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Polymer 126 (2017) 248e258

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Polymer

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Graphene oxide reduces the hydrolytic degradation in polyamide-11

Samuel J.A. Hocker a, c, Natalie V. Hudson-Smith b, Patrick T. Smith b,Christopher H. Komatsu b, Laura R. Dickinson a, Hannes C. Schniepp a,David E. Kranbuehl b, *

a Department of Applied Science, The College of William & Mary, Williamsburg, VA, United Statesb Department of Chemistry, The College of William & Mary, Williamsburg, VA, United Statesc Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, VA, United States

a r t i c l e i n f o

Article history:Received 15 June 2017Received in revised form11 August 2017Accepted 14 August 2017Available online 16 August 2017

Keywords:Polyamide-11Graphene oxideHydrolysis

* Corresponding author.E-mail address: dekran@wm.edu (D.E. Kranbuehl)

http://dx.doi.org/10.1016/j.polymer.2017.08.0340032-3861/© 2017 Published by Elsevier Ltd.

a b s t r a c t

Graphene oxide (GO) was incorporated into polyamide-11 (PA11) via in-situ polymerization. The GO-PA11 nano-composite had elevated resistance to hydrolytic degradation. At a loading of 1 mg/g, GO toPA11, the accelerated aging equilibrium molecular weight of GO-PA11 was higher (33 and 34 kg/mol at100 and 120 �C, respectively) compared to neat PA11 (23 and 24 kg/mol at 100 and 120 �C, respectively).Neat PA11 had hydrolysis rate constants (kH) of 2.8 and 12 ( � 10�2 day�1) when aged at 100 and 120 �C,respectively, and re-polymerization rate constants (kP) of 5.0 and 23 ( � 10�5 day�1), respectively. Thehigher equilibrium molecular weight for GO-PA11 loaded at 1 mg/g was the result of a decreased kH, 1.8and 4.5 ( � 10�2 day�1), and an increased kP, 10 and 17 ( � 10�5 day�1) compared with neat PA11 at 100and 120 �C, respectively. The decreased rate of degradation and resulting 40% increased equilibriummolecular weight of GO-PA11 was attributed to the highly asymmetric planar GO nano-sheets thatinhibited the molecular mobility of water and the polymer chain. The crystallinity of the polymer matrixwas similarly affected by a reduction in chain mobility during annealing due to the GO nanoparticles'chemistry and highly asymmetric nano-planar sheet structure.

© 2017 Published by Elsevier Ltd.

1. Introduction

Polyamide-11 (PA11) is a widely-used engineering polymer.PA11 comprises the pressure sheath liner in contact with the pro-duction fluid at elevated temperature and pressure in flexible hosesfor underwater transport of gas, oil, and crude to offshore plat-forms. As such, PA11 fulfills a vital role in the global economy.Degradation and failure of these hoses have far-reaching financialand environmental implications. Currently, themolecular weight ofPA11 is used to predict the operational lifetimes of the PA11 liner[1,2] where hydrolysis of PA11 is the mechanism for degradation inanaerobic environments such as the condition of underwater crudeoil risers. Improving the properties of PA11 as a pressure sheathmotivates research on the hydrolytic degradation of graphene ox-ide (GO) loaded PA11: GO-PA11 polymer nano-composite.

Polymer nano-composites have become high potential alterna-tives to traditionally filled polymers or polymer blends [3]. Since

.

Usuki et al. [4], polymer nano-composites have shown valuableperformance property enhancements such as stiffness and thermalmanagement at much lower percent mass loadings than often usedglass and carbon fiber micron scale fillers. This lower percent massloading can lead to cheaper processing and novel material solutionsto engineering problems.

Graphene is an atomic single layer of carbon, a 2-D carbon-honeycomb crystal structure with remarkable properties. Itwould be advantageous if graphene's properties; an extremely highmodulus of 1 terapascal (TPa), an ultimate strength of 130 giga-pascal (GPa), a gas impermeable honeycomb network, and superiorelectrical and thermal conductivity [5,6], could be lent to polymersby means of creating graphene-polymer nano-composites[3,7e10]. The challenge is to disperse graphene as single nano-sheets into polar solvents, then into the polymer matrix.

A wide interest in graphene's exceptional strength involves theanalogous compound GO. GO has nearly the same strength andbarrier properties as pristine graphene, given the very high aspectratio planar honeycomb structure. GO has the same honeycombstructure as graphenewith oxygen-containing functional groups onthe surface of the planar carbon. These oxygen containing

Fig. 1. AFM height image from GO-water dispersion distributed on mica.

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258 249

functional groups are ketones, 6-membered lactol rings, alcohols,epoxides, and hydroxyl groups [11]. As a result, GO is a polar hy-drophilic material compared to graphene, which is nonpolar andhydrophobic. GO, unlike graphene, can be dispersed into water andsolvents [8] commonly used in polymer precursor resins or mix-tures to create GO-polymer nano-composites. Also, the oxidegroups on the GO surface can be reacted with functional groups forimproved dispersion and incorporation into a polymer matrix.

