Step‐like melting mechanisms of isothermally crystallized isotactic polypropylene

The complex melting behavior of isotactic polypropylene, after isothermal crystallization, was studied within the context of step‐like melting mechanisms which were previously proposed for high temperature polymers. The morphological characteristics of the melting process were also studied as a function of molecular weight, and close similarities were observed with respect to high temperature polymers. Positive birefringence crystals of low molecular weight samples developed double melting behavior in three steps. The first melting step was assigned to continuous melting of secondary crosshatch reversing lamellae, together with recrystallization of the remaining isothermal crystals. In the second melting step (first melting endotherm), crystals tended to lose their original coarse negative birefringence due to melting of secondary reversing branching. This effect rendered new, finer texture, but still negative birefringence crystals. In the third melting step (second melting endotherm), there was a combination of melting of two crystal populations, one consisting of the remaining fraction of reversing primary crystals, and the other consisting of nonreversing primary crystals. A crosshatch secondary branching model was therefore proposed to explain the overall results. Mixed birefringence spherulites of high molecular weight samples displayed similar, although proportional, behavior under identical crystallization and melting conditions corroborating the proposed melting mechanism. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2188–2200, 2008     

Changes in the melting behavior with the radial distance in isotactic polypropylene spherulites

The melting of highly tactic i‐polypropylene occurs in two stages even for crystallization at 145 °C, a temperature at which reorganization during scanning is negligible. A comparison of two such polypropylenes, one nucleated and the other not nucleated, together with fractions from the latter, has been made with electron microscopy following permanganic etching, in addition to differential scanning calorimetry. This has allowed the two melting stages to be assigned to two components of the lamellar morphology, with progressive changes in both occurring with increasing radial distance within a spherulite. The highest melting temperature is for dominant radial lamellae far from a spherulite center. The lowest melting regions are the evenly crosshatched spherulite centers and a narrow peripheral band. Lower melting is attributed to the suppression of isothermal lamellar thickening paralleling recent direct demonstrations in polyethylene. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2342–2354, 2003     

Crystallization and melting of isotactic polypropylene in response to temperature modulation

Isotactic polypropylene (iPP) was crystallized using temperature modulation in a differential scanning calorimeter (DSC) to thicken the crystals formed on cooling from the melt. A cool-heat modulation method was adopted for the preparation of the samples under a series of conditions. The effect of modulation parameters, such as temperature amplitude and period was monitored with the heating rate that followed. Thickening of the lamellae as a result of the crystallization treatment enabled by the cool-heat method lead to an increase in the peak melting temperature and the final traces of melting. For instance, iPP melting peak shifted by up to 3.5°C with temperature amplitude of 1.0°C while the crystallinity was increased from 0.45 (linearly cooled) to 0.53. Multiple melting endotherms were also observed in some cases, but this was sensitive to the temperature changes experienced on cooling. Even with a slower underlying cooling rate and small temperature amplitudes, some recrystallization and reorganization occurred during the subsequent heating scan. The crystallinity was increased significantly and this was attributed to the crystal perfection that occurred at the crystal growth surface. In addition, temperature modulated differential scanning calorimetry (TMDSC) has been used to study the melting of iPP for various crystallization treatments. The reversing and non-reversing contribution under the experimental time scale was modified by the relative crystal stability formed during crystallization. Much of the melting of iPP was found to be irreversible.This revised version was published online in November 2005 with corrections to the Cover Date.     

Crystallization and melting behaviors of isotactic polypropylene nucleated with compound nucleating agents

Crystallization and melting behaviors of isotactic polypropylene (iPP) nucleated with compound nucleating agents of sodium 2,2′‐methylene‐bis (4,6‐di‐tert‐butylphenyl) phosphate (hereinafter called as NA40)/dicyclohexylterephthalamide (hereinafter called as NABW) (weight ratio of NA40 to NABW is 1:1) were studied by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD), the relative β‐amount of iPP nucleated with these compound nucleating agents was also calculated in Turner‐Jones equation by using wide‐angle X‐ray diffraction data. Under isothermal crystallization, there exists a temperature range favorable for formation of β‐iPP. When the concentration of compound nucleating agents is 0.2 wt %, the temperature range is from 100 to 140 °C. While in nonisothermal crystallization, lower cooling rate is favorable for form of β‐iPP and the relative β‐amount of iPP increases with the decreasing of cooling rate in crystallization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 911–916, 2008     

Crystallization and melting of isotactic polypropylene crystallized from quiescent melt and stress‐induced localized melt

