Influence of Spacer Length between Actuator and Sensor on Their

Dec 11, 2012 - JAPAN. •S Supporting Information. ABSTRACT: In living bodies, specific ... The mutual communications were found to have quite differe...
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Influence of Spacer Length between Actuator and Sensor on Their Mutual Communications in Poly(N‑Isopropylacrylamide-co-βCyclodextrin), an Autonomous Coordinative Shrinking/Swelling Polymer Hidenori Ohashi, Tomoaki Abe, Takanori Tamaki, and Takeo Yamaguchi* Chemical Resources Laboratory, Tokyo Institute of Technology, R1-17, 4259, Nagatsuta-cho, Midori-ku, Yokohama, 226-8503, JAPAN S Supporting Information *

ABSTRACT: In living bodies, specific molecular recognition triggers various kinds of autonomous behaviors. To artificially reconstruct one such distinct phenomenon, an autonomous shrinking and subsequent reswelling of poly(N-isopropylacrylamide) (pNIPAM) having a β-cyclodextrin (CD) pendant group was induced by a specific signal of a guest molecule for CD, 8anilino-1-naphthalenesulfonic acid. The autonomous phenomenon is realized as a consequence of mutual influences between the pNIPAM main chain as actuator and the CD moiety as sensor. The mutual communications were found to have quite different dependence on the length of the spacer connecting pNIPAM and CD; a long spacer hinders the communication from CD to pNIPAM, whereas a short spacer does not. However, the reverse communication from pNIPAM to CD is maintained even with long spacers, possibly because of the pNIPAM main chain spatially close to the CD moiety. This knowledge is useful for the design of highly functional polymeric materials conjugating sensors and actuators.

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INTRODUCTION In living bodies, many complex and attractive phenomena can be realized by coordinating distinct processes of molecular recognition, structural change, chemical reaction, and mass and electron transfer. Many researchers have sought to reconstruct some of the extraordinary biological functions using artificial materials; for example, polymer gels shrinkable in response to specific signals, including temperature,1,2 pH,3,4 electric field,5,6 light,7,8 and chemical signals,9−12 have been investigated. Further effort has even led to the realization of nonlinear oscillation, which is analogous to functions of living bodies, by combining the Belousov−Zhabotinsky reaction,13,14 laser irradiation,15 or an electric field;16 furthermore, a polymer exhibiting coordinative autonomous behavior induced by molecular recognition has been developed.17 We developed a smart polymer exhibiting autonomous behavior by molecular recognition by coupling a sensor moiety of β-cyclodextrin (CD), which can recognize a specific molecule, and an actuation moiety of poly(N-isopropylacrylamide) (pNIPAM), which activates upon recognition (Scheme 1a).17 pNIPAM is a well-known thermosensitive polymer that undergoes abrupt volume change at a certain temperature, called the crowd point, and the crowd point shifts toward higher/lower temperature in the presence of a hydrophilic/ hydrophobic comonomer.18−22 CD is a cyclic oligosaccharide with a cavity inside, which can include a variety of guest © 2012 American Chemical Society

Scheme 1. (a) Schematic Representation of Polymer Exhibiting Nonlinear Behavior Induced by Molecular Recognition. (b) CD/ANS Complexation and Hydrophobically Protruding Phenyl Ring of ANS

molecules; in this study, 8-anilino-1-naphthalenesulfonic acid (ANS) was selected as the guest molecule. When ANS forms a complex with CD, a hydrophobic phenyl ring of ANS protrudes from the CD cavity (Scheme 1b). Received: September 5, 2012 Revised: November 18, 2012 Published: December 11, 2012 9742

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It was expected that it would be different from the response to the temperature shooting investigated in a previous study,17 in that a different stimulus is imposed for triggering the autonomous phenomenon. Therefore, the first part of the present study is devoted to investigating the response of poly(NIPAM-co-CD) to ANS concentration shooting. The influence of spacer length on the phase transition behavior, and the response to the ANS concentration shooting and the temperature shooting are then observed and discussed.

