Differential thermal analysis of polyethylene under high pressure

The design of a differential thermal analysis apparatus for use at elevated pressure is described. Experiments on melting and crystallization of folded-chain crystals of polyethylene and poly(ethylene–butene-1) copolymer, and melting of extended-chain polyethylene crystals have been conducted at pressures up to 4200 bars. The precision in transition temperature measurement was ±1°C. The Clausius-Clapeyron equation predicts the melting point increase with pressure at atmospheric pressure to be 32.0°C/kb. The melting point depression due to copolymerization remained constant over the complete pressure range analyzed on the poly(ethylene–butene-1) used in this study. Crystallization of polyethylene is retarded at elevated pressures, and a 50% larger degree of supercooling is necessary at 5000 bars to give a crystallization rate equal to that observed at atmospheric pressure. The difference in melting point between folded-chain and extended-chain polyethylene increases from 8.4°C at 1 bar to 25.6°C at 3000 bars.     

Melting behavior of linear polyethylene crystallized by solution stirring

Calorimetric and dilatometric studies have been made of the fusion process of linear polyethylene crystals precipitated by high speed stirring from solution. It is shown that long-time annealing at elevated temperatures alleviates the superheating observed when rapid heating rates are employed. By the annealing procedures that have been adopted, a small but demonstrable fraction of high melting material can be produced whose melting temperature depends on the crystallization temperature. For crystallization at 105°C, followed by annealing at 142°C, a melting temperature of 146.0 ± 0.5°C is observed. The dissolution temperature in xylene, determined for the same sample, is consistent with the high melting temperature observed for the pure polymer. It is recognized that a state of high axial orientation need not necessarily be identified with extended chain crystals. Consequently, the increased melting temperature can result from either an increase in the crystallite size or a reduced interfacial free energy relative to crystallites produced by the more conventional mode of crystallization.     

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.     

Application of the high-tension multiannealing to nylon 46 fibers

Nylon 46 fibers produced by the high-temperature zone-drawing treatment were treated by repeating high-tension annealing treatments, that is, a high-tension multiannealing (HTMA) treatment to improve their tensile properties. The HTMA treatment was carried out at a repetition time of 10 times and treating temperature of 110°C under high tension (538.2 MPa) close to the tensile strength at break. Although the HTMA treatment was carried out at 110°C, which is much lower than the crystallization temperature of 265°C for nylon 46, the degree of crystallinity increased up to 59%. The orientation factor of crystallites increased dramatically up to 0.949 by the first high-temperature zone-drawing treatment and slightly during the subsequent treatments. This observation indicated that the orientation of crystallites due to slippage among molecular chains did not occur during the HTMA treatment. The treatments shifted the melting peak to slightly higher temperatures, and the HTMA fiber has a melting endotherm peaking at 285°C. The fiber obtained finally had a storage modulus of 12.5 GPa at 25°C. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2737–2743, 1998     

Annealing and melting of poly-4-methylpentene-1 single crystals

The effects of thermal treatment at temperatures in the 177–250°C. range for 2 hr. on solution-grown single crystals of poly-4-methylpentene-1 have been studied by using electron microscopy. Crystals were grown both from 0.02% pentyl acetate solution at 110°C. and from 0.02% toluene solutions at temperatures less than 80°C. A number of distinct types of crystals have been obtained in the same or in separate solutions depending on the crystallization conditions. These crystals have some morphological characteristics in common such as a square outline with well-defined faces. The contrasting features include such things as a marked difference in relative size, with the smaller crystals showing extensive overgrowth and collapse markings. Observations by means of differential scanning calorimetry (DSC) on the melting behavior of these various crystals show the presence of distinct and characteristic melting points. The effects of a given thermal treatment depend on the type of crystals used, with the larger crystals showing greater stability. The first effects are the appearance of lines, notches at the edges, and holes. An increase in temperature results in an increase in these effects with the formation of fibrillar structures. Consideration is given to the influence of molecular conformation and molecular chain folding on the observed results.     

