Carbon nanocapsules-reinforced syndiotactic polystyrene nanocomposites: Crystallization and morphological features

Syndiotactic polystyrene (sPS) composites filled with well-dispersed carbon nanocapsules (CNC) were prepared through solution blending along with ultrasonication. Several analytic techniques, including DSC, FTIR, PLM, WAXD, TEM, and TGA were performed to reveal the CNC effects on the crystallization, morphology and the thermal degradation of the as-prepared sPS/CNC composites. Addition of CNC was found to favor the crystalline modification of β-form sPS and depress the α-form ones. For the dynamic crystallization, a gradual reduction of cold-crystallization temperature of the α-form sPS was observed by increasing the CNC content although the glass transition temperature remained unchanged (∼96 °C). In contrast, the melt-crystallization temperature of the β-form sPS was elevated from 238 °C for the neat sPS to 251 °C for the 99/5 composite in spite of the fact that the equilibrium melting temperature (∼290 °C) determined from the linear Hoffman-Weeks plot was irrelevant with CNC concentrations. The former was attributable to the formation of an effective heat-conduction path to trigger an earlier overall crystallization. On the other hand, the latter resulted from the enhanced nucleation sites due to the presence of uniformly dispersed CNCs. Results of the isothermal crystallization of the β-form sPS concluded that the presence of 1% CNCs led to a significant increase in the crystallization rate as much as an order of magnitude. Moreover, the Avrami exponent changed to ∼2.0 from a value of 2.8 for the neat sPS, suggesting a different crystallization mechanism involved. At a given crystallization temperature, PLM results showed a negligible variation in the crystal growth rates and a decrease in spherulitic sizes, indicating that nucleation played the key role in enhancing the crystallization rate. For samples isothermally crystallized at 260 °C, the lamellar thickness was constant to be ∼7.2 nm regardless of the CNC content. Due to the enhanced nucleation, however, lamellar stacks were more randomly oriented and its lateral dimensions became shorter with increasing CNC contents. For composites with more than 1 wt% CNC, the crystallizability of sPS chains was reduced and the annealing peak located ca. 4 °C higher than the crystallization temperature became more evident, suggesting the plausible formation of micro-crystals in between the lamellar stacks. The TGA results illustrated that a better thermal stability was reached for the CNC-filled sPS composites.     

Isothermal crystallization, melting behavior and crystalline morphology of syndiotactic polystyrene blends with highly-impact polystyrene

Syndiotactic polystyrene (sPS) blends with highly-impact polystyrene (HIPS) were prepared with a twin-screw extruder. Isothermal crystallization, melting behavior and crystalline morphology of sPS in sPS/HIPS blends were investigated by differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarized optical microscopy (POM). Experimental results indicated that the isothermal crystallization behavior of sPS in its blends not only depended on the melting temperature and crystallization temperature, but also on the HIPS content. Addition of HIPS restricted the crystallization of sPS melted at 320 °C. For sPS melted at 280 °C, addition of low HIPS content (10 wt% and 30 wt%) facilitated the crystallization of sPS and the formation of more content of α-crystal. However, addition of high HIPS content (50 wt% and 70 wt%) restricted the crystallization of sPS and facilitated the formation of β-crystal. More content of β-crystal was formed with increase of the melting and crystallization temperature. However, α-crystal could be obtained at low crystallization temperature for the specimens melted at high temperature. Addition of high HIPS content resulted in the formation of sPS spherulites with less perfection.     

Melting and crystallization behavior of poly(trimethylene 2,6-naphthalate)

The melting and crystallization behavior of poly(trimethylene 2,6-naphthalate) (PTN) are investigated by using the conventional DSC, the temperature-modulated DSC (TMDSC), wide angle X-ray diffraction (WAXD) and polarized light microscopy. It is observed that PTN has two polymorphs (α- and β-form) depending upon the crystallization temperature. The α-form crystals develop at the crystallization temperature below 140 °C while β-form crystals develop above 160 °C. Both α- and β-form crystals coexist in the samples crystallized isothermally at the temperature between 140 and 160 °C. When complex multiple melting peaks of PTN are analyzed using the conventional DSC, TMDSC and WAXD, it is found that those arise from the combined mechanism of the existence of different crystal structures, the dual lamellar population, and melting-recrystallization-remelting. The equilibrium melting temperatures of PTN α- and β-form crystals determined by the Hoffman-Weeks method are 197 and 223 °C, respectively. When the spherulitic growth kinetics is analyzed using the Lauritzen-Hoffmann theory of secondary crystallization, the transition temperature of melt crystallization between regime II and III for the β-form crystals is observed at 178 °C. Another transition is observed at 154 °C, where the crystal transformation from α- to β-form occurs.     