The copolymer of pNIPAM and CD is not a mere copolymer of the two components; rather, the components have mutual influences on each other. One influence that CD has on pNIPAM is as follows: CD with ANS has a hydrophobic nature due to the protruding phenyl ring of ANS compared with CD without ANS, and thus complexation of ANS with CD lowers the crowd point of pNIPAM. The influence that pNIPAM has on CD is that, when pNIPAM shrinks, the protruding ANS suffers from steric hindrance of shrunken polymer.23 As a consequence, the complexation constant between ANS and CD is lowered,24 and ANS detaches from CD. As a result of these mutual influences, nonlinear autonomous behavior is induced (Scheme 2). Now, suppose the copolymer

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EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (NIPAM) monomer was kindly provided by Kohjin Co., Ltd. (Tokyo, Japan) and was used after purification by recrystallization from hexane and acetone to remove inhibitors. The following chemicals were purchased from Wako Pure Chemicals (Osaka, Japan), and used as received: p-nitrophenol (>99%), potassium hydroxide (>85%), acryloyl chloride (>98%), βcyclodextrin (>97%), p-toluenesulfonyl chloride (>97%), sodium hydroxide (>96%), lithium chloride (>96%), hydrochloric acid (35− 37%), ethylenediamine (>99%), and 2,2′-azobis(isobutylonitrile) (>98%). 2,2′-(Ethylenedioxy)bis(ethylamine) (>98%) and diethylene glycol bis(3-aminopropyl) ether (>98%) were purchased from SigmaAldrich Japan (Tokyo, Japan), and Tokyo Kasei Kogyo (Tokyo, Japan), respectively, and were also used as received. The solvents used were also purchased from Wako Pure Chemicals as ACS reagent grade: ethanol, toluene, acetone, methanol, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and deuterated dimethyl sulfoxide (DMSO-d6). Synthesis of Poly(N-isopropylacrylamide-co-β-cyclodextrin) (Poly(NIPAM-co-CD)). It has been reported that, in order to induce the autonomous phenomenon, a sufficient amount of cyclodextrin has to be introduced into the polymer to ensure sufficient influence from CD/ANS to pNIPAM.17 As it is difficult to achieve the required yield by direct copolymerization of CD monomer and NIPAM monomer,24 in the present study NIPAM and p-nitrophenyl acrylate (PNPA) were copolymerized to synthesize poly(NIPAM-co-PNPA), and then the PNPA moiety was substituted by aminated CD to introduce more CD (Scheme 3a). The aminated CD was synthesized from tosylated CD

Scheme 2. Autonomous Phenomenon of the Poly(NIPAMco-CD)/ANS Systema

a

Key: (i) Decomplexation of ANS from CD triggered by polymer shrinkage. (ii) Polymer swelling due to hydrophilic CD without ANS. (iii) Complexation of CD/ANS triggered by polymer swelling. (iv) Polymer shrinkage due to hydrophobic CD/ANS complex.

is in its swollen state and ANS is present (Scheme 2, upper left). When some force to shrink the pNIPAM is added to the system, the following takes place. (i) Shrinking of pNIPAM lowers the complexation constant of CD/ANS, and ANS detaches from CD. (ii) When ANS is detached from CD, the environment around CD becomes hydrophilic because of the disappearance of the hydrophobic environment around the ANS phenyl ring. This induces an increase in the crowd point of the main chain, resulting in polymer swelling. (iii) The polymer swelling releases steric hindrance around CD, and ANS forms a complex with CD. (iv) The hydrophobic ANS/ CD complex formation again induces lowering of the polymer’s crowd point, and the cycle continues. It has been reported that in a closed system that does not allow additional energy inflow, only one cycle of the autonomous phenomenon can be induced.17 When an open system is constructed, successive phenomena can occur autonomously. In a system in which the sensor and actuator interact closely, it is particularly crucial to understand the mutual communications between the elements. It is certain that communications are significantly affected by the spacer connecting the actuator and the sensor; they might be hindered by a sufficiently long spacer. In the present study, to clarify the influence of the spacer on the mutual communications, the effect of spacer length between CD and the main chain was investigated; that is, the phase transition behavior of poly(NIPAM-co-CD) and the autonomous phenomenon of the polymer in response to an abrupt temperature increase (called temperature shooting) and an abrupt ANS concentration increase (called ANS concentration shooting) to trigger the phenomenon were examined. However, prior to the above investigation, the response to the ANS concentration shooting itself had not been observed.

