Mechanical properties and shape memory effect of short fiber ...

Mechanical properties and shape memory effect of short fiber reinforced SMP composite Kai Yua, Haibao Lva, Guo Yib, Yanju Liub, Jinsong Lenga P.O. Box 3011, Centre for Composite Materials and Structures, No. 2 YiKuang Street, Science Park of Harbin Institute of Technology (HIT), Harbin 150080, P.R. China b Department of Aerospace Science and Mechanics, No. 92 West Dazhi Street, Harbin Institute of Technology (HIT), Harbin 150080, P.R. China

a

ABSTRACT By adding randomly distributed short fiber into a shape memory polymer (SMP) matrix, both the mechanical properties and the shape memory behavior are improved significantly, overcoming some traditional defects of SMP composite reinforced by long fiber and particles. In this paper, the short fiber reinforced SMP composite are developed for the improvement of the mechanical and thermal properties of styrene-based SMP bulk. The specimens with different chopped fiber weight fractions are prepared, and then their mechanical behavior and electrical properties are investigated. As a result, the resistance against mechanical and thermal mechanical loads in the developed materials increases due to the role of reinforcement fiber. For the conducting composite filled with short carbon fiber, not only the actuation of SMP composite can be driven by low voltage, but also its tensile, bending strength, glass transition temperature, storage modulus and thermal conductivity increase by a factor of filler content of carbon fiber increasing. The results show meaningful guidance for further design and the performance evaluation of such composite materials. Keywords: Short fiber, mechanical properties, shape memory polymer, conducting composite

1. INTRODUCTION Smart materials attract much interest in recent years. Their applications cover non-destructive evaluation [1] dynamics control and shape control [2], etc. Shape-memory polymers (SMPs) are one of a candidate actuation material in the field of shape control. SMPs are second important group of shape-memory materials. Like SMAs, they have the capability of changing their shapes in response to an external stimulus. The most common ones are temperature or thermo-responsive SMPs, which typically consist of two polymer components and two phases: one with a higher melting temperature than the other. The glass transition temperature (Tg) is the reference point where the higher temperature component starts to melt. When heated above Tg, the SMPs are soft and rubbery, and easy to change the shapes. When subsequently cooled below Tg, they will retain the given shape (shape fixing characteristic). When heated again above Tg, the materials autonomously return to the original “parent” shape [3, 4]. SMPs have gained substantial interest in the community of deployable space structures, owing to their superior structural versatility, lower manufacturing cost, easier pretreatment procedure, larger recoverable deformation and lower recovery temperature [5-9]. However, these materials in the unreinforced form are unsuitable for the proposed applications, which require high stiffness and recovery force [10]. And the need to induce SMPs by non-thermal stimuli has led to increased usage of conductive fillers, such as conductive particles, carbon particles and conductive fibers [11]. In comparison to micro-sized reinforcements, nano-reinforcements dispersing into SMPs hold great promise as materials that provide nonlinear improvements to the thermo-mechanical and dynamic mechanical properties [12, 13]. The short fiber reinforced SMP composite (SFR-SMP) consists of short fibers dispersed into SMP matrix. Compared with SMP composites reinforced by conductive particles or long conductive fibers, SFR-SMP has many unique advantages, such as low cost, easy of fabricating complex parts and isotropic nature, and hence it is an ideal choice for large-scale production. Consequently, it is also suitable for lightly loaded component manufacturing. Aligned SFC-SMP can be produced by injection molding and extrusion processes which have excellent in-plane mechanical properties, Active and Passive Smart Structures and Integrated Systems 2010, edited by Mehrdad N. Ghasemi-Nejhad, Proc. of SPIE Vol. 7643, 76430G · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.847365