Transitions in trans-1,4-polyisoprene

The melting transitions of both crystalline forms of trans-1,4-polyisoprene, as detected by differential thermal analysis, have been identified by attendant studies with optical microscopy and x-ray diffraction. The lower-melting (LM) form melts initially at a temperature which depends upon the crystallization temperature but which, under our experimental conditions, is between 45 and 53°C. If recrystallization is allowed to occur, the apparent final melting point, which depends upon the recrystallization temperature, is about 58°C. The initial melting point of the higher-melting (HM) form, also crystallization temperature-dependent, is upwards of 57°C. Under the most easily accessible experimental conditions, it may be obscured by the final melting of the LM-form. The apparent final melting point of the HM form is approximately 66°C. Conversion of the LM form into the HM form occurs only by fusion and crystallization. No evidence of a solid-solid transition was found. The rate of conversion is governed principally by the rate of nucleation at the conversion temperature. If fusion of the LM form is incomplete, recrystallization of the LM form takes place instead of conversion to the HM form.     

Higher aliphatic polyaldehydes. II. Polymer structure and polymer stability of poly(n-heptaldehyde)

Poly(n-heptaldehyde) has been prepared by anionic and cationic polymerization at ?60°C in methyl cyclohexane. The anionic polymer is more crystalline and of a higher degree of isotactic structure than the somewhat rubbery but still crystalline cationic polymer. The polymers have been acetate-endcapped to improve their thermal stability. Cationic polymer, when endcapped and purified, begins to degrade above room temperature; even crystalline anionic polymer degrades at a reasonable rate at 100°C. The crystallinity of poly(n-heptaldehyde) is caused by crystallization of the acetalic main chain as well as the side chain. Two regions of melting have been recognized by DSC analysis and by microscopic observations. The low melting region between 80 and 100°C has been identified as the melting of the paraffinic side chains of poly(n-heptaldehyde). It consists of three clearly definable endotherms at 78, 87, and 101°C.     

Achieving highly crystalline rate and crystallinity in Poly(l-lactide) via in-situ melting reaction with diisocyanate and benzohydrazine to form nucleating agents

The drawback of the application for poly(_l-lactide) (PLLA) is the low crystalline rate and crystallinity obtaining via normal processing methods. Modifying crystallization of PLLA has been found to be an efficient way to improve its mechanical and heat resistance properties. In this wok, 4, 4′-diphenylmethane diisocyanate (M) and benzohydrazine (P) were employed into PLLA melt to in-situ form nucleating agents. The in-situ melting reaction was confirmed by a nuclear magnetic resonance spectroscopy. The crystallization behavior and crystalline morphology were investigated by a differential scanning calorimetry, a polarized optical microscopy and a field emission scanning electron microscope. The crystalline rate of PLLA was abruptly enhanced by adding (M+P) and melting reaction with PLLA. The crystallization half-time of PLLA dramatically decreased from 42.0 to 1.1 min at 130 °C by the in-situ formation of nucleating agents. The crystallinity of PLLA increased from 10.3 to 42.1 by adding 0.25% (M+P) and melting reaction for 8 min. Furthermore, the size of PLLA crystals was dramatically reduced because of the nucleating effect. Accompanied with improvement on crystallinity, the Vicat softening temperature of PLLA shifted from 57.4 °C to 93.7 °C by the in-situ reaction with 6.00% (M+P), and indicating heat resistance enhancement.     

Calorimetric study of blends of poly(butylene terephthalate) and liquid crystalline copolyester

A calorimetric study of blends of poly(ethylene terephthalate-co-p-oxybenzoate), PET/PHB, with poly(butylene terephthalate), PBT has been carried out in the form of as-spun and drawn fibres. DSC melting and crystallization results show that PBT is compatible with LCP and the crystallization of PBT decreases by the addition of LCP in the matrix. The crystallization behaviour of blend fibres is investigated as a function of temperature of crystallization. A detailed analysis of the crystallization course has been made utilizing the Avrami expression. The isothermal calorimetric measurements provide evidence of decrease of rate of crystallization of PBT on addition of the liquid crystalline component up to about 50% by weight. The values of the Avrami exponents change in the temperature range from 200° to 215°C. Dimensionality changes in crystallization could be due to LCP mesophase-transition.     

