Influence of Progressive Cross-Linking on Dewetting of Polystyrene Thin Films
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- 作者：Samer Al Akhrass ; Roxana-Viorela Ostaci ; Yves Grohens ; Eric Drockenmuller ; Gü ; nter Reiter
- 刊名：Langmuir
- 出版年：2008
- 出版时间：March 4, 2008
- 年：2008
- 卷：24
- 期：5
- 页码：1884 - 1890
- 全文大小：446K
- 年卷期：v.24,no.5(March 4, 2008)
- ISSN：1520-5827
文摘
We present dewetting experiments on thin polymer films as a function of cross-linking density. Covalent cross-linkswere obtained in the glassy state on the basis of azide photochemistry of linear random copolymers of styrene andp-(azidomethyl)styrene, i.e., 106 and 2500 kg/mol with 7% and 1% azide functionality among the polymer backbone,respectively. Upon ultraviolet radiation, azides generate highly unstable nitrene radicals which react with the surroundingpolymer backbone, yielding covalent cross-links. We determined the probability for film rupture, defined by thenumber of holes formed per unit area, and the relaxation time (
w) of residual stresses which resulted from the filmpreparation process. For the lower molar mass polymer studied and for azide conversion rates lower than 60%, onlypartial cross-linking occurred. The effective molar mass of the polymer increased, and consequently, an increase inw was observed. The increase in w was accompanied by a decrease in hole density, indicating that the still presentresidual stresses in the films were not able anymore to rupture the films at the high probability of un-cross-linkedpolymers. For high conversion (>60%), cross-linking was significant enough to lead to the formation of a three-dimensional rubbery network which, in turn, generated an elastic force that counteracted the driving forces. This elasticforce eventually inhibited dewetting and the relaxation of residual stresses. Thus, at high conversions, the relaxationtime w grew exponentially and the number of holes tended toward zero. For the higher molar mass polymer, nochanges in the relaxation time w were observed for low conversion (<30%). However, at a higher conversion rate,w increased drastically, suggesting an almost infinitely long relaxation time at 100% conversion. Consequently, tosuccessfully stabilize thin polymer films by cross-linking, it is preferable to use long polymer chains.
w) of residual stresses which resulted from the filmpreparation process. For the lower molar mass polymer studied and for azide conversion rates lower than 60%, onlypartial cross-linking occurred. The effective molar mass of the polymer increased, and consequently, an increase inw was observed. The increase in w was accompanied by a decrease in hole density, indicating that the still presentresidual stresses in the films were not able anymore to rupture the films at the high probability of un-cross-linkedpolymers. For high conversion (>60%), cross-linking was significant enough to lead to the formation of a three-dimensional rubbery network which, in turn, generated an elastic force that counteracted the driving forces. This elasticforce eventually inhibited dewetting and the relaxation of residual stresses. Thus, at high conversions, the relaxationtime w grew exponentially and the number of holes tended toward zero. For the higher molar mass polymer, nochanges in the relaxation time w were observed for low conversion (<30%). However, at a higher conversion rate,w increased drastically, suggesting an almost infinitely long relaxation time at 100% conversion. Consequently, tosuccessfully stabilize thin polymer films by cross-linking, it is preferable to use long polymer chains.