Trans. Mat. Res. Soc. Japan 38[4] 589-592 (2013)
Water State in Methyl cellulose Thermo Reversible Hydrogels Containing Salt and Polyethylene Glycol
Yuzuru Uehara，Eita Shimoda, Yuichi Iitaka, Yuko Nishimoto* Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan Fax: 81-463-59-4111, e-mail: [email protected]
Methyl cellulose (MC) and Polyethylene Glycol (PEG) were considered to be environmentally and biologically friendly materials, and have been applied to industrial or biological use as a highly hydrophilic and viscous polymer. PEG-water systems in various molar ratios of ethylene oxide (constitutional repeating unit of PEG) to H2O were investigated by DSC, NIR (Near infrared spectroscopy) and 17O NMR, in order to investigate the behavior of water molecules in PEG hydrogel. The melting behavior of eutectic of PEG and water by DSC shows good agreement with those of NIR, NMR measurements. Alkali chloride aqueous solution forms a eutectic at low temperature. In this study, we focused on the MC -PEG- water system and MC-alkali chloride- water system at the water-rich region. The water state or the dynamic mechanical property was investigated. The PEG-MC-water system and MC-alkali chloride-water system form thermo reversible gel in the heating process. We found that the memory of the gel state was kept for 3 days after gelation at room temperature. The period of keeping the memory of the gel state after gelation agreed with the strength of PEG or Alkali chloride -water interaction. It was found that the interaction was detectable by DSC. Key words: thermo reversible hydrogel, water state, eutectic, methyl cellulose, polyethylene glycol, DSC
1. INTRODUCTION 2. EXPERIMAENTAL Aqueous solutions of methyl cellulose (MC) become MC50, MC400, MC4000, NaCl, KCl, RbCl and CsCl a gel state when the temperature increases. The gelation were purchased from Wako pure chemical industries, of the MC solution occurs because of hydrophobic PEG6000 was purchased from MERCK, and used interaction between molecules having methoxy without further purification. PEG (molar fraction of functional group mainly. The change of the heat of the ethylene oxide (EOX): 0.01 or 0.07) and MC (molar sol gel transition of MC-water system is small 1,2. The fraction of MC: 1×10-3) or alkali chloride (molar hydrophobic group associates in water solutions and fraction of alkali chloride : 1.8×10-3，0.1 mol/L) and MC forms cross-linked structure. The viscosity rises by the (molar fraction of MC: 1×10-3) were used in this study. formation of the three dimensional network. These gels Measurements were made with an SIINT DSC Exstar are completely reversible. Polyethylene glycol (PEG) is 6000 DSC100, X-DSC7000, a JEOL NMR JNM EX400, a simple synthetic and water soluble polymer. PEG is ECA400, a Jasco NIR V570, AND Sine wave Vibro considered environmentally and biologically friendly Viscometer SV-10, and TA Instruments rheometer materials, and has been applied to industrial or AR-G2, AR2000, ARES-G2. biological use as a highly hydrophilic and viscous polymer. MC-PEG-water system forms thermo 3. RESULTS AND DISCUSSION reversible gel in heating3. PEG-water system forms a 3.1 Viscoelastic properties eutectic of PEG and water at lower temperature 4. In the MC-water system is classified in a sol (G’
589 590 Water State in Methyl cellulose Thermo Reversible Hydrogels Containing Salt and Polyethylene Glycol
viscoelastic characteristic of MC50 and MC400 returned period kept at 277 K on absorbance of peak near 17 to the value before gelation in 72 hours. 1200nm (a) and T2 of O NMR (b) of PEG6000 (0.01) -MC400 (1.00×10-3)-water sample. The absorption peak 1.0 of a hydrogen-bonded-water appears in 1200 nm. The number of hydrogen-bonded-water molecule was decreased after gelation and returned to the original 0.8 17 value gradually. The value of T2 of O NMR changes reflecting the mobility of the water. The mobility of 0.6 water decreased after gelation and returned to the original value gradually. From the above results, the water state in G'' / Pa 0.4 methylcellulose thermo reversible hydrogel containing PEG can be detected by using melting enthalpy of 0.2 eutectic of PEG and water.
