Melting Of Lamellae Research Articles

Entropic barrier theory of polymer melting

AbstractWe present a theory of melting kinetics of semicrystalline polymers at temperatures above the equilibrium melting temperature, by accounting for conformational entropy of chains during melting. We have derived free energy landscapes for escape of individual chains from a lamella into the amorphous phase as a function of the characteristics of the initial lamella, such as the lamellar thickness, number of chain folds, fold‐ and lateral‐surface free energies, and mean energy of a monomer inside the lamella. We show that melting of lamellae is always accompanied by a free energy barrier which is entirely entropic in origin. In terms of the parameters characterizing the lamellae and the extent of superheating, closed‐form formulas are presented for the equilibrium melting temperature, driving force for crystallization, free energy barrier height, average expulsion time of a single chain from a lamella, and the melting velocity of lamellae. The present entropic barrier theory predicts that the dependence of melting velocity on superheating is nonlinear and non‐Arrhenius, in qualitative agreement with experimental observations reported in the literature. The derived formulas open an opportunity to further explore the role of various molecular features of semicrystalline polymers on their melting kinetics.

To view on evolution of primary texture of Aliabad chromite (NE Forumad), petrogenesis and tectonic setting

Ali abad chromites mines (north east of Forumad) are as a part of chromites assemblage dependent on ophiolite Sabzevar that are located in NW Sabzevar city. Chromitites deposits in these mines have been located as irregular lenses and layered or banded along with pods of dunite in depleted harzburgite. In attention to mineralogy observation, incongruent melting of orthopyroxenes and exsolution lamellae of clinopyroene in the orthopyroxenes, indicate that these rocks were originally formed at mantle pressures and temperatures conditions. Mineralography study on the textures and structures of Ali abad chromites indicate two types main textures that are primary and secondary. The primary textures and structures include massive and disseminated or separated grain. The milonitic, brecciated and tensional textures are from secondary types in chromites. On the basis of geochemical results the structural formula are calculated as the chromite minerals [(Mg,Fe2+)O (Cr,Al,Fe3+)2O3].These studies represent height percent Cr2O3 chromite mineral in disseminated texture than massive textures. Geochemical study on the chromitite area, site them in height Cr types and associated to podiform type that are originated from a parent melt with boninitic character resulting from high degree of partial melting of mantle peridotite in Supra Subduction Zone (SSZ).

Confined Crystal Melting in Edgeless Poly(l-lactic acid) Crystalsomes.

Polymer single crystals tend to be quasi two-dimensional (2D) lamellae and their small lateral surfaces are the starting points of lamella melting and thickening. However, the recently discovered crystalsomes, which are defined for hollow single crystal-like spherical shells, are edgeless, self-confined, and incommensurate with translational symmetry. This work concerns the structure and melting behavior of these edgeless crystalsomes. Poly(l-lactic acid) crystalsomes were grown using a miniemulsion solution crystallization method. Differential scanning calorimetry and in situ wide-angle X-ray diffraction were used to follow the structural evolution of the crystalsomes upon heating. Our results demonstrated that the structure and melting behavior of crystalsomes are curvature-dependent and significantly different from their flat crystal counterpart.

Influences of molecular weight and thermal history on partial melting of polyethylene: Existence of non-lamellar crystallite

The effects of molecular weight and thermal history on the partial melting of polyethylene (PE) were investigated by in-situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. In the partial melting process, both surface melting and melting of small lamellae were found to play prominent roles. However, in some cases that could not be explained by these mechanisms, the concept of “non-lamellar crystalline” (NLC) was proposed. We argue that melting of the NLC domains can also contribute to the partial melting, especially at low temperatures. The event would decrease the crystallinity but does not perturb the lamellar structure, which are not seen in surface melting and melting of small lamellae. Upon further heating, structural changes, including the extension of lamellar length and reduction of lamellar width, were also observed. The experimental results in the temperature range dominated by the surface melting mechanism could be described by the model proposed by Albrecht and Strobl.

