Thermo-responsive cellulose papers grafted with poly ( di ( ethylene glycol ) methyl ether methacrylate )

Novel thermo-responsive cellulose papers were prepared via grafting poly (di(ethylene glycol) methyl ether methacrylate) (PDEGMA) by activators regenerating electron transfer (ARGET) and atom transfer radical polymerization (ATRP). Attenuated total refraction Fourier-transform infrared spectroscopy (ATR-FTIR) and scanning electron microscopy (SEM) measurements of the modified paper showed that PDEGMA brushes were successfully grafted on the paper surface. The thermal stability of the papers before and after grafting was evaluated by thermogravimetric analysis (TGA). The PDEGMA-grafted paper exhibited a two-step thermal degradation process, and presented thermo-responsive characteristics. It was hydrophilic at room temperature but changed rapidly to highly hydrophobic when the temperature rose above 50 °C.


INTRODUCTION
Cellulose paper is one of the commonly used materials in our daily life.][13][14][15][16] Poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) is a biocompatible, thermo-responsive polymer with a stable lower critical solution temperature (LCST) of 27 °C. 17,18 hen its temperature is above the LCST, PDEGMA changes from hydrophilic to hydrophobic.There has been no research on the synthesis of PDEGMA-grafted papers and the thermo-responsive properties of paper surface.This report presents a versatile, synthetic strategy to fabricate cellulose paper grafted with thermo-responsive PDEGMA via surface-initiated activators regenerated by electron transfer for atom transfer radical polymerization (SI-ARGET-ATRP). 19,20 he hydrophobic characteristic and thermo-responsive behavior of the resulting modified paper were investigated.

Immobilization of Initiator on Filter Paper
Filter paper (6 × 6 cm) was washed with ethanol, acetone, and THF via ultra-sonication for 3 min prior to initiator immobilization.The filter paper, BiB (2.81 g), TEA (1.36 g), DMAP (0.15 g), and THF (80 mL) were added to a round bottom flask.Polymerization proceeded for 24 h with shaking at room temperature.Thereafter, the filter paper was removed from the flask, washed by ultra-sonication with acetone and THF, Soxhlet-extracted with ethanol for 24 h, and finally dried under vacuum.

Grafting of PDEGMA in 2-Propanol/H 2 O Mixed Solvent
A piece of initiator-modified filter paper (2 × 2 cm) was immersed into a round bottom flask containing CuBr 2 (1.1 mg, 5.0 µmol), PMDETA (8.6 mg, 50 µmol), AsAc (8.8 mg, 50 µmol), DEGMA (3.2 g, 17 mmol), and ethanol (4 mL).The flask was degassed with nitrogen for 15 min and finally sealed.The flask was placed in a 25 °C water bath for 8 h, and then the reaction mixture was exposed to air and diluted with methanol to terminate the polymerization reaction.The filter paper was removed and purified by ultrasonic washing with dichloromethane, THF, ethanol, and methanol.Thereafter, the filter paper was dried under vacuum and stored in a desiccator.

Methods
X-ray photoelectron spectroscopy (XPS) was performed on a PHI 5000 VersaProbe spectrometer (ULVAC-PHI, Tokyo, Japan) with a monochromatized Al-Kα X-ray source, and all binding energies were referenced to the neutral C1s peak at 284.6 eV to compensate for the surface charging effects.Fourier-transform infrared spectroscopy (FTIR-650) and attenuated total refraction Fourier-transform infrared spectroscopy (ATR-FTIR) (Nicolet Nexus 870, Thermo Scientific, Waltham, USA) was utilized to identify the functional groups of the modified cellulose papers.The surface morphology of papers was observed by FE-SEM (Hitachi model S-4800, Tokyo, Japan).Thermogravimetric analysis (TGA) was run on a Shimadzu DTG-60 AH instrument (TA Instruments, Tokyo, Japan) at a heating rate of 10 °C/min.Static water contact angle (CA) measurements were conducted on a CA instrument (model JC2000D, Powereach, Shanghai, China) at room temperature or 50 ± 1 °C.

