Epoxy Resin Composite Bilayers with Triple-Shape Memory Effect

Triple-shape memory epoxy composites with bilayer structures of well-separated glass transition temperatures have been successfully prepared.Thedifferent glass transition temperatures of the epoxy composites were obtained by physically incorporating various amounts of nanosilica particles, which were introduced into the epoxy by utilizing polyethylene glycol. A scanning electron microscope and a transmission electron microscope were used to analyze the dispersibility of the nanosilica particles. The effects of nanosilica particles on the mechanical properties as well as on the dual-shape memory effects (DSME) and triple-shape memory effects (TSME) of the nanocomposites were studied. The nanosilica particles were homogenously dispersed in the matrix and well incorporated into the epoxymatrix.The resulting nanocomposites exhibited excellent TSME, and their shape fixity properties were significantly improved by nanosilica particles.

In recent years, triple-SMPs (TSMPs) have received even greater attention because shape changes are no longer limited to being unidirectional but could now potentially offer unique opportunities in many applications, including morphing aircrafts, fasteners, and medical devices [1,11].Unlike conventional dual-SMPs, which can recover from a temporary shape to a permanent shape, TSMPs can fix two temporary shapes and recover sequentially from one temporary shape to the other and eventually to the permanent shape [12].TSMPs have either more than one switching thermal transition [1] or a single switching transition with a broad thermal transition range [13].Triple-shape memory effects (TSME) can be achieved through many ways, including polymers blends, grafting and blocking copolymers, SMP hybrids, and polymer laminates [14].Bellin et al. [15] first reported a SMP with a two-step TSME by copolymerizing poly(ethylene glycol) monomethyl ether monomethacrylate with poly(caprolactone) dimethacrylate.The grafting and blocking copolymers of different soft segments result in more than one well-separated multiple phase in a single SMP.Bae et al. prepared SMPU bilayer films of different molecular weights with nanosilica particles acting as multifunctional cross-links and reinforcing fillers [16].Xie et al. prepared TSMP bilayer epoxy by curing the high   epoxy layer on top of the low   epoxy layer [1].
Herein, novel triple-shape memory epoxy nanocomposites with two different glass transition temperatures have been prepared through casting nanosilica/epoxy nanocomposites layer by layer.By varying the amounts of polyethylene glycol (PEG) and nanosilica in the epoxy system, we achieved a series of composite layers with different glass transition temperatures.The dispersibility of the nanosilica particles was evaluated via field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM).The effects of the nanosilica particles on the mechanical and shape memory properties of the composites were analyzed and discussed.[17].Typical procedure was as follows: a known amount of SiO 2 was dispersed in PEG by mechanical stirring for 3 h.Then epoxy resin and curing agent were dissolved in the SiO 2 dispersion under mechanical stirring for another 0.5 h.Subsequently, the mixture solution was degassed placed in a vacuum oven.And then, the mixture solution was poured into a Teflon mold, cured at 60 ∘ C for 4 h.Changing the mass ratio of the SiO 2 and PEG in the mixture and according to the above method, the SiO 2 /epoxy nanocomposites were obtained with different mass ratio of 1 wt% and 2 wt%, respectively.Meanwhile, the pure epoxy and the PEG mixed epoxy composites were also prepared in the absence of nanosilica particles via the method described above.The stoichiometric amounts of every material used to prepare the SiO 2 /epoxy nanocomposites have been listed in Table 1.Bilayer SiO 2 /epoxy nanocomposite was prepared as follows: the mixture solution A 02 was poured into a Teflon mold, cured at 60 ∘ C for 0.5 h.And then the mixture solution A 10 was poured on top of the A 02 , cured at 60 ∘ C for 3.5 h.Finally, the bilayer SiO 2 /epoxy nanocomposite was obtained with the weight ratio of A 02 to A 10 , that is, 2 : 1, and the sample was designated as B 21 .