A Synchrotron in situ X-ray Study on the Multiple Melting Behaviors of Isomorphous Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(HB-co-HV)) with Middle HV Content

The multiple endothermic peaks without observable recrystallization phenomenon of isomorphous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(HB-co-HV)] with the middle HV content of 19.4 mol% and 28.7 mol% were confirmed by differential scanning calorimetry (DSC), and the evolutions of crystal structure and lamellar morphology in the heating and melting process were tracked by in situ synchrotron wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. The emergence of asymmetric features of both the diffraction peaks and scattering curves indicates the coexistence of different lamellar crystals with varied unit cell parameters. Based on the in situ WAXD and SAXS measurements, we calculated the evolutions of the unit cell parameters a and b as well as the long period and lamellar thickness upon heating. The comparative analysis of WAXD and SAXS data confirms that the multiple endothermic peaks of P(HB-co-19.4%HV) and P(HB-co-28.7%HV) result from the melting of different lamellae rather than the melting/recrystallization. The thinner, unstable uniform lamellae with HV counits total inclusion melt first and the thicker, stable sandwich lamellae with HV counits partial inclusion melt last. In addition, the large second melting peak in P(HB-co-19.4%HV), differing from that of samples with HV content of 28.7 mol% and 36.2 mol%, is due to the unique state of HV content leading to a transition of sandwich lamellae to uniform lamellae. The present study establishes the relationship between the different lamellae structure and multiple melting behaviors of isomorphous copolymer.

In-situ investigation of multiple endothermic peaks in isomorphous poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with low HV content by synchrotron radiation

Multiple endothermic peaks of isomorphous P(HB-co-HV) with low HV content were investigated by DSC, in-situ WAXD and SAXS measurements. The multiple endothermic peaks are confirmed to be caused by two different kinds of crystal structures of P(HB-co-HV), namely uniform lamellae with HV units complete exclusion and sandwich lamellae with HV units partial inclusion. With the increase of the temperature, the thinner uniform lamellae melt at the lower melting peak and the thicker sandwich lamellae melt at the higher melting peak. Comparing with P(HB-co-4.9%HV), P(HB-co-9.4%HV) has the larger value of unit cell parameters and the higher distribution and fusion enthalpy of higher melting peak, which confirm that P(HB-co-HV) with higher HV content contains more sandwich lamellae and more HV units included in crystals for P(HB-co-HV) with HV content lower than 10 mol%.

Melting behavior and structural and morphological changes of isotactic polypropylene from heat treatment

Time-resolved X-ray measurements during melting and detailed structural and morphological analyses of samples heat treated near their melting temperature for a short time revealed a structural change of isotactic polypropylene (iPP) in which the melt-crystallized at a cooling rate (CR) of 20 K/min. During melting at a heating rate (HR) of 5 K/min, we observed the appearance of a peak with a higher melting temperature (Tm); however, we could not clearly observe −231, −161 reflections that are typical for an α2 crystal. The α2 crystal was a minor component of the heat-treated sample with elevated Tm in which reorganization was almost completed during the heat treatment. These results suggest that lamellar thickening of the α1 crystal is the main mechanism of reorganization. Lamellar thickening was accompanied by an increase in central diffuse scattering of small-angle X-ray scattering (SAXS), suggesting that thickening occurred at randomly distributed positions and resulted in the disordering of lamellar stacking observed based on observations with a transmission electron microscope (TEM) for the heat-treated samples. The drastically increased long period (L) above 160 °C, which can be attributed to the partial melting of thin lamellae, was maintained after the heat treatments by the aforementioned reorganization. Time-resolved X-ray measurements during melting and analyses of samples heat treated near their melting temperature revealed a structural change of the melt-crystallized isotactic polypropylene. During melting at a heating rate of 5 K/min, we observed the appearance of a higher melting temperature peak due to lamellar thickening of the α1 crystal. Lamellar thickening was accompanied by an increase in central diffuse scattering of SAXS, suggesting that thickening occurred at random positions and caused disordering of lamellar stacking.

