Melting Of Crystallites Research Articles

STARCH GELATINIZATION PHENOMENA STUDIED BY DIFFERENTIAL SCANNING CALORIMETRY

ABSTRACTDifferential Scanning Calorimetry (DSC) was used to study gelatinization phenomena of corn, potato, acid‐modified corn and smooth pea, and various legume starches. Two endothermic transitions were observed for starches heated in the presence of limited amounts of water (starch/water = 46–48% w/w). The effect of starch concentration on the shape of these two endotherms was studied for three legume starches. Considering the starch granule as a spherulite, the experimental data were treated thermodynamically by applying equations describing phase transitions of semi‐crystalline polymers. Experimental evidence suggested that a solvation‐assisted melting of starch crystallites took place during gelatinization and that the process was facilitated by the presence of the amorphous parts of the granule. The enthalpy change, ΔH, associated with the gelatinization varied among these starches within a range of 2.6–4.4 Cal/g. A comparison between DSC transition temperatures and characteristic gelatinization temperatures obtained by other methods was also made.

Small‐angle x‐ray scattering studies of melting

AbstractThe course of melting of melt‐crystallized polyethylene fractions and of a poly(ethylene oxide)‐polystyrene‐poly(ethylene oxide) triblock copolymer has been followed by small‐angle x‐ray scattering (SAXS). Changes in the intensity and shape of the SAXS curves indicated that both surface melting and melting over the full crystallite thickness (full‐strand melting) take place. Full strand melting is the final, irreversible process. Comparison with an analytical model indicates that in the earlier stages of the irreversible, full‐strand process the crystallites melt out randomly throughout the bulk. Later stages may occur by the simultaneous melting of a larger stack of crystallites.

Thermal behavior of co[poly(ethylene terephthalate)‐p‐oxybenzoate

AbstractIn a previous investigation of a co[poly(ethylene terephthalate)‐p‐oxybenzoate] containing 30 mole % oxybenzoate units (referred to as T2/30), Krigbaum and Salaris identified the endotherm at tIII = 244°C as a nematic → isotropic transition. Subsequent investigation revealed that the tIII endotherm disappeared if the polymer is heated to the isotropic melt or dissolved and reprecipitated. The loss of the tIII transition might be due to molecular weight reduction, sequence randomization, or erroneous identification of the transition. Viscosity and high‐field NMR data eliminate the first two explanations. Annealing studies at temperatures higher than those of the earlier work demonstrate that the tIII transition, in fact, represents the melting of crystallites formed during a high‐temperature annealing operation. Moreover, we cannot determine the thermodynamic melting temperature of the copolymer from solid‐state annealing studies. Hence, the principal objective of the study of Krigbaum and Salaris, to compare the enthalpies and entropies of the nematic → isotropic and crystal → isotropic transitions, must be fulfilled in future work.

Phase‐separation phenomena in solutions of poly(2,6‐dimethyl‐1,4‐phenylene oxide). III. Pulse‐induced critical scattering of solutions in toluene

AbstractFor the polymer‐solvent system poly(phenylene oxide) in toluene the mechanism and kinetics of crystallization have been studied with the Pulse Induced Critical Scattering technique. It was found that after a delay‐time τ the growth mechanism was diffusion controlled. The delay‐time is thought to be connected with the nucleation of the crystallites and it disappeared in the “seeded” crystallizations studied. After incomplete melting of crystallites the first stages of growth resemble a condensation reaction.

Luminescence in warming and cooling from γ-irradiated polyethylene

Anomalous luminescence during warming and cooling has been found in previously oxidized polyethylene following γ irradiation in air at room temperature. During warming, one of the thermoluminescence peaks appears at about 384 °K, close to the crystallite melting temperature. Further luminescence during a subsequent cooling appears, rising abruptly at about 370 °K near the recrystallization temperature. Such a luminescence is observable by repeating the warming and cooling cycle, although the luminescence intensity decreases with successive cycles. It appears that the presence of crystallites is essential for an appearance of the luminescence, and an existence of some long-lived trapped electrons above the melting temperature is suggested.

