Langmuir-Blodgett monolayers of isotactic PMMA exhibit a pressure-induced transition upon compression, that can be described in terms of a two-dimensional crystallization process, analogous to a normal melt crystallization.
These water surface crystallized monolayers can be used to prepare highly crystalline thin films of isotactic PMMA with tailor-made orientational characteristics.

Brinkhuis, R.H.G.,

Schouten, A.J.,

English

R. H. G. Brinkhuis

A. J. Schouten

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Makromolekulare Chemie Macromolecular Symposia

Crystallization of isotactic poly(methylmethacrylate) in monolayers and thin films

Langmuir-Blodgett monolayers of isotactic PMMA exhibit a pressure-induced transition upon compression, that can be described in terms of a two-dimensional crystallization process, analogous to a normal melt crystallization.These water surface crystallized monolayers can be used to prepare highly crystalline thin films of isotactic PMMA with tailor-made orientational characteristics.</p

Brinkhuis, R.H.G.

Schouten, A.J.

University of Groningen

Crystallization of Isotactic Poly(methylmethacrylate) in Monolayers and Thin Films

ARTS repository - University of Groningen

Langmuir-Blodgett monolayers of isotactic PMMA exhibit a pressure-induced transition upon compression, that can be described in terms of a two-dimensional crystallization process, analogous to a normal melt crystallization. These water surface crystallized monolayers can be used to prepare highly crystalline thin films of isotactic PMMA with tailor-made orientational characteristics

