Glass transition and cold crystallization in carbon dioxide treated poly(ethylene terephthalate)

An amorphous poly(ethylene terephthalate) (aPET) and a semicrystalline poly(ethylene terephthalate) obtained through the annealing of aPET at 110°C for 40 min (aPET‐110‐40) were treated in carbon dioxide (CO₂) at 1500 psi and 35°C for 1 h followed by treatment in a vacuum for various times to make samples containing various amount of CO₂ residues in these two CO₂‐treated samples. Glass transition and cold crystallization as a function of the amount of CO₂ residues in these two CO₂‐treated samples were investigated by temperature‐modulated differential scanning calorimetry (TMDSC) and dynamic mechanical analysis (DMA). The CO₂ residues were found to not only depress the glass‐transition temperature (T_g) but also facilitate cold crystallization in both samples. The depressed T_g in both CO₂‐treated poly(ethylene terephthalate) samples was roughly inversely proportional to amount of CO₂ residues and was independent of the crystallinity of the poly(ethylene terephthalate) sample. The nonreversing curves of TMDSC data clearly indicated that both samples exhibited a big overshoot peak around the glass transition. This overshoot peak occurred at lower temperatures and was smaller in magnitude for samples containing more CO₂ residues. The TMDSC nonreversing curves also indicated that aPET exhibited a clear cold‐crystallization exotherm at 120.0°C, but aPET‐110‐40 exhibited two cold‐crystallization exotherms at 109.2 and 127.4°C. The two cold crystallizations in the CO₂‐treated aPET‐110‐40 became one after vacuum treatment. The DMA data exhibited multiple tan δ peaks in both CO₂‐treated poly(ethylene terephthalate) samples. These multiple tan δ peaks, attributed to multiple amorphous phases, tended to shift to higher temperatures for longer vacuum times. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermal memory of polyethylenes analyzed by temperature modulated differential scanning calorimetry

Temperature modulated differential scanning calorimetry (TMDSC) was employed to study the melting and crystallization behavior of various polyethylenes (PEs). Samples of high density PE (HDPE), low density PE (LDPE), linear low density PE (LLDPE), and very low density PE (VLDPE) with different crystal structures and morphologies were prepared by various thermal treatments (isothermal crystallization and slow, fast, and dynamic cooling). The reversing and nonreversing contributions, measured on the experimental time scale, were varied, depending on the crystal stability. A relatively large reversing melt contribution occurs for unstable crystals formed by fast cooling compared to those from slow cooling treatments. All samples of highly branched LDPE, LLDPE, and VLDPE showed a broad exotherm before the main melting peak in the nonreversing curve, suggesting crystallization and annealing of crystals to more stable forms. Other samples of HDPE, except when cooled quickly, did not show any significant crystallization and annealing before melting. The crystallinity indicated that dynamically cooled polymers were much more crystalline, which can be attributed to crystal perfection at the lamellar surface. A reversible melting component was also detected during the quasiisothermal TMDSC measurements. Melting is often accompanied by large irreversible effects, such as crystallization and annealing, where the crystals are not at equilibrium. Such phenomena during a TMDSC scan provide information on the polymer thermal history. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 681–692, 2003     

X‐ray diffraction studies on reverse‐annealed polyethylenes

Linear low and high density polyethylene sheets were compression molded and crystallized at a 5–10°C/min cooling rate. Parts of the sheets were annealed at different temperatures up to 2°C below the melting temperature. The small angle X‐ray scattering (SAXS) and the wide angle X‐ray scattering intensities of the annealed samples were studied. SAXS intensities showed particle scattering with a bimodal size distribution. The estimated radii of gyration were 15–17 nm and 5–7 nm, respectively. The crystallinity and the radius of gyration increased slightly with increasing annealing temperature for some samples; others did not show any change. No peaks characteristic of intercorrelated lamellar crystallinity in the SAXS intensities developed during the annealing. The original broad peak of high density polyethylene disappeared from the SAXS recordings on annealing. The length of the perfect chain versus melting temperature was calculated by the Thomson‐Gibbs formula and Flory's concept of melting temperature depression where methyl groups and tertiary carbon atoms at the branches were regarded as second components (solvent). Linear relationships were found for both cases. Experimental data for a linear low density polyethylene obtained from the literature were in between the two functions. A lamellar model of crystallization corresponding to the data is proposed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 340–349, 2001     

