Article pubs.acs.org/Biomac
Lignin-Based Triple Shape Memory Polymers Gopakumar Sivasankarapillai,† Hui Li,‡ and Armando G. McDonald* Renewable Materials Program, Department of Forest, Rangeland, and Fire Sciences, University of Idaho, Moscow, Idaho 83844-1132, United States S Supporting Information *
ABSTRACT: Lignin-based triple shape memory polymers comprised of both permanent covalent cross-links and physical cross-links have been synthesized. A mixing phase with poly(ester-amine) and poly(ester-amide) network having two distinct glass transitions was hot mixed with more structurally homogenized methanol soluble lignin fraction by one-pot, two-step method. Triple shape properties arise from the combined effect of the glass transition of polyester copolymers and lignin and the dissociation of self-complementary hydrogen bonding and cross-link density. The percentage of recovery in each stage was investigated and it was proved that the first recovery is related with lignin-poly(ester-amine) rich network and the second recovery stage is related with ligninpoly(ester-amide) rich network. The thermal and mechanical properties of the lignin-copolymer networks were also investigated using differential scanning calorimetry and dynamic mechanical analysis.
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INTRODUCTION Lignin-based side streams of the pulp and paper industry are currently incinerated to produce energy while being a potential large volume feedstock resource for producing higher addedvalue products such as fibers, polymers, resins and platform chemicals. The exploitation of these streams requires the application of new technologies to arrive at novel and economically viable solutions able to compete with fossilbased alternatives. Significant commercial potential exists in the conversation of lignin to high-value end products, but lignin remains a highly difficult and challenging material to convert into such useful products. Although functional materials have been produced from lignin, these materials are generally highly cross-linked materials.1 Furthermore, due to their rigid and brittle character, the known lignin-based materials generally lack the required flexibility, strength, and toughness for use in many industrial and commercial applications. Novel methods for the fabrication of smart materials are especially welcome if they can produce samples of both from readily available and inexpensive starting materials. Lignin from industrial waste has come into such a category.2−5 The use of lignin from industrial waste for obtaining new classes of engineering plastics is highly motivated by the abundance of this raw material and advantageous properties.6 Of late, many studies based on biobased materials having the properties of memorizing one temporary shapes, corresponding to dual-shape memory effects have been reported.7−11 Recently, my group has developed and studied toughening agents for lignin-copolymeric materials.12 In this work, a highly branched lignin-based poly(ester-amine) copolymer was developed by reactions of triethanolamine (TEA), adipic acid (AA), and lignin.12 Lignin appears to be especially compatible with © XXXX American Chemical Society
polyester materials, both as a reactant to form covalently linked lignin composites as well as in forming lignin−polyester blends.12 Dicarboxylic acid (e.g., AA) and hydroxyl groups formed ester linkages, and each component of final polymer was controlled by insertion amount. Lignin was added up to 50%, and the final product showed good miscibility with a single glass transition temperature (Tg). In addition, mechanical properties were comparably strong as compared with commonly used commercial polymers: polyethylene, polypropylene, and polyurethane.13 These lignin-copolymers were shown to have dual shape memory effect (SME) at various thermal transitions.14−16 It has been suggested that the shape-memory ability of shape memory polymers (SMP) is dominated by the thermal and mechanical properties of polymer networks.17 The precise design of polymer networks seems to be necessary to achieve the desired SME. In order to obtain improved properties and new functions of SMPs, the toughening agent, i.e., poly(esteramine), prepolymer was modified by incorporating another poly(ester-amide) network. This network has thermomechanical properties especially Tg in between the poly(ester-amine) and lignin.18 Thus, a synthetic approach was designed for hyperbranched polyesters (HBP) as a new toughening agent with amine cores and amide linkages by a one-pot, two-step method. The final lignin-copolymers were prepared by chemical linking of hydroxyl on lignin with terminal carboxyl on HBP and physical cross-linking between various functional groups. In this work, it was demonstrated that the ligninReceived: May 14, 2015 Revised: July 16, 2015
