Synthesis and characterization of nylon 18 18 and nylon 18 adamantane

Synthesis and Characterization of Nylon 18 18 andNylon 18 Adamantane

CARL BENNETT, ETHEM KAYA, ALLISON M. SIKES, WILLIAM L. JARRETT, LON J. MATHIAS

Department of Polymer Science, University of Southern Mississippi, 118 College Drive 10076,Hattiesburg, Mississippi 39406-0076

Received 13 March 2009; accepted 30 April 2009DOI: 10.1002/pola.23494Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Nylon 18 18 and nylon 18 1,3-adamantanedicarboxylic acid (ADA) havebeen synthesized via melt polycondensation and characterized by thermal and spec-troscopic techniques. Good film forming behavior combined with film toughness andflexibility indicate reasonable molecular weights for both. The higher aliphatic con-tent of nylon 18 18 leads to increased resistance to common organic solvents overcommercial nylons. Crystallization of nylon 18 18 combines hydrogen bonding of theamide units with a more significant contribution from van der Waals forces than pos-sible for lower aliphatic content nylons due to the greater aliphatic chain lengths.Solid state 15N CP/MAS NMR indicates a mostly amorphous polymer, with crystal-line regions comprised of the thermodynamically stable a-form generally adopted byeven–even nylons. Nylon 18 ADA as-produced is completely amorphous as deter-mined by differential scanning calorimetry. However, solution cast samples of nylon18 ADA shows some ordered structures that can grow into more stable crystals withannealing. These crystals, once melted, do not recrystallize possibly due to chainrearrangement inhibited by bulky adamantly-groups. VVC 2009 Wiley Periodicals, Inc.

J Polym Sci Part A: Polym Chem 47: 4409–4419, 2009

Keywords: amorphous; annealing; crystal structures; high aliphatic content; nylon

INTRODUCTION

Nylons with low amide density and high methyl-ene content are desirable due to improved proper-ties (such as radiation shielding) and applicationsin thermoplastic processing. These polymers actas ‘‘functionalized polyethylene,’’ combining thehydrophobic character of polyethylene with thethermal properties afforded by the hydrogenbonding amide units of polyamides. While therehave been reports of high aliphatic content ABnylon systems,1,2 there are only a few reports ofhigh aliphatic content AABB nylons with regu-

larly placed amide units, such as nylons X 16,3

nylons X 18,4,5 nylons X 20,6 nylons X 22,7 andnylons X 34.8 Of these, the highest aliphatic con-tent is for nylon 12 34,8,9 which contains 44 meth-ylene units per two nylon repeat units. While thisis an extremely high aliphatic content overall,there is a marked imbalance in the spacing of theamide groups, which are 12 and 32 methyleneunits apart, a factor that may impact solid phaseorder and packing.

To further evaluate effects of high aliphaticcontent on nylon properties, there is need for asystem with evenly spaced polymethylene seg-ments between amide units. Figure 1 representsthe progression to higher aliphatic content withregular spacing of the amide units, ranging fromnylon 6 6 to nylon 18 18. The highest contentreported that retains similar length aliphatic

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 47, 4409–4419 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: L. J. Mathias (E-mail: [email protected])

4409

segments are nylons 10 14,10 12 12,11 and 1313.12,13 The majority of cases reported in the liter-ature incorporate 1,12-dodecanediamine (as theonly commercially available high aliphatic dia-mine), combined with higher methylene contentin the diacid moiety.

Adamantane, a highly symmetrical tricyclichydrocarbon, has been incorporated into manypolymer structures as a pendant group or directlyinto the polymer’s main chain. The incorporationof the adamantane as a pendant group increasesthe solubility, decreases the crystallinity, enhancesthe glass transition temperature, and improvesthermal stability.14–16 There have also been a num-

ber of reports on incorporation of the adamantanemoiety in aromatic polyamides and polyimides.17,18

We report here the synthesis and evaluation ofnylon 18 18, which contains amide units consis-tently 16 and 18 methylene units apart. This isthe highest degree of spacing reported to date fora regular AABB nylon. In addition, we describe anew type of amorphous nylon based on an ada-mantane segment in the repeat unit. This moietypossesses all sp3 carbons, a relatively high hydro-gen-to-carbon ratio, and a rigid structure thatprevents symmetrical packing of the amidegroups attached to the adamantane core.