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whereas randomly oriented SCF-SMP composite shows quasi-isotropic nature in macroscopic scale. The most widely used composite material should be the randomly oriented SCF-SMP composite for its comparatively easy production process. The randomly oriented SCF-SMP composite can be regarded as conductive polymer which has been found extensive potential application in the fields of micro-sensors and micro-actuators, providing cheaper, accurate and faster alternatives to devices already on the market. Based on these, various chemical sensors and biosensors with remarkable specifications will be developed and applied in medicine for fast bio-diagnostic systems. Conductive polymer-based actuators also seem to hold great promise for the future applications, owing to their high strength and low energy consumption [9, 16]. In the sections to come, specimens with different chopped fiber weight fractions are prepared firstly and their morphologies are observed by scanning electron microscope (SEM). Then, their mechanical properties are studied by isothermal static stress-strain tests and bending stress-strain tests. Finally, the shape memory effect of the SCF-SMP composite is also investigated on the base of experimental data.

2. MATERIALS PREPARATION The SMP material used in this study is a thermosetting styrene-based shape-memory resin with a density of 0.92 g/cm3 and curing temperature of 75oC (Cornerstone Research Group Inc, OH). Carbon black (CB) (AX-010) particles are purchased from GuangZhou Sunny Plaza Trading Co., LTD. The mean aggregate size is 4 μ m, the mean domain size (x-ray diffraction) is 18-20 nm, and the domain content is 65 wt.%. The particles have an electrical volume resistivity of 5×10-2 Ω ·m at 20oC. The density is in the range of 1.80-1.85 g·cm-3. The short carbon fiber (SCF) cut from carbon fiber (T700) is of 0.5 to 3 mm length with a diameter of 7 μm, stress strength of 4900 MPa and volume resistivity of 1.5×104 Ω ·m at 20oC. The materials preparation follows four steps: (1) Shape-memory resin blended with crosslink agent in the fixed proportion at room temperature. (2) Carbon particles and carbon fiber are added in with necessary proportion, and followed by adding solution to reduce the viscosity of mixture, that is because viscosity problem always made the mixture difficult to release the air bubble and fabricate; and then mixed by mechanical stir, followed by high-energy sonication by VCX750 (purchased from Sonics&Materials, INC.). (3) The mixture is treated with vacuum pump for more than 3h. (4) Transferred mixture to open-mould, vaporized the solvent and cured at the temperature of 70oC to a constant weight in oven.

3. RESULTS AND DISCUSSION 3.1 Morphologies of fillers Figure 1 shows the morphologies of the developed SCF-SMP composite specimen with a SCF content of 2 wt% and CB particles content of 5 wt%. From the image of figure 1(a), it is found that the SCF fillers distribute randomly and separately from each other in the thermoset styrene-based SMP matrix. SCFs may be considered to provide the conductive pathways and promote relatively long distance charge transfers, leading to easy formation of continuous conductive networks, resulting in electrical resistance significantly drop. Normally, these SCFs play a more prominent role in comparison with the particles aggregates in single particle-filler system. The numerous interconnections between the SCF fibers form the conductive networks in the composite, and can be used to explain the excellent electrical conductivity of the composites filled with SCF. Meanwhile, figure 1(b) shows the morphology of CB particles in the specimen. Clearly, the particle fillers distribute uniformly in the SMP matrix, aggregating as clusters instead of absolutely separating from each other. In this way, the CB particles will act as the nodes among the fibers, and the local conductive pathways will also be formed in the composite, which will make orientation of short fiber improve on. Because of the large amount of conductive channels formed in the composite, the resistivity may be relatively low and stable.

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(a) (b) Fig. 1. The morphologies of SCF-SMP composite specimen observed by SEM (SMP with 2 wt% SCF and 5 wt% CB particles) (a) Morphologies of SCF fillers; (b) Morphologies of CB particles

3.2 Mechanical properties 3.2.1 Theoretical analysis Unlike continuous fiber SMP composites, the external loads are not directly applied to the fibers in SCF-SMP composites. The load applied to matrix materials is transferred to the fibers via fiber ends and the surfaces of fibers. As a consequence, the mechanical properties of SCF-SMP composites greatly depend on the fiber length and diameter (i.e., fiber aspect ratio = length/diameter of fiber) of the fibers. Further, several factors such as fiber orientation, volume fraction, fiber spacing, fiber packing arrangement, and curing parameters also significantly influence the properties of the SCF-SMP composites [17]. During the past few decades, there has been a significant development in property prediction models of short fiber reinforced composites based on micro and macro mechanics. Some empirical relationships are also available to be used in the determination of approximate properties of random short fiber composites [18]. Typically, the modified Halpin_Tsai [19] equation can be expressed as follows:

