X-Ray Study of Isothermal Thickening of Lamellae in Bulk Polyethylene at the Crystallization Temperature

Hoffman, John D.; Weeks, James J.

doi:10.1063/1.1695935

Cited by 260 publications

(85 citation statements)

References 10 publications

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“…The results have been elaborated according to the Hoffman-Weeks approach 18,19) in order to calculate T 0 m or T 0 mb . In the PVME-rich blend the presence of a shoulder has not allowed a correct resolution of the peak and indicates that the scan speed was not sufficient to prevent reorganisation phenomena within the crystals; however, the melting temperatures of this blend were taken in the same way as for the other compositions, i. e., identifying the melting temperature of the system with the temperature which corresponds to the maximum of the endothermic peak.…”

Section: Resultsmentioning

confidence: 99%

Thermodynamics of polymer blends based on poly(ε-caprolactone)/poly(vinyl methyl ether)

Bisso

Casarino²,

Pedemonte

1999

Macromol. Chem. Phys.

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SUMMARY: The miscibility of the system poly(e-caprolactone)/poly(vinyl methyl ether), PCL/PVME, was investigated using differential scanning calorimetry (DSC), solution calorimetry, optical microscopy (OM), Fourier transform infrared spectroscopy (FTIR), dilatometry, and internal pressure. The system is semicrystalline and the PCL crystallinity is sensitive to the composition of the blend and to the thermal treatment performed. The presence of a single glass transition temperature for all compositions is considered as an index of miscibility in the amorphous phase, but the evolution of crystallization and melting peaks shows that phase separation phenomena occur upon heating the system just above the PCL melting point, according to a 'lower critical solution temperature' behaviour. The process of mixing is markedly endothermic and this means that the system is immiscible in the liquid phase, i. e. just above the PCL melting point. FTIR experiments demonstrate the absence of specific interactions between the two components. OM was used combined with DSC to investigate the occurrence of a melting point depression which can be used to evaluate the v 12 parameter. Finally, dilatometric and internal pressure techniques were used to obtain the required data for the evaluation of the interaction parameter, v 12 , following the Patterson approach. The resulting value of the v 12 parameter is always positive and higher than the critical value in the considered temperature range. This behaviour indicates, too, that the system is not miscible when it is heated above the PCL melting point.

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Section: Resultsmentioning

confidence: 99%

Thermodynamics of polymer blends based on poly(ε-caprolactone)/poly(vinyl methyl ether)

Bisso

Casarino²,

Pedemonte

1999

Macromol. Chem. Phys.

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“…This is /A fcos 0 > Ac~e -2~i sin 0 (14) where Af is the change in bulk free energy and A~ is the change in the basal-surface free energy on transformation, ~i is the interracial surface free energy (that is that of the h-o surface for PE) and 0 is half the wedge angle. If, in addition to Equation 14, the right-hand side is also zero or negative, that is,…”

Section: Wedge-shaped Crystalsmentioning

confidence: 99%

“…Rastogi and Ungar [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] have demonstrated the reality of a crystal-size-determined phase transition in the case of 1-4 poly-trans-butadiene (1 4 PTBD), following some preliminary pointers by Finter and Wegner [19]. Their experiments are of obvious significance for the present discussion, and they can be represented in terms of the presently adopted schematics as follows.…”

Section: Direct Evidence Of the Size Dependence Of The Phase Transitimentioning

confidence: 99%

An approach to the formation and growth of new phases with application to polymer crystallization: effect of finite size, metastability, and Ostwald's rule of stages

Keller

Hikosaka

Rastogi

et al. 1994

JOURNAL OF MATERIALS SCIENCE

235

145

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This article aims to link the mainstream subject of chain-folded polymer crystallization with the rather speciality field of extended-chain crystallization, the latter typified by the crystallization of polyethylene (PE) under pressure. Issues of wider generality are also raised for crystal growth, and beyond for phase transformations. The underlying new experimental material comprises the prominent role of metastable phases, specifically the mobile hexagonal phase in polyethylene which can arise in preference to the orthorhombic phase in the phase regime where the latter is the stable regime, and the recognition of "thickening growth" as a primary growth process, as opposed to the traditionally considered secondary process of thickening. The scheme relies on considerations of crystal size as a thermodynamic variable, namely on melting-point depression, which is, in general, different for different polymorphs. It is shown that under specifiable conditions phase stabilities can invert with size; that is a phase which is metastable for infinite size can become the stable phase when the crystal is sufficiently small. As applied to crystal growth, it follows that a crystal can appear and grow in a phase that is different from that in its state of ultimate stability, maintaining this in a metastable form when it may or may not transform into the ultimate stable state in the course of growth according to circumstances. For polymers this intermediate initial state is one with high-chain mobility capable of "thickening growth" which in turn ceases (or slows down) upon transformation, when and if such occurs, thus "locking in" a finite lamellar thickness. The complete situation can be represented by a P, 7", 1/1(I-crystal thickness) phase-stability diagram which, coupled with kinetic considerations, embodies all recognized modes of crystallization including chain-folded and extended-chain type ones. The task that remains is to assess which applies under given conditions of Pand T. A numerical assessment of the most widely explored case of crystallization of PE under atmospheric pressure indicates that there is a strong likelihood (critically dependent on the choice of input parameters) that crystallization may proceed via a metastable, mobile, hexagonal phase, which is transiently stable at the smallest size where the crystal first appears, with potentially profound consequences for the current picture of such crystallization. Crystallization of PE from solution, however, would, by such computations, proceed directly into the final stage of stability, upholding the validity of the existing treatments of chain-folded crystallization. The above treatment, in its wider applicability, provides a previously unsuspected thermodynamic foundation of Ostwald's rule of stages by stating that phase transformation will always start with the phase (polymorph) which is stable down to the smallest size, irrespective of whether this is stable or metastable when fully grown. In the case where the phase transformation is nucleation contro...

