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graphene composites by in-situ polymerization of cyclic butylene terephthalate
Polymer 53 (2012) 897e902
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Polymer journal homepage: www.elsevier.com/locate/polymer
Preparation and characterization of poly (butylene terephthalate)/graphene composites by in-situ polymerization of cyclic butylene terephthalate Paola Fabbri a, *, Elena Bassoli a, Silvia Bittolo Bon b, Luca Valentini b a b
Faculty of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy Department of Civil and Environmental Engineering, University of Perugia, Strada Pentima Bassa 4, 05100 Terni, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2011 Received in revised form 11 January 2012 Accepted 11 January 2012 Available online 17 January 2012
The ultra-low viscosity of cyclic butylene terephthalate oligomers has been exploited to perform their insitu ring-opening polymerization in the presence of graphene, to obtain homogeneously dispersed poly(butylene terephthalate)/graphene (PBT/G) composites containing 0.5 to 1.0 %wt of graphene. The results of gel permeation chromatography show that increasing amounts of graphene causes a decrease in the average molecular weight of PBT if the time of polymerization is kept constant, and morphological investigations performed by electron microscopy and x-rays diffraction show that high levels of dispersion of the G sheets are easily obtained by this method of composites processing. Thermal properties of the composites were studied by differential scanning calorimetry and thermogravimetric analysis; results indicate that increasing amounts of G do not strongly influence the degree of crystallinity and the crystallization temperature of PBT, while its thermal stability is significantly increased by the presence of G. All the PBT/G composites demonstrated to be electrically conductive; we found that the electric field assisted thermal annealing of the PBT/G composites induces an increase in conductivity. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Poly(butylene terephthalate) Graphene Cyclic monomers
1. Introduction 1.1. Graphene Novel graphene-based polymer composites are presently emerging as a new class of highly functional advanced materials that hold promise for a more versatile and cheaper alternative to carbon nanotubes-based composites. In fact, while the addition of carbon nanotubes to polymer matrices has already been shown to improve their mechanical, electrical and thermal properties [1,2], the challenge is now to exfoliate graphite to single graphene sheets to be used as an inexpensive and feasible substitute to carbon nanotubes. Graphene is the name given to a single layer of carbon atoms densely packed into a benzene ring structure. Graphene sheets are one-atom thick, 2D layers of sp2-bonded carbon atoms and can be considered as the building blocks for carbon materials of all other dimensionalities [3], and therefore the mother of all graphitic materials: the 2D material can be wrapped up into fullerenes, rolled into nanotubes or stacked into graphite. Graphene was demonstrated to have extraordinary electronic transport properties [4e8], combined with a wide set of other
* Corresponding author. E-mail address: [email protected] (P. Fabbri). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.015
unusual properties [9]: its thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (3,000 Wm1K1 and 1060 GPa, respectively); its fracture strength should be comparable to that of carbon nanotubes for similar types of defects [10e12]. On the basis of these exemplary physicalemechanical properties, it results clear that the transfer of such features to polymeric materials usable to fabricate devices, holds a deep scientific interest. In fact, the use of graphene as filler in polymeric composites produces exceptional improvements to electrical, thermal, electromagnetic, mechanical and gas barrier properties [13e15]. Very small graphene amounts are sufficient to obtain significant changes in the polymer matrix, since the high surface area of exfoliated graphene causes an extensive interface between the two phases. Moreover, in comparison to carbon nanoparticles or nanotubes, the use of graphene strongly reduces potential hazards associated with handling and inhalation of nanoparticles [16] and cost. However, for the time being, a relatively limited number of studies have been done on the harnessing of these properties into functional graphene-base polymer composites, and there is an absolute need to perform comprehensive basic studies on their possible preparation methods and properties evaluation. One important step was already done in understanding that graphene-based polymer composites exhibit extraordinarily low electrical percolation threshold (<0.1 vol%) due to the large conductivity and aspect ratio of graphene sheets; this
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percolation threshold is about three times lower than reported for any other two-dimensional filler, and it is also due to the highly homogeneous dispersion achieved in the reported composites [17e19]. So far, the successful manufacturing of graphene-polymer composites requires a highly efficient dispersion of single graphene sheets into the polymer matrices. Among the possible routes of preparation of polymer-graphene composites, recently reviewed by Kim et al. [13], melt blending results to be the most economically attractive and scalable method for dispersing graphene sheets into polymers; successful melt compounding of graphene into elastomers [20] and different thermoplastic polymers [21,22] has been reported. On the other hand, melt blending presents severe drawbacks due to the high melt viscosity of the vast majority of industrial-grade polymers, that prevents it from being an always-effective strategy for achieving well dispersed, high-performance polymer/graphene composites. This is the main reason why a very limited number of studies have been reported till now on the preparation and characterization of engineering plastics/graphene composites, while they should certainly attract a wide interest in the field of materials for advanced applications; one of the few studies in this field was recently reported by Zhang and coworkers [23] on the preparation of electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding. Therefore, the insitu polymerization approach should certainly draw several advantages with respect to melt blending with comparable costs, as recently demonstrated for polyesters [24], acrylates [25,26] and epoxies [27]. 1.2. PBT from cyclic monomer Recently, cyclic butylene terephthalate (CBT) oligomers stimulated a great interest because of their waterlike viscosity and ability to be readily polymerized to form the engineering thermoplastic poly(butylene terephthalate) (PBT). PBT is commonly obtained through industrial processes based on the polycondensation of dimethylterephthalate and 1,4 butanediol, operating in two stages under controlled temperature and pressure to get high molecular weights, but entropically-driven ring-opening polymerization of CBT into linear high molecular weight PBT can be obtained in a shorter time scale under isothermal conditions [28]. Therefore CBT oligomers represent the best alternative to polymerize PBT under milder conditions, and their ultra-low viscosity makes them well suited for numerous processes, such as composite processing. Recent literature demonstrated that improved dispersion of clay nanoplatelets in PBT is easily obtained by in-situ ring-opening polymerization of CBT, respect to the standard melt intercalation process [29], and notable advantages were also observed in the field of fibre-reinforced composites [30]. In fact, fibre impregnation with CBT can be achieved as easily as with thermosets, and the advantages typical of thermoplastics can be exploited at the same time: higher toughness, faster manufacturing, recyclability, easier storage of raw materials, weldability and possibility to post-form moulded parts [31]. CBT in-situ polymerization in the presence of graphene should then provide an ideal solution for the preparation of homogeneously dispersed PBT/graphene composites. 1.3. Potential of PBT-graphene composites Kim and colleagues studied the functional properties of graphene composites with the engineering polyester poly(ethylene-2,6naphthalate) (PEN), obtaining intense improvements in electrical conductivity, melt viscoelasticity and gas barrier properties [32].
