Implementation of two distinct wavelengths to induce multistage polymerization in shape memory materials and nanoimprint lithography

Abstract

Here, a process is introduced for forming dual stage thiol-Michael/acrylate hybrid networks photocured by two different wavelengths, demonstrating its use in nanoimprint lithography (NIL) and shape memory materials. Initiated with a visible light sensitive photobase and a UV-sensitive radical initiator, thiol-Michael-acrylate hybrid polymerizations were programmed to proceed sequentially and orthogonally, with base-catalyzed thiol-Michael photopolymerization as the first stage and radical mediated acrylate photopolymerization as the second stage. By regulating the photopolymerization formulations, i.e. thiol-to-acrylate ratios, initiator loadings and irradiation conditions, a series of materials with highly tunable mechanical performance was achieved, with ultimate T_g values ranging from 23 to 70 °C. With a photopatternable first stage and a readily reconfigurable second stage, its implementation in nanoimprint lithography (NIL) enabled surface features on the scale of 10 nm to be formed on a photopatterned substrate. Additionally, the dual stage polymer results in a relatively homogenous polymer network with a narrow glass transition temperature (Tg), which enables rapid response in applications as shape memory materials, with shape-fixity values above 95% and shaperecovery values above 99%. With its unique photocuring process and programmable mechanical properties, the two color light controlled photopolymerization can be exploited as a useful tool in a wide range of materials science applications.

Graphical Abstract

1. Introduction

The thiol-Michael addition reaction has served as a powerful “click” reaction tool since the introduction of thiol chemistry into the “click” reaction concept.[1, 2] With attributes of high efficiency, rapid reaction rates and selective product generation, the thiol-Michael reaction has seen wide implementation in polymer science,[1] surface modification,[3] biomaterial construction,[4, 5] and particle synthesis[6]. Initiated with proper catalysts, including bases or nucleophiles, the thiol-Michael reaction proceeds rapidly between a thiol and an electron-deficient vinyl group, such as an acrylate, methacrylate or maleimide, as well as others, and achieves quantitative yields at mild, solventless reaction conditions.[7] Thiol-Michael polymerizations of multifunctional monomers are step-growth reactions, allowing for the generation of uniform polymer networks with narrow glass transition temperatures. However, due to the relatively soft thioester linkage and the relatively low crosslink density associated with the step growth polymer network character, thiol-Michael based materials exhibit relatively low glass transition temperatures (Tg), which restricts the potential applications of this approach.[8] Innovation in the available monomers (e.g., vinyl sulfone or vinyl sulfane) and the implementation of rigid cores containing reactants has greatly improved the stiffness of the thiolMichael based resins/ composites and expands its versatility in applications that vary from lithographic substrates to dental restoratives.[9, 10]

Further, there is a growing interest in using multi-component systems and hybrid polymerization to produce structurally diversified materials to increase the glass transition temperature and crosslink density of the otherwise soft, flexible polymer matrix. Hybrid polymer materials not only inherit the attributes of the distinct polymer components, but also exhibit unique properties as a result of both chemical and physical interactions within the system itself.[11] For example, free radical-cationic hybrid photopolymerizations (e.g., epoxy/acrylate hybrid systems) successfully use these advantages to achieve improved abrasion resistance, reduced toxicity, and low shrinkage from robust, reactive acrylate/methacrylate radical polymerizations. Decker and coworkers demonstrated that the epoxide/acrylate hybrid system also reduces oxygen inhibition in radical polymerizations, due to the increased viscosity and the concomitantly reduced oxygen diffusion associated with the simultaneous epoxide polymerization.[12] Previous research achieved a variety of hybrid polymer systems through different curing strategies such as amine/ epoxy thermosets,[13, 14] epoxide/methacrylate hybrid networks,[15], thiol-ene/thiol-epoxide based polymers[16, 17].

Building upon these advances in hybrid networks, this work will demonstrate the application of two distinct wavelengths to induce sequential thiol-Michael/ free-radical hybrid polymer formation in nano-imprinting lithography (NIL) and shape memory materials. With different absorption characteristics, photobases and radical initiators are capable of initiating base-catalyzed thiol-Michael and radical-mediate acrylate homopolymerization sequentially with appropriate induction wavelengths.[18, 19] By implementation of non-stoichiometric thiol and acrylate monomers (acrylate in excess), a photobase catalyzed thiol-Michael polymer matrix was first constructed upon 400–500 nm light irradiation, followed by a second stage acrylate polymerization initiated upon a UV light irradiation. The initiator cleavage and polymerization initiation mechanism of the distinct stages are described in Scheme 1. With the same functional groups (the acrylate moieties) involved in both photopolymerization stages, issues often associated with multi-component polymerization, such as entropically driven demixing and phase separation, were prevented in the dual cure system. Furthermore, the sequential initiation with lights at different wavelengths enables accurate spatiotemporal control and photopatterning at each photopolymerization stages.

