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Shape Memory Properties of Polystyrene-block-Poly(ethylene-co-butylene)block-Polystyrene (SEBS) ABA Triblock Copolymer Thermoplastic Elastomers Marcos Pantoja, Pei-Zhen Jian, Miko Cakmak, and Kevin A. Cavicchi ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00139 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Applied Polymer Materials

Shape Memory Properties of Polystyrene-block-Poly(ethylene-co-butylene)block-Polystyrene (SEBS) ABA Triblock Copolymer Thermoplastic Elastomers Marcos Pantoja1, Pei-Zhen Jian1, Miko Cakmak1,2, Kevin A. Cavicchi1* 1Department

of Polymer Engineering, University of Akron, Akron, OH 44325-0301

2Departments

of Materials and Mechanical Engineering, Purdue University, West Lafayette, IN 47907

*Corresponding Author. (E-mail: [email protected]) Keywords: Shape memory polymer, dynamic mechanical analysis (DMA), polystyrene-blockpoly(ethylene-co-butylene)-block-polystyrene (SEBS), chain pull-out, triblock copolymer, thermoplastic, microphase separation

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ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract This work demonstrates that neat polystyrene-block-poly(ethylene-co-butylene)-block-poly styrene (SEBS) displays thermally-responsive shape memory properties. The shape memory properties were quantitatively investigated under uniaxial tension using a dynamic mechanical analyzer and manual stretching. The shape memory properties of SEBS were found to depend on both the molecular weight of the polymer and on the shape programming conditions, including the programming temperature, applied strain, and annealing time at elevated temperature under load. The shape memory mechanism is proposed to be a result of partial stress relaxation of the block copolymer network under load and the formation of a second network with a lower glass transition temperature. This second network counterbalances the initially stretched network producing fixity and weakens first on heating allowing recovery. Due to the unique mechanism of shape memory where the secondary network is generated from the initial network achieving higher fixities generally occurs at the expense of high recovery and vice versa. Introduction Styrenic block copolymers (SBCs) are thermoplastic elastomers that have been extensively studied due to their distinctive properties derived from their synergistic ABA triblock copolymer architecture, where A is a glassy thermoplastic block (i.e. polystyrene) and B is an elastomeric block (e.g. polydiene or hydrogenated polydiene).[1-22] Typical SBCs include polystyrene-block-polyisoprene-block-polystyrene (SIS), polystyrene-block-polybutadiene-block- polystyrene (SBS), and polystyrene-block-poly(ethylene-cobutylene)-block-polystyrene (SEBS) block copolymers.[2] These materials are known to microphase separate due to the immiscibility between the shorter polystyrene (PS) blocks and the midblock, producing dispersed spherical, cylindrical, or alternating lamellar PS domains as a function of PS content.[39]

As a result, SBCs are physically crosslinked due to the bridging of the entangled midblock chains across


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the glassy PS domains. Once the copolymer is heated above the glass transition temperature (Tg) of the PS blocks, the lifetime of the physical crosslinks decreases rapidly and the entire elastomer can be processed as a thermoplastic.[9-14] Since the 1960s, extensive efforts have been devoted to understanding the intrinsic properties of SBCs. As outlined by the reviews of Adhikari,[9] Honeker,[11] and Kelterborn,[15] these studies have analyzed the structure-property correlations of SBCs to understand how aspects such as molecular weight, chain architecture, molecular orientation, and microdomain morphology influence their phase behavior, tensile properties, deformation mechanisms, and hardness. Through the understanding of these structureproperty relationships, SBCs have been engineered into advanced materials with applications in adhesives, sealants, coatings, the footwear industry, automotive parts, and wire insulation.[2] The elastomeric properties of SBCs have also been used as a base material to construct shape memory polymers (SMPs).[16-30] SMPs are responsive smart materials capable of holding a deformed shape over a long time period, a property known as fixity, and recovering their original shape upon the application of an external stimulus, known as recovery, with thermal initiation being the most widely used stimulus.[31-37] At least two structural elements are required for a polymer to exhibit shape memory properties. The first element is an elastic, crosslinked polymer network. Known as the permanent network, this network prevents extensive chain relaxation under load and drives shape recovery.[31] The second structural element is a temporary network that restricts the elastic recovery of the deformed permanent network until it is weakened or vanishes by the application of an external stimulus. For thermally activated SMPs, this temporary network is typically an amorphous or crystalline solid network, where shape recovery is triggered by heating the SMP above the Tg or melting temperature (Tm) of the reversible network.[32]



The past decade has seen a large number of studies investigating SBC SMPs generated by blending,[16-19,29,30] grafting,[20,21] swelling,[22-27] or laminating[28] an SBC with a reversible network. Here the SBC was always used to construct the permanent network, while the second component was added to form the reversible network necessary for the shape memory effect. Similar to other SMP blends,[38-43] the secondary network in most of these examples is a small molecule crystalline additive which forms a percolating, solid network. The yield strength of this network is strong enough to withstand the elastic force applied by the stretched elastomer, where shape recovery in blend SMPs is governed by the phase transition of the crystalline additive upon melting. However, control experiments on neat SBCs have demonstrated that the single network SBC is able to display shape memory although its fixity is typically poor. Widely different results were obtained ranging from 0 to ca. 40% shape fixity using different types of SBCs and different programming conditions.[16-18,20,30] These experiments are summarized in Table 1. Prior to their use as SMPs, mechanical hysteresis studies of SBCs had measured non-zero tensile set (i.e. fixity) and thermal recovery of the residual strain. [1,7,15,44-58] However, in these reports their shape memory properties were not explored further.


