Jianrong Feng

Characterization of latex blend films by atomic force microscopy

Abstract Analysis of latex blend films, made possible by TappingMode™ atomic force microscopy, has provided insight into the interaction properties of hard and soft latex particles. Looked at in isolation, the hard latex forms a cracked and opaque film with no detectable particle deformation, while the soft latex coalesces into a transparent coating showing small domains with face-centred-cubic arrays. In blends where the soft component is present in amounts greater than 40% by volume, smoothed bumps are observed which appear larger than either the hard or soft particles alone. The smoothness of each bump, supported by other evidence, suggests that the soft particles have coalesced into a virtual continuum at the surface, while the overall surface unevenness is thought to be indicative of underlying hard particles. We interpret the submersion of the hard particles as arising because of the lower surface energy of the soft polymer.

Study of polymer diffusion across the interface in latex films through direct energy transfer experiments

Efforts have been made to improve the method of using direct nonradiative energy transfer to follow the interdiffusion of polymer molecules across an interface. Two data analysis techniques are developed. The first one presents the extent of interdiffusion in terms of an acceptor concentration distribution; the second one recovers the diffusion coefficient and the concentration profile based on a Fickian diffusion model. The first method is model independent while the second method can be readily extended for other diffusion models. Polydisperse systems are characterized by a distribution of molecular mobilities. The energy transfer experiment provides a mean diffusion coefficient averaged over the sample history. For such systems, the concept of ‘‘instantaneous’’ effective diffusion coefficient and a method to derive it are discussed. These techniques are examined using computer simulations and tested in the study of molecular diffusion process of a poly(butyl methacrylate) latex system.

Formation and Crosslinking of Latex Films through the Reaction of Acetoacetoxy Groups with Diamines under Ambient Conditions

We investigated the processes of film formation, polymer diffusion, and crosslinking of latex films at ambient temperature, using low Tg methacrylate latex bearing acetoacetoxy groups, and curing the systems with 1,6-hexanediamine as the crosslinker. The addition of diamine induces floc formation, which modifies the rheological properties of the dispersion and increases its drying rate when coated onto a substrate. The crosslinking reaction between diamine and acetoacetoxy groups occurs at a rapid rate, even in the dispersed state. Although the crosslinking reaction precedes polymer diffusion in the two systems we examined, latex films with relatively good solvent resistance are obtained.

Polymer diffusion in latex films at ambient temperature

Polymer diffusion across interfaces at room temperature (21°C) was analyzed by direct nonradiative energy transfer (DET) in labeled latex films. Two modellatex polymers were examined: poly(butyl methacrylate) [PBMA, Mw = 3.5 × 104, Tg (dry) = 21°C] and a copolymer of 2-ethylhexyl methacrylate with 10 wt % (acetoacetoxy)-ethyl methacrylate [P(EHMA-co-AAEM), Mw = 4.8 × 104, Tg (dry) = −7°C]. Little energy transfer due to polymer diffusion was detected for the P(EHMA-co-AAEM) latex samples in the dispersed state or dried to solids content below ca. 90%, but above 90% solids, diffusion occurs among particles. For PBMA, diffusion occurs only after the film is dried (>97% solids) and aged. In the dry PBMA films, it requires 4–5 days at 21°C to reach a significant extent of mixing (fm = 0.3–0.4). This corresponds to an estimated penetration depth dapp of 30–40 nm and a mean apparent diffusion coefficient (Dapp) of 5 × 10−4 nm2/s. The corresponding Dapp value for the dry P(EHMA-co-AAEM) sample is 5 × 10−2 nm2/s, and it takes about 25–40 min for this polymer to reach fm of 0.3–0.4 with dapp of 20–30 nm.

Interface characterization in latex blend films by fluorescence energy transfer

Immiscible polymer blend films were formed by air drying aqueous dispersions containing mixtures of a high-T g latex, poly(methyl methacrylate), and a film-forming low-T g latex, poly(butyl methacrylate-co-butyl acrylate). Fluorescence energy transfer experiments were used to characterize the interfaces in these films, in which one component was labeled with a donor dye and the other with an acceptor. The quantum efficiency of energy transfer (Φ ET ) between the donors and acceptors is influenced by the interfacial contact area between the two polymer phases. As the amount of soft component in the blend is increased, Φ ET approaches an asymptotic value, consistent with complete coverage ofthe hard polymer surface with soft polymer. This limiting extent of energy transfer is very sensitive to the total surface area in the film, with correspondingly more energy transfer at constant volume fraction for small hard particles. Some of the details of the energy transfer are revealed through a fluorescence lifetime distribution analysis. The presence of ionic surfactant (sodium dodecyl sulfate) in the dispersion from which the latex blend film is prepared reduces the cross-boundary energy transfer by 30%, which implies that in these films the surfactant decreases the interfacial contact. After annealing the surfactant-free blends above 100°C, we observe an increase in energy transfer, consistent with a broader interface between the two polymers.

Effect of oligomers on the polymer diffusion rate in poly(butyl methacrylate) latex films

The aqueous phase of a poly(butyl methacrylate) (PBMA) latex dispersion contained an oligomeric component that was isolated after sedimentation of the PBMA latex particles. The component contained both water-soluble PBMA oligomer and some longer chain species that were present as a very fine colloidal dispersion. We describe the isolation and characterization of this component. This component was then added to a purified PBMA latex dispersion from which the aqueous component was previously removed. Latex films were prepared, and in the presence of the oligomeric material, the rate of polymer diffusion in the latex film was strongly enhanced. The magnitude of the enhancement was fit quantitatively to the Fujita–Doolittle equation, indicating that the oligomers acted like a traditional plasticizer to increase the free volume in the system.

Direct non-radiative energy transfer across a sharp polymer interface

Abstract A simple interfacial model system is chosen in which fluorescent donors and acceptors attached to incompatible polymer chains are randomly distributed in respective phases separated by a sharp interface. Direct non-radiative energy transfer (DET) across the interface between the labeled polymer phases is observed by fluorescence decay measurements. The quantum efficiency of energy transfer ( Φ ET ) is used to quantify DET. We show that Φ ET increases as the interfacial contact area between the phases increases and is inversely proportional to the diameter of the dispersed spherical donor-labeled particles. The data indicate that the degree of interpenetration at the interface is less than 2 nm.

Thermal decomposition behavior of naphthalene-labeled polyethylene

Pyrolysis–GC/mass spectrometry experiments reveal that naphthalene groups attached to maleated polyethylene as the 1-naphthylethyl ester are stable for relatively long periods of time at 170°C. Decomposition can be detected for samples heated for 2.0 min at 200°C, but even at that temperature, the extent of decomposition is very small. At higher temperatures, two of the decomposition products from the labeled polymer are readily understood: 1-vinylnaphthalene and 1-naphthylethanol can form by reactions that are well-precedented in the organic chemistry literature. At 200°C, only naphthalene is formed, which requires scission of the bond between the naphthyl ring and the C1 carbon of the ethyl group. We suggest two possible pathways for this reaction.