In situ studies on the temperature‐related deformation behavior of isotactic polypropylene spherulites with uniaxial stretching: The effect of crystallization conditions

The deformation behavior of isotactic polypropylene (iPP) spherulites with uniaxial stretching was investigated at different drawing temperatures via in situ polarized optical microscope (POM) observation. The iPP spherulites were prepared by two procedures: cooled to the room temperature from melt and annealed at 135, 140, and 145°C for 3 h. It was found that the crystallization conditions dominate the crystalline morphology and even the tensile properties of iPP. For iPP which crystallized during cooling progress, the spherulites were imperfect and the boundaries of the spherulites were diffuse, displaying good toughness at various drawing temperatures. For iPP annealed at high temperatures displayed the brittle fracture‐modes and the crack happened between spherulites, which due to the large and perfective spherulites have thick lamellas and weak connection at interspherulitic boundary. The shape and size of the iPP spherulites formed at 140 and 145°C are affected with uniaxial stretching till to the fracture of the samples at different drawing temperatures. The spherulites obtained at 135°C are deformed along the drawing direction at 100°C but not affected at low drawing temperatures, indicating the toughness increased with the increase of the drawing temperatures. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Crystallinity morphology and dynamic mechanical characteristics of PBT polymer and glass fiber‐reinforced composites

The crystalline morphologies of PBT (poly butylene terephthalate) and its glass fiber reinforced composite systems were investigated in a thin‐film form by polarized optical microscopy and wide‐angle X‐ray diffraction. Three different types of PBT morphology were identified in the Maltese cross pattern: 45° cross pattern (usual type) by solvent crystallization, 90° cross pattern (unusual type) by melt crystallization at low crystallization temperature, and mixed type by melt crystallization at crystallization temperatures higher than 160°C. The glass fibers increased the number density of spherulites and decreased the size of crystallites acting as crystallization nucleation sites without exhibiting trans‐crystallinity at the vicinity of the glass fiber surfaces. Finally, the storage modulus was analyzed by using a dual‐phase continuity model describing the modulus by the power‐law sum of the amorphous‐ and crystalline‐phase moduli. The crystalline‐phase modulus was extracted out from the PBT polymer and composite systems containing different amount of crystallinity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 478–488, 2002     

Crystal morphology,melting behaviors and isothermal crystallization kinetics of SCF/PTT composites

Isothermal crystallization kinetics, subsequent melting behavior, and the crystal morphology of short carbon fiber and poly(trimethylene terephthalate) composites (SCF/PTT) were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The crystal morphology of the composites isothermally crystallized at T_c = 205°C is predominantly banded spherulites observed under polarizing micrographs, while the pattern of banded spherulites changed from ring to serration as the SCF content added into the PTT. Moreover, nonbanded spherulites formed at T_c = 180°C. The commonly used Avrami equation was used to fit the primary stage of the isothermal crystallization. The Avrami exponents n are evaluated to be 1.6–2.0 for the neat PTT and 2.7–3.0 for SCF/PTT composites, and the SCF acting as nucleation agents in composites accelerates the crystallization rate with decreasing the half‐time of crystallization and the sample with SCF component of 2% has the fastest crystallization rate. The crystallization activation energy calculated from the Arrhenius formula suggests that the adding SCF component improved the crystallization ability of the PTT matrix greatly, and the sample with of 2% SCF component has the most crystallization ability. Subsequent melting scans of the isothermally crystallized composites all exhibited triple melting endotherms, in which the more the component of SCF, the lower temperature of the melting peak, indicating the less perfect crystallites formed in those composites. Furthermore, the melting peaks of the same sample are shifted to higher temperature with increasing T_c, suggesting the more perfect crystallites formed at higher T_c. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Effects of shear stress on the crystallization of linear polyethylene and polybutene-1

