Atomic force microscopy study of the lamellar growth of isotactic polypropylene

The growth behaviors of cross-hatched and lath-like lamellae of α-form spherulites and flat-on lamellae of β-form pherulites of isotactic polypropylene were studied with a high-temperature atomic force microscopy in situ and in real time. The growth rates of crystal lamellae in types I, II and mixed α-form spherulites and in β-form spherulites, as well as the spatial frequency of tangential branching, were measured. The frequency of tangential branching increases with decreasing crystallization temperature, while the growth rates of leading radial and tangential lamellae are approximately the same at a given temperature. Observations of as-crystallized materials demonstrated that the spacing and length of transverse lamellae is sufficient to differentiate among spherulite types. Height measurements in the melt near the growth surface indicate roles of molecular transport in the crystallization process.     

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.     

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.     

Real time in situ X-ray diffraction studies on the melting memory effect in the crystallization of β-isotactic polypropylene

The melting memory effect during the crystallization and heating of semi-crystalline polymers was clearly demonstrated using β-isotactic polypropylene (β-iPP). Differential scanning calorimetry and real-time in situ X-ray diffraction using a synchrotron radiation source were employed to investigate the role of the newly formed α-form crystals via phase transformation from the metastable β-form during the melting process, and to elucidate the memory effect of these new α-form crystals during the crystallization process. The evolution of the memory effect in β-iPP during the crystallization and melting processes is ideally based on the existence of locally ordered α-form in the melt. We monitored the role of this local order by preparing the melt state using a range of hold temperatures and hold times. It was found that the final melt temperature and hold time greatly affect the crystallization behavior during cooling and the phase transformation behavior during heating. Lower hold temperatures and shorter hold times lead to samples rich in α-modification, whereas longer hold times generate samples rich in β-modification during crystallization. At higher hold temperatures even a short hold time is sufficient to destroy the local order in the melt, and the resulting sample exhibits more β-modification. The results are explained on the basis of the existence of local order in the amorphous melt along with external nucleating agent during the crystallization process.     

Effects of crystallization temperature on the polymorphic behavior of syndiotactic polystyrene

The influence of crystallization temperature on formation of the α- and β-form crystals of syndiotactic polystyrene (sPS) was investigated by X-ray diffraction and non-isothermal differential scanning calorimetry analysis. For sPS samples without any thermal history, the crystallization temperature must be the intrinsic factor controlling the formation the α and β-form crystals. Being crystallized at different cooling rate from the melt, sPS forms the β-form crystal until the temperature cooled down to about 230 °C, and α-form crystal can only be obtained when the temperature was below about 230 °C.     

Study of the Polymorphism of Saturated Monoacid Triglycerides I: Melting and Crystallization Behaviour of Tristearin

The crystallization and melting behaviour of tristearin has been considered in detail. Both the thermal and structural characteristics of the β'- and β-crystal forms have been found to be largely dependent on the crystallization conditions. For the α-form, crystallization takes place very fast at low undercooling (T about 3°C). Upon melting, the α-form transforms directly to β, without any detectable appearance of a β'-form. The β'-form can only be obtained properly from the isotropic melt within a narrow temperature range (54 to 57°C). Above 57°C, β-crystallization becomes dominant. The main difference between the β'- and β-crystallization process is the induction time for crystallization. The decrease in β'-crystallization kinetics with increasing crystallization temperature is expressed in a longer induction time as well as in a slower rate of crystallization. In the case of the β-crystallization, the decrease in the overall crystallization rate mainly results from the sharp increase in induction time. The experimental data do not support the existence of distinct multiple subforms for the β'-form. The difference between β'₂ and β'₁ seems to be due to differences in the degree of ordering of the molecules in the β'-form. No significant differences have been observed between tristearin and tripalmitin with respect to their polymorphic behaviour. Both triglycerides only differ from each other in the kinetics of the crystallization and transformation.     

Study of the Polymorphism of Saturated Monoacid Triglycerides II: Polymorphic Behaviour of a 50/50 Mixture of Tripalmitin and Tristearin

The polymorphic behaviour of a 50/50 blend of tripalmitin and tristearin has been investigated in detail using differential scanning calorimetry and X-ray diffraction. The blend is characterized by a greater tendency to β'-crystallization as compared to the pure triglycerides. Tripalmitin and tristearin, when mixed in a 1:1 ratio, are miscible in both the α-form and the β-form. In the β-form, however, demixing occurs, resulting in a 2-phase solid state. The characteristics of the α-form are considerably affected by the crystallization conditions, due to the formation of concentration gradients during crystallization. The β'-form can be obtained from the melt as well as via recrystallization of the α-form, and is characterized by a much higher stability as compared to the pure triglycerides. The X-ray diffraction data of the β'-form of the blend reveal a β₁-crystal structure. The β'-form of the pure triglycerides, however, is characterized by a β'₂-crystal structure. On the basis of the present data, however, no clear structural distinction can be made between β'₂ and β'₁.     