Temperature dependency of crystalline lamellar thickness during crystallization and subsequent melting in isotactic polypropylene crystallized from both quiescent molten state and stress‐induced localized melt was investigated using small angle X‐ray scattering technique. Both cases yield well‐defined crystallization lines where inverse lamellar thickness is linearly dependent on crystallization temperature with the stretching‐induced crystallization line shifted slightly to smaller thickness direction than the isothermal crystallization one indicating both crystallization processes being mediated a mesomorphic phase. However, crystallites obtained via different routes (quiescent melt or stress‐induced localized melt) show different melting behaviors. The one from isothermal crystallization melted directly without significant changing in lamellar thickness yielding well‐defined melting line whereas stress‐induced crystallites followed a recrystallization line. Such results can be associated with the different extent of stabilization of crystallites obtained through different crystallization routes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 957–963     

Melting behavior of drawn polypropylene

The melting behavior of drawn, compression-molded isotactic polypropylene has been ex-amined in terms of the infuence of drawing conditions on the observed properties. Two endothermic peaks were observed on differential scanning calorimetry (DSC) for samples when high draw ratios and high heating rates were used during DSC tests. The peak at lower temperature is influenced by draw ratio, temperature, and rate, and exhibits a strong superheating effect. The species associated with this peak can partially recrystallize into another species associated with the peak at higher temprature during DSC measurements. The position of the peak at higher temperature depends only on draw ratio. It is proposed that the doubel-melting peaks at lower and higher temperature result from extremely thin quasi-amorphous or crystalline layers between microfibrils and the lamellar crystals within microfibrils, repectively. © 1993 John Wiley & Sons, Inc.     

Drawing behavior and characteristics of laser‐drawn polypropylene fibers

Drawing behavior, flow drawing, and neck drawing, was studied for isotacticpolypropylene fibers in CO₂ laser drawing system, and the fiber structure and the mechanical properties of drawn fibers were analyzed. For a certain laser power, flow drawing of polypropylene (PP) was possible up to draw ratio (DR) 19.5. Though the drawing stress was very low, the flow‐drawn PP fiber exhibited oriented crystal structure and improved mechanical properties. On the other hand, neck‐drawing was accomplished from DR 4 to 12, with significant increase in drawing stress that enhanced the development of fiber structure and mechanical properties. Unlike PET, the drawing stress depends not only on the DR, but on irradiated laser power also. The 10–12 times neck‐drawn fibers were highly fibrillated. The fibers having tensile strength 910 MPa, initial modulus 11 GPa, and dynamic modulus 14 GPa were obtained by single‐step laser drawing system. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 398–408, 2006     

Partial melting and recrystallization of isotactic polypropylene

<正>A relatively high predetermined crystallization temperature(135℃) was chosen to grow well developed iPP spherulites,then the partial melting was carried out at a temperature of 165℃,where the preformed spherulites were seen to only decrease their size but not completely melted.The crystallization behavior of partially melted isotactic polypropylene (iPP) has been carefully examined by different scanning calorimetry(DSC) and polarized light microscopy(PLM).The experimental results show that at a special annealing temperature(165℃) the melting behavior of iPP includes two parts with different mechanism,one part is the melting of iPP spherulite outside,another is the partial lamellae perfection during longer annealing time in the unmelted spherulite.The conformational orders of the iPP melt decrease with the increase of the annealing temperature.     

Molecular weight dependency of crystallization and melting behavior of β‐nucleated isotactic polypropylene

Compounds of isotactic polypropylene (iPP) and β‐nucleating agent were used to investigate the relationship between the development of β phase and molecular weight in iPP under quiescent crystallization conditions by using wide angle X‐ray diffraction and differential scanning calorimetry techniques. In all cases, the dependency of the formation of β phase in iPP on molecular weight of iPP at a defined crystallization temperature range was found. The iPP with high molecular weight possessed a wide range of crystallization temperature in inducing rich β phase. However, poor or even no β phase was obtained for the samples with low molecular weight in the same range. In addition, an upper critical crystallization temperature of producing dominant β phase was found at 125 °C. Beyond this temperature, a phenomenon of prevailing α phase became obvious. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1301–1308     

Thermal analysis and X‐ray scattering study of metallocene isotactic polypropylene prepared by partial melting

