Nonintegral and integral folding crystal growth in low-molecular mass poly (ethylene oxide) fractions. III. Linear crystal growth rates and crystal morphology

Correlations of the linear crystal growth rates with the change in crystal morphology for poly (ethylene oxide) (PEO) (MW 3000) and α,ω--methoxy-poly (ethylene oxide) (MPEO) (MW 3000) fractions have been established over a wide range of supercooling (ΔT = 25 K). Two linear crystal growth rates were measured, namely, the linear crystal growth rate of spherulites or hedrites and the lateral (linear) crystal growth rate of single lamellar crystals along different crystalline planes (below ΔT = 11–12 K). At a low supercooling of 5 K, the crystal growth rate of the MPEO fraction passes through a minimum. Subsequently, the rate increases abnormally and reaches a maximum at even lower supercooling. This crystal growth retardation has been attributed to the competition between differing chain conformations during crystal growth. In this case, particularly, these conformations are the extended chain and the once-folded chain conformations. This retardation is not observed in the PEO fraction, since in this low supercooling region any once-folded chain conformations formed during crystal growth are of the double-layer lamella type. The change in morphology in this region supports this judgement. Above ΔT = 7.6 K, the crystal growth behavior in these two fractions can be described by the present nucleation theory on flat crystal growth faces. Below that ΔT, however, different crystal growth mechanisms can be clearly seen. This may be associated with the crystal growth on the highly serrated growth faces in these two fractions.     

Crystal growth of polyethylene from dilute solution: Growth kinetics of {110} twins and diffusion-limited growth of single crystals

We have studied the growth kinetics of {110} twins and single crystals of polyethylene in dilute solution of tetrachlorethylene. In terms of {110} twins, we succeeded in obtaining twins without {100} sectors, using a relatively high molecular weight fraction M_w > 10⁴. It is confirmed that the growth is enhanced at the reentrant corner of the twins, and the enhanced growth face inclines to the {110} face because of consecutive generation of steps at the corner. These facts are strong evidence for nucleation-controlled growth of single crystals. The growth rates and obliquity are measured at various supercoolings and concentrations. From consideration of kinetics of steps on the growth face, the following rates and velocity are independently determined from the experimental data: nucleation rate on a flat face, velocity of step propagation, and generation rate of steps at the reentrant corner. The supercooling dependence strongly supports regime II growth. The results on concentration dependence show that the velocity of steps is proportional to concentration over the whole range examined, and the nucleation rate is independent of it in the usual range and becomes proportional to it in the lower range. This concentration dependence of nucleation rate is attributed to the density of adsorbed polymer on the growth face. From this evidence, it is suggested that the rate of travel of steps is limited by volume diffusion of solute polymer, whereas the growth face is saturated with adsorbed polymer at ordinary concentrations. This contradictory situation could be explained by the hypotheses that the saturation density is rather low and that surface diffusion of adsorbed polymer is much slower than volume diffusion of solute polymer. The lower limit of the rate of folding is also determined for the first time from the velocity of step propagation. As regards the single crystals, it is found that the habit maintains a lozenge shape with sharpened points, even at very high supercooling (δT < 50°C) if the concentration is very dilute. Diffusion-limited growth is verified for the first time at the higher supercoolings, where the growth rate is almost independent of supercooling. The growth rate becomes almost equivalent to the velocity of steps determined in the experiments with twins, and this fact will support the accuracy of the evaluation of the step velocity. The order of magnitude of the growth rate obtained agrees with the value which is calculated from the balance between the flux of solute polymer to the growth face and the rate of growth of single crystals.     

