Polymer molecular-weight distribution

First, in composites with high fiber concentrations, there is little matrix in the system that is not near a fiber surface. Inasmuch as polymerization processes are influenced by the diffusion of free radicals from initiators and from reactive sites, and because free radicals can be deactivated when they are intercepted at solid boundaries, the high interfacial area of a prepolymerized composite represents a radically different environment from a conventional bulk polymerization reactor, where solid boundaries are few and very distant from the regions in which most of the polymerization takes place. The polymer molecular weight distribution and cross-link density produced under such diffusion-controlled conditions will differ appreciably from those in bulk polymerizations. 

OS 61] ]R 20] ]P 44] The polymer molecular-weight distribution of a static mixer-based processing, which was determined both by UV and refractive index analysis. 

In the absence of any nonsize exclusion effects or branching effects, the concentration distribution within the chromatographic peak describes the polymer molecular weight distribution. The concentration distribution is deduced from the response of a suitable detector. Detectors which respond to physicochemical properties other than concentration may also be of use in GPC. Over the past 25 years, a variety of detectors have been developed. This section reviews detectors available to the chromatographer. The detectors used in GPC can be grouped as... 

Initially the polymer molecular weight distribution obeys a Poisson distribution, typical of a chain growth reaction without chain transfer. Since the reactions are reversible, at a later stage, also the equilibration between the polymers becomes important and a broad distribution of molecular weights is obtained. As can be seen from Figure 16.5 the presence of linear alkenes causes chain termination (chain transfer) and thus low molecular weights are produced if the cycloalkenes are not sufficiently pure. 

Further evidence from the mechanism of the polymerization reactions arose from using two initiators with different reactivity, Al(TPP)Me and Al(EtioP)Me (EtioP = etioporphyin). A unimodal polymer molecular weight distribution... 

Size exclusion chromatography (SEC) polymer elution profiles yield information regarding the molecular size distributions of polydisperse macromolecules. Polymer molecular weight distribution (MWD) represents an intrinsic property which provides direct correlation with many end-use physical properties and a universal criterion for polymer characterization (1). In order to convert elution profiles or chromatograms into MWD information proper calibration methods are required. SEC molecular weight calibration techniques represent experimental approaches for transformation of polymer elution profiles into MWD information and are dependent upon instrumentation, columns, and the polymer/solvent system under study. 

A more sensitive viscometer than the drop-time glass capillary method is also needed in size exclusion chromatography (SEC) such as the gel permeation chromatographic (GPC) analysis of polymer molecular weight distribution (MWD). In an SEC system, a concentration detector is commonly used for providing the weight concentration profile of the polymer elution curve. 

Thin-layer chromatography (TLC) is a common laboratory technique for separating complex mixtures of solutes, usually by an adsorbtion mechanism. Several laboratories have applied the technique to the separation of polymer fractions and characterization of polymer molecular weight distributions. This work reviews the experimental results and theoretical approaches to the fractionation mechanisms. 

The major side reactions in lactide ROP are intramolecular and intermolecular transesterification (Fig. 3). Intramolecular transesterification leads to formation of mixtures of linear and cyclic polymers, ultimately reducing the molecular weight of the polymer sample. Intermolecular transesterification, or chain transfer, leads to a broadening of the polymer molecular weight distribution. The extent of these processes can be experimentally verified using mass spectrometry, most commonly using ESI or MALDI-ToF mass spectrometry and by l3C hi) NMR spectroscopy [24, 25]. 

The relationship can also be cast in terms of distributions of molecular weights for polymer systems. This transformation is achieved by relating the decay constant to its corresponding diffusion coefficient D which in turn is related to molecular weight through a Mark-Houwink like relationship. Equation 3 shows the fundamental relationship between the autocorelation function and polymer molecular weight distributions. 

The possibility of improving selectivity or polymer molecular weight distribution by periodically changing the composition of the reactor feed was pointed out by Bailey and Horn and co-workers on one hand O, 2, 3) and by Douglas and Rippen (4,5) and others (6) in the late 1960 s and early 1970 s. Experimental investigations in the following years (T, 8) confirmed the predictions of these theoretical studies. In a series of contributions beginning in 1973 (9,10,11), our... 

The disengagement rate was assumed to be proportional to ywhere the parameter m is related to the elasticity of the medium and depends on the polymer molecular weight distribution. An empirical equation for m is... 

Table 4. Influence of polymerization technique on polymer molecular weight distribution Monomer L ...

Polymer molecular weight distributions and averages seem to me to be widely quoted and little understood, even though there is nothing particularly difficult in the topic. This may be because many textbooks present the basic equations for... 

To this end, knowledge of the principles which govern polymer molecular weight distribution (MWD) and the methods by which it can be controlled, is of extreme importance both from a theoretical and industrial point of view. 

The donors present in the catalyst system play an active role in the formation or modification of isospecific sites, and the polymer molecular weight distribution depends on the relative contribution and hydrogen response (i.e., sensitivity to chain transfer with hydrogen) of each type of active site. The characteristics of different catalyst systems with regard to PP molecular weight distribution are as follows ... 

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