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Crystallization of Poly(butylene succinate) on Rapid Cooling and Heating : Towards Enhanced Nucleation by Graphene Nanosheets Nicolas Bosq, Nathanael Guigo, Duangdao Aht-Ong, and Nicolas Sbirrazzuoli J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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The Journal of Physical Chemistry

Crystallization of Poly(butylene succinate) on Rapid Cooling and Heating : Towards Enhanced Nucleation by Graphene Nanosheets

Nicolas Bosq,‡ Nathanaël Guigo,† Duangdao Aht-Ong,‡* Nicolas Sbirrazzuoli†*

‡

Chulalongkorn University, Faculty of Science, Materials Science Department, 10330 Bangkok, Thailand †

Université Côte d’Azur, Institut de Chimie de Nice, UMR CNRS 7272, 06108 Nice, France

ABSTRACT Fast Scanning Calorimetry (FSC) was employed to analyse the crystallization kinetics of Poly(butylene succinate) (PBS) graphene nanocomposites. The emphasis of the present study is made on the heterogeneous nucleation of the PBS crystals in the presence of graphene nanosheets. Graphene oxide nanosheets were first synthesized, then reduced and homogeneously dispersed into PBS matrix. The internal structure of neat PBS and of the nanocomposite was investigated by Transmission Electron Microscopy (TEM) and Wide Angle X-ray Diffraction (WAXD). The crystallization from the melt and from the glass studied at high cooling and heating rates via FSC appeared to be significantly promoted in the presence of graphene. The presence of secondary crystallization on heating and the higher crystal perfection observed was attributed to the formation of graphene network inside the PBS matrix. The activation energies of melt and glass crystallization were combined in order to obtain one set of Hoffman-Lauritzen * corresponding authors : [email protected], Phone: +66 (0) 221 855 59; [email protected], Phone: +33 (0) 492 076 179 1 ACS Paragon Plus Environment


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parameters for each system. Finally, the growth rate of PBS crystals appeared to be highly enhanced in the presence of graphene nanosheets.

1. Introduction The environmental problems due to the intense use of polymers lead to a high consideration for polyesters that low impact on environment. For these reasons, biodegradable polymeric materials have been developed at the industrial and commercial scales for many years.1 The Poly(butylene succinate) (PBS), a biodegradable polyester synthesized from the polycondensation between biobased succinic acid and 1,4 propanediol. It appears to be the most promising polymer from the family of biodegradable materials.2 This semi crystalline aliphatic polyester is expected to be a part of the growth market demand since a large production of PBS is currently performed. Indeed, the PBS is employed in a wide range of applications due to its good processability and its many suitable properties. Several types of fillers such as clay3 or carbon nanotubes4 were used to modify the PBS micro-structure and consequently their thermomechanical properties. Since the micro-structure of a semi-crystalline polymer is tightly linked to its crystallization,5,6 a careful attention has to be paid on the crystallization kinetics of polymers. Thus, the effect of fillers on the crystallization of PBS was previously investigated in the literature. The variation of PBS crystallization behavior in the presence of surface treated carbon nanotubes was highlighted by Tan et al.7. Additionally, Bian et al.8 showed that silica nanoparticles induce a nucleating effect on the crystallization of PBS. Currently, graphene is widely involved in the preparation of polymer nanocomposites due to its unique structure and its outstanding properties.9 The graphene displays a high young’s

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modulus and fracture strength10 but also presents a good thermal11 and electrical12 conductivities. Graphene can be obtained from the oxidation of pure graphite (i.e. graphite oxide) that presents a structure formed of several layers. Each layer is defined as a graphene oxide sheet that is highly oxygenated via the presence of hydroxyl and epoxide groups on its basal plane, presenting also carbonyl and carboxyl groups on its edges.13 Graphene sheets (i.e. graphene oxide sheets after reduction) are known to be the strongest material developed so far,14 and attract attention with their ability to be dispersed into various polymer matrixes.15,16 Thus, the exfoliation of graphite oxide that allows to produce stable suspension of those two-dimensional carbon sheets and their insertion as a filler into polymer matrixes are largely employed.17 Klonos et al.18 investigated the effect of graphene oxide (GO) on the glass transition of poly(l-lactide acid). The authors remarked the variation of α-relaxation that appear to be broader and faster in the presence of GO. Besides Wang et al.19 investigated the crystallization kinetics of syndiotactic polystyrene in the presence of graphene oxide and observed a higher conductivity threshold in the case of the nanocomposite. Jin et al.9 studied the crystallization and the mechanical performance of PBS/GO nanocomposite obtained by in-situ polymerization. They consequently found out a variation of PBS performance with the insertion of low content of GO and showed that this latter can act as a nucleating agent. Finally the work of Papageorgiou et al.20 showed that the GO tends to aggregate in the PBS matrix but also modifies strongly its crystallization kinetics. Indeed, the interplay between the matrix and the filler appears to be highly governed by the morphology and the surface chemistry of this latter. The PBS displays both hydrophobic and hydrophilic properties, and many attempts were carried out to control the balance between its hydrophobicity and its hydrophilicity.21,22,23 However the graphene oxide is rather hydrophilic due to numerous hydroxyl groups present onto its surface. It is well known that the nature at molecular scale of



