The phase transition of urea complexes whose guest components range from n-paraffin to polyethylene was investigated by DSC thermal analysis. The transition temperatures increased with increasing the chain length; the related heats did not change much with the chain length. An X-ray powder diffractometry for urea-polyethylene complex revealed that the complex undergoes the same kind of phase transition between orthorhombic and hexagonal as urea-n-paraffin complex. The transition is interpreted in terms of an order-disorder transition with
respect to the orientation of the guest molecule

Monobe, Kazuo

Yokoyama, Fumiyoshi

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Order-Disorder Transition in Urea-Polyethylene Complex

Okayama University Scientific Achievement Repository

Memoirs of the School of Engineering, Okayama University Vol. 19, No.2, February 1985Order-Disorder Transition ln Urea-Polyethylene ComplexFumiyoshi YOKOYAMA* and Kazuo MONOBE*(Received January 31, 1985)SYNOPSISThe phase transition of urea complexes whoseguest components range from n-paraffin to poly-ethylene was investigated by DSC thermal analysis.The transition temperatures increased with increasingthe chain length; the related heats did not changemuch with the chain length. An X-ray powder diffrac-tometry for urea-polyethylene complex revealed thatthe complex undergoes the same kind of phasetransition between orthorhombic and hexagonal asurea-n-paraffin complex. The transition is inter-preted in terms of an order-disorder transition withrespect to the orientation of the guest molecule.1. INTRODUCTIONFrom heat capacity measurements for some urea complexes with n-paraffins whose carbon numbers range from n = 10 to n = 20, Pembertonand parsonage1 ,2) first showed that in each case the heat capacitycurve had at least one region of high heat anomaly in a low tempera-ture. They considered that this phenomenone was attributed to anorder-disorder transition of the guest molecule around the chain axis.However, they did not take account of the structural change in the* Department of Industrial Chemistry.2 Fumiyoshi YOKOYAMA and Kazllo MONOBEhost urea lattice. It was found that the transition temperaturesincreased with the chain lengths of the guest molecules.Recently, from X-ray and differential thermal analyses, Chataniet al. 3 ,4) indicated that the ordinary hexagonal complexes (denotedas high-temperature form) transform commonly into orthorhombiccomplexes (low-temperature form) except for a few complexes. Thetransition was regarderd as an order-disorder transition of the guestchain molecules, accompanied by structural changes in the host urealattice. In the former formS), the guest chains in the tunnelsrotate around the long axis independently of each other; while in thelatter form3 ,4), they are disposed in a definite orientation aboutthe tunnel axis. In our previous work6 ), urea-polyethylene complexwas shown to exhibit the similar phase transition to urea-n-paraffincomplex. Umemoto and Danyluk7 ) indicated that the markedline narrowing temperatures in broad line NMR of several urea-d4complexes with n-paraffins correspond nearly to the transitiontemperature obtained by the heat capacity measurements, indicating themotional changes of the guest molecules around the tunnel axis. Horiet al. 8 ) investigated molecular motion of polyethylene molecules inurea-polyethylene complex by broad line NMR, related to the phasetransition.In this study, the phase transition of urea complex withn-paraffin and that of urea complex with polyethylene are studied bythermal analysis and X-ray powder diffractometry. The transitionphenomena for a series of the urea complexes will be discussed froma structural point of view.2. EXPERIMENTAL2.1 MaterialsThe complexes of urea with n-tetracontane, n-tetratetracontane,and those with polyethylenes were prepared by the method presented byus 9 ). Here, polyethylenes used were Hiwax 200P of Mitsui Petro-chemical Industries, Ltd. (denoted as PE(A), Mv 2300), Hiwax 400P(PE(B), Mv ~ 5800) and Sholex 6050 of Showa Denko Co. (PE(C), Mv ~59000). All the complexes were provided for DSC thermal analysis.The urea complex with polyethylene (PE(C)) was provided for X-rayanalysis. The complex of urea with n-paraffin(mp.42~44·C) was alsoprepared and used for obtaining the X-ray diffraction pattern.Order-Disorder Transition in Urea-Polyethylene Complex2.2 DSC thermal analysisA Perkin Elmer differential scanning calorimeter (DSC-1) was usedfor the measurements of the thermal properties of the complexes. Bothtemperature and amplitude scales were calibrated by using mercury(melting point: -38.9°C, heat of fusion :43.2 cal/g10 ) ) and water(0 °C,79.8 cal/g), with the same scanning rate 10°C/min as the sample.2.3 X-ray analysisPreceding the X-ray measurements, urea-polyethylene (PE(C»complex was milled to fine powder of the particle size less than 145mesh. An X-ray powder diffractometer with Ni-filtered Cu-~radiation,the usual optical arrangement, and a vacuum chamber for the setting ofthe sample was used for the identification of the crystal types at 23,-12, and -190°C; determination of the unit cell constants at -190°C;evaluation of the integral breadth of line profiles at 23 and -190°C.The known diffraction peaks of hexagonal urea-polyethylene complex at23°C were used to correct diffraction angles. The evaluation of theintegral breadth of line profiles at 23 and -190·C for the complexof urea with n-paraffin (mp.42~44 °C) was also carried out.3. RESULTS AND DISCUSSION3.1 Thermal analysisIn Fig. 1, we show the transition temperatures of urea complexeswith n-paraffins and polyethylenes, together with those of pure n-paraffins and melting points of polyethylenes, plotted against thecarbon number. The transition temperatures of urea complexes with n-paraffins increase with the chain length in an irregular manner,significantly differing from those of even or odd-membered pureparaffin crystals 11 ). This feature was ascribed to the fact thatthe increment of paraffin chain length of about 2.5 Aor the fiberperiod does not fit into the integral of urea spiral repeatingdistance, 1.83 A (=11.0 A/G) along the urea tunnel axis. The varioustemperature-widths of transition about urea-n-paraffin complexesindicated in Tab. 1 also may be related to such a structuralcharacter.The transition temperatures of three urea-polyethylene complexesapproach the room temperature; DSC thermograms of these complexes showendothermic peaks at 5 ...... 8 ·C, which agrees nearly with the marked34 Fumiyoshi YOKOYAMA and Kazuo MONOBE150J>000100.oeo•""'• 0.0U 0.0L- 0.0.50 o.QJ o.o•...•:)+" •~ • 0 0 0QJ O~a. •E •QJ+"•c-50 ~.& 00 0~ .&'iii /::,.~-100tf'.&.&.&I--150 /::,...10 20 30 40 102 103 104Carbon numberFiure 1. Transition temperature versus carbon number of the guestmolecule (b.: even, A : odd, 0: polyethylene). Transition temper-atures of pure n-paraffins 11 ) (0: even, • : odd) and meltingtemperatures of polyethylenes (c) are also shown. The latters wereestimated by the relation11 ): Tm(oK) = 414.3(n - 1)/(n + 5).line-narrowing temperature in broad line NMR8).We find from the figure that the transition temperatures of thecomplexes are situated nearly 100 QC lower than those of the corre-sponding pure paraffin crystals or melting points of the correspond-ing pure polyethylene crystals. This will be attributed to a veryweak interaction resulting from a large interguest distance, 8.2 A,which is about twice the intermolecular distance in pure paraffin orpolyethylene crystals.Table 1. Comparison of thermal properties with respect to thetransition among the urea complexes.cal/deg·g of complex transition Reference(cal/deg·mole of guest) (OC)* *n=lO -162 0.32 0.003 4(203) (1. 94)* 0.003 *n=12 -150 0.35 4(265) (2.26)* 0.003 *n=16 -121 0.43 2(422) (2.91)* 0.003 *n=20 -84 0.56 8(667) (3.61)n=40 -61 0.36 0.002 5n=44 -50 0.39 0.002 5PE(A) +5 0.48 0.002 7PE(B) +7 0.51 0.002 7PE(C) +8 0.53 0.002 8Guest Ttmolecule (OC)AHcal/g of complex(cal/mole of guest)AS **Width ofPembertonandParsonageThis workS?i}":'~~"~~:";;';:::~;pi~1;1~~~* We derived these values from the corresponding ones in the parenthesesgiven by Pemberton and Parsonage; using the re1ationship~ mole ratio ofurea/n-paraffin = 0.6848(n - 1) + 2.181, n: carbon number.** We estimated this quantity as the half-width of the endothermic peak profile.Ul6 Fumiyoshi YOKOYAMA and Kazlio MONOBE(11·0)(20·0)(10·0)a (10-0'---------(20·1) ...... (11·2)+(lO·3)30(l30)(220)(040)/;;221)(131) .(20'Y -(202)+{132)\ +(11 3) +(023)(200)20 2528(degree)(l12).(022)15------(022), ,(l10) 011).