Scheme 1.

The initiation mechanism of first stage thiol-Michael polymerization and the second stage acrylate homopolymerization irradiated by lights at 400–500 nm and 365 nm.

Here, two color light initiated, dual stage polymer materials are developed with desirable properties and characteristics for nanoimprint lithography (NIL) and shape memory polymers (SMP). Nanoimprinted surfaces created with lithographic techniques play an important role in many emerging applications. General lithographic methods developed with deep-UV, E-beam, or lasers, are expensive, energy consuming and also challenged to generate features smaller than the diffraction limit of light. NIL is thus a promising alternative to pattern nanoscale (dimension under 10 nm) features by imposing surface deformation on the materials via direct mechanical contact at a controlled pressure and irradiation conditions.[6, 20] In this work, large-scale geometries are first created upon visible light irradiation by thiol-Michael polymerization, and NIL is subsequently used to generate small lithographic patterns on the surface. Different photopatterned nanoimprinted resins, with varied reactant formulation, were developed with the NIL method and the topography of the lithographic structures was evaluated by atomic force microscopy (AFM). Considering the dual patternable nature and the reconfigurability of the second stage, the implementation of the dual stage material in SMP is also promising. SMPs are materials that, after deformation, are able to recover their original shape upon application of a specific stimulus (light or heat).[21] The rapid development of SMPs is driven by a number of practical applications, including electronics, robotics, biomedical devices and optical materials.[22, 23] When used as a SMP, the dual stage photopolymerization is advantageous in generating a glassy material with high modulus, and able to sustain a high strain deformation due to the uniform matrix created by the thiol-Michael chemistry. More interestingly the two color light initiation enables two-stage patterning during network formation. After first stage patterning, the intermediate is reshaped into a permanent 3D structure at the second stage curing. As such, SMPs with simple 3D structures are programmed. The shape memory polymer properties of the materials were evaluated by tensile strength, the glass transition region, free strain recovery, and shape fixity. The goal of this work is to evaluate the efficacy of two color light initiated photopolymerization as an alternative methodology for generating photopatterned nanoimprinted substrates and 2D or 3D structured shape memory materials. With complete photo-induced spatiotemporal control over the polymerization process, this dual stage photopolymerizaiton allows for the preparation of materials with programmable appearance and tunable mechanical behaviors, which is suitable for a wide range of applications.

2. Experimental

2.1. Materials.

Pentaerythritol tetra(3-mercaptopropionate) (PETMP) was purchased from Bruno Bock. Tricyclo[5.2.1.02,6] decanedimethanol diacrylate (TCDDA), 2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone (Irgacure 2959) were purchased from Sigma-Aldrich without further purification. The photobase generator, 2-(2-nitrophenyl)propyloxycarbonyl-1,1,3,3- tetramethylguanidine (NPPOC-TMG) was synthesized according to previous reports.[24]

2.2. Sample preparation.

All samples were mixed in the desired ratios of thiol to acrylate based on the functional group concentrations. The mixture of monomer and initiators was injected into the space between two glass slides and 250 μm spacers were used to control the film thickness during the photopolymerization process. In the systems where initiators were difficult to dissolve in the monomer systems, a small amount of DCM was used to dissolve the imitators before adding into the monomer mixture. The samples were then vacuumed to remove the small amount of DCM. The first stage photopolymerization reactions were achieved in the presence of 2 wt % NPPOC-TMG with 400–500 nm irradiation. The second stage polymerizations were performed with 2 wt % Irgacure 2959 in the presence of 365 nm irradiation. The irradiation process was conducted using a Acticure 4000 mercury lamp with 365 nm or 400–500 nm bandgap filters and the light intensities were measured with a calibrated radiometer (Model IL400A from International Light company). All photopolymerized samples were irradiated for 5–10 min, followed by post-curing at 80 °C for 8–10 h to increase the final conversions of the monomers.

