Poly(thiodiethylene adipate): Melting behavior,crystallization kinetics,morphology, and crystal structure

The melting behavior and crystallization kinetics of poly(thiodiethylene adipate) (PSDEA) were investigated with differential scanning calorimetry and hot‐stage optical microscopy. The observed multiple endotherms, commonly displayed by polyesters, were influenced by the crystallization temperature (T_c) and ascribed to melting and recrystallization processes. Linear and nonlinear treatments were applied to estimate the equilibrium melting temperature for PSDEA with the corrected values of the melting temperature. The nonlinear estimation yielded a higher value by about 9 °C. Isothermal crystallization kinetics were analyzed according to Avrami's treatment. Values of Avrami's exponent close to 3 were obtained, independently of T_c, in agreement with a crystallization process originating from predetermined nuclei and characterized by three‐dimensional spherulitic growth. As a matter of fact, space‐filling spherulites were observed by optical microscopy at all T_c's. The rate of crystallization became lower as T_c increased, as usual at a low undercooling, the crystallization process being controlled by nucleation. Moreover, the crystal structure of PSDEA was determined from powder X‐ray diffraction data by full profile fitting. A triclinic unit cell containing two polymer chains arranged parallel to the c axis was found. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 553–566, 2004     

Neopenthyl glycol containing poly(propylene azelate)s: Synthesis and thermal properties

Poly(neopenthyl azelate) (PNAz) and poly(propylene/neopenthyl azelate) random copolymers (PPAz-PNAz) (NAz unit content from 5 to 20 mol%) were synthesized and characterized in terms of chemical structure and molecular weight. Afterwards, the polyesters were examined by TGA, DSC and X-ray diffractometry. Good thermal stability was found for each sample. The thermal analysis showed that the T_m of the copolymers decreased with the increment in NAz unit content, differently from T_g, which on the contrary increased. X-ray diffraction measurements allowed the identification of the PPAz crystalline structure in all the copolymers. Multiple endotherms were shown in the PPAz-PNAz samples, due to melting and recrystallization processes, similarly to PPAz. The of the copolymers was derived from the application of the Hoffman-Weeks’ method. Baur’s equation described well the T_m-composition data. The isothermal crystallization kinetics were analyzed according to Avrami’s treatment. The introduction of NAz units decreased the crystallization rate compared to pure PPAz. Values of the Avrami’s exponent n close to 3 were obtained in all cases, regardless of T_c, in agreement with a crystallization process originating from predeterminated nuclei and characterized by a three dimensional spherulitic growth.     

Crystallization behavior and morphology of poly(propylene terephthalate) copolymers containing neopenthyl glycol moieties

The melting behavior and the crystallization kinetics of random poly(propylene/neopenthyl terephthalate) copolymers (PPT‐PNT) were investigated by means of differential scanning calorimetry and hot‐stage optical microscopy. Multiple endotherms were evidenced in the PPT‐PNT samples, due to melting and recrystallization processes, similarly to PPT. By applying the Hoffman‐Weeks' method, the T_m° of the copolymers was derived. Baur's equation described well the T_m‐composition data. The isothermal crystallization kinetics was analyzed according to the Avrami's treatment. The introduction of NT units decreased the crystallization rate in comparison to pure PPT. Values of the Avrami's exponent close to three were obtained in all cases, regardless of T_c, in agreement with a crystallization process originating from predeterminated nuclei and characterized by three dimensional spherulitic growth. As a matter of fact, space‐filling spherulites were observed by optical microscopy at all T_cs. Banded spherulites were found for PPT‐PNT5 and PPT‐PNT10, the band spacing being affected by both T_c and composition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 818–830, 2008     

Crystallization and melting behaviors of polystyrene-b-poly(ethylene-co-butene) block copolymers

