Glass transition temperature, polymer packaging materials

If the two different polyamic acid solutions were mixed and then cured, the glass transition temperatures of the cured polyimides are different from those corresponding to separately cured polyimides. These results are interpreted as transamidization which gives random co-polymer(s). C. Feger in Polymeric Materials for Electronic Packaging and Interconnections J.H. Lupinski and R.S. Moore Eds American Chemical Society Washington, DC, 1989 p 114. 

Ethylene-norbornene copolymers, which have thermoplastic properties when heated above their glass transition temperatures of ca. 200-250°C, have been commercialized by Ticona GmbH under the trade name TOPAS (Tliermoplas-tic Olefin Polymer of Amorphous Structure). Their properties - exceptional transparency, low double refraction, high stiffnes and hardness, low permeability for moisture and excellent biocompatibility - make these ethylene-norbornene copolymers particularly valuable as engineering polymers, for optical applications and as materials for food and medical packaging. 

PVA is not suitable as the base of packaging materials since its physical and mechanical properties are impaired abruptly above the glass-transition temperature (28°C). It is used, nevertheless, along with polyvinyl butyral (PVB), polyvinyl alcohol (PVAL), alcohol-soluble polyamides and polyacrylates as a layer-carrier of Cl in multilayered films. The layer is formed via application of emulsions, suspensions or solutions of named polymers containing Cl onto the base film [23-26]. 

Polymers that have a high glass transition temperature (Tg) are difficult to be blown into films or they are too brittle for their use as a packaging material. In contrast, polymers that with a low glass transition temperature show low softening and melting temperatures. This makes the fabrication of sheets and films difficult without selfadhesion. In addition, such sheets may lack adequate an water permeation, which is necessary for packaging applications (49). 

The low heat deflection temperature of PLA limits its use for several application fields, such as in packaging materials and electronic components. The introduction of rigid building blocks [63] or cross-links [64] is known, for instance, to increase the glass transition temperature and/or heat resistance of lactic acid based polymers. The effect of different amounts of comonomers in the prepolymers on the Tg and mechanical properties of poly(ester-urethane)s is demonstrated in Table 3.2. The heat resistance of poly(ester-ure-thane)s can be improved by the copolymerization of lactic acid with D,L-mandehc acid. This broadening of the operating temperature range is of clear practical importance. The incorporation of other comonomers that impede rotation and make polymer chains less mobile also causes an increase in Tg, even if the same comonomers can depress the rate of poly condensation [50]. 

During the last decades interest in membrane separation processes and in solutions for packaging problems has given a substantial boost to research on sorption and mass transport in polymeric materials. Several interpretative models for the solubility and diffusivity in rubbery polymers are now available which, at least in principle, allow for the prediction of the permeability of low molecular weight species in polymeric films above their glass transition temperature. 

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