Several methods of making graphitic oxide have been devel-oped, starting with Brodie [12] in 1859, modified Brodie [13,14],Staudenmaier [15], Hofmann [16], Hummers [17], modified Hum-mers [18], andmost recently Tour [19] in 2010. Each synthesis routeresults in a different graphitic oxide chemistry: in depth studiesand reviews have been done illustrating such differences [11,19,20].Importantly, synthesis of graphitic oxide is suitable for industrialscale GO production [21,22].

GO-polymer nano-composites are easily made by exfoliatinggraphitic oxide into GO nanosheets using a compatible solvent in-situ with the polymer. Many GO-polymer nano-composites havebeenmade using polyurethanes, polyimides, polyamides, and otherpolymers [3,7e10]. Well dispersed GO nanoparticles in a polymercan change the chemical and morphological properties of thepolymer, to result in significantly improved performance properties[23e44].

In previous research on GO-PA11, Jin et al. [39] extruded ther-mally reduced GO into commercial PA11 at loadings ranging from 1to 30 mg/g reduced GO-PA11. They showed that water vapor andoxygen permeation resistance of PA11 films were increased by 49%at a 1 mg/g loading and reported on the tensile properties offunctionalized graphene loaded PA11. Yuan et al. [45] prepared 1.25to 5 mg/g GO-PA11 by in-situ melt polycondensation and discussedimproved toughness and correlated changes in crystal structuresthat were formed when GO was loaded into PA11.

In this work, the hydrolytic degradation of GO-PA11 and theeffect GO had on the PA11 matrix crystallization was explored. Thelarge immobile nano thin GO sheets and the intermolecular inter-action between the GO's surface C¼O groups with the polyamide'sN-H groups significantly reduced molecular mobility in the GO-polymer and resulted in a reduction in the rate of crystallizationand most importantly the rate of degradation by hydrolysis.

1.1. Methods and measurements

GO was incorporated into PA11 via in-situ melt condensationpolymerization (GO-PA11). Accelerated aging of PA11 and GO-PA11was performed by immersing 0.3 mm thick � 1 cm2 samples in15 mL of deionized water at elevated temperatures of 100 and120 �C. The extent of hydrolytic degradation was measured bymonitoring the change in mass averaged molecular weight (Mm) ofPA11 over time. For PA11, a hydrolytic aging equilibrium molecularweight (Mme) was established after extended aging times inanaerobic conditions.

GO was synthesized using Hummers method [17] for a C/O ratioof 2.3, measured by Galbraith laboratories via elemental analysis.The GO was dispersed in water by ultrasonication (Cole-ParmerEW-04711-40 homogenizer 750-Watt) for 30 min. The resultingsolution of GO in water was filtered using G4 glass fiber filter paper(Fisher Scientific) to remove unreacted graphitic particles. The GOparticle water dispersion was measured by mass to have a con-centration of 0.3 mg/g. A Laurell: WS-650Sz-6NPP-Lite spin pro-cessor was used to apply a single layer of GO particles to a V5cleaved sheet of mica (Ted Pella). A NT-MDT NTEGRA atomic forcemicroscope was used to characterize the dimensions of the GOsheets. The sheet sizes in the collected filtered GO/water dispersionwere observed to be 0.3e1 mm laterally and 1 nm in height, Fig. 1.

Dry 11-aminoundecanoic acid powder, Sigma Aldrich, wasmixed with the filtered GO/water dispersion at a concentration of 1and 5 mg of GO per gram of powder. Water was added to themixture for a total volume of 15 mL. An ultrasonication tip wassubmerged in the mixture and sonication at 35% of maximum po-wer proceeded for 30 min with continuous stirring at 300 rpm anda maximum temperature of 30 �C.

The homogeneous golden-brown mixture of GO-monomersettled with a clear water supernatant. The mixture was pouredinto a Teflon® lined vessel and placed at 100 �C in an oven for60 min. The dried powder was uniformly brown. The GO-monomerpowder was placed under argon at 235 �C for 4 h of melt conden-sation static polymerization, and thereby GO-PA11 was made. PA11fabrication followed the same method. For convention, a concen-tration of 1 mg/g GO-PA11 was termed 1-GO-PA11 and 5 mg/g GO-PA11 was 5-GO-PA11.

Previous work had found that GO will thermally reduce thecarbon to oxygen ratio in-situ in a polymer matrix [46]. GO washeated under nitrogen gas to 250 �C and found to have a C/O ratio of5.3 via elemental analysis. GO was assumed to have increased inhydrophobicity by reducing within PA11 in-situ duringpolymerization.

