Isothermal crystallization study on a biodegradable segmented copolymer constituted by glycolide and trimethylene carbonate units

The isothermal crystallization behavior of a segmented copolymer constituted by hard blocks of polyglycolide and soft segments derived from the copolymerization of glycolide and trimethylene carbonate was investigated. This polymer has applied relevance because it is one of the most widely used for bioabsorbable surgical sutures. Calorimetric, optical microscopy, and infrared techniques were combined to understand the thermal properties and the different factors that influence the crystallization process. Basically, only the hard blocks crystallized, although certain processing conditions allowed performing an additional crystallization associated with small lamellar domains of the soft segment. Crystallization from both the melt and the glass state rendered positive spherulites with a fibrillar texture. The observed unusual sign of birefringence was a consequence of the close packing structure of polyglycolide, which was also corroborated by electron diffraction patterns. Crystallization was characterized by an athermal nucleation, which allowed accurate estimation of the secondary nucleation parameter by using the calorimetric data only. Significant differences in the Avrami exponent (from 2.32 to 1.45) were found between the cold and hot isothermal crystallizations. The stronger geometric constraints observed in the crystallization from the glass state were also corroborated by FTIR analyses. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Crystallization kinetics of glass fiber reinforced PBT composites

The effect of glass fibers on the crystallization of poly(butylene terephthalate) (PBT) was investigated by crystallization kinetics analysis under isothermal and nonisothermal conditions. From the crosspolar optical micrographs of melt‐ and solvent‐crystallized PBT composites, the glass fibers were found to increase the number density and decrease the size of crystallites. The glass fibers provided heterogeneous nucleation sites, and thus enhanced the overall rate of PBT crystallization in isothermal experiments. However, the Avrami exponent and the regime transitions were not significantly affected by the presence of glass fibers. For the nonisothermal kinetics of PBT composites, the model prediction was excellent in most ranges of crystallization, but it deviated above 70% of crystallization especially at fast cooling rates (>40°C/min). This discrepancy of the model seemed to result from the growth regime transitions, which were clearly observed especially at high undercoolings. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 576–585, 2000     

Isothermal crystallization of poly(glycolic acid-alt-6-hydroxyhexanoic acid) studied by DSC and real time synchrotron SAXS/WAXD

The thermal properties and the isothermal hot crystallization behavior of a new regular polyester constituted of glycolic acid and 6-hydroxyhexanoic acid units were studied by differential scanning calorimetry (DSC). The morphological development during isothermal crystallization was also investigated using simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques with synchrotron radiation. A crystalline-amorphous two-phase lamellar system was assumed and interpreted using correlation functions.Kinetic parameters deduced from the Avrami analysis indicated a three-dimensional spherulitic growth from heterogeneous nuclei, whereas optical micrographs revealed the formation of small spherulites with a negative birefringence and a fibrillar texture. The Lauritzen and Hoffman analysis of crystallization regimes was performed taking into account the kinetic constants of the overall crystallization process deduced from DSC and also the reciprocal crystallization halftimes evaluated by means of DSC, SAXS and WAXD experiments. Data were consistent with a single crystallization regime and yielded similar values for the transport activation energy and the nucleation constant in all cases.The evolution of long period and lamellar thickness was evaluated during crystallization. Changes with both crystallization time and crystallization temperature were significant.     

Cold‐crystallization kinetics of syndiotactic polystyrene

The kinetics of the isothermal and nonisothermal cold crystallization of syndiotactic polystyrene (s‐PS) were characterized with differential scanning calorimetry. A Johnson–Mehl–Avrami analysis of the isothermal experiments indicated that the cold crystallization of s‐PS at a constant temperature followed a diffusion‐controlled growth mode with a decreasing nucleation rate. Furthermore, the slow nucleation rate was the controlling step of the entire kinetic process. For nonisothermal cold‐crystallization kinetics, we used a simple model based on a combination of the well‐known Avrami and Ozawa models. The analysis revealed that, unlike for melt crystallization, the Avrami and Ozawa exponents were not equal. The activation energies for the isothermal and nonisothermal cold crystallizations of s‐PS were 792.0 and 148.62 kJ mol^?1, respectively, indicating that the smaller motion units in cold crystallization had a weaker temperature dependence than those in melt crystallization. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3464–3470, 2003     