Non-isothermal and isothermal crystallization behaviors of polystyrene-b-poly(ethylene-co-butene) (PSt-b-PEB) block copolymers with different compositions and chain lengths were investigated by differential scanning calorimetry (DSC). The results show that crystallization of PEB block is strongly dependent on the composition. Crystallization temperature (T_c), melting temperature (T_m) and fusion enthalpy (ΔH_f) increase rapidly with PEB volume fraction (V_E) for block copolymers with V_E below 50%, but there is little change when PEB block becomes the major component. Glass transition temperature (T_g) of the PSt block and order-disorder transition temperature (T_ODT) of block copolymers also have a weak effect. The isothermal crystallization kinetics results show that Avrami exponent (n) was strongly dependent on the composition and crystallization temperature. For the block copolymers with V_E below 38.7 vol%, the values of n vary between 0.9 and 1.3, indicating that crystallization is confined. For the PSt-b-PEB block copolymers with V_E higher than 50%, fractionated crystallization behavior is usually observed. A two-step isothermal crystallization procedure is applied to these block copolymers. It is found that breakout crystallization occurs at higher T_c, but confined at lower T_c. Two overlapped melting peaks are observed for the block copolymers with fractionated crystallization behavior after two-step crystallization, and only the higher melting peak corresponding to breakout crystallization can be used to derive equilibrium melting temperature.     

Crystallization kinetics and melting behavior of metallocene short‐chain branched polyethylene fractions

Metallocene polyethylene (mPE) fractions are recognized as being more homogeneous with respect to short‐chain branch (SCB) distribution as compared with unfractionated mPEs. Differential scanning calorimetry and polarized optical microscopy (POM) were used to study the influences of SCB content on the crystallization kinetics, melting behavior, and crystal morphology of four butyl‐branched mPE fractions. The parent mPE of the studied fractions was also investigated for comparative purposes. mPE fractions showed a much simpler crystallization behavior as compared with their parent mPE during the cooling experiments. The Ozawa equation was successfully used to analyze the nonisothermal crystallization kinetics of the fractions. The Ozawa exponent n decreased from about 3.5 to 2 as the temperature declined for each fraction, indicating the crystal‐growth geometry changed from three‐dimensional to two‐dimensional. For isothermal crystallization, the fraction with a lesser SCB content exhibited a higher crystallization temperature (T_c) window. The results from the Avrami equation analysis showed the exponent n values were around 3 (with minor variation), which implied that the crystal‐growth geometry is pseudo‐three‐dimensional. Both of the activation energies for nonisothermal and isothermal crystallization were determined for each fraction with Kissinger and Arrhenius‐type equations, respectively. Double melting peaks were observed for both nonisothermally or isothermally crystallized specimens. The high‐melting peak was confirmed induced via the annealing effect during heating scans. The Hoffman–Weeks plot was inapplicable in obtaining the equilibrium melting temperature (T_m°) for each fraction. The relationship between T_c and T_m for the fractions is approximately T_m = T_c (°C) + 8.3. The POM results indicated that the crystals of parent or fractions formed under cooling conditions did not exhibit the typical spherulitic morphology as a result of the high SCB content. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 325–337, 2002     

Poly(butylene terephthalate)/poly(ε-caprolactone) blends: Influence of PCL molecular mass on PBT melting and crystallization behavior

The isothermal crystallization kinetics and melting behavior of poly(butylene terephthalate) (PBT) in binary blends with poly(ε-caprolactone) (PCL) was investigated as a function of PCL molecular mass by differential scanning calorimetry and optical microscopy. The components are miscible in the melt when oligomeric PCL (M_w = 1250) is blended with PBT, whereas only partial miscibility was found in mixtures with higher molecular mass (M_w = 10,000 and 50,000). The equilibrium melting point of PBT in the homopolymer and in blends with PCL was determined through a non-linear extrapolation of the T_m = f(T_c) curve. The PBT spherulitic growth rate and bulk crystallization rate were found to increase with respect to plain PBT in blends with PCL1250 and PCL10000, whereas addition of PCL50000 causes a reduction of PBT solidification rate. The crystallization induction times were determined by differential scanning calorimetry for all the mixtures through a blank subtraction procedure that allows precise estimation of the crystallization kinetics of fast crystallizing polymers. The results have been discussed on the basis of the Hoffman-Lauritzen crystallization theory and considerations on both the transport of chains towards the crystalline growth front and the energy barrier for the formation of critical nuclei in miscible and partially miscible PBT/PCL mixtures are widely debated.     

