Preparation of Photochromic Waterborne Polyurethane Films:Effect of Spiropyran Concentration on Photochromic Properties

BAO LihongWANG Shixian

1 School of Materials Design and Engineering,Beijing Institute of Fashion Technology,Beijing 100029,China 2 Beijing Key Laboratory of Clothing Materials R &D and Assessment,Beijing 100029,China

Abstract:Incorporation of photochromic compounds to the polymer through chemical bonding will result in an enhancement of photoactivity and stabilization of optical properties. Here,spiropyran (SP) was introduced into polyurethane (PU) through semi-continuous emulsion copolymerization to get waterborne polyurethane dispersion (SPU). Studies on photochromic properties by UV-vis technique demonstrated the absorption intensity increased with the increasing of SP concentration and the colorless film turned to purple under UV irradiation. The kinetics for ring-opening and ring-closing reaction against SP were both first order,but the rate constants decreased with the increasing of SP concentration. The prepared SPU films showed reasonable photostability and photoreversibility according to the convenient test methods under UV-vis irradiation. Temperature dependence studies demonstrated that the photobleaching rate increased at higher temperatures with the same SP concentration. The chemical combination between SP and isocyanate (NCO) end group of polyurethane was confirmed through swelling experiments. Morphology studies were conducted by scanning electron microscope (SEM) micrographs and showed good compatibility between SP and PU.

Key words:spiropyran concentration;waterborne polyurethane film;photochromic properties;kinetics

Introduction

Stimuli-responsive materials have received increasing attention for their special utilities.Spiropyrans (SPs) are a set of well-known photochromic compounds.These compounds are characterized by a marked photochromism.In this case,UV irradiation leads to a usually colorless,nonplanar closed SP form to a colored,planar open zwitterionic merocyanine (MC) form.The MC makes an appearance of an intensive absorption peak at the visible spectral region (550±20) nm,and this process is thermally and photochemically reversible.

SP can be attached directly into a linear polymer chain[1-3]or function as cross-linking groups[4-5].Conformational changes and properties of the polymer can be affected.The presence of photosensitive units allows such copolymers to be used for advanced devices,such as chemical sensors[6-9],rewritable data storage[10-11],biological imaging[12],and optical devices[13].This is due to that different SP isomers could interact with a wide range of materials.Zhangetal.[14]synthesized a kind of poly (N-acryloylglycine-co-N-acryloylglycine SP ester) block,which is a pH-,light- and UCST-type multi-responsive block.Adelamannetal.[15]synthesized P(BMA-co-HEMA-SP) macromolecules compounds and studied the UV-induced conformational changes.They found the polymer backbone of P (BMA-co-HEMA-SP) obviously shielded the SP moieties and was responsible for a less polar microenvironment.Grimm and Schacher[16]presented the solution and film properties of dual stimuli-responsive poly (N-isopropylacrylamide-co-SP acrylate) (P(NIPAAm-co-SPA)) copolymers of varying composition.They found these materials exhibited reversible responses to irradiation in polymeric films for a minimum of three cycles and the increasing of SPA content led to a more pronounced difference between both states.

We herein reported on the characterization of photochromic waterborne polyurethane films containing SP.SPUs with different SP concentrations were synthesized via polycondensation.We were able to further show that the resulting copolymers were photo responsive in the solid polymeric films.The kinetics of the photo-response film were investigated in detail.

1 Experiments

1.1 Materials

Polytetrahydrofuran glycol (PTMG,Mw= 1 000,Aladdin) was dried and degassed for 5 h at 80 ℃ under 0.09 MPa before use.Triethylamine (TEA),acetone，trimethylolpropane(TMP) and 1,4-butanediol (BD) (Fuchen Chemical Reagents Factory,Tianjin,China)were used after being treated with a molecular sieve.Other first grade reagents,i.e.isophorone diisocyanate (IPDI,Aladdin),1-(2-hydroxyethyl)-3,3-dimethylindolino-6′-nitrobenzopyrylospiran (SP,J&K Scientific Ltd.),2,2-bis(hydroxymethyl)propionic acid (DMPA，Aladdin) and N-methyl-2-pyrrolidinone (NMP,Aladdin),were used without further purification.Water was distilled and deionized.

