Melting Of Poly Research Articles

Raman spectroscopic measurements in self-pressurized aqueous solutions above 100�C: The melting of poly(G) and poly(G) � poly(C)

Raman spectroscopic measurements in self-pressurized aqueous solutions above 100�C: The melting of poly(G) and poly(G) � poly(C)

Long-ranged relaxations in poly(ethylene oxide) melts: Evidence for network behavior

Two slow, long-ranged relaxations in melts of poly(ethylene oxide) (PEO) were studied using the noninvasive dynamic light-scattering technique of photon correlation spectroscopy (PCS). Measurements were made for two PEO samples, one with a molecular weight distinctly above the entanglement value and a second sample with a molecular weight distinctly below the entanglement value. Based on several lines of evidence, it is concluded that the faster of these relaxations, which can be identified with the “cluster” or “Fischer” mode observed earlier in other homopolymer melts, results from the gel-like behavior of a physical network formed by the polymer in the melt, while larger-scale rearrangements in the polymer network structure are responsible for the slower relaxation.

Observations of crystallization and melting in poly(ethylene oxide)/poly(methyl methacrylate) blends by hot-stage atomic-force microscopy

The binary blend of poly(ethylene oxide)/atactic poly(methyl methacrylate) is examined using hot-stage atomic-force microscopy (AFM) in conjunction with differential scanning calorimetry and optical microscopy. It was found possible to follow in real time the melting process, which reveals itself to be nonuniform. This effect is ascribed to the presence of lamellae having different thicknesses. The crystallization process of poly(ethylene oxide) from the miscible melt is also followed in real time by AFM, affording detailed images of the impingement of adjacent spherulites and direct observation of lamellar growth and subsequent polymer solidification in the interlamellar space.

Structure and properties of tussah silk fibers graft-copolymerized with methacrylamide and 2-hydroxyethyl methacrylate

Tussah silk fibers were graft-copolymerized with methacrylamide (MAA) and 2-hydroxyethyl methacrylate (HEMA) in aqueous media, using a chemical redox system as an initiator. High weight gain values were obtained with both grafting agents (up to 175%). The extent of homopolymerization was negligible for the MAA grafting system over the entire range of monomer–silk ratios examined, while polymer deposition on the fiber surface occurred when the HEMA–silk ratio exceeded 0.5% (w/w). The moisture content of poly(MAA)-grafted silk fibers was enhanced by grafting. Breaking load, elongation at break, and energy decreased at low weight gain (0–20%) and then remained rather constant. The DSC curves of poly(MAA)-grafted silk showed a new endotherm at about 280°C, due to the melting of poly(MAA) chains. The loss modulus peak of poly(HEMA)-grafted silk fibers broadened and shifted to a lower temperature, showing a tendency to split into two peaks at high weight gain. On the other hand, grafting with poly(MAA) induced a noticeable upward shift of the loss peak. The TMA curves showed that grafting with poly(MAA) resulted in a higher extent of fiber contraction from room temperature to about 250°C. Moreover, the intensity of the final contraction step at about 350°C decreased with increasing weight gain and shifted to a lower temperature. The Raman spectra of grafted fibers were characterized by overlapping of the characteristic lines of both silk fibroin and polymer, the latter showing an intensity proportional to the amount of weight gain. Among the conformationally sensitive vibrational modes of tussah silk fibroin, the amide III range was significantly modified by grafting with both poly(MAA) and poly(HEMA). © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1393–1403, 1998

The (010)–(120) crystal growth face transformation in poly(ethylene oxide) spherulites

The crystallization and melting of poly(ethylene oxide) (PEO) have been examined in detail using polarized light microscopy (PLM), polarized infrared microspectroscopy (PIRM) and differential scanning calorimetry (DSC). Examination of the orientation of the crystalline stems within the spherulites by PIRM supports the hypothesis that the dominant crystal growth face of PEO changes from the (010) crystallographic plane at crystallization temperatures ( T c) > 51°C to the (120) plane at T c > 51°C. The cusp in the logarithmic growth rate—temperature plot appears to be the result of this phenomenon. Analysis of spherulitic growth rate data for the monodisperse sample ( M w = 1.8 × 10 5) fails to provide evidence in support of the regime II/III transition proposed in a previous literature study and indicates that in the range 45–56°C crystallization occurs solely within regime III. The larger value for the product of the lateral and fold surface free energies in the case of (010) growth appears to arise from the higher fold surface free energ' of this growth face. The difference in the fold surface free energies of the different growth faces, 41 erg/cm 2 for the (010) face as compared with 29 erg/cm 2 for the (120) face, can be accounted for by the corresponding differences in the minimum chain length required for chain folding in the case of adjacent re-entry.

