Dispersoidal Analyses of Hevea Latex

1947 ◽  
Vol 20 (4) ◽  
pp. 1109-1123 ◽  
Author(s):  
J. H. E. Hessels
Keyword(s):  
Specific Gravity ◽  
Weight Distribution ◽  
Distribution Curve ◽  
Visible Spectrum ◽  
Sedimentation Analysis ◽  
Significant Percentage ◽  
Total Percentage ◽  
Rubber Particles

Abstract For the purpose of determining the weight-distribution curve of the rubber particles in Hevea latex according to their diameter, two methods were employed, namely, photomicrographic dispersoidal analysis and sedimentation analysis. In addition to the fact, that the first method is very laborious, its precision does not prove to be high, mainly because a significant percentage of the particles is too small to be determined microscopically in the visible spectrum. Sedimentation analysis with the help of a modified pipette method according to Andreasen was found, however, to be very suitable for the purpose. By an accurate regulation of the viscosity, specific gravity and temperature of the suspension it became possible to determine the weight-distribution curve of latex to a diameter of about 0.4µ within a creaming time of 10 to 14 days, while the total percentage of still smaller particles was also determined at the same time. This method is accurate, not too laborious, and lends itself very well to a series of investigations.

The Structure of Neoprene. I. The Molecular-Weight Distribution of Neoprene Type GN

1949 ◽  
Vol 22 (3) ◽  
pp. 680-689
Author(s):  
W. E. Mochel ◽  
J. B. Nichols ◽  
C. J. Mighton
Keyword(s):  
Molecular Weight ◽  
Natural Rubber ◽  
Osmotic Pressure ◽  
Molecular Weight Distribution ◽  
Weight Distribution ◽  
Distribution Curve ◽  
Pressure Measurements ◽  
Dilute Solution ◽  
Molecular Weights ◽  
Good Agreement

Abstract Polychloroprene rubber (Neoprene Type GN) was fractionated by partial precipitation from dilute solution in benzene and the fractions were examined both osmotically and viscometrically in benzene solutions. The molecular-weight distribution curve for Neoprene Type GN based on osmotic pressure measurements shows a pronounced maximum at 100,000, but has a long extension to molecular weights of over one million, indicating the presence of branched or cross-linked material which is still soluble. The uniformity is somewhat less than that of sol natural rubber, while in shape the Neoprene distribution curve resembles more closely that of peptized natural rubber than fresh sol rubber. Observed variations in the slopes of the π/c vs. c and the ηsp/c vs. c curves also indicate the presence in solution of complex, branched and (or) cross-linked molecules. Calibration of the intrinsic viscosity-molecular weight relationship by osmotic pressure measurements gave good agreement with the equation: [η]=KMa, where K=1.46×10−4 and a=0.73.

Gel Permeation Chromatography of Natural Rubber

1972 ◽  
Vol 45 (1) ◽  
pp. 346-358 ◽  
Author(s):  
A. Subramaniam
Keyword(s):  
Molecular Weight ◽  
Natural Rubber ◽  
Weight Distribution ◽  
Hevea Brasiliensis ◽  
Distribution Curve ◽  
Gel Permeation Chromatography ◽  
Bimodal Distribution ◽  
Fractional Precipitation ◽  
Molecular Weights ◽  
Gel Permeation

Abstract The Waters Model 200 Gel Permeation Chromatograph has been used to study the molecular weight distribution of natural rubber. The cumulative weight distribution curve of synthetic cis-polyisoprene from the GPC method showed fair agreement with the distribution obtained by fractional precipitation. For natural rubber the agreement was not so good. Natural rubber samples from six clones of Hevea Brasiliensis were examined with the GPC. Differences were observed in their distributions. Five clones showed a distinct bimodal distribution. The weight and number average molecular weights from the GPC were found to be too low. Some possible reasons for this have been suggested.

Characterization of the Large and Small Rubber Particles in Fresh Hevea Latex

2002 ◽  
Vol 75 (2) ◽  
pp. 179-185 ◽  
Author(s):  
Jitladda T. Sakdapipanich ◽  
Kanjanee Nawamawat ◽  
Seiichi Kawahara
Keyword(s):  
Molecular Weight ◽  
Particle Size ◽  
Physical Properties ◽  
Weight Distribution ◽  
Rubber Latex ◽  
Mature Trees ◽  
Branching Points ◽  
Rubber Particles ◽  
Linear Molecules

