Graphene oxide reduces the hydrolytic degradation in polyamide-11

Accepted Manuscript Graphene oxide reduces the hydrolytic degradation in polyamide-11 Samuel J.A. Hocker, Natalie Hudson-Smith, Patrick Smith, Chris Komatsu, Laura R. Dickinson, Hannes C. Schniepp, David E. Kranbuehl PII:

S0032-3861(17)30809-1

DOI:

10.1016/j.polymer.2017.08.034

Reference:

JPOL 19936

To appear in:

Polymer

Received Date: 15 June 2017 Revised Date:

11 August 2017

Accepted Date: 14 August 2017

Please cite this article as: Hocker SJA, Hudson-Smith N, Smith P, Komatsu C, Dickinson LR, Schniepp HC, Kranbuehl DE, Graphene oxide reduces the hydrolytic degradation in polyamide-11, Polymer (2017), doi: 10.1016/j.polymer.2017.08.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Samuel JA Hocker,a,c Natalie Hudson-Smith,b Patrick Smith,b Chris Komatsu,b Laura R. Dickinson,a

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Hannes C. Schniepp,a and David E. Kranbuehlb*

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a

Department of Applied Science, The College of William & Mary, Williamsburg VA

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b

Department of Chemistry, The College of William & Mary, Williamsburg VA

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c

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E-mail: [email protected]

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Phone: 1-757-221-2542

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Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, VA

Abstract Graphene

oxide

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Graphene Oxide Reduces the Hydrolytic Degradation in Polyamide-11

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(GO)

was

incorporated

into

polyamide-11 (PA11)

via

in-situ

polymerization. The GO-PA11 nano-composite had elevated resistance to hydrolytic

13

degradation. At a loading of 1 mg/g, GO to PA11, the accelerated aging equilibrium molecular

14

weight of GO-PA11 was higher (33 and 34 kg/mol at 100 and 120 ˚C, respectively) compared to

15

neat PA11 (23 and 24 kg/mol at 100 and 120 ˚C, respectively). Neat PA11 had hydrolysis rate

16

constants (kH) of 2.8 and 12 (×10-2 day-1) when aged at 100 and 120 ˚C, respectively, and re-

17

polymerization rate constants (kP) of 5.0 and 23 (×10-5 day-1), respectively. The higher

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equilibrium molecular weight for GO-PA11 loaded at 1 mg/g was the result of a decreased kH,

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1.8 and 4.5 (×10-2 day-1), and an increased kP, 10 and 17 (×10-5 day-1) compared with neat PA11

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at 100 and 120 ˚C, respectively. The decreased rate of degradation and resulting 40 % increased

21

equilibrium molecular weight of GO-PA11 was attributed to the highly asymmetric planar GO

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nano-sheets that inhibited the molecular mobility of water and the polymer chain. The

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crystallinity of the polymer matrix was similarly affected by a reduction in chain mobility during

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annealing due to the GO nanoparticles’ chemistry and highly asymmetric nano-planar sheet

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structure.

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Introduction

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Polyamide-11 (PA11) is a widely-used engineering polymer. PA11 comprises the pressure

28

sheath liner in contact with the production fluid at elevated temperature and pressure in flexible

29

hoses for underwater transport of gas, oil, and crude to offshore platforms. As such, PA11 fulfills

30

a vital role in the global economy. Degradation and failure of these hoses have far-reaching

31

financial and environmental implications. Currently, the molecular weight of PA11 is used to

32

predict the operational lifetimes of the PA11 liner 1,2 where hydrolysis of PA11 is the mechanism

33

for degradation in anaerobic environments such as the condition of underwater crude oil risers.

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Improving the properties of PA11 as a pressure sheath motivates research on the hydrolytic

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degradation of graphene oxide (GO) loaded PA11: GO-PA11 polymer nano-composite.

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Polymer nano-composites have become high potential alternatives to traditionally filled

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polymers or polymer blends 3. Since Usuki, et al. 4, polymer nano-composites have shown

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valuable performance property enhancements such as stiffness and thermal management at much

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lower percent mass loadings than often used glass and carbon fiber micron scale fillers. This

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lower percent mass loading can lead to cheaper processing and novel material solutions to

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engineering problems.

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Graphene is an atomic single layer of carbon, a 2-D carbon-honeycomb crystal structure

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with remarkable properties. It would be advantageous if graphene’s properties; an extremely

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high modulus of 1 terapascal (TPa), an ultimate strength of 130 gigapascal (GPa), a gas 2

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impermeable honeycomb network, and superior electrical and thermal conductivity, 5,6 -could be

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lent to polymers by means of creating graphene-polymer nano-composites 3,7-10. The challenge is

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to disperse graphene as single nano-sheets into polar solvents, then into the polymer matrix. A wide interest in graphene’s exceptional strength involves the analogous compound GO.

