Polymer blends, barrier property

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. 

Immiscible Blends. When two polymers are blended, the most common result is a two-phase composite. The most interesting blends have good adhesion between the phases, either naturally or with the help of an additive. The barrier properties of an immiscible blend depend on the permeabihties of the polymers, the volume fraction of each, phase continuity, and the aspect ratio of the discontinuous phase. Phase continuity refers to which phase is continuous in the composite. Continuous for barrier appHcations means that a phase connects the two surfaces of the composite. Typically, only one of the two polymer phases is continuous, with the other polymer phase existing as islands. It is possible to have both polymers be continuous. 

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. 

The modification of PET with naphthalene-2,6-dicarboxylic acid and other additional comonomers is a common measure in bottle manufacturing. Copolyesters based on this compound show excellent barrier properties. Such materials can be produced by addition of the desired amount of comonomer during polymer processing or by blending PET with poly(ethylene naphthalate) (PEN). Additionally, PEN can also be modified by other comonomers such as isophthalic acid (IPA) to improve the flow properties and reduce the melting point. The high price of naphthalene dicarboxylic acid is the reason for its limited application. The overall cost may be reduced by using TPA or IPA as comonomers. 

Furthermore, another advantage of nanofillers is not only to reinforce the rubber matrix but also to impart a number of other properties such as barrier properties, flammability resistance, electrical/electronic and membrane properties, and polymer blend compatibility. In spite of tremendous research activities in the field of polymer nanocomposites during the last two decades, elastomeric nanocomposites... 

A wide variety of polymeric membranes with different barrier properties is already available, many of them in various formats and with various dedicated specifications. The ongoing development in the field is very dynamic and focused on further increasing barrier selectivities (if possible at maximum transmembrane fluxes) and/ or improving membrane stability in order to broaden the applicability. This tailoring of membrane performance is done via various routes controlled macro-molecular synthesis (with a focus on functional polymeric architectures), development of advanced polymer blends or mixed-matrix materials, preparation of novel composite membranes and selective surface modification are the most important trends. Advanced functional polymer membranes such as stimuli-responsive [54] or molecularly imprinted polymer (MIP) membranes [55] are examples of the development of another dimension in that field. On that basis, it is expected that polymeric membranes will play a major role in process intensification in many different fields. 

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. 

This new product was called Skin Exposure Reduction Paste Against Chemical Warfare Agents (SERPACWA), SERPACWA consisted of fine particles of polytetrafluoroethylene (PTFE) solid dispersed in a fluorinated polyether. The excellent barrier properties of this polymer blend were related to the low solubility of most materials in it. SERPACWA is now a standard issue item to U.S. forces when there is a threat of CWA use. 

Earlier work done in these laboratories has shown that polymer blends wherein the barrier polymers are dispersed as thin platelets parallel to the surface of the fabricated article, have significantly improved permeability barrier properties than the conventional "homogeneous", uniform, dispersions (1,2). The high barriers demonstrated were then achieved by blending 15-20% polyamides with a linear polyethylene, which achieved performance comparable to that obtained by adding as much as 50% nylon by conventional blending. While this performance has been satisfactory for a variety of applications, some of the more demanding uses require that the barrier polymer be used more efficiently. Described here are such blend compositions which show substantial resistance to permeation of hydrocarbon solvents and their mixtures. 

Mixtures of polar polymers, such as PVC, PC, PMMA, TPU, PA, PEST, PGI, SAN, or ABS could be compatibilized by incorporation of two copolymers, the first containing vinyl alcohol, the second an anhydride. For example, blends of TPU with Phenoxy, EVAl, COPO, modified cellulose, and/or polyalkylene oxide, had attractive physical, optical and barrier properties, and were melt-processable without degradation. They could be transformed into films, sheets, or bottles with good barrier properties. Blends containing PA were used for films, tubes, toys, gears, bearings, shafts, curtain sliders, door rollers, etc. The blends with elastomers were reported suitable for improved wiper blades [La Fleur et al., 1994]. 

For packaging, there are a variety of requirements. The foodstuff must be preserved for the customer until the moment of consumption in a form that is palatable and wholesome. This means the polymer will usually require good barrier properties, both to prevent loss of moisture from the food, and to prevent ingress of undesirable, possibly odorous contaminants from outside. In order to obtain optimum preservative properties, polymers are often employed in multilayers, each layer being a different polymer, so that the whole polymer system presents a barrier that is apprcjpriately impervious. Unfortunately, blended films and used containers of this kind are difficult to recycle, and thus present a potential environmental problem (see Chapter 10). 

The barrier properties of PLA have not been extensively studied. The first articles treating the permeability of PLA film have been published in 1997 [120, 121], when PLA started to be considered for packaging applications. PLA films with various L/D ratios, different crystallinity degree and blends with numerous additives and polymers have been tested in recent years with gases, water vapour and organic compounds. 

The well-known examples of blends are impact modified, toughened polymers, where polymers with different glass transition temperatures are blended, such as a rubber with a thermoplastic. Many other blends are known, such as barrier polymers for packaging, where specific polar or nonpolar polymers improve the properties of polymer blends, combined to increase the resistance against transport of water and a certain gas (oxygen, carbon dioxide, etc.), such as PA (barrier to oxygen) with a polyolefin (barrier to water vapor). 

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