Phase separation and crystallization behavior in extruded polypropylene/ethylene–propylene rubber blends

Liquid–liquid (L–L) phase separation and its effects on crystallization in polypropylene (PP)/ethylene–propylene rubber (EPR) blends obtained by melt extrusion were investigated by time‐resolved light scattering (TRLS) and optical microscopy. L–L phase separation via spinodal decomposition (SD) was confirmed by TRLS data. After L–L phase separation at 250°C for various durations, blend samples were subjected to a temperature drop to 130°C for isothermal crystallization, and the effects of L–L phase separation on crystallization were investigated. Memory of the L–L phase separation via SD remained for crystallization. The crystallization rate decreased with increasing L–L phase‐separated time at 250°C. Slow crystallization for the long L–L phase‐separated time could be ascribed to decreasing chain mobility of PP with a decrease in the EPR component in the PP‐rich region. The propylene‐rich EPR exhibited good affinity with PP, leading to a slow growth of a concentration fluctuation during annealing. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 695–700, 2001     

Primary and secondary crystallization kinetic analysis of nylon 1212

The isothermal and non‐isothermal melt‐crystallization kinetics of nylon 1212 were investigated by differential scanning calorimetry. Primary and secondary crystallization behaviors were analysed based on different approaches. The results obtained suggested that primary crystallization under isothermal conditions involves three‐dimensional spherulite growth initiated by athermal nucleation, while under non‐isothermal conditions, the mechanism of primary crystallization is more complex. Secondary crystallization displays a lower‐dimensional crystal growth, both in the isothermal and non‐isothermal processes. The crystallite morphology of nylon 1212, isothermally crystallized at various temperatures, was observed by polarized optical microscopy. The activation energies of crystallization under isothermal and non‐isothermal conditions were also calculated based on different approaches. Copyright © 2004 Society of Chemical Industry     

Crystallization behavior of biodegradable poly(L‐lactic acid) filled with a powerful nucleating agent: N,N′‐bis(benzoyl) suberic acid dihydrazide

N,N′‐Bis(benzoyl) suberic acid dihydrazide (NA) as nucleating agent for poly(L ‐lactic acid) (PLLA) was synthesized from benzoyl hydrazine and suberoyl chloride, which was deprived from suberic acid via acylation. PLLA/NA samples were prepared by melt blending and a hot‐press forming process. The nonisothermal and isothermal crystallization, spherulite morphology, and melting behavior of PLLA/NA with different contents of NA were investigated with differential scanning calorimetry, depolarized‐light intensity measurement, scanning electron microscopy, polarized optical microscopy, and wide‐angle X‐ray diffraction. With the incorporation of NA, the crystallization peak became sharper and shifted to a higher temperature as the degree of supercooling decreased at a cooling rate of 1°C/min from the melt. Nonisothermal crystallization indicated that the presence of NA accelerated the overall PLLA crystallization. In isothermal crystallization from the melt, the presence of NA affected the isothermal crystalline behaviors of PLLA remarkably. The addition of NA led to a shorter crystallization time and a faster overall crystallization rate; this meant that there was a heterogeneous nucleation effect of NA on the crystallization of PLLA. With the addition of 0.8% NA, the crystallization half‐time of PLLA/NA decreased from 26.5 to 1.4 min at 115°C. The Avrami theory was used to describe the kinetics of isothermal crystallization of the PLLA/NA samples. Also, with the presence of NA, the spherulite number of PLLA increased, and the spherulite size decreased significantly. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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.     

Crystallization kinetics and crystalline morphology of poly(ethylene naphthalate) and poly(ethylene terephthalate‐co‐bibenzoate)