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GO has nearly the same strength and barrier properties as pristine graphene, given the very high

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aspect ratio planar honeycomb structure. GO has the same honeycomb structure as graphene with

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oxygen-containing functional groups on the surface of the planar carbon. These oxygen

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containing functional groups are ketones, 6-membered lactol rings, alcohols, epoxides, and

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hydroxyl groups 11. As a result, GO is a polar hydrophilic material compared to graphene, which

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is nonpolar and hydrophobic. GO, unlike graphene, can be dispersed into water and solvents

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commonly used in polymer precursor resins or mixtures to create GO-polymer nano-composites.

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Also, the oxide groups on the GO surface can be reacted with functional groups for improved

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dispersion and incorporation into a polymer matrix.

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Several methods of making graphitic oxide have been developed, starting with Brodie 13,14

, Hofmann

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, Hummers

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1859, modified Brodie

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and most recently Tour

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chemistry: in depth studies and reviews have been done illustrating such differences

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Importantly, synthesis of graphitic oxide is suitable for industrial scale GO production 21,22.

, modified Hummers

in 18

,

in 2010. Each synthesis route results in a different graphitic oxide 11,19,20

.

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, Staudenmaier

15

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12

8

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GO-polymer nano-composites are easily made by exfoliating graphitic oxide into GO

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nanosheets using a compatible solvent in-situ with the polymer. Many GO-polymer nano-

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composites have been made using polyurethanes, polyimides, polyamides, and other polymers

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3,7-10

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properties of the polymer, to result in significantly improved performance properties 23-44.

. Well dispersed GO nanoparticles in a polymer can change the chemical and morphological

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In previous research on GO-PA11, Jin, et al.

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extruded thermally reduced GO into

commercial PA11 at loadings ranging from 1 to 30 mg/g reduced GO-PA11. They showed that

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water vapor and oxygen permeation resistance of PA11 films were increased by 49 % at a

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1 mg/g loading and reported on the tensile properties of functionalized graphene loaded PA11.

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Yuan, et al. 45 prepared 1.25 to 5 mg/g GO-PA11 by in-situ melt polycondensation and discussed

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improved toughness and correlated changes in crystal structures that were formed when GO was

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loaded into PA11.

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In this work, the hydrolytic degradation of GO-PA11 and the effect GO had on the PA11 matrix crystallization was explored.

The large immobile nano thin GO sheets and the

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intermolecular interaction between the GO’s surface C=O groups with the polyamide’s N-H

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groups significantly reduced molecular mobility in the GO-polymer and resulted in a reduction

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in the rate of crystallization and most importantly the rate of degradation by hydrolysis.

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Methods and Measurements

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GO was incorporated into PA11 via in-situ melt condensation polymerization (GO-PA11).

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Accelerated aging of PA11 and GO-PA11 was performed by immersing 0.3 mm thick × 1 cm2

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samples in 15 mL of deionized water at elevated temperatures of 100 and 120 ˚C. The extent of

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hydrolytic degradation was measured by monitoring the change in mass averaged molecular

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weight (Mm) of PA11 over time. For PA11, a hydrolytic aging equilibrium molecular

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weight (Mme) was established after extended aging times in anaerobic conditions.

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GO was synthesized using Hummers method

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for a C/O ratior of 2.3, measured by

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Galbraith laboratories via elemental analysis.

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ultrasonication (Cole-Parmer EW-04711-40 homogenizer 750-Watt) for 30 minutes. The

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resulting solution of GO in water was filtered using G4 glass fiber filter paper (Fisher Scientific) 4

The GO was dispersed in water by

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to remove unreacted graphitic particles. The GO particle water dispersion was measured by mass

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to have a concentration of 0.3 mg/g. A Laurell: WS-650Sz-6NPP-Lite spin processor was used to

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apply a single layer of GO particles to V5 cleaved sheet of mica (Ted Pella). A NT-MDT

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NTEGRA atomic force microscope was used to characterize the dimensions of the GO sheets.

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The sheet sizes in the collected filtered GO/water dispersion were observed to be 0.3-1 µm

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laterally and 1 nm in height, Figure 1.

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Figure 1: AFM height image (from GO-water dispersion) distributed on mica.

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Dry 11-aminoundecanoic acid powder, Sigma Aldrich, was mixed with the filtered

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GO/water dispersion at a concentration of 1 and 5 mg of GO per gram of powder. Water was

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added to the mixture for a total volume of 15 mL. An ultrasonication tip was submerged in the

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mixture and sonication at 35 % of maximum power proceeded for 30 minutes with continuous

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stirring at 300 rpm and a maximum temperature of 30 ˚C.

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The homogeneous golden-brown mixture of GO-monomer settled with a clear water

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supernatant. The mixture was poured into a Teflon® lined vessel and placed at 100 ˚C in an oven

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for 60 minutes. The dried powder was uniformly brown. The GO-monomer powder was placed

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under argon at 235 ˚C for 4 hours of melt condensation static polymerization, and thereby GO5

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PA11 was made. PA11 fabrication followed the same method. For convention, a concentration of

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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 carbon oxygen ratio in-situ in a

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polymer matrix 46. GO was heated under nitrogen gas to 250 ˚C and found to have a C/O ratio of

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5.3 via elemental analysis. GO was assumed to have increased in hydrophobicity by reducing

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within PA11 in-situ during polymerization.

