Temperature-induced changes in chain-folded lamellar crystals of aliphatic polyamides. Investigation...

Temperature-Induced Changes in Chain-Folded LamellarCrystals of Aliphatic Polyamides. Investigation ofNylons 2 6, 2 8, 2 10, and 2 12

N. A. JONES, S. J. COOPER, E. D. T. ATKINS, M. J. HILL, L. FRANCO

H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK

Received 17 June 1996; revised 16 September 1996; accepted 20 September 1996

ABSTRACT: Four members of the even-even nylon 2 Y series, for Y Å 6, 8, 10, and 12,have been crystallized in the form of chain-folded lamellar single crystals from 1,4-butanediol and studied by transmission electron microscopy (imaging and diffrac-tion) , x-ray diffraction, and thermal analysis. The structures of these 2 Y nylons aredifferent from those of nylon 6 6 and many other even-even nylons. At room tempera-ture, two strong diffraction signals are observed at spacings 0.42 and 0.39 nm, respec-tively; these values differ from the 0.44 and 0.37 nm diffraction signals observed fornylon 6 6 and most even-even nylons at ambient temperature. Detailed analyses ofthe diffraction patterns show that all these 2 Y nylons have triclinic unit cells. Thediamine alkane segments of 2 Y nylons are too short to sustain chain folds; thus, thechain folds must be in the diacid alkane segments in all cases. On heating the crystalsfrom room temperature to the melt, the triclinic structures transform into pseudo-hexagonal structures and the two diffraction signals meet at the Brill transitiontemperature which occurs significantly below the melting point. The room tempera-ture structures of these 2 Y nylons are similar to the unit cell of nylon 6 6 at elevatedtemperature, but below its Brill temperature. The room temperature structures andbehavior on heating of the nylon 2 Y family is noticeably different from that of theeven-even nylon X 4 family, although the only difference between these families ofpolyamides is the relative disposition of the amide groups within the chains. Theresults show that in order to understand the structure, behavior and properties ofcrystalline nylons, especially as a function of temperature, the detailed stereochemis-try needs to be taken into account. q 1997 John Wiley & Sons, Inc. J Polym Sci B: PolymPhys 35: 675–688, 1997Keywords: nylons; lamellar crystals; diffraction; Brill transition temperatures

INTRODUCTION of lengths which differ between the various ny-lons. Nylons of the 2 Y series have only two meth-ylene units in the diamine alkane segments, asAliphatic polyamides, or nylons, are important in-shown in Figure 1. The length of the diacid alkanedustrial materials, valued for strength and pro-segment is in the range 4–10 methylene unitscessability. The chains contain amide groups, sep-for the nylons reported here. Directionally specificarated, at regular intervals, by alkane sequencesinterchain hydrogen bonds are formed betweenneighboring amide groups and it is these hydro-

Correspondence to: E. D. T. Atkins gen bonds which give the nylons improved physi-Contract grant sponsor: EPSRC cal properties and substantially enhanced meltingContract grant sponsor: EU Human Capital Mobility Pro-

points in comparison with those of the polyolefinsgramand polyesters.q 1997 John Wiley & Sons, Inc. CCC 0887-6266/97/040675-14

675

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676 JONES ET AL.

the case of nylon 6 6 single crystals, hydrogenbonds can be linear only if the chains progres-sively shear within each hydrogen-bonded sheet,as illustrated in Figure 2, and this is the structurefound experimentally.1–3 This hydrogen bondingpattern will give linear hydrogen bonds for alleven-even nylons.7,8 However, in some instances,for example, nylons 4 65 and 6 86 a different ar-rangement of hydrogen bonds—a nonprogressivescheme—can give linear hydrogen bonding, in ad-dition to the progressive scheme generally found.This is a consequence of the matching in lengthof the diamine and diacid alkane sequences whichtakes place in 2n 2(n / 1) nylons. In lamellarsingle crystals of nylons 4 6 and 6 8 the nonpro-gressive hydrogen bonding pattern has beenfound to occur within the sheets.5,6

