A difusividade térmica é uma das propriedades físicas que está entre as mais importantes em um material. Sua determinação apresenta inúmeras dificuldades, o que requer o uso de técnicas precisas que forneçam valores confiáveis. Neste trabalho será apresentado à viabilidade de utilização de espectroscopia de lente térmica (ELT) na determinação da difusividade térmica (D) de materiais transparentes. Como exemplo discute-se os valores da difusividade térmica de materiais policristalinos, vítreos, poliméricos e cristais líquidos. Quando comparada com técnicas convencionais de medidas de difusividade térmica, a ELT apresenta a vantagem de ser rápida, medir amostras de tamanho reduzido, e permitir a obtenção de valores precisos em temperatura ambiente. Quando usada com outras técnicas permite a determinação de outros parâmetros térmicos e ópticos. Ou seja, ELT é uma poderosa ferramenta na determinação de parâmetros termo-ópticos de materiais transparentes.

The thermal diffusivity is one of the physical properties that are among the most important in a material. Its determination presents several difficulties that require the use of accurate techniques that supply trustworthy values. In this work will be present the utilization of the thermal lens spectroscopy (TLS) as a technique to determine the thermal diffusivity (D) of transparent materials. As example are discussed the values of the thermal diffusivity of polycrystalline materials, glasses, polymer and liquid crystals. When compared with conventional techniques, the TLS presents the advantage to be rapid, to measure small sample size and to give accurate values at room temperature in the process of determination of the thermal diffusivity. When in concomitant use with other techniques it allows the determination of other thermo-optical parameters. In other words, the thermal lens spectroscopy is a powerful tool in the determination of thermo-optical parameters of transparent materials.

In this paper, the application of photoacoustic methods to study thermo-optical and spectroscopic properties of polymers is described. The Photoacoustic Spectroscopy, the Two-Beam Phase-Lag and also the so-called Open Photoacoustic Cell methods will be presented. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations of the methods as compared with conventional measurements. Applications for spectroscopic and depth profile analysis and also for thermal properties measurements in several polymers samples are discussed.

In this work we propose a photoacoustic model based on the Rosencwaig-Gersho theory to study dyed polymer samples. The model is similar to a two layer system and it is used to obtain the thermal diffusivity and the optical absorption coefficient of one side dyed polymer foils. The tested sample was the polyethylene terephthalate dyed with disperse dye, the Blue Samarom HGS. The results showed that the dye depth penetration can be accessed from the tted data and the dyeing process can be better controlled by using the retrieved data.

In this work thermal lens spectrometry is applied to investigate the thermo-optical properties of complex fluids as a function of the temperature. The method is applied in poly(vinyl chloride), in polycarbonate and in lyotropic liquid crystals as the testing samples. The focus of the discussion will be in the temperature range where the phase transitions occur. The perspective of future studies in this area will be discussed.

In this work we used the Photoacoustic Spectroscopy (PAS) to evaluate the crosslinking of the copolymer from ethylene vinyl trimethoxysilane (EVS) and the grafted vinyl trimethoxysilane (VTS) on low density polyethylene (LDPE). PAS is used for sounding the overtone bands and stretching frequencies combinations of the groupings -Si-OH, =CH2, -CH3 and -CH2 -CH3, in the near and medium infrared range. The samples were typically prepared with 3%, 5% and 7% of catalyst and crosslinked in the temperatures of 70, 80 and 90ºC. Using the overtone bands of -OH groups the PAS shows the optimum combination, pointing a better crosslinking effect for 80ºC and in the range 5% to 7% of catalyst, typically.

We describe applications of Thermal Lens Spectrometry to study optical and thermal properties of lyotropic liquid crystal. In particular, we refer to methods for measurements of thermal diffusivity anisotropy at different temperatures and as a function of the ferrofluids doping. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations as compared with conventional methods. Finally, future developments of photothermal techniques for these applications are assessed.