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Aging samples were prepared by slowly hot pressing the PA11 or GO-PA11 between two

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Teflon® surfaces at pressures up to 16 MPa at 250 ˚C under argon. The system was allowed to

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cool under flowing argon to room temperature. The approximate dimension of each film was 60

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cm2 with a 300 µm thickness. Coupons of 1 cm2 by 300 µm thick were cut from the 60 cm2 by

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300 µm thick PA11 or GO-PA11 films. For each material tested, ten coupons were immersed in

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deionized water over a period of 3 - 4 months at elevated temperature to accelerate aging. Two

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independent sets were studied at the temperatures of 100 and 120 ˚C. High-pressure rated #40

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glass tubes with Teflon® plugs (Ace Glass) were used as containment vessels. Before sealing the

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pressure tubes, dissolved oxygen was removed by sparging the water with argon in-situ until the

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measured oxygen concentration dropped below 50 ppm (Oakton Waterproof DO 450 Optical

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Dissolved Oxygen Portable Meter). Samples were removed periodically from each pressure

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tube, and then each pressure tube was degassed and resealed for continued aging of the

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remaining samples.

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Mass average molecular weight (Mm) was measured by in-line Shodex size exclusion

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chromatography columns (HFIP-LG, HFIP-805, and HFIP-803) paired with a Wyatt miniDAWN

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light scattering detector and Wyatt Optilab 803 dynamic refractive index detector. The

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miniDAWN had a laser wavelength of λ = 690 nm and three detection angles θ of 45, 90, and

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135˚. The solvent used to dissolve the PA11 samples was 1,1,1,3,3,3-hexafluroisopropan-2-ol

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(HFIP) doped with 0.05 M potassium tri-fluroacetate (KTFA) salt to remove nylon

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agglomerations

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helium sparge under atmospheric pressure. The polymer solution concentration was 2 mg/mL

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(PA11/KTFA-HFIP). A 100 µL aliquot of solution was injected for measurement. A

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0.235 mL/g determined the concentration of the PA11 at each fraction per the refractive index

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measurement. The

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concentrations. The measured Mm values had a 6% error margin, determined by the standard

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deviation divided by the mean for six consecutive Mm measurements on a single solution of

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PA11.

dn

/dc of

/dc was experimentally measured for PA11/KTFA-HFIP using the solution

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. The KTFA doped HFIP was degassed in the pump reservoir via constant

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The GO-PA11/KTFA-HFIP solutions were darkened by the presence of GO. All visible GO

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in each solution was removed by a 0.45 µm porosity polytetrafluoroethylene filter. Additionally,

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the 0.2 µm stainless steel frit column guard on the SEC-MALLS was unaffected by injections of

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clear solutions that were passed through the 0.45 µm filter. The unaffected 0.2 µm frit indicated

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that the polymer solutions being measured on the SEC-MALLS were free of GO contamination.

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Differential scanning calorimetry (DSC) was used to measure the heat of fusion and re-

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crystallization in PA11. The DSC (TA Instruments Q20) was calibrated using indium. A ramp

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rate of 3 ˚C/min was used to heat, cool, and re-heat the samples under nitrogen from 40 to

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220 ˚C. The integration limits were 140 to 200 ˚C for the heating ramps and 130 to 190 ˚C for

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the cooling ramp. The root of the peak onset maximum slope with the peak baseline set to zero

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was the fusion or crystallization onset temperature.

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Optical microscopy images were taken with an IX71 Olympus Inverted microscope and a

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LUCPLFLN, 40x (0.6 NA) objective. Transmission bright field and cross polarized light images

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were taken. For PA11 and 1-GO-PA11, optical glass microscope slides were prepared by melt

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pressing the PA11 or GO-PA11 for 30 seconds at 250 ˚C under argon, then ambient cooling to

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room temperature under argon.

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Microtome to obtain a sample less than 150 nm, for optical transmission.

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The 5-GO-PA11 was sliced using a Leica Ultracut UCT

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was used

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to probe the chemical functional groups and intermolecular interactions. Spectra were taken

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using an IR Tracer-100 Shimadzu FTIR with a MIRacle 10 Single Reflectance ATR accessory at

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a resolution of 2 cm-1 from 600 to 4000 cm-1 and 32 scans were averaged. The spectra were

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adjusted to a common baseline and normalized to the 2851 cm-1 peak. To analyze the carbonyl

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oxygen peak, the work of Skrovanek, et al.

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peak was de-convoluted using MagicPlot software, and the relative concentration of free and

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hydrogen bonded amide oxygen groups were measured at 1684, 1654, and 1635 cm-1. The

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heights of the de-convoluted peaks were used to compare the relative concentration of hydrogen

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bonded carbonyl oxygen groups. The half-height peak width of the 3304 cm-1 N-H absorption

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was also used to probe the hydrogen bonding behavior of the amide hydrogen.