Synthesis and characterization of novel random copolymers based on PBN: Influence of thiodiethylene naphthalate co-units on its polymorphic behaviour

Poly(butylene/thiodiethylene naphthalate) copolymers (PBN-PTDEN) were synthesized in bulk according to the usual polycondensation procedure and examined by NMR, GPC, TGA, DSC and XRD techniques. At room temperature they appeared as semicrystalline materials; the copolymerization caused a lowering in the T_g value, a decrement of T_m and of the crystallization rate. Pure α- or β′-form was obtained at low and high TDEN unit content, respectively; crystalline form transition never occurred in the solid state, analogously to PBN. After cooling from the melt, the pure α-form was always evidenced in PBN-PTDEN10, whereas the pure β′ crystal phase develops in the copolymers containing 30 and 40 mol% TDEN units, independently on the cooling rate. In the case of PBN-PTDEN20 a pure α- or β′-form was obtained at low and high cooling rate, respectively.     

Compared structure and morphology of nylon-12 and 10-polyurethane lamellar crystals

A comparative study of the structure and morphology of nylon-12 and 10-polyurethane (10-PUR) lamellar crystals, was carried out. Lamellar crystals were obtained by isothermal crystallization from diluted solution. Electron diffraction of lamellae combined with WAXS data recorded from crystal sediments indicated that nylon-12 crystallized in either α-form or γ-form according to the solvent chosen for crystallization. The α-form was the crystal modification predominant in doubly oriented films of nylon-12 prepared by epitaxial crystallization. On the contrary, 10-PUR invariably crystallized in α-form regardless crystallization conditions. The α-form of nylon-12 and 10-PUR shared the same crystal structure with hydrogen-bonded sheets made of antiparallel chains and stacked with progressive shifting along both b and c directions. Lamellar crystals of nylon-12 in γ-form and 10-PUR in α-form displayed similar morphological features but only the former appeared to be sensitive to temperature. Upon heating the nylon-12 crystals near to melting, the real-time WAXS analysis evidenced the occurrence of a partial γ-to-α crystal transition, and in situ AFM observations revealed the appearing of more or less regular ridges on the crystal surface. None of these changes were observed in 10-PUR crystals when subjected to similar treatment.     

Isotactic polypropylene crystallized from the melt. I. Morphological study

The spherulitic structure of isotactic polypropylene (iPP) from the melt was studied by polarized light and scanning electron microscopy. From the crystallization morphology, it can be observed that crystallization of iPP from the melt below 132°C forms two types of spherulites, termed α- and β-spherulites. The structure of iPP isothermally crystallized above 132°C shows α-type only. The α-spherulites have a complex crosshatched array of radial and tangential lamellar structures, while β-spherulites have, to some extent, simpler lamellar morphology with lower crosshatching content compared with α-type. However, in α-spherulites the radial lamellar thickness is greater than that of tangential lamellae, but in β-spherulites the radial and tangential lamellae have approximately the same thickness. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1259–1265, 1998     

Higher order structures and mechanical properties of bacterial homo poly(3-hydroxybutyrate) fibers prepared by cold-drawing and annealing processes

Pure bacterial homo poly(3-hydroxybutyrate) (PHB) fibers were prepared by melt spinning, followed by cold-drawing in an amorphous state at a temperature just above its glass transition temperature. Cold drawn fibers obtained were further drawn at higher temperatures, followed by annealing at various temperatures under tension. Relations among the processing conditions, higher order structures and mechanical properties were investigated using wide- and small-angle X-ray diffractions (WAXD and SAXD, respectively), birefringence, differential scanning calorimetry (DSC), and tensile measurements. PHB has two different crystalline forms, 2₁ helix conformation (α-form) and planar zigzag conformation (β-form). A single broad reflection of β-form was detected even in a PHB fiber drawn once at a temperature just above its T_g immediately after quenching and it tended to be stronger after 2nd drawing at higher temperatures. Annealing under low temperature and high tension facilitates the occurrence of β-form. It is suggested that the β-form crystal is formed not only from the tie chains between α-form lamella, but also from completely free amorphous chains. Changes in the amount of two types of crystals were analyzed using the WAXD integrated intensity. Birefringence of these fibers shows negative and positive values, depending on process conditions. Changes in higher order structure on the mechanical properties are also discussed.     

Circular dichroism of poly(β-benzyl-l-aspartate) films in the α, β and ω conformations

The conformations of poly(β-benzyl l-aspartate) (PBLA) in film were studied by means of circular dichroism (c.d.). Films of PBLA show c.d. which is related to the benzyl chromophores of this polypeptide. The magnitude and sign of the c.d. of the benzyl chromophores depend on the conformation i.e. α-helix, ω-helix or β-form. The aromatic c.d. of the left-handed α-helix or the left-handed ω-helix is negative in sign, and that of the β-form is positive in sign. The absolute magnitude of the c.d. of PBLA is much larger in the ω-helical conformation than in the α-helical or the β-conformations. In the left-handed α-helical PBLA (α-PBLA), the positive dichroism band of the $\text{n?π1}$ peptide electronic transition is observed at about 226 nm. When the α-PBLA is converted to the left-handed ω-helical PBLA (ω-PBLA), the band at 226 nm shifts to 224 nm, and its magnitude becomes much smaller. In the β-form PBLA (β-PBLA), this band is negative in sign and observed at 223 nm. The absolute magnitude of the c.d. relative to this electronic transition follows the order: β-form < ω-helix < α-helix.     