Temperature modulated DSC and DSC studies on the origin of double melting peaks in poly(ether ether ketone)

Crystallization kinetics and melting behavior of poly(ether ether ketone) were studied by differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC). The isothermal crystallization was performed between 290 and 320 °C. The Avrami exponents and the level off time were determined from the Avrami analysis. The minimum induction time required for the occurrence of double melting peaks was obtained by increasing the isothermal crystallization time in steps of one minute. It was found that the level off time did not represent the delimitation of single- or double-melting peak behavior. To elucidate the behavior of double melting peaks, the samples were crystallized isothermally between 280 and 320 °C for 10 min, and then they were heated to 380 °C at 2 °C/min. From the TMDSC results, the exothermic behavior in the non-reversing curves supports the mechanism of melting–recrystallization at T_c≤310 °C. On the other hand, no exothermic flow for T_c at 320 °C supports the mechanism of two different morphologies. As the isothermal crystallization temperature increased from 280 to 320 °C, the contribution of melting–recrystallization to the upper melting peak gradually decreased, and finally disappeared.     

In situ fibrillar reinforced PET/PA‐6/PA‐66 blend

Ternary fibrillar reinforced blends are obtained by melt‐blending of poly(ethylene terephthalate) (PET), polyamide 6 (PA‐6) and polyamide 66 (PA‐66) (20/60/20 by weight) in the presence of a catalyst, followed by cold drawing of the extruded bristles to a draw ratio of about 3.4 and additional annealing of the drawn blend at 220 or 240°C for 4 or 8 h. The blend samples are studied by DSC, X‐ray diffraction, SEM, and static and dynamic mechanical testing (DMA). SEM and DMA show that PA‐6 and PA‐66 form a homogeneous, continuous matrix in which PET regions are dispersed. X‐ray and DSC measurements of the drawn and annealed at 220°C samples suggest mixed crystallization (solid solubility) of PA‐6 and PA‐66, and cooperative crystallization of PET with the two polyamides. After annealing at 240°C (above the melting point of PA‐6 and below that of PET), the polyamide matrix becomes partially disoriented, while the oriented, fibrillar PET is preserved and plays the role of a reinforcing element. The DSC results for the same samples suggest in situ generation of an additional amount of copolymer. This additional copolymerization, together with that generated during blend mixing in the extruder, improves the compatibility of the blend components (mostly at the PET‐polyamide interface) and alters the chemical composition of the blend.     

The crystallization behavior and mechanical properties of polylactic acid in the presence of a crystal nucleating agent

The crystallization behavior of polylactic acid (PLA) was studied in the presence of a crystal nucleating agent, ethylenebishydroxystearamide (EBH). The crystallization rate and crystallinity were significantly increased with addition of EBH. The isothermal crystallization half-time at 105°C was decreased from 18.8 minutes for neat PLA to 2.8 minutes for PLA with 1.0 wt % of EBH. The crystallinity of PLA with 1.0 wt % EBH was about 35% after 5-minute annealing at 105°C. Like neat PLA, the double melting peaks were also observed for nucleated PLA. The changes of the double melt peaks were investigated with various crystallization temperatures, heating rates, and annealing times. The heat deflection temperature (HDT) of nucleated PLA was up to 93°C after annealing. The correlation between crystallinity and HDT was demonstrated. A percolation threshold of crystallinity was found corresponding to HDT. The crystal size of nucleated PLA was significantly decreased with addition of EBH. The mechanical properties of annealed PLA blends simultaneously; showed improved modulus and impact strength. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of annealing on the structure and properties of polyvinylidene fluoride hollow fiber by melt‐spinning

Polyvinylidene fluoride hollow fibers were prepared by melt‐spinning technique under three spinning temperatures. The effects of annealing treatment on the structure and properties of hollow fiber were studied by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), tensile test, and scanning electron microscopy (SEM) measurements. DSC and WAXD results indicated that the annealing not only produced secondary crystallization but also perfected primary crystallization, and spinning and annealing temperature influenced the crystallinity of hollow fiber: the crystallinity decreased with the increase of spinning temperature; 140°C annealing increased the crystallinity, and hardly influenced the orientation of hollow fiber; above 150°C annealing increased the crystallinity as well, and furthermore had a comparative effect on the orientation. The tensile tests showed that the annealed samples, which did not present the obvious yield point, exhibited characteristics of hard elasticity, and all the hollow fiber had no neck phenomenon. Compared with the annealed sample, the precursor presented a clear yield point. In addition, the annealed samples had a higher break strength and initial modulus by contrast with the precursor, and the 140°C annealed sample showed the smallest break elongation. SEM demonstrated the micro‐fiber structure appeared in surface of drawn sample. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 935–941, 2007     