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was followed. The 1635 cm-1 carbonyl oxygen

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The equilibrium molecular weight (Mme) hydrolysis rate constant (kH), and solid state

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polymerization (kP) rate constant were determined during accelerated molecular weight

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degradation by fitting the literature mathematical model of amide hydrolysis to the measured Mm

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per aging time

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hydrolysis kinetics described by Equation 2 and the associated first order ordinary differential

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Equation 7. The values kH and kP were fit to the changing values of molecular weight over time

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in days and reflected the effect of the aqueous aging environment on the hydrolysis process.

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49-51

. The previously published model was derived from the condensation-

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In the model, Equation 10, was derived from the hydrolysis kinetics of Equation 1 - 3, and

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the associated first order ordinary differential solution, Equation 9. The values kH and kP were fit

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to the changing values of molecular weight over time via the relationship described in Equation

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10 where at was the average number of amide bonds at time t. Subscript “0” referred to the

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starting value, “e” referred to the equilibrium value, and “t” referred to the value associated at

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time t.

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Equation 1

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184 185 186

Equation 2

Equation 3

190 191 Equation 4

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Equation 5

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Equation 6

Equation 7

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Equation 8

204 205 Equation 9

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Equation 10

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The hydrolysis degradation model in terms of the mass average number of amide bonds at

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time t (at) was defined by Equation 10. The derivation of Equation 10 started with the steady

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state approximation at equilibrium, Equation 3

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assumed to be constant and thereby Equation 6 was used to form the first order ordinary

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differential Equation 7. At the steady state, equilibrium, dx/dt was defined to be zero and thereby

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rearranging Equation 7 in terms of of kP resulted in Equation 8. Equation 9 was the solution of

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the differential equation that resulted from substituting Equation 8 into Equation 7. Equation 10

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was Equation 9 rearranged in terms of at. Equation 5 established that at was proportional to the

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measured Mm by the molecular weight of the monomer. Using Equation 5, the changes in the

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Mm data over time were fit to Equation 10 to determine kH using a non-linear least squares fit.

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The model fit determined both the Mme and kH. Then, the best fit of Mme and kH were used in

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Equation 8 to calculate the re-polymerization rate constant kP.

.

Then the concentration of water was

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Results and Discussion The Mme of PA11 is very important to the performance properties of PA11. If the Mme is

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above the ductile-brittle transition Mm then PA11 can retain its ductility 1. The Mme of 1-GO-

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PA11 was 10 kg/mol larger than that obtained for PA11 at both 100 and 120 ˚C, respectively.

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This was an impressive increase of 40 % at both temperatures.

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Using Equation 10 and a non-linear least squares fit, the kH and solid state kP rate constants

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were determined and listed in Table 1. Figure 2 and Table 1 showed that the kH of 1-GO-PA11

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was reduced by a factor of 2 and the Mme of 1-GO-PA11 was increased by 40 % at both

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temperatures. At 120 ˚C, both kH and kP were reduced for 1-GO-PA11 compared with PA11.

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The higher loading, 5-GO-PA11 showed no change in the rate of hydrolytic degradation

234

compared with PA11 at both temperatures. The absence of this effect at the higher concentration,

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5-GO-PA11, was attributed to a poor nano-particle dispersion as shown in Figure 3. The optical

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images of 5-GO-PA11 showed that GO was highly agglomerated, the GO sheets were in contact

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with each other, Figure 3. The 1-GO-PA11 lacks sharp features in the optical micrograph,

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compared with the defined sharp features sized from 3 µm to 15 µm in 5-GO-PA11, GO

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agglomerates. This agglomeration resulted in no effect on the Mme compared with the finely

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dispersed GO in the 1-GO-PA11 nano-composite.

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Figure 2: Dependence of Mm on time when aged in water at 100 and 120 ˚C for PA11, 1-GO-PA11, and

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5-GO-PA11. Mme of 1-GO-PA11 was approximately 10 kg/mol higher than PA11 and 5-GO-PA11 when

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aging in water at 100 ˚C and 120 ˚C, respectively.

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Table 1: Rate constants and Mme for GO-PA11 aged in water; determined using Equation 8 and a non-

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linear least squares fit.

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PA11 1-GO-PA11 5-GO-PA11 PA11 1-GO-PA11 5-GO-PA11

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Temperature, °C 100

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kH,×10-2 day-1 2.8 ±0.5 1.8 ±0.5 2.9 ±0.6 12 ±2 4.5 ±1.1 13 ±2

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kP, ×10-5 day-1 5.0 ±1.5 10.0 ±4.0 6.8 ±2.4 23 ±7 17 ±5 19 ±5

Mme, kg/mol 24 ±3 34 ±4 24 ±3 24 ±3 34 ±3 23 ±3

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Figure 3: Transmission optical microscopy of PA11 bright field (a), cross-polarized (b), 1-GO-PA11

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bright field 6c), and 5-GO-PA11 bright field (d). Each image is sized at 60 x 60 µm.