3 5 n = E1 + E2 8 8

(1)

Ef

) −1

Ef

) −1 1 + ξη1v f 1 + ξη2 v f Em Em Where E1 = Em ( ) , E2 = Em ( ) ,η1 = and η 2 = . Ef Ef 1 − η1v f 1 − η2v f ( ) + 2(l d ) ( )+2 Em Em (

(

However, the Young’s modulus in the former empirical relationships is based on one component materials, which can be regarded as a basic material constant. However, in our study, the CB particles are doped into the SMP resin before it is used as the composite matrix. The mole number per unit of the material will be subsequently changed. Because of the great influence of CB particles on the material properties of SMP, especially the tensile strength and elastic modulus, the Young’s modulus of the SMP doped with CB particles will be recalculated in the following study. If the CB particles are treated as micromolecules in solvent, a swelling effect will be generated in the mixed solution after the particle fillers are doped into SMP. Based on the Rubber Elasticity Theory, the force per unit area can be expressed as following:

σ = nRT

ri 2 ⎛ 1 ⎞ − α ⎜ α 2 ⎟⎠ r02 ⎝

(2)

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In this equation, L0 is increased to L, α= L/L0 is the elongation ratio, and RT is the gas constant times absolute 2

2

temperature. The two quantities r0 and ri represent the same chain in the network and uncross-linked states, respectively. Generally, the quantity ri

2

r02 approximately equals unity. The quantity n represents the network made up

of n chains per unit volume. Several other relationships may be derived immediately, since Young’s modulus is the function of state,

ri 2 ∂σ E = L( )T ,V ≅ 3n RT ∂L r02

(3)

Therefore,

E = nRT

ri 2

2

(α + 2

α

r0

) ≅ 3n 2

ri 2 r02

RT

(4)

If a polymer network is swollen with a “solvent” (it does not dissolve), the detailed effect has two parts: 1. The effect on ri

2

r02 . The quantity ri 2 increases with the volume V to the two-third power, while r02 remains

constant. The work done on the polymer network is

⎛ ri 2 ⎞ ⎛ V ⎞ ⎜ 2 ⎟=⎜ ⎟ ⎜ r ⎟ ⎝ V0 ⎠ ⎝ 0 ⎠

2

3

∗

⎛ ri 2 ⎞ 1 ⎜ 2⎟ = 2 ⎜r ⎟ v 3 ⎝ 0 ⎠ 2

⎛ ri 2 ⎞ ⎜ 2⎟ ⎜r ⎟ ⎝ 0 ⎠

∗

(5)

Where “*”represents the swollen state and v2 is the volume ration of polymer in the no swollen and swollen stages. Of course, v2 is less than unity, as is commonly experienced. 2. The effect on the number of network chain segments concentration, n. The quantity n gives the expression:

⎛V ⎞ ⎜ ⎟ n = v2 n = ns ⎝ V0 ⎠

(6)

Where ns is the chain segment concentration in the swollen stage. By substituting equation (5) and (6) into equation (4) we obtain empirical relationships of 1

E = 3n ⋅ v2 3

ri 2 r02

RT

(7)

Until now, the equivalent modulus of the SMP material doped with CB particles is worked out. By submitting it into the existing property prediction theories of short fiber reinforced composites, the elastic modulus of the SCF-SMP composite can be calculated. 3.2.2 Isothermal static stress-strain tests Tensile tests are performed by using a Zwick/Roell servo-mechanical test frame with a series digital controller. An Instron clip-on extensometer is used for strain measurement in tensile tests, while the test frame’s extension sensor is used for the shear tests. A forced air convective environmental chamber is used for elevated temperature tests. In calculation of the experimental data, the engineering stress and strain are used. Figure 2 shows the stress–strain curves at the testing temperatures of 298 K. The strain is calculated by the ratios of the elongation obtained by the crosshead displacement to the gage length (50 mm).