Lamellar morphologies and crystal stability of syndiotactic polystyrene in α-crystalline form

Syndiotactic polystyrene (sPS) samples melt-crystallized into neat α″ hexagonal modifications were prepared at various temperatures thoroughly for the extensive morphological studies. Lamellar morphologies of the as-prepared sPS samples were investigated using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Absence of a discernible scattering peak was found for SAXS conducted at room temperature, resulting from a negligible difference in the electron density between the lamellar and amorphous layers. To enhance the scattering contrast and strength, SAXS was carried out at 180 °C to obtain more reliable morphological parameters. Due to the broad thickness distribution of morphological features as revealed from the TEM observations, a pronounced variation is found for the long periods derived from the Bragg's law, one-dimensional correlation function, and interface distribution function of the SAXS data. In addition, relatively irregular packing of lamellar stacks with short lateral dimensions was detected in the as-prepared α″-form sPS, leading to the absence of spherulitic birefringence under polarized optical microscopy. Based on the interface distribution function analysis of the SAXS intensity profiles, the lamellar thicknesses were estimated. Using the Gibbs-Thomson relation, the ratio of fold surface free energy (σ_e) to the fusion enthalpy for α″-form sPS was successfully deduced to be ca. 0.057 nm, which is lower than that of β′-form sPS, ca. 0.12 nm. On this basis, a comparison of critical lamellar thicknesses for α″- and β′-form sPS at various crystallization temperatures is provided and the crystal stability associated with the lamellar thickness is discussed as well.     

Polymorphism and β-form structure of poly(octamethylene 2,6-naphthalate)

It was revealed that poly(octamethylene 2,6-naphthalate) (PON) existed in two different crystal structures, α- and β-form, depending on crystallization process: The α-form crystal was dominantly developed from the cold-crystallization, whereas the β-form was from the melt-crystallization. The apparent melting temperatures of α- and β-form crystals were characterized to be 175 and 183 °C, respectively. On the basis of X-ray diffraction and molecular modeling studies, the crystal structure of β-form, developed dominantly from the melt-crystallization, was identified to be triclinic with dimensions of a = 0.601 nm, b = 1.069 nm, c = 2.068 nm, α = 155.68°, β = 123.25°, γ = 52.85°, and with the space group of . The calculated crystal density was 1.243 g/cm³, supporting that one repeating unit of PON exists in a unit cell. The octamethylene units in the PON backbone take largely all-trans conformation in the β-form unit cell.     

Thermal analysis, X-ray and electron diffraction studies on crystalline phase transitions in solvent-treated poly(hexamethylene terephthalate)

The crystal polymorphism, transformation, and morphologies in chloroform solvent-cast poly(hexamethylene terephthalate) (PHT) were examined by using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and temperature in situ transmission electron microscopy (TEM). Solvent-induced crystallization of PHT at room temperature yielded an initial crystal of γ-form, as confirmed by WAXD. Upon DSC scanning, the original γ-form in PHT exhibited three endothermic peaks, whose origins and association were carefully analyzed. The first peak, much smaller than the other two, is in the temperature range of ca. 100-120 °C. It was found that the solvent-induced γ-form was transformed to β-form at 125 °C via a solid-to-solid transformation mechanism. In addition, WAXD showed that γ- and β-forms co-existed in the temperature range of 100-125 °C. These mixed crystal forms were further identified using TEM, and the selected-area electron diffraction (ED) patterns revealed that both γ- and β-form crystals co-existed and were packed within the same spherulite. Solid-solid transformation from the solvent-induced γ-form to β-form in PHT upon heat scanning was presented with evidence and discussed.     

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.     