Copolymers of ethylene glycol with 4,4′‐bibenzoic acid and terephthalic acid are known to crystallize rapidly to surprisingly high levels of crystallinity. To understand this unusual behavior, the isothermal crystallization of poly(ethylene bibenzoate‐co‐terephthalate) in the molar ratio 55:45 (PETBB55) was studied. Poly(ethylene naphthalate) (PEN) was included in the study for comparison. The kinetics of isothermal crystallization from the melt and from the amorphous glass was determined using differential thermal analysis. The results were correlated with the crystalline morphology as observed with atomic force microscopy (AFM). Crystallization of PEN exhibited similar kinetics and spherulitic morphology regardless of whether it was cooled from the melt or heated from the glass to the crystallization temperature. The Avrami coefficient was close to 3 for heterogeneous nucleation with 3‐dimensional crystal growth. The copolymer PETBB55 crystallized much faster than did PEN and demonstrated different crystallization habits from the melt and from the glass. From the melt, PETBB55 crystallized in the “normal” way with spherulitic growth and an Avrami coefficient of 3. However, crystallization from the glass produced a granular crystalline morphology with an Avrami coefficient of 2. A quasi‐ordered melt state, close to liquid crystalline but lacking the order of a recognizable mesophase, was proposed to explain the unusual crystallization characteristics of PETBB55. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 98–115, 2002     

Estimation of thermal transitions in poly(ethylene naphthalate): Experiments and modeling using isoconversional methods

Thermal transitions of PEN, such as the glass transition temperature and those occurring during isothermal or nonisothermal crystallization were investigated based on careful experiments and modeling with isoconversional methods. The latter was applied to DSC data to determine the effective activation energy for the glass transition in PEN. Using the same data and different thermal methods the dynamic fragility of PEN was evaluated. The Lauritzen-Hoffman (LH) parameters K_g and U^∗ were estimated using the secondary nucleation theory from both PLM and isothermal DSC after self-nucleation measurements. Regime II to I and III to II transition at about 253 °C and 243 °C were concluded. Elliptical-shaped hedrite-like morphology was observed above 253 °C. Finally, isoconversional analysis was applied to both melt and glass non-isothermal crystallization data and the combined set of activation energies was found to be described by the theoretical Vyazovkin-Sbirrazzuoli equation using a single set of LH parameters coming from PLM measurements.     

Crystallization behavior and crystallization kinetic studies of isotactic polypropylene modified by long‐chain branching polypropylene

In this article, we discuss the crystallization behavior and crystallization kinetics of isotactic polypropylene (iPP) modified by long‐chain‐branching (LCB) high‐melt‐strength iPP over a wide composition range, that is, LCB‐iPP from 10 to 50 wt %. Over the entire range we investigated, the presence of LCB‐iPP accelerated crystallization in both the isothermal crystallization process and nonisothermal crystallization process, even when the LCB‐iPP content was as low as 10%, and both crystallization processes were enhanced more significantly as the LCB‐iPP content increased. Hoffman–Lauritzen theory analysis revealed that the fold‐free energy decreased effectively with the occurrence of the LCB structure, although the growth rate of spherulites was depressed, as shown by polarized optical microscopy. Meanwhile, the regime III–regime II transition temperature was about 15° higher for all of the LCB‐iPP compositions than that of iPP because the LCB structure reduced the mobility of the polypropylene chains. Furthermore, the γ‐form crystal structure was favored by LCB compared to the β form, which was supported by wide‐angle X‐ray diffraction. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Investigation on the Isothermal Crystallization of Poly(phenlene sulfide) by Rheology and Polarized Light Microscopy

Multiwave frequency experiments were used to determine the critical gel behavior of the crystallizing melt of poly(phenylene sulfide) (PPS), which allowed for the simultaneous measurement of the viscoelastic properties at different oscillation frequencies. Therefore it was possible to determine the apparent critical gel point because of the shorter measuring time. The resulting critical gel time was related to the crystallization kinetics during the early stages. The isothermal crystallization kinetic of PPS was calculated by rheometer and hot stage-polarized light microscopy (PLM) experiments. The Avrami exponent n from PLM and rheological techniques was 2.9 and 2.7, therefore the two methods were around 3. The half crystallization time was 2025s for optical microscopy and 2112s for the rheological techniques, and were in good agreement with each other.     

On melt‐crystallization of polytetrafluoroethylene and of random fluorinated copolymers of tetrafluoroethylene

