This paper describes a novel hydrogel concept, which is based on self-assembling of enantiomeric lactic acid oligomers (stereocomplex formation) grafted to dextran. The hydrogels are prepared in an all-aqueous environment. For this purpose, l- and d-lactic acid oligomers were coupled to dextran, yielding dex−(l)lactate and dex−(d)lactate, respectively. Upon dissolving each product in water separately and mixing the solutions, we observed that a hydrogel is formed at room temperature as demonstrated by rheological measurements. The storage modulus of the obtained hydrogel strongly decreased upon heating to 80 °C, while it was restored upon cooling to 20 °C, demonstrating the thermoreversibility and the physical nature of the cross-links. Rheological experiments with monodisperse lactic acid oligomers grafted to dextran showed that the degree of polymerization (DP) of the lactic acid oligomers must be at least 11 to obtain a hydrogel. The hydrogel characteristics can be modulated by varying the degree of polymerization (DP, number of lactate units per oligomer) and the degree of substitution (DS, number of lactic acid side chains per 100 glucopyranose units) of the dex−lactate products, as well as the water content of the dex−lactate solutions. Stronger gels were obtained by increasing the DP and DS and by decreasing the water content. FTIR−photoacoustic (PA) analysis demonstrated that in the hydrogels stereocomplexes were formed between the lactic acid oligomers of opposite chirality.

The model of Rosencwaig and Gersho for the photoacoustic effect is tested. The use of carbon black samples as a phase reference to estimate the apparatus phase φapp from the experimental signal phase φexp turned out to be ambiguous. A new method is presented to correct the experimental signal phases φexp of polymer films for the apparatus phase φapp. The photoacoustic signal phase of some well-defined poly(ethyleneterephthalate) (PET) films is obtained. As a phase reference, transparent thermally thick polymer films were found to be more convenient and dependable than carbon black samples. The resulting phase spectra of thermally thick and thermally thin PET films confirm the Rosencwaig–Gersho theory.

Experimental photoacoustic (PA) magnitude spectra of a coated paper and the uncoated basepaper are presented. The normalized and scaled PA magnitude spectra are used to calculate difference magnitude spectra. It was decided to scale all PA magnitude spectra to (low) equal intensity at the approximately optically thin spectral range before subtraction. Then no infrared (IR) bands of identical band shape and height (as needed for common difference spectroscopy) in either PA magnitude spectrum are needed. Contributions of the two individual layers to the IR-PA magnitude spectrum of the coated paper are separated in the difference spectrum by their sign. An increasing relative contribution of the coating layer with an increasing phase modulation frequency is found. On decreasing the thermal length to a value near the coating thickness, the difference spectra increasingly show positive coating bands and negative bulk signals. The extension of the Rosencwaig–Gersho theory to a double-layered system introduced by N. C. Fernelius [J. Opt. Soc. Am. 70, 480 (1980) and J. Appl. Phys. 50, 650 (1980)] applied to synthetic spectra confirms the experimental observation. It is found that photoacoustic difference spectroscopy may provide quantitative depth-resolved spectral information due to the presented scaling procedure, and photoacoustic difference magnitude spectra of any polymeric laminate may therefore be calculated.

Photoacoustic Fourier transform infrared spectroscopy of nanoporous SiO x ∕ Si thin films with varying porosities J. Appl. Phys. 98, 114310 (2005); 10.1063/1.2138376 X-ray photoemission spectroscopy and Fourier transform infrared studies of dye molecule doped conducting polymer films J. Vac. Sci. Technol. A 23, 869 (2005); 10.1116/1.1914816The (n00), n=3, 4, and 6, local mode states of H 3 SiD : Fourier transform infrared and laser photoacoustic spectra and ab initio calculations of spectroscopic parameters Photoacoustic ͑PA͒ magnitude and phase spectra of, respectively, polyethyleneterephthalate/ polyethyleneterephthalate ͑PET/PET͒, polypropylene/polyethyleneterephthalate ͑PP/PET͒, and polyethyleneterephthalate/polypropylene ͑PET/PP͒ laminates are presented. The extension of the Rosencwaig-Gersho theory to a double-layered system introduced by N. C. Fernelius ͓J. Opt. Soc. Am. 70, 480 ͑1980͒ and J. Appl. Phys. 51, 650 ͑1980͔͒ is applied to compare the experimental data with simulated PA magnitudes and phases. The simulations for the double-layered systems indicate that the absorption coefficient ␤ at the optically transparent region of the single layers-estimated from photoacoustic data-strongly influences the phase spectra of the polymeric laminate. In particular, the relative magnitude of these residual absorptions in the two layers is important. The optical absorption spectra of the polypropylene and polyethyleneterephthalate single layers ͑accessible from infrared absorbance and infrared photoacoustic spectra͒ were used to simulate the photoacoustic magnitude and phase spectra of the two possible double layers, i.e., PP/PET and PET/PP. A reasonable agreement of experimental and simulated photoacoustic data is observed.

The influence of various metal substrates on the photoacoustic response of a double-layered polymer coating (polypropylene on a polybutylacrylate-adhesive) is demonstrated. The extended Rosencwaig–Gersho theory by Fernelius for a double-layered sample is used to discuss the step-scan Fourier transform infrared photoacoustic spectroscopy (FT-IR PAS) experimental results. A clear dependence of the photoacoustic phase φ on the thermal diffusivity α of the metal substrate is found. Therefore, the quality of the thermal contact between the polymer coating and the metal substrate influences the photoacoustic phase spectrum.