The effects of shear stress on the crystallization kinetics and morphology of linear polyethylene and polybutene-1 were studied with the aid of a specially designed apparatus. With this equipment, it was possible to heat a thin polymer sample between glass slides to a melt temperature, quench the sample to a crystallization temperature, and then deform the sample in shear by applying a constant load to one of the glass slides. During the deformation, the crystallization process was observed and photographed under a polarizing microscope. Also, the displacement of the glass slide was simultaneously recorded which made possible a determination of the shear strain as a function of time. The results demonstrate that two phenomena may occur in the initially supercooled polymer samples in response to the applied shear stress. In one case, the sample deformed until it fractured, generally exhibiting no evidene of crystallization; in the other, the sample deformed until an inflection point was reached after which the sample became rigid. This latter phenomenon was attributed to crystallization. At low shear stresses, the inflection point was associated with the growth of spherulites which simply became large enough to bridge the glass slides and prevent further deformation of the sample. This generally occurred prior to the completion of the radial growth of the lamellae. At high shear stresses, however, no evidence of crystallization was seen in the microscope until the inflection point was reached. At this point, birefringence was observed in the sample. The resulting structure generally could not be resolved in the microscope, thereby indicating very profuse nucleation. The results obtained clearly demonstrate that the application of a sufficiently high shear stress to an initially supercooled melt has a substantial effect on the rates of crystallization of both polyethylene and polybutene-1. This was shown most dramatically at temperatures close to the melting point, e.g., both polyethylene at 130°C and polybutene-1 at 113°C, which require over 10⁴ sec to crystallize under quiescent conditions, crystallized at approximately 0.05 seconds. The application of a shear stress to a polymer melt is envisaged as resulting in molecular orientation. In accord with the theories of Flory, and Krigbaum and Roe, the associated decrease in entropy of the melt may be considered to increase the supercooling. Under high stresses at which large increases in supercooling result, crystallization occurs more rapidy at the high temperatures and with polymers of lower molecular weight. At low shear stresses, the influence of temperature and molecular weight on the crystallization kinetics is essentially the same as that obseved under quiescent conditions. Observations through the microscope have shown that the application of a shear stress to a polymer melt leads to large increases in the number of crystalline structures formed and to the formation of oriented morphologies. This latter phenomenon arises due to nucleation lines formed by impurities and spherulites in the deforming melt. The impurities and spherulites apparently cause a disturbance which is thought to result in a local increase in stress of the melt and, hence, a local increase in supercooling. Lamellae then nucleate on these lines and grow out radially.     

Crystallization of polyformals: 2. Influence of molecular weight and temperature on the morphology and growth rate in poly(1,3-dioxolane)

The morphology and growth rates of crystallized molecular weight fractions of poly(1,3-dioxolane) covering the range M_n = 8 800 to 120 000 have been studied by polarized light microscopy. Two different supermolecular structures, dependent on molecular weight and crystallization temperature have been found. Spherulites are formed after rapid crystallization and a more disordered morphology is formed at the lowest undercoolings but there is a temperature region where both forms are observed. The disordered form appears first and a consecutive spherulitic growth takes place. The crystallization kinetics were analysed over the temperature range 10°C to 36°C. At crystallization temperatures lower than 15°–18°C, the growth rate is linear and only spherulites are found. In the temperature range from 18°C to 36°C a well defined break is observed in the growth rate but the spherulitic growth rate is always higher than that of the irregular form. The growth rate temperature coefficient was studied and the usual plots are not linear in the whole range of crystallization temperatures. For the high crystallization temperature region, the slope is about twice as great as the low crystallization temperature slope. This is the region where regular spherulites are formed. The comparison between dilatometric and growth rate data has shown that the overall rate and growth rate temperature coefficients are the same.     

Morphology and melt crystallization of poly(L‐lactide) obtained by ring opening polymerization of L‐lactide with zinc catalyst

The morphology and melt crystallization of zinc catalyzed poly(L ‐lactide) (PLLA) were investigated by using differential scanning calorimetry (DSC), polarized optical microscopy, and scanning electron microscopy. Isothermal melt crystallization performed at 95–135°C showed that the morphology depends on the degree of supercooling, as illustrated by crystallite perfection and lamellar thickening behaviors. Double melting peak was observed on DSC thermograms and attributed to the melt‐recrystallization mechanism, small and imperfect crystals becoming gradually more stable ones. Circumferential and hexagonal cracks were detected in PLLA spherulites, which were formed during melt‐crystallization at 135°C and quenching in liquid nitrogen. Rhythmic growth and thermal shrinkage are suggested to be the two main factors accounting for the formation of periodic cracks. Spherulite growth rates of PLLA were evaluated by using combined isothermal and nonisothermal procedures, and were analyzed by the secondary nucleation theory. The maximum growth rate reached 9.1 μm/min at 130°C. The temperature range investigated (120–155°C) belongs to the Regime II of crystallization. The value of U* was found to be 1890 cal/mol, instead of 1500 cal/mol commonly used in literature, and K_g and σ were estimated to be 3.03 × 10⁵ K² and 1.537 × 10^?4 J/m², respectively. As a result, no distinct difference between PLLA catalyzed by zinc metal and those prepared with stannous octoate catalyst exists in this work. POLYM. ENG. SCI., 46:1583–1589, 2006. © 2006 Society of Plastics Engineers.     

