Different deswelling behavior of temperature-sensitive microgels of poly(N-isopropylacrylamide) crosslinked by polyethyleneglycol dimethacrylates

Journal of Colloid and Interface Science 276 (2004) 53–59 www.elsevier.com/locate/jcis

Different deswelling behavior of temperature-sensitive microgels of poly(N-isopropylacrylamide) crosslinked by polyethyleneglycol dimethacrylates Xiaomei Ma,a,b Yanjun Cui,a Xian Zhao,a Sixun Zheng,a and Xiaozhen Tang a,∗ a School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China b School of Chemical Engineering, Qingdao University, Qingdao 266071, People’s Republic of China

Received 2 December 2003; accepted 9 March 2004 Available online 10 April 2004

Abstract Polymerization of N-isopropylacrylamide (NIPAM) with polyethyleneglycol dimethacrylates (nG, n representing the number of –CH2 CH2 O– units in polyethyleneglycol dimethacrylates) through surfactant-free radical polymerization was used to prepare the temperature-sensitive microgels. The morphology, dispersity, and deswelling behavior of the microgels were investigated by means of transmission electron microscopy (TEM), ultraviolet–visible spectroscopy, differential scanning calorimetry (DSC), and dynamic light scattering (DLS) techniques. TEM micrographs revealed that it was feasible to obtain regular spherical microgels for crosslinking agents with short chain. Turbidity, DSC, and DLS analysis showed that in marked contrast to 1G and 3G crosslinked microgels, the collapse of microgels crosslinked by 9G, 14G, and 23G proceeded in a two-step mechanism. The amide groups dehydrated at the lower temperature leading to the first-step transition. In the transition, the hydrophilic long –(–CH2 CH2 O–)n – segments could be enriched on the surface of the microgels, which was further verified by variable temperature 1 H NMR spectroscopy. The hydrophilic long –(–CH2 CH2 O–)n – segments can be dehydrated at the higher temperature.  2004 Elsevier Inc. All rights reserved. Keywords: Microgels; pNIPAM; Temperature-sensitive; Volume-phase transition

1. Introduction Microgels are a class of crosslinked lattices swollen by good solvents, the average sizes of which are in the range of diameter 50 nm–5 µm [1]. The specific structural feature motivates ones to explore the broad application of the class of materials; e.g., they can be used as coatings owing to their small size, larger ratio of area to volume, ease of becoming swollen, and good fluidity in comparison with bulk gels [2]. The physicochemical properties of some microgels can vary in response to features of the surrounding environments, such as pH values [3] and temperature [4], and these are called environment-responsive or intelligent microgels. The synthesis and characterization of the first environment-responsive microgel was reported by Pelton and Chi* Corresponding author. Fax: +86-21-54743264.

E-mail address: [email protected] (X. Tang). 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.03.023

bante [4] in 1986. This environment-responsive microgel was temperature-sensitive poly(N -isopropylacrylamide) (pNIPAM). Since then temperature-sensitive microgels have attracted considerable attention to the correlation between structure and properties and to their potential applications in many fields, such as drug delivery [5], sensing [6], catalysis [7], and pollution control [8]. PNIPAM microgels are temperature-sensitive spongelike particles, which can shrink and/or swell in response to changes in temperature and even undergo a large-magnitude volume change at a certain temperature that is called the volume-phase transition temperature (VPTT). The VPTT of pNIPAM microgels is ∼34 ◦ C in water [9]. Similarly to macroscopic pNIPAM hydrogels, the phase-transition behavior of pNIPAM microgels is generally attributed to the reversible formation and breakage of hydrogen-bonding interactions between water and the polymer. In the hydrophilic swollen state, water molecules can surround and fill individual microgels via hydrogen-bonding interactions between

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X. Ma et al. / Journal of Colloid and Interface Science 276 (2004) 53–59

Scheme 1. Structure of polyethyleneglycol dimethacrylates (nG).