Aging samples were prepared by slowly hot pressing the PA11 orGO-PA11 between twoTeflon® surfaces at pressures up to 16MPa at250 �C under argon. The systemwas allowed to cool under flowingargon to room temperature. The approximate dimension of eachfilm was 60 cm2 with a 300 mm thickness. Coupons of 1 cm2 by300 mm thick were cut from the 60 cm2 by 300 mm thick PA11 orGO-PA11 films. For each material tested, ten coupons wereimmersed in deionized water over a period of 3e4 months atelevated temperature to accelerate aging. Two independent setswere studied at the temperatures of 100 and 120 �C. High-pressurerated #40 glass tubes with Teflon® plugs (Ace Glass) were used ascontainment vessels. Before sealing the pressure tubes, dissolvedoxygenwas removed by sparging the water with argon in-situ untilthe measured oxygen concentration dropped below 50 ppm(Oakton Waterproof DO 450 Optical Dissolved Oxygen PortableMeter). Samples were removed periodically from each pressuretube, and then each pressure tube was degassed and resealed forcontinued aging of the remaining samples.

Mass average molecular weight (Mm) was measured by in-lineShodex size exclusion chromatography columns (HFIP-LG, HFIP-805, and HFIP-803) paired with a Wyatt miniDAWN light scat-tering detector and Wyatt Optilab 803 dynamic refractive indexdetector. The miniDAWN had a laser wavelength of l¼ 690 nm andthree detection angles q of 45, 90, and 135�. The solvent used todissolve the PA11 samples was 1,1,1,3,3,3-hexafluoro-2-propanol(HFIP) doped with 0.05 M potassium tri-fluroacetate (KTFA) salt to

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258250

remove nylon agglomerations [47]. The KTFA doped HFIP wasdegassed in the pump reservoir via constant helium sparge underatmospheric pressure. The polymer solution concentration was2 mg/mL (PA11/KTFA-HFIP). A 100 mL aliquot of solution wasinjected for measurement. A dn/dc of 0.235 mL/g determined theconcentration of the PA11 at each fraction per the refractive indexmeasurement. The dn/dc was experimentally measured for PA11/KTFA-HFIP using the solution concentrations. The measured Mmvalues had a 6% errormargin, determined by the standard deviationdivided by the mean for six consecutive Mm measurements on asingle solution of PA11.

The GO-PA11/KTFA-HFIP solutions were darkened by the pres-ence of GO. All visible GO in each solution was removed by a0.45 mm porosity polytetrafluoroethylene filter. Additionally, the0.2 mm stainless steel frit column guard on the SEC-MALLS wasunaffected by injections of clear solutions that were passed throughthe 0.45 mm filter. The unaffected 0.2 mm frit indicated that thepolymer solutions being measured on the SEC-MALLS were free ofGO contamination.

Differential scanning calorimetry (DSC) was used tomeasure theheat of fusion and re-crystallization in PA11. The DSC (TA In-struments Q20) was calibrated using indium. A ramp rate of 3 �C/minwas used to heat, cool, and re-heat the samples under nitrogenfrom 40 to 220 �C. The integration limits were 140 to 200 �C for theheating ramps and 130 to 190 �C for the cooling ramp. The root ofthe peak onset maximum slope with the peak baseline set to zerowas the fusion or crystallization onset temperature.

Optical microscopy images were taken with an IX71 OlympusInverted microscope and a LUCPLFLN, 40� (0.6 NA) objective.Transmission bright field and cross polarized light images weretaken. For PA11 and 1-GO-PA11, optical glass microscope slideswere prepared by melt pressing the PA11 or GO-PA11 for 30 s at250 �C under argon, then ambient cooling to room temperatureunder argon. The 5-GO-PA11 was sliced using a Leica Ultracut UCTMicrotome to obtain a sample less than 150 nm, for opticaltransmission.

Attenuated total reflectance Fourier transform infrared spec-troscopy (ATR-FTIR) was used to probe the chemical functionalgroups and intermolecular interactions. Spectra were taken usingan IR Tracer-100 Shimadzu FTIR with a MIRacle 10 Single Reflec-tance ATR accessory at a resolution of 2 cm�1 from 600 to4000 cm�1 and 32 scans were averaged. The spectra were adjustedto a common baseline and normalized to the 2851 cm�1 peak. Toanalyze the carbonyl oxygen peak, the work of Skrovanek et al. [48]was followed. The 1635 cm�1 carbonyl oxygen peak was de-convoluted using MagicPlot software, and the relative concentra-tion of free and hydrogen bonded amide oxygen groups weremeasured at 1684, 1654, and 1635 cm�1. The heights of the de-convoluted peaks were used to compare the relative concentra-tion of hydrogen bonded carbonyl oxygen groups. The half-heightpeak width of the 3304 cm�1 N-H absorption was also used toprobe the hydrogen bonding behavior of the amide hydrogen.

The equilibrium molecular weight (Mme) hydrolysis rate con-stant (kH), and solid state polymerization (kP) rate constant weredetermined during accelerated molecular weight degradation byfitting the literature mathematical model of amide hydrolysis to themeasured Mm per aging time [49e51]. The previously publishedmodel was derived from the condensation-hydrolysis kineticsdescribed by Equation (2) and the associated first order ordinarydifferential Equation (7). The values kH and kP were fit to thechanging values of molecular weight over time in days and re-flected the effect of the aqueous aging environment on the hy-drolysis process.