Crystallization kinetics and morphology of poly(ethylene suberate)

The crystallization kinetics and morphology of poly(ethylene suberate) (PESub) were studied in detail with differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray diffraction. The Avrami equation could describe the overall isothermal melt crystallization kinetics of PESub at different crystallization temperatures; moreover, the overall crystallization rate of PESub decreased with increasing crystallization temperature. The equilibrium melting point of PESub was determined to be 70.8°C. Ring‐banded spherulites and a crystallization regime II to III transition were found for PESub. The Tobin equation could describe the nonisothermal melt crystallization kinetics of PESub at different cooling rates, while the Ozawa equation failed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43086.     

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

The confined crystallization of poly(?‐caprolactone) (PCL) block in poly(?‐caprolactone)–poly(l ‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry     

Study on the crystal morphology and melting behavior of isothermally crystallized composites of short carbon fiber and poly(trimethylene terephthalate)

The spherulites of the short carbon fiber(SCF)/poly (trimethylene terephthalate) (PTT) composites formed in limited space at designed temperatures, and their melting behaviors were studied by the polarized optical microscopy, atomic force microscopy (AFM), and scanning electron microscopy (SEM), respectively. The results suggest that SCF content, isothermal crystallization temperatures, and the film thicknesses influence the crystal morphology of the composites. The dimension of the spherulites is decreased with increasing SCF content, but whether banded or nonbanded spherulites will form in the composites is not dependent on SCF content. However, the crystal morphology of the composites depends strongly on the temperature. When the isothermal crystallization temperatures increase from 180°C to 230°C, the crystal morphology of SCF/PTT composites continuously changes in the following order: nonbanded → banded → nonbanded spherulites. Discontinuous circle lines form in the film when the film thickness increases from 30 to 60 μm. Basing on the SEM observation, it is found that these circle lines are cracks formed due to the constriction difference of the different parts of the spherulites. These cracks are formed when the film is cooled from the isothermal crystallization temperature to the room temperature at a slow cooling rate; while they will disappear gradually at different temperatures in the heating process. The crack will appear/disappear first around the center of the spherulite when the film was cooled/heated. The nontwisted or slightly twisted lamellas will reorganize to form highly twisted lamellas inducing apparent banded texture of the spherulites.     

Cracking in poly(trimethylene terephthalate) spherulites

The crack behavior of poly(trimethylene terephthalate) spherulites was studied mainly by polarizing optical microscopy, along with scanning electron microscopy (SEM) and atomic force microscopy (AFM). In addition to the effects of temperature and constraining substrate, another important factor, film thickness, on the formation of crack was first put forward and investigated. The emergence of the first crack occurred at 120°C during cooling after crystallization at 190°C for the sample with a thickness of 31.0 μm. For the spherulites growing between glass sheets, it was interesting that the sample with a thickness of 26.0 μm exhibited the largest number of cracks measured per 200 μm of radius, whereas samples thicker than 100 μm or thinner than 1 μm did not induce the formation of crack. Also, spherulites growing between two polyimide and two Teflon sheets showed no crack. Glass sheets lubricated with silicon oil restrained the number of cracks but did not eliminate cracks. SEM revealed that the cracking was about 900 nm in width. In addition, the AFM results suggest that the cracks had a depth of at least 150 nm. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Determination of the α, β, and γ crystalline phases of poly(vinylidene fluoride) films prepared at different conditions

