(233h) Major Impact of Initiator Fragments Located at Chain Ends on the Glass Transition Temperature and Fragility of Low Molecular Weight Polystyrene

Major
impact of initiator fragments located at chain ends on

the
glass transition temperature and fragility of low molecular weight polystyrene

Lanhe
Zhang¹ and John M. Torkelson^1,2

¹Dept. of Materials Science and Eng.,
Northwestern University, Evanston, IL 60208, USA

²Dept. of Chemical and Biological Eng.,
Northwestern University, Evanston, IL 60208, USA

Contact
information

Lanhe Zhang
– lanhezhang2012@u.northwestern.edu

John M.
Torkelson – j-torkelson@northwestern.edu

Abstract

Important, but unexplored effects of chemically
distinct initiator fragments incorporated as chain ends during free radical
polymerization (FRP) of styrene on material properties are investigated in
depth. While the effect of different chain ends on T_g is
negligible for high molecular weight (MW) PS,
it is substantial in ultralow MW PS due to the
molar enrichment of chain ends. Such an investigation is critical in light of
the rising technological and scientific interests in developing novel
photoresist materials and functional coatings that are based on relatively low
MW polymers. Particularly, the need for high resolution and low line-edge roughness of features formed during microlithographic
processing has led to the use of polymers and copolymers with MW typically well
below 10 kg/mol. In addition, low MW functionalized polymers exhibit higher
grafting density/better adsorption to solid/liquid interfaces due to higher
interfacial diffusion rate and interfacial reaction rate.

Changes in material properties are expected with
decreasing MW of a polymeric system as a consequence of increased chain-end
concentration. However, almost no studies have investigated the influence of
initiator fragments as chain ends on the bulk material properties of polymers
synthesized by FRP. We study for the first time the effect of chemically
distinct initiator fragments incorporated as chain ends during RP on T_g,
fragility, and surface wettability of ultralow MW PS. Polystyrene samples with
a wide range of MW down to ~4,000 g/mol were
synthesized via free radical and pseudo-living polymerization. Initiators
ranging from highly polar and bulky 4,4'-azobis(4-cyanovaleric acid) (ACVA) to
highly flexible, nonpolar n-dodecyl mercaptan (RSH) were employed to obtain a variety of
chain-end structures. NMR and FTIR spectroscopy confirmed the molar enrichment
of initiator fragments at chain ends in ultralow MW PS samples. For example,
NMR analysis determined that nearly all chain ends in PS/BPO/3.5k were
initiator fragments, consistent with the fact that PS undergoes termination
predominantly via combination.

The influence that such chemically distinct
chain ends have on the MW dependence of glass transition temperature and
fragility, commonly understood to be a result of excess free volume/configurational
entropy conferred by chain ends, has not been explored beyond speculation. For
ultralow MW PS of M_n ~4,000 g/mol, the chain-end structures can tune the T_g,onsetvalues over a 33 °C range going from 61 °C for PS/RSH/3.7k to 94 °C for PS/ACVA/4.1k. The presence of long, flexible
dodecane ends at substantial levels can explain the dramatically lowered T_g for
PS/RSH/3.7k, while bulky chain ends with hydrogen bonding capabilities cause
the near MW independence of T_g as exhibited in PS/ACVA. Fragility (m) was also measured on samples with M_n
~4,000 g/mol to understand the impact of such chain
ends on the cooperativity and chain packing effectiveness. The presence of
non-polar, flexible dodecane ends in PS/RSH/3.7k makes it a stronger glass
former, experiencing much less packing frustration (m = 65) compared to PS/Anionic/3.6k (m = 90). On the other hand, incorporation of other bulky end groups
such as cyano isopropyl and benzoyloxy
resulted in moderate to no increase in fragility, respectively compared to the
PS standard. This can be explained based on the minor contribution offered by
these chain-end structures to the overall steric hindrance of the rigid styrene
repeat unit. Interestingly, swapping the isopropyl groups in 2,2'-azobis(2-methylpropionitrile)
with cyclohexane ring in 1,1'-azobis(cyclohexanecarbonitrile) resulted in a larger m compared to PS standard, possibly due to the larger van der Waals
volume possessed by the ring structure. The largest m of 130 was measured in PS/ACVA/4.1k due to the ability for the
-COOH groups in the cyano pentanoic acid ends to
hydrogen bond. The additional interchain forces arising from such interactions
make the material a more fragile glass former with fragility that is twice that
of PS/RSH/3.7k and similar to 20,000 g/mol PS standard
reported in literature.    

These results further shine light on the distinction
between entanglement MW and the MW below which bulk T_g becomes MW
dependent. Researchers have often attempted to attribute the molecular weight
dependence of T_g
in polymers to chain entanglement. However, the current study definitively demonstrates
that the MW dependence of T_g in PS has no relation with chain entanglement.
Regardless of chain-end structure, the MW between entanglements (M_e) for PS is 18,000 g/mol. At
this MW, PS/ACVA has a T_g
of ~102 °C and maintains the same T_g (within
experimental error) until M_n ~5,200 g/mol.
On the other hand, PS/RSH with M_n ~18,000
g/mol has a T_gof ~90 °C, almost 10 °C lower than that seen with its high MW counterpart.

Lastly, these ultralow MW materials are examined on
their surface wettability, a parameter sensitive to changes in surface chemical
composition. By incorporating polar initiator fragments at chain ends, films of
ultralow MW PS exhibit tunable surface wettability. Due to an enrichment of -COOH
chain ends, θ_R, an indicator for
surface hydrophilicity, is decreased by almost 30° for PS/ACVA/4.1k compared to
its high MW counterpart. This makes evident of the strong retention of water on
the polymer surface due to the presence of substantial levels of          -COOH
groups at chain ends that undergo hydrogen bonding with water.