Citric Acid

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Materials and Interfaces

Anisotropic Shape Memory Behaviors of Polylactic Acid/Citric Acid-Bentonite Composite with a Gradient Filler Concentration in Thickness Direction Lihua Fu, Fudong Wu, Chuanhui Xu, Tinghua Cao, Ruimeng Wang, and Shihao Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00602 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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Industrial & Engineering Chemistry Research

Anisotropic Shape Memory Behaviors of Polylactic Acid/Citric Acid-Bentonite Composite with a Gradient Filler Concentration in Thickness Direction Lihua Fu, Fudong Wu, Chuanhui Xu*, Tinghua Cao, Ruimeng Wang, Shihao Guo Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China Corresponding Author: Chuanhui Xu [email protected] ABSTRACT Until now, polylactic acid (PLA) composites with a uniform dispersion of fillers have been extensively studied. However, there is few research report about the shape-memory (SM) behavior of a PLA composite in which the nanoclays have a non-uniform dispersion. In this paper, the alkaline calcium bentonite (ACBT) was firstly modified by citric acid (CA) to improve the compatibility between bentonite (BT) and PLA. The structure of CA-modified bentonite (CABT) were characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray Diffraction (XRD) and scanning electron microscopy (SEM). Then the CABT/PLA composite with gradient concentration of CABT in thickness direction was fabricated. Because of the good affinity between the grafted CA and the ester groups, the CABT imposed powerful restrictions on the PLA chains, which contributed to the anisotropic driving force for shape recovery. The relationship between structure and properties and the mechanism of anisotropic SM behavior of composite were studied. Keyword: non-uniform dispersion; composite; shape memory; structure and property 1. Introduction Shape memory materials (SMs) are a kind of stimuli-responsive materials that have the ability to remember their original shapes1. There are several classes of SMs such as shape memory alloys (SMAs)2, shape memory polymers (SMPs)3, shape memory ceramics (SMCs)4 and shape memory gels (SMGs)5,6. Among these, SMAs and SMPs are most widely used. Compared with SMAs, SMPs receive more interests due to the advantages such as low weight, low cost, and easy manufacturing. These materials can be processed a temporary shape, then restore to their original shapes under the external stimulation such as heat7,8, electromagnetism9, solvent10-12, light13 and so on and thus are widely used in aerospace, biological medicine and smart sonar. So far, the most extensively investigated SMPs are thermally-induced SMPs14. The thermally-induced SMP is usually composed of fixing phase and reversible phase. The fixing phase refers to the crosslinked or partially crystalline structure of polymer which maintains the initial shape, while the reversible phase can be softened or hardened to ensure the shape changing15,16. The thermally-induced SMP is usually shaped above a transition temperature Ttrans (glass transition temperature Tg17 or melting temperature Tm18) and then fixed below the Ttrans. When it was reheated to the Ttrans, the elastic force stored in the reversible phase turns it back to the initial shape. In recent years, biodegradable SMPs are of increasing interest for applications in various areas19-21. Many works have reported that polylactic acid (PLA) and its copolymers have a mild SM effect22,23. In order to further improve the performances, clays such as kaolinite24, montmorillonite25, halloysite26, etc were employed to prepare PLA-based SM composites. Before the composite preparation, organic modification for clays is necessary to be done because the surface treatment not only enhances the compatibility between clays and PLA but also help to achieve a uniform dispersion of clays. This is critical to improve the performance of materials. For example, Krikorian et al.27 found that the degree of peeling of MMT-C30B in PLA was higher than that of other OMMTs due to that the two -OH of the MMT-C30B had a strong interaction with C=O on the PLA backbone. The fine 1

