Single crystals of two crystatalline forms, α and γ, in nylon 12 were obtained from dilute solutions. The single crystals of γ-form from diols solutions are ribbon like lamellae in shape, and that of α-form obtained from solution in water-formic acid are parallelogramic. Electron diffraction study shows that chain packings in single crystals of two crystalline forms are very similar to those of nylon 6, respectivel

Fujiwara, Yasuo

Monobe, Kazuo

Okayama University Scientific Achievement Repository

MEMOIRS OF THE SCHOOL OF ENGINEERING, OKAYAMA UNIVERSITY, Vol. 2, No.1, APRIL, 1967A Study on single Crystals of Nylon 12Kazua MONOBE and Yasua FUJIWARADepartment of Industrial Chemistry(Received December 28, 1966)Single crystals of two crystatalline forms, a and "'I, in nylon 12 were obtainedfrom dilute solutions. The single crystals of "'I-form from diols solutions areribbon like lamellae in shape, and that of a-form obtained from solution inwater-formic acid are parallelogramic. Electron diffraction study shows that chainpackings in single crystals of two crystalline forms are very similar to those ofnylon 6, respectively.§ 1. IntroductionIt is well known that the crystalline state inbulk polymers plays an important role to thephysical properties of the matter. The crystal-line structures of nylons have been investigatedby several workersl - 4) because of their commer-cial importance and varieties of crystallinestructures.The usual a- and 13-forms1) of nylon 6 areeasily converted to a new crystalline form whentreated with iodine2,31. This new crystallineform was called the r-form by Kinoshita4). Hereported also that in the series of nylons fromw-amino acids with even numbers of carbon,nylons 4 and 6 usually contain the a- and 13-forms, but nylons 8 and 10 appear in the r-form.The usual r-form of nylons 8 and 10 can beconverted into the a-form by phenol treatment.Thus, it is easy to obtain the a ((3) and T-crys-talline forms in fibers of nylons 6, 8 and 10.Geil51 obtained at first lozenge single crystalsof the r-form of nylon 6 from dilute solution inglycerine, and Nagai et a1.6, 7) obtained paral-lelogramic single crystals of the a-form fromdilute solution in water.The present authors studied on the fibersof nylon 12 by x-ray method and recognizedthat the x-ray fiber diagrams showed thecharacteristics of the r-form and the r-formcould not be easily converted to the a-form byphenol treatment.During the investigation of single crystals ofnylon 12, we are interested in following points;(a) whether the single crystal of nylon 12 of a-form can be obtained or not, (b) what are the fac-tors to cantroll the growth of these two crystal-line forms. This paper deals with the results of88our study on nylon 12 single crystals by electronmicroscopy and electron diffraction method.§ 2. Materials and Experimental TechniquesThe material used in this study was unfrac-tionated nylon 12 and the solvents used weretriethylene glycol, water and water-formic acid.Single crystals were prepared by the followingtwo methods.(a) A small amount of nylon 12 was dissolvedin triethylene glycol at 180°C at concentrationranging from 0.01 to 0.001% and was preci-pitated in the form of single crystal by coolingslowly or by maintaining the solution at suitablecrystallization temperature (170-150°C).(b) A small amount of the polymer was sealedinto a glass tube with water or water-formicacid. The tube was heated in a electric furnanceat 250°C until the polymer was dissolved inhot solvents and was cooled slowly.Drops of resulting crystal suspension wereplaced on carbon-coated grids and the solventwas evaporated in vacuo at 80-120°C. Thespecimens were shadowed with Pt-Pd and ex-amined by direct transmission in ]EM-6A elect-ron microscope. Selected area electron diffrac-tion works were carried out with the sameinstrument. Calibration of the diffraction spotswas made with the aid of a thin layer of almi-nium on the same specimen.§ 3. Results(1) MorphologyIt was found that the crystal habit is verystrongly dependent upon the nature of solvents.A lamellar crystal obtained from sJlution intriethylene glycol and its selected area electrondiffraction pattern are shown in Fig. 1. TheA Study on single Crystals of Nylon 12 89lamellar thickness deduced from shadow angleis about 80A. The overgrowth depending uponthe screw dislocation is observed on both sidesof lamella. Both of mother crystal and daughtercrystal show linear long edge and short zigzag,which have an obtuse angle in larger crystal.At higher concentration, single crystals showthe remarkable difference of the growth ratebetween long edge and short one. Thus theygrow as ribbons.Two types of single crystals, which are diffe-rent in morphology and structure, were ob·tained from water and water-formic acid solu-tion. The first type shown in Fig. 2 is lozenge-like parallelogram and is similar to the r-formsingle crystals of nylon 6 reported by Geil6) andNagai et a1.6) The lamellar thickness is about3OOA, and the another electron micrographsshow that these lamellae are stacks of thinnerlamellae.Fig. 3 shows another type of single crystalfrom water-formic acid solution and its selectedarea electron diffraction pattern. The singlecrystal is very thin lamella and no dislocationwas observed on the lamella. The longer edgesof parallelogram are linear and have somegrains, but shorter ones are zigzag.