Microphase separation and rheology of a semicrystalline poly(ether-ester) multiblock copolymer

A microphase separation transition (MST) of a thermoplastic elastomer based on soft segments of poly(tetra methylene oxide) and hard crystalline segments of poly(tetra methylene terephthalate) has been studied by means of rheological measurements, differential scanning calorimetry (DSC), and wide-angle X-ray scattering (WAXS), showing that the MST is entirely caused by melting/crystallization, and that no separate segmental mixing/demixing transition is involved. DSC and WAXS measurements show that melting starts at 190°C, leading to crystal reorganization effects up to above 200°C, and that a gradual decrease in crystallinity occurs from below 210°C up to 224°C, above which temperature no crystals are left. Rheological measurements reveal a wide MST (207–224°C) upon heating, which coincides perfectly with the melting range. From this coincidence together with the Maxwell fluid behavior directly following the MST, it is concluded that melting leads to a one-phase liquid, and that no separate segmental mixing transition occurs. Similar results are obtained upon cooling, indicating that crystallization is the driving force for phase separation and that no separate segmental demixing step precedes crystallization. The wide MST implies a large processing window over which the rheological properties change from highly elastic, with a distinct yield stress, to normal pseudoplastic, enabling application in preparation of structured blends. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1795–1804, 1998     

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     

Morphology of polyethylene crystallized by solution stirring

Dilatometric and calorimetric studies have been made of the fusion process of linear polyethylene crystallized by stirring xylene solutions at elevated temperatures. It is shown that the melting point of the crystals increases rapidly from 139.5°C to 145°C in the crystallization temperature range of 100–103°C and levels off to 146 ± 0.5°C, provided that very slow heating rates are employed. Stirrer-crystallized samples treated with fuming nitric acid show higher crystalline contents. Comparison of their enthalpies of fusion and melting points indicate that higher molecular order along the fiber axis is associated with higher crystallization temperatures. This is in general agreement with corresponding results of other modes of crystallization. The attack of fuming nitric acid on stirrer crystals is characterized by weight-loss curves similar to those of dilutesolution crystals and bulk polyethylene. The linear molecular weight dependence on time of exposure to nitric acid suggests that the oxidation proceeds mainly from the chain ends at a constant rate for samples stirred in the lower crystallization range, but an increased rate is observed for a sample stirred from xylene at 105°C. It is suggested that the lamellar overgrowths, most evident at low crystallization temperatures, are epitaxially attached to the fiber axis, whereas the smaller crossbandings observed at higher crystallization temperatures are possibly made up of elements of chains that are only partly incorporated in the highly ordered fibrous core.     

Melting and crystallization DSC profiles of milk fat depending on selected factors

The aim of this study was to test selected factors, such as sample preparation and measurement procedure, potentially influencing repeatability of DSC analysis of milk fat melting and crystallization. The study investigated the effect of such factors as scanning rate, type of sample pans, method of butter dehydration, and final temperature in the cooling experiment. Based on recorded results, it was observed that cooling rate has a considerable effect on temperature, enthalpy, and height of peaks in the process of milk fat crystallization, as well as peak height and enthalpy in the melting process. By contrast, in the melting process no significant differences were observed in all measured temperatures in the range of heating rate of 2–20 °C min^?1 (p > 0.05). No statistically significant effect on thermodynamic parameters was found for sample pan type, the applied butter dehydration method and various final cooling temperatures (?60, ?50, and ?40 °C) either in the melting or crystallization processes. Only temperature of the second peak (T _c2) in the crystallization process constituted an exception in this respect, with significant differences (p ≤ 0.05) being recorded depending on the applied pan and dehydration method. With regard to the dehydration method, for the extraction and centrifugation methods the first peak forming during crystallization was characterized by high instability, manifested by various peak shape. Generally, it was found that the analysis of the melting and crystallization processes in milk fat, despite its complex composition, is characterized by high repeatability. Mean values of RSD calculated from all the experiments were very low, i.e., 1.8 % for the temperature in the melting process and 1.5 % in crystallization, 0.9 % for melting enthalpy, and 3.2 % for crystallization enthalpy, whereas for peak heights in melting it was 2.9 % and for crystallization it was 9.3 %, respectively.     

