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The mechanical properties of single poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) molecules under different solvent conditions were investigated using a new atomic force microscope (AFM) based techniquesingle-molecule force spectroscopy. The single-molecule force spectrum of PVA shows a kink at around 200 pN, deviating from the entropic elasticity of a random coil predicted from the WLC model. Since the single-molecule force spectra of PVAc and PVA in 8 M urea indicate that both behave like ideal entropic springs, the hydrogen bonding of hydroxyl groups in PVA must govern the elastic behaviors of PVA in water. Because of hydrogen bonding, PVA assumes a multiple stranded suprastructure in water. Upon stretching, the suprastructure undergoes a conformational transition to an overstretched state. This conformational change is fully reversible and is thus in thermodynamic equilibrium. This study demonstrates the unique utility of AFM in exploiting of suprastructures and conformational changes of synthetic polymers.
Li et al. (Sat,) studied this question.