Synthesis

All reagents were used as received. The mono-mers 1,18-octadecanediamine and 1,18-octadeca-nedioic acid were supplied by Cognis Corp. 1,18-octadecanediamine contains �1% of the 18 carbonmonofunctional amine, and 1,18-octadecanedioicacid contains no impurity detected by NMR anddifferential scanning calorimetry (DSC). 1,3-Ada-mantanedicarboxylic acid (ADA, 98%) was pur-chased from Aldrich. All solvents were purchasedfrom Acros, with the exception of 1,1,1,3,3,3-hexa-fluoroisopropanol (HFIP), which was purchasedfrom Regis Technology. All chemicals were usedwithout further purification.

Nylon 18 18 was produced by a melt polycon-densation technique. The monomers were mixedtogether in equalmolar amounts and heatedunder nitrogen purge using a silicon oil bath at170–180 �C for 4 h, then 220–230 �C for 4 h. Atthis point, no bubbling was visible in the melt anda vacuum was applied for 0.5 h. The sample wascooled under vacuum. The resulting material waswhite in color.

Nylon 18 ADA was synthesized in a similarmelt polycondensation reaction except the totalreaction time was 22–24 h. This polymer wasobtained as a transparent, yellow-brown plug thatwas very tough and hard to cut or grind.

Characterization

Dilute solution viscometry was performed using aCannon-Ubbelohde 1C C628 viscometer withdichloroacetic acid as the solvent. Efflux timeswere recorded at a temperature of 35 � 0.2 �C.

Transmission Infrared measurements werecarried out on a Mattson Galaxy Series FTIR5000 using solution cast films and 256 transients.ATR measurements were made using a Spectra-

Figure 1. Repeat units of nylons with progressivelyhigher aliphatic content. As amide density decreasesand the spacing becomes more uniform, the polymerchain more resembles a functionalized polyethylene.

4410 BENNETT ET AL.

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Tech HATR cell with a ZnSe 45� crystal and meltpressed films in a Nicolet Protege FTIR.

Thermogravimetric analysis (TGA) was per-formed on a TA Instruments Q500. The tempera-ture was ramped at a heating rate of 10 �C/minunder nitrogen. The temperature at which a 5%loss in weight occurred was recorded as thedecomposition onset temperature. In addition, thepeak maximum of the derivative of the weightloss curve was recorded as the peak decomposi-tion temperature.

DSC experiments were performed on a TAInstruments 2920 using pierced-lid crimped alu-minum pans. Nonisothermal DSC experimentswere recorded at a ramp rate of 10 �C/min. Eachsequence of scans was recorded twice for reprodu-cibility. The melting temperature was taken asthe endotherm maximum, and the crystallizationtemperature was taken as the exotherm maxi-mum. Annealing was conducted in the DSC. For atypical DSC analyses, small pieces (�4 mg) werecut and placed in an aluminum DSC pan. Thesamples were heated at 10 �C/min to a given tem-perature and annealed at that temperature for2 h. They were cooled down to �10 �C with a cool-ing rate of 10 �C/min, and heated again with aheating rate of 10 �C/min.

Tensile testing of nylons was performed on aMTS Alliance RT/10 according to the standardprotocol of ASTM 882. DMA measurements wereobtained on a Seiko Instruments SDM 5600 seriesdynamic mechanical spectrometer (DMS 210).

Wide angle x-ray diffraction (WAXD) measure-ments were obtained with a Rigaku Ultima IIIdiffractometer operated at 40 kV and 44 mA andrun with a scan speed of 0.2� per minute from 2 to45�. Samples for XRD analysis were placed in asealed aluminum holder and annealed using aconventional oven.