New insights into the multiple melting behaviors of the semicrystalline ethylene‐hexene copolymer: Origins of quintuple melting peaks

A semicrystalline ethylene‐hexene copolymer (PEH) was subjected to a simple thermal treatment procedure as follows: the sample was isothermally crystallized at a certain isothermal crystallization temperature from melt, and then was quenched in liquid nitrogen. Quintuple melting peaks could be observed in heating scan of the sample by using differential scanning calorimeter (DSC). Particularly, an intriguing endothermic peak (termed as Peak 0) was found to locate at about 45 °C. The multiple melting behaviors for this semicrystalline ethylene‐hexene copolymer were investigated in details by using DSC. Wide‐angle X‐ray diffraction (WAXD) technique was applied to examine the crystal forms to provide complementary information for interpreting the multiple melting behaviors. Convincing results indicated that Peak 0 was due to the melting of crystals formed at room temperature from the much highly branched ethylene sequences. Direct heating scans from isothermal crystallization temperature (T_c, 104–118 °C) were examined for comparison, which indicated that the multiple melting behaviors depended on isothermal crystallization temperature and time. A triple melting behavior could be observed after a relatively short isothermal crystallization time at a low T_c (104–112 °C), which could be attributed to a combination of melting of two coexistent lamellar stack populations with different lamellar thicknesses and the melting‐recrystallization‐remelting (mrr) event. A dual melting behavior could be observed for isothermal crystallization with both a long enough time at a low T_c and a short or long time at an intermediate T_c (114 °C), which was ascribed to two different crystal populations. At a high T_c (116–118 °C), crystallizable ethylene sequences were so few that only one single broad melting peak could be observed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2100–2115, 2008     

Preparation of small poly(butylene terephthalate) spheres by crystallization from solution

The crystallization of poly(butylene terephthalate) (PBT) from moderately dilute solutions of PBT in a diglycidyl ether of bisphenol-A epoxy has been investigated. PBT dissolves in this epoxy approximately 35°C below its usual melting temperature of 227°C to form a one-phase solution. Cooling this solution below 165°C leads to rapid crystallization of the PBT. The resulting mixture of liquid epoxy and crystalline PBT has a low viscosity and contains highly birefringent, individual PBT spherulites. The PBT spherulites have a narrow size distribution and a high surface-to-volume ratio. These particles are suggested to arise from a rapid crystallization that follows liquid–liquid phase separation. © 1994 John Wiley & Sons, Inc.     

Thermodynamics of crystalline linear high polymers. V. Extended-chain copolymers of polyethylene

Ethylene—propylene and ethylene—butene-1 copolymers with up to 1.7 side groups per 100 carbons have been crystallized at 227°C. and under 4100–4900 atm. pressure. The resulting crystalline polymers are at least partially of extended-chain crystal morphology. Comparison with the same polymers crystallized at atmospheric pressure, which gives folded-chain crystal morphology, revealed: (1) a density higher by 0.008–0.019 g./cm.³ depending on copolymer content; (2) a similar decrease of crystallinity with side group concentration; (3) a similar decrease of the beginning of melting from 125°C. for homopolymer to 65°C. for 1.7 side groups per 100 carbons; (4) a higher (138 ± 0.8°C.) experimental maximum melting point which, in contrast, is independent of copolymer content and seems to vary only with the fraction of low molecular weight material; (5) a decreasing amount of high-melting crystals with increasing copolymer content (72–8%) and an increasing amount of low-melting crystals (27–53%) with increasing copolymer content. In addition, superheating, which reached 5.5°C. for 50°C./min. heating rates, was detected. It was concluded that high-pressure crystallization leads, at least for part of the crystals, to solid solution formation, while atmospheric pressure crystallization does not. Which mode of crystallization is achieved seems kinetically determined. Experimental techniques were dilatometry, DTA, and calorimetry.     

