Bret H. Calhoun

Thermal analysis: the next two decades

Abstract As some of the most important analytical methods, thermal analysis techniques have been greatly improved and perfected due to the requirements of materials characterization. Over the past four decades, the major developments have been associated with computerizing thermal analysis techniques. In the coming 20 years, thermal analysis techniques will continue to develop in at least two different directions. First, more precise measurements can be carried out using traditional thermal analysis techniques by making a small extra effort. This will lead to a variety of new information regarding a material’s structure interface and morphology obtained from the results. Second, thermal analysis techniques can be combined with other in-situ temperature-controlled experiments, such as diffraction, scattering, microscopy, and spectroscopy, for investigation of the structure and dynamics of materials. This will generate information of the structural evolution with changing thermal properties and therefore, greatly aid in the understanding of structure–property relationships of the materials.

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

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].

Phase structures, transition behavior and surface alignment in polymers containing rigid-rod backbones with flexible side chains. Part VI Novel band structures in a combined main-chain/side-chain liquid crystalline polyester : From liquid crystal to crystalline states

Physical origins of banded structures appearing on different length scales have been investigated using polarized light and atomic force microscopies (PLM and AFM), polarized Fourier Transform infrared spectroscopy (FT-IR) and wide angle X-ray diffraction (WAXD) in a combined main-chain/side-chain liquid crystalline (LC) polyester, PEFBP(n). This series of PEFBP(n) polymers was synthesized from the polycondensation of 2,2′-bis(trifluoromethyl)-4,4′-biphenyldicarbonyl chloride with 2,2′-bis{ω-[4-(4-cyanophenyl)-phenyoxy]-n-alkoxycarbonyl]}-4,4′-biphenyl diol. In this paper, we focus on one polymer [PEFBP(n = 11)] of this series to illustrate the band structural formation on different length scales during the evolution from liquid crystal to crystalline states. Alternating bands of the films mechanically-sheared at 190 °C are formed with a spacing of 3 ± 0.5 μm in PLM, and recognized to be primary bands. PLM and AFM results show that these bands are seen due to the change of optical birefringence constructed mainly by alternating film thickness (and thus, retardation). Based on polarized FT-IR results, both the backbones and side chains of the polymers are orientated parallel to the shear direction. Secondary fibrillar bands develop within the primary bands after the sample is subsequently crystallized at 105 °C. These bands show a zigzag arrangement and possess a lateral size of 250 ± 50 nm determined by AFM. High resolution AFM observations illustrate that these bands consist of aggregated edge-on crystal lamellae having a thickness of approximately 20 nm. The lamellar crystals are assembled together and lie across the film thickness direction. The mechanism for the formation of these secondary zigzag bands originates from the expansion of the lattice dimension along the chain direction on a molecular scale during the nematic to crystalline phase transition and crystallization in the partially confined LC primary bands, which form macroscopic zigzag buckling.