Reversible Melting of Semi-Crystalline Polymers 2. Annealing Near to the Melting Point

Annealing experiments have been carried out at a few degrees below the melting point of different polyethylenes (LDPE, LLDPE, HDPE), of polypropylene (PP) and of Nylon-6. The heat capacities decrease during the annealing, within a 2-4 min time scale, to a lower value which corresponds to the extrapolated heat capacity values obtained for the cooling cycle when the polymer is cooled from the melt. Heat capacities in the heating cycle following the cooling cycle of PP, Nylon-6 and HDPE have the same value as during the cooling section. This is not the case for LDPE and LLDPE. Exothermic total heat flow in the cooling section following the annealing indicates that the crystallisation takes place during the cooling rather than during the annealing period. The total melting enthalpy measured before and after the annealing cycle is the same. The reversing heat flow shows an excellent fit to the change of the crystallinity measured by small angle scattering of synchrotron radiation during a heating cycle at temperatures below the melting peak. A coupled thermodynamic interaction of the crystalline and the amorphous phases is concluded from this study. This kind of interaction is possible at the lateral end of polymeric chains incorporated into the crystalline phase. This is an indication of the portion of tie molecules in the system, i.e. the portion of fringed micelle type of crystalline morphology with respect to that of folded chain lamellae. This revised version was published online in July 2006 with corrections to the Cover Date.     

Microhardness studies of chain-extended PE. I. Correlations to microstructure

The hardness–microstructure correlation of various polyethylene (PE) samples crystallized at high pressure from the melt (chain-extended), with different molecular weights, has been investigated and compared to melt crystallized samples at atmospheric pressure (chain-folded). The hardness, H, of melt crystallized PE is confirmed to increase linearly with the logarithm of the annealing time, t_a, at a constant annealing temperature. The H increase with t_a is discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values are obtained for samples crystallized or annealed at high pressure as a consequence of the resulting high degree of crystallinity and large crystalline lamellar thickness values. However, it is shown that the high surface free energy value of the chain-extended crystals considerably lowers the hardness values from that of an ideal infinitely thick PE crystal. Analysis of the crystal hardness and the melting temperature data of different polymeric materials emphasizes the close existing relationship between both quantities. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3151–3158, 1999     

Structural changes and chain radius of gyration in cold-drawn polyethylene after annealing: small- and wide-angle X-ray scattering and small-angle neutron scattering studies

Samples made from linear polyethylene were drawn at room temperature and subsequently annealed at high temperatures below the melting point. The structural changes of the crystalline lamellae and lamellar superstructures as well as the single chain radius of gyration were studied by means of combined small- and wide-angle X-ray scattering and small-angle neutron scattering (SANS). After drawing, the polymeric chain segments in the crystalline phase are preferentially oriented along the drawing direction with a high degree of orientation whereas the lamellae in the samples are found to be slightly sheared exhibiting oblique surfaces as evidenced by X-ray scattering. SANS indicates that the chains are highly elongated along the drawing direction. Annealing the deformed samples at temperatures where the mechanical alpha-process of polyethylene is active leads to a thickening of both crystalline lamellae and amorphous layers. The chains in the crystalline phase retain their high degree of orientation after annealing while the lamellae are sheared to a larger extent. In addition, there is also lateral growth of the crystalline lamellae during high-temperature annealing. Despite the structural changes of the crystalline and amorphous regions, there is no evidence for global chain relaxation. The global anisotropic shape of the chains is preserved even after prolonged annealing at high temperatures. The results indicate that the mobility of polyethylene chains-as seen, e.g., by 13C NMR-is a local phenomenon. The results also yield new insight into mechanical properties of drawn PE, especially regarding stress relaxation and creep mechanisms.     

