Controlled Variation of Spacer Segment in Hyperbranched Polymers:From Densely Branched to Lightly Branched Systems

摘要

A series of hyperbranched polyethers containing different types(and lengths)of spacer segments that link the branch points were prepared by a solvent-free melt transetherification methodology.Three types of spacers,namely n-alkylene,cycloalkylene,and oxyethylene,were incorporated into the hyperbranched structures in order to examine the role of spacer length(and flexibility)on the relative compactness of the chain conformation as well as to study its effect on their T_g.The distinctive feature of this investigation is that the branching density(number of branch-points per unit volume)is varied in a controlled manner by utilizing AB_2-type monomers that contain spacer segments of different lengths.GPC studies using a dual detector(RI-DV)system revealed that most of the polymers were of fairly high molecular weight,with M_w values ranging from 24 000 to 90 000.It is seen that as the branching density decreases,i.e.,when the spacer segment length increases,the Mark-Howink exponent"a"increases;for instance,it increased from 0.37 for the polymer without a spacer to 0.54 in the case of the polymer having a 10-carbon spacer segment.This suggests that the relative compactness of the hyperbranched structures diminishes with increasing length of the spacer segment and might be expected to approach the behavior of linear macromolecules for even longer spacers.Furthermore,comparison of the molecular weights of several fractionated samples,determined by both the standard polystyrene calibration plot as well as by the universal calibration plot,reveals that the extent of molecular weight underestimation using the PS calibration becomes significant only at higher molecular weights.Thus,as seen in dendrimers,in hyperbranched polymers also compact structures are formed only beyond a certain molecular weight.The glass transition temperatures of the hyperbranched polymers were seen to decrease monotonically with increasing spacer length within each series,and hyperbranched structures with relatively flexible spacers exhibited lower T_g values when compared to ones with relatively rigid spacers of similar length.