Isotactic poly(butene‐1) trigonal crystal growth in the melt

Crystal growth of the trigonal form of isotactic poly(butene‐1) (it‐PB1) was successfully observed in the melt at atmospheric pressure. The growth rate of trigonal crystals was obtained by in situ optical microscopy. It is one hundredth that of it‐PB1 tetragonal crystals. The growth rate of trigonal crystals, as well as that of tetragonal crystals, shows supercooling dependence derived from the nucleation theory. The value of the kinetic constant K of trigonal crystals is about 3.3 times larger than that of tetragonal crystals. The value of the pre‐exponential factor G₀ of trigonal crystals was found to be 41 times as large as that of tetragonal crystals. The difference between these K values can be attributed to the conformational entropy of the ethyl side groups in a nucleating stem. The discrepancy found in the values of G₀ could be explained by introducing pinning and nucleation barriers, which originate from the crystal thickness δl_c, which does not depend on the crystallization temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 684–697, 2007     

An isochronous decoration method for measuring linear growth rates in polymer crystals

The long spacing l of lamellar crystals of linear polyethylene increases with the crystallization temperature T_c. For low degrees of supercooling, the ratio Δl/ΔT is around 0.5 nm K^?1 for PE single crystals obtained from solution in xylene. In the restricted situation where only conduction in the crystallization vessel is involved, a heat transfer analysis shows that about 20 s is needed to change by 5 K the crystallization temperature T_c in a cylindrical vessel of 1.5 mm radius. Such rapid change of the crystallization temperature induces a sharp increase or decrease of the thickness of the single crystals. After conventional shadowing with palladium–gold alloy, the steps on the crystals are observed by conventional bright-field electron microscopy. A pioneering work was performed in this way by Bassett and Keller in 1962. Our technique allows one to determine both the shape and the dimensions of single crystals or twinned crystals of polyethylene as a function of the time of crystallization, and therefore give the quasi-instantaneous growth rates at various times.     

Nucleation-controlled growth in polyethylene single crystals: Growth and shape of {100} sectors

The nucleation rate and propagation rate of steps on the {100} faces of polyethylene crystals have been determined. For single crystals, under conditions where the width of the {100} sectors remains constant during growth, it is confirmed that the growth is in regime I or the crossover region between regime I and II. In {110} twinned crystals, the {100} sectors are well developed and the width increases linearly with time; therefore, the growth in the twins must be in regime II. It is shown that the differing growth regimes of {100} faces in single crystals and twins allow the independent determination of the nucleation rate and the propagation rate of steps. The nucleation rate and propagation rate of steps on the {100} faces were determined from measurements of the constant width of the {100} faces in single crystals and the growth rate of the {100} faces in single crystals and twins. The observed rates show abnormal dependence on supercooling and concentration. The results are attributed to a weaker dependence of the constant width of {100} sectors on supercooling and concentration than predicted.     

Fast growth rates of polyethylene single crystals grown at high temperatures and their relevance to crystallization theories

The rates of growth of polyethylene single crystals grown from dilute solution in hexadecane and tetradecanol have been measured over the temperature range T_c = 98–120°C by following the change in turbidity during crystallization of a suspension of crystals of known shape and final size. The rates decrease similarly with T_c in each solvent, but for a given supercooling crystals grow much faster in tetradecanol where the corresponding crystallization temperature is higher. Similarly, the rates are much higher in hexadecane than those previously reported from xylene at equivalent supercoolings but lower T_c. Changes in the corresponding crystal morphologies as T_c is raised are quantified in terms of the axial ratio and the degree of curvature of the nominally {100} faces, both of which increase with T_c. The results can be interpreted as showing a transition from regime I to regime II growth in both solvents, which agrees both qualitatively and quantitatively with the predictions of the nucleation-based kinetic theories. Such a transition has never before been reported for solution crystallization. Using this analysis, reasonable values are obtained for the crystal side-surface energy σ of 7.4–7.5 erg cm^?2 and for the regime I substrate length L of 0.14 μm. No correlation is found between crystal morphology and growth rate and there are no discontinuous changes in morphology at the proposed transition points. The occurrence of curved crystal edges raises the fundamental issue of how to reconcile noncrystallographic growth surfaces with nucleation-controlled growth. A new approach to polymer crystal growth based on equilibrium surface roughening, which does not require nucleation, is therefore very pertinent in this respect and this is discussed.     