Morphological Changes of Linear,Branched Polyethylenes and their Blends during Crystallization and Subsequent Melting by Synchrotron SAXS and DSC

Summary: The crystalline structure and phase morphology of linear, branched polyethylenes and their blends during crystallization and subsequent melting were investigated, using a combination of differential scanning calorimetry (DSC), and synchrotron small angle X-ray scattering (SAXS). A linear polyethylene (PE1) with weight-average molecular weight (M_w) of 114 000 g/mol, and two branched polyethylene copolymers, containing 4.8 mol% (PE4) and 15.3 mol% (PE10) hexane, with molecular weights of 93 000 g/mol and 46 000 g/mol were used as pure samples. Two blends, PE1-4 and PE1-10, each with a weight ratio of 50/50, were prepared by solution blending. Our results indicate that in PE4 a phase separation within the branched component itself occurred, forming a broad distribution of lamellar thicknesses during the crystallization process. PE10 on the other hand did hardly crystallize because of the high degree of branching. Co-crystallization of both components took place in blend PE1-4 and liquid-liquid phase separation occurred in the melt of PE1-10. Morphological parameters were determined by using Bragg's law and the correlation function, respectively. The detected semicrystalline morphology can be well described by the lamellar insertion mode where thin lamellae develop between thicker primary lamellae. During subsequent heating, lamellae melted in the reversed sequence of their formation. The evolution of the structural parameters as a function of temperature revealed that surface melting began at first, and then the complete melting of stacks occurred until the final melting temperature was reached.     

High-melting bisphenol-A polycarbonate from annealing of vapor-induced crystals

Polycarbonate is known to crystallize thermally, but only slowly and to a limited (25%) extent. The melting points reported exhibit a wide variation. We have found that the melting temperature of polycarbonate may be drastically increased by employing a sequence of vapor-induced crystallization and annealing treatments. The crystals formed by the treatment with organic vapor act as a nucleation or precursor state for further crystallization into larger, more perfect lamellae. An initial peak melting temperature of 195°C has been annealed up to 239°C, and then to 295°C by a double-heat treatment. This sample is 60% crystalline, based on heat-of-fusion calculations. An equilibrium melting point of 335°C has been obtained for PC from an extrapolation of reciprocal lamellar thickness.     

Transformation of isotactic polypropylene droplets from the mesophase into the α‐phase

The structural transformation of homogeneously nucleated metastable mesophase of polypropylene (PP) particles was investigated in this study. We demonstrated the formation of heterogeneity‐free mesophase by slow cooling of the droplets unlike mesophase formation by quenching of the PP melt, which contained large number of bulk nuclei. Submicron size PP droplets were produced by thermal break up of PP and polystyrene layered film assembly. When cooled from melt, the PP droplets crystallized into mesophase at 44 °C revealing granular morphology. Subsequent heating thermogram of the PP particles showed a broad exotherm, which was attributed to the transformation of mesophase into α‐phase. This transformation was investigated during heating by annealing the PP particles at different temperatures. Annealed PP particles were analyzed by means of thermal, morphological and structural properties measurements. Results revealed a two step process for the transformation process. In the first step, the internal rearrangement of PP chains, as against melting and recrystallization of the mesophase, was observed. Since granular morphology was not affected significantly up to 120 °C, it was suggested that translational and rotational motions of PP helices produced ordered α‐phase. In the second step, increment in grain size distribution was observed, when the droplets were annealed at 140 °C. The results were attributed to enhanced chain mobility and merging of the grain boundaries. Annealing at 160 °C revealed the formation of short lamellar structures. Crystal thickening, melting and recrystallization of α‐phase were suggested at high temperature annealing. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Nucleation and overgrowth of PE on PTFE/iPP interfaces

Using an experimental setup with a three‐phase intersecting boundary of PTFE/PE/iPP, the nucleation power of PTFE compared to iPP on the PE was studied by TEM. It was found that the nucleation of the PE on the PTFE interface started at a higher temperature than epitaxial nucleation of the PE onto the iPP interface. During cooling of the melt, the growth direction of the PE crystalline lamellae changes in a continuous manner from the transcrystallization direction of the PTFE/PE interface into the heteroepitaxial “crosshatched” orientation of the iPP/PE interface. A (still highly speculative) self‐assembly of the PE macromolecules at the respective interface just in front of the actual crystallization edge is used to explain this observed phenomenon. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 80–83, 2000     

Melting behavior of drawn films from polyethylene single-crystal mats

Drawing of mats of linear polyethylene single crystals prepared from dilute solution is possible at temperatures above about 90°C. The structure and properties of the drawn specimens are much different from those ordinary drawn bulk polymer. Drawn mats have been investigated by differential scanning calorimetry. The characteristic experimental results are: (a) a broad melting curve, (b) considerable superheating depending on the rate of heating, (c) constancy of the melting point and the heat of fusion with annealing, (d) deviation from the relation between the heat of fusion and the density obtained for the drawn bulk specimens, (e) appearance of two melting peaks in samples annealed at temperatures above about 130°C. These results imply that the structure of the drawn mat is characterized by a larger number of the tie chains connecting the neighboring crystals (the structure postulated in earlier papers) than is the case in ordinary drawn bulk polymer. It can be concluded that the transformation of a fringed micellar type of structure to the folded lamellar structure may be difficult during annealing unless crystals melt and then recrystallize during cooling.     

