Swelling and mechanical properties of thermoresponsive/hydrophilic conetworks with crosslinked domain structures prepared from various triblock precursors

Shohei Ida *, Miki Morimura , Hironobu Kitanaka , Yoshitsugu Hirokawa and Shokyoku Kanaoka *
Department of Materials Science, The University of Shiga Prefecture, 2500 Hassaka, Hikone, Shiga 522-8533, Japan. E-mail: ida.s@mat.usp.ac.jp; kanaoka.s@mat.usp.ac.jp

Received 20th September 2019 , Accepted 14th October 2019

First published on 15th October 2019


A designed amphiphilic conetwork (APCN) having thermoresponsive polymer chains is attractive for the development of novel stimuli-responsive materials with a controlled responsive behavior. We have recently proposed a novel APCN having crosslinked domain (CD) structures prepared by the post-polymerization crosslinking of controlled triblock precursor polymers with reactive sites in the outer blocks. In the current study, we evaluated the effects of the structures of the triblock precursors including the sequence, molecular weight, and composition on the gelation reaction and the swelling properties of the obtained gels in detail. The gelation reaction and the volume at the swelling state at a low temperature were strongly affected by the molecular weight of the middle block of a precursor, whereas the temperature and the sharpness of the response were controlled by the composition of a precursor. Interestingly, the gel consisting of thermoresponsive CDs and hydrophilic bridging chains had improved elastic modulus and elongation ability upon heating in air without external water, probably because water flowed between the thermoresponsive CDs and the domains of the hydrophilic bridging chains in response to temperature change.


Introduction

Thermoresponsive polymer hydrogels, which reversibly swell and shrink in water in response to temperature change, are hopeful materials for various applications such as actuators,1 drug delivery systems,2 and biomedical devices.3,4 For example, a poly(N-isopropylacrylamide) (PNIPAAm) gel, one of the most representative thermoresponsive gels,5–7 has been studied extensively for its applications. To further broaden the availability of thermoresponsive gels, it is imperative to develop various types of gels with differing response temperatures. A direct method would be to design and synthesize a new monomer with a suitable hydrophilic/hydrophobic balance for a thermoresponsive polymer with a desired response temperature; however direct methods involve individual monomer syntheses, which often require bothersome synthesis procedures.

A more facile way of producing a series of gels with varying properties is gel synthesis using two different components. A typical example is a gel composed of two types of polymers with clearly different properties, or an amphiphilic conetwork (APCN),8,9 which expresses interesting properties reflecting the properties of each constituent. For example, a gel consisting of a hydrophilic polymer and a hydrophobic polymer is swellable both in water and in organic solvents.10–20 In addition, the unique structure and properties of APCNs have been recently attracting attention from application viewpoints.21–26 For example, a contact lens made from hydrophilic polymers combined with hydrophobic silicone polymers has high water retainability derived from hydrophilic polymers and high oxygen permeability from hydrophobic components.27,28

The incorporation of a stimuli-responsive polymer as a component of APCN is attractive for the design of unique responsive properties different from gels prepared from a single monomer. For example, the introduction of hydrophilic chains into a thermoresponsive network accelerates the volume change in water,29–33 much faster than that with a simple PNIPAAm gel at the macro-scale. Sakai et al. reported a “non-swellable” gel, which keeps the original volume at the preparation state even when immersed in water, by connecting hydrophilic polymers and thermoresponsive polymers with an appropriate response temperature.34 Marcellan et al. constructed a grafting structure in an APCN of a hydrophilic polymer and a thermoresponsive polymer, and this gel showed a thermoresponsive toughening behavior with keeping the original volume.35,36 These examples clearly indicate the usefulness of the construction of APCN structures for the realization of unique responsive behaviors.

