A sample of low-density polyethylene containing 1.4 long chain branches per 100 carbon atoms was chemically modified by the inclusion of phosphonic acid side groups. Dynamic mechanical and differential scanning calorimetry (dsc) studies were carried out on the unmodified polyethylene and on samples containing 0.8, 1.8, 2.8, and 7.4 phosphonic acid side groups per 100 carbon atoms. The dsc results show that the weight per cent crystallinity is a decreasing function of the phosphonic acid concentration, no crystallinity being detectable by dsc in the material with highest phosphonic acid content. The effect is partly kinetic as illustrated by the fact that the weight per cent crystallinity may be considerably enhanced by prolonged annealing. However, no crystal­linity was observable in the highest phosphonic acid content material under any thermal history investigated. The dynamic mechanical spectra exhibit four distinct dispersion regions. The unmodified polyethylene shows three loss peaks labeled a, fl, and 'Y in order of decreasing temperature. The phosphonic acid containing polymers all have the {:J and 'Y dispersions in common with the polyethylene and those samples of appreciable degrees of crystallinity also have the polyethylene a relaxation. The sample containing 7.4 phosphonic acid side groups per 100 carbon atoms has a new relaxation, designated a' and occurring at 50° (110 Hz). It is postulated that the a' relaxation arises from motion occurring within domains formed from the clustering of hydrogen bonded phosphonic acid groups.