Extended-chain crystals. II. Crystallization of polyethylene under elevated pressure

A report on crystallization of polyethylene at elevated pressures to an extended-chain morphology is presented. The crystals have been characterized by electron microscopy and density determination. Pressure, supercooling (temperature), and crystallization time have been varied to find the best conditions for production of perfect crystals. At 10–30°C supercooling completely crystallized polyethylene was obtained between 4.5 and 7 kb crystallization pressure in 1–8 hr. Analysis of fracture surfaces of samples crystallized for different lengths of time shows an increase in size and number of crystal lamellae and an improvement of extended chain crystals in the early stages of crystallization. A further improvement of the less well crystallized material between the lamellae occurs after 15 min of crystallization time.     

Crystal structure and morphology of phenyl‐capped tetraaniline in the leucoemeraldine oxidation state

A series of crystals of phenyl‐capped tetraaniline in the leucoemeraldine oxidation state were obtained at different isothermal temperatures and were observed directly under transmission electron microscope. The crystals obtained at higher temperatures exhibit more perfect structures than those obtained at lower temperatures. Both the lamella thickness and the crystal size increase with crystallization temperature. The tetraaniline is apt to form larger scale crystals under lower degree of supercooling. However, their crystal structures keep steady with the crystallization temperature. The tetramer was found to adopt a monoclinic lattice with unit cell parameter of a = 13.93 Å, b = 8.82 Å, c = 23.20 Å, and β = 95.03°, as determined using electron diffraction tilting method combined with wide‐angle X‐ray diffraction experiment. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 764–769, 2006     

High-pressure phases in polymers. I. Spherulitic growth kinetics in cis-polyisoprene

This is the first paper in a series dealing with the crystallization of high polymers at elevated pressures. Through an extension of the thin-film technique using osmium tetroxide staining, the measurement of lamellar growth kinetics at high pressures as well as the observation of micromorphological changes has proved possible. Three different morphological species of cis-polyisoprene have been detected in the pressure–temperature plane. A review is given of the conditions governing their formation and the growth kinetics of lamellar crystals are discussed in detail. Secondary nucleation theory is obeyed, the major effect of pressure being to alter the kinetics through the thermodynamic transitions. Variations in lamellar thickness with pressure may be explained simply in terms of the elevation by pressure of the equilibrium melting point.     

Experimental criterion for the crystallization regime in polymer crystals grown from dilute solution: Possible limitation due to fractionation

By integration of equations previously derived by Frank, the growth rate of polymer crystals is shown to be dependent on their size, provided that the persistence length L_p or the kinetic length L_k = (2g/i)^1/2 are significantly larger than the primary nucleus. A new method of decorating the fold surface (isochronous decoration) allows the measurement of the quasi-instantaneous growth rate of very small crystals obtained from dilute xylene solution of a sharp polyethylene fraction of moderate molecular weight (M_w = 17,000, M_w/M_n = 1.11). Although the theory predicts that the growth rate increases with the size of the crystals as long as its dimension is smaller than the persistence length and/or the kinetic length, such an increase is not observed experimentally with the sharp PE fraction presently used. Therefore it appears that both the kinetic length and the (hypothetical) persistence length are beyond the resolution limit of electron microscopy and that crystallization occurs in the polynucleation regime. An upper bound is obtained for the rate g at which a locally new layer spreads in two directions on the substrate. The rate is lower than is estimated by the commonly Accepted theories. These theories lead also to an abnormally high value for the lateral surface free energy. The possibility that the observed initial linearity of the growth-rate curve may results from a balance of opposite effects (an increase with the size of the crystals on the one hand, a decrease with decreasing concentration and possible fractionation on the other) is thoroughly examined and ruled out. In fact, it must be stressed that at the early beginning of crystallization, negligible parts of the sample are crystallized and it is only at the end of crystallization that these effects appear. The fall in the growth rate as crystallization ends is due neither to progressive exhaustion of the solution alone nor to a depletion of the concentration by diffusion for this sharp fraction of low-molecular-weight PE. The major effect comes from fractionation. This segregation of the various molecular weights is predicted on the basis of a simple model and is verified by gel permeation chromatography (GPC). The fact that in such a sharp fraction significant fractionation occurs precludes any accurate determination of the supercooling and of the concentration of the polymer actually crystallizing. Subtle differences in the molecular weight distributions may result in significant variation of the growth rate. In conclusion, as the data used in the first part of this work were obtained with only a small percentage of the dissolved polymer sample crystallized, the observed constancy of the growth rate does not result from mutual compensation of opposite effects, and our conclusions about crystallization regime, order of magnitude of the kinetic and persistence lengths, and value of the rate of lateral spreading of a secondary nucleus are well founded.     