Structure evolution of polyethylene-plasticizer film at industrially relevant conditions studied by in-situ X-ray scattering: The role of crystal stress

Structure evolution of slightly oriented high density polyethylene (HDPE)-plasticizer film is recorded with small- and wide- angle X-ray scattering (SAXS/WAXS) during stretching. The HDPE has number average molecular weight (Mn) of 300 kg/mol and polydispersity index (Mw/Mn) of 20, respectively. The plasticizer is mixture of aliphatic hydrocarbon with chain length of 11–13 carbon atoms. Experiments are carried out at industrially relevant conditions, with film thickness of 650 μm, plasticizer content of 56%, strain rate of 0.5 s−1 and stretch temperature from 80 °C to 100 °C, respectively. An interesting evolution of SAXS patterns from anisotropic ring, four-point, six-point, and two-lobule is observed. From WAXS data, a monotonous increasing after the first decline of crystallinity is obtained, by which melting-recrystallization is validated to occur during stretching. With peak shift of WAXS pattern at different azimuthal angle (Φ), triaxial local stresses exerted on lamellae oriented along different directions are acquired. As consequence, stress exerted along and that perpendicular to chain direction are revealed to induce melting of lamellae at azimuthal angle from 0° to 60° and that from 60° to 90°, respectively. Due to the heterogeneous stress distribution of the system, the appearance of four-point SAXS pattern stems from the sequential melting of lamellae at different azimuthal angles. Through thermodynamic analysis, the physical essence of stress induced melting of lamellae is validated to be elastic energy driven crystal to amorphous phase transition.

Raman Identification of Multiple Melting Peaks of Polyethylene

Multiple melting peaks of high-density polyethylene produced by complex thermal pretreatment are investigated with variable temperature Raman spectroscopy and differential scanning calorimetry (DSC). We examine the origin of the observed multiple melting peaks with three possible model mechanisms: melting–recrystallization, premelting via mesophases, and melting of multithickness lamellae. We quantitatively analyze Raman spectral change associated with each melting peak with respect to temperature. The Raman spectral change is represented by a least squares moving-window (LSMW) method, which is effective in suppression of high-frequency derivative noise. The conformational similarity among multiple melting peaks strongly suggests that dominant melting occurs from the crystalline phase directly to the amorphous phase at all transitions. The observed multiple melting transitions are consistent with one-step melting of multithickness lamellae as the dominant origin. However, two-dimensional Raman correlation...

Relationship between melting behavior and morphological changes of semicrystalline polymers

The present study on the case of poly(hexamethylene succinate) is to provide a basis for a better understanding of the subtle relationship between melting behavior and morphological changes of semicrystalline polymers. The melting behavior and morphological changes of poly(hexamethylene succinate) during both isothermal secondary crystallization and annealing processes were investigated by DSC and SAXS. DSC results showed that, with increasing crystallization time or annealing time, the melting endotherm continuously shifted to higher temperature, which suggested that some minor structural or morphological changes must occur. However, almost no changes at all on the crystal thickness were observed from SAXS measurements. The observed evidence confirmed that the increase in the melting temperature is not attributed to crystal thickening but crystal perfection. More exactly, the rearrangement and smoothing of tie molecules at the folding surface result in the reduction of the fold surface free energy, which dominantly contributes to the increase in the melting peak temperature. The origin of the new endothermic peak observed after annealing at elevated temperature was also discussed. TMDSC results indicated that the annealing peak resulted from the enthalpy relaxation and devitrification transition of rigid amorphous fraction formed by the driving force of thermodynamic nonequilibrium, rather than usually regarded as the melting of thin lamellae or imperfect crystals formed by annealing secondary crystallization.

Temperature dependence of interfacial thickness and conductivity of SIO2/LDPE composite films

A melt-blending process was adopted to prepare a 5wt% and 150μm thick SiO2/LDPE (Low-Density Poly-Ethylene) composite film, to study the influence of the microstructure evolution of the composite film on its conductivity when the temperature changes. The results show that there is an interface layer between SiO2and PI polymer matrix in composite film. When temperature varies from 30°C to 100°C, the interfacial thickness increases from 2.8nm to 3.5nm, and remains constant beyond 100°C. DSC (Dynamic Stability Control) shows that this increase is mainly caused by the melting of lamellae of polyethylene polymer chains. DMA (Dynamic Mechanical Analyzer) indicates the bound effect of the interface on the polymer chains of the composite film. When temperature changes from 30°C to 70°C at the heating rate of 10°C/min, the conductivity values of LDPE and SiO2/LDPE change from 1×10−16 to 1×10−14s/m, 1.5×10−16 to 1.7×10−16s/m, respectively. The conductivity of the composite film becomes 2 orders of magnitude smaller than that of pure polyethylene film. The increases of interface layer limit the space charge accumulation effectively, and lowers the sensitiveness of conductivity to temperature, resulting in significant improvement of insulating property.