Lamellar crystals of all the even-even nylonslisted above are found in hydrogen-bonded sheetsat room temperature. The sheets stack together,and are held by van der Waals forces. It is foundthat they can either stack with a progressiveshear; nylon 6 6 behaves in this way,1 as do ny-lons 4 4, 6 4, 8 4, 10 4, and 12 4,7 or stack in analternating fashion, one sheet up parallel to thechain axis, the next sheet down; nylons 4 6,5,6 68,6 have this structure. In some nylons we findboth the progressive intersheet shear (termed a-Figure 1. Computer-generated space-filling models

of nylons 2 6, 2 8, 2 10, and 2 12. The nitrogen atoms phase) and the staggered intersheet shearare labelled on nylon 2 12 only. The diamine alkane (termed b-phase). Lamellar single crystals of ny-segment (labelled) contains only two CH2 groups and lons 4 8, 4 10, 4 12, 6 10, 6 12, 6 18, 8 12,8 andis too short to form a hairpin fold. Note that the chains 12 109 have been found to have both structures.have no polarity in these synchephalic even-even ny- For these lamellar single crystals consisting oflons, i.e., they are apolar. Color code: hydrogen, white; chain-folded, hydrogen-bonded sheets, held to-carbon, pale grey; nitrogen, medium grey; oxygen, dark

gether by van der Waals forces, two strong andgrey.characteristic diffraction signals are observed atspacings of 0.44 and 0.37 nm; they represent theprojected interchain distance within a hydrogen-Nylon 6 6 has been extensively investigated

(e.g., refs. 1–4) because of its commercial promi- bonded sheet (actual value 0.48 nm) and the in-tersheet distance, respectively. However, all thesenence, but, in comparison, there has been rela-

tively little work on other nylons. There has been nylons exist in a different crystal phase at hightemperatures.a tendency to assume that all other even-even ny-

lons will behave in a similar manner to nylon 66. It is known that as nylon 6 6 crystals are heatedthe two characteristic diffraction signals move to-However, recent studies of lamellar single crys-

tals of nylons 4 6,5,6 6 8,6 4 4, 6 4, 8 4, 10 4, and gether and meet.10 The Brill transition tempera-ture, TB , (named after Brill, who first observed12 47 and 4 8, 4 10, 4 12, 6 10, 6 12, 6 18, and 8

128 have shown that although these materials are this behavior with x-rays in nylon 6 6 fibers; theterm ‘‘transition’’ is commonly used, but it is notsimilar to nylon 6 6 in some respects, there are

noticeable differences. certain that it is a thermodynamic transition) isdefined as the lowest temperature for which theThe requirement that the hydrogen bonds be-

tween adjacent chains should be linear (or close spacings of the two signals are identical, abovethe Brill temperature the single spacing increasesto linear) is a crucial feature in determining the

crystal structure adopted by aliphatic polyamides. slightly as a consequence of thermal expansion.(This single diffraction signal can also change inThe pattern of hydrogen bonds that develops is a

function of the stereochemistry of each nylon. In sharpness and relative intensity indicating a


TEMPERATURE-INDUCED CHANGES IN POLYAMIDES 677

steady improvement in the structural organiza- toward the pseudo-hexagonal phase on heating.In some cases the Brill temperature is well belowtion of the pseudo-hexagonal lattice as a function

of temperature above TB .) The high temperature the melting point (nylons 6 6,4,10 4 6,6 6 8,6 4 4,6 4, 8 4, 10 4, and 12 47), for others the Brillphase of nylon 6 6 is pseudo-hexagonal.4,10

All the even-even nylons discussed above move temperature and the melting temperature are co-incident (nylons 4 8, 4 10, 4 12, 6 10, 6 12, 6 18,and 8 128). It has been shown that crystals ofnylons 6 8,6 6 6,4 and the even nylon 811 (whichhas a structure similar to that of nylons 4 6 and6 8) grow in the pseudo-hexagonal form at hightemperature. On cooling, the crystal structuresrevert to the higher density room temperaturestructures; triclinic (for nylon 6 6) or monoclinicfor nylons 6 8 and 8.

There is an approximate linear relationship be-tween the hydrogen bond density along the chainand melting temperature.12 As the polymer isheated the alkane chains become mobile and areeffectively molten by about 1407C. There is roomfor considerable movement of the alkane seg-ments because the hydrogen bonds hold thechains apart (in the ac-sheets; see Fig. 2) at aspacing greater than is observed for alkane chainswith no hydrogen-bonding; for example, polythel-yene. At 1407C and above, the hydrogen bondsstill hold the structure together; although the al-kane segments become increasingly mobile push-ing the sheets apart. If the hydrogen bonds areclosely packed along the chain with only shortalkane sequences between them, the meltingpoint will be higher. It has been observed thatwhen nylons have the same linear density of hy-drogen bonds, e.g., nylons 6 6 and 6, those thatreach their Brill temperatures before meltinghave the highest melting points.13

There is little reported work on 2 Y nylonswhere Y is an even number. Dreyfuss14 synthe-sized the polymers of the present study, amongother nylons, and compared the projected c -repeatspacing with the lamellar stacking periodicity foreach, using wide- and small-angle x-ray diffrac-tion data from sedimented mats. However, nei-ther the unit cells nor their structural changes onheating were studied.