Thermal stability and nonisothermal crystallization of short fiber-reinforced poly(ether ether ketone) composites

The thermal stability of a short carbon-fiber-reinforced PEEK composite was assessed by thermogravimetry and by a Rheometrics dynamic analyzer. The results indicated that holding for 10 min at 380°C was a suitable melting condition to avoid the thermooxidative degradation under air. After proving that the heating rate of 50°C/min can be used to evaluate the crystallinity, a heating stage was used to prepare nonisothermally crystallized specimens using cooling rates from 1 to 100°C/min after melting at 400°C for 3 or 15 min. The degree of crystallinity and the melting behavior of these specimens were investigated by DSC at a heating rate of 50°C/min. The presence of three or four regions indicated that the upper melting temperature, T_m, changed with the crystallization temperature. The first region with the highest T_m, which corresponded to the cooling rate of 1°C/min, can be associated with the crystallization in regime II. There was a second region where T_m decreased as the amount of crystals formed in regime II decreased with increasing cooling rate from 5 to 20°C/min. The third region, a plateau region, corresponded to regime III condition in which the crystals were imperfect. In the fourth region, the cooling was so fast that crystallization was incomplete during the cooling for the melting condition of 400°C for 15 min. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2225–2235, 1998     

Effect of silver nanoparticles on the dynamic crystallization behavior of nylon‐6

The effect of introducing silver nanoparticles on the rheological properties and dynamic crystallization behavior of nylon‐6 was investigated. The nanocomposites showed slightly higher viscosity than pure nylon‐6 in the low‐frequency range even at an extremely low loading level of the silver particles (0.5–1.0 wt %). The nanoparticles had a more noticeable effect on the storage modulus than on the loss modulus of a nylon‐6 melt and reduced its loss tangent. They increased the crystallization temperature of nylon‐6 by about 14 °C and produced a sharper crystalline peak. The silver nanoparticles promoted the crystallization of nylon‐6, and their effect on the dynamic crystallization of nylon‐6 at 200 °C was more notable at a lower shear rate and at 190 °C at a higher frequency. Nylon‐6 produced large spherulitic crystals, but the nanocomposites showed a grainy structure. In addition, the silver nanoparticles reduced the fraction of the α‐form crystal but increased that of the γ‐form crystal. The nanocomposites crystallized at 190 °C showed a lower melting temperature than nylon‐6 by about 3 °C, whereas the nanocomposites crystallized at 200 °C showed almost the same melting temperature. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 790–799, 2004     

Solution crystallization of nylon 12

Electron microscopy and x-ray diffraction data have been obtained on nylon 12 crystallized from 1-hexanol, 1,6-hexanediol, and hexylene glycol. Ribbonlike lamellar crystals of the γ form are obtained by crystallization from all the solutions and elongated flat crystals of the α form by crystallization from the 1-hexanol and hexylene glycol solutions. The direction of the hydrogen bond in these crystals is almost parallel to that of maximum crystal elongation. α- and γ-form crystals both grow from 1-hexanol and hexylene glycol at appropriate crystallization temperatures. γ-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 7.6–10.6 nm. The melting behavior of the solution-grown crystals is examined and discussed. The melting temperatures of the γ form may be lower than that of the α form. An equilibrium melting temperature of 208.4°C for γ-form crystals is obtained by using a relation between thickness of lamellar crystals and their melting temperatures observed by differential scanning calorimeter measurements. Solvents affect the growth of the two crystalline forms in solution crystallization.     

Phase transition of syndiotactic polypropylene in electrospun nanofibers during progressive heating

By means of high-temperature electrospinning process, syndiotactic polypropylene (sPP) nanofibers with an average diameter of 127 nm were obtained using a rotating disc as a collector. The aligned fibers were subjected to progressive heating for fiber melting. During heating, structural evolution of the sPP nanofibers was investigated in situ by means of two-dimensional wide-angle and small-angle X-ray scattering with synchrotron radiation sources. It was found that the as-spun fibers consist of the antichiral form I (9 %), mesophase (31 %), and amorphous phase (60 %), in the absence of isochiral form II. Upon heating, the mesophase started to melt and completely disappeared at 90 °C. The melting of the mesophase directly produced amorphous chains at 35–60 °C, and brought up the isochiral form II at low temperatures (60–70 °C), as well as the antichiral form I at high temperatures (70–110 °C). These events were in accordance with the DSC heating curve, which exhibited a small endotherm centered at 52 °C for the mesophase melting, followed by a shallow and broad exotherm associated with two phase-transition events, i.e., the crystal reorganization and the crystallization of supercooled liquid. The former is likely due to the solid–solid transition of meso→II phase as suggested by Lotz et al. (Macromolecules 31:9253, 1998), and the latter is relevant with crystallization of amorphous chains to develop the thermodynamic stable form I phase at high temperatures.