Routine solution 13C nuclear magnetic reso-nance (NMR) spectroscopy was performed on aVarian MercuryPLUS 300 MHz spectrometer oper-ating at 75.5 MHz for carbon, using a pulse widthof 7.8 ls, acquisition time of 1.8 ms, and no relax-ation delay. The sample was prepared in a mix-ture of HFIP and CDCl3, and referenced to thecenter peak of the deuterated solvent triplet atd ¼ 77.23 ppm.

CP/MAS solid state 15N NMR experimentswere performed on a Bruker MSL 200 MHz spec-trometer operating at 20.28 MHz with a 7.5 mmChemagnetics double resonance probe, with asample spinning speed of 2.2 kHz. Cross-polariza-tion was conducted using a 3.5 ls 1H 90� pulse fol-

lowed by a mixing time of 2 ms. An acquisitiontime of 42 ms using high powered decoupling wasused, with a relaxation delay of 3 s between scans.Peaks were referenced to the amide nitrogen of15N labeled glycine at d ¼ 0.0 ppm as an externalstandard.

RESULTS AND DISCUSSION

Nylon 18 18

Nylon 18 18 exhibits similar but enhanced solventresistance to typical organic solvents seen in theseries of nylons X 18; that is, it was insoluble inchloroform, 2,2,2-trifluoroethanol (TFE), or mix-tures of TFE with chloroform.19,20 It was alsoinsoluble in carbon tetrachloride. For the nylon X18 series, m-cresol was found to dissolve all butnylons 2 18 and 12 18; it also does not dissolvenylon 18 18. In addition, nylon 18 18 is not solublein HFIP, which is a good solvent for all othernylons X 18 examined to date. Thus, trifluoroace-tic acid or an 80–20 mixture of HFIP-CDCl3 wasused for solution state NMR characterization.Dichloroacetic acid is also a suitable solvent, andwas used in viscosimetric characterization. Theintrinsic viscosity was determined to be 0.54 dL/g.

Infrared spectra for nylon 18 18 prepared inthis study displayed the characteristic peaks ofamide groups and methylene groups, as shown inFigure 2. The peaks were found at 3324 cm�1

(amide A, H-bonded NAH stretching), 3087 cm�1

(amide B, overtone of NAH in-plane bending),2905 cm�1 (CAH stretching), 1643 cm�1 (amide I,

Figure 2. FTIR spectrum of nylon 18 18.

NYLON 18 18 AND NYLON 18 ADAMANTANE 4411


C¼¼O stretch), 1538 cm�1 (amide II, CAN stretchand COANAH bend), 1273 cm�1 (amide III), 943cm�1 (amide IV, C-CO stretch), 721 cm�1 (CH2

rocking), and 591 cm�1 (amide VI, C¼¼O out-of-plane bend).

The solution state 13C NMR spectrum (Fig. 3)of Nylon 18 18, shows peaks typical of aliphaticnylons, which are summarized in Table 1. Previ-ous work in our group19 has shown the observa-tion of cis-amide units when the sample is dis-solved in a fluorinated alcohol and chloroalkane.It was also shown that the relative population ofcis-amide units (obtained by integration of thetrans and cis-carbon peaks a to the amide NH)increases with increasing aliphatic content, fromnylon 4 (0.76% cis) to nylon 13 13 (2.70% cis), andthis behavior continues to nylon 18 18 (5.18% cis).The 18 carbon diamine contained �1% of the 18carbon monofunctional amine so that end groupsare a combination of carboxylic acid and terminalalkanes, as can be seen by the small carbonyl car-bon peak at d ¼ 180.3 ppm and the terminalmethyl group at d ¼ 13.6 ppm (not shown). Thepeaks associated with amine end groups were notobserved, so the tabulated values of end groupsinclude only the acid and alkyl group, denoted byend1, end2, and end3 for the terminal C18 alkanestarting from the methyl end group.

DSC indicates a Tm of 163 �C and a Tc of126 �C, taken from the scan obtained at 10 �C/min.