Crystallization Behaviors and Regime Kinetics Analysis of Poly(<Emphasis Type="Italic">L</Emphasis>-lactide)-poly(butylene adipate)-poly(<Emphasis Type="Italic">L</Emphasis>-lactide) Based Multiblock Thermoplastic Polyurethanes

Poly(L-lactide)-based (PLLA) poly(ester-urethane)s are particularly relevant and gain significant attention due to their environment-friendly degradability and elastomeric shape memory capability. The tensile properties, resilience and degradation are strongly affected by their crystallization. This work was to investigate crystallization behaviors of the poly(L-lactide)-poly(butylene adipate)-poly(L-lactide) (PLLA-PBAPLLA) based thermoplastic polyurethane elastomers (PLAEUs) we synthesized previously. Dynamic scanning calorimetry (DSC) and polarized optical microscopy (POM) in combination with Avrami, Jezioney and Hoffman-Weeks models were used to analyze the impact of the PLLA block length on the crystallization temperature T_c, degree of crystallinity X_c, nucleation and spherulite growth mode and crystallization regime kinetics of the PLAEUs. The results indicate the low melting point poly(butylene adipate) (PBA) block resides in the amorphous domains while the PLLA block resides in both crystalline and amorphous phases. The X_c of the PLAEUs increase with the increased length of the PLLA block (i.e. higher content of PLLA block). The analyses with Avrami and Jezioney models show the PLAEU copolymers follow a disc-like spherulite growth. The covalently bonded PBA block decreases both nucleation velocity and spherulite growth rate in the isothermal crystallization. Such an impact is lessened as PLLA block length increases. The PLLA homopolymers demonstrate crystallization regime transition from II to III at a certain T_c of isothermal crystallization, while the crystallization regime kinetics of PLLA block in the PLAEUs are explained by a single regime III at low molecular weights of PLLA and the transition is restored as the PLLA block length increases (i.e. regime II to III).     

Calorimetric analysis of the multiple melting behavior of melt-crystallized poly(l-lactic acid) with a low optical purity

The polymorphous crystallization and multiple melting behavior of poly(l-lactic acid) (PLLA) with an optical purity of 92 % were investigated after isothermally crystallized from the melt state by wide-angle X-ray diffraction and differential scanning calorimetry. Owing to the low optical purity, it was found that the disordered (α′) and ordered (α) crystalline phases of PLLA were formed in the samples crystallized at lower (<95 °C) and higher (≥95 °C) temperatures, respectively. The melting behavior of PLLA is different in three regions of crystallization temperature (T _c) divided into Region I (T _c < 95 °C), Region II (95 °C ≤ T _c < 120 °C), and Region III (T _c ≥ 120 °C). In Region I, an exothermic peak was observed between the low-temperature and high-temperature endothermic peaks, which results from the solid–solid phase transition of α′-form crystal to α one. In Region II, the double-melting peaks can be mainly ascribed to the melting–recrystallization–remelting of less stable α crystals. In Region III, the single endotherm shows that the α crystals formed at higher temperatures are stable enough and melt directly without the recrystallization process during heating.     

