Isotactic Polystyrene Research Articles

Detailed interpretation of the results of two-dimensional correlation analysis of infrared spectra obtained during isothermal crystallization of isotactic polystyrene and poly(3-hydroxybutyrate)

Two sets of IR spectral data obtained during the isothermal melt-crystalization process of two different polymers are respectively analyzed by two-dimensional (2D) correlation approach. The aim of this study is to inspect this details of the obtained correlation matrices, remove the correlation artifacts and produce reliable 2D correlation results. Particular attention is thus paid to the representation of the correlation data, contour plots and correlation slices. An example of misinterpretation caused by improper thresholding of the asynchronous data is given. Correlation coefficients are also employed to confirm the simplicity of the correlation data. Finally, sample–sample correlation matrix is calculated to additionally confirm that simple two-component systems can immediately be recognized through their correlation pattern.

Polymerization of styrene in γ-cyclodextrin channels: Lightly rotaxanated polystyrenes with altered stereosequences

Modeling of polystyrene (PS) with various stereosequences in γ-cyclodextrin (γ-CD) channels has been conducted and it was found that only isotactic PS stereoisomers can fit into the γ-CD cavity. Thus, based on the modeling of stereoisomeric polystyrenes in narrow γ-CD channels, it was suggested that PSs with unusual microstructures might be produced via constrained polymerization of styrene monomer in its γ-CD-IC crystals. The in situ polymerization of styrene inside the narrow channels of its γ-CD-IC crystals suspended in aqueous media was performed. Alternatively, the solid-state polymerization of styrene/γ-CD-IC has also been conducted by exposure to γ-radiation. It was found that most host γ-CD molecules slip off during polymerization and the channel structure was not preserved. Consequently, much of the guest styrene monomer polymerizes outside of the host γ-CD channels, where the constrained environment is absent. Yet, a lightly rotaxanated structure has been obtained, where some threaded γ-CD molecules ∼15 wt% (∼1 γ-CD per 70 PS repeat units) are permanently entrapped along the PS chains after polymerization. 13C NMR spectra of PSs synthesized from styrene/γ-CD-IC and homogeneously in toluene show some differences, which are presumably due to variations in the stereosequences of PSs obtained from the partially constrained polymerization of styrene/γ-CD-IC.

Influence of thermal history on primary nucleation and crystal growth rates of isotactic polystyrene

The influence of thermal history on primary nucleation and crystal growth rate of isotactic polystyrene (i-PS) was studied in a wide range of time and temperature. Samples were melted at several temperatures from 230 to 250°C and then crystallized from those molten states and also crystallized from the glassy state. The primary nucleation rate is strongly influenced by the thermal history but the crystal growth rate is mainly governed by the crystallization temperature. Below a melt temperature of 230°C, the nucleation density was attributed to the seed nuclei, which result from incomplete melting of the spherulites. Above 250°C, a limited number of heterogeneous nucleation sites remain, and these sites are activated on the surface of impurities or foreign bodies in the melt. The primary nucleation is controlled heterogeneously both from the molten and the glassy states. The nucleation rate from the glassy state is faster than that from the molten state. A linear relationship between the nucleation rate and the inverse of induction time was found in each experimental condition and their slopes are related to the saturation density of the nuclei. The activation energy for the molecular transport and the primary nucleation energy were smaller than those of the crystal growth. The primary nucleation energy from the glass was lower than that from the melt. These energies are discussed in the present work.

Exploring Time-Dependent Structural Changes during the Cold Crystallization Process of Isotactic Polystyrene by Infrared Spectroscopy and Multivariate Curve Resolution

The present study attempts an application of Fourier transform infrared (FT-IR) spectroscopy in conjunction with multivariate curve resolution (MCR) techniques to explore the structural evolution of isotactic polystyrene (iPS) during the cold crystallization process. The focus of the present study is placed on the performance of MCR techniques, e.g., orthogonal projection (OP), alternating least squares (ALS), and fixed-size moving window evolving factor analysis (FSMWEFA), and the interpretability of spectral changes in the investigated chemical process. As a result, valuable information and conclusions about the structural evolution of iPS during the crystallization process can be extracted: when the amorphous phase of iPS changes, the ordering of the phenyl rings takes place first, and then the polymer chains adjust their local conformations to form short 3(1) helix structures. Furthermore, according to intensity profiles of the spectral variations, the ordering of the phenyl rings proceeds more intensely than the formation of ordered local chains, and the structural evolution of iPS occurs even during the induction period. The spectral variations resulting from the conformational changes in the 3(1) helical structures depend on the sequence length of the helical chains: the longer the polymer chain is, the smaller the corresponding band variations are. It has been demonstrated that the combination of FTIR spectroscopy and chemometric MCR techniques is very promising for the analysis of the crystallization process of polymers. MCR is a powerful tool for analyzing and visualizing spectral data and integrating them with other information, making spectral intensity variations more amenable to interpretation in order to explore the molecular dynamics of polymers.

