Solution grown isotactic polystyrene crystals. II. Kinetics of thermal changes by x-ray diffraction

The present paper is a study of the annealing, melting, and recrystallization behavior of solution grown isotactic polystyrene crystals in order to elucidate changes both in total lamellar thickness and in the thickness of the crystalline “core” as a function of temperature. The lamellar thickness was obtained from x-ray long spacings and the core thickness from the broadening of appropriate reflections, both assessed by a high sensitivity x-ray detector system able to produce records within a few minutes during time dependent processes. The x-ray results were correlated with differential scanning calorimetry (DSC) measurements. On heating, first the usual annealing effects with little net melting were observed, but on increasing the temperature large changes in long spacing, which were reversible with temperature, were seen accompanied by only small changes in crystal core thickness. The second effect was found to be associated with a substantial degree of melting. Even after heating above the temperature where complete melting was indicated by DSC, the recrystallization rate was found to decrease as the melt temperature was increased and to be much faster than on cooling from the usual melts. Strikingly, this recrystallization was accompanied by a decrease in long period with time even under isothermal conditions. It is inferred that the memory of the preceding crystalline stack is preserved throughout the melting range and even beyond this, far into the molten state. This “seeding” effect is intrinsic to the polymer even if its nature cannot be specified. The principal effects in question can then be explained by envisaging that randomly placed lamellas gradually disappear within the stack on melting and successively reappear on crystallization during cooling. These ideas agree well with previous work on reversible long spacing changes in polyethylene (see ref. 9) and are likely to be of wider generality for melting and recrystallization phenomena in systems having stacked lamellar morphologies.     

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.

     

Development of the crystallinity and rigid amorphous fraction in cold‐crystallized isotactic polystyrene

The phase structure of crystalline isotactic polystyrene (iPS) has been investigated with temperature‐modulated differential scanning calorimetry (TMDSC), wide‐angle X‐ray scattering (WAXS), and Fourier transform infrared (FTIR) spectroscopy. Quenched amorphous samples have been cold‐crystallized at 140 or 170 °C for various crystallization times. The degree of crystallinity obtained from WAXS, with the ratio of the crystal peak intensity to the total peak intensity, shows excellent agreement with the crystallinity determined from TMDSC total heat flow endotherms. For the first time, FTIR results show that the absorbance peak ratio (I/I) has a linear correlation with the crystalline mass fraction (χ_c) for cold‐crystallized iPS according to the following relation: I/I = 0.54χ_c + 0.16. This relationship allows the crystallinity of iPS to be determined from infrared spectroscopy analyses in cases in which it is difficult to perform thermal or X‐ray measurements. On the basis of the measurements of the heat capacity increment at the glass transition, we find that a significant amount of the rigid amorphous fraction (RAF) coexists with the crystalline and mobile amorphous phases in cold‐crystallized iPS. The RAF increases systematically with the crystallization time, and a greater amount is formed at a lower crystallization temperature. A three‐phase model (crystalline phase, mobile amorphous phase, and rigid amorphous phase) is, therefore, appropriate for the interpretation of the structure of cold‐crystallized iPS. The origin of the low‐temperature endothermic peak (annealing peak) has been investigated with TMDSC and FTIR spectroscopy and has been shown to be due to irreversible relaxation of the RAF. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 3026–3036, 2003     

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.     

Structure of single-molecule single crystals of isotactic polystyrene and their radiation resistance

The structure of single-molecule single crystals of isotactic polystyrene (i-PS) was investigated by electron diffraction (ED). The nanoscale single-molecule single crystals were found to be more resistant to electron irradiation when compared to the larger crystals of many molecules, as indicated by both observation of ED and high-resolution electron microscopy with increasing radiation dose. It is proposed that since the single-molecule single crystals are very small, the secondary electrons escape more frequently from the crystal so that the radiation damage is reduced. Lattice imaging was achieved at room temperature in the case of single-molecule single crystals because of their stability to electron irradiation. Published 1998 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.

J Polym Sci B: Polym Phys 36 : 105–112, 1998     

Extended-chain crystals. III. Size distribution of polyethylene crystals grown under elevated pressure

Extended-chain crystals of high molecular weight polymethylene, a polyethylene with a broad molecular weight distribution, and three fractions of polyethylene were grown from the melt under elevated pressure. Comparison of the crystal size distribution in the molecular chain direction (measured on fracture surfaces by electron microscopy) with the molecular weight distribution (measured by gel-permeation chromatography) gave the following results. Up to molecular weight 10,000 all samples showed eutectic separation into fully extended chain crystals of narrow molecular weight distribution. Above molecular weight 10,000 mixed crystals were formed. Under the chosen crystallization conditions larger chain extension was achieved with higher molecular weights. However, an increase in molecular weight by a factor of 1000 led only to a tenfold increase in chain extension. These facts are discussed in the light of a proposed mechanism of crystal growth.     

