Phase transformations in a chiral main-chain liquid crystalline polyester involving double-twist helical crystals

doi:10.1016/S0032-3861(00)00238-X

Polymer

Volume 41, Issue 25, 1 December 2000, Pages 8953-8960

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Abstract

A main-chain non-racemic chiral liquid crystalline polymer has been synthesized from (R)-(−)-4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-nonyloxy}-4-biphenyl carboxylic acid, abbreviated PET(R^∗)-9. Based on differential scanning calorimetry, wide angle X-ray diffraction (WAXD) and polarized light microscopy experiments, this polymer undergoes at least three liquid crystalline (LC) transitions in addition to crystallization. The LC transition sequence is $I ↔ 3.21 kJ/mol 185° C TGBA^{∗} ↔ 1.27 kJ/mol 175° C S_{A}^{∗} ↔ 0.35 kJ/mol 130° C S_{C}^{∗} ↔ 37° C T_{g}$ The transition sequence is reversible since they are LC phases, which are close to thermodynamic equilibrium. Among these phases, the twisted grain boundary smectic A (TGBA^∗) phase is, for the first time, found in a main-chain LC polymer. The TGBA^∗ is only stable within a temperature region of 10°C between the Smectic A^∗ (S^∗_A) phase and the isotropic melt. Crystallization that takes place in the (S^∗_A) phase can form both flat-elongated and double-twist helical single lamellar crystals, as observed by transmission electron microscopy (TEM). Analysis of the WAXD fiber patterns of this polymer indicates that the crystal structure of PET(R^∗)-9 in the bulk is orthorhombic, identical with that determined using selective area electron diffraction in TEM [Macromolecules 32 (1999) 524; Phys Rev B 60 (1999) 12 675].

Introduction

Over the past two years, we have focused on the study of one non-racemic chiral main-chain liquid crystalline (LC) polyester synthesized from (R)-(–)-4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-nonyloxy}-4-biphenyl carboxylic acid, abbreviated PET(R^∗)-9 [1], [2], [3], [4], [5]. The polymer repeat unit chemical structure is:

For the first time, both flat-elongated and helical single lamellar crystals have been thermotropically grown under the same crystallization condition as shown in Fig. 1[1], [2]. Surprisingly, these crystals possess the same structure, which is an orthorhombic lattice with the three dimensional sizes of the crystal unit cell a=1.07, b=0.48 and $c=5.96 nm$ [1], [2]. The polymer chain folding direction in both flat and helical lamellar crystals is also determined to be identical, and it is always along the long axis of the lamellar crystals [1], [2].

Recent dark field (DF) image, bright field image, and selective area electron diffraction (SAED) experiments in transmission electron microscopy (TEM) provide chain orientation information for both the flat-elongated and helical lamellar crystals [3], [4]. In the flat-elongated lamellar crystals, the chain direction is only perpendicular to the substrate surface in a near center zone along the long axis of the crystals [4]. Moving away from this zone along the short axis of the crystal, the chain direction continuously tilts in the ac-plane. A small tilt angle of approximately 0.002° per molecular layer can be estimated using the SAED results [4]. In the helical lamellar crystals, the main twist direction is parallel to the long helical axis, and the rotating angle for each molecular layer is approximately 0.05° [3], [4]. However, specifically designed DF experiments using the entire and partial (205) and (206) diffraction arcs show that the chain orientation direction is also twisted along the short helical axis of the lamellar crystal. The rotating angle is approximately 0.01° per molecular layer. This leads to a second twist direction in addition to the twist along the long helical axis of the crystal [3], [4]. Based on these experimental observations, the concept of double-twist molecular orientation in the helical lamellar crystal can be established, although in principle, the macroscopic translation symmetry is broken along both the long and short axes of the helical lamellar crystals. This crystal can therefore be recognized as symmetrically “soft”, rather than a true crystal based on the traditional crystal definition in Euclidean space. Mathematically, double-twist crystals can only be true crystals in Riemannian space [3], [4]. Note that the double-twist was first proposed in description of condensed states in small molecular liquid crystals and biopolymers [6], [7].