Melting and crystallization behaviours of the electrons irradiated LDPE/MWCNTs composites

Electrons are an important constituent part of radiation space of charged particles and could damage microelectronic devices. Therefore, effective radiation protection is very important for the electronic device. So, a radiation protecting material with lightweight, high performance and low cost is urgently needed. Polyethylene (PE) with high hydrogen content/carbon nanotube (CNT) composite as a space shielding material is very promising for spacecraft application in the future. In order to meet the requirement of space applications for PE/CNT composite, it is of important academic value and practical significance to explore melting and crystallization behaviors of LDPE/MWCNT composites. In this paper, melting and crystallization behaviours of the irradiated LDPE/MWCNT composites irradiated by 110 keV electrons are studied by differential scanning calorimetry (DSC), synchrotron radiation X-ray small angle scattering (SAXS) and wide angle diffraction (WAXD). Experimental results show that the irradiation by 110 keV electrons does not affect the thermal characteristics of the LDPE, but can enhance the initial melting temperature and the melting-terminating temperature of LDPE/2% MWCNT composites during melting. Also, the radiation by 110 keV electrons could reduce the initial crystallization temperature and the crystallization-terminating temperature during crystallization. MWCNTs could enhance the initial melting temperature and reduce the melting-terminating temperature of LDPE/2% MWCNT composites during melting. Moreover, MWCNTs could enhance the initial crystallization temperature and crystallinity, and reduce the crystallization-terminating temperature during crystallization. SAXS and WAXD analyses show that with increasing the temperature, long periods of the irradiated/unirradiated LDPE increase during melting. Compared with that of the unirradiated LDPE, at a given temperature, long period of the irradiated LDPE is small. During crystallization, with reducing the temperature, long period of the irradiated/unirradiated LDPE begins to appear and gradually decreases. At the same temperature, long period of the irradiated LDPE is larger than that of the unirradiated one. For LDPE/2% MWCNT composites, long periods of the irradiated/unirradiated samples during melting and crystallization do not exist. The 110 keV electron irradiation mainly influences LDPE matrix of LDPE/2% MWCNT composites during melting and crystallization. The 110 keV electron irradiation can slow down the amorphous region expansion and the initial melting of lamellae of the LDPE matrix during melting. The 110 keV electron irradiation can slow down the amorphous region shrinkage and inhibit crystal from growing up during crystallization. During melting, MWCNTs can hinder the amorphous and crystalline molecular chains of LDPE from moving, which hinders the LDPE matrix from initially melting, but promotes the melting process after the initial melting has begun. During crystallization, MWCNTs could promote the formation of crystal of the LDPE matrix and inhibit the crystal from growing up.

Influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane

The influence of annealing temperature on the lamellar and connecting bridge structure of stretched polypropylene microporous membrane was investigated using small-angle X-ray scattering, temperature-modulated differential scanning calorimetry and scanning electron microscopy. It is found that with increasing annealing temperature from 105 to 145 degrees C, the main lamella melting peak combines with that from connecting bridges and a uniform pore arrangement is obtained in the microporous membrane. The annealed lamella thickness is increased and lamellar structure is improved, due to the occurrence of melting and recrystallization during annealing. At the same time, more secondary crystals are formed. The melting and recrystallization and secondary crystals contribute to the appearance of an annealing peak in the differential scanning calorimetry curve of annealed film. During the following cold and hot stretching, the secondary crystals disappear and convert to initial connecting bridges. The improved lamellar structure can support the scaffold of pore structure, resulting in a uniform connecting bridge arrangement. But further increasing the temperature to 150 degrees C degrades the initial lamellar structure, leading to a decrease of pore arrangement in the stretched microporous membrane. Annealing leads to the difference of lamellar structure: the initial lamellar structure is improved and some weak secondary crystals are formed in the amorphous region. (c) 2014 Society of Chemical Industry

