Glass-transition temperature internal plasticization

ISO 11357-2 1999. (1999). Plastics. Differential scanning calorimetry (DSC) - part 2 Determination of glass transition temperature. International Standards Organization. 

The increase in the length of the side chain results normally in an internal plasticization effect caused by a lower polarity of the main chain and an increase in the configurational entropy. Both effects result in a lower activation energy of segmental motion and consequently a lower glass transition temperature. The modification of PPO with myristoyl chloride offers the best example. No side chain crystallization was detected by DSC for these polymers. 

To investigate the internal plasticization of polystyrene (Tg = 105 C) by insertion of methyl acrylate (Tg = 4°C),the samples are run on a DSC.Therefore, approximately 15 mg of each of the well dried polymers are weighed into small aluminum pans and measured by two heat-cool runs in a DSC apparatus.The glass transition is found as a characteristic jump in the heat capacity in the system.The glass transition temperature is evaluated after the second heating from the DSC plot. 

Lowering the Glass Transition Temperature by External or Internal Plasticization... 

Internal plasticizers are synthesized by copolymerization of suitable monomers. Polymeric non-extractable plasticizers, mostly copolymers having substantially lower glass transition temperatures due to the presence of plasticizing ( soft ) segments such as poly(ethylene-co-vinyl acetate) with approximately 45 % vinylacetate content, ethylene-vinyl acetate-carbon monooxide terpolymer, or chlorinated PE, are available for rather special applications in medicinal articles (Meier, 1990). In this case, the performance of the internally plasticized polymers is the principal advantage. However, copolymerization may account for worse mechanical properties. A combination with external plasticizers may provide an optimal balance of properties. For example, food contact products made from poly(vinylidene chloride) should have at most a citrate or sebacate ester based plasticizers content of 5 % and at most 10 % polymeric plasticizers. 

Poly(vinyl acetate) dispersions form lightfast, dry, hard, brittle films. Plasticizers therefore have to be used (external plasticization), which are, however, volatile and lead to embrittlement of the films after a relatively short time. Internally plasticized dispersions of copolymers of vinyl acetate with vinyl laurate, butyl maleate, Versatic Acid esters, or ethylene form permanently flexible, nonaging films that are not, however, always sufficiently resistant to hydrolysis. Terpolymer (vinyl acetate-ethylene-vinyl chloride) dispersions form films that are more resistant to hydrolysis than homopolymer and copolymer dispersions. The films also have a higher mechanical strength and lower flammability. The glass transition temperature of the terpolymer can be varied within wide limits and properties can be matched to requirements by using a suitable choice of comonomers. The same is true of vinyl propionate copolymer dispersions. 

The influence of a stiff and bulky substituent on the thermal and the solution properties is governed by the size, polarity, position, and number of the substituents [24]. The main differences between flexible and bulky substituents are their influence on the thermal stability and the glass transition temperature. Whereas flexible chains act as an internal plasticizer and lower the thermal stability, most bulky substituents increase Tg and, depending on the chemical nature, do not lower the thermal stability. 

A shift in the glass transition temperature results from copolymerization of plastic A with B (A-co-B), in our example of butadiene with polystyrene. This process is also known as internal plasticization. By contrast, in block and graft copolymerization and in polyblends of two plastics C and D, their glass transition temperatures remain essentially unchanged. 

Internal plasticization is copolymerization of two compatible mmiomers with widely disparate glass transition temperatures. 

Example of internal stress due to cooling Parabolic temperature profiles form across the thickness of a warm, cooling sheet of amorphous plastic. Fig. 71. If the initial temperature is above the glass transition temperature and cooling is rapid (quenching), the internal stress distribution across the cross-section of the material is nearly parabolic as well. 

Comonomer having hexyl side chain was added to N-vinylpyrrolidone to improve its properties. It was fonnd that the inclusion of comonomer rednces glass transition temperature of copolymer because it acts as an internal plasticizer. Polyimides are internally plasticized with alkyl 3,5-diaminobenzoate compounds. Without internal plasticization a polymer has too high a glass transition temperature that makes processing veiy difficult. 

Acrylic copolymers may be internally plasticized by the selection of an appropriate composition of monomers which give the copolymer the required glass transition temperature. 

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