Photoinscription of Chain Anisotropy into Polymer Networks

Nov 21, 2016 - Fixing chain orientation within polymeric materials can impart anisotropic mechanical, optical, and electrical properties. Although mac...
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Photoinscription of Chain Anisotropy into Polymer Networks Yuan Meng,† Jeh-Chang Yang,† Christopher L. Lewis,† Jisu Jiang,† and Mitchell Anthamatten*,†,‡ †

Department of Chemical Engineering, University of Rochester, Rochester, New York 14627-0166, United States Laboratory of Laser Energetics, University of Rochester, Rochester, New York 14623-1212, United States

‡

S Supporting Information *

ABSTRACT: Fixing chain orientation within polymeric materials can impart anisotropic mechanical, optical, and electrical properties. Although macroscopic anisotropy in amorphous or liquid crystalline phases has been achieved by cross-linking or by thermoreversible bond shuffling under strain, these methods lack spatial and temporal resolution. Here, we demonstrate a method to controllably write chain anisotropy into polymer networks containing both permanent and light-sensitive bonds. While held under mechanical stress or strain, light initiates a cascade of addition−fragmentation chain transfer reactions, causing photosensitive functional groups to reshuffle, thereby stabilizing the deformed network. Photoinscription of chain anisotropy allows for simplified processing on fully cross-linked networks with spatial and temporal control over chain orientation, thus enabling a spectrum of anisotropic polymeric materials. As an example, we demonstrate how built-in anisotropy of a semicrystalline network encourages crystallization along a preferred direction, leading to fully reversible shape actuation.

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reaction,14 chain orientation of different degrees can be written into a stretched covalent network. The AFCT reaction has been recently utilized to demonstrate photomediated, temporal control of localized stress relaxation,15,16 self-healing,17 and molecular diffusion.18 During irradiation, the number density of covalent bonds within and between chains remains nearly constant, and only the network topology changes. The achieved configurational chain bias can guide polymer crystallization along a preferred direction, enabling thermoreversible selfstretching.11,19−21 The described light-based methodology represents a new approach for spatiotemporal control of network anisotropy and architecture for delicate shaping and patterning processes.

INTRODUCTION Elastic networks consisting of chemically cross-linked polymer chains are widely employed for their high failure strain and tunable viscoelastic properties. Covalent cross-linking is usually accompanied by shrinkage stress that introduces an uncontrollable level of configurational chain bias, altering a network’s physical properties. However, a number of alignment-crosslinking methods effectively trap anisotropy by first aligning a partially cross-linked network with an external field and then capturing chain alignment by introducing new covalent crosslinks. These methods have been judiciously employed to achieve permanent, long-range anisotropy of polymer chains, 1−6 liquid crystal mesogens,7−11 and embedded particles12 within a covalent network bringing about optical, electrical, and mechanical anisotropy of liquid crystalline elastomers and double networks. There, anisotropy is achieved by chemically fixing oriented domains while simultaneously applying an external field. However, the aligning-cross-linking method faces a contradiction: effective alignment requires first establishing a percolated cross-linked structure beyond the gel point, but this first-stage cross-linking introduces disorder, reducing the achievable anisotropy in the later phase. Also, the alignment-cross-linking approach lacks flexibility because chemical cross-linking is conventionally irreversible. Although thermoreversible cross-linking methods, such as transesterification,10,13 present new options toward macroscopic anisotropy, such thermal-based methods lack precise temporal and spatial resolution. In the present study, we demonstrate a novel approach to establish configurational chain bias within a soft, photoreconfigurable polymer network. By employing a photoinduced cascading addition−fragmentation chain transfer (AFCT) © XXXX American Chemical Society

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PREPARATION OF PHOTORECONFIGURABLE NETWORKS The three macromer and linker reagents shown in Figure 1 were used to produce a well-defined, photoreconfigurable network (see Experimental Section). Macromolecular polycaprolactone (PCL) trithiol (1) provides a semicrystalline backbone with reactive thiol end-groups and is coupled with two small molecule diacrylates (1,3-butanediol diacrylate and MBTA, 2 and 3) to link together PCL strands. Reagents were stoichiometrically mixed by balancing the number of thiol and acrylate groups (Table 1). Diacrylate 2 serves to establish a covalent network that is inert to radical attack, and MBTA offers radical-sensitive allyl−sulfide linkages. Diacrylate 3, Received: September 9, 2016 Revised: October 21, 2016