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dispersion of MMT-C30B in PLA contributed to the improved mechanical properties of the composites. McLauchlin et al.28 prepared cocoamidopropyl betaine modified montmorillonite/PLA composites and found that the introduction of a polar group -COOH on the modified montmorillonite enhanced the interfacial interactions of PLA/montmorillonite and improved the dispersion of montmorillonite. However, there is few research report about the SM behavior of a PLA composite in which the clays have a non-uniform dispersion. It is well known that the strong interaction between clays and PLA will restrict the chain movement of PLA, which consequently improves its response ability to shape recovery. This suggests that, if a PLA composite is composed by several layers which have different filler concentrations, and then it possibly exhibits an anisotropic SM behavior. Based on above idea, in this paper, we designed a PLA/bentonite (BT) composite in which the BT had gradient concentration in the thickness direction. Before the composite preparation, the BT was modified by citric acid (CA) through the neutralization between the -COOH groups of CA and reactive -Ca-OH groups of alkaline calcium bentonite (ACBT)29,30. As a result, ACBT had a strong interaction with PLA molecules via the hydrogen bonding between the ester group of PLA and -COOH groups of grafted CA. As expected, the CA-modified bentonite (CABT)/PLA composite exhibited different SM behavior on the opposite (up and down) directions. The relationships between structure and properties of the CABT/PLA composite were investigated and discussed. We envision that this developed material cloud be used as a temperature-responsive switch where requires the different responsive ability at opposite directions. 2. Experimental section 2.1 Materials Bentonite was purchased from Guangxi long’an Ruifeng Industrial and Trading Company (Nanning, China). The raw bentonite was composed of 61.2% SiO2, 17.3% Al2O3, 4.0% Fe2O3, 1.7% CaO, 2.8% MgO, 1.0% K2O and 4.1% Na2O. Cyclohexane (analytical purity), chloroform (analytical purity) and anhydrous ethanol (analytical purity) were purchased from KeLong Chemical Company (Chengdu, China). Citric acid (CA, analytical purity) was purchased from Sinopharm Chemical Reagent Co., Ltd (China). Polylactic acid (PLA), Mw=2.5×105 g/mol, density=1.24 g/cm3, Tg= 62.5°C and Tm=165°C (measured by differential scanning calorimetry (DSC), was purchased from NatureWorks LLC (USA). Other chemicals were of analytical purity and used as received. 2.2. Preparation of CABT The ACBT was firstly prepared as described by Wei29,30. Next, 5.0g ACBT was added into 60 mL cyclohexane under a vigorous stirring of 400 r/min at 80°C. Then, keeping the vigorous stirring and temperature, a pre-prepared mixture of 5.0g CA and 7.0g anhydrous ethanol was slowly added into above system. The reaction was lasted about 2h. Finally, the resultant CABT was washed with superfluous anhydrous ethanol, followed by pumping filtration and drying at 55 °C until the weight was constant. The prepared CABT was stored at dried condition before characterization. 2.3. Preparation of CABT/PLA composite First, 10g PLA granules were completely dissolved in 64 mL chloroform at room temperature, and then 1.5g CABT (well dispersed in 10mL cyclohexane) was added into PLA solution under vigorous stirring. The temperature was kept at 80°C. The stirring time was 12h. The solvent evaporation in this process increased the viscosity of the solution to about 2000Pa/s, which was the key to make sure that the CABT had a gradient concentration in the gravity direction rather than all of them precipitated at the bottom during next drying. The resultant viscous CABT/PLA mixture was poured into a custom mold and dried in a vacuum oven for 24h at 60°C. The thickness of final CABT/PLA composite was controlled to be ~2mm by changing a suitable base area of the custom mold. CABT/PLA-1, CABT/PLA-2 and CABT/PLA-3 represented the three different sections from top to bottom of the CABT/PLA composite. As shown in Figure 1, the CABT/PLA-1 contains less CABT, the CABT/PLA-2 has a 2


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middle CABT content and the CABT/PLA-3 has more CABT.

