Hydrocarbon polymers surface properties

In 1938, while attempting to prepare fluorocarbon derivatives, Roy J. Plunkett, at DuPont s Jackson Laboratory, discovered that he had prepared a new polymeric material. The discovery was somewhat serendipitous as the TFE that had been produced and stored in cylinders had polymerized into poly(tetra-fluoroethylene) (PTFE), as shown in Eigure 4.2. It did not take long to discover that PTFE possessed properties that were unusual and unlike those of similar hydrocarbon polymers. These properties include (1) low surface tension, (2) high Tm, (3) chemical inertness, and (4) low coefficient of friction. All of these properties have been exploited in the fabrication of engineering materials, wliich explains the huge commercial success of PTFE. 

It is believed that polymer surface fluorination proceeds via a free radical mechanism, where fluorine abstracts hydrogen atoms from the hydrocarbon, and fluorine atoms are substituted.11 Of course, the precise conditions depend on the nature of the polymer in question and the surface properties required. 

Surface fluorination changes the polymer surface drastically, the most commercially significant use of polymer surface direct fluorination is the creation of barriers against hydrocarbon permeation. The effectiveness of such barriers is enormous, with reductions in permeation rates of two orders of magnitude. Applications that exploit the enhanced barrier properties of surface-fluorinated polymers include (1) Polymer containers, e.g., gas tanks in cars and trucks, which are produced mostly from high-density polyethylene, where surface fluorination is used to decrease the permeation of fuel to the atmosphere and perfume bottles. (2) Polymeric membranes, to improve selectivity commercial production of surface-fluorinated membranes has already started.13... 

Before fluorination, the dielectric constant ofpoly(bisbenzocyclobutene) was 2.8, and this value was reduced to 2.1 after plasma treatment. No data were reported in the paper on characterization of structure or properties, except for the dielectric constant of the modified poly(bisbenzocyclobutene). The authors did report that the thermal stability offluorinatedpoly(vinylidenefluoride) was inferior to the original poly(vinylidenefluoride) when treated in a similar way. One of the probable reasons for the low thermal stability is that the NF3 plasma degraded the polymer. According to their results, the thickness of fluorinated poly(bisbenzo-cyclobutene) was reduced by 30%. The same phenomenon was observed for other hydrocarbon polymers subjected to the NF3 plasma process. A remaining question is whether plasma treatment can modify more than a thin surface layer of the cured polymer Additionally, one of the side products generated was hydrogen fluoride, which is a serious drawback to this approach. 

With values in the range of about 10-18 mN m 1 perfluorinated liquids have the lowest surface tensions among the known organic liquids, and will completely wet any solid surface. Increasing amounts of hydrogen in the molecule increase the surface tension. Fluorinated solid surfaces, e.g. fluoropolymers, possess very low critical surface tensions yc, which relates to their antistick and low frictional properties, whereas hydrocarbon polymers have substantially higher values (PTFE yc = 16.0 mN m-1 PE yc = 31.0 mN m-1).7... 

If solid polymer objects are fluorinated or polymer particles much larger than 100 mesh are used, only surface conversion to fluorocarbon results. Penetration of fluorine and conversion of the hydrocarbon to fluorocarbon to depths of at least 0.1 mm is a result routinely obtained and this assures nearly complete conversion of finely powdered polymers. These fluorocarbon coatings appear to have a number of potentially useful applications ranging from increasing the thermal stability of the surface and increasing the resistance of polymer surfaces to solvents and corrosive chemicals, to improving friction and wear properties of polymer surfaces. It is also possible to fluorinate polymers and polymer surfaces partially to produce a number of unusual surface effects. The fluorination process can be used for the fluorination of natural rubber and other elastomeric surfaces to improve frictional characteristics and increase resistance to chemical attack. 

The wetting properties of polyacetylene have been studied by Schonhom et al. 380) who measured a critical surface tension of 51 mN m 1, considerably higher than for other hydrocarbon polymers. This value was attributed to oxidation of the surface as no change was observed on further oxidation. Treatment of polyacetylene with aqueous potassium permanganate renders it hydrophilic, reduces the contact angle for water from 72° to 10° and renders the structure more water-permeable 381)... 

