Environmental Engineering Reference
In-Depth Information
4.3
characterization and property analysis of Multifunctional nanocoMposites
As their nature indicates, multifunctional nanocomposites generally demonstrate a variety of unique properties associated with
the integration of multicomponents in the entity and because of the synergy induced during material fabrication and processing.
In this section, we mainly focus on the thermal, mechanical, magnetic, and rheological properties relevant to our research
interests.
4.3.1
thermal properties analysis with tga and dsc
Thermal stability of multifunctional nanocomposites can be analyzed by thermal gravimetric analysis (TGA) either in air or of
inert gases such as nitrogen. Through TGA, changes in the mass of nanocomposites over a wide range of temperatures will
reflect the thermal behavior of integrated components within the composites as well as those adsorbed on the surface, such as
moisture and residual solvents. It typically involves both thermal degradation and oxidative decomposition of the polymer if
analyzed in air, but merely thermal degradation in inert gases. The residual mass is usually metal or ceramic species that will
not decompose and be released into the gas phase at the final high temperature. Incorporation of nanofillers has, in general, been
proven to enhance thermal stability; that is, the polymer or carbon matrix can only be degraded at higher temperatures.
Differential scanning calorimetry (DSC) is another important thermal analysis technique that is widely used to determine glass
transition temperature (
T
g
), melting temperature (
T
m
), as well as crystalline properties of nanocomposite samples. The inherent
characteristic temperatures
T
g
and
T
m
reflect the atomic, molecular, and stereomicrostructures of the respective polymer and/or
carbon matrix in multifunctional composites, and nanofiller components may affect both temperatures by interaction with the
matrix, thereby affecting the corresponding rigidity as well as the crystallinity of the polymer and/or carbon.
In the PP/iron core-shell nanocomposite system, thermal behavior was affected not only by the molecular weight of the PP,
but also by the relative density of the grafting maleic anhydride (MA) functional groups on the polymer chain. In Figure 4.4,
both neat low molecular weight (lM)-PP and high molecular weight (HM)-PP show different thermal decomposition profiles
in air and in nitrogen; neat lM-PP demonstrated slower degradation kinetics than HM-PP in both air and nitrogen, which can
be attributed to the relatively higher density of MA groups on lM-PP compared to HM-PP, and thus much higher intermo-
lecular interactions (mostly notably through hydrogen bonding) between polymer chains for lM-PP. The peaks of melting
temperature recrystallization in the DSC graph indicate that HM-PP is highly crystalline while lM-PP is amorphous.
4.3.2
Mechanical properties analysis: young's Modulus and tensile strength
Mechanical properties are critical evaluation aspects for multifunctional nanocomposites, no matter what type of application
they are targeting, which may include electronic devices, magnetic sensors, drug delivery vehicles, and so on. Generally
speaking, the mechanical properties in consideration mainly focus on Young's modulus, which is the ratio of the tensile stress
(MPa) and the tensile strain (dimensionless) in the elastic portion of the stress-strain curve. Figure 4.5 shows the tensile stress-
strain curves for polyurethane/iron nanocomposites formed through either the direct method (DM) or the SIP method. It dem-
onstrates that the product derived from the former method showed large cracks, while that derived from the latter was much
more flexible and showed no cracks. Figure 4.5 presents a quantitative measurement of the mechanical behaviors via the tensile
stress-tensile strain curves of the Fe
2
O
3
/polyurethane (Pu) composites fabricated from the DM and SIP methods, respectively.
Young's moduli and tensile strengths are similar in both composites. However, the elongation of the SIP composite is about four
times that of the DM composite. The strong chemical bonding between Fe
2
O
3
NPs and polyurethane as well as the uniform
particle distribution within the polymer matrix contribute toward the mechanical behavior of the SIP composites.
4.3.3
Magnetic properties, Mr, and giant Magnetoresistance in Multifunctional nanocomposites
Multifunctional magnetic composites have found wide applications in electronics, magnetic sensing, and environmental reme-
diation. Compared to their metallic counterpart, polymer- and carbon-based multifunctional nanocomposites have the advan-
tages of light weight, flexibility, processability, and anticorrosion. The high specific area of these structured materials offers
enhanced adsorption capacity, while their magnetic property enables recycling of the new family of adsorbents. Figure 4.6
shows the magnetic properties of magnetic graphene nanoplatelet composites (MGNCs); a large coercivity of 496 Oe was
observed in core-double shell MGNCs, which indicates that they are ferromagnetic and harder than bare Fe NPs (5.0 Oe) of
comparable size [17, 24] at room temperature after they were placed on the graphene sheet. MGNCs are tested to be stable after
a 4-h immersion in 1 M HCl [25]. This further confirms the formation of a protective carbon shell around the magnetic core.
This unique magnetic property enables easy recycling of MGNC adsorbents for environmental remediation, leaving no
secondary pollution, and simultaneously improving the economic value of MGNCs.