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the interface between the filler and the matrix induces major variation of the polymer physical transitions. Additionally, to our knowledge no investigations were performed on PBS crystallization kinetics at high cooling and heating rates in the presence of chemically reduced graphene oxide (rGO). The originality of the present work lies in the fact that the GO was synthesized by the improved method developed by Marcano et al.24. This method provides many advantages compared to the conventional Hummers’ methods, such as a higher yield of GO production with a more stable structure presenting less defects in the basal plan. Moreover, as a previous study mentions the aggregation of GO in the PBS matrix,20 the chemical reduction of the GO was hereby performed ex-situ using ascorbic acid25,26,27 in order to improve its dispersion. The reduction was then verified by Fourier Transform Infra-Red (FTIR) spectroscopy and solid state Nuclear Magnetic Resonance (NMR). The nanocomposite was prepared by solvent casting since the melt-blending method leads to poor dispersion results.9 Firstly, TEM was employed to verify the quality of reduced GO dispersion that strongly affects the crystallization of PBS matrix. WAXD analysis was also performed in order to obtain information on the crystal structure and on the crystallinity of the samples. For the first time, the crystallization of this nanocomposite was analyzed from the melt and from the glass at high cooling and heating rate using Fast Scanning Calorimetry (FSC). The thermoanalytical data were computed by the advanced isoconversional method in order to obtain kinetic parameters of crystallization. Finally, Hoffman-Weeks28 and Hoffman Lauritzen29 theories were employed to calculate the parameters U*, Kg and G/Go related to the diffusion, the nucleation and the crystal growth, respectively.

2. Experimental


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2.1 Material PBS was purchased from PTT-MCC BioChem (Thailand) presenting a density of 1.26 g cm-3 and a melt flow index of 5 g/10 min (preheated at 190 °C and using a standard weight of 2.16 kg). Graphite powder (< 20µm, m.p. = 3652-3697 oC, Mw = 12.01 g.mol-1) was provided by Aldrich Chemicals. Sulfuric acid (b.p. = 330 oC, purity > 98%), Phosphoric acid (b.p. = 158 oC, purity > 85%), Dimethylformamide (b.p. ~ 153 oC, Mw = 73.09 g.mol-1, purity > 99.8 %) and Chloroform (b.p. ~ 61 oC, Mw = 119.38 g.mol-1, purity > 99.8 %) were purchased from RCI Labscan. Potassium Permanganate (m.p. = 240 oC, Mw = 158.03 g.mol-1) was obtained from Thermo Fisher Scientific and Hydrogen Peroxide (b.p. ~ 108 oC, Mw = 34.02 g.mol-1, purity > 35 %) was provided by QReC. Ascorbic Acid (b.p. ~ 190 oC, Mw = 176.19 g.mol-1, purity > 99 %) was purchased from Chem-Supply Pty. Ltd.

2.2 Synthesis of GO The graphene oxide (GO) was synthesized according to the improved method of Marcano et al.24. The graphite commercial powder (10 g) was inserted into a mixture of H2SO4/H3PO4 (400:50 mL) and KMnO4 (60 g). The blend was magnetically stirred at 50 oC for 12 h and cooled down to ambient temperature. The solution was then placed in an ice bath and 18 mL of H2O2 was gently added. After 24 h at room temperature the supernatant was decanted away. The remaining solid residue was washed several times by centrifugation (Senova, TD5M-WS) at 1800 g for 30 min with deionized water until reaching a pH ~ 7, and was bath-sonicated (Wisd Laboratory Instrument, 425 W, 60 Hz) 3 h to ensure the good separation of graphene nanosheets. The solution was finally dried overnight at 60 oC to obtain the final product.



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2.3 Reduction of GO The reduction of GO was conducted with ascorbic acid.25,26,27 A solution containing 460 mg of GO and 300 mL of deionized water was firstly bath-sonicated (Wisd Laboratory Instrument, 425 W, 60 Hz) for 3 h. Ascorbic acid (2 g) was then added to the blend under magnetically stirring, then maintained at 95 oC for 7 h. The reduced graphene oxide (rGO) was separated from the blend and washed several times by centrifugation (Senova, TD5M-WS) at 1800 g for 30 min, then vacuum filtrated using deionized water until reaching a pH ~ 7. The mixture was finally placed in the oven at 60 oC overnight to obtain dry rGO.

2.4 Preparation of PBS/rGO nanocomposite The nanocomposite was prepared by solvent casting as indicated in the literature.30 rGO was dispersed in DMF by bath-sonication (Wisd Laboratory Instrument, 425 W, 60 Hz) for 1h allowing to obtain an homogeneous rGO/DMF dispersion with a concentration of 0.5 wt/vol %. Separately, a PBS/chloroform solution (1 wt/vol %) was prepared at ambient temperature by dissolving the polymer pellets in chloroform. The chloroform solution containing the polymer was mixed with the rGO/DMF solution in order to obtain a concentration of rGO at 2 wt % into the polymer matrix. The blend was kept stirring for 4 h and finally cast into Petri dishes at room temperature.

2.5 Experimental techniques FTIR spectra were measured on a Thermo Scientific Nicolet 6700 spectrometer. A total of 64 scans were recorded for each spectrum ranging from 4000 to 400 cm-1 with a resolution of 4 cm-1. The GO and rGO were dried at 60 oC under vacuum prior to FTIR analysis and were


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finally dispersed in KBr pellet. This pellet was disposed on a frame and the FTIR analysis was conducted in transmittance mode. Solid-state Single Pulse Magic Angle Spinning (SP-MAS) NMR measurements were performed at ambient temperature on a Bruker Avance-400 MHz NMR spectrometer (magnetic field 9.4 T) operating at a 13C resonance frequency of 100.7 MHz. 13C CPMAS experiments were obtained with a commercial Bruker 4 mm double channel probe. Samples with a mass of ~50 mg were placed in zirconium dioxide rotors of 4-mm outer diameter and spun at a Magic Angle Spinning with a spinning speed set at 10 kHz. The CP technique31 was applied using a ramped 1H-pulse starting at 100% power and decreasing until 50% during the contact time in order to circumvent Hartmann-Hahn mismatches.32,33 The contact time was set at 2 ms for 13C CPMAS. To improve the resolution, a 1H dipolar decoupling pulse sequence was applied during the acquisition time. To obtain a good signal-to-noise ratio, 2048 scans were accumulated using a delay of 5 s in