EXPERIMENTALFigure 2. A view orthogonal to a single hydrogen-

Crystallizationbonded sheet (ac-plane) of nylon 6 6, illustrating theprogressive shear of chains by 137. The hydrogen bonds

Samples of nylon 2 6, 2 8, 2 10, and 2 12 wereare indicated with dashed lines. Color code: hydrogensprepared by Dr. P. Dreyfuss in the form of crystaland carbons black; nitrogens and oxygens grey. Thesuspensions in 1,4-butanediol and oriented mats,evidence will show that the 2 Y nylons adopt similarsedimented from the crystal suspensions. Theprogressive shear hydrogen-bonded sheet-like struc-

tures. samples have been stored in the dark; single crys-


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tal mats dry, in bottles and crystals in suspension and 40 mA. The x-ray diffraction patterns wererecorded on film using an evacuated flat-platein 1,4-butanediol. The details of sample synthesis

have been reported previously.14 Crystals of ny- camera. Calcite (dBÅ 0.3035 nm) was dusted ontoselected samples for calibration purposes. Abovelons 2 6, 2 8, and 2 10 were recovered from suspen-

sion by centrifugation and repeatedly washed room temperature an Enraf-Nonius TN-20 set op-erating at 40 kV and 20 mA was used with awith n -butanol. The crystals of nylon 2 12 did not

need washing. Metler FR-52 hotstage for nylon 2 6 only. A point-collimated monochromatized beam from an ElliotGX-21 rotating target x-ray generator with a Lin-

Electron Microscopy kam T300 Hotstage was used for the other poly-mers. The information was collected using a Sie-Samples for Transmission Electron Microscopy

(TEM) were made by placing drops of crystal sus- mens GADDS two-dimensional detector. Small-angle x-ray diffraction patterns were obtained atpension on carbon-coated copper grids. Nylons 2

6, 2 8, and 2 10 were dried (from n -butanol) at room temperature using flat-plate, evacuated,point-collimated x-ray cameras.room temperature. Nylon 2 12 crystals were dried,

from 1,4-butanediol, in a vacuum oven at 1007C.All samples were annealed in the vacuum oven

Computer Modeling and Simulated X-rayfor 24 h at 1007C. Samples were examined in bothDiffraction Patternsimaging and diffraction modes, using a Philips

301 TEM operating at 80 kV. The crystals were The computer graphics, model building, and simu-decorated with platinum/palladium to calibrate lated x-ray diffraction patterns were performedthe diffraction patterns and to shadow the images. on a Silicon Graphics Iris 4D20 machine usingSome crystals were decorated with polyethylene, Cerius, Release 1.6, program (Molecular Simula-following the technique of Wittmann and Lotz.15 tions Inc.) . The Lorentz and polarization correc-Some samples were heated to melting using a hot tion factors were incorporated in the displayedstage. Diffraction patterns were taken at inter- intensities. The degree of arcing and the tempera-vals between room temperature and the melting ture factor were chosen to match the observed x-temperature for each nylon. Relative tempera- ray diffraction photographs.tures were accurate to within {27C and absolutetemperatures to within {57C. The hot stage was

Differential Scanning Calorimetry (DSC)calibrated by observing the melting of substancesof known melting point. Melting temperatures of the sedimented mats were

measured using a Perkin Elmer DSC 7 flushed withnitrogen. The heating rate was 107C/min.X-Ray Diffraction

The sedimented mats were annealed in a vacuumoven for 24 h at 1007C. Wide- and small-angle RESULTSx-ray diffraction patterns were obtained using apoint-collimated beam directed parallel to the sur- Electron Microscopy and Diffractionface of the crystal mat. At room temperature, dif-fraction patterns were obtained using a Ni-fil- TEM imaging shows that the crystals of nylons 2

6, 2 8, 2 10, and 2 12 are lath-like, similar totered CuKa radiation from a Phillips PW2213sealed beam x-ray generator operating at 35 kV crystals of nylon 6 6 and many other nylons (com-

Figure 3. Transmission electron micrographs of lamellar single crystals of nylons 26, 2 8, 2 10, and 2 12. All photographs are printed to the same scale; the scale bar is1 mm. Nylons crystallized from solution in 1,4-butanediol give long sheaves with lath-shaped extremities; (a) 2 6, (b) 2 8, (c) 2 10, and (d) 2 12. (e) Crystals of nylon 2 6prepared as in (a) but decorated with polyethylene using the method of Wittmann andLotz.15 The polyethylene fibrils are distributed randomly on the supporting carbon film,but orient normal to the long faces of the individual lamellar crystals. This indicatesthat the fold planes are aligned parallel to the long axis of the lamellae, which corre-sponds to the hydrogen bond direction deduced from electron diffraction. (f ) Electrondiffraction pattern obtained from nylon 2 12 lamellar single crystals. The a-phasecrystals give rise to a strong pair of 100 diffraction signals at 0.414 nm spacing.