Through differential scanning calorimetry, isothermal crystallization from the melt of polytetrafluoroethylene (PTFE) has been investigated. PTFE was regarded as one of the polymers for which crystallization is so rapid that the samples crystallize during the cooling from the melt to the selected crystallization temperature. By contrast, we now report that a stochastic behavior is observed for isothermal melt‐crystallization of PTFE. In fact, on cooling very quickly the samples from the molten state to the selected crystallization temperature, crystallization during the cooling is randomly observed. Therefore, repeating the experiments until crystallization on cooling was absent, it was possible to investigate isothermal melt‐crystallization of PTFE. However, crystallization is very fast; in fact, crystallization kinetics can be followed just for very low undercoolings, while as the undercooling becomes as large as about 15°C, only secondary crystallization is observed. In both cases, the data have been examined through the well‐known Avrami analysis, taking into account the different physical meaning of the obtained parameters. For the first cases (actual crystallization kinetics) very low, noninteger Avrami exponents have been obtained. They have been related to the fractal dimension of the crystallites and their values to the morphological observations on PTFE. For the second cases, the typical low values of Avrami exponents of secondary crystallization are obtained. Moreover, isothermal melt‐crystallization of random fluorinated copolymers of tetrafluoroethylene with either hexafluoropropylene or perfluoromethylvinylether as comonomers has been studied and compared with that of PTFE. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1607–1613, 1999     

Crystallization behavior of poly(ϵ‐caprolactone) in poly(ϵ‐caprolactone) and poly(vinyl methyl ether) mixtures

The morphological development and crystallization behavior of poly(?‐caprolactone) (PCL) in miscible mixtures of PCL and poly(vinyl methyl ether) (PVME) were investigated by optical microscopy as a function of the mixture composition and crystallization temperature. The results indicated that the degree of crystallinity of PCL was independent of the mixture composition upon melt crystallization because the glass‐transition temperatures of the mixtures were much lower than the crystallization temperature of PCL. The radii of the PCL spherulites increased linearly with time at crystallization temperatures ranging from 42 to 49°C. The isothermal growth rates of PCL spherulites decreased with the amount of the amorphous PVME components in the mixtures. Accounting for the miscibility of PCL/PVME mixtures, the radial growth rates of PCL spherulites were well described by a kinetic equation involving the Flory–Huggins interaction parameter and the free energy for the nuclei formation in such a way that the theoretical calculations were in good agreement with the experimental data. From the analysis of the equilibrium melting point depression, the interaction energy density of the PVME/PCL system was calculated to be ?3.95 J/cm³. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Crystallization kinetics and morphology of nylon 1212

The isothermal and non-isothermal crystallization processes of nylon 1212 were investigated by polarized optical microscopy. The crystal growth rates of nylon 1212 measured in isothermal conditions at temperatures ranged from 182 to 132 °C are well comparable with those measured by non-isothermal procedures (cooling rates ranged from 0.5 to 11 °C/min). The kinetic data were examined with the Hoffman-Lauritzen nucleation theory on the basis of the obtained values of the thermodynamic parameters of nylon 1212. The classical regime I→II and regime II→ III transitions occur at the temperatures of 179 and 159 °C, respectively. The crystal growth parameters were calculated with (100) plane assumed to be the growth plane. The regime I →II→ III transition is accompanied by a morphological transition from elliptical-shaped structure to banded spherulite and then non-banded spherulite. The development of morphology during isothermal and non-isothermal processes shows a good agreement.     

Effect of lignin particles as a nucleating agent on crystallization of poly(3‐hydroxybutyrate)

The effect of lignin fine powder, as a new kind of nucleating agent, on the crystallization process of poly(3‐hydroxybutyrate) (PHB) was studied. The kinetics of both isothermal and nonisothermal crystallization processes from the melt for both pure PHB and PHB/lignin blend was studied by means of differential scanning calorimetry. Lignin shortened the crystallization half‐time t_1/2 for isothermal crystallization. The activation energy ΔE for PHB/lignin and pure PHB in the isothermal crystallization process was ?237.40 and ?131.22 kJ/mol, respectively, clearly indicating that the crystallization of the PHB/lignin blend was more favorable than that of pure PHB from a thermodynamic perspective. At the same time, according to polarized optical microscopy, the rate of spherulitic growth from the melt increased with the addition of lignin, which is ascribed to the reduction of surface fold energy σ_e, that is, σ_e is 59.2 × 10^?3 and 41.6 × 10^?3 J m^?2 for pure PHB and PHB/lignin, respectively. Polarized optical microscopy also showed that the spherulites found in PHB with lignin were smaller in size and greater in number than those found in pure PHB. The wide‐angle X‐ray diffraction indicated that an addition of lignin caused no change in the crystal structure and degree of crystallinity. These results indicated that lignin is a good nucleating agent for the crystallization of PHB. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2466–2474, 2004     