13

C CPMAS experiment. The

13

C chemical shifts were referenced to

tetramethylsilane (TMS) and calibrated with glycine carbonyl signal, set at 176.5 ppm. WAXD analysis was performed with a Wide Angle X-ray Diffractometer (Bruker AXS Diffractometer D8, Bruker, Karlsruhe, Germany). The CuKα radiation (k = 0.1542 nm) was employed for the analysis and the sealed tube was operated at 40 kV and 40 mA. The diffraction patterns were recorded at the values of 2θ ranging from 2 ° to 40 ° with a scanning rate of 1 °.min-1. The overall degree of the crystallinity for each sample was measured as indicated in the literature:34

X c (WAXD) =

Ac × 100% Ac + Aa

Equation 1

Where Ac and Aa are the integral area of crystal peak and the amorphous area respectively.



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The morphology and dispersion of graphene nanosheets was analyzed by TEM with a JEOL JEM-1400 using an accelerator voltage of 120 kV. Ultrathin sections (~80 nm) of samples were cut with an ultramicrotome prior to analysis. The graphene nanosheets appear black/grey colored on the micrographs. Fast scanning calorimetry (FSC) was performed on the Mettler-Toledo Flash DSC1 using the UFS1 chip calorimeter. Detail about the instrumental setup and chip calibration is mentioned elsewhere.35,36,37,38 Each sensor is calibrated by Mettler-Toledo and displays a specific serial number that is registered in the STARe software used for data treatment. The calibration of FSC is regularly checked by using indium samples. Prior to analysis the sensors were conditioned and temperature-corrected as required by the instrument feature. To run FSC experiments, a fraction of the materials was placed directly on the sample area of the sensor while the reference area remains free. Several heating and cooling scans were performed to obtain a uniform sample that remains stuck to the sensor and to optimize the surface contact between the sample and the sensor. In order to investigate the crystallization from the melt, the samples were firstly heated at 180 oC (i.e. T m0 + 65 °C) and held 1 s at this temperature to erase their thermal history. The samples were then cooled until -95 oC using cooling rates ranging from 10 until 5000 oC.s-1. In order to investigate the crystallization from the glass, the samples were firstly cooled down from the melt to -95 oC at the cooling rate of 5000 oC.s-1 to reach the amorphous state. The samples were then heated at various heating rates ranging from 10 until 1000 oC.s-1. The mass of the samples was about 300 ng in accordance to FSC technical specifications35 and was quantitatively deduced by comparing the melting enthalpy obtained in FSC (J) with the melting enthalpy obtained in conventional DSC (J.g−1). Indeed, the samples previously crystallized via those two techniques using the same cooling rate of 0.334 oC.s−1 (i.e. 20 oC.min−1) present comparable


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crystalline parts. The computations of kinetics parameters were performed using the temperature programs of 10, 20, 30, 40 and 50 oC.s−1 for the crystallization from the melt, and 20, 30, 40, 50 and 70 oC.s−1 for the crystallization from the glass.

3. Theoretical approaches 3.1. Kinetics DSC is widely employed to study the crystallization kinetics of polymer. This technique allows to measure the rate of heat released that is proportional to the macroscopic rate of extent of crystallization. The relative extent of crystallization or relative degree of crystallinity at time t defined by αt can be computed according to Equation 2: t

αt

(dH / dt) dt α ∫ = = ∫ (dH / dt) dt α 0

∞

c( t )

Equation 2

c ( ∞)

0

In this equation αc(t) and αc(∞) correspond to the relative degree of crystallinity at time t and at the end of crystallization (time t → ∞), respectively. The general form of the basic rate equation is usually written as: 39,40 dα = k (T ) f (α ) dt

Equation 3

where k(T) corresponds to the overall macroscopic rate coefficient, f(α) defines the function that represents the reaction model related to the crystallization mechanism, T is the temperature and α corresponds to the relative degree of crystallinity which varies from 0 to 1. The dependence of k(T) with temperature is given by Arrhenius law:41

k (T ) = Ae

− E / RT

Equation 4

where E is the activation energy, A the pre-exponential factor and R the universal gas constant. 9 ACS Paragon Plus Environment


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In order to overcome the drawbacks of integral methods and to take into account the variation of E in the computation of the temperature integral, an advanced isoconversional method was applied in this study. Isoconversional methods are among the more reliable kinetic methods for the treatment of thermoanalytical data.39,40,42,43,44 For crystallization kinetics, the information produced by these methods allows to obtain the effective activation energy Eα for each value of the relative extent of crystallization α. The value of Eα is determined as the value that minimizes the equation described below 45 J [Eα , Ti (tα )] i =1 j ≠ i J Eα , T j (tα ) n

n

Φ ( Eα ) = ∑ ∑

[

]

Equation 5

where J is evaluated over small intervals of E variation according to Equation 6: tα  − Eα  J [Eα , Ti (tα )] ≡ ∫ exp   dt t α − ∆α  RTi (t ) 

Equation 6

For complex crystallization processes a dependence of Eα with the relative extent of crystallization α is obtained. This dependence is then very well suited to detect and treat multistep kinetics. Additionally, the Eα-dependencies evaluated by isoconversional methods allow to disclose and handle the complexity of thermally simulated processes and to perform reliable kinetic predictions on the related mechanisms.42 One of the main advantages of these methods is that they provide the estimation of kinetic parameters without making any assumption on the crystallization (or reaction) mechanism, and they also can be applied to any temperature program. A non-linear procedure46 was used to determine the Eα values for kinetic computations. The computation of Eα values for each value of α lying in between 0.02 to 0.98 with a step of 0.02 was performed using the software developed by N. Sbirrazzuoli.47,48 This non-linear method was applied in the present study.