Solid-state reorganization, melting and melt-recrystallization of conformationally disordered crystals (α′-phase) of poly (l-lactic acid)

Conformationally disordered α′-crystals of poly (l-lactic acid) were formed by crystallization of the melt at high supercooling at 95 °C. Analysis of their melting temperature as a function of the crystallinity revealed absence of crystal thickening during isothermal crystallization. Annealing of α′-crystals between the crystallization temperature of 95 °C and their zero-entropy production melting temperature of 150 °C leads to their stabilization, mainly by solid-state reorganization. Heating faster than 30 K s−1 suppresses reorganization and permits superheating of the α′-phase. Consequently, isothermal melting followed by melt-recrystallization becomes accessible. Melting is completed within few hundreds of milliseconds, and melt-recrystallization is about two orders of magnitude faster than crystallization of the isotropic melt at identical temperature. The time required for melting decreases with superheating and increases with the lateral dimension of the lamellar crystals. Laterally extended lamellae require long time for melting of the outer crystal layers, which allows stabilization of the central crystal part. These crystal remnants then serve as seed for immediate recrystallization. In case of complete melting of smaller lamellae, melt-recrystallization is retarded but still distinctly faster than cold- and melt-crystallization, due to incomplete isotropization of the melt.

Insight into the annealing peak and microstructural changes of poly(l-lactic acid) by annealing at elevated temperatures

An endothermic annealing peak appears in DSC heating scans of cold-crystallized poly(l-lactic acid) after annealing above the cold-crystallization temperature. The annealing peak shifts to higher temperature and its magnitude increase with increasing annealing temperature or time. Furthermore, the origin of the annealing peak and microstructural changes of poly(l-lactic acid) were investigated by temperature-modulated differential scanning calorimetry (TMDSC), Wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). During TMDSC scan, the observed annealing peak on the total heat flow (THF) is separated into an endothermic peak of enthalpy relaxation on the nonreversible heat flow (NHF) and a glass transition (devitrification) step on the reversible heat flow (RHF). SAXS results showed that the significant increase in long period mainly arises from the increase in the amorphous layer after annealing. The storage modulus is enhanced with increasing annealing temperature, accompany with glass transition temperature shifts to lower temperature, and the softening is also observed above annealing temperature in DMA measurements. It is showed that the annealing at elevated temperature promotes the chain mobility of the constraint amorphous phase and divides them into more orderly rigid amorphous fraction (RAF) and more disorderly mobile amorphous fraction (MAF). The above results indicate that the annealing peak is attributed to the nonreversible enthalpy relaxation of RAF, which formed by annealing process, rather than the melting of thin lamellae or imperfect crystals formed by secondary crystallization.

Shear-induced alignment of lamellae in thin films of diblock copolymers

The shear alignment of lamellae-forming diblock copolymers in thin films is examined using coarse-grained Langevin dynamics simulations. We investigate how lamellar orientation is affected by the shear rate, and interactions between polymer segments and the confining surfaces. For neutral confining surfaces, we find that above a critical stress, lamellae melt and reform in the direction of the shear (perpendicular to the walls), irrespective of their initial orientation. The time needed for reorientation was four times longer when the initial configuration was parallel to the walls, relative to lamellae initially oriented perpendicular to both the walls and the shear direction. When the surface–block interactions become non-neutral, there is critical interaction strength above which the perpendicular orientation is no longer favored, because of enthalpic contributions of one block preferentially wetting the non-neutral surface. The formation of parallel lamellae in films under shear requires a smaller degree of preferential interfacial interactions if both confining surfaces are non-neutral. The film thickness affects the rate of lamellar reorientation as well as the lamellar periodicity. Overall, our results are in good agreement with experimental findings and provide insights into how to use shear to control alignment of lamellar structures in thin films.

