8 Epitaxial Polymerization as a Tool for Molecular Engineering JEROME B. LANDO, ERIC BAER, SCOTT E. RICKERT, HEMI ΝΑΕ, and STEPHEN CHING
Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch008
Case Western Reserve University, Department of Macromolecular Science, Cleveland, OH 44106 Epitaxial crystallization is the oriented over growth of a substance on a crystalline substrate. The interaction between the two is usually highly specific and has profound effects on the morphology and struc ture of the crystallizing material. Epitaxial polymeri zation is a new phenomenon in the field of macromolecu lar science. It combines the epitaxial crystallization of a monomer on a crystalline substrate, followed by solid-state polymerization, which is controlled by the epitaxial crystallization of the monomer. A study of the vapor phase epitaxial polymerization of disulfurni tride to polythiazyl, (SN) , resulted in three new crystalline phases of (SN) , and a new appreciation of the catastrophic effect of water on this polymer. The epitaxial polymerization of hexachlorocyclotriphospha zene to poly(dichlorophosphazene), (NPCl ) has been investigated. Deposition from both the vapor phase and solution has been studied. The polymer structure and morphology depend upon the monomer epitaxial crystals. The actual monomer morphology and structure have been found to be dependent on the geometry of the substrate. The application of this method to the epitaxial poly merization of diacetylenes will also be discussed. x
x
2
x
A topochemical effect in a solid state reaction is any effect on the structure and properties of the product or the kinetics of the reaction that can be directly attributed to the geometric arrangement of the reacting groups or the distance between those groups. The degree of topochemical control in a solid state reaction can vary greatly depending upon the particular system investigated.(1) Reactions to be discussed here, in which there is a crystallographic correlation between the reactant and the resulting product, can occur in solid solution or with the nuclea tion and growth of a product phase. Systematic investigation of solid state polymerization reactions began with the discovery that crystalline acrylamide polymerizes when exposed to ionizing radiation. (2,3) Since that 0097-6156/83/0212-0089$06.00/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch008
90
INITIATION OF POLYMERIZATION
time i n v e s t i g a t o r s have been i n t r i g u e d by the p o s s i b i l i t y of pro ducing polymers with unusual s t r u c t u r e s , morphologies and p r o p e r t i e s through s o l i d s t a t e p o l y m e r i z a t i o n . Although these goals have been f u l f i l l e d to some extent i n the past twenty-five years, a p e r v a s i v e problem has been a l a c k of c o n t r o l over monomer morphology and s t r u c t u r e . In simple terms we are "stuck" with the s t r u c t u r e s that nature gives us. In the f o l l o w i n g paper, a method w i l l be discussed that allows v a r i a t i o n of monomer s t r u c t u r e and morphology. This i n v o l v e s a technique we have termed e p i t a x i a l polymerization* Monomers are c r y s t a l l i z e d on c r y s t a l l i n e sub s t r a t e s . L a t t i c e matching allows the v a r i a t i o n of monomer s t r u c ture and morphology, y i e l d i n g d i f f e r e n t polymer s t r u c t u r e s and morphologies upon p o l y m e r i z a t i o n . Three types of monomers w i l l be discussed i n t h i s paper, c y c l i c s u l f u r - n i t r o g e n compounds, c y c l i c phosphazenes and d i a c e t y l e n e s . Experimental T e t r a s u l f u r Tetramide. T e t r a s u l f u r tetramide (S^N,) was sublimed at 100°C and 10" t o r r . The vapor was passed through Ag S wool at 210°C forming S^^. The hot S , ^ vapor condensed and c r y s t a l l i z e d on a l k a l i h a l i d e s i n g l e c r y s t a l s at -78 0.^±-*Α) Polymerization occurred during heating to room temperature. 5
ο
Polyphosphazene. Hexachlorocyclotriphosphazene (N P^Cl^) was obtained as p u r i f i e d c r y s t a l s from the Army Research L a b o r a t o r i e s . This trimer was deposited as t h i n f i l m s on a l k a l i - h a l i d e substrates from the vapor, melt, and s o l u t i o n . Vapor phases-deposition was accomplished by subliming Ν P^Cl^ at 80°C i n 10 t o r r vacuum. I t was found that d e p o s i t i o n times of around 20 minutes yielded incomplete f i l m s s u i t a b l e f o r e l e c t r o n microscopy. P r i o r to sublimation the a l k a l i h a l i d e c r y s t a l was always annealed at room temperature i n 10"^ t o r r vacuum i n order to ensure removal of adsorbed water from the c r y s t a l s u r f a c e . C r y s t a l l i z a t i o n from s o l u t i o n i n v o l v e d the p r e p a r a t i o n of a 20 wt % s o l u t i o n of the trimer i n decane. The p r e c i p i t a t i o n temperature f o r t h i s s o l u t i o n was 23°C, and a l l e p i t a x i a l d e p o s i t i o n s were done at 29°C, or 6°C above t h i s cloud p o i n t . Deposi t i o n times on i n s i t u cleaved s a l t c r y s t a l (100) surfaces were near 10 minutes f o r f i l m thicknesses s u i t a b l e f o r e l e c t i o n micros copy. Another method used i n v o l v e d the c a s t i n g of a t h i n f i l m of N^P^Cl^ from decane on a f r e s h l y cleaved a l k a l i h a l i d e surface at room temperature, followed by h e a t i n g to 130°C. The molten trimer f i l m was then slow cooled to 110°C (4°C below the bulk m e l t i n g temperature) and h e l d at that temperature f o r 1 hour. A l l e p i t a x i a l f i l m s were e i t h e r prepared d i r e c t l y f o r micros copic examination, or polymerized using a post-polymerization technique f o r the f i r s t time on phosphazene monomer. The trimer f i l m s were i r r a d i a t e d with 2.5 Mrad of γ-radiation from a Co60
Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
8.
LANDO ET AL.
Epitaxial
91
Polymerization
source (24 hours exposure time) at 37°C. The now-activated trimer f i l m s were reacted by annealing i n an i n e r t atmosphere at 140°C f o r two hours. P o l y d i a c e t y l e n e s . Dimethanol-diacetylene (HOCH^CEC-CECCH OH) (DMDA) was used as received from Farehan Chemical Co. Diphenyf urethane-diacetylene (
