Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
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Chapter 2.<br />
<str<strong>on</strong>g>Processes</str<strong>on</strong>g> to Manufacture Foams and to Functi<strong>on</strong>alize <str<strong>on</strong>g>the</str<strong>on</strong>g> Surface<br />
where X is given by:<br />
X<br />
*<br />
v1<br />
* * *<br />
( P1<br />
P2<br />
2<br />
12<br />
)<br />
(2.13)<br />
RT<br />
1<br />
P<br />
For a pure comp<strong>on</strong>ent, where = 1, <str<strong>on</strong>g>the</str<strong>on</strong>g> chemical potential can be reduced to:<br />
P<br />
1 <br />
R<br />
P1<br />
R<br />
(1 <br />
R<br />
) ln(1 <br />
R<br />
) ln <br />
R<br />
<br />
r1<br />
<br />
<br />
<br />
RT T1<br />
R<br />
T1<br />
R<br />
<br />
R<br />
r1<br />
<br />
(2.14)<br />
Equati<strong>on</strong>s neglecting <str<strong>on</strong>g>the</str<strong>on</strong>g> solubility <str<strong>on</strong>g>of</str<strong>on</strong>g> polymer in CO 2 , have been solved for <str<strong>on</strong>g>the</str<strong>on</strong>g> chemical potential<br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 in <str<strong>on</strong>g>the</str<strong>on</strong>g> fluid phase. As menti<strong>on</strong>ed above, <str<strong>on</strong>g>the</str<strong>on</strong>g> values <str<strong>on</strong>g>of</str<strong>on</strong>g> three characteristic parameters for each pure<br />
comp<strong>on</strong>ent and <str<strong>on</strong>g>of</str<strong>on</strong>g> <strong>on</strong>e binary interacti<strong>on</strong> parameter for polymer-CO 2 mixture are required in <str<strong>on</strong>g>the</str<strong>on</strong>g> SL model.<br />
<strong>The</strong> pure comp<strong>on</strong>ent parameter values were all found in <str<strong>on</strong>g>the</str<strong>on</strong>g> literature, as reported in Table 2.3.<br />
Table 2.3: SL-EOS characteristic parameters for CO 2 and PLGA 50:50 .<br />
Comp<strong>on</strong>ent P * (bar) T * (K) ρ * (kg / m 3 ) Reference<br />
CO 2 5745.0 305 1.510 [Kiszka et al., 1988]<br />
PLGA 50:50 5727.4 649.63 1.4516 [Liu and Tomasko, 2007a]<br />
Below <str<strong>on</strong>g>the</str<strong>on</strong>g> critical point, <str<strong>on</strong>g>the</str<strong>on</strong>g> behaviour predicted by SL equati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> state is that typical <str<strong>on</strong>g>of</str<strong>on</strong>g> a cubic<br />
equati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> state: at a given pressure, up to three roots, those are density values, and can be found from<br />
equati<strong>on</strong> 2.4. <strong>The</strong> Mathcad program proposed by Kennedy [2003] has been used to solve <str<strong>on</strong>g>the</str<strong>on</strong>g> equati<strong>on</strong> set<br />
2.4−13 and it is presented in Annex A-1.1<br />
4.2 Plasticizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Polymers by CO 2<br />
With <str<strong>on</strong>g>the</str<strong>on</strong>g> excepti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> a few polymers, such as poly(dimethylsiloxane) and some specially<br />
syn<str<strong>on</strong>g>the</str<strong>on</strong>g>sized fluoropolymers, most high molecular weight polymers show poor dissoluti<strong>on</strong> in supercritical<br />
CO2 [Adamsky and Beckman, 1994; Desim<strong>on</strong>e et al., 1992]. In those circumstances, carb<strong>on</strong> dioxide acts as a<br />
diluent ra<str<strong>on</strong>g>the</str<strong>on</strong>g>r than a solvent. As <str<strong>on</strong>g>the</str<strong>on</strong>g> c<strong>on</strong>tent <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 is increased in <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer phase, <str<strong>on</strong>g>the</str<strong>on</strong>g> sorpti<strong>on</strong> and<br />
subsequent swelling <str<strong>on</strong>g>of</str<strong>on</strong>g> an amorphous polymer can cause <str<strong>on</strong>g>the</str<strong>on</strong>g> depressi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> glass-to-rubber transiti<strong>on</strong><br />
temperature (Tg) <str<strong>on</strong>g>of</str<strong>on</strong>g> a polymer by 30°C or more [Tomasko et al., 2003; C<strong>on</strong>do et al., 1992; Wissinger and<br />
Paulaitis, 1987].<br />
Gas foaming takes advantage <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> plasticizing properties <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> dioxide. It is qualitatively<br />
known for many years that <str<strong>on</strong>g>the</str<strong>on</strong>g> compressi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> solid materials with gases alter <str<strong>on</strong>g>the</str<strong>on</strong>g> phase equilibrium <str<strong>on</strong>g>of</str<strong>on</strong>g> pure<br />
comp<strong>on</strong>ent, in particular, <str<strong>on</strong>g>the</str<strong>on</strong>g> dissoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> carb<strong>on</strong> dioxide lowers <str<strong>on</strong>g>the</str<strong>on</strong>g> T g <str<strong>on</strong>g>of</str<strong>on</strong>g> amorphous polymers, and in<br />
some cases, significantly. <strong>The</strong> reducti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> glass transiti<strong>on</strong> temperature is a <str<strong>on</strong>g>the</str<strong>on</strong>g>rmodynamic effect due to<br />
intermolecular interacti<strong>on</strong>s between carb<strong>on</strong> dioxide and <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer. Str<strong>on</strong>ger interacti<strong>on</strong>s enhance T g<br />
depressi<strong>on</strong>, as does chain flexibility. <strong>The</strong> use <str<strong>on</strong>g>of</str<strong>on</strong>g> this technique is, however, limited to amorphous polymers<br />
or semi-crystalline polymers with low T g .<br />
It was assumed that polymer segments remain completely immobile below Tg, while small<br />
plasticizers (e.g., gas molecules) are able to move and fill <str<strong>on</strong>g>the</str<strong>on</strong>g> holes within <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer matrix. Tomasko et<br />
al. [2003] have shown that if a polymer is exposed to a pressurized gas, <str<strong>on</strong>g>the</str<strong>on</strong>g> glass transiti<strong>on</strong> temperature <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
this <strong>on</strong>e decreases m<strong>on</strong>ot<strong>on</strong>ically. This analysis is based <strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> assumpti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Wissinger and Paulaitis [1991<br />
and Dimarzio and Gibbs [1963] that <str<strong>on</strong>g>the</str<strong>on</strong>g> c<strong>on</strong>formati<strong>on</strong> entropy is zero.<br />
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