Using a procedure based on group additivity val-ues from van Krevelen,21 the percent crystallin-ity was calculated from DSC data and theoreticalDHf. The equilibrium heat of fusion, DHf, was

Table 1. 13C Solution NMR Chemical Shifts forNylon 18 18

Nylon 18 18

Trans Cis

1 40.36 43.772 28.88 32.183 26.72 –4 29.195–9 29.46–29.77 –10 36.77 33.9320 25.88 25.4330 29.07 –40 29.1950–80 29.46–29.77 –90 177.38 179.73

End groupsEnd1 13.57 –End2 22.68 –End3 32.02 –aCO2H 33.55 –bCO2H 24.90 –90CO2H 180.28 –

Figure 3. 13C solution NMR of the aliphatic and carbonyl (inset) regions of nylon18 18. Sample was dissolved in a mixture of HFIP: CDCl3.

4412 BENNETT ET AL.


calculated by adding two amide groups (67.4 J/g)and thirty-four methylene groups (at 270.8 J/g),to give a combined total of 338.2 J/g. Using aDSC heating rate of 20 �C/min, the experimentalenthalpy of fusion after melt crystallizationwas 68.0 J/g, giving a calculated crystallinity of20%. This value is lower than for typical com-mercial polyamides, perhaps owing to conforma-tional and/or librational motion of the polymethy-lene chains inhibiting crystallization (entropyinhibition).

From nonisothermal DSC studies, it can beseen that, as the heating rate increases, a broadmelting endotherm develops from 120 to 140 �C(Fig. 4), most likely associated with ‘‘melting’’ ofaligned polymethylene segments.22 DSC experi-ments on polyethylene have shown a similarbroadening of the melting endotherm at higherheating rates.23 This endotherm is not as well-defined at lower heating rates, perhaps becausethere is sufficient time for reorganization and(re)crystallization as the sample is heated. Inaddition, as ramp rate increases, a small crystalli-zation exotherm develops at �148 �C. This behav-ior is dependent on the cooling rate from abovethe melt. When the cooling rate increases, thelibrational mobility of the polymethylene chainsprecludes high order for the formed crystals, andthe recrystallization event reflects a population ofless perfect crystallites. Thus, there are three fac-tors leading to a broad population of imperfectcrystallites. The first is the random occurrence ofgauche configurations in the polymethylenechains. The second is the formation of randomswitchboard-type lamellae due to the length of thealkane segments involved in the folding processduring crystallization. The third is cis-amide resi-dues which can not pack into crystal domains.

It has been reported that the predominantmechanism of lamellar formation for nylons isthrough folding of the alkane segments.24 As thelength of the alkane chain involved in foldingincreases, these lamellar folds act as impuritiesbecause of a more random reentry into the lamel-lar structure. During heating of the sample, reor-ganization leads to a higher degree of perfectionin the crystallites, demonstrated by the recrystal-lization event, followed by melting of the moreordered domains. It is not clear how cis-amidegroups influence lamellar packing.

When comparing the melting behavior of nylon18 18 to the even–even series of nylons X 18, thebroad melting endotherm at 120–140 �C that ispresent in nylon 18 18 could also be seen in nylon12 18, but is not observed with lower methylenecontent diamines, indicating that high aliphaticcontent in both the diacid and diamine segmentsis necessary for this transition to be observable.Thus, the broad crystallization and meltingcurves are considered to be the result of two lessertransitions attributed to two different portions ofthe chains, the amide linkages and the C16 andC18 polymethylene segments.

Polymethylene segments in polyolefins meltand crystallize at much lower temperatures thanthose for high-aliphatic nylons. As a result, thebroad peaks may involve configurations of amidesand CH2 segments in imperfectly ordereddomains in which first, the van der Waals forcesof the polymethylene segments are overcome, andat higher temperatures, the hydrogen bonds ofthe amide linkages are broken up. In the coolingscans at different cooling rates (Fig. 5), the crys-tallization exotherms begin to show two overlap-ping exotherms at slower cooling rates. The lowtemperature exotherm is the predominant peak,with a shoulder on the high temperature side. As

Figure 4. Nonisothermal DSC heating traces ofnylon 18 18 recorded at different heating rates.