Separate hydrophilic and thermoresponsive domains in an APCN, if evenly dispersed, would allow reversible water flow just inside a gel. In general, the presence of external water is required for a thermoresponsive gel because the volume change of a gel is induced by transfer of water between the inside and outside of a gel. This external water often causes the gels to swell more than necessary, reducing the mechanical strength of the gels. Thus, the development of a thermoresponsive gel system without external water is critical to achieving its practical use in air. In order to enhance the dual functions of constituent polymer chains in APCN, clear separation and relatively even dispersion of the two domains are desirable, which may be achieved by an appropriate design of the network structure concerning not only the choice of the constituent chains but also the spatial arrangement of the chains in the network.

From this point of view, we have recently proposed the design of an APCN having a crosslinked domain (CD) structure by the incorporation of the crosslinking points into one part of the two component chains (Fig. 1a).37 This structure consists of the CDs dispersed in the network and the polymer chains bridging the CDs. In this network, the two kinds of component chains are distinctly compartmentalized. Such a hierarchical structure including a microgel-like crosslinked structure in a macro network would contribute to the significant expression of the dual functions of the constituent chains such as the characteristic responsive behavior. This new class of designed gels can be synthesized by post-polymerization crosslinking of ABA triblock precursor polymers embedding reactive sites only in the outer blocks. The gel obtained from the precursors with hydrophilic and thermoresponsive blocks showed a larger and sharper volume change against temperature change compared to the gel with a random sequence of monomers and crosslinkers in the same composition. In addition, a very fast volume change was observed.


image file: c9py01417a-f1.tif
Fig. 1 (a) Schematic representation of APCNs with functional CDs prepared by post-polymerization crosslinking of controlled triblock precursors, and (b) thermoresponsive change of the internal structure without macroscopic volume change of the designed APCN with the thermoresponsive CD structure in the absence of external water.

The properties of such a designed gel are considered to depend on the structure of the triblock precursor polymers. Therefore, this study focuses on the effects of the structural factors of the triblock precursors including the composition, block sequence, and molecular weight on the properties of the obtained APCN, which leads to the establishment of the design criteria of novel functional gel materials. Triblock precursors having a variety of molecular weights and compositions in the inverse orders, hydrophilic/thermoresponsive/hydrophilic and thermoresponsive/hydrophilic/thermoresponsive, were prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization,38–40 and were employed for gel synthesis by post-polymerization crosslinking utilizing activated ester chemistry.41 The gelation behavior and the thermoresponsive swelling behavior in water were examined to discuss the correlation between the precursor structure and gel properties. Furthermore, a gel with thermoresponsive CDs is expected to express a unique responsive behavior in the absence of external water (Fig. 1b). In such a gel, the CDs can swell and shrink inside of a network with the temperature change even in the absence of external water, since water would flow in and out between the CDs and hydrophilic bridging chains, which leads to a change of internal structures without volume change at the macro scale. Thus, we also investigated the temperature dependence of mechanical properties of the APCN having CD structures without external water.

Experimental

Materials

NIPAAm (Wako, 98%) was purified by recrystallization from toluene/n-hexane. N-(Acryloyloxy)succinimide (NHSA)42 and a chain transfer agent for RAFT polymerization carrying two trithiocarbonate groups in the molecule (CTA-1)43 were prepared as reported in the literature. N,N-Dimethylacrylamide (DMAAm; Wako, 98%), ethylenediamine (EDA; Wako, 98%), azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70; Wako, 95%), 1,2,3,4-tetrahydronaphthalene (tetralin; Aldrich, 99%), N,N-dimethylformamide (DMF; Wako, 99.5%), 1,4-dioxane (Wako, 99%), and tetrahydrofuran (THF; Wako, 99.5%) were used as received.