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3.2. Hoffman–Lauritzen theory of crystallization The Hoffman–Lauritzen theory describes the temperature dependence of the growth rate G measured microscopically,29 and is presented in the Equation 7 :

 − Kg  −U *   exp G = G0 exp  R (T − T∞ )   T∆Tf

  = G 0 term (U * )term ( K g ) 

Equation 7

where G0 is the pre-exponential factor, U* is the activation energy of the segmental jump (associated with diffusion process), T0m is the equilibrium melting temperature, ∆T=T0m-T is the undercooling, f=2T/(T0m+T) is the correction factor, T∞ is a hypothetical temperature where motion associated with viscous flow ceases that is taken 30K below the glass transition temperature (i.e. T∞ = Tg -30 K). Term(U*) and term(Kg) are the terms corresponding to the contribution of diffusion and nucleation to the growth rate, respectively. According to this theory, the crystallization rate G passes through a maximum for a given temperature Tmax. For a crystallization temperature Tc lying within the region Tmax – Tm the sample will follow the antiArrhenius behavior. This behavior is characterized by negative values of the temperature coefficient of the crystallization rate. It is also reflected by negative values of the effective activation energy computed via the isoconversional methods if the Arrhenius law is used. In the temperature domain defined by Tmax – Tm, the crystallization rate is controlled by the nucleation and displays a negative temperature coefficient. Below Tmax, the regular Arrhenius behavior should be observed and the temperature coefficient of the crystallization rate is characterized by positive values, that are reflected by positive values of the effective activation energy. For the temperatures Tc < Tmax , the crystallization rate becomes controlled by diffusion.49 Thus, the rate coefficient (or effective activation energy) should be equal to zero when T=Tmax, and should present positive decreasing values in the glass crystallization region until E=0 at T=Tmax.



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However it should present negative increasing values in the melt crystallization region until E=0 at T=Tmax. Moreover, Toda et al.50 showed that the logarithmic derivative of the microscopic growth rate G is equal to the logarithmic derivative of the overall crystallization rate defined by Ф : d ln(G ) d ln(Φ) = dT −1 dT −1

Equation 8

where Ф = ∆hSG with ∆h the volumetric heat of crystallization and S the total area of the growth surface. Vyazovkin and Sbirrazzuoli49 have proposed to compute the Hoffman–Lauritzen parameters directly from non-isothermal DSC data. This can be performed with combining Equation 7 and Equation 8 to obtain a new equation that gives the temperature dependence of the effective activation energy of crystallization:

Eα (T ) = − R

d (ln G) T2 (Tm0 ) 2 − T 2 − Tm0T =U* + K R g d (1 / T ) (T − T∞ )2 (Tm0 − T ) 2 T

Equation 9

where U* corresponds to the activation energy of molecular diffusion at the interfacial boundary between melt and crystals, and Kg corresponds to the activation energy for the nucleation of a crystal with critical size. The dependencies of Eα vs. T obtained in the present study were fitted to Equation 9. Origin 8.5 software was used to perform the non-linear fitting to the experimental Eα-dependence. The validity of Equation 9 has been recently tested by Papageorgiou et al.43 and in several papers using different polymers such as PTFE51, gelatin52 and PDMS.53

4. Results and discussion 4.1 Characterization of GO reduction


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FTIR spectroscopy was performed to verify the reduction of the graphene nanosheet surface. The FTIR spectra of GO and rGO are presented in Figure 1.

rGO

Absorbance / a.u.

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GO

3500

3000

2500

2000

Wavenumber / cm

1500

1000

-1

Figure 1. FTIR spectra of graphene. Blue line: rGO, Black line : GO.

For the surface-modified samples, the data clearly show a decrease of OH stretching vibrations at ~3500 cm-1 and a decrease of the OH bending vibration at ~1450 cm-1.25 Additionally, a decrease of the CO epoxy stretching vibration at ~1220 cm-1 and a decrease of the CO alkoxy stretching vibration at ~1050 cm-1 are observed.27 These observations confirm that most of the chemical functions composed of oxygen groups were removed after the reduction of graphene surface. The

13

C MAS NMR spectra of GO and rGO are displayed in Figure 2 and show

significant structural variations associated to the reduction of graphene oxide.



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rGO

GO

Figure 2 : CP/MAS 13C solid-state NMR spectra. Blue line: rGO. Black line: GO. All spectra were collected at a spin rate of 10 kHz. The chemical shift value at the top of the peak is indicated by the curve.

The spectra of GO displays two typical resonances of graphene oxide structure at 61 and 72 ppm corresponding to the

13

C nuclei of the epoxide and hydroxyl groups respectively.54,55,56,57 The

resonance observed at 133 ppm is assigned to the un-oxidized sp2 carbons present in the graphene network. It is worthy to note here that the resonance of the oxygenated carbons are unambiguously absent in the spectrum of rGO. However, a prominent and broad resonance is present at 120 ppm and is attributed to the variations of carbon atom environments.58 The concordance of results obtained by FTIR and NMR shows clearly that the reduction of graphene was successfully performed with the removal of the oxygenated chemical functions.

4.2 Morphology of PBS/rGO nanocomposite


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The TEM pictures of the PBS/rGO nanocomposite are displayed in Figure 3.

Figure 3: TEM pictures of PBS/rGO nanocomposite at (left) 5 µm scale and (right) 200 nm scale.