{021)(020)/(110)+(020)b (111)+(021)cFigure 2. X-ray powder diffraction patterns of urea-polyethylenecomplex: (a) at 23°C, (b) at -12°C, (c) at -190°C.Pemberton and Parsonage1 ) showed that urea-2-methyl penta-decane complex had not the phase transition; the bulky methyl grouphinders the rotational jumping from one site to another around thechain axis. Chatani et al. 4 ) reported that the absence of thephase transition in urea-sebasic acid complex and in urea-poly(1,4-butadiene) complex is resulted from the cylindrical shape of theseguest molecules in the urea tunnel. In this study, urea-polyethylenecomplex did show the phase transition. Then, the conformation of theguest polyethylene molecules in the tunnel is expected to be zigzagone as a whole.Order-Disorder Transition in Urea-Potyethylene ComplexIn Tab. 1, the thermal properties about the solid-phase transi-tion of urea complexes are summarized. The heat and entropy oftransition of any of three urea-polyethylene complexes are 0.5 cal/gand 0.002 cal/deg g, respectively, which are on the same order asthose for urea-n-paraffin complexes. It is expected from such smallvalues for urea-polyethylene complex that changes in the host latticestructure and those in thermal motion of the guest molecule accompa-nied with the transition will be as small as in the case of urea-n-paraffin complexes. In fact, it has been shown by Chatani et al. 4 )that the orthorhombic form of urea-n-hexadecane complex is attainedby a small deformation of urea tunnel from hexagonal, and that thetemperature factor of the guest molecule is significantly high evenbelow the transition temperature, 2.8 A-2 at _175°C12).3.2 X-ray analysisIn Fig. 2 are shown the X-ray diffraction patterns of urea-polyethylene complex at 23, -12 and -190 PC; the similar patterns tothose at 23°C and -190°C were obtained for urea-n-paraffin complex,at the respective temperatures. We find that the pattern at 23°C in(a) is ascribed to the hexagonal form as already shown in our work9 ),while the patterns at -12°C in (b) and -190°C in (c) are ascribed tothe orthorhombic one. The transition from hexagonal to orthorhombicresults in the splitting of the diffraction peaks, because of thereduction of symmetry from the space group P6122 to P212121,4)e.g. (10·0) in the former form becomes (110) and (020) in the latterform. The splittings of the other reflexions are found in the figure.Evaluations of integral breadths of the following equatorialreflexions are presented later: (11'0) and (20'0) in the hexagonalform and the corresponding reflexions in the orthorhombic form~ apair of (200) and (130), and of (220) and (040).By using the following reflexions in Fig. 2 (c): (112), (200),(130), (201), (131), (220), (040), and (221), orthorhombic unitcell constants of urea-polyethylene complex at -190°C were obtained: a= 8.30 A, b= 13.81 A, c = 11.00 A. The superposition of many pairs ofthe splitting reflexions at -12 ~ makes it difficult to evaluate thecell constants. The above constants agree well with those of urea-n-hexadecane complex at the same temperature: a = 8.272 A, b = 13.824 A,c = 10.988 A, in which a and b dimensions were derived from thether~al expansiion equations, given by Chatani et al. 4 )78abFwniyashi YOKOYAMA and Kazua MONOBE8.178[1 - 6.08.10- 5 (T - 273.15)]14.091[1 + 9.96.10- 5 (T - 273.15)][ 1a][1 b]where T is expressed inoK; they reported that a drastic change in thec-dimension at the phase transition temperature (-175°C) was notrecognized within the experimental accuracy. The above agreementimplies that the orthorhombic crystal of urea-polyethylene complex hasnot only the similar host lattice structure but slso the similarmolecular orientation of the guest about the tunnel axis, to those inurea-n-hexadecane complex; in the latter complex4 ), the host latticeis deformed gradually with lowering the temperature by such aanisotropic thermal expansion that the linear coefficient for a-axisis negative and that for b-axis is positive, as indicated in Eq. 1;then the orientational change of the guest molecules, whose overalrotation about the chain axis is forbidden by the relatively highenergy barriers, proceeds cooperatively with the deformation and withthe zigzag plane being almost parallel to the long axis of theellispe-shaped cross section as shown in Fig. 3. In structuralstudies of the complex13 ), on the other hand, we have indicated bythe X-ray and potential energy analyses that the hexagonal form ofurea-polyethylene complex is characterized by the overall rotation ofthe guest molecule around the long axis.In Tab. 2, integral breadths of X-ray line profiles of urea-polyethylene complex and those of urea-n-paraffin complex betweencorresponding equatorial reflexions about orthorhombic and hexagonalcrystal forms are compared; these were evaluated after theelimination of the ~,d~ doublet by Keating's method14 ). In eachcomplex, we cannot recognize the difference between crystal typeswithin the experimental errors; smaller breadths for all reflexionsin urea-n-paraffin complex will be due to a larger crystallite size.Then, it is expected that in the orthorhombic form, there is nogeneration of lattice distortion to cause the line broadening,even when longer polyethylene molecules are enclosed as guests. Inthe melt-crystallized polyethylene15 ), the lattice