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PA11 is a semicrystalline polymer. Amide hydrolysis in PA11 is understood to occur in the

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amorphous regions rather than the impermeable crystalline domains. A key component of the

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crystalline formation is hydrogen bonding. Hydrogen bonding in the PA11 matrix is an

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intermolecular interaction between the amide hydrogen and the carbonyl oxygen of a

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neighboring amide bond. Hydrogen bonding also occurs between nearby amide bonds in the

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amorphous regions. The hydrogen bonding between adjacent amide groups is responsible for the

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higher melting temperatures of polyamide crystalline structures versus a non-polar

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semicrystalline polymer such as polyethylene. Commercially prepared PA11 is plasticized with N-n-butylbenzenesulfonamide (BBSA) and 49,52

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shows a similar increase in the Mme over neat PA11

. The BBSA increases the amorphous

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content in PA11 by disrupting hydrogen bonding. BBSA inserts itself to replace the amide/amide

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interaction with a sulfonamide/amide interaction. The C-N-H – O=C hydrogen bond is replaced

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by a stronger S-N-H – O=C hydrogen bond, and a weaker C-N-H – O=S hydrogen bond.

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Effectively, BBSA blocks intermolecular hydrogen bonding between neighboring amide bonds

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for lower crystallinity and increased amorphous content 53.

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In the 1-GO-PA11, there were three species involved in the amide hydrolysis reaction:

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polyamide chains, GO, and water. The elementary chemical mechanisms for chain scission were

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identified: base catalyzed

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energies had been determined to be 21, 31, and 99 kJ/mol for the base catalyzed, acid catalyzed,

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and water assisted mechanisms of amide hydrolysis, respectively

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found activation energies of 66, 78, and 147 kJ/mol. Thus, given equivalent concentrations of

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OH- and H+ the base catalyzed pathway was favored; and the water assisted hydrolysis of an

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amide bond was the least favored pathway.

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59-62

, and water assisted

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63,64

. The activation

. Computationally, Zahn

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, acid catalyzed

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54-58

Equation 11: Elementary chemical mechanism of resonance stabilization of the amide bond.

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14

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Equation 12: Base catalyzed elementary chemical mechanism of amide hydrolysis.

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Equation 13: Acid Catalyzed elementary chemical mechanism of amide hydrolysis.

Equation 14: Elementary chemical mechanism for water assisted Hydrolysis of the amide bond.

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Equation 11 shows the initial resonance stabilization of the amide bond. stabilization chemically shortens and strengthens the amide bond.

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Resonance

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The base catalyzed hydrolysis mechanisms were drawn in Equation 12. Zahn

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computationally studied the mechanism of base catalyzed hydrolysis of the amide bond, and

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found that the base catalyzed Pathway II was favored. Zahn

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reaction led to the simultaneous protonation of the amide nitrogen and deprotonation of the

293

hydroxyl group. Since the amide anion was a poorer leaving group than an alkoxide ion, the base

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catalyzed hydrolysis the proton transfer to the amide anion was considered the rate limiting step

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66,67

also found that a proton transfer

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The four-step acid catalyzed hydrolysis mechanism was drawn in Equation 13. The first step

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was the protonation of the carbonyl oxygen of the amide bond. Zahn 68 found in a computational

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study that the second step, nucleophilic attack of a water molecule to the carbon atom of the

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amide group, was the rate determining step. The third step was the intermediate de-protonation

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of the carbonyl oxygen and protonation of the amide nitrogen, quickly followed by the

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dissociation of the protonated amine and the carboxylic acid.

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The water assisted hydrolysis mechanism was drawn in Equation 14. The water assisted

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mechanism involved two water molecules that acted in a concerted process and utilized a

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Grotthuss mechanism of proton hopping through hydrogen bonding. The first step and rate

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determining step was the nucleophilic attack of the amide carbon and simultaneous protonation

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of the amide nitrogen 63.

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The crystalline regions in PA11 were impermeable to water molecules: therefore, hydrolysis

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took place predominately in the molecularly mobile amorphous regions. Unaged PA11 and GO-

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PA11 had the same crystalline content and therefore no change in hydrolysis was expected.

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However, the well dispersed 1-GO-PA11 showed unexpected decreases in the kH at both

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temperatures and a 40 % higher Mme than PA11, Figure 2.

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From the amide bond hydrolysis elementary mechanisms for base hydrolysis Equation 12,

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acid hydrolysis Equation 13, and water assisted hydrolysis Equation 14, it was clear that both the

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availability of water molecules and hydrogen bonding were required for amide hydrolysis. Thus,

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the BBSA’s disruption of inter-chain hydrogen bonding also inhibits hydrolysis. Changes in the

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hydrogen bonding behavior was another possible explanation for GO inhibiting the hydrolysis

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process in 1-GO-PA11, in addition to its large immobile nano-sheet structure that inhibited

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polymer chain and water mobility. Skrovanek, et al.

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first studied the use of FTIR to detect hydrogen bonding interactions 48

found de-convoluting the 1635 cm-1 peak of the

within a PA11 polymer. Skrovanek, et al.

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carbonyl oxygen revealed three peaks that could be characterized at 1635, 1654, and 1684 cm-1.