As observed at low magnification, the rGO is well dispersed into the PBS matrix and forms a network. Moreover there is no evidence of highly aggregated graphene. This can be due to the decrease of van der Waals interactions after the modification of GO.20 After the reduction of the graphene oxide the number of epoxy and hydroxyl groups decrease and lead to a structure presenting a much lower dipole value that induce a decrease of van der Waals interactions. This good dispersion consequently results in homogeneous properties inside the material. The observation at higher magnification shows that the thickness of the graphene nanosheets inserted into the nanocomposite is ~ 50 nm.

4.3 Crystal structure and crystallinity of PBS and PBS/rGO The WAX diffractograms of the samples are displayed in Figure 4.



90 80 70

Intensity / counts

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PBS

60 Pure graphite

50 40

PBS/rGO

30 20 10 0 10 12 14 16 18 20 22 24 26 28 30 32 34

2θ / degrees

Figure 4: WAXD spectra of neat PBS (black line), PBS/rGO (blue line) and pure graphite (red line).

Firstly it is worthy to notice that the characteristic peak of graphite59 located at 2θ ~ 26.6o and corresponding to a d-spacing of 3.34 Å between the sheets layers is not apparent on the spectrum of PBS/rGO showing that the graphene nanosheets are well exfoliated into the PBS matrix.14 According to the pattern of each sample, the diffraction peaks are located at 2θ values of 19.8, 22.2, and 22.8° and are attributed respectively to the diffractions from (020), (021), and (110) planes of the PBS crystal α-form.60 This shows that the PBS crystal form is not influenced by the presence of graphene nanosheets. However, the crystallinity was calculated from the WAXD patterns and was found to be ~45% for the neat PBS and ~49% for PBS/rGO. This result is due to the presence of graphene nanosheets that probably enhance the crystallization, which


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results in a higher crystallinity of PBS. In addition, the present difference in crystallinity is larger than the difference observed in the presence of 2.5 %wt graphene oxide.20

4.4 Crystallization kinetics of PBS and PBS/rGO 4.4.1 Melt crystallization The normalized heat flow curves measured by FSC upon cooling are presented in Figure 5. For both materials, the data are collected from the melt and the resulting relative degree of crystallinity is obtained after the integration of FSC curves. The thermodynamic parameters corresponding to the crystallization of neat PBS and of PBS/rGO performed under the same conditions are displayed in Table 1 and Table 2 respectively.

0.008 50

20

30

10

1.0 0.5

0.006 -1

Exo 0.0

30 50

0.004

20 30

20

-0.5

10

α

Heat flow / W g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50

-1.0

10

0.002

-1.5 0.000 -10

0

10

20

30

40

50

60

-2.0 70

Temperature / °C

Figure 5 : FSC heat flow and relative degree of crystallinity vs. temperature during the nonisothermal crystallization from the melt of PBS (black, line) and PBS/rGO (blue, dot) presenting a mass of ~ 300 ng. The cooling rate of each experiment (in oC.s-1) is indicated by each curve.



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Table 1 : Thermodynamic Parameters of the Melt Crystallization of PBS

βa

Tonset b

Tendset c

Tc d

∆cH° e

Tmelt f

/ K.s-1

/ °C

/ °C

/ °C

/ J.g-1

/ °C

10

54.8 ± 0.5

27.2 ± 0.5

42.4 ± 0.5

9±1

79.0 ± 0.5

20

54.9 ± 0.5

15.1 ± 0.5

35.1 ± 0.5

8±1

81.6 ± 0.5

30

52.1 ± 0.5

-6.3 ± 0.5

28.8 ± 0.5

8±1

81.7 ± 0.5

50

51.7 ± 0.5

-7.7 ± 0.5

23.9 ± 0.5

5±1

81.4 ± 0.5

a

Cooling rate. b Temperature of the upper integration limit of the FSC peak. c Temperature of the lower integration limit of the FSC peak. d Peak maximum temperature. e Melt crystallization enthalpy related to the mass of neat PBS. f Melting temperature (peak maximum temperature) after further heating at 1000 K.s-1.

Table 2 : Thermodynamic Parameters of the Melt Crystallization of PBS/rGO

βa

Tonset b

Tendset c

Tc d

∆cH° e

Tmelt f

/ K.s-1

/ °C

/ °C

/ °C

/ J.g-1

/ °C

10

67.4 ± 0.5

34.8 ± 0.5

49.2 ± 0.5

8±1

83.9 ± 0.5

20

60.3 ± 0.5

19.9 ± 0.5

41.7 ± 0.5

10 ± 1

83.2 ± 0.5

30

57.0 ± 0.5

5.8 ± 0.5

35.7 ± 0.5

10 ± 1

84.4 ± 0.5

50

52.9 ± 0.5

-7.0 ± 0.5

26.9 ± 0.5

7±1

85.3 ± 0.5

a

Cooling rate. b Temperature of the upper integration limit of the FSC peak. c Temperature of the lower integration limit of the FSC peak. d Peak maximum temperature. e Melt crystallization enthalpy related to the mass of neat PBS. f Melting temperature (peak maximum temperature) after further heating at 1000 K.s-1.