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Table 1: Summary of neat SBC shape memory studies. All heating/cooling mediums were water baths except for reference 20 where samples were heated using an oil bath. Starred entries in reference 20 correspond to thermo-mechanical data obtained using DMA. Reference 20 also cooled samples to -170 or -100 C but that data was excluded as this table only includes cases where the samples were maintained above the Tg of the midblock. Table 1. Previously reported shape memory properties of neat SBC SMPs. Ref

16 17 18 20

30

Polymer Information SBC Properties SBS YH-791 SEBS Kraton G1654 SEBS Kraton G1654 m-SEBS

SEBS Kraton MD6684 SEBS Kraton G1651

30 wt% PS

Programming Conditions Hold Time Hold Time Under Load Without Load (min) (min) 100 @ 80 C 5 @ 20 C 0 @ 20 C

31 wt% PS

100 @ 70 C

5 @ 10 C

31 wt% PS

50 @ 70 C 50 @ 45 C

30 wt% PS 1.85 x 10-4 mol maleic acid/g of m-SEBS MFI = 5 g/10 min (230 C for 2.16 kg)

N/A @ 120 C

50 @ 120 C*

N/A

Strain (%)

Shape Memory Fixity Recovery (%) (%) ca. 40

ca. 100

0 @ 10 C

11

99

5 @ 45 C N/A @ 0 C

0 @ 10 C N/A

N/A

N/A

0 @ 80 C

1 @ 80 C

14.6

100

Cool to 80 C @ 20 C/min*

0 @ 80 C*

15.2*

74.2*

100 @ 70 C

10 sec @ 25 C

30 sec @ 25 C

6

100

100 @ 70 C

10 sec @ 25 C

30 sec @ 25 C

14

98

Given that SBCs are commercially available, the ability to shape memory program the neat polymer would be widely useful in a variety of industries. There have been many studies showing examples of device fabrication using shape memory polymers for biomedical, aerospace, and textiles 5 ACS Paragon Plus Environment


applications[59-61] Since these SMPs require deformation as part of the shape memory cycle they are best when used in one-time applications, such as remote deployment of a structure or a one-time sensing event. The evidence of non-zero fixity and wide variation in the reported shape memory properties, especially from similar polymer systems under different processing conditions, motivated a comprehensive study of the shape memory behavior of a series of neat SEBS SBCs. In this paper it is shown that significant fixity (81%) and recovery (89%) are obtained through the precise shape memory programming of SEBS SBCs by controlling both the temperature and time of deformation during shape programming, presenting a more systematic method of controlling their shape memory properties compared to the results summarized in Table 1. This initially counter-intuitive result of shape memory in a single network thermoplastic elastomer is proposed to be due to the physically crosslinked nature of the permanent network. A mechanism for the shape memory behavior is put forward where the stress relaxation of the network through chain pull-out and the formation of new, plasticized glassy domains constructs a second, reversible network in-situ during deformation facilitating both shape fixity and recovery. This mechanism is supported by the variation of the shape memory properties as functions of different programming and material parameters controlling stress relaxation, including the deformation temperature, applied strain, hold time under load at elevated temperature, and molecular weight. In addition, the utility of this single component material to display shape memory is demonstrated by the fabrication of different shape morphing materials fabricated from large area films prepared by compression molding one SEBS SBC. Experimental Procedures: Materials Specification: G1650, G1652, and G1654 SEBS triblock copolymers with 30 wt% styrene content were provided by Kraton® Polymers. The melt flow index (MFI) of the different SEBS grades (measured at 230 C) are