Synthesis of triblock precursor polymers

Triblock precursor polymers were prepared by sequential RAFT block polymerization, followed by end-modification with a radical initiator. As a typical example, the synthesis of NDN1 is given below. DMAAm (4.12 mL, 40.0 mmol), CTA-1 (163 mg, 0.400 mmol), AIBN (6.7 mg, 0.040 mmol), tetralin (1.0 mL), and 1,4-dioxane (14.9 mL) were added to a 50 mL round-bottomed flask equipped with a three-way stopcock, and bubbled with nitrogen for 10 minutes. The flask was placed in an oil bath kept at 60 °C for 24 h. The reaction was terminated by cooling the reaction mixture to −60 °C. Monomer conversion was determined from the concentration of the residual monomer measured by 1H NMR as a reference of the internal standard (tetralin). Then, the reaction mixture was poured into diethyl ether to obtain the purified PDMAAm (D1; 2.53 g).

Then, the obtained PDMAAm was employed as a macro-CTA for block polymerization. The PDMAAm macro-CTA (DPn = 101 and Mn = 10[thin space (1/6-em)]400, which were determined by 1H NMR analysis; 2.09 g, 0.200 mmol), NIPAAm (2.26 g, 20.0 mmol), NHSA (338 mg, 2.00 mmol), AIBN (3.3 mg, 0.020 mmol), tetralin (0.5 mL), and 1,4-dioxane (9.5 mL) were added to a 50 mL round-bottomed flask equipped with a three-way stopcock, and bubbled with nitrogen for 10 minutes. The flask was placed in an oil bath kept at 60 °C for 42 h. The reaction was terminated by cooling the reaction mixture to −60 °C. Monomer conversion was determined from the concentration of the residual monomer measured by 1H NMR. Then, the reaction mixture was poured into diethyl ether to obtain the purified triblock polymer, P[(NIPAAm/NHSA)-b-DMAAm-b-(NIPAAm/NHSA)] (3.57 g). The monomer composition of the polymer was determined by 1H NMR analysis for NIPAAm and the calculation from monomer conversion for DMAAm and NHSA because of the overlapping of the signals in the 1H NMR spectrum.

Finally, the trithiocarbonate end-groups were deactivated by the radical coupling reaction with V-70. The triblock polymer (DPn, NIPAAm = 100, DPn, DMAAm = 101, DPn, NHSA = 10, Mn = 23[thin space (1/6-em)]500; 1.76 g, 0.0750 mmol), V-70 (0.93 g, 3.0 mmol), and 1,4-dioxane (15.0 mL) were added to a 50 mL round-bottomed flask equipped with a three-way stopcock. The solution was stirred at 40 °C under nitrogen for 24 h. Then, the solution was concentrated by evaporation and was poured into diethyl ether to obtain the purified NDN1 (1.61 g).

Gel synthesis by post-polymerization crosslinking

As a typical example, the synthesis of NCDD1 gel is given below. NDN1 (DPn, NIPAAm = 100, DPn, DMAAm = 101, DPn, NHSA = 10; Mn = 23[thin space (1/6-em)]400; 326 mg, 0.014 mmol) was dissolved in 0.90 mL of THF. To this solution, 0.10 mL of THF solution of EDA (containing 0.070 mmol of EDA; amino groups were set equimolar to NHSA units in the precursors) was added, and glass capillaries (internal diameter: 1300 μm, volume: 40 μL) were added to the reaction vessel. The mixture was kept at room temperature for 24 h to complete the crosslinking reaction. Then, the cylindrical gels were taken out from the capillaries and washed twice with THF for 24 h and three times with distilled water for 24 h by means of immersion to remove the residual molecules and replace the solvent in the gel.