Figure 5. Nonisothermal DSC cooling traces ofnylon 18 18 at different cooling rates.



the cooling rate increases, the two endothermsmerge and become indistinguishable. Overall, theTc shows a significant decrease (�10 �C) as thecooling rate increases. As the sample is cooledfrom the melt, it would be expected that thehydrogen bonds form first, and act as nucleationsites, or pinning points, for the subsequent crys-tallization of the polymethylene segments. Thus,unlike traditional nylons, this system seemed toresemble a polyethylene backbone with periodichydrogen bonding that preorganizes the melt forcrystal formation.

Further comparison of the melting and crystal-lization temperatures of nylon 18 18 with the restof the series nylons X 18 shows that these valuesapproach those of polyethylene in a nonlinearfashion, as shown in Figure 6. Similar trendshave been found for both Tc and Tm of nylons X20,6 as well as the Tm of nylons X 34,8 for whichno Tc values were given. The Tm for nylon 6 18 is197 �C, and for nylon 12 18, with an addition of 6diamine methylene units, the Tm is 172 �C, a dif-ference of 25 �C. The difference in Tm afteranother addition of six more diamine methyleneunits, from nylon 12 18 to 18 18 (Tm ¼ 163 �C) isonly 9 �C. There comes a point at which themelting point is decreasing in progressivelysmaller steps as the diamine segment lengthincreases beyond 12 methylene units, and the am-ide unit spacing becomes more uniform along thebackbone.

Typically, AABB nylons melt at significantlyhigher temperatures than their AB analogs withcomparable aliphatic content. However, thereported melting temperatures of nylon 161 and

nylon 182 are 164 �C and 158 �C, respectively,which are not significantly different from the Tm

of nylon 18 18. It is of note that the enthalpies ofboth melting and crystallization for nylon 18 18are significantly higher than those of the rest ofthe nylons X 18 (Table 2). This suggests that thereis an additional component contributing to thecrystallization involving the C16 and C18 poly-methylene chains, which augments the observedenthalpy values related mainly to behavior of theamide ordered units.

The solid state 15N CP/MAS NMR spectrum,shown in Figure 7, contains a broad peak result-ing from a highly amorphous system, observed at87 ppm. A significant shoulder to this amorphouspeak is found at 84 ppm, associated with the acrystalline structure, the thermodynamically sta-ble form for even–even nylons.5 The even–evennylons X 18 previously reported show a trend

Table 2. Thermal Data for Nylons X 18, Recordedat a Scan Rate of 10 �C/min5

Nylon Tc (�C) DHc (J/g) DHm (J/g) Tm (�C)

2 18 208 37.9 34.4 2293 18 166 60.5 51.3 1984 18 197 42.5 40.9 2186 18 178 50.4 58.3 1978 18 159 51.2 51.1 1859 19 153 61.9 57.5 17912 18 131 48.4 34.9 17218 18 126 65.8 68.0 163

Figure 6. Graph of Tm and Tc for nylons X 18.5 TheAB analogs nylon 161 and nylon 182 are shown forcomparison.

Figure 7. Solid state 15N CP/MAS NMR of nylon18 18.

4414 BENNETT ET AL.


toward higher amorphous content as the aliphaticcontent increases. This can be attributed to thelibrational motion of the polymethylene segmentsin both the diacid and diamine moieties, whichaffects both the kinetics and thermodynamics ofcrystallization. It is most likely also related tothe higher cis-amide content of higher aliphaticnylons which also inhibits crystalline packing.