Melting and reorganization of the crystalline fraction and relaxation of the rigid amorphous fraction of isotactic polystyrene on fast heating (30,000 K/min)

For polymers the origin of the multiple melting peaks observed in DSC curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. Developments in thin-film (chip) calorimetry allow scanning rates as high as several thousand Kelvin per second. We utilized a chip calorimeter based on a commercially available vacuum gauge, which is operated under non-adiabatic conditions. The calorimeter was used to study the melting of isothermally crystallized isotactic polystyrene (iPS). Our results on melting at rates up to 30,000K/min (500K/s) give evidence for the validity of a melting–recrystallization–remelting process for iPS at low scanning rates (DSC). At isothermal conditions iPS forms crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of multimodal populations of crystals with significantly different stability (melting temperatures). Furthermore, relaxation (devitrification) of the rigid amorphous fraction occurs in parallel to melting. Superheating of the crystals is of the order of 25K at 30,000K/min.

Relationship between magnetic alignment and the crystallization condition of isotactic polystyrene

The relationship between a magnetic alignment and crystallization conditions of isotactic polystyrene (iPS) was studied by means of DSC, POM, WAXD, FT-IR, and birefringence measurements. The onset temperature for crystallization during the cooling process was found to depend on the melting temperature. From observations using a polarized optical microscope, theproportion of spherulite decreased in cases where melting occurred at greater than 265 °C. These results suggest that melting at less than 265 °C causes self-seeding, which is known as the ‘melt memory effect’ in iPS, leading to a high nucleation density. The maximum intensity of transmitted light that could be observed by birefringencemeasurements was also dependent on the melting temperature. The high nucleation density that was obtained by melting at less than 265 °C was suitable for achieving higher orientations. In situ infrared measurements in a magnetic field were carried out in a home-built apparatus into which an autorotation polarizer unit had been installed. This allowed us to automatically acquire simultaneous polarized infrared spectra both parallel and perpendicular to the magnetic field. Orientation was confirmed for the crystalline band, while no orientation was observed for the amorphous band. The dichroic difference increased during isothermal crystallization with increasing crystallinity. No orientation effects were observed in the melted state in the present study.

Evaluation of the reversible contribution to the reversing heat capacity in isotactic polystyrene

We report the results of time dependent analysis of the reversing heat capacity peaks in isotactic polystyrene (iPS). Solution grown crystals (SGCs) and bulk films of iPS were examined using quasi-isothermal temperature modulated differential scanning calorimetry (QI-TMDSC). Both types of iPS samples showed two reversing heat capacity peaks as reported before [H. Xu, P. Cebe, Macromolecules 37 (2004) 2797; H. Xu, P. Cebe, Macromolecules 38 (2005) 770], and the two peaks are associated with melting of different portions of crystal lamellae with different degree of thermal stability [T. Liu, J. Petermann, Polymer 42 (2001) 6453]. Here, the behavior of the reversing heat capacity is studied as a function of holding time from the establishment of steady state, until the end of the quasi-isothermal holding period. The reversing heat capacity decreases with time at each of the two peak positions. For both types of iPS (SGC and bulk film) the time dependence of each of the reversing heat capacity peaks is fitted using Cp(t)=Cp∞+A1exp(−t/τ1)+A2exp(−t/τ2). The fit parameters (equilibrium reversible heat capacity, Cp∞; prefactors, A1 and A2; relaxation times, τ1 and τ2) provide a means to compare the decay of the dual reversing heat capacity peaks of iPS solution grown crystals with bulk cold-crystallized film. Results show that iPS bulk films have longer relaxation times (τ1 and τ2) and smaller Cp∞ value than the iPS SGCs. Both results suggest that the iPS bulk films have higher degree of crystal perfection than SGCs.

Crystal Structure of the Trigonal Form of Isotactic Polypropylene as an Example of Density-Driven Polymer Structure

Propene-hexene copolymers crystallize in a new polymorphic form of isotactic polypropylene when the concentration of hexene is higher than nearly 10-15 mol %. The hexene units are included in the crystals, inducing an increase of density that allows crystallization of 3-fold helical chains in a trigonal unit cell according to the space group R3c or Rc, where the helical symmetry of the chains is maintained in the crystal lattice. The structure of this new form is similar to those of isotactic polybutene and polystyrene and does not crystallize in polypropylene homopolymer because it would have too low density. The crystal structure of isotactic polypropylene is therefore no longer an exception to the principles of polymer crystallography, but the new structure represents the fulfillment of these principles and indicates that the packing of polymer molecules is mainly driven by density.