Figure 1 Schematic of the CABT/PLA composite preparation 2.4. Characterizations Fourier transform infrared spectroscopy (FT-IR) The FT-IR test of CABT was taken from 4000 to 500 cm−1 using a FT-IR 100 (Perkin Elmer) in the background of KBr with a resolution of 4 cm-1and 32 scans. The FT-IR test of the composite was directly measured under the attenuated total reflectance (ATR) model of the FT-IR 100, resolution 4 cm-1and 32 scans. X-ray Diffraction (XRD) The ACBT and CABT were tested in the scanning range of 2θ = 5-60° on the X'Pert PRO type X-ray diffractometer (Panantiia), with Cu anode (λ = 0.154 nm), the tube voltage was 40 kV, Current 200 mA, scanning speed 5 °/min. Thermal Gravimetric Analysis (TGA) Thermal decomposition behavior of the ACBT and CABT was studied by a TGA50 (TA, USA) which was carried out from 23 to 800 °C at 10 °C min-1 in nitrogen atmosphere. Particle size analysis Particles size was determined using a laser scattering particle size analyzer (NanoBrook Omni, Brookhaven). Samples for particles size analysis are taken directly from the ACBT or CABT suspension in wet analysis mode at 23°C. Scanning Electron Microscopy (SEM) The surface morphologies of ACBT and CABT were investigated using an S3400 scanning electron microscope (Hitachi Company). The cross-sectional morphology of CABT/PLA composite was observed using the SU8020 field emission scanning electron microscope (Hitachi Company). Before morphological observation, the surface of samples was coated with a thin layer of gold to prevent electrostatic charge build-up during observation. Condition: Tube voltage 15.00 KV. Differential Scanning Calorimetry (DSC) The test was carried out using a Q2000 differential scanning calorimeter (TA, USA). The samples were heated from -60 ºC to 200 ºC at a heating rate of 5 ºC· min-1 in flowing argon gas. Rheological behaviors The dynamic rheological behaviors of the CABT/PLA composite were recorded by a MCR302 Anton-Paar rheometer at 180°C. A plane-plane geometry (diameter of 50 mm) was used. The angular frequency (ω) ranged 3



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from 0.01 to 100 rad/s and the strain was fixed at 1% to make the measurements were in a linear viscoelastic strain range. Dynamic mechanical behavior The dynamic mechanical behavior of the CABT/PLA composite was determined using a DMA242C (NETZSCH, Germany) with tensile mode at 1 Hz and a heating rate of 3°C /min in the temperature range from 0 to 100°C. SM behaviors The CABT/PLA composite was cut into a rectangular strip with 5 cm (length) × 1 cm (width) × 2 mm (thickness). The strip was then placed under a deformation temperature for 1 min to soften the material. After that, the sample was bended to a certain angle, and quickly cooled to fix the temporary shape. The temporary shaped sample was placed at room temperature for 2 hours to investigate its shape fixity. When it was quickly reheated to 65 ºC, the shape recovery behavior was triggered. The final recovered angles were recorded to evaluate the SM effect. The above test was done on opposite directions to evaluate the anisotropic SM behavior. Shape fixity ratio (SF) and shape recovery ratio (SR) were calculated according to the followed equations.

SF =

fixed angle（2h later) × 100% initial fixed angle

SR =

recovered angle × 100% 180°

(1)

Mechanical Properties The tensile properties of the CABT/PLA composite were measured using a universal tester (Shimadzu AG-1, 10kN, Japan) according to GBT529-2008 standard at a crosshead speed of 10 mm/min. Test temperature: 23°C, and the average value was calculated from 5 test specimens 3. Results and discussion

Figure 2 (a) FTIR spectrum of ACBT and CABT; (b) schematic for the citric acid-modification of ACBT The successful CA-modification of ACBT was confirmed by FTIR. As shown in Figure 2a, the absorption peaks of ACBT at 872 and 1792 cm-1 were ascribed to the reactive hydroxyl group of BT-Ca-OH31,32. After the treatment with CA, the absorption peaks at 872 cm-1 and 1792 cm-1 disappeared. Additionally, compared with ACBT, the appearance of new absorption peak at 1715 cm-1 in the CABT spectrum was attributed to the symmetric stretching vibration peaks of carbonyl (C=O)33. These suggested that the -COOH groups of CA had reacted with the reactive -OH groups of ACBT. The Si-O-Si antisymmetric stretching vibration peak at 1037 cm-1 (ACBT)34 was shifted to 1046 cm-1 (CABT), at the same time, the peak became sharp. These suggested that the chemical environment of crystals in BT had been changed after modification. The chemical bonding came from the neutralization of -COOH of CA and BT-Ca-OH of ACBT. As a result, the -COOH and -OH groups of the CA were chemically bonded onto the surface of BT (Figure 2b). The absorption at 3621cm-1 being shifted to 3661 cm-1 4


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suggested that the reactive -OH groups of ACBT was replaced by the -OH groups of CA. The grafted CA was expected to have strong interactions with the ester groups of PLA chains, which improved the compatibility between the BT and PLA matrix.