A
DOI: 10.1021/acs.biomac.5b00655 Biomacromolecules XXXX, XXX, XXX−XXX
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constant during the subsequent cooling stage. All strain recovery experiments were carried out under stress free condition. The strain recovery upon staged heating was conducted by holding the polymer at the target temperatures (recovery temperatures, Tr’s) equal to the corresponding deformation temperatures (Td’s). Shape Memory Characterization. All the thermomechanical analysis (TMA) experiments were conducted in a tensile mode using a DMA Q800 (TA Instruments). Stress controlled deformation was performed by applying a deformation force at a target deformation temperature (Td) and the force was maintained constant during the subsequent cooling stage. All quantitative shape memory properties were evaluated in a tensile and force controlled mode. The heating and cooling rates were both 5 °C min−1. The triple-shape memory properties were investigated by stretching the samples at target temperature (Td1) and cooled to lower temperature (Td2) under a force (F1), then the sample was equilibrated at Td2 for several minutes unloading to afford the temporary shape, S1. Then the sample was further stretched at the same temperature (Td2) and cooled to lower temperature (>Td2) under another force (F2, F2 > F1), then the external force was removed to obtain second temporary shape, S2. In the recovery step, the sample was reheated and equilibrated at Td1 to obtain shape ll (LS1,rec) and Td2 for shape l (LS1,rec). The programming conditions for each sample were chosen based on its physical properties.
copolymer networks were able to show TSME behavior at programmed deformation temperatures. The general concept of the triple shape memory effect (TSME) is based on polyester networks, poly(ester-amine), and poly(ester-amine-amide) segments. The coexistence of these two networks with lignin created broadened Tg and mico-level inhomogeneity. The protocols to construct triple-shape cycles followed the recently reported work on triple-shape polymers.19 Although most of the triple-shape memory polymer systems have been reported from petroleum-derived polymers, we anticipate that more biobased polymers will be developed in the near future.
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EXPERIMENTAL SECTION
Materials. Protobind 1000 soda lignin was supplied by ALM India. The methanol soluble fraction of the soda lignin (lignin content = 93.3%, Mw = 958 g mol−1 (ESI-MS), aromatic/aliphatic hydroxyl group ratio = 0.93)15 was used for copolymer synthesis. 1,1,1Triethanolamine (TEA, B3), tris(hydroxymethyl)aminomethane (THAM, CB31), adipic acid (AA, A2), methanol, and tetrahydrofuran (THF) were obtained from Acros Organics and used as received. Synthesis of Lignin-copoly(ester-amine-amide). Synthesis and characterization of hyperbranched poly(ester-amine-amide) (HBPEAA) prepolymers was described in our previous publication.18 In brief, AA (0.740 g, 5.02 mmol), THAM (0.305 g, 2.51 mmol), and TEA (0.375 g, 2.51 mmol) were melt condensed in a crystallizing dish for 10 h under reduced pressure (650 mmHg) at 45 °C in a vacuum oven, then the temperature was increased to 100 °C and kept for another 10 h. The prepolymer product was used for mixing with lignin. Lignin was dissolved in minimum amount of THF and proportionally mixed with HBPEAA prepolymer, to synthesize the lignincopoly(ester-amine-amide) (lignin-co-HBPEAA). Then THF was removed under a stream of N2 at 80 °C. The highly viscous mixture was then transferred to a prepared aluminum pan (15 × 15 cm2) and was placed into vacuum oven at 80 °C and 750 mmHg to remove residual solvent and moisture (about 30 min). Then the reaction temperature was increased to 120 °C at 650 mmHg and the reaction proceeded for 20 h. The lignin copolymer was tough and flexible at ambient temperature and insoluble in common organic solvents. The preparation was repeated with varying THAM contents from 20% to 80% to total required amount of TEA and THAM with respect to AA moles at 