Measurement of residual water and crystallinity in dry samples
This protocol is extracted from research article:
Anti-fatigue-fracture hydrogels
Sci Adv, Jan 25, 2019; DOI: 10.1126/sciadv.aau8528

We measured the crystallinities of the resultant PVA hydrogels in their dry state by DSC (DSC/cell: RCS1-3277 Cooling System: DSC1-0107). For as-prepared chemically cross-linked PVA and freeze-thawed PVA, we used excess chemical cross-links to fix the amorphous polymer chains before air-drying, minimizing the formation of further crystalline domains during the air-drying process. We first soaked the samples (thickness of 1 mm) in the aqueous solution consisting of 10 ml of glutaraldehyde (25 volume %; Sigma-Aldrich, G6257), 500 μl of hydrochloric acid (36.5 to 38 wt %; J.T. Baker, 9535-02), and 50 ml of DI water for 2 hours. Thereafter, we soaked the samples in a DI water bath for 2 hours to remove the residual hydrochloric acid. The samples were further dried in an incubator (New Brunswick Scientific, C25) at 37°C for 2 hours.

Thereafter, we measured the mass of residual water mresidual, the mass of crystalline domains mcrystalline, and the total mass of the dry samples (with residual water) m using DSC. In a typical DSC measurement, we first weighed the total mass of the dry sample (with residual water) m. The sample was thereafter placed in a Tzero Pan and heated up from 50° to 250°C at a rate of 20°C/min under a nitrogen atmosphere with a flow rate of 30 ml/min. The curve of heat flow shows a broad peak from 60° to 180°C, indicating that the sample contains a small amount of residual water. The integration of the endothermic transition ranging from 60° to 180°C gives the enthalpy for evaporation of the residual water per unit mass of the dry sample (with residual water) Hresidual. Therefore, the mass of the residual water mresidual can be calculated asmresidual=mHresidualHwater0(3)where Hwater0=2260J/g is the latent heat of water evaporation. The curve of heat flow shows another narrow peak ranging from 200° to 250°C, corresponding to melting of the crystalline domains. The integration of the endothermic transition ranging from 200° to 250°C gives the enthalpy for melting the crystalline domains per unit mass of the dry sample (with residual water) Hcrystalline. Therefore, the mass of the crystalline domains mcrystalline can be calculated asmcrystalline=mHcrystallineHcrystalline0(4)where Hcrystalline0=138.6J/g is the enthalpy of fusion of 100 wt % crystalline PVA measured at the equilibrium melting point Tm0 (30). Therefore, the crystallinity in the ideally dry sample Xdry (without residual water) can be calculated asXdry=mcrystallinemmresidual(5)

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