Figure 3 (a) TGA curves and (b) bulk densities of ACBT and CABT; SEM images of (c) ACBT and (d,e) CABT The grafting of CA was also semi-quantitatively evaluated by TGA. The CABT was extracted by anhydrous ethanol for 3d to remove the free CA as possible before TGA measurement. As shown in Figure 3a, ACBT has a mild weight losing between 100 and 700°C due to the loss of interlayer water in this temperature zone35,36. Then, it left a constant residual ratio of 86% above 700°C. At this time, the interlayer water was removed completely and the residual should be the inorganic BT. As for CABT, it showed a sharp weight loss when the temperature exceeded 200°C due to the degradation of grafted CA. Then it exhibited dramatic weight loss containing the degradation of CA and the loss of interlayer water until 700°C. After that, the weight loss of CABT was unobvious and its residual ratio was about 51%. This implied that the grafting of CA in ACBT was more than 35% since the weight percent of interlayer water in CABT was lower than that in ACBT. The TGA measurements were repeated for two times and the results curves were completely overlapped. However, it was still possible that some CA molecules remained in the interlayer of the BT particles with physical interactions. In addition, the molecule weight of CA is relative larger than other small organic molecules which is usually used in modification of clays. This resulted in a high organic grafted ration of CABT. Nonetheless, in the case of CABT, a major weight loss was observed in the temperature range of 200-700 °C mainly corresponding to the grafted CA. Furthermore, the weight percentage of CA in CABT, namely the grafting degree was higher than 35% by considering the differences in weight loss rate of ACBT and CABT. The successful CA-modification resulted in a looser appearance with deeper color for CABT, as shown in the inset image of Figure 3a. Apparently, the results of bulk densities of ACBT (1.84g/cm3) and CABT (1.72g/cm3) in Figure 3b were consistent with their apparent appearances. The micro-morphology of ACBT and CABT was investigated by SEM. As shown in Figure 3c, ACBT exhibited a particle gathered morphology that the BT layers are tightly packed together. In comparison, CABT (Figure 3d and e) showed a loose morphology that some of the BT were exfoliated to be small layers with thickness of 20~30nm. The exfoliation was possibly due to the strong acid-base neutralization reaction between the CA inserted into the ACBT layers and the active -OH between the ACBT layers33. The volume effect of the modifier and the heat-release from 5



the neutralization reaction facilitated the enlargement of the interlayer spacing of BT, which resulted in the exfoliation of CABT. However, the exfoliated CABT could be re-aggregated again in dried powder state due to their large specific surface area, which resulted in large aggregates as observed in Figure 3d and e. To further confirm the exfoliation of CABT, XRD and particle size analysis for ACBT and CABT were done, and the results are provided in supporting information (Figure S1 and S2).

