1~H NMR Study of Polyvinylalcohol Irradiated by Ultra-violet

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【刊名】Chinese Chemical Letters


Polymers may be aged by many physical and chemical factors1-4. Reduction of theirmolecular weights, the change of colors, and worse the mechanics properties are all thesigns of their aging. Ultra-violet light (UV) is a higher energy source, it can causestronger effect on the aging of the polymer. We studied the motion of polyvinyl alcohol( PVA ) chains after irradiated of UV with a wavelength of 310 nm, the formation ofhydrogen bonds between hydroxyl groups and the residual water in the solvent DMSOby H NMR spectra, spin-lattice relation time (T1) measurements and infrared (IR) 1spectrometry.ExperimentalPVA (DP=1750 50) is a commercial product of the Chemical Plant of Beijing East City.It was irradiated in a box with a lamp of 365 nm in distance of 10 cm, which type isUV-A of the lamps' plant of Tianjin. The irradiation time varies from 1 to 5 hours. The irradiation polymers were then dissolved in dimethyl sulfoxide (DMSO) at aconcentration of 5g/L. NMR experiments were performed on a Varian Inova 400 NMRspectrometer at 30°C at a proton frequency of 399.74 MHz. Spin-lattice relaxationtimes were measured by the inversion recovery method, with anerrorless than 5%. IRspectra were recorded with potassium bromide pellets on Bruker Equinox 55 IRspectrometer.Results and DiscussionFigure 1 shows that the H NMR spectra of PVA before and after irradiation by UV at 365 nm at 1different time. Figure 1 H NMR spectra of PVA before and after irradiation by UV light 1 (a)0h (b) 1 h (c) 2 h (d) 3 h (e)4 h (f) 5 h Table1 The spin-lattice relaxation times( T1) of PVA at various irradiation time Irradiation time (h) Spin-lattice relaxation times (s) CH CH2 0 0.57 0.36 1 0.62 0.35 2 0.59 0.35 3 0.69 0.35 4 0.70 0.36 5 0.77 0.36 Peaks at δ 1.33-1.45, 3.84 and 3.30 ppm are the proton of CH2, CH and the residualwater protons, respectively. The group of peaks at δ 4.19-4.64 ppm is the resonance ofthe OH group of the polymer originating from the "head-tail" sequence structure5 of thePVA main chain. The peak at δ 2.50 ppm belongs to the proton of the solvent DMSO.It is obvious that the OH proton peak of the polymer becomes wider and featureless asthe duration of irradiation increases, and finally vanishes. Meanwhile, the water protonpeak is widened simultaneously, and shifts to the high field, while those of the CH2 andCH remain unchanged. These results suggest that the amount of hydroxyl groups inthe polymer decreased, probably owning to the self-condensation, forming the etherbonds. These oxygen atoms bearing lone pair electrons, which can form hydrogenbonds with water molecules, resulting the shift of the water proton peak towards lowfield in H NMR. 1 Meanwhile, the widening of this peak further indicates that themotion of the water protons is seriously restricted by the bonding with the soft-solid likepolymer. Table 1 shows the variation of the spin-lattice relaxation times (T1) of the CH andCH2 protons with the increase of irradiation time. T1 of CH2 remains unchanged, whilethat of CH1 connected with the hydroxyl group increases monotonously.Wavenumber cm-1 (a) before irradiation (b) after irradiation 4 hours As shown in Table 1, T1 of CH2 remains almost unchanged, while that of CH,nnected with the hydroxyl group, increases monotonously. This indicates theistence of OH groups in the PVA chain which connect with CH group. These OHoups can not form hydrogen bond between themselves, but it can form hydrogen bondth water molecule. So the movement of CH group is somehow inhibited and the axation times T1 increase. In contrast, CH2 group cannot be affected by the hydrogennd of OH group. So the relaxation times did not change obviously. Figure 2 shows that the IR spectra before and after UV irradiation, the strongeraks of IR adsorption at wave number of 3400 cm-1 indicated the formation of the ermolecular hydrogen bond. Usually the peaks of hydroxyl group are at 3600 cm-1. In conclusion, for PVA, before and after irradiation by UV, the formation of hydro-n bond between hydroxyl group and water proton and their exchange not only make peak of water proton moves to lower field, but also effect the movement of CH group.AcknowledgmentsThe authors are grateful for the financial support received from the Laboratory of MagneticResonance and Atomic and Molecular Physics (Wuhan institute of Physics and Mathematics,Chinese Academy of Sciences).1~H NMR Study of Polyvinylalcohol Irradiated by Ultra-violet1. D. H. Choi, Y. K. Cha, Polym. Bull., 2002, 48, 4. 2. B Piyush S., Bandopadhyay, Jayesh R. Bellare, Polym. Degr. Stab., 1995, 7. 3. L. J, Arnaud, R, Gardette, J., Polym. Degr. Stab., 1991, 33. 4. Dave Louden, Alan Handley, Eva Lenz. et al., Anal.Bio. Chem., 2002, 373, (6).. 5. Q. H. Chu, B. Ma, Liaoning Chemical Industry (in Chinese), 1997,26(2), 113. 6. J. Q. Li, Spectral Analysis of Organic Structure (in Chinese), The press of Tianjin University, Fenhe Yang, 2002,1, 91.