In the model, Equation (10), was derived from the hydrolysiskinetics of Equations (1)e(3), and the associated first order

ordinary differential solution, Equation (9). The values kH and kPwere fit to the changing values of molecular weight over time viathe relationship described in Equation (10) where at was theaverage number of amide bonds at time t. Subscript “0” referred tothe starting value, “e” referred to the equilibrium value, and “t”

referred to the value associated at time t.

R0NH2 þ RCO2H ) *kP

kHR0NHCORþ H2O (1)

�dðR0NH2Þdt

¼ �dðRCO2HÞdt

¼ kP ½R0NH2�½RCO2H� � kH½R0NHCOR�½H2O� (2)

0 ¼ kP ½R0NH2�e½RCO2H�e � kH½R0NHCOR�e½H2O�e (3)

X ¼ ½R0NH2� ¼ ½RCO2H� (4)

a ¼ ½R0NHCOR� ¼ Mm

2$M0� 1 (5)

k0H ¼ kH$½H2O� (6)

dxdt

¼ k0Hða0 � xeÞ � kP$X2 (7)

kP ¼ k0Hða0 � xeÞx2e

(8)

k0Ht ¼xe

2a0 � xe$ln

a0xe þ xða0 � xeÞa0ðxe � xÞ (9)

at ¼a20 þ a0aee

a0þaea0�ae

tk0H

ae þ a0ea0þaea0�ae

tk0H(10)

The hydrolysis degradation model in terms of the mass averagenumber of amide bonds at time t (at) was defined by Equation (10).The derivation of Equation (10) started with the steady stateapproximation at equilibrium, Equation (3) [1,50]. Then the con-centration of water was assumed to be constant and therebyEquation (6) was used to form the first order ordinary differentialEquation (7). At the steady state, equilibrium, dx/dt was defined tobe zero and thereby rearranging Equation (7) in terms of kP resultedin Equation (8). Equation (9) was the solution of the differentialequation that resulted from substituting Equation (8) into Equation(7). Equation (10) was Equation (9) rearranged in terms of at.Equation (5) established that at was proportional to the measuredMm by the molecular weight of the monomer. Using Equation (5),the changes in the Mm data over time were fit to Equation (10) todetermine kH using a non-linear least squares fit. The model fitdetermined both the Mme and kH. Then, the best fit of Mme and kHwere used in Equation (8) to calculate the re-polymerization rateconstant kP.

2. Results and discussion

The Mme of PA11 is very important to the performance proper-ties of PA11. If the Mme is above the ductile-brittle transition Mmthen PA11 can retain its ductility [1]. The Mme of 1-GO-PA11 was10 kg/mol larger than that obtained for PA11 at both 100 and 120 �C,respectively. This was an impressive increase of 40% at both

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258 251

temperatures.Using Equation (10) and a non-linear least squares fit, the kH and

solid state kP rate constants were determined and listed in Table 1.Fig. 2 and Table 1 showed that the kH of 1-GO-PA11 was reduced bya factor of 2 and the Mme of 1-GO-PA11 was increased by 40% atboth temperatures. At 120 �C, both kH and kP were reduced for 1-GO-PA11 compared with PA11. The higher loading, 5-GO-PA11showed no change in the rate of hydrolytic degradation comparedwith PA11 at both temperatures. The absence of this effect at thehigher concentration, 5-GO-PA11, was attributed to a poor nano-particle dispersion as shown in Fig. 3. The optical images of 5-GO-PA11 showed that GO was highly agglomerated, the GOsheets were in contact with each other, Fig. 3. The 1-GO-PA11 lackssharp features in the optical micrograph, compared with thedefined sharp features sized from 3 mm to 15 mm in 5-GO-PA11, GOagglomerates. This agglomeration resulted in no effect on the Mmecompared with the finely dispersed GO in the 1-GO-PA11 nano-composite.

PA11 is a semicrystalline polymer. Amide hydrolysis in PA11 isunderstood to occur in the amorphous regions rather than theimpermeable crystalline domains. A key component of the crys-talline formation is hydrogen bonding. Hydrogen bonding in thePA11 matrix is an intermolecular interaction between the amidehydrogen and the carbonyl oxygen of a neighboring amide bond.Hydrogen bonding also occurs between nearby amide bonds in theamorphous regions. The hydrogen bonding between adjacentamide groups is responsible for the higher melting temperatures ofpolyamide crystalline structures versus a non-polar semicrystallinepolymer such as polyethylene.