distortion ofequatorial direction increased with lowering the temperature, indi-cating the increase in the broadening of X-ray line profile. In theOrder-Disorder Transition in Urea-Polyethylene Complex 9Orthorhombic(Low-Temp. Form)----Hexagonal(High-Temp. Form)Figure 3. Schematic representation of structural changeof the urea complex in the solid-phase transition. Theorthorhombic and hexagonal host lattices are symbolizedby ellispes and circles, respectively. The planarzigzag molecules are denoted by lines.Table 2. Comparison of integral breadths(degree) ofX-ray line profiles between hexagonal and orthorhombic,for urea-n-paraffin complex and urea-polyethylenecomplex6 ). The values for the former complex aregiven in the parentheses.Hexagonal (23°C)(hk·O) BreadthOrthorhombic (-190·C)(hkO) Breadth(11.0)(20.0)0.32 (0.25)0.32 (0.25)(200)(130 )(220)(040)0.29 (0.25)0.33 (0.23)0.34 (0.22)0.34 (0.24)10 Fumiyoshi YOKOYAMA and Kazuo MONOBEurea-poly-ethylene complex, on the contrary, the accumulation ofdistortion energy of lattice in a low temperature is avoided throughthe cooperative deformationn process mentioned above. The narrownessof transition given in the fifth column in Tab. 1 suggests that thetransition is strongly cooperative irrespective of chain length. Thereason why the transition for urea-n-hexadecane complex is very sharpis now obscure.Since the equatorial reflexions of urea-polyethylene complexoriginate from the combined structure factor of both host and guestlattices, the X-ray diffraction line profiles invlove the informationabout degrees of oderings of the combined lattice. The conformationof polyethylene molecules around which the host urea lattice isconstructed should be strictly related to disorders of the complexlattice. In this respect, the results given in Tab. 2 suggest thatpolyethylene molecules in the orthorhombic complex are as fairlyaccommodated to the tunnel cavity in the form of extended-chains, muchas in the hexagonal complex.4. CONCLUSIONSThe solid-phase transition of urea-polyethylene complex wasrevealed by thermal analysis and X-ray powder diffractometry, and wehad the following conclusions.1) Urea-polyethylene complex exhibited the phase transition betweenorthorhombic (low-temperature form) and hexagonal (high-temperatureform), at-BoC. The heat and entropy of transition were about 0.5cal/g and 0.002 cal/deg g, respectively, which were on the same orderas those in urea-n-paraffin complexes.2) The transition has been regarded as an order-disorder transitionof the guest molecule, accompanied by the structural change in thehost urea lattice; in the orthorhombic form, the guest molecule hasa definite orientation about the tunnel axis, and while in thehexagonal form, its overall rotation around the axis is facilitated.3) The existence of the transition of urea-polyethylene complexsuggested that even a long polyethylene molecule has a planar zigzagconformation as a whole in an urea tunnel.4) No generation of the host lattice distortion near liquid nitrogentemperature implied that the guest polyethylene molecules in theorthorhombic complex are as fairly accommodated to the tunnel cavityOrder-Disorder Transition in Urea-Polyethylene Complexin the form of extended-chains, much as in the hexagonal complex.5) The narrowness of transition suggested that the transition isstrongly cooperative regarding structural changes in the host andguest lattices, irrespective of chain length.REFERENCES1) R. C. Pemberton and N. G. Parsonage, Trans. Faraday Soc., 61(1965),2112.2) R. C. Pemberton and N. G. Parsonage, Trans. Faraday Soc., 62(1966), 553.3) Y. Chatani, Y. Taki and H. Tadokoro, Acta Crystallogr., B33(1977), 309.4) Y. Chatani, H. Anraku and Y. Taki, Mol. Cryst. Liq. Cryst., 48(1978), 219.5) A. E. Smith, Acta Crystallogr., 5 (1952), 224.6) F. Yokoyama and K. Monobe, Polymer Letters, 19 (1981), 91.7) K. Umemoto and S. S. Danyluk, J. Mol. Phys., 17 (1976), 3757.8) Y. Hori, T. Tanigawa, S. Shimada and H. Kashiwabara, Polymer J.1113 (1981), 293.9) F. Yokoyama and K.10) P. E. Slade and L.Evaluation, vol 2,Dekker, New York,11 ) M. G. Broadhurst,241.·Monobe, Polymer, 21 (1980), 968.T. Jenkins, "Techniques and Methods of PolymerThermal Characterixzation Techniques", Marcelp.9, 1970.J. Research Natl. Bur. Standards, 66A (1962),12) Y. Chatani, H. Anraku and H. Tadokoro, Polym. Prepr. Jpn., 26,1306 (1977).13) F. Yokoyama and K. Monobe, Polymer, 24 (1983), 149.14) D. T. Keating, Rev. Sci. Instrum., 30 (1959), 725.15) A. Kawaguchi, Dr. Thesis, Kyoto Univ. (1980).

Order-Disorder Transition in Urea-Polyethylene Complex

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