Amorphous phase permeability

A more polar comonomer, eg, an AN comonomer, increases the water-vapor transmission more than VC when other factors are constant. For the same reason, AN copolymers are more resistant to penetrants of low cohesive energy density. AH VDC copolymers, however, are very impermeable to ahphatic hydrocarbons. Comonomers that lower T and increase the free volume in the amorphous phase increase permeability more than the polar comonomers higher acrylates are an example. Plasticizers increase permeabiUty for similar reasons. 

When the crystallinity of polyethylenes is increased, the gas permeability through the film decreases. The factors involved are the tortuosity of the gas path through the amorphous phase, and the effect of the crystals in restricting the mobility of the amorphous polymer chains (chain immobilisation factor). The logarithm of the permeability of nitrogen, argon and carbon dioxide decreased almost linearly with increased crystallinity of PE, with the ratio of the gas values remaining almost constant for a particular PE. 

Measurements of the permeability and diffusivity have been undertaken for helium and oxygen for a range of highly oriented polyethylene films The deduced solubilities for both gases are proportional to the amorphous volume fraction showing that the non-crystalline regions are the transport medium in all instances. The solubility obtained for oxygen is about ten times that for helium. This result is consistent with the proposal of Michaels and Bixler which relates the amorphous phase... 

Water permeates a flawless polymer film through the amorphous phase via solution-diffusion mechanisms. Therefore, water permeability is inversely proportional to the volume fraction of the crystalline phase (crystallinity). Water molecules are first dissolved into the polymer matrix at the interface the dissolved water molecules diffuse through the polymer according to the chemical potential... 

The remainder of this chapter will focus on the oxygen permeabilities of amorphous polymers (or the amorphous phase, in the case of semicrystalline polymers). See Chapter 20 for a discussion of methods for the prediction of the permeabilities of heterogeneous materials (such as blends, composites and oriented semicrystalline polymers) in the much broader context of the prediction of both the thermoelastic and the transport properties of such materials. 

Miscible Blends. Sometimes a miscible blend results when two polymers are combined. A miscible blend has only one amorphous phase because the polymers are soluble in each other. There may also be one or more crystal phases. Simple theory (26) has supported the empirical relation for the permeability of a miscible blend. Equation 18 expresses this relation where Pmb is the permeability of the miscible blend and and are the volume fractions of polymer 1 and 2. 

In Figure 6, one can see, as with amorphous materials, that the more unbalanced the biaxial orientation is the greater are the reductions in permeability. Interpretations of the transport data for biaxially and uniaxially drawn PET samples can be explained by observing conformational changes in the polymer backbone itself. Apparently, the chain packing efficiency of the amorphous phase improves as the number of trans isomers in the ethylene glycol unit increases. Polarized infrared analysis of uniaxially and biaxially oriented systems indicates that the fraction of... 

Rubber consumption is dominated by tyre production. In these, conveyor belts, and pressure hoses, thin layers of either steel wire or polymeric fibre reinforcement take the main mechanical loads. These layers, with rubber interlayers, allow flexibility in bending, whereas the reinforcement limits the in-plane stretching of the product. The applications are dominated by natural rubber and styrene butadiene copolymer rubber (SBR). Other rubbers have specialised properties butyl rubbers have low air permeability, nitrile rubbers have good oil resistance, while silicone rubbers have high and low temperature resistance. Rubbers play a relatively small role in this book, but the rubbery behaviour of the amorphous phase in semi-crystalline thermoplastics is important. 

No change of the sorption properties of the amorphous phase is observed by thermal treatment. A low gas permeability measured at the biaxially stretched films is related to both a change of the free volume sizes distribution and a tortuosity effect. The barrier properties of biaxially stretched films are kept even after annealing the film at 250°C. 

The diffusivity D is a kinetic parameter related to polymer mobility, while the solubility coefficient is a thermodynamic parameter which is dependent upon the strength of the interactions in the polymer-penetrant mixture. Chemical modifications of polymers affect the coefficients of diffusion and of solubility. Changes in material structure have a greater effect on diffusion coefficient, whereas the solubility coefficient depends mainly on the character of the low-molecular-mass compound. Permeability is determined by factors such as the magnitude of the free volume, and crosslinking which reduces the segmental mobility and the free volume and diminishes the permeability coefficient. A reduction of interchain cohesion and of crystallinity increases the permeability coefficient. The transition from the amorphous to the crystaUine state usually decreases the permeability. A decrease in crystallinity may increase the permeability. The permeability of polymers is determined primarily by the amount of the amorphous phase [62,300, 301]. 

Althongh the crystalline regions of PMP were fonnd to be penetrable, the CO2 and CH4 permeabilities are still dependent on 4>a, since the solubilities and diffn-sivities in the more structured phase were lower than in the amorphons phase. It was concluded that gases such as CO2 and CH4 dissolve in the crystalline regions of PMP at about 1/3 to 1/4 the extent in the amorphons phase (58). Moreover, the diffusion coefficients of CO2 and CH4 are about 37% and 64% lower, respectively, in the crystalline phase than the amorphous phase. 

The good barrier properties of VDC copolymers are a consequence of crystallinity and low free volume in the amorphous phase. The sjunmetric nature of the VDC unit in the polymer leads to nested packing that is adequate for crystallization and that leaves very little dead volume in the amorphous phase. Both polyisobutylene and PVDC have unusually low permeability to water compared to their monosubstituted counterparts, polypropylene and PVC (86). The values listed in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

For PLA, the absorption of water from the atmosphere has also been reported to affect O2 permeability. The effect of moisture on the values of P, S, and D for O2 of PLA films (94% and 98% L-lactide) has been characterized by Auras et al. [54]. In this study, PLA films were exposed to different water activities (a = 0-0.9) at different temperatures. PET films were studied for comparison. P of the PLA films decreased with increasing water activity, and this reduction was more pronounced at 40°C and a = 0 (from 11 x 10 to 8.5 X 10 kg m/(m s Pa)) than at 23°C. This behavior is in contrast to the increase in P of several conventional plastics under wet conditions mentioned above and shown in Table 12.2. The behavior of those polymers is due to their hydrophilic nature, whereas PLA and PET films are both relatively hydrophobic. The value of D for O2 in PLA and PET films showed an exponential increase as a function of for ail temperatures (Figure 12.3), which was attributed to the plasticization effect of the water molecules on the amorphous phase. Moreover, the lower value of D for O2 of the PLA with a higher L-lactide content has been attributed to the higher crystallinity and, thus, more tortuous path for the O2 molecules. The value of S for O2 decreased linearly with increasing a , as illustrated for one example in Figure 12.4, due... 

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