Polymer interdiffusion

In numerous applications of polymeric materials multilayers of films are used. This practice is found in microelectronic, aeronautical, and biomedical applications to name a few. Developing good adhesion between these layers requires interdiffusion of the molecules at the interfaces between the layers over size scales comparable to the molecular diameter (tens of nm). In addition, these interfaces are buried within the specimen. Aside from this practical aspect, interdififlision over short distances holds the key for critically evaluating current theories of polymer difllision. Theories of polymer interdiffusion predict specific shapes for the concentration profile of segments across the interface as a function of time. Interdiffiision studies on bilayered specimen comprised of a layer of polystyrene (PS) on a layer of perdeuterated (PS) d-PS, can be used as a model system that will capture the fundamental physics of the problem. Initially, the bilayer will have a sharp interface, which upon annealing will broaden with time. 

Polymers interpenetration of polymer chains, phase separation, compatibility between polymers, interdiffusion of latex particles, interface thickness in blends of polymers, light-harvesting polymers, etc. 

Sahlin, J.J., and Peppas, N.A., Enhanced hydrogel adhesion by polymer interdiffusion Use of linear poly(ethylene glycol) as an adhesion promoter, J. Biomater. Sci., Polym. Ed., 8 421-436 (1997). 

For bioadhesive applications, anionic polymers appear to provide the most effective balance between adhesiveness and toxicity, with carboxylic materials preferred over sulfonic polymers [400]. Polyfacrylic acid) microparticles have been identified as particularly effective bioadhesive materials [402]. Studies with poly(acrylic acid) microparticles have indicated that, while water-swollen particles exhibit good bioadhesion, dry polymer particles give no adhesion at all. In addition, adhesive strength increases as the degree of ionization of the polymer is increased [402]. Thus the expanded nature of the polymer network is important to mucoadhesion, probably via polymer interdiffusion and entanglement with mucin [403],... 

A 1 1 mixture of a carbodiimide containing latex polymer (t-butylcarbodiimidoethyl methylmethacrylate) and a carboxylic acid containing latex polymer (2-ethylhexyl methacrylate) showed that polymer interdiffusion occurs faster than crosslinking. Also, cyclohexylcarbodiimidoethyl methacrylate and t-butylcarbodiimidoethyl methacrylate have been prepared, and these monomers are used to prepare carbodiimide functionalized latex particles for emulsion polymerization. ... 

The time dependence of the early stages of polymer interdiffusion at interfaces is indicative of the diffusion process. The normal approach to study such interdiffusion by neutron reflectivity is to use an anneal/quench cycle where the sample is heated for a given time above the glass transition temperature (Tg) of the polymer, then quenched rapidly to room temperature, after which the reflectivity profile is measured. This has proved to be highly effective for a number of systems, but is difficult to apply when Tg is room temperature, or for small molecule ingress into a higher molecular weight polymer layer. 

Once the polymer particles have deformed, any free polymer chains will diffuse across particle boundaries. The strength of polymer films has been shown to increase with the depth of polymer interdiffusion, up to size of a single polymer chain. ... 

Polymer interdiffusion above Tg was then monitored [93] by using technique (ii) at ambient probe temperature following various times during which the polymer system was held at 190°C. Initially, the sample consisted of stacks of discs cut from PMMA and PVDF sheet (average thicknesses 92.5 and 88.2 p,m, respectively) alternately. Evaluation of the diffusion equation gave intrinsic diffusion coefficients (7 5) x 10 and (15 5) x 10... 

The solid phase of bread crumb can be viewed as a composite material where amylose, amylopectin and protein form separated phases due to poor thermodynamic miscibility of the different polymers. Composites are characterized by exhibiting mechanical properties that cannot be achieved with the individual constituents alone, but are dependent on the interface between the components. A sharp interface as found between starch and protein provides strong evidence of little polymer interdiffusion and weak interfacial adhesion.14 The present results suggest that starch forms a continuous phase in bread which has also been confirmed with confocal scanning laser microscopy.15 The presence of a protein phase reduces the continuity of the starch phase and, thus, reduces the cohesion of the material as revealed by a comparison of the breaking stresses of aged flour and starch gels (data not shown).16... 

Kausch, H. and M. Tiorrell, Polymer interdiffusion. Annual Review of Material Science, 1989, 19, 341-377. 

In trying to answer these questions it is certainly instructive to examine previous fundamental experimental works in which such thin films have been employed. In particular, we have to mention studies on polymer interdiffusion in which thin polymer films were used for testing theoretical concepts of polymer physics [102-106]. Spin-coating was typically used to prepare the films because this process presents an easy way of obtaining smooth film of precisely controllable thickness, even in the nanometer range. This is an essential criterion for investigating, for example, polymer diffusion across an interface between two films. While these experiments successfully supported the model of chain reptation [102-106], they also indicated some deviations that hinted, for example, at the enrichment of chain ends at surfaces [104, 106]. 

Virtually all end-use formulations of latex polymers of MFT much above 5 °C (40 °F) contain a moderately low volatility solvent for the polymer. These act as temporary plasticizers to ensure low MFT and enhance polymer interdiffusion. The solvent should diffuse and evaporate as quickly as possible for film hardness development, but not at the expense of completeness of film strength development A complication is that compositions of high Tg which contain solvent... 

There is a need for extensive studies of polymer inteidiffusion in films formed from core-shell latexes. Only a veiy small number of experiments have beat reported. Chevalier et al. [5] suggested in the case of poly[styrene-co-(butyl aciylate)] latex containing an acrylic acid-ridi shell, that the membrane phase formed by the shell in the newly formed film would act as a barrier to prevent polymer interdiffusion. In their view, only at tempmtures above the Tg of the membrane diase would the membranes break, bringing the core pol3rmer into intimate contact. By microscopy they could observe the break-up of the membranes once the sample was heated above 110°C. 

The general problem of polymer-polymer interdiffusion is studied and used to illustrate several unique aspects of the orientation and motion of macromolecules at interfaces. Specific new results are obtained for the short time evolution of the Interfacial concentration profile which is shown, following the reptation... 

Amorphous polymers Interdiffusion Surface glass transition Vitrified bulk... 

Welp et al. (43) continued the ripple experiment, going to 400,000 g/mol polystyrene to obtain a better definition of the ripple. They concluded that the reptation model proposed by de Gennes (44) with parallel development by Doi and Edwards (45) was the best model to describe the dynamics of polymer interdiffusion. There were six dominant ripple characteristics that were examined ... 

Dhinojwala A, Torkelson JM (1994) A reconsideration of the measurement of polymer interdiffusion by fluorescence nonradiative energy-transfer. Macromolecules 27 (17) 4817-4824. doi 10.1021/ma00095a024... 

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