Characterization.
The dispersibility of nanosilica particles dispersed in PEG and the fracture surfaces of some samples were characterized by field emission scanning electron microscope (ZEISS ULTRA55) and transmission electron microscope (TEM, JSM-2100).
Tensile properties of the composites were measured by using a universal testing machine (UTM) (Instron 3367R4415, Canton, MA, USA) at a crosshead of 5 mm/min.The dimensions of the rectangular film were 50 mm × 5 mm × 2 mm, and the length gripped the sample was 30 mm.
Dynamic mechanical analysis (DMA) was performed with the use of DMA Q800 (TA Instruments, New Castle, USA) in a uniaxial tension mode at 1 Hz and a heating rate of 3 ∘ C/min.The dimensions of the rectangular film were 10 mm × 5 mm × 2 mm.
A thermomechanical cycle test was conducted with a UTM with a temperature controlling chamber.To investigate the dual-shape memory effect (DSME), the sample was stretched to the maximum strain (  ) of 10% at  1 ( 1 >   , where   is the glass transition temperature), followed by cooling the sample to  2 ( 2 <   ) and maintaining the load for several minutes.And then, the sample was unloaded at  2 with an unloading strain (  ).The permanent strain (  ) was reached during the reheating of the sample from  2 to  1 .This completes a thermomechanical cycle and four circles were conducted to examine the shape recovery capacity of the composites.Shape fixity (  ) and shape recovery (  ) ratios for the cycle are defined as follows [16,18]: where   =   −   . 1 and  2 were 42 ∘ C and 15 ∘ C for A 10 and A 00 and 72 ∘ C and 42 ∘ C for A 02 and A 01 , respectively.The triple-shape memory was performed through eight thermomechanical loading steps using a DMA Q800 in a uniaxial tension mode, as shown in Figure 1 [19].The analysis was conducted under strain controlled programming and the heating and cooling rates were 3 ∘ C/min.At step 1 ( 1 ), the sample was stretched from  0 to  1 at  ℎ ( ℎ >  trans1 >  trans2 , where  trans1 and  trans1 are two thermal transition temperatures); the corresponding strain was recorded as   .At step 2 ( 2 ), the sample was cooled to   ( trans1 >   >  trans2 ) and the load was maintained, where A 10 and A 02 were glassy and rubbery states, respectively.At step 3 ( 3 ), the external load was removed, and the sample was fixed at the first temporary shape , with the strain of   : where   and   are the maximum and the loading strains, respectively.At step 4 ( 4 ), the sample was further stretched to the strain of   ( 2 ) at   .At step 5 ( 5 ), the sample was cooled to   while keeping the strain at   .At step 6 ( 6 ), the second temporary shape  with a strain of   was fixed at  1 after the removal of the external force: where   and   are the maximum and unloading strains, respectively.At step 7 ( 7 ), the sample was reheated to   , and it was recovered to the first temporary shape , with the strain of   .At step 8 ( 8 ), the sample was recovered to shape  with the strain of   by heating back to  ℎ .The recovery ratio of shape band shape  was obtained as follows: ,   , and  ℎ are set to 15 ∘ C, 42 ∘ C, and 72 ∘ C, respectively;   = 2.5% and   = 5%.

Results and Discussion
3.1.Morphology of Nanocomposites.Figure 2 shows the morphology evolution of nanosilica dispersed in PEG. Figure 2(a) and Figure 2(b) are the TEM and FE-SEM images of nanosilica dispersed in PEG, respectively.It confirms that the nanosilica particles are well distributed and no agglomerates present in the PEG.The cryofractured cross section of the silica/epoxy nanocomposites was observed by FE-SEM.In Figure 3, the samples A 01 (Figure 3(a)) and A 02 (Figure 3(b)) images reveal that the nanosilica particles are also well dispersed in the epoxy matrix.Through the typical process, it is noted that the obtained silica/epoxy nanocomposites   4(a)), as shown in Table 2.It is also noted that the   of sample was increased with the increasing amount of nanosilica and decreased with the increasing amount of PEG, which was attributed to the segmental motion of polymer that was hindered by nanosilica particles and PEG acted as a plasticizer.
In view of the well phase mixing of nanocomposites, A 02 and A 01 show a relatively narrow switching transition, implying that sufficient chain movement is achieved within that short temperature regime.The tan  peaks of all of samples are relatively high (higher than 0.6) indicating that the significant difference between viscous and elastic components of SMP in the  trans regime, which is of great benefit to shape recovery ratio of sample [20][21][22].Moreover, in Figure 4(a), two tan  peaks can be clearly observed in the curve of B 21 , and it is also found that the sample B 21 possessed the two well-separated glass transitions, which is shown in Figure 4(b).This phenomenon could be attributed to the formation of the phase-separated bilayer structure and provided the conditions to achieve TSME.