Anomalous molecular orientation of isotactic polypropylene sheet containing N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide

Small amount of N,N′-dicyclohexyl-2,6-naphthalenedicarboxamide as a β-form nucleating agent is dissolved beyond 280 °C in a molten isotactic polypropylene (iPP) and appears as needle crystals around at 240 °C during cooling procedure. Further, iPP molecules crystallize on the surface of the needle crystals, in which c-axis of the β-form iPP crystals grows perpendicular to the long axis of the needle crystals. Under flow field at extrusion processing, the needle crystals orient to the flow direction prior to the crystallization of iPP. As a result, c-axis of the β-form iPP crystals orients perpendicular to the applied flow direction with a small amount of α-form iPP. Moreover, the vertical molecular orientation of the extruded sheet sample is responsible for unique mechanical anisotropy; the fracture occurs along the transversal direction.     

The crystallization characterization of bulk syndiotactic polystyrene sample: immediate evidence from IR spectroscopy

The crystallization characterization of bulk syndiothactic polystyrene (s-PS) sample is thoroughly studied using the Fourier transform infrared spectroscopy (FTIR). The WAXD is further used to identify the s-PS crystal formation to confirm the specific absorbance in FTIR spectra. Both melt and cold-crystallization behavior are quantitatively determined using FTIR spectra ranging from 870 to 820 cm⁻¹ at 264 °C. Fitting curves to IR spectra provides direct evidence of bulk s-PS crystallization behavior in quantification. The melt-crystallization process yields the β-form only; while the cold-crystallization process yields both the α and the β-form crystal in bulk s-PS sample. The β-form crystal is generated from the phase-transformation of the α-form crystal by cold-crystallization process, the α-form crystal is the initial phase. The activity energy of the α-form formation is lower that that of the β-form, suggesting that the α-form crystal is kinetically favorable while the β-form crystal is thermodynamically favorable.     

Influence of crystallization temperature on the morphologies of isotactic polypropylene single-polymer composite

The supermolecular structures of iPP fiber/matrix composites as a function of crystallization temperature were studied by means of optical microscopy. The results show that, even though partial melting of the iPP fibers is in favor of initiating the β-iPP crystal growth, the interfacial morphology of iPP single-polymer composites induced by its own fiber depends strongly on the crystallization temperature. It was found that transcrystalline structures of negative radial β_III-iPP or banded β_IV-iPP can be produced within the crystallization temperature range 105-137 °C, while transcrystallization zone of pure negative radial α_II-iPP crystals is observed at higher crystallization temperature, e.g. 141 °C. On the other hand, the surrounding iPP spherulites grown from the bulk are composed of α-iPP in the whole crystallization temperature range. However, the optical character of the spherulites is controlled by the thermal condition.     

Isothermal crystallization of isotactic polypropylene blended with low molecular weight atactic polypropylene. Part I. Thermal properties and morphology development

The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T_c=120 °C. However, below this transition temperature, imperfect α-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect α- and γ-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the γ-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of α- and γ-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of γ-form coexisting with (0 4 0) of α-form, and crystallinity were obtained for blended with 20% of aPP, the γ-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization.     

Crystallization behavior and morphology of poly(ethylene 2,6-naphthalate)

The crystallization behavior and morphology of poly(ethylene 2,6-naphthalte) (PEN) were investigated by means of differential scanning calorimetry (DSC), polarized optical microscopy (POM) and transmission electron microscopy (TEM). POM results revealed that PEN crystallized at 240 °C shows the coexistence of α and β-form spherulite morphology with different growth rates. In particular, when PEN crystallized at 250 °C, the morphology of spherulites showed a squeezed peanut shape. The Avrami exponents decreased from 3 to 2.8 above the crystallization temperature of 220 °C, indicating a decrease in growth dimension. Analysis from the secondary nucleation theory suggests that PEN crystallized at 240 °C has crystals with both regime I and regime II. In TEM observation, the ultra-thin PEN film crystallized at 200 °C showed the spherulitic texture with characteristic diffractions of α-form, while PEN crystallized at 240 °C generated an axialite structure with only β-form diffraction patterns. In addition, despite a long crystallization time of 24 h, amorphous regions were also observed in the same specimen. It was inferred that the initiation of PEN at 240 °C generates only β-form crystals from axialite structures.     