The possibility to industrialize the production of graphene-filled engineering polymers paves the way to applications in electronic devices and communication instruments for commercial, military and scientific purposes [33]. In all these uses electromagnetic interference (EMI) shielding of radio frequency radiation represents a serious concern and the replacement of metal-based materials with conducting polymer composites could offer many advantages, such as weight reduction, higher corrosion resistance and easier processing. Many other applications in the automotive and aerospace sector, requiring sufficient conductivity to provide electrostatic discharge, have been considered so far as ideal uses for nanotubes reinforced composites and could be interested by PBT with graphene [34]. In addition, shrinkage and corrosion resistant conductive coatings, light emitting devices, energy storage and energy conversion devices, gas barrier devices, fuel cells, hydrogen storage, field emission displays, chemical or biological sensors, are only few of the possible applications of advanced functional plastics with conductive fillers [35e37]. PBT is a semicrystalline polymer widely used as structural material in the automotive, electrical, and electronic industries: its use as a matrix for electrically conductive polymer nanocomposites could be of great industrial relevance [38]. In this study, we prepared PBT/graphene composites by the insitu polymerization of cyclic butylene terephthalate oligomers in the presence of graphene nanoplatelets. Composites containing 0.5 to 1.0 %wt of graphene were prepared and their morphological, thermal and mechanical properties were explored. In view of the potential application in the field of advanced plastics for electronic applications, the electrical properties were also evaluated for the PBT/graphene composites. 2. Experimental 2.1. Materials Graphene nanoplateletes were purchased from Cheap Tubes Inc. (Vermont, USA) and used as received without further chemical modification. The average lateral dimension of this graphene sheets was ranging between 2 and 5 mm. CBT oligomers (CBTÒ 500, USA) and the polymerization catalyst butyl tin chloride dihydroxide were purchased from Cyclics Corp. (New York, USA) and used as received. Tetrahydrofuran (99%, THF) was purchased from Sigma Aldrich (Milan, Italy) and used as received to prepare homogeneous dispersions of CBT and graphene before the polymerization. Chloroform (CHCl3) and 1,1,1,3,3,3 hexafluoroisopropanol (HFIP) were cromatography grade solvents purchased from Sigma Aldrich (Milan, Italy). 2.2. Preparation of PBT/graphene composites 10 g of CBT were completely dissolved at room temperature under vigorous mechanical stirring in 60 ml of THF, and then the desired amount of graphene nanoplatelets was homogeneously dispersed in that solution by mechanical stirring coupled with ultrasonication for 3e4 h. THF was completely evaporated under vacuum at 50 C, and then the solid mixture was heated to 200 C for 30 min keeping it under vacuum. Afterwards, the tin-based polymerization catalyst was added to the liquid mixture (0.45 % wt with respect to CBT) and mechanical stirring kept at 500e600 rpm for 30e40 min at 200 C. Finally, PBT/graphene composites were collected from the reactor and coded accordingly to the weight content of graphene as PBT/G0.5, PBT/G0.75 and PBT/G1.0, respectively for composites containing 0.5, 0.75 and 1.0 %wt of graphene. As a reference, PBT
P. Fabbri et al. / Polymer 53 (2012) 897e902
without graphene was polymerized from CBT under identical experimental conditions, and the sample was coded PBT.
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The samples were heated up to 230 C (heating rate 10 C/min) and exposed to a DC electric voltage of 1 V applied between the aluminium electrodes. The samples were then cooled down to room temperature leaving the electric field applied.
2.3. Characterization The average molecular weights and polydispersity of PBT polymerized in the presence of graphene were determined by gel permeation chromatography (GPC). The composites were dissolved in a mixture solvent of CHCl3/HFIP (90:10 vol/vol) at room temperature over 3e4 h, and filtering over Teflon filters with mash 0.2 mm efficiently separated graphene. The clear PBT solutions were analysed by GPC by injection of 20 ml of the above mentioned solutions into a conventional MiniMIX-D GPC column, using a mixture of CHCl3/HFIP (98:2 vol/vol) as mobile phase. Average molecular weights were determined using a universal calibration method based on the MarkeHouwink k and alpha parameters; commercial monodispersed polystyrene standards were used with molecular weights ranging between 2000 and 111,000 Da. Morphology and homogeneity of graphene dispersion were investigated by field emission scanning electron microscopy using an FESEM-Supra 25 (Zeiss SMT). The stacking and interlayer distance of graphene nanoplatelets dispersed in PBT were investigated by X-rays diffraction (XRD) by using a Philips PW3710 instrument, in the 2q degree range 10e70, step size 0.02 2q degree, acquisition time 1 s/step. Differential scanning calorimetry (DSC) was carried out under nitrogen on a TA 2010 DSC instrument equipped with a cooling system and calibrated with Indium standard. The samples were heated from room temperature to 280 C at 10 C/min and kept under isothermal conditions for 2 min to ensure a complete melting of the crystallites. Then, they were cooled to 20 C at 10 C/min to obtain the crystallization temperature (Tc). Finally, samples were heated again to 280 C at 10 C/min to obtain the melting temperatures (Tm). The degree of crystallinity (Xc) for the PBT and PBT/G composites was determined from the DSC traces by the enthalpy variation during the second melting scan using the following equation:
Xc ð%Þ ¼
0 DHm =DHm 100
(1)
where DHm is the measured melting enthalpy for the sample and 0 is the heat of melting of 100% crystalline PBT, reported to be DHm 140 J/g [39]. Thermogravimetric analysis (TGA) was performed both under oxidant and inert atmosphere with a Netzsch STA 429 CD instrument; samples were heated at 20 C/min from room temperature to 700 C. Mechanical properties were investigated through instrumented indentation tests by using a CSM nano-indentation tester (CSM Instruments SA) compliant to ASTM E2546 standard. A Berkovich tip was used providing a maximum load of 50 mN, reached with a rate of 0.5 mN/s and held for 5 s. The instrument ensures resolutions of 40 nN on the loading system and 0.04 nm on depth. Load versus depth curves were recorded for each indentation, then indentation hardness (HIT) and indentation modulus (EIT) were calculated according to the method proposed by Oliver and Pharr [40]. Three valid tests were performed on each sample. For the measurement of electrical resistivity the samples were molten at 230 C and deposited by drop casting onto microscopy slides where aluminum electrodes (1 mm 2 mm) had been prefabricated by thermal evaporation up to a thickness of 60 nm, about 1 mm away from each other. The electrical resistance was measured by four-point probe method using a Keithley 4200 SCS.