Scheme 3. Synthesis of Poly(NIPAM-co-CD) with High CD Content and Aminated CD Having Different Spacer Lengths

and diamines with different spacer lengths. The syntheses of pnitrophenyl acrylate and tosylated CD are described elsewhere.17 The synthesized materials were characterized by 1H NMR spectroscopy (JNM-LA400; JEOL Ltd., Tokyo, Japan). DMSO-d6 was used as solvent for all NMR measurements. Synthesis of Aminated Cyclodextrin (Aminated CD). To introduce spacer arms of different lengths between CD and the polymer main chain, three diamines having different spacer arms were used for the synthesis of aminated CD (Scheme 3b). Amino-EDA-CD, 9743

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amino-(EG)2-CD, and amino-(EG)3-CD were synthesized using ethylenediamine, 2,2′-(ethylenedioxy)bis(ethylamine), and diethylene glycol bis(3-aminopropyl) ether, respectively. They include two, eight, and 13 C or O atoms between terminal amines, respectively. Diamine (126 mL) was added to tosylated CD (12.0 g), and the mixture was stirred at 40 °C for 2 days. The crude aminated CD was precipitated by adding acetone dropwise to the reaction mixture. After filtration, the precipitate was dissolved in a mixed solvent of methanol (75 mL) and reverse osmosis (RO) water (25 mL). The pure aminated CD was obtained after adding acetone dropwise to the solution, washing the precipitate, and then drying the filtrate in vacuum at room temperature for 1 day. Synthesis of Poly(N-isopropylacrylamide-co-p-nitrophenyl acrylate) (Poly(NIPAM-co-PNPA)). NIPAM (35.00 g) and PNPA (5.96 g) were dissolved in DMF (350 mL) and the solution was purged by bubbling with N2 for 20 min. AIBN initiator (0.5243 g) was added to the solution and the reaction was allowed to proceed at 70 °C for 18 h. After concentration of the reaction mixture by rotary evaporation, the solution was added dropwise to diethyl ether to precipitate the polymer. The precipitated polymer (poly(NIPAM-coPNPA)) was filtered and then dried under vacuum for 1 day. The molecular weight of the polymer was determined by gel permeation chromatography (GPC) using a column (GF-7 M HQ; Shodex, Tokyo, Japan) and RI detector (RI-71; Shimadzu, Tokyo, Japan). The elution solvent was DMF with lithium chloride. The molar ratio of NIPAM/PNPA was determined with 1H NMR, from the number of protons of the isopropyl group in NIPAM to the number of the phenyl ring in PNPA. Synthesis of Poly(N-isopropylacrylamide-co-β-cyclodextrin) (Poly(NIPAM-co-CD)). Poly(NIPAM-co-PNPA) (about 5 g) and a designated quantity of aminated CD (molar ratio of aminated CD to PNPA in poly(NIPAM-co-PNPA) was 0.1−3) were dissolved in DMSO (180 mL) and then reacted at 50 °C for 2 days. The reaction mixture was diluted with a 10× volume of RO water and then introduced into a cell dialysis tube (cutoff molecular weight: 12 000− 14 000) and dialyzed. The dialyzed sample was freeze-dried. The obtained sample was redissolved in RO water and then again fully dialyzed in order to remove ingredients and PNPA from the polymer. The final product was freeze-dried. The copolymers synthesized using amino-EDA-CD, amino-(EG)2-CD, and amino-(EG)3-CD are referred to in the present study as poly(NIPAM-co-EDA-CD), poly(NIPAMco-(EG)2-CD), and poly(NIPAM-co-(EG)3-CD), respectively. Collectively, they are called poly(NIPAM-co-CD). Absorbance Measurement of Aqueous Polymer Solutions. A UV−vis spectrophotometer (Model U-3310; Hitachi, Ibaraki, Japan) with a peltier temperature controller option (Model SDR-30) was used for all absorbance measurements in the present study. The wavelength of visible light was 650 nm and the cell light path length of a quartz cell was 1 cm. A magnetic stirring tip was placed in the bottom of the quartz cell to mix the solution homogeneously. An increase/decrease in absorbance corresponds to shrinkage/swelling of the polymer. Measurement of the Crowd Point of Poly(NIPAM-co-CD) with and without ANS. A 0.4 wt % poly(NIPAM-co-CD) aqueous solution (2000 μL) with and without ANS was placed in the quartz cell of the UV−vis spectrophotometer. An ANS concentration of between 0 and 10 mM was used. In the case of poly(NIPAM-co-CD) without ANS, the equilibrated absorbance of an aqueous polymer solution at each temperature was recorded. The absorbance of poly(NIPAM-co-CD) with ANS increased and subsequently decreased in response to each temperature rising step, due to the abovementioned autonomous phenomenon. When each maximum absorbance was observed, the next temperature rising was carried out and the maximum absorbance was recorded. The detailed procedure and an explanation can be found elsewhere.17 Each measurement was conducted in 1.0 or 2.0 °C steps around the crowd point of each sample. Temperature Shooting Experiment of Poly(NIPAM-co-CD). A 0.4 wt % aqueous poly(NIPAM-co-CD) solution (2000 μL) with 1 mM ANS was placed in the quartz cell of the UV−vis