The crystallization behavior of the metastable melts of poly(ethylene terephthalate) from the multiple melting process

The crystallization behavior of the metastable melts of poly(ethylene terephthalate) from the multiple melting process

Crystallization and melting of poly(oxyethylene) analyzed by temperature‐modulated calorimetry

Temperature-modulated calorimetry, TMC, is used to evaluate the temperature region of metastability between crystallization and melting. While crystals like indium can be made to melt practically reversibly during a TMC cycle of low amplitude so that sufficient crystal nuclei remain unmelted, linear macromolecules cannot, because of their need to undergo molecular nucleation. Modulation amplitudes varying from ±0.2 to ±3.0 K are used to assess the temperature gap between the slow crystallization region and the melting of metastable crystals of poly(oxyethylene) (PEO) of molar mass 1500 Da. This low molar mass PEO serves as a model compound with a metastable gap of melting/crystallization that can be bridged by TMC with a large modulation amplitude. © 1997 John Wiley & Sons, Inc.1 This article is a US Government work and, as such, is in the public domain in the United States of America. J Polym Sci B: Polym Phys 35: 1877–1886, 1997

Crystallization and melting of poly(oxyethylene) analyzed by temperature-modulated calorimetry

Temperature-modulated calorimetry, TMC, is used to evaluate the temperature region of metastability between crystallization and melting. While crystals like indium can be made to melt practically reversibly during a TMC cycle of low amplitude so that sufficient crystal nuclei remain unmelted, linear macromolecules cannot, because of their need to undergo molecular nucleation. Modulation amplitudes varying from ±0.2 to ±3.0 K are used to assess the temperature gap between the slow crystallization region and the melting of metastable crystals of poly(oxyethylene) (PEO) of molar mass 1500 Da. This low molar mass PEO serves as a model compound with a metastable gap of melting/crystallization that can be bridged by TMC with a large modulation amplitude. © 1997 John Wiley & Sons, Inc.1 This article is a US Government work and, as such, is in the public domain in the United States of America. J Polym Sci B: Polym Phys 35: 1877–1886, 1997

Imaging of Melting and Crystallization of Poly(ethylene oxide) in Real-Time by Hot-Stage Atomic Force Microscopy

Crystallization and melting in poly(ethylene oxide) are followed in real-time using hot-stage atomic force microscopy (AFM). Hedritic morphology is observed at various stages of development. Lamellar growth rates are estimated and found to agree with values obtained by conventional optical microscopy. The presence of a depletion zone is detected at the crystal/melt interface.

Melting of Poly(oxyethylene) Analyzed by Temperature-Modulated Calorimetry

Recently a small amount of locally reversible melting was observed in semicrystalline poly(ethylene terephthalate) during temperature-modulated differential scanning calorimetry (TMDSC). To further study the reversibility of melting, poly(oxyethylene) (POE) is analyzed. Low molar mass POE is known to be able to form extended-chain, equilibrium crystals, while at higher molar mass and less favorable crystallization conditions, nonequilibrium, folded-chain crystals grow. The TMDSC of POE reveals variable amounts of reversible melting depending on crystallization conditions and molar mass. The crystals closest to equilibrium show no reversible melting, proving the inherently irreversible nature of polymer melting. Crystals of high molar mass show a small amount of the prior discovered locally reversible melting. Poorly crystallized POE of low molar mass have, because of their lower zero-entropy-production melting temperature, a sufficiently smaller gap between crystallization and melting temperature to show ...

Time-Resolved SAXS Study of the Crystallization and Melting of Poly(ethylene oxide) Molecular Complexes

Time-Resolved SAXS Study of the Crystallization and Melting of Poly(ethylene oxide) Molecular Complexes

Reversible local melting in polymer crystals

AbstractThe melting of poly(ethylene terephthalate) is analyzed by quasi‐isothermal, temperature‐modulated differential scanning calorimetry. The measurement is done by sinusoidally changing the temperature in the melting range (± 1.0 K). In the melting range a small portion of the sample melts reversibly. This observation is taken as a direct observation of the reversibility of melting of specific macromolecules as long as they are melting only partially and need no molecular nucleation for recrystallization.