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Figure 1. Formation and reconfiguration of a photoreconfigurable network: (a) network precursors include PCL-trithiol, 1; 1,3-butanediol diacrylate, 2; and 2-methylenepropane-1,3-bis(thioethyl acrylate), 3. (b) Illustration of network photoreconfiguration performed under uniaxial deformation. Initially, network strands are isotropically configured. When tensile stress is applied along z, all chains stretch along z. While sustaining a stress, light induces AFCT reactions, enabling light-sensitive strands to recoil back to their isotropic configurations while the strands lacking allyl sulfides remain distorted along z. Finally, upon removing the applied stress, the network relaxes into its final “state of ease”. This state minimizes free energy by balancing (i) compression of strands containing photosensitive groups into oblate coils with (ii) stretching of strands lacking photosensitive groups into prolate coils.

properties as an experimental control network that was prepared exclusively with covalent linkages (Table 1). Our strategy to enable light inscription of chain anisotropy is outlined in Figure 1. Partially reconfigurable networks are exposed to light while imposing mechanical strain. Since the network contains semicrystalline PCL strands, the material is first heated above PCL’s melting point, e.g., to 60 °C, where it behaves like a rubbery elastomer. When relaxed, network subchains adopt entropically preferred, random-coil configurations, but they can be uniaxially stretched by applying an external load. While holding an applied load, and maintaining T > Tm, the material is irradiated with UV light, causing reversible linkages to reshuffle. This network reconfiguration allows elongated strands to recoil back to their isotropic states and react with nearby allyl sulfide moieties, forming new network strands that bear no load. The AFCT reactions also generate other “unloaded” dangling and radicalized chain ends which are capable of continuing the cascading reaction. Upon load removal, the reconfigured network locates its “state of ease”, which minimizes the total stretching free energy of all strands. The final shape represents a balance of internal stress: network subchains without photosensitive groups remain configurationally stretched in tension, whereas subchains containing photosensitive groups are deformed into oblate configurations, in compression. The photoreconfigurable network can be programmed into new topologies using two different methods, each of which represents a distinctive network reformation pathway. Figure 2 shows the creep response of a network irradiated with UV light at constant stress, and Figure 3 shows the stress relaxation of a

Table 1. Composition, High Temperature Storage Modulus, Calorimetry Results, and Gel Fraction of Prepared Photoreconfigurable Network and Control diacrylate fractiona (%) network

2

3

E0b (MPa)

Tm (°C)

ΔHm (J/g)

gel fractionc (%)

control P-RCN

100 60

0 40

2.5 2.8

35.07 37.39

35.84 35.17

98.5 97.9

a

Molar percentage of diacrylate species used to formulate networks. High-temperature storage modulus measured at 70 °C using DMA. Gel fraction tests were performed in acetone to determine the mass of extractable species; the masses of added catalyst and photoinitiator were excluded from this calculation. b c

containing the ally−sulfide linkage, is susceptible to radical attack through AFCT cascading reactions which actively reshuffle the network, but only at the photoreversible nodes.14 In a typical AFCT reaction, thiyl radicals add to allyl sulfides groups to yield unstable carbon-centered radical intermediates which can fragment into different allyl sulfides and thiyl radicals.22 Thus, network reshuffling can be triggered and sustained by UV exposure in the presence of photoinitiator. The fraction of reversible linkers is chosen to be 40 mol % of the total diacrylates to maximize the effect of AFCT-induced network reconfiguration, while ensuring the covalent network is always above its percolation threshold (>50%) throughout the programming phase (see Supporting Information, Section 1, for supporting calculation). Prior to UV exposure, the photoreconfigurable network (P-RCN) exhibits similar physical B