spectrophotometer and preheated to the starting temperature. Under stirring, the temperature was rapidly increased (temperature shooting) from the starting temperature to the end temperature. The time course of the absorbance of the solution in response to the temperature shooting was observed. ANS Concentration Shooting Experiment of Poly(NIPAM-coCD). A 0.44 wt % aqueous poly(NIPAM-co-CD) solution (1800 μL) was placed in the quartz cell of the UV−vis spectrophotometer and preheated to the designated temperature. The concentration of ANS was rapidly increased to the designated concentration (ANS concentration shooting) by quickly adding concentrated ANS solution (200 μL) to the polymer solution, using a micropipet. The time course of the absorbance in response to the ANS concentration shooting was observed. The added ANS solution was separately preheated to the same temperature in a temperature bath.

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RESULTS AND DISCUSSION Synthesis of Poly(NIPAM-co-PNPA), Aminated CD, and Poly(NIPAM-co-CD). The successful synthesis of all the materials was confirmed by 1H NMR. The number-average molecular weight of poly(NIPAM-co-PNPA) and the polydispersity were determined by GPC to be about 18 000 g/mol and 2.15, respectively. The molar ratios of NIPAM and PNPA in poly(NIPAM-co-PNPA) were determined by 1H NMR to be 89.5 mol % and 10.5 mol %. The yields of the syntheses of amino-EDA-CD, amino-(EG)2-CD, and amino-(EG)3-CD were 92.4%, 81.5%, and 76.1%, respectively. As for the introduction of aminated CD into poly(NIPAM-co-PNPA), the relationship between the molar ratio in the feed (aminated CD to PNPA in poly(NIPAM-co-PNPA)) and product (amount of CD introduced into poly(NIPAM-co-CD)) is summarized in Figure 1. Note that poly(NIPAM-co-PNPA) containing 10.5 mol %

Figure 1. Relationship between molar ratio of aminated CD to PNPA in poly(NIPAM-co-PNPA) in feed and CD amount introduced into the poly(NIPAM-co-CD) product. Poly(NIPAM-co-PNPA) contained 10.5 mol % PNPA.

PNPA was used in all cases. The figure shows that, in the case of poly(NIPAM-co-EDA-CD), the introduced CD amount reached a plateau (but not in the cases of poly(NIPAM-co(EG)2-CD) and poly(NIPAM-co-(EG)3-CD)). This may be because amino-EDA-CD has a short spacer, which suffers strong steric hindrance of a large CD molecule during the insertion reaction. In contrast to poly(NIPAM-co-EDA-CD), more CD can be introduced in the case of poly(NIPAM-co(EG)2-CD) and poly(NIPAM-co-(EG)3-CD), probably because the existence of a sufficiently long spacer arm mitigates the steric hindrance on the reaction. The result suggests that a long spacer is advantageous for the introduction of a large group into the polymer. 9744

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Phase Transition Behavior of Poly(NIPAM-co-EDACD). The phase transition behavior of poly(NIPAM-co-EDACD) containing 3.3 mol % CD with various ANS concentrations was observed with UV−vis spectroscopy. Results are shown in Figure 2. With an increase in ANS

Scheme 4. Schematic Illustration of Autonomous Phenomenon of Poly(NIPAM-co-EDA-CD) in Response to (a) Temperature Shooting and (b) ANS Concentration Shooting

