DYNAMIC LIGHT-SCATTERING OF POLY(N-LAURYLMETHACRYLATE) IN THE MELT AND IN CONCENTRATED ETHYL-ACETATE SOLUTIONS

Dynamic light-scattering measurements have been made on the ''soft'' p olymer: poly(n-laurylmethacrylate) (PLMA) in solution in ethyl acetate over the concentration range 0.1 < C < 0.9 g/mL as well as melt PLMA. With the concentrated solutions, the hydrodynamic screening length (x i(h)) scales with an exponent of -0.7 below C approximate to 0.3 g/mL. Importantly, xi(h) decreases above 0.3 g/mL, and the concentration de pendence increases strongly above this concentration. The latter contr asts with the behavior in a variety of well-studied poly( styrene) sol utions, which are inevitably investigated close to the glass point, an d for which there is a pronounced increase of xi(h) over the same conc entration range. In PLMA solutions, the decrease in xi(h) parallels th e behavior of the static screening length (xi(s)) which has been obser ved in other flexible polymer systems. Dynamic ''clusters'' (also term ed ''long-range density fluctuations'') contribute a large part of the scattered intensity above the concentration 0.6 g/mL and also in melt PLMA which contrasts with poly( styrene) systems in which they are ab sent. In common with the clusters recently observed in another polymer ic melt, poly(methyl-p-tolylsiloxane) (ref 9) this component is diffus ive. While, however, in the latter system the clusters ''melt out'' at 90 degrees C, we find in the well-matured PLMA systems that the clust er amplitude is temperature-independent (within the temperature-time r ange studied) in the solutions up to at least 80 degrees C and up to a t least 150 degrees C in the melt, which were the highest temperatures examined. The clusters in the PLMA systems have a correlation length of the magnitude 65 nm in the melt, whereas in the solutions this dime nsion varies between 73 nm at 0.9 g/mL to 126 nm at 0.7 g/mL. In PLMA. the clusters are presumed to arise from local segregation owing to th e large paraffinic side groups on the chain. (C) 1997 John Wiley & Son s, Inc.