TRANSIENT MESOMORPHIC PHASES IN CRYSTALLIZATION:
SPECIALTY LIPIDS, ALIPHATIC POLYAMIDES AND
POLYETHYLENE
S. Cabus1*, M. Smet2, B. Goderis1
1
2
Molecular and Nanomaterials, Katholieke Universiteit Leuven, Heverlee, Belgium
Molecular Design and Synthesis, Katholieke Universiteit Leuven, Heverlee, Belgium
Keywords: mesomorphic, polyethylene, polyamide, phase separation
*) e-mail: steven.cabus@chem.kuleuven.be
About one decade ago a number of features has
been found in polymer crystallization, which cannot be
placed within the – widely accepted - crystallization
scheme of Hoffman and Lauritzen [1]. These new
features include: the granular substructure of the
crystalline lamellae [2], the existence of a transient
hexagonal phase at high temperature and pressure [3],
an all-trans configuration of the polyethylene chains
prior to crystallization in the orthorhombic phase[4].
Based on these findings, a transient mesomorphic
phase is believed to be formed prior to the conversion
of a poly(ethylene) melt into semicrystalline matter
even at ambient pressure [2,5]. The life time of this
mesomorphic phase is very short and it is believed to
be present only as a small interphase at the crystal
growth front. As a result, evidence on its existence is
rather circumstantial and its role in the crystallization
of poly(ethylene) is largely hypothetical and a matter of
debate.
Knowledge on the mechanisms involved in these
liquid-solid and solid-solid transitions is crucial in view
of controlling polymer crystallization kinetics,
morphology and properties.
In order to increase the life time of this presumed
mesomorphic precursor phase for systematic studies,
polyethylene-like aliphatic poly(amides) have been
synthesized in our group and the crystallization
behavior is being investigated. These aliphatic
poly(amides) are synthesized by the condensation of
1,4 diamino butane (NH2-(CH2)4-NH2) with long
aliphatic di-acids (HOOC-(CH2)(y-2)-COOH), with y
typically larger than 30 (figure 1).
Figure 1. Typical structure of the aliphatic poly(amides)
studied.
More specifically, methylene sequences (y) as long
as the thickness of typical polyethylene crystals are
aimed for. The amide groups are – because of their
ability to form hydrogen bonds at high temperature and
because of their more polar character – supposed to
induce a mesomorphic layer like arrangement of the
long aliphatic moieties. The conversion of this poorly
ordered material into well-ordered polyethylene-like
crystallites is studied by means of time resolved
synchrotron X-ray scattering methods.
Besides aliphatic poly(amides) also a set of lipid
like model molecules, as illustrated in Figure 2, were
investigated. While type B caries amide moieties (just
like the polyamides discussed above), type A more
closely resembles fats. Fats are triglycerides with 3
alkyl chains bonded to a glycerol moiety, while the
model compounds only have 2.
Figure 2. Specialty lipids with ester (A) or amide (B) links.
Fats usually pass through different (metastable)
crystalline phases before entering the final stable
crystal structure, and are therefore used as a model to
study polymer crystallization [6,7]. Their morphology –
just like synthetic polymers - displays hierarchical
features, including a crystalline subcell, a lamellar-type
packing and spherulitic crystal aggregates.
References
[1] Lauritzen, J.I., Hoffman, J.D., Journal of research of the
National Bureau of Standards Section A – Physics and
Chemistry, 1960, 64A, 73-102
[2] Strobl, G., Prog. Polym. Sci., 2006, 31, 398 - 442
[3] Kolnaar, J.W.H., Keller, A., Journal of Non-Newtonian
Fluid Mechanics, 1997, 69, 71-98
[4] Tashiro, K., Sasaki, S., Gose, N., Kobayashi, M., Polymer
Journal, 1998, 30, 485 - 491
[5] Rabiej S., Goderis B., Janicki J., Mathot V.B.F., Koch
M.H.J., Groeninckx G., Reynaers H., Gelan J., Wlochowicz
A. Polymer, 2004, 45, 8761 – 8778
[6] Sato K., Chemical Engineering Science, 2001, 56, 22552265