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The 1635 cm-1 sub-peak was a lower energy carbonyl vibration correlated to aligned hydrogen

323

bonding. The 1654 cm-1 peak was un-aligned amorphous hydrogen bound carbonyl oxygens. The

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1684 cm-1 peak was correlated with carbonyl oxygen groups that were free of hydrogen bonding.

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The amide bond 1635 cm-1 carbonyl oxygen peak in PA11, 1-GO-PA11 and 5-GO-PA11 was

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deconvoluted based on the results of Skrovanek, et al.

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carbonyl oxygen peaks for PA11 and GO-PA11 from deconvolution of the 1635 cm-1 amide

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C=O. Further details were provided in the supplementary information.

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. No difference was observed at the

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

Infrared spectroscopy was used to probe the PA11 inter-molecular interactions, and

333

hydrogen bonding behavior at the amide bonds. Figure 4 shows the FTIR spectra for PA11 and

334

1-GO-PA11, pristine and aged. The characteristic peaks were: 1158 cm-1, an interaction of the N-

335

H stretch and O=C-N deformation; 1539 cm-1, amide II C-N stretch; 1635 cm-1, amide I C=O

336

stretch; 2851 cm-1, symmetric C-H aliphatic vibration; 2919 cm-1, asymmetric C-H aliphatic

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vibration; and 3304 cm-1, amide III N-H stretch.

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Figure 5: The peak widths centered at 3304 cm-1 for PA11, 1-GO-PA11, and 5-GO-PA11; and grouped

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by unaged and aged at 120 ˚C for 90 and 120 days, respectively. The 3304 cm-1 peak widths are

342

associated with the distribution of hydrogen bonding orientations of the amide bond hydrogen.

Figure 5 compared the half height peak widths of the 3304 cm-1 amide (N-H) stretching peak

344

for the PA11 and 1-GO-PA11, pristine and aged. The peak widths were associated with the

345

distribution of hydrogen bonding orientations of the amide bond hydrogen. With aging, PA11

346

showed a decline in the distribution of hydrogen bonding orientations; from 54 to 34 ± 2 cm-1.

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The 1-GO-PA11 showed less of a decline in the 3304 cm-1 peak widths; from 54 to 39 ± 2 cm-1.

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The broader distribution of hydrogen bonding orientations in the GO-PA11 had also been

349

correlated previously to the lower overall crystallinity content 48.

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In aged 1-GO-PA11 the N-H stretch had a broader distribution than aged PA11, Figure 5.

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Further 1-GO-PA11 had an increased intensity at the 1158 cm-1 peak, an interaction of the N-H

352

stretch and O=C-N deformation. These results showed that the GO with C=O groups present on

353

the surface

69

were interacting with the N-H amide hydrogen of polyamide. This interaction

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reduced the molecular mobility of the PA11 chain and thereby reduced the rate of hydrolysis and

355

rate of the crystallization. A transmission optical microscope was used in cross-polarization mode and the

357

birefringence of the crystalline regions within PA11 and 1-GO-PA11 was observed. In

358

transmission mode, the light passed through the sample. With cross polarizers, the birefringent

359

and therefore visible crystalline regions of PA11 and 1-GO-PA11 were imaged. Figure 6 showed

360

that well dispersed GO within the polymer matrix altered the crystalline morphology of PA11.

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The 1-GO-PA11 showed less birefringence than PA11. The aged PA11 showed the most

362

birefringence, and banded spherulites, broader bands of birefringence, for both the 100 and 120

363

˚C with widths of 0.9 ± 0.3 and 1.0 ± 0.2 µm, respectively. The banded spherulites were

364

associated with bundling of laminar stacks and higher crystallinity

365

laminar stacks in aged PA11 were absent in the aged 1-GO-PA11 optical images. The bundles of

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laminar stacks in the 100 and 120 ˚C aged 1-GO-PA11 appeared much thinner with widths of 0.7

367

± 0.2 and 0.7 ± 0.1 µm, respectively. Zhang, et al.

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montmorillonite-PA11 nano-composites. These smaller crystalline domains agreed with the

369

lower crystallinity DSC results for 1-GO-PA11 in Table 2.

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. The larger bundled

reported a similar crystalline behavior in

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Figure 6: Cross polarized transmission optical microscopy images of 100 and 120 ˚C 80-90 days aged

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PA11 and 1-GO-PA11. Each image is sized at 60 x 60 µm.

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When fully dissociated and dispersed, the GO nanoparticles acted as large immobile 2-D

374

nano-thin barriers within the PA11 matrix. Consequently, they were predicted to have inhibited

375

molecular mobility within the polymer. As such, the results suggested that the GO nano-sheets

376

reduced molecular mobility for both PA11 chains and water for less accessible hydrolysis sites.

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GO’s effect on the rate of crystallization was characterized in PA11 and GO-PA11 - unaged and

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aged using DSC, which provided evidence for GO’s role in reducing chain mobility.

