A series of styrene-d 8/4-hydroxystyrene graft and block copolymers has been prepared by “living” radical and anionic techniques for use in interfacial strengthening studies at the polystyrene/poly(2-vinylpyridine), PS/PVP, interface. The following copolymers in which A and B segments represent poly(styrene-d 8) and poly(4-hydroxystyrene), respectively, have been prepared: poly(A-graft-B), poly(B-graft-A), poly(B-block-A-block-B), poly(A-block-B-block-A-block-B-block-A). The poly(4-hydroxystyrene) segments were obtained by “living” radical polymerization of 4-acetoxystyrene or anionic polymerization of 4-methoxystyrene, followed by conversion to the phenolic derivative. In general, the amphiphilic copolymers when placed at the PS/PVP interface acted as interfacial reinforcers but were susceptible to the formation of microphases such as lamellae or micelles, and therefore the measured fracture toughness depended on both the copolymer/homopolymer interfacial strength and the toughness of the copolymer phase structure itself. The pentablock copolymer showed better strengthening behavior than the triblock copolymer especially at very low areal chain density. The strengthening ability of the graft copolymers was found to depend on the lengths of the polystyrene, PS, and poly(4-hydroxystyrene), PS(OH), segments. In both graft and block copolymers the PS(OH) segments were found to resist pull-out from the bulk PVP even at low degrees of polymerization (N PS(OH) = 29). The H-bonding interaction between the phenolic and pyridyl groups combined with the severe immiscibility of poly(4-hydroxystyrene) and polystyrene is the most likely cause for pull-out resistance.
We have studied the segregation of a block copolymer of poly(d8‐styrene‐b‐2‐vinylpyridine) (dPS‐PVP) at the interface between polystyrene and a random copolymer of poly(styreneran‐4‐hydroxystyrene) (PS‐r‐PPHS). Forward recoil spectrometry (FRES) was used to measure the equilibrium excess (z*) of the dPS‐PVP chains at the interface as a function of its volume fraction in the bulk PS phase (ϕ∞). It was found that there is a sharp increase in z* at a critical value of ϕ∞. This upturn indicates the formation of a microemulsion of PS and the random copolymer PS‐r‐PPHS due to a vanishing of the interfacial tension caused by the strong adsorption of the block copolymer. Cross‐sectional transmission electron microscopy (TEM) of the interface shows that this microemulsion starts to form at the interface by forming a deeply corrugated structure where the “wavelength” of the corrugations is of the order of 50 nm. © 1995 John Wiley & Sons, Inc.
We have used a random copolymer of deuterated styrene and p-hydroxystyrene [dPS(1-f)r-PPHSf], where f is the molar fraction of PHS, to strengthen the weak phase boundary between PS and poly(2-vinylpyridine) (PVP). The fracture toughness (Gc) of the phase boundary was measured using an asymmetric double cantilever beam method, and the areal chain density (Σ) of copolymer at the phase boundary was determined by forward recoil spectrometry (FRES). The interfacial strength was extremely sensitive to the composition of the random copolymer with an optimum value found for f ≈ 0.03. The maximum measured fracture toughness was around 250 J/m 2 at f ) 0.022, while no significant strengthening was observed at f ) 0.01 and f ) 0.066. Such a strong composition dependence of Gc is in marked contrast to the case of dPS(1-f)-r-PVPf copolymers, for which the maximum strengthening is seen at f ≈ 0.5 with significant strengthening still observed at f ) 0.4 and f ) 0.6. The differences in the compositions for optimum strengthening and in the composition sensitivity of strengthening in these two cases are attributed to the hydrogen bonding between different PHS units and between PHS units and the PVP homopolymer as well as the absence of composition drift in our dPS(1-f)-r-PPHSf random copolymers. Using FRES, we determined the fracture mechanism to be chain pull-out for f ) 0.01 and f ) 0.066 over the entire range of Σ probed. For copolymers with f ) 0.022 and f ) 0.045, a mixture of chain scission and chain pull-out prevailed at low areal chain densities while craze formation followed by craze breakdown was observed at higher areal chain densities.
Two series of random copolymers, poly(styrene-d 8-co-4-vinylbenzamide) and poly(styrene-d 8-co-4-vinyl-N-ethylbenzamide), were prepared with varying compositions. The functionalized random copolymers were tested for their abilities to reinforce the weak interface between immiscible polymers: polystyrene and poly(2-vinylpyridine). The effect of the hydrogen-bonding groups with different interaction strengths (primary or secondary benzamide) was studied through the evaluation of interfacial fracture toughness and fracture surface characteristics. For the compositions investigated, the copolymers with the primary benzamide functionality were shown to attain higher fracture toughness values than the substituted benzamide copolymers. Additionally, the composition at which maximum interfacial strengthening was attained was much lower in the primary benzamide case (f max = 0.06) than in the substituted benzamide case (f max = 0.25). However, in both cases the observed strengthening was lower than our previous results using copolymers bearing phenolic groups. The effect of the copolymer functionality, including such variables as steric constraints and degree of self-association, and composition drift on the measured interfacial properties are discussed.
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