Ultralow Stress, Thermally Stable Cross-Linked Polymer Films of Polydivinylbenzene (PDVB)

Abstract

Although closely related to polystyrene, poly(divinylbenzene) (PDVB) has found limited utility due to the difficulties associated with its synthesis. As a highly cross-linked polymer, PDVB is infusible and insoluble and thus nearly impossible to shape into films by either melt or solvent-based processes. Here, we report the initiated chemical vapor deposition (iCVD) of nearly stress-free, highly transparent, free-standing films of PDVB up to 25 μm thick. Films initially grow under tensile intrinsic stress but become more compressive with thickness and eventually converge to zero-stress values once they reach ≥10 μm in thickness. Upon initial heating, the evaporative loss of unreacted monomer left in the polymer matrix induces between 35 and 45 MPa of tensile stress in the films. Afterward, subsequent heating cycles induce reversible stress and film expansion behaviors. We estimate the degree of cross-linking to be 44%, resulting in high thermal stability (up to 300 °C) and mechanical stiffness (Young's modulus of 5.2 GPa). The low stress combined with high cross-linking makes iCVD PDVB an excellent candidate for protective coatings in harsh environments.

Enhancing the Oxidation Stability of Polydivinylbenzene Films via Residual Pendant Vinyl Passivation

“…Differential scanning calorimetry (DSC) on as‐synthesized PDVB in Figure provides further clues on the passivation mechanisms occurring in the polymer during thermal treatment. We observed an evolution of an endothermic peak occurring during the first heating of PDVB between 43 and 109 °C consistent with the evaporation of unreacted monomer . The energy associated with this transition of 4 J/g corresponds to an evaporation of 1.2 wt.% of pure‐DVB monomeric units (ΔH vap = 43 kJ/mol) and matches closely with the measured 1.6 wt.% loss occurring at the same temperature described in our previous work .…”

“…We observed an evolution of an endothermic peak occurring during the first heating of PDVB between 43 and 109 °C consistent with the evaporation of unreacted monomer . The energy associated with this transition of 4 J/g corresponds to an evaporation of 1.2 wt.% of pure‐DVB monomeric units (ΔH vap = 43 kJ/mol) and matches closely with the measured 1.6 wt.% loss occurring at the same temperature described in our previous work . We were unable to identify any concurrent exothermic processes below 109 °C that could explain the 9% depletion of vinyl groups (observed by FTIR spectroscopy in Figure ) via crosslinking or other chemical reactions.…”

“…About 67% of the pendant CH=CH 2 groups are lost after annealing at 200 °C as depicted in Figure (quantification methodology is detailed in the Experimental section). Unreacted DVB monomer evaporates from the polymeric matrix at around the same temperature, but since it only accounts for a 1.6 wt.% loss , it is unlikely that volatile species are responsible for the spectral change seen in the polymer. Annealing at 300 °C drives the concentration of unreacted vinyls down to values that are indistinguishable from the background noise (see Figure B ), suggesting their complete removal from the polymer.…”

“…Our previous studies showed that at high filament temperatures (> 270 °C) during deposition, the only entrained species present in the PDVB was unreacted monomer. No tert ‐butoxy or related initiator species were observed by mass spectrometry.…”

“…While iCVD polymer polydivinylbenzene (PDVB) can reproduce the high crosslink density and thermal stability of plasma polymers,, only limited information exists regarding its long‐term stability at room conditions or light exposure . We previously demonstrated that the absence of intrinsic photoactive centers in PDVB is responsible for its larger stability towards short‐term photo‐oxidation (minutes) than plasma polymers.…”