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.     

Degree of crystallinity and lattice thermal conductivity of polyethylene at low temperatures

The lattice thermal conductivity of a semicrystalline polymer was studied at low temperatures by calculating the total lattice thermal conductivities of four samples of polyethylene with different degrees of crystallinity between 0.43 and 0.81 and temperatures between 0.4 and 20 K. The contributions of the crystalline and noncrystalline natures and their percentage contributions were taken into account. The predicted lattice thermal conductivity of polyethylene was in fairly good quantitative agreement with the experimental value, and showed a strong crystallinity dependence, with a distinctive cross-over point at about 2 K.     

The crystalline core of the row structures in isotactic polystyrene. I. Nucleation and growth

The nucleation and growth of the crystalline core in the row structures of isotactic polystyrene were investigated by transmission electron microscopy. The number of core crystals, nucleated at a specific temperature, depends on the external strain. Their length was found to increase with time if the sample is kept at the straining temperature. If a strained sample is cooled to room temperature and subsequently reheated, no further growth of the core crystals is observed. Obstacles in the path of growth were circumvented by local changes of the growing direction. Melt-soluble noncrystallizable molecules are rejected by the growing core into the surrounding melt. The observations suggest a growth mechanism of the cores based on the successive self-induced alignment of molecules at the tip of the growing cores.     

Unique thermal behaviors of collective single and few-chain compact globules of isotactic polystyrene

Collective single- and few-chain compact globules of isotactic polystyrene (i-PS) were prepared from very dilute solution via the freeze-drying technique. A small exothermic peak was found in the differential scanning calorimetry (DSC) traces of freeze-dried samples. This is a manifestation of the existence of ‘cohesional entanglements’.     

Electron microscopic investigation of the molecular mechanism of plastic deformation of polyethylene and isotactic polystyrene crystals

The molecular process involved in the plastic deformation of crystals of linear polyethylene and isotactic polystyrene by necking was investigated by transmission electron microscopy (TEM). The results for the draw ratio, the height, and the shape of the neck and the molecular structure of the deformed material suggest that plastic deformation occurs basically by the unfolding and bending of the molecules in a deformation region only a few nm wide. The observations are at variance with deformation models involving complete unfolding of the molecules or the breaking off of folded-chain blocks and the incorporation of these blocks into the deformed material.     

FTIR and thermal studies on gel grown neodymium tartrate crystals

The growth of neodymium tartrate crystals was achieved in silica gel by single diffusion method. Optimum conditions were established for the growth of good quality crystals. Fourier transform infrared (FT-IR) spectroscopic study indicates the presence of water molecules and tartrate ligands and suggests that tartrate ions are doubly ionised. The thermal behaviour of the material was studied using thermogravimetry (TG), differential thermal analysis (DTA), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). Thermogravimetric analysis support the suggested chemical formula of the grown crystal to be Nd₂(C₄H₄O₆)₃·7H₂O, and the presence of seven water molecules as water of hydration. It is shown that the material is thermally stable up 45 °C beyond which it decomposes through many stages till the formation of neodymium oxide (Nd₂O₃) at 995 °C. The decomposition pattern is reported to be typical of a hydrated metal tartrate.     

Degradation of polymers and morphology: Photo-oxidative degradation of isotactic polystyrene in presence of sulfur dioxide as function of polymer crystallinity

The photo-oxidative chain scission of isotactic polystyrene films has been studied as a function of the degree of crystallinity, SO₂, and NO₂ pressures, and temperature. The rate of chain scission increases in the presence of SO₂ with extent of crystallinity. It is assumed to be faster due to strain in and near the folds in the crystalline areas than in the amorphous regions. In the presence of NO₂, chain scission increases up to about 8% crystallinity but subsequently becomes constant with further increase in crystallinity. It is suggested that the diffusion rates of oxygen and nitrogen dioxide into the films decrease with increasing crystallinity. These two processes compensate each other.     

The important role of crystallinity and amylose ratio in thermal stability of starches

Journal of Thermal Analysis and Calorimetry - The thermal features of potato, banana, corn and cassava starches were correlated with structural properties. These starches were characterized by SEM,...     

