We describe the synthesis and characterization of a number of polymers with well-defined structures that serve as models for polyethylene with long chain branching. All of them have been made by using anionic polymerization techniques and controlled chlorosilane chemistry to give nearly monodisperse polybutadienes with precise control of the number, length, and placement of long (M h w > 1500 g/mol) branches on each chain. This was followed by hydrogenation to give saturated polymers with the same well-defined long chain branching and the local structure of a typical linear low-density polyethylene. That is, both the backbones and the long branches had 17-25 ethyl branches per 1000 total carbons. Among the structures made were some with no long branches ("linears"), some with a single long branch ("stars"), others with exactly two branch points (the R-ω type, "H's", "super-H's", and "pom-poms"), and some with several long branches randomly distributed along the backbone ("combs"). Essentially all types of branching from a linear backbone can be made by the techniques described herein. While linear and symmetrical star models of polyethylene have been made previously, the other structures are the first examples of polyethylene models with multiple branches and precise control of the molecular architecture. We use the results given here to discuss how long chain branching can be detected in polyethylene. We also show how the branching structure controls chain dimensions. The Zimm-Stockmayer model works well to describe the sizes of the lightly branched molecules, but its predictions are too small for those with many long branches. This is presumably due to crowding of the branches. The rheological properties of these polymers will be described in subsequent publications.
The morphology of a family of four 3-miktoarm star terpolymers of polystyrene (PS),
polyisoprene (PI), and polymethyl methacrylate (PMMA) was studied. For three of the samples, the
molecular weight of the PMMA block was systematically varied while that of the PS and PI blocks were
held fixed. In the fourth sample, the PS and PMMA blocks were approximately of the same molecular
weight and the PI was the majority component. In all terpolymers, the system displayed a three-phase,
two-dimensionally periodic microstructure of an inner PI column with a surrounding PS annulus in a
matrix of PMMA. For the two samples with the longer PMMA blocks (218 and 186 kg/mol), the PI−PS
and PS−PMMA interfaces were cylindrical, whereas a unique nonconstant mean curvature (non-CMC)
concentric diamond prism shape of the PI and PS microdomains occurred for the sample with the lowest
molecular weight PMMA block (144 Kg/mol) as well as the polymer with the longer PI block. In these
star terpolymers there are three chemically different chains emanating from the same junction point.
The interaction parameter between PS and PMMA is relatively low, whereas that between PI and PMMA
is the highest. The star molecular architecture gives the molecule the ability to “choose” which arms
directly interact in the microphase segregate state. In the present systems, the junctions lie on the PI−PS interface causing partial mixing of the PS and PMMA blocks, while minimizing the highly unfavorable
contact between the PI and PMMA blocks.
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