1.1:1 molar ratio of COOH/OH and NH2 groups. For example, sample ID, L25T20 represent 20% THAM (T20) and 80% TEA used for prepolymer preparation and the final lignin-co-polymer contains 25% lignin (L25). Instrumental Methods. All Fourier transform infrared (FTIR) spectra were collected using a ThermoNicolet Avatar 370 spectrometer operating in the attenuated total reflection (ATR) mode (SmartPerformer, ZnSe crystal). Differential scanning calorimetry (DSC) was performed on a TA Instruments model Q200 DSC equipped with a refrigerated cooling unit to monitor the thermal behavior of materials. The instrument was calibrated using indium. The polymeric materials (5−7 mg) were analyzed using a temperature profile after cooling to −70 °C then ramped to 70 °C at a heating rate of 10 °C min−1 and held isothermally for 5 min, followed a cooling ramp of −10 °C min−1 to −70 °C, and then a second heating cycle to 70 °C at a heating rate of 10 °C. All the data collected was from the second heating cycle. Dynamic mechanical analysis (DMA) experiments were performed using a TA Instruments model Q800 DMA, both under isothermal and nonisothermal conditions in nitrogen atmosphere. The copolymers (12 × 4 × 0.2 mm3) were tested in the tensile mode. The heating scans were carried out at 1 Hz at a heating rate of 3 °C min−1. The tensile properties of the lignin copolymers (five replicates per sample) were also measured using DMA at 27 °C. All the shape memory experiments were conducted in a tensile mode using a DMA Q800 (TA Instruments). Stress-controlled deformation was performed by applying a deformation force at a target deformation temperature (Td) and the force was maintained
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RESULTS AND DISCUSSION Physical Properties of Lignin-co-HBPEAA and Prepolymers. The shape memory properties of the lignin-coHBPEAA are addressed by the possibilities of combined covalent/noncovalent effect and inhomogeneity in the polymer networks that could affect the entire material properties. In our previous work, amide formation was detected in the HBPEAA prepolymer formed below 45 °C.18 Under feasible conditions, dicarboxylic acids would react with primary amine in THAM to form amide dominant intermediate in the presence of TEA and further increased temperature (100 °C) condition created various ester-amide and ester-amine polymeric networks.18,20−23 According to the reported structural analysis data in our previous article,18 the lignin-co-HBPEAA is a mixed polymeric network of poly(ester-amine) (Tg = −45 °C), poly(esteramide) (Tg = 8 °C), and lignin (Tg = 119 °C) with distinct Tg (inset of Figure 1). DSC analysis of the lignin copolymers show
Figure 1. DSC thermograms of (a) L25T50 and (b) L35T20; In inset, DSC thermograms of prepolymers; (a) poly(ester-amine) (TEA:AA = 1:1.1), (b) poly(ester-amine-amide) (TEA:THAM:AA = 1:1:1.1) from two temperature (45 and 100 °C), (c) poly(ester-amide) (THAM:AA = 1:1.1). B
DOI: 10.1021/acs.biomac.5b00655 Biomacromolecules XXXX, XXX, XXX−XXX
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HBPEAA segments as well as the covalent cross-links between lignin-co-HBPEAA. Tensile Properties of Lignin-co-HBPEAA. The tensile properties for the lignin-co-HBPEAA at 27 °C are shown in Figure 2 and values are given in Table 1. The figure shows that
Table 1. Thermal and Mechanical Data and Crosslink Density of Lignin-co-HBPEAA sample
E″ peak (Tg °C)
L25T20 L35T20 L45T20 L25T50 L35T50 L40T50 L50T50 L25T40 L25T60
−2 14 39 20 35 46 70 15 25
Young’s modulus (MPa)
elongation (%)
cross-link density (ρ × 102 mol cm−3)
7 16 630 30 27 34
71 200 19 145 152 147
21 35
154 121
2.73 2.57 3.53 2.06 1.73 1.56 0.79 2.69 1.94
example, in the case of L35T20 from around −20 °C (Figure 1) and followed by the major transition which begins from around −10 °C. The transition temperature around −20 °C corresponds to poly(ester-amine-amide) phase (inset of Figure 1b). This indicates that microlevel inhomogeneity exists in the copolymeric system. Moreover, the Tg of the lignin-coHBPEAA increased with lignin and even at low wt % of THAM. DSC analysis also shows that the presence of THAM contributes NHCO cross-links that enhance Tg of HBPEAA, thereby Tg of lignin-co-HBPEAA. Our previous studies18 indicate that various types of poly(ester-amine) and poly(ester-amide) networks (Scheme1) are existing in the lignin-co-
Figure 2. Tensile stress−strain curves of various lignin-co-HBPEAAs at 27 °C.