water and amide groups. Hydrogen-bonding interactions can be disrupted when the local solution temperature is increased to the VPTT of microgels. Water can be expelled from the matrix of microgels as a result of disruption of hydrogen bonds and hydrophobic interactions among isopropyl groups of neighboring polymer chains, and at the same time the particles of microgels can undergo morphological transition from a loosely swollen network to rigid spheres; i.e., a large-magnitude volume change (volume-phase transition) occurs [10]. In marked contrast to discontinuous volume-phase transitions of macroscopic pNIPAM hydrogels, crosslinked pNIPAM microgels undergo almost continuous volume change induced by temperature. Wu and Zhou [11] proposed that this is due to the inhomogeneity of the subchain length between two neighboring crosslinking points inside the gel. In fact, a number of other factors, such as structure and concentration of the crosslinking agent used [12,13], incorporation of co-monomers [14,15], properties of solvents [16,17], and surfactants [18] also have a strong influence on phase behavior of microgels. K. Kratz et al. [12] have investigated the volume-phase transition of pNIPAM microgels crosslinked by triethyleneglycol dimethacrylate (TREGDMA), ethyleneglycol dimethacrylate (EGDMA), and N, N  -methylenebis(acrylamide) (BIS) and found that EGDMA- and TREGDMA-crosslinked microgels showed similar swelling behavior, whereas the swelling capacity of the BIS-crosslinked microgels was much lower. In addition, they have noted that the BIS-crosslinked particles displayed a quite broad transition, whereas a relatively sharp and nearly discontinuous transition was observed for the other two systems. They thought that EGDMA and TREGDMA had larger bridging chains with a higher flexibility than BIS, which endows the microgels with higher network flexibility. In this work, we synthesized a series of pNIPAM microgels crosslinked by polyethyleneglycol dimethacrylates with various chain lengths (see Scheme 1), abbreviated as nG; n corresponds to the number of –CH2 CH2 O– units in polyethyleneglycol dimethacrylates. Five nGs (n = 1, 3, 9, 14, 23, respectively) were used in the experiment. The relationship between the structure of the crosslikers and the deswelling behavior of the microgels was investigated.

2. Experimental 2.1. Materials and preparation of samples N -isopropylacrylamide (NIPAM) (Acros, purity
99%), nG (Shin-Nakamura Chemical Co. Ltd., Japan), and potas-

sium persulfate (KPS) (Aldrich, 99.99%) were all used as received. The microgel preparation was based on the procedure described by Pelton and Chibante [4]. We employed a conventional stirring technique for the synthesis of the microgels. A sample of 0.72 g of NIPAM and the desired amount of crosslinking agents (9G, 14G, 23G) were dissolved in 50 ml of triple-distilled degassed water and the mixture was transferred into a 250-ml four-necked roundbottom flask equipped with a condenser, a thermometer, a N2 inlet, and a mechanical stirring paddle. The desired amount of 1G and 3G was injected into the flask because they were insoluble in water. The synthesis of the microgels was performed in an inert gas atmosphere to avoid side reactions with oxygen. At 373 K, a 10-ml KPS aqueous solution (0.0053 M) was added to initiate the reaction of polymerization. Within a couple of minutes, the mixture became turbid, suggesting the formation of emulsion particles. The reaction was allowed to continue for an additional 6 h at this temperature. After that, under continuous stirring the system was cooled to room temperature. To remove possible unreacted monomers as well as other low-molecular-weight impurities, the product was extensively dialyzed using triple-distilled water for 3 weeks. The purified microgels were collected and further characterized. 2.2. Measurement and characterization In order to investigate the effect of the lengths of the crosslinkers on the deswelling behavior of the microgels, stable dispersions of microgels crosslinked by nG of 2 mol% concentration (coded as pNIPAM–nG-2%) were measured. 2.2.1. Transmission electronic microscopy (TEM) Morphological observation of the microgels was carried out on a JEM-100X (II) transmission electron microscope. In order to obtain clear TEM images of the microgels, the samples were first stained with solution of phosphotungstic acid. A few droplets of the samples at a certain temperature were spread onto the surface of a 300-mesh copper grid and were allowed to dry for 24 h at the same temperature. The dried specimen was clamped onto a TEM specimen rod, inserted into the sample chamber, and observed at 100 kV. 2.2.2. Dynamic light scattering (DLS) A Malvern Zetasizer 1000 dynamic light-scattering apparatus equipped with a helium–neon laser (λ = 633 nm) with a detector placed at 90◦ was used to investigate the deswelling behavior of the microgel. All the samples were diluted with pure water. The hydrodynamic diameters of the particles of microgels were measured at various temperatures. Before the data were collected, the sample was held at each temperature for at least 15 min to attain swelling/ deswelling equilibrium.

