Reversible networks in supramolecular polymers

Non–covalent interactions between low molecular weight polymers form the basis of supramolecular polymers. The material properties of such polymers are determined by the strength and lifetime of the non–covalent reversible interactions. Due to the reversibility of the interactions between the low molecular weight polymers, stress relaxation is not exclusively determined by reptation but also by the lifetime of the non– covalent interaction. The aim of this thesis is to get more insight in the relationship between material properties, dynamics, and network structure of hydrogen bonded supramolecular polymers. The supramolecular polymers used in this thesis are all based on hydrogen bonding, a commonly used non–covalent interaction. The self– complementary quadruple hydrogen bonding ureidopyrimidinone (UPy) unit is the basis of the majority of the discussed polymers. In Chapter 2, the effect of chain structure (supramolecular random copolymer vs. supramolecular segmented copolymer) on material properties of UPy end–functionalized polyesters is studied. Mixing of miscible UPy homopolymers led to supramolecular segmented copolymers while functionalization of random copolymer diols resulted in supramolecular copolymers. The (co)polymers were prepared by enzymatic (co)polymerization of e–caprolactone and d–valerolactone using Novozym 435, followed by end–functionalization with UPy moieties. Thermal analysis of the functionalized (co)polymers showed two melting transitions. Variable temperature infrared spectroscopy was used to attribute the lower transition to the melting of the polyester part, while it was shown that the higher transition corresponded to melting of the UPy moieties. The materials can be considered as supramolecular thermoplastic elastomers with a hard phase of microphase separated UPy dimers, giving mechanical strength to the material. Mixing of UPy functionalized homopolymers gave better control over the mechanical properties than using UPy functionalized copolymers. A simple correlation was found between the Young’s modulus and the fraction of d–valerolactone polymer in the mixtures, whereas no correlation was observed for the supramolecular copolymers with varying d–valerolactone fraction. The effect of stacking of UPy end groups on the rheological behavior of supramolecular polymer melts is discussed in Chapter 3. Oscillatory shear experiments in the transition zone from the pseudo rubber plateau to the flow region of polycaprolactones (PCL’s) with UPy end groups directly attached to PCL can be fitted with a single Maxwell element. This demonstrates that dimerization of the UPy groups is unidirectional and that reversible chain scission is faster than reptation. A low–frequency plateau in G’ is observed if the UPy groups are connected to the polymer via a urethane linker. This is ascribed to the formation of a network of stacked UPy dimers, aided by urethane hydrogen bonding. Below their melting point, these stacks form long fibers in the urethane linked supramolecular poly(4–methylcaprolactone), which were observed with Summary 136 atomic force microscopy. Steric hindrance interferes with stacking, since the plateau in G’ is lower in a urethane linked polymer with bulky adamantyl–UPy end groups. The relation between the degree of polymerization, controlled by the presence of chain stoppers, and the rheological properties is investigated for linear supramolecular PCL in Chapter 4. The viscosity and extrapolated relaxation time were independent of stopper fraction when the fraction of stopper molecules was below 0.05. Above the critical chain stopper fraction, a strong decrease in both viscosity and relaxation time with respect to chain stopper fraction is observed. The slopes of this plot confirm the flexible and fast– breaking nature of the supramolecular PCL. Finally, an apparent association constant of 800 M–1 was estimated from the critical chain stopper fraction at a temperature of 70 °C. The effect of polymer architecture on the melt behavior of supramolecular polymers is discussed in Chapter 5. Oscillatory shear experiments were performed on trifunctional UPy PCL in which the UPy moieties were connected via an ester linker to the polymer. Additional lateral interactions were provided in a polymer with a urethane group between the UPy moiety and the PCL backbone. Denser network formation was achieved by incorporation of the UPy moiety in the polymer main chain, resulting in a material, which showed pronounced shear–thinning behavior. Relaxation time, (zero–shear) viscosity, and activation energy for flow were determined for the network forming polymers and compared with the corresponding properties of the linear equivalents. It was found that the rheological properties of the supramolecular polymers were strongly influenced by the chemistry used for linking the UPy unit to the polymer, whereas the presence of physical cross–links in the trifunctional polymers resulted in a plateau in the storage moduli at low frequencies, longer relaxation times and higher viscosities. Only minor differences between