the tensile strength and modulus increased with increasing lignin wt % and −NHCO linkage density. At high lignin contents, the number of free OH groups will be higher, thus, contributing to hydrogen-bonding with HBPEAA.18 Hence, the higher values of tensile modulus are likely the result of combined effect of covalent cross-links and increased level of intermolecular associations between lignin and HBPEAA. Higher THAM ratio causes the formation of amide linkages, which are likely to be cross-linked with HBPEAA branches. As the THAM/TEA molar ratio changes, the NHCO concentration reaches an optimum level at which proper branching and cross-links occurs. This could be the case with T20L35 with 35% lignin. Thus, the tensile properties show a decrease as THAM/TEA ratio reduced from the optimum value. A common trend observed in the tensile behavior of all the samples except L35T0 (100%TEA) is that they undergo an initial elastic deformation to some extent. At the end of the deformation, the curve enters the plastic region where it abruptly stops. The initial elastic elongation is due to the HBPEAA component in the copolymer which is present as the major part (i.e., > 60 wt %). In the later part of the tensile curve, this is attributed to the plastic component in the copolymer. The plastic region is due to the cross-linked ligninHBPEAA chains. Thermomechanical Properties of Lignin-co-HBPEAA. The thermal properties of lignin-co-HBPEAA were studied using DMA, and the results are presented in Figure 3. It was observed that all the samples exhibited an increase in storage modulus (E′) and Tg of the lignin-co-HBPEAA increased with increasing THAM as well as lignin contents, indicating the thermomechanical properties of lignin-co-HBPEAA can be tuned by altering the material composition. The effect of cross-linking and molecular interaction between lignin and HBPEAA was observed by DMA. For a good shapememory material, a large and sharp drop in the E′ around the Tg is important. A drop in E′ was observed over a temperature range (−60 to 125 °C) and also the temperature dependency of loss modulus (E″) of lignin copolymers are shown in Figure 3. The copolymers give rise to only single loss modulus relaxation
Scheme 1. Possible Chemical Linkages among Monomers in HBPEAA18
HBPEAA that contributed various microlevel transitions. This leads to broadened glass transition temperature. From FTIR and DSC studies, functional groups such as OH, COONHCO-, and tertiary-amine from the HBPEAA were involved in the physical cross-links with lignin. Hence, the stronger interaction between them would make the molecular movements constrained, which led to the hysteresis of lignin copolymer chain segments relaxation when the glass transition was initiated.24−28 Therefore, the broadened Tg range was produced by strong physical interactions from various C
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where ρ is the cross-link density expressed in moles of elastically effective network chains per cubic centimeter of sample. G′ is the shear storage modulus of the cured network at a temperature well above Tg, R is the gas constant, and T is the absolute temperature at which the experimental modulus is determined. As shown in Table 1, the cross-linking density decreases from 0.0388 to 0.0079 mol cm−3 as the HBPEAA content increases to 50 wt %. High cross-links values were shown in all T20 samples (20 wt % THAM) and are in the range of 0.036 to 0.0257 mol cm−3 at the same time T50 samples have values < 0.021 mol cm−3, respective of their high lignin content. The cross-link density was the highest in the L45T20 sample, that is, sample with high TEA wt % and high lignin wt % samples (low THAM). The mechanical property study shows that the L45T20 sample has a high Young’s modulus (Table 1). The modulus values obtained strongly support the trend of crosslink density values of each sample. As far as the chemical structures of these networks are concerned, this behavior was expected. The analysis of the height and width of the relaxation peak shows the trends in the cross-linking density and network homogeneity.33,34 The height of the tan δ peak, which is associated with the cross-linking density decreases as lignin content increases (Figure 4). It can be assumed that the decreased height is associated with the restricted segmental mobility and less relaxing species, and is therefore indicative that the copolymers with strong physical and chemical interactions were formed. In this work, the methanol soluble lignin (MSL) fraction was used to homogenize its structure and molar mass. It was found through FTIR spectroscopy that the intensity of the band around 2800 cm−1 in MSL is strong compared to methanol insoluble lignin fraction (MIL). The intensity of the bands at 1704 and 1205 cm−1 corresponds to the unconjugated stretching of the CO and C−O bonds, respectively.35 The intensity of the carbonyl band is much higher for MSL compared to MIL which is likely responsible for the solubility of MSL in methanol due to the formation of hydrogen bonds between carbonyl and alcohol (Figure S1). The MSL fractions were shown to contain fewer condensed structures, lower aromatic/aliphatic OH ratio, higher β-O-4 linkages, lower Mw and Tg, and lower thermal degradation properties than the original lignin and MI lignin fractions.36 These observations revealed that the MSL have a more aliphatic nature with nonconjugated carbonyl groups. This structural environment is more favorable for very strong chemical interactions as well as the physical cross-linking with the polyester. According to our earlier work based on lignin model compounds and spectroscopic studies of the sol-fraction of lignin-poly(ester-amine) confirmed that the formation of cross-links (ester formation) with aliphatic hydroxyl groups in lignin.12 According to our recent studies, lignin-co-poly(ester-amineamide) and lignin-co-poly(ester-amine) were found to exhibit excellent dual-shape memory performance with both Rf and Rr over 90%.14 Consecutive shape memory cycling experiments show that shape memory performance is highly reproducible (Figure S2). Additionally, upon consecutive cycling at different temperatures the copolymer experienced almost no deterioration confirming that the cyclic process did not affect the thermomechanical response of lignin-copolymer up to moderately high temperature (Figure S3).
Figure 3. DMA thermograms showing storage modulus (E′) and loss modulus (E′′, inset) of various lignin-co-HBPEAAs.
peak which corresponds to the Tg of the copolymers. Peaks in the E″ and in the damping term (tan δ) of L35T20 are observed around 13 and 36 °C, respectively (Figures 3 and 4).
Figure 4. DMA thermograms showing tan δ values of various ligninco-HBPEAAs.
For instance, the breadth of the Tg region obtained from E″ peak of L25T20 was 78 °C, but the range reached up to 96 °C in the case of the L35T20 sample. According to the structure of HBPEAA, the hyperbranched prepolymers have a lot of chain ends which contribute to the finite free volume to the polymer system and the excess free volume has a proportional effect on the Tg of the lignin-co-HBPEAA.29 Moreover, the presence of various polar groups increases intermolecular interactions such as hydrogen bonding between lignin and HBPEAA, which limits the segmental mobility of the polymer. The introduction of end groups, free volume, and possibilities of physical network are the reason for the damping property of lignin-co-HBPEAA. The cross-linking density of a polymer was estimated from the plateau of E′ in its rubbery state.30,31 According to the theory of rubber elasticity, the equilibrium E′ is given by32 ρ = G′/RT = E′/3RT
(1) D
DOI: 10.1021/acs.biomac.5b00655 Biomacromolecules XXXX, XXX, XXX−XXX
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Biomacromolecules Triple Shape Memory (TSM) Study. A triple-shape memory cycle constructed under stress-controlled programming and staged heating recovery conditions were performed. The deformation temperature for each sample was chosen to obtain optimal strain recovery. The optimal deformation temperature range is obtained by selecting achievable strains greater than 50%. The TSME can thus be quantified on the basis of the percentage of shape fixity (Rf) and shape recovery (Rr), calculated using the eqs 2 and 3.19 R f (S1 → S2) = 100% × (LS2 − LS1)/(LS2load − LS1) (2)
R r(S1 → S2) = 100% × (LS2 − LS1rec)/(LS2 − LS1)
(3)
where S1 and S2 denote two different shapes, respectively, LS2,load represents the maximum strain under load, LS2 and LS1 are fixed strains after cooling and load removal, and LS1,rec is the strain after recovery. From eqs 2 and 3, the quantitative analysis of SME of the material to memorize its permanent shape; Rf1 (S1) and Rf2 (S2) of L35T20 could reach 41 and 92%, respectively (Figure 5). Furthermore, the Rr1 (LS1,rec) and Rr2 (S0,rec) could reach 40
Figure 6. Shape deformation and recovery processes of triple shape memory test in L25T20 (Td1 = Tr2 = 38 °C, Td2 = Tr1 = 28 °C; Rf1 = 22%, Rf2 = 98%; Rr1 = 9% and Rr2 = 44%).