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2.2.3. Turbidimetric analysis The temperature dependence of the conformational transition of microgel dispersions was turbidimetrically determined at 547 nm using a Cintra 10e UV–visible spectrophotometer equipped with a GBC thermocell. The scan range of temperature was from 25 to 90 ◦ C, with a heating rate of 1 ◦ C min−1 . 2.2.4. Differential scanning calorimetry (DSC) The volume-phase transition behavior of the microgels was investigated by DSC. The calorimetric analysis was carried out on a Perkin–Elmer Pyris-1 differential scanning calorimeter. The samples were scanned from 10 to 90 ◦ C at the scan rate of 2 ◦ C min−1 . In the experiments, the baselines were first measured by using triple-distilled water in the range of 10–90 ◦ C. 2.2.5. 1 H nuclear magnetic resonance spectroscopy (NMR) A Varian Mercury plus 400 NMR spectrometer (400 MHz for protons, Varian, Inc., USA) was used to determine the conformation transition of the microgels before and after deswelling. The microgels were freeze-dried at −40 ◦ C and were redispersed in D2 O. The spectra were recorded at 25 and 40 ◦ C, respectively. To attain equilibration, the dispersion was kept at each temperature for 15 min.

3. Results and discussion 3.1. Effect of crosslinker concentration Table 1 summarized the effect of crosslinker concentration on the stability of the microgels. For 1G and 3G, the stable dispersion of microgels can be obtained in a relatively

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Table 1 Stability dependence of the microgels on the crosslinker concentration Crosslinker 1G 3G 9G 14G 23G

Concentration (mol%) 2

3.5

4

5

7.5

S S S S S

S S S G G

S S G G G

S S G G G

S S G G G

Note. S: stable microgels; G: macroscopic gels.

wide range of concentration of the crosslinker (2–7.5 mol%). For 9G, 14G, and 23G the stable dispersion of microgels can only be obtained at the lower crosslinker concentration (less than 3.5 mol%) and the formation of macroscopic gels shows the stable dispersion of microgels can not be obtained when the concentration of crosslinker is more than 3.5 mol%. In comparison with the crosslinker with the lower n, the crosslinking agents of the higher n afford a lower crosslinking density at the same concentration, i.e., a greater average length of chains between the crosslinking points. The greater average length of chains between the crosslinking points can possibly make the size of the microgels greater. Larger microgel particles collide with each other and aggregate more easily than smaller ones. The dependence of the formation of the stable microgels on the values of n and the concentration of the crosslinkers suggest that the average length of chains between the crosslinking points could be a significant factor in preserving the stability of microgels. In other words, the optimum degree of crosslinking is crucial to the formation of stable microgels. In addition, the reactivity of the crosslinkers could have an effect on the stability of the dispersion of the microgels.

Fig. 1. Comparative TEM micrographs (25 ◦ C (left) and 40 ◦ C (right)) of microgels crosslinked by different crosslinking agents.

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Fig. 2. Turbidity dependence of pNIPAM–nG-2% microgels on temperature.