3. Results and discussion 3.1. Effect of graphene on the polymerization of PBT from cyclic oligomers Preparation and polymerization of butylene phthalate cyclic oligomers have been described in details by the inventor D. Brunelle and coworkers [41]; CBT can be efficiently polymerized using tin or titanate catalysis well below the melting point of the resulting polymer, leading to formation of high molecular weight polyester, which subsequently crystallizes at the reaction temperature. These initiators are thought to operate by Lewis acid activation of the ester group and then transferring a ligand and forming a new ester and an active chain end. Propagation continues until all the cyclic oligomers are depleted. Initiation, propagation, and chain transfer have nearly the same rates; polydispersities therefore are typically high, in the range 2.0e3.0. Polymerization proceeds at a high enough rate that polymerization results essentially complete before crystallization occurrs; for this reason polymerization of CBT can be easily performed at temperatures lower than the melting temperature of PBT. To investigate the role played by graphene during the in-situ polymerization of PBT from CBT oligomers, the number and weight-average molecular weights for the polymer were determined by GPC with an universal calibration method based on the MarkeHouwink k and alpha parameters for the monodispersed polystyrene standards (7.16 103 ml/g; 0.76) and the broad PBT samples (1.17 102 ml/g; 0.87) in the used solvent mixture [42]. As shown by GPC results listed in Table 1, the presence of graphene nanoplatelets during the polymerization of PBT strongly influences the ability of CBT to undergo ring-opening and polycondensation; an almost linear decrease of PBT molecular weight was observed for increasing amounts of graphene during the insitu polymerization, probably due to the presence of the carbon sheets which are unreactive and represent a physical obstruct for the crystalline organization of the polymer segments. A considerable retard in the ring-opening polymerization of CBT due to the presence of carbon nanotubes was also recently observed by Wu et al. [43]; in that work a negligible effect on the final molecular weight of PBT was reported for the filled system if longer polymerization times were allowed. In our work the polymerization time was kept equal for CBT polymerized alone or in the presence of graphene. The significant increase in polydispersity for increasing G contents also indicates that G plays a role in the mechanism of PBT chains growth; it can be assumed that G interacts with the reactive center of propagation, or with the catalyst, but correct evaluations will need further kinetic studies.
Table 1 Number-average and weight-average molecular weights of PBT in the PBT/G composites. Sample
Mn (Da)
Mw (Da)
Polydispersity (Mw/Mn)
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
8400 4100 2100 1550
22,700 18,800 8400 6400
2.7 4.6 4.0 4.1
900
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Fig. 1. FESEM images of (a) PBT/G0.5, (b) PBT/G0.75 and (c) PBT/G1 samples.
3.2. Dispersion of graphene in the composites All the prepared PBT/G composites showed a good dispersion of graphene into the polymer matrix; FESEM images reported in Fig. 1 demonstrate the absence of macroscopic G stacking even for the highest G content (1.0 %wt, Fig. 1c). Interfacial adhesion between polymer and G nanoplatelets resulted very efficient thanks to the applied in-situ polymerization of the ultra-low viscosity CBT oligomers; no voids were visible at the grapheneepolymer interface even at higher magnifications. In order to evaluate possible variations in the interlayer distance between G nanoplatelets dispersed in the PBT matrix, i.e. the efficiency of the applied polymerization method in promoting the exfoliation of G and intercalation of the polymer chains within the carbon stacks, XRD analysis was used. Due to the low content of G into the prepared composites, relatively close to the detection limit of the used instrument, results reported in Table 2 should be intended as a bare indication of a trend, which would need to be further investigated by using a more sensitive technique. Table 2 reports the shift in the 2q position of the diffraction peak relative to the pristine G, and G dispersed in PBT in different amounts, with the respective interlamellar d-spacing (Å) calculated by the Bragg’s Law; as expected, the in-situ polymerization of CBT in the presence of G promotes a partial intercalation of PBT in the interlamellar space of graphene for the lower G contents. When G is present in the composite in higher amounts (1.0 %wt) the pep interactions between graphene sheets are probably too strong to be overcome by PBT intercalation, and a higher stacking of graphene is observed.
