Crystallization and melting behavior in syndiotactic polypropylene: Origin of multiple melting phenomenon

The melting behavior of syndiotactic polypropylene (s‐PP) after isothermal crystallization from the melt state was studied using differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) techniques. Three melting endotherms were observed for isothermal crystallization at high degrees of undercooling. The minor endotherm, located closed to the corresponding crystallization temperature, was postulated to be the melting of the secondary crystallites formed at the crystallization temperature. The low‐temperature melting peak was found to be the melting of the primary crystallites formed, and the high‐temperature melting peak was a result of the melting of the crystallites recrystallized during a heating scan. The triple‐melting behavior observed in subsequent melting endotherms of s‐PP was therefore described as contributions from melting of the secondary crystallites and their recrystallization, partial melting of the less stable fraction of the primary crystallites and their recrystallization, melting of the primary crystallites, and remelting of the recrystallized crystallites formed during the heating scan. In addition, determination of the equilibrium melting temperature for this s‐PP resin according to the linear and nonlinear Hoffman–Weeks extrapolations provided values of 143.1 and 185.6°C, respectively. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 1083–1097, 2001     

Crystallization kinetics and melting behavior of poly[(trimethylene terephthalate)‐co‐(29 mol % ethylene terephthalate)] copolyester

The copolyester was characterized as having 71 mol % trimethylene terephthalate units and 29 mol % ethylene terephthalate units in a random sequence according to the NMR spectra. Differential scanning calorimeter (DSC) was used to investigate the isothermal crystallization kinetics in the temperature range (T_c) from 130 to 170°C. The melting behavior after isothermal crystallization was studied using DSC and temperature‐modulated DSC by varying the T_c, the crystallization time, and the heating rate. The DSC thermograms and wide‐angle X‐ray diffraction patterns reveal that the complex melting behavior involves melting‐recrystallization‐remelting and different lamellar crystals. As the T_c increases, the contribution of recrystallization gradually falls and finally disappears. A Hoffman‐Weeks linear plot yields an equilibrium melting temperature of 198.7°C. The kinetic analysis of the growth rates of spherulites and the change in the morphology from regular to banded spherulites indicate that a regime II→III transition occurs at 148°C. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Multiple melting and partial miscibility of ethylene‐vinyl acetate copolymer/low density polyethylene blends

Multiple melting behaviors and partial miscibility of ethylene‐vinyl acetate (EVA) copolymer/low density polyethylene (LDPE) binary blend via isothermal crystallization are investigated by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). Crystallization temperature T (°C) is designed as 30, 50, 70, 80°C with different crystallization times t (min) of 10, 30, 60, 300, 600 min. The increase of crystallization temperature and time can facilitate the growth in lateral crystal size, and also the shift of melting peak, which means the completion of defective secondary crystallization. For blends of various fractions, sequence distribution of ethylene segments results in complex multiple melting behaviors during isothermal crystallization process. Overlapping endothermic peaks and drops of equilibrium melting points of LDPE component extrapolated from Hoffman–Weeks plots clarify the existence of partial miscibility in crystalline region between EVA and LDPE. WAXD results show that variables have no perceptible influence on the predominant existence of orthorhombic crystalline phase structure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Sulfur‐containing polymers: Effect of composition on melting behavior and crystallization kinetics of poly(butylene terephthalate)

The melting behavior and crystallization kinetics of poly(butylene terephthalate/thiodipropionate) (PBT) copolymers were investigated using the differential scanning calorimetry technique. Multiple endotherms typical of PBT were observed in the copolymers under investigation and were found to be influenced both by crystallization temperature (T_c) and composition. Wide‐angle X‐ray diffraction measurements permitted the identification of the crystalline structure of PBT in all the copolymers investigated. By applying the Hoffman–Weeks method, the equilibrium melting temperature of the copolymers was derived. Isothermal crystallization kinetics were analyzed according to Avrami's treatment. Values of the exponent n close to 3 were obtained, independent of T_c and composition, results in agreement with it being a crystallization process originating from predetermined nuclei and characterized by three‐dimensional spherulitic growth. The introduction of butylene thiodipropionate units was found to decrease the PBT crystallization rate. The heat of fusion (ΔH_m) was correlated to the specific heat increment (Δc_p) of samples of different degrees of crystallinity, and the results were interpreted based on there being an interphase, whose amount was found to increase as the sulfur‐containing unit content was increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2003–2009, 2003     