Miscibility, crystallization and morphologies of syndiotactic polystyrene blends with isotactic polystyrene and with atactic polystyrene

Two-component blends of differing polystyrene (PS), one syndiotactic (sPS) and the other isotactic (iPS) or atactic (aPS), were discussed. The phase behavior, crystallization and microstructure of binary polystyrene blends of sPS/iPS and sPS/aPS with a specific composition of 5/5 weight ratio were investigated using optical microscopy (OM), differential scanning calorimetry, wide-angle X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM). Based on the kinetics of enthalpy recovery, complete miscibility was found for the sPS/aPS blends where a single recovery peak was obtained, whereas phase separation was concluded for the sPS/iPS blends due to the presence of an additional recovery shoulder indicating the heterogeneity in the molten state. These findings were consistent with OM and SEM observations; sPS/iPS exhibits the dual interconnectivity of phase-separated phases resulting from spinodal decomposition. Both iPS and aPS have the same influence on the sPS crystal structure, i.e., dominant β-form sPS and mixed α-/β-form sPS obtained for melt-crystallization at high and low temperatures respectively, but imperfect α-form sPS developed when cold-crystallized at 175 °C. Co-crystallization of iPS and sPS into the common lattice was not observed regardless the thermal treatments, either cold or melt crystallization. Due to its slow process, crystallization of iPS was found to commence always after the completion of sPS crystallization in one-step crystallization kinetics. Segregation of rejected iPS component during sPS crystallization was extensively observed from TEM and SEM images which showed iPS pockets located between sPS lamellar stacks within spherulites, leading to the interfibrillar segregation, which was similar with that observed in the sPS/aPS blends. The addition of iPS (or aPS) component will reduce the overall crystallization rate of the sPS component and the retardation of crystal growth rates can be simply accounted by a dilution effect, keeping the surface nucleation intact. The phase-separated structure in the sPS/iPS blend shows a negligible effect on sPS crystallization and the signature of phase separation disappears after sPS crystallization. Depending on the relative dimensions of the segregated domains and iPS lamellar nucleus, subsequent crystallization of iPS can proceed to result in a crystalline/crystalline blend, or be inhibited to give a crystalline/amorphous blend morphology similar with that of sPS/aPS blends.

Crystallization and morphological features of syndiotactic polystyrene induced from glassy state

Spherulitic growth rates and microstructure of syndiotactic polystyrene (sPS) cold-crystallized isothermally at various temperatures, T c (115–240 °C), have been investigated by small-angle light scattering (SALS), optical microscopy and transmission electron microscopy. The derived activation energy for sPS chain mobility at the crystal growing front is 5.4 kJ/mol, which is relatively lower than that of isotactic polystyrene, 6.5 kJ/mol. In addition, the Hv scattering invariant ( Q Hv) measured by SALS on the crystallized sPS samples displays a pronounced minimum at ∼150 °C. Despite a wide range of T c used, however, the sample crystallinity estimated by Fourier transformation infrared spectroscopy remains unchanged. Prior to crystallization, the correlation length derived from the Vv patterns on the basis of Debye–Bueche model is ca. 1.13 μm regardless of T c used. Interconnected domains with a width of ca. 1.8±0.5 μm are readily observed in all the crystallized samples under phase contrast microscopy and the phase-separated structure is conserved within sPS spherulites whose diameters are increased with increasing T c. Based on the above facts, we conclude that the presence of a Q Hv minimum is ascribed to the resultant events of the two competitive transitions i.e. liquid–solid crystallization, and liquid–liquid demixing resulting from the spinodal decomposition (SD). At lower T c, the unstable SD transition overwhelms the crystallization. Despite the low chain mobility, the coarsening process driven by the interfacial energies has reached a certain level before crystalline nucleation takes place. At higher T c, on the other hand, cold crystallization becomes the dominant process due to the enhanced chain mobility, leading to the suppression of ongoing SD coarsening process. At an intermediate T c range, comparable competition of the phase separation and crystallization prohibits the development of ordered symmetry within spherulites, giving the presence of Q Hv minimum.