Impact of Nanosilicates on Poly(vinylidene fluoride) Crystal Polymorphism: Part 2. Melt-crystallization at Low Supercooling

Nanocomposites of poly(vinylidene fluoride) (PVDF) filled with Lucentite STN^TM organically modified silicate (OMS) were investigated upon melt-crystallization at temperatures near its melting point (i.e., at low supercooling temperatures). Previously, we showed that the addition of extremely small amounts of OMS into PVDF causes the polar beta phase formation in cold-crystallized samples, and causes polar gamma phase formation in melt-crystallized samples under high supercooling. The current study focused on the impact of OMS on polymorphic behavior of PVDF crystallized from the molten state, or annealed, at low supercooling temperatures. Nanocomposites with 0–4.0 wt% concentration were prepared from solutions. The existence of α or γ phase was verified by using Fourier transform infrared spectroscopy, wide-angle X-Ray scattering or Differential Scanning Calorimetry (DSC). Morphology of α- and γ-spherulites was observed by polarizing optical microscopy (POM). In annealed PVDF/OMS nanocomposites, gamma crystals were observed to dominate at all clay compositions except 0.01 wt%. DSC and POM data show that two types of gamma crystals, γ and γ’, exist when PVDF/OMS nanocomposites were annealed.     

A Study of Ca-Mg Silicate Crystalline Glazes ——An Analysis on Forms of Crystals

In the study on Ca-Mg silicate crystalline glazes, we found some disequilibrated crystallization phenomena,such as non-crystallographic small angle forking and spheroidal growth, parasitism and wedging-form of crystals, dendritic growth, secondary nucleation, etc. Those phenomena possibly resulted from two factors:(1) partial temperature gradient, which is caused by heat asymmetry in the electrical resistance furnace,when crystals crystalize from silicate melt; (2) constitutional supercooling near the surface of crystals. The disparity of disequilibrated crystallization phenomena in different main crystalline phases causes various morphological features of the crystal aggregates. At the same time, disequilibrated crystallization causes great stress retained in the crystals, which results in cracks in glazes when the temperature drops. According to the results, the authors analyzed those phenomena and displayed correlative figures and data.     

A Study of Ca-Mg Silicate Crystalline Glazes--An Analysis on Forms of Crystals

In the study on Ca-Mg silicate crystalline glazes, we found some disequilibrated crystallization phenomena,such as non-crystallographic small angle forking and spheroidal growth, parasitism and wedging-form of crystals, dendritic growth, secondary nucleation, etc. Those phenomena possibly resulted from two factors:(1) partial temperature gradient, which is caused by heat asymmetry in the electrical resistance furnace,when crystals crystalize from silicate melt ; (2) constitutional supercooling near the surface of crystals. The disparity of disequilibrated crystallization phenomena in different main crystalline phases causes various morphological features of the crystal aggregates. At the same time, disequilibrated crystallization causes great stress retained in the crystals, which results in cracks in glazes when the temperature drops. According to the results, the authors analyzed those phenomena and displayed correlative figures and data.     