1.2 General preparation of SPU films

Synthesis was generally carried out in four steps as follows.Firstly,in a 250-mL four-necked flask equipped with a mechanical stirrer and a condenser,calculated amount of IPDI and polytetrahydrofuran glycol were added and heated at 60 ℃ under N2atmosphere until the theoretical NCO value was reached (determined by the di-n-butylamine titration method) to get prepolymer.Secondly,DMPA,BD and TMP dissolved in NMP were added and the mixture was heated to 70 ℃ until the theoretical NCO value was reached.During the reaction process,acetone was added to decrease the mixture’s viscosity.Thirdly,the temperature was allowed to drop below 30 ℃ and the mixture was kept in the dark.SP dissolved in dichloromethane was added and the mixture remained under 30 ℃ for 30 min.Then,the carboxylic acid groups were neutralized by the addition of TEA and stirred for further 60 min to ensure the neutralization was fully completed.Finally,distilled water (25 ℃) was added to the mixture with vigorous stir.After the addition of water,further 30 min stir was needed to get polyurethane dispersion with SP (SPU dispersion,shown in Scheme 1).Films were obtained by casting the SPU dispersions on a levelled surface and allowing them to dry at room temperature in the dark for 7 d,and then dryinvacuofor 4 h.The resulting films were stored away from light.

Scheme 1 Schematic process for the preparation of SPU dispersion

1.3 Characterization

An 18 W Hg lamp (Philips) with a wavelength of 365 nm was used as excitation UV source for the isomerization between SP and MC.Absorption spectra were obtained on a UV-visible absorption spectrophotometer (Thermo Scientific,EVOLUTION 600) ranged from 400 nm to 700 nm.Firstly,SPU films were irradiated with the Hg lamp for a certain time and plotted the graph of absorbancevs.time.Secondly,the Hg lamp was taken away and the graph of absorbancevs.time was plotted until the absorbance recovered to the initial state.Finally,the absorbance atλmaxvs.time was recorded.

2 Results and Discussion

2.1 Photochromic properties

2.1.1PhotocolorationprocessofSPUfilms

It is well known that the SP species have two stable forms:ring-closed (SP form) and ring-opened (MC form).These two forms can be reversibly interconverted upon irradiation of UV and visible light.Generally,the absorption spectrum of colorless SP does not show any absorption larger than 400 nm,whereas MC with purple color displays an absorption at (550±20) nm.Therefore,we can study the reversible interconversion of the SP units incorporated in the polymer chains via their UV-vis absorption spectra.To compare the isomerization difference between the SP moieties incorporated in polymer chains,the UV-vis absorption spectra of the polymer films upon irradiation of UV are shown in Fig.1.Before UV light illumination,the polymer film did not show any color and the absorption bands from 500 nm to 700 nm (Fig.1) were a horizontal line,which indicated the existence of SP moieties in SP form.After UV irradiation,an intense absorption band around 555 nm appeared as shown in Fig.1,and the absorption peak increased with irradiation time extended,which was ascribed to the existence of high absorption capacity of the metastable ion formed after UV irradiation.This phenomenon demonstrated that the cleavage degree of SP was very high after irradiation,as it was expected,and most of SP turned to open-ring MC.Similar results have been obtained during the photocoloration of poly (methyl methacrylate) (PMMA) with different concentrations of spirooxazines[18].Photographs of the corresponding films were recorded and shown in Fig.2,displaying an apparent corresponding color upon irradiation with UV light.The colorless film turned to purple under UV irradiation,identifying the conversion of the SP to MC.After illuminated with visible light,the purple film recover to colorless.These changes are reversible and can be cycled many times.

(a) 0.05%

(b) 0.10%

(c) 0.30%

(d) 0.50%

Fig.1 UV-vis absorption spectra of SPU films with different SP concentrations versus UV irradiation time

Fig.2 Photos of SPU films with different SP concentration irradiated with UV and visible light

Fig.3 Photocoloration kinetics (determined at 555 nm) of SPU films exposured to UV light (λ=365 nm)

The kinetics of photocoloration process of SPU films were investigated and the result was shown in Fig.3,whereA0,AtandA∞r(nóng)epresent absorbance of SPU film exposed to UV light for zero,tand ∞ second.κdenotes the slope of the line resulted from the plot ln [(A∞-A0)/(A∞-At)] tot.Figure 3 showed that for all concentrations,the kinetics of the phtocoloration of SPU films were first order.The rate constants for the arise of the colored form (open ring form) were summarized in Table 1 and ranged from 4.913×10-2s-1in 0.05% to 2.700×10-2s-1in 0.50%,which indicated that with the increasing concentration of SP,the photocoloration rate decreased.

Table 1 Rate constants of the photocoloration of SPU films with different SP concentrations

2.1.2Photobleachingprocess

The SPU films were irradiated under UV light for 2 min,then each absorption spectrum was recorded after irradiated with visible light for certain time,and the photobleaching rates of the photochromic films were identified.The dependence of ln[(A∞-A0)/(A∞-At)] with time for the films was shown in Fig.4.The kinetic behaviors were first order too.The photobleaching rates of the SPU photochromic films presented significant variation in the rate constant,and they werek0.50%=0.361 44 min-1，k0.30%=0.456 50 min-1，k0.10%=0.808 32 min-1andk0.05%=2.224 36 min-1,respectively.The results showed that with the increasing of SP concentration,the ring closing reaction declined,just the same as the ring opening reaction.This may be due to more interaction occurred between dyes and urethane groups in polymer chains,and thus decreased the photoisomerization of SP and MC.