In situ AFM study of the growth of banded hedritic structures in thin films of isotactic polystyrene

The growth of banded hedrites of isotactic polystyrene (iPS) is followed in situ, using atomic force microscopy (AFM). The melt layer surrounding the growing hedritic front is much thinner than both the hedrite front and the far-field melt. Growth occurs simultaneously in three dimensions: radially, circumferentially and height. The height of the hedrite growth front being so much higher than the adjacent molten pool, the observed propagation of the front requires that it be covered by a thin film of molten material, likely drawn up the face of the front by capillarity. As the hedritic front grows in height, it demands material at a higher rate than can be delivered from the far-field melt and the stacked lamellae stop growing, layer by layer, from the top downward, until only the basal lamella continues to grow (at a constant velocity). The kinetics of the position of the lowest point ahead of the growth front slows with time during this process. Supplying only the basal layer, the adjacent molten pool is replenished and now begins to feed new layers of growing lamellae as the process repeats itself. The creation of the new lamellar layers appears to be coupled to morphological instability of the basal layer, in the form of growth front serrations, likely causal of the giant screw dislocations.

Phase-field modeling on morphological landscape of isotactic polystyrene single crystals

Spatio-temporal growth of isotactic polystyrene single crystals during isothermal crystallization has been investigated theoretically based on the phase field model by solving temporal evolution of a nonconserved phase order parameter coupled with a heat conduction equation. In the description of the total free energy, an asymmetric double-well local free energy density has been adopted to represent the metastable melt and the stable solid crystal. Unlike the small molecule systems, polymer crystallization rarely reaches thermodynamic equilibrium; most polymer crystals are kinetically stabilized in some metastable states. To capture various metastable polymer crystals, the phase field crystal order parameter at the solidification potential has been treated to be supercooling dependent such that it can assume an intermediate value between zero (melt) and unity (perfect crystal), reflecting imperfect polycrystalline nature of polymer crystals. Two-dimensional simulations exhibit various single crystal morphologies of isotactic polystyrene crystals such as faceted hexagonal patterns transforming to nonfaceted snowflakes with increasing supercooling. Of particular interest is that heat liberation from the crystallizing front influences the curvature of the crystal-melt interface, leading to directional growth of lamellar tips and side branches. The landscape of these morphological textures has been established as a function of anisotropy of surface energy and supercooling. With increasing supercooling and decreasing anisotropy, the hexagonal single crystal transforms to the dense lamellar branching morphology in conformity with the experimental findings.

Transitions from solid to liquid in isotactic polystyrene studied by thermal analysis and X-ray scattering

A three-phase model, comprising mobile amorphous fraction (MAF), rigid amorphous fraction (RAF) and crystalline fraction (C), has been applied to interpret the thermal transitions and structure of cold-crystallized isotactic polystyrene (iPS) from below the glass transition temperature, Tg, to above the melting point. Quenched amorphous iPS films were isothermally crystallized at different temperatures for 12h. The fraction of crystalline phase, ϕc, was measured by differential scanning calorimetry (DSC), wide angle X-ray scattering and Fourier Transform infrared spectroscopy. The fraction of the mobile amorphous phase, ϕMAF, was obtained from the heat capacity increment at the glass transition temperature. In the three-phase model, the fraction of the rigid amorphous phase, ϕRAF, was found from 1−ϕMAF−ϕc. Specific heat capacity measurements by standard DSC confirm that the experimental baseline heat capacity conforms to a three-phase model for temperatures ranging from below Tg, up to the relaxation of RAF. The relaxation of RAF appears as a sigmoidal change in heat capacity accompanied by excess enthalpy, in which solid-like RAF is converted to an identical amount of liquid-like MAF.At temperatures above the relaxation of RAF, either one or two major crystal melting endotherms are observed in standard DSC, dependent upon crystallization temperature. However, using quasi-isothermal temperature modulated DSC, we always observed two reversing melting endotherms. The effects of annealing on iPS structure during the quasi-isothermal measurement were assessed using small angle X-ray scattering (SAXS). Combining the DSC and SAXS results, a model for the melting of iPS lamellae at low heating rates is presented.

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.