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45,70,73-81

Nanoparticles had been known to affect the crystalline morphology of PA11

. Figure 7

380

showed the DSC curves of the un-aged and 80-90 days aged samples in three columns from left

381

to right to indicate the heat (fusion), cool (crystallization), and heat (fusion) curves. The peak

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integration values of specific heat of fusion (∆Hfus) and crystallization (∆Hc) were listed in Table

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2.

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In Table 2, a lower total ∆Hfus indicated a decrease in the crystallinity of the GO-PA11 after

385

heat, cool, and re-heat ramps. Peak and onset temperatures were respectively tabulated in Table 3

386

and Table 4 for the fusion and crystallization of PA11 and GO-PA11. The effect of the previous

387

thermal history on the semi-crystalline behavior of PA11 was revealed in the first heat ramp. The

388

first heat ramp both revealed the previous thermal history on the semi-crystalline behavior of

389

PA11 and melted the PA11 to erase that previous thermal history. The 3 ˚C/min controlled

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cooling ramp measured the affect that GO has upon the crystallization behavior of PA11, both

391

nucleation and growth. The second heating ramp compared how the total crystallization growth

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was affected by the GO loading.

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During aging at 100 and 120 ˚C the PA11 matrix was annealed resulting in an increase in

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crystallinity. Quantified by the peak total area, the first heating ramp revealed the extent of

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annealing for the PA11 and GO-PA11 samples via their ∆Hfus. Before aging, PA11 had 25%

396

crystallinity, using 189 J/g as 100% crystalline PA11

397

crystallinity increased to 34 and 48% at 100 and 120 ˚C; a relative increase of 36 and 92%,

398

respectively. The 1-GO-PA11 started with a crystallinity of 25% and had lower relative increases

399

of 12 and 76% at the same annealing conditions.

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. Upon aging and annealing PA11 the

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Figure 7: DSC scans for un-aged, 100 ˚C, and 120 ˚C for 80-90 days aged PA11, 1-GO-PA11, and 5-

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GO-PA11. The 3 columns from left to right were 1st heat ramp from 40 ˚C to 220 ˚C, followed by a cool

403

ramp to 40 ˚C, and a 2nd heat ramp to 220 ˚C. The rows top to bottom were un-aged, 100 ˚C, and 120 ˚C

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aged samples.

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Table 2: Integration values of DSC heat flow peaks from 140 to 200˚C for GO-PA11 and PA11, aged

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(80-90 days) and un-aged samples. The error margin was 2.5 J/g per measurement and peak integration.

120 ˚C

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Cooling Ramp ∆Hc, J/g 50.0 42.5 44.0 60.0 50.0 53.5 65.5 53.5 49.5

2nd Heat Ramp ∆Hfus, J/g 48.0 49.0 49.0 60.0 55.0 56.0 61.0 59.0 51.0

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1st Heat Ramp ∆Hfus, J/g 47.5 46.5 46.0 63.0 53.0 57.5 91.0 82.0 83.0

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GO Loading, mg/g 0 1 5 0 1 5 0 1 5

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Table 3: DSC peak temperatures for GO-PA11 and PA11, aged (80-90 days) and un-aged samples. The

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peak temperatures for melting(Tm,fus) and re-crystallization (Tm,c) were accurate within 0.5 ˚C.

100 ˚C

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1st Heat Ramp Tm,fus, ˚C 187.5 190.0 187.5 189.5 191.0 189.5 184.5, 190.5 182.0, 187.0, 191.0 183.0, 188.5

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Cooling Ramp Tm,c, ˚C 166.5 169.5 169.0 168.5 173.0 174.0 170.0 173.5 174.5

2nd Heat Ramp Tm, fus, ˚C 187.5 188.5 187.0 189.5 190.0 187.5 190.0 191.0 187.5

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These results showed that the GO nano-sheets hindered annealing in PA11 during aging at

414

these temperatures. The 1-GO-PA11 samples had the lowest ∆Hfus in both 100 and 120 ˚C

415

conditions and indicated a large immobilizing effect on the polymer chains. The decreased effect

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of the 5-GO-PA11 was attributed to agglomeration and poor dispersion of 2-D nano-particles as

417

previously described.

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Table 4: DSC onset melting and formation temperatures for GO-PA11 and PA11, aged (80-90 days) and

419

un-aged samples. The onset melting(To,fus) and re-crystallization (To,c) temperatures were accurate within

420

0.5 ˚C.