Characterization,crystallization kinetics,and melting behavior of poly(ethylene succinate) copolyester containing 10 mol % butylene succinate

Copolyester was synthesized and characterized as having 89.9 mol % ethylene succinate units and 10.1 mol % butylene succinate units in a random sequence, as revealed by NMR. Isothermal crystallization kinetics was studied in the temperature range (T_c) from 30 to 73 °C using differential scanning calorimetry (DSC). The melting behavior after isothermal crystallization was investigated using DSC by varying the T_c, the heating rate and the crystallization time. DSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are associated primarily with the melting of lamellar crystals with various stabilities. As the T_c increases, the contribution of recrystallization slowly decreases and finally disappears. A Hoffman‐Weeks linear plot gives an equilibrium melting temperature of 107.0 °C. The spherulite growth of this copolyester from 80 to 20 °C at a cooling rate of 2 or 4 °C/min was monitored and recorded using an optical microscope equipped with a CCD camera. Continuous growth rates between melting and glass transition temperatures can be obtained after curve‐fitting procedures. These data fit well with those data points measured in the isothermal experiments. These data were analyzed with the Hoffman and Lauritzen theory. A regime II → III transition was detected at around 52 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2431–2442, 2008     

Crystal orientation and melting behavior of poly(ɛ-Caprolactone) under one-dimensionally “hard” confined microenvironment

Crystal orientation and melting behavior of poly(ε-caprolactone) in a diblock copolymer of poly(ε-caprolactone)-block-poly(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PCL-b-PMPCS) was investigated. The degrees of polymerization of the PCL and PMPCS block are 200 and 98, respectively. With the PMPCS in a columnar liquid crystalline phase, the diblock is rod-coil one, which exhibits a lamellar phase morphology with the PCL layer thickness of 15.2 nm. Since the glass transition temperature of PMPCS block is much higher than the melting temperature of PCL, the crystallization of PCL is in a one-dimensionally "hard" confinement environment. Mainly on the basis of two-dimensional wide-angle X-ray diffraction experiments, we identified the orientation of PCL isothermally crystallized at various crystallization temperatures (Tcs). At high Tcs (Tc≥10℃), the c-axis of the PCL crystal is along the layer normal of the microphase-separated sturcture. Decreasing Tc can result in the tilting of PCL c-axis with respect to the layer normal. The lower the Tc is, the more the c-axis inclines. Meanwhile, the b-axis of PCL remains perpendicular to the layer normal. At a very low Tc of -78℃, the orientation of the PCL crystals is completely random. For the samples isothermally crystallized at Tc≤10℃, double melting behavior can be observed. While the low temperature endotherm reflects the melting of the crystals originally formed at the Tc applied, the high temperature one is associated with the crystals subjected to the process of recrystallization/reorganization upon heating due to the annealing effect.     

Crystallization,morphology, and thermal behavior of poly(p‐phenylene sulfide)

This article deals with the structure, crystallization, morphology, and thermal behavior of poly(p‐phenylene sulfide) (PPS) with low‐molecular mass, probed by DSC, optical, and electron microscopy. The growth rates of spherulites were measured over the temperature range 235–275°C. A regime II–III transition was found at T = 250°C. The regime transition was accompanied by a morphological change from sheaflike structure to classical spherulites. The Avrami equation poorly described the isothermal crystallization of PPS, for the occurrence of mixed growth mechanisms and secondary crystallization, in agreement with the morphology and the thermal behavior. Two melting peaks were detected on DSC curves and attributed to the melting of crystals formed isothermally at T_c by primary and secondary crystallization. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 415–424, 2001     

Study of thermo-oxidative chemical pre-treatment of isotactic polypropylene

A thermo-oxidative pre-treatment with chemical solutions is required in order to provide the adherence of inorganic semiconductor to the isotactic polypropylene (iPP) surface. A few thin films of iPP were treated with oxidizing solution at 90 °C. The crystalline properties were analyzed using XRD, and it had shown the presence of the α-monoclinic phases. The ATR-FTIR spectra had indicated that characteristic iPP peaks after thermo-oxidative chemical pre-treatment diminished sharply. Moreover, the new carbonyl groups (C = O) were observed, which signified oxidation. The UV–Vis spectra had showed a blue shift in the absorption edge, which corresponded to decrease in the optical band gap. The non-isothermal decomposition and crystallization kinetics of iPP films were studied and compared by means of thermogravimetric analysis and differential scanning calorimetric measurement. The values of the melting temperature T _m and the crystallization temperature T _c were found to be iPP surface structure and heating/cooling rate dependent. The activation energy of crystallization E _c was determined.     

