Crystallization in PEG networks: The importance of network topology and chain tilt in crystals

We synthesized poly(ethylene glycol) (PEG) networks by Cu(I) catalyzed azide-alkyne cycloaddition (CuAAC) using three-arm star-PEG with azide end groups and linear PEG oligomers of different molar masses with alkyne end groups. After extensive characterization of the networks by C-13 MAS and H-1 multiple-quantum (MQ) NMR spectroscopy, their crystallization behavior is studied by differential scanning calorimetry (DSC), temperature dependent small and wide angle X-ray scattering (SAXS, WAXS) as well as l fl FID NMR. Since CuAAC introduces 1,4-disubstituted 1,2,3-triazole (TR) rings as chain defects into the network, we compare our results with those obtained from linear PEG which contained a purposely introduced TR ring in the middle of the polymer chain. PEG network chains are able to form the characteristic 7(2) helices upon cooling to - 20 degrees C and crystallize into the monoclinic unit cell in complete analogy to the homopolymer. The crystallinity of the networks, though it is lower as compared to the linear samples, reached values of about 30-50%. Given the fact that the network chains are highly constrained in their configurational freedom, this is very surprising. However, cross-links and the TR defects have only a minor effect on the molecular dynamics of the PEG chains in the crystals since the characteristic alpha(c)-relaxation process (helix jumps) is also detected in the PEG networks by analysis of the H-1 FIDs. The SAXS data reveal that randomly arranged stacks of lamellae are formed upon crystallization and the chains are tilted in the crystals at large angles of about 40 degrees-55 degrees. Supported by all experimental data, we propose a model to explain the formation of polymer crystals in the networks and concluded that primary and secondary loops which extent the network chains beyond the length of the linear precursors play an essential role for the crystallization.