Figure 7. Shape deformation and recovery processes of triple shape memory test with L25T60 (Td1 = Tr2 = 55 °C, Td2 = Tr1 = 35 °C; Rf1 = 61%, Rf2 = 97%; Rr1 = 63% and Rr2 = 110%).
= 63%). The portion that was not recovered in the first recovery stage was recovered at the second recovery stage, thus, leading to a Rr1 value above 100%.37 In the first set of characterizations, including thermomechanical analysis, L35T20 showed better performance among the synthesized copolymers. From DSC and DMA data, the L35T20 have a wide transition temperature from −27 to 69 °C (Figure 4). Hence, the rest of the current investigation is mainly concentrated on to study the effect of various shape memory programs in L35T20. A series of experiments were thus conducted in which different holding time at (i) first fixing step (Td1) and (ii) recovery steps (Rr1 and Rr2); (ii) different fixing and recovery temperatures (Td1 and Rr2). Figure 8 is an experiment under conditions identical to the first test done in L35T20 (see Figure 5) except that longer holding time at first fixing stage from 10 to 20 min. The Rf1 value is not much changed during the process. The above result imply that full fixing could be possible at minimum holding time. In another experiment, the same sample was subjected to TSM test under identical conditions of Figure 5 except longer recovery time. The corresponding recovery curves are shown in Figure 9. At the longer recovery time of 30 min, Rr1 and Rr2 values increased to 64 and 107% with better separation in recovery stages. Such a separation becomes much more evident that the presence of two different thermo responsive shapes exist in the lignin-coHBPEAA. The shape memory experiment in L35T20 at high
Figure 5. Shape deformation and recovery processes of triple shape memory test in L35T20 (Td1 = Tr2 = 55 °C, Td2 = Tr1 = 35 °C; Rf1 = 41%, Rf2 = 92%; Rr1 = 40%, and Rr2 = 86%).
and 86%, respectively. In case of L25T20, values of Rr1 is 9% and Rr2 is 44%. From these Rf and Rr values it was observed that the triple shape memory properties of lignin-co-HBPEAA improved from L25T20 to L25T60. It is apparent that although lignin-poly(ester-amine) shows an excellent dual-shape performance of >90%,16 it exhibits poor TSME. This is clearly seen from the shape memory test in L25T20 and L25T60 samples (Figures 6 and 7). Compared with an individual network existing in the ligninco-polymer, the two networks, that is, poly(ester-amine) and poly(ester-amide), lignin-co-HBPEAA shows high Rr ratio in materials with both high lignin and THAM content. This may be ascribed to the combined effect of additional cross-links (NHCO- chemical links) and distinct hydrogen-bonding evolved in polyester networks and with lignin which can be confirmed by the high tensile strength of lignin-co-HBPEAA. On the basis of such properties, the broad thermal transition of lignin copolymer can be viewed as the collective contribution of both chemical cross-links and various kinds of physical crosslinks. However, the memorizing ability is increased to Rr1 = 63% and Rr2 = 110% in L25T60. Here, Rr2 above 100% is due to the incomplete recovery of the previous recovery event (Rr1 E
DOI: 10.1021/acs.biomac.5b00655 Biomacromolecules XXXX, XXX, XXX−XXX
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bonds in the first deformation process.38 Overall, the shape fixing ratio, Rf1 of all these samples did not change much during these conditions. The stability of the shape at high temperature fixing stage is due to the chemical cross-links formed from THAM, which can provide good thermal properties. Moreover, we note that no well separated strain plateau is observed between the two recovery events in all the above experiments. This might be the reason for continuous transitions of polymeric networks in lignin-co-polymer under the applied heating rate (5 °C min−1). Further design of the copolymer with more widened thermal transitions between the networks might show clear distinguished recovery events with good shape fixing ratio. In the evaluation of triple shape properties, slope change in strain evolution curve of lignin-co-HBPEAA at different THAM content was noticed. A smooth change in slope of the strain curve is shown in the L35T20 samples irrespective of testing protocol (Figure 11; line a−b,xintersect). This change appears to
Figure 8. Shape deformation and recovery processes of triple shape memory test of L35T20 with increased first shape fixing time at Td1 from 10 to 20 min (Figure 5; Td1 = Tr2 = 55 °C, Td2 = Tr1 = 35 °C; Rf1 = 35%, Rf2 = 92%; Rr1 = 40% and Rr2 = 72%).