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Figure 2. Elevated temperature (60 °C > Tm) mechanical response of a photo-reconfigurable network maintained at constant stress during UV exposure: (a) cartoon illustrating the programming protocol; (b) stress and strain plotted against time before and during UV irradiation (beginning at 175 s). The upper dotted line corresponds to applied stress; the bottom red curve corresponds to strain, and the bottom dashed curve corresponds to a fit based on eq 1 (see Supporting Information, Section 3); model fit parameters are υ1 = 192 mol/m3, υ2 = 128 mol/m3, and k = 0.0040 s−1. (c) A series of images taken during photoinduced creep with multistage UV exposure (initial gauge length: 2.5 cm; load: 50.0 g; temperature: 60 °C). (d) Stepwise creep curve observed at constant stress with three irradiation periods; the upper dotted curve reflects applied stress; the bottom red curve is recorded strain change in response to UV exposure (purple windows); the bottom dashed curve indicates a fit based on eq 1, the fit was generated using the same parameters except a slightly higher rate constant k = 0.0042 s−1 (see Supporting Information, Section 3).

⎛ ⎛3 ⎞ 1 ⎞ 2 ⎜ p − 1⎟(ν + ν )⎜λ(t ) − ⎟ + ν2 2 ⎝2 ⎠ 1 λ(t ) ⎠ ⎝ ⎡ λ(t )2 λ(t − τ ) ⎤ σ − ×⎢ ⎥ dτ = 2 λ(t ) ⎦ kBT ⎣ λ(t − τ )

network irradiated at constant strain. Each method will be sequentially discussed.

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INSCRIPTION OF ANISOTROPY AT CONSTANT STRESS Anisotropy can be programmed into a partially reconfigurable network by first establishing mechanical equilibrium at a constant stress, followed by UV irradiation. Remarkably, photoinduced mechanical creep (Figure 2b) occurs only during active irradiation, and creep promptly ceases when the light is turned off. The observed creep is complex and is attributed to topological rearrangement of network strands. Initially, the applied stress is distributed equally among all network strands, i.e. those with both permanent and photosensitive linkages. As UV irradiation commences, photosensitive strands begin to reconfigure. While reconfigured strands experience lower stress, their stress is transferred to other network strands. At irradiation time t, the total applied stress is distributed onto three types of strands: (i) strands belonging to the permanent network, (ii) strands with light-sensitive linkages that have not yet reconfigured, and (iii) strands with light-sensitive linkages that have already reconfigured at least once since irradiation started. A model was developed to capture the time history evolution of these three populations during irradiation. The model combines Green and Tolbosky’s classical polymer stress relaxation theory,23 which depends explicitly on strain history, with a correction to account for the loss of stiffness as the percolation threshold is approached from above the gel point.24 The model derivation is provided in the Supporting Information (Section 2) and can be expressed as

∫0

t

k exp(− kτ )

(1)

where ν1 and ν2 are the number densities of covalent and reversible strands, λ(t) is the network’s time-dependent elongation ratio, σ is the applied (engineering) stress, t is experiment time, τ is reversible bond age, and p is the fraction of stress-bearing strands that have not reconfigured since stress was applied. The first term in parentheses corrects the stiffness of the initial load-bearing network, as the network approaches its percolation threshold from the gel state according to the percolated-network law.24 This fraction p of non-reconfigured bonds can be expressed as p=

ν1 + ν2 exp( −kt ) ν1 + ν2

(2)

where k describes the rate of the AFCT reaction. The model can be fit to experimental data λ(t) by systematically varying the three fit parameters (ν1, ν2, k). The ability to enable network reshuffling only during light exposure provides effective temporal control of mechanical creep. For example, stagewise UV exposure under constant stress leads to multistep creep that can be accurately modulated with light. Before UV exposure, the partially reconfigurable network maintains a stretched length of about 10% with 0.5 MPa of applied tensile stress. Network reconfiguration, causing mechanical creep, only occurs with active UV irradiation. Whenever the UV source is switched off, the sample is static C

