Elsevier

Polymer

Volume 46, Issue 15, 11 July 2005, Pages 5556-5568
Polymer

Polymer thin film photodegradation and photochemical crosslinking: FT-IR imaging, evanescent waveguide spectroscopy, and QCM investigations

https://doi.org/10.1016/j.polymer.2005.05.050Get rights and content

Abstract

Photodegradation and photocrosslinking of benzophenone blended polystyrene (PS) thin films were investigated primarily using Fourier transform infrared (FT-IR) imaging, evanescent waveguide spectroscopy (WS), and quartz crystal microbalance (QCM) methods. The main objective is to observe the changes (spectral and chemical) indicative of these competitive processes in an ultrathin polymer film. This also serves as a model study in the application of combined spectroscopic, optical, and acoustic methods towards understanding crosslinking and degradation phenomena within the same time frame of observation. To induce photocrosslinking, 1,12-dodecanediylbis(oxy-4,1-phenylene)] [bis[phenylmethanone] (2BP12), a small molecule with two benzophenone groups, was blended with PS in solution, spincast onto glass and silicon substrates, and irradiated with ultraviolet light. Photodegradation and benzophenone-mediated crosslinking were observed both directly via functional group spectroscopies and indirectly via their effects on thin film surface properties and morphologies. Atomic force microscopy (AFM) and QCM were used to elucidate local morphology change and mass-uptake kinetics in the presence of O2 in air, respectively. All results correlated well with the two photoprocesses occurring simultaneously and competitively on these films with the refractive index, thickness, and mass change differing with the presence of 2BP12. Crosslinking was observed to cause an increase in thickness while photodegradation gave a decrease. Both processes resulted in an increase in the refractive index and mass. While various methods have separately identified these observations, this is the first instance that the in situ chemical, optical, and structural dimensionality of the photodegradation and photocrosslinking phenomena in a thin polymer film is correlated within the same time frame of observation.

Introduction

Polysytrene (PS) is one of the most ubiquitous of commercial polymers. Due to the huge variety of its applications, photoinitiated degradation and oxidation in air are important issues. These decomposition processes have been reported to typically occur over the range of 250–400 nm [1], and can be initiated even by ordinary sunlight [2]. According to accepted terminology, photodegradation refers to light-initiated chain scission, crosslinking and other processes under vacuum or inert atmosphere. On the other hand, oxidative photodegradation (or degradative photooxidation) occurs in air and includes, in addition to the processes described above, the formation of various oxygen-containing functional groups such as peroxides or carbonyls. In this work, the term photodegradation will be used in general, and the environment in which occurs will be obvious from context.

Numerous mechanisms for PS photodegradation have been proposed over the years [3], but a totally consistent theory is yet to be agreed upon, due to the complexity of the kinetics and the formation of various photodegradation products. An accepted classical mechanism [1], [4] for PS degradation under ultraviolet (UV) light is given in Scheme 1. When irradiated at λ<300 nm, PS forms the PS radical (Eq. (1)), which in air leads to a peroxyradical, and eventually a PS hydroperoxide (Eq. (2)). The latter photolyzes into an alkoxy radical (Eq. (3)), leading to chain scission with the formation of shorter chain PS radicals and carbonyl species (Eq. (4)). In addition to chain scission, conjugated alkenes also form in the aliphatic portion of PS through stable polyene radical intermediates (Eq. (5)). Polyene formation (which causes, in addition to other factors, yellowing in degrading PS) is the major decomposition process occurring when PS is irradiated under vacuum.

