Physical Aging of Photo-Crosslinked Poly(ethyl acrylate) Observed in the Nanometer Scales by Mach-Zehnder Interferometry

Abstract

Deformation in the nanometer scales of photo-crosslinked poly(ethyl acrylate) (PEA) was in situ monitored by MachZehnder interferometry (MZI). Upon irradiation with 365 nm, anthracene labeled on the PEA chains undergoes photodimerization, generating the PEA networks. The cross-link kinetics induced by different light intensities was monitored by using a UV-visible spectrometer. On the other hand, the deformation in the nanometer scales of the photocross-linked polymer was measured in situ by using a Mach-Zehnder interferometer (MZI) under the same experimental conditions. The aging data observed by ceasing irradiation at different stages of the photocuring process reveal a general relationship for the kinetics of aging under different cross-link densities. Upon approaching the glassy region, mobility of polymer quickly decreases and eventually almost vanishes as the polymer is vitrified. This phenomenon is often observed in polymers under cooling, thermally curing or photocuring. A number of interesting physical phenomena such as the Kohlrausch-Williams-Watts (KWW) relaxation behavior 1,2 have been observed in the vicinity of this vitrification process. For this particular case, diffusion becomes size-dependent and the kinetics is no longer expressible by a simple exponential function of time.3 As the cross-link reaction proceeds to a certain point, the polymer enters the glassy state and falls out of equilibrium, exhibiting the so-called physical aging phenomena whose time-evolution is much slower than the time scales of observation.4 It is well known that photochemical reactions in the bulk state of polymers often proceed non-uniformly due to the inhomogeneity of the local environments.5 This inhomogeneous reaction kinetics, particularly for the case of cross-link, could generate a local transient strain field in these reacting polymers. Depending upon the relative distance from the glass transition temperature (T g ), this local strain field can either completely relax or persist for a long time inside the sample during the course of the reaction. The latter case would be enhanced by the glassy state where the relaxation of the reaction-induced strain becomes extremely slow.Physical aging of polymer has been extensively investigated by a number of experimental methods. Among them, dilatometry has been widely used to monitor the changes in volume of polymers at different temperatures in the glassy state. [6][7][8] Recently, fluorescence method using probe molecules with the internal rotations sensitive to viscosity was also utilized to examine the aging phenomena of polymers.9 Furthermore, an X-ray diffraction method was recently proposed for monitoring the strain distribution in amorphous materials, 10 which would be able to provide a potential tool for studies on physical aging.Recently, we have developed a Mach-Zehnder interferometer (MZI) equipped with a high-pressure Mercury lamp (350 W, Moritex, Japan) to in situ monitor the time-evolution process of the strain in photoreactive polymers.11 In this study,...

Reaction‐Induced Phase Separation of Polymeric Systems under Stationary Nonequilibrium Conditions

“…Owing to this inhomogeneity, a transient local strain field can be generated and developed in the sample. By performing in situ experiments using a Mach-Zehnder interferometer [51], the relaxation of this strain field in both photo-cross-linked homopolymers [45] and in the bulk state of binary polymer mixtures was reported with its consequence on the morphology emerging in photo-cross-linked polymers [52,53]. It should be noted that, in contrast to the bulk state, the effects of elastic strain are much more significant in liquid mixtures where the morphological regularity greatly decreases by periodic irradiation [54].…”

“…These results were obtained from the changes in the optical path length difference (OPLD) and in the refractive index between the irradiated and the nonirradiated parts in the same sample. Note that the strain or deformation ε induced by irradiation can be calculated from the following equation [45,51]:…”

“…For polymer mixtures undergoing phase separation, this reaction-induced strain can couple to fluctuations, leading to several interesting phenomena [52]. It is also found that the elastic deformation emerging under irradiation can be expressed by a universal function of the nondimensionalized aging rate, which is defined as the product between the aging rate k a and the elapsed time t e [45]. This aging rate is obtained by analyzing the decay of the elastic strain ε with the elapsed time after stopping irradiation.…”

“…Relaxation of the elastic strain generated in a poly(ethyl acrylate) film observed by a Mach-Zehnder interferometer at 25 • C. γ : the mean cross-link density; dashed curve with open symbols: the elastic strain observed during irradiation; solid curves with closed symbols: elastic strain monitored after stopping irradiation[45].…”

Applications of Mach-Zehnder Interferometry to Studies on Local Deformation of Polymers Under Photocuring

“…From the plot of normalized shrinkage ( ε / ε max ) vs. non-dimensionalized elapse time deined as ( t e . k a ) where t e is the elapse time and k a is the characteristic time of the aging process, it was found that all the aging data obtained with a constant irradiation intensity can be expressed by a master curve [17].…”