Macromolecules 1988,21,55-59
mixture was stirred and heated for 24 h under nitrogen in the dark at 70 "C. The red polymer was precipitated from the cooled solution with methanol and purified by repeated precipitation with methanol from a benzene solution. The purified Rose Bengal red polymer was then dried in vacuo at 45 O C overnight.
Acknowledgment. This work was supported by the National Science Foundation (DMR 8404001). We are grateful for their support. We also acknowledge the experimental assistance of S. R. Wu. References and Notes (1) Neckers, D. C.; Paczkowski, J.; Lamberts, J. J. M.; Paczkowska, B. Free Radicals Biol. Med. 1985,1,345.
55
(2) Neckers, D. C.; Linden, S. M.; van Loon, A.; Xu, D. J . Photo-
chem. 1987,38,357. (3) Neckers, D. C.; Xu, D. J . Photochem.; Valdes-Aquiler, 0.; Neckers, D. C. (4) Lamberts, J. J. M.; Neckers, D. C. 2. Naturforsch., B 1984, 39B, 474. (5) Linden, S. M.; Neckers, D. C., presented at the 18th Central Regional Meeting of the American Chemical Society, June, 1986. Linden, S. M.; Neckers, D. C. Photochem. Photobiol., in press. (6) Koizumi, M. Mol. Photochem. 1972,4,57. (7) Gordon, A. J.; Ford, R. A. The Chemist's Companion; Wiley: New York, 1972; pp 429-436. (8) Brandcup, J.; Immergut, E. H. Polymer Handbook; Wiley: New York, 1975; p IV-17, Tables of Viscosity-Molecular
Weight Relations.
Morphology Changes and Volume Deformation of Individual Phases of Polymer Blends: Fluorescence Studies of Polymer Colloids. 9 Mitchell A. Winnik,* Onder Pekcan, and Liusheng Chenf Department of Chemistry, and Erindale College, University of Toronto, Toronto, Canada M5S 1Al
Melvin D. Croucher Xerox Research Centre of Canada, Mississauga, Ontario, Canada L 5 K 2LI. Received February 17,1987
ABSTRACT Fluorescence decay measurements were carried out on micron-sized nonaqueous dispersions of poly(methy1 methacrylate) (PMMA) particles sterically stabilized with polyisobutylene (PIB) and labeled in the PMMA phase with naphthalene (N) groups. The decays were nonexponential and mean decay times ( 7 ) were intermediate between those of similar N groups in pure PMMA (53 ns) or PIB (43 ns) samples. These mean lifetimes ( 7 ) decrease with increasing local N concentration. When samples of the dispersions in isooctane or hexadecane are annealed above 60 "C, their room temperature ( T ) values increase,and when powder samples are so annealed, ( 7 ) decreases. These results point to the presence of an extensive PMMA-PIB interphase, with solvent swelling, and annealing leading to changes in the extent of interphase formation. Both environmental factors and the extent of naphthalene self-quenching [N* + N 2N] affect the value of ( 7 ) .
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Many polymer systems of commercial importance are composed of separate phases of different compositions. These come about through blending or phase separation in graft or block copolymers. Although the mechanical properties of such materials depend intimately on the nature and size of the phases and their interface characteristics, measurements of these properties or other bulk responses of the system provide only indirect insights into the origins of these properties. It would be advantageous to have experiments sensitive to phenomena occurring only within certain phases of the material. Labeling techniques permit such experiments. With judicious use of isotopic or heavy atom labeling, one could examine individual phases by NMR or by neutron or X-ray scattering.' In this report we describe experiments directed toward developing such a method based upon fluorescence spectroscopy. We examine a material with a morphology best described as an interpenetrating network of a rubbery polymer (polyisobutylene, PIB) in a glassy matrix [poly(methyl methacrylate), PMMA], with the PMMA chains labeled with naphthalene (N) groups. This material is prepared as a sterically stabilized nonaqueous dispersion
* To whom correspondence should be addressed.