Fig. 3. Plots of first derivative of absorbance as functions of temperatures.

3.2. Morphology below and above VPTT To observe the morphology of the microgel particles, TEM was employed. Representatively shown in Fig. 1 are the TEM micrographs of the samples of pNIPAM–14G-2% and pNIPAM–23G-2% dried at 25 and 40 ◦ C, respectively. For all the microgels obtained, it is noted that the particles dried at 25 ◦ C have larger sizes in diameter than those dried at 40 ◦ C; i.e., the microgels exhibit typical temperature sensitivity. Moreover, this reveals that spherical microgel particles could be obtained using 1G, 3G, 9G, and 14G as crosslinking agents. However, spherical microgel particles were not formed for the pNIPAM–23G-2% sample, due possibly to the higher flexibility of long –(–CH2 CH2 –)n – segments. It should be pointed out that the sizes observed by TEM could not correspond to those of the microgels in reality, since there is a tendency for the particles to flatten on the TEM grid during the sample drying process. 3.3. Volume-phase transition behavior The volume-phase transition behavior of pNIPAM–nG2% microgels was investigated by means of UV spectroscopy. Fig. 2 shows the plots of the turbidity (absorbance) of pNIPAM–nG-2% as functions of temperature. It is observed that the phase-transition temperature of the microgels varied with n values of the crosslinking agents. The turbidity of pNIPAM–1G-2% and pNIPAM–3G-2% microgels arrives at plateau at relatively lower temperatures than that of pNIPAM–9G-2%, pNIPAM–14G-2%, and pNIPAM–23G2% microgels. On the other hand, the turbidity of pNIPAM– 9G-2%, pNIPAM–14G-2%, and pNIPAM–23G-2% microgels changes more slowly in the broader temperature ranges than that of pNIPAM–1G-2% and pNIPAM–3G-2% microgels. This suggests that these microgels exhibit broader, more continuous phase transition ranges, and the VPTTs of these microgels increase with increasing n. It is plausible to propose that the increase of the VPTTs with the increase

Fig. 4. DSC scan of pNIPAM microgels crosslinked by nG (at a heating rate of 2 ◦ C min−1 ).

of the values of n is caused by the increasing hydrophilicity of the crosslinking agents, since the –(–CH2 CH2 O–)n – segment of nG is quite hydrophilic. It is noted that the turbidity curves of pNIPAM–9G-2%, pNIPAM–14G-2%, and pNIPAM–23G-2% microgels displayed two-step transitions with the range of the experimental temperature; at lower temperature the turbidity changes faster than at higher temperature. This two-step transition of turbidity could be seen clearly from the derivative curves of turbidity as a function of temperature, as shown in Fig. 3. It shows that the first transition temperatures for pNIPAM–9G-2%, pNIPAM–14G2%, and pNIPAM–23G-2% microgels are quite close to those of pNIPAM–1G-2% and pNIPAM–3G-2% microgels, whereas the second transition temperatures increase with the increase of n values; i.e., T23G > T14G > T9G (see Fig. 3). The behavior of the two-step transition was further confirmed by the results of DSC and DLS. In order to investigate the two-step volume-phase transition behavior of pNIPAM–9G-2%, pNIPAM–14G-2%, and pNIPAM–23G-2%, the temperature-sensitive microgels were subjected to calorimetric analysis. The dialyzed mi-

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Fig. 5. Size distribution of pNIPAM–nG-2% microgels at 20 ◦ C.

