Polymer blends, barrier property enhancement

One of the main advantages of the nanostructured polymer blends is their enhanced barrier properties compared with pure polymeric matrix. The impermeable clay layers force a tortuous pathway for a permeate transverse the nanocomposites. The presence of nanoparticles is believed to greatly reduce the permeability of the pure polymer [40]. It was reported that the gas permeability through the polymer films can be reduced with small loadings of nanoclays [41]. 

The primary motivation for blending immiscible polymers is to create materials with combinations of properties superior to its components [1], Cocontinuous morphologies are characterized by the mutual interpenetration of phases and can form over a range of compositions, depending largely on the relative polymer viscosities, elasticity, and interfacial tension. These morphologies offer a variety of applications mechanical property enhancement, electrically conductive blends, barrier property improvement, and tissue scaffolds. 

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

Pawlowski and Schartel92 have added 1 or 5 wt % of boehmite to blends of PC/ABS with PTFE and RDP or bisphenol A bis(diphenylphosphate). The release of water from AlOOH influences the decomposition of the material by enhancing the hydrolysis of PC and RDP. Consequently, the condensed action of RDP or BDP is perturbed. The reaction of the arylphosphate with boehmite replaces both the formation of anhydrous alumina and alumina phosphate on the one hand, and the cross-linking of arylphosphate with PC on the other hand, since less phosphate is available to perform condensed-phase action. The reaction with arylphosphate therefore decreases the char formation, but the formation of aluminum phosphate could enhance barrier properties. On the whole, even high levels of fire retardancy can be achieved (V-0 ratings) the combination of boehmite with arylphosphates acting in the condensed phase seems very complex, particularly when the host polymer can undergo hydrolysis reactions due to water release. 

One approach to enhancing the PO layer s properties in the pouches has been proposed by TOPS Advanced Polymers. It involves blending amorphous COCs in the PO film layer. (COCs are composed of ethylene linkages interspersed with norbomene cyclic hydrocarbons these bridged ring elements are said to enhance stiffness, heat resistance, and barrier properties.)... 

A modified melt blending method has been developed for preparing exfoliated nanocomposites of poly(m-xylylene adipamide) with sodium montmoril-lonite [100]. There, an aqueous solution of sodium montmorillonite was blended with the polymer in a twin-screw extruder. This kind of mixing ensures that the silica nanoparticles are exfoliated in the polymer matrix through fixing the nanoparticles within the polymer matrix just as they are in water. Oxygen permeation data show enhanced the barrier properties of the nanocomposites. 

Besides mechanical performance, the introduction of nanoclay to the proper location and with the proper phase morphology can also be utilized to enhance the permeation properties of polymer blends in barrier film applications. The high aspect ratio and platelike shape of impermeable montmorillonite inclusions allow them to disrupt the diffusion of permeants through a polymer, thereby increasing the tortuosity of the path through the film and reducing its permeability. The component polymers comprising blends are often selected for reasons other than... 

Wang Z, Zhou J, Wang X, Zhang N, Sun X, Ma Z (2014) The effects of ultrasonic/microwave assisted treatment on the water vapor barrier properties of soybean protein isolate-based oleic acid/stearic acid blend edible films. Food Hydrocolloids 35 51-58 Wihodo M, Moraru Cl (2013) Physical and chemical methods used to enhance the structure and mechanical properties of protein films a review. J Food Eng 114(3) 292-302 Woehl MA, Canestraro CD, Mikowski A, Sierakowski MR (2010) Bionanocomposites of thermoplastic starch reinforced with bacterial cellulose nanofibers effect of enzymatic treatment on mechanical properties. Carbohydr Polym 80 866-873 Xu YX, Kim KM, Hanna MA, Nag D (2005) Chitosan-starch composite film preparation and characterization. Ind Crops Prod 21 185-192... 

For the enhancement of gas-barrier properties, the most commonly used nanoparticles in polymer blends, are fillers ... 

Nanostructured polymer blends have drawn considerable interest because of their enhanced properties, including flame resistance, mechanical properties, gas-barrier properties [37-39], thermal stability, and biodegradability when compared with pristine polymers. Nanostructured polymer blends have also been used in various consumer products and in construction and transportation industries, with specific impact on technologies, such as barrier layer materials, drinks packaging applications, bottle applications, protective coatings, and adhesive molding compounds. 

The decreased permeability of the nanocomposites arises from the longer diffusion pathway that the penetrants must travel in the presence of clay nanolayers. In general, there are two reasons for the enhancement of gas-barrier properties in nanostructured polymer blends with clay. First, gas-impermeable nanoclay layers dispersed in the polymer matrix form tortuous pathways, which retard the diffusion of the gas molecules through the composites. Second, exfoliated and intercalated clay-layer bundles strongly restrict the motions of the polymer chain probably reducing the coefficient of diffusion of the gas molecules. 

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