Chapter IV Structure-Property Relationships for Hetero- Complementary Hydrogen-Bonding Partners
4.3 Discussions
4.3.2 Reconciliation between 1 H NMR Results and Viscosity Data
(Figures 4.13 and 4.14): While the HR/CA pair failed to provide enough strength to hold polymer chains of Mw ~ 200 kg/mol together as supramolecules stable at low shear rates, the elegantly simple DA/DB pair was found effective in driving these polymer chains to form supramolecular aggregates stable at moderate shear rates. The above comparison also implicates that an OHB-based hetero-complementary associative pair that contains 8 OHBs and has a D/A site arrangement free of repulsive SEIs may possess sufficient binding strength for affording supramolecules stable at shear rates relevant to the MCK application. The problem is, however, that it is already difficult enough to synthesize an array of hydrogen-bond acceptors containing 4 acceptor sites and to use it as a polymer end group.72 To double the number of acceptors from 4 to 8 is a daunting task, and the accompanying cost and instability issues would further render the implementation of OHB-based hetero-complementary association in the development of mist-control polymers for fuels impractical.
explained through the interplay between the equilibrium nature of such hetero- complementary association and hydrodynamic force. Take the HR/CA pair for example, and consider the complementary association of HR and CA units as an equilibrium reaction:
Complex
CA HR
Kasso can thus be expressed as follows:
disso assoasso k
K k
CA HR Complex
Where [HR] and [CA] are the concentrations of free HR and CA end-groups, [Complex]
is the concentration of bound end groups, kasso is the rate constant of the association process, and kdisso is the rate constant of the dissociation process. In this case, Arrhenius equation can be used to quantitate kdisso as follows:
RT
A E
kdisso dissoexp disso
Where Adisso and Edisso are the pre-exponential factor and the activation energy of the dissociation process, respectively. In the 1H NMR study of ~1wt% CDCl3 solution of the mixture of 240K di-HR and 200K di-CA 1,4-PBs, the only energy source contributing to the dissociation process was the thermal energy of the polymer chains. The absence of the imide proton signal of the CA end-groups (Figure 4.11) indicates that HR and CA could still bind with each other, which suggests that the thermal energy of the polymer chains may not be sufficient to surpass Edisso. In shear viscometric study, however, mechanical energy was introduced into the 1wt% CDD solution of the polymer mixture as it was subjected to shear deformation, and the amount of such external energy was positively correlated to the applied shear deformation (expressed in terms of shear rate in
the present study). We believe it is possible that at higher shear rates (>10 s-1), the amount of introduced mechanical energy, combined with the thermal energy of the telechelic chains, may be sufficient to surpass Edisso. Consequently, the dissociation process could take place readily, leading to the absence of viscosity enhancement by mixing the 1wt% CDD solutions of 240K di-HR and 200K di-CA 1,4-PBs (Figure 4.14).
We believe the above rationale can also apply to the case of the complementary pair of 288K di-THY and 219K di-DAAP 1,4-PBs (Figure 4.13).
The other feature of the equilibrium nature of THY/DAAP and HR/CA complementary associations concerning the discrepancy between the conclusions of 1H NMR and shear viscometry data is that both the size of polymer backbone and the end- group concentration can affect the strength of complementary association. Numerous reports have revealed that the values of Kasso of HR/CA pair in polymeric systems (~
2×104 M-1) can be lower than those in oligomeric and small-molecule systems (~105-106 M-1) by an order of magnitude,21,23,29,37 possibly due to the steric hindrance from the polymer chains during complexation.49 A similar decrease in Kasso has also been reported on the stronger UG/DAN pair, which has a value of Kasso ~107 M-1 in small-molecule systems and a Kasso ~2×104 M-1 in polymeric systems.14,21 As shown in Figure 4.2, PA (Mw = 8.5 kg/mol, PDI = 1.30) and PB (Mw = 11.4 kg/mol, PDI = 1.75) prepared by Yang and coworkers can form supramolecular diblock copolymer (evidenced by increase in ηsp at low shear rates (<10 s-1)) at 1.4-2.8 wt% in CH2Cl2 via HR/CA complementary association.21 In the present study, the average size of backbone is 20 times as high as those of PA and PB, and the concentration used in 1H NMR and shear viscometry study is also lower (~1wt%). Consequently, [HR] and [CA] in this study is ~ 1/20 of those in
Yang and coworkers’ work. From the viewpoint of equilibrium reactions, the above difference in [HR] and [CA] can be treated as dilution of HR and CA end-groups.
According to Le Chatelier's principle, the equilibrium shifts to the left in order to counteract the effect of dilution, resulting in more free-standing HR and CA end groups.
In other words, the binding strength of HR/CA end-association is weaker when a longer backbone and a lower polymer concentration are used. Supporting results for the above hypothesis are reported in Park and Zimmerman’s work on supramolecular multi-block alternating copolymers from DAN-terminated telechelic poly(n-butylmethacrylate) (Mw
= 100 kg/mol) and UG-terminated telechelic poly(ethylene oxide) (Mw = 2 kg/mol):30 The shear rheology data of the polymer mixture in 1:1 DAN:UG end-group ratio in CHCl3 at 26 oC showed that enhancement in ηsp was observable only at concentrations higher than a total polymer concentration of 6.25 g/dL (or 4wt%).