Commercially prepared PA11 is plasticized with N-n-butylben-zenesulfonamide (BBSA) and shows a similar increase in the Mme

Table 1Rate constants and Mme for GO-PA11 aged in water; determined using Equation (8) and

Temperature,�C

kH, � 10�2 da

100 PA11 2.8 ± 0.51-GO-PA11 1.8 ± 0.55-GO-PA11 2.9 ± 0.6

120 PA11 12 ± 2.01-GO-PA11 4.5 ± 1.15-GO-PA11 13 ± 2.0

over neat PA11 [49,52]. The BBSA increases the amorphous contentin PA11 by disrupting hydrogen bonding. BBSA inserts itself toreplace the amide/amide interaction with a sulfonamide/amideinteraction. The C-N-H e O¼C hydrogen bond is replaced by astronger S-N-H e O¼C hydrogen bond, and a weaker C-N-H e O¼Shydrogen bond. Effectively, BBSA blocks intermolecular hydrogenbonding between neighboring amide bonds for lower crystallinityand increased amorphous content [53].

In the 1-GO-PA11, there were three species involved in theamide hydrolysis reaction: polyamide chains, GO, and water. Theelementary chemical mechanisms for chain scission were identi-fied: base catalyzed [54e58], acid catalyzed [59e62], and waterassisted [63,64]. The activation energies had been determined to be21, 31, and 99 kJ/mol for the base catalyzed, acid catalyzed, andwater assisted mechanisms of amide hydrolysis, respectively [65].Computationally, Zahn [63] found activation energies of 66, 78, and147 kJ/mol. Thus, given equivalent concentrations of OH� and Hþ

the base catalyzed pathway was favored; and the water assistedhydrolysis of an amide bond was the least favored pathway.

Elementary chemical mechanism of resonance stabilization ofthe amide bond.

(11)

Base catalyzed elementary chemical mechanism of amide hy-

drolysis.

a non-linear least squares fit.

y�1 kP, � 10�5 day�1 Mme, kg/mol

5.0 ± 1.5 24 ± 310.0 ± 4.0 34 ± 46.8 ± 2.4 24 ± 3

23 ± 7.0 24 ± 317 ± 5.0 34 ± 319 ± 5.0 23 ± 3

(12)

Fig. 2. Dependence of Mm on time when aged in water at 100 and 120 �C for PA11, 1-GO-PA11, and 5-GO-PA11. Mme of 1-GO-PA11 was approximately 10 kg/mol higher than PA11and 5-GO-PA11 when aging in water at 100 �C and 120 �C, respectively.

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258252

Acid catalyzed elementary chemical mechanism of amide hy-drolysis.

(13)

Elementary chemical mechanism for water assisted hydrolysisof the amide bond.

(14)

Equation (11) shows the initial resonance stabilization of theamide bond. Resonance stabilization chemically shortens andstrengthens the amide bond.

The base catalyzed hydrolysis mechanisms were drawn inEquation (12). Zahn [54] computationally studied the mechanism

of base catalyzed hydrolysis of the amide bond, and found that thebase catalyzed Pathway II was favored. Zahn [54] also found that aproton transfer reaction led to the simultaneous protonation of the

amide nitrogen and deprotonation of the hydroxyl group. Since theamide anion was a poorer leaving group than an alkoxide ion, thebase catalyzed hydrolysis the proton transfer to the amide anionwas considered the rate limiting step [66,67].

The four-step acid catalyzed hydrolysis mechanism was drawnin Equation (13). The first step was the protonation of the carbonyloxygen of the amide bond. Zahn [68] found in a computationalstudy that the second step, nucleophilic attack of a water moleculeto the carbon atom of the amide group, was the rate determiningstep. The third step was the intermediate de-protonation of thecarbonyl oxygen and protonation of the amide nitrogen, quickly

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258 253

followed by the dissociation of the protonated amine and the car-boxylic acid.

The water assisted hydrolysis mechanism was drawn in Equa-tion (14). The water assisted mechanism involved two water mol-ecules that acted in a concerted process and utilized a Grotthussmechanism of proton hopping through hydrogen bonding. The firststep and rate determining step was the nucleophilic attack of theamide carbon and simultaneous protonation of the amide nitrogen[63].

The crystalline regions in PA11 were impermeable to watermolecules: therefore, hydrolysis took place predominately in themolecularly mobile amorphous regions. Unaged PA11 and GO-PA11had the same crystalline content and therefore no change in hy-drolysis was expected. However, the well dispersed 1-GO-PA11showed unexpected decreases in the kH at both temperatures and a40% higher Mme than PA11, Fig. 2.

From the amide bond hydrolysis elementary mechanisms forbase hydrolysis Equation (12), acid hydrolysis Equation (13), andwater assisted hydrolysis Equation (14), it was clear that both theavailability of water molecules and hydrogen bonding wererequired for amide hydrolysis. Thus, the BBSA's disruption of inter-chain hydrogen bonding also inhibits hydrolysis. Changes in thehydrogen bonding behavior was another possible explanation forGO inhibiting the hydrolysis process in 1-GO-PA11, in addition to itslarge immobile nano-sheet structure that inhibited polymer chainand water mobility.