2.3. Real-Time Fourier Transform Infrared (FT-IR) Spectroscopy.

Photopolymerization kinetics and final monomer conversions were measured by FT-IR (Nicolet 670) in transmission mode. The of functional group conversions (thiol or acrylate) were determined by monitoring the disappearance of the corresponding IR peaks. NaCl plates were used and the thiol peak was monitored in the range between 2500 and 2600 cm⁻¹ while the acrylate peak was monitored in the range between 780 and 820 cm⁻¹. The functional group conversions were calculated by the ratio of peak area to the peak area prior to the polymerization reaction.

2.4. Shape memory material testing

The glass transition temperature (Tg) of the material was measured with a dynamic mechanical analyzer (DMA) (Q800, TA Instruments). The temperature was ramped at 3 °C/min with a frequency of 1 Hz. The Tg was determined as the maximum of the tan δ profile. The half width of the glass transition was taken as the width of the tan delta peak at half the maximum value. DMA experiments were thermally cycled and replicated two times. Tensile strength tests were performed on an MTS Exceed Electromechanical Systems (Exceed ™ Model 43).

Shape memory properties of the material were also characterized with DMA by measuring the free strain recovery and shape fixity. For the free strain recovery tests, the polymers were first held at heating conditions (20 to 30 °C above the Tg of the polymers) and strained in tension approximately 10 %. The strain of the sample at this point was measured and recorded as the maximum strain (ε_m). The sample was then held at constant stress and rapidly cooled to a temperature below the Tg of the material at a rate of 3 °C / min. Then, the force was released and maintained at zero and after reaching the static state, and the strain on the unloaded sample was recorded as ε_u. In the next heating step, strain recovery behavior was observed as the temperature above the Tg of the material, and the final strain of the system post recovery was recorded as ε_p. Free strain recovery was defined as R_r(%) = (ε_m – ε_p)/(ε_u – ε_p) ×100. Shape fixity is given by R_f(%) = (ε_u / ε_m) ×100. All shape memory material cycles were repeated three times, and the parameters were calculated from the data in the third cycle.

2.5. Nanoimprint lithography.

NIL processes were performed on a nanoimprinter (Eitre 3, Obducat) at 50 °C under controlled pressures (3 MPa) for different durations of time. During imprinting, specimens were irradiated with 300–400 nm UV light at an intensity of 20 mW/cm². Prior to the NIL process, the silicon mold (with line-and-space grating patterns with a 840 nm periodicity and 220 nm height) used for imprinting was first cleaned using piranha solution and coated with a low surface-energy, self-assembled monolayer of CF₃(CF₂)₅(CH₂)₂SiCl₃ (tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane, Sigmae Aldrich) by vapor deposition to facilitate mold release from the imprinted polymer after NIL. A transparent poly(ethylene terephthalate) (PET) film covers the sample stage during imprinting to help seal the imprinting chamber and provide uniform pressure across the sample surface. The topographical features patterned were characterized via AFM (DI3100, Bruker) using tapping mode, with scan directions perpendicular to the pattern lines.

3. Results and discussion

3.1. Photopolymerization kinetics

Since their introduction into material synthesis, photobase generators (PBGs) have enabled surface modification and base-catalyzed reactions, opening new possibilities in the fields of surface etching, photo-induced polymerization/ depolymerization and photoimaging. During photolysis, many PBGs produce radical species as intermediates or/and products in the excited stage. When applied in thiol-Michael reaction, these radicals can easily induce homopolymerization of the vinyl moieties in the system, which negatively influences the orthogonality of the polymer matrix. As such, not all photobase generators are suitable for the photoinitiation of the stoichiometric thiol-Michael reaction. In previous work, it was demonstrated that NPPOC-TMG (2-(2-nitrophenyl) propyloxycarbonyl/tetramethylguanidine) not only generates negligible radical side products due to its photolysis mechanism, but also produces a strong organic base, tetramethyl guanidine (TMG), allowing for efficient deprotonation in the photoinitiation step.[18] In this two color light initiated polymerization system, the NPPOC-TMG and a UV sensitive radical generator I2959 were identified as an appropriate initiator combination to initiate orthogonally the sequential photopolymerization reactions. The respective mechanisms and absorption differences of the NPPOC-TMG (absorption ends around 405 nm) and I2959 (absorption ends around 370 nm) allow the thiol-Michael polymerization and acrylate homopolymerization to be triggered in a sequential, non-interfering fashion. The photo-generated base is not expected to react with the I2959 during the first stage photopolymerization process.