In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m‐iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic α and γ phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self‐nucleation temperatures (T_n's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160 °C. The relative amounts of α and γ crystals are determined from the area under the unique wide‐angle X‐ray reflections. The lower and upper endotherms are caused by the melting of γ and α crystals, respectively. Four distinct regions of T_n were identified on the basis of the thermal and structural parameters of m‐iPP. In region I, T_n is below the peak melting temperature of the γ phase. Here, γ crystals are annealed and α crystals are barely affected by T_n. In region II, T_n is above the peak of the lower endotherm but below the peak of the upper endotherm. γ crystals melt, and α crystals anneal. In both regions I and II, the portion of the sample melted at T_n recrystallizes epitaxially with existing parent α lamellae as the substrates, and the amount of α always exceeds the amount of γ. In region III, T_n is above the peak of the upper endotherm, and all γ crystals and some or all α crystals are melted at T_n. The number of α‐crystal nuclei steadily decreases as T_n increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, T_n exceeds the upper endotherm, and only small self‐nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of α and γ is observed during cooling from T_n. Because of the effective nucleating ability of α toward γ, as the temperature drops, the amount of γ increases and then exceeds the amount of α. With subsequent reheating, the reverse crossover occurs because of the lower melting point of γ. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1644–1660, 2002     

Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization

The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 °C/min. The WAXD experiments proved that only α-form crystal was formed during SIC process and no transition from α1- to α2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into γ-form crystal having a low melting temperature whereas the long sequences crystallized into α-form crystal having high melting temperature.     

A WAXD/SAXS/DSC study on the melting behavior of Ziegler–Natta and metallocene catalyzed isotactic polypropylene

A small- and wide-angle X-ray scattering study was performed on two metallocene catalyzed isotactic polypropylene (miPP) resins. The results were compared with two similar molecular weight Ziegler–Natta catalyzed isotactic polypropylene (zniPP) materials. Wide-angle X-ray diffraction (WAXD) results showed the existence of two crystalline structures in the metallocene samples, the α-monoclinic and γ-orthorhombic crystal structure, with increasing relative amounts of γ-orthorhombic phase as the lamellae thickness increased. Differential scanning calorimetry (DSC) scans exhibited a melting peak for each crystal structure. The metallocene resins had the same equilibrium melting temperature (186 ± 2 °C) as the high tacticity Ziegler–Natta (ZNHT) resin, whereas a second Ziegler–Natta resin had a lower equilibrium melting temperature (178 ± 2 °C). The equilibrium melting temperature for the γ-orthorhombic crystal structure in the metallocene resins was found to be 178 ± 4 °C. The results were explained by the distribution of defects within the miPP chains, generating higher fold surface free energies for the miPP resins. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3050–3064, 1999     

Crystallization and equilibrium melting behavior of PBT/PETG blends

The results of studies of equilibrium melting point and crystallization behavior of PBT/PETG blends are reported for the first time. A single composition‐dependent glass‐transition temperature is observed in the DSC studies. The isothermal crystallization studies of the blends indicate retardation in crystallization rate as evidenced by the increase in crystallization half time. The retardation in crystallization rate has been attributed to the miscibility in the molten state and the hindrance to the diffusion of crystallizable units. This assumption is further supported by the composition dependence of the crystallization half time. A composition‐dependent melting point depression has been observed which has been attributed to the possible thermodynamic and morphological effects. The interaction parameter calculated by analyzing equilibrium melting point depression shows composition‐dependent negative values confirming the miscibility of the systems. These results are in good agreement with our earlier results on mechanical and dynamic mechanical properties of PBT/PETG blends. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2439–2444, 1999     

The equilibrium melting points of random ethylene‐octene copolymers: A test of the Flory and Sanchez–Eby theories

Ethylene/l‐octene copolymers produced with metallocene catalysts are believed to have a homogeneous comonomer content with respect to molecular weight. Two series of copolymers of different molecular weights with a 1‐octene content ranging from 0 to 39 branches per 1000 carbon atoms were studied. The influence of branch content on structure and melting behavior as well as on isothermal and nonisothermal bulk crystallization was studied. In this article, the equilibrium melting temperatures of ethylene/l‐octene random copolymers is the focus. The principal techniques used were thermal analysis and small‐angle X‐ray scattering. The use of Hoffman–Weeks plots to obtain the equilibrium melting temperatures of ethylene/l‐octene random copolymers resulted in nonsensical high values of the equilibrium melting point or showed behavior parallel to the T_m = T_c line, resulting in no intercept and, hence, an infinite equilibrium melting point. The equilibrium melting temperatures of linear polyethylenes and homogeneous ethylene/l‐octene random copolymers were determined as a function of molecular weight and branch content via Thompson–Gibbs plots involving lamellar thickness data obtained from small‐angle X‐ray scattering. This systematic study made possible the evaluation of two equilibrium melting temperature depression equations for olefin‐type random copolymers, the Flory equation and the Sanchez–Eby equation, as a function of defect content and molecular weight. The range over which the two equations could be applied depended on the defect content after correction for the effect of molecular weight on the equilibrium melting temperature. The equilibrium melting temperature, T(n, p_B), of the ethylene/l‐octene random copolymers was a function of the molecular weight and defect content for low defect contents (p_B ≤ 1.0%). T(n, p_B) was a weak function of molecular weight and a strong function of the defect content at a high defect content (p_B ≥ 1.0%). The Flory copolymer equation could predict T(n, p_B) at p_B ≤ 1.0% when corrections for the effect of molecular weight were made. The Sanchez–Eby uniform inclusion model could predict T(n, p_B) at a high defect content (1.6% ≤ p_B ≤ 2.0%). We conclude that some defects were included in the crystalline phase and that the excess free energies (18–37 kJ/mol) estimated in this study were within the theoretical range. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 154–170, 2000     

