No abstract

Adsorbed poly(ethylene‐stat‐vinyl acetate) (PEVAc) on fumed silica was studied using temperature‐modulated differential scanning calorimetry (TMDSC) and FT‐IR spectroscopy. The properties of the copolymers were compared with poly(vinyl acetate) (PVAc) and low density polyethylene (LDPE) as references. TMDSC analysis of the copolymer‐silica samples in the glass transition region was complicated for the copolymers because of the ethylene crystallinity. Nevertheless, examination of the glass transition region for small adsorbed amounts of these copolymers indicated the presence of tightly‐ and loosely‐bound polymer segments, similar to other polymers which have an attraction to silica. Compared with bulk polymers with the same composition, the tightly‐bound polymers showed an increased glass transition temperature (Tg) and a loosely‐bound fraction with a lower Tg than bulk. FT‐IR spectra of the surface copolymers indicated that the fraction of bound carbonyls (p) increased as the fraction of vinyl acetate in the copolymers decreased, consistent with the notion that the carbonyls from vinyl acetate preferentially find their way to the silica surface. Spectra from samples with different adsorbed amounts of polymer were used to obtain the amount of bound polymer (Mb) and the ratio of molar absorption coefficients of bound carbonyls to free carbonyls (X). The copolymers had very large p values (up to 0.8) at small adsorbed amounts and dependent on the composition of the polymer. However, an analysis of the bound fractions, based on only the vinyl acetate groups, superimposed the data, suggesting that the ethylene units simply dilute the vinyl acetate groups in the surface polymer. The sample with the smallest fraction of vinyl acetate did not show this behavior and may be considered to be “carbonyl poor.” © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 727–736

Enhancement of the hydrophobic nature of natural rubber latex (NRL) films will open up a wide range of applications for them. Incorporating hexadecyltrimethoxysilane treated superhydrophobic diatomaceous earth (DE) in NRL, resulted in films with increased hydrophobicity. The nano‐scale surface roughness for the superhydrophobic surface was provided by DE and low surface energy was achieved by treating DE with hexadecyltrimethoxysilane (treated DE). Surface wettability and morphology were analyzed by water contact angle (WCA) measurements and scanning electron microscopic analysis, respectively. About 38% wt/wt hexadecyltrimethoxysilane to DE ratio could make superhydrophobic DE with WCAs larger than 150°. A dispersion of treated DE was incorporated into NRL resulting in hydrophobic NRL films possessing WCAs greater than 110° with 41% treated DE particle loading. The resulting NRL films were harder and had smaller tensile strength compared to normal NRL film.

Variable temperature, solid-state deuterium (H) NMR spectroscopy has been used to probe the rather complex head group dynamics of the surfactant cetyltrimethylammonium bromide- d (CTAB- d) in bulk. Heating and cooling runs were made as the surfactant underwent supercooling. H NMR line shape simulations were used to identify the hierarchy of the molecular motions of CTAB as a function of temperature. Fast continuous methyl rotations about the N-C axes and 3-fold jumps about the main chain C-N axis were present at all of the temperatures from -40 to 120 °C. With heating, the spectra were consistent with CTAB molecules starting 180° flips about the hydrocarbon chain molecular axis around 0 °C, which continued to flip with increasing flip rates up to 80 °C. At 90 °C, the flips changed to rotation of the CTAB molecules about the hydrocarbon chain axis and that rotation continued to 120 °C. Comparison of spectra of bulk CTAB at different temperatures from heating and cooling runs revealed that the rotation about the long axis of the hydrocarbon chains started at around 90 °C on heating, however, it does not freeze out until between 70 and 80 °C because of supercooling.

Vinyl acetate (VAc) was polymerized to about 90% conversion in 9 h at 40°C from the colloidal microstructure of the VAc/fumed silica/cetyltrimethylammonium bromide (CTAB) system. The glass transition ( T g ) of poly(vinyl acetate) (PVAc) polymerized in these emulsion gels with silica was higher ( T g = 41 ° C ) than those of PVAc made from bulk polymerization at 60°C ( T g = 31 ° C ) and the weight average molar mass ( M w ) was also larger ( M w about 300 kg/mol) than those from bulk polymerization ( M w = 125   kg / mol ). Increased M w , T g , and lowered processing temperature for these composites could facilitate new applications for PVAc.