ASTM_D_3750_-_79_1985_scan.pdf

ASTM D3750 77 I 0759530 00337B7 2 Designation:D 3750-79(Reapproved 1985)Standard Practice for WEIGHT OF POLYMERS BY MEMBRANE OSMOMETRY DETERMINATION OF NUMBER-AVERAGE MOLECULAR T h i s standard i s issued under the fixed designation D 3750;the number immediatelyfollowing the designation indicates the year of original adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.A superscript epsilon(6)indicates an editorial change since the last revision or reapproval.i.Scope 1.1 This practice describes the test procedure for determining the number-average molecular weight of polymers by membrane osmometry.It is applicable to aii polymers that dissolve completely without reaction or degradation to form stable solutions.1.2 The molecular-weight range for mem-brane osmometry is,to some extent,a function of the particular sample-solvent-membrane sys-tem in question.A range frequently stated is 20 O00 to 500 000,but this may be extended in either direction with suitable systems and by the use of special techniques.1.2.1 The upper limit for molecular weight results from the reciprocal relationship between the quantity of interest(the molecular weight)and the quantity measured(the osmotic pres-sure).As the molecular weight is increased,the measured quantity becomes less,until it can no longer be measured with the required precision.1.2.2 The lower limit for molecular weight results from diffusion of low-molecular weight species(other than the solvent)present in the sample into the solvent compartment of the osmometer.Here they do not contribute to the osmotic pressure,but do affect the equilibrium setting of the osmometer.The effect is further discussed in Section 7.1.3 The greatest difficulty with the technique is experienced with polymer samples having a broad molecular-weight distribution that ex-tends well into the low-molecular weight range.Such samples may exhibit the effect referred to as permeation discussed in Section 7.Note the precaution stated in 7.6.1.4 This standard may involve hazardoirs ma-terials,operations,and eauiument.This standard does not piirport to address all o f the safity prob-lems associated with its use.It is the responsibil-ity o f whoever uses this standard to consult aiid establish appropriate sajty and health practices and determine the applicability o f regulatorylmi-tations prior to use.A specific precautionary statement is given in 4.1.2.Significance and Use 2.I Molecular weight is a fundamental-structure parameter of polymers that is related to many physical properties of the bulk mate-rial,such as,tensile strength,melting point,and elasticity.2.2 To the extent that membrane behavior is understood,membrane osmometry is an ab-solute measurement and may therefore be ap-plied to polymers that have not been previously synthesized or studied.See,however,the dis-cussion in Section 7 and Appendix XI.3.Apparatus venient size.3.1 Volumetric Flasks,100-mL or other con-3.2 Transfer Pipets.3.3 Osmometer-Both automated and non-automated instrument designs are in common use and differ considerably in their methods of operation.Equally valid data may be obtained with either type when properly used;the auto-mated instruments offer considerable advan-tage over the nonautomated ones in conveni-ence,ease of operation,and speed.This practice is under the jurisdiction of ASTM Committee D-20 on Plastics and is the direct responsibility ofsubcommittee D20.70 on Analytical Methods(Section D20.70.05).Current edition approved March 15,1979.Published May 1979.NOTICE:This standard has either been superseded and replaced by a new version or discontinued.Contact ASTM International(www.astm.org)for the latest information.ASTM D3750 79 M 0759530 0033990 9-3.3.1 Automated Osmometer-These operate either by direct measurement of the pressure developed in a suitable cell,or by balancing the pressure developed against a calibrated pressure source,usually a column of the sol-vent.For operational details,the literature and operating manuals of the manufacturers should be consulted,The pressure-balancing instru-ments are absolute in the same sense as the nonautomated osmometers.The pressure-mea-suring instruments must be calibrated against a suitable pressure source and,ifthis is properly done,are capable of equally accurate measure-ment.The automated instruments use very small amounts of sample;data are obtained much more rapidly with them than with the nonautomated osmometers,and some form of recorder output is often introduced.NOTE 1-Automated osmometers should incor-porate either a strip-chart recorder or have provision for output to an external recorder.It is strongly recommended that a recorder be used,particularly when working with unfamiliar systems or at low molecular weights.3.3.2 Nonautomated Osmometer-It is pref-erable if this is of the Stabin-Immergut or similar design in which the membranes are securely held between clamps to minimize dis-to