Samples containing the three crystalline phases of poly(vinylidene fluoride), α, β, and γ, have been obtained under distinct crystallization conditions. Samples containing exclusively unoriented β phase have been obtained by crystallization from dimethylformamide (DMF) solution at 60°C. Oriented β phase has been obtained by uniaxial drawing, at 80°C, of an originally α phase sample. Samples containing exclusively α phase have been obtained by melting and posterior cooling at room temperature. Samples containing both α and γ phases have been obtained by melt crystallization at 164 °C for 16 and 36 h. Presence of the crystalline phases in each sample were confirmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXD), polarized light optical microscopy (PLOM), and scanning electron microscopy (SEM). Infrared absorption bands identifying unequivocally the presence of β and γ phases in a sample are presented. It is shown that solution crystallization at T < 70°C always results in the β phase, regardless of the solvent used. Melt temperatures of the respective phases have also been determined. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3272–3279, 2006     

Phase structure,morphology and phase boundary diagram in an aromatic polyimide (BPDA–PFMB)/m-cresol system

An organo-soluble aromatic polyimide has been synthesized from 3,3′, 4,4′-bis(phenyltetracarboxylic) dianhydride (BPDA) and 2,2′-bis (trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) via a one-step polymerization in m-cresol. The phase boundary diagram for this system has been established by differential scanning calorimetry, polarized light microscopy (PLM) and wide angle X-ray diffraction (WAXD) experiments. A crystallosolvate form I has been found over the entire concentration region at low temperatures. When the temperature is increased, an isotropic phase has been observed below concentrations of about 40%. In the relatively high concentration region between 45 and 95%, a transition from the crystallosolvate from I to a crystallosolvate form II has been observed. Form II exhibits a different WAXD pattern. In a narrow concentration region between the isotropic and the crystallosolvate form II (40–45%), a biphase behavior has been found. The birefringence of this region may be an indication of a liquid crystalline phase. The BPDA–PFMB polymer crystal has been found in the very high concentration region (>95%) for temperatures >250°C. The morphologies of these phases have been investigated via PLM and transmission electron microscopy. Above the gel/sol transition temperature, the form I shows negative birefringent spherulites consisting of thin lamellae. The form II exhibits a tendency of positive birefringent spherulites. Possible mechanisms of the formation of the metastable phase morphology and their associations with the mechanical gel/sol transition are also discussed.     

Modification of rheological properties of a thermotropic liquid crystalline polymer by melt-state reactive processing

Thermotropic main-chain liquid crystalline polymers typically have very low melt viscosity with strong temperature dependence compared to other common thermoplastics. While this is beneficial in some processing applications, such as injection molding, it presents challenges for others, such as coextrusion. In this study, the rheological properties of a thermotropic main-chain liquid crystalline polymer (Vectra A950) were enhanced by melt-state reactive processing with triphenyl phosphite (TPP), which can react with up to three polymer chain-ends through their chain-end functionalities. The influence of processing time and TPP content on the shear viscosity and other important material properties were investigated. Optimal conditions, which increased the shear viscosity by nearly a factor of 20 over the neat polymer, were found to be 4 wt% TPP and 30 min of reaction time at 290 °C. Further results from differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy confirmed that coupling with TPP did not affect the microstructure, melting/crystallization behavior or liquid crystallinity. The stability of TPP-modified samples was also studied at 80 °C in air and following melt reprocessing at 290–300 °C under N₂ or air. Samples were stable (as measured by shear viscosity) for more than one month at 80 °C in air or when reprocessed in N₂ at 290 °C for up to 10 min. However, when reprocessed at 300 °C in air, the viscosity enhancement was partially reversed due to scission of P–O bonds that were formed during the initial reaction between the polymer chain-ends and TPP.     

Solid‐state structure of thermally crystallized syndiotactic polystyrene

The solid‐state structure of syndiotactic polystyrene (s‐PS) after crystallization from the melt and the glassy state was examined by differential scanning calorimetry (DSC), density, and X‐ray diffraction analysis. It was possible to prepare semicrystalline s‐PS containing either the pure α‐ or the pure β‐crystalline form by melt crystallizing s‐PS from 280 or 330°C. The measurements confirmed the low density of both crystalline forms, which in the case of α‐crystalline form was smaller and in the case of β‐crystalline form was only slightly larger than the density of the glassy amorphous s‐PS. An endeavor to introduce the crystalline phase in s‐PS through cold crystallization at constant temperature above the glass transition resulted in a complex ordered phase. This ordered phase, depending on the crystallization temperature, contained the planar chain mesomorphic phase and the α‐crystalline phase with a low degree of perfection (cold crystallization in the range 120–175°C) or a mixture of the α‐ and β‐crystalline forms with a high degree of perfection (cold crystallization in the range 210–260°C). The combination of DSC and X‐ray measurements enabled us to resolve the complex ordered structure in semicrystalline s‐PS after cold crystallization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2705–2715, 2002     