Nylon 18 ADA

Nylon 18 ADA was readily soluble in CHCl3,HFIP and m-cresol. It is partially soluble in meth-ylene chloride, 2,2,2-trifluoroethanol (TFE), andtrifluoroacetic acid leading to a cloudy suspension.It is also partially soluble in a 80-20 mixture ofTFE-CDCl3. Films cast from solution were clearand tough. Infrared spectra of nylon 18 ADAshowed characteristic absorption bands of amidegroups and methylene segments as seen in Figure8. Peaks were found at 3345 cm�1 (NAH stretch-ing), 2921-23 cm�1 (CAH stretching), 1633-35cm�1 (amide I, C¼¼O stretch), 1535 cm�1 (amideII, CAN stretch and COANAH bend), 1284 cm�1

(amide III), and 736 cm�1 (CH2 rocking).On the basis of Coleman’s previous work25 on

nylon 11, NAH stretching bands can be dividedinto three parts. The band at 3456 cm�1 is due tofree NAH, the band at 3362 cm�1 is due to disor-dered hydrogen bonded NAH in the amorphousphase, and the band at 3320 cm�1 is due toordered hydrogen bonded NAH in the crystallinephase. The frequency of hydrogen bonded NAHindicates the strength, while the breadth of thisband results from the conformational distribution

of hydrogen bonded groups. The NAH stretchingband of nylon 18 18 is broad, from 3146 to 3480cm�1, while the band for nylon 18 ADA is morenarrow from 3256 to 3480 cm�1. The peak posi-tions and the breadth of the peaks suggests thatnylon 18 18 is mostly amorphous with some or-dered structures, whereas nylon 18 ADA is almosttotally amorphous. It is interesting to note thatthe amide II overtone band at 3084 cm�1 (nylon18 18, Fig. 2) is not observed for nylon 18 ADA.

Figure 9 shows solution 13C spectra for Nylon18 ADA. All peaks are assigned based on thestructure of the polymer except the carbonyl car-bon which shows up at 176.92 ppm. The adaman-tane carbons next to the carbonyl carbon are seenat 39.48 ppm (2C), while the carbon atom betweenthese two carbons is observed at 35.45 ppm (1C).The other adamantyl carbons show up at 29.29(2C), 38.42 (4C), and 40.48 ppm (1C). Peaks asso-ciated with amine end groups were not observed.Like nylon 18 18, only carboxylic acid and termi-nal alkanes were assigned for nylon 18 ADA. Thecarbonyl carbon peak of the end groups is seen at178.71 ppm. The other peaks associated with ada-mantane end groups were observed at 40.78,40.19, 37.98, 27.95 ppm. End1 and end2 of termi-nal alkanes were observed at 13.98 and 22.58ppm, respectively, due to the presence of previ-ously mentioned C18 monoamine. The molecularweight of nylon 18 ADAwas calculated from NMRdata using the following equation26:

Mnðg=molÞ ¼ M0ðI1=2Þ=½IbCOOH þ Iend2Þ=2�where M0 is the molecular weight of the repeatingunit for nylon 18 ADA (472.75 g/mol), I1 is the

Figure 8. FTIR spectrum of nylon 18 ADA.

Figure 9. 13C solution NMR of Nylon 18 ADA.Sample was dissolved in CDCl3.



intensity of the methylene carbons a to the nitro-gen atom, while IbCOOH and Iend2 are the inten-sities of the methylene carbon b to acid endgroups and the terminal methyl group donated asend2, respectively. The intensity of the methylenecarbon b to acid end groups was calculated basedon the assumption that the ratio of the intensitiesof carbonyl end groups to carbonyl main chainpeaks will be the same as the ratio of the inten-sities of the methylene carbon b to acid endgroups to the methylene carbons a to nitrogen.The number-average molecular weight of nylon18 ADAwas then calculated to be 21,000 g/mol.

It has been shown that the chemical shift ofamide nitrogens in 15N CP/MAS NMR spectraindicates the crystalline form of the nylon sam-ples. The solid state 15N CP/MAS NMR spectrumof nylon 18 ADA is shown in Figure 10 and con-tains only a broad peak resulting from a highlyamorphous system, with a peak value at 77 ppm.