The DSC curves on Figure 5 clearly show the shift of crystallization to lower temperatures with increasing cooling rates. The temperatures corresponding to the crystallization temperature (Tonset) and the crystallization endset temperature (Tendset) are higher in the case of the nanocomposite. The enthalpies of crystallization appear to be higher for the nanocomposite at high cooling rates. This shows that the degree of crystallinity for melt crystallization at 30 and 50 K.s-1 is higher for the PBS/rGO. According to the values of Tmelt displayed in Table 1 and Table 2, the PBS/rGO displays globally a higher melting temperature compared to the neat PBS. These 18 ACS Paragon Plus Environment

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results show consequently that the graphene promotes the crystallization from the melt and induces higher crystallization temperature of the nanocomposite upon cooling. Moreover, the width and the height of the crystallization peaks are different between neat PBS and PBS/rGO. The variation of the width and the height shows that the crystallization mechanism is different in the presence of graphene. This corroborates the variation of crystallization peak height. Higher crystal perfection is obtained for the nanocomposite as reflected by the higher melting temperature (Figure S1 in Supporting Information). At lower cooling rates the height of PBS/rGO peak is lower than the neat PBS, but the opposite behavior is observed for the higher cooling rates.

4.4.2 Glass crystallization Figure 6 shows the normalized heat flow curves measured from the glass by FSC upon heating. The resulting relative degree of crystallinity is obtained after integration of the FSC data. The corresponding thermodynamic parameters are reported in Table 3 and Table 4.

0.014

20 20

0.012

40 50

30

Exo

1.0

30

0.5

40

0.010 50

-1

50

0.008 0.006

0.0

40

50 40

α

Heat flow / W g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

-0.5

30

0.004

20 20

-1.0

0.002 0.000

-1.5 -10

0

10

20

30

40

50

Temperature / °C


60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 6 : FSC heat flow and relative degree of crystallinity vs. temperature during the nonisothermal crystallization from the glass of PBS (black line) and PBS/rGO (blue dot) presenting a mass of ~ 300 ng. The heating rate of each experiment (in oC.s-1) is indicated by each curve.

Table 3 : Thermodynamic Parameters of the Glass Crystallization of PBS

βa

Tonset b

Tendset c

Tcc d

∆cH° e

Tmelt f

/ K.s-1

/ °C

/ °C

/ °C

/ J.g-1

/ °C

20

0.4 ± 0.5

39.8 ± 0.5

18.7 ± 0.5

11 ± 1

91.2 ± 0.5

30

2.8 ± 0.5

47.7 ± 0.5

23.2 ± 0.5

13 ± 1

89.8 ± 0.5

40

5.6 ± 0.5

58.2 ± 0.5

27.9 ± 0.5

10 ± 1

88.9 ± 0.5

50

6.7 ± 0.5

63.7 ± 0.5

30.8 ± 0.5

10 ± 1

88.7 ± 0.5

a

Heating rate. b Temperature of the lower integration limit of the FSC peak. c Temperature of the upper integration limit of the FSC peak. d Peak maximum temperature. e Glass crystallization enthalpy related to the mass of neat PBS. f Melting temperature (peak maximum temperature).

Table 4 : Thermodynamic Parameters of the Glass Crystallization of PBS/rGO

βa

Tonset b

Tendset c

Tcc d

∆cH° e

Tmelt f

/ K.s-1

/ °C

/ °C

/ °C

/ J.g-1

/ °C

20

-11.6 ± 0.5

43.4 ± 0.5

3.1 ± 0.5

10 ± 1

94.4 ± 0.5

30

-7.5 ± 0.5

48.4 ± 0.5

7.5 ± 0.5

10 ± 1

93.4 ± 0.5

40

-6.0 ± 0.5

54.4 ± 0.5

10.2 ± 0.5

9±1

92.5 ± 0.5

50

-5.6 ± 0.5

56.4 ± 0.5

12.7 ± 0.5

10 ± 1

91.7 ± 0.5

a

Heating rate. b Temperature of the lower integration limit of the FSC peak. c Temperature of the upper integration limit of the FSC peak. d Peak maximum temperature. e Glass crystallization enthalpy related to the mass of neat PBS. f Melting temperature (peak maximum temperature).

According to Figure 6 the crystallization normally shifts to higher temperature with increasing the heating rate. The values of Tonset are lower for the PBS/rGO compared to neat 20 ACS Paragon Plus Environment

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PBS. A difference of about 10 °C in favor of PBS/rGO is observed whatever the heating rate. The glass crystallization temperatures are much lower for the nanocomposite and consequently show that the graphene promotes the nucleation of the crystals, as for the melt crystallization. As for the crystallization from the melt, the observation of such a difference at high cooling and heating rates is possible due to the reduction of graphene oxide allowing its good dispersion in the PBS matrix, whereas the non-modified graphene oxide displays aggregates that allow to observe variations in crystallization temperature for lower cooling rates only.20 It is also worthy to note here that this nucleation effect is observed with a lower quantity of filler compared to the work of Papageorgiou et al.61 that use a percent weight of 2.5% wt. On the other hand, this does not affect the amount of crystalline phase since the enthalpies of crystallization from the glass are globally similar for the PBS and the PBS/rGO. Additionally, the shape, the width and the height of the crystallization peak are different between the PBS/rGO and the neat PBS. This observation strongly suggests that the two samples present different crystallization mechanisms. The PBS/rGO crystallization peak shows a shoulder, especially at low cooling rates suggesting the presence of a second crystallization peak. This shoulder is located at higher temperatures and is marked by an exothermic effect lasting on a wider temperature range. This variation was already observed in a previous study and was attributed to the secondary crystallization process.53 On the α-curves the presence of the shoulder is reflected by a variation of the slope. The presence of a double cold crystallization peak was also observed by Pingping et al.62 in the case of pre-treated poly(ethylene terephthalate) filled with calcium carbonate. Additionally, the melting behavior appears to be different between the neat PBS and the PBS/rGO. The values of Tmelt are globally higher in the case of the nanocomposite and show that the thermal stability of



the crystals formed increases. These crystals formed in the presence of the graphene consequently melt at higher temperature as observed on Figure S2 in the Supporting Information.