Crystallization from the melt of poly(lactide)s with different optical purities and their blends

AbstractPoly(lactide)s (PLA) with different optical purities (OP) were synthesized by copolymerization of D‐lactide and L‐lactide. Crystallization of D‐ and L‐lactide‐rich copolymers before and after 1 : 1 blending was studied from the melt over a wide range of annealing temperatures (Ta) using differential scanning calorimetry and polarizing microscopy. Melting temperature and crystallinity increased with an increase in OP for both the non‐blended and the 1 : 1 blended PLAs and they lost their crystallizability when their OP became smaller than 76%, suggesting that the critical isotactic sequence length of PLA for crystallization was approximately 15 isotactic lactate units. Normal spherulite formation was observed for both the non‐blended and the blended PLAs even when the OP of PLAs decreased to 76%, whereas the spherulitic size of the non‐blended PLAs was larger than that of the 1 : 1 blended PLAs when compared at the same OP and Ta. The highest Ta for crystallization increased with increasing OP and was higher for the 1 : 1 blended PLAs than for the non‐blended PLAs at a fixed OP, indicating that the crystallizable Ta range was extended to higher temperature by blending D‐ and L‐lactide‐rich PLAs. This is ascribed to stereocomplex (racemic crystallites) formation at high Ta from the 1 : 1 blended PLAs. Based on these results, the phase diagram for crystallization was depicted for the non‐blended and the 1 : 1 blended PLAs.

Morphological Changes during Secondary Crystallization and Subsequent Melting in Poly(ether ether ketone) as Studied by Real Time Small Angle X-ray Scattering

In this paper, we present results of morphological studies during long time melt crystallization and subsequent melting in poly(aryl ether ether ketone) (PEEK). Morphological changes were monitored via small angle X-ray scattering (SAXS). SAXS data were analyzed via a combination of the correlation and interface distribution functions. Our analysis indicates the following: (1) The semicrystalline morphology is best described by a three-phase, dual lamellar stack model. Stacks of a finite number of lamellae and interlamellar amorphous layers are separated from each other by interstack regions of amorphous material (liquid pockets). (2) Secondary crystallization occurs via the formation of secondary lamellar stacks within the liquid pockets. Secondary lamellae are thinner than primary lamellae (70 A vs 120 A), and the amorphous layer thicknesses are about 47 A in both stacks. (3) The low endotherm observed during a heating scan is associated with the melting of the secondary lamellae. (4) At room temperatu...

Miscibility, crystallization and melting of poly(3-hydroxybutyrate)/ poly(l-lactide) blends

The spherulitic structure, growth rates and melting behaviour of blends of bacterially produced poly(d(−)-3-hydroxybutyrate) (PHB) and poly(l-lactide) (PLLA) were investigated using polarized light microscopy. Results indicated that low-molecular-weight PLLA (Mn = 1759) was miscible in the melt over the whole composition range, whereas a blend of high-molecular-weight PLLA (Mn = 159 400) with PHB showed biphasic separation, in accordance with calculations based on Flory-Huggins theory. Two types of spherulite were formed on cooling, relating to the crystallization of PHB and PLLA respectively. In some blends, spherulites of opposite type interpenetrated when the growth fronts met. It is proposed that lamellae belonging to one type of spherulite continued to grow in the interlamellar regions of the other type of spherulite.

Time-resolved synchrotron small angle X-ray scattering studies of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate- co-3-hydroxyvalerate) polymers

Time-resolved synchrotron small-angle X-ray scattering measurements have been performed during the heating and melting of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate- co-3-hydroxyvalerate). A rapid, continuous rise in d-spacing was observed during melting, consistent with the disappearance of thinner, unstable crystals from stacks of lamellae possessing a broad distribution of thicknesses. The thickening of the remaining crystals has been characterized during the annealing of a copolymer close to its melting temperature. Differential scanning calorimetry measurements have indicated the presence of multiple melting endotherms in the case of the copolymer but not in the homopolymer. These multiple endotherms have been related to the effects of the hydroxyvalerate units in limiting the thickness of the lamellae at any given temperature.