Abstract The rubbers from small and large rubber particles were separated from the serum and cream phases of centrifuged fresh Hevea latex, respectively. The small rubber particles (SRP) consist of rubber molecules showing a unimodal molecular weight distribution (MWD), with a peak-top value falling between the two peak values of the bimodal MWD observed in rubbers from the large rubber particles (LRP). The rubbers in LRP from mature trees and SRP from seedlings are polyisoprene molecules terminated by phospholipids comprising branching points, while those in SRP from mature trees are presumed to be linear molecules containing no phospholipid terminal. The serum rubber was found to be soft and viscous rubber differing from the cream rubber containing in LRP. Thus, it can be presumed that the particle size of rubber latex is an important parameter influencing the MW, MWD and physical properties of Hevea rubber.

scholarly journals Fatty Acid Composition of Eucommia Rubber Particles, Content and Molecular Weight Distribution of Eu-rubber in Eucommia Ulmoides Leaves and Samaras

2020 ◽  
Author(s):  
Li-Jun Qin ◽  
Ting Yuan ◽  
ZhengWei Yang ◽  
DeGang Zhao
Keyword(s):  
Molecular Weight ◽  
Fatty Acid Composition ◽  
Fatty Acid ◽  
Molecular Weight Distribution ◽  
Acid Composition ◽  
Weight Distribution ◽  
Lipid Membrane ◽  
Eucommia Ulmoides ◽  
Rubber Biosynthesis ◽  
Rubber Particles

Abstract [Backgroud] Eucommia ulmoides rubber particles (EuRPs) wrapped by a monolayer protein-lipid membrane are speculated to an important site for the biosynthesis of E. ulmoides rubber (Eu-rubber), and the types and contents of these protein and lipid varied in different E. ulmoides organs which might indicate the difference of Eu-rubber biosynthesis in EuRPs from different organs and tissues. In this paper, the morphological structure and fatty acid composition of EuRPs in leaves and samaras collected in October, 2019 were compared, and molecular weight (Mw) and molecular weight distribution (MWD) were also analyed by GPC system. [Results] The results showed that small rubber particles (SRPs, 1~4 μm) and medium rubber particles (MRPs, 4~7 μm) from leaves and samaras respectively accounted for 39%, 53% and 38%, 34%, while the amount of the large rubber particles (LRPs, 7~11 μm) were 23% and 13%, which indicated there was no significance in leaf and samara SRPs while the amount of MRPs and LRPs in leaf both were more than that of in samara. Synchronously, the determination results of fatty acids (FAs) compositon showed three of the 35 kinds FAs were uniquely detected in leaf rubber particles (LeRPs), which included eleven carbonic (C11:0), thirteen carbonic (C13:0) and arachidic acids (C20:0), while γ-linolenic acid (C18:3n6) was exclusively found in samara rubber particles (SaRPs) with a lower content. Additioanlly, five characteristic distribution peaks (CDPs) of Eu-rubber from leaves were detected while there were only 2-3 CDPs in samaras; Further, the MWD analysis suggested that the MWD of Eu-rubber in E. ulmoides samaras and leaves were 40.61 and 6.76 respectively, which indicated the more Eu-rubber molecules with various Mw existed in samara compared with those in leaf. [Conclusions] Together, we concluded that average size of LeRPs was bigger than SaRPs in October, and 3 kinds of saturated fatty acid (SAF) were unique to the leaves while only one of unsaturated fatty acid (UFA) was sole to SaRPs. Simultaneously, the Eu-rubber in samara in October possessed more rubber molecules with various molecular weights compared with the contemporaneous leaves.

The Specific Gravity of Rubber in Hevea Latex

1940 ◽  
Vol 13 (1) ◽  
pp. 130-132
Author(s):  
O. de Vries
Keyword(s):  
Specific Gravity ◽  
Reasonable Agreement ◽  
Serum Proteins ◽  
Interesting Problem ◽  
Prolonged Action ◽  
Rubber Content ◽  
Rubber Particles ◽  
Average Figure ◽  
Acid Coagulation ◽  
Corrected Figure