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faster. Thus, all networks discussed in remainder of the article were programmed under constant strain. These photo-reconfigurable networks can be programmed into a series of mechanically distinct new materials, depending on imposed strain and UV exposure. Figure 4a shows stress− strain behavior of samples that were first strained to different lengths (0%, 100%, 200%, and 300%) and irradiated with UV light under constant strain (see Figure 4b). The specimens were from the same network and share the same stretch history prior to irradiation as indicated by their overlapping curves in Figure 4a,b for the initial drawing phase. Irradiation causes stress relaxation to be observed as the network reconfigures, and each specimen stretched to a different strain follows a different path during irradiation. Specimens irradiated at lower strain tend to take longer time to reach equilibrium. This may be due to changes in the film’s optical thickness as the sample is stretched or, possibly, the AFCT reaction is activated with stress. After allowing a long enough time for stress relaxation under constant irradiation to occur, the external tensile stress was removed, and samples did not return to their original shape. Instead, samples remained elongated to a new unloaded length, representing a mechanical “state of ease” that balances the internal stress and locks chain bias of varying degrees into each sample. As will be shown later, this different degree of chain bias introduced through programming allows for tuning of actuation performance. Samples programmed at constant strain exhibited different unloaded lengths, and the resulting stress−strain characteristic curves are displayed as dashed characteristic curves in Figure 4a. During irradiation, a fraction of the molecular junctions were reconfigured by reversible AFCT reactions. Network reconfiguration effectively removes stress from the population of reversible network subchains. The newly formed molecular junctions connect the relaxed reactive chain ends together, weaving through the elastically deformed network, and stabilizing the elongated state. Interestingly, the specimen cured without strain exhibits exactly the same stress−strain behavior as the original network (see Figure 4a, black dashed curve), suggesting strain-free programming does not change network topology. There, relaxed polymer strands simply reshuffle into new relaxed states with no significant change in the number of entanglements. The three characteristic stress−strain curves after programming, as well as the pristine stress−strain curve, were fitted using Mooney−Rivlin and neo-Hookean models, respectively (Supporting Information, Section 5). The Mooney−Rivlin model better fits the pristine network’s stress−strain data, and the Neo-Hookean model better fits the reshuffled network’s strain-softening behavior. This difference is especially apparent for samples processed at high curing strain. This trend suggests that topological entanglements are lost during reconfiguration at higher strains, shedding insight onto the molecular rearrangements that occur during photoreconfiguration.25 The initial subchains are highly constrained by a large number of permanent entanglements that contribute to elastic free energy of stretching. As these subchains are stretched and irradiated, they undergo AFCT, recoil to a relaxed configuration, and form new covalent bonds. While AFCT exchange reactions may generate new entanglements, they are not as elastically effective, and consequently, more strain softening is observed. The observation of fewer effective entanglements following irradiation at high strain is analogous to changes

Figure 3. Elevated temperature (60 °C > Tm) response of a photoreconfigurable network held at constant strain during UV exposure: (a) cartoon illustrating the constant strain programming protocol; (b) strain and stress plotted against time. At the experimental time of 175 s, the UV light source was turned on. The upper dotted line corresponds to sample strain; the bottom red curve corresponds to measured stress; the bottom dashed curve is a fit to the photoinduced relaxation behavior based on eq 1. Model fit parameters are υ1 = 192 mol/m3, υ2 = 128 mol/m3, and k = 0.020 s−1 (Supporting Information, Section 4).

and resists additional creep. Importantly, accumulated creep should eventually arrive at equilibrium where only the permanent fraction of the network remains elastically active and withstands the load. This limit is suggested by the photoinduced creep curve (Figure 2c) which approaches an equilibrium strain after about an hour of UV exposure. Fitting our reconfigurable network model to the creep data captures the main features of the experimental curve (Figure 2d).

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INSCRIPTION OF ANISOTROPY AT CONSTANT STRAIN Irradiation at constant strain allows strands with photoresponsive linkages to quickly reconfigure into their elastically inactive, unloaded states, leaving only permanent strands under stress. The time scale to reach network equilibrium for photoprogramming at constant strain is considerably shorter (3500 s, Figure 2b). This observation is explained as follows: at constant stress, relaxed network strands repeatedly undergo bond exchange, and newly formed bonds are continuously subjected to new stresses as the sample deforms toward its ultimate equilibrium strain. At constant strain, on the other hand, reversible junctions need only reconfigure once because when a reversible strand breaks, new bonds should reform in relaxed configurations that do not contribute to the overall stress. Thus, compared to constant stress programming, light inscription of anisotropy at constant strain is inherently D