Figure 9. Shape deformation and recovery processes of triple shape memory test of L35T20 with increased shape recovery at Rr1 and Rr2 from 10 to 30 min (Figure 5) in the first fixing stage (Td1 = Tr2 = 55 °C, Td2 = Tr1 = 35 °C; Rf1 = 33%, Rf2 = 94%; Rr1 = 64% and Rr2 = 107%).
Figure 11. Slope change in strain evolution curve of lignin-coHBPEAA at testing conditions; l L35T20 (from Figure 8); ll L35T20 (from Figure 9); lll L35T20 (from Figure 10).
occur when the strain reaches a value in the vicinity of the first recovery plateau. In the other set of strain evolution curve of L25T20 and L25T50, the latter one has a clear slope change at the near the level of %strain to the first recovery plateau (Figure 12; line a−b,xintersect). However, the former sample has no such changes in its strain curve. The results shown in Figures 11 and 12 that the lignin-co-polymer with increased amount of THAM and lignin content showing this property. In these types of samples, more cross-links was formed. This has provided better thermomechanical properties. Here, the change of strain curve nature in middle of the elongation process indicates that some part of network responded differently to the applied testing programs. Detailed study is required to establish the reason for such kind of characteristics. Effect of Polyester Networks in Shape Memory Characteristics. To understand the role of monomers in the TSME, the percentage of recovery in each stage was calculated from the total recovery of the materials that underwent triple shape studies. The values obtained correspond to the wt % changes of THAM or TEA monomer used for the preparation of HBPEAA. The copolymer with high TEA content (L25T20, 80 wt % TEA and 20 wt % THAM) shows high percentage of recovery in first stage (LS1,rec), but in the case of L25T50 (50 wt % TEA and 50 wt % THAM), a high percentage of recovery was observed in the second stage (S0,rec). The percentage of recovery of L25T20 in first stage is 81% and second stage is
deformation and second recovery step was also done to study the shape memory effect at higher Td1 (Td1 = Rr2 = 60 °C). Figure 10 shows that poor recovery was noticed at Rr1 and an incomplete Rr2 was also observed. The incomplete recovery at the recovery stage may be due the breaking of some chemical
Figure 10. Shape deformation and recovery processes of triple shape memory test of L35T20 with increased deformation time at Rf1 and Rr2 from 55 to 60 °C (Figure 5; Td1 = Tr2 = 60 °C, Td2 = Tr1 = 35 °C; Rf1 = 31%, Rf2 = 90%; Rr1 = 29% and Rr2 = 79%). F
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networks by the applied synthetic methods. These findings were an indication of the existence of poly(ester-amine) and poly(ester-amine-amide) as two phases with minimum overlap,18 when melt mixed with lignin, the Tg of lignin-co-HBPEA appeared in the broadened temperature region with major transitions of various individual microlevel. This indicates that both networks might have linked together by lignin and this Tg is a combined effect of microtransitions. In all the shape memory events, the stress removal leads to spring back of the materials (strain recovery). This spring back strain is reducing with increased THAM contents as well as lignin wt %, as shown in Figure 14. This can be explained since
Figure 12. Slope change in strain evolution curve of lignin-coHBPEAA at different THAM content, l L25T20 (from Figure 6); ll L25T50 (Td1 = Rr2 = 60 °C, Td2 = Rr1 = 28 °C).