Mechanical Properties of the Composites.
Figure 5 shows the stress-strain behaviors of the composites at different temperatures.It can be seen from the results of Figures 5(a), 5(c), and 5(e) that the break strength and elongation of the silica/epoxy nanocomposites were improved by the addition of nanosilica particles, except for the sample A 00 that has better break elongation than the samples A 01 and A 02 at 15 ∘ C. It may be attributed to the reinforcing function and homogenous dispersion of the nanosilica particles as observed in the FE-SEM images (Figure 3).Moreover, the samples A 01 and A 02 also show good strain energy storage capacity which can be obtained from the area under the stress-strain curve, implying that the silica/epoxy nanocomposites may have better shape recovery effect [16,20].Meanwhile, the break strength and the elongation of the composites decreased with the increasing amount of PEG, indicating that PEG serves as plasticizer.
The stress-strain behaviors of B 21 depend on the temperature as illustrated in Figures 5(b), 5(d), and 5(f).Comparing with the stress-strain behaviors of B 21 at 15 ∘ C and 72 ∘ C, B 21 at 42 ∘ C exhibits an interesting phenomenon of the stress maintaining almost constant when the strain increased from 31% to 34%.This is mainly because the sample B 21 is consisted of two layers, including A 10 layer in the rubbery state and A 02 layer in the glassy state at 42 ∘ C, respectively.Additionally, A 10 layer of B 21 does not break at the strain of approximately 25%, whereas the sample A 10 breaks.This result might be due to the powerful interfacial adhesion of the two layers.For comparison, stress-strain behavior for pure EP at different temperatures is displayed in Figure 6.A notable difference between EP composites and pure EP is that the failure strain of EP composites is greatly improved due to the addition of PEG in EP matrix.data are summarized in Table 3.On the basis of DMA curves (Figure 6(a)), the rubbery state and glassy state of the samples A 02 and A 01 can be achieved at 72 ∘ C and 42 ∘ C, and those of the samples A 00 and A 10 can be achieved at 42 ∘ C and 20 ∘ C, respectively.The composites deformed in their rubbery state and generated a decrease in conformational entropy of the constituent polymer network chains.Then, the cooling down of the deformed material triggered vitrification, which kinetically traps the SMP in its low entropy state as a result of a significant reduction in chain mobility.Shape recovery is later initiated by reheating the material under stressfree conditions and allowing for the relaxation of polymer chain segments (with regained mobility) to their original entropically favored conformational state [11].Additionally, it should be noted that the shape fixities of A 00 and A 10 are relatively low while the samples A 01 and A 02 display better shape fixities at 87-93% for the four cycles, which may be contributed from the addition of nanosilica particles.Furthermore, The shape recovery effect of all the samples was 96%-99%, especially the sample A 02 that is better than the others.This finding is coincident with previous analysis.

TSME.
The triple-shape memory capability for B 21 is shown in Figure 8.The sample was first strained to 2.5% at 72 ∘ C, which was followed by cooling down to 42 ∘ C.After the stress was released, the strain instantaneously shrunk to 1.96%.At this step, the shape fixity was decided by the molecular frozen of A 02 because rubbery state of A 10 tended to recoil the sample.At last, the shape fixity ratio   of 78% was achieved.Then, the sample was stretched to 5% at 42 ∘ C, followed by cooling down to 15 ∘ C and the subsequent releasing of the stress, and the strain shrunk from 5% to 4.5%.At 15 ∘ C, although both A 02 and A 10 were in glassy state, A 02 had a tendency to keep shape , resulting in   of 84%.Finally, the sample was reheated to 42 ∘ C and 72 ∘ C; the strain was 2% and 0.04% at 42 ∘ C and at 72 ∘ C, respectively.  = 97% and   = 100% were calculated based on (4).
The TSME is further demonstrated visually in Figure 9. B 21 was cut into a rectangular film of dimensions 50 mm × 5 mm × 2 mm (shape ).Shape  was first heated in an oven at 72 ∘ C and then quickly deformed to a "" shape and quenched in a water bath at 42 ∘ C; some minor recovery occurred due to the not so perfect shape fixing ratio, but most of the deformation was kept and fixed at a temporary shape  (progress 1).After this, shape  was transformed into shape  and immersed in a water bath of 15 ∘ C; shape  was fixed at the same time (progress 2).Reheating to 42 ∘ C and 72 ∘ C, shape  and shape  recovered in turn (progress 3 and 4).Shape  and shape  behaved just the corresponding shape at the right temperature due to their excellent shape recovery ratio.

Conclusions
A novel bilayer epoxy system shape memory composites with TSME have been successfully synthesized and designed.The triple-shape memory nanocomposites were prepared by casting together two different shape memory composites exhibiting outstanding shape recovery effect.By varying the amount of PEG and nanosilica particles, we prepared a series of nanocomposites layers with different   .The dispersibility of the nanosilica particles and the failure strains of the nanocomposites were significantly improved by the addition

Figure 1 :
Figure 1: Schematic of a temperature ()-loading stress ()length () plot showing the eight-step thermomechanical cycle for attaining triple-shape memory effect.

Figure 4 :
Figure 4: DMA curves for the samples.

Figure 8 :
Figure 8: Triple-shape memory cycles of the sample for B 21 .

Table 1 :
Raw material ratios of pure EP and A 10 , A 00 , A 01 , and A 02 .
3.2.Dynamic Mechanical Properties.The dynamic mechanical properties of samples from the DMA test are shown in Figure4.The samples A 10 , A 00 , A 01 , and A 02 prepared in this study possess glass transition temperatures (  s) of 36 ∘ C, 38 ∘ C, 48 ∘ C, and 65 ∘ C, respectively, based on their tan delta peaks in the DMA curves (as shown in Figure

Table 3 :
Shape fixity and shape recovery of A 02 , A 01 , A 00 , and A 10 ( = number of thermomechanical cycles).