Skrovanek et al. [48] first studied the use of FTIR to detecthydrogen bonding interactions within a PA11 polymer. Skrovaneket al. [48] found de-convoluting the 1635 cm�1 peak of the carbonyloxygen revealed three peaks that could be characterized at 1635,

Fig. 3. Transmission optical microscopy of PA11 bright field (a), cross-polarized (b), 1-GO-PA

1654, and 1684 cm�1. The 1635 cm�1 sub-peak was a lower energycarbonyl vibration correlated to aligned hydrogen bonding. The1654 cm�1 peak was un-aligned amorphous hydrogen boundcarbonyl oxygens. The 1684 cm�1 peak was correlated withcarbonyl oxygen groups that were free of hydrogen bonding. Theamide bond 1635 cm�1 carbonyl oxygen peak in PA11, 1-GO-PA11and 5-GO-PA11 was deconvoluted based on the results of Skrova-nek et al. [48]. No difference was observed at the carbonyl oxygenpeaks for PA11 and GO-PA11 from deconvolution of the 1635 cm�1

amide C¼O. Further details were provided in the supplementaryinformation.

Infrared spectroscopy was used to probe the PA11 inter-molecular interactions, and hydrogen bonding behavior at theamide bonds. Fig. 4 shows the FTIR spectra for PA11 and 1-GO-PA11,pristine and aged. The characteristic peaks were: 1158 cm�1, aninteraction of the N-H stretch and O¼C-N deformation; 1539 cm�1,amide II C-N stretch; 1635 cm�1, amide I C¼O stretch; 2851 cm�1,symmetric C-H aliphatic vibration; 2919 cm�1, asymmetric C-Haliphatic vibration; and 3304 cm�1, amide III N-H stretch.

Fig. 5 compared the half height peak widths of the 3304 cm�1

amide (N-H) stretching peak for the PA11 and 1-GO-PA11, pristineand aged. The peak widths were associated with the distribution ofhydrogen bonding orientations of the amide bond hydrogen. Withaging, PA11 showed a decline in the distribution of hydrogenbonding orientations; from 54 to 34 ± 2 cm�1. The 1-GO-PA11showed less of a decline in the 3304 cm�1 peak widths; from 54 to39 ± 2 cm�1. The broader distribution of hydrogen bonding orien-tations in the GO-PA11 had also been correlated previously to thelower overall crystallinity content [48].

In aged 1-GO-PA11 the N-H stretch had a broader distribution

11 bright field (c), and 5-GO-PA11 bright field (d). Each image is sized at 60 � 60 mm.

Fig. 4. ATR-FTIR spectra of PA11 and 1-GO-PA11, and aged PA11 and aged 1-GO-PA11.

Fig. 5. The peak widths centered at 3304 cm�1 for PA11, 1-GO-PA11, and 5-GO-PA11; and grouped by unaged and aged at 120 �C for 90 and 120 days, respectively. The 3304 cm�1

peak widths are associated with the distribution of hydrogen bonding orientations of the amide bond hydrogen.

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258254

than aged PA11, Fig. 5. Further 1-GO-PA11 had an increased in-tensity at the 1158 cm�1 peak, an interaction of the N-H stretch andO¼C-N deformation. These results showed that the GO with C¼Ogroups present on the surface [69] were interacting with the N-Hamide hydrogen of polyamide. This interaction reduced the mo-lecular mobility of the PA11 chain and thereby reduced the rate ofhydrolysis and rate of the crystallization.

A transmission optical microscope was used in cross-polarization mode and the birefringence of the crystalline regionswithin PA11 and 1-GO-PA11 was observed. In transmission mode,the light passed through the sample. With cross polarizers, thebirefringent and therefore visible crystalline regions of PA11 and 1-GO-PA11were imaged. Fig. 6 showed that well dispersed GOwithinthe polymer matrix altered the crystalline morphology of PA11. The1-GO-PA11 showed less birefringence than PA11. The aged PA11showed the most birefringence, and banded spherulites, broaderbands of birefringence, for both the 100 and 120 �C with widths of

0.9 ± 0.3 and 1.0 ± 0.2 mm, respectively. The banded spheruliteswere associated with bundling of laminar stacks and higher crys-tallinity [70,71]. The larger bundled laminar stacks in aged PA11were absent in the aged 1-GO-PA11 optical images. The bundles oflaminar stacks in the 100 and 120 �C aged 1-GO-PA11 appearedmuch thinner with widths of 0.7 ± 0.2 and 0.7 ± 0.1 mm, respec-tively. Zhang et al. [72] reported a similar crystalline behavior inmontmorillonite-PA11 nano-composites. These smaller crystallinedomains agreed with the lower crystallinity DSC results for 1-GO-PA11 in Table 2.

When fully dissociated and dispersed, the GO nanoparticlesacted as large immobile 2-D nano-thin barriers within the PA11matrix. Consequently, they were predicted to have inhibited mo-lecular mobility within the polymer. As such, the results suggestedthat the GO nano-sheets reduced molecular mobility for both PA11chains and water for less accessible hydrolysis sites. GO's effect onthe rate of crystallization was characterized in PA11 and GO-PA11 -

Fig. 6. Cross polarized transmission optical microscopy images of 100 and 120 �C 80e90 days aged PA11 and 1-GO-PA11. Each image is sized at 60 � 60 mm.

