Model for athermal enhancement of molecular mobility in solid polymers by light

Abstract

Molecular motion in polymers is frozen below the glass transition temperature T g and changes of viscoelastic functions are most spectacular near T g. Exceptional enhancement of molecular mobility and a decrease of polymer viscosity, by several orders of magnitude, down to the viscous flow regime, are observed way below T g by light absorption. Relaxation processes, which take decades to centuries in some high-T g polymers, are reduced to minute timescales by sub-T g light absorption. Here we develop a model for this intriguing albeit spectacular action of light on glass forming materials and we propose experiments to relate light absorption to materials properties. The model provides a solution to a long-lasting problem of how molecular mobility is enhanced in solid polymers by photoisomerization and provides a tool for a better understanding of the relationship between light absorption and material properties and developing photosensitive polymers for light to mechanical energy transduction.

Optical Trapping of Photosoftened Solid Polymers

“…Taking u r = 0 and f r = 0.024, which correspond to a polymer, poly­(methyl methacrylate) with DR1 as a side chain (PMMA-DR1), close in structure to PMA-DR1, in the dark, i.e., without green irradiation, and γ f ∼ 0.01 J –1 , and u ≅ 0.5575 J , given the experimental conditions, we find ln (η/η r ) ≅ −0.232, implying (η ≅ 0.79 η r ), i.e., ∼20% athermal decrease in the viscosity of the film by green irradiation. The model used in ref assumes an athermal change of viscosity by photoisomerization of the azo dyes. Reference also provides a relationship of a fictitious temperature increase Δ T of the azo-polymer upon photoisomerization, Δ T = ( e γ f ( u – u r ) – 1) f r /β with the coefficient of thermal expansion β of the polymer, assuming that all of the absorbed energy is hypothetically transformed into heat.…”

“…The relationship between η and the absorbed energy dose u is given by where η r , u r , and f r are the reference viscosity, absorbed energy dose, and fractional free volume, respectively, and γ f is the coefficient of photoexpansion of the polymer. Taking u r = 0 and f r = 0.024, which correspond to a polymer, poly­(methyl methacrylate) with DR1 as a side chain (PMMA-DR1), close in structure to PMA-DR1, in the dark, i.e., without green irradiation, and γ f ∼ 0.01 J –1 , and u ≅ 0.5575 J , given the experimental conditions, we find ln (η/η r ) ≅ −0.232, implying (η ≅ 0.79 η r ), i.e., ∼20% athermal decrease in the viscosity of the film by green irradiation. The model used in ref assumes an athermal change of viscosity by photoisomerization of the azo dyes.…”

“…Sub-glass-transition-temperature (sub- T g ) mobility enhancement of azo-polymers by photoisomerization was first reported and used to model photoassisted poling . Recently, a model of athermal mobility enhancement, up to visco-fluidic motion, of solid polymers was reported, relating photoisomerization to viscosity change of solid polymers . Owing to the photoselectivity of azo-polymers, the polymers move in the direction of the gradient of the light intensity with an efficiency that depends on the cosine square of the angle between the vector of the light gradient and the polarization vector, a feature that forms the basics of photoinduced vectorial mobility of matter (PVMM) .…”

Optical Characterization of Ultra-Thin Films of Azo-Dye-Doped Polymers Using Ellipsometry and Surface Plasmon Resonance Spectroscopy

“…Azo-dye-containing materials have been studied extensively in past decades for applications in holography and optical data storage [28,29], nonlinear optics (NLO), electro-optic modulation (EO), second-harmonic generation (SHG) [30][31][32][33], photomechanical actuation and matter motion, and so on [34][35][36][37][38][39][40]. Azo dyes also have potential applications in dye chemistry and bio-photonics [41,42], driving a natural interest in their optical characterization, namely the determination of the real and imaginary parts of n c as a function of the wavelength of light.…”

Одно- И Двулучевое Оптическое Формирование Рельефных Дифракционных Микроструктур В Пленках Карбазолсодержащего Азополимера

“…Разработаны методы формирования трехмерных периодических массивов [10][11][12][13], двумерных поляризационных решеток [14,15], а также одиночных микронеровностей [16][17][18]. Развитие указанных технологий дает предпосылки для реализации управляемых дифракционных оптических элементов, например, для устройств оптической памяти [19].…”