Crystallization behaviour of poly(butylene succinate) copolymers

Poly(butylene succinate‐co‐butylene 2‐methyl succinate) (PBSMS) random copolymers were synthesized with various comonomer compositions and their crystallization behaviour and morphology were investigated by differential scanning calorimeter, small angle X‐ray scattering and polarized optical microscopy. The equilibrium melting temperature obtained by the Hoffman–Weeks plot significantly decreased with increasing comonomer concentration containing methyl side‐groups. Spherulitic growth rates were strongly dependent on comonomer concentration and were analyzed using the Lauritzen–Hoffman kinetic theory. The surface free energy (σσ_e) dramatically decreased with comonomer contents. From analysis of the SAXS data, the dependence of the lamellar thickness on crystallization temperature decreased with increasing comonomer concentration. © 2002 Society of Chemical Industry     

Crystallization and morphology of metallocene polyethylenes with well‐controlled molecular weight and branching content

The crystallization and morphology of some metallocene polyethylenes with well‐controlled molecular weight and branching content were investigated by DSC, WAXD, PLM and SALS. The banded spherulites observed in linear PE are not seen in crystallization of branched PEs. The small spherulites with small lamellae or fringed micelle crystals are formed when branching content is higher, as suggested by PLM and SALS. The expansion of the unit cell was observed by WAXD as the molecular weight and branching content increased. At even higher branching content (more than 7 mol%), a shrinkage of the unit cell was seen, probably due to a change of crystal morphology from lamellar‐like crystals to fringed micelle‐like crystals. Crystallization temperature, melting point and crystallinity are greatly decreased for branched PEs compared with linear PEs. The equilibrium melting temperature cannot be determined via the Hoffman–Weeks approach for branched PEs since T_m is always 5–6 °C higher than T_c and there is no intercept with the T_m = T_c line. Our results show a predominant role of branches in the crystallization of polyethylene. © 2003 Society of Chemical Industry     

Differential scanning calorimetry of copolymer of isotactic polypropylene backbone with grafted poly(ethylene‐co‐propylene) branches

A series of graft polymers having polypropylene (PP) backbone and poly(ethylene‐co‐propylene) (EPR) side chains was prepared. PP backbone molecular weight (M_n) was 28–98 kg/mol, EPR side chain M_n was 2.6–17 kg/mol, and EPR content was 0–16 wt %. In this work, thermal analysis of the copolymers was performed using differential scanning calorimetry (DSC). Nonisothermal crystallization was performed at different cooling rates. The DSC thermograms revealed multiple melting peaks for slowly cooled samples, most likely the result of the melting of thinner tangential lamellae followed by the melting of thicker radial lamellae. Equilibrium melting temperature (T_m⁰) was determined using the linear Hoffman–Weeks method. Another approach was also used for determining T_m⁰: melting temperature (T_m) and crystallization temperature (T_c) were plotted as functions of logarithmic cooling rate. Linear relationships were observed for all samples with the cross points as T_m⁰'s. As cooling rate decreased, T_c, T_m, and enthalpy of fusion (ΔH_f) increased. T_m and T_m⁰ increased with increasing PP M_n. T_c and T_m were unaffected by the grafting of EPR onto the PP backbone. T_m⁰ and ΔH_f decreased as EPR content increased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3380–3388, 2006     

The effects of atacticity,comonomer content,and configurational defects on the equilibrium melting temperature of monoclinic isotactic polypropylene

A series of isotactic polypropylenes were investigated to account for total defect content using xylene fractionation and carbon‐13 NMR experimental methods. The defects of interest were percent atactic content, copolymer content, and configurational defects. Experimental equilibrium melting temperatures were obtained for each material using the Gibbs‐Thomson equation and extrapolation to infinite crystal thickness or the Hoffman‐Weeks analysis. The experimental equilibrium melting temperature was then compared with the theoretical equilibrium melting temperature predicted by Flory's melting point depression model. Flory's model was found to fit the experimental data using an equilibrium melting temperature of 186°C when configurational defects are ignored. However, to account for all defects, the equilibrium melting temperature for 100% isotactic polypropylene must be increased from 186 to 192°C. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 229–236, 2001     

Crystallization of 2‐methyl‐1,3‐propanediol substituted poly(ethylene terephthalate). I. Thermal behavior and isothermal crystallization

