ma9b02103_si_001.pdf (3.37 MB)
Molecular Aspects of Film Formation of Partially Cross-Linked Water-Borne Secondary Dispersions that Show Skin Formation upon Drying
journal contribution
posted on 2019-12-12, 18:40 authored by Yang Liu, Pietro Paolo de Oliveira Silva, Kenneth Tran, Hang Zhou, Jessica Emsermann, Margaret Zhang, Kevin Ho, Yijie Lu, Mohsen Soleimani, Mitchell A. WinnikIn a previous publication [Macromolecules 2019, 52, 5245–5254], we described
the synthesis of surfactant-free latex dispersions of nanoparticles
(NPs) based on emulsification of a preformed proprietary BASF polymer
(MnGPC = 5000 g/mol, D̵ = 3), in which
the −COOH groups were partially neutralized by using ammonia.
The NPs in these dispersions were then partially cross-linked with
neopentyl glycol diglycidyl ether (NGDE) to increase the molecular
weight, followed by reaction with monoglycidyl ether to reduce the
acid number and lower the glass transition temperature (Tg). In the work reported here, we used fluorescence resonance
energy transfer (FRET) measurements to examine polymer diffusion rates
in the films formed from these dispersions. We compared films formed
from the uncross-linked NPs, with ones containing the NPs partially
cross-linked with NGDE but not reacted with the monoepoxide. In this
way, both the cross-linked and noncross-linked polymers had similar Tg values. We also examined films formed from
a similar polymer with MnGPC = 4000 g/mol, D̵ = 3. Because of the high Tg of these
polymers (ca. 65 °C), films were formed on heated substrates,
and this led to skin formation at the film surface. We used FRET measurements
to monitor the extent of polymer diffusion at both the film–air
(F–A) and film–substrate (F–S) interfaces. We
found that the onset of polymer diffusion occurred more rapidly within
the skin at the F–A interface at elevated temperatures, but
this was quickly surpassed by polymer diffusion at the bottom of the
film because of the hydroplasticization effect. The presence of the
skin layer retarded water evaporation and extended the time needed
for the efficiency of energy transfer to reach its plateau value.
We also found that the extent of chain diffusion in the partially
cross-linked (XL) films was reduced compared to the non-XL samples
because of limited interdiffusion of the polymer that formed the gel
content. Dynamic mechanical analysis was employed to investigate the
viscoelastic behavior of the samples using time–temperature
superposition to generate master curves. We calculated apparent activation
energies in the temperature range of the FRET experiments that were
consistent with the strong dependence of polymer diffusion rates on
the difference between the annealing temperature and glass transition
temperature.