Contraction upon heating

ELP-based triblock copolypeptides have also been used to produce stimulus-responsive micelles, and Chaikof and coworkers envisioned the possible application of these micelles as controlled drug delivery vehicles. These amphiphilic triblock copolymers were constructed from two identical hydrophobic ELP endblocks and a hydrophilic ELP midblock. Below the transition temperature, loose and monodispersed micelles were formed that reversibly contracted upon heating, leading to more compact micelles with a reduced size [90]. 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

Sol-gel methods are used to prepare a heterogeneous, nanometer composite of calcium zirconium phosphate and strontium zirconium phosphate, Cao.sSro.sZrsOn. In this composite, individual domains both expand and contract upon heating, leading to a material with virtually zero thermal expansion over the range 0-500 °C. 

Aluminous Refractories.—It can bo readily seen that alumina itself, owing to its high fusion point, 2,050°C., is inherently an excellent refractory. The only difficulty in tlici way of utilizing it is its large contraction upon heating. 

The expansion of chemical bonds with increasing temperature leads the vast majority of known solids to expand with heating (positive thermal expansion, PTE), a property once thought to be an immutable law of nature. A relatively small number of materials are known that defy this expectation and contract upon heating i.e. display negative thermal expansion, NTE) or are temperature-invariant i.e. display zero thermal expansion, ZTE). These novel behaviours arise due to a range of physical mechanisms that include magnetostriction and, most... 

Negatively, temperature-sensitive hydrogels possess a LCST and contract upon heating above the LCST. This type of swelling behavior is known as the inverse (or negative) temperature dependence. Inverse temperature-dependent hydrogels are comprised of polymer chains which either possess moderately hydrophobic groups or a mixture of hydrophilic... 

Charette has reported a highly effective carbohydrate-derived auxiliary for diastereoselective cyclopropanation reactions of olefins with high induction (Scheme 6.13) (see also Chapter 15, Section 15.4) [42]. Cyclopropane 69 produced by treatment of olefin 68 under Furukawa s conditions [43] could be smoothly liberated on treatment of the adduct with triflic anhydride under these conditions the derived 2-0-triflate intermediate undergoes ring contraction upon heating, releasing 70 in 98.4% ee, together with aldehyde 71 [42]. 

Expansion and contraction of siUca gel monoliths show hysteresis upon heating and cooling. As long as the sample was cycled below approximately 500°C, there was no hysteresis and the monolith was thermally stable (6,23). 

The distances between anionic sites in the ordered hydrocalumite [76-80], i.e., 9.96 A and 5.74 A along the a and b axes, respectively, are suitable for a syndiotactic polymerization process with monomers alternating above and below the central plane, running along the a axis as illustrated in Fig. 18. However, in order to connect with each other, the VBS monomers must incline, which is consistent with the lamellar contraction observed upon heating and associated with the polymerization process (discussed in Sect. 2.2.3). 

Certainly one of the most novel isomerizations involving heterocyclic five-membered rings is the ring contraction and structural rearrangement of perfluoro-U -bipyrimidine derivative 11, which upon heating at 150 C for 16 hours in the presence of cesium fluoride yields per-fluoro(4-methyl-l.2,5,7-tetraazatrieyclo[5.3.1.02 6]undec-4-ene) (12).8... 

The homopolymer showed an enantiotropic nematic mesophase, whereas the diblock copolymer generated microphase-separated lamellae, in which the SCLCP block possessed a nematic-isotropization transition similar to the homopolymer (Table 17). Upon heating, the nematic microphase decreased continuously in the nematic phase from 38.5 nm to 27 nm and showed a constant value of about 26 nm after the nematic-isotropization transition. Therefore, materials in which these block copolymers are macroscopically aligned are expected to show reversible contraction in one dimension, making this polymer system an interesting candidates for an artificial muscle or actuator. 

FIGURE 13-50 Schematic diagram depicting the contraction of a rubber chain upon heating. 

Both copolymers were cross-linked by copolymerization with (1-2%) ethylene dimethacrylate the effect of irradiation was followed by measuring the change of elastic retractive force at constant elongation. In the first case the contraction upon irradiation of the gel is mainly due to changes of chain conformation and swelling equilibrium induced by trans-cis photoisomerization. In the second case by comparing the retractive force for irradiated and unirradiated samples the heat effect was evaluated, and the photoinduced contraction estimated to 1%. 

Both materials were subjected to the same processing conditions. The cure profile consisted of heating from room temperature to 2 60°C at 5°C/min, holding at 260°C for 2 hours, then cooling to room temperature at 5°C/min. As can be seen in Figure 3, their stress-temperature profiles are quite different. Both films left the spin-coater with approximately zero stress. Upon heating, the polyimide film developed substantial tensile stress due to film contraction from solvent evaporation while the BCB film exhibited only mild tensile stress buildup. The stress in the BCB film relaxed at 260°C while the stress in the polyimide did not. 

The BCB exhibits minimal solid-like character upon heating contraction due to solvent evaporation only slightly outpaces thermal expansion. The Tg remains significantly below the sample temperature, resulting in a low modulus and thus low stress. Upon reaching 260°C, the tensile stress in the BCB film relaxes because of only minimal network formation, which would inhibit the relaxation, during the heating cycle. 

Heat is absorbed by the rubber band, so A// is positive. Since the contraction occurs spontaneously, AG is negative. For the reaction to be spontaneous, AS must be positive meaning that the rabber becomes more disordered upon heating. This is consistent with what we know about the stracture of mbber The rabber molecules become more disordered upon contraction (See the Figure in the Chemistiy in Action Essay on p. 826 of the text). 

Figure 3. Volume relaxation in poly(vinylacetate) upon cooling from 40 C, through Tg to 30 C ( ), contraction upon cooling from 40 C, through Tg to 25 C holding for 90 hr and heating to 30 C (o), memory and upon cooling from 40 C, through Tg to 25 C holding until quasi-equilibrium is reached and heating to 30 C (—) expansion, (Adapted from ref. 24.)...

---