4.4.3 Eα dependences and evaluation of Hoffman-Lauritzen parameters The Eα vs. α dependencies obtained by application of the advanced isoconversional methods during melt and glass crystallization of PBS and PBS/rGO are presented in Figure 7.

100 80 60

-1

40

Eα / kJ.mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 0 -20 -40 -60 -80 0.0

0.2

0.4

0.6

0.8

1.0

α

Figure 7 : Eα-dependence vs. relative degree of crystallinity computed from FSC data. Black circles: PBS, blue triangles: PBS/rGO, open: crystallization from the melt (cooling), solid: crystallization from the glass (heating).

In the case of the crystallization Eα is not related to an energy barrier but corresponds to the temperature coefficient of the Arrhenius law. Figure 7 shows the existence of an Eα-dependence on the relative degree of crystallinity, indicating that the crystallization mechanism is complex


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and involves various activation energies and different steps such as nucleation and growth. Those steps contribute to the overall crystallization rate measured by FSC that will vary both with the temperature and the relative degree of crystallinity. The values of Eα computed from FSC data are thus a function depending of these two variables. The melt crystallization energies for the two systems display negative increasing values that is characteristic of a nucleation control.51,53 In concordance with the Hoffman-Lauritzen theory of crystallization this variation of Eα in the melt crystallization region corresponds to an anti-Arrhenius behavior. It is worthy to note that the value of Eα for PBS/rGO is lower than the neat PBS when α < 0.94, however the Eα dependencies of both samples present a monotonous increase without large variations. According to Figure 7 the behavior of Eα computed with the data of glass crystallization is different compared to the case of melt crystallization. The values are positive but decrease with α and display thus an Arrhenius behavior in agreement with the Hoffman-Lauritzen theory. Unlike crystallization from the melt, the Eα values of PBS/rGO are always higher compared to the neat PBS during glass crystallization. This observation is logical since Eα values during glass crystallization decrease with T and it was shown that PBS/rGO crystallize at lower temperature compared to PBS (Figure 6). However, the PBS/rGO dependence shows two deviations compared to the neat PBS Eα-curve. For α ~ 0.3 a slight increase is observed while at the end of crystallization, a significant increase of Eα values (~ 34 kJ.mol-1) for the PBS/rGO is clearly apparent. In general, the Hoffman-Lauritzen theory predicts an acceleration of glass crystallization with increasing the temperature, due to the decrease of the diffusion constraints. However, these increasing deviations of Eα observed for the PBS/rGO is not consistent with this prediction and could correspond to secondary crystallization processes that may occur in the presence of the graphene nanosheets.53 The secondary crystallization is also correlated with the 23 ACS Paragon Plus Environment


exothermic shoulder located after the main crystallization peak of PBS/rGO (Figure 6). Figure 8 shows Eα vs. Τ dependencies obtained from the melt and glass crystallization of neat PBS and PBS/rGO.

120 80 Glass Crystallization

-1

40

Eα / kJ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 -40 Melt Crystallization -80 -120

0

5

10

15

20

25

30

35

40

45

50

Temperature / °C

Figure 8 : Eα-dependence vs. temperature computed from FSC data. Black circles: PBS, blue triangles: PBS/rGO, open: crystallization from the melt (cooling), solid: crystallization from the glass (heating).

In the case of the crystallization from the melt, the values of Eα are negative and increase with the decrease in temperature, corresponding to an anti-Arrhenius behavior of the crystallization for both samples.51 Figure 8 shows that the dependence of PBS/rGO corresponds to the dependence of PBS that have been shifted to higher temperatures. The same result was observed in a previous study53 and confirms the nucleation effect on cooling induced by the graphene nanosheets. In the case of the crystallization from the glass, the values of Eα for the two


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samples are positives and globally decrease with increase in temperature corresponding to an Arrhenius behavior of the crystallization. For the lower temperatures of crystallization the variations in Eα are similar between the PBS and the PBS/rGO. In this temperature range the rate of nucleation is fast and consequently not rate determining. The initial crystallization rate is poorly affected by the presence of graphene nanosheets that only act as a nucleating agent allowing to initiate the crystallization at lower temperatures. While higher, the initial Eα−values of PBS/rGO are aligned with those of PBS. Indeed, the crystallization of PBS/rGO is closer to the glass transition of PBS and the diffusion of PBS chains more constrained. As shown in Figure 8, two deviations at respectively 10 °C and 30 °C are noticed in the Eα−curves of PBS/rGO. As mentioned earlier, these deviations might be the sign of secondary crystallization that could occur in the neighborhood of the graphene nanosheet where the crystal lamellar arrangement is probably modified. Moreover, the secondary crystallization has enough time to occur since, due to the presence of graphene nanosheets, the glass crystallization of PBS appears at lower temperature, i.e. far from the melting of secondary crystals. The fitting of truncated Eα versus T-dependencies obtained from the melt and glass crystallization was performed by using Equation 9 and allows to obtain the Hoffman-Lauritzen parameters U* and Kg.63 Firstly, Hoffman-Weeks theory was employed to deduce the equilibrium melting temperatures ( T m0 ).28 It was experimentally determined that Tm° ~ 388 K (115 oC) for the neat PBS and T m0 ~ 390 K (117 oC) for the PBS/rGO. The higher value of Tm° obtained in the presence of the graphene nanosheets shows that the crystal perfection is improved. In order to perform the Hoffman-Lauritzen fits presented in Figure 9, the value of T∞ is fixed to Tg – 30 K ~ 203 K for both neat PBS and PBS/rGO. To our knowledge, this is the first 25 ACS Paragon Plus Environment