Permanent address: Institute of Chemistry, Academia Sinica, Beijing, China. 0024-9297/88/2221-0055$01.50/0
in isooctane,2with particle size ranging from 1 to 3 pm. Chemical and spectroscopic analysis indicates a mole ratio of IB/MMA to be 13/100, which corresponds to a particle composition of 9% PIB dry v01ume.~ The particle morphology was originally inferred from fluorescence quenching experiment^,^ with further supporting evidence from X-ray scattering studies of particles doped with tetraphenyl lead.5 The particles contain a thin surface shell of PIB which serves as the steric barrier against flocculation. The remainder of the PIB penetrates throughout the particle enveloping PMMA domains. This morphology is created because chemical bond formation (grafting) occurs between the PIB and PMMA during particle preparation. Thus, while PIB and PMMA are incompatible polymers and phase separate, they are still held together by the covalent bonding between them. Furthermore, we anticipate the existence of an extensive interphase between the two discrete homopolymer phases. In a different system dispersed in cyclohexane, we have documented6 that the presence of 7-870 of a low Tgstabilizer transforms 50% of the glassy particle into an interphase containing both polymers as well as solvent. A pictorial representation of this morphology is shown in Figure 1. Operationally, the PMMA phase in the particle is labeled by incorporating a comonomer containing a naph0 1988 American Chemical Society
56 Winnik e t al.
Macromolecules, Vol. 21, No. I , 1988
sample 1
2 3 4 5 6 7 8
9 10
mol % N 0.04 0.12 0.59 1.1 2.4
4.7 6.2 9.7 15 23
Table I N, mol/L 104Mn M,/M, 0.005 4.2 1.95 0.014 4.9 1.71 0.070 6.1 1.76 0.13 5.5 1.98 0.28 4.8 1.71 0.52 6.0 1.75 0.65 5.2 1.84 1.0 1.79 5.4 1.5 4.4 1.89 2.2 4.1 2.36
(7).ns
52.9 51.5 51.0 49.3 47.5 46.4 45.0 43.2 40.5 37.6
Experimental Section
in 12-mm-0.d.quartz tubes fitted with a graded seal, were degassed by several freeze-pump-thaw cycles and sealed under vacuum. Powder samples were placed in 3-mm-i.d. quartz tubes and sealed under vacuum. Fluorescence decay profiles were measured by the time-correlated single photon counting technique,' exciting the samples a t 280 mm and observing the emission through an interference filter a t 337 nm. The samples were turbid to opaque. While the excitation and emission optics were at 90° to one another, samples often had to be positioned so that one measured essentially front-face fluorescence. Fluorescence decays for these samples were nonexponential but could be fitted to a sum of two exponential terms. The fits were reasonable (x2< 1.3). The individual decay parameters here do not have particular significance. We simply calculate ( T ) = StI(t)dt/JI(t)dt. Values of (7)are rather insensitive to the quality of the fit and do not change when the decay curves are fitted with a sum of three exponential terms. During the experiments, the samples were heated to the annealing temperature and kept there for 2-3 h. Each sample was allowed to cool to room temperature, and the fluorescence decay profiles Z ( t ) were remeasured. The samples were then reheated to the next higher annealing temperature, allowed to cool, and remeasured. All measurements reported here were carried out a t room temperature.22 We did not examine thermal history effects in any systematic way. The results we report are reproducible within the protocol we report. Samples of copolymers poly(MMA-co-NMA) were prepared by free radical polymerization of MMA 1 in benzene solution at 80 "C under nitrogen and initiated by AIBN. Reactivity ratios were determined to be r,(MMA) = 0.89, r2(1) = 0.83. The polymers were reprecipitated 3 times from chloroform into methanol and analyzed by gel permeation chromatography (GPC) using microstyragel columns (toluene solvent) in conjunction with tandem UV and refractive index detectors. Molecular weights were calculated by using PMMA standards (Polymer Laboratories, London, UK), assuming that the N-units did not affect the hydrodynamic volume of the polymer. This assumption becomes increasingly poor for larger extents of N-incorporation. Our experimental results do not, however, depend upon the molecular weights of the copolymers. The extent of N-group incorporation was determined by UV absorption in chloroform using 2 as a model from which eZm 6800 was determined. Table I summarizes the characteristics of these copolymers. GPC analysis indicates no detectable (