Effect of poly(butylene succinate) on the morphology evolution of poly(vinylidene fluoride) in their blends

The effect of PBS on the morphological features of PVDF has been investigated by optical and atomic force microscopies under various conditions.It was found that neat PVDF forms largeγform spherulites with extraordinarily weak birefringence at 170℃.Adding 30%PBS makes PVDF exhibit intrigued flower-like spherulitic morphology.The growth mechanism was explained by the decrease of the supercooling and the materials dissipation.Increasing the PBS content to 70%favors the formation of ring banded spherulites.Temperature dependent experiments verify theα→γphase transition occurs from the junction sites of theαandγcrystals,while starts from the centers ofαspherulites in the blends.Ring banded structures could be observed in neat PVDF,70/30 blend and 30/70 blend when crystallized at 155℃,withoutγcrystals.The band period of PVDFαspherulites increases with crystallization temperature as well as the amount of PBS content.At 140℃,spherulites in neat PVDF lose their ring banded feature,while coarse spherulites consisting of evident lamellar bundles could be found in 30/70 blend.     

Reinforcing thermosets with crystallizable solvents

This study demonstrates an approach to generate reinforcement in thermosetting polymers through crystal growth of crystallizable solvents. Emphasis is to identify the reaction conditions, which lead to suitable reinforcement in selected compounds. Crystallization behavior and miscibility of dimethylsulfone (DMS) in diglycidylether of bisphenol‐A epoxy monomer was investigated. Small angle laser scattering and optical microscopy were utilized to monitor phase separation and crystallization of DMS at different isothermal conditions during the cure process. It is shown that DMS crystals grow anisotropically to form faceted geometries and demonstrate possible structures to anchor into the epoxy matrix. The growth mechanism and the agility of crystals are shown to be affected by the cure reaction as well as depth of supercooling. A completely cured sample with 15 wt % DMS shows a broad map of rich morphologies from nanoscale particles to uniformly distributed macroscale, discontinuous fiber‐like crystals generated only by altering the curing conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 840–849, 2010     

The phase morphology and crystal growth habits of DNA fractions. Optical microscopy and light scattering

Polydisperse DNA of reasonable molecular weights was prepared from a mammalian source via sonication and fractionation. A method for characterizing the molecular weight using gel electrophoresis is described. Quiescent crystallization was studied in thin films of one of the fractions induced by rapidly changing the hydration state isothermally. We report the occurrence of the semicrystalline nature of DNA. The crystal growth occurring via aggregates is best described as sheaves and spherulites from DNA gels in the relative humidity range (RH) corresponding to A-DNA. These habits exhibit primary nucleation and secondary growth, which closely resemble those of melt-crystallized, synthetic macromolecules and, in a follow-up report, will be shown to be lamellar in nature. Small, needle-like crystals are observed for B-DNA hydration levels, and are unstable at lower hydration levels. A transformation from needle to lamellar crystals can occur, even when the primary nucleation of lamellar forms is otherwise absent at that hydration level, through a cylindrical phase exhibiting selective reflection of colored bands. The hydration level plays, in part, the role of the supercooling in this system and the long-known hysteresis in the formation and dissolution of the A-DNA (crystals) can now be viewed in light of those factors known to operate in semicrystalline systems. A morphological phase diagram is developed and is in accord with the known physical evidence. Because this preparation and these morphological observations are without precedence, substantial detail into methodology is included for this first article in the series. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1843–1854, 1997     

Gelation-crystallization in isotactic polystyrene solutions and its implications to crystal morphology,to the origin and structure of gels,and to the chemical homogeneity of polyolefins