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5 (G1652), < 1 (G1650), and zero (G1654) g/10min. The polymers were used as provided without further modification. A two-part cyanoacrylate Loctite® Super Glue Plastics Bonding System kit (Henkel Corporation) was used as received for fabricating SEBS bilayer samples. Compression Molding: SEBS samples were compression molded using a Carver® hydraulic compression press. The asreceived SEBS was placed inside a 0.9 mm thick square metal frame spacer with a 100 mm x 150 mm compression area (50 mm frame width). This assembly was set between two 1 mm thick Teflon sheets and compressed for 30 minutes at 3,000 psi. The G1650 and G1652 SEBS grades were compressed at 150 C and the G1654 grade at 250 C. The resulting films were cut into rectangular film specimens with dimensions of 7.5 mm wide x 15 mm long x 0.9 mm thick for dynamic mechanical analysis (DMA) shape memory testing and with dimensions of 7.5 mm wide x 40 mm long x 0.9 mm thick for water bath shape memory testing. DMA Shape Memory Testing: The shape memory properties were measured using a TA Instruments Q800 dynamic mechanical analyzer (DMA) with a tensile film fixture operating using the Strain Rate Mode. Stress-controlled experiments were conducted by choosing a Custom test. The film samples were first heated to a temperature above room temperature at a rate of 10 C/min under a preload force of 0.005 N and an initial set strain of 1.0%. After stabilizing at the programming temperature for 20 minutes, a stress was applied at a rate of 1 MPa/min and kept constant for one minute once reached. The samples were then cooled to 25 C at a rate of 10 C/min and were held under load for 10 minutes. The stress was then lowered to 0.0 MPa at a rate of 10 MPa/min and the samples were held in that state for 10 minutes to allow the strain to stabilize. Finally, the samples were heated to the initial programming temperature at a rate of 10 C/min and were held isothermally for 20 minutes to allow for shape recovery. This procedure 7 ACS Paragon Plus Environment


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was repeated two more times or until sample failure. Table S1 in the Supporting Information summarizes the programming temperatures and the applied stresses used for G1650 SEBS samples. G1652 and G1654 SEBS samples were tested with a programming temperature of 80 C. G1652 SEBS samples had applied stresses of 0.5 and 0.7 MPa whereas G1654 SEBS samples had applied stresses of 0.8, 1.0, 1.3, and 1.5 MPa. Strain-controlled experiments were performed on G1650, G1652, and G1654 SEBS using a procedure similar to the stress-controlled procedure outlined above. In this case, a Strain Ramp test was specified and the experiment was begun by entering an equilibrium temperature of 80 C, a strain rate of 100 %/min, and a final strain of 50, 100, or 150%. Only after beginning this experiment were the rest of the shape memory cycle steps manually entered into the running segment description. After equilibrating at 80 C for 20 minutes, a strain ramp was applied at 100 %/min to 50, 100, or 150% strain and was held for 1 minute. (G1652 and G1654 SEBS were only subjected to 100% strain). Upon cooling, the applied strain was then released by setting the force to 0.001 N. The second cycle was performed following this same procedure (with the same applied strain used in the first cycle) but with a hold time of 1, 30, 60, or 300 minutes (or until failure). Compared to the Custom test description used in stress controlled experiments where a “heating ramp” was used to initially heat the sample, choosing a Strain Ramp test uses the “equilibrate” function by default to begin initial heating which does not collect data. As a result, the data collection of strain controlled experiments begins once the DMA has moved onto the 20 minute isothermal step. The strain data provided by the DMA was used to calculate the percent fixity (F) and recovery (R) as,

𝐹=

𝜀𝑓 ― 𝜀𝑖 𝜀𝑎 ― 𝜀𝑖

𝑥 100

1


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𝑅=

(𝜀𝑓 ― 𝜀𝑟) (𝜀𝑓 ― 𝜀𝑖)

𝑥 100

2

where the initial strain (εi) refers to the strain at the beginning of the shape memory cycle, applied strain (εa) refers to the strain obtained prior to stress/strain release, fixed strain (εf) refers to the strain stored after releasing the applied stress/strain, and residual strain (εr) refers to the strain after shape recovery. Water Bath Shape Memory Testing: The water bath shape memory testing setup consisted of an immersion heater (VWR Model: LXC) mounted on a polypropylene Sterilite® bin with dimensions of 29.2 cm width x 42.9 cm length x 23.5 cm height filled with tap water to construct a reservoir of hot water. The heater was used to set the temperature of the water in the reservoir to 80 C (this setup is shown in Figure S1 in the Supporting Information). Sample deformation was strain controlled using a modified crescent wrench (Figure S2 in the Supporting Information) and was performed inside the hot water reservoir. G1650, G1652, and G1654 SEBS samples were immersed in the hot water bath for 1 minute prior to the deformation to ensure thermal equilibrium of the sample. Samples were then manually deformed inside the hot water bath to different strains using the modified wrench, taking less than 5 seconds to reach the deformations. The samples were held at these strains for 0.1, 1, or 10 minutes. After this hold period, the samples were immediately immersed in a separate room temperature water bath for 1 minute. After the room temperature quenching, the samples were released from the modified wrench and left to equilibrate under ambient conditions for 10 minutes. The programmed samples were then reheated in the hot water bath (at the same temperature used during programming) for 1 minute to recover the original shape. Samples programmed using a temperature of 80 C and a hold time of 10 minutes were subjected to a second programming cycle.



