Chemistry Reference
In-Depth Information
Hydrogen bonding has improved mechanical properties in a number of systems
and is often grouped with other noncovalent cohesive intermolecular interactions,
performing as the “glue” that sticks chains. Hydrogen bonding supramolecular
assembly in conjunction with covalent polymer synthesis often leads to a positive
impact on the mechanical properties such as the stress at break and percentage of
elongation (Reith et al. 2001) as well as the elastic modulus (Shandryuk et al.
2003). Nissan (1976) theorized that for a hydrogen bonded solid the Young's
modulus scales with the number of hydrogen bonds per unit volume to the 1/3
power. Hydrogen bonding results in an increased glass-transition temperature (T
g
),
which is due to restricted molecular mobility and temporary “cross-links”
(Yamauchi, Lizotte, Hercules, et al. 2002). Hydrogen bonding interactions are cred-
ited, in part (along with microphase separation), with the outstanding mechanical
properties of polyurethanes. Sheth and coworkers recently demonstrated that hydro-
gen bonding in model single unit hard segment poly(urethane urea)s leads to the
development of ribbonlike hard phases containing stacks of hydrogen bonding
urethanes or ureas that are disrupted upon the addition of branching units in the
hard phase or addition of hydrogen bond screening agents such as lithium chloride
(Sheth, Wilkes, et al. 2005). Further studies of single hard segment polyurethanes
and polyureas revealed that the symmetry of the hard segment precursor, which
affects the packing ability of hydrogen bonding groups, has a dramatic influence
on the thermal integrity of the hard phases. Model polyureas based on p-phenylene
diisocyanate and poly(tetramethylene oxide) possess rubbery plateaus extending to
250 8C, whereas meta substitution leads to rubbery plateaus extending only 100 8C
(Sheth, Klinedinst, et al. 2005). McKeirnan et al. (2002) utilized variable temperature
FTIR to show that the hydrogen bonds in polyurethanes persist up to temperatures
beyond 100 8C and that, even in the melt,
75% of the urethane linkages were
involved in hydrogen bonding.
Hydrogen bonding interactions confer unique melt (Yamauchi et al. 2003) and
solution (Lele and Mashelkar 1998) rheological behavior. The temperature sensitivity
of the hydrogen bond holds promise for increasing the temperature dependence of the
melt viscosity while providing an effectively higher molecular weight at room temp-
erature. Thus, lower molecular weight hydrogen bond containing polymers may be
employed. However, increased melt and solution viscosities are typically observed
at temperatures where hydrogen bonds still exist. Lillya and colleagues (1992)
showed that the zero shear melt viscosity of a 650 g/mol carboxylic acid terminated
poly(tetrahydrofuran) [poly(THF)] was 2.5 times that of the corresponding protected
(ester) version telechelic carboxylic acid functionalized poly(THF). This high melt
viscosity dropped above 62 8C for the 650 g/mol polymer and above 50 8C for the
1000 g/mol polymer. Sivakova et al. (2005) also observed sudden decreases in vis-
cosity in hydrogen bonding systems. Based on the sticky reptation theory that Leibler
and colleagues (1991) developed, the terminal relaxation time (t
D
) for a thermorever-
sible, hydrogen bonded network is longer than for a corresponding nonfunctionalized
polymer. Rogovina et al. (1995) observed reversible network formation in self-
associating carboxyl functionalized poly(dimethylcarbosiloxane) with 0.5-2.5
mol% of the carboxylic acid group. M¨ ller et al. (1995) conducted some of the
