Form ⅡStart-up Shear Flow and Nanoscale Structure of a Thermotropic Liquid Crystal Polymer
Koya Suzuki, Masataka Sugimoto, Sathish K. Sukumaran*Graduate School of Organic Materials Science, Yamagata University, Yonezawa, Japan*sa.k.sukumaran@gmail.com
Liquid crystalline polymers (LCP) exhibit complex response to flow due to a combination of the features oflow molecular weight liquid crystals and high molecular weight polymers. While aspects of the steady flow behavior
remain to be elucidated, understanding of their transient flow characteristics lags far behind. Although certain of thestart-up flow characteristics exhibit some similarity to those of flexible polymer melts, there are also significantdifferences. For instance, under fast start-up shear, flexible polymer melts exhibit a stress overshoot at a shear strain(γ) in the range 1 < γ < 10, followed by a steady state. Thermotropic liquid crystalline polymers (TLCP) also exhibit
a stress overshoot at a similar 𝛾. In addition, before the steady state is reached, a second stress overshoot at γ ≈ 100
has been observed in some TLCP [1] but not in others [2]. The reasons for the difference, the molecular origin of
the phenomenon and its relation to changes in the underlying morphology remain unclear. To address these questions,
we began our investigations with the start-up shear flow of the liquid crystalline state of p‐hydroxybenzoate/2‐hydroxy‐6‐naphthoic acid (HBA/HNA), a widely used random copolyester with a chemical structure different to
the one used in [1].
Fig.1 exhibits the γ dependence of the shear stress, normalized using the height of the first maximum, forseveral shear rates, 𝛾˙. It is clear that two stress maxima can be observed. For the investigated 𝛾˙, the first stressmaximum occurs at γ ≈ 3, independent of 𝛾˙. For entangled flexible polymer melts and at 𝛾˙ less than the inverse of
the chain Rouse time, a similar superposition is observed but for γ ≈ 2. However, as the TLCP exhibits a domain
morphology in the quiescent state, it is unclear if the underlying physics is similar in both cases. Fig. 1 also indicatesthat a second stress maximum can be observed at a larger γ. Upon increasing 𝛾˙, this maximum progressively shiftedto larger γ and appeared to saturate at γ ≈ 200. In order to investigate the effect of the shear flow on the nanoscalestructure, samples rapidly quenched at several strains were examined using small angle x-ray scattering. As can beseen from Fig. 2, the orientation seen in the two dimensional scattering patterns exhibit a 90° rotation upon
traversing the second maximum. Interestingly, sector averages of the two dimensional scattering patterns indicatedthat the nanostructure was not otherwise affected. This suggests that, at the nanoscale, the flow results in a 90°
rotation of the underlying structure but does not otherwise perturb it not unlike observations in low molecular weightliquid crystals.
2310

210s-1
3.0s-1
1.0s-10.1s-1

 / Pa

1.5

10

1

0.5

010-1

100

101

102103Fig. 1 Dependence of shear stress, normalizedwith the height of the first peak, on 𝛾 forseveral 𝛾˙ under start-up shear flow at 310 ºC.

101102103104105Fig. 2 SAXS results under start-up flow at a shear
rate of 3.0 𝑠 −1 and a temperature of 310 ℃.

References[1] Guo, et al. Polym. Eng. sci. 45, 187 (2005).
[2] I. Pilar, M.E. Munoz, J.J. Pena, and A. Santamaria, Polym. Eng. Sci., 36, 721 (1996).