Table 2Integration values of DSC heat flow peaks from 140 to 200 �C for GO-PA11 and PA11,aged (80e90 days) and unaged samples. The error margin was 2.5 J/g per mea-surement and peak integration.

Sample GO Loading,mg/g

1st Heat RampDHfus, J/g

Cooling RampDHc, J/g

2nd Heat RampDHfus, J/g

Unaged 0 47.5 50.0 48.01 46.5 42.5 49.05 46.0 44.0 49.0

100 �C 0 63.0 60.0 60.01 53.0 50.0 55.05 57.5 53.5 56.0

120 �C 0 91.0 65.5 61.01 82.0 53.5 59.05 83.0 49.5 51.0

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258 255

unaged and aged using DSC, which provided evidence for GO's rolein reducing chain mobility. Nanoparticles had been known to affectthe crystalline morphology of PA11 [45,70,73e81]. Fig. 7 showedthe DSC curves of the unaged and 80e90 days aged samples inthree columns from left to right to indicate the heat (fusion), cool(crystallization), and heat (fusion) curves. The peak integrationvalues of specific heat of fusion (DHfus) and crystallization (DHc)were listed in Table 2.

In Table 2, a lower total DHfus indicated a decrease in the crys-tallinity of the GO-PA11 after heat, cool, and re-heat ramps. Peakand onset temperatures were respectively tabulated in Table 3 andTable 4 for the fusion and crystallization of PA11 and GO-PA11. Theeffect of the previous thermal history on the semi-crystallinebehavior of PA11 was revealed in the first heat ramp. The firstheat ramp both revealed the previous thermal history on the semi-

crystalline behavior of PA11 and melted the PA11 to erase thatprevious thermal history. The 3 �C/min controlled cooling rampmeasured the affect that GO has upon the crystallization behaviorof PA11, both nucleation and growth. The second heating rampcompared how the total crystallization growth was affected by theGO loading.

During aging at 100 and 120 �C the PA11 matrix was annealedresulting in an increase in crystallinity. Quantified by the peak totalarea, the first heating ramp revealed the extent of annealing for thePA11 and GO-PA11 samples via their DHfus. Before aging, PA11 had25% crystallinity, using 189 J/g as 100% crystalline PA11 [71]. Uponaging and annealing PA11 the crystallinity increased to 34 and 48%at 100 and 120 �C; a relative increase of 36 and 92%, respectively.The 1-GO-PA11 started with a crystallinity of 25% and had lowerrelative increases of 12 and 76% at the same annealing conditions.

These results showed that the GO nano-sheets hinderedannealing in PA11 during aging at these temperatures. The 1-GO-PA11 samples had the lowest DHfus in both 100 and 120 �C condi-tions and indicated a large immobilizing effect on the polymerchains. The decreased effect of the 5-GO-PA11 was attributed toagglomeration and poor dispersion of 2-D nano-particles as pre-viously described.

The melting temperatures (Tm,fus) in Table 3 for PA11 and GO-PA11 were indicative of the thermal stability of the polymer crys-talline regions. A lower pre-peak Tm,fus, 184.5 �C, became increas-ingly prominent for the PA11when aged at 120 �C. The 184.5 �C pre-peak was attributed to poorly formed crystalline structures[71,72,77,82,83]. As seen in Fig. 7, the PA11 had the largest pre-peakat 184.5 �C in the first heating ramp after aging at 120 �C.Conversely, the poorly formed crystals were not favored in the 1-GO-PA11 and there was an additional intermediate melting peak

Fig. 7. DSC scans for unaged, 100 �C, and 120 �C for 80e90 days aged PA11, 1-GO-PA11, and 5-GO-PA11. The 3 columns from left to right were 1st heat ramp from 40 �C to 220 �C,followed by a cool ramp to 40 �C, and a 2nd heat ramp to 220 �C. The rows top to bottom were unaged, 100 �C, and 120 �C aged samples.

Table 3DSC peak temperatures for GO-PA11 and PA11, aged (80e90 days) and unagedsamples. The peak temperatures for melting (Tm,fus) and re-crystallization (Tm,c)were accurate within 0.5 �C.

Sample GO Loading,mg/g

1st Heat RampTm,fus,�C

Cooling RampTm,c,�C

2nd Heat RampTm, fus,�C

Unaged 0 187.5 166.5 187.51 190.0 169.5 188.55 187.5 169.0 187.0

100 �C 0 189.5 168.5 189.51 191.0 173.0 190.05 189.5 174.0 187.5

120 �C 0 184.5, 190.5 170.0 190.01 182.0, 187.0, 191.0 173.5 191.05 183.0, 188.5 174.5 187.5

Table 4DSC onset melting and formation temperatures for GO-PA11 and PA11, aged (80e90days) and unaged samples. The onset melting (To,fus) and re-crystallization (To,c)temperatures were accurate within 0.5 �C.