Studies on the isothermal crystallization kinetics and crystal morphology of PTT/SGF composites

The kinetics of isothermal crystallization and crystal morphology of poly(trimethylene terephthalate)/short glass fibers (PTT/SGF) composites were investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM). DSC data was analyzed by the Avrami equation and Hoffman‐Lauritzen theory. The results show that SGF plays a role as the nucleating agent, which largely accelerates the crystallization rate of PTT. SGF below 20% will increase the crystallinity of the composites but 30% SGF will decrease the crystallinity, which is also verified by the crystallinity results calculated from the wide‐angle x‐ray diffraction (WAXD) experiments. The crystal morphology of the composites exhibits so much microcrystallites because of the fast crystallization rate and the strong interaction between SGF and polymers, which is consistent with the results analysized by the Avrami theory. The nuclei exponent K_g is increased sharply as the SGF added into polymer, and SGF makes PTT easier to crystallize during isothermal crystallization process. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Effect of alkane chain segment length on the crystallization kinetics of nylon 1010, 1013 and 1014

Isothermal and non‐isothermal crystallization kinetics of long alkane chain segment nylon 1010, 1013 and 1014 were investigated by differential scanning calorimetry. The commonly used Avrami equation and that modified by Jeziorny were employed to fit the isothermal and non‐isothermal crystallizations of nylon 1010, 1013 and 1014, respectively. It was found that the crystallization rate of nylon with a longer alkane chain segment was slower than that of nylon with a shorter one at a given cooling rate. The activation energies for the isothermal and non‐isothermal crystallizations determined by the Arrhenius and the Kissinger methods, respectively, decreased with increase of the alkane chain segment length of nylon 1010, 1013 and 1014. Furthermore, the activation energy of the non‐isothermal crystallization process of these nylons, determined by the isoconversional methods of Flynn and Wall and Ozawa, was found to be a decreasing function of the relative degree of crystallinity. © 2014 Society of Chemical Industry     

Crystallization and melting behavior of zenite thermotropic liquid crystalline polymers

The crystallization and melting behavior of a DuPont Zenite^TM series, namely, Z 6000 and Z 8000B, thermotropic liquid crystalline polymer (TLCP) have been investigated by differential scanning calorimetry (DSC). Both, non‐isothermal and isothermal crystallizations were carried out. From the non‐isothermal experiments, the crystallization temperature was found to be 234°C for a cooling rate of 10°C/min whereas it was only 228°C for 40°C/min for Z 8000B, and was found to be 296°C and 290°C, respectively, for Z 6000. In the isothermal experiment both the thermal and crystallization behaviors were studied as a function of the annealing temperature and annealing time. Two types of transition processes were evidence in the low temperature region of the isothermal crystallization. One is fast transition, which may be regarded as liquid crystal transition, and is characterized by the enthalpy, which is independent of annealing time. The other is slow process, related to crystal perfection, and it shows increases in the transition temperature and enthalpy, which is dependent on annealing time.     

Nucleation‐controlled dual semicrystalline morphology of polyamide 11

Crystallization of polyamide 11 at low supercooling of the melt proceeds via heterogeneous nucleation and spherulitic growth of lamellae, while at temperatures close to the glass transition homogeneous nucleation prevails, preventing spherulite formation and leading to formation of a large number of nanometer‐sized mesophase domains. It is shown that spherulitic and non‐spherulitic crystallization at low and high supercooling of the melt, respectively, can be enforced by tailoring the cooling conditions, causing a twofold semicrystalline morphology at ambient temperature. Analysis of non‐isothermal crystallization as a function of the cooling rate, using fast scanning chip calorimetry, reveals that in the case of polyamide 11 such twofold semicrystalline morphology is predicted when cooling at rates between about 20 and 200 K s^?1, since then two separate crystallization events are observed. The prediction has been confirmed by preparation of films crystallized during ballistic cooling at different rates which then were analyzed regarding their structure using optical microscopy, X‐ray diffraction and calorimetry. The study is completed by discussion of implications of twofold non‐isothermal crystallization for structure evolution in polymer processing, as well as by providing information that such behavior is not only typical for polyamide 11 but also for isotactic polypropylene or poly(butylene terephthalate) as two further examples. © 2018 Society of Chemical Industry