Ultratough Yet Dynamic Crystalline Poly(thiourethane) Network Directly from Low Viscosity Precursors

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES1 Mar 2024Ultratough Yet Dynamic Crystalline Poly(thiourethane) Network Directly from Low Viscosity Precursors Haijun Feng†, Yi Sheng†, Guancong Chen, Binjie Jin, Zizheng Fang, Bo Yang, Xiaorui Zhou, Wenxuan Wu, Tao Xie and Ning Zheng Haijun Feng† State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Yi Sheng† State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Guancong Chen State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Binjie Jin State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Zizheng Fang State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Bo Yang State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Xiaorui Zhou State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Wenxuan Wu State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 , Tao Xie State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 Department of Colorectal Surgery and Oncology, Key Laboratory of Cancer Prevention and Intervention, Ministry of Education, The Second Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou 310027 and Ning Zheng *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310058 https://doi.org/10.31635/ccschem.023.202303402 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Thermosets based on direct curing of multifunctional monomers offer processing flexibility that thermoplastics cannot provide. However, this type of thermoset is typically amorphous since it is difficult to meet the stringent requirement of long-range molecular regularity for crystallization. However, we report here a crystallizable poly(thiourethane) thermoset synthesized directly from the curing of low-viscosity liquid precursors that does not employ any solvent. Its crystalline nature results in superior toughness, comparable to commercial high-density polyethylene, in sharp contrast to the brittleness of the typical, rigid, glassy thermoset materials. Beyond that, the network polymer exhibits shape-memory behavior in which the crystalline transition is utilized for temporary shape fixing/recovery whereas the dynamic thiourethane bonds can be activated for bond exchange as a mechanism for complex shape manipulation. Materials with these combined features represent attractive options for demanding engineering applications due to their high performance and multifunctinality. Download figure Download PowerPoint Introduction Thermosets are a significant part of the materials used in modern society. In comparison to thermosets obtained by post-crosslinking of thermoplastic polymers, thermosets synthesized directly from multifunctional monomer precursors are particularly attractive, given their unique processing flexibility. For simplicity only, thermosets refer to the latter class in the current context. For this type of thermoset, the low viscosity of the precursors makes them easy to use for molding and filling. Most importantly, their material properties are relatively insensitive to the formulation (due to slight stoichiometric imbalance, monomer purity, etc.). This allows the synthesis and processing of thermosets to be conducted in one combined step, in sharp contrast to thermoplastic polymer that typically demands synthesis in highly controlled conditions to ensure adequate molecular weight, which can then be processed afterward. Combined with other advantages such as dimensional and chemical stability, thermosets have been widely used as adhesives,1 coatings,2 structural composites,3 and functional materials such as shape-memory polymers.4 Traditional thermosets are mostly amorphous polymers. Their chemistries include epoxy, polyurethane, amine-formaldehyde, silicone, and vinyl ester resins among others.5 Depending on the specific chemistry used and the crosslinking density, their thermomechanical and mechanical properties can be tuned, but the mechanical toughness, as an exception, is often restricted. Specifically, they are either brittle, rigid polymers with high modulus but low elongation or soft rubbers with high elongations but low modulus. Various strategies have been implemented to enhance the toughness of rubbers with good levels of success.6–9 For rigid thermosets, this task is particularly challenging. One well-known approach is to construct a multiphase structure in the polymer matrix. To achieve this, various additives have been used in the process of thermoset curing, including both organic10–12 and inorganic13–15 fillers. However, these additives inevitably increase the viscosity of the precursors, leading to processing complexity. Another toughening method lies in the delicate design of precursors, which includes synthesizing hyperbranched polymers,16 introducing dangling side chains,17 incorporating molecular interactions (e.g., hydrogen bonding18 and supramolecular interaction19), and