Melting Of Crystallites Research Articles

Reversible Surface Melting of PE and PEO Crystallites Indicated by TMDSC

Quasi-isothermal temperature-modulated differential scanning calorimetry was employed in a study of thermoreversible structural changes in the melting range of semicrystalline polymers. The results indicated a reversible melting and crystallization process occurring at the fold surfaces of crystallites of polyethylene and poly(ethylene oxide). For polyethylene, good agreements were found with reported small-angle X-ray scattering data and Fischer's theory. Surface melting and crystallization depend on the ability of the chains in the crystals to carry out a sliding diffusion. This was shown by a comparison of polyethylene and poly(ethylene oxide) with other polymers such as poly(ethylene terephthalate), polycaprolactone, isotactic polypropylene, and syndiotactic polypropylene. When the longitudinal chain mobility in the crystals is much reduced or completely absent, only a small or no excess reversing heat capacity is observed. The special performance of short-chain poly(ethylene oxide) is indicative for the metastability of the crystals built of once-folded chains.

Crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene

The changes in crystallite orientation during melting of oriented ultra-high-molecular-weight polyethylene (UHMW PE) were investigated by means of wide-angle X-ray scattering. The orientation distribution of crystallites in drawn UHMW PE is composed of two components differing in width. The narrow and broad components revealed in this study indicate the existence of two classes of crystallites with different orientability. Some of the crystallites are oriented almost perfectly even at low-draw ratios, while the others do not orient so effectively. The analysis of melting behaviour of such a texture composed of orthorhombic crystals indicates that highly oriented crystallites are formed by taut molecules and transform first to the hexagonal phase, while the molecules constituting low-oriented crystallites melt directly to the typical amorphous phase. The increase in orientation of highly oriented crystallites during their partial melting, observed in the samples kept at constant length and even those allowed to shrink under constant load, can be explained by the kinetic factor proposed by Ziabicki.

Carbon black–filled immiscible blends of poly(vinylidene fluoride) and high density polyethylene: The relationship between morphology and positive and negative temperature coefficient effects

AbstractConductive polymer composites were prepared by melt‐mixing of an immiscible blend of poly(vinylidene fluoride) (PVDF), high density polyethylene (HDPE), and carbon black (CB). Three major factors—the carbon black content, the carbon black type, and the composite morphology—were shown to have remarkable effects on the positive temperature coefficient (PTC) and negative temperature coefficient (NTC) effect of the composites. The relationship between the morphology and the PTC and NTC effects of the composites was investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The OM micrographs indicated that CB was selectively located in the HDPE phase and the SEM micrographs showed that there were some gaps between the two phases. The PTC effect of the composites is caused by the thermal expansion as a result of the melting of the HDPE crystallites. The morphology of the composites greatly affects the PTC and NTC behaviors of the composites. When the CB‐filled HDPE formed a continuous phase and the PVDF formed a dispersed phase, the PTC and NTC behaviors of the composites were similar to those of CB‐filled neat HDPE composite without crosslinking. When the composite exhibited an interlocking structure, a normal PTC effect could also be observed, but the NTC effect was delayed to higher temperatures. A mechanism was proposed to explain this new physical phenomenon, and the mechanism was verified by another CB‐filled polymer blend comprising an alternating copolymer of tetrafluoroethylene‐ethylene and HDPE.

Synthesis and thermal properties of poly(arylene ether ketone)s containing phthalazinone moieties

A series of novel poly(arylene ether ketone)s were synthesized from the reaction of hydroquinone and 4-(4-hydroxyphenyl)-2,3-phthalazin-1-one with 4,4′-difluorobenzophenone in N-cyclohexylpyrrolidinone containing anhydrous potassium carbonate. The polymers exhibited high glass transition temperatures together with excellent thermooxidative stability. The chain structure of these polymers was studied by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction techniques (WAXD), and matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). The experimental results indicated that these “as-made” copoly(aryleneketone)s containing hydroquinone moieties exhibited a block chain structure with segments which mainly consisted of hydroquinone and 4,4′-difluorobenzophenone. These chain segments resulted in crystallites in the polymers although they are thermodynamically unstable. The polymers showed thermal properties comparable to commercial PEEK, but the conditions for synthesis are much milder. The glass transition temperatures and solubilities of the copoly(arylene ketone)s tended to increase with increasing phthalazinone moiety content, while the crystallite melting points and crystallinity appeared to decrease. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1781–1788, 1999