Fig. 6. Hydrodynamic diameters of pNIPAM–nG-2% microgels against temperature.

crogels were concentrated to ∼1.35 wt%. Fig. 4 presents the DSC thermograms of pNIPAM–nG-2% microgels. It is seen that for pNIPAM–1G-2% and pNIPAM–3G-2% microgels there is only one endothermic transition, whereas there are two for pNIPAM–9G-2%, pNIPAM–14G-2%, and pNIPAM–23G-2% microgels. It is found that the peak temperatures of the first peak of these microgel dispersions are 33.7, 34.1, and 34.0 ◦ C, respectively, which are comparable with those of the microgels of pNIPAM–1G-2% and pNIPAM–3G-2%. The quite identical transition temperatures could correspond to the VPTTs of pNIPAM. It should be pointed out that the second transition could be related to dehydration of –(–CH2 CH2 O–)n – segments (we will return to this issue later on). The hydrodynamic diameter and polydispersity of the microgels can be determined quantitatively using the DLS technique. For pNIPAM–1G-2% and pNIPAM–3G-2% microgels, nearly monodisperse particles (see Fig. 5) with a polydispersity index less than 0.2 were obtained. The size distributions are broader for pNIPAM–9G-2% and pNIPAM–14G2% microgels. In addition, it is noted that the sizes of particles of pNIPAM–23G-2% displayed a bimodal distribution. The broader distribution of pNIPAM–9G-2% and pNIPAM– 14G-2% and the bimodal distribution of pNIPAM–23G-2% suggest the ununiformity of the microgels, which may be caused by the lower reactivity and the higher flexibility of –(–CH2 CH2 O–)n – segments of the crosslinkers. Because of the bimodal distribution it was difficult to investigate the deswelling behavior of pNIPAM–23G-2% microgels using the DLS technique. Fig. 6 is the plot of the hydrodynamic diameter of pNIPAM–nG-2% (n = 1, 3, 9, 14) microgels changing with temperature. The deswelling behavior of pNIPAM–nG-2% microgels monitored by DLS was in good agreement with other methods. In addition, it could be seen from Fig. 6 that the hydrodynamic diameters of pNIPAM–9G-2% and pNIPAM–14G-2% microgels are much larger than those of pNIPAM–1G-2% and pNIPAM– 3G-2% microgels in the swollen state, which could be ascribed to the difference of the microgels in chain length

between crosslinking points. The chain lengths between two crosslinking points for pNIPAM–9G-2% and pNIPAM– 14G-2% microgels were longer than those of pNIPAM–1G2% and pNIPAM–3G-2% microgels, and thus pNIPAM–9G2% and pNIPAM–14G-2% microgels could have larger pore sizes than pNIPAM–1G-2% and pNIPAM–3G-2% microgels. This structural feature is favorable for selective separation of molecules with different sizes. The two-step deswelling behavior for pNIPAM–9G2%, pNIPAM–14G-2%, and pNIPAM–23G-2% microgels could be explained in terms of the intermolecular (or intramolecular) specific interactions. Below VPTT both amide groups and long –(–CH2 CH2 O–)n – segments in the microgels are hydrophilic and they can associate with water molecules via intermolecular hydrogen bonding interactions [19]. The –(–CH2 CH2 O–)n– segments which act as a part of crosslinking agents between pNIPAM segments are mainly located in the interiors of microgels under this condition. As the temperature of the dispersion increases, the hydrogen bonds of amide groups are firstly disrupted and hydrophobic isopropyl groups can simultaneously associate around the VPTT of usual pNIPAM microgels and thus the pNIPAM segments become hydrophobic. The change could contribute to the first-step deswelling of these microgels. The network is loosely crosslinked because the concentration of the crosslinker is low and hence many hydrophilic –(–CH2 CH2 O–)n– segments could be exposed to the surfaces of the microgels. These long hydrated –(–CH2 CH2 O–)n – segments will be dehydrated with increasing temperature and the dehydration process could give rise to the second-step deswelling of the microgels. The enrichment of the hydrophilic –(–CH2 CH2 O–)n– segments on the surface of microgels upon heating was verified by 1 H NMR (see below). 3.4. 1 H NMR spectroscopy Shown in Fig. 7 is the representative 1 H NMR spectrum for pNIPAM–14G-2% microgels at 20 and 40 ◦ C. The res-

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Fig. 7. 1 H NMR of pNIPAM–14G-2% microgels at 20 and 40 ◦ C.