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When the temperature is at T= 298 K, the 5CB/0.5SCF, 5CB/1.0SCF, 5CB/1.5SCF and 5CB/2SCF wt% composite specimens have small fracture strains relatively (from 1.730 to 1.877), while the pure SMP bulk do not rupture within the strain range of 3.90%. The yielding phenomenon is observed in all specimens with different fiber weight fractions. The uniform elongation and considerable large fracture stresses are observed in the composite specimens. And the maximum stress increases considerably with the increment of fibrous filler content. It seems that the failure in pure SMP specimen mainly depended on the ductile deformation. On the other hand, the fractographs in the composite specimens are relatively flat and the matrix resin fractured bristly. The reason for these differences may be considered as that the occurrence and propagation of cracks initiated by the debonding between matrix resin and fillers are the major failure characterization in the filler-filled specimens due to high fiber weight fraction. The cracks caused by the debonding between matrix and fibers may not propagate easily since the viscous matrix resin became hard to flow and the composite materials became brittle [20].

Fig. 2. Stress-strain curves of composites filled with different chopped carbon fiber content in the tensile model [20]

Figure 3 shows a comparison between the experimental results and theoretical predictions in the mechanical analysis of SCF-SMP composite. In this image, the normalized composite modulus donates the composite modulus divided by modulus of pure SMP material doped with CB particles. Each value in the theoretical models is calculated by the former developed analysis method. Meanwhile, the four experimental data correspond with the maximum strain in the isothermal static stress-strain tests of each specimen. It is clearly that the composite modulus is greatly enhanced by adding short carbon fibers, and it increases significantly with increasing relative short carbon fibers volume fraction. Moreover, the variation trend of the experimental values is similar to the prediction values. When the short carbon fibers volume fraction is relatively lower, the difference between the experimental data and theoretical prediction is small. But with the volume fraction increasing, the results get divergent. While the calculation results in Rom, Cox and Clyne models are relatively deviated with the experimental values, the Halpin_Tsai model shows a closer prediction with the experimental results, indicating that it can provide a more precise prediction during the mechanical analysis of the SCFSMP composite.

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Fig. 3. Curves of normalized composite modulus versus fiber volume fraction of short carbon fiber

3.2.3 Isothermal bending stress-strain tests The flexural strength of the composites is measured by the three-point bending test on a Zwick/Roell servo-mechanical test frame with a series digital controller. An Instron clip-on extensometer is used for strain measurement in the treepoint bending model. A forced air convective environmental chamber is used for elevated temperature tests. In calculation of the experimental data, the flexural strength and conductive filler content are used at the testing temperatures of 298 K. When the content of SCF is relatively high, say 2 wt%, the flexural strength is much higher than that of composite filled with 0.5 wt% SCF. While, when the content of SCF increases, the strength gradually increases. Figure 4 shows the variations of flexural strength of the composites with respect to the filler content of chopped carbon fiber. As we can see, the flexural strengths of the four composites are 95.06, 130.30, 138.03 and 152.13 MPa, respectively. The flexural strength of the composite can be enhanced by adding small amount of SCF. Therefore, the flexural strength increases from 95.06 to152.13 MPa with only 1.5 wt% SCF increment. It can be stated that SCF available from enhancement of composite could be applied to have good mechanical properties. This phenomenon may be mainly attributed to the natively mechanical properties of fibrous filler.