680 JONES ET AL.

Table Ia. Comparison of Electron and X-ray Diffraction Spacings, in Nanometers, for the Nylon 2 Y Family

Electron Diffraction Wide-angle X-ray DiffractionSmall-

Index 100 010/110 100 010/110 001 002 angle

Nylon Spacing Spacing Spacing Spacing Spacing Spacing Spacing{ 0.003 { 0.003 { 0.003 { 0.003 { 0.02 { 0.01 { 0.04

2 6 0.420 0.390 0.419 0.390 1.02 0.51 5.462 8 0.424 0.395 0.423 0.387 1.22 0.62 5.482 10 0.425 0.391 0.423 0.388 1.47 0.73 5.712 12 0.414 0.388 0.414 0.388 1.71 0.86 6.74

pare Figures 3a–d with figures in refs. 5–10). tron diffraction data in Table Ia. Figure 4a showsthe wide-angle diffraction photograph obtainedThe Wittmann and Lotz polyethylene decoration

technique15 shows that the chain-folded sheets from mats of nylon 2 12. The prominent 001 diffrac-tion signal is centered on the meridian (a weak 002run parallel to the long dimension of the crystals;

this is also the direction of the hydrogen bonds. is also seen). In addition a single weak subsidiarymaximum is observed2 at 3.4 nm. The 100 and 010/An example is shown in Figure 3e.

A representative TEM diffraction pattern, from 110 diffraction signals at spacings 0.414 and 0.388nm, respectively, are split either side of the equator;a single crystal, is shown in Figure 3f. The pat-

terns from all the 2 Y nylons in the group under 57 for the 100 and 307 for the 010/110 combination).A similar x-ray diffraction pattern was obtainedexamination are similar. They show a pair of

strong diffraction signals at spacing É 0.42 nm. from nylon 2 10 sedimented mats. Figure 4b showsthe x-ray diffraction photograph from sedimentedIf a lamella is tilted, or in cases where lamellar

aggregates occur in which some lamellae are nat- mats of nylon 2 8. The relative intensities of the001 and 002 are different from that of nylon 2 12urally tilted, an additional diffraction signal at

spacingÉ0.39 nm is observed. These spacings are (Fig. 4a); the 002 is relatively stronger comparedwith the 001. Furthermore, the 001 is actually splitcloser together than the corresponding values of

0.44 and 0.37 nm, respectively, for other even- into two peaks; a strong inner and weak outer. Inaddition, two subsidiary maxima occur between theeven nylons with longer diamine alkane seg-

ments.1–11 The observed spacings for the 2 Y ny- 001 and 002 diffraction signals.2 Two more subsid-iary maxima also occur between the 001 and thelons are collated in Table Ia. These results indi-

cate that the crystals have triclinic structures.3,7 origin; the second of which is visible between theinner 001 peak and the beam stop in Figure 4b. Theinterlamellar stacking periodicity, at a measured

X-ray Diffraction spacing of 5.48 nm (see Table I), is shown as aninsert in the lower left-hand corner. The spacingsThe x-ray diffraction patterns from sedimented

mats are in overall agreement with the electron of the meridional diffraction signals for nylon 2 8are listed in Table II. A similar x-ray diffractiondiffraction results. The unit cell parameters are

given in Table Ib and the comparison with the elec- pattern was obtained from nylon 2 6.

Table Ib. a-Phase Triclinic Unit Cell Parameters for the Nylon 2 Y Family

Nylon 2 6 2 8 2 10 2 12

a { 0.01 nm 0.49 0.49 0.49 0.49b { 0.01 nm 0.53 0.53 0.51 0.51c { 0.01 nm 1.23 1.48 1.73 1.98a { 27 56 56 58 60b { 17 76 77 77 75g { 17 59 60 60 59Angle between chain axis