Sample GO Loading,mg/g

1st Heat RampTo,fus,�C

Cooling RampTo,c,�C

2nd Heat RampTo,fus,�C

Unaged 0 175.5, 180.5 167.0 177.5, 178.51 178.5, 180.0 175.0 181.05 175.0, 176.5 176.0 178.0

100 �C 0 182.5, 183.5 170.5 178.0, 183.01 179.5, 183.5 179.0 181.55 177.5, 179.5 180.0 177.5

120 �C 0 181.0, 186.5 172.0 180.0, 184.51 174.0, 177.5, 181.5 179.5 182.55 171.0 181.0 171.0

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258256

at Tm,fus 187.0 �C. The highest Tm,fus of 191.5 �C indicated that theannealed GO-PA11 had the most stable crystalline structure. Theincreased annealed crystalline melt temperature after aging sug-gested intermolecular attractive interactions between the GOsheets and the polyamide matrix as was seen in the ATR-FTIRspectra.

After the first heating ramp on the DSC, the polymer samplemelted and its thermal history erased. With controlled cooling theheat of re-crystallization (DHc) can be characterized. The decreasein the DHc for GO-PA11 compared to PA11 indicated that the chainmobility was inhibited by the inclusion of GO nanoparticles.

The GO-PA11 had a higher onset temperature of re-crystallization during the cooling ramp (To,c) and lower DHc value

than PA11, Table 4. The pristine PA11 had a To,c of 167 �C thatincreased to 170 (þ2%) and 172 (þ3%) �C after aging at 100 and120 �C. The increase in To,c after aging correlated to lowermolecularweights. For a polyamide, crystalline regions form more quickly atlower molecular weights due to increased mobility and ease ofalignment of the shorter polymer chains [84]. Both GO-PA11showed a 9 �C higher To,c with a broader reaction peak thanunloaded PA11. The 1-GO-PA11 samples initially had a To,c of 175 �C.After aging at 100 �C, the To,c increased to 179 (þ2%) �C; and agingat 120 �C increased the To,c to 180 (þ3%) �C. The increased To,c of theGO-PA11 samples were evidenced that the GO nano-particles werenucleating sites resulting in higher crystallization onsettemperatures.

S.J.A. Hocker et al. / Polymer 126 (2017) 248e258 257

In summary the large immobile GO, a 2-dimensional sheet inthe amorphous phase, in addition to inhibiting molecular mobility,competed for hydrogen bonding with the polyamide's amide hy-drogens. Both effects inhibited hydrolysis of the amide bond. It isthe decrease in chain and water molecular mobility combined withhydrogen bonding interactions between GO and the N-H bonds ofthe polymer chains that led to a lower hydrolysis rate for the welldispersed 1-GO-PA11, Table 1. The effects of GO's large sheet shapeand its surface functional groups interacting with the polymerchain, were similarly shown to have reduced polymer chainmobility in polyimide [85].

3. Summary and conclusion

A concentration of 1 mg/g GO in PA11 produced a 40% increaseover PA11's equilibrium molecular weight when aged in water at100 and 120 �C, respectively. No changewas observed in the 5mg/gGO-PA11 due to GO agglomeration. Fitting the previously reportedkinetic model to aging results showed an increase in equilibriummolecular weight, when the GO particles were well dispersed. Thiswas due to a reduction in the model's hydrolytic rate constantrelative to that for re-polymerization.

Measured by the heat of fusion, GO-PA11's decreased crystalli-zation indicated that the GO decreased molecular mobility withinthe nanocomposite, specifically the polymer chain molecularmobility during the annealing process. Both 1-GO-PA11 and 5-GO-PA11 nanocomposites showed a slight decrease in the amount ofPA11 crystallinity prior to aging. Upon aging at both 100 and 120 �C,the GO-PA11 had significantly less crystallinity and demonstrated areduction in the rate of annealing. After removing the thermalhistory by heating above the melting temperature and cooling, thematerials were nearly comparable and suggested no effect on thecrystallinity. The decrease in the annealing rate of crystallization inthe aged samples was attributed to GO's large immobile nanothickness sheet structure, which inhibited chainmolecular mobilitywithin the polymer nano-composite.

ATR-FTIR spectra showed that the graphene oxide's surface C¼Ogroups hydrogen bond with the polyamide's N-H groups. Thisinteraction further inhibited polymer mobility.

Two factors significantly reduced molecular mobility in the GO-polymer and resulted in a reduction in the rate of crystallizationand most importantly the rate of degradation by hydrolysis: thelarge immobile nano thin GO sheets and the intermolecular inter-action between the GO's surface C¼O groups with the polyamide'sN-H groups.

Acknowledgements

DEK acknowledges support from the ACS Petroleum ResearchFund under Grant No. 54253-ND7. DEK and HCS acknowledgesupport from the Center for Innovative Technology under Grant No.MF145-034-AM. HCS acknowledges support from the NationalScience Foundation under Grant No. 1534428.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymer.2017.08.034.

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