The effect of high temperature annealing process on crystallization process of polypropylene,mechanical properties,and surface quality of plastic parts

This work studied the effects and action mechanism of high‐temperature annealing process parameters, such as annealing temperature, annealing duration and cooling speed, on the microstructural evolution of polypropylene (PP) on different thickness layers, the surface quality, and mechanical properties of PP plastic parts. The results show that when the PP plastic parts are annealed at slightly higher than 100°C, the resin on the surface and internal layers of plastic parts just generates the relaxation and rearrangement at the molecular level. Only at an enough high annealing temperature, the secondary crystallization and phase transformation process can be observed. The crystallinity of all annealed samples is higher than that of unannealed samples, but the crystallinity is decreased with the increase of cooling speed after annealing duration, and the annealing duration exceeding 60 min almost has no effect on the crystallinity. The microstructural change of PP on the internal layer of plastic parts is weaker than that on the surface layer. The surface hardness of the plastic parts mainly depends on the crystallinity of the surface layer, whereas the surface roughness of the plastic parts depends on not only the crystallinity, but also the space conformation of molecular chains and the residual stress. With the change of annealing process parameters, the tensile and impact strengths of plastic parts show a non‐monotonic change law. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42773.     

Effect of different thermal treatments on the mechanical performance of poly(L‐lactic acid) based eco‐composites

PLLA‐based eco‐composites reinforced with kenaf fiber and rice straw and containing red or yellow pigments have been studied. The mechanical behavior of the composites was tested by DMTA at two different annealing temperatures (65°C and 85°C) and times (15 min and 120 min) as well as at two preparation conditions: vacuum drying and long time at room temperature. A decrease of microhardness was observed during the water absorption tests. Moreover, the rice straw‐based composites absorbed more water than the kenaf‐ones. Generally, the dyed NFs composites presented better water resistance than undyed ones. The pigments improved the adhesion and led to better mechanical performance. The natural fibers favored the cold crystallization process of PLLA and shifted the cold crystallization peak temperature to lower values, as it was confirmed by DSC measurements. The values of tensile storage modulus obtained after different preparation condition were strongly affected by the process of physical ageing. According to, tan δ parameter, the samples stored at room temperature for a long time showed the highest amorphous content. The PLLA eco‐composite reinforced with kenaf fibers, dyed with the red pigment, and annealed at 85°C for 2 h displays the best mechanical properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Temperature modulated DSC studies of melting and recrystallization in poly (oxy‐1,4‐phenyleneoxy‐1,4‐phenylenecarbonyl‐1,4‐phenylene) (PEEK)

The thermal and crystal morphological properties of amorphous and melt crystallized poly(oxy‐1,4‐phenyleneoxy‐1,4‐phenylenecarbonyl‐1,4‐phenylene) (PEEK) were investigated. Two different molecular weights were studied by Temperature Modulated DSC (TMDSC) over a broad range of annealing times and temperatures. The lower molecular weight PEEK under all crystallization conditions was found to exhibit secondary crystal melting in the low endotherm region, followed by melting of primary crystals melting in the low endotherm region, followed by melting of primary crystals superimposed with a large recrystallization contribution. Primary crystal melting broadly overlapped with melting of the recrystallized species and contributed to the broad highest endotherm. Recrystallization contributions and the interpretation of TMDSC were partially confirmed by independent rapid heating rate melting point determinations and variable heating rate DSC. The higher molecular weight PEEK showed many similarities but generally had smaller levels of reorganization above the annealing temperature under most higher temperature crystallization conditions. TMDSC provides excellent resolution of recrystallization and related events compared to standard DSC. The broad and substantial exothermic recrystallization in amorphous samples was also examined, showing that recrystallization continues through the final melting region.     

