Mesomorphism, Polymorphism, and Semicrystalline Morphology of Poly(Di-n-propylsiloxane) Raluca I. Gearba, ‡,§ Denis V. Anokhin, ‡,§ Alexander I. Bondar, ‡ Yuli K. Godovsky, ², ⊥ Vladimir S. Papkov, | Natalia N. Makarova, | Sergei N. Magonov, # Wim Bras, O Michel H. J. Koch, X Francis Masin, ¶ Bart Goderis, [ and Dimitri A. Ivanov* ,‡,§ Laboratoire de Physique des Polyme ` res, CP-223, UniVersite ´ Libre de Bruxelles, Bld. du Triomphe, B-1050, Brussels, Belgium, KarpoV Institute of Physical Chemistry, 10 VorontsoVo Pole, 103064 Moscow, Russia, NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences, 28 VaViloV Str., 117813 Moscow, Russia, Veeco Metrology Group, 112 Robin Hill Road, Santa Barbara, California 93117, Netherlands Organization for Scientific Research (NWO), DUBBLE-CRG/ ESRF, B.P. 220, F-38043 Grenoble Cedex, France, European Molecular Biology Laboratory, EMBL c/o DESY, Notkestrasse 85, 22603 Hamburg, Germany, Matie ` re Condense ´ e et Re ´ sonance Magne ´ tique, CP-223, UniVersite ´ Libre de Bruxelles, Bld. du Triomphe, B-1050, Brussels, Belgium, and Chemistry Department, Catholic UniVersity of LeuVen, Celestijnenlaan 200F, 3001 HeVerlee-LeuVen, Belgium ReceiVed June 22, 2005; ReVised Manuscript ReceiVed NoVember 10, 2005 ABSTRACT: The crystalline structure and semicrystalline morphology of poly(di-n-propyl siloxane), PDPS, were studied with powder and fiber X-ray diffraction, differential scanning calorimetry, solid-state nuclear magnetic resonance and atomic force microscopy (AFM). PDPS exhibits two crystalline phases, R (-45.5 °C < T < 62 °C) and  (T <-45.5 °C), and a hexagonal columnar mesophase which is stable in a broad temperature range between 62 and 197 °C. The low-temperature crystalline phase  is found to be monoclinic with lattice parameters a ) 20.60 Å, b ) 19.22 Å, c ) 4.95 Å, γ ) 93.1°. The R crystalline phase has a pseudo-tetragonal unit cell with a ) b ) 19.15 Å and c ) 5.00 Å and is characterized by monoclinic C2/c (No. 15) group symmetry. In the refined unit cell (R wp ) 0.127) the PDPS chains adopt a planar cis-trans conformation with the plane of the chain parallel to the b axis. The semicrystalline morphology of the R crystal was studied with AFM. The crystallization of PDPS from the hexagonal mesophase results in very thick (100-150 nm thick) crystalline lamellae, which implies that the chains are fully extended in the crystal. This feature, which is found in few polymers, makes PDPS similar to such systems as HDPE at high pressure or 1,4-trans-poly(butadiene). I. Introduction Inorganic polymers have attracted a continuously growing interest during the past decades due to their various technological applications. In particular, siloxanes are widely employed as lubricants, resins, greases, and elastomers. Apart from their practical importance these polymers display interesting thermal behavior. For instance, poly(di-n-alkysiloxanes), with two to six carbon atoms per side chain, form hexagonal columnar mesophases although there are no mesogenic moieties in the chemical structure. Linear poly(di-n-propylsiloxane), PDPS, is the second member in this family of flexible mesomorphic nonmesogenic poly(di-n-alkylsiloxanes). 1,2 The thermal stability interval of the mesomorphic state in PDPS is about 100 °C broader than that in poly(di-n-ethylsiloxane), PDES, which confirms the very strong dependence of the isotropization temperature on the side-chain length already documented for the first members of the family. 3,4 PDPS exhibits two crystalline modifications 3-10 and a mesophase for which a hexagonal columnar packing of the chains was suggested. 2,9 The high- temperature crystalline modification of PDPS was first described by Petersen et al. 11 as tetragonal with a ) b ) 9.52Å and c ) 9.4Å, while the low-temperature modification was assumed to be monoclinic, but no lattice parameters were reported. 3-9 It was also suggested that the low-temperature modification could be a mixture of a tetragonal and a monoclinic phase. 8 Early studies 11 established that in the semicrystalline structure PDPS forms chain-folded lamellar crystals with thicknesses around 20 nm. However, more recent small-angle neutron scattering work has shown existence of partly disentangled chains in the columnar mesophase, 12 which suggests a tendency to form extended or nearly-extended chain crystals. Upon cooling from the isotropic melt, crystallization in bulk PDPS is rather fast making it difficult to obtain amorphous samples even by rapid quenching. Similarly, the rate of mesophase formation upon cooling of PDPS is so high that bulk crystallization always proceeds from the mesomorphic state. The critical molecular mass for mesophase formation in PDPS is about 10 kg/mol 3,4 or, about three times lower than that for PDES (28 kg/mol). 13,14 This difference is likely to result from an increase in chain stiffness as a function of the length of the side groups. 1 As for PDES, a linear relationship between the isotropization temperature and reciprocal molecular mass was established, 1-3 which is closely related to the low isotropization enthalpy and the rather high interfacial energy of the mesophase * To whom correspondence should be addressed. E-mail: dimitri.ivanov@uha.fr. ² Unexpectedly passed away on May 23, 2005. ‡ Laboratoire de Physique des Polyme `res, CP-223, Universite ´ Libre de Bruxelles. § Present address: Institut de Chimie des Surfaces et Interfaces (ICSI), UPR CNRS 9069, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France. ⊥ Karpov Institute of Physical Chemistry. | Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. # Veeco Metrology Group. O Netherlands Organization for Scientific Research (NWO). X European Molecular Biology Laboratory. ¶ Matie `re Condense ´e et Re ´sonance Magne ´tique, CP-223, Universite ´ Libre de Bruxelles. [ Catholic University of Leuven. 988 Macromolecules 2006, 39, 988-999 10.1021/ma0513315 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/11/2006