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time that the crystallization of PBS performed at high cooling and heating rates is simultaneously described by the set of Hoffman-Lauritzen parameters U* and Kg via the data issued from both melt and glass crystallization. The fit of the PBS activation energy (correlation coefficient r2 ~ 0.997) leads to the values of U* = 9667 ± 100 J.mol-1 and Kg = 4.0 ± 0.1 x 105 K2. For the PBS/rGO, the fit (r2 ~ 0.996) leads to the values of U* = 7105 ± 100 J.mol-1 and Kg = 2.7 ± 0.1 x 105 K2. According to the results, the values of U* are at the same magnification than the universal value set at 6270 J.mol-1, and appear to be similar for the two samples (U* is slightly lower for the nanocomposite). The PBS displays thus a similar molecular diffusion in the presence or in the absence of graphene nanosheets. However, the results clearly show a lower value of Kg for the PBS/rGO attributed to the decrease of the nucleation barrier in the presence of the graphene nanosheets. This shows that the energy required to form the critical size of PBS nuclei is lower for the nanocomposite than for the neat PBS64 and is correlated with the analysis of crystallization kinetics performed above. The values of U* and Kg determined for the PBS and the PBS/rGO allow to plot G/G0 vs. temperature as presented in the inset of Figure 9.


Page 27 of 39

100

-9

1.5x10

PBS/rGO

80

-13

2.0x10

-9

60

G/G0

G/G0

1.0x10

-13

1.0x10

-10

5.0x10

-1

40

Eα / kJ.mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PBS

20

0.0

0.0 -40

0

-20

0 20 40 60 o Temperature / C

80

100

-20 -40 -60 -80 0

10

20

30

40

50

60

o

Temperature / C

Figure 9 : Fit of Eα-dependence vs. temperature by Equation 9 for glass and melt crystallization data. Black circles: PBS, Blue triangles: PBS/rGO. Inset: Plot of G/G0 computed using Hoffman– Lauritzen parameters. In the case of the nanocomposite the crystallization starts earlier and the maximum value of the G/G0 is higher. If it is considered that the value of G0 is similar for the two systems it consequently means that the linear growth rate from the melt and from the glass is favored in the presence of the graphene nanosheets. The G/G0 values being always much higher for the nanocomposite (different scales on Figure 9). This observation is correlated with the appearance of secondary crystallization and higher crystal perfection in the case of the nanocomposite. According to this computation, the temperature corresponding to the maximum value of the peak, defined as the temperature of the maximum growth rate (Tmax), is estimated. The value of Tmax is observed at 29 oC for the neat PBS and 33 oC for the PBS/rGO.



Figure 10 displays the contribution of nucleation and diffusion, defined by term(Kg) and term(U*) respectively that are involved in the linear growth rate G for PBS and PBS/rGO.

10

-4

10

-5

10

-6

10

-7

10

-8

10

-9

PBS/rGO

PBS 0.004

0.003

term(U*)

term(Kg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

PBS/rGO

0.002

0.001

PBS

0.000 0

20

40

60

80

100

o

10

Temperature / C

-10

-40

-20

0

20

40

60

80

o

Temperature / C

Figure 10 : Schematic representation of the exponential terms term(Kg) and term(U*) (inset) function with the temperature. Black circles: PBS, Blue triangles: PBS/rGO. The results obviously show that the values of term(Kg) are higher for the nanocomposite at the temperatures close to the glass transition. The contribution of nucleation to the crystal growth rate is then enhanced in the presence of the filler in this temperature range. Additionally, the same behavior is observed for term(U*) at the temperatures close to the melting indicating that the graphene nanosheets promote also the contribution of crystal chains diffusion. This behavior of term(Kg) and term(U*) consequently confirms the higher value of G for the PBS/rGO.


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5. Conclusion The present work proposes a study of PBS crystallization filled with graphene nanosheets. The graphene oxide nanosheets were originally synthesized via the improved method and were reduced with ascorbic acid as an eco-friendly reducing agent. Structural investigations by FTIR and solid state NMR confirmed the reduction of graphene oxide while TEM observations allowed to confirm the homogeneous dispersion of the graphene. The results from WAXD highlighted the good exfoliation of graphene nanosheets into the PBS matrix and an increase of crystallinity in the presence of the filler. For the first time, the crystallization from the melt and from the glass of PBS/rGO nancomposite was studied by FSC. The overall results obtained by fast calorimetry showed that the presence of graphene modifies the crystallization behavior of PBS. The nucleating effect induced by the filler allows to observe higher crystallization temperatures upon cooling (melt crystallization) and lower crystallization temperature upon heating (glass crystallization). The secondary crystallization on heating is promoted with the insertion of the graphene and a higher crystal perfection is obtained. The Hoffman-Lauritzen crystallization parameters were obtained by fitting the Eα vs. T-dependencies that were previously computed via the advanced isoconversional method. The results show that the two materials share the same relaxation dynamics, however a decrease of nucleation barrier and an increase of crystal growth rate are observed in the presence of graphene. This increase in G was finally explained by the simultaneous enhancement of nucleation and diffusion contributions in the presence of graphene nanosheets.



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Supporting Information. Melting behavior of PBS and PBS/rGO after crystallization from the melt (Figure S1) and after crystallization from the glass (Figure S2). This information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors gratefully thank the Rachadapisek Sompot Fund for Postdoctoral Fellowship, Chulalongkorn University and the Rachadapisek Sompot Endowment under Outstanding Research Performance Program (GF-58-08-23-01) for financial support of this research. Scientific exchanges from fruitful collaboration with Mettler-Toledo Inc. on Flash-DSC 1 are gratefully acknowledged. The authors thank the Microscopy Center of Nice Sophia Antipolis University.


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