Grafting of PDEGMA via ARGET-ATRP
The immobilization of BiB on the paper surface was attained via the reaction between 2-bromoisobutyryl bromide and the hydroxyl groups of cellulose.The resultant product was then used as the ARGET-ATRP agent to graft PDEGMA, as illustrated in Scheme 1. 11,14 XPS analysis showed that the bromide content on the surface was about 0.62% after a 24-h reaction.Compared to conventional ATRP, ARGET-ATRP utilized stable Cu (II) as a catalyst rather than Cu (I), which was sensitive to oxygen in the air.The amount of catalyst required for ARGET-ATRP was also much less than ATRP.In addition, ARGET-ATRP could be conducted in the presence of a small amount of oxygen because of the addition of excessive reducing agent.Therefore, the process of deoxygenation for polymerization was simplified.In this paper, the grafting of DEGMA on filter paper was conducted with 300 ppm of catalyst, and the modified paper was easily purified by ultrasonic washing.Figure 1 showed the ATR-FTIR spectra of the filter papers before and after grafting.The characteristic absorption bands of cellulose were observed in all spectra.The peaks at 3322 cm -1 , 2895 cm -1 , 1440 cm -1 , and 1000 to 1150 cm -1 were attributed to the stretching vibration of OH groups, stretching vibration of saturated hydrocarbon bonds, bending vibration of saturated hydrocarbon bonds, and stretching vibration of C-O bonds, respectively. 16The peak at 1725 cm -1 in the PDEGMA-grafted filter paper spectrum, which represented esters, indicating the successful grafting of PDEGMA on the filter surface.To eliminate the effect of physical absorption of polymers on the surface, a reference blank paper without initiator was processed under the same conditions.The ATR-FTIR spectrum of the blank paper was almost the same as that of the virgin paper, indicating that the PDEGMA cannot be grafted to filter paper without initiator immobilized on the surface.Figure 2 shows SEM images of the virgin and PDEGMA-grafted paper.The unmodified cellulose fiber surface was relatively smooth (Fig. 2a), and became rough after PDEGMA grafting due to the presence of the grafting polymer (Fig. 2b).
Scheme 1 Synthesis route for the immobilization of initiator on paper and subsequent surface grafting of PDEGMA Fig. 1 ATR-FTIR spectra of the papers before and after grafting The thermal degradation behaviors of the modified filter papers were investigated by TGA (Fig. 3).There was only one decomposition step for virgin and initiator-immobilized filter paper.The onset decomposition temperature was Fig. 2 SEM images of the virgin paper before grafting (a) and PDEGMA-modified paper after grafting (b) Fig. 3 TGA (a) and DTG (b) traces of the papers before and after grafting 339 °C for virgin paper, which was reduced to 308 °C for initiator-immobilized paper.The decrease in the degradation temperature can be explained as follows.First, the esterification of cellulose hydroxyl groups destroyed the crystal structure to some extent, which further reduced the thermal stability according to previous literature. 21,22 condly, HBr was produced during the thermal degradation process, which probably accelerated the decomposition of cellulose. 21The thermal stability of the filter paper decreased after the grafting of PDEGMA, showing two main peaks on DTG profiles, which were located at 220 and 364 °C, respectively.It meant the degradation behavior of PDEGMA-grafted filter paper was completed in two steps.The first step with lower decomposition peak might be attributed to the low stability of grafted PDEGMA side chains. 14,22 he later step with higher decomposition peak reflected the degradation behavior of cellulose.Fig. 4 Thermo-responsive property of the PDEGMA-grafted paper Fig. 5 CA photographs of the PDEGMA-grafted paper at room temperature and high temperature

Thermo-Response
Static water contact angle (CA) measurements were utilized to evaluate the wettability of the cellulose surfaces at different temperatures (Fig. 4).When a water droplet was placed on the surface of PDEGMA-grafted paper, the initial 10µm a 10µm b CA at room temperature and at 50 °C was the same (128°).However, the CA stability differed at different temperatures.At room temperature, the modified filter paper was relatively hydrophilic.The CA decreased to 62.5° at 10 s; the water droplet was completely absorbed in 15 s.In contrast, at 50 °C, the modified filter paper became hydrophobic, and the CA remained at above 120°, even after 19 s.Photographs of water droplets at different temperatures and times are shown in Fig. 5.The process was reversible.When the temperature dropped from 50°C to room temperature, the modified paper switched back to hydrophilicity from hydrophobicity.This thermal responsive cycle can be repeated many times.The underlying mechanism of this thermal responsive behavior has been explained in our previous publications. 23,24 CONCLUSIONS Thermo-responsive cellulose paper was successfully fabricated by graft filter paper with PDEGMA via ARGET-ATRP.Initiator was first immobilized on the filter paper, followed by grafting polymerization.The surface grafting of PDEGMA was confirmed by ATR-FTIR and SEM analyses.The PDEGMA-grafted paper had a two-step thermal degradation process, and presented a thermo-responsive characteristic.It exhibited hydrophilicity at room temperature and highly hydrophobicity at a higher temperature.