Crystallization kinetics of isotactic polystyrene from isotactic-atactic polystyrene blends

In an attempt to facilitate a better understanding of the role of noncrystallizable components on the crystallization kinetics, spherulitic growth rates as well as the morphology and melting behavior of isotactic polystyrene in blends with various molecular weight atactic polystyrenes (900 to 1,800,000) over a wide range of concentrations have been studied. The growth rates are uniformly depressed with increasing amounts of atactic diluent. In addition, they are dependent on the molecular weight of the added polystyrene, generally decreasing in the molecular weight ranges between 4800 and 19,800 and between 51,000 and 1,800,000. However, between these two ranges, anomalous growth rates showing a sudden increase are observed, which may be explained by an increase in the entrapment of the noncrystallizable diluent. An explanation based on morphological observations, which showed an increase in coarseness of the spherulites with increasing molecular weight of the added atactic polystyrene, is offered.     

Crystallization of isotactic polystyrene from dilute solutions

The overall rate of crystallization of isotactic polystyrene from dilute solutions, 1% by weight, in trans-decalin and benzyl alcohol was studied as a function of temperature using dilatometry. These solvents were chosen because the dissolution temperatures of crystalline isotactic polystyrene are practically the same in both solvents. The overall rate of crystallization as a function of crystallization temperature showed a maximum in both solvents at about 50°C. At lower crystallization temperatures the rate of crystallization is much lower. The overall rate of crystallization of isotactic polystyrene in benzyl alcohol is far larger than in trans-decalin at the same undercooling throughout the temperature range, which is in apparent contradiction to present crystallization theories. At very large undercooling (T_c lower than about 0°C) the solutions of isotactic polystyrene in both solvents quickly become “rigid” gels. Surface replicas of freeze-etched gels indicate that a fringed micelle type of crystallization takes place at these low temperatures. The transition from folded chain crystallization to fringed micelle crystallization may be due to a stiffening of the polymer chain below about 50°C, with a reduced rotational mobility of the phenyl groups on the chain. If very dilute solutions, below 0.5% by weight, are crystallized at these low temperatures no gels were formed but fibrous crystals are produced which could be observed under the polarizing microscope.     

Crystallization kinetics of isotactic polypropylene blended with atactic polystyrene

Crystallization kinetic parameters, such as spherulitic growth rates, nucleation densities, and Avrami-exponents, have been determined by optical microscopy for isotactic polypropylene blended with atactic polystyrene. It is found that the crystallization of iPP is strongly influenced by the presence of polystyrene. With increasing PS concentration in the blend, the nucleation densities decrease, while the spherulitic growth rates as well as the positions of thermal peaks, measured by DSC, remain independent of sample composition. Due to the formation of interfaces as a consequence of increasing dispersion of polystyrene the nucleation changes from preferentially thermal to athermal.     

Crystallization behavior of single or pauci chain aggregates of isotactic polystyrene

Single and pauci chain aggregates of isotactic polystyrene (i-PS) were prepared by the freeze-drying process from dilute solutions with the concentration from 1×10⁻³ to 2×10⁻⁵ g/mL. It was found by DSC measurements that the melting point of samples gradually shifted to lower temperatures with the decrease of the solution concentration used for sample preparation. As a result, the lamella thickness of bulk samples and the samples prepared by the freeze-drying process from a solution of 2×10⁻⁵ g/mL was 19.3 and 12.6 nm, respectively. At 468.3 K the half crystallization time (t _1/2) of samples freeze-dried from a solution of 1×10⁻⁴ g/mL was about 36 s, which was merely one tenth of that of the bulk sample. In addition, the growth rate of spherulite (dr/dt) of samples prepared from a solution of 2×10⁻⁵ g/mL was faster than that of the bulk sample annealed at 478.3 K. All these results should be attributed to the fewer entanglements in samples prepared by freeze-drying process from dilute solutions, and presented clear evidence for the influence of chain entanglements on the crystallization behavior of polymers. __________ Translated from Chemical Journal of Chinese Universities, 2005, 26(10) (in Chinese)     

Crystallization kinetics of isotactic polystyrene. I. Spherulitic growth rate

The spherulitic growth rate of isotactic polystyrene has been measured in a wide range of temperature by means of a polarizing microscope provided with a hot stage. It was possible to fit the experimental data to theory by choosing a value of 75 for the constant C₂ of the WLF equation. The growth rate parameters were compared with those of polyethylene and polychlorotrifluoroethylene. The slowness of crystallization of isotactic polystyrene is mainly a consequence of the lower mobility of the molecules caused by the bulky phenyl groups.     