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Figure 4. Stress−strain and stress-relaxation behavior of samples reconfigured with light at specified strains. (a) Stress−strain behavior of four samples during elongation to different strains (0%, 100%, 200%, and 300%) followed by irradiation while holding strain (vertical solid curves); after irradiation, the tensile load was removed and stress−strain tests (dashed curves) were collected beginning at each specimen’s new unloaded length. (b) The same data plotted against time to show stress-relaxation behavior of samples irradiated at different strains. (c) Stress−strain behavior of a sample elongated to 200% strain and subjected to three successive UV exposure cycles (20, 25, and 800 s); between each exposure, the sample was unloaded and stretched back to 200% for the next irradiation; the blue dashed curve is the stress−strain characteristic curve of the sample after the third irradiation cycle. (d) The same data plotted against time to show stress relaxation during irradiation; the positions of the second and third relaxation cycles were shifted right to form a continuous decay curve; the black dotted curve represents stress-relaxation behavior of a sample strained to 200% and continuously irradiated until equilibrium was approached. All strains are relative to the sample’s original prestretched length.

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SHAPE ACTUATION Specimens photoprogrammed at different levels of fixed strains exhibit different unloaded lengths as well as thermal actuation capability. For example, specimens in Figure 4 show unloaded lengths that are 90%, 160%, and 230% greater than the network’s original length for samples programmed at 100%, 200%, and 300%, respectively. Thus, an increasing degree of configurational chain bias is introduced by programming at higher levels of strains. When cooled in the absence of stress, the configurational chain bias enables strain-induced crystallization to occur along the prestrained axis, causing the sample to elongate to a new length.11,19−21 This effect is fully reversible; upon heating, the biased crystals melt, and the network strands shrink back to an equilibrium coiled state that corresponds to the sample’s state-of-ease, unloaded length. This process can be repeated multiple times without loss in actuation performance. Figure 5 shows heating and cooling cycles, and the corresponding shape changes of samples that were programmed by irradiation at various strains relative to the samples’ prestretched length. In Figure 5, the reported actuation strain (y-axis) is relative to the network’s unloaded length in the melt state. Reversible shape changes of about 2.5%, 6%, 13%, and 19% were observed for samples programmed at 100%, 200%, 250%, and 300%, respectively.

occurring in entanglement networks upon chemical crosslinking at high strain.26−28 Networks stretched to the same strain can be programmed into materials with different degrees of anisotropy simply by varying the UV exposure time. Figure 4c shows stepwise stress relaxation achieved by successively UV-treating a network under constant strain. The unloaded length grows every time the specimen is exposed to UV, suggesting that a greater fraction of the network has reconfigured, and therefore network chains possess greater configurational bias. The shapes of the resulting stress−strain curves also show an increasing amount of strain softening which is better captured by the neo-Hookean model (Supporting Information, Section 5). This indicates that permanent entanglements are growing scarce as reshuffling proceeds. If the three stress-relaxation curves are superimposed and plotted against time (Figure 4d), the resulting curve emulates the single-step stress-relaxation curve under irradiation. Thus, network reshuffling at constant strain only depends on light exposure and can be viewed as time stackable. This multistage exposure test demonstrates outstanding temporal control over network reconfiguration. E

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Figure 5. Shape changes during heating and cooling cycles for samples that were programmed by irradiation at various strains: (a) 100%, (b) 200%, (c) 250%, and (d) 300% relative to the samples’ prestretched length. The black dashed lines correspond to temperature on the right axis, and the red solid curves correspond to actuation strain on the left axis.