and lamellar normal(for nylon 6 6 Å 427) 347 347 327 307



Figure 4. Wide-angle x-ray diffraction photographs obtained from sedimented matsof two of the nylons investigated with the main diffraction signals labelled. Note theangular splitting of the 100 and 010/110, especially the latter, above and below theequator indicating progressive shear between chains within a hydrogen-bonded sheetand intersheet shear, respectively. (a) Nylon 2 12; note the 001 is substantially strongerthan the 002 (which is hardly visible in this exposure). An x-ray diffraction patternwith similar features was obtained from nylon 2 10. (b) Nylon 2 8; the meridional regionis rich with diffraction information; between the split 001 (indicated by two dashes)and 002 there are two subsidiary maxima (marked ‘s’ ) indicating four repeats withinthe chain-folded lamellar crystals.2 Subsidiary maxima also occur in the region betweenthe 001 peak and the origin; one of these (marked ‘s’ ) is visible between the inner 001peak and the beam stop. The 5.48 nm small-angle diffraction signal, representing theinterlamellar stacking periodicity, is shown as an inset in lower left-hand corner. Anx-ray diffraction pattern with similar features was obtained from nylon 2 6.

Changes on Heating 5a,b for nylons 2 6 and 2 12; related data for ny-lons 6 6 and 4 4 lamellae are shown in FigureChanges in the strong spacings of nylon lamellar 5c,d. Each of the nylons tested exhibits a Brillcrystals as a function of increasing temperature, temperature. Beyond TB the spacing of the singlemeasured using both wide-angle x-ray diffraction peak did not change apart from the slight increaseand electron diffraction, are recorded in Figure owing to thermal expansion. Note that once thespacings are equal they do not move through each

Table II. Measured Spacings, in Nanometers, of the other but remain equal. The Brill temperaturesMeridional Series for Nylon 2 8 (from wide-angle x-ray diffraction and electron

diffraction) and melting temperatures (fromSmall-angle 5.48 DSC) for the four nylon 2 Y polymers are listed

in Table III.001 (split) inner 1.34

outer 1.10DISCUSSIONsubsidiary 1 0.90

subsidiary 2 0.76 The electron diffraction patterns of the nylon 2 Y002 0.61 family, at room temperature, usually show only


682 JONES ET AL.

Figure 5. Graphs showing variations in the 100 and 010/110 spacings as the nylonsingle crystal lamellae are heated from room temperature to the melt for: (a) Nylon 26, (b) nylon 2 12, (c) nylon 6 6, and (d) nylon 4 4. j Wide-angle x-ray diffraction data;h Electron diffraction data. TB is the temperature where the two spacings meet.



Figure 5. (Continued from the previous page )

100 diffraction signals. The absence of other hk0 sively,1 so that the sheets incline at a substantialangle to the lamellar surface normal. This is con-diffraction signals, the 010 in particular, indicates

that the polymer chains are substantially tilted firmed by the wide-angle diffraction pattern ineach case, since the strong 0.39 nm (010) diffrac-from the lamellar surface normal, consistent with

a triclinic unit cell. Like the nylon 6 6 a-phase tion signal (intersheet spacing) occurs in therange 30–347 off the equator. In each case, thestructure, the chain-folded sheets shear progres-


684 JONES ET AL.

Table III. Thermal Data and Brill Spacings for the sory level by dividing the appropriate d001 value2 Y Nylon Family into the interlamellar stacking value to find the

largest integer; thus for nylon 2 8 we have: 5.48Melting nm/1.22 nm Å 4.5, i.e., four repeats plus space for

Brill Brill Temperature folds and nonperfect interlamellar packing. SuchTemperature Spacing (single crystals) a small number of repeats in a lattice gives riseNylon {57C {0.003 nm {27C

to observable subsidiary maxima.2 The diffractionrule is that for N repeats there are (N0 2) subsid-2 6 174 0.415 315iary maxima; thus, in the case of nylon 2 8 (and2 8 177 0.409 294nylon 2 6) this means two subsidiary maxima.2 10 181 0.427 280

2 12 158 0.419 262 These can be seen on the meridian in the x-raydiffraction photograph shown in Figure 4b. Sincethe principal scattering planes that generate the00l meridional series (and their associated sub-d001 is substantially less than the chemical repeat

distance, c , of the polymer, confirming that the sidiary maxima) emanate from the oxygen atomsin the amide carbonyl groups, the four repeatschains are tilted to the chain-folded crystal sur-

face. As for other even-even nylons, the chain-fold are basically four pairs of scattering planes2 asindicated on the actual nylon 2 8 chain-foldeddirection is parallel to the long dimension of the

lath-like lamellar crystals. structure shown in Figure 6. The salient featuresof the meridional diffraction pattern from such aThe chains must fold in an alkane segment in

order to maintain the intrasheet hydrogen-bond- limited lattice (four repeats convoluted with apair of scattering planes) are illustrated in Figureing pattern (see Fig. 2). This is true for all even-

even nylons with triclinic unit cells.1,7 Alkane seg- 7. This also shows why, in this particular nylon,the 001 is split into two unequal (stronger inner/ments with less than four methylene units are