The melting of small particles. I. Lead

The melting points of individual lead crystallites have been investigated in detail in the electron microscope, by using a technique which allows accurate measurement of the specimen temperature. The experiments have shown that the melting points of the lead crystallites were not determined solely by their size, as the simple melting point theories predict, but also depended on a second variable effect. These results are consistent with a liquid skin model of the melting of crystallites containing imperfections.

Investigation of the aging process of a polyvinyl chloride gel by the measurement of its dynamic moduli

During the aging process of a 10%-solution of polyvinyl chloride in di-(2-ethylhexyl)phthalate at temperatures between −20 and 110°C its storage and loss moduli were measured in a frequency range from 1.6×10−4 to 40 cycles/sec. The specially defined rate of aging plotted against the aging temperature closely resembles the crystallization curve of a normally crystallizable polymer. Although the internal structure of the system changes, when the temperature is changed, the time-temperature reduction could still be applied provided some precautions were taken.Bueche's theory for networks, combined withFlory's post-gelation treatment, has been used to calculate some gel parameters: the molecular weight between cross-links, the sol fraction, the monomeric friction coefficient etc. It is shown that the aging process strongly depends on the thermal history. The cross-links formed by crystallites, differ very much in functionality. Continued aging, after previous aging at lower temperature, causes, after a first melting of unstable crystallites, a recrystallization process. During this process entanglements are formed. This results in a pseudo-equilibrium rubber plateau at higher frequencies, followed by a dispersion region and a lower equilibrium rubber plateau at lower frequencies.

Viscosity and shear modulus of PVC‐plastisols during gelation and fusion

AbstractThe Contraves balance rheometer has been used in a study of the gelation and fusion process of three DOP‐based PVC‐plastisols and the results compared with those obtained with the Brabender plastograph. The tensile properties of samples fused at different temperatures have also been determined. The rheometer results relate to the temperature dependence of the viscosity, shear modulus, and loss angle of the plastisols used. The viscosity‐ and modulus‐temperature curves are to some extent reminiscent of the torque‐temperature curves obtained with the plastograph; the fusion temperatures are slightly different. The loss‐angle‐temperature curves measured with the rheometer exhibit sharp maxima in the vicinity of the Tg‐point of the base polymer. The shear modulus decreases with time when the temperature is kept constant (above the fusion point). This effect is possibly associated with crystallite melting. The temperature necessary to reach the maximum strength plateau of films fused at various temperatures was found to agree (within 10°C) with the maximum in the modulus‐temperature curves (minimum in loss‐angle‐temperature curves) when the rheometer shear rate was sufficiently low. On the whole, the rheometer allows for better temperature control. Also, the fact that the results are given in terms of viscosity and shear modulus may have some advantage in interpreting gelation and fusion data.

Melting of crystallites in polymers with low degree of crystallinity under shear flow conditions I: Poly(vinyl chloride)

AbstractThe aggregates found in dilute solutions of nearly atactic PVC have been shown to consist of 10 to 15 single molecules held together by a crystalline nucleus. Based on the postulate that the last crystallites in a PVC melt have the same structure as found in dilute solution, a relation between the melting temperature in shear (Tdyn), the static melting temperature (Tm) and the shear stress (τ) is found: where Q is a constant. The model predictions are in accordance with data from a Brabender rheometer and a Rheometrics mechanical spectrometer. Q is found to be a constant for samples with weight average molecular weights in the interval between 70,000 and 160,000 corresponding to polymerization temperatures between 40 and 70°C.

Thermal analyses of polymers: XIV. Multiple melting phenomena in poly(ethylene terephthalate)

The observation of multiple melting phenomena in poly(ethylene terephtha- late) (PET) by DSC or DTA has been confirmed to arise from recrystallization by the changes in birefringence occurring in the thermal regime of the polymer. Isothermal birefringence changes illustrate the melting of crystallites around 230–248 °C, with the formation of more perfect and higher melting crystallites at temperatures of annealing above 248°C. The formation of diethylene glycol residues in the PET during solid state polymerization may be a cause of the higher melting endotherm occurring at lower temperatures when annealed under vacuum.