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Importantly, there is a clear trend that greater strain present during programming leads to greater amount chain bias within the network, and consequently, enhanced actuation can be achieved. Moreover, photoinscription of chain bias appears to be more efficient than the traditional alignment-cross-linking method which achieved less than 10% reversible actuation under similar programming strain (300%).19

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EXPERIMENTAL SECTION

Materials. ε-Caprolactone (CL) was purchased from SigmaAldrich and distilled before use. All the other chemicals were obtained from Sigma-Aldrich and were used as received. The synthesis of 2methylenepropane-1,3-bis(thioethyl acrylate) (MBTA, 2) was reported previously.29 PCL-Triol. The hydroxyl-terminated, three-arm poly(caprolactone) was synthesized by ring-opening polymerization of CL in the presence of SnOct2 as a catalyst and trimethylolpropane as a trifunctional initiator. To a dried, silanized 100 mL flask, distilled CL (40.0 g, 39 equiv), trimethylolpropane (1.208 g, 1 equiv), and SnOct2 (88.8 mg, 1/40 equiv) were added. The reaction was carried out neat, at 120 °C, and under continuous N2 purge for 24 h. The resulting PCL polymer was purified by precipitation into methanol followed by vacuum-drying for 12 h at 60 °C. Yield 92%. The number-averaged molecular weight was determined to be 4400 g/mol (1H NMR end-group analysis) and 5000 g/mol (GPC), and the molar mass dispersity from GPC was 1.17. PCL-Triacrylate. The chain ends of hydroxyl-terminated prepolymer were acrylated by nucleophilic substitution with acryloyl chloride. To PCL-triol (30.0 g, 1 equiv) under N2 at 0 °C was added freshly distilled toluene and potassium carbonate (4.23 g, 4.5 equiv). After 30 min of degassing with N2, acryloyl chloride (2.48 mL, 2.78 g, 4.5 equiv) was added dropwise over 15 min. The reaction was warmed to 80 °C and allowed to stir for 48 h. The mixture was filtered, and the liquid fraction was precipitated into methanol to afford a white powder. The product, a PCL-triacrylate, was vacuum-dried at 60 °C overnight. Yield 90%. 1H NMR indicated a molecular weight of 5000 g/mol and complete end-group conversion; GPC showed molecular weight of 5200 g/mol and a molar mass dispersity of 1.18. PCL-Trithiol (1). The chain ends of acrylate-terminated prepolymer were further modified by thiol−acrylate reaction with excessive 1,2-ethanedithiol. The procedure was similar to the end-functionalization of prepolymers described earlier in our lab.30 Freshly distilled triethylamine (1 equiv) was added to PCL-triacrylate (10 g, 1 equiv) dissolved in 30 mL of toluene under N2 purge. Ten times excess of stoichometric 1,2-ethanedithiol was dissolved in 10 mL of toluene and added dropwise into the reaction flask. The reaction was carried out at room temperature for 24 h. The mixture was precipitated into methanol to afford a white powder. The product was vacuum-dried at

CONCLUDING REMARKS

By combining stress and light, chain anisotropy can be written into polymer networks containing both covalent cross-links and photoreconfigurable, allyl sulfide linkages. To write chain anisotropy, semicrystalline poly(caprolactone) networks containing allyl−sulfide linkages only need to be melted, strained to various elongations, and irradiated. Light initiates a cascade of reaction events, resulting in rupture of some network strands, configurational relaxation of dangling ends, and re-formation of network bonds. After irradiation, the resulting network assumes a mechanical state of ease that has trapped permanent configurational bias into many chains. The network’s final mechanical properties are path-dependent on both the level of applied stress and light exposure; results are explained by treating the network as a transient network during irradiation or, otherwise, a static network. When a previously lightprogrammed semicrystalline network is cooled, the built-in anisotropy encourages chains to crystallize in a preferred direction, leading to fully reversible shape actuation. This study offers a new framework to lock-in chain bias that could be extended to align morphological features, enhance liquid crystalline order, or arrange mesoscopic particles within soft, elastomeric solids. The spatiotemporal control inherent to light programming will likely enable new 2D and 3D actuation patterns to be programmed into films or in the bulk. F