Characterization

M n and Mw/Mn of polymers were determined by size-exclusion chromatography (SEC) in DMF containing 10 mM LiBr as the eluent at 40 °C using three polystyrene gel columns (Shodex KF-805L) that were connected to a Shimadzu LC-10AD precision pump and a Shimadzu RID-10A refractive index detector. The columns were calibrated against standard poly(methyl methacrylate) samples (Agilent). 1H NMR spectra were recorded on a JEOL JNM-LA400 spectrometer, operating at 399.65 MHz. The swelling degree of gels was determined by measuring the diameter of the cylindrical gels. The gels were immersed in water at a predetermined temperature, and the equilibrium diameter at a given temperature, d, was measured using a digital microscope (MOTICAM2000, Shimadzu). The swelling degree was calculated by using (d/d0)3; d0 is the internal diameter of the glass capillary (1300 μm), which could be regarded as the diameter of the as-prepared gel. The uniaxial tensile test was conducted with Shimadzu EZ-SX using the rectangular specimens with the dimensions of ca. 1 × 10 × 20 mm. The cross-head speed was 5.0 mm min−1. The sample at the heating state was prepared by immersing the gels in water at 50 °C to the equilibrium state, and was quickly employed for the test. The sample at the cooling state was prepared by leaving the gel at the heating state in the vessels at room temperature under nitrogen for 24 h.

Results and discussion

Synthesis of triblock precursor polymers

Triblock precursor polymers for gel synthesis were prepared by RAFT block polymerization of the hydrophilic monomer, DMAAm, and thermoresponsive monomer, NIPAAm (Scheme 1). The triblock polymers synthesized in this paper have two types of block sequences: a hydrophilic/thermoresponsive/hydrophilic sequence, which is denoted by “DND” using the initial letter of the monomers, and a thermoresponsive/hydrophilic/thermoresponsive counterpart, denoted by “NDN”. For each sequence, precursors with various molecular weights and compositions were prepared.
image file: c9py01417a-s1.tif
Scheme 1 Synthesis of the triblock precursor, NDN, by RAFT block polymerization and subsequent end-modification by the radical coupling reaction with V-70.

First, the polymerization of NIPAAm and DMAAm was conducted using the bifunctional RAFT agent, CTA-1, respectively. With each of PNIPAAm and PDMAAm, three polymer samples with different molecular weights were synthesized by changing the feed ratio of the monomer and CTA-1 (PNIPAAm: N1–N3, PDMAAm: D1–D3; Table 1). All polymerizations proceeded to a high monomer conversion (∼90%), yielding polymers with a narrow molecular weight distribution (Fig. 2). Moreover, the signals derived from the RAFT agent were observed in each 1H NMR spectra (Fig. S1 and S2). The molecular weights were calculated from the integral ratio of these peaks and the peaks attributed to the monomeric unit. The observed molecular weights were close to the values calculated from the feed concentration and conversion, indicating that polymers with a defined structure were obtained.


image file: c9py01417a-f2.tif
Fig. 2 SEC curves of triblock precursors obtained by RAFT polymerization (synthesis conditions: see Tables 1 and 2).
Table 1 Synthesis of macro-CTA (1st block) by RAFT polymerization
Code Monomer [Monomer]/[CTA-1] Time [h] Conversion [%] DPn[thin space (1/6-em)]a M n[thin space (1/6-em)]a M w/Mn[thin space (1/6-em)]b
a Determined by 1H NMR analysis. b Determined by SEC analysis. c Polymerization condition: [NIPAAm] = 2000 mM, [CTA-1]/[AIBN] = 10 in 1,4-dioxane at 60 °C. d Polymerization condition: [DMAAm] = 2000 mM, [CTA-1]/[AIBN] = 10 in 1,4-dioxane at 60 °C. e Polymerization condition: [DMAAm] = 3000 mM, [CTA-1]/[AIBN] = 10 in 1,4-dioxane at 60 °C.
N1 NIPAAm 50 21 92 49 5900 1.16
N2 100 24 89 88 10[thin space (1/6-em)]400 1.15
N3 200 24 90 185 21[thin space (1/6-em)]400 1.14
D1 DMAAm 100 48 89 101 10[thin space (1/6-em)]400 1.12
D2 200 48 92 200 20[thin space (1/6-em)]200 1.25
D3 300 48 93 277 27[thin space (1/6-em)]800 1.23