Morphology of segmented polyester thermoplastic elastomers

AbstractThe morphology of a new family of copolyester thermoplastic elastomers is described and the physical properties of these polymers are interpreted in terms of their unusual two‐phase domain structure. The elastomers were synthesized by the equilibrium melt transesterification of dimethyl terephthalate with 1, 4‐butanediol and poly (tetramethylene ether) glycol, resulting in a molecule in which crystallizable tetramethylene terephthalate (4GT) segments and amorphous poly (tetramethylene ether) glycol terephthalate segments are randomly distributed along the backbone. The 4GT sequences were found to crystallize as lamellar domains measuring approximately 100 Å in width and up to several thousand angströms in length. These crystalline regions are not discrete as are the polystyrene domains in the typical polystyrene‐polybutadiene‐polystyrene (SBS) triblock polymers, but rather are continuous and highly interconnected. Like the glassy regions in the SBS polymers, however, they serve as intermolecular tiepoints for the amorphous portions of the molecules and lead to the formation of a three‐dimensional network structure. At temperatures below the crystallite melting point, the material exhibits many properties characteristic of conventionally cured elastomers, but at elevated temperatures the melting of these thermally labile noncovalent junctures allows the material to flow and be processed as a thermoplastic. The stress‐strain behavior of these polymers is discussed in terms of their morphology, and thermal analysis, electron microscopy, and x‐ray diffraction results are presented.

Multiple melting transitions in fractions of partially isotactic poly(propylene oxide)

AbstractThree transitions are detected dilatometrically when partially isotactic poly(propylene oxide) melts. One transition, the temperature of which is independent of the crystallization temperature over a wide range below 60°C, is ascribed to the melting of lamellar crystallites which are limited in thickness by the average isotactic sequence length alone. The other two transitions, the temperatures of which vary with the crystallization temperature, are ascribed to the melting of lamellar crystallites with thickness determined predominantly by three‐ and two‐dimensional primary nucleation acts. The theory of Flory is adapted and applied quantitatively to the melting points of three crystalline fractions of poly(propylene oxide), obtained from a polymer produced via the zinc diethyl and water catalyst system. This method leads to a thermodynamic melting point of isotactic poly(propylene oxide) near 82°C.

Structure and physical properties of polyurethanes

AbstractThe relations between structure and physical properties of polyurethane elastomers, particularly elastic properties, were studied. Polyurethanes with various structures were prepared by a prepolymer technique. Among the conceivable structural factors the substituted urea group, which was introduced by the use of a diamine‐type curing agent, had a remarkable reinforcing effect. This was attributed to the strong hydrogen‐bonding effect between urea groups. In this connection, softening occurred during repeated deformation of diamine‐cured polyurethane elastomers. It was proved that this is due not to the melting of crystallite but to the slip of hydrogen bonding between urea groups. On the other hand, urea groups did not have much influence on equilibrium rubber elasticity. The contribution of the force caused by the internal energy to the total force was independent of curing agents, being determined by the principal segments of network chains.

Thermographic Study of Elastomers III. Peculiarities of Crystallization of Natural Rubber

Abstract Thermographic (DTA) study reveals two independent melting regions of natural rubber crystallites and a gap between them which is determined by a sharp difference in the rates of the crystallization process. Crystallites formed at a low temperature (− 25° C) and at room temperatures are distinguished by the degree of development, but pertain to the same crystallographic form. High temperature crystals play the role of seed in low temperature crystallization. During the melting of low temperature crystallites of natural rubber, they recrystallize with a rise in the melting point (without reaching, however, room temperatures). The appearance of this phenomenon on the thermograms is determined by the relation between rates of crystallization and of heating in the course of the experiment. The rate of crystallization of natural rubber and the relative amount of crystallites formed at − 25° C were studied by DTA; the effect of rubber plasticization on these magnitudes was noted.

Growth Rate of Natural Rubber Crystallites

Abstract The melting range and growth rate curves for natural rubber crystallites have been studied by the differential thermal analysis technique. The results obtained compare favorably with the growth rate curves obtained by dilatometric techniques. The use of differential thermal analysis revealed details of the crystallite melting which were not apparent when the dilatometer was used; however, no quantitative data on heat effects or the degree of crystallinity were calculated.