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The sample length throughout the shape memory cycle was determined by marking two lines ca. 5 or 10 mm apart with a permanent marker and measuring the length after each step of the shape memory cycle from the inside of the lines. These measurements include the initial length of the sample (li), the length after quenching at room temperature (while still under tension from the wrench and outside the quenching water) (la), the length after equilibrating for 10 minutes under ambient conditions (lf), and the length after recovery and removal from the hot water bath (lr). These lengths were then used to calculate εi, εa, εf, and εr as follows,

𝜀𝑥 =

(𝑙𝑥 ― 𝑙𝑖)

3

𝑙𝑖

where lx is li, la, lf, or lr, with the same subscript as x. Gravimetric Creep: Using a preload force of 0.005N and an initial strain of 0.0%, G1650 SEBS samples were heated to 60, 80, or 100 C at a rate of 10 C/min and held isothermally for 200 min without applied deformations. Step Heating: Following the stress-controlled shape memory procedure previously described, G1650 SEBS samples heated to 80 C and subjected to a 1.0 MPa applied stress were exposed to a stepwise recovery treatment consisting of 20 minute heating steps at 40, 60, and 80 C. Temperature Sweep: The G1650 SEBS temperature sweeps were performed on the DMA Q800 using the MultiFrequency Strain mode. The samples were heated from room temperature to 200 C at a rate of 1 or 3 C/min using a frequency of 1.0 Hz and a strain of 1.0%. Temperature sweeps were performed on an undeformed sample and on fixed samples programmed using a variety of stress- and strain-controlled 10 ACS Paragon Plus Environment

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DMA shape memory cycling conditions. Samples programmed to different extents of strain were prepared using the stress-controlled DMA shape memory cycle procedure previously described with a programming temperature of 80 C, a 1 minute hold time, and applied stresses of 1.0 and 1.3 MPa. Samples programmed at different temperature were prepared using the strain-controlled DMA shape memory cycle procedure previously described with an applied strain of 100%, a 1 minute hold time, and programming temperatures of 60, 80, and 90 C. Samples programmed with different hold times were prepared using the strain-controlled DMA shape memory cycle procedure previously described with a programming temperature of 80 C, an applied strain of 100%, and hold times of 1, 10, and 60 minutes. All the samples programmed using DMA were left clamped on the DMA between shape memory programming and temperature sweep testing, where the sample dimensions were re-measured prior to the temperature sweep. Additionally, all the samples were programmed using a single shape memory cycle. Bilayer Samples Preparation: G1650 SEBS strips 7.5 mm wide x 40 mm long x 0.9 mm thick were programmed using the hot water method previously described with a programming temperature of 90 C and a hold time inside the hot water bath of 1 minute. Separate samples were strained to 80, 120, or 160% strain to obtain fixed strain (εf) values of 50, 70, and 90% relative to the original 10 mm length between the two marked lines. Once fixed, the portion between the marked lines was cut from the entire sample (with lf values of 15, 17, and 19 mm, respectively), where two samples were prepared for each strain selection. These two pieces were then glued onto another unstrained strip of G1650 SEBS 7.5 mm wide x 40 mm long x 0.9 mm thick acting as the passive layer and whose excess length was cut to allow the placement of the active layer strips end-to-end with a 2mm separation distance. The two active layer pieces were placed on the same side of the passive layer or on opposite sides as shown in Figure S3 in the Supporting Information.



A large G1650 SEBS square film 100 mm wide x 100 mm long x 0.9 mm thick was made by cutting 50 mm off the length of a film obtained using the compression molding frame previously described. This film was uniaxially stretched to 100% strain at 80 C using a Brückner Karo IV biaxial stretcher. Once stretched, the biaxial stretcher’s oven was turned off and the sample was fan cooled to room temperature while maintaining the applied strain (the sample was held stretched for 80 minutes to allow proper cooling). The uniaxially stretched film was then released from the clamps to obtain a εf value of 74% relative to the original 100 mm length of the film (lf value of 174 mm). As shown in Figure S4 in the Supporting Information, rectangular samples 7.5 mm wide x 70 mm long x 0.9 mm thick were cut at 0, 30, 45, and 90 relative to the stretch direction (i.e. 0 is parallel to the stretch direction and 90 is perpendicular). The samples were cut from the middle of the uniaxially stretched film. These samples were then glued onto another unstrained strip of G1650 SEBS 7.5 mm wide x 70 mm long x 0.9 mm thick to form a bilayer. Dome Shape Processing: A 60 mm radius x 1.4 mm thick circular sample was cut out of a sheet of G1650 SEBS and inserted inside the concave portion of a 62 mm radius Chemglass aluminum hemisphere (designed for heating a 100 ml round bottom flask) to form a dome shape. This assembly was heated for 1 hour inside an oven set to 150 C, after which the oven’s temperature controller was turned off to allow the sample to gradually cool to room temperature overnight. The dome-shaped sample was manually inverted and inserted inside a hot water bath set to 80 C for 1 minute followed by cooling inside a room temperature water bath for another minute. The inverted programmed dome was placed inside a petri dish and water at 100 C was slowly poured inside the dish until the sample recovered. Figure S5 in the Supporting Information illustrates these processing steps.