Upon careful examination of the crystallization behavior of PET(R^∗)-9, it is not difficult to find that the crystallization takes place in a LC phase instead of the isotropic melt. The crystallization behavior and crystal morphology formed may critically depend upon the molecular orientation and structural order in this LC phase. Therefore, a clear understanding of the phase structures and transformations of this polymer is necessary in order to make connections with crystal morphology. In this publication, investigations on this topic of PET(R^∗)-9 are reported. At least three LC phases can be identified in this polymer based on differential scanning calorimety (DSC) experiments. Their structures are characterized using techniques of wide angle X-ray diffraction (WAXD) and SAED in TEM. Morphological investigations in PLM also aid in phase identification of the LC phases.

Section snippets

Materials and sample preparation

The polymer reported here is synthesized from (R)-(−)-4′-{ω-[2-(p-hydroxy-o-nitrophenyloxy)-1-propyloxy]-1-nonyloxy}-4-biphenyl carboxylic acid. The detailed synthesis routes of the monomers and polymers are published elsewhere [1], [2], [3], [4], [5]. The number of methylene units in the polymer of this study is nine, and this polymer possesses right-hand chiral centers (R^∗) along the main-chain backbone. The specific rotation of the monomers is [α]_D=−28.5°. Due to the head-to-tail connection

Thermodynamic transition behaviors

Fig. 2, Fig. 3show a set of cooling and subsequent heating DSC thermal diagrams for PET(R^∗)-9 at different rates between 2.5 and 40°C/min. A glass transition temperature (T_g) at 37°C is evident. When the cooling and heating rates are equal or faster than 10°C/min, there is apparently a first-order transition appearing at an onset transition temperature of 185°C during cooling and 175°C during heating. Of interest is that the onset transition temperature during cooling is higher than during

Conclusion

In summary, phase structural identifications for a non-racemic chiral PET(R^∗)-9 have been carried out. This polymer possesses a complicated LC phase behavior. In addition to the normal $S_{A}^{∗}$ and $S_{C}^{∗}$ phases, it is the first example for chiral main-chain LC polymers that possesses the TGBA^∗ phase. When crystallization is carried out from the $S_{A}^{∗}$ phase, both the flat-elongated and double-twisted single helical lamellar crystals can be observed. Based on the WAXD fiber pattern, the structural

Acknowledgements

This work was supported by the NSF (DMR-9617030) and the NSF ALCOM Science and Technology Center (DMR-8920147). Thoughtful and in-depth discussions with Dr F. Khoury at NIST are gratefully acknowledged. Over the past two years, Professor A. Keller had extensive discussions with us regarding this topic. Many brilliant thoughts from him have and will continue to enlighten and encourage us to further pursue this research.

References (18)

B Lotz et al.
J Mol Biol
(1982)
C.Y Li et al.
Macromolecules
(1999)
C.Y Li et al.
Phys Rev B
(1999)
C.Y Li et al.
Phys Rev Lett
(1999)
C.Y Li et al.
J Am Chem Soc
(2000)
F Bai et al.
Chem Mater
(1999)
M Kleman
Phys Scr
(1987)
M Kleman
Rep Prog Phys
(1989)
J.J Ge et al.
Macromol Chem Phys
(1999)

There are more references available in the full text version of this article.

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^☆: In memory of Professor Andrew Keller for his great lifetime contributions to polymer physics and many years of friendship and collaborations.

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Phase transformations in a chiral main-chain liquid crystalline polyester involving double-twist helical crystals☆

Abstract

Introduction

Section snippets

Materials and sample preparation

Thermodynamic transition behaviors

Conclusion

Acknowledgements

J Mol Biol

Macromolecules

Phys Rev B

Phys Rev Lett

J Am Chem Soc

Chem Mater

Phys Scr

Rep Prog Phys

Macromol Chem Phys