not able to form (sharp, adjacent re-entry) chain weaker outer) peaks.Comparison of the experimental x-ray diffrac-folds;7,16 thus, the diamine alkane segments of all

tion photograph, shown in Figure 4b, with thethe nylons of the 2 Y series (with only two methyl-simulated x-ray diffraction pattern (Fig. 8b) forene units) are certainly too short to sustain chainour proposed structure of the nylon 2 8 chain-folds; hence all the chain folds must be in thefolded lamellar crystal shows the fit is good. Thediacid alkane segments.structure of nylon 2 6 is similar to that of nylonThe unit cells of nylon 2 Y single crystals at2 8 apart from the contraction in the c -dimension.room temperature, calculated from the diffraction

In the case of nylon 2 12 only three repeats occurdata in Table Ia, are listed in Table Ib. The spac-in the chain-folded lamellar and one subsidiaryings of the 100 and 010 diffraction signals differmaximum occurs. Because the pairs of carbonylby 0.03 nm for the nylon 2 Y structures, and theoxygen atoms are relatively much closer togethervalues of a lie within {27 of 587. This should bethan for nylon 2 8 the 002 diffraction signal is sup-compared with the data for nylon 6 6, which haspressed and only just observable; thus, its subsid-a 0.07 nm difference between the 100 and 010 spac-iary maximum is even weaker and not observedings and a Å 48.57. The distance corresponding toin Figure 4a. However a single weak subsidiarythe 001 diffraction signal means that the chainsmaximum is observed at 3.4 nm in the small-anglelie at angles of 30–347 (see Table Ib) to the normalregion. The simulated x-ray diffraction pattern forof the chain-folded surface, whereas, in the nylonnylon 2 12 is shown in Figure 8a, which should6 6 a-phase triclinic the angle of tilt is 4271.be compared with experimental x-ray diffractionphotograph in Figure 4a. The structure of nylon 2

Analyses of the X-ray Diffraction Patterns from 10 is similar to that of nylon 2 12 apart from theSedimented Mats contraction in the c-dimension.

Comparison of d001 values and the interlamellarspacing from the small-angle peak, both listed in Behavior on HeatingTable Ia, shows that there are only a small num-ber of crystallographic repeats along the chain di- Changes in melting temperature for all the 2 Y

nylons are in accordance with changes in hydro-rection within each lamella. The four 2 Y nylonsstudied fall into two categories. Nylons 2 6 and 2 gen bond density,12 and all have Brill tempera-

tures between 1587C and 1817C, well below their8 have four crystallographic repeats in their re-spective lamella. This can be ascertained at a cur- melting temperatures (despite the hydrogen bond



Figure 6. Computer-generated structure of a single sheet (ac-plane) of nylon 2 8folding with a periodicity compatible with the experimental data. There are 31

2 straightstem crystallographic repeats plus folds on both lamellar surfaces. This generates alamellar of thickness 4.88 nm, allowing 0.6 nm extra, per lamella, for nonperfect interla-mellar stacking which is consistent with the observed small-angle diffraction features.The four pairs of scattering planes through the carbonyl oxygen atoms are shown. Colorcode as in Figure 1.

density along the chain nearly halving from nylon 6 6. The strong and characteristic 100 and 010/110 spacings of nylon 6 6 hardly change (see2 6 to nylon 2 12). The melting points of the 2 Y

nylons fit in with the generally observed linear Fig. 5c ) as a function of increasing temperatureuntil about 907C, at which temperature the tworelationship between the hydrogen bond density

along the chain and melting temperature.12 How- spacings move rapidly toward each other andmeet at TB . Brill temperatures of between 165ever, this relationship breaks down at very high

bond densities. Nylons with very high hydrogen and 2207C have been reported for nylon 6 6, de-pending on the crystallization conditions. Forbond densities thermally degrade rather than

melt.17,18 Nylons with very low hydrogen bond single crystals of all members of the nylon 2 Yfamily, the two spacings start off much closerdensities (beyond nylon 18) have not been stud-

ied. We would, however, expect these polymers to together and move together gradually fromroom temperature to meet at TB . It has pre-behave more like polyethylene than nylons.