Figure 4 (a) FTIR spectrum of neat PLA and CABT/PLA composites; (b) schematic of sampling for the CABT/PLA composite; (c) DSC curves and (d) crystallinity of neat PLA and CABT/PLA composites The infrared spectrum of PLA and CABT/PLA composite are shown in Figure 4a. Unfortunately, the characteristic absorption peaks of CABT were shielded by the strong absorption peaks of the PLA matrix, which turned out similar FT-IR spectra for neat PLA and CABT/PLA composite. The absorption peak at 1453 cm-1 and 1356 cm-1 were assigned to the bending vibration of -CH3 group and -CH2 group, respectively. The strong absorption peak at 1753 cm-1 was corresponded to the stretching vibration of -C=O bond. The adsorption peak of -CO- stretching vibration in the -CH-O- group appeared at about 1178 cm-1 and the absorption peaks at 1128 cm-1, 1089 cm-1 and 1040 cm-1 were attributed to –OC=O. These results were consistent with the reports of FT-IR characterization of PLA in the literatures36. Therefore, the addition of CABT did not change the structure of PLA, suggesting that the compatibilization was originated from the physical interactions between the grafted CA and PLA chains. According to the design, the content of CABT had an ascending gradient from the top to the bottom of CABT/PLA composite, which resulted in different structure and the properties from the top to the bottom of material. We selected three sampling positions from the top to the bottom of CABT/PLA composite, as schematically illustrated in Figure 4b. To give an intuitionistic understanding of the dispersion of CABT in PLA matrix, we sampled CABT/PLA-1, CABT/PLA-2 and CABT/PLA-3 and heated them in the Muffle furnace at 800°C. As shown in Figure 4b, the residual rate of CABT/PLA-1, CABT/PLA-2 and CABT/PLA-3 were 5.9wt%, 7.6wt% and 12.8wt%, showing increased filler content from the top to the bottom of CABT/PLA composite. Although the residual rate was not the real CABT concentration due to the degradation of grafted CA and the loss 6


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of interlayer water, the residual rate had a very instructive meaning for the gradient CABT concentration. Figure 4c shows the DSC thermograms of neat PLA and three different sampling of CABT/PLA composite (CABT/PLA-1, CABT/PLA-2 and CABT/PLA-3). Three replicate DSC measurements were made and the values in DSC figures are the average values. The Tg of PLA was mainly affected by the amorphous phase transition energy while the BT was stable in the test temperature range. With the increase of CABT concentration, the Tg exhibited an increase from 62.5 ºC of the neat PLA to 64.6±0.2 ºC of the CABT/PLA-3, which was attributed to two factors: on the one hand, the movement of PLA molecules was obstructed by filler effect37,38; on the other hand, the interaction between PLA chains and grafted CA made PLA partially immobilized, and their micro-Brownian motion was constrained by the CABT lamellae or particles39,40. It is well known that the SM behaviors of thermo-SMPs are closely related to the Tg of material, therefore the CABT/PLA composite had different responses to the shape recovery on opposite directions, which will be discussed later. Meanwhile, the increasing CABT concentration resulted in a decrease from 124.7 to 115.2±0.1 ºC ºC in the Tcc (cold-crystallization temperature) of PLA, which was attributed to that the grafted CA provided additional nucleation sites in the process of PLA crystallization41. Thus, the CABT served as nucleating agents to induce a faster crystallization of PLA42. As a result, the crystallization of PLA occurred rapidly and consequently resulted in less perfect crystals, which led to two sharp melting peaks between 165 ºC and 170 ºC43. The crystallinity (Xc%) of neat PLA and three different sections of CABT/PLA composite are shown in Figure 4d. With the addition of CABT, a significant improvement was achieved in the Xc% due to that more nucleation sites were available for the growth of PLA crystal. As seen, the CABT/PLA-1 showed an improved crystallinity of 16.1±1.1 ºC % (neat PLA 12.2%), and it was further increased up to 19.3±1.3 ºC % for CABT/PLA-2. However, the increased CABT concentration in the CABT/PLA-3 started to influence the crystalline structure as defects, and a small decrease in crystallinity (18.9±0.9 ºC %) was observed. Such a result suggested that the mutual competition of heteregeneous nucleation and chains-constraint of CABT worked on the PLA matrix, which induced the final crystallization.

