INTERACTION OF I'OLV.M i:i{S AND M KC 11 ANICA L WAVES 333 



some observations at low fretiuencies on isoviscous i>roperties of polyiso- 

 l)ut\ienes A" and C indicate nenrW identical retardation times. Thus A" 

 at '2o°C and (' at ()1°C have 77., = 39. () poises. The 77 vahies at 2()() and 

 1(100 cps arc also al)ont 39 poises, jjl at 'iOO cps is 3800 dynes per cm" 

 tor l)()th li(iuids, and at 1(500 cps is from 3..") X 10* to 4.8 X 10* dynes 

 per cm". 



Ill the filial section, mechanical waves have been used to explore 

 dilute pol>'mcr solutions, to see how isolated molecules behave, free of 

 interaction with cacii oth(>r. 



DIHTH I'OLYMKR SOLUTIONS 



Physical Principles in McdsKroncnIs 



Precise information on dynamics of solutions aiiproachiii<>; iniinite 

 dilution (and thus complete separation of the polymer chains) is desired 

 here. Again these must be shear dynamics; bulk rigidity of orduiary 

 liciuids is so high that a few polymer molecules added cause little effect. 

 Dilution is emphasized because even at 1 per cent by volume, high 

 polymer molecule coils frequently interact, especially in "good" solvents. 

 Thus, several workers have detected shear rigidity in polymer solutions, 

 in one case for polymethyl methacrylate of average molecular weight 

 320,000, at 1 per cent concentration in o-dichlorobenzene. Very low 

 frequencies used ('^10 cycles) there and in an earlier study suggest, 

 however, that even here, appreciable entangling of the molecules created 

 a temporary network such as studied b}^ Ferry."'' "'' Such was certainly 

 present in the 5 to 18 per cent solutions of cellulose acetate m dioxane 

 measured in one of the earliest observations of shear rigidity in polymer 

 solutions. "" 



Accordingly, shice strictly linear, and hence non-interacting, mechanics 

 are sought for the macromolecules in dilute solution, careful evaluation 

 of experiments is essential. Since already it appears that important 

 over-all (quasi-configurational) relaxations occur for, say, polyisobutyl- 

 ene in the kilocycle range, and it is suspected that not all of the inter- 

 actions involved are between chains, the torsional crystal technicjues 

 are attractive. The absolute viscosity of these solutions is very low, so 

 the ammonium dihydrogen pho.sphate crystal whose piezoelectric (lual- 

 ities are appropriate for polymer licjuids in the circuits pre\'iously 

 noted"' ' ' is advantageously replaced by quartz. 



Detailed electromechanical behaviour of such crystals in the pure 

 liquids cyclohexane and benzene is of first concern. The electric field 

 applied to electrodes on the suspended crystal produces mechanical 