Subsequently, to observe the guest-molecule-induced autonomous phenomenon, absorbance change in response to ANS concentration shooting was observed. Experimentally, the temperature was set at 36 °C, just below the crowd point of the polymer, without ANS. Results are shown in Figure 4. At all

Figure 2. Phase transition behavior of poly(NIPAM-co-EDA-CD) containing 3.3 mol % CD. Each symbol represents absorbance of each ANS concentration.

concentration (up to 1 mM ANS), the crowd point moves to a lower temperature, probably because of the hydrophobicity of the ANS phenyl ring protruding from the CD cavity. There was not much difference between the crowd point with 1 mM ANS and that with 10 mM ANS. A possible explanation for this is the following: even in the case of 1 mM ANS, almost all of the CD forms a complex with ANS and the additional ANS could not further increase the amount of CD/ANS and its hydrophobicity, which therefore could not shift the crowd point of the polymer. It should be noted that the crowd point of pNIPAM was hardly influenced by the existence of ANS (Figure S1 in Supporting Information) Temperature Shooting and ANS Concentration Shooting of Poly(NIPAM-co-EDA-CD). On the basis of the results of the phase transition behavior, the absorbance change in response to temperature shooting was investigated. Experimentally, the ANS concentration was set to be 1 mM and the starting temperature was set at 29 °C, just below the crowd point of the polymer with 1 mM ANS. Results are shown in Figure 3. The higher the end temperature, the steeper is the absorbance decrease after an initial increase. The observed behavior is similar to results reported in a previous study.17 The phenomenon is schematically shown in Scheme 4a.

Figure 4. Time course of absorbance of poly(NIPAM-co-EDA-CD) containing 3.5 mol % CD in response to ANS concentration shooting at 36 °C. Each symbol represents the absorbance change of each ANS concentration shooting.

ANS concentrations, there was an initial increase and then a subsequent decrease in absorbance. Up to 1 mM ANS concentration shooting, the higher the ANS concentration, the steeper is the absorbance change, probably because of higher energy feeding to the system. On the other hand, 5 mM ANS concentration shooting could not induce a steep absorbance decrease. At such a high ANS concentration, ANS could not be detached from CD, even when the polymer is in a shrunken state, as indicated in Figure 2; hence, subsequent polymer shrinkage was restricted. The first event applied in the closed system is temperature increase in the case of temperature shooting, and is ANS concentration increase in the case of ANS concentration shooting. The autonomous behavior induced by ANS concentration shooting is different in quality from that induced by temperature shooting, as schematically shown in Scheme 4. In the case of temperature shooting, the first phenomenon to occur is the shrinkage of the main chain (step (iv)′ in Scheme 4a). In the case of ANS concentration shooting, the first phenomenon is the complexation of ANS in CD (step (iii)′ in Scheme 4b). The occurrence of the autonomous shrinking/ swelling in response to ANS concentration shooting indicates

Figure 3. Time course of absorbance of poly(NIPAM-co-EDA-CD) containing 3.3 mol % CD and 1 mM ANS in response to temperature shooting. Each symbol represents the absorbance change of each temperature shooting. 9745

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that the autonomous phenomenon can also be induced by a concentration stimulus (besides a heat stimulus). On the basis of the above discussion of the response to the ANS concentration shooting, we next addressed the effect of the spacer length between CD and pNIPAM on the phase transition behavior and autonomous phenomenon of poly(NIPAM-co-CD) in response to the ANS concentration shooting and temperature shooting observed, in order to clarify the influence of the spacer connecting the elements on their mutual communications. The Effect of Spacer Length between pNIPAM and CD/ANS on Phase Transition Behavior. The phase transition behavior of poly(NIPAM-co-CD) having different spacer lengths was investigated by determining the absorbance change of a polymer solution. The behaviors of poly(NIPAMco-EDA-CD) containing 3.3 or 7.6 mol % CD, poly(NIPAM-co(EG)2-CD) containing 3.0 or 7.1 mol % CD, and poly(NIPAM-co-(EG)3-CD) containing 3.2 or 9.4 mol % CD, with and without ANS, are shown in Figures 5, 6, and 7, respectively.