Differential scanning calorimetry (DSC) was used to evaluate the thermal behavior and isothermal crystallization kinetics of poly(ethylene terephthalate) (PET) copolymers containing 2‐methyl‐1,3‐propanediol as a comonomer unit. The addition of comonomer reduces the melting temperature and decreases the range between the glass transition and melting point. The rate of crystallization is also decreased with the addition of this comonomer. In this case it appears that the more flexible glycol group does not significantly increase crystallization rates by promoting chain folding during crystallization, as has been suggested for some other glycol‐modified PET copolyesters. The melting behavior following isothermal crystallization was examined using a Hoffman–Weeks approach, showing very good linearity for all copolymers tested, and predicted an equilibrium melting temperature (T_m⁰) of 280.0°C for PET homopolymer, in agreement with literature values. The remaining copolymers showed a marked decrease in T_m⁰ with increasing copolymer composition. The results of this study support the claim that these comonomers are excluded from the polymer crystal during growth. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2592–2603, 2006     

Study on the thermal behaviors and the morphology in PVP and nylon 6 blends

In this report, polyamides were solution blended in the formic acid with poly(vinyl pyrrolidone)(PVP), an amorphous polar polyamide. The thermal behaviors and morphological change in the blends of Nylon 6 (PA6) and PVP were investigated in details via WAXD, DSC, FT‐IR and POM methods. The equilibrium melting temperatures for PA6 in the blends were estimated based on the linear and nonlinear Hoffman‐Weeks (LHW and NLHW) extrapolative methods. With increasing the mass ratio of PVP to PA6, T_m (melting temperature) and T_c (crystallization temperature) of PA6 in blends both decreased as well as that of the spherulite size of PA6. The interaction mode between PVP and PA6 was investigated by FT‐IR spectroscopy, and the spectral changes indicated that the carbonyl groups of PVP had formed hydrogen bonding with the N? H groups of PA6 molecules in the molten state, which resulted in the variation of the morphology and thermal behaviors. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Isothermal crystallization of poly(vinyl alcohol–co–ethylene)

Isothermal melt crystallization of poly(vinyl alcohol–co–ethylene) with different ethylene contents was studied in the temperature range of 140°C–160°C. A differential scanning calorimeter was used to follow the energy of the crystallization process. The results were analyzed by Avrami and Hoffman–Laurizten methods. The Avrami exponent was close to 2, indicating two‐dimensional growth with a linear growth rate and crystals nucleating athermally. The equilibrium melting temperature was determined by the Hoffman–Weeks method. The rate of crystallization depended on ethylene content and temperature. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1071–1077, 2003     

Effect of monomer composition on properties of copolyimides derived from 3,3′,4,4′‐biphenyltetracarboxylic dianhydride/4,4′‐oxydianiline/1,3‐bis (4‐aminophenoxy)benzene

A series of uncontrolled molecular weight homopolyimides and copolyimides based on 3,3′,4,4′‐biphenyltetracarboxylic dianhydride (s‐BPDA)/4,4′‐oxydianiline (4,4′‐ODA)/1,3‐bis(4‐aminophenoxy)benzene (TPER) were synthesized. All the polyimides displayed excellent thermal stability and mechanical properties, as evidenced by dynamic thermogravimetric analysis and tensile properties testing. A singular glass transition temperature (T_g) was found for each composite from either differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA), but the values determined from tan δ of DMA were much different from those determined from DSC and storage modulus (E′) of DMA. The Fox equation was used to estimate the random T_g values. Some composites exhibited re‐crystallization after quenching from the melt; upon heating, multi‐melting behavior was observed after isothermal crystallization at different temperatures. The equilibrium melting temperature was estimated using the Hoffman‐Weeks method. Additionally, DMA was conducted to obtain E′ and tan δ. Optical properties were strongly dependent on the monomer composition as evidenced by UV‐visible spectra. X‐ray diffraction was used to interpret the crystal structure. All the results indicated that composites with TPER composition ≥ 70% were dominated by the TPER/s‐BPDA polyimide phase, and ≤40% by the 4,4′‐ODA/s‐BPDA polyimide phase. When the ratio between the two diamines was close to 1:1, the properties of the copolyimides were very irregular, which means a complicated internal structure. Copyright © 2011 Society of Chemical Industry     

Interaction of self‐nucleation and the addition of a nucleating agent on the crystallization behavior of isotactic polypropylene

