Wide angle X-ray diffraction and force field constrained Rietveld modelling of m-linked fully aromatic copolyesters. 2. Poly(p-phenylene terephthalate- co-p-phenylene isophthalate) P. Friedel * ,c , J. Bergmann a , T. Taut b , J. Tobisch c , D. Pospiech c , D. Jehnichen c a Ludwig-Renn-Allee 14, D-01217 Dresden, Germany b Seifert-FPM GmbH Freiberg/Sa., Am St. Niclas Schacht 13, D-09599 Freiburg, Germany c Institute of Polymer Research Dresden, Hohe Str. 6, D-01069 Dresden, Germany Received 16 June 1998; received in revised form 25 May 1999; accepted 17 August 1999 Abstract Fully aromatic copolyesters with regular chain structure containing kinked monomer units were investigated by powder diffraction experiments combined with structure modelling. The proposed series of copolyester structures including isophthalic acid was continued with (TPA–HQ–IPA–HQ) n (containing 104 atoms as repeating unit). The monoclinic unit cell has a  0:781 nm; b  0:557 nm; c  5:03 nm and b  89:2° as atom coordinates. The symmetry of the unit cell was determined by means of a force field constrained Rietveld refinement algorithm including nonbonding interactions (Lennard-Jones potentials) and chain continuation enforcement (penalty functions) in the force field. A reliability factor R Rietveld of 9.5% was calculated.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Kinked aromatic copolyesters; Solid phase structure; Powder diffraction 1. Introduction The solid state structure and phase behaviour that deter- mine material properties are influenced by primary chemical structures and molecular characteristics. Relations between the structure of aromatic liquid-crystalline main chain copolyesters containing either random or regular chain structure and their anisotropic material properties are of basic interest and have been discussed widely [1–5]. Two views exist pertaining to the symmetry of these crystalline structures. Orthorhombic [1–3] and monoclinic [4] lattices have been assumed, in agreement with the measured diffrac- tion patterns. The assumption of a monoclinic unit cell seems to be correct for all previously investigated structures [4,5]. This is related to almost same values of a and b-axes, regarding this class of copolyesters, whereas the length of the c-axis depends on the length of the chemical as well as the crystallographic repeating unit. The general calculation formula is |c| i × 1:25 nm; with i 2; 4; 6; …: The aim of this paper is to continue structural refinement of aromatic main chain copolyesters containing regular chain sequences, that was discussed in the last publication [5]. Here, a new force field constrained Rietveld refinement procedure is applied. Using the (IPA–HQ) n philosophy of two kinked IPA units, the c-axis for poly(p-phenylene terephthalate-co-p-phenylene isophthalate) (TPA–HQ– IPA–HQ) n had to be expanded up to 5.0 nm in order to realise an extended polymer chain. This concept shall apply to the determination of the structure parameters of the above polymer. 2. Experimental 2.1. Synthesis and characterisation of poly(p-phenylene terephthalate-co-p-phenylene isophthalate) Poly(p-phenylene terephthalate-co-p-phenylene isophtha- late), (TPA–HQ–IPA–HQ) n , was synthesised by polycondensation of terephthaloyl chloride with bis(p-hydro- chinoyl)-isophthalate in 1-chloronaphthaline at 240°C as described earlier [4]. The following functional groups were identified using I–R spectroscopy (KBr): 1731 (CyO ester,ar ), 1508/1498 (n C–C ar ), 1173 (n C–O–C ar ), 1014, 721 (d (C–H) ar ) cm -1 . Additionally, the substance was characterised by elemental analysis: C, 69.43 wt% (calc. 70.00 wt%); H, 3.36 wt% Polymer 41 (2000) 3791–3795 JPOL 4297 0032-3861/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(99)00595-9 * Corresponding author. Tel.: +49-351-4658-289; fax +49-351-4658- 284. E-mail address: friedel@argos.ipfdd.de (P. Friedel).