As part of a wider study on the crystallization of isotactic polystyrene solutions it was observed that at sufficient concentrations (> 3–5%) gelation sets in below a certain (very high) supercooling in competition with the usual single crystal formation which in itself produces turbid suspensions. It was established that gelation is a form of crystallization (mode A) which must be of fringed micellar type to provide the connectedness as opposed to the chain folded lamellae (mode B) which gives rise to discrete particles. The gel crystals (A) display sharp melt endotherms and produce distinct x-ray diffraction patterns both of which, however, differ decisively from those provided by crystals B, a distinction which can be preserved even after removal of the solvent. The melting points of A are significantly lower than those of B and the x-ray diffraction patterns of A are incompatible with the recognized structure of polystyrene (3₁ helix) possessed by B; they point to a broadly planar zig-zag arrangement of the chain. This strongly suggests that we have blocks of chemically distinct sequences which could be syndiotactic or head-to-head tail-to-tail (presently with substantial support for the latter) which is responsible for the gel forming crystallization. However, so far the C¹³ nuclear magnetic resonance (NMR) results do not provide the evidence for these distinct species but explanations for our observations on any other basis seem to lead to unsuperable difficulties from other points of view. Consequently, the paper is left open ended with the possibilities discussed. Amongst these the existence of a very few but long, chemically distinct sequences seems most attractive. The wider implications of the facts as they stand for crystal morphology (fringed micelles versus lamellae), for the origin and structure of gels in general, for the crystallization of block copolymers and for issues relating to chemical homogeneity (tacticity, head-to-head tail-to-tail) are discussed and preliminary effects are quoted which indicate that these issues may also be relevant to the usual atactic polymers.     

Thermal analysis of paraffins as model compounds for polyethylene

The n‐paraffins C₅₀H₁₀₂, C₄₄H₉₀, and C₂₆H₅₄ were analyzed with standard and temperature‐modulated differential scanning calorimetry. Crystallization and ordering from the melt to the condis phase showed practically no supercooling. These observations were confirmed with hot‐stage microscopy and a melting‐point apparatus. Only the organization of the C₂₆H₅₄ condis crystals to fully ordered crystals showed a supercooling of 4.0 K. The measurement of the apparent reversing heat capacity with a 0.05‐K modulation amplitude revealed that the melting of C₅₀H₁₀₂ was completed within 1.0 K and the isotropization of C₂₆H₅₄ was completed within less than 0.6 K, but 62–78% of the total transitions occurred over a much narrower temperature range of 0.1 K or less. The link to polyethylene was made with fractions of the masses 15,520, 2150, and 560 Da. The 560‐Da sample corresponded to C₄₀H₈₂ and showed also almost no supercooling, whereas the others, with folded and extended‐chain crystals, supercooled by about 10 K. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2810–2822, 2000     

Morphology of polypropylene crystals. I. Dilute solution-grown lamellar crystals

The dependence of crystalline morphology of isotactic polypropylene crystallized from dilute solutions on its molecular weight and growing conditions and the mechanism of crystal growth were studied by electron microscopy and electron diffraction. Lathshaped lamellar crystals 150–300 A. in thickness are obtained from fractionated polypropylene powders of M _w (average molecular weight) = 600,000 and 240,000, but not from the samples of M _w = 82,000 and 44,000, by means of isothermal crystallization at 130°C. for 20 hr. in dilute α-chloronaphthalene solution (0.005 wt.-%). Precipitation of the fractionated polypropylene sample of M _w = 82,000 from a dilute solution of carbitol gives typical dendritic crystals under the same isothermal crystallizing conditions as mentioned above. The mode of chain folding in these crystals based on the orientation and the crystal structure of the lamellar crystals agrees with that proposed by Sauer, Morrow, and Richardson. From the morphological observations, the mechanism of growth pertinent to polypropylene lamellar crystals is presumed to be as follows: fibrils at first aggregate, then the molecular chains are folded to form small lamellae, and then these small lamellae accumulate compactly to grow to large, lath-shaped, lamellar crystals.     

Formation of crystal nuclei near critical supersaturation in small volumes

This work deals with the nucleation of crystals in confined systems in response to the recent high interest in research on crystallization in emulsion and microemulsion droplets. In these confined systems, crystallization often occurs at high supercooling; thus, nucleation determines the overall crystallization process. A decrease in the volume of the confined mother phase leads to the higher supercooling needed for the phase transition. We have numerically solved kinetic equations in order to determine the conditions under which the first crystal nuclei are formed by homogeneous and heterogeneous nucleation from supercooled melt and supersaturated solution, depending on the volume of the mother phase. Supersaturation (or supercooling) increases with decreasing volume of the mother phase. The nucleation barrier depends linearly on the logarithm of volume of the mother phase in all cases under consideration, as follows from the numerical solution of kinetic equations.