The relationship between heterogeneous or homogeneous nucleation and self‐nucleation of polypropylene (PP) and PP nucleated by an organic phosphate salt (PPA) was studied by DSC. For pure PP, it homogeneously nucleated during cooling after melting at the selected temperature (T_s) of 170–200°C for 3 min, but at the T_s of 160–168°C self‐nucleation occurred; PPA only nucleated heterogeneously at the T_s of 168–200°C, and there existed self nucleation at the T_s of 160–168°C. The double melting peaks of PP and PPA at the T_s of 162°C were observed. Once the self‐nucleation occurred, the change of the crystallization temperature and heat of fusion of PP is more significant than that of PPA with the change of the T_s, depending upon the crystallization conditions. Results were explained by homogeneous nucleation, heterogeneous nucleation, self‐nucleation, and annealing crystallization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 78–84, 2001     

Isothermal crystallization and melting behavior of polypropylene with lanthanum complex of cyclodextrin derivative as a β‐nucleating agent

A novel highly active β‐nucleating agent, β‐cyclodextrin complex with lanthanum (β‐CD‐MAH‐La), was introduced to isotactic polypropylene (iPP). Its influence on isothermal crystallization and melting behavior of iPP was investigated by differential scanning calorimeter (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized light microscopy (PLM). WAXD results demonstrated that β‐CD‐MAH‐La was an effective β‐nucleating agent, with β‐crystal content of iPP being strongly influenced by the content of β‐CD‐MAH‐La and the isothermal crystallization temperature. The isothermal crystallization kinetics of pure iPP and iPP/β‐CD‐MAH‐La was described appropriately by Avrami equation, and results revealed that β‐CD‐MAH‐La promoted heterogeneous nucleation and accelerated the crystallization of iPP. In addition, the equilibrium melting temperature (T) of samples was determined using linear and nonlinear Hoffman‐Weeks procedure. Finally, the Lauritzen‐Hoffman secondary nucleation theory was applied to calculate the nucleation parameter (K_g) and the fold surface energy (σ_e), the value of which verify that the addition of β‐CD‐MAH‐La reduced the creation of new surface for β‐crystal and then led to faster crystallization rate. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Crystallization and melting characteristics of metallocene isotactic polypropylenes

The measurement of crystallization and melting behaviors under constant rates of cooling and heating and the Avrami's analysis of isothermal crystallization were carried out for various metallocene‐ (MET‐PP) and Ziegler‐ Natta‐catalyzed (ZN‐PP) random polypropylene copolymers with various ethylene contents. Both the melting point T_m and the crystallization temperature T_c decrease with increasing ethylene content. Compared at the same ethylene content, both are lower for MET‐ PP because of the higher uniformities of stereoirregular bonds and copolymerization bonds. T_m and T_c show a linear correlation, and compared at the same T_c, T_m of MET‐PP is lower than that of ZN‐PP. This is because MET‐PP has narrower distributions of isotactic sequence length and hence of lamellar thickness than ZN‐PP. The heat of fusion and the latent heat of crystallization decrease with ethylene content. At the same ethylene content, both are lower for MET‐PP, owing also to the higher uniformities of stereoirregular bonds and copolymerization bonds. From the Avrami's analysis, it is assumed that there is little difference in crystallization modes of both catalyst PPs. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1851–1857, 2002     

Synthesis and thermotropic liquid‐crystalline behaviour of novel main‐chain poly(ether ether ketone ketone)s containing a lateral tert‐butyl group

The synthesis of novel poly(ether ether ketone ketone)s containing a lateral group via the random copolymerization of 4,4′‐biphenol, tert‐butylhydroquinone and 1,4‐bis(p‐fluorobenzoyl)benzene is described. The copolymers were characterized by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and polarized optical microscopy (POM) observation. The results showed that the thermotropic liquid‐crystalline properties were achieved in the copolymers containing 30 mol% and 50 mol% tert‐butylhydroquinone, which have relatively lower melting temperatures due to the copolymerization effect. Both the crystalline–liquid‐crystalline transition (T_m) and the liquid‐crystalline–isotropic phase transition (T_i) were observable in the DSC thermograms, while the biphenol‐based poly(aryl ether ketone) has only one melting transition. The hydroquinone‐based polymer was shown to be amorphous. Thermogravimetric analysis (TGA) results showed that these copolymers are all high‐temperature resistant with higher glass transition temperature between 147 and 149 °C, and higher decomposition temperature T_d in the range 480–520 °C. © 2000 Society of Chemical Industry     