Figure 7. Phase transition behavior of poly(NIPAM-co-(EG)3-CD) containing 3.2 mol % or 9.4 mol % CD. Closed symbols and open symbols represent the absorbance without ANS and with 1 mM ANS, respectively.

the normal sense, the crowd point of the polymer seems to increase with the amount of CD and to decrease with the addition of ANS in all the polymers investigated in this study. However, a close examination of the graphs shows the situation to be somewhat different. Considered on a smaller scale, when the absorbance of the polymer solution becomes more than zero, more than submicrometer-sized polymer aggregate is already formed, which is sufficient to scatter visible light having a wavelength of 650 nm. In this sense, the highest temperature whose absorbance is ≤0.002 is defined hereafter, in the present study, as the crowd point, and is plotted as triangles in Figures 8. The value of 0.002 was selected after considering the experimental error of absorbance, with the minimum step size of absorbance of 0.001. Although the definition may appear somewhat arbitrary, it should be noted that this minor change in the definition does not change the overall trend. Middle and right figures of Figures 8 show that the crowd point of poly(NIPAM-co-(EG)2-CD) and poly(NIPAM-co(EG)3-CD) is almost unchanged by the addition of ANS. This means that CD/ANS complex formation has no influence on the crowd point of the polymers. In other words, a change in hydrophobic environment around CD/ANS does not extend to the pNIPAM main chain, probably because of the existence of overlong spacers between CD and pNIPAM. The introduction of comonomer with an ethylene glycol chain into pNIPAM is known to increase the crowd point of pNIPAM.25,26 Therefore, the increase in the crowd point with an increase in the amount of CD can be attributed not to the increase in CD itself but to the increase in the ethylene glycol side chain. As described in left figure of Figure 8, contrary to poly(NIPAM-co-CD) having long spacers, the crowd point of the poly(NIPAM-co-EDA-CD) is strongly reduced by the addition of ANS. Besides, when the CD content in the polymer is increased, the shift in the crowd point with the addition of ANS becomes larger (as also shown in left figure of Figure 8). This indicates that CD/ANS complexation can have an influence on the pNIPAM main chain, probably because of the proximity between CD and ANS in the case of poly(NIPAM-co-EDA-CD). Some research has been reported on the influence of spacer length of the comonomer on phase transition behavior in poly(NIPAM-co-comonomer); The monomer contains alkyl spacer between main chain and methyl ester/carboxyl group, or ethylene glycol spacer between main chain and methyl group.18,19,25 However, as far as we know, this is the first research work focusing on the influence of

Figure 5. Phase transition behavior of poly(NIPAM-co-EDA-CD) containing 3.3 mol % or 7.6 mol % CD. Closed symbols and open symbols represent the absorbance without ANS and with 1 mM ANS, respectively.

Figure 6. Phase transition behavior of poly(NIPAM-co-(EG)2-CD) containing 3.0 mol % or 7.1 mol % CD. Closed symbols and open symbols represent the absorbance without ANS and with 1 mM ANS, respectively.

It should be noted that all the poly(NIPAM-co-CD)s were prepared using the same poly(NIPAM-co-PNPA) containing 10.5 mol % PNPA, except that poly(NIPAM-co-PNPA) containing 17.0 mol % PNPA was used to introduce as much as 7.6 mol % CD into the poly(NIPAM-co-EDA-CD) for the purpose of comparison. Usually, the crowd point of a thermosensitive polymer is defined as the inflection point of absorbance or the intercept of the absorbance line. When the crowd point is defined as the intercept of the absorbance line in 9746

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Figure 8. Enlarged views of Figures 5−7 showing the phase transition behavior of poly(NIPAM-co-CD)s. The spacer and content of pendant CD in poly(NIPAM-co-CD) and ANS concentration are shown. Triangles represent the crowd point of the polymer.

spacer length on the molecular interaction between sensor and actuator. Another important feature of poly(NIPAM-co-CD)s evident in Figure 8 is that the polymer without ANS is less prone to aggregate than that with ANS at a temperature above the crowd point. This can be attributed to the hydrophilic feature of CD. pNIPAM, having a hydrophilic comonomer, is known to include much water inside and to be less prone to aggregate, even at higher temperatures than the crowd pointit is called a coacervate.27−30 It is probable that the polymer with hydrophilic CD forms a water-rich structure, whereas the polymer with hydrophobic CD/ANS does not. The Effect of Spacer Length between pNIPAM and CD/ANS on the Autonomous Phenomenon (Response to ANS Concentration Shooting). The influence of spacer length on the response to ANS concentration shooting was also investigated. The time courses of absorbance change in response to ANS concentration shooting of poly(NIPAM-coEDA-CD) having 3.3 mol % CD, poly(NIPAM-co-(EG)2-CD) having 3.0 mol % CD, and poly(NIPAM-co-(EG)3-CD) having 3.2 mol % CD are shown in Figures 9, 10, and 11, respectively.