Abstract Since my former communication on this subject several papers have appeared which form valuable contributions to the interesting problem of the actual specific gravity of the rubber particles in (original or preserved) Hevea latex. Rhodes recalculated his data, and came to an average figure of 0.9064 for the specific gravity of rubber in ammoniated latex, preserved during several weeks in the East; the rubber content being determined after coagulation by acetic acid in the usual way. Using the term proposed in my former paper, this may be called the specific gravity of “crepe rubber in preserved latex”, which may differ from that of “crepe rubber in original latex” by the effect of possible changes by the prolonged action of ammonia, by the settling out of the sludge (ammonium magnesium phosphate, mixed with protein-like substances), and other changes that occur in preserved latex. Leaving these unknown factors out of consideration, we have to take into account the fact that the rubber content was determined by acid coagulation, which means that a certain amount of serum substances, principally proteins, was precipitated with the rubber, and may have influenced its specific gravity. In my former communication I have shown that the specific gravity of “crepe rubber in latex” is found lower, the higher the rubber content of the original latex, i.e., the smaller the ratio of serum to rubber, and the smaller, therefore, the amount of acid-precipitated serum proteins in per cent of the rubber. Plotting, in this line of thought, Rhodes' figure in the graph (Figure 1), it will be seen that it corresponds to 45 grams rubber per 100 cc, or about 46.6 in percentage of weight, not abnormal for a preserved latex. Protein, precipitated with the rubber, has a still smaller effect in another figure given by Rhodes, namely, the specific gravity of rubber from a centrifuged cream of 56.7 per cent rubber content (acid coagulation). The corrected figure for the specific gravity of this rubber is given by Rhodes as 0.9011, which in Figure 1 corresponds to 55.5 grams of rubber per 100 cc, or about 58.5 per cent by weight; this is in reasonable agreement with the real figure of 56.7, taking into account the unavoidable errors of an extrapolation, such as in Figure 1.

Valence electron spectroscopy of inhomogeneous media

1992 ◽  
Vol 50 (2) ◽  
pp. 1150-1151
Author(s):  
A. Howie ◽  
D.W. McComb
Keyword(s):  
Specific Gravity ◽  
Loss Function ◽  
Electron Spectroscopy ◽  
Valence Electron ◽  
Peak Height ◽  
Loss Functions ◽  
Loss Peak ◽  
High Energies ◽  
Valence Electron Density ◽  
Electron Microscopist

The bulk loss function Im(-l/ε (ω)), a well established tool for the interpretation of valence loss spectra, is being progressively adapted to the wide variety of inhomogeneous samples of interest to the electron microscopist. Proportionality between n, the local valence electron density, and ε-1 (Sellmeyer's equation) has sometimes been assumed but may not be valid even in homogeneous samples. Figs. 1 and 2 show the experimentally measured bulk loss functions for three pure silicates of different specific gravity ρ - quartz (ρ = 2.66), coesite (ρ = 2.93) and a zeolite (ρ = 1.79). Clearly, despite the substantial differences in density, the shift of the prominent loss peak is very small and far less than that predicted by scaling e for quartz with Sellmeyer's equation or even the somewhat smaller shift given by the Clausius-Mossotti (CM) relation which assumes proportionality between n (or ρ in this case) and (ε - 1)/(ε + 2). Both theories overestimate the rise in the peak height for coesite and underestimate the increase at high energies.

Classifying Disfluencies in Preschool- and School-Age Spanish–English Bilinguals Who Do Not Stutter: An Exploratory Study

2020 ◽  
Vol 5 (1) ◽  
pp. 119-130
Author(s):  
Raúl Rojas ◽  
Farzan Irani
Keyword(s):  
Exploratory Study ◽  
Evidence Base ◽  
Medium Effect ◽  
Bilingual Children ◽  
Linear Regression Models ◽  
Cross Sectional ◽  
Total Percentage ◽  
Narrative Production ◽  
Language Measures ◽  
Mean Length Of Utterance

Purpose This exploratory study examined the language skills and the type and frequency of disfluencies in the spoken narrative production of Spanish–English bilingual children who do not stutter. Method A cross-sectional sample of 29 bilingual students (16 boys and 13 girls) enrolled in grades prekindergarten through Grade 4 produced a total of 58 narrative retell language samples in English and Spanish. Key outcome measures in each language included the percentage of normal (%ND) and stuttering-like (%SLD) disfluencies, percentage of words in mazes (%MzWds), number of total words, number of different words, and mean length of utterance in words. Results Cross-linguistic, pairwise comparisons revealed significant differences with medium effect sizes for %ND and %MzWds (both lower for English) as well as for number of different words (lower for Spanish). On average, the total percentage of mazed words was higher than 10% in both languages, a pattern driven primarily by %ND; %SLDs were below 1% in both languages. Multiple linear regression models for %ND and %SLD in each language indicated that %MzWds was the primary predictor across languages beyond other language measures and demographic variables. Conclusions The findings extend the evidence base with regard to the frequency and type of disfluencies that can be expected in bilingual children who do not stutter in grades prekindergarten to Grade 4. The data indicate that %MzWds and %ND can similarly index the normal disfluencies of bilingual children during narrative production. The potential clinical implications of the findings from this study are discussed.

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