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1.
Wenbing Hu Alexander Buzin Jar‐Shyong Lin Bernhard Wunderlich 《Journal of Polymer Science.Polymer Physics》2003,41(4):403-417
The annealing at 373 K of ultrastrong, gel‐spun polyethylene (PE) has been studied. At this temperature, the fibers show no significant shrinkage. Still, a significant decrease in the mechanical properties is observed. The fibers have been analyzed with differential scanning calorimetry (DSC), temperature‐modulated differential scanning calorimetry (TMDSC), atomic force microscopy (AFM), and small‐angle X‐ray scattering (SAXS). During the annealing, the glass transition of the intermediate phase is exceeded, as shown by DSC. When split for structure analysis by AFM, the annealed fibers undergo plastic deformation around the base fibrils instead of brittle fracture. The quasi‐isothermal TMDSC experiments are compared to the minor structural changes seen with SAXS and AFM. The loss of performance of the PE fibers at 373 K is suggested to be caused by the oriented intermediate phase, and not by major changes in the structure or morphology. The overall metastable, semicrystalline structure is shown by TMDSC to posses local regions that can melt reversibly. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 403–417, 2003 相似文献
2.
In this work, the melting behaviors of nonisothermally and isothermally melt‐crystallized poly(L ‐lactic acid) (PLLA) from the melt were investigated with differential scanning calorimetry (DSC) and temperature‐modulated differential scanning calorimetry (TMDSC). The isothermal melt crystallizations of PLLA at a temperature in the range of 100–110 °C for 120 min or at 110 °C for a time in the range of 10–180 min appeared to exhibit double melting peaks in the DSC heating curves of 10 °C/min. TMDSC analysis revealed that the melting–recrystallization mechanism dominated the formation of the double melting peaks in PLLA samples following melt crystallizations at 110 °C for a shorter time (≤30 min) or at a lower temperature (100, 103, or 105 °C) for 120 min, whereas the double lamellar thickness model dominated the formation of the double melting peaks in those PLLA samples crystallized at a higher temperature (108 or 110 °C) for 120 min or at 110 °C for a longer time (≥45 min). © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 466–474, 2007 相似文献
3.
Bernhard Wunderlich 《Thermochimica Acta》2003,403(1):1-13
Linear, flexible macromolecules are long recognized as phase structures limited to micrometer and nanometer dimensions with covalent bonds crossing the interfaces. This special, usually non-equilibrium structure leads to unique properties and a multitude of changes for different thermal and mechanical histories. Analyses that enable the study of these properties are temperature-modulated calorimetry and related techniques which allow the separation of equilibrium and non-equilibrium responses. Research on these topics is reviewed and combined to a model for the nanophases. The new approach to the complex nanophase systems yields a better understanding of the relationship between structure and thermodynamic properties. Special emphasis is placed on the size and surface effects on the glass and melting transitions, the development of rigid-amorphous phases, and the reversible melting within the globally metastable structure. 相似文献
4.
5.
Annealing experiments have been carried out just below the melting temperature of both polyethylene (LLDPE) and polypropylene
(PP) and their blends. The total melting enthalpy measured after the annealing cycle was greater by 10-15% with respect to
the value having been measured before it. During the annealing period the heat capacity decreases to a lower value within
the first 2-3 min. Heat capacities of PP (either in pure form or in the blends) measured during the heating cycle following
the annealing cycle have the same value as during the cooling section. The heat capacities of the LLDPE in the heating cycle
following the annealing were those of the preceding heating cycle. The total heat flows in the cooling section following the
annealing cycle were greater than those in another cooling cycle at the same temperatures indicating that the crystallisation
takes place during the cooling rather than during the annealing periods.
The presence of LLDPE decreases the crystallisation temperature of PP. The presence of SEBS in the blend results in a greater
crystallisation temperature than that of pure PP. The crystallisation temperature of LLDPE increases with increasing levels
of PP.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
A three-phase model, comprising crystalline, mobile amorphous, and rigid amorphous fractions (χ
c, χ
MA, χ
RA, respectively) has been applied in the study of semicrystalline Nylon-6. The samples studied were Nylon-6 alpha phase prepared
by subsequent annealing of a parent sample slowly cooled from the melt. The treated samples were annealed at 110°C, then briefly
heated to 136°C, then re-annealed at 110°C. Temperature-modulated differential scanning calorimetry (TMDSC) measurements allow
the devitrification of the rigid amorphous fraction to be examined.
We observe a lower endotherm, termed the ‘annealing’ peak in the non-reversing heat flow after annealing at 110°C. By brief
heating above this lower endotherm and immediately quenching in LN2-cooled glass beads, the glass transition temperature and χ
RA decrease substantially, χ
MA increases, and the annealing peak disappears. The annealing peak corresponds to the point at which partial de-vitrification
of the rigid amorphous fraction (RAF) occurs. Re-annealing at 110°C causes the glass transition and χ
RA to increase, and χ
MA to decrease. None of these treatments affected the measured degree of crystallinity, but it cannot be excluded that crystal
reorganization or recrystallization may also occur at the annealing peak, contributing to the de-vitrification of the rigid
amorphous fraction.
