synopsisThis manuscript reports on an experimental investigation of the chemical-initiated (AIBN) bulk polymerization of methyl methacrylate to limiting conversion a t temperatures of 50°, 70") and 90°C. The change in the cumulative differential molecular weight distribution (CDMWD) with respect to conversion was measured by gel permeation chromatography (GPC). These CDMWD's were differentiated to determine the instantaneous differential molecular weight distribution (instantaneous DMWD) over the range of conversions investigated. These experimental instantaneous DMWD's were found to agree with theoretical distributions predicted by classical free-radical kinetics over the entire conversion range and where diffusion control of the termination reactions is dramatic. A correlation of the dimensionless group a (where a = ktdR,/ kP2M2) with free volume is proposed. This correlation appears to adequately account for diffusion control of the termination reaction. A kinetic model for the bulk polymerization of methyl methacrylate has been developed. This model should find use in the design, simulation, and optimization of PMMA reactors. MODEL DEVELOPMENTFurther detail of the material to be discussed here may be found elsewhere.' The reactions which are significant in the bulk polymerization of MMA with a free radical initiator follow. Initiation:I = rate of formation of free radicals of chain length unity, Ri.I n order to keep the model as general as possible at this stage, we shall not consider the detailed mechanism of initiation, but rather will use the net production of radicals Ri in the analysis. Propagation:h i R; + M -R;+1 (isotactic placement) R; + M k,. R;+' (syndiotactic placement)

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SynopsisPolypropylene was degraded by injection of a free-radical initiator during extrusion. Molecular weight distribution, molecular weight averages, and melt flow index were measured to see the effect of initiator concentration (0.00 to 0.04 wtW), temperature (200 and 220"C), and screw rpm (31 and 44). Initiator concentration was the most significant variable. In all cases, increased initiator concentration degraded the high molecular weight tail of the polypropylene and narrowed the molecular weight distribution. Melt flow index varied linearly with initiator concentration beyond 0.01 wt% initiator. Reaction temperature had no effect on the measured properties of the extrudate. This was attributed to the minimum residence time in the extruder being sufficient for all degradation reactions to be completed. It also implied that the same reactions occurred at both temperatures. Increased screw rpm slightly increased molecular weight. No interaction among initiator concentration, temperature, and screw rpm was observed. INTRODUCTIONPolypropylene can be intentionally degraded with a free-radical initiator during extrusion. This reactive extrusion process causes the removal of the high molecular weight tail and the narrowing of the molecular weight distribution. The "specialty polymer" product ("controlled rheology polypropylene") is known to exhibit less swell, draw resonance, and melt fracture than the parent "commodity polymer".'-5 Mechanical properties such as stiffness, brittleness, and tensile strength are unchanged.6 This is the first paper in a series describing an investigation of this process. Our main objective is to show the effect of processing variables on the molecular weight distribution of the product. Following papers will present a kinetic model of the degradati~n,~ and size exclusion chromatography (SEC) method d e v e l~p m e n t .~,~ Preliminary results have already been presented and published. lo-l 2 BACKGROUND INFORMATION

SynopsisA kinetic model for the free-radical-initiated molecular weight degradation of polypropylene was further developed. The model has a single variable parameter, the initiator efficiency, f . Assumptions were detailed, new comparisons with experimental data presented, and model sensitivity to the value of f evaluated. The model was found to provide a good description of both molecular weight distribution and molecular weight average data from degradations carried out in a single-screw extruder at 200 and 220°C. Data at 0.04 wt% initiator feed concentration were fit and the resulting f value used to predict results at 0.01 wt% and 0.02 wt%. In accordance with observation, the model predicted that temperature would have no effect on the molecular weight of the extrudate because the comparatively long ( 2 2.8 min) residence time in the extruder permitted degradation reactions to go to completion. The model predictions were found to depend upon the change in molecular weight distribution rather than the absolute value of the distribution data. Predictions were therefore unaffected by concentration correction in size exclusion chromatography interpretation. The model was determined to have a low sensitivity to the value of f . Hence, f was estimated to only +25% and it is anticipated that attempts to use the model for comparing different initiators would be limited by this characteristic. Also, because the yninimum residence time in the extruder was 2.8 min, the model has yet to be tested against data at times less than this value.

Films composed of a mixture of poly(methyl methacrylate) (PMMA, My, = 306 000) and polystyrene (PS, My, = 234 000) were prepared by solvent casting. In these films, the PMMA component, covalently labeled with a fluorescent dye, comprised 10 wt % of the mixture. These films were examined by laser confocal fluorescence microscopy (LCFM). At depth greater than 5-6 pm below the surface, the blend morphology was one of tiny PMMA spheres dispersed in a continuous PS matrix. These spheres were randomly distributed in space and were characterized by a broad size distribution. At the surface, the morphology was very different. The PMMA was present in the form of large spheres, 5-6 pm in diameter, characterized by not only a very narrow size distribution but a strong periodicity in their location. The broad distribution of small particles in bulk can be attributed to phase separation by a nucleation and growth mechanism coupled with the small surface energy between PS and PMMA. At the surface, two other and larger surface energies come into play, those of PS/air and PMMA/air. In addition, more rapid evaporation of solvent from the surface may lead to a spinodal decomposition mechanism for surface phase segregation.