Thermal conductivity and structure of non-covalent functionalized graphene/epoxy composites Chih-Chun Teng a , Chen-Chi M. Ma a, * , Chu-Hua Lu b , Shin-Yi Yang a , Shie-Heng Lee a , Min-Chien Hsiao a , Ming-Yu Yen a , Kuo-Chan Chiou c , Tzong-Ming Lee c a Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30043, Taiwan b Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu 30010, Taiwan c Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsin-Chu 31040, Taiwan ARTICLE INFO Article history: Received 24 January 2011 Accepted 24 June 2011 Available online 23 July 2011 ABSTRACT Non-covalent functionalization was used to functionalize graphene nanosheets (GNSs) through p–p stacking of pyrene molecules with a functional segmented polymer chain, which results in a remarkable improvement in the thermal conductivity of GNS-filled poly- mer composites. The functional segmented poly(glycidyl methacrylate) containing local- ized pyrene groups (Py-PGMA) was prepared by atom transfer radical polymerization, and Py-PGMA was characterized by nuclear magnetic resonance spectroscopy. Raman spec- tra, X-ray photoelectron spectroscopy and thermogravimetric analysis reveal the character- istics of Py-PGMA–GNS. Differential scanning calorimetry indicated that the functional groups on Py-PGMA–GNSs can generate covalent bonds with the epoxy matrix, and further form a cross-linked structure in Py-PGMA–GNS/epoxy composites. The Py-PGMA on the GNS surface not only plays an important role to facilitate a homogeneous dispersion in the polymer matrix but also improves the GNS–polymer interaction, which results in a high contact area. Consequently, the thermal conductivity of integrated Py-PGMA–GNS/epoxy composites exhibited a remarkable improvement and is much higher than epoxy rein- forced by multi-walled carbon nanotubes or GNSs. The thermal conductivity of 4 phr Py- PGMA–GNS/epoxy has about 20% (higher than that of pristine GNS/epoxy) and 267% (higher than pristine MWCNT/epoxy). Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Polymer composites with carbon nanofillers have many poten- tial applications, including thermal management, electronics, green energies, and transportation [1]. With the growing de- mand for high density electronic devices, developing poly- mer-based composites with high thermal conductivity and low fabrication cost is of primary importance. On the basis of previous investigation on polymer-based composites, poly- mers filled with thermally conductive particles such as alu- mina [4], boron nitride [5], and alumina nitride [6] are conventional to enhance the performance of polymer compos- ites. However, they relied on excessively high quantity of fillers (about 30–60 vol%) to achieve thermal conductivity values of 1– 2 W/mK. Carbon-based nanofillers, such as carbon nanotubes and carbon nanofibers, possess unique nanostructures, high aspect ratio, and superior thermal conductivity, they were expected to be the potential fillers for improving the thermal conductivity of polymer composites [7–9]. However, there are two main reasons limiting the applications of polymer com- posites with carbon nanofillers: (1) the poor dispersion of carbon nanofillers in polymeric matrices, which limited the 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.06.095 * Corresponding author: Fax: +886 3 571 5408. E-mail address: [email protected](Chen-Chi M. Ma). CARBON 49 (2011) 5107 – 5116 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon
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Thermal conductivity and structure of non-covalentfunctionalized graphene/epoxy composites
Chih-Chun Teng a, Chen-Chi M. Ma a,*, Chu-Hua Lu b, Shin-Yi Yang a, Shie-Heng Lee a,Min-Chien Hsiao a, Ming-Yu Yen a, Kuo-Chan Chiou c, Tzong-Ming Lee c
a Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30043, Taiwanb Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu 30010, Taiwanc Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsin-Chu 31040, Taiwan
A R T I C L E I N F O
Article history:
Received 24 January 2011
Accepted 24 June 2011
Available online 23 July 2011
0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.06.095
Table 1 – Analysis of the deconvoluted C1s peaks from XPS and their relative atomic percentage in terms of graphite, GO,graphene, and Py-PGMA–graphene.