Annealing of Nylon 18 ADA

Figure 11 shows the DSC scans of nylon 18 ADAsamples annealed at different temperatures for2 h in the range of 60 to 100 �C. Annealing tem-peratures higher than 100 �C gave similar ther-mograms to that obtained at 100 �C. Two distinctpeaks are observed for the sample annealed at60 �C. The lower endothermic peak disappearsand the heat of fusion increases once the anneal-ing temperature is 70 �C. The heat of fusion ofmelting decreases with higher annealing temper-atures and no melting peak is observed once theannealing temperature is 100 �C or above. On theother hand, the melting peak shifts slightly to

higher temperatures with increasing annealingtemperatures.

Two peaks for the sample annealed at 60 �Cmay be related to two distinct crystal populationswith different degrees of perfection and size.Smaller and less perfect crystals may grow intomore stable and thicker crystals. This mayexplain the increase in the heat of fusion for thesample annealed at 70 �C. It is interesting to notethat once the melting of these crystals occurs, thepolymer chains do not rearrange to form crystalsunder the conditions indicated, possibly due tobulky adamantane groups in the backbone inhibi-ting chains getting closer in the time frame ofDSC analysis. During solvent casting, nylon 18ADA chains are more mobile and have enoughtime and flexibility to form limited amounts of or-dered structures which they do not from the melt.This may explain why there is only a small endo-thermic peak with higher melting point observedonce the sample is annealed at 90 �C. At this tem-perature, most of the crystals melt, and the peakobserved is only for the most perfect crystalswhich have higher melting points and are stillstable at this temperature. The glass transitionfor nylon 18 ADA is always observed at 64 �Cwhich is higher than the other long alkyl chainnylons examined to date, possibly due bulky ada-mantane groups inhibiting segmental motion.

Figure 12 shows the DSC traces of nylon 18ADA annealed at 80 �C for different periods oftime. The heat of fusion and the melting pointslightly increase with annealing time. The

Figure 10. Solid state 15N CP/MAS NMR of nylon18 ADA.

Figure 11. DSC thermograms of solution-cast nylon18 ADA annealed at various temperatures for 2 h.

4416 BENNETT ET AL.


melting point shifts from 99 to 102, while theheat of fusion of the melting increases from 15 to18 J/g. These changes are not significant. Thereare also slight differences observed for the sam-ples annealed at 90 �C for different time periods.These results indicate that there is little or nochange in crystallinity with time using theannealing conditions described here.

A series of WAXD pattern, recorded at roomtemperature, of solution-cast nylon 18 ADA sam-ples annealed at various temperatures for 2 h isgiven in Figure 13. The as-cast sample (without

further thermal treatment) shows an amorphoushalo in addition to a sharp peak at 21.04� andanother small but sharp peak at 28.08�. TheWAXD patterns follow the same trends as theDSC thermograms (Fig. 11). The peak intensitiesare almost unchanged with annealing up to 70 �C,and then the intensity of the peak at 21.04�

decreased for the sample annealed at 80 �C. Oncethe annealing temperature is 90 �C and higher,only an amorphous halo is observed.

Thermogravimetric Analysis

Both polymers showed good thermal and thermo-oxidative stability. Figure 14 shows typical TGAtraces for nylon 18 18 and nylon 18 ADA withheating rates of 10 �C/min in N2. The temperatureat 5 wt % loss is 429 �C, and the temperature at10 wt % loss is 440 �C for nylon 18 18 in N2. Nylon18 ADA shows a slightly enhanced thermal stabil-ity compared to nylon 18 18. The temperature at5 wt % loss is 434 �C , and the peak decompositiontemperature is 485 �C which is about 18 �C higherthan the peak decomposition temperature ofnylon 18 18. The change in thermal stability ofnylon 18 18 in air is insignificant, and no changeis observed in thermal stability of nylon 18 ADA.

Mechanical Analysis

Figure 15 shows typical stress–strain curves fornylon 18 18 and nylon 18 ADA. The initial modu-lus values are measured as 820 MPa and 486

Figure 12. DSC thermograms of solution-cast nylon18 ADA annealed at 80 �C for various times.