Melting of Gelatin Crystals below Glass Transition Temperature: A Direct Crystal-Glass Transition as Revealed by Microhardness

In the present paper the effect of crystallization conditions on the melting temperature (Tm ) of chemically cross-linked gelatin is studied by means of differential scanning calorimetry (DSC) and microhardness (H) measurements as extension of our recent findings (Ref. [11]). A rather unusual situation when polymer crystals melt below the glass transition temperature (Tg ) is confirmed by DSC. This is highlighted on dry gelatin which is characterized by relatively high, and close to each other, Tg and Tm values (217 and 230°C, respectively). By depressing Tg , using water as plasticizer, rather imperfect crystallites are obtained which melt well below the Tm of dry gelatin. It is shown that H increases with temperature mostly due to the drying out of the room-conditioned gelatin. In the 160–180°C range H reaches values of about 390MPa. In this temperature range the imperfect crystallites melt (according to DSC), however without formation of a typical liquid phase (since H remains constant). It is expected that this direct crystal-glass transition can be used to obtain highly ordered polymer glasses. The unusual high H values as well as the low coefficient of thermal softening of gelatin, in contrast to those common for synthetic polymers, makes this material attractive for practical applications.

The influence of annealing on thermal transitions in a nematic copolyester

Thermal transitions of a glassy, main chain, liquid crystalline, random copolyester, HIQ-40, have been characterized. HIQ-40 is made from 40 mol percent p-hydroxybenzoic acid (HBA) and 30 mol % each of p-hydroquinone (HQ) and isophthalic acid (IA). This polymer is soluble in organic solvents, permitting the preparation of thin, solution-cast films that are in a glassy, metastable, optically isotropic state. On first heating of an isotropic HIQ-40 film in a calorimeter, one glass transition is observed at low temperature (approximately 42°C), and is ascribed to the glass/rubber transition of the isotropic polymer. A cold crystallization exotherm centered near 150°C is observed. This is associated with the development of low levels of crystalline order. A broad melting endotherm is centered at about 310°C; this endotherm marks the melting of crystallites and the transformation to a nematic fluid. A nematic to isotropic transition was not observed by calorimetry. After quenching from the nematic melt, a T g is observed in the range of 110-115°C and is associated with the glass/rubber transition of the nematically ordered polymer. Annealing optically isotropic films at temperatures above the isotropic glass transition results in the systematic development of axial order. In these annealed samples, T g increases rapidly until it is near the annealing temperature, then T g increases more slowly at longer annealing times. In as-cast films annealed at 120-135°C, the light intensity transmitted through a sample held between crossed polarizers in an optical microscope (a qualitative measure of birefringence and, in turn, axial order) initially increases rapidly and uniformly throughout the sample and, at longer annealing times, approaches asymptotic values that are higher at higher annealing temperatures. The increase in transmitted intensity is ascribed to the development of axial order. The uniform increase in transmitted intensity suggests that ordering occurs by a rather global process and not via a nucleation and growth mechanism.

Melting of polymer crystals observed by temperature modulated d.s.c. and its kinetic modelling

Irreversible melting of poly(ethylene terephthalate) crystals on heating has been examined by temperature modulated differential scanning calorimetry (t.m.d.s.c.). The apparent heat capacity of complex quantity obtained by t.m.d.s.c. showed a strong dependence on frequency and heating rate during the melting process. In order to explain this behavior, a kinetic modelling of melting has been presented. The modelling considers the melting of an assembly of fractions having a continuous distribution of non-equilibrium melting points. Three cases of the superheating dependence of melting rate coefficient have been examined: constant rate coefficient, linear dependence and exponential dependence. The modelling predicts frequency response functions similar to Debye's type with a characteristic time dependent on heating rate. The response function successfully explains the dependence on frequency and heating rate of the apparent heat capacity obtained experimentally. The characteristic time of melting of crystallites has been evaluated as a fitting parameter of the response function, and the superheating dependence of melting rate coefficient has been distinguished by the heating rate dependence of the characteristic time. Taking account of the relatively insensitive nature of crystallization to temperature modulation, it is further suggested that the ‘reversing’ heat flow is related to the pure endothermic heat flow of melting and the ‘non-reversing’ heat flow corresponds to the exothermic heat flow of re-crystallization and reorganization when extrapolated to ω → 0. The behavior of the apparent heat capacity will be an important characteristic feature of the melting kinetics, and hence the modelling will develop a new applicability of t.m.d.s.c. to the melting of polymer crystals.