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Makromol. Chem., Macromol. Symp. 46,335-339 (1991) 335
CRYSTALLIZATION OF ISOTACTICPOLY(METHYLMETHACRYLATE) IN MONOLAYERS
ANDTHIN FILMS
R.H.G. Brinkhuis, A.J. Schouten
Laboratory of Polymer Chemistry, University of Groningen, Nijenborgh 16,
9747 AG Groningen, The Netherlands
Abstract. Langmuir-Blodgett monolayers of isotactic PMMAexhibit a pressure-
induced transition upon compression, that can be described in terms of a two-
dimensional crystallization process, analogous to a normal melt
crystallization.
These water surface crystallized monolayers can be used to prepare highly
crystalline thin films of isotactic PMMA with tailor - made orientational
characteristics.
INTRODUCTION
It is a well- known phenomenon that hydrocarbon (side) chains in amphiphilic
molecules can crystallize upon compression. In this paper, we will describe a
completely different crystallization process, that can occur in Langmuir-Blodgett
monolayers of isotactic PMMAupon compression, involving the polymeric main chain
instead of a long side chain.
We will also focus on the use of this water surface crystallization process in
preparing highly oriented crystalline thin films of isotactic PMMA.
EXPERIMENTAL
Apparatus and sample characteristics are described in ref. 1.
RESULTS AND DISCUSSION
Monolayers The monolayer behaviour of isotactic PMMAis strongly different from
that of syndiotactic PMMA. The pressure area isotherm indicates that isotactic
PMMA forms an expanded type monolayer in contrast to syndiotactic PMMA, and
reveals a characteristic transition taking place upon compression at about 7-8
mN/m. The isotherm in the transition region can be seen to be strongly dependent
on compression speed, with a surface pressure dip remaining clearly observable
even at low compression speeds (figure I).
The nature of this transition was elucidated by FTIR studies on multilayers on
solid substrates transferred at different surface pressures: before the
transition, monolayers are transferred in an amorphous, non-oriented conformation,
whereas at higher surface pressures (beyond the transition regime), the films
exhibit spectral features characteristic for a conformation similar to that of
© 1991Hüthig & Wepf Verlag, Basel CCC 0258-0322/91/$ 02.00336
Figure 1. Pressure area isotherms of isotactic PMMA.Compression speed 12 (al, 2
(b), and 0.25 (c) Å2/monomeric unit'minute. T=22°C.
crystalline isotactic PMMA(1). It was concluded that upon compression, structures
are formed similar to the 10/1 double helices proposed for the bulk crystalline
structure (2).
The analogy between the monolayer transition and a "normal" bulk crystallization
can be extended further. The "surface pressure" dip observable in the isotherm
suggests the existence of an induction period, similar to the bulk crystallization
process. The auto accelerating nature of the monolayer transition is clearly
demonstrated by monitoring its conversion at constant surface pressures in the
transition region (figure 2). The results indicate the transition to be a
nucleated crystallization process, in which two - dimensional crystallites are
formed. From Avrami analyses of the stabilization curves it was inferred that this
process can be described by an activated nucleation stage, followed by a one-
dimensional (maybe lamellar type) growth, perpendicular to the direction of the
helix axis (1).
Support for this hypothesis was obtained from the IR dichroism exhibited by thin
films built from these LB layers. The data indicate the helices to be oriented
parallel to the substrate, generally with a clear preferential orientation in the
transfer direction, as has been observed for many helical molecules (3). The
orientation of the helices within the plane parallel to the substrate can easily
be changed. The use of lower molecular weight samples, or the use of lower
crystallization pressures, results in an inversion of the preferential orientation
of the helices from parallel to, to perpendicular to the transfer direction. This
observation can be rationalized by assuming that the transfer process results in a337
Figure 2. Monolayer area at constant surface pressure (6.5 mN/m), T=22°C.
flow induced orientation of anisotropic rigid structures in the monolayer, these
rigid structures being the monolayer crystallites rather than the individual
helices. The resulting lateral orientation is determined by the monolayer
viscosity and the anisotropy in the shape of the crystallites, a parameter that
can be varied through the crystallization pressure, or through the use of lower
molecular weight samples in which the nucleation process is slowed down due to
critical chain length limitations (1).
An intriguing observation is the fact that, whereas using too low molecular
weights leads to a suppression of the monolayer crystallization process, the use
of very high molecular weights does not appear to affect the crystallization
process at all. The pressure area isotherms for molecular weights from 30 000 up
to one million do not differ significantly in the transition region. This
contrasts sharply with the bulk crystallization behaviour of isotactic PMMA,where
an increase of the molecular weight and the melt viscosity slows down the rate of
crystallization dramatically (4). Apparently, the local segmental mobility
determines the rate of the monolayer crystallization process rather than the
monolayer viscosity.
In general, the monolayer crystallization process proceeds very rapidly,
especially when compared to the slow bulk crystallization kinetics of i-PMMA, even
for favorable molecular weights. The crystallization process evidently benefits
from the typical characteristics of the monolayer conformation, like the absence
of entanglements, as present in the three -dimensional melt, and the high segmental
mobility (the segments only have neighbours in the plane of the interface, with
weak cohesive forces (1)).338
Thin films Crystallization of amorphous thin films «500 Å) of isotactic PMMA
(even of favorable molecular weights) proves to be almost impossible, due to the
combination of the geometric constraints of the thin film and the low nucleation
density in amorphous i-PMMA (4,5). Crystalline thin films can be prepared by
transfering water surface crystallized LB layers of isotactic PMMA onto solid
substrates. The LB layers retain their crystalline conformation upon transfer, and
the helical structures rapidly attain a three - dimensional crystalline packing upon
annealing at 120°C. The orientational characteristics of the as deposited
multilayer appear to be enhanced during this annealing process. This way, highly
oriented, highly crystalline thin films of i-PMMA can be prepared, even from high
molecular weight samples.
There is another approach which succesfully uses monolayer crystallized LB films
of i-PMMA, and which takes advantage of the low nucleation density of amorphous
i-PMMAat 120°C. Crystallization of amorphous thin films of i-PMMA proved to be
possible using Langmuir-Blodgett overlayers of monolayer crystallized isotactic
PMMA as surface crystallization nuclei (figure 3). Upon annealing at the
crystallization temperature, crystallization is instantaneously nucleated at the
surface, and starting from there the crystalline phase grows to fill the entire
film. This approach allows the rapid preparation of highly crystalline thin films,
without having to build the entire film monolayer by monolayer.
An important aspect of the overlayer technique is the fact that the resulting
surface nucleated crystalline structure adopts the orientational characteristics
of the nucleating overlayers. Since the orientation of these overlayers can easily
be controlled, this approach appears to offer a versatile technique for preparing
highly crystalline films with tailor-made orientational characteristics.
The overlayer may be very thin to effectively nucleate an amorphous film: one
monolayer suffices to induce crystallization, although the orientational
(4). This is still very thin when compared to the thickness of the amorphous films
characteristics are best retained when the overlayer thickness is at least 50 A
Figure 3. Schematic representation of the overlayer approach. An amorphous thin
film of i-PMMA is covered with a thin overlayer of monolayer crystallized i-PMMA
(A), and can subsequently be crystallized (B).339
that can be crystallized: for favorable molecular weight samples, 10 micron thick
films present no problem. In contrast to films completely built from monolayer
crystallized LB layers, the molecular weight of the amorphous material to be
crystallized does present an limiting factor: high molecular weight samples of
course suffer from the same viscosity suppressed crystal growth as observed in
"normal" melt crystallization experiments. Especially in the case of thicker
films, the overlayer approach is only effective for rather favorable molecular
weights.
CONCLUSIONS
Isotactic PMMAexhibits a pressure induced two -dimensional crystallization process
at the air water interface. The monolayer crystallization characteristics compare
favorably to those of the normal melt crystallization, due to specific features
associated with the monolayer conformation, like a high segmental mobility and the
absence of entanglements. It is very easily conceivable that other potentially
crystalline polymers may also benefit from these advantages.
The water surface crystallized LB layers can effectively be used to prepare highly
oriented, highly crystalline thin films of isotactic PMMA.
REFERENCES
(I) R.H.G. Brinkhuis, A.J. Schouten, accepted for publication in Macromolecules.
(2) H. Tadokoro, Y. Chatani, H. Kusanagi, M. Yokoyama, Macromolecules 3, 441
(1970).
(3) F. Takeda, M. Matsumoto, T. Takenaka, Y. Fujijoshi, N. Uyeda, 1,. Coil.
Interface Sci. 267, 91 (1983).
G. Duda, Thesis, Max Planck Institut fUr Polymerforschung, Mainz, BRD (1988).
(4) R.H.G. Brinkhuis, A.J. Schouten, in preparation for publication.
(5) N. Billon, J.M. Haudin, ColI. Polvm. Sci 267, 1064 (1989).

accepted for publication in Macromolecules.

Thesis, Max Planck Institut fUr Polymerforschung,

University of Groningen Research Database

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Crystallization of Isotactic Poly(methylmethacrylate) in Monolayers and Thin Films

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