The observed reduction of the melting temperature (Tm) associated with an increase in the crystallization temperature (Tc) are consistent with the observed values of molecular weights, that are decreasing for increasing G content; the lower crystallinity and Tc observed for PBT/G1.0 are consistent with the lower molecular weight of PBT in this composite, and support the results of XRD that suggest a worse dispersion of G into the polymer matrix, which obstructs crystalline organization of the polymer segments. The role played by graphene nanoplatelets on the thermal stability of PBT was evaluated by thermogravimetric analysis, performed under oxidant (air) as well as inert atmosphere (He); results are collected in Table 4, where temperatures corresponding to the initial thermal composition (Ti) and to maximum rate of weight loss (Tmax) are reported for both cases. Tmax values demonstrated a clear effect of stabilization played by G towards the polymer matrix, with an increase of approximately 20 C in the temperature of maximum weight loss rate for the PBT/ G0.75 with respect to unfilled PBT. The lower molecular weights of PBT in the composites containing G with respect to unfilled PBT must also be taken into account, because this would induce a decreasing in Tmax for the polymer matrix; this observation supports the significant action of thermal stabilization played by G towards PBT. 3.4. Functional properties of PBT/graphene composites Due to the relatively low molecular weights obtained by polymerization of PBT in the presence of G, all the PBT/G composite
Table 3 DSC data of the PBT/G composites.
3.3. Thermal properties of PBT/graphene composites DSC was used to evaluate the effect of G nanoplateletes dispersed in the PBT on its melting and crystallization behavior. The results are reported in Table 3.
Table 2 XRD diffraction peaks of G in the PBT/G composites.
Sample
Tm ( C)
DHm (J/g)
Tc ( C)
DHc (J/g)
Xc (%)
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
228.4 223.6 214.7 212.3
39.3 41.4 36.1 35.2
191.6 198.8 192.2 189.7
43.1 47.4 38.8 40.5
28.1 29.6 25.8 25.1
Table 4 TGA data of the PBT/G composites.
Sample
Position (2q)
d-spacing (Å)
Sample
Ti,
G PBT/G0.5 PBT/G0.75 PBT/G1.0
26.403 26.348 26.326 26.408
3.373 3.380 3.383 3.372
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
389 388 404 390
ox
( C)
Tmax, 404 413 425 431
ox
( C)
Ti,
He
397 395 389 395
( C)
Tmax, 406 412 412 411
He
( C)
P. Fabbri et al. / Polymer 53 (2012) 897e902
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Table 5 Results of instrumented nano-indentation tests on PBT/G composites. Sample
EIT (GPa) mean (SD)
HIT (MPa) mean (SD)
HV mean (SD)
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
3.34 4.66 4.24 2.36
163.2 287.3 249.8 102.4
15.1 26.6 23.1 9.5
(0.27) (0.37) (0.30) (0.24)
(12.7) (19.9) (18.4) (28.2)
(1.2) (1.8) (1.7) (2.6)
materials were quite brittle and standard specimens for mechanical testing were impossible to mold without defects. For this reason, the mechanical properties were investigated on small specimens by depth-sensing nano-identation tests. The results are listed in Table 5, where standard deviation is shown between brackets next to the mean values. The corresponding value of Vickers Hardness (HV) is calculated for every HIT measurement and listed in the last column, since HV is a more widespread index and ensures an easy comparison of the results. EIT and HIT for the sample PBT/G0.5 are considerably higher than for unfilled PBT, with increases of 40 and 76%, respectively. Slightly lower values were obtained for the sample with 0.75 wt% of graphene, even if it is still much stiffer and harder than PBT. The sample PBT/G1.0, instead, exhibited values of indentation modulus 30% lower than PBT and a high scattering of indentation hardness, which on average is 37% less than unfilled PBT. Mechanical properties inferior than the theoretically expected ones for graphene reinforced composites have been also reported in many previous studies, where the result is usually ascribed to defects such as wrinkles in the exfoliated structure [44]. An alternative explication is related to a primary effect of the lower molecular weight and lower degree of crystallinity obtained in the samples with the higher graphene percentages. Electrical conductivity of an insulating media containing conducting inclusions is described by the percolation theory in the vicinity of insulatoreconductor transition. Percolation theory [45] proposes that below a critical concentration conducting inclusions are individually isolated in the insulating matrix. As the concentration of inclusions increases, connection probability of inclusions also increases until a network construction of the particles appears corresponding to a sharp change in the conductivity of the system from insulator to semimetal conductor. The PBT/G0.5 composite showed a sheet resistance of about 760 MU, and increasing graphene content to 0.75 and 1.0 %wt takes the sheet resistance values down to 200 and 50 MU, respectively (Table 6). The currentevoltage correlation observed for PBT/G composites can be related to a characteristic semi-metallic behavior, as shown by Fig. 2 for PBT/G0.75 and PBT/G1.0 samples. As demonstrated by the observed trend of the electric current vs. time recorded on our samples and reported in Fig. 3, it is interesting to note that the application of an electric field during the thermal annealing of the PBT/G composites induces an increase of the current passing through the samples (z106 A) with respect to that measured for the composites at room temperature (z1011 A). The electric current decreases with time when the samples are cooled down to room temperature. Being G an electrically conductive filler dispersed in an insulating PBT polymer matrix, it is Table 6 Resistance values for PBT/G composites. Sample
Resistance [MU]
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
e 760 200 50
Fig. 2. Currentevoltage curves for PBT/G0.75 (black) and PBT/G1 (red) composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Current vs. time curves of PBT composites under the application of an external bias of 1 V at 230 C. Above the time indicated by the dashed line the samples were cooled down to 25 C.
