DOI: 10.25349/d9xw4d
title: Hydrogen Bonding Bottlebrush Networks: Self-healing Materials from Super-soft to Stiff

description: The impact of polymer architecture on network dynamics and self-healing is
 presented using bottlebrushes containing side chains that are
 end-functionalized with 2-ureido-4[1H]-pyrimidinone (UPy). The synthesis
 of these materials is straightforward through a three-step process: (1)
 synthesizing rubbery poly(4-methylcaprolactone) macromonomers (pMCL−OH)
 with a norbornene-based initiator, (2) functionalizing the terminal
 hydroxyl group with UPy−isocyanate (pMCL−UPy), and (3) statistically
 copolymerizing pMCL−OH and pMCL−UPy via ring-opening metathesis
 polymerization (ROMP) to form hydrogen-bonding bottlebrushes having a
 fraction (p) of side chains functionalized with UPy. Attaching UPy to the
 free end of bottlebrush side chains dilutes the impact of friction from
 complementary UPy interactions on segmental dynamics, leading to a much
 weaker dependence of the glass-transition temperature (Tg) on p than
 observed in linear analogues, while the activation energy to dissociate
 UPy−UPy bonds (41−47 kJ/mol) remains mostly unchanged. Longer side chains
 result in a competition between reducing Tg and inducing entanglements
 that influence hydrogen-bonded network dynamics. Increasing the backbone
 length extends the sticky Rouse region without affecting the network
 modulus (Gx) or UPy−UPy dissociation time (τs). Gx scales linearly with p
 and ranges from 27 kPa to 1.6 MPa, while τs remains nearly constant in
 contrast to linear telechelic ionomers, implying a similar self-
 healability across bottlebrush networks containing different amounts of
 UPy. These stretchable networks with p ≤ 0.25 undergo self-healing upon
 repeated rupture and melt pressing at ≤100 °C while retaining similar
 tensile properties. In summary, decorating bottlebrush polymers with
 hydrogen bonds creates opportunities to independently manipulate
 associative network dynamics and mechanical moduli.
description: All polymer 1H nuclear magnetic resonance (NMR) spectra were
 collected using a Varian Unity Inova AS600 600 MHz equipped with a 5 mm
 Varian triple resonance 1H/13C/15N inverse detection probe with z-axis
 pulsed field gradient (PFG). Small molecule 1H and 13C NMR were collected
 using a Bruker Avance NEO 500 MHz equipped with a CryoProbe Prodigy BBO probe with z-axis PFG. Chemical shifts (δ) are reported in ppm relative to residual undeuterated CHCl3 in CDCl3 (7.26 ppm), CD2Cl2 (5.32 ppm), or acetone-d6  (2.05 ppm) for 1H NMR spectra and CDCl3 (77.16 ppm) for 13C NMR spectra. The following abbreviations (or combinations thereof) were used to describe multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet). 1H diffusion-ordered spectroscopy (DOSY) experiments were performed on a Varian Unity Inova AS600 600 MHz using bipolar gradient pulses stimulated echo sequence with convection compensation. The sample solution was kept at approximately 20 mg/mL with no spinning to avoid convection. Acquisition was performed with 16 gradient steps and 16 at each scan. DOSY analysis used MestreNova software and Bayesian data fitting. Size exclusion chromatography (SEC) was performed on a Waters e2695 Separations Module system using Agilent PLgel columns and a differential refractive-index detector with tetrahydrofuran (THF) as the mobile phase at 35 °C with a flow rate of 0.3 mL/min. Molar masses and molar-mass dispersities (Ɖ) were determined against narrow polystyrene standards (Agilent Technologies, EasiVial). Infrared (IR) spectra were collected using a Nicolet is10 FTIR equipped with a Smart Diamond attenuated total reflectance (ATR) accessory. Data were collected between 4000 to 500 cm–1 with 64 scans. Time-of-flight matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) was performed on a Bruker Microflex LRF MALDI-TOF equipped with a 60 Hz nitrogen laser at 337 nm. Calibration was performed with a peptide standard mixture containing Bradykinin Fragment 1-7, Angiotensin II, Angiotensin I, Substance P, Bombesin, Renin Substrate, ACTH clip 1-17, ACTH clip 18-39, and Somatostatin 28. Samples were prepared in a dithranol/CHCl3 solution. The laser intensity was set at 30%. Differential Scanning Calorimetry (DSC) was performed on a TA instrument Q2000 DSC with three repeated cycles between −100 and +100 °C at a constant rate of 10 ℃/min. Only the second heating scan is shown in this manuscript. Uniaxial tensile testing of pNb300-pMCL17-UPy25 was performed with a TA.XTplusC texture analyzer equipped with A/MTG tensile grips. Samples were clamped without additional adhesive. Uniaxial tension data for pNb300-pMCL17-UPy5 was collected on an ARES-G2 rheometer from TA Instruments. The strain rate was set at 2 s−1, which was chosen based on the rheological master curve to occur within the network plateau region where elasticity dominates. A strain-controlled ARES-G2 rheometer equipped with a liquid nitrogen dewar from TA Instruments was used to investigate the linear viscoelastic and tensile properties of hydrogen-bonded bottlebrush polymers. For generating master curves, isothermal frequency sweeps between 100 and 0.1 rad/s were collected using a strain amplitude of 1% − 5% from a high (100 or 70 °C) to low (−40 or −50 °C) temperature with increments ranging from 30 to 10 °C. To verify thermal reversibility, a temperature ramp was performed by heating from −50 to 100 °C and then cooling at a rate of 5 °C/min using an auto strain amplitude ranging between 1% and 50% within the linear region and a frequency of 1 rad/s. Rheological master curves were constructed using time–temperature superposition (TTS) following the Williams–Landel–Ferry (WLF) relation.
description: All data can be opened with any application that can read .csv
 files. An exception is 2D DOSY NMR which needs to be
 fitted with NMR software that can operate fitting using the Bayesian
 method.