Then, the obtained polymers (N1–N3 and D1–D3) were employed for RAFT block copolymerization as a macro-CTA. DMAAm was polymerized with a PNIPAAm macro-CTA (N1–N3) and NIPAAm was polymerized with a PDMAAm-based CTA (D1–D3) to obtain triblock polymers in conjunction with a small amount of a crosslinkable monomer, NHSA, embedding an activated ester group as a comonomer. After the second polymerization, the trithiocarbonate end groups were deactivated by a radical coupling reaction with an azo initiator, V-70. We synthesized eight kinds of polymers having different block sequences, molecular weights, and compositions by changing the feed ratios (Table 2). The GPC curves of all the polymers shifted to higher molecular weight regions from those of macro-CTAs with molecular weight distributions maintained monodisperse and narrow (Fig. 2). In addition, 1H NMR spectra of the obtained polymers exhibited the peaks attributable to the constituent monomers (Fig. S3 and S4). These results indicated the successful synthesis of the triblock precursors. Fig. 3 shows schematic structures of the synthesized polymers. The four types of the polymers were DND polymers having hydrophilic/thermoresponsive/hydrophilic sequences (DND1–DND4), and the others were NDN polymers with thermoresponsive/hydrophilic/thermoresponsive sequences (NDN1–NDN4). Among them, DND1, DND3, DND4, NDN1 and NDN3 had almost the same composition (NIPAAm[thin space (1/6-em)]:[thin space (1/6-em)]DMAAm = 1[thin space (1/6-em)]:[thin space (1/6-em)]1). DND3 and NDN1, and DND4 and NDN3 had almost the same molecular weight but different block sequences, respectively. Furthermore, DND2 and DND3, and NDN2 and NDN3 had the middle block of the same molecular weight but the outer blocks of different molecular weights, respectively. NDN3 and NDN4 possessed the same total number of monomeric units but different compositions.


image file: c9py01417a-f3.tif
Fig. 3 Schematic illustrations of triblock precursors synthesized in this study.
Table 2 Triblock precursor polymers synthesized by RAFT block polymerizationa
Code Macro-CTA Time [h] Conversionb [%] DPn[thin space (1/6-em)]c M n[thin space (1/6-em)]c M w/Mn[thin space (1/6-em)]d
NIPAAm DMAAm NHSA
a All the polymerizations were conducted in 1,4-dioxane at 60 °C. End-deactivation was conducted with the condition: [polymer] = 5.0 mM, [V-70] = 200 mM in 1,4-dioxane at 40 °C for 24 h. b Conversion of NIPAAm or DMAAm. NHSA was consumed quantitatively. c Determined by 1H NMR analysis and calculations from feed concentration and monomer conversion. d Determined by SEC analysis. e Polymerization condition: [DMAAm] = 1000 mM, [NHSA] = 100 mM, [macro-CTA] = 20 mM, [AIBN] = 4.0 mM. f Polymerization condition: [DMAAm] = 1000 mM, [NHSA] = 200 mM, [macro-CTA] = 20 mM, [AIBN] = 2.0 mM. g Polymerization condition: [monomer] = 2000 mM, [NHSA] = 200 mM, [macro-CTA] = 20 mM, [AIBN] = 2.0 mM. h Polymerization condition: [monomer] = 2000 mM, [NHSA] = 200 mM, [macro-CTA] = 10 mM, [AIBN] = 1.0 mM. i Polymerization condition: [DMAAm] = 1000 mM, [NHSA] = 200 mM, [macro-CTA] = 10 mM, [AIBN] = 1.0 mM.
DND1 N1 24 92 48 46 5 11[thin space (1/6-em)]200 1.24
DND2 N2 48 89 87 44 10 16[thin space (1/6-em)]300 1.32
DND3 N2 48 91 88 91 10 21[thin space (1/6-em)]100 1.32
DND4 N3 48 87 181 175 20 41[thin space (1/6-em)]600 1.47
NDN1 D1 42 89 100 101 10 23[thin space (1/6-em)]400 1.20
NDN2 D2 24 88 99 200 10 33[thin space (1/6-em)]100 1.25
NDN3 D2 48 86 200 200 20 44[thin space (1/6-em)]500 1.31
NDN4 D3 45 87 83 279 20 40[thin space (1/6-em)]800 1.33