constructing interpenetrating network structures.20,21 Despite different levels of success of the above methods, we hypothesize that an alternative attractive approach is to develop a thermoset chemistry that directly yields tough materials via crystallization. Unlike glassy, rigid polymers that are often brittle, crystalline thermoplastics such as high-density polyethylene (HDPE) show superior mechanical performance with high modulus and elongation.22 HDPE's outstanding mechanical properties originate from its crystalline multiphase structure, which can resist deformation and dissipate energy through inter- and inner-lamellar slipping upon mechanical loading. Indeed, crystalline thermosets obtained by post-crosslinking of linear crystalline polymers are known to be tough. However, they come at the expense of losing the much desired processability from low-viscosity precursors. Crystalline thermosets from direct curing of multifunctional monomers are ideal given the processing convenience and eminent mechanical performance. However, the long-range molecular regularity required for crystallization is difficult to achieve with such an approach since the curing process is often accompanied by unpredictable side reactions, and the crosslinking points are statistically random. Nevertheless, a few exceptions have been reported in the last few years.23–27 The crystalline network derived directly from the pyrazole-isocyanate reaction possesses superior mechanical properties,23 while a large amount of chloroform was required in the synthesis, compromising the processing benefits. Indeed, with the assistance of solvents, crystalline structures or other strong intermolecular forces were easily introduced into polymer networks, leading to tough thermosets. However, the use of solvents is not environmentally friendly. More importantly, unwanted voids and defects might appear during the evaporation of solvents, which will place strong restrictions on applications. Using aromatic diallyl-based thiol-ene photochemistry, Bowman's group24,25 successfully synthesized a crystalline thermoset without the use of solvents. Replacing aromatic diallyl with aliphatic diallyl also yielded a crystalline thermoset as demonstrated by Ware's group,26,27 but the maximum elongation was not reported. All the above examples exhibit crystalline melting temperatures around 65 °C and lose their mechanical performance when exposed to elevated temperatures (e.g., in the ground in summer and in hot water) in actual applications. Here, we report a polythiourethane thermoset synthesized directly from commercially available low-viscosity monomers. Thiol-isocyanate click chemistry provides molecular regularity, while the resulting thiourethane bonds offer strong hydrogen bonding, leading to a crystallizable network polymer. The discovery of the crystalline behavior in poly(thiourethane) presented here contrasts with the conventional noncrystalline poly(thiourethane) elastomers reported in the literature.28–30 In its crystalline state, this material shows thermomechanical and mechanical performance (melting temperature: 103 °C; elongation: 459%, toughness: 130 MJ/m3) comparable to and even tougher than commercial HDPE22 (melting temperature: 120 °C; elongation: 325%, toughness: 55 MJ/m3). In addition to the easy processing and superior mechanical performance, the intrinsic reversible nature of dynamic thiourethane bonds allows for materials with shape-memory functions for which both the temporary and permanent shapes can be reconfigured. In this polymer network, the crystalline transition is utilized for temporary shape fixing/recovery, while the dynamic thiourethane bonds can be activated for permanent shape reconfiguration through a covalent bond exchange mechanism.4,31–35 Experimental Methods Materials 3,6-Dioxa-1,8-octanedithiol (DODT; TCI, Shanghai, China, 97%), pentaerythritol tetrakis (3-mercaptopropionate) (PTMP; Sigma Aldrich, Shanghai, China, 95%), hexamethylene diisocyanate (HDI; Aladdin, Shanghai, China, 99%), dibutyltin dilaurate (TCI, 95%), 1,2-propanedithiol (TCI, 95%), isophorone diisocyanate (J&K, Shanghai, China, 95%), diallyl ether (TCI, 98%), and Kevlar fabrics (Dupont, Wilmington, Delaware, USA; 3000D400G). All solvents for solvent resistance tests were obtained from Sinopharm (Shanghai, China). All chemicals were used as received. Synthesis of poly(thiourethane) films HDI, DODT, and PTMP were added into a glass bottle according to Supporting Information Table S1 and mixed evenly. Afterward, 1 wt % dibutyltin dilaurate (DBTDL) was added to the mixture, followed by mixing. The reactant liquid was then put into a vacuum oven of 70 °C for 10 min for degassing. The mixture was then poured into an aluminum plate and cured at 100 °C for half an hour. After curing, the liquid turned into a transparent film. The film was annealed at 70 °C for 48 h and at room temperature (25 °C) for another 48 h. Materials