![]() ![]() The relevance of any of the averages for a property depends on the extent to which the average in question takes into account the segment of the molar mass distribution that mainly influences the property. The impact of the higher molecular weights on the average increases along with the exponent in the numerator (Equations (1)–(3)). Thus, the historical background of cellulose research should be considered, at least to a certain degree.įor a uniform sample, all these values are the same. In fact, a fundamental knowledge of different methods for determining molecular weight can be beneficial in understanding the cellulose molecule. Other methods can provide complementary information, however. In industrial settings, viscometry (without separation) still plays a major role since it is quick and, at least in theory, does not require much expertise. In cellulose research, the contemporary method of choice for determining molecular weight is size exclusion chromatography (SEC) coupled with a light scattering detector and a refractive index (RI) detector. The appearance and ever-increasing role of synthetic polymers in daily life has led to a greater scope of analytical methods for polymer characterization. Staudinger investigated cellulose and rubber by viscometry. Initial attempts to determine the molecular weight of polymers were performed with natural polymers. Averages offer an incomplete summarized description of the molecules molar mass. A polymer consists of molecules with different chain lengths (proteins are not included in this characterization), so their molecular weights are always average values. As a consequence, most polymers do not have individual molecular weights in the same way that glucose, for example, always has a molecular mass of 180 g∙mol −1. Today, it is generally accepted that polymers consist of repeating units that are bound together covalently. When Hermann Staudinger first described the molecular structure of macromolecules, there was considerable doubt concerning his hypothesis. After nearly 200 years of polymer chemistry, the determination of polymer molecular weights (averages and distributions) remains a challenge. In this context the ability to determine cellulose’s molecular weight and its chemical uniformity is crucial. The properties of cellulose can vary significantly depending on the origin, the isolation process and/or the treatment. The term bio refers to plants, algae, bacteria, and animals basically, anything that lives and is thus (at least in principle) a renewable resource. Here, size exclusion chromatography (SEC) is unquestionably the most powerful and most commonly-applied method in modern laboratories and industrial settings.Ĭellulose is a commercially important biopolymer. The primary benefit of performing a pre-separation step on the molecules is the discovery of the molecular weight distribution (MWD). In the final section, coupling methods are described. Regardless of an absolute or relative approach, the outcome is a molecular weight average (MWA). The first part of the paper reviews methods, either absolute or relative, for the estimation of average molecular weights. However, older methods, such as osmometry or ultracentrifuge experiments, were the first analytical methods used in polymer chemistry and continue to serve as sources of fundamental information (such as the cellulose structure in solution). Many of the methods described are primarily of historical interest since they have no use in modern cellulose chemistry. Methods that employ direct dissolution of the cellulose polymer are described hence methods for investigating the molecular weight of cellulose in derivatized states, such as ethers or esters, only form a minor part of this review. The purpose of this article is to provide the reader with an overview of the methods used to determine the molecular weights of cellulose. ![]()
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