Homopolymer chains temperature

MACA as a hydrophobic comonomer can be used to modify PNIPAM. Copolymers, PNIPAM-co-MACA with different amounts of MACA can be synthesized by free-radical copolymerization of NIPAM and MACA in a mixture of methanol and chloroform with AIBN as the initiator. The resulting copolymers after purification can be dried in vacuum at 40 °C for 24 h. Hereafter, these copolymers are denoted as PNIPAM-co-x-MACA, where x denotes the molar percent of MACA. As expected, their solubility in water decreases as the MACA content or the solution temperature increases. It is also expected that the copolymer chains with a higher MACA content would have a lower LCST in comparison with PNIPAM homopolymer chains. In order to prepare a true solution, one has to dissolve these copolymers in water at low temperatures. The chemical structure of PNIPAM-co-MACA is as follows (Scheme 7). 

Fig. 2 Schematic of four thermodynamically stable states (random coil, crumpet coil, molten globule and collapsed globule) of a homopolymer chain in the coil-to-globule and the globule-to-coil transitions. There exists a hysteresis between the two transitions around the 0-temperature ( 30.6 °C) of the PNIPAM solution [37]...

Figure 24 shows that both (i g) and (jq,) decrease as the temperature increases. Each data point was obtained only after the solution had reached thermodynamically equilibrium and the measured value was stable. Note that in each curve there exists a small kink at 29.4 °C and that (Rg)/(Rh) remains constant at 1.15 in the range 29-30.6 °C, representing an additional transition prior to the collapse of the PNIPAM chain segments. The decreases of both (Rg) and (R ) after the kink become faster. As shown before, the coil-to-globule transition of PNIPAM homopolymer chains do not present such a kink [32-34,37-40]. The sharp decrease of (Rg)/(Rh) from 1.5 to 0.6 in the inset confirms the coil-to-globule transition of individual copolymer chains. However, a careful examination of Fig. 24 raises a number of questions. 

Fig. 2. Mole fraction in dilute phase versus the inverse reduced temperature for homopolymer chains of length 1 to 5 (points top curve is r = 1). The top scale gives the corresponding real temperature, assuming that e = 60 K. Experimental data for n-hexane and n-nonane are shown as solid lines (top curve is for n-hexane).

Ttp 4 Chain microstructure and propagation reactions. Propagation reactions are mainly responsible for the development of polymer chain microstructure (and control chain composition and sequence length distribution in copolymerizations). In free radical polymerization, the stereoregularity of a high molecular weight homopolymer chain depends on polymerization temperature almost exclusively. It is usually independent of initiator type and monomer concentration. Calculations on stereoregularity... 

Figure 5.19 shows that lowering the solution temperature from 60 to 10 °C has nearly no effect on Af of hyperbranched copolymer chains but leads to a very sharp increase of Af of hyperbranched homopolymer chains at -28 °C, signaling the interchain association, which is expected because unlike the copolymer chains there is no soluble PtBA blocks to stabilize those collapsed PS subchains and prevent them to phase out of the solution. Such intrachain contraction and interchain association are also directly reflected in the decrease of both R and (7 h). It is also understandable that the hyperbranched homopolymer chains shrink much... 

Figure 5.19c shows that for the hyperbranched homopolymer chains, Rg)/ Rh) decreases as the temperature decreases before the chains start to associate because Rg) decreases faster than (Rh). Note that (Rg) describes how mass is distributed in space, while Rh) contains the hydrodynamic draining. Such a size decrease as the chain shrinks was also observed for linear homopolymer chains before because of a less change in Rh) [45, 46], especially in the low temperature region. However, an opposite trend was observed for the hyperbranched copolymer chains, presumably because the shrinking of each PS block makes the subchain less flexible, which has an opposite effect on (Rg). 

In section III we discussed the diffusion of a homopolymer chain comprised of N monomer segments into a highly entangled melt where it was shown that its tracer diffusion coefficient, D, varied as and that the temperature dependence of D /T was virtually the same as that of the inverse of the zero shear-rate viscosity of the homopolymer. Here we discuss the diffusion of homopolymers into microphase-separated diblock copolymer structures. 

The temperature dependent measurements corroborate the SAXS findings that the copolymers are microphase separated. This data also strongly suggests that the d-PS chains diffuse within the PS domains and the d-PMMA chains diffuse within the PMMA domains. The phases of PS and PMMA are continuous, at least on length scales on the order of 800nm, the distance probed by the FRES experiment. It appears that the difference between the magnitudes of Dhc D for the diffusion of the homopolymer chains is related to the structure of the copolymer and not due to the... 

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