The detailed behavior on heating single crys- viously been shown that, for crystals of nylon8, 11 nylon 6, 19 and nylon 6 6, 4 the measuredtals of each of these 2 Y nylons is different from

that observed on heating single crystals of nylon value of TB is dependent on the crystallization


686 JONES ET AL.

and meet, i.e., the general shape of the curvesshown in Figure 5, is not observed to changewith crystallization conditions.4,11

Although the room temperature crystal struc-tures are different from that of nylon 6 6 andthose of other even-even nylons, the high tem-perature crystal phases of all the 2 Y nylons arepseudo-hexagonal, as are those of other even-even nylons. Nylons 8, 6 8, and 6 6 have beenshown to crystallize in the pseudo-hexagonalphase 4,6,11 and then to revert to the triclinic a-phase, for nylon 6 6, or monoclinic phase, fornylons 8 and 6 8, as they cool. We expect thischange to involve a movement of hydrogenbonds, from a two-dimensional network (a mix-ture of inter- and intrasheet hydrogen bonds) ,in the high temperature pseudo-hexagonalphase, to an intrasheet only arrangement in the

Figure 7. One-dimensional Fourier transform of a four room temperature structure. We believe thatunit repeat of the appropriate scale for nylon 2 8 to illus- the 2 Y nylons crystallize in the pseudo-hexago-trate the salient features of the diffraction. The curve has nal phase as do other nylons. However, on cool-to be mentally multiplied by the dotted curve (Fourier

ing freshly crystallized nylons of the 2 Y group,transform for a pair of scatterers at appropriate scale)it appears that the usual room temperature hy-and the resultant squared to obtain the diffracted inten-drogen-bonded sheet structure is not attained.sity profile. The important ingredient is that the dottedThe room temperature structure of the 2 Y ny-curve has zero value within the 001 peak and thereforelons, with characteristic strong signals at 0.39splits it into two (strong inner/weak outer).and 0.42 nm, lies between the high temperature(characteristic single 0.42 nm signal ) structure

conditions; this may also be the case for nylons and the room temperature (characteristic 0.44of the 2 Y family. However, the way in which and 0.37 nm signals ) found for even-even nylons

with longer diamine alkane segments.the two characteristic spacings come together

Figure 8. (a) Simulated x-ray diffraction pattern of the proposed chain-folded lamel-lar crystal structure for nylon 2 12. The pattern should be compared with the experimen-tal photograph in Figure 4a. (b) Simulated x-ray diffraction pattern of the proposedchain-folded lamellar crystal structure for nylon 2 8. The pattern should be comparedwith the experimental photograph in Figure 4b.



Figure 9. Computer-generated space-filling models of ac sheets of nylons 2 6 (left)and 4 4 (right). The nitrogen atoms are marked on one chain in each. The figurehighlights the equal distribution of the amide groups along the chains in these twonylons; effectively, the only difference is the reversal of the amide groups (interchangeof C|O with N{H). Color code as in Figure 1.

We believe that the hydrogen bonds form a two- lons 2 6 and 4 4 might be thought to be similar,since the overall lengths of the repeating unitsdimensional network in the pseudo-hexagonal

phase in which these materials crystallize, how- and periodic distribution of amide groups arethe same, as illustrated in Figure 9. At roomever, in the case of the 2 Y nylons, these bonds do

not all rearrange on cooling to give the usual temperature the structures of both 2 Y and X 4lamellar crystals are triclinic. The X 4 crystalschain-folded, hydrogen-bonded sheets held to-

gether by van der Waals forces. Some of the in- (which all behave like nylon 6 6 7) show strongdiffraction signals at 0.44 and 0.37 nm (see Fig.tersheet hydrogen bond network still remains at

room temperature. These intersheet hydrogen 5d) , whereas for nylon 2 Y crystals the spacingsare 0.42 and 0.39 nm, respectively (Fig. 5a,b) .bonds hold the chain-folded sheets apart, increas-

ing the distance between 010 planes (and 110 Both nylon families have a pseudo-hexagonalstructure at high temperatures and Brill tem-planes) and reducing the magnitude of the shear

between adjacent sheets; this offers an explana- peratures well below their melting points, butthe details of the way in which the strong sig-tion for the unusual room temperature structure.

The short diamine alkane segment is the only nals move together on heating are different be-tween the two groups (compare Fig. 5a with 5d) .common factor within the nylon 2 Y series, hence

we consider that the unusual room temperature Each of the X 4 nylons has hydrogen-bondedsheets at room temperature, held together bystructures of these nylons, and the details of their

behavior on heating to TB , must be determined van der Waals forces. At room temperaturethe 2 Y nylons have a structure between theby diamine segments in the chains, rather than

the diacid segments. high (pseudo-hexagonal ) and room temperaturestructures of the X 4 nylons. We believe that theIt is instructive to compare the nylon 2 Y fam-

ily with the nylon X 4 family.7 For instance, ny- reason for the difference between the behavior