Synthesis,characterization, and thermal properties of phosphorus‐containing,wholly aromatic thermotropic copolyesters

A series of phosphorus‐containing, wholly aromatic thermotropic copolyesters from acetylated 2‐(6‐oxide‐6H‐dibenz〈c,e〉〈1,2〉oxa phosphorin‐6‐yl)‐1,4‐dihydroxy phenylene, p‐acetoxybenzoic acid, terephthalic acid, and isophthalic acid were prepared by melting polycondensation. The structure and basic properties of the polymers, such as the glass‐transition temperature (T_g), melting temperature (T_m), thermal stability, crystallinity, and liquid crystallinity, were investigated with Fourier transform infrared, elemental analysis, differential scanning calorimetry (DSC), thermogravimetric analysis, wide‐angle X‐ray diffraction, and hot‐stage polarizing optical microscopy. The copolyesters had relatively high T_g values ranging from 183 to 192°C. The T_m values obtained from DSC curves for samples P‐20 and P‐25 were 290 and 287°C, respectively (where the number in the sample name indicates the molar fraction of the phosphorus‐containing monomer in the reactants). The initial flow temperatures of other samples observed with hot‐stage polarizing microscopy were 271–290°C. The 5% degradation temperatures in nitrogen ranged from 431 to 462°C, and the char yields at 640°C were 41–52%. All the copolyesters, except P‐40, were thermotropic and nematic. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1278–1284, 2002     

Synthesis and thermotropic liquid‐crystalline behavior of novel main‐chain poly(aryl ether ketones)

Novel aromatic poly(ether ketones) containing bulky lateral groups were synthesized via nucleophilic substitution reactions of 4,4′‐biphenol and (4‐chloro‐3‐trifluoromethyl)phenylhydroquinone (CF‐PH) with 1,4‐bis(p‐fluorobenzoyl)benzene. The copolymers were characterized by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction, and polarized light microscopy observation. Thermotropic liquid‐crystalline behavior was observed in the copolymers containing 40, 50, 60, and 70 mol % CF‐PH. The crystalline–liquid‐crystalline transition [melting temperature (T_m)] and the liquid‐crystalline–isotropic phase transition appeared in the DSC thermograms, whereas the biphenol‐based homopolymer had only a melting transition. The novel poly(aryl ether ketones) had glass‐transition temperatures that ranged from 143 to 151°C and lower T_m's that ranged from 279 to 291°C, due to the copolymerization. The polymers showed high thermal stability, and some exhibited a large range in mesophase stability. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1347–1350, 2003     

Isothermal cold crystallization and melting behaviors of poly(L‐lactic acid)s prepared by melt polycondensation

The isothermal cold crystallization and melting behaviors of poly(L ‐lactic acid)s (PLLAs, weight average molecular weight, M_w, 6000–80,000) prepared via melt polycondensation were studied with differential scanning calorimeter in this work. It is found that the crystallization rate increased with decreasing M_w, reached a maximum at M_w of ca. 21,000 and then decreased again. The crystallinity of PLLA can be controlled in the range 30–50% by crystallization temperature (T_c) and time to fulfill the requirement of subsequent solid state polycondensation. The melting behavior strongly depends on T_c. The samples crystallized at high T_c melted with a single peak but those crystallized at low T_c melted with double peaks. The higher melting point (T_mH) kept almost constant and the lower melting point (T_mL) increased clearly with T_c. But the T_mL changed in jumps and a triple melting peak appeared at the vicinity of a characteristic crystallization temperature T_b, possibly because of a change of crystal structure. The equilibrium melting temperature of PLLA with M_w of 21,300 was extrapolated to be 222°C with nonlinear Hoffman‐Weeks method. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of self‐nucleation on crystallization and melting behavior of polypropylene and its copolymers

The effect of self‐nucleation on the crystallization and melting behavior of isotactic polypropylene (i‐PP) and low ethylene content propylene–ethylene copolymers were investigated. Isothermal crystallization kinetics were studied using the Avrami equation and Lauritzen‐Hoffman nucleation theory. It was found that self‐nucleation can enhance the crystallization. The surface free energy ς_e decreased for the self‐nucleated sample. The melting behavior was affected by the preselected temperature, T_s, at which the polymer was partially melted. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1559–1564, 1999