Moisture‐induced plasticization of amorphous polyamides and their blends

Abstract

SynopsisThe moisture-induced plasticization of some amorphous polyamides in pure and blended form has been determined by calorimetric methods. The compositional dependence of the glass transition temperature of these polymer-diluent systems has been adequately accounted for Using an existing predictive expression derived from both a conformational entropy and a thermodynamic treatment of the glass transition phenomenon. The success of this approach to account for the behavior of amorphous polyamide blends containing hydrophobic and hydrophilic constituents exemplifies the importance of compositional parameters in determining the overall response of morphologically complex hydrophilic polymers. The results add further credence to the simple plasticizing action of water in polar polymers irrespective of their chemical and physical constitution. INTRODUCTIONThe sustained incursion of polymeric materials into demanding applications necessitates increased vigilance with regard to their long-term behavior and durability. The glass transition temperature (T,) often denotes a temperature threshold of utility mark& by a precipitous change in some mechanical properties (e.g., modulus). This is particularly true for completely amorphous materials, and any agent, for example water, which can induce a substantial reduction of Tg requires special consideration. In this context the ubiquitous presence of moisture and the hydrophilic nature of many polymers is of considerable importance. Therefore it is often useful to know, a priori, how a hydrophilic polymer responds in a humid environment. These concerns are manifest by only a scant inspection of recent literature detailing a continued interest in the interactions of hydrophilic polymers with water. '-4 The polar nature of water, and the chemical functionalities often present in polymers in which water acts as a plasticizer, has often been the source of speculation with regard to the role of water-polymer interactions in the depression of T ' in these polymer^.^^^-' Typically, water acting as a plasticizer at low concentration (1% by wt) can induce a 15-20°C reduction of Tg compared to a value of 4 4°C for common polymers plasticized by organic diluents. Discussion of observed behavior has sometimes centered on the "disruptive" potential of water molecules in hydrogen-bonded environments and the possibility that water molecules may exist in "free" or "bound" states within a polar polymer m a t r i~.~,~,~,~, ' It has also been demonstrated, however, that water has an effective but rational plasticizing action in hydrophilic polymers without resorting to the imposition of "disruptive" polymer-diluent interactions," or the effects alluded to above. Similarly sub-Tg One of the supporting arguments from the aforementioned studies relies on the application of a theoretical description of the composition-dependent Tg in polymer-diluent binary systems to account successfully for observed behavior. Similar treatment of plasticization data from nonpolar polymer-diluent system^'^-'^ h...

Hydrogen bonding, mobility, and structural transitions in aliphatic polyamides

“…119,120 As an explanation for such effects, water is said to loosen up the structure 117,121 or soften nylons, in effect acting as a plasticizer. 122,123 It is possible to gain a better understanding of the solventinduced changes and thus enhance the material properties with the results published over the last 20 years on the effects of water on the relaxation processes, chain mobility, amorphous orientation, phase transitions, diffusion of gases, and mechanical properties. Because the crystalline regions are impermeable to the host molecules (low-molecular-weight compounds), their transport (diffusion behavior) is determined almost entirely by the permeability and distribution of the amorphous regions.…”

Influences of the Glassy and Rubbery States on the Thermal, Mechanical, and Structural Properties of Doughs and Baked Products

“…(From Slade andLevine, 1988a, with permission.) et al, 1984;Jin et al, 1984;Ellis, 1988) . Free volume theory (Ferry, 1980) provides the general concept that free volume is proportional to inverse Mn , so that the presence of a plasticizing diluent of low MW leads to increased free volume, allowing increased backbone chain segmental mobility.…”

“…For synthetic polymers, it is well known that the ability of a diluent to depress Tg decreases with increasing diluent MW (Boyer et al, 1985), as predicted by free volume theory. Recent reports have demonstrated that the effectiveness of water as a plasticizer of synthetic polymers primarily reflects its low molar mass Qin et al, 1984;Ellis et al, 1984;Ellis, 1988). These workers have discounted older concepts of specific interactions, such as disruptive water-polymer hydrogen bonding in polymer hydrogen-bonded networks, or plasticizing molecules becoming "firmly bound" to polar sites along a polymer chain, in explaining the plasticizing ability of water.…”

Chemical deterioration and physical instability of food powders

“…1%) can largely decrease the T g (i.e. by 15-20 °C) (Ellis, 1988). Figure 22.5 demonstrates T g depression as a result of water plasticization in milk powder.…”

“…Only a small amount of water is required to cause collapse in those products (Ellis, 1988). It was found that a 1% increase in moisture content of freeze-dried carbohydrates was enough to decrease the collapse temperature by 5 °C (Bellows and King, 1973).…”