The crystallization and melting of copolymers I—The effect of the crystallization temperature upon the apparent melting temperature of polymethylene copolymers

The apparent melting temperatures for a series of polymethylene copolymers were determined as a function of crystallization temperature and level of crystallinity. The copolymers contained from 1·2 to 6·4 mole per cent of either methyl or n -propyl side groups arranged in random sequence distribution. The data could be analysed to yield extrapolated equilibrium melting temperatures by assuming the formation and melting of finite size crystallites. The size of the crystallites is taken to be dependent solely on the crystallization temperature. In accord with the previous observations of Richardson, Flory and Jackson, based on directly observed melting temperatures, it is concluded that at equilibrium the crystalline phase remains pure for n -propyl side group copolymers while solid-solution occurs in the methyl copolymers.

EFFECTS OF Co<sup>60</sup> GAMMA-RAY ON THE VISCO-ELASTIC DISPERSION OF SOME TEXTILE MATERIALS. (1)

BS>The results obtained were (1) saran showed degradation with increasing radiation doses, its dynamic modulus decreased with melting of fine crystallites, and the disper sion peak of the loss-tangent shifted to a lower temperature; (2) nylon 6 showed an increase in the dynamic modulus up to 5 x 10/ sup 7/ rep at 25 deg C and a decrease at 10/sup 8/ rep when compared with that of an unirradiated sample. Hence, the crosslinking effect of nylon 6 was dominant up to a dosage of 10/sup 7/ rep. The dynamic modulus of crystalline polyethylene terephthalate increased with irradiation dosages up to 10/sup 8/ rep and with practically no change in the loss tangent. It was concluded that the annealing effect induced by irradiation was not related to crosslinking or degradation. (OID)

Polymer–diluent interactions. I. A new micromethod for determining polyvinyl chloride‐diluent interactions

AbstractThe changes that are observed under magnification when a single polyvinyl chloride particle is immersed in a drop of diluent and slowly heated have been explained on the basis of a three‐dimensional network system in which the polymer crystallites act as junction points for the network. The existence of a temperature at which a sharp gel‐to‐sol transformation is observed has been attributed to the melting of the polymer crystallites. The temperature at which this transformation occurs was found to be rate‐independent, reproducible, and characteristic for each polyvinyl chloride–diluent system. At this apparent melting temperature, the volume fraction of the diluent in the swollen particle was shown to approach unity. These findings made it possible, for the first time, to apply to polyvinyl chloride–diluent systems Flory's treatment of the effect of diluents on the crystal melting temperature of semicrystalline polymers. The apparent melting temperatures of polyvinyl chloride with 27 diluents were determined by the above method. The interaction parameter χ for each of the diluents used had been previously reported by Doty and Zable. A linear relationship was obtained when the reciprocal melting temperature was plotted against (1–χ)/V1 for all of the above polyvinyl chloride–diluent systems (V1 is the molar volume of the diluent at the melting temperature). From this plot the values for the extrapolated melting temperature of pure polyvinyl chloride and for the average heat of fusion were calculated and found to be 176°C. and 656 cal./mole, respectively. Based on these findings a micromethod has been developed which can be used to determine the parameter χ of a diluent with polyvinyl chloride from a single determination of this apparent melting temperature. This method also yields the value for the energy of interaction parameter B.

Transitions in ethylene polymers

AbstractA study of polyethylenes and ethylene–vinyl acetate copolymers by dynamic mechanical mesurements gives new information on the transitions in polyethylene polymers. The melting points agree with the predictions of Flory's theory. The alpha transition is shown to be due to the melting of crystallites containing ethylene sequences of lengths near the average sequence lengths; this transition is related to the lengths of crystallites. The beta transition is not necessarily due to branch points, as a number of noncrystallizable groups produce beta transitions at the same temperature. The beta transition degenerates into the gamma transition if the crystallinity is destroyed.

Scattering of Light in Crystalline Polymers. I. Formation and Melting of the Crystalline Phase in Neoprene

Abstract The process of formation and melting of neoprene crystallites was investigated by the polarized light scattering method. Conclusions about the nonreversibility of the crystallization and melting processes were drawn on the basis of angular distribution measurements of depolarized and polarized components of light scattered from polymer films. The appearance of a relatively small number of large crystallites is characteristic for the initiation of crystallization; further development of crystallization is accompanied by a decrease in the size of the crystallites. All crystallites disappear simultaneously during the melting process. The size of the crystallites determined by microphotography corresponds approximately to the particle size determined by the formula for isotropic, spherical particles. A method for the determination of the melting range of crystalline polymers is proposed.