Superheating effects during the melting of crystallites of syndiotactic polypropylene analysed by temperature-modulated differential scanning calorimetry

Temperature modulated differential scanning calorimetry (TMd.s.c.) was applied in a study of syndiotactic polypropylene. The crystallites melt from their lateral surfaces only and the kinetics shows up in the imaginary part c″ of the complex specific heat. Theoretical analysis predicts and experiments confirm that c″ increases linearly with the underlying mean heating rate and the modulation period. Furthermore, it can be shown that c″ is inversely related to the superheating effective during melting. Use of the relation yields for syndiotactic polypropylene values in agreement with direct measurements employing conventional d.s.c.

Consequences of antisense RNA inhibition of starch branching enzyme activity on properties of potato starch

Antisense constructs containing cDNAs for potato starch branching enzyme (SBE) were introduced into potato ( Solanum tuberosum L.). A population of transgenic plants were generated in which tuber SBE activity was reduced by between 5 and 98% of control values. No significant differences in amylose content or amylopectin branch length profiles of transgenic tuber starches were observed as a function of tuber SBE activity. Starches obtained from low SBE activity plants showed elevated phosphorous content. 31P n.m.r. analysis showed that this was due to proportionate increases in both 3- and 6-linked starch phosphates. A consistent alteration in starch gelatinisation properties was only observed when the level of SBE activity was reduced to below ~5% of that of control values. Starches from these low SBE activity plants showed increases of up to 5 °C in d.s.c. peak temperature and viscosity onset temperature. Studies on melting of crystallites obtained from linear (1 → 4)-α- d-glucan oligomers suggest that an average difference of double helix length of about one glucose residue might be sufficient to account for the observed differences in gelatinisation properties. We speculate that the modification of gelatinisation properties at low SBE activities is due to a subtle alteration in amylopectin branch patterns resulting in small changes in double helix lengths within granules.

Temperature Modulated DSC of Irreversible Melting of Nylon 6 Crystals

A new method is presented to analyze the irreversible melting kinetics of polymer crystals with a temperature modulated differential scanning calorimetry (TMDSC). The method is based on an expression of the apparent heat capacity, \(\Delta \tilde C{e}^{---{i\alpha }} = mc_p + i(1/{\omega }F'_{T} \), with the true heat capacity, mcp, and the response of the kinetics, \(F'_{\text{T}} \). The present paper experimentally examines the irreversible melting of nylon 6 crystals on heating. The real and imaginary parts of the apparent heat capacity showed a strong dependence on frequency and heating rate during the melting process. The dependence and the Cole-Cole plot could be fitted by the frequency response function of Debye's type with a characteristic time depending on heating rate. The characteristic time represents the time required for the melting of small crystallites which form the aggregates of polymer crystals. The heating rate dependence of the characteristic time differentiates the superheating dependence of the melting rate. Taking account of the relatively insensitive nature of crystallization to temperature modulation, it is argued that the ‘reversing’ heat flow extrapolated to ω → 0 is related to the endothermic heat flow of melting and the corresponding ‘non-reversing’ heat flow represents the exothermic heat flow of re-crystallization and re-organization. The extrapolated ‘reversing’ and ‘non-reversing’ heat flow indicates the melting and re-crystallization and/or re-organization of nylon 6 crystals at much lower temperature than the melting peak seen in the total heat flow.