Fig.4 Photobleaching kinetics of SPU films under visible light (determined at 555 nm)

2.1.3Fatigueresistance

Instability of the SP dyes under UV irradiation caused by the degradation of dyes are generally observed.To investigate the photostability of SP in polymer films,the SPU films were exposed to UV and visible light for 2 min and 30 min alternatively and measured their absorptions at 555 nm.The results were shown in Fig.5.Figure 5 demonstrated that the photochromic properties of all SPU films faded away gradually with the increase of loop counts.Similar results are reported with PMMA and epoxy resin[19],and this may be attributed to the available polymer-free volume decrease due to increasing interactions of polymer chains.

(a) 0.05%

(b) 0.10%

(c) 0.30%

2.1.4EffectoftemperatureonphotobleachingofSPUfilms

(a) 25 ℃

(b)60 ℃

(c) 80 ℃Fig.6 First-order kinetic plots for photobleaching of SPU films at 25 ℃,60 ℃ and 80 ℃

Table 2 Photobleaching kinetic parameters of SPU films at different temperatures

The results indicated that the photobleaching rate increased with the increasing of temperature for the same SP concentration.The photobleaching rate was 47 times faster at 60 ℃ than that at 25 ℃,and 468 times faster at 80 ℃ than that at 25 ℃ for 0.50% concentration.This may be due to more segmental motion of polymer chains at higher temperatures and increase photoisomerization of SP,resulting in higher photobleaching rates.

2.2 Solvent swelling experiments

To verify SP was chemically combined into polymer,the solvent swelling experiments were carried out.The SPU film was immersed into C2H5OH and CH2Cl2respectively for 24 h (shown in Fig.7).Comparison between the appearance of the solution before [Figs.7 (a) and (c)]and after [Figs.7 (a′) and (c′)] UV light irradiation reveals the diffusion and resistance of SPU film in solvent.The colorless transparent uniformity of the solution demonstrated a good interaction and compatibility between SP and polymer.This returned to the possible reaction between hydroxyl groups (in SP) with isocyanate groups of PU chain during polycondensation and formed a compatible covalent bond.In some reports,the photochromic compound is doped into the polymer latex without any chemical bonding[20-21],which will cause a lack of sufficient stability of the optical properties and worse compatibility of composites.On the other hand,if the photochromic compound is added to the solid polymer film directly,its smart behavior regarding reversible isomerization is sacrificed.The main advantage of our work is to cover both of the deficiencies mentioned above by designing an appropriate copolymer matrix.

(a),(a′),(c) and (c′)— C2H5OH and CH2Cl2 respectively before and after UV light(λ=365 nm) irradiation that have been used to swell SPU films;(b),(b′),(d) and (d′) — SPU films that have been swelled in C2H5OH and CH2Cl2 before and after UV light (λ=365 nm) irradiationFig.7 Results of SPU films swelled in C2H5OH and CH2Cl2(0.05% of SP concentration)

2.3 Scanning electron microscope (SEM)

Morphology and SP distribution of SPU films were studied by an SEM as shown in Fig.8.The images showed that the increase of SP concentration resulted in some aggregation of SP and the aggregation degree increased with the increasing concentration of SP.Therefore,the higher concentration of SP will increase the instability of SPU films.It can be also seen from Fig.2 that the higher content of SP in the polymer will make the film yellowish.So,it is necessary to optimize the SP content to obtain appropriate SPU films with excellent photochromic properties.

(a) 0.05%

(b)0.10%

(c) 0.30%

(d)0.50%Fig.8 SEM microphotographs of SPU films with different SP concentrations

3 Conclusions

A series of photochromic waterborne polyurethane was synthesized by incorporating photochromic compounds SP into the polymer through chemical bonding.The photochromic properties of SPU films were studied by UV-vis technique.The results demonstrated that the absorption intensity increased with the increasing of SP concentration and the colorless film turned to purple under UV irradiation.The kinetics of ring opening and ring closing process were both first order,but the rate constants decreased with the increasing of SP concentration.The prepared SPU films showed reasonable photoreversibility and their photostability needed to be improved further according to cycle tests under UV-vis irradiation.The temperature had a big influence on the photobleaching of SPU films.The swelling experiments in solvent identified that chemical bonds formed between SP and other components of the polymer.SEM photos indicated good compatibility between SP and polymer and SP aggregation would increase at a higher SP concentration.