Melting behavior of isotactic polystyrene

The melting behavior of isotactic polystyrene, crystallized from the melt and from dilute solutions in trans-decalin, has been studied by differential scanning calorimetry and solubility measurements. The melting curves show 1, 2, or 3 melting endotherms. At large supercooling, crystallization from the melt produces a small melting endotherm just above the crystallization temperature T_c. This peak originates from secondary crystallization of melt trapped within the spherulites. The next melting endotherm is related to the normal primary crystallization process. Its peak temperature increases linearly with T_c, yielding an extrapolated value for the equilibrium melting temperature T_c° of 242 ± 1°C as found before. By self-seeding, crystallization from the melt could be performed at much higher temperature to obtain melting temperatures as high as 243°C, giving rise to doubt about the value of T_c° found by extrapolation. For normal values of T_c and heating rate, an extra endotherm appears on the melting curve. Its peak temperature is the same for both melt-crystallized and solution-crystallized samples, and independent of T_c, but rises with decreasing heating rate. From the effects of heating rate and partial scanning on the ratio of peak areas and of previous heat treatment on dissolution temperature, it is concluded that this peak arises from the second one by continuous melting and recrystallization during the scan.     

Phase behavior and crystal morphology in poly(ethylene succinate) biodegradably modified with tannin

Specific interactions, growth kinetics, and dendritic morphology in poly(ethylene succinate) (PESu) biodegradably modified with various contents of tannic acid (TA) were characterized using differential scanning analysis, Fourier-transform infrared (FTIR) spectroscopy, polarized-light optical microscopy, and atomic force microscopy. Strong interactions and highly retarded growth between PESu and a macromolecular ester with polyphenol groups, TA, interaction-induced highly retarded growth rates for the PESu/TA (80:20) composition are proven to lead to single-crystal-like dendrites when crystallized at high crystallization temperature (T _c). At T _c = 70 °C, the growth rate for neat PESu is 12 μm/min while it is dramatically depressed to one tenth-fold at 1.5 μm/min with 20 wt.% TA in the blend. Strong specific interactions between the carbonyl group of polyesters and the phenolic hydroxyl group of TA are confirmed by (1) the blend’s glass transition temperature (T _g)–composition relationship exhibits a sigmoidal curve, well fitted by the Kwei T _g model for miscible blends with large negative q = −90; (2) thermal analysis on crystal melting revealed an interaction parameter χ = −0.64 between PESu and TA; and (3) IR peak shifting analyzed using two-dimensional FTIR technique. A comparative blend of another polyester poly(hexamethylene sebacate) with TA, lacking the specific interactions, does not exhibit such single crystals upon similar melt crystallization.     

SAXS/WAXS/DSC studies on crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene triblock copolymer with lamellar morphology and low glass transition temperature

Crystallization of a polystyrene-b-poly(ethylene oxide)-b-polystyrene (S-EO-S) triblock copolymer, S₄₀EO₁₃₆S₄₀, with lamellar morphology in the melt and low glass transition temperature (T_g=47 °C) of the S block was studied. The triblock copolymer was cooled from ordered melt and isothermal crystallization was conducted at crystallization temperatures (T_c) near the T_g of the S block. It is found that crystallization behavior of S₄₀EO₁₃₆S₄₀ strongly depends on T_c. When T_c is far below T_g, an Avrami exponent n=0.5 is observed, which is attributed to diffusion-controlled confined crystallization. As T_c slightly increases, the Avrami exponent is 1.0, indicating that crystallization is confined and crystallization rate is determined by the rate of homogeneous nucleation. When T_c is just below the T_g of the S block, crystallization tends to become breakout and accordingly Avrami exponent changes from 1.0 to 3.2. Crystallinity and melting temperature of the EO block in breakout crystallization are slightly higher than those in confined crystallization. Time-resolved small and wide angle X-ray scattering (SAXS/WAXS) were used to monitor isothermal crystallization of S₄₀EO₁₃₆S₄₀. It shows that the long period is constant in confined crystallization, but it gradually increases during breakout crystallization. WAXS result reveals that confined or breakout crystallization has no effect on the crystal structure of the EO block.     

