buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 61
3. Results and Discussion
3.1. Morphology
The detailed morphology of both investigated polymer and the
corresponding nanocomposites subjected to shearing was examined by atomic
force microscopy. The topography of the PS film obtained by shearing its
solution in methylene chloride is shown in Fig. 2. AFM image of multiwalled
carbon nanotubes introduced into the PS matrix is illustrated in Fig. 3. AFM
phase contrast image of the PS/CNT nanocomposites films is presented in Fig.
4, and their morphology in Fig. 5.
From AFM data the following observations can be extracted:
i) casting and shearing polystyrene solution leads to macroscopically
oriented films, as shown in Fig. 2;
ii) AFM phase contrast image shows that carbon nanotubes are oriented
in the matrix of PS, with the length perpendicular to the composite film surface;
iii) organization of carbon nanotubes in PS is ideal for improving the
thermal conductivity, resulting in copolymers with materials designed to protect
electrical components overheating circuit.
3.2. Thermal Properties
Thermal properties of PS/CNT composites are theoretically evaluated
using a formalism based on connectivity indices developed by Bicerano
(Bicerano, 1996). The glass transition temperature, Tg
, is calculated using an
equation with terms related the cohesive energy and van der Waals volume. The
estimated value of Tg
is 108.85°C, which is in agreement with experimental
value obtained by differential calorimetry data namely of 103.2°C. In the case
of nanocomposite films the glass transition temperature is high, i.e. 116°C
revealing a good thermal stability induced by the presence of CNT.
The thermal conductivity, λ , of these materials is estimated by
considering it as an additive function of the PS and CNT compositions. First,
the thermal conductivity of PS at room temperature was predicted using
1
0.135614 0.126611 0.108563( 0 0.125 )
BB
+ χ
N N + N − NH
λ = +
. (1)
N N
1 BB
where χ denotes the portion of the first –order connectivity index contributed
by the bounds between pairs of backbone atoms.
1
BB
For polystyrene the value of χ is 0.8165. The thermal conductivity
of PS calculated from Eq. (1) is 0.135 W/m K. Secondly, considering the
thermal conductivity of the multiwalled carbon nanotubes, measured by the