Formation and stability of β-structure in biodegradable ultra-high-molecular-weight poly(3-hydroxybutyrate) by infrared, Raman, and quantum chemical calculation studies

Infrared (IR) and Raman spectra were measured for four kinds of ultra-high-molecular-weight poly[(R)-3-hydroxybutyrate] (UHMW-PHB) films: a solvent-cast film, a cold-drawn film, a two-step-drawn film, and a hot-drawn film. Quantum chemical calculations were made for octamer models of UHMW-PHB with a helix conformation (α-form) and a planar zigzag conformation (β-form). Comparison of the results between the Raman spectra of four kinds of films and the quantum chemical calculations of octamer models revealed that only two-step-drawn film yields additional bands at 1735, 966, 935, 908, and 858 cm⁻¹ assignable to the β-structure, suggesting that it contains the β-form as well as the α-form. Detailed comparison of the frequencies and intensities of Raman bands between the observed and calculated values for the β-form indicates that the amount of β-form is relatively small and that the β-structure has a less ordered structure. The infrared and Raman spectra of two-step-drawn film also indicate that it has more amorphous parts than other films. When the two-step-drawn film was further heated up to 130 °C and then cooled down to room temperature, the above additional bands due to the β-form disappeared in the Raman spectra, suggesting that the β-form is less stable than α-form.     

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.     

Rod-like crystal growth of dioctyl substituted polyfluorene from nematic and isotropic states

The growth behavior of the rod-like α-form crystals from both the liquid crystal and the isotropic melt state was studied in situ and in real time using a hot-stage atomic force microscopy. The growth rate and direction of the poly(9,9-dioctylfluorene) (PFO) α-form crystals are greatly affected by the orientation of the polymer chains in the liquid crystal matrix. The pre-existence of chain alignment in the liquid crystal matrix serves as a precursor for crystallization, which greatly reduces the growth energy barrier and promotes the overall growth rate of PFO α-form crystals. Our results provide some valuable information for understanding the relationship between morphology and photoexcited emission behaviors.     

Effects of polyoxyethylene nonylphenol on dynamic mechanical properties and crystallization of polypropylene

Polypropylene (PP) was blended with polyoxyethylene nonylphenol (PN) in a twin-screw extruder and injection moulded. The dynamic mechanical properties of PP/PN blends were characterized by dynamic mechanical analyser (DMA). The glass transition temperature (T_g) of PP showed a slight decrease with incorporation of PN. Differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and polarized optical microscopy (POM) were employed to investigate the effects of PN on crystallization of PP. In a study of nonisothermal crystallization of PP and PP/PN blends, crystallization parameter analysis showed the addition of PN reduced the peak temperature of crystallization. β-form crystals of PP coexisted with α-form crystals in PP/PN blends, and oriented on the surface layer of injection moulded bar as revealed by WAXD. The degree of orientation was determined using Hermans orientation function. The thermal stability of β-form crystals was evaluated using high temperature WAXD and POM.     

Higher-order structural analysis of bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanote] highly oriented films

Bacterial poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] (PHBH) highly oriented films were prepared by the combination of roll and uniaxial drawing processes. The change in the higher-order structure of PHBH films was investigated by wide-angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). Extended films, which show superior mechanical properties and high ductility, have complicated structures. By roll extension, both deformed and undeformed spherulites co-exist, the former inclined to the direction perpendicular to the film surface. The latter were destroyed by further uniaxial extension. The tie-molecules between uniaxially oriented lamellae were extended and transformed to the β-form with a planar zig-zag conformation. Three kinds of structures, c-axis parallel to the uniaxial drawing direction, c-axis inclining to the normal vector of the film surface, and the β-form between lamellae, are intermingled in the film.     

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.     

Optimization of temperature swing strategy for selective cooling crystallization of α-form l-glutamic acid crystals

l-glutamic acid can be crystallized as metastable α-form and stable β-form crystal. The α-form is desired because of its prismatic shape. Production of α-form of l-glutamic acid by cooling crystallization is not well-defined and α-form solid is commercially not available. In this study, an optimal cooling strategy to selectively produce large and narrowly distributed α-crystals is found by modeling and optimizing the crystallization and polymorphic transformation of l-glutamic acid. The optimal temperature profile is found to be cooling-heating-cooling concept where short nucleation period is followed by growth period in metastable zone. The obtained α-form of L-glutamic acid by optimal strategy had improved mean size, distribution, and purity compared to constant cooling.