The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

Qian, Chen; Baird, Donald G.

doi:10.1016/j.polymer.2016.08.056

Cited by 11 publications

(4 citation statements)

References 38 publications

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“…These phenomena are correlated to the development of the interchain orientation and the domain texture of the LCP, and can be used as an indicator for the relaxation behavior. 65 In order to confirm that the experiments conducted on both LCPs are performed in the regime where the LCP plateau viscosity is constant as a function of shear rate, 66 the LCPs were subjected to a constant shear stress until a plateau value was reached. Figure 8 (left) shows the plateau shear rate as a function of the applied shear stress.…”

Section: Resultsmentioning

confidence: 99%

“…The shear response of LCPs typically shows a minimum in viscosity at low strain, followed by a maximum, and at sufficiently large strains, a plateau is reached. These phenomena are correlated to the development of the interchain orientation and the domain texture of the LCP, and can be used as an indicator for the relaxation behavior . In order to confirm that the experiments conducted on both LCPs are performed in the regime where the LCP plateau viscosity is constant as a function of shear rate, the LCPs were subjected to a constant shear stress until a plateau value was reached.…”

Section: Resultsmentioning

confidence: 99%

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Controlling Processing, Morphology, and Mechanical Performance in Blends of Polylactide and Thermotropic Polyesters

2019

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Thermoplastic composites based on thermotropic liquid crystalline polymer (LCP) materials are interesting candidates for reinforced composite application due to their promising mechanical performance and potential for recyclability. In combination with a societal push toward the more sustainable use of materials, these properties warrant new interest in this class of composites. Though numerous studies have been performed in the past, a coherent set of design rules for LCP design for the generation of injection-molded reinforced thermoplastic composites is not yet available, likely due to the complex interplay between LCP and matrix components. In this study, we report on the processing of poly(l-lactide) with two different LCPs, at relatively low processing temperatures. The study focuses on critical parameters for the morphological development and mechanical performance of LCP-reinforced composites. The influence of blend composition and the processing conditions, on the mechanical response of the composites, is investigated using rheology, wide-angle X-ray diffraction, mechanical analysis, and microscopy techniques. The study conclusively demonstrates that both the matrix viscosity and viscosity ratio between the dispersed and matrix phase, determine the deformation and breakup of the dispersed LCP droplets during extrusion. In addition, the thermal dependence of the viscosity ratio appears to be a critical parameter for the composite performance after injection molding. For example, during injection molding, stretching and molecular orientation of the LCP phase into highly oriented fibrils are prevented when the viscosity ratio increases rapidly upon cooling. In contrast, melt drawing proves to be a more effective processing route as the extensional flow field stabilizes elongated droplets, independent of the viscosity ratio. Overall, these findings provide valuable insights in the morphological development of LCP-reinforced blends, highlighting the importance of the development of viscoelastic properties as a function of temperature, and provide guidelines for the design of new LCP polymers and their thermoplastic composites.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Controlling Processing, Morphology, and Mechanical Performance in Blends of Polylactide and Thermotropic Polyesters

2019

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“…In-situ rheo-WAXD provides a powerful tool that can aid in the elucidation of the complex transient behavior of LCPs and the correlated structural changes. Due to their correlation with the structural evolution, the shear stress response of the LCP to the inception of shear, in combination with a controlled relaxation period, can provide information on the textural relaxation via the characteristics in the transient behavior [22,48].…”

Section: Resultsmentioning

confidence: 99%

Effect of Shear Rate on the Orientation and Relaxation of a Vanillic Acid Based Liquid Crystalline Polymer

et al. 2018

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In this study, we report on the visco-elastic response during start-up and cessation of shear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzed at different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction. Upon inception of shear, the polydomain texture is initially stretched, at larger strain break up processes become increasingly important, and eventually a steady state texture is obtained. The shear stress response showed good coherence between optical and rheo-X-ray data. The evolution of the orientation parameter coincides with the evolution of the texture: the order parameter increases as the texture stretches, drops slightly in the break up regime, and reaches a constant value in the plateau regime. The relaxation of the shear stress and the polydomain texture showed two distinct processes with different timescales: The first is fast contraction of the stretched domain texture; the second is the slow coalescence of the polydomain texture. The timescale of the orientation parameter’s relaxation matched with that of the slow coalescence process. All processes were found to scale with shear rate in the tested regime. These observations can have far reaching implications for the processing of liquid crystal polymers as they indicate that increased shear rates during processing can correspond to an increased relaxation rate of the orientation parameter and, therefore, a decrease in anisotropy and material properties after cooling.

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“…At the G 0 and G 00 cross-over point, the temperature can be taken as the onset of TLCP crystallization. [31] After cooling down to below its onset of crystallization, the TLCP primarily presented viscoelastic behavior and solidified. Based on the overlapping cross-over points shown in Figure 1, cooling from different temperatures within the range of 290-320 C did not make much difference in the TLCP's solidification temperature.…”

Section: Primary Processing Temperature Selection With Rheological Measurementsmentioning

confidence: 99%

The influence of processing conditions on tensile properties of wholly thermoplastic composites reinforced with nanotubes

Han

Baird

2021

Polymer Composites

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This paper is concerned with novel wholly thermoplastic composites (WTCs) reinforced with supercritical carbon dioxide-aided exfoliated multi-walled carbon nanotubes (MWCNTs) and the enhancement of mechanical properties.The bicomponent in situ injection molded plaques consist of polyamide 6 (PA6) and microfibers of thermotropic liquid crystalline polymer (TLCP) with miss-matched melting temperature. Multiple mixing histories with MWCNTs and processing temperatures were studied and tested for minimizing matrix thermal degradation and maximizing tensile properties of the multiscale WTCs. The optimum in situ injection molding temperature was found at 300 C with melt blending nylon 6 matrix and exfoliated MWCNTs in advance.Exfoliated MWCNTs acted as bridging elements between the TLCP and the nylon 6 matrix, which lead to effective stress transfer from the nylon 6 matrix to the TLCP fibrils. Compared to the WTCs without MWCNTs reinforcement, tensile modulus and tensile strength in longitudinal and latitudinal directions were both improved. Furthermore, with the reinforcement of TLCP and exfoliated MWCNTs, the tensile modulus and tensile strength of nylon 6 were enhanced by 173% and 35% in the fluid flow direction, respectively. These experimental results for 20 wt% TLCP/PA6 were even better than the mechanical properties of 20 wt% carbon fiber (CF)/PA6 reported in the literature. K E Y W O R D Sexfoliated multi-walled carbon nano-tubes, tensile properties enhancement, thermotropic liquid crystalline polymer, wholly thermoplastic composites

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The transient shear rheology of a thermotropic liquid crystalline polymer in the super-cooled state

Cited by 11 publications

References 38 publications

Controlling Processing, Morphology, and Mechanical Performance in Blends of Polylactide and Thermotropic Polyesters

Controlling Processing, Morphology, and Mechanical Performance in Blends of Polylactide and Thermotropic Polyesters

Effect of Shear Rate on the Orientation and Relaxation of a Vanillic Acid Based Liquid Crystalline Polymer

The influence of processing conditions on tensile properties of wholly thermoplastic composites reinforced with nanotubes

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