Preparation, characterization and novel photoregulated rheological properties of azobenzene functionalized cellulose derivatives and their α-CD complexes
Introduction
Photoresponsive polymers, which can change their structure, conformation and properties reversibly in response to light, have been the focus of research interests for many years. These polymers will find potential applications in various systems such as photonics, optical storage, surface relief gratings, as well as photoswitchable biomaterials [1], [2], [3], [4], [5].
It is well known that azobenzene undergoes photoinduced trans–cis isomerization under UV irradiation, while the reverse cis–trans isomerization can be effected photochemically or thermally. When azobenzene chromophores are incorporated in the main chain or attached to the side chain of polymers, the conformation changes of azobenzene induced by the isomerization shall produce a concomitant change in the physical properties of the azobenzene-containing polymers [6]. This has led to the use of azobenzene derivatives as a photoresponsive trigger for control of polymers properties. Photoinduced phase separation of polymer solutions is a typical example [7]. At a temperature close to the phase transition temperature, the system is in an unstable state, and hence a small perturbation by photochromic reactions of chromophores may bring about phase separation.
Hydroxypropyl methylcellulose (HPMC) is a hydrophobically modified nonionic cellulose derivative which undergoes thermoreversible sol–gel transition in aqueous solutions [8], [9], [10]. The structure of HPMC is illustrated in Scheme 1. Although the gelation behavior of HPMC has been extensively investigated, there is no report on the preparation of photoresponsive HPMC. Arai et al. [11] prepared azobenzene-containing methylcellulose. However, photoregulation of the sol–gel transition behavior was not possible due to the small temperature window before and after UV irradiation.
α-Cyclodextrin (α-CD) is well known in supermolecular chemistry as a molecular host [12], [13]. Since α-CD is shaped like a truncated cone with a hydrophilic exterior and a hydrophobic interior cavity, it can selectively form inclusion complexes with a variety of guest molecules in aqueous solutions [14]. Molecules that have the azobenzene moiety can be easily included into the cavity of CDs [15], [16], [17], [18], [19], [20], [21], [22]. It has been reported that the trans azobenzene can form stable inclusion complex with α-CD, while the cis azobenzene cannot. In 1990, Harada et al. [23] first reported the formation of complexes of CDs with polymers. Due to their unique architectures and important applications in biomedical fields, inclusion complexes of CDs and polymers have been extensively studied [24], [25], [26]. CDs can also form inclusion complexes with the hydrophobic tails of the hydrophobically modified polymers, which cancel the hydrophobic association and reduce the viscosity of the solution [27], [28], [29]. On the basis of these studies, we have recently designed a series of novel azobenzene functionalized hydroxypropyl methylcellulose (AZO-HPMC) and constructed supramolecular complexes with α-CD [30]. The azobenzene moieties on the side chain of AZO-HPMC not only act as photoresponsive triggers, but also as hydrophobic tails. The sol–gel transition behavior of these polymers can thus be regulated by photoirradiation.
In this study, AZO-HPMC polymers were systematically characterized by FT-IR, 1H NMR, FT-Raman and UV–visible spectroscopy. The effect of α-CD on the water solubility of AZO-HPMC polymers was investigated. Rheological measurements were carried out to study the thermoreversible sol–gel transition behavior of AZO-HPMC and its α-CD complex in aqueous solutions. A possible gelation mechanism for AZO-HPMC was proposed. The effect of photoirradiation on the rheological behavior was also investigated.
Section snippets
Materials
Hydroxypropyl methylcellulose (HPMC, Metolose 90SH100) was kindly provided by Shin-Etsu Chemical Co. Ltd., Japan. The degree of substitution (DS) of methyl groups is 1.4 and the molar substitution (MS) of hydroxypropyl groups is 0.2. The weight average molecular weight is reported by the supplier to be 110,000, determined by light scattering. HPMC powder was dried at 60 °C under vacuum for 24 h before use. 4-Phenylazobenzoyl chloride (97%) was purchased from Aldrich and used without further
Preparation and characterization of AZO-HPMC
The AZO-HPMC polymers with different azobenzene contents were prepared by esterification reaction of the hydroxyl groups on HPMC with the acid chloride groups on 4-phenylazobenzoyl chloride, as illustrated in Scheme 2. AZO-HPMC polymers with DSazo ranging from 0.0035 to 0.059 were prepared by varying the molar feed ratio of 4-phenylazobenzoyl chloride to the anhydroglucose unit of HPMC. Fig. 1 shows the DSazo as a function of molar feed ratio of two reactants. It is found that the DSazo is not
Conclusions
Novel AZO-HPMC polymers and their α-CD complexes have been successfully prepared. FT-IR, 1H NMR, FT-Raman and UV–visible spectroscopy confirm both qualitatively and quantitatively the grafting of azobenzene chromophores onto AZO-HPMC polymers. Addition of α-CD improves the water solubility of AZO-HPMC by formation of inclusion complexes with azobenzene side groups. Red-shift in UV–visible spectra confirms the formation of AZO-HPMC/α-CD complexes.
Rheological studies substantiate the
Acknowledgements
This work is financially supported by the Ministry of Education through an ARC research grant. One of the authors (PJZ) would like to thank Nanyang Technological University for providing research scholarship.
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