with different chemical compositions and stoichiometric imbalance were synthesized according to Supporting Information Figure S5 and Table S2. Fabrication of the poly(thiourethane) and Kevlar composite The polymer precursor was prepared using the aforementioned method. Kevlar fabrics were placed on an aluminum plate. The precursor was then poured into the aluminum plate slowly to ensure the fabrics were fully soaked by the precursor. Afterward, the composite was taken out and underwent thermal curing and annealing under the same conditions as the above method. X-ray diffraction The X-ray diffraction (XRD) test was carried out by the UltimaIV system (Rigaku, Tokyo, Japan). The crystallinity (Xc) samples was calculated by the following equation: X c = I c I c + I a × 100 % where Ic refers to the area of the crystalline part in the XRD results and Ia refers to the area of the amorphous part. Calculation of crosslinking density The crosslinking (d) was calculated by the following equation: d = E r 3 R T where Er refers to the rubbery state modulus, R refers to the gas constant, and T refers to the absolute testing temperature. It is worth noting that the rubbery state moduli of each sample was tested at 50 °C, as there is almost no dynamic bond exchange at this temperature. PTU 4 to PTU 6 are in the rubbery state under these conditions. For PTU 1 to PTU 3, they were first heated to 120 °C until all crystalline structures melted and then thermally quenched to 25 °C before testing them for their rubbery state modulus, ensuring that no crystalline structures were present during testing. Swelling and solvent resistance test Around 0.1 g of PTU2 was submerged in 10 mL of different solvents for 48 h. The sufficiently swelled samples were taken out and weighed. The swelling ratio (Rs) was calculated by the following equation: R s = W s − W 0 W 0 × 100 % where Ws refers to the weight after swelling, and W0 refers to the original weight. To measure the gel content, the swelled sample was fully dried in the vacuum oven at 70 °C. Gel content (Rg) was calculated by the following equation: R g = W 0 − W d W 0 × 100 % where Wd refers to the weight of the fully dried sample. Estimation of toughness from literature Since the literature does not directly show the toughness of the materials, the toughness was estimated by the following methods. If the materials showed no yielding in the tensile test, and the stress–strain curves were nearly linear, the toughness was calculated by the following equation: Toughness = M × l m 2 / 2 where M refers to the Young's modulus of the materials, and lm refers to the maximum strain of the materials. If the materials showed yielding or nonlinear tensile behavior, 30 data points were read from the tensile curves. And the toughness was calculated by the area under stress–strain curves. Calculation of the activation energy for thermal curing The activation energy is calculated by the Kissinger's equation as follows: ln ( β T p 2 ) = ln ( A R E a ) − E a R T p where β is the heating rate, A is the preexponential factor, R is the Avogadro constant, Tp is the temperature of the exothermic peak, and Ea is the activation energy. Shape-memory behavior measurement To measure the shape fixity and recovery ratio of PTU2, the films were first cut in a rectangle of 20 × 3 × 0.5 mm. Afterward, the films were annealed at 70 °C for a period of time (0.5–6 h) for shape fixing. To recover to the original shape, the files were put into the oven at 130 °C for 5 min. The shape fixity (Rf ) and recovery ratio (Rr) were calculated by the following formulations: R f = l f − l 0 l m − l 0 R r = l f − l r l f − l 0 where l0 refers to the original length of the sample, lm refers to the length under external load, lf refers to the fixed length after retreating the external force, and lr refers to the length after shape recovery at 130 °C. Other analysis methods The polarized microscopic picture was taken by an Eclipse E600W POL (Nikon, Tokyo, Japan). The infrared spectrum was carried out by a Nicolet iS50 (Thermo Fisher, Waltham, Massachusetts, USA) in the reflection mold (attenuated total reflection, ATR). The thermal gravimetric analysis was carried out by a TA-Q500 (TA Instruments, New Castle, Delaware, USA) at a heating rate of 10 °C/min. The viscosity of the reactants and the reacting mixture was measured by a digital viscometer (NDJ-5S, Shanghai, China) produced by Shanghai Hengping Instrument and Meter Factory. The tensile experiments were performed by a Zwick/Roell Z005 (Ulm, Germany) materials testing machine at a strain rate of 10 mm/min. Each sample for the tensile test was cut into a rectangular shape of 12 × 3 × 0.5 mm. The rotational rheological tests were performed on a HAAKE RS6000 rheometer (Vreden, Germany) at a temperature of 120 °C. The relaxation tests were carried out by a DMA Q800 (TA Instruments, New Castle, Delaware, USA) in the "stress relaxation" model with a