T

(

SMP with 5 wt% CB and 0.5 wt% SCF SMP with 5 wt% CB and 1 wt% SCF SMP with 5 wt% CB and 1.5 wt% SCF SMP with 5 wt% CB and 2 wt% SCF σmax=138.03

σmax=152.13

σmax=130.30 σmax=95.06

(\o)

Fig. 4. The flexural strength of composites vs. the filler content of chopped carbon fiber

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3.3 Shape recovery effect 3.3.1Thermal response shape memory behavior Polymeric materials show an intrinsically low thermal conductivity in the range of 0.15< k< 0.30 W/mK for most cases, which makes them good insulators for numerous applications. However, thermally stimulated SMPs demand comparatively high thermal conductivity to enhance response time. Fig. 5 shows the effect of filler on the thermal conductivity of the SCF-SMP composites with different weight fractions of conductive fillers. The thermal conductivity of the pure polymer at room temperature is around 0.17 W/mK. However, with the addition of 2 wt% SCF and 5 wt% particles, a 100% increase in thermal conductivity is observed. So, this increase in thermal conductivity suggests that hybrid fillers are an effective component for increasing the thermal conductivity of SMP composites. Meanwhile, a significant increase in thermal conductivity is also observed for all the samples with increase in temperature. However, the effect of temperature on thermal conductivity is negligible after 85°C. This might be because of the opposing effects of temperature on the specific heat and the thermal diffusivity.

Recovery Ratio, (%)

TOO

Pure SMP SMP with 5 wt% CB and 0.5 wt% SCF SMP with 5 wt% CB and 1 wt% SCF SMP with 5 wt% CB and 1.5 wt% SCF SMP with 5 wt% CB and 2 wt% SCF

Fig. 5. Shape recovery ratio curves of the pure SMP and the SMP composites against temperature

3.6 Shape memory behavior driven by electricity The shape recovery behavior of SCF-SMP composite materials being driven by electrical current have been carried out in this section. It is found that the chopped carbon fiber significantly reduced the electrical resistivity, and then the particulate filler inhomogeneously improve the electrical properties and thermal conductivity. This synergic effect has been testified to drive shape recovery of SMP composite being recorded by an infrared video camera (AGEMA, Thermovision 900) to monitor the temperature distribution and shape recovery simultaneously in figure 6. The used specimen which contained 5 wt% CB and 2 wt% SCF (the length of chopped carbon fiber is 3 mm) is about 112 mm × 23.2 mm × 4 mm. And the electrical conductivity 2.32 S·cm of composite which can be considered as conductor is calculated by the Van De Pawn method. The shape transition in the DC field of 25 V is documented with a digital camera. In 55 s, the starting conversion of the flexural shape is observed to be completed. The final shape is close to the original plane shape with some remaining flexion due to friction between the soft polymer and the glass plate. The rate of shape recovery is strongly dependent on the magnitude of the applied voltage and the electrical resistivity of composite. Furthermore, all the composites filled with chopped carbon fiber (content of 0.5 wt% to 2 wt%) behave like conductor in this study, and shape recovery of which with dimension of 112 mm × 23.2 mm × 4 mm can be driven by low voltage from 20 V to 100 V [20].

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Fig. 6. Series of photographs showing the macroscopic shape-memory effect of SMP/5CB/2SCF composite. The permanent shape is a plane stripe of composite material, and the temporary shape is deformed as right-angled shape [20].

4. CONCLUSIONS This paper presents a systematic study on mechanical and electrical properties of conducting SCF-SMP composites. The following conclusions are obtained. (1) The conductive network is virtually formed resulting in a sharp transition from insulating to electrical conductive. While the inherent fibrillar form of SCF fillers have a higher tendency to form a three-dimensional network in the conductive composites and promote relatively long distance charge transfers, the CB particles will act as the nodes among the fibers and attribute to form the local conductive pathways, which will make orientation of short fiber improve on. (2) As mechanical analysis and experiments shown, the mechanical properties of SCF-SMP composite are influenced by the conductive fillers. The fibrous fillers enhance the mechanical properties of SCF-SMP composites more obviously than particulate fillers, and the elastic modulus and strength of the composite material increase with the fiber volume fraction increasing. (3) Due to the synergic effect of fibrous and particular fillers, the thermal conductivity of SCF-SMP composite is improved significantly. Leading to shape recovery behavior of SMP can be driven by effective temperature and electricity, moreover fast responsive time in either approach. The meanings of this research on conductive SCF-SMP composites attributes to further design and the performance evaluation of such composite materials.

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