evident that the electric field assisted thermal annealing segregates G in a conductive pathway that promotes the electron charge carrier flow. 4. Conclusions PBT/Graphene composites were prepared by in-situ polymerization of cyclic butylene terephthalate oligomers in the presence of graphene. Results demonstrate that G plays a constraining role in the mechanism of PBT chains growth, probably by interaction with the catalyst and/or the active center of propagation. Electron microscopy and XRD investigations revealed that the applied method of in-situ composites preparation is highly efficient in promoting the intercalation of PBT chains inside the interlamellar channels of graphene, thus giving rise to homogeneously dispersed composites that turns into electrically conductive engineering plastics. Mechanical testing by nano-indentation showed that elastic modulus and indentation hardness of PBT are significantly increased for G contents lower than 0.75 %wt, while higher loading induces stronger interactions between the carbon nanoplatelets and decreases the overall properties of PBT/G composites. Acknowledgements Professor Francesco Pilati (University of Modena and Reggio Emilia, Italy) is gratefully acknowledged for the inspiring scientific
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Preparation and characterization of poly (butylene terephthalate)/graphene composites by in-situ polymerization of cyclic butylene terephthalate Paola Fabbri a, *, Elena Bassoli a, Silvia Bittolo Bon b, Luca Valentini b a b
Faculty of Engineering “Enzo Ferrari”, University of Modena and Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy Department of Civil and Environmental Engineering, University of Perugia, Strada Pentima Bassa 4, 05100 Terni, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2011 Received in revised form 11 January 2012 Accepted 11 January 2012 Available online 17 January 2012
The ultra-low viscosity of cyclic butylene terephthalate oligomers has been exploited to perform their insitu ring-opening polymerization in the presence of graphene, to obtain homogeneously dispersed poly(butylene terephthalate)/graphene (PBT/G) composites containing 0.5 to 1.0 %wt of graphene. The results of gel permeation chromatography show that increasing amounts of graphene causes a decrease in the average molecular weight of PBT if the time of polymerization is kept constant, and morphological investigations performed by electron microscopy and x-rays diffraction show that high levels of dispersion of the G sheets are easily obtained by this method of composites processing. Thermal properties of the composites were studied by differential scanning calorimetry and thermogravimetric analysis; results indicate that increasing amounts of G do not strongly influence the degree of crystallinity and the crystallization temperature of PBT, while its thermal stability is significantly increased by the presence of G. All the PBT/G composites demonstrated to be electrically conductive; we found that the electric field assisted thermal annealing of the PBT/G composites induces an increase in conductivity. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Poly(butylene terephthalate) Graphene Cyclic monomers
1. Introduction 1.1. Graphene Novel graphene-based polymer composites are presently emerging as a new class of highly functional advanced materials that hold promise for a more versatile and cheaper alternative to carbon nanotubes-based composites. In fact, while the addition of carbon nanotubes to polymer matrices has already been shown to improve their mechanical, electrical and thermal properties [1,2], the challenge is now to exfoliate graphite to single graphene sheets to be used as an inexpensive and feasible substitute to carbon nanotubes. Graphene is the name given to a single layer of carbon atoms densely packed into a benzene ring structure. Graphene sheets are one-atom thick, 2D layers of sp2-bonded carbon atoms and can be considered as the building blocks for carbon materials of all other dimensionalities [3], and therefore the mother of all graphitic materials: the 2D material can be wrapped up into fullerenes, rolled into nanotubes or stacked into graphite. Graphene was demonstrated to have extraordinary electronic transport properties [4e8], combined with a wide set of other
* Corresponding author. E-mail address: [email protected] (P. Fabbri). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.015
unusual properties [9]: its thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (3,000 Wm1K1 and 1060 GPa, respectively); its fracture strength should be comparable to that of carbon nanotubes for similar types of defects [10e12]. On the basis of these exemplary physicalemechanical properties, it results clear that the transfer of such features to polymeric materials usable to fabricate devices, holds a deep scientific interest. In fact, the use of graphene as filler in polymeric composites produces exceptional improvements to electrical, thermal, electromagnetic, mechanical and gas barrier properties [13e15]. Very small graphene amounts are sufficient to obtain significant changes in the polymer matrix, since the high surface area of exfoliated graphene causes an extensive interface between the two phases. Moreover, in comparison to carbon nanoparticles or nanotubes, the use of graphene strongly reduces potential hazards associated with handling and inhalation of nanoparticles [16] and cost. However, for the time being, a relatively limited number of studies have been done on the harnessing of these properties into functional graphene-base polymer composites, and there is an absolute need to perform comprehensive basic studies on their possible preparation methods and properties evaluation. One important step was already done in understanding that graphene-based polymer composites exhibit extraordinarily low electrical percolation threshold (<0.1 vol%) due to the large conductivity and aspect ratio of graphene sheets; this
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percolation threshold is about three times lower than reported for any other two-dimensional filler, and it is also due to the highly homogeneous dispersion achieved in the reported composites [17e19]. So far, the successful manufacturing of graphene-polymer composites requires a highly efficient dispersion of single graphene sheets into the polymer matrices. Among the possible routes of preparation of polymer-graphene composites, recently reviewed by Kim et al. [13], melt blending results to be the most economically attractive and scalable method for dispersing graphene sheets into polymers; successful melt compounding of graphene into elastomers [20] and different thermoplastic polymers [21,22] has been reported. On the other hand, melt blending presents severe drawbacks due to the high melt viscosity of the vast majority of industrial-grade polymers, that prevents it from being an always-effective strategy for achieving well dispersed, high-performance polymer/graphene composites. This is the main reason why a very limited number of studies have been reported till now on the preparation and characterization of engineering plastics/graphene composites, while they should certainly attract a wide interest in the field of materials for advanced applications; one of the few studies in this field was recently reported by Zhang and coworkers [23] on the preparation of electrically conductive polyethylene terephthalate/graphene nanocomposites prepared by melt compounding. Therefore, the insitu polymerization approach should certainly draw several advantages with respect to melt blending with comparable costs, as recently demonstrated for polyesters [24], acrylates [25,26] and epoxies [27]. 1.2. PBT from cyclic monomer Recently, cyclic butylene terephthalate (CBT) oligomers stimulated a great interest because of their waterlike viscosity and ability to be readily polymerized to form the engineering thermoplastic poly(butylene terephthalate) (PBT). PBT is commonly obtained through industrial processes based on the polycondensation of dimethylterephthalate and 1,4 butanediol, operating in two stages under controlled temperature and pressure to get high molecular weights, but entropically-driven ring-opening polymerization of CBT into linear high molecular weight PBT can be obtained in a shorter time scale under isothermal conditions [28]. Therefore CBT oligomers represent the best alternative to polymerize PBT under milder conditions, and their ultra-low viscosity makes them well suited for numerous processes, such as composite processing. Recent literature demonstrated that improved dispersion of clay nanoplatelets in PBT is easily obtained by in-situ ring-opening polymerization of CBT, respect to the standard melt intercalation process [29], and notable advantages were also observed in the field of fibre-reinforced composites [30]. In fact, fibre impregnation with CBT can be achieved as easily as with thermosets, and the advantages typical of thermoplastics can be exploited at the same time: higher toughness, faster manufacturing, recyclability, easier storage of raw materials, weldability and possibility to post-form moulded parts [31]. CBT in-situ polymerization in the presence of graphene should then provide an ideal solution for the preparation of homogeneously dispersed PBT/graphene composites. 1.3. Potential of PBT-graphene composites Kim and colleagues studied the functional properties of graphene composites with the engineering polyester poly(ethylene-2,6naphthalate) (PEN), obtaining intense improvements in electrical conductivity, melt viscoelasticity and gas barrier properties [32].