strain of 20%. The slow crystallization behavior of PTU2 was also monitored by DMA Q800 at a constant temperature. The thermal properties of samples were measured by TA-Q200 (TA Instruments, New Castle, Delaware, USA) at a heating rate of 10 °C/min. Results and Discussion Poly(thiourethane) thermosets were synthesized through the click reaction of three commercial chemicals, that is, DODT, PTMP, and HDI (Figure 1a). We note that all the monomers were low viscous liquids and were sufficiently mixed without any solvents. The crosslinked network was formed under the catalysis of dibutylin dilaurate with a fast curing process at 100 °C for 30 min. Afterward, the polymer was thermally annealed to ensure complete crystallization. In this network, the properties are strongly dependent on the crosslinking densities, which can be easily tuned by the content of the crosslinker (i.e., PTMP). Detailed formulations of crosslinked poly(thiourethane) are listed in Supporting Information Table S1 and named PTU1 to PTU6 with the increase of PTMP. In addition, formulation without crosslinker, named PTU0, was also studied for the linear poly(thiourethane) model. In these formulations, the molar content of the thiol and isocyanate groups was equivalent. The chemical reaction was analyzed by Fourier transform infrared spectroscopy, showing that all formulations were fully reacted with the appearance of thiourethane peak (1635 cm−1) and no residual isocyanate (NCO; 2248 cm−1) or thiol (SH; 2537 cm−1) peaks ( Supporting Information Figure S1a,b). It can be seen from Supporting Information Figure S1b that with the increase of the crosslinker, the peak of the ester bond (1740 cm−1) in the system increases, while the thiourethane bond (1635 cm−1) remains consistent. In other words, the concentration of thiourethane bonds in the system remains nearly identical across PTU1 to PTU6, with the exception of variations in crosslinking density. Further, all the formulas show good thermal stability with degradation temperature above 250 °C ( Supporting Information Figure S2). Figure 1 | The synthesis and crystalline structure of polythiourethane. (a) Illustrator shows the synthesis procedure; (b) photo of the samples with different formulations; (c) the polarized microscopic picture of PTU2; (d) XRD results; (e) DSC curves of PTU1 to PTU6. Download figure Download PowerPoint When adjusting the crosslinking density of the material, we unexpectedly found that this type of material is crystallizable. The pictures of PTU1 to PTU6 are shown in Figure 1b, with their appearance gradually transforming from obscure white to transparent. As a representative white sample, PTU2 shows an obvious birefringence effect under polarized light (Figure 1c). The similarity of the Maltese crosses between PTU2 and the image documented in the polymer spherocrystal literature36,37 suggests that this polymer possesses a spherulite-like crystal structure. The size of the spherocrystals calculated in Figure 1c is around 25 μm. The changes in transparency likely result from the differences in crystallinity, which are further characterized by both XRD and differential scanning calorimeter (DSC). The XRD results show that PTU1, PTU2, and PTU3 have a crystalline diffraction angle of 20.1° (Figure 1d). The degree of crystallinity (Xc) of each sample is calculated from the area of XRD data and summarized in Supporting Information Figure S3. As a reference, the linear sample PTU0 without crosslinking exhibits the same crystalline diffraction angle with the highest Xc of 49.7% ( Supporting Information Figure S4a). In contrast, Xc of PTU1 and PTU2 is around 37.1%, and Xc for PTU3 drops drastically when the crosslinking density increases. In comparison, PTU4, PTU5, and PTU6 are amorphous. The DSC curves show similar results (Figure 1e and Supporting Information Figure S4b). The melting transition temperature of PTU0 is around 106 °C. PTU1 and PTU2 are around 103 °C, while it is situated at 50 °C for PTU3, which results from the relatively small size of the crystalline grain. Further, the glass transition temperature gradually increases from 1 °C to 19 °C in DSC curves, which is attributed to the increase of crosslinking density. The crystalline behavior is likely to come from two sources. First, it is the click nature of thiol-isocyanate chemistry that helps to construct regular polymer chains which are necessary for crystallization. Second, the hydrogen bonding of thiourethane bonds offers the polymer chains stronger interactions that accelerate crystallization. To support our conjecture about the regular structure requirement, two other poly(thiourethane)s synthesized from another symmetrical thiol and another asymmetrical isocyanate are respectively shown in Supporting Information Figure S5. In these two formulations, the content of thiourethane bonds is consistent with PTU2. The results indicate that crystallization similar to PTU2 occurs