A demonstration of the polymerization reaction kinetics of the dual stage polymerization is shown in Fig. 2. Real-time Fourier transform infrared (FT-IR) was used to monitor the thiol (2500–2600 cm⁻¹) and acrylate peaks (790–830 cm⁻¹) in off stoichiometric PETMP/ TCDDA polymerization systems (thiol to acrylate molar ratio equals to 1:2) during the photopolymerization process. Fig. 2B shows that the initiation of the thiol-Michael polymerization (first stage) and acrylate homopolymerization (second stage) were well controlled with the light stimulus (400–500 nm and 365 nm respectively) and both polymerization reactions proceed orthogonally. The photo deprotection mechanism of the NPPOCTMG involves little to any radical formation, according to previous studies. As such, no radicalmediated acrylate homopolymerization is detectable in the first stage during the 400–500 nm light irradiation period. Initiated with different wavelengths, precise spatiotemporal control over both stages of the polymer formation was readily achieved, allowing for various processing techniques to be conducted during the photopolymerization process.

Fig. 2.

(A) The evolution of thiol and acrylate infrared absorption peaks during the thiol-Michael and radical photopolymerizations at different reaction stages. (B) Thiol and acrylate functional group conversion versus time for 1:2 PETMP/TCDDA dual stage photopolymerization. All samples were composed of 2 wt % NPPOC-TMG and 1 wt % Irgacure 2959 and cured with 400–500 nm, 20 mW/cm² light during the first thiol–Michael stage and 365 nm, 10 mW/cm² light during radical homopolymerization stage.

3.2. Nanoimprint lithography (NIL)

The patterning process, including a first stage photopatterning and a second stage nanoimprinting, is schematically illustrated in Scheme 2. In the first stage photopolymerization, the monomer mixture was injected between two glass slides (with 0.25 mm spacer) and then irradiated through a photomask with 20 mW/cm², 400–500 nm light. Tetrafunctional thiol, PETMP and difunctional acrylate, TCDDA (molar ratio equals to 1:2 and 1:1.5) were used to generate two different dual stage polymer materials. After 10 minutes of irradiation, the unreacted, non-cross-linked reactants were removed by washing with ethyl acetate and the intermediate material was visually formed (shape as shown in Fig. 3D), indicating the successful thiol–Michael addition polymerization achieved via the photobase generator. In the photo-induced thiol-Michael polymerization, rapid formation of the crosslinked networks significantly reduces the catalyst diffusion and improves fidelity of the photoprinted features. Moreover, adding a moderate amount of weak acid will also quench the initiation and/or propagation anion species in the photo thiol-Michael polymerization, which serves as an alternative way to improve the resolution.

Scheme 2.

Schematic illustration of the imprinting process to create nanoscale features on an already photopatterned substrate. (a) Irradiated with 20 mW/cm² 400–500 nm light, a photopatterned, loosely crosslinked resin is prepared during first stage thiol-Michael photopolymerization, followed by a washing step to remove unreacted monomer from the dark region. (b) The intermediate resin was reset on a PET support film and then compressed with a Si mold. Nanoscale features were created by the surface deformation during the second stage radical photopolymerization when exposed to UV irradiation.

Fig. 3.

AFM analysis of nanoimprint lithography on the surface of dual stage photopolymer films. Cross-sectional plots of a representative AFM surface scan of PETMP/TCDDA dual stage samples with a molar ratio of (a) 1:1.5 and (b) 1:2. (c) Height profile of the imprinted surface of 1:2 PETMP/TCDDA sample after the mold was removed (d) Photograph of the photopatterned and nanoimprinted film, placed at an angle to demonstrate the effect of the diffraction grating. The first stage photopattern was performed with photomask and 400–500 nm, 20 mW/cm² light irradiation, and the nanoscale features were created by nanoimprint lithography with 365 nm, 20 mW/cm² light irradiation.