(2) Electron DiffractionSelected area electron diffraction patternshown in Figs. 1 and 2 are fully interpreted,when the same structure of the r-form as innylon 6 is adopted. That is, the four of the sixinnermost spots with spacing 4.1A, and thetwo with spacing 4.0A will be assigned toreflections of the plane of [(001) and (201)J and(200) respectively, and remaining ones are otherequatorials.Fig. 4 shows a schematic representation ofthe diffraction patterns with respect to thesingle crystals. a (a*) and c (c*) are the real(reciprocal) axis based on a modeI8). Thisdiagram shows that the long edge of ribbon·like lamellae from triethylene glycol solutioncorresponds to (200) plane, and the hydrogenbonded planes are parallel to this direction.In the r-form crystal it should be noticedthat hydrogen bonds are formed betweenparallel segments, and that the fold~ng of seg-ments are permitted in (001) and (201) planes.Thus the shorter zigzag edge of the ribboncorresponded to (001) plane can be the growingface, but the longer linear edge assigned to(200) planes can not be the fold planes. Thegrowth of longer edge will be assumed to depe~dupon either alternate folding in (001) and (201)."f' .'Fig. 1 Electron micrograph of a single crystal of nylon 12 (1·form) crystallized from dilutesolution in triethylene glycol and its corresponding electron diffraction pattern. Remarkablespiral growth centered on screw dislocations. X 7.500.90 K. MONOBE and Y. FUJIWARA (Vol. 2Fig.2 Electron micrograph of a lozenge single crystal of nylon 12 ('I-form) crystallized fromdilute solution in water and its corresponding electron difTraction. Rings in the electrondilJraction patterns are the reflections from thin evaporated layer of Al used for references.Calculated spacings agree with those of single crystals from the solution in triethyleneglycol. X7,500.•Fig. 3 Electron micrograph of a parallelogramic ingle crystal of nylon 12 (a-form) crystallizedfrom dilute solution in water-formic acid and its corresponding electron diffraction pattern.X13,OOO.1967) A Study on single Crystals of Nylon 12Fig. 4 Schematical representation of two morphologically different single crystals of 'l'-formand their reciprocal lattice. Dots show observed reflections in the electron diffraction patterns.91planes or the stacks of (001) fold planes.On the other hand, in the case of crystals fromwater and water-formic acid solutions, theminor axis of the lozenge corresponds to c axisand is parallel to the hydrogen bonded plane(200). The four edges are assigned to (001)and (201) planes. Two pairs of edges of thiscrystal have not equal length. This fact suggeststhat they are not same crystallographic plane,and that the growth rates are different betweenthese two planes.We shall consider that the shape of 7-formsingle crystals of nylon 12_depends on thegrowth rates in (001) and (201) planes. In thecase of lozenge type crystals, these two growthrates are nearly equal, while in the ribbon likecrystals the growth r~te of (001) plane islarger than that of (201) plane. These diffe-rences depend on the nature of solvents andother crystallization conditions.A selected area electron diffraction patternfrom a parallelogramic crystal obtained fromwater-formic acid solution is shown in Fig. 3.The spacings of the strong six reflections are4.5A, 3.7 A and 3.5A, which agree with thespacings of (200), (002) and (202) planes of IX(or S) form of nylon 6 proposed by BunnI),respectively. The correlation between the mor-phology and selected area electron differactionpattern in Fig. 5 shows that the long and shortedges of r:arallelogram correspond to (002) and(200) planes, respectively. These relationsagree with those of nylon 6 by us9). As thechain folding is possible in (002) and (202)planes in the IX-form, the similar questionsto the 7-form arise. The long edge can be thefold plane. On the other hand, the shortedge cannot be the fold plane as in the case of(200) plane in the 7-form. The growth of thisface, therefore, depends upon either alternatefolding in (002) and (202) planes or stacks of(002) fold planes only.§ 4. DiscussionTwo kinds of crystal modification in nylon12, IX- and 7-forms, were obtained in the formof single crystals. Electron diffraction patternshows that the side by side packing of molecularchains in these two single crystals closelyresemble with those of nylon 6, respectively.Although we did not succeed to obtain theIX-form crystals in nylon 12 by phenol treat-ment on fiber, we obtained them in the formof single crystals from dilute solution in water·formic acid. From various kinds of diol solvents,however, we obtained only the 7-form single92 K. MONOBE and Y. FunwARAaFig. 5 Schematical representation of a paraUelogramic single crystal of a·form and its re-ciprocal latticecrystals.Nylon 12 has longer C''!2 chains betweenamide groups than nylon 6. The stable crys-talline form of nylon 12 is usually the r-form,while the a-form is more stable than the r-formin nylon 6. The longer CH2 chains per therepeating unit in nylon homologue decrease theeffect of the hydrogen bonding and increasethe influence of the intermolecular van derWaals force. This seems to be the reason whythe r-form is stable in the even nylons whichhave longer CH2 chains in the repeating unit.We used formic acid aqueous solutions assolvent for the a-form single crystals of nylon12. We shall suggest that polar solvent such asformic acid aqueous solution plays an importantrole to produce a-crystalline form in nylon 12.References1) D. R. HOLMES, C. W. BUNN and T. D. SMITH:]. Polymer Sci., 19, (1956) 401.2) S. UEDA and T. KIMURA: Kobunshi Kagaku,15. (1958) 243.3} M. TSURUTA, H. ARIMOTO and M. ISllIBASHI:Kobunshi Kagaku, 15, (1958) 619.4} Y. KINOSHITA: Makromol. Chem.. 33, (1959) 1.5} P. H. GEIL: ]. Polymer Sci., 44, (1960) 449.6} M. OGAWA, O. YOSHIZAKI, T.OTA and E. NAGAI:]. Polymer Sci., Bl. (1963) 57.7} E. NAGAI and M. OGAWA: ]. Polymer Sci., B3.(1965) 295.8) H. ARIMOTO, M. ISHIBASHI and M. HIRAI: ].Polymer Sci., A3, (1965) 317.9} N. HIRAI, and K. MONOBE: ]. Chem. Soc. lap.pure Chem. Sect. 86, 460 (1965)10} F. J. BALTA CALLEJA and A. KELLER: ]. PolymerSci., A2. (l964) 2151.

A Study on single Crystals of Nylon 12

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