Figure 5 SEM images of (a, a’) CABT/PLA-1, (b, b’) CABT/PLA-2 and (c, c’) CABT/PLA-3 The morphology of CABT dispersion in PLA was observed from the cryofractured surface of CABT/PLA composite. As shown in Figure 5a, at the upper layer of CABT/PLA composite (CABT/PLA-1), most CABT were in loose aggregates (white arrows in SEM images) which had an unconspicuous boundary with PLA. The formation of loose aggregates was attributed to the self-assembly behavior of exfoliated CABT in PLA solution during sample preparation. Although the grafted CA had good affinity with PLA molecules, their direct interactions in solution were not strong enough to stop the aggregation of CABT. As a result, the exfoliated CABT with large specific surface area had an aggregate tendency during solvent evaporation process. However, it is clearly seen from Figure 7



5a’ that the loose aggregates of CABT exhibited excellent compatibility with the PLA matrix, which was strongly in accordance with the Tg changes in Figure 4c. As seen in Figure 5b and Figure 5c, more larger CABT aggregates with several micrometers (red arrows in SEM images) were found in the lower layer of CABT/PLA composite (CABT/PLA-3). The larger aggregates were originated from the incompletely exfoliation of CABT, and most of them precipitated at the bottom in the PLA solution during coagulation. Nevertheless, no holes were observed in Figure 5b and 5c, which confirmed the excellent compatibility between CABT and PLA matrix. It is worthy of pointing that the loose aggregates of exfoliated CABT were also found in Figure 5b’ and 5c’, which suggested that most of exfoliated CABT were in the form of loose aggregates dispersed uniformly in the whole PLA matrix. The SEM images of cryofractured surface of ACBT/PLA composite can be found in Figure S3.

Figure 6 (a) dynamic complex viscosity and (b) storage modulus of neat PLA and CABT/PLA composite Rheological behaviors are quite useful in detecting the filler dispersion state. The gradient concentration of CABT resulted in an increased complex viscosity (η*) for the composite, as shown in Figure 6a. In the low frequency region, the η* of CABT/PLA composite samples were insensitive to the frequency dependence, showing a Newtonian plateau. Then, the samples presented shear thinning behavior with increasing shear frequency. The zero shear-rate viscosity of CABT/PLA composite is higher than neat PLA due to the interactions between CABT and PLA molecules. As expected, CABT/PLA-3 exhibited the highest η*, suggesting that the bottom of CABT/PLA composite had the maximum CABT concentration. According to Figure 5, the increased η* was due to the precipitation of larger incompletely exfoliated CABT at the bottom of material. Similar increasing tendency can be found for the storage modulus (G’) in Figure 6b. Incorporation of CABT brought an improvement in G’ over the whole frequency range, which was more pronounced at low frequency region. As seen, the G’ increased with increasing CABT content at low frequency and a low-frequency plateau appeared in the G′ curve of CABT/PLA-3, which usually indicated the formation of a rheological percolation network44. This further suggested that the PLA chain relaxations were significantly restricted by CABT at the bottom of composite. The comparison of rheological behaviors of CABT/PLA and ACBT/PLA composites is shown in Figure S4.

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Figure 7 (a)Storage modulus (E’) V.S temperature of neat PLA and CABT/PLA composites; (b) E'23°C/E'65°C ratio: the indicator for SM capability of CABT/PLA composites Figure 7a shows the temperature dependence of storage modulus (E’) of neat PLA and CABT/PLA composite in the range of 0~100 ºC. As expected, the E’ increased dramatically from the top to the bottom of CABT/PLA composite due to the increasing CABT content in this direction. As seen, the E’ (0 ºC) of CABT/PLA-1 (~2500MPa) was slightly higher than that of neat PLA (~2250MPa), while the E’ (0 ºC) of CABT/PLA-3 increased up to ~3600MPa, nearly 1.5 times that of the CABT/PLA-1. Obviously, this dramatic difference of E’ revealed that majority of CABT was concentrated at the bottom of composite, which was consistent with the residual rate in Muffle furnace (Figure 4b). Furthermore, the enhanced compatibility between CABT and PLA turned out strong effect on the segmental motion of PLA chains, which also resulted in an increase in the strength of PLA (see Table S1, the strength was increased from 59.2±2.6 of neat PLA to 62.8±1.9MPa of CABT/PLA composite). Note that all of samples reached a similar rubbery plateau above 65 ºC, which suggested that the glass transition of CABT/PLA composite had been completed at 65 ºC. Therefore, 65 ºC was selected as Ttran for this CABT/PLA composite to realize SM behavior. It is well known that the E’ below the Ttran is usually associated with the shape fixing for a thermally-induced SMP while the E’ above the Ttran will govern the shape recovery rate. Then, the E’23°C/E’65°C ratio was used as an indicator for the SM performance of CABT/PLA composite. According to the E’-temperature curve in Figure 7a, the E'23°C/E'65°C ratios of PLA, CABT/PLA-1, CABT/PLA-2 and CABT/PLA-3 are calculated as 86, 91, 94 and 101, respectively, as shown in Figure 7b. The E'23°C/E'65°C ratio of CABT/PLA composite was higher than that of the neat PLA, suggesting that an improved SM performance of PLA was achieved after the addition of CABT. Since the shape recovery driving force of PLA mainly come from the release of entropic unstable oriented amorphous molecular chains45,46, the chains restriction resulted from CABT played a positive effect on the SM behavior of PLA. As a result, the E'23°C/E'65°C ratios were increased from the top to the bottom of the CABT/PLA composite. However, this is what we wanted. The unique gradient responsiveness for CABT/PLA composite tuned out anisotropic SM behaviors on opposite directions.