Study of an initial transient relaxation in XLPE cable insulation by TSDC and PEA

Space charge profiles in electrets of cable XLPE insulation have been determined by pulsed electroacustic method (PEA). A single positive peak close to the cathode, which decreases with annealing time, is observed. Typically, thermally stimulated depolarization currents (TSDC) of those electrets show heteropolar peaks initially. A new transient homopolar peak emerges in TSDC profiles at 99°C with annealing at high temperatures. No direct correlation can be established between PEA and TSDC results, since the case of the largest value of the maximum of this TSDC peak corresponds to the smallest charge peak observed in PEA profile. The transient behavior is explained by means of charge trapped on crystals surfaces due to the differences between the conductivities of amorphous and crystalline phases. The homoplar peak is associated with dipolar contaminants located in the interlamellar amorphous phase, which are reoriented by the field produced by charge trapped on crystal surfaces, during the TSDC discharges. Detrapping of charge trapped on crystal surfaces occurs when lamellae melt, which contributes homopolarly to the current. Both contributions result in the peaks observed in the TSDC discharge.

Amniotic membrane transplantation in corneal melting after anterior lamellar keratoplasty assisted by femtosecond laser in children

To evaluate the efficacy of amniotic membrane transplantation in lamella melting after anterior lamellar keratoplasty (ALK) assisted by femtosecond laser in a pediatric patient. An 11-year-old girl with progressive keratoconus underwent ALK assisted by femtosecond laser on the right eye. The surgical procedure was performed under general anesthesia. The 60-KHz IntraLase femtosecond laser (Abbott Medical Optics) created both the donor (thickness 350 µm; diameter 8.1 mm; side cut 70°) and recipient (thickness 260 µm; diameter 8.0 mm; side cut 70°) lamellae. The recipient lamella was then gently removed and donor was fitted into place and sutured using 4 interrupted sutures added to a running suture in nylon 10-0. The surgery was uneventful. Two months after surgery, a lamella melting was observed. One month after topical steroid treatment, amniotic membrane transplantation was performed. When lamella melting was observed, the best-corrected visual acuity (BCVA) was 0.1 and 0.5 with foramen and no inflammatory reaction in the anterior chamber was reported. Eleven months after amniotic membrane transplantation, BCVA was 0.9 and no visual acuity increase was recorded with foramen. Our findings show that amniotic membrane transplantation could be considered in lamella melting after ALK assisted by femtosecond laser in children.

Crystallization and melting behavior of poly(ethylene oxide) and its blend with styrene-based ionomer using time-resolved SAXS/WAXS experiments

Time-resolved synchrotron wide- and small-angle X-ray scattering experiments were used to investigate the crystallization behavior and microstructure development of neat poly(ethylene oxide) (PEO) and its 50/50 blend with ionomer containing 6.4 mol% of sodium acrylate. The apparent lateral crystal sizes D (1 2 0) and D (1 1 2)/(0 0 4) were derived from the WAXS profiles. It was found that D (1 2 0) and D (1 1 2)/(0 0 4) of PEO in the blend are almost independent of temperature and are smaller when compared to those of neat PEO sample. The evolution of morphological parameters extracted from time-resolved SAXS profiles such as the long period L, the lamellar crystal thickness l C and the amorphous layer thickness l A , shows that the crystallization process of neat PEO follows the nucleation theory. The lamellar crystal thickness l C shows a single linear dependence on inverse supercooling, over the whole temperature range investigated. In contrast, the crystallization process of PEO in the blend (i.e. in the presence of interactions with the ionomer) follows the nucleation theory only in the narrow supercooling range. It was found also that the morphology of the blend consists of a broad population of lamellar crystal thicknesses. During heating lamellae melt in the reversed sequence of their formation.

DSC analysis of isothermally melt-crystallized bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) films

The multiple melting behavior of isothermally melt-crystallized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) from its melt was investigated using differential scanning calorimetry (DSC). PHBHHx exhibits a fourfold endothermic melting phenomenon, which were expressed as A, I, II, and III from low to high temperature, and attributed to the melting of secondary lamellae formed at room temperature, the melting of secondary lamellae at crystallization temperature, the melting of primary lamellae, and the melting of the recrystallized lamellae of different stabilities, respectively. Secondary crystallization is much slower than the primary crystallization and needs a relatively long period of time to occur. Furthermore, secondary crystallization at room temperature is heterogeneous, which depends on the presence of the primary lamellae and the secondary lamellae formation.