Polymorphic and miscibility behavior in crystalline/crystalline blends of poly(pentamethylene terephthalate) with poly(heptamethylene terephthalate)

BACKGROUND: The phase behavior of blends of semicrystalline aryl polyesters with long methylene segments (? (CH₂)_n? with n = 5 or 7) in the repeat units has not been much studied. Thus, crystalline/crystalline blends comprising monomorphic poly(pentamethylene terephthalate) (PPT) and polymorphic poly(heptamethylene terephthalate) (PHepT) were prepared and the crystal growth kinetics, polymorphism behavior and miscibility in this blend system were probed using polarized‐light optical microscopy, differential scanning calorimetry and wide‐angle X‐ray diffraction. RESULTS: The PPT/PHepT blends of all compositions were first proven to be miscible in the melt state or quenched amorphous phase, whose interaction strength was determined (χ₁₂ = ? 0.35), showing favorable interactions and phase homogeneity. Although the spherulites of neat PPT and PHepT could exhibit ring bands at different crystallization temperature (T_c) ranges (100–110 and 50–65 °C, respectively), the spherulites of PPT/PHepT (50/50) blend became ringless in the range 50–110 °C. Growth analysis and polymorphic behavior in the crystalline phases of the blends provided extra evidence for the miscibility between these two crystalline polymers. Spherulitic growth rates of PPT in the PPT/PHepT blends were significantly reduced in comparison with those of neat PPT. In addition, miscible blending of a small fraction of monomorphic PPT (20 wt%) with polymorphic PHepT altered the crystal stability and led to the originally polymorphic PHepT exhibiting only the β‐crystal form when melt‐crystallized at all values of T_c. CONCLUSION: The highly intimate mixing in polymer chains of crystalline PPT and PHepT causes significant disruption in ring‐band patterns and reduction in crystallization rates of PPT as well as alteration in the polymorphic behavior of PHepT. Copyright © 2009 Society of Chemical Industry     

Phase stability and melting behavior of the α and γ phases of nylon 6

The phase stability and melting behavior of nylon 6 were studied by high‐temperature wide‐angle X‐ray diffraction and differential scanning calorimetry (DSC). The results show that most of the α phase obtained by a solution‐precipitation process [nylon 6 powder (Sol‐Ny6)] was thermodynamically stable and mainly melted at 221°C; the double melting peaks were related to the melt of α crystals with different degrees of perfection. The γ phase formed by liquid nitrogen quenching (sample LN‐Ny6) melted within the range 193–225°C. The amorphous phase converted into the γ phase below 180°C but into the high‐temperature α phase at 180–200°C. Both were stable over 220°C. α‐ and γ*‐crystalline structures were formed by annealing but were not so stable upon heating. Typical double melting peaks were shown on the DSC curve; melt recrystallization happened within the range 100–200°C. The peak at 210°C was mainly due to the melting of the less perfect crystalline structure of the γ phase and a fraction of the α phase; the one at 219°C was due to the high‐temperature α‐ and γ‐phase crystals. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of nylon 6 inclusions on the crystalline morphology of polypropylene–nylon 6 blends

Crystallization behavior and crystalline morphology of plain polypropylene (PP) and its blend with 0 to 30 wt % nylon 6 were studied by the hot‐stage polarized light microscopy method. Radial growth rate and the size and number of PP spherulites were measured as a function of both the isothermal crystallization temperature and the nylon 6 content of the blend. The study revealed that a reduction in the isothermal crystallization temperature from 135 to 120°C, for both the plain PP and its blend with nylon 6, leads to the formation of a large number of fast‐growing, small spherulites. Moreover, the size and growth rate of PP spherulites decreased on increasing the nylon 6 content of the blend; whereas the number of PP spherulites decreased sharply on initial addition of 10% nylon 6 and, thereafter, increased slightly by further addition of nylon 6. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1769–1775, 2000     