Field-Cycling NMR Relaxation Spectroscopy of Poly(di-n-alkylsiloxanes) in Solid, Mesomorphic Liquid, and Isotropic Liquid Phases

The frequency dependence of the proton spin-lattice relaxation times T1 and T1., in the laboratory and rotating frames, respectively, is reported for solid and liquid phases of poly(diethylsiloxane) (PDES) and in melts of poly(dimethylsiloxane) (PDMS). The total frequency range is 5 X 102 -3 X 108 Hz and is mainly covered by field-cycling NMR relaxation spectroscopy. The relaxation behavior of PDES in the liquid but ordered mesophase is compared to that of isotropic melts of PDES and PDMS and also to that of nematic main-chain liquid-crystal polymers. The frequency dependences of PDES and PDMS liquids can be represented at low and high frequencies by power laws, section by section. The relaxation behavior in the isotropic melts is entirely equivalent to that previously reported for other polymer species. In the PDES mesophase, the exponents of the power laws are significantly larger and the crossover frequency between the two regimes is reduced. The dynamics in this phase are discussed with respect to the influence of chain modes and order director fluctuations. The main conclusion is, on the whole, that data of the liquid phases are determined by chain modes rather than by local segment fluctuations. The chain dynamics in the PDES mesophase resemble the chain modes in isotropic melts modified for a microstructure with reduced randomness, whereas the influence of order director fluctuations can neither be confirmed nor ruled out.

Solidification behavior of the theta system 2-propanol/poly(n-butyl methacrylate) I. Influences of thermoreversible gelation on stationary flow

Zero shear viscosities, η0, were determined by means of a magnetoviscometer for melts of poly(n-butyl methacrylate) (M = 8.7 to 450 kg/mol, T =53.5 to 200°C) and for concentrated solutions of the highest molecular weight sample in isopropanol (T = 34.8 to 131.5 °C). Master curves can be constructed in both cases if the reference temperature is set proportional to the gelation temperature of the particular fluid. Special intersegmental interactions (eventually leading to thermoreversible gelation) can above all be felt in η0 (T) and in M c , the critical molecular weight determined in plots of log η0 vs. log M. As the temperature is lowered, the behavior changes from WLF to Arrhenius, and M c declines considerably. The former observation is explained by analogy to the transition from fragile to hard glasses resulting from increased crosslinking. The latter effect can also be rationalized in terms of a physical network reducing the molecular weight that is required for the formation of entanglements.

Rheological behavior of poly(1,4‐butylene isophthalate) containing sodium sulfonate groups

AbstractThe steady‐state rheological behavior in the melt of poly(1,4‐butylene isophthalate)s containing various amounts of sodium sulfonate groups has been investigated using a capillary rheometer. It has been found that the presence of SO3Na groups strongly enhances the melt viscosity and this result cannot be explained as due to the small increase of the glass transition temperature, but has to be attributed to the relevant interactions between ionic groups. Measurements carried out at different temperatures in the range 190–220°C have shown that the “Newtonian” viscosity of sulfonated poly(1,4‐butylene isophthalate) is well correlated to temperature by an Arrhenius‐type equation. The activation energy for melt flow has been found to increase linearly with SO3Na group content in the polymer. Fur‐thermore, it has been found that for samples having a content of ionic groups of 5% or higher and a sufficiently high molecular weight the time–temperature superposition principle does not hold. © 1993 John Wiley & Sons, Inc.

Effect of Temperature History upon Stability and Melting of Poly(ethylene oxide)

The effect of temperature history upon the distribution of molecular weights and melting points has been investigated with high-molecular poly(ethylene oxides). The results obtained by the size exclusion chromatography (SEC) and differential scanning calorimetry (DSC) have shown that a long-term annealing of high-molecular poly(ethylene oxides) results in a distinct degradation of polymeric chains. The change in distribution of molecular weights and the presence of oligomers formed by degradation markedly affect the melting points of poly(ethylene oxide). The diffusion of oligomers from crystalline into amorphous regions is manifested by a time change of melting point and crystallinity degree of poly(ethylene oxide). The degradation is also observed during X-ray measurements.