Figure 10. Time course of absorbance of poly(NIPAM-co-(EG)2-CD) containing 3.0 mol % CD in response to ANS concentration shooting. Each symbol represents absorbance of each ANS concentration shooting at each designated temperature.

Figure 11. Time course of absorbance of poly(NIPAM-co-(EG)3-CD) containing 3.2 mol % CD in response to ANS concentration shooting. Each symbol represents absorbance of each ANS concentration shooting at each designated temperature.

state and forms a coacervate at the beginning. The ANS concentration shooting induces CD/ANS complex formation, and the coacervate becomes hydrophobic because of the disappearance of the hydrophilic environment around CD, and forms an aggregate. However, the hydrophobic environment formed by the CD/ANS complexation does not extend to the pNIPAM main chain and cannot affect polymer phase transition due to the long and hydrophilic spacer, as discussed in the previous section. As a consequence of the unchanged polymer phase, ANS cannot be detached from CD, and the subsequent aggregate reswelling into a coacervate state cannot occur (the absorbance was kept high). This phenomenon is different from that of poly(NIPAM-co-EDA-CD); the effect of the CD/ANS complexation can reach, and shrink, the pNIPAM main chain due to the short and hydrophobic spacer, and

Figure 9. Time course of absorbance of poly(NIPAM-co-EDA-CD) containing 3.3 mol % CD in response to ANS concentration shooting. Each symbol represents absorbance of each ANS concentration shooting at each designated temperature.

Compared with the response of poly(NIPAM-co-EDA-CD), almost no absorbance decrease after an increase in response to ANS concentration shooting was observed in the case of poly(NIPAM-co-(EG)2-CD) and poly(NIPAM-co-(EG)3-CD) having long spacers, even though the polymers contain almost same amount of CD (about 3 mol %). The feature of poly(NIPAM-co-CD) having long spacers can be explained as follows (see Scheme 5b). At a temperature higher than the crowd point, the polymer is in its shrunken 9747

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Scheme 5. Schematic Illustration of (a) Response of Poly(NIPAM-co-CD) Having Long Spacers to ANS Concentration Shooting below the Crowd Point, (b) Response of poly(NIPAM-co-CD) having Long Spacers to ANS Concentration Shooting above the Crowd Point, (c) Response of poly(NIPAM-co-CD) Having Long Spacers to Temperature Shooting, and (d) Response of Poly(NIPAM-co-CD) Having Short Spacers to Temperature Shooting

to Temperature Shooting). Temperature shooting can induce an autonomous absorbance decrease after an increase, even in the case of poly(NIPAM-co-(EG)2-CD) and poly(NIPAM-co-(EG)3-CD), as shown in Figures 12 and 13; this is

subsequent CD/ANS decomplexation induces autonomous polymer reswelling and an absorbance decrease (Scheme 4b). It can be imagined that ANS concentration shooting of poly(NIPAM-co-(EG)2-CD) and poly(NIPAM-co-(EG)3-CD) at a temperature lower than the crowd point could not induce even the first polymer shrinkage and absorbance increase. The hydrophobic environment induced by the CD/ANS complexation cannot extend to the main chain, and the swollen state of the polymer is maintained (Scheme 5a). The Effect of Spacer Length between pNIPAM and CD/ANS on the Autonomous Phenomenon (Response

Figure 13. Time course of absorbance of poly(NIPAM-co-(EG)3-CD) containing 3.2 mol % CD in response to temperature shooting. Each symbol represents absorbance of each temperature shooting.

different from their response to ANS concentration shooting shown in Figures 10 and 11. As discussed in the previous section, without ANS detachment from CD, the subsequent absorbance decrease should not occur. Therefore, it is strongly suggested that there is ANS detachment from CD induced by forced polymer shrinkage by the temperature shooting, even in the case of poly(NIPAM-co-CD) having long spacers.