0.0 290 300 310 320 330 340 350 360 40.0 T / K (a)
Fig.1 Temperature dependency of Elastic modulus 30.0 of PEG6000 (0.07)-MC50 (1.0×10-3) -water (1). ●：original sample, □：sample kept at 277 K for 48 hours after gelation (Pa) 20.0 G' 5.0 10.0
2.5 0.0 0 50 100 150 200
δ 0.0 Keeping period at 277K (hours)
tan 40.0 -2.5 (b) 30.0 -5.0 0.01 0.1 1 10 100 -1 angular frequency (rad s )
Fig. 2 Influence of keeping period at 277K on tanδ (Pa) 20.0 -3
PEG6000 (0.07)-MC50 (1.00×10 )-water sample G'' ▽：original sample ○：kept at 277 K for 24 hours after gelation 10.0 □：kept at 277 K for 48 hours after gelation ◇：kept at 277 K for 72 hours after gelation ×：kept at 277 K for 96 hours after gelation 0.0 △： 0 50 100 150 200 kept at 277 K for 120 hours after gelation Keeping period at 277K (hours)
Fig.3 Influence of the keeping period on viscoelastic 3.2 Water state analysis properties of samples after gelation. (a):Gʹ, (b):Gʺ The water affects the characteristic of the hydrophilic ■： -3 polymers. Several techniques can be used to examine a PEG6000(0.07)-MC400 (1.0×10 )-water sample -3 different state of the water of the hydrogels. DSC ●：PEG6000(0.07)-MC50 (1.0×10 )-water sample (differential scanning calorimetry) has been used for estimation of bound water restrained by various kinds of polymeric materials and biomaterials. Fig.4 shows the influence of a period kept at 277 K after gelation on melting enthalpy of eutectic of PEG and water. The melting enthalpy of the eutectic of PEG and water was decreased after gelation. After gelation, sample was kept at 277 K, the melting enthalpy value returned to the original value gradually. Fig.5 shows the influence of a Y. Uehara et al. Trans. Mat. Res. Soc. Japan 38[4] 589-592 (2013) 591
6.0 system obtained from NIR spectrum. It was considered that the hydrophobic interaction of MC with cesium ion (a) was related to the mechanism of the memory of the gel state.
4.0 0.004 (J/g)
H (a) Δ 2.0 0.002 Abs 0 0 50 100 150 200 0 Keeping period at 277K (hours)
30.0 -0.002 (b) 0 50 100 150 200 Keeping period in 277K (hours)
28.0 8.0 (J/g)
H (b) Δ 26.0 7.0 (msec) 2
24.0 T 0 50 100 150 200 6.0 Keeping period at 277K (hours)
Fig.4 Influence of the keeping period on melting enthalpy of PEG-water eutectic after gelation. 5.0 (a): PEG6000 (0.01)-MC400 (1.0×10-3)-water sample, 0 50 100 150 200 (b): PEG6000 (0.07)-MC400 (1.0×10-3)-water sample Keeping period in 277K (hours)
Fig. 5 Influence of the keeping period on absorbance of 17 3.3 Methylcellulose hydrogel containing salt peak near 1200nm (a) and T2 of O NMR (b) of The results cited above applied to MC hydrogel PEG6000(0.01)-MC400 (1.0×10-3)-water sample containing salt. Fig.6 shows the DSC melting curves of eutectics of water and alkali chloride 6, 7. The melting temperature of eutectic in the MC-salt -water system without salt was agreed with that of eutectic in the salt-water system. Fig.7 shows the influence of the keeping period on containing NaCl melting enthalpy of salt-water eutectic after gelation. In the case of KCl, RbCl and CsCl, the melting enthalpy of containing KCl the eutectic of PEG and water was decreased in after 252.6 K gelation. After gelation, the sample was kept at 277 K, containing RbCl the value of melting enthalpy returned to the original endo. dq/dt (mW) containing CsCl value gradually. On the other hand, in the case of NaCl, 263.3 K the melting enthalpy of the eutectic of PEG and water 257.2 K was increased in after gelation. The keeping period was 250 .7K influenced by cation. The keeping period of MC-CsCl-water system was the longest. The period of (mW) 4.0 keeping the memory of the gel state after gelation agreed 223.0 233.0 243.0 253.0 263.0 273.0 283.0 with the strength of MC-salt interaction in the gel state. T (K) It was revealed that the period when MC hydrogel memorized a state of gel was not related to molecular Fig. 6 DSC melting curves of eutectic of water and salt weight of MC. These data agreed with the changes in of alkali chloride (1.8×10-3)-MC4000 (1.0×10-3)- water. 960Abs of free water with keeping period of MC-water 592 Water State in Methyl cellulose Thermo Reversible Hydrogels Containing Salt and Polyethylene Glycol
10
8
6 (J/g) H 4 Δ
2
0 0 20 40 60 80 100 keeping period (h)
Fig.7 Influence of the keeping period on melting enthalpy of salt-water eutectic after gelation. ◆：Nacl, ●：KCl, ▼：RbCl, ▲：CsCl
4．CONCLOUSION MC-PEG-water system forms thermo reversible gel in heating. We focused on the eutectics of water and PEG in the PEG-MC-water system and the eutectics of water and salt in the salt-MC-water system. The water state in methylcellulose thermo reversible hydrogel containing PEG or alkali chloride can be detected by using melting enthalpy of eutectic of PEG and water. It was considered that the hydrophobic interaction of MC with PEG or cesium ion was related to the mechanism of the memory of the gel state.
ACKNOWLEGEMENTS The authors wish to thank Mr. T. Aikawa of TA Instruments for the high sensitive rheological measurements.
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(Received May 15, 2013; Accepted August 17, 2013)