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Results and Discussion: The ability of a neat G1650 SEBS block copolymer to exhibit shape memory is shown qualitatively in Figure 1. A compression molded 7.5 mm wide x 60 mm long x 0.9 mm strip of G1650 SEBS was wrapped around a metal rod (8 mm diameter) and clamped on its ends using 32 mm binder clips. Once secured, the assembly was heated at 80 C for 2 minutes, cooled to 23 C for another two minutes, and removed from the rod to produce a programmed spiral shape. Recovery of the initial shape and the unspiraling of the sample occurred once immersed in a hot water bath at 100 C. While some permanent deformation was observed in the recovered shape, this test demonstrates the ability of a neat SEBS film to display shape memory behavior. A video of the recovery is shown in the Supporting Information (Video S1).

Figure 1: a) Initial, b) fixed, and c) recovered strips of G1650 SEBS strips programmed by being wrapped around a metal rod. More quantitative shape memory characterization was conducted using a dynamic mechanical analyzer. A representative three cycle shape memory experiment on a G1650 SEBS sample is shown in Figure 2. A significant unrecovered strain (r,1) was repeatedly observed at the end of the first cycle. In subsequent cycles the unrecovered strain did not typically increase much compared to the value obtained 13 ACS Paragon Plus Environment


after the first cycle, resulting in more consistent fixity and recovery values during the second and third cycles. For example, in Figure 2 the unrecovered strain at the end of the second and third cycles compared to the first cycle were r,1 = 0.86r,2 = 0.84r,3. This unrecovered strain is attributed to two factors. First the creep of the sample under the force of the bottom clamp of the DMA. As discussed in a previous publication, it is difficult to entirely neutralize the weight of the DMA clamp over the entire testing temperature range, resulting in a non-zero force on the sample and creep when the strain is not constrained.[62] Figure S6 in the Supporting Information shows DMA curves from heating the sample with no applied stress or fixed strain resulting in plateau strain of ca. 10% at 80 C. This creep would occur during the shape recovery step when the sample is annealed at elevated temperature at zero applied stress. As the extent of creep is significant in this 20 minute period, it is less apparent in subsequent cycles leading to a more accurate measure of the shape fixity and recovery. Since the observed tensile set in the first cycle (24%) is larger than the 10% attributed to creep, additional plastic deformation must occur during the first shape memory cycle. During the initial stretching of the block copolymer, chain pull-out of the PS endblocks, voiding and the break-up, orientation, and plastic or viscoleastic deformation of the glassy PS domains may all contribute to tensile set, which is either irreversible or only recovered after extremely long times when annealing at the same temperature as the deformation temperature.[50,53,63,64] Similar to the results of room temperature hysteresis testing the increase in tensile set was negligible in subsequent cycles deformed to the same strain.[11] Therefore, the fixity and recovery of the second cycle were used to characterize the shape memory properties of the neat SEBS block copolymers. Tables of the fixity and recovery values for all of the subsequent DMA tests are included in the Supporting Information. It is also noted that a complex stress-strain response is observed in Figure 2 in the first heating step prior to the application of stress (time < 25 min). The overall change in strain is small, but a repeatable increase and decrease to zero strain was observed and is shown in a magnified view of this region in Figure S7 in the Supporting information. The exact origin of this behavior is unknown but is attributed to the 14 ACS Paragon Plus Environment

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instrument response to requirement to keep zero strain under a combination of thermal expansion and non-zero loading force.

Figure 2: Three-step DMA shape memory cycling of G1650 SEBS using a programming temperature of 80 C, an applied stress of 1.0 MPa. The shape memory behavior was studied as both a function of the applied strain, by varying the applied stress in the shape programming step, and the programming temperature. As shown in Figure 3a, the fixity increases with programming temperature and decreases with applied strain. Figure 3b shows the recovery data, with overlapping recovery values at 60, 80, and 90 C programming temperatures and significantly lower recovery values with 40 and 100 C programming temperatures. The decrease in fixity with applied strain should be due to the larger retraction force generated by the stretched PEB chains, which the second network producing fixity must counterbalance. At 100 C the PS domains are no longer glassy which limits the amount of strain that can be applied to the sample in the DMA before failure. This also results in lower recovery due to the disruption of the permanent network and therefore the reduction in the elastic force driving recovery. The low recovery at 40 C is an artifact of the recovery equation, 15 ACS Paragon Plus Environment


where the low fixity at 40 C means that the fixed strain is low and accentuates any small residual strain. Overall, a programming temperature of 90 C yielded the highest combined fixity and recovery values of 81 and 89%, respectively.