The effect of melt processing conditions on the hydrogen-bonded sheet orientation and polarization of nylon 11 films

It is known that melt-quenched, cold-drawn, and then annealed nylon 11 films possess a particular doubly oriented hydrogen-bonded sheet structure: the hydrogen-bonded sheets being in the plane of the film and the molecular chain direction being in the direction of draw. These films have been shown to be ferroelectric. In order to clarify the role of this special double orientation on polarization mechanisms and, in addition, provide insight into the reasons for this unique orientation, a systematic study was undertaken. Nylon 11 was melted at 210°C in a hot press for different melting times (ranging from 30 s to 20 min) prior to quenching into an ice-water bath. The resulting orientation of the hydrogen-bonded sheet structure in these films was examined using wide-angle X-ray diffraction and FTIR spectroscopy. For undrawn, melt-quenched films with short times in the melt, a degree of preferred orientation of the hydrogen-bonded sheets in the plane of the film was observed. As time in the melt increased, this preferred orientation in the plane decreased. This was also observed for films which were cold drawn before annealing. However, following cold drawing and several cycles of polarization using a maximum field of 150 MV/m at room temperature, the uniaxially drawn films with different times in the melt possessed the same remanent polarization and the same final orientation of the hydrogen-bonded sheets in the film thickness direction. © 1996 John Wiley & Sons, Inc.     

Preparation of Oriented Liquid‐Crystalline/Isotropic Block Copolymer Films with Parallel or Perpendicular Arrangement of Smectic Layers and Lamellar Microdomains

Films of a symmetric liquid‐crystalline/isotropic block copolymer consisting of a smectic LC side‐chain polymer and polystyrene were prepared by solvent casting from solution and from the isotropic melt. By annealing the solvent‐cast film in the S_A phase an oriented microphase‐separated film of lamellar morphology was obtained in which both the lamellae of the block copolymer and the smectic layers of the LC block were oriented parallel to the film surface. A lamellar morphology with perpendicular orientation of lamellae and smectic layers was generated by cooling the block copolymer from the melt.     

Effect of annealing on the structure and properties of poly(vinylidene fluoride) β‐form films

Oriented poly(vinylidene fluoride) (PVDF) films consisting of β crystals were prepared by the solid‐state coextrusion (SC) of a gel film near the melting temperature (T_m) and by conventional cold tensile drawing (TD) of a melt‐quenched film. These films were annealed over the temperature range of 75–190 °C (below and above the static T_m) while the sample length was kept constant or constant loads were applied. After annealing with the sample length kept constant, the dynamic Young's modulus markedly decreased because of the relaxation of oriented amorphous chains, as shown by infrared spectroscopy. In contrast, annealing under a constant load improved the chain orientation in both the crystalline and amorphous regions, resulting in an increase in the modulus from an initial 10.5 to 14.3 GPa for the SC and from an initial 3.3 to 4.8 GPa for the TD. The SC, annealed at 190 °C with a constant load corresponding to an initial tension of 200 MPa, exhibited an extreme crystalline‐chain orientation of 0.998 and a modulus of 14.3 GPa, among the highest values ever reported for PVDF. Although the remanent polarization (P_r) of the TD increased slightly from the initial 62 to 68 mC/m², P_r of the SC stayed constant at 100 mC/m² independently of the annealing conditions. This suggests that the P_r value of 100 mC/m² approached the equilibrium value for this PVDF sample containing 3.5 mol % structural defects. Therefore, although the modulus and P_r of the TD increased slightly with annealing, the maximum values achieved by annealing were markedly lower than those of the SC and annealed SC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1701–1712, 2003     

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.     

