Fracture of molecular glasses under tension and increasing their fracture resistance with polymer additives

• Fracture resistance has been measured for molecular glasses for the first time.
• Theories describe fracture conditions over wide range of film thickness in open and sandwich geometries.
• Polymer additives increase fracture resistance, possibly by increasing fracture surface area around pervaded volumes.

The fracture of single-component molecular glasses has been characterized under tension. In-plane tension is created by cooling a liquid film below its glass transition temperature constrained on a substrate that is less thermally expansive. The fracture produces a network structure whose cell size is comparable to film thickness. For each system studied (indomethacin, o-terphenyl, and sucrose benzoate), the condition of fracture is well described by the fracture models for supported films and a characteristic energy release rate Gc. Agreement is found for a wide range of film thickness (10–250 μm) in both open and sandwich geometries. For the molecular glasses studied, Gc ≈ 1 J/m2. o-Terphenyl glasses become significantly more resistant to fracture with the addition of polystyrene; at 10 wt.%, Gc increases with the molecular weight of polystyrene, by a factor of 5 at 1 million g/mol. The molecular-weight dependence is consistent with an increase of fracture surface area as the crack tip goes around the pervaded volume of each polymer chain encountered (sphere defined by the radius of gyration).