Liquid-Crystalline Polymers

Keller, A.; Ungar, Goran; Percéc, Virgil

doi:10.1021/bk-1990-0435.ch023

Cited by 40 publications

(21 citation statements)

References 9 publications

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“…As in [10][11][12], we start with a schematic freeenergy, G, versus temperature plot for a system capable of displaying the C-, M-and L-phases. (Figs 1, 2 and 3).…”

Section: Free-energy Considerationsmentioning

confidence: 99%

“…Following the argument in [10][11][12] Such an "uncovering" of the M-phase can be most conveniently visualized in physical terms by taking two extreme cases, namely, where the principal change is that in GL, or alternatively that in Gc, with the smaller changes in GM disregarded in both cases. As seen from Figs 2 and 3, the M-phase can be uncovered if either GL (Fig.…”

Section: Free-energy Considerationsmentioning

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%

“…This arises from the fact that, for a given T, it is intrinsic that (AT)m~t. < (AT)~t~ble (12) In view of the fact that, on decreasing T, the ratio…”

Section: On Rates Involving Metastable Phasesmentioning

confidence: 99%

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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

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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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“…As in [10][11][12], we start with a schematic freeenergy, G, versus temperature plot for a system capable of displaying the C-, M-and L-phases. (Figs 1, 2 and 3).…”

Section: Free-energy Considerationsmentioning

confidence: 99%

Section: Free-energy Considerationsmentioning

confidence: 99%

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

confidence: 99%

“…This arises from the fact that, for a given T, it is intrinsic that (AT)m~t. < (AT)~t~ble (12) In view of the fact that, on decreasing T, the ratio…”

Section: On Rates Involving Metastable Phasesmentioning

confidence: 99%

See 2 more Smart Citations

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

Self Cite

235

145

View full text Add to dashboard Cite

show abstract

“…65 , 66 Virtual mesophases exist only below the melting and crystallization temperature and therefore can not be observed. Monotropic mesophases can be observed only during cooling due to the fact that the crystallization process is kinetically controlled and therefore is supercooled, while the liquid crystalline phase is thermodynamically controlled and is not supercooled.…”

Section: Introduction To Low Molar Mass Liquid Crystals and Definitionsmentioning

confidence: 99%

Molecular Engineering of Liquid Crystalline Polymers

Percéc

Tomazos

1989

Comprehensive Polymer Science and Supplements

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Transformation of a kinetically prohibited mesophase of a linear polymer into an enantiotropic mesophase via cyclization

Percéc

Kawasumi

1992

Advanced Materials

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MATERIALS(0.29g, estimated 0.96mmol of Br based on the molecular formula [C,,H,,Br].), copper(]) iodide (0.04 mmol, 4.0 mol.-%), and dichlorobis-(triphenylphosphine)palladium(n) or tetrakis(triphenylphosphine)palladium(o) (0.05 mmol, 4.8 mol.-%) in THF (3 ml) was added at room temperature a terminal acetylene (2.9 mmol, 3.5 eq) and diisopropykdmine (2.9 mmol, 3.5 eq). The black reaction solution was stirred at room temperature for 1 h then warmed to 60 "C for 10 h and 100 "C for 5 h. The reaction mixture was allowed to cool to room temperature and poured into water (20 ml). The organic layer was separated and the aqueous portion extracted with dichloromethane (3 x ) . The combined organic layers were washed with aqueous ammonium chloride (3 x). Filtration and removal of the solvent in vacuo gave a brown oil which was usually stirred with ether and the ether-insoluble solid was removed by filtration. No attempt was made to recover the catalyst, although the palladium catalyst would be soluble in the ether layer.

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Liquid-Crystalline Polymers

Abstract: Savitz • Semor _ce Ihe,qdent Southwest Re,,carch Inshlule D_rectur Itomeqake Mmmg Company P.(). l)ra'_er 28510 Garrett Ceramic Components 651) ('ahfornm Slreet San Antonio. TX 78284 Divi-,ton San Ft'ant_t,,co. ('A 9410g Allied-Signal Aerospace Co. l)r. Ronald M. Latarnsmn

Cited by 40 publications

References 9 publications

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

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

Molecular Engineering of Liquid Crystalline Polymers

Transformation of a kinetically prohibited mesophase of a linear polymer into an enantiotropic mesophase via cyclization

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