the activation energies of the linear and trifunctional equivalents were observed, indicating that the relaxation mechanism is not influenced by the polymer architecture. Based on an associative network model, the lifetime of the UPy dimer is estimated to be 2.8 ms at 70 °C. In Chapters 2 and 3 it was shown by differential scanning calorimetry and variable temperature infrared spectroscopy that phase separation occurs between the polyester backbone and UPy dimer and in Chapter 6 a detailed investigation is performed using fluorescent spectroscopy. For this purpose, a highly fluorescent quinolinium probe was functionalized with an UPy moiety, yielding a supramolecular probe. This probe was mixed with supramolecular polyesters and the emission wavelength and intensity of this supramolecular probe were investigated during heating and cooling of the polymers. The supramolecular probe was successfully used in determination of phase transitions attributed to the UPy moiety present in supramolecular polyesters. However, similar results were obtained by the use of the quinolinium reference probe, due to its preference for the polar microenvironment of the UPy moieties. Fluorescent spectroscopy provided the possibility to not only detect phase transitions, but also to cover the whole transition regime as changes in intensity indicated either the start of a melting transition or the end of a crystallization transition. In Chapter 7, quadruple hydrogen bond formation between UPy moieties was investigated in micellar environment. The distribution coefficients of four UPy model compounds were determined by micellar electrokinetic chromatography (MEKC). The formation of UPy dimers could not be elucidated from the MEKC measurements on pure UPy compound neither from mixing of UPy compounds with different distribution coefficients. Hydrogen bonding was therefore investigated by experiments on mixtures of UPy and naphthyridine (Napy) compounds on the assumption that upon heterocomplexation the distribution coefficient of the UPy would change. Further experiments were performed using the UPy model compound with the highest distribution coefficient to enhance hydrogen bond formation in micellar phase. UV absorption spectra in different micellar environments showed that the dimerizing 4[1H] tautomer of this UPy model compound was present in PEG–PCL polymer solutions and absent in SDS, CTAB, and MEGA–10 micellar solutions. The formation of UPy–Napy heterocomplexes was monitored by UV titrations and UPy–Napy association constants were determined using a 1:1 binding model. The highest apparent UPy–Napy association constant was observed for PEG–PCL polymer micellar solutions (2·105 M–1), while the lowest apparent association constants were observed for the ionic micelles CTAB and SDS. When the apparent association constants were corrected for the volume fraction of the micellar phase where complexation takes place, values below 100 M–1 for the UPy– Napy association constant were found for CTAB and MEGA–10. The highest UPy–Napy association constant with a value of 700 M–1 was found for the PEG–PCL polymer solutions and a value of 148 M–1 for the UPy dimerization constant was determined in these polymer solutions. A hydrophilic bifunctional polymer with UPy end groups was shown to have a similar binding strength with Napy as the monofunctional UPy’s, which opens opportunities to create transient networks based on multiple hydrogen bonding within micelles. Association of a 16–fold excess of a monodisperse telechelic oligo(THF) (Mw = 1270g/mol) containing two end groups that selectively bind to the 32 binding sites of a fifth generation dendritic host (Mw = 18511 g/mol and radius Rh = 2.4 nm) results in the formation of reversible and dynamic supramolecular complexes as was shown in Chapter 8. The structure of these complexes in solution depends strongly on the concentration. At low concentration, the two end groups of one guest are proposed to complex to the same host and flower–like structures are formed with a radius of Rh = 3.7 nm. At higher concentrations, both end groups of one guest are complexed to different hosts, forming a bridge between them. This gives rise to the formation of larger associates and eventually to a transient supramolecular network. Dynamic light scattering unequivocally showed that three distinct relaxation processes, associated to the proposed structures, are present in this system. In addition to the dynamics related to the flower–like (fast) and the transient network structures (slow), an intermediate dynamic process is attributed to thecooperative motion of a few (~6) connected flower–like structures. Rheological data elucidate the nature of the intermittent network responsible for the slowest process. A monofunctional guest – not capable of forming a network structure – was used as a reference and star–like supramolecular structures are formed at all concentrations, indeed. In conclusion, the results described in this thesis add significantly to the insight in the material properties, dynamics, and network structures of hydrogen bonded supramolecular polymers.