Influence of the Molecular Ordering on the Wetting ofSiO2/Air Interfaces by Alkanes

Alkanes from 16 to 50 C atoms show three wetting topologies at ${\mathrm{SiO}}_{2}/\mathrm{air}$ interfaces. Below the bulk freezing temperature ${T}_{\mathrm{b}}$, small crystallites (frozen droplets which can be annealed into mesas of uniform height) are observed. A monolayer of homeotropically oriented molecules covers the air/substrate interface in between. At ${T}_{\mathrm{b}}$, the crystallites melt into droplets. The ordered monolayer remains up to ${T}_{\mathrm{S}}^{\mathrm{SV}}\ensuremath{\approx}{T}_{\mathrm{b}}+3\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, probably due to fluctuation-stabilized ``surface freezing.'' Above ${T}_{\mathrm{S}}^{\mathrm{SV}}$, a liquid alkane film wets the surface. The decoupling of ${T}_{\mathrm{b}}$ and ${T}_{\mathrm{S}}^{\mathrm{SV}}$, and the difference between the ordered and liquid surface energies, causes the droplet-to-film transition.

IR-spectroscopic study of the deformation of alkane molecules on a metal surface at high temperature

An increase of the valence angle in the CH2 groups of alkane molecules is detected when the crystallites in samples of ceresin grease melt on an aluminum substrate. This effect is explained by compression of the sample because the adhesion to the substrate is strengthened as the crystallites melt.

Recrystallization Kinetics and Glass Transition of Rice Starch Gel System

The effect of storage temperature on recrystallization and glass transition temperature (Tg) of nonwaxy and waxy rice starch gel systems containing 60% moisture content was investigated by differential scanning calorimetry to understand the relationship between them. The nucleation and propagation for the recrystallization process were determined by recrystallization degree obtained from crystallite melting enthalpy changes during storage. The recrystallization rate for both rice starch gels within 3 days of storage and its temperature dependence were analyzed by Avrami and Arrhenius equations. The maximum nucleation and propagation for recrystallization of both rice starch gel systems occurred at 4 °C and 30 °C, respectively. The Tg slightly increased with increasing recrystallization degree, and the highest Tg was observed in the maximum recrystallization temperature ranges. The Tg and recrystallization rate of nonwaxy rice starch gel were changed more than those of the waxy one, while the higher activation energy and Q10 value were shown in waxy rice starch gel. Keywords: Recrystallization; glass transition; rice starch gel; nucleation; propagation; Avrami equation; Arrhenius equation

A Four-State Scheme for Treating Polymer Crystallization and Melting Suggested by Calorimetric and Small Angle X-ray Scattering Experiments on Syndiotactic Polypropylene

As shown by time- and temperature dependent SAXS experiments, crystals of s-PP do not change their thickness during isothermal crystallization and a subsequent heating to melting. This allows an accurate determination of the relations between crystallization temperature, crystallite thickness, rate of crystallization, and melting points. There are five main results obtained in a comprehensive SAXS and DSC investigation: (i) Crystals have greatly varying stabilities, in spite of their uniform thickness; the first crystals melt close to the crystallization temperature. (ii) Melting points are affected by the distance to neighboring crystals. (iii) Crystals perfect during heating and annealing. (iv) Recrystallization after melting, as observed for low enough heating rates, starts with crystal growth rates that are at least 2 orders of magnitude higher than for a primary crystallization, and then slows down progressively, being accompanied by an increase in crystal thickness. (v) The dynamic signals observed in a MDSC run are indicative of a smooth change between crystallization and melting at the growth front. Data evaluation yields the average time required for a melting of individual crystallites. A four-state scheme allows a description of the behavior. It is based on the following assumptions: (i) Primary crystallization from the melt produces in a first step imperfect “native” crystals, which are subsequently stabilized by structural relaxation processes. These affect both the interior and the surface region. Crystallization proceeds under a small driving force, near to the equilibrium between melt and native crystals. (ii) The amount of structural relaxation is nonuniform thus producing crystals of different stability. (iii) With increasing crystallization temperature, i.e., decreasing growth rate, native crystals continuously approach the equilibrium state. (iv) The main part of fusion takes place near the equilibrium between the relaxed crystals and the disentangled melt.