After the first stage photopatterning with 400–500 nm light, the dual stage sample with a molar ratio of 1:2 gave a soft, loosely crosslinked polymer with a glass transition temperature (Tg) of −2 °C. Before the second stage photocuring, the rubbery intermediate material is put on a transparent PET film and then brought into contact with a patterned Si mold to form a sandwich structure. An appropriate pressure (3 MPa) was applied to the sample to induce a surface deformation, and then the intermediate polymer was exposed to 365 nm light through the PET film side for controlled durations. Generally, performing the NIL process on the surface of a cross-linked polymer is capable of creating surface features in the rubbery state and temporarily trapping them there by cooling the deformed sample. However, heating the material above its Tg will remove the temporary deformation and the surface features of the material. In contrast, the intermediate substrate from the dual stage polymerization still contains residual, reactive acrylate moieties, which enables permanent surface transformation and patterning in the NIL process. Fig. 3 shows that the formulation of this dual stage photopolymer, more specifically, the ratio of the thiol to acrylate greatly affects this surface reconfiguration during the NIL process. In Fig. 3C, the AFM scan shows a 3D representation of the surface features of the 1:2 dual stage polymer after it was removed from the mold, and the imprinted pattern height (peak-to-peak amplitude) was 156 ± 4 nm. The pattern exhibits excellent regularity across the surface of the sample. The pattern height is smaller than the master mold height of 220 nm, a phenomenon attributed to a significant resistance to extruding the thermoset into the cavity. Fig. 3D shows the photopatterned and nanoimprinted dual stage film, positioned at an angle to demonstrate the effective diffraction grating that has been written. The resulting structure highlights the first instance of sequential, spatially resolved photopatterning generating hierarchical structures with length scales varying by up to four orders of magnitude. During the second stage polymerization, NIL created a surface deformation on the intermediate material, balancing out the original thiol-Michael matrix to form the final morphology. As such, the dual cure systems with a higher thiol-to-acrylate ratio result in a polymer matrix with less change in crosslink density, Tg and modulus in the second stage, making it difficult for the NIL process to result in high fidelity of the final structure to the mold.

A control experiment on the 1:1.5 sample was also conducted. Even with similar photopatterned first stage image, the nanoimprinted features of the second stage were measured at 15 ± 4 nm. (Fig. 3A) The intermediate polymer of the 1:1.5 sample contains less unreacted acrylate moieties and thus, the final network more closely resembles the initial thiol-Michael network, with a smaller modulus and property change in the second step. Specifically, in terms of the first stage intermediate polymer, the 1:1.5 sample has a higher elastic modulus (5 MPa) compared to that of the 1:2 sample (2 MPa), which also makes it more challenging to fill the Si mold completely during the imprinting process.

3.3. 3D structured shape memory materials

In addition to creating nanoscale features on photopatterned substrates, the two color light controlled photopolymerization also serves as a promising method to generate thermally responsive shape memory materials. SMPs have been formed from thiol-ene based thermosets by using urethane containing multifunctional monomers.[25] Here, the low stiffness problem resulting from thiol-Michael polymerizations is overcome by the second stage acrylate homopolymerization. Photo controlled dual stage polymerization generates rigid materials with high crosslink density but also reduces the homogeneity of the thiol-Michael matrix to some extent.

Table 1 shows the mechanical performance and shape memory behavior of the PETMP-TCDDA systems. The mechanical experiments were characterized with glass transition temperature and tensile tests to evaluate the homogeneity of the polymer matrix, as indicated by the half width of the glass transition, and also determine the response of the dual stage systems in practical processing and manufacturing conditions. The dynamic mechanical analysis results of a series of PETMP-TCDDA (with functional group molar ratio of 1:1, 1:1.5 and 1:2) are shown in Fig. S1. With increased acrylate content, the dual stage polymer tends to have a higher Tg (> 25 °C) and becomes a glassy material at ambient temperature. Accordingly, the half-width of the glass transition region (Tg1/2_width) of the material also increases, indicating the formation of a less homogeneous polymer network. In contrast, the dual stage polymer formulation also influences the tensile modulus and strain at break of the polymer film. Compared with the pure acrylate homopolymerized matrix with an elongation of 2.4 ± 0.5%, all hybrid polymer networks broke at a later elongation point and exhibited a tensile strength varying from 1 to 30 MPa, which had a tensile modulus of 10 to 290 MPa By adjusting the ratio of thiol and acrylate in the system, different materials with tunable extensibility and stiffness are generated.

Table 1.

Tensile strength, strain at break, Tg and the half width of glass transition, shape fixity (R_f), free strain recovery (R_r), of a series of dual stage photopolymerization with different PETMP-TCDDA ratios.