Initial crystallization mechanism of isotactic polystyrene from different States

The isothermal crystallization processes of isotactic polystyrene at 160 degrees C from different initial states (quenched glassy state and melt state), i.e., cold- and melt-crystallization processes, have been investigated by infrared (IR) and generalized two-dimensional (2D) IR correlation spectroscopy. It has been found that not only the crystallization kinetics and crystallinity but also the sequential changes of the amorphous and crystalline sensitive bands are quite different for the cold- and melt-crystallization processes. This leads to the conclusion that the physical origins for spinodal decomposition prior to polymer crystallization may be different for different crystallization processes.     

Secondary crystallization of isotactic polystyrene

When isotactic polystyrene (i-PS) is crystallized from the melt or from the glassy state at rather large supercooling an additional melting peak appears on the curve during scanning in a differential calorimeter. The overall rate of crystallization deduced from the total peak areas as a function of crystallization time did not fit the Avrami equation well. When we omit the area of the additional melting peak in the kinetic analysis a much better fit is obtained. We also observed that no lamellar thickening occurs during isothermal crystallization. In view of the low degree of crystallinity of i-PS these results lead to the idea that a secondary crystallization process takes place within the amorphous parts of the spherulites resulting in this additional melting peak on the DSC curve. The large supercooling needed and the increase in peak area with increasing molecular weight make us suppose that intercrystalline links are probably responsible for the additional melting peak of bulk-crystallized i-PS. Electron microscopic studies of surface replicas of i-PS support this view.     

Melting behavior of isotactic polystyrene

The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans-decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature T_c. This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with T_c, yielding an extrapolated value for the equilibrium melting temperature T_c° of 242 ± 1°C as found before. By self-seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of T_c° found by extrapolation. For normal values of T_c and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt-crystallized and solution-crystallized samples, and independent of T_c, but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.     

Conformation analysis of freeze-dried isotactic polystyrene from dilute solutions

Fourier transform infrared spectra of freeze-dried samples of isotactic polystyrene (i-PS) from dilute solutions were analyzed. The 698, 1452, 1493 and 1601 cm⁻¹ bands increased in sharpness and intensity as the solution concentration decreased, indicating that the freeze-dried sample is in a more dilated state. When the freeze-dried i-PS was annealed at 100°C, all aforementioned bands broadened and their heights depressed, accompanied with a significant change of the bands at 981 and 906 cm⁻¹.     

Crystallization kinetics of isotactic polystyrene. II. Influence of thermal history on number of nuclei

The influence of various thermal pretreatments on the nucleation of isotactic polystyrene has been studied quantitatively by dilatometry. A distinction can be made between nuclei still present above the melting point (“resistant” nuclei) and nuclei created by severe supercooling (“induced” nuclei). The number of spherulites formed has been determined for different combinations of supercooling and crystallization temperatures. The results are interpreted in a satisfactory manner by assuming that in severe supercooling induced nuclei are created, which may grow into effective nuclei at higher temperatures. The crystallization of a severely supercooled polymer is completely governed by these induced nuclei, because they outnumber the resistant nuclei by some orders of magnitude. The number of induced nuclei can be decreased by purifying the polymer (removing catalyst residues). When cooled polymer is heated to temperatures just above the melting point, the induced nuclei are destroyed (“reversible melting”), so that only the resistant nuclei, which are few in number, remain. These resistant nuclei govern the crystallization behavior of a polymer which has not previously been cooled. Their number decreases on heating to temperatures far above the melting point.     