Ordered structure in polyester elastomers containing quaterphenyl group

By use of DSC, X-ray diffraction and dynamic mechanical spectroscopies, phase separation and ordered structure were investigated for 4, 4′″-dihydroxy-p-quaterphenyl/ethylene glycol/adipic acid copolymers (xDHQ copolyesters, x: DHQ content in mol%, 0<x≦10). The xDHQ copolyesters are distinctly phase-separated into two phases, rigid DHQ and soft PEA phases. The PEA phase were partially crystalline, and showed glass transition near 240 K and melting of PEA crystallites near 280 K. In the DHQ phase, DHQ segments were ordered in layered structure at room temperature.

Origin of the bimodal ?melting? endotherm of ?-form crystallites in wool keratin

The melting endotherm of α-form crystallites in wool keratin appears often to be bimodal. The origin of the bimodal endotherm has been considered to correspond to the differential melting of the α-form crystallites in the domains of ortho- and para-cortical cells. An alternative interpretation is presented, in which the bimodal peak arises from the overlapping of the melting endotherm of α-form crystallites and the thermal degradation of other wool histological components. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 411–415, 1997

Gelation and retrogradation of concentrated starch systems: 1 Gelation

Small deformation properties of potato and wheat starch suspensions were studied during heating and cooling at rest by a small amplitude dynamic rheological test method. Starch concentrations used were 10 to 30% w/w. The temperature to which the suspensions were heated varied from 65 to 90°C. During heating the moduli of the starch sytems at first increase and subsequently decrease. On prolonged heating the moduli depend greatly on heating temperature: the higher the temperature, the lower the moduli. Properties of concentrated starch systems during heating are related to swelling of granules, melting of crystallites, separation of amy lose from amylopectin and disentanglement of non-covalent bonds. The differences between the rheological properties of potato and wheat starch suspensions are discussed, and compared with those of chemically cross-linked potato starches. Our hypothesis for the explanation of the observed phenomena is that physical entanglements or chemical cross-links, naturally or artificially present in starch granules, reduce the swelling capacity of the granules and increase the stiffness of the swollen granules; as a consequence, they affect the rheological properties of concentrated starch suspensions.

Melting of metastable crystallites in charge-stabilized colloidal suspensions.

Melting of metastable crystallites in charge-stabilized colloidal suspensions.

Phase and Thermal Behavior of Liquid-Crystalline Block Copolymers

The phase behavior of liquid—crystalline (LC) block copolymers containing mesomorphic blocks with main chain (MC) units and side-group (SG) substituents was investigated. The MC block consisted of an LC semiflexible polyester and the SG block was an LC polymethacrylate. Four different samples were prepared with the content of the SG blocks varying from 35 to 79 wt%. The thermal behavior of the copolymers was investigated using differential scanning calorimetry, and X-ray diffraction was used to characterize the phase structure. Both the MC and SG blocks formed smectic A and nematic mesophases. Thus, the occurrence of distinct and well-defined phase transitions clearly indicated that MC and SG blocks were phase-separated and showed their own phase behavior. Two different endothermic processes were identified in annealed samples that followed different kinetics. The low-temperature process is thought to involve the MC blocks in their own domains and was observed up to the smectic A to nematic transition temperature of these blocks. The hightemperature process involves the interface between MC and SG domains and is probably due to melting of crystallites grown during annealing. It is shown that the crystallization develops with kinetics affected by the degree of order of the LC state of the SG domains.

Influence of glycerol on the melting of potato starch

The gelatinization and melting of granular and recrystallized starch have been studied in the presence of low and high levels of glycerol or water by differential scanning calorimetry. The gelatinization onset temperature is increased in the presence of glycerol, whereas the excess gelatinization enthalpy is not affected. A broadening of the transition or change in shape is observed by lowering the amount of plasticizer. The increase of the melting onset temperature by the addition of glycerol is explained by an increase in the activation energy of the melting of the starch crystallites. The rise in activation energy is the result of the higher glass transition temperature of the amorphous phase and by the higher interaction forces of glycerol with the starch polymers. The changes in the shape of the endotherm at low plasticizer content is interpreted as an entropic effect. With less plasticizer the change in entropy during melting is low as a result of the lower mobility and viscosity of the starch polymers and the diluent molecules leading to less conformational freedom.