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“…In the analysis of the experimental data with eq 7, values of M =270.28 gmol-1 , d= 1.3872 gcm-3 ,1 3 !1Hm =7883 cal mol-1 , 1 7 and Tm O = 294°C are used. The equilibrium melting temperature for PBN was determined by using the method of Hoffman and Weeks, 18 shown in Figure 4 6. The samples crystallized isothermally below 215°C exhibit double melting peaks of PBN crystal for a mechanism based on melting, recrystallization, and subsequent remelting.…”

Section: G=xag0 Exp[ -U* ]Exp[~]mentioning

confidence: 99%

Crystallization Kinetics of Poly(butylene 2,6-naphthalate) and Its Copolyesters

Lee

Yoon

Kim

1997

Polym J

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ABSTRACT:The crystallization kinetics of poly(butylene 2,6-naphthalate) and its copolyesters has been investigated by differential scanning calorimetry at large undcrcoolings. Wide angle X-ray diffraction patterns show that the copolymers crystallize purely in crystalline phase of poly(butylene 2,6-naphthalate). In contrast to the typical crystallization behavior of copolymer, the crystallization isotherms follow the A vrami equation up to high degree of conversion, and the Avrami exponents are mostly close to 2, independent of copolymer composition and crystallization temperature. The analysis of kinetic data with a modified Lauritzen-Hoffman equation indicates that the product of the lateral surface free energy and the fold one ( (J(J ,) is 141 0 ± 100 erg 2 cm -4 and this value is not dependent on copolymer composition. The surface free energies and the work of chain folding are also discussed in comparison with those of poly(butylene terephthalate) and poly(ethylene terephthalate).KEY WORDS Poly(butylene 2,6-naphthalate) / Large Undercooling / Copolyesters / Crystallization Rate/ Surface Free Energies / Work of Chain Folding/ The temperature dependence of the crystallization rate for polymeric systems has been understood based on a classical nucleation-and-growth process. 1 · 2 In the region of melting temperature, the growth rate is rapid compared to the nucleation rate so that the crystallization rate is governed by nucleation process and hence depends strongly on the crystallization temperature. At lower crystallization temperature, i.e., large undercoolings, the growth process, related to the transport of crystallization units onto the crystal growth surface, becomes important. Over a wide range of crystallization temperature, the crystallization kinetic data of many polymers have been analyzed with the Turnbull-Fisher equation,3 formulated in terms of nucleation and growth rates. This equation has been modified by Lauritzen and Hoffman using a molecular model, and successfully applied to a large number of homopolymers. 4 -8 For copolymers where one kind of component units can crystallize and the other units remain in the noncrystalline phase, there are two characteristic crystallization behaviors in contrast to homopolymer crystallization. First, the isotherms change with the crystallization temperature and do not follow the A vrami equation. Second, a noncrystallizable component strongly reduces the crystallization rate. These phenomena have been understood based on a kinetic theory for random copolymers proposed by Gornick and Mandelkern. 9 According to this theory, the addition of a noncrystallizable component significantly attenuates the nucleation rate, and the nucleation rate decreases with the conversion of crystallization. Recently, Alamo and Mandelkern10 reported that several ethylene copolymers show the above two characteristics of copolymer crystallization at 25-60° undercoolings.In this study, the crystallization kinetics of poly-(butylene 2,6-naphthalate)(PBN) and three poly(butylene 2...

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X-Ray Study of Isothermal Thickening of Lamellae in Bulk Polyethylene at the Crystallization Temperature

Cited by 260 publications

References 10 publications

Thermodynamics of polymer blends based on poly(ε-caprolactone)/poly(vinyl methyl ether)

Thermodynamics of polymer blends based on poly(ε-caprolactone)/poly(vinyl methyl ether)

An approach to the formation and growth of new phases with application to polymer crystallization: effect of finite size, metastability, and Ostwald's rule of stages

Crystallization Kinetics of Poly(butylene 2,6-naphthalate) and Its Copolyesters

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