Radiation effects and re-crystallization mechanism of syndiotactic polystyrene with β′-crystalline form

The double melting behavior of syndiotactic polystyrene (sPS) with β′-form crystallites was systematically investigated by several analytical techniques, including differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), as well as wide-angle and small-angle X-ray scattering (WAXD, SAXS). For preventing the possible chain re-organization during intermediate melting, a high-energy electron beam (e-beam) radiation was carried out on the melt-crystallized samples to chemically cross-link the amorphous chains between lamellar crystals. The WAXD intensity profiles of the irradiated sPS samples revealed that no crystal transformation took place, and the crystallinity fraction remained unchanged for a received dose up to 2.4 MGy. As the received dose was increased, however, the high melting temperature peak was gradually diminished and finally disappeared after 1.8 MGy e-beam radiation, suggesting that the double melting phenomenon was mainly attributed to the melting/re-crystallization/re-melting behavior. The re-crystallization mechanism of sPS samples was studied using DSC and PLM to reveal the effects of heating rate and annealing temperature on the Avrami exponent and re-crystallization rate constant. In addition, the lamellar morphologies of the re-crystallized samples were also investigated by means of SAXS and TEM. With increasing heating rate or annealing temperature, the derived Avrami exponent was slightly decreased from 1.4 to 1.1; in comparison, the re-crystallization rate showed a shallow maximum at a rate of 10 °C/min, but it became evidently reduced at high annealing temperatures. Based on the morphological observations, we proposed that the re-crystallization of β^′-form sPS crystals involved with the presence of broad lamellar thickness distribution as well as abundant irregular loose folding chains on the lamellar surfaces, which became tightened and crystallized into the un-melted lamellae when the neighboring thinner lamellae trapped in-between were melted. Thus, the high melting temperature is dependent on the average thickness of lamellae consisting of the un-melted lamellae developed initially and thickened ones associated with re-crystallization.     

X-ray studies of multiple melting behavior of poly(butylene-2,6-naphthalate)

Multiple melting behavior of poly(butylene-2,6-naphthalate) (PBN) was studied with X-ray analysis and differential scanning calorimetry (DSC). Double endothermic peaks L and H attributed to the α-form crystal modification, a small peak attributed to the β-form crystal modification, and a new shoulder peak S at a lower temperature of peak H appeared in the DSC melting curves. Wide-angle X-ray diffraction patterns of the samples isothermally crystallized at 200 and 220 °C were obtained at a heating rate of 1 K min⁻¹, successively. In this heating process, change of crystal structure and increase of quantity of the β-form crystallites could not be observed up to the final melting. With increasing temperature, the diffraction intensity decreased gradually and then increased distinctly before a steep decrease due to the final melting. The X-ray analysis clearly proved the melt-recrystallization during heating. The β-form crystal modification was formed during slow heating process in the high temperature region.     

Thermal, spectroscopy, and morphological studies on polymorphic crystals in poly(heptamethylene terephthalate)

Polymorphism and its influential factors in poly(heptamethylene terephthalate) (PHepT) were probed using differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR) spectroscopy, and wide angle X-ray diffraction (WAXD). PHepT exhibits two crystal types (α and β) upon crystallization at various isothermal melt-crystallization temperatures (T_cs) by quenching from different T_maxs (maximum temperature above T_m for melting the original crystals). Melt-crystallized PHepT with either initial α- or β-crystal by quenching from T_max lower than 110 °C leads to higher fractions of α-crystal, but crystallization from T_max higher than 140 °C leads to higher fractions of β-crystal. In addition to T_max, polymorphism in PHepT is also influenced by crystallization temperature (T_c = 25-75 °C). When PHepT is melt-crystallized from a high T_max = 150 °C (completely isotropic melt), it shows solely β crystal for higher T_c, and solely the α-crystal for T_c < 25 °C; in-between T_c = 25 and 35 °C, mixed fractions of both α- and β-crystals. However, by contrast, when PHepT is melt-crystallized from a lower T_max = 110 °C, it shows α-crystal only at all T_cs, high or low.     

