Handbook of Size Exclusion Chromatography and Related ...

336 Sjöholm
significantly(134).Asaconsequenceofthederivatization,theprofileoftheMMD
is changed to become more like those of hardwood kraft pulps, possibly due to
decreased association between glucomannan and cellulose, that is, a better
chromatographic separation between hemicellulose/lignin and cellulose.
The MMD corresponding to the cellulose portion of high molecular mass
hardwood kraft pulps commonly has ashoulderon the high Mend (196,199).This
may be more or less pronounced depending on the Mof the cellulose, and is
commonly not seen for underivatized softwood pulp samples dissolved in LiCl/
DMAc. Asystematic study of the origin of this appearance revealed also that the
MMD of acotton linter (DP 8000) having about the same elution range as wood
pulp cellulose also had asimilar shoulder (199). The shoulder was attributed to
aggregation/association of the cellulose. Using light scattering, stable aggregates
havebeendemonstratedinconcentratedLiCl/DMAcsolutionsofcellulosesamples
with lower M(DP ,1500) (63,69,222). It was shown that even if aggregates were
present in the stock solution, molecular dispersed,that is, nonaggregated,solutions
for most of the samples could be obtained after dilution to 0.9% LiCl and 0.1%
cellulose, that is, at concentrations used in SEC (69,222). Considering the
additional dilution that occurs during chromatography,it was concluded that true
molecular dispersed solutions exist under common SEC conditions.
Differentapproacheswereinvestigatedtoavoidtheformationofaggregates
by using off-line LS and deconvoluting the MMD obtained by SEC (199). An
optimized mechanical treatment by shaking the solutions was the only possible
waytobreaktheaggregates.Thetreatmentonlyinfluencedtheshoulderatthehigh
molecular mass end of the cellulose MMD. The LiCl concentration during
dissolutionhadapronouncedeffectontheformationofaggregates,butat6%,the
lowest concentration possible for dissolution of the studied samples, the shoulder
still remained. Different activation–dissolution procedures (I orII), urea addition,
thermal treatment, decrease in sample concentration or dissolution time did not
influence the shape of the MMDs. It should be pointed out that the used columns
were packed with 20 mm PS/DVB particles. Using smaller particles, the sample
solutions will experience a higher shear force during chromatography and
aggregated cellulose may thus be disrupted.
Fundamental studies concerning the influence of different treatments of a
fibrous sample on the MMD profiles of its polymers are of interest in order to
interpret the effect of different types of degradation. In this context, hardwood
pulps and pure cellulose samples have been studied. The relation between fiber
strength andMMD obtained with LiCl/DMAchasbeenstudiedafterdegradation
of unbleached hardwood kraft pulp with gamma irradiation, oxygen/alkali or
alkali (210), and by ozone or acid hydrolysis (200), the latter study also included
degradation of cotton linters. By comparing the profiles of hardwood kraft pulp
with those of cotton linter, it was concluded that the MMD profiles depend on the
typeofdegradationaswellastypeoffiber.AbimodalMMDprofile(Fig.4)ofthe
© 2004 by Marcel Dekker, Inc.