Molecular Surface Structural Changes of Plasticized PVC Materials after Plasma Treatment

Abstract

In this research, a variety of analytical techniques including sum frequency generation vibrational spectroscopy (SFG), coherent anti-Stokes Raman spectroscopy (CARS), and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the surface and bulk structures of phthalate plasticized poly(vinyl chloride) (PVC) at the molecular level. Two types of phthalate molecules with different chain lengths, diethyl phthalate (DEP) and dibutyl phthalate (DBP), mixed with PVC in various weight ratios were examined to verify their different surface and bulk behaviors. The effects of oxygen and argon plasma treatment on PVC/DBP and PVC/DEP hybrid films were investigated on both the surface and bulk of films using SFG and CARS to evaluate the different plasticizer migration processes. Without plasma treatment, SFG results indicated that more plasticizers segregate to the surface at higher plasticizer bulk concentrations. SFG studies also demonstrated the presence of phthalates on the surface even at very low bulk concentration (5 wt %). Additionally, the results gathered from SFG, CARS, and XPS experiments suggested that the PVC/DEP system was unstable, and DEP molecules could leach out from the PVC under low vacuum after several minutes. In contrast, the PVC/DBP system was more stable; the migration process of DBP out of PVC could be effectively suppressed after oxygen plasma treatment. XPS results indicated the increase of C═O/C-O groups and decrease of C-Cl functionalities on the polymer surface after oxygen plasma treatment. The XPS results also suggested that exposure to argon plasma induced chemical bond breaking and formation of cross-linking or unsaturated groups with chain scission on the surface. Finally, our results indicate the potential risk of using DEP molecules in PVC since DEP can easily leach out from the polymeric bulk.

The molecular interfacial structure and plasticizer migration behavior of “green” plasticized poly(vinyl chloride)

“…1 displays the chemical structures of TBAC and PVC. 31,33,34 The ssp SFG spectrum of pure TBAC is very similar to that of phthalate molecules, i.e. PVC is a linear polymer with chlorine atoms on alternating carbon centers.…”

“…Here, a second-order nonlinear optical technique, sum frequency generation vibrational spectroscopy (SFG), was utilized to study citrate plasticized PVC. 31,33 Our previous results show that phthalates segregate to the PVC/air surface even at a very low concentration (5 wt%) in the bulk, 31,33,34 and the phthalate plasticized PVC surfaces restructure in water, leading to the migration of phthalates into water. [22][23][24][25][26][27][28][29][30][31] Recently, we developed a methodology which combines different analytical techniques including SFG, coherent anti-Stokes Raman spectroscopy (CARS), and Fourier transform infrared spectroscopy (FTIR) to probe the molecular structural changes on the surface, at the buried interface, and in the bulk for the same material.…”

“…[22][23][24][25][26][27][28][29][30][31] Recently, we developed a methodology which combines different analytical techniques including SFG, coherent anti-Stokes Raman spectroscopy (CARS), and Fourier transform infrared spectroscopy (FTIR) to probe the molecular structural changes on the surface, at the buried interface, and in the bulk for the same material. Here, a second-order nonlinear optical technique, sum frequency generation vibrational spectroscopy (SFG), was utilized to study citrate plasticized PVC.…”

“…30,32 This methodology was successfully applied to examine the molecular structures of phthalate (e.g. 31,33,34 Here, we investigate the molecular structural changes of TBAC plasticized PVC system before and after water exposure. 31,33 Our previous results show that phthalates segregate to the PVC/air surface even at a very low concentration (5 wt%) in the bulk, 31,33,34 and the phthalate plasticized PVC surfaces restructure in water, leading to the migration of phthalates into water.…”

“…S1 (ESI †). 31,33,34 TBAC and phthalate functional groups have similar chemical environments. S1a, ESI †), SFG signals in the C-H stretching frequency region from the polymer/substrate are quite weak and can be ignored, so the collected ssp SFG signals were mainly generated from the polymer/air surface.…”