Confined crystallization phenomena in immiscible polymer blends with dispersed micro-and nanometer sized PA6 droplets part 4: polymorphous structure and (meta)-stability of PA6 crystals formed in different temperature regions

The genesis and stability of different PA6 crystalline polymorphs, dispersed as micro- and submicrometer sized droplets inside an amorphous polymer matrix, are discussed over a very broad temperature range. Different PA6 droplet sizes lead to different PA6 crystallization events in a 100 °C wide temperature window that extends down to 85 °C. Static WAXD and DSC experiments on micrometer sized PA6 droplets indicate the formation of a stable γ-crystal phase in the region between 175 and 130 °C. Sub-micrometer sized PA6 droplets only crystallize at 85 °C in the β-phase. Upon heating above the PA6 glass transition, these crystals progressively increase their perfection and ultimately transform into the α-phase around 170 °C.     

High pressure crystallization of random propylene-ethylene copolymers: α-γ Phase diagram

Two random propylene copolymers with low ethylene content synthesized by Ziegler-Natta catalysts were used is this study to investigate the formation of γ-crystal phase during isothermal crystallization at high pressures. At atmospheric pressure these copolymers crystallize in a mixture of α- and γ-crystals. The content of the γ-phase in the copolymer crystals increased with increasing defect content, crystallization temperature and pressure. Wide-angle X-ray diffraction studies showed that crystallization of these copolymers at pressures above 88 MPa and temperature above 142 °C leads to formation of pure γ-phase. The equilibrium melting temperature of the γ-phase has been determined as a function of defect content and crystallization pressure. Temperature-pressure-composition α-γ phase diagram of isotactic polypropylene was constructed based on the Gibbs free energy approach. This diagram enabled the extrapolation of the equilibrium melting temperatures of both phases for defect free isotactic polypropylene. They were found to be 186.9°C for the α-phase and 189.9°C for the γ-phase.     

Crystallization behavior of nano-composite based on poly(vinylidene fluoride) and organically modified layered titanate

To understand the effect of the nano-filler particles on the crystallization kinetics and crystalline structure of poly(vinylidene fluoride) (PVDF) upon nano-composite formation, we have prepared PVDF/organically modified layered titanate nano-composite via melt intercalation technique. The layer titanate (HTO) is a new nano-filler having highly surface charge density compared with conventional layered silicates. The detailed crystallization behavior and its kinetics including the conformational changes of the PVDF chain segment during crystallization of neat PVDF and HTO-based nano-composite (PVDF/HTO) have been investigated by using differential scanning calorimetric, wide-angle X-ray diffraction, light scattering, and infrared spectroscopic analyses. The neat PVDF predominantly formed α-phase in the crystallization temperature range of 110-150 °C. On the other hand, PVDF/HTO exhibited mainly α-phase crystal coexisting with γ- and β-phases at low T_c range (110-135 °C). A major γ-phase crystal coexists with β- and α-phases appeared at high T_c (=140-150 °C), owing to the dispersed layer titanate particles as a nucleating agent. The overall crystallization rate and crystalline structure of pure PVDF were strongly influenced in the presence of layered titanate particles.     

Halloysite nanotubes as a novel β-nucleating agent for isotactic polypropylene

The nucleating ability of halloysite nanotubes (HNTs) towards isotactic polypropylene (iPP) was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM). HNTs are identified to have dual nucleating ability for α-iPP and β-iPP under appropriate kinetics conditions. The formation of β-iPP is dependent on the HNTs loading in the iPP/HNTs composites. The composite with 20 phr of HNTs is found to have the highest content of β-iPP. Under non-isothermal crystallization the content of β-iPP increases with decreasing of the cooling rate. The maximum β-crystal content is obtained at cooling rate of 2.5 °C/min. The supermolecular structure of the β-iPP is identified as β-hedrites with flower-cup-like and axialite-like arrangements of the lamellae. Under isothermal crystallization the β-crystal can be formed in the temperature range of 115-140 °C. Outside the temperature range, no β-iPP can be observed. The content of β-crystal reaches the maximum value at crystallization temperature of 135 °C. The formation of the β-iPP in the composites is correlated to the unique surface characteristics of the HNTs.