Failure mechanisms of polymer interfaces reinforced with block copolymers

Abstract

The fracture toughness (characterized by the critical energy release rate Gc) of interfaces between polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with poly(styrene-6-2-vinylpyridine) was measured with a double cantilever beam test geometry. The effect of the PVP block degree of polymerization (Wpvp) and the areal density of block copolymer chains at the interface (2) on the measured Gc and on the fracture mechanisms was investigated quantitatively. The PS degree of polymerization (Alps) was kept >280, while Alpvp was varied between 45 and 870. For Afpvp below 200 the interfaces showed only a small increase in Gc with increasing 2 and failed by pull-out of the short PVP chain. In this regime Gc increases linearly with 2 and scales roughly with New2, in reasonable agreement with a recently proposed model of failure by chain pull-out.1 If Npw was increased well above 200, corresponding roughly to the average molecular weight between entanglements for the PVP, two separate fracture mechanisms could be distinguished. At low values of 2, Gc increased only slowly with 2 and the interfaces failed by scission of the copolymer chains near the joint between the two blocks. At higher values of 2, the interfaces fractured by first forming a stable craze ahead of the propagating crack tip, giving rise to much higher values of the measured fracture toughness.In this regime, Gc scaled with 2,^, an areal density of chains with at least one "effective" entanglement, in very good agreement with a model recently proposed by Brown2 for failure by craze fibril breakdown.

Model for the fracture energy of glassy polymer–polymer interfaces

“…A value of 2 nN was used for f b . 7 From eq 10, ⌺ bulk for PS was calculated as 1.32 ϫ 10 18 chains/nm 2 , and for PMMA, it was calculated as 1.77 ϫ 10 18 chains/nm 2 . 13 c was taken as 55 MPa for PS and 80 MPa for PMMA.…”

“…These interfaces would only exhibit high values of G c when the molecular weight of each block exceeded M e . [3][4][5][6][7][8] The low G c values measured for interfaces reinforced with unentangled diblock copolymers suggested that entanglement formation was the primary mechanism of reinforcement.…”

The reinforcement of polystyrene and poly(methyl methacrylate) interfaces using alternating copolymers

“…This makes sense in terms of the miscibility of the copolymers in the homopolymers, as these three alternating copolymers are significantly miscible in PMMA (at least up to 10 wt%), thus they are able to expand into and entangle with the homopolymers. Our interpretation of this result is that the increase in molecular weight, increases the effective number of entanglements per chain, N eff ; which has been shown to be directly responsible for the fracture toughness of a biphasic interface [13]. Also supporting this interpretation is an ADCB study by Bernard that showed that the effectiveness of P(S 0.7 -ran-MMA) as an interfacial strengthener increases with M w for four copolymers ranging from 1.60 £ 10 5 to 4.50 £ 10 5 g/mol [22] as well as work by Cho which also observed an increase in G c as a function of M w for compositionally symmetric P(S-ran-MMA) [38].…”

Importance of Molecular Friction in a Soft Polymer−Nanotube Nanocomposite

“…In this case, the failure occurs by chain scission. Values in the range of 1 to 3 nN have been reported experimentally 9,27,28 as being required to break a C-C bond in a chain scission process. The range of F frict values found in the data in Fig.…”

“…We also note that a similar dependence on interfacial chain density was found in the interaction of diblock copolymer brushes at the interfaces in immiscible polymer blends. [9][10][11] When the Σ became sufficiently large, the interfacial strengths were found to level off at a constant value. In Brown's experiments, 13 sliding friction was increased by thin brush layers (i.e.…”