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Impact of the α‑Methyl Group (α-CH3) on the Aggregation States and Interfacial Isotherms of Poly(acrylates) Monolayers at the Water Surface
journal contribution
posted on 2017-08-28, 00:00 authored by Yongming Hong, Hao Zhou, Wenhao Qian, Biao Zuo, Xinping WangPolymer
monolayers at liquid surfaces have attracted considerable
attention as tools to construct molecular devices with unique properties.
The aggregation states of chains within the monolayers must be understood
as the first benchmark to establish molecular designs for the devices.
It has been reported that large differences exist in surface pressure–area
isotherms of polyacrylates and polymethacrylates at the water surface,
although they differ chemically only by the absence of an alpha methyl
group (α-CH3) in the former. Herein, we carried out
experiments using sum-frequency generation vibrational spectroscopy,
atomic force microscopy and infrared reflection absorption spectroscopy
to explore the changes of the aggregation state and interaction of
four poly(acrylates) (i.e., poly(methyl acrylate), PMA; poly(ethyl
acrylate), PEA; poly(methyl methacrylate), PMMA; poly(ethyl methacrylate),
PEMA) at the water surface during compression. In the case of PMA
and PEA without α-CH3, the polymer chains adopt an
expanded 2-D random-coil conformation. The methyl group in the ester
side chain (s-CH3), which is the origin of the surface
pressure variation, is increasingly ordered with compressing of the
surface, leading to the expanded-type isotherms.
Nevertheless, since chains of PMMA and PEMA were collapsed into compact
globules, the s-CH3 orientation becomes invariant on compression,
giving rise to more condensed isotherms with faster
increase of surface pressure with compression. Weaker hydration effects
and suppressed segmental mobility due to introduction of an α-CH3 group was responsible for the condensed molecular conformation
and reduced structural compressibility of the monolayers of PMMA and
PEMA.