Critical surface tension of polymers

Table 3.4 Critical surface tension of polymer solids [10] ...

Figure 2. Comparison of the observed and the calculated critical surface tensions of polymers (on the basis of solubility parameter)...

The critical surface tension of all known solid polymers is lower than the surface tension of water at 72 X 10 N/cm (Table 12-2). All polymers are therefore relatively poorly wetted by water. The critical surface tension of polymers containing fluorine is particularly low, and they are poorly wetted... 

The wetting behavior of polymers is reviewed beginning with the thermodynamic conditions for contact angle equilibrium. The critical surface tension of polymers is discussed followed by some of the current theories of wettability, notably the theory of fractional polarity and theories of contact angle hysteresis. The nonequilibrium spontaneous and forced spreading of polymer liquids is reviewed from two points of view, the surface chemical perspective and the hydrodynamic perspective. There is a wide di.sperity between these two viewpoints that needs to be resolved inorder to establish the predictive relations that govern spreading behavior. 

CALCULATION OF CRITICAL SURFACE TENSIONS OF POLYMERS AND SURFACE TENSIONS OF LIQUIDS FROM CHEMICAL STRUCTURE ONLY. 

Partially fluorinated oligomers derived from hexafluoropropylene oxide je.g., F[C(CF3)FCF20]9C(CF3)FC00CH2CH2(0CH2CH2)60CH3 lower the critical surface tension of polymers on which they are adsorbed [e.g., polystyrene, poly (methyl methacrylate), and a vinyl chloride-vinylidene copolymer] [282]. 

PDMS based siloxane polymers wet and spread easily on most surfaces as their surface tensions are less than the critical surface tensions of most substrates. This thermodynamically driven property ensures that surface irregularities and pores are filled with adhesive, giving an interfacial phase that is continuous and without voids. The gas permeability of the silicone will allow any gases trapped at the interface to be displaced. Thus, maximum van der Waals and London dispersion intermolecular interactions are obtained at the silicone-substrate interface. It must be noted that suitable liquids reaching the adhesive-substrate interface would immediately interfere with these intermolecular interactions and displace the adhesive from the surface. For example, a study that involved curing a one-part alkoxy terminated silicone adhesive against a wafer of alumina, has shown that water will theoretically displace the cured silicone from the surface of the wafer if physisorption was the sole interaction between the surfaces [38]. Moreover, all these low energy bonds would be thermally sensitive and reversible. 

Organofunctional group Chemical structure OSi-Specialties Germany GmbH product Critical surface tension of glass with silane treatment [dyne/cm] Applied polymers (abbreviations according ASTM 1600)... 

Complete wetting of a solid is only possible if a drop of the liquid spreads spontaneously at the surface, i.e. for 9 = 0 or cos 9=1. The limiting value cos 6 = 1 is a constant for a solid and is named critical surface tension of a solid y... Therefore, only liquids with yl < Vc have the ability to spontaneously spread on surfaces and wet them completely. Tab. 4.2 gives an overview of critical surface tension values of different polymer surfaces [40]. From these data it can be concluded that polytetrafluoroethylene surfaces can only be wetted by specific surfactants with a very low surface tension, e.g. fluoro surfactants. 

The viscosity or resistance to flow increases as the number of repeat units increases, but physical properties, such as surface tension and density, remain about the same after a DP of about 25. The liquid surface tension is lower than the critical surface tension of wetting, resulting in the polymer spreading over its own absorbed films. The forces of attraction between polysiloxane films are low resulting in the formation of porous films that allow oxygen and nitrogen to readily pass though, but not water. Thus, semipermeable membranes, films, have been developed that allow divers to breath air under water for short periods. 

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)... 

Allcock and Smith118 have prepared a series of poly(organophosphazenes) containing siloxane grafts and/or trifluoroethoxy side groups (31-34). Polymers 31 and 32 had critical surface tensions of 16-17 rn N rn 1 and surface layers which were enriched in fluorine. In the case of 32, silicone was not observed at the surface. Polymer 33 had a critical... 

Critical Oxygen Index (COI), 853 Critical size, 704-705 Critical spherical nucleus, 710, 711 Critical strain, 867, 868 Critical stress energy factor, 474 Critical surface tension of wetting, 232 Critical temperature, 655 Cross-linked polymers, 29 Cross-linking, 148 Cross model, 731 Cross polarisation, 376, 377 Crystallinity, 728, 732, 815 Crystallites/Crystallisation, 690, 725 of rigid macromolecules, 739 Cyclical chain length, 782... 