Fig. 8. 1 H NMR of pNIPAM–1G-2% microgels at 20 and 40 ◦ C.

4. Summary Table 2 Area ratio of peak d to peak a at different temperatures Microgels pNIPAM–9G-2% pNIPAM–14G-2% pNIPAM–23G-2%

Area ratio of peak d to peak a Theoretical

20 ◦ C

40 ◦ C

0.12 0.19 0.31

0.09 0.14 0.24

0.33 0.49 0.75

onance at 0.99 ppm is attributed to the methyl proton of N -isopropyl. The broad peaks b and c are mainly due to the backbone of polymer. The peak d is ascribed to the protons of –(–CH2 CH2 O–)n – segments and the peak e to the protons in methylene of isopropyl group. The integration area ratio of peak d to peak a is 0.14 at 20 ◦ C, which is quite close to the theoretical value of 0.19, indicating that most of the crosslinking agents took part in the polymerization. Fig. 6 also shows that the relative intensity of peak d increases as temperature increases to 40 ◦ C, since the integration area ratio of peak d to peak a has attained a value of 0.49 at 40 ◦ C. This observation suggests that more hydrophilic –(–CH2 CH2 O–)n– segments have protruded to the outer part of the network upon heating, due to the hydrophilicity and the increasing mobility of –(–CH2 CH2 O–)n– segments. The situations of pNIPAM–9G-2% and pNIPAM–23G-2% microgels are similar to those of pNIPAM–14G-2% microgels and the NMR integration area changes at different temperature are listed in Table 2. For pNIPAM–1G-2% microgels, there is no such resonance in 1 H NMR spectroscopy in 20 and 40 ◦ C, because the contents of –(–CH2 CH2 O–)n – segments are too low to detect by means of NMR (see Fig. 8). For pNIPAM–3G-2% microgels, the integration area ratio of peak d to peak a in 20 (0.02) and 40 ◦ C (0.024) is small, indicating little enrichment of –(–CH2 CH2 O–)n – segments on the surface of pNIPAM–3G-2%.

Temperature-sensitive microgels crosslinked by polyethyleneglycol dimethacrylates with various lengths (nG) were prepared through surfactant–free radical polymerization. The experimental results show that at the lower length of nG (viz. 1G and 3G), the stable microgels could be obtained up to the higher crosslinker concentrations, whereas at the greater length of nG (i.e., 9G, 14G, and 23G), the stable microgels could be obtained only at lower crosslinker concentrations. TEM results show that the lengths of –(–CH2 CH2 O–)n– segments in the polyethyleneglycol dimethacrylates have significant effect on the morphology of the microgels. When the length of nG is less, spherical particles of microgels can be formed. Due possibly to the higher flexibility of the –(–CH2CH2 O–)n – segment, spherical particles of microgels were not formed for the pNIPAM–23G2% samples. The comparison of the deswelling behavior of pNIPAM–nG-2% microgels reveals that the pNIPAM microgels crosslinked by 1G and 3G undergo a one-step phase transition in the neighborhood of the VPTT, while the pNIPAM microgels crosslinked by 9G, 14G, and 23G occur in the two-step mechanism. The first-step transitions of the microgels are caused by disruption of H-bonds between water and the amide group and the hydrophobic interactions between isopropyl groups; the occurrence of the second-step transition is due to dehydration of the long –(–CH2CH2 O–)n – segments in the crosslinking agents. The long hydrophilic –(–CH2 CH2 O–)n– segments could protrude from the interior to the exterior of the microgel particles after the first transition, since the network is loosely crosslinked. The hydrophilic long –(–CH2 CH2 O–)n – segments can be dehydrated at the higher temperature. References [1] R. Pelton, Adv. Colloid Interface Sci. 85 (2000) 1. [2] K. Ishii, Colloids Surf. A Physicochem. Eng. Asp. 153 (1999) 591.

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