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Figure 8 Digital photograghs of the shape recovery behaviors at upwards and downwards (V shapes) of neat PLA (a, a’) and CABT/PLA composite (b, b’) Table 1 Shape memory recovery of the angles (V shapes) samples Neat PLA CABT/PLA

Shape upwards (top side) fixed angle recovered angle 64º 52º

Shape downwards (bottom side) fixed angle recovered angle

83º 106º

51º 32º

82º 71º

According to Figure 7a, the deformation temperature (Td) was set at 65 ºC since PLA had been softened at this temperature. The anisotropic SM behaviors of CABT/PLA composite were evaluated from the recovered angles of “V” shapes. As shown in Figure 8a and a’, neat PLA was unable to fully recover its original shape (180º), which retained a high residual angle. Note that the fixed angles of “V” shapes toward two opposite sides were different: the shape upwards was 64º while the shape downwards was 51º. However, the final recovered angles were basically the same (82~83º) in different bending directions, suggesting that the initial fixed shape of PLA did not influence its final degree of shape recovery. As for CABT/PLA composite (Figure 8b and b’), the final recovered angle of the shape upwards was increased to 106º (initial fixed angle 52º) while the one of the shape downwards was only 71º (initial fixed angle 32º), exhibiting an obvious anisotropic SM behavior at opposite sides. The above recovered angles were also summarized in Table 1. This anisotropic SM behavior was attributed to the different responses to structure changes from top to bottom of the composite. According to the previous analysis, more CABT precipitated at the bottom of PLA matrix, which increased the interactions between PLA chains and CABT. Because of the excellent affinity between the grafted CA and the ester groups of PLA, CABT imposed powerful restrictions on PLA chains, serving as physical cross-linkages. Furthermore, the increased crystals contributed to the physical cross-linkages effect (Figure 4d), which enhanced the recovery ability of the PLA chains in amorphous regions. When the sample was up-bended, the oriented PLA chains in the bottom of material suffered the maximum stretching force. This endowed PLA chains with a stronger driving force for shape recovery. In contrast, the lower CABT concentration at the top of composite resulted in less physical cross-linkages, which led to a relative weaker driving force for shape recovery when the sample was down-bended. The schematic of structure changes in the amorphous regions of CABT/PLA composite is shown in Figure 9. The anisotropic SM behavior of CABT/PLA composite was originated from the gradient CABT concentration from the top to the bottom of the composite.

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Figure 9 Schematic of structure changes in the amorphous regions of the CABT/PLA composite and the driving forces of shape recovery.