Gel synthesis

The gel syntheses by the reaction of the obtained triblock precursors with EDA as a crosslinker were examined. At all the reactions, EDA was employed in the equimolar concentration of amino groups to NHSA units, and THF and DMF were used as solvents because they are good solvents for both PNIPAAm and PDMAAm. The results are summarized in Table 3.
Table 3 Synthesis of gels by crosslinking of triblock precursors
Entry Precursor Monomer concentration [mM] EDA concentration [mM] Solvent Result Gel code
1 DND1 2800 75 THF No gelation
2 4000 106 THF Gelation DCDN1
3 DND2 2100 80 THF Gelation DCDN2
4 DND3 2800 78 THF Gelation DCDN3T
5 78 DMF Gelation DCDN3D
6 DND4 2800 79 DMF Gelation DCDN4
7 NDN1 2800 70 THF Gelation NCDD1
8 70 DMF No gelation
9 NDN2 2800 47 THF No gelation
10 4200 70 THF Gelation NCDD2
11 NDN3 2800 70 THF Gelation NCDD3
12 NDN4 2800 76 THF Gelation NCDD4


Among the DND polymers with the same composition, the polymers with a higher molecular weight (DND3 and DND4) gave gels at 2800 mM of the monomer unit concentration, while no gelation proceeded when DND1 with a lower molecular weight was used at the same monomer unit concentration (entries 1 and 4–6 in Table 3). DND1 afforded a gel at as high as 4000 mM of the monomer unit (entry 2), but the obtained gel was very soft, indicating that the crosslinking efficiency was not high and the network was difficult to expand to the whole system due to the low molecular weight of the precursor. By contrast, DND2, which had the same middle block as DND3, gelled the whole system under the constant concentration condition of the NIPAAm unit (ca. 1400 mM for NIPAAm unit: entry 3). Since DND1 and DND2 possessed the outer PDMAAm block with nearly the same molecular weight, these results indicated the importance of the molecular weight of the middle block for efficient gelation.

Interestingly, NDN1, which had the same molecular weight but a different sequence with DND3, induced gelation in THF, but not in DMF (entries 7 and 8), although DND3 gave gels in both solvents (entries 4 and 5). This solvent dependence would result from the chain conformation of the triblock polymers in the solvent. We have recently reported that the gelation behavior of the end-crosslinking of telechelic PNIPAAm was strongly influenced by the reaction solvent and the reaction efficiency in DMF was not so high probably because a PNIPAAm chain was not in the spreading conformation in DMF.44 A similar phenomenon occurred in the triblock polymer system, particularly in NDN polymers having reactive sites in the outer PNIPAAm blocks. This might be related to the aggregation behavior and cluster formation during the crosslinking reaction, similar to the phase separation structure, hence affecting the swelling behavior, which is discussed later.

Furthermore, NDN3 with the same composition and the higher molecular weight than NDN1 produced a gel, whereas NDN2 having a longer middle block did not undergo gelation under 2800 mM of the monomer unit concentration (entries 9 and 11). These reaction conditions were set at the same concentration of total monomeric units, and therefore the concentration of the NHSA unit was relatively low in entry 9. Then, the higher concentration condition was also examined (entry 10). Gelation occurred under this condition where the total concentration of the monomer unit was 4200 mM and the concentration of the NIPAAm unit was equal to the condition using NDN1 (entry 7), but the obtained gel was very soft, indicative of the low crosslinking efficiency. Such gelation behavior was also likely attributed to the solvent dependence of the chain conformation. NDN3 and NDN4 with the larger overall molecular weight gave stable gels (entries 11 and 12).