Figure 3: Second cycle a) fixity and b) recovery of G1650 SEBS. The exact experimental conditions are outlined in Table S1 in the Supporting Information. While the SEBS is initially composed of a single physically crosslinked network, a second temporary network must be generated in-situ during the shape programming for the material to exhibit shape memory properties. Under constant strain SEBS will undergo stress relaxation below the glass transition temperature of the PS domains. The mechanism for this stress relaxation has previously been proposed to primarily occur through chain pull-out of the PS endblocks and insertion into another PS domain.[54,65] Partial stress relaxation will result in a counterbalanced, double network structure where the newly developed network resulting from stress relaxation (i.e. temporary network) spanning the entire sample is at equilibrium and will resist the entropic retraction force of the remaining, stretched original network (i.e. permanent network). The structure spanning nature of both of these networks is due to both the bridging of chains across different glassy domains and the trapping of the entanglements from the immobilization of the end blocks in both bridged and looped chains. Therefore, these are not likely two 16 ACS Paragon Plus Environment

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independent networks, but would have to share some net-points, such as the cylinders containing endblocks which form bridges and loops in the original permanent network and chains where the other end has undergone pull-out to form a new domain defining the second, reversible network. This mechanism would only produce a shape memory effect if PS domains are formed during stress relaxation that have a lower glass transition temperature than the original PS domains. This could occur if new, less well-defined PS domains were formed from the pulled-out chains that were plasticized by PEB midblock chains rather than their complete reinsertion into existing PS domains. Two routes for this to occur are the association and vitrification of multiple endblocks to nucleate a new domain or the growth of domains at the surface of the existing PS domains. Upon heating the programmed shape the sample will undergo recovery as the low Tg domains soften allowing the stretched permanent network to recover. This will then deform the secondary network, however at this elevated temperature these chains will now undergo stress relaxation to form new domains, reinsert into existing domains, or be trapped as free chains. The imperfect recovery of the network is consistent with the residual strain in cycles 2 and 3 and the increase in the applied strain with cycle number in Figure 2. An illustration of the shape memory mechanism is shown schematically in Figure 4. This is a simplified picture showing cross-section slice of two end-on cylinders. Mechanisms such as cylinder break-up and reorientation, that are likely most active in the first cycle, are not shown in the schematic.



1

2

Stretch

Temporary Domain

Heat

Stress Relaxation

4

3

Cool Release

Heat

Reannealed

5 Free Chain

New Domain

2

Temp Stress Strain

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3 4

5

1 Time

Figure 4: Shape memory mechanism of styrenic block copolymers. This mechanism is different from other block copolymers, such as thermoplastic polyurethanes (TPUs). In typical shape memory TPUs the soft blocks have a transition temperature above room temperature, where depending on the chemistry of the soft block this transition temperature could either result from melting or a glass transition temperature.[66] The crystallization or vitrification of this soft block is what forms the reversible network and provides the shape fixing mechanism. Shape memory polyurethanes have been extensively studied and reviewed.[32] To examine the effect of stress relaxation on the shape memory behavior, samples were deformed to a constant strain of 50% and the annealing time under load at 80 C during the second cycle


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was varied. Figure 4 shows DMA data for an annealing time of 45 minutes. DMA data for 15, 75 and 315 minutes are shown in Figure S8 in the Supporting Information. There are four keys points in this plot. First, stress relaxation is observed during the annealing of the sample at 80 C. This stress relaxation continues as the sample is cooled to room temperature. Second, as the annealing time increases the fixed strain, after cooling and unloading the sample, also increases. This result is in agreement with the strained permanent network being consumed by the formation of the second temporary network, which will shift the force balance towards the temporary network at longer time. Third, the unrecovered strain measured after a constant recovery time increases (i.e. larger r) with increasing annealing time due to the consumption of the original permanent network reducing the driving force for recovery. This appears to reduce both the rate of recovery and the extent of recovery. Fourth, with increasing cycle number the applied stress to achieve the same strain decreases, while r increases, which are both in agreement with the consumption of the original permanent network, reduction of its crosslink density, and a gradual increase in the crosslink density of the temporary network. Figure 5 shows the fixity and recovery vs. annealing time for three different applied strains. In general, there is a tradeoff between fixity and recovery with longer annealing time. As for the effect of applied strain, the fixity increases with larger deformations. The fixities at 100 and 150% applied strains closely overlap each other, with the fixity at 150% applied strain being only slightly larger at longer annealing times. This small increase compared to the increase between 50 and 100% applied strain could result from a smaller amount of additional pulledout chains obtained by increasing the applied strain from 100 to 150% forming reversible domains compared to the additional stored elastic force from that additional strain. Recovery appears to be generally independent of applied strain. These results suggest an optimum set of programming conditions which is probably defined by the intersection between the fixity and recovery curves.