Effect of PEG on the crystallization of PPDO/PEG blends

A new biodegradable polymer system, poly(p-dioxanone) (PPDO)/poly(ethylene glycol) (PEG) blend was prepared by a solvent casting method using chloroform as a co-solvent. The PPDO/PEG blends have different weight ratios of 95/5, 90/10, 80/20 and 70/30. Crystallization of homopolymers and blends were investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). When 5% of PEG was blended, the crystallization exothermal peaks (T_c) of PPDO increased sharply and the crystallization exothermal peaks (T_c) of PEG decreased slightly compared with the homopolymers. The crystallization rates of both components increased, and caused greater relative crystallization degree (X_t%). But when the content of PEG was more than 5%, the crystalline behaviors of blends had no more significant changes accordingly. The melting points of each sample varied little over the entire composition range in this study. The nonisothermal crystallization of PPDO homopolymer and blend (PPDO/PEG = 70/30) were also studied by DSC. The crystallization began at a higher temperature when the cooling rates were slower. The nonisothermal crystallization kinetics of blends was analyzed by Ozawa equation. The results showed that the Ozawa equation failed to describe the whole crystallization of the blend, but Mo equation could depict the nonisothermal crystallization perfectly.     

Studies of non-isothermal crystallization and melting of ultra high molecular weight polyethylene

The non-isothermal crystallization and melting of ultra high molecular weight polyethylene (UHMWPE) were observed by means of differential scanning calorimetry and compared with those of ordinary high-density polyethylene (HDPE). The crystallization temperature (T _c ) and melting point (T _m ) of UHMWPE were found to be higher thanT _c andT _m of HDPE, and the latent heat of crystallization (δH _c ) and fusion (δH _m ) of UHMWPE are smaller thanδH _c andδH _m of HDPE. The results were explained in terms of the theory of polymer crystallization and the structure characteristics of UHMWPE. The relationships between the parameters (T _c ,T _T ,δH _c andδH _m ) and the molecular weight (M) of UHMWPE are discussed. Processing of the experimental data led to the establishment of four expressions describing the above relationships.     

Nonisothermal crystallization kinetics and melting behavior of bimodal medium density polyethylene/low density polyethylene blends

Nonisothermal crystallization kinetics and melting behavior of bimodal-medium-density- polyethylene (BMDPE) and the blends of BMDPE/LDPE were studied using differential scanning calorimetry (DSC) at various scanning rates. The Avrami analysis modified by Jeziorny and a method developed by Mo were employed to describe the nonisothermal crystallization process of BMDPE. The BMDPE DSC data were analyzed by the theory of Ozawa. Kinetic parameters such as the Avrami exponent (n), the kinetic crystallization rate constant (Z_c), the peak temperatures (T_p) and the half-time of crystallization (t_1/2) etc. were determined at various scanning rates. The appearance of double melting peaks and the double crystallization peaks in the heating and cooling DSC curves of BMDPE/LDPE blends indicated that the BMDPE and LDPE could crystallize respectively.     

Unusual recrystallization and melting behaviour in DSC scans of isotactic polypropylene samples

The unusual dependence of the melting peak temperature of the species recrystallized during DSC scans on the isothermal crystallization temperature (T _c ) is discussed for isotactic polypropylene samples. It is pointed out that such a phenomenon is not due to superheating effects; it is believed to be accounted for by assuming that the rates of the recrystallization phenomena at the same temperature are higher for samples obtained at higherT _c values.