The mechanism above shows that the reactive PS hydroperoxide is formed upon exposure to atmospheric oxygen. However, the photodegradation process can be initiated by in-chain structural defects [5], such as hydroperoxides formed during the polymerization of styrene and subsequent processing. Mechanistic studies have also been performed using external photoinitiators such as peroxides, benzophenone (BP) [6], and other ketones and aldehydes [7]. Such initiators increase the rate and efficiency of photodegradation by the process below, shown for BP in Scheme 2. BP forms a triplet ketyl radical upon UV irradiation (Eq. (6)), which abstracts a hydrogen from PS (Eq. (7)). The singlet BP radical in the presence of oxygen forms the highly reactive hydroperoxide radical (Eq. (8)). The latter upon reaction with a PS radical produces a PS hydroperoxide (Eq. (9)), which decomposes as discussed above in Eqs. (3)-(4). Kaczmarek and co-workers have shown photoinitiation as the dominant function of BP (and other initiators) at relatively low concentration (0.1%) in a PS matrix when irradiated in air [6]. At a higher concentration (0.5%), photodegradation efficiency decreases. This change is due to phase-incompatibility between PS and BP, and decrease in their surface contact. In addition to this, benzopinacol formation from ketyl radical combination (Eq. (10)) competes with the initiation and acceleration of PS degradation. Crosslinking can also become a major process when a singlet BP radical and a PS radical (Eq. (11)) combine although this has not been thoroughly investigated before [8].

PS degradation processes can be observed by a number of techniques, including thermal and dynamic mechanical analyses. For example, simple viscometric measurements of irradiated films of PS–BP blends [4] show a decrease in viscosity-average molecular weight, Mv, as chain scission occurs at low initiator concentrations. As BP concentration is increased incrementally, Mv eventually increases as crosslinking becomes more pronounced. These physico–mechanical methods are important for analyses of the bulk polymers. Spectroscopic methods are also common, for bulk as well as film samples. Electron spin resonance spectroscopic measurements of PS films irradiated both under vacuum and in air give spectra assigned to the PS radical and the peroxyradical [8]. Pioneering work by Gueskens and co-workers shows IR peaks at 3300–3500 and ∼1700 cm−1 for hydroperoxide and carbonyl species, respectively [9]. The C6-point double bondC stretch of degradation polyolefin products is not observed directly in the IR, due to overlap with the strong phenyl ring stretch at 1601 cm−1. However, they have reported these chromophores using UV–vis spectroscopy. While much investigation has been done on bulk samples, only a few detailed studies have focused on thin films using combined spectroscopic, microscopic, and acoustic methods within the same time frame of observation.

There is a lot of interest in polymers (including PS) cast as thin films. In this form, many novel applications present themselves for mechanical, optical, and electronic thin film devices and coatings. It is also known that the physical properties of polymers can deviate significantly from those of bulk samples when the film thickness is less than 150–100 nm [10], [11]. Therefore, techniques that are sensitive to the physico–chemical processes occurring with ultrathin films are of great interest for structure–property correlation and understanding of phenomena. Thus, we have sought in this work to investigate the analysis of PS photodegradation and photocrosslinking using such methods.

FT-IR Imaging [12] is a technique for obtaining spatially and temporally-resolved chemical and structural information. As such, it can provide a ‘still picture’ of specific functional groups, e.g. a carbonyl moiety, over a relatively large sample area at specific periods of time. As a non-destructive technique, it has been applied to numerous polymeric systems for observing processes as diverse as oxidation [13], crosslinking [14], and crystallization [15]. Physical blends [16], and dissolution of polymers in various solvents [17] including supercritical CO2 [18] have also been monitored. The use of synchrotron radiation as the IR source drastically improves spectral quality [19a,b], and new sampling techniques like internal reflection IR imaging (IRIRI) or micro-ATR provide enhanced spatial resolution [19c,d]. Since FT-IR imaging allows real-time observation of chemical changes in functional groups, we have utilized it to observe the photochemical processes in a PS-initiator polymer blend degradation and cross-linking study.

WS can be used to determine the thickness and refractive indices of polymer films when deposited on a noble metal surface. Typically, gold (45 nm) is coated on a glass slide, the polymer film is cast on the gold side, and the glass side of the substrate is then put into optical contact with a glass prism using a refractive index matching liquid, using the Kretschmann coupling technique [21].