Figure 5: a) G1650 SEBS shape memory DMA data of 50% applied strain with a 45 minute second cycle annealing time. b) Fixity and c) recovery of G1650 SEBS with various second cycle hold times obtained using a programming temperature of 80 C during shape memory cycling. The legend indicates the applied strain during programming. 20 ACS Paragon Plus Environment

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The second requirement for chain pull-out to produce shape memory properties is for the formation of new PS domains with a lower glass transition temperature than the original domains. Direct observation of these domains by small angle X-ray scattering (SAXS) or transmission electron microscopy (TEM) will be difficult. These new domains are likely have poor long-range ordering and low electron density contrast, which would limit SAXS measurements. TEM measurements would require careful staining and a large number of measurements to distinguish these new, low Tg domains. Therefore, the thermomechanical properties were characterized by DMA in the fixed, unrecovered state by a low frequency, oscillatory temperature sweep to infer the presence of these domains. Figure 6 shows tan  vs. temperature with varied fixed strain (Figure 6a), programming temperature (Figure 6b) and annealing time (Figure 6c). A large peak is observed at ca. 110 C and is assigned to the glass transition of the PS domains. (The higher than expected temperature of this peak is attributed to the fast heating rate (3 C/min) of the DMA measurement and an apparent temperature lag in the measurement. Figure S9 in the Supporting Information compares sweeps ran at two different heating rates and shows that the temperature at which the tan  peak occurs decreases at lower heating rate (1 C/min)). The peak at ca. 142 C has been previously observed for SEBS with 30 wt% PS.[67] This could be due to an additional ordering process (e.g. order-order transition), but was not explored further as it was outside of the temperature range of interest for the shape memory behavior. Compared to the undeformed sample a low temperature shoulder is observed, which could be attributed to less well-defined PS domains plasticized by PEB blocks. Above the PS Tg the data shows good overlap without additional normalization. As the area of the shoulder increases there is concomitant decrease of the Tg peak in agreement with the pull-out of the PS blocks to form new domains and reducing the fraction of the original PS domains. A non-monotonic behavior in the height of the PS Tg peak is observed as a function of annealing time in Figure 6c. This is likely due to the annealing of the temporary network during programming. Initially, chain pull-out produced less well-defined PS domains, further annealing will increase the Tg of these domains. 21 ACS Paragon Plus Environment


Therefore, the relative concentration of low Tg PS domains to high Tg PS domains will increase initially as these new domains are formed, but then reach a steady-state level as the existing temporary domains are annealed to a more well-defined, higher Tg state and new temporary domains are formed. Figure S10 in the Supporting Information shows the results of step heating of a programmed sample. Partial recovery is observed with each step change in temperature. This is attributed to the softening of an increasing number of domains as the sample is heated.


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Figure 6: Temperature sweeps of G1650 SEBS samples programmed a) at 80 C with a 1 minute hold time using various applied strains, b) at 100% strain with a 1 minute hold time using various programming temperatures, and c) at 80 C with 100% strain using different hold times. These experiments were performed with a heating rate of 3 C/min. 23 ACS Paragon Plus Environment


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As the chain dynamics have a strong molecular weight dependence, the effect of molecular weight was also studied. Figure 7a-b shows fixity and recovery data of G1652, G650, and G1654 SEBS, with molecular weight increasing as follows: G1652 < G1650 < G1654. Overall, lower molecular weight increases fixity and decreases recovery. As previously shown, fixity and recovery decrease with increasing applied strain. Figure S11 in the Supporting Information shows the shape memory properties of these three polymers as a function of annealing time at an applied strain of 100%. As the molecular weight increases a lower fixity and higher recovery is observed at constant annealing time. The dynamics of stress relaxation will slow down with increasing molecular weight. This is due to both the molecular weight scaling for the diffusion coefficient of a single chain in an entangled network (Do

M2.3),[68] and the

additional enthalpic penalty for chain pull-out, D ̴Doexp(-NPS), which depends on the degree of polymerization of the endblock (NPS) and the interaction parameter , where  ̴1 in the strongly segregated state.[69-71] As a result, lowering χN decreases phase segregation and facilitates PS chain pull out, decreasing recovery due to stress relaxation and increases fixity due to the formation of more secondary domains. Examining the data in Figure S11, while decreasing the molecular weight of the SEBS improves the fixity, it has a more deleterious effect on the recovery. Therefore the intermediate molecular weight SEBS (G1650) provides the best balance between fixity and recovery in the shape memory properties.


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Figure 7: Second cycle a) fixity and b) recovery of SEBS polymers varying in molecular weight. The exact experimental conditions are outlined in Table S1 in the Supporting Information. There are a number of other factors that could affect the chain dynamics and therefore the rate of chain-pull out, diffusion, formation and annealing of domains, which control this process such as the Flory-Huggins interaction parameter, entanglement molecular weight, bare diffusivity of disordered chains, and the glass transition temperatures of the end and midblocks. While it would be possible to explore these other parameters using different polymer chemistries, some of which are commercially available, it is difficult to isolate individual contributions of these different parameters, since more than one will vary as either the chemistry of the midblock or endblock are varied. Compared to the results of the shape memory behavior of neat SBCs in Table 1, these current results display significantly higher fixities. The main difference is the amount of time the samples were annealed at high temperature. Based on Figure 5 and Figure S11a shorter annealing times give lower fixity, however the DMA measurements are limited by the heating and cooling rate (10 C/min) of the measurement. To analyze the effect of much shorter annealing times, shape memory experiments were performed on G1650, G1652, and G1654 SEBS samples using hot and cold water baths to quickly heat and cool the samples under controlled strain conditions as described in the experimental procedures section.