Crystallization of oxidized isotactic polypropylene

The effect of processes accompanying thermal oxidation of the polymer on the characteristics of its isothermal crystallization has been revealed. It has been shown that crystallization decelerates with a rise in the degree of PP oxidation. The higher the concentration of functional groups, the stronger the deceleration. The energy of nucleation increases when passing from virgin to oxidized PP samples. The higher the concentration of carbonyl groups accumulated in polymer chains, the more pronounced this effect, although the degradation of the chains must lead to a reduction in this parameter. It has been concluded that the kinetic and thermodynamic parameters of the isothermal crystallization are applicable to investigation of processes accompanying thermal oxidation of the crystallizable polymer.     

Crystallization and melting of the system isotactic polystyrene + poly(2,6-dimethyl phenylene oxide)

From glass transition T_g measurements on isotactic polystyrene (IPS)–poly(2,6-dimethyl phenylene oxide) (PPO) blends, it was concluded that thoroughly annealed, freeze-dried samples, or samples evaporated from solution at high temperature, are homogeneous. Without annealing, the freeze-dried blends show two to three T_g's characteristic of the presence of different phases. The overall crystallization rate of these samples is much higher than that observed with annealed samples. The presence of dissolved PPO in annealed samples reduces the overall crystallization rate and the spherulitic growth rate, compared to IPS. The melting behavior of the blends is influenced by the extent of mixing of both polymers. Without annealing, isothermally crystallized, freeze-dried blends show the same melting behavior as IPS (i.e., multiple melting). In homogeneous annealed samples the rate of reorganization is strongly reduced and multiple melting only occurs at low scanning rate (e.g., 1°C/min). This behavior is influenced by the crystallization temperature and by the composition of the blends. The addition of PPO has no influence on the relation between melting point and crystallization temperature and the same equilibrium melting point is found by extrapolation.     

Small-angle x-ray scattering of isotactic polystyrene

Isothermal crystallization process of isotactic polystyrene at 167°C has been studied by smallangle x-ray scattering. The observed SAXS intensities consist of the twophase lamellar structure component, the density fluctuation, and the foreign particle components. The profile of lamellar structure component remains unchanged during crystallization while its intensity increases with crystallization. The lamellar structure of isotactic polystyrene is investigated on the basis of the interface distribution function. An interface distribution function is obtained from the lamellar structure component after correcting the effect of the finite thickness of boundary regions between crystalline and amorphous phases. In order to obtain the structure parameters, the Gaussian correlation model is used, in which the correlation between the distributions of neighboring crystal and amorphous thicknesses is taken into account. Agreement is satisfactory between the experimental results and the calculations. The structure parameters of isotactic polystyrene are determined for isothermal crystallization at 167°C as follows: the average and the standard deviation of crystal thickness are 40 A and 10 A, respectively, those of amorphous thickness are 70 A and 23 A, and the standard deviation of long period is 31 A.     

Single-Molecule single crystals of isotactic polystyrene

Single-molecule single crystals were grown from amorphous droplets of fractionated isotactic polystyrene. The crystals were analyzed by electron microscopy and electron diffraction. The molecular mass distribution could be matched with a statistical analysis of single-molecule particles (amorphous and crystals). Proof was brought that single molecules of isotactic polystyrene do not reach equilibrium dimensions on crystallization, rather assume the lamellar morphology with chain-folded macroconformation, also known from crystallization of polymolecular crystals. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America. US Government contract No. DE-AC05-840R-21400.

     

Crystallization,thermal behavior,and compatibility in isotactic polystyrene/poly(o-chlorostyrene-co-p-chlorostyrene) 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.     

Miscibility of isotactic polypropylene with atactic polystyrene

The miscibility of isotactic polypropylene-atactic polystyrene blends using dilatometric, TOA, DAT and stress-strain methods were studied. It appears that the blends are heterogeneous, having two glass transition temperatures. However, some interactions between components are evident to change theTgs of their amorphous phases and theTm of isotactic polypropylene crystals. The results are discussed on the basis of the blends' thermal and tensile properties.