Blends of isotactic polypropylene and natural terpene resins. I. Phase structure,thermal, and dynamic‐mechanical properties

This article discusses the influence of two natural terpene resins (NTR), poly(α‐pinene) (PαP A115) and poly(d‐limonene) (PL C115), on morphology, miscibility, thermal, and dynamic‐mechanical properties of their blends with isotactic polypropylene (iPP). The NTR have interesting physical and chemical properties, and they are approved for food contact application. From the results of differential scanning calorimetry and dynamic‐mechanical thermal analysis it was deduced that both the resins were completely miscible with the amorphous iPP up to the composition investigated here (70/30 wt %). Scanning electron microscopy (SEM) analysis instead showed that the 70/30 iPP/PαP A115 blend and 80/20 and 70/30 iPP/PL C115 blends contained very small domains homogeneously distributed into the matrix. It is hypothesized that the domains are likely formed by the terpene‐rich phase, and the matrix by the iPP‐rich phase (besides the crystallized iPP phase). The iPP‐rich phase and the NTR‐rich phase would have the glass transition temperatures so close that they cannot be resolved by DSC and DMTA. Finally, for the iPP/PαP A115 system an upper critical solution temperature (UCST) is proposed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 867–878, 1999     

The effect of morphology on thermal stability of isotactic polypropylene in air

Summary The studies on thermooxidative degradation of isotactic polypropylene films with different morphological structure were carried out in the air in temperature range 85–145 °C. The various crystallinity degrees and morphologies of PP films were obtained by cooling the melt and crystallization at different temperatures. The studies on structural changes going together with degradation and on the kinetics of chain scission have shown that this reaction occurs in two stages. The first is connected with consumption of oxygen dissolved in the initial films and the second is controlled by its diffusion into the films. The rates of degradation and structural changes occurring simultaneously depend not only on the initial crystallinity but also on morphology of the systems under study.With 8 figures and 1 tableDedicated to Prof. G. Rehage on occasion of his 60th birthday     

Infrared spectra of polymer solutions. I. Conformational stability of isotactic polymer chains in solution

The infrared spectra of isotactic polystyrene, polypropylene, and poly-p-chlorostyrene were measured in dilute solutions or gels at various temperatures ranging from room temperature to ?100°C. For isotactic polystyrene and polypropylene, all the absorption bands characteristic of the helical conformation of the molecules increase in intensity with decreasing temperature, and show intensities at low temperature as strong as in the highly crystallized samples. This suggests that the molecules can assume very regular conformations even in solution. Similar temperature dependence of the spectra was observed for a homogeneous gel of isotactic poly-p-chlorostyrene, which is believed to be noncrystalline. These experimental facts lead to the conclusion that the stability of the TG type helical conformation of these isotactic polymers may be ascribed to the intramolecular forces within a molecule. The fact that these spectral changes are reversible has permitted a thermodynamic treatment of the conformational regularity in solution on the basis of the infrared data. The temperature dependence of the absorption intensities has been interpreted quantitatively by a simple statistical mechanical model, and the enthaply and the entropy differences between the helical and the random states of the monomeric residue in the molecular chain have been estimated.     

Thermal and photochemical stability of polystyrene and its blends with polyvinyl chloride I.

The influence of a 1–20% content of PVC in PS films on their thermal stability was investigated. It was found that the thermal stabilities of these blends are higher than that of either of the two pure polymers. This indicated the mutual stabilizing effects of these polymers on each other. The effect is significant when the PVC content in the blends is 1–5%. A higher amount of PVC causes either no significant change or a lowering of the thermal stability. The miscibility of the polymer components in the blends clearly has an important influence on the course of the thermal processes.

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Zusammenfassung Der Einfluß von 1–20% PVC in PS-Filmen auf deren thermische Stabilität wurde untersucht. Es wurde festgestellt, daß die thermische Stabilität dieser Gemische höher ist, als die der beiden reinen Polymere für sich. Dies verdeutlicht den gegenseitigen Stabilisierungseffekt der beiden Polymere. Zu einem signifikanten Effekt kommt es bei einem PVC Gehalt der Gemische von 1–5%. Ein höherer Gehalt an PVC führt entweder zu keiner signifikanten Veränderung oder zu einer Abnahme der thermischen Stabilität. Die Mischbarkeit der Polymerkomponenten des Gemisches verfügt eindeutig über einen wichtigen Einfluß auf den Ablauf der thermischen Prozesse.

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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.