Quantitative structural characterization of the melting behavior of isotactic polypropylene

The melting behavior of restrained isotactic polypropylene fibers is examined quantitatively in terms of the influence the anisotropic structural state of the polymer has on the observed properties. Two endotherm peaks are observed to occur in some of the samples. The formation and location of the multiple peaks are determined by the orientation of the noncrystalline chains, and is independent of the fabrication path used to achieve that orientation. Above a certain minimum orientation of the noncrystalline chains, multiple endotherm peak formation occurs. The high-temperature endotherm (T_2M) extrapolates to an ultimate melting point for fully oriented noncrystalline chains of 220°C, while the lower-temperature endotherm (T_1M) extrapolates to an ultimate melting point of 185°C. Noncrystalline chain orientation influences the endotherm temperature through its changing configurational entropy. It is shown quantitatively that the noncrystalline polymer must be considered as plastically deformed, since rubber elasticity theory is not followed as predicted. The melting behavior of isothermally crystallized samples are also reported to further elucidate the nature of the observed endotherms.     

Mechanical properties and structure of the zone‐drawn poly(L‐lactic acid) fibers

The zone‐drawing (ZD) method was applied three times to the melt‐spun poly(L ‐lactic acid) (PLLA) fibers of low molecular weight (M_v = 13,100) at different temperatures under various tensions. The mechanical properties and superstructure of the ZD fibers were investigated. The resulting ZD‐3 fiber had a draw ratio of 10.5, birefringence of 37.31 × 10⁻³, and crystallinity of 37%, while an orientation factor of crystallites remarkably increased to 0.985 by the ZD‐1. The Young's modulus and tensile strength of the ZD‐3 fiber respectively attained 9.1 GPa and 275 MPa, and the dynamic storage modulus was 10.4 GPa at room temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 991–996, 1999     

Nonisothermal melting and crystallization of polypropylene in model composites: Kinetic analysis

The kinetics of nonisothermal melting and the crystallization of polypropylene (PP) in polypropylene/carbon‐fiber (C/PP) composites were studied by differential scanning calorimetry with the Nedkov and Atanasov method. Characteristic parameters such as the lamellar thickness, the transport energy through the phase boundary, and the surface free energy were determined and analyzed. In nonisothermal melting, the nucleation effect of carbon fibers was confirmed by decreasing transport energy (79 and 41 kJ/mol for PP and C/PP, respectively) and surface free energy (8 × 10^?4 and 7.9 × 10^?5 J/m² for PP and C/PP, respectively). Depending on the carbon‐fiber content, the lamellar thickness changed from 6.7 × 10^?9 m to 9.05 × 10^?9 m. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 66–73, 2005     

Crystallization kinetics and melting behavior of syndiotactic 1,2‐polybutadiene

Differential scanning calorimetry was used to investigate the isothermal crystallization, subsequent melting behavior, and nonisothermal crystallization of syndiotactic 1,2‐polybutadiene (st‐1,2‐PB) produced with an iron‐based catalyst system. The isothermal crystallization of two fractions was analyzed according to the Avrami equation. The morphology of the crystallite was observed with polarized optical microscopy. Double melting peaks were observed for the samples isothermally crystallized at 125–155 °C. The low‐temperature melting peak, which appeared approximately 5 °C above the crystallization temperature, was attributed to the melting of imperfect crystals formed by the less stereoregular fraction. The high‐temperature melting peak was associated with the melting of perfect crystals formed by the stereoregular fraction. With the Hoffman–Weeks approach, the value of the equilibrium melting temperature was derived. During the nonisothermal crystallization, the Ozawa method was limited in obtaining the kinetic parameters of st‐1,2‐PB. A new method that combined the Ozawa method and the Avrami method was employed to analyze the nonisothermal crystallization of st‐1,2‐PB. The activation energies of crystallization under nonisothermal conditions were calculated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 553–561, 2005