688 JONES ET AL.

of the 2 Y nylons and the X 4 nylons is, at least planar amide units distributed along thepolymer backbone. This change has sub-in part, a result of the restriction in movement

of the diamine alkane segments. This hinders stantial effects on the structure and the be-havior on heating; thus, the detailed chem-the transition from the high temperature,

pseudo-hexagonal, growth phase, with its mixed istry strongly influences the properties ofnylons.inter- and intrasheet ( two-dimensional net-

work) arrangement of hydrogen bonds, to theS. C. and N. A. J. would like to thank the EPSRC forroom temperature sheet structure ( found forfinancial support. L. F. acknowledges financial supportmany even-even nylons) with intrasheet hydro-from the EU through a grant from the Human Capitalgen bonds only. The differences between the 2Mobility program. We are all grateful to Dr. Pat Drey-Y and X 4 groups of nylons highlights the needfuss who made the nylons used for this study.

to grasp the subtleties in configuration, confor-mation, and distribution of the amide groups inorder to understand the structure of nylons and REFERENCES AND NOTESpredict their properties and behavior as a func-

1. C. W. Bunn and E. V. Garner, Proc. Roy. Soc. (Lon-tion of temperature.don), 189, 39 (1947).

2. E. D. T. Atkins, A. Keller, and D. M. Sadler, J.Polym. Sci., A2, 863 (1972).

CONCLUSIONS 3. V. F. Holland, Makromol. Chem., 71, 204 (1964).4. H. W. Starkweather, J. F. Whitney, and D. R. John-

son, J. Polym. Sci., A1, 715 (1963).Lamellar single crystals of nylons 2 6, 2 8, 2 10,5. E. D. T. Atkins, M. J. Hill, S. K. Hong, A. Keller,and 2 12, were crystallized from 1,4-butanediol

and S. J. Organ, Macromolecules, 25, 917 (1992).and investigated.6. M. J. Hill and E. D. T. Atkins, Macromolecules, 28,

604 (1995).• For these nylons of the 2 Y series, the di- 7. N. A. Jones, E. D. T. Atkins, M. J. Hill, S. J. Cooper,

amine alkane segments are too short to sus- and L. Franco, Macromolecules 29, 6011 (1996).8. N. A. Jones, E. D. T. Atkins, M. J. Hill, S. J. Cooper,tain (adjacent re-entry, hairpin-like) chain

and L. Franco, Polymer ( in press).folds; all the chain folds must be in the diacid9. L. Franco and J. Puiggali, J. Polm. Sci., Part B,alkane segments.

33, 2065 (1995).• The room temperature structure of these ny-10. R. Brill, Z. Physik. Chem., 1353, 61 (1943).lons is different from that of nylon 6 6 and11. E. D. T. Atkins, M. J. Hill, and K. Veluraja, Poly-many other even-even nylons. At room tem-

mer, 36, 35 (1995).perature each nylon in the 2 Y series forms 12. R. S. Williams and T. Daniels, Polyamides, RAPRAcrystals with unit cells that are triclinic, but Review Reports, RAPRA Technology Ltd., 3, 33the characteristic room temperature signals (1990).are at 0.42 and 0.39 nm (compared with 0.44 13. E. D. T. Atkins, Macromolecules ’92, Canterbury,and 0.37 nm for even-even nylons with longer UK, September 1992, Abstract 10; E. D. T. Atkins,

Macromolecular Reports, A31 (Suppls. 6 & 7),diamine alkane segments) and the chains691–694 (1994).are inclined at about 327 to the fold surface

14. P. Dreyfuss, J Polym. Sci., Polym Phys., 11, 201normal, compared with an angle of 427 for(1973).nylon 6 6).

15. J. C. Wittmann and B. Lotz, J Polym. Sci., Polym.• On heating lamellar single crystals of the ny-Phys., 23, 200 (1985).lon 2 Y series, from room temperature to

16. M. A. Bellinger, A. J. Waddon, E. D. T. Atkins, andmelting, the triclinic structures are trans- W. J. Macknight, Macromolecules, 27, 2130 (1994).formed to pseudo-hexagonal structures, with 17. A. Xenopoulos, B. Wunderlich, and J. A. Subirana,Brill temperatures between 158 and 1817C. Euro. Polym. J., 29, 927 (1993).

• Nylons of the 2 Y family differ from those 18. L. Franco, E. Navarro, J. A. Subirana, and J. Puig-of the X 4 family only in the interchange gali, Macromolecules, 27, 4284 (1994).

19. N. A. Jones and M. Hill, to be published.of the C|O and N{H groups within the


Temperature-induced changes in chain-folded lamellar crystals of aliphatic polyamides. Investigation...

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