Thiol to acrylate ratio	Strain (%)	Tensile strength (MPa)	Tg (°C)	Tg 1/2width (°C)	Rf (%)	Rr (%)
1:1	25 ± 4	1.4 ± 0.3	26 ± 2	16 ± 1	96 ± 2	97 ± 2
1:1.5	28 ± 5	9 ± 2	34 ± 2	33 ± 3	96 ± 1	99 ± 1
1:2	9 ± 2	29 ± 3	62 ± 3	38 ± 3	97 ± 1	99 ± 1
0:1a	2.4 ± 0.5	22 ± 4	220 ± 10	120 ± 10	-	-

^aHomopolymerized acrylate control samples were not suitable for shape memory tests due to their wide Tg and brittleness

The shape memory characteristics of the polymer network formed from PETMP-TCDDA systems were also evaluated by the strain fixity and recovery parameters, R_f and R_r as shown in Table 1. The shape memory testing was completed with three sequential runs used to evaluate the efficacy of the programming and shape recovery. Fig. 4 shows the strain-temperature plot of the 1:1.5 PETMP-TCDDA polymer system. The corresponding shape memory profile is shown as Fig. S2. Upon heating to 50 °C, the polymer is deformed in its rubbery state under an applied external stress, followed by cooling to preserve the temporary shape. With a relatively narrow Tg, the polymer film recovered its original strain over a short temperature range upon heating. Repeated shape memory cycles were performed and high R_f and R_r values (>95%) were observed in all systems. More interestingly, photo thiol-Michael polymerization generates photopatterned 2D intermediates, which can be further programed into permanent 3D shapes by the second stage curing. As a demonstration, a 1:2 PETMP-TCDDA based polymer was programmed into a 1 cm³ cubic structure as its permanent shape. Deformed into a flat film, this polymer recovered its original structure upon heating in a short period of time as shown in Fig. 5. By regulating the photopatterning at the first stage and the reprograming and folding process at the second stage, a series of SMPs with simple 3D structures are readily generated.

Fig. 4.

A representative example of free strain recovery of 1:1.5 PETMP/TCDDA dual-cure polymer (Tg= 36 °C) over 3 cycles. The permanent shape was heated to 50 °C and deformed by a tensile force, followed by a cooling process to program the temporary shape. The original permanent shape was recovered upon heating. Fixity and recovery parameters in Table 1 are reported for the third cycle.

Fig. 5.

Demonstration of shape memory actuation in a 1:2 PETMP-TCDDA thin film. A ca. 1 cm³ cube was programmed to close into 1 cm³ cubic structure from a flat film upon heating. Shape recovery behavior of the material was induced by gentle heating with a heat gun. All black scale bars represent 1 cm.

4. Conclusions

Two color light induced sequential photopolymerizations based on thiol-Michael/ acrylate hybrid network were used as a facile methodology to form polymeric materials with desirable physicochemical behavior. The photobase and photo radical initiator system catalyzes the thiol-Michael and acrylate radical polymerization sequentially and orthogonally, enabling photopatterning and structure reconfiguration at both polymerization stages. In addition to being initiated with lights of different wavelengths, this dual stage photopolymerization also benefits from both the homogeneous step-growth polymerization and highly crosslinked chain growth polymerizations. As such, by tuning the polymer formulation and photocuring conditions, a variety of polymers are generated with desirable material properties. In PETMP–TCDDA networks incorporating the NPPOC-TMG/I2959 initiation system, different NILs were demonstrated not only for simple photopatterning but also creating nanoscale features on the surface. With narrow glass transition temperatures and appropriate mechanical strength, these materials exhibited temperature sensitive shape memory behavior. The shape memory performance together with tunable mechanical properties and reconfigurable second stage polymerization make this dual curing system a prime candidate for the use as SMPs in various practical applications.

Fig. 1.

Chemical structures of monomers and initiators used in the two color light initiated dual stage polymerization.

Highlights:

• Photobase catalyzed thiol-Michael photopolymerization was incorporated with radical mediated acrylate photopolymerization to achieve a multiple wavelength-induced multistage polymerization system.

• Nanoscale features (less than 10 nm) were created on the photopatterned surfaces by nanoimprint lithography via the multistage hybrid system.

• With shape fixity values of 95 % and shape recovery values of 99 %, the dual curing polymers were demonstrated to produce excellent shape memory polymers incorporating simple, three-dimensional structures.