Transformation kinetics of liquid-crystallines based on 1,4-phenylene 4-n-alkylbenzoate-4-allyloxybenzoate by DSC

Bo, Zhonglin; Shen, Yiping

doi:10.1002/(sici)1097-4601(1997)29:5<317::aid-kin1>3.0.co;2-x

Cited by 2 publications

(1 citation statement)

References 12 publications

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“…7. SEM micrograph showing the structure of Material C; In addition to the spherulitic lamellar texture of the polymer, aggregates of MMT can also be seen as bright inclusions; Scale bar: 10 µm In this, V c is the volume fraction of crystallinity at time t c , V inf is the maximum crystallinity attained, K exp is the experimental rate constant and n is the so-called Avrami exponent, which can be interpreted as a dimensionality (31) . However, it is generally assumed that non-integral values of n are caused by factors such as secondary crystallization and mixed nucleation modes, rather than being a genuine reflection of non-three dimensional development.…”

Section: Crystallization Behaviourmentioning

confidence: 99%

Polyethylene Nanodielectrics: The Influence of Nanoclays on Structure Formation and Dielectric Breakdown

2006

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Polymeric composites containing nanometric fillers have attracted considerable interest because of the attractive properties that such materials can display. In the area of nanodielectrics, many workers have stressed the significance of the interfaces that exist between such fillers and the host polymer as being critical in influencing properties. Indeed, Tanaka (1) proposed a concentric shell model in which interactions can be thought of as occurring at a range of dimensional levels; Nelson and Fothergill (2) considered nanodielectrics in terms of an interaction zone. In this work, we set out to prepare a range of nanocomposites based on polyethylene (PE) and montmorillonite nanoclay (MMT), to examine the effect of MMT dispersion on properties and structure formation and, thereby, to explore both short-range molecular effects and the extent of the polymer/nanoclay interaction zone. Figure 1 compares AC ramp breakdown results obtained from 3 materials at a ramp rate of 50 V s -1 . Where dispersion of the MMT is poor (C Quenched), the breakdown strength is greatly reduced compared with a comparable system containing well-dispersed nanoclay (E Quenched). This is in line with behaviour reported for nano-and microcomposites (2) , suggesting that it is attributable to aggregation of the MMT in C. However, attempts to improve the breakdown strength of E by controlled crystallization were unsuccessful; systems prepared by rapid quenching from the melt or by prolonged isothermal crystallization both behaved in an equivalent manner. This is in contrast to the same polymer, but without MMT (B 117 o C), where controlled crystallization can lead to improved properties, as reported elsewhere (3) . Thus, when quenched, equivalent polyethylene materials with and without MMT exhibit similar breakdown behaviour, provided the MMT is well dispersed. However, in the absence of MMT, isothermal crystallization can result in improved performance whereas, when MMT is present, this appears not to occur.To explore the origin of the above effects, the influence of MMT on structural evolution in PE was examined. In the scanning electron microscope (SEM), all the materials where MMT was absent or poorly dispersed were found to exhibit spherulitic morphologies. In contrast, Fig.2 shows a sample of Material E, which contains 5% by weight MMT, following crystallization at 117 o C. An MMT aggregate ~1 µm in size can be seen near the centre of this image but, elsewhere, it is not possible to distinguish MMT platelets from polymer lamellae. Although the material is highly nucleated, the lamellar texture also appears highly disrupted. Thus, in the absence of MMT or when the MMT is poorly dispersed, crystallization of the polymer leads to conventional morphologies, whereas well-dispersed MMT appears to promote nucleation but, subsequently, to disrupt the processes by which ordered structures form. This interpretation of fig.2 is supported by quantitative studies of crystallization kinetics and the thermodynamics of crystallization. MMT increases...

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Section: Crystallization Behaviourmentioning

confidence: 99%

Polyethylene Nanodielectrics: The Influence of Nanoclays on Structure Formation and Dielectric Breakdown

2006

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Crystallization behavior of a propylene/ethylene copolymer: Nucleation and a clarifying additive

Martin

Vaughan

Sutton

et al. 2002

J Polym Sci B Polym Phys

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The isothermal crystallization behavior of a propylene/ethylene copolymer containing a clarifying additive has been studied in detail and compared with the equivalent unclarified grade. Differential scanning calorimetry was used to obtain crystallization exotherms for both the unclarified system and the clarified analogue. Avrami analysis of these data was then performed, using both linear and nonlinear data‐fitting techniques. Linear analysis revealed a change from a primary to a secondary crystallization process in the clarified system at about 50% relative crystallinity. Nonlinear techniques, however, led to more reliable estimates of the Avrami parameters and provided estimates of crystallization‐induction times. By combining the preceding with isothermal crystal‐growth‐rate data, the nucleation density in each material was obtained as a function of crystallization temperature. In the unclarified case, this fell exponentially with temperature. The nucleation density in the sorbitol‐clarified copolymer was 103–106 times greater than in the unclarified material, but decreased only slowly with increasing crystallization temperature throughout the temperature range investigated here. This final result appears entirely contradictory to previous morphological work in which a distinct morphological transition was observed at 128 °C and associated with a marked reduction in the nucleating efficiency of the sorbitol. Possible explanations for this apparent contradiction are considered. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2178–2189, 2002

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Transformation kinetics of liquid-crystallines based on 1,4-phenylene 4-n-alkylbenzoate-4-allyloxybenzoate by DSC

Cited by 2 publications

References 12 publications

Polyethylene Nanodielectrics: The Influence of Nanoclays on Structure Formation and Dielectric Breakdown

Polyethylene Nanodielectrics: The Influence of Nanoclays on Structure Formation and Dielectric Breakdown

Crystallization behavior of a propylene/ethylene copolymer: Nucleation and a clarifying additive

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