Figure 10 (a) SM properties of the samples shaped at different Td; (b) Repeated SM behavior at deformation temperature of 61°C and 65°C To further reveal the SM mechanism of CABT/PLA composite, we deformed the samples at a series of temperatures from 58 to 72°C to fix a temporary angle at 50º. Then, the SM behavior of “V” shape samples was triggered at 65°C. The representative SF and SR data for shape upwards and downwards at various Td are shown in Figure 10a. As seen, the SR had an apparent dependence with the Td while the SF was insensitive to the temperature dependence. The excellent 100% SF was attributed to the high rigidity of PLA matrix which forced an excellent shape fixing for the composite. When the Td was below 60°C, the stress whitening occurred at the bending section due to that the PLA chains were unable to move timely to adapt the deformation47. This resulted in structure rupture at the bending position where suffering the maximum stress, which seriously reduced the shape fixing and the shape recovery. When the Td was above 60°C, the enhanced movement of PLA chains in amorphous regions made material to be easily shaped at a mild deformation rate without any stress whitening. It is clearly seen 11



that the SR of shape upwards and shape downwards respectively increased up to 90% and 81%, which were much higher than that obtained at 65°C. The remarkable improvement of SR was attributed to that the suitable movement of PLA chains facilitated the orientation of PLA chains in amorphous regions, which consequently provided stronger driving force for shape recovery. Since the CABT in the bottom of CABT/PLA composite were denser than that in the top of composite, the restriction effect resulted in more and higher orientation of PLA chains. Therefore, the shape upwards exhibited a higher SR than the shape downwards, showing an anisotropic SM behavior. However, when the Td was above 70°C, the SR was reduced to only about 25%. We noticed that the material was quite soft at this temperature, which demonstrated that the fast rearrangement of PLA molecules could be realized timely during deformation at this time. Additionally, the weakened physical interactions between CABT and PLA at elevated temperatures also released more removable length of PLA chains. As a result, CABT lost his job on restricting PLA chains. Consequently, the SM behavior was reduced and even lost at a higher Td. Finally, we evaluated the repeated SM effect of CABT/PLA composite at two deformation temperatures: 61°C and 65°C, which are shown in Figure 10b. Although the 2nd and 3rd SR were slightly reduced due to the rupture of interactions between CABT and PLA molecules, the values were still maintained at an acceptable scope, in particular at a Td of 61°C. The repeated SF was always maintained at 100%, which were not shown here. The comparison of SM effect among neat PLA, CABT/PLA and ACBT/PLA composites (Td=61°C) is provided in Figure S5. 4. Conclusions In this paper, the ACBT was treated with CA to improve the compatibility between BT and PLA. The grafting of CA on the CABT was more than 35% and some of CABT were exfoliated to be small layers with thickness of 20~30nm during modification. The CABT/PLA composite was prepared via solution mixing method. Although the grafted CA had good affinity with PLA molecules, their direct interactions in solution were not strong enough to stop the aggregation of CABT. As a result, the exfoliated CABT with large specific surface area had an aggregate tendency during solvent evaporation process. Consequently, most of them precipitated at the bottom in the viscous PLA solution during coagulation. This resulted in an ascending gradient CABT concentration from the top to the bottom in the CABT/PLA composite, which turned out different structure and the properties from the top to the bottom of the material. Because of the excellent affinity between the grafted CA and the ester groups of PLA, the CABT imposed powerful restrictions on the PLA chains, which contributed to the driving force for shape recovery. Since the CABT in the bottom of CABT/PLA composite are denser than that in the top of composite, the restriction effect resulted in more and higher orientation of PLA chains during deformation. Consequently, the shape upwards exhibited a higher SR than the shape downwards, showing an anisotropic SM behavior. It was found that deformation temperature had a significant influence on the SM behavior of the CABT/PLA composite. When the composite was shaped at 60~61°C with a mild deforming rate, the SR of shape upwards and shape downwards increased up to ~90% and 80%, respectively. The authors remarked that there were drawbacks in the preparation of CABT/PLA composite with precise gradient concentration of ACBT. However, the authors hoped that this work would open up specific applications for the polymer composites with non-uniform dispersion of fillers. Supporting Information. XRD patterns and particle size analysis of ACBT and CABT; SEM images of cryofractured surface of ACBT/PLA composite; rheological behaviors and SM effects of neat PLA, CABT/PLA and ACBT/PLA composites; mechanical properties of PLA and CABT/PLA composite; Shape memory recovery of the angles (V shapes) Acknowledgments 12


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Citric Acid

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