120 ˚C

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2nd Heat Ramp To,fus, ˚C 177.5, 178.5 181.0 178.0 178.0, 183.0 181.5 177.5 180.0, 184.5 182.5 171.0

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Cooling Ramp To,c, ˚C 167.0 175.0 176.0 170.5 179.0 180.0 172.0 179.5 181.0

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1st Heat Ramp To,fus, ˚C 175.5, 180.5 178.5, 180.0 175.0, 176.5 182.5, 183.5 179.5, 183.5 177.5, 179.5 181.0, 186.5 174.0, 177.5, 181.5 171.0

GO Loading, mg/g 0 1 5 0 1 5 0 1 5

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The melting temperatures (Tm,fus) in Table 3 for PA11 and GO-PA11 were indicative of the

423

thermal stability of the polymer crystalline regions. A lower pre-peak Tm,fus, 184.5 ˚C, became

424

increasingly prominent for the PA11 when aged at 120 ˚C. The 184.5 ˚C pre-peak was attributed

425

to poorly formed crystalline structures

426

pre-peak at 184.5 ˚C in the first heating ramp after aging at 120 ˚C. Conversely, the poorly

427

formed crystals were not favored in the 1-GO-PA11 and there was an additional intermediate

428

melting peak at Tm,fus 187.0 ˚C. The highest Tm,fus of 191.5 ˚C indicated that the annealed GO-

429

PA11 had the most stable crystalline structure. The increased annealed crystalline melt

430

temperature after aging suggested intermolecular attractive interactions between the GO sheets

431

and the polyamide matrix as was seen in the ATR-FTIR spectra.

. As seen in Figure 7, the PA11 had the largest

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After the first heating ramp on the DSC, the polymer sample melted and its thermal history

433

erased. With controlled cooling the heat of re-crystallization (∆Hc) can be characterized. The

434

decrease in the ∆Hc for GO-PA11 compared to PA11 indicated that the chain mobility was

435

inhibited by the inclusion of GO nanoparticles. 25

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The GO-PA11 had a higher onset temperature of re-crystallization during the cooling ramp

437

(To,c) and lower ∆Hc value than PA11, Table 4. The pristine PA11 had a To,c of 167 ˚C that

438

increased to 170 (+2%) and 172 (+3%) ˚C after aging at 100 and 120 ˚C. The increase in To,c

439

after aging correlated to lower molecular weights. For a polyamide, crystalline regions form

440

more quickly at lower molecular weights due to increased mobility and ease of alignment of the

441

shorter polymer chains 84. Both GO-PA11 showed a 9 ˚C higher To,c with a broader reaction peak

442

than unloaded PA11. The 1-GO-PA11 samples initially had a To,c of 175 ˚C. After aging at 100

443

˚C, the To,c increased to 179 (+2%) ˚C; and aging at 120 ˚C increased the To,c to 180 (+3%) ˚C.

444

The increased To,c of the GO-PA11 samples were evidenced that the GO nano-particles were

445

nucleating sites resulting in higher crystallization onset temperatures.

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In summary the large immobile GO, a 2-dimensional sheet in the amorphous phase, in

447

addition to inhibiting molecular mobility, competed for hydrogen bonding with the polyamide’s

448

amide hydrogens. Both effects inhibited hydrolysis of the amide bond. It is the decrease in chain

449

and water molecular mobility combined with hydrogen bonding interactions between GO and the

450

N-H bonds of the polymer chains that led to a lower hydrolysis rate for the well dispersed 1-GO-

451

PA11, Table 1. The effects of GO’s large sheet shape and its surface functional groups

452

interacting with the polymer chain, were similarly shown to have reduced polymer chain

453

mobility in polyimide 85.

454

Summary & Conclusion

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A concentration of 1 mg/g GO in PA11 produced a 40% increase over PA11’s equilibrium

456

molecular weight when aged in water at 100 and 120 ˚C, respectively. No change was observed

457

in the 5 mg/g GO-PA11 due to GO agglomeration. Fitting the previously reported kinetic model

458

to aging results showed an increase in equilibrium molecular weight, when the GO particles were 26

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459

well dispersed. This was due to a reduction in the model’s hydrolytic rate constant relative that

460

for re-polymerization. Measured by the heat of fusion, GO-PA11’s decreased crystallization indicated that the GO

462

decreased molecular mobility within the nanocomposite, specifically the polymer chain

463

molecular mobility during the annealing process.

464

nanocomposites showed a slight decrease in the amount of PA11 crystallinity prior to aging.

465

Upon aging at both 100 and 120 ˚C, the GO-PA11 had significantly less crystallinity and

466

demonstrated a reduction in the rate of annealing. After removing the thermal history by heating

467

above the melting temperature and cooling, the materials were nearly comparable and suggested

468

no effect on the crystallinity. The decrease in the annealing rate of crystallization in the aged

469

samples was attributed to GO’s large immobile nano thickness sheet structure, which inhibited

470

chain molecular mobility within the polymer nano-composite.

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Both 1-GO-PA11 and 5-GO-PA11

ATR-FTIR spectra showed that the graphene oxide’s surface C=O groups hydrogen bond

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with the polyamide’s N-H groups. This interaction further inhibited polymer mobility. Two factors significantly reduced molecular mobility in the GO-polymer and resulted in a

474

reduction in the rate of crystallization and most importantly the rate of degradation by

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hydrolysis: the large immobile nano thin GO sheets and the intermolecular interaction between

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the GO’s surface C=O groups with the polyamide’s N-H groups.

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1. Graphene oxide reduces extent of hydrolytic degradation in polyamide-11. 2. Graphene oxide reduces molecular mobility in polyamide-11. 3. Polyamide's amide hydrogen bonds with the graphene oxide surface