Pathway‐Dependent Melting in a Low‐Molecular‐Weight Polyethylene‐block‐Poly(ethylene oxide) Diblock Copolymer

Summary: In a low‐molecular‐weight polyethylene‐block‐poly(ethylene oxide) (PE‐b‐PEO) diblock copolymer, two pathway‐dependent melting processes were observed: Upon slow heating, the PE lamellar crystals melted at ≈97 °C into a disordered state. However, when the temperature rapidly jumped to above the melting point (e.g., 100 °C), the PE lamellar crystals transformed directly into an ordered lamellar melt, followed by an isothermal conversion into a disordered melt. This isothermal order‐to‐disorder transition was explained by superheating of the PE crystals using a G‐T diagram.

A schematic G‐T diagram explaining the pathway‐dependent double melting for a crystalline polyethylene‐block‐poly(ethylene oxide) copolymer.     

Surface properties of hydrosilylated polyolefins annealed in supercritical carbon dioxide

Two hydrosilylated polyolefin compounds are obtained by reacting polypropylene (PP) and polyethylene (PE) with di‐ and multi‐functional hydride‐terminated poly(dimethylsiloxane) (dH‐PDMS and mH‐PDMS), respectively. The PDMS‐rich surface layers on these two samples show different Si concentrations but similar thicknesses. Samples of these materials are annealed in supercritical carbon dioxide (scCO₂) at various temperatures and pressures for different periods of time. On the PP/dH‐PDMS sample, an increase in the annealing temperature does not affect the Si concentration up to 120°C. However, the Si concentration is sharply reduced at T = 150°C at which point the surface appears to be covered by SiO₂ particles. Annealing the PP/dH‐PDMS sample for short times leads to submicron scale SiO₂ particle formation on the surface. The particles form aggregated clusters that spread all over the surface uniformly when the annealing time is extended. However, Si concentration on the PE/mH‐PDMS sample surface is enhanced as the annealing temperature increases, reaching a maximum at an annealing temperature of 100°C. No particle formation is observed on the PE/mH‐PDMS sample surface. The contact angle of both samples is found to increase with annealing temperature. Increasing the scCO₂ pressure leads to a higher Si concentration on the surfaces of both samples. On the other hand, increasing the CO₂ pressure leads to opposite trends in contact angle with the PP/dH‐PDMS sample exhibiting an increasing contact angle and the PE/mH‐PDMS sample exhibiting a decreasing one. Copyright © 2007 John Wiley & Sons, Ltd.     

Fabrication of microporous membranes from melt extruded polypropylene precursor films via stretching: Effect of annealing

In this work, the effects of annealing conditions on the microstructure of polypropylene(PP) precursor films and further on the porous structure and permeability of stretched membranes were investigated. Combinations of WAXD, FTIR, DSC and DMA results clearly showed the crystalline orientation and crystallinity of the precursor film increased with annealing temperature, while the molecular chain entanglements in the amorphous phase decreased. Changes in the deformation behavior suggested more lamellar separation occurred for the films annealed at higher temperatures. Surface morphologies of the membranes examined by SEM revealed more pore number and uniform porous structure as the annealing temperature increased. In accordance with the SEM results, the permeability of the membranes increased with annealing temperature. On the other hand, it was found that 10 min was almost enough for the annealing process to obtain the microporous membranes with an optimal permeability.     

The double melting behavior of a liquid crystalline polyimide derived from PMDA and 1,3‐bis[4‐(4′‐aminophenoxyl) cumyl] benzene

The double melting behavior of a thermotropic liquid crystalline polyimide was studied by means of differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering (SAXS). This liquid crystalline polyimide exhibited a normal melting peak around 278 °C and transformed into a smectic A phase. The smectic A phase changed to nematic phase upon heating to 298 °C, then became isotropic melt around 345 °C. The samples annealed or isothermally crystallized at lower temperature showed double melting endotherms during heating scan. The annealing‐induced melting endotherm was highly dependent on annealing conditions, whereas the normal melting endotherm was almost not influenced by annealing when the annealing temperature was low. Various possibilities for the lower melting endotherm are discussed. The equilibrium melting points of both melting peaks were extrapolated to be 283.2 °C. Combined analytical results showed that the double melting peaks were from the melting of the two types of crystallites generated from two crystallization processes: a slow and a fast one. Fast crystallization may start from the well‐aligned liquid crystal domains, whereas the slow one may be from the fringed or amorphous regions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3018–3031, 2000     

Cumulative erasing effect in the melt memory of nylon 6

The effect of melt annealing over a wide range of temperatures (240–300°C) and durations (0.5–120 min) on the induction period and the heat of crystallization of nylon 6 was studied by means of differential scanning calorimetry. It was found that the crystallization characteristics are determined by the total annealing time experienced by the melt at a given temperature; i.e., a cumulative effect on the erasure of the melt memory is observed. Complete destruction of the crystalline nuclei is achieved after annealing for 90 min at 280°C, the equilibrium melting point of nylon 6; the nuclei cannot be regenerated by nonisothermal crystallization. All the effects observed are explained by the existence of a wide distribution of size and perfection of nuclei.     

