Chapter IV Structure-Property Relationships for Hetero- Complementary Hydrogen-Bonding Partners
4.1 Introduction
4.1.2 Methods to Characterize Noncovalent Bonding
strong acid relative to the conjugated acid of the acceptor (i.e., pKa <-4, see Table 4.2 for pKa values), a single hydrogen bond can meet or exceed the 14 kBT (8.3 kcal/mol) requirement in non-polar aprotic solvents (e.g., chloroform and toluene), that is, a CAHB (which typically occurs when the donor is a carboxylic, phosphonic, or sulfonic acid and the acceptor is a primary, secondary or tertiary amine). The strength of CAHBs allows us to build supramolecular polymer structures using associative groups that are readily accessible and lend themselves naturally to construction of homologous series to elucidate structure-property relationships.2,6,7,39-45 Although CAHB-based hetero- complementary associative pairs have not received as much attention as those based on multiple OHBs, recent advances in polymer synthesis and the associated ability to prepare well-defined CAHB-based systems have led to a renaissance in this area.4 Representative examples of this category of supramolecular polymer structures include miktoarm supramolecular star copolymers of polystyrene (PS) and polyisoprene (PI),6 PI- PS-PI, supramolecular triblock copolymer and its thermo-responsive lamellar/cylindrical nanostructures,7,46 PS-PMMA supramolecular diblock copolymer and its application as anti-reflective coatings,47 and supramolecular polymer gels via blending two polymers that are liquids at room temperature.48
chemical shifts of these protons.11,49 Capillary (or “Ubbelohde”) viscometry at low shear rates (<101 s-1) has been used in few cases in conjunction with 1H NMR spectroscopy to relate formation of supramolecular multiblock copolymers with their effects on macroscopic properties. Supramolecular block copolymers resulting from self-assembly are expected to possess higher solution viscosity compared to that of their individual blocks.12,21,22,30,50,51 Specifically, viscosity enhancement at low shear rates occurs when the product of the lifetime of the noncovalent bonds and the modulus of the associated structures is greater than the solvent viscosity, whereas the usual analysis of the peak shift observed in 1H NMR spectra presumes “fast exchange” on a timescale of 1 ms.
The independent nature of the two observables (peak shift in 1H NMR and low- shear-rate viscosity) is illustrated by recent results of Yang, Ambade and Weck on a non- covalent ABC triblock in which the three blocks were (A) poly(norbornene ester) with Mn = 6.5 kg/mol, (B) poly(norbornene imide) with Mn = 6.5 kg/mol, and (C) poly(ethylene oxide) with Mn = 2.0 kg/mol, such that all three blocks have similar degrees of polymerization of approximately DP = 25-45 (Figure 4.2).21 The A-chains (PA) were monotelechelic with one end capped with CA (Table 4.1); the B-chains (PB) were heterotelechelic with a Hamilton receptor (HR) installed at one end (to connect with PA) and a 2,7-diamido-1,8-naphthyridine (DAN) group at the other end (to connect with PC), the C-chains were monotelechelic with a UG group at one end (to connect with the DAN end of PB). Association of HR and CA was demonstrated using 1H NMR, which showed a roughly 5ppm shift of the protons of the CA unit in a stoichiometric solution of PA and PB (5 mM total polymer in CDCl3 at 25oC), indicating that >99% of HR and CA were paired. Similarly, association of DAN and UG was evident in a shift of approximately 4
ppm of amide protons of the UG unit in a stoichiometric solution of PB and PC (5 mM total polymer in CDCl3 at 25oC, which indicates >99% of DAN and UG were paired).
These shifts occurred to the same extent when all three polymers were mixed at stoichiometric ratios, confirming that the HR/CA and DAN/UG associations were mutually orthogonal. Nevertheless, the specific viscosity (ηsp, ≡ ηsolution/ηsolvent – 1) values only increased slightly upon association, consistent with approximately 20% of chains being connected (Figure 4.2 b). This apparent contradiction with respect to the high degrees of association suggested by 1H NMR may simply reflect the rapid exchange among the associative groups.
Thus, the literature highlights the challenge of producing supramolecular polymers that confer effects provided by ultralong linear chains (>106 g/mol).
Associations that give only a 10%-20% increase in the apparent molecular weight are not sufficient. Even if the non-associated chains are 250 kg/mol (over 20 times longer than those examined in prior literature on supramolecules), it is necessary to increase their apparent molecular weight by 800% to mimic effects that are conferred by chains of 2×106 g/mol. Furthermore, the concentration regime in which the effects of ultralong polymer are of particular interest is in the range of 1wt% or less, which has not been examined in the prior literature on supramolecules.
The literature is essentially silent in relation to long telechelic polymers (the longest we have found reported are <100 kg/mol). Nor are there reports of associative groups that have exchange times longer than approximately 0.1s. And the scant rheological data is limited to concentrations >1% and, with few exceptions, to shear
viscosity reported for only a single, low shear rate. As for CAHB-based supramolecular multiblock copolymers, to the best of our knowledge there are no reports on their applications as rheology modifiers in non-polar media. The lack of literature renders it unclear if the desired modification of rheological properties by hydrogen-bonding-based supramolecular multiblock copolymers can be achieved. In other words, it is uncertain whether the binding strengths of hydrogen-bonding-based complementary associative units discussed above are sufficient to hold longer polymer chains (Mw > 100 kg/mol) together at lower concentrations (1% or less) and comparatively higher shear rates (>10 s-
1). Motivated by the predictions of our previous theoretical work, we entered this unexplored regime of low concentrations of long telechelic chains and relatively strong associations (16-18 kBT, see Chapter 1) to test the expectation that ultralong linear polymers could indeed be achieved, opening the way to various applications-including fire-safer fuels.