Lamellar structure of non-integer folded and extended long-chain n-alkanes by small-angle X-ray diffraction Xiangbing Zeng and Goran Ungar* University of Sheffield, Department of Engineering Materials, Sir Robert Hadfield Building, Mappin Street, Sheffield S1 3JD, UK (Received 12 August 1997; revised 17 October 1997; accepted 24 October 1997) A small-angle X-ray scattering (SAXS) study was conducted on melt-crystallized long-chain n-alkanes C 162 H 326 , C 194 H 390 and C 246 H 494 . The main objective was to clarify the structure of lamellar crystals with the layer period a non-integer fraction (NIF) of the period of extended-chain crystals; the NIF form is the product of primary chain- folded melt crystallization (Ungar, G. and Keller, A., Polymer, 1986, 27, 1835) and it transforms subsequently to one of the integer folded forms. The extended-chain form crystallized from melt was also studied. Electron density profiles along the layer normal were reconstructed by inverse Fourier transformation using discrete diffraction intensities. The profiles were compared with the data computed from models. The best fitting models consist of alternating high- and low-density layers with relatively sharp boundaries. The high-density layers, representing the crystalline regions, have thicknesses which closely match either the calculated thickness of extended-chain layers with 358 chain tilt (for the extended-chain form) or half that value (for the NIF form). In the extended- chain form a low density intercrystalline layer is observed with thickness between 1 and 3 nm, depending on chain length and temperature. In contrast, the non-crystalline layer in the NIF form is 6 to 8 nm thick. It has constant density and is truly amorphous, as indicated by the close match between SAXS and DSC crystallinities, which are less than two thirds for the as-formed NIF structure. While less than half the chains are folded in the latter structure, those that are have a fold in the middle (‘integer folding’). Non-folded chains traverse the crystal layer only once, while their uncrystallized ends (cilia) comprise the amorphous layer. Within the crystalline layer of the NIF form chains are tilted at 358. The fraction of folded, fully crystallized chains increases with decreasing temperature at the expense of the amorphous layer, resulting in a reduction in amorphous layer thickness and overall lamellar periodicity (lamellar ‘thinning’). q 1998 Published by Elsevier Science Ltd. All rights reserved. (Keywords: polymer; oligomer; monodisperse) INTRODUCTION Since the first synthesis of monodisperse long n-alkanes 1–3 their physical behaviour has been studied extensively, as it was recognised that these model polymer materials hold important clues for a better understanding of polymer crystallization and structure. Folded as well as extended- chain lamellar crystals were obtained. It has been shown that mature crystals have fold lengths equal or very close to integer fractions (IF) of the extended-chain length 4–6 . This is in contrast to the behaviour of polydisperse polymers where lamellar thickness changes continuously with crystal- lization temperature T c 7 . Quantized values of lamellar thickness were also found in oligo(ethyleneoxide) frac- tions in the pioneering studies in this area by Kovacs and co-workers 8 . However, early real-time small-angle X-ray scattering (SAXS) experiments on long alkanes crystal- lizing from the melt have shown the long period corres- ponding to primary crystal formation to be a non-integer fraction (NIF) of the long period for extended-chain crystals L 0 9 . Primary NIF crystals with a long period value L between L 0 and L 0 /2 were found to transform subsequently to IF forms either via thickening (to the extended form) or thinning (to the once-folded form with L ¼ L 0 /2). More recent work on poly(ethyleneoxide) fractions by Cheng and Chen 10 also showed the presence of NIF as the primary form. The main purpose of the present work is to determine the structure of the NIF form and to explain the process of formation of chain-folded alkane crystals from the melt. Here we also investigate extended-chain crystals as reference. We limit our study to the NIF form in the most accessible range L 0 /2 , L NIF , L 0 . The information on this NIF form, available from our previous studies on alkanes 9 , can be summarized as follows. (1) The SAXS long period L is significantly different from the values corresponding to either extended (L 0 ) or once-folded (L 0 /2) chain crystals, even if correction for chain tilt (see later) is applied. (2) The first-order diffraction intensity is considerably higher than those of extended or once-folded chain crystal forms. In contrast, higher order diffraction lines are weak and were rarely observed in the time- resolved experiments. (3) NIF is always the primary form to appear in melt crystallization of long alkanes in the temperature range delimited roughly by the respective melting points of once- and twice-folded chain forms. It is unstable and normally disappears if the material is kept at elevated temperature, either by transforming to extended-chain or to strictly once-folded chain crystals. POLYMER Volume 39 Number 19 1998 4523 Polymer Vol. 39 No. 19, pp. 4523–4533, 1998 q 1998 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0032-3861/98/$19.00+0.00 PII: S0032-3861(97)10319-6 * To whom correspondence should be addressed