Reversible local melting in polymer crystals

Okazaki, Iwao; Wunderlich, Bernhard

doi:10.1002/marc.1997.030180407

Cited by 88 publications

(53 citation statements)

References 10 publications

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“…A chain segment is schematically shown as being ªpeeledº back from the crystal growth face into the drawn amorphous phase. This is consistent with the recent work of Okazaki and Wunderlich [74] on reversible local melting of PET where parts of the chains comprising the stems in the crystals reversibly melted and crystallized without nucleation. While chains comprising crystal stems and amorphous segments are expected to exist in rather substantial number in unoriented bulk semicrystalline polymers, peeling off crystal growth surfaces is expected to occur more seldom, mostly under externally applied stresses.…”

Section: Amorphous Phases In Semicrystalline Polymerssupporting

confidence: 91%

Increased glass transition temperature in motionally constrained semicrystalline polymers

Aharoni¹

1998

Polym. Adv. Technol.

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A large number of experimental results in the literature support and illuminate a model of behavior of chains and chain segments in the amorphous phase of semicrystalline polymers connecting the elevation of the glass transition temperature (Tg) above its normal value to several kinds of motional restrictions imposed on the chains and parts thereof. Accordingly, polymer chain, chain‐segment and chain‐fragment motions of all kinds comprise one or more torsions around main‐chain bonds from one stable conformation to another, known as rotational isomerizations. When impediments are placed in front of thermal fluctuations and larger transversal and longitudinal motions of polymer chains, segments and shorter fragments in the amorphous phase, and the motions are thus restricted, the glass transition temperature is elevated relative to that of the same amorphous phase in the bulk under normal conditions. The obstructions may prevent either the onset of rotational isomerizations or of their completion once started. The completion of the torsional isomerizations and larger motions may be prevented by eliminating the free spaces necessary to accommodate the volumes of the interconverting chain fragments and segments even when they move in concert, or by preventing the creation of such free spaces. Another way to hinder the completion of such motions is by the introduction into the system of many rigid walls and other interfaces with strong attractive interactions with the polymer, that by geometrical constraints and attractive interactions suppress the rotational and larger motions and prevent their completion. Elimination of the necessary free volume is achievable by the application of compressive pressure, while the introduction of rigid attractive walls may be accomplished by the incorporation of crystallites, as in semicrystalline polymers, or by the addition of rigid finely comminuted foreign additives with very large surface areas or confining voids with high tortuosity. It is believed that motional restrictions imposed on the amorphous phase by the growth faces of polymer crystallites, especially in oriented semicrystalline polymers, are more effective than the restrictions imposed by the fold surfaces of these crystallites. The prevention of the onset of rotational isomerizations and larger motions may be achieved by stretching the polymer chains and chain segments in the amorphous phase and, by one means or another, pinning down the taut chains such that essentially all their rotational isomers are in the trans conformation: they cannot interconvert to the gauche conformation since it requires the chain’s end‐to‐end distance to decrease. Parallel alignment of relatively taut chain‐segments may impose additional geometrical restrictions on both the onset and completion of rotational isomeric torsions and, of course, on longer‐range motions. In all cases, the Tg of the motionally constrained parts of the amorphous phase, especially in semicrystalline polymers, is expected to rise. It is likely that the cha...

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Section: Amorphous Phases In Semicrystalline Polymerssupporting

confidence: 91%

Increased glass transition temperature in motionally constrained semicrystalline polymers

Aharoni¹

1998

Polym. Adv. Technol.

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“…They are then able to rapidly recrystallize due to templating of the just melted chains as they recrystallize on existing crystals. Crystallization exotherms only contribute to the nonreversing signal, making this a very powerful technique for separation of exotherms from glass transitions, reversible melting, or other heat capacity related events [20].…”

Section: Resultsmentioning

confidence: 99%

The influence of molecular weight on the properties of polyacetal/hydroxyapatite nanocomposites. Part 1. Microstructural analysis and phase transition studies

Pielichowska

2012

J Polym Res

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In this work the influence of molecular weight of three polyoxymethylene (POM) copolymers on the properties of (POM)/hydroxyapatite (HAp) nanocomposites for long-term bone implants have been investigated by various physicochemical methods. Electron microscopy observations confirmed uniform dispersion of HAp in the polymer matrix, whereby DSC results show that HAp influences the degree of crystallinity of POM matrix. Temperature modulated differential scanning calorimetry (TMDSC) was applied to study the melting and recrystallization processes of POM matrix-it was found that with decreasing of POM molecular weight the amout of reversible melting increases. WAXD results show no shift in 2θ for pure POM copolymers and all POM/hydroxyapatite nanocomposites, indicating that the addition of HAp does not change the hexagonal system of POM. For all three copolymers, Young's modulus increases with increasing hydroxyapatite concentration, whereby elongation at break decreases. On the contrast, HAp concentration does not have a significant influence on the tensile strength.

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“…Sauer et al [35] used MT-DSC technique to characterise melting and recrystallisation of poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK)-for the interpretation of MT-DSC results three main rules have been applied: (i) the non-reversing endothermic signal is typically due to complete melting of separate lamellae or stacks of lamellae; sometimes perfected crystals cannot recrystallise fast enough because of a low degree of undercooling, (ii) the reversing endothermic signal is due to partial melting of lamellae which are then able to recrystallise quite rapidly due to templating of the just melted chains as they recrystallise on existing crystals (the physics of this process was discussed in detail in Ref. [36]) and, (iii) crystallisation exotherms contribute only to the non-reversing signal which is used for separation of exotherms from e.g. reversible melting.…”

Section: Influence Of the Heating Ratementioning

confidence: 99%

Step-scan alternating DSC study of melting and crystallisation in poly(ethylene oxide)

Pielichowski

Flejtuch

Pielichowski

2004

Polymer

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Reversible local melting in polymer crystals

Cited by 88 publications

References 10 publications

Increased glass transition temperature in motionally constrained semicrystalline polymers

Increased glass transition temperature in motionally constrained semicrystalline polymers

The influence of molecular weight on the properties of polyacetal/hydroxyapatite nanocomposites. Part 1. Microstructural analysis and phase transition studies

Step-scan alternating DSC study of melting and crystallisation in poly(ethylene oxide)

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