Wettability of Elastomers and Copolymers. The wettability of elastomers (37, 38) in terms of critical surface tension was reported previously. The elastomers commonly used for the reinforcement of brittle polymers are polybutadiene, styrene-butadiene random and block copolymers, and butadiene-acrylonitrile rubber. Critical surface tensions for several typical elastomers are 31 dyne/cm. for "Diene rubber, 33 dyne/cm. for both GR-S1006 rubber and styrene-butadiene block copolymer (25 75) and 37 dyne/cm. for butadiene-acrylonitrile rubber, ( Paracril BJLT nitrile rubber). The copolymerization of butadiene with a relatively polar monomer—e.g., styrene or acrylonitrile—generally results in an increase in critical surface tension. The increase in polarity is also reflected in the increase in the solubility parameter (34,39, 40) and in the increase of glass temperature (40). We also noted a similar increase in critical surface tensions of styrene-acrylonitrile copolymers with the... 

In Table III, critical surface tensions of thirty-nine polymers are reported, calculated on the basis of Equation 5 by assuming < = 1. In other words, these calculated results are equivalent to the calculated solid surface tensions (y8). Solubility parameters of several polymers listed in the table were calculated on the basis of Small s constants (35), and all molar volumes were calculated on the basis of the molecular weight of the repeat unit and the density of the polymer. The results in Table III were used to prepare a graph (Figure 2) for the comparison between the calculated and the observed critical surface tensions of these polymers. The data are rather scattered, and the calculated values are generally lower than those observed directly. The following factors may be contributing to the deviations ... 

Critical surface tensions of functional polymers were experimentally determined. This set of data and the data on elastomers obtained previously were used to elucidate the proposed solubility parameter-surface tension relationship and the proposed parachor-surface tension relationship. The results show that the former has a higher correlation coefficient than the latter. The correlation coefficients, including three highly hydrogen-bonded polymers, are 0.731 for the former and 0.299 for the latter. Otherwise, they are 0.762 for the former and 0.178 for the latter. For the size of samples examined, we can conclude that the proposed solubility parameter-relationship is more effective than the proposed parachor-relationship in calculating critical surface tension of a polymer. 

Surface tension studies of the most common fluorosilicone, poly(3,3,3-trifluoropropylmethylsiloxane) (PTFPMS), give unexpected results. Compared with (PDMS), PTFPMS has a higher liquid surface tension, a similar critical surface tension of wetting, and a considerably lower solid surface tension, as determined by water and methylene iodide contact angles and the method of Owens and Wendt (67). These results are summarized in Table X (7, 67, 72-74, 76, 77), in which PTFPMS is compared with two other fluorocarbon polymers, poly(tetrafluoroethylene) (PTFE) and poly(chlorotrifluoroethylene) (PCTFE). PTFE behaves like PTFPMS, whereas PCTFE behaves like PDMS. 

Polymer Equilibrium Liquid Surface Tension Ref. Critical Surface Tension of Wetting Solid Surface Tension"" Ref... 

TgS for polymers with architectures similar to PDMS are given in Table XII 21, 81). The TgS of poly(dimethylsilazane) and poly(dimethyl-silmethylene) lie between those of PDMS and poly(isobutylene) (critical surface tensions of wetting of 24 and 27 mN/m, respectively [70]). These values suggest that poly(dimethylsilazane) and poly(dimethylsilmethylene) will have critical surface tensions of wetting in the 25-26-mN/m range. 

The lattice fluid equation-of-state theory for polymers, polymer solutions, and polymer mixtures is a useful tool which can provide information on equa-tion-of-state properties, and also allows prediction of surface tension of polymers, phase stability of polymer blends, etc. [17-20]. The theory uses empty lattice sites to account for free volume, and therefore one may treat volume changes upon mixing, which are not possible in the Flory-Huggins theory. As a result, lower critical solution temperature (LCST) behaviors can, in principle, be described in polymer systems which interact chiefly through dispersion forces [17]. The equation-of-state theory involves characteristic parameters, p, v, and T, which have to be determined from experimental data. The least-squares fitting of density data as a function of temperature and pressure yields a set of parameters which best represent the data over the temperature and pressure ranges considered [21]. The method,however,requires tedious experiments to deter-... 

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