The possibility to industrialize the production of graphene-filled engineering polymers paves the way to applications in electronic devices and communication instruments for commercial, military and scientific purposes [33]. In all these uses electromagnetic interference (EMI) shielding of radio frequency radiation represents a serious concern and the replacement of metal-based materials with conducting polymer composites could offer many advantages, such as weight reduction, higher corrosion resistance and easier processing. Many other applications in the automotive and aerospace sector, requiring sufficient conductivity to provide electrostatic discharge, have been considered so far as ideal uses for nanotubes reinforced composites and could be interested by PBT with graphene [34]. In addition, shrinkage and corrosion resistant conductive coatings, light emitting devices, energy storage and energy conversion devices, gas barrier devices, fuel cells, hydrogen storage, field emission displays, chemical or biological sensors, are only few of the possible applications of advanced functional plastics with conductive fillers [35e37]. PBT is a semicrystalline polymer widely used as structural material in the automotive, electrical, and electronic industries: its use as a matrix for electrically conductive polymer nanocomposites could be of great industrial relevance [38]. In this study, we prepared PBT/graphene composites by the insitu polymerization of cyclic butylene terephthalate oligomers in the presence of graphene nanoplatelets. Composites containing 0.5 to 1.0 %wt of graphene were prepared and their morphological, thermal and mechanical properties were explored. In view of the potential application in the field of advanced plastics for electronic applications, the electrical properties were also evaluated for the PBT/graphene composites. 2. Experimental 2.1. Materials Graphene nanoplateletes were purchased from Cheap Tubes Inc. (Vermont, USA) and used as received without further chemical modification. The average lateral dimension of this graphene sheets was ranging between 2 and 5 mm. CBT oligomers (CBTÒ 500, USA) and the polymerization catalyst butyl tin chloride dihydroxide were purchased from Cyclics Corp. (New York, USA) and used as received. Tetrahydrofuran (99%, THF) was purchased from Sigma Aldrich (Milan, Italy) and used as received to prepare homogeneous dispersions of CBT and graphene before the polymerization. Chloroform (CHCl3) and 1,1,1,3,3,3 hexafluoroisopropanol (HFIP) were cromatography grade solvents purchased from Sigma Aldrich (Milan, Italy). 2.2. Preparation of PBT/graphene composites 10 g of CBT were completely dissolved at room temperature under vigorous mechanical stirring in 60 ml of THF, and then the desired amount of graphene nanoplatelets was homogeneously dispersed in that solution by mechanical stirring coupled with ultrasonication for 3e4 h. THF was completely evaporated under vacuum at 50 C, and then the solid mixture was heated to 200 C for 30 min keeping it under vacuum. Afterwards, the tin-based polymerization catalyst was added to the liquid mixture (0.45 % wt with respect to CBT) and mechanical stirring kept at 500e600 rpm for 30e40 min at 200 C. Finally, PBT/graphene composites were collected from the reactor and coded accordingly to the weight content of graphene as PBT/G0.5, PBT/G0.75 and PBT/G1.0, respectively for composites containing 0.5, 0.75 and 1.0 %wt of graphene. As a reference, PBT
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without graphene was polymerized from CBT under identical experimental conditions, and the sample was coded PBT.
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The samples were heated up to 230 C (heating rate 10 C/min) and exposed to a DC electric voltage of 1 V applied between the aluminium electrodes. The samples were then cooled down to room temperature leaving the electric field applied.
2.3. Characterization The average molecular weights and polydispersity of PBT polymerized in the presence of graphene were determined by gel permeation chromatography (GPC). The composites were dissolved in a mixture solvent of CHCl3/HFIP (90:10 vol/vol) at room temperature over 3e4 h, and filtering over Teflon filters with mash 0.2 mm efficiently separated graphene. The clear PBT solutions were analysed by GPC by injection of 20 ml of the above mentioned solutions into a conventional MiniMIX-D GPC column, using a mixture of CHCl3/HFIP (98:2 vol/vol) as mobile phase. Average molecular weights were determined using a universal calibration method based on the MarkeHouwink k and alpha parameters; commercial monodispersed polystyrene standards were used with molecular weights ranging between 2000 and 111,000 Da. Morphology and homogeneity of graphene dispersion were investigated by field emission scanning electron microscopy using an FESEM-Supra 25 (Zeiss SMT). The stacking and interlayer distance of graphene nanoplatelets dispersed in PBT were investigated by X-rays diffraction (XRD) by using a Philips PW3710 instrument, in the 2q degree range 10e70, step size 0.02 2q degree, acquisition time 1 s/step. Differential scanning calorimetry (DSC) was carried out under nitrogen on a TA 2010 DSC instrument equipped with a cooling system and calibrated with Indium standard. The samples were heated from room temperature to 280 C at 10 C/min and kept under isothermal conditions for 2 min to ensure a complete melting of the crystallites. Then, they were cooled to 20 C at 10 C/min to obtain the crystallization temperature (Tc). Finally, samples were heated again to 280 C at 10 C/min to obtain the melting temperatures (Tm). The degree of crystallinity (Xc) for the PBT and PBT/G composites was determined from the DSC traces by the enthalpy variation during the second melting scan using the following equation:
Xc ð%Þ ¼
0 DHm =DHm 100
(1)
where DHm is the measured melting enthalpy for the sample and 0 is the heat of melting of 100% crystalline PBT, reported to be DHm 140 J/g [39]. Thermogravimetric analysis (TGA) was performed both under oxidant and inert atmosphere with a Netzsch STA 429 CD instrument; samples were heated at 20 C/min from room temperature to 700 C. Mechanical properties were investigated through instrumented indentation tests by using a CSM nano-indentation tester (CSM Instruments SA) compliant to ASTM E2546 standard. A Berkovich tip was used providing a maximum load of 50 mN, reached with a rate of 0.5 mN/s and held for 5 s. The instrument ensures resolutions of 40 nN on the loading system and 0.04 nm on depth. Load versus depth curves were recorded for each indentation, then indentation hardness (HIT) and indentation modulus (EIT) were calculated according to the method proposed by Oliver and Pharr [40]. Three valid tests were performed on each sample. For the measurement of electrical resistivity the samples were molten at 230 C and deposited by drop casting onto microscopy slides where aluminum electrodes (1 mm 2 mm) had been prefabricated by thermal evaporation up to a thickness of 60 nm, about 1 mm away from each other. The electrical resistance was measured by four-point probe method using a Keithley 4200 SCS.
