The effect of three hydrotropic molecules, phenol, resorcinol and phloroglucinol, on the thermodynamic equilibrium and the dynamics of poly(N-isopropylacrylamide) (PNIPA) hydrogels is reported. With increasing hydroxyl number of this molecular sequence both the depression of the temperature and the width of the volume phase transition (VPT) increase, reflecting the nonuniform distribution of these molecules inside the gel. Small-angle X-ray scattering detects no notable change in the local structure of the polymer chains in the presence of phenol. Dynamic light scattering observations, however, show that phloroglucinol, unlike phenol and resorcinol, produces a small but significant decrease in the mobility of the polymer in the solvent. Isothermal microcalorimetry reveals an anomalous increase in the exothermic enthalpy of mixing just below the transition, both with phenol and phloroglucinol, which indicates a pretransition state in the gel in which the aromatic molecules partly replace the bound water. This finding is corroborated by small-angle neutron scattering measurements under contrast matched conditions showing that at low phenol concentration the aromatic molecules are uniformly dispersed in the solvent, but in pretransition conditions a sparsely populated layer of phenol forms close to the polymer chains.

Measurements of optical transmission and Rutherford backscattering experiments (RBS) are carried out to investigate the correlation between different defect structures and the near edge optical properties of nitrogen implanted GaAs. The defect structures detected with these methods can be classified into two groups. A point defect‐like structure connected with an exponential long tail to the absorption edge and a weak contribution to the dechanneling without any damage peak in the RBS spectra preferably occurs for low dose implantation at Ti = 80 K and for room temperature implantation. A transition to heavily damaged regions connected with a short tail to the absorption edge is formed for higher fluences at Ti = 80 K. In this case damage peaks occur in the RBS spectra and the refractive index change reaches a value of about 10% after formation of a continuous amorphous layer.