14%. However, the values of L25T50 are 28 and 63%, respectively. These changes corresponding to wt % of THAM and TEA in the materials shows that the first recovery is related to lignin-poly(ester-amine)-rich network and the second recovery stage is related to lignin-poly(ester-amide)-rich network (Figure 13). Furthermore, the structure−property Figure 14. Strain recovery (spring back strain %) at stress-free condition in the two shape fixing stages.
both the various physical cross-links and the chemical crosslinks formed in HBPEAA and between lignin and HBPEAA can serve as net points during the recovery process. From these results, the amide cross-links in polyester network affects the Rf and Rr properties. The shape recovery of the lignin-co-HBPEAA were increased with the increased multifunctional moiety, which resulted in interhydrogen bonding between lignin and polyester and intrahydrogen bonding in HBPEAA.37,39
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CONCLUSION The formation of lignin copolymers with triple shape properties could be achieved by (i) selective use of a lignin fraction which give maximum homogeneity in lignin behavior, (ii) the selection of monomers for the preparation of mixing phase, and finally, (iii) mode of preparation. The DSC and DMA data also showed that the polyester networks in the lignin-copolymer have good compatibility with lignin and with its own thermal behavior. The appearance of dual Tgs on the prepolymer is an evidence of two distinct networks in the HBPEAA due to applied synthetic methods. The structure− property studies of HBPEAA revealed that the two networks, poly(ester-amine) and poly(ester−amide) can exist with minimum overlap. The Tg of lignin-co-HBPEAA appeared in the broadened temperature region as single Tg. This indicates that both networks have been linked together by lignin and this Tg is a combined effect of numerous micro transitions. Different thermoresponsive lignin-based materials can be made by altering the compositions and synthetic approach. The shape memory effect from dual to triple demonstrated in lignincopolymer suggest that the polymer has an adaptable polymer architecture to develop the adjustable multiple SMPs.
Figure 13. Percentage of recovery in first and second stage of various lignin-copolymers (L25T20, L35T20, and L25T50) based on different networks.
studies of HBPEAA revealed that more branches were derived from TEA because of the bulky nature of the THAM unit.18 On the other hand, the THAM unit extends the linearity of the prepolymer, but at the same time, TEA leads the branching structure. The evaluation of thermomechanical properties also shows that TEA in the prepolymer and lignin are mainly involved in the cross-linking. Hence, the monomers TEA and THAM have distinct roles on its chemical and physical properties of HBPEAA. As shown in the Figure 1, the appearance of dual Tgs in L35T20 and L25T50 may be due to the early formation of amide cross-links and its arrangements in the HBPEAA G
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ASSOCIATED CONTENT
S Supporting Information *
Figure S1. FTIR spectra of lignin methanol soluble (a) and insoluble (b) fractions. Figure S2. Consecutive shape memory thermomechanical cycles of lignin-HBPEAA (L35T20). Figure S3. Consecutive shape memory thermomechanical cycles of lignin-HBPEAA (L35T20) at different temperatures (40 to 70 °C). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.biomac.5b00655.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +1 (208) 885 9454. Fax: +1 (208) 885 6564. Present Addresses †
Freiburg Materials Research Center, Albert-Ludwigs University of Freiburg, D-79104 Freiburg, Germany. ‡ Composites Materials and Engineering Center, Washington State University, Pullman, WA 99164−1806, U.S.A. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to acknowledge (i) the financial support from a USDA-NIFA Wood Utilization Research Grant No. 201034158-20938 and Dr. Bob Stillinger Scholarship and (ii) USDA-CSREES Grant 2007-34158-17640 for supporting the DSC and DMA.
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DOI: 10.1021/acs.biomac.5b00655 Biomacromolecules XXXX, XXX, XXX−XXX
