ASTMD375077I075953000337B72Designation:D3750-79(Reapproved1985)StandardPracticeforWEIGHTOFPOLYMERSBYMEMBRANEOSMOMETRY'DETERMINATIONOFNUMBER-AVERAGEMOLECULARThisstandardisissuedunderthefixeddesignationD3750;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginaladoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscriptepsilon(6)indicatesaneditorialchangesincethelastrevisionorreapproval.i.Scope1.1Thispracticedescribesthetestprocedurefordeterminingthenumber-averagemolecularweightofpolymersbymembraneosmometry.Itisapplicabletoaiipolymersthatdissolvecompletelywithoutreactionordegradationtoformstablesolutions.1.2Themolecular-weightrangeformem-braneosmometryis,tosomeextent,afunctionoftheparticularsample-solvent-membranesys-teminquestion.Arangefrequentlystatedis20O00to500000,butthismaybeextendedineitherdirectionwithsuitablesystemsandbytheuseofspecialtechniques.1.2.1Theupperlimitformolecularweightresultsfromthereciprocalrelationshipbetweenthequantityofinterest(themolecularweight)andthequantitymeasured(theosmoticpres-sure).Asthemolecularweightisincreased,themeasuredquantitybecomesless,untilitcannolongerbemeasuredwiththerequiredprecision.1.2.2Thelowerlimitformolecularweightresultsfromdiffusionoflow-molecularweightspecies(otherthanthesolvent)presentinthesampleintothesolventcompartmentoftheosmometer.Heretheydonotcontributetotheosmoticpressure,butdoaffecttheequilibriumsettingoftheosmometer.TheeffectisfurtherdiscussedinSection7.1.3Thegreatestdifficultywiththetechniqueisexperiencedwithpolymersampleshavingabroadmolecular-weightdistributionthatex-tendswellintothelow-molecularweightrange.SuchsamplesmayexhibittheeffectreferredtoaspermeationdiscussedinSection7.Notetheprecautionstatedin7.6.1.4Thisstandardmayinvolvehazardoirsma-terials,operations,andeauiument.Thisstandarddoesnotpiirporttoaddressallofthesafityprob-lemsassociatedwithitsuse.Itistheresponsibil-ityofwhoeverusesthisstandardtoconsultaiidestablishappropri...