When the polymer is of sufficient thickness, guided optical waves can propagate in a direction parallel to the substrate plane. These waveguide modes exist subject to the following condition [22]:kzd=mπ+β1+β2where kz is the component of the wave vector normal to the substrate plane, and d is the thickness of the polymer film, m is the order of the waveguide mode, and βi (i=1,2) are the phase shifts of the propagating wave at the film boundaries. βI are dependent on the polarization of light, and are defined by Fresnel formulas.

For very thin layers of polymer, a surface plasmon resonance (SPR) mode (m=0) can be observed using p-polarized light. With increased thickness, two possible waveguide modes can be excited depending on the polarization of light: (i) a transverse magnetic (TM) mode under p-polarization, which is sensitive to both the out-of-plane refractive index nz and the in-plane ny in the direction of guided wave propagation, and (ii) a transverse electric (TE) mode under s-polarization, sensitive to the in-plane nx perpendicular to the guided wave direction. When reflectivity (R) is measured versus angle of incidence θ, minima are observed beyond a critical angle θc according to the number of modes (m=integer) excited. The angle θ of these minima depend on the dielectric constants ε and the thicknesses of the waveguide layers, including the glass, gold, and polymer [23]. The thickness(es) and ε of the gold layer (and any underlying metal) can be determined from SPR data (using the same Kretschmann set-up) by simulating the spectrum of the bare gold-coated substrate using Fresnel calculations. The experimental waveguide spectra can then be fit to generate thickness d and refractive index n (n2=ε) for the polymer layer. WS can be a powerful tool to determine thickness, optical properties, and even molecular orientation of thin films. For instance, anisotropy in the film can be detected by non-identical values of nx, ny, and nz. Again, a series of spectra periodically obtained can be used to observe changes as a function of time.

The QCM is a highly sensitive technique for observing mass changes due to physical or chemical changes occurring in a thin layer of material deposited on a quartz crystal [24]. The piezoelectric properties of the crystal changes with mass adsorption at the interface of the crystal and air, causing a change in the oscillating frequency Fq. The resulting change in frequency ΔF can be used to measure a mass change Δm in the nanogram scale, using the Sauerbrey equation [25], tailored to the parameters in the QCM experiment:ΔF=2Fq2ΔmAρqμqwhere Fq is the resonant frequency of the QCM crystal, A is the area of the electrode, ρq is the density of quartz (2.65 g/cm3) and μq is the shear modulus of quartz (2.95×106 N/cm2). The values pertinent to our work are Fq=5 MHz, and A=1.227 cm2. Plugging in the appropriate values, the change in mass can be determined directly from the equation:Δm=2.17×108ΔF(Hz)This equation is taken to be valid for measurements taken both in air and in vacuum [26]. By measuring the frequency or mass changes as a function of time, kinetic data can be obtained.

Section snippets

Materials

Polystyrene (average Mw 250,000) was purchased from Acros Organics. All other materials were purchased from Aldrich and used without further purification. All solvents were purchased from EM Science and used without further purification.

Results and discussion

For all FT-IR, IR imaging, and UV–vis work described below, a higher concentration of 2BP12 (50 mg/mL 50:50 wt% with PS: 2BP12) was used than is typical of most photodegradation studies [4], [6]. Our objective in using this PS:2BP12 ratio was to ensure a significant signal attenuation within the time of observation for the BP carbonyl suitable for all FT-IR imaging, UV–vis, WS, and QCM measurements (Fig. 1(a)). Our early experiments showed that at lower amounts of 2BP12 or PS:2BP12 ratio of 90:10 

Conclusions

In this work, we have investigated systematically the photodegradation and photocrosslinking of benzophenone modified PS thin films. While there are myriad techniques for the physico–chemical analysis of photodegradation and photocrosslinking in the bulk, relatively fewer spectroscopic and microscopic methods have been combined and applied to polymer thin films within the same time frame of observation. Since the oxidation and chain scission processes occur only within a limited distance (100s

Acknowledgements

We acknowledge partial financial support from NSF-CTS (0330127) and DMR-0315565. We would also like to acknowledge technical support from Molecular Imaging Inc., Maxtek Inc., and Optrel GmBH.

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