when the monomer is symmetric, not vice versa. To the requirement of hydrogen bonding, another regular polymer without any hydrogen bonds was synthesized in Supporting Information Figure crystalline transition is in this polymer. As the content of thiourethane bonds remains almost constant from PTU1 to PTU6, the in crystallinity is due to the introduced crosslinking points that the network The crystalline nature can to the of mechanical This is in tensile where the material exhibits during its appearance from obscure white to ( Supporting Information Figure The mechanical properties of the crystallization are due to the of that dissipate energy through during Detailed mechanical properties are summarized in Figure temperature: 25 the in the the modulus and toughness first from and to 10 and PTU1 to and gradually increase to and to We note that the PTU0 sample also exhibits mechanical properties with the modulus and toughness of and respectively ( Supporting Information Figure In addition, PTU1 to PTU6 are soaked in and then taken into a tensile test to the resistance ( Supporting Information Figure The result that crystalline PTU1 to PTU3, have relatively with almost no mechanical after In contrast, the noncrystalline samples in and have a in maximum This suggests that the stability of the material can be through to the nature of the crystalline Figure 2 | The mechanical performance of (a) curves of PTU1 to (b) mechanical upon the of (c) mechanical comparison between PTU2 and other (d) tensile curves of PTU2 and and thermosets. Download figure Download PowerPoint The first in mechanical performance results from the in crystallinity of the while the latter can be attributed to the increase in crosslinking density of It can be seen from Supporting Information Figure that the crosslinking density from moduli in the rubbery increases gradually from PTU1 to PTU6. We that our crystalline polymer exhibits both high modulus and toughness with other as summarized in the (Figure the method of toughness is shown in we PTU2 for further with the commercial polymers, the mechanical of PTU2 to high crystallinity polyethylene with even and toughness (Figure We that the mechanical properties of thermoplastics are highly dependent on their molecular weight and of the In contrast, our system as a thermoset gel content in chloroform is and the swelling in different solvents are shown in Supporting Information Figure does not conditions. even a slight of the reactants from the stoichiometric will not the performance of the As by Supporting Information Table and Figure 2 wt and 4 wt % and not the crystalline Figure 3 | The processing properties of PTU2. (a) Viscosity of the precursors at 25 °C; (b) DSC curves of the PTU2 curing process with different heating from 5 to 20 (c) linear of Kissinger's equation to the curing (d) different shapes by PTU2; (e) composite from PTU2 and Kevlar Download figure Download PowerPoint of the mechanical performance by the crystalline structure does not processing The polymers can be directly synthesized from small without solvents viscosity of each material is shown in Supporting Information Figure Figure that the mixed precursors a low-viscosity of at the which is comparable to Afterward, viscosity gradually due to the slight and into crystalline at the temperature of 25 °C for 6 h. We note that, after the first 2 the viscosity of the precursors remains 100 which a time for heating at a rate of 10 the DSC test in Figure the precursors can be cured into thermosets 20 min. The activation energy of the curing is after by Kissinger's equation (Figure which is similar to the curing of The low and nature of this material it with in including the to complex shapes through molding and polymer Figure shows that the material can be into shapes such as a a and a To the of the with from 0.5 to are The of the structure is by the of the mold and when a mold with is the of or is this polymer can be into Kevlar (Figure The poly(thiourethane) thermoset offers Kevlar fabrics with and In addition to the of thermoset curing, the intrinsic dynamic thiourethane bonds other processing The dynamic nature of thiourethane bonds was through relaxation experiments as shown in Figure °C, all external 2 h. The relaxation rate with the following the equation with an activation energy of to the dynamic bond all the crosslinked samples similar moduli at the high temperature ( Supporting Information Figure This dynamic bond exchange results in the of poly(thiourethane) after the network is different complex shapes such as a and a can be obtained by after by external and thermal annealing at °C for 1 h (Figure Figure 4 | The dynamic nature and shape-memory behavior of PTU2. (a) relaxation curves of PTU2 at the temperature from 130 to °C with a strain of (b) of the permanent shape of PTU2 through bond exchange at °C. 1 (c) modulus of melted PTU2

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