Promoting effect of crystallization on the foaming behavior in polypropylene homopolymer/polypropylene block copolymer blends

Polypropylene (PP) is a kind of semi‐crystalline polymer so it is hard to foam with supercritical carbon dioxide (SCCO₂). We used polypropylene block (PP‐B) copolymer as a modifier to improve the crystallization behaviors and foaming performance of polypropylene homo polymer (HPP). HPP, PP‐B, and a series of HPP/PP‐B blends were characterized by differential scanning calorimeter (DSC), X‐ray diffraction, and polarized optical microscope. Results show that both the crystallization behaviors and melt strength have influence on the cell structure. The crystallization temperature of PP‐B is about 7°C higher than that of HPP and that the crystallization behavior of HPP/PP‐B blends is similar to that of PP‐B. Much denser and smaller size spherulites are observed in PP‐B and HPP/PP‐B blends than in HPP, and the crystal structure is unchanged after blending. Scanning electron microscope results show that much more uniform, smaller cells can be obtained for the HPP/PP‐B blends. The crystal nuclei formed earlier can act as physical crosslink points, increasing the melt strength and improving dramatically the cell structure and morphology of the HPP/PP‐B blends. Furthermore, the best cell structure and morphology was achieved for HPP/PP‐B blends with the ratio of 70/30 under the same foaming conditions. POLYM. ENG. SCI., 56:1175–1181, 2016. © 2016 Society of Plastics Engineers     

Crystallization of Stoichiometric SbSI Glass

Congruent crystallization of antimony sulphoiodide (SbSI) glass of stoichiometric composition, which is prepared successfully for the first time using rapid melt‐quenching, has been investigated using differential scanning calorimetry (DSC). The results for glass powder show a glass transition at 127°C and two separate exothermal peaks with maxima around 140°C and 190°C. The ratio of the intensities of the exothermal peak at ~190°C to the peak at ~140°C increases as the particle size and heating rate are increased, but their total enthalpy remains constant at 62 ± 2 J/g for all DSC runs. Surface heating of the glass induced by a 520 nm CW laser shows two contracted regions: needle‐like crystalline formations at low temperature and bulk crystallization at high temperature. The observed phenomena and DSC results suggest two different kinds of crystallization of the SbSI phase: one‐dimensional crystallization at low temperature which starts from the sample surface and three‐dimensional bulk crystallization that continues the transformation to crystalline state at higher temperatures. The origin of the two different crystallizations can be traced to the strong anisotropy of the SbSI crystal structure due to the weak van der Waals interaction between covalent‐ionic chains (Sb₂S₂I₂)_n.     

Effect of poly(propylene carbonate) on the crystallization and melting behavior of poly(β‐hydroxybutyrate‐co‐β‐hydroxyvalerate)

The crystallization and melting behavior of poly(β‐hydroxybutyrate‐co‐β‐hydroxyvalerate) (PHBV) and a 30/70 (w/w) PHBV/poly(propylene carbonate) (PPC) blend was investigated with differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR). The transesterification reaction between PHBV and PPC was detected in the melt‐blending process. The interaction between the two macromolecules was confirmed by means of FTIR analysis. During the crystallization process from the melt, the crystallization temperature of the PHBV/PPC blend decreased about 8°C, the melting temperature was depressed by 4°C, and the degree of crystallinity of PHBV in the blend decreased about 9.4%; this was calculated through a comparison of the DSC heating traces for the blend and pure PHBV. These results indicated that imperfect crystals of PHBV formed, crystallization was inhibited, and the crystallization ability of PHBV was weakened in the blend. The equilibrium melting temperatures of PHBV and the 30/70 PHBV/PPC blend isothermally crystallized were 187.1 and 179°C, respectively. The isothermal crystallization kinetics were also studied. The fold surface free energy of the developing crystals of PHBV isothermally crystallized from the melt decreased; however, a depression in the relative degree of crystallization, a reduction of the linear growth rate of the spherulites, and decreases in the equilibrium melting temperature and crystallization capability of PHBV were detected with the addition of PPC. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2514–2521, 2004