic forces in parent phase

Summary The applicability of SMA requires a matching of the properties with the technological needs. The metastability effects and the phase coexistence can produce, via atomic diffusion, some changes in the expected properties against time. The careful analysis of the external thermodynamic forces (stress) on the parent phase of a Cu-Al-Zn single crystal alloy establishes a proportional minor change in critical transformation stress (near 1 per cent of the external stress). Via the Clausius-Clapeyron coefficient, the stress effects show a similar behaviour but faster than the room temperature effects on the transformation temperature M_S. In parent phase, the effect of the thermodynamic force regarded as an M_S shift lies between 10 to 15 percent of temperature change.     

Initial stages of soot formation in thermal pyrolysis of acetylene. II. A model for the incipience and growth of soot particles

A model is developed which describes simultaneously occurring processes of the initial hydrocarbon pyrolysis, nucleation, surface growth, and coagulation of soot particles. The model permits one to find the size distribution of the primary soot particles up to size 30–40 nm using a relatively small set of equations. The computed time dependence of soot particle concentration agrees satisfactorily with available experimental data. The existence of two limiting stages of the soot formation is revealed.