HBA/HNA copolyester

The HBA/HNA system provides a more suitable system for study, since it is prepared by melt polymerization of the two monomers and is far more stable at elevated temperatures compared to the PHBA/PET. The HBA/HNA copolymers are soluble in pentafluorophenol permitting use of NMR techniques to characterize diad sequences. In Fig. 13b,c the 13CNMR spectrum of the carboxyl carbon region of the HBA/HNA copolyesters of the 73/27 and 48/52 systems is shown [34]. Also shown in Fig. 13a,d are the spectra of 13C enriched HBA and HNA containing copolymers permitting unique identification of the diad sequences. As a result of this technique it was possible to determine the reactivity ratios of the two monomers by analyzing the 50/50 copolymer after polymerization to a molar mass value of 2000 [35]. Examination of the copolymer by 13C NMR showed the same ratio of monomers as in the starting... 

Fig. 20. Transition temperatures of ordered and randomized HBA/HNA copolyesters as a function of the composition...

Fig. 24. Viscosity of the 24/76 HBA/HNA copolyester vs shear rate with temperature...

Fig. 26. Change in lap shear strength with temperature A 24/76 HBA/HNA copolyester, 73/27 HBA/HNA...

The melt viscosity of LCPs is sensitive to thermal and mechanical histories. Quite often, instrumental influences are important in the value of viscosity measured. For example, the viscosity of HBA/HNA copolyesters are dependent on the die diameter in capillary flow (59). LCP melts or solutions are very efficiently oriented in extensional flows, and as a result, the influence of the extensional stresses at the entrance to a capillary influence the shear flow in the capillary to a much greater extent than is usually found with non-LC polymers. 

HBA/HNA, copolyester of 4-hydroxybenoic acid and 6-hydroxy-2-naphthalic acid. 

The aim of this work is to investigate the structure of these copolyesters at the molecular level both in terms of the sequence distribution and conformation of the individual molecules and how these pack in three dimensions. This paper describes X-ray diffraction work on these copolymers, and primarily addresses the problem of the chain structure. X-ray diffraction patterns of melt-spun fibers of HBA/HNA copolyesters (fig. 1) show a high degree of molecular orientation parallel to the fiber axis. Meridional maxima can be seen which are aperiodic, i.e. they are not orders of a simple fiber repeat these maxima also change in number and position with the monomer ratio. This is seen clearly in fig. 2 which shows 6/20 meridional X-ray diffractometer scans for five HBA/HNA compositions. We have calculated the theoretical diffraction patterns of random chain copolymers of HBA and HNA, and details of these calculations are given below. 

Figure 9.28 DSC thermograms for compression-molded 73/27 HBA/HNA copolyester specimens annealed at different temperatures ...

Figure 9.30 (a) Temperature protocols employed, (b) plots of log G versus log co, and (c) plots of log f] versus log a> for a compression-molded 73/27 HBA/HNA copolyester specimen having thermal histories as indicated in the temperature protocol (O) step 2, (A) step 3, (0) step 4, and (A) step 5. (Reprinted from Han et ah, Molecular Crystals and Liquid Crystals 254 335. Copyright 1994, with permission from Taylor Francis Group.)... 

Figure 9.38 Trace of first normal stress difference of a compression-molded 73/27 HBA/HNA copolyester specimen during transient and steady-state shear flow, and dnring the relaxation after cessation of steady-state shear flow. The normal stress before applying a sudden shear flow to the specimen is taken to be zero. (Reprinted from Han and Chang, Journal of Rheology 38 241. Copyright 1994, with permission from the Society of Rheology.)...

Figure 9.40 Plots of (t, y) versus yt for compression-molded 73/27 HBA/HNA copolyester specimens at 290°C for various shear rates (s ) (O) 0.05, (A) 0.1, and ( ) 0.3. A fresh specimen was used for each applied shear rate. Each specimen, after being cooled from 320 to 290 °C, was annealed for 5 min before being subjected to shear flow. The unrelaxed normal...

An unexpected degradation process was observed in attempting to step-order the HBA/HNA copolyester system. Stepwise heating of the HBA/HNA 73/27, 50/50 and 24/76 compositions resulted in a complete loss of crystallinity, even though the annealing temperatures were well below the melting transitions and well below the original polymerization temperatures at 350°C. 

Figure 8 As received 50/50 HBA/HNA copolyester annealed at 2(X)°C for 24 hours followed by 234 C for 24 hours, 270 C for 24 hours and then heated at 3(X)°C for the indicated times...

---