Crystallinity of uniform triblock oxyethylene/methylene/oxyethylene (E_mC_nE_m) oligomers studied by X‐ray diffraction, differential scanning calorimetry and Raman spectroscopy

Abstract

Uniform triblock oligomers with a central methylene block and outer oxyethylene blocks terminated by methoxy groups, CH,(OCH,CHz)mO(CH,)n(OCHzCHz)~OCH, (E,CnE,), were prepared and studied in the solid state by X-ray diffraction, differential scanning calorimetry and Raman spectroscopy. Both fully crystalline and partly-crystalline structures were found, with chains in wholly trans-planar and mixed trans-planar/helical conformations. Comparison is made with previous results for triblock oligomers of type CnE,Cn and diblock oligomers of type CnEm.The crystallinities o b s e r~e d~-~) in stable layer crystals of uniform C,E,,C, triblock oligomers are listed below. POE: n 5 2 Completely crystalline helical oligo(oxyethy1ene) chains. I: n < 10 Crystalline oxyethylene blocks, liquid-crystalline methylene blocks, helical oxyethylene blocks and trans-planar methylene blocks orientated normal to the end-group plane. 11: 10 5 n 5 26 Completely crystalline, helical oxyethylene blocks orientated normal to the end-group plane, trans-planar methylene blocks tilted at an angle of 61 111: n 2 26Crystalline methylene blocks, liquid-crystalline oxyethylene blocks, trans-planar oxyethylene and methylene blocks orientated normal to the end-group plane.Chain folding is observed in type I11 structures of series C,E,,C,, the fold being in the trans-planar, liquid-crystalline oxyethylene block. Oligomers C,E,C, with m ranging from 9 to 45 have been studied, and oligomers C,E,,C, can form chain folded type I1 structures with the fold in the helical, crystalline oxyethylene block7). Nonequilibrium structures may be formed, depending on the crystallisation conditions. These include structures in which the whole oligomer chain is tilted7).The structures of uniform diblock oligomers are less well documented. Several authors9* lo, 12) have reported Raman spectroscopic studies, which give valuable information on chain conformation. Diffraction studies 8, indicate similar structures to those established for the uniform C,E,C, triblock oligomers, but with some distinctly 'diblock' features, e. g. bilayer structures in the C,E,OH oligomers.In this paper we report on the crystallinity of uniform triblock oligomers with a central methylene block and outer oxyethylene blocks terminated by methoxy groups: with respect to the end group plane. CH3(OCHzCHJ,0 (CH,),(OCH2),0CH3So far as we are aware, uniform block oligomers of this type have not been studied before. TWO series of samples were prepared E,C,,E, m = 3, 6, 9, 12, 15 E,Cz3E, m = 3, 6, 9, 12The general methods of preparation have been described previously13). In this work an a,wdibromo-n-alkane was reacted under alkaline conditions (Williamson reaction) with either (a) an a-methyl-o-hydroxy-oligo(oxyethy1ene) or (b) an a-hydro-whydroxy-oligo(oxyethy1ene) and the product then methylated. In both procedures the triblock product was purified by preparative-scale gel permeation chromatography followed by recrystallisation. Experimental partDibromoalkanes 1,12-Dibromododecane (n = 12) was purchased ...

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