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Macromolecules 60 °C overnight. Yield 97%. 1H NMR indicated complete end-group conversion and a molecular weight of 5300 g/mol; GPC showed molecular weight of 5200 g/mol and molar mass dispersity of 1.18. PCL Networks Containing Covalent and Photoreversible Linkages. Cross-linked films were prepared by thiol−acrylate Michael addition between trifunctional PCL-trithiol and difunctional small molecule linkers (both MBTA and 1,3-butanediol diacrylate, 2 and 3). The thiol−acrylate coupling reaction was conducted in the presence of 4-(dimethylamino)pyridine (DMAP) (e.g., 0.5 wt %) as a base catalyst. Phenothiazine was added into the mixture as a radical scavenger at 0.5−0.8 wt % to protect the reagent mixture from undue radicalinduced polymerization. PCL-trithiol prepolymer and scavenger were first mixed on a glass slide using a spatula, and the mixture was melted on a hot plate at 60 °C. After 1−2 min, a homogeneous mixture was obtained, and a stoichiometrically balanced amount of diacrylate linker (MBTA (40 mol %) and 1,3-butanediol diacrylate (60 mol %)) was added. The mixture was melted again for 1−2 min; while melted, the finely ground base (DMAP) and photoinitiator 2,2-dimethoxy-2phenylacetophenone (DMPA, 0.5 wt %) were added and stirred into the melt for 1−2 min. The mixed and melted reagents were briefly degassed under vacuum at 60 °C until no bubbles were observed. The mixture was then quickly sandwiched between two glass slides that were separated by a 0.70 mm thick Teflon spacer. After assembly, the sandwich was placed in a curing oven (convection) for 2 days at 60 °C. A similar control network with only covalent linkage was prepared by adopting the same method, but using 1,3-butanediol diacrylate (3) as the only difunctional small molecule linker. Characterization. 1H NMR spectra were acquired on a Brüker AVANCE-III 400 NMR spectrometer system operating at 400.13 MHz. Molecular weight and polydispersity were measured by gel permeation chromatography (PolyAnalytik PAS103-L and PAS104-L GPC columns and Viscotek TPA301 detector) using THF as an eluent and polystyrene standards. Differential scanning calorimetry was performed using a TA Instruments, Q2000 DSC. Five to six mg samples were placed in a hermetically sealed pan and subjected to heating and cooling at 10 °C/min over the temperature range of −30 to 130 °C. Dynamic mechanical analysis (DMA) experiments were performed in tension at 1 Hz and 5 °C/min over the temperature range 25−65 °C using a Rheometrics RSAII solids analyzer. IR spectroscopy (8000S, Shimadzu) was employed to confirm complete consumption of the acrylate functional groups. Network Photoprogramming. Network photoprogramming was performed at 60 °C on a DMA (RSA-G2, TA Instruments). For constant-strain programming, samples were uniaxially stretched and held at a specified strain, and engineering stress was monitored. Next, a UV lamp (OmniCure Series 2000) was powered to deliver 325 nm light to the DMA chamber though fiber optics. The output of the lamp was set to a constant 5 mW/cm2. For constant-stress programming, a sample was stressed to a predetermined level, and the sample strain was monitored until equilibrium was achieved. UV irradiation was then introduced using the same exposure apparatus to induce photoinduced creep. Multistage creep was performed under the same conditions by cycling the UV source on and off for a certain amount of time. Actuation tests were achieved on the same DMA instrument. Samples were subjected to cooling−heating cycles between 0 and 70 °C under a negligible 0.01 N tensile force. All stresses are reported as engineering stresses.

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of stress−strain behavior following fixed-strain photoprogramming (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; tel (585) 273-5526, fax (585) 273-1348 (M.A.). ORCID

Mitchell Anthamatten: 0000-0002-7763-9465 Present Address

C.L.L.: Department of Mechanical Engineering Technology, Rochester Institute of Technology, Rochester, NY 14623-5603. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from funding provided by the National Science Foundation under Grant ECCS-1530540. M.A. was supported in part by the Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302 with Laboratory for Laser Energetics (LLE) at the University of Rochester. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article. Y.M. was partially supported by an Elon Huntington Hooker Fellowship, and J.Y. was supported by a Horton Fellowship from LLE.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01990. Calculation of percolation threshold; transient network model with external stress; fitting photoinduced creep data with transient network model; fitting photoinduced stress relaxation with transient network model; analysis G

DOI: 10.1021/acs.macromol.6b01990 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.6b01990 Macromolecules XXXX, XXX, XXX−XXX