Figure 13. The WAXD patterns of solution-cast nylon18 ADA annealed at various temperatures for 2 h.

Figure 14. TGA thermograms in N2 with heatingrates of 10 �C/min. Dashed line is nylon 18 18, solidline is nylon 18 ADA.



MPa for nylon 18 ADA and nylon 18 18, respec-tively. The Young’s modulus for nylon 18 18 is inthe range of the nylon X 34 series which wasgiven as 500–700 MPa.27 The analysis also indi-cates elongation at the breaking point increasedby 300% for nylon 18 ADA, while initial modulusis almost doubled compared to nylon 18 18. Thisremarkable improvement in mechanical proper-ties confirms the reinforcing effect of the adaman-tyl moiety in polyamide backbones. Fully aliphaticpolyimides from adamantane-based diamides wasreported to illustrate increases in either elonga-tion at break or modulus.28 The same observationwas reported for polyamide-imides containingpendent adamantyl groups.29 However, none ofthose samples showed increases in both elonga-tion at break and modulus. The stress–straincurves of nylon 6 18 and nylon 8 18 are also givenin Figure 15 for comparison. The elongations atbreak for nylon 6 18 and 8 18 are 550 and 750%,respectively.

Figure 16(A) shows the storage module as afunction of temperature for nylon 18 18. For com-parison, the data are given for two other nylons,nylon 6 18 and nylon 8 18, also synthesized in ourlaboratory. As expected, the storage modulusincreases with increasing amide density for thesenylons. The increase is significant for nylon 6 18,and is almost 300% higher compared to nylon 1818. Figure 16(B) shows the loss tangent delta de-pendence on temperature for these nylons. Theglass transitions obtained by DMA are in thesame range of 43–45 �C. The small increase in Tg

for nylon 18 18 may be a result of lower amidedensity of this polymer which leads to lower waterabsorption.

CONCLUSIONS

Two novel aliphatic nylons have been successfullysynthesized. The high-aliphatic nylon with near-uniform spacing of the amide units, nylon 18 18,approaches the properties expected for polyethyl-ene with periodic hydrogen bonds substitutedalong the backbone. Resistance to typical organicsolvents increases over other aliphatic nylons as aresult of the increased polymethylene content.Quantitative 13C solution NMR analysis con-firmed a continued increase in the population ofcis-amide units as the aliphatic content increases.Nonisothermal DSC experiments point to a two-component system with two major melting andcrystallization phenomena, the lower associatedwith van der Waals forces and the higher withamide hydrogen bonds. Combined, the regularplacement of amide groups and long methylenesegment lengths results in a higher-than-expected

Figure 16. DMA curves of (a) nylon 6 18, (b) nylon8 18, and (c) nylon 18 18 [(A) Storage modulus vs.temperature, (B) Tan delta vs. temperature].

Figure 15. Stress–strain curve for (a) nylon 18 18,(b) nylon 18 ADA, (c) nylon 8 18, and (d) nylon 6 18.

4418 BENNETT ET AL.


Tm, with a higher enthalpy (and higher degree ofcrystallinity) than most of the other nylons X 18.Nylon 18 ADA, on the other hand, is totally amor-phous from the melt. However, solution-cast sam-ples of nylon 18 ADA shows some ordered struc-tures that can grows into more stable crystalswith annealing. Incorporation of adamatyl moi-eties in the polymer’s main chain enhanced boththe thermal stability and mechanical properties.

The authors thankCognis Corp. for funding andmateri-als. Partial support was also provided by the NSFIGERT grant 0333136. We also acknowledge NSF-MRIgrant award 0079450 for funding to purchase the Var-ian UNITYINOVA 500 MHz NMR spectrometer andupgrade other university NMR systems.

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Synthesis and characterization of nylon 18 18 and nylon 18 adamantane

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Transcript of Synthesis and characterization of nylon 18 18 and nylon 18 adamantane