Swelling behavior in water

The temperature dependence of the swelling degree in water was measured for the obtained gels. Here, a DND polymer gives a gel with a hydrophilic PDMAAm CD structure bridged by PNIPAAm chains: this type of gel is denoted as “DCDN”, and a NDN polymer yields a gel with PNIPAAm CD bridged by PDMAAm: this type of gel is denoted as “NCDD”. Sample codes of the obtained gels are listed in Table 3. Fig. 4 shows the results of the gels with the same composition ratio of NIPAAm and DMAAm units. The gels swelled to a high extent at a low temperature, and started shrinking largely and sharply at around 30–40 °C. This behavior was significantly different from the gels with the same composition but random monomer sequence as we have recently reported.37 The large swelling degree stemmed from the concentrated crosslinking points which reduced the apparent crosslinking density at the macro scale, and the effect of the domain formed by thermoresponsive polymer chains effectively expressed to induce a large and sharp volume change.
image file: c9py01417a-f4.tif
Fig. 4 Temperature dependence of the swelling degree of the gels synthesized from triblock precursors with the same composition of NIPAAm and DMAAm: (a) DCDN gels and (b) NCDD gels.

Fig. 4a and b show the swelling curves of the gels obtained from DND and NDN precursors with the same composition, respectively. DCDN1, which was prepared at a high concentration condition (4000 mM) as aforementioned, exhibited a larger swelling degree than the others because of the high concentration during the preparation and insufficient crosslinking. However, the response temperature was close to that of the gels from the other DND polymers probably due to the same composition. Moreover, DCDN3 and DCDN4 gels showed similar shrinkage behavior although the molecular weights of the precursors differ by nearly double. The same tendency was observed in NCDD gels; NCDD1 and NCDD3 showed similar behavior. In detail, the gel obtained from the precursor with a large molecular weight gave a sharper shrinkage at a lower temperature, compared to the gel from the precursor with a lower molecular weight for each sequence. It is considered that the effect of hydrophilic domains on PNIPAAm domains became relatively weak and the property of PNIPAAm domains expressed more effectively as the molecular weight of the precursors increased.

The effect of the preparation solvent can be estimated from the results of DCDN3T and DCDN3D. The gel synthesized in THF (DCDN3T) swelled much more than the gel synthesized in DMF (DCDN3D), while the shrinking temperature did not change. This suggested that the crosslinking reaction and the average number of polymer chains consisting of CDs was affected by the solvent. As discussed in the section of gelation behavior, PNIPAAm chains were supposed to be relatively contracted in DMF. Therefore, it would cause the aggregated structure of PNIPAAm during the crosslinking and hence densely packed CDs were formed from more number of chains, resulting in a lower swelling degree of the obtained gel.

Then, the effect of the composition on the swelling behavior is discussed. Fig. 5 shows the swelling behaviors of the gels with different compositions for both DND and NDN sequences. DCDN3T and DCDN2, which were obtained from the precursors with the same molecular weight of the middle PNIPAAm block and different molecular weights of the outer PDMAAm blocks, showed almost the same swelling degree at a low temperature. It can be understood that the molecular weight of the middle block of the precursor strongly influenced not only the gelation behavior but also the volume at the swelling state. On the other hand, when increasing temperature, DCDN2 with a higher content of NIPAAm units shrunk more largely and sharply than DCDN3T.


image file: c9py01417a-f5.tif
Fig. 5 Effect of the composition on the temperature dependence of the swelling degree of the gels synthesized from triblock precursors: (a) DCDN gels and (b) NCDD gels.