PA6／PET共混物的非等温结晶动力学研究

采用等速升温和等速降温ＤＳＣ法对ＰＡ６／ＰＥＴ共混物的非等温结晶动力学进行了研究。在升温和降温ＤＳＣ相变曲线上，ＰＡ６／ＰＥＴ共混物具有双重熔融峰和双重结晶峰，表明ＰＡ６和ＰＥＴ组分可形成各自的结晶体。给出了各组分的结晶峰温度、结晶峰的半高宽、结晶半时间等表征结晶行为的参数，并讨论了影响结晶的因素。     

Crystallization and melting behavior of polyester/clay nanocomposites

Polyester/clay nanocomposites were prepared by melt compounding with different clay loadings. Comparing against neat polyester resins, the crystallization and multiple melting behavior of the nanocomposites was investigated by differential scanning calorimetry (DSC) and X‐ray diffraction (XRD). Nanoclay filler is an effective heterogeneous nucleating agent, as evidenced by a decrease and an increase in the crystallization temperature for both cold and melt crystallization of polyesters, respectively. The degree of crystallinity was found to increase with increasing clay content, due to heterogeneous nucleation effects by the addition of a nanofiller. For the annealed samples, multiple melting peaks were always observed for both neat polyester and its nanocomposites. The origins of the multiple melting behavior are discussed, based on the DSC and XRD results. Interestingly, an ‘abnormal’ high‐temperature endothermic peak (T_{m, 3}) at about 260 °C was observed when the nanocomposite samples were annealed at higher temperatures (eg ≥240 °C). The constrained polyester crystals formed within intercalated clay platelets due to confinement effects were probably responsible for this melting event at these higher temperatures. Copyright © 2004 Society of Chemical Industry     

Thermal,mechanical, and barrier properties of polyethylene terephthalate‐platelet nanocomposites prepared by in situ polymerization

This study used in situ polymerization to prepare polyethylene terephthalate (PET) nanocomposites incorporating Ethoquad‐modified montmorillonite (eMMT), unmodified hectorite (HCT), or phenyl hectorite (phHCT) particles to study the impact of platelet surface chemistry and loading on thermal, mechanical, and gas barrier properties. eMMT platelets reduced the PET crystallization rate without altering the ultimate degree of crystallinity. In contrast, HCT and phHCT platelets accelerated the polymer's crystallization rate and increased its crystallinity. DMA results for thermally‐quenched samples showed that as T increased past glass transition temperature (T_g), HCT and phHCT nanocomposites (and control PET) manifested precipitous drops in G′ followed by increasing G′ due to cold crystallization; in contrast, eMMT nanocomposites had much higher G′ values around T_g. This provides direct evidence of eMMT reinforcement in thermally‐quenched eMMT nanocomposites. These results suggest that eMMT has a strong, favorable interaction with PET, possibly through Ethoquad‐PET entanglement. HCT and phHCT have a fundamentally different interaction with PET that increases crystallization rate and T_g by 11 to 17°C. Water barrier improvement in eMMT nanocomposites agrees with previously published oxygen barrier results and can be rationalized in terms of a tortuous path gas barrier model. POLYM. ENG. SCI., 52:1888–1902, 2012. © 2012 Society of Plastics Engineers     

Radiopaque iodinated poly(ester-urethane)s based on poly(butylene succinate): Retarded crystallization and dual recrystallization behaviour

A series of biodegradable radiopaque iodinated poly(ester-urethane)s (IPEUs) were synthesized by chain-extension of dihydroxylated poly(butylene succinate) (PBS–OH) and isophorone diisocyanate (IPDI) with iodinated bisphenol-A (IBPA). The effects of IBPA on the crystallization and melting behaviour of IPEUs were investigated by wide-angle X-ray diffraction (WAXD), polarized optical microscope (POM), differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC). WAXD results suggest that the PBS soft segments form only one crystal modification, and that the crystallinity of the samples decreases with increasing the amorphous hard segments. POM observation indicates that the ring-bands of spherulites disappear and then the spherulitic texture is disturbed with increasing the IBPA content. The analysis of isothermal crystallization kinetics shows that the crystallization of IPEUs is retarded by the introduction of IBPA chain extender with bulky pendent groups. In the DSC heating curves, at most four endothermic peaks were observed and their origins were examined. Two recrystallization exothermic peaks were observed for IPEUs in the nonreversible signals of TMDSC. The twice sequential melting–recrystallization–remelting model could be used to explain the multiple melting behaviour of IPEUs.     