Using a combined approach of thermal analysis with wide and small angle X-ray scattering, we analyze the location of the rigid
amorphous and mobile amorphous fractions within the context of the Heterogeneous and Homogeneous Stack Models. Results show
the homogeneous stack model is the correct one for Nylon-6. The cooperativity length (ξA) increases with a decrease of rigid amorphous fraction, or, increase of the mobile amorphous fraction. Devitrification of
some of the RAF leads to the broadening of the glass transition region and shift of T
g. 相似文献
7.
Different grades of linear low density polyethylenes (LLDPEs) have been quenched cooled step-wise and crystallised isothermally
at (a series of increasing) temperatures in a DSC (thermal fractionated samples). These samples have been investigated by
temperature modulated DSC (MDSC). The heat flow curves of the thermal fractionated materials were compared with those obtained
from samples crystallised at a relatively slow cooling rate of 2 K min-1(standard samples).
The melting enthalpy obtained from the total heat flow of the thermal fractionated samples was 0-10 J g-1higher than those of standard samples. The melting enthalpy obtained from the reversing heat flows was 13-31 J g-1lower in the thermal fractionated samples than in the standard samples. The ratio of the reversing melting enthalpy to the
total melting enthalpy increased with decreasing density of the PE. The melting temperature of the endotherms formed by the
step-wise cooling was 9 K higher than the crystallisation temperature.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
R. Androsch B. Wunderlich T. Lüpke A. Wutzler 《Journal of Polymer Science.Polymer Physics》2002,40(12):1223-1235
The effect of uniaxial deformation and subsequent relaxation at ambient temperature on irreversible and reversible crystallization of homogeneous poly(ethylene‐co‐1‐octene) with 38 mol % 1‐octene melt‐crystallized at 10 K min was explored by calorimetry, X‐ray scattering, and Fourier transform infrared spectroscopy. At 298 K, the enthalpy‐based crystallinity of annealed specimens increased irreversibly by stress‐induced crystallization from initially 15% to a maximum of, at least, 19% when a permanent set of more than 200% was attained. The crystallinity increased by formation of crystals of pseudohexagonal structure at the expense of the amorphous polymer, and as a result of destruction of orthorhombic crystals. The stress‐induced increase of crystallinity was accompanied by an increase in the apparent specific heat capacity from 2.44 to about 2.59 J g?1 K?1, which corresponds to an increase of the total reversibility of crystallization from, at least, 0.10 to 0.17% K?1. The specific reversibility calculated for 100% crystallinity increased from 0.67 to 0.89% K?1 and points to a changed local equilibrium at the interface between the crystal and amorphous phases. The deformation resulted in typical changes of the phase structure and crystal morphology that involve orientation and destruction of crystals as well as the formation of fibrils. The effect of the decrease of the entropy of the strained melt on the reversibility of crystallization and melting is discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1223–1235, 2002 相似文献
9.
A study of the effects of changing sample mass and purge gas on the phase lag and calculation of the out of phase (kinetic)
component for polystyrene in MTDSC was undertaken. The results confirm those from an earlier study that suggested the kinetic
signal is unchanged by altering these parameters thus it is probably a correct quantitative measure of this signal. The effects
of long annealing times were also studied and it was shown that all the signals of MTDSC are affected in contrast to a previous
study showing that for moderate annealing the reversing and kinetic signals are substantially invariant. Nevertheless the
non-reversing signal remains useful for characterising relative enthalpy losses.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
Madhubhashini Maddumaarachchi Frank D. Blum 《Journal of Polymer Science.Polymer Physics》2014,52(10):727-736
Adsorbed poly(ethylene‐stat‐vinyl acetate) (PEVAc) on fumed silica was studied using temperature‐modulated differential scanning calorimetry (TMDSC) and FT‐IR spectroscopy. The properties of the copolymers were compared with poly(vinyl acetate) (PVAc) and low density polyethylene (LDPE) as references. TMDSC analysis of the copolymer‐silica samples in the glass transition region was complicated for the copolymers because of the ethylene crystallinity. Nevertheless, examination of the glass transition region for small adsorbed amounts of these copolymers indicated the presence of tightly‐ and loosely‐bound polymer segments, similar to other polymers which have an attraction to silica. Compared with bulk polymers with the same composition, the tightly‐bound polymers showed an increased glass transition temperature (Tg) and a loosely‐bound fraction with a lower Tg than bulk. FT‐IR spectra of the surface copolymers indicated that the fraction of bound carbonyls (p) increased as the fraction of vinyl acetate in the copolymers decreased, consistent with the notion that the carbonyls from vinyl acetate preferentially find their way to the silica surface. Spectra from samples with different adsorbed amounts of polymer were used to obtain the amount of bound polymer (Mb) and the ratio of molar absorption coefficients of bound carbonyls to free carbonyls (X). The copolymers had very large p values (up to 0.8) at small adsorbed amounts and dependent on the composition of the polymer. However, an analysis of the bound fractions, based on only the vinyl acetate groups, superimposed the data, suggesting that the ethylene units simply dilute the vinyl acetate groups in the surface polymer. The sample with the smallest fraction of vinyl acetate did not show this behavior and may be considered to be “carbonyl poor.” © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 727–736 相似文献