Sample name C1s fitting binding energy (eV; relative atomic percentage, %)
Fig. 10 – Thermal conductivity with various filler contents of
MWCNTs/epoxy, graphene/epoxy, and Py-PGMA–graphene.
C A R B O N 4 9 ( 2 0 1 1 ) 5 1 0 7 – 5 1 1 6 5115
1 phr Py-PGMA–GNS/epoxy composites about 208.7%, being
much higher than that of 1 phr pristine MWCNTs. It was
noticeable that the thermal conductivity of Py-PGMA–GNS/
epoxy with only 4 phr loading reached 1.91 W/mK; it usually
required about 20 times graphite content (80–200 phr) to
achieve the comparable thermal conductivities. Three rea-
sons were proposed to explain this significant enhancement:
(i) the better graphitic integrity of GNS can possess better con-
ductance. The XPS and Raman results revealed that thermal
exfoliation could reduce GO efficiently, and Py-PGMA modi-
fied GNS through non-covalent functionalization can pre-
serve the structure integrity of GNS; (ii) the DSC analysis
indicated that the functional groups on Py-PGMA–GNS could
generate covalent bonds with epoxy matrix, and further
formed the cross-linked structure of Py-PGMA–graphene/
epoxy composites, which could enhance the interfacial inter-
action between GNS and epoxy considerably. The strong
interaction between nanofillers and polymer could reduce
the thermal interfacial resistance effectively and improve
the phonon transport in composite; (iii) the excellent solubil-
ity of Py-PGMA modified GNS in solvent can facilitate GNS to
disperse in polymer composites homogeneously, resulting in
an increased contact surface area between Py-PGMA–GNS
and the polymer. The homogeneous Py-PGMA–GNS possess
a large contact area with polymer permitting ease of heat
flows and promoting phonon diffusion in Py-PGMA–GNS/
epoxy composites.
In summary, the theoretic performance of GNS would be
reduced significantly due to the nanosheet aggregation and
poor compatibility with polymer, which is the critical issue
in relation to the potential of GNS in polymer composites.
Consequently, pyrene molecule with functional segmented
polymer chain can be a good approach to improve the perfor-
mance of GNS in polymer composite though non-covalent
functionalization.
4. Conclusions
This study demonstrated a non-destructive approach to im-
prove the thermal conductivity of GNS-filled epoxy compos-
ites through non-covalent functionalization of pyrene
molecules with a functional segmented polymer chain on
the thermally exfoliated graphene. The thermal conductivity
of Py-PGMA–GNS/epoxy composite increased more than
800% with low GNS loading (4 phr), compared with neat
epoxy, which was superior to the epoxy composites with indi-
vidual MWCNTs or GNS. At loading 4 phr Py-PGMA–GNS has
about 20% higher thermal conductivity than pristine GNS.
The remarkable improvement originated from the Py-PGMA
functionalization. The Py-PGMA on GNS surface plays an
important role in inhibiting their aggregation and facilitating
dispersion within polymer matrix homogeneously. Further-
more, Py-PGMA on GNS could generate covalent bonds with
the epoxy to form a cross-linked structure of Py-PGMA–
GNS/epoxy composites; the integrated Py-PGMA–GNS/epoxy
composite can possess a large contact area with polymer per-
mitting ease of heat flows and promoting phonon diffusion.
Consequently, the non-destructive approach can maintain
the high graphitic integrity of GNS and improve the GNS-
epoxy interaction, which is of critical importance for the po-
tential of graphene-based materials in enhancing thermal
conductivity of polymer-based composite.
Acknowledgment
The financial support from the Industrial Technology
Research Institute and National Science Council of Taiwan
ROC under contract no. NSC-99-2221-E-007-005 and the boost
program of the Low Carbon Energy Research Center of
National Tsing Hua University, are gratefully acknowledged.
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