Annealing times of 0.1, 1 and 10 minutes were used. The first cycle results are shown for the 0.1 and 1 minute annealing time whereas the first and second cycle results are shown for the 10 minute annealing time. The results of these measurements are shown in Figure 8a-f. Figure S12 in the Supporting Information specifies the data collected with li values of 5 and 10 mm demonstrating that there is no dependence on the initial sample length. Similar to the results shown in Figures 5 and S11a, fixity increases and recovery decreases with longer annealing times. Furthermore, the overall fixity increases with decreasing molecular weight and is independent of cycle number. The independence on the cycle number is consistent with the DMA results from Figure 2. The recovery decreases with the hold time used for the first cycle. At 10 minute annealing time a higher recovery was observed in the second cycle, which is also consistent with the DMA results in Figure 2. In addition, the magnitude of the recovery for the second cycle at 10 minute annealing for both the DMA and water bath experiments were similar indicating that the second cycle results give an accurate measure of the shape memory properties.


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Figure 8: Fixity and recovery of a-b) G1652, c-d) G1650, and e-f) G1654 SEBS obtained using the water bath shape memory testing method. The DMA data is the same data from Figure 8. Since these polymers can be processed by standard thermoplastic extrusion and molding processes they lend themselves to the facile fabrication of more complex shape memory articles. One 27 ACS Paragon Plus Environment


example is the transformation of flat sheets to bent and twisted shapes through the fabrication of bilayer structures.[72] As shown in Figure S13 in the Supporting Information, SEBS bilayer hinges were prepared by adhering strips of SEBS programmed under uniaxial extension to an undeformed strip of SEBS. Upon heating, the programmed active layer will retract and cause the bilayer to bend to minimize the overall stress in the sample, where the extent of curvature is proportional to the fixed strain in the active layer. Two examples are shown, to generate an S-shape and a U-shape. In a second example a large sheet was programmed by uniaxial deformation and strips were cut at different angles relative to the strain direction and adhered to undeformed strips of SEBS. Figure 9 shows how these bilayers exhibit twisting motion when there is a non-zero angle between the initial strain direction and the long-axis of the strip. A third example is the snap-through instability of the dome shape SEBS sample shown in Figure S5 in the Supporting Information.[73-76] Once inverting the dome and programming the inverted shape using the hot water programming method previously described, a snap-through recovery is obtained. A video showing the recovery in hot water can be found in the Supporting Information (Video S2). These example shows that through the combination of its thermoplastic molding properties and its intrinsic shape memory properties, SEBS can be fabricated into complex shape morphing materials using conventional polymer processing techniques.


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Figure 9: Directional recoveries of SEBS bilayers. Conclusion: Neat SEBS triblock copolymer, thermoplastic elastomers were shown to have inherent shape memory properties. In contrast to most reported shape memory polymers, where shape memory is achieved through the fabrication of materials with separate solid networks for shape fixing and recovery, the shape fixing, secondary network is formed in-situ during deformation from the original physically crosslinked polymer network. The formation of the secondary network is primarily attributed to partial stress relaxation of the triblock copolymer and the formation of new domains with a lower glass transition temperature. Compared to the range of fixity and recovery values summarized in Table 1, under optimum



sample and processing conditions a shape fixity of 81% and shape recovery of 89% were obtained under uniaxial tension. The ability to apply shape memory programming using the straightforward processing of a commercial thermoplastic should open up design opportunities in device fabrication using shape memory polymers. While it is known that blending of SEBS with commercial additives, such as wax can further improve the shape memory properties and tune the transition temperature, using the neat SEBS removes this blending step, simplifying the fabrication of these materials. Furthermore, this mechanism is likely not unique to SEBS triblock copolymers, but may be more generally applicable to physically crosslinked polymer networks where new crosslinks formed during rearrangement require annealing to fully develop thereby differentiating them from the original crosslinks under an appropriate external stimulus. Therefore other commercial polyurethane or polyolefin thermoplastic elastomers would be interesting to study as potential shape memory polymers. One reason the shape memory behavior of neat SBCs may have been overlooked until now is that the shape memory effect is overlaid with the processes that produce tensile set in cylinder-forming block copolymers even under room-temperature deformation. Therefore, sphere-forming systems, which are known to show different mechanical properties compared to cylinder-forming systems, would be an interesting alternate system to study the mechanism of shape memory and its dependence on stress-relaxation and chain pull-out. This may lead to further optimization of the shape memory properties of neat SBCs. Acknowledgments: The authors thank KratonTM Corporation for supplying the SEBS polymers used in this investigation. Supporting Information: (1) Water bath shape memory cycling setup; (2) bilayer and dome processing schematics; (3) gravimetric creep measurements at different temperatures; (4) DMA temperature sweeps at different heating rates; (5) tables of fixity and recovery data; (6) videos of various actuations. 30 ACS Paragon Plus Environment

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