Structure and morphology of nanocomposite films prepared from polyvinyl alcohol and silver nitrate: Influence of thermal treatment

Hybrid organic/inorganic films have been prepared from an aqueous solution of polyvinyl alcohol (PVA) and silver nitrate (AgNO₃). The silver nanoparticles have been generated in the PVA matrix by thermal treatments. The structure and the morphology of the hybrid films have been studied as a function of the silver precursor concentration and of the annealing conditions for a wide range of annealing temperatures. It was shown that in the uncured hybrid film most of the silver ions were initially coordinated with the polymer OH groups to form a chelate structure. A nanostructuration effect leading to the formation of crystalline silver nanoparticles was evidenced for annealing treatments performed at temperatures higher than 90 °C. For a curing temperature equal to 110 °C, the sizes of the formed nanoparticles were only slightly increasing as a function of annealing time and the effect of AgNO₃ complex amount in this curing condition was also significant, but slight. Annealing at a temperature equal to 160 °C thus at a temperature for which a part of the crystalline phase of PVA was melt led to an important increase of the size of the generated metal nanoparticles. The evolution of the morphology was discussed for each curing temperature as a function of the kinetics of the nanostructuration, of the size of the matrix amorphous lamellae and of the polymer chain mobility. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2657–2672, 2007     

Effect of annealing on thermal properties and crystalline structure of polyamides. Nylon 11 (polyundecaneamide)

Summary The effect of prolonged annealing at a low super-cooling on the thermal properties and crystalline structure of nylon 11 has been investigated. Annealed nylon 11 formed the lamellar spherulites of the alpha-crystalline modification. Thickness of the lamellae increased with time of annealing from 50 to 165 Å for the samples annealed for 20 and 2000 hours respectively. Corresponding values of the melting temperature and the heat of melting increased from 190 °C and 10 cal/g to 211 °C and 20 cal/g.The extrapolated heat of melting of the crystalline phase of nylon 11 is 47 cal/g.

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Zusammenfassung Untersucht wurden die thermischen Eigenschaften und kristallinen Strukturen des 11-Polyamid nach langfristiger Temperung mit Temperaturen nahe dem Schmelzpunkt.Getempertes 11-Polyamid bildete lamellenartige Sphärolithe und kristallisiert in alpha-Modifikation.Die Dicke der Lamellen vergrößert sich mit der Temperungszeit von 50 Å bei einer Temperungszeit von 20 Stdn. bis auf 165 Å bei einer Zeit von 2000 Stdn.Schmelzpunkt und Schmelzwärme dieser getemperten Proben erhöhen sich entsprechend von 190 °C und 10 cal/g auf 211 °C und 20 cal/g.Die extrapolierte Schmelzwärme der kristallinen Phase von 11-Polyamid beträgt 47 cal/g.

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With 8 figures

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Presented in part at the 8th Europhysics Conference on Macromolecular Physics, Bristol, 1978.     

Influence of the order of polymer melt on the crystallization behavior: I. Double melting endotherms of isotactic polypropylene

The influence of the order of polymer melt on the subsequent crystallization and melting has been carefully studied. The experimental data show that the order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature. For an iPP sample isothermally crystallized at 130 °C for half an hour, the degree of order of melt is higher when the fusion temperature is lower than about 170.5 °C, hence the lamellae formed in a rapid cooling process are perfect. If the fusion temperature is not higher than 167 °C, some thicker lamellae can exist in the melt. The melting of these unmelted lamellae and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when the fusion temperature is higher than 170.5 °C, the order of the iPP melt decreases greatly; thus, the lamellae formed in the following cooling process are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae also leads to double melting endotherms. Received: June 16, 2000 Accepted: October 16, 2000