Chapter 9
Manipulating Copolymers with Confinement and Interfacial Interactions
Downloaded by UNIV MASSACHUSETTS AMHERST on September 8, 2012 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0736.ch009
O . K . C . Tsui, L . Rockford, E . Huang, and T . P. Russell
1
Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003
Fully utilizing the morphologies present in polymeric materials requires simple, robust methods for manipulating the spatial organization o f the morphology over many length scales. In the absence o f an external applied field, control over the morphology is achieved by tailoring the interfacial interactions and f i l m thickness. In thin f i l m s , incommensurability between the natural polymer length scale and film thickness will tend to disorder the polymer, resulting in a corresponding shift in the ordering transition temperature. The phase transition behavior o f a symmetric diblock copolymer is used to demonstrate this point. In addition, two different cases are discussed which demonstrate the influence o f interfacial interactions. In the first, the surface is laterally homogeneous but can be made effectively neutral which results in an orientation o f the morphology normal to the surface. In the second, the surface is laterally heterogeneous where the chemical composition o f the surface is altered in a periodic manner. The wavelength o f the period is made comparable to the size o f the polymer chain and, hence, manipulates the polymer on a molecular scale. Molecular recognition o f the surface by the polymer is shown to yield control over the morphology both parallel and normal to the surface.
The morphologies observed in polymeric materials span length scales from the nanoscopic to macroscopic. Taking advantage o f naturally developing structures, however, requires the ability to manipulate their spatial orientation. Bates and coworkers (7) have described the use of shear to achieve this end and have shown that mechanical forces can be effectively coupled into the polymer, to produce w e l l defined, highly oriented morphologies with very high persistence. In the case o f thin 1Corresponding author.
140
© 1999 American Chemical Society
In Supramolecular Structure in Confined Geometries; Manne, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.
141 films, however, the use o f mechanical force is difficult, i f not impossible, to utilize in any practical manner. Consequently, other routes are needed to manipulate the spatial arrangement o f the polymer morphology. While electric fields have been shown to be ineffective in orienting bulk polymers (2,3), in thin films, very high field strengths can be achieved at low applied voltages (due to the small separation distance between the electrodes) to produce highly oriented morphologies (4). Magnetic fields, though not studied in detail, can also be used (5). However, practical use o f magnetic fields
Downloaded by UNIV MASSACHUSETTS AMHERST on September 8, 2012 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0736.ch009
for controlling polymer morphology may be limited due to the cost o f the magnets which produce sufficiently high fields, and the need to have non-magnetic processing equipment. A n attractive, alternative route is to tailor both the film thickness and the interfacial interactions, such that the polymer morphology w i l l spontaneously orient in a predetermined manner without the need o f any applied, external field. diblock copolymers o f poly(styrene), P S , and poly(methyl methacrylate),
Here, PMMA,
denoted P ( S - 6 - M M A ) , and o f PS and poly(n-butyl methacrylate), P n B M A , denoted P ( S - 6 - n B M A ) , are used as simple model systems to demonstrate the idea and its feasibility. Diblock copolymers, in the bulk, can microphase separate into morphologies ranging from spherical to cylindrical to lamellar having a characteristic period, L , on 0
the tens o f nanometer size scale (6).
In the particular case o f a symmetric, diblock
copolymer, the copolymer microphase separates into a lamellar microdomain morphology.
L o c a l l y , the lamellar microdomains are parallel to one another, as
shown schematically in Figure l a . However, the local orientation is only confined to single grains w h i c h are randomly oriented in the sample forming an isotropic structure globally. Adjacent to an interface, the preferential interactions o f one o f the blocks w i l l force an orientation o f the lamellar microdomains parallel to the interface (7-JO).
This orientation w i l l persist into the bulk o f the sample for
some
characteristic distance, whereupon, defects w i l l cause a loss o f a preferred orientation. If a thin film is prepared where there is a strong segregation o f the blocks to both interfaces, as shown in Figure l b , a global orientation o f the block copolymer morphology parallel to the surface occurs (11-13). If the initial film thickness, /, is not commensurate with L , (nL 0
0
for the case where the same component preferentially
segregates to both interfaces or (n+l/2)L
0
when one block segregates to one interface
and the other block to the other interface), the diblock copolymer is frustrated as is evident from the observed difference between the measured period and the bulk value LQ for P ( S - 6 - M M A ) lamellae confined between two solid interfaces (75).
For this
system, when nL < t
