A verification by FTIR spectroscopy of the effect of some local chain conformations on the specific molecular interactions of solvents, esters, and polyesters with PVC
Authors: Tiemblo Pilar , Martínez Gerardo , Millán José Luis 
Published: April 2003

The molecular interactions of poly(vinylchloride) (PVC) with some solvents [cyclohexanone (CH), methyl ethyl ketone (MEK) and N‐methylpirrolidone (MP)], esters [dioctylphthalate (DOP) and butyl stearate (BuSt)], and polyesters [poly(ethylene adipate) (PEA) and poly(ε‐caprolactone) (PCL)] have been investigated by FTIR spectroscopy. In all cases the band of the carbonyl group is found to shift to lower frequencies, but significant differences between the solvent and the esters, whether polymeric or not, are evidenced. For PVC‐solvent systems, the shift proves to increase linearly as PVC/solvent ratio increases, what suggests that only a definite number of polymer sites is involved. From the slopes of the straight lines this effect of composition is shown to increase in the order MP < MEK < CH, i.e., as the basicity of the solvent decreases. In contrast, for the PVC‐esters or polyester blends, a nonlinear behavior consisting of two distinct interaction processes, is obtained. The increase of shift as PVC/ester ratio increases is faster in the first process for all PVC‐ester systems and it is particularly enhanced for BuSt and, to a lesser extent, for DOP. Instead, during the second process, that increase is of little significance for BuSt relative to DOP and PCL. These results account for the saturation of the polymer structures that are capable of interacting, at different rates depending on the type of ester. Besides, the whole number of those structures appears to be lower than in the case of solvents.

The results are discussed on the ground of, on one side, the mechanism of nucleophilic substitution on PVC, in the same solutions and blends, which, as found previously, is of a stereospecific nature, and, on the other, the electron‐donor‐acceptor concept (EDA) and the hard‐soft‐acid‐base concept (HSAB) as applied to both the interacting agents (solvents and esters) and the isotactic GTGT and GTTG− triad conformations as well as the heterotactic GTTT. In the light of the resulting conclusions it is suggested that: (i) the linear behavior shown by the solvents obeys the solvent ability to ensure a dynamic equilibrium between the two possible conformations of ‐mmr‐ sequence, i.e., GTGTTT and GTTG−TT, through the preferential interaction with the little likely GTTG− conformation, the content of which happens so to be constant as long as there are ‐mmr‐ sequences in solution; (ii) the nonlinear behavior of PVC‐ester or polyester binary systems reveals a nonequilibrium situation and so the conformational change GTGTTT ⇒ GTTG−TT, which is highly hindered, will occur occasionally depending on the ester nature. This enables one to attribute the fast and the slow interaction processes to the permanent GTTG−TT conformations derived from the polymerization and to the same conformations formed as the result of the conformational changes, respectively.

Strong support for the above novel finding that PVC … OC interaction is of a local conformational nature is given by two additional investigations. First, a similar study with a PVC sample prepared at −50°C, shows that the carbonyl band shifts of CH and PCL are appreciably lower than those of PVC prepared at 70°C. The same holds for the blendof PCL with the latter PVC sample after substitution reaction (0.6%) at −15°C in CH with sodium benzenethiolate (NaBT). Since the PVC obtained at −50°C and the 0.6% substituted polymer exhibit a lower content of both permanent GTTG−TT conformations ad ‐mmr‐ sequence, these results agree with expectatins and confirm the above suggestions. Secondly, the changes in the CCl stretchign frequencies of PVC with increasing amounts of solvent or ester, as extensively studied, clearly indicate the occurrence of the aforementioned conformational change, and so they are consistent with our proposals as to the actual conformational nature of PVC…OC interactions. © 1995 John Wiley & Sons, Inc.