Figure 12. Time course of absorbance of poly(NIPAM-co-(EG)2-CD) containing 3.0 mol % CD in response to temperature shooting. Each symbol represents absorbance of each temperature shooting. 9748

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Scheme 6. Schematic Representation of the Relationship between Spacer Length and Influences between CD/ANS and pNIPAM

However, the response is more moderate than in the case of poly(NIPAM-co-EDA-CD), as shown in Figure 3. This can be attributed to differences in the polymer solubilization step (ii) and/or the preceding CD/ANS decomplexation step (i) in Scheme 4b. The former offers the following possibility: in the case of poly(NIPAM-co-EDA-CD), the detachment of ANS from CD induces polymer swelling. On the other hand, in the cases of poly(NIPAM-co-(EG)2-CD) and poly(NIPAM-co(EG)3-CD), it induces coacervate formation, because CD/ ANS decomplexation cannot swell the pNIPAM main chain, as discussed above. This difference is one possible reason for the moderate response of the autonomous phenomenon in the case of poly(NIPAM-co-CD) with longer spacers (Scheme 5, c and d). There might be a further reason for the moderate response: the precedent CD/ANS decomplexation is partially hindered by long spacers, which will be examined in further investigations. Nevertheless, even if it is partial, the CD/ANS decomplexation occurs even in the case of poly(NIPAM-coCD) having long spacers. This may be because, in a shrunken polymer state, the CD/ANS is influenced by the hydrophobic environment or steric hindrance, not by it being structurally close to the main chain but by it being spatially close to the main chain. This is quite opposite to the influence from CD/ ANS to pNIPAM, which is strongly restricted by the long spacers. In essence, as schematically shown in Scheme 6, the experimental results of the influence of spacer length on the phase transition behavior suggest that long spacers restrict the communication from CD/ANS to pNIPAM. In the case of the poly(NIPAM-co-CD) system investigated in the present study, a spacer arm of an 11-atom spacer length was found to be too long to convey communication from sensor to actuator, whereas communication is maintained with a five-atom spacer length. It should be noted that the atomic length from the main chain to the isopropyl group of NIPAM is also five. Only the CD/ANS structurally close to the polymer main chain exhibited this influence. On the other hand, the influence of spacer length on the autonomous behavior suggests that the interaction from pNIPAM to CD/ANS is maintained to a certain degree even in the case of long spacers. This may be because CD/ANS is influenced by a hydrophobic environment or steric hindrance, not by being structurally close to the main chain, but by being spatially close to the main chain. This

knowledge is necessary for the design of further functional polymeric materials comprising sensor and actuator.

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CONCLUSION The autonomous polymer shrinking/swelling behavior of poly(NIPAM-co-CD) was induced by ANS concentration shooting. The higher the ANS concentration, the steeper is the autonomous behavior, up to 1 mM ANS. By contrast, with 5 mM ANS, the autonomous behavior was not steep, probably due to insufficient ANS detachment from CD, because such high ANS concentration stabilized the CD/ANS complex even in a shrunken polymer state. On the basis of this knowledge, the influence of spacer length between pNIPAM and CD on the phase transition behavior and on the autonomous phenomenon in response to ANS concentration/temperature shooting was investigated. A very different behavior was observed when a long spacer was used instead of a short spacer; in the case of a long spacer, CD/ANS complexation did not influence the phase transition temperature of poly(NIPAMco-CD), and almost no autonomous polymer swelling after shrinking was induced in response to ANS concentration shooting, although it was induced in response to temperature shooting. These results strongly suggest that the communication from CD to pNIPAM is hindered by long spacers, whereas communication from pNIPAM to CD is maintained to a certain extent, even with long spacers. This fact will be useful for designing further highly functional polymeric materials composed of sensor and actuator.

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ASSOCIATED CONTENT

S Supporting Information *

Phase transition behavior of pNIPAM with/without 1 mM ANS as a control. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (+81)45-924-5254. Fax: (+81)45-924-5253. Email: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. 9749

dx.doi.org/10.1021/ma3018603 | Macromolecules 2012, 45, 9742−9750

Macromolecules

Article

Notes

The authors declare no competing financial interest.

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REFERENCES

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