1、Standard Recommended Practice for DETERMINING MOLECULAR WEIGHT BY VAPOR PRESSURE OSMOMETRY This Standard is issued under the fixed designation D 3592;the number immediately following the designation indicates the year of original adoption or,in the case of revision,the year of last revision.A number
2、 in parentheses indicates the year of last reapproval.1.Scope 1.1 This recommended practice covers the determination of the number average molecu-lar weight,&I,by means of vapor pressure osmometry(VPO).It is applicable to all poly-mers that dissolve completely without reac-tion or degradation.1.2 Th
3、e practical molecular weight range for vapor pressure osmometry is generally considered to be between 10 O00 and a lower limit that is determined by the requirement that the solute have negligible vapor pressure.The upper molecular weight limit can be ex-tended by the use of special techniques.1.2.1
4、 The use of low molecular weight standards to determine the calibration con-stant is only an approximate procedure(1,2);2 this approximation becomes less accurate as the molecular weight difference between the calibration material and the unknown solute is increased.Therefore,for samples having numb
5、er average molecular weights larger than 10 000,a revised calibration procedure must be employed.1.2.2 The precision with which fi,of high molecular weight solutes can be determined is critically dependent on the non-ideality of the solute-solvent system.For non-ideal polymer systems,in which the so
6、lvent is a relatively good solvent for the solute,the differential thermistor resistance,AR,of the system is represented as follows:AR=A I C +A2C2+A3C3+.*-(1)A is proportional to the reciprocal of M,C is the weight concentration,and A2 and A3 are the second and third viria1 coefficients respec-tivel
7、y.The higher order terms become more significant(3)as the molecular weight is in-creased,necessitating a polynomial fit of the data to determine the coefficient A,from which&I,is obtained.The precision of the measurements is improved for high polymers as the apparatus sensitivity is increased(4)so t
8、hat lower concentrations can be used;this use of low concentrations decreases the impor-tance of the nonlinear terms.Another limiting factor in determining A?,of high molecular weight polymers is the potential presence of small amounts of low molecular weight impur-ities.2.Significance 2.1 The numbe
9、r average molecular weight,M,is a molecular weight value that is quite sensitive to the low molecular weight homo-logs present in a polydisperse polymeric mate-rial.&I,may be defined simply as follows:where ni is the number of moles of molecular weight Mi.2.2 The number average molecular weight is a
10、n important parameter of polymers that can be related both to the reaction kinetics of polymerization and physical properties of the bulk material such as tensile strength,melting This recommended practice is under the jurisdiction of ASTM Committee D-20 on Plastics,and is the direct re-sponsibility
11、 of Subcommittee D 20.70 on Analytical Meth-ods.Current edition approved June 24,1977.Published Au-gust 1977.*The boldface numbers in parenthesis refer to the list of references at the end of this recommended practice.-.I 867 NOTICE:This standard has either been superseded and replaced by a new vers
12、ion or discontinued.Contact ASTM International(www.astm.org)for the latest information.9009276 0029025 459 point,glass transition temperature,impact strength,and elasticity.2.3 Vapor pressure osmometry fills the im-portant gap in the determination of M,since membrane osmometry experiences problems d
13、ue to membrane permeability for molecular weights below approximately 20 000.For molecular weights above 20 000,membrane osmometry is the preferred technique.3.Apparatus 3.1 VPO InstrumenP-The main compo-nents of a VPO instrument are a pair of high-temperature coefficient thermistors or ther-mocoupl
14、e junctions suspended in a tempera-ture-controlled cell that is saturated with sol-vent vapor.A drop of solution is placed on one thermistor or thermocouple and a drop of solvent is placed on the other.The vapor pressure of the solvent in the solution droplet is lowered by the presence of the solute
15、;con-densation of the solvent vapor in the tempera-ture-controlled cell onto the solution droplet takes place.This causes a temperature rise of the junction containing the solution droplet and an electrical output signal is produced by incorporation of the thermistors into a Wheat-stone bridge arran
16、gement or by measuring the output of a pair of thermocouple junctions directly with a sensitive microvolt meter.The above picture of how the VPO functions is accurate(2)only for low bridge voltages,and changes markedly for cases where thermistor self-heating is significant compared to the heat generated by mass transfer to the drops.3.2 Balance,having a sensitivity of 0.1 mg.3.3 Timer,1-s graduations.3.4 Recorder,10-mV,full scale.The use of the recorder is optional and the desirability of its ut