3. Results and discussion 3.1. Effect of graphene on the polymerization of PBT from cyclic oligomers Preparation and polymerization of butylene phthalate cyclic oligomers have been described in details by the inventor D. Brunelle and coworkers [41]; CBT can be efficiently polymerized using tin or titanate catalysis well below the melting point of the resulting polymer, leading to formation of high molecular weight polyester, which subsequently crystallizes at the reaction temperature. These initiators are thought to operate by Lewis acid activation of the ester group and then transferring a ligand and forming a new ester and an active chain end. Propagation continues until all the cyclic oligomers are depleted. Initiation, propagation, and chain transfer have nearly the same rates; polydispersities therefore are typically high, in the range 2.0e3.0. Polymerization proceeds at a high enough rate that polymerization results essentially complete before crystallization occurrs; for this reason polymerization of CBT can be easily performed at temperatures lower than the melting temperature of PBT. To investigate the role played by graphene during the in-situ polymerization of PBT from CBT oligomers, the number and weight-average molecular weights for the polymer were determined by GPC with an universal calibration method based on the MarkeHouwink k and alpha parameters for the monodispersed polystyrene standards (7.16 103 ml/g; 0.76) and the broad PBT samples (1.17 102 ml/g; 0.87) in the used solvent mixture [42]. As shown by GPC results listed in Table 1, the presence of graphene nanoplatelets during the polymerization of PBT strongly influences the ability of CBT to undergo ring-opening and polycondensation; an almost linear decrease of PBT molecular weight was observed for increasing amounts of graphene during the insitu polymerization, probably due to the presence of the carbon sheets which are unreactive and represent a physical obstruct for the crystalline organization of the polymer segments. A considerable retard in the ring-opening polymerization of CBT due to the presence of carbon nanotubes was also recently observed by Wu et al. [43]; in that work a negligible effect on the final molecular weight of PBT was reported for the filled system if longer polymerization times were allowed. In our work the polymerization time was kept equal for CBT polymerized alone or in the presence of graphene. The significant increase in polydispersity for increasing G contents also indicates that G plays a role in the mechanism of PBT chains growth; it can be assumed that G interacts with the reactive center of propagation, or with the catalyst, but correct evaluations will need further kinetic studies.
Table 1 Number-average and weight-average molecular weights of PBT in the PBT/G composites. Sample
Mn (Da)
Mw (Da)
Polydispersity (Mw/Mn)
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
8400 4100 2100 1550
22,700 18,800 8400 6400
2.7 4.6 4.0 4.1
900
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Fig. 1. FESEM images of (a) PBT/G0.5, (b) PBT/G0.75 and (c) PBT/G1 samples.
3.2. Dispersion of graphene in the composites All the prepared PBT/G composites showed a good dispersion of graphene into the polymer matrix; FESEM images reported in Fig. 1 demonstrate the absence of macroscopic G stacking even for the highest G content (1.0 %wt, Fig. 1c). Interfacial adhesion between polymer and G nanoplatelets resulted very efficient thanks to the applied in-situ polymerization of the ultra-low viscosity CBT oligomers; no voids were visible at the grapheneepolymer interface even at higher magnifications. In order to evaluate possible variations in the interlayer distance between G nanoplatelets dispersed in the PBT matrix, i.e. the efficiency of the applied polymerization method in promoting the exfoliation of G and intercalation of the polymer chains within the carbon stacks, XRD analysis was used. Due to the low content of G into the prepared composites, relatively close to the detection limit of the used instrument, results reported in Table 2 should be intended as a bare indication of a trend, which would need to be further investigated by using a more sensitive technique. Table 2 reports the shift in the 2q position of the diffraction peak relative to the pristine G, and G dispersed in PBT in different amounts, with the respective interlamellar d-spacing (Å) calculated by the Bragg’s Law; as expected, the in-situ polymerization of CBT in the presence of G promotes a partial intercalation of PBT in the interlamellar space of graphene for the lower G contents. When G is present in the composite in higher amounts (1.0 %wt) the pep interactions between graphene sheets are probably too strong to be overcome by PBT intercalation, and a higher stacking of graphene is observed.
The observed reduction of the melting temperature (Tm) associated with an increase in the crystallization temperature (Tc) are consistent with the observed values of molecular weights, that are decreasing for increasing G content; the lower crystallinity and Tc observed for PBT/G1.0 are consistent with the lower molecular weight of PBT in this composite, and support the results of XRD that suggest a worse dispersion of G into the polymer matrix, which obstructs crystalline organization of the polymer segments. The role played by graphene nanoplatelets on the thermal stability of PBT was evaluated by thermogravimetric analysis, performed under oxidant (air) as well as inert atmosphere (He); results are collected in Table 4, where temperatures corresponding to the initial thermal composition (Ti) and to maximum rate of weight loss (Tmax) are reported for both cases. Tmax values demonstrated a clear effect of stabilization played by G towards the polymer matrix, with an increase of approximately 20 C in the temperature of maximum weight loss rate for the PBT/ G0.75 with respect to unfilled PBT. The lower molecular weights of PBT in the composites containing G with respect to unfilled PBT must also be taken into account, because this would induce a decreasing in Tmax for the polymer matrix; this observation supports the significant action of thermal stabilization played by G towards PBT. 3.4. Functional properties of PBT/graphene composites Due to the relatively low molecular weights obtained by polymerization of PBT in the presence of G, all the PBT/G composite
Table 3 DSC data of the PBT/G composites.