Recent developments in thermal analysis of polymers: Calorimetry in the limit of slow and fast heating rates

AbstractThis review focuses on new insights into the crystal melting transition and the amorphous glass transition of polymers that have been gained through recent advances in thermoanalytical methods. The specific heat capacity can now be studied under two extreme limits, that is, under quasi‐isothermal conditions (limit of zero heating rate) and, at the other end of the scale, under rapid heating conditions (heating rates on the order of thousands of degrees per second), made possible through nanocalorimetry. The reversible melting, and multiple reversible melting, of semicrystalline polymers is explored using quasi‐isothermal temperature modulated differential scanning calorimetry, TMDSC. The excess reversing heat capacity, above the baseline, measured under nearly isothermal conditions is attributed to locally reversible surface melting and crystallization processes that do not require molecular nucleation. Observations of double reversible melting endotherms in isotactic polystyrene suggest existence of two distinct populations of crystals, each showing locally reversible surface melting. The second subject of the review, nanocalorimetry, is utilized to study samples of small mass under conditions of very fast heating and cooling. The glass transition properties of thin amorphous polymer films are observed under adiabatic conditions. The glass transition temperature appears to be independent of film thickness, and is observed even in ultra‐thin films. Recrystallization and reorganization during rapid heating are studied by nanocalorimetry of semicrystalline polymers. The uppermost endotherm seen under normal DSC scanning of poly(ethylene terephthalate) is caused by reorganization, and vanishes under the rapid heating conditions (3000K/s) provided by nanocalorimetry. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 629–636, 2005

Heat Capacity Study of Solution Grown Crystals of Isotactic Polystyrene

We have performed measurements of the specific heat capacity on isotactic polystyrene (iPS) crystals grown from dilute solution. Solution grown crystal (SGC) samples had larger crystal fractions and greatly reduced rigid amorphous fractions compared to their bulk cold-crystallized counterparts. Heat capacity studies were performed from below the glass transition temperature to above the melting temperature by using quasi-isothermal temperature modulated differential scanning calorimetry (TMDSC) and standard DSC. Two or three endotherms are observed, which represent the melting of crystals. The small rigid amorphous fraction relaxes in a wide temperature range from just above the glass transition temperature to just below the first crystal melting endotherm. As in bulk iPS, 1 multiple reversing melting was found in iPS SGCs, supporting the view that double melting in iPS may be due to dual thermal stability distribution existing along one single lamella.2 The impact of reorganization and annealing on the melt endotherms was also investigated. Annealing occurs as a result of the very slow effective heating rate of the quasi-isothermal measurements compared to standard DSC. The improvement of crystal perfection through annealing causes the reversing melting endotherms to occur at a temperature higher than the endotherms seen in the standard DSC scan.

Comparison of Miscibility and Morphology in Two Ternary Blend Systems: Effects of Binary Interactions

Experimental and theoretical studies on two ternary blend systems comprised of polystyrene (PS), isotactic polystyrene (iPS) and poly(cyclohexy methacrylate) (PCHMA) or poly(2,6-dimethyl-p-phenylene oxide) (PPO) are described. The differences in phase behavior and morphology of PS/iPS/PCHMA and PS/iPS/PPO ternary blends were investigated and compared by using differential scanning calorimeter (DSC), polarized-light optical microscopy (POM), and scanning electron microscopy (SEM). These two ternary blend systems were found to be completely miscible within the entire composition range at ambient temperature. However, cloud-point transition and LCST behavior was observed for the PS/iPS/PCHMA blend system, but not for the PS/iPS/PPO blend system. Phase diagrams for ternary systems of PS/iPS/PCHMA vs. PS/iPS/PPO at elevated temperatures of cloud points were constructed with the experimental data. Theoretical calculations were found to agree well with the data. Interaction parameters in the PS/iPS/PCHMA vs. PS/iPS/PPO ternary blends were evaluated to assess possible Δχ among all the binary pairs that constitute these two ternary blends. The melting point depression was used to investigate interactions between pairs of PS/PCHMA vs. PS/PPO, and it was found that asymmetry in the binary interactions led to more ternary phase instability, which was manifested as lowering the LCST and/or cloud points.

Depletion-Induced Nonbirefringent Banding in Thin Isotactic Polystyrene Thin Films

Depletion-Induced Nonbirefringent Banding in Thin Isotactic Polystyrene Thin Films

Molecular structure of polyelectrolyte/surfactant complexes vs. polymer tacticity

The molecular structure of polystyrene sulphonate/CTAB stoichiometric complexes has been studied by small-angle neutron scattering in solutions and in gels for two different tacticities of the polystyrene moiety ( atactic and isotactic polystyrene). It is found that tacticity has no influence on the molecular structure in solution but plays a role in the gel state.

Near-field polarimetric characterization of polymer crystallites

We use near-field polarimetry (NFP) to investigate thin-film crystallites of isotactic polystyrene (iPS). NFP micrographs enable quantitative optical characterization of the birefringence in these specimens with subdiffraction-limited resolution, resulting in observations that give: (1) evidence for radial strain in the depletion boundary surrounding the growth front, and (2) a map of local tilt in the crystal axis and/or strain in the amorphous layers above and below the growth plane of the crystallites.