Crystallization and melting behaviors of poly(aryletheretherketone) (PEEK) on origin of double melting peaks

The crystallization and melting behaviors of poly(aryletheretherketone) (PEEK) films were investigated, using differential scanning calorimetry and metallurgy concepts. The shape of the time–temperature–transformation (TTT) diagram, established for PEEK, results from both nucleation and growth phenomena. The double melting behavior exhibited by isothermally crystallized PEEK samples are discussed through the TTT diagram and the influence of the thermal history in the molten state. The upper melting peak arises first and the lower melting peak is developed later. The location of such a second endotherm is shifted toward the higher temperature with increasing either the crystallization temperature or the annealing time while the location of the upper melting peak seems to be unchanged. The double melting behavior is related to a bimodal distribution in size and/or perfection of lamellae developed in a two-step crystallization. With increasing temperature and/or annealing time in the molten state, the pattern of the endothermic curves is modified. The observed changes are discussed through two origins: the progressive disappearance of remnants of the former crystals and a thermal degradation leading to a cross-linking of the polymer. © 1994 John Wiley & Sons, Inc.     

Influence of syndiotacticity and annealing temperature on the double melting peak behaviour of syndiotactic polypropylene

Fractionated syndiotactic polypropylene (sPP) samples with homogeneous tacticity were annealed at different temperatures. The influence of syndiotacticity and annealing temperature on the double melting peak phenomena were investigated. It is found that all fractions show double melting peaks at 75°C annealing temperature, while the low peak disappears when the fractions with higher syndiotacticity are annealed at 85°C and above. The fraction with the lowest syndiotacticity remains the same at any annealing temperature. In combination with wide-angle X-ray diffraction experiments, the double peaks are believed to correspond to the melting of cell II and III. The results indicate that higher temperature and syndiotacticity are the external conditions and internal structural factor that permit cell II to transform into cell III. © 1999 Society of Chemical Industry     

Thermal properties of poly(vinyl alcohol)–solute blends studied by TMDSC

A thermal analysis study of blends of semicrystalline poly(vinyl alcohol) (PVA) with a pharmaceutical substance, buflomedil pyridoxal phosphate (BPP) is presented. Temperature‐modulated DSC (TMDSC) was used to determine the T_g as well as the crystallinity of blends with various polymer to drug ratios, for different annealing procedures. Positive deviations from a simple expression for the composition dependence of the glass transition of the blend were found. This result, together with the increased thermal stability of PVA–BPP blends, evidenced by TGA analysis, indicates the existence of specific interactions between the polar groups of the two components. The incorporation of dispersed BPP in the PVA matrix results in a composition‐dependent lowering of the polymer's T_m and degree of crystallinity. In addition, we found that, while melting of pure PVA is predominantly reversing, its melting in the blends acquires an increasingly higher nonreversing component with increasing BPP content in the blend. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1151–1156, 2004     

Numerical simulation of DSC and TMDSC curves as well as reversing and nonreversing curve separation

The basic physical meaning of temperature modulation for DSC is an arguable research topic, and its interpretation affects the development of thermal analysis and polymer science. This article studies the basic physical meaning of TMDSC by numerical simulation. DSC and TMDSC output curves are computed for a sample with step changes in its specific heat and for a sample with crystallites melting over the temperature range. The TMDSC curves are further analyzed to obtain the reversing and nonreversing components. It is shown that separation of the reversing and nonreversing components from the underlying heat flow cannot be justified. Some common misconceptions regarding TMDSC are discussed as well. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Annealing effects on the crystallinity of polyetheretherketone (PEEK) and its carbon fiber composite

The degree of crystallinity of polyetheretherketone (PEEK) has been measured using both the density gradient technique (DGT) and differential scanning calorimetry (DSC). The difference in results between the methods was shown to depend on crystallization taking place during the heating scan in the DSC. By freezing the sample at different stages of the DSC thermogram and measuring its crystallinity in the density gradient column, the existence of induced crystallization for PEEK was established. Though this induced crystallization is not visible in the DSC thermogram, it must be taken into account when comparing the degree of crystallinity measured by the two methods. The induced crystallization was in turn interpreted as a result of an increase in crystal perfection that is also commonly observed during the initial stages of the annealing process. Accordingly, the effect of annealing on the crystallinity was also investigated. DSC scans on annealed samples exhibited a small endothermic peak at approximately 10°C above the annealing temperature. This peak was observed in both neat PEEK and its carbon fiber-reinforced composite. Annealed PEEK shows, therefore, two melting transitions, a low one which depends on the annealing temperature and a high one which is independent of annealing temperature conditions. Collectively, the results of this study demonstrate that processing conditions and morphological features must be considered in characterizing semicrystalline-based matrix polymers for high performance composites.