Finally, it is highly desirable to improve the ability to calculate the properties of surfaces and interfaces involving polymers by means of fully atomistic simulations. Such simulations can, potentially, account for much finer details of the chemical structure of a surface than can be expected from simulations on a coarser scale. It is, currently, difficult to obtain quantitatively accurate surface tensions and interfacial tensions for polymers (perhaps with the exception of flexible, saturated hydrocarbon polymers) from atomistic simulations, because of the limitations on the accessible time and length scales [49-51]. It is already possible, however, to obtain very useful qualitative insights as well as predictions of relative trends for problems as complex as the strength and the molecular mechanisms of adhesion of crosslinked epoxy resins [52], Gradual improvements towards quantitative accuracy can also be anticipated in the future. 

A considerable amount of work has focused on the design and synthesis of macromolecules for use as emulsifiers for lipophilic materials and as polymeric stabilizers for the colloidal dispersion of lipophilic, hydrocarbon polymers in compressed CO2. It has been shown that fluorinated acrylate homopolymers, such as PFOA, are effective amphiphiles as they possess a lipophilic acrylate-like backbone and C02-philic, fluorinated side chains, as indicated in Figure 4.5-1 [100]. Furthermore, it has been demonstrated that a homopolymer which is physically adsorbed to the surface of a polymer colloid precludes coagulation due to the presence of loops and tails [110]. These fluorinated acrylate homopolymers can be synthesized homogeneously in CO2 as described in an earlier section. The solution properties [111,112] and phase behavior [45] of PFOA in SCCO2 have been thoroughly examined. Additionally, the backbone of these materials can be made more lipophilic in nature by incorporating other monomers to make random copolymers [34]. 

The chemical nature of the base polymer is an important aspect in membrane development. There has been preference for the thermally stable fluorinated polymers over hydrocarbon polymers. Fluorine-containing polymers, characterized by the presence of carbon-fluorine bonds, are widely used as the base matrices owing to their outstanding chemical and thermal stability, low surface energy, and the ease of modiflcation of various properties by the grafting method. Per fluorinated polymers and partially fluorinated polymers combining hydrocarbon and fluorocarbon structures are excellent candidates as base polymers. For instance, fluorinated FEP has drawn wide attention due to its reasonably good radiation stabiUty [58]. 

This is another version of Biospan with surface modification by an oligomeric hydrocarbon covalently bonded to the base polymer during synthesis. The additive has a pronounced effect on the polymer surface chemistry but little effect on the bulk properties of the base polymer according to the manufacturer (Tables 4.3, 4.12, 4.13, and 4.14). 

The practical applications of fluoropolymer membranes especially in the areas of purification and separation related to potable water production, wastewater treatment and bioprocessing, have been limited to some extent by their hydrophobic and inert surface properties. Among the different modification techniques, graft copolymerization of hydrophilic monomers, or inimers for further surface reactions, from fiuoropolymers has been useful and effective in improving the physicochemical properties of the parent fluoropolymer with minimum alteration of their desirable bulk properties. Apart from fiilly fluorinated polymers, most of the partially fluorinated polymers can dissolve in polar organic solvents, such as Ai,Ai-dimethylformamide (DMF), A,A-dimethylacetamide (DMAc), NMP, and dimethyl sulfoxide (DMSO), but are insoluble in water, alcohols, and hydrocarbons. 

Static deflection AFM can be used to measure local mechanical properties of polymer surfaces, but only after consideration of the relative stiffness of the cantilever and the surface under study. Cantilevers with stiffness in excess of 50 N/m are necessary to indent materials with a bulk modulus in excess of 1 GPa (10 N/m ). Soft levers with a spring constant less than 1 N/m are sufficient to indent elastomers. Conventional staining techniques used in electron microscopy provide a viable way to harden unsaturated, hydrocarbon elastomers for imaging with soft cantilevers. Alternatively, low bulk modulus polymers (E < 1 MPa or 10 N/m ) require resonant imaging techniques such as Tapping Mode for direct imaging. 

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