Comparing the behavior of NCDD3 and NCDD4, which were prepared from the precursors with the same total number of monomer units but different compositions (NDN3 and NDN4), the swelling degree at a low temperature increased with a higher composition of DMAAm as a middle block. It also indicated that the length of the middle block determined the swelling degree, which corresponded to the distance between the CDs. Moreover, NCDD4 with a higher DMAAm content kept a high swelling degree even in the shrunken state and the shrinking behavior was relatively gentle compared to NCDD3 gel. Nevertheless, the volume was about 20% of that of the maximum swelling state, indicating that the thermoresponsive crosslinked domains contributed to the macroscopic volume change effectively.

Thermoresponsive mechanical behavior

Formation of the CD structure is considered to realize a unique thermoresponsiveness not only in the swelling properties but also in the mechanical properties. Particularly, a NCDD gel, in which the CDs are expected to swell and shrink against temperature change, potentially changes its mechanical properties without macroscopic volume change by the water flowing between the CDs and the hydrophilic bridging chains inside the network (Fig. 1b). In order to explore such responsiveness, a NCDD gel immediately after swelling to the equilibrium in water at 50 °C (heating state), and a gel cooling to room temperature in the air without external water (cooling state) were prepared for uniaxial tensile tests (Fig. 6).
image file: c9py01417a-f6.tif
Fig. 6 Stress–strain curves of (a) NCDD3 and (b) NCDD4 gels just after heating to 50 °C in water (heating state) and cooling to room temperature in the air without external water (cooling state).

Although both NCDD3 and NCDD4 gels showed little change in weight and volume between the heating and cooling states, the elastic modulus at the heating state was higher than that at the cooling state for both gels. Furthermore, the heating state of NCDD3 with a high composition of the thermoresponsive unit showed a higher elongation and enhanced toughness compared to the cooling state. In NCDD gels, the thermoresponsive CDs shrunk when heated, which led to simultaneous physical crosslinking by the aggregation of thermoresponsive chemically crosslinked domains. The physical crosslinking of the CDs is likely to result in a higher elastic modulus. Consequently, multiple hydrophilic bridging chains were bound to the aggregated and contracted CDs. When a stress was applied to the network, such multiple chains remained to restrain from breaking at the macro scale even if a break in network chains started to occur in the network. Furthermore, the aggregated CDs also played a key role as a filler in the formation of a hard particle structure.35,36 These effects are considered to improve the toughness of the NCDD3 gel, but are difficult to emphasize in NCDD4 with a lower composition of thermoresponsive units, affording almost no change in breaking elongation. The designed gels in this study exhibited relatively low elastic modulus (10–40 kPa) and elongation ability (<40%) because the synthesis system required solvent substitution into water from the reaction solvent (THF). This process led to the swelling state with a low strength even at high temperature. In order to improve the strength of the designed gels, the development of a synthesis strategy is critical, which we are currently investigating.

Conclusions

In this study, we evaluated the effects of the structures of the triblock precursor polymers on the gelation reaction and the swelling properties of the gels with CD structures prepared by post-polymerization crosslinking of the triblock precursors having the reactive sites in the outer blocks. In addition, thermoresponsive mechanical properties in air without external water were explored. We prepared two kinds of networks, DCDN and NCDD gels, with a variety of precursors having different molecular weights, compositions, and sequences. The gelation reaction and the swelling degree at the swelling state at low temperature were strongly affected by the molecular weight of the middle block of the precursors, and the temperature and the sharpness of the response were controlled by the composition of the precursors. Interestingly, the NCDD gel improved its elastic modulus and elongation ability upon heating without external water probably because water flowed between thermoresponsive CDs and the domains of the hydrophilic bridging chains in response to temperature change. This unique behavior would contribute to the design of novel thermoresponsive gel materials functioning in an isochoric manner.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was partially supported by the Japan Society for the Promotion of Science through a Grant-in-aid for Young Scientists (B) (No. 16K17962) and a Grant-in-aid for Scientific Research (C) (No. 19K05602) and (A) (No. 19H00739), for which the authors are grateful.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9py01417a

This journal is © The Royal Society of Chemistry 2019