3.3. Thermal properties of PBT/graphene composites DSC was used to evaluate the effect of G nanoplateletes dispersed in the PBT on its melting and crystallization behavior. The results are reported in Table 3.
Table 2 XRD diffraction peaks of G in the PBT/G composites.
Sample
Tm ( C)
DHm (J/g)
Tc ( C)
DHc (J/g)
Xc (%)
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
228.4 223.6 214.7 212.3
39.3 41.4 36.1 35.2
191.6 198.8 192.2 189.7
43.1 47.4 38.8 40.5
28.1 29.6 25.8 25.1
Table 4 TGA data of the PBT/G composites.
Sample
Position (2q)
d-spacing (Å)
Sample
Ti,
G PBT/G0.5 PBT/G0.75 PBT/G1.0
26.403 26.348 26.326 26.408
3.373 3.380 3.383 3.372
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
389 388 404 390
ox
( C)
Tmax, 404 413 425 431
ox
( C)
Ti,
He
397 395 389 395
( C)
Tmax, 406 412 412 411
He
( C)
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Table 5 Results of instrumented nano-indentation tests on PBT/G composites. Sample
EIT (GPa) mean (SD)
HIT (MPa) mean (SD)
HV mean (SD)
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
3.34 4.66 4.24 2.36
163.2 287.3 249.8 102.4
15.1 26.6 23.1 9.5
(0.27) (0.37) (0.30) (0.24)
(12.7) (19.9) (18.4) (28.2)
(1.2) (1.8) (1.7) (2.6)
materials were quite brittle and standard specimens for mechanical testing were impossible to mold without defects. For this reason, the mechanical properties were investigated on small specimens by depth-sensing nano-identation tests. The results are listed in Table 5, where standard deviation is shown between brackets next to the mean values. The corresponding value of Vickers Hardness (HV) is calculated for every HIT measurement and listed in the last column, since HV is a more widespread index and ensures an easy comparison of the results. EIT and HIT for the sample PBT/G0.5 are considerably higher than for unfilled PBT, with increases of 40 and 76%, respectively. Slightly lower values were obtained for the sample with 0.75 wt% of graphene, even if it is still much stiffer and harder than PBT. The sample PBT/G1.0, instead, exhibited values of indentation modulus 30% lower than PBT and a high scattering of indentation hardness, which on average is 37% less than unfilled PBT. Mechanical properties inferior than the theoretically expected ones for graphene reinforced composites have been also reported in many previous studies, where the result is usually ascribed to defects such as wrinkles in the exfoliated structure [44]. An alternative explication is related to a primary effect of the lower molecular weight and lower degree of crystallinity obtained in the samples with the higher graphene percentages. Electrical conductivity of an insulating media containing conducting inclusions is described by the percolation theory in the vicinity of insulatoreconductor transition. Percolation theory [45] proposes that below a critical concentration conducting inclusions are individually isolated in the insulating matrix. As the concentration of inclusions increases, connection probability of inclusions also increases until a network construction of the particles appears corresponding to a sharp change in the conductivity of the system from insulator to semimetal conductor. The PBT/G0.5 composite showed a sheet resistance of about 760 MU, and increasing graphene content to 0.75 and 1.0 %wt takes the sheet resistance values down to 200 and 50 MU, respectively (Table 6). The currentevoltage correlation observed for PBT/G composites can be related to a characteristic semi-metallic behavior, as shown by Fig. 2 for PBT/G0.75 and PBT/G1.0 samples. As demonstrated by the observed trend of the electric current vs. time recorded on our samples and reported in Fig. 3, it is interesting to note that the application of an electric field during the thermal annealing of the PBT/G composites induces an increase of the current passing through the samples (z106 A) with respect to that measured for the composites at room temperature (z1011 A). The electric current decreases with time when the samples are cooled down to room temperature. Being G an electrically conductive filler dispersed in an insulating PBT polymer matrix, it is Table 6 Resistance values for PBT/G composites. Sample
Resistance [MU]
PBT PBT/G0.5 PBT/G0.75 PBT/G1.0
e 760 200 50
Fig. 2. Currentevoltage curves for PBT/G0.75 (black) and PBT/G1 (red) composites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Current vs. time curves of PBT composites under the application of an external bias of 1 V at 230 C. Above the time indicated by the dashed line the samples were cooled down to 25 C.
evident that the electric field assisted thermal annealing segregates G in a conductive pathway that promotes the electron charge carrier flow. 4. Conclusions PBT/Graphene composites were prepared by in-situ polymerization of cyclic butylene terephthalate oligomers in the presence of graphene. Results demonstrate that G plays a constraining role in the mechanism of PBT chains growth, probably by interaction with the catalyst and/or the active center of propagation. Electron microscopy and XRD investigations revealed that the applied method of in-situ composites preparation is highly efficient in promoting the intercalation of PBT chains inside the interlamellar channels of graphene, thus giving rise to homogeneously dispersed composites that turns into electrically conductive engineering plastics. Mechanical testing by nano-indentation showed that elastic modulus and indentation hardness of PBT are significantly increased for G contents lower than 0.75 %wt, while higher loading induces stronger interactions between the carbon nanoplatelets